glass. [PERSON] et al. [10] investigated the ultrasonic microdrilling of glass to produce deep microholes. [PERSON] et al. [11] used microrotary ultrasonic machining to produce microholes in borosilicate. However, this technique has limitations, such as tool wear and low feedrates.  
This study presents a mechanical drilling process that is cost-effective, simple, and well-suited for rapid prototyping with high-dimensional accuracy. However, since the machinability of mechanical drilling is limited by crack formations at the entrance and exit of holes, studies aimed at reducing and preventing crack formation have been conducted. [PERSON] et al. [12] used a back-up glass plate in the microdrilling of soda-lime glass. They proposed a method to attach soda-lime glass to a back-up plate with water in order to more effectively prevent exit cracks. [PERSON] et al. [13] used a sintered diamond abrasive core tool and an electroplating diamond abrasive core tool in the drilling of soda-lime glass. They investigated the effect of drilling parameters on edge collapse at hole exits. [PERSON] and [PERSON] [14] used two kinds of electroplated tools to minimize crack sizes in the through-hole drilling of soda-lime glass. [PERSON] and [PERSON] [15] developed a drilling system with feedback control based on cutting forces in order to prevent cracks in the drilling of borosilicate glass. [PERSON] et al. [16] used a diamond tool with negative back rake angles to prevent crack formation at the entrance and a sacrificial pad to prevent exit crack formation in the drilling of optical glass and quartz. [PERSON] et al. [17] used a back-tapered electroplated diamond tool when drilling soda-lime glass. The back-tapered tool was effective in reducing crack sizes and improving chip discharge efficiency.  
In order to improve surface quality and accuracy in micromechanical machining, harder tools with high wear resistance are needed, because tool wear increases cutting force during machining. For this reason, PCD as the hardest tool material (except for a single crystal diamond tool) is suitable for the micromachining of hard, brittle materials [18]. [PERSON] et al. [19] used both conical and cylindrical PCD tools to machine microgrooves in soda-lime glass and micropockets in ultra-low expansion glass. They obtained smooth surfaces under ductile-regime machining and evaluated the wear of PCD tools. [PERSON] et al. [20] fabricated microspheric molds and dies in tungsten carbide using a PCD micromilling tool. Tungsten carbide was machined in the ductile mode, and the surface roughness was R\({}_{2}\) 15 nm. [PERSON] et al. [21] used a PCD straight edge micromill to machine slots in tungsten carbide and silicon wafers. [PERSON] et al. [22] used a square PCD end mill to machine silicon carbide. [PERSON] et al. [18] used a PCD grinding tool to machine 3D microstructures in alumina and zirconia.  
The research mentioned above mainly studied the mechanical drilling of general glasses, such as soda-lime and borosilicate. In contrast, the microdrilling of harder glass, such as fused silica, has yet been studied extensively. Fused silica has high hardness, low thermal expansion, and high chemical resistance [23]. Therefore, in this paper, the microdrilling of fused silica using PCD microtools was used to produce microholes with good edge quality and to improve drilling efficiencies.