Piezoelectric Pb(Zr_x, Ti_1 x)O₃thin film cantilever and bridge acoustic sensors for miniaturized photoacoustic gas detectors

Abstract

Novel, highly sensitive piezoelectric acoustic sensors based on partially unclamped Pb(Zrx, Ti1−x)O3 (PZT) coated cantilever and bridge have been fabricated by silicon micromachining. High sensitivity at low frequencies (5–100 Hz) has been achieved by patterning very narrow slits (3 to 5 µm) around the structures. A typical response of 100 mV Pa−1 and a noise equivalent pressure of 1.6 mPa Hz1/2 at 20 Hz have been measured using a 10 pF charge amplifier. Stress compensation, dry etching and integration of high performance piezoelectric thin films were the key issues. PZT/Pt/SiO2 stacks have been patterned by reactive ion etching and stress compensation has been achieved by compensating the PZT film's tensile stress by adjusting the thickness of a thermal SiO2 layer. The integration of sol–gel PZT films with a transverse piezoelectric coefficient e31,f of −12.8 C m−2 has been realized without any degradation of the properties. The microphones were successfully integrated into a miniature photoacoustic detector and tested for CO2 detection. Concentrations down to 330 ppm could be measured with significant signals.

Characterisation of PZT thin film micro-actuators using a silicon micro-force sensor

“…Devices were made from doublesided polished 100 mm (1 0 0) silicon on insulator (SOI) wafers, n-type (10-20 cm) with a handle layer thickness of 400 m and three device layers of 50, 100 and 200 m. A 600 nm thick silicon oxide layer is grown by wet oxidation at 1000 • C. Piezoresistor windows are opened using mask 1 and the oxide is etched by reactive ion etching (RIE) in a mixture of CHF 3 and Ar gases. Another thinner layer of silicon oxide is grown at 950 • C in oxygen.…”

“…Piezoelectric micro-electro-mechanical systems (MEMS) have been studied in many areas related to precision position control [1], acoustic [2], pressure and gas sensors [3]. Lead zirconate titanate (PZT) ceramics have been extensively used due to their superior piezoelectric properties [4] for a wide range of sensors and actuators.…”

“…Ewt 3 δ 4l 3 Expression of the blocking force (1) where F is the blocking force, E the Young's modulus, l, w and t, respectively, the length, width and thickness of the beam and δ is the displacement. This, however, does not give a true representation of the blocking force as already shown on macroscale piezoelectric actuators [14].…”

Analog Front-End Circuitry in Piezoelectric and Microphone Detection of Photoacoustic Signals

“…There are many kinds of detectors used in photoacoustic equipment [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Some of them are quite sophisticated solutions-e.g., optical microphones [6,7], quartz resonator-based detectors [13][14][15], and microcantilevers for which the deflection is measured with interferometric techniques [16][17][18].…”

“…Quite often they are also patented, which prevents their wider use, particularly in commercial applications. As a result, a great part of practical implementations of photoacoustic equipment still uses detectors much simpler, and T. Starecki (B) Institute of Electronic Systems, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warsaw, Poland e-mail: tomasz@starecki.com; t.starecki@ise.pw.edu.pl thus much cheaper and easier to use, based on piezoelectric transducers [9][10][11] or microphones [1][2][3][4].…”