Fabricating a field effect transistor utilizing a dummy gate
DESCRIPTION 
     1. Technical Field 
     The invention relates to a process of fabricating a fully self-aligned field effect transistor (FET), the transistor being formed on a semiconductor substrate and comprising a current channel and associated source, gate and drain electrodes. The process is particularly suited for technologies using III-V compound semiconductors such as gallium arsenide (GaAs). 
     2. Background Art 
     The FET has become an established element not only in microwave technology but also in switching and data processing system applications. For high speed integrated circuits there is a growing need for submicron FET&#39;s, i.e., transistors with gate lengths of below one micron. Furthermore, GaAs structures have received increased attention because of the inherent material properties, particularly the high carrier mobility. 
     A variety of FET structures and processes for their fabrication have already been proposed, among others there are those for metal-semiconductor field effect transistors (MESFET) using GaAs. Basically, there are two types of proposed processes for the fabrication of self-aligned GaAs MESFET&#39;s: one type of process uses a refractory gate metal as a mask for the implantation of the ohmic contact regions, the other type of process uses a &#34;dummy&#34; gate as a mask for the contact implant, the dummy gate being replaced by a Schottky gate after ohmic contact metal deposition. 
     Refractory gate structures and processes have been described by N. Yokoyama et al. in articles &#34;TiW Silicide Gate Self-Alignment Technology for Ultra-High-Speed GaAs MESFET LSI/VLSI&#39;s,&#34; in IEEE Transactions, ED-29, p. 1541, 1982, and &#34;A GaAs 1K Static RAM Using Tungsten Silicide Gate Self-Aligned Technology,&#34; in IEEE JSC, SC-18, p. 520, 1983. The advantage of this type of process is its simplicity. It allows the fabrication of a simple FET structure with a relatively small number of lithographic steps. The prime concern with this process however is the stability of the metal-semiconductor interface on annealing: since the gate deposition takes place prior to the ohmic contact region implant, the Schottky barrier at the gate-channel interface is exposed to high-temperature cycles required to anneal the implanted material and to alloy the ohmic contacts. During these high-temperature processes the metal-semiconductor interface undergoes changes due to interdiffusion, lateral diffusion, stress, shrinkage, etc. This affects the Schottky barrier height and the threshold voltage of the transistor. 
     Dummy gate structures and processes have been proposed by K. Yamasaki et al. in articles &#34;GaAs LSI-Directed MESFET&#39;s with Self-Aligned Implantation for n +  - Layer Technology (SAINT),&#34; in IEEE Transactions, ED-29, p. 1772, 1982, and &#34;Self-Align Implantation for n +  -Layer Technology (SAINT) for High-Speed GaAs ICs,&#34; in Electronics Letters, 18, p. 119, 1982. The use of a dummy gate makes the fabrication process for MESFET&#39;s more complicated compared to the refractory-gate process, however, it offers a number of advantages: 
     the metal-semiconductor interface is not exposed to a high-temperature cycle and, therefore, is not distorted since the actual gate metal deposition is done after the contact region implant anneal process, 
     there is more freedom in the choice of the gate material, the requirements being determined by the final process steps such as chip mounting, packaging, etc., which are significantly less stringent than with the high-temperature implant anneal, 
     the sequence of process steps permits FET channel measurements (between ohmic contacts) and characterization prior to gate metal deposition, i.e., a final adjustment of the channel threshold can be achieved before device completion, e.g., by channel recessing, thereby improving the yield, 
     the gate metal can be used as the first-level wiring. 
     The SAINT process referred to above is based on the formation of a dummy &#34;T&#34;-gate structure and the use of this T-structure to mask the contact implantation. Embedding of the T-structure in selectively etchable material like SiO 2  and subsequent etching and removal leads to the replication of the foot of the T-structure into a dielectric like Si 3  N 4 , thus defining the actual gate area. In the resulting MESFET, the gate area is self-aligned with respect to the implanted contact regions, the interdistance being determined by the overhang of the T-shaped dummy gate. There are a few concerns regarding this process: 
     the gate length of the MESFET is defined by the foot of the dummy T-structure that is formed by an isotropic, or partially isotropic, etching process following an anisotropic definition of the cap of the T, the foot dimension being hard to control to the extent required by the device design, in particular for dimensions in the submicron regime, and 
     the gate metallization and the ohmic metallization are not self-aligned with respect to the implanted contact regions. Truly self-aligned metallizations are, however, highly desirable to keep parasitic effects, in particular parasitic series resistances, small. Again, this effect will be of increasing importance when reducing the device dimensions. 
     DISCLOSURE OF THE INVENTION 
     It is an object of the present invention to provide a MESFET fabrication process which allows self-alignment of all three critical device parts, namely, the gate and ohmic contact metallizations and the implanted contact regions, with respect to each other. 
     A further object of this invention is to disclose a process that provides for improved dimensional control of the gate length and gate separation, thereby making the process applicable to the fabrication of submicron structures and permitting a substantial reduction in the required device area. 
     The invention as claimed is intended to meet the above objectives and to remedy the drawbacks encountered with hitherto known FET fabrication processes. It solves the self-alignment problem by forming a multi-layer mask which includes an &#34;Inverted-T&#34; structure for the contact region implantation. The middle part of the inverted T represents the dummy gate that is replaced by the Schottky gate after self-aligned ohmic contact metal deposition. Depending on the embodiment chosen, the source-gate and drain-gate separation distances can be determined either by sidewall spacers formed at both sides of the dummy gate or by the overhang of an initial &#34;double-T&#34; structure. 
     The advantages offered by the invention are mainly that, due to the full self-alignment, the source-to-drain spacing given by the gate length and the gate-source and gate-drain separations can be minimized. In fact, the process can be extended to the fabrication of submicron structures whereby high speed devices and high packaging densities can be achieved. The process furthermore allows GaAs surface passivation throughout the whole process thereby providing for better and more reliable process control. 
    
    
     The foregoing and other objects, features and advantages of the invention will be apparent for the following and more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional representation of an embodiment of a MESFET produced in accordance with the invention. 
     FIGS. 2A-2I, 2K-2N, 2P and 2Q are illustrations of the steps of the process used to fabricate the structure shown in FIG. 1, 
     FIGS. 3A-3C are illustrations of alternative process steps that result in graded contact region implantations, and 
     FIGS. 4A-4D are illustrations of process steps resulting in a &#34;double-T&#34; mask structure. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring now to the drawings in more detail and first to FIG. 1 thereof, there is shown a first embodiment of a MESFET produced in accordance with the present invention. The MESFET 10 is formed on a semi-insulating (SI) GaAs substrate 11. The self-aligned device 10 comprises a relatively shallow channel 12G, its doping concentration resulting in either a n- or n -  -channel depending on whether enhancement-type (E) or depletion-type (D) devices are required, i.e., it depends on the desired value of the threshold voltage V th . Arranged on the channel 12G is a metal gate electrode 15G forming a Schottky barrier at the gate-channel interface. Positioned at both sides of the channel 12G but separated from gate electrode 15G are heavily doped n +  GaAs contact regions 13S and 13D forming source and drain regions, respectively. Source and drain electrodes 14S and 14D, respectively, are deposited on their respective n +  contact regions forming ohmic contacts therewith, 16S and 16D are passivating layers. Sections 15S and 15D of the gate metallization layer cover the underlying ohmic contact metallization (14S, 14D) lowering the ohmic metal wiring resistance. 
     In FIGS. 2A through 2Q the successive steps of the process used to fabricate the MESFET of FIG. 1 in accordance with the present invention are illustrated in detail. These successive steps are listed in Table I indicating the correspondence between the steps and the drawings. 
     
                       TABLE I                                                     
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Step No.  Description of Process Step                                     
                                FIG.                                      
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1         Passivation layer deposition                                    
                                2A                                        
2         Channel implantation (n/n.sup.-)                                
                                2B                                        
          through the passivation layer                                   
3         Deposition of a dielectric layer                                
                                2C                                        
4         Deposition of dummy gate layer                                  
                                2D                                        
5         Patterning the dummy gate                                       
                                2E                                        
6         Formation of sidewall spacers                                   
                                2F                                        
7         Definition of the mask for the                                  
                                2G                                        
          n.sup.+ -implant                                                
8         n.sup.+ -implant to form contact regions                        
                                2H                                        
9         Removal of sidewall spacers and                                 
                                2I                                        
          ion implant annealing                                           
10        Lithography for the definition of                               
                                2K                                        
          ohmic contacts                                                  
11        Deposition of ohmic contact                                     
                                2L                                        
          metallization                                                   
12        Removal of the dummy gate                                       
                                2M                                        
13        Opening of self-aligned via hole                                
                                2N                                        
          for gate definition                                             
14        Deposition of gate metallization                                
                                2P                                        
15        Removal of remaining mask portions                              
                                2Q                                        
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     While the following description is primarily directed to the fabrication of the MESFET shown in FIG. 1, this description is only exemplary of the inventive method. It should be noted that the thickness and other dimensions, the materials used as well as the process parameters given herein are selected for clarity of illustration and are not to be interpreted in a limiting sense. Most individual process steps that are used to evaporate, to implant, to etch and to pattern are well known and can be performed by employing conventional equipment and techniques. They are, therefore not explained in greater detail. 
     Referring now to FIG. 2A, the fabrication process is initiated starting from a semi-insulating GaAs wafer representing the substrate 11. In step 1 of the process its surface is covered with a passivation layer 16 which, in the example, consists of sputter-deposited aluminum nitride (A1N) of 80 nm thickness. This layer is a dielectric and permits through-implant ions into the underlying semiconductor substrate 11. 
     Next, photoresist 20 is applied and patterned as shown in FIG. 2B to define the substrate surface area where the channel 12 is to be formed. Subsequently (step 2), the wafer is subjected to an ion beam (arrows 21). The ions, after passing through passivation layer 16, are implanted in substrate 11 forming n-(or n -  -) channel 12. In the example, Si (29) is implanted at 90 kV, the dose being in the range of 3 to 6×10 12  cm -2 , depending on whether a D- or E-type device is being fabricated. 
     After stripping photoresist 20, another dielectric layer 22 is deposited on top of passivation layer 16 as illustrated in FIG. 2C. Silicon nitride (Si 3  N 4 ) is chosen for layer 22 and a layer of 250 nm thickness is deposited using a Plasma Enhanced Chemical Vapor Deposition (PECVD) method (step 3). Thereafter, in step 4, a further dielectric layer 23 is evaporated (FIG. 2D) consisting, in the example, of calcium fluoride (CaF 2 ), its thickness being 350 nm. 
     Subsequently, reference is now made to FIG. 2E, a dummy gate 23G is patterned from layer 23 using photoresist lithography (step 5): after applying and patterning a photoresist 24, the exposed portion of the CaF 2  layer 23 is sputter-etched, the Si 3  N 4  layer 22 serving as an etch stop. 
     Upon stripping photoresist 24, sidewall spacers 25 are formed at both edges of dummy gate 23G as shown in FIG. 2F. This is done by first depositing an insulation layer consisting of a polymeric material in a Chemical Vapor Deposition (CVD) or sputter process--the thickness of this layer eventually determining the source-gate and gate-drain distances of the completed MESFET device. The layer of polymeric material is then aniostropically etched in a reactive ion etching (RIE) process until the planar surface portions of the underlying structure are exposed. As a result, portions of the insulation layer remain at the vertical edges of the dummy gate 23G forming sidewall spacers 25 (step 6). 
     In the next process step 7, a multi-layer mask structure 26 of FIG. 2G is formed which comprises portion 22G of the dielectric Si 3  N 4  layer, the dummy gate 23G and the sidewall spacers 25. This structure is achieved by using a further RIE process during which the exposed dielectric layer 22 is etched, the A1N passivation layer 16 acting as an etch stop. 
     As will already be apparent now and will become clearer from the following description, some of the process steps rely on a distinct etch selectivity, i.e., the etch rates of some of the materials used, namely, that of passivation layer 16, dielectric layer 22, dummy gate 23G and that of the sidewalls 25, must be sufficiently different as to permit preferred etching of one of the materials. A broad spectrum of materials is available for this purpose, examples being polysilicon, silicon, silicon oxide, silicon nitride, silicon oxynitride, aluminum nitride, polyimide, etc. 
     FIG. 2H illustrates step 8, i.e., the n +  ion implant (arrows 27) to form the source and drain contact regions 13S and 13D, respectively: after applying and patterning photoresist 28, the Si (27) implant is again made through the A1N layer 16, the implant dose being 5×10 13  cm -2  at 130 kV. The channel region 12G is shielded by the multi-layer mask structure 26, the implant being self-aligned to the dummy gate. 
     Subsequently, photoresist 28 is stripped using acetone, the sidewall spacers 25 are dissolved in a photoresist stripper, and the ion implants annealed in a forming gas (N 2  /H 2 ) at 850° C. for 20 minutes (step 9). The annealing takes place with the substrate surface protected by the A1N passivation layer 16 and prior to the deposition of the gate metallization. At this stage, the multi-layer mask structure 26 has been reduced to an &#34;inverted-T&#34; structure 30 as illustrated in FIG. 2I. 
     In the next step 10, substrate surface areas are defined where the source and drain electrodes forming ohmic contacts with the N +  contact regions 13S and 13D are to be deposited. First, photoresist 31 is applied and patterned as shown in FIG. 2K. Thereafter, via holes for the ohmic contacts are etched into passivation layer 16, the patterned photoresist and the inverted-T shaped structure 30 serving as a mask. A wet chemical etch process with hydrogen fluoride (HF) is used for this purpose, to form 16S, 16Y and 16D. The remaining photoresist 31 is then stripped. 
     In the succeeding step 11 of the process, the ohmic contact metallization, in the example consisting of an AuGeNiAu layer of 80 nm thickness, is evaporated using a photoresist mask not shown in the drawing. This metallization is deposited on the planar surfaces forming the various sections 14S, 14SG, 14G, 14GD and 14D as illustrated in FIG. 2L. Contacts 14S and 14D are self-aligned to the dummy gate. Alloying takes place at 430° C. for 30 seconds. 
     As shown in FIG. 2M, the CaF 2  dummy gate 23G is now removed together with section 14G of the ohmic contact metallization by dissolving the CaF 2  in HCl/H 2  O (step 12). This is followed by step 13 during which the self-aligned via hole for the gate contact is opened by etching the Si 3  N 4  layer 22G (RIE using CF 4 ) to form 22 5G and 22GD and, subsequently, the A1N layer 16G (RIE using CCl 4 ). This results in the structure shown in FIG. 2N. 
     Next, in step 14, the gate metallization, in the example consisting of a palladium-gold (PdAu) layer of 100 nm thickness, is evaporated using a photoresist mask 17. This metallization forms the various sections 15S, 15SG, 15GD and 15D, as illustrated in FIG. 2P. Gate contact 15G is self-aligned to both the ohmic contacts 14S and 14D and to the n +  -implant regions 13S and 13D. 
     In a final step 15, photoresist 17 is stripped and the remaining parts 22SG and 22GD of the Si 3  N 4  layer are etched off using plasma etching in a CF 4  atmosphere. This results in the MESFET structure of FIG. 2Q which corresponds to that of FIG. 1 except for the thin A1N insulations 16SG and 16GD that separate the gate electrode 15G from the source and drain electrodes 14S and 14D, respectively. These may or may not be removed before completing the integrated circuit structure, i.e., before adding further insulations and metallizations. 
     The source resistance (n +  -contact region-channel) of the MESFET of FIG. 1 (or FIG. 2Q) can be further decreased by employing a two-step graded contact region implantation. The required process steps are illustrated in FIGS. 3A through 3C. In the complete fabrication process, they replace steps 2F through 2H. The successive steps required are listed in Table II where the correspondence between the steps and the drawings are indicated. 
     
                       TABLE II                                                    
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Step No.                                                                  
       Description of Process Step                                        
                            FIG.                                          
______________________________________                                    
       Same as steps 1 to 5 of Table                                      
                            2A-2E                                         
1&#39;     n-implant            3A                                            
2&#39;     Formation of sidewall spacers                                      
                            3B                                            
3&#39;     Definition of the mask for the                                     
                            3C                                            
       n.sup.+ -implant                                                   
4&#39;     n.sup.+ -implant to form contact regions                           
                            3C                                            
       Same as steps 9 to 15 of Table I                                   
                            2I-2N, 2P and 2Q                              
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     The modified fabrication process is initiated starting from the structure shown in FIG. 2E. The next process step (step 1&#39; of Table II) is an intermediate ion implant to form n-implant regions 42S and 42D adjacent channel 12G. This is illustrated in FIG. 3A: after stripping photoresist 24 in FIG. 2E and applying and patterning photoresist 40, the Si (27)-implant (arrows 41) is made through the Si 3  N 4  layer 22 and through the passivation layer 16 with an implant dose of 1×10 13  cm -2  at 200 keV. The channel region 12G is shielded by dummy gate 23G. The implant is self-aligned to the dummy gate. 
     Subsequently, the multi-layer mask structure 26 of FIG. 3B is formed in virturlly the same way as in steps 6 and 7 of the initial process sequence, i.e., after removal of photoresist 40 sidewall spacers are formed (step 2&#39;) and, thereafter, the exposed regions of Si 3  N 4  layer 22 are etched (step 3&#39;). 
     The n +  -implant step 4&#39; masked by a photoreist layer 43 illustrated in FIG. 3C, then corresponds to step 8 of the initial process sequence, i.e., n +  -contact regions 13S and 13D are formed. The resulting MESFET structure of FIG. 3C differs from that of FIG. 2Q in that n-zones 42S and 42D are arranged in between the n -  -channel 12G and the n +  -source and drain contact regions 13S and 13D, respectively. 
     In the processes described above, a multi-layer mask structure 26 has been formed and thereafter gradually reduced through selective etching processes to provide the masks needed in the fabrication of the self-aligned gate and of the source and drain electrodes and n +  - contact regions. Mask structure 26 consisted of an inverted-T structure 30 comprising layers 22G and 23G and sidewall spacers 25, the thickness of the latter eventually determining the gate-ohmic contact separations. A different &#34;double-T&#34; multi-layer mask structure 53, shown in FIG. 4D, consisting of an inverted-T structure 22G and 52, as shown in FIG. 4C, topped by an &#34;extending&#34; thin film 50 may be used instead of the mask structure designated 26 in, e.g., FIGS. 2G or 3B. 
     The required process steps for the formation of such a &#34;double-T&#34; structure are illustrated in FIGS. 4A through 4D which, in the complete fabrication process, replace the steps indicated in FIGS. 2E through 2I. The successive steps required are listed in Table III where the correspondences between the steps and the drawings are indicated. 
     
                       TABLE III                                                   
______________________________________                                    
Step                                                                      
No.  Description of Process Step                                          
                           FIG.                                           
______________________________________                                    
     Same as steps 1 to 4 of Table I                                      
                           2A-2D                                          
1&#34;   Formation of the thin film                                           
                           4A                                             
     &#34;topping&#34; the inverted-T structure                                   
2&#34;   Selective removal of the two di-                                     
                           4B                                             
     electric layers (22, 23) to define                                   
     an initial multi-layer mask                                          
     structure                                                            
3&#34;   Dummy gate (or &#34;double-T&#34;) shaping                                   
                           4C                                             
4&#34;   n.sup.+ -implant to form contact regions                             
                           4D                                             
5&#34;   Removal of &#34;topping&#34; thin film to                                    
     obtain &#34;inverted-T&#34; structure                                        
     Same as steps 10 to 15 of Table I                                    
                           2K-2N, 2P and 2Q                               
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     This modified fabrication process is initiated starting from the structure in FIG. 2D. The next process step (step 1&#34; of Table III) provides the thin film 50 that eventually &#34;tops&#34; the double-T multi-layer mask structure. This step is illustrated in FIG. 4A: Film 50 is deposited on top of the CaF 2  layer 23 and then patterned using photoresist lithographic techniques involving photoresist 51 and an etch process. 
     Next, layers 22 and 23 are selectively removed (step 2&#34;) in an RIE process using film 50 as a mask. The resulting multi-layer structure is shwon in FIG. 4B. In the following process step 3&#34;, the dummy gate 52 is tailored by underetching whereby the double-T structure 53 (FIG. 4C) is obtained. It serves the same purposes as the mask structure 26 in FIGS. 2G or 3B, the undercut eventually determining the distance between the gate and the ohmic source and drain contacts. 
     The subsequent ion implant (arrows 54) for forming the n +  -contact regions (step 4&#34;) corresponds to that described above in connection with FIG. 2H, the difference being that the &#34;topping&#34; film 50 of mask structure 53 (FIG. 4D) determines the contact region boundaries rather than the sidewall spacers 25 of mask structure 26 in FIG. 2H. 
     In step 5, the film 50 is removed and the ion implant annealed. At this stage, the inverted-T structure 30 illustrated in FIG. 2I is obtained and the fabrication process can proceed with the steps described above with the aid of FIGS. 2K to 2N, 2P and 2Q. 
     One further deviation from the initially described process sequence which should be mentioned is a process and device in which the passivation layer 16 is omitted. This layer, providing for better process control and reproducibility, may not be needed in less critical applications and does not form an essential feature of the inventive process. 
     In summary, the new process allows self-aligning both, the n +  -contact region implant as well as the ohmic contacts with respect to the gate. This minimizes the source-to-drain spacing which is important for high packaging densities. For submicron gate length structures requiring shallow channels it is also of advantage that the ohmic contact can be moved as near as possible to the gate especially when the channel doping is high. Particularly, the process employing sidewall spacers for determining the n +  -implant-to-gate spacing provides for better control of the critical spacing and dimensions than the hitherto known processes. 
     From the above it will be evident that many other alternatives, modifications and variations still falling within this scope and spirit of the present invention will be apparent to those skilled in the art.