Evaluation of Bessel beam machining for scalable fabrication of conductive channels through diamond

Scalable methods must be developed for fabricating high-density arrays of conductive microchannels through similar to 0.5 mm-thick synthetic diamonds in order to form radiation-hard 3D particle tracking detectors for use in future high-energy particle physics experiments, such as those beyond the scheduled 2022 high-luminosity upgrade of the Large Hadron Collider. Prototype detectors with small-area arrays of graphitic columns, each written by slowly translating a femtosecond laser beam focus through the diamond, have established proof of concept, but much faster procedures are needed to manufacture large arrays of electrodes with <100 micron spacing over diameters of similar to 10 cm. We have used a Bessel beam, formed using a 10 degrees axicon and 0.68 NA aspheric lens, to very quickly write micron-diameter columns through similar to 0.5 mm-thick electronic grade CVD diamonds without axially translating the diamond with respect to the beam. We employ an optical microscope to visualize columns, Raman spectroscopy to ascertain the degree of graphitization, and cat-whisker probes to test overall conductivity. Bessel focusing enables formation of a complete column with just a few femtosecond laser pulses, and so provides a scalable manufacturing method. However, reduction in the electrode resistivity is desired. To this end, expulsion of material from the column is probably needed, as carbon plasma will otherwise condense back into diamond, due to the disparate densities of graphite and diamond. We describe the use of several different femtosecond laser systems to evaluate a range of pulse parameters with the goal of increasing the level of graphitization and improving the conductivity of the electrodes.