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Fabrication of microchannels in the glass substrate using electrochemical discharge machining process

Name: Dileep Kumar Mishra

Department: Mechanical Engineering

Program: Ph.D (4th  year)

Name of supervisor: Prof Pradeep Dixit

 

Microchannels fabricated in the glass substrates have applications in microfluidics, biomedical devices, lab-on-a-chip etc., due to their optical transparency, biocompatibility, and chemical resistance properties. However, due to the brittle nature of the glass substrates, micromachining using conventional techniques is a challenge. Micromachining of the glass substrates using unconventional methods such as ultrasonic machining, abrasive jet machining, chemical etching, and laser machining either has a high setup cost or has a slow material removal rate. In the last two decades, electrochemical discharge machining (ECDM) has emerged as an economical and faster technique to fabricate microfeatures in the glass substrates. 

ECDM is a hybrid micromachining process of electrochemical machining (ECM) and electric discharge machining (EDM). In the ECDM process, a thin layer of hydrogen gas film is formed around the cathode tool electrode when a potential difference is applied across the cathode and the auxiliary electrode (anode). The glass workpiece is kept very close to the cathode (< 20 µm). Electrochemical discharges are generated when the applied electric field strength exceeds the breakdown strength of the hydrogen film. Due to the electrochemical discharge, high-velocity electrons strike the glass substrate (kept in close vicinity of the cathode) while moving towards the anode/counter electrode. Therefore, the temperature of the glass substrate increases and subsequent material removal occurs due to thermal melting and vaporization as well chemical etching of the glass substrate by the hydroxyl ions at elevated electrolyte temperature.

Several parameters that affect the ECDM process i.e. machining voltage, electrolyte type and concentration, duty cycle, pulse frequency, tool feed rate, etc. Fabrication of deeper microchannels in the glass substrate can be achieved by using a multi-pass approach as demonstrated by Mishra et al. [1]. Fig. 1a-b shows the cross-section image of the microchannel obtained using KOH electrolyte at 50 V as machining voltage, 10 kHz pulse frequency, and 70 % duty cycle. The microchannels depth was > 800 µm obtained after the 12th pass. Using the same technique through or deeper microchannels can be fabricated in the glass substrates. Further, line-array tool electrodes can be used to create many microchannels simultaneously. The use of line-array tool electrodes increases the overall productivity of the ECDM process.  The line-array tool electrodes can be customised by varying their configurations i.e. tip number, tip size and pitch. Mishra et al. demonstrated the fabrication spiral (Fig. 2a-b) and zigzag (Fig. 2c) microchannels using 1×5 and 1×10 line-array tool electrode [2]. Moreover, the microchannels made by the ECDM process can be used to create embedded redistribution lines (RDL) by filling the channels with conductive materials required in the MEMS packaging applications.

 

Fig. 1: Optical image showing the cross-section of the microchannel.

Fig. 2: Fabrication of spiral and zigzag microchannels using the line-array tool electrode.

 

References:

[1] Mishra, D.K., Arab, J., Magar, Y. and Dixit, P., 2019. High aspect ratio glass micromachining by multi-pass electrochemical discharge based micromilling technique. ECS Journal of Solid State Science and Technology, 8(6), p. P322.

[2] Mishra, D.K., Arab, J., Pawar, K. and Dixit, P., 2019, December. Fabrication of deep microfeatures in glass substrate using electrochemical discharge machining for biomedical and microfluidic applications. In 2019 IEEE 21st Electronics Packaging Technology Conference (EPTC) (pp. 263-266). IEEE.