Basic arrangement of a pressure sensor

Differential pressure measurement - between input1 and input2,

Gauge pressure measurement - if the input2 is closed under the atmospheric pressure conditions,

Absolute pressure measurement - when the input2 is vacuum sealed.

Sensor fundamental part - plate (membrane) of silicon or polysilicon equipped with one or several piezoresistives gauges.

The pressure difference induces a stress variation in the plate.

The resulting stress is then measured by piezoresistive gauges that are placed in specific places of a plate.

Piezoresistors can be diffused in silicon plates with different crystal orientations, along many crystal directions.

In order to make an optimal choice of a piezoresistive sensor location, the stress pattern on the surface of the plate deformed by the pressure difference should be known.

Typical geometrical forms of the sensor membrane

• Circular form
  The simplest in terms of stress estimation,
  Symmetrical and easy to analyse,
  Difficult to obtain in (100) planes by micromachining.
• Square form is more easy to obtain by micromachining
  Behaves similarly to a circular plate in the centre, but at the edges, the stress is more concentrated towards their centers.
  This fact may have as a consequence ruptures of square plates in lower pressure differences.
• Ring form
  Obtained if the centre part of a plate is not etched,
  Can be considered for certain applications where only small pressure differences have to be measured,
  Better nonlinear behaviour.