GaAs-based micro-electro-mechanical systems (MEMS) are emerging technologies, which will lead to integration of new sensing and actuating devices with microwave and millimeter-wave electronics. However, developing the capability of selective deep etching of GaAs is a critical issue to be solved. This paper presents a study of a highly anisotropic deep GaAs etching. Two different plasma etching methods were studied for GaAs etching with chlorine-based chemistries: reactive ion etching (RIE) with Cl2/BCl3/SiCl4mixtures and high-density inductive coupled plasma (HDICP) etching with Cl2/BCl3 mixtures. A negative acting photoresist, SU-8, which allows high resolution, high aspect ratio patterning with strong resistance to etchants, was selected as the etching mask for this work. Process parameters were varied for best results, and the effects of the process parameters are presented. Issues such as etch selectivity of SU-8 and GaAs, reflow of SU-8, and loading effects were also investigated. An optimized etching process was developed to achieve a deep etch in excess of 300μm of GaAs with a single layer of SU-8 photoresist.

Recent research has established that for silicon structural films used in microelectromechanical systems (MEMS), the susceptibility to premature failure under cyclic fatigue loading originates from a degradation process that is confined to the surface oxide. In ambient air environments, a sequential, stress-assisted oxidation and stress-corrosion cracking process can occur within the native oxide on polycrystalline silicon (referred to as reaction-layer fatigue); for the structural films of micron-scale dimensions, such incipient cracking in the oxide can lead to catastrophic failure of the entire silicon component. Since the degradation process is intimately linked to the thin reaction layer on the silicon, modification of this surface and the access of the environment to it can dramatically alter the fatigue resistance of the material. The purpose of this paper is to evaluate the efficacy of modifying the fatigue behavior of polycrystalline silicon with alkene-based monolayers. Specifically, 2-μm thick polysilicon fatigue structures were coated with a monolayer film based on 1-octadecene and cyclically tested to failure in laboratory air. By applying the coating, the formation of the native oxide was prevented. Compared to the fatigue behavior of untreated polysilicon, the lives of the coated samples ranged from 105 to >1010 cycles at stress amplitudes greater than ∼90% of the ultimate strength of the film. The dramatic improvement in fatigue resistance was attributed to the monolayer inhibiting the formation of the native oxide and stress corrosion of the surface. It is concluded that the surprising susceptibility of thin structural silicon films to premature fatigue failure can be inhibited by such monolayer coatings.

The rapid development of MicroElectroMechanical Systems (MEMS) implied monolithic integration of these devices with the driving, controlling and signal processing electronics on the same CMOS substrate as this can improve performance, yield and reliability as well as lower the manufacturing, packaging and instrumentation costs. The post-processing route (fabricating MEMS on top of CMOS) is preferred as it avoids introducing any changes in standard foundry CMOS processes. Depending on the specific metallization type and the design requirements, post-processing limits the maximum fabrication temperature of MEMS to 450°C or 520°C [1], as this avoids introducing any damage or degradation to interconnects. This temperature constraint is quite strict for post processing surface micromachined MEMS, as it might affect relevant physical properties of the active element of MEMS such as crystallization, growth rate, mechanical properties, dopant activation, electrical resistivity, etc.. Polycrystalline silicon germanium (poly SiGe) is an attractive material for such applications due to its low amorphous to polycrystalline transition temperature, which can be as low as 400°C (with the appropriate germanium content). Furthermore, the mechanical properties of poly SiGe proved to be suitable for surface micromachining application [2]. In addition it is compatible with standard IC fabrication process, and hence it enables monolithic integration of MEMS with the driving electronics.

Au/Cr and Au/Ti films were deposited on Si (100) substrates using DC planar magnetron sputtering to assess residual stress in high reflectivity coatings. The dependence of stress on argon sputter pressure, component materials, and film thickness is discussed. Stress evolution as a function of thickness for individual Cr, Ti and Au films is also investigated to identify high-stress components of these two-layer coatings. Near-zero stress Au/Ti films were achieved with a particular set of sputtering parameters. Using the same process conditions, films were deposited onto pre-released MEMS mirrors having a number of different shapes and sizes. Optical interferometry demonstrates minimal change in the bow of 500, 250, and 125 μm diameter mirrors, consistent with a λ/40 flatness (λ = 1319 nm).

Due to their excellent piezoelectric properties, PZT ceramics are attractive materials for many MEMS applications. By depositing a PZT film on a micro-machined platinized silicon substrate, membranes can be actuated in the flexural mode. In this work, 2D arrays of PZT actuated membranes, which can be used as ultrasonic transducers, have been designed, fabricated, and tested for their ferroelectric response. The device behavior was also modeled using FEA. Data from FEA runs were used to evaluate the device performance based on its effective coupling coefficient (efficiency), acoustic impedance, and resonance frequency. Results show that the device-coupling factor is significantly affected by the PZT and silicon layers' thickness and the top electrode configuration. Results also indicate a considerable flexibility in optimizing the device performance under a wide range of operational requirements, which makes such devices attractive for different applications.

Microfabricated patch clamping devices comprising planar arrays of individually addressable nozzles, fluidic channels and electrodes have been developed. Patch clamp based electrophysiological techniques are among the most widespread methods in neurophysiology and are used to address a broad range of cellular physiology and quantitative biological questions. Among the limitations of the technique are the difficulty of obtaining multiple patches on connected cells or on the same cell, limited stability of patches, and constraints on chemical and optical access to the patched membrane. The parallel array device will enable the formation of multiple seals simultaneously. The structure facilitates visualization of the interior of the patched membrane during electrical recording, as well as delivery of chemicals. The microfabrication technique gives precise control over the capacitive and resistive characteristics of the electrode channels, as well as the flow resistance, which are important factors in patch clamp recording. The device is fabricated using an SOI wafer and Deep Reactive Ion Etching to create an array of cylindrical nozzles, each of which has a core of silicon dioxide and interior walls of silicon nitride. Vertical channel segments and plumbing holes are fabricated by deep reactive ion etching through the wafer. Important electrical properties of the device were characterized, and patch clamping was attempted.

Micro-electro-mechanical systems (MEMS) are batch fabricated miniature systems with both electrical and non-electrical (e.g. mechanical) functionalities. Examples of such MEMS are miniature mechanical switches, acceleration sensors, gyroscopes etc… RF-MEMS devices are an emerging class of MEMS devices used for RF signal processing in telecommunication applications. For example, metal surface micro-machined switches are a promising alternative for current solid state switches or mechanical reed relays due to their potential for miniaturization, integration and improved performance. RF-MEMS switches normally consist of a metal bridge or cantilever electrode, which can be pulled in towards a bottom electrode by electrostatic forces [1]. The bridge material is often chosen to be Al or an Al-alloy because of its low resistivity [2]. During each switch cycle the metal top electrode goes through a mechanical stress cycle. These successive stress cycles might lead to reliability problems such as fatigue and/or creep. This is certainly the case for Al, which is known to have a low creep resistance [3]. Several alternatives have been investigated as alloying Al with other materials [4], or replacing Al with metals having better mechanical properties, specifically low creep, such as Tungsten (W) or Tantalum (Ta). Especially high melting point materials are interesting alternatives as the creep resistance is expected to increase with the melting point [5]. If we consider surface micromachined RF switches, the pull in voltage and sheet resistance are two important parameters that should be taken into consideration while selecting the switch material. In general, the pull in voltage depends on mean stress, lateral dimensions, layer thickness and Young's modulus. In case of stress free films, a material having a low Young's modulus is preferred for low pull-in voltage. As W is known to have high intrinsic stress [6], and a higher Young's modulus compared to Ta [7], it is instructive to consider Ta as a structural material for surface micromachining applications.

Dielectrophoresis, the movement of particles in non-uniform AC electric field, was used to separate live and heat-treated Listeria innocua cells with great efficiency on micro-fabricated devices with interdigitated electrodes by utilizing the difference of dielectric properties between live and dead cells. Both live and dead cells are found to be only able to collect either at the centers of the electrodes in negative dielectrophoresis or at the electrode edges in positive dielectrophoresis. Cell viability was characterized by a rapid method using epifluorescence staining. The dependency of the applied AC signal's frequency on the dielectrophoresis of different cells, as well as the orientation of the cells on the electrodes in the dielectrophoresis, is also observed and discussed. This on-electrode manipulation and separation of cells can prove to be useful in micro-scale diagnostic applications in biochips.

Bilayer microbridges of aluminum and hydrogenated amorphous silicon are fabricated using thin film technology and surface micromachining at low temperatures on glass substrates. The microstructure is electrostatically actuated by applying a voltage between the bridge and a metal gate counter electrode placed beneath it. The movement is measured with a precision close to 0.1 Å by sensing the magnetic field of a permanent magnet, deposited and patterned on top of the microbridge, with an integrated spin valve magnetic sensor. The deflection of the bridge is at the same time monitored using an optical setup. The deflection of the structures is studied as a function of the driving applied gate voltage and bridge length and experimental results are analyzed with an electromechanical model.

Five different methods were explored for the formation of the lift-off mold. In the first method, a tri-level resist scheme was used to generate a lift-off mold. The lift-off mold has a vertical wall slope. In the second method, an aluminum/photoresist double layer was used as the mold material. An overhang structure is obtained by undercutting the underlying aluminum layer. In the third method, a composite layer of two different photoresists (AZ 1811 and LOR 10A) was used to form the lift-off mold. With the fast dissolution rate of the underlying LOR 10A photoresist an overhanging mask structure was obtained. Since regular photoresists were used as part of the mold material in these three lift-off methods, they are suitable for thin film materials with thickness of ≤2 μm and processing temperatures ≤100°C. For thicker device film materials and higher processing temperature (∼200°C), methods 4 and 5 using negative resists, Futurrex NR7-3000PY (∼3 μm) and SU-8 (∼50 μm), respectively, were experimented with to form the lift-off mask. The fabrication processes used in these two methods were relatively simple and a negative wall slope was readily obtained. While Futurrex photoresist is easily stripped by the resist remover provided by the manufacturer, there is no effective chemical solution available for the removal of SU-8. We found that plasma etching with a mixture of O2 and C2F6 can be used for the removal of SU-8.

Here we describe the evolution of a silicon, MEMS-based chip design developed for infrared gas and chemical detection. The “Sensor-Chip,” with integrated photonic crystal and reflective optics, employs narrow-band optical emission/absorption for selective identification of gas and chemical species. Gas concentration is derived from attenuated optical power, which results in a change in device set point. This change in temperature results in a change in device resistance, via the TCR of the Si. Thermal non-uniformity across the device results in optical “noise” and accelerates localized thermal and electrical failures. This paper reports the influence of processing and design, on achieving uniformly heated, high reliability devices. Specifically, we examine the role of contacts, drive scheme, and device thermal distribution on chip design. Experimentally the temperature uniformity was characterized using an infrared camera. Experimental results indicate that the design of the contact areas in combination with the device design is essential for the reliable performance of the Sensor-Chip. Redesigned devices were fabricated and demonstrated as highly-selective gas and chemical sensors.

Nanogap capacitors are fabricated for DNA hybridization detection. Without labeling, the nanogap capacitors on a chip can function as DNA microarray sensors. The difference in dielectric properties between single-stranded DNA and double-stranded DNA permits use of capacitance measurements to detect hybridization. To obtain high detection sensitivity, a 50 nm gap capacitor was fabricated using a Si-nanotechnology. To ensure proper measurement of DNA's dielectrical properties, the probe ssDNA was first immobilized onto the electrode surface using self-assembly monolayers and allowed to hybridize with the target ssDNA. The capacitance changes were measured for 35-mer homonucleotides. The self-assembly monolayer and DNA immobilization events were verified independently by contact angle measurement and FTIR. Capacitance values are measured at frequencies ranging from 75 kHz to 5 MHz, using 0 VDC bias and 25 mVAC signals. Approximately 9% change in capacitance was observed after DNA hybridization at 75 kHz.

Cyclic Olefin copolymers (COC) are a new class of polymers that may prove to be extremely useful in injection molding of micron scale fluidic devices. In microfluidic devices it is desirable to have a hydrophilic surface such as in flow driven by capillary action. However, such hydrophilic surfaces tend to display protein deposition when contacted with blood unlike hydrophobic surfaces. Alternatives to capillary action are then needed to control fluid flow for hydrophobic surfaces. Our goal in this research is to tune the slightly hydrophobic COC surfaces through simple surface modifications that are amenable to injection molding and other processing methods. In this study, the surface of an injection molded microfluidic component made from COC was modified in order to change the surface properties important to bio-fluidic devices. Some of the techniques used in this study were plasma treatments and ASG (aerosol gel) coating. Plasma treatments were conducted by using O2, CF4 and their combination gas. O2 treated surfaces became hydrophilic with increasing time of treatment. Combining O2 and CF4 made the surfaces more hydrophobic compared to CF4 only. The structural changes after the plasma treatments were examined by ATR (Attenuated Total Reflectance) spectroscopy. Titania and silica particles from the ASG process were synthesized from titanium iso-propoxide and tetraethoxysilane, respectively. Titania coated surfaces became more hydrophilic and the silica coated surfaces did not have much change in their surface characteristics. The hydrophobicity of the plastic surfaces was measured by their contact angle with water. The implication of these treatments on bio-fluidic devices and their adaptation to the injection molding process will also be discussed.

The micromachined Planar High Field Asymmetric Waveform Ion Mobility Spectrometer (PFAIMS) is a novel detector for chemical and biological sensing applications. This detector fills an unmet market need, providing spectrometer capabilities and extremely high sensitivity, at a cost comparable to stand-alone sensors. The PFAIMS is quantitative, and has detection limits down to the parts-per-trillion. The performance of the PFAIMS in a number of applications ranging from industrial to biomedical, where it is used as both a stand alone sensor, and as a gas chromatographic detector are demonstrated. These applications include the detection of xylene isomers and non-invasive medical diagnosis through breath analysis.