## 2 Novel G-THGEMs Manufacturing Process  
The manufacturing process for G-THGEMs is composed of several distinct steps. A visual representation is shown in Figure 1. First, a substrate is selected. The substrate must generally be nonductile, favouring materials such as glass or ceramics. In this work, fused silica and Schott Borofloat 33 substrates were tested. The exact substrate can be tailored according the requirements of the application. For example, synthetic fused silica can be made especially radiopure [22] and would therefore be ideally suited for experiments requiring low backgrounds. Schott Borofloat 33 is relatively lower cost and may be well suited for applications requiring high resistance to thermal shock. For this investigation, all substrates were selected to be 1 mm thick, identical to that of the traditional FR4 THGEM. The next step in the manufacturing process is the formation of electrodes. In this work, a vacuum deposited 150 \(\Omega\)/Sq indium tin oxide (ITO) coating is applied on both sides of the substrate. A simple mask, as shown in orange in Figure 1, is used during the coating process to define the shape of the electrode--in this case, the electrode area is a 163 mm diameter circle on the 200 mm diameter substrates. A photograph of the substrate with ITO coating is shown in Figure 2. ITO was selected predominantly for its relatively low cost and good availability, although many different electrode materials could be applied. The controllable sheet resistance of the ITO coating may provide additional benefits when compared to highly conductive coatings--this is discussed further in Section 3.1. More intricate electrode geometries may also be possible via laser etching of the electrode coating [23]. Figure 1: A visual representation of the novel, patent pending, abrasive machining technique for the production of G-THGEMs. The substrate, now with electrodes on both sides, is ready for the through hole machining process. The position and shape of the through holes are defined using a 2D CAD drawing, such as a drawing exchange format (DXF) file. For the best results during the machining process, the diameter of the holes should typically be at least half of the substrate thickness. For example, for a 1 mm thick substrate, hole diameters \(\geq\)0.5 mm are preferred. Figure 3 shows a schematic of two hole layouts which were tested in this work. One design uses the conventional THGEM dimensions: 0.5 mm diameter holes on a hexagonal array with a hole-to-hole pitch of 0.8 mm. The second design resembles a honeycomb structure, with hexagonal holes on a hexagonal array. This design highlights new capabilities provided by the manufacturing process--no longer being limited to geometries which can be produced by drilling. Figure 3: The hole topographies for (**a**) circular and (**b**) hexagonal masks, with the repeat unit overlayed. These masks dictate the hole structure of the G-THGEMs, formed via abrasive machining. The hole sizes and pattern dimensions are given in Table 1.  
Figure 2: A 1 mm thick, 200 mm diameter glass wafer, coated with a 163 mm diameter indium tin oxide (ITO) electrode on both the top and bottom faces. The resulting mask, shown in purple in Figure 1, which defines the location and shape of the through holes, is applied to both sides of the substrate. This mask is patterned via a photolithography technique, selectively producing areas which resist the abrasive machining process. Then, a series of abrasive delivery nozzles traverse the substrate, selectively abrading the substrate in the regions which are not masked. Typical abrasive materials includes aluminium oxide or silicon carbide, with a particle size of around 20-25 \(\upmu\)m. When performed from one side, the abrasion process produces holes which taper inwards with increasing depth into the substrate. The taper angle (controllable between 6-35 degrees) was measured to be approximately 12 degrees in this work.