Srinivas Tadigadapa

Piezoelectric MEMS sensors: state-of-the-art and perspectives

Over the past two decades, several advances have been made in micromachined sensors and actuators. As the field of microelectromechanical systems (MEMS) has advanced, a clear need for the integration of materials other than silicon and its compounds into micromachined transducers has emerged. Piezoelectric materials are high energy density materials that scale very favorably upon miniaturization and that has led to an ever-growing interest in piezoelectric films for MEMS applications. At this time, piezoelectric aluminum-nitride-based film bulk acoustic resonators (FBAR) have already been successfully commercialized. Future innovations and improvements in inertial sensors for navigation, high-frequency crystal oscillators and filters for wireless applications, microactuators for RF applications, chip-scale chemical analysis systems and countless other applications hinge upon the successful miniaturization of components and integration of piezoelectrics and metals into these systems. In this article, a comprehensive review of micromachined piezoelectric transducer technology will be presented. Piezoelectric materials in bulk and thin film forms will be reviewed and fabrication techniques for the integration of these materials for microsensor applications will be presented. Recent advances in various piezoelectric microsensors will be presented through specific examples. This review will conclude with a critical assessment of the future trends and promise of this technology.

Intrinsic doping and gate hysteresis in graphene field effect devices fabricated on SiO2 substrates

We have studied the intrinsic doping level and gate hysteresis of graphene-based field effect transistors (FETs) fabricated over Si/SiO(2) substrates. It was found that the high p-doping level of graphene in some as-prepared devices can be reversed by vacuum degassing at room temperature or above depending on the degree of hydrophobicity and/or hydration of the underlying SiO(2) substrate. Charge neutrality point (CNP) hysteresis, consisting of the shift of the charge neutrality point (or Dirac peak) upon reversal of the gate voltage sweep direction, was also greatly reduced upon vacuum degassing. However, another type of hysteresis, consisting of the change in the transconductance upon reversal of the gate voltage sweep direction, persists even after long-term vacuum annealing at 200 °C, when SiO(2) surface-bound water is expected to be desorbed. We propose a mechanism for this transconductance hysteresis that involves water-related defects, formed during the hydration of the near-surface silanol groups in the bulk SiO(2), that can act as electron traps.

Lead-zirconate-titanate-based piezoelectric micromachined switch

A piezoelectric microelectromechanical switch actuated by lead zirconate titanate (PZT) is reported. A PZT unimorph cantilever actuator, fabricated on a sacrificial polysilicon layer and released using a xenon difluoride dry etch, was used to realize the switch. The PZT thin film was poled and driven with interdigitated electrodes to exploit the d33 coefficient for switching actuation. Preliminary dc and rf switching characteristics are reported. Measurements indicate a fast switching “on” time of 25 dB was achieved up to 100 MHz.

Thermal characterization of liquids and polymer thin films using a microcalorimeter

This paper presents an integrated microfluidic thermal sensor that can be used to characterize the thermal properties of nanoliter quantities of fluids and polymer thin films. The device consists of a polycrystalline silicon (polysilicon) heater located in close proximity to the hot junctions of p+-polysilicon/gold microthermopiles fabricated on a thermally isolated membrane. ac calorimetric measurements were performed by introducing a periodic heat signal using the heater and detecting the frequency-dependent thermal signal response in the presence of various fluids and polymers. The thermal conductivity of different fluids and five typical polymers used in microfabrication was measured using this device.This paper presents an integrated microfluidic thermal sensor that can be used to characterize the thermal properties of nanoliter quantities of fluids and polymer thin films. The device consists of a polycrystalline silicon (polysilicon) heater located in close proximity to the hot junctions of p+-polysilicon/gold microthermopiles fabricated on a thermally isolated membrane. ac calorimetric measurements were performed by introducing a periodic heat signal using the heater and detecting the frequency-dependent thermal signal response in the presence of various fluids and polymers. The thermal conductivity of different fluids and five typical polymers used in microfabrication was measured using this device.

Application of Micromachined

This paper presents the design, fabrication, and characterization of thermal infrared (IR) imaging arrays operating at room temperature which are based on Y-cut-quartz bulk acoustic wave resonators. A novel method of tracking the resonance frequency based upon the measurement of impedance is presented. High-frequency (240-MHz) micromachined resonators from Y-cut-quartz crystal cuts were fabricated using heterogeneous integration techniques on a silicon wafer. A temperature sensitivity of 22.16 kHz/°C was experimentally measured. IR measurements on the resonator pixel resulted in a noise equivalent power of 3.90 nW/Hz1/2, a detectivity D* of 1 × 105 cm · Hz1/2/W, and a noise equivalent temperature difference of 4 mK in the 8- to 14-μm wavelength range. The thermal frequency response of the resonator was determined to be faster than 33 Hz, demonstrating its applicability in video-rate uncooled IR imaging. This work represents the first comprehensive thermal characterization of micromachined F-cut-quartz resonators and their IR sensing response.

Y

Miniaturized tonpilz transducers are potentially useful for ultrasonic imaging in the 10 to 100 MHz frequency range due to their higher efficiency and output capabilities. In this work, 4 to 10-/spl mu/m thick piezoelectric thin films were used as the active element in the construction of miniaturized tonpilz structures. The tonpilz stack consisted of silver/lead zirconate titanate (PZT)/lanthanum nickelate (LaNiO/sub 3/)/silicon on insulator (SOI) substrates. First, conductive LaNiO/sub 3/ thin films, approximately 300 nm in thickness, were grown on SOI substrates by a metalorganic decomposition (MOD) method. The room temperature resistivity of the LaNiO/sub 3/ was 6.5 /spl times/ 10/sup -6/ /spl Omega//spl middot/m. Randomly oriented PZT (52/48) films up to 7-/spl mu/m thick were then deposited using a sol-gel process on the LaNiO/sub 3/-coated SOI substrates. The PZT films with LaNiO/sub 3/ bottom electrodes showed good dielectric and ferroelectric properties. The relative dielectric permittivity (at 1 kHz) was about 1030. The remanent polarization of PZT films was larger than 26 /spl mu/C/cm/sup 2/. The effective transverse piezoelectric e/sub 31,f/ coefficient of PZT thick films was about -6.5 C/m/sup 2/ when poled at -75 kV/cm for 15 minutes at room temperature. Enhanced piezoelectric properties were obtained on poling the PZT films at higher temperatures. A silver layer about 40-/spl mu/m thick was prepared by silver powder dispersed in epoxy and deposited onto the PZT film to form the tail mass of the tonpilz structure. The top layers of this wafer were subsequently diced with a saw, and the structure was bonded to a second wafer. The original silicon carrier wafer was polished and etched using a Xenon difluoride (XeF/sub 2/) etching system. The resulting structures showed good piezoelectric activity. This process flow should enable integration of the piezoelectric elements with drive/receive electronics.

-Cut-Quartz Bulk Acoustic Wave Resonator for Infrared Sensing

Heavily doped p-type silicon substrates were etched to form thick porous layers (~170 µm) and impregnated with magnesium perchlorate to form reactive composites. Characterization of the reactive wave propagation by high speed photography and spectroscopic methods indicated slow propagation rates between 1 and 8 m s−1. Multiscale structures were formed on the same substrates using microfabrication techniques followed by an electrochemical etch based on a random micro-crack pattern observed in lower doped substrates which yielded faster propagating composites. These organized multiscale composites exhibited flame propagation speeds up to 500 m s−1 indicating that reaction propagation can be controlled by structural modifications.

Fabrication and characterization of micromachined high-frequency tonpilz transducers derived by PZT thick films

This paper reviews the fabrication and performance of micromachined quartz resonator arrays. Using inductively coupled plasma etching techniques, we have successfully fabricated micromachined quartz resonator arrays with fundamental frequencies in the range of 25?85 MHz in an array format. These resonators have been experimentally evaluated for their performance in viscous (liquid) and viscoelastic (a biomolecular film in liquid) loading conditions. The paper discusses the ultimate sensitivity to mass and other properties of the adsorbates/contacting materials onto high-frequency quartz resonator surfaces. Measuring the frequency and Q-factor changes at the fundamental and third overtone of a 66 MHz resonator upon adsorption of immunoglobulin G (IgG) protein film on a hexadecanethiol functionalized surface, we were able to deduce: (i) the film thickness = 18 nm, (ii) density = 1040 kg m?3, (iii) elastic modulus = 6.7 MPa and (iv) viscosity = 5.5 mPa s. Furthermore, from the adsorption isotherm for the IgG film, two different Langmuir equilibrium constants (K) were deduced. In the low-concentration region K = 2.13 ? 108 M?1 and in the high-concentration region K = 6.53 ? 106 M?1 were obtained. The thickness and density values obtained for IgG are consistent with the bilayer model predicted from interfacial packing of spherical protein molecules as a function of the molecular weight, and K values are consistent with earlier reported values for adsorption of IgG films. This is the first reporting of the elastic modulus and viscosity of IgG films in phosphate buffer solution.

Control of nanoenergetics through organized microstructures

This paper presents a discussion of some of the major issues that need to be considered for the successful commercialization of MEMS products. The diversity of MEMS devices and historical reasons have led to scattered developments in the MEMS manufacturing infrastructure. A good manufacturing strategy must include the complete device plan including package as part of the design and process development of the device. In spite of rapid advances in the field of MEMS there are daunting challenges that lie in the areas of MEMS packaging, and reliability testing. CAD tools for MEMS are starting to get more mature but are still limited in their overall performance. MEMS manufacturing is currently at a fragile state of evolution. In spite of all the wonderful possibilities, very few MEMS devices have been commercialized. In our opinion, the magnitude of the difficulty of fabricating MEMS devices at the manufacturing level is highly underestimated by both the current and emerging MEMS communities. A synopsis of MEMS manufacturing issues is presented here.

Fabrication and performance characteristics of high-frequency micromachined bulk acoustic wave quartz resonator arrays

This paper presents a tin-based solder bonding technology for microelectromechanical systems (MEMS) fabrication and packaging applications. Electroplated tin on a lithographically patterned seed layer of chrome–gold was used for bonding two Pyrex substrates. Average values of the tensile strength and the shear strength obtained were 14.6 MPa and 5.78 MPa, respectively. Shear strength data were analysed using Weibull statistics, which revealed presence of multiple failure mechanisms and an average value of β greater than 3. Failure occurred not only at the tin–tin bond interface but also at the tin–chrome interface. Highest values for the bond strength were obtained when the samples failed due to delamination at the tin–tin bond interface. Acoustic images of the bonded interface were used to qualitatively study the degradation in the bond interface when subjected to tensile stresses. Hermetic testing of the samples using a conventional He-leak detector showed leak integrity of better than 1 × 10−11 mbar l s−1.