Characterization of masking layers for deep wet etching of glass in an improved HF/HCl solution Ciprian Iliescu a, * , Ji Jing b , Francis E.H. Tay a,b , Jianmin Miao c , Tietun Sun d a Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, 138669 Singapore b Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore c Micromachines Center, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore d Department of Physics, Shanghai Jiaotong University, Shanghai, China Available online 26 November 2004 Abstract The paper presents a solution for improving the quality of the surface generated during deep wet etching of glass using an HF (49%)/HCl (37%) solution in a volumetric ratio 10:1. Pyrex glass (Corning 7740) and soda lime glass were analyzed. In addition, the characterization of the main masking layers, including photoresist, amorphous silicon, polysilicon and Cr/Au for deep wet etching in the optimal solution, is described. Published by Elsevier B.V. Keywords: Wet etching of glass; Surface roughness; Masking layers 1. Introduction Glass is a commonly used material for micromachining. Glass offers high chemical and heat resistance, high electrical isolation, large optical transmission range and low optical absorption. Some of the applications of glass include bio-MEMS, RF-MEMS and optical MEMS devices. Glass can also be used as a packaging material. The classical processing of glass includes drilling with diamond bits, ultrasonic drilling [1], electrochemical dis- charge [2], powder blasting [3] and laser machining. These methods are commonly used for performing holes through the glass wafers. However, they tend to generate a rough surface. Plasma was also used to etch glass, and this typically resulted in a low etch rate of 10 nm/min [4]. Deep reactive ion etching (DRIE) using the ICP system can produce high aspect ratio glass microstructures [5], but it is a very expensive process and the masking layer of 20 Am thick electroplated Ni is relatively difficult to achieve. The maximal etch depth reported was 200 Am. Wet etching of glass is one of the most common and easiest ways for glass micromachining. Wafer thinning [6], via holes etching for wafer level packaging [6] or microchannels for bio-MEMS applications [7], requires long processing time in highly concentrated HF solution (40–49%). The masking layer used for glass micromachining are photoresist [8,9], metal layer Cr/Au [8–10], amorphous silicon [8,11], PECVD amorphous silicon [8], LPCVD polysilicon [8,12], silicon carbide [13] or even bulk single crystal silicon anodically bonded on glass [14]. After etching, the generated surface becomes rough or nonuniformities can be observed at the bottom of the etched cavities. We present in this paper a solution for improving the quality of generated surface using a HF/HCl solution. In addition, the characterization of the main masking layers, photoresist, amorphous silicon, polysilicon and Cr/Au for deep wet etching in the optimal solution, is described. 2. Experiments, conditions and materials In this study, 4U Pyrex glass (Corning 7740) and soda lime glass wafers were used. The initial value of surface roughness for the tested glass wafers was, in both cases, less 0257-8972/$ - see front matter. Published by Elsevier B.V. doi:10.1016/j.surfcoat.2004.10.094 * Corresponding author. Tel.: +65 68247137; fax: +65 64789082. E-mail address: ciliescu@ibn.a-star.edu.sg (C. Iliescu). Surface & Coatings Technology 198 (2005) 314– 318 www.elsevier.com/locate/surfcoat