Laser drilling with a top-hat beam of micro-scale high aspect ratio holes in silicon nitride

High aspect ratio micro holes are very important functional features in many products, in particular in electronics industry. Especially, the critical requirements that such holes in electronic devices should satisfy, concern their morphology and quality that can impact directly the products' functional performance. At the same time, ultra-short lasers have shown that they have the capabilities to match such tight requirements due to their unique processing characteristics. The typical beam spatial profile of the laser sources used for drilling is Gaussian and this entails some constraints and limitations. In this study, a beam shaping system for laser micro drilling has been designed and implemented to achieve a top-hat spatial profile. The morphology of the high aspect ratio holes in terms of cylindricity, circularity, tapering angle, heat affected zone (HAZ) and penetration depth was investigated by a high resolution X-Ray Computed Tomography (XCT). The capabilities and limitations of such beam shaping solutions for producing micro-scale high aspect ratio holes has been discussed, i.e. their sensitivity to defocusing, and compared to Gaussian beam spatial distribution. Conclusions were made regarding the effects of top-had beams on morphology of high aspect ratio holes and trade-offs when deploying them for laser micro drilling.