Deep Reactive Ion Etching Technology Research Articles

Voltage- and Metal-assisted Chemical Etching of Micro and Nano Structures in Silicon: A Comprehensive Review.

Sculpting silicon at the micro and nano scales has been game-changing to mold bulk silicon properties and expand, in turn, applications of silicon beyond electronics, namely, in photonics, sensing, medicine, and mechanics, to cite a few. Voltage- and metal-assisted chemical etching (ECE and MaCE, respectively) of silicon in acidic electrolytes have emerged over other micro and nanostructuring technologies thanks to their unique etching features. ECE and MaCE have enabled the fabrication of novel structures and devices not achievable otherwise, complementing those feasible with the deep reactive ion etching (DRIE) technology, the gold standard in silicon machining. Here, a comprehensive review of ECE and MaCE for silicon micro and nanomachining is provided. The chemistry and physics ruling the dissolution of silicon are dissected and similarities and differences between ECE and MaCE are discussed showing thattheyare the two sides of the same coin. The processes governing the anisotropic etching of designed silicon micro and nanostructures are analyzed, and the modulation of etching profile over depthis discussed. The preparation of micro- and nanostructures with tailored optical, mechanical, and thermo(electrical) properties is then addressed, and their applications in photonics, (bio)sensing, (nano)medicine, and micromechanical systems are surveyed. Eventually, ECE and MaCE are benchmarked against DRIE, and future perspectives are highlighted.

Design and fabrication of a tiny micro-pressure sensor with improved linearity and sensitivity

With the rapid development of key industries such as new energy vehicles, smart homes, smart phones and consumer grade drones, the demand for high performance miniature micro-pressure sensors is also growing promptly. At prssent, most of the pressure sensors of this type adopt a C-type structure, and the thickness of the sensitive diaphragm needs to be continuously thinned in order to improve the sensitivity of this type sensor, which leads to its sensitivity to etching errors and makes it difficult to ensure the linearity of the pressure sensor. Based on this, we designed a tiny micro-pressure half active Wheatstone bridge piezoresistive pressure sensor with C-type square diaphragm structure, whose area is reduced by shortening the number of leads and pads, through ion impantation on the surface of the diaphragm to create piezoresistors, using deep reactive ion etching (DRIE) technology to release the pressure sensitive diaphragm with length of 460 μm × 460 μm, thinkness of 7.5 μm. The buried oxygen layer in the SOI (silicon on insulator) can play a role of stopping the etching, which assures the uniformity of the released sensitive diaphragm and the linearity of the sensor while maintains its sensitivity. Before preparing a pressure sensor, simulation data is used to predict whether the test results meet the design requirements. In this paper, the influence of piezoresistors position offset on the performance of the pressure sensor is analyzed by means of simulation, which improves the accuracy of the designed model and guarantees its performance parameters can meet the design indicators. Finally, the static characteristics of the sensor is calibrated. The experimental results show that the sensitivity of the sensor is up to 0.538 mV/kPa in the range of 40 kPa at room temperature, and the nonlinear error is only 0.386% FS. This study provides some new ideas and references for the design work of high performance micro-pressure sensors.

Deep reactive ion etching of silicon using non-ICP-based equipment

Deep reactive ion etching (DRIE) technology is one of the most important technologies in the processing of microelectronic devices and microelectromechanical system. As a necessary process in semiconductor integration, it has been widely studied in the past decades. It is known that the traditional DRIE process typically uses a plasma etching reactor equipped with inductively coupled plasma (ICP) sources to generate a high-density plasma so as to achieve high aspect ratio trenches with relatively small roughness. A cryogenic temperature control unit is typically employed as well. Here, however, we use a parallel plate RIE with rather simple structure, which is not usually used for DRIE, to obtain high aspect ratio silicon etching. With no ICP sources and no sophisticated temperature control unit, the system and experiment are now much more cost effective. Through the optimization of the processing, the etching rate of silicon can reach 440 nm/min. Finally, a 45 μm deep trench is etched in silicon with good perpendicularity. This method will greatly reduce the equipment related cost, especially for those applications that do not have extremely stringent requirement on the final etching accuracy.

Fabrication and application of flexible AlN piezoelectric film

A fabrication method for high quality flexible aluminum nitride piezoelectric thin films based on micro-nano fabrication technology is reported in this paper. Molybdenum (Mo)/aluminum nitride (AlN)/aluminum (Al) structure was prepared on silicon(100) by sputtering. Then, deep-reactive ion etching technology was used to remove the silicon material used for support, then the Mo/AlN/Al flexible sandwich film was obtained. The full-width at half-maximum of the AlN film (002) peak before and after removal of silicon was only 1.47° and 1.49°, respectively. No obvious cracks in the film under bending conditions were observed with a scanning electron microscope. Capacitance–voltage (C–V) test results showed that the capacitance curve of the sandwich structure was consistent with that of the double-sided Schottky junction. The flexible piezoelectric film with an active area of 2 × 2 mm2 exhibited an average peak-to-peak generated voltage of 0.6 V in energy harvesting tests, which indicates that the method could be used to fabricate high quality flexible energy collectors based on AlN film. The preparation scheme in this paper is compatible with integrated circuit processes and simple to implement, which lays a foundation for the wide application of flexible piezoelectric devices.

Scalable Superomniphobic Surfaces

Superomniphobic surfaces that repel liquids of extremely low surface tension rely on carefully fabricated doubly re-entrant topographies, typically made by silicon deep reactive ion etching technology. However, previously published processes have depended on critically timed etching steps, which are difficult to downscale. We present a scalable process that eliminates the critically timed etching steps. It is based on the use of silicon-on-insulator wafers and a silicon oxide foot of the micropillar, which makes the isotropic silicon release step non-critical. The process allows easy downscaling of pillars from 20 $\mu \text{m}$ to 10 $\mu \text{m}$ and $5~\mu \text{m}$ . The downscaling increases the stability of the Cassie state. Based on the process, we are able to create superomniphobic surfaces that sustain perfluorohexane (FC-72), which has the lowest surface tension of the known liquids at room temperature ( $\gamma _{\text {lv}}=11.91$ mN/m at 20 °C), in the Cassie state at droplet diameters down to 200 micrometers. These are the smallest perfluorohexane droplets repelled to date. [2019-0207]

Micro-fabrication and hermeticity measurement of alkali-atom vapor cells based on anodic bonding

A vapor cell provides a well-controlled and stable inner atmosphere for atomic sensors, such as atomic gyroscopes, atomic magnetometers, and atomic clocks, and its hermeticity affects the stability and aging of atomic sensors. We present the micro-fabrication of a micro-electromechanical system wafer-level hermit vapor cell based on deep reactive ion etching and vacuum anodic-bonding technology. The anodic-bonding process with the voltage increasing in steps of 200 V had a critical influence on vapor cell hermeticity. Further, the silicon–glass bonding surface was experimentally investigated by a scanning electron microscope, which illustrated that there were no visual cracks and defects in the bonding surface. The leak rate was measured using a helium leak detector. The result shows that the vapor cells with different optical cavity lengths comply with the MIL-STD-883E standard (5 × 10−8 mbar·L/s). Moreover, D2 absorption spectroscopy was characterized via optical absorption. The bonding strength was determined to be 13 MPa, which further verified the quality of the vapor cells.

Development of the MEMS-Based Nozzle Using DRIE of Tapered Hole Technology for Cube Satellite

In this paper, we proposed a 3D MEMS nozzle using deep reactive ion etching (DRIE) to improve the performance of MEMS thruster. The design, fabrication, and performance evaluation of the proposed nozzle were presented. Prior to fabrication, CFD analyses were performed to predict the performance enhancement of the tapered nozzle compared with the hole-type nozzle. Flow characteristics were studied under ambient condition at an operating pressure of 10 bar. The DRIE technology for fabricating the hole nozzle was established, and hole nozzles with a throat diameter of 50–350 μm was fabricated. Thrust was measured through the use of quartz force sensor based on piezo-resistive sensor. We compared experimental results with CFD analysis result at various operating pressure. From the experimental results, CFD and experimental results showed about 10% thrust difference.

In-plane coupled Fabry-Perot micro-cavities based on Si-air Bragg mirrors: a theoretical and practical study.

In-plane Fabry-Perot cavities based on deeply etched Bragg mirrors are used in many microphotonic applications including sensing, telecom, and swept laser devices. A main limitation to their performance is the small free spectral range (FSR) and low finesse. The FSR limits the dynamic range or the wavelength tuning range, while the linewidth limits the resolution. In this work, we propose coupled Fabry-Perot micro-cavities that greatly enhance the FSR, besides reducing the linewidth, which lead to higher finesse and better performance. The proposed structure is modeled and etched on Si substrate to a depth of 150μm using the deep reactive ion etching technology. Optical measurements indicate an enhanced FSR of more than 140nm and a quality factor of 3152 using coupled cavities as compared to only 9nm FSR for a single cavity of the same length. The over-etching and surface roughness, being the main effective fabrication tolerances, are modeled and extracted from the measurements.

Innovative Silicon Microgrippers for Biomedical Applications: Design, Mechanical Simulation and Evaluation of Protein Fouling

The demand of miniaturized, accurate and robust micro-tools for minimally invasive surgery or in general for micro-manipulation, has grown tremendously in recent years. To meet this need, a new-concept comb-driven microgripper was designed and fabricated. Two microgripper prototypes differing for both the number of links and the number of conjugate surface flexure hinges are presented. Their design takes advantage of an innovative concept based on the pseudo-rigid body model, while the study of microgripper mechanical potentialities in different configurations is supported by finite elements’ simulations. These microgrippers, realized by the deep reactive-ion etching technology, are intended as micro-tools for tissue or cell manipulation and for minimally invasive surgery; therefore, their biocompatibility in terms of protein fouling was assessed. Serum albumin dissolved in phosphate buffer was selected to mimic the physiological environment and its adsorption on microgrippers was measured. The presented microgrippers demonstrated having great potential as biomedical tools, showing a modest propensity to adsorb proteins, independently from the protein concentration and time of incubation.

Improvement of Deep Reactive Ion Etching Process For Motional Resistance Reduction of Capacitively Transduced Vibrating Resonators

Motional resistance is one of the most important performance metrics for high qualitfy factor, low power, and complementary metaloxide semiconductor (CMOS)-compatible capacitively transduced vibrating micromechanical resonators. The motional resistance is primarily set by the electrode-to-resonator air gap that can be formed by deep reactive ion etching (DRIE) process. Although the state-of-the-art DRIE technologies can achieve a narrow capacitive air gap down to 1 m or less, the effective gap tends to be larger than designed values due to the sidewall roughness known as scalloping. Systematic modifications of all key process parameters are presented in this article for lowering the sidewall roughness to result in an up to 2x reduction of the effective capacitive transducer gap, which could lead to an up to 16x decrease of the effective motional resistance.

Influence of the side etching effect in DRIE on performance of electrostatic linear comb-drive actuators

Side etching and scalloping are unwanted but unavoidable phenomenon occurring during fabrication process of micro electro-mechanical system devices with the deep reactive ion etching technology. This paper reports the influence of deep reactive ion etching fabrication errors on the geometrical dimension (as a comb finger shape, width of the suspension beam), electrostatic force and displacement of electrostatic linear comb-drive actuators. A number of deep reactive ion etching experiments were carried out in order to find out the optimal etching and passivation time, which allows to achieve vertical and smooth sidewall, as well as acceptable side etching value. Discussion and evaluation of deviation between the theoretically calculated and measured displacements of the electrostatic linear comb-drive actuator were also reported.

MEMS and FOG Technologies for Tactical and Navigation Grade Inertial Sensors-Recent Improvements and Comparison.

In the following paper, we present an industry perspective of inertial sensors for navigation purposes driven by applications and customer needs. Microelectromechanical system (MEMS) inertial sensors have revolutionized consumer, automotive, and industrial applications and they have started to fulfill the high end tactical grade performance requirements of hybrid navigation systems on a series production scale. The Fiber Optic Gyroscope (FOG) technology, on the other hand, is further pushed into the near navigation grade performance region and beyond. Each technology has its special pros and cons making it more or less suitable for specific applications. In our overview paper, we present latest improvements at NG LITEF in tactical and navigation grade MEMS accelerometers, MEMS gyroscopes, and Fiber Optic Gyroscopes, based on our long-term experience in the field. We demonstrate how accelerometer performance has improved by switching from wet etching to deep reactive ion etching (DRIE) technology. For MEMS gyroscopes, we show that better than 1°/h series production devices are within reach, and for FOGs we present how limitations in noise performance were overcome by signal processing. The paper also intends a comparison of the different technologies, emphasizing suitability for different navigation applications, thus providing guidance to system engineers.

Fabrication and characterization of a tungsten microneedle array based on deep reactive ion etching technology

In this paper, the authors report on a novel tungsten microneedle array fabricated using deep reactive ion etching. The fabricated sample was 10 mm long, 10 mm wide, and 40 μm in pitch. Each microneedle had a top-end diameter of 7.7 μm, a bottom-end diameter of 30 μm, a length of 60 μm, and a sidewall tilt angle of approximately 14°. The mechanical strength, hydrophobicity and contact impedance of the tungsten microneedle array were characterized. For the mechanical strength, a rubbing test was conducted, which involved moving the tungsten microneedle array structure on a 800-grit abrasive paper with an equivalent slide friction force of 2 × 10−3 μN. Results indicated no obvious damage to the microneedles at the scanning electron microscopy level. Hydrophobicity test results showed that the surface of the tungsten microneedle array was uniform and hydrophobic, with an average contact angle of 137.9° and a maximum contact angle variation of 5.9° for the best sample. The contact impedance of the tungsten microneedle array sample to skin was found to be stable after 1 h of contact at a value of less than 2000 Ω in the range of 50–100 kHz.

In-Line Optical MEMS Phase Modulator and Application in Ring Laser Frequency Modulation

Optical phase modulators are essential building components in a wide range of applications, for instance, optical communication systems, tunable lasers, optical phase locked loops, and optical sensors. The production of in-plane MEMS-based optical phase modulator with self-aligned mirrors, actuator, and fiber grooves enables the low cost and easy integration with fiber-based lasers and sensors or photonic microsystems. In this paper, we report an in-plane transmission type MEMS-based optical phase modulator fabricated by deep reactive ion etching technology on a silicon-on-insulator (SOI) substrate. Detailed optical analysis of the MEMS phase modulator considering the diffraction of the single-mode fiber output beam, the asymmetric truncation of the beam by the limited aperture of the micromirrors and the tilt angle of the deeply etched mirrors is presented. The device layer height of the fabricated SOI wafer is 100 μm, and the sidewalls are etched with verticality that is better than 89.98°. The micromechanical system is characterized experimentally using electrical technique, and the resonance frequency and quality factor are 11.3 kHz and 163, respectively. The MEMS device is integrated into fiber ring laser (FRL) enabling the achievement of low- and high-frequency modulation indices. The frequency modulation of the FRL using the presented phase modulator is supported with numerical analysis and experimental results.

Chromatographic separation of simulants of nerve and blister agents by combining one- and two-channel columns with different stationary phases.

A two-channel gas chromatography column and a single-channel column were made by deep reactive-ion etching technology. The two short columns were coated with different stationary phases, and then linked without a modulator. This is to aim at increasing the sample capacity and achieving a higher separation efficiency in complex environments. The results show that the capacity of the connected column is approximately 4 and 1.5 times larger than that of the single- and two-channel columns, respectively. The linked column was utilized to separate a six-component mixture, composed of three simulants of nerve and blister agents and three interfering vapors. The results demonstrate that the combined column has a remarkably higher separation efficiency than the individual columns, and an acceptable resolution is achieved although the total length of the linked column is only 1.5 m.

In-plane monolithic microscanner with two synchronized, self-aligned flat mirrors and compliant springs

This work proposes an architecture for a wide-angle, self-aligned, in-plane monolithic scanner fabricated using the deep reactive ion etching technology. The fabricated microscanner provides an optical scanning in-plane angle of about 86 deg and operates at the speed of 2.73 kHz. The scanning is achieved using two synchronized, flat mirrors coupled mechanically to allow for wide-angle scanning and connected through a compliant structure to allow the use of a linear comb actuator. This wide-angle, in-plane scanning opens the door for many applications, especially for handheld optical displays.

Wideband Optical MEMS Interferometer Enabled by Multimode Interference Waveguides

A novel optical MEMS interferometer is proposed based on spatial splitting and combining of optical beams using the imaging properties of multimode interference (MMI) waveguides. The light propagates in air, allowing operation over wide spectral range covering both the infrared and the visible ranges. The optical propagation in the structure is modeled and the interferometer is analyzed by incorporating the modal analysis technique for the waveguides and the angular spectrum approach for free-space propagation. The beam splitter and the overall interferometer are fabricated using deep reactive ion etching technology on silicon-on-insulator wafer. The MMI waveguide sidewalls are aluminum metalized to improve the insertion loss of the interferometer. The fabricated splitter and interferometer are characterized in the visible and near-infrared spectral ranges. The splitter output intensity profile is recorded to verify its wideband proper operation. The interferometer is characterized versus the wavelength and tested as a Fourier transform spectrometer, thanks to a monolithically integrated corner mirror driven by a comb-drive actuator. The spectral resolution obtained is 2.5 nm at 635-nm wavelength.

In-plane deeply-etched optical MEMS notch filter with high-speed tunability

Notch filters are used in spectroscopy, multi-photon microscopy, fluorescence instrumentation, optical sensors and other life science applications. One type of notch filter is based on a fiber-coupled Fabry–Pérot cavity, which is formed by a reflector (external mirror) facing a dielectric-coated end of an optical fiber. Tailoring this kind of optical filter for different applications is possible because the external mirror has fewer mechanical and optical constraints. In this paper we present optical modeling and implementation of a fiber-coupled Fabry–Pérot filter based on dielectric-coated optical fiber inserted into a micromachined fiber groove facing a metallized micromirror, which is driven by a high-speed MEMS actuator. The optical MEMS chip is fabricated using deep reactive ion etching (DRIE) technology on a silicon on insulator wafer, where the optical axis is parallel to the substrate (in-plane) and the optical/mechanical components are self-aligned by the photolithographic process. The DRIE etching depth is 150 μm, chosen to increase the micromirror optical throughput and improving the out-of-plane stiffness of the MEMS actuator. The MEMS actuator type is closing-gap, while its quality factor is almost doubled by slotting the fixed plate. A low-finesse Fabry–Pérot interferometer is formed by the metallized surface of the micromirror and a cleaved end of a standard single-mode fiber, for characterization of the MEMS actuator stroke and resonance frequency. The actuator achieves a travel distance of 800 nm at a resonance frequency of 89.9 kHz. The notch filter characteristics were measured using an optical spectrum analyzer, and the filter exhibits a free spectral range up to 100 nm and a notch rejection ratio up to 20 dB around a wavelength of 1300 nm. The presented device provides batch processing and low-cost production of the filter.

Structural parameter analysis and experimental study of micropumps with saw-tooth microchannel

The changing trends of vortex, pressure loss, microchannel efficiency and instantaneous flow rate along with Reynolds number are discussed based on the structural parameters of a saw-tooth microchannel that include diffuser angle, sectional width, length, taper angle and depth. Through finite element analysis it is shown that the nozzle flow is steadier than diffuser angle. The optimal range of diffuser angle is 5–9°. The channel efficiency decreases with an increase of sectional width, and the optimal value is 0.04mm according to the technology conditions. The pressure loss increases with increasing length. The channel performance is best when the taper angle is in the range 20–40°. The channel efficiency increases with an increase of depth, but, with this increasing depth, the wall friction force increases, and the fabrication processing becomes more difficult. Finally, five micropump structures were designed and fabricated with deep reactive ion etching technology based on silicon wafers. The five micropumps were irreversibly sealed with polydimethylsiloxane (PDMS) on both sides using ultraviolet irradiation of the PDMS. For testing the flow rate and pressure of the five micropumps, a test-bed was established. The results show that the flow rate and pressure of one of the micropumps are maximal values under sine, square and triangular driving waves. This agrees well with the results of the structural parameter analysis. When the driving frequency is 200Hz, the maximum flow and pressure are 63.6μl/min and 1.3kPa respectively.

Curved Silicon Micromirror for Linear Displacement-to-Angle Conversion With Uniform Spot Size

This paper reports a novel class of deeply etched curved micromirrors enabling linear conversion between the reflection angle of incident light beam and displacement of the beam axis with respect to the curved mirror principal axis. Moreover, the mirror provides phase-transformation of the light beam independent of the inclination angle of the incident light on the mirror surface. The micromirrors are fabricated on SOI substrate by deep reactive ion etching technology. The profile of the curved surface is optimized and controlled precisely, thanks to the photolithographic process. High optical throughput micromirrors exhibiting submillimeter focal lengths are fabricated with 200-μm etching depth and with a sidewall angle deviation from perfect verticality, which is smaller than 0.1°. Optical measurements at wavelengths of 675 and 1550 nm show transformation of the optical beam with high optical spot size stability during a beam steering process with less than ±5% dependence on the inclination/reflection angle over a scanning angle range of 120°. The presented micromirror has applications in MEMS scanners, displacement/rotation sensing, and optical imaging.