ThermaHead™ Temperature Controlled Vacuum Sensor Heads
If you can't stand the heat, get a ThermaHead?
Ever wondered why the manual for your quartz crystal film thickness monitor instructed you to keep the sensor head at 20º C when the monitor itself had no provision of measuring it in the first place? Or did you ever wonder why you should keep the crystal at 20ºC in the first place? Why not 30 or 40 or 50º?
It's surprising that the effect of temperature on the measurement properties of a quartz crystal sensor has not been given the attention it deserves. A device that can measure mass in the picogram levels absolutely needs to be temperature monitored at the very least! But even that is only going halfway. We believe it should be temperature controlled continually, in order to obtain the most accurate film thickness values. And if you want to know more, we suggest you read the papers on temperature measurement in our technical bulletin section.
To obtain the most accurate film thickness measurements, or the most stable crystal environment for monitoring dielectric coatings, we are offering the ThermaHead™ sensor system (image to the left).
Comprised of a 304 stainless steel quartz crystal sensor head (suitable for 0.550" 5 or 6 MHz quartz crystals, the standard variety), this recently patented design combines a cartridge heater with a water-cooed crystal holder. The action of these two elements allows a set point controller, which receives temperature input from a type K thermocouple embedded in the sensor housing, to add heating as needed to maintain the crystal from a range of 25 to 90º C. If you operate your cooling from a refrigerated recirculating chiller, you can hold the sensor to a tolerance of +/-1ºC easily. If you use tap water, the fluctuating input temperature and pressure leads to a more realistic +/-3ºC. In either case, you are a quantum leap from the uncontrolled sensor environment that currently exists.
The water-cooling is an essential ingredient of this system, incidentally. With so much radiant heat present in a typical vacuum coating chamber, there is always the problem of thermal runaway. And since we wanted to use environmentally benign coolants, we stuck to water! That explains the 90ºC upper limit. We could always use refrigerated air-cooling, but we had to start somewhere...
The typical ThermaHead™ system is comprised of the following elements:
- A temperature controlled sensor head, suitable for one (1) crystal, and complete with heater, water-cooling lines (up to 30" long), embedded type K thermocouple, and shutter (if desired).
- A 2.75" ConFlat™ vacuum flange, with water lines, electrical connections for heater, crystal cable, thermocouple and (if needed) shutter air.
- A 19" rack mounted or freestanding set point temperature controller (we currently use Eurotherm™ controllers, but that is subject to change), mated to a variable DC power supply (24VDC max). The controller has an RS232 communications output, but other protocols are also available.
- All connecting cables and an instruction manual.
Since many people have unusual requirements for sensor head configurations, most units are custom made. Our base price, however, of $4500 for bare sensors (part number TAN-THSYS) and $5500 for shuttered sensors (part number TAN-THSYS-SH), will get you a turnkey heated sensor, suitable for use to 10-7 torr operation, and with a 1 year money back warranty. And of course, significantly better production results!
In the near future, we will have this in a dual sensor head version as well. But, we always welcome customer requests if you need anything special, right now. We will do our best to meet your requirements..
ThermaHead™ systems will work with any standard 5 or 6 MHz quartz crystal controller. All you need to do is hook up the crystal cable from your control unit to the BNC connector on the sensor head feedthrough. Nothing could be simpler.
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