Abstract:
Embodiments disclosed herein generally relate to thin film transistors with one or more trenches to control the threshold voltage and off-current and methods of making the same. In one embodiment, a semiconductor device can include a substrate comprising a surface with a thin film transistor formed thereon, a first passivation layer formed over the thin film transistor, a trench formed within the first passivation layer and a second passivation layer formed over the first passivation layer and within the trench.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/823,336, filed May 14, 2013 (APPM/17011USL), which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention generally relate to improving the threshold voltage in a thin film transistor (TFT). 
     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 employed 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. 
     The current driven through the TFTs (i.e., the on-current) is limited by the channel material (often referred to as the active material, semiconductor material or semiconductor active material) as well as the channel width and length. Additionally, the turn-on voltage is determined by the accumulation of the carrier in the channel area of the semiconductor layer which could change as the shift of the fixed charge in the semiconductor material or the charge trapping in interfaces and the threshold voltage shifts after bias temperature stress or current temperature stress. Current MO-TFTs, such as indium gallium zinc oxide (IGZO), zinc oxide (ZnO) and zinc oxynitride (ZnON) TFT devices, have interface problems which can include mobility problems and offset turn on voltages. 
     Therefore, there is a need in the art for better control of the threshold voltage of TFTs. 
     SUMMARY OF THE INVENTION 
     The present invention generally relates to controlling the threshold voltage and off-current of a TFT. In one embodiment, a semiconductor device can include a substrate comprising a surface with a thin film transistor formed thereon; a first passivation layer formed over the thin film transistor; a slot or trench formed in the first passivation layer; and a second passivation layer formed over the first passivation layer and within the trench. 
     In another embodiment, a semiconductor device can include a substrate comprising a surface with a thin film transistor formed thereon; a silicon nitride layer formed over the source electrode, the drain electrode and the semiconductor layer; one or more trenches formed through the silicon nitride layer; and a silicon oxide layer formed over the silicon nitride layer and within the one or more trenches. The thin film transistor can include a gate electrode disposed over a substrate; the gate dielectric layer disposed over the gate electrode; the semiconductor layer disposed over the gate dielectric layer; a source electrode disposed over the semiconductor layer; and a drain electrode disposed over the semiconductor layer. 
     In another embodiment, a method for forming a thin film transistor can include forming a source electrode and a drain electrode over a semiconductor layer that is formed over a gate dielectric layer and a gate electrode, a first portion of the semiconductor layer is exposed between the source electrode and the drain electrode; depositing a first passivation layer over the source electrode, the drain electrode and the exposed first portion of the semiconductor layer; forming at least one trench in the first passivation layer between the source and the drain to expose a second portion of the semiconductor layer; and depositing a second passivation layer on the first passivation layer and within the trench. 
    
    
     
       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. 
         FIG. 1  is a cross-sectional schematic view of a PVD chamber according to one embodiment of the invention; 
         FIGS. 2A-2C  are schematic cross-sectional views of a TFT at various stages of production; and 
         FIGS. 3A-3C  depict TFT devices incorporating one or more slots or trenches according to one 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 using trenches in the passivation layer to control the threshold voltage of a TFT. A TFT has a threshold voltage which is the voltage at the gate which is required for current to flow between the source and the drain. By forming one or more slots or trenches through the passivation layer, and then filling the slots or trenches with additional passivation material, the threshold voltage can be corrected such that current flow is better controlled by the gate when the gate is either on or off based on voltage received. 
     The invention is illustratively described below utilized in a processing system, such as a plasma enhanced chemical vapor deposition (PECVD) system available from AKT America, a division of Applied Materials, Inc., located in Santa Clara, Calif. However, it should be understood that the invention has utility in other system configurations, including those sold by other manufacturers. 
       FIG. 1  is a schematic, cross sectional view of an apparatus that may be used to perform the operations described herein. The apparatus includes a chamber  100  in which one or more films may be deposited onto a substrate  120 . The chamber  100  generally includes walls  102 , a bottom  104  and a showerhead  106  which define a process volume. A substrate support  118  is disposed within the process volume. The process volume is accessed through a slit valve opening  108  such that the substrate  120  may be transferred in and out of the chamber  100 . The substrate support  118  may be coupled to an actuator  116  to raise and lower the substrate support  118 . Lift pins  122  are moveably disposed through the substrate support  118  to move a substrate to and from the substrate receiving surface. The substrate support  118  may also include heating and/or cooling elements  124  to maintain the substrate support  118  at a desired temperature. The substrate support  118  can also include RF return straps  126  to provide an RF return path at the periphery of the substrate support  118 . 
     The showerhead  106  can be coupled to a backing plate  112  by a fastening mechanism  140 . The showerhead  106  may be coupled to the backing plate  112  by one or more fastening mechanisms  140  to help prevent sag and/or control the straightness/curvature of the showerhead  106 . 
     A gas source  132  can be coupled to the backing plate  112  to provide process gases through gas passages in the showerhead  106  to a processing area between the showerhead  106  and the substrate  120 . The gas source  132  can include a silicon-containing gas supply source, an oxygen containing gas supply source, and a nitrogen-containing gas supply source, among others. Typical process gases useable with one or more embodiments include silane (SiH 4 ), disilane, N 2 O, ammonia (NH 3 ), H 2 , N 2  or combinations thereof. 
     A vacuum pump  110  is coupled to the chamber  100  to control the process volume at a desired pressure. An RF source  128  can be coupled through a match network  150  to the backing plate  112  and/or to the showerhead  106  to provide an RF current to the showerhead  106 . The RF current creates an electric field between the showerhead  106  and the substrate support  118  so that a plasma may be generated from the gases between the showerhead  106  and the substrate support  118 . 
     A remote plasma source  130 , such as an inductively coupled remote plasma source  130 , may also be coupled between the gas source  132  and the backing plate  112 . Between processing substrates, a cleaning gas may be provided to the remote plasma source  130  so that a remote plasma is generated. The radicals from the remote plasma may be provided to chamber  100  to clean chamber  100  components. The cleaning gas may be further excited by the RF source  128  provided to the showerhead  106 . 
     The showerhead  106  may additionally be coupled to the backing plate  112  by showerhead suspension  134 . In one embodiment, the showerhead suspension  134  is a flexible metal skirt. The showerhead suspension  134  may have a lip  136  upon which the showerhead  106  may rest. The backing plate  112  may rest on an upper surface of a ledge  114  coupled with the chamber walls  102  to seal the chamber  100 . 
       FIGS. 2A-2C  are schematic cross-sectional views of a TFT  200  at various stages of production. As shown in  FIG. 2A , a gate electrode  204  is formed over a substrate  202 . Suitable materials that may be utilized for the substrate  202  include, but not limited to, silicon, germanium, silicon-germanium, soda lime glass, glass, semiconductor, plastic, steel or stainless steel substrates. Suitable materials that may be utilized for the gate electrode  204  include, but are not limited to, chromium, copper, aluminum, tantalum, titanium, molybdenum, and combinations thereof, or transparent conductive oxides (TCO) such as indium tin oxide (ITO) or fluorine doped zinc oxide (ZnO:F) which are commonly used as transparent electrodes. The gate electrode  204  may be deposited by suitable deposition techniques such as PVD, MOCVD, a spin-on process and printing processes. The gate electrode  204  may be patterned using an etching process. 
     Over the gate electrode  204 , a gate dielectric layer  206  may be deposited. Suitable materials that may be used for the gate dielectric layer  206  include silicon dioxide, silicon oxynitride, silicon nitride, aluminum oxide or combinations thereof. The gate dielectric layer  206  may be deposited by suitable deposition techniques including plasma enhanced chemical vapor deposition (PECVD). 
     A semiconductor layer  208  is then formed over the gate dielectric layer  206  as shown in  FIG. 2B . Suitable materials that may be used for the semiconductor layer  208  include Indium Gallium Zinc Oxide (IGZO), Zinc Oxynitride (ZnON) ZnO x N y , SnO x N y , InO x N y , CdO x N y , GaO x N y , ZnSnO x N y , ZnInO x N y , ZnCdO x N y , ZnGaO x N y , SnInO x N y , SnCdO x N y , SnGaO x N y , InCdO x N y , InGaO x N y , CdGaO x N y , ZnSnInO x N y , ZnSnCdO x N y , ZnSnGaO x N y , ZnInCdO x N y , ZnInGaO x N y , ZnCdGaO x N y , SnInCdO x N y , SnInGaO x N y , SnCdGaO x N y , InCdGaO x N y , ZnSnInCdO x N y , ZnSnInGaO x N y , ZnInCdGaO x N y , and SnInCdGaO x N y . Each of the aforementioned semiconductor films may be doped by a dopant. The semiconductor layer  208  may be deposited by suitable deposition methods, such as PVD. In practice, the semiconductor layer  208  is oftentimes referred to as the channel layer, the active layer or the semiconductor active layer. 
     As shown in  FIG. 2C , over the semiconductor layer  208 , the source electrode  210  and the drain electrode  212  are formed. The exposed portion of the semiconductor layer  208  between the source and drain electrodes  210 ,  212  is referred to as the slot or trench  214 . Suitable materials for the source and drain electrodes  210 ,  212  include chromium, copper, aluminum, tantalum, titanium, molybdenum, and combinations thereof, or TCOs mentioned above. The source and drain electrodes  210 ,  212  may be formed by suitable deposition techniques, such as PVD followed by patterning through etching. 
       FIGS. 3A-3C  depict TFT devices incorporating a slot according to one or more embodiments. In this depiction, the substrate  302  has a stack with one or more layers which are deposited and etched as described with reference to  FIGS. 2A-2C , including a gate electrode  305 , a gate dielectric layer  306 , a semiconductor layer  308 , a source electrode  311  and a drain electrode  312 . 
     Depicted in  FIG. 3A , a first passivation layer  318  is deposited over an exposed semiconductor material  316 , the source electrode  311  and the drain electrode  312 . In one embodiment, the first passivation layer  318  is a silicon oxide or silicon nitride layer, such as SiO x , SiN, SiON or combinations thereof. The first passivation layer  318  can be deposited to a thickness of from 20 Å to 3000 Å. The first passivation layer  318  can be deposited using CVD, PECVD, ALD or other deposition techniques known in the art. Deposition gases for depositing the first passivation layer  318  can include silanes, such as SiH 4 , N 2 O, O 2 , N 2 , an inert carrier gas, such as Ar, or combinations thereof. As depicted, the deposition of the first passivation layer  318  is substantially conformal across the surface of the exposed semiconductor material  316 , the source electrode  311  and drain electrode  312 . The first passivation layer  318  can have a low flat band voltage. In one embodiment, the flat band voltage of the first passivation layer  318  can be lower than −10 V. In another embodiment, the flat band voltage of the first passivation layer  318  can be approximately 0 V. 
     A trench  314  is then formed in the first passivation layer  318  between the source electrode  311  and the drain electrode  312  to expose the semiconductor layer  308 . The trench  314  can formed by patterning the first passivation layer  318 . The first passivation layer  318  can be patterned by forming either a photolithographic mask or a hard mask over the first passivation layer  318  and exposing the first passivation layer  318  to an etchant. The first passivation layer  318  may be patterned by exposing the exposed portions of the first passivation layer  318  to a wet etchant or to an etching plasma. In one embodiment, the etching plasma can comprise gases selected from SF 6 , O 2 , Cl 2 , or combinations thereof. 
     The trench  314  is generally a slot or trench which extends at least the length of the source electrode  311  and drain electrode  312 . In one embodiment, the source electrode  311  and the drain electrode  312  are both approximately 40 microns wide and the trench  314  extends approximately 50 microns to 60 microns. Thus, the ratio of the source/drain electrode width to the slot or trench length can be from 1:1 to 1:2, such as between 1:1 and 1:1.5. In this embodiment, the width of the slot or trench can be from about 1 micron to about 3 microns, such as about 2 microns. In further embodiments, the trench  314  can extend to multiple TFTs such that the trench is formed above the active channel region for each of the TFTs involved. 
     The trench  314  can be parallel to the edge of either the source electrode  311  or the drain electrode  312 . The trench  314  can be positioned at one or more locations in the portion of the first passivation layer  318  which is above the exposed semiconductor material  316 . Depicted here, the trench  314  is positioned approximately in the center of the first passivation layer  318 . However, the positioning of the trench  314  may be shifted within the region of the exposed semiconductor material  316 . 
     Once the trench  314  is etched, the exposed semiconductor material  316  can be treated with an activated gas. The activated gas can include oxygen, nitrogen or combinations thereof. The activated gas can be activated by plasma and delivered to the substrate to expose the exposed semiconductor material  316 , where the activated gas can be incorporated into the exposed portion of the exposed semiconductor material  316 . After the trench  314  is etched into the first passivation layer  318  and any treatment performed, a second passivation layer  319  is then formed over the surface of the first passivation layer  318  and the trench  314 . The second passivation layer  319  can be deposited generally in the same manner as the first passivation layer  318 . The second passivation layer  319  is composed of a separate passivation material from that of the first passivation layer  318 . In one example, the first passivation layer  318  is composed of silicon nitride and the second passivation layer  319  is composed of silicon oxide. In one or more embodiments, the material deposited in the trench  314  is the same material used to form the second passivation layer  319 . The first passivation layer  318  or the second passivation layer  319  may be deposited with or treated with either p-type dopants or n-type dopants. 
     Further, the first passivation layer  318 , the second passivation layer  319  or combinations thereof, can be composed of one or more sublayers, such that the first passivation layer  318  or the second passivation layer  319  are composed of a plurality of sublayers, shown at  360 . The sublayers may be composed of silicon oxide or silicon nitride, such as SiO x , SiN, SiON or combinations thereof. The sublayers of the first passivation layer  318  or the second passivation layer  319  may have different compositions between them. The sublayers which interface between the first passivation layer  318  and the second passivation layer  319  should be of a different composition than one another. In one example, the first passivation layer  318  is composed of a single layer of SiN and the second passivation layer is composed of three layers, where the first layer is SiO, the second layer is SiON and the third layer is SiO. The first layer of the second passivation layer  319  forms the interface with the first passivation layer  318 . Further permutations are envisioned without further specific recitation. 
       FIG. 3B  depicts a first passivation layer  338  deposited over the exposed semiconductor material  316 , the source electrode  311  and drain electrode  312 . The first passivation layer  338  can be substantially similar to the passivation layer  318  described with reference to  FIG. 3A . In this embodiment, the passivation layer  338  has a trench  334  formed therein. The trench insert can be formed using the photomask/etch method described with reference to  FIG. 3A . The trench  334  is wider in this embodiment and offset toward the drain electrode  312 . After the trench  334  is etched into the first passivation layer  338 , a second passivation layer  339  is then formed over the surface of the first passivation layer  338  and in the trench  334 . The second passivation layer  339  can be substantially similar to the second passivation layer described with reference to  FIG. 3A . 
       FIG. 3C  depicts a first passivation layer  358  deposited over the exposed semiconductor material  316 , the source electrode  311  and drain electrode  312 . The first passivation layer  358  can be substantially similar to the passivation layer  318  described with reference to  FIG. 3A . In this embodiment, the passivation layer  358  has two trenches  354  formed therein. The trenches  354  are formed near both the source electrode  311  and the drain electrode  312 . After the trench  354  is etched into the first passivation layer  358 , a second passivation layer  359  is then formed over the surface of the first passivation layer  358  and in the trench  354 . The second passivation layer  359  can be substantially similar to the second passivation layer described with reference to  FIG. 3A . 
     The trenches described above are believed to improve the threshold voltage (V th ) of the TFT. The V th  is the value of the gate-source voltage when the conducting channel just begins to connect the source and drain contacts of the transistor, allowing significant current to flow. Though, optimally, this voltage would be zero, most modern TFTs do not achieve an optimal V th . Thus, many modern TFT can have a low steady current between the source electrode and the drain electrode, even when the gate is not receiving voltage. The trench is believed to shift the actual V th  closer to the optimal V th  through the creation of a second field which interferes with the first field. 
     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.