Method of Reducing Moisture Content of Hermetic Packages Containing Semiconductor Devices

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.

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 5000 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.

According to the present invention there is provided a method of reducing the water moisture content which comprises incorporating within the package a small but effective amount of a cyanate ester. The cyanate ester is reacted with water moisture to produce an intermediate imidocarbonic acid which becomes molecularly rearranged upon formation into a stable carbamate molecular structure. Upon exposure of the molecular carbamate structure to an elevated temperature such as would be used to cure the die attach adhesive securing the semiconductor device to the substrate, the carbamate further reacts with water to produce carbon dioxide. The presence of carbon dioxide within the hermetic package is not deleterious or damaging to the semiconductor device and corrosion of the device, as may be induced by the presence of water moisture within the package, is thereby avoided.

FIG. 1 is a description of the chemical reactions which the cyanate ester undergo to reduce the moisture content within a hermetic package; and

FIG. 2 is a graph showing the effects of aging at 150.degree. C. comparing the amount of moisture and carbon dioxide which is released.

To reduce the moisture content of a hermetic package it is necessary to incorporate within the package a small but effective amount of cyanate ester. The cyanate ester undergoes the chemical reactions described in FIG. 1. As can be seen, the cyanate ester reacts with the water moisture to produce an imidocarbonic acid as an intermediate which quickly rearranges upon formation into a stable carbamate. The carbamate, upon exposure to an elevated temperature of at least 100.degree. C. yields carbon dioxide which is not injurious to semiconductor device in the package.

The cyanate ester may be incorporated within the package in any suitable manner. One manner of incorporating cyanate ester in the package is to incorporate it in a die attach adhesive. Examples of suitable die attach adhesive compositions in which cyanate ester has been incorporated appear below in Table I.

The moisture content in a hermetic pack in which the cyanate ester has been incorporated has been found to be extremely low. In fact, the moisture content is even lower than that of a blank cavity or of package which does not contain any adhesive or other source of moisture.

The data shown in Table II compares the moisture content for polyimide, epoxy die attach materials, a blank hermetic package, and a package in which cyanate ester has been incorporated. As can be seen, the moisture content in a package employing epoxy resin but which does not include cyanate ester within it, is 57,500 ppm. as compared to less than 100 ppm. moisture in a comparably sized ceramic package in which cyanate ester has been incorporated.

To illustrate additional advantages of incorporating cyanate ester in a package that contains moisture, further thermal cycling tests were conducted in which the packages were thermally cycled from -65.degree. C. to 150.degree. C. over 1000 cycles and then stored at 150.degree. C. for 1000 hours. The results of these tests indicate that the packages containing cyanate ester still retain low moisture content even under these tests conditions.

Two common methods of sealing hermetic packages containing semiconductor devices are to braise a lid, such as a gold plated "Kovar" lid, onto the package with a suitable gold alloy such as an Au-Sn alloy. Another method is glass sealing. In glass sealing a glass is used to form a sandwiched seal between a ceramic lid and a ceramic substrate. Generally, glass sealing is more popular because of its lower cost. However, current techniques of glass sealing require sealing temperatures well above 400.degree. C. and are therefore not suitable for many applications because such elevated temperatures may damage some semiconductor devices.

Another method of incorporating a cyanate ester into the standard ceramic hermetic package is to apply a small amount of cyanate ester to the backside of the lid, such as either a "Kovar" lid or ceramic lid. Such a formulation may comprise a cyanate ester compound. The formulation may also include fillers, alumina and/or silicon carbide. The package and lid is then sealed at an elevated temperature, e.g., 275.degree. C. to 400.degree. C. For the cyanate ester to adhere to the lid best, a small amount of alkylphenol and/or a metal curing catalyst may be added to the cyanate ester to increase adhesiveness.

It is apparent from the foregoing that by including cyanate ester within a hermetic package containing a semiconductor device, the moisture content within the package may be significantly reduced so as to avoid injury or damage to the semiconductor device contained in the package. It is also apparent that within the purview of the invention the cyanate ester may be included in various ways to achieve this function.