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

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