What You Need to Know About Quartz Crystals

By Paula L. Becker
When process problems occur, crystals are often the first and only ingredient in the coating process that gets the blame. Low activity readings, rate spikes, and/or crystal failures do not always indicate crystal problems—these problems may actually reveal other hidden process issues. By focusing on the crystal, you may be overlooking other more significant process problems.

Fil-Tech recommends the following: Use Longer Life Gold R or Alloy coated crystals instead of Gold coated when depositing SiO2, magnesium fluoride, or similar high stress dielectric materials. Alloy and Longer Life Gold coatings dissipate the stresses of deposited films by plastic yielding or flowing. This results in a longer life, more stable crystal with a longer period of steady, jump-free oscillation.

Move the sensor head away from deposition splatter and shield the crystal from direct exposure to radiant heat sources. Material splatter on crystal surface can register as a significant mass increase and cause positive or negative rate spikes and/or crystal failure.

1. Do not rely solely on Activity Data to determine a “good” or “bad” crystal sensor. Activity is the ability of the crystal measurement circuit to conduct current. Poor contact between the quartz crystal, sensor head, feedthrough and/or cables can impede the ability of the crystal measurement circuit to conduct current. Low Activity readings can be caused by excessive coating on the sensor head, damaged cables, shorts in the feedthroughs, and/or contact misalignment in the sensor head.

2. Frequently inspect your sensor head and feedthrough hardware. Check the finger spring contacts in the sensor head and adjust or replace when bent, broken or flattened. Keep the sensor head cap clean. Keep cables in good condition and snug to the sensor head and feedthrough.

3. Do not touch crystals with fingers or hands. Always use plastic tweezers around the edge of the crystal during handling. Never touch the center of the crystal—oil, dirt, dust or scratches will degrade the ability of the crystal to vibrate.

4. Maintain the crystal sensor water temperature at a constant between 20 and 50 degrees Celsius to promote better film adhesion. The hotter the deposition, the closer the crystal should run to 50 degrees. Keep your water temperature stable to within 1-2 degrees Celsius to give superior results. Check water lines for clogs (from high mineral content of water) to maintain cooling water temperatures.

5. Store your unused crystals at standard room temperature between 20 and 24 degrees Celsius and standard room humidity between 40 and 60 percent. Do not store crystals in particulate laden environments, in the presence of volatile material, oils, sulfur, halides, ozone, iodine, or oxidizing agents, or in areas with excessive mechanical vibrations.

Help solve your process problems with Scanning Electron Microscope (SEM) analysis. Your “problem” coated or uncoated crystals can be used to help solve your process problems through SEM analysis for examining and investigating surface morphology of thin and thick film coatings. SEM analysis can magnify the surface of the crystal up to 100,000 times and can identify evaporant splatter, fingerprints, and other causes of crystal failures on the crystal surface. A SEM Report, complete with surface analysis conclusions, electrical findings and Digital Microscopy Imaging, can provide significant process results. Written by Paula L. Becker, vice president and co-owner of Fil-Tech, Inc. and Cold Springs R&D, Inc. Fil-Tech and Cold Springs are the leading manufacture and supplier of Consistent Quality Quartz Crystals R, electron beam gun and ion source replacement parts, ion, convection and thermocouple gauges, and mechanical and vacuum pump fluids. For additional Technical Information, visit www.filtech.com.

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May/June LabTalk 2017