Abstract:
Disclosed is a method of forming an opaque coating on an integrated circuit or multichip module. A coating composition is prepared and then heated to a temperature sufficient to transform the coating composition to a molten state. Next, the molten coating composition is applied to a surface of the integrated circuit device to form an opaque coating that overlies active circuitry on the surface, to prevent optical and radiation based inspection and reverse engineering of the active circuitry.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to opaque coatings for electronic devices. In particular, the present invention is an opaque protective coating and method of applying the coating to integrated circuits and multichip modules. The 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 wafer level process that includes selectively applying a coating composition comprising a silica precursor resin and a filler onto the surface of complete IC wafer. A wafer is defined here as a slice of semiconductor crystalline ingot used for substrate material when modified by the addition, as applicable, of impurity diffusion (doping), ion implantation, epitaxy, etc., and whose active surface has been processed into arrays of discreet devices or ICs by metallization and passivation. 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 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.&#39;s method is not particularly cost effective or efficient on a mass production level. Also, the Bearinger 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 IC or MCM packages. Therefore, protection is not provided for packaging components such as wire bonds, bond pads, and inteconnects. 
     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 limited with respect to security and is also time consuming and therefore not particularly cost effective nor efficient on a mass production level. 
     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 abrasion resistant, adherent, radiopaque and optically opaque to prevent inspection and/or reverse engineering of the topology of the integrated circuits and multichip modules. 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. 
     SUMMARY OF THE INVENTION 
     The present invention is an opaque coating and a method of forming an opaque coating on a semiconductor integrated circuit device. To form the opaque coating on the integrated circuit device a coating composition is prepared. The coating composition is then heated to a temperature sufficient to transform the coating composition to a molten state. Next, the molten coating composition is applied to a surface of the integrated circuit device to form an opaque coating that overlies active circuitry on the surface so as to prevent optical and radiation based inspection and reverse engineering of the active circuitry. 
     This protective opaque coating can be applied to semiconductor integrated circuit devices, such as integrated circuits and multichip modules, in a time efficient and cost effective process to permit coating application on a mass production level. The protective coating can be applied in whole or in part to assembled MCM and IC devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of an integrated circuit or multichip module prior to the application of a protective opaque coating in accordance with the present invention. 
     FIG. 2 is a schematic elevational view of the protective opaque coating being applied to the integrated circuit or multichip module shown in FIG.  1 . 
     FIG. 3 is a sectional view similar to FIG. 1 of the integrated circuit or multichip module with the protective opaque coating applied thereto. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A semiconductor integrated circuit device, such as an integrated circuit (IC) or multichip module (MCM)  10  to be coated in accordance with the present invention is illustrated generally in FIG.  1 . The IC or MCM  10  includes a single, active circuitry semiconductor chip  12  (in the case of an IC) or multiple, active circuitry semiconductor chips  12  (in the case of a MCM). The semiconductor chip(s)  12  is mounted on a surface  13  of a substrate  14  and includes lead wires  16  that are connected to pads  18  also mounted on the surface  13  of the substrate  14 . The pads  18  serve as ports for electrical connection to external sources (not shown). The substrate  14  with the chip(s)  12  and pads  18  mounted thereto is housed within a ceramic package  20  defined by a base member  22 , a lid member  24  and a lid seal  26  (the lid member  24  and lid seal  26  not being shown in FIG.  1 .). 
     The IC or MCM  10  is coated with a protective opaque coating  28  (see FIG. 3) by a thermal spray process  29  illustrated in FIG.  2 . The thermal spray process  29 , of the present invention, is a line of sight coating process that includes a thermal spray gun  30  having a nozzle  31 . A heat energy source  32  is delivered to the nozzle  31  (in a known manner) to heat a ceramic particle based coating composition  33  also delivered to the nozzle  31  (in a known manner). The heat energy source  32  uses a flame  34  to heat the coating composition  33  to a molten state defined by molten liquefied particles  35 . The molten liquefied particles  35  defining the coating composition  33  are carried to the IC or MCM  10  by a carrier gas jet  36  also delivered to the nozzle  31  (in a known manner). The IC or MCM  10  is supported on a support element  38  that may act as heat sink during the coating process. 
     The thermal spray process  29  first requires the preparation of the ceramic particle based coating composition  33 . It is desirable that the chemistry of the coating composition  33  be similar to the chemistry of the materials of the IC or MCM  10 , such that attempted removal of the protective opaque coating  28  (formed from the coating composition  33 ) from the IC or MCM  10  (for inspection and/or reverse engineering of the topology of the IC or MCM) via chemical methods will simultaneously destroy the IC or MCM  10 . In the present invention, the coating composition  33  may be a single chemical component or a multi chemical component composition, partially or entirely formed from any one of alumnina, beryllia, silica, silicon carbide, aluminum nitride, fused alumina-titanium oxide, fused alumina-titanium dioxide and nylon or alumina-titanium oxide, barium titanate, or other ceramic oxides or silicates and teflon. In one preferred embodiment fused alumina-titanium oxide was found to provide a desirable coating composition  33  for the protective opaque coating  28 . 
     The coating composition  33  is prepared by manufacturing the chemical materials of the coating composition  33  into a powder or sintered rod having particle sizes within the range of ten microns to sixty microns. Particle sizes in excess of sixty microns tend to cause mechanical damage to the IC or MCM  10  due to that force at which the carrier gas jet  36  delivers the molten liquefied particles  35  to the IC or MCM  10 . Particle sizes less than ten microns tend to cause transformation of the particle based coating composition  33  into a liquid stream (rather than molten liquefied particles  35 ) that may be difficult to control during the application process. In one preferred embodiment, a coating composition  33  prepared in the form of a sintered rod with the coating composition  33  having a particle size within the range of ten microns to twenty microns is desirable. 
     Once the coating composition  33  is prepared, the coating composition  33 , the heat energy source  32  and the carrier gas jet  36  are simultaneously delivered to the nozzle  31  of the thermal spray gun  30 . The heat energy source  32  can take the form of a plasma arc, an electric arc or a fuel gas. In one preferred embodiment, the heat energy source is a fuel gas  40  (preferably acetylene) which is combined with oxygen  42  to create that flame  34  that is of a temperature sufficient to transform the ceramic particle based coating composition  33  to molten liquefied particles  35 . In one preferred embodiment, this temperature is in the range of between 200° C. and 2500° C. The molten liquefied particles  35  are applied to the IC or MCM  10  via the carrier gas jet  36  which carries the molten liquefied particles  35  to the IC or MCM  10  and causes the particles  35  to impact upon the IC or MCM  10 . The molten liquefied particles  35  undergo a “splat” upon impact with the surface of the IC or MCM  10 , and then coalesce to form a contiguous coating that thickens with continued successive depositions of the molten liquefied particles  35  to form the lamellar protective opaque coating  28 . In one preferred embodiment, the carrier gas jet  36  is pressurized nitrogen which is delivered to the nozzle  31  of the thermal spray gun  30  in the range of 10-100 cfm. 
     As seen in FIG. 2, in practice, the nozzle  31  of the thermal spray gun  30  is positioned above the IC or MCM  10  which is held in place by the support element  38  which can draw heat away from the IC or MCM  10  during the application process. Typically, the nozzle  31  is positioned from the IC or MCM  10  within the range of between five inches and seven inches. In one preferred embodiment, the nozzle  31  is positioned six inches from the IC or MCM  10 . The molten liquefied particles  35  can be applied in successive layers or as a single burst depending upon the desired coating thickness and the thermal limitations of the IC or MCM  10 . In one preferred embodiment, the thickness of the formed protective coating  28  is in the range of between 1 mils and 100 mils. The molten liquefied particles  35  are applied by moving the nozzle  31  of the thermal spray gun  30  back and forth over the surface of the IC or MCM  10 , or by moving the IC or MCM  10  relative to the nozzle  31 , or by moving both the nozzle  31  and the IC or MCM  10  relative to one another. In one preferred embodiment, the nozzle  31  is moved relative to a moving IC or MCM  10 . 
     Once the molten liquefied particles  35  are applied, they form a lamellar protective opaque coating that adhesively bonds to the surface of the IC or MCM  10  and is abrasion resistant, provides a hermetic seal, and prevents both active and passive, chemical, optical and radiation based inspection and/or reverse engineering of the active and inactive circuitry of the IC or MCM  10 . As seen in FIG. 3, the formed protective opaque coating  28  completely covers the semiconductor chip(s)  12 , lead wires  16 , pads  18  and the surface  13  of the substrate  14  housed within the base member  22 . However, the protective, opaque coating  28  may be applied so as to only partially or completely cover any one of or more of the semiconductor chip(s)  12 , leads  16 , pads  18  and/or surface  13 . Once the protective opaque coating  28  is formed, the lid seal  26  and the lid member  24  are mounted on the base member  22  to further hermetically seal the IC or MCM  10 . 
     The molten liquefied particles  35  can be applied to the surface of the IC or MCM  10  (to achieve complete coverage as shown in FIG. 3) in 15 to 600 seconds. The protective opaque coating  28  can be fully applied and cooled and the IC or MCM  10  ready for use in only 1 to 70 minutes. Therefore, the thermal spray process is capable of producing inspection and/or reverse engineering proof IC&#39;s or MCM&#39;s  10  in a time efficient and cost effective manner that permits coating application on a mass production level. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.