Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of application Ser. No. 09/477,203, filed Jan. 4, 2000, pending, which is a continuation of application Ser. No. 08/900,906, filed Jul. 28, 1997, now U.S. Pat. No. 6,140,160, issued Oct. 31, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a method for fabricating a Complementary Metal Oxide Semiconductor (“CMOS”) Thin Film Transistor (“TFT”). More particularly, the present invention relates to fabricating a CMOS TFT with a simplified method which eliminates the ion implantation and annealing steps used in present methods.  
           [0004]    2. State of the Art  
           [0005]    The development of new portable electronic products, such as notebook computers and personal interface devices, is currently receiving a great deal of attention in the consumer products market. Substantial research and development have been focused in the field of active matrix liquid-crystal displays. Active matrix displays generally consist of flat panels of liquid crystals or electroluminescent materials which are switched “on” and “off” by electric fields. Every liquid crystal picture element (“pixel”) is charged by thin film transistors (“TFTs”). One of the most common components in the circuit for the liquid crystal display is the inverter, which has a CMOS structure constructed with a pair of n-type and p-type TFTs.  
           [0006]    Although both polysilicon and amorphous silicon may be used in the fabrication of TFTs, a polysilicon semiconductor layer generally provides greater mobility of electrons and holes than does amorphous silicon. Furthermore, the CMOS structure can be easier to construct with polysilicon since the n-type and p-type TFTs can be formed by the same implant and anneal process. The CMOS inverter constructed with polysilicon TFTs also offers excellent characteristics in terms of operating frequency and power consumption.  
           [0007]    In the fabrication of a TFT, the semiconductor-layer source and drain regions are formed by introducing an impurity element into the semiconductor layer (see U.S. Pat. No. 5,514,879 issued May 7, 1996 to Yamazaki). The controlled introduction of impurities enable good transistor characteristics. Typically, the introduction of impurities for a CMOS TFT requires two masking and implantation steps. As shown in FIG. 26, a substrate  202  is coated with a layer of oxide  204 . A non-doped polysilicon layer  206  is applied to the oxide layer  204 , and a resist layer  208  is applied to the non-doped polysilicon layer  206  in a predetermined pattern. The non-doped polysilicon layer  206  is then etched to form a non-doped polysilicon ledge  210 , as shown in FIG. 27. A portion of the non-doped polysilicon ledge  210  is masked with a first mask  212  and an n-type impurity is introduced into the unmasked portion of the non-doped polysilicon ledge  210  to form an n-type area  214 , as shown in FIG. 28. The first mask  212  is removed and a second mask  216  is applied to the non-doped polysilicon ledge  210  so as to cover the n-type area  214  and a portion  220  of the non-doped polysilicon ledge  210 . A p-type impurity is introduced to the unmasked portion of the non-doped polysilicon ledge  210  to form a p-type area  218 , as shown in FIG. 29. The second mask  216  then is removed to form a fundamental CMOS TFT gate structure  222  with the portion  220  acting as an insulating barrier between the n-type area  214  and the p-type area  218 , as shown in FIG. 30.  
           [0008]    The impurities can be introduced by thermal diffusion or ion implantation. By using thermal diffusion, the impurities are introduced from the surface of the semiconductor layer. By using ion implantation, impurity ions are implanted into the semiconductor layer. The ion implantation method provides a more precise control with respect to the total impurity concentration and depth that the impurities can be implanted into the semiconductor layer, and thus allows impurities to be implanted into a shallow, thin film. Furthermore, ion implantation can be performed at low temperatures. For the above reasons, ion implantation is a preferred technique for introducing impurities into a semiconductor layer in the fabrication process of the TFT.  
           [0009]    In the above-described fabrication process of the TFT, impurities are implanted by a conventional ion implantation apparatus using an ion beam having a diameter of only several millimeters. When the ions are to be implanted over a large substrate using the above conventional ion implantation apparatus, it is necessary to either move the substrate mechanically or scan the ion beam electrically over the substrate since the area of the substrate is larger than the diameter of the ion beam. The necessity of having a mechanical moving means for the ion beam causes a problem in that the ion implantation apparatus becomes complicated, large, and expensive.  
           [0010]    One technique for solving the above problem, wherein ions can be easily implanted into a large area, is an ion shower-doping method. According to this technique, ions generated by using a plasma discharge are dispersed in a cone shape and accelerated at a low voltage without mass separation to implant in the substrate. Although this technique allows ions to be implanted simultaneously over a large portion of the entire semiconductor layer, it does not result in uniform implantation.  
           [0011]    Once the implantation is complete, the structure is annealed at about 600° C. to activate the impurities. However, the temperature of annealing is detrimental to any temperature-sensitive portion of the entire structure.  
           [0012]    Therefore, it would be advantageous to develop a technique to form a CMOS TFT which eliminates the need for introducing impurities during the fabrication of the CMOS TFT, while using state-of-the-art semiconductor device fabrication techniques employing known equipment, process steps, and materials.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention relates to a method for fabricating a CMOS TFT using doped and activated n-type and p-type polysilicon layers, which eliminates ion implantation and annealing steps used in present methods.  
           [0014]    In one embodiment of the present invention, the CMOS TFT is constructed by first coating a base substrate with a layer of oxide. A pre-doped and pre-activated n-type polysilicon layer is applied over the oxide layer. A resist layer is applied to the n-type polysilicon layer such that when the n-type polysilicon layer is etched and the resist layer is removed, n-type regions are formed. An isolation material layer is then applied over the oxide layer and the n-type regions. The isolation material layer is etched such that a portion of the isolation material layer, which resides in the corners formed at the junction between the oxide layer and the n-type regions, forms isolation caps. A pre-doped and pre-activated p-type polysilicon layer is then applied over the oxide layer, n-type region, and isolation caps.  
           [0015]    The p-type polysilicon layer is then planarized down to form the p-type regions having a height substantially equal to that of the n-type regions. The p-type regions and the n-type regions are isolated from one another by the isolation caps, hereafter referred to as the isolation barriers. It is, of course, understood that the p-type layer may be applied to the oxide layer prior to applying the n-type layer.  
           [0016]    The n-type regions and p-type regions may alternately be formed by again coating a substrate with a layer of oxide. A pre-doped and pre-activated n-type polysilicon layer is applied to the oxide layer. A pad oxide is applied over the n-type polysilicon layer and a nitride layer is applied over the pad oxide to form a layered assembly. A resist layer is patterned on the nitride layer. The layered assembly is etched and the resist layer removed to form an n-type region stack. The nitride layer is then removed and a thin layer of silicon dioxide is formed over the n-type polysilicon layer. A pre-doped and pre-activated p-type polysilicon layer is then applied over the oxide layer and the thin layer of silicon dioxide. The p-type polysilicon layer is then planarized down to the height of the n-type region to form p-type regions, wherein the thin silicon dioxide layer may act as a planarization stop. The p-type and the n-type regions are isolated from one another by the portions of the thin silicon dioxide layer, which become the isolation barriers.  
           [0017]    Once the p-type and n-type regions (separated by the isolation barriers) are fabricated, resist masks are formed over each isolation barrier such that open areas are positioned over the n-type regions and the p-type regions. The n-type regions and the p-type regions are then simultaneously etched and the resist masks removed such that the n-type region and the p-type region are each substantially bifurcated into n-type areas and p-type areas, respectively.  
           [0018]    A layer of semiconductive, undoped material is disposed over the exposed portions of the oxide layer, the n-type areas, the p-type areas, and the isolation barriers. A layer of insulative material is placed over the semiconductive, undoped material. A layer of metal gate material is then placed over the insulative material layer. A layer of barrier oxide is then disposed over the metal gate material layer.  
           [0019]    The barrier oxide layer is masked and etched down to each of the n-type areas, the p-type areas, and the isolation barriers to form gates. An isolation material is applied over the gates, the n-type areas, the p-type areas, and the isolation barriers. The isolation material is etched, leaving end caps abutting edges of the gates.  
           [0020]    A layer of passivation material is disposed over the gates, the n-type areas, the p-type areas, and the isolation barriers. The passivation layer is then masked and etched to expose a portion of the n-type areas, the p-type areas, and the isolation barriers.  
           [0021]    Poly plugs are disposed within the etched areas in the passivation layer to contact the n-type areas, the p-type areas, and the isolation barriers to form a CMOS TFT structure. Thus, the entire CMOS TFT structure is built with only four mask steps with the n-type and p-type areas being formed by two of these mask steps, which eliminates time-consuming, multiple masking steps required by known fabrication techniques. This, in turn, reduces the cost of manufacturing the CMOS TFT.  
           [0022]    As discussed above, the present invention utilizes deposited layers of pre-doped and pre-activated n-type and p-type polysilicon layers. However, it is understood that ion implant steps can be used to vary device characteristics.  
           [0023]    Furthermore, it is, of course, understood that, although the present invention is described in terms of a CMOS TFT, the techniques can be applied to other MOS structures such as PMOS and NMOS structures. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0024]    While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings in which:  
         [0025]    FIGS.  1 - 15  are side cross-sectional views of a CMOS TFT fabrication technique of the present invention;  
         [0026]    [0026]FIG. 16 is an electrical schematic of a CMOS TFT;  
         [0027]    FIGS.  17 - 19  are side cross-sectional views of an alternate gate formation method for a CMOS TFT fabrication technique of the present invention;  
         [0028]    FIGS.  20 - 25  are side cross-sectional views of a method of forming n-type and p-type regions of the present invention; and  
         [0029]    FIGS.  26 - 30  are side cross-sectional views of a prior art method of fabricating a CMOS TFT. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    [0030]FIGS. 1 through 15 illustrate a preferred method of the present invention for forming a CMOS TFT. As shown in FIG. 1, a base substrate  102 , such as a semiconductor substrate, glass substrate, or the like, is coated with a layer of oxide  104 , such as silicon dioxide. An n-type polysilicon layer  106 , which has previously been doped and activated, is applied to the oxide layer  104 . A resist layer  108  is applied to the n-type polysilicon layer  106  in a predetermined pattern. The n-type polysilicon layer  106  is etched and the resist layer  108  removed to form an n-type region  110 , as shown in FIG. 2. An isolation material layer  112 , preferably silicon dioxide or silicon nitride, is applied over the oxide layer  104  and the n-type region  110 , as shown in FIG. 3. The isolation material layer  112  is etched, leaving isolation caps  114  abutting edges  116  of the n-type region  110 , as shown in FIG. 4. Optionally, a thin layer of silicon dioxide  118  can be deposited or grown (e.g., by thermal oxidation) over the oxide layer  104 , n-type region  110 , and isolation caps  114 , as shown in FIG. 5. The thin layer of silicon dioxide  118  is used to prevent contamination and act as an etch stop during subsequent processing. A p-type polysilicon layer  120 , which has been doped and activated, is then applied over the oxide layer  104 , n-type region  110 , and isolation caps  114 , or over the thin layer of silicon dioxide  118 , if formed, as shown in FIG. 6.  
         [0031]    The p-type polysilicon layer  120  is then planarized down to the n-type region  110  wherein the thin silicon dioxide layer  118  may act as a planarization stop. Planarization ensures that the n-type polysilicon layer  106  and the p-type polysilicon layer  120  will be on the same horizontal plane, similar to a twin-tub process. This planarization forms p-type regions  122  and is preferably achieved using an abrasive process such as chemical mechanical planarization (“CMP”). The p-type regions  122  and the n-type region  110  are isolated from one another by the isolation caps  114  and such portions of the thin silicon dioxide layer  118  as may exist. This combination will hereafter be referred to as isolation barriers  124 , as shown in FIG. 7.  
         [0032]    The n-type regions  110  and p-type regions  122  may alternately be formed by the process illustrated in FIGS.  20 - 25 . As shown in FIG. 20, a substrate  160 , such as a glass substrate or the like, is coated with a layer of oxide  162 . An n-type polysilicon layer  164  which has previously been doped and activated is applied to the oxide layer  162 . A pad oxide  166  is applied over the n-type polysilicon layer  164  and a mask layer  168 , preferably a nitride layer, is patterned over the pad oxide  166  to form a layered assembly  170 . The layered assembly  170  is etched to form an n-type region stack  174 , as shown in FIG. 21. The mask layer  168  is removed, as shown in FIG. 22. If the mask layer  168  is a nitride layer, it is preferably removed with phosphoric acid at about 155° C. A thin layer of silicon dioxide  176  can be deposited or grown over the n-type polysilicon layer  164 , as shown in FIG. 23. A p-type polysilicon layer  178 , which has been doped and activated, is then applied over the oxide layer  162  and the thin layer of silicon dioxide  176 , as shown in FIG. 24.  
         [0033]    The p-type polysilicon layer  178  is then planarized down to the n-type polysilicon layer  164  wherein the thin silicon dioxide layer  176  may act as a planarization stop. This planarization forms p-type regions  180 . The p-type regions  180  and the n-type polysilicon layer  164  are isolated from one another by non-etched portions  182  of the thin silicon dioxide layer  176 , as shown in FIG. 25. It is, of course, understood that the order of forming the n-type polysilicon layer  164  and the p-type regions  180  is not important. Thus, the p-type polysilicon layer  178  may be formed first, followed by n-type polysilicon layer  164 .  
         [0034]    As shown in FIG. 8, once the p-type and n-type regions (separated by the isolation barriers) are fabricated, resist masks  126  are formed over each isolation barrier  124  such that open areas  128  are positioned over the n-type regions  110  and the p-type regions  122 . The n-type regions  110  and the p-type regions  122  are simultaneously etched and the resist masks  126  removed to form n-type regions  110  and p-type regions  122  that are each substantially bifurcated into n-type areas  130  and p-type areas  132 , as shown in FIG. 9.  
         [0035]    As shown in FIG. 10, a layer of undoped semiconductive material  134 , preferably undoped polysilicon, is disposed over the exposed portions of the oxide layer  104 , the n-type areas  130 , the p-type areas  132 , and the isolation barriers  124 . A layer of insulative material  136  is placed over the undoped semiconductive material layer  134 . A layer of conductive gate material  138  is then placed over the insulative material layer  136 . A layer of barrier oxide or nitride  140  is then disposed over the conductive gate material layer  138 . The conductive gate material layer  138  is preferably planarized, such as by CMP, prior to the deposition of the barrier oxide/nitride layer  140 . It is, of course, understood that the conductive gate material layer may be comprised of metal, metal alloys, conductive polymer material, or a layered combination of both.  
         [0036]    The barrier oxide layer  140  is masked and etched down to each of the n-type areas  130 , the p-type areas  132 , and the isolation barriers  124 , as shown in FIG. 11, to form gates  142 . An isolation material (not shown), preferably silicon dioxide or silicon nitride, is applied over the gates  142 , the n-type areas  130 , the p-type areas  132 , and the isolation barriers  124 . The isolation material is spacer etched, leaving end caps  144  abutting edges  146  of the gates  142 , as shown in FIG. 12.  
         [0037]    A layer of passivation material  148 , such as a low eutectic glass (e.g., borophosphosilicate glass or “BPSG”) or other material known in the art, is disposed over the gates  142 , the n-type areas  130 , the p-type areas  132 , and the isolation barriers  124 , as shown in FIG. 13. The passivation layer  148  is then masked and etched to exposed a portion of the n-type areas  130 , a portion of the p-type areas  132 , and the isolation barriers  124 , as shown in FIG. 14.  
         [0038]    As shown in FIG. 15, metal or poly plugs  150 , preferably doped polysilicon, are disposed within the etched areas in the passivation layer to contact the n-type areas  130 , the p-type areas  132 , and the isolation barriers  124 , thus forming a CMOS TFT structure  152 , which comprises an NMOS (N-type Metal Oxide Semiconductor) section  190  and a PMOS (P-type Metal Oxide Semiconductor) section  192 . The CMOS TFT structure  152  forms the CMOS TFT circuit  154  shown in FIG. 16.  
         [0039]    An alternate method for gate fabrication is illustrated in FIGS.  17 - 19 . Elements common between FIGS.  1 - 15  and FIGS.  17 - 19  retain the same numeric designation. After the formation of the n-type areas  130  and p-type areas  132  (separated by isolation barriers  124 ) on the oxide layer  104 , as shown in FIG. 9, a layer of undoped semiconductor material is deposited over the structure shown in FIG. 9. The layer of undoped semiconductor material is then planarized to form undoped semiconductor material plugs  194  between the n-type areas  130  and p-type areas  132 , as shown in FIG. 17. A layer of insulative material  136  is placed over the undoped semiconductive material plugs  194 , the n-type areas  130 , the p-type areas  132 , and the isolation barriers  124 . A layer of conductive gate material  138  is then placed over the insulative material layer  136 . A layer of barrier oxide or nitride  140  is then disposed over the conductive gate material layer  138 , as shown in FIG. 18. The barrier oxide layer  140  is masked and etched down to each of the n-type areas  130 , the p-type areas  132 , and the isolation barriers  124 , as shown in FIG. 19, to form gates  142 . A CMOS TFT is formed from the structure shown in FIG. 19 in a similar manner as illustrated in FIGS.  12 - 15 .  
         [0040]    As discussed above, the present invention utilizes deposited layers of pre-doped and pre-activated n-type and p-type polysilicon layers. However, it is understood that the above process can be complimented by ion implant and anneal steps to vary device characteristics.  
         [0041]    Having thus described in detail preferred embodiments of the present invention, it is to be understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description, as many apparent variations thereof are possible without departing from the spirit or scope thereof.  
         [0042]    It is, of course, understood that the above described technique can be used to form an NMOS TFT by replacing the p-type polysilicon layer  120  with an n-type polysilicon layer and to form a PMOS TFT by replacing the n-type polysilicon layer  106  with a p-type polysilicon layer.

Technology Category: 5