Materials with low moisture outgassing properties and method of reducing moisture content of hermetic packages containing semiconductor devices
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 08/187,797 filed Jan. 25, 1994, now abandoned, which is a division of application Ser. No. 08/032,679 filed Mar. 17, 1993, now abandoned which is a continuation-in-part of application Ser. No. 07/843,735 filed Feb. 28, 1992, now U.S. Pat. No. 5,195,299 issued Mar. 23, 1993. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method of reducing the water moisture content in a hermetic package containing a semiconductor device to minimize the possibility of damage to the semiconductor device which may be caused by the presence of water in the package. 
     BACKGROUND OF THE INVENTION 
     Hermetic packages for semiconductor devices are typically ceramic packages sealed with caps or lids using glass or metal seals. Hermetic sealing is employed to prevent entry into the package of undesirable chemicals which could damage the semiconductor device contained therein. However, the presence of moisture, even in very small or trace amounts, within the hermetic package can also damage the semiconductor device. To avoid any damage to the semiconductor device from moisture contained within the package after hermetic sealing, it is necessary to have the moisture content less than 5,000 ppm, otherwise moisture induced corrosion failure of the semiconductor device may result. 
     One of the sources of moisture in the package can be the inorganic adhesive used to attach the semiconductor device to a substrate. To avoid the possibility of introducing moisture to the package from the adhesive, inorganic adhesive such as silver filled glass paste have been used. These adhesives do not release moisture upon heating for curing. However, common die attach adhesive containing polymers, epoxy, polyimide, etc., give off moisture upon heating for curing and therefore cannot be used where low moisture conditions within the hermetic package are required. 
     The present invention provides a method of reducing the water moisture content in a hermetic package containing a semiconductor device to prevent damage to the semiconductor. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided materials with low moisture outgassing properties which generally comprise monomers with at least one OCN functional group. In order to have good adhesiveness to ceramic substrates and to be capable of rapid curing, it is desirable to include a curing catalyst. The monomer containing an OCN functional group must also be reactive with moisture so that the moisture content of a package to be hermetically sealed can be reduced as the OCN group is polymerized to produce a low moisture resin upon curing. It is also possible to use a partially polymerized monomer with an OCN group as well as an unpolymerized monomer for this purpose. The low moisture outgassing material may be formulated as an adhesive composition by blending with reactive diluents and incorporating in addition to a curing catalyst other known components which aid in imparting rapid curability to the composition such as metal curing catalyst, alkylphenols, etc. Presently preferred monomers with OCN groups include bismaleimide-triazine and phenolic triazine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to 4 are diagrams of curing reactions for (1) the monomer function group --O--C.tbd.N-- and the formation of a triazine ring; (2) dicyanate monomer; (3) bismaleimide-triazine; and (4) multifunctional phenolic triazine. 
     FIG. 5 is a diagram of the mechanism for low-moisture outgassing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The moisture content of a hermetic package may be reduced by incorporating therein a small but effective amount of a resin that contains a least one functional group (O--C.tbd.N) having low moisture characteristics. Such material may be conventionally incorporated in the hermetic package as an adhesive in which case it should include a curing catalyst to assure that the adhesive composition is capable of being rapidly cured. Where a blended composition is used it is important to form a homogeneous mixture before curing. 
     The resin containing at least one OCN functional group may additionally be blended with reactive diluents which will also maintain low moisture properties. The reactive diluents may additionally assist in controlling or adjusting viscosity and facilitating the reaction with the OCN group of the monomer to form a thermally stable network to maintain thermal stability. As an adhesive formulation, the composition should be adjusted so that it may be cured rapidly, i.e., within less than five minutes at 200° C. Other filler materials such as thermal and/or electrical conductive fillers as are known for die attach adhesives may be included as well. 
     It has been determined that all resins that contain the functional group (O--C.tbd.N) have very low moisture outgassing during curing and subsequent processes. During the curing reaction, all resins with this functional group form some type of polytriazine network. Examples of suitable commercially available resins are: Dicyanate monomers, i.e., supplied by Ciba Geigy (formerly Rhone-Poulenc and Hi-Tek); bismaleimide-triazine available from Mitsubishi Gas Chemical, which is similar to dicyanate, and multifunction phenolic triazine, available from Allied Signal and Ciba Geigy. The curing reaction is of the polymerization type with two main constituents, bismalimide and triazine. The resin is cured by heating without evolution of gas by forming triazine rings, triazine-imidazole rings, oxazole rings, etc. and multifunctional phenolic triazine. All cyanate esters are cured by additional polymerization. There are no outgassing byproducts in the reaction. The repeated unit of the cured polymer has the same molecular weight as the monomer, i.e., the monomer is not split up. Curing reactions of the above-described resins are shown in FIGS. 1-4. 
     The mechanism for low moisture outgassing is shown in FIG. 5. As indicated the cyanate reacts with moisture to form a carbamate and at temperatures about 200° C. the carbamate reacts to produce carbon dioxide. 
     In addition to the foregoing, resins containing at least one OCN group, as described above, can be blended with other compatible polymer resins and still maintain low moisture characteristics. Resins that are compatible include the following: 
     Epoxy--conventional epoxy curing reaction yields unacceptably high levels of moisture during curing. However, by blending as little as 20% of a resin containing at least one OCN functional group, the moisture outgassing can be reduced to acceptable levels. 
     Polyester (saturated), polyether. 
     Diene rubbers--acrylonitrile butadiene rubber, acryl rubber, polybutadiene, polyvinyl butyral, high polymer natural resins. 
     Thermoplastics--including polyetherimide, polysulfone, polyether sulfone, polyarylate, polyimide, polyimidesiloxane. 
     In addition to the foregoing, monomers containing at east one OCN functional group may be blended with diluents. For example, to adjust the viscosity the resin may be blended with suitable diluents while still maintaining the low moisture characteristics. Typical diluents useful for adjusting viscosity include: 
     Acrylic acid esters, such as trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, polyethyleneglycol dimethacrylate, etc. 
     Triallyl isocyanurate, diallyl phthalate, castor oil. 
     Glycidylethers. 
     As can be seen from the foregoing, resins that contain at least one functional group OCN yielded low moisture characteristics. Also, blends of such resins with compatible polymers will also result in compositions with low moisture characteristics as do blends of the resin with reactive diluents. 
     A convenient form of the material is as an adhesive formulation which can then accomplish the dual purpose of serving as an adhesive to bond the hermetically sealed package as well as to incorporate the low moisture material in the package. Examples of die attach adhesive formulations of this kind appear below. The electrically and/or thermally conductive filler in an amount not to exceed 90 wt. % may also be present. 
     
         ______________________________________                                    
EXAMPLES OF DIE-ATTACH                                                    
ADHESIVE FORMULATIONS                                                     
                  A        B        C                                     
______________________________________                                    
Dicyanate (LIO)   20                                                      
Bismaleimide Triazine (BT 3109)                                           
                           20                                             
Phenolic Triazine (PT 30)           20                                    
Copper Naphthenate                                                        
                  0.01     0.01     0.01                                  
Silver Flake      80       80       80                                    
Total             100      100      100                                   
Evaluation: 600 × 600 MIL Die Attached on Alumina                   
Adhesive Strength (lbs)                                                   
                  &gt;800     &gt;800     &gt;800                                  
Cavity Moisture (ppm) (RGA)                                               
                  100      280      170                                   
______________________________________                                    
 Note                                                                     
 the cavity moisture level for A, B and C are equivalent and well exceeded
 military specification of &lt;5000 ppm                                      
 
    
     
         BLENDS OF CYANATE ESTER WITH OTHER RESINS                                 
1) EPOXY: EXAMPLES OF ADHESIVE FORMULATION                                
                                    100%                                  
EXAMPLE           D        E        Epoxy                                 
______________________________________                                    
Cyanate Resin     10       5                                              
Epoxide Resin     10       15                                             
AG Flake          80       80                                             
Copper Naphthenate                                                        
                  0.01     0.01                                           
Total             100      100                                            
Evaluation: 600 × 600 MIL DIE Attached on Alumina                   
Adhesive Strength (lb)                                                    
                  &gt;800     &gt;800     400                                   
Cavity Moisture (ppm) (RGA                                                
                  1500     2500.    50000                                 
______________________________________                                    
 Note:                                                                    
 Examples D &amp; E will meet the military specification requirement, while th
 100% epoxy does not.                                                     
 
    
     
         Example           F        G        H                                     
______________________________________                                    
Cyanate Resin     15       15       15                                    
Polyester         15                                                      
Acrylonitrile Butadiene    15                                             
Polyimidesiloxane                   15                                    
Copper Naphthenate                                                        
                  0.01     0.01     0.01                                  
AG Flake          70       70       70                                    
Total             100      100      100                                   
Adhesive Strength (lbs)                                                   
                  &gt;800     &gt;800     &gt;800                                  
Cavity Moisture (ppm)                                                     
                  500      700      600                                   
______________________________________                                    
 Cavity moisture reduced by blending cyanate resin.                       
 
    
     
         BLENDS OF CYANATE WITH DILUENTS                                           
Example           I        J        K                                     
______________________________________                                    
Cyanate Resin     6.5      9        7.5                                   
Trially Isocyanurate       9        3.75                                  
Trimethylolpropane                                                        
                  6.5               3.75                                  
Trimethacrylate                                                           
Cu Naphthenate    0.01     0.01     0.01                                  
AG Flake          87       87       85                                    
Total             100      100      100                                   
Evaluations 600 × 600 MIL DIE on Alumina                            
Adhesive Strength (lb)                                                    
                  &gt;800     &gt;800     &gt;800                                  
Cavity Moisture (ppm)                                                     
                  172      288      456                                   
______________________________________                                    
 I, J &amp; K still maintained low moisture.                                  
 
    
     It is apparent from the foregoing that various changes and modifications may be made without departing from the invention. Accordingly, the scope of the invention should be limited only by the appended claims wherein what is claimed is.