Influence of stoichiometry on the performance of MIM capacitors from plasma‐assisted ALD SrxTiyOz films

Abstract

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 α of about 400 ppm V−2, while the negative α‐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.