US20140145186A1 - Offset electrode tft structure - Google Patents

Abstract

The present invention generally relates to an offset electrode TFT and a method of its manufacture. The offset electrode TFT is a TFT in which one electrode, either the source or the drain, surrounds the other electrode. The gate electrode continues to be below both the source and the drain electrodes. By redesigning the TFT, less voltage is necessary to transfer the voltage from the source to the drain electrode as compared to traditional bottom gate TFTs or top gate TFTs. The offset electrode TFT structure is applicable not only to silicon based TFTs, but also to transparent TFTs that include metal oxides such as zinc oxide or IGZO and metal oxynitrides such as ZnON.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application is a continuation of U.S. patent application Ser. No. 13/289,033 (APPM/15544), filed Nov. 4, 2011, which claims benefit of U.S. Provisional Patent Application Ser. No. 61/448,429 (APPM/15544L), filed Mar. 2, 2011, which is herein incorporated by reference.
GOVERNMENT RIGHTS IN THIS INVENTION
• This Invention was made with Government support under Agreement No. DAAD19-02-3-0001 awarded by ARL. The Government has certain rights in this Invention.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• Embodiments of the present invention generally relate to a thin film transistor (TFT) and a method of its manufacture.
• 2. Description of the Related Art
• Current interest in TFT arrays is particularly high because these devices may be used in liquid crystal active matrix displays (LCDs) of the kind often employee for computer and television flat panels. The LCDs may also contain light emitting diodes (LEDs), such as organic light emitting diodes (OLEDs) for back lighting. The LEDs and OLEDs require TFTs for addressing the activity of the displays.
• Bottom gate TFTs made with amorphous silicon have been utilized for the flat panel display industry for many years. Unfortunately, the on and off-current driven through source and drain electrodes of the TFT is limited by its channel material as well as the channel width and length. Additionally, the turn-on gate voltage, or a threshold voltage, under a voltage between the source and drain electrodes, is determined by the accumulation of the carrier in the active channel area of the semiconductor active layer which could change as the change of the charge in the channel material, dielectric material as well as interfaces between the materials after bias temperature stress or current temperature stress.
• Therefore, there is a need in the art for a TFT that utilizes an additional source-drain current controlling layer beneath the source or drain electrode or both to miminize the off-current when the gate voltage sets at a turn-off voltage. Because the additional control of the source-drain current control, the channel length between the source-drain can be significantly reduced for high on-current when the TFT turns on, keeping the off-current low when the TFT turns off. In addition, the TFT design keeps the turn-on voltage, or threshold gate voltage, unchanged after operations under different conditions.
SUMMARY OF THE INVENTION
• The present invention generally relates to an offset electrode TFT and a method of its manufacture. The offset electrode TFT is a TFT in which one electrode, either the source or the drain, surrounds the other electrode. The gate electrode continues to be below both the source and the drain electrodes. By redesigning the TFT, less voltage is necessary to transfer the voltage from the source to the drain electrode as compared to traditional bottom gate TFTs or top gate TFTs. The offset electrode TFT structure is applicable not only to silicon based TFTs, but also to transparent TFTs that include metal oxides such as zinc oxide or IGZO and metal oxynitrides such as ZnON.
• In one embodiment, a TFT is disclosed. The TFT comprises a gate electrode disposed above a substrate; a gate dielectric layer disposed over the gate electrode; a channel semiconductor layer disposed over the gate dielectric layer; a first electrode disposed over the channel semiconductor layer and at least partially defining a via; and a second electrode disposed over the channel semiconductor layer, within the via and extending over at least a portion of the first electrode.
• In another embodiment, a TFT includes a gate electrode, a source electrode disposed over the gate electrode and a drain electrode disposed over the source electrode with dielectric or semiconductor layer in between.
• In another embodiment, a TFT fabrication method is disclosed. The method includes depositing a gate dielectric layer over a gate electrode, depositing a channel semiconductor layer over the gate dielectric layer, depositing a source electrode over the channel semiconductor layer and depositing a first source dielectric layer over the source electrode. The method also includes depositing a second source dielectric layer over the first source dielectric layer and removing at least a portion of the source electrode, the first source dielectric layer and the second source dielectric layer to form a via that is bound by edges of the source electrode, the first source dielectric layer and the second source dielectric layer and expose at least a portion of the control semiconductor layer. The method also includes depositing a spacer layer over at least a portion of the control semiconductor layer and the edges of the source electrode, the first source dielectric layer and the second source dielectric layer. The method additionally includes depositing a first control semiconductor layer over the exposed channel semiconductor layer, depositing a second control semiconductor layer over the first control semiconductor layer and depositing a drain electrode over the second control semiconductor layer.
BRIEF DESCRIPTION OF THE DRAWINGS
• So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
• FIGS. 1A-1V are schematic cross-sectional and top views of anoffset TFT100 at various stages of production.FIGS. 1A,1C,1E,1G,1I,1K,1M,1O,1Q,1S and1U are cross-sectional views whileFIGS. 1B,1D,1F,1H,1J,1L,1N,1P,1R,1T and1V are top views.
• FIGS. 2A and 2B are schematic cross-sectional and top views of anoffset TFT200 according to another embodiment.
• FIGS. 3A and 3B are schematic cross-sectional and top views of anoffset TFT300 according to another embodiment.
• FIGS. 4A and 4B are schematic cross-sectional and top views of anoffset TFT400 according to another embodiment.
• To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.
DETAILED DESCRIPTION
• The present invention generally relates to an offset electrode TFT and a method of its manufacture. The offset electrode TFT is a TFT in which one electrode, either the source or the drain, surrounds the other electrode. The gate electrode continues to be below both the source and the drain electrodes. By redesigning the TFT, less off-current from the source to the drain electrode is achieved when the gate voltage sets at an off voltage as compared to traditional bottom gate TFTs or top gate TFTs. It will also make the threshold voltage of the gate electrode, at which TFT starts to turns on or off, less sensitive of the change of active layer, dielectric layer and their interfaces. The offset electrode TFT structure is applicable not only to silicon based TFTs, but also to transparent TFTs that include metal oxides such as zinc oxide or IGZO and metal oxynitrides such as ZnON.
• Current TFTs have a gate electrode, a gate dielectric layer, a semiconductor channel, a source electrode and a drain electrode. A vertical TFT structure was presented in a paper that published in 2005. The vertical TFT has a supercapacitor, a thin rough electrode, a semiconductor, and a drain electrode. In the vertical TFT, the supercapacitor is made of LiF. In the invention discussed herein, the supercapacitor comprises metal oxides or metal oxynitride or a combination with thin film semiconductors such as a-Si or possible with a dielectric layer, but not LiF specifically, or other electrolytic materials. Therefore, the capacitor will be made using CVD or PVD processes commonly utilized in semiconductor fabrication and equipment. In addition, the electrodes will be made with oxide or oxynitride based composite materials, instead of thin rough electrodes. The invention includes the materials to make the TFT, the materials for the capacitor, the materials for the electrodes and the material configuration; the film stack and process; and a solution for high off current. The invention is a new way to make a TFT and could provide unique features to address the stability issues with are encountered in current TFT structures.
• In the state of the art TFT, when gate voltage is set at an off voltage, certain level current still drives through TFT source and drain electrodes under a source-drain voltage, depending on its channel material as well as the channel width and length. Because of possible high off-current, the short channel device is not practical although it can provide high on-current at a low voltage. In addition, its turn-on gate voltage, or threshold voltage, under a voltage between the source and drain electrodes is determined by the accumulation of the carrier in the channel layer, which could change with the change of the electrical charge in the channel material, dielectric materials, and their interfaces composed of the TFT after bias temperature stress or current temperature stress. The TFT proposed herein has a gate, a gate dielectric, a channel semiconductor layer, and one metal electrode in direct contact with the semiconductor layer serves as source, which is similar as the state-of-the-art TFT. However, the TFT also has a spacer and a control semiconductor on the top of the channel semiconductor but beneath the drain electrodes. The new TFTs proposed herein will provide a higher current than the state-of-the-art TFTs since the actual channel length will be much shorter when the device is turned on. Since the source-drain current is controlled by the control semiconductor layer(s), off-current of the short-channel device can be minimized. In addition, the charge trapping at the interfaces in the channel region and bulk (the channel layer under the spacer) becomes less critical for the turn on voltage for the device, since the turn-on voltage the device will be also determined by the accumulation in the control region which has channel semiconductor, control semiconductor and drain electrode. Therefore, threshold gate voltage shift caused by a change of charge in interface and bulk layers should be significantly minimized. The invention includes: a TFT configuration comprising the TFT composed of a gate, gate dielectric, channel semiconductor, source electrode, control semiconductor or semiconductors, drain electrode; a TFT configuration comprising the TFT has a channeling region and one or more controlling regions. The channel semiconductor and the control semiconductor material can be different or the same. Additionally, an additional barrier layer may be present in between the semiconductor layers. For example, metal oxide or metal oxynitride is used as channel semiconductor, and amorphous Si is used as control semiconductor. Alternatively, the metal oxynitride may be used as both the channel semiconductor and control semiconductor; however the carrier concentration or the band gap will be different or a thin SiO or SiN layer will be present in between the semiconductor layers. The spacer is made of SiO or SiN, or other dielectric materials. The spacer is made from a conformal deposition and etch back process. The controlling region for on-off can result from a barrier between the channel semiconductor and control semiconductor, or a barrier created inside the control semiconductor material, or a barrier between the source metal and the channel semiconductor or between the drain metal and the control semiconductor. The barrier can result from the Fermi level difference, work function difference, band gap difference, or others film property difference.
• FIGS. 1A-1V are schematic cross-sectional and top views of aTFT100 at various stages of production.FIGS. 1A,1C,1E,1G,1I,1K,1M,1O,1Q,1S and1U are cross-sectional views whileFIGS. 1B,1D,1F,1H,1J,1L,1N,1P,1R,1T and1V are top views. As shown inFIGS. 1A and 1B, agate electrode104 is formed over asubstrate102. Suitable materials that may be utilized for thesubstrate102 include, but may not be limited to, silicon, germanium, silicon-germanium, soda lime glass, glass, semiconductor, plastic, steel or stainless steel substrates. Suitable materials that may be utilized for thegate electrode104 include, but may not be limited to, chromium, copper, aluminum, tantalum, titanium, molybdenum, and combinations thereof, or conductive transparent oxides (TCO) such as ITO (indium tin oxide) or ZnO:F commonly used as transparent electrodes. Thegate electrode104 may be deposited by suitable deposition techniques such as physical vapor deposition (PVD), MOCVD, spin-on process, and printing processes. If necessary, thegate electrode104 may be patterned using an etching process.
• Over thegate electrode104, agate dielectric layer106 may be deposited as shown inFIGS. 1C and 1D. Suitable materials that may be used for thegate dielectric layer106 include silicon dioxide, silicon oxynitride, silicon nitride, aluminum oxide or combinations thereof. Thegate dielectric layer106 may be deposited by suitable deposition techniques including plasma enhanced chemical vapor deposition (PECVD).
• Achannel semiconductor layer108 is then formed over thegate dielectric layer106 as shown inFIGS. 1E and 1F. Thechannel semiconductor layer108 may comprise traditional semiconductor material used in TFTs such as amorphous silicon or polysilicon. Additionally, next generation semiconductor materials are also contemplated such as metal oxides including zinc oxide (ZnO) and indium-gallium-zinc oxide (IGZO) as well as oxynitrides such as indium-gallium-zinc-oxynitride (IGZON) and zinc oxynitride (ZnON). Other materials that are contemplated include ZnO_xN_y, SnO_xN_y, InO_xN_y, CdO_xN_y, GaO_xN_y, ZnSnO_xN_y, ZnInO_xN_y, ZnCdO_xN_y, ZnGaO_xN_y, SnInO_xN_y, SnCdO_xN_y, SnGaO_xN_y, InCdO_xN_y, InGaO_xN_y, CdGaO_xN_y, ZnSnInO_xN_y, ZnSnCdO_xN_y, ZnSnGaO_xN_y, ZnInCdO_xN_y, ZnInGaO_xN_y, ZnCdGaO_xN_y, SnInCdO_xN_y, SnInGaO_xN_y, SnCdGaO_xN_y, InCdGaO_xN_y, ZnSnInCdO_xN_y, ZnSnInGaO_xN_y, ZnInCdGaO_xN_y, and SnInCdGaO_xN_y. Each of the aforementioned semiconductor films may be doped by a dopant such as Al, Sn, Ga, Ca, Si, Ti, Cu, Ge, In, Ni, Mn, Cr, V, Mg, Si_xO_y, Si_xN_y, Al_xO_y, and SiC. Thechannel semiconductor layer108 may be deposited by a suitable deposition method such as PVD, PECVD, chemical vapor deposition (CVD), or atomic layer deposition (ALD), spin-on or printing processes. Thechannel semiconductor layer108 permits the current to flow between the source and drain electrodes once thegate electrode104 is biased. Thechannel semiconductor layer108 may be patterned, if desired, by a wet etching process.
• As shown inFIGS. 1G and 1H, over thechannel semiconductor layer108, aconductive layer110 is deposited that will eventually form the first electrode which can be either the source or the drain electrode, depending upon the electrical connection. Hereinafter, it is to be understood that reference to the first electrode includes either the source electrode or the drain electrode. Suitable materials for theconductive layer110 include chromium, copper, aluminum, tantalum, titanium, molybdenum, and combinations thereof, or TCOs mentioned above. Theconductive layer110 may be deposited by suitable deposition techniques, such as PVD. The conductive layer could be patterned through etching or a printing process as the first electrode, or not patterned, or partially patterned. The following gives the example that the first electrode is not patterned or partially patterned, (i.e., outside the first electrode is defined but the control portion of the first electrode is not defined or formed yet).
• Afirst dielectric layer112 is then deposited over theconductive layer110 as shown inFIGS. 1I and 1J. Suitable materials that may be used for thefirst dielectric layer112 include silicon dioxide, silicon oxynitride, silicon nitride, or combinations thereof. Thefirst dielectric layer112 may be deposited by suitable deposition techniques, including PECVD. Asecond dielectric layer114 is then deposited over thefirst dielectric layer112 as shown inFIGS. 1K and 1L. Suitable materials that may be used for thesecond dielectric layer114 include silicon dioxide, silicon oxynitride, silicon nitride, or combinations thereof. Thesecond dielectric layer114 may be deposited by suitable deposition techniques, including PECVD. In one embodiment, thefirst dielectric layer112 and thesecond dielectric layer114 comprise separate, distinct layers that comprise different materials. In another embodiment, thefirst dielectric layer112 and thesecond dielectric layer114 may comprise a single layer.
• Thesecond dielectric layer114, thefirst dielectric layer112 and theconductive layer110 are then patterned as shown inFIGS. 1M and 1N to create thefirst electrode116, patterned firstdielectric layer118 and patterned seconddielectric layer120. As shown inFIG. 1N, a via119 is present such that thefirst electrode116, the patterned firstdielectric layer118 and the patterned seconddielectric layer120 bound the via119. In the embodiment shown inFIG. 1N, thefirst electrode116 has aslot121 therein such that thefirst electrode116 does not completely encircle the exposedchannel semiconductor layer108, but does at least partially encircle the exposedchannel semiconductor layer108. However, it is to be understood that thefirst electrode116 could completely encircle thechannel semiconductor layer108 or with a large opening cross several sides. After the patterning, a portion of thechannel semiconductor layer108 is exposed. While the patterning is shown as occurring after thesecond dielectric layer114, thefirst dielectric layer112 and theconductive layer110 have all been deposited, it is contemplated that the patterning may occur after the deposition of thesecond dielectric layer114, the deposition of thefirst dielectric layer112 and the deposition ofconductive layer110. Additionally, it is contemplated that the patterning may occur after the deposition of theconductive layer110 and then again after the collective deposition of thefirst dielectric layer112 and thesecond dielectric layer114. The patterning may occur by forming a mask over the uppermost layer (i.e., thesecond dielectric layer114 inFIGS. 1K and 1L) and then etching the exposed surfaces. Different etching conditions may be necessary for each layer etched.
• A spacer layer is then deposited over the exposed surfaces and selectively removed so that aspacer122 remains along the edges of thefirst electrode116, the patterned firstdielectric layer118 and the patterned seconddielectric layer120. Thespacer122 is also present on the now exposedchannel semiconductor layer108. However, as shown inFIGS. 1O and 1P, thespacer122 does not cover the entirechannel semiconductor layer108 as the spacer layer has been removed from selected portions of thechannel semiconductor layer108. Thus, a portion of thechannel semiconductor layer108 remains exposed after the formation of thespacer122. Suitable materials that may be used for thespacer122 include silicon dioxide, silicon oxynitride, silicon nitride, or combinations thereof. Thespacer122 may be deposited by suitable conformal deposition techniques including PECVD, CVD and ALD. After the conformal deposition process, a spacer etch or photo-resist patterning plus an etch process occurs to form thespacer122. After the spacer formation, the second source dielectric layer may or may not exist. If the second dielectric layer is removed during the spacer etch, the thickness of the first source dielectric may or may not be reduced. Thedielectric layers118,120 andspacer122 can insulate the first andsecond electrodes116,128 from each other. Thespacer layer122 can be used for selective etching. For example, thespacer122 can be formed by a conformal deposition process formed over thesecond dielectric layer120 and the sidewalls that surround thechannel semiconductor layer108. Thesecond dielectric layer120 is harder to etch than thespacer122, thespacer122 can be over etched without any fear of losing dielectric material over theelectrode116 during the spacer etch process.
• A firstcontrol semiconductor layer124 is then formed over the exposedchannel semiconductor layer108, thespacer122 and the patterned seconddielectric layer120. The firstcontrol semiconductor layer124 is formed by blanket depositing a layer and then etching the layer to leave the resulting structure shown inFIGS. 1Q and 1R. The firstcontrol semiconductor layer124 may comprise traditional semiconductor material used in TFTs such as amorphous silicon or polysilicon. Additionally, next generation semiconductor materials are also contemplated such as IGZO and ZnON. Other materials are contemplated such as boron doped or phosphorous doped, or no doped amorphous silicon and its combination with these or with others. Additionally materials that are contemplated include ZnO_xN_y, SnO_xN_y, InO_xN_y, CdO_xN_y, GaO_xN_y, ZnSnO_xN_y, ZnInO_xN_y, ZnCdO_xN_y, ZnGaO_xN_y, SnInO_xN_y, SnCdO_xN_y, SnGaO_xN_y, InCdO_xN_y, InGaO_xN_y, CdGaO_xN_y, ZnSnInO_xN_y, ZnSnCdO_xN_y, ZnSnGaO_xN_y, ZnInCdO_xN_y, ZnInGaO_xN_y, ZnCdGaO_xN_y, SnInCdO_xN_y, SnInGaO_xN_y, SnCdGaO_xN_y, InCdGaO_xN_y, ZnSnInCdO_xN_y, ZnSnInGaO_xN_y, ZnInCdGaO_xN_y, and SnInCdGaO_xN_y. Each of the aforementioned semiconductor films may be doped by a dopant such as Al, Sn, Ga, Ca, Si, Ti, Cu, Ge, In, Ni, Mn, Cr, V, Mg, Si_xN_y, Al_xO_y, and SiC. The firstcontrol semiconductor layer124 may be deposited by suitable deposition methods such as PVD, PECVD, CVD, or ALD. The firstcontrol semiconductor layer124 is disposed over thesource electrode116, the patterned first sourcedielectric layer118 and the patterned second sourcedielectric layer120.
• The purpose of a control semiconductor layer is to create an additional barrier for electrons to flow easily in one direction, yet difficult for electrons to flow in the opposite direction, such as a diode or rectifying effect. The control semiconductor layer lets electrons flow only under certain source-drain voltage differences. A controlling semiconductor layer has a different composition than the adjacent channel semiconductor layer. Without a controlling semiconductor layer, the barrier could still be created based upon the difference of work functions, surface charge trapping or even surface defects that are created by intentionally creating defects in the semiconductor layer.
• Over the firstcontrol semiconductor layer124, a secondcontrol semiconductor layer126 is formed. The secondcontrol semiconductor layer126 is formed by blanket depositing a layer and then etching the layer to leave the resulting structure shown inFIGS. 1S and 1T. The secondcontrol semiconductor layer126 may comprise traditional semiconductor material used in TFTs such as amorphous silicon or polysilicon. Additionally, next generation semiconductor materials are also contemplated such as IGZO and ZnON. Other materials that are contemplated include ZnO_xN_y, SnO_xN_y, InO_xN_y, CdO_xN_y, GaO_xN_y, ZnSnO_xN_y, ZnInO_xN_y, ZnCdO_xN_y, ZnGaO_xN_y, SnInO_xN_y, SnCdO_xN_y, SnGaO_xN_y, InCdO_xN_y, InGaO_xN_y, CdGaO_xN_y, ZnSnInO_xN_y, ZnSnCdO_xN_y, ZnSnGaO_xN_y, ZnInCdO_xN_y, ZnInGaO_xN_y, ZnCdGaO_xN_y, SnInCdO_xN_y, SnInGaO_xN_y, SnCdGaO_xN_y, InCdGaO_xN_y, ZnSnInCdO_xN_y, ZnSnInGaO_xN_y, ZnInCdGaO_xN_y, and SnInCdGaO_xN_y. Each of the aforementioned semiconductor films may be doped by a dopant such as Al, Sn, Ga, Ca, Si, Ti, Cu, Ge, In, Ni, Mn, Cr, V, Mg, Si_xN_y, Al_xO_y, and SiC. Other materials are contemplated such as boron doped or phosphorous doped, or no doped amorphous silicon and its combination with these or with others. The secondcontrol semiconductor layer126 may be deposited by suitable deposition methods, such as PVD, PECVD, CVD, or ALD. The secondcontrol semiconductor layer126 is disposed over the firstcontrol semiconductor layer124, thesource electrode116, the patterned first sourcedielectric layer118 and the patterned second sourcedielectric layer120.Dielectric layer120 could be removed partially or fully during formation of thedielectric spacer122. The secondcontrol semiconductor layer126 is used for tuning the barrier.
• Finally, over the secondcontrol semiconductor layer126, thesecond electrode128 is formed as shown inFIGS. 1U and 1V. Thesecond electrode128 is formed by blanket depositing a conductive material and then etching the conductive material to form the final structure of thesecond electrode128. Suitable materials for thesecond electrode128 include chromium, copper, aluminum, tantalum, titanium, molybdenum, and combinations thereof, or TCOs mentioned above. Thesecond electrode128 may be deposited by suitable deposition techniques, such as PVD. As shown inFIGS. 1U and 1V, thefirst electrode116, while disposed below thesecond electrode128, in essence surrounds thesecond electrode128 because thefirst electrode116 forms at least a portion of the via119 in which thesecond electrode128 is deposited. However, it should be noted that a portion of thesecond electrode128 is disposed over thefirst electrode116, the patterned firstdielectric layer118, the patterned sourcedielectric layer120, the firstcontrol semiconductor layer124 and the secondcontrol semiconductor layer126.
• Electrode128 covers thechannel semiconductor layer108 as shown inFIGS. 1U and 1V to protect thechannel semiconductor layer108 from light. If thechannel semiconductor layer108 is exposed to light, then thechannel semiconductor layer108 becomes conductive.
• Thechannel semiconductor layer108 and the control semiconductor layers124,126 may comprise different materials. For example, thechannel semiconductor layer108 may comprise a metal oxide or a metal oxynitride while the control semiconductor layers124,126 may comprise amorphous silicon. In one embodiment, thechannel semiconductor layer108 and the control semiconductor layers124,126 comprise the same material. For example, the control semiconductor layers124,126 and thechannel semiconductor layer108 may comprise a metal oxynitride. It is contemplated that a barrier layer (not shown) may be present between thechannel semiconductor layer108 and the firstcontrol semiconductor layer124. Suitable materials for the barrier layer include silicon oxide or silicon nitride, or not doped or doped amorphous silicon. The controlling region of the on-off for theTFT100 results from a barrier between thechannel semiconductor layer108 and the firstcontrol semiconductor layer124, a barrier created inside the control semiconductor material or a barrier between thesource electrode116 and the firstchannel semiconductor layer124, or a barrier between thedrain electrode128 and the secondcontrol semiconductor layer126. The barrier created inside the control semiconductor material can result from the Fermi level difference, work function difference, band gap difference, or other film property differences.
• FIGS. 2A and 2B are schematic cross-sectional and top views of an offsetTFT200 according to another embodiment. TheTFT200 includes asubstrate202,gate electrode204,gate dielectric layer206 andchannel semiconductor layer208. However, there is both a firstcontrol semiconductor layer210,222 and a secondcontrol semiconductor layer212,224 that are present adjacent to eachelectrode214,226. Eitherelectrode214,226 may function as the source electrode while theother electrode214,226 functions as the drain electrode. Thefirst electrode214 could completely encircle thechannel semiconductor layer108 or with a large opening cross several sides not shown. Both the firstcontrol semiconductor layer210,222 and the secondcontrol semiconductor layer212,224 are below each of the first andsecond electrodes214,226. Additionally, multipledielectric layers216,218 and aspacer220 are present.Dielectric layer218 could be removed partially or fully during formation of thedielectric spacer222. Thus, in the embodiment shown inFIGS. 2A and 2B, control semiconductor layers are present at both the first andsecond electrodes214,226 whereas in the embodiment shown inFIGS. 1A-1V, the control semiconductor layers are present at only one electrode. The control semiconductor layers210,212,222,224 are spaced apart byspacer220.
• FIGS. 3A and 3B are schematic cross-sectional and top views of an offsetTFT300 according to another embodiment. TheTFT300 includes a substrate302,gate electrode304,gate dielectric layer306 andchannel semiconductor layer308. Over thechannel semiconductor layer308, a firstcontrol semiconductor layer310 is present. Additionally, there is both a secondcontrol semiconductor layer312,324 and a thirdcontrol semiconductor layer314,326 that are present. Both the secondcontrol semiconductor layer312,324 and the thirdcontrol semiconductor layer314,326 are below each of the first andsecond electrodes316,328. Thefirst electrode316 could completely encircle thechannel semiconductor layer108 or with a large opening cross several sides not shown. Additionally, multipledielectric layers318,320 and aspacer322 are present.Dielectric layer320 could be removed partially or fully during formation of thedielectric spacer322. Thus, in the embodiment shown inFIGS. 3A and 3B, control semiconductor layers are present at both the first andsecond electrodes316,328, just as inFIGS. 2A and 2B. Control semiconductor layers312,314,324,324 are spaced apart byspacer322, but there is an additionalcontrol semiconductor layer310 that spans across the entirechannel semiconductor layer308.
• FIGS. 4A and 4B are schematic cross-sectional and top views of an offsetTFT400 according to another embodiment. InTFT400, no control semiconductor layers are present. Rather, theTFT400 includes asubstrate402, agate electrode404, agate dielectric layer406, achannel semiconductor layer408 and first andsecond electrodes412,414 spaced apart by aspacer410. Thefirst electrode414 could completely encircle thechannel semiconductor layer108 or with a large opening cross several sides not shown. Note that in each ofFIGS. 2A,2B,3A,3B,4A and4B, between 20 percent and 100 percent of the active channel (i.e., channel semiconductor layer between the first and second electrodes) is covered. A metal oxide TFT is more stable when the active channel is covered because the active channel is not exposed to light which can make the active channel conductive rather than semiconductive.
• The offset electrode TFT structure can be used widely on many electronic applications such as OLED TV or other devices that require high current and stable threshold voltages. The TFTs disclosed herein have a higher current as compared to the state of the art TFTs because the actual channel length is much shorter when the device is turned on. The distance between the source and drain electrodes is reduced because of the location of the drain electrode relative to the source electrode. Additionally, the charge trapping at the interfaces in the channel region (the channel layer under the spacer) becomes not-critical for turning on the device since turning on the device will be determined by the accumulation in the control region which has channel semiconductor material, control semiconductor material and the drain electrode.
• The TFTs described herein are beneficial for the next generation high definition displays. Due to one of the electrodes being over the top of the channel semiconductor layer, the TFT is smaller in size as compared to the traditional bottom gate TFT. Because the TFT is smaller in size, more pixels may be contained within a smaller space (i.e., higher density of pixels). Additionally, because the TFT is offset whereby an electrode covers the channel semiconductor layer, less energy is needed to illuminate the pixel. Finally, because one of the electrodes is formed over the channel semiconductor layer, metal oxides are not exposed to light which would otherwise render the metal oxides conductive rather than semiconductive. Therefore, the TFTs described herein are much more stable and reliable as compared to bottom gate and top gate TFTs.
• While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. Additionally, the term “over” as used herein is meant to include objects that are both above and in contact with another object as well as objects that are above and not in contact with the other object.

Claims (16)

1. A thin film transistor, comprising:

a gate electrode disposed above a substrate;

a gate dielectric layer disposed over the gate electrode;

a channel semiconductor layer disposed over the gate dielectric layer;

a first electrode disposed over the channel semiconductor layer and at least partially defining a via, wherein the first electrode is either a source electrode or a drain electrode; and

a second electrode disposed over the channel semiconductor layer, within the via and extending over at least a portion of the first electrode, wherein the second electrode is either a source electrode or a drain electrode and is different from the first electrode.

2. The thin film transistor ofclaim 1, wherein the channel semiconductor layer comprises a metal oxide or a metal oxynitride.

3. The thin film transistor ofclaim 2, wherein the channel semiconductor layer comprises indium, gallium and zinc.

4. The thin film transistor ofclaim 2, wherein the channel semiconductor layer comprises zinc oxynitride.

5. The thin film transistor ofclaim 1, further comprising a spacer layer disposed over the first electrode and the channel semiconductor layer, wherein the second electrode is disposed at least partially over the spacer layer.

6. The thin film transistor ofclaim 1, wherein at least a portion of the gate electrode, the first electrode and the second electrode are vertically aligned.

7. A thin film transistor, comprising:

a gate electrode;

a first electrode disposed over the gate electrode, wherein the first electrode is either a source electrode or a drain electrode; and

a second electrode disposed over the first electrode, wherein the second electrode is either a source electrode or a drain electrode and is different from the first electrode.

8. The thin film transistor ofclaim 7, further comprising a first semiconductor layer disposed between the gate electrode and the first electrode.

9. The thin film transistor ofclaim 7, wherein the channel semiconductor layer comprises indium, gallium and zinc.

10. The thin film transistor ofclaim 7, wherein the channel semiconductor layer comprises zinc oxynitride.

11. The thin film transistor ofclaim 7, wherein at least a portion of the gate electrode, the first electrode and the second electrode are vertically aligned.

12. The thin film transistor ofclaim 7, wherein at least a portion of the first electrode encircles at least a portion of a semiconductor layer disposed between the first electrode and the second electrode.

13. A thin film transistor fabrication method, comprising:

depositing a gate dielectric layer over a gate electrode;

forming a channel semiconductor layer over the gate dielectric layer;

forming a first electrode over the channel semiconductor layer, wherein the first electrode is either a source electrode or a drain electrode; and

forming a second electrode over the channel semiconductor layer and over at least a portion of the first electrode, wherein the second electrode is either a source electrode or a drain electrode and is different from the first electrode.

14. The method ofclaim 13, wherein the channel semiconductor layer comprises indium, gallium and zinc.

15. The method ofclaim 13, wherein the channel semiconductor layer comprises zinc oxynitride.

16. The method ofclaim 13, wherein the first electrode encircles at least a portion of the channel semiconductor layer.

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