Anisotropic Etching Process Research Articles

Micro membrane reactor: a flow-through membrane for gas pre-combustion

A reactor element consisting of a permeable membrane fabricated in silicon micromachining technology will be presented. To circumvent a reaction chamber with a high dead volume, we have realised a permeable membrane in conjunction with porous silicon to achieve a high surface to volume ratio impregnated with different catalytic materials (depending on application and target gases). The complete device includes a heating element around the membrane which is thermally decoupled with a porous silicon well of the surrounding bulk material.The gas flows through the membrane and reacts during the passing time. The time of reaction for the gases depends on the membrane thickness. For optimised gas reaction, heating with an integrated temperature element is essential.The fabrication process is a combination of anisotropic silicon etch processes to create a silicon membrane with porous silicon technology to perforate the membrane. Porous silicon will also be used as thermal isolation of the active membrane and the heater.

Fabrication of single-crystal Si cantilever array

Abstract For the purpose of improvement in resolution of force gradient and mass detection in atomic force microscopy (AFM), we are developing cantilevers measuring a few microns. We succeeded in fabrication of a single-crystal Si cantilever with several microns size and measurement of its mechanical characteristics. A silicon-on-insulator (SOI) wafer was used for the fabrication. Fabrication was based on three anisotropic etching processes by KOH and two local oxidation processes of Si. Without depending on precision of lithography technique, triangular cantilevers measuring a few microns with tetrahedral tips on their ends were fabricated with high uniformity. The thickness of the cantilever could be chosen from 20 to 200 nm. Typical spring constants, resonance frequencies and Q factors of the single-crystal Si cantilevers were 0.01–10 N/m, 1–100 MHz and around 104 in vacuum. The density of the cantilever is up to 1 million cantilevers per cm2. One cantilever can be used for high sensitivity AFM or used in parallel to image a large area with high time efficiency.

A novel bulk micromachined gyroscope based on a rectangular beam-mass structure

Abstract Described in this paper is a micromachined vibratory gyroscope fabricated by combining anisotropic etching and deep reactive ion etching (DRIE) process. The basic structure of the device is a cantilever beam-mass structure. The cross section of the beam is rectangular. The gyroscope employs electrostatic driving and piezoresistive sensing. The gyroscope can operate in an atmospheric pressure environment due to the high quality factor in the sensing direction. Piezoresistive sensing avoids the difficulties caused by small capacitance detection. The packaging and testing costs can be reduced significantly. A novel operation mode of two-dimensional excitation and phase detection for vibratory gyroscopes is also proposed. The gyroscope is measured by both of the amplitude detection and phase detection. Experiment results show that the sensitivity for angular rate signal is about 11.45 μV/°/s/5 V in the range of −120–120°/s in an atmospheric pressure environment when the amplitude detection is used. The phase change is about 0.152°/°/s in the range of −120–120°/s when the phase detection is used.

The influence of atomic configuration of (h k l) planes on adsorption processes associated with anisotropic etching of silicon

Abstract The effect of isopropyl alcohol (IPA) on anisotropic etching process of monocrystalline silicon in KOH solution has been considered. The differences in planes developing during the etching in KOH and KOH+IPA solutions has been analysed. The atomic configuration of different ( h k l ) planes was modelled with a computer program “Cerius”. Dangling bonds configuration, their spacings and inclination angles towards the etched ( h k l ) surface were used for explanation of the differences in etching rates and surface smoothness among different planes. The theoretical analysis, confirmed with the experimental results, allowed us to establish the role of isopropyl alcohol (IPA) in the etching process. Obtained results can be very useful for selection of other modifiers of etching solutions to create new shapes of 3D atructures in silicon for micromechanical applications.

Convex corner undercutting of {100} silicon in anisotropic KOH etching: the new step-flow model of 3-D structuring and first simulation results

In this paper, the mechanism of convex corner (CC) undercutting of Si/sub {100}/ in pure aqueous KOH solutions is revisited by proposing the step-flow model of 3-D structuring as a proper description of the observed phenomena. The basic idea is to conceive the Si/sub {100}/ anisotropic etching process, on the atomic scale, as a peeling process of terraced {111} planes at [110] oriented steps to understand also the arising shape in Si/sub [100]/ etching. On the basis of our new model, we are able to predict the microscopic three-dimensional (3-D) structure of the characteristic CC undercutting without any compensation etchmask structures. Furthermore, the theoretical description has been implemented in a new 3-D simulation tool. Its ability to calculate the shape of simple beam structures of different orientation is experimentally shown.

Maskless epitaxial lateral overgrowth of GaN layers on structured Si(111) substrates

GaN layers are laterally overgrown by metalorganic chemical vapor deposition on structured Si(111) substrates in a single growth process. The substrates are structured with parallel grooves along the Si 〈1–10〉 or perpendicular to the Si 〈1–10〉 direction by standard photolithography and subsequent dry etching. Due to the anisotropic chemical dry etch process, the remaining Si ridges are underetched. The GaN layer grows nearly exclusively on the bottom of the grooves and on the top of the ridges between the grooves. These two growth fronts are completely separated from each other. As a consequence, the GaN layer growing from the ridge area between grooves can extend over the grooves. This process is similar to the so called pendeo-epitaxy process, but is completely mask free during growth and does not require any growth interruption. The improvement of the crystalline and the optical quality of the GaN layer is demonstrated by atomic force microscopy and cathodoluminescence spectroscopy.

Fabrication of TiNi shape memory micropump

We have developed a shape memory alloy (SMA) actuated micropump as a component for use in micro analysis or micro dosage systems. In this paper, we will discuss the fabrication process and dynamic actuation properties of an SMA actuator, as well as the first result of pumping properties of a completed SMA micropump. TiNi thin film of about 6 μm in thickness was deposited onto a Si wafer with a square recess on its reverse side, and memorized an initial flat shape. The TiNi thin film and a Pyrex glass cap with a square recess were then anodically bonded together in a vacuum to form a chamber to which a bias pressure was to be applied to deform the TiNi thin film. After removing the remaining Si layer beneath the TiNi thin film by RIE in SF 6 plasma, a shape memory actuator of 5 mm square in size was completed. A Si check valve structure was also fabricated through a process of anisotropic etching and fusion bonding and was assembled with the actuator. The fabricated micropump with a size of 10 mm×20 mm×1.4 mm was driven by thermal cycles of resistive heating and air-cooling under bias pressure which was applied by a nitrogen gas flow. The completed SMA micropump proved to give a flow rate of 0.4 μl per cycle at a bias pressure of 100 kPa.

Materials Issues in the Application of Silicon Nitride Films in Silicon MEMS

AbstractThis paper reviews bulk micromachining of Si, with particular reference to the microassembly and packaging of optical and electronic components. Taking as an example the use of 2.3 [.proportional]m thick silicon nitride microclips to hold an optic fibre in place in a silicon V-groove, the potential of thin film packaging is briefly assessed. The fabrication process is to: [i] deposit a controlled stress silicon nitride layer onto a blank (100) oriented silicon wafer by chemical vapour deposition (standard LPCVD but silicon rich); [ii] pattern the silicon nitride clip mask shape by optical lithography and reactive ion etching; and [iii] remove the underlying silicon to form a V-shaped groove with an anisotropic liquid etch process using the silicon nitride as a mask. From the measured deflection of a silicon nitride beam when a force is applied, the Young's modulus is 400 GPa. The beams are rugged and a clipping force of 10 N per metre of optic fibre length is achieved.This approach to applying the mechanical properties of thin films is compatible with standard thin film technology, enables precise microassembly, and has great potential to reduce manufacturing costs in a wide range of sensors and microsystems. In some applications high temperature processing must be avoided, even early on in the manufacturing process. Using low temperature processing, robust clips from silicon nitride made by plasma enhanced chemical vapour deposition (PECVD and hydrogen rich) are made, but the Young's modulus of this material is typically 40 GPa and the clipping force is correspondingly small. The paper concludes by assessing the prospects for other thin film materials in mechanical MEMS and packaging.

Silicon anisotropic etching in alkaline solutions IV: The effect of organic and inorganic agents on silicon anisotropic etching process

Silicon anisotropic etching process in organic and inorganic solutions has been studied. Experimental results on silicon etching in KOH and TMAH solutions with and without IPA addition have been presented. Etching rate curves versus solution concentration were plotted for different crystallographic planes with special emphasis on high-indexed ones. A comparison of our own experimental results with the results of other authors, concerning silicon anisotropic etching process in hydroxides of other metals from the first group of periodic table, revealed that their ions and molecules contained in the solution play an important role in the process and influence its anisotropy. The observations led us to some general conclusions regarding physical and chemical phenomena associated with the silicon anisotropic etching. We have suggested that the lowering of etching rates is connected with adsorption of cations and organic additions on some crystallographic planes.

A novel micromachined vibrating rate-gyroscope with optical sensing and electrostatic actuation

This paper reports the fabrication and preliminary characterization of a novel vibratory rate-sensor. The sensor employs the modulated integrative differential optical sensing (MIDOS) to detect the output mode amplitude. The mechanical part of the sensor is fabricated by bulk micromachining using a double side anisotropic wet etching process. A CMOS chip containing the detecting photodiodes and their readout electronics is fabricated through MOSIS. Electrical and mechanical integration of the two parts is achieved by using the indium bumps technology. The proof mass is aligned with the detecting photodiodes, so that when at rest, equal portions of the two photodiodes are exposed. Several prototypes have been fabricated and tested on a rotating table. Good linearity (<1%) was shown on a −1000–1000°/s range and so far, a minimum detectable rate (MDR) below 1°/s was demonstrated. Future research will focus on lowering the MDR below 0.01°/s.

Investigation of a solder bumping technique for flip‐chip interconnection

As the demand for flip‐chip products increases, the need for low cost high volume manufacturing processes also increases. Currently solder paste printing is the wafer bumping method of choice for device pitches down to 150‐200μm. However, limitations in print quality and stencil manufacture mean that this technology is not likely to move significantly below this pitch and new methods will be required to meet the demands predicted by the technology roadmaps. This paper describes experiments conducted on carriers made from silicon for bumping of die using solder paste. An anisotropic etching process was used to generate pockets in the silicon surface into which solder paste was printed. Die were then placed against the carrier and reflowed to transfer the solder directly to the bondpads. An assessment was carried out of the potential application and limitations of this technique for device pitches at 225 and 127μm.

Anisotropic plasma etching of polymers using a cryo-cooled resist mask

An anisotropic etching process of a perfluorocyclobutene polymer that makes use of a spin-coated photoresist mask instead of a commonly used thin metal layer is reported. We demonstrate that such masking can be applied to advantage for anisotropic reactive ion etching of polymers if the wafer is cooled down to T=−50 °C. For the fabrication of integrated optical waveguides, the choice of an appropriate photoresist is very important if the edge roughness needs to be low in order to avoid optical scattering losses. For such applications, the Microresist Technology ma-P 1275 has been found to be very suitable.

410 機械加工を併用した大規模高精度異方性エッチング(第 3 報)

Crystal regularity governs final accuracy in anisotropic etching process since the final shape consists of specific crystal planes. However the size is limited up to millimeter order, this process is not suitable for processing the large size. This paper proposes a combined process of lapping and anisotropic etching to achieve large-scale regular shape with high accuracy. Removal amount and rate of specific crystal planes were analyzed and it was proven that the single crystal silicon was anisotropically machined under the control of lapping pressure. The maximum ratio of the removal rate to one of (111) plane was 2.1. It was shown the ability of the combined process to make surface of the (111) planes smoother in large scale.

Biochemical sensors with structured and porous silicon capacitors

Chemical sensors with structured and porous semiconductor/insulator capacitors have been developed. The sensors have been prepared using standard techniques of semiconductor processing, i.e. by means of an anisotropic and anodic etching process, respectively. Both sensor types exhibit a high, near-Nernstian pH sensitivity of about 54 mV per pH decade. Due to the enlargement of the active sensor area, the measured capacitance values increase up to a factor of 30. These structured and porous electrolyte–insulator–semiconductor (EIS) sensor substrates are suitable as transducer material for biosensors which has been demonstrated by depositing the enzyme penicillinase through physical adsorption onto the respective pH-sensitive EIS capacitors.

Applications and simulation of unconventional bulk-micromachining using underetching of {100} silicon planes

In this paper we present the principle and feasibility of bulk-micromachining using underetching of vertical {1 0 0} Silicon planes. Beside the demonstration of realized devices we investigate the fabrication reproducibility of these devices by simulating the anisotropic etching process for different cases of mask misalignment and crystal misorientation of the silicon wafer. Devices fabricated by the proposed etching technique can be reproduced within variations in mechanical performance lower than 3% if precautions such as an adapted design or pre-etching are considered.

The non equilibrium band model of silicon in TMAH and in anisotropic electrochemical alkaline etching solutions

This study presents the energy band diagram of silicon in contact with an aqueous solution of tetra-methyl ammonium hydroxide (TMAH) during the anisotropic electrochemical etch process. The band diagram is constructed from the measured open circuit potential (OCP) and the measured total band bending of the silicon at the OCP which is determined by impedance measurements. Current–voltage as well as capacitance–voltage characteristics of silicon in contact with TMAH solutions are presented. The present study emphasizes the non-equilibrium operation of the electrochemical cell even at the open circuit potential (OCP). In previous studies of the band diagram of silicon in alkaline anisotropic etch solutions, the Fermi levels of the silicon and the electrolyte are aligned at OCP. In contrast, in the present study the Fermi levels are not aligned since steady state rather than equilibrium prevails at this operation point. The value of the measured OCP is used to determine the separation between the two quasi Fermi levels. The assumptions leading to the construction of the band diagram are elaborated. The relationships between OCP, flat band voltage, passivation potentials and Fermi potentials are discussed. The predictions of the model and the effect of conductivity type, doping level, temperature and illumination are compared with experiments. The new model and methodology presented in this study is applicable to additional anisotropic alkaline etch solutions such as KOH and EDP. It is very useful for the appropriate design of the process required for electrochemical etch stop for the fabrication of membranes and controlled micromachining of silicon.

Optimization of dry etch process conditions for HgCdTe detector arrays

A key technology required for fabricating single and multi-band mesa photodiodes with pixel sizes less than or equal to 25 µm is the development of an anisotropic etch process for HgCdTe. The primary approach investigated for this purpose has been electron cyclotron resonance (ECR) dry etching. This paper reviews an experiment used to optimize the ECR etch process at Lockheed Martin IR Imaging Systems, Inc. and then the use of the process to produce state-of-the-art LW photodiodes. In this work, a Ar:H plasma was used in a Plasma Therm series 700 ECR plasma etcher. Reactor variables were optimized by a designed experiment against the following response parameters: anisotropy, etch uniformity and “damage,” as measured by the photodiode zero bias and reverses bias impedance characteristics. The critical process variables of Ar:H gas pressure, lower magnet current, and electrode height were all optimized. The optimized process parameters were then utilized to fabricate arrays with 80K cut-off wavelengths in excess of 11 µm, R0As of 29 Ω-cm2, Rd20mV/Rd0mV>13 and quantum efficiency>71%.

Coulomb blockade in VLSI-compatible multiple-dot and single-dot MOSFETs

Silicon-MOSFET-based single electron devices are promising for the future of extremely low power VLSI applications. Very narrow wire MOSFET and point contact MOSFET, which act as multiple-dot and single-dot single electron devices respectively, are fabricated using a VLSI-compatible anisotropic etching process. The Coulomb blockade phenomena in these devices have been extensively investigated. The devices show the Coulomb blockade oscillations at room temperature. At low temperatures, the coupling effects between dots play an important role in electron transport in the multiple-dot MOSFETs, while the quantum mechanical effects strongly affect the transport properties in the single-dot MOSFETs.

Improved anisotropic etching process for industrial texturing of silicon solar cells

An experimental study on the surface texturisation of monocrystalline wafers with solutions containing sodium-hydroxide and isopropanol was carried out. Both the composition and the temperature of the etching solution were optimised on the basis of etch-rate measurements on silicon samples having different crystallographic directions. It was found that the density and the size of the pyramids are influenced by the etch-rate of silicon in the 〈1 0 0〉 direction and also by the anisotropy factor of the solutions being the quotient of the etch rate in the 〈1 0 0〉 to 〈1 1 1〉 directions. Design of experiments and response surface methods were used to extract the etch rate as a function of different input parameters, such as the sodium hydroxide and isopropanol concentrations and the temperatures of the solutions. Optimum texturing conditions were found at a temperature of 80°C and a composition which causes a relatively high etch rate in the 〈1 0 0〉 direction with an anisotropy factor of 10. At the starting point of the etching process, the inhomogeneity of pyramid nucleation can be avoided by mixing an additive to the texturing solution. With such a solution, the pyramid size can be tuned by varying the etching time in order to obtain a low reflectivity from the textured silicon wafers. Based on our results the texturing process could be stabilised with respect to the reproducibility on a large scale of wafers.

ETCHING OF SILICON NITRIDE IN CCL2F2, CHF3, SIF4, AND SF6 REACTIVE PLASMA : A COMPARATIVE STUDY

Silicon nitride is an important material layer in various types of microelectronic devices. Because of continuous integration of devices, patterning of this layer requires a highly selective and anisotropic etching process. Reactive ion etching is one of the most simple and popular plasma processes. The present work is an experimental analysis of primary etch characteristics in reactive ion etching of silicon nitride using chlorine- and/or fluorine-based organic and inorganic chemistries (CCl2F2+O2, CHF3+O2, SiF4+O2, SF6+O2, and SF6+He) in order to obtain a simultaneous etch selectivity against polysilicon and silicon dioxide. A recipe, in CCl2F2/O2plasma chemistry, which provides acceptable etch characteristics, along with a reasonable simultaneous selectivity against polysilicon and silicon dioxide, has been formulated.