This invention relates to a method and technique of metallizing an organic layer and particularly to chrome metallization of such an organic layer.
Chrome or chromium has been widely employed in the prior art as an electroplate material as having an excellent corrosion resistance and a lustrous appearance. Chrome is relatively inexpensive and has a very hard abrasion resistive surface with a low coefficient of friction. Chrome is widely employed for many applications considering its durability and also considering the ability to utilize chrome for wire bonding to electrical circuitry.
The chrome metallization of an organic layer generally results in a multitude of cracks in the chrome due to stress and other factors. These stresses are due to the fact that the chrome has a different coefficient of expansion than the organic layer. A further problem is that due to the elasticity of the organic layer, wire bonding is difficult to implement. Such organic layers, for example, constitute organic polymers. These polymers provide electrical insulation and are widely employed in many areas. Thus, if one attempts to deposit chrome on such an organic layer, the above-noted problems exist. In addition circuit patterns cannot be reliably reproduced due to cracks in the chrome. There are two causes of stress in the chromium. One is intrinsic stress due to defects in crystal structure during deposition. The other is thermal stress. The latter is due to the fact that the coating is typically deposited or processed at a high temperature, 150.degree. C. for example. As the coating and substrate heat or cool, they expand and contract at different rates. This causes stress. Organic polymers typically expand or contract ten or more times as fast as chrome. Silicon dioxide expands only about 1.5 times the rate of chrome. As will be explained, and according to this invention, the highest thermal stress interface has been moved away from the chrome to the silicon dioxide - polymer interface.
Thus, as one can ascertain from the above, while it is extremely desirable to utilize chrome as a conductor and as a reliable element it is extremely difficult, utilizing prior art techniques, to deposit the chrome on an organic polymer.
It is therefore an object of the present invention to describe a method of applying a submicron layer of silicon dioxide to the organic material, followed by a submicron layer of chrome which is deposited upon the layer of silicon dioxide. It is indicated that after patterning of the chrome on the silicon dioxide no stress cracks were evident. It was also discovered that wire bonding to the chrome layer, as deposited on a silicon dioxide layer, was easily accomplished.