Chemical Physics Letters 368 (2003) 183–188 www.elsevier.com/locate/cplett
Low optical loss germanosilicate planar waveguides by low-pressure inductively coupled plasma-enhanced chemical vapor deposition Q.Y. Zhang a
a,*
, K. Pita a, Charles K.F. Ho a, N.Q. Ngo a, L.P. Zuo b, S. Takahashi b
Photonics Research Group, Microelectronics Division, School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang 639798, Singapore b HOYA Photonics Singapore Pte Ltd, 83 Science Park Drive, Singapore Received 7 August 2002; in final form 6 November 2002
Abstract This Letter reports on the preparation and properties of germanium-doped silica thin films by a new technique namely inductively coupled plasma-enhanced chemical vapor deposition (ICP-CVD). 13.56 MHz ICP was generated inside the chamber upon the supplying of 1 KW r.f. power at 2.5 Pa. The Films have been deposited on Si-wafers from the tetraethoxysilane, tetramethoxygermane and oxygen system at 400 °C. The behaviors of 200 and 240 nm UV absorptions under different treatment conditions comprising annealing temperatures and excimer laser exposure have been systematically investigated. A low propagation loss of 0.1 dB/cm at 1550 nm has been achieved. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Over the past several decades the inductively coupled plasma (ICP) method has been studied. Low-pressure ICP has got unique characteristics such as 101 –103 times higher electron, ion, and radical densities than the glow plasma utilized in conventional plasma aided materials processing techniques [1,2]. Application of high-density plasma operating at low pressure below 10 Pa has been
*
Corresponding author. Fax: +65-6790-4161. E-mail address:
[email protected] (Q.Y. Zhang).
performed for films deposition because of the fact such films obtained through plasma reactors would offer several advantages, i.e., lower hydrogen content, higher quality films at lower process temperature, void-free gap filling profiles, in contrast with typical deposition pressure on the order of 103 –104 Pa [2–6]. Germanium-doped silica (Ge:SiO2 or GeO2 – SiO2 ) has extensively been investigated in the past one decade due to the significant UV photosensitivity of this material, where intrinsic or induced chemical defects are photobleached resulting in a modulation of the refractive index (RI) [4,7–11]. This modulation of the RI has been used to
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 8 3 7 - 7
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fabricate diffraction gratings, i.e., narrowband reflectors for use in wavelength division multicomplexing (WDM) such as multichannel fiber Bragg grating (FBG), optical amplifiers in planar waveguides and fibers [7–9]. At present, GeO2 –SiO2 planar waveguides are generally fabricated by flame hydrolysis deposition (FHD) [12,13] and chemical vapor deposition (CVD) [14–16]. Although a great deal of progress has been made pertaining to the FHD technique, which has been a complicated processing and high consolidation temperatures facing certain hindrances for its full commercialization. Regarding the conventional CVD and the plasma-enhanced CVD (PECVD) are having some disadvantages such as low deposition rate and relatively high propagation loss, those constraints have so far been causing some difficulties in the fabrication of commercial optical waveguides. The present work describes the study of GeO2 –SiO2 thin films by a new low temperature, low-pressure inductively coupled plasma-enhanced chemical vapor deposition (ICP-CVD) for planar waveguides application. Characterization includes
Fourier transform infrared spectroscopy (FTIR), UV absorption, RI changes and attenuation coefficient measured at 632.8 and 1550 nm by a prism coupling technique as a function of Ge concentration change and annealing temperatures employed. The behaviors of 200 and 240 nm absorption peaks under different treatment conditions, i.e., annealing temperature and KrF excimer laser exposure have been investigated.
2. Experiments A schematic of the ICP-CVD experimental setup is shown in Fig. 1. The ICP-CVD reactor consists of a stainless steel cylinder with a helical coil antenna located above a quartz window. RF power (13.56 MHz) was supplied to the coil up to 1 kW via an impedance matching unit. The mixture of tetraethoxysilane (TEOS), tetramethoxygermane (TMOG) and a small amount of oxygen as the source gases were introduced into the chamber through the lower inlet near the substrate holder, and a larger amount of oxygen was intro-
Fig. 1. Schematic diagram of ICP-CVD experimental setup.
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duced through the upper inlet below the powered coil. All gases were controlled by electronic mass flow controllers. The flow rate of x mol.% (x ¼ 0; 6; 10 and 24) TMOG diluted with TEOS was 5 sccm (where sccm denotes cubic centimeters per minute at standard temperature and pressure), and that of oxygen was O2 /(TEOS + TMOG) ¼ 30. The oxygen-rich mixture used in this work was to prevent CH radicals those were incorporated into the deposited GeO2 –SiO2 films. A mirror-polished SiO2 buffered Si wafer substrate (h100i direction, 100 mm in diameter, SiO2 buffer layer thickness of 1.5 lm) or quartz was fixed mechanically on a substrate electrode with a resistive heater. RF power was applied between the ground and a substrate holder in order to control ion bombarding energy. Temperature of the substrate heater was measured by a thermocouple and regulated below 400 °C during deposition processes. The operating pressure and the substrate temperature were maintained at 2.5 Pa and 400 °C in this work, respectively. The samples were heat-treated for 30 min. The plasma source and deposition chamber were pre-evacuated to 6 103 Pa by a turbo molecular pump and backfilled with the required gases. The structures of the as-deposited and the treated films were studied by a FTIR (Perkin– Elmer Spectrum 2000 spectrometer, wavenumber range from 4000 to 400 cm1 , increment of 1 cm1 ) in transmittance mode, using a bare SiO2 =Si wafer as a reference. Absorption spectra of the samples were measured on a HP8453 UV– Vis spectrophotometer. The refractive index (RI) and the thickness of GeO2 –SiO2 films were measured for both transverse electric (TE) and transverse magnetic (TM) polarization on a Metricon 2010 by means of the prism coupling technique [17]. A 0.8 mW He–Ne laser tube with standard silicon detectors and a 2 mW diode laser tube with germanium detectors were employed operating at 632.8 and 1550 nm, respectively.
3. Results and discussion Fig. 2 shows the FTIR absorption spectra of GeO2 –SiO2 thin films annealed at different tem-
185
Fig. 2. FTIR patterns of GeO2 –SiO2 films annealed at different temperature in argon.
peratures in an argon atmosphere. Three prominent peaks, which are located at around 1080, 810 and 460 cm1 have been attributed to the absorption of Si–O and Ge–O bond stretching [18]. A broad-band at around 3500 cm1 , has been due to the stretching mode of OH group, and the bands at 1600–1650 cm1 , are ascribed to the bending mode of physically adsorbed water, are very weak both in the case of as-prepared and the 1000 °C treated samples as well. There exists no absorption peak corresponding to organic radicals as is shown in Fig. 2. These results suggest that GeO2 –SiO2 thin films prepared by ICP-CVD exhibit low OH and CH content, thus they have significant potentiality as the low loss waveguide. The optical absorption spectra of the as-deposited and treated GeO2 –SiO2 films are shown in Fig. 3. It has been observed that there exists a strong ultraviolet absorption at the short wavelength of 200 nm along with a prominent shoulder peak at 240 nm. There is an agreement over the assignment of the absorption band at around 200 nm to the Si EÕ and Ge EÕ centers [7–9]. The absorption at 240 nm has also been analyzed well and found to be arising due to an increase in the concentration of neutral oxygen monovacancies (NOV) defects [7–9]. Approximately a linear relationship between the 240 nm absorption and the TMOG inflow gas concentrations has been
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observed and shown in Fig. 3a. Fig. 3b shows ultraviolet absorption spectra of GeO2 –SiO2 films, in which TMOG inflow gas concentrations was 24 mol.%, under different treatment conditions. A
(a)
(b) Fig. 3. Ultraviolet absorption spectra of GeO2 –SiO2 films with different inflow gas concentrations (a), and films under different treatment conditions (b). Inset in (a) shows a roughly linear relationship between the 240 nm absorption and the inflow gas concentrations. Inset in (b) shows relationships between the 240 nm absorption and annealing temperatures or UV laser exposure (UVLE) times.
remarkable decrease in 240 nm absorption is observed with the annealing temperature increase from room temperature to 1000 °C, due to the reduction of NOV defects. In order to evaluate the effects of UV laser exposure (UVLE), samples were irradiated by KrF excimer laser (k ¼ 248 nm, operating at a 10 Hz repetition rate with a 25 ns pulse duration) at a UV fluency of 350 mJ/cm2 per pulse. Clearly, it is noticed that there is a proportional relationship between the 240 nm absorption and UV laser illumination times as shown in Fig. 3b curve d (before exposure), e (UVLE 2.5 min) and f (UVLE 5 min). Intensity of absorption band centered at 240 nm is found decreased while intensity of absorption above 200 nm increases with UV laser illumination duration. This is obviously because that the absorption band at 240 nm is bleached and absorption above 200 nm is induced. The photobleachable Ge–Ge/Ge–Si bonds (centered at 240 nm), associated with NOV defects, have been converted to Ge EÕ center (200 nm) upon UV laser illumination and that could be explained as follows:
Optical parameters of the ICP-CVD GeO2 – SiO2 thin films under different treatment conditions measured at 632.8 and 1550 nm are presented in Table 1 and also in Fig. 4. Smaller differences observed for the RI obtained for TE and TM modes both at 632.8 nm (Dnmax ¼ 0:0005) and 1550 nm (Dnmax ¼ 0:0009) are given in Table 1 indicating the fact that the birefringence in the present waveguide is negligible. Fig. 4a shows that the RI of the as-prepared and 1000 °C treated GeO2 –SiO2 films increased roughly linearly from 1.4622 to 1.4757 at 632.8 nm, 1.4502 to 1.4610 at 1550 nm, and from 1.4573 to 1.4694 at 632.8 nm, 1.4442 to 1.4560 at 1550 nm, respectively, as the TMOG inflow gas concentrations was increased from 0 to 24 mol.%. The UV absorption results shown in Fig. 3a reveal the similar trends when the concentration of germanium in the deposited films
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Table 1 Optical parameters of the ICP-CVD GeO2 –SiO2 thin films under different treatment conditions measured at 632.8 and 1550 nm Samplea
Thickness
Refractive index @632.8 (nm)
0.1 lmc
0:0001c
S1
2.1
S2
2.2
S3
2.1
S4
2.3
1.4693 1.4692 1.4695 1.4693 1.4729 1.4728 1.4757 1.4752
(TE) (TM) (TE) (TM) (TE) (TM) (TE) (TM)
@1550 (nm)
Number of modes
Attenuation coefficientb
@632.8 (nm)
@632.8 (nm)
@1550 (nm)
@1550 (nm)
0.1 (dB/cm)c 1.4560 (TE) 1.4558 (TM) 1.4564 (TE) 1.4557 (TM) 1.4567 (TE) 1.4560 (TM) 1.4610 (TE) 1.4601(TM)
3
3
0.9
0.1
3
3
1.0
0.1
3
3
1.3
0.4
3
3
2.1
1.0
a
Samples annealed at 1000 (S1 ), 800 (S2 ) and 600 °C (S3 ) in argon and as-prepared (S4 ). Attenuation coefficient was measured by exciting TE0 mode. c Measurement error. b
is proportional to the TMOG inflow gas concentrations. It should be mentioned that the RI of the GeO2 –SiO2 films (24 mol.% inflow gas of TMOG) decrease initially from 1.4757 to 1.4694 at 632.8 nm, and from 1.4610 to 1.4560 at 1550 nm with increasing annealing temperature from room temperature to 1000 °C. This is accompanying with the decrease of 240 nm absorption band as
(a)
discussed-above in Fig. 3b, due to the reduction of NOV defects. Above 1000 °C annealing temperature, the RI of GeO2 –SiO2 films slightly increases due to the films densitification, possibly. The optical propagation losses at 632.8 and 1550 nm, for the TE0 mode, were measured by a fiber photometric detection. It scans down the length of the propagating streak records the light
(b)
Fig. 4. The refractive index of GeO2 –SiO2 films as function of TMOG inflow gas concentration (a), and films annealing temperature (b).
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intensity scattered out of waveguide plane. The losses were evaluated by fitting the intensity to an exponential decay function, assuming a homogenous distribution of the scattering centers in the waveguides. The total loss of a planar waveguide consists of absorption and scattering contributions, the latter being usually predominant at the wavelength considered in the integrated optics. The scattering optical loss measured for an amorphous waveguide is the sum of two contributions: surface scattering, due to the surface roughness of sample, and volume scattering, due to local fluctuation in the refractive index resulting from density and compositional variation [19]. Optical propagation loss of 1.0 dB/cm at 632.8 nm and 0.1 dB/cm at 1550 nm of our present 800 °C annealed GeO2 –SiO2 planar waveguide by ICP-CVD have been obtained. It is quite interesting to report that the GeO2 –SiO2 planar waveguide fabricated through ICP-CVD technique has exhibited a low loss at the suitable wavelengths employed in the integrated optics (IO) and thus we identify its potentiality for promising applications in optical communications.
4. Conclusion In summary, we conclude that low temperature, low-pressure ICP-CVD technique has successfully been employed in the production of germaniumdoped silica planar waveguides by using TEOS, TMOG and oxygen as the source gas. FTIR spectra of the present GeO2 –SiO2 films have demonstrated low OH and CH contents. A large ultraviolet absorption region at short wavelength of 200 nm, associated with Si EÕ and Ge EÕ centers, accompanied with a strong shoulder peaked at 240 nm, due to NOV defects, have been analyzed. The behaviors of 200 nm and 240 nm absorption peaks under different conditions have been examined. Low optical propaga-
tion loss of 0.1 dB/cm at 1550 nm has been achieved in the present work. Acknowledgements The authors would like to thank Professor S. Buddhudu for his cooperation and useful discussion in our present work. References [1] J. Amorim, H.S. Maciel, J.P. Sudano, J. Vac. Sci. Technol. B 9 (1991) 362. [2] T. Ichiki, T. Yoshida, Appl. Phys. Lett. 64 (1994) 851. [3] K. Teii, Appl. Phys. Lett. 74 (1999) 4067. [4] T. Hattori, S. Semura, N. Akasaka, Jpn. J. Appl. Phys. 38 (1999) 2775. [5] C.S. Yang, K.S. Oh, J.Y. Ryu, D.C. Kim, Thin Solid Films 390 (2001) 113. [6] J.W. Lee, K.D. MacKenzie, D. Johnson, J.N. Sasserath, S.J. Pearton, F. Ren, J. Electrochem. Soc. 147 (2000) 1481. [7] M.G. Sceats, G.R. Atkins, S.B. Poole, Annu. Rev. Mater. Sci. 23 (1993) 381. [8] P.J. Hughes, A.P. Knights, B.L. Weiss, S. Kuna, P.G. Coleman, S. Ojha, Appl. Phys. Lett. 74 (1999) 3311. [9] Y. Miyake, H. Nishikawa, E. Watanabe, D. Ito, J. NonCryst. Solid 222 (1997) 266. [10] M. Essid, J.L. Brebner, J. Albert, K. Awazu, J. Appl. Phys. 84 (1998) 4193. [11] H. Hosono, Y. Abe, D.L. Kinser, R.A. Weeks, K. Muta, H. Kawazoe, Phys. Rev. B 46 (1992) 11445. [12] J.M. Ruano, V. Benoit, J.S. Aitchison, J.M. Cooper, Anal. Chem. 72 (2000) 1093. [13] M. Kawachi, M. Yasu, T. Edahiro, Electron. Lett. 19 (1983) 583. [14] I.A. Shareef, G.W. Rubloff, M. Anderle, W.N. Gill, J. Cotte, D.H. Kim, J. Vac. Sci. Technol. B 13 (1995) 1888. [15] N. Nourshargh, E.M. Starr, T.M. Ong, Electron. Lett. 25 (1989) 981. [16] Q. Lai, J.S. Gu, M.K. Smit, J. Schmid, H. Melchior, Electron. Lett. 28 (1992) 1000. [17] R. Syms, J. Cozens, in: Optical Guided Waves and Devices, McGraw-Hill International, London, 1992, p. 150. [18] F.X. Gan, in: Optical and Spectroscopic Properties of Glass, Springer-Verlag, Berlin, 1992, p. 33. [19] S.J.L. Ribeiro, Y. Messaddeq, R.R. Goncalves, M. Ferrari, M. Montagna, M.A. Aegerter, Appl. Phys. Lett. 77 (2000) 3502.
