Pyramidal texturing of silicon solar cell with TMAH chemical anisotropic etching

The fabrication of LiNbO3 ridge waveguides etched by a mixture of HF and HNO3 using chromium (Cr) stripes as masks is reported. Smooth etched surfaces are obtained by adding some ethanol into the etchant. Under-etching is nearly avoided by annealing the sample with the Cr masks before the wet etching process. Low-loss monomode ridge guides with a height of up to 8 mum and a width between 4.5 and 7.0 mum are demonstrated. As an example, the propagation losses in a 6.5-mum-wide and 8-mum-high structure are 0.3 dB/cm for transverse-electric and 0.9 dB/cm for transverse-magnetic polarization, respectively, at 1.55-mum wavelength

Lithium Niobate Ridge Waveguides Fabricated by Wet Etching

This review-oriented paper presents a simplified model of anisotropic etching of crystalline silicon for the three principal orientations (1 1 1), (1 1 0) and (1 0 0), including their vicinal surfaces. The model combines pit nucleation and step flow with micromasking and diffusion phenomena in order to explain the major morphologic features and their changes with concentration. It also qualitatively explains the orientation and concentration dependence of the etch rate. We conclude that the shallow round pits on (1 0 0) and the elongated zigzag structures on (1 1 0), each of which constitutes the basic morphology of the corresponding surface, are actually the result of the same physical phenomenon, diffusion, disguised by a different underlying symmetry. It is also shown that the formation of hillocks on the two surfaces at different concentrations is a related process. We also describe and support the idea that the rotation of the triangular pits on (1 1 1) is due to a selective blocking mechanism by the etchant cations and explain how the formation of polygonal steps and/or step bunches on miscut (1 1 1) surfaces can occur as a result of diffusion phenomena and not only due to micromasking. Finally, the particular features of Cu as a micromasking agent are explained.

https://iopscience.iop.org/article/10.1088/0960-1317/17/4/S01/pdf

An atomistic introduction to anisotropic etching

Silicon bulk micromachining using the wet anisotropic etching process is widely employed for the development of commercial products such as an inkjet printer head, a pressure sensor, accelerometers, infrared sensors, etc using (1 0 0) silicon wafers. In wet anisotropic etching, the resultant shape and size of the microstructures are restricted by crystallographic properties of silicon. If structures such as seismic mass in an accelerometer are required to be created, convex corners will emerge in the etching process. Considerable deformation occurs at convex corners resulting in poor control on the shape and size of the microstructure. Various methods/techniques are developed to overcome the problem of undercutting at convex corners in a (1 0 0) silicon wafer. Here, we have reviewed the fabrication techniques for the realization of convex corners in silicon bulk micromachining technology. The review is restricted to the wet anisotropic etching process which is usually performed in potassium hydroxide solution, ethylenediamine pyrocatechol solution, tetramethylammonium hydroxide, etc. The corner compensation method is the most widely used technique for the fabrication of convex corners. Various types of corner compensating design have been proposed by different research groups. The corner compensation method gives nearly sharp corners. Recently developed techniques, which do not use any compensating design, give perfect convex corners. The limitations and advantages of all the techniques have been discussed. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0960-1317/17/10/R01/pdf

Fabrication techniques of convex corners in a (1?0?0)-silicon wafer using bulk micromachining: a review

Wet anisotropic etching based silicon micromachining is an important technique to fabricate freestanding (e.g. cantilever) and fixed (e.g. cavity) structures on different orientation silicon wafers for various applications in microelectromechanical systems (MEMS). {111} planes are the slowest etch rate plane in all kinds of anisotropic etchants and therefore, a prolonged etching always leads to the appearance of {111} facets at the sidewalls of the fabricated structures. In wet anisotropic etching, undercutting occurs at the extruded corners and the curved edges of the mask patterns on the wafer surface. The rate of undercutting depends upon the type of etchant and the shape of mask edges and corners. Furthermore, the undercutting takes place at the straight edges if they do not contain {111} planes. {100} and {110} silicon wafers are most widely used in MEMS as well as microelectronics fabrication. This paper reviews the fabrication techniques of convex corner on {100} and {110} silicon wafers using anisotropic wet chemical etching. Fabrication methods are classified mainly into two major categories: corner compensation method and two-steps etching technique. In corner compensation method, extra mask pattern is added at the corner. Due to extra geometry, etching is delayed at the convex corner and hence the technique relies on time delayed etching. The shape and size of the compensating design strongly depends on the type of etchant, etching depth and the orientation of wafer surface. In this paper, various kinds of compensating designs published so far are discussed. Two-step etching method is employed for the fabrication of perfect convex corners. Since the perfectly sharp convex corner is formed by the intersection of {111} planes, each step of etching defines one of the facets of convex corners. In this method, two different ways are employed to perform the etching process and therefore can be subdivided into two parts. In one case, lithography step is performed after the first step of etching, while in the second case, all lithography steps are carried out before the etching process, but local oxidation of silicon (LOCOS) process is done after the first step of etching. The pros and cons of all techniques are discussed.

/pdf/a-comprehensive-review-on-convex-and-concave-corners-in-5f1mlgjztz.pdf

A comprehensive review on convex and concave corners in silicon bulk micromachining based on anisotropic wet chemical etching

Agitation is a key method that significantly affects silicon wet anisotropic etching quality including the etching rate and surface roughness. This study introduced the ultrasonic agitation to improve the roughness quality of etched (1 0 0) silicon plane in 30 wt.% KOH solution. These etching characteristics have been compared with those using magnetic stirring and no agitation. In ultrasonic agitation condition, the etching rate increases linearly with agitating power, but the surface roughness worsens. Although the etching rate always exceeds 1.2 μm/min and the average roughness, Ra is below 15 nm, the membrane microstructures are damaged easily by ultrasonic agitation. Moreover, the KOH solution with added anionic surfactant, dihexyl ester of sodium sulfosuccinic acid, is also used to evaluate the etching properties of (1 0 0) silicon under without agitation. Owing to increasing hydrophilic ability between the hydrogen bubble and silicon surface and the effect of silicon wettability, the etching properties are promoted drastically. Experimental results show that the average roughness, Ra can be reduced to 7.5 nm in aqueous KOH with anionic surfactant, which is about eight times better than achieved by the pure KOH solution without agitation for etching temperature of 100 °C. Meanwhile, the etching rate can be enhanced to 4.9 μm/min, which is 1.44 times better than that is obtainable in a pure KOH solution with ultrasonic agitation. The etching rate of the KOH solution with added surfactant is about twice that in the KOH solution with isopropyl alcohol (IPA) for etching temperature of 80 °C. Furthermore, this study also illustrates the reflectivity of etched surface for visible wavelength and the fabrication of thin film microstructures.

Effects of mechanical agitation and surfactant additive on silicon anisotropic etching in alkaline KOH solution

Three ion-typed surfactants, including anionic SDSS, cationic ASPEG and non-ionic PEG, which are powerful wetting agents in electroforming, were added to 30 wt.% KOH and 10 wt.% TMAH solutions to evaluate the silicon anisotropic etching properties of the (1 0 0) silicon plane without agitation and no IPA additive. The results indicate that the surfactant ion-types are not the main determinants of the silicon anisotropic etching properties in KOH and TMAH solutions. The wetting capacity of the etchants causes the efficacies of the etchants on the roughness to follow the order anionic SDSS, cationic ASPEG, non-ionic PEG and pure solution in KOH solutions, and the order cationic ASPEG, non-ionic PEG, pure solution and anionic SDSS in TMAH solutions, especially at higher etching temperatures. Moreover, the chemical activities of etchants differ so that the etching rates follow the order anionic SDSS, pure solution, non-ionic PEG and cationic ASPEG in KOH solutions, and the order anionic SDSS, pure solution, cationic ASPEG and non-ionic PEG in TMAH solutions at a given etching temperature. Anionic SDSS has the highest etching rate of 5.4 μm/min and the lowest surface roughness of 7.5 nm, which are about 1.69 times higher and 7.87 times lower, respectively, than those obtained in pure KOH solution. The cationic ASPEG has a reasonable etching rate of 0.7 μm/min and the lowest surface roughness of 4 nm in TMAH solutions for etching temperature of 100 °C. Furthermore, the surfactants used here were demonstrated to allow the utilization of usual mask materials and anionic SDSS can even increase the selectivity of silicon dissolution toward silicon dioxide in KOH solutions. A drastic reduction of the undercutting of the convex corners is obtained in TMAH solutions with non-ionic PEG surfactant. This finding reveals that the addition of non-ionic PEG to TMAH solutions is ideal when accurate profiles are required without extremely deep etching.

Effects of various ion-typed surfactants on silicon anisotropic etching properties in KOH and TMAH solutions

Three ion-typed surfactants, including anionic sodium dihexyl sulfosuccinate (SDSS), cationic ammonium salt of poly(ethylene glycol) (ASPEG) and non-ionic poly(ethylene glycol) (PEG), were added to 10 wt% tetramethyl ammonium hydroxide water (TMAHW) solutions to evaluate the silicon anisotropic etching properties of the (1 0 0) silicon plane without agitation and no isopropyl alcohol (IPA) additive The results indicate that the wetting capacity of the etchants cause the efficacies of the etchants on the roughness reduction to follow the order cationic ASPEG, non-ionic PEG, pure solution and anionic SDSS in TMAHW solutions, especially at high etching temperatures Moreover, the chemical activities of the etchants cause the efficacies of the etchants on the etching rates to follow the order anionic SDSS, pure solution, cationic ASPEG and non-ionic PEG in TMAHW solutions at a given etching temperature The cationic ASPEG has a reasonable etching rate of 07 µm min−1 and the lowest surface roughness of 4 nm in TMAHW solutions at an etching temperature of 100 °C ASPEG and PEG in TMAHW solutions markedly affect aluminum passivation The undercutting of the convex corners in PEG-added TMAHW solutions can be drastically reduced without using corner compensation; the undercutting ratio obtained using a PEG surfactant is about 45% lower than that obtained in pure TMAHW solution This finding reveals that non-ionic PEG should be added to TMAHW solutions when accurate profiles are required without extremely deep etching This study also demonstrated that non-ionic PEG is more appropriate than IPA for anisotropic silicon TMAHW etching

https://iopscience.iop.org/article/10.1088/0960-1317/15/11/006/pdf

Po Ying Chen

Papers

Effects of mechanical agitation and surfactant additive on silicon anisotropic etching in alkaline KOH solution

Effects of various ion-typed surfactants on silicon anisotropic etching properties in KOH and TMAH solutions

Study on anisotropic silicon etching characteristics in various surfactant-added tetramethyl ammonium hydroxide water solutions