Optical waveguide fabrication by ion beams in the PADC-polymer

NuclearInstruments and Methodsin Physics Research B62(1991) 103-108 North-Holland

Nuclear Inshummnts Methods iniphysics: Research

a

Smt- B

Optical waveguide fabrication by ion beams in the PARC-polymer B. Bennamane, J.L. Decossas, C. Gagnadre and J.C . Vareille Laboratoire d'Électronique desPolymères sous Faisceaux Ioniques, Université de Limoges, 123, avenue Albert Thomas, 87060 Limoges Cedex, France Received 11 April and in revised form 10 June 1991 Ion beams and y rays are used to modify refractive index of polyallyldiglycol carbonate (PADC) . Ion irradiations (H,Li,B) lead to optical waveguides the properties of which depend on the kind of ions, ion fluences and energy . Radiation damage consistently increases the refractive index. Measured variations of refractive index are given as function of deposited energy density (by ions or y rays). It appears that index variations are principally linked to structural modification due to deposited energy. 1. Introduction Polymers subjected to irradiation undergo structural modifications [1] that cause, among other consequences, variations of the refractive index. In the present case, an ion beam sufficiently increases the index to permit optical guidance in the irradiated layer. In earlier studies we demonstrated the feasibility of planar guidage using a proton beam [2]. Here we present results obtained using characteristically different H, Li and B ion beams. The systematic study of index variations 8n induced as a function of the various irradiation parameters permits us to probe the evolution laws of Sri when the energy and nature of the ion flux are modified . We use the experimental results to establish a relationship between observed index variation and the energy volume density deposited by the ion beam. 2. Experimental conditions 2.1. Material a-Polyallyldiglycol carbonate (PADC), commercially known as CR 39 with the following monomer formula, is used : O O

CH 2 -CH2-O-C-O-CH2-CH=CH 2 CH 2 -CH2-O-C--O-CH2-CH=CH 2

O We have chosen this material for two reasons: firstly for its optical quality, which has led to its use by the

optical industry. Secondly, because earlier work has shown its great sensitivity to ion beams since it is an excellent solid particle track detector [3-5]. The material has a refractive index of 1.493 (A =0.6328 gm). 2.2. Irradiations An HVEE 400 kV accelerator is used to irradiate the material with H, Li, and B ions having energies between 50 and 300 keV, with ion fluences ranging from 10 13 to lo ts ions cm -2 . Considering the nature of the ions (Z < 5) and the energies used in our experiments, the predominant physical phenomena of the material interaction process are: - The interaction of the incident ion with the atomic electrons of the target, which is related to the electronic power; - nuclear collisions (ion incident on nuclei of the target material) which are related to nuclear stopping power. We define the total stopping power as St. _ -dEs /dx, where dEs represents the average energy lost per ion with energy Es over an elementary path dx in the target . S L is the sum of both the electronic and nuclear contributions just noted. We also define the linear energy transfer TL = dEL/dx, where dE L is the local energy transfer in the target medium over a length dx along the path of the incident ion. The concept of stopping power is different than that of linear energy transfer since the former concerns en ,, rgy lost by the incident particle without considering the volume in which the energy is lost, while the latter refers to energy absorbed by the target in a given volume . To distinguish between these two values, a

0168-583X/91/$03.50 0 1991 - Elsevier Science Publishers B.V. All rights reserved

104

B. Bennamane et al. / Optical waveguide fabrication

Table 1 Average range and r.m .s. deviation of 200 keV ions in CR 39 Ion R (WM) AR (pm) Rp (Win) AY,- (P in) H Li

34

B

2+

E1keV)

Fig. 1 . Range of H, Li, and B ions in CR39 as a function of energy.

better knowledge of the interactions process is necessary; given the actual state of knowledge and in order not to complicate the study, our calculations are based upon the assimilation of linear energy transfer and stopping power. We utilise the results of a computer program used at LEFOFI by Hejduk and Bahna [6]. It is based upon the work of Biersack and Ziegler [7]. Some basic hypotheses have been introduced : - elastic ion-atom collisions are independent of inelastic ion-electron collisions; - electronic interactions cause a negligible deviation of the ion trajectory; - nuclear diffusion can bring about a trajectory deviation; - the number of implanted ions does not change the target nature, and the statistical behavior relative to ion energy transfer in the target is assumed to be symmetric around the trajectory. Nuclear stopping power is determined from the Moli6re potential [8] . For electronic stopping power we call attention to the results of Northcliffe and Schilling [9] .

3.29 1 .67 1 .13

0.05 0.12 0.11

3.09 1 .51 1.02

0.20 0.16 0.13

Once given the stopping power, the range of an ion having initial energy En is calculated from dE R(Eo) IEaSL(E) For ions used in our experiments, fig. 1 shows the evolution of ion range as a function of energy . More rigorously, one can distinguish the range from the penetration depth; in effect, the ion can undergo directional changes which preclude considering the trajectory rectilinear. Experimentally, one cannot access the actual R but a projected path Rp can be determined given the initial direction of the incident ion. Further, since energy losses are statistical in nature, the range undergoes fluctuations leading to the distribution shown in fig. 2. From the distribution one can define the most probable range, the average range, and standard deviation. The results are given in table 1. As an example fig. 3. gives the evolution of TL versus the projected range Rp for protons as well as Li and B ions having initial energy of 250 keV. We have also used y irradiation in our experiments. The irradiation took place at SIDR (CEA-Fontenay aux Roses), and used a cobalt 60 source delivering a dose rate of 3 x 103 Gy h- ) . The material was irradiated for 18 to 670 h leading to a total dose of 0.54 x 10 5 to 2 x 106 Gy. dEM (teVAao)

Fig. 2. Ion range distribution of a monoenergetic beam of 300 keV protons.

Fig. 3. Linear energy transfer as a function of depth: 250 keV H, Li, and B ions .

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B. Bennamane et al. / Optical waueguide fabrication

Wrsdiated Sample) Light beam

U,_ _, ~tUnirradiated Semple)

Fig. 4. Schematic of the method used to measure 8n in the case of y irradiation. 2.3. Refractive index measurements

The surface indicies are measured by two classical methods : limiting angle of refraction and Brewster angle. We were also interested in the refractive index profile of the guide whenever we could measure it . The wavelength used was An = 0.6328 Wm. In the case of multimode waveguides (see section 3) the determination of index profile is possible from the measured effective index distribution by inversion of the WKB approximation of the characteristic equation . White's method [10], used here, recovers the profile by approximating it with straight line segments and by computing the depths where the refractive index of the guiding is equal to one of the effective indicies . However, in contrast with White et al ., we measured the surface index rather than estimating it recursively from a test of minimum curvature. After irradiation the effective indices of the waveguide modes in each sample were determined by means of the well known "m lines" method [11] . In our experiments, only the TE modes were excited because of the polarization of the incident light . The glass prism couplers had a refractive index n P= 1.738 . The precision of effective index measurements was about 2 X 10 -3 . Finally, the small index variations 8n due to the y irradiations were measured by an interferometric method (fig. 4) which determined Sn =xAlie

(2)

where e is the sample thickness, A is the wavelength used, i is the interfringe and x the displacement of the fringes. In the less sensitive arrangement, the technique has a precision of 10 -4 . In our experiments the achromatic raywas displaced toward the slit having the irradiated sample, indicating a positive Sn .

5 X 10'5 ions cm - Z. It supports six modes, which permits us to calculate the index profile (fig. 6). The implantation distribution from fig. 2 shows an absence of implanted ions at the beginning of the range. Almost all ions are implanted at a depth near the most probable range RS = 4.2 Am . Significantly, the refractive index increase has its maximum at the surface of the guide, with no other peaks appearing down range. Consequently, the index modification cannot be attributed to the presence of implanted ions. On the other hand, a comparison of the refractive index profile (fig . 6a) with the linear energy transfer curve (fig. 6b) shows that the two are very similar. For the protons used here, it appears therefore that the increase in the refractive index can be attributed to structural modifications that are linked to the deposition of ion energy [12], even if, as we will see below, it depends en the method of depositing energy . So far we have only considered proton irradiation. The generalization to other ions cannot be made a priori . 3.2. Irradiation with different ions

We have made a large number of measurements that permit us to follow the evolution of the surface index n s when, for a given ion, the energy and fluence vary. Fig. 7 permits a comparison of the surface index evolution for irradiation by protons as well as Li and B ions of energy 100 keV, when the fluences varies from 10'3 to 5 X 10'4 ions cm -Z. An index variation 8ns = 0.15 is achieved with boron ions . In fig. 8 we have plotted the evolution of the surface index as a function of implantation energy for several proton fluences. The curves show that there is very little influence due to proton energy in this domain . The samples irradiated by H, Li, and B permit both multimode and monomode optical guidance in certain cases . In table 2 we summarize the results obtained . 3.3. Evaluation of guide attenuation

It is also interesting to evaluate the attenuation of the guides, which have losses of 2 dB cm'. From fig. 9 IONS

3. Experimental results 3.1. Index profile

We determined the index profile for a guide created (fig . 5) with a 300 keV proton beam having a fluence of

Fig. 5. Implanted guide.

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B. Bennamane et al. / Optical waueguide fabrication

Fig. 6. (a) Guide index profiles using 300 keV proton irradiation. (b) Linear energy transfer as a function of depth (300 keV protons, target CR39) : duc to electronic collisions, ------ : due to nuclear collision.

one notes that attenuation varies weakly with the ion energy used to make the guide when the flux remains below 10 14 ions cm -2 ; above that, attenuation levels become prohibitively high since they exceed 5 dB cm -1 for proton energies of 300 keV . This problem needs further studies in order to obtain attenuation values compatible with optical guides . 3.4. Deposited energy density considerations t

to

too

Fl- (1013ion .em2)

Here we examine the link between volume energy density and the resulting surface index variation . We have calculated the average deposited energy density D in a superficial substrate layer, arbitrarily chosen to

Fig. 7. Surface index variation vs fluence for various ions at 100 keV.

Reftxtive index 1.56 0 1 .54 .

1015 H+/cm2

5.1014 H"/cm2

1 .52 ____________

10--H"/cm2

A_ __ _ _____ ,~

o

1 .50

5 . IOt3 H./c.2 1013 H*/cm2

100

200 E (keV)

300

Fig. 8 . Surface index as a function of proton energy for various fluences .

B. Bennamane et ai. / Opticai waueguide fabrication

107

have a thickness of A 0/4. For an ion having initial energy E0 , the density D is given by D,( E0) -

50

100

200 Energy (keV)

300

Refrxetive index .f asw.v ~,1 9 ~lp v

c=.i~ aou v -i -

:d uaw" "1 =ov.

Fig . 10 . Evolution of the refractive index of CR39 vs D, for three ions . Index vxnxtion (&n)

.r . v.

0 .04

i

. u"

0.02

11

0 .01

1

I

0.1

(x))

dx,

(3)

..

Our experiments establish that optical guides can be fabricated in polymers [1,14] by direct implantation f H, Li, v shwn d B. Whae o that index varitone ons Sri were, for our experimental conditions, linked to material structural modifications resulting from the

ot.+AW.wn-0~

1

(A/4 dE ( dx

4. Conclusion

/1 '

tom,--.-----u,-'"r~'

0

where F is the fluénce and (dE/dxXx) is the linear energy transfer at a depth x for an ion having initial energy E,) . From fig . 10, where we plot the refractive index versus D for the three ion's, two interesting points are brought to light: - the surface index increases with D for a given ion ; - the refractive index increases in proportion to the ion mass for a given D . We name this phenomenon "mass effect" . These results can be extrapolated by applying our earlier work using y irradiation [13]. We emphasize that one should be cautious in extending the y ray work to the present study since the interaction mechanisms, for equal deposited energy, are not the same for the ions and y photons, even if one considers that for the ions the interaction is essentially due to secondary electrons. However, it is interesting to examine fig. 11 in which we compare the refractive index increase obtained using y irradiation to the increase obtained using H, He, and Li beams. The Sri variations due to y photons are extremely weak but the curves confirm that the index change evidently is not related to implantation, but instead to structural modifications correlated with deposited energy. Nevertheless, the fact that, at equal D , ions implantation has greater influence on the index change than y radiation clearly shows the role of the kind of interaction on the index evolution .

Fig. 9. Attenuation of guides in CR39 as a function of proton energy, for various fluences.

1 .70

4

AF!0

loo

10

Fig . 11 . Evolution of index variation 8n as a function of deposited energy per unit volume.

Table 2 Number of modes detected according to the ion nature, energy, and fluence Ions fluences (ionscm -2 ) 1013

5x 10 13

10 14

5x10 14

10 15

H 100 (keV) 1 1

1

1

2

Li 200

300

1

1

(keV) 0

1 2 3

2 3 4

1 1 1

1

1

100

1

B 200

250

1 not realized 1 2 3

1 1 2 3

100

(keV)

150

250

not realized 0 1 0 0 1 0 1 1 0 1 1 samples clearly become yellow

108

B. Bennamane et al. /Optical waueguide fabrication

ion-material interaction, and not due to the doping effect of the implanted ions. This is corroborated by the index variation observed using y rays. Nevertheless, a "mats effect" is clearly observed when once compares the 8n obtained for H, Li, and B ions and y rays, as a function of deposited energy per unit volume. We consistently observe an increase of the refractive index, until the material becomes very lossy (high absorption). Further study is needed to understand the nature of the damage created by irradiation and the correlation between the material damage and index variations . This studies will help to optimize the fabrication of low loss optical guides. References [1) Ion Beam Modification of Insulators, eds P . Mazzoldi and G.W. Arnold (Elsevier, 1987). [21 B . Bennamane, J .L. Decossas, J. Marcou and J.C. Vareille, Optical guides in CR39 irradiated by ion beams, SPIE Micro-optics, 1014 (1988) 132.

[3) B .G . Cartwright and E. Shirk, Nucl. Instr. and Meth. 153 (1978) 457. [4) R.M. Cassou and E .V. Benton, Nuclear Track Detector 2 (1978) 173. [51 J.C. Vareille, J.L. Decossas, S. Sadaka and J .L. Teyssier, Nucl Instr. and Meth. 17 (1986) 280. 161 Z. Bahna, Thesis no. 41-88 Limoges (1988) . [7) J .P. Biersack and J .F. Ziegler, Ion Implantation Techniques (Springer, 1982) p . 171 . [8) W .D. Wilson, L.G . Haggmark and J.P. Biersack, Phys. Rev. 15 (1977) 2458. [91 L.C. Northcliffe and R.F. Schilling, Nucl. Data Tables 7 (1970)253. [101 J.M. White and P.F. Heidrich, Appl . Opt. 15 (1976) 151 . [111 P .K. Tien, R . Ulrich and J.R. Martin, Appl . Phys. 4 (1969) 291 . [121 J.C. Vareille, J.L. Decossas and J.L. Teyssier, and Rad. Prot . Dos ., 13 (1985) 49 . [131 B. Bennamane, J.L . Decossas, C . Gagnadre and J.C. Vareille, Anna]. de Phys. 14 (1989) 243. [141 J.R . Kulish, H. Franke, A. Singh, R.A. Lessard, and E.J. Knystautas, J. Appl. Phys. 63 (1988) 2517.