Figure 1. Schematic diagram of over etched trench Study on Compensation Method for Vertical Trench Using Anisotropic Wet Etching Mingquan Yuan 1, a , Kan Yu 1, b , Xiaomei Yu 12, c 1 Institute of Microelectronics, Peking University, Beijing 100871, China 2 National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Beijing 100871, China a. jsyuanmq@gmail.com , b. yukan@pku.edu.cn , c. yuxm@pku.edu.cn Abstract This paper investigates anisotropic wet etching characteristics of (110)-oriented silicon wafers in KOH etchant for obtaining vertical trench. To obtain a near rectangled trench instead of parallelogrammic trench with concave corners of 70 and 110, a new compensation method was proposed. With this design, a trench with all sidewalls vertical was obtained by an over etching of silicon. Due to different widths of the trench, the over etching rate in releasing inclined crystal plane {111} varies from 0.32 / min m  to 0.43 / min m  . 1. Introduction To realize varieties of structures, both the dry and wet etching methods are used widely. Compared with dry etching, wet etching method has many advantages, such as low surface roughness, good selectivity, and most importantly low cost. All anisotropic etchants are aqueous alkaline solutions, where the main component can be either organic or inorganic [1]. Wet anisotropic etching etchant applied in our experiment is KOH. To compensate for the undercut of convex corners during the wet etching process, researchers have designed some ways to overcome it [2, 3]. In this paper, a novel compensation method was proposed to obtain a near rectangle trench with (110) orientation wafer. High aspect ratio vertical trenches have many uses in MEMS. The most common way to obtain an ideal vertical trench is deep reactive ion etching(DIRE) technology [4]. Besides, LIGA is also a perfect technology to realize vertical trench. By using LIGA, a vertical trench with very high aspect ratio can be obtained [5]. Due to the advantages mentioned above, anisotropic wet etching could be taken as an alternative for obtain vertical trench since etching rate is strongly dependent of direction and there are {111} planes perpendicular to the (110) plane in (110)-oriented silicon wafer. In our research, anisotropic wet etching was used to obtain a near vertical trench. Oblique {111} planes are supposed to be removed by over etching. 2. Compensation theory The etching rates of (111) crystal planes are approximately two orders smaller than other principal crystal orientations using anisotropic etchant, and a model to explain this was proposed by Seidel [6]. Due to the etching characteristics and silicon crystal structure, it is possible to obtain vertical sidewalls in (110) silicon wafer when etching-window is specially designed. Except the four {111} planes perpendicular to the (110) plane, there also exist two inclined {111} planes, and they could be observed in Figure 1. A schematic diagram of an over etched vertical trench at a size of L×a×H is shown in Figure 1, and four vertical {111} planes and two inclined {111} planes can be seen. Inclined {111} plane intersects with (110) plane at an angle () of 35. To ensure the wafer with the thickness of 450 m  can be etched through, the trench length (L) should be designed greater than a critical value. In figure 1, the length of BX, XY, BD, AG and AB are defined as L , 1 L , 2 L , H, a. BF is added to AB as a auxiliary line at the same direction of AB, which make AF and AE has a same length. Compensating triangle plane ACG  is made to bisect isoceles triangles EAF  and EGF  , then AC EF  , CG EF  . In the case that  is the intersection angle of EAF  and EGF  , and 90 CAG    , it can be obtained that, AG H AC tg tg     (1) 978-1-4244-5798-4/10/$26.00 ©2010 IEEE