Effects of Metal Gates and Back-End-of-Line Materials on X-Ray Dose in ${\rm HfO}_{2}$ Gate Oxide

The effects of 10-keV and 400-keV endpoint-energy bremsstrahlung x-rays have been studied using the Monte Carlo simulator, MRED, for MOS capacitors with HfO₂ gate dielectrics and TiN and TaN metal gates. We compute the reduction in dose that occurs for 10-keV x-ray irradiation of thin HfO₂ gate dielectrics sandwiched between the metal gate and the Si substrate. We quantify the effects of back-end-of-line metallization layers, including copper interconnects, W vias, and borophosphosilicate glass (BPSG) and SiO₂ passivation layers for low and medium energy x-rays. For thick metallization stacks irradiated by 10-keV x-rays, dose enhancement or attenuation can occur, depending on material type and overlayer thicknesses. For similar stacks irradiated with 400-keV endpoint-energy bremsstrahlung x-rays, significant dose enhancement is observed.