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Definitions

• This invention generally relates to stable MOTFTs with high mobility and a process of fabrication at low temperature.
• MOTFT metal oxide thin film transistors
• Poly-Si cannot be used for such active layer: the high mobility in poly-Si TFTs is achieved by an increase in the grain size to a level comparable to the channel length. Only a small number of grains exist in the channel region which leads to non- uniformity of devices because of its large statistical fluctuation. Moreover, the high mobility in poly-Si TFTs can only be achieved at relatively high temperature (typically, beyond 500°C). A similar trend also existed in CdSe based TFTs, in which high mobility is achieved when the active layer is formed (or post baked) over a certain temperature and when grain size becomes substantial compared to the channel length.
• each different TFT can include from one or two poly-silicon crystalline grains to several crystalline grains and the different number of crystals in the conduction area will produce different characteristics.
• the dimensions and their physical characteristics among different grains are also different.
• the channel length of presently standard thin film transistors is less than approximately 5 microns, especially for portable display applications.
• amorphous is defined as a material with grain size, along the channel length, much less than the channel length of presently standard thin film transistors, e.g. approximately 100 nanometers or less.
• a MOTFT with a channel layer formed of amorphous or nanocrystalline metal oxide insures uniformity similar to a-Si TFT by having a very large number of grain boundaries which results in much smaller performance fluctuations between devices.
• a typical MOTFT based on amorphous In-Ga-Zn-0 has a mobility less than 15 cm 2 /Vs.
• a high quality Poly-Si TFT has a mobility around 40 to 100 cm 2 /Vs.
• Many display applications call for mobility and stability as good as those demonstrated by poly-Si TFTs. It would, therefore, make a MOTFT even more attractive to improve the mobility beyond 40 cm 2 /Vs.
• the mobility strongly depends on the volume carrier concentration of the channel layer. To achieve high mobility, the volume carrier concentration has to be equal to or greater than 10 18 /cm 3 . But there is a constraint on how high the volume carrier concentration can be raised.
• V th a threshold voltage
• OLED organic light emitting diodes
• inorganic LEDs typically operate in a desired range of 0-10V and in a 0-15V range for AMLCDs.
• the charge under control by the gate voltage is C g (V g -V th ), where C g is the gate capacitance, V g is the gate voltage, and Va, is the threshold voltage.
• the volume carrier concentration is constrained by C g (V g -V th )/d, where d is the thickness of the carrier transport layer (MOTFT channel).
• the thickness 'd' of the carrier transport layer should be made as small as possible.
• the thickness d is constrained by the surface quality (such as roughness and uniformity) of the underlying substrate. Typical surface roughness under the channel layer is around 0.2-2nm for the cases of dielectric layers on glass or polymer based substrates.
• the stability may be or is easily compromised by the environment.
• the very thin active layer can be attacked and damaged or even destroyed by various processing materials.
• the operating stability may be compromised if the very thin active layer is exposed to oxygen or water during device fabrication and operation.
• MOTFT metal oxide thin film transistor
• MOTFT metal oxide thin film transistor
• MOTFT metal oxide thin film transistor
• MOTFT metal oxide thin film transistor
• the desired objects of the instant invention are achieved in accordance with a method of fabricating a stable high mobility amorphous MOTFT including the steps of providing a substrate with a gate formed thereon and a gate dielectric layer positioned over the gate. The method further includes the steps of depositing a carrier transport structure on the gate dielectric layer.
• the carrier transport structure includes a layer of amorphous high mobility metal oxide adjacent the gate dielectric and a protective layer of material relatively inert compared to the layer of metal oxide, and depositing source/drain contacts on the protective layer.
• the desired objects of the instant invention are achieved in accordance with a specific method of fabricating a stable high mobility amorphous MOTFT including steps of providing a substrate with a gate formed thereon and a gate dielectric layer positioned over the gate.
• the method further includes a carrier transport structure deposited by sputtering on the gate dielectric layer.
• the carrier transport structure includes a layer of amorphous high mobility metal oxide adjacent the gate dielectric and a relatively inert protective layer of material deposited on the layer of amorphous high mobility metal oxide both deposited without oxygen and in situ.
• the layer of amorphous metal oxide has a mobility above 40 cm 2 /Vs and a carrier concentration in 18 -3 19 -3
• Source/drain contacts are positioned on the protective layer and in electrical contact therewith.
• a stable high mobility amorphous MOTFT including a substrate with a gate formed thereon and a gate dielectric layer positioned over the gate.
• a carrier transport structure is sputtered on the gate dielectric layer.
• the carrier transport structure includes a layer of amorphous high mobility metal oxide adjacent the gate dielectric with a thickness in a range of equal to or less than approximately 5nm and preferably approximately 2nm, a protective layer of relatively inert material deposited on the layer of amorphous high mobility metal oxide with a thickness in a range of equal to or less than approximately 50nm, and the layer of amorphous metal oxide having a mobility above 40 cm 2 /Vs and a carrier concentration in a range of approximately 10 18 cm “3 to approximately 5xl0 19 cm “3 .
• Source/drain contacts are positioned on and in electrical contact with the protective layer.
• FIG. 1 shows a simplified layer diagram illustrating several steps in a process of fabricating a high mobility stable amorphous metal oxide transport layer for use in a high mobility metal oxide thin film transistor (MOTFT), in accordance with the present invention
• FIG. 2 illustrates an example of a stable high mobility metal oxide thin film transistor (defined as an "etch-stop" MOTFT) incorporating the high mobility stable transport layer of FIG. 1 , in accordance with the present invention
• FIG. 3 illustrates an example of another stable high mobility metal oxide thin film transistor (defined as a "back-channel etch" MOTFT) incorporating the high mobility stable transport layer of FIG. 1 , and the source/drain contacts positioned on and in electrical contact with the protective layer in accordance with the present invention;
• a back-channel etch MOTFT another stable high mobility metal oxide thin film transistor
• FIG. 4 illustrates graphically Id-Vgs and mobility- Vgs data sets from the MOTFT disclosed in this invention.
• Data in FIG. 4A are from a MOTFT with etch- stop structure described in FIG. 2, and data in FIG. 4B are from a MOTFT with back- channel-etching structure illustrated in FIG. 3 ; and
• FIG. 5 illustrates graphs of Id-Vgs and mobility- Vgs curves from a MOTFT made in accordance with the present invention on a flexible PET substrate.
• MOTFT channel the carrier transport layer
• a substrate 12 which may be any convenient supporting material and in a preferred embodiment is a material transparent to a radiation wavelength used in a self-alignment procedure, or transparent in the case of bottom emission light emitting displays or transmissive liquid crystal displays.
• Typical materials for the transparent substrate 12 include glass, plastic film, etc. In applications that do not require substrate transparency, polished stainless-steel sheets can also be used.
• Substrate 12 can be in rigid, conformable, or flexible forms. Fabrication of MOTFTs on a thin flexible plastic substrate requires all process temperatures below its glass temperature, Tg (above which permanent deformation occurs so that pattern alignment among different layers becomes impractical). Tg in typical polymer substrates are in a 160°C(PET)-390°C (PI) range.)
• a gate metal layer 14 is deposited on the surface of substrate 12 in any well known manner.
• a thin layer 16 of gate dielectric material is formed over gate metal 14 and may be any convenient material that provides the desired dielectric constant for TFT operation. Suitable materials for the gate dielectric layer include Si0 2 , SiN,
• metal-oxide dielectric above can also be made with surface oxidation from the corresponding metal. Examples of surface oxidation include heating in oxygen- rich ambient, anodization, or their combination in sequence.
• a carrier transport structure 18 includes a lower active transport layer dl of semiconductor metal oxide deposited over the upper surface of layer 16 and a protective layer d2 deposited directly on layer dl.
• layer d2 is deposited right after layer dl without breaking vacuum in the deposition chamber so that for purposes of this disclosure layers dl and d2 are considered in combination as a carrier transport structure.
• sputter depositing the layers without oxygen presence is preferred to achieve the high carrier mobility as is described in more detail presently.
• the best materials for active transport layer dl are transparent metal oxide semiconductors such as indium-tin-oxide (ITO), indium oxide (InO), tin oxide (SnO), cadmium oxide (CdO), zinc oxide (ZnO), indium-zinc-oxide (IZO), and the like.
• Composite films comprising multiple metal-oxide compositions listed above can also be used. Control of chemical purity and intentional chemical doping can optimize the carrier mobility at the desired carrier concentration level.
• carrier transport structure 18 along with the proper selection of the dielectric constant of the gate dielectric material forming layer 16, a MOTFT with a carrier concentration in active transport layer dl as high as 5xl0 19 cm ⁇ 3 can be fabricated with effective current switching between source and drain electrodes.
• the carrier concentration in active transport layer dl is in a range of approximately 10 18 cm “3 to approximately 5xl0 19 cm "3 .
• the thickness of the carrier transport layer should be made as small as possible.
• the thickness of active transport layer dl is in a range of less than approximately 5nm and may conveniently be as low as approximately 2nm.
• thin transport layer dl has to be protected by some more inert layer, such as protective layer d2.
• inert is defined to mean that layer d2 has a much lower carrier mobility and carrier concentration, or is either an insulating material or is much closer to an insulating material.
• Protective layer d2 is preferably deposited directly on and substantially in the same deposition process (i.e. in situ or without breaking vacuum in the deposition chamber) as thin transport layer dl .
• protective layer d2 By depositing layer d2 directly on layer dl , thin transport layer dl is not exposed to ambience or processing chemicals.
• the best materials to use in protective layer d2 are metal oxides that are more inert than transport layer dl, such as M-Zn-0 or M-In-0 or their combination in which M includes at least one of Al, Ga, Ta, Ti, Si, Ge, Sn, Mo, W originate Cu, Mg, V, Zr, or the like, with sufficient M content to ensure a desired inert state.
• M component possesses generally higher bonding strength to oxygen that makes the protection layer d2 substantially more inert than the Dl layer.
• the bilayer structure and the corresponding carrier transport within the bilayer structure guarantees uniform conduction over large substrate areas even on a gate insulator with a relatively rough surface.
• thin amorphous transport layer dl is deposited by sputtering at low temperatures, preferably room temperature but no more than 160°C. Such low temperature process enables the TFT disclosed in this invention to be made on plastic substrates (such as PET, PEN, PAN, PAS, PI, etc).
• plastic substrates such as PET, PEN, PAN, PAS, PI, etc.
• the residual carrier concentration in the thin transport layer dl should be as high as possible.
• no oxygen is introduced in the sputtering process. In a sputtering process incorporating oxygen, the oxygen is negative charged by the deposition system and accelerated toward the substrate.
• oxygen-free sputter process disclosed in the present invention enables one to achieve the needed carrier concentration and high mobility with a nominal thickness for transport layer dl thinner than 5nm.
• the inert metal oxide of protective layer d2 is deposited by sputtering directly on layer dl at low temperatures, preferably room temperature, but no more than 160°C.
• Protective layer d2 is formed with a thickness in a range of up to approximately 20nm or greater.
• the same problems are prevalent as discussed above for active layer dl. If oxygen is introduced into the sputtering gas, the negatively charged oxygen will be accelerated toward the substrate. Some of the accelerated oxygen may penetrate into active transport layer dl and create metastable oxygen in active transport layer dl, resulting in mobility and stability degradation. Therefore, in the preferred process the inert metal oxide of protective layer d2 is deposited by sputtering without oxygen.
• protective layer d2 may become too conductive and move the threshold voltage of the MOTFT too negative.
• an oxidizing ambience e.g. >160°C
• protective layer d2 can be oxidized to move the threshold toward the positive direction.
• the problem with annealing in ambient atmosphere is that the process will generally be too slow, especially for temperatures below 200°C. Therefore, the desired oxidized result is achieved in a two step process that is very effective at low temperatures (i.e. ⁇ 160°C).
• a first step of the oxidizing process the surface of the inert metal oxide of protective layer d2 is treated by a chemically oxidizing process.
• the surface oxygen is driven into protective layer d2 at an elevated temperature. It has been confirmed that by driving oxygen into protective layer dl a MOTFT with a switch threshold voltage larger than 0V can be achieved at a temperature at or below approximately 160°C.
• the forming of an oxygen source at the top surface of the metal oxide of protective layer d2 can include any one of a variety of possible options.
• the surface oxidizing can include, for example, the use of a high pressure (>100mtorr) oxygen plasma that would not include any high energy ions that can generate a metastable state.
• Another oxidizing option is the use of a high pressure (>100mtorr) N 2 0 plasma.
• Another oxidizing option is the use of ultraviolet ozone.
• protective layer d2 with a self-assembled monolayer such as 4- chlorophenyl trichlorosilane (4-CPTS), chloromethyl trichlorosilane (CMTS), 4- chlorophenyl phosphonic acid (4-CPPA), 3-nitrophenyl phosphonic acid (3-NPPA), and 2-chloroethyl phosphonic acid (2-CEPA).
• a self-assembled monolayer such as 4- chlorophenyl trichlorosilane (4-CPTS), chloromethyl trichlorosilane (CMTS), 4- chlorophenyl phosphonic acid (4-CPPA), 3-nitrophenyl phosphonic acid (3-NPPA), and 2-chloroethyl phosphonic acid (2-CEPA).
• a self-assembled monolayer such as 4- chlorophenyl trichlorosilane (4-CPTS), chloromethyl trichlorosilane (CMTS), 4- chlorophenyl phosphonic acid (4-CPPA), 3-nitrophen
• the purpose of the surface modification is to deposit a concentrated oxygen source/reservoir on the surface of protective layer d2 which is much more concentrated than the oxygen in the ambience.
• Such deposition process is done at low temperature, for example at room temperature without intentional substrate heating. Oxygen diffusion during this process is negligible.
• the concentrated oxygen source on the surface of protective layer d2 is then subjected to an elevated temperature (e.g. at ⁇ 160°C, or higher) which causes the oxygen to migrate into protective layer d2.
• the migration of oxygen into protective layer d2 makes the inert metal oxide more oxidized and moves the threshold voltage closer to zero.
• the oxygen will stay in protective layer d2 because the composition of the inert metal oxide selected for protective layer d2 is more stable than the material forming active transport layer dl.
• the oxidizing process for protective layer d2 enables a lower elevated temperature step because of the very concentrated source of oxygen at the surface (i.e. the oxygen diffusion is concentration dependent).
• the process above provides a method to make a high mobility, amorphous bi- layer on plastic substrates (such as PET, PEN, PAN, PAS, and PI).
• plastic substrates such as PET, PEN, PAN, PAS, and PI.
• the maximum temperature of ⁇ 160°C fits with PET substrates.
• PEN, PAN, and PAS the temperature for the diffusion process can be increased to a 180-220°C range.
• higher treatment temperature can be chosen to reduce the process times.
• the treatment temperature can be selected in an even broader range.
• the oxygen diffusion process is preferred to be carried out below 350°C.
• MOTFT 10 is defined as an "etch-stop" MOTFT.
• etch-stop MOTFT 10 includes transparent substrate 12, which may be any convenient material transparent to radiation (i.e. self-alignment exposure) wavelengths used in the self- alignment procedure, such as glass, plastic, etc.
• Gate metal layer 14 is patterned on the upper surface of substrate 12 by any convenient means. Since the position of gate metal layer 14 is not critical virtually any non-critical patterning technique can be used.
• gate metal layer 14 in addition to or instead of forming gate metal layer 14 with a physical vapor deposition process (such as sputter, a-beam, thermal deposition, etc.) and patterned by photolithography with a proximity or a projection tool, the gate layer can be formed with any of the various printing processes known to experts in the field, including ink jetting, dispensing, imprinting, transfer printing or off-set printing methods.
• the gate metal can also be formed with a plating method known in the art.
• traditional photolithography layer 14 can also be patterned with laser writing lithography. While a single gate metal 14 (i.e. single MOTFT) is illustrated for convenience in understanding, it will be understood that this might represent one or more (even all) of the TFTs used in a backplane or other large area applications.
• Thin layer 16 of gate dielectric material is formed over gate metal 14 and the surrounding area.
• the term "surrounding area" includes at least the area illustrated in FIG. 2 (i.e. the gate and channel areas and the source/drain areas).
• layer 16 may be a blanket layer covering the entire large area application and no alignment is required.
• the gate dielectric material may be any convenient material that provides the desired dielectric constant for TFT operation. Typical inorganic materials include Si0 2 , SiN, AI2O 3 , Ta 2 Os, Ti0 2 , Hf0 2 , Zr0 2 , SrO and the like. Organic dielectrics can also be used for layer 16. For example, a metal-oxide TFT with organic gate dielectric is disclosed in U.S. Pat. 7,772,589. In addition to single compounds, gate dielectric layer 16 can be constructed with these materials in mixed composite form, or in a multiple-layer stack.
• typical materials for active transport layer dl are transparent metal oxide semiconductors such as indium-tin-oxide (ITO), indium oxide (InO), tin oxide (SnO), cadmium oxide (CdO), zinc oxide (ZnO), indium-zinc-oxide (IZO), and the like.
• ITO indium-tin-oxide
• InO indium oxide
• SnO tin oxide
• CdO cadmium oxide
• ZnO zinc oxide
• IZO indium-zinc-oxide
• Composite films comprising multiple metal-oxide components listed above can also be used.
• a desired carrier concentration can be achieved by chemical doping and by the bilayer dl/d2 structure deposited by a sputter process sequentially in an oxygen- free environment and without substrate heating.
• the completed d2 layer is more inert chemically and more resistive electrically
• the pristine deposited d2 layer in an oxygen-free environment is selected (through the proper selection of the metal components) to have a higher tendency to attract oxygen from both the top surface and the bottom surface contacting layer dl .
• Attracting oxygen from active transport layer dl results in optimized carrier concentration and carrier mobility in active transport layer dl.
• Carrier transport structure 18 can then be patterned with standard photolithography and a surface oxidation process is performed followed with an oxygen diffusion process at an elevated temperature.
• the resulting structure 18 is amorphous/nanocrystalline without crystalline structure beyond lOOnm.
• a passivation/etch-stop layer 20 is then deposited on carrier transport structure 18 and patterned.
• a photo-patternable organic material can also be used for the layer 20.
• the selection principle for layer 20 is that the material and the corresponding forming process do not create damage in the underlying carrier transport structure 18.
• Layer 20 is used as an etch stop/passivation layer during the following processes.
• the patterning can be accomplished with regular photolithography or by a self-aligned process using the gate pattern as a built-in mask. Details on the self- aligned process are disclosed in U.S. Patents 7,605,026 and 7,977,151.
• the source/drain areas 22 can be formed by physical vapor deposition and by standard etching methods known to artisans in the field. Areas 22 can alternatively be formed with self-aligned process by means of gate layer pattern and a non-crucial lithography mask, or by means of an add-on/printing method (such as plating) as disclosed in U.S. Patent 7,977,151 and U.S. Patent Application 13/406,824.
• Optional cleaning/treatment/etching processes could be inserted after the processing of etch stop/passivation layer 20 and before deposition of source/drain areas 22 to improve the electrical contact between carrier transport structure 18 and S/D electrodes (source/drain areas 22).
• Etch stop/passivation layer 20 provides the needed protection to the channel area during these processes.
• MOTFT 30 includes a substrate 34 with a bottom gate 36 formed thereon and a gate dielectric layer 38 overlying gate 36.
• An active layer 40 of metal oxide is formed on gate dielectric layer 38 and source/drain metal contacts 42 are positioned in a spaced apart relationship on active layer 40 to define a channel area therebetween in a well known manner.
• Active layer 40 is a carrier transport structure constructed similar to carrier transport structure 18 of FIG. 1 and includes a lower active transport layer dl of high mobility amorphous semiconductor metal oxide deposited over the upper surface of layer 38 and a protective layer d2 deposited directly on layer dl.
• This type of MOTFT is known as a 'back-channel etch' MOTFT and illustrates that a stable high mobility amorphous MOTFT can also be fabricated using the back-channel-etching process.
• Source/drain metal contacts 42 are preferably formed by first depositing a blanket layer of contact metal. After blanket depositing the S/D metal layer, the layer is patterned by either a dry or wet etchant to open the channel area, designated 44. The surface of channel area 44 can be cleaned by a washing procedure and followed with a surface treatment procedure. In 'back-channel etch' MOTFT 30 configuration, channel area 44 is available for the step or steps of oxidation and oxygen drive-in after the S/D patterning and is preferred at this point in the fabrication process.
• additional organic/inorganic passivation layer(s) and electrode layers may be needed subsequent to the formation and patterning of the S/D layer. Due to the improved chemical resistance disclosed in this invention, a larger process window, more process methods, and broader material selection can be used for the following processes.
• FIG. 4 two typical Id-Vgs data sets are illustrated from MOTFTs fabricated in accordance with the present invention.
• a data set is illustrated from a TFT with etch-stop structure as described in conjunction with FIG. 2.
• a data set is illustrated from a TFT with BCE structure as described in conjunction with FIG. 3.
• the corresponding linear mobility and saturate mobility are also shown at the right side of each FIG.
• the gate metal for these TFTs was an Al-Nd alloy.
• the gate insulator was formed by surface anodization at room temperature.
• the transport layer was ITO with an In:Sn ratio of 90: 10.
• the thickness of layer dl was 2.5nm.
• Protection layer d2 was In-Al-Zn-0 with a thickness of 30nm.
• Layers dl and d2 were deposited by sputtering without oxygen at room temperature.
• An oxygen source zone was formed at the surface of layer d2 by means of oxygen plasma as disclosed above.
• a process of driving the oxygen into layer d2 from the oxygen source zone was carried out at 300°C for 30 minutes.
• a photopatternable polyimide was used as the passivation/etch-stop layer 20 in FIG. 2.
• the oxygen surface forming and driving- in processes were carried out after S/D deposition and patterning.
• the current switch voltage is close to 0V.
• Sub-threshold voltage swing is - 0.1V, as good as that in high-end LTPS-TFT.
• TFT with BCE structure FIG. 4B
• TFTs made with the fabrication methods disclosed in this invention show superb stability.
• the total charge passing through the TFT was over 70 Coulombs.
• TFT stability under backlight illumination also needs to be considered.
• Such bilayer channel structure has intrinsic stability under whiye light illumination.
• Experimental yesy results under a backlight unit with light intensity close to that in LCD TV and portable device applications confirmed the stability shifts less than IV after 2 hours of testing.
• the mobility beyond 40cm 2 /Vsec enables displays with 8000 columns and 4000 rows with frame rates up to 480Hz.
• Such high mobility and stability MOTFT can also be used for thin film electronics beyond display arrays. Examples include high pixel density and high frame rate imager sensor arrays, pressure sensor arrays, touch sensor arrays, chemical sensor arrays, or biosensor arrays.
• the process method and thin film process tools for large size substrates enables many applications in-capable with electronic circuits based on silicon wafers.
• FIG. 5 graphs of Id-Vgs and mobility- Vgs curves from a MOTFT made on a flexible PET substrate are illustrated.
• the biasing stability and current operation stability are close to that shown in the examples above.
• a new and improved process for fabricating a stable high mobility metal oxide thin film transistor is disclosed. Further, the new and improved process for fabricating a stable amorphous metal oxide thin film transistor (MOTFT) describes the fabrication of a MOTFT with mobility at or above 40 cm 2 /Vs. Also, a new and improved stable amorphous high mobility metal oxide thin film transistor (MOTFT) is disclosed.
• the fabrication process includes steps for depositing a carrier transport structure with a high mobility layer and a relatively inert protective layer deposited in situ with the high mobility layer to protect the high mobility layer from damage due to subsequent processing steps and ambient gasses. Also, the MOTFT is fabricated at room temperature and no greater than 160°C. Further, in the steps of depositing a carrier transport structure with a high mobility layer and a relatively inert protective layer, no oxygen is present.
• the examples described include various transparent and opaque layers for purposes of self-alignment, it will be understood that if self-alignment is not used in the fabrication process the various layers may be transparent or opaque depending upon the specific material used in the formation thereof and the application of the final product.
• the various applications require no transparency from the substrate so that, for example, flexible stainless steel foil can be used as the substrate material.
• the MOTFT disclosed in this invention enables conformable or flexible electronic devices and apparatus.

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• 2013
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