Aluminum germanium ohmic contacts to gallium arsenide

Ohmic contacts are attached to n-type Gallium Arsenide with an alloy of Aluminum-Germanium. The contact is prepared by depositing by evaporation a sequence of 400 Angstroms of Germanium, 300 Angstroms of Nickel, and 2000 Angstroms of Aluminum and subsequent alloying.

BACKGROUND OF THE INVENTION 
This invention relates to the fabrication of ohmic contacts to n-type 
Gallium Arsenide. More particularly, this invention relates to such ohmic 
contacts which are fabricated from an alloy of Aluminum and Germanium. 
Contact resistance to n-type Gallium Arsenide is an important process and 
design parameter for the advancing Gallium Arsenide integrated circuit 
technology. The Ni-capped, Gold Germanium (12%) eutectic ohmic contact 
alloy was first introduced by Braslau (N. Braslau, J.B. Gunn and J. L. 
Staples, "Metal-Semiconductor Contacts for GaAs Bulk Effect Devices." 
Solid State Electronics, Vol. 10, page 381, 1967) and is now extensively 
used for Au-Ge-Ni contacts with a variety of techniques and compositions. 
The eutectic temperature is listed as 356.degree. C. (M. Hansen, 
Constitution of Binary Alloys, page 97, McGraw Hill Book Company, Inc., 
New York, 1958) and alloy temperatures up to 450.degree. C. are used to 
form the ohmic contacts. To complete the ohmic contacts and form the first 
level of metal interconnects, an overlay of 2500 Angstroms of Au is 
employed with a thin interspaced layer of Pt or Ti for improved adhesion. 
While these contacts have good ohmic properties for device and integrated 
circuit application, there is room for improvement. For example, Au as a 
high atomic mass element (197) will tend to absorb relatively more X-ray 
energy than would a lighter element, leading to deterioration of 
conductivity. This can have significant consequences in a high energy 
radiation environment. 
SUMMARY OF THE INVENTION 
The ohmic contacts of this invention have been fabricated to n-type Gallium 
Arsenide with an alloy of Aluminum Germanium which has a eutectic 
temperature of 424.degree. C. with 53 weight percent Germanium. The lowest 
contact resistance of 1.4.times.10.sup.-6 ohm-cm.sup.2 for the contact was 
measured with a transfer length transmission line structure. In the 
specific embodiment, the substrate material was LEC grown semi-insulating 
Gallium Arsenide without intentional doping, with upper Si.sup.+ ion 
implanted n-type layers. A typical peak impurity concentration is in the 
range of 10.sup.17 to 10.sup.18 cm.sup.-3. Rapid thermal anneal at 
825.degree. C. was used to activate the ion implantations. The contact 
itself was prepared by a series of evaporations in the sequence of 400 
Angstroms of Germanium, 300 Angstroms of Nickel and 2000 Angstroms of 
Aluminum. A contact resistance of 1.4.times.10.sup.-6 ohm-cm.sup.2 was 
obtained at 500.degree. C. After alloying, another layer of Aluminum 
approximately 2500 Angstroms thick was deposited on top of the alloyed 
contact and serves as the first level interconnection.

DETAILED DESCRIPTION OF THE INVENTION 
Although the invention will be described hereinafter in the context of a 
preferred embodiment, the true scope of the invention will be found in the 
appended claims The invention was implemented in a matrix of 50 JFET's 
with devices of 1 micron channel length and 15 micron channel width. The 
matrix was fabricated by a lift-off process utilizing the Aluminum 
Germanium contacts of this invention. The Gallium Arsenide JFET's were 
fabricated by a known process. See G. Troeger and J. Notthoff, "A 
Radiation-Hard Low-Power GaAs Static Ram Using E-JFET Spaced DCFL." GaAs 
IC Symp. Technical Digest, page 78, 1983. Only a first level metal 
interconnect was used, the devices were measured by point contact probes 
to source entering the contacts in the array, and only the gates were 
connected with one micron lines to a common large area pad. The threshold 
voltage of a typical enhancement mode JFET in the array was +0.3 volts, 
and the transconductance was 120 ms/mm. The average values of transistor 
values in the matrix and their standard deviations were: 
EQU V.sub.T =(+0.36.+-.0.04) volts 
EQU I.sub.DS =(503.+-.76) micro amps (at V.sub.G =+1 v) 
EQU g.sub.m =(106.+-.8) mS/mm (at V.sub.G =+1 v) 
The contacts were prepared by conventional evaporation techniques in the 
sequence of 400 Angstroms of Germanium, 300 Angstroms Nickel and 200 
Angstroms of Aluminum For the substrate material, LEC grown 
semi-insulating Gallium Arsenide without intentional doping was used with 
Si.sup.+ ion implanted n-type layers. A typical peak impurity 
concentration is in the range of 10.sup.17 to 10.sup.18 cm.sup.-3 . This 
sequence is shown in FIG. 1 in which region 10 is the semi-insulated 
substrate, the region 12 is the ion implanted n-type layer, layer 16 is 
the Germanium layer, layer 18 is the Nickel layer, and layer 20 is the 
Aluminum layer. The region 14 bounded by the dotted line enclosing the 
notation "n.sup.+ " indicates the region of the layer 12 into which the 
Germanium will migrate after alloying to form an especially high 
concentration of n impurities. The interface between the Germanium layer 
16 and the n-type layer of Gallium Arsenide 12 is marked as 15. Rapid 
thermal annealing at 825.degree. C. was used to activate the ion 
implantations. By increasing alloying temperature, a contact resistance of 
1.4.times.10.sup.-6 ohm-cm.sup.2 was obtained at 500.degree. C. The Nickel 
layer 18 is required to prevent balling up of the Al-Ge after alloying and 
leads to smooth surface texture of the contact layer. The alloying step is 
carried out in the reducing atmosphere of hydrogen in a graphite strip 
heater. The time of alloying ranges from 1 to 30 minutes. After the 
alloying procedure, an overlay of 2500 Angstroms of pure Aluminum was 
evaporated and patterned utilizing a conventional photoresist liftoff 
method. This layer of Al is not shown in FIG. 1. 
While the bulk resistivity of Aluminum is 2.7.times.10.sup.-6 ohm-cm, and 
is slightly higher than the value of 2.2.times.10.sup.-6 ohm-cm for Gold, 
the sheet resistivity of 2500 Angstroms of Aluminum is 120 
milli-ohms/.sup.2 and less than that for 2500 Angstroms of magnatron 
sputtered Gold which has a sheet resistivity of 160 milli-ohms per square. 
At the second interconnect level, 5000 Angstroms of Gold has a sheet 
resistivity of 80 milli-ohms per square (sputtered) while the Aluminum is 
again less with 60 milli-ohms per square (evaporated). For a thickness of 
one micron (10000 Angstroms) the sheet resistivity of an Aluminum layer of 
this thickness drops to 30 milli-ohms per squire. Thus it would be 
advantageous to use a thicker layer of Aluminum for the second metal 
interconnect layer than is presently used for Gold and similar 
applications which is typically 5000 to 6000 Angstroms. 
The Al-Ge alloy could also be deposited by other techniques besides the 
conventional sequential deposition by evaporation of the respective Ge, Ni 
and Al layers followed by alloying For example, an already alloyed mixture 
of Al-Ge in the form of pellitized material could be deposited onto the 
GaAs by flash evaporation Also, co-evaporation of Al and Ge could be 
conducted to deposit the correct eutectic alloy mixture on the GaAs. The 
phase diagram for the Al-Ge system can be found in Hansen's book, 
Constitution of Binary Alloys, referenced above. 
FIG. 2 is a graph which demonstrates the dependence of the specific contact 
resistance, R.sub.C, in ohms-cm.sup.2 as a function of the level of n-type 
ions in substrate (cm.sup.-3). The graph clearly demonstrates the benefits 
of high levels of n-type impurities in the diffused contact region as 
shown by the region 14 in FIG. 1. 
It was also found that contact resistance decreased as the alloying 
temperature increased for a peak n-type impurity concentration of about 
10.sup.18 cm.sup.-3 .