Ge-doped SiO2glassfilms prepared by plasma enhanced chemical vapor deposition for planar waveguides
Jeong Woo Lee,Sang Sub Kim*,Byung-Taek Lee,Jong Ha Moon
Department of Materials Science and Engineering,Photonic and Electronic Thin Film Laboratory,
Chonnam National University,Gwangju500-757,South Korea
Received13December2002;received in revised form3December2003;accepted7January2004
Abstract
Ge-doped SiO2glassfilms on Si(001)wafers were deposited by a plasma enhanced chemical vapor deposition(PECVD) technique.Then the effects of processing parameters on their growth and properties were systematically investigated.An increase in GeH4flow results in a gradual rise in refractive index of the resultingfilms.Film growth rate significantly increases with increasing in working pressure as well as in input power.This increased growth rate produces a rougher surface and a lower refractive index.A channel waveguide,fabricated using a Ge-doped SiO2film prepared under an optimized deposition condition as a core-waveguiding layer,shows a very low propagation loss suitable
to waveguide applications.
#2004Published by Elsevier B.V.
PACS:42.82.-m;68.55.-a;78.66.-w
Keywords:SiO2film;Plasma enhanced chemical vapor deposition;Optical waveguide;Planar lightwave circuit
1.Introduction
Planar light circuit(PLC)has been received great attention as an alternative to a bulk-type optical circuit due to its promising potential for economically produ-cing an optical device where all the passive and active functions can be integrated onto a single planar wave-guide chip.Micron-order-thick SiO2-basedfilms are usually used in the fabrication of PLC and thus their deposition behavior has been widely investigated[1–3].For the growth of SiO2-basedfilms,various deposi-tion methods have been tested.Among them,flame hydrolysis deposition(FHD)is most frequently attempted[4–6].However,FHD has a drawback of consolidating silica soot at higher temperatures,which makes it incompatible with the well-developed silicon semiconductor chip process.
In contrast,chemical vapor deposition(CVD)has an advantage of being applied much lower processing temperatures than FHD.Recently,plasma enhanced chemical vapor deposition(PECVD),by which faster film growth is realized than conventional CVD,is often used for the growth of SiO2-basedfilms[7,8]. Meanwhile,Ge-doped SiO2glass has been largely used as a material for the core in a bulk-type optical fiber.Therefore,afilm of Ge-doped SiO2can be considered as a core-waveguiding layer in SiO2-based PLC and its improvement in optical loss is highly
required.
Applied Surface Science228(2004)271–276
*Corresponding author.Tel.:þ82-62-530-1702;
fax:þ82-62-530-1699.
E-mail addresses:sangsub@chonnam.ac.kr(S.S.Kim),
jhmoon@chonnam.ac.kr(J.H.Moon).
0169-4332/$–see front matter#2004Published by Elsevier B.V.
doi:10.1016/j.apsusc.2004.01.013
In PECVD,the change of deposition parameters,in general,affectsfilm properties greatly.For a success-ful fabrication of PLC,a precise control of the proper-ties such as refractive index,thickness,composition, and smoothness of both a core-waveguiding layer and cladding layers is necessary.In this respect,investiga-tion of the deposition characteristics and properties of Ge-doped SiO2films depending on processing para-meters of PECVD appears of importance in order to expedite their use in PLC.
In this article,the effects of processing parameters on thefilm properties and characteristics were system-atically investigated when Ge-doped SiO2films were deposited on Si(001)wafers by PECVD.Also we tested the Ge-doped SiO2films grown under opti-mized conditions as a core layer in PLC waveguides by fabricating a channel waveguide structure and then we confirmed its excellent loss property.
2.Experimental
Ge-doped SiO2films were grown on commercially available n-type Si(001)wafers of10cm in diameter by a PECVD system with a parallel-plate type reactor. In the PECVD reactor,the top electrode was supplied by380kHz radio frequency(RF)power through a matching network and the bottom electrode
on which the Si wafers were placed was electrically grounded and resistively heated.During deposition,the Si wafers were maintained at3208C.According to the previous report[9],undesirable Si–OH bonds which are generated during SiO2deposition in PECVD process are mostly prevented at that temperature. The diameter of the top and bottom electrodes was 250mm and the spacing between them was22mm. The precursors used for Si,O,and Ge were silane (99.999%SiH4),nitrous oxide(99.999%N2O),and 10%GeH4diluted in Ar,respectively.Each gaseous precursor,regulated precisely by massflow control-lers,was separately introduced into the reactor passing through a showerhead.
Thefilms deposited were thermally annealed using a conventional quartz tube furnace in N2atmosphere at 11008C for various durations.Their refractive indices and thicknesses were measured using a prism coupler at a wavelength of632.8nm and surface morphologies were observed using atomic force microscopy.Che-mical bonding nature was investigated using IR spec-troscopy.The refractive index inhomogeneity and the variation of thefilm thickness were below1Â10À4 and1.0%,respectively,over a70mmÂ70mm area
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Fig.1.Variation in refractive index as a function of GeH4flowing quantity.Note that GeH4was diluted in Ar in the manner of GeH4/ Ar¼10/90%.The other deposition conditions are3208C(growth temperature),17/2000sccm(SiH4/N2
Oflow ratio),700W(RF power),and300mTorr(working pressure).deposition
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Fig.2.Variations in(a)refractive index and(b)film thickness as a function of annealing time.The as-grownfilms were deposited under the following conditions:3208C(growth temperature),20/ 17/2000sccm(GeH4(10%diluted in Ar)/SiH4/N2Oflow ratio), 700W(RF power),and300mTorr(working pressure).
272J.W.Lee et al./Applied Surface Science228(2004)271–276
on the10cm wafers,indicating that the overall PECVD process adopted in this work produced very uniform and homogeneousfilms in quite a large area.
3.Results and discussion
First of all,the change in refractive index of the films depending on theflow of GeH4was investigated and the results are shown in Fig.1.During the deposition process,the growth temperature,working pressure,and input power werefixed at3208C, 300mTorr,and700W,respectively.Theflow of GeH4(10%diluted in Ar)was changed in the range of4–22.5sccm,while theflows of SiH4and N2O source werefixed at17and2000sccm,respectively. As seen in Fig.1,as the GeH4flow increases,the refractive index of thefilms deposited linearly increases.It is known that Ge incorporation in a SiO2glass increases its refractive index[10].In fabrication of SiO2-based PLC,an accurate control of refractive indices of both core and cladding layers is essential.This linear change in refractive index with varying the GeH4flow suggests that we can grow SiO2-basedfilms tuned precisely in refractive index with ease by simply changing the GeH4flowing quantity.
Hydrogen species usually incorporated in SiO2-basedfilms when prepared by a PECVD technique cause O–H,N–H,and Si–H bondings.These hydro-gen bondings consequently result in a serious optical loss in waveguides.Therefore,thermal annealing in N2or O2atmosphere is often attempted to remove such unwanted hydrogen bondings[11–15].We investigated the effects of annealing duration on thefilm characteristics.The results are summarized in Fig.2.The as-grownfilms were prepared
under Fig.3.Surface morphologies,observed by atomic force microscopy,of the Ge-doped SiO2films after being thermally annealed at11008C for (a)3h,(b)6h,(c)9h,and(d)12h.
J.W.Lee et al./Applied Surface Science228(2004)271–276273
the following conditions:3208C (growth tempera-ture),20/17/2000sccm (GeH 4(10%diluted in Ar)/SiH 4/N 2O flow ratio),700W (input power),and 300mTorr (working pressure).As seen in Fig.2(a),the refractive index of the as-grown film signi ficantly decreases only after the 3h annealing treatment.But it almost remains exhibiting a value of 1.470for longer thermal annealing than 3h.On the other hand,the film thickness shows a tendency of decreasing somewhat with increasing the annealing time as seen
in Fig.2(b),indicating that the as-grown film under-goes a slight densi fication during the thermal anneal-ing.In the case of surface roughness,the longer the annealing time,the smother the film surface progres-sively,as displayed in Fig.3.The as-deposited film
shows a root mean square (rms)roughness of 37A ˚.This value is much lowered even down to 4A
˚after the 12h thermal annealing.We attribute this smooth-ness to re flow and/or densi fication of the
Ge-doped SiO 2films during the thermal annealing process.According to our infrared (IR)spectroscopy results,data not presented here,the spectrum for the ther-mally annealed film for just 3h showed no mean-ingful O –H,Si –OH,and Si –H stretches,indicating that the thermal annealing condition used in this work (11008C and N 2atmosphere)was suitable to effuse out hydrogen species contained in the as-grown film.
Both working gas pressure and radio frequency power are important parameters that should be con-trolled with care in PECVD to grow a film of aimed properties.Fig.4shows the effects of working pres-sure on the growth characteristics and properties of Ge-doped SiO 2films.All the films were prepared under the following conditions:3208C (growth tem-perature),20/17/2000sccm (GeH 4(10%diluted in Ar)/SiH 4/N 2O flow ratio),700W (input power),and 6h thermal annealing.As shown in Fig.4(a),with increasing the working pressure,the growth rate first increases signi ficantly up to 0.22m m/min at 300mTorr,then saturates above that pressure.We speculate that in this working pressure range the higher the pressure,the more reaction products by the gaseous precursors arrive on the substrate surface,consequently resulting in an increase in the growth rate.This increase in the growth rate may lead to a relatively rougher surface,as shown in Fig.4(b).In contrast,the refractive index gradually decreases with increasing the working pressure.
Fig.5shows the effects of RF input power.The films were deposited at a working pressure of 300mTorr keeping the other conditions identical with the case of Fig.4.The growth rate increases with increasing the input power.This may be due to a high dissociation ef ficiency of the gaseous precursors under higher RF powers.Similar to the results in Fig.4,the surface roughness and the refractive index show a converse behavior with increasing the input power.
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0.12Fig.4.Variations in (a)growth rate,(b)surface roughness,and (c)refractive index as a function of working pressure for the Ge-doped SiO 2films grown by PECVD under the following conditions:3208C (growth temperature),20/17/2000sccm (GeH 4(10%diluted in Ar)/SiH 4/N 2O flow ratio),700W (RF power),and thermally annealed for 6h.
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J.W.Lee et al./Applied Surface Science 228(2004)271–276
We fabricated a channel waveguide structure to test the Ge-doped SiO 2films as a core layer in PLC waveguides.First,for an under-cladding layer a SiO 2film with a thickness of 22m m was grown on a Si(001)wafer by PECVD.Then,a Ge-doped SiO 2film with a thickness of 10m m was deposited for a core-waveguiding layer on the SiO 2/Si(001)by using the same PECVD technique.On the basis of the deposition parameters investigated in this study,we chose the following condition to deposit the Ge-doped SiO 2film:3208C (growth temperature),20/17/2000sccm (GeH 4(10%diluted in Ar)/SiH 4/N 2O flow ratio),700W (input power),300mTorr,and 6h annealing.Next,the channel was de fined by an induc-tively coupled plasma (ICP)etching process.A typical channel waveguide after being etched is shown in Fig.6(a).Finally,a SiO 2film codoped with boron and phosphorus was deposited for an upper-cladding layer by PECVD.The fabricated channel waveguide,observed by optical microscopy,is displayed in
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Fig.5.Variations in (a)growth rate,(b)surface roughness,and (c)refractive index as a function of RF input power for the Ge-doped SiO 2films grown by PECVD under the following conditions:3208C (growth temperature),20/17/2000sccm (GeH 4(10%diluted in Ar)/SiH 4/N 2O flow ratio),300mTorr (w
orking pressure),and thermally annealed for 6h.
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