ZrO2 Thin Films on Glass Substrates

SpectroycopicElipsometry Characterisation of ZrO2 Thin Filmson Glass Substrates Celine Marchand， David Sheppard - Application Scientists - Thin Film Division Zirconia films are good material for use in many applications such as optical filters， laser mirrors，thermal barrier coatings， and gate dielectrics in microelectronic devices. The films possess high heat resistance with low thermal conductivity， and have a high dielectric con-stant and refractive index. They are very transparent in the visible and near-IR region of the spectrum.Zirconia films are also chemically inert and have a high laser damage threshold. Many methods have been reported for the production of zirconia films including sol-gel spin coating，atomic layer deposition (ALD)， reactive sputtering and pulsed laser deposition. For the samples de-scribed here the ZrO2 films were prepared by the electron beam evaporation of tablets of ZrO2 in areactive oxygen atmosphere. For all applications of zirconia films it is necessary to have an accurate measurement of their thicknessand refractive index， especially when the films are used as the high refractive index material in high-lowindex stacks for antireflection coatings or optical filters. Spectroscopic Ellipsometry (SE) is an ideal technique for this task as it is non-destructive and able to ex-tract both the film thickness and optical constants (refractive index and extinction coefficient) from theobserved data simultaneously. ZrO2 films were prepared by electron beam evap-oration of tablets of ZrO2 in a reactive oxygen at-mosphere. The glass substrates were 5 mm thick.The substrates were rotated during deposition toimprove the uniformity of the deposited films. Results The work was performed using a Jobin YvonUVISEL Visible Spectroscopic Phase ModulatedEllipsometer. The Ellipsometric measurementswere m e at an angle of incidencnceacross the spectral range 300-830 nm. Boththe refractive indexes and thicknesses were ex-tracted simultaneously from the SE data anal-ysis. The analysis were carried out in the transpar-ent region of the material， and the LorentzOscillator formula was used to determine itsoptical constants. This formula minimisesthe number of parameters to calculate. Ifthe analysis is extended to the semi-ab-sorbing part of the spectrum then a Tauc-Lorentz oscillator modelnmaybepre-terred. When compared with RAE / RPE ellipsometers thephase modulated ellipsometer configuration isunique in that it allows high accuracy determina-tion of the Aangle across the full [0°， 360°] range，even when A it is close to 0°. The reason for this isthat A is determined from tanA whereas the RAE /RPE ellipsometer allows only the determination ofcosA. Characterisation of Zirconium Dioxide on Glass Films directly evaporated from the ZrO2 target can be de-scribed accurately using a two-layer model with a higherretractive index at the bottom and a lower retractive indexon the top. The structure employed for the characterisationof the zirconia film is shown below. The best model was found to have the top layer describedby a mixture of 92% ZrO2 and 8% void calculated using theEffective Medium Approximation. The experimental dataand fitted curve are shown in the figure on the right. Thisbehaviour was expected as non-stabilised zirconia films，such as prepared here， have a tendency to exhibit surfaceroughness. For applications where better surface rough-ness is necessary， so that there is les optical scatter， thefilms are modified by the addition of a stabilising materialssuch as Y2O3 ， SiO2，MgO， CaO or CeO2. The opticalproperties calculated for the ZrO2 film areshown opposite. A value of k=0 shows that the film is trans-parent across the wavelength range studied. ZrO2 optical properties Conclusion The deposition method and conditions used for preparation of the ZrO2 films have resulted in generation of an inhomoge-neous porous layer. Owing to the sensitivity of the UVISEL Spectroscopic Phase Modulated Elllippsometer and sottware it ispossiblle to detect this layer， and to characterise its composition while at the same time measuring the film thicknesses andoptical constants. This document is not contractually binding under any circumstances - Printed in France -02/2004 EMISSION·FLUORESCENCE·FORENSICS·GRAIINGS E OEM·OPIICALSPECIROSCOPY·RAMAN·IHIN FILM