Bulk Micromachining Research Articles

Wedge Mode Propagation Characteristics of Triangular–shaped Surface Plasmon Waveguide

Abstract: In this paper, we investigate wedge mode propagation characteristics of a triangular-shaped surface plasmon waveguide. This structure consists of a thin metal layer deposited onto the surface of a triangular–shaped silicon waveguide which could be fabricated on a silicon–on–insulator wafer by wet–bulk micromachining. These elements are embedded in a dielectric medium (such as air) to form metal–dielectric interface and the surface plasmon wave propagates at the wedge of metal layer. The influence of structural parameters such as metal layer, height of silicon waveguide and effect of fabrication on the wedge mode propagation characteristics of the waveguide is analyzed by using numerical simulation.
 Keywords: V–shaped waveguide, Surface plasmon polariton, plasmonic waveguide.

Surface micromachining of polydimethylsiloxane for microfluidics applications.

Polydimethylsiloxane (PDMS) elastomer has emerged as one of the most frequently applied materials in microfluidics. However, precise and large-scale surface micromachining of PDMS remains challenging, limiting applications of PDMS for microfluidic structures with high-resolution features. Herein, surface patterning of PDMS was achieved using a simple yet effective method combining direct photolithography followed by reactive-ion etching (RIE). This method incorporated a unique step of using oxygen plasma to activate PDMS surfaces to a hydrophilic state, thereby enabling improved adhesion of photoresist on top of PDMS surfaces for subsequent photolithography. RIE was applied to transfer patterns from photoresist to underlying PDMS thin films. Systematic experiments were conducted in the present work to characterize PDMS etch rate and etch selectivity of PDMS to photoresist as a function of various RIE parameters, including pressure, RF power, and gas flow rate and composition. We further compared two common RIE systems with and without bias power and employed inductively coupled plasma and capacitively coupled plasma sources, respectively, in terms of their PDMS etching performances. The RIE-based PDMS surface micromachining technique is compatible with conventional Si-based surface and bulk micromachining techniques, thus opening promising opportunities for generating hybrid microfluidic devices with novel functionalities.

Micro thermal conductivity detector with flow compensation using a dual MEMS device

A generic method to reduce the in-line flow dependence of thermal conductivity detectors (TCDs) is presented. The principle is based on a dual-MEMS device configuration. Two thin-film sensors on membranes in parallel in the gas stream on the same chip are differentially operated. Both micro-TCDs are designed to be identical in terms of contact with the main gas flow, however a different depth of the detection chamber results in a different response to the thermal conductivity of the sample gas. Static and dynamic simulations have been performed to characterize the design of the fabricated structures. Devices have been fabricated in a MEMS process using a combined surface- and bulk micromachining process. The devices have been characterized statically and dynamically. Measurements on prototypes show that depending on the range of gases, device size and flow range device the effect of flow on the thermal conductivity can be reduced by a factor 4–15.

Rotation Control and Characterization of High-Speed Variable-Capacitance Micromotor Supported on Electrostatic Bearing

This paper presents the rotation control design and experimental performance of a microelectromechanical systems (MEMS) variable-capacitance motor where a free-spinning rotor is suspended and centered in an evacuated vacuum cavity by a contactless electrostatic bearing. The micromachined device is based on a glass/silicon/glass bonding structure, fabricated by bulk micromachining, driven by a three-phase electrostatic motor, and used as an angular rate gyroscope by spinning-up the rotor rate over 104 r/min. A closed-loop phase commutation scheme is proposed in our rotation design where three-phase drives are switched depending on one channel of the rotor's angular position. The design of the micromotor spin-up and constant-speed operation is described based on the proposed electronic commutation. Experimental results of the motor spin-up process under different vacuum settings, static, and dynamic characteristics of the constant-speed control loop together with scale factor of the spinning-rotor gyroscope are described for the device operated in vacuum. It is indicated that the rotor can be spun up to 2.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> r/min within 400 s, and up to 2.96 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> r/min in steady state under a drive voltage of 11.8 V. Measurement data in constant-speed control mode show that the standard deviation of the spin rate error is 0.07 r/min at a rated speed of 1.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> r/min.

Fringing capacitance and tolerance of DRIE effect on the performance of bulk silicon comb-drive actuator

A bulk silicon comb-drive actuator with low driving voltage and large displacement is presented in this paper. The bulk silicon comb-drive actuator is fabricated by a simple bulk micromachining process based on the low temperature Au–Au bonding technology. A cascade folded beam is designed to improve the displacement of comb-drive actuator at low driving voltages. The instability of the whole system decreases by utilizing unequal wide comb fingers design. The fringing capacitance and the fabrication tolerances together with their effects on the performances of the comb-drive actuators are also discussed. The measurement results show that the capacitance change rate and the displacement change rate of the comb-drive actuator are 1.5 fF/V2 and 0.125 μm/V2, respectively. The displacement of the actuator can reach 28.5 μm at 15 V driving voltages. The experimental results of the comb-drive actuator are in good agreement with the modified theoretical predictions.

Determination of precise crystallographic directions for mask alignment in wet bulk micromachining for MEMS

In wet bulk micromachining, the etching characteristics are orientation dependent. As a result, prolonged etching of mask openings of any geometric shape on both Si{100} and Si{110} wafers results in a structure defined by the slowest etching planes. In order to fabricate microstructures with high dimensional accuracy, it is vital to align the mask edges along the crystal directions comprising of these slowest etching planes. Thus, precise alignment of mask edges is important in micro/nano fabrication. As a result, the determination of accurate crystal directions is of utmost importance and is in fact the first step to ensure dimensionally accurate microstructures for improved performance. In this review article, we have presented a comprehensive analysis of different techniques to precisely determine the crystallographic directions. We have covered various techniques proposed in the span of more than two decades to determine the crystallographic directions on both Si{100} and Si{110} wafers. Apart from a detailed discussion of each technique along with their design and implementation, we have provided a critical analysis of the associated constraints, benefits and shortcomings. We have also summed up the critical aspects of each technique and presented in a tabular format for easy reference for readers. This review article comprises of an exhaustive discussion and is a handy reference for researchers who are new in the field of wet anisotropic etching or who want to get abreast with the techniques of determination of crystal directions.

A MEMS torsion magnetic sensor with reflective blazed grating integration

A novel magnetic sensor based on a permanent magnet and blazed grating is presented in this paper. The magnetic field is detected by measuring the diffracted wavelength of the blazed grating which is changed by the torsion motion of a torsion sensitive micro-electromechanical system (MEMS) structure with a permanent magnet attached. A V-shape grating structure is obtained by wet etching on a (1 0 0) SOI substrate. When the magnet is magnetized in different directions, the in-plane or out-of-plane magnetic field is detected by a sensor. The MEMS magnetic sensor with a permanent magnet is fabricated after analytical design and bulk micromachining processes. The magnetic-sensing capability of the sensor is tested by fiber-optic detection system. The result shows the sensitivities of the in-plane and out-of-plane magnetic fields are 3.6 pm μT−1 and 5.7 pm μT−1, respectively. Due to utilization of the permanent magnet and fiber-optic detection, the sensor shows excellent capability of covering the high-resolution detection of low-frequency signals. In addition, the sensitive direction of the magnetic sensor can be easily switched by varying the magnetized direction of the permanent magnet, which offers a simple way to achieve tri-axis magnetic sensor application.

Modelling and characterisation of a micromachined resonant pressure sensor with piezoelectric excitation and sensing

A resonant pressure sensor with piezoelectric excitation and sensing is presented, which is realised by combining a micromachined diaphragm and a quartz resonator. The sensor performance is improved by introducing double enhancement beams into the diaphragm. Finite element method is utilised to reveal the validity and optimise the structural dimensions. A self-excitation circuit is designed to drive the resonator into steady oscillation. The sensor chip is fabricated by bulk micromachining technologies and packaged in a metal shell in the inert gas environment for characterisation. The performance characteristics including the short-term drift, temperature characteristic and quality factor are investigated. The experimental results demonstrate that the sensor features a high sensitivity of 100.45 Hz/kPa and a short-term drift <14 ppm in the operating range of −10∼10 kPa at room temperature. Owing to the excellent performance in sensitivity and accuracy, the sensor can be applied for micropressure measurement.

Double-meander spring silicon piezoresistive sensors as microforce calibration standards

A transferable force calibration standard based on a silicon microelectromechanical sensor has been designed, fabricated, and characterized for micrometrology applications. Two essential elements of double-meander springs and full piezoresistive etched p-silicon-on-insulator Wheatstone bridges (WBs) are integrated to the sensor for enhancing the device’s sensitivity and eliminating the current leakage during an active sensing operation, respectively. The design process is supported by three-dimensional finite element modeling to select the optimal proposed sensors as well as simulating their mechanical and electrical properties in the desired force range (≤1000 μN). To fabricate the microforce sensors, a bulk micromachining technology is used by frequently involving an inductively coupled plasma deep reactive ion etching at cryogenic temperature. Several optical and electrical characterization techniques have been utilized to ensure the quality of the fabricated WBs, where their measured offset voltage can be down to 0.03±0.071 mV/V. In terms of its linearity, the fabricated device exhibits a small nonlinearity of <3%, which leads this sensor to be appropriate for precise microforce standard.

Design and Manufacturing of a Passive Pressure Sensor Based on LC Resonance.

The LC resonator-based passive pressure sensor attracts much attention because it does not need a power source or lead wires between the sensing element and the readout system. This paper presents the design and manufacturing of a passive pressure sensor that contains a variable capacitor and a copper-electroplated planar inductor. The sensor is fabricated using silicon bulk micro-machining, electroplating, and anodic bonding technology. The finite element method is used to model the deflection of the silicon diaphragm and extract the capacitance change corresponding to the applied pressure. Within the measurement range from 5 to 100 kPa, the sensitivity of the sensor is 0.052 MHz/kPa, the linearity is 2.79%, and the hysteresis error is 0.2%. Compared with the sensitivity at 27 °C, the drop of output performance is 3.53% at 140 °C.

Design, analysis, and characterization of stress-engineered 3D microstructures comprised of PECVD silicon oxide and nitride

Microelectromechanical systems (MEMS) are typically 2D or quasi-3D structures fabricated using surface and bulk micromachining processes. In this work, an approach for 3D structure fabrication based on stress engineering is demonstrated. Specifically, sub-mm 3D spherical cage-like structures are realized through the deformation of bilayers of residually-stressed silicon oxide and silicon nitride with micrometer-scale thicknesses. Analytical and finite models to predict the shape of stress-engineered structures based on geometry and residual stress are described and used for structure design. A systematic experimental study was performed to quantify residual stresses in silicon nitride films made by plasma-enhanced chemical vapor deposition (PECVD). The measurements show that the residual stress of PECVD silicon nitride can be tuned over a wide range of tensile stresses through the control of deposition parameters, such as flow rate and power. Stress engineered 3D cage-like structures comprised of PECVD silicon nitride and oxide films were fabricated. 3D structures with a range of curvatures were demonstrated. The measured geometry of the fabricated structures are in good agreement with predictions from analytical and finite element models.

Development of Multi-Degree-Of-Freedom Piezoelectric Energy Harvester Using Interdigital Shaped Cantilevers.

A piezoelectric vibration energy harvester with interdigital shaped cantilever was developed by using silicon bulk micromachining technology. The proposed energy harvester was designed to obtain multi degree-of-freedom (m-DOF). Most of the piezoelectric vibration energy harvesters are comprised of mass-loaded cantilever beams having several resonant frequencies. The second resonant frequency of such a device has lower amplitude compared to its first resonant frequency (fundamental frequency). Therefore, the interdigital shaped cantilever has been proposed for multiple fundamental resonant frequencies. The fabricated piezoelectric vibration energy harvester is composed of main cantilever (MC), sub-main cantilever (SMC), and secondary cantilevers (SC). MC surrounds SMC and SC which have same dimension of 5600 x 800 x 10 μm3. The fabricated piezoelectric energy harvester can generate 51.4 mV(p-p) and 11 mV(p-p) of output voltages at 24.2 Hz and 33 Hz of its resonant frequencies by MC. Moreover, it can generate 8 mV(p-p) and 6.6 mV(p-p) of output voltages at 24.2 Hz and 33.2 Hz of its resonant frequencies by SMC; and 364 mV(p-p) of output voltage at 33.6 Hz of its resonant frequency by SC.

Micromachined multiple focused-ion-beam devices

Micromachined multiple focused-ion-beam devices were developed, which were composed of an array of ion sources, electrostatic extractors, and lenses to achieve concurrent processing of microdevices with high throughput. All of the parts of the devices were fabricated and packaged on the scale of a microchip, employing Si bulk micromachining technologies. Ionic liquids (ILs) were used to generate the ion species instead of conventional liquid metals such as Ga and In. Three types of testing devices were fabricated to comprehensively evaluate the system. The first, device 1, consisted of the emitter arrays and a biased Si target, whereby the etching characteristics of beams generated with several ILs were evaluated. The peaks of SiF+, SiF2+, SiF3+, and SiF4+, which provide evidence for reactive etching of Si, were obtained by in situ monitoring of the gas composition. Compared with a conventional Ga ion source, the ion beams emitted from the IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide exhibited a 5.4-fold sputtering yield of 11.9 atom/ion at an acceleration voltage of 5 kV. In device 2, the electrostatic extractor and lens were mounted on the emitter array, and control of the divergence angle of the ion beam was confirmed. In device 3, which has a separated extractor on the electrostatic lens plate, an individual control of ion beams was demonstrated.

The MEMS four-leaf clover wideband vibration energy harvesting device: design concept and experimental verification

In this contribution, we discuss a novel design concept of a high-performance wideband MEMS vibration energy harvester (EH), named four-leaf clover (FLC EH-MEMS) after its circular shape featuring four petal-like mass-spring systems. The goal is to enable multiple resonant modes in the typical range of vibrations scattered in the environment (i.e., up to 4---5 kHz). This boosts the FLC conversion capability from mechanical into electrical energy exploiting the piezoelectric effect, thus overcoming the common limitation of cantilever-like EHs that exhibit good performance only in a very narrow band of vibration (i.e., fundamental resonant mode). The FLC concept is first discussed framing it into the current state of the art, highlighting its strengths. Then, after a brief theoretical introduction on mechanical resonators, the FLC EH-MEMS device is described in details. Finite Element Method (FEM) analyses are conducted in the ANSYS Workbench™ framework. A suitable 3D model is built up to perform modal simulations, aimed to identify mechanical resonant modes, as well as harmonic analyses, devoted to study the mechanical and electrical behaviour of the FLC EH-MEMS (coupled field analysis). The work reports on experimental activities, as well. Physical samples of the FLC EH-MEMS device are fabricated within a technology platform that combines surface and bulk micromachining. Thereafter, specimens are tested both with a laser doppler vibrometer measurement setup as well as with a dedicated shaker-based setup, and the results are compared with simulations for validation purposes. In conclusion, the FLC EH-MEMS exhibits a large number of resonant modes scattered in the tested range of vibrations (up to 15 kHz) already starting from frequencies as low as 200 Hz, and expected levels of converted power better than 10 µW.

Etchant wettability in bulk micromachining of Si by metal-assisted chemical etching

Wet bulk micromachining of Si by metal-assisted chemical etching (MaCE) has successfully been demonstrated. Based on the mechanism of defective etching results from Ag and Au metal catalyst experiments, the wettability of etchant solution, in addition to metal type, has been found to have profound effect on the etching process. Addition of low surface tension co-solvent, ethanol in this work, into conventional etchant formulation has enabled complete wetting of etchant on surface, which prevents hydrogen bubble attachment on sample surface during the etching. The complete elimination of bubble attachment guarantees very uniform etch rate on all over the sample surface, and thus prevents premature fragmentation/rupture of catalyst metal layer. Under the optimized etching conditions, the MaCE could be done for up to 12h without any noticeable film rupture and thus etching defects. Thanks to very smooth surface of the etched patterns, conformal contact and direct bonding of elastomer on such surface has been easily accomplished. The method demonstrated here can pave the way for application of simple, low-cost MaCE process in the bulk micromachining of Si for various applications.

Fabrication and characterization of a microaccelerometer based on resonant-tunneling diodes

A microaccelerometer based on gallium arsenide (GaAs) resonant-tunneling diodes (RTDs) is demonstrated. The input acceleration signal can be transformed into an output electrical signal using the meso-piezoresistive effects of the RTDs located at the root of the detection beams. Finite element simulations were performed to design, analyze, and optimize the structures of the accelerometer. The accelerometer was fabricated using a combination of GaAs IC surface and bulk micromachining techniques. Vibrating tests and shock tests were conducted to investigate the accelerometer characteristics. The experimental results revealed that the sensitivity of the RTD accelerometer was 7.91 mV/g. The noise resolution was ∼1.264 mg/√Hz, and the working frequency was up to 3 kHz.

Precise identification of directions on Si{0 0 1} wafer using a novel self-aligning pre-etched technique

Micromirrors with a tilt angle of 45° are widely used in optical switching and interconnect applications which require 90° out of plane reflection. Silicon wet bulk micromachining based on surfactant added TMAH is usually employed to fabricate 45° slanted walls at the direction on Si wafers. These slanted walls are used as 45° micromirrors. However, the appearance of a precise 45° wall is subject to the accurate identification of the direction. In this paper, we present a simple technique based on pre-etched patterns for the identification of directions on the Si surface. The proposed pre-etched pattern self-aligns itself at the direction while becoming misaligned at other directions. The direction is determined by a simple visual inspection of pre-etched patterns and does not need any kind of measurement. To test the accuracy of the proposed method, we fabricated a 32 mm long rectangular opening with its sides aligned along the direction, which is determined using the proposed technique. Due to the finite etch rate of the plane, undercutting occurred, which was measured at 12 different locations along the longer edge of the rectangular strip. The mean of these undercutting lengths, measured perpendicular to the mask edge, is found to be 13.41 μm with a sub-micron standard deviation of 0.38 μm. This level of uniform undercutting indicates that our method of identifying the direction is precise and accurate. The developed method will be extremely useful in fabricating arrays of 45° micromirrors.

A Torsion MEMS Magnetic Sensor with Permanent Magnet and Fiber-optic Detection

A novel microelectromechanical system (MEMS) magnetic sensor based on fiber-optic detection is presented in this paper. The magnetic field is detected by measuring the tilting motion of a mechanical suspension with permanent magnet attached. When the magnet is magnetized in different directions, the magnetic sensor can detect the in-plane or out-of-plane magnetic field. The MEMS structure was designed analytically, fabricated by using a bulk micromachining process, and assembled with the permanent magnet manually. Finally, the magnetic-sensing capability of an as-prepared sensor was tested by assembling it into the fiber-optic detection system. The testing result shows a sensitivity of 2.86 mV/ $\mu \text{T}$ for the in-plane magnetic field and 6.57 mV/ $\mu \text{T}$ for the out-of-plane. According to the mechanical–thermal noise theory, estimated resolutions are, respectively, close to 142 and 62 nT for the in-plane and out-of-plane magnetic field, which suggest the adequacy for measurement in earth magnetic field range. The reported magnetic sensor shows the capability to fulfill the high-resolution detection of low-frequency signals, benefitted from the utilization of permanent magnet and fiber-optic detection. Moreover, the sensitive direction of reported magnetic sensor can be easily switched by varying its magnetized direction, which, therefore, paves the way to the integrated tri-axis magnetic sensor.

A Compact CMOS Compatible micro-Pirani Vacuum Sensor with Wide Operating Range and Low Power Consumption

A micro-Pirani vacuum sensor with an operating pressure range of more than 5 decades is described. The device is fabricated by applying a low-resolution and potentially low-cost front-side bulk micromachining step to a chip produced with a commercial CMOS technology. Maximization of the thermally coupled surfaces has been obtained by stacking all layers available by default in the CMOS process. This design choice and the integration of a low-noise, low-power readout interface allowed achievement of state-of-art performances with a fabrication approach affordable even to SMEs and small University laboratories.

Non-initial overlap bulk silicon comb-drive actuator with low-voltage and large-displacement

Two different kinds of bulk-Si comb-drive actuators are designed in this work: initial overlap comb-drive actuator and non-initial overlap comb-drive actuator. They are fabricated by a simple post-CMOS bulk micromachining process. A cascade folded beam is designed to achieve large displacement at low driving voltages. And non-initial overlap and unequal wide comb fingers are used to reduce the side instability and improve the pull-in voltage. The measurement results show that the non-initial overlap comb-drive actuator improves the pull-in voltage by 73.3% than the initial overlap comb-drive actuator, and the maximum displacement of the non-initial overlap actuator is larger by 78.9% than that of the initial overlap actuator.