TMAH Based High Speed Silicon Bulk Micromachining for MEMS

Veerla, Swarnalatha and Pal, Prem (2019) TMAH Based High Speed Silicon Bulk Micromachining for MEMS. PhD thesis, Indian Institute of Technology Hyderabad.

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Abstract

Various process steps such as oxidation, diffusion, etching, lithography, etc. are employed for the fabrication of microstructures used in microelectromechanical systems (MEMS). In addition to these processes, micro-stereolithography (MSL), LIGA (a German acronym for Lithographie, Galvanoformung, Abformung), and micromachining are used in MEMS fabrication. Among these methods, micromachining is most widely used. It is further classified into two categories: surface micromachining and bulk micromachining. In these two techniques, bulk micromachining is a popular technique in MEMS fabrication and further divided into wet and dry bulk micromachining based on the type of chemical/process (wet chemical or gas/plasma/LASER) is employed. Wet anisotropic etching based micromachining is extensively used to fabricate various MEMS structures including suspended (e.g., microcantilever, diaphragm, etc.) and fixed (e.g., grooves, trenches, channels, etc.) structures. Wet anisotropic etching is used owing to its several benefits such as low cost, easy handling, orientation dependent etching, and bulk production capability over other techniques. Most importantly, it provides unique shape structures, which may not be possible by dry etching, for examples, the fabrication of freestanding microstructures using the undercutting process, slanted sidewalls for optical mirror application, etc. Tetramethylammonium hydroxide (TMAH) and potassium hydroxide (KOH) are the most widely employed etchants for silicon wet anisotropic etching. In these two etchants, TMAH is preferred when complementary metal oxide semiconductor (CMOS) compatibility is a major concern, and the oxide layer is used as a mask material. Although wet anisotropic etching has many advantages, industrial vii throughput is still limited due to the slow etch rate. In addition, slow etch rate increases the etching time and therefore mask material such as SiO2 is affected. Hence increasing the etch rate is an important research problem for both academic and industrial applications. In order to reduce the etching time to increase productivity, etchant must provide high etch rate. However, the etch rate attainable using the conventional etchants is limited and hence affects industrial productivity. Several methods have been proposed to increase the etch rate such as ultrasonic agitation and microwave irradiation during etching, adding some additives, oxidizing agents, various ion and non-ionic typed surfactants, etching at the boiling point of the etchant. Each method has its own pros and cons such as the ultrasonic method may rupture the fragile structures, and microwave irradiation causes damage. The present thesis work is focused on investigating a non-conventional etchant in the form of NH2OH-added in 5 wt% TMAH to determine its etching characteristics. A systematic and parametric analysis with concentrations of NH2OH varying from 5% to 20% in step of 5%, all in 5 wt% TMAH, to obtain the optimum concentration for achieving improved etching characteristics including higher etch rate, higher undercutting at convex corners, and smoother etched surface morphology is performed. To study different etching characteristics, various kinds of mask patterns are used on Si{100}, Si{110}, and Si{111} wafers. As the lower concentration TMAH (2-5 wt%) provides high etch in comparison to higher concentration TMAH (20-25 wt%), 5 wt% TMAH is selected to improve its etching characteristics. Average surface roughness (Ra), etch depth, and undercutting length are measured using 3D scanning laser microscope. Surface morphology of the etched surfaces is examined using a scanning electron microscope (SEM), and the thickness of the oxide layer is determined using spectroscopic ellipsometry. The etch rate of silicon with the addition of NH2OH in TMAH solution enhances viii significantly. Additionally, the incorporation of NH2OH significantly improves the etched surface morphology of Si{100} and the undercutting at the convex corner, which is highly desirable for the quick release of microstructures from the substrate. Moreover, the addition of NH2OH in TMAH increases etch selectivity between thermal oxide and silicon. 10% NH2OH is found to be an optimal concentration for addition in 5 wt% TMAH to achieve favorable etching characteristics. The optimal etchant composition (i.e., 10% NH2OH + 5 wt% TMAH) is used to study the effect of etchant age on the etching characteristics. Moreover, the effect of different concentrations of Triton in 10% NH2OH + 5 wt% TMAH is investigated. The etching mechanism in NH2OH-added alkaline solution is investigated. An in-depth analysis of possible etching mechanism in modified alkaline solutions is presented by considering NH2O- and OH- ions as catalysts and H2O as the reactive molecule. It accounts for the rise in etch rate in NH2OH-added alkaline solution. It also justifies the reasonable explanation of the etching mechanism in pure alkaline solution. In the fabrication of freestanding structures, the undercutting mechanism is used to remove the underneath material. In addition, it can be used to create a preetched pattern to determine precise crystallographic directions. In this work, a novel self-aligning pre-etched pattern to precisely identify the <100> directions on Si{100} wafers is presented. The proposed pre-etched patterns self-align itself at the <100> direction while becoming misaligned at directions away from the <100>. This self-aligned pattern distinguishes the precise <100> direction by making it appear quite obvious among the cluster of patterns. The aligned patterns can be easily located using a simple optical microscope. Additionally, the proposed technique does not require any measurement to identify the correct direction. ix In order to demonstrate the applications of NH2OH-added TMAH in MEMS fabrication, different kinds of suspended microstructures are fabricated successfully. Based on this study it can be stated that the NH2OH-added TMAH is an appropriate etchant composition for high speed silicon bulk micromachining for MEMS fabrication and therefore it is a potential candidate to replace pure TMAH for industrial applications. The results presented in this paper are extremely useful for engineering applications and will open a new direction of research for scientists in both academic as well as industrial laboratories.

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