FET having minimized parasitic gate capacitance
This application is a division, of application Ser. No. 08/176,599 filed Jan. 5, 1994, now U.S. Pat. No. 5,461,244. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention pertains to the fabrication of field effect transistors (FETs), particularly heterostructure FETs (HFETs). More particularly, the invention pertains to a self-aligned gate heterostructure insulated gate (HIG) FET having a parasitic capacitance between the gate pad and the remaining FET structure, and a method and structure for minimizing such capacitance. 
     In the related art there is a significant capacitance between the gate pad and the FET structure. Attempts to reduce that capacitance include ion implanting at the surface of the heterostructure wafer before the gate pad is formed. However, this results in the production of HFETs that are not adequate or uniform, especially relative to one another. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to produce a heterostructure FET having a capacitance between the gate pad and the channel layer that is much less than similar type capacitances of related art FETs. This object is achieved by a process which utilizes an ion implant isolation of the channel layer beneath the gate pad without affecting or damaging the surface of the heterostructure wafer. 
     In the HIGFET fabrication method of the related art, the first step in the fabrication process includes the deposition of a single thick gate metal layer including the gate pad area. This layer is too thick to permit an ion isolation implantation to affect the FET channel in the heterostructure wafer. If the surface is ion isolation implanted prior to the deposition of the gate metal layer, then the wafer is affected in other ways that degrades the surface which results in poor quality FETs or a non-uniform group of FETs. However, in accordance with the invention, the gate metal deposition is accomplished in two separate steps, the first one of which is depositing thin gate metal layer that protects the surface of the wafer but still permits ion isolation implantation of the channel layer under the gate metal. The second deposition step is depositing, after the implantation, a thick gate metal layer to bring the gate and its pad to a thickness required for adequate conduction for satisfactory FET performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a cross-sectional view of a typical prior art heterostructure wafer. 
     FIG. 2 is a plan view showing the gate finger and pad of a heterostructure FET. 
     FIG. 3 is a cross-sectional view of a heterostructure FET having a thick gate metal layer of the related art. 
     FIG. 4 is a cross-sectional view showing an ion implantation through a thin gate metal layer. 
     FIG. 5 is a cross-sectional view revealing a plurality of gate metal layers. 
    
    
     DESCRIPTION OF THE EMBODIMENT 
     An insulated gate FET is fabricated on a heterostructure wafer 10 such as the one shown in FIG. 1. Wafer 10 is composed of a semi-insulating GaAs substrate 12, an unintentionally doped and therefore slightly less insulating buffer GaAs layer 14 with a &#34;pulse&#34; doping charge 15, an InGaAs channel layer 16, and an AlGaAs gate insulator layer 18. These layers, with the exception of layer 15, are not intentionally doped and have the attributes which are the same as those layers typically used in the prior art for desired purposes. Layer 15 is a &#34;pulsed doped layer&#34; which is inserted into layer 14. Layer 15 is a &#34;sheet&#34; of dopant, typically Si, that is used to adjust the threshold of HIGFETs. Layer 15 is a fraction of a monolayer of Si dopant embedded in a single atomic layer. The typical dose of layer 15 is 1×10 11  to 1×10 12  per cm 2 . 
     A typical thickness of layer 20 (WSi or W or WN or a combination) is 3000 to 10,000 angstroms; the thickness of layer 18 (AlGaAs) is 100 to 500 angstroms; the thickness of layer 16 (InGaAs) is 70 to 250 angstroms; and the thickness of layer 14 (GaAs) is 1000 to 10,000 angstroms. 
     In one method of HIGFET fabrication, heterostructure wafer 10 is covered with a layer 20 of gate metal, typically tungsten (W), and/or tungsten silicide (WSi), and/or tungsten nitride (WN). In FIG. 2, gate metal layer 20 is patterned and etched to form at least one gate finger 22, typically measuring 0.8 micron by 3 microns. Gate finger 22 extends over active region 30 of the FET, and a gate pad 24, typically measuring at least 5 microns by 5 microns, which allows an upper layer metal or wire to electrically contact gate finger 22. This configuration may be utilized in the present invention. 
     In FIG. 3, after gate metal layer 20 has been patterned, an ion implantation step, prior to forming photoresist layer 55 but using a photoresist layer on wafer 10 that covers all of wafer 10 except for the area that subsequent photoresist layer 55 covers, is used to form source region 32 and drain region 34 for horizontal conduction in channel layer 38, and source and drain contacts, respectively, for vertical conduction in layer 18 of the FET in a manner known in the art (i.e., silicon ions for an N-channel FET and beryllium ions for a P-channel FET). Ohmic contacts (not shown) are formed on the source and drain contacts of layer 18, also typical in the related art. Gate finger 22 blocks ion implantation over a channel region 36 of the FET, so that region 36 is not affected by the implantation and region 62 remains an insulator for gate 22. Channel region 36 is self-aligned to implanted source region 32 and drain region 34. 
     Next, active FET region 30 is masked by a photoresist layer 55, and wafer 10 is bombarded with high energy ions, such as oxygen, hydrogen or argon ions. This step, known as ion isolation implantation 64 and 66, damages channel layer regions 38 around active region 30, rendering regions 38 and pulse doped regions 15 which are not underneath photoresist layer 55 and therefore non conducting. Ion implantation makes regions of layers 18, 16, 15 and 14 more non conducting. This implantation electrically isolates the FET from adjacent devices on wafer 10. 
     However, the ion isolation implantation 66 does not penetrate the thickness of gate pad 24, so that the portion of channel layer 16 and/or 18 under gate pad 24 is not affected and thus not electrically isolated from pad 24. Because this portion of channel layer 16 remains conductive and pulse doped layer 15 remains conductive, there is significant parasitic capacitance between gate pad 24 of metal layer 20 and channel layer 16 underneath gate pad 24, with layer 18 functioning as the capacitance dielectric. While high capacitance per unit area between gate finger 22 and channel 36 permits fast operation of the FET, high capacitance between gate pad 24 and channel layer 16 slows down FET operation and is undesirable. A better process of FET fabrication is one which allows ion isolation implantation of the channel layer beneath gate pad 24 without reducing transistor quality or significantly increasing the number of steps in the fabrication sequence. 
     A fabrication method that solves the problem of gate pad 24 parasitic capacitance involves implanting the surface of the region of channel layer 16 beneath gate pad 24 before gate metal layer 20 is deposited. However, this method exposes wafer 10 to abuse and reduces the uniformity of structure and resulting performance among the produced HFETs. This is because the heterostructure wafer layers are very thin, and the removal of even a few atomic layers from heterostructure wafer 10 negatively affects the HFETs produced thereon. Any process step (e.g., cleaning) which occurs before deposition of gate metal layer 20 results in the removal of some material from the surface of heterostructure wafer 10. Therefore, deposition of gate metal layer 20, which protects the surface of heterostructure wafer 10, might best be kept as a step before ion isolation implantation 64 and 66 in the fabrication process, after growing the layers of heterostructure wafer 10. 
     FIG. 4 illustrates the present method of fabrication. The layers of heterostructure wafer 10 are grown, and a gate metal (WSi) layer 40 is deposited. However, this gate metal layer 40 is very thin (200-500 angstroms) so that the ions in the ion isolation implantation 66 can pass through it. Thin gate metal layer 40 serves to protect the surface of heterostructure wafer 10 while still allowing ion implantation 64 and 66 of the wafer. In other words, layer 40 is thin enough to pass ions but thick enough to protect the surface of wafer 10. Incidentally, thin layer 40 may be used to form other circuit elements such as &#34;thin film&#34; resistors or &#34;metal-insulator-metal&#34; capacitors. 
     But prior to ion isolation implantation 64 and 66, photoresist layer 55 is deposited on gate metal layer 40 over active FET portion 30, and wafer 10 is implanted with ions. After implantation, channel layer regions 38 and pulse doped regions 15 which are not underneath photoresist layer 55 are damaged and therefore non conducting so as to prevent layer 16 being a parasitic capacitor in conjunction with metal layer 40. 
     As shown in FIG. 5, photoresist layer 55 is removed after the ion implantation. An additional gate metal layer 60 of WSi, at a thickness of 3500-3800 angstroms, is then deposited. This brings the total thickness of gate metal layers 40 and 60 to approximately 4000 angstroms, a thickness which provides sufficient gate conductivity for efficient operation of the FET. This minimum gate metal thickness is determined by the resistance of the gate finger; too much resistance means slow operation. 
     By comparing FIG. 5 with FIG. 3, it may be noted that at this point the wafer configurations are essentially the same, except that all of channel layer regions 38 outside active area 30 have been implant isolated. After deposition, combined gate metal layers 40 and 60 are fashioned into a gate pad 24 and a gate finger 22, like those of FIG. 2. Further fabrication may be conventional.