The alloy electrode crystal is the single most important innovation in dealing with the issue of thin film stresses on a crystal. For optical coatings utilizing silicon dioxide or other similar dielectric materials, this crystal can extend the useful life of the sensor by 100 to 1000% or more when compared to the industry standard gold crystal. Under a microscope, film stress essentially causes a cracking of the thin gold electrode layer. As this layer is damaged the crystal’s resonance frequency can be erratic leading to rate “noise.” The extreme durability of the alloy layer minimizes the stress build up on the crystal, which is the leading cause of frequency life decreases and crystal failure.
Furthermore, resonance frequency shifts due to electrode adhesion failure are reduced by 90% or more under standard laboratory conditions. This improvement appears to be material specific, and in some cases, deposition specific, since it does not eliminate early crystal failure for all dielectrics. However, generally speaking, alloy crystals are an excellent choice for use in most optical coating applications. The only reason gold crystals are widely used today is simply because they were the original crystal designed for use in a film-thickness monitoring system and therefore have become very commonplace and accepted in the industry. Unfortunately the industry can be very slow to adapt and therefore the alloy crystal has a much under-appreciated potential for many processes.
