There are 2 technologies being used in semiconductor mass flow measurement:
• Thermal distribution
• Differential pressure
Both measurements are pressure sensitive. It means a change in applied pressure will affect the mass flow measurement accuracy. Device makers are using independent pressure sensor to compensate mass flow measurement error induced by applied pressure. It seems that current trend is integrating pressure sensor to mass flow meter or mass flow controller. It is claimed as pressure insensitive mass flow meter or mass flow controller.
The advantage of using pressure sensor in MFC is the response:
With differential pressure measurement, the MFC can achieve the set/targeted mass flow much faster than thermal distribution measurement.
In differential pressure MFC, PID control can be used during flow increase ①, steady state ②, and flow decrease ③.
While in thermal distribution MFC, PID control can only be used during flow increase①.