Ion Etching Of Silicon Research Articles

Realization of Three-Dimensionally MEMS Stacked Comb Structures for Microactuators Using Low-Temperature Multi-Wafer Bonding with Self-Alignment Techniques in CMOS-Compatible Processes.

A high-aspect-ratio three-dimensionally (3D) stacked comb structure for micromirror application is demonstrated by wafer bonding technology in CMOS-compatible processes in this work. A vertically stacked comb structure is designed to circumvent any misalignment issues that could arise from multiple wafer bonding. These out-of-plane comb drives are used for the bias actuation to achieve a larger tilt angle for micromirrors. The high-aspect-ratio mechanical structure is realized by the deep reactive ion etching of silicon, and the notching effect in silicon-on-insulator (SOI) wafers is minimized. The low-temperature bonding of two patterned wafers is achieved with fusion bonding, and a high bond strength up to 2.5 J/m2 is obtained, which sustains subsequent processing steps. Furthermore, the dependency of resonant frequency on device dimensions is studied systematically, which provides useful guidelines for future design and application. A finalized device fabricated here was also tested to have a resonant frequency of 17.57 kHz and a tilt angle of 70° under an AC bias voltage of 2 V.

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.

Capabilities and Limits to Form High Aspect-Ratio Microstructures by Molding of Borosilicate Glass

This paper presents an experimental study of capabilities to fabricate high aspect-ratio microstructures in glass. The fabrication was done by molding borosilicate glass in deep silicon trenches created by deep reactive ion etching in silicon and releasing the structures by potassium hydroxide (KOH) and Xenon difluoride (XeF2) etching of the Si mold. Subsequent over-etching of the glass can also be used to increase the aspect ratio of the glass structures. By using this molding method, glass structures have been successfully fabricated achieving an aspect ratio of 42:1, a height of 212 $\mu \text{m}$ , and vertical sidewall angle up to 89.7°. We also investigated the effect of molding parameters (heat treatment time, temperature, and pressure) on the process capabilities and limitations for defect free structures. This work demonstrates the capabilities to fabricate high aspect-ratio glass structures that cannot be achieved by conventional etching techniques to date, opening new perspectives for the use of glass in MEMS and microfluidics.

Monte Carlo Simulation of Defects of a Trench Profile in the Process of Deep Reactive Ion Etching of Silicon

A numerical model of the evolution of a 2D profile during cryogenic etching of Si in SF6/O2 plasma is proposed and implemented. To calculate the fluxes of species a Monte Carlo method is used. The etch profile is presented with the help of square cells. The model is meant to investigate diverse defects in a profile of stochastic nature. For this, the state of a model cell is presented as a combination of states of several subcells randomly chosen upon each interaction of the species with the surface, which makes it possible to study small-scale profile defects without loss of calculation performance. They are compared with the experimental data, and good qualitative agreement is observed. Surface defects typical of high aspect ratio cryogenic etching are investigated numerically. They include the depth-dependent sidewall roughness, formation of cavities, trench splitting, and formation of “nanograss.”

Dependence of the Resistance of the Negative e-Beam Resist HSQ Versus the Dose in the RIE and Wet Etching Processes

This article studied the resistance of the negative electron resist based on hydrogen-silsesquioxane (HSQ) depending on the dose of exposure in the process of Reactive Ion Etching (RIE). These studies showed the strong dependence of resistance on irradiation dose (in case of full development of the e-beam resist) even after annealing the resist 30 minutes 400°C in air. Selectivity up to 14 was obtained in the process of reactive ion etching of silicon in a mixture of gases SF6: C4F8. These results can be used to manufacturing of silicon nanoscale structures. It was shown that the resistance to wet etching in a 5% solution of hydrofluoric acid (HF) is also determined by irradiation dose. Additionally, taking into account the obtained results, silicon nanowires of width 10 nm with an aspect ratio of 1: 10 was manufactured.

Soft thermal nanoimprint of PMMA doped with upconverter nanoparticles

Upconversion of low-energy photons into higher energy photons is a non-linear effect that can be strongly enhanced by increasing the intensity of impinging light. Such an enhanced intensity can also be achieved by near-field optical effects in photonic structures. In this paper, we investigate photonic structures in the form of linear gratings with varying periods realised in poly-methyl-methacrylate (PMMA) layers using soft stamps in a thermal nanoimprint process. Master structures were fabricated using e-beam lithography and reactive ion etching in silicon. These master structures were replicated into bi-layer polydimethylsiloxane (PDMS) stamps. PMMA layers with embedded β-NaYF4:25% Er3+ upconverter nanoparticles were applied to glass substrates using spin coating. The PDMS stamps were used to imprint the PMMA layer with very accurate pattern fidelity. The upconversion luminescence around 980nm was enhanced more than three times by the photonic structure under 1523nm laser excitation with 0.43±0.02W/cm2 irradiance, in comparison to a reference sample containing the same amount of upconverter material.

Fabrication of two-dimensional hard X-ray diffraction gratings

Hard X-ray grating interferometry has shown promising results in phase and scattering imaging, as well as in metrology applications. Recently, the technique has been extended to two dimensions, recording the full phase gradient vector and a directional scattering signal. Here, we present a process for fabricating the key optical elements required for this technique: phase and absorption gratings with periods of few micrometers and high aspect ratios, with a particular focus on two-dimensional grating structures. The fabrication process is based on deep reactive ion etching in silicon and electroplating of gold.

Techniques of cryogenic reactive ion etching in silicon for fabrication of sensors

Cryogenic etching of silicon, using an inductively coupled plasma reactive ion etcher (ICP-RIE), has extraordinary properties which can lead to unique structures difficult to achieve using other etching methods. In this work, the authors demonstrate the application of ICP-RIE techniques which capitalize on the cryogenic properties to create different sensors geometries: optical, electrical, magnetic, and mechanical. The three techniques demonstrated are (1) single step deep etches with controllable sidewall profiles. Demonstrating this, silicon pillars with over 70μm depth and less than 250nm sidewall roughness were etched using only 1.6μm of photoresist for use as solar cells. (2) Using the cryogenic etch for thick metallization and liftoff with a thin photoresist mask. Demonstrating this second technique, a magnetic shim was created by deposition of 6.5μm of iron into 20μm deep etched trenches, using the remaining 1.5μm photoresist etch mask as the liftoff mask. Using the same technique, 15μm of copper was lifted off leaving a 20μm deep plasma enhanced chemical vapor deposition silicon oxide coated, silicon channel with copper. (3) Use of a two step cryogenic etch for deep etching with reduced sidewall undercutting. This was demonstrated by fabrication of deep and anisotropic microelectromechanical systems structures; a mechanical resonator was etched 183μm deep into silicon with less than 3μm of undercutting. This work also describes the etch parameters and etch controls for each of these sensors.

Deep Vertical Etching of Silicon Wafers Using a Hydrogenation-Assisted Reactive Ion Etching

A novel hydrogenation-assisted deep reactive ion etching of silicon is reported. The process uses sequential hydrogen-assisted passivation and plasma etching at low-density plasma powers to stimulate the vertical removal of the exposed Si substrate. The main feature of this technique is the sequential alternation of the electrodes while switching between different gases. Three-dimensional structures with aspect ratios in excess of 40:1 and features as small as 0.7 mum have been realized. The net etch rate is about 0.25 mum/min, although higher rates are expected to be achievable.

Revisiting micro hot-embossing with moulds in non-conventional materials

Manufacturing moulds in non-metallic tooling materials using non-conventional structuring techniques is explored for rapid and/or alternative tooling for hot-embossing of polymers. Some challenges and advantages of producing embossing tools by deep reactive ion etching in silicon and/or replica-casting in poly(dimethylsiloxane) and utilizing them in the replication process are highlighted.

An interconnected 2D-TM EBG structure for millimeter and submillimeter waves

Due to their unique properties, electromagnetic bandgap (EBG) materials are of high interest for applications in communication technology for many frequency bands from microwave up to optical frequencies. We have investigated in both simulation and experiment a two dimensionally periodic EBG structure made by reactive ion etching of silicon with a bandgap for transverse magnetic waves in the millimeter wave range around 100 GHz. The structure comprises both a large bandgap and a high mechanical stability due to interconnecting dielectric bridges.

Bidirectional pumping properties of a peristaltic piezoelectric micropump with simple design and chemical resistance

A planar bidirectional valveless peristaltic micropump for controlling biosamples and reagent fluids was designed with a very simple structure and fabricated employing microelectromechanical system technologies, which include deep reactive ion etching in silicon and silicon–glass anodic bonding techniques. The proposed micropump consists of piezoelectric disks, silicon, and glass, thereby having the advantage of being resistant to aggressive media. The pump diaphragms are excited by applying pulse signal voltages to three lead zirconate titanate (PZT) disks on glass plates. For the channel and chamber in the silicon, the deep reactive ion etching process is utilized and a glass wafer is bonded to the pump-cavity side of the silicon wafer using an anodic bonding machine. The entire pump chip is 30×20×1 mm 3. The proposed micropump was able to control bidirectionally the flow rate of sample water at ∼12 μl/min (20 nl/stroke) and sample air at ∼60 μl/min (100 nl/stroke) based on a frequency of 10 Hz at 120 peak-to-peak voltage. In addition, actuation modeling of the PZT–glass actuator was performed using a Coventor simulator, CoventorWare, and the simulation results agreed well with the experimental measurements.

Microfabrication of Piezoelectric MEMS

In this paper we present an overview of processes for fabrication of piezoelectric thin film devices using PZT (Pb(ZrxTi1−x)O3) in planar structures. These structures are used in cantilever-like and membrane configurations for sensing and actuation. Elaboration of a compatible wet and dry etching sequence for patterning of PZT, electrodes, SiO2 and silicon substrate is the key issues. The method for compensation of mechanical stresses to obtain flat, multilayer structures is demonstrated. Definition of membrane thickness and release of the structures are obtained by Deep Reactive Ion Etching of silicon or by surface micromachining. The complete process has been used for fabrication of cantilever arrays, ultrasonic transducers and pressure sensors. Excellent permittivity and transverse piezoelectric coefficient of PZT have been obtained with the final devices.

Using polydimethylsiloxane as a thermocurable resist for a soft imprint lithography process

Nanoimprint lithography has been investigated using polydimethylsiloxane as a thermocurable resist. This novel process allowed us to reduce both pressure (<10 bars) and temperature (80 °C) when compared to a conventional imprinting process with a thermoplastic polymer resist such as polymethylmethacrylate. Using a new formulation of the elastomeric material, we have demonstrated high quality imprinting of both micronic and nanometric structures with no evidence of any viscous flow problems. The excellent etching resistance of the polydimethylsiloxane structures to a reactive ion etching silicon process and the compatibility with a lift-off procedure for pattern transfer are also presented.

Bilayer resist used in e-beam lithography for deep narrow structures

A bilayer reist system developed by Olin Microelectronic Materials, consisting of a UV curable polystyrene bottom coat and a silicon loaded UV sensitive positive tone top coat ('bamboo resist'), has been investigated for its sensitivity and resolution in electron beam lithography, and its etch selectivity during subsequent use in Reactive Ion Etching of silicon. The ultimate resolution obtainable with the top coat appears to be around 80 nm, with a Dose-to-Clear of 28 @mC/cm^2 and a contrast value above 25. The mechanical and chemical stability of the bottom coat and its adhesion on silicon are excellent, allowing the fabrication of 30 nm thick free standing resist 'fences'. The best resolution obtained here in e-beam lithography, indicates that the resolution of this system will most probably not be limited by the resist performance down to at least 80 nm in optical lithography as well.

Application of Al Mask for SF6/N2 Reactive Ion Etching of Silicon

We researched SF6/N2 reactive ion etching of silicon using Al mask. It was established, that Al mask prevent formation of polymeric layers and rate of etching is higher, than using photoresistive mask. Etching directionality amount to 20 units, selectivity - 400 units.

Surface modelling of reactive ion etching of silicon–germanium alloys in a SF6 plasma

Abstract Reactive ion etching (RIE) of amorphous Si x Ge 1− x alloys (0≤ x ≤100%) in a low-pressure SF 6 plasma has been investigated by means of quasi in situ X-ray photoelectron spectroscopy. The reactive layer on the etched alloys mainly consists of SiF, GeF, SiS and GeS 2 species. The sulfur and fluorine coverages on both Ge and Si atoms depend on the SiGe alloy stoichiometry in agreement with the nonlinear evolution of the alloy etch rates. A simple etching model where the etch rate is assumed to be proportional to the fluorine coverages has been developed for the description of the Si x Ge 1− x etching mechanism.

Vertical mirrors fabricated by deep reactive ion etching for fiber-optic switching applications

We report on vertical mirrors fabricated by deep reactive ion etching of silicon. The mirror height is 75 /spl mu/m, covering the fiber core of a single-mode fiber when the latter is placed into a groove of equal depth and etched simultaneously with the mirror. To obtain a uniform etch depth, etching is stopped on a buried oxide layer. Using the buried oxide as a sacrificial layer allows to fabricate mirrors with suspension and actuation structures as well as fiber-alignment grooves in one and the same processing step. A minimal mirror thickness of 2.3 /spl mu/m was achieved, resulting in an aspect ratio higher than 30. The verticality was better than 89.3/spl deg/. In the upper part of the mirror a surface roughness below 40 nm rms was obtained. At a wavelength of 1300 nm the reflectivity of the aluminum-coated mirrors was measured to be higher than 76%. Using a reactive ion etched mirror we have fabricated an optical fiber switch with electrostatic actuation. The coupling loss in the bar state of two packaged prototypes was between 0.6 and 1.7 dB and between 1.4 and 3.4 dB in the cross state. The switching time is below 0.2 ms.

Small free energy barrier and postdesorption collisions: The keys towards the understanding of reactive ion etching of silicon.

Reactive ion etching of Si with C${\mathrm{l}}_{2}$ as the reactive gas has been examined with molecular dynamics simulations. The volatility of ${\mathrm{SiCl}}_{x}$ molecules is determined by Gibbs free energy calculations as a function of temperature. The high entropy of activation of the desorbing molecules results in an unexpected low desorption barrier under etching conditions. Etching simulations show that many sputtered molecules undergo one or more collisions with protrusions on the corrugated surface. This results in a redistribution of the kinetic energy and a consistent interpretation of experimental results.

Temperature dependent electron-beam-induced-current investigation of electronic damages in silicon due to reactive ion etching

The damage introduced by reactive ion etching in silicon was investigated by the planar-electron-beam-induced-current (PEBIC) method. A reduction of the EBIC signal in the etched areas is detected and studied of temperature dependency in the range of 5 K&amp;lt;T&amp;lt;300 K. The EBIC contrast between etched and unetched areas increases with decreasing temperature. Our results are interpreted by a reduction of the net acceptor dopant impurity near the etched surface.