CHARACTERIZATION OF ANISOTROPIC ETCHING PROPERTIES OF SINGLE-CRYSTAL SILICON: EFFECTS OF KOH CONCENTRATION ON ETCHING PROFILES

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1 CHARACTERIZATION OF ANISOTROPIC ETCHING PROPERTIES OF SINGLE-CRYSTAL SILICON: EFFECTS OF KOH CONCENTRATION ON ETCHING PROFILES Kazuo Sato, Mitsuhiro Shikida, Yoshihiro Matsushima, Takashi Yamashiro, Kazuo Asaumi", Yasuroh Iriye*, and Masaharu Yamamoto** Nagoya University, Froh-cho, Chikusa-ku, Nagoya , Japan Phone: , Fax: Fuji Research Institute Corporation*, and IRI Aichi Prefectural Government** ABSTRACT We have evaluated the orientation dependence in chemical anisotropic etching of single-crystal silicon. Etch rates for a number of crystallographic orientations were measured for a wide range of etching conditions, including KOH concentrations of 30 to 50% and temperatures of 40 to 90 C. Though the etchants all consisted of the same components KOH and water, the orientation dependence varied considerably with change in etchant temperature and concentration. The resulting etch rate database allows numerical prediction of etch profiles of silicon, necessary for the process design of microstructures. Changing the KOH concentration yielded different etch profiles both analytically and experimentally. INTRUCUCTION Anisotropic chemical etching using KOH water solution has long been usedfor fabricating microstructures such as diaphragms and cantilevers on a silicon wafer. This process is chosen because of its excellentrepeatability and uniformity in fabrication and its low production cost. However in the past, there was a limitation as to the shape of the microstructures fabricated by this etching system. The main reasons for this limitation are as follows. First, a conventional single-step process provides only a limited set of crystallographic planes appearing on the etched profile. Second, the etch rates of individual crystallographic planes having a higher Miller-index order were not known; thus it was hard to design a multistep anisotropic etching process that could produce complicated 3-D structures. A research group including the first author proposed using a multistep chemical anisotropic etching process in order to achieve 3-D microstructures on single-crystal silicon, in MEMS'91.[1] They &dually obtained smooth, round etch profiles composed of a number of crystallographic planes by applying two of the several steps of the anisotropic etching process. Designing the multisteps of an etching process reqyires an etchng simulator. The etching simulator requires the etch rates of all the crystallographic orientations for analysis. Hence we decided to construct a database of etching rates related to crystallographic orientation and the etchant recipe including its concentration and temperature. Some work has been reported on the characterization of orientation-dependence in the etching of single-crystal materials. Ueda measured the decrease in the thickness of quartz wafers having different surface orientations. [2] This was a direct method of evaluating anisotropy. However, a dense net of data is hard to accumulate because of the numerous experiments required for a single etching condition. Weirauch [3] measured the side-etch around the periphery of a circular mesa of silicon whose top was masked Seidel [4] and Kendall [5] also measured the side-etch along straight mask edges that were aligned with a number of different directions on a silicon wafer. Though these side-etch measurements were qualitatively effective in visualizing anisotropy, the obtained data did not necessarily show correct etch rates, because the sideetched planes were not always perpendicular to the wafer surface. The first author previously proposed the use of a hemispherical silicon specimen [ 13 for the measurement of orientation dependence. The etch rates were calculated for some orientations from the dimensional change of the hemisphere, by comparing the surface profiles before and after etching. Though some other researchers have also used a spherical specimen of single-crystal silicon,[6] no attention seems to have been paid to the effects of specimen size on measurement accuracy. They sometimes etched the sphere to such an extent that some orientations disappeared from the specimen surface /97/$ IEEE 406

2 In this study, we investigated the size parameter of the spherical specimen to determine which size allows reliable measurement of the orientation dependence in anisotropic etching. Then, we carried out a series of experiments measuring the etch rates of all the orientations. This resulted in the construction of an etch rate database covering a wide range of KOH etching conditions. The database that we obtained clarified that orientation dependence varies considerably with change in KOH concentration and etching temperature. Finally, we analyzed etch profiles under different KOH concentrations. The etch profiles varied according to the change in orientation dependence. according to their own etching rates. When plane B advances beyondpoint C, the etching front directed toward the orientation A disappears. In such a case, the etchrate in the Orientation A cannot be correctly evaluated. We investigated the dimensional parameters of the specimen to find those with which the etch rate can be correctly evaluated without letting a desirable orientation disappear from the hemispherical surface. We concluded that, in terms of crystallographic orientation, to evaluate the etching rates for the desired dense net of data the following relationship must be satisfied. EXPERIMENTAL METHODS Effect of the size of the hemisphere We used a hemispherical specimen of single-crystal silicon, as shown in Fig. 1. It was mechanically ground, lapped, and polished into a mirrored surface. The radius of the hemisphere was 22 mm. The sphericity was less than 10 pm. The top of the hemisphere was oriented toward (1 lo), andorientation flats weremadeat the periphery. All crystallographic orientations appeared on the hemispherical surface. Measuring the profile before and after etching andnoting the change enables calculation of the etch rate in any orientation. When the etching advances, the specimen surface takes on a polygonal profile. We considered the problem of interference between neighboring orientations. This problem is illustrated in Fig. 2. Points A and B, which have differing crystallographic orientations, advance R is the radius of the hemisphere, 1 and 2 are the measured etch depths at two different points A andb on a same hemisphere, and 8 is the angular distance between these measuring points (Fig. 2). This equation implies the following requirements for the experiments. (1) A large-radius specimen is desirable in measuring etch rates with that allows a dense net of orientations. (2) The etch depth should be small enough in comparison with the specimen radius. We used the specimen having a radius R=22 mm. The equation above was proved to be satisfied in experiments by restricting the maximum etch depth to a range of 100 to 150 pm and the probing density to 2 degin angle. The etch depth was measured using a UPMCSSO-CARAT (Carl Zeiss Co.) three dimensional measuring machine contact probe silicon surface &I, &2: etch depth B Fig. 1 Etch rate measurement scheme using a hemispherical specimen of single-crystal silicon. Fig. 2 Geometrical conditions in which two neighboring etching fronts interfere with each other, related to the density of the probing network. 407

3 with contact probe. The surface profile was probed every 2 deg in angle in the latitude ranging from 20 to 90 deg and in the longitude ranging from 0 to 360 deg. The total number of the probe points was Etching conditions We used a solution of KOH and water as an etchant. The KOH, produced by Kishida Chemical Co., Ltd, was supplied in 85-wt% pellets. The KOH concentration of the etchant was varied within a range of 30 to 50% according to pellet weight. The temperatures selected were within a range of 40 to 90 C. The stability of the temperature during etching was k 0.5"C. The hemispherical silicon specimen was immersed in 1 liter of etchant. A silicon chip was thrown into the etchant prior to the experiments, to estimate the time necessary to etch the hemisphere. Fresh etchant was used in every subsequent experiment. EXPERIMENTAL RESULTS Orientation dependence Examples of measured etch rate distributions are shown with isolines in Fig. 3 as projections of the hemispheres. According to the symmetry of the crystal lattice, the plots are folded into a quarter of a circle. All of the orientations appear in this quarter circle. Three perpendicular (100) planes appear at the origin and the end of the X, Y axes. Three (1 10) are located in the middle of the X, Y axes and on the periphery of the circle. The (1 1 1) is in the middle of the quarter circle. Some etch rates are already known for such orientations having low-order Miller indexes. This measurement yields etch rates for orientations of higherorder indexes. The isolines indicate relative, rather than absolute, etch rates between these projections. Note the change in isoline patterns caused by the change in temperature and KOH concentration. For example, with a KOH concentration of 40%, at 40 C, the maximum etch rate appears in (110). This peak splits in two when the temperature increases. The location of the maximum-value moves to near (320) at 90 C via (540) at 70%. A similar tendency is observed with a KOH concentration of 30%. Though two peaks appear even at 40 C, they tend to move apart with increase in temperature, to (210) at 90 C via (530) at 70 C. With a KOH concentration of 50%, we observed the split two peaks also near (320). However, the drift of peak location with change in temperature was not as obvious in this case. It is known that the orientation dependence varies according to changes in etching temperature, with a consequent change in the etch profile.[l] The results in Fig. 3 show that the orientation dependence varies also with changes in the concentration of the KOH. The location of the maximum etch rate drifted in accordance with change in KOH concentration. Influence of KOH concentration These results suggest that the etch profile will vary between different KOH concentrations. We demonstrated the difference in etching profile by changing the KOH concentration. We chose the groove profile on a (1 10) silicon wafer as an example. A deep groove can be etched through a narrow aperture of a mask aligned toward the (1 12) direction. The side wall is composed of (1 1 l), which is perpendicular to the wafer surface of (1 IO). The profile of the etching front at the bottom of the groove is adjustable by changing the KOH concentration. The overall etching rate distribution is again plotted in Fig. 4 as a projection of a hemisphere whose center is aligned toward (1 IO), so the (1 11) are located toward the left and right ends of the circle. This figure compares the change in isoline patterns between KOH concentrations of 30 % and 40 %. The etch rates on lines C-C and D-D determine the groove profiles. The etch rate distribution along these lines is shown below each circle in Fig. 4. A significant change in etch rate distribution is observed by the change in KOH concentration. We analxzed the etch profiles using Wulff-Jaccodine's method[7] An etch profile is determined by the distribution of the etch rate vectors existing in the analyzed plane. Such a two-dimensional analysis is applicable for etch profile prediction in the case of long and narrow groove etching. Figure 5 shows the analytical results obtained using the etch rate distribution in Fig. 4. It can be clearly observed that the bottom surface of the groove is flat in case of 30% KOH, and wedge-shapedin case of the 40% concentration. The wedge profile is composed of (331). Experiments were carried out for the groove formation on a (1 10) silicon wafer in the same condition as described above. Figure 6 shows the resulting cross-sections. Note 408

4 that the actual profiles agree very closely with the analytical results. It is clear that the orientation dependence of the etch rate varies according to the change in KOH concentration, with consequent variation in the etch profiles. DISCUSSION Both temperature and concentration of KOH etchant influence orientation dependence. It is important to know the change in orientation dependence under a wide range of etching conditions. We caniedout a series of experiments covering a temperature range of 40 to 90 C and a KOH concentration in a range of 30 to 50%. Our efforts continue toward covering a wider range of conditions, including different etchants such as TMAH. We have summarized the results as a database to support numerical simulation of anisotropic etching. The simulation is of importance, especially for the process design, in that it introduces a series of multisteps in the anisotropic etching, using different mask patterns, to provide complicated three-dimensional microstructures on single-crystal silicon. 90" min max X location of the maximum value ( ' 4 0 C 30' 0" 0' 30" 60" 90" 0" 30" 90' 90" 90" 7 0 C 60' 60" 30" 30" n' 0' - 0; 30" )t, 60' 90' 0' 30" 90" 90" 90" 9 0 C 60" 60" 30" 30' n' 0" 3 O%KOH 4 O%KOH Fig. 3 Comparison of orientation dependence in the etch rate of single-crystal silicon between different etching temperatures and KOH concentrations. 409

5 cross-section C-C 0.0 J (a) 30% KOH (b) 40% KOH Fig. 4 Comparison of etch rate distribution between 30 and 40% KOH concentrations at 70 deg C. The circles above are projections of the surface of a hemispherical specimen whose top is directed toward (11O).Below are the normalized etch rate distributions along lines C-C and D-D of the circle. (a) 30% KOH (b)40% KOH Fig. 5 Difference in etch profile of a deep groove on a (1 10) wafer between two different KOH concentrations of 30 and 40%. Profiles (a) and (b) are calculated using the respective etch rates of the C-C and the D-D cross-section in Fig

6 (a) 30% KOH (b) 40% KOH Fig. 6 Actual etch profiles of a deep groove etched on a (1 10) wafer under different KOH concentrations of 30 and 40%. The etching temperature was the same (70 C). CONCLUSION Measurement was carried out on the etch rates of singlecrystal silicon as a function of crystallographic orientation, using hemispherical specimens. The results are summarized as follows. (a) The orientation dependence of the etch rate of silicon for a KOWwater solution was evaluatedfor adense net of orientations. (b) It was observed that the Orientation dependence changes according to changes in etching temperature and also in KOH concentration. (c) A change in KOH concentration causes a change in orientation dependence and, consequently, different etch profiles These measurement results on orientation dependence were accumulated as an etching database. Etch rates in arbitrary orientations were described as functions of the etchant, its concentration, and the etching temperature. This database allows the simulation of etching profiles for arbitrary etching conditions. Combined with an etching simulator, the database is of great importance for the development of microfabrication processes for bulk silicon microstructures. ACKNOWLEDGEMENTS This work was supported by a grant-in-aid for Scientific Research (B) No from the Ministry of Education, Science and Culture of the Japanese govemment. It was also supported by Miaomachining- Process Consortium organized by Fuji Research Institute Corporation. The silicon ingots were the donation of Sumitomo Sitix Co., Ltd The authors express their thanks for all of this support. REFERENCES [l] Koide, A., Sato, K., et al.: Proc. of IEEE MEMS 91 (Nara, 1991) [2] Ueda, T., Kohsaka, F., et al.: Trans. of the SICE, (1987) (in Japanese). [3] Weirauch, D. F.: J. of Appl. Phys (1975) [4] Seidel, H., Csepregi, L., et al.: J. of Electrochem. Soc (1990) [5] Kendall, D. L., de Guel, G. R., et al.: Electrochem. Soc. Extended Abstracts, 82-1 (1982) [6] Ju, C. and Hesketh, P. J.: Sensors and Actuators A-33 (1992) [7] Jaccodine, R. J.: J. of Appl. Phys (1962)