Nicolas Ledermann

{1 0 0}-Textured, piezoelectric Pb(Zrx, Ti1−x)O3 thin films for MEMS: integration, deposition and properties

Pb(Zr-x, Ti1-x)O-3 (PZT) piezoelectric thin films are of major interest in MEMS technology for their ability to provide electro-mechanical coupling. In this work, the effective transverse piezoelectric coefficient e(31,f) of sol-gel processed films was investigated as a function of composition, film texture and film thickness. Dense, textured and crack-free PZT films have been obtained on silicon substrates up to a thickness of 4 mum. Crystallization anneals have been performed for every 0.25 mum. Nucleation on the previous perovskite layer combined with directional growth leads to a gradient of the compositional parameter x of +/-20% (at x = 0.53 average composition). Best properties have been achieved with {100}-textured film of x = 0.53 composition. Large remanent e(31,f) values of -11 to -12 C/m(2) have been obtained in the whole thickness range of 1-4 mum. These values are superior to values of undoped bulk ceramics, but smaller than in current, optimized (doped) bulk PZT

Piezoelectric Pb(Zrx, Ti1−x)O3 thin film cantilever and bridge acoustic sensors for miniaturized photoacoustic gas detectors

Novel, highly sensitive piezoelectric acoustic sensors based on partially unclamped Pb(Zr-x, Ti1-x)O-3 (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 mum) around the structures. A typical response of 100 mV Pa-1 and a noise equivalent pressure of 1.6 mPa Hz(1/2) 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 films tensile stress by adjusting the thickness of a thermal SiO2 layer. The integration of sol-gel PZT films with a transverse piezoelectric coefficient e(31,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.

Sputtered silicon carbide thin films as protective coating for MEMS applications

There is a need for chemically resistant coatings that protect the exposed surface of microfluidics components. Pinhole free films with low stress and a good uniformity on flat and inclined surfaces are required. In this study, amorphous silicon carbide (SiC) thin films have been deposited by RF magnetron sputtering on hat surfaces and into micromachined cavities of Si (100). The variation of RF power, deposition pressure and substrate bias voltage have been studied. Depending on the deposition conditions, the film stress can be adjusted from - 1400 MPa to + 100 MPa. Modifications of the deposition rate and the morphology between normal and inclined (54.7 degrees) planes have been observed. Optimal chemical stability was found with slightly compressive (-100 MPa) SiC thin films. No degradation of the protective layer has been observed after 3 h in KOH at 80 degrees C

Processing optimization of solution derived PbZr1−xTixO3 thin films for piezoelectric applications

Abstract Processing optimization allowed the sol-gel fabrication of 1 μm thick, phase pure perovskite thin films with identical grain size and controlled texture. This made it possible to fabricate stress compensated beams to measure the transverse piezoelectric coefficient over the whole composition range of PbZr1−xTixO3. The highest value of -12.1 C/m2 was measured for (100)/(001) textured PZT53/47. For (111) textured films the maximum value of -8.7 C/m2 was found to be in the tetragonal phase field at 55% Ti.

Piezoelectric cantilever microphone for photoacoustic GAS detector

Abstract New micromachined pressure sensors based on PZT coated silicon cantilevers have been fabricated and integrated in a photoacoustic gas detector. PZT Sol-gel thin films texture and composition were optimized with respect to the transverse piezoelectric coefficient e31,f. A best value of -12 C/m2 was obtained with (100)/(001) textured thin films at the MPB composition. Optimum stress compensation between the different layers composing the cantilever has been studied in order to yield flat cantilevers. A high response of 150 mV/Pa with a S/N of 700 at 1 Pa and 1 Hz bandwidth has been measured. The influence of the damping chamber under the cantilever is also reported.

Piezoelectric micromachined transducers (PMUT's) based on PZT thin films

Test structures for piezoelectric micromachined ultrasonic transducers have been fabricated and investigated. The basic element consisted of a silicon membrane coated with a 2 /spl mu/m thick (100)-textured Pb(Zr,Ti)O/sub 3/ (PZT) thin film deposited by sol-gel techniques. SOI wavers 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.

Etching of RuO2 and Pt thin films with ECR/RF reactor

Ferroelectric thin films for memory and MEMS applications require noble metal or refractory metal oxide electrodes. In this paper, physical and chemical parameters during etching of RuO2 and Pr by a dual frequency ECR/RF reactor have been investigated. The removal characteristics of blanket films and films with a patterned mask (photoresist or SiO2 masks) were investigated as a function of gas chemistry (Ar, O-2, halogen gases), ion beam energy (ion extraction/acceleration voltages), substrate bias RF power and working pressure (from 5 x 10(-3) to 5 x 10(-1) Pa). The etch processes were characterized in terms of etch rate, selectivity, critical dimension, lateral uniformity and mask stability. High etching rate processes (up to 70 nm/min for RuO2 and 60 nm/min for Pt with removable photoresist mask) were obtained and a micron scale patterns demonstrated. Patterning of a multilayer stack PZT/Pt/SiO2 could be achieved with a single photolithography step

SIMULATION AND CHARACTERIZATION OF PIEZOELECTRIC MICROMACHINED ULTRASONIC TRANSDUCERS (pMUTs) BASED ON PZT/SOI MEMBRANES

Piezoelectric micromachined ultrasonic transducers for airborne and immersed applications in the frequencies from 20 kHz in liquid up to 750kHz in air have been simulated by finite element approach, 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. The unclamping of the silicon membrane at the border increases drastically the coupling factor. For unclamped structures the membranes completely covered with top electrode show the highest coupling coefficient (k2 = 5.6%). Immersed (in FluorinertTM) membranes show 7 times smaller quality factor, while the coupling factor k2 remains the same. The obtained structures were enough sensitive to detect acoustic waves in air and in liquid emitted from the same type of elements at a distance of few centimeters. Good agreemen...

Mechanisms of Pb(zr0.53Ti0.47)O3 thin film etching with ECR/RF reactor

Abstract Ferroelectric capacitive devices for memory and MEMS applications require patterned ferroelectric thin films with high anisotropic etched features. In this paper, physical and chemical parameters during etching of Pb(Zr0.53Ti0.47)O3 (PZT) by a dual frequency ECR/RF reactor have been investigated. The removal characteristics of blanket films and films with a patterned mask were investigated as a function of gas chemistry (Ar, halogen gases), substrate bias RF power and working pressure (from 5 × 10−4 Pa to 1Pa). The etch processes were characterized in terms of etch rate, selectivity and mask stability. High etching rate processes (up to 70 nm/min with removable photoresist mask) were obtained and micron scale patterns were demonstrated. The impact of the etch process on the PZT surface layer modification was characterized by AFM, SEM, TEM and XPS. A strong influence of process chemistry and RF bias power on etching selectivity and surface topography (roughness, involatile residues) was observed. No surface damage layer was detected by Transmission Electron Microscopy. However, XPS revealed fluorine (up to 34%) and chlorine radicals (below 10%) in a 10nm thick surface layer.

New Material for Thin Film Filament of Micromachined Hot-plate

Ta5Si3, a new material for the resistive thin film of micro hot plates was investigated and tested. The material was obtained by dynamic co-sputtering and crystallized in the finished device by resistive heating (burn-in) to 900 °C. The transformation temperature is 650 °C. Encapsulated into SiO2 films, this material allows for operation temperatures of up to 1000 °C. The resistance increases linearly with temperature. The TCR amounts to (5.5 ± 0.6)* 10−4 °K−1, the specific resistivity to 90 µΩcm at 20 °C.