Study of Fast Etching of Silicon for the Fabrication of Bulk Micromachined MEMS Structures

Authors: V. Swarnalatha, A. V. Narasimha Rao, P. Pal

Abstract:

The present research reports the investigation of fast etching of silicon for the fabrication of microelectromechanical systems (MEMS) structures using silicon wet bulk micromachining. Low concentration tetramethyl-ammonium hydroxide (TMAH) and hydroxylamine (NH₂OH) are used as main etchant and additive, respectively. The concentration of NH₂OH is varied to optimize the composition to achieve best etching characteristics such as high etch rate, significantly high undercutting at convex corner for the fast release of the microstructures from the substrate, and improved etched surface morphology. These etching characteristics are studied on Si{100} and Si{110} wafers as they are most widely used in the fabrication of MEMS structures as wells diode, transistors and integrated circuits.

Keywords: KOH, MEMS, micromachining, silicon, TMAH, wet anisotropic etching.

Digital Object Identifier (DOI): doi.org/10.5281/zenodo.1314660

References:

[1] P. Pal, and K. Sato, Silicon wet bulk micromachining for MEMS. Pan Stanford Publishing, Singapore, 2017.
[2] I. Zubel, and M. Kramkowska, “Possibilities of extension of 3D shapes by bulk micromachining of different Si (hkl) substrates,” J. Micromech. Microeng., vol. 15, pp.485-93, Dec. 2005.
[3] P. Pal, K. Sato, M. A. Gosalvez, B. Tang, H. Hida, and M. Shikida, “Fabrication of novel microstructures based on orientation dependent adsorption of surfactant molecules in TMAH solution,” J. Micromech. Microeng., vol. 21, pp. 015008 (11pp), Dec. 2011.
[4] P. Pal, and K Sato, “Fabrication methods based on wet etching process for the realization of silicon MEMS structures with new shapes,” Microsyst. Technol., vol. 16, pp. 1165-1174, Jul. 2010.
[5] P. Pal, K. Sato, and S. Chandra, “Fabrication techniques of convex corners in (100)-silicon wafer using bulk micromachining: A Review,” J. Micromech. Microeng., vol. 17, no. 10, pp. R111-R133, Sep. 2007.
[6] P. Pal, and K. Sato, “A comprehensive review on convex and concave corners in silicon bulk micromachining based on anisotropic wet chemical etching,” Micro Nano Syst. Lett., vol. 3, no. 1, pp.1-42, Dec. 2015.
[7] J. Chen, L. Liu, Z. Li, Z. Tan, Q. Jiang, H. Fang, Y. Liu, “Study of anisotropic etching of (100) Si with ultrasonic agitation,” Sens. Actuators A, vol. 96, no. 2, pp.152-156, Feb. 2002.
[8] J. A. Dziuban, “Microwave enhanced fast anisotropic etching of monocrystalline silicon,” Sens. Actuators A, vol. 85, no.1, pp.133-138, Aug. 2000.
[9] H. Tanaka, S. Yamashita, Y. Abe, M. Shikida, and K. Sato, “Fast etching of silicon with a smooth surface in high temperature ranges near the boiling point of KOH solution,” Sens. Actuators A, vol. 114, no. 2-3, pp. 516-520, Sep. 2004.
[10] B. Tang, K. Sato, D. Zhang, and Y. Cheng, “Fast Si (100) etching with a smooth surface near the boiling temperature in surfactant modified tetramethylammonium hydroxide solutions,” Micro Nano Letts., vol. 9, no. 9, pp. 582-584, Sep. 2014.
[11] R. Sotoaka, “New etchants for high speed anisotropic etching of silicon,” J. Surf. Finish. Soc. Jpn., vol. 59, no. 2, pp. 104-106, 2008.
[12] H. Tanaka, M. Takeda, and K. Sato, “Si (100) and (110) etching roperties in 5, 15, 30 and 48 wt% KOH Aqueous solution containing Triton-X-100,” Microsyst. Technol., 1-8, 2017.
[13] P. Pal, A. Ashok, S. Haldar, Y. Xing, and K. Sato, “Anisotropic etching in low-concentration KOH: effects of surfactant concentration,” Micro & Nano Letts., vol. 10, no. 4, pp. 224-228, Mar. 2015.
[14] C. R. Yang, P. Y. Chen, C. H. Yang, Y. C. Chiou, and R. T. Lee, “Effects of various ion-typed surfactants on silicon anisotropic etching properties in KOH and TMAH solutions,” Sens. Actuators A, vol. 119, pp. 271-281, Mar. 2005.
[15] I. Zubel, I. Barycka, K. Kotowska, and M. Kramkowska, “Silicon anisotropic etching in alkaline solutions IV: the effect of organic and inorganic agents on silicon anisotropic etching process,” Sens. Actuators A, vol. 87, pp. 163-171, Jan. 2001.
[16] I. Zubel, and M. Kramkowska, “The effect of isopropyl alcohol on etching rate and roughness of (100) Si surface etched in KOH and TMAH solution,” Sens. Actuators A, vol. 93, no. 2, pp. 138-147, Sep. 2001.
[17] P. Pal, K. Sato, M. A. Gosalvez, and Shikida, “Study of rounded concave and sharp edge convex corners undercutting in CMOS compatible anisotropic etchants,” J. Micromech. Microeng., vol. 17, no. 11, pp. 2299-2307, Oct. 2007.
[18] Y. W. Xu, A. Michael, and C. Y. Kwok, “Formation of ultra-smooth 45◦ micromirror on (100) silicon with low concentration TMAH and surfactant: Techniques for enlarging the truly 45◦ portion,” Sens. Actuators A, vol. 166, pp. 164-71, Mar. 2011.
[19] P. Pal, K. Sato, M. A. Gosalvez, Y. Kimura, K. Ishibashi, M. Niwano, H. Hida, B. Tang, and S. Itoh, “Surfactant adsorption on single crystal silicon surfaces in TMAH solution: orientation-dependent adsorption detected by in-situ infra-red spectroscopy,” J. Microelectromech. Syst., vol. 18, pp. 1345-1356, Dec. 2009.
[20] P. Pal, M. A. Gosalvez, and K. Sato, “Etched profile control in anisotropic etching of silicon by TMAH+Triton,” J. Micromech. Microeng., vol. 22, pp. 065013 (9pp), May. 2012.
[21] B. Tang, P. Pal, M. A. Gosalvez, M. Shikida, K. Sato, H. Amakawa, and S. Itoh, “Ellipsometry study of the adsorbed surfactant thickness on Si{110} and Si{100} and the effect of pre-adsorbed surfactant layer on etching characteristics in TMAH,” Sens. Actuators A, vol. 156, pp. 334-341, Dec. 2009.
[22] A. V. Narasimha Rao, V. Swarnalatha, A. Ashok, S. S. Singh, and P. Pal, “Effect of NH2OH on etching characteristics of Si {100} in KOH solution,” ECS J. Solid State Sci. Technol., vol. 6, no. 9, P609-P614, Jan. 2017.
[23] V. Swarnalatha, A. V. Narasimha Rao, A. Ashok, S. S. Singh, and P. Pal, "Modified TMAH based etchant for improved etching characteristics on Si {100} wafer," J. Micromech. Microeng,. vol. 27, no. 8, p. 085003, Aug. 2017.
[24] P. Pal, and K. Sato, “Complex three dimensional structures in Si{100} using wet bulk micromachining,” J. Micromech. Microeng., vol. 19, p. 105008, (9pp), Sep. 2009.
[25] P. Pal, and K. Sato, “Various shapes of silicon freestanding microfluidic channels and microstructures in one step lithography,” J. Micromech. Microeng., vol. 19, no. 5, p. 055003, (11pp), Apr. 2009.
[26] P. Pal, and S. Chandra, “Bulk-micromachined structures inside anisotropically etched cavities,” Smart Mater Struct., vol. 13, pp.1424-1429, Oct. 2004.
[27] G. T. Kovacs, N. I. Maluf, and K. E. Petersen, “Bulk micromachining of silicon,” IEEE Pro. 1998, vol. 86, pp. 1536- 551.