Deposition of Nb₂O₅ Film on Large Wafer by Flashing Spray CVD Method

Abstract

A Nb₂O₅ thin film is a DRAM capacitor material with over 4 Gbit because the specific inductive capacity is 60. In CVD process, pentaethoxyniobium (PENb) is usually used as precursor. There are several problems attributed to this such as the thermal decomposition of the superheating, and it is necessary to keep the highest temperature in the evaporator and transport process because the PENb vapor pressure is very low, such as 406 K at 13.3 Pa. To improve the problems, a flashing spray CVD (FS-CVD) method is proposed. In FS-CVD, the precursor is supplied through the injector, which is set at the top of the CVD chamber as a pulsed spray into lower ambient than the precursor vapor pressure in the chamber. The spray is evaporated rapidly and forms a homogene vapor by flash boiling. Also the precursor and low boiling point organic solvent mixed solution is used as a raw material. The high boiling point precursor is converted to lower boiling point material by mixing low boiling point organic solvent with vapor liquid equilibrium. In the experiment, PENb was used as the precursor. The low boiling point material for PENb was selected from vapor-liquid equilibrium considering the mutual solubility and vapor pressure. As a result, ethanol was selected for PENb. Nb₂O₅ film and was deposited on an 8 inch Si wafer by the FS-CVD method. The influence of the film thickness distribution by the distinction of the mixing fraction of ethanol was observed. The mixing molar fraction of ethanol, X was 0.4 and 0.9. Then, the relationship between the injection cycle and film thickness was observed. As a result, the film thickness distribution on an 8 inch wafer is evaluated as +/-14% or less at X=0.4 and +/-2% or less at X=0.9. The distribution of X=0.9 was better than 0.4 due to the rapid and homogeneous PENb and the ethanol vapor formation. The film thickness with molecular scale size might be accomplished by a FS-CVD process.