Thin Film Deposition - why you need to control for temperature
Film thickness measurement is based on the principle that the frequency of vibration of a quartz disc decreases continuously as material is coated on its surface. As long as you know the density of the film, a process computer can calculate the thickness versus time. Unfortunately this vibration frequency is also sensitive to: 1) temperature changes, and 2) stress in the growing film. Any frequency change in the quartz crystal is interpreted as a film thickness change by the controller or monitor, no matter what causes it.
To minimize temperature dependence, monitor crystals are cut at a specific angle called the AT cut. This cut has a near zero frequency-temperature dependence at 20°C. So, for a change of a few degrees in either direction, the quartz crystal will not exhibit any frequency shift. Of course, when you get much past 20°C, this goes out the window, and the frequency does change, sometimes by significant amounts.
To further minimize temperature-related frequency changes, quartz sensors are traditionally placed in water-cooled housings, with the cooling water set at 20°C. In theory, this configuration results in the frequency-temperature behavior being minimized or even eliminated.
The trouble is…no one REALLY knows the temperature of their crystal…unless they’re using xtronix-supplied temperature-controlled sensor heads and an Eon™ monitor with temperature compensation. Not only that, but shifts of 50-100°C can occur, and because water cooling cannot sufficiently compensate, errors of 20 Angstroms or more can occur! This is especially critical for thin films of less than 500 Angstroms.
More importantly, if you're operating in a high temperature environment such as CVD (semi), ALD, and OVPD, there currently is NO solution that will work for you, as standard crystals fail in such environments. For this reason, the pioneering development of high temperature crystal sensors (RC & SuperQuartz), high temperature sensor heads (Oasis, a water-cooled, temperature controlled sensor for up to 90°C and Tempe, a waterless sensor rated up to 500°C), and the patent-pending Eon™ instrument with Total Temperature Compensation, a PC-based film thickness monitor.
When it comes to film thickness measurement and control for high temperature processes, your search is over. Call X-TRONIX LTD +41 21 802 54 90
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