TECHNICAL PAPER Investigations into surface topography of glass microfeatures formed by pulsed electrochemical discharge milling for microsystem applications Aman Kumar Verma 1 • Dileep Kumar Mishra 1 • Karan Pawar 1 • Pradeep Dixit 1 Received: 20 January 2020 / Accepted: 1 February 2020 Ó Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract Experimental investigations into the effect of duty cycle and electrolyte concentration on the surface topography in the microchannel formation in the soda-lime glass substrate by a pulsed electrochemical discharge machining are reported. These microstructures are required in various microsystems applications such as microfluidics, integrated-passive-devices for radio-frequency MEMS, 3-D glass interposer. No significant machining happened at lower duty cycles (\ 50%) and lower electrolyte concentration (10%), which was due to the lower surface temperature generated as predicted by the numerical simulation. At these parameters, the tool electrode locally adhered to the glass substrate and prevented the underneath glass surface from being etched periodically. The non-homogeneous etching leads to the uneven surface topography and pillar-like structures, which is termed as the ‘localized adhesion and fast releasing effect. It was observed to be severe when the tool electrode was in contact with the glass substrate, i.e., zero-gap. At this condition, high temperature led to the thermal expansion of the tool electrode, which resulted in the intermittent machining and pillar-like structures in the middle of the microchannel. As the gap was increased, this effect was subsidized, and the surface topography of microchannel improved. Variation in microchannel depth and heat-affected zone (HAZ) at various elec- trolyte concentrations and duty cycles were investigated. Experimental results revealed that the microchannel depth and the HAZ increase with an increase in the duty cycle and electrolyte concentration. Deeper microchannels ([ 500 lm) having complex shapes were demonstrated by using a multi-pass milling technique. The capability of the presented method in making buried redistribution lines in 3D MEMS inductor is also shown. 1 Introduction Due to its optical transparency, biocompatibility, high thermal, and electrical stability, glass-based materials are widely used in various optical, microfluidics, and biomedical applications such as micro-pumps, micro-ac- celerometers, micro-fuel cells, and micro-tool analysis systems (l-TAS) (Zheng et al. 2007). Being an electrically non-conductive and brittle nature, the formation of microstructure in the glass-based materials has remained a challenging task. Processes such as abrasive jet machining (AJM), ultrasonic machining (USM), plasma dry etching by deep reactive ion etching (Kolari et al. 2008) and wet chemical etching, laser processing, etc., used for glass micromachining have specific limitations such as expen- sive tool cost, slow etching rate, high tool wear, limited aspect ratio, etc. (Zheng et al. 2007; Jui et al. 2013). Though wet chemical etching is used for Pyrex glass, it is limited to lower aspect ratio microchannel (Iliescu et al. 2008) and also requires thick hard masks, such as PECVD amorphous silicon as masking materials masking (Iliescu and Chen 2007). In recent years, electrochemical discharge machining (ECDM) has emerged as a low-cost alternative to popularly used laser ablation process. During the ECDM process, the material removal occurs due to the combined action of melting and vaporization and temperature-as- sisted chemical etching (Han et al. 2017; Saranya et al. 2017; Arab et al. 2019; Huang et al. 2019). Compared to AJM, USM, wet etching, the etching rate is higher in the ECDM and also does not require any hard mask layers as in the plasma etching. & Pradeep Dixit pradeep.dixit@iitb.ac.in 1 Electrochemical Microfabrication Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India 123 Microsystem Technologies https://doi.org/10.1007/s00542-020-04770-4