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Intrinsic stress in dielectric thin films for micromechanical components
Surface and Coatings Technology 116–119 (1999) 116–120 www.elsevier.nl/locate/surfcoat
Intrinsic stress in dielectric thin films for micromechanical components H. Kupfer *, T. Flu¨gel, F. Richter, P. Schlott Technische Universita¨t Chemnitz, Institut fu¨r Physik, 09107 Chemnitz, Germany
Abstract The film stress in coated micromechanical elements may cause bending of such elements and thus impair their performance. In these cases, stress reduction within a single layer by proper choice of deposition parameters or stress compensation within multilayer systems is necessary. In this paper, possibilities for stress reduction in high-reflection (Nb O /SiO ) quarterwave 2 5 2n multilayers for thin silicon laser mirrors have been investigated. Film deposition was performed by reactive direct-current (Nb O ) and non-reactive radio-frequency magnetron sputtering 2 5 (SiO ), respectively. The film stress was investigated as a function of process gas pressure, substrate temperature and ion 2 bombardment of the growing film. At zero bias voltage, a total stress of about −30 MPa was obtained in the Nb O films. 2 5 Utilization of an additional electrode to reduce the plasma density in front of the substrate did change the stress to a small tensile value. SiO films show a compressive stress that could not be reduced below 100 MPa within the parameter range investigated. 2 Complete stress compensation in the multilayer film systems was only possible by application of an additional tensile-stressed metal interlayer. Chromium films deposited prior to the growth of a (4×2) stack of Nb O and SiO did compensate — within 2 5 2 the error of measurement of ±25 MPa — the average stress in the multilayer system to zero. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Dielectric multilayer; Magnetron sputtering; Mechanical stress; Micromechanical mirrors
1. Introduction Modern information technologies make use of microoptical components for display and data transmission purposes. To obtain high reflection of light on surfaces, special thin-film systems are usually utilized. For that purpose dielectric multilayers are used, which are mostly deposited by electron-beam evaporation [1] or sputtering methods [2–4]. In this work, the mechanical stress in highly reflecting thin-film systems on micromechanical silicon mirror plates for laser-beam deflection has been investigated. The mirror plates have a typical size of about 4 mm×4 mm and a thickness of approximately 30 mm [5]. The thin-film system investigated consists of stacks of alternating quarterwave films of Nb O and SiO 2 5 2 deposited by magnetron sputtering. Without special measures, sputtered oxide films exhibit a compressive * Corresponding author. Tel: +49-371-531-8259; fax: +49-371-531-3042. E-mail address: [email protected] (H. Kupfer)
film stress of several hundred MPa, which would bend a 30 mm thick silicon plate to a radius of curvature of a few tenths of a metre. This would lead to an unacceptable divergence of the reflected laser beam. In order to influence the film stress we have varied the substrate bias voltage, substrate temperature and operating gas pressure. Since low compressive stress or even tensile stress is likely to occur for low ion impact, a special electrode was applied to reduce the plasma density in front of the substrate. Finally, stress compensation by means of a metal underlayer having tensile stress was investigated as well.
2. Experimental The oxide deposition was performed in a turbomolecular-pumped vacuum system having a residual gas pressure below 5×10−5 Pa. Two circular magnetron sources (4 in. diameter) were fixed side-by-side in front of a rotable substrate holder
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 11 4 - 0
H. Kupfer et al. / Surface and Coatings Technology 116–119 (1999) 116–120
which could be biased by a separate radio-frequency (RF ) power supply. By using movable shutters having a special shape, the inhomogeneity of the film thickness on a 4 in. silicon wafer could be improved to ≤5%. The process gas flow was controlled by mass-flow controllers. If not stated otherwise, the total pressure was adjusted to 0.4 Pa. The SiO films were deposited by an RF sputtering 2 process from an SiO target at an RF power of 200 W 2 in pure argon. A deposition rate of about 11 nm min−1 was found. In contrast, the Nb O films 2 5 were direct-current (DC ) sputtered using a niobium target in an oxygen/argon atmosphere (1:1). At a DC power of 650 W, a deposition rate of 17 nm min−1 was obtained. No considerable arcing was observed during the deposition process. The thickness of single layers was measured with an accuracy of ±1 nm with an optical film-thickness probe (FTP 500, Sentech Instruments). Optical parameters (index of refraction, n, and coefficient of absorption, k) were measured by spectral ellipsometry. At standard deposition conditions mentioned above, we obtained refractive indices similar to the bulk material values and sufficiently low absorption coefficients (<104; cf. Fig. 1). Moreover, these values were almost constant when varying the process parameters within the range investigated. Elastic recoil detection analysis ( ERDA) was applied to measure the composition of the films [6 ]. It could be shown that the films were stoichiometric oxides containing typically a few per cent of hydrogen due to the residual gas atmosphere. The mass density of the films was measured both by gravimetry and X-ray reflectometry. Both methods yielded about 2.2 g cm−3 for SiO 2 and 4.3 g cm−3 for Nb O films — i.e., for both films, 2 5 approximately 95% of the bulk density. To characterize the structure of the films, X-ray diffraction was used. Film stress was calculated from the curvature of bent substrates. For these experiments we used 1.5 in. silicon
Fig. 1. Refractive index, n, and absorption coefficient, k, of magnetronsputtered Nb O and SiO in the visible wavelength range as a function 2 5 2 of the wavelength.
117
(111) wafers with a thickness of 200 mm. The curvature of the deformed substrates was measured by means of a mechanical surface profiler (Rank Taylor Hobson) with an accuracy of ±10 nm. From that, the film stress could be calculated by using the Stoney equation [7] for the rotational-symmetric case and setting E/(1−n)= 212 GPa for our substrates. A stress resolution of about ± 25 MPa was achieved.
3. Results 3.1. Variation of the substrate temperature: niobium oxide At increasing substrate temperature a distinct increase of the compressive film stress in Nb O was found 2 5 ( Fig. 2). Moreover, between 250°C and 400°C, the slope of the curve was significantly larger than in the lowtemperature range. This is an indication of a modification of the film structure resulting in varied thermal expansion and/or a change in the intrinsic film stress. However, this assumption has to be proved by further investigations of the film structure and by measurements of the thermal expansion coefficient of the Nb O films. 2 5 X-ray diffraction ( XRD) investigations have shown that films were amorphous within the temperature range investigated; i.e., up to a substrate temperature of 400°C. Recrystallization to a hexagonal phase (the so-called TT phase [2]) was found after annealing at 600°C for 7 h. Unlike Nb O , the film stress of the SiO layers was 2 5 2 only slightly influenced by the substrate temperature. Fig. 3 shows the total stress measured for the SiO films 2 together with a thermal stress curve that has been calculated from bulk values of the material parameters. From the comparison of the measured total and calculated thermal film stresses it can be concluded that the intrinsic stress decreases with increasing substrate temperature. One reason for this might be a reduced incor-
Fig. 2. Residual stress versus substrate temperature during the deposition process for sputtered Nb O . 2 5
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Fig. 3. Residual stress versus substrate temperature during the deposition process for sputtered SiO compared with the calculated ther2 mal stress.
poration of water into the film. It is known that impurity incorporation in silicon oxide films due to residual water vapour may cause intrinsic compressive film stress [8]. This is probably the case also in our vacuum chamber and it is likely that this effect is reduced with increasing substrate temperature. XRD investigations have shown amorphous films. We did not find any recrystallization during annealing at temperatures up to 1000°C. 3.2. Variation of the substrate bias voltage: niobium oxide As a consequence of the RF substrate power, a negative DC voltage component was formed which essentially influences the energy of the ions hitting the substrate surface. In the following, this DC component is referred to as the substrate bias voltage, U . B Fig. 4 gives the dependence of the film stress of Nb O on U for total process gas pressures of 0.4 Pa 2 5 B (squares) and 2.0 Pa (triangles). The stress is generally compressive and grows with increasing U — i.e., B increasing ion energy — in a progressively decreasing manner. This is in agreement with the work of
Fig. 4. Dependence of the residual stress of Nb O films on the DC 2 5 component of the RF substrate bias voltage.
Windischman [9], who showed that compressive stress due to ion impact in thin films increases with the square root of the ion energy. For higher pressure the stress is smaller at a given U for voltages higher than −100 V, B probably due to a higher energy loss of ions due to scattering with gas atoms. In order to decrease the ion flux towards the substrate further, a ring-shaped electrode was applied which surrounded the plasma near the substrate surface. The electrode was subject to a negative voltage which was assumed to extract ions from the region near the substrate. This should decrease the plasma density in that region and therefore reduce the ion flux towards the substrate. The potential, U , of the ‘extraction extract electrode’ was varied between −40 V and −250 V. As can be seen from Fig. 5, the compressive stress is reduced with increasing U . For the highest value applied, extract even a slight tensile stress of about 20 MPa could be achieved. Supplementary measurements using a DC-biased metal probe in place of the substrate revealed that the substrate current is reduced from 125 mA cm−2 (U =−40 V ) to 25 mA cm−2 extract (U =−250 V ). extract 3.3. Variation of the substrate bias voltage: silicon dioxide The substrate bias voltage, U , was varied in a range B from zero to −100 V. Upon applying a bias voltage of −125 V or higher, resputtering of the growing film exceeded the deposition rate. The film stress was again compressive but, in contrast to Nb O , it had its highest value at zero bias and 2 5 decreased slightly with increasing substrate bias voltage ( Fig. 6). Similar behaviour was observed at a total process gas pressure of 2 Pa. 3.4. Stress compensation of a dielectric multilayer by a metal interlayer The results of the stress measurements on single layers of Nb O and SiO illustrate that even under 2 5 2
Fig. 5. Compressive stress reduction due to ion extraction by application of a negatively charged electrode.
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As can be seen, chromium is the most appropriate material. The compensation of the residual compressive stress of a (2×4) Nb O /SiO quarterwave stack by a chro2 5 2 mium underlayer was demonstrated on a thin silicon substrate (thickness 200 mm), cf. Fig. 7. After deposition of the dielectric film system onto the chromium layer, the curvature of the uncoated silicon substrate was reproduced within the error of measurement.
4. Conclusions
Fig. 6. Dependence of the residual stress of SiO films on the DC 2 component of the RF substrate bias voltage.
Film stress in magnetron-sputtered Nb O and SiO 2 5 2 films has been investigated as a function of deposition parameters. The final goal of this work was to grow low-stress films of these materials as part of dielectric multilayers suitable for coating very thin silicon mirror plates. In Nb O films, stress is usually compressive and 2 5 grows with increasing substrate bias voltage. Low compressive stress and even slight tensile stress (+20 MPa) could be achieved by using an additional ring-shaped electrode which decreases the plasma density in front of the substrate. In general, the dependence of stress in
optimum deposition conditions complete stress compensation in Nb O /SiO multilayers could not be obtained. 2 5 2 Therefore it is advisable to use an underlayer of a third material having high tensile stress to compensate the compressive stress in the dielectric multilayer. In addition, good reflectivity of the underlayer should improve the optical performance of the complete film system. For instance, a model calculation [10] showed that a (4×2) Nb O /SiO quarterwave stack designed for l= 2 5 2 486 nm and having the n and k values of our films (cf. Fig. 1) should have a reflection of 98.3% when deposited directly on silicon. In case that the stack is deposited on material with a reflection coefficient against vacuum of 64% at 486 nm, the reflection of the layer system is improved to 99.0%. For application as underlayer, metal films are appropriate. In principle, every additional layer increases the total mass of the layer system and hence the dynamic properties of the mirror plate. Because of the relatively high mass density of most metals, this effect may not be neglected. To minimize it, metal films with a high tensile stress but low mass density are required. We have tested several metals in this respect and deposited thin films of those metals by magnetron sputtering using floating substrates to minimize ion impact and hence favour the development of tensile stress. Table 1 lists the film stress, mass density and density-to-stress ratio for the metal films investigated.
Fig. 7. Compensation of the residual compressive stress of an oxide multilayer by an additional cromium underlayer having tensile stress; (2×4) quarterwave stack multilayer of Nb O /SiO , designed for a 2 5 2 laser wavelength of 446 nm at normal beam incidence, total thickness of 540 nm.
Table 1 Film stress, mass density and density-to-stress ratio of different metal layers Material
Film stress (MPa)
Mass densitya (g cm−3)
Density/stress [g (MPa)−1 cm−3]
Niobium Titanium Zirconium Chromium Stainless steel Cu/Ni/Zn alloy Copper Aluminium
−1050 −790 −280 1400 400 50 145 55
8.57 4.50 6.49 7.10 7.90 8.72 8.96 2.67
−0.0082 −0.0057 −0.0232 0.0051 0.0198 0.1744 0.0618 0.0485
a Density of bulk materials [11].
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Nb O films on the deposition conditions can be 2 5 explained with the model of stress formation by ion impact in vapour-deposited films developed by Windischman [9]. In the case of SiO , increasing sub2 strate bias voltage causes a slight decrease of the compressive stress which cannot be explained yet. We assume that impurity incorporation due to water vapour in the vacuum chamber plays a major role in this respect. Even under optimum deposition conditions, complete stress compensation within the (Nb O /SiO ) multilayer 2 5 2 could not be achieved. Therefore we tested several metals with regard to their ability to form thin films with high tensile stress but low mass per unit area, and chose chromium for further investigations. It could be shown that a chromium underlayer of 55 nm thickness did completely balance out the stress of the dielectric multilayer and made possible a practically stress-free, highreflecting multilayer system.
Acknowledgements Financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. The authors
are indebted to the Zentrum fu¨r Mikrotechnologien of the Technische Universita¨t Chemnitz for preparation of laser mirrors and S. Collard for X-ray investigations.
References [1] Z.L. Wu, E. Matthias, SPIE 1992 (1848) 210–223. [2] D. Rosenfeld, R. Sanjine´s, F. Levy et al., J. Vac. Sci. Technol. A 12 (1) (1994) 135. [3] W.T. Pawlewicz, R. Busch, Thin Solid Films 63 (1979) 251–256. [4] B.E. Kempf, H.W. Dinges, A. Po¨cker, Mater. Res. Soc., Symp. Proc. 354 (1995) 529. [5] U. Breng, T. Gessner, C. Kaufmann, R. Kienscherf, J. Markert, J. Micromech. Microeng. 2 (1992) 256. [6 ] C. Spaeth, F. Richter, S. Grigull, U. Kreissig, Nucl. Instrum. Methods B 140 (1998) 243. [7] G.G. Stoney, Proc. Roy. Soc. (London) A 82 (1909) 172. [8] H. Leplan, B. Geenen, J.Y. Robic, Y. Pauleau, SPIE 2253 (1994) 1263. [9] H. Windischman, J. Appl. Phys. 62 (1987) 1800. [10] FilmStar software, FTG Software Associates, Princeton, NJ (1995). [11] Goodfellow Catalogue, Goodfellow GmbH, Bad Nauheim, Germany, 1995.
Intrinsic stress in dielectric thin films for micromechanical components H. Kupfer *, T. Flu¨gel, F. Richter, P. Schlott Technische Universita¨t Chemnitz, Institut fu¨r Physik, 09107 Chemnitz, Germany
Abstract The film stress in coated micromechanical elements may cause bending of such elements and thus impair their performance. In these cases, stress reduction within a single layer by proper choice of deposition parameters or stress compensation within multilayer systems is necessary. In this paper, possibilities for stress reduction in high-reflection (Nb O /SiO ) quarterwave 2 5 2n multilayers for thin silicon laser mirrors have been investigated. Film deposition was performed by reactive direct-current (Nb O ) and non-reactive radio-frequency magnetron sputtering 2 5 (SiO ), respectively. The film stress was investigated as a function of process gas pressure, substrate temperature and ion 2 bombardment of the growing film. At zero bias voltage, a total stress of about −30 MPa was obtained in the Nb O films. 2 5 Utilization of an additional electrode to reduce the plasma density in front of the substrate did change the stress to a small tensile value. SiO films show a compressive stress that could not be reduced below 100 MPa within the parameter range investigated. 2 Complete stress compensation in the multilayer film systems was only possible by application of an additional tensile-stressed metal interlayer. Chromium films deposited prior to the growth of a (4×2) stack of Nb O and SiO did compensate — within 2 5 2 the error of measurement of ±25 MPa — the average stress in the multilayer system to zero. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Dielectric multilayer; Magnetron sputtering; Mechanical stress; Micromechanical mirrors
1. Introduction Modern information technologies make use of microoptical components for display and data transmission purposes. To obtain high reflection of light on surfaces, special thin-film systems are usually utilized. For that purpose dielectric multilayers are used, which are mostly deposited by electron-beam evaporation [1] or sputtering methods [2–4]. In this work, the mechanical stress in highly reflecting thin-film systems on micromechanical silicon mirror plates for laser-beam deflection has been investigated. The mirror plates have a typical size of about 4 mm×4 mm and a thickness of approximately 30 mm [5]. The thin-film system investigated consists of stacks of alternating quarterwave films of Nb O and SiO 2 5 2 deposited by magnetron sputtering. Without special measures, sputtered oxide films exhibit a compressive * Corresponding author. Tel: +49-371-531-8259; fax: +49-371-531-3042. E-mail address: [email protected] (H. Kupfer)
film stress of several hundred MPa, which would bend a 30 mm thick silicon plate to a radius of curvature of a few tenths of a metre. This would lead to an unacceptable divergence of the reflected laser beam. In order to influence the film stress we have varied the substrate bias voltage, substrate temperature and operating gas pressure. Since low compressive stress or even tensile stress is likely to occur for low ion impact, a special electrode was applied to reduce the plasma density in front of the substrate. Finally, stress compensation by means of a metal underlayer having tensile stress was investigated as well.
2. Experimental The oxide deposition was performed in a turbomolecular-pumped vacuum system having a residual gas pressure below 5×10−5 Pa. Two circular magnetron sources (4 in. diameter) were fixed side-by-side in front of a rotable substrate holder
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 11 4 - 0
H. Kupfer et al. / Surface and Coatings Technology 116–119 (1999) 116–120
which could be biased by a separate radio-frequency (RF ) power supply. By using movable shutters having a special shape, the inhomogeneity of the film thickness on a 4 in. silicon wafer could be improved to ≤5%. The process gas flow was controlled by mass-flow controllers. If not stated otherwise, the total pressure was adjusted to 0.4 Pa. The SiO films were deposited by an RF sputtering 2 process from an SiO target at an RF power of 200 W 2 in pure argon. A deposition rate of about 11 nm min−1 was found. In contrast, the Nb O films 2 5 were direct-current (DC ) sputtered using a niobium target in an oxygen/argon atmosphere (1:1). At a DC power of 650 W, a deposition rate of 17 nm min−1 was obtained. No considerable arcing was observed during the deposition process. The thickness of single layers was measured with an accuracy of ±1 nm with an optical film-thickness probe (FTP 500, Sentech Instruments). Optical parameters (index of refraction, n, and coefficient of absorption, k) were measured by spectral ellipsometry. At standard deposition conditions mentioned above, we obtained refractive indices similar to the bulk material values and sufficiently low absorption coefficients (<104; cf. Fig. 1). Moreover, these values were almost constant when varying the process parameters within the range investigated. Elastic recoil detection analysis ( ERDA) was applied to measure the composition of the films [6 ]. It could be shown that the films were stoichiometric oxides containing typically a few per cent of hydrogen due to the residual gas atmosphere. The mass density of the films was measured both by gravimetry and X-ray reflectometry. Both methods yielded about 2.2 g cm−3 for SiO 2 and 4.3 g cm−3 for Nb O films — i.e., for both films, 2 5 approximately 95% of the bulk density. To characterize the structure of the films, X-ray diffraction was used. Film stress was calculated from the curvature of bent substrates. For these experiments we used 1.5 in. silicon
Fig. 1. Refractive index, n, and absorption coefficient, k, of magnetronsputtered Nb O and SiO in the visible wavelength range as a function 2 5 2 of the wavelength.
117
(111) wafers with a thickness of 200 mm. The curvature of the deformed substrates was measured by means of a mechanical surface profiler (Rank Taylor Hobson) with an accuracy of ±10 nm. From that, the film stress could be calculated by using the Stoney equation [7] for the rotational-symmetric case and setting E/(1−n)= 212 GPa for our substrates. A stress resolution of about ± 25 MPa was achieved.
3. Results 3.1. Variation of the substrate temperature: niobium oxide At increasing substrate temperature a distinct increase of the compressive film stress in Nb O was found 2 5 ( Fig. 2). Moreover, between 250°C and 400°C, the slope of the curve was significantly larger than in the lowtemperature range. This is an indication of a modification of the film structure resulting in varied thermal expansion and/or a change in the intrinsic film stress. However, this assumption has to be proved by further investigations of the film structure and by measurements of the thermal expansion coefficient of the Nb O films. 2 5 X-ray diffraction ( XRD) investigations have shown that films were amorphous within the temperature range investigated; i.e., up to a substrate temperature of 400°C. Recrystallization to a hexagonal phase (the so-called TT phase [2]) was found after annealing at 600°C for 7 h. Unlike Nb O , the film stress of the SiO layers was 2 5 2 only slightly influenced by the substrate temperature. Fig. 3 shows the total stress measured for the SiO films 2 together with a thermal stress curve that has been calculated from bulk values of the material parameters. From the comparison of the measured total and calculated thermal film stresses it can be concluded that the intrinsic stress decreases with increasing substrate temperature. One reason for this might be a reduced incor-
Fig. 2. Residual stress versus substrate temperature during the deposition process for sputtered Nb O . 2 5
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H. Kupfer et al. / Surface and Coatings Technology 116–119 (1999) 116–120
Fig. 3. Residual stress versus substrate temperature during the deposition process for sputtered SiO compared with the calculated ther2 mal stress.
poration of water into the film. It is known that impurity incorporation in silicon oxide films due to residual water vapour may cause intrinsic compressive film stress [8]. This is probably the case also in our vacuum chamber and it is likely that this effect is reduced with increasing substrate temperature. XRD investigations have shown amorphous films. We did not find any recrystallization during annealing at temperatures up to 1000°C. 3.2. Variation of the substrate bias voltage: niobium oxide As a consequence of the RF substrate power, a negative DC voltage component was formed which essentially influences the energy of the ions hitting the substrate surface. In the following, this DC component is referred to as the substrate bias voltage, U . B Fig. 4 gives the dependence of the film stress of Nb O on U for total process gas pressures of 0.4 Pa 2 5 B (squares) and 2.0 Pa (triangles). The stress is generally compressive and grows with increasing U — i.e., B increasing ion energy — in a progressively decreasing manner. This is in agreement with the work of
Fig. 4. Dependence of the residual stress of Nb O films on the DC 2 5 component of the RF substrate bias voltage.
Windischman [9], who showed that compressive stress due to ion impact in thin films increases with the square root of the ion energy. For higher pressure the stress is smaller at a given U for voltages higher than −100 V, B probably due to a higher energy loss of ions due to scattering with gas atoms. In order to decrease the ion flux towards the substrate further, a ring-shaped electrode was applied which surrounded the plasma near the substrate surface. The electrode was subject to a negative voltage which was assumed to extract ions from the region near the substrate. This should decrease the plasma density in that region and therefore reduce the ion flux towards the substrate. The potential, U , of the ‘extraction extract electrode’ was varied between −40 V and −250 V. As can be seen from Fig. 5, the compressive stress is reduced with increasing U . For the highest value applied, extract even a slight tensile stress of about 20 MPa could be achieved. Supplementary measurements using a DC-biased metal probe in place of the substrate revealed that the substrate current is reduced from 125 mA cm−2 (U =−40 V ) to 25 mA cm−2 extract (U =−250 V ). extract 3.3. Variation of the substrate bias voltage: silicon dioxide The substrate bias voltage, U , was varied in a range B from zero to −100 V. Upon applying a bias voltage of −125 V or higher, resputtering of the growing film exceeded the deposition rate. The film stress was again compressive but, in contrast to Nb O , it had its highest value at zero bias and 2 5 decreased slightly with increasing substrate bias voltage ( Fig. 6). Similar behaviour was observed at a total process gas pressure of 2 Pa. 3.4. Stress compensation of a dielectric multilayer by a metal interlayer The results of the stress measurements on single layers of Nb O and SiO illustrate that even under 2 5 2
Fig. 5. Compressive stress reduction due to ion extraction by application of a negatively charged electrode.
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As can be seen, chromium is the most appropriate material. The compensation of the residual compressive stress of a (2×4) Nb O /SiO quarterwave stack by a chro2 5 2 mium underlayer was demonstrated on a thin silicon substrate (thickness 200 mm), cf. Fig. 7. After deposition of the dielectric film system onto the chromium layer, the curvature of the uncoated silicon substrate was reproduced within the error of measurement.
4. Conclusions
Fig. 6. Dependence of the residual stress of SiO films on the DC 2 component of the RF substrate bias voltage.
Film stress in magnetron-sputtered Nb O and SiO 2 5 2 films has been investigated as a function of deposition parameters. The final goal of this work was to grow low-stress films of these materials as part of dielectric multilayers suitable for coating very thin silicon mirror plates. In Nb O films, stress is usually compressive and 2 5 grows with increasing substrate bias voltage. Low compressive stress and even slight tensile stress (+20 MPa) could be achieved by using an additional ring-shaped electrode which decreases the plasma density in front of the substrate. In general, the dependence of stress in
optimum deposition conditions complete stress compensation in Nb O /SiO multilayers could not be obtained. 2 5 2 Therefore it is advisable to use an underlayer of a third material having high tensile stress to compensate the compressive stress in the dielectric multilayer. In addition, good reflectivity of the underlayer should improve the optical performance of the complete film system. For instance, a model calculation [10] showed that a (4×2) Nb O /SiO quarterwave stack designed for l= 2 5 2 486 nm and having the n and k values of our films (cf. Fig. 1) should have a reflection of 98.3% when deposited directly on silicon. In case that the stack is deposited on material with a reflection coefficient against vacuum of 64% at 486 nm, the reflection of the layer system is improved to 99.0%. For application as underlayer, metal films are appropriate. In principle, every additional layer increases the total mass of the layer system and hence the dynamic properties of the mirror plate. Because of the relatively high mass density of most metals, this effect may not be neglected. To minimize it, metal films with a high tensile stress but low mass density are required. We have tested several metals in this respect and deposited thin films of those metals by magnetron sputtering using floating substrates to minimize ion impact and hence favour the development of tensile stress. Table 1 lists the film stress, mass density and density-to-stress ratio for the metal films investigated.
Fig. 7. Compensation of the residual compressive stress of an oxide multilayer by an additional cromium underlayer having tensile stress; (2×4) quarterwave stack multilayer of Nb O /SiO , designed for a 2 5 2 laser wavelength of 446 nm at normal beam incidence, total thickness of 540 nm.
Table 1 Film stress, mass density and density-to-stress ratio of different metal layers Material
Film stress (MPa)
Mass densitya (g cm−3)
Density/stress [g (MPa)−1 cm−3]
Niobium Titanium Zirconium Chromium Stainless steel Cu/Ni/Zn alloy Copper Aluminium
−1050 −790 −280 1400 400 50 145 55
8.57 4.50 6.49 7.10 7.90 8.72 8.96 2.67
−0.0082 −0.0057 −0.0232 0.0051 0.0198 0.1744 0.0618 0.0485
a Density of bulk materials [11].
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Nb O films on the deposition conditions can be 2 5 explained with the model of stress formation by ion impact in vapour-deposited films developed by Windischman [9]. In the case of SiO , increasing sub2 strate bias voltage causes a slight decrease of the compressive stress which cannot be explained yet. We assume that impurity incorporation due to water vapour in the vacuum chamber plays a major role in this respect. Even under optimum deposition conditions, complete stress compensation within the (Nb O /SiO ) multilayer 2 5 2 could not be achieved. Therefore we tested several metals with regard to their ability to form thin films with high tensile stress but low mass per unit area, and chose chromium for further investigations. It could be shown that a chromium underlayer of 55 nm thickness did completely balance out the stress of the dielectric multilayer and made possible a practically stress-free, highreflecting multilayer system.
Acknowledgements Financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. The authors
are indebted to the Zentrum fu¨r Mikrotechnologien of the Technische Universita¨t Chemnitz for preparation of laser mirrors and S. Collard for X-ray investigations.
References [1] Z.L. Wu, E. Matthias, SPIE 1992 (1848) 210–223. [2] D. Rosenfeld, R. Sanjine´s, F. Levy et al., J. Vac. Sci. Technol. A 12 (1) (1994) 135. [3] W.T. Pawlewicz, R. Busch, Thin Solid Films 63 (1979) 251–256. [4] B.E. Kempf, H.W. Dinges, A. Po¨cker, Mater. Res. Soc., Symp. Proc. 354 (1995) 529. [5] U. Breng, T. Gessner, C. Kaufmann, R. Kienscherf, J. Markert, J. Micromech. Microeng. 2 (1992) 256. [6 ] C. Spaeth, F. Richter, S. Grigull, U. Kreissig, Nucl. Instrum. Methods B 140 (1998) 243. [7] G.G. Stoney, Proc. Roy. Soc. (London) A 82 (1909) 172. [8] H. Leplan, B. Geenen, J.Y. Robic, Y. Pauleau, SPIE 2253 (1994) 1263. [9] H. Windischman, J. Appl. Phys. 62 (1987) 1800. [10] FilmStar software, FTG Software Associates, Princeton, NJ (1995). [11] Goodfellow Catalogue, Goodfellow GmbH, Bad Nauheim, Germany, 1995.