Piezoelectric Micromachined Ultrasonic Transducers Based on PZT Films
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PIEZOELECTRIC MICROMACHINED ULTRASONIC TRANSDUCERS BASED ON PZT FILMS Jacek Baborowski, Nicolas Ledermann, Paul Muralt Ceramics Laboratory, Swiss Federal Institute of Technology EPFL, CH-1015 Lausanne, Switzerland. ABSTRACT Test structures for piezoelectric micromachined ultrasonic transducers have been fabricated and investigated. The basic element consisted of a oxidized and platinized silicon membrane coated with a 2 µm thick (100)-textured Pb(Zr,Ti)O3 (PZT) thin film deposited by sol-gel techniques. SOI wafers have been applied to obtain a good definition of the silicon part of the membrane. Test devices have been characterized in air and in an insulating liquid.
INTRODUCTION Micromachined ultrasonic transducers (MUT) are investigated for phased arrays in high frequency acoustic imaging to overcome resolution and frequency limits of reticulated bulk PZT transducers applied today. The basic element consists of a micromachined membrane that is driven by either capacitive [1] or piezoelectric actuation [2, 3] (pMUT). This paper deals with piezoelectric MUT’s using Pb(ZrxTi1-x)O3 (PZT) thin films of 2 µm thickness. In a previous contribution [4] we presented thin film processing, pMUT fabrication, and characterization of single elements by means of admittance and optical interference measurements. In this article we focus on improvements of fabrication, characterization of linear arrays, and acoustic experiments in air and liquid.
FABRICATION ISSUES Two µm thick PZT52/48 thin films have been deposited by means of the sol-gel technique, using 4 annealing steps per micrometer (see [4] for further details). The films were highly {100} textured and exhibited a dense microstructure. Wet etching of the films reveals interfaces due to the annealing steps (see fig. 1(a)) because of a [Zr]/[Ti] gradient that develops during crystallization. The transverse coefficient e31,f was determined by means of the cantilever E bending method [5], the longitudinal effective coefficient d 33, f = e33 / c33 was measured by means of a double side interferometer [6]. The two coefficients are linked through the relation: E e31, f = e31 − c13d33, f 0 Berlingcourt's PZT ceramics
-10 -15
e
31,f
-2
(Cm )
-5
-20
1 µm
2 µm
3 and 4 µm
Thin films of this work
-25
3203 HD
-30 60
(a)
80
100 120 140
d
(pm/V)
33,f
(b)
Figure 1. (a) Scanning Electron Microscopy image of a wet etched 2 µm thick PZT film. (b) The effective e31,f coefficient vs. d33,f for the obtained thin films in comparison with the early PZT ceramics of Berlincourt and a commercial modern ceramics (3203HD).
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The remanent values of both coefficients are plotted in fig. 1(b) for a series of 1 to 4 µm thick films. It is interesting to note that d33,f improves with film thickness, whereas e31,f does not. The quality of the thin film material is superior to early PZT ceramics, but still a factor 2 away from modern optimized PZT ceramics (3203 HD in fig. 1(b)).
SiO2 PVD
Au/Cr
PZT _Pt /Ti _SiO2 burried _Si bulk _SiO2 wet
Figure 2. Schematic design
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