KOH Anisotropic Etching Research Articles

A piezoresistive accelerometer with a novel vertical beam structure

A micromachined piezoresistive accelerometer sensitive to an acceleration component in the chip plane and vertical to the beam direction has been developed. To sense a lateral acceleration, a beam with a cross section vertical to the (001) wafer surface has been developed by anisotropic etching in aqueous KOH. The vertical beam is along the 〈100〉 direction and has a high aspect ratio. Two n-type piezoresistors instead of p-type ones are positioned on the top edge (wafer top surface) of the beam with one resistor on each side of the neutral line of the beam. The top edge of the beam is widened slightly to make enough room to accommodate the piezoresistors and their interconnections so that, in fact, the beam has a T-shaped cross section. The sensitivity of the device fabricated is about 0.5 mV g −1 5 V 1 and the resonance frequency is about 1.2 kHz. As the sensitivity is comparable to and the fabrication process is compatible with the conventional piezoresistive accelerometers for normal acceleration, this design leads the way to developing a monolithic piezoresistive multi-axis accelerometer.

Anisotropic etching of inverted pyramids in the sub-100 nm region

We have investigated the generation of inverted pyramids in the sub-100 nm region. A process based on e-beam lithography and anisotropic etching in aqueous KOH was developed which allows the generation of sub-100 nm pyramidal etch pits in a reproducible manner. Inverted pyramids as small as 75 nm with a period of 130 nm were produced. AFM-measurements have shown that such etch pits possess a very sharp tip curvature radius which was typically below 5 nm. In order to demonstrate that these structures can be further processed, the pyramidal etch pits were filled with metal and the supporting silicon was finally removed.

Sub-10-nm Si Lines Fabricated Using Shifted Mask Patterns Controlled with Electron Beam Lithography and KOH Anisotropic Etching

We developed a new method for Si nanofabrication that provides sub-10-nm resolution using electron beam lithography combined with KOH anisotropic etching of a Si(1\\overline10) substrate. The method, called shifted mask pattern method, uses two shifted mask patterns to reduce the Si linewidth. The two shifted mask patterns delineated in the <11\\overline2> direction are used as a KOH etching mask where the masks are shifted vertically to each other in the <11\\overline2> direction. During KOH anisotropic etching, Si linewidth under the shifted part of the masks is reduced due to the etching's smoothing resulting from the low etching rate of the Si(111) plane. The linewidth can be controlled by changing the mask shift. An image reversal process using electron cyclotron resonance (ECR) plasma oxidation was used to form a plasma oxide mask for KOH etching. This mask was obtained by ECR oxygen plasma irradiation using a ZEP520 resist mask pattern. Highly accurate position and linewidth c ontrol was achieved with a 70-kV electron beam nanolithography system that has a minimum deflection step of 2 nm. The variation of pattern position and linewidth was about 1 nm. Using this method, we have been able to fabricate lines as narrow as 2 nm.

Maskless etching of three-dimensional silicon structures in KOH

A novel micromechanical technique using maskless etching of three-dimensional anisotropically etched silicon structures is investigated. The structures investigated are convex prismatic edges included by {100} and {111} planes and the convex corners of a rectangular m both formed by a previous masked anisotropic etching in KOH. Experimental results verify that the cutting planes developed at the mesa edges are {311} planes that make the contour change obviously. Analytical relations have been found to predict the evolution of the contour and the undercutting rate at the convex edges and corners based on the fact that the cutting planes at the convex edge are {311} planes while the cutting planes at the convex corner are {411}. The relations have been confirmed by experiments and the etching rate of {311} p found for various KOH concentrations. As an example of application, a symmetric cantilever beam-mass structure with beams horizontally located at the central plane of the wafer has been fabricated for a differential capacitive accelerometer with minimized cross-axis sensitivity.

Vibration modes of a resonant silicon tube density sensor

We present an investigation of the resonance parameters for a new sensor for on-line measurements of fluid density. The sensor consists of a tube system made of single crystalline silicon. The tube system is excited electrostatically into mechanical resonance and the vibration is detected optically. Using a simplified theoretical analysis, the resonance frequency can be shown to be proportional to 1/spl radic//spl rho/, where /spl rho/ is the density of the silicon and the fluid weighted according to their areas in a cross section of the tube. Thus, a change in fluid density results in a change in the resonance frequency. This dependence is demonstrated by measurements for four different vibrations modes. The quality of the vibration is also investigated through measurements of the Q-values of the vibration modes. The tubes are made using anisotropic silicon KOH etching and silicon-to-silicon fusion bonding micromachining techniques. The dimensions of the tube system are 8.6/spl times/17.7 mm with an outer tube thickness of 1 mm and a wall thickness of 100 /spl mu/m. Total tube length is 61 mm, and the sample volume is 0.035 ml. The sensor has a very good density sensitivity of the order of -200 ppm/(kgm/sup -3/) and a high Q of the order of 3000 for air in the tube.

Si nanostructures fabricated by electron beam lithography combined with image reversal process using electron cyclotron resonance plasma oxidation

A new image reversal process has been developed for Si nanodevice fabrication that uses electron beam lithography and electron cyclotron resonance (ECR) plasma techniques. This process is based on Si oxidation with an ECR oxygen plasma through the openings in resist mask patterns. Si on SiO2 is selectively etched by either Cl2-based ECR plasma etching or KOH anisotropic etching by using a plasma oxide mask. ECR plasma formed silicon oxide with a thickness of 2–3 nm was found to be an excellent etch mask for these etching techniques. Highly directional ECR oxygen plasma keeps the change in the resist linewidth and edge roughness small enough for nanofabrication. Furthermore, the linewidth of reversed Si patterns can be reduced by SF6 addition to Cl2 in ECR plasma etching. This image reversal process successfully achieves 10-nm-scale Si wires and pillars.

Silicon on insulator Mach–Zehnder waveguide interferometers operating at 1.3 μm

Mach–Zehnder (MZ) waveguide interferometers integrated on SOI (silicon on insulator) for 1.3 μm operation are studied on the basis of the large cross-section single-mode rib waveguide condition and the free-carrier plasma dispersion effect in Si wafer direct bonding SOI by back-polishing. And the MZ interferometers are fabricated by using KOH anisotropic etching. Their insertion losses and modulation depths are measured to be 4.81 dB and 98%, respectively, at the wavelength of 1.3 μm when a forward bias voltage applied to a p+n junction is 0.95 V and the active zone length of the MZ interferometers is 816.0 μm.

Monolithic bridge-on-diaphragm structure for pressure sensor applications

Monolithically clamped bridge-on-diaphragm (BOD) structures for pressure sensor applications were fabricated by means of Nd: YAG-laser micromachining and anisotropic KOH-etching techniques. The pressure/frequency-dependence of the BOD structures was measured by acoustical resonance excitation and optical detection of the microbridge and applying an external pressure between-0.8 bar and+1 bar to the diaphragm. In this vacuum/atmospheric pressure range the pressure/frequency-characteristic is quite linear with a sensitivity of about 4.5 kHz/bar and a fundamental bridge resonance frequency of 82 kHz. Extensive finite-element modelling has been carried out to optimize the geometrical dimensions of the BOD structures with respect to maximum sensitivity and pressure range. Using the same BOD structure layout it is possible to realize pressure sensors with applications ranging from 0.5 to 12 bar by only varying the thickness of the diaphragm. Varying the BOD structure layout to smaller dimensions the pressure sensors can be operated up to 100 bar with sensitivities of about 141 Hz/bar.

Micromachined millimeter-wave SIS-mixers

Micromachined SIS-mixers for 90-115 GHz and for 180-230 GHz have been fabricated and tested. A micromachined SIS-mixer consists of an antenna-coupled superconducting tunnel junction fabricated on a thin SiN membrane which is placed inside a pyramidal horn structure. The horn is formed by anisotropic KOH-etching of [100] Si along the (111) crystal planes. The high accuracy of the etching and the ease of whole-wafer fabrication make this type of SIS mixers attractive for applications both at high frequencies and in imaging arrays. Experimental results show that high-quality tunnel junctions can be reliably fabricated on thin SiN membranes. Experiments also show that the junctions are adequately cooled on the membrane and that sufficient LO-power can be coupled. >

New Fabrication Method and Electrical Characteristics of Conical Silicon Field Emitters

A new fabrication method for a silicon field emitter has been proposed, which combines the techniques of Si KOH anisotropic etching and local oxidation of silicon (LOCOS) to form the emitter tip. The fabricated conical silicon emitter had an extremely well-defined structure indicative of good reproducibility and uniformity of the fabrication process. The emission characteristics were examined under DC and AC conditions, and indicated high performance of the emitter. Successful application to a digit display has been demonstrated.

Electron beam nanolithography with image reversal by ECR plasma oxidation

A new image-reversal process has been developed for electron beam nanolithography. This process is based on Si oxidation with ECR oxygen plasma through openings in resist patterns. Si on SiO 2 is selectively etched by either Cl 2-based ECR plasma etching or KOH anisotropic etching. This image-reversal process achieves 10-nm-scale Si line and dot patterns.

Fabrication of visibly photoluminescent Si microstructures by focused ion beam implantation and wet etching

A technique is reported for the fabrication of optically active Si microstructures embedded in a crystalline Si (c-Si) substrate. The process combines Si microstructure fabrication by localized high dose Ga+ (1016/cm2) focused ion beam (FIB) implantation at 30 kV into n-type (100) Si followed by anisotropic etching in KOH:H2O (1:5 by volume). Self-selective porous Si (PoSi) formation of the microstructures is obtained by stain etching in HF:HNO3:H2O (1:3:5 by volume). Upon UV 365 nm or Ar+ 488 nm excitation, selective visible room-temperature photoluminescence (PL) was observed from the Si microstructures only. The PL, peaked at ∼670 nm with a full width at half-magnitude (FWHM) of ∼130 nm, is similar to that of PoSi obtained from c-Si substrate.

Electronically controlled etch-mask for silicon bulk micromachining

Wafers that are to be submitted to anisotropic etching in aqueous KOH are conventionally passivated with a silicon dioxide or nitride layer in which backside windows are etched to define the microstructures. A different method to mask the backside of a silicon wafer for this purpose is presented. The method makes use of the phenomenon that silicon is not etched in KOH when biased above the passivation potential. The mask is defined by applying a set of bias voltages to the front of the wafer instead of patterning a deposited passivation layer at the backside, for which an accurate double-sided alignment is required. The feasibility of the method was demonstrated with the fabrication of membranes and suspended masses of various sizes. >

PHET, an electrodeless photovoltaic electrochemical etchstop technique

A novel etchstop technique for the fabrication of micromechanical components in single-crystal silicon is presented. The photovoltaic electrochemical etchstop technique (PHET) was characterized for anisotropic etching in 7M KOH in the 60/spl deg/ to 80/spl deg/C temperature range, but can in principle be extended towards other anisotropic etchants. PHET is based on the photovoltage generated across a p/n junction illuminated during etching. In contrast with the established two-, three- or four-electrode electrochemical etchstop on n-epi, the technique does not require any external electrodes or connections to be made to the wafer, hence alleviating the need for a mechanical etchholder that protects one wafer side from the etchant. These etch-fielders are known to introduce stress and to substantially reduce yield in the production of the more fragile microstructures. Moreover, PHET is suited for the realization of diaphragm type structures in p-epi as well as for undercutting boron-diffused beam or bridge type of structures. In contrast with the established high boron dose etchstop, with its known adverse mechanical and electrical side effects, PHET does not require excessive boron concentrations. PHET therefore merges the fabrication possibilities offered by the conventional electrochemical etchstop and the conventional high boron dose etchstop, without being subject to the respective drawbacks of these techniques. The only technology required in addition to those used in the conventional etchstops is platinum sputtering. The possibilities that are offered for postprocessing standard p-well or twin-tub CMOS substrates are considered to be a major asset of the presented photovoltaic etchstop.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Scalable Fabrication and Optical Characterization of Nm Si Structures

ABSTRACTObservations of efficient room temperature photoluminescence (PL) from porous Si have generated a great deal of interest in the optical properties of nm-scale Si structures. The stochastic character of porous-Si fabrication results in a distribution of crystal sizes and shapes. We report on a scalable (to large areas) and manufacturable (to high volumes) fabrication technology for uniform, nm-linewidth Si structures providing an important testbed for controlled studies of these optical properties. Large areas ( ∼ 1 cm2) of extreme sub-μm structures (to ∼ 5 nm) are re-producibly fabricated. Both walls (1-D confinement) and wires (2-D confinement) are reported. The fabrication process includes: interferometric lithography, highly anisotropic KOH etching, and structure dependent oxidation. For the walls, nearly perfect &lt;111&gt; crystal planes form the sidewalls and very high width/depth aspect ratios (&gt; 50) have been achieved. Raman scattering results on the walls demonstrate three regimes: 1) lineshapes and cross sections similar to bulk Si for line widths, W &gt; 200 nm; 2) electromagnetic resonance enhancement of the cross section ( to - 100x) for W from 50-200 nm; and 3) highly asymmetric lineshapes and splittings from W &lt; 30 nm. Photoluminescence is observed for the thinnest samples (W &lt; 10 nm) and is as intense as that observed from porous Si with a spectral linewidth ∼ 50 % smaller than that of porous Si.

Piezoresistive accelerometer with overload protection and low cross-sensitivity

Abstract A piezoresistive silicon bridge-type accelerometer is described. The application of a standard bipolar process allows the mechanical structure to be realized by anisotropic wet etching in KOH with an electrochemical etchstop at selectively diffused n-type layers. A novel glass-bonding technology offers complete encapsulation of the sensor with silicon top and bottom caps, including wafer-level assembly and simultaneous overload protection. Optimization of the mechanical structure by finite-element modelling (FEM) with respect to the resonance frequency results in the application of transverse piezoresistors with excellent cross-sensitivity compensation. The sensor features a sensitivity of 40 μV/N g with a non-linearity of less than 0.5% up to 350 g, first resonance frequency of about 5 kHz, and cross-sensitivity in lateral directions of less than 0.2% with an overall chip size of 3.7 × 3.7 × 1.0 mm3.

A New Evaluation Method of Silicon Wafer Bonding Interfaces and Bonding Strength by KOH Etching

KOH anisotropic etching is used to study the interfaces of bonded silicon wafers. This method has been developed to detect the presence of bubbles or unbonded areas with a gap of less than 0.27 µm, which is the detection limit of an IR camera. Because of the anisotropic etching properties of KOH and the presence of interfacial oxide layers, the etching surface morphology is dependent on the crystallographic orientation of the cross section. Correlation of etching results to the bonding strength of silicon wafers with and without a thermally oxidized silicon layer is also discussed.

Integrated optics combined with micromechanics on silicon

The combination of integrated optics and micromechanics on silicon offers new integration potentials for sensor applications. Silicon oxinitride (SiON) optical strip waveguides are fabricated by low-temperature (570 K) PECVD and RIE processes on microbridges, cantilevers and resonators based on field oxide. The single-mode strip waveguides with a refractive index n=1.52 of the SiON layer show 0.5 dB/cm attenuation at 0.633 μm wavelength. The micromechanical elements are delineated by anisotropic KOH etching. They are built up with SiO 2SiONSiO 2 triple layers and exhibit low internal stress.

A new technology for micromachining of silicon: dopant selective HF anodic etching for the realization of low-doped monocrystalline silicon structures

The sharp selectivity of HF anodic etching between p-Si and n-Si is used to realize monocrystalline silicon microstructures, in this case suspended beams, making use of masked implantation of phosphorous for the definition of the geometry. This technology offers opportunities in the field of micromachining of silicon for micromechanical applications. The technology is complementary to bulk micromachining (anisotropic KOH or EDP etching, or isotropic HF/HNO/sub 3/ etching) and surface micromachining with sacrificial-layer techniques.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The potential dependence of silicon anisotropic etching in KOH at 60°c

The electrochemistry and potential dependent etching of silicon in 40% KOH at 60 ° C is investigated. A new technique, which produces an almost perfect (111) silicon crystal face, was used to obtain the true electrochemical characteristics of the (111) crystal plane. From i-V sweep rate dependence, it was discovered that the process of anodic oxide formation on (111) silicon uses a constant quantity of charge, 2.4×10 −3 C cm −2, and that the potential denoted classically as the passivation potential on n-type (100) silicon does not correspond to the potential of anodic oxide formation. The oxide formation potentials determined from i-V behavior are confirmed by oxide etch-back experiments.