Deep Reactive Etching Research Articles

Transparency and icephobicity of moth eye-inspired tailored omniphobic surface

ABSTRACT In this work, we presented a facile method to introduce a transparent and anti-icing omniphobic surface fabricated on the glass substrate. The uniform moth eye-inspired structure was generated by the Deep Reactive Ion Etching method and followed by an additional wet etching step to enhance the sharpness of the nanostructure. A hydrophobe compound was assembled on the rough substrate by chemical vapor coating to achieve remarkable liquid-repellent properties. The tensile strength test was conducted using a designed apparatus to evaluate the anti-icing performance of functional surfaces compared to the as-received one. In addition, the optical performance in terms of transmittance and reflectance also was tested and revealed significant optical enhancement owing to the unique design of nanoarrays. The results demonstrated the great potential and proposed design for multifunctional surfaces.

Design and fabrication of piezoelectrically driven deformable reflective micromirrors based on MEMS technology

As the core component of the adaptive optics system, the deformable mirror is developing towards miniaturization and integration through the combination of MEMS technology. In this paper, based on the inverse piezoelectric effect of piezoelectric materials, a piezoelectrically driven deformable reflective micromirror is designed and fabricated by physical vapor deposition, UV lithography, eutectic bonding, mechanical cutting, deep reactive ion etching (RIE) and other processes to complete the preparation of micromirrors. The problems related to the eutectic bonding process of PZT and Si in the fabrication of micromirrors are emphatically studied through tensile experiments. The micromirror was tested with a laser interferometer. The test results showed that the prepared micromirror could achieve a maximum deformation of 0.6 μm in the positive direction and 1.7 μm in the negative direction when a voltage of ±1.5 kV was applied.

Investigation of Achieving Ultrasonic Haptic Feedback Using Piezoelectric Micromachined Ultrasonic Transducer

Ultrasound haptics is a contactless tactile feedback method that creates a tactile sensation by focusing high-intensity ultrasound on human skin. Although air-coupled ultrasound transducers have been applied to commercial products, the existing models are too bulky to be integrated into consumer electronics. Therefore, this study proposes a piezoelectric micromachined ultrasonic transducer (pMUT) with a small size and low power consumption to replace traditional transducers. The proposed pMUT has a resonance frequency of 40 kHz and a radius designed through the circular plate model and finite element model. To achieve better performance, lead zirconate titanate was selected as the piezoelectric layer and fabricated via RF sputtering. The cavity of the pMUT was formed by releasing a circular membrane with deep reactive ion etching. The resonance frequency of the pMUT was 32.9 kHz, which was close to the simulation result. The acoustic pressure of a single pMUT was 0.227 Pa at 70 Vpp. This study has successfully demonstrated a pMUT platform, including the optimized design procedures, characterization techniques, and fabrication process, as well as showing the potential of pMUT arrays for ultrasound haptics applications.

Groovy Electrodes Enable Facile O2 and H+ Transport in PEMFCs

Polymer electrolyte membrane fuel cells (PEMFCs) are promising alternative to internal combustion engines, owing to their capability to produce on-demand power with zero local carbon emissions. However, PEMFCs suffer from limitations related to performance, particularly for heavy-duty vehicle applications [1]. A major contributor to the performance loss is the cathode electrode; uncontrolled conventional electrode structures lead to non-ideal transport pathways of O2 and H+ [2], subsequently leading to performance loss. Here, we report an alternative electrode structure termed groovy electrodes, fabricated via patterned Si templates. Si templates with desired patterns are fabricated through photolithography and deep-reactive ion etching techniques (Fig. 1a), and an electrode layer is coated onto the template and subsequently transferred to the membrane or the gas diffusion layer. The resulting electrode features ordered grooves with consistent depth and width (Fig. 1b-c). We will present how this alternative structure enables improved H+ transport (enabled by higher ionomer content) with grooves facilitating effective O2 transport to reaction sites. We will also demonstrate the benefits of a groovy electrode for the durability of PEMFCs. Reference Cullen et al., Nat. Energy, 6, 462 (2021).Ramaswamy et al., J. Electrochem. Soc., 167, 064515 (2020). Figure 1

Development of Bipolar Electrostatic Chuck with a Beam-Array Assembly Fabricated by Lithography

This technical paper demonstrates the influence of the probe-tip surface smoothness of a bipolar electrostatic chuck (ESC) on electrostatic force. ESC, which has a silicon-based beam-array microstructure, aims to pick and place a dielectric object with a curved surface owing to the compliance of its elastically deformable beams. The ESC was fabricated using a lithography technique, specifically deep reactive ion etching (DRIE), to smooth the surface of the beam tip. The surface roughness of the beam tips was observed using a field-emission scanning electron microscope (FE-SEM), and the adhesional force was experimentally evaluated. The results show that by the smoothing process, the adhesional force per unit area is significantly increased compared to the previous study reported by Choi (one of the authors). This suggests that the proposed bipolar ESC device has great potential for use in various industries.

Micro-heat sink based on silicon nanowires formed by metal-assisted chemical etching for heat dissipation enhancement to improve performance of micro-thermoelectric generator

This work demonstrates the micro-heat sink based on silicon nanowires formed by metal-assisted chemical etching (MACE) for heat dissipation enhancement to improve the performance of the micro-thermoelectric generator (µ-TEG). The heat dissipation through the micro-heat sink is enhanced by increasing the surface-to-volume ratio, which can be achieved by combining deep reactive ion etching (RIE) and MACE. Silicon nanowires with a diameter of 100 nm and a height of 9 µm are successfully formed in both horizontal and vertical surface directions. The micro-heat sink effectiveness is 8.3 times better than that of without employing the micro-heat sink. In addition, the performance of the µ-TEG has been significantly enhanced by utilizing the micro-heat sink. The maximum output power of the µ-TEG with and without the micro-heat sink are 93 µW and 18.5 µW, respectively, under the same evaluation conditions. The findings in this work may be useful not only for the µ-TEG, but also other applications such as micro-supercapacitors, micro-sensors, chemical analysis, and biological processes, which require a large surface-to-volume ratio.

Improvement of imaging performance of silicon micropore X-ray optics by ultra long-term annealing.

We have been developing a light-weight X-ray telescope using micro electro mechanical systems technologies for future space missions. Micropores of 20 µm width are formed in a 4-inch Si wafer with deep reactive ion etching, and their sidewalls are used as X-ray reflection mirrors. The flatness of the sidewall is an important factor to determine the imaging performance, angular resolution. It is known that hydrogen annealing is effective to smooth a Si surface. We tested 150 hour annealing to achieve the ultimately smooth sidewalls. After 50 hour, 100 hour, and 150 hour annealing, the angular resolution improved 10.3, 4.0, and 2.6 arcmin in full width at half maximum (FWHM) and 17.0, 14.5, and 10.8 arcmin in half-power width (HPW). In spite of this improvement, the edge shapes of the sidewall were rounded. Therefore, both edges of the sidewall were ground and polished, and then the angular resolution was improved further to 3.2 arcmin (FWHM) and 5.4 arcmin (HPW).

An all-silicon microfabricated gas chromatographic column with a high aspect ratio

Increasing the aspect ratio of the microchannels of microfabricated gas chromatographic columns (μGCCs) can allow analytes to quickly reach equilibrium between the stationary phase and mobile phase, which helps improve their separation performance. In this paper, an all-silicon microfabricated gas chromatographic column (AS-μGCC) with a high aspect ratio was proposed and fabricated using deep reactive-ion etching (DRIE) and silicon direct bonding. Two AS-μGCCs with the lengths of 2 m and the aspect ratios of 6.6 (100 µm-width × 660 µm-depth) and 25 (30 µm-width × 750 µm-depth) were successfully fabricated. The effects of different aspect ratios on the column efficiency of AS-μGCCs were studied. A separation experiment showed that the column efficiency of the AS-μGCC with an aspect ratio of 25 (AS-μGCC-25) was 12 times higher than that of the AS-μGCC with an aspect ratio of 6.6 (AS-μGCC-6.6). The column efficiency of the AS-μGCC-25 was 1961 plates/m, and a heavy hydrocarbon mixture (C8–C20) could be separated using AS-μGCC-25 within 4.5 min

The Development of Monolithic Silicon Carbide Intracortical Neural Interfaces for Long-Term Human Implantation

Silicon Carbide (SiC) has been demonstrated as both a bio- and neuro-compatible wide-band-gap semiconductor with a high thermal conductivity and magnetic susceptibility and may be potentially compatible with human brain tissue. Two single-crystal, solid-state forms of SiC have been used to create monolithic intracortical neural implants (INI) without using physiologically exposed metals or polymers, thus eliminating many known reliability challenges in-vivo through a single, homogenous material. Amorphous SiC (a-SiC) was used to insulate 16-channel functional INI and the electrochemical and MRI compatibility (7T) performance were measured. 4H-SiC interfaces were fabricated using homoepitaxy,alternating epitaxial films of n-type and p-type forming an isolating PN junction which prevents substrate leakage current between the 16 adjacent electrodes and traces fabricated which were formed using deep-reactive ion etching (DRIE). 3C-SiC interfaces were fabricated in a similar fashion, but the epitaial conductive layers were grown on on both bulk crystalline (100) silicon and SOI wafers. In both cases a conformal coating of a-SiC was used as the top-side insulator and windows opened using RIE to allow electrochemical interaction. Electrochemical charaterization achieved through electrochemcial impedance spectroscopy (EIS) and cyclic voltammetry (CV) indicates performance on par, or exceeding, that of Pt reference electrodes with the same form fit. While magnetic resonance imaging (MRI) is an essential, non-contact method used to investigate issues with the nervous system, the high field MRI (e.g., 3 T and higher) necessary for proper diagnosis can be a safety issue for patients with INI due to inductive coupling between the powerful electromagnetic fields and the implanted device. This results in having to use lower electromagnetic field power (less than 1.5T), and therefore lower resolution, which hinders diagnostic prognosis for these patients. In this work the MRI compliance of epitaxial, monolithic SiC INI was studied. The specific absorption rate (SAR), induced heating, and image artifacts caused by the portion of the implant within a brain tissue phantom located in a 7 T small animal MRI machine were estimated and measured via finite element method (FEM) and Fourier-based simulations. Both the simulation and experimental results revealed that free-standing 3C-SiC films had no observable image artifacts compared to silicon and platinum reference materials inside the MRI at 7 T while FEM simulations predicted an ~30% SAR reduction for 3C-SiC compared to Pt. These initial simulations and experiments indicate a SiC monolithic INI may effectively reduce MRI induced heating and image artifacts in high field MRI.

Design and fabrication of a micro-opto-mechanical-systems accelerometer based on intensity modulation of light fabricated by a modified deep-reactive-ion-etching process using silicon-on-insulator wafer

Accelerometers that work based on intensity modulation of light are more sensitive, economically feasible, and have a simpler fabrication process compared to wavelength modulation. A micro-opto-electro-mechanical-system accelerometer based on intensity modulation of light is designed and fabricated. A movable shutter that is attached to the proof mass is designed to change the intensity of light. Moreover, the mechanical part is designed to improve the overall sensitivity and linear behavior in the measurement range. The designed accelerometer is fabricated by a deep-reactive-ion-etching (DRIE) process. The DRIE process used in this report is based on a Bosch-like process, which uses SF6 and a mixture of H2, O2, and SF6 gases in etching and passivation subsequences, respectively. This method has a lower plasma density and a higher base pressure and causes higher verticality, which results in lower optical efficiency loss in comparison with the Bosch process. Furthermore, the functional characteristics of the accelerometer are derived with analytical and numerical methods, and the results are compared. The functional characteristics of the accelerometer are as follows: a resonant frequency of 0.56 kHz, a mechanical sensitivity of 0.6μm/g, an optical sensitivity of 16%/nm, an overall sensitivity of 9.6%/g, a footprint of 2000×2000μm2, a measurement range of 3g, a mechanical cross axis sensitivity of 0.058μm/g, and an overall cross axis sensitivity of 0.00029%/g. These functional characteristics make the design appropriate for a large range of applications.

A carbon nanotube@silicon-based three-dimensional porous photo-supercapacitor for self-powered UV detection

Miniaturized self-powered energy devices are indispensable units for next-generation portable electronics. Photo-supercapacitors (PSCs), as a reliable energy harvesting device, have been explored for self-powered applications by utilizing the photovoltaic effect of micro-supercapacitors. In this paper, we propose a self-powered PSC with bulk micro-machined three-dimensional (3D) porous asymmetric electrodes with a high-aspect ratio via deep reactive-ion etching (DRIE) technique for the detection of ultra-violet (UV) radiance. The as-fabricated porous CNT@TiC/CNT@TiO2 PSC delivers an areal capacitance of 2.69 mF/cm2 (@5 mV/s) and 7.35 mF/cm2 (@20 μA/cm2) before and after photo-charge process, respectively. The optical response of the PSC is linearly proportional to the UV power intensity with a photoresponsivity of 14.8 mA/W. A photocurrent density of 354 μA/cm2 and a photo-to-dark current ratio of 12 are experimentally obtained under a UV power intensity of 15 mW/cm2. The as-prepared PSCs are further integrated on-chip to implement a UV leakage warning system that can drive a white LED for more than 60 s under a UV power intensity of 10 mW/cm2, fully demonstrating the practical application of the 3D silicon-based device in UV monitoring systems.

A Low-Temperature-Sensitivity Resonant Pressure Microsensor Based on Eutectic Bonding

This paper presented a temperature-insensitive resonant pressure microsensor where silicon based resonators anchored on a pressure-sensitive diaphragm were vacuum packaged by a silicon cap based on eutectic bonding. Incoming pressures deformed the pressure-sensitive diaphragm and built stresses around resonators for frequency modulation while under temperature challenges, deformations of silicon based resonators and the vacuum cap were consistent and thus no stresses were generated on resonators. The temperature-insensitive resonant pressure microsensor was analyzed in both theoretical analysis and numerical simulations with confirmed high pressure sensitivities and low temperature disturbances. The resonant pressure microsensor was then fabricated by key steps of photolithography, deep reactive ion etching, and eutectic bonding and characterized in both open-loop and close-loop testing systems. Characterization results showed that the quality factors of resonators were <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 10000$ </tex-math></inline-formula> with pressure sensitivity of 82.98 Hz/kPa and temperature disturbance of −0.63 Hz/° (the lowest result among previously reported resonant pressure microsensors). In summary, the temperature-insensitive resonant pressure microsensor developed in this study exhibited a fitting accuracy better than 0.02% FS within the pressure range of 10 to 120 kPa and the temperature range of −45 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$85^{\circ }\text{C}$ </tex-math></inline-formula> .

Stamping colors with solid-state superionic stamping (S4)

Traditional top-down approaches to producing metallic nanostructures are usually indirect; involve ultra-high vacuum processing steps and deep sub-micron lithography such as electron-beam lithography (EBL). Therefore, such pathways are difficult to scale up to high production rates. Additionally, they are too expensive to deploy in the production of single-use, low-cost products like plasmonic biological or chemical sensors, security tags or decorative patterns, all of which involve metallic nanopatterns. Hence, there is a need for more scalable and affordable manufacturing pathways for producing metallic nanostructures. In this work, we demonstrate a scalable manufacturing pathway for patterning silver nanostructures for plasmonic color generation. The pathway begins with the production of a single master pattern. We use commercially available Two-Photon Lithography (TPL) to direct-write a mask on a silicon substrate. Shallow etching of the silicon substrate with this mask by the Reactive Ion Etching (RIE) process (or Deep Reactive-Ion Etching for deeper patterns) produces a silicon master mold with the desired pattern. This master is used for hot embossing AgI-AgPO3 glass, a chemically stable, pure Ag-ion conductor with a low glass transition temperature (of about 100 °C, depending on its stoichiometry), into a stamp. This glass stamp, in turn, is used in the Solid-State Superionic Stamping (S4) process to electrochemically imprint the desired pattern into several substrates. Here, with this direct, facile, and efficient process, we report that a single nano-patterned stamp is capable of producing several high-fidelity replicas of the master.

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.

Fabrication of a fractal pattern device for focus characterizations of X-ray imaging systems by Si deep reactive ion etching and bottom-up Au electroplating.

Precisely aligned optical components are crucial prerequisites for X-ray tomography at high resolution. We propose a device with a fractal pattern for precise automatic focusing. The device is etched in a Si substrate by deep reactive ion etching and then filled by a self-terminating bottom-up Au electroplating process. The fractal nature of the device produces an X-ray transmission image with globally homogeneous macroscopic visibility and high local contrast for pixel sizes in the range of 0.165 µm to 11 µm, while the high absorption contrast provided between Au and Si enables its use for X-ray energies ranging from 12 keV to 40 keV.

New Fabrication Method of Silicon Sub-Micron Beams with Monolithic Contacts for Thermoelectric Transport Properties Analysis.

Micromachined devices were developed and fabricated using complementary metal-oxide-semiconductor (CMOS)/micro-electro-mechanical systems (MEMS) technology allowing for the analysis of transport properties of silicon sub-micron beams having monolithic contacts. The beams were fabricated by a combination of deep reactive ion etching (RIE) and potassium hydroxide (KOH) etching techniques on standard p and n silicon bulk and silicon-on-insulator (SOI) wafers. Simultaneous fabrication of many devices on one wafer allows for the extraction of statistical information to properly compare the different layers and contacts. Fabricated devices are presented, underlining the feasibility of the proposed microdevice. The methods used to manipulate the geometry and the surface roughness of the single crystalline silicon beams are described. The presented measurement device offers the possibility to determine simultaneously all the main transport values, thermal, and electrical conductivities as well as the Seebeck coefficient.

Etch mechanism of an Al2O3 hard mask in the Bosch process

The etching of high aspect ratio structures in silicon via the Bosch process is essential in modern technologies such as microelectromechanical systems (MEMS) and through‑silicon vias (TSV) fabrication. The process can be very demanding on the mask selectivity due to long etching times, and it has been shown that an Al 2 O 3 hard mask is very suitable in this regard, as it offers significantly higher selectivity compared to the conventional SiO 2 or resist masks. In this work, we employ a combination of Scanning Electron Microscopy (SEM), Spectroscopic Ellipsometry (SE) and X-Ray Photoelectron Spectroscopy (XPS) depth profiling to scrutinize the Al 2 O 3 mask etching mechanism and therefore the origin of the extraordinary high selectivity. We demonstrate that by increasing the passivation step time, a thicker fluorocarbon polymer layer is formed on the Al 2 O 3 , and Al 2 O 3 is then removed with a minuscule average etch rate of ~0.01 nm/min. XPS depth profiling reveals that during Deep Reactive Ion Etching (DRIE) using the Bosch process, an AlF x layer is formed between the polymer and Al 2 O 3 . As AlF x is non-volatile, it requires sputtering to be removed. If the polymer layer is thick enough to attenuate the incoming ions such that their energy is not sufficient to lead to desorption of AlF x , such as when using a longer passivation time, the mask is not eroded. By investigating the surface after different amounts of DRIE cycles, we also obtained information about the formation rate of AlF x and the changes in the Al 2 O 3 and polymer thicknesses over the course of a DRIE process. These findings further expand the knowledge of DRIE and can help process engineers to tailor the processes accordingly. • Selectivity of an Al 2 O 3 mask in the Bosch process becomes practically infinite with longer passivation step time • XPS depth profiling reveals that an AlF x layer is formed during DRIE between Al 2 O 3 and the fluorocarbon polymer on top • This non-volatile AlF x cannot be removed by sputtering if the polymer is thick enough, i.e. with longer passivation time • The dynamics of AlF x formation during DRIE was observed with time resolved measurements

Bio-Inspired Hierarchical Micro/Nanostructured Surfaces for Superhydrophobic and Anti-Ice Applications.

We report a scalable and cost-effective fabrication approach for constructing bio-inspired micro/nanostructured surfaces. It involves silicon microstructure etching using a deep reactive ion etch (DRIE) method, nanowires deposition via glancing angle deposition (GLAD) process, and fluorocarbon thin film deposition. Compared with the smooth, microstructured, and nanostructured surfaces, the hierarchical micro/nanostructured surfaces obtained via this method showed the highest water contact angle of ∼161° and a low sliding angle of <10°. It also offered long ice delay times of 2313 s and 1658 s at −5°C and −10°C respectively, more than 10 times longer than smooth surfaces indicating excellent anti-icing properties and offering promising applications in low-temperature environments. These analyses further proved that the surface structures have a significant influence on surface wettability and anti-icing behavior. Hence, the GLAD process which is versatile and cost-effective offers the freedom of constructing nanostructures on top of microstructures to achieve the required objective in the fabrication of micro/nanostructured surfaces when compared to other fabrication techniques.

Improvement on the uniformity of deep reactive ion etch for electrically isolated silicon-based substrates

Substrate-free micro-electro-mechanical systems (MEMS) devices are becoming the hotspots for microsystems. The fabrication of substrate-free MEMS devices usually involves the release of backside silicon by the inductively coupled plasma deep reactive ion etch (ICP-DRIE) process. However, when using DRIE to etch electrically isolated samples, significant non-uniformity in the etch profile were often observed. Compared to grounded silicon samples, the electrically isolated counterparts after DRIE showed a faster etch rate at the edge and a slower one in the center. This phenomenon is believed to be caused by the interaction between the deflection of charge-bearing ions entering the aperture region and the accumulated charges on the sidewall during DRIE. Simulation results with ICP showed that the electric field and ion distribution can be affected in electrically isolated substrates. After the isolated samples were electrically grounded, the charge accumulation on the sidewall was reduced and 12% etch uniformity was obtained. This technique helps in the fabrication of substrate-free MEMS devices.

High aspect ratio tilted gratings through local electric field modulation in plasma etching

The anisotropic nature of plasma etching is usually exploited to realize vertical nano-/micro- silicon structures by deep reactive ion etching. However, some applications require tilted instead of perpendicular profiles with respect to the substrate. Here, a controlled tilted etching is realized by introducing a set of metal electric field modulator(s), which modify the near sample surface potential. The ions from the plasma body are accelerated under the influence of the distorted electric field, and hit the silicon surface with a certain incident angle. A model is built with finite elements method, taking into account the geometry of the experiment and the chamber conditions during the etching process. The thickness and the inter-distance of Al slabs have been varied in a range of 0.5–3 mm and 10–25 mm, respectively. A tilt angle ranging from 0° to 22.6° has been measured and validates the simulation results, showing that a desired tilt profile can be achieved with a proper parameters tuning. Examples of 1D and 2D modulations are reported with linear and chessboard slanted gratings for X-ray imaging applications.