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Example F001

A single axis (Z-axis) accelerometer is being designed. Suppose the chosen transduction principle is electrostatic sensing. Compare the pros and cons of the following two configuration options. Compare the following perspectives: (1) ease of fabrication; (2) sensitivity; (3) compatibility with low cost manufacturing.

Answer:

  Pros Cons
1 parallel capacitor introduces large capacitance value; since the bottom metal electrode is to be covered by LPCVD sacrificial and structural layers, the material integrity is questionable;

on the flip side, the sacrificial and structural layers may dictate the choice of metal electrode on the bottom.

2 the electrodes are both made of polycrystal silicon, and hence the processing does not involve metal layer as in option 1. the sensing is based on comb drive capacitors, which may be smaller than the parallel plate capacitor depending on the design.