Capacitive micromachined ultrasonic transducers using commercial multi-user MUMPs process: Capability and limitations

Abstract

The objective of this work is to construct capacitive micromachined ultrasouind transducers (cMUTs) using multi-user MEMS (MicroElectroMechanical Systems) process (MUMPs) and to analyze the capability of this process relative to the customized processes commonly in use. The MUMPs process has the advantages of low cost and accessibility to general users since it is not necessary to have access to customized fabrication capability such as wafer-bonding and sacrificial release processes. While other researchers have reported fabricating cMUTs using the MUMPs process none has reported the limitations in the process that arise due to the use of standard design rules that place limitations on the material thicknesses, gap thicknesses, and materials that may be used. In this paper we explain these limitations, and analyze the capabilities using 1D modeling, Finite Element Analysis, and experimental devices. We show that one of the limitations is that collapse voltage and center frequency can not be controlled independently. However, center frequencies up to 9 MHz can be achieved with collapse voltages of less than 200 volts making such devices suitable for medical and non-destructive evaluation imaging applications. Since the membrane and base electrodes are made of polysilicon, there is a larger series resistance than that resulting from processes that use metal electrodes. We show that the series resistance is not a significant problem. The conductive polysilicon can also destroy the cMUT if the top membrane is pulled in the bottom. As a solution we propose the application of an additional dielectric layer. Finally we demonstrate a device built with a novel beam construction that produces transmitted pressure pulse into air with 6% bandwidth and agrees reasonably well with the 1D model. We conclude that cMUTS made with MUMPS process have some limitations that are not present in customized processes. However these limitations may be overcome with the proper design considerations that we have presented putting a low cost, highly accessible means of making cMUT devices into the hands of academic and industrial researchers. Keywordscapacitive micromachined ultrasonic transducer; MUMPS; finite element analysis

Performance Evaluation of a Loeb-Eiber Mass Filter at 1 Torr

“…The Loeb-Eiber mass filter used in this work was produced using the multi-user MEMS process (MUMPS) (MEMSCAP Inc., Durham, NC, USA) [43][44][45][46]. The MEMS based mass filter had square cross-section electrodes of 25 μm×25 μm with an 8 μm interelectrode spacing.…”

Characterization and Modelling of Circular Piezoelectric Micro Speakers for Audio Acoustic Actuation

“…Advantages of piezoelectric type transducers include smaller size, lower costs, batch fabrication, and integration with CMOS, which may allow for fabrication with required electronics such as amplifier or other signal conditioning circuits. Their main disadvantage is the difficulty of finding foundry standard processes which include piezoelectric layers for research and inexpensive prototyping, in contrast with the other main types of micro ultrasonic transducers: the capacitive type (cMUT), which can be fabricated with multiuser MEMS processes [18] or simply with standard CMOS foundry processes [19]. Impedance matching with radiating media properties, in our case air, is also an important issue: the acoustic impedance of air (400 kg/m 2 s) is considerably smaller than that of piezoelectric materials commonly used in ultrasound imaging (30 × 10 6 kg/m 2 s), for which matching techniques have to be employed.…”

Air-coupled linear and sparse cMUT array manufactured using MUMPs process

“…Previous cMUT designs have successfully been made using MUMPs (Oppenheim et al 2003;Liu et al 2009;Octavio et al 2006;Octavio et al 2007a). Oppenheim et al (2003) studied the behaviour of cMUT device manufactured using MUMPs in both gas and solid coupled arrangements in order to analyse the membrane dynamics in the different coupling arrangements.…”

“…Oppenheim et al (2003) studied the behaviour of cMUT device manufactured using MUMPs in both gas and solid coupled arrangements in order to analyse the membrane dynamics in the different coupling arrangements. Liu et al (2009) reviewed the limitations and capabilities of cMUTs made with MUMPs; an additional dielectric layer was proposed in order to mitigate shorting between the membrane and substrate. Previous work by one of the authors (Octavio et al 2006) analysed the layer combinations of the MUMPs process in order to realise an efficient transducer design.…”

“…The forgoing art demonstrates the capability of the MUMPs process for the realisation of efficient devices-analysis of more complex 1-D and 2-D array configurations and their associated behaviour is merited and forms the basis of this paper. Some of the electrical problems (electrical breakdown, electrical losses, shorts at pull-in voltage, parasitic capacitances) that were observed in initial designs (Oppenheim et al 2003;Liu et al 2009;Octavio et al 2006Octavio et al , 2007a are addressed in the current context.…”