Anisotropic Wet Etching Of Si Research Articles

Monodisperse Micro-Droplet Generation in Microfluidic Channel with Asymmetric Cross-Sectional Shape.

Micro-droplets are widely used in the fields of chemical and biological research, such as drug delivery, material synthesis, point-of-care diagnostics, and digital PCR. Droplet-based microfluidics has many advantages, such as small reagent consumption, fast reaction time, and independent control of each droplet. Therefore, various micro-droplet generation methods have been proposed, including T-junction breakup, capillary flow-focusing, planar flow-focusing, step emulsification, and high aspect (height-to-width) ratio confinement. In this study, we propose a microfluidic device for generating monodisperse micro-droplets, the microfluidic channel of which has an asymmetric cross-sectional shape and high hypotenuse-to-width ratio (HTWR). It was fabricated using basic MEMS processes, such as photolithography, anisotropic wet etching of Si, and polydimethylsiloxane (PDMS) molding. Due to the geometric similarity of a Si channel and a PDMS mold, both of which were created through the anisotropic etching process of a single crystal Si, the microfluidic channel with the asymmetric cross-sectional shape and high HTWR was easily realized. The effects of HTWR of channels on the size and uniformity of generated micro-droplets were investigated. The monodisperse micro-droplets were generated as the HTWR of the asymmetric channel was over 3.5. In addition, it was found that the flow direction of the oil solution (continuous phase) affected the size of micro-droplets due to the asymmetric channel structures. Two kinds of monodisperse droplets with different sizes were successfully generated for a wider range of flow rates using the asymmetric channel structure in the developed microfluidic device.

Microconical silicon mid-IR concentrators: spectral, angular and polarization response.

It is widely discussed in the literature that a problem of reduction of thermal noise of mid-wave and long-wave infrared (MWIR and LWIR) cameras and focal plane arrays (FPAs) can be solved by using light-concentrating structures. The idea is to reduce the area and, consequently, the thermal noise of photodetectors, while still providing a good collection of photons on photodetector mesas that can help to increase the operating temperature of FPAs. It is shown that this approach can be realized using microconical Si light concentrators with (111) oriented sidewalls, which can be mass-produced by anisotropic wet etching of Si (100) wafers. The design is performed by numerical modeling in a mesoscale regime when the microcones are sufficiently large (several MWIR wavelengths) to resonantly trap photons, but still too small to apply geometrical optics or other simplified approaches. Three methods of integration Si microcone arrays with the focal plane arrays are proposed and studied: (i) inverted microcones fabricated in a Si slab, which can be heterogeneously integrated with the front illuminated FPA photodetectors made from high quantum efficiency materials to provide resonant power enhancement factors (PEF) up to 10 with angle-of-view (AOV) up to 10°; (ii) inverted microcones, which can be monolithically integrated with metal-Si Schottky barrier photodetectors to provide resonant PEFs up to 25 and AOVs up to 30° for both polarizations of incident plane waves; and iii) regular microcones, which can be monolithically integrated with near-surface photodetectors to provide a non-resonant power concentration on compact photodetectors with large AOVs. It is demonstrated that inverted microcones allow the realization of multispectral imaging with ∼100 nm bands and large AOVs for both polarizations. In contrast, the regular microcones operate similar to single-pass optical components (such as dielectric microspheres), producing sharply focused photonic nanojets.

Etching of Uncompensated Convex Corners with Sides along and in 25 wt% TMAH at 80 °C.

This paper presents etching of convex corners with sides along <n10> and <100> crystallographic directions in a 25 wt% tetramethylammonium hydroxide (TMAH) water solution at 80 °C. We analyzed parallelograms as the mask patterns for anisotropic wet etching of Si (100). The sides of the parallelograms were designed along <n10> and <100> crystallographic directions (1 < n < 8). The acute corners of islands in the masking layer formed by <n10> and <100> crystallographic directions were smaller than 45°. All the crystallographic planes that appeared during etching in the experiment were determined. We found that the obtained types of 3D silicon shape sustain when n > 2. The convex corners were not distorted during etching. Therefore, no convex corner compensation is necessary. We fabricated three matrices of parallelograms with sides along crystallographic directions <310> and <100> as examples for possible applications. Additionally, the etching of matrices was simulated by the level set method. We obtained a good agreement between experiments and simulations.

Two-Layer Microstructures Fabricated by One-Step Anisotropic Wet Etching of Si in KOH Solution.

Anisotropic etching of silicon in potassium hydroxide (KOH) is an important technology in micromachining. The residue deposition from KOH etching of Si is typically regarded as a disadvantage of this technology. In this report, we make use of this residue as a second masking layer to fabricate two-layer complex structures. Square patterns with size in the range of 15–150 μm and gap distance of 5 μm have been designed and tested. The residue masking layer appears when the substrate is over-etched in hydrofluoric acid (HF) solution over a threshold. The two-layer structures of micropyramids surrounded by wall-like structures are obtained according to the two different masking layers of SiO2 and residue. The residue masking layer is stable and can survive over KOH etching for long time to achieve deep Si etching. The process parameters of etchant concentration, temperature, etching time and pattern size have been investigated. With well-controlled two-layer structures, useful structures could be designed for applications in plasmonic and microfluidic devices in the future.

Fracture characterization of brittle thin-films by membrane testing

The continuous miniaturization process in the microelectronic industry, along with the introduction of Interlayer Dielectrics (ILDs) with poorer mechanical properties, makes necessary the development of characterization techniques to evaluate the mechanical performance of very thin films. This work presents a mechanical characterization technique for thin films based on membrane testing. Membranes, micromachined with anisotropic wet etching of Si, are tested to fracture using a nanoindenter to apply the load and register the provoked deflection. The technique is applied to the fracture characterization of two different ILDs with four thicknesses ranging from 100nm to 500nm. Combination of experiments and finite element simulations allows for the calculation of the strength of the materials from the fracture load. The technique permits to discriminate both ILDs and to establish clear thickness dependence: for both materials, 100nm films show a significant lower strength while no effect of film thickness on strength is observed in the range between 200 and 500nm. A sensitivity analysis of the outcome of the technique, the fracture stress, to the variability of the input parameters is presented, showing the robustness of the proposed approach: the experimental error in the fracture stress is smaller than the variation in the input parameters.

Site Selective Integration of III–V Materials on Si for Nanoscale Logic and Photonic Devices

Integrating high electron mobility III–V materials on an existing Si based CMOS processing platform is considered as a main stepping stone to increase the CMOS performance and continue the scaling trend. Owing to the polar nature of III–V materials versus the nonpolar nature of Si, antiphase boundaries (APBs) arise in epitaxially grown III–V materials on Si. Here, we demonstrate an approach to restrict the generation of APBs by selectively depositing a III–V material in narrow Si-trenches as formed within the shallow trench isolation (STI) patterned Si(001) wafers. Based on the detailed crystal structures of Si and III–V materials, a concept has been developed comprising the deposition in “v-grooves” with {111} facets in the Si wafer. The grooves are formed by anisotropic wet etching of Si. When InP is deposited selectively into these “v-grooves”, the crystallographic alignment between the Si and InP restricts the APBs nucleation to the corners of the “v-grooved” trench. This approach offers a promising method of large-scale integration of III–V materials on Si as required for the fabrication of novel logic and photonic devices.

Evaluation of the soft x-ray reflectivity of micropore optics using anisotropic wet etching of silicon wafers

The x-ray reflectivity of an ultralightweight and low-cost x-ray optic using anisotropic wet etching of Si (110) wafers is evaluated at two energies, C K(alpha)0.28 keV and Al K(alpha)1.49 keV. The obtained reflectivities at both energies are not represented by a simple planar mirror model considering surface roughness. Hence, an geometrical occultation effect due to step structures upon the etched mirror surface is taken into account. Then, the reflectivities are represented by the theoretical model. The estimated surface roughness at C K(alpha) (approximately 6 nm rms) is significantly larger than approximately 1 nm at Al K(alpha). This can be explained by different coherent lengths at two energies.

Atomic force microscope based nanofabrication of master pattern molds for use in soft lithography

The authors have developed a technique that couples nanolithographic patterning using an atomic force microscope with the parallel patterning abilities of soft lithography. Master pattern generation is accomplished using local anodic oxidation as a mask pattern for anisotropic wet etching of Si(110). The resulting nanostructures are then used as master patterns for the molding of polymeric stamps to be used for microcontact printing of alkanethiols. Analysis of the resulting patterns demonstrates the validity of this method as a simple, effective, and low cost alternative to conduct and prototype nanoscale patterning in a parallel fashion.

Fabrication of smooth amorphous carbon micro-cantilevers by lift-off

The amorphous carbon films prepared by filtered cathodic vacuum arc (FCVA) deposition system are superior in nature with very smooth surface morphology, relatively high hardness, exceptional tribological behavior and excellent bio-compatibility. These exceptional qualities make them suitable for many device applications. However, these superior quality films prepared at relatively lower substrate bias such as −80 V exhibits high compressive stress. The stress reduction is achieved by preparing the film in conjunction with high substrate pulse biasing. In the present study, pure a-C films of about 1 μm in thickness were prepared, on highly doped n-type 〈1 0 0〉 Si wafer, using FCVA system in conjunction with high substrate pulse biasing of −5 kV, 600 Hz and 25 μs. The intrinsic compressive stress in the film, calculated from the change in radius of curvature from the substrate and film-substrate sandwich is approximately 300 MPa. Free-standing amorphous carbon (a-C) film structures were successfully fabricated by photolithography technique together with anisotropic wet etching of Si in 40% KOH. However, it has been noticed that etching in KOH for a longer duration at room temperature or for a relatively shorter duration at higher temperatures creates relatively larger holes on the surface of the film. This directed us also to analyze the micro-structural and morphological changes of pure a-C films as a function of etching temperature and duration, for two different concentrations (20 and 40%) of KOH and which are discussed in detail.

FABRICATION OF SMOOTH DIAMOND-LIKE CARBON MICROCANTILEVER ARRAYS

International Journal of Computational Engineering ScienceVol. 04, No. 03, pp. 671-674 (2003) Poster PapersNo AccessFABRICATION OF SMOOTH DIAMOND-LIKE CARBON MICROCANTILEVER ARRAYSD. SHEEJA, B. K. TAY, L. J. YU, J. MIAO, Y. Q. FU, D. H. C. CHUA, and W. I. MILNED. SHEEJASchool of Electrical and Electronic Engineering, Nanyang Technological University, Singapore Search for more papers by this author , B. K. TAYSchool of Electrical and Electronic Engineering, Nanyang Technological University, Singapore Search for more papers by this author , L. J. YUSchool of Electrical and Electronic Engineering, Nanyang Technological University, Singapore Search for more papers by this author , J. MIAOSchool of Mechanical and Production Engineering, Nanyang Technological University, Singapore Search for more papers by this author , Y. Q. FUSchool of Mechanical and Production Engineering, Nanyang Technological University, Singapore Search for more papers by this author , D. H. C. CHUAUniversity of Cambridge, Engineering Dept, England, UK Search for more papers by this author , and W. I. MILNEUniversity of Cambridge, Engineering Dept, England, UK Search for more papers by this author https://doi.org/10.1142/S1465876303002015Cited by:0 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail AbstractThe amorphous carbon films prepared by filtered cathodic vacuum arc (FCVA) deposition system are superior in nature with very smooth surface morphology, relatively high hardness, favorable tribological behaviour and excellent bio-compatibility. These exceptional qualities make them suitable for many device applications. However, the superior quality films prepared at relatively lower substrate bias such as -80V exhibits high compressive stress and thus limit the thickness of the film. This constraint is overcome by stress reduction, which is achieved by preparing the film in conjunction with high substrate pulse biasing.Free-standing Diamond-like Carbon (DLC) micro-cantiliver arrays were successfully fabricated by a single photolithography step. The relatively thick (~1μm), smooth (Rms ~0.75nm), low stress (<300MPa) DLC films that exhibits low friction (<~0.08) and wear rate (~10-9mm3/N-m), were deposited, on patterned highly doped n-type Si <100> substrates, by the FCVA deposition system in conjunction with high substrate pulse biasing of -5kV, 900Hz and 30μs. The undecutting of the microstructure was carried out by anisotropic wet etching of Si in 40 wrt% KOH at 23°C for 31 hrs. The SEM image of the free-standing DLC micro-cantiliver arrays reveals that the maximum transverse deflection of the cantilever at the free end is quite low. The quantitative measurements of deflections were also carried out using an optical profiler to verify.Keywords:micro-cantilever arraysdiamond-like carbonlow stress filmsMEMS References X. Shiet al., J. of Appl. Phys. 79, 7234 (1996). Google ScholarG. M. Pharret al., Appl. Phys. Lett. 68, 779 (1996). Crossref, Google ScholarD. Sheejaet al., Wear 249, 433 (2001). Crossref, Google Scholar FiguresReferencesRelatedDetails Recommended Vol. 04, No. 03 Metrics History Keywordsmicro-cantilever arraysdiamond-like carbonlow stress filmsMEMSPDF download

Surface diffusion limited nucleation of Ge dots on the Si(001) surface

The formation of Ge islands during MBE growth is a spontaneous process and these islands, i.e. dots, are usually randomly arranged. In order to implement these nanoscaled islands into device applications, ordering of epitaxial dots is a crucial step. We report a study on the MBE growth of Ge islands on Si(001) substrates, containing 〈110〉-oriented square and long stripe type patterns defined by anisotropic wet etching of Si, in order to provide more understanding of how surface diffusion of Ge atoms would influence the formation of Ge islands on various types of surfaces. It has been found that there were preferential nucleation sites for Ge islands along the bottom edges of the Si ridges. The Ge islands at the edge positions were larger than those formed on the free surface and they could be regularly spaced. Due to the consumption of Ge at the bottom edges of ridge patterns, the density of Ge dots on the free surface varied between ≈3×10 8 and ≈1×10 9 cm −2 when changing the spatial separation between two adjacent Si ridges (2–100 μm). A Ge mean diffusion length of ≈7.5 μm has been determined for Ge growth at 700 °C.

Experiments for microphotonic components fabrication using Si 〈1 1 1〉 etching techniques

Abstract This paper presents the experiments we have performed to study the anisotropic wet etching of Si 〈1 1 1〉-oriented wafers. The anisotropic etching was combined with an isotropic pre-etch step, followed or not by side-wall passivation. We studied influence of the mask orientation and layout, and of the technological parameters (temperature, concentration of the etching solution) on the shape of the etched cavity. The aim of our experiments was to obtain microphotonic components by anisotropic wet etching of Si 〈1 1 1〉 wafers. We experimented fabrication of SiO 2 or c-Si free-standing structures that can be used as movable micromirrors or waveguides.

ANALYSIS OF SUBSTRATE EFFECTS ON THE RESPONSE OF PYROELECTRIC THIN-FILM INFRARED SENSORS

An infrared (IR) sensor with a lead titanate (PbTiO3) thin film using the technology of microelectromechanical systems (MEMS) has been designed, fabricated, and developed. Anisotropic wet etching of Si(111) orientation in ethylenediamine/pyrocatechol (EDP) solution is used to process a micromachined cantilever beam for better thermal isolation and the array form available. The major IR-sensing part on the cantilever beam consists of a 5-nm PbTiO3 layer deposited by RF sputtering, and a gold (Au) black layer evaporated as an IR radiation absorber, with the active cantilever dimensions of 200 x 100 x 5 mu m3 formed by the etching processes. The substrate effect on the performance of the IR sensor has been studied in both experimental and theoretical analysis. The cantilever structure exhibits much superior performance in both theory and experiment to that of a traditional IR-sensing bulk structure under incident radiation at a wavelength of 970 nm.

White-light emitting thin-film electroluminescent device using micromachined structure

A white-light emitting electroluminescent (EL) device with newly developed ZnS:Pr, Ce, F phosphor layer was fabricated inside a micromachined well having four-sided Si mirrors, prepared by anisotropic wet etching of Si (100) wafer. Highly luminant EL was achieved using the Si micromirrors. Furthermore, the EL device utilizing the metallized mirrors incorporated into the glass substrates also exhibited enhanced brightness when compared to the conventional face-emitting EL device.