Thursday, November 3, 2022

Selecting the best crystal for your coating process

A quartz crystal microbalance (QCM) takes advantage of the piezoelectric effect found in quartz crystals. Application of an electric potential across the quartz crystal induces mechanical shear strain in the crystal. If the polarity of this electric potential is reversed, the strain direction reverses. Rapid oscillation of the electric potential polarity leads to vibrational motion of the quartz crystal. Under the proper conditions, this vibration can induce an acoustic standing wave between the two crystal faces. The frequency of the standing wave is proportional to the thickness of the quartz crystal. If additional material is uniformly deposited on the face of the crystal, the additional thickness will decrease the resonant frequency of the acoustic wave. This frequency shift due to mass deposition may be correlated to the absolute mass deposited via the following substituted form of the Sauerbrey equation:

where pq is the density of quartz, Aq is the area of resonance, Nq is a frequency constant for AT-cut quartz crystals (1.668 X 105 Hz cm), Fq is the frequency of quartz prior to deposition, and F is the frequency at any point during the deposition process. This equation is only valid if the total frequency shift is kept within 2% of the starting frequency. 

In summary, the thickness of the added layer changes the wavelength of the standing wave resonance. In essence, a deposited film acts as if the quartz is increasing in thickness. The thicker the crystal, the longer the resonance wavelength. This is measured as a frequency shift at the monitor. The film density value is input in order to compensate the density difference between the film deposited and the density of quartz which is 2.648 g/cc.

The most common diameter of a quartz crystal is 14 mm, which is seen with Inficon, Balzers, Satis, Maxtek, Sycon, Intellimetrics and Colnatec. However, 12.5 mm is the second most common size, and this is seen with Ulvac.

The average thickness of a crystal is: 0.01076″ at the center.  The typical shape of a crystal is described as Plano-Convex. Meaning the crystal is flat on one side and curved on the other side. 

Given the wide variety of coating technologies available to the thin film engineer and scientist, it can be difficult to determine the best monitor crystal type for a given process. We will attempt to generally categorize processes and the appropriate crystals in the following:

Low-Stress Metallizing

The most common thin film process is the deposition of metals such as aluminum, gold, copper, and silver to provide electrical contacts or optical reflectance. These films are relatively free of tensile or compressive stress and are deposited at room temperature. They are soft and easily scratched but do not tend to flake off or damage substrates. 

Such films can easily be monitored using either gold, silver, or alloy electrode crystals. One can deposit over 30’000Å of gold or 200’000Å of aluminum on 6 MHz X-TRONIX Quality Quartz Crystals before changing to a new sensor. 

Recommended crystal: XQI-8010G (6 MHz / Gold)


High-Stress Metallizing

Please see details on our web site: X-TRONIX | Quality Quartz Crystals

Dielectric (Optical) Material Coating

Please see details on our web site: X-TRONIX | Quality Quartz Crystals

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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