Radiation hard 3D diamond sensors for vertex detectors at HL-LHC

Diamond is a good candidate to replace silicon as sensor material in the innermost layer of a tracking detector at HL-LHC, due to its high radiation tolerance. After particle fluences of $10^{16}\,{\rm protons/cm^2}$, diamond sensors are expected to achieve a higher signal to noise ratio than silicon. In order to use low grade polycrystalline diamonds as sensors, electrodes inside the diamond bulk, so called 3D electrodes, are produced. Typically, this kind of diamond material has a lower charge collection distance (CCD) than higher grade diamond, which results in a decreased signal amplitude. With 3D electrodes it is possible to achieve full charge collection even in samples with low CCDs by decoupling the spacing of the electrodes from the thickness of the diamond bulk. The electrodes are produced using a femtosecond laser, which changes the phase of the diamond material. The phase changed material is conductive and identified as nanocrystalline graphite using Raman spectroscopy. Due to a crater like structure of the channels on one side of the diamond, contacting of the channels is difficult. With the femtosecond laser setup conductive channels are produced at a laser power of $150\,{\rm mW}$ and a velocity of the diamond sample of $2$ - $10\,{\rm mm/s}$.