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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 2, APRIL 2014
Surface-Micromachined CMUT Using Low-Temperature Deposited Silicon Carbide Membranes for Above-IC Integration Qing Zhang, Student Member, IEEE, Paul-Vahé Cicek, Student Member, IEEE, Karim Allidina, Student Member, IEEE, Frederic Nabki, Member, IEEE, and Mourad N. El-Gamal
Abstract— This paper presents a surface-micromachining technology to fabricate silicon carbide (SiC)-based capacitive micromachined ultrasonic transducers (CMUTs). The use of dc-sputtered amorphous SiC as a structural layer allows the fabrication process to limit the temperature to a thermal budget of 200 °C, which is the lowest reported to date, making this technology ideally suited for above-IC integration. The high Young’s modulus of the deposited SiC film, along with its very low residual stress, results in high strength and resilient CMUT membranes. The placement of the suspended aluminum electrode directly at the bottom side of the membrane reduces the effective size of the electrostatic transduction gap, resulting in superior electro-mechanical coupling. Fabricated transducers are tested in air with both continuous-wave and pulsed signals, using a pitch-and-catch configuration. The transducer pair, composed of 110-µm-diameter membrane arrays, exhibits a resonant frequency of 1.75 MHz, a 3 dB-bandwidth of 0.15 MHz, and a transmission gain of −38 dB. The CMUT prototypes showcase the versatility of low-temperature dc-sputtered SiC films applied in the field of MEMS. [2013-0137] Index Terms— Capacitive transducers, CMUT, MEMS, integration.
micromachined silicon carbide,
ultrasonic monolithic
I. I NTRODUCTION
T
ODAY, ultrasonic transducers are seeing widespread use ranging from low-intensity applications such as medical imaging, non-destructive testing and ranging, to highintensity applications like cleaning and liquid emulsification. For decades, piezoelectric transducers have been the workhorse in ultrasonics. However, capacitive-based transducers are currently attracting significant interest. A unique advantage over their piezoelectric counterpart lies in their lower mechanical impedance, offering the potential for a better impedance
Manuscript received April 30, 2013; revised August 20, 2013; accepted September 1, 2013. Date of publication September 24, 2013; date of current version March 31, 2014. Subject Editor J. A. Yeh. Q. Zhang, P.-V. Cicek, K. Allidina, and M. N. El-Gamal are with the Department of Electrical and Computer Engineering, McGill University, Montreal, QC H3A 0E9, Canada (e-mail:
[email protected];
[email protected];
[email protected]; mourad.
[email protected]). F. Nabki is with the Computer Science Department, Université du Québec à Montréal, Montreal, QC H3C 3P8, Canada (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JMEMS.2013.2281304
match with low-density fluidic media such as air. The operation of capacitive transducers is also much less sensitive to temperature. Miniaturization efforts to fabricate capacitive ultrasonic transducers date back to as early as 1993 [1]. During the past two decades, there has been intensive research and increasing interest towards capacitive micromachined ultrasonic transducers (CMUTs), e.g. [2]–[12]. They can be conveniently customized into 1- or 2-dimensional arrays, and the fact that they are mass-produced using microfabrication techniques shared with the semiconductor industry opens the door for low-cost batch fabrication and a highly compact formfactor. CMUTs must always be interfaced with driving or sensing electronics to enable their use, and the electronics are typically in the form of an integrated circuit (IC). For this purpose, the integration can be done in a system-in-package fashion where two dies (i.e., the CMUT die and the IC die) are stacked together, or monolithically, such that the CMUTs are fabricated within the integrated circuits die. In general, monolithic integration presents multiple benefits: i ) reduced system footprint due to the direct proximity of the circuitry and CMUTs, ii) reduced interconnection parasitics through the removal of interconnects such as bondwires, iii) simplified packaging due to the centralization of all system elements within one die. These advantages can yield systems that are more compact, have lower production costs, and exhibit higher performance and lower power consumption. Moreover, monolithically integrating a CMUT array with electronics allows for a very dense number of interconnects to be present between the CMUTs and the electronics. This enables the possibility of individually controlling the actuation phase of each CMUT element to leverage beam-forming techniques. Such tailored control would require a prohibitive number of interconnections if implemented with wirebonds between the transducer and the driving electronics in a system-in-package solution. Accordingly, this work is aimed at developing CMUT arrays which are amenable to IC monolithic integration. Although wafer–bonding techniques have been successfully demonstrated to fabricate CMUTs, most bonding processes are high-temperature in nature and require highly planar surfaces to ensure bonding quality [13]–[15]. Therefore,
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ZHANG et al.: SURFACE-MICROMACHINED CMUT USING LOW-TEMPERATURE DEPOSITED SiC MEMBRANES
surface-micromachined CMUTs constitute a preferable option when targeting monolithic integration with electronics on a single chip. A possible path for such integration is to build the transducer side by side with the electronic components on the same substrate using a standard IC foundry process for co-fabrication. For instance in [11], [16], CMOS dielectric layers are used as CMUT membranes, while the metallization layer serves as the sacrificial material. However, this limits the material types and parameters available for building the transducers to the ones used in the IC fabrication process, which may not be optimal for the CMUT performance requirements. Alternatively, CMUTs have been successfully integrated above an IC die using silicon nitride membranes formed by plasma-enhanced chemical vapor deposition (PECVD) and the use of a chromium sacrificial layer [17]. This approach is very attractive because it does not require any alteration of the semiconductor fabrication process, and the CMUT fabrication can be implemented as a subsequent independent process module with a more optimal choice of materials and CMUT structure. Given these considerations, this work develops a process that is compatible with aboveIC integration to fabricate surface micromachined CMUT devices. Naturally, this scheme requires that the technology limit itself to IC-compatible materials and chemicals, and that the process steps be carried out at temperatures well below a specific thermal budget to avoid degrading IC performance. In addition to presenting a CMUT fabrication process that is compatible with above-IC monolithic integration, this work uses a superior silicon carbide (SiC) structural material to fabricate the devices. In the field of micro-electromechanical systems (MEMS), SiC as a structural material is generating significant interest for building strain sensors, pressure sensors, resonators, and inertial sensors because of its superior mechanical properties and resistance to harsh environments [18]–[20]. However, to the authors’ best knowledge, SiC-based CMUT membranes have yet to be reported. Since SiC has a high Young’s modulus compared to PECVD silicon nitride, it can be used to fabricate sturdier membranes to improve device reliability. At the same time, the high Young’s modulus maximizes the electro-mechanical conversion and bandwidth of the ultrasonic transducers [21]. To enable above-IC integration, SiC needs to be deposited at a low temperature. It has been shown that deposition of amorphous SiC (a-SiC) thin films by magnetron sputtering is achievable with a total thermal budget below 200 °C [22], [23]. In addition to IC compatibility, low temperature deposition allows for a MEMS fabrication process comprised of aluminum metallization and polymer materials. The deposited aSiC film has a very low stress (
