Abstract:
A method of manufacturing a bottom gate transistor comprises depositing a first microcrystalline silicon layer ( 40 ) over the gate insulator layer ( 22   a ) and exposing the microcrystalline silicon layer to a nitrogen plasma ( 42 ), thereby forming silicon nitride with a crystalline structure. A plurality of microcrystalline silicon nitride layers are formed in this way. A further microcrystalline silicon layer is deposited over the exposed layers defining the semiconductor body ( 14 ) of the transistor. This method enables the bottom of the transistor body to have a microcrystalline structure, improving the mobility of the semiconductor layer, even at the interface with the gate insulator layer. The exposed silicon nitride layers become part of the gate insulator layer, and there is improved structural matching between the gate insulator layer and the semiconductor transistor body, which layers derive from the same microcrystalline silicon structure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a divisional of application Ser. No. 09/881,601, filed Jun. 14, 2001, now U.S. Pat. No. 6,410,372. 
     This invention relates to thin film transistors, for example for forming the transistor substrate used in the manufacture of liquid crystal displays. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     There is much interest in improving arrays of TFTs which are used to form the switching elements for flat panel liquid crystal displays. These TFT devices may be fabricated with portions of an amorphous, polycrystalline or microcrystalline semi-conductor film to form the body of the transistor devices. 
     Hydrogenated amorphous silicon is currently used as the active layer in thin film transistors (TFTs) for active matrix liquid crystal displays. This is because it can be deposited in thin, uniform layers over large areas by plasma enhanced chemical vapour deposition (PECVD). However, due to its amorphous structure, it has a very low carrier mobility, which reduces the switching speed of devices and prevents the use of these transistors in display driver circuitry. Amorphous silicon TFTs are also relatively unstable and are useful for display applications only because the duty cycle is relatively low. 
     Crystalline silicon is required for the higher speed driver circuitry, which necessitates both a driving circuit panel and a display panel within a display device, with interconnections between these two circuit types. 
     It has been recognised that microcrystalline silicon may offer a solution to these problems because transistors having microcrystalline silicon as the active layer have improved carrier mobility and yet can still be deposited using a PECVD process. Microcrystalline silicon films deposited in this way consist of small crystals, for example up to 100 nm, embedded within an amorphous matrix. If the crystal grains are large enough, then extended state conduction is enhanced and the mobility increased, approximately by a factor of 10 compared to amorphous silicon layers. 
     However, deposition by PECVD tends to result in grains being produced with a conical structure. This results in the lower 5-10 nm of material being predominantly amorphous. In a bottom gate TFT structure, the bottom part of the silicon film defines the boundary between the gate insulator and silicon body of the transistor. Therefore, in a bottom gate TFT structure the advantage of the crystalline material is largely lost, whereas top gate TFT structures do exhibit improved mobility and significantly improved stability. These improvements in performance have not been achieved for bottom gate structures for the reasons above. 
     According to a first aspect of the present invention, there is provided a method of manufacturing a transistor, comprising: 
     (i) defining a gate conductor over an insulating substrate; 
     (ii) forming a gate insulator layer over the gate conductor; 
     (iii) depositing a first microcrystalline silicon layer over the gate insulator layer; 
     (iv) exposing the microcrystalline silicon layer to a nitrogen plasma, thereby forming silicon nitride, and substantially maintaining the crystalline structure; 
     (v) repeating steps (iii) and (iv) for a plurality of microcrystalline silicon layers; 
     (vi) forming a further microcrystalline silicon layer over the exposed layers, the further layer defining the semiconductor body of the transistor; and 
     (vi) defining a source and drain structure over the transistor body. 
     This method enables the bottom of the transistor body to have a microcrystalline structure, improving the mobility of the semiconductor layer, even at the interface with the gate insulator layer. The exposed layers which form silicon nitride become part of the gate insulator layer, and there is improved structural matching between the gate insulator layer and the semiconductor transistor body, which layers derive from the same microcrystalline silicon structure. 
     The microcrystalline silicon layer deposited in steps (iii) and (vi) may be formed by a PECVD process, and the plurality of layers deposited in these steps typically have a combined thickness of between 5 and 25 nm. The individual layers deposited may each have a thickness of between 0.5 and 2 nm. 
     The exposure in step (iv) is preferably exposure to a dense nitrogen plasma produced by electron cyclotron resonance PECVD. 
     According to a second aspect of the invention, there is provided a bottom gate thin film transistor comprising: 
     a gate conductor disposed over an insulating substrate; 
     a gate insulator layer over the gate conductor; 
     a silicon nitride layer over the gate insulator layer, the silicon nitride layer having a substantially crystallised structure at the top of the layer, and a substantially amorphous structure at the bottom of the layer; 
     a microcrystalline silicon layer over the silicon nitride layer which defines the semiconductor body of the transistor; and 
     a source and drain structure over the transistor body. 
     The crystal structure within the silicon nitride layer enables the semiconductor body of the transistor to have the desired microcrystalline structure throughout the thickness of the layer, and particularly at the semiconductor/insulator interface. 
     A thin film transistor active plate for an active matrix liquid crystal display may use transistors of the invention 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     The invention will now be described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 shows in plan view a pixel of a liquid crystal display device incorporating a transistor-capacitor arrangement using a bottom gate transistor; 
     FIG. 2 illustrates the components of a liquid crystal display pixel, for explaining the operation of the display device; 
     FIG. 3 shows a liquid crystal display using a transistor substrate having transistors manufactured according to the invention; 
     FIG. 4 shows the manufacturing steps of the invention for forming a bottom gate microcrystalline TFT. 
     It should be noted that these figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. 
    
    
     DETAILED DESCRIPTION 
     Transistor substrates according to the invention, and manufactured in accordance with the invention, form the switching elements of a liquid crystal display device. By way of example, FIG. 1 shows the whole area of one pixel of an active matrix display device using bottom gate transistors, to which the invention may be applied. A pixel comprises an electrode pattern  9  formed on an insulating substrate  10 . The substrate  10  may comprise a back plate of the display, for example a glass plate or polymer film. 
     The electrode pattern  9  defines the row conductors of the matrix array, and also defines the gate electrode  12  of the switching transistor TFT. The semiconductor transistor body  14  overlies the gate electrode, spaced by a gate insulator. An upper electrode layer  16   a ,  16   b  defines the source and drain electrodes of the transistor TFT, which are connected to a column electrode  18  and a connector pad  20  for the liquid crystal material, respectively. 
     The column conductors  18  are defined by part  16   a  of the upper electrode layer, and these column.conductors  18  define the source electrodes of the TFTs. Part  16   b  of the upper electrode layer forms a drain electrode of the TFT and also forms the bulk of the upper electrode layer, forming a pixel electrode  20 . This pixel electrode  20  is integral with the drain electrode and also, in this example, with a part  16 c which forms the top electrode of a pixel storage-capacitor, the bottom electrode being defined by a row conductor  9  of a neighbouring pixel. 
     The switching TFT of each cell comprises a silicon transistor body  14 . The invention is concerned specifically with the structure and processing of transistor body and the gate insulator layer, as will be described further below. 
     Liquid crystal material is provided over the transistor substrate, the components of which are shown in FIG.  1 . Above the liquid crystal material, an additional substrate is provided defining a ground plane, as will be described with reference to FIG.  3 . 
     FIG. 2 shows the electrical components which make up the pixels shown in FIG.  1 . The row conductor  9  is connected to the gate of the TFT  30 , and the column electrode  18  is coupled to the source electrode, as explained with reference to FIG.  1 . The liquid crystal material provided over the pixel effectively defines a liquid crystal cell  32  which extends between the drain of the transistor  30  and a common ground plane  34 . The pixel storage capacitor  36  is connected between the drain of the transistor  30  and the row conductor  9   a  associated with the next row of pixels. 
     During operation of the display device, signals are applied to rows of pixels in turn. In order to address a row of pixels, an appropriate signal is applied to the associated row conductor  9  to turn on the transistors  30  of the row of pixels. This enables a display signal applied to the column conductor  18  to be fed to the liquid crystal cell  32 , which results in charging of the liquid crystal cell to the desired voltage. The storage capacitor  36  is also charged and is provided to ensure that the signal on the liquid crystal cell  32  remains constant even after the addressing of that particular row has been completed, and the transistors  30  have been turned off. During addressing of the row of pixels, the row conductor  9   a  of the subsequent row of pixels is held at ground potential so that the storage capacitor  36  is charged to a voltage corresponding to that which is to be applied across the liquid crystal cell  32 . 
     When the next row of pixels is addressed, there will be an increase in the voltage of the row conductor  9   a , which will feed through the capacitor  36  by capacitive coupling to the liquid crystal cell  32 . However, this increased voltage on the next row conductor  9   a  only lasts for one row address period, after which that row conductor  9   a  returns to ground. The liquid crystal material has slower response time and does not respond to these instantaneous voltage changes. 
     FIG. 3 shows in cross section (line III—III of FIG. 1) a transistor substrate for use within a liquid crystal display. 
     A gate electrode pattern  9  is provided on the substrate  10 , and also defines the lower terminal  37  of the storage capacitor  36 . The gate of the transistor forms part of the respective row conductor, and the lower terminal  37  of the storage capacitor  36  forms part of the row conductor for the next adjacent row of pixels. 
     To define the pattern  9 , a conductor layer may be deposited on a glass substrate  10 , and wet etching may be performed in order to define the conducting pattern. 
     A gate dielectric layer  22  is then deposited. This layer  22  extends beyond the body of the transistor, and defines the dielectric layer for the storage capacitor  36 . 
     The silicon layer  14  forming the body of the transistor is deposited over the gate insulator layer. The invention is concerned specifically with the processing of the gate insulator layer and the deposition of the silicon layer  14 , as will be described below. 
     An etch stop plug  24  is patterned overlying and aligned with the gate  9 , and an n+ contact layer overlies the transistor body, over which the source and drain electrodes  26 , 28  are then deposited. The layer  16  defining the source and drain electrodes also defines the top contact  38  of the storage capacitor  36 . The source  26  forms part of a respective column conductor  18 , and the drain  28  is integral with the liquid crystal contact pad  20  as well as the top contact  38  of the storage capacitor  36 . Of course, additional layers to those described may be desired, for example planarising layers. 
     These layers complete the transistor substrate for the liquid crystal display. A layer of liquid crystal material  50  is provided over the transistor substrate, and a further substrate  52  overlies the layer of liquid crystal material. This further substrate  52  may be provided on one face with an arrangement of colour filters  54  and a plate defining the common electrode  34 . A polarising plate  56  is provided on the opposite side of the substrate  52 . 
     The liquid crystal display structure is known to the extent described above. One commonly used insulator for the gate insulator layer  22  is silicon nitride, which also can be deposited by a PECVD process. It has been shown, for example in the article of W N Singer et al in Appl. Phys. Lett. 72,1164,1998, that a 25 nm thick layer of silicon nitride can be produced on a crystalline silicon wafer by exposure to a dense nitrogen plasma produced by electron cyclotron resonance (ECR) PECVD. In order to implant nitrogen atoms to such a depth, a high substrate bias has to be used. 
     The invention is based on the recognition that this conversion of the silicon layer into silicon nitride can be used to obtain improved crystallization of the silicon body of the transistor particularly at the base of the silicon layer. In particular, a layer-by-layer deposition technique is used to convert the lower amorphous layer of a microcrystalline silicon sample into a silicon nitride layer which then forms part of the gate insulator layer within a bottom gate TFT. 
     FIG. 4 shows the processing steps provided by the invention and which form part of the overall transistor manufacturing process. 
     FIG. 4 a  shows the substrate  10  already provided with the patterned gate electrode layer  9  and a silicon nitride gate insulator layer  22 . Initially, a thin microcrystalline silicon layer  40  is deposited by a PECVD process (FIG. 4 b ). This layer  40  preferably has a thickness of between 0.5 nm and 2 nm. As shown in FIG. 4 c , this layer is exposed to a dense nitrogen plasma containing a significant population of atomic nitrogen ions, represented by arrows  42 , for example such as produced by ECR-PECVD, very high frequency RF-PECVD or any other appropriate technique. With a suitably low ion energy, the microcrystalline silicon layer will be converted into silicon nitride, while preserving any crystalline structures in the layer. This ion energy is likely to be in the range 0-100 eV and, in the case of ECR-PECVD, it may be defined though application of an rf bias to the substrate. 
     Further thin layers of microcrystalline silicon are deposited and treated with the dense nitrogen plasma, so that a layer-by-layer process is produced. This process is repeated until the top surface is predominantly crystalline. The thickness of the combined layers is likely to be of the order of 10 nm. As shown in FIG. 4 d , the resulting gate insulator layer comprises a base part  22   a  and a top part  22   b  derived from the layer-by-layer process. The symbols  44  in FIG. 4 d  are intended to represent schematically how the state of crystallization increases towards the top of the gate insulator layer  22   a ,  22   b  as a result of the conical grain structure of microcrystalline silicon deposited by PECVD. 
     A further layer of microcrystalline silicon is then deposited to define the body  14  of the transistor. 
     This process provides an improved crystalline structure of the microcrystalline transistor body, particularly at the semiconductor-gate insulator interface. This gives rise to improved carrier mobility. Furthermore, the mechanical interface between the layers  22  and  14  will be improved as a result of the excellent structural match between these layers. 
     Conventional techniques can be used to define the source and drain structure over the insulated gate structure shown in FIG. 4 e  as well as for the liquid crystal layer and associated further substrate  52 . These processes will therefore not be described in detail in this application. 
     In the preferred example described, the gate insulator layer  22  comprises silicon nitride, so that the additional gate insulator layers resulting from the nitrogen plasma have the same structure as the underlying insulator layer, although in partly crystallized form. However, the lower gate insulator layer  22   a  does not necessarily need to be silicon nitride. 
     The transistor of the invention has been described as used to form the active plate of a liquid crystal display. However, the transistor of the invention may equally be applied to other devices, and in particular to any existing integrated circuit devices using amorphous silicon transistors. The invention enables bottom gate transistors to be manufactured using chemical vapour deposition processes whilst giving improved transistor performance resulting from increased carrier mobility within the silicon transistor channel. 
     Various modifications to the specific layers used in the manufacture of the TFT substrate will be apparent to those skilled in the art, which do not prohibit the use of the invention in those transistor substrates.