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74 The PCB Magazine • November 2015 UV Solid State Lasers In addition to CO 2 lasers, diode-pumped solid-state (DPSS) lasers operating in the ultra- violet (UV) at 355 nm are a well-established source for drilling microvias, and are employed in other microelectronic fabrication tasks, such as wafer and micro-SD card singulation. The UV output of these lasers delivers two impor- tant benefits for micro-via drilling applications. First, with a wavelength that is over ten times smaller than CO 2 and CO lasers, plus their ex- cellent beam quality, these lasers can be focused down to even smaller spot sizes. Second, shorter wavelengths are absorbed more strongly than infrared light by the vast majority of materials, including both metals and dielectrics. Because they don't penetrate as far into the bulk mate- rial during processing, they deliver the ability to more precisely control the depth of the re- moved material, and produce a much smaller heat affected zone (HAZ). Together, these factors make UV DPSS lasers the first choice for producing microvias in the 25–35 µm diameter range. These applications also benefit from the ability of the UV laser to drill both copper and composite lasers. They are generally not used for vias above about 70 µm, since these lasers typically have a higher cost of ownership than infrared (CO and CO 2 ) lasers. One major limiting factor in the overall life- time and service frequency for UV DPSS lasers is the frequency tripling crystal. Specifically, this is the non-linear optical element that converts the native, near infrared output of the solid-state la- ser crystal (typically at about 1 µm wavelength) into the UV. There are two basic configurations in which this crystal can be used. It can be placed intracavity (within the laser resonator) or exter- nal to the laser cavity. Intracavity frequency tri- pling more readily delivers high output power than external cavity tripling. However, expos- ing the crystal to the higher optical powers ex- perienced within the cavity also significantly shortens its lifetime and reduces service intervals for a given power rating. Instead, Coherent UV DPSS lasers for microvia drill- ing use external cavity tripling in order to ensure the highest possible reliability. Using this approach, Coherent is able to supply UV lasers with up to 40W of power that operate for more than 10,000 hours with - out a crystal change, which is more than sufficient for cur- rent microelectronics process- ing tasks. excimer Lasers Excimer lasers are an- other, even deeper UV source (usually at 308 nm or 248 nm), although they have vastly different output char- acteristics than DPSS lasers, leading them to be employed for microvia drilling in a very different manner. Specifical- ly, the pencil-shaped beam AS VIAS SHrInK, oPPorTUnITIeS For LASer DrILLInG exPAnD ArTiCle Figure 2: Schematic illustrating the difference between the direct write and mask-based writing technique for laser via drilling.