Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Packaging or treatment of packaged semiconductor
Reexamination Certificate
1995-10-27
2001-11-20
Talbot, Brian K. (Department: 1762)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Packaging or treatment of packaged semiconductor
C427S077000, C427S096400, C427S126200, C427S453000, C427S454000, C438S064000, C438S106000
Reexamination Certificate
active
06319740
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to opaque coatings for electronic devices. In particular, the present invention is a multilayer opaque protective coating and method of applying the multilayer coating to integrated circuits and multichip modules. The multilayer coating inhibits inspection and reverse engineering of integrated circuits and multichip modules.
Opaque coatings and methods of applying opaque coatings to electronic devices to inhibit inspection and reverse engineering are generally known. U.S. Pat. No. 5,399,441 to Bearinger et al. discloses one such method of forming an opaque coating on an integrated circuit. In Bearinger et al., an opaque ceramic coating is formed on an integrated circuit by a process which includes selectively applying a coating composition comprising a silica precursor resin and a filler onto the surface of the integrated circuit. A liquid mixture that includes the silica precursor resin and the filler is selectively applied to the integrated circuit by (1) masking the circuit, applying the liquid mixture and removing the mask, (2) selectively “painting” the circuit or (3) silk screening the circuit.
The coated integrated circuit is then heated at a temperature sufficient to convert the coating composition (i.e., liquid mixture) to a silica containing ceramic matrix having the filler distributed therein. Preferably, the integrated circuit with coating composition thereon is heated in a Lindberg furnace at a temperature within the range of about 50° C. to 425° C. for generally up to six (6) hours, with less than about three (3) hours being preferred, to convert the coating composition to a silica containing ceramic matrix. In Bearinger et al. the preferred silica precursor resin is hydrogen silsesquioxane resin (H-resin). To achieve a coating opaque to radiation, a filler comprising insoluble salts of s heavy metals is combined with the silica precursor resin. To achieve a coating impenetrable to visual light, an optically opaque filler is combined with the silica precursor resin.
Because the method of applying the opaque coating to an integrated circuit of Bearinger et al. requires an extensive heating time period to transform the coating composition to a silica containing ceramic matrix, Bearinger et al.'s method is not particularly cost effective or efficient on a mass production level. The Bearinger, et al. coating does not provide full protection since the liquid mixture is applied to the integrated circuit at the wafer level and before assembly of the actual devices into integrated circuit or multichip module packages. Therefore, protection is not provided for packaging components such as wire bonds, bond pads, and inteconnects. In addition, though the opaque coating formed by the process of Bearinger et al. may prevent reverse engineering of the underlying circuit, Bearinger et al.'s opaque coating does not protect the underlying integrated circuit from the effects of electromagnetic interference and/or high energy radiation (such as may exist in outer space, in sensor and control electronics of commercial nuclear power generating plants and/or in equipment designed to survive nuclear explosions).
The U.S. Pat. No. 5,258,334 to Lantz, II discloses another process of applying an opaque ceramic coating to an integrated circuit. In Lantz, II, visual access to the topology of an integrated circuit is denied via an opaque ceramic produced by first mixing opaque particulate with a silica precursor. This mixture is then applied to the surface of the integrated circuit. The coated integrated circuit is then heated to a temperature in the range of 50° C. to 450° C. in an inert environment for a time within the range of one (1) second to six (6) hours to allow the coating to flow across the surface of the integrated circuit without ceramifying. The coated integrated circuit is then heated to a temperature in the range of 20° C. to 1000° C. in a reactive environment for a time in the range of two (2) to twelve (12) hours to allow the coating to ceramify. As with the above described Bearinger et al. patent, the method of applying the opaque coating of Lantz, II is time consuming and therefore not particularly cost effective or efficient on a mass production level. Likewise, as with the above described Bearinger et al. patent, the resulting coating does not provide full protection since the liquid mixture is applied to the integrated circuit at the wafer level and before assembly of the actual devices into integrated circuit or multichip module packages. Therefore, protection is not provided for packaging components such as wire bonds, bond pads, and inteconnects. In addition, as with the Bearinger et al. opaque coating, the opaque coating formed by the Lantz, II process does not protect the underlying integrated circuit from the effects of electromagnetic interference and/or high energy radiation.
There is a need for improved protective coatings for integrated circuits and multichip modules. In particular, there is a need for an improved protective coating that is both radiopaque and optically opaque to prevent inspection and/or reverse engineering of the topology of the integrated circuits and multichip modules. In addition, the protective coating should be capable of sheilding an underlying integrated circuit or multichip module from the adverse affects of electromagnetic interference an/or high energy radiation. Moreover, the protective coating should be capable of being applied to integrated circuits and multichip modules in a time efficient and cost effective process to permit coating application on a mass production level. Finally, there is a need to apply the protective coatings to the wire bond and interconnects in integrated circuit and multichip module packages. These areas are unprotected using a wafer level coating.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method for applying thermal coatings to integrated circuits in a way that does not damage the circuit elements and connections so that multilayers of coatings can be applied to make such circuits resistant to inspection (reverse engineering) and damage from electromagnetic energy and radiation.
According to the invention, coating composition particles are selected so that they can enter a molten state in a flame spray and impact the circuit without causing damage to the circuit components. If the particles are too large they will damage the circuit. If they are too small they will cool too quickly and the coating will be imperfect. The flame spray with the molten particles is passed over the circuit at a distance at which the particles are still molten but heating to the circuit is at a safe level. Successive passes are made, building up the coating layer.
According to the invention, coating materials are selected to absorb different electromagnetic frequencies, such as visible light and X-rays, and the layers are built-up to a thickness that provides a desired level of attenuation.
According to the invention, a coating composition of a radiation resistant material is built-up to a thickness that protects the circuit from the damaging effects of the radiation.
A feature of the invention is that, by applying layers of different coatings, the circuit can be protected from reverse engineering as well as damage from external energy sources.
Another feature of the invention is that the process can be performed quickly on small circuits without damaging the delicate components and the thickness of the coatings is uniform along the circuit.
Other objects, benefits and features of the invention will be apparent to one of ordinary skill in the art from the drawings and detailed description.
REFERENCES:
patent: 3170813 (1965-02-01), Duncan et al.
patent: 5258334 (1993-11-01), Lantz, II
patent: 5290354 (1994-03-01), Haluska
Anderson Curtis W.
Heffner Kenneth H.
Greenstien Robert E.
Honeywell International , Inc.
Rendos Thomas A.
Talbot Brian K.
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