Investigation of High Speed Silicon Wet Bulk Micromachining in KOH-based Solution

Avvaru, V N and Pal, Prem (2019) Investigation of High Speed Silicon Wet Bulk Micromachining in KOH-based Solution. PhD thesis, Indian Institute of Technology Hyderabad.

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Abstract

Micromachining is the most widely used technique for the fabrication of various types of microelectromechanical systems (MEMS) components such as cantilever, diaphragm for different kinds of devices. It is classified in two categories: surface micromachining and bulk micromachining. In surface micromachining, microstructures are fabricated using deposited layers and the substrate is used as support material, while in bulk micromachining the microstructures are realized by selective removal of bulk material. Bulk micromachining is further subdivided into dry and wet bulk micromachining, depending on the type of etchant/etching is used. Although wet bulk micromachining is performed using wet etching, the etching may be anisotropic or isotropic. Wet anisotropic etching is one of the most popular etching methods for silicon bulk micromachining for the fabrication of different kinds of microstructures such as cantilever, diaphragm, cavity, etc. Wet anisotropic etching has several advantages over dry etching including low cost, simple experimental setup, easy handling, batch processing, orientation dependent etch rate, unmatched capability to release mechanical structures, etc. Potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) are most extensively used etchants for wet anisotropic etching process. Amongst these two etchants, KOH is a low cost etchant and provides high etch rate anisotropy between Si{111} and Si{100} (or Si{110}) planes. Despite the advancements in characterizing etchants and the advantage of batch fabrication in wet anisotropic etching, the industrial throughput is still limited due to the slow etch rate. So, increasing industrial throughput is still an area of active research. The slower etch rates of commonly used etchants not only 1imits the throughput but also affects the frequently used masking layer (e.g. SiO2) due to an increased time of contact with the etchants (say KOH). Thus, increasing the etch rate is the need of the hour for enhancing the industrial production. There have been a few attempts toward enhancing the etch rate. The ultrasonic agitation and microwave irradiation have been used to increase the etch rate. However, these techniques on one hand usually damage the structures and on the other hand do not improve the etch rate significantly. Another direction which was explored toward improving the etch rate was the addition of additives (e.g. redox-system or complexants, oxidizing agents, various ion-typed surfactants, metallic impurities (Ni, Al, Cu, Zn, Fe, Cr and Na), etc.) and etching at boiling point of the etchant. The present thesis work focuses on the study of the effect of hydroxylamine (NH2OH) in 20 wt% KOH on the etching characteristics of Si{100}, Si{110} and Si{111}, which are known as principal crystalline planes. A systematic parametric analysis of various concentrations of hydroxylamine (from 0 to 20% in step of 5%) added 20 wt% KOH is carried out and its effect on the etching characteristics is discussed. Mainly three etching characteristics including etch rate, etched surface morphology and undercutting at convex corners are systematically studied. In addition, the etch rate of silicon dioxide and its selectivity with silicon are investigated. The concentration of NH2OH is varied to optimize the concentration to achieve best etching characteristics. 15% NH2OH-added 20 wt% KOH provides improved etching characteristics in which etch rate and undercutting increases significantly. The effect of etchant age on the etching characteristics is investigated. The etchant composition optimized to achieve high etch rate and undercutting is exploited to fabricate various kinds of suspended structures to demonstrate its application in MEMS fabrication. To explain the high speed etching of silicon in NH2OH-added KOH, a simple model is presented to describe the etching mechanism in KOH in the presence of NH2OH. In wet bulk micromachining, the alignment of mask edges along crystallographic direction plays a significant role to control the dimensions of fabricated structures. The mask edges aligned with the direction comprising {111} planes exhibit least undercutting as the etch rate of Si{111} planes are slowest in all kinds of wet anisotropic etchant. Hence the precise identification of crystallographic direction is very important in wet bulk micromachining. Various kinds of pre-etched designs have been reported to identify the crystallographic directions (e.g. <110>) on Si{110} wafer surface. To the best of our knowledge, no pre-etched design has been reported to identify crystal directions on Si{111} wafer. In this thesis, a simple and measurement free technique based on pre-etched pattern is developed and demonstrated for the identification of <110> directions on Si{110} and Si{111} wafer surfaces. The thesis concludes by suggesting the scope of further research in the area of MEMS.

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