The hydrogen in PECVD-SiOxNy was studied with IR spectroscopy and ERD analysis as a function of the O/N ratio and the annealing treatment up to 1150°C. The results were compared with measured spectral waveguide losse

Albers, H.

Hilderink, L.T.H.

Lambeck, P.V.

Passti, F.

Popma, Th.J.A.

Snilagyi, E.

English

Lambeck, Paul

Popma, T.J.A.

University of Twente Research Information

4:30pm - 4:45pm OFPW3.4 Reduction of hydrogen induced losses in PECVD-SiO,N, optical waveguides in the near infrared H. Albers'), L.T.H. Hilderink'), E. SzilBgyi'), F. Paszti'), P.V. Lambeck') and Th. J.A. Popma') ') MESA Research Institute, University of Twente, P.0.Box 217, 7500 AE Enschede, The Netherlands, ') KFKI-RMKI, Research Institute for Particle and Nuclear Physics, P.O.Box 49, H-1525 Budapest, Hungary. The hydrogen in PECVD-SiOxNy was studied with IR spectroscopy and ERD analysis as function of the O/N ratio and the annealing treatment up to 1150 "C. The results were compared with measured spectral waveguide losses. Introduction Silicon oxynitride is a flexible material for use as planar optical waveguides because by changing the composition ( O N  ratio) the refractive index can be tuned from 1.46 to 2.0'. Several methods can be used for deposition: PECVD, LPCVD and sputtering. The advantages of PECVD are easy control of composi- tion, high deposition rate and low stress. A disad- vantage is the high hydrogen content, up to 30 at%. Absorption bands from Si-OH and N-H at resp. 1380 nm and 1510 nm have to be reduced for telecommu- nication applications '. It is known that the amount of hydrogen can be reduced by annealing, but this complicated process depends strongly on the deposition conditions '. Experimental SiOxNy films were deposited with a parallel plate PECVD reactor (Electrotech 3 10) with dual fre- quency plasma generator, 187.5 kHz and 13.56 Mhz. The deposition conditions are given in table I. Process pressure 86 Pa Si&+ NZ flow (2% Si&) I 1000 sccm N H 3  flow 3 sccm NzO flow I 0-1OOOsccm The substrates were bare 3" silicon wafers for the analysis and oxidised wafers for waveguide struc- tures. Samples of the wafers were annealed in a tube oven at 700, 1000 and 1150 "C under nitrogen flow. The composition of thin (140 nm) films was deter- mined with RBS and ERDA. Furthermore the hydro- gen content and bonding type was determined by transmission IR spectroscopy using films of around 1 pm thick. The hydrogen concentration was evalu- ated from the IR peak area of the N-H peak at about 3370 an' and the Si-H peak at about 2230 cm' using the calibration factors from Lanford and Rand '. Waveguide loss spectrum In figure 1 a loss spectrum of a SiO,N, waveguide on top of a thermal oxide buffer layer is given. The peak at 15 10 nm is the first overtone of the N-H vibration. The shoulder at about 1400 nm corre- sponds with the Si-OH absorption band, but no Si- OH absorption is observed in the transmission spectrum. It is therefore likely that for some reason the background has changed. Si-OH absorption are only observed in PECVD SiOz films. 20.0 I A ~n.1 1110-3 Fig.1. Loss spectrum of the zeroorder mode of a SiGN,, waveguide with refractive index of 1.7 and a thickness of 0.9 pm. Bufferlayer 2.8 pm thermal oxide. The single point is a single wavelength measurement at 1523 nm. The losses measured at 632.8 nm were about 1 dEVcm for as deposited waveguides and below 0.5 dB/cm after a moderate anneal at 500 OC '. Composition as depositedficrns The O/N ratios of SiO,N, films have been varied by changing the N20 flow. The refractive index can be varied from 2.0 for no N,O flow to 1.52 for a N20 flow of 1000 sccm. The composition of the different films prepared according to the conditions in table I, is presented in figure 2. The amount of hydrogen clearly depends on the OM ratio in the film. The amount of Si-H in the nitride is high. It reduces sharply upon incorporation of 0 in the film. 88 In table I1 we present the hydrogen concentration as determined with IR spectroscopy of films with n=1.7 for two plasma generator frequencies and pressures. 1 U) 5 0.8 = 0 e 0.6 0.2 0 200 400 600 800 1000 Fig.2. Composition as function of N2O flow. The atomic fraction of each element is given by the distance between the lines below and above its name. The hydrogen fraction is split into two fractions bonded to resp. nitrogen and oxygen. Table IL Hydrogen content (at%)for different deposition conditions. Cfiessure + I 1 i N20 flow (sccm) I 13.56- I 9 I 1 8  I 2 7  1 1 8  I 1 2  1 3 0 1  The lowest hydrogen content is reached with low frequency and high pressure, our standard process. Furthermore the Si-H content is high for the high frequency and pressure. As we will see in the follow- ing this influences the annealing behaviour strongly. Annealing We observed several effects after annealing, a reduction of the hydrogen content (fig.3), a decrease of thickness and refractive index and a change in stress from compressive to strong tensile. Films with a refractive index above 1.6 cracked at 1000 and 1150 "C annealing. The initial Si-H amounts in these films are larger than 4 atyo, while the Si-H amounts in the not cracked films are < 1 YO. The same trend was observed after annealing of the samples mentioned in table 11, all samples cracked at 1000 OC, except for the one made with 187.5 kHz and 40 Pa, which cracked only at 1150 "C. Conclusions for waveguides The above results show that there is a combination of deposition parameters and annealing treatment possible for which the final hydrogen content is rather low. On bases of the measured loss in figure 1 and the amount of N-H in that film we expect that the waveguide absorption loss can be calculated from the fraction N-H by the next equation: loss(dB/cm) = O.Sxw-Hl(at%) = S 0.3 '11 ~ 0 1.4 1.6 1.8 2 2.2 Refractive index Fig.3. The total hydrogen fraction before annealing (A) and after annealing at 700 "C (o), 1000 "C (U) and 1 150 "C (0). The grey area covers the conditions were cracking of the film occurs. The expected losses at the absorption maximum at 1510 nm for two annealed films are given in table 111. These values are rough estimates. At a wave- length of 1550 nm the values will be substantial lower. Table HI. Expected absorption losses for annealed Our conclusion is that we have shown that low loss PECVD SiO,N, waveguides for use in telecom- munication applications are possible in spite of the high initial hydrogen concentration in these films. F.H.P.M. Habraken, 'Characterization of LPCVD and PECVD silicon oxynitride films', Appl. Surf. Sc., 30, 186- 196, ( 1987). M. HORIMIUI, E. Voges, 'LOW temperature, nitrogen doped waveguides on silicon with small core dimensions fabricated by PECVDRIE', Proc. 7th Em. Conf. on Int. Opt. ( ECIO '95), 299-302, (1 995). van der Weg, 'Annealing of plasma silicon oxynitride films', J. Appl. Phys., 60,2536-2542, (1986). W.A. Lanford, M.J. Rand, 'The hydrogen content of plasmadeposited silicon nitride', J. Appl. Phys., 49,2473- 2477, (1 978). W.S. Bijlsma, H.Albers, P.V. Lambeck, Th.J.A. Popma, 'PECVD SiO,N, films as planar optical waveguides', in: Sensors & Actuators, Optical Transducers, Symposium proceedings 1990, ed. A. Driessen, Kluwer Technical Books, Deventer. C.M.M. Denisse, K.Z. Troost, F.H.P.M. Habraken, W.F. 89 

Reduction of hydrogen induced losses in PECVD-SiOxNy optical waveguides in the near infrared

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Reduction of hydrogen induced losses in PECVD-SiOxNy optical waveguides in the near infrared

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