Influence of stochiometry on the performance of MIM capacitors from plasma-assisted ALD SrxTiyOz films

Strontium titanate, SrxTiyOz (STO), thin films with various cation stoichiometries were deposited by plasma-assisted atomic layer deposition (ALD) using cyclopentadienyl-based metal precursors and oxygen plasma as counter-reactant. [Sr]/([Sr]¿+¿[Ti]) compositions ranging from 0.46 to 0.57 were obtained by changing the (SrO)/(TiO2) ALD cycle ratios. As-deposited 15–30¿nm thick SrxTiyOz films prepared at 350¿°C on Pt-coated silicon substrates were amorphous. Post-annealing at 600/650¿°C for 10¿min under N2 gas resulted in a crystallization into the perovskite phase. Stoichiometric STO and Sr-rich STO films exhibited a certain degree of (111) texture while the Ti-rich STO films showed a lower degree of crystallization. Crystallized layers exhibited a smaller band gap Eg than amorphous ones, while within the stoichiometry series the value of Eg increased with increasing Sr-content. Within the stoichiometry series Pt/STO/Pt structures with Sr-rich STO films showed the lowest leakage current densities. At 1.0¿V values of about 2¿×¿10-8 and 5¿×¿10-6¿A¿cm-2 were obtained for the as-deposited and the annealed films, respectively. Highest capacitance density was obtained for 15¿nm polycrystalline stoichiometric SrTiO3 films resulting in a capacitor equivalent thickness CET of about 0.7¿nm. Pt/SrxTiyOz/Pt capacitors with the STO being in amorphous state exhibited a positive voltage nonlinearity factor a of about 400¿ppm¿V-2, while the negative a-values for crystallized films showed a systematic variation with the stoichiometry, the degree of crystallization and the thickness of the STO layer. This demonstrates that a broad performance range of MIM capacitors is accessible by controlling the stoichiometry and the degree of crystallization of plasma-assisted ALD SrxTiyOz thin films.