Design and fabrication of PZT microcantilevers with freestanding structure

For developing freestanding piezoelectric microcantilevers with low resonant frequency, some critical mechanical considerations, especially cantilever bending, were given in this study. Two strategies, using piezoelectric thick films and adding a stress compensation layer, were calculationally analyzed for mitigating the cantilever bending, and then was applied for the fabrication of PZT freestanding microcantilevers. (100) oriented PZT thick films with the thickness of 6.93 μm were grown on the Pt/SiO₂/Si substrate by chemical solution deposition (CSD), and the SiO₂ layer with the thickness of 1.0 μm was kept under the PZT layer as a stress compensation layer of the freestanding microcantilevers. The freestanding microcantilevers fabricated with the micromachining process possessed the resonant frequency of 466.1 Hz, and demonstrated no obvious cantilever bending.     

High-aspect-ratio microstructures fabricated by X-ray lithography of polymethylsilsesquioxane-based spin-on glass thick films

 In this paper, a process for 200 μm high-aspect-ratio micro-optical (HARM) structures fabricated by deep X-ray lithography (DXRL) of polymethylislesuioane-based spin-on glass (SOG) thick films is presented. The SOG material used in the whole procedures is polymethylsilsesquioxane (GR650), which is a kind of sol-gel derived material and can be cured at a reasonable low temperature (75 °C). A technique to cast thick GR650 films was established in the overall process. After consolidation, the GR650 thick films were machined to reach 200 μm uniformly. Then, as negative resists, the GR650 thick films were patterned directly by DXRL. X-ray irradiated regions can be selectively retained with high structural resolution by development in an organic solvent, such as methanol. Parameter screening was done to find minimum and maximum doses needed for patterning/cross-linking, to vary development time, and to explore different film thickness. The whole process is a novel of technique to create HARM structures based on SOG materials without using molds. This technique can be extended to considerably larger structural heights. Surface and bulk compositions of the irradiated films were measured by XPS and Fourier transform infrared spectroscopy. Surface quality by roughness testing system (WYKO RST) was investigated to fabricate the microstructure with a high-accuracy surface. Received: 31 October 2001/Accepted: 23 January 2002 This work was partially supported by NSF/LEQSF (2001-04)-RII-02 grant “Micro/Nanodevices for Physical, Chemical and biological Sensors”.     

Thin-film sensor based tip-shaped split ring resonator metamaterial for microwave application

The artificially constructed materials based split ring resonators (SRRs) may have exotic electromagnetic properties and have received growing interest in recent years. Moreover, the resonance frequency shift of this material is extraordinarily sensitive to the changes in the capacitance of SRR, which makes SRR suit for microwave thin-film sensing applications. Based on such principle, the tip-shaped SRR metamaterial is presented as thin-film sensor in this paper to reduce device size and resonance frequency as well as to improve the Q-factor. The structure is placed inside an X-band waveguide with dimensions of 22.86 mm × 10.16 mm × 12.8 mm to investigate resonance frequency shift in different cases by numerical method. In contrast to the traditional structures, the tip-shaped design exhibits a miniaturization and sharper dip on resonance in their transmission spectra. Furthermore, the proposed sensor can deliver the sensitivity level of 16.2 MHz/μm and less than a 2 μm nonlinearity error when the uniform benezocyclobutene films from 100 nm to 50 μm thick are coated onto the fixed structure. These results indicate that the proposed thin-film sensor has high sensitivity and low nonlinearity error, and make it great promising application for wireless sensors in future.     

Morphology and magnetic properties of electroplated Co-rich Co-Zn thin films

This work explores the microstructure and magnetic properties of electrodeposited Co-Zn thin films. Using pulse-reverse electroplating technique, Co-rich Co-Zn films are deposited 0.4–1.9 μm thick from aqueous sulfate-based baths at low temperature (55°C). The influence of current density (25–100 mA/cm²) and electrolyte Zn concentration (0–0.28 M) on the microstructure and magnetic properties are investigated. All of the Co-Zn films exhibit higher out-of-plane coercivity, as compared to in-plane. With increasing current density, the out-of-plane coercivity decreases from 50 to 40 kA/m (628–500 Oe). The influence of the Zn concentration in the electrolyte is more pronounced, affecting the grain size, film composition, and magnetic properties. The best magnetic properties were obtained from a bath with 0.21 M Zn and an average current density of 25 mA/cm², resulting in a Co₉₇Zn₃ composition and an out-of-plane coercivity of 92 kA/m (1,160 Oe).     

Fabrication of compliant high aspect ratio silicon microelectrode arrays using micro-wire electrical discharge machining

This paper reports on the fabrication of high aspect ratio silicon microelectrode arrays by micro-wire electrical discharge machining (μ-WEDM). Arrays with 144 electrodes on a 400 μm pitch were machined on 6 and 10 mm thick p-type silicon wafers to a length of 5 and 9 mm, respectively. Machining parameters such as voltage and capacitance were varied for different wire types to maximize the machining rate and to obtain uniform electrodes. Finite element analysis was performed to investigate electrode shapes with reduced lateral rigidity. These compliant geometries were machined using μ-WEDM followed by a two step chemical etching process to remove the recast layer and to reduce the cross sections of the electrodes.     

Design, fabrication and testing of a high performance silicon piezoresistive Z-axis accelerometer with proof mass-edge-aligned-flexures

This paper presents design, fabrication and testing of a quad beam silicon piezoresistive Z-axis accelerometer with very low cross-axis sensitivity. The accelerometer device proposed in the present work consists of a thick proof mass supported by four thin beams (also called as flexures) that are connected to an outer supporting rim. Cross-axis sensitivity in piezoresistive accelerometers is an important issue particularly for high performance applications. In the present study, low cross-axis sensitivity is achieved by improving the device stability by placing the four flexures in line with the proof mass edges. Various modules of a finite element method based software called CoventorWare^™ was used for design optimization. Based on the simulation results, a flexure thickness of 30 μm and a diffused resistor doping concentration of 5 × 10¹⁸ atoms/cm³ were fixed to achieve a high prime-axis sensitivity of 122 μV/Vg, low cross-axis sensitivity of 27 ppm and a relatively higher bandwidth of 2.89 kHz. The designed accelerometer was realized by a complementary metal oxide semiconductor compatible bulk micromachining process using a dual doped tetra methyl ammonium hydroxide etching solution. The fabricated accelerometer devices were tested up to 13 g static acceleration using a rate table. Test results of fabricated devices with 30 μm flexure thickness show an average prime axis sensitivity of 111 μV/Vg with very low cross-axis sensitivities of 0.652 and 0.688 μV/Vg along X-axis and Y-axis, respectively.     

Fabrication of sub-micron structures for MEMS using deep X-ray lithography

Fabrication techniques of microstructures with high resolution and high aspect ratio are necessary for practical microelectromechanical systems (MEMS) that have high performance and integration. In order to fabricate microstructures with sub-micron resolution and high aspect ratio, deep X-ray lithography has been investigated using the compact synchrotron radiation (SR) light source called “AURORA”. An X-ray mask for sub-micron deep X-ray lithography, which is composed of 1 μm thick Au as absorbers, 2 μm thick SiC as a membrane and 625 μm thick Si as a frame, was designed. In preliminary experiments, the following results were achieved: EB resist microstructures with an aspect ratio of 22 corresponding with 0.07 μm width and 1.3 μm height were formed; a 10 μm thick PMMA resist containing no warp was formed by direct polymerization, enabling more precise gap control.     

Fabrication of metallic micromolds by laser and electro-discharge micromachining

A method of creating metallic micromolds with features that have high-aspect ratios is described in this paper. The proposed manufacturing process utilizes laser micromachining to cut the negative two-dimensional profiles of the desired microfeatures and fluidic network patterns on a 100 μm thick brass sheet. The positive relief of the cut pattern is then created by using electro-discharge micromachining (micro-EDM) die-sinking the metallic mask onto a brass substrate. The final substrate with the desired relief pattern becomes the molding tool used for either elastomer casting or thermoplastic hot embossing. To validate the proposed fabrication methodology and evaluate the quality of surface finishes, a brass mold master of a T-channel micromixer (50 μm width, 25 μm height) is developed and multiple replicated devices are cast on this mold using poly-di-methyl-siloxane (PDMS). The surface finish of both the original micromold master and final molded channels on PDMS are measured using an optical profiler and found to have a roughness of approximately 400 nm Ra. The ability of the proposed fabrication technique to create high-aspect ratio features is illustrated by manufacturing a Y-channel micromixer with an aspect ratio of 4. Experimental results are discussed and suggestions for improvement are presented.     

Micro-electroforming metallic bipolar electrodes for mini-DMFC stacks

This paper describes the development of metallic bipolar plate fabrication using micro-electroforming process for mini-DMFC (direct methanol fuel cell) stacks. Ultraviolet (UV) lithography was used to define micro-fluidic channels using a photomask and exposure process. Micro-fluidic channels mold with 300 μm thick and 500 μm wide were firstly fabricated in a negative photoresist onto a stainless steel plate. Copper micro-electroforming was used to replicate the micro-fluidic channels mold. Following by sputtering silver (Ag) with 1.2 μm thick, the metallic bipolar plates were completed. The silver layer is used for corrosive resistance. The completed mini-DMFC stack is a 3.5 × 3.5 cm² fuel cell stack including a 1.5 × 1.5 cm² MEA (membrane electrode assembly). Several MEAs were assembly into mini-DMFC stacks using the completed metallic bipolar plates. All test results showed the metallic bipolar plates suitable for mini-DMFC stacks. The maximum output power density is 9.3 mW/cm² and current density is 100 mA/cm² when using 8 vol.% methanol as fuel and operated at temperature 30°C. The output power result is similar to other reports by using conventional graphite bipolar plates. However, conventional graphite bipolar plates have certain difficulty to be machined to such micro-fluidic channels. The proposed micro-electroforming metallic bipolar plates are feasible to miniaturize DMFC stacks for further portable 3C applications.     

Use of a PDMS spring element for ink jet printing applications

Results of the design, microfabrication and testing of a proof-of-concept, diaphragm-type silicone sealing joint are presented. DRIE-etched cavities were filled with a flexible sealing element made of polydimethylsiloxane that supports a silicon piston. A series of sealing joints were produced with variable widths, and the displacement of the piston was measured after applying pressures of up to 1 bar above atmospheric pressure in 0.2 bar increments. Two masks were designed to produce several sets of silicone springs with widths of 2–10.5 μm, each consisting of a 10 μm thick silicon piston that is 2 mm long. Tests performed on the shear spring joints were found to give a displacement of 0.5 μm at 1 bar when the sealing width is 6 μm or more. The sealing joint with a 10 μm width was found to give a displacement of 0.9 μm and an elastic recovery of 88%. The results showed this type of joint in the form of an elastically-deforming seal provides sufficient displacement for propelling liquid droplets as part of a liquid propulsion system.     

Fabrication of RF MEMS variable capacitors by deep X-ray lithography and electroplating

Radio frequency micro electro-mechanical systems (RF MEMS) vertical cantilever variable capacitors fabricated using deep X-ray lithography and electroplating are presented. Polymethylmethacrylate (PMMA) layers of 100 μm and 150 μm have been patterned and electroplated with 70 μm and 100 μm thick nickel. A 3 μm thick titanium layer was used as plating base as well as etch time-controlled sacrificial layer for the release of the cantilever beam. The parallel plate layout includes narrow gaps and cantilever beams with an aspect ratio in nickel of up to 60 for 1 mm long features. Auxiliary structures support the beams and gaps during the processing. Room temperature electroplating significantly reduces the risk of deformations compared to the standard process temperature of 52°C. The capacitors operate in the 1–5 GHz range, and demonstrate good RF performance, with quality factors on the order of 170 at 1 GHz for a 1 pF capacitance.     

Micromechanical characterization of electroplated permalloy films for MEMS

In order to improve the reliability of MEMS designs, evaluating the mechanical properties of soft magnetic materials is needed. In this paper, we present a tensile testing method to characterize the mechanical properties of microscale electroplated permalloy (80 wt% Ni, 20 wt% Fe) films. The gauge section of the specimen is 50 μm wide, 100 μm long and 5 μm thick. The measured Young’s modulus of permalloy films is 96.4 GPa, and the tensile strength is 1.61 GPa. The fracture strain measured by the images of specimens is about 2%.     

Microfabrication of thick tungsten films for use as absorbers of deep X-ray lithography masks

 We fabricated thick (5 μm) tungsten (W) film patterns by sputtering and dry etching, and realized a new deep X-ray lithography mask. The X-ray mask with 5-μm-thick W absorbers could expose about 1-mm-thick resist structures. In the deposition process of W films, the column structure of about 0.2 μm grain size, from which pattern edge roughness originates, disappeared by adding nitrogen into the sputtering gas. W film etching was carried out by reducing gas pressure and cooling the substrate (−40 °C), and a side etch width of below 0.2 μm was obtained. From the results of the pattern edge roughness and the side etch width, a pattern fabrication accuracy below ±0.5 μm was achieved. Furthermore, film stress, which induces pattern distortion, was reduced to below 50 MPa by controlling the sputtering gas pressure. The obtained mask achieved a pattern distortion below ±0.3 μm. Received: 7 July 1999/Accepted: 29 May 2000     

Uncooled matrix IR detector based on optoacoustic cells and optoelectronic reading system

The matrix structure 200 × 200 of optoacoustic cells (OAC) for uncooled IR imager with optoelectronic reading system is fabricated and investigated at the first time. Cells 100 μm in diameter shaped on ZnSe window and filled by xenon. Photosensitive layer consisted of SiO₂ film with adsorption range 8–14 μm. Flexible membrane 0.1 μm of thick consisted of SiO₂ and Al films. Radiation temperature sensitivity and noise equivalent power with optics f/1 were 0.15 K/Hz^1/2 and 10 nW/Hz^1/2, respectively, and the thermal response time was below 30 ms. The article is published in the original.     

Process conditions in X-ray lithography for the fabrication of devices with sub-micron feature sizes

This article describes the fabrication of polymer structures with lateral dimensions in the sub-micron regime using hard X-rays (λ_c ≈ 0.4 nm) from the electron storage ring ANKA. Spincoated polymethylmethacrylate (PMMA) grades have been analyzed with respect to development rates and contrast. The contrast has been determined to be constant over a wide dose regime but rapidly decreases for dose values below 1 kJ/cm³. Films with a thickness from 2 to 11 μm have been patterned using a high resolution X-ray mask consisting of 2 μm thick gold absorbers on a suspended 1 μm thick silicon nitride membrane. The fabrication of sub-micron X-ray lithography structures with feature sizes down to 400 nm is confined by the mechanical parameters of the resist material and the process conditions. Surface tension after development limits the achievable aspect ratio of isolated pillars and walls, depending on the actual resist height. PMMA structures have been successfully used as template for electroplating of 1 μm thick gold to demonstrate the fabrication capability of sub-micron scale metal parts.     

Tensile testing of microfabricated thin films

 Mechanical properties of titanium thin films of 0.5 μm thickness and aluminum thin films of 1.0 μm thickness, microfabricated by magnetron sputtering, were measured by using a novel tensile machine. These thin films are difficult to handle because they are fragile, so the thin film specimens were fabricated by using semiconductor manufacturing technology in a silicon frame to protect them. The test section of these specimens was 300 μm in width and 1400 μm in gauge length. By gripping the thin film specimen with a new device using a micrometer, it could be mounted on the tensile machine easily. The stress-strain diagrams of both thin films were measured continuously in the atmosphere at room temperature. The experimental results indicated that the titanium thin film and the aluminum thin film had a smaller breaking elongation although they had a larger tensile strength than bulk pure titanium and bulk pure aluminum, respectively. Received: 31.10.96/Accepted: 14.11.96     

Super permeable nano-channel membranes defined with laser interferometric lithography

We report the design, fabrication, and testing of super permeable nano-channel membranes, characterized by the absolute control in the pore size at the nano-scale dimensions, large surface area, very high permeability, mechanical stability and durability. The membranes were fabricated using a unique nanotechnology process that combines laser interferometric lithography to define nano-channels (pores) and micro-machining to produce free-standing amorphous silicon membranes, allowing rapid and cost-effective mass production. The suspended membranes were defined as 50 nm thick a-Si, characterized by a very high porosity of approximately 20%, achieved by definition of large arrays of nano-channels. The dimensions of each individual nano-channel was 65 nm wide, 250 nm long. The measured apparent permeability was 0.14 ± 0.05 cm/min for each individual 70 μm × 70 μm membrane, representing one of the highest permeability values ever reported for this scale.     

Laser micro-structuring of magnetron-sputtered SnOx thin films as anode material for lithium ion batteries

SnO_x electrode thin films for lithium ion batteries were deposited by reactive and non-reactive rf magnetron sputtering of a SnO₂ target in an argon–oxygen atmosphere. Amorphous and nano-crystalline SnO_x films could be synthesized, with regard to the O₂:Ar volume ratio in the sputter gas which was adjusted to 0, 3.5 or 10%. Laser micro-structuring using a KrF excimer laser operating at a wavelength of λ = 248 nm was applied to create free-standing microstructures. Thus, the active surface of the anode material was significantly increased. Furthermore, it was expected that the large volume changes during electrochemical cycling of SnO_x could be better compensated by a microstructured surface. The laser parameters were optimized in a way which leads to structures without any defects and little debris. Depending on the laser fluence and pulse number, free-standing conical structures could be formed with a horizontal spacing of <0.5 μm up to 2 μm. The structured and unstructured thin films were cycled in a battery tester against metallic lithium. The structured SnO_x thin films exhibited significantly better battery performance with respect to cycling stability.     

Modeling and analysis of a 2-DOF bidirectional electro-thermal microactuator

In this paper, a four hot-arm U-shape electro-thermal actuator that can achieve bidirectional motion in two axes is introduced. By selectively applying voltage to different pairs of its four arms, the device can provide actuation in four directions starting from its rest position. It is shown that independent in-plane and out-of-plane motions can be obtained by tailoring the geometrical parameters of the system. The lumped model of the microactuator was developed using electro-thermal and thermo-mechanical analyses and validated using finite element simulations. The device has been fabricated using PolyMUMPs and experimental results are in good agreement with the theoretical predictions. Total in-plane deflections of 4.8 μm (2.4 μm in either direction) and upward out-of-plane deflections of 8.2 μm were achieved at 8 V of input voltage. The large achievable deflections and the higher degree-of-freedom of the proposed device compared to its counterparts, foresee its use in diverse MEMS applications.     

A piezoelectric micro-cantilever bio-sensor using the mass-micro-balancing technique with self-excitation

A biosensor was developed for using in a Lab-On-a-Chip (LOC). The sensor detects the change in the resonance frequency of a micro-cantilever with a piezoelectric film. This is the mass micro-balancing technique, which has been successfully used for detecting bio-materials in the quartz crystal microbalance (QCM). The PZT film, a piezoelectric film, is designed to act as both sensor and actuator. The geometry of the micro cantilever is optimized to maximize the sensitivity and minimize the environmental effects such as viscous damping and added mass effect in liquid. The fabricated sensor is composed of a 100 μm long, 30 μm wide, and 5 μm thick cantilever with a 2.5 μm thick piezoelectric (PZT) layer on it. The ratio of thickness to length of the micro cantilever is very high compared to others in micro cantilever-based studies. This high aspect ratio is the key to maximize the sensitivity and minimize the environmental effects. The fabricated micro sensor was tested by detecting the mussel gluing protein, the insulin-anti insulin binding protein and the poly T-sequence DNA.