Contamination - Instrutech The Worker Bee CVG101 User Manual

Instruction Manual CVG101 Worker Bee

5.4 Contamination

The most common cause of all vacuum gauge failures is contamination of the sensor. Noisy or erratic readings,
the inability to set zero or atmosphere and total gauge failure, are all possible indications of gauge
contamination.
Contamination can be generally characterized as either:
A) a reaction of process gases with sensor elements, or
B) an accumulation of material on the sensor elements. Sensors that fail due to chemical reaction are generally
not salvageable. Sensors that fail due to condensation, coatings, or particles may possibly be restored by
cleaning.
A) Reactive Gases
If process gases react with the materials of construction of the sensor, the result is corrosion and disintegration
of the sensor over time. The chemistry of the gases used for plasma etching and other reactive semiconductor
processes are examples where this failure mode is possible. In this case, cleaning can't solve the problem
because the sensor has been destroyed. The CVG101 must be replaced.
If you experience this failure mode quickly or frequently, you should consider a different vacuum gauge for your
application. Thermal vacuum gauges may be available with different sensor materials that are not as reactive
with your particular process gases. The standard gold plated tungsten sensor used in the InstruTech convection
gauge is offered for use with air and inert gases such as N , argon, etc. InstruTech also offers modules with
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platinum sensors for applications not compatible with gold plated tungsten.
There is no material that is universally chemical resistant; your choice of vacuum gauge (as well as all other
vacuum components) should take into consideration the potential reactions between your process gases and the
materials of construction. Consider what effect water vapor will have when combined with your process gases
because a finite amount of water will enter the chamber during venting to atmosphere with air.
B) Oil, Condensation, Coatings, and Particles
If the failure is due to an accumulation of material in the gauge, we may be able to restore your gauge or
module by cleaning. Contamination may be as simple as condensed water, or as difficult as solid particles.
Oils and hydrocarbons: Exposure of the gauge internal surfaces to oils and hydrocarbons can result in sensor
contamination. Some of these types of contamination may be removed by cleaning the gauge. If there is the
possibility of oil back streaming from wet vacuum pumps, it is recommended that a filter or trap be installed to
prevent contamination of components of your vacuum system.
Condensation: Some gases (such as water vapor) can condense on sensor surfaces, forming a liquid coating that
changes the rate at which heat is removed from the sensor (which changes the calibration). The sensor can
often be restored simply by pumping on the gauge between process cycles. A dry N purge will help speed up
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drying, or the gauge may be gently heated provided temperature doesn't exceed the specified limit of 40 C,
operating.
InstruTech, Inc.
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