Nanowires of a diameter less than 50nm have been predicted to exhibit a higher thermoelectric figure of merit in
comparison to their bulk equivalent. In order to experimentally measure the thermoelectric power in nanowires it is
necessary to design and fabricate a measurement platform that is ideally matched in thermal and physical size and
capable of testing a large number of individual nanowires in a high throughput manner. In this paper we present the
design, fabrication, and characterization of a MEMS thermoelectric workbench with a high density of testing locations.
Characteristic measurements of the thermoelectric power of Au nanowires are presented as demonstration of the
This paper presents a unique solution to the inaccuracies produced when thermally scanning various micro and nano
systems with thermistor tip scanning thermal microscopy (SThM). Under dc measurement conditions, thermistor tip
heating induces perturbations in the measured system that change with sample properties like material and
geometry. As a result, normal SThM scans are affected by errors that make it difficult to interpret the 2D-temperature
scans of such systems. By coating the SThM tips with a thermally resistive material (100nm of Si3N4)
we demonstrate that the temperature dependence on sample material and geometry can be minimized and the tip
heating problem can be mitigated to that of a constant temperature offset problem. Included are the first images of
coated scanning thermal microscopy (C-SThM) as well as a lumped model that describes the basis of the
improvement seen in the thermal images.
We report the design and fabrication of a micromachined quartz crystal balance (QCM) array for self
assembled monolayers (SAMs) and protein adsorption studies. The microQCM was fabricated using recently
developed inductively coupled plasma etching process for quartz to realize resonators with 60 &mgr;m thickness
and electrode diameters of 0.5 mm. The reduction in the thickness and lateral pixel size has resulted in a
sensitivity improvement by factor of 1700 over a commercially available macro-sized QCM. Adsorption of
hexadecanethiol on the gold electrode of the QCM in ethanol at a concentration of 1 mM was recorded in real
time and a frequency shift of 3650 Hz was obtained. Modeling the SAMs layer as an ideal, rigid mass layer
the expected frequency shift was calculated to be 1031 Hz. This was followed by a study of the adsorption of
human serum albumin (HSA) protein on the SAMs layer. For 1.5×10-10 moles/ml concentration of protein
solution in phosphate buffer solution (PBS) we obtained a frequency change of 13.28 kHz. Modeling the
protein layer as a viscoelastic layer in a viscous Newtonian fluid, for saturation protein surface coverage, the
frequency change was calculated to be 17.27 kHz whereas the experimentally obtained frequency change was
51.82 kHz. In both rigid and viscoelastic film adsorption experiments, we find the microQCM to exhibit three
times greater sensitivity than the predicted value when operated at the third overtone. These results show that
the micromachined QCM in array format is a very sensitive gravimetric sensor capable of mass resolutions
into the femtograms range.
We present results of room temperature studies of the electrical characteristics of back-gated ultrathin graphite films
prepared by mechanical transfer of thin sections of Highly Oriented Pyrolytic Graphite (HOPG) to a Si/SiO2
substrate. The films studied were quite thin, exhibiting only a few graphene layers (n). Films with thickness in the
range 1 < n < 20 were studied, where n has been deduced by Atomic Force Microscopy (AFM) z-scans. The n value
deduced by AFM z-scan data was correlated with the n value deduced by Raman scattering data. We discuss at some
length, the issue of whether or not Raman scattering can provide a standalone measure of n. Electrical contacts were
made to a few of the low n (n = 1,2,3) graphene films. Most graphene films exhibited a nearly symmetric resistance
(R) anomaly vs. gate voltage (VG) in the range 25 < VG < 110 V; some films exhibited as much as a factor of ~50
decrease in R (relative to the maximum R) with changing VG. An interesting low bias shoulder on the negative side
of the resistance peak anomaly was also observed. The devices were fabricated with a lithography free process.
Lead Zirconate Titanate (PZT) is a high energy density active material with good piezoelectric coefficient and electromechanical coupling constant making it highly suitable for microsystems applications. In this paper, we present a rapid anisotropic high aspect ratio etching process for defining micron size features in PZT. We used an inductively coupled plasma reactive ion etching (ICP-RIE) system employing sulfur hexafluoride (SF6) and argon (Ar) based chemistry. A seed layer of Au/Cr was lithographically patterned onto fine lap finished PZT-4 substrates followed by electrodeposition of a thick 2-5 μm nickel on the seed layer, which acts as a hard mask during the etching process. The demonstrated technique was used to etch bulk PZT ceramic substrates, thereby opening possibilities for integration of bulk PZT substrates and structures into microsystems. A maximum etch rate of 19 μm/hr on PZT-4 and 25 μm/hr for PZT-5A compositions was obtained using 2000 W of ICP power, 475 W of substrate power, 5 sccm of SF6, and 50 sccm of Ar on PZT substrate. We have also demonstrated a high aspect ratio etch (>5:1) on a 3 μm feature size. Detailed analysis of the effects of ICP power, substrate power, and the etch gas composition on the etch rate of PZT are also presented in this article.
Etching of quartz and glass for microsystems applications requires optimization of the etch process for high etch rates, high aspect ratios and low rms surface roughness of the etched features. Typically, minimum surface roughness of the etched feature accompanied with maximum etch rate and anisotropy are desired. In this article, we investigate the effect of different gas chemistries on the etch rate and rms surface roughness of the Pyrex(R) 7740 in an inductively coupled plasma reactive ion etching (ICP-RIE) system. The gases considered were SF6 and c-C4F8, with additives gases comprising of O2, Ar, and CH4. A standard factorial design of experiment (DOE) methodology was used for finding the effect of variation of process parameters on the etch rate and rms surface roughness. By use of 2000 W of ICP power, 475 W of substrate power, SF6 flow rate of 5 sccm, Ar flow rate of 50 sccm, substrate holder temperature of 20°C, and distance of substrate holder from ICP source to be 120 mm, we were able to obtain an etch rate of 0.536 μm/min and a rms surface roughness of ~1.97 nm. For an etch process optimized for high etch rate and minimum surface roughness using C4F8/SF6/O2/Ar gases, an etch rate of 0.55 μm/min and a rms surface roughness of ~25 nm was obtained for SF6 flow rate of 5 sccm, C4F8 flow rate of 5 sccm, O2 flow rate of 50 sccm, Ar flow rate of 50 sccm. Keeping all other process parameters the same, increasing the SF6 flow rate to 50 sccm resulted in an etch rate of 0.7 μm/min at an rms surface roughness of ~800 nm whereas increasing the C4F8 flow rate to 50 sccm resulted in an etch rate of 0.67 μm/min at an rms surface roughness of ~450 nm . Addition of CH4 did not contribute significantly to the etch rate while at the same time causing significant increase in the rms surface roughness. Regression or least square fit was used define an arbitrary etch rate number (Wetch) and rms surface roughness number (Wrms). These numbers were calculated by least square fit to the data comprising of ten correlated etch variables and enable quantization of etch parameters in terms of process parameters. The etch numbers defined in this work as function of process parameters present a very useful tool for the optimization, quantification and characterization of the dielectric etch processes developed in this work for MEMS fabrication and packaging applications.
It has been shown that the addition of single walled carbon anotubes (SWNTs) cause an increase in the resonance frequency of micromachined clamped-clamped structures. This is believed to be due to an increase in the effective stiffness of the micromachined structures due to the high Young's modulus of carbon nanotubes. These results were obtained in spite of a relatively poor control over the orientation and aerial density of the deposited SWNTs. Finite element simulations showed an increase in the resonance frequency of up to ~25% for the simulated devices. This increase in the resonance frequency of the bridges can be attributed to the high Young's modulus (~1TPa) of the carbon nanotubes.
A polymer microfluidic pump has been developed using electrostrictive poly(vinylidene fluoride-trifluoroethylene) based polymer, which possesses a large electrostrictive strain (5-7%) and high elastic energy density (1 J/cm3), as the driving microactuator. The microfluidic pump was realized by integrating a nozzle/diffuser type fluidic mechanical-diode structure with the polymer microactuator, which shows an actuation deflection of 80 mm for a pumping chamber of 2.2x2.2 mm2. The microfluidic pump could pump methanol at a flow rate of 25 mL/min at 63 Hz with a backpressure of 350 Pa. The flow rate of this pump could be easily controlled by external electrical field. Results from both analytical and numerical analysis show that, due to the high load capability of the microactuator, the frequency response of this nozzle/diffuser pump is mainly limited by the resonance of the fluid in the fluid channel.
This paper presents a novel micro flextensional actuator design consisting of bulk PZT bonded to a silicon microbeam. The fabrication process includes boron doping, EDP etching, dicing, and bonding to produce actuators with large displacements (8.7 μm) and gain factors (32) at 100V. The theoretical model predicts that a thin beam with properly designed initial imperfection maximizes the actuator displacement and gain factor. For large PZT displacement, small imperfection maximizes gain factor but may not gaurantee, the desired (up or down) displacement direction. For small PZT displacement, large initial imperfection improves performance and guarantees displacement in the (desired) direction of the initial imperfection. The theoretical model, based on the measured initial beam shape, predicts the experimentally measured direction and magnitude of beam displacement for two devices.
Low temperature bonding techniques with high bond strengths and reliability are required for the fabrication and packaging of MEMS devices. Indium and indium-tin based bonding processes are explored for the fabrication of a flextensional MEMS actuator, which requires the integration of lead zirconate titanate (PZT) substrate with a silicon micromachined structure at low temperatures. The developed technique can be used either for wafer or chip level bonding. The lithographic steps used for the patterning and delineation of the seed layer limit the resolution of this technique. Using this technique, reliable bonds were achieved at a temperature of 200°C. The bonds yielded an average tensile strength of 5.41 MPa and 7.38 MPa for samples using indium and indium-tin alloy solders as the intermediate bonding layers respectively. The bonds (with line width of 100 microns) showed hermetic sealing capability of better than 10-11 mbar-l/s when tested using a commercial helium leak tester.
MEMS fabrication and packaging requires a bonding technology that is universal for all substrates, has high resolution, requires relatively lower temperatures, is reliable and is low cost to implement. The bonding technology presented meets the above standards. The process is substrate independent and involves aligned bonding of two similarly patterned wafers using tin solder as the bonding material. The technique can be used for whole wafer or selected area bonding. The resolution of this technique is only limited by the resolution that can be achieved in the patterning and delineation of the seed metal.
The high piezoelectric effect of lead zirconate titanate (PZT) films enables improved performance in microelectromechanical systems (MEMS). The material's reliable integration into current and mainstream MEMS microfabrication processes is then of great interest. In this paper we report on high reliability fabrication processes that can be used for producing PZT based MEMS devices. Pattern definition and release of PZT, low stress silicon nitride, platinum, and/or zirconia structures via wet and dry chemical etching and ion beam etching, including their affects on the piezoelectric properties of PZT are reported. Ion beam etching results in appreciable imprint in the polarization - electric field hysteresis loop of the PZT, which can be ameliorated by annealing in ambient air at 450 degree(s)C. PZT on silicon nitride cantilever structures were defined and released by dry xenon difluoride silicon sacrificial etching. The advantages and difficulties of wet release etching versus xenon difluoride are also presented.
Reliability is a key parameter for the eventual prevalence of microelectromechanical systems (MEMS) as either sub-components or as standalone products. Traditionally, micromachined components have been made by separating the micromachined chip design and fabrication processes from the packaging and reliability issues.
Piezoelectric accelerometers fabricated from Lead-Zirconate-Titanate (PZT) thin films are expected to achieve higher sensitivities and better signal-to-noise ratios (SNR) in comparison with capacitive and piezoresistive accelerometers. This paper will present, for the first time, the fabrication and performance of piezoelectric, bulk-micromachined accelerometers using PZT thin films operating in the d33-mode. Using sol-gel techniques, 0.6 mm thick PZT films with high piezoelectric coefficients were deposited. Measurements on these PZT films show a remnant polarization Pr < 19 (mu) C/cm2, dielectric constants Er > 800, and d33 coefficient of 120 pC/N. The PZT accelerometers operating in the d33 mode were successfully fabricated. Interdigitated capacitors were used to achieve the d33 mode of operation and deep reactive ion etching was used to define the proof-mass of the accelerometers. Measurements on these accelerometers show sensitivities ranging from 0.85 - 1.67 mV/g with resonance frequencies ranging from 22.4 - 15.4 kHz respectively. In addition to the improved sensitivity, advantages of d33-mode accelerometers include use of thinner PZT films, and the ability to optimize the impedance of the device to achieve a higher SNR. The performance of MEMS d33-mode accelerometers will also be compare with the previously reported d31-mode accelerometers using PZT thin films.
A sensor fabricated from microminiature free-standing structures which is capable of simultaneous measurements of several different inputs in real-time is described. The small thermal mass and good thermal isolation of free-standing structures have been used to advantage for sensing infrared radiation, ambient pressure and gas flow. The sensory element in all of these detectors is a microthermopile. The hot junctions of the device are made of free-standing wires whereas the cold junctions are thermally attached to the substrate. Energy dissipated in the microthermopile causes a rise in the temperature of the hot junctions relative to the cold junctions and thus produces a thermovoltage across the device. Monitoring the thermovoltage caused by the absorption of incident infrared radiation has resulted in a fast and sensitive thermal infrared detector which can be used for noncontact temperature measurements. For small temperature differences from ambient, the rise in the temperature of hot junctions is determined by the magnitudes of conductive and convection heat losses from the free- standing wires and therefore is a function of the ambient pressure, gas composition and gas flow. These dependencies have been used for sensing pressure, flow and gases. The sensors made from free-standing structures can be monolithically integrated into a sensor microsystem because the techniques used in their fabrication are compatible with silicon microfabrication technology. It should therefore be possible to integrate these sensors with active electronic circuits to make a smart microsystem.