Anodically Bonded Photoacoustic Transducer: An Approach towards Wafer-Level Optical Gas Sensors.

Simon Gassner,Matthias Eberl,Jürgen Wöllenstein,Mohammadamir Ghaderi,Carsten Von Koblinski,Katrin Schmitt,Rainer Schaller,Simon Essing

doi:10.3390/s22020685

Abstract

We present a concept for a wafer-level manufactured photoacoustic transducer, suitable to be used in consumer-grade gas sensors. The transducer consists of an anodically bonded two-layer stack of a blank silicon wafer and an 11 µm membrane, which was wet-etched from a borosilicate wafer. The membrane separates two cavities; one of which was hermetically sealed and filled with CO2 during the anodic bonding and acts as an infrared absorber. The second cavity was designed to be connected to a standard MEMS microphone on PCB-level forming an infrared-sensitive photoacoustic detector. CO2 sensors consisting of the detector and a MEMS infrared emitter were built up and characterized towards their sensitivity and noise levels at six different component distance ranging from 3.0 mm to 15.5 mm. The signal response for the sample with the longest absorption path ranged from a decrease of 8.3% at a CO2 concentration of 9400 ppm to a decrease of % at a concentration of 560 ppm. A standard deviation of the measured values of 18 ppm was determined when the sensor was exposed to 1000 ppm CO2.

Highlights

Over the last decade, the public interest in air pollution measurement has gradually increased, giving rise to high demand for air quality sensors [1,2]
There are various different approaches to classify the air quality based on the concentration measurement of air pollutants including either a specific or a mix of volatile organic compounds (VOC), total suspended particles (TSP), or relative humidity (RH) [3,4,5,6]
It was ensured that no IR light could hit and potentially disturb the microphone membrane and, that reflection caused a second pass of the light through the absorption cavity, increasing the absorption inside

Summary

Introduction

The public interest in air pollution measurement has gradually increased, giving rise to high demand for air quality sensors [1,2]. PAS sensors, on the other hand, make use of the photoacoustic effect, discovered by Bell and Roentgen in 1881, and sense CO2 indirectly by measuring sound [17,18] These sensors detect gas concentrations by introducing periodically modulated light of a specific wavelength into a detection chamber in which a target gas absorbs precisely that wavelength. As these sensors use microphones to detect the photoacoustic signal, optimized gas port geometries need to be used in order to prevent acoustical interference such as banging doors or loud conversations These gas inlets cause an increase in the response time of PAS CO2 sensors [19,20]. The approach resolves the issue of acoustic interference by concept, as the microphone is embedded within a closed cavity, without increasing the response time of the sensor

Sensor Concept

Manufacturing

Experimental Setup

Results and Discussion