Intro4u2u

Characteristic of aluminium oxide sensor

Sensitivity over wide dynamic range
Dependable Accuracy & Reliability
Fast Response
Long term calibration stability
Specific to moisture only
No calibration compensation
Suitable for in-situ use under varying process condition

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 rq 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 percent 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.

Our crystals are plano-convex, meaning they are thicker at the center than the edges.

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