About

How does a Quartz Crystal Microbalance work?

A quartz crystal microbalance is a very sensitive scale, able to weigh as little as a few billionths of a gram. A QCM measures a mass per unit area by measuring the change in frequency of a quartz crystal oscillator. The frequency increases or decreases by the addition or removal of a small amount of mass deposited on the surface of the quartz sensing crystal. The QCM is generally used under vacuum, in gas phase to detect molecular contamination. Frequency measurements can be made to very high precision. Using 15MHz base frequency crystals allows mass measurements in the nanogram range to be made.

Quartz naturally exhibits the piezoelectric effect. This effect causes a thin wafer of quartz, plated with metallic electrodes, to mechanically deform when a voltage is applied across it. An AC voltage applied across the crystal will induce it to oscillate at its resonant frequency with great precision. Any molecules landing and sticking to the surface of the oscillating crystal will cause its frequency to decrease. By measuring this frequency change, the amount of mass deposited on the surface can be determined.

The Sauerbrey equation can be used to determine the mass change per unit change in frequency:

Where f_o is the fundamental crystal frequency, ρ_q is the density of quartz, μ_q is the shear modulus, and A is the active crystal area between the electrodes.

Operating Guidelines

The TQCM is shipped with a base frequency of approximately 1500 Hz. The output frequency of the sensor is produced by taking the difference, or beat frequency, between the “sense” crystal and a second “reference” crystal. The sense and reference crystals operate at 15MHz. Each one Hertz change represents 1.96 x 10-9 g/cm² being deposited on the surface.

The sensor has a dynamic range for non-solid films of about 15 kHz. Non-solid contaminants can be residues from coatings, adhesives, lubricants or cleaning agents. As these non-solid contaminants collect, they will damp the oscillation of the quartz sense crystal. At about 15kHz, the sensor may “damp out” and stop producing an output frequency due to an overload of contamination. If this occurs, the temperature of the sensor should be raised so that these contaminants can be driven off. This “baking off” of contaminants should return the sensor frequency back to the vicinity of its base frequency. Once the contaminants are driven off, the sensor crystal will resume oscillating and the frequency output should reappear. As a general rule, it is best to keep the sensor temperature high, i.e +60 to +80°C, until you are ready to make an actual contamination measurement. This will keep the sensor relatively clean until the measurement is made. Typical measurement temperatures are in the range of -20°C to 0°C where contaminants will collect much more readily.

The ability of molecular contaminants to collect on the surface of the TQCM is dependent on its temperature. High temperatures drive the contaminants off the sensor surface, and lower temperatures allow the contaminants to stick and collect.

Two 15Mhz crystals are used in the sensor. The output frequency is produced by taking the beat frequency (difference) of these two oscillating crystals. The reference crystal is set approximately 1500 Hz higher than the sense. As contamination collects on the sense crystal, its frequency will naturally drop, but by taking the beat frequency relative to the fixed and slightly higher (1.5kHz) reference crystal, the output frequency of the QCM electronics module registers a frequency increase as contamination collects. Working in the kHz range allows the output signal of the sensor to be transmitted easily over long cables.

Changing the temperature of the sensor will cause the output frequency to change regardless of any contamination present on the sensor. (AT cut quartz crystals are used to minimize this to approximately a few hundred Hz between -20°C and +80°C). This is caused by the natural frequency vs. temperature dependence of the individual quartz crystals in the sensor. The two crystals used in the sensor are temperature matched (i.e chosen with similar freq/temp curves) in order to minimize this natural effect. By temperature matching the two crystals and then taking their beat frequency, much of the temperature/frequency dependence is subtracted out. When making a contamination measurement, as long as the sensor is kept at a fixed temperature, then the measurement will be a true measure of the collected contamination and it will not be affected by the frequency/temperature dependence of the individual crystals.

The TQCM is able to heat and cool by using a thermoelectric device (heat pump). During cooling, the device pumps heat out of the sensor head and into the sensor base. This creates a temperature difference, or ΔT between the sensor head and base. The TQCM can achieve a maximum ΔT of 70°C. The waste heat is removed via the heat sink clamped to the sensor base. Therefore, the sensor must be bolted to a mounting bracket in the vacuum chamber that provides a good thermally conductive path (approx 5W @ 20°C) to ambient, or an active cooling loop can be used. The maximum ΔT must not be exceeded or damage to the sensor's thermoelectric device will result. To achieve the absolute sensor minimum of -59°C a mounting bracket of < +10°C must be provided.

When mounting the TQCM sensor/electronics/heat sink assembly to a bracket inside the vacuum chamber, clamp the perimeter heat sink to the base of the sensor first, then tighten the four mounting bolts to the bracket. A good thermal conductive path is critical to allow the thermoelectric device (heat pump) inside the sensor to pump waste heat out when in cooling mode. Improper heat sinking can damage the sensor!