Protection Layer Influence on Capacitive Micromachined Ultrasonic Transducers Performance

Abstract

For MEMS technology, reliability is of major concern. The implementation of a protection and passivation layer, that may easily enhance reliability of capacitive Micromachined Ultrasonic Transducers (cMUTs) must be done without degrading device performance. In this work, realization, simulation and characterization of passivated cMUT are presented. Two materials, SiNx and Parylene C, were selected with regard to their mechanical and physical properties as well as their compatibility with device processing. Particular attention was paid on layer deposition temperature to avoid a structural modification of the top aluminium electrode and, hence, a membrane bulge. The characterization results are in good agreement with the simulations. The SiN passivation layer clearly impact device performance while Parylene C effectiveness is clearly pointed out even through ageing characterizations. If SiNx layer can be used for passivation with particular precautions, Parylene is definitely an interesting material for cMUT passivation and protection.

Evaluation of Parylene as Protection Layer for Capacitive Micromachined Ultrasonic Transducers

“…Reliable performance over time requires a passivation layer mechanically and thermally stable providing effective protection against the environment. Several materials may be considered suitable (9). With regard to the above detailed requirements, Parylene seems to be the most suitable material.…”

Investigation of PDMS as coating on CMUTs for imaging

“…Parylene C [1], [3], which gives good results and has the advantage of being cleanroom compatible, but is deposited using Vapor Deposition Polymerization (VDP). Silicon nitride has also been proposed due to cleanroom compatibility, however, the stress in the nitride highly affects the device performance [4]. Different types of PDMS have also been investigated, and it is seen that some will increase the output signal, due to increased mass loading, and others will decrease the influence of the echo from the coating-water interface, due to better impedance matching [2], [3].…”

CMUTs with high-K atomic layer deposition dielectric material insulation layer

“…In low temperature surface micromachining processes for CMUTs, it is convenient to use the same PECVD SixNy (abbreviated as SiN) as the dielectric insulation material [7], [11], [16], [17] and membrane structure material. However, as demonstrated below, higher performance CMUTs can be realized by choosing a high-κ dielectric material (high dielectric constant κ as compared with silicon dioxide) with minimal adverse effects in terms of achievable device capacitance and electric field.…”