Waveguiding structures in polymers by ion implantation

NOMIB

Nuclear Instruments and Methods in Physics Research B 89 (1994) 373-378 North-Holland

Beam Intomctions with MatdaIs & Atoms

Waveguiding structures in polymers by ion implantation S. Brunner

*, D.M. Riick

Gesebchaft fiir Schwerionenforschung,Darmstadt, Germany

W.F.X. Frank, F. Linke, A. Schiisser German Telekom Research Center, Darmstadt, Germany

U. Behringer, IBM Deutschland GMTC, Stuttgart,Germany

Optical waveguides have been produced by ion irradiation of the surface of polymer blocks of poly-(methyl-meth-acrylate) (PMMA). ion energies in the range of keV and MeV have been used. Mask construction is described. Strip waveguides, branches and Mach-Zehnder

interferometers

were generated. The refractive index as a function of the depth will be discussed.

1. Introduction The demand for passive optical devices is growing rapidly since fiber optical networks tend to extend the optical domain throughout to the home of the customer. Waveguides, couplers, multi- and demultiplexers integrated on a common substrate are necessary in large amounts. Structure formation in polymers as low cost materials is easy and so these materials are promising to have good chances for mass production. in this paper is reported how in bulk sheets of poly(methyl-meth-acrylate) (PMM.4) strip waveguides in the polymer surface can be generated by ion irradiation. In this method energy is transfered to the material which changes the chemical structure of the polymer. The processes in the material has been described earlier [1,2]. The most interesting property as a consequence of the chemical changes, is the modification of the refractive index of the material. The processes using ion beam irradiation together with mask exposure technology allows the generation of waveguide structures without any additional chemical treatment.

ished in a two step procedure: a block of 10 sheets is clamped in a polishing machine and first ground with 2400 enamal paper for 10 min, followed by a polishing process with a mixture of lubricant-red and diamondpowder (grains below 0.25 km) on felt, for 10 min. The end faces are obtained with sharp edges and with a roughness below 500 nm.

3. Method of waveguide generation In the waveguiding regions the refractive index increase by 0.02 (nsubs= 1.489 at 633 nm). This is produced by hydrogen ions (protons) of an energy of 230 keV with an ion fluence of 5 X lOi ions/cm’ with an exposure time below 15 min. These parameters have been determined as the best values. To obtain single mode waveguides in the region of 850 nm up to 1550 nm higher ion fluences are needed [2]. In order to obtain strip waveguides (see Fig. 11 a special mask was constructed, which will be described in the following. 3.1. The transmission shadow mask

2. Sample preparation Sheets of bulk material are obtained from RGhm GmbH, Darmstadt in a thickness of 1.5 mm. They were cut in pieces of 32 x 23 mm’, the end faces are pol-

* Corresponding author.

In a 4 in. silicon wafer a window of 27 x 27 mm2 with a thickness of approximately 2 pm is etched. The wafer is then covered by a 500 nm layer of silica. The bottom of the wafer is covered by a photo resist of 1.8 urn thickness. In this resist the lay out of the mask shown in Fig. 2 is written by electron beam lithography. After developing of the resist the structures in the

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VIII. BEAM MODIFICATION

S. Brunner et al. / Nucl. Instr. and Meth. in Phys. Res. B 89 (1994) 373-378

374

pal difficulties in fabricating a free standing shadow mask: (1) The structure of 25 mm in length and 7 pm in width requires further stabilization of the 2 p,rn silicon window. (2) Closed loopes like e.g. Mach-Zehnder interferometers can not be realized since the inner part of the interferometer would break out. Therefore the structures in the window are not continously open over the full lenght; they are interrupted after each 15 pm distance by a bar of 3 km. These bars stabilize the structure and prevent the inner part of the MachZehnder from breaking off. Experiments have revealed that the method of stabilizing the mask structure by bars of 3 km does not affect the light guiding properties.

mask substrate

b) Fig.

1. Scheme

of ion

irradiation through shadow mask.

a transmission

window of the silicon wafer are etched with KOH. In order to protect the mask against sputtering induced by ion bombardment, the whole mask is covered by a 200 nm aluminum layer. The cross section of the mask is shown schematically in Fig. 3. There are two princi-

/

4. Generated waveguides In Fig. 4a an enlarged detail of the Y-branch region on the mask is shown. Fig. 4b shows the corresponding

/ 25 mm

,i

,:.

,.,’

..j

Fig. 2. Lay out of the transmission

shadow

mask, area 25 X 25 mm’, waveguide

3.2#:

I 1.5:i

width 7 km.

S. Brunner et al. /Nucl.

Instr. and Meth. in Phys. Res. B 89 (1994) 373-378

Fig. 3. Schematic representation

sample area after irradiation. The light guiding properties are demonstrated in Fig. 5a and 5b, where the end face of a Y-branch with the bright dots is shown.

5. Index profile of waveguides

In order to measure the index profile of the generated waveguides it is suitable to have planar slabs since the mode spectrum is easy to measure by a prism coupler (PC 2000 from Metricon Corp., Pennington, NJ). Former experiments in order to determine the

375

of cross section of the shadow mask.

index profile of waveguides are based on the method of stepwise reduction of waveguide thickness by successive grinding of the surface and subsequent determination of the remaining mode spectrum. This procedure is rather awkward and of poor accuracy, because the surfaces normally are not plane enough. In order to avoid this, some 2 in. silicon wafers were covered with silica as a buffer layer. Then PMMA layers (KTI 950kresist) in thicknesses between 400 nm up to 4400 nm with successive layers of approximately 400 nm thickness were spin-coated. All this samples were irradiated with hydrogen ions under identical conditions. The mode spectrum of each layer system was then determined using the prism coupler technique. The values

(W Fig. 4. Detail of a Y-branch: (a): on the mask; (b): on the sample surface.

Fig. 5. (a): Functional principle of coupling light to a waveguiding structure. (b): Light emitting from the end face of the structure. VIII. BEAM MODIFICATION

376

S.Brunner etal. / Nucl. Instr. and Meth. in Phys. Res. B 89 (1994) 373-378

for the layer thicknesses d are measured by a Dektak 3030ST surface profilometer. Assuming that the refractive index within a 400 nm layer is constant the profile has been determined in the following steps, see Fig. 6. (1) The index of the first layer (425 nm) can be directly deduced from the prism coupler results. (2) The second slab, approx. 800 nm thick, contains

._

SiOz

in its top layer the index which has been determined in step 1. The index of the underlying slap has to be chosen so that the mode spectrum determined by the prism coupler is matched. It has to be taken into account that the layers during the irradiation undergo a compactation process. The mode spectrum determined by the prism coupler is simulated by a n-layer program [3]. The index profile so obtained is shown in

. _ _.

-4

b) Fig. 6. Method of successive determination

of the refractive index profile.

” Layer 1,550 1,540 i

Fig. 7. Index of refraction of irradiated PMMA as a function of depth, calculated by TRAMAX simulation programm.

dE/dx [eV/A]

ii suho* Irnd_

1.5

2 depth

2.5

3

3,5

4

4,5

Fml

Fig. 8. Electronic energy loss of hydrogen ions regarding the compactation of the surface (TRIM simulation).

S. Brunner et al. /Nucl. Instr. and Meth. in Phys. Rex B 89 (1994) 373-378

377

dE/dx[eV/A]

L-T 0

.;....:....:....;.,..:....:.’ 10

5

15

20

25

“I

30

35

40

depth Cm1

Fig. 9. Electronic energy loss of helium ions of 5.6 MeV as a function of depth (TRIM simulation).

Fig. 7. This can be compared

with the energy loss, calculated by a simulation program TRIM [4] (TRansport of Ions in Matter). From Fig. 8 it can be seen that beyond a depth of 4 p,rn (taking into account the recess of the surface of approximately 1 pm) the index of the substrate is not further influenced. Comparison of Fig. 7 and Fig. 8 leads to the conclusion, that the increased index is predominantly a function of the electronic energy loss.

6.

High

energy

irradiation

experiment

The facilities of Gesellschaft fiir Schwerionenforschung (GSI) in Darmstadt provide the possibility to irradiate with an energy of 1.4 MeV/nucleon. The maximum electronic energy loss happens in a depth

Fig. 10. Buried planar waveguide (bright line) in PMMA by 5.6 MeV helium irradiation.

900 800 700

d

P

500

g 400

f

300

0

10

20

30

40

50

60

depth wm]

Fig. 11. Light intensity of the waveguide of Fig. 10 perpendicular to the surface. VIII. BEAM MODIFICATION

378

S. Brunner et al. /NucL Instr. and Meth. in Phys. Res. B 89 (1994) 373-378

which is a function of the ion energy. So it can be expected that in a single irradiation process a buried waveguide can be written directly into the polymer. The final step of covering the waveguide structure by a cladding can be avoided. The TRIM program predicted for helium of 5.6 MeV a penetration depth of 35 vrn, see Fig. 9. The experiment has confirmed this prediction. The result is shown in Fig. 10, the planar waveguide in a depth of approx. 35 km. In this planar waveguide light has been coupled in by a fiber from the opposite surface and the light intensity perpendicular to the surface has been recorded in Fig. 11. Of course, these waveguides can not be treated with the prism coupler.

7. Conclusions The optical measurements reported here are made with 633 nm (HeNe lasers). Mode spectra reported in a

previous paper [Z] has been measured up to a wavelength of 1520 nm. The mode spectra reveal good transparency even at these high wavelengths. The profil of the refractive index as a function of depth was measured by a new method. Taken into account the compaction of the material the calculated energy loss of the ions is similar to the obtained profil. Planar buried waveguides are generated by a MeV helium irradiation.

References [l] W.F.X. Frank, J. Kulisch, H. Franke, D.M. Riick, S. Brunner, R.A. Lessard, SPIE 1559-35 (1991) p. 344. [2] W.F.X. Frank, Optical properties of waveguiding structures in polymers, SPIE 1774 (1992) p. 268. [3] L. Leine and Ch. WIchter, University of Jena, Germany (1993). [4] J.P. Biersack, Simulation program TRIM, Berlin, 1989.