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Journal of Crystal Growth 288 (2006) 171–175

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Silicon oxynitride prepared by chemical vapor depositionas optical waveguide materials

C.K. Wonga, Hei Wonga,�, C.W. Kokb, M. Chanb

aDepartment of Electronic Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong KongbDepartment of Electrical and Electronic Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

Available online 18 January 2006

Abstract

This work explores the technology for preparing low hydrogen-content silicon oxynitride film for integrated optical waveguide

applications. Plasma-enhanced chemical vapor deposition with N2O, NH3 and SiH4 precursors was used for the oxynitride preparation.

The flow 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 film can be readily tuned between 1.47 and 1.92 by varying the gas flow rates. The composition and

the bonding structure of the oxynitride films were investigated with Fourier transform infrared (FTIR) spectroscopy. Results showed

that the silicon oxynitride deposited with gas flow rates of NH4/N2O/SiH4 ¼ 10/400/10 (sccm) has favorable properties for integrated

waveguide applications. The refractive index of this layer is about 1.5 and 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 film in nitrogen ambient.

Annealing at temperature of 1000 1C or above which can significantly 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 2005 Published by Elsevier B.V.

PACS: 42.82.�m; 78.20.�e; 78.30.�j

Keywords: B1. Oxynitride; B3. Waveguide

1. Introduction

It has been demonstrated that the silicon oxynitride is apromising material to replace the existing silica-basedpassive optical components such as branches, interferom-eters, filters, couplers, and splitters for downsizing andsystem integration [1,2]. Silicon oxynitride has a wide rangeof tunable refractive index from 1.45 (SiO2) to 2.0 (Si3N4).The minimum allowable bending radius for this high-index-contrast material could be one order of magnitudesmaller than that of the silica ones. This would enable agreat reduction in the size of the integrated components [2].However, the propagation loss of this material is still amajor concern. The optical loss in plasma-enhancedchemical vapor deposition (PECVD) or low-pressurechemical vapor deposition (LPCVD) oxynitride is mainly

e front matter r 2005 Published by Elsevier B.V.

rysgro.2005.12.022

ing author. Fax: +8522788 7722.

ess: heiwong@ieee.org (H. Wong).

due to the absorption loss (particular in the 1460–1620 nmband) of inherent N–H bonds [2,3]. Several methods forsolving this drawback have been proposed. The hydrogencontent of oxynitride film was found to decrease by morethan 40% by thermal oxidation of LPCVD silicon-richsilicon nitride film [4,5]. The hydrogen content can also bereduced significantly by increasing the nitric oxide flow rateduring the PECVD growth of silicon oxynitride andconducting 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 lackof systematic approach on the process optimization andmaterial property, especially the hydrogen bonds, analysisfor the oxynitride film. In this work, we reported attempton the process optimization for preparing silicon oxynitridefilms using PECVD method with nitric oxide, ammoniaand silane as the precursor gas sources. Special emphasis isplaced on the hydrogen content and hydrogen bondingstructures in the oxynitride layers.

ARTICLE IN PRESSC.K. Wong et al. / Journal of Crystal Growth 288 (2006) 171–175172

2. Experimental procedures

The oxynitride films were deposited in a STS 310PECVD reactor with silane (SiH4), ammonia (NH3), andnitrous oxide (N2O) as reactant gases at temperature350 1C and a pressure of 1 T. To control the chemicalcomposition and then the refractive index, several differentflow rates of N2O and NH3 were used. The resultingrefractive indices of these films varied from 1.48 to 1.65which is suitable for using as the core material for opticalwaveguides operating at wavelength of 1550 nm. Effects ofthe thermal annealing were investigated in detail. Thesamples were annealed in nitrogen ambient at temperaturesranging from 800 to 1100 1C for duration from 30min to3 h. The refractive index and the thickness characterizationwere done with a Rudolph Auto EL II ellipsometer with632.8 nm light source. The compositional and structuralproperties of the as-grown layers were analyzed by makinguse of a Bio-Rad Fourier transform infrared (FTIR)spectrometer FTS 6000 whose wavelength resolution is2 cm�1.

3. Results and discussion

Fig. 1 depicts the refractive index variation as functionsof NH3 and N2O flow rates. The refractive index changesfrom 1.65 to 1.48 as the N2O flow rate decreases from 100to 500 sccm. Higher refractive indices up to 1.92 were alsoobtained by using a lower N2O/SiH4 ratio or using agreater NH3 flow rate. This range of refractive index is notsuitable to waveguide application and is not the interest ofthis study. At low N2O flow rates and with the absence ofammonia, large index films were produced because of theformation of silicon-rich films [3]. At large N2O flow rates,large amount of oxygen (with trace amount of nitrogen)will be incorporated into the film, resulting in refractiveindex closer to that of stoichiometric SiO2. Increasing the

N2O Flow Rate (sccm)100 200 300 400 500 600

Ref

ract

ive

Inde

x

1.48

1.50

1.52

1.54

1.56

1.58

1.60

1.62

1.64

1.66

NH3 = 10 sccm

NH3 = 20 sccm

NH3 = 40 sccm

Fig. 1. Plot of refractive index variation of oxynitride film as a function of

N2O and NH3 flow rates. Flow rates of SiH4 and N2 were 10 and 490 sccm,

respectively.

NH3 flow rate would enhance the refractive index due tothe increase in nitrogen and hydrogen contents. Therelationship between the refractive index and the N2O flowrate is quite linear, which enables an easy process controlfor a specific film. Annealing conditions, both temperatureand duration, have profound effects on the opticalproperties of the oxynitride film. Fig. 2 plots the changesof refractive index and film thickness as a function ofannealing temperature. The film thickness almost decreaseslinearly as the annealing temperature or duration increaseswhich is attributed to the densification effects involvingboth the removal of microvoids and hydrogen atoms in thefilm. However, as shown in Fig. 2, the relation between therefractive index and annealing temperature is morecomplicated.High hydrogen content was found with oxynitride films

prepared by chemical deposition method [4]. Hydrogenbonds in oxynitride films do not only affect the refractiveindex but also are the major sources for absorption loss ofoptical transmission in the oxynitride waveguide. To studythe processing and the annealing effects on the oxynitridefilms, FTIR measurements were conducted. Fig. 3 depictsinfrared absorbance of various oxynitride films depositedwith 10 sccm NH3 and several different N2O flow 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 areindicated. Particularly, all samples show a dominantabsorption peak at around 1050 cm�1 which is due to thestretching vibrations of Si–O groups. This value is smallerthan that of a stoichiometric SiO2 film which has an Si–Opeak at 1080 cm�1 [9,10]. As the flow rate of N2O increases,the Si–O peak intensity increases because of higher Si–Ocontent. The peak skews to higher frequency side becauseof the increasing symmetric stretching vibration of theSi–O bonds and the reduction of asymmetric Si–Ostretching.

Annealing Temperature (°C)

700 800 900 1000 1100

Ref

ract

ive

Inde

x C

hang

e (%

)

-2.0

-1.5

-1.0

-0.5

0.0

As-Deposited

Thi

ckne

ss C

hang

e (%

)0.0

-5

-10

-15

-20

As-Deposited Film:Thickness = 0.766 µmRefractive Index = 1.495

Fig. 2. Variation of refractive index and film thickness as a function of

annealing temperature. The samples were furnace annealed in nitrogen

ambient for 3 h.

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bsor

banc

e (a

.u.)

N2O=100 sccm

N2O=200 sccm

N2O=300 sccm

N2O=400 sccm

N2O=500 sccm

3600 3400 3200Wavenumber (cm-1)6

5

4

32 1

5000 4000 3000 2000 1000 0

Wavenumber (cm-1)

Fig. 3. Infrared absorbance of silicon oxynitride films deposited with

10 sccm NH3 flow rate and various N2O flow 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)30003200340036003800

Abs

orba

nce

(a.u

.)

MeasuredN-H ... N stretchingN-H stretchingH-O-H stretchingSiO-H stretchingSiO-H stretching

N2O = 400 sccm

Fig. 4. Gaussian deconvolution of O–H and N–H absorption bands of

silicon oxynitride film prepared by PECVD with gas flow rates of NH4/

N2O/SiH4 ¼ 10/400/10 (sccm).

N2O Flow Rate (sccm)100 200 300 400 500

Are

a of

Abs

orpt

ion

Pea

k (a

.u.)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

N-H stretchingN-H...N stretchingH-O-H stretchingSiO-H stretchingSiO-H stretching

Fig. 5. Variation of the areas of O–H and N–H stretching bands derived

from the Gaussian deconvolution as a function of N2O flow rate.

C.K. Wong et al. / Journal of Crystal Growth 288 (2006) 171–175 173

Clear N–H and O–H stretching absorption band rangingfrom 3200 to 3800 cm�1 was found for all samples. Thisband is the main cause of the optical absorption at 1550 nm[11]. Gaussian deconvolutions of this band in varioussamples were conducted (see Fig. 4 for example). With thistechnique, the concentration of various absorbance bandswhich is proportional to the area of the band can bedetermined using the Lanford and Rand’s formula [12]. Asshown in Fig. 4, the N–H absorption bands decreaseremarkably whereas the O–H bands increase slightly forlarge N2O flow rate but the total hydrogen contents insamples prepared with high N2O flow rates are still smallerthan those prepared with low N2O flow rates. Theseobservations can be explained with the higher reactivity ofO than N. A peak value of N–H?N bonding relatedabsorption was found at N2O flow rate of 300 sccm. Thisobservation can be attributed to the fact that hydrogenatoms at high N2O flow rate are bonded mainly to oxygenatoms and the amount of N–H and thus N–H?N bondsare smaller [6] (Fig. 5).

To eliminate the N–H bond, thermal annealing wasconducted at several different temperatures. Shift of theSi–O–Si stretching frequency to high-frequency side isnotable, indicating that film becomes more ordered and thebond lengths were shortened. The annealing has alsoaccompanied with the densification effect as depicted inFig. 2. Another obvious change in the FTIR spectra is thesignificant reduction of the vibration bands related tohydrogen bond. The as-deposited film has N–H and O–Hbond densities of 6.05� 1021 and 1.04� 1022 cm�3, respec-

tively. Gaussian decompositions of the spectra reveal thatthe annealing is particularly high effective for removing theO–H related absorption bands, annealing at 800 1C for 3 his already able to reduce the O–H bond density todetectable limit. For N–H stretching vibration band, highertemperature (41000 1C) is required. The hydrogen contentof sample annealed at 1100 1C has reduced down to3.57� 1020 cm�3. With this connection SIMS measure-ments were conducted to probe the hydrogen variations atdifferent annealing conditions. Fig. 6 plots the change ofhydrogen as a function of annealing temperature andannealing duration. The hydrogen content was reduced by

ARTICLE IN PRESS

Annealing Temperature (°C)700 800 900 1000 1100

N-H

Con

cent

ratio

n (x

1021

cm

-3)

0

1

2

3

4

5

6N2O Flow Rate = 500 sccm

Fig. 6. Effects of annealing temperature on N–H concentration of the

oxynitride films. The annealing duration is 3 h.

5000

4000

3000

2000

1000

00 10 20 30 40 50 60 1.0 0.5 0.0

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

SiO2/oxynitride/thermal SiO2 channel waveguide structure. The separa-

tion width is 3mm and the coupler length is 356.5mm.

C.K. Wong et al. / Journal of Crystal Growth 288 (2006) 171–175174

about 18% (compared to as-deposited sample) after 2 hannealing at 1100 1C. Measurements on the oxynitridechannel waveguide found that the propagation loss isabout 0.1 dB/cm [13].

With the available process for thickness, refractive indexand hydrogen content control, making a low-loss channeloxynitride waveguide in the 1550-nm wavelength region ispossible. Based on the data obtained in previous section,some waveguide devices are designed. The guiding corelayer (oxynitride) has a refractive index of 1.500 and isenclosed with PECVD oxide cladding layer with arefractive index of 1.480. The oxynitride core layer can befabricated with PECVD with gas flow rates of NH4/N2O/SiH4 ¼ 10/400/10 (sccm). To reduce the hydrogen content,the oxynitride layer should be annealed in nitrogenambient at 1000 1C for 3 h. According to an effective indexcalculation and Beamprop (Rsoft) simulation, to enablesingle mode transmission, the cross-section of the corelayer should not be less than 3� 3 mm2. Since a largerefractive index difference between core and cladding layersis achieved, this kind of waveguide devices has advantagesof high contrast and compact (because of small waveguidebending curvature). Fig. 7 demonstrates an example of a3 dB coupler design using this technology. The separationbetween the channel waveguide is 3 mm and coupling lengthis 356.5 mm only. This size is much smaller than theconventional doped silica technology.

4. Conclusion

We have conducted comprehensive study on the PECVDgrowth of silicon oxynitride film for integrated opticalwaveguide applications. The flow rates of N2O and NH3

precursor gases are varied to study processing effects on therefractive index and the content of hydrogen bonds. Thecomposition and bonding structure of the oxynitride filmswere investigated with FTIRspectroscopy. Special atten-tion was given to the hydrogen bonds related absorption

band in the frequency ranging from 3300 to 3800 cm�1.Results showed that silicon oxynitride deposited with gasflow rates of NH4/N2O/SiH4 ¼ 10/400/10 (sccm) hasfavorable properties for using as core layer of integratedwaveguide. 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 1000 1C or above which cansignificantly suppress both the N–H bonds and O–H bondsis preferred. Waveguide devices built with oxynitrideprepared at those conditions would have properties oflow propagation loss and small size.

Acknowledgment

The work described in this paper was fully supported bya project (Project no. 7001513) funded by City Universityof Hong Kong.

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