R. Haouari, R. Jansens, V. Rochus, B. Figeys, X. Rottenberg
Imec (Belgium) ; KU Leuven (Belgium),
Keywords: MEMS, pressure sensor, microphone, photonic
Summary:With a trend towards a plethora of ‘smart’ devices, the MEMS microphone sector has maintained an exponential growth in recent years. For many applications microphones need to survive harsh environments or be bio-compatible. Micro-Opto-Mechanical-Pressure Sensor (MOMPS) have been demonstrated to have promising performance, as well as characteristics which enables these applications, such as insensitivity to EM, radiation hardness and the ability for remote sensing. In this work, we investigate the use of similar devices as Micro-Opto-Mechanical-Microphones (MOMM), and list the design optimizations required for this purpose. Based on the Mach-Zehnder interferometry (MZI) principle, the microphone photonic sensor consists of a multimode interferometer (MMI) splitter, two waveguide arms and a MMI combiner (Figs.1 - 2). One of the arms consists of a spiral waveguide on a flexible membrane subjected to a pressure wave, while the other acts as a fixed reference. When the membrane deforms, the radial strain changes the spiral length. This leads to a phase difference between both MZI arms, which results in a varying output intensity from the MZI. We implemented the new design in a novel high quality integrated photonics platform for applications in near-infrared and visible (cross-section in Fig.3). Figure 4a shows a SEM cross-section of the patterned SiN waveguide. Circular membranes were defined through a backside DRIE of the substrate (Fig.4b). The membranes show little buckling due to the remaining compressive stress (~28MPa). Typical dimensions for such membranes are 200 to 1.600µm in diameter and the oxide thickness is varied from 2 to 6µm. Figure 5 shows a typical measurement. Microphones have three main characteristics that need to be optimized: the bandwidth, the dynamic range and the sensitivity. To meet the requirements of a microphone in the audible range we need to optimize the mechanical behavior of the MOMM along with the readout circuit. To meet the Nyquist criteria a bandwidth of 40kHz is required for audible applications, which introduces a lower limit to the first resonance frequency of the membrane. We have demonstrated a MOMPS with a dynamic range of 2kPa with a resolution of 7Pa (Figs. 6-7), using a simple setup comprising of a photodetector, a 10bit ADC and a basic amplifier circuit. However as this is insufficient for speech applications which requires 100 mPa resolution, a higher performance readout is required. With an improved amplifier design and a higher bit encoder (24bit), a 10µPa resolution is possible, which is lower than the human audibility threshold (20µPa). Hence, the dynamic range is expected to cover the audible amplitude range. Further crucial aspects to be considered are noise sources (mechanical and electronic) and thermal effects. In summary, we investigate design optimization and system adaptions required to move from MOMPS to MOMM, and show promising predicted performance. Considering the characteristics of the photonic sensor that allows deployment in harsh environments, MOMM are of interest for a wide range of applications.