Silicon Micromachining Research Articles

Millimeter-Wave Planar Antenna Array Based on Modified Bulk Silicon Micromachining Technology

In this communication, a planar antenna array is proposed in millimeter-wave bands by using a modified bulk silicon micromachining technology. The proposed antenna array is designed and fabricated based on air-filled microstrip line structures. By periodically blocking the microstrip line, fringing electric fields along the microstrip lines are excited in phase, which contributes to achieving high gain performance with a relatively compact feeding structure. Moreover, the proposed antenna array is designed and fabricated by a modified bulk silicon micromachining technology. The process is quite suitable for designing low loss antennas in millimeter-wave bands. In the process, the silicon wafers are deeply etched and gold-plated. As a result, microstrip line structures in the proposed antenna array are filled with air medium, which mitigates dielectric loss and improves radiation efficiency. A prototype of the proposed antenna array is fabricated and tested to verify the design strategy. The dimension of the antenna array is 49.40 × 20.00 × 0.75 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . The simulated -10 dB bandwidth is 58.8-59.7 GHz and the measured bandwidth is 58.8-59.5 GHz. In the operating band, the maximum simulated gain of the proposed antenna is 19.8 dBi and the minimum simulated gain is 18.3 dBi. The measurement results illustrate that the maximum gain of the proposed antenna array is 18.8 dBi and the minimum gain is 17.0 dBi.

CPW-Fed Wide Band Micro-machined Fractal Antenna with Band-notched Function

In this paper, a straightforward yet effective design methodology to design wideband antenna with band notched characteristics has been proposed. Sierpinski carpet fractal geometry has been used to realize the antenna structure. Co-planar waveguide feed is used with a novel structure to achieve larger impedance bandwidth and band notching characteristics. Proposed antenna is designed using High Frequency Structure Simulator (HFSS) on a low cost FR4 substrate (ɛr=4.4) which resonates at three frequencies 1.51 GHz (1.19-2.06GHz), 6.53 GHz and 8.99 GHz (4.44-9.54 GHz) while a band is notched at 10.46 GHz (9.32-11.92 GHz). The proposed antenna has an electrical dimension of 0.36 λm× 0.24 λm, here λm is the wavelength with respect to lowest resonating frequency of the antenna. The resonating and radiation characteristics of the antenna are verified experimentally. Further, investigations are made to achieve easy integration of the antenna to the monolithic microwave integrated circuits. For that the antenna has been designed on micro-machined high index Silicon substrate which improve matching and gain of the antenna. The results of the micro-machined Sierpinski carpet fractal antenna are highly convincing over the conventional FR4 based antenna.

3D-electrode integrated microsieve structure as a rapid and cost-effective single neuron detector

Using integrated silicon micromachining and thin-film technology, the fabrication of electrically functionalized microsieves for the study of 3D neuronal cell networks in vitro was a major challenge and is still very expensive at the current scale of device production, which is limited to fundamental research. Also, thin-film sidewall electrodes are in contact with the neurons and the microsieves need to be rigorously cleaned prior to reuse or the expensively integrated culture platform must be discarded. To simplify such microsieve studies on neuronal cell networks, we started analysis by optical techniques on polymer microsieves, which also proved to be valuable in our previous studies. Knowing the distribution of cells throughout the pores of the sieve, however, will enhance statistical relevance of these biological experiments. Hence, here, we present the feasibility study on a new technical concept for a cost-effective, fast, and reusable electrical platform to monitor the cell placement distribution in single-use 3D microsieves by a hybrid assembly approach in a label-free manner. The proposed system, having 3D electrodes integrated with microsieves, was compared with the thin-film sidewall electrodes that touch cells in a 3D simulation platform. Although a relatively thick and tapered insulating layer exists between cells and electrodes in the proposed 3D pluggable system, an impedance variation ratio of 3.4% on a measurable based impedance of ∼59 kΩ was obtained in these simulations and is very similar to the values for sidewall electrodes.

Isolation Enhancement for W-Band Coplanar Array Antennas Based on Silicon Micromachining Technology

A W-band decoupling structure sandwiched between a pair of high-gain step-profiled horn array antennas using the silicon micromachining technology is proposed. To suppress the surface wave, 18 choke slots with a depth of one quarter wavelength at 94 GHz are arranged in a triangular-grid array and the isolation can be averagely improved by about 7 dB with a range of 2-20 dB. This decoupling structure and two W-band coplanar array antennas are fashioned and fully integrated together by the silicon micromachining technique. Five 400 μm thick and one 650 μm thick silicon wafers are utilized and the total thickness of the antennas is 2.65 mm. Thanks to this technology, light weight, compact size, and high integration of the decoupling structure and array antennas can also be achieved. Additionally, the two array antennas share a common ground plane within a small edge-to-edge spacing of 6.05 mm where TM surface waves exist. Experimental studies validate the performance of the proposed triangular-grid choke slot array.

Silicon Micromachined D-Band Diplexer Using Releasable Filling Structure Technique

A D-band waveguide diplexer, implemented by silicon micromachining using releasable filling structure (RFS) technique to obtain high-precision geometries, is presented here for the first time. Prototype devices using this RFS technique are compared with devices using the conventional microfabrication process. The RFS technique allows etching large waveguide structures with nearly 90° sidewall angles for the 400-μm-tall waveguides. The diplexer consists of two direct-coupled cavity six-pole bandpass filters, with the lower and the upper band at 130-134 and 141-148.5 GHz, respectively. The measured insertion loss of the two bands is 1.2 and 0.8 dB, respectively, and the measured return loss is 20 and 18 dB, respectively, across 85% of the passbands. The worst case adjacent channel rejection is better than 59 dB. The unloaded quality factors of a single cavity resonator are estimated from the measurements to reach 1400. Furthermore, for the RFS-based micromachined diplexer, an excellent agreement between measured and simulated data was observed, with a center frequency shift of only 0.8% and a bandwidth deviation of only 8%. In contrast to that, for the conventionally micromachined diplexer of this high complexity, the filter poles are not well controllable, resulting in a large center frequency shift of 3.5%, a huge bandwidth expanding of over 60%, a poor return loss of 6 and 10 dB for the lower and the upper band, respectively, and an adjacent channel rejection of only 22 dB.

Fabrication of a MEMS Micromirror Based on Bulk Silicon Micromachining Combined With Grayscale Lithography

A 1D MEMS (Micro-Electro-Mechanical Systems) mirror for LiDAR applications, based on vertically asymmetric comb-drive electrostatic actuators, is presented in this work employing a novel fabrication process. This novel micromachining process combines typical SOI-based bulk micromachining and grayscale lithography, enabling the fabrication of combs actuators with asymmetric heights using a single lithography step in the active layer. With this technique, the fabrication process is simplified, and the overall costs are reduced since the number of required lithography steps decrease. The fabricated mirrors present self-aligned electrodes with a $2.8~\mu \text{m}$ gap and asymmetric heights of the movable and the fixed electrodes of 20 $\mu \text{m}$ and 50 $\mu \text{m}$ , respectively. These asymmetric actuators are an essential feature for the operation mode of this device, enabling both in resonant and static mode operation. A mirror field of view (FOV) of 54° at 838 Hz was achieved under low-pressure, when resonantly operated, and a FOV of 0.8° in the static mode.

Employing a MEMS plasma switch for conditioning high-voltage kinetic energy harvesters

Triboelectric nanogenerators have attracted wide attention due to their promising capabilities of scavenging the ambient environmental mechanical energy. However, efficient energy management of the generated high-voltage for practical low-voltage applications is still under investigation. Autonomous switches are key elements for improving the harvested energy per mechanical cycle, but they are complicated to implement at such voltages higher than several hundreds of volts. This paper proposes a self-sustained and automatic hysteresis plasma switch made from silicon micromachining, and implemented in a two-stage efficient conditioning circuit for powering low-voltage devices using triboelectric nanogenerators. The hysteresis of this microelectromechanical switch is controllable by topological design and the actuation of the switch combines the principles of micro-discharge and electrostatic pulling, without the need of any power-consuming control electronic circuits. The experimental results indicate that the energy harvesting efficiency is improved by two orders of magnitude compared to the conventional full-wave rectifying circuit.

Development of a Thin Film Gas Microsensor of Low Power Consumption for Monitoring of Environmental Pollutants

In this work the development and microfabrication of a gas sensor is described. The microsensor consists of a thin film of SnO2 deposited on a micro-machined silicon substrate [1] . This film is heated by a microheater integrated in the microsensor to obtain a temperature between 100 and 400 °C (microhot plates). With the aim to analyze the proposed design, simulations of the mechanical (structural rigidity, maximum and minimum sizes of the structures) and thermal behaviors (distribution of temperatures and electrical powers involved) were performed [2].The manufacturing process carried out in a clean-room area involved a series of steps that include: the design and mastering of masks, photolithography operations, sputtering metal deposition, chemical attacks with dry (RIE) and wet etching (with KOH), dicing of the silicon wafer in individual sensors of 3mm x 3mm and finally, the encapsulation of the sensors by means of wire bonding (see Fig.1)It is important to note that the silicon substrate is micromachined in order to reduce its thermal inertia and the power necessary to heat the sensor film. Such a film is deposited on a silicon nitride insulating membrane that is only 250 nm thick, which significantly reduces the consumption in terms of electrical power, with respect to the known commercial sensors.The SnO2 thin films were fabricated using the technique Rheotaxial Growth and Thermal Oxidation technique (RGTO) on a microhot plate[3]. RGTO is a technique composed of two steps: depositionof a Sn thin film on the substrate maintained at the temperature slightly above the Sn melting point and oxidation at a temperature up to 400 oC. In the first step of the RGTO technique tin was deposited by sputtering DC on the central part of the membrane by means of a shadow mask. During the deposition, substrates were heated at 310 oC . Thickness of the deposited tin was 100 nm. The second step, involving the thermal oxidation of Sn thin layer, was performed in two sub-steps: 2 h at 250 oC and 60 h at 450 oC in wet air. The final result is a granular film with quasi-spherical grain shape. The microstructure was characterized by X-ray diffraction analysis and scanning electron microscopy (SEM).The characterization of the sensors with gases at different concentrations was done. In the case of ammonia in air, a linear response between 50ppm (maximum permissible limit value) and 6ppm were obtained, the latter being its detection limit (Fig 2). Also H2S were tested (Fig. 3) . The measurements were made by applying a voltage of 5 V to the heater, obtaining dissipated power between 40 and 50 mW. The operating temperature of the sensor film under these conditions was approximately 180°C. On the other hand, the sensor film is connected to a potential of 1 V. The sensor response is then measured as the variation of the electrical resistance as a function of the response time.Additionally, electronic control and signal processing was developed, suitable for incorporating sensors into portable instruments. [4]The results of this development also allow us to have a platform for the microfabrication of gas sensors at the request of other public institutions or the private sector.

Study on the mechanism of ultrasonic-assisted water confined laser micromachining of silicon

The ultrasonic-assisted water confined laser micromachining is a novel micromachining method. This paper reports the experimental studies of ultrasonic power and water layer thickness on the laser (1064 nm nanosecond pulse laser) micromachining of silicon. Besides, this reports a systematic discussion on the influence of the acoustic streaming and bubbles (induced by laser) dynamic change on the machining process. The acoustic streaming and flow pattern under different ultrasonic power application and different water layer thickness using Particle Image Velocimetry (PIV). A finite element (FE) heat transfer model was employed to gain a better understanding of the effect of the acoustic streaming velocity on the temperature field change in laser processing. The bubbles (induced by laser) dynamic behavior is observed by the high-speed camera. The results showed that the ultrasonic-assisted laser ablation can yield much higher the material removal rate than laser cutting in water without ultrasonic-assisted. Besides, the increase in the ultrasonic power helps to improve the etching efficiency but is not apparent. When the water layer thickness is 1 mm, the cutting effect is significantly different from the laser ablation in the other three water layers. Through the PIV images and simulate results, it has been found that the influence of acoustic streaming on heat transfer in laser processing is small. Through high-speed camera imaging observations, the dynamic change of bubbles is an essential mechanism for the enhanced material removal rate and affect the machining performance in this study. This study helps understand the mechanism of ultrasonic-assisted water confined laser micromachining better and promoting this technique potential application in the micromachining of hard brittle materials.

Diamond turned micro machined metal diaphragm based Fabry Perot pressure sensor

Micro-machined Silicon (Si) diaphragm is extensively studied in Fabry Perot Interferometer (FPI) based pressure sensors, but its applications are limited to non-corrosive media due to poor corrosion resistance. Metal alloys such as Copper-Beryllium Alloy (CBA), Stainless Steel (SS) and Ni based alloys have comparable mechanical properties and much better corrosion resistance than Si. But their use as diaphragm for FPI based pressure sensor is much under reported. In this paper, design, analysis and fabrication of an FPI based pressure sensor with CBA diaphragm is discussed. Its sensing probe has two parts, a sensing head and a sleeve, welded together. The sensing head is a three-dimensional monolithic component which consists of pressure sensitive diaphragm and a step for FPI cavity. The sleeve guides the optical fiber to a fixed position such that the cleaved end of the fiber forms an FPI with the diaphragm. Both the sensing head and the sleeve are manufactured by Diamond Turning Machining (DTM) process and have CBA as material of construction. This manufacturing process ensures that flatness on interferometric side of the diaphragm and its parallelism with the fiber end are achieved within 2 µm; surface finish (Ra) on same side of the diaphragm is less than 10 nm. The sensor showed sensitivity of more than 1 µm/bar within the measurement range of 0–7 bar. DTM machined sensing probe with all essential FPI features (flatness, surface finish and parallelism) and welded diaphragm in FP pressure sensor are being reported for the first time to the best of our knowledge.

A Simple and Robust Fabrication Process for SU-8 In-Plane MEMS Structures.

In this paper, a simple fabrication process for SU-8 in-plane micro electro-mechanical systems (MEMS) structures, called “border-bulk micromachining”, is introduced. It aims to enhance the potential of SU-8 MEMS structures for applications such as low-cost/disposable microsystems and wearable MEMS. The fabrication process is robust and uses only four processing steps to fabricate SU-8 in-plane MEMS structures, simplifying the fabrication flow in comparison with other reported attempts. The whole fabrication process has been implemented on copper-polyimide composites. A new processing method enables the direct, laser-based micromachining of polyimide in a practical way, bringing in extra processing safety and simplicity. After forming the polymeric in-plane MEMS structures through SU-8 lithography, a copper wet etching masked by the SU-8 structure layers is carried out. After the wet etching, fabricated in-plane MEMS structures are suspended within an open window on the substrate, similar to the final status of in-plane MEMS devices made from industrial silicon micromachining methods (such as SOIMUMPS). The last step of the fabrication flow is a magnetron sputtering of aluminum. The border-bulk micromachining process has been experimentally evaluated through the fabrication and the characterization of simple in-plane electrically actuated MEMS test structures. The characterization results of these simple test structures have verified the following process qualities: controllability, reproducibility, predictability and general robustness.

MEMS Chip With Amplifier for 4-W Power Combining up to 100 GHz

Millimeter-wave and terahertz bands are very attractive for high-resolution radars and fifth-generation communication and beyond; nevertheless, it is very challenging to achieve high output power in silicon. To address this crucial issue, this article first presents an in-house developed silicon micromachining process. Using this process, a power combiner is formed in silicon waveguide, by dry etching and bonding two 8-in silicon wafers. Benefiting from the hollow, symmetrical, and accurately assembled waveguide structure, the power combiner features a measured low loss of 0.2 dB and a high amplitude/phase balance of ±0.15 dB/±2° from 92 to 96 GHz. Moreover, based on this power combiner and an in-house fabricated power amplifier in gallium nitride (GaN), this article further demonstrates a power combing prototype with a typical output power of 4 W, a power-added efficiency of higher than 12.9%, and a combining efficiency of higher than 92%.

An Electret-Augmented Low-Voltage MEMS Electrostatic Out-of-Plane Actuator for Acoustic Transducer Applications.

Despite the development of energy-efficient devices in various applications, microelectromechanical system (MEMS) electrostatic actuators yet require high voltages to generate large displacements. In this respect, electrets exhibiting quasi-permanent electrical charges allow large fixed voltages to be integrated directly within electrode structures to reduce or eliminate the need of DC bias electronics. For verification, a biased electret layer was fabricated at the inner surface of a silicon on insulator (SOI) structure facing a 2 μm gap owing to the high compatibility of silicon micromachining and the potassium-ion-electret fabrication method. A electret-augmented actuator with an out-of-plane motion membrane reached a sound pressure level (SPL) of 50 dB maximum with AC input voltage of alone, indicating a potential for acoustic transducer usage such as microspeakers. Such devices with electret biasing require only the input signal voltage, thus contributing to reducing the overall power consumption of the device system.

MEMS-based Micro-scale Wind Turbines as Energy Harvesters of the Convective Airflows in Microelectronic Circuits

As an alternative to conventional batteries and other energy scavenging techniques, this paper introduces the idea of using micro-turbines to extract energy from wind forces at the microscale level and to supply power to battery-less microsystems. Fundamental research efforts on the design, fabrication, and test of micro-turbines with blade lengths of just 160 µm are presented in this paper along with analytical models and preliminary experimental results. The proof-of-concept prototypes presented herein were fabricated using a standard polysilicon surface micro-machining silicon technology (PolyMUMPs) and could effectively transform the kinetic energy of the available wind into a torque that might drive an electric generator or directly power supply a micro-mechanical system. Since conventional batteries do not scale-down well to the microscale, wind micro-turbines have the potential for becoming a practical alternative power source for microsystems, as well as for extending the operating range of devices running on batteries.

Effect of pulse duration on the energy delivery under nonlinear propagation of tightly focused Cr:forsterite laser radiation in bulk silicon

We have investigated nonlinear laser ̶ matter interaction inside silicon under tight focusing conditions by continuously tuning driving pulse duration from femtosecond to picosecond timescales. Such tailoring of laser pulse width provides a new route for energy delivery into a microvolume avoiding two-photon absorption and plasma defocusing in the pre-focal region. As a result, we have achieved values of saturated deposited energy density and plasma electron concentration of as high as 1 kJ cm−3 and 1019 cm−3 respectively, which is lower than the threshold of irreversible structural transformation. For further increase of energy delivery inside silicon, a two-color technique supported by extremely tight focusing can be realized, forming a roadmap to the 3D industrial micromachining of planar bulk silicon.

Development of isoporous microslit silicon nitride membranes for sterile filtration applications.

The widely used 0.2/0.22 µm polymer sterile filters were developed for small molecule and protein sterile filtration but are not well-suited for the production of large nonprotein biological therapeutics, resulting in significant yield loss and production cost increases. Here, we report on the development of membranes with isoporous sub-0.2 μm rectangular prism pores using silicon micromachining to produce microslit silicon nitride (MSN) membranes. The very high porosity (~33%) and ultrathin (200 nm) nature of the 0.2 µm MSN membranes results in a dramatically different structure than the traditional 0.2/0.22 µm polymer sterile filter, which yielded comparable performance properties (including gas and hydraulic permeance, maximum differential pressure tolerance, nanoparticle sieving/fouling behavior). The results from bacteria retention tests, conducted according to the guidance of regulatory agencies, demonstrated that the 0.2 µm MSN membranes can be effectively used as sterile filters. It is anticipated that the results and technologies presented in this study will find future utility in the production of non-protein biological therapeutics and in other biological and biomedical applications.

Design and Optimization of a Low Power and Fast Response Viscometer Used for Determination of the Natural Gas Wobbe Index

This paper reports on the design of a microsystem-based thermal gas viscometer used for the determination of the quality of natural gas and its optimization through modification of its two main components: the hotplate and the capillary tube. Features, such as ultra-low power consumption ( 95%), robustness, and cost-effective mass production are the key outcome from this optimization. Two different technologies were investigated for the integrated hotplate: thick film technology using screen printing on a ceramic substrate and silicon technology based on thin films’ deposition and silicon micromachining. The sensors have been evaluated both numerically and experimentally for gas mixtures composed of nitrogen, methane, and ethane. The sensor with silicon hotplate exhibited superior performances in terms of power consumption, thermal time constant, and reproducibility of manufacturing process. The impact of the geometry of the hotplate on the performance of the sensor was investigated and the optimum design identified. The electro-thermo-mechanical tests confirmed its long-term stability. Finally, the reduction of the response time of the sensor was evaluated by modifying the diameter of its capillary tube. The experimental results showed that by increasing the diameter of capillary tube from 20 to $30~\mu \text{m}$ , the response time of the sensor decreased from 60 to 25 s.

Compact, 3D-Printed Electron Impact Ion Source with Microfabricated, Nanosharp Si Field Emitter Array Cathode

We report the design, fabrication, and characterization of a novel miniature electron impact gas ionizer manufactured via silicon micromachining and high-resolution 3D printing. The ionizer uses an array of 2,500 gated silicon nano-sharp field emitters (20 μm emitter pitch) as cathode and a set of additively manufactured, finely featured polymer and metallic parts as three-dimensional ion-generating structure. Finite element simulation of a gated field emitter tip predicts a start-up bias voltage equal to 64 V –close to the 62 V turn-on bias voltage estimated via experiments. The Si field emitter array emits 124.5 μA of electron current with ~60% gate transmission while operating at 200 V. The ionization efficiency of the gas ionizer is linear with pressure, reaching values as high as ~0.4% at 5×10−4 Torr.

A Low-Profile and High-Gain Frequency Beam Steering Subterahertz Antenna Enabled by Silicon Micromachining

A very low-profile sub-THz high-gain frequency beam steering antenna, enabled by silicon micromachining, is reported for the first time in this paper. The operation bandwidth of the antenna spans from 220 to 300 GHz providing a simulated field of view of 56°. The design is based on a dielectric filled PPW leaky-wave antenna fed by a pillbox. The pillbox, a two-level PPW structure, has an integrated parabolic reflector to generate a planar wave front. The device is enabled by two extreme aspect ratio, 16 x 16 mm large perforated membranes, which are only 30 μm thick, that provide the coupling between the two PPWs and form the LWA. The micromachined low-loss PPW structure results in a measured average radiation efficiency of -1 dB and a maximum gain of 28.5 dBi with an input reflection coefficient below -10 dB. The overall frequency beam steering frontend is extremely compact (24 x 24 x 0.9 mm) and can be directly mounted on a standard WM-864 waveguide flange. The design and fabrication challenges of such high performance antenna in the sub-THz frequency range are described and the measurement results of two fabricated prototypes are reported and discussed.

Fiber-Interconnect Silicon Chiplet Technology for Self-Aligned Fiber-to-Chip Assembly

A passive self-alignment and assembly approach for optical fibers is proposed and demonstrated using a combination of silicon micromachining and 3D printing to achieve efficient and accurate near-vertical coupling to a silicon-on-insulator (SOI) substrate with monolithic ridge waveguides and grating couplers. The alignment process is repeatable and the peak efficiency is comparable to the active fiber alignment efficiency (86% or -0.67 dB of that obtained from active alignment). The overall alignment accuracy is in the submicron range due to the formation of mechanical self-aligning structures. The overall approach is compatible with numerous applications, including fiber-to-chip and fiber-to-package assemblies as well as in the testing of photonic devices.