Optical waveguides in polymeric material by ion implantation

318

Surface and ('omin.~'.~ '/'echnohtey. 51 11~1921 318 323

Optical waveguides in polymeric material by ion implantation D, M. Riick and S. Brunner Ge.wll.~chaJ'tflit Schwerionet!li~rschung. POB 110552. t~I00 Darnt~tadt ~l"R(i ~'

W. Frank i"or,whun,~,~institut der Deutschen Bundespost Teh.kom, POB IOt~L¢, 6 IrMJDarms'tudt ~'i.'R(i.

J, Kulisch and H, Franke FB Phy.~ik-'l'echnoh~gie. Uni-GltS I)uishlo',~, POB 101503. 4100 Duixhur.t, ~I " R G

;

Abstract Poly(methyl-methacrylatel (PMMAI substrates were implanted with He ' ions of energies between 511 keV and 2511 keV and at 1 MeV and 2 MeV. The ion dose was varied between 10~2 and 10ts ions cm -'. We measured the increase in refractive index of the implanted surface layer, which acts us an optical waveguidc, by measuring the effective index of refraction n~, for different optical modes. According to the range of ions in the polymer, surface waveguides and buried waveguides could be generated. We fimnd that the number of m~lcs which a waveguide can carry depends on the ion dose. To obtain single-mode waveguides, low ion doses were used. Waveguides with optical losses b¢lo~ 1 dB cm - t were found. The chemical effect induced by ion implantation was measured by IR spectroscopy and residual gas analysis, physical mt~lilication of the surface was studied by scanning ek.'ctron microscopy. The refractive index profiles are discussed using simulation calculations.

I. latro4~ti~t Optical signal prt~zessing in future telecommunications requires waveguiding, modulating and switching devices. Strong efforts have been made during the last two decades to construct components which can satisfy the,~ requirea-nents on bases of inorganic crystals, e.g. LiNbO~ [I], Several years ago polymeric materials gained more and more interest as suitable materials for these purpo~s. Since polymeric materials exhibit a wide variety of molecular architecture, they are expected to be able to meet the requirements and the device specifications of integrated optics for signal processing, for example low optical losses, easy to structure, mechanical and thermal stability. In addition, polymeric materials promise a relatively low cost prc~luction process. Planar and strip waveguides have been produced by diffusion of photosensitive substances [2] and photoresist etching technique [3]. Ion implantation into polymeric materials has been used for ~rface modification [4]. Also, an increase in the refractive index of a thin layer in the surface of the implanted sample has been found [5]. This may allow the u ~ of these layers as waveguides [6]. This work is focused on the generation of optical waveguides in the polymeric material poly(methylmethacrylate) IPMMA) by ion implantation. Because the projected range of an ion, implanted into the material, is a function of the ion energy [7], the use of different ion

0257 89729255.00

energies provides the possibility of producing waveguides at a variable depth in the bulk material (buried iaycrs). We used ion energies from 50 keV up to 2 MeV, We investigated the influence of the ion dose on the quality of the waveguiding structure produced, which is characterized by the loss value and the profile of the refractive index, For these investigations we studied multimodc as well as single-mode waveguides, whereas for applications the single-waveguide mode is required. Additional methods such as residual gas analysis (RGA). attenuated total reflectance {ATR}-IR and scanning electron microscopy (SEMi were used to understand the underlying prtx:css of the increase in refractive index induced by ion beam treatment. A combil-l~l experimental and theoretical study was started to obtain information about the refractive index profile in the surface htyer.

2. Exl~rimeatwl details The polymeric material PMMA was chosen because of its high optical transparency, This material was used in three different sample preparations: the liquid monomer methyl-methacrylate was polymerized to form blocks of volume 44) x l0 × 3 mm "~ [8]: commercially available PMMA (R6hm GmbH) was cut into pieces of volume 45 x 20 x 2 mm 3, PMMA solution was spin coated onto glass substrates in thicknesses of 2 up to

~ 19~)2

Elsevier Sequoia. All rights reserved

D, M, Rfick et al, / Optical waveguides in polymeric mmeriai

319

TABLE l, Implantationparametersusedat the differentimplantationfacilities

Ion Energy Ion current

Ion dose

GSI

KFA Jiilich

Univexsit~Laval

He + U p to 250 keV 100 nA cm - z 10tz-10 =s

He + I, 2 M e V 100-300 nA cm -z 101~-!014

O+ 100 keV 100 nA cm -z 101z--101~

Electrostatically

Scanningsystem

Magnetically

Electrostatically

!)1 situ m c a s u r e m c n t s

-

-

neff

Masking

Yes

No

No

RGA Temperature me,asurement

Yes Yes

No No

No No

15 pro, Ion implamation of the PMMA samples was carried out at GSI, Darmstndt [9] KFK, Jiilich [10] and the Universit6 Laval, Quebec, The parameters used arc summarized in Table 1, Temperature measurement was performed with a Rayt¢c pyrometer through a germanium window,

3. ltesd~ In addition to visible changes, the irradiated PMMA samples showed a change in refractive index. Using a prism coupler device we found optical wavcguidcs in the surface layer. The measurements were performed using a home-made prism coupling device and a commercially available automatic prism coupler PC 2000 (Mctricon Corp,) [11, 12], The change in surface refractive index was in the range of 1 0 - z . We measured the effective refractive index n , . as a function of the implantation parameters ion dose and ion energy, As a representative of a series of measurements with different ion doses at different ion energies, Fig, I shows the effective refractive index n=fr as a function of ion dose for 100 keV He + ions, We found two mode groups, TM and TE, For the ion dose range

between I0 xa cm -a and I0Is cm -2, proportionality botwceAl thc ion dose a n d th~ neff was found, At higher ion doses the PMMA became dark brown and black and was not suitable for waveguiding even though the temperature was kept below 4,5 °C. The PMMA samples implanted with a mask of 20 x 5 mm 2 showed remarkable loss of material in the implanted region compared with the unimplanted region. Precise measurement with a DEKTAC 3030 protilomcter of the obtained devpening revealed proportionality between this deepening and the ion dose. A comparison of n,rr of the wave.guiding area and the value of the deepening lea to the relation shown in Fig. 2. n,, was measured at higher ion energies of ! MeV and 2 MeV. We observed a relationship hetween n , . and the ion dose. The n , . values at various ion doses for an ion energy of i MeV are shown in Fig. 3, It can he seen that at a low ion dose the wavcguiding structure is single mode, whereas with higher ion doses additional modes appeared, indicated by an increased layer index. Attenuation measurements of the waveguides obtained with the high energy ions led to losses of 5-8 dB c m - l whereas the waveguides obtained with low ion energies showed losses below I dB cm-z. Figure 4 shows the long-term be~aviour of an implanted sample, Sample a was stored under vacuum for several months and sample b was

1,6

t~O0 1,SO

TE0

S c

100 Go0

~.$Z 1,5 No I,~II

i t

i Ill

i tZ

i 16

* Z0

__ Z~

DI10t¢lcm z

I

2

__..I

=

~.

I

-

-

-

5

I

I

6

tO

12

D/lOl~lcm:Z Fig, l , R c f r a v t i v e i n d e x n ~ - as a f u n c t i o n

o f t h e i m p l a n t e x i i o n dose,

Two modesTo and T= could be found.TE and TM are the different polarization directions, H e * ions ~t 100 keV were used,

FiB,2, The functional relation between the deepening d and the refractiveindex nm, He- ions at 100k©V weze us~l,

3211

D . M . Riick et al.

Opm'al Wal"C.vuides m i~oh'merh " m , t c r h d

1.58

1,S'/S :

1.52

I S'~

',b6S

,':

¢2.

1,50 1,49

1,48

LS~5 ' I

IO

II'l'

~lll0"zhn ~

Fig. 3. The refractive index n,.,t as a function of the implanted ion dose. He" ions at I MeV were used• At Io~ ion dose one optical mode could be detected. ~vhel"¢as if the ion dose xver¢ raised additional modes could bc detected.

stored in air, both at room tempcratu~, i:k~lh samples showed a decrease in n~. during the lirst month, but remained stable after thai time.

4, F~lher stedies In order to find the rea~n for the increase in refractive index due to the implantation process, we studied the mechanical modification of the implanted surface• The effect of compaction has already been mentioned above. Additionally, we studied the boundary between the mask and the implanted region. In Fig. 51a} the area ~en in the scanning electron microscope is depicted• The SEM picturc is shown in Fig, 5(b), It ix obvious that the implanted region showed a very smt~th and clear surface region. This has also been ~ n for other polymeric materials [4]. The reason for the rim between the mask and the implanted area is not clear,

I'ig 4. NtabiliD. oxcr thllc o1"the x~a~¢guiding .Mruclul¢ uudcr dillk'rent treatnlentx. ,";alllpl¢ a xxax slor~.'d under xacuum and xanlpl¢ b xxilx MOl'cd

ill a

IIo, rlllil] i i t l l l l o s p h e l ' i f

~,.~n~irollnlenl.

b~.lth

~11 l-OOul Ii2111~

peral ii i-~d,

In site RGA during the implantation process showed outgassing of several volatile products..A.x an example the lime dependence of the devdopmcnt of H.~('=O lalotnic mass unit 311 is depicted m Fig. f~• As an additional method of studying dlemical changes in the implanted p M M A we look IR absorption spectra using the ATR technique. The absorption speetra are shown in Fig, 7 b r implanted and unimphmted samples, Two changes are seen in the sp,ectra: in the 1600 crn region the appearance of N O groups is seen, and in the C H stretching region additionally O t t groups {about 32(~)cm I I are fi~rmed, The appearance of the N O groups must be caused by reaction with nitrogen (N:). In order to study the intlucnce of nitrogen on the change in refractive index after the implantation was tinished, we performed an experiment to measure the rcfl'activc index in ,~itu. During the implantation process the formation of a special optical mode represented by thc index of refraction was continuously recorded and

(a) (b) Fig. 5, SEM image Ibl of lhe boundary urea between implanted and unitupluntcd P M M A , shovol ,xchemalically m lal,

D, M, Rgick et ai / Optical wace,euides in polymeric mmerial

9

waveguide was not buried is in agre.en-w.nt with tbe observation that PMMA was compacted under ion beam treatmont This led to a change in ion range in the bulk material during the implantation process

7,5

oj

321

4,5

~ a

5. C,oadasioa

i.I

0

60

180

120

240

300

t./s Fig, 6 Quantitative tendency of residual gas analy~s of mass 31 (COH~) as a function of the time At the markod time the ion beam

was stopp~ after exposing the sample to nitrogen a further increase in naf was observed in order to study the refractive index profile we proceeded as follows We compared the results of a numerical simulation of two kinds of profiles, an open waveguide and a buried waveguide The numerical simulation was done because analytical solution of the wave equation for the waveguides is not possible [13l The refractive index function was fitted according to the measured no. (Fig 8(a), (b)) The best result was obtained assuming a smoothed step function By mechanical grinding, the surface of a waveguiding area was carried off step by step, and am was measured after each grinding step (Fig, 8(c)). We conclude that the obtained refractive index profile can be described by a monotonically decreasing function. The fact that the obtained

From our results we can propose a hypothesis of how the implamation of ions leads to a layer of increased refractive index, The elementary step caused by the electronic energy loss is the ionization and bond scission along the polymer chains. Short cut pieces were outgassed, while longer chains linked and densified. As a consequence of this process the density is increased, which leads to an increase in the index of refraction, Additionally, the formation of double bonds in N = O groups supported the increase in refractive index. The actual • site of the waveguidc could be centre|led by the ion energy, choosing a suitable range of ions in the polymer. The number of modes as well as the loss depended mainly on the ion dose, For single-mode waveguktcs of low loss, low ion doses were required.

6. Smmmacy

Single-mode waveguides were obtained by implamation of He + ions into PMMA with low ion dose (less than or equal to i0 t4 ions cm-2). These waveguides exhib/ted loss values below 1 dB cm ~ . From the in situ RGA, ATR-IR spectroscopy and in situ measurements

&50

25(

UNIWPLANIE..~_P M M ~ , ~

130

O6', "

~10

--

-

I

WAVENUMBER

Fig, 7. ATR-IR spectra of implamcd and unimplanted PMMA sample,

9~0

sh0

D.M.

322

ct ~1.

Riick

Optic~d wm'¢,gl~ide,~ i. p.Irmn'ic mat(,ri~d

1,,53

1,52

t--.., b -b -t~.

1,Sl

-I-

%

~ ...~

~" ~ -..~.

1,50

-I 1,49

>i_.

I:/

1,48 t .......

= ............. J. . . . . . . . 0.5 1,0

0,0

I

I .

.

.

I

.................

I .........

.

1,,5

2,0

2,5

3,0

3,5

dx/pm lit )

.....

TEO

I-

TEl

- ~ - - TE2

"~'-"

TE3

~

TE4

1,53

1,52 I

- I

I

I,- ..... 4,.

"b

1

~'~ 1.51

15o1

-i-

i

k II

1.49 [

II

1,48

l

t

I

I

I

0.0

0.5

1,0

1,5

2,0

2,5

3,0

dxlpm (b)

........ TEO

1,53

.....t - T E l

....~- TE2

*~

TE3

"

TE4

I i

........

-

1,52

r--

1.Sl]-

~.

1,50 [ " - .

.1

.........

t ..........

t .......

!

1

t

I

2

1

3

dx/pm (¢)

.......... TEO

TEl

TE2

. . . . TE3

~

TE4

I:ig. 8. Calculated and measured dependence of ",.,r with depth in P M M A matcriah {al and (hi are results from calculatio.s. (~'1 is the result from the ¢×pcrimentally estimated refractive index prolil¢ (I McV He" I.

D. M, Riick et al, / Optical wm~eguides in tadymeric material

of the refractive index we found that the refractive index increase was related to radiation-induced chemical reactions. This leads to omgassing of volatile products, compaction in the implanted zone and the formation of N = O groups and O - H groups, Analysis of the implanted multimod¢ wavcguidc, s resulted in a linearly de.creasing profile in the bulk material,

The authors are indebted to the technical staff at GSI, especially D, Vogt for technical support at the GS! ion implantation facility. Thanks also to R, lrmscher and Ch, guchal ISI at Kernforschungsinstitm KFK Jiilich and the group of R, A. Lessard, Universit6 Laval, Quebec for implantations and helpful discussions, N, Skoulas took the IR-spectra and W. getz the SEM pictures at the Forschungsinstitut der Deutschen Bundespost Telekom. The work was financially supported by the Deutsc2~ Bundespost Telckom.

323

Rdereaees I P, Townsand, J, C, Kelly and N, E, W, HarUey, hm Implaauuioa, Spmterin.~ and their Application, Academic Press, London, 1976, 2 H. Franke and W. Hcuer, Conf on lawgrated Optical Circui~ Engineerir~e !!1, SPIE, 615 (1986) p. 120. 3 E, W, Becket, W. Ehrfeld, P, Hagmann, A, Maner and D. Miinchmeyer, Microelectron, lin~,. 4 (1986) 35, 4 E, H, Lee, M, B, Lewis, P, J. Blau and L. K, Mansur, J, Mater, Res,. 6 13) (1991) 610, 5 J, P, Biersack and R, Kallweit, Nucl, Instrum, Methods B. 445(1990) 309, 6 J, Kulisch and H, Franke~ AppL Phyx, A. 50 11990l 425. 7 J. E. Ziegler, J. P. Biersack and U. Littmark. The Stopping aml Range.s of hms in Solid.s. Vol. I, Pergamon. New York, 1985. 8 M. Koppitz, M. D. Lcchner, D. G. Steinmeier, J. Marotz, H. Franke and E. K~tzig, Polym. Photochem.. 5 (1984) 109. 9 D. M. Riick, N. Angert, H. Emig. K. D. L¢ible, P. Sp;~dtke, D. Vog! and B. H, Wolf, Proc, Int, Con,l~ on Nuclear Tracks anti Radiati~m Measurements, Pergamon. Oxford, 1991, in the pr¢~. 10 R. Irrascher, D. Fluck, Ch. Buchal. B. Stritzker and P. Guemer. Mater. Res. Soc. Syrup. Proc,, in the press. I1 R, Ulrich and R, Torg¢, Appl, Opt,, 12 (1976) 2901, 12 R. Th. Kersten, Opti.~he Nachrichtenlechaik. Springer. Berlin, 1983. 13 J. Kulisch, Thesis. 1991, in preparation.