Microwave machining of glass substrate for micro-hole fabrication

Microwave energy finds widespread utility across industrial, scientific, and medical realms for a variety of purposes such as processing, drying, and heating [ 1–3 ]. Yet, a significant challenge arises in the form of thermal runaway, wherein microwave-induced heating triggers a swift and irregular rise in localized temperature, generating detrimental hot spots. While typically undesirable, this phenomenon was deliberately harnessed by Jerby et al. for the first time in 2002 for the drilling of various engineering materials [ 3 ]. Jerby et al. demonstrated the application of microwave-based drilling in silicon, glass, ceramics, rock, etc. [ 3–8 ]. In this process, microwave drilling functions as a heat-assisted machining technique, where a metallic tool converts electromagnetic energy into localized thermal energy. This converted thermal energy is subsequently employed for the machining process on the workpiece [ 3 , 9–11 ]. The experimental drilling system consisted of a magnetron, tuner, coupler, transmission line, and antenna. By focusing microwaves locally, a hot spot was generated beneath the tungsten tool, allowing it to penetrate materials such as glass, concrete, ceramics, basalt, and silicon. This technique successfully drilled a 2-cm-deep hole in concrete within one minute, delivering accuracy comparable to that of conventional mechanical drilling [ 3 ]. Later, Meir et al. reported that thermal-runaway instability can be induced even at low input microwave power ranging between 10 and 100 W from solid-state devices, leading to localized melting within seconds in various materials in 2012. Stepwise drilling progress was demonstrated in a glass plate of 4 mm thickness at an input power of 70 W. The formation of two successive hotspots at different depths was observed, which was evident from a drop in microwave reflection. The initial hotspot facilitated penetration, with further drilling contingent upon the formation of a second hotspot below. To address the issue of removing soft, porous debris during the drilling process, it was suggested to employ a slowly rotating reamer. This reamer could be utilized either independently or integrated as a hollow reamer on the outer cylinder of the coaxial structure [ 6 ]. Lautre et al. used a standard domestic multi-mode microwave applicator (440 × 420 × 210 mm) and a metallic concentrator for material drilling. The drilling process was performed at power levels between 90 and 900 W, with duty cycles ranging from 20% to 100%, on materials such as soda-lime glass. Metallic tools like copper and stainless steel were employed to concentrate microwave energy at their tips, where plasma formation enabled material removal [ 11–13 ]. Likewise, Kumar and Sharma drilled borosilicate glass at a microwave input power of 700 W using graphite and WC tools, incorporating transformer oil as a dielectric medium to improve hole quality. Techniques including controlled feed rate, a constant machining gap, and preheating were adopted to improve hole quality, while immersing the tool and the tool–workpiece interface in transformer oil to a depth of 45 mm significantly reduced HAZ, minimized overcut, and improved hole circularity [ 10 , 14 , 15 ]. The subsequent section discusses the development of the experimental setup of microwave drilling of glass, the theory of microwave drilling, the selection of process parameters, and the effect of process parameters on the surface integrity of the hole.