Three-axis piezoresistive accelerometer:

Basic features:

• Commercial 0.8 μm CMOS technology was used
• Three-layer structure formed by cap glass, micromachined silicon and bottom glass
• Detection of three-axis acceleration, and signal processing to extract each component of acceleration,
  are performed by the on-chip CMOS circuitry
• Signal processing circuits are formed by a commercial 0.8 μm CMOS technology
• For detection of acceleration, p-MOSFETs are used as stress sensitive elements
• Sensing principle is based on piezoresistive effect of p-type inversion layer in p-MOSFETs
• Since p-MOSFETs are standard elements in CMOS circuits, it is convenient to use them
  in CMOS integrated sensors as sensing elements
• Piezoresistive coefficient of p-MOSFETs does not strongly depend on the CMOS technology used
• Piezoresistive coefficient of conventional piezoresistors is quite sensitive to their impurity concentration
• One of four sets of folded beams surrounding the seismic mass is shown in the part b of the preceeding figure
• Folded beam was designed to have the maximum sensitivity in a die size
• Four sensing p-MOSFETs are formed on the folded edges of each beam where the largest strain
  is generated by beam deflection
• Sensing elements are put at a distance from the boundary in order to minimize the strong variation of the sensitivity
  that occurs near the folded edge of beams
• Device performance of this accelerometer is not so sensitive to device fabrication error
• Sensing p-MOSFETs in this accelerometer are standard p-MOS devices formed in the commercial
  CMOS fabrication process
• Gate size of p-MOS piezoresistor devices is 45 μm in length and 140 μm in width, respectively
• Area of gate was designed to be relatively large so as reduce 1/f noise from them.
• Stress detection circuit using the four sensing p-MOSFETs on each beam is shown in following figure:

• Each detection circuit can detect the direction (polarity) and amplitude of deflection at the centre of each beam structure

• Detected strain signal is amplified by the detection circuit at the same time

• If the centre of the beam (connection point with the mass) is moved upwards from the chip surface,
  generated stress becomes:

  • Tensile on MOS-A and MOS-B

  • Compressive on MOS-C and MOS-D

• Differential mobility change occurs between the two MOSFET pairs in the input stage of the differential amplifier
  due the piezoresistive effect in p-MOS inversion layer

• Output voltage of the detection circuit for each beam V_n (n = 1..4) is expressed by the following expression:

• In this expression:

  • A total voltage gain of the amplifier

  • V_offset input offset voltage of the amplifier induced by stress

  • C constant which depends on circuit design

  • p_pMOS piezoresistive coefficient of the p-MOSFET

  • s_AB induced stress on MOS-A and MOS-B

  • s_CD induced stress on MOS-C and MOS-D

  • T_in translation constants between induced differential stress on each beam, n
    and each component of input acceleration in the direction i

  • a_i component of input acceleration vector in the direction i

• Three-axis components of acceleration are calculated from the signals of the four detection circuits
  by using the following expressions:

• Accelerometers reported in literature:
  • Accelerometer with dimensions 3 x 3 mm2 having the sensitivity of 2.45 mV/g in x and y axis and the sensitivity of 12.5 mV/g in z axis
  • Accelerometer with dimensions 6 x 6 mm2 having the corresponding sensitivities are 31 mV/g and 167.5 mV/g, respectively.