Abstract:
A method of manufacturing a thin-film transistor or like structure provides conductive “tails” below an overhang region formed by a top gate structure. The tails increase in thickness as they extend outward from a point under the overhang to the source and drain contacts. The tails provide a low resistance conduction path between the source and drain regions and the channel, with low parasitic capacitance. The thickness profile of the tails is controlled by the deposition of material over and on the lateral side surfaces of the gate structure.

Description:
CROSS REFERENCE TO RELATED CASES 
       [0001]    The present disclosure is a divisional application based on U.S. patent application Ser. No. 12/130,347, titled “Self-Aligned Thin-Film Transistor and Method of Forming Same”, filed May 30, 2008, which is incorporated by reference herein and to which priority is hereby claimed. The present disclosure also relates to U.S. patent application Ser. No. 12/130,261, titled “Isolated Sensor Structures Such As For Flexible Substrates”, filed May 30, 2008, which is also incorporated by reference herein and to which priority is also hereby claimed. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    Work related to this disclosure was performed with the support of U.S. Government under contract number 70NANB3H3029 awarded by the United States Department of Commerce, National Institute of Standards and Technology (NIST). Therefore, the U.S. Government has certain rights herein. 
     
    
     BACKGROUND 
       [0003]    The present disclosure is related to solid-state devices such as thin-film transistors, and more specifically to methods of forming such devices utilizing a self-aligning technique. 
         [0004]    Current methods for manufacturing arrays of amorphous silicon thin film transistors (a-Si:H TFTs) typically begin with the deposition of a metal on the substrate on which a layer of a-Si:H is deposited. Additional layers of conducting and insulating materials are formed and patterned by photolithographic processes to create source, gate and drain regions for each TFT. These photolithographic processes typically involve the deposition of layers of photosensitive or photoresistive materials. The photoresistive materials are exposed through a mask, developed to remove portions of the materials, then the structure is etched to remove portions of the conducting and/or insulating layers not protected by the remaining photoresistive materials, to thereby form electrically connected and isolated or semi-isolated regions. Through multiple photolithographic and deposition steps, an array of layered semiconductor devices and interconnections may be formed on the transmissive substrate. 
         [0005]    More specifically, a typical TFT  10 , for example of the type shown in  FIG. 10 , comprises a source region  12  and drain region  14  formed over a dielectric layer  16 , a semiconductive layer  18 , and a gate region  20 , all formed over a substrate  22 . In order for current to flow in a channel in layer  18 , there must be an amount of overlap, x 1 , x 2 , between the lateral edges of source region  12  and drain region  14  on the one hand, and gate region  20  on the other hand. A certain amount of such an overlap is required in order to provide conductivity between the source/drain and channel (for the injection of carriers). However, too much overlap results in parasitic capacitance between the gate region and the source/drain regions, resulting is device switching speed degradation and latency. 
         [0006]    A number of techniques have been developed for creating self-aligned structures, for example using the gate region  20  as a mask when exposing photocurable etch resist, doing laser recrystallization, etc. However, it has heretofore been difficult to form the desired overlap x 1 , x 2 , using such techniques. 
       SUMMARY 
       [0007]    Accordingly, the present disclosure is directed to a TFT structure and method for providing that structure which may be produced using a form of a self-aligning, self-patterning process in which a desirable amount of overlap is formed between source/drain and gate regions. Specifically, the present disclosure provides the advantage of being able to pattern an overlying layer without special added processing steps. Isolated island structures are produced which serve to form overhang regions. The overhang regions partially mask regions therebelow, such that material may be deposited on top of the island structures and partly in gaps between the island structures. The material partly deposited in the gaps form a conduction path for carriers during operation of the TFT. The gaps formed between the discontinuous island structures also electrically isolate the island structures, and act as strain relief regions to accommodate stresses formed during manufacture. 
         [0008]    According to one embodiment disclosed herein a structure is provided which comprises a substrate onto which is formed a gate, conductive channel, dielectric region, and a patterned gate metal. Selective etching of the dielectric/gate metal stack results in an over-etching of the dielectric and an undercutting of the gate metal, forming overhang regions. A subsequently deposited doped conductive layer forms source and drain regions, with “tails” which extend into the overhang regions. Patterned contact metal for source and drain contacts completes the structure. 
         [0009]    According to another embodiment disclosed herein, a method is provided according to which a gate metal structure is patterned over a dielectric layer. The structure is selectively etched such that at least one lateral edge of the gate metal extends beyond the corresponding lateral edge(s) of an underlying dielectric layer to define an overhang region. The overhang region may, for example, be defined by the difference in etching rates between the gate metal and the dielectric layer. 
         [0010]    A conductive layer such as doped silicon is then deposited over the gate metal such that it covers the top surface and at least a portion of a lateral edge(s) of the first doped sensor layer. For each TFT in an array of such devices, this structure is, as deposited, physically discrete from adjacent TFT structures, separated by gaps, discussed further below. In the process of depositing the conductive layer over the gate metal, material is also deposited in regions laterally adjacent the dielectric/gate metal stack. These regions form the source and drain of the TFT. Due to the partial masking of the overhang region, tails of the conductive layer extend into the overhang region. These tails provide a conduction path for operation of the TFT. 
         [0011]    The deposition processes are performed at a relatively high temperature, causing expansion of the materials, including the substrate. The aforementioned gaps may therefore provide a strain relief region to accommodate thermally-induced expansion and contraction and provide a free surface at which crack propagation may terminate. 
         [0012]    The above is a summary of a number of the unique aspects, features, and advantages of the present disclosure. However, this summary is not exhaustive. Thus, these and other aspects, features, and advantages of the present disclosure will become more apparent from the following detailed description and the appended drawings, when considered in light of the claims provided herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    In the drawings appended hereto like reference numerals denote like elements between the various drawings. While illustrative, the drawings are not drawn to scale. In the drawings: 
           [0014]      FIG. 1  is a cross-sectional view of an initial stage in the manufacture of a thin-film transistor or the like according to one embodiment disclosed herein; 
           [0015]      FIG. 2  is a cross-sectional view of an intermediate stage in the manufacture of a thin-film transistor or the like according to one embodiment disclosed herein; 
           [0016]      FIGS. 3A and 3B  are a cross-sectional views of a further intermediate stage in the manufacture of a thin-film transistor or the like according to two embodiments, respectively, disclosed herein; 
           [0017]      FIG. 4  is a cross-sectional view of a still further intermediate stage in the manufacture of a thin-film transistor or the like according to one embodiment disclosed herein; 
           [0018]      FIG. 5  is a cross-sectional view of a final stage in the manufacture of a thin-film transistor or the like according to one embodiment disclosed herein; 
           [0019]      FIG. 6  is a flow chart showing the steps in the manufacture of a thin-film transistor or the like according to one embodiment disclosed herein; 
           [0020]      FIG. 7  is a plan view of a final stage in the manufacture of a thin-film transistor or the like according to one embodiment disclosed herein; 
           [0021]      FIG. 8  is a micro-photograph of a cross-section of a partial thin-film transistor structure according an embodiment disclosed herein; 
           [0022]      FIG. 9  is a cross-sectional view of a final stage in the manufacture of a thin-film transistor or the like according to another embodiment disclosed herein; and 
           [0023]      FIG. 10  is a cross-sectional view of a thin-film transistor according to the prior art. 
       
    
    
     DETAILED DESCRIPTION  
       [0024]      FIGS. 1-5  illustrate various stages in the production of a thin film transistor (TFT)  30  according to the present disclosure. While each of  FIGS. 1-5  illustrate the structure during stages of its manufacture, its manufacture follows a process such as that illustrated in  FIG. 6 , although the individual steps of such a process are not expressly further discussed in detail herein. 
         [0025]    While the actual composition and arrangement of TFT  30  may vary without departing from fundamental concepts of the present disclosure, in one example TFT  30  comprises a top gate TFT. With reference to  FIG. 1 , a conductive layer  34 , such as hydrogenated amorphous silicon (a-Si:H) which is about 500 Å to about 700 Å thick, is formed over a substrate  32 . Substrate may be any of a variety of materials such as a rigid glass substrate, a flexible plastic substrate, etc. A dielectric layer  36  is thereafter formed over conductive layer  34 . Dielectric layer  36  may be, for example, silicon nitride (SiNx) about 2000-3000 Å thick. 
         [0026]    With reference next to  FIG. 2 , a gate metal layer  38  is thereafter deposited over the structure and patterned such to produce a gate metal structure  38   a.  The gate metal comprises refractory metals, such as aluminum-chrome (AlCr), titanium-tungsten (TiW), etc., about 2000 Å (angstroms) thick. 
         [0027]    A selective etch is next performed. In this embodiment, gate metal structure  38   a  has a higher resistance to certain etching than dielectric layer  36 . Accordingly, when an appropriate etchant is presented to the surface of dielectric layer  36 , it is etched at a different (typically much faster) rate that gate metal structure  38   a,  which may advantageously be employed as follows. 
         [0028]    With reference to  FIG. 3A  a wet etch (or alternatively, a dry etch) is performed to remove portions of layers  36  and  38 . Importantly, layer  36  is selected to have a relatively low selectivity to the etchant (it is relatively easily etched), and layer  38  is selected to have a relatively high selectivity to the etchant (resists etching). Accordingly, material forming layer  36  will be removed to a greater extent than the material forming layer  38 . This creates structure  36   a  and an overhang region  42  on one or both lateral edges of the stack  40  of structures  36   a  and  38   a.  Optionally, structure  38   a  may be covered with a gate mask (not shown) prior to etching to enhance the formation of the overhang region(s)  42 . This gate mask would remain during the etching process and can be removed after etch. The dielectric etch is selective so that it will not remove the mask, metal or Si layers. Alternatively, and with reference to  FIG. 3B , the gate metal layer,  38 , may be a bi-layer metal stack  38   b/   38   c,  composed for example of chromium (Cr) over aluminum (Al). The Cr layer may be undercut, for example, by over etching the Al layer thereunder. The dielectric can then be etched using the same mask, resulting in a second, staggered overhang  43 . 
         [0029]    While the precise lateral depth, x, of the overhang region is not critical, and is generally controlled by the etch parameters and material, as a general rule we have found it useful to form the overhang (or undercut) to have a depth approximately equal to the thickness of a conductive layer ( 44 , shown in  FIG. 4 ) formed over structure  38   a.  An exemplary depth x of overhang  42  would be 300 nm (approximately the same depth as the nitride thickness). Other methods of forming overhang  42  may also be employed without modification of the benefits of its presence in the ultimate structure. Accordingly, the bi-layer and etch process described form just one exemplary embodiment of an aspect of the present disclosure. 
         [0030]    As shown in  FIG. 4 , once structures  36   a  and  38   a  are deposited and patterned, and overhang  42  defined, a conductive layer  44  may next be formed over the structure. In one embodiment of the present disclosure, layer  44  is n+ phosphorous doped amorphous silicon deposited by Plasma-enhanced chemical vapor deposition (PECVD). The thickness of layer  44  is important as it controls the formation of structures referred to herein as “tails”  46 , described further below. An exemplary thickness for layer  44  is on the order of 1000 Å. 
         [0031]    We have discovered, that by providing a structure with overhangs  42 , the deposition of layer  44  forms three distinct, simultaneously formed regions: region  44   a  over stack  40 , and regions  44   b  and  44   c  to the sides of stack  40 . The material comprising region  44   a  attaches not only to the upper surface  50  of structure  38   a,  but also to its lateral edges  52 ,  54  as well. As the region  44   a  is deposited, and accumulates on lateral edges  52 ,  54 , the material so accumulating masks an area therebelow, beginning in the overhang regions  42 , from further deposition. As the material forming region  44   a  on lateral edges  52 ,  54  grows larger, more area is masked. A portion of the material forming layer  44  in regions  44   b  and  44   c,  the tails, begins with an area in the overhang regions  42  and extend outwardly to a point at which there is no further masking. Importantly, it is the overhang regions  42  which facilitate the formation of the masking portions of region  44   a  on lateral edges  52 ,  54 , which in turn produce tails  46 . It will also now be appreciated that the thickness of layer  44  (as well as, in some embodiments, the rate of deposition) controls the width of region  44   a  and hence the amount of masking of regions thereunder. 
         [0032]    Regions  44   b  and  44   c  of layer  44  which are not masked by region  44   a  form to the same thickness as region  44   a  above stack  40 . A portion of region  44   b  can then serve as the source for TFT  30 , and a portion of region  44   c  can serve as the drain for TFT  30 . Structure  38   a  then serves as the gate for TFT  30 , and region  36   a  serves as the gate dielectric isolating structure  38   a  from the channel which may form in the a-Si:H layer  34  therebelow. In order to facilitate electrical connection to the structure, source and drain metal regions  56 ,  58 , for example Al (100 nm)/Cr(100 nm), are deposited and patterned. A cut-away view of a completed device is shown in  FIG. 5 . 
         [0033]    In operation, tails  66  serve as conduction paths for current to enter channel  60  under control of gate formed by gate structure  38   a.  We have found that the shape and size of tails  46  can be controlled to achieve optimum current injection, while minimizing the parasitic capacitance usually associated with an overhanging source/drain. In one embodiment, tails  46  have a non-linear profile, resulting from the material accumulating at the lateral edges of region  44   a.    
         [0034]    With reference next to  FIG. 7 , there is shown therein a plan view of TFT  30 . It can be seen from  FIG. 5  and  FIG. 7  that region  44   a  is both mechanically and electrically isolated from adjacent structures by gaps  66 . Gaps  66  help to accommodate stress that may accumulate in layer  40  when deposited. As a part of the deposition process for certain layers, the TFT structure is exposed to relatively large thermal cycling, resulting in expansion and contraction during manufacturing. This is especially true for devices formed on flexible substrates, sensor structures with relatively thick sensor layers overlying the TFT, etc. Gaps  66  allow for some movement of the various portions of the structure relative to one another without the cracking and failure. The width of gap  66  may vary, but in certain embodiments may be as large as the thickness of n+ Si layer  44 , assuming the lateral deposition is the same as the vertical direction, for example between 20-100 nm. 
         [0035]      FIG. 8  is a micro-photograph of a cross-section of a partial structure according to the present disclosure. Overhang region  42  is formed adjacent stack  40 . Structure  44   a  forms such that it creates tail  46 . Based on this, a self-aligned TFT  30  can be produced with a improved carrier injection from source  44   b  and reduced parasitic capacitance. 
         [0036]    While a plurality of preferred exemplary embodiments have been presented in the foregoing detailed description, it should be understood that a vast number of variations exist, and these preferred exemplary embodiments are merely representative examples, and are not intended to limit the scope, applicability or configuration of the disclosure in any way. For example,  FIG. 9  shows a structure  70  similar to that previously described, but for there being only a single overhang portion of region  44   a,  producing a single tail region  46  of region  44   b.  Thus, it will be appreciated that the overhang-tail arrangement of the present disclosure is useful for creating a variety of structures, and is not to be limited by the embodiments disclosed herein. Therefore, the foregoing detailed description provides those of ordinary skill in the art with a convenient guide for implementation of the disclosure, by way of examples, and contemplates that various changes in the functions and arrangements of the described embodiments may be made without departing from the spirit and scope of the disclosure defined by the claims thereto.