Table of contents

The application of anisotropic alkaline etching for the fabrication of micromechanical parts is mostly based on the strong angular dependence of the etch rates in single-crystal silicon. In some applications, the smooth, defect-free etched surface is of high importance and from this point of view the strong anisotropy is disadvantageous. With this consideration, the composition of the alkaline etching solution has been changed by the addition of strong oxidizing agent. In such an etching solution, a mirror-like and defect-free silicon surface has been obtained, even after long etching times. In parallel, the reduction of the anisotropy, i.e. the decrease of the ratio (R_⟨001⟩/R_⟨111⟩) of the etch rates has occurred. In (001) oriented wafers, when the pattern alignment follows the ⟨100⟩ directions, this type of anisotropic etching produces vertical walls. The absolute values of the etch rates in all crystallographic directions are relatively low and they change with the doping concentration. The strong oxidizing component is supposed to ensure homogeneous oxidation (i.e. passivation layer formation) on the whole surface even though there are differences in the activation energy of the surface states. In this process the dissolution of the passivation layer is the rate-limiting chemical reaction.

Micromachining arbitrary 3D silicon structures for micro-electromechanical systems can be accomplished using gray-scale lithography along with dry anisotropic etching. In this study we have investigated two important design limitations for gray-scale lithography: the minimum usable pixel size and maximum usable pitch size. Together with the resolution of the projection lithography system and the spot size used to write the optical mask, the maximum range of usable gray levels can be determined for developing 3D large area silicon structures. An approximation of the minimum pixel size is shown and experimentally confirmed. Below this minimum, gray levels will be developed away due to an excessive amount of intensity passing through the optical mask. Additionally, oscillations in the intensity are investigated by the use of large pitch sizes on the optical mask. It was found that these oscillations cause holes in the photoresist spaced corresponding to the pitch used on the gray-scale mask and penetrate the thickness of the photoresist for thin gray levels. From the holes in the photoresist, significant surface roughness results when used as a nested mask in reactive ion etching, and the very thin gray levels are lost.

We investigate the electromechanical side instability and the stable travel range of comb-drive actuators. The stable travel range depends on the finger gap spacing, the initial finger overlap, and the spring stiffness ratio of the compliant suspension. Proper design of the suspension structure is the most effective way to stabilize the actuator and therefore to achieve a large deflection. In this paper, we propose an improved suspension design, the so-called tilted folded-beam suspension. Using such suspension, the stability of the comb-drive actuator is improved and the stable travel range is enhanced. We give the expressions for the spring constants of the proposed suspension, both in the stroke direction and perpendicular to it. The suspension designs are also studied numerically using the finite element method (FEM), in which the geometric nonlinearities, such as large deflections and stress stiffening, are considered. Analytical calculations and FEM simulations are compared. The results demonstrate that an enhanced stable travel range, compared to that of a comb-drive actuator with the most commonly used folded-beam flexure, can be achieved by using the proposed suspension design. Comb-drive actuators with various tilted folded-beam suspensions have also been fabricated using the standard surface micromachining technology and their operational performances have been characterized. The experimental results are in good agreement with the theoretical predictions.

In this paper, single deeply corrugated diaphragms (SDCDs) with various corrugation depths and initial stresses are studied extensively for applications to micromachined high-sensitivity devices. Finite-element model simulation results show that significant improvement in mechanical sensitivity can be achieved using a SDCD with larger corrugation depth. The diaphragm has been applied to the fabrication of a high-sensitivity microphone. The measurements show that the SDCD structure is promising in its applications to high-sensitivity devices.

The design and parameters of a new electrostatic micromotor with high energy output are described. The motor is created by means of microelectronic technology. Its operation is based on the electromechanic energy conversion during the electrostatic rolling of the metallic films (petals) on the ferroelectric film surface. The mathematical simulation of the main characteristics of the rolling process is carried out. The experimentally measured parameters of the petal step micromotors are shown. The motor operation and its efficiency are investigated.

Microfluidic cassettes that perform integrated biological sample preparation and DNA analysis require fluidic control and transport mechanisms built into the device. In this study, pneumatically actuated diaphragm pumps and valves were employed to achieve precise fluidic manipulation and enabled the execution of several sample-processing steps within a single cassette. However, the design of the microfluidic cassette to accomplish this multi-step fluidic protocol required a complex three-dimensional fluid path through valves, bends, various sized passageways and a porous filter for cell capture. In order to understand the fluidic behavior in such a device, measurements were taken of the pneumatic pressure delivered to the diaphragm pump as it pushed sample through the complicated fluidic pathway. Simultaneously monitored were the resulting volumetric flow rate, and the corresponding pre- and post-filter fluid pressures. The data enabled the construction of a model that simulated the fluidic action through the device using established fluid mechanics theory that closely matched flow rate and pressure data. The ability to simulate the behavior of diaphragm pumping and resulting fluidic movements in complex microfluidic devices provides a greater comprehension of this phenomenon and a useful tool in the application to future devices for biochemical analysis.

Microgenerators that could extract energy from the environment would be very attractive for powering some types of microsystems. One approach uses a mechanical resonant element to transfer seismic vibrations into useful motion. A novel configuration for an electromagnetic microgenerator and an electrostatic microgenerator has been modelled and confirmed by experiments at a small macroscopic scale. This design allows the possibility of a 'stacked' array to increase the output. However it is concluded, as with other studies, that few applications are plausible with the outputs that can be achieved. A power output of 6 nW is predicted for a typical single-element electromagnetic microgenerator but at a very low voltage. The electrostatic form delivers more useful voltage but, at the small scale, its reactive impedance is much too high for useful power delivery.

In this paper, anodic bonding between silicon wafer and glass wafer (Pyrex 7740) has been achieved at low temperature. The bond strength is measured using a tensile testing machine. The interfaces are examined and analyzed by scanning acoustic microscopy (SAM), scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS). The effects of the bonding parameters on bond quality are investigated using the Taguchi method. The bonding temperature used ranges from 200 °C to 300 °C. Almost bubble-free interfaces have been obtained. The bonded area increases with increasing bonding temperature. The unbonded area is less than 1.5% within the whole wafer for bonding temperature between 200 °C and 300 °C. The bond strength is higher than 10 MPa and increases with the bonding temperature. Fracture mainly occurs inside the glass wafer other than in the interface when the bonding temperature is higher than 225 °C. Higher bonding temperature results in more oxygen migration to the interface and more Si–O bonds. The bonding mechanisms consist of hydrogen bonding and Si–O chemical reaction.

This paper describes rotational infrared polarization modulators using a micro-electromechanical-system-based (MEMS-based) air turbine with different types of journal bearing. Three types of journal bearing, circular, symmetrical two-lobed and asymmetrical seven-lobed journal bearing, were compared. Using an optical displacement meter and a high speed camera, it was confirmed that all turbines exhibit three modes of rotation: (a) low speed mode, (b) intermediate mode and (c) high speed mode in this order, when decreasing N₂ flow rate to an aerostatic thrust bearing. In the low speed mode, the rotor is lifted up by excess flow to the thrust bearing, making a contact with an upper layer. In the high speed mode, the rotor is levitated without any contact to the upper and lower layers by balanced flow to the thrust bearing, and the maximum rotational speed of 74000 rpm was achieved using the asymmetrical seven-lobed bearing. The rotation in this mode is, however, discontinuous due to the collision between the rotor and the journal bearing. It was concluded that a journal bearing with sufficient load capacity is necessary for continuous high speed rotation.

This paper presents results of structural and thermal modeling of a z-axis rate integrating gyroscope. A strain energy method is used to obtain a structural model of the device, which is verified using finite element analysis. Based on a parametric analysis, an appropriate micromachining technology suitable for the fabrication of the gyroscope is identified. A sensitivity study shows that the operational modes of the proposed gyroscope remain matched under thermal and stress fluctuations, whereas a device with a commonly used H-type suspension shows a 31% frequency mismatch under thermal loading of 23.67 °C or 9 MPa of compressive stress.

Technologies for GaSb dry etching employing Cl₂/Ar-plasma discharges are reported. Etch rates higher than 2 μm min⁻¹ are achieved in a conventional reactive ion etching (RIE) chamber. To our knowledge these are the highest etch rates reported in GaSb dry-etch technology with conventional RIE. Also, three different single-layer soft mask processes are described and compared with respect to suitability for deep RIE of GaSb. Soft masks have many advantages over (metal) hard masks, such as easy and inexpensive processing, low pinhole density and high etching reproducibility. The well known AZ5214E resist from Clariant allows for GaSb etch depths of up to 6.4 μm keeping dimensional accuracy. Using TI 35ES photoresist, developed by MicroChemicals, GaSb etch profiles of up to 51 μm depth are obtained revealing considerable dimensional stability. This photoresist material is reported in the literature for the first time. By applying optimized GaSb dry-etch parameters, SU-8–50, an epoxy-type resist, developed by MicroChem Corp., will show an outstanding resist mask lifetime of up to 420 min, if SU-8–50 is spin-coated to a thickness of 80 μm. Thus it is possible to achieve GaSb etch depths beyond 100 μm. Deep dry etching of GaSb can be exploited to fabricate fiber or capillary connections, or to create substrate windows for backside-illuminated photodetectors for the mid-infrared (MIR) wavelength range.

In this paper, the design, analysis and experimental results of the self-sustained oscillation loop for a tunable surface micromachined resonant accelerometer, ACRC-RXL are presented. The fabrication process of the mechanical structure is also illustrated. For the oscillation loop analysis, an operator-theoretical approach is applied based on the describing function technique. Using the analytical results, feedback parameters are designed and the expected loop performance is characterized. Then the accelerometer system is practically implemented using the mechanical structure and signal processing electronics. The experimental results show that the developed accelerometer has a performance of bias stability of about 0.7 mg and a dynamic range over 10 g, which satisfies the navigation-graded sensor performance.

In this paper we propose a new algorithm to fit interatomic potentials. In the new algorithm, molecular dynamics simulations are applied to calculate the material properties which are used to match the experimental data during the fitting procedure. This includes the effect of atom relaxations in fitting calculations. An inter-generation projection genetic algorithm is used to optimize the fitting parameters until the error between the calculated and experimental material properties is within tolerance. This leads to a global optimal solution. The new algorithm significantly improves the accuracy and transferability of the fitted potential. It has been demonstrated by a numerical example of fitting potential of nickel.

In this paper we describe the design, construction and operation of a micropump that delivers continuous, ultra-low flow velocities at ∼100 μm s⁻¹. The pumping concept is based on the commonly observed phenomenon of transpiration in plant leaves. A liquid meniscus is pinned inside a microchannel by selective hydrophobic patterning and the evaporation rate of the liquid at the meniscus is controlled. The controlled evaporative flux results in a regulated flow of the liquid from a reservoir to the meniscus. Using this technique, precise flow control (5 nl min⁻¹) has been achieved in several channel geometries for extended periods of time (∼2 h). Various factors affecting the performance of the pump were studied and theoretical predictions along with experimental results are presented. Such a micropump could find applications in emerging biological assays such as single-molecule studies of DNA and cell adhesion analyses.

A new technology to pattern surface charges, either negatively or positively, using a standard photolithography process is introduced. A positively charged poly(allylamine hydrochloride) (PAH) layer is coated onto a negatively charged silicon oxide surface by electrostatic self-assembly (ESA). Combined with photolithography in a lift-off-based process, several different surface charge patterns were successfully produced. Due to definition of the pattern by photolithography, no limitations in the pattern geometry exist. Any surface charge pattern can be created to enable fine control of fluid motion in microfluidic devices. Physical properties of this PAH layer were characterized. The generation of a bi-directional shear flow was demonstrated by using alternating longitudinal surface charge pattern with a single driving force, i.e. an externally applied electric field inside a microchannel.

This paper reports on a multi-purpose two-axis micropositioner with sub-nanometer position sensing for precise feedback control. Along each axis it has an electrothermal actuator, a capacitive position sensor and a displacement amplifier that provides a gain of 3.37 for the sensor. It is fabricated from custom SOI wafers using dry etching, and each component is electrically and thermally isolated by silicon nitride. For a fabricated device of 65 μm thickness, the measured displacement sensitivity is 0.333 fF nm⁻¹, which corresponds to 0.3 nm resolution with available laboratory instrumentation. The range is ≈19 μm along each axis for the positioner, which corresponds to 66 μm travel in the sense combs. Using an external parallel inductor, a positioning displacement of 9.6 μm offers a shift of 240 kHz in L–C resonance, corresponding to a sensitivity of 25 Hz nm⁻¹.

In this paper, a phase-change type micropump is presented. This micropump consists of a pair of aluminum flap valves and a phase-change type actuator. The actuator is composed of a heater, a silicone rubber diaphragm and a working fluid chamber. The diaphragm is actuated by the vaporization and the condensation of the working fluid. The micropump is fabricated by the anisotropic etching, the boron diffusion and the metal evaporation. The dimension of the micropump is 8.5 mm × 5 mm × 1.7 mm. The forward and the backward flow characteristics of the flap valve illustrate the appropriateness as a check valve. The flow rate of the micropump is measured for various voltages, frequencies and duty cycles of the square-wave input. When the square-wave input voltage of 10 V is applied to the heater, the maximum flow rate of the micropump is 6.1 μl min⁻¹ at 0.5 Hz and the duty ratio of 60% for zero pressure difference. The maximum backward pressure when the flow rate is zero is 10 mm H₂O.

This paper presents the design of an ultra precision positioning system, which consists of a coarse stage and a fine stage. Two servo motors moving on recirculating ball screw are used to drive the coarse stage and three piezo actuators are used to provide the nanoscale positioning. The static and dynamic performances of the positioning system are formulated for designing the micro stage. Based on an evaluation of the system's natural frequency, a dual sever loop approach is used as the control mechanism. The main noise caused by the ambient environment is reduced by a vibration-suppressing table, and in the system control software, a digital Chebyshev filter is used to remove the noise caused by the magnetic chuck on the table. To correct the hysteresis and nonlinearity of PZT, Exact Model Matching (EMM) control law has been used, and therefore repeatability of the fine stage's motion can be improved considerably, the positive and negative movement can follow exactly the same path. A positioning accuracy of 8 nm is achieved over a traveling range of 200 mm with this system.

To directly acquire three-dimensional geometrical parameters of a micro-electro-mechanical system (MEMS) microstructure, an image measurement method has been developed and fully tested in this paper. By establishing the relationship between the luminosity of the measured surface and the gray level of the relevant image, the depth size can be obtained with an absolute error of 2 μm and a relative error found to be within 1%. And with the same system but a different imaging lens and light sources, the planar size can also be measured by utilizing the perspective projection imaging theory and relevant algorithms. The relative error of the planar size measured is less than 2% and the repeatable precision is higher than 0.01 mm. Compared with present measurement technologies for MEMS microstructures, this new proposed method has demonstrated adequate accuracy and reliability, more flexible in applications and convenient measurement conditions. As a practical application of this method, experimental results of micro cantilever beam measurement have been analyzed and discussed in this paper.

We have successfully fabricated and characterized a micro-cavity fluidic dye laser with metallic mirrors, which can be integrated with other microfluidic systems without adding further process steps. A laser dye solution is pumped through a microfluidic channel containing the laser cavity. The microfluidic channel structure, which is formed in SU-8 photoresist, is sandwiched between Pyrex glass wafers, bonded together at low temperature by means of SU-8. The laser was characterized using Rhodamine 6G laser dye dissolved in ethanol as the active medium, and optically pumped by a frequency doubled Nd:YAG laser. The dye solution was optimized, and lasing was observed at a wavelength of 570 nm with a full width half maximum linewidth of 5.7 nm and the optical pumping power density threshold for lasing was 34 mW cm⁻².

Electrothermal actuators have a very promising future in MEMS applications since they can generate large deflection and force with low actuating voltages and small device areas. In this study, a lumped model of a two-hot-arm horizontal thermal actuator is presented. In order to prove the accuracy of the lumped model, finite element analysis (FEA) and experimental results are provided. The two-hot-arm thermal actuator has been fabricated using the MUMPs process. Both the experimental and FEA results are in good agreement with the results of lumped modeling.

We describe a frontside Si micromachining process for the fabrication of suspended silicon oxide or nitride membranes for thermal sensors. Membrane release is achieved by means of lateral nearly isotropic dry etching of the bulk silicon substrate, the etching being optimized for high rates and high selectivity with respect to the photoresist used to protect the device and the membrane material. Lateral Si etch rates of the order of 6–7 μm min⁻¹ have been achieved in a high-density F-based plasma, which permit a reasonable etching time for the release of the membrane and the simultaneous formation of the cavity underneath ensuring thermal isolation of the final device. The proposed process can enhance the flexibility of device design and reduce the complexity of the fabrication process, since it does not require any additional steps other than the photoresist lithography for the protection of the active elements (e.g. polysilicon heaters and catalytic materials) that are formed on top of the membrane, due to the high selectivity of the process for Si etching with respect to the photoresist. We attempt to explain the observed dependencies of etch rates and selectivities on the plasma parameters and the dimensions of the released membranes by means of a simulator of the mechanisms involved in etching of structures.

In this paper, we present the fabrication process of a shape memory alloy (SMA) thin film in both monolithic and hybrid configurations. This provides an effective actuation part for a gripper made of SU-8 thick photoresist. We also extensively describe and discuss the assembly of the SMA thin film with the SU-8 mechanism. Measurements show that the SU-8 gripper is able to achieve an opening action of 500 μm in amplitude at a frequency of 1 Hz. Finite element model simulations indicate that a force of 50 mN, corresponding to 400 μm of opening amplitude, should be produced by the SMA actuator. Although the assembly of the TiNi SMA thin film with the SU-8 mechanism is demonstrated, the bond reliability needs further development in order to improve the thermal behavior of the interface. In this paper, we show that SU-8 is well suited as a structural material for microelectromechanical systems (MEMS) applications. An attractive feature in the MEMS design is that the SMA generated force is well matched with the elastic properties of SU-8. From the application point of view, a SMA-actuated SU-8 high-aspect-ratio microgripper can serve as a secure means to transport microelectronics device, because it provides good grasping and safe insulation. This is also a preliminary result for the future development of biogrippers.

Reliable microfluidic interconnectors are one of the basic building blocks of integrated fluidic and chemical reaction systems-on-chip. Though many ideas have been proposed and implemented in the literature for creating different kinds of macro-to-micro fluidic connections, development of integrated on-chip connectors for high temperature and pressure microfluidic applications has not been properly studied. Such connectors will be indispensable in true on-chip chemical processing applications for reactions which require more severe operating conditions than those possible using currently available interconnection techniques. In this paper we present novel microfluidic interconnects that can be used in applications involving operating temperatures of up to 275 °C and pressures in excess of 315 Psi (21.43 atm). The only wetted surfaces in this design are teflon, silicon and pyrex glass, making the design inert to most chemicals. High-pressure leakage, pull-out and high-temperature durability tests conducted on the interconnect show that the connections obtained are superior to those reported in the literature using other techniques. Structural analysis of the interconnect is carried out to illustrate the effect of interconnect geometry on strength and high-pressure performance.

We present experimental details of anodic bonding of optical fibers to silicon wafers having conventional thickness using an ultra-thin silicon layer as a stress-reducing layer. These results are expected to play a significant role in integrating microelectromechanical systems (MEMS) devices with optical fibers to form a new class of devices. Tensile bond strength measurements are presented as a function of bonding temperature that indicate optimum temperatures for both the optical-fiber to silicon bonding and silicon-to-silicon bonding. These measurements indicate that the maximum average tensile strength of the fiber-to-silicon bonds is 4.25 MPa at 400 °C and the maximum average strength of the thick silicon-to-silicon bonds is 7.93 MPa at 350 °C. We also demonstrate the simultaneous anodic bonding of up to five layers of silicon, each 250 μm thick. A 2D ANSYS simulation reveals that ultra-thin silicon does play a role as a stress-reducing layer.