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Ex situ and in situ spectroscopic ellipsometry of MF and DC-sputtered TiO2 and SiO2 films for process control
Thin Solid Films 351 (1999) 42±47
Ex situ and in situ spectroscopic ellipsometry of MF and DC-sputtered TiO2 and SiO2 ®lms for process control M. VergoÈhl a,*, N. Malkomes a, T. Staedler a, T. MattheÂe a, U. Richter b a
Fraunhofer Institute for Surface Engineering and Thin Films, Bienroder Weg 54E, D-38108 Braunschweig, Germany b Sentech Instruments GmbH, Rudower Chaussee 6, D-12484 Berlin, Germany
Abstract Titanium oxide and silicon oxide ®lms were deposited on ¯oatglass substrates by magnetron sputtering employing both the DC and the MF (mid-frequency) technique. The ®lms were grown at different working points between transition and oxide mode, target power densities up to 7.5 W/cm 2. Ex-situ ellipsometry at different angles of incidence was applied to study the optical properties and the morphology of the ®lms. For modeling the spectra, the Lorentz model with one single oscillator was used within the spectral range between 380 and 850 nm. For SiO2, the ellipsometric data could be ®tted using a model with one single homogeneous ®lm. The ellipsometric investigation of TiO2 ®lms with thicknesses below 150 nm show that, in general inhomogeneities of the refractive index in the growth direction have to be taken into account. It is concluded that both plasma heating as well as ion bombardment are responsible for these inhomogeneities. Refractive indices of 2.43 for DC sputtered ®lms up to 2.58 for MF sputtered ®lms are observed. A four-layer SiO2±TiO2 antire¯ective coating on glass was fabricated using both plasma and ellipsometric control. The re¯ectivity, calculated from the in situ ellipsometric analysis, is in a good agreement with the measured re¯ectivity. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Ellipsometry; Optical coatings; Silicon oxide; Titanium oxide
1. Introduction Antire¯ective coatings on glass are of essential importance in optical systems since the re¯ectance of a plane glass substrate is about 8.5%. This relatively high re¯ectance often causes a disturbance in various cases. In most applications of broadband antire¯ective coatings, sequences of thin layers with alternating high and low index of refraction are used. For the deposition of those systems, the technique of magnetron sputtering is convenient because of its possibility to produce coatings with high optical quality and a good homogeneity. On architectural glass of large scales, the recently developed mid-frequency (MF) sputtering technique [1] offers decisive advantages over the conventional direct current sputtering (DC) technique because of the improved sputter rate due to the suppression of oxidation of the cathodes. Therefore, the process can be driven in the transition mode with high deposition rates. Furthermore, highly insulating materials like SiO2 can be deposited with drastically reduced arcing rate, compared to the conventional DC process.
* Corresponding author. Fax: 149-531-215-5900. E-mail address: [email protected] (M. VergoÈhl)
In contrast to the oxide mode the transition mode is not stable. Recently, some effort has been made for the stabilization of the MF process in the transition mode. This was successfully done with partial pressure measurements [1] and optical emission spectroscopy [2], respectively. However, the plasma properties are subjected to temporal drifts leading to changes of the ®lm properties during the deposition process. In order to reach constant layer properties during the whole lifetime of the target, optical methods like photometry or ellipsometry are extremely useful. This paper reports on ex situ and in situ spectroscopic ellipsometry for the investigation of the ®lm properties during the magnetron sputter deposition in order to measure and control the optical properties of TiO2 and SiO2 layers on ¯oatglass substrates. 2. Experiment The ®lms were deposited at different oxygen partial pressures (from metallic to oxide mode) and target powers up to 7.5 W/cm 2 in a standard high vacuum chamber (Balzers± Pfeiffer Boxcoater PLS 580), equipped with MF and DC magnetron sputter sources (PK 500 cathodes) [3,5]. In addition a spectroscopic ellipsometer (SENTECH Instruments SE 801) and a computer controlled sample holder were
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0015 2-2
M. VergoÈhl et al. / Thin Solid Films 351 (1999) 42±47
43
V 0, and G are the amplitude, resonance frequency and the damping of the oscillator, respectively. The second term corresponds to the free electron contribution to the dielectric function, with v p being the plasma frequency. v is the angular frequency of the light. 3.2. Silicon oxide
Fig. 1. Refractive index and absorption index of SiO2 ®lms deposited at different target voltages. The error bar of the refractive index is ^0.01.
adapted. The ellipsometer is a polarizer-sample-analyzer (PSA) system with rotating analyzer. In the present work, the step-scan modus was applied. With an acquisition time of 1 s for a complete spectrum of 1024 data points (280±850 nm), an acceptable signal to noise ratio was reached. For ex situ ellipsometry, a SENTECH SE800 gonio-spectroellipsometer was used. The ellipsometer arms are directed between the two cathodes of the MF magnetron, so the growth of the layer on the MF side can be examined in situ. The angle of incidence was determined by a ®t to the ellipsometric C-spectrum of the uncoated ¯oatglass sample with a reproducibility of F0 65 ^ 0:028. For the coating on the DC position the samples can be moved to the opposite side of the vacuum chamber. In the present paper, only the main features of the stabilization scheme for the reactive MF plasma process will be given, details will be published elsewhere. For the stabilization on short time scales (,1 s) the electrical parameters (voltage, power) and the intensity of the optical emission lines were used. For the SiO2 process with silicon targets, the target voltage was stabilized. All process setpoints from metallic mode throughout the transition mode to the oxide mode could be stabilized. The TiO2 process stabilization, however, was performed with the an optical emission system (OES). Up to power densities of 5.5 W/cm 2, the complete hysteresis could be stabilized.
From the ex situ ellipsometric analysis, it was found out that a model which includes a single homogeneous layer is suitable for the description of the ellipsometric data. In Fig. 1, the refractive index and the absorption index at l 600 nm as a function of the target voltage are shown. Up to 450 V (transition mode), transparent layers with an absorption index of 10 24 were obtained. At higher target voltages (metallic mode), absorbing layers result. At the lowest voltages (oxide mode) the absorption again increases. Potentially, absorption of free electrons could be taken into consideration for that infrared absorption, as can be concluded from an appearing Drude term in the ®t. The background of this absorption will be part of further investigations. From the in situ ellipsometric spectra, the thickness and the optical indices at the different working points were determined as depicted in Fig. 2. From the thickness, the rate was determined by numerical derivation, where the last ®ve points were used for the polynomial ®t. This polynomial was also used for the extrapolation of the thickness, in order to estimate the residual time required for the exact thickness. This is very useful in thin layers, where coating time of less than one minute were applied. At larger target voltages (transition and metallic mode) the linear ®ts of the ®lm thickness indicate a constant growth rate. At lower voltages, deviations from the linear ®ts occur, indicating a drift of the working point in power and in voltage. This is mainly due to the growing oxide layer on the silicon targets. This drift can also be seen in the inset of Fig. 2, where different rates at the same target voltage of 290 V are obtained. As the voltage is
3. Ex situ and in situ ellipsometry 3.1. Introduction For modeling the optical dispersion, the well-known Drude±Lorentz oscillator model was employed. Within this model, the complex dielectric function can be written as
1~ 1inf 1
V2pk v2p 1 2 v2 2 iGFE v V20 2 v2 2 iGv
1
with 1 inf being the high frequency dielectric constant, V pk,
Fig. 2. Thicknesses determined by in situ ellipsometry versus time of the SiO2 layers deposited at different voltages. The lines are linear ®ts. Inset: Growth rate as a function of target voltage.
M. VergoÈhl et al. / Thin Solid Films 351 (1999) 42±47
44
Table 1 Deposition parameters and properties of the deposited TiO2 ®lms onto unheated substrates. The ®lm thicknesses are typically 100 nm. For Sample (2e), for ex situ ellipsometry a two-layer model, for in situ ellipsometry a single layer model is recommended. The Samples (2a±e) are deposited in the oxide mode next to the transition mode (minimum oxygen ¯ow), therefore the rate is already enlarged by a factor of two±three compared to the ®lms deposited with completely oxidized targets. The error of n is ^0.01, the error of k is ^0.005. Note that the value of n for Sample (4) is the mean value Sample
Target power (W/cm 2)
Deposition mode
Rate (nm/s)
n (550 nm)
k (550 nm)
Ar ¯ow (sccm)
Recommended model
1 2a 2b 2c 2d 2e 3a 3b 3c 4
1.0 1.0 4.0 5.0 6.0 7.5 2.9 4.0 5.2 6.0
DC, oxide MF, oxide MF, oxide MF, oxide MF, oxide MF, oxide MF, transition MF, transition MF, transition MF, oxide
0.038 0.05 0.16 0.24 0.27 0.42 0.397 0.67 0.79 0.23
2.440 2.480 2.475 2.500 2.520 2.540 2.515 2.510 2.510 2.620
0.008 0.008 0.006 0.009 0.011 0.013 0.004 0.005 0.004 0.017
60 90 90 90 90 90 90 90 90 50
One layer One layer One layer One layer One layer One±two layer One layer One layer One layer Two layer
increased further, the rate increases continuously up to 2 nm/s. Low values of absorption are obtained in this region. Since the absorption increases at higher target voltages, the working point for the deposition of transparent layers is at 450 V. The rate is a monotone function of the process voltage, therefore it can be used as a control parameter for the SiO2 process. 3.3. Titanium oxide Different TiO2 ®lms with thicknesses of typically 100 nm were deposited on ¯oatglass in the DC and the MF mode, respectively, where the target power was limited to 1 and 7.5 W/cm 2 target area, respectively. The deposition parameters and results of the investigations are summarized in Table 1. Fig. 3 shows an ex situ ellipsometric measurement at different angles of incidence between 65 and 758 of
Fig. 3. Ex situ ellipsometric data of a TiO2 ®lm on glass (Sample (2e)). The spectra were acquired at angles of incidence of 65, 70, and 758. For the ®t within the one layer model (dotted line), a mean square error of 9.3 (compare to the data in the inset of Fig. 4) was reached. Within the two layer model (solid line), the MSE could be reduced to 5.8.
Sample (2e) which was deposited at 7.5 W/cm 2. In that ®t, the implementation of an interface layer with a smaller refractive index than that of the upper TiO2 layer was necessary. This interface layer was modelled with a Bruggeman effective medium of TiO2 and voids. The thickness of the bottom layer amounts 71 nm, the void concentration 6%. These inhomogeneities in the growth direction were observed to depend on the plasma conditions. This sample was deposited at lower Ar ¯ux, therefore the inhomogeneity was the largest. For other samples grown at higher pressures (e.g. Samples (2a±c)), the single layer model is adequate. Hence, a model which involves one or two TiO2 layers is required for thicknesses below 150 nm. For thinner ®lms below 70 nm, in any case the simple model is adequate. The layers deposited at lower power in the oxide mode (Samples (2a±c)) could be described within the simple model, as shown by the low values of the void concentration in Fig. 4, and by the values of the mean square error (MSE) shown in the inset. The MSE were plotted for the two models as a function of the target power. Primary at 6 W/ cm 2 (Sample (2d)), a difference of the MSE between the two models becomes signi®cant. A surface roughness layer of 1.7 nm could improve the ®t only for the ®lms sputtered in MF oxide mode, as can be seen from the reduction of the MSE from 5.5 (O) down to 3 (B) at 1 W/cm 2. This may be due to the larger grain size and the higher surface roughness of the ®lms deposited in the MF oxide mode compared to the ®lms deposited in the DC mode, as found in AFM investigations. This may explain the lower MSE value of the ®t of the DC sputtered samples at 1 kW power (Sample (1)). For the in situ application with the higher noise level and the lower acquisition time, such slight improvements due to very thin surface layers will be neglected. In contrast to the MF oxide mode, all samples deposited in the transition mode could be treated within the simple model, i.e. no difference in the MSE values were observed between the two models. In Fig. 5, the optical constants of the different samples determined from the ex situ ellipsometric analysis were
M. VergoÈhl et al. / Thin Solid Films 351 (1999) 42±47
Fig. 4. Void concentration of the TiO2 Samples (2a±e) (MF, oxide mode). Inset: Mean square error (MSE) of the ®ts of the ex situ ellipsometric spectra within the one layer model and within the two layer model. The symbols are: (X), MF one layer model, oxide mode; (O), MF two layer model, oxide mode; (B), MF two layer model, oxide mode and surface layer; (1), DC one layer model; (£), DC two layer model. Note that for all samples deposited in the transition mode (Samples (3a±c), not shown here), no difference in the MSE values were observed between the two models.
plotted as a function of the process power (a), (b) and of the temperature, respectively, (c), (d). The DC sputtered ®lm at 1.0 W/cm 2 shows the lowest refractive index (n 2:43). The MF ®lm sputtered with the same power density shows n 2:48. With increasing power density, both the refractive index and the absorption increase. At 7.5 W/cm 2, the refractive indices of the bottom and the top layer are shown. While the refractive index of the bottom layer nearly corresponds to that of the ®lm sputtered at 1 W/cm 2 in the MF mode, the refractive index of the top layer agrees with the rutile value (open square in Fig. 5a). The refractive index and the absorption index of the TiO2 layers deposited in the transition mode are also shown in
Fig. 5. Refractive index n (a), (c) and absorption index k (b), (d) of different TiO2 layers deposited on ¯oatglass substrates as a function of the target power (a), (b) and of temperature, respectively, (c), (d). Symbols are: (O), DC mode; (B), MF oxide mode; (V), MF transition mode.
45
Fig. 6. In situ ellipsometric spectra of Sample (4), measured at different time positions. The ®ts are also shown.
Fig. 5a,b, respectively. The normalized OES intensity ratio of the titanium line to the oxygen line was 0.5. It can be seen that the refractive indices are slightly larger than that of the TiO2 ®lms sputtered in the oxide mode. The growth rate of the sample deposited at 4 W/cm 2 was determined as 0.67 nm/s, which is a factor of three larger than the corresponding sample deposited in the oxide mode. Note that the Samples (2a±e) are deposited with not completely oxidized targets, as the OES (normalized intensity ratio of 0.1) show. Therefore, the rate is already enlarged by a factor of three compared to the ®lms deposited with completely oxidized targets (OES intensity ratio ,0.02). Two reasons may be responsible for this increasing refractive index in the oxide mode: First, the higher energy of the ions in the MF process [5], secondly an increase of the temperature due to plasma heating. In Fig. 5c, ®lms of the same thickness of 100 nm were deposited in the DC mode onto heated substrates. The refractive index also increases from 2.43 corresponding to the anatase value for unheated substrates to a value of 2.58 for substrate temperatures of 2008C, which is similar to the measured refractive index of a rutile wafer. For the MF process, the substrate temperature was estimated not to be higher than about 1008C under the same deposition conditions. Therefore nearby the plasma heating also the higher ion energy of the MF plasma seem to be responsible for the higher refractive index. With increasing power density, the absorption index of the ®lms deposited in the oxide increases. This dependency was not observed in the ®lms deposited in the transition mode, where lower values were determined. EPMA investigations on thicker ®lms delivered oxygen to titanium ratios between 1.99 and 2.01, which is near to the ratio of a TiO2 standard (1.99), so that absorption due an understochiometry should be neglectible. XRD measurements of the MF oxide ®lms reveal both a sharp anatase (101)-peak and a broader rutile (110)-peak with half the intensity of the anatase peak. For the ®lms deposited in the MF transition mode, no peaks were found, indicating an amorphous ®lm structure, in agreement with [1,4]. Therefore, it can be
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M. VergoÈhl et al. / Thin Solid Films 351 (1999) 42±47
model shows a constant MSE value. Therefore, it can be summarized that for all TiO2 ®lms with thickness below 65 nm, the simple model should be used while for thicker ®lms, the two layer model is superior under speci®c deposition conditions. 3.4. SiO2/TiO2 multilayer antire¯ective coating
Fig. 7. Results of the ®t of the in situ ellipsometric data of Sample (4) (see Fig. 6). Shown are the thicknesses of the bottom and the top layer, and the full thickness, respectively. The interface layer was assumed to have 10% voids. Inset: Mean square error of the two models versus full thickness.
concluded that the lower absorption of the ®lms deposited in the transition mode is mainly due to the amorphous structure with lower losses at the grains. Depending on the deposition technique and the process parameters, the refractive index of TiO2 ®lms shows large differences. The lowest values are found in evaporation techniques (l 550 nm: n 2:33 [9], n 2:2 [11]), while for ion plating (n 2:52±2:61 [11]), ion assisted deposition (n 2:50 at 508C, n 2:43 at 1378C [11]), RF diode sputtering (n 2:1±2:5 [10], 2.49 [11]) and RF magnetron sputtering (n 2:55 [6], 2.1±2.4 [7]) higher refractive indices are found. In general, higher substrate temperature as well as the energy of bombarding ions [8,12] increase the refractive index. In [1,4], the TiO2 ®lms deposited by the MF sputtering show signi®cantly higher values the ®lms deposited with the DC technique. This is also found here. In addition, the refractive indices reported in [1,4] are in a good agreement with our data. In Fig. 6, some in situ ellipsometric measurements at different deposition times of Sample (4) are shown together with the ®ts within the two layer model. As ®t parameters, the thickness of the two layers were used. In Fig. 7, the ®tted thicknesses are plotted versus the full thickness of Sample (4). In this plot, the onset of the inhomogeneous growth can be observed at 65 nm full thickness. This value is in very good agreement with the ex situ ellipsometric results. At lower full thickness (i.e. ,65 nm), there is a large correlation between the thickness of the bottom layer to that of the top layer. Thus, for an optimum of sample information, in region I the one layer model should be used, whereas in region II the two layer model should be applied. It should be emphasized that the resulting thickness remains constant, due to the low correlation with the void concentration. The inset of Fig. 7 shows the mean square error of this ®t. At 65 nm ®lm thickness, a signi®cant increase of the MSE is observed for the one layer model. Only the two layer
A four layer SiO2±TiO2 broadband antire¯ective coating was deposited on a ¯oatglass substrate. The desired sample geometry is shown in the inset of Fig. 8. As mentioned before, only at the MF side an in situ ellipsometric control could be applied during the deposition process. This is the case for the SiO2 layers. For the control of the thickness on the DC side (TiO2), an intermediate ellipsometric measurement was performed after stopping the process and rotating the sample in the measuring position. The deposition time left was then estimated by comparing the desired to the measured thickness. From the ellipsometric analysis it was found that the thicknesses are in a good agreement with the desired thicknesses of the layer stack and the calculated re¯ectivity is in a good agreement with the measured re¯ectivity, as shown in Fig. 8. 4. Conclusions The present investigations on TiO2 and SiO2 single layers and SiO2±TiO2 multilayers show that in situ ellipsometry is an excellent technique for monitoring and control of the deposition process of optical functional layers like antire¯ective coatings. Due to its simplicity, the Lorentz model is convenient as a good and fast description for the optical dispersion within the spectral range below the fundamental absorption edge. While in SiO2, a one layer model could be used, some of the TiO2 layers display inhomogeneities
Fig. 8. Re¯ectivity of a four layer antire¯ective coating. Also shown is the calculated re¯ectivity, as a result of the in situ ellipsometry data. Inset: Desired (left) and determined (right) thicknesses of the antire¯ective coating. Note that the ellipsometric measurements could only be performed on the SiO2 side, the TiO2 thicknesses were estimated by an intermediate measurement near the end of the process.
M. VergoÈhl et al. / Thin Solid Films 351 (1999) 42±47
which depend on the plasma conditions. These layers can be modelled within a two layer model. The onset of the inhomogeneous growth was observed via in situ ellipsometry, in excellent agreement with the ex situ ellipsometric results. Further improvements of the in situ ellipsometric system may be achieved by a reduction of both the acquisition and ®tting time, which should result in a better performance of the thickness estimator. Acknowledgements The authors are pleased to acknowledge the BMBF for the ®nancial support under contract 13N6911. References [1] J. Szczyrbowski, G. BraÈuer, G. Teschner, A. Zmelty:, J. Non-Cryst. Solids 218 (1997) 25.
47
[2] S. Schiller, U. Heisig, K. Steinfelder, J. StruÈmpfel, A. Friedrich, R. Fricke, 6th Int. Conf. on Ion and Plasma Assisted Techniques, Vol. 23, 1987, pp. xi, 481. [3] B. Szyszka, S. JaÈger, J. Non-Cryst. Solids 218 (1997) 74. [4] G. BraÈuer, J. Szczyrbowski, G. Teschner, J. Non-Cryst. Solids 218 (1997) 19. [5] S. JaÈger, B. Szyzska, J. Szczyrbowski, G. BraÈuer, Surf. Coat. Technol. 98 (1998) 1303. [6] W.J. Coleman, Appl. Opt. 13 (4) (1974) 946. [7] S. Ben, Amor, G. Baud, J.P. Besse, M. Jacquet, Mater. Sci. Eng. B 47 (1997) 110. [8] N. Martin, C. Rousselot, D. Rondot, F. Palmino, R. Mercier, Thin Solid Films 300 (1997) 113. [9] J.V. Grahn, M. Linder, E. Fredsiksson, J. Vac. Sci. Technol. A 16 (4) (1998) 2495. [10] W.T. Pawlewicz, P.M. Martin, D.D. Hays, I.B. Mann, Optical Thin Films (1982) 105. [11] J.M. Bennet, E. Pelletier, G. Albrand, et al., Appl. Opt. 28 (15) (1989) 3302. [12] P.J. Martin, J. Mater Sci. 21 (1986) 1.
Ex situ and in situ spectroscopic ellipsometry of MF and DC-sputtered TiO2 and SiO2 ®lms for process control M. VergoÈhl a,*, N. Malkomes a, T. Staedler a, T. MattheÂe a, U. Richter b a
Fraunhofer Institute for Surface Engineering and Thin Films, Bienroder Weg 54E, D-38108 Braunschweig, Germany b Sentech Instruments GmbH, Rudower Chaussee 6, D-12484 Berlin, Germany
Abstract Titanium oxide and silicon oxide ®lms were deposited on ¯oatglass substrates by magnetron sputtering employing both the DC and the MF (mid-frequency) technique. The ®lms were grown at different working points between transition and oxide mode, target power densities up to 7.5 W/cm 2. Ex-situ ellipsometry at different angles of incidence was applied to study the optical properties and the morphology of the ®lms. For modeling the spectra, the Lorentz model with one single oscillator was used within the spectral range between 380 and 850 nm. For SiO2, the ellipsometric data could be ®tted using a model with one single homogeneous ®lm. The ellipsometric investigation of TiO2 ®lms with thicknesses below 150 nm show that, in general inhomogeneities of the refractive index in the growth direction have to be taken into account. It is concluded that both plasma heating as well as ion bombardment are responsible for these inhomogeneities. Refractive indices of 2.43 for DC sputtered ®lms up to 2.58 for MF sputtered ®lms are observed. A four-layer SiO2±TiO2 antire¯ective coating on glass was fabricated using both plasma and ellipsometric control. The re¯ectivity, calculated from the in situ ellipsometric analysis, is in a good agreement with the measured re¯ectivity. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Ellipsometry; Optical coatings; Silicon oxide; Titanium oxide
1. Introduction Antire¯ective coatings on glass are of essential importance in optical systems since the re¯ectance of a plane glass substrate is about 8.5%. This relatively high re¯ectance often causes a disturbance in various cases. In most applications of broadband antire¯ective coatings, sequences of thin layers with alternating high and low index of refraction are used. For the deposition of those systems, the technique of magnetron sputtering is convenient because of its possibility to produce coatings with high optical quality and a good homogeneity. On architectural glass of large scales, the recently developed mid-frequency (MF) sputtering technique [1] offers decisive advantages over the conventional direct current sputtering (DC) technique because of the improved sputter rate due to the suppression of oxidation of the cathodes. Therefore, the process can be driven in the transition mode with high deposition rates. Furthermore, highly insulating materials like SiO2 can be deposited with drastically reduced arcing rate, compared to the conventional DC process.
* Corresponding author. Fax: 149-531-215-5900. E-mail address: [email protected] (M. VergoÈhl)
In contrast to the oxide mode the transition mode is not stable. Recently, some effort has been made for the stabilization of the MF process in the transition mode. This was successfully done with partial pressure measurements [1] and optical emission spectroscopy [2], respectively. However, the plasma properties are subjected to temporal drifts leading to changes of the ®lm properties during the deposition process. In order to reach constant layer properties during the whole lifetime of the target, optical methods like photometry or ellipsometry are extremely useful. This paper reports on ex situ and in situ spectroscopic ellipsometry for the investigation of the ®lm properties during the magnetron sputter deposition in order to measure and control the optical properties of TiO2 and SiO2 layers on ¯oatglass substrates. 2. Experiment The ®lms were deposited at different oxygen partial pressures (from metallic to oxide mode) and target powers up to 7.5 W/cm 2 in a standard high vacuum chamber (Balzers± Pfeiffer Boxcoater PLS 580), equipped with MF and DC magnetron sputter sources (PK 500 cathodes) [3,5]. In addition a spectroscopic ellipsometer (SENTECH Instruments SE 801) and a computer controlled sample holder were
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0015 2-2
M. VergoÈhl et al. / Thin Solid Films 351 (1999) 42±47
43
V 0, and G are the amplitude, resonance frequency and the damping of the oscillator, respectively. The second term corresponds to the free electron contribution to the dielectric function, with v p being the plasma frequency. v is the angular frequency of the light. 3.2. Silicon oxide
Fig. 1. Refractive index and absorption index of SiO2 ®lms deposited at different target voltages. The error bar of the refractive index is ^0.01.
adapted. The ellipsometer is a polarizer-sample-analyzer (PSA) system with rotating analyzer. In the present work, the step-scan modus was applied. With an acquisition time of 1 s for a complete spectrum of 1024 data points (280±850 nm), an acceptable signal to noise ratio was reached. For ex situ ellipsometry, a SENTECH SE800 gonio-spectroellipsometer was used. The ellipsometer arms are directed between the two cathodes of the MF magnetron, so the growth of the layer on the MF side can be examined in situ. The angle of incidence was determined by a ®t to the ellipsometric C-spectrum of the uncoated ¯oatglass sample with a reproducibility of F0 65 ^ 0:028. For the coating on the DC position the samples can be moved to the opposite side of the vacuum chamber. In the present paper, only the main features of the stabilization scheme for the reactive MF plasma process will be given, details will be published elsewhere. For the stabilization on short time scales (,1 s) the electrical parameters (voltage, power) and the intensity of the optical emission lines were used. For the SiO2 process with silicon targets, the target voltage was stabilized. All process setpoints from metallic mode throughout the transition mode to the oxide mode could be stabilized. The TiO2 process stabilization, however, was performed with the an optical emission system (OES). Up to power densities of 5.5 W/cm 2, the complete hysteresis could be stabilized.
From the ex situ ellipsometric analysis, it was found out that a model which includes a single homogeneous layer is suitable for the description of the ellipsometric data. In Fig. 1, the refractive index and the absorption index at l 600 nm as a function of the target voltage are shown. Up to 450 V (transition mode), transparent layers with an absorption index of 10 24 were obtained. At higher target voltages (metallic mode), absorbing layers result. At the lowest voltages (oxide mode) the absorption again increases. Potentially, absorption of free electrons could be taken into consideration for that infrared absorption, as can be concluded from an appearing Drude term in the ®t. The background of this absorption will be part of further investigations. From the in situ ellipsometric spectra, the thickness and the optical indices at the different working points were determined as depicted in Fig. 2. From the thickness, the rate was determined by numerical derivation, where the last ®ve points were used for the polynomial ®t. This polynomial was also used for the extrapolation of the thickness, in order to estimate the residual time required for the exact thickness. This is very useful in thin layers, where coating time of less than one minute were applied. At larger target voltages (transition and metallic mode) the linear ®ts of the ®lm thickness indicate a constant growth rate. At lower voltages, deviations from the linear ®ts occur, indicating a drift of the working point in power and in voltage. This is mainly due to the growing oxide layer on the silicon targets. This drift can also be seen in the inset of Fig. 2, where different rates at the same target voltage of 290 V are obtained. As the voltage is
3. Ex situ and in situ ellipsometry 3.1. Introduction For modeling the optical dispersion, the well-known Drude±Lorentz oscillator model was employed. Within this model, the complex dielectric function can be written as
1~ 1inf 1
V2pk v2p 1 2 v2 2 iGFE v V20 2 v2 2 iGv
1
with 1 inf being the high frequency dielectric constant, V pk,
Fig. 2. Thicknesses determined by in situ ellipsometry versus time of the SiO2 layers deposited at different voltages. The lines are linear ®ts. Inset: Growth rate as a function of target voltage.
M. VergoÈhl et al. / Thin Solid Films 351 (1999) 42±47
44
Table 1 Deposition parameters and properties of the deposited TiO2 ®lms onto unheated substrates. The ®lm thicknesses are typically 100 nm. For Sample (2e), for ex situ ellipsometry a two-layer model, for in situ ellipsometry a single layer model is recommended. The Samples (2a±e) are deposited in the oxide mode next to the transition mode (minimum oxygen ¯ow), therefore the rate is already enlarged by a factor of two±three compared to the ®lms deposited with completely oxidized targets. The error of n is ^0.01, the error of k is ^0.005. Note that the value of n for Sample (4) is the mean value Sample
Target power (W/cm 2)
Deposition mode
Rate (nm/s)
n (550 nm)
k (550 nm)
Ar ¯ow (sccm)
Recommended model
1 2a 2b 2c 2d 2e 3a 3b 3c 4
1.0 1.0 4.0 5.0 6.0 7.5 2.9 4.0 5.2 6.0
DC, oxide MF, oxide MF, oxide MF, oxide MF, oxide MF, oxide MF, transition MF, transition MF, transition MF, oxide
0.038 0.05 0.16 0.24 0.27 0.42 0.397 0.67 0.79 0.23
2.440 2.480 2.475 2.500 2.520 2.540 2.515 2.510 2.510 2.620
0.008 0.008 0.006 0.009 0.011 0.013 0.004 0.005 0.004 0.017
60 90 90 90 90 90 90 90 90 50
One layer One layer One layer One layer One layer One±two layer One layer One layer One layer Two layer
increased further, the rate increases continuously up to 2 nm/s. Low values of absorption are obtained in this region. Since the absorption increases at higher target voltages, the working point for the deposition of transparent layers is at 450 V. The rate is a monotone function of the process voltage, therefore it can be used as a control parameter for the SiO2 process. 3.3. Titanium oxide Different TiO2 ®lms with thicknesses of typically 100 nm were deposited on ¯oatglass in the DC and the MF mode, respectively, where the target power was limited to 1 and 7.5 W/cm 2 target area, respectively. The deposition parameters and results of the investigations are summarized in Table 1. Fig. 3 shows an ex situ ellipsometric measurement at different angles of incidence between 65 and 758 of
Fig. 3. Ex situ ellipsometric data of a TiO2 ®lm on glass (Sample (2e)). The spectra were acquired at angles of incidence of 65, 70, and 758. For the ®t within the one layer model (dotted line), a mean square error of 9.3 (compare to the data in the inset of Fig. 4) was reached. Within the two layer model (solid line), the MSE could be reduced to 5.8.
Sample (2e) which was deposited at 7.5 W/cm 2. In that ®t, the implementation of an interface layer with a smaller refractive index than that of the upper TiO2 layer was necessary. This interface layer was modelled with a Bruggeman effective medium of TiO2 and voids. The thickness of the bottom layer amounts 71 nm, the void concentration 6%. These inhomogeneities in the growth direction were observed to depend on the plasma conditions. This sample was deposited at lower Ar ¯ux, therefore the inhomogeneity was the largest. For other samples grown at higher pressures (e.g. Samples (2a±c)), the single layer model is adequate. Hence, a model which involves one or two TiO2 layers is required for thicknesses below 150 nm. For thinner ®lms below 70 nm, in any case the simple model is adequate. The layers deposited at lower power in the oxide mode (Samples (2a±c)) could be described within the simple model, as shown by the low values of the void concentration in Fig. 4, and by the values of the mean square error (MSE) shown in the inset. The MSE were plotted for the two models as a function of the target power. Primary at 6 W/ cm 2 (Sample (2d)), a difference of the MSE between the two models becomes signi®cant. A surface roughness layer of 1.7 nm could improve the ®t only for the ®lms sputtered in MF oxide mode, as can be seen from the reduction of the MSE from 5.5 (O) down to 3 (B) at 1 W/cm 2. This may be due to the larger grain size and the higher surface roughness of the ®lms deposited in the MF oxide mode compared to the ®lms deposited in the DC mode, as found in AFM investigations. This may explain the lower MSE value of the ®t of the DC sputtered samples at 1 kW power (Sample (1)). For the in situ application with the higher noise level and the lower acquisition time, such slight improvements due to very thin surface layers will be neglected. In contrast to the MF oxide mode, all samples deposited in the transition mode could be treated within the simple model, i.e. no difference in the MSE values were observed between the two models. In Fig. 5, the optical constants of the different samples determined from the ex situ ellipsometric analysis were
M. VergoÈhl et al. / Thin Solid Films 351 (1999) 42±47
Fig. 4. Void concentration of the TiO2 Samples (2a±e) (MF, oxide mode). Inset: Mean square error (MSE) of the ®ts of the ex situ ellipsometric spectra within the one layer model and within the two layer model. The symbols are: (X), MF one layer model, oxide mode; (O), MF two layer model, oxide mode; (B), MF two layer model, oxide mode and surface layer; (1), DC one layer model; (£), DC two layer model. Note that for all samples deposited in the transition mode (Samples (3a±c), not shown here), no difference in the MSE values were observed between the two models.
plotted as a function of the process power (a), (b) and of the temperature, respectively, (c), (d). The DC sputtered ®lm at 1.0 W/cm 2 shows the lowest refractive index (n 2:43). The MF ®lm sputtered with the same power density shows n 2:48. With increasing power density, both the refractive index and the absorption increase. At 7.5 W/cm 2, the refractive indices of the bottom and the top layer are shown. While the refractive index of the bottom layer nearly corresponds to that of the ®lm sputtered at 1 W/cm 2 in the MF mode, the refractive index of the top layer agrees with the rutile value (open square in Fig. 5a). The refractive index and the absorption index of the TiO2 layers deposited in the transition mode are also shown in
Fig. 5. Refractive index n (a), (c) and absorption index k (b), (d) of different TiO2 layers deposited on ¯oatglass substrates as a function of the target power (a), (b) and of temperature, respectively, (c), (d). Symbols are: (O), DC mode; (B), MF oxide mode; (V), MF transition mode.
45
Fig. 6. In situ ellipsometric spectra of Sample (4), measured at different time positions. The ®ts are also shown.
Fig. 5a,b, respectively. The normalized OES intensity ratio of the titanium line to the oxygen line was 0.5. It can be seen that the refractive indices are slightly larger than that of the TiO2 ®lms sputtered in the oxide mode. The growth rate of the sample deposited at 4 W/cm 2 was determined as 0.67 nm/s, which is a factor of three larger than the corresponding sample deposited in the oxide mode. Note that the Samples (2a±e) are deposited with not completely oxidized targets, as the OES (normalized intensity ratio of 0.1) show. Therefore, the rate is already enlarged by a factor of three compared to the ®lms deposited with completely oxidized targets (OES intensity ratio ,0.02). Two reasons may be responsible for this increasing refractive index in the oxide mode: First, the higher energy of the ions in the MF process [5], secondly an increase of the temperature due to plasma heating. In Fig. 5c, ®lms of the same thickness of 100 nm were deposited in the DC mode onto heated substrates. The refractive index also increases from 2.43 corresponding to the anatase value for unheated substrates to a value of 2.58 for substrate temperatures of 2008C, which is similar to the measured refractive index of a rutile wafer. For the MF process, the substrate temperature was estimated not to be higher than about 1008C under the same deposition conditions. Therefore nearby the plasma heating also the higher ion energy of the MF plasma seem to be responsible for the higher refractive index. With increasing power density, the absorption index of the ®lms deposited in the oxide increases. This dependency was not observed in the ®lms deposited in the transition mode, where lower values were determined. EPMA investigations on thicker ®lms delivered oxygen to titanium ratios between 1.99 and 2.01, which is near to the ratio of a TiO2 standard (1.99), so that absorption due an understochiometry should be neglectible. XRD measurements of the MF oxide ®lms reveal both a sharp anatase (101)-peak and a broader rutile (110)-peak with half the intensity of the anatase peak. For the ®lms deposited in the MF transition mode, no peaks were found, indicating an amorphous ®lm structure, in agreement with [1,4]. Therefore, it can be
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M. VergoÈhl et al. / Thin Solid Films 351 (1999) 42±47
model shows a constant MSE value. Therefore, it can be summarized that for all TiO2 ®lms with thickness below 65 nm, the simple model should be used while for thicker ®lms, the two layer model is superior under speci®c deposition conditions. 3.4. SiO2/TiO2 multilayer antire¯ective coating
Fig. 7. Results of the ®t of the in situ ellipsometric data of Sample (4) (see Fig. 6). Shown are the thicknesses of the bottom and the top layer, and the full thickness, respectively. The interface layer was assumed to have 10% voids. Inset: Mean square error of the two models versus full thickness.
concluded that the lower absorption of the ®lms deposited in the transition mode is mainly due to the amorphous structure with lower losses at the grains. Depending on the deposition technique and the process parameters, the refractive index of TiO2 ®lms shows large differences. The lowest values are found in evaporation techniques (l 550 nm: n 2:33 [9], n 2:2 [11]), while for ion plating (n 2:52±2:61 [11]), ion assisted deposition (n 2:50 at 508C, n 2:43 at 1378C [11]), RF diode sputtering (n 2:1±2:5 [10], 2.49 [11]) and RF magnetron sputtering (n 2:55 [6], 2.1±2.4 [7]) higher refractive indices are found. In general, higher substrate temperature as well as the energy of bombarding ions [8,12] increase the refractive index. In [1,4], the TiO2 ®lms deposited by the MF sputtering show signi®cantly higher values the ®lms deposited with the DC technique. This is also found here. In addition, the refractive indices reported in [1,4] are in a good agreement with our data. In Fig. 6, some in situ ellipsometric measurements at different deposition times of Sample (4) are shown together with the ®ts within the two layer model. As ®t parameters, the thickness of the two layers were used. In Fig. 7, the ®tted thicknesses are plotted versus the full thickness of Sample (4). In this plot, the onset of the inhomogeneous growth can be observed at 65 nm full thickness. This value is in very good agreement with the ex situ ellipsometric results. At lower full thickness (i.e. ,65 nm), there is a large correlation between the thickness of the bottom layer to that of the top layer. Thus, for an optimum of sample information, in region I the one layer model should be used, whereas in region II the two layer model should be applied. It should be emphasized that the resulting thickness remains constant, due to the low correlation with the void concentration. The inset of Fig. 7 shows the mean square error of this ®t. At 65 nm ®lm thickness, a signi®cant increase of the MSE is observed for the one layer model. Only the two layer
A four layer SiO2±TiO2 broadband antire¯ective coating was deposited on a ¯oatglass substrate. The desired sample geometry is shown in the inset of Fig. 8. As mentioned before, only at the MF side an in situ ellipsometric control could be applied during the deposition process. This is the case for the SiO2 layers. For the control of the thickness on the DC side (TiO2), an intermediate ellipsometric measurement was performed after stopping the process and rotating the sample in the measuring position. The deposition time left was then estimated by comparing the desired to the measured thickness. From the ellipsometric analysis it was found that the thicknesses are in a good agreement with the desired thicknesses of the layer stack and the calculated re¯ectivity is in a good agreement with the measured re¯ectivity, as shown in Fig. 8. 4. Conclusions The present investigations on TiO2 and SiO2 single layers and SiO2±TiO2 multilayers show that in situ ellipsometry is an excellent technique for monitoring and control of the deposition process of optical functional layers like antire¯ective coatings. Due to its simplicity, the Lorentz model is convenient as a good and fast description for the optical dispersion within the spectral range below the fundamental absorption edge. While in SiO2, a one layer model could be used, some of the TiO2 layers display inhomogeneities
Fig. 8. Re¯ectivity of a four layer antire¯ective coating. Also shown is the calculated re¯ectivity, as a result of the in situ ellipsometry data. Inset: Desired (left) and determined (right) thicknesses of the antire¯ective coating. Note that the ellipsometric measurements could only be performed on the SiO2 side, the TiO2 thicknesses were estimated by an intermediate measurement near the end of the process.
M. VergoÈhl et al. / Thin Solid Films 351 (1999) 42±47
which depend on the plasma conditions. These layers can be modelled within a two layer model. The onset of the inhomogeneous growth was observed via in situ ellipsometry, in excellent agreement with the ex situ ellipsometric results. Further improvements of the in situ ellipsometric system may be achieved by a reduction of both the acquisition and ®tting time, which should result in a better performance of the thickness estimator. Acknowledgements The authors are pleased to acknowledge the BMBF for the ®nancial support under contract 13N6911. References [1] J. Szczyrbowski, G. BraÈuer, G. Teschner, A. Zmelty:, J. Non-Cryst. Solids 218 (1997) 25.
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[2] S. Schiller, U. Heisig, K. Steinfelder, J. StruÈmpfel, A. Friedrich, R. Fricke, 6th Int. Conf. on Ion and Plasma Assisted Techniques, Vol. 23, 1987, pp. xi, 481. [3] B. Szyszka, S. JaÈger, J. Non-Cryst. Solids 218 (1997) 74. [4] G. BraÈuer, J. Szczyrbowski, G. Teschner, J. Non-Cryst. Solids 218 (1997) 19. [5] S. JaÈger, B. Szyzska, J. Szczyrbowski, G. BraÈuer, Surf. Coat. Technol. 98 (1998) 1303. [6] W.J. Coleman, Appl. Opt. 13 (4) (1974) 946. [7] S. Ben, Amor, G. Baud, J.P. Besse, M. Jacquet, Mater. Sci. Eng. B 47 (1997) 110. [8] N. Martin, C. Rousselot, D. Rondot, F. Palmino, R. Mercier, Thin Solid Films 300 (1997) 113. [9] J.V. Grahn, M. Linder, E. Fredsiksson, J. Vac. Sci. Technol. A 16 (4) (1998) 2495. [10] W.T. Pawlewicz, P.M. Martin, D.D. Hays, I.B. Mann, Optical Thin Films (1982) 105. [11] J.M. Bennet, E. Pelletier, G. Albrand, et al., Appl. Opt. 28 (15) (1989) 3302. [12] P.J. Martin, J. Mater Sci. 21 (1986) 1.