Vapor Concentration Monitoring

Problems with vapor delivery are often sporadic, appearing only for certain parameter settings and resulting in time-varying vapor delivery. It is difficult to diagnose such problems by examining deposition results.

It is possible to use non-invasive techniques to monitor vapor concentration in the gas flow. One method is to measure the speed of sound in the gas. A schematic of the test setup is shown below:


Recall that the velocity of sound is proportional to the square root of (1/M) where M is the molecular mass. Adding heavy vapor molecules slows the sound velocity and thus lengthens the transit time in the cell. Shawn Murphy of ASM Thermal Systems has reported that 1% resolution for TEOS concentration in N2 is achievable. The availability of real time measurements of vapor concentration is a powerful tool for diagnosing and correcting vapor delivery problems.

A commercial sensor based on this principle, the Piezocon vapor sensor, is available from Veeco (

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