1. Field of the Invention
This invention relates to glass compositions and methods for use in encapsulating electronic devices and, more particularly, to improved glass compositions having controlled expansion properties and fusion temperatures which can be used in glass compression seals for insulating the electric leads of semiconductor device packages; improved methods for making seals, and, further, to improved semiconductor devices utilizing these glass compositions and methods.
2. Description of the Prior Art
Insulating glasses which melt or soften at temperatures exceeding 500.degree. C. are conveniently used to provide insulated electrical feedthroughs in the walls of many semiconductor device enclosures. The glass serves to insulate the electrical leads from the metal body or base of the device enclosure, and at the same time to prevent atmospheric contaminants from reaching the interior of the enclosure where they might adversely affect the semiconductor chip. Electrical leads and device enclosures sealed so as to be impervious to atmospheric contaminants are said to be "hermetic." As used herein the word "semiconductor device" is intended to include the semiconductor chip, the sealed electrical leads, and the surrounding enclosure.
Compression glass seals are well known for providing hermetic insulating feedthroughs in electrical enclosures, particularly semiconductor device enclosures. U.S. Pat. No. 4,128,697 to W. M. Simpson describes a typical compression seal structure, which includes an outer metal casing member composed of a relatively high expansion coefficient metal, a lead-in conductor of a relatively low expansion coefficient metal passing through a hole in the outer casing member, and a glass seal of relatively low or intermediate expansion coefficient glass filling the annular opening between the casing and the metal lead. This selection of relative coefficients of expansion tends to keep the glass in compression, hence the designation of "compression seal."
Several glass compositions have been developed in the prior art which provide useful hermetic compression seals. For example, TO-3 type semiconductor packages are widely used. These packages typically have a 1010-alloy steel base in which are mounted two or more 52-alloy lead-in wires sealed with Corning Type 9013 glass. Corning Type 9013 glass is manufactured by Corning Glass Works, Corning, N.Y. 52-alloy and 1010-alloy are American National Standard designations defined in ANSI/ASTM Standard Specifications F30-77 and A29-79 respectively. These materials are well known in the art.
A compression glass seal is typically made by heating the assembled pieces (base, lead-in wire, and glass) above the softening and sealing temperature of the glass, usually about 1000.degree. C. This high softening and sealing temperature is a significant disadvantage of prior art glasses, since it precludes the use of base members or lead-in wires which are composed of, or have been plated with, lower melting point materials. For example, in order to improve the corrosion resistance of a 1010-alloy steel base, it is important to be able to use a protective plating. However, some desirable protective platings, such as electroless nickel, melt at temperatures less than the sealing temperature of the prior art glass. Electroless nickel is typically a nickel-phosphorous alloy having a melting point less than that of pure nickel. Other alloying materials besides phosphorous are also useful. With the prior art glass, the nickel alloy plating must be applied following the glass sealing operation. This production sequence is undesirable since the leads are frequently bent during the plating operation. Packages with bent leads will jam automated handling equipment unless the leads are straightened. Straightening the leads is time consuming and costly. In addition the bending and straightening degrades the integrity of the hermetic glass seal between the leads and base. Manufacturing yield decreases and cost increases.
Prior art attempts to lower the sealing temperature have resulted in glasses which are either mechanically weaker, chemically less stable, or both. Thus, a need has continued to exist for compression sealing glasses which have a lower sealing temperature while at the same time continuing to exhibit good electrical, mechanical, and chemical properties, particularly resistance to atmospheric corrosion, chemical reduction, and thermal cycling.
Accordingly, it is an object of the present invention to provide an improved glass composition for the fabrication of compression glass seals on electrical devices.
It is a further object of this invention to provide a sealing glass composition with a lower sealing temperature than heretofore achieved without sacrifice of other desirable electrical, chemical, and mechanical properties.
It is an additional object of this invention to provide an improved method for making enclosures for electrical devices, particularly with hermetic sealed lead-in wires for semiconductor devices.
It is a further object of this invention to provide an improved method for making enclosures for electrical devices having hermetic glass sealed lead-in wires wherein a low temperature protective plating is applied to the metal parts prior to glass sealing of the lead-in wires.
It is a still further object of this invention to provide a semiconductor device enclosure of improved properties, including lower cost.