Optical property modification of PMMA by ion-beam implantation

Optical property modification of PMMA by ion-beam implantation

Applied Surface Science 169±170 (2001) 428±432 Optical property modi®cation of PMMA by ion-beam implantation Wan Hong*, Hyung-Joo Woo, Han-Woo Choi, ...

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Applied Surface Science 169±170 (2001) 428±432

Optical property modi®cation of PMMA by ion-beam implantation Wan Hong*, Hyung-Joo Woo, Han-Woo Choi, Young-Suk Kim, Gi-dong Kim Korea Institute of Geology, Mining and Materials, Yusong-ku, Gajung-dong 30, Taejon 305-350, South Korea Received 5 August 1999; accepted 15 December 1999

Abstract Polymeric waveguides were fabricated by proton implantation on poly(methyl methacrylate) (PMMA). Depth pro®les of the refractive indices of modi®ed regions were obtained and were found to be in good agreement with the stopping power curve of protons in PMMA. It means that the waveguides are formed at the depths where the stopping power is the maximum value. Light losses for 635 nm wavelength were measured using planar waveguides to verify if the transmittance is enough for the application of the technique to optical devices. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Waveguide; Ion implantation; PMMA; Optical properties; Refractive index; Index depth pro®le; Light loss

1. Introduction Since many devices used in optical communication are composed by waveguides, waveguides play an important role in optical devices such as optical switches, interferometers, couplers and signal-branching devices. A waveguide is characterized by a region of high refractive index bounded by regions of lower index. Historically, the diffusion doping method has been a high priority for fabricating waveguides, because this method can be done with relatively low cost and simple equipments. However, it has some disadvantages; limited choice of dopant materials and index pro®les, high temperature processing and disturbances of dopant materials with each other. In the case of diffusion into crystal material, diffusion *

Corresponding author.

is sensitive to dislocation and grain boundaries, and lateral spreading of dopants beneath a mask. Epitaxial growth and ion exchange have been used also to increase the refractive index of the surface layer of a few micrometers. High energy ion implantation, which is a surfacemodi®cation technique, can be applied to form waveguide structures [1±5]. This technique has become a common tool to modify surfaces of semiconductors, crystals and optical materials in order to obtain certain electrical, mechanical and optical properties. Compared with other waveguide fabrication methods, ion implantation has some unique advantages [6]. This method can be applied to produce waveguide structures in the most of optical materials. It has a superior controllability of depth of the waveguide to other techniques since incident energy of ion can be selected freely. Ion implantation is a low temperature process, and is a great advantage in the case of the ferroelectric

0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 6 9 8 - X

W. Hong et al. / Applied Surface Science 169±170 (2001) 428±432

optical material since its phase is more stable at the lower temperature. Inorganic materials such as LiNbO5, InP have been developed and tested for the production of waveguides [6]. The high cost of these materials, however, is a barrier to the widespread use for optical devices. Polymeric materials offer a solution of the materialcost problem. Polymeric materials are very interesting for the production of optical devices because of their proper wavelength-band characteristics, rapid responses, high optical damage thresholds and low permittivity constants. Especially, poly(methyl methacrylate) (PMMA) is a very attractive material, because it is easy to form a structure with desired optical properties. Many investigations of PMMA have been done for waveguide fabrication [7±11]. In this work, high energy protons were implanted to PMMA to make waveguide structures, and the changes of optical properties were observed. 2. Sample preparation and measurement A PMMA sheet (Goodfellow, ME307901) of 1 mm thickness was cut into three pieces of 5 cm  2 cm size. The samples were irradiated by 350 keV protons with ion ¯uences of 2  1014 , 6  1014 and 1  1015 ions per cm2, respectively. The number of modes and the effective refractive indices of them for 633 nm light were measured by the `m-lines' technique [12]. Samples for obtaining refractive-index depth pro®le were prepared for two proton energies of 1 MeV and 350 keV. A PMMA plate of 2 mm thickness was irradiated by 1 MeV protons with a ¯uence of 5  1013 ions per cm2. All end faces of the sample were ground using 600, 1000, and 2400 mesh sand papers, and polished with diamond paste of 0.4 mm grain size. Index depth pro®le was measured using Refracted Near Field (RNF) technique [13]. The measurement system was preform analyzer (York Technology, LTD., Model P104). Light scattering on the sample surface was suppressed by refractiveindex matching oil (n ˆ 1:4587). After measuring of index pro®le, the planar waveguide of this sample was observed with a CCD camera. The range of 350 keV proton in PMMA is about 5.5 mm. Since it was too short to pro®le the refractive index directly, a unique method was attempted to

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obtain index pro®le. Seven PMMA ®lms of about Ê thickness were prepared on a Si wafer by spin 500 A coating. The range of 350 keV proton was divided into seven slabs and the proton energies at each slab were calculated. Each sample was irradiated by protons having the calculated energy to simulate the implantation to the thick PMMA with 350 keV proton. The ¯uence for the sample representing the surface was 6  1014 ions per cm2. For other six samples, ¯uences were corrected along with depths using simulated data. Average refractive indices of the PMMA ®lms were measured by an ellipsometer. To measure light losses of waveguides, four samples were irradiated by 350 keV protons with irradiation ¯uences of 2  1014 , 4  1014 , 6  1014 and 8  1014 ions per cm2, respectively. Light from a diode laser (635 nm) was coupled to the samples using a prism. The intensity of the light scattered out from the prism was scanned along the sample surface by a photodiode. To reduce the scattered light from other points, the photodiode was covered with a cylinder contacted tightly with the sample surface. 3. Results and discussion The relation between the number of mode and ion implantation ¯uence in PMMA samples was obtained by `m-lines' measurement. Fig. 1 shows that the number of mode increases along with the ¯uences of 350 keV protons. The PMMA sample with higher

Fig. 1. Variation of effective index of PMMA for 633 nm along with implantation ¯uences of 350 keV protons.

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W. Hong et al. / Applied Surface Science 169±170 (2001) 428±432

ion ¯uence has more modes, which means that high ¯uence introduces large variation of refractive index. The effective refractive index of each mode for each sample is also shown in Fig. 1. Considering that the refractive index of pristine PMMA is known to be 1.490, it is clear that refractive index was increased rapidly and this technique is superior to other waveguide fabricating techniques from the point of view of refractive index treatment. Two effects, cross-linking and scission arise by proton implantation. Cross-linking/scission decrease/increase the molecular weights of the polymer molecules. Since the scission effect is dominant in this energy range of proton [14,15], compaction of irradiated layer was observed. Measurement of the compaction and the change of molecular weight distribution of substrate material PMMA are presented elsewhere in this conference [16]. The compaction increases with ¯uence. In the case of 1  1015 protons per cm2 ¯uence, the modi®ed layer shrinks to more than half in depth. Molecular weight distribution spectra also show that the distribution of the scission products moves lower mass along with proton ¯uence. This results show that long main chains of PMMA molecules be broken into shorter ones and compacts to small volume. The refractive index of isotropic polymers is related to the physical and chemical properties by the Lorentz±Lorenz relation [17]. Refractive index increases along with the increase of density and the decrease of molecular weight. The density of modi®ed layer increases effectively and molecular weight decreases, therefore, result in the rapid increase of the refractive index. The variation of refractive index is related to the energy transfer ratio from implanted protons to polymer molecules. Energy transfer ratio is directly related to the stopping power of proton. Since the stopping power of the ion has a pro®le along with distance through a material, it can be guessed easily that the refractive index will have a similar pro®le along the depth direction. A depth pro®le of refractive index of PMMA irradiated by 1 MeV protons with a ¯uence of 5  1013 ions per cm2 was compared with the stopping power curve in Fig. 2. Stopping power was calculated using SRIM2000 code, the new version of TRIM by Ziegler [18]. The maximum value of the stopping power appears at the depth of 24.5 mm from the sample surface. The depth is just before the range (25 mm) of 1 MeV proton in PMMA. The stopping

Fig. 2. Comparison of the refractive index depth pro®le and the simulated stopping power curve of 1 MeV protons in PMMA. The solid line represents stopping power curve. Fluence was 5  1013 ions per cm2. The index curve was obtained by the RNF technique.

power, then, decreases rapidly. The index pro®le resembles the stopping power curve until the maximum point but deviates thereafter. The index decreases slowly until 45 mm where it goes back to the pristine value. The reason may be that protons lose a large portion of their kinetic energy within the several micrometer range just before they stop. Whereas, the energy from protons may transfer deeper. The simulated depth pro®le of refractive index of PMMA irradiated by 350 keV protons with a ¯uence of 6  1014 ions per cm2 and the stopping power curve of 350 keV proton in PMMA were shown in Fig. 3. The stopping power curve was calculated using

Fig. 3. Comparison of the refractive index depth pro®le and the simulated stopping power curve of 350 keV protons in PMMA. The solid line represents stopping power curve. Fluence was 6  1014 ions per cm2. Index curve was obtained by ellipsometry measurement of samples irradiated by simulated energies described in the main text.

W. Hong et al. / Applied Surface Science 169±170 (2001) 428±432 Table 1 Light attenuations (in ions per cm2) of PMMA waveguides for 635 nm along with implantation ¯uence

Loss (dB/cm)

2  1014

4  1014

6  1014

8  1014

1.2

1.7

2.0

2.3

SRIM2000. The index curve recorded the maximum value at the position where the stopping power becomes maximum. This result is similar to the case of index pro®le of the PMMA sample irradiated by 1 MeV protons. It was found that this new method can be successfully applied to obtain index pro®le within a shorter range than 10 mm. The pro®le shown in Fig. 3 may be quite different from real index pro®le in the waveguide region because of the compaction effect. However, the simulated pro®le still gives useful information on the approximate index pro®le in the buried waveguide. Light loss of waveguide is another important factor of optical devices. The loss L is de®ned as [19] Lˆ

10 log10 …I=I0 † x ÿ x0

(1)

Light losses for several implantation conditions are shown in Table 1. Considering that the light loss of pristine PMMA was measured to be 0.03 dB/cm for 635 nm, the index increased to two or four times of

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pristine by implantation for the ¯uence used for this experiment. The general criterion of the light loss in waveguiding element in telecommunication networks is below 1 dB. However, since some of optical devices are often less than a centimeter and ¯uence can be reduced because the index change is suf®ciently large, the implantation technique may still be applied to manufacture some optical devices by optimizing irradiation conditions. A planar waveguide is shown in Fig. 4, obtained with 1.8 MeV proton implantation. It can be clearly seen that the waveguiding layer is formed in the substrate material PMMA. 4. Conclusion Proton implantation was applied to the fabrication of waveguides in PMMA. Increase of mode number and refractive index of irradiated region was observed. Depth pro®les of refractive index were also obtained. Especially, the refractive index pro®le in short range was successfully obtained by a unique method. Increase of refractive index is the result of compaction of irradiated layer. Although the light loss increase is rather large with ¯uence, it can be still controlled to a proper level for some optical devices. Acknowledgements The authors would like to thank Prof. Changkwon Hwangbo in the Inha University for `m-line' technique and his kind advice about optics measurement. We also would like to thank Mr. Young-Tark Lee and Mr. Jeong-U Jeon in the Access Network Laboratory of Korea Telecom for the index depth pro®ling measurement. References

Fig. 4. Buried planar waveguide in PMMA formed by 1.8 MeV proton implantation.

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