RF transistor package and mounting pad

Abstract: An improved semiconductor package is provided wherein the mounting pad for the semiconductor is made from a material selected from the group consisting of aluminum nitride, diamond, alumina, and boron nitride.

Description: This invention provides a novel mounting package for RF transistors including a novel thermally conductive electrically insulating mounting pad, and relates to the semiconductor packaging industry. The use of semiconductor devices for RF (radio or high frequency) applications has increased dramatically as heat dissipation, size, dependability, and other characteristics have made them particularly suited for many applications. Semiconductor transistors are very small and very difficult to use; therefore, they are generally incorporated into packages. Radio frequency packages are used for holding semiconductor components, particularly transistors, and for providing readily available terminals for connection to other components. In designing such packages, the tendency of semiconductors to generate heat must be considered. To this end, the semiconductor is frequently mounted on a thermally conductive pad to act as a heat sink and dissipate heat generated by the transistor. However, semiconductors are sensitive to electrical energy, and therefore the thermally conductive pad should also be electrically insulating. Hence, a thermally conducting, electrically insulating material is used for the mounting pad. Commonly, this material is beryllia (beryllium oxide - BeO), although alumina (aluminum oxide - Al 2 O 3 ) has occasionally been used. Beryllia (BeO) has better thermal conductivity than Al 2 O 3 , and is therefore more commonly used. However, care must be taken in handling and processing BeO. BeO is highly toxic. It can be hazardous to the human respiratory system when in powder form. Therefore, care must be exercised if the BeO pad is to be machined or ground. Proper equipment and safeguards are needed to insure that the BeO is handled safely and that improper contact with humans is not made. Another disadvantage of the prior art packages is that when BeO is used, a "thin" pad of BeO is employed. This pad is attached to the heat sink through the use of a preform. The preform is generally made of gold and tin, or silver and copper. The preform is placed between the mounting pad and the heat sink and the structure is heated. This melts the preform and brazes the pad to the heat sink. This creates two interfaces: pad-to-preform, and preform-to-heat sink. These interfaces decrease the thermal conductivity of the connection. Further, BeO has a high thermal conductivity up to about 200°C to 250°C, but its performance decreases with increasing temperature. The BeO pad used is generally about lmm to 1.5mm (.040" to .060")thick. Such thickness is required since the mechanical strength of BeO is low. If the pad is not sufficiently thick, the pad may crack or split during attachment, or later upon heating from use. EP 0297569 discloses the joining of an aluminium nitride substrate of 1300µm thickness to a Cu-Sn alloy radiating member. It teaches that a stress relieving member should be interposed between the aluminium nitride substrate and the radiating member. Patent Abstracts of Japan vol 009284, 11.12.85 & JP-A-60-127750 discloses a diamond heat sink in which layers of metal are insulated from each other by a diamond plate of 300µm thickness. These plates of materials in the prior art do not, however, confer to the prior art semiconductor package the advantageous effect that thin slivers and coatings of diamond or aluminium nitride confer to the present invention. Other materials have been used for their thermally conductive, electrically insulating properties. In GB-A-2,084,796 the use of plates of materials including BeO, polymide resin, mica, epoxy resin, BN and laminated resin, is disclosed. These plates of materials do not however confer to the prior art semiconductor package the advantageous effects that slivers of diamond or aluminium nitride confer to the present invention. It has now been discovered that other thermally conductive, electrically insulating materials may be used as mounting pads for mounting transistors and other semiconductors in packages. These materials may be applied directly to the surface of the heat sink in very thin films, or mounted on the heat sink as thin slivers. This increases the thermal conductivity of the package and decreases the overall height of the finished transistor package. The materials which are useful for the present invention are diamond and aluminum nitride (AlN). The AlN may be deposited by plasma deposition. The diamond thin coating may be deposited by vacuum deposition. Any of the above materials may alternatively be mounted on the heat sink by placing a preform between the heat sink and a sliver of the material and heating the combination to braze the two together. These coatings (or films) or slivers can be substantially thinner than the BeO pads of the prior art, since the materials possess higher mechanical strength. Further, the coatings (or films) are deposited directly on the heat sink, thus eliminating one material-to-material interface. According to the present invention there is provided a semiconductor package having an input, output and a common terminal for connection therewith of respective terminals of a semiconductor which comprises a support member, a thermally conducting, electrically insulating pad mounted on and secured to said support member, for receiving said semiconductor, a semiconductor mounted on said pad and a protective lid, said pad comprising: a sliver of material selected from the group consisting of aluminium nitride and diamond, said sliver having a thickness of between 203µm and 305µm (8-12mils) in the case of aluminium nitride and a thickness of between 76µm and 102µm (3-4mils) in the case of diamond or said pad comprises a thin coating of material selected from the group consisting of aluminium nitride and diamond, said thin coating having a thickness of between 178µm and 203µm (7-8mils) in the case of aluminium nitride and a thickness of between 7.6µm and 12.7µm (.3 - .5mils) in the case of diamond. The figure is a three dimensional expanded view of a package of the present invention. Referring now to the Figure, package 10 of the present invention includes heat sink 20 with capacitor 30. Heat sink 20 is preferably made from tungsten and copper (W+Cu) which have qualities desirable for a heat sink. Heat sink 20 acts as a support for package 10. Capacitor 30 is mounted directly on heat sink 20 and inside of the assembly package 10. Mounting pad 40 is attached to heat sink 20. Semiconductor 50 is then attached to mounting pad 40. These attachments will be explained further presently. Preform 60 is placed on heat sink 20, around capacitor 30, pad 40 and semiconductor 50. Lamina 70 and 80 are then placed on preform 60. The lamina 70 and 80.act as spacers to allow proper connections to be made to semiconductor 50. Lamina 70 and 80 are co-fired with leads 75 (for connection to semiconductor 50 on the inside) already in place. A second preform 90 is placed on lamina 70 and 80, and lid 100 caps the entire package. Lamina 70 and 80 are generally made from alumina and allow proper positioning of leads 75 with respect to semiconductor 50. Lid 100 is generally made from kovar, an iron-cobalt alloy. Preforms 60 and 90 are made from gold and tin, or copper and silver. This allows low temperature brazing. Mounting pad 40 may be made from any of several different materials and may be a sliver of material or a very thin coated layer. Pad 40 may be made of diamond or aluminum nitride. Pad 40 is attached to heat sink 20 in different ways depending on the form of the pad. A pad 40 made from diamond may take two different forms: a single crystal substrate, or a thin coating (film). A single crystal substrate would be from 76µm to 102µm (3 to 4mils (.003"-.004") thick. Such a single crystal substrate would be bonded to heat sink 20 through the use of a gold and tin, or copper and silver preform. The preform is placed between the silver and heat sink 20. Heat is then applied and the sliver is brazed to the heat sink 20. The preferred method of using diamond for pad 40 is through vacuum deposition of minute diamond particles onto heat sink 20. This eliminates one material-to-material interface, allowing greater thermal conductivity between the semiconductor and the heat sink, across pad 40. This also reduces the size of pad 40. As previously mentioned, a diamond sliver is usually 76 to 102µm (3 to 4mils (.003"-.004") thick, but when vacuum deposited, the diamond layer is preferably 10µm (.00039") and generally between 7.6µm and 12.7µm (.3 and .5mils (.0003"-.0005'')) thick. Whether a diamond sliver or diamond coating is used, the diamond pad 40 is then preliminarily plated with an appropriate metal and then gold plated. Semiconductor 50 is then mounted on the gold plating by applying heat and pressure. The gold plating and silicon semiconductor combine to form a gold-silicon euctectic which holds semiconductor 50 in place. If pad 40 is made from aluminium nitride it may also be applied to heat sink 20 in two different forms: as a thin layer, or a as a thin sliver. A sliver of aluminium nitride would be approximately 203µm-305µm (8-12mils ie .008"-.012") thick. Use of such a sliver would necessitate the use of a preform to braze the sliver (pad 40) to heat sink 20. Aluminium nitride may also be applied by plasma deposition. Plasma deposition produces a layer of material about 178-203µm (7-8mils ie .007"-.008") thick. Plasma deposition requires no preform to secure the coating. Whichever form of aluminium nitride is used, it must be plated with layers of molybdenum, nickel and gold. Semiconductor 50 is then secured to the gold plating as described above. Using the techniques described above, the overall thickness of the RF package can be decreased by one-third from about 3mm (.120") to about 2mm (.080"). However, if necessary, a Kovar spacer may be included between heat sink 20 and lamina 70 to maintain an overall package thickness of 3mm (.120"). The invention has been described as an RF transistor package and mounting pad in the best mode known to applicant, however, it will be apparent that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the appended claims.

Claims: A semiconductor package having an input, output and a common terminal for connection therewith of respective terminals of a semiconductor which comprises: a support member; a thermally conducting, electrically insulating pad mounted on and secured to said support member, for receiving said semiconductor; a semiconductor mounted on said pad; and a protective lid, characterised in that : said pad comprises a sliver of material selected from the group consisting of aluminium nitride and diamond, said sliver having a thickness of between 203µm and 305µm (8 and 12 mils) in the case of aluminium nitride and a thickness of between 76µm and 102µm (3 and 4 mils) in the case of diamond or said pad comprises a thin coating of material selected from the group consisting of aluminium nitride and diamond, said thin coating having a thickness of between 178µm and 203µm (7 and 8 mils) in the case of aluminium nitride and a thickness of between 7.6µm and 12.7µm (.3 and .5 mils) in the case of diamond. The packaging of claim 1 wherein said material is diamond. The package of claim 1 wherein said material is aluminium nitride. The package as claimed in any one of claims 1 to 3 wherein the pad comprises a sliver of material. The package as claimed in any one of claims 1 to 3 wherein the pad comprise a thin coating of material.

Inventors: BUTERA GASPER A, BUTERA, GASPER A.
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS, INC.
Application year: 1990
Main classification: H01L23
IPC codes: H01L23