30 nm thick silicon photodiodes with 100% internal quantum efficiency for VUV spectroscopy of tokamak plasmas

It is known that the knowledge gained with and the performance of present fusion machines is directly linked to its diagnostic capability. Vacuum ultraviolet (VUV) spectroscopy has played an especially important role in plasma diagnostics leading to new tokamak designs and techniques. Thus, the proposed VUV arrays when successfully developed will help improving the burning plasma machines helping DOE in its goal of seeing commercial fusion pilot plant in the 2030s. The harsh environmental conditions expected in future reactors like ITER will severely constrain or even eliminate many key diagnostics and measurements that are presently used in D-D magnetically confined fusion plasma devices. Conventional silicon detectors used presently for plasma diagnosis withstand maximum neutron fluences of about 1013/cm2 of around 1 MeV neutrons. Future detectors will have to withstand fluences in excess of 1017 neutrons/cm2. Thus, development of new kinds of radiation-hardened silicon detectors is needed. Development of 30 nm effective silicon thickness VUV photodiode arrays is proposed during phase 1 research. Because of their minuscule active silicon thickness, these are expected to survive 1019/cm2 neutron fluence levels of future burning plasma machines. A direct VUV filter deposition process will be developed during phase I on test devices. This process will be used to fabricate arrays with directly deposited filter during phase II. These arrays can then be used to measure hydrogen Lyman alpha line intensity with high temporal resolution. Filters used for this application presently are on magnesium fluoride substrate, which absorbs wavelengths below about 115 nm. The direct filter deposition technique will enable the detection of wavelengths as short as 60 nm and lower (EUV) if silicon thickness is increased from 30 nm to 100 nm. The semiconductor market in 2023 was over 580 billion dollars. The entire modern semiconductor wafer processing plants use VUV lasers (the current workhorse, 193 nm machines) or EUV plasma sources (for linewidths 13 nm or less). All these machines need VUV/EUV detectors for light intensity monitoring. The scientific market segments will be all the burning plasma machines, satellites for VUV radiation measurement and synchrotron radiation.