Oasis™ Dual or Single Sensor Head
Heated Sensor Head with Embedded Thermocouple
The Oasis™ sensor head embodies a revolutionary approach to quartz crystal film thickness monitoring — temperature measurement and control. Traditionally a crystal is water-cooled to minimize the frequency drift of AT-cut quartz when it is exposed to heat. This drift adds an error to the thickness measurement. But how do you know if the cooling is actually working? The simple truth is, you don’t. Conventional sensor heads don’t have a means of measuring temperature, let alone controlling it. In a world of nanogram measurements, it seems odd that any world-class manufacturer would tell you to keep your crystal “around 20°C.”
Quartz crystals are temperature, stress and mass sensitive devices. To get an accurate measurement, you need to eliminate the temperature and stress effects, leaving only the mass change (which is converted to thickness by knowing the density). Water-cooling helps, but only as long as you can keep that temperature constant, which it never is, unless you use a dedicated chiller. But that still leaves stress, the source of all crystal headaches.
Stress is the reason crystals get noisy after coating them with dielectrics like SiO2, MgF2 and TiO2. It leads to erratic measurements and early crystal failure. Cooling the crystal exacerbates the problem. So not only do traditional sensors poorly control the variations caused by heat, they also shorten the life of the crystal — bad news all the way around. Our experiments revealed that heating the sensor to 90°C increased life by 300% or more and decreased noise up to 100%. Problems solved.
But there’s more…the Colnatec Eon™ monitor has a built-in frequency vs. temperature algorithm to smooth out the thermal drift to increase accuracy by an order of magnitude, and our RC™ quartz crystal has been independently proven to be stress and thermal shock resistant. Coat this with aluminum alloy and noise disappears, creating the ultimate film thickness sensor solution.
For additional information, refer to the following datasheets:
Oasis™ Datasheet
Eon™ Datasheet
