Two clover-shaped piezoresistive silicon microphones for photo acoustic gas sensors

Abstract

Low cost CO 2 gas sensors for demand-controlled ventilation can lower the energy consumption and increase comfort and hence productivity in office buildings and schools. The photo aoustic principle offers very high sensitivity and selectivity when used for gas trace analysis. Current systems are too expensive and large for in-duct mounting. Here, the design, modeling, fabrication and characterization of two micromachined silicon microphones with piezoresistive readout designed for low cost photo acoustic gas sensors are presented. The microphones have been fabricated using a foundry MPW service. One of the microphones has been fabricated using an additional etching step that allows etching through membranes with large variations in thickness. To increase sensitivity and resolution, a design based on a released membrane suspended by four beams was chosen. The microphones have been characterized for frequencies up to 1 kHz and 100 Hz, respectively. Averaged sensitivities are measured to be 30 μV/(V × Pa) and 400 μV/(V × Pa). The presented microphones offer increased sensitivities compared to similar sensors.

QEPAS based ppb-level detection of CO and N_2O using a high power CW DFB-QCL

“…However, most of conventional microphone-based photo-acoustic cells have a resonance at low frequency values (<2 kHz), which makes them more sensitive to environmental and sample gas flow noise. Moreover, the size of the typical photo-acoustic cell is still considered to be large [14,15]. A recent modification of the conventional PAS is the quartz-enhanced photoacoustic spectroscopy (QEPAS) technique that was first reported in 2002 [16].…”

A miniaturized bulk micromachined triaxial accelerometer fabricated using deep reactive etching through a multilevel thickness membrane

“…It is a bulk micromachining process with 14-16 mask layers offering buried-and surface-conductors and resistors, a metal layer and anodic glass bonding with two layers of micromachined glass. The resulting triple stack chip set offers the capability to form cavities and openings on both sides which can been used for a broad range of applications such as microfluidic flow sensors, pressure sensors, out-of-plane accelerometers, microphones (Grinde et al 2010) or various combinations of these (Ferreira et al 2009). The bulk micromachining is performed with a four-electrode electrochemical etch stop (Kloeck et al 1989), which provides the possibility to fabricated membranes from n-wells of two different thicknesses with great reproducibility.…”

“…Finally, the wafers are stripped for aluminum and the structures are released. The combination of processes has also been used to successfully fabricate piezoresistive microphones for photoacoustic gas sensor systems (Grinde et al 2010), although not with etch-through of the full wafer thickness. An extensive description of the process is given in (Grinde et al 2010).…”

Analytic Model and FEM Characterization of Two Piezoresistive Microphone Membrane

“…The first one consists of a circular membrane with 3µm thickness, while the second microphone has a square membrane with 23µm thickness. Both of them have four piezoresistive sensing elements placed on the four suspending beams (a more detailed description of the fabrication process and the design can be found in [1]). In particular we created a FEM model of the membranes by using COMSOL software and we simulated the structures in order to find the Eigen frequencies (Fig.…”

“…The time constant of the exponential solution, gave us the possibility to estimate the minimum working frequencies of these devices. We compared the results that we found with both the mechanical resonance estimates from FEM models and the initial measurements of the devices presented in [1].…”