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76 The PCB Magazine • November 2015 from a DPSS UV laser is typically utilized in a drill, step, and repeat mode. In this mode, the focused laser beam is moved to a desired hole location, the laser drills the hole and, once fin- ished, the laser beam is moved to the next hole location, all in sequence. In this approach, the throughput is linearly dependent on the num- ber of vias being drilled. In contrast, excimer lasers produce a large rectangular-shaped beam that is ideal for use in a mask-based writing process. A photomask containing the pattern of vias for a panel, or re- gion of a panel, is illuminated with the laser. This photomask is then re-imaged onto the work surface, and all holes within the beam sec- tio n are drilled simultaneously. In mask-based writing, laser fluence and pulse frequency dictate the maximum field size that can be exposed at once, but not the total number of holes that can be produced within this field. Thus, as pitch size decreases (and the number of holes produced in a given area increases), paral - lel, mask-based drilling becomes increasingly ef- ficient. In fact, the parallel drilling rate increases with the square of the pitch size. This makes it an increasingly attractive alternative as feature size and spacing decreases, and tends to "future proof" the technique as via diameter and pitch decrease over time. Because of the higher capital cost of excimer lasers, these tools typically make economic sense at production rates around 50– 100 panels per hour or higher, or if the desired feature sizes get down to 5–10 µm. Because of their short wavelength output, excimer lasers have similar proc essing charac- teristics to UV DPSS lasers. Specifically, there is strong absorption by most materials, both metal and dielectrics, which allows them to produce micron-scale feature sizes with nearly zero HAZ. Our laboratory has also investigated the use of excimer lasers for via drilling in glass inter- posers. In these tests, 25 µm diameter holes, with a pitch (hole-to-hole spacing) of 50 µm, were produced in glass substrates ranging in thickness from 100–300 µm. The laser wave- length of 193 nm was used, with a 600 mJ pulse energy, in a mask-based process that produced a fluence of 7 J/cm² at the work surface. The 193 nm wavelength was chosen because glass exhib- its strong absorption at this wavelength. Clean, round, symmetric vias were success- fully produced in all the thicknesses tested us- ing a total of 700 pulses or less. Hole taper was seen in the higher thicknesses, but sequential drilling from both sides of the glass reduced this effect substantially. This is relatively easy, since the transparent glass makes it easy to register fiducial marks on one side of the glass when it is flipped over for drilling from the second side. Overall, this testing showed that vias down to 5 µm diameter could be successfully produced. In addition, mask-based excimer laser abla- tion provides excellent control of feature depth and wall angle. Unlike CO 2 laser via drilling, which usually takes just three laser pulses, ex- cimer laser drilling utilizes numerous pulses, each of which removes just a small amount of material. Thus, via depth is precisely controlled by varying the number of laser pulses delivered. Wall angle is highly dependent upon laser flu- ence, so this parameter can also be varied to produce exactly the desired results. In conclusion, most of the advanced pack- aging techniques that are currently on line, or becoming popular, require microvias that are beyond the capabilities of mechanical drills. In AS VIAS SHrInK, oPPorTUnITIeS For LASer DrILLInG exPAnD ArTiCle Figure 3: excimer laser (193 nm) vias drilled into glass. a) the 25 µm diameter entrance hole; B) the exit hole; and c) the cross-sectional view.

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