Examination of two-step fabrication methods forsingle-mode fiber compatible ion-exchangedglass waveguides

Ad Tervonen, Pekka Pbyhbnen, Seppo Honkanen, Markku Tahkokorpi, and Simo Tammela

A computer model was used to study important two-step ion-exchange processes for fabrication of single-mode optical waveguides with fiberlike mode field distributions. As a first process step, a field-assistedmethod was calculated to have a better controllability than a diffusion method. As a second process step,field-assisted burial was found to optimize the coupling loss to fiber to 0.05 dB, but losses lower than 0.2 dBwere calculated also for less complex diffusion process steps. On the basis of modeling, a solid statefabrication process was proposed, and the near field distributions of fabricated waveguides were measured toconfirm the model.

1. Introduction

For passive integrated optical components in tele-communications applications, low cost large scale fab-rication of low loss waveguide devices is needed. Ion-exchange processes for fabrication of glasswaveguides' 2 have a strong potential for fulfilling thisrequirement. With branching devices the low lossesfor both propagation and coupling to optical fibers areimportant. In wavelength division multiplexing com-ponents based on directional couplers, for example, anaccurate control of propagation constants in wave-guides is also necessary. To obtain ion-exchangedwaveguides with mode fields resembling those in opti-cal fibers, two-step fabrication processes are generallyneeded. The first step involves the exchange for theoriginal ions in the glass of ions that locally increase therefractive index. The second step is used to modifythe waveguide refractive index distribution by trans-port of the ions in the glass. This often includes ex-change from a source containing the original ionic spe-cies of the glass to bury the waveguide under thesurface.

This paper is mainly concerned with the develop-ment of an ion-exchange process for fabrication of

S. Tammela is with Technical Research Center of Finland, Semi-conductor Laboratory, SF-02150 Espoo, Finland; the other authorsare with Nokia Research Center, P.O. Box 156, SF-02101 Espoo,Finland.

Received 5 January 1990.0003-6935/91/030338-06$05.00/0.© 1991 Optical Society of America.

single-mode optical waveguides with good couplingproperties to telecommunication fibers. Thus the em-phasis is on the field distributions of the waveguidemodes. Additionally, attention is paid to the simplic-ity, reproducibility, and controllability of the process.

11. Methodology

We studied different two-step processes with a com-puter model3 that can be applied to all the commonlyused process configurations. This model calculatesthe ion concentration distributions resulting from thecombinations of successive process steps. For eachprocess step the concentration is calculated from thediffusion equation, which includes the contribution ofthe electric field:

ac/at = DV2 C +C(M 1) +1I

D(M- 1)(VC)2 + MJ0 . VC(1)

[C(M- 1) + 1]2

Here C is relative concentration C = c/c0 , where c is theconcentration of ions exchanged into the glass, and c isthe original concentration in glass of the exchangeablecations. D is the diffusion constant of the exchangedions, and Mis the ratio of this to the diffusion constantof the original ions. Jo = /ec0 is the relative ionic fluxcarried by the electrical current i in the glass with thecharge of proton e. The diffusion equation (1) issolved in a finite difference form with a modified Du-fort-Frankel method. The parameters, boundary con-ditions, and electric current distribution are chosenaccording to the process configuration. The materialparameters obtained for Ag+-Na+ exchange in Cor-ning 0211 glass at the temperature of 616 K fromexperiments with multimode planar waveguides4 wereused in comparison of the different processes. These

338 APPLIED OPTICS / Vol. 30, No. 3 / 20 January 1991

parameters are DAg = 0.0020 Am 2/s, M = 0.7, and c, =

5.0 X 10-3 mol/cm3 . A detailed description of themodel is in Ref. 3.

To transform the concentration distribution to thedistribution of a refractive index, a model for refractiveindices of oxide glass compositions 5' 6 was used. Thismodel is based on Gladstone-Dale relations and givesthe approximate linear dependence of the refractiveindex on the concentration of exchanged ions. Themeasured values in Corning 0211 glass at the wave-length of 0.633 /im are 1.523 for the substrate index and0.049 for the maximum index change, 4 which is consid-ered to correspond to the total replacement of sodiumions in the glass by silver ions. The refractive indexmodel was used to calculate the dispersion of the glass,and, for the wavelength of 1.523 Aim, values of 1.507and 0.047 were obtained for the substrate index andmaximum index increase, respectively.

The scalar mode field distributions E(x,y) and modepropagation constants # in the waveguides were solvedfrom the wave equation for the chosen vacuum wave-number k:

a2E/ax2 + a2E/0y2 = {d2 - [kn(xsy)]2

}E,

E

'N10U)

5.2-

4.8-

4.4-

4.03.50 3.75 I |4.00 4.25 4.50

w / m

Fig. 1. Dependence of the quantity of Ag exchanged into the glass

on the stripe width for a field-assisted process (solid line, 3-V volt-

age, 60-s duration) and for a diffusion process (dashed line, 1000-sduration) at 616 K.

5.2

nFE

L

'nCY

(2)

using the calculated refractive index profiles n(x,y).The method of solution was an iterative finite differ-ence method7 based on the Gauss-Seidel method witha suitable overrelaxation factor. The coupling coeffi-cient with an optical fiber was calculated from theoverlap integral for mode field distributions of thewaveguide and fiber.

Ill. Results

A. First Process Step

There are fundamentally two choices for the firstprocess step: In the purely thermal diffusion process8

the transfer of ions into the glass is by ion exchange,and the transport of ions in the glass is by diffusion. Inthe field-assisted process9 the transfer of ions into theglass by electrolysis and the transport in the glass bymigration in the electric field are dominating. Forchannel waveguide fabrication the geometry of the ionsource is a narrow stripe defined by lithography. InRef. 3, we have shown with computer model calcula-tions that concentration distributions produced bytwo-step processes, in which the first step was field-assisted, can be quite accurately reproduced with aprocess combination of a properly chosen first diffu-sion step and a second step similar to that in theoriginal process but with changed duration. General-ly in two-step processes the choice of the second pro-cess step determines the nature of the refractive indexprofile of the waveguide as long as this is considerablymodified by the second step. For the first processstep, the factors of main importance are thus the sim-plicity of the process and the reproducibility of theresulting concentration distribution as long as it has asignificant effect on the second step. For a given firstprocess step, reproducibility is mainly represented bythe accurate amount of ions exchanged into the glass.

4.8-

4.4-

4.0- l E

10.0 -5.0 0.0

A(Dt) / Z5.0 10.0

Fig. 2. Dependence of the quantity of Ag exchanged into the glasson the change in product Dt for the field-assisted process (solid line)

and for the diffusion process (dashed line) of Fig. 1.

This is shown in the calculations, since small variationsin parameters change the concentration distributiononly slightly if the process duration is adjusted so thatthe total quantity of ions does not change. During thesecond process step these differences in distributionare further reduced.

An almost steplike concentration profile resultsfrom the field-assisted process in which ionic migra-tion dominates the effect of diffusion. As a startingprofile for the second process step, this can be approxi-mated by a rectangular step distribution under thesource stripe that is twice its depth wider than thestripe and contains the correct quantity of ions.10 Fordiffusion processes the resulting concentration pro-file 1' decreases steeply with depth from the maximumconcentration value at the surface.

We studied the effect of the variation in the ionsource stripe width. The calculated dependence onthe stripe width w of the quantity of silver exchangedto the glass NAg is shown in Fig. 1 for typical diffusionand field-assisted processes. With the diffusion pro-cess NAg is almost directly proportional to w. In thefield-assisted process NAg is obtained from the chargecarried by the electric current, which is not affectedmuch by the small changes in the anode dimensions,since it depends on the resistance of the glass betweenthe electrodes. The diffusion first step is seen to be bya factor of -4 more sensitive to a change in stripe widththan the field-assisted first step.

For the same two representative processes, the cal-culated dependence of NAg on the product of diffusion

20 January 1991 / Vol. 30, No. 3 / APPLIED OPTICS 339

I I

.

I I I

Table 1. Comparison of First Process Steps

Diffusion step Field-assisted step

dNAg/dw g/m2 ) 0.81 0.20

Dt[dNAg/d(Dt)] 2.5 4.5(g/m)

Typical NAg 3.6 2.2variation (%)

Charge control - Possible

constant and process time Dt is shown in Fig. 2. In thediffusion first step NAg is, typically for a diffusionprocess, proportional to VE. In the field-assisted firststep the conductivity of the glass has the same expo-nential temperature dependence as D, and thus NAg isproportional to Dt. This explains that the field-assist-ed process step is by a factor of 2 more sensitive to thechange of Dt than the thermal process step. Thevariation of Dt is mainly due to the variation in D fromthe instability of temperature during the process. InCorning 0211 glass a 10% change in the diffusion con-stant of Na+ corresponds to a change in temperature Tby -3 K. The shorter process time in the field-assist-ed step can be controlled precisely by connecting anddisconnecting the electric voltage.

To compare the two examined first steps, typicalvalues of 0.2 jAm and 0.6 K were chosen for the indepen-dent variations of w and T, respectively. This givesthe variation in NAg by 3.6% for the diffusion step and2.2% for the field-assisted step. The significance ofthese variations depends on the needed controllabilityof waveguides. However, the field-assisted methodadmits the possibility that the quantity of exchangedions can be controlled directly from the measured elec-tric charge, as demonstrated in Ref. 12.

Table I is a summary of the first process step com-parison.

B. Second Process Step

For the second process step we made a comparison ofthe three common alternatives. In the postbakingprocess the ion concentration distribution is modifiedwith diffusion at an elevated temperature." 1 In thethermal burial process the substrate is immersed in amolten salt containing the original ionic species in theglass, so that diffusion occurs in the glass and the ionsat the surface are exchanged back for the original ionicspecies.13 In the field-assisted burial process a similarmolten salt is used as an anode, and the exchanged ionsare made to migrate deeper into the glass.9

We studied the mode field distributions of wave-guides for different durations of the second processsteps. The mode mismatch loss for coupling to anoptical fiber, which has a Gaussian mode field distribu-tion with an le radius of 4.17 ,jm, was calculated. InFig. 3, the coupling losses as functions of the secondstep duration are shown for typical three alternativeprocesses. The first step was a field-assisted processwith a w = 4-,jm source stripe width for all three secondstep alternatives. The amount of Ag in the glass NAg

1.2

1.0-

cn

-o0

0.8 -

0.6 -

0.4 -

0.2 -

0.0 -0.0

I -I

20.0 40.0 60.0 80.0"Vt / 100.0

Fig. 3. Coupling losses for different second process step durationsfor postbaking, thermal burial, and field-assisted burial processes at

616 K. Lines are third-order polynomial fits.

a)

Ea

'N)

0.0

10.0

x / pm

b) 0.0

E

:y)

10.0

C)

EaL

'N1

0.1

10.

x / pm

-10.0 0.0 10.0x / pm

Fig. 4. Optimum refractive index distributions of waveguides forthe three processes of Fig. 3: (a) postbaking, t = 3200 s; (b) thermalburial, t = 1600 s; (c) field-assisted burial, t = 1400 s, 20-V voltage.The contours are for an index increase from the substrate index at

intervals of 0.001.

after the first step was 6.1 jig/m for the postbaking andfield-assisted burial cases and 10.5 jig/m for the ther-mal burial case, since in this process a considerableamount of Ag diffuses back into the melt during the

340 APPLIED OPTICS / Vol. 30, No. 3 / 20 January 1991

I I

post-bakea03

l l l E

I 'm " "-SE // I/

- - - -

/ PM

-5.0 0.0x / pm

,/ - - - _ ._ -

,, / / - .. % ., , , '/ - - ' ' '

, ,, I , .~ .i r / / _ I/ I} A

\s v _ _ _ ~- /

-5.0 0.0x / pm

-5.0 0.0

Table II. Comparison of Second Process Steps

Thermal Field-assistedPostbaking burial burial

Coupling loss (dB) 0.17 0.16 0.05

Temperature change 5.7 5.1 5.4for 0.01-dB lossincrease (K)

Complexity simple quite rathersimple complex

* U burial, and field-assisted burial steps, respectively.Since the effect of proportional change in process du-ration is again the same as the effect of proportionalchange in the diffusion constant, these numbers repre-sent also the sensitivity to process temperature. Thissensitivity corresponds to a change of temperature by-5 K for the 0.01-dB increase in mode mismatch loss.

The refractive index distributions for the optimumprocess durations are shown in Fig. 4, and the corre-sponding mode intensity distributions are shown inFig. 5. There is not much difference in the depth fromthe glass surface of the mode fields in the postbakingand thermal burial processes. In the field-assisted

5.0 burial process the asymmetry of the mode field distri-bution due to the air-glass interface is largely eliminat-ed, and also the surface scattering is considerably low-ered.

Table II is a summary of the second process stepcomparison.

IV. Discussion and Conclusions

5.U

x / pmFig. 5. Intensity distributions of waveguide modes for the refrac-tive index distributions of Fig. 4 at a 1.523-um wavelength. The

contours are for normalized intensities of 0.3, 0.5, 0.7, and 0.9.

second step. The voltage in the field-assisted burialprocess was 20 V. The waveguides were confirmed tobe single mode with the finite difference method calcu-lation.

In agreement with Ref. 14, the lowest coupling lossesare similar for both the postbaking and thermal burialprocesses, here 0.17 and 0.16 dB, respectively. Thecoupling loss achieved with the field-assisted burialprocess is 0.05 dB. From the third-order polynomialfits to the calculated data we estimated the proportion-al lengths of the time intervals within which the lossincrease from minimum was <0.01 dB. The ratio ofthis interval length to the optimum process durationwas 0.36, 0.32, and 0.34 for the postbaking, thermal

The results above indicate that the optimum processis a combination of first and second steps that are bothfield-assisted. However, if the losses are low enough,other characteristics of the process gain more impor-tance than the extreme optimization of the losses. Forexample, with the purely thermal process of Ref. 13,there is no need for a special process setup where saltmelts with different potentials need to be isolated fromeach other. Alternatively, with Ag+-Na+ exchangethere is the possibility of carrying out the field-assistedfirst step with a metal thin-film electrode configura-tion,1 5 which is very uncomplicated. This process stepalso offers the possibility of controlling the amount ofAg exchanged into the glass by the source film thick-ness.'6

Silver ion exchange has the advantage that relativelyhigh maximum index changes can be achieved, whichis important, since in the second process step the con-centration is much decreased. The main drawback ofthe Ag+-Na+ exchange is considered to be the lossesbrought about by the reduction of the Ag+ ions intometal colloid. However, with good quality substrateglasses' and low Ag+ concentrations, the losses can beexpected to be quite low at the IR wavelengths'7 usedin telecommunications. From the other ions in com-mon use Tl can produce high index changes, but itstoxicity adds complication to the processes.' With Csindex changes are lower and a special glass for Cs+-K+exchange is necessary for good results.' 8 Potassium

20 January 1991 / Vol. 30, No. 3 / APPLIED OPTICS 341

a) 0.0

S:

n'N

7.0

b)

E

=7)

C )

-I"I'N

0.0

7.0

0.0

3.0

6.0

9.0

--

- -

/ - -- .- '.� - -=

j / , II I I � I� I

I I / I� I\ � - - - // / I

- / I'. '. -. - / // I

,-- �

I

-I

n

a)

x / pm

x / pm5.0

a) o.C

SI

'N1

5.C

b) o.r

n

'N1

7.C

-5.0 0.0 5.0x / m

Fig. 6. Measured near field intensity distributions of fabricatedsingle-mode waveguides at 1.523-pm wavelength. The postbakingduration was (a) 1600 s, (b) 3100 s, (c) 6200 s at 620 K. The contoursare for normalized intensities of 0.3, 0.5, 0.7, and 0.9. The scale onthe vertical axis is the same as in the horizontal axis, although the

position of the glass surface is not known accurately.

exchange results only in a very small index increase,'9which limits the use of the second process step.

We chose as the single-mode waveguide fabricationprocess a combination that has a field-assisted firstprocess step with an Ag thin film source and a postbak-ing second process step. The second step is consider-ably uncomplicated to carry out and also more control-lable than a thermal burial process, where asubstantial amount of Ag diffuses out of the glass, andsmall variations in the initial concentration distribu-tion can have an effect on this. Additionally, since thediffusion of Ag from metal film into the glass withoutthe electric field is very small,'6,2 0 the two steps can becombined in the same oven by proceeding directlyfrom the first to the second step by disconnecting thevoltage to the electrodes. This eliminates the uncon-trollability of cooling and heating between the processsteps. Also, during the heating before the first stepvery little exchange occurs, and after the process thesmoothed ion distribution will not change much duringthe cooling.

Figure 6 shows the measured near field intensitydistributions at a 1.523-,jm wavelength for single-mode waveguide fabricated into Corning 0211 glass.Openings 4 jm wide had been patterned on a TiW filmsputtered on the glass, and a silver source film was thendeposited on top of that. The first process step was

c )

l

:y)

0.0 -

3.0 -

v - . ,

/ pm~(

,,, ', . .

' -

-5.0

6.0 -

9.0

0.0/ m

-5.0 0.0x / m

.0

5.0

5.0

Fig. 7. Intensity distributions of waveguides calculated from themodel for the postbaking process of Fig. 2. The postbaking durationwas (a) 1600 s, (b) 3200 s, (c) 6400 s. The contours are for normalized

intensities of 0.3, 0.5, 0.7, and 0.9.

carried out at a temperature of 620 K with a 5-Vvoltage for an 80-s duration. Second process stepdurations were 1600,3100, and 6200 s at a temperatureof 620 K. The measurement was done using an IRvideo camera with the light of an IR He-Ne lasersource coupled to the waveguide from an optical fiber.The picture was digitized and processed using a micro-computer with a video digitizer board. Figure 7 showsthe calculated intensity distributions for the postbak-ing process of Fig. 2 for second step durations of 1600,3200, and 6400 s. The actual fabrication conditionsare only approximately similar to the modeling param-eters, since the blocking anode current under the metalthin film mask makes the process more complicated.A resistant mask would also be necessary for accuratecharge control.'2 The experimental mode sizes are

342 APPLIED OPTICS / Vol. 30, No. 3 / 20 January 1991

..%

/ - 'I

I I II I� / I I

'. / I I

' /

/I-

l

somewhat smaller than the calculated ones, but theaspect ratios and vertical symmetry, which are impor-tant for coupling losses, are quite similar. Thus themeasurements confirm that Fig. 3 describes quite wellthe evolution of coupling loss for this process. It is alsoknown that the diffusion coefficient of silver ions isreduced at low Ag+ concentrations.2' This explainsdifferences between calculated and theoretical resultsfor longer postbake times.

In conclusion, we have used a computer model tostudy important two-step ion-exchange processes forthe fabrication of single-mode glass waveguides withfiberlike mode field distributions. For the first pro-cess step the field-assisted process step is, comparedwith the thermal process step, less sensitive for thevariation in width of lithographically patterned stripesbut more sensitive to the temperature variation.However, the possibility of directly controlling theamount of ions exchanged into the glass by measuringan electric charge during the process gives the field-assisted first step an advantage in reproducibility.For the second process step the optimum coupling andsurface-scattering losses are obtained with a field-as-sisted burial process, although quite low couplinglosses can be achieved with postbaking and thermalburial processes too. Based on the modeling consider-ations a fabrication process was proposed based on thefield-assisted first step with an Ag thin film ion sourceand postbaking second step. The mode field distribu-tions of single-mode waveguides fabricated with thisprocess were measured, and these measurements con-firm the theoretical predictions.

When this work was done Ari Tervonen was workingat the Technical Research Centre of Finland.

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20 January 1991 / Vol. 30, No. 3 / APPLIED OPTICS 343