Silicon oxynitride prepared by chemical vapor deposition as optical waveguide materials

Journal of Crystal Growth 288(2006)171–175

Silicon oxynitride prepared by chemical vapor deposition

as optical waveguide materials

C.K.Wong a ,Hei Wong a,?,C.W.Kok b ,M.Chan b

a

Department of Electronic Engineering,City University of Hong Kong,Tat Chee Avenue,Kowloon,Hong Kong

b

Department of Electrical and Electronic Engineering,Hong Kong University of Science and Technology,Clear Water Bay,Kowloon,Hong Kong

Available online 18January 2006

Abstract

This work explores the technology for preparing low hydrogen-content silicon oxynitride ?lm for integrated optical waveguide applications.Plasma-enhanced chemical vapor deposition with N 2O,NH 3and SiH 4precursors was used for the oxynitride preparation.The ?ow rates of the precursor gases are varied to study processing effects on the refractive index and the content of hydrogen bonds.The refractive index of the oxynitride ?lm can be readily tuned between 1.47and 1.92by varying the gas ?ow rates.The composition and the bonding structure of the oxynitride ?lms were investigated with Fourier transform infrared (FTIR)spectroscopy.Results showed that the silicon oxynitride deposited with gas ?ow rates of NH 4/N 2O/SiH 4?10/400/10(sccm)has favorable properties for integrated waveguide applications.The refractive index of this layer is about 1.5and the layer has a comparative low density of N–H bonds.The high content of O–H bond can be readily eliminated with high-temperature annealing of the as-deposited ?lm in nitrogen ambient.Annealing at temperature of 10001C or above which can signi?cantly suppress both the N–H bonds and O–H bonds is preferred.Waveguide devices built with oxynitride prepared at those conditions would have properties of low propagation loss and small size.r 2005Published by Elsevier B.V.

PACS:42.82.àm;78.20.àe;78.30.àj Keywords:B1.Oxynitride;B3.Waveguide

1.Introduction

It has been demonstrated that the silicon oxynitride is a promising material to replace the existing silica-based passive optical components such as branches,interferom-eters,?lters,couplers,and splitters for downsizing and system integration [1,2].Silicon oxynitride has a wide range of tunable refractive index from 1.45(SiO 2)to 2.0(Si 3N 4).The minimum allowable bending radius for this high-index-contrast material could be one order of magnitude smaller than that of the silica ones.This would enable a great reduction in the size of the integrated components [2].However,the propagation loss of this material is still a major concern.The optical loss in plasma-enhanced chemical vapor deposition (PECVD)or low-pressure chemical vapor deposition (LPCVD)oxynitride is mainly

due to the absorption loss (particular in the 1460–1620nm band)of inherent N–H bonds [2,3].Several methods for solving this drawback have been proposed.The hydrogen content of oxynitride ?lm was found to decrease by more than 40%by thermal oxidation of LPCVD silicon-rich silicon nitride ?lm [4,5].The hydrogen content can also be reduced signi?cantly by increasing the nitric oxide ?ow rate during the PECVD growth of silicon oxynitride and conducting a high-temperature annealing after the deposi-tion [6].Although several attempts for making oxynitride-based waveguide devices were reported [7–9],there is a lack of systematic approach on the process optimization and material property,especially the hydrogen bonds,analysis for the oxynitride ?lm.In this work,we reported attempt on the process optimization for preparing silicon oxynitride ?lms using PECVD method with nitric oxide,ammonia and silane as the precursor gas sources.Special emphasis is placed on the hydrogen content and hydrogen bonding structures in the oxynitride layers.

https://www.wendangku.net/doc/155161646.html,/locate/jcrysgro

0022-0248/$-see front matter r 2005Published by Elsevier B.V.doi:10.1016/j.jcrysgro.2005.12.022

?Corresponding author.Fax:+852********.

E-mail address:heiwong@https://www.wendangku.net/doc/155161646.html, (H.Wong).

2.Experimental procedures

The oxynitride ?lms were deposited in a STS 310PECVD reactor with silane (SiH 4),ammonia (NH 3),and nitrous oxide (N 2O)as reactant gases at temperature 3501C and a pressure of 1T.To control the chemical composition and then the refractive index,several different ?ow rates of N 2O and NH 3were used.The resulting refractive indices of these ?lms varied from 1.48to 1.65which is suitable for using as the core material for optical waveguides operating at wavelength of 1550nm.Effects of the thermal annealing were investigated in detail.The samples were annealed in nitrogen ambient at temperatures ranging from 800to 11001C for duration from 30min to 3h.The refractive index and the thickness characterization were done with a Rudolph Auto EL II ellipsometer with 632.8nm light source.The compositional and structural properties of the as-grown layers were analyzed by making use of a Bio-Rad Fourier transform infrared (FTIR)spectrometer FTS 6000whose wavelength resolution is 2cm à1.

3.Results and discussion

Fig.1depicts the refractive index variation as functions of NH 3and N 2O ?ow rates.The refractive index changes from 1.65to 1.48as the N 2O ?ow rate decreases from 100to 500sccm.Higher refractive indices up to 1.92were also obtained by using a lower N 2O/SiH 4ratio or using a greater NH 3?ow rate.This range of refractive index is not suitable to waveguide application and is not the interest of this study.At low N 2O ?ow rates and with the absence of ammonia,large index ?lms were produced because of the formation of silicon-rich ?lms [3].At large N 2O ?ow rates,large amount of oxygen (with trace amount of nitrogen)will be incorporated into the ?lm,resulting in refractive index closer to that of stoichiometric SiO 2.Increasing the

NH 3?ow rate would enhance the refractive index due to the increase in nitrogen and hydrogen contents.The relationship between the refractive index and the N 2O ?ow rate is quite linear,which enables an easy process control for a speci?c ?lm.Annealing conditions,both temperature and duration,have profound effects on the optical properties of the oxynitride ?lm.Fig.2plots the changes of refractive index and ?lm thickness as a function of annealing temperature.The ?lm thickness almost decreases linearly as the annealing temperature or duration increases which is attributed to the densi?cation effects involving both the removal of microvoids and hydrogen atoms in the ?lm.However,as shown in Fig.2,the relation between the refractive index and annealing temperature is more complicated.

High hydrogen content was found with oxynitride ?lms prepared by chemical deposition method [4].Hydrogen bonds in oxynitride ?lms do not only affect the refractive index but also are the major sources for absorption loss of optical transmission in the oxynitride waveguide.To study the processing and the annealing effects on the oxynitride ?lms,FTIR measurements were conducted.Fig.3depicts infrared absorbance of various oxynitride ?lms deposited with 10sccm NH 3and several different N 2O ?ow rates.Various features related to Si–O rocking,Si–O bending,Si–N stretching,Si–O symmetric stretching,Si–O asym-metric stretching,and band related to hydrogen bonds are indicated.Particularly,all samples show a dominant absorption peak at around 1050cm à1which is due to the stretching vibrations of Si–O groups.This value is smaller than that of a stoichiometric SiO 2?lm which has an Si–O peak at 1080cm à1[9,10].As the ?ow rate of N 2O increases,the Si–O peak intensity increases because of higher Si–O content.The peak skews to higher frequency side because of the increasing symmetric stretching vibration of the Si–O bonds and the reduction of asymmetric Si–O stretching.

N 2O Flow Rate (sccm)

100

200

300400500

600

R e f r a c t i v e I n d e x 1.48

1.501.521.541.561.581.601.621.641.66

Fig.1.Plot of refractive index variation of oxynitride ?lm as a function of N 2O and NH 3?ow rates.Flow rates of SiH 4and N 2were 10and 490sccm,respectively.Annealing Temperature (°C)

8009001000

1100

R e f r a c t i v e I n d e x C h a n

g e (%)

-2.0

-1.5

-1.0-0.50.0T h i c k n e s s C h a n g e (%)

Fig.2.Variation of refractive index and ?lm thickness as a function of annealing temperature.The samples were furnace annealed in nitrogen ambient for 3h.

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Clear N–H and O–H stretching absorption band ranging from 3200to 3800cm à1was found for all samples.This band is the main cause of the optical absorption at 1550nm [11].Gaussian deconvolutions of this band in various samples were conducted (see Fig.4for example).With this technique,the concentration of various absorbance bands which is proportional to the area of the band can be determined using the Lanford and Rand’s formula [12].As shown in Fig.4,the N–H absorption bands decrease remarkably whereas the O–H bands increase slightly for large N 2O ?ow rate but the total hydrogen contents in samples prepared with high N 2O ?ow rates are still smaller than those prepared with low N 2O ?ow rates.These observations can be explained with the higher reactivity of O than N.A peak value of N–H ?N bonding related absorption was found at N 2O ?ow rate of 300sccm.This observation can be attributed to the fact that hydrogen atoms at high N 2O ?ow rate are bonded mainly to oxygen atoms and the amount of N–H and thus N–H ?N bonds are smaller [6](Fig.5).

To eliminate the N–H bond,thermal annealing was conducted at several different temperatures.Shift of the Si–O–Si stretching frequency to high-frequency side is notable,indicating that ?lm becomes more ordered and the bond lengths were shortened.The annealing has also accompanied with the densi?cation effect as depicted in Fig.2.Another obvious change in the FTIR spectra is the signi?cant reduction of the vibration bands related to hydrogen bond.The as-deposited ?lm has N–H and O–H bond densities of 6.05?1021and 1.04?1022cm à3,respec-

tively.Gaussian decompositions of the spectra reveal that the annealing is particularly high effective for removing the O–H related absorption bands,annealing at 8001C for 3h is already able to reduce the O–H bond density to detectable limit.For N–H stretching vibration band,higher temperature (410001C)is required.The hydrogen content of sample annealed at 11001C has reduced down to 3.57?1020cm à3.With this connection SIMS measure-ments were conducted to probe the hydrogen variations at different annealing conditions.Fig.6plots the change of hydrogen as a function of annealing temperature and annealing duration.The hydrogen content was reduced by

A b s o r b a n c e (a .u .)

N 2O=100 sccm N 2O=200 sccm

N 2O=300 sccm N 2O=400 sccm

N 2O=500 sccm

360034003200

Wavenumber (cm -1

)

6

5

432

1

5000

4000

3000

2000

1000

Wavenumber (cm -1)

Fig.3.Infrared absorbance of silicon oxynitride ?lms deposited with 10sccm NH 3?ow rate and various N 2O ?ow rates.Numbers indicate the major features of the absorbance spectra.(1)Si–O rocking,(2)Si–O bending,(3)Si–N stretching,(4)Si–O symmetric stretching,(5)Si–O asymmetric stretching,and (6)band related to H bonds.

Wavenumber (cm -1)

3000

3200

3400

3600

3800

A b s o r b a n c e (a .u .)

Fig.4.Gaussian deconvolution of O–H and N–H absorption bands of silicon oxynitride ?lm prepared by PECVD with gas ?ow rates of NH 4/N 2O/SiH 4?10/400/10(sccm).

N 2O Flow Rate (sccm)

100

200

300400

500

A r e a o f A b s o r p t i o n P e a k (a .u .)

Fig.5.Variation of the areas of O–H and N–H stretching bands derived from the Gaussian deconvolution as a function of N 2O ?ow rate.

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about 18%(compared to as-deposited sample)after 2h annealing at 11001C.Measurements on the oxynitride channel waveguide found that the propagation loss is about 0.1dB/cm [13].

With the available process for thickness,refractive index and hydrogen content control,making a low-loss channel oxynitride waveguide in the 1550-nm wavelength region is possible.Based on the data obtained in previous section,some waveguide devices are designed.The guiding core layer (oxynitride)has a refractive index of 1.500and is enclosed with PECVD oxide cladding layer with a refractive index of 1.480.The oxynitride core layer can be fabricated with PECVD with gas ?ow rates of NH 4/N 2O/SiH 4?10/400/10(sccm).To reduce the hydrogen content,the oxynitride layer should be annealed in nitrogen ambient at 10001C for 3h.According to an effective index calculation and Beamprop (Rsoft)simulation,to enable single mode transmission,the cross-section of the core layer should not be less than 3?3m m 2.Since a large refractive index difference between core and cladding layers is achieved,this kind of waveguide devices has advantages of high contrast and compact (because of small waveguide bending curvature).Fig.7demonstrates an example of a 3dB coupler design using this technology.The separation between the channel waveguide is 3m m and coupling length is 356.5m m only.This size is much smaller than the conventional doped silica technology.4.Conclusion

We have conducted comprehensive study on the PECVD growth of silicon oxynitride ?lm for integrated optical waveguide applications.The ?ow rates of N 2O and NH 3precursor gases are varied to study processing effects on the refractive index and the content of hydrogen bonds.The composition and bonding structure of the oxynitride ?lms were investigated with FTIRspectroscopy.Special atten-tion was given to the hydrogen bonds related absorption

band in the frequency ranging from 3300to 3800cm à1.Results showed that silicon oxynitride deposited with gas ?ow rates of NH 4/N 2O/SiH 4?10/400/10(sccm)has favorable properties for using as core layer of integrated waveguide.The refractive index of this layer is about 1.5.The layer has a comparative low density of N–H bonds.Annealing at temperature of 10001C or above which can signi?cantly suppress both the N–H bonds and O–H bonds is preferred.Waveguide devices built with oxynitride prepared at those conditions would have properties of low propagation loss and small size.Acknowledgment

The work described in this paper was fully supported by a project (Project no.7001513)funded by City University of Hong Kong.References

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Annealing Temperature (°C)

700

80090010001100

N -H C o n c e n t r a t i o n (x 1021 c m -3)

Fig.6.Effects of annealing temperature on N–H concentration of the oxynitride ?lms.The annealing duration is 3h.

5000

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1000

0102030405060

1.0

0.50.0

Pathway,Monitor:

1. Mode 0

2. Mode 0

Z (μm )

X (μm)

Monitor Value (a.u.)

Fig.7.Simulation result of a designed 3-dB coupler based on PECVD SiO 2/oxynitride/thermal SiO 2channel waveguide structure.The separa-tion width is 3m m and the coupler length is 356.5m m.

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