Thursday, January 6, 2022

Quartz Crystal Longevity

Even if you water-cool your crystal sensor to exactly 20°C, you can encounter an even bigger problem. Ever had a crystal abruptly fail when you were coating a substrate with a high stress film such as magnesium fluoride or silicon dioxide? You probably thought it was the dreaded “bad crystal” problem. Well, regardless that millions are made, periodically a few get past QC. But even when a “good crystal” is used, abrupt failure still occurs. In some cases it is simply a spatter caused by the deposition source. That will kill any crystal. But by and large, the greatest source of early crystal failure is stress build-up in the film being monitored. This problem got minimized when the “alloy” quartz crystal was invented back in 1987. This is actually an ultra thick aluminum coated crystal. The extreme thickness of the aluminum minimizes the stress build up in the crystal, leading to significantly longer life, often 100 to 200 percent longer. It works, and most optical coating laboratories use aluminum instead of the thinner gold electrode crystals. You can make gold thicker, but it tends to dampen out the crystal vibration if you get too extreme.


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