Ž . Sensors and Actuators 76 1999 425–430 www.elsevier.nlrlocatersna Optical monitoring and control of Si wet etching U.M. Mescheder ) , Ch. Koetter Institute for Applied Research, Department of Mechatronics and Microsystems, FH Furtwangen, UniÕersity of Applied Sciences, Postfach 1152, Furtwangen 78113, Germany Received 23 October 1998; received in revised form 24 March 1999; accepted 26 March 1999 Abstract Light can be used to monitor and control anisotropic etching of silicon. For in situ monitoring of the Si-thickness during anisotropic wet etching in KOH, a technique based on transmission spectroscopy has been investigated. Using special self-calibrating, recursive algorithms, thickness of Si membranes can be measured in the range between 10 and 500 mm. Illuminating the Si-surface during etching with a high light intensity, etch rate of c-Si in KOH can be controlled by illumination of the silicon surface. This method might be used for maskless fabrication of simple micromechanical devices. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Silicon micromachining; Light-controlled etch rate; Optical monitoring of etch rate 1. Introduction Anisotropic wet etching of c-Si is a standard technique of bulk-micromachining, e.g., for the fabrication of Si pressure sensors. Thickness control is a crucial point for a Ž reliable production process e.g., for pressure sensors sen- 2 . sitivity depends on membrane thickness d as 1rd . Elec- trochemical etch stop at a suitable biased p–n junction or the use of highly boron-doped silicon layers are practicable fabrication methods only for membrane thickness well below 20 mm. Therefore, often, thickness of relatively thick membranes is still controlled by process time. Ž Changes and fluctuations during process temperature, concentration of etch medium and solved Si, native oxide . thickness will cause variations in the resulting membrane thickness. An optical measurement technique is described which allows the in situ monitoring of membrane thickness during anisotropic wet etching of crystalline silicon. In bulk micromachining of c-Si with anisotropic etchants Ž . Ž KOH , masking layers such as Si N deposited by chem- 3 4 w x ical vapor deposition CVD and structured by photolithog- . raphy and subsequent etching of the thin layer are nor- mally used to protect those parts of the silicon surface which should not be etched. However, deposition of Si N 3 4 by CVD has to be done prior to process steps with limited Ž . temperature budget e.g., metal, sensitive doping . Addi- ) Corresponding author. Tel.: q49-7723-920232; Fax: q49-7723- 920610; E-mail: mescheder@fh-furtwangen.de  4 tionally, orientation and angle of the 111 -planes with respect to the etched surface determine the shape of the resulting devices. Thus, circular structures and structures with convex corners are hardly difficult to achieve with anisotropic wet etching of c-Si. An optical technique is presented which allows to form such irregular shapes with an illumination assisted etch process. 2. Optical in situ monitoring of Si-thickness during wet etching 2.1. Measurement principles Only optical methods can be used for continuous and contactless thickness measurement in aggressive solutions Ž . e.g., 30% aqueous KOH at 808C and for sensitive sur- faces like Si-membranes. In principle, measurements can be performed in emission-, reflection- and transmission- mode. Emission of Si is strongly influenced by temperature and thermal conductivity of the etch solution, thus, emis- sion spectroscopy is not suitable for in situ monitoring. Reflection spectroscopy has been used by Tosaka et al. wx wx 1 and Steinsland et al. 2 for in situ monitoring of Ž . membrane thickness. For thick membranes, infrared IR - light sources and special expensive detectors are necessary. Furthermore, the proposed interference method measures only the change in thickness and not absolute thickness. 0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. Ž . PII: S0924-4247 99 00212-5