Selective etching process
FIELD OF THE INVENTION 
     The invention relates to a process for fabricating semiconductor devices. In particular, it relates to an etching process used in the fabrication of semiconductor devices. 
     BACKGROUND OF THE INVENTION 
     Great advancements have been made in semiconductor technology in the last few years largely due to the discovery of new materials and the discovery of new methods of making better materials. These advances have led to new device applications for semiconducting materials and such applications often require different fabrication techniques. Typically, these techniques are aimed toward obtaining smaller size, more precise location of various geometrical features in the device, more accurate shapes for various geometrical features in the structure, greater adherence of metallic substances to the semiconductor surfaces, etc. 
     An important aspect of semiconductor fabrication technology is etching procedures and solutions. Etching solutions should be stable, have predictable rates and produce uniform etching rates over wide areas of a semiconductor wafer. A particularly important problem in the fabrication of many III-V semiconductor devices is the uniform etching of layers (generally epitaxial layers) of aluminum containing III-V semiconductor compounds (e.g. aluminum gallium arsenide, aluminum indium arsenide, etc.). Such compounds often exhibit problems both as to the stability of the etching solution and uniformity of the etch rate across the semiconductor wafer. In addition, in the fabrication of many devices, it is highly advantageous to selectively etch one semiconductor (e.g. an aluminum containing semiconductor compound) without etching (or etching only to a small extent) another semiconductor compound. 
     A particular case in point is the development of a variety of heterojunction devices involving different III-V semiconducting compounds. A typical example is heterojunction devices featuring aluminum gallium arsenide and gallium arsenide. The inclusion of the aluminum is done for a variety of reasons including altering the band gap, providing lattice matching to another III-V semiconductor compound, improve etch unformity and changing optical properties of the semiconductor. 
     In fabricating such devices, it is highly desirable to be able to selectively etch one III-V semiconductor compound without etching the other semiconductor compound. For example, in fabricating various gallium arsenide heterojunction devices, it is often highly desirable to etch aluminum gallium arsenide selectively without etching the gallium arsenide. In many device fabrication procedures, it is desirable that such selective etching be effective even for very low aluminum contents. Often, this is desirable because crystal morphology is easily degraded with high aluminum content material. Also, various electronic properties change as aluminum content increases beyond 20 or 30 mole percent. 
     A variety of references describe etching solutions and processes for III-V semiconductor compounds. Particularly noteworthy is U.S. Pat. No. 4,049,488 issued on Sept. 20, 1977 to R. P. Tigburg. This reference describes selective etching of aluminum gallium arsenide in the presence of gallium arsenide using various redox systems at specific pH valves. Also described is selective etching of gallium arsenide in the presence of aluminum gallium arsenide. 
     Other references also describe etching of gallium aluminum arsenide including &#34;GaAs-GaAlAs Heterojunction Transistor for High Frequency Operation&#34; by W. P. Dumke et al, Solid State Electronics, 1972, Volume 15, pp. 1339-1343; &#34;Selective Etch Characteristics of HF for Al x  Ga 1-x  As/GaAs&#34;, by X. S. Wu et al, Electronic Letters, 21, 558 (1985), &#34;Extreme Selectivity in the Lift-Off of Epitaxial GaAs Films&#34;, by Eli Yablonovitch et al, Applied Physics Letters, Volume 51, No. 26 (1987); and &#34;The Etching of Ga 0 .7 Al 0 .3 As Using KI-I 2  &#34; by A. C. Wismayer et al, Materials Letters, Volume 6, page 284-286 (1988). 
     Aqueous dichromate ion solutions have been used in etching III-V semiconductor compounds in a number of references including &#34;Chemical Etching of (100) GaAs in the (NH 4 ) 2  Cr 2  O 7  --H 2  SO 4  --NH 4  Cl--H 2  O Systems&#34;, by I. Barycka et al, Journal of Material Science 21 (1986) 2153-2158; &#34;A New Etchant System, K 2  Cr 2  O 7  --H 2  SO 4  --HCl, for GaAs and InP&#34; by S. Adachi et al, Journal of Materials Science 16, 2449-2456 (1981); &#34;Defect Selective Etching of GaAs in CrO 3  --HCl Solutions&#34; by J. van de Ven et al, Chemtronics, 1986, Vol. 1, March, pp. 19-26; &#34;The Mechanism of GaAs Etching in CrO 3  --HF Solutions&#34; by J. van de Ven et al, Journal of the Electrochemical Society, 132, 3020 (1985) and &#34;Chemical Etching on GaAs&#34; by S. Adachi et al, Journal of the Electrochemical Society: Solid- State Science and Technology 131, No. 1, pp. 126-130 (1984) 
     SUMMARY OF THE INVENTION 
     The invention is a process for fabricating a device, said device comprising an aluminum containing III-V semiconductor compound in which the aluminum containing III-V semiconductor compound is etched using an acidic dichromate solution. The amount of aluminum in the III-V semiconductor may vary over large limits, typically from 5 mole percent to 100 mole percent and is usually determined by the device application rather than the performance of the etch solution. The concentration of dichromate ion (Cr 2  O 7   = ) may vary over large limits including 0.0001M to saturation but concentrations from 0.001M to 0.5M yield excellent etch rates. 
     Various acids may be used in the practice of the invention. Preferred are sulfuric acid and phosphoric acid with phosphoric acid most preferred. Acid concentrations may vary over large limits including 0.01M to 10M but best results are obtained in the range from 0.25M to 10M. Typical aluminum containing III-V compounds are aluminum gallium arsenide, aluminum indium arsenide and aluminum arsenide. 
     An extremely important aspect of the invention is the selectivity of the etching process for etching aluminum-containing III-V semiconductor compounds without etching significantly non-aluminum containing III-V semiconductor compounds. Particularly important is etching aluminum gallium arsenide in the presence of gallium arsenide and etching aluminum indium arsenide in the presence indium gallium arsenide (typically in an indium phosphide device). Selectivity (ratio of etch rates for aluminum compound and non-aluminum compound) varies with dichromate ion concentration and acid concentration. Useful selectivity is obtained for dichromate ion concentrations from 0.001M to 0.5 and acid concentrations from 0.1M to 10M, with 2-10 molar preferred. Both sulfuric acid and phosphoric acid yield useful selectivity with phosphoric acid most preferred. A variety of devices are usually made using this etch process including heterojunction bipolar transistors, (HBT), heterojunction field effect transistors (HFET) and self-enhanced electro optic devices (SEED). 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows in graphical form data on etch rate as a function of aluminum content in aluminum gallium arsenide for an etch solution of 0.06 molar K 2  Cr 2  O 7  in 7 molar phosphoric acid; and 
     FIG. 2 shows a similar graph to FIG. 1 for an etch solution of 0.0068M K 2  Cr 2  O 7  in 7 molar phosphoric acid. 
    
    
     DETAILED DESCRIPTION 
     The invention is based on the discovery that acidic dichromate solution is an extremely convenient and effective etch solution for aluminum containing III-V semiconductor compounds and that this etch, particularly when phosphoric acid is used as the acid, shows a high degree of selectivity when the etching process is carried out in the presence of non aluminum containing III-V semiconductor compounds. Typical examples are the etching of aluminum gallium arsenide in the presence of gallium arsenide and the etching of aluminum arsenide (e.g. In 0 .53 Al 0 .47 As) in the presence of indium gallium arsenide (e.g. In 0 .53 Ga 0 .47 As). Highly advantageous is the large range of aluminum content in the III-V semiconductor compound that can be etched using this process. For example, aluminum contents as low as 5 mole percent are etched by the etch solution with good etch rates and excellent selectivity. Aluminum content may vary from 5 mole percent or 10 mole percent to 100 mole percent (AlAs) with excellent results. Indeed, one of the major advantages of this etch system is that it can selectively etch very low aluminum content aluminum gallium arsenide in the presence of gallium arsenide which is required in fabricating a variety of devices. Applications are numerous largely because of the extensive use of aluminum gallium arsenide in gallium arsenide devices and the use of aluminum indium arsenide in indium gallium arsenide-indium phosphide devices. 
     The data shown in Table 1 and FIGS. 1 and 2 represent examples of individual etch experiments and show the great versatility of this etching solution. 
     The etch solution is made up by conventional means. Generally, a source of dichromate is added to the aqueous acid (e.g. phosphoric acid or sulfuric acid). Any source of dichromate ion may be used including an alkali-metal dichromate (e.g. K 2  Cr 2  O 7 ) and ammonium dichromate. Also, mixtures of sulfuric acid and phosphoric acid may be used in the practice of the invention. 
     A large number of examples were carried out and the resulting data given below in graphical form and in tabular form. The experiments were carried out by immersing the samples in etching solution at room temperature. Room temperature is used as a matter of convenience. Other temperatures are useful typically from the freezing point of the solution to the boiling point of the solution. Also, etch rate can be adjusted by varying the temperature of the etch solution. 
     FIG. 1 shows some etch rate data of an acid dichromate solution in graphical form. The plot shows Etch Rate in Angstroms/minute as a function of aluminum content (as mole fraction) in the III-V semiconductor compound Al x  Ga 1-x  As x  in a 0.06 molar K 2  Cr 2  O 7  in 7 molar H 3  PO 4 . Notice the extremely low etch rate for pure gallium arsenide and the extremely high selectivity of the etch solution. 
     FIG. 2 shows a similar plot for a different etch solution composition, namely 0.0068M K 2  Cr 2  O 7  in 7 molar H 3  PO 4 . Here, etch rates are somewhat higher but selectivity not as great. By proper adjustment of the composition of the acid dichromate solution, the etch rate and selectivity can be tailored to particular needs and compositions. 
     Table 1 shows further data on etch rate and selectivity as a function of acid concentration and dichromate concentration. Here, etch rate for the compound Al 0 .35 Ga 0 .65 As and selectivity for Al 0 .35 As in the presence of GaAs is shown for a variety of phosphoric acid concentrations and dichromate ion concentrations. 
     Etch rate is shown in terms of depth etched in 30 minutes in Al 0 .35 Ga 0 .65 As and selectivity in terms of depth etched in Al 0 .35 Ga 0 .65 As in 30 minutes divided by depth etched in GaAs in 30 minutes. 
     
                       TABLE I                                                     
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Etch rate and Selectivity as a Function of Dichromate and Acid            
Concentrations for Al.sub.0.35 Ga.sub.0.65 As in the Presence of GaAs.    
Dichromate Conc.                                                          
            Acid Conc. Etch Rate                                          
(Molar)     (Molar)    (Å/30 min)                                     
                                  Selectivity                             
______________________________________                                    
0.0068      0.5        2391       16                                      
0.0068      1.0        4653       11                                      
0.0068      1.5        8802       15                                      
0.0017      2.0        16989      11                                      
0.0034      2.0        13968      12                                      
0.0051      2.0        12774      18                                      
0.0068      2.0        9894       12                                      
0.010       2.0        7314       12                                      
0.014       2.0        6480       20                                      
0.017       2.0        4894        9                                      
0.026       2.0        3538       17                                      
0.034       2.0        2781       12                                      
0.05        2.0        1928       --                                      
0.06        2.0        1789       29                                      
0.068       2.0        1555       --                                      
0.0068      3.0        20442      14                                      
0.0051      4.0        20550      14                                      
0.0068      4.0        18192      15                                      
0.017       4.0        12942      15                                      
0.034       4.0        5949       18                                      
0.05        4.0        3822       25                                      
0.06        4.0        3659       27                                      
0.068       4.0        2550       34                                      
0.085       4.0        1965       39                                      
0.1         4.0        1978       40                                      
0.0034      6.0        12930       4                                      
0.0051      6.0        28110      17                                      
0.0068      6.0        21510      18                                      
0.01        6.0        21474      16                                      
0.017       6.0        15900      17                                      
0.034       6.0        6400       22                                      
0.05        6.0        3622       31                                      
0.06        6.0        2967       25                                      
0.068       6.0        2598       52                                      
0.051       7.0        24618      41                                      
0.0068      7.0        30270      21                                      
0.010       7.0        18300      20                                      
0.017       7.0        7926       16                                      
0.034       7.0        5703       25                                      
0.05        7.0        3573       33                                      
0.06        7.0        3415       73                                      
0.068       7.0        2243       45                                      
0.0017      8.0        8298        3                                      
0.0034      8.0        23604       9                                      
0.0051      8.0        27252      17                                      
0.0068      8.0        29058      22                                      
0.01        8.0        16680      18                                      
0.017       8.0        7480       14                                      
0.05        8.0        2541       51                                      
0.06        8.0        1807       36                                      
0.0051      10.0       14760      15                                      
0.0068      10.0       25680      23                                      
0.01        10.0       10332      51                                      
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     The Table shows large variations in etch rate and selectivity for different dichromate concentrations and acid concentrations. Optimum etch composition often depends on the nature of the application. For example, where high selectivity is desired, an etch solution composition near 0.06M (e.g. 0.05-0.1M) dichromate, 7M (e.g. 5-10M) phosphoric acid might be desirable; where high etch rate and only modest selectivity is desired, a composition near 0.007M (e.g. 0.003 to 0.015) dichromate and 7M (e.g. 3-10M) might be used. 
     The acid dichromate solution also etches other aluminum-containing III-V semiconducting compounds such as indium aluminum arsenide and also shows great selectivity in etching indium aluminum arsenide in the presence of indium gallium arsenide. For example, for 0.06M K 2  Cr 2  O 7  in 7M H 3  PO 4  etches In 0 .53 Al 0 .47 As at a rate of 340 Å/min while etching In 0 .53 Ga 0 .47 As at less than 7 Å/hr for a selectivity of greater than 2900. 
     Etching procedures carried out in dichromate solutions with sulfuric acid also yielded excellent etch rates. Etch rates of 2000 to 12,000 Å in a 10 minute period are obtained with 1-60 millimolar dichromate in 2M sulfuric acid. Good selectivities are also obtained, often in the range from 5 to 10. 
     A variety of conventional procedures may be used to carry out the etching process. Generally, wetting the surface to be etched with the etching solution, is sufficient to achieve etching. Spraying the solution onto the surface to be etched is particularly convenient. Also, immersion in the etching solution yields excellent results. Stirring the solution insures greater etching uniformity over the surface being etched (e.g. a semiconuctor wafer) and rapidly replaces spent etching solution close to the surface. Also, both with the spray procedure and immersion procedure, spinning the surface around the axis perpendicular to the surfaces insures highly uniform etch rates. 
     Another advantage of the acid dichromate etch is that this etch does not attack gold. Thus, etching can be carried out on semiconductor structures with gold electrodes. Of much greater importance is the fact that gold electrodes and the acid dichromate solution can be used to make self-aligned type structures using the gold electrode as an etch mask.