THE BLACK SILICON METHOD VI: HIGH ASPECT RATIO TRENCH ETCHING FOR MEMS APPLICATIONS - Micro Electro Mechanical Systems, 1996, MEMS '96, Proceedings. 'An Investigation of Micro Structur

Etching high aspect ratio trenches (HART’s) in silicon is becoming increasingly important for MEMS applications. Currently, the most important technique is dry reactive ion etching (ME). This paper presents solutions for the most notorious problems during etching HART’s: tilting and the aspect ratio dependent etching effects such as bowing, RIE lag, bottling, and micrograss or black silicon. To handle these problems submicron HART’s are etched and the black silicon method is used to direct the pressure, power, ion energy, or f lows of a fluorine-based RIE into the preferred settings. The influence of ion energy and -trajectory is found to be most critical. The behaviour of the HART process is explained with the help of a set of variables and used to optimise the final profile. After this optimisation the RIE setting found is used for etching supermicron HART’s which are characteristic for MEMS applications. INTRODUCTION In the first years after the introduction of dry plasma etching in microelectronics, the technique was mainly used for the ashing of photoresist because of its cleanliness and high selectivity [ I ] . In the beginning, the isotropic nature of these so-called plasma etchers (PE) was not a problem but the ever decreasing dimensions in the integrated circuits forced the research institutes to develop dry plasma systems for anisotropic etching and the ion beam etching (IBE) was bom [2]. However, the etch selectivity between mask-substrate was rather poor and special IBE reactors e.g. chemical assisted ion beam etching (CAIBE) were designed [3]. A major step into the direction of large scale integration was taken after the reactive ion etching (ME) came available [4]. In RIE it is possible to get a very high selectivity with a perfect anisotropy [5]. Nowadays, to increase the electronic circuit density, the use of the surface area of the silicon wafers only is not sufficient and the research is focused to use the third dimension: depth. So, there is an increasing demand for processes which are able to create not only a high selectivity and anisotropy but also high aspect ratio trenches (HART’s). The aspect ratio (AR) is defined as the maximum depth divided by the maximum width. Such HART’s, e.g. a quarter micron in width and three micron in depth, are needed for e.g. transistor trench isolation and trench capacitors. HART’s are not only important for silicon etching. Especially the etching of HART’s in polymers is a prime technology [6] . In micromechanics, as in microelectronics, profile control of HART’s is increasingly important. These trenches give rise to specific problems: The aspect ratio dependent etching (ARDE) effects and tilting [7-91. In figure 1 these effects are shown. RIE lag, bowing, and trench area dependent tapering of profiles (TADTOP, fig. la), bottling (fig. 1 b), micrograss ( f iglc) , and tilting (fig.ld). The main subject of this study is to explain and control these effects. The N E process is analysed with the help of four sets of variables: i) The RIE setting, ii) the equipment parameters, iii) the plasma characteristics, and iv) the trench forming mechanisms. These figures are controlling the final HART profile. The study continuous with information concerning the equipment: a conventional RIE of the STS company and a cryogenically cooled RIE with a high density source of the Plasma Technology company. After information about the sample preparation, results and discussions are given for the HART’s which are etched. Because the tapering of the trenches is changing with RIE setting, the black silicon method (BSM) is used to be able to change the setting while keeping the profile anisotropic [lo]. Figure 1 : important ARDE effects. a) RIE lag, bowing, and TADTOP, b) bottling, c) micrograss, and d) tilting. 0-7803-2985-6/96 $5.00