Influence of sputtering power and the substrate–target distance on the properties of ZrO2 films prepared by RF reactive sputtering

Thin Solid Films 377᎐378 Ž2000. 557᎐561

Influence of sputtering power and the substrate ᎐target distance on the properties of ZrO 2 films prepared by RF reactive sputtering Pengtao Gao a , L.J. Meng b,U , M.P. dos Santos a , V. Teixeiraa , M. Andritschky a b

a Departamento de Fisica, Uni¨ ersidade do Minho, 4710 Braga, Portugal Departamento de Fisica, Instituto Superior de Engenharia do Porto, Rua de Sao ˜ Tome, ´ 4200 Porto, Portugal

Abstract ZrO 2 films have been prepared by RF reactive sputtering using different substrate ᎐target distances and RF powers. The films have been characterized by X-ray diffraction ŽXRD., scanning electron microscopy ŽSEM., and optical spectroscopy. Both monoclinic and tetragonal phases have been found in the films. All the films show a random orientation. The crystallite size increases as the substrate ᎐target distance decreases and the RF power increases. It has been found that the residual stress of the films is mainly caused by the intrinsic stress. Also, the influence of substrate ᎐target distance and RF power on the optical properties of the films has been discussed. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Thin film; ZrO 2 ; Sputtering

1. Introduction ZrO 2 films are widely used as optical coatings due to their excellent properties, such as a high refractive index, a broad region of low absorption from the nearUV Žabove 240 nm. to the mid-IR Žbelow 8 m., and a high pulse-laser damage threshold. Pure ZrO 2 films have been found to exist in the monoclinic phase w1,2x, tetragonal phase w3x, cubic phase w2,4x and an amorphous w5x structure. The structure depends on the method and conditions of preparation. ZrO 2 films prepared by reactive electron beam evaporation w6x, sputtering w3,7,8x, ion-assisted deposition w9,10x, pulsed laser deposition w11x and sol᎐gel processing w12,13x have been reported. In the case of reactive sputtering, it is well known that the microstructure and properties of the films are strongly influenced by the deposition conditions. In previous work, we have studied the influence of the O 2 concentration in the sputtering gases on the U

Corresponding author.

microstructure and optical properties ZrO 2 films w14x. In this work, the influence of the sputtering power and of the substrate ᎐target distance on the microstructure and optical properties of ZrO 2 films prepared by RF reactive sputtering have been studied. In addition, the residual stress of the films, which can strongly affect the film adhesion and other properties, plays an important role in many applications. Hence the influence of sputtering power and the substrate ᎐target distance on the residual stress of ZrO 2 films have also been studied. 2. Experimental ZrO 2 films were prepared by the RF reactive magnetron sputtering method ŽAlcatel SCM 650.. The purity of the Zr metal target was 99.5%, with a diameter of 200 mm. Microscope glass slides were used as substrates. Before deposition, the vacuum chamber was evacuated, using a turbomolecular pump, to 6 = 10y4 Pa. The sputtering gas, Ar with a purity of 99.99%, and the reactive gas, O 2 with a purity of 99.99%, were

0040-6090r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 2 9 1 - 8

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introduced to the chamber separately and controlled by standard mass-flow controllers. An O 2 gas concentration of 5%, defined as the ratio of O 2 mass flow rate to the total mass flow rate ŽAr and O 2 ., was used during the film deposition. The sputtering pressure was 5 = 10y1 Pa and substrates were unheated for all the samples. Before deposition, the target was cleaned by sputtering with an RF power of 500 W for 5 min within a pure Ar atmosphere while the substrate was covered with a shield. Two sets of films were prepared. One was prepared at a fixed RF power of 1000 W, with changing the substrate ᎐target distance from 60 to 90 mm. Another was prepared at a fixed substrate ᎐target distance of 60 mm, changing the RF power from 600 W to 1000 W. The microstructure of the films was analyzed by X-ray diffraction with the 2␪ angle in the range of 20᎐70⬚ using CuK ␣ Ž40 kV, 20 mA. radiation in steps of 0.02⬚. Peak position, and full width of peak at half maximum intensity ŽFWHM., were obtained by fitting the measured peaks with two Gaussian curves in order to find the true peak position and width corresponding to monochromatic K ␣1 radiation. The average crystallite dimension of films, D, was calculated using the formula of D s 0.9␭rBcos␪ neglecting the micro strain w15x, where ␭ is the X-ray wavelength, ␪ is the Bragg diffraction angle and B is the FWHM after correction for instrument broadening. The residual stress of the films was calculated by Eq. Ž1. w16x, ␴1 q ␴2 s y

Ec dy d o ⭈ ␥c do

Ž1.

where d is the crystallite plane spacing of the films, d o is the standard plane spacing from ZrO 2 powder X-ray diffraction files Ž0.3698 nm for the monoclinic phase of ZrO 2 Ž110. plane., Ec s 170 GPa w17x is Young’s modulus and ␥c s 0.28 w18x is the Poisson ratio for ZrO 2 films. The surface and cross-section microstructure of the films were observed by SEM. Specular transmittance spectra were measured by a Shimadzu double beam spectrophotometer UV-3101PC, which can perform various measurements over a wavelength range of 200᎐3200 nm. Thickness, refractive index and the extinction coefficient of films were calculated from the specular transmittance spectra using Swanepoel’s theory w19x. Slit widths of 1 and 20 nm were selected for the measurement of specular transmittance and integral diffuse reflectance spectra, respectively. 3. Results and discussion Fig. 1 shows X-ray diffraction pattern of samples prepared using different substrate ᎐target distances and RF powers. The peak at approximately 30⬚ is attributed to diffraction from Ž111. planes of the tetragonal phase

Fig. 1. X-Ray diffraction spectra of the films prepared at different substrate ᎐target distances Ža. and RF powers Žb..

of ZrO 2 . The others are attributed to diffraction from the monoclinic phase. No other diffraction peak from the tetragonal phase can be found. The intensity of the peak from the tetragonal phase is very weak, which indicates that the fraction of tetragonal phase in the films is very low. Thus, the dominant phase in all our films is the monoclinic phase. The microstructure of films prepared by electron beam evaporated ZrO 2 films w1x and the results of Kim w3x and Mehner w12x do not agree with ours. In all cases, these films showed random orientation. The intensity of monoclinic peaks m Žy111. and m Ž111. Žat approx. 28 and 31⬚, respectively. increase as the RF power decreases and the substrate ᎐target distance increases. On the other hand, the peak m Ž220. Žat approx. 34⬚. disappears as the RF power decreases and the substrate ᎐target distance increases. It seems that the films have favored growth orientations along m Žy111. and m Ž111., as the average energy of condensed particles is low. In the case of a high average energy of the condensed particles on the other hand, the films have a favored orientation along m Ž220.. Fig. 2 shows the surface and cross-section scanning electron micrographs of ZrO 2 films prepared using different substrate ᎐target distances and RF powers. It is seen that the surface of the films becomes rougher and the grain size along the surface increases with the substrate ᎐target distance increase and the RF power decrease. A normal columnar and dense structure can be found for films prepared at a low substrate ᎐target distance and high RF power Žsee Fig. 2b. from the cross-section view. A random packed structure, which means more grain boundaries, is evident for the films prepared at a large substrate ᎐target distance and low RF power Žsee Fig. 2a,c.. By fitting the measured X-ray diffraction peak of m Ž110. Žat approx. 24⬚., the average crystallite size of the films has been calculated. The average crystallite size increases as the RF power increases and the substrate ᎐target distance decreases, as can be seen in Fig. 3. This can be related to the energy of the condensed particles. When the substrate ᎐target distance decreases, the energy of condensed particles increases

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Fig. 2. SEM micrographs of ZrO 2 films prepared at different substrate ᎐target distances and RF powers Ža. 600 W, 60 mm; Žb. 1000 W, 60 mm; Žc. 1000 W, 90 mm.

due to the decrease of scattering. On the other hand, the energy of sputtered particles increases with an increase in sputtering power. High-energy condensed particles have a high surface mobility to form large crystallites. In addition, the temperature of the substrate was higher during film deposition in the case of low substrate ᎐target distance and high power, because that part of the energy of the condensed particles transforms to thermal energy, and thus favors the surface mobility to form large crystallites. From the fitting result of the m Ž110. peak, the residual stress of the films has been calculated by using Eq. Ž1.. The variation in residual stress of the films with substrate ᎐target distance and RF power can be seen in Fig. 4. All calculated values are negative, which indicates a compressive stress in all our films. In general, thermal and intrinsic stresses contribute to the residual stress of the films. The thermal stress can be calculated from ␴thermal s Ž ␣ s y ␣ c .ŽT y To . EcrŽ1 y ¨c ., where ␣ s s 4 = 10y6 Ky1 w20x and ␣ c s 10 = 10y6 Ky1 w21x are the thermal expansion coefficient of the substrate and the films, respectively; T and To are the substrate temperature after and during the film deposition; Ec and ¨c are Young’s modulus and the Poisson ration for ZrO 2 films. It can be found that ␣ s - ␣ c and T - To . Thus the stress calculated from this equation would be a positive value, which indicates the temperature-induced stress in the films is tensile stress. The final residual stress calculated from Eq. Ž1. is compressive stress. This means that the intrinsic stress caused by grain boundaries, dislocations, crystallite size, etc., is compressive stress, and is larger than the thermal stress. The dominant factor on the residual stress in the films

is the intrinsic stress. Considering Fig. 3 and Fig. 4 together, it can be found that films having a bigger crystallite size, which means fewer crystallite boundaries and dislocations, have a lower residual stress. It also has been found that the residual stress increases as the substrate ᎐target distance increases and the RF power decreases. In the case of a low substrate ᎐target distance and high RF power, the temperature of the substrate is high during deposition, because the energy of condensed particles is high. According to the equation mentioned above, the thermal stress is higher for films deposited at high temperature than that of films deposited at low temperature. The thermal stress in our case is a tensile one, therefore its contribution to the overall residual stress is to counteract the compressive intrinsic stress. This is the reason that the residual stress increases as the substrate ᎐target distance increases and the RF power decreases. Fig. 5 shows the variation in refractive index at 550 nm with the substrate ᎐target distance and RF power. The refractive index decreases and increases, respec-

Fig. 3. The variation of average crystallite size with substrate ᎐target distance Ža. Ž60 mm. and RF power Žb. Ž1000 W..

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P. Gao et al. r Thin Solid Films 377᎐378 (2000) 557᎐561

Fig. 4. The variation of residual stress with substrate ᎐target distance Ža. and RF power Žb..

tively, with increasing substrate ᎐target distance and RF power. These refractive index values are lower than that of monoclinic ZrO 2 crystal Ž2.19. w22x and films prepared by the CVD method w23x. One reason for the variation in refractive index is probably the change in microstructure for the films prepared at different substrate᎐target distances and RF powers. On the other hand, the film refractive index is proportional to the film density w24x. The packing density of the films prepared at a large substrate ᎐target distance and low sputtering power are low Žthis can be seen in Fig. 2.. Thus, this is probably another reason that the refractive index decreases and increases, respectively, with increasing substrate ᎐target distance and RF power. The variation in extinction coefficient at 550 nm with substrate ᎐target distance and RF power is shown in Fig. 6. This shows that the extinction coefficient increases and decreases, respectively, with increasing substrate ᎐target distance and RF power. The extinction coefficient can be affected by two factors, one is absorption and the other is scattering. The absorption can be neglected in the visible region, due to the high optical band energy for ZrO 2 films. Thus, optical loss in the films is mainly caused by scattering. A large crystallite size is always associated with low dislocation and crystallite boundaries. Considered together with Fig. 3, films having a large crystallite size have a low extinction coefficient

Fig. 6. The variation of extinction coefficient Ž550 nm. with substrate᎐target distance Ža. and RF power Žb..

distances and RF powers have a dominant structure of the monoclinic phase and a small fraction of the tetragonal phase. All the films have random orientation. The average crystallite size increases with the increasing sputtering power and decreases with increasing substrate᎐target distance, because the surface mobility of the condensed particles is higher in the case of a higher sputtering power and shorter substrate ᎐target distance. The surface of the films becomes rougher as the sputtering power decreases and the substrate ᎐target distance increases. The residual stress of the films increases as the substrate ᎐target distance increases and the RF power decreases, and the intrinsic stress is the dominant factor on the residual stress of the films. The refractive index of the films decreases and the extinction coefficient increases, respectively, as sputtering power decreases. The refractive index of the films decreases and the extinction coefficient increases as the substrate ᎐target distance increases. Acknowledgements Pengtao Gao is grateful to the Orient Foundation for providing a scholarship and also wishes to thank A. Azevedo and Fernanda Guimaraes ˜ from the University of Minho for XRD and SEM measurements. References

4. Conclusion ZrO 2 films prepared using different substrate ᎐target

Fig. 5. The variation of refractive index Ž550 nm. with substrate ᎐ target distance Ža. and RF power Žb..

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