Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor
Reexamination Certificate
1999-06-07
2001-09-25
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
C438S121000, C438S125000
Reexamination Certificate
active
06294402
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for attaching an integrated circuit chip to a substrate, e.g., a housing or other next level structure, and an integrated circuit chip useful in such a method. In particular, the method is useful for attaching a high power integrated circuit, e.g., a microwave monolithic integrated circuit (MMIC) to a housing.
Integrated circuit chips can be attached to a housing using a manual eutectic attachment technique. In such a technique, a eutectic material preform, e.g., a gold-tin eutectic material preform, is provided between the integrated circuit chip and the housing. Scrubbing is generally required for the component attachment and there has been a lack of eutectic material flow control when using a manual eutectic attachment technique. In addition, most gold-tin eutectic attachments are performed with gold-tin preforms and gold plated surfaces. The gold from the gold plated surface tends to diffuse into the preform. The resultant gold rich composition causes the rework temperature to increase.
Integrated circuits, e.g., high power integrated circuits, can be attached to a housing or other next level structure by an automated assembly system using an epoxy adhesive. However, the thermal conductivity of the epoxy material which is used to attach the integrated circuits, especially GaAs or InP integrated circuits, with the automated assembly is about one tenth the thermal conductivity of a eutectic material, e.g., gold-tin, gold-germanium, tin-lead. Heat generated by high power integrated circuits, e.g., MMIC's, must be removed to keep the operation temperature compatible with the device limitation. The lower thermal conductivity of the epoxy adhesive limits the device power or increases the operation temperature.
SUMMARY OF THE INVENTION
Applicants have found that there is a need to replace the low thermal conductivity adhesive with a better thermal conductivity material to enhance heat dissipation and enable high power output. There is also a need to provide a method for attaching an integrated circuit chip to a substrate with a method which is susceptible to automated assembly systems.
The invention provides an integrated circuit chip which includes a substrate, a plurality of transistors provided in the substrate, a circuit pattern provided on a top surface of the substrate and a metal layer comprising at least two metals in substantially eutectic proportions provided on a bottom surface of the substrate, the bottom surface of the metal layer being exposed. The integrated circuit chip can be attached to a further substrate, e.g., a housing, using automated attachment techniques. The chip can be attached to the housing by picking up the integrated circuit chip with the metal layer provided on the bottom surface thereof and placing the integrated circuit chip, e.g., under programmed control, onto a housing so the bottom surface of the integrated circuit chip faces the housing with the metal layer there between; and then heating the metal layer to a temperature above its eutectic temperature to melt the metal layer and attach the integrated circuit chip to the housing.
The metal layer can be provided on the bottom surface of the integrated circuit chip by depositing at least one layer of each of the at least two metals and inter diffusing the at least two metals, e.g., in an inert atmosphere to a temperature below the melting point of the metals and at or above a temperature sufficient to inter diffuse the at least two metals. In order to economically deposit the metal layer, especially when larger thicknesses are required, e.g., up to 3.0 mil, the layer can be provided by plating at least one layer of each of the at least two metals on the bottom surface of the integrated circuit chip and inter diffusing the at least two metals. Alternatively, they can be simultaneously plated or vacuum deposited.
The housing is preferably made of a material, e.g., AlSiC, having a coefficient of thermal expansion close to that of the integrated circuit chip. The present invention is especially applicable to high power integrated circuit chips, e.g., microwave monolithic integrated circuits, e.g., those made of GaAs or InP, which generate considerable heat which, according to the present invention, can be conducted away from the chip through the eutectic material.
The eutectic material can be, e.g., a gold-tin eutectic. When the housing has a gold layer on a surface to which the integrated circuit chip is attached, the eutectic layer, as deposited, can be slightly tin rich (e.g., gold:tin ratio of 78:22) in order to accommodate diffusion of gold from the housing surface to provide the eutectic proportions (80:20). A metal barrier layer, e.g., nickel for preventing diffusion of materials from said integrated circuit chip into the eutectic metal layer is preferably provided on the back surface of the chip.
The chip is attached to the housing by heating the metal layer to a temperature above its eutectic temperature, e.g., by heating the metal layer to a temperature 30-55° C. above its eutectic temperature in an inert atmosphere. No scrubbing is required.
REFERENCES:
patent: 3802065 (1974-04-01), Vosburgh
patent: 5027997 (1991-07-01), Bendik et al.
patent: 5089439 (1992-02-01), Lippey
patent: 5156998 (1992-10-01), Chi et al.
patent: 5470787 (1995-11-01), Greer
patent: 5668033 (1997-09-01), Ohara et al.
Lau James Chung-Kei
Pilkington Geoffrey
Antonelli Terry Stout & Kraus LLP
Jones Josetta I.
Niebling John F.
TRW Inc.
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