Hafnium Nitrate Precursor Synthesis and HfO 2 Thin Film Deposition

Abstract

The paper will introduce a simple new method for the synthesis of both hafnium and zirconium nitrate precursors. The intermediate product, dinitrogen pentoxide produced from the extraction of water from fuming nitric acid via phosphorus pentoxide, was condensed with a liquid nitrogen trap into a flask containing either hafnium or zironium tetrachloride. To achieve a high yield, the mixture of fuming nitric acid and phosphorus pentoxide was heated to a certain temperature, from which a large quantity of dinitrogen pentoxide had been generated. In the following steps hafnium or zirconium tetrachloride was refluxed over dinitrogen pentoxide at 30 to 35 C for about a half-hour. The product was purified by sublimation. Yields above 95% were obtained. The cost for the hafnium nitrate precursor synthesis was estimated. The precursor was not stable at room temperature, and should be stored in refrigerator in sealed vials. No chlorine was detected from both EDS and chemical analysis. The volatility was evaluated by thermal gravity analysis. For high k thin film applications, the precursors were evaluated for hafnium oxide thin film deposition via an ALD process. High quality hafnium oxide thin films were obtained and the properties were verified to be consistent over several batches of our synthesized hafnium nitrate precursor. X-ray diffraction analysis indicated the films were smooth, uniform, amorphous as deposited and monoclinic after post annealing. For as-deposited hafnium oxide thin films, XPS studies showed that the films were rich in oxygen and contained NO x residues, which could be eliminated with a post deposition forming gas anneal process. In the electrical property measurement, the 57Å hafnium oxide thin film showed a dielectric constant of k ∼ 10.6 and a capacitive equivalent thickness of approximately 21Å.