PIN diodes for digital X-ray detection as a single photon counting sensors were fabricated with a guard ring structure with p+ doping for reducing the leakage current. The efficiency of the guard ring was verified by significantly reduced leakage current compared to the Si-PIN diodes without guard ring structure and the gap distance between the active area and the guard ring was optimized as the leakage currents showed strong dependency on it. In this paper, secondary ion mass spectroscopy (SIMS) profile was measured and characterized to investigate potential process improvement. Since a large transient enhanced diffusion (TED) as the broadening of 200 nm at the tail is observed in the boron SIMS profile, it is suggested to reduce the annealing process time of RTA or to use spike annealing process. Also, in order to investigate the effect of reduced TED or other possible process to achieve shorter junction depth for improving device performance, it is in progress to fully optimize the process simulation incorporating the transient enhanced diffusion model of boron in Si.