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We are often times asked for the "one minute explanation" of how a film thickness monitor works. I cannot speak for all of the products of all manufacturers, but the following is a simplified explanation of the operation of the SPI #12162-AB SPI-Module™ Quartz Crystal Thickness Monitor (QTM).
Quartz crystals, when electrically energized, have the property of producing an electrical current which oscillates at a fixed frequency; the frequency depends on the mass of the crystal, and the stability of the quartz crystal allows the design of, for example, very precise time pieces, like wrist watches. The QTM basically (I am ignoring a lot of electronics here for simplicity) consists of two crystals, one in the instrument and the other in the stage. Each crystal oscillates at a slightly different frequency. When a coating is applied to a sample, it is also applied to the sensor of the QTM, increasing the mass of the crystal, which decreases the crystal's resonant frequency of vibration.
The electronics of the system treat each oscillation cycle of each crystal as a pulse and count the difference in pulses per unit time between the two crystals. This is sometimes referred to as the "frequency counter". At the start of coating, the difference is set as "zero" thickness . The difference between the two oscillation frequencies (I hope you're still following me) is compared with the zero value. The system is calibrated for the density of the material being deposited by setting the number of counts per unit time which equal 1 angstrom (0.1 nm) of coating thickness (the instrument makes this calculation for you. The readout is in increments of 1 nm. The built- in calibration can be further refined by coating a test sample, sectioning it and measuring the actual thickness of material deposited; it is possible to adjust the instrument internally, but it is much easier to construct a calibration curve which accounts for the material you deposit and the way you deposit it.
Is the QTM the most precise way to measure coating thickness? No, it is not. It is intended to allow the process to be controlled while it is going on, but it is no substitute for sectioning a witness sample to determine what actually happened.
It is actually the mass of the coating per unit area, and not its thickness, which is of concern. Unlike other methods, the QTM actually is measuring the mass of material deposited. Thus it is actually more relevant to the concern of the analyst than is the normal method of assuming a density and measuring the thickness of deposited material. I know that this sounds like hair splitting, but it is precisely this kind of hairs that are the difference between good quantitation and unreliable results.
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