From “Experimental Characterization of Collapse-Mode CMUT Operation” (2006):

conventional operation

1. CMUT DC bias and AC excitation controlled so membrane doesn’t make contact with substrate
2. DC bias is used to create an electrostatic force to attract the membrane closer
   1. strain in the deflection of membrane balances electrostatic force
3. TX: AC excitation used to create ultrasound
4. RX: receiving waves create changes in membrane to electrode gap thereby changing capacitance and creating current output

collapse operation

1. CMUT DC bias applied so that membrane sticks to bottom electrode
   1. electrostatic force > mechanical force stopping membrane from collapsing
   2. DC bias set larger than snapback voltage to ensure membrane is in contact with bottom electrode
2. AC excitement causes membrane displacement in ring around the center (in contact with bottom electrode)

From “A New Regime for Operating CMUTs” (2003):

Why lower voltage after reaching collapse voltage?

Left shows that electromechanical efficiency (k2) is maximized at a bias voltage lower than the collapse voltage

⇒ implies that we could use high AC signal for larger ultrasound signals without worrying about collapse compared to conventional

Right shows the sensitivity of the CMUT at collapse (marked as 2)

⇒ larger displacement (50A/V) and increased output power (displacement^2) in collapse mode VS 10A/V in conventional

⇒ maximum (86V) can be used to operate membrane with high displacement with no risk of snapback

Comparison Between Collapsed Mode and Conventional Mode CMUT

Capacitance

• Capacitance increases when in collapsed mode
  • higher electrostatic force due to smaller gap
  • hysteresis occurs due to charging effect

Resonance Frequency

• Collapse mode

  • Resonance Frequency (Center Frequency) higher than in conventional mode

    • center frequency shift

    ⇒ effective membrane size (ring shaped) decreases in collapse mode

    ⇒ decrease in membrane size increases resonance frequency due to higher electrostatic force

  • After collapse mode (before snapback), width of ring shaped membrane increases, causing resonant frequency to decrease

• Conventional Mode
  • Resonance Frequency decreases due to spring softening: (Huang 2005)

Fractional Bandwidth (FBW)

• Huang (2005): collapse mode has higher FBW than conventional

  • 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