Micro-Opto-Mechanical Pressure Sensor (MOMPS) in Sin Integrated Photonics Platform

R. Jansen, V. Rochus, J. Goyvaerts, G. Vandenboch, B. van de Voort, P. Neutens, J. O’Callaghan, H.A.C. Tilmans, X. Rottenberg
imec,
Belgium

Keywords: MOMPS, Integrated Photonics, Pressure sensor, SiN, MZI

Summary:

This paper presents a Micro-Opto-Mechanical Pressure Sensor (MOMPS) manufactured in a SiN integrated photonic platform for short wavelength implementation. It implements a single wavelength Mach Zehnder Interferometer (MZI) system allowing a simple readout system. We report a measured resolution of 9 Pa at 1.2 Hz sampling rate for a 0.7mm diameter device, to our knowledge the first reported measured resolution for MOMPS MZI systems. The devices have a variety of attainable sensitivity and operating ranges. MOMPSs systems are predicted to have improved sensitivity and noise performance compared to their piezoelectric and capacitive counterparts [1]. MOMPS have been demonstrated using MZIs [2] or ring resonators [3,4]. These approaches typically use either tunable sources or broadband sources with spectroscopy to obtain a readout. In this paper, we demonstrate a single wavelength system based on a SiN integrated photonic MZI in the near-IR (NIR) that relies on a single readout detector that could be integrated on CMOS. We implement MOMPSs in a novel high quality integrated photonics platform for applications in near-infrared (NIR) and visible. Using low temperature PECVD SiN as waveguide core and manufactured with 200 mm DUV lithography [5], it can be post-processed on CMOS imager/detector wafers. To create a pressure sensitive device the silicon substrate is etched away with a DRIE process to define SiO membrane, which carries the SiN photonic structures. Subjected to a differential pressure, the membranes deform, modifying the optical characteristics of the devices/circuits. The effective refractive index of the waveguide (neff) changes as a function of the stress in core and cladding according to their respective intrinsic opto-elastic coefficients. Importantly, the physical deformation itself also causes a change in optical path length. MOMPS MZIs consist of a multimode interferometric (MMI) splitter, two waveguide arms and a MMI combiner. One of the MZI arms is placed on a membrane subjected to a differential pressure while the other one acts as a fixed reference. The optical intensity emerging from MOMPS MZI depends on the phase difference between arms, and thus on the differential pressure it is subjected to. We show in this work that for our design space the physical path length change dominates performance. Further, we show that, although there is an optimum location for a single ring, the maximum response is obtained with a spiral to maximize the length of the guide on the membrane. Using a custom vacuum chamber we measure the differential pressure response of the MOMPS MZIs. We report on three different designs, illustrating that the range and sensitivity of the devices can be engineered. Finally, using a fine fine-grained measurement we extract the system resolution better than 9 Pa for at a 1.2Hz rate for a 0.7mm diameter membrane, currently limited by the measurement setup and the laser source. These preliminary results are comparable or better than those of commercial capacitive and piezoelectric sensors, but leave space for improvements currently under investigation.