US2970044A

US2970044A - Solution and process for etching indium dots

Info

Publication number: US2970044A
Application number: US705826A
Authority: US
Inventors: Peter L Ostapkovich
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1957-12-30
Filing date: 1957-12-30
Publication date: 1961-01-31
Anticipated expiration: 1978-01-31

Description

United States Patent SOLUTION AND PROCESS FOR ETCHmG INDIUM DOTS Peter L. Ostapkovich, Poughkeepsie, N.Y., assignor to International Business Machines Corporation, New York, N.Y., a corporation of New York No Drawing. Filed Dec. 30, 1957, Ser. No. 705,826

3 Claims. (Cl. 41-42) This invention relates to a process for cleaning indium dots of etching, preparatory to using the indium dots in the manufacture of semi-conductive devices, typically transistors.

In the manufacture of transistors by the alloy junction method, a main body of semi-conductive material, typically germanium or silicon, is alloyed at the interface with a smaller body of a donor or acceptor material, commonly called a dot. The usual technique is to place the dots on the surface of the main bodies and fire them in a furnace for a period of time and at a temperature sufficient to melt the dot and weld or alloy it at the interface with the underlying body. For the manufacture of PNP transistors, the material commonly used for the dot is indium.

The present invention relates to the cleaning of the dots preparatory to the firing step of the manufacturing process. The purpose of the cleaning is to remove all foreign materials from the surface of the dot, so as to secure a more uniform alloy junction or weld between the indium and the germanium.

An object of the present invention is to provide an improved method for etching indium.

Another object of the invention is to provide an improved etchant solution for use with indium.

The foregoing and other objects of the invention are attained by immersing the indium clots in a solution consisting essentially of /2 to 20 parts hydrofluoric acid, /2 to 20 parts acetic acid and 20 to 100 parts water, all parts being by volume. The immersing of the dots is maintained for a time suflicient to clean the dots, and at a temperature which may be varied inversely with the time, and wherein the time varies more or less inversely with the concentration of the etching solution.

The two principal ingredients of the novel etching solution are hydrofluoric acid and acetic acid. The process works well at room temperature (25 C.). The temperature may be increased up to 45 C. for the purpose of reducing the etching time. Above that temperature, acid vapor pressures are high and may give trouble, and in any case, the conditions are such as to be highly objectionable to the operators of the process.

The time may be varied from a few seconds to several .minutes, depending upon the acid concentrations used. It is usually best to select acid concentrations which allow a convenient operating time.

The presently preferred method of carrying out the invention is set forth in the following example:

Example 1 Mix together 5 parts hydrofluoric acid, 5 parts acetic acid in 100 parts of water, all parts being by volume. Dip the indium dots to be etched in the acid etching solution so prepared for a period of about one minute at room temperature (25 C.). The dots are thereafter rinsed with water and dried in a current of warm air.

Other illustrative examples of processes embodying the invention are set forth below:

Example 2 Use the same etching solution as in Example 1, but dip the dots in the solution for 7 seconds at a temperature of 45 C.

Example 3 Mix together 20 parts hydrofluoric acid, 20 parts acetic acid and 20 parts water. Dip the indium dots in the acid solution for 10 seconds at room temperature.

Example 4 Mix together /2 part hydrofluoric acid, /2 part acetic acid and parts water. Dip the indium dots in the acid solution for 10 minutes at room temperature. This is a presently preferred embodiment of the invention.

In all of the examples, conventional rinsing and drying processes follow the immersion.

While, in the foregoing examples, equal proportions of hydrofluoric acid and acetic acid have been indicated, and those proportions give the best results, nevertheless it is possible to secure a commercially acceptable product by varying the proportions of the two acids within the range from about 1 part acetic to about 5 parts hydrofluoric, on the one hand, to about 5 parts acetic to about 1 part hydrofluoric, on the other hand.

In the foregoing examples the hydrofluoric acid is a commercially available solution of 48% by weight HF, and the acetic acid is a commercially available solution of 99.79% by weight CH COOH, hereinafter termed glacial acetic acid.

The indium dots etched in accordance with any of the preceding examples may then be placed on a germanium body and heated in a furnace for 5 to 10 minutes at a temperature of between 560 C. and 610 C. The temperature selected is a function of the thickness of the germanium body. However, the particular furnace treatment step per se, is no part of the present invention.

It has been found that transistors made with indium dots which have been etched according to the process described above have superior electrical characteristics as compared to transistors of the prior art. In particular, the cutoff frequencies of the transistors are higher and more uniform from one transistor to another. Furthermore, the breakdown voltages across the alloy junctions are also higher and more uniform.

While I have described certain preferred embodiments of my invention, other modifications thereof will readily occur to those skilled in the art, and I therefore intend my invention to be limited only by the appended claims.

I claim:

1. The method of preparing indium dots for use in alloying with germanium to form semi-conductor devices, comprising immersing said dots in an aqueous etching solution, the active ingredients of which consist of /2 to 20 parts commercial hydrofluoric acid (about 48% by weight HP) /2 to 20 parts glacial acetic acid 20 to 100 parts water,

present in equal proportions by volume. V

a s3 3. The method of claim 1, in which the active ingredients of said solution consist of /2 par-t of said commercial hydrofluoric acid /2 part acetic acid 100 parts water said temperature is 25 C. and said time is 10 minutes.

References Cited in the file of this patent UNITED STATES PATENTS Heidenreich Nov. 25, 1952 Spanos Ian. 24, 1956 Armstrong Sept. 17, 1957 Certa Aug. 26, 1958 UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Pa ten no 2 970.0 1 T c L -Q Peter L Ostapiaovicl': It is hereby certified that error a ppears in the above numbered patent reqiiring correction and that the said Letters Patent should read as corrected below.

Col-mam 1, line 16, for "of etching" read by etching column 3, line 4, after "part" insert Signed and sealed this 13th day of June 1961 SEA L) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents of said glacial

Claims

1. THE METHOD OF PREPARING INDIUM DOTS FOR USE IN ALLOYING WITH GERMANIUM TO FORM SEMI-CONDUCTOR DEVICES, COMRPISING IMMERSING SAID DOTS IN AN AQUEOUS ETCHING SOLUTION, THE ACTIVE INGREDIENTS OF WHICH CONSIST OF 1/2 TO 20 PARTS COMMERCIAL HYDROFLUORIC ACID (ABOUT 48% BY WEIGHT HF) 1/2 TO 20 PARTS GLACIAL ACETIC ACID 20 TO 100 PARTS WATER, ALL PARTS BEING BY VOLUME, WHEREIN THE PROPORTION OF EITHER ACID TO THE OTHER ACID IS NOT MORE THAN ABOUT FIVE TO ONE, AT A TEMPERATURE OF NOT OVER ABOUT 45*C. AND FOR A TIME SUFFICIENT TO CLEAN THE DOTS, BUT NOT OVER ABOUT 10 MINUTES.