PRODUCTLASER
Fig. 6: holes with 3mm, 5mm and 10mm diameter in 4mm thick semi-tempered glass
Fig. 4: Schematic of laser drilling of a cylindrical hole with radius r by three dimensional lining-up of single dots
lower than the cost of ownership for mechanical drilling or sandblasting systems. This advantage will pay back the investment during mass production. The laser glass drilling system needs low-maintenance and has almost no consumable parts in comparison to the other two methods.
Process principle
At low intensities glass is transparent for visible wavelengths. This enables focusing of green laser light inside bulk glass. However, if the intensity exceeds the threshold for non-linear absorption (range of GW/cm2) the glass starts to absorb the laser light. In our process the intensity exceeds this threshold in the focal area and the light is transferred into heat. This leads to melting and evaporation of a small amount of glass. Due to the stress induced by the melting and evaporation process micro dots with a size of approximately 100µm occur (Fig. 3). To achieve the needed intensity for non-linear absorption 4JET uses a green short pulse diode pumped solid state lasers with high beam
quality.The next step to a drilled through-hole is lining-up overlapping multiple single dots, layer by layer (Fig. 4). For spatial movement of the focus a scanner with focus shifter
is used. The process direction is from the underside of the glass to the upside of the glass. The distance between the single dots is one of the key-parameters of the laser drilling process and is defined by the speed of the scanner and the frequency of the laser. One layer consists of multiple contours with a spatial distance (dc) as shown in Fig. 4 top view. The number of layers is defined by the thickness (t) of the glass and the optimal distance between the single layers (dl).
This layer wise flexible ablation process allows the creation of endless hole geometries by free spatial placement of dots inside the glass substrate. For example a conic shape hole can be drilled by starting with small circles at the underside of the glass and ending with larger circles at the upside of the glass (Fig. 5 B). Holes with a high aspect ratio (Fig. 5 A) and holes with changing cross sections (Fig. 5 C & D) are other possibilities of this drilling method.
In case there is a coating on top of the glass (e.g. CIGS solar panel) this coating has to be removed before drilling. Local removing of the coating can be done by the drilling laser as well.
Processable glass types
Fig. 5: examples of different through-hole geometries in glass (cross section view)
The method described in this paper can be used to process float glass with various thicknesses at high speed and with high quality. Table 1 shows the time needed for cylindrical holes with different diameters through 3mm float glass. Unique is the possibility to drill holes into semi-
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