CA1281536C - Fabrication of optical waveguides - Google Patents

Abstract

FABRICATION OF OPTICAL WAVEGUIDES

ABSTRACT OF THE DISCLOSURE

The guiding layers of optical waveguides are formed of arsenosilicate glass (ASG). By varying the arsenic content from 2 to 13 mole percent it is possible to vary the refractive index in the range 1.45 to 1.53.
Pure silica or less heavily doped ASG can be used for the cladding layers. The ASG is preferably formed as the result of a heterogeneous reaction between silane and oxygen in the presence of arsine. Such a reaction can be carried out at temperatures down to 390°C, allowing the ASG to be used on substrates of group III-V compounds.

Description

1~81536 FABRICATION OF OPI'IC~L WAVEGUIDES
The present invention relates to the fabrication of optieal waveguides, and in particular to the fabrieation of optieal waveguides in integrated optical circuits.
In integrated optical circuits it is necessary to provide op-tical waveguides of controlled refraetive index to provide optical connection between the various optieal components.
Such op-tical waveguides have been made by flame lo hydrolysis, as described in US patent no 3,806,2~3 of Keck et al. In this method fine glass particles are produced as a soot Erom the fl.ame of a gas burner which is fed with SiC14. The soot is deposi.ted on a substrate, sueh as fused silica, having the appropriate optical and mechanieal properties. A less heavily doped silica layer is then formed over the layer of doped soot. Finally, the structure is sintered at about 1500C to vitrify and densify the so~ty layers.
UK published patent application no 2066805A
teaehes the use of a furnaee in plaee of a gas burner to hydrolyse halides of Si and Ti, B, P or Ge, together with oxygen or steam to procluee fine glass partieles on a substrate heated to 600-11.00C. Vitrifieation involved heating the substrate to 1300-1.600C.
The disadvantage o both these teehniques is that they recluire that the substrate be exposed to very high temperatures, partieularly duri.ng the vitrifieation step, whieh limits the ehoiee of substrate materials (effee-tively just to high temperature glasses: semieondue-tors eannot be used) and dopant materials (no volatile speeies sueh as arsenie or phosphorus).

~g ~281536 Moreover, differential thermal expansion be-tween the thin deposited films and the substrate is more detrimental in cooling from high temperatures. Such extreme heating can also lead to poor flatness, rendering subsequent micro lithography dif:Eicult.
An alternative approach has been taken by Stutius and Streifer, Applied Optics, Vol 16, No 12, December 1977, pages 3218-3222. They experimented with chemical vapour deposition (CVD) of silicon nitride onto thermally lo oxidised silicon, and favoured low pressure CVD in preference to atmospheric pressure CVD and plasma enhanced CVD, both of which produced high loss films which were subject to cracking. No details of reaction conditions are given, but low pressure CVD of silicon nitride is usually performed a-t 800-900C using dichlorosilane and ammonia.
The advantages of using CVD instead of flame deposition are that the process is carried out at a lower temperature, and by virtue of the reaction r.lechanism CVD
gives rise di.rectly to a film with better coverage and greater integrity (with flame deposition the :Ei.lm is not formed until the sooty layer has been vitrified). In particular, the vitrification step, wi-th its very high temperatures, is avoided.
D K W Lam, ~pplied Optics, Vol. 23, No 16, August 1984, page 2744 to 2746, proposes the use oE plasma enhanced chemical vapour deposition (PECVD) as a means o depositing silicon oxynitride (SiXOyN ) from silane (SiH4) and nitrous oxide (N2O) at a very low temperature of 200C. Because of the low temperature used, the process is said to be suitable for use over group III-V semiconductor compounds such as GaAs and InP which decompose at 500C, unwanted drive-in diffusion of dopants already in the semiconductor substrate is also avoided.
In spite of the advanta~es inherent in CVD
processes, disadvantages remain with the approaches of both Stutius and Lam. The use of silicon ni-tride (n 2.01) presents problems in coupling to optical fibres. With Stutius differential thermal expansion can be expected to be problematic. In the Lam process. although the use of plasma enhancement allows the temperature to be dropped to 200C, energy from the plasma also leads to the formation of Si-Si bonds, extending -the UV absorption edge. Indeed in the Stutius and Streifer paper, PECVD i9 rejected because the films produced in that way contained excess silicon, and no guided mode could be launched at 6328A.
Additionally, N-H and 0-H bonds may be Eormed adversely affecting absorption in the near infra-red. Also, silicon-oxynitride, produced by CVD, has an unacceptable degree of surface roughness which must be reduced by reflowing at high temperature to reduce scatter loss.
Internal defects also occur in CVD silicon nitride and oxynitride, and these too can give rise -to high losses unless annealed out ~a C02 laser was uYed for this by Lam~.
Thu~ it can be seen that there exists a need for a wave~uide fabrication process which does not re~uire the use of plaYmas or excessively hig}l temperatures. It would also be desirable if the fabrication process produced a smooth surface, and hence avoided the need for laser annealing or high temperature baking to reduce scatter loss to an acceptable level.
According to one aspect of the present invention there is provided an optical waveguide comprising a guiding layer of arsenosilicate glass consisting essentially of arsenic, silicon and oxygen or a doped silica glass in which arsenic is -the principal dopant, the arsenosilicate glass containing arsenic, the arsenic content of the glass bein~ at most 17 mole %, and the guiding layer being formed on and in direct contact with a ~,~

1~:8~S36 cladding layer having a refractive index lower than tha-t of the ~uiding layer.
Ano-ther aspect of -the invention provides a method of Eabricating an optical waveguide comprising the step of forming a guiding layer of arsenosilicate glass consisting essentially of arsenic silicon and oxygen or a doped silica glass in which arsenic is the principal dopant the arsenosilica-te glass contairling arsenic -the arsenic content of the glass being a-t most 1~ mole % on and in direct contact with a cladding layer having a -efractive index lower than that of the guiding layer.
The invention will be fur-ther described by way of example only with reference to the accompanying drawings in which:
Figure 1 is a diagrammatic representation of apparatus suitable for use in depositing arsenosilicate glass;
Figure 2 is a graph showir1g how changes in the arsine and silane flow rates affect the deposition rate with temperature;
Figure 3 is a graph of deposi-tion rate against temperature for the silane-oxygen system and the silane-arsine-oxygen system;
Figure 4 i8 a micrograph showirlg surface roughness typical of converltiorlally deposited low temperature CVD oxide;
Figure 5 is a contrastirlg micrograph showing the smooth surface attainable with arsenosilicate glass produced as the result of a heterogeneous reaction:
Figure 6 is a micrograph showing a step covered by a conformal coating of arsenosilicate glass produced as the result of a heterogeneous reaction;
Figure ~ is a contrasting micrograph showing poor step coverage with phosphosilicate glass as in Figure 5;
Figure ~ is a graph of deposition rate against temperature for the silane-arsine-oxygen system;

1~815~

Figure 9 is a micrograph showing the effect of reflowing arsenosili.cate glass at ~00C;
Figure 10 is a similar micrograph showing the effect of reflowing at 900C;
Figure 11 shows the RBS spectra of as-deposited and densified films of arsenosilicate glass;
Figure 12 shows the compositional changes experienced by arsenosilica-te glass subjected to a typical fabrication sequence; 0 Figure 13 is a depth profile of a 0.6 ~m densified filmof arsenosilicate glass;
Fi.gure 14 is a graph showing the relationshi.p between refracti.ve index and arsenic content in arsenosilicate glass; 5 Figure 15 is a micrograph showing the smooth surface of an arsenosilicate glass waveguide;
Figure 16 is a micrograph showing -the ripple-:Eree sides oE an arsenosi.licate glass waveguide.
We have discovered that arsenosilicate gl.ass (ASG) is suitable for use as a waveguide, and, moreover, offers surprising~y low loss in the near inra red, despite the fact that the glass is made from hydrides (SiH4 and AsH3) which would be expected to give rise to ]arge losses in the near infra-red. The refractive i.ndex of the as deposited film varies with arsenic content, between about 1.53 for 12% As to about 1.45 to 1.7% As. This is a very convenient range, making ASG suitable for use as a guiding layer with silica (N 1.46) cladding, and also enabling the production of waveguides in which the cladding and guiding layers are each made of ASG. Arsenosilicate glass is thus preferable to silicon nitride (n 2.01) for use with SiO2, since a large refractive F P ~ 3 ~J ~ J ' -' ' ' ' ~Z8i536 1ndex dlfference necess1tates the use of very thln waveguldes 1f only low order modes are to be supported.
For asymmetrlc wavegu1des, where the gu1d~ng layer, w1th refract1ve 1ndex n2 1s bounded by two layers whose refractlve 1ndexes dlffer w1dely ~n3~nl) the followlng equat10n allows calculatlon of the th1ckness, t, requ1red for wavegu1dlng of any part1cular mode, m ~ O, 1, 2,...... ,:-/on ~ n2 - n3 > (2m ~ I)2 A20 132n2 t ) where ~o 1s the vacuum wavelength ~see chapters 2 and 3 of Hlntegrated Opt1cs : Theory and Technology" by R G
Hunsperger, publlshed by Sprlnger-Verlag). A d1sadvantage ~5 of very th1n waveguldes ls that they are d1ff1cult to couple to optlcal fibres.
Arsenosll1cate glass 1s an arsen1c doped form of sll1con d10x1de, and ls conventlonally produced by react1ng s11ane (S1H4), arslne (AsH3) and oxygen 1n a CVD reactor. However, the react10n ls a homogeneous gas phase reactlon and hence the ASG 1s depos~ted wtth a rough surface whlch needs to be reflowed to reduce scatter loss to an acceptably low level. Arsenlc reduces the melttng po1nt of slltcon dloxlde, the meltlng po1nt decreas1ng progresslvely wlth an tncreas1ng concentrat10n of dopant, so that ASG wlth more than about 10/o As can be reflowed at temperatures as low as 800/900-C I tn steam~.
Arsen k 1s present ln ASG ln the form of arsenlc tr10xlde (As203) wh k h has a slgnlflcant vapour pressure over ASG and 1t can readlly be lost by evaporatlon when the glass ls heated. Because of th1s, the arsen~c content of the glass drops dur1ng reflowlng .

F F. ~ M r" ~ ~ ~3 r~ n ~ 1 I! t. . 1~ J ~

~8~S36 as a result of whlch the refract1ve 1ndex also falls. The loss of arsen~c 1s not un1form throughout the thlckness of the film; the greatest amount ls lost from the surface layers, as wlll be expla1ned ln greater detall below.
Although wavegu1des can be Made by depos1t~ng ASG
uslng conventlonal processes, 1t 1s preferably deposlted accord1ng to the method descrlbed ln our copendlng European appl1cat10n number 85300172.5 flled 10 January 1985, and publlshed under the number 0150088.
o In that appllcatlon we descrlbe how, by us~ng the appropr~ate reactlon cond1tlons, lt ls posslble to modlfy the react10n mechan~sm so that the ASG 1s formed as a conformal coatlng as the result of a heterogeneous react~on. The reactlon can be carr1ed out at temperatures down to below 400 C w1thout the use of a plasma, and by vlrtue of the react10n mechan1sm, the ASG ls depos1ted w1th a much smoother surface than 1s ach1eved wlth convent10nal processes. the as depos~ted dens~ty 1s also lmproved compared to that of conventlonal ASG. However, should lt be reqùlred, the ASG can be baked at 600-900 C
to further dens1fy and reflow the layer.
The advantages of uslng ASG depos1ted as the result of the mod1fled, heterogeneous react10n are that the substrate need not be exposed to h1gh temperatures, no plasma 1s needed; and the smooth surface produced by the reactlon g1ves rlse to low scatter loss, even w1thout belng reflowed.
The process by whlch ASG can be deposlted conformally w111 now be descrlbed, and examples w~ll be glven of the reactlon condlt10ns used to produce ASG films of v~r10us composlt~ons.
The ASG ls produced ln a chem~cal vapour deposltlon 1~81536 ~CVD) process such as ray be carr1ed out ln a commerc~al CYD mach1ne. Mach~nes des1gned for the s11ane-oxygen react10n for CVD of slllcon dloxlde, such as the PYROX
Reactor produced by Tempress-Xynet1cs, are partlcularly sultab1é for carrylng out the ASG depos1t~on, although other machlnes may also be sultable. For the purposes of descrlpt10n ~t w~11 be assumed that d PYROX 216 Reactor ls to be used, and such a reactor 1s shown d1agrammatlcally Flgure 1.
The PYROX Reactor, whlch prov~des for batch processlng of wafers, has a water cooled 102 reactor head 100 wlthin ~h1ch there ~s a rotatable c~rcular table 101 upon wh1ch are placed wafers 103 to be treated. The table 101, wh1ch supports a graph1te wafer carrier 104, ls heated from underneath dur1ng processlng, the temperature of the table 101 and hence of the wafers belng measured by means of a thermocouple. ~n the experlments to be reported, three-lnch wafers were used. The wafers were held on s111con carblde coated graphlte succeptors 99, arranged ln a c1rcle of twelve around the outer rlng of an elghteen wafer carr~er.
The reactor head cons~sts of four concentrlc zones whlch, movlng out from the centre, are termed A. 8, C, and D. The gas flow to each of these zones can be adJusted to vary the condltlons wlth1n the reactor head. Separate flow control valves 105, 106 and 107 and pressure gauges 108, 109, and 110 are provlded for zones A, B, and C; flow to Zone D 1s not lndependently controllable. The compos~t10n of the gas fed to the reactor head can be ad~usted by flow control valves 111-116 ln each of slx flowl1nes, the flowrates ln each of the flow11nes be~ng mon1tored by means of rotameters 117-122, conta1n1ng floats 123-128. In the present case, only flve flowl~nes are requ1red:
* trademark F P O PI r~ J 81 IJ

3LZ 8~L5 3 6 FlowllneGas Rotameter Float Ident1tyCompos1tlon Porter Model Type Humber Main N2 B250-8 Stain7ess Steel Hitrogen Sllane 5/oS1H4 B125-40 ~n N2 Dopant ~ 1/oAsH3 B125^40 ln N2 Oxygen 2 ~125-40 u D11ution Nitrogen N2 B250-8 n rhroughout the experiments the zone pressures were maintAined at values rout~rely used when deposlting USG or PSG:
ZONE A 13 psi ZONE ~ 11 pS1 ZONE C 12 psl ~ONE D - not dlrectly measurable Results for reactions carried out at plate Iwafer) temperatures between 400 and 450 degrees C are shown ln Figure 2. This figure shows how temperature affects the th~n fllm deposlt~on rate for flve dlfferent tot~l hydrlde flow r~tes 119,29,75,110,130 cc/mlnute) with the oxygen flow rate held constant at 2500 cc/mlnute, and with the ma~n nitrogen and dllut~on nltrogen flow rates each held at 3~ litres/mlnute.
It is instructlve to compare the deposition vs temperature curves obta1ned ~ith sllane-arsine-oxygen wlth those obtalned wlth the silane-oxygen system. In Flgure 3 examples of each are compared. The curve for ~ 8~536 sllane-arslne-oxygen ~for 75cc total hydrlde flow, SlH4:
Arslne ratlo = 61:14) shows the two reglons whlch characterlse lt as a heterogeneous reactlon. In the low temperature reglon, where there ls k~netlc control, the deposltlon ra~e ls reactlon rate llmlted and shows the exponentlal rlse wlth temperature predlcted by the Arrhenlus rate equat10n:-D . Ae - /oE/RT

wlth some temperature varlatlon of A (as predlcted by the Eyring rate equatlon). In the second reglon (the mass transport llmlted region) the deposltlon rate 15 llmlted by the dlffuslon rate of the reactants through a very thin lS depleted zone near the surface whlch wlll follow the contours of the surface. By comparlson, the sllane and oxygen system shows a very small dependence of deposltlon rate on temperature (ln the example lllustrated lt ~s practlcally constant at 9A per C, whlch ls small when compared to the 29A per C to 63 A per C for the arslne-sllane-oxygen example shown) and the lack of any dlffuslon llmlt lndlcates that lt ls homogeneous gas phase reactlon.
The reactlon mechanlsm determlnes the type of specles that wlll arrlve at the surface. If the reactlon ls homogeneous, the oxygen and sllane react to form slllcon dloxlde, or a slmllar specles, ln the gas phase. These molecules may condense ln the gas phase to form colloldal partlcles, The slllcon dloxlde wlll arrlve at the surface as partlcles ranglng 1n slze from the monomer to collo~dal partlcles, glv~ng rlse to the characterlst~c rough pebble - llke texture of low temperature CVD ox~des, as shown ln Figure 4. The reaction parameters such as pressure and gas composition will control the particle size distribution and hence the surface tex-ture. The mobility of these particles will be small and decrease with increasing particle size.
The significance of the heterogeneous reaction is that the deposition rate is controlled by the surface temperature and not by the geometry of the surface, and hence one may expect such a reaction to give conformal lo oxide coatings. Moreover in a heterogeneous reac~ion the silane and oxygen are absorbed onto a surface where they subsequently react to form silicon dioxide. ~s these absorbed species will be very mobile, good step coverage and smooth surfaces should result. Figure 5 shows the smooth surface produced as a result of heterogeneous re-action, and should be compared with Figure 4 which shows a typical equlvalent (PSG) deposited from a homogeneous reaction. Figure 6 shows a conformal coating 50 of ~SG
over a 1 ~m high aluminium track 51 wlth near vertical slde walls 52. This should be compared with Fiyure 7 which shows a typical non-conEormal coating 60 produced as a result of a homogeneous reaction between silane and oxygen. The results oE the homogeneous reaction can be seen as overhangs 61 at the sides of the track 51; such overhangs are typical of the non-conformal deposition which characterises homogeneous reactions.
The conformal coating of ASG as shown in Figure 6 was produced with the instrument settings given in the following example:

I:F.UII ~J 1 3~:Y IJ~ J .:~, . 11~.. 1~. 1 1l: ',~:
~ 8 1~;3 6 EXAMPLE
Gas Rotameter read1ng helght ln mm Maln N2 56 5/o SlH41nN2 40 1 /AsH31nN2 44 D11ut10n N2 56 Oxygen 9S
Thls equals 61 cc/m~nute of pure S~4 14 cc/mlnute of pure ASH3 2500 cc/m1nute of pure 2 The zone pressures were malnta1ned as above at 13 ps1 Zone A; 11 ps1 Zone B; 12 ps~ ~one C Plate temperRture - 450 C. 3-1nch s111con wafers placed 1n outer c1rcle of an 18 wafer plate. Th1n f11m depos1tlon of rate of A
575 A/mlnute.
Glass depos1ted under these condlt10ns was found to have an lntrlnslc stress of 5 x 10 Dynes cm tens11e.
~he glass had a compslt10n of 12 mo1 /o AS203, 88 mol /o S102.
Sat1sf~ctory conformal coatlngs have been prcduced wlth sllane:ars1ne rat10s between about 3.8:1 and 11.7:1.
A deposltlon rate versus temperature curve for a s11ane flow rate of 60 cc m1nute~l and an arslne flow rate of lO cc m1n~1 (w1th 2500 cc m1n~l02, ma1n N2 ; 38.
L1tres m1n 1, and d11ution N2 ~ 38 L1tres mln~ ) 1s shown 1n Flgure 8.
It has been found that 1n general an 1ncrease 1n oxygen and/or sllane concentrat10n favours a homogeneous react10n and an 1ncrease ln ars1ne concentrat10n favours heterogeneous react10n.

F p l l l l u 1 .; ~:: IJ IJ .~

1'~3~;3~;
~3 The followlng gas m1xtures, used under the cond1t10ns set out above, have been found to 91ve the reactlon type 1nd kated:

Ars1ne S11ane Oxygen H1trogen Runml/m1nml/m1n 1/m1n 1/m1n React10n A 6.5 61 1.4 76 heterogeneous B 3 .1 3~ 1. 4 76 homogeneous C 6.5 76 1.4 76 heterogeneous D 6.5 113 1.4 76 homogeneous E 6.5 148 1.4 76 homogeneous The As203 content of the glasses produced under heterogeneous react~on cond1tions were as follows:
Run A 6/o when depos1ted at 400 C, 4/o when depos1ted at 450-C
Run C 3/o when deposlted at 400 C, 2/o when depos1ted at 450 C
the f11ms w111 reflow 1n 2' steam and POC13/02. ~e have found th~t the f11m flow ls greatest 1n steam. To ach1Qve the same degree of planar1zat10n f11ms that ~re reflowed 1n steam requlre temperatures about 100 C lower than those reflowed ln POC13 /2' Wlth 17 mole /o As203 (not produced as a conformal fllm), complete reflow has been ach1eved ln 3 mlnutes at 800C ln steam. Flgures 9 and lO show the cross sectlon o~ ASG fllms 18000A 17 001e /oAs203) that have been deposlted on polys111con steps (5000A hlgh wlth plasma etched near vertlcal walls) and reflo~ed ln steam for 15 m1nutes. The fllm ln Flgure 9 was flowed at 800 C and that ln Flgure 10 at 900 C.

~Z8~S36 Following re~l~wing! in steam a bake in oxygen at 600C is advisable to remove water from the film.
Films deposited directly onto silicon should not be baked in pure nitrogen or argon, as this may lead to the ormation of elemental arsenic at the ASG/silicon interface.
As noted above, arsenic trioxide (As2O3) has a significant vapour pressure over ~SG and it can readil.y be lost by evaporation when the glass is heated. Since the melting point of the glass decreases progressively with an increasing concentration of dopant oxide, it should be possible to flow a glass containing a high proportion of As2O3 at a relatively low temperature and in the same process remove some of the As2Q3.
Many analytical techniques have been used to determine the composition of doped oxide films, but infra red spectrophotometry (IRS) is the popular technique for rapid routine determinations. A calibration graph for ASG films, which relates absorption peak intensiti.es to film composition has been generated by Wong and Ghezzo (J Electrochem Soc V118, No 9, pl540, 1.971) who used the X-ray microprobe anal.ysis o:E the Ei].ms and the Elame spectro-photometric analysis of the dissol.ved :Eilms as calibration standards.
In a subsequent investigation of densified ASG
films Wong :Eound that I~ analysis, which indicated that the As2O3 content had decreased during densification, did not agree with the X-ray microprobe analysis, which indicated that no change had occurred (J Electrochem Soc 30 V120 p 122 1973). He concluded that it was a physical change in the glass matrix and not a compositional change that was causing the change to the IR spectrum.

1Z8~536 To investigate the matrix e.ffect for our ASG
layers we deposited films of ASG (2000A, 10 Mole % As2O3) on silicon wafers (1000 ohm cm p type~ some of which had a 400A layer of oxide grown on the surface to prevent the diffusion of arsenic (As) into the silicon. The films were analysed by IR and Rutherford Backscattering (RBS) both before and a:Eter ~ensification (980C, 15 min, 10 2 in N2) and -the results compared.
The RBS spectra of a film deposited on a grown oxide is shown in Figure 11. Although a brief inspection of the spectra, which were taken before and after densification, shows that As was .lost during this processing more information can be obtained from the interpretation of all the spectra tabulated below:
15 S~BSTRATE MOLE % As O

AS DEPOSITED AFTER DENSIFICATION
IR RBS IR RBS

Silicon 10.5 - 5.7 6.9 Grown Oxide 10.5 12.7 6.2 7.5 The IR results were calcul.ated using Wong and Ghezzo's calibration chart or as-deposited ASG fllms.
It can be seen that in each case where the IR
result is compared to the RBS result the RBS result gives a value that is consistently about 20% higher. The further conclusi.ons from these results are:-1 The changes in peak intensi-ties observed in the IR spectrum are due to compositional changes and not physical changes in the glass matrix so that IR can 3 be used to monitor As2O3 content during the densification process.

~81536 2 During densiEication As2O3 leaves the film by evaporation Erom the surface and if the densification is performed on a bare silicon wafer As will diffuse into the substra-te, but the quantity will be small compared to that lost by evaporation.
To confirm these results a second series of samples was prepared which had a thicker film of deposited oxide (6000 At. The experiments were repeated, but in this case the films were analysed by IR and atomic absorption lo spectroscopy of the dissolved films. The conclusions from the previous experiments were confirmed.
FILM DENSIFICATION
To investigate the changes that occur during densification a series of 6000 A ASG films, each with a different composition, were deposited onto silicon substrates with a 400 A oxide film grown on the surface. The composition of the deposi-ted films varied from 1.6 to 17 mole % ~s2O3.
The samples were analyzed by IR beEore and aEter a typical fabrication sequence that would be used between ~SG deposi-tion and first metal deposition. The sequence includeda back gettering step using POC13 at 980C.
The results from these e~periments are plo-tted on the graph in Flgure 12. It can be seen from the graph that if the inltial concentratLon oE ~s2O3 was below 6.5 mole % tllen about 25~ of it was lost during processing.
If the lnitial concentration was greater than 6.5 mole % then the final concentratlon always fell to a plateau level of 4.7 mole % (this limit only applies to the particular processing sequence used here: further heating would lead to greater loss). This result indicated that there were two mechanisms by which the ~X81~;36 As2O3 left the fi]m. The first mechanism which was kinetically fast only occurred if the initial concentration was greater than 6.5 mole %, and should be interpreted as a super-saturated solution expelling the dopant oxide until -the saturation limit was reached. The second, slower, mechanism appeared to be the diffusion of the As2O3 to the surface where it was lost by evaporation to the ambient atmosphere.
To verify the latter mechanism a depth profile of the As2O3 concentration was required. As it has been shown that errors can occur when Auger electron spectroscopy/depth profiling is used to evaluate doped oxide films it was decided to obtain an IR depth profile. Once the IR spectrum of a densified film has been recorded it was etched in a solution of 5% HF to remove a layer 500 to 1000 A thick from the surEace of the film. The IR spectrum of the thinner remaining film was recorded and by comparing the two spectra the composition of -the dlssolved layer could be calculated. By repeating this sequence of etching and recording -the IR spectrum a depth profile of the whole film could be obtained.
A typical result from a film that contained an initial concentration of 13 mole % ~s2O3 is shown in Figure 13. The graph clearly shows the concentration gradient caused by the dif~usion of As2O3 From the surface.
The P2O5 generated during the back gettering step can be seen difusing from the surace into the film creating a near-surface layer of arsenophosphosilicate glass which should give the ilm better gettering properties.
Although loss of arsenic occurs on heating the film, there need not be a corresponding fall in refractive F l ' U I I '~ J , ~ ~

~153 1ndex. The reason for th~s ~s apparently that the denslflcat10n wh1ch occurs on heat1ng ~at least by 600 C) ralses the refractlve lndex, offsettlng the fall due to arsenlc loss. Other workers have observed slmllar lncreases ln refractlve 1ndex on heatlng and denslfylng Sl02 For example, Pllskln and Lehman, ln J Electrochem Soc Vol 112, No 10, pages 1013-1019, report an lncrease 1n refract1ve lndex from 1.43 to 1.46 for S102 heated 1n steam at 850 C for lS m1nutes. It should be reallsed however that once the f~lm has been dens~fied, whlch can be done at 600 C, contlnued or repeated heat1ng wlll dr~ve off more arsenlc, and the refractlve 1ndex would be expected to fall.
Arsenosll~cate glass contaln1ng 12 mole /o arsenlc was depos~ted, under heterogeneous reaction condltlons (mass transport llm~ted reglon) on a 3u sll~ca wafer Itype Q2 3W55.10.C, made by the Uoya corporatlon) to a depth of 2~,m at a temperature of 450 C, at a rate of 400A per mlnute. A reference slllcon wafer was also ooated at the same tlmel and the ASG thlckness measured on that by optlcal lnterference technlques. The thtckness was found to vary by about ~ 3/o across the 3u wafer. A pr1sm coupler ~nd hellum neon (6328A) laser were used to determlne the fll~th~ckness ~nd refractlve lndex of the fllm on the slllca wafer. The thlckness measurements agreed wlth that c~rrled out on the sll~con wafer. The f~lm was found to be b~modal tat 63Z8A). The effect1ve ~ndex of the zero order TE and TM modes was found to be 1.502, and that of the flrst order TE and tM modes 1.417.
the bulk refractlve lndex of the fllm calculated to be about 1.53, about 0.07 above that of slllca, and agrees well wlth that measured on the s1mllar fllm on s~llcon.

F P ~ r~ 3 ~ J

lX81536 The decay of the scattered llght (6328A) along the propagat1ng beam was measured and losses of between 0.3 and 0.5dB cm~l for the fundamental mode, and 0.8 to 1.2dB cm~l for the flrst order mode were deter~ned.
Exam1nat1On of the uncoupled reflected llght beam 1ndlcate a very low level of scatter loss.

Slm11ar samples were baked at 600 C for 15 mlnutes 1n an o~ygen atmosphere. The va1ues for effect1ve refract~ve IU 1ndex were: zero order, l.5087; f1rst order, 1.479. The very close agreement between these f1gures and those obtalned on the as deposlted samples are, w1th1n the 11m1ts of exper1mental error, the same. Th1s result 1s a 11ttle surpr1s1ng 1n that the fall 1n 1ndex due to arsen1c loss was apparently closely balanced by the 1ncrease ln lndex due to denslflcatlon~
The substant1ally 11near relatlonsh1p between refractlve 1ndex and arsenlc content for as deposlted f11ms ls shown ln Flgure 14.
A s1mple wavegulde can be made w1th ~ust a guld1ng layer of ASG on a s11lca or other su1t~ble substrate.
However, ASG ls also well su1ted to manufacture of bur1ed wavegu1de dev1ces. Because surface scatter loss 1s dependent upon the refract1ve 1ndex d1fference across the surface, bur~ed waveguldes~ whlch reduce that d1fference can have reduced scatter loss. As can be seen from F1gure 14, lt 1s posslble to vary the refractlve lndex of ASG between about 1.45 and 1.53 by varylng the arsenlc content between about 2 and 12/o, A2.5~ layer of ASG conta1nlng 10 ~ole percent AS203 (n 1.52) was deposited on a s111ca sllce. A

~8~;36 500 A layer of aluminium was evaporated onto the back of the substrate to facili-tate mask align~ent. Nex-t a 1 ~m layer of positive resist was spun on-to the ASG layer.
The waveguide pattern was then printed using a contact mask, and the resist developed to define the pattern.
The ASG layer was plasma etched in a c2F6/cllF3/E~e atmosphere (oxygen being excluded to prevent erosion of the resist mask). The remaining resist was stripped. Finally the aluminium was removed from the rear side. Waveguides produced by this technique are shown in Figures 15 and 16. of particular note are the waveguide's very smooth top and its freedom from edge ripple. Figure 16 clearly shows the waveguide's smooth sides.
The increased loss of arsenic from the surface layers on heating ASG, gives rise to the possibility of producing a pseudo buried waveguide s-tructure. The more heavily doped region adjacent the subs-trate would form the guiding layer, while the depleted surface layers would produce a more gradual index change between the guiding layer and air, reducing scatter loss.
~ dopting a conventiona~ approach to the fabrication of a buried waveguide structure, two separate ASG layers may be provided. A heavily doped .layer, with 10-.12 mole ~ arsenic, 2 ~m thick is depos:Lted For the guiding ]ayer.
Using standard microlithographic techniques (coat with photo~electron beam resist, expose, develop resist to define pattern, wet or plasma etch) a passive device structure such as a ring resonator, beam splitter, or coupler etc.
can be formed in the first layer. Next, a second layer, with 2-4~ arsenic, 3 ~Im thick is deposited to bury the first.
If plasma etching is used it may be necessary to briefly reflow the first ASG layer to remove wall rough-ness before depositing the second layer.
Depending upon the desired arsenic content, the reaction temperature can be reduced to about 390C.
However, to ensure uniform thickness it is advisable to ~81S3~, operate in the mass transport limited region - where deposition rate is ]ess sensitive to temperature ehanges.
Although the heterogeneous reaction produces ASG films of low stress, some cracking has been observed with films of 5 llm thickness. Where more than one layer is deposited, reflowing earlier layers should reduce the incidence of cracks in later layers, enabling grea-ter overall thicknesses to be built up.
Since the ASG ean be deposited at temperatures between about 390 and 450C, provided no reflowing is required it ean be used in conjunction with group III-V semiconductors. With a suitable buffer layer (which eould be a low arsenic ASG layer) between the ASG III-V compound, ASG may be deposited on a substrate of group III-V material. Sueh an approach would enable monolithic optical integrated eireuits to be fabricated, using active (light generating) components sueh as lasers fabricated from group III-V compounds.
It would also be possible -to integrate ASG with a lower index eladding o an opt:ieally aetive mater:La, e.g., an optically aetive organie materia]..

~, , . . j.

Claims (19)

1. An optical waveguide comprising a guiding layer of arsenosilicate glass consisting essentially of arsenic, silicon and oxygen or a doped silica glass in which arsenic is the principal dopant, the arsenosilicate glass containing arsenic, the arsenic content of the glass being at most 17 mole %, and the guiding layer being formed on and in direct contact with a cladding layer having a refractive index lower than that of the guiding layer.

2. An optical waveguide as claimed in claim 1, wherein the cladding layer is formed on a substrate having a higher refractive index than that of the guiding layer.

3. An optical waveguide as claimed in claim 2, wherein the cladding layer lying between the guiding layer and the substrate comprises arsenosilicate glass having a refractive index lower than that of the guiding layer.

4. An optical waveguide as claimed in claim 1, 2 or 3, wherein the guiding layer is buried by an arsenosilicate glass layer having a lower refractive index than the guiding layer.

5. An optical waveguide as claimed in claim 1, 2 or 3, wherein the guiding layer is overlaid with a further layer of an optically active material of lower refractive index than the guiding layer.

6. An optical waveguide as claimed in claim 1, 2 or 3, wherein the guiding layer has a bulk refractive index of between 1.53 and 1.47 at 6328A.

7. An optical waveguide as claimed in claim 1, 2 or 3, wherein the guiding layer contains between 3 and 6 mole percent arsenic.

8. An optical waveguide comprising a layer of arsenosilicate glass consisting essentially of arsenic, silicon and oxygen or a doped silica glass in which arsenic is the principal dopant, the arsenosilicate glass containing arsenic, the arsenic content of the glass being at most 17 mole %, on a substrate, the refractive index of the arsenosilicate glass layer decreasing gradually towards the surface remote from the substrate, and the substrate having a refractive index less than the highest refractive index present in the arsenosilicate glass layer.

9. An optical waveguide as claimed in claim 8, wherein the layer of arsenosilicate glass provides a guiding layer and a cladding region spaced from the substrate by the guiding layer, the arsenic content of the glass varying from between 3 to 6 mole percent in the guiding layer to less than 1 mole percent at the surface of the cladding region remote from the substrate.

10. An optical waveguide as claimed in claim 1 or 2, in which the arsenosilicate glass contains about 2 to about 13 mole % arsenic and has a bulk refractive index in the range of about 1.45 to about 1.53 at 6328A.

11. An optical waveguide as claimed in claim 1, wherein the cladding layer forms the substrate of the waveguide.

12. A method of fabricating an optical waveguide, comprising the step of forming a guiding layer of arsenosilicate glass consisting essentially of arsenic, silicon and oxygen or a doped silica glass in which arsenic is the principal dopant, the arsenosilicate glass containing arsenic, the arsenic content of the glass being at most 17 mole %, on and in direct contact with a cladding layer having a refractive index lower than that of the guiding layer.

13. A method as claimed in claim 12, wherein the arsenosilicate glass is produced in a chemical vapour deposition process involving reactions between silane and oxygen in the presence of arsine, wherein the reactions between silane and oxygen are predominantly of a heterogeneous nature.

14. A method as claimed in claim 13, wherein the volume ratio of silane to arsine lies in the range 3.8 to 1 to 11.7 to 1, and the volume ratio of oxygen to silane lies in the range 18.5:1 to 41:1.

15. A method as claimed in claim 13 or 14, wherein the reaction is carried out at a temperature below 500°C.

16. A method as claimed in claim 13 or 14, wherein the reaction is carried out at a temperature of between 390 and 450°C.

17. A method as claimed in claim 12, 13 or 14, comprising the additional step of forming a second layer of arsenosilicate glass over the first.

18. A method as claimed in claim 12, 13 or 14, comprising the additional step of forming a second arsenosilicate glass layer over the first layer, said second layer having a lower arsenic content that the first.

19. A method as claimed in claim 12, 13 or 14, comprising the step of heat treating the arsenosilicate glass layer to produce an arsenic concentration gradient with the arsenic content of the glass layer decreasing gradually towards the surface remote from the substrate.