LapTech
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Laptech Precision Inc. news

QCM Quartz Microbalance for sensing applications. - May 2012

Quartz blanks have been used for decades to detect mass in vacuum thin film deposition processes. Typically an AT cut quartz crystals operating at 5MHz or 6 MHz fundamental mode are used for this as they are both robust and sensitive.

The frequency shift of the crystal can be calibrated to output an equivalent thickness of whatever metal is to be deposited.
This phenomenon is also useful in detecting mass increases in liquids and gases. Those elements are commonly referred to as QCMs or Quartz Crystal Microbalances.

The demands for more sophisticated measurements using QCM require quartz blanks made to a very high standard. Typically an optically polished blank is preferred as it produces the most even and consistent surface. Care is taken to ensure the removal of pits and scratches as well as ensuring the lapped surface prior to polish is perfect.

This ensures the most efficient and consistent electrical coupling.
Electrodes are plated to allow electrical connection, most commonly Gold with a Chrome adhesion layer underneath.

Once again care is taken in the pre cleaning and plating to ensure very good adhesion.
Some available design options from LapTech Precision:

Contouring.
? Applying a curve to one side of the blank increases the Q factor and short term and thermal stability. Especially useful at lower operating frequencies.
Wrap around plating.
? A special plating technique that allows connection to one side of the sensor. Makes connections more simple especially in liquids
Integrated heating elements.
? Heating elements can be applied as part of the plating process in order to add self heat functionality. This can be used for thermal compensation or degassing etc.
Overtone operation.
? A well designed blank can operate on several overtones as a way of increasing resolution and improving signal to noise ratio.
Doubly rotated quartz.
? Both IT and SC cuts have found uses in QCM applications where better thermal stability is desired at higher than ambient temperatures.
High frequency blanks.
? As a general rule quartz blanks become a problem to handle when thinner than 75 microns. Blanks can be designed with a thinner center section leaving a support ring to aid handling. Fundamental frequencies above 100 MHz can be realized with this approach. These are referred to as inverted mesa blanks.


 

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