Fabrication of a MZI device based on waveguides of SiN for biological detection

Abstract

Integrated Optical (IO) devices have been gained much attention for chemical or biological sensing applications due to high sensibility, mechanical stability and the integration in Silicon [1–3]. In addition, the Si technology in conjunction with techniques of micromachining has enabled the creation of a variety of novel functions in Micro-Opto-Electro-Mechanical Systems (MOEMS). biological sensing uses the evanescent wave theory for detecting change in the refractive index or light absorption induced by the analyte contact. Spectrophotometric analysis is used to determine the concentration or amount of a particular component in an analyte [6]. Usually, the samples need to be sent to a laboratory for the analysis, and the results take time. The need for rapid and on-line measurements led to the development of microsystems with the substance, the detection and the readout electronics circuits all integrated in a single system with advantages that include: small sample volume, system integration, automation of measurement, short response time, improved analytical performance and reduction of time and cost. The most IO devices based in MZI have been fabricated SiN using high temperature (750–900°C) process in LPCVD system, which results in a challenge for the optical-electronic device integration. In contrast, PECVD SiN films for the MZI waveguides used in this work were fabricated at low temperature (∼300 °C) with some similar optical properties to LPCVD and without requirements of a post-annealing temperature. In this work is presented a simple sensor based in Mach-Zehnder Interferometric (MZI) configuration using silicon nitride waveguides deposited by Plasma Enhanced Chemical Vapor Deposition. The aim of this work is to use this micro-interferometer device for biological sensing using the absorbance of light. The light travels through an area of an arm of the waveguide called micro-cavity where the analyte under study is attached. This light is compared with a reference arm of the waveguide, which contain the reference substance. The device was fabricated by standard CMOS technology and using bulk micromachining in order to obtain the micro-cavities. The low temperature deposition of SiN thin films for the waveguide is attractive to have an integrated optic device with the read out circuits. The waveguide showed attenuation of 3% at 0.632¼m. The integrated interferometric sensor is a two arm multilayered optical integrated waveguide structure with a core layer of SiN film. For sensing purposes, in the MZI device one arm is used as the reference arm and the other arm is the sensing arm, with two micro-cavities between those, one for the analyte to be measured and the other used for a reference substance or a “blank” solution. The schematic of this simple MZI sensor is depicted in Figure 1. The micro-cavities, placed in each arm of the MZI were fabricated using KOH as an anisotropic etching solution (45% solution at 80°C) which results in an etch rate of 1.2 mm per min, in this way for the (111) substrate a membrane with a thickness of 50 mm could be produced from a wafer with a nominal thickness of 300 mm. The thermal oxide layer served as an etch-mask. The final micro-interferometric device has a total length of about 10mm, width of 4mm, with a separation of the reference and measuring arms of 3.5mm, as is shown in the figure 2.