Structure, morphology and optical properties of SiO2−x thin films prepared by plasma-assisted pulsed laser deposition

Structure, morphology and optical properties of SiO2−x thin films prepared by plasma-assisted pulsed laser deposition

Applied Surface Science 254 (2008) 1730–1735 www.elsevier.com/locate/apsusc Structure, morphology and optical properties of SiO2x thin films prepare...

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Applied Surface Science 254 (2008) 1730–1735 www.elsevier.com/locate/apsusc

Structure, morphology and optical properties of SiO2x thin films prepared by plasma-assisted pulsed laser deposition Xiliang He a,b, Jiehua Wu c, Lingnan Wu c, Lili Zhao c, Xiangdong Gao a, Xiaomin Li a,* a

State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, People’s Republic of China b Graduate School of the Chinese Academy of Sciences, People’s Republic of China c The key Laboratory of Inorganic Coating Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, People’s Republic of China Received 27 March 2007; received in revised form 5 July 2007; accepted 17 July 2007 Available online 20 July 2007

Abstract The amorphous silicon oxide SiO2x thin films were prepared by the plasma-assisted pulsed laser deposition (PLD) method. X-ray diffraction spectrometry (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-VIS-NIR scanning spectrophotometry and ellipsometry were used to characterize the crystallinity, microscopic morphology and optical properties of obtained thin films. The influences of substrate temperatures, oxygen partial pressures and oxygen plasma assistance on the compositions of silicon oxide (SiO2x) thin films were investigated. Results show that the deposited thin films are amorphous and have high surface quality. Stoichiometric silicon dioxide (SiO2) thin film can be obtained at elevated temperature of 200 8C in an oxygen plasma-assisted atmosphere. Using normal incidence transmittance, a novel and simple method has been proposed to evaluate the value of x in transparent SiO2x thin films on a non-absorbing flat substrate. # 2007 Elsevier B.V. All rights reserved. Keywords: Pulsed laser deposition; Plasma assistance; Silicon oxide; Thin films; Optical properties

1. Introduction Pulsed laser deposition (PLD) is a versatile physical vapor deposition coating technique for the production of thin films with complex chemical compositions, which has been successfully applied to an extremely wide range of materials including dielectrics, semiconductors, metals, superconductors and so forth [1–8]. Compared with other physical vapor deposition techniques such as magnetron sputtering, e-beam evaporation, PLD possesses several unique advantages: wide and easy composition control, easy deposition for refractory materials, favorableness for multilayer and superlattice deposition using several targets, etc. Silicon dioxide (SiO2) thin film is an important material for semiconductor industry [9–12] as the gate dielectric material, the low refractive index choice of the multilayer optical thin

* Corresponding author. Tel.: +86 21 52412554; fax: +86 21 52413122. E-mail address: [email protected] (X. Li). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.127

films [13–18] as the antireflective coating or the active waveguide, and so on. Up to now, several physical vapour deposition methods have been explored to prepared silicon oxide (SiO2x) coatings, including plasma ion-assisted deposition [19], magnetron sputtering deposition [20], and ebeam deposition [19,21] etc. However, little attention has been paid to the deposition of silicon–oxygen coatings by PLD [12,22]. Especially, PLD method had a great difficulty in fabricating stoichiometric SiO2 thin film when silicon target was used. Therefore, it is very important to study the parameters influencing the x value in sub-stoichiometric SiO2x thin film deposited by PLD technique using silicon target. The adoption of reactive atmosphere instead of the vacuum or inert process gas atmosphere is expected to be an effective measure to control the stoichiometry of SiO2x thin film deposited by PLD. In this paper, the stoichiometric SiO2 thin films ware deposited on BK7 glass and silicon wafer substrates by PLD method using oxygen plasma-assistance. Effects of the substrate temperatures, the oxygen pressures and oxygen

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plasma-assistance on the compositions of SiO2x thin films were discussed in detail. Furthermore, a simple method to estimate the x value of SiO2x thin films using the transmittance spectra has been proposed and demonstrated. These studies can offer useful inspiration for other PVD techniques. 2. Experimental 2.1. Film deposition High purity silicon target (99.95%) was used for the ablation experiments using a Lambda Physik KrF (l = 248 nm) excimer laser system (COMPex201). The focused laser fluence was set at 3 J/cm2 and the target-substrate distance was 50 mm (optimized). The BK7 glass and silicon substrates were thoroughly cleaned to remove the surface contamination before loading into the chamber. The target was mounted to a holder which would rotate to avoid the formation of deep craters for laser irradiation. The emitted species were deposited directly onto the substrate mounted vertically to the target, see Fig. 1. Prior to the deposition of thin films, the stainless-steel vacuum chamber was evacuated with a turbomolecular pump to pressure below 2  104 Pa. The flow of the oxygen process gas was adjusted by means of electronic mass flow controller. The deposition ambient parameters employed in this work are listed in Table 1. Variation of deposition temperatures and oxygen pressures and oxygen plasma-assistance were used to explore how much the process conditions influence the compositions of sub-stoichiometric SiO2x thin films and find the best growth condition for stoichiometric SiO2 thin films. Fig. 1 is the schematic view of the plasma-assisted PLD system for thin film deposition. The feature of the PLD system used in this study is its plasma-assisted component, which can ionize the process gas of oxygen through the voltage applied between the vacuum chamber wall and the process gasintroducing-gun. That process can elevate the activation energy of process gas and highly enhance the activity of oxygen, thus resulting in the easy bonding between silicon and oxygen. In that way, the compositions of SiO2x thin films can be easily changed, and the x value of SiO2x can be decreased. One important trait of the plasma-assisted component is that the position of the process gas-introducing-gun can be changed, which allows the changing of the distance between the ionized process gas and the substrate. In this work, the distance between

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Table 1 The process conditions of Sample1 to Sample7 (S1–S7) Samples S1 S2 S3 S4 S5 S6 S7

Temperature (8C) RT 300 400 RT 300 RT 200

Oxygen pressure (Pa) 2

2  10 2  102 2  102 20 20 3.6 2.4

Plasma-assistance Unused Unused Unused Unused Unused Used Used

the gas-introducing-gun and the substrate was optimized as 20 mm. The discharging voltage was set up as 0.6 KV and the discharging current was about 40 mA. Supposed that the pressure in the stainless-steel chamber is equilibrium, the ionized proportion of the oxygen is about 10–20% as a function to the whole oxygen in the chamber. Especially, that part of ionized oxygen plasma is around the substrate which would probably react with the ablated species. The study of oxygen plasma-assistance can provide beneficial help to other PVD method such as e-beam evaporation. 2.2. Film characterization The film structure and surface quality were examined by Xray diffraction spectrometry (XRD), scaning electron microscopy (SEM) and atomic force microscopy (AFM). The optical property of the thin film was determined by UV-VIS-NIR scanning spectrophotometry in the spectral range from 600 to 1200 nm wavelengths with 1 nm spectral resolution. The chemical bonding state and chemical composition of thin film were measured by X-ray photoelectron spectroscopy (XPS). The refractive index of the films deposited on silicon wafers was determined by fixed wavelength ellipsometry (l = 633 nm). 2.3. Theoretical considerations Considering a uniform BK7 glass substrate without film on it, at normal incidence, the reflection coefficient r, transmission coefficient t, reflectance R and transmittance T of the substrate (for a parallel beam of light of unit amplitude and of wavelength l) are given by Fresnel formulas [23] as follows: n0  n1 r¼ (1) n0 þ n1 2n0 n0 þ n1  2  2 2  n0  n1 1  ðn1 =n0 Þ 1  n10 R¼ ¼ ¼ 1 þ ðn1 =n0 Þ n0 þ n1 1 þ n10 t¼

T ¼1R

Fig. 1. Schematic view of the plasma-assisted PLD system.

(2)

(3) (4)

where n0, n1 are the refractive indices of air, substrate, and n10 represents the relative refractive index of substrate-to-air system, actually equivalent to the substrate refractive index n1. For Eq. (4) the dispersion and absorption are neglected.

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Then consider a transparent homogeneous film of uniform thickness, d, and refractive index, n, is bounded on such a substrate of either side by two semi-infinite non-absorbing layers. Define optical admittance of the assembly (including air-tofilm boundary, film, film-to-substrate boundary, substrate, substrate-to-air boundary) Y as [24]: Ht K  Et



(5) 3. Results and discussion

where Ht, Et are the tangential component of magnetic vector, electric vector of the light, and K is the tangential unit vector. Since the tangential components of Ht and Et are continuous across a boundary with different refractive indices, Y is continuous through the assembly. Defining the film and substrate as a new substrate, Y is the equivalent refractive index of the new substrate, which can be deduced from the definition of optical admittance of the assembly. As for the boundary between incidence medium (refractive index n0) and the new substrate (equivalent refractive index Y), the reflection coefficient r0, transmission coefficient t0 , reflectance R0 and transmittance T0 of the assembly are now given by Fresnel formulas as follows: r0 ¼

n0  Y n0 þ Y

(6)

t0 ¼

2n0 n0 þ Y

(7)

R0 ¼



n0  Y n0 þ Y

T 0 ¼ 1  R0

other hand, because of some applications’ needs, more and more SiO2 thin films are required to be deposited on glass substrates, in which circumstances the refractive indices of SiO2 thin films are difficult to be measured owing to the appropinquity of their refractive indices to the glasses. Thus the simplified theoretical treatment has special advantage to judge how much the deposited SiO2x thin films close to the stoichiometric SiO2 thin films in chemical compositions.



2 ¼

1  ðY=n0 Þ 1 þ ðY=n0 Þ



2 ¼

1  Y0 1 þ Y0

2 (8) (9)

where Y0 is the relative refractive index from new substrate to air, actually equivalent to the equivalent refractive index of the new substrate. For Eq. (9) the dispersion and absorption are neglected. Therefore, Eq. (3) and Eq. (8) deal with the relations from the relative refractive indices n10 or Y0 to the reflectance R or R0 , and they possess the same expressions. Since the expressions of R and R0 are proper fractions, R < R0 can be deduced when n10 < Y0, and R > R0 can be deduced when n10 > Y0. Hence, according to Eq. (4) and Eq. (9), T > T0 exists when n10 < Y0, and T < T0 exists when n10 > Y0, vice versa. Then the relation between relative refractive index n10 and Y0 can be determined by the relation between T and T0. Since the refractive index of BK7 glass substrate (1.52 at l = 550 nm) is higher than that of SiO2 (where X value equals to 0), the deviation degree of x value away from 0 can be estimated using the relation between n10 and Y0. Here the simplified theoretical treatment neglected the wave character of light and corresponding interference effects and the effects of film thickness. The main intention here using this treatment was to judge the relation between the trends of transmittance curves to the films’ refractive indices which have a relation to the films’ chemical compositions for SiO2x. Hence, the neglecting factors here do not influence the judgment. On the

3.1. Film structure Fig. 2 shows XRD results of SiO2x thin films deposited at room-temperature (Sample 1) and 300 8C (Sample 2). No peak value of crystallined silicon oxide thin films was found, which is consistent with results by Lackner et al. [22]. As it can be seen from SEM photographs (Fig. 3), all samples have amorphous compact structure almost without obvious defects. Film Sample 6 by plasma-assistance at room-temperature has the most compact structure compared with other three samples. However, Sample 7 by plasma-assistance at elevated temperature has a little defect due to the bombardment and the high substrate temperature. The roughness of mean square (RMS) of these four samples’ surfaces measured by atomic force microscopy (AFM) was 0.9522 nm, 1.044 nm, 1.475 nm, 3.189 nm for Sample 1 (S1), Sample 5 (S5), Sample 6 (S6), Sample 7 (S7), respectively. From these above results it can be obtained that the oxygen plasma improved the compactness of thin films significantly which has important influence on the optical properties of thin films, although it resulted in a little rough surface quality. 3.2. Optical properties and their relations to chemical compositions 3.2.1. Films deposited at low oxygen pressure Fig. 4 shows the transmittance curves of SiO2x thin films deposited at different temperatures (i.e., room temperature

Fig. 2. XRD patterns of SiO2x thin films of S1 and S2: (S1) RT 2  102 Pa, (S2) 300 8C 2  102 Pa.

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Fig. 3. SEM photographs of (a) S1, (b) S5, (c) S6, (d) S7: (S1) RT 2  102 Pa O2 without plasma, (S5) 300 8C 20 Pa O2 without plasma, (S6) RT 3.6 Pa O2 plasmaassistance, (S7) 200 8C 2.4 Pa O2 plasma-assistance.

(S1), 300 8C (S2), and 400 8C (S3)) at an oxygen pressure of 2  102 Pa (the transmittance of BK7 glass substrate without film was taken as the reference). The transmittance curves show a step at the wavelength of 800 nm because of the changing of detector from PbS based infrared detector to photomultiplier tube of the shortwave band in the UV-VIS-NIR scanning spectrophotometry. It can be seen that the transmittance of all three samples was lower than that of BK7 glass substrate. Based on the theoretical considerations discussed above, the

Fig. 4. Transmittances of S1–S3 compared with BK7 substrate: (S1) RT 2  102 Pa, (S2) 300 8C 2  102 Pa, (S3) 400 8C 2  102 Pa.

inequation Y0S3 > Y0S2 > Y0S1 > n1 can be deduced, where Y0S3, Y0S2, Y0S1 represent the equivalent refractive indices of new substrates (see the definition in the theoretical considerations part above) for S1, S2 and S3, respectively. It can be deduced from above relation that along with the increase of substrate temperature the value of x in SiO2x increases from S1 to S3. With the increase of the substrate temperature, the migration of surface atoms was enhanced, the film growth was speeded up, so the oxygen deficiency became more serious due to the low oxygen pressure, thus leading to the increase of x value. On the other hand, as a result of the low oxygen pressure x value of S1–S3 had an obvious deviation from zero judged from their transmittance curves compared with that of BK7 glass irrespective of substrate temperature. And that the chemical compositions of S1, S2, and S3 were identified by XPS to be SiO1.36, SiO1.29 and SiO1.23, respectively, in good agreement with the results determined by the simple method through transmittance curves. 3.2.2. Films deposited at high oxygen pressure Fig. 5 indicates the transmittance curves of the SiO2x thin films deposited at different temperatures (i.e., room temperature (S4), 300 8C (S5)) at an oxygen pressure of 2  101 Pa. Similarly, inequation Y0S1 > Y0S4 > Y0S5 > n1 can be determined according to the theoretical considerations discussed above, where Y0S4, Y0S5 represent the equivalent refractive indices of new substrates for S4 and S5, respectively. The

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Fig. 5. Transmittances of S4–S5 compared with BK7 substrate: (S4) RT 20 Pa, (S5) 300 8C 20 Pa.

varying rules of x value along with substrate temperatures were the same as that for low oxygen pressure. However, the transmittances of S4 and S5 were higher than that of S1 and close to that of BK7 glass substrate. That is to say the x value of S4 and S5 had an evident amount of reduction compared with that of S1–S3 and were close to 0, at which circumstances the compositions of SiO2x was very close to the SiO2 stoichiometry as a result of the high oxygen pressure which offered sufficient oxygen sources for the bonding of Si–O–Si. The refractive index of S5 deposited at silicon substrate was 1.54 at 633 nm wavelength determined by the Ellipsometry, in good agreement with the result by the simple method through transmittance curve. Thereby SiO2x thin film with high optical quality was obtained with stoichiometry close to SiO2 thin film at oxygen pressure of 2  101 Pa ambient even at substrate temperature of 300 8C. 3.2.3. Films deposited by oxygen plasma-assistance Fig. 6 presents the transmittance curves of SiO2x thin films deposited in oxygen plasma-assisted ambient at different temperatures and oxygen pressures (i.e., room temperature,

Fig. 7. XPS spectra of the thin film S7 (200 8C 2.4 Pa oxygen plasmaassistance).

3.6 Pa O2 (S6); 300 8C, 2.4 Pa O2 (S7)). Parallel results of Y0S4 > Y0S6 > n1 > Y0S7 can be obtained according to the theoretical considerations discussed above, where Y0S6, Y0S7 represent the equivalent refractive indices of new substrate for S6 and S7, respectively. Owing to the enhancement of activation energy for oxygen plasma compared with atmosphere without oxygen plasma, the bonding of Si–O–Si became easier in the oxygen plasma-assisted ambient, resulting in the decrease of x value in SiO2x thin films. The refractive index of S7 deposited on silicon substrate was 1.50 at 633 nm wavelength determined by ellipsometry. Additional XPS investigation for S7, see Fig. 7, was carried out in order to study the nature of chemical bonding and chemical composition of Sample 7. The XPS spectra indicated the presence of Si4+ (103.0 eV), oxygen (532.5 eV) and carbon (285.0 eV). The contamination of carbon came from the probe handling or storage in air or from the vacuum chamber. In addition, the XPS measurement gave the O/Si ratio of 2 in obtained SiO2x thin film, indicating that stoichiometric SiO2 thin film was obtained with oxygen plasma-assistance at elevated temperature. Note that the x value became smaller along with the increase of substrate temperature and at 200 8C atmosphere x value of Sample 7 equaled to 0. That was different from the varying rules of x value along with substrate temperature variation in ambient without plasma-assistance. Two aspects may result in the rapid and sufficient bonding of Si–O–Si at elevated temperature in oxygen plasma-assisted atmosphere: (1) the atoms deposited onto substrate at elevated temperature possess higher activity and migration; (2) the oxygen plasma with the ionized oxygen anion has the enhanced activation energy. Based on above results, it can be concluded that the oxygen plasma can improve significantly the oxygen proportion of SiO2x thin films, which has prominent influence on the transmittance of silicon oxide thin films. 4. Conclusions

Fig. 6. Transmittances of S6–S7 compared with BK7 substrate: (S6) RT 3.6 Pa oxygen plasma-assistance, (S7) 200 8C 2.4 Pa oxygen plasma-assistance.

Using PLD method, SiO2x and SiO2 thin films were deposited on BK7 glass and silicon wafer substrates at various atmospheres. Results indicate that the deposited thin films are

X. He et al. / Applied Surface Science 254 (2008) 1730–1735

amorphous and have high surface quality. A simple and novel method for estimating stoichiometry of SiO2x thin film has been proposed based on normal incidence transmittance or reflectance data. The x value in SiO2x thin films decreased along with the increase of substrate temperatures in oxygen plasma-assisted ambients which was different from the case without the oxygen plasma-assiatance. At low oxygen pressure of 2  102 Pa the compositions of SiO2x thin films had higher deviations away from stoichiometry SiO2 thin film irrespective of elevated substrate temperature. At high oxygen pressure of 2  101 Pa SiO2x thin films with compositions close to stoichiometric SiO2 thin films were fabricated. Using the oxygen plasma-assistance, stoichiometric SiO2 thin film was deposited at 200 8C substrate temperature. The proposed method for evaluating the stoichiometry of deposited thin film by the transmittance in this paper can be extended to the estimation of all type of transparent thin films deposited on non-absorbing flat substrates. Acknowledgements This work is financially supported by National Natural Science Foundation of China (10576036). Thanks to professor Jun Shen and M.D. Zhiyong Xie for the help of ellipsometric investigation. References [1] D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. [2] T.L. Chen, X.M. Li, K.S. Wan, W.L. Zhu, G. Pezzotti, Appl. Phys. Lett. 87 (2005) 181914. [3] D. Dijkkamp, T. Venkatesan, X.D. Wu, S.A. Shaheen, N. Jisrawi, Y.H. Min-Lee, W.L. McLean, M. Croft, Appl. Phys. Lett. 51 (1987) 619.

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