Influence of process parameters on the structure and the properties of ZrO2 coatings deposited by reactive pulsed magnetron sputtering (PMS)

Thin Solid Films 377᎐378 Ž2000. 37᎐42

Influence of process parameters on the structure and the properties of ZrO 2 coatings deposited by reactive pulsed magnetron sputtering ŽPMS. Klaus Goedicke, Jorn-Steffen LiebigU , Olaf Zywitzki, Hagen Sahm ¨ Fraunhofer-Institut fur ¨ Elektronenstrahl- und Plasmatechnik, Winterbergstrasse 28, D-01277 Dresden, Germany

Abstract Thin ZrO 2 layers in the thickness range between 100 and 150 nm were deposited by reactive pulsed magnetron sputtering ŽPMS.. The influence of the sputtering Ar pressure and of the target to substrate distance on structure and properties of the films were investigated. The structure of the layers was determined by grazing angle XRD. At low sputtering pressure of 0.3 Pa, the low temperature stable monoclinic modification of ZrO 2 is deposited, while at a sputtering pressure of 3.5 Pa, the high-temperature cubic phase of ZrO 2 can be obtained. Atomic force microscopy investigations have shown that with higher sputtering pressures the roughness of the deposited layers is increased. The residual stresses were also drastically influenced by the sputtering pressure. In the layers deposited at low sputtering pressures of approximately 0.3 Pa very high compressive stresses of up to 1800 MPa are present. With further increase of sputtering pressure these high compressive residual film stresses were decreased down to low tensile stresses of approximately 150 MPa. After a storage time of 1 month in air only small changes in the film stress values were measured. The hardness and Young’s modulus of the layers were determined by nanoindentation techniques at an indentation depth of 20 nm. The results show that with increasing sputtering pressure the hardness and the Young’s modulus of the layers are decreased from approximately 12.3 to 6.2 GPa and from approximately 173 GPa to 150 GPa, respectively. The refractive index Žat ␭ s 550 nm. of the layer deposited at low sputtering pressure is 2.2. With increasing sputtering pressure the refractive index is shifted to 2.1. The thickness of a surface roughness layer calculated by effective medium approximation is increased with increasing sputtering pressure from 2.5 to 11.5 nm. 䊚 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Ziconium oxide; Process parameters; Reactive pulsed magnetron sputtering

1. Introduction Zirconium oxide is widely used as layer material, e.g. for thermal barrier coatings on turbine blades w1,2x and as intermediate buffer layer for superconductive YBa 2 Cu 3 O 7yx layers w3x. For optical applications ZrO 2 layers are used because of their high refractive index and the low dispersion in the visible and infrared

U

Corresponding author. Tel.: q49-35973-23257; fax: q49-3512586-55623. E-mail address: [email protected] ŽJ. Liebig..

regions and the small optical absorption. Applications include laser mirrors, edge or broad band filters w4,5x. Processes used for the deposition of optical layers are reactive evaporation, reactive low-voltage ion plating and magnetron sputtering. Without plasma assistance the layers have generally a high microporosity and therefore a comparably low and often also inhomogeneous refractive index. The microporosity is further connected with the absorption of water in humid atmospheres, which can shift the optical and mechanical properties. Drastic improvements of the microstructure can be achieved by plasma-assisted processes. Coatings de-

0040-6090r00r$ - see front matter 䊚 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 3 8 1 - X

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K. Goedicke et al. r Thin Solid Films 377᎐378 (2000) 37᎐42

posited by reactive low-voltage ion plating have a dense microstructure and therefore a high and also homogeneous refractive index and good chemical stability w6,7x. In the present paper we present some results concerning structure and properties of thin ZrO 2 layers deposited by reactive pulsed magnetron sputtering ŽPMS.. The PMS operated in the medium frequency range of 10᎐100 kHz overcomes the well-known process instabilities of the conventional dc magnetron sputtering, such as drifting potentials caused by disappearing anode and arcing w8x. Recent results have shown that by pulsed magnetron sputtering some additional interesting effects on the structure of the layers can be achieved. For example the thermodynamically stable phases of TiO 2 Žrutile. w9,10x and of Al 2 O 3 Žcorundum. w11x can be deposited at lower substrate temperatures than with conventional DC or RF magnetron sputtering. An explanation for the observed behavior is the high power density within the pulses. This high power density is connected with a higher energy and mobility of the condensing particles w11x. The main interest of the present work was the deposition of ZrO 2 layers by pulsed magnetron sputtering with a relatively high density and with moderate residual stresses for sufficient adhesion to the substrate. As shown in this presented work sputtering pressure and the target-to-substrate distance have a significant influence on the intrinsic stresses of thin oxide layers. We have varied these process parameters to investigate their influence on optical and mechanical film properties. In addition we present some XRD and AFM results with respect to structure and surface topography of the ZrO 2 layers. 2. Experimental 2.1. Deposition Zirconium oxide thin films were deposited by reactive PMS in a multi-chamber in-line sputtering equipment. A dual magnetron system consisting of two magnetron sources with 610 mm= 160 mm targets was powered by a bipolar sine wave generator at a medium frequency Žmf. of approximately 50 kHz. The sputtering was carried out from metallic zirconium targets at a controlled argon pressure. To ensure the stoichiometry and the transparency of the layers we used an additional gas manifold for the oxygen in the vicinity of the dual magnetron system and controlled the oxygen gas inlet by an optical emission monitor. As substrates we used glass slides and silicon wafers without substrate heating. The observed increase of the substrate temperature was at most 30⬚C, measured by thermo strips. The thickness of the zirconium oxide films varied between 100 nm and 150 nm. The process parameter target to substrate distance TSD was set to

Table 1 Process parameters for the deposition of ZrO 2 by reactive pulsed magnetron sputtering ŽPMS. Target material Target area Substrates Substrate temperature Power Argon pressure Flow rate of argon Flow rate of oxygen Target-to-substrate distance

Zr, purity 99.7% Two times 610 = 160 mm2 Glass, Si-wafers 20᎐50⬚C 15 kW Žmf. 0.3 . . . 3.5 Pa 8 . . . 80 sccm 20 . . . 27 sccm 90 and 170 mm

90 mm and 170 mm, respectively. The sputtering pressure was modified by increasing Ar-flow between 0.3 and 3.5 Pa. The thin films were deposited by controlling the medium frequency power. All relevant deposition parameters are summarized in Table 1. 2.2. Characterization A Dektak profilometer was used to measure the layer thickness on partly masked glass substrates. The structure of the ZrO 2 layers was investigated by grazing angle XRD ŽRD 7, Seifert-FPM. using Cu K ␣ radiation with an angle of incidence of 1⬚. Atomic force microscopy was applied to characterize the surface topography and the roughness of the layers. The AFM ŽTopometrix, Explorer. was operated in the non-contact mode. Hardness and Young’s modulus of zirconium oxide thin films on silicon substrates were measured by nanoindentation techniques ŽNanoindenter XP. using a Berkovich indenter. For calculation of hardness and Young’s modulus the method of Oliver and Pharr was applied w12x. The stiffness of the contact was measured continuously during the whole indentation process. This enables the exact calculation of the mechanical properties as a function of the indentation depth. To eliminate the substrate influence, the hardness and Young’s modulus are evaluated at an indentation depth of 20 nm. Residual stress of the coatings was determined by measurement of the deflection of the substrates before and after coating. To determine the alteration of residual stress with storage time in air at room temperature, measurements 1 week and 1 month after deposition were performed. The optical properties of the ZrO 2 layers were investigated by Perkin-Elmers spectrometer Lambda-19 in the range from 380 to 2000 nm immediately after deposition to ensure layers with very low absorption. Spectroscopic ellipsometry ŽSentech, SE850. in the spectrum range of 350᎐1000 nm was used to evaluate the refractive index and the thickness of a surface

K. Goedicke et al. r Thin Solid Films 377᎐378 (2000) 37᎐42

Fig. 1. XRD diagram of ZrO 2 layer deposited by PMS at sputtering pressure of 0.3 Pa.

roughness layer by effective medium approximation ŽEMA.. 3. Results 3.1. Structure All coatings investigated by grazing angle XRD contain crystalline phases of ZrO 2 . The results show a significant influence of the sputtering pressure on the observed crystalline phases. The layer deposited at low sputtering pressure of 0.3 Pa shows only one strong and relatively broad diffraction peak ŽFig. 1., which can be assigned to the Ž111. reflex of the monoclinic phase of ZrO 2 ŽASTM 36-420.. Some additional very small peaks couldn’t be assigned to any ZrO 2 modification. In comparison at higher sputtering pressure of 3.5 Pa

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Fig. 2. XRD diagram of a ZrO 2 layer deposited by PMS at sputtering pressure of 3.5 Pa.

the high-temperature cubic phase of ZrO 2 could be clearly identified by XRD ŽFig. 2.. All reflections of the cubic phase are present and show a good conformity with the published values ŽASTM 27-997.. The full widths at half maximum of the Ž111. reflex of the monoclinic phase and of the Ž111. reflex of the cubic phase were used for an additionally rough estimation of the grain sizes. Thereafter the mean grain size of the monoclinic phase amounts to 15 nm and of the cubic phase 18 nm, respectively.

3.2. Surface topography

The layer deposited at low sputtering pressure of 0.3 Pa has a very smooth surface topography with a maximum height of 5 nm. The roughness average R a amounts to 0.3 nm ŽFig. 3..

Fig. 3. AFM image of a ZrO 2 layer deposited by PMS at sputtering pressure of 0.3 Pa.

K. Goedicke et al. r Thin Solid Films 377᎐378 (2000) 37᎐42

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Fig. 4. AFM image of a ZrO 2 layer deposited by PMS at sputtering pressure of 3.5 Pa.

At the highest investigated sputtering pressure of 3.5 Pa the maximum height is increased to 10 nm and the corresponding R a value is 1.4 nm, respectively ŽFig. 4.. The width of the visible columns were estimated in the top view of the AFM images. For the layers deposited at sputtering pressure of 0.3 Pa the width of columns is between 15 and 20 nm and for the layers deposited at a sputtering pressure of 3.5 Pa it is between 50 and 100 nm. 3.3. Mechanical properties Fig. 5 shows that the residual stress of the ZrO 2 layers is strongly influenced by the sputtering pressure. Starting at low sputtering pressures of approximately 0.3 Pa high compressive stresses of approximately 1800

MPa are present. With increasing sputtering pressure the stress is monotonously decreased. At a sputtering pressure between 2.5 and 3 Pa the residual stresses approach zero. A further raising of the pressure to 3.5 Pa is connected with low tensile stresses. Furthermore, the results suggest that at a constant sputtering pressure the compressive stress is decreased by a larger target-to-substrate distance. After a storage time of 1 week and 1 month only small changes in the stresses are observed. A tendency of small reduction of the tensile stresses can be observed for the layers deposited at high sputtering pressures Žsee Table 2.. The hardness of the ZrO 2 layers deposited at low sputtering pressure of 0.3 Pa amounts to 12.3 GPa and the Young’s modulus to 173 GPa, respectively. The increase of sputtering pressure to 3.5 Pa is connected with a decrease of the hardness to approximately 6.2 GPa and the Young’s modulus to 150 GPa, respectively. Table 2 Time behavior of the residual stresses of ZrO 2 layers Žstorage at room temperature in air. deposited by pulsed magnetron sputtering in dependence of sputtering pressure at a target-to-substrate distance of 170 mm

Fig. 5. Residual stresses of ZrO 2 layer deposited by PMS in dependence on the sputtering pressure and the target-to-substrate distances.

Sputtering pressure ŽPa.

As-deposited

Stress ŽMPa. 1 week

1 month

0.33 1.2 1.6 3 3.5

y1840 y380 y144 61 139

y1838 y382 y144 46 103

y1830 y393 y161 36 103

K. Goedicke et al. r Thin Solid Films 377᎐378 (2000) 37᎐42

Fig. 6. The diagram shows results of spectroscopic ellipsometry for ZrO 2 layers deposited by PMS in dependence on the sputtering pressure.

3.4. Optical properties All ZrO 2 layers deposited by reactive pulsed magnetron sputtering are, in the visible and near infrared regions of the spectrum, practically free of absorption with a k value of approximately 10y5 . The measured dispersion curves show a dependence of the refractive index on the sputtering pressure ŽFig. 6.. With increasing sputtering pressure from 0.3 Pa to 3.5 Pa the refractive index Ž ␭ s 550 nm. is decreased from 2.2 to 2.1. Furthermore, the thickness of surface roughness layers were calculated by effective medium approximation. The results show that the thickness of a surface roughness layer is increased with increasing sputtering pressure from 2.4 nm at 0.3 Pa to 8 nm at 1.2 Pa to 11.4 nm at 3.5 Pa. 4. Discussion ZrO 2 exists in three different crystallographic modifications: monoclinic, tetragonal and cubic. In the equilibrium state up to temperatures of 1200⬚C the monoclinic phase is dominant. At a temperature of approximately 1200⬚C the phase transformation from monoclinic into the tetragonal phase takes place. There is further a transformation from tetragonal to cubic at approximately 2370⬚C. The high-temperature phases, tetragonal and cubic ZrO 2 , can be stabilized at low temperatures by addition of the oxides Y2 O 3 , CeO 2 , and MgO w13,14x. It is well known that the deposition of thin films from the vapor phase is far away from the equilibrium state. Therefore non-stabilized ZrO 2 layers deposited by DC magnetron sputtering without substrate heating are reported to be amorphous. Only after an annealing procedure of 1 h at 400⬚C the monoclinic phase can be found w15x. ZrO 2 layers deposited at substrate temperature of 150⬚C by RF magnetron sputtering contain still

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a phase mixture of the amorphous and the monoclinic phase w16x. In contrast, non-stabilized ZrO 2 layers deposited by pulsed magnetron sputtering contain predominately the monoclinic phase and a minor volume of the metastable tetragonal phase w13,17x. This is in agreement with our results. In a ZrO 2 layer deposited at low sputtering pressure of 0.3 Pa and with a substrate temperature below 50⬚C we identified predominately the monoclinic phase. We assume that the higher energy of the condensing particles, caused by rapid increase of power densities within the pulse, can favor the formation of the thermodynamically stable monoclinic ZrO 2 phase. The measured hardness of 12.3 GPa, the Young’s modulus of 173 GPa and the refractive index of approximately 2.2 are completely comparable with the bulk material baddeleyite, which has a hardness of 10᎐12 GPa; a Young’s modulus of 168.7 GPa and also a refractive index of 2.2 w18x. It is especially remarkable such properties are already achieved without application of an additional mf bias voltage. At high sputtering pressure the layers completely consist of the high-temperature cubic phase. The higher sputtering pressure reduces the mean free path of the sputtered particles and therefore the mobility of the incoming particles on the substrate. This lower mobility is connected with a microporosity of the layers and therefore a reduced hardness and lower compressive residual stress. The lower energy of the condensing particles is the reason for the microporosity and also probably for the stabilization of the metastable high temperature cubic phase. The microporosity in the coatings deposited at higher sputtering pressures is further connected with an increase of surface roughness and a relatively small reduction of the refractive index. 5. Conclusions The pulsed magnetron sputtering ŽPMS. enables the long time stable deposition of oxides without process instabilities. The results have further shown that by PMS ZrO 2 layers can be deposited with a high quality for optical applications. Layers with bulk-like properties with respect to refractive index, hardness and Young’s modulus can be deposited at low sputtering pressures. The residual compressive stresses of the layers can be drastically reduced by an increasing sputtering pressure, which is connected with a higher microporosity of the layers, but still acceptable optical properties. References w1x W. Beele, G. Marijnissen, A. van Lieshout, Surf. Coat. Technol. 120r121 Ž1999. 61.

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w2x E. Reinhold, P. Botzler, C. Deus, Surf. Coat. Technol. 120r121 Ž1999. 77. w3x H. Tomaszewski, J. Haemers, N. De Roo, J. Denul, R. De Gryse, Thin Solid Films 293 Ž1997. 67. w4x K.H. Guenther, B. Loo, D. Burns, J. Edgell, D. Windham, K.H. Muller, J. Vac. Sci. Technol. A7 Ž3. Ž1989. 1436. ¨ w5x M.A. Russak, C.V. Jahnes, E.P. Katz, J. Vac. Sci. Technol. A7 Ž3. Ž1989. 1248. w6x S. Pongratz, A. Zoller, J. Vac. Sci. Technol. A 10 Ž4. Ž1992. ¨ 1897. w7x H.K. Pulker, Surf. Coat. Technol. 112 Ž1999. 250. w8x S. Schiller, K. Goedicke, V. Kirchhoff, in: Pulsed Technology ᎏ A New Era of Magnetron Sputtering, SVC 38th Annual Technology Conference, Chicago, 1995. w9x J. Szcyrbowski, G. Brauer, M. Ruske, J. Bartella, J. Schroeder, ¨ A. Zmelty, Surf. Coat. Technol. 112 Ž1999. 261.

w10x O. Treichel, V. Kirchhoff, Surf. Coat. Technol. 123 Ž2000. 268. w11x O. Zywitzki, G. Hoetzsch, F. Fietzke, K. Goedicke, Surf. Coat. Technol. 82 Ž1996. 169. w12x W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 Ž1992. 1564. w13x K. Koski, J. Holsa, P. Juliet, Surf. Coat. Technol. 120r121 ¨ Ž1999. 303. w14x J.P. Abriata, R. Versaci, J. Garces, Bull. Alloy Phase Diagrams 7 Ž2. Ž1986.. w15x M.H. Suhail, G. Mohan Rao, S. Mohan, J. Vac. Sci. Technol. A 9 Ž1991. 2675. w16x M. Boulouz, A. Boulouz, A. Giani, A. Boyer, Thin Solid Films 323 Ž1998. 85. w17x W.D. Sproul, M.E. Graham, M.S. Wong, P.J. Rudnik, Surf. Coat. Technol. 89 Ž1997. 10. w18x G.V. Samsonov, The Oxide Handbook, IFIrPlenum, New York, Washington, London, 1982.