US 3654694 A
An alloy of silicon and metal (e.g., gold) formed at the eutectic temperature thereof permits the formation of a bonded contact of the metal (e.g., gold) to silicon carbide. Such a contact is useful for securing a wire to the silicon carbide, for forming alloy sites thereon, etc. A nail head bonder with a threadable heated tip may be utilized in the use of this method.
Claims available in
Description (OCR text may contain errors)
United States Patent Dunlap  Apr. 11, 1972 54 METHOD FOR BONDING CONTACTS 3,028,663 4/1962 lverson et a1 ..29/472.9 ux TO AND FORMING ALLOY SITES 0N 3,075,282 1/1963 McConville ..29/590 UX 3,091,849 6/1963 Cohen ..29/472.9 SILICONE cARBIDE 3,228,104 1/ 1966 Emeis ..29/472.9 X  Inventor: Howard L. Dunlap, Granada Hills, Calif. 3,298,093 1/1967 Cohen ..29/590 X 3,316,628 5/1967 Lang, Jr ..29/473.1 X  "8" Alma Culver CIY, 3,397,451 8/1968 Avedissian m1... ..29/589 Cahf- 3,341,938 9/1967 Hoppe el al ..29/589 x  Filed; APR 23 9 3,492,719 2/1970 Zeitman et a1 ..29/473.1 X
1 PP N041 819,832 Primary Examiner-John F. Campbell Assistant Examiner-Ronald J. Shore 152 0.8. CL ..29/587,29/473.1, 29/504 Mme-14am Haskell and  Int. Cl. ..B0lj 17/00, H0l15/04,H0117/60, A T
H0119/08, H011 11/02, H011 11/04, H01115/08  ABSTR C  Field 61 Search ..29/472.9, 473.1, 589, 590, An alloy of silicon and metal 8- g formed t e eutec- 29/621 504 537 tic temperature thereof permits the formation of a bonded contact of the metal (e.g., gold) to silicon carbide. Such a con- R f tact is useful for securing a wire to the silicon carbide, for  e erences cued forming alloy sites thereon, etc. A nail head bonder with 21 UNITED STATES PATENTS threadable heated tip may be utilized in the use of this th d. 2,918,396 12/1959 Hall ..29/472.9 ux me 0 2,937,324 5/1960 Kroko ..29/472.9 UX 18 Claims, 1 Drawing Figure g ,I /Z M 0- /3 4:47 Pan/98 .Sadec' METHOD FOR BONDING CONTACTS TO AND FORMING ALLOY SITES ON SILICONE CARBIDE The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).
The present invention relates to a method for making bonded electrical contacts on silicon carbide.
With the increasing need to utilize semiconductor devices in high temperature environments, materials, which will satisfactorily function without deterioration at high temperatures, have been required. One such material is silicon carbide, whose melting point is approximately 2,700 C. and which is relatively insoluble in most substances. The fabrication of semiconductor devices utilizing silicon carbide, however, has presented difficulties. One serious problem is the difficulty in forming electrical contacts to precise locations or sites on a silicon carbide sample with ease and without producing a poor contact. Existing processes have not satisfactorily overcome these problems.
In one prior process, a silicon carbide wafer, with a pellet of material placed at the desired contact site, is placed in a furnace and heated to approximately 2,000 C. to form an alloy site and thereafter cooled. After properly made alloy sites are formed, a wire lead is attached to the wafer at the site. However, the pellet might be improperly or inaccurately placed or accidentally moved and, therefore, the contact site would not be located where desired. Jigs have been utilized to hold the pellet in place but are cumbersome and are still liable to accidental movement or to imprecise placement. Even when proper alloy sites are formed, the high temperatures required can adversely affect the silicon carbide wafer or cause undesired penetration thereof by the material of pellet. This penetration damages the wafer and may even make it useless for its intended purpose.
The present invention overcomes these and other problems by providing a method which can be carried out under atmospheric conditions in a low temperature environment to secure a wire lead to silicon carbide, to form alloy sites, and the like. A pulse of heat is utilized simultaneously to form an alloy of silicon and metal from the lead at their eutectic temperature and to cause adhesion of the alloy and the lead to the silicon carbide sample or specimen.
A nail head bonder having a heated tip is preferably utilized in the practice of the present invention and the wire to be bonded to the silicon carbide sample is threaded through the tip. From a prior use of the bonder, the wire is equipped with a bead. A small particle of silicon, used as a wetting agent, is placed on the substrate at the point or site where the contact is to be made. When a pulse of heat is produced by the bonder tip, an alloy is formed from the bead and the silicon at the eutectic temperature thereof. For a gold-silicon alloy, the tem perature is approximately 370 C. In addition, the silicon carbide sample is heated, for example, to approximately 100 C., in order to prevent heat flow out of the bead-silicon-silicon carbide interface area. Once the molten alloy is formed, the surface of the silicon carbide is wetted and affixation of the wire to the silicon carbide occurs.
In the use of the present inventive method, when a nail head bonder is utilized, some correlation is required between the wire used, the temperature of the bonder tip, the wetting agent, and the substrate surface. The tip temperature must not be allowed to approach the melting point of the wire; otherwise, the wire would melt within the tip. The quantity of silicon and the size of metal bead must be such that the eutectic ratio of silicon to the metal permits formation of the alloy only at interface and not within the bonder tip. For a gold-silicon alloy, the eutectic ratio is 31 atomic percent of silicon to gold. Regardless of whether a nail head bonder is used, the interface temperature at the silicon carbide sample cannot be below the eutectic temperature of the wire and wetting agent. Once the proper parameters have been obtained, the wire may be easily placed at any specific desired site on the silicon carbide sample with a high degree of accuracy.
The silicon used as a wetting agent may be placed on the silicon carbide sample manually or by any other method and, in most cases when great accuracy is required, the silicon can be applied as a pad by a conventional evaporation process or sputtering process in conjunction with photo masking methods.
The use of the present invention does not cause the substrate to enter into the eutectic condition, thereby avoiding penetration of the substrate. After the wire has been fixed in position, it may be used in a variety of ways such as a mechanical hold-down device or as electrical contacts to silicon carbide of sufficiently low carrier concentration, in the realm of 10 carriers per cubic centimeter, or as alloy sites. In the latter use the wire lead is removed and the wafer is heated to a sufiiciently high temperature to permit penetration of the wafer by the molten metal. For this use as an alloy site, if the wetting agent has been predoped or if multilayers of materials have been applied, this alloying operation into the substrate can be efiected without the need for jigs to locate the defined site. In addition, except for alloy sites, once the contact has been formed, it can be removed mechanically, electronically, or chemically and reapplied, if needed or desired, without changing the characteristics of the silicon carbide sample because no penetration thereof has occurred. Mechanical removal can be effected by cutting, electronic by ion or electron beam machining, and chemically by such reagents as aqua regia.
It is, therefore, an object of the present invention to provide a method for forming bonded contacts on silicon carbide samples.
Another object is the provision of a simple, yet accurate, method for making such silicon carbide contacts.
Another object is to provide such a method whereby the silicon carbide sample is not deleteriously affected by the formed contact.
Another object is to provide a method whereby a contact on silicon carbide, after having been made, may be removed and relocated to another position on the sample.
Other aims and objects as well as a more complete understanding of the present invention will appear from the following explanation of exemplary embodiments and the accompanying drawing thereof, in which:
The FIGURE depicts a preferred apparatus for carrying out the present invention.
Accordingly, a nail head bonder 10 is provided with a tip 12 which is adapted to be heated. A feeder hole 14 is provided in tip 12 so that a wire 16 may be threaded and fed therethrough from bonder 10. A bead 18 terminates wire 16 and is generally formed by a prior use of the bonder. A silicon carbide wafer or sample 20 is shown placed on a work surface 22 and a pad or particle of silicon 24 is placed on the sample in readiness for formation and bonding of the contact. An electrical coil 26 is placed adjacent to sample 20 in order to heat the sample; however, other suitable heating means may be utilized.
In operation, when it is desired to join wire 16 to sample 20, bead 18 is placed in contact with pad 24 and a pulse of heat, formed by bonder l0 and tip 12, produces an alloy between bead l8 and pad 24 at their eutectic temperature. Upon cooling of the alloy, a bond exists between wire 16 and sample 20.
EXAMPLE A silicon carbide specimen of crystal quality, with designated sites at which contacts were to be made, was placed in readiness for bonding of a gold wire. The wafer may have already undergone prior processing, such as by ion implantation or thin layer diffusion. A silicon chip in the form of a flake was placed on the sample. A gold wire of 1 mil thickness having a bead of approximately 2 mil diameter had been extended from the bonder tip from a prior use of the bonder. The silicon carbide wafer was heated to approximately C. and the bead of gold was brought into contact with the silicon chip and pressed thereto. Heat was applied by the tip and, at the eutectic temperature of gold and silicon, which was estimated at approximately 370 C., an alloy was formed to bond the gold wire to the silicon carbide wafer.
Testing of the bonded contact thereby made showed that no discemable penetration of the wafer occurred, that the bond was mechanically strong, and that electronic noise was not present. The contact was made at the precise site desired.
Although the invention has been described with reference to particular embodiments thereof, it should be realized that various changes and modifications may be made therein without departing from the spirit and scope of the invention.
What is claimed is:
l. A method for forming a bonded electrical contact of a gold wire lead to a silicon carbide crystal without the need of special holding fixtures comprising the steps of:
placing silicon on the crystal,
placing the wire lead in contact with the silicon, and
heating the wire lead and the silicon in an ambient atmospheric environment to a temperature sufficient to form a gold-silicon alloy at the eutectic temperature to bond the lead to the crystal.
2. A method as in claim 1 further including the step of heating the crystal.
3. A method as in claim 1 further including the step of utilizing a nail head bonder having a heating tip for supplying a heat pulse to accomplish said heating step.
4. A method of forming bonded electrical contacts of wire leads to a silicon carbide specimen without the use of special holding fixtures comprising the steps of:
placing silicon on the silicon carbide specimen at the sites where the contacts are to be made,
placing the leads in contact with the silicon at the sites, and
forming an alloy of the silicon and the material of the lead on the specimen at each site to bond the lead to the specimen in an ambient atmosphere.
5. A method of fabricating a bonded electrical contact between a wire lead and a silicon carbide specimen comprising the step of forming an alloy of the lead material and silicon on the specimen at the site thereon where the contact is to be made, said forming step being performed under ambient atmospheric conditions at a temperature sufficient to form the alloy and, consequently, to bond the lead to the specimen.
6. A method as in claim 5 including the step of utilizing predoped silicon.
7. A method as in claim 5 including the step of utilizing a photo masked specimen.
8. A method as in claim 5 wherein said forming step comprises the steps of placing silicon on the specimen, locating an end of the lead on the silicon, and heating the lead and the silicon to the eutectic temperature thereof.
9. A method as in claim 8 wherein said placing step includes the step of attaching the silicon particle to the specimen.
10. A method as in claim 8 wherein said placing step consists of the steps selected from the methods of sputtering, evaporation and manually positioning the silicon on the specimen.
11. A method as in claim 8 wherein said heating step comprises the step of utilizing a nail head bonder having a wire lead feed and a heating tip for supplying a heat pulse to the silicon particle and an end of the wire lead.
12. A method as in claim 5 further comprising the step of heating the specimen.
13. A method as in claim 5 further comprising the step of pressing the alloy to the specimen.
14. A method as in claim 8 further comprising the step of forming a bead of the wire at the end thereof.
15. A method for bonding gold to a silicon carbide wafer at a specified site thereon including the steps of placing silicon on the wafer at the site,
placing the gold on the silicon,
heating the wafer to approximately 100 C., and
heating the silicon and gold bead to their eutectic temperature to form a bond between the gold and the wafer under ambient atmospheric conditions. 16. A method as in claim 15 including the step of forming a silicon-gold utectic ratio of 31 atomic percent of silicon.
17. A method as in claim 15 further including the step of removing the bond from the wafer.
18. A method of forming alloy sites on a metal silicon carbide specimen comprising the steps of forming on the specimen at each of the sites an alloy of the metal and silicon at the eutectic temperature thereof under ambient atmospheric conditions and further heating the specimen sufficiently to cause penetration of the wafer by the alloy and to form a bond therebetween.