WO2006003619A1 - A thin film transistor, method of producing same and active matrix display - Google Patents

A thin film transistor, method of producing same and active matrix display Download PDF

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Publication number
WO2006003619A1
WO2006003619A1 PCT/IB2005/052143 IB2005052143W WO2006003619A1 WO 2006003619 A1 WO2006003619 A1 WO 2006003619A1 IB 2005052143 W IB2005052143 W IB 2005052143W WO 2006003619 A1 WO2006003619 A1 WO 2006003619A1
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WO
WIPO (PCT)
Prior art keywords
layer
gate
thin film
film transistor
semiconductor
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PCT/IB2005/052143
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French (fr)
Inventor
Dirkjan B. Van Dam
Johan Marra
Martinus P. J. Peeters
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Koninklijke Philips Electronics N.V.
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Application filed by Koninklijke Philips Electronics N.V. filed Critical Koninklijke Philips Electronics N.V.
Publication of WO2006003619A1 publication Critical patent/WO2006003619A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
    • H01L29/76—Unipolar devices, e.g. field effect transistors
    • H01L29/772—Field effect transistors
    • H01L29/78—Field effect transistors with field effect produced by an insulated gate
    • H01L29/786—Thin film transistors, i.e. transistors with a channel being at least partly a thin film
    • H01L29/7869—Thin film transistors, i.e. transistors with a channel being at least partly a thin film having a semiconductor body comprising an oxide semiconductor material, e.g. zinc oxide, copper aluminium oxide, cadmium stannate
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/66007—Multistep manufacturing processes
    • H01L29/66969—Multistep manufacturing processes of devices having semiconductor bodies not comprising group 14 or group 13/15 materials

Definitions

  • a thin film transistor method of producing same and active matrix display
  • This invention relates to an array of thin film transistors, a method of producing same, and a flat panel display, such as an active matrix display.
  • Active matrix technology is a method of sending electric charges to the pixel electrodes of selected pixels of an LCD display for the purpose of selective pixel activation or deactivation, and for maintaining pixels in an actuated or deactivated state for a predetermined, limited period of time.
  • the most common example of an active matrix display is a TFT, or thin-film transistor, LCD.
  • a passive matrix uses a simple conductive grid composed of othogonal sets of row and column electrodes, respectively, to deliver electric charges to certain pixels, both for addressing purposes and for activation purposes
  • an active matrix display uses a grid of transistors, each transistor being associated with a single pixel, with the ability to selectively activate or deactivate individual pixels for a limited period of time.
  • Fig. 1 illustrates schematically the construction of a general conventional liquid crystal display using bottom-gate thin film transistors (hereinafter referred to as TFTs), and illustrates one example of the structure of a TFT array board.
  • TFTs bottom-gate thin film transistors
  • gate lines 50 and source lines 51 are arranged on a transparent substrate in a matrix pattern.
  • Each of the areas surrounded by the gate lines and the source lines 51 serves as one pixel 52, and a TFT 53 is provided for each pixel 52.
  • Fig. 2 is a schematic cross-sectional illustration of the TFT 53, in which a gate electrode 55 led out of the gate line 50 is formed on a transparent substrate 54, and the gate insulating film 56 is formed in covering relation to the gate electrode 55.
  • a semiconductor active film 57 made of amorphous silicon is formed on the gate insulating film 56 at a position above the gate electrode 55.
  • Asource electrode 59 led out of the source line 51 and a drain electrode 60 are formed to extend over the semiconductor active film 57 through an ohmic contact layer 58 which is made of amorphous silicon containing an n-type impurity such as phosphorous, and then on the gate insulating film 56.
  • a passivation film 61 is formed in covering relation to the TFT 53 made up of the source electrode 59, the drain electrode 60, the gate electrode 55, etc., and a contact hole 62 is formed in the passivation film 61 at a position above the drain electrode 60. Further, a pixel electrode 63 formed of a transparent conductive film, such as indium tin oxide (ITO), is filled in the contact hole 62 for electrical connection to the drain electrode 60.
  • ITO indium tin oxide
  • Additional advantages of printing type methods are that they ideally require only two process steps: a deposition step and a curing step, and they use much less material and clean room space than lithography.
  • the semiconductor layer is the most difficult layer to print, because the printing of many square-shaped isolated structures (see the conventional array of substantially square semiconductor islands 57 in the structure described with reference to Fig. 1 of the drawings), without a previous patterning of the surface, is difficult to do.
  • a thin film transistor comprising a substrate on which is provided a gate layer, a dielectric layer covering said gate layer, and a layer of semiconductor material disposed over said dielectric layer substantially in alignment with and facing said gate layer.
  • said gate layer comprises a gate line and a gate electrode led out of said gate line.
  • the semiconductor layer is preferably in the form of a line, beneficially provided with a lateral protrusion which is beneficially of substantially the same size and shape as the gate layer.
  • At least one of the gate layer and the semiconductor layer, and most preferably the gate layer is deposited using an inkjet printing method.
  • at least one of the gate layer and the semiconductor layer, and most preferably the semiconductor layer is deposited using an electrostatic aerosol deposition process and more preferably a patterned electrostatic aerosol deposition process.
  • the semiconductor material could be deposited using any suitable method, including electrohydrodynamic spraying (e.g. using the Taylor-cone method, the general principle of which is described in, for example, US Patent No. 6,454,193), any aerosol deposition process including a patterned aerosol deposition process or other guided aerosol deposition process, inkjet printing, dispensing, spray pyrolysis, etc.
  • electrohydrodynamic spraying e.g. using the Taylor-cone method, the general principle of which is described in, for example, US Patent No. 6,454,193
  • any aerosol deposition process including a patterned aerosol deposition process or other guided aerosol deposition process
  • inkjet printing dispensing
  • spray pyrolysis etc.
  • the gate layer can be deposited using any suitable method including electrohydrodynamic spraying (e.g. using the Taylor-cone method), any aerosol deposition process, including a patterned aerosol deposition process or other guided aerosol deposition process, inkjet printing, dispensing etc.
  • the semiconductor material is beneficially a high-mobility semiconductor material, such as Indium Sulfide (InS), CdS, CdO, ZnS or ZnO.
  • the semiconductor material may comprise a high mobility organic semiconductor material, i.e.
  • a polymer such as polythiophene, poly(alkylthiophene), pentacene, copolymer of fluorine and bithiophene, polythienylenevinylene, thiophene-based oligomers, phthalocyaiine, lutecium diphthalo-cyanine (Pc2Lu), thullium diphthalo-cyanine (Pc2Tm), Fullerene (C60/C70), TetraCyano-p-Quinodimethane (TCNQ), perylene-tetracarboxylic diimide (PTCDI)-Ph, TetraCyano-n-Quinodimethane (TCNNQ), naphthaline-tetracarboxylic-diimide (NTCDI), naphthalene-tetracarboxylic-dianhydride (NTCDA), perylene-tetracarboxylic-dianhydride (PTCDA), perylene-t
  • a method of manufacturing a thin film transistor comprising depositing a gate layer on a substrate, depositing a layer of dielectric material over said gate layer, and depositing a layer of semiconductor material in the form of a line on said dielectric layer substantially in alignment with and facing said gate layer.
  • the step of depositing said gate layer on said substrate comprises depositing a gate line and a gate electrode led out of said gate line.
  • the layer of semiconductor material is of substantially the same size and shape as the gate layer.
  • the method preferably includes the step of depositing at least one of the semiconductor layer or, more preferably, the gate layer by means of an inkjet printing process.
  • the method preferably also includes the step of depositing at least one of the gate layer or, more preferably, the semiconductor layer by means of an electrostatic aerosol deposition process, and more preferably a patterned electrostatic aerosol deposition process.
  • the method preferably further includes the steps of forming at least source and drain lines after deposition of said semiconductor layer.
  • the present invention extends further to an active matrix display device comprising a plurality of thin film transistors as defined above.
  • Fig. 1 is a schematic plan view of a known active matrix display
  • Fig. 2 is a schematic cross-sectional view of a known TFT structure
  • Fig. 3 a is a partial schematic illustration of a TFT structure according to a first exemplary embodiment of the present invention
  • Fig. 3b is a partial schematic illustration of a TFT structure according to a second exemplary embodiment of the present invention
  • Fig. 4a is a schematic diagram illustrating apparatus with which an electrostatic aerosol deposition of a thin film material can be performed according to a first exemplary embodiment of the present invention
  • Fig. 4b is a schematic diagram illustrating apparatus with which an electrostatic aerosol deposition of a thin film material can be performed, according to a second exemplary embodiment of the present invention.
  • Figs. 5 and 6 are schematic diagrams illustrating a patterned aerosol deposition process which can be used in accordance with an exemplary embodiment of the present invention.
  • a gate layer 12 made of a metal or metal colloid is formed on a transparent substrate 14 which may comprise, for example, glass or plastic.
  • a dielectric layer 16 (or gate insulating film) is provided over the gate layer 12 and a semiconductor layer 18 is provided on the dielectric layer 16, the semiconductor layer 18 being embodied as a straight line (instead of having the conventional 2D square shape) having a shape which mirrors and faces the shape of the gate layer 12 and being in alignment therewith.
  • the remaining layers (not shown), i.e. source line, drain electrode, etc., are deposited after.
  • a TFT structure according to a second exemplary embodiment of the invention is similar in many respects to that of Fig. 3 a, except in this case a gate layer comprising a gate line 12 and a gate electrode 13 led out of the gate line 12 is formed on the substrate 14, and the semi-conductor layer 18 is embodied as a substantially straight line having a lateral protrusion, and has a shape which mirrors and faces the shape of the gate layer formed by the gate line 12 and the gate electrode 13 led out of the gate line 12.
  • a TFT in accordance with the present invention has been found to display good electrical characteristics.
  • the gate layer structure formed by the gate line 12 and the gate electrode 13 led out of the gate line 12 is preferably inkjet printed onto the substrate 14, then the dielectric layer 16 is deposited over the gate layer 12,13 and an electrostatic aerosol deposition method is beneficially used to perform a patterned deposition of the semiconductor layer 18 on top of the dielectric layer 16 substantially in alignment with and facing the gate layer 12,13.
  • InkJet printing is a well known deposition process in which extremely small dots of "ink” are placed via a nozzle onto a substrate.
  • the "ink" in this case, would comprise a suitable conductive (for the gate line and the gate electrode) or semiconductor material dissolved or suspended in a carrier fluid, and the process involves two steps: applying droplets of the ink to the substrate to create the desired form of the deposition pattern, and causing or allowing the ink to dry to form the final layer pattern.
  • Two, similar, electrostatically-augmented aerosol deposition processes, which could be used to patternwise deposit the semiconductor layer of the present invention, will now be described with reference to Figs. 4 to 6 of the drawings.
  • Electrostatic aerosol deposition as referred to herein may comprise a method in which either solid aerosolised particles or fluid aerolised particles can be generated and dispersed in a carrier gas stream, then size-classified, then unipolar electrostatically charged in an internal high-voltage corona section, then concentration-homogenised in an expansion chamber, and finally either homogeneously or patternwise deposited on a substrate.
  • the aerosolised particles are either composed of or comprise semiconductor materials and/or semiconductor precursor materials.
  • a particle feeder 100 consisting of a transportation piston 101 in communication with a volume of powder 102 in a powder reservoir 103.
  • the particle feeder 100 further comprises a dispersion head 104 carrying a rotating brush 105 which transfers dry powder particles in the powder reservoir 103 from a compacted state to an airborne state with the help of both the rotating brush 105 and a dispersion air stream 106 which blows the partcles out of the brush and carries them away in aerosolised form 107.
  • a particle feeder 100 of this type enables the dispersion of powders with particle sizes down to well below 1 micrometer in diameter in a carrier gas stream.
  • a liquid aerosol generator such as a spinning disk generator, a vibrating orifice generator, an ultrasonic nebuliser, an ordinary compressed-air nebuliser, or a compressed-air nebuliser 200 comprising a Laskin nozzle 201 (from which is emitted an air-jet 202, as shown in Fig. 4b of the drawings, for the dispersion in a carrier gas stream of liquid particles comprising dissolved or suspended solid semiconductor materials and/or semiconductor precursor materials (derived from a body of liquid 203 containing dissolved or suspended semiconductor materials or semiconductor precurser materials).
  • the dispersion of the liquid aerosol is performed in a first solvent-saturated gas stream 204.
  • Size classification of the liquid aerosol is performed by means of a baffle plate 206, after which the liquid aerosol, present in the first gas stream, is mixed with a second gas stream.
  • the volume flow of the gas streams, the temperature, and the size of the expansion chamber 207 determine the evaporation kinetics that affects the size of the liquid aerosol particles 205:- the evaporation kinetics and the size and/or composition of the liquid aerosol particles can thus be tuned.
  • size classification of said aerosolised particles involves the passage of the aerosol in a carrier gas stream through a cyclone 108 from where the aerosol cloud 109 passes into a glass expansion chamber 110 and through a dust filter which may be a simple mechanical filter or, as in this case, a dielectric filter 111, such as a course fibrous dielectric filter sandwiched between two conductive metal gauzes between which a voltage differential V f iite r has been applied.
  • the filter 111 removes the larger particles and transmits the smaller particles, such that large solid particle aggregates are removed.
  • a liquid aerosol generator generates a mist of liquid drops, each drop containing finely dispensed or dissolved semiconductor (precursor) material.
  • the baffle plate 206 ensures a proper size classification.
  • the evaporation speed of the aerosol droplets 205, either in the aerosol phase or on the substrate, can be tuned by regulating the support dry air stream and/or the temperature and/or the size of the top expansion chamber 207.
  • the size-classified, solid or liquid, aerosolised particles then pass to a high- voltage corona charging section comprising a high- voltage needle electrode 112 supported by two high- voltage gauzes at V corona and defining an ionising needle tip 112a at V coro na, and a water-irrigated counter electrode 113 defined, for example, by a water-irrigated earther aluminium surface covered with hygroscopic glass- fibre paper.
  • the water irrigation of the counter electrode surface is important for removing deposited particles from the counter electrode surface, thus avoiding trouble with back-corona events and electrical polarisation effects in the deposited particle layer on the counter electrode.
  • the illustrated high-voltage corona charging section further comprises insulator plates 114.
  • a top glass expansion chamber 115 is provided for the enhanced concentration homogenisation of the charged aerosol in the carrier gas stream under the influence of the space charge effect in the charged aerosol cloud.
  • the charged particles dispersed in the carrier gas stream leave the top expansion chamber 115 and enter into the deposition chamber via an aerosol outlet provided by a porous (metal) gauze 116 in, and electrically connected to, a high- voltage (metallic) deposition electrode plate 117 which is set at a voltage V dep o s i t i o n- Some distance above the deposition electrode plate 117, the substrate plate 118, whereon aerosol deposition is required to occur, is positioned substantially in parallel with the deposition electrode plate 117.
  • the deposition chamber is physically bounded by the substrate plate 118 and the deposition electrode plate 117 facing each other but left substantially open to the outside environment at all other sides, thus the carrier gas stream can freely flow away to the sides and along the entire side surface of the substrate plate 118 facing the deposition electrode plate 117.
  • the substrate plate 118 is, in this case, backed by, and capacitively coupled to, an earthed backing electrode plate 119 such that the charged particles 120 are always drawn towards the substrate plate 118 by means of the electric field existing between the substrate plate 118 and the deposition electrode plate 117, the idea being to make the electric field sufficiently high to remove virtually all charged aerosol particles from the carrier gas flow and deposit them onto the substrate plate 118 during their residence time inside the deposition chamber, while the lateral flow of the carrier gas along the surface of the side of substrate plate 118 facing the deposition electrode plate 117 ensures a lateral spreading of the depositing particles across the entire substrate surface.
  • the substrate plate 118 is positioned upside down (anti-gravitationally) during aerosol deposition, which makes the substrate plate 118 much less susceptible to become contaminated by depositing dust particles.
  • a xy translator stage in order to allow the substrate plate 118 to undergo a controlled series of lateral movements with respect to the aerosol outlet provided by the porous metal gauze 116 during the aerosol deposition process, thus improving the lateral homogeneity and thickness uniformity of the thin layer formed by the deposited aerosol particles 120 across the surface of the substrate plate 118.
  • the side of the substrate plate 118 facing the deposition electrode plate 117 carries a set of gate lines 11, 12 and associated gate electrodes (not shown) led out of gate lines 11,12.
  • a gate insulating film 56 is present in covering relation with all gate lines 11, 12 and with all gate electrodes led out of gate lines 11, 12.
  • a first voltage (V 1 ) is imposed on gate lines 11, while a different second voltage (V 2 ) is imposed on gate lines 12.
  • the sign and magnitude of the voltages Vi and V 2 with respect to Vdeposition are chosen such that substantially all charged aerosolised particles are deposited only on the parts of the gate insulating film 56 that cover the gate lines 11 and the gate electrodes led out of gate lines 11, resulting in a patterned deposition of semiconductor material or semiconductor precursor material on the parts of the gate insulating film 56 that are aligned with and in covering relation with the gate lines 11 and the gate electrodes led out of gate lines 11, (as shown I Fig. 5).
  • Vi is chosen the same as the voltage on the backing electrode plate 119 e.g. earth potential.
  • the above process is repeated by imposing the first voltage Vi on the gate lines 12 and the different second voltage V 2 on gate lines 11.
  • a patterned deposition of semiconductor material or semiconductor precursor material is accomplished on the parts of the gate insulating film 56 that are aligned with and in covering relation with the gate lines 11, 12 and the gate electrodes led out of gate lines 11 , 12.
  • the advantages of the above-described aerosol deposition set-up include its versatility and reliability, the possibility of generating and processing much smaller aerosol particles than is possible with conventional electrostatic spraying equipment, the economic use of coating material, the upside-down positioning of the substrate plate so as to avoid a ready deposition of contaminating dust particles onto the (tacky) substrate plates under the influence of gravity, and the possibility of allowing for a patterned deposition of aerosol material onto a substrate.
  • Electrostatic aerosol deposition such as that described above has the additional advantage, in the case of the present invention, that it can use previously deposited gate lines and gate electrodes led out of gate lines to manipulate the local electric field close to the substrate surface.
  • the charged aerosol particles can be electrically guided to be deposited right on top of the parts of the dielectric layer that cover the gate lines and the gate electrodes led out of the gate lines.
  • the deposited semiconductor layer faces and becomes self-aligned with respect to the gate lines and the gate electrodes led out of the gate lines.
  • Suitable high-mobility semiconductor materials which can be used to form the semiconductor line of the invention include Indium Sulfate (InS), CdS, CdO, ZnS and ZnO, using for example spray pyrolysis. It will be appreciated by a person skilled in the art that spray pyrolysis involves the spraying of particles of the material onto a substrate which is held at a high temperature within some predetermined temperature range depending on the substrate material and the material being deposited.
  • organic semiconductors i.e. polymers, which include but are not limited to polythiophene, poly(alkylthiophene), pentacene, copolymer of fluorine and bithiophene, polythienylenevinylene, thiophene-based oligomers, phthalocyainine, Pc2Lu, Pc2Tm, C60/C70, TCNQ, PTCDI-Ph, TCNNQ, NTCDI, NTCDA, PTCDA, P 16CuPc, NTCDI-C8F, DHF (Dilute Hydrofluoric acid)-6T and PTCDI-C8.
  • suitable semiconductor materials will be evident to a person skilled in the art.

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Abstract

A thin film transistor (10) for use in, for example, an active matrix LCD display device, comprising an electrode layer structure (12,13), formed by a gate line (12) and a gate electrode (13), that is deposited on a substrate (14) with a dielectric layer (16) disposed thereover, and a layer (18) of high-mobility semiconductor material in this case, in the form of a line provided with a lateral protrusion (18a) disposed on the dielectric layer (16) in alignment with and facing the electrode structure comprising the gate line (12) and the gate electrode (13). The proposed structure has the advantage that the semiconductor line (18) can be deposited by, for example, an inkjet printing process or by a patterned electrostatic aerosol deposition process. The semiconductor material may be organic.

Description

A thin film transistor, method of producing same and active matrix display
This invention relates to an array of thin film transistors, a method of producing same, and a flat panel display, such as an active matrix display.
Active matrix technology is a method of sending electric charges to the pixel electrodes of selected pixels of an LCD display for the purpose of selective pixel activation or deactivation, and for maintaining pixels in an actuated or deactivated state for a predetermined, limited period of time. The most common example of an active matrix display is a TFT, or thin-film transistor, LCD. Whereas a passive matrix uses a simple conductive grid composed of othogonal sets of row and column electrodes, respectively, to deliver electric charges to certain pixels, both for addressing purposes and for activation purposes, an active matrix display uses a grid of transistors, each transistor being associated with a single pixel, with the ability to selectively activate or deactivate individual pixels for a limited period of time. Because transistors can be electrically switched independently from each other, only the pixel electrodes of selected pixels receive electric charges, thereby improving the image quality over that of a passive matrix wherein only a reduced degree of pixel-switching selectivity can be accomplished. Furthermore, because of the thin film transistor's ability to hold a charge, the pixel remains either activated or deactivated until the next refresh. Fig. 1 illustrates schematically the construction of a general conventional liquid crystal display using bottom-gate thin film transistors (hereinafter referred to as TFTs), and illustrates one example of the structure of a TFT array board. In such a TFT array board, as shown in Fig. 1, gate lines 50 and source lines 51 are arranged on a transparent substrate in a matrix pattern. Each of the areas surrounded by the gate lines and the source lines 51 serves as one pixel 52, and a TFT 53 is provided for each pixel 52.
Fig. 2 is a schematic cross-sectional illustration of the TFT 53, in which a gate electrode 55 led out of the gate line 50 is formed on a transparent substrate 54, and the gate insulating film 56 is formed in covering relation to the gate electrode 55. A semiconductor active film 57 made of amorphous silicon is formed on the gate insulating film 56 at a position above the gate electrode 55. Asource electrode 59 led out of the source line 51 and a drain electrode 60 are formed to extend over the semiconductor active film 57 through an ohmic contact layer 58 which is made of amorphous silicon containing an n-type impurity such as phosphorous, and then on the gate insulating film 56. A passivation film 61 is formed in covering relation to the TFT 53 made up of the source electrode 59, the drain electrode 60, the gate electrode 55, etc., and a contact hole 62 is formed in the passivation film 61 at a position above the drain electrode 60. Further, a pixel electrode 63 formed of a transparent conductive film, such as indium tin oxide (ITO), is filled in the contact hole 62 for electrical connection to the drain electrode 60. The manufacture of electronic structures such as TFT' s on flat displays, such as LCDs and PoIyLEDs for example, has conventionally tended to happen by means of photolithographic techniques. Lithography requires expensive, bulky machines. In addition, lithography requires a large number of process steps. It is obviously highly desirable to achieve cost reductions in respect of active matrix displays, and this may be achieved using new printing type methods for manufacturing the above-mentioned electronic structures.
Additional advantages of printing type methods are that they ideally require only two process steps: a deposition step and a curing step, and they use much less material and clean room space than lithography.
However, of all of the TFT layers, the semiconductor layer is the most difficult layer to print, because the printing of many square-shaped isolated structures (see the conventional array of substantially square semiconductor islands 57 in the structure described with reference to Fig. 1 of the drawings), without a previous patterning of the surface, is difficult to do.
It is therefore an object of the present invention to provide an improved TFT structure and a method of producing same, in which the above-mentioned problem is alleviated. Also provided is a flat panel display, such as an active matrix LCD display, and a thin film forming apparatus adapted to perform the above-mentioned production method. Thus, in accordance with the present invention, there is provided a thin film transistor comprising a substrate on which is provided a gate layer, a dielectric layer covering said gate layer, and a layer of semiconductor material disposed over said dielectric layer substantially in alignment with and facing said gate layer. As a result of this structure, the need to print many islands of semiconductor material is eliminated without any significant adverse effect on the performance of the resulting thin film transistor.
Beneficially, said gate layer comprises a gate line and a gate electrode led out of said gate line.
The semiconductor layer is preferably in the form of a line, beneficially provided with a lateral protrusion which is beneficially of substantially the same size and shape as the gate layer.
Beneficially, at least one of the gate layer and the semiconductor layer, and most preferably the gate layer, is deposited using an inkjet printing method. Preferably at least one of the gate layer and the semiconductor layer, and most preferably the semiconductor layer, is deposited using an electrostatic aerosol deposition process and more preferably a patterned electrostatic aerosol deposition process.
However, it will be appreciated that the semiconductor material could be deposited using any suitable method, including electrohydrodynamic spraying (e.g. using the Taylor-cone method, the general principle of which is described in, for example, US Patent No. 6,454,193), any aerosol deposition process including a patterned aerosol deposition process or other guided aerosol deposition process, inkjet printing, dispensing, spray pyrolysis, etc. Equally, the gate layer can be deposited using any suitable method including electrohydrodynamic spraying (e.g. using the Taylor-cone method), any aerosol deposition process, including a patterned aerosol deposition process or other guided aerosol deposition process, inkjet printing, dispensing etc.
The semiconductor material is beneficially a high-mobility semiconductor material, such as Indium Sulfide (InS), CdS, CdO, ZnS or ZnO. Alternatively, the semiconductor material may comprise a high mobility organic semiconductor material, i.e. a polymer, such as polythiophene, poly(alkylthiophene), pentacene, copolymer of fluorine and bithiophene, polythienylenevinylene, thiophene-based oligomers, phthalocyaiine, lutecium diphthalo-cyanine (Pc2Lu), thullium diphthalo-cyanine (Pc2Tm), Fullerene (C60/C70), TetraCyano-p-Quinodimethane (TCNQ), perylene-tetracarboxylic diimide (PTCDI)-Ph, TetraCyano-n-Quinodimethane (TCNNQ), naphthaline-tetracarboxylic-diimide (NTCDI), naphthalene-tetracarboxylic-dianhydride (NTCDA), perylene-tetracarboxylic-dianhydride (PTCDA), perylene 16 copper phthalcyanine (P16CuPc), NTCDI-C8F, DHF-6T and PTCDI- C8. Yet another option would be to use semiconductor nanowires, for example, silicon nanowires.
Also in accordance with the present invention, there is provided a method of manufacturing a thin film transistor, comprising depositing a gate layer on a substrate, depositing a layer of dielectric material over said gate layer, and depositing a layer of semiconductor material in the form of a line on said dielectric layer substantially in alignment with and facing said gate layer.
Beneficially the step of depositing said gate layer on said substrate comprises depositing a gate line and a gate electrode led out of said gate line. Preferably, the layer of semiconductor material is of substantially the same size and shape as the gate layer.
The method preferably includes the step of depositing at least one of the semiconductor layer or, more preferably, the gate layer by means of an inkjet printing process. The method preferably also includes the step of depositing at least one of the gate layer or, more preferably, the semiconductor layer by means of an electrostatic aerosol deposition process, and more preferably a patterned electrostatic aerosol deposition process.
The method preferably further includes the steps of forming at least source and drain lines after deposition of said semiconductor layer.
The present invention extends further to an active matrix display device comprising a plurality of thin film transistors as defined above.
These and other aspects of the present invention will be apparent from, and elucidated with reference to, the embodiments described herein.
Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which:
Fig. 1 is a schematic plan view of a known active matrix display; Fig. 2 is a schematic cross-sectional view of a known TFT structure; Fig. 3 a is a partial schematic illustration of a TFT structure according to a first exemplary embodiment of the present invention;
Fig. 3b is a partial schematic illustration of a TFT structure according to a second exemplary embodiment of the present invention; Fig. 4a is a schematic diagram illustrating apparatus with which an electrostatic aerosol deposition of a thin film material can be performed according to a first exemplary embodiment of the present invention;
Fig. 4b is a schematic diagram illustrating apparatus with which an electrostatic aerosol deposition of a thin film material can be performed, according to a second exemplary embodiment of the present invention; and
Figs. 5 and 6 are schematic diagrams illustrating a patterned aerosol deposition process which can be used in accordance with an exemplary embodiment of the present invention.
One exemplary embodiment of a TFT structure according to the invention is illustrated schematically in Fig. 3 of the drawings. In the illustrated TFT 10, a gate layer 12 made of a metal or metal colloid is formed on a transparent substrate 14 which may comprise, for example, glass or plastic. A dielectric layer 16 (or gate insulating film) is provided over the gate layer 12 and a semiconductor layer 18 is provided on the dielectric layer 16, the semiconductor layer 18 being embodied as a straight line (instead of having the conventional 2D square shape) having a shape which mirrors and faces the shape of the gate layer 12 and being in alignment therewith. The remaining layers (not shown), i.e. source line, drain electrode, etc., are deposited after.
Referring to Fig. 3b of the drawings, a TFT structure according to a second exemplary embodiment of the invention is similar in many respects to that of Fig. 3 a, except in this case a gate layer comprising a gate line 12 and a gate electrode 13 led out of the gate line 12 is formed on the substrate 14, and the semi-conductor layer 18 is embodied as a substantially straight line having a lateral protrusion, and has a shape which mirrors and faces the shape of the gate layer formed by the gate line 12 and the gate electrode 13 led out of the gate line 12.
It is relatively easy to perform deposition of a semiconductor layer of the above-mentioned form by inkjet printing or by means of a patterned electrostatically- augmented aerosol deposition process, for example. Furthermore, a TFT in accordance with the present invention has been found to display good electrical characteristics.
In a preferred embodiment, the gate layer structure formed by the gate line 12 and the gate electrode 13 led out of the gate line 12 is preferably inkjet printed onto the substrate 14, then the dielectric layer 16 is deposited over the gate layer 12,13 and an electrostatic aerosol deposition method is beneficially used to perform a patterned deposition of the semiconductor layer 18 on top of the dielectric layer 16 substantially in alignment with and facing the gate layer 12,13.
InkJet printing is a well known deposition process in which extremely small dots of "ink" are placed via a nozzle onto a substrate. The "ink", in this case, would comprise a suitable conductive (for the gate line and the gate electrode) or semiconductor material dissolved or suspended in a carrier fluid, and the process involves two steps: applying droplets of the ink to the substrate to create the desired form of the deposition pattern, and causing or allowing the ink to dry to form the final layer pattern. Two, similar, electrostatically-augmented aerosol deposition processes, which could be used to patternwise deposit the semiconductor layer of the present invention, will now be described with reference to Figs. 4 to 6 of the drawings.
Electrostatic aerosol deposition as referred to herein may comprise a method in which either solid aerosolised particles or fluid aerolised particles can be generated and dispersed in a carrier gas stream, then size-classified, then unipolar electrostatically charged in an internal high-voltage corona section, then concentration-homogenised in an expansion chamber, and finally either homogeneously or patternwise deposited on a substrate. The aerosolised particles are either composed of or comprise semiconductor materials and/or semiconductor precursor materials. A basic exemplary equipment set-up for the homogeneous or patterned deposition of solid-particle aerosols onto a substrate plate is illustrated schematically in Fig. 4a of the drawings. It comprises a particle feeder 100 consisting of a transportation piston 101 in communication with a volume of powder 102 in a powder reservoir 103. The particle feeder 100 further comprises a dispersion head 104 carrying a rotating brush 105 which transfers dry powder particles in the powder reservoir 103 from a compacted state to an airborne state with the help of both the rotating brush 105 and a dispersion air stream 106 which blows the partcles out of the brush and carries them away in aerosolised form 107. A particle feeder 100 of this type enables the dispersion of powders with particle sizes down to well below 1 micrometer in diameter in a carrier gas stream. Instead of a particle feeder dispersing solid particles, one can alternatively use a liquid aerosol generator such as a spinning disk generator, a vibrating orifice generator, an ultrasonic nebuliser, an ordinary compressed-air nebuliser, or a compressed-air nebuliser 200 comprising a Laskin nozzle 201 (from which is emitted an air-jet 202, as shown in Fig. 4b of the drawings, for the dispersion in a carrier gas stream of liquid particles comprising dissolved or suspended solid semiconductor materials and/or semiconductor precursor materials (derived from a body of liquid 203 containing dissolved or suspended semiconductor materials or semiconductor precurser materials). Preferably, the dispersion of the liquid aerosol is performed in a first solvent-saturated gas stream 204. Size classification of the liquid aerosol is performed by means of a baffle plate 206, after which the liquid aerosol, present in the first gas stream, is mixed with a second gas stream. The volume flow of the gas streams, the temperature, and the size of the expansion chamber 207, among other things, determine the evaporation kinetics that affects the size of the liquid aerosol particles 205:- the evaporation kinetics and the size and/or composition of the liquid aerosol particles can thus be tuned.
In the arrangement of Fig. 4a, size classification of said aerosolised particles involves the passage of the aerosol in a carrier gas stream through a cyclone 108 from where the aerosol cloud 109 passes into a glass expansion chamber 110 and through a dust filter which may be a simple mechanical filter or, as in this case, a dielectric filter 111, such as a course fibrous dielectric filter sandwiched between two conductive metal gauzes between which a voltage differential Vfiiter has been applied. The filter 111 removes the larger particles and transmits the smaller particles, such that large solid particle aggregates are removed.
In summary, in the arrangement of Fig. 4b of the drawings, a liquid aerosol generator generates a mist of liquid drops, each drop containing finely dispensed or dissolved semiconductor (precursor) material. The baffle plate 206 ensures a proper size classification. The evaporation speed of the aerosol droplets 205, either in the aerosol phase or on the substrate, can be tuned by regulating the support dry air stream and/or the temperature and/or the size of the top expansion chamber 207.
In the case of the arrangements of both Figs. 4a and 4b, the size-classified, solid or liquid, aerosolised particles then pass to a high- voltage corona charging section comprising a high- voltage needle electrode 112 supported by two high- voltage gauzes at Vcorona and defining an ionising needle tip 112a at Vcorona, and a water-irrigated counter electrode 113 defined, for example, by a water-irrigated earther aluminium surface covered with hygroscopic glass- fibre paper. The water irrigation of the counter electrode surface is important for removing deposited particles from the counter electrode surface, thus avoiding trouble with back-corona events and electrical polarisation effects in the deposited particle layer on the counter electrode. The illustrated high-voltage corona charging section further comprises insulator plates 114. A top glass expansion chamber 115 is provided for the enhanced concentration homogenisation of the charged aerosol in the carrier gas stream under the influence of the space charge effect in the charged aerosol cloud. The charged particles dispersed in the carrier gas stream leave the top expansion chamber 115 and enter into the deposition chamber via an aerosol outlet provided by a porous (metal) gauze 116 in, and electrically connected to, a high- voltage (metallic) deposition electrode plate 117 which is set at a voltage Vdeposition- Some distance above the deposition electrode plate 117, the substrate plate 118, whereon aerosol deposition is required to occur, is positioned substantially in parallel with the deposition electrode plate 117. The deposition chamber is physically bounded by the substrate plate 118 and the deposition electrode plate 117 facing each other but left substantially open to the outside environment at all other sides, thus the carrier gas stream can freely flow away to the sides and along the entire side surface of the substrate plate 118 facing the deposition electrode plate 117. The substrate plate 118 is, in this case, backed by, and capacitively coupled to, an earthed backing electrode plate 119 such that the charged particles 120 are always drawn towards the substrate plate 118 by means of the electric field existing between the substrate plate 118 and the deposition electrode plate 117, the idea being to make the electric field sufficiently high to remove virtually all charged aerosol particles from the carrier gas flow and deposit them onto the substrate plate 118 during their residence time inside the deposition chamber, while the lateral flow of the carrier gas along the surface of the side of substrate plate 118 facing the deposition electrode plate 117 ensures a lateral spreading of the depositing particles across the entire substrate surface. The substrate plate 118 is positioned upside down (anti-gravitationally) during aerosol deposition, which makes the substrate plate 118 much less susceptible to become contaminated by depositing dust particles. In case of a large-sized substrate plate 118, it is advantageous to mount the substrate plate 118 onto a xy translator stage, in order to allow the substrate plate 118 to undergo a controlled series of lateral movements with respect to the aerosol outlet provided by the porous metal gauze 116 during the aerosol deposition process, thus improving the lateral homogeneity and thickness uniformity of the thin layer formed by the deposited aerosol particles 120 across the surface of the substrate plate 118. In this example (see Figs. 5 and 6 of the drawings) illustrating a patterned aerosol deposition process), the side of the substrate plate 118 facing the deposition electrode plate 117 carries a set of gate lines 11, 12 and associated gate electrodes (not shown) led out of gate lines 11,12. A gate insulating film 56 is present in covering relation with all gate lines 11, 12 and with all gate electrodes led out of gate lines 11, 12. A first voltage (V1) is imposed on gate lines 11, while a different second voltage (V2) is imposed on gate lines 12. The sign and magnitude of the voltages Vi and V2 with respect to Vdeposition are chosen such that substantially all charged aerosolised particles are deposited only on the parts of the gate insulating film 56 that cover the gate lines 11 and the gate electrodes led out of gate lines 11, resulting in a patterned deposition of semiconductor material or semiconductor precursor material on the parts of the gate insulating film 56 that are aligned with and in covering relation with the gate lines 11 and the gate electrodes led out of gate lines 11, (as shown I Fig. 5). Preferably Vi is chosen the same as the voltage on the backing electrode plate 119 e.g. earth potential. In the next step (see Fig. 6), the above process is repeated by imposing the first voltage Vi on the gate lines 12 and the different second voltage V2 on gate lines 11. In this way, a patterned deposition of semiconductor material or semiconductor precursor material is accomplished on the parts of the gate insulating film 56 that are aligned with and in covering relation with the gate lines 11, 12 and the gate electrodes led out of gate lines 11 , 12. The advantages of the above-described aerosol deposition set-up include its versatility and reliability, the possibility of generating and processing much smaller aerosol particles than is possible with conventional electrostatic spraying equipment, the economic use of coating material, the upside-down positioning of the substrate plate so as to avoid a ready deposition of contaminating dust particles onto the (tacky) substrate plates under the influence of gravity, and the possibility of allowing for a patterned deposition of aerosol material onto a substrate.
Electrostatic aerosol deposition such as that described above has the additional advantage, in the case of the present invention, that it can use previously deposited gate lines and gate electrodes led out of gate lines to manipulate the local electric field close to the substrate surface. As a result, the charged aerosol particles can be electrically guided to be deposited right on top of the parts of the dielectric layer that cover the gate lines and the gate electrodes led out of the gate lines. Then, the deposited semiconductor layer faces and becomes self-aligned with respect to the gate lines and the gate electrodes led out of the gate lines. Suitable high-mobility semiconductor materials which can be used to form the semiconductor line of the invention include Indium Sulfate (InS), CdS, CdO, ZnS and ZnO, using for example spray pyrolysis. It will be appreciated by a person skilled in the art that spray pyrolysis involves the spraying of particles of the material onto a substrate which is held at a high temperature within some predetermined temperature range depending on the substrate material and the material being deposited.
Another option would be to use organic semiconductors, i.e. polymers, which include but are not limited to polythiophene, poly(alkylthiophene), pentacene, copolymer of fluorine and bithiophene, polythienylenevinylene, thiophene-based oligomers, phthalocyainine, Pc2Lu, Pc2Tm, C60/C70, TCNQ, PTCDI-Ph, TCNNQ, NTCDI, NTCDA, PTCDA, P 16CuPc, NTCDI-C8F, DHF (Dilute Hydrofluoric acid)-6T and PTCDI-C8. Other suitable semiconductor materials will be evident to a person skilled in the art.
Yet another option would be to use, for example, silicon nanowires. All of the above examples of suitable semiconductor materials are suitable for use in respect of the above-mentioned deposition methods. Further suitable materials will be apparent to a person skilled in the art.
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:
1. A thin film transistor (10) comprising a substrate (14) on which is provided a gate layer (12,13), a dielectric layer (16) covering said gate layer (12,13), and a layer (18) of semiconductor material disposed over said dielectric layer (16) substantially in alignment with and facing said gate layer (12,13).
2. A thin film transistor (10) according to claim 1, wherein said gate layer comprises a gate line (12) and a gate electrode (13) led out of said gate line (12).
3. A thin film transistor (10) according to claim 1, wherein said semiconductor layer (18) is of substantially the same size and shape as the gate layer (12,13).
4. A thin film transistor (10) according to claim 2, wherein said semiconductor layer (18) is provided with a lateral protrusion (18a) which substantially mirrors the shape and size of said gate electrode (13) led out from said gate line (12).
5. A thin film transistor (10) according to claim 1, wherein the semiconductor material is a high mobility semiconductor material.
6. A thin film transistor (10) according to claim 1, wherein the semiconductor material comprises a polymer.
7. A method of manufacturing a thin film transistor (10), comprising depositing a gate layer (12,13) on a substrate (14), depositing a layer of dielectric material (16) over said gate layer (12,13), and depositing a layer of semiconductor material (18) on said dielectric layer (16) substantially in alignment with and facing said gate layer (12,13).
8. A method according to claim 7, wherein said gate layer comprises a gate line (12) and a gate electrode (13) led out of said gate line (12).
9. A method according to claim 8, wherein the layer (18) of semiconductor material is of substantially the same size and shape as the gate line (12,13).
10. A method according to claim 8, wherein said semiconductor layer (18) is provided with a lateral protrusion (18a) which substantially mirrors the size and shape of said gate electrode (13) led out from said gate line (12).
11. A method according to claim 7, comprising depositing the semiconductor layer (18) by means of an inkjet printing process, electrohydrodynamic spraying, an aerosol deposition process, dispensing or spray pyro lysis.
12. A method according to claim 7, comprising depositing the gate layer (12,13) by means of electrohydrodrynamic spraying, an aerosol deposition process, inkjet printing or dispensing.
13. A method according to claim 7, comprising depositing the gate layer (12,13) on said substrate (14) by means of an inkjet printing process and depositing said layer (18) of semiconductor material on said dielectric layer (16) by means of a patterned electrostatic aerosol deposition process.
14. A method according to claim 7, further comprising forming at least source and drain lines after deposition of said semiconductor layer (18).
15. A thin film transistor (10) manufactured according to the method of claim 7.
16. An active matrix display device comprising a plurality of thin film transistors (10) according to claim 1.
PCT/IB2005/052143 2004-07-02 2005-06-28 A thin film transistor, method of producing same and active matrix display WO2006003619A1 (en)

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WO2010004271A1 (en) * 2008-07-08 2010-01-14 Imperial Innovations Limited Low-voltage thin-film field-effect transistors
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