Quartz crystal microbalance
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A quartz crystal microbalance (QCM) (or piezoelectric microbalance) measures mass by measuring the change in frequency of a piezoelectric quartz crystal when it is disturbed by the addition of a small mass such as a virus or any other tiny object intended to be measured. It can work under vacuum or liquid environment thus making it useful to determine the properties of polymers and adhesion of proteins. Frequency measurements are easily made to high precision, hence, it is easy to measure small masses. Correlation between mass and frequency is achieved by means of the Sauerbrey equation. In addition to measuring the frequency, the dissipation is often measured to help analysis. The dissipation is a dimensionless quantity inversely related to the resonance frequency and decay time constant. As such it gives a qualitative measure of the damping in the system.
A very common use of quartz crystal microbalances is as a thickness monitor in thin film technology, mostly under vacuum. There the QCM sensor head is placed near to the sample and deposited as well. The ratio of the amount of deposition on the sample to that on the sensor is called the 'tooling factor'. One way to determine the tooling factor is to move the sensor head between the sample position and the measurement position; the ratio between the deposition rates in the two positions gives the estimated tooling factor. Another way is to use a laboratory technique such as scratch tests or white light interference to measure the actual thickness deposited on the sample after the deposition is over; the ratio of this thickness to what the sensor measured is the estimated tooling factor.
Other techniques for measuring the properties of thin films include Dual Polarisation Interferometry and SPR.
