Anisotropic Etching Of Silicon Research Articles

A novel batch-processing method for accurate crystallographic axis alignment

A new method for the accurate alignment of lithographically-defined patterns to the crystallographic axes of substrates is presented. We provide a lower (worst-case) limit of the achievable high aspect ratio using anisotropic wet chemical silicon etch for deep trenches. The method uses the fact that the intensity of light reflected from two sets of gratings, one on the photomask and the other on the substrate, is a sharp function of their relative angular misalignment. By using pre-etched gratings on the substrate formed by wet anisotropic etching, alignment accuracies better than 50 millidegrees with respect to silicon crystallographic axes have been demonstrated. Two types of microstructures—trenches with an aspect ratio >90:1 and silicon nanowires with widths <50 nm with atomically smooth etched facets—have been fabricated using i-line lithography to illustrate some applications of this alignment method. This all-optical method is readily applicable to industry-standard optical lithography and avoids the need for any individualized process steps, enabling cost-effective micro/nanostructure manufacturing.

On-Demand Hydrogen Generation using Nanosilicon: Splitting Water without Light, Heat, or Electricity

We demonstrate that nanosize silicon (~10 nm diameter) reacts with water to generate hydrogen 1000 times faster than bulk silicon, 100 times faster than previously reported Si structures, and 6 times faster than competing metal formulations. The H(2) production rate using 10 nm Si is 150 times that obtained using 100 nm particles, dramatically exceeding the expected effect of increased surface to volume ratio. We attribute this to a change in the etching dynamics at the nanoscale from anisotropic etching of larger silicon to effectively isotropic etching of 10 nm silicon. These results imply that nanosilicon could provide a practical approach for on-demand hydrogen production without addition of heat, light, or electrical energy.

Fabrication of nano-porous silicon using alkali etching process

The best conditions for the production of porous silicon (PS) using alkali etching process have been defined and discussed. The surface chemistry of anisotropic etching of n-type Si-wafer is reviewed and the anisotropic chemical etching of silicon in alkaline solution using wetting agents were discussed. The main factors which affect the production of porous silicon from crystalline silicon as a result of anisotropic etching are the concentration of etching solution (KOH) and wetting agent (n-propanol), temperature and time of the etching process. Transformation of crystallographic plane of n-Si (211) to n-PS (100) is produced by using {(2 wt%) KOH+(15 vol%) n-propanol} at etching temperature 80°C and etching time 5h. These results are confirmed using FTIR which detect the peak at 1015cm−1. A particle size of 15nm for porous nano-silicon calculated from the XRD data. Pores of (49–349) nm in diameter and 25μm in depth and a pitch of (4.203×10−6–6.92×10−6) μm per layer are obtained for the same conditions explained above.

J026033 微小孔アレイを有するSiN製円形細胞培養膜の開発([J026-03]細胞および分子のマイクロ・ナノスケール解析(3))

We developed circular cell-culture membranes which are made of SiN (silicon nitride) films of 1 μm in thickness and have multiple square or circular holes of several micrometers in size. The membranes were fabricated using SiN film formation with PECVD (plasma-enhanced chemical vapor deposition) and anisotropic wet etching of silicon with a surfactant-added TMAH (tetramethylammonium hydroxide) solution. Mechanical properties of the membranes were evaluated using a bulge test, and it was found that a circular membrane with circular holes has much larger fracture strength than a square membrane with square holes. Also, we demonstrated that optical transmittance of the membranes decreases with decreasing size of microholes and increasing porosity of the membranes, which gives useful design rules for their practical applications to cell culturing.

Fabrication of Congo Red/Oxidized Porous Silicon (CR/OPS) pH-Sensors

The fabrication of nano porous silicon, nPSi, using alkali etching process has been studied and carried out. The surface chemistry of anisotropic etching of n-type Si-wafer is reviewed and the anisotropic chemical etching of silicon in alkaline solution using wetting agents is discussed. Transformation of crystallographic plane of n-Si (211) to nPSi (100) has occurred on using n-propanol as wetting agent. The rate of pore formation was 0.02478 - 0.02827 μm/min, which was heavily dependent upon the concentration of the etchant containing wetting agents, allowing patterned porous silicon formation through selective doping of the substrate. A particle size of 15 nm for porous nano-silicon was calculated from the XRD data. Porosity of PS layers is about 10%. Pore diameter and porous layer thickness are 0.0614 nm and 16 μm, respectively. The energy gap of the produced porous silicon is 3.3 eV. Furthermore, the combination of PS with Congo Red, which are nanostructured due to their deposition within the porous matrix is discussed. Such nano compounds offer broad avenue of new and interesting properties depending on the involved materials as well as on their morphology. Chemical route was utilized as the host material to achieve pores filling. They were impregnated with Congo Red, which gave good results for the porous silicon as a promising pH sensor.

6PM3-PMN-006 低侵襲シリコン神経プローブの作製と刺入特性評価(OS3 マイクロ・ナノ生体医工学,ポスターセッション)

Recently, many neural probes with various materials and shapes have been developed for treatments of cerebropathy and analyses of the brain function. Among these probes, silicon neural probe attracts much attentions because various kinds of functional structures such as microfluidic channel and optical waveguides can be fabricated by semiconductor micro- and nano-fabrication technologies. On the other hand, it was reported that the recording quality of the neuronal signals deteriorated when nervous tissues were damaged due to insertion and placement of the silicon neural probes. In this research, lower invasive Si neural probes with small shank cross-sections and sharpened tips were successfully fabricated using silicon anisotropic etching techniques. Also, insertion characteristics of the probes were carefully evaluated, indicating that the probe will cause less damages to nervous tissues in the brain.

Fabrication of curved sub-micron Si structures by a combination of anisotropic etching using surfactant-added TMAH solution and FIB direct-drawn mask

Silicon anisotropic wet etching is applied for fabricating round-shaped micro-structures in a size range of sub-microns. In this work, we demonstrate that arbitrary 2-D mask patterns having curved profile can be successfully transferred to deep-etched cavity profiles on a Si {100} wafer. The sub-micron mask is directly drawn on the Si wafer by irradiating focused ion beam to the wafer surface. Anisotropy in etch rate of Si using tetra-methyl ammonium hydroxide solution was modified and controlled by adding a surfactant Triton X-100 to the solution. Etched profile was conformal to etch mask patterns having smooth curvatures.

Research and development of deep anisotropic plasma silicon etching process to form MEMS structures

Process characteristics of deep silicon etching as a function of its operational parameters are investigated. A process of deep anisotropic plasma etching process is developed and optimized to form MEMS silicon structures. The results of the work are used in forming MEMS working structures.

Surface tension and its role for vertical wet etching of silicon

In this paper, we present for the first time a systematic investigation of the role of the surface tension in anisotropic wet etching of silicon, focusing on the sidewall angle. We show that in the KOH and NaOH solutions with high surface tension a reliable fabrication of vertical sidewalls is possible. Etching experiments on (1 0 0)-Si and surface tension measurements via bubble pressure tensiometry were conducted using inorganic etchants in a wide range of concentrations. Finally, the surface tension of the etching solutions was identified as an important quantity that determined the etching behavior, while concentration dependences were eliminated by controlling the surface tension using temperature.

Pyramid array substrates for biomedical studies

Cellular reactions to structured surfaces are the subject of intense studies in biomedical research, e.g., for implant technology. Substrates with well-defined structures in relevant materials such as titanium (Ti) are required for these investigations. The pyramidal arrays presented here offer very sharp edges and tips, which appeared to be the main adhesion spots in our earlier cell-surface-interaction studies. Using an etch process based on anisotropic etching of silicon in alkaline solution, the shape of the pyramids is strictly determined by the crystal structure of silicon, while the height as well as the pitch of the pyramids can be precisely controlled and, therefore, be varied in a systematic fashion. Being made from silicon nitride with a thin cover layer of titanium, they offer mechanical stiffness, inertness to all chemicals used in the cell experiments, and a high degree of biocompatibility.

Development of nanocrystalline diamond windows for application in synchrotron beamlines

A multistep growth and masking method allowed developing windows with controlled geometry inside a silicon frame. In this paper, we present a new method to produce nanocrystalline diamond windows with thickness of about 200 nm to 40 μm, with different areas and shapes (circular, rectangular and rounded rectangle). The nanocrystalline diamond (NCD) films deposited on a silicon substrate (100) p-type, had a nucleation density of about 1011 part/cm2. Electrostatic self-assembly of nanodiamond seeds (4 nm powder) improved nucleation. Silicon anisotropic etching reveals the window geometry. The high nucleation density enabled smooth surfaces on both sides without the need for polishing the window. Pressure tests were performed in windows of varying thickness. The windows with thickness larger than 10 μm supported a pressure gradient of 1 atm.

Facile fabrication of sub-20-nm nanochannels based on crystallinity-dependent anisotropic etching of silicon

We developed a novel method to fabricate nanochannels with simple and facile process utilizing the crystallinity-dependent anisotropic wet etching of silicon. The nanochannels were formed in bulk (111)-oriented single-crystalline silicon wafer by single step of microscale lithography and sealed by thin film deposition. Hence the massively parallel and wafer scale nanochannel fabrication was easily achieved without the necessity of nanolithography or complicated process. The dimension of the fabricated nanochannels was down to 16nm in depth and 175nm in width, and up to 8cm in length.

Bubble Jet agent release cartridge for chemical single cell stimulation

We present a new method for the distinct specific chemical stimulation of single cells and small cell clusters within their natural environment. By single-drop release of chemical agents with droplets in size of typical cell diameters (d <30μm) on-demand micro gradients can be generated for the specific manipulation of single cells. A single channel and a double channel agent release cartridge with integrated fluidic structures and integrated agent reservoirs are shown, tested, and compared in this publication. The single channel setup features a fluidic structure fabricated by anisotropic etching of silicon. To allow for simultaneous release of different agents even though maintaining the same device size, the second type comprises a double channel fluidic structure, fabricated by photolithographic patterning of TMMF. Dispensed droplet volumes are V = 15pl and V = 10pl for the silicon and the TMMF based setups, respectively. Utilizing the agent release cartridges, the application in biological assays was demonstrated by hormone-stimulated premature bud formation in Physcomitrella patens and the individual staining of one single L 929 cell within a confluent grown cell culture.

Evolutionary determination of kinetic Monte Carlo rates for the simulation of evolving surfaces in anisotropic etching of silicon

This study explores for the first time the possibility of using an evolutionary algorithm (EA) for the determination of atomistic rates for use in kinetic Monte Carlo (KMC) simulations of anisotropic etching of silicon for the manufacture of microelectromechanical systems (MEMS). Traditionally, KMC rates are determined based on (i) computationally expensive density functional theory (DFT) calculations or, when possible, (ii) a combination of physical insight and a labor-intensive, manual procedure where, e.g., experimental and simulated surface morphologies are visually matched for a collection of surface orientations. Compared to these approaches, the evolutionary KMC method proposed in this study provides a more flexible, autonomous procedure to describe correctly a wide variety of etching conditions. We focus on the use of a functional representation of the atomistic rates, referred to as the removal probability function (RPF). This simplifies the EA search space to just a few parameters and reduces the number of required experimental data points to just a few etch rates. By proposing two alternative RPFs with four and six parameters, respectively, we show that the ability to explain the orientation dependence of the etch rate for a wide variety of etchants increases with the number of parameters and conclude that the six-parameter RPF provides sufficiently good simulations for a wide range of etching conditions, including KOH, KOH+IPA, TMAH and TMAH+Triton at different concentrations and temperatures. By uncovering the relationships between the parameters and the concentration of the etchant, it is possible to extend the simulations to nonmeasured etching conditions. Although the use of an RPF effectively restricts the search to a subspace of the atomistic rates, the present results suggest that physically meaningful KMC rates can probably be determined in the near future by direct comparison of macroscopic experiments and simulations through the use of evolutionary strategies.

Silicon isotropic and anisotropic etching for MEMS applications

In MEMS technology there is an increasing interest in developing high aspect ratio silicon columns having rounded corners, slightly positive tapered shafts, sharp tips, and smooth surfaces. A precise control of the profile can be used for different applications, such as for molds used in polymer hot embossing processes, micro needles used for drug delivery and blood sampling, and neural probes used for controlling motor or sensory prosthetic devices. The mixture of hydrofluoric acid (HF) and nitric acid (HNO3) is an isotropic etchant and is used in MEMS technology to etch silicon. We present a novel way of isotropically and anisotropically etch MEMS structures using the HF–HNO3 etchant. The shape and size of the structure is controlled by the dynamics of acid solution to yield highly repeatable and reproducible needle geometry.

Implications of alkaline solutions-induced etching on optical and minority carrier lifetime features of monocrystalline silicon

In this work, we search to optimize the surface textures of monocrystalline silicon (c-Si) intended to be used in silicon solar cells. For this purpose, we studied the morphology of formed etch hillocks during anisotropic etching of silicon using alkaline solutions based on sodium hydroxide (NaOH), potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH). Such treatments lead to the formation of various pyramids-like textures that can be well optimized to improve the photocurrent of c-Si-based solar cells. The produced textures were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–visible optical reflectivity and minority carrier lifetime measurements. These investigations allow evaluating the size and density of the formed pyramidal textures; the apex angles vary between 75° and 82°, while the heights and bases of the pyramids range from a few hundred nanometers to several micrometers. A minimum reflectivity value of about 6% was obtained at specific conditions using NaOH, whereas it was found that the apparent effective minority carrier lifetime (τeff) is sensitive to the injection level (Δn), which shows an apparent increase from 1.2μs to 2.4μs for a minority carrier density of about Δn=21014cm−3.

Microneedle-based electrodes with integrated through-silicon via for biopotential recording

The fabrication of a novel ultrasharp silicon microneedle array for use as a physiological signal monitoring electrode is described. This work uses double-sided silicon wafer patterning and anisotropic potassium hydroxide wet etching to simultaneously create a microneedle on the front side of the wafer, and a through-silicon via from the backside. Metal deposition on both the front and the back of the wafer then establishes electrical contact through this via between both sides of the electrode. This technique eliminates the limitations associated with other approaches that are used to create front-to-back electrical contact and that may be slow or cumbersome. Wearable electrode prototypes have been assembled using these arrays, and electrocardiography (ECG) and electromyography (EMG) recordings have been carried out to verify the functionality of the technique.

Etched profile control in anisotropic etching of silicon by TMAH+Triton

The adverse effect of mechanical agitation (magnetic bead stirring) as well as galvanic interaction between the evolving facets of the etch front on the amount of undercutting during anisotropic etching of Si{1 0 0} wafers in surfactant-added tetramethylammonium hydroxide (TMAH) is studied by etching different mask patterns in magnetically stirred and nonstirred solutions. Triton X-100, with formula C14H22O(C2H4O)n, where n = 9–10, is used as the surfactant. The stirring results conclude that the adsorption of the surfactant on the etched silicon surfaces is predominantly physical in nature rather than chemical (physisorption versus chemisorption). The proposed model to account for the galvanic interaction between the evolving facets indicates that the underlying chemical etching process can be significantly surpassed by the onset of an electrochemical etching contribution when the relative area of the exposed {1 0 0} surface becomes relatively small in comparison to that of the developed {1 1 1} sidewalls. This study is useful for engineering applications where surfactant-added TMAH is used for the fabrication of silicon MEMS structures that should contain negligible undercutting.

Feasibility to Fabricate the 45° Slant with Anisotropic Silicon Etching in NaOH Solution

The experiments to study the feasibility to fabricate the 45 slant on p-type (100)-oriented silicon wafer were done. The various mask shapes, rectangular, cross, circle and boomerang, were patterned on the SiO2 mask by utilizing the conventional photolithography and dry etching process for investigating the anisotropic wet etch characteristic. The edge of masks were align in two crystal direction, 110 and 100 that is allowable to get a better understanding about the crystal orientation and the angle between planes in a crystal system. The very low etch rate,  50 nm/min, process regime was selected to fabricate the 45 slant with the concept is the lowest of an overall etch rate in the system to reach the level that is possible to detect the (110) plane. The etch recipe can be used for the next development work to built a housing of the laser light source for applying in a data storage technology.

Silicon anisotropic etching in TMAH solutions containing alcohol and surfactant additives

Wet chemical etching of Si(100) and Si(110) wafers in TMAH solutions containing Triton X-100 and alcohol additives was studied in a wide range of alcohol concentrations. TMAH solutions containing butanol-2 and ternary solutions composed of TMAH+Triton+alcohol, used in the experiments, had not been investigated before. The phenomena associated with the presence of surfactant and alcohol additives in etching solutions were analyzed and the mechanism including co-solving of surfactant molecules in alcohol and interplay in the adsorbing mechanism was proposed. The influence of these phenomena on the etching rates and morphologies of Si(100) and Si(110) was discussed. The addition of alcohol, especially butanol-2, to a TMAH solution resulted in significant improvement of the etched surface finish. The roughness of Si(110) surfaces etched in TMAH+butanol solutions was superior to the roughness of the surfaces etched in TMAH+Triton. The best surface roughness parameters of Si(110) (Ra=0.0322μm and RMS=0.0549μm), measured with an optical profilometer on the area of 1.6mm×1.6mm, were achieved after etching in a ternary solution of TMAH+Triton+butanol. This solution appears to be a good candidate in applications where high quality finish of etched surfaces is required on a substantially high area.