Dana Pitzer

First results on label-free detection of DNA and protein molecules using a novel integrated sensor technology based on gravimetric detection principles.

A novel integrated bio-sensor technology based on thin-film bulk acoustic wave resonators on silicon is presented and the feasibility of detecting DNA and protein molecules proofed. The detection principle of these sensors is label-free and relies on a resonance frequency shift caused by mass loading of an acoustic resonator, a principle very well known from quartz crystal micro balances. Integrated ZnO bulk acoustic wave resonators with resonance frequencies around 2 GHz have been fabricated, employing an acoustic mirror for isolation from the silicon substrate. DNA oligos have been thiol-coupled to the gold electrode by on-wafer dispensing. In a further step, samples have either been hybridised or alternatively a protein has been coupled to the receptor. The measurement results show the new bio-sensor being capable of both, detecting proteins as well as the DNA hybridisation without using a label. Due to the substantially higher oscillation frequency, these sensors already show much higher sensitivity and resolution comparable to quartz crystal micro balances. The potential for these sensors and sensors arrays as well as technological challenges will be discussed in detail.

Solidly mounted ZnO shear mode film bulk acoustic resonators for sensing applications in liquids

Solidly mounted film bulk acoustic resonators (FBAR) operating at 850 MHz in the shear vibration mode have been fabricated. C-axis inclined zinc oxide (ZnO) thin films realized by modified reactive magnetron sputtering were used. Coupling factors k/sup 2/ of 1.7% and Q-factors of 312 were determined in air. Q-factors of 192 were measured in water, making these devices attractive for sensing applications in liquids, e.g., biosensing.

Electro-acoustic hysteresis behaviour of PZT thin film bulk acoustic resonators

Abstract Thin film bulk acoustic resonators (FBARs) based on Pb(Zr x Ti (1− x ) )O 3 (PZT) with varying compositions, ranging from x =0.25 to 0.6, were fabricated to investigate hysteresis-like dependencies of the resonance frequency and electro-mechanical coupling constant on bias voltage. The resonators, formed by a simple sandwich structure consisting of bottom electrode, PZT thin film and top electrode, arranged on a planar acoustic mirror, were designed to give a resonance frequency of about 2 GHz. PZT thin films were deposited in a planar multi target sputtering system using three metallic targets in a reactive Ar/O 2 mixture. For low Zr-content, where PZT is grown in the tetragonal phase, the parallel resonance frequencies are strongly dependent on the applied electric field, while the series resonance frequency is practically unaffected. This behaviour is completely different for rhombohedral PZT at higher Zr-content. Here the series resonance frequency becomes strongly field dependent, which can be attributed to 109°/71° domain switching. As a potential application based on the observed strong field dependence of the acoustic properties, a bandwidth-tuneable or programmable RF filter based on PZT FBARs is proposed.

Novel integrated FBAR sensors: a universal technology platform for bio- and gas-detection

In this paper the feasibility of thin film bulk acoustic resonators (FBAR), for applications in bio- and gas-detection, is shown for the first time. Solidly mounted, ZnO FBARs with frequencies around 2 GHz have been fabricated on silicon substrates. The dependence of the FBAR mass sensitivity on the design of the layer stack has been investigated exhibiting an optimized sensitivity of 2.5 Hz cm/sup 2//pg. Using a common protein assay the capability of detecting bio-molecules has successfully been proved. Gas sensing has been demonstrated by coating the FBAR with a humidity absorbing polymer. A strong non-linear dependence of the humidity sensitivity on the thickness of the polymer coating has been found. When the polymer thickness is far less than the acoustic wavelength, a pure mass dependent response occurs, leading to a negative shift in resonance frequency. Moreover, as the polymer thickness becomes significant, acoustic influences affect the response and the shift becomes large and positive. A sensitivity to humidity of up to two orders of magnitude higher than that of comparably coated quartz crystal micro-balances has been observed.

Deposition of self-polarized PZT films by planar multi-target sputtering

Abstract A planar multi target sputtering approach was used to deposit self polarized PZT films on TiO2/Pt bottom electrodes for the use in thin film pyroelectric IR detector arrays. By using elevated substrate temperatures of about 450°C “in situ” growth of tetragonal PZT could be achieved. The films exhibited pyroelectric currents without poling. The pyroelectric coefficient was 2×10−4 C/m2K, the dielectric constant was 300 and dielectric loss tan δ was 0.01. The self polarization disappears after heating the sample to 600°C. Stresses were studied in the thin film processing for the bottom electrode and the PZT film. The TiO2/Pt electrode is under high tensile stress of 900 MPa after preparation. PZT has a small compressive stress of -60 MPa, the whole TiO2/Pt/PZT stack has a tensile stress of +80 MPa. This low stress level together with the self polarization and the good electrical properties makes the films suitable for the use in pyroelectric detector arrays.

Optimized PZT Thin Films for Pyroelectric IR Detector Arrays

A planar multi target sputtering technology was used to deposit highly (111) oriented Pb(ZrxTi1−x)O3 (PZT) thin films with x ranging from 0–0.6. The preparation of a stable Pt/ZrO2 electrode is described and analyzed in terms of stress and stress-temperature behavior. The PZT films with low Zr content are under compressive stress after deposition. The dielectric constant and loss peaks occur at a composition close to the morphotropic phase boundary. Films on the tetragonal side of the phase diagram with a Zr content up to about 25% exhibited a strong self polarization and strong voltage shifts in the C(V) curves. High pyroelectric coefficients of >2×10−4 C/(m2K) have been measured on these films without additional poling. The self polarization fades out with increasing Zr content. The low values of the pyroelectric coefficient for the PZT film with 60% Zr is discussed in terms of the possible crystallographic variants after distortion and the tensile stress state during the phase transition. Based on the systematic study of stress and electrical properties of PZT films with a wide range of composition presented in this paper, films with a Zr content up to about 25% turned out to give the best properties for the use in pyroelectric detector arrays.

Deposition of ferroelectric PZT thin films by planar multi-target sputtering

Abstract A planar multi-target sputtering system was used to deposit ferroelectric lead zirconate titanate (PZT) films. At substrate temperatures of about 450°C “in-situ” deposition of single phase perovskite PZT was obtained. By investigation with SEM and TEM films exhibited a columnar structure with crystallites of about 200 nm in size. The permittivity of the films varied from 450-600. The films exhibited hysteresis loops, remanent polarization of 15-20 μC/cm2 and coercive field strength of 70-85 kV/cm.

Sputtering of self-polarized PZT films for IR-detector arrays

Within a 3-year project a pyroelectric detector array based on a tetragonal Pb(Zr,Ti)O/sub 3/ (PZT) thin film on a silicon substrate was developed. Compositions of about 25-30% Zr showed best properties for the application in pyroelectric sensing systems. For the film deposition, planar multi target sputtering from 3 metallic targets in a reactive Ar/O/sub 2/ mixture is used. The advantage of this method is that the perovskite phase can be grown down to relatively low substrate temperatures of about 500/spl deg/C, a strong self polarization for compositions Zr/(Zr+Ti)<0.3 and a convenient control of the stoichiometry simply by varying the power supplied to the individual targets. The developed chip has (11/spl times/6) elements, his overall size is (8/spl times/4,7) mm/sup 2/. The array pixels of 280/spl times/280 /spl mu/m/sup 2/ have a noise equivalent power NEP of less than 0.7 nW at 1 Hz. The achieved specific detectivity D/sup */ at a modulation frequency of 10 Hz is 3/spl middot/10/sup 8/ cm/spl middot/Hz/sup 1/2//W.

PZT thin films grown by multi-target sputtering: Analysis of thin film stress

Abstract A planar multi target sputtering approach was used to deposit Pb(Zr, Ti)O3 (PZT) films with different composition on Pt bottom electrodes for the use in thin film pyroelectric IR detector arrays. Low stress ferroelectric capacitor stacks are needed in these devices, because the sensing elements are arranged on thin micromachined membranes. PZT films with a Zr content of 10 at% (10/90) and 48 at% (48/52) were analyzed in terms of thin film stress. PZT (10/90) films were found in a slight compressed state and PZT (48/52) under tensile stress after deposition. Stress temperature curves were used to evaluate the Curie temperature of the films. From the stress temperature curves the spontaneous distortion of the ferroelectric given by the lattice parameters can be extracted. The lower Curie temperature in combination with the smaller spontaneous distortion of the PZT (48/52) film is the reason for the difference in the stress state after deposition.

A 11×6 element pyroelectric detector array utilizing self-polarized pzt thin films grown by sputtering

Abstract A planar multi target sputtering approach was used to deposit PbTiO3 (PT) and Pb(Zr, Ti)O3 (PZT) films on TiO2/Pt bottom electrodes for the use in thin film pyroelectric IR detector arrays. PZT films with a Zr content of 28 at% (PZ28T) exhibited the best pyroelectric coefficient of typically 2×10−4 Cm−2K−1. The PZ28T films have been used for fabricating a two dimensional 11×6 pixel pyroelectric detector array on Si wafers. The array pixels with a sensitive area of 280 ×280 μm2 have a noise lequivalent power NEP of less than 0.7 nW at 1 Hz. It is planned to use the detector array in systems for motion detection.