Bulk Micromachining Research Articles

Silicon flow stop frame for encapsulation of CMOS microsensor chips by wet anisotropic etching in KOH

Bulk micromachining in silicon is governed by the etching process where anisotropic (wet) etching in KOH can yield complex structures beyond that achievable with isotropic (dry) etching techniques. One example is the miniaturised frame reported herein with an area of 2.9 to 7.5 mm2, walls that are 1/10 mm thick, a height of 525 μm equipped with sloping walls that takes advantage of the 54.7° angle of the (111) planes to the horizontal (100) top surface of the wafer. Convex corners liable to damage are protected by sacrificial bridge structures which are etched thin to a point where the frame can be easily removed from the bulk substrate material. Frames made from isotropic (dry) etching processes have been made for comparison. Although the frame structure has different applications in microfabrication, the intended use is a flow stop barrier preventing liquid resins from entering the active area of a CMOS chemical sensor chip during encapsulation for use in aqueous or gaseous media. Beyond this specific proof-of-concept, the strategy is expected to be of general interest for all who treasures KOH etching and wants to explore new avenues based on this process.

A high aspect ratio surface micromachined accelerometer based on a SiC-CNT composite material

Silicon carbide (SiC) is recognized as an excellent material for microelectromechanical systems (MEMS), especially those operating in challenging environments, such as high temperature, high radiation, and corrosive environments. However, SiC bulk micromachining is still a challenge, which hinders the development of complex SiC MEMS. To address this problem, we present the use of a carbon nanotube (CNT) array coated with amorphous SiC (a-SiC) as an alternative composite material to enable high aspect ratio (HAR) surface micromachining. By using a prepatterned catalyst layer, a HAR CNT array can be grown as a structural template and then densified by uniformly filling the CNT bundle with LPCVD a-SiC. The electrical properties of the resulting SiC-CNT composite were characterized, and the results indicated that the electrical resistivity was dominated by the CNTs. To demonstrate the use of this composite in MEMS applications, a capacitive accelerometer was designed, fabricated, and measured. The fabrication results showed that the composite is fully compatible with the manufacturing of surface micromachining devices. The Young’s modulus of the composite was extracted from the measured spring constant, and the results show a great improvement in the mechanical properties of the CNTs after coating with a-SiC. The accelerometer was electrically characterized, and its functionality was confirmed using a mechanical shaker.

A C-shaped hinge for displacement magnification in MEMS rotational structures

The design, analysis, fabrication, and characterization of two distinct MEMS rotational structures are provided; these structures include a classical symmetrical lancet structure and a novel symmetrical C-shaped structure provided with a tilted arm, and both are actuated by thermal actuators. Our proposed C-shaped structure implemented a curved beam mechanism to enhance the movement delivered by the thermal actuators. The geometrical parameters of our proposed device were optimized using the design of experiment (DOE) method. Furthermore, the analytical modeling based on Castigliano’s second theorem and the simulations based on the finite element method (FEM) were used to predict the behavior of the symmetrical C-shaped structure; the results were in good agreement with each other. The MEMS-based rotational structures were fabricated on silicon-on-insulator (SOI) wafers using bulk micromachining technology and deep reactive ion etching (DRIE) processes. The fabricated devices underwent experimental characterization; our results showed that our proposed MEMS rotational structure exhibited a 28% improvement in the delivered displacement compared to the symmetrical lancet structure. Furthermore, the experimental results showed good agreement with those obtained from numerical analysis. Our proposed structures have potential applications in a variety of MEMS devices, including accelerometers, gyroscopes, and resonators, due to their ability to maximize displacement and thus enhance sensitivity.

Analytical Review on Enhancing Sustainability in microsystems by Integrating MEMS for Compact Design

By combining silicon-based microelectronics with micromachining technology, microelectromechanical systems (MEMS) have been identified as one of the most promising technologies for the 21st Century. With its microsystem-based devices and technologies, it will have a dramatic impact on the way we live and the way we live our lives. With an emphasis on both commercial applications and device fabrication methods, the paper provides an introduction to the field of MEMS. As well as discussing the range of MEMS sensors and actuators, the phenomena that MEMS devices can sense and act upon, and the major challenges facing the industry, the presentation discusses the challenges faced by the MEMS industry. An introduction to the field of MEMS is presented in this paper, which is divided into four sections. A section on MEMS introduces the reader to its definitions, history, current applications, and miniaturization-related issues. Photolithography, bulk micromachining, surface micromachining, and high-aspect-ratio micromachining are among the fundamental fabrication methods discussed in the second section, as well as assembly, system integration, and packaging of MEMS devices. A brief description of the basic principles of sensing and actuation mechanisms is provided in the third section, which discusses the range of MEMS sensors and actuators, as well as the phenomena that can be sensed or acted upon with MEMS devices. Toward the commercialization and success of MEMS, the final section illustrates the challenges facing the industry.

Development of a new MEMS resonant differential pressure sensor with high accuracy and high stability

This paper presents the design method and manufacture of a new resonant differential pressure sensor where the sensitive element is assembled with packaging components through an embedded boss type stress isolation structure. The responses of differential pressure, temperature and static pressure are theoretically modeled, which provide guidance on sensor design. And a method based on finite-element is used to conduct dimensional optimization of sensitive elements and embedded boss type stress isolation structures based on device sensitivities and isolation effect. Fabrication based on bulk micromachining and packaging based on Au-Au bonding and Au/Sn welding are employed to manufacture the sensor. Experimental results indicate that the fabricated sensor yields a high differential pressure sensitivity of − 87.06 Hz/kPa, a low temperature sensitivity of 5.24 Hz/°C and a low static pressure sensitivity of − 0.48 Hz/kPa. Additionally, a high accuracy of 0.05 %FS is reported under full scale. Furthermore, the sensor demonstrates an excellent long-term stability with no decrease on accuracy within a 15-day experiment.

A Ka-Band Silicon-Based Antenna-in-Package Design Using Characteristic Mode Analysis for Bandwidth Enhancement

With the emergence of 5G and satellite communication applications, where millimeter-wave (mm-wave) active phased arrays play an important role, the demand for a highly integrated and cost-effective method to achieve mm-wave antennas is an inevitable trend. Antenna-in-package (AiP) design is therefore becoming a hotspot. This paper presents the design procedure for a broadband silicon-based stacked patch antenna in Ka-band, which realizes a practical AiP structure for phased-array module integration requirements. A stacked-patch antenna on a high-resistivity silicon (HRSi) substrate is demonstrated to effectively extend the bandwidth with the guidance of characteristic mode analysis (CMA).The proposed antenna element and its 2 × 2 array were designed and fabricated using silicon bulk micromachining and wafer-level bonding technology. The measured results from the fabricated antenna prototypes showed that (1) the antenna element had an impedance bandwidth of 13.8% from 26.2 to 30.1 GHz, and the peak gain was 6.1 dBi at 28.9 GHz; (2) the 2 × 2 array realized an impedance bandwidth of 11.4% from 27.2 to 30.5 GHz, and the peak gain was 9.3 dBi at 28.5 GHz.

Fabrication and development of novel micromachined parylene-based electroactive membranes with embedded microfluidic architectures

This work describes the design, fabrication, modeling, and testing of monolithic micromachined parylene-based electroactive membranes (µPEMs) with embedded microfluidic channels. The design and modeling employed analytical plate theory to determine the optimal membrane dimensions and structural shapes for various microsystem designs. The µPEMs were fabricated using a combination of surface and bulk micromachining techniques incorporating Parylene C as a biocompatible polymeric structural material combined with patterned electrodes for actuation. Experimental actuation of the electroactive membranes demonstrated reliability with minimal voltage shifts, and theoretical pull-in voltages closely matching experimental results. Different structural parameters of the µPEMs were also tested, such as varying the overall membrane thickness/structural rigidity and actuation chamber depth. Dynamic actuation of the membrane, including, the deflection and system response to various actuation frequencies, was observed and quantified via optical coherence tomography techniques. Microfluidic architectures were monolithically integrated with the membrane actuator and successfully perfused, with no signs of leakage. This compact microsystem has potential applications in microfluidics and Lab/System-On-a-Chip devices, for use in micromixers, particle manipulators, and applying strain to adherent cells cultured on top of the membrane.

Optimization and fabrication of MEMS based piezoelectric acoustic sensor for wide frequency range and high SPL acoustic application

This paper reports finite element model (FEM) simulation and fabrication of a square shaped diaphragm along with microtunnel for MEMS acoustic sensor which can be used for measurement of wide operational frequency range and high sound pressure level (SPL) 100 dB–180 dB measurement in launching vehicle and aircraft. The structure consists of a piezoelectric ZnO layer sandwiched between two aluminum electrodes on a thin silicon diaphragm. There is a microtunnel in the structure which relates the cavity to the atmosphere for pressure compensation. The microtunnel decides the lower cut-off frequency of device. Analytical and simulation approaches are used to optimize microtunnel dimension and simulation approach for diaphragm structure optimization. The change in displacement, stress, sensitivity and resonance frequency due to different diaphragm sizes with diaphragm thickness variation is also analyzed. The optimized diaphragm structure of 1750 × 1750 μm2 and microtunnel of 100 μm wide and 24 μm deep have been fabricated using bulk micromachining technique. The fabricated device response has been tested using LDV and sensitivity measurement system.

Development and Research of the Sensitive Element of the MEMS Gyroscope Manufactured Using SOI Technology

In this article, based on the developed methodology, the stages of designing the sensitive element of a microelectromechanical gyroscope with an open-loop structure are considered. This structure is intended for use in control units for mobile objects such as robots, mobile trolleys, etc. To quickly obtain a ready-made gyroscope, a specialized integrated circuit (SW6111) was selected, for the use of which the electronic part of the sensitive element of the microelectromechanical gyroscope was developed. The mechanical structure was also taken from a simple design. The simulation of the mathematical model was carried out in the MATLAB/Simulink software environment. The mechanical elements and the entire structure were calculated using finite element modeling with ANSYS MultiPhysics CAD tools. The developed sensitive element of the micromechanical gyroscope was manufactured using bulk micromachining technology−silicon-on-insulator−with a structural layer thickness equal to 50 μm. Experimental studies were carried out using a scanning electron microscope and a contact profilometer. Dynamic characteristics were measured using a Polytec MSA-500 microsystem analyzer. The manufactured structure has low topological deviations. Calculations and experiments showed fairly accurate results for the dynamic characteristics, with an error of less than 3% for the first iteration of the design.

Study of masking layer behavior towards bulk micromachining of Borofloat glass

In this work, a study of different masking materials for the development of deep grooves in a Borofloat glass wafer using bulk micromachining is presented. Masking layers such as photoresist (AZ1512HS), Chromium (Cr)/photoresist, and Chromium (Cr)/Gold (Au)/photoresist are deposited on the Borofloat glass and investigated in 20% hydrofluoric acid (HF) for isotropic etching. Among all the masking layers, the photoresist layer is not sustainable in the etchant solution and is peeled off immediately due to poor surface imperfections. Metal masking layers with the combination of photoresists are therefore investigated for the formation of deep grooves in the Borofloat glass. Here the metal layers are deposited by DC magnetron sputtering. By utilizing thin masking layers of Cr (30 nm)/Au (200 nm)/photoresist and Cr (250 nm)/photoresist over the glass wafer, a significant etch depth with a good etch rate of 5 µm/min is observed along with good sustainability of the masking layer in the etchant. For a longer duration of the etching time, thin metal masking layers are not sufficient to provide a good barrier to the etchant. Further, an etch depth of greater than 400 µm is achieved using a combination of thicker layers of Cr (250 nm), Au (250 nm and 1000 nm), and photoresist with minimal surface imperfection compared to other thin masking layers. The present study of masking layers paves a new way toward the deep cavities and through holes in the Borofloat glass.

Wet bulk micromachining characteristics of Si{110} in NaOH-based solution

Silicon wet bulk micromachining is an extensively used technique in microelectromechanical systems (MEMS) to fabricate variety of microstructures. It utilizes low-cost etchants and suitable for batch process that made it popular for industrial production. The etch rate and the undercutting at convex corner significantly affect the productivity. In wet anisotropic etching-based micromachining, Si{110} wafer is employed to fabricate unique shape geometries such as the microstructures with vertical sidewalls. In this research, we have investigated the etching characteristics of Si{110} in 10 M sodium hydroxide without and with addition of hydroxylamine (NH2OH). The main objective of the present work is to improve the etch rate and the undercutting at convex corners. Average surface roughness (R a), etch depth, and undercutting length are measured using a 3D scanning laser microscope. Surface morphology of the etched Si{110} surface is examined using a scanning electron microscope. The incorporation of NH2OH significantly improves the etch rate and the corner undercutting, which are useful to enhance the productivity. Additionally, the effect of etchant age on the etch rate and other etching characteristics are investigated. The etch rate of silicon and the undercutting at convex corners decrease with etchant aging. The results presented in this paper are very useful to scientists and engineers who use silicon wet anisotropic etching to fabricate MEMS structures using bulk micromachining. Moreover, it has great potential to promote the application of wet etching in MEMS.

Inductively coupled plasma etching of bulk tungsten for MEMS applications

As a promising material for bulk micromachining for MEMS applications, tungsten can be used as microneedles for medicine injection and tools for micro-inject molding, ultrasonic machining, and electrical discharge machining. However, it is difficult to fabricate microstructures with high aspect ratios and small feature sizes due to the excellent material of tungsten properties, such as chemical stability, high hardness, and high melting point. Here, an inductively coupled plasma (ICP) deep etching process is developed for bulk tungsten, and its process parameters are studied. After optimizing the process parameters, three recipes were developed for different MEMS applications. A patterned tungsten structure with a depth of more than 300 µm was etched by tungsten (W) ICP deep etching (WIDE) at an etch rate of 2.73 µm/min and etch selectivity of 35. The WIDE process produced a tungsten microstructure with a high aspect ratio (> 20), a small feature size (< 3 µm), and a surface roughness of Ra 30 nm. To demonstrate the potential application prospect of deep etching bulk of tungsten, tungsten microneedles with a positive sidewall angle and several non-silicon substrates for MEMS applications were achieved based on the WIDE process. • Bulk tungsten ICP deep etching process development based SF 6 +O 2 +C 4 F 8 . • Effect of process parameters on ER, ES, DUR, roughness, sidewall angle. • μ-structures with > 300 µm deep, > 20 AR, and Ra 30 nm roughness. • Die steel, glass, and polymer μ-structures machined by etched tungsten. • Metal-based and non-silicon-based MEMS application prospect.

Design and Fabrication of a Differential MOEMS Accelerometer Based on Fabry–Pérot Micro-Cavities

In this paper, a differential MOEMS accelerometer based on the Fabry-Perot (FP) micro-cavities is presented. The optical system of the device consists of two FP cavities and the mechanical system is composed of a proof mass that is suspended by four springs. The applied acceleration tends to move the PM from its resting position. This mechanical displacement can be measured by the FP interferometer formed between the proof mass cross-section and the optical fiber end face. The proposed sensor is fabricated on a silicon on insulator (SOI) wafer using the bulk micromachining method. The results of the sensor characterization show that the accelerometer has a linear response in the range of 1g. Also, the optical sensitivity and resolution of the sensor in the static characterization are 6.52 nm/g and 153ug. The sensor sensitivity in the power measurement is 49.6 mV/g and its resonant is at 1372 Hz. Using the differential measurement method increases the sensitivity of the accelerometer. Based on experimental data, the sensor sensitivity is two times as high as that of a similar MOEMS accelerometer with one FP cavity.

Microrobotic Leg With Expanded Planar Workspace

A microrobotic leg actuated with amplifying mechanisms is developed in this letter to mimic the motion of legs in natural organisms on the small scale. The leg device has a planar structure and is fabricated with bulk micromachining. Two electrothermal actuators are used to drive the leg back and forth in the planar workspace. The actuator motion is transmitted elastically to the leg using flexible arms. The leg mechanism has a symmetrical structure similar to standard parallel planar manipulators but with deformable parts instead of joints. The motion of the leg at the tip side is further amplified by blocking its motion at the other side using stoppers, resulting in an extended range of motion at the leg tip. Finite element simulations and experimental tests on fabricated prototypes demonstrated the efficient operation of the proposed design. The unique characteristics of the proposed leg mechanism, including large force and displacement, multiple degrees of freedom, and compatibility with micromachining, enhance the power, control, and mobility autonomy of legged microbots.

Closed-loop MOEMS accelerometer.

In this paper, a closed-loop micro-opto-electro-mechanical system (MOEMS) accelerometer based on the Fabry-Pérot (FP) interferometer is presented. The FP cavity is formed between the end of a cleaved single-mode optical fiber and the cross-section of a proof mass (PM) which is suspended by four U-shaped springs. The applied acceleration tends to move the PM in the opposite direction. The arrays of fixed and movable comb fingers produce an electrostatic force which keeps the PM in its resting position. The voltage that can provide this electrostatic force is considered as the output of the sensor. Using a closed-loop detection method it is possible to increase the measurement range without losing the resolution. The proposed sensor is fabricated on a silicon-on-insulator wafer using the bulk micromachining method. The results of the sensor characterization show that the accelerometer has a linear response in the range of ±5 g. In the closed-loop mode, the sensitivity and bias instability of the sensor are 1.16 V/g and 40 µg, respectively.

Single-shot femtosecond bulk micromachining of silicon with mid-IR tightly focused beams

Being the second most abundant element on earth after oxygen, silicon remains the working horse for key technologies for the years. Novel photonics platform for high-speed data transfer and optical memory demands higher flexibility of the silicon modification, including on-chip and in-bulk inscription regimes. These are deepness, three-dimensionality, controllability of sizes and morphology of created modifications. Mid-IR (beyond 4 µm) ultrafast lasers provide the required control for all these parameters not only on the surface (as in the case of the lithographic techniques), but also inside the bulk of the semiconductor, paving the way to an unprecedented variety of properties that can be encoded via such an excitation. We estimated the deposited energy density as 6 kJ cm−3 inside silicon under tight focusing of mid-IR femtosecond laser radiation, which exceeds the threshold value determined by the specific heat of fusion (~ 4 kJ cm−3). In such a regime, we successfully performed single-pulse silicon microstructuring. Using third-harmonic and near-IR microscopy, and molecular dynamics, we demonstrated that there is a low-density region in the center of a micromodification, surrounded by a “ring” with higher density, that could be an evidence of its micro-void structure. The formation of created micromodification could be controlled in situ using third-harmonic generation microscopy. The numerical simulation indicates that single-shot damage becomes possible due to electrons heating in the conduction band up to 8 eV (mean thermal energy) and the subsequent generation of microplasma with an overcritical density of 8.5 × 1021 cm−3. These results promise to be the foundation of a new approach of deep three-dimensional single-shot bulk micromachining of silicon.

Fabrication and flow rate characterization of a DRIE process based valveless piezoelectric micropump

Micropumps have become one of the major research topics in the field of microfluidics. Different actuators have been used in active micropumps including piezoelectric ones, which convert electrical energy to mechanical energy. In this study, a piezoelectric disc was designed as an actuator and was integrated into a fabricated piezoelectric micropump. Unlike wet etching methods, which is generally used to etch layers of relatively high thickness in the silicon wafer, dry etching was used in this study. Aluminum was used as a mask material during the fabrications-steps, and the fabrication process was shortened with the use of the masking steps. The etching process of the slot, in which the piezoelectric disc was placed, and etching of the inlet/outlet channels were performed simultaneously. Thus, the processing time was significantly shortened. The fabrication of this silicon-based valveless micropump was accomplished by using the DRIE (deep reaction-ion etching) technique, which provided controlled etching. Experiments were conducted on the fabricated micropump with the use of the bulk micromachining technology to deliver the desired pumping action. The leakage and the air entrapment between the consecutive micropump structural layers were satisfactorily eliminated. This micropump is capable of delivering a promising flow rate of 52 (µl min−1) for deionized water, which corresponds to a 150 [Hz] square wave type and a peak-to-peak voltage of 60 (V) Vp -p . No moving mechanical valves were included so that the risks of clogging, reduced performance, and reduced reliability due to wear and fatigue were minimized. Not only gases and liquids but also fluids containing particles could be used as the working fluid for this micropump. The piezoelectric micropump design was optimized to achieve a high time-averaged flow rate (>50 (µl min−1)) with a relatively low excitation voltage (<100 (V) Vp -p ) as opposed to the use of a high excitation voltage.

"Analytical Modelling of the Boron Diffusion and Etching Process for the Accurate Design and Fabrication by Bulk Micromachining Technology of the Silicon Capacitive Pressure Sensors for Biomedical Applications"

"Analytical Modelling of the Boron Diffusion and Etching Process for the Accurate Design and Fabrication by Bulk Micromachining Technology of the Silicon Capacitive Pressure Sensors for Biomedical Applications"

Sensitive Transfer-Free Wafer-Scale Graphene Microphones.

During the past decades micro-electromechanical microphones have largely taken over the market for portable devices, being produced in volumes of billions yearly. Because performance of current devices is near the physical limits, further miniaturization and improvement of microphones for mobile devices poses a major challenge that requires breakthrough device concepts, geometries, and materials. Graphene is an attractive material for enabling these breakthroughs due to its flexibility, strength, nanometer thinness, and high electrical conductivity. Here, we demonstrate that transfer-free 7 nm thick multilayer graphene (MLGr) membranes with diameters ranging from 85–155 to 300 μm can be used to detect sound and show a mechanical compliance up to 92 nm Pa–1, thus outperforming commercially available MEMS microphones of 950 μm with compliances around 3 nm Pa–1. The feasibility of realizing larger membranes with diameters of 300 μm and even higher compliances is shown, although these have lower yields. We present a process for locally growing graphene on a silicon wafer and realizing suspended membranes of patterned graphene across through-silicon holes by bulk micromachining and sacrificial layer etching, such that no transfer is required. This transfer-free method results in a 100% yield for membranes with diameters up to 155 μm on 132 fabricated drums. The device-to-device variations in the mechanical compliance in the audible range (20–20000 Hz) are significantly smaller than those in transferred membranes. With this work, we demonstrate a transfer-free method for realizing wafer-scale multilayer graphene membranes that is compatible with high-volume manufacturing. Thus, limitations of transfer-based methods for graphene microphone fabrication such as polymer contamination, crack formation, wrinkling, folding, delamination, and low-tension reproducibility are largely circumvented, setting a significant step on the route toward high-volume production of graphene microphones.

Synergy Effect and Symmetry-Induced Enhancement Effect of Surface Multi-Defects on Nanohardness by Quasi-Continuum Method.

The quasicontinuum method has been applied to probe the thin film with surface multi-defects, which is commonly seen in nanoimprint technique and bulk micromachining. Three unilaterally distributed multi-defect models and six bilaterally distributed multi-defect models of Pt thin film have been carried out in nanoindentation. The results show that the nanohardness gradually decreases as the number of unilaterally distributed multi-defects increases, along with the increasingly low decline rate of the nanohardness. The synergy effect of the unilaterally distributed multi-defects has been highly evidenced by the critical load revision for dislocation emission of Pt thin film, and it is predicted into a universal form with the synergy coefficient among the existing multi-defects for FCC metals. Moreover, the nanohardness obviously increases when the bilaterally distributed multi-defects form into symmetrical couple, and it could be even greater than the one with defect-free surface, due to the symmetry-induced enhancement effect on nanohardness. The symmetry-induced enhancement coefficient has been brought out and has well explained the symmetry-induced enhancement effect of bilaterally distributed multi-defects on the nanohardness by a prediction formula. Furthermore, the characteristic length of symmetric relations has been brought out to calculate the symmetry-induced enhancement coefficient and it has been effectively predicted to equal to the sum of the adjacent distance between the surface defect and the indenter, the defect depth near the indenter, and the defect width for FCC metal.