Dual-wafer intergrinding thinning by bipolar-discharge EDM with a capacity-coupled pulse generator considering large gap capacitance and minimization of discharge energy

The noncontact electrical discharge grinding technique has the potential to effectively grind thin, increasingly large-diameter wafers. This study proposes a bipolar electrical discharge machining (bipolar-EDM) method to realize the intergrinding of dual wafers, in which both wafers serve as the working electrode. A capacity-coupled (CC-type) pulse generator generates regular and identical bipolar discharges, ensuring symmetrical and stable removal of both wafers. Thus, both the energy efficiency and wafer thinning yield are doubled. The process characteristics are evaluated through equivalent circuit analysis and experiments, demonstrating that two thinned wafers exhibit high consistency in surface integrity and material removal rate (MRR). Moreover, the dual-wafer system causes an increase in the circuit resistance and gap capacitance, leading to a much lower discharge energy. This is beneficial for precision wafer grinding and thinning by electrical discharge machining (EDM) while keeping damage minimal. As a result, two phi 20 mm 4H-SiC wafers were thinned simultaneously with a maximum thinning rate of 3 mu m/min. A mirror-like smooth surface (Ra < 80 nm) was obtained under finishing conditions, demonstrating its capability for precise wafer processing.