Bulk silicon etching techniques, used to selectively remove silicon from substrates, have been broadly applied in the fabrication of micromachined sensors, actuators, and structures. Despite the more recent emergence of higher resolution, surface-micromachining approaches, the majority of currently shipping silicon sensors are made using bulk etching. Particularly in light of newly introduced dry etching methods compatible with complementary metal-oxide-semiconductors, it is unlikely that bulk micromachining will decrease in popularity in the near future. The available etching methods fall into three categories in terms of the state of the etchant: wet, vapor, and plasma. For each category, the available processes are reviewed and compared in terms of etch results, cost, complexity, process compatibility, and a number of other factors. In addition, several example micromachined structures are presented.

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Bulk micromachining of silicon

Comparison of chemical cleaning methods of glass in preparation for silanization

A method for fabricating Si nanostructures with an air‐operated atomic force microscope (AFM) is presented. An electrically conducting AFM tip is used to oxidize regions of size 10–30 nm of a H‐passivated Si (100) surface at write speeds up to 1 mm/s. This oxide serves as an effective mask for pattern transfer into the substrate by selective liquid etching. The initial oxide growth rate depends exponentially on the applied voltage which produces an effective ‘‘tip sharpening’’ that allows small features to be produced by a relatively large diameter tip.

Fabrication of si nanostructures with an atomic force microscope

Pyramidal texturing of silicon solar cell with TMAH chemical anisotropic etching

We compared the anisotropic etching properties of KOH and TMAH solutions. We used hemispherical specimens of single-crystal silicon whose surface exhibited every crystallographic orientation, in order to evaluate the etching properties as a function of the orientation. We carried out a series of experiments using different etchant concentrations and etching temperatures. The orientation dependence in the etching rates of the surface crystals significantly differed between the two etchants, especially for the (111) and (221) planes. We conclude that the two etchants have different etching mechanisms at least in an area including these two planes. The etching rates varied with etchant concentration and etching temperature. The concentrations that maximized the etching rate were 25 wt.% for KOH and 20 wt.% for TMAH. The activation energies in KOH and TMAH were almost the same for the (100), (110), and (320) planes but not for the (221) and (111) planes. Etchant circulation had a significant effect on the etching rates in diluted TMAH solution but not in KOH solution. The roughness of the (100) plane in KOH was one order smaller than that in TMAH.

Differences in anisotropic etching properties of KOH and TMAH solutions

The quality of vertical roughness produced by the etching of Si in aqueous KOH has been studied by varying several experimental parameters such as molarity, time of etching, temperature, and stirring. We note that at room temperature, unstirred etching is smoother at low and high molarities, and etch rate and roughness both peak near 5–6 M. With no stirring, roughness increases as a function of etch time, then levels off. With stirring, roughness decreases, especially around the peak etch rate near 5–6 M. For a fixed molarity like 5 M, unstirred etching becomes smoother with increasing temperature even as the etch rate increases rapidly. Such results suggest that masking by hydrogen bubbles or silicate etch products is the principle origin of vertical roughness. Bubble properties as a function of molarity and stirring (as determined from electrolysis experiments) are used to suggest a pseudo‐masking model to explain some roughness properties.

I. Heard

Papers

Etching roughness for (100) silicon surfaces in aqueous KOH

Study of Bias‐Dependent Etching of Si in Aqueous KOH

Current‐Voltage Characteristics of Heavily Doped p‐Si in Aqueous KOH