PROCESS FOR FORMING A METALLURGICAL SYSTEM COMPRISING A BOTTOM LAYER OF NICKEL AND A TOP LAYER OF GOLD

Abstract: The process comprises placing a nickel layer on the metallized portion of the substrate and then covering the nickel with an immersion layer of gold. The gold immersion layer is chemically stripped followed by depoiting thick gold over said nickel and diffusing said thick gold into said nickel. The process is particularly applied in making thermocompression bonds to a metallized ceramic substrate on which semiconductor chips are to be mounted and eletrically contacted.

Description: The invention relates to a process for forming a metallurgical system comprising a top layer of gold and a bottom layer of nickel on a semiconducting or insulating substrate. In the metallization of ceramic and other substrates for the purpose of providing electrical contact terminals, nickel is often deposited as a base layer. Nickel, however, is known to oxidize rather readily and the resulting oxide interferes with the establishment of sound electrical contact. Accordingly, steps have been taken in the prior art to inhibit or prevent the oxidation of nickel by the application of a material, usually gold, over the nickel as soon as the nickel layer has been formed. In some instances where a thick layer of gold is desired, such as where thermocompression bonding is required, a thick layer of gold is electroplated on the nickel. This is disclosed in the IBM Technical Disclosure Bulletin "Process For Preventing Chip Pad Corrosion" by M.M. Haddad et al, Vol. l9, No. l2, May l977, page 458l. A similar thick gold electroplating process is described in the Technical Disclosure Bulletin "Selectively Electroplating Pad Terminals on an MLC Substrate" by M.M. Haddad, Vol. 20, No. 9, February l978, page 3443. In the Technical Disclosure Bulletin "Nickel/Gold Diffusion Barrier" by J.R. Lynch, Vol. l4, No. 4, September l97l, page l099, a diffusion barrier is formed between a layer of electroplated gold and an adjacent layer of nickel in order to prevent the diffusion of the nickel into a silicon device which is to be bonded to the gold layer. The diffusion barrier is formed by a layer of electroplated gold placed on the nickel and then fired in a hydrogen atmosphere at about 700° C. The barrier layer is left in place and later covered by an additional layer of gold to which the silicon device is bonded. It has been found that the process of the abovecited 1971 Technical Disclosure Bulletin also is suitable for use in applications where thermocompression bonding is desired and where a diffusion barrier, as such, is not needed. More recently, however, it has been noted that the "pull strength" of a thermocompression bond establixhed upon a metallurgical combination of a second gold layer upon a first gold layer (diffusion barrier), upon a nickel layer, upon a substrate, is insufficient. In some cases, it is necessary that the pull strength be increased by a factor of 50% or more. It is the object of the invention to provide a simple, reliable process for reproducibly producing an enhanced bond strength between a gold layer and an underlaying nickel layer deposited on a semiconducting or insulating substrate. This object is achieved by a process as defined in claim 1. The pull strength of a metallurgical system comprising a bottom layer of nickel and a top layer of heavy gold is increased by the application of the thin immersion layer of gold on the nickel layer and the specific stripping of the undiffused immersion gold by a chemical etchant before the top layer of heavy gold is placed down. According to another feature of the invention, the final structure of nickel and heavy gold is subjected to a 550° C anneal which softens the heavy gold deposit and diffuses the gold into the nickel and improves the pull strength or bondability of the assemblage. Other advantages embodiments of the inventive process are disclosed in the subclaims. The invention which became more apparentfrom the following detailed description taken in conjunction with the accompanying figure. The sole Figure is a simplified cross-sectional view of a metallurgical system, in accordance with the present invention for making electrical contact to metallized ceramic substrate. Ceramic substrate 1 is metallized by a layer 2 of, for example, molybdenum which is personalized or shaped by conventional lithography techniques into a pattern of desired electrical contact areas (not shown), as is well understood in the semiconductor chip packaging art. A nickel layer 3 typically is placed over the molybdenum contact area in preparation for a thick or heavy gold deposit 4 which is required for thermocompression bonding of a wire 5, as generally described in the aforementioned February l978 Technical Disclosure Bulletin. It is known that nickel quickly becomes oxidized causing its surface to become passive to subsequent plating. Such oxidation may be inhibited by the application of a "flash" coating of a different metal, for example gold, according to the teaching of U.S. Patent 3,362,85l to D. Dunster, issued on January 9, l968. However, while a flash coating of gold acts as an oxidation inhibiter, it has been found that the pull strength of the bond between the heavy or thick gold and the nickel is limited unnecessarily when an intervening flash coating of gold is left intact and completely diffused into the final composite structure. In accordance with the present invention, an immersion gold layer of about 70 to about 120 nm thickness placed upon the nickel without any further step to enhance the diffusion of the gold into the nickel. Selective masking is applied to cover those nickel areas which are not to receive any thermocompression bonding. Then, the undiffused immersion gold is removed from the the un masked areas by a chemical stripper immediately before the heavy gold is electroplated upon the unmasked nickel. It is preferred to use a chemical stripper for removing the immersion gold characterized by the following constituents and approximate concentrations: 1. gold complexing agent such as potassium cyanide - 16%;*) 2. oxidizing agent such as sodium meta-nitro benzoate - 49%; 3. sodium carbonate (to make a solution PH of about 11.5) - 26%; 4. sodium sulfate - 9%; 5. lead ion - 0.05%; 6. water. *) all the percentages are % by weight Finally, the selective masking is removed and the heavy gold is diffused into the nickel with preferably a 550° C heat treatment cycle to produce a metallurgical system characterized by reduced hardness and enhanced pull strength relative to that attainable using the prior art processes cited. The following detailed example (which contrasts the process steps of a prior process similar to the above-cited 1971 Technical Disclosure Bulletin with those of the present invention) will facilitate a more complete understanding of the present invention. It will be noted that the first 5 steps of the present invention are the same as those of the prior process. However, the gold diffusion step 6 of the prior process is omitted in the present invention and replaced by the later-occurring heavy gold diffusion step 16. Steps 6-9 of the present invention are similar to steps 7-10 of the prior process. The undiffused thin gold layer resulting from step 5 is etched away in step 10 of the present invention and rinsed (step 11). Then the remaining steps 11-14 of the prior process are used as steps 12-15 of the present present invention. Finally, the postponed gold diffusion step is exercised in step 16 and is followed by a plasma ash to form a Au-Ni diffusion barrier necessary to assure proper metallurgy and subsequent bondability in the use of the present invention. The following average bond pull strength results were obtained in a comparison test between 3105 control samples made with the prior process and 423 samples made with the present invention, using the steps described above: In addition, it was found that non-repairable failures (such as delamination at the metallurgical interfaces) were eliminated while repairable failures (such as rupturing of the gold bulk) were reduced in specimens constructed with the method of the present invention relative to specimens constructed with the prior process.

Claims: 1. Process comprising depositing nickel on a substrate, forming a thin layer of gold over said nickel, chemically stripping said gold layer, depositing thick gold over said nickel, and diffusing said thick gold into said nickel. 2. Process according to claim l wherein said thick gold diffusing step is done at a temperature of about 550° C. 3. Process according to claim l or 2 wherein said substrate is a ceramic material. 4. Process as described in claim 3 and further including a heating step for diffusing said nickel into said substrate. 5. Process according to any one of claims 1 to 4, wherein said thin layer of gold is formed by an immersion process to a thickness in the range of about 70 to about 120 nm. 6. Process according to any one of claims 1 to 5, wherein a gold stripper is used comprising a gold complexing agent, like potassium cyanide, an oxidizing agent, like sodium meta-nitro benzoate, sodium carbonate, sodium sulfate, lead ion and water. 7. Process according to any one of claims 1 to 6 and further including the step of thermocompression bonding an electrode to said thick gold. 8. Process according to any one of claims 1 to 7, wherein prior to the chemical stripping a masking material is selectively placed to cover certain areas of said thin gold layer and wherein said masking material is removed after depositing said thick gold.

Inventors: ADWALPALKER, AVINASH SRIPAD, HARVILCHUCK, JOSEPH MATTHEW, RANALLI, JOSEPH ROCCO, RICH, DAVID WILLIAM
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: C04B41
IPC codes: H01R43, C04B41, H05K3, H01L21, B23K20, B23K35