Hafnium Nitrate Precursor Synthesis and HfO2 Thin Film Deposition

The paper will introduce a simple new method on the synthesis of both hafnium and zirconium nitrate precursors. The intermediate product, dinitrogen pentoxide produced from the water extraction from fume nitric acid via phosphorus pentoxide, was condensed by liquid nitrogen trap into a flask equipped with hafnium or zirconium tetrachloride. To give the high yield, the mixture of fume nitric acid and phosphorus pentoxide was heated to a certain temperature, from which large quantity of dinitrogen pentoxide had been generated. In the following step, hafnium or zirconium tetrachloride was refluxed over dinitrogen pentoxide at 30 to 35 oC for half-hour. The product was purified by sublimation. High yield, above 95%, was 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 through the hafnium oxide thin film deposition via ALD process. High quality hafnium oxide thin films were obtained. The hafnium oxide thin film property consistence using different batches of our synthesized hafnium nitrate precursor was also verified. 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 the films were rich in oxygen and contained nitrogen residues,- 2-which could be eliminated from forming gas post-annealing process. On the electrical property measurement, the 57Å hafnium oxide thin film showed a dielectric constant of k ~ 10.5 and a capacitive equivalent thickness of approximately 21Å