Classifications

• • H—ELECTRICITY
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  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/02104—Forming layers
  • H01L21/02365—Forming inorganic semiconducting materials on a substrate
  • H01L21/02518—Deposited layers
  • H01L21/0257—Doping during depositing
  • H01L21/02573—Conductivity type
  • H01L21/02576—N-type
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
  • C23C16/22—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the deposition of inorganic material, other than metallic material
  • C23C16/30—Deposition of compounds, mixtures or solid solutions, e.g. borides, carbides, nitrides
  • C23C16/40—Oxides
  • C23C16/407—Oxides of zinc, germanium, cadmium, indium, tin, thallium or bismuth
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
  • C23C16/44—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating
  • C23C16/455—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating characterised by the method used for introducing gases into reaction chamber or for modifying gas flows in reaction chamber
  • C23C16/45502—Flow conditions in reaction chamber
  • C23C16/45504—Laminar flow
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
  • C23C16/44—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating
  • C23C16/455—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating characterised by the method used for introducing gases into reaction chamber or for modifying gas flows in reaction chamber
  • C23C16/45523—Pulsed gas flow or change of composition over time
  • C23C16/45525—Atomic layer deposition [ALD]
  • C23C16/45544—Atomic layer deposition [ALD] characterized by the apparatus
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
  • C23C16/44—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating
  • C23C16/455—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating characterised by the method used for introducing gases into reaction chamber or for modifying gas flows in reaction chamber
  • C23C16/45523—Pulsed gas flow or change of composition over time
  • C23C16/45525—Atomic layer deposition [ALD]
  • C23C16/45544—Atomic layer deposition [ALD] characterized by the apparatus
  • C23C16/45548—Atomic layer deposition [ALD] characterized by the apparatus having arrangements for gas injection at different locations of the reactor for each ALD half-reaction
  • C23C16/45551—Atomic layer deposition [ALD] characterized by the apparatus having arrangements for gas injection at different locations of the reactor for each ALD half-reaction for relative movement of the substrate and the gas injectors or half-reaction reactor compartments
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
  • C23C16/44—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating
  • C23C16/455—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating characterised by the method used for introducing gases into reaction chamber or for modifying gas flows in reaction chamber
  • C23C16/45563—Gas nozzles
  • C23C16/45574—Nozzles for more than one gas
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/02104—Forming layers
  • H01L21/02365—Forming inorganic semiconducting materials on a substrate
  • H01L21/02518—Deposited layers
  • H01L21/02521—Materials
  • H01L21/02551—Group 12/16 materials
  • H01L21/02554—Oxides
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/02104—Forming layers
  • H01L21/02365—Forming inorganic semiconducting materials on a substrate
  • H01L21/02518—Deposited layers
  • H01L21/02521—Materials
  • H01L21/02565—Oxide semiconducting materials not being Group 12/16 materials, e.g. ternary compounds
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/02104—Forming layers
  • H01L21/02365—Forming inorganic semiconducting materials on a substrate
  • H01L21/02612—Formation types
  • H01L21/02617—Deposition types
  • H01L21/0262—Reduction or decomposition of gaseous compounds, e.g. CVD
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B33—ADDITIVE MANUFACTURING TECHNOLOGY
  • B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3-D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3-D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
  • B33Y80/00—Products made by additive manufacturing

Definitions

• This invention generally relates to the deposition of thin film zinc-oxide- based materials and, more particularly, to a process using atomic layer deposition.
• the present invention relates to a process of making a film of an indium-doped zinc-oxide-based semiconductor material for thin film transistors, which can be used in electronic devices, particularly in displays, the making of which at low temperature and near atmospheric pressure is advantageous.
• Thin film transistors are widely used as switching elements in electronics, for example, in active-matrix liquid-crystal displays, smart cards, and a variety of other electronic devices and components thereof.
• the thin film transistor is an example of a field effect transistor (FET).
• FET field effect transistor
• MOSFET Metal-Oxide-Semiconductor-FET
• a critical step in fabricating the thin film transistor involves the deposition of a semiconductor onto the substrate.
• most thin film devices are made using vacuum deposited amorphous silicon as the semiconductor.
• Amorphous silicon as a semiconductor for use in TFTs still has its drawbacks.
• the deposition of amorphous silicon, during the manufacture of transistors, requires relatively difficult or complicated processes such as plasma enhanced chemical vapor deposition and high temperatures (typically 360 0 C) to achieve the electrical characteristics sufficient for display applications.
• high processing temperatures disallow deposition on substrates made of certain plastics that might otherwise be desirable for use in applications such as flexible displays.
• a useful example of a flexible substrate is polyethylene terephthalate. Such plastics, however, limit device processing to below 200 0 C.
• thin film transistors made of easily deposited semiconductor materials can be viewed as a potential key technology for circuitry in various electronic devices or components such as display backplanes, portable computers, pagers, memory elements in transaction cards, and identification tags, where ease of fabrication, mechanical flexibility, and/or moderate operating temperatures are important considerations.
• metal oxide semiconductors are known that constitute zinc oxide, indium oxide, gallium indium zinc oxide, tin oxide, or cadmium oxide deposited with or without additional doping elements including metals such as aluminum.
• Such semiconductor materials which are transparent, can have an additional advantage for certain applications, as discussed below.
• a semiconductor material useful in a TFT must display several characteristics. In typical applications of a thin film transistor, the desire is for a switch that can control the flow of current through the device. As such, it is desired that when the switch is turned on a high current can flow through the device. The extent of current flow is related to the semiconductor charge carrier mobility.
• the current flow be very small. This is related to the native charge carrier concentration. Furthermore, it is desired that the device be weakly or not at all influenced by visible light to avoid a light-protection layer. In order for this to be true, the semiconductor band gap must be sufficiently large (> 3 eV) so that exposure to visible light does not cause an inter-band transition. Zinc oxide based materials are capable of delivering these features. Furthermore, in a real high volume web based atmospheric manufacturing scheme, it is highly desirable that the chemistries used in the process be both cheap and of low toxicity, which can be satisfied also by the use of ZnO-based materials and the majority of its precursors.
• Another transistor characteristic that is relevant for useful operation is the steepness of the transistor turn on, represented by the subthreshold slope.
• the subthreshold slope As the gate voltage of a transistor is varied, the transistor will start in an off state, characterized by low current flow, and transition to an on state, characterized by high current flow.
• This increase called the subthreshold slope, is measured in volts of gate voltage per decade of drain current. This expression therefore represents the number of volts of gate voltage required to produce a 10-fold increase in drain current. Lower values of the subthreshold slope indicate faster device turn on and are desirable.
• Various processes for making zinc oxide films have been disclosed, both high temperature and low temperature processes, including radio frequency magnetron sputtering or modified reactive planar magnetron sputtering.
• Ohya et al. Japanese Journal of Applied Physics, Part 1 , January 2001, vol. 40, no.1 , pages 297-8 disclose a thin film transistor of ZnO fabricated by chemical solution deposition.
• Transparent conducting oxides are reviewed in the August 2000 issue of the Materials Research Bulletin, Volume 25 (8) 2000, devoted to materials and properties of transparent conducting oxide compounds.
• One low temperature process for deposition of such oxide semiconductors is disclosed in US 2004/0127038 to Carcia et al.
• This publication discloses a semiconductor deposition process that uses magnetron sputtering of a metal oxide (ZnO, In 2 O 3 , SnO 2 , CdO) or metal (Zn, In, Sn, Cd) target in an atmosphere with a controlled partial pressure of oxygen in an inert gas.
• This is a low temperature process that is compatible with temperature sensitive substrates and components, for example, drive circuits for displays on flexible, polymer substrates.
• ALD Atomic layer deposition
• ALD can be used as a fabrication step for forming a number of types of thin-film electronic devices, including semiconductor devices and supporting electronic components such as resistors and capacitors, insulators, bus lines, and other conductive structures.
• ALD is particularly suited for forming thin layers of metal oxides in the components of electronic devices.
• General classes of functional materials that can be deposited with ALD include conductors, dielectrics or insulators, and semiconductors.
• Examples of useful semiconducting materials are compound semiconductors such as gallium arsenide, gallium nitride, cadmium sulfide, zinc oxide, and zinc sulfide.
• a capacitor results from placing a dielectric between two conductors.
• a diode results from placing two semiconductors of complementary carrier type between two conducting electrodes. There may also be disposed between the semiconductors of complementary carrier type a semiconductor region that is intrinsic, indicating that that region has low numbers of free charge carriers.
• a diode may also be constructed by placing a single semiconductor between two conductors, where one of the conductor/semiconductors interfaces produces a Schottky barrier that impedes current flow strongly in one direction.
• a transistor results from placing upon a conductor (the gate) an insulating layer followed by a semiconducting layer. If two or more additional conductor electrodes (source and drain) are placed spaced apart in contact with the top semiconductor layer, a transistor can be formed. Any of the above devices can be created in various configurations as long as the critical interfaces are created.
• ALD steps are self-terminating and can deposit precisely one atomic layer when conducted up to or beyond self-termination exposure times.
• An atomic layer typically ranges from 0.1 to 0.5 molecular monolayers, with typical dimensions on the order of no more than a few Angstroms.
• deposition of an atomic layer is the outcome of a chemical reaction between a reactive molecular precursor and the substrate.
• the net reaction deposits the desired atomic layer and substantially eliminates "extra" atoms originally included in the molecular precursor.
• ALD involves the adsorption and reaction of each of the precursors in the complete absence of the other precursor or precursors of the reaction.
• a metal precursor molecule comprises a metal element, M that is bonded to an atomic or molecular ligand, L.
• M could be, but would not be restricted to, Al, W, Ta, Si, Zn, etc.
• the metal precursor reacts with the substrate when the substrate surface is prepared to react directly with the molecular precursor.
• the substrate surface typically is prepared to include hydrogen-containing ligands, AH or the like, that are reactive with the metal precursor. Sulfur (S), oxygen (O), and Nitrogen (N) are some typical A species.
• S sulfur
• O oxygen
• N Nitrogen
• the gaseous precursor molecule effectively reacts with all of the ligands on the substrate surface, resulting in deposition of a single atomic layer of the metal: substrate-AH+ML x ⁇ substrate-AML x- ⁇ +HL
• HL is a reaction by-product.
• the initial surface ligands, AH are consumed, and the surface becomes covered with AML x-I ligands, which cannot further react with metal precursor ML x . Therefore, the reaction self- terminates when all of the initial AH ligands on the surface are replaced with AML x-I species.
• the reaction stage is typically followed by an inert-gas purge stage that eliminates the excess metal precursor and the HL by-product species from the chamber prior to the separate introduction of the other precursor.
• a second molecular precursor then is used to restore the surface reactivity of the substrate towards the metal precursor. This is done, for example, by removing the L ligands and re-depositing AH ligands.
• the second precursor typically comprises the desired (usually nonmetallic) element A (i.e., O, N, S), and hydrogen (i.e., H 2 O, NH 3 , H 2 S).
• desired element A i.e., O, N, S
• hydrogen i.e., H 2 O, NH 3 , H 2 S.
• an ALD process requires alternating in sequence the flux of chemicals to the substrate.
• the representative ALD process as discussed above, is a cycle having four different operational stages:
• ALD deposition cycle This repeated sequence of alternating surface reactions and precursor- removal that restores the substrate surface to its initial reactive state, with intervening purge operations, is a typical ALD deposition cycle.
• a key feature of ALD operation is the restoration of the substrate to its initial surface chemistry condition.
• a film can be layered onto the substrate in equal metered layers that are all identical in chemical kinetics, deposition per cycle, composition, and thickness.
• Self-saturating surface reactions make ALD insensitive to transport non- uniformities, which might otherwise impair surface uniformity, due either to engineering tolerances and the limitations of the flow process or related to surface topography (that is, deposition into three dimensional, high aspect ratio structures).
• ALD atomic layer deposition
• a non-uniform flux of chemicals in a reactive process generally results in different completion times at different areas.
• each of the reactions is allowed to complete on the entire substrate surface.
• differences in completion kinetics impose no penalty on uniformity. This is because the areas that are first to complete the reaction self- terminate the reaction; other areas are able to continue until the full treated surface undergoes the intended reaction.
• an ALD process deposits 0.1 - 0.2 nm of a film in a single ALD cycle (with numbered steps 1 through 4 as listed earlier).
• a useful and economically feasible cycle time must be achieved in order to provide a uniform film thickness in a range of from 3 nm to 300 nm for many or most semiconductor applications, and even thicker films for other applications.
• Industry throughput standards dictate that substrates be processed in 2 minutes to 3 minutes, which means that ALD cycle times must be in a range from 0.6 seconds to 6 seconds.
• An ALD process must be able to execute this sequencing efficiently and reliably for many cycles in order to allow cost-effective coating of many substrates.
• pulsed ALD a substrate sits in a chamber and is exposed to the above sequence of gases by allowing a first gas to enter the chamber, followed by a pumping cycle to remove that gas, followed by the introduction of a second gas to the chamber, followed by a pumping cycle to remove the second gas.
• This sequence can be repeated at any frequency and variations in gas type and/or concentration.
• the net effect is that the entire chamber experiences a variation in gas composition with time, and thus this type of ALD can be referred to as time dependent ALD.
• time dependent ALD The vast majority of existing ALD processes are time dependent ALD.
• U.S. Patent No. 6,821,563 entitled “GAS DISTRIBUTION SYSTEM FOR CYCLICAL LAYER DEPOSITION” to Yudovsky describes a spatially dependent ALD processing system, under vacuum, having separate gas ports for precursor and purge gases, alternating with vacuum pump ports between each gas port.
• Each gas port directs its stream of gas vertically downward toward a substrate.
• the separate gas flows are separated by walls or partitions, with vacuum pumps for evacuating gas on both sides of each gas stream.
• a lower portion of each partition extends close to the substrate, for example, 0.5 mm or greater from the substrate surface. In this manner, the lower portions of the partitions are separated from the substrate surface by a distance sufficient to allow the gas streams to flow around the lower portions toward the vacuum ports after the gas streams react with the substrate surface.
• a rotary turntable or other transport device is provided for holding one or more substrate wafers.
• the substrate is shuttled beneath the different gas streams, effecting ALD deposition thereby.
• the substrate is moved in a linear path through a chamber, in which the substrate is passed back and forth a number of times.
• a gas flow array is provided with alternating source gas openings, carrier gas openings, and vacuum exhaust openings. Reciprocating motion of the substrate over the array effects ALD deposition, again, without the need for pulsed operation.
• sequential interactions between a substrate surface and reactive vapors are made by a reciprocating motion of the substrate over a fixed array of source openings.
• Diffusion barriers are formed by a carrier gas opening between exhaust openings.
• U.S. Patent Publication No. 2005/0084610 to Selitser discloses an atmospheric pressure atomic layer chemical vapor deposition process. Selitser states that extraordinary increases in reaction rates are obtained by changing the operating pressure to atmospheric pressure, which will involve orders of magnitude increase in the concentration of reactants, with consequent enhancement of surface reactant rates.
• the embodiments of Selitser involve separate chambers for each stage of the process, although Figure 10 shows an embodiment in which chamber walls are removed.
• a series of separated injectors are spaced around a rotating circular substrate holder track. Each injector incorporates independently operated reactant, purging, and exhaust gas manifolds and controls and acts as one complete mono-layer deposition and reactant purge cycle for each substrate as is passes there under in the process.
• a spatially dependent ALD process can be accomplished with other apparatus or systems described in more detail in commonly assigned US Application No. 11/392,007, US Application No. 1 1/392,006, US Application No. 11/620,744, and US Application No. 1 1/620,740.
• These systems attempt to overcome one of the difficult aspects of a spatial ALD system, which is undesired intermixing of the continuously flowing mutually reactive gases.
• US Application No. 1 1/392,007 employs a novel transverse flow pattern to prevent intermixing
• US Application No. 1 1/620,744 and US Application No. 11/620,740 employ a coating head partially levitated by the pressure of the reactive gases of the process to accomplish improved gas separation.
• amorphous films are more robust than poly-crystalline or crystalline films.
• the advantages over polycrystalline films are a decreased surface roughness and an absence of grain boundaries that act as both charge trapping sites and diffusion pathways. While single crystalline films would generally have the highest mobilities, the deposition of a film that is single crystalline over a large area is very difficult, especially if flexibility in the choice of substrate is desired.
• Amorphous zinc oxide-based semiconductors are especially advantaged because the penalty in carrier mobility going from the crystalline to the amorphous state is much less compared to crystalline and amorphous silicon.
• the conduction band of ZnO is comprised of s-type wave functions that are spherically symmetric, whereas p-type wave functions make up the conduction band of Si.
• Indium-doped zinc-oxide films and indium zinc oxides (IZOs) have received attention both as conductive transparent oxides and as transparent semiconducting materials for use in devices such as thin film transistors (TFTs).
• TFTs thin film transistors
• these types of films are typically made by vacuum processes such as rf magnetron sputtering.
• a common approach to chemically modifying the nature of a semiconductor is to include in the matrix of the semiconductor other atoms or molecules which vary the electrical properties of the semiconductor. These additional atoms typically operate by accepting or donating mobile charge to the system.
• acceptor dopants can be used to trap electrons, thus driving the semiconductor to have increased hole concentration and toward a p-type semiconductor.
• donor dopants tend to release electrons and in a semiconductor like zinc oxide this can produce high numbers of conduction band electrons, a desirable effect for applications requiring conductivity.
• Indium is a donor dopant for a zinc-oxide-based material.
• incorporating both donor and acceptor dopants into a zinc-oxide-based material can increase the control one has in selecting carrier type and concentration.
• Chen et al. found that for zinc oxide films co-doped with indium and nitrogen they could vary the films from p-type to n-type by controlling the substrate temperature (Chen, et al., Applied Physics Letters 89, 2521 13 (2006)).
• Chen et al. used direct current (DC) reactive magnetron sputtering for their experiments, varying the substrate temperatures between 44O 0 C and 600 0 C. It is, thus, known that indium incorporation in zinc-oxide-based films can provide the benefit of decreased coating crystallinity and increased conductivity. There remains a need to provide a useful and cost effect process for producing indium-doped a zinc-oxide-based films.
• defects in the materials may lead to two deleterious effects. First, such materials may tend to have too high a concentration of native charge carrier, leading to high off currents. Secondly, such materials may tend to contain defect sites that can cause bulk charging effects, leading to poor subthreshold slope.
• the present invention provides doping methods that permit zinc-oxide-based semiconductor materials produced in a spatially dependent ALD system to effectively work in practice and perform more robustly.
• the present invention relates to a process of making an indium-doped n-type zinc-oxide-based thin film semiconductor by depositing a thin film material on a given surface area of a substrate (either an entire surface area or a selected portion thereof) wherein the deposition process is an atomic layer deposition process that is carried out substantially at or above atmospheric pressure, wherein the temperature of the substrate during deposition is under 300 0 C, and wherein the atomic layer deposition process comprises simultaneously directing a series of gas flows along elongated substantially parallel channels comprising, in order, at least a first reactive gaseous material containing a zinc- containing compound, an inert purge gas, and a second reactive gaseous material through a plurality of output openings spaced apart from the substrate, and transporting the substrate in a direction relative to the plurality of output openings, such that any point on the surface area of the substrate experiences a sequence of the first, second and third gaseous materials, whereby the sequence causes a thin film to be formed by
• the process can be operated with continuous movement of a substrate being subjected to thin film deposition, wherein the process is capable of conveying the support on or as a web past the deposition device, preferably in an unsealed environment to ambient at substantially atmospheric pressure. It is an advantage of the present invention that it provides an advantageous process for atomic layer deposition of indium doped zinc-oxide- based thin film onto a substrate, well suited to a number of different types of substrates and deposition environments. It is yet a further advantage of the present invention that it is adaptable for deposition on a web or other moving substrate, including deposition onto a large area substrate.
• the invention is also directed to a transistor comprising a zinc-oxide-based semiconductor, preferably on a flexible substrate, made by the present process.
• Semiconductor films made by the present method are capable of exhibiting, in the film form, excellent field-effect electron mobilities of greater than 0.1 cm 2 /Vs and on-off ratios of greater than 10 4 , in which performance properties are sufficient for use in a variety of relevant technologies, including active matrix display backplanes.
• a TFT structure includes, in addition to the zinc-oxide-based semiconductor, conducting electrodes, commonly referred to as a source and a drain, for injecting a current into the zinc-oxide-based semiconductor.
• the n- channel semiconductor films made by the present invention can be used in thin film transistors each comprising spaced apart first and second contact means connected to an n-channel semiconductor film.
• a third contact means can be spaced from said semiconductor film by an insulator, and adapted for controlling, by means of a voltage applied to the third contact means, a current between the first and second contact means through said film.
• the first, second, and third contact means can correspond to a drain, source, and gate electrode in a field effect transistor.
• Figure 1 is a flow chart describing the steps of the present process
• Figure 2 is a cross-sectional side view of one embodiment of a deposition device for atomic layer deposition that can be used in the present process
• Figure 3 is a cross-sectional side view of an embodiment, for one exemplary system of gaseous materials, of the distribution of gaseous materials to a substrate that is subject to thin film deposition;
• Figures 4A and 4B are cross-sectional side views of one embodiment of the distribution of a system of gaseous materials, schematically showing the accompanying deposition operation;
• Figure 5 is a perspective view, from the output face side, of a portion of one embodiment of a deposition device, showing the orientation of output channels relative to the substrate and reciprocating motion, which can be used for deposition on the substrate;
• Figure 6 is a perspective view as in Figure 5, showing one exemplary arrangement of gas flow in the deposition device
• Figures 7A, 7B, 7C, and 7D are cross-sectional views taken orthogonally to the cross-sectional views of previous Figures 2-4B, showing gas flow directions for output channels in various embodiments;
• Figures 8A and 8B are cross-sectional views showing the impact of reduced distance from the output face, of a deposition device for providing gaseous materials, to the substrate surface;
• Figure 9 is a plan view of the output face of a deposition device such as shown in Figure 3 that can be used in the present process, showing movement of gaseous materials through an arrangement of output channels according to one embodiment of the invention
• Figure 10 is a perspective view of one embodiment of a deposition device formed from stacked plates that can be used in the present process;
• Figures 1 I A and 1 IB are exploded views of the construction of a deposition device that employs the stacked-plate structure shown in Figure 10, the exploded views showing two different output channels for different gases used in the process;
• Figure 12 is a perspective view showing an embodiment using an enveloping shroud of inert gas directed along the perimeter of the deposition device;
• Figure 13 is a schematic diagram showing an alternative motion pattern for reciprocating and orthogonal movement;
• Figure 14 is a block diagram of one embodiment of a deposition system that uses the process according to the present invention.
• Figure 15 is a block diagram showing one embodiment of a deposition system applied to a moving web in accordance with the present process
• Figure 16 is a block diagram showing another embodiment of a deposition system applied to a moving web in accordance with the present process, with the deposition device stationary;
• Figure 17 is a cross-sectional side view of one embodiment of a deposition device with an output face having curvature that can be used in the present process;
• Figure 18 is a block diagram of the source materials for a thin film deposition process according to the Examples.
• Figure 19 is a cross-sectional side view of a deposition device used in the present process, showing the arrangement of gaseous materials provided to a substrate that is subject to the thin film deposition process of the Examples;
• Figure 20 illustrates a cross-sectional view of a typical thin film transistor having a bottom-gate/bottom-contact configuration
• Figure 21 illustrates a cross-sectional view of a typical thin film transistor having a bottom-gate/top-contact configuration
• Figure 22 illustrates a cross-sectional view of a typical thin film transistor having a top-gate/bottom-contact structure
• Figure 23 illustrates a cross-sectional view of a typical thin film transistor having a top-gate/top-contact structure
• Figure 24 illustrates a typical active matrix pixel design comprising a select transistor and capacitor representing the capacitance due to display design
• Figure 25 illustrates a typical pixel layout comprising data lines, control lines, thin film transistors, and pixel conductor pads;
• Figure 26 shows overlaid X-ray diffraction spectra of a series of Indium- doped ZnO films made using the atmospheric pressure ALD process herein at a range of Indium dopant levels, wherein at the highest Indium dopant levels the peak intensity is diminished as the crystallinity of the ZnO film approaches an amorphous state, as described in the Examples;
• Figure 27 shows comparative changes in the CESR spectra of a series of Indium-doped ZnO layers as the Indium level was varied from zero to a higher level, as described in the Examples.
• the semiconductor films made according to the present method exhibit a field effect electron mobility that is greater than 0.01 cm 2 /Vs, preferably at least 0.1 cm 2 /Vs, more preferably greater than 1 cm 2 /Vs.
• n-channel semiconductor devices made according to the present invention are capable of providing on/off ratios of at least 10 4 , advantageously at least 10 5 or 10 6 .
• the device be weakly or not at all influenced by visible light.
• the semiconductor band gap must be sufficiently large (> 3 eV) so that exposure to visible light does not cause an inter- band transition.
• Zinc-oxide-based materials are capable of yielding a high mobility, low carrier concentration, and high band gap.
• the chemistries used in the process be both cheap and of low toxicity, which can be satisfied by the use of ZnO-based materials and the majority of its precursors.
• the present invention employs an n-type zinc-oxide-based thin film semiconductor that is deposited as a thin film material on a surface area of a substrate, either an entire surface area or a selected portion thereof, by an atomic layer deposition process.
• the atomic layer deposition process comprises simultaneously directing a series of gas flows along elongated substantially parallel channels comprising, in order, at least a first reactive gaseous material containing a zinc-containing compound, an inert purge gas, and a second reactive gaseous material through a plurality of output openings spaced apart from the substrate, and transporting the substrate in a direction relative to the plurality of output openings, such that any point on the surface area of the substrate experiences a sequence of the first, second and third gaseous materials.
• a volatile indium-containing compound is introduced into the first reactive gaseous material or a supplemental gaseous material such that the indium-containing compound has a molar flow at a level of greater than 20 percent of the molar flow of the zinc-containing compound.
• the weight ratio of zinc to indium in the zinc-oxide-based thin film semiconductor is 0.05 to 0.7, preferably 0.2 to 0.5.
• the introduction of the indium results in the crystallinity of the zinc-oxide-based film being less than 85 percent as measured by x-ray diffraction.
• the volatile indium-containing compound suitably can be a trialkylindium compound, such as trimethylindium, or cyclopentadienylindium(I), wherein the alkyl has 1 to 4 carbon atoms, preferably 1 to 2 carbon atoms.
• the resistivity of the zinc oxide based film can be reduced relative to the resistivity of a similar film not containing indium.
• the presence of incorporated indium dopant derived from an indium-containing compound or precursor can provide a decrease in resistivity by a factor of at least 10 in the thin film material. Consequently, the zinc-oxide-based thin film semiconductor can effectively serve as the channel layer of a thin film transistor or as one or more conductive electrodes of a thin film transistor such as a gate material.
• the zinc- oxide-based thin film semiconductor can also serve as an electrical conduit in an electronic circuit in some applications such as displays.
• the zinc-oxide-based semiconductor material can contain minor amounts of other metals capable of forming semiconducting oxides such as tin or cadmium, and combinations thereof.
• other metals capable of forming semiconducting oxides such as tin or cadmium, and combinations thereof.
• Chiang, H. Q. et al. "High mobility transparent thin-film transistors with amorphous zinc tin oxide channel layer," Applied Physics Letters 86, 013503 (2005) discloses zinc tin oxide materials.
• the term "zinc-oxide-based” refers to a composition comprising mostly zinc and indium as the metal, but (excluding oxygen) allowing up to 20 percent by weight of other elements or dopants , as will be appreciated by the skilled artisan.
• the highest In/Zn ratio in the coated indium-doped zinc oxide layer herein is 45 weight % (mass of In atoms to Zn atoms).
• the Indium-doped zinc oxide may have a different preferable concentration of Indium, depending on whether the material is used as a conductive material or a semiconductive material, for example, a conductive electrode or the semiconductor of a transistor.
• the preferable In/Zn ratio by weight for a semiconductor in a transistor is 10 weight % Indium, and the preferable In/Zn ratio for a conductive material, for example a gate electrode, is 30 weight %.
• the preferable range of In/Zn weight ratio is between 5% and 60%, more preferably from 10 to 50 percent.
• Nominally undoped zinc oxide shows n-type conductivity because of the presence of shallow donors.
• Lattice imperfections due to a deviation from stoichiometry are thought to play a role in the creation of shallow donor states, although unintentionally incorporated hydrogen is also considered as a source of shallow donor states.
• These shallow donors have in common that they introduce occupied energy levels that are less than 100 meV below the conduction band edge of zinc oxide. Electrons occupying these shallow donor states can be promoted into the conduction band via thermal excitation and lead to intrinsic conductivity.
• the resistivity of the ZnO can be enhanced by substitutional doping with an acceptor dopant made from a volatile organic compound, for example, volatile compounds comprising an acceptor dopant such as N, P, As, Li, Na, K, Cu, Ag, or mixtures thereof.
• the acceptor dopant comprises a Group V element, more preferably nitrogen, for example, using an acceptor dopant precursor comprises nitrogen in the form of NO, N 2 O, NO 2 , or ammonia.
• an acceptor dopant has fewer valence electrons than the atom it replaces, whether a Group (V) element substituting for oxygen or a Group (IA) or (IB) element substituting for zinc.
• the on-off ratio of the semiconductor without an acceptor dopant is less than 1.0 x 10 6 .
• the presence of the acceptor dopant provides an increase in resistivity of at least 10 2 wherein the resistivity of the semiconductor without the acceptor dopant is less than 500 ohm*cm.
• the presence of an acceptor dopant also provides an increase in mobility of at least 25 percent, more preferably at least 50 percent.
• Volatile compounds used in the present invention are defined as compounds that can exist to some extent in the vapor phase at room temperature, so that they may be delivered to a reaction chamber in the form of a gas.
• the volatile compound In order for such a gas to contain sufficient volatile materials to usefully affect the deposition process, the volatile compound must have a vapor pressure at room temperature of greater than 0.1 mmHg, preferably greater than 1 mmHg.
• Dopants such as nitrogen, for example are preferably present in the final semiconductor in the amount of 0.001 % to 5%, more preferably 0.01 % to 1 %.
• An acceptor dopant is typically an impurity that introduces an energy level within the band-gap of ZnO-based materials that is below the energy level of shallow donors and is able to accept an electron.
• Such an acceptor dopant can be an impurity atom that has fewer valence electrons than the atom it replaces.
• ZnO-based materials are group (V) elements substituting for oxygen, e.g., nitrogen, phosphorous or arsenic or Group (I) and (IB) elements substituting for zinc, e.g. lithium, sodium, potassium, copper or silver. See U. ⁇ zgur, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doan, V.
• the thickness of the channel layer may vary, and according to particular examples it can range from 5 nm to 500 nm, 10 nm to 50 nm.
• the length and width of the channel is determined by the pixel size and the design rules of the system under construction. Typically, the channel width may vary from 10 to 1000 ⁇ m. The channel length may vary, and according to particular examples it can range from 1 to 100 ⁇ m.
• the entire process of making the thin film transistor or electronic device of the present invention, or at least the production of the thin film semiconductor, can be carried out below a maximum support temperature of 300° C, more preferably below 250° C, or even at temperatures around room temperature (25° C to 70 0 C).
• the temperature selection generally depends on the support and processing parameters known in the art, once one is armed with the knowledge of the present invention contained herein. These temperatures are well below traditional integrated circuit and semiconductor processing temperatures, which enables the use of any of a variety of relatively inexpensive supports, such as flexible polymeric supports. Thus, the invention enables production of relatively inexpensive circuits containing thin film transistors with significantly improved performance.
• gas or “gaseous material” is used in a broad sense to encompass any of a range of vaporized or gaseous elements, compounds, or materials.
• the figures provided are not drawn to scale but are intended to show overall function and the structural arrangement of some embodiments of the present invention.
• the process of the present invention offers a significant departure from conventional approaches to making a film of zinc-oxide-based n-type semiconductor, employing a system for delivery of gaseous materials to a substrate surface that can be adaptable to deposition on larger and web-based substrates and capable of achieving a highly uniform thin-film deposition at improved throughput speeds.
• the process of the present invention employs a continuous spatially dependent ALD (as opposed to pulsed or time dependent ALD) gaseous material distribution.
• the process of the present invention allows operation at atmospheric or near-atmospheric pressures and is capable of operating in an unsealed or open-air environment.
• Figure 1 is a generalized step diagram of one embodiment of a process for making a film of zinc-oxide-based n-type semiconductor according to an embodiment of the present invention, in which two reactive gases are used, a first molecular precursor and a second molecular precursor.
• Gases are supplied from a gas source and can be delivered to the substrate, for example, via a deposition device.
• Metering and valving apparatus for providing gaseous materials to the a deposition device can be used.
• Step 1 a continuous supply of gaseous materials for the system is provided for depositing a thin film of material on a substrate.
• the Steps in Sequence 15 are sequentially applied.
• Step 2 with respect to a given area of the substrate (referred to as the channel area), a first molecular precursor or reactive gaseous material is directed to flow in a first channel over the channel area of the substrate and reacts therewith.
• Step 3 relative movement of the substrate and the multi-channel flows in the system occurs, which sets the stage for Step 4, in which second channel (purge) flow with inert gas occurs over the given channel area.
• Step 5 relative movement of the substrate and the multi-channel flows sets the stage for Step 6, in which the given channel area is subjected to atomic layer deposition in which a second molecular precursor now (in this particular embodiment, transversely and substantially parallel to the surface of the substrate) over the given channel area of the substrate and reacts with the previous layer on the substrate to produce (theoretically) a monolayer of a desired material.
• a first molecular precursor is a zinc-containing compound in gas form
• the material deposited is a zinc-containing compound, for example, an organozinc compound such as diethylzinc.
• the second molecular precursor can be, for example, a non-metallic oxidizing compound.
• a volatile acceptor doping compound can be supplied to the system in any way in which allows it to react to sites in the growing film.
• it can be delivered with the first or second molecular precursors, or with the purge gas.
• the volatile acceptor is delivered in common with the oxidizing molecular precursor.
• Step 7 relative movement of the substrate and the multi-channel flows then sets the stage for Step 8 in which again an inert gas is used, this time to sweep excess second molecular precursor from the given channel area from the previous Step 6.
• Step 9 relative movement of the substrate and the multichannels occurs again, which sets the stage for a repeat sequence, back to Step 2.
• the cycle is repeated as many times as is necessary to establish a desired film.
• the steps are repeated with respect to a given channel area of the substrate, corresponding to the area covered by a flow channel.
• the various channels are being supplied with the necessary gaseous materials in Step 1.
• Simultaneous with the sequence of box 15 in Figure 1 other adjacent channel areas are being processed, which results in Multiple channel flows in parallel, as indicated in overall Step 1 1.
• Parallel flow can be either substantially orthogonal or substantially parallel to the output face of the deposition device.
• the primary purpose of the second molecular precursor is to condition the substrate surface back toward reactivity with the first molecular precursor.
• the second molecular precursor also provides material from the molecular gas to combine with metal at the surface, forming an oxide with the freshly deposited zinc-containing precursor.
• This particular embodiment does not need to use a vacuum purge to remove a molecular precursor after applying it to the substrate. Purge steps are expected by most researchers to be the most significant throughput-limiting step in ALD processes.
• AX and BY are used, for example.
• the reaction gas AX flow is supplied and flowed over a given substrate area, atoms of the reaction gas AX are chemically adsorbed on a substrate, resulting in a layer of A and a surface of ligand X (associative chemi sorptions) (Step 2).
• the remaining reaction gas AX is purged with an inert gas (Step 4).
• the flow of reaction gas BY, and a chemical reaction between AX (surface) and BY (gas) occurs, resulting in a molecular layer of AB on the substrate (dissociative chemisorptions) (Step 6).
• the remaining gas BY and by-products of the reaction are purged (Step 8).
• the thickness of the thin film can be increased by repeating the process cycle (steps 2-9) many times.
• the film can be deposited one monolayer at a time it tends to be conformal and have uniform thickness.
• Oxides that can be made using the process of the present invention include, but are not limited to: ZnO, indium oxide, tin oxide.
• Mixed structure oxides that can be made using the process of the present invention can include, for example, InZnO.
• Doped materials that can be made using the process of the present invention can include, for example, ZnO:Al, Mg ⁇ Zn ⁇ -x O, and LiZnO doped with indium.
• alloys of two, three, or more metals may be deposited, compounds may be deposited with two, three, or more constituents, and such things as graded films and nano-laminates may be produced as well.
• oxide substrates provide groups for ALD deposition
• plastic substrates can be used by suitable surface treatment.
• FIG. 2 there is shown a cross-sectional side view of one embodiment of a deposition device 10 that can be used in the present process for atomic layer deposition of a zinc-oxide-based semiconductor onto a substrate 20 according to the present invention.
• Deposition device 10 has a gas inlet port 14 for accepting a first gaseous material, a gas inlet port 16 for accepting a second gaseous material, and a gas inlet port 18 for accepting a third gaseous material. These gases are emitted at an output face 36 via output channels 12, having a structural arrangement described subsequently.
• the arrows in Figure 2 and subsequent Figures 3-4B refer to the diffusive transport of the gaseous material, and not the flow, received from an output channel. In this particular embodiment, the flow is substantially directed out of the page of the figure, as described further below.
• gas inlet ports 14 and 16 are adapted to accept first and second gases that react sequentially on the substrate surface to effect ALD deposition, and gas inlet port 18 receives a purge gas that is inert with respect to the first and second gases.
• Deposition device 10 is spaced a distance D from substrate 20, provided on a substrate support, as described in more detail subsequently. Reciprocating motion can be provided between substrate 20 and deposition device 10, either by movement of substrate 20, by movement of deposition device 10, or by movement of both substrate 20 and deposition device 10. In the particular embodiment shown in Figure 2, substrate 20 is moved across output face 36 in reciprocating fashion, as indicated by the arrow R and by phantom outlines to the right and left of substrate 20 in Figure 2.
• reciprocating motion is not always required for thin-film deposition using deposition device 10.
• Other types of relative motion between substrate 20 and deposition device 10 could also be provided, such as movement of either substrate 20 or deposition device 10 in one or more directions, as described in more detail subsequently.
• each output channel 12 is in gaseous flow communication with one of gas" inlet ports 14, 16 or 18 seen in Figure 2.
• Each output channel 12 delivers typically a first reactant gaseous material O, or a second reactant gaseous material M, or a third inert gaseous material I.
• Figure 3 shows a relatively basic or simple arrangement of gases. It is envisioned that a plurality of non-metal deposition precursors (like material O) or a plurality of metal-containing precursor materials, including at least a zinc- containing precursor (like material M) may be delivered sequentially at various ports in a thin-film single deposition.
• a mixture of reactant gases for example, a mixture of metal precursor materials or a mixture of metal and non- metal precursors may be applied at a single output channel when making complex thin film materials, for example, having alternate layers of metals or having lesser amounts of dopants admixed in a metal oxide material.
• the inter-stream labeled I separates any reactant channels in which the gases are likely to react with each other.
• First and second reactant gaseous materials O and M react with each other to effect ALD deposition, but neither reactant gaseous material O nor M reacts with inert gaseous material I.
• the nomenclature used in Figure 3 and following suggests some typical types of reactant gases.
• first reactant gaseous material O could be an oxidizing gaseous material
• second reactant gaseous material M could be a metallic zinc-containing compound.
• Inert gaseous material I could be nitrogen, argon, helium, or other gases commonly used as purge gases in ALD processes. Inert gaseous material I is inert with respect to first or second reactant gaseous materials O and M. Reaction between first and second reactant gaseous materials could form a metal oxide or other binary compound, such as zinc oxide ZnO, in one embodiment. Reactions between more than two reactant gaseous materials could form other zinc-oxide-based materials such as a ternary compound, for example, ZnAlO.
• FIGS. 4 A and 4B show, in simplified schematic form, the ALD coating operation performed as substrate 20 passes along output face 36 of deposition device 10 when delivering reactant gaseous materials O and M.
• the surface of substrate 20 first receives an oxidizing material from output channels 12 designated as delivering first reactant gaseous material O.
• the surface of the substrate now contains a partially reacted form of material O, which is susceptible to reaction with material M.
• the reaction with M takes place, forming a metallic oxide or some other thin film material that can be formed from two reactant gaseous materials.
• inert gaseous material I is provided in every alternate output channel 12, between the flows of first and second reactant gaseous materials O and M.
• Sequential output channels 12 are adjacent, that is, share a common boundary, formed by partitions 22 in the embodiments shown.
• output channels 12 are defined and separated from each other by partitions 22 that extend at a perpendicular to the surface of substrate 20.
• deposition device 10 directs a gas flow (preferably substantially laminar in one embodiment) along the surface for each reactant and inert gas and handles spent gases and reaction by-products in a different manner, as described subsequently.
• the gas flow used in the present invention is directed along and generally parallel to the plane of the substrate surface. In other words, the flow of gases is substantially transverse to the plane of a substrate rather than perpendicular to the substrate being treated.
• Figures 5 and 6 show perspective views of one embodiment of a deposition device 10 that can be used in the present process, from the output face 36 (that is, from the underside with respect to Figures 2 - 4B). Partitions 22 that define and separate the adjacent output channels 12 in this embodiment are represented as partially cut away, to allow better visibility for the gas flows flowing from gas outlet ports 24. Figures 5 and 6 also show reference x.y.z coordinate axis assignments used in the figures of this disclosure. Output channels 12 are substantially in parallel and extend in a length direction that corresponds to the x coordinate axis. Reciprocating motion of substrate 20, or motion relative to substrate 20, is in the ⁇ coordinate direction, using this coordinate assignment.
• Figure 6 shows the gas flows Fi, Fo, and F M for the various gaseous materials delivered from deposition device 10 with this embodiment.
• Gas flows Fi, Fo, and F M are in the x-direction, that is, along the length of elongated output channels 12.
• the cross-sectional views of Figures 7A, 7B, 1C, and 7D are taken orthogonally to the cross-sections of Figures 2 - 4B and show gas flows in one direction from this view.
• the corresponding gaseous material flows from a gas output port 24, shown in phantom in the views of Figures 7 A, 7B, 7C, and 7D.
• gas flow Fl directs the gaseous material along the length of output channel 12 and across substrate 20, as was described with reference to Figures 5 and 6. Flow Fl continues past the edge of deposition device 10 in this arrangement, flowing outward into the environment or, if desirable, to a gas collection manifold (not shown).
• Figure 7B shows an alternative embodiment for gas flow F2 in which output channel 12 also provides an exhaust port 26 for redirection or drawing off of the gas flow.
• Figure 7C shows an alternative embodiment for gas flow F3, in which gas output port 24 is centrally located within output channel 12 and directs gaseous material in gas flows along the channel in both directions.
• Figure 7D shows an alternate embodiment for gas flow F4, in which gas output port 24 is also centrally positioned, with multiple exhaust ports 26 suitably placed near the extreme ends of output channel 12.
• gas output port 24 is also centrally positioned, with multiple exhaust ports 26 suitably placed near the extreme ends of output channel 12.
• a particular deposition device 10 may use output channels 12 configured using any one of the gas flow configurations or combinations thereof, either the Fl flow of Figure 7A, the F2 flow of Figure 7B, the F3 flow of Figure 7C, the F4 flow of Figure 7D, or some other variation in which gaseous material is directed to flow across substrate 20 along output channel 12, preferably in a substantially laminar or smooth fashion with controlled mixing.
• one or more exhaust ports 26 are provided for each output channel 12 that delivers a reactant gaseous material.
• output channels 12 for first and second reactant gaseous materials, labeled O and M are configured with exhaust ports 26 to vent or draw off the reactant substances, following the pattern of flow F2 ( Figure 7B). This allows some recycling of materials and prevents undesirable mixing and reaction near the end of the manifold.
• Output channels 12 for inert gaseous material, labeled I do not use exhaust ports 26 and thus follow the pattern of flow Fl ( Figure 7A). Although laminar flows are preferred in some embodiments, some degree of mixing can occur and even may be beneficial to some extent, depending on the flow rates and other circumstances involved in a particular application.
• Exhaust port 26 is not a vacuum port, in the conventional sense, but is simply provided to draw off the gaseous flow in its corresponding output channel 12, thus facilitating a uniform gas flow pattern within the channel.
• a negative draw just slightly less than the opposite of the gas pressure at gas output port 24, can help to facilitate an orderly gas flow.
• the negative draw can, for example, operate at a pressure of between 0.9 and 1.0 atmosphere, whereas a typical vacuum is, for example, below 0.1 atmosphere.
• An optional baffle 58 as shown in dotted outline in Figures 7B and 7D may be provided to redirect the flow pattern into exhaust port 26.
• output face 36 can be positioned very closely, to within 1 mil (approximately 0.025 mm) of the substrate surface.
• an earlier approach such as that described in the US Patent No. 6,821 ,563 to Yudovsky, cited earlier, required gas flow around the edges of channel sidewalls and was thus limited to 0.5 mm or greater distance to the substrate surface.
• Positioning the deposition device 10 closer to the substrate surface is preferred in the present invention.
• distance D from the surface of the substrate can be 0.4 mm or less, preferably within 0.3 mm, more preferably within 0.25 mm of the output face of the deposition device or the bottom of the guide walls that provide the flow channels.
• FIG. 8A and 8B show why it is desirable to have the distance D relatively small, consistent with the operation of the invention.
• deposition device 10 is moving over substrate 20 from left to right, as the arrow indicates.
• output channel 12 carrying a reactive gaseous material M moves to the right over an area, it encounters a diffusion layer 72 from the next adjacent (previous-in-time) output channel that is primarily inert gaseous material I.
• reactive gaseous material M In order to react on the surface of substrate 20, reactive gaseous material M must diffuse through diffusion layer 72, which has a thickness proportional to distance D.
• Figure 8B shows what happens when distance D is reduced: diffusion layer 72 is reduced proportionately.
• Diffusion through diffusion layer 72 happens more quickly and more efficiently, allowing less waste and reducing the overall amount of time needed for reaction on the surface of substrate 20.
• the lower partition walls 22 also prevent less gas to remain from the previous-in time output channel gas.
• the flow of the gases in the channels are perpendicular to the page of the Figures 8A and 8B, as shown by the back of the arrow, which flow maintains a concentration gradient that aids the diffusion through the thin diffusion layer 72 to the surface of the substrate 20.
• the surface is exposed to the gas flow of M for sufficient time for diffusion and any mixing to replace the previous-in-time output channel gas.
• the flow of gas is across the surface rather than directly into the surface so that it limits the undesirable mixing of reactant gases between outlet channels, which might otherwise be exacerbated by relative oscillation of the manifold and/or substrate.
• gas output port 24 may be inclined at an angle away from normal, as indicated in Figures 7A and 7B.
• some type of gas flow redirecting structure may also be employed to redirect a downward flow from gas output port 24 so that it forms a gas flow that runs substantially in parallel to output face 36.
• FIG. 9 shows output face 36 of a portion of a deposition device 10 that can be used in one embodiment.
• redirecting plates 38 may be positioned in each of the output channels 12 for directing reactant gaseous materials.
• only output channels 12 that deliver reactant gaseous materials are provided with redirecting plates 38 and exhaust ports 26.
• This particular arrangement may be advantageous in some applications, where it is advantageous to surround deposition device 10 with inert gas, such as to reduce unwanted intake of ambient gases.
• redirecting plates 38 could be used on all output channels 12.
• exhaust ports 26 could be used on some or all output channels 12.
• redirecting plates can be used on all channels, but the outlet edge of the redirecting plate may be at a different x-axis position depending upon which channel is considered.
• Figure 9 also shows the pattern of output channels in one embodiment.
• inert gas channels I as the outermost channels of deposition device 10.
• Oxidation channels with first reactant gaseous material O are next to the outermost channels, since these condition the surface for ALD reaction with the metallic component of second reactant gaseous material M.
• Figure 10 shows one embodiment of a deposition device 10 that can be used in the present process in which the channels of width W, length L, and height H are formed from stacked metal plates 42 and 44 having apertures 40 forming ducts 46a,b,c.
• Figure 1 IA shows an exploded view of a single output channel 12 section of deposition device 10 formed in this way, showing an arrangement of apertures 40 in alternating partition plates 42 and delivery plates 44.
• Figure 1 IB shows a similar exploded view for an adjacent output channel 12.
• Ducts 46a, 46b, and 46c formed by alignment of apertures 40, extend through deposition device 10 and provide input conduits for gaseous flow communication, accepting the different reactant and inert gaseous materials from external sources and providing the redirecting structures that provide gas flows along output face 36 described earlier.
• Baffles and other redirecting structures are not shown in these figures, but could be provided using stacked plates suitably structured, or applied after the device is assembled.
• Figures 1 IA and 11 B each show a single output channel 12 formed from stacked plates 42 and 44.
• output channel 12 provides gaseous material provided from duct 46b.
• Ducts 46a and 46c conduct other gases past this channel in the embodiment shown in Figure 1 IA.
• Delivery plate 44 dimensioned and apertured differently than the partition plates 42 that bound output channel 12, contains a redirecting chamber 48 that redirects a portion of the gas in duct 46b into gas flow Fl .
• a redirecting chamber 48 that redirects a portion of the gas in duct 46b into gas flow Fl .
• output channel 12 provides gaseous material provided from duct 46a.
• Ducts 46b and 46c conduct other gases past this channel in the embodiment shown in Figure 1 IB.
• Plates 42 and 44 should be of a suitable metal for conducting the reactive gaseous materials, such as stainless steel or other metal for example.
• the gas flow delivered to the substrate is uniform across all of the channels delivering a particle flow (I, M, or O). This can be accomplished by proper design of the plates, such as having restrictions in some part of the flow pattern for each plate which are accurately machined to provide a reproducible pressure drop for each channel.
• the method of stacking plates is a particularly useful way of constructing a deposition device for use in this invention
• the deposition device for use in the present process may be constructed by direct machining of a metal block, or of several metal blocks adhered together.
• molding techniques involving internal mold features can be employed, as will be understood by the skilled artisan.
• a deposition device can also be constructed using any of a number stereolithography techniques.
• deposition device 10 can be constructed of very small size, with each output channel 12 having the width of a sheet of metal plating.
• output channel 12 is 0.034 inches (0.86 mm) in width W.
• Output channels 12 for different materials could be fabricated at different thicknesses to obtain width W ranges, preferably from 0.01 inches (0.25 mm) to 0.1 inches (2.5 mm) for a compact arrangement.
• the length L of output channel 12 can vary, depending on the needed uniformity and desired gas pressure. In one embodiment, output channel length L is 3 inches (75 mm).
• the height H of output channels 12 formed from extending portions of stacked plates 42 is 0.1 inches in one embodiment. Because gas flow can inadvertently draw ambient gases, due to low- pressure zones that are created, it may be useful to provide additional protection barrier from an inert layer. Referring to Figure 12, there is shown an envelopment gas flow F E by which an additional flow of inert gas is used on one or more sides of deposition device 10 to prevent ambient gases from contaminating the process gases.
• deposition device 10 requires movement relative to the surface of substrate 20 in order to perform its deposition function.
• This relative movement can be obtained in a number of ways, including movement of either or both deposition device 10 and substrate 20, such as by movement of a process that provides a substrate support. Movement can be oscillating or reciprocating or could be continuous movement, depending on how many deposition cycles are needed. Rotation of a substrate can also be used, particularly in a batch process, although continuous processes are preferred.
• ALD requires multiple deposition cycles, building up a controlled film depth with each cycle.
• a single cycle can, for example in a simple design, provide one application of first reactant gaseous material O and one application of second reactant gaseous material M.
• the distance between output channels for O and M reactant gaseous materials determines the needed distance for reciprocating movement to complete each cycle.
• reciprocating motion (along the y axis as used herein) of at least 0.20 inches would be required.
• an area of substrate 20 would be exposed to both first reactant gaseous material O and second reactant gaseous material M with movement over this distance.
• consideration for uniformity may require a measure of randomness to the amount of reciprocating motion in each cycle, such as to reduce edge effects or build-up along the extremes of reciprocation travel.
• a deposition device 10 may have only enough output channels 12 to provide a single cycle. Alternately, deposition device 10 may have an arrangement of multiple cycles, enabling it to cover a larger deposition area or enabling its reciprocating motion over a distance that allows two or more deposition cycles in one traversal of the reciprocating motion distance.
• a given area of the substrate is exposed to a gas flow in a channel for less than 500 milliseconds, preferably less than 100 milliseconds.
• the relative movement of the substrate to a channel during oscillation is at a speed of at least 0.1 cm/sec, and the gas flow in the channels is at least 1 cm/sec.
• the temperature of the substrate during deposition is under 300 0 C, more preferably under 250 0 C.
• FIG. 13 shows schematically how this broader area coverage can be effected, using reciprocating motion along the y axis as shown by arrow R and also movement orthogonal or transverse to the reciprocating motion, relative to the x axis.
• motion in either the x or y direction can be effected either by movement of deposition device 10, or by movement of substrate 20 provided with a substrate support 74 that provides movement, or by movement of both deposition device 10 and substrate 20.
• the relative motion of the deposition head and the substrate are perpendicular to each other. It is also possible to have this relative motion in parallel. In this case, the relative motion needs to have a nonzero frequency component that represents the oscillation and a zero frequency component that represents the displacement of the substrate.
• This combination can be achieved by: an oscillation combined with displacement of the deposition device over a fixed substrate; an oscillation combined with displacement of the substrate relative to a fixed substrate deposition device; or any combinations wherein the oscillation and fixed motion are provided by movements of both substrates.
• the present ALD process can be performed at or near atmospheric pressure and over a broad range of ambient and substrate temperatures, preferably at a temperature of under 300 0 C.
• a relatively clean environment is needed to minimize the likelihood of contamination; however, full "clean room” conditions or an inert gas-filled enclosure would not be required for obtaining good performance when using preferred embodiments of the process of the present invention.
• Figure 14 shows an Atomic Layer Deposition (ALD) 60 process, for making a zinc-oxide-based semiconductor, having a chamber 50 for providing a relatively well-controlled and contaminant-free environment.
• Gas supplies 28a, 28b, and 28c provide the first, second, and third gaseous materials to deposition device 10 through supply lines 32.
• the optional use of flexible supply lines 32 facilitates ease of movement of deposition device 10.
• an optional vacuum vapor recovery process and other support components are not shown in Figure 13 but could also be used.
• a transport subsystem 54 provides a substrate support that conveys substrate 20 along output face 36 of deposition device 10, providing movement in the x direction, using the coordinate axis system employed in the present disclosure.
• control logic processor 56 controls an actuator 30 for providing reciprocating motion to deposition device 10 and also controls a transport motor 52 of transport subsystem 54.
• Figure 15 shows an alternate embodiment of an Atomic Layer Deposition
• ALD process 70 for thin film deposition of zinc-oxide-based semiconductor onto a web substrate 66 that is conveyed past deposition device 10 along a web conveyor 62 that acts as a substrate support.
• a deposition device transport 64 conveys deposition device 10 across the surface of web substrate 66 in a direction transverse to the web travel direction.
• deposition device transport 64 uses a lead screw that traverses the width of web substrate 66.
• multiple deposition devices 10 are used, at suitable positions along web 62.
• FIG 16 shows another Atomic Layer Deposition (ALD) system 70 for depositing a zinc-oxide-based semiconductor in a web arrangement, using a stationary deposition device 10 in which the flow patterns are oriented orthogonally to the configuration of Figure 14.
• ALD Atomic Layer Deposition
• Reciprocating motion could also be used in this environment, such as by repeatedly reversing the direction of rotation of a web roller to move substrate 66 forward and backwards relative to deposition device 10.
• Reciprocation motion can also be obtained by allowing a reciprocating motion of the deposition device across an arc whose axis coincides with the roller axis, while the web is moved in a constant motion.
• FIG 17 an embodiment of a portion of deposition device 10 is shown in which output face 36 has an amount of curvature, which might be advantageous for some web coating applications. Convex or concave curvature could be provided.
• the present process can be accomplished with other apparatus or systems described in more detail in commonly assigned US Application No. 1 1/392,007, US Application No. 1 1/392,006, US Application No. 11/620,744, and US Application No. 1 1/620,740.
• a delivery device having an output face for providing gaseous materials for thin-film material deposition onto a substrate comprises elongated emissive channels in at least one group of elongated emissive channels, of the three groups of elongated emissive channels (namely, at least one group of: (i) one or more first elongated emissive channels, (ii) one or more second elongated channels, and (iii) a plurality of third elongated channels) that is capable of directing a flow, respectively, of at least one of the first gaseous material, second gaseous material, and the third gaseous material substantially orthogonally with respect to the output face of the delivery device, which flow of gaseous material is capable of being provided, either directly or indirectly from each of the elongated emissive channels in the at least one group, substantially orthogonally to the surface of the substrate.
• apertured plates are disposed substantially in parallel to the output face, and apertures on at least one of the apertured plates form the first, second, and third elongated emissive channels.
• the apertured plates are substantially perpendicularly disposed with respect to the output face.
• the deposition device comprises exhaust channels, for example, a delivery device for thin-film material deposition of a zinc-oxide-based semiconductor onto a substrate comprising: (a) a plurality of inlet ports comprising at least a first inlet port, a second inlet port, and a third inlet port capable of receiving a common supply for a first reactive gaseous material, a second reactive gaseous material, and a third (inert purge) gaseous material, respectively; (b) at least one exhaust port capable of receiving exhaust gas from thin-film material deposition and at least two elongated exhaust channels, each of the elongated exhaust channels capable of gaseous fluid communication with the at least one exhaust port; and (c) at least three pluralities of elongated output channels, (i) a first plurality of first elongated output channels, (ii) a second plurality of second elongated output channels, and (iii) a third plurality of third elongated output channels, each of the first
• the delivery head has elongated output openings in the output face of the delivery head for providing the gaseous materials to the substrate.
• each elongated output opening can be connected to a corresponding elongated output/emissive channel and optionally, in some embodiments, can form the opening thereof in the output face.
• FIG. 1 can depict a gas diffuser associated with at least one group of the three groups of elongated emissive channels such that at least one of the first, second, and third gaseous material, respectively, is capable of passing through the gas diffuser prior to delivery from the delivery device to the substrate, during thin-film material deposition onto the substrate, and wherein the gas diffuser maintains flow isolation of the at least one of first, second, and third gaseous material downstream from each of the elongated emissive channels in the at least one group of elongated emissive channels.
• such a gas diffuser is capable of providing a friction factor for gaseous material passing there through that is greater than 1x10 , thereby providing back pressure and promoting equalization of pressure where the flow of the at least one first, second and third gaseous material exits the delivery device.
• the gas diffuser comprises a porous material through which the at least one of the first, second, and third gaseous material passes.
• the gas diffuser comprises a mechanically formed assembly comprising at least two elements comprising interconnected passages, for example, in which nozzles are connected to a flow path provided by a thin space between parallel surface areas in the two elements.
• the one or more of the gas flows from the deposition devices provides a pressure that at least contributes to the separation of the surface of the substrate from the face of the delivery head, thereby providing a "floating head” or “air bearing” type deposition head, which can help to stabilize the gas flows and limit intermixing of the gas flows.
• the process of the present invention is advantaged in its capability to perform deposition onto a substrate over a broad range of temperatures, including room or near-room temperature in some embodiments.
• the process of the present invention can operate in a vacuum environment, but is particularly well suited for operation at or near atmospheric pressure.
• a substrate is provided, a film or layer of the semiconductor material as described above can be applied to the substrate, and electrical contacts made to the layer.
• the exact process sequence is determined by the structure of the desired semiconductor component.
• a gate electrode can be first deposited on a substrate, for example a vacuum or solution deposited metal or organic conductor. The gate electrode can then be insulated with a dielectric and then source and drain electrodes and a layer of the n-channel semiconductor material can be applied on top.
• a gate electrode can be deposited first, followed by a gate dielectric, then the semiconductor can be applied, and finally the contacts for the source electrode and drain electrode deposited on the semiconductor layer.
• a third structure could have the source and drain electrodes deposited first, then the semiconductor, with dielectric and gate electrode deposited on top.
• a field effect transistor comprises an insulating layer, a gate electrode, a semiconductor layer comprising a ZnO material as described herein, a source electrode, and a drain electrode, wherein the insulating layer, the gate electrode, the semiconductor layer, the source electrode, and the drain electrode are in any sequence as long as the gate electrode and the semiconductor layer contact opposite sides of the insulating layer, and the source electrode and the drain electrode both contact the semiconductor layer.
• a thin film transistor is an active device, which is the building block for electronic circuits that switch and amplify electronic signals.
• Attractive TFT device characteristics include a high ratio of the on current to the off current, and a steep subthreshold slope.
• a voltage applied between the source and drain electrodes establishes a substantial current flow only when the control gate electrode is energized. That is, the flow of current between the source and drain electrodes is modulated or controlled by the bias voltage applied to the gate electrode.
• the relationship between material and device parameters of the zinc-oxide-based semiconductor TFT can be expressed by the approximate equation (see Sze in Semiconductor Devices — Physics and Technology, John Wiley & Sons (1981)):
• I a is the saturation source-drain current
• C is the geometric gate capacitance, associated with the insulating layer
• W and L are physical device dimensions
• ⁇ is the carrier (hole or electron) mobility in the zinc-oxide-based semiconductor
• V g is the applied gate voltage
• V, h is the threshold voltage.
• the TFT allows passage of current only when a gate voltage of appropriate polarity is applied.
• t is the zinc-oxide-based semiconductor layer thickness and V S d is the voltage applied between source and drain. Therefore, for the TFT to operate as a good electronic switch, e.g. in a display, with a high on/off current ratio, the semiconductor needs to have high carrier mobility but very small intrinsic conductivity, or equivalently, a low charge carrier density. On/off ratios >10 are desirable for practical devices.
• the TFT structure described herein includes a transparent zinc-oxide- based semiconductor with conducting electrodes, commonly referred to as a source and a drain, for injecting a current into the zinc-oxide-based semiconductor and a capacitance charge injection scheme for controlling and/or modulating the source-drain current.
• a transparent zinc-oxide-based semiconductor with conducting electrodes commonly referred to as a source and a drain
• a capacitance charge injection scheme for controlling and/or modulating the source-drain current.
• Zinc oxide semiconductor transistors and/or transistor arrays are useful in applications including, but not limited to, flat panel displays such as active matrix imagers, sensors, rf price labels, electronic paper systems, rf identification tags and rf inventory tags.
• the inclusion of indium doping in these transistors can be effective for both improving the robustness of the semiconductor layer when used as the channel material and the conductivity of the electrodes when used as a conducting electrode.
• the present invention is preferably used for making an "enhancement- mode transistor” which means a transistor in which there is negligible off-current flow, relative to on-current flow, between a source and a drain at zero gate voltage.
• the transistor device is "normally off.”
• a depletion-mode transistor is "normally on” meaning that more than a substantially negligible current flows between a source and a drain at zero gate voltage. Enhancement-mode devices are typically preferred.
• the terms “over,” “above,” “under,” and the like, with respect to layers in the thin film transistor refer to the order of the layers, wherein the thin film semiconductor layer is above the gate electrode, but do not necessarily indicate that the layers are immediately adjacent or that there are no intermediate layers.
• the descriptors "top” and “bottom” refer to the disposition of the contact with respect to the semiconductor, with bottom representing closer to the substrate and top representing further from the substrate. "Vertical” means substantially perpendicular to the surface of a substrate.
• Described in the embodiments of Figures 20 to 25 are enhancement-mode, field effect transistors wherein at least a portion of the transistor structure may be substantially transparent. Accordingly, an optional characteristic of a transistor structure made using the present invention is that selected embodiments of the construct, or a subset thereof that include the semiconductor channel layer and the gate insulator layer, may exhibit an optical transmission of at least 90%, more particularly at least 95%, across the visible portion (and/or infrared portion in certain variants) of the electromagnetic spectrum.
• Each of the additional components of the structure i.e., substrate, gate electrode, source/drain terminals
• the transistor structure as a whole (and/or individual components of the transistor) may exhibit an optical transmission of at least 50%, more particularly at least 70%, and most particularly at least 90%, across the visible portion (and/or infrared portion in certain variants) of the electromagnetic spectrum. Because of the optional transparency, transistors made according to the present invention may be advantageously included in optoelectronic display devices as switches coupled to at least one display element, as described in greater detail below.
• Figures 20 - 25 Cross-sectional views of typical deposited thin film transistors are shown in Figures 20 - 25.
• Figure 20 illustrates a typical bottom contact configuration
• Figure 21 illustrates a typical top contact configuration.
• Each thin film transistor (TFT) in the embodiments of Figures 20 and 21 contains a source electrode 120, a drain electrode 130, a gate electrode 144, a gate dielectric 156, a substrate 128, and the semiconductor 170 of the invention in the form of a film connecting the source electrode 120 to drain electrode 130.
• the TFT operates in an enhancement-mode, the charges injected from the source electrode into the semiconductor are mobile and a current flows from source to drain, mainly in a thin channel region within 100 Angstroms of the semiconductor-dielectric interface. See A. Dodabalapur, L. Torsi H. E. Katz, Science 1995, 268, 270.
• the charge need only be injected laterally from the source electrode 120 to form the channel.
• the channel ideally has few charge carriers; as a result there is ideally no source-drain conduction when the device is in off mode.
• the off current is defined as the current flowing between the source electrode 120 and the drain electrode 130 when charge has not been intentionally injected into the channel by the application of a gate voltage. This occurs for a gate-source voltage more negative, assuming an n-channel, than a certain voltage known as the threshold voltage. See Sze in Semiconductor Devices— Physics and Technology, John Wiley & Sons (1981), pages 438-443.
• the on current is defined as the current flowing between the source electrode 120 and the drain electrode 130 when charge carriers have been accumulated intentionally in the channel by application of an appropriate voltage to the gate electrode 144, and the channel is conducting. For an n-channel accumulation-mode TFT, this occurs at gate-source voltage more positive than the threshold voltage.
• this threshold voltage is desirable for this threshold voltage to be zero, or slightly positive, for n-channel operation. Switching between on and off is accomplished by the application and removal of an electric field from the gate electrode 144 across the gate dielectric 156 to the semiconductor-dielectric interface, effectively charging a capacitor.
• a further (third) specific transistor structure is shown in Figure 22, in which a third variation of a TFT structure includes an insulating substrate 128 upon which is disposed a source electrode 120 and a drain electrode 130. A semiconductor film 170 is provided such that contact between the source and drain electrodes is made. A gate electrode 144 is disposed on the top surface (from a vertical perspective) of the gate dielectric 156. In other words, the gate electrode 144 and the semiconductor film 170 are provided on opposing surfaces of the gate dielectric 156.
• the TFT structure of Figure 22 may be fabricated, for example, by depositing and patterning a film that defines the source electrode 120 and the drain electrode 130. For instance, a 500 Angstrom ITO source/drain electrode film may be sputtered onto the glass substrate 128. The source and drain patterning may be accomplished via shadow masking or photolithography. The source/drain electrode film could optionally be annealed. The semiconductor film 170 may then be deposited and patterned over the source electrode 120, the drain electrode 130, and the substrate 128. For example, a ZnO-based film may be deposited, and then patterned via photolithography or patterning during deposition.
• the gate dielectric 156 may then be deposited and patterned over the semiconductor film 170.
• a 2000-Angstrom Al 2 O 3 film may be sputter deposited, and then patterned via shadow masking or photolithography. Vias (not shown) may be formed through the gate dielectric 156 to electrically connect to the source electrode 120 and the drain electrode 130.
• the Al 2 O 3 film could optionally be annealed.
• the gate electrode 144 may then be deposited and patterned over the gate dielectric 156.
• a 2000 Angstrom ITO film may be sputter deposited, and then patterned via shadow masking or photolithography.
• the ITO film could optionally be annealed.
• This TFT structure includes a substrate 128 upon which is disposed a semiconductor channel layer 170.
• a source electrode 120 and a drain electrode 130 are provided on a surface of the semiconductor channel layer 170 opposing the surface that is adjacent to the glass substrate 128.
• a gate dielectric 156 is disposed over the semiconductor channel layer 170, the source electrode 120, and the drain electrode 130.
• a gate electrode 144 is disposed on the top surface (from a vertical perspective) of the gate dielectric 156. In other words, the gate electrode 144 and the semiconductor channel layer 170 are provided on opposing surfaces of the gate dielectric 156.
• the TFT structure of Figure 23 may be fabricated, for example, by the deposition and patterning of a film that defines the semiconductor channel layer 170. For instance, a 500- Angstrom ZnO-based film may be deposited, and patterned via photolithography. The source electrode 120 and the drain electrode 130 may then be deposited and patterned. For example, a 500- Angstrom
• Aluminum or Silver, or other metal source/drain electrode film may be deposited in the desired pattern.
• the gate dielectric 156 may then be deposited and patterned over the semiconductor channel layer 170, the source electrode 120, and the drain electrode 130.
• a 2000 Angstrom dielectric may be deposited and patterned or selectively deposited.
• Vias (not shown) may be formed through the gate dielectric 156 to electrically connect to the source electrode 120 and the drain electrode 130.
• the gate electrode 144 may then be deposited and patterned over the gate dielectric 156.
• a 2000 Angstrom ITO or metal film may be deposited and/or patterned.
• the contacts in the above configuration of Figure 23 may be made by selectively doping the ends of the semiconductor channel layer with In, Al. or any other suitable n-type dopant.
• a substrate for supporting the TFT during manufacturing, testing, and/or use can include organic or inorganic materials.
• the support may comprise inorganic glasses, ceramic foils, polymeric materials, filled polymeric materials, coated metallic foils, acrylics, epoxies, polyamides, polycarbonates, polyimides, polyketones, poly(oxy-l ,4- phenyleneoxy-l,4-phenylenecarbonyl-l,4- phenylene) (sometimes referred to as poly(ether ether ketone) or PEEK), polynorbornenes, polyphenyleneoxides, poly(ethylene naphthalenedicarboxylate) (PEN), polyethylene terephthalate) (PET), poly(ether sulfone) (PES), poly(phenylene sulfide) (PPS), and fiber-reinforced plastics (FRP).
• inorganic glasses ceramic foils, polymeric materials, filled polymeric materials, coated metallic foils, acrylics, epoxies,
• Flexible substrates can also be thin metal foils such as stainless steel provided they are coated with an insulating layer to electrically isolate the thin film transistor.
• a flexible support in some embodiments of the present invention, allows for roll processing, which may be continuous, providing economy of scale and economy of manufacturing over flat and/or rigid supports.
• the flexible support chosen preferably is capable of wrapping around the circumference of a cylinder of less than 50 cm diameter, more preferably 25 cm diameter, most preferably 10 cm diameter, without distorting or breaking, using low force as by unaided hands.
• the preferred flexible support may be rolled upon itself.
• the substrate may be a wafer or sheet made of materials including glass and silicon.
• the thickness of the substrate may vary, and according to particular examples it can range from 100 ⁇ m to 1 cm.
• the support may be optional.
• the support in a top construction as in Figure 21 , when the gate electrode and/or gate dielectric provides sufficient support for the intended use of the resultant TFT, the support is not required.
• the support may be combined with a temporary support.
• a support may be detachably adhered or mechanically affixed to the support, such as when the support is desired for a temporary purpose, e.g., manufacturing, transport, testing, and/or storage.
• a flexible polymeric support may be adhered to a rigid glass support, from which support could be removed.
• the gate electrode can be any useful conductive material.
• the gate electrode may comprise doped silicon, or a metal, such as aluminum, chromium, gold, silver, nickel, copper, tungsten, palladium, platinum, tantalum, and titanium.
• Gate electrode materials can also include transparent conductors such as indium-tin oxide (ITO), ZnO, SnO 2 , or In 2 O 3 .
• Conductive polymers also can be used, for example polyaniline, poly(3,4- ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOTrPSS).
• PEDOTrPSS poly(styrene sulfonate)
• alloys, combinations, and multilayers of these materials may be useful.
• the thickness of the gate electrode may vary, and according to particular examples it can range from 50 to 1000 nm.
• the gate electrode may be introduced into the structure by chemical vapor deposition, sputtering, evaporation and/or doping, or solution processing.
• the same material can provide the gate electrode function and also provide the support function of the support.
• doped silicon can function as the gate electrode and support the TFT.
• the gate dielectric is provided in contact with the gate electrode. This gate dielectric electrically insulates the gate electrode from the balance of the TFT device.
• the gate dielectric comprises an electrically insulating material.
• the gate dielectric should have a suitable dielectric constant that can vary widely depending on the particular device and circumstance of use. For example, a dielectric constant from 2 to 100 or even higher is known for a gate dielectric.
• Useful materials for the gate dielectric may comprise, for example, an inorganic electrically insulating material.
• the gate dielectric may comprise a polymeric material, such as polyvinylidenedifluoride (PVDF), cyanocelluloses, polyimides, etc.
• PVDF polyvinylidenedifluoride
• the gate dielectric may comprise a plurality of layers of different materials having different dielectric constants.
• materials useful for the gate dielectric include strontiates, tantalates, titanates, zirconates, aluminum oxides, silicon oxides, tantalum oxides, titanium oxides, silicon nitrides, barium titanate, barium strontium titanate, barium zirconate titanate, zinc selenide, and zinc sulfide.
• alloys, combinations, and multilayers of these examples can be used for the gate dielectric.
• aluminum oxides, silicon oxides, and zinc selenide are preferred.
• polymeric materials such as polyimides, polyvinyl alcohol, poly(4-vinylphenol), polyimide, and poly(vinylidene fluoride), polystyrene and substituted derivatives thereof, poly(vinyl naphthalene) and substituted derivatives, and poly(methyl methacrylate) and other insulators having a suitable dielectric constant.
• the gate dielectric can be provided in the TFT as a separate layer, or formed on the gate such as by oxidizing the gate material to form the gate dielectric.
• the dielectric layer may comprise two or more layers having different dielectric constants. Such insulators are discussed in U. S Patent No. 5,981 ,970 and copending US Patent Publication No. 2006/0214154, both cited herein.
• Gate insulator materials typically exhibit a band-gap of greater than 5 eV.
• the thickness of the gate insulator layer may vary, and according to particular examples it can range from 10 to 300 ran.
• the gate dielectric layer may be introduced into the structure by techniques such as chemical vapor deposition, sputtering, atomic layer deposition, or evaporation, solution.
• Source/drain terminals refer to the terminals of a TFT, between which conduction occurs under the influence of an electric field. Designers often designate a particular source/drain terminal to be a "source” or a “drain” on the basis of the voltage to be applied to that terminal when the TFT is operated in a circuit.
• the source electrode and drain electrode are separated from the gate electrode by at least the gate dielectric, while the zinc-oxide-based semiconductor layer can be over or under the source electrode and drain electrode.
• the source and drain electrodes can be any useful conductive material.
• Useful materials include most of those materials described above for the gate electrode, for example, aluminum, barium, calcium, chromium, gold, silver, nickel, palladium, platinum, titanium, copper, tungsten, polyaniline, PEDOT:PSS, other conducting polymers, alloys thereof, combinations thereof, and multilayers thereof.
• Other illustrative materials include transparent, n-type conductors such as indium-tin oxide (ITO), ZnO, SnO 2 , or In 2 O 3 .
• Preferred electrodes are silver, gold, PEDOT:PSS, or aluminum.
• the source electrode and drain electrode can be provided by any useful means such as chemical or physical vapor deposition (e.g., thermal evaporation, sputtering), evaporation, ink jet printing, or doping of the semiconductor channel layer material via diffusion or ion implantation and solution deposition.
• the patterning of these electrodes can be accomplished by known methods such as shadow masking, additive photolithography, subtractive photolithography, printing, microcontact printing, and pattern coating.
• the source and drain terminals may be fabricated such that they are geometrically symmetrical or nonsymmetrical. Electrical contact to the gate electrode, source, drain and substrate may be provided in any manner.
• metal lines, traces, wires, interconnects, conductors, signal paths, and signaling mediums may be used for providing the desired electrical connections.
• the related terms listed above, are generally interchangeable, and appear in order from specific to general.
• Metal lines generally aluminum (Al), copper (Cu), or an alloy of Al and Cu, are typical conductors that provide signal paths for coupling or interconnecting, electrical circuitry. Conductors other than metal may also be utilized.
• connection to the electrical contact can be made by creating a "via" that penetrates to the contact.
• vias can be made by convenient patterning operations such as lithography, etching, or laser based processes.
• An illustrative n-channel operation of the transistor involves applying a positive voltage to the gate electrode, grounding the source, and applying a positive voltage to the drain. For example, a voltage of 5 to 40 V may be applied to the gate electrode and the drain during operation.
• the threshold voltage may range from -minus 10 to 20 V, although devices can operate with larger ranges.
• Electrons flow from the source, along the semiconductor thin film, and out of the transistor through the drain. The effective mobility of the electrons may vary depending upon the specific structure, but typically should be greater than 0.01 cm V " s "1 for useful practical applications. Simply by removing the positive voltage applied to the gate electrode turns the transistor off when the transistor is an enhancement-mode transistor.
• TFTs and other devices include, for example, more complex circuits, e.g., shift registers, integrated circuits, logic circuits, smart cards, memory devices, radio-frequency identification tags, backplanes for active matrix displays, active-matrix displays (e.g. liquid crystal or OLED), solar cells, ring oscillators, and complementary circuits, such as inverter circuits, for example, in combination with other transistors made using available p-type organic semiconductor materials such as pentacene.
• a transistor according to the present invention can be used as part of voltage hold circuitry of a pixel of the display.
• TFTs are operatively connected by means known in the art.
• a microelectronic device is an active-matrix liquid-crystal display (AMLCD).
• AMLCD active-matrix liquid-crystal display
• One such device is an optoelectronic display that includes elements having electrodes and an electro-optical material disposed between the electrodes.
• a connection electrode of the transparent transistor may be connected to an electrode of the display element, while the switching element and the display element overlap one another at least partly.
• An optoelectronic display element is here understood to be a display element whose optical properties change under the influence of an electrical quantity such as current or voltage such as, for example, an element usually referred to as liquid crystal display (LCD).
• LCD liquid crystal display
• the presently detailed transistor has sufficient current carrying capacity for switching the display element at such a high frequency that the use of the transistor as a switching element in a liquid crystal display is possible.
• the display element acts in electrical terms as a capacitor that is charged or discharged by the accompanying transistor.
• the optoelectronic display device may include many display elements each with its own transistor, for example, arranged in a matrix. Certain active matrix pixel designs, especially those supplying a display effect that is current driven, may require several transistors and other electrical components in the pixel circuit.
• the AMLCD cell circuit includes a transistor 100 as presently described, and an LCD pixel 102 electrically coupled thereto.
• the transistor 100 and the LCD pixel 102 together form a transistor/pixel cell 104.
• the transistor 100 is electrically coupled to the LCD pixel 102 via the drain electrode 130.
• the gate electrode 144 of the transistor 100 is electrically coupled to a row or control line 108 (also referred to as a select or gate line) that receives on/off input for the transistor 100.
• the source electrode 120 of the transistor 100 is electrically coupled to a column or data line 106 that receives a signal for controlling the LCD pixel 102.
• Each LCD pixel 102 can also be viewed as a capacitor representing the capacitance according to display design.
• Figure 25 shows a typical pixel layout in which data lines 106 lead to individual source electrodes 120, control lines 108 lead to individual gate electrodes 144, semiconductors 170, and drain electrodes 130 each forming a pixel conductor pad.
• the flow setup is supplied with nitrogen gas flow 81 that has been purified to remove oxygen and water contamination to below 1 ppm.
• the gas is diverted by a manifold to several flow meters which control flows of purge gases and of gases diverted through bubblers to select the reactive precursors.
• air flow 90 is also delivered via flow meter 91 to the apparatus.
• the air is pretreated with molecular sieves to remove moisture.
• the following flows are delivered to the ALD coating apparatus: metal
• (zinc) precursor flow 92 containing metal precursors diluted in nitrogen gas; oxidizer-containing flow 93 containing non-metal precursors or oxidizers diluted in nitrogen gas; and nitrogen purge flow 95 composed only of the inert gas.
• the composition and flows of these streams are controlled as described below.
• Gas bubbler 82 contains liquid diethylzinc.
• Gas bubbler 83 contains liquid trimethylaluminum. Both bubblers are kept at room temperature.
• Gas bubbler 99 contains solid trimethylindium. This bubbler is heated by means of a water bath to 60 0 C, in order to increase the vapor pressure of the trimethylindium.
• Flow meters 85, 86, and 100 deliver flows of pure nitrogen to the diethylzinc bubbler 82, trimethylaluminum bubbler 83, and trimethylindium bubbler 99, respectively.
• the output of the bubblers now contains nitrogen gas saturated with the respective precursor solutions.
• These output flows are mixed with a nitrogen gas dilution flow delivered from flow meter 87 to yield the overall flow of metal precursor flow 92.
• the flows will be as follows: Flow meter 85: To Diethylzinc Bubbler Flow
• Flow meter 87 To Metal Precursor Dilution Flow Gas bubbler 84 contains pure water for the control (or ammonia in water) at room temperature.
• Flow meter 88 delivers a flow of pure nitrogen to gas bubbler 84, the output of which represents a stream of saturated water vapor.
• the water bubbler output and nitrogen streams are mixed with dilution stream from flow meter 89 to produce the overall flow of oxidizer-containing flow 93 which has a variable water vapor composition, ammonia composition, nitrogen composition, and total flow.
• the flows will be as follows:
• Flow meter 88 To Water Bubbler Flow meter 89: To Oxidizer Dilution Flow
• Flow meter 94 controls the flow of pure nitrogen that is to be delivered to the coating apparatus.
• Streams or Flows 92, 93, and 95 are then delivered to an atmospheric pressure coating head where they are directed out of the channels or microchamber slots as indicated in Figure 19.
• a gap 96 of approximately 30 microns exists between the elongated channels and the substrate 97.
• the microchambers are approximately 2.5 mm tall, 0.86 mm wide, and run the length of the coating head, which is 76 mm.
• the reactant materials in this configuration are delivered to the middle of the slot and flow out of the front and back.
• the coating head In order to perform a deposition, the coating head is positioned over a portion of the substrate and then moved in a reciprocating fashion over the substrate, as represented by the arrow 98.
• the length of the reciprocation cycle was 32 mm.
• the rate of motion of the reciprocation cycle is 30 mm/sec.
• Transistor characterization of the fabricated devices fabricated using the present invention was performed with a Hewlett Packard HP 4156 parameter analyzer. Device testing was done in air in a dark enclosure.
• the drain current (Id) was measured as a function of source-drain voltage (Vd) for various values of gate voltage (Vg). Furthermore, for each device the drain current was measured as a function of gate voltage for various values of source- drain voltage. For most devices, Vg was swept from -10 V to 30 V for each of the drain voltages measured, typically 10 V, 20 V, and 30 V. Mobility measurements were taken from the 30 V sweep.
• Parameters extracted from the data include field-effect mobility ( ⁇ ), threshold voltage (Vth), and the ratio of Ion/Ioff for the measured drain current. These measurements can be taken without any protective film on the semiconductor thin film, yielding substantially the same results with or without the protective film.
• Parameters extracted from the log I d plot include the I on /I of r ratio.
• the I 0 Jl 0 ( T ratio is simply the ratio of the maximum to minimum drain current.
• the on/off ratio is measured by the ratio of the on current, defined as the drain current at a gate voltage of 10 V in excess of the threshold voltage, to the off current, defined as the drain current at a gate voltage for which the transistor is in an off state. The measurements in the present examples were taken in excess of 10 V, which would result in minor changes to the measurement. Materials Used:
• Conductive ITO substrate 100 nm thick ITO on soda lime glass, commercially available from MBC, product #255
• Ammonium Hydroxide (aqueous ammonia), (reagent grade, ⁇ 29 weight % ammonia, commercially available from J. T. Baker Chemical Co.)
• Table 1 The data in Table 1 indicate that substantial amounts of In 2 O 3 have been doped into the ZnO host using this experimental approach and that it is possible to achieve a ratio of In/Zn, between 0 and 0.50 by controlling the respective metal 0 precursor flow rates.
• This example shows the change in crystallinity of the ZnO based semiconductor layer with increased incorporation of In 2 O 3 .
• X-ray diffraction was used to determine the relative crystallinity of the same coatings used in the 5 previous Example 1.
• X-ray diffraction analysis is capable of determining if a material is non-crystalline (amorphous), since a perfectly amorphous material would generate no lattice peaks.
• profile fitting summed the relative areas of the lattice peaks in each trace. The results are tabulated in Table 2 below.
• Figure 26 is an overlaid plot of the X-ray diffraction spectra of the series in Table 2.
• Table 2 shows that as In 2 O 3 content in the film is increased, the degree of crystallinity is significantly lowered. Because crystal regions in semiconductor films can be a source of film non-uniformity, a lower degree of crystallinity is associated with more uniform, robust films.
• This example describes the preparation of a thin film transistor device (TFT) that is comprised of a glass substrate, an ITO (indium tin oxide) gate electrode, a 1 100 A thick Al 2 O 3 dielectric layer, and a 200 A thick, undoped ZnO layer having evaporated Al metal source/drain contacts.
• TFT thin film transistor device
• ITO indium tin oxide
• Al 2 O 3 and ZnO layers have been described in detail in Figures 5 to 12 herein.
• a 2.5 X 2.5 inch square (62.5 mm square) piece of ITO coated glass was positioned on the platen of this device, held in place by a vacuum assist, and heated to 200 0 C.
• the platen with the glass substrate was positioned under the coating head that directs the flow of the active precursor gasses.
• the spacing between the ITO substrate and the coating head was adjusted using a micrometer to 30 microns.
• the coating head has isolated channels through which flow: (1) inert nitrogen gas; (2) a mixture of nitrogen, and water vapor; and (3) a mixture of 5 active metal alkyl vapor (Me 3 Al or Et 2 Zn) in nitrogen.
• the flow rate of the active metal alkyl vapor was controlled by bubbling nitrogen through the pure liquid (Me 3 Al or Et 2 Zn) contained in an airtight bubbler by means of individual mass flow control meters.
• the flow of water vapor was controlled by adjusting the bubbling rate of nitrogen passed through pure water in a bubbler.
• the temperature 0 of the coating head was maintained at 40 0 C. Adjusting the flow rates of the individual gasses to the settings shown in Table 3 below, the coating process was initiated by oscillating the coating head across the substrate for the number of cycles specified.
• a common layer OfAl 2 O 3 dielectric was deposited on the 5 ITO substrate using the process parameters for layer A in Table 3 below.
• the preparation of the indium-doped ZnO layer in sample 4 is identical to that in sample 3 except that a flow of trimethylindium vapor is added to the flow 0 of diethyl zinc, at the concentration specified in Table 3 below.
• Table 4 shows that the In-doping of the ZnO semiconductor layer provides a similar or increased mobility relative to samples that do not contain indium, while providing increased robustness due to the lower crystallinity of the layers as indicated in Example 2
• Electron Paramagnetic Resonance has been used as a technique to study donor states in ZnO based semiconductors. To our knowledge, there is no EPR data in the literature on either thin films of ZnO or mixed phases of In x Zn y O. Therefore, our interpretation of the EPR data is based on prior publications on shallow donor states in ZnO single crystals and nanoparticles. In the ZnO system, EPR can detect the conduction electron spin resonance (CESR) signal originating from shallow donor states of various intrinsic and extrinsic dopants. A highly n- doped ZnO matrix typically exhibits a CESR signal in the dark.
• CESR conduction electron spin resonance
• CESR signal intensity increases monotonically with the trimethylindium flow rate as shown in Figure 27. This increase is concomitant with a decrease in resistivity of the samples as measured with the 4-point probe technique.
• Changes in the spectral characteristics of the EPR spectrum with increasing In content are: A substantial increase in the EPR line-width and a slight increase in the g- value towards the value of the free electron. The former could be due to the fact that In, at the center of a shallow donor state, will exhibit hyperfine interaction between the donor electron and the In nucleus.
• the increase in g-value coincides with an increase in the optical band-gap of the In-doped ZnO layer.
• the purpose of this example was to prepare a TFT device that had a transparent, conductive gate electrode prepared from a coating of In-doped ZnO.
• the devices on the sample had the following cross-sectional composition: glass substrate/In-doped ZnO gate electrode/Al 2 ⁇ 3 dielectric/ZnO semiconductor/Al source and drain electrodes.
• the In-doped ZnO gate electrode, layer 7A was prepared in a similar fashion to sample 4A.
• the resistivity of the In-doped ZnO gate electrode layer was measured to be 3.3 x 10 "02 ohm*cm.
• An Al 2 O 3 dielectric layer (7B) and a ZnO semiconductor layer (7C) were then coated onto the top of layer 7A using the settings specified in Table 8.
• Example 7 Aluminum source and drain contacts were evaporated onto the top of layer 7C through a shadow mask, yielding thin film transistors with channel lengths of 50, 100 or 150 ⁇ m and a channel width of 500 ⁇ m. A photolithographic process was used to isolate devices on the substrate, producing islands of ZnO. Electrical test results for the above device (Sample 7) having the In-doped ZnO gate electrode are shown below in Table 9 and are compared to a check sample (3B) which was identical except that it had an ITO gate electrode.
• first channel flow of first molecular precursor over channel are of substrate
• ALD Atomic Layer Deposition

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