Barium, Strontium and Bismuth Contamination in CMOS Processes

摘要

Contamination aspects of ferroelectric (SrBi_2Ta_2O_9) and high dielectric constant (Ba,Sr)TiO_3 materials in CMOS processes were investigated in this paper. TXRF, VPD-TXRF, and ToF-SIMS were used to study the desorption and diffusion properties of Sr, Bi, and Ba contamination on Si at 800℃ anneals. Sr and Ba dissolve in native or thermal oxide, and only very small quantities diffuse into silicon, whereas Bi - if annealed in N2 - evaporates and its diffusion into silicon cannot be detected; in case of O_2 annealing, all Bi is incorporated in the thermal oxide grown. The diffusion constants for Ba and Sr in Si are in the order of 10~(-16) cm~2/s at 800℃. Measurements of minority carrier recombination lifetime of contaminated n and p-type wafers showed no drastic decrease in lifetime for the three elements up to contamination levels of 10~(14) at/cm~2. The leakage currents of diodes contaminated after device processing by Ba, Sr, and Bi up with concentrations between 10~(12) and 10~(14) at/cm~2 have nearly the same value as for non-contaminated diodes. This leads to the conclusion that Sr, Bi, and Ba do not present a drastic risk on minority carrier lifetime and diode leakage current in future Gbit-scale non-volatile Ferroelectric Random Access Memories (NVFeRAMs) or high dielectric constant Dynamic Random Access Memories (high k-DRAMs) generations.