area of oxide posts should be small enough that charges trapped in them have negligible effect on device operation
the configuration of the posts should prevent the devices from shorting in the entire mode of operation
if post-contact-operation is required, the configuration of posts should provide, without hysteresis, the frequency response desired after the membrane makes contact with the post
central contact area of conventional CMUT is larger than for postCMUT
after contact ⇒ resonance frequency of a conventional CMUT (smaller equivalent membrane size) was higher than that of postCMUT
(a) conventional CMUT (b) 5-point post pattern
no hysteresis in postCMUT (very minimal charging occurs at membrane-post contact)
contact force during membrane-post contact affects a wider operating voltgae mode in postCMUT with cross pattern than in point-pattern
hysteresis in cross-pattern is more severe
CV curves of postCMUTs (a) with point-pattern (b) with cross-pattern
severe hysteresis in conventional CMUT
may cause permanent sticking of membrane to bottom electrode due to field from induced charges trapped in oxide layer (prevent snapback)
may cause CMUT CV curve to shift along bias axis (> 20V)
postCMUT allowed application of higher bias voltage
gain a corresponding performance increase without suffering from hysteresis or charging
less transmission and reception efficiency compared to CMUT in collapsed mode
after contact, high frequency cur-off of postCMUT doesn’t change with bias (membrane shape doesn’t change after contact)
normal CMUT high frequency cut-off increases with bias because membrane shape decreases
