This paper presents the fabrication, testing and analysis of the MIT micro-rocket combustion chamber structure. The structure is a rocket chamber with a closed throat and pressure feeds. It is fabricated with the same processes as the full micro-rocket builds. Mechanical test results are correlated with inspection of failed devices for flaws and with finite element modeling of the test condition. This analysis provides recommendations for modifications to improve the strength of the micro-rocket chamber. These recommendations are discussed in the context of the current status of the micro-rocket.

Piezoelectric diaphragms were fabricated using bulk micromachining. The diaphragms had a unimorph structure, where Pb(Zr0.52Ti0.48)O3 (PZT) and thermally grown silicon oxide (SiO2) films were used as the active and passive layers, respectively. To actuate the diaphragms, two modes were designed: d31 and d33-mode. For d31-mode diaphragms, a Si wafer with Pt/Ti/SiO2 (0.5 νm) was coated with ∼1.2 νm PZT. A Cr/Au top electrode was then evaporated. Each layer including the bottom electrode was patterned into a circular shape. To fabricate d33-mode diaphragms, a Si wafer with thermal SiO2 (0.5 νm) was coated with ∼0.3 νm ZrO2 and ∼1.6 νm of PZT. On top of these layers, a Cr/Au top electrode was deposited and patterned into a ring- shaped interdigitated transducer. Finally, both d31 and d33-mode diaphragms were released using deep reactive ion etching. Diameters of the fabricated diaphragms were in the range of 600 νm and 1000 νm. For d31-mode diaphragms, the dielectric constant and loss of the released piezoelectric layer at 1 kHz were > 800 and < 2%, respectively. The remanent polarization was ∼20 νC/cm2and the coercive field was ∼61 kV/cm. Ferroelectric measurements showed well- developed hysteresis loops for the d33-mode diaphragms. Both d31 and d33-mode diaphragms behave as membranes rather than plates. Their measured resonance frequencies were consistent with calculations from an analytic model for circular membranes and ANSYS finite element analysis.

Fatigue crack initiation is shown to be associated with the stress-assisted evolution of a surface silica layer that forms during the normal exposure of unpassivated polysilicon surfaces to lab air In-situ atomic force microscopy (AFM) techniques are used to reveal the evolution of overall surface topology during incremental cyclic deformation to failure. Linear perturbation analysis of stress-assisted dissolution is then utilized to predict the evolution of the surface morphology. The predictions from the perturbation analysis are shown to be consistent with measured surface morphologies obtained using AFM techniques.

A micro-molding process was used to fabricate parts in the 0.1 to 10 mm size range from a stainless steel nano-powder. The two types of molds used were both produced from parts fabricated using the LIGA process so that they had precise dimensional tolerance and straight sidewalls. Rigid PMMA molds were made by injection molding and flexible silicone rubber molds were made by casting. Mold filling was accomplished by mixing the powder with epoxy to form a putty-like material that was then pressed into the mold cavities and allowed to cure. After pyrolysis of the epoxy, the parts were sintered in forming gas. The densification kinetics were measured in situ using a video system. Full densification was achieved after 1 hour at 1350°. The microstructure of the sintered parts was examined using the SEM. The mechanical strength, hardness, dimensional tolerance and surface roughness of the sintered parts were also measured.

We report on comprehensive characterization of piezoelectric shear mode inkjet actuators micromachined into bulk Pb(Zr0.53Ti0.47)O3 (PZT) ceramics. The paper starts with an overview of different inkjet technologies such as continuous jet and drop-on-demand systems, whereat main attention is turned on piezoelectric systems particularly Xaar-type shear mode inkjet color printheads. They are an example of complex microelectromechanical systems (MEMS) and comprise a ferroelectric array of 128 active ink channels (75νm wide and 360νm deep). Detailed information about manufacturing and principles of operation are given. Several techniques to control manufacturing processes and to characterize properties of the piezoelectric material are described: dielectric spectroscopy to measure dielectric permittivity ε and loss tanσ; ferroelectric hysteresis P-E loop tracing to get remnant polarization Pr and coercive field Ec, and a novel pulsed technique to quantify functional properties of the PZT actuator such as acoustic resonant frequencies and electromechanical coupling factor. Stroboscope technique has been employed to find correlation between the degradation of ink-jet performance and heat/high voltage treatment resulting in ferroelectric fatigue.

A study is made of radiation-induced expansion/compaction in Pyrex and Hoya SD-2 glasses, which are used as substrates for MEMS devices. Glass samples were irradiated with a neutron fluence composed primarily of thermal neutrons, and a flotation technique was employed to measure the resulting density changes in the glass. Transport of Ions in Matter (TRIM) calculations were performed to relate fast (∼1 MeV) neutron atomic displacement damage to that of boron thermal neutron capture events, and measured density changes in the glass samples were thus proportionally attributed to thermal and fast neutron fluences. The trend for strain with thermal neutron fluence (n/cm2) was found to be a linear compaction of -2.8×10−20 for Pyrex and -1.0×10−21 for Hoya SD-2. For fast neutron fluence, the trend for strain was also linear: -6.1×10−21for Pyrex and -7.9×10−22 for Hoya SD-2. The measured radiation-induced compaction of Pyrex is found to agree with that of previous studies. To our knowledge, this work represents the first study of compaction in Hoya SD-2 with neutron fluence. Hoya SD-2 is of considerable importance to MEMS, owing to its close thermal expansivity match to silicon from 25-500 C.

The design of a novel spectrometer based on a thin film diode in combination with an electro statically tunable micro machined mirror will be presented. The semi transparent diode is introduced into a standing wave created in front of a reflector to sample the profile of the standing wave. Varying the position of the reflector results in a shift of the phase of the standing waves and thus in a change of the optical generation profile within the detector. The spectral information of the incoming light can be determined by the Fourier transformation of the transient response of the sensor. An analytical optical model will be presented which facilitates the evaluation of different detector concepts. The model will be applied to discuss different device designs regarding the resolution of the spectrometer, the spectral range and the linearity.

An overview is given of methods for testing thin-film tensile specimens of either MEMS materials or surface coatings. MEMS specimens are deposited in a final shape and need only to be released for testing, while specimens of coating materials must be extracted. Very brief descriptions of the specimen designs, force application approaches, and strain measurements are given along with a limited number of references. Representative stress-strain curves for polysilicon, silicon nitride, silicon carbide, electroplated nickel and diffusion aluminide bond coating are presented. Results at high temperatures are presented for the latter two materials.

Silicon carbide is a very attractive material for a variety of applications. Originally considered for use in high power and high temperature electronics because of its large bandgap, designers of MEMS are now considering use of silicon carbide because of its stability at high temperatures, resistance to corrosives, high stiffness, and radiation resistance. However, as with any new structural material, its mechanical properties must be measured for design information. This research measures the elastic modulus, strength, and Poisson's ratio of two different silicon carbides using microtensile testing. One material is a 0.5-1νm thick film from Case Western Reserve University. Preliminary results give an average of 420 GPa for elastic modulus, a strength of 1.2 GPa, and a Poisson's ratio of 0.19. The second material is from Massachusetts Institute of Technology with an average thickness of 30 microns. Preliminary results show an elastic modulus of 430 GPa, a strength of 0.49 GPa, and a Poisson's ratio of 0.24. In addition to the most recent results, techniques used to obtain these results, microstructure investigations, and a comparison of the materials are detailed.

Polycrystalline silicon thin films that are permeable to fluids, known as permeable polysilicon, have been reported by several researchers. Such films have great potential for the fabrication of difficult to make MEMS structures, but their use has been hampered by poor process repeatability and a lack of physical understanding of the origin of film permeability and how to control it. We have completed a methodical study of the relationship between process, microstructure, and properties for permeable polysilicon thin films. As a result, we have determined that the film permeability is caused by the presence of nanoscale pores, ranging from 10-50 nm in size, that form spontaneously during LPCVD deposition within a narrow process window. The unusual microstructure within this process window corresponds to the transition between a semicrystalline growth regime, exhibiting tensile residual stress, and a columnar growth regime exhibiting compressive residual stress. A simple kinetic model is proposed to explain the unusual morphology within this transition regime. It is determined that measurements of the film residual stress can be used to tune the deposition parameters to repeatably produce permeable films for applications. The result is a convenient, single-step process that enables the elegant fabrication of many previously challenging structures.

Silicon in single crystal form has been the material of choice for the first demonstration of the MIT microengine project. However, because it has a relatively low melting temperature, silicon is not an ideal material for the intended operational environment of high temperature and stress. In addition, preliminary work indicates that single crystal silicon has a tendency to undergo localized deformation by slip band formation. Thus it is critical to obtain a better understanding of the mechanical behavior of this material at elevated temperatures in order to properly exploit its capabilities as a structural material. Creep tests in simple compression with n-type single crystal silicon, with low initial dislocation density, were conducted over a temperature range of 900 K to 1200 K and a stress range of 10 MPa to 120 MPa. The compression specimens were machined such that the multi-slip <100> or <111> orientations were coincident with the compression axis. The creep tests reveal that response can be delineated into two broad regimes: (a) in the first regime rapid dislocation multiplication is responsible for accelerating creep rates, and (b) in the second regime an increasing resistance to dislocation motion is responsible for the decelerating creep rates, as is typically observed for creep in metals. An isotropic elasto-viscoplastic constitutive model that accounts for these two mechanisms has been developed in support of the design of the high temperature turbine structure of the MIT microengine.

Tonpilz transducers consist of a heavy tail mass, a piezoelectric spring, and a light head mass. Miniaturized tonpilz transducers are potentially interesting for the 10 to100 MHz frequency range in imaging transducers due to their high efficiency and output capabilities. Piezoelectric thin films can be used as the active element in the construction of miniaturized tonpilz structures. A 4-10 νm film is necessary for the mass-spring-mass system to resonate at these high frequencies. In this work, fabrication and characterization of lead zirconate titanate (PZT) thick films on conductive oxide LaNiO3 (LNO) coated silicon on insulator (SOI) substrates will be reported for this application. First, conductive LNO thin films, approximately 300 nm in thickness, were grown on SOI substrates by a metal-organic decomposition (MOD) method. The room temperature resistivity of the LNO was 6.5×10-4 ωcm. Randomly oriented PZT (52/48) films up to 7 νm thick were then deposited using a sol-gel process on the LNO coated SOI substrates. 20 mol.% excess lead was added to the solutions to compensate for lead volatilization during film heat treatments. PZT films with LNO bottom electrodes showed good dielectric and ferroelectric properties. The dielectric permittivity (at 1 kHz) was over 1000. The remanent polarization of PZT films was larger than 26 νC/cm2. The e31,f coefficient of PZT thick films was larger than –6.5 C/m2 when poled at -75 kV/cm for 15 minutes. A silver layer approximately 40 νm thick was screen- printed onto the PZT film to form the tail mass of the tonpilz structure. Elements were diced and the bulk silicon was removed by dry and wet-etching methods to leave the p-type silicon layer as tonpilz head mass. Fabrication of MEMS tonpilz microstructures will also be presented.

Field dependent bulk charge transport in Si-rich, low stress silicon nitride thin films is studied in correlation to the local atomic Si-N bond strain. Across a range of film compositions varying from fully stoichiometric Si3N4 to Si-rich SiN0.54, Poole-Frenkel emission is determined to be the dominant charge transport mechanism with the Poole- Frenkel barrier height found to decrease concomitantly from 1.10 to 0.52 eV. Across the same composition range the local residual Si-N bond strain, as measured by FTIR spectroscopy, is observed to vary from 0.006 to –0.0026. Comparison of the barrier height to the residual strain reveals a direct correlation between the two quantities. It is concluded that reductions in the Poole-Frenkel barrier height are a manifestation of compositionally induced strain relief at the molecular level. Reductions in the barrier height result in increased Poole-Frenkel emission detrapping rates and consequently higher leakage currents in Si-rich films.

According to market surveys automotive microsensors will evolve into a multi-billion dollar business by 2005. Key roles are attributed to inertial sensors for passenger safety systems, and mass flow and pressure sensors for engine management systems. Thin film techniques together with silicon bulk or surface micromachining have been established as preferential processes to achieve reduction of sensor size, weight and cost along with improvements of sensor functionality and reliability. Enhanced sensor performance often pushes the limits of process technology and therefore the need arises very early in the MEMS design process to identify materials and geometry related parameters which are critical with respect to their tolerance band specifications. In order to control these critical parameters, automated wafer level test procedures need to be developed (based preferentially on electrical quantities) and additionally considered for in the sensor design phase (design for test). In analogy to microelectronics 2D wafer maps of critical parameters may give hints on how to improve process stability and how to adapt the sensor design in order to optimize yield. Examples of critical model parameter variations include thermal conductivity, thickness, and shear modulus of thin films.

In this study, thick sol-gel PZT(Pb(Zr1−xTix)O3) films were fabricated and their characteristics were investigated for angular rate sensor applications. The crack-free thick sol-gel PZT films were successfully deposited on silicon substrates by multiple spin-coating technique combined with rapid thermal annealing(RTA). The thickness of the PZT films is 1.5 νm, which is required by a vibration angular rate sensor for a good actuation and sensing. The remnant polarization of the PZT films is 12.0νC/cm2. The PZT films were applied to the vibration angular rate sensor structure, and the vibration of 1.78νm in amplitude at the resonant frequency of 35.8kHz was obtained. In this case, the driving voltage of 5Vp-p by bulk piezoelectric materials was applied to the driving electrode with out of phase signal through voltage and inverting amplifier.

The oscillating output voltage obtained by external actuation using a stacked piezo-actuator showed the values of 0.76V and 0.87V in outer/inner driving electrode at driving voltage of 5Vp-p.

Our practical experiments highlight that aluminum-silicon (Al-Si) and gold-silicon (Au- Si) eutectics are fairly inert to the attack of both anisotropic and isotropic wet etchants of Si (e.g. KOH or HF/HNO3 solutions). Therefore, these interfacial eutectics can be used as etch-stop layers in wet etching-based bulk micromachining. This paper presents how Al-Si and Au-Si eutectic layers may be employed for applications whose production involves such operations (e.g. high-pressure sensors) and will discuss three major directions of interest: eutectic formation, diaphragm generation and application of the eutectic layer in a pressure sensor.

A novel MEMS technology using multi-layer poly-silicon (poly-Si) is proposed. The poly-Si film is formed from the new Nickel-Induced-Lateral-Crystallization (NILC) method and has very large grain (>10νm) and near crystal quality. 700 nm thermal oxide was grown on a Si wafer. 100 nm LPCVD amorphous Si was deposited and followed by a 50 Å Ni deposition. The a-Si was crystallized at 550°C for 65 hours and subsequent 800°C for 2 hours to form the first (lower) NILC poly-Si layer. N-channel TFTs were fabricated on the NILC polysilicon layer. The process was repeated and a second (upper) polysilicon layer and TFTs were formed on top of the first polysilicon layer.

The lower polysilicon has slightly larger grains and better material quality. Thin-film- transistors (TFT) fabricated on the 3-dimensional (3-D) poly-Si layers have I-V characteristics similar to (>40%) silicon-on-insulator TFTs. While TFTs on lower layer have better mobility and device properties, TFTs on upper layer have better uniformity. The accumulated heating and other effects have also been studied.

In most MEMS applications, dust and particles are avoided with considerable endeavor. However, for many applications such as gas sensors, powders of functional ceramics would often provide better performance than corresponding thin film layers. There are abundant ways to synthesize powders with well defined chemical composition, phase and size distribution, whereas the processing parameters for thin-film preparation often are limited. Specifically where the functionality is based on a chemical reaction on surfaces, nanoscaled powders with a high specific surface area have proven useful. This is the case for tin oxide gas sensors that exhibit a drop in electrical resistivity induced by the combustion of the analyte gas. Soft lithography was used for the fabrication of mesoscaled powder-based ceramic structures. In this paper, we present the integration of small tin oxide microstructures with an effective gas-sensing area of 10 by 30 νm2 on a micro-hotplate substrate. Such a substrate can heat the ceramic sensor to operating temperatures quickly with low power consumption. A whole array of sensors can be integrated on one micro-hotplate. Processing steps to prepare the sensor array on the micro hot plate are presented and discussed concerning processing sequence, sensitivity towards 1000 to 1500 ppm hydrogen and power consumption. Additionally, effects of grain growth due to on-chip annealing of the ceramic nanostructure were observed.

Recent advances in surface nanomachining have allowed the fabrication of mechanical structures with dimensions reaching 20 nm, and resonant frequencies in the 100s of MHz. Structural issues prevent the “top-down” surface machining of high-quality NEMS resonators. Such systems are alternatively to be bestowed by “bottom-up” manufacturing technologies. We report the surface assembly of RF-range NEMS. Using electrofluidic assembly, we have successfully positioned Rh mechanical beams onto specific sites of a silicon circuit. With diameters as small as 250 nm and lengths varying from 2 to 3 [.proportional]m, preliminary results show mechanical resonances ranging from 5 MHz to 80 MHz, and quality factors reaching 500. We also report the development of nanostructured NEMS for sensor applications, and present strategies for their deployment in integrative nanosystems.

We evaluated the mechanical properties of a very thin single-crystal-silicon film 140 nm thick fabricated from a SIMOX wafer using an on-chip tensile testing method. The silicon specimen oriented in the <110> direction was integrated with a loading mechanism on the same chip, which was fabricated in three etching steps. The strain in the film was measured using a newly developed tensile testing system having a two-field-of-view microscope that allowed us to observe the elongation of the specimen directly. The measured fracture strain was distributed in the range of 2.4-3.2%, which is narrower than the range of 1.3-3.5% for a thicker 5-νm film.