Quartz Crystal Microbalance

The quartz crystal microbalance (QCM) is a very sensitive instrument for measuring mass, capable of measuring nanogram quantities of substances deposited on specially prepared quartz crystals, which are typically disk-shaped. Quartz is used due to its strong piezoelectric response: vibrations in the crystal create electrical pulses and vice versa. As with the quartz crystal in a watch or crystal radio set, the crystal in a QCM has a resonant frequency where it has the greatest response to an electrical stimulus. This resonant frequency depends on the size, shape and mass of the crystal. If additional mass is deposited onto the quartz disk, the resonant frequency changes, analogous to the change in the sound frequency of a struck water glass when water is added to it. Measuring the shift in the resonant frequency provides a measurement of the mass. QCM measurements are often made in a vacuum, making semiconductor applications one of the primary uses of the technique. As a thin film is being deposited under a vacuum onto a semiconductor substrate, a QCM sensor inside the chamber measures the mass deposited onto it. The mass deposited on the sensor is measured as a proxy for the thickness of the deposited film. Around the turn of the millennium, QCM was adapted for use with liquids, which has broadened its applications substantially. With specially prepared sensors, the QCM can be used as a biosensor, measuring the mass of biological molecules that bind to the substrate. Other applications focus on surface chemistry, polymer analysis and biofilms. In addition to measuring mass, the damping of the resonant motion provides a way to measure other properties of the system, such as the viscoelasticity of the adhering film. Many QCM systems also measure electrical or electrochemical properties of the sample. Having started mainly as a process tool in the semiconductor industry, QCM is now being used more and more in the laboratory, making it one of the rare techniques that has migrated from process to lab. Field applications are also being pursued, such as detecting bacteria and viruses for security purposes. QCM sensors have also been installed in airborne and spaceborne platforms. The total market for process and lab QCM was around $9 million in 2006, with a roughly even split between process and lab applications. In addition to systems, the consumable quartz crystals themselves provide another revenue stream for participants in the market. The primary vendor of laboratory QCM tools is Biolin AB, the owner of both KSV Instruments and Q-Sense, which pioneered the use of QCM for biointerfaces in liquids. Another leading supplier of QCM products, Maxtek, was acquired in May by Inficon AG (see IBO 6/15/07). Maxtek is more involved in semiconductor applications of QCM, although they also offer laboratory systems. Other suppliers include QCM Research, Sigma Instruments and Elbatech.

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