Spectroscopic analysis of silica soot deposited by flame hydrolysis deposition

Vibrational Spectroscopy 46 (2008) 14–21 www.elsevier.com/locate/vibspec

Spectroscopic analysis of silica soot deposited by flame hydrolysis deposition Dongwook Shin * Division of Material Science & Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea Received 4 April 2007; received in revised form 1 August 2007; accepted 3 August 2007 Available online 8 August 2007

Abstract Flame hydrolysis deposition is widely used to fabricate SiO2/Si passive optical waveguide devices for optical communication network. In flame hydrolysis deposition, silica soot doped with various elements such as B, P, and Ge is formed in the form of thin film layer and the composition of this layer is not predictable due to complex process variables. In this research, it was intended to clarify the quantitative relationship between simple process parameters and resulting chemical composition. In FHD process, compositional analysis of soot by FTIR and ICP-AES under the control of the amount of dopant is carried out to obtain the compositional information, which will affect the optical and thermo-mechanical properties of films. By measuring absorbance spectra with FTIR, the variation of B–O, Si–O, OH (H2O) composition in FHD soot was investigated and could be quantified in B–O absorbance by ICP-AES analysis. # 2007 Elsevier B.V. All rights reserved. Keywords: Flame hydrolysis deposition; Silica waveguide; Planar lightwave circuit; Silica soot

1. Introduction It has been passed long time since optical communication network became a major backbone communication network for high data transfer rate responding the rapidly increasing data traffic by voice telephone service, Internet, multimedia data communications. Optical communication itself has been challenging for the wider bandwidth and faster transfer rate, and eventually the wavelength division multiplexing (WDM) technology became a worldwide accepted standard in optical communication industry. In WDM optical communication, the bandwidth of erbium doped fiber amplifier (EDFA) is divided into sub-band of 100 GHz (1.6 mm) or 200 GHz (0.8 mm) and each of these subband serve as a data channel. To implement WDM technology, it is required to develop the optical devices enabling accurate manipulation of each wavelength, such as AWG MUX/ DEMUX and Add/Drop multiplexer. As the number of channel increases (data transfer capacity increases), the structure of device becomes more complex and, hence, the price of device increases quickly. Furthermore, it is very unpractical to * Tel.: +82 2 2220 0503; fax: +82 2 2299 3851. E-mail address: [email protected]. 0924-2031/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2007.08.001

fabricate these devices by traditional optical fiber device technology, since the productivity and reproducibility of fiber devices get worse as the number of channel increases. Therefore, the planar waveguide device based on SiO2/Si becomes a replacement of the fiber devices since it can be produced in mass by employing semiconductor fabrication technology. The device size, price, productivity, and reproducibility of waveguide devices are better than fiber devices. Therefore, SiO2/Si waveguide devices have drawn a large attention in the industries producing passive integrated optical devices used in optical communication. As a common process for fabricating passive integrated optical devices used in optical communication, flame hydrolysis deposition (FHD) method is widely employed to deposit SiO2 films, in which the hydrolysis of SiCl4 in high-temperature H2–O2 flame is utilized. It is known that the precise control of deposition rate and the accurate compositional control in FHD process are difficult compared with CVD process and additional densification process of porous soot into amorphous SiO2 film is required. However, FHD method is extensively and preferably employed to deposit thick SiO2 amorphous film up to 30 mm since deposition rate is fast enough and material quality produced is already proven in VAD or OVD process for optical fiber.

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To form a waveguide by FHD process, a core layer where light is guided, and undercladding and overcladding layers that shield the core layer are needed. For the successful fabrication of SiO2 waveguide, the refractive index of the core layer should be higher than those of both cladding layers. Furthermore, in order to complete densification process, undercladding should be densified at the highest temperature, core at a moderate temperature, and overcladding at the lowest temperature. Some dopants such as B, P, and Ge are added to control the refractive index and the process temperature. The amount of dopant added is normally larger compared to the optical fiber processes since Si wafer is used as a substrate and the maximum process temperature is limited. In FHD process, the one of the important goals is to control the compositions in each layer. The composition of the soot layer (the deposited layer before the densification process) can be sensitively changed by the following factors: the temperature of the source materials in vaporization chamber, the flow rate of carrier gases, the temperature of H2–O2 flame, the geometrical structure of H2– O2 flame torch, the angle and distance between the torch and the substrate, the structure of ventilation system, and the rate of ventilation to remove soot which is not deposited. Practically, it is very difficult to systematically control these numerous experimental parameters, and thus it is common to achieve desired control by trial and error method. Among many process parameters, the flow rate of source gases are generally controlled to achieve the compositional variation, but it is not easy to relate the above experimental parameters to thermal and optical properties of optical waveguide because other experimental parameters can easily affect to a major parameter under control. Furthermore, a technique easily analyzing compositions of soot has not been established yet mainly due to B, which is the most important dopant. Many structural and compositional studies on silica have been conducted by FTIR method, which is well known as an excellent analysis for structural and compositional variations of hydroxyl in silica and physically adsorbed H2O [1]. In addition, FTIR is a relatively simple and non-destructive method for analyzing composition since it is easy to prepare samples in the form of powder, bulk, and thin film. To apply FTIR method for quantitative analysis of composition, it is important to identify every peak of dopants and the quantitative relationships between the absolute concentration of chemical species and the area or height of relevant peaks. In this research, there are two objectives. One is to identify the structural peaks of silica and dopants such as B, P, and Ge in silica soot deposited by FHD and measure quantitatively the variation of these peaks as a function of flow rate of source gases. The other is obtaining molar absorption coefficient, which shows the relationship between the peak height of FTIR and the absolute chemical concentration. Among the doped element mentioned above, B is the most abundant in the amorphous silica waveguide film. Therefore, it is the first step to analyse this dopant. In this report, the quantitative relationship between FTIR peak height and absolute chemical concentration of B will be discussed and the molar absorption coefficient of the element will be reported.

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In this research, the molar absorption coefficient of B–O band could be obtained by determining the absorbance of B–O band from FTIR absorbance spectrum and the concentration of B from ICP-AES analysis. Furthermore, the structural change of soot deposited by FHD along with processing factors (the various flow rate of the dopant) of FHD process could be determined. 2. Experimental For FTIR analysis, the grinded powder samples of silica (optical fiber cladding, Suprasil, Shin-Etsu Glass Co., Japan) and silica soot obtained by FHD method were used. FHD soot samples were prepared at various flow rate of B. The flow rates of each source gases are SiCl4 90 sccm, POCl3 40 sccm, BCl3 10, 30, 50, 70, 90 sccm. The prepared powder or soot samples were mixed with KBr. The mixed samples were then grinded into fine powder, and uniaxially pressed into disc of 1.1 cm diameter and thickness of 0.43–0.45 mm. Nicolet Magna-IR 760 spectrometer was used for FTIR analysis, and the measurement conditions were resolution 4 cm1, scan number 64, and wavenumber range 400–4000 cm1. In IR spectroscopy, transmittance of IR, T, passing through the sample is given by,   I T ¼ 100 ; (1) I0 where I is the intensity of light after passing through the sample and I0 is the intensity of light incident to the sample. For quantitative analysis, transmittance can be changed to absorbance A, by the following expression:     100 T A ¼ log10 ¼ log10 : (2) T 100 The absorbance of a peak was measured as two-point baseline method [1]. When IR passes through the sample, the absorbance of each band is shown as below A ¼ eCL;

(3)

where A is the absorbance, e the molar absorption coefficient, C the concentration of absorbing species and L is the distance of propagation of light through the sample. Therefore, theoretically, the concentration of absorbing species can be determined by knowing the absorbance of absorbing species from the FTIR spectrum, the molar absorption coefficient, and the thickness of the sample. To determine the molar absorption coefficient, the absolute chemical compositions of FHD soot samples were measured by ICP-AES. The FHD soot samples with various flow rate of B were dissolved in solution of HF:HNO3 = 2:1, and then maintained for an hour at 150 8C for complete dissolution. After adding D.I. water into the dissolved solution of FHD soot, the HF in solutions were completely evaporated and removed with water at 150 8C. For ICP-AES, JOBIN YVONJY 138 Ultrace model was used for analysis.

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D. Shin / Vibrational Spectroscopy 46 (2008) 14–21

3. Results 3.1. Silica for optical fiber As presented in Fig. 1, IR absorption bands of silica are almost identical with the previous reports [2–6]. H2O band is reported to be found at 3417 cm1, and three types of Si–O structural bands is reported to be observed at 1105 cm1 (antisymmetric stretching), 795 cm1 (symmetric stretching), and 467 cm1 (rocking motion). Both overtone band and combination band of Si–O structural band are reported to be observed at 1628 cm1, 1870 cm1, and 1995 cm1 [7]. The peak positions measured in this study are summarized in Table 1; H2O band at 3410 cm1, Si–O stretching (antisymmetric stretching) band at 1060–1114 cm1, Si–O stretching (symmetric stretching) band at 800–850 cm1, and Si–O rocking band at 450–468 cm1. OH band (3673 cm1) found in a commercial silica glass could not be observed due to overlapping with the H2O band [8–10].

Table 1 Comparison between FTIR peaks of pure silica (optical fiber cladding) measured in this work and previous reports [2–7] Fundamental band

Previous result (cm1)

Current result (cm1)

H2O band SiO2 (antisymmetric stretching) band SiO2 (symmetric stretching) band SiO2 (rocking motion) band

3410 1060–1114 800–850 450–468

3417 1105 795 467

Combination or overtone 2nss(SiO2), nAs,2(SiO2) + nR(SiO2)* nAs,1(SiO2) + nR(SiO2)* nss(SiO2) + nAs,2(SiO2)*

1633 1873 1990

1627 1870 1995

flow rate of BCl3. Since the peak of OH group could not be separated into individual single peaks, the change of absorbance of OH stretching band could not be measured quantitatively. However, the increased absorbance of OH stretching band could be expected from the positional change of Si–O stretching band.

3.2. FHD soot 3.2.1. Si–O structural bands The FTIR absorption spectrums of FHD soots with various flow rate of BCl3 are shown in Fig. 2. Si–O antisymmetric stretching band is found at 1112–1115 cm1, Si–O symmetric stretching band at 807–811 cm1, and Si–O rocking motion band at 476  1 cm1. As summarized in Table 2, structural bands (Si–O antisymmetric stretching band, Si–O symmetric stretching band, Si–O rocking band) of FHD soot are located at higher wavenumbers compared to those of high purity silica for optical fiber. 3.2.2. OH, H2O bands Fig. 3 represents the results from the absorbance spectrum of OH group with various flow rate of BCl3. H2O band was located at 3432 cm1 and 3216–3223 cm1. Also another H2O band was found at 3223–3216 cm1 which is not found in high purity silica. It was observed in Fig. 2 that the absorbance of H2O band at 3432 cm1 and at 3216–3223 cm1 could increase with the

Fig. 1. FTIR absorption spectrum of silica glass for optical fiber cladding.

3.2.3. B–O bands Fig. 4 provides the absorption spectra and the change of absorbance of B–O stretching band at the various flow rate of BCl3. As summarized in Table 3, B–O stretching band is found at 1394–1414 cm1. Compared to B–O stretching band of borosilicate film produced by CVD process (1370 cm1) [11], that of FHD soot is located at higher wavenumber. 3.2.4. Other bands The absorbance spectra of B–OH band, Si–H band, and overtone band of B–O at various flow rate of BCl3 are presented in Fig. 5. As shown in Table 3, B–OH band is found at 1193 cm1, and Si–H band at 649–651 cm1. Additionally, combination/overtone band of B–O is found at 884–885 cm1, 674–674 cm1, and 547.69 cm1 [12]. Since the absorbance of these bands is small, it is difficult to see the changes of absorbance without the correction of baseline. However one could find a tendency to increase and then saturates with

Fig. 2. FTIR absorption spectrum of FHD soot at various BCl3 flow rate.

D. Shin / Vibrational Spectroscopy 46 (2008) 14–21

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Table 2 FTIR peak positions of fundamental SiO2 bands found in FHD soot BCl3 flow rate (sccm)

10 30 50 70 90 a

H2O (free water) (cm1)

– 3223 3220 3219 3216

Si–O(A,S)a (cm1)

Si–O(S,S)a (cm1)

Si–O(R)a (cm1)

Pure silica

FHD soot

Pure silica

FHD soot

Pure silica

FHD soot

1105 1105 1105 1105 1105

1115 1114 1114 1114 1112

795 795 795 795 795

811 810 807 807 807

467 467 467 467 467

476 476 476 476 476

A,S: antisymmetric stretching, S,S: symmetric stretching, R: rocking motion.

Fig. 3. Absorption spectrum of OH group at various BCl3 flow rate.

increasing flow rate of BCl3, which is similar to the case of B–O stretching band. 4. Discussion It is known that the position of Si–O structural bands of silica may be different depending on the fabrication process of silica and the amount of OH (H2O) in silica. The higher fictive

temperature (higher process temperature) produces the peak shift to higher wavenumber [13] and the higher content of OH leads to the peak shift to lower wavenumber [1]. According to Davis [1], Si–O structural band of silica shifts to higher wavenumber when silica deposited by evaporation process is heat treated at a high-temperature so that the dehydration reaction resulting in decreased amount of OH occurs. As shown by Fig. 2, the absorbance of OH band of silica soot by FHD method is higher than that of silica optical fiber. Therefore, it is believed that the shift of Si–O structural bands to higher wavenumber is not due to the amount of OH, but due to the process temperature of fabricating silica. When producing silica by FHD process, it is expected the fictive temperature of silica is higher than that of silica cladding used in this study since silica cladding is consolidated after soot formation and this consolidation temperature is lower than flame used in FHD. Therefore the positions of structural bands of silica would shift to higher wavenumber. On other hand, FHD soot produced with various flow rate of BCl3, the amount of OH (H2O) incorporated during deposition is changed depending upon the flow rate of BCl3. As shown by Fig. 2, the absorbance of OH (H2O) bands increases with the increase of flow rate of BCl3, therefore the amount of OH (H2O) in silica soot increases. Because of increased amount of OH (H2O) in FHD soot, Si–O structural bands shift to lower wavenumber with increasing flow rate of BCl3.

Fig. 4. Absorption spectrum and absorbance of B–O stretching band at various BCl3 flow rate. (a) FTIR absorption spectrum of B–O band and (b) the variation of absorbance of B–O band.

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Table 3 Peaks of various bands except Si–O, OH, H2O bands BCl3 flow rate

B–O (cm1)

B–OH (cm1)

B–O–Si (cm1)

B–O (overtone or combination) (cm1)

Si–H (cm1)

10 30 50 70

1394 1402 1406 1406

– – 1193 1193

– 919 918 919

– 885 885 885

– – 649 651

Since B in FHD silica soot is so hygroscopic that the water would adhere to the surface of soot particles during deposition or handling in air after deposition. Such physically adsorbed H2O molecules were mostly removed during heat treatment or sintering process. However, the water molecules could react with silica network and can change to chemically adsorbed OH, which accelerates crystallization during densification process (sintering) of soot. This crystallization certainly is very harmful for optical transmittance and one of the major origins of optical loss in waveguide devices. In the process of synthesizing silica soot, the water related species incorporated with B can react with silica network in various forms. Therefore, the shift of B–O stretching band to higher wavenumber seems to be related with the resonating frequency change by [OH] bond formed in [Si–O  O–B] network [11]. In the meanwhile, the absorbance of B–O stretching band shows a tendency to increase and then saturate with increasing flow rate of BCl3 (Fig. 4(b)). This tendency could be resulted from B not being added proportionally to the flow rate of BCl3 or from the changes of molar absorption coefficient of B–O bond (the ability of B–O bond to absorb IR). The exact reason of this tendency will be discussed later in conjunction with ICPAES results. In Fig. 6, both the absorbance spectra and the change of the absorbance of B–O–Si band with the flow rate of BCl3 are shown. B–O–Si band at 918  1 cm1 is in agreement with the

673 674 674 674

– 546 547 548

band at 917 cm1 of borosilicate film by CVD. Like B–O stretching band, the absorbance of B–O–Si band increased and then saturated as the flow rate of BCl3 increased. Based on the report for the absorbance spectrum of P–O band [14], the P–O band in FHD soot could not be observed because the position of this peak was almost overlap with Si–O peak, which is due to the similar atomic masses of Si and P, as well as small amount of P doped. Table 4 and Fig. 7 represent the absolute amount of B and P found in FHD soot measured by ICP-AES method. The amount of B added to FHD soot proportionally increased as the flow rate of BCl3 increased. Moreover, the amount of P in FHD soot decreased in proportion to the flow rate of BCl3. Since the vapor pressure of P2O5 is higher than that of B2O3, P could not be added proportional to the flow rate of source materials and the amount of P in FHD soot was smaller than that of B in the same flow rate of BCl3 and POCl3. Furthermore, increasing flow rate of BCl3 seems to affect the doping amount of P even though the flow rate POCl3 was fixed. This phenomenon adds a difficulty to control of dopant in FHD process. Fig. 8(a) represents the relationship between the absorbance of B–O stretching band from Fig. 4(b) and the absolute concentration of B from Fig. 7(a). The absorbance of B–O stretching band tends to increase and then reaches a saturated value with the increasing concentration of B. This is believed to be due to the molar absorption coefficient changes as shown in Fig. 8(b).

Fig. 5. FTIR spectrum of various bands found in FHD silica soot. (a) Absorption spectrum of B–OH band, (b) absorption spectrum of overtone of B–O band, (c) absorption spectrum of overtone (combination) band of B–O and Si–H band, and (d) absorption spectrum of overtone (combination) band of B–O.

D. Shin / Vibrational Spectroscopy 46 (2008) 14–21

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Fig. 6. Absorption spectrum and absorbance of B–O–Si band at various BCl3 flow rate. (a) FTIR absorption spectrum of B–O–Si band and (b) absorbance of B–O–Si as a function of BCl3 flow rate.

As mentioned earlier, due to small variation in the conditions of specimens such as the ratio of KBr, the thickness, and the particle size, it is required to correct the absorbance of B–O stretching band to a reference peak, which is chosen to Si– O(A,S*) in this work. Furthermore, it would be convenient if the absorbance of B–O peak is correlated directly to the B concentration. Taking these two requirements into accounts, the normalization of B–O band is necessary. Fig. 9(a) shows the values of B–O stretching band normalized by Si–O(A,S*) as a function of the concentration of B. This diagram shows that the Table 4 Amount of B, P in FHD soot measured by ICP-AES as a function of BCl3 flow rate BCl3 flow rate

B (wt%)

P (wt%)

10 30 50 70 90

1.16 2.36 4.10 4.79 5.86

1.81 1.62 1.35 1.22 1.11

normalized absorbance of B–O stretching band increases in proportion to the concentration of B. On the basis of the corrected absorbance of B–O stretching band, Beer Lambert law can be changed as following, assuming that the concentration of Si–O band is the total concentration of FHD soot (CSi–O = 1). ABO eB CB ¼ ffi eBO M B ; ASiO eSiO CSiO

(4)

where eBO is the corrected molar absorption coefficient of B, MB the molar fraction of B and Ci is the molar concentration of i species. The speculation that the concentration of Si–O(A,S) band equals the total concentration of silica soot is generally incorrect. Especially, when the concentration of B is large, this assumption results in errors and cannot be applied. But in this research, as B’s concentration is relatively small compared to the concentration of Si as can be seen in Table 4, CSi–O = 1 can be assumed within the experimental error. From Fig. 9(a), it can be seen that the relationship between the normalized

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D. Shin / Vibrational Spectroscopy 46 (2008) 14–21

Fig. 7. The concentration of B and P in FHD soot measured by ICP-AES at various BCl3 flow rate. (a) B concentration and (b) P concentration.

Fig. 8. Relation between absorbance of B–O band and B–O concentration in FHD soot. (a) Absorbance of B–O band vs. B–O concentration and (b) molar absorption coefficient of B–O band.

Fig. 9. Relation between absorbance of B–O/Si–O(A,S) band and B–O concentration in FHD soot. (a) Absorbance of B–O/Si–O band vs. B–O concentration and (b) the corrected molar absorption coefficient of B ðeBO Þ.

D. Shin / Vibrational Spectroscopy 46 (2008) 14–21

absorbance of B–O stretching band and concentration of B follow a linear relationship, though it deviates from linearity at a high concentration of B. Fig. 9(b) shows eBO as a function of B concentration in FHD soot. Compared to the molar absorption coefficient in Fig. 8(b), the normalized molar absorption coefficient has smaller variance in the BCl3 flow rate range of 30–70 sccm and has an average value of 0.0622(0.001).

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This result suggests that, in FHD process to fabricate SiO2/Si waveguide devices, the composition of a film could not be simply controlled by changing flow rate of a source gas since the flow rate of certain source gas affects the concentration of other constituents. Therefore, the flow rate of a source gas could not be controlled independently and the effect of a source gas should be analyzed in conjunction with other variables. Acknowledgement

5. Conclusion From the FTIR absorption spectra of FHD soot and high purity silica for optical fiber preform, various peaks by silica structural bonds and dopants in the FHD soot could be identified and could be found to be well matched with previous reports. Moreover, by comparing the FTIR analysis and ICPAES analysis of each peak as functions of flow rate of BCl3, the quantitative relationship between the concentration of B and peak height is clarified. It was found that FTIR peak of B–O does not increase in proportion to the concentration of B and, hence, the molar absorption coefficient of B–O peak is not a constant in the B concentration range of 0.1–0.6 mol/L. The normalized molar absorption coefficient of B–O peak, which is linearly proportional to the concentration of B, is found to be 0.0622(0.001) in average. It was also found that the flow rate of BCl3 could affect not only the concentration of B, but also OH, H2O, Si–O, and P–O band. Namely, as the concentration of B increases, the amounts of OH or H2O in soot increase due to hygroscopic nature of B. In contrary, the amount of P in soot decreases in proportion to the B concentration even though the flow rate of P source gas is maintained to a constant rate.

This work was supported by the Seoul Research & Business Development program (Grant No. 10583). References [1] K. Davis, Ph.D. Thesis, Rensselaer Polytechnic Inst., 1993. [2] F.M. Ernsberger, J. Am. Ceram. Soc. 60 (1977) 91. [3] E.R. Lippincott, A.V. Valkenburg, C.E. Weir, E.N. Bunting, J. Res. Nat. Bureau Stand. 61 (1958) 61. [4] I. Simon, in: J.D. Mackenzie (Ed.), Infrared Studies of Glass in Modern Aspects of the Vitreous State, vol. 1, Butterworth Inc., Washington, 1960, p. 120. [5] R.J. Bell, N.F. Bird, P. Dean, J. Phys. C 1 (1968) 299. [6] F.L. Galeener, Phys. Rev. B 8 (1979) 4292–4297. [7] R. Hanna, J. Am. Ceram. Soc. 48 (1965) 595–599. [8] J.E. Shelby Jr., J. Vitko, R.E. Benner, Commun. Am. Ceram. Soc. (1982) C59. [9] P.F. McMillan, R.L. Remmele, Am. Min. 71 (1986) 772. [10] H. Wakabayashi, M. Tomozawa, J. Am. Ceram. Soc. 72 (1989) 1850. [11] W. Kern, RCA Rev. 32 (1971) 429. [12] J.L. Parsons, M.E. Milberg, J. Am. Ceram. Soc. 43 (1960) 326–340. [13] A. Agarwal, Ph.D. Thesis, Rensselaer Polytechnic Inst., 1995. [14] P.Y. Shih, S.W. Yung, T.S. Chin, J. Non-Cryst. Sol. 244 (1999) 211–222.