⇒ OVERAL SUMMARY:

• collapse mode provide gains in transduction efficiency

  ⇒ due to higher electric field & higher capacitance

• fractional bandwidth higher in collapse mode operation

  ⇒ higher frequency cut-off

  • collapse mode causes effective membrane size to decrease → higher frequency cut-off
  • note: CMUT also acts as low pass filter to electrical signal (TX, RX) → total capacitance increases → cut-off frequency can decreases

  ⇒ lower frequency cut-off

  • collapse mode causes effective membrane size to fall → spring constant increase → low-frequency cut-off increases

• pronounced hysteresis behavior

  • explained below

⇒ TX in collapse mode

• higher amplitude than conventional by 91%
• uncorrected bandwidth of received one-cycle signal increase by 78%
  • 2.01 MHz at 35 V DC
• linearity of device increase by 0.1 percentage point

⇒ RX in collapse mode

• higher amplitude than conventional by 75%
• uncorrected bandwidth of received one-cycle signal increase by 13%
  • 1.06MHz at 54 V DC

⇒ Pulse-Echo

• higher amplitude than conventional by 255%
• absolute bandwidth of received one-cycle sine signal increase by 236%
• corrected fractional bandwidth 140%

⇒ Device Linearity

linearity: how much data points deviate from a linear model

nonlinear distortion: ratio of power in second harmonic to power in fundamental frequency of TX or RX signal

• CMUT in reception (RX): maximum deviation from linear input-output behavior lower than 0.7%
• CMUT in transmission (TX): largest deviation from a line through 0V AC and 16V AC data (normalized by 16V AC) points was less than 4.5%
  • measured improvement of collapse mode linearity - 0.1 percentage point

DISCUSSION:

• impedance of liquid medium > impedance of CMUT membrane within transducer bandwidth
  • average displacement of CMUT membrane dominated by bulk properties of immersion medium (oil)
  • RX:
    • membrane displacement (x) small compared to average gap (d) between electrodes (less movement of membrane)
    • nonlinear distortion is proportional to (x/d)
      • in collapse mode, d decreases and x increases due to higher electrostatic forces
      • increase is nonlinear distortion
  • TX:
    • nonlinear distortion is proportional to ratio of amplitude of AC signal and DC bias
    • in collapse mode, DC bias is large and AC excitation could be large too

⇒ Hysteresis

• collapse mode has most hysteresis
  • charging might be a cause ⇒ this only translates bias voltage with no clear impact on hysteric behavior
  • material and properties
    • in another paper, single crystal membrane was used ⇒ collapse takes abruptly and snapback takes place in stages
    • in this study (different material) ⇒ not evident (snapback occurs suddenly)
      • type of material in contact with bottom electrode might have effect
  • non-uniform adhesion
    • adhesive hysteria: deformation of material due to adhesion
    • non-uniform adhesion between membrane face and bottom electrode along membrane surface causes adhesive hysteria