Evaluation of barriers for B and P diffusion into As-doped polysilicon

Abstract Diffusion barriers are used mainly to prevent ionic contamination and diffusion of interconnect metals into the Si substrate in complementary metal oxide semiconductor devices (CMOS). However, diffusion barriers are equally important in order to prevent B and P diffusion from boro-phospho-silicate glass (BPSG) into highly conductive films like As-doped polysilicon. BPSG is commonly used as a contact level dielectric material because of its superior planarization properties in advanced dynamic random access memory devices (DRAM). Secondary ion mass spectrometry (SIMS) data indicate that B and P from BPSG can diffuse easily into polysilicon during routine annealing without any diffusion barrier. The diffusion of these species affects the sheet resistance of polysilicon. The effectiveness of plasma-enhanced chemical vapor deposited tetraethyl ortho silicate (TEOS) and low-pressure chemical vapor deposited Si 3 N 4 films with varying thickness have been evaluated as barriers to B and P diffusion. The B and P diffusion profiles in polysilicon after annealing have been obtained using SIMS analysis. The experimental data are compared with simulated diffusion profiles. We found a 50 nm thick layer of TEOS to be an effective barrier to B and P diffusion into As-doped poly silicon based on experimental data. The SIMS profiles show an order of magnitude higher concentration of As, B and P at the TEOS and poly-Si interfaces which is attributed to the segregation of dopants at grain boundaries. The SIMS profiles indicate faster diffusion of P towards the interface from the bulk of BPSG in comparison with the results obtained by simulation. Revised diffusion coefficients for B and P in BPSG and poly-Si are determined using the experimental data. The calculated diffusion coefficients for B and P in BPSG are at least 3—4 orders of magnitude higher than the values of these coefficients in SiO 2 .