Journal of the Korean Physical Society, Vol. 42, February 2003, pp. S947∼S951

Optical Characterization of Silica Based Waveguide Prepared by PlasmaEnhanced Chemical Vapor Deposition

S. M. Cho, Y. T. Kim and D. H. Yoon

Department of Advanced Materials Engineering, Sungkyunkwan University, Suwon 440-746

J. M. Kim, H. D. Yoon and Y. M. Im

Optical Telecommunication Research Center, Korea Electronics Technology Institute, Pyungtaek 451-865

G. E. Jang

Department of Materials Science & Materials Engineering, Chungbuk University, Cheongju 361-763

Silicon dioxide (SiO2) and silicon oxynitride (SiON) thick films used for cladding and core layerof silica based waveguides were fabricated by plasma enhanced chemical vapor deposition (PECVD)method, at a low temperature (320 ◦C) and from (SiH4+N2O+N2) gas mixtures. The effects ofdeposition parameters on properties of SiO2 and SiON thick films were investigated by variationsof SiH4, N2, N2O flow ratio and RF power. The obtained SiO2 films had a high thickness andrefractive index of 1.460±0.001 with the annealing process. FTIR and XPS revealed that depositedfilms were nearly stoichiometric composition with no presence of hydrogen content. SiON films hada 0.75 % higher refractive index than the cladding layer and a high thickness(> 5 µm).

PACS numbers: 81.15.Gh, 78.66.-wKeywords: PECVD, Optical waveguide, Silicon dioxide, Silicon oxynitirde

I. INTRODUCTION

Various kinks of optical components are requiredfor implementation optical communication systems withhigher transmission efficiency. These componentsinclude optical beam splitters/combines for branch-ing/combining optical signals, optical switches for chang-ing optical paths for flexible network utilization, andwavelength-division multi/demultiplexers for introduc-ing multiple optical channels into a single optical fiber[1]. Silica waveguide systems with glass composition aresimilar to that of optical fibers. Silica waveguides can beformed on crystal silicon substrates by a combination ofglass film deposited and etching. Their simple and clearcore structures, low propagation loss are almost perfectfield matching to optical fibers. The formation techniqueof the SiO2 based waveguide is required for the fabrica-tion of a planar waveguide device which is necessary forbroad-band optical communication technique [2,3].

Flame hydrolysis deposition (FHD) [4], chemical va-por deposition (CVD) [5] and plasma enhanced chemi-cal vapor deposition (PECVD) [6,7] are usually used forfabrication of silica waveguides. The FHD process canfabricate a thick SiO2 layer in a short time and has theadvantage of low optical loss and easier coupling withoptical fiber. But it is difficult to control the dopant andflame. Moreover, the FHD process is required the con-solidation process at a higher temperature about 1300

◦C. Chemical vapor deposition (CVD) and its variants,such as modified chemical vapor deposition (MCVD) andplasma enhanced chemical vapor deposition, can all pro-duce thin layers of deposited silica or doped silica byusing admixtures of either vapor or gases. MCVD relieson thermal heating for the creation and deposition of theglass soot, whereas PECVD and its variants rely on ioniccatalysts. One advantage of PECVD, over both FHDand other CVD processes, is that it is a low-temperatureprocess, compatible with temperatures occurring in mi-croelectronics processing.

In this study we discuss the fabrication of silicawaveguides on silicon substrate, describing the low-temperature PECVD process and the relationship of be-tween optical characterizations and processing parame-ters.

II. EXPERIMENT

Silicon dioxide (SiO2) and silicon oxynitride (SiON)films were deposited by PECVD having a radio frequencyplasma using a standard 13.56 MHz source in a popu-lar parallel plate electrode. P-type Si (100) wafers werecleaned by the RCA process consisting of immersion ina piranha (H2SO4:H2O2=4:1) solution for 10 min at 80◦C, a deionized water (DI) rinse and immersion in a 1

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Fig. 1. Refractive index and deposition rate of SiO2 filmsas a function of N2O/SiH4 flow raito.

% HF for 40 s. After a final DI rinse and a nitrogenblow dry, the substrates were loaded to chamber andcleaned by N2plasma sputtering. For the growth SiO2

films on the substrate, high purity silane (SiH4, 90 %dilution H2), nitrogen (99.99 % N2) and nitrous oxide(99.999 % N2O) were used. Gas input was controlled bythe Mass Flow Controller (MFC) and low evacuation wasdone through a double stage rotary pump and boosterpump. Pre-deposition evacuation was pumped at the10−5 torr range by using a diffusion pump.

The effects of the deposition parameters on propertiesof SiO2 and SiON thick films were investigated by varia-tions of the SiH4, N2O, N2 flow ratio and RF power. Af-ter the deposition process, the samples were annealed ina furnace at 1150 ◦C for 2h. Lithography processes werethen used to deposit a predesigned mask on the top ofthe core layer. Chrome was used for the mask materials.The silica waveguide layer was etched by Inductive Cou-pled Plasma (ICP) etching system. The thickness andthe refractive index measurements were measured by aprism coupler. X-ray photoelectron spectroscopy (XPS)and fourier transform infrared spectroscopy (FTIR) wereused to determine the chemical states. The cross-sectionof films was observed by a scanning electron microscopy(SEM).

III. RESULTS AND DISCUSSION

1. Silicon dioxide films

In a typical fabrication process, a uniform buffer layerof pure silica is deposited on the surface of the waferto a depth of the order of 10 or more. The effects ofthe reactant gas flow ratio on the growth characteristicsand optical properties were investigated by changing theN2O/SiH4flow ratio and RF power. Fig. 1 shows thevariation of the growth rate and refractive index with

Fig. 2. Refractive index and deposition rate of SiO2 filmsas a function of RF power.v

the N2O/SiH4 flow ratio. The pressure, RF power andtemperature were 0.9 torr, 120W and 320◦, respectively.It is observed that the deposition rate was decreasedby the depletion of silane as the N2O/SiH4flow ratioincreased. Refractive index decreased approaching thethermally grown SiO2(1.46) as the flow ratio increasedfrom 3 to 22. After annealing process, Si-O-Si bondswere increased and SiO2 films having a stoichiometriccomposition were formed [8].

Fig. 2 shows the variation of the growth rateand refractive index with RF power. The pressure,N2O/SiH4flow ratio and temperature were 0.9 torr, 8and 320 ◦, respectively. RF power varied from 60 to180W. Deposition rate was increased from 4.7 /h to 6.9/h resulting in a higher fractional ionization and a denserplasma as the RF power increased. Refractive index hadan index range of 1.460 ± 0.001 at the RF power above120 W. After the annealing process, the deposition rateand the refractive index were decreased. It can be ex-plained by a desorption of Si-N and Si-H bondings ad-sorbed in the surfaces of samples as a function of anneal-ing.

The structural and bonding properties of SiO2 thickfilms as an annealing process are revealed in Fig. 3. Hy-drogen content induces bad optical characteristics intothe silica waveguide [9–11]. O-H, Si-OH and Si-H bond-ings can be detected by fourier transform infrared spec-troscopy (FTIR). It is observed that Si-H, Si-O-H bond-ings are detected at 2300 cm−1 , 960 cm −1 respectivelyfor non-annealing. After the annealing process, Si-H,Si-O-H bondings are not detected and Si-O-Si stretch-ing and bending mode are detected at 1068 cm−1, 845cm−1. It is known that the stretching frequency of theSi-O bonds is related to the films stoichiometry.

Films stoichiometry is also identified by the XPS spec-trum using the O 1s and Si 2p binding energy. In case ofstoichiometry SiO2, the silicon atom surrounded by fouroxygen atoms has a characteristic binding energy (103.3eV). Because carbon appeared at the oxide surface dueto the contamination from the atmosphere, carbon was

Optical Characterization of Silica Based· · · – S. M. Cho et al. -S949-

Table 1. Deposition Condition of SiO2 films.

Sample Temp. (◦) Deposition pressure (torr) RF power (W) N2O/SiH4flow ratio Variable constant

SO-1

320 0.9 150

3

N2O/SiH4 flow ratio

SO-2 6

SO-3 8

SO-4 10

SO-5 14

SO-6 22

SO-7

320 0.9

60

8 RF powerSO-8 90

SO-9 120

SO-10 180

used for reference (284.5 eV). Fig. 4 shows the chemicalbonding state of silicon and oxygen. The binding en-ergy of Si 2p and O 1s was 103 and 532 eV, respectively.It is confirmed that deposited films has a stoichiometric

Fig. 3. IR spectrum of SiO2 films as annealing process (a)before annealing and (b) after annealing.

Fig. 4. XPS Si 2p and O 1s line shape of SiO2 films.

composition.As the condition of N2O/SiH4 flow ratio and RF power

are 8 and 120 W, respectively high quality SiO2 thickfilm having an 18 µm thickness was deposited and itscross-section is revealed in Fig. 5.

2. Silicon oxynitride films

The core index must be slightly higher than thecladding index in order to provide light guidance, andalso to be compatible with the core-cladding index differ-ence of standard telecommunications single mode fiberswhich are spliced to the planar devices. The higher indexof the core layer can be achieved by doping with nitro-gen. The effects of the reactant gas flow ratio on thegrowth characteristics and optical properties were inves-tigated by changing the (N2O + N2)/SiH4flow ratio andRF power. Fig. 6 shows the variations of the growth rateand the refractive index with the (N2O + N2)/SiH4 flowratio. The pressure, RF power and temperature were0.9 torr, 150 W and 320 ◦C, respectively. The deposi-tion rate was decreased from 8.7 µm/h to 2.1 µm/h as

Fig. 5. SEM photograph of SiO2 thick film.

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Table 2. Deposition Condition of SiON films.

Sample Temp. (◦C) Deposition pressure (torr) RF power (W) (N2O+N2)/SiH4flow ratio Variable constant

ON-1

320 0.9 150

3 (N2O+N2)/ SiH4 flow ratio

ON-2 4

ON-3 6

ON-4 10

ON-5 20

ON-6

320 0.9

60

6 RF powerON-7 90

ON-8 120

ON-9 180

the (N2O + N2)/SiH4 flow ratio increased. For a small(N2O + N2)/SiH4 flow ratio, Si-N and N-H bonds whichformed by preference reaction between silane radical andoxygen radical are incorporated into the films, so refrac-tive index has a high value. As the (N2O + N2)/SiH4

flow ratio increased, the refractive index decreased ap-proaching the 1.4710 that had a 0.75 % difference withthe cladding index.

Fig. 7 shows the variations of the growth rate andthe refractive index with the RF power. The pressure,(N2O+N2)/SiH4 flow ratio and temperature were 0.9torr, 6 and 320 ◦C, respectively. As the RF power in-creased, the deposition rate was increased from 4.4 µm/hto 6.7 µm/h by the high dissociation. For lower RFpower, silane decomposes more easily than nitrous ox-ide [12], so Si-H and N-H bonds are preformed and therefractive index is increased. As RF power increased,the refractive index was decreased by higher nitrous ox-ide dissociation. As the condition of (N2O+N2)/SiH4

flow ratio and RF power are 6 and 150W, SiON thickfilm having an 8 µm thickness was deposited and its re-fractive index was 1.4705.

Having a above results, we fabricated the cladding andcore layer which were 15 µm and 8 µm in thickness, re-

Fig. 6. Refractive index and deposition rate of SiON filmsas a function of (N2O + N2)/SiH4 flow raito.

Fig. 7. Refractive index and deposition rate of SiON filmsas a function of RF power.

spectively. The silica waveguide layer was etched by ICPetching system. Fig. 8 shows the SEM photograph of theSiON thick film and etched core ridge of a single-modewaveguide. The waveguide in Fig. 8 has a relative refrac-tive index difference ∆ between the core and cladding of0.75 %.

IV. CONCLUSIONS

In this study, we have deposited SiO2 and SiON thickfilms by PECVD at low temperature and have shown therelationship between the optical properties and the pro-cess parameters. The obtained SiO2 films had a highthickness and refractive index of 1.460 ± 0.001 withthe annealing process. FTIR and XPS revealed thatdeposited films were nearly stoichiometric compositionwith no presence of hydrogen content. SiON films usinga core layer had a 0.75 % higher refractive index thanthe cladding layer and a high thickness(> 5 µm). Wecould note that film thickness and refractive index werecontrolled by the flow ratio of the reactant gas, the RFpower and the annealing process.

Optical Characterization of Silica Based· · · – S. M. Cho et al. -S951-

Fig. 8. (a) SEM photograph of SiON thick film. (b) SEMphotograph of etched core ridge of a single-mode waveguideby ICP etching system.

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