Selectively implanting GaAs semiconductor substrates through a metallic layer
BACKGROUND OF THE INVENTION 
     This invention relates to semiconductor processing and in particular to ion implantation techniques. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a process for implanting ions into a semiconductor substrate comprising the steps of depositing a metallic layer on a surface of the substrate, providing a photoresist implantation mask on the metallic layer, and bombarding the substrate with ions whereby ions become selectively implanted into regions of the substrate via corresponding windows of the mask and the metallic layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: 
     FIGS. 1 to 5 show successive stages in the fabrication of discrete field effect transistors. 
    
    
     DETAILED DESCRIPTION 
     The drawings illustrate the fabrication of field effect transistors, i.e., a semi-insulating gallium arsenide substrate 1. The active regions required for the transistors are achieved by ion implantation. A thin titanium layer 2 (approximately 400 A thick) is evaporated onto a surface of the substrate (FIG. 1). A layer 3 of a positive photoresist is spun onto the titanium layer 2. Small windows 4 are opened in the photoresist layer 3 and through these windows 4 registration marks 5 are etched in the gallium arsenide surface. The layer 3 is removed and a fresh layer 6 (FIG. 2) of a positive photoresist applied. Large windows 7 are opened in the photoresist layer 6, thus defining a photoresist implantation mask. 
     The photoresist masked surface of the gallium arsenide substrate I is then subjected to bombardment with, for example, Si 29  ions, as indicated by the arrows, which are thus selectively implanted through the titanium layer 2 into regions of the substrate in order to provide conductive, in this case n-type, regions 8. 
     The photoresist layer 6 is removed by a suitable solvent and the titanium layer is removed by etching with HF thus achieving the selectively doped gallium arsenide structure shown in FIG. 3. In order to remove implantation induced damage of the structure it is subjected to heat treatment at approximately 850° C. by means, for example, of a capless annealing technique, that is a heat treatment without the provision of a layer on top of the GaAs substrate, thus the structure as shown in FIG. 3 is heat treated. 
     Source and drain electrodes 9 and regions 10 are then provided (FIG. 4). This may be achieved by means by metallization lift-off process comprising applying a relatively thick photoresist layer to the annealed substrate; providing windows in the photoresist layer corresponding to the required source and drain electrodes and regions; metallizing the photoresist layer and the regions 8 where exposed by the windows with an electrode/dopant metal such as a gold/germanium/nickel alloy, the photoresist is relatively thick in comparison with the alloy layer to ensure that the alloy layer breaks at the edges of the windows; stripping off the photoresist layer together with the surplus metal, leaving the metal electrodes 9 disposed on the regions 8; and heating the assembly to a sufficient temperature to diffuse the germanium from the metal electrodes 9 into the region 8, thus providing source and drain regions 10 with a high doping level, in this case n + , in the regions 8. 
     Using a similar metallisation lift-off process, this time employing a Schottky metal, for example chromium/gold, a Schottky barrier gate electrode 11 is disposed between the source and drain electrodes 9 of each transistor. Finally the produced assembly is divided into individual devices or circuits which are then contacted or packaged. 
     The thin layer of titanium acts as a buffer layer between the photoresist and the gallium arsenide surface, and prevents any possible contamination of the gallium arsenide surface, which may result from photoresist residues when such a buffer layer is not used. Such residues are particularly undesirable when employing ion implantation techniques since the substrate must be subjected to heat treatment, the annealing process, after ion implantation and the surface of the substrate, if the photoresist is not fully cleaned off, will become contaminated and the electrical properties of the wafer will be adversely affected. These residues are particularly encountered with gallium arsenide substrates since it is not possible to rigorously clean photoresist from a surface thereof without adversely affecting the surface. 
     Whereas the invention has basically been described with respect to discrete devices it is also applicable to the fabrication of integrated circuits on gallium arsenide substrates. Whereas only a titanium buffer layer has been described above other metallic layers of, for example, tin, chromium, aluminum, manganse, magnesium or indium, may be similarly employed. Such metallic layers, and any photoresist residues therein, can be simply and effectively removed by HF or HCL etching, for example, without damaging the substrate surface. Whereas the semiconductor body into which ions are implanted, has been specifically described as gallium arsenide, other semiconductors, for example, other compound semiconductors or silicon may be similarly processed. It is possible to merely use a patterned photoresist layer for the selective ion implantation of silicon, since the latter can be subjected to a very rigorous cleaning process, to ensure that no photoresist residues remain before annealing, without adversely affecting the surface of the silicon. However in certain instances it may be preferable to use the metallic buffer layer described above.