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Ellipsometric, XPS and FTIR study on SiCN films deposited by hot-wire chemical vapor deposition method
Materials Science in Semiconductor Processing 42 (2016) 373–377
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Ellipsometric, XPS and FTIR study on SiCN films deposited by hot-wire chemical vapor deposition method Md. Momtazur Rahman a,n, Syed Kamrul Hasan b a b
BCMC College of Engineering & Technology, Jessore 7400, Bangladesh Kyushu Institute of Technology Fukuoka, Kitakyushu, Fukuoka Prefecture 804-0015, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 7 July 2015 Received in revised form 14 October 2015 Accepted 10 November 2015 Available online 18 November 2015
SiCN films were deposited by hot-wire chemical vapor deposition (HWCVD) method using hexamethyldisilazane (HMDS). These films contain a lot of oxygen. Using HMDS with NH3, low oxygen content films can be obtained. It is found from the structure determination that Si‐N bonds are the vital bonds of SiCN films. It is also found that the highest amount of Si‐N bonds content SiCN has the highest amount of nitrogen and the amount of nitrogen is directly related to the properties of the films. The amount of oxygen, film density, the refractive index and optical band gap are strong functions of the amount of nitrogen in the films. With increasing nitrogen, the amount of oxygen decreases and with decreasing nitrogen, the amount of oxygen increases. The film density and optical band gap also increase with increasing nitrogen. On the other hand with increasing nitrogen, the refractive index decreases. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Hot-wire CVD SiCN Si‐N bonds Nitrogen Film structure Film density Refractive index Optical band gap
1. Introduction Silicon carbon nitride (SiCN) films are well known as a transparent hard coating film [1–7]. Therefore, SiCN films are very attractive as a coating material for stainless steel, construction materials and for bodies of information technology (IT) devices, such as mobile phone, digital camera and computer. SiCN films have been generally expected in various applications because we can use them as not only for barrier films, insulating films, and passivation films for ultra large scale integration (ULSI) but also for general industrial uses. Hot-wire chemical vapor deposition (HWCVD) attracts attention as a method of replacing by plasma enhanced chemical vapor deposition (PECVD) for a low temperature film deposition [1–4]. In general, Silane (SiH4) is required for deposition of SiCN films but SiH4 is so dangerous that it is impossible for general industries to use these films for the issue of safety. Recently we have deposited SiCN films by HWCVD using hexamethyldisilazane (HMDS) and NH3 [1,2]. HMDS is a non explosion organic liquid source instead of SiH4. As a result, it is possible to use the SiCN films even by general industries. At the beginning, we mentioned SiCN films are transparent hard coating films. But uniting oxygen from environment with SiCN films after deposition is inevitable. Our previous work and other papers also n
Corresponding author.
http://dx.doi.org/10.1016/j.mssp.2015.11.006 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
reported about uniting oxygen with SiCN films [2,4,5–10]. Low density SiCN films are unstable and are easily united with oxygen. The electrical, optical and barrier properties of such low density SiCN films are also not satisfactory. As uniting oxygen is inevitable, we have to inhibit and minimize the amount of uniting to increase the performance of SiCN films. In this work, we prepared different SiCN films changing the HMDS flow rate by HWCVD and investigated the film structures and optical properties to obtain high density SiCN films.
2. Experimental SiCN films were deposited by HWCVD method [1,2]. Si (100) substrates were installed into the vacuum chamber whose pressure was below 1.0 10 5 Torr. Before installing into the processing chamber, the substrates were cleaned by 10% HF acid to remove the native oxide and then the substrates were washed in deionized water and were dried in nitrogen atmosphere. After that the HMDS and NH3 were supplied into the chamber. Typical deposition conditions are shown in Table 1. We used the zigzag shaped tungsten filament with a diameter of 0.5 mm and placed it 35 mm below the substrate. The temperature of tungsten filament was kept at 1750 °C. When the deposition was started, then the initial substrate temperature was 120 °C and at the end of the deposition the substrate temperature was 270 °C. The substrate
374
Md.M. Rahman, S.K. Hasan / Materials Science in Semiconductor Processing 42 (2016) 373–377
temperature was monitored using a thermocouple attached to the substrate holder. The chemical structure of the deposited film was investigated with Fourier transform infrared spectroscopy (HORIBA, FT-710). The composition and chemical bonding states of surface and bulk were investigated by X-ray photoelectron spectroscopy (XPS) using a Shimadzu KRATOS Axis Nova system. The AlKα radiation of energy 1486.69 eV was used. Quantitative analysis of composition was performed using the following elemental sensitivity factors: Si ¼0.328, C ¼0.278, N ¼0.477 and O ¼0.78 [Shimadzu KRATOS Softwere dictionary]. After the deposition of SiCN films, the sample was immediately transferred from the HWCVD apparatus to the spectroscopic ellipsometry and the SE characterization was performed. The spectroscopic ellipsometric (SE) characterization of the films was carried out using a variable angle of incidence spectroscopic ellipsometer (VASE) manufactured by SOPRA S.A. model, GESP-5. The light source was comprised of a Xe lamp and a monochoromator which allows optical measurements in the ultraviolet (UV)–visible (VIS) range of wavelength (250–800 nm). Then the ellipsometic angles (Ψ, Δ) related to the complex reflectance ratio ρ by;
ρ = RP /Rs = tan Ψ eiΔ where, Rp and Rs are the measured complex reflectance of the pplane and the s-plane respectively. From these, the complex dielectric function can be derived. Further optical analysis was carried out through the application of a dispersive model for the dielectric constant. The spectral dependence of the refractive index and the extinction co-efficient of the deposited film was then obtained assuming multilayer plane-parallel structures (void/SiCN film/SiO2 interfacial layer/ Si substrate) [6,11]. The film density was determined by the weight-gain method using a micro-balance with accuracy to 10 μg [12]. Seven 16 cm2 silicon substrates were weighed before and after SiCN deposition. The films thicknesses were determined by the VASE analysis. The calculated substrate area and the measured film thickness provide the material volume from which a simple density calculation was performed for each sample using the equation, density ¼weight/volume.
3. Results and discussion The FTIR spectrum serves as an indication of the bond vibration in the thin films. The absorbance spectra of SiCN films deposited at different flow rate of HMDS and NH3 (100 sccm) are shown in Fig. 1. The absorption at approximately 450 cm 1 could be assigned to the Si–N asymmetric stretching vibration or Si–O rocking vibration [13,14]. The absorption at approximately 700 cm 1 could be assigned to Si–H rocking and wagging vibrations [13].The absorption at approximately 830 cm 1 could be assigned to the Si–C stretching and wagging vibrations [13].The absorption at approximately 940 cm 1 could be assigned to the Si–N symmetric
Si-H
N-Si/Si-O
Si-N Si-C
Si-O
Si-CH
NH 3.0 sccm
NH
N-H
HMDS, NH3 Si (100) 120–275 1750 800, 0.50 35 1.0–3.0 100 2.4 10 1 30
2.5 sccm 2.0 sccm 1.5 sccm 1.4 sccm 1.3 sccm 1.0 sccm
4000
3000
2000
1000
400
-1
Wavenumber (cm ) Fig. 1. The absorbance spectra of SiCN films deposited at different flow rate of HMDS.
stretching vibration. [13,14]. The absorption at approximately 1030 cm 1 could be assigned to Si–O stretching vibrations. [13]. The absorption at approximately 1160 and 1250 cm 1 could be attributed to the N–Hx and Si–CH3 bending vibrations [13]. The absorption at approximately 1500 cm 1 could be assigned to NH2 wagging vibration [13]. The absorption at approximately 2200 cm 1 could be assigned to C≡N stretching vibration [14,17,18]. The absorption at approximately 3300 cm 1 could be assigned to N–H stretching vibration [13]. As shown in the Fig. 1. the peak Si–C, Si–N (symmetric), Si–O, N–Hx and Si–CH3 bonds are significant while Si–N (asymmetric stretching)/Si–O (rocking), Si– H, NH2, C≡N, and N–H bonds are less significant. To obtain significant bonds’ relative intensity, position and area, an enlargement and deconvolution were performed in the spectra of region between 600 and 1400 cm 1 as shown in Fig. 2. From Fig. 2 we can observe that each component in the ternary system is bonded to the other two components forming a complex network. We can also observe that Si‐N peak is the most intense and wide. Fig. 3 we can observe that at HMDS flow rate 1 sccm, the amount of Si‐N bonds is the lowest and at HMDS flow rate 1.4 sccm, (or very near to 1.4 sccm) the amount of Si‐N bonds increases abruptly and further increasing HMDS flow rate, the amount of Si‐N bonds decreases. The Si‐N bonds decreased from 70.6% to 48.8%. On the other hand, at HMDS flow rate 1 sccm, the amount of Si‐C and Si‐O bonds are the highest and at HMDS flow rate 1.4 sccm, (or very near to 1.4 sccm) the amount of Si‐C and Si‐ O bonds decrease abruptly and further increasing HMDS flow rate,
Si-N
Intensity (a.u)
Deposition sources Substrate type Substrate temperature (°C) Tungsten filament temperature (°C) Filament length, diameter (mm) Distance between filament and substrate (mm) HMDS flow rate (sccm) NH3 flow rate (sccm) Gas pressure (Torr) Deposition time (minute)
Absorbance (a.u.)
Table 1 Deposition conditions
1400
Si-O
Si-C
NH x Si-CH 3
1200 1000 800 Wave number (cm -1 )
600
Fig. 2. The typical deconvoluted FTIR spectrum of the SiCN thin films between 600 cm 1 and 1400 cm 1.
Md.M. Rahman, S.K. Hasan / Materials Science in Semiconductor Processing 42 (2016) 373–377
Fig. 3. The relative area of different bonds of SiCN thin films deposited with different flow rates of HMDS and NH3 (100 sccm) determined by FTIR.
the amount of Si‐C and Si‐O bonds increase with increasing HMDS flow rate. Si‐C bonds increased from 17.9% to 26.7% and Si‐O bonds increased from 1% to 18.9%. And the amount of NHx and Si–CH3 bonds of all the seven samples were approximately around 8.8% and 0.5% respectively. It is also observed that the highest Si–N bonds content SiCN films has the lowest amount of Si‐O and Si‐C bonds and the lowest Si‐N bond content SiCN films has the highest amount of Si‐O and Si‐C bonds. As shown in our previous work, SiCN films can be deposited using only HMDS. But the amount of Si‐O bonds of the films prepared by using only HMDS increases [2]. We found that the amount of SiO2 layer of SiCN films prepared by using only HMDS is larger than that of SiCN films prepared by using HMDS with NH3 assuming the structure described in the previous section determined by spectroscopic ellipsometry. It is also found that SiO2 layer of SiCN films prepared by using only HMDS and HMDS with NH3 are almost constant with elapsed time. From this observation, it can be resolve that most of the oxygen uniting happen when the wafer is introduced outside of the chamber just after deposition. Fig. 4 shows SiO2 layer of SiCN films versus elapsed time. Therefore, it is resolved that SiCN films prepared only HMDS has large amount of oxygen than that of SiCN films prepared by using HMDS with NH3. From this observation, it is found that to obtain less oxygen content SiCN films surely NH3 have to flow with HMDS. From this observation it is also resolved that NH3 inhibits to increase the amount of SiO2 layer. It is assumed that SiCN films prepared only HMDS has a lot of dangling bonds and voids. The role of NH3 is considered to terminate the dangling bonds forming Si‐H, C‐H and N‐H bonds [7,8]. NH3 also increases the Si‐N and C‐N
Fig.4. SiO2 layer (at%) of SiCN thin films deposited with the flow rate of HMDS (1.5 sccm) with NH3 (100 sccm) and without NH3 determined by VASE.
375
bonds [2]. It is also found that NH3 can not completely terminate all the dangling bonds as Fig. 3 shows the presence of Si‐O bonds in every flow rate of HMDS with NH3. Again from Fig. 3, at HMDS flow rate 1 sccm, the amount of Si‐N bonds is the lowest and at HMDS flow rate 1.4 sccm, (or very near to 1.4 sccm) the amount of Si‐N bonds increases abruptly and further increasing HMDS flow rate, the amount of Si‐N bonds decreases. This happens because at HMDS flow rate 1 sccm, interaction of NH3 with HMDS is the lowest and for that dangling bonds termination are the lowest because NH3 decomposed species terminate the dangling bonds and also increases Si‐N bonds. These dangling bonds are easily united with atmospheric oxygen. As shown previously oxygen unites only with Si atoms. As a result, the amount of Si‐O bonds increases and consequently the amount of Si‐N bonds decreases as the interaction of NH3 with HMDS is the lowest. The amount of Si‐ C bonds proportionally increases because the lowest interaction of NH3 with HMDS forms the lowest amount of Si‐N bonds. At HMDS flow rate 1.4 sccm, (or very near to 1.4 sccm) the interaction of NH3 with HMDS is the highest and for that dangling bonds are utter mostly terminated by NH3. As a result, the amount of Si‐N bonds increases abruptly and Si‐O bonds decrease utter mostly. The amount of Si‐C bonds proportionally decreases because the highest interaction of NH3 with HMDS forms the highest amount of Si‐ N bonds. Further increasing HMDS flow rates, more carbon atoms generate as HMDS has six methyl groups. So more carbon atoms react with silicon and nitrogen atoms that make the Si‐C bonds increased and Si‐N bonds decreased. Fig. 3 shows that with increasing and decreasing Si‐N bonds, in due orders Si‐O bonds are also decreasing and increasing and the highest Si‐N bonds content SiCN has the lowest amount of Si‐O bonds. Therefore, it can be resolved that Si‐N bonds are very important bonds of SiCN films. XPS analysis gives direct information about the composition and the bonding type of the atoms in the films. The wide-scan XPS spectra of the deposited films showed the presence of Si, C, N and a small amount of O in the films. Typical high-resolution XPS narrow scans of Si 2p, C 1s, N s and O 1s peaks of SiCN films are shown in Fig.5. For the photoelectron peak of Si 2p, the peaks centered at 101.8, 102.7 and 103.6 eV were attributed to Si‐C, Si‐N, and Si-O bonds respectively [5]. The photoelectron peak of C 1s, the peaks centered at 282.5, 284.5 and 285.9 eV were attributed to C‐Si, C‐C and C–N bonds respectively [5]. The photoelectron peak of N 1s, the peaks centered at 398, 398.8 and 399.9 eV were attributed to N–Si, N–C, and N ¼C bonds respectively [5]. The/photoelectron peak of O 1s centered at 533 eV was attributed to O‐Si bonds [16]. Therefore, it is found that the deposited films are constituted by Si‐N, Si‐O, Si‐C, C‐C, C‐ Si, C‐N, N‐Si, N‐C, N ¼C and O‐Si bonds. It is again also found that oxygen uniting happens only with Si atom. From Fig. 6 it is found that at HMDS flow rate 1 sccm, the amount of nitrogen is the lowest and the amount of oxygen and carbon are the highest and at HMDS flow rate 1.4 sccm (or very near to 1.4 sccm), the amount of nitrogen is the highest and the amount of oxygen and carbon are the lowest. Further increasing HMDS flow, the amount of nitrogen decreases and the amount of oxygen and carbon increase. The amount of Si is almost same in every flow rate of HMDS and NH3 (100 sccm).This observation can explain by the same explanation from FTIR result. At HMDS flow rate 1 sccm, the interaction between HMDS and NH3 is the lowest and for that the amount of nitrogen is the lowest and oxygen is the highest. The amounts of carbon proportionally increase because the lowest interaction of NH3 with HMDS forms the lowest amount of nitrogen. At HMDS flow rate 1.4 sccm, (or very near to 1.4 sccm) the interaction between HMDS and NH3 is the highest and for that the amount of nitrogen is the highest and oxygen is the lowest. The amount of carbon proportionally decreases because the highest interaction of NH3 with HMDS forms the highest
Md.M. Rahman, S.K. Hasan / Materials Science in Semiconductor Processing 42 (2016) 373–377
Si-O
98
C-C C-N
290
C-Si
288 286 284 282 Binding Energy (eV)
N 1s N-C
N=C 404
Si-C
106 104 102 100 Binding Energy (eV)
C1s Intensity (a.u.)
Si-N
Intensity (a.u.)
108
Si 2p
280
O1s Intensity (a.u)
Intensity (a.u.)
376
O-Si
N-Si
402 400 398 396 Binding Energy (eV)
394
538
536 534 532 530 Binding Energy (eV)
528
3
Film density (g/cm )
Fig. 5. The typical deconvoluted XPS spectra of SiCN thin films.
Nitrogen content Fig. 6. The atomic ratio of SiCN thin films with different flow rates of HMDS and NH3 (100 sccm) determined by XPS.
Fig. 7. Nitrogen content (at%) versus Film density (g/cm3) of SiCN thin films of different flow rate of HMDS and NH3 (100 sccm).
amount of nitrogen. Further increasing HMDS flow rates, more carbon atoms generate as HMDS has six methyl groups. So, more carbon atoms react with silicon and nitrogen atoms that make the amount of nitrogen decreased and the amount of carbon increased. From Figs. 3 and 6, it is found that the highest nitrogen content SiCN films has the highest amount of Si‐N bonds and the lowest nitrogen content SiCN films has the lowest amount of Si‐N bonds. Film density determination shows that the highest nitrogen content SiCN film shows the highest film density. Because the highest nitrogen content SiCN films has the highest amount of Si‐ N bonds and the highest Si‐N bonds content SiCN films has the lowest amount of oxygen which means the lowest amount of
dangling bonds that increases the film density. Fig. 7 shows film density versus nitrogen content in SiCN films. Further, the amount of nitrogen is directly related to the optical properties. Fig. 8 shows the relationship between the amount of nitrogen and refractive index. It is found that refractive index decreases with increasing the amount of nitrogen [6]. The optical band gap Eg, is also related to the amount of nitrogen of the films. Optical band gap can be determined using Tauc’s relation, (αhʋ) 2 ¼ A (hʋ Eg) where A is a constant, α ( ¼4πK/λ) is the absorption coefficient, K is the extinction co-efficient and hʋ is the photon energy [11].
Md.M. Rahman, S.K. Hasan / Materials Science in Semiconductor Processing 42 (2016) 373–377
2. 3.
4. 5. Fig.8. Nitrogen content (at%) versus refractive index of SiCN thin films of different flow rate of HMDS and NH3 (100 sccm).
6.
377
elements (Si, C, N) and it is also found from the structure that Si–C, Si–N (symmetric) Si–O (stretching) NHx and Si–CH3 bonds are significant and dominate in the SiCN films. On the other hand, Si–H, Si–N (asymmetric), NH2, C≡N, and N–H bonds are less significant. It is also found that Si–N bonds peak is the most intense and wide. Low oxygen SiCN films can obtain using HMDS with NH3. Si‐N bonds are important bonds of SiCN films and increasing or decreasing Si‐O bonds are directly related to the Si‐N bonds. The highest Si‐N bonds content SiCN films have also the highest amount of nitrogen. The highest nitrogen content SiCN films show the highest density. The highest nitrogen content SiCN films show the lowest refractive index. The highest nitrogen content SiCN films show the widest band gap. From these results, we can resolve that flowing proper ratio of HMDS and NH3, low oxygen content, high dense, high transparent and wide band gap SiCN films can be prepared.
References
Fig.9. Nitrogen content (at%) versus band gap of SiCN thin films of different flow rate of HMDS and NH3 (100 sccm).
Fig. 9 shows that optical band gap increases with increasing the amount of nitrogen. Other work also found with increasing nitrogen, the band gap also increases and with decreasing nitrogen, the band gap also decreases [5]. Besides from Fig. 6, it is found that the highest amount of nitrogen content SiCN films has the lowest amount of carbon and the lowest amount of carbon content SiCN films show the highest band gap [7].
4. Conclusion The following results were obtained. 1. The structure of SiCN thin films has been revealed and it is found the formation of complex networks among the three
[1] Akira Izumi, Koshi Oda., Thin Solid Films 501 (2006) 195–197. [2] T. Nakayamada, K. Matsuo, Y. Hayashi, A. Izumi, Y. Kadotani., Thin solid Films 516 (2008) 656–658. [3] Amornrat Lamamanee, Michio Otsubo, Tsutomu Sugiura, Takehiko Sato, Shinsuke Miyajima, Akira Yamada, Makoto Konagai., Thin Solid Films 516 (2008) 652–655. [4] Bibhu P. Swain, Nong M. Hwang., Appl. Surf. Sci. 254 (2008) 5319–5322. [5] Peng Gao, Jun Xu, Yong Piao, Wanyu Ding, Dehe Wang, Xinlu Deng, Chuang Dong., Surf. Coatings Technol. 201 (2007) 5298–5301. [6] L.C. Chen, H.Y. Lin, C.S. Wong, K.H. Chen, S.T. Lin, Y.C. Yu, C.W. Wang, E.K. Lin, K. C. Ling., Diam. Relat. Mater. 8 (1999) 618–622. [7] C.W. Chen, C.C. Huang, Y.Y. Lin, L.C. Chen, K.H. Chen, W.F. Su., Diam. Relat. Mater. 14 (2005) 1010–1013. [8] C.W. Chen, C.C. Huang, Y.Y. Lin, L.C. Chen, K.H. Chen., Diam. Relat. Mater. 14 (2005) 1126–1130. [9] K.H. Chen, J.-J. Wu, C.Y. Wen, L.C. Chen, C.W. Fan, P.F. Kuo, Y.F. Chen, Y.S. Huang., Thin Solid Films 355–356 (1999) 205–209. [10] Wenjuan Cheng, Jinchun Jiang, Yang Zhang, Hesun Zhu, Dezhong Shen., Mater. Chem. Phys. 85 (2004) 370–376. [11] Hiroyuki Fujiwara, Spectroscopic Ellipsometry, Principles and Applications, John Wiley and Sons Ltd, England (2007), p. 253–265. [12] Steven M. Smith, Tim Tighe, Diana Convey, Jaime Quintero, Yi Wei., Thin Solid Films 516 (2008) 885–890. [13] I. Ferreira, E. Fortunato, P. Vilarinho, A.S. Viana, A.R. Ramos, E. Alves, R. Martins., J. Non- Cryst. Solids 352 (2006) 1361–1366. [14] Xing-cheng Xiao, Ya-wen Li, Li-xin Song, Xiao-feng Peng, Xing-fang Hu., Appl. Surf. Sci. 156 (2006) 155–160. [16] Erqing Xie, Ziwei Ma, Hongfeng Lin, Zhimin Zhang, Deyan He, 23, 2003, pp. 151–156. [17] Y. Gao, J..Wei, D.H. Zhang, Z.Q. Mo, P. Hing, X. Shi, 377–378, 2000, pp. 562–566. [18] V.I. Ivashchenkoa, A.O. Kozaka, O.K. Poradaa, L.A. Ivashchenkoa, O.K.O. S. Lytvynb, Sinelnichenkoa, T.V. Tomilaa, V.J. Malakhova, Thin Solid Films 569 (2014) 57–63.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Ellipsometric, XPS and FTIR study on SiCN films deposited by hot-wire chemical vapor deposition method Md. Momtazur Rahman a,n, Syed Kamrul Hasan b a b
BCMC College of Engineering & Technology, Jessore 7400, Bangladesh Kyushu Institute of Technology Fukuoka, Kitakyushu, Fukuoka Prefecture 804-0015, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 7 July 2015 Received in revised form 14 October 2015 Accepted 10 November 2015 Available online 18 November 2015
SiCN films were deposited by hot-wire chemical vapor deposition (HWCVD) method using hexamethyldisilazane (HMDS). These films contain a lot of oxygen. Using HMDS with NH3, low oxygen content films can be obtained. It is found from the structure determination that Si‐N bonds are the vital bonds of SiCN films. It is also found that the highest amount of Si‐N bonds content SiCN has the highest amount of nitrogen and the amount of nitrogen is directly related to the properties of the films. The amount of oxygen, film density, the refractive index and optical band gap are strong functions of the amount of nitrogen in the films. With increasing nitrogen, the amount of oxygen decreases and with decreasing nitrogen, the amount of oxygen increases. The film density and optical band gap also increase with increasing nitrogen. On the other hand with increasing nitrogen, the refractive index decreases. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Hot-wire CVD SiCN Si‐N bonds Nitrogen Film structure Film density Refractive index Optical band gap
1. Introduction Silicon carbon nitride (SiCN) films are well known as a transparent hard coating film [1–7]. Therefore, SiCN films are very attractive as a coating material for stainless steel, construction materials and for bodies of information technology (IT) devices, such as mobile phone, digital camera and computer. SiCN films have been generally expected in various applications because we can use them as not only for barrier films, insulating films, and passivation films for ultra large scale integration (ULSI) but also for general industrial uses. Hot-wire chemical vapor deposition (HWCVD) attracts attention as a method of replacing by plasma enhanced chemical vapor deposition (PECVD) for a low temperature film deposition [1–4]. In general, Silane (SiH4) is required for deposition of SiCN films but SiH4 is so dangerous that it is impossible for general industries to use these films for the issue of safety. Recently we have deposited SiCN films by HWCVD using hexamethyldisilazane (HMDS) and NH3 [1,2]. HMDS is a non explosion organic liquid source instead of SiH4. As a result, it is possible to use the SiCN films even by general industries. At the beginning, we mentioned SiCN films are transparent hard coating films. But uniting oxygen from environment with SiCN films after deposition is inevitable. Our previous work and other papers also n
Corresponding author.
http://dx.doi.org/10.1016/j.mssp.2015.11.006 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
reported about uniting oxygen with SiCN films [2,4,5–10]. Low density SiCN films are unstable and are easily united with oxygen. The electrical, optical and barrier properties of such low density SiCN films are also not satisfactory. As uniting oxygen is inevitable, we have to inhibit and minimize the amount of uniting to increase the performance of SiCN films. In this work, we prepared different SiCN films changing the HMDS flow rate by HWCVD and investigated the film structures and optical properties to obtain high density SiCN films.
2. Experimental SiCN films were deposited by HWCVD method [1,2]. Si (100) substrates were installed into the vacuum chamber whose pressure was below 1.0 10 5 Torr. Before installing into the processing chamber, the substrates were cleaned by 10% HF acid to remove the native oxide and then the substrates were washed in deionized water and were dried in nitrogen atmosphere. After that the HMDS and NH3 were supplied into the chamber. Typical deposition conditions are shown in Table 1. We used the zigzag shaped tungsten filament with a diameter of 0.5 mm and placed it 35 mm below the substrate. The temperature of tungsten filament was kept at 1750 °C. When the deposition was started, then the initial substrate temperature was 120 °C and at the end of the deposition the substrate temperature was 270 °C. The substrate
374
Md.M. Rahman, S.K. Hasan / Materials Science in Semiconductor Processing 42 (2016) 373–377
temperature was monitored using a thermocouple attached to the substrate holder. The chemical structure of the deposited film was investigated with Fourier transform infrared spectroscopy (HORIBA, FT-710). The composition and chemical bonding states of surface and bulk were investigated by X-ray photoelectron spectroscopy (XPS) using a Shimadzu KRATOS Axis Nova system. The AlKα radiation of energy 1486.69 eV was used. Quantitative analysis of composition was performed using the following elemental sensitivity factors: Si ¼0.328, C ¼0.278, N ¼0.477 and O ¼0.78 [Shimadzu KRATOS Softwere dictionary]. After the deposition of SiCN films, the sample was immediately transferred from the HWCVD apparatus to the spectroscopic ellipsometry and the SE characterization was performed. The spectroscopic ellipsometric (SE) characterization of the films was carried out using a variable angle of incidence spectroscopic ellipsometer (VASE) manufactured by SOPRA S.A. model, GESP-5. The light source was comprised of a Xe lamp and a monochoromator which allows optical measurements in the ultraviolet (UV)–visible (VIS) range of wavelength (250–800 nm). Then the ellipsometic angles (Ψ, Δ) related to the complex reflectance ratio ρ by;
ρ = RP /Rs = tan Ψ eiΔ where, Rp and Rs are the measured complex reflectance of the pplane and the s-plane respectively. From these, the complex dielectric function can be derived. Further optical analysis was carried out through the application of a dispersive model for the dielectric constant. The spectral dependence of the refractive index and the extinction co-efficient of the deposited film was then obtained assuming multilayer plane-parallel structures (void/SiCN film/SiO2 interfacial layer/ Si substrate) [6,11]. The film density was determined by the weight-gain method using a micro-balance with accuracy to 10 μg [12]. Seven 16 cm2 silicon substrates were weighed before and after SiCN deposition. The films thicknesses were determined by the VASE analysis. The calculated substrate area and the measured film thickness provide the material volume from which a simple density calculation was performed for each sample using the equation, density ¼weight/volume.
3. Results and discussion The FTIR spectrum serves as an indication of the bond vibration in the thin films. The absorbance spectra of SiCN films deposited at different flow rate of HMDS and NH3 (100 sccm) are shown in Fig. 1. The absorption at approximately 450 cm 1 could be assigned to the Si–N asymmetric stretching vibration or Si–O rocking vibration [13,14]. The absorption at approximately 700 cm 1 could be assigned to Si–H rocking and wagging vibrations [13].The absorption at approximately 830 cm 1 could be assigned to the Si–C stretching and wagging vibrations [13].The absorption at approximately 940 cm 1 could be assigned to the Si–N symmetric
Si-H
N-Si/Si-O
Si-N Si-C
Si-O
Si-CH
NH 3.0 sccm
NH
N-H
HMDS, NH3 Si (100) 120–275 1750 800, 0.50 35 1.0–3.0 100 2.4 10 1 30
2.5 sccm 2.0 sccm 1.5 sccm 1.4 sccm 1.3 sccm 1.0 sccm
4000
3000
2000
1000
400
-1
Wavenumber (cm ) Fig. 1. The absorbance spectra of SiCN films deposited at different flow rate of HMDS.
stretching vibration. [13,14]. The absorption at approximately 1030 cm 1 could be assigned to Si–O stretching vibrations. [13]. The absorption at approximately 1160 and 1250 cm 1 could be attributed to the N–Hx and Si–CH3 bending vibrations [13]. The absorption at approximately 1500 cm 1 could be assigned to NH2 wagging vibration [13]. The absorption at approximately 2200 cm 1 could be assigned to C≡N stretching vibration [14,17,18]. The absorption at approximately 3300 cm 1 could be assigned to N–H stretching vibration [13]. As shown in the Fig. 1. the peak Si–C, Si–N (symmetric), Si–O, N–Hx and Si–CH3 bonds are significant while Si–N (asymmetric stretching)/Si–O (rocking), Si– H, NH2, C≡N, and N–H bonds are less significant. To obtain significant bonds’ relative intensity, position and area, an enlargement and deconvolution were performed in the spectra of region between 600 and 1400 cm 1 as shown in Fig. 2. From Fig. 2 we can observe that each component in the ternary system is bonded to the other two components forming a complex network. We can also observe that Si‐N peak is the most intense and wide. Fig. 3 we can observe that at HMDS flow rate 1 sccm, the amount of Si‐N bonds is the lowest and at HMDS flow rate 1.4 sccm, (or very near to 1.4 sccm) the amount of Si‐N bonds increases abruptly and further increasing HMDS flow rate, the amount of Si‐N bonds decreases. The Si‐N bonds decreased from 70.6% to 48.8%. On the other hand, at HMDS flow rate 1 sccm, the amount of Si‐C and Si‐O bonds are the highest and at HMDS flow rate 1.4 sccm, (or very near to 1.4 sccm) the amount of Si‐C and Si‐ O bonds decrease abruptly and further increasing HMDS flow rate,
Si-N
Intensity (a.u)
Deposition sources Substrate type Substrate temperature (°C) Tungsten filament temperature (°C) Filament length, diameter (mm) Distance between filament and substrate (mm) HMDS flow rate (sccm) NH3 flow rate (sccm) Gas pressure (Torr) Deposition time (minute)
Absorbance (a.u.)
Table 1 Deposition conditions
1400
Si-O
Si-C
NH x Si-CH 3
1200 1000 800 Wave number (cm -1 )
600
Fig. 2. The typical deconvoluted FTIR spectrum of the SiCN thin films between 600 cm 1 and 1400 cm 1.
Md.M. Rahman, S.K. Hasan / Materials Science in Semiconductor Processing 42 (2016) 373–377
Fig. 3. The relative area of different bonds of SiCN thin films deposited with different flow rates of HMDS and NH3 (100 sccm) determined by FTIR.
the amount of Si‐C and Si‐O bonds increase with increasing HMDS flow rate. Si‐C bonds increased from 17.9% to 26.7% and Si‐O bonds increased from 1% to 18.9%. And the amount of NHx and Si–CH3 bonds of all the seven samples were approximately around 8.8% and 0.5% respectively. It is also observed that the highest Si–N bonds content SiCN films has the lowest amount of Si‐O and Si‐C bonds and the lowest Si‐N bond content SiCN films has the highest amount of Si‐O and Si‐C bonds. As shown in our previous work, SiCN films can be deposited using only HMDS. But the amount of Si‐O bonds of the films prepared by using only HMDS increases [2]. We found that the amount of SiO2 layer of SiCN films prepared by using only HMDS is larger than that of SiCN films prepared by using HMDS with NH3 assuming the structure described in the previous section determined by spectroscopic ellipsometry. It is also found that SiO2 layer of SiCN films prepared by using only HMDS and HMDS with NH3 are almost constant with elapsed time. From this observation, it can be resolve that most of the oxygen uniting happen when the wafer is introduced outside of the chamber just after deposition. Fig. 4 shows SiO2 layer of SiCN films versus elapsed time. Therefore, it is resolved that SiCN films prepared only HMDS has large amount of oxygen than that of SiCN films prepared by using HMDS with NH3. From this observation, it is found that to obtain less oxygen content SiCN films surely NH3 have to flow with HMDS. From this observation it is also resolved that NH3 inhibits to increase the amount of SiO2 layer. It is assumed that SiCN films prepared only HMDS has a lot of dangling bonds and voids. The role of NH3 is considered to terminate the dangling bonds forming Si‐H, C‐H and N‐H bonds [7,8]. NH3 also increases the Si‐N and C‐N
Fig.4. SiO2 layer (at%) of SiCN thin films deposited with the flow rate of HMDS (1.5 sccm) with NH3 (100 sccm) and without NH3 determined by VASE.
375
bonds [2]. It is also found that NH3 can not completely terminate all the dangling bonds as Fig. 3 shows the presence of Si‐O bonds in every flow rate of HMDS with NH3. Again from Fig. 3, at HMDS flow rate 1 sccm, the amount of Si‐N bonds is the lowest and at HMDS flow rate 1.4 sccm, (or very near to 1.4 sccm) the amount of Si‐N bonds increases abruptly and further increasing HMDS flow rate, the amount of Si‐N bonds decreases. This happens because at HMDS flow rate 1 sccm, interaction of NH3 with HMDS is the lowest and for that dangling bonds termination are the lowest because NH3 decomposed species terminate the dangling bonds and also increases Si‐N bonds. These dangling bonds are easily united with atmospheric oxygen. As shown previously oxygen unites only with Si atoms. As a result, the amount of Si‐O bonds increases and consequently the amount of Si‐N bonds decreases as the interaction of NH3 with HMDS is the lowest. The amount of Si‐ C bonds proportionally increases because the lowest interaction of NH3 with HMDS forms the lowest amount of Si‐N bonds. At HMDS flow rate 1.4 sccm, (or very near to 1.4 sccm) the interaction of NH3 with HMDS is the highest and for that dangling bonds are utter mostly terminated by NH3. As a result, the amount of Si‐N bonds increases abruptly and Si‐O bonds decrease utter mostly. The amount of Si‐C bonds proportionally decreases because the highest interaction of NH3 with HMDS forms the highest amount of Si‐ N bonds. Further increasing HMDS flow rates, more carbon atoms generate as HMDS has six methyl groups. So more carbon atoms react with silicon and nitrogen atoms that make the Si‐C bonds increased and Si‐N bonds decreased. Fig. 3 shows that with increasing and decreasing Si‐N bonds, in due orders Si‐O bonds are also decreasing and increasing and the highest Si‐N bonds content SiCN has the lowest amount of Si‐O bonds. Therefore, it can be resolved that Si‐N bonds are very important bonds of SiCN films. XPS analysis gives direct information about the composition and the bonding type of the atoms in the films. The wide-scan XPS spectra of the deposited films showed the presence of Si, C, N and a small amount of O in the films. Typical high-resolution XPS narrow scans of Si 2p, C 1s, N s and O 1s peaks of SiCN films are shown in Fig.5. For the photoelectron peak of Si 2p, the peaks centered at 101.8, 102.7 and 103.6 eV were attributed to Si‐C, Si‐N, and Si-O bonds respectively [5]. The photoelectron peak of C 1s, the peaks centered at 282.5, 284.5 and 285.9 eV were attributed to C‐Si, C‐C and C–N bonds respectively [5]. The photoelectron peak of N 1s, the peaks centered at 398, 398.8 and 399.9 eV were attributed to N–Si, N–C, and N ¼C bonds respectively [5]. The/photoelectron peak of O 1s centered at 533 eV was attributed to O‐Si bonds [16]. Therefore, it is found that the deposited films are constituted by Si‐N, Si‐O, Si‐C, C‐C, C‐ Si, C‐N, N‐Si, N‐C, N ¼C and O‐Si bonds. It is again also found that oxygen uniting happens only with Si atom. From Fig. 6 it is found that at HMDS flow rate 1 sccm, the amount of nitrogen is the lowest and the amount of oxygen and carbon are the highest and at HMDS flow rate 1.4 sccm (or very near to 1.4 sccm), the amount of nitrogen is the highest and the amount of oxygen and carbon are the lowest. Further increasing HMDS flow, the amount of nitrogen decreases and the amount of oxygen and carbon increase. The amount of Si is almost same in every flow rate of HMDS and NH3 (100 sccm).This observation can explain by the same explanation from FTIR result. At HMDS flow rate 1 sccm, the interaction between HMDS and NH3 is the lowest and for that the amount of nitrogen is the lowest and oxygen is the highest. The amounts of carbon proportionally increase because the lowest interaction of NH3 with HMDS forms the lowest amount of nitrogen. At HMDS flow rate 1.4 sccm, (or very near to 1.4 sccm) the interaction between HMDS and NH3 is the highest and for that the amount of nitrogen is the highest and oxygen is the lowest. The amount of carbon proportionally decreases because the highest interaction of NH3 with HMDS forms the highest
Md.M. Rahman, S.K. Hasan / Materials Science in Semiconductor Processing 42 (2016) 373–377
Si-O
98
C-C C-N
290
C-Si
288 286 284 282 Binding Energy (eV)
N 1s N-C
N=C 404
Si-C
106 104 102 100 Binding Energy (eV)
C1s Intensity (a.u.)
Si-N
Intensity (a.u.)
108
Si 2p
280
O1s Intensity (a.u)
Intensity (a.u.)
376
O-Si
N-Si
402 400 398 396 Binding Energy (eV)
394
538
536 534 532 530 Binding Energy (eV)
528
3
Film density (g/cm )
Fig. 5. The typical deconvoluted XPS spectra of SiCN thin films.
Nitrogen content Fig. 6. The atomic ratio of SiCN thin films with different flow rates of HMDS and NH3 (100 sccm) determined by XPS.
Fig. 7. Nitrogen content (at%) versus Film density (g/cm3) of SiCN thin films of different flow rate of HMDS and NH3 (100 sccm).
amount of nitrogen. Further increasing HMDS flow rates, more carbon atoms generate as HMDS has six methyl groups. So, more carbon atoms react with silicon and nitrogen atoms that make the amount of nitrogen decreased and the amount of carbon increased. From Figs. 3 and 6, it is found that the highest nitrogen content SiCN films has the highest amount of Si‐N bonds and the lowest nitrogen content SiCN films has the lowest amount of Si‐N bonds. Film density determination shows that the highest nitrogen content SiCN film shows the highest film density. Because the highest nitrogen content SiCN films has the highest amount of Si‐ N bonds and the highest Si‐N bonds content SiCN films has the lowest amount of oxygen which means the lowest amount of
dangling bonds that increases the film density. Fig. 7 shows film density versus nitrogen content in SiCN films. Further, the amount of nitrogen is directly related to the optical properties. Fig. 8 shows the relationship between the amount of nitrogen and refractive index. It is found that refractive index decreases with increasing the amount of nitrogen [6]. The optical band gap Eg, is also related to the amount of nitrogen of the films. Optical band gap can be determined using Tauc’s relation, (αhʋ) 2 ¼ A (hʋ Eg) where A is a constant, α ( ¼4πK/λ) is the absorption coefficient, K is the extinction co-efficient and hʋ is the photon energy [11].
Md.M. Rahman, S.K. Hasan / Materials Science in Semiconductor Processing 42 (2016) 373–377
2. 3.
4. 5. Fig.8. Nitrogen content (at%) versus refractive index of SiCN thin films of different flow rate of HMDS and NH3 (100 sccm).
6.
377
elements (Si, C, N) and it is also found from the structure that Si–C, Si–N (symmetric) Si–O (stretching) NHx and Si–CH3 bonds are significant and dominate in the SiCN films. On the other hand, Si–H, Si–N (asymmetric), NH2, C≡N, and N–H bonds are less significant. It is also found that Si–N bonds peak is the most intense and wide. Low oxygen SiCN films can obtain using HMDS with NH3. Si‐N bonds are important bonds of SiCN films and increasing or decreasing Si‐O bonds are directly related to the Si‐N bonds. The highest Si‐N bonds content SiCN films have also the highest amount of nitrogen. The highest nitrogen content SiCN films show the highest density. The highest nitrogen content SiCN films show the lowest refractive index. The highest nitrogen content SiCN films show the widest band gap. From these results, we can resolve that flowing proper ratio of HMDS and NH3, low oxygen content, high dense, high transparent and wide band gap SiCN films can be prepared.
References
Fig.9. Nitrogen content (at%) versus band gap of SiCN thin films of different flow rate of HMDS and NH3 (100 sccm).
Fig. 9 shows that optical band gap increases with increasing the amount of nitrogen. Other work also found with increasing nitrogen, the band gap also increases and with decreasing nitrogen, the band gap also decreases [5]. Besides from Fig. 6, it is found that the highest amount of nitrogen content SiCN films has the lowest amount of carbon and the lowest amount of carbon content SiCN films show the highest band gap [7].
4. Conclusion The following results were obtained. 1. The structure of SiCN thin films has been revealed and it is found the formation of complex networks among the three
[1] Akira Izumi, Koshi Oda., Thin Solid Films 501 (2006) 195–197. [2] T. Nakayamada, K. Matsuo, Y. Hayashi, A. Izumi, Y. Kadotani., Thin solid Films 516 (2008) 656–658. [3] Amornrat Lamamanee, Michio Otsubo, Tsutomu Sugiura, Takehiko Sato, Shinsuke Miyajima, Akira Yamada, Makoto Konagai., Thin Solid Films 516 (2008) 652–655. [4] Bibhu P. Swain, Nong M. Hwang., Appl. Surf. Sci. 254 (2008) 5319–5322. [5] Peng Gao, Jun Xu, Yong Piao, Wanyu Ding, Dehe Wang, Xinlu Deng, Chuang Dong., Surf. Coatings Technol. 201 (2007) 5298–5301. [6] L.C. Chen, H.Y. Lin, C.S. Wong, K.H. Chen, S.T. Lin, Y.C. Yu, C.W. Wang, E.K. Lin, K. C. Ling., Diam. Relat. Mater. 8 (1999) 618–622. [7] C.W. Chen, C.C. Huang, Y.Y. Lin, L.C. Chen, K.H. Chen, W.F. Su., Diam. Relat. Mater. 14 (2005) 1010–1013. [8] C.W. Chen, C.C. Huang, Y.Y. Lin, L.C. Chen, K.H. Chen., Diam. Relat. Mater. 14 (2005) 1126–1130. [9] K.H. Chen, J.-J. Wu, C.Y. Wen, L.C. Chen, C.W. Fan, P.F. Kuo, Y.F. Chen, Y.S. Huang., Thin Solid Films 355–356 (1999) 205–209. [10] Wenjuan Cheng, Jinchun Jiang, Yang Zhang, Hesun Zhu, Dezhong Shen., Mater. Chem. Phys. 85 (2004) 370–376. [11] Hiroyuki Fujiwara, Spectroscopic Ellipsometry, Principles and Applications, John Wiley and Sons Ltd, England (2007), p. 253–265. [12] Steven M. Smith, Tim Tighe, Diana Convey, Jaime Quintero, Yi Wei., Thin Solid Films 516 (2008) 885–890. [13] I. Ferreira, E. Fortunato, P. Vilarinho, A.S. Viana, A.R. Ramos, E. Alves, R. Martins., J. Non- Cryst. Solids 352 (2006) 1361–1366. [14] Xing-cheng Xiao, Ya-wen Li, Li-xin Song, Xiao-feng Peng, Xing-fang Hu., Appl. Surf. Sci. 156 (2006) 155–160. [16] Erqing Xie, Ziwei Ma, Hongfeng Lin, Zhimin Zhang, Deyan He, 23, 2003, pp. 151–156. [17] Y. Gao, J..Wei, D.H. Zhang, Z.Q. Mo, P. Hing, X. Shi, 377–378, 2000, pp. 562–566. [18] V.I. Ivashchenkoa, A.O. Kozaka, O.K. Poradaa, L.A. Ivashchenkoa, O.K.O. S. Lytvynb, Sinelnichenkoa, T.V. Tomilaa, V.J. Malakhova, Thin Solid Films 569 (2014) 57–63.