Effect of Ion Implantation on the Refractive Index of Glass

Pram~i0a, Vol. 6, No 2, 1976, pp. 102-108. © Printed in India.

Effect of ion implantation on the refractive index of glass

P K BHATTACHARYA, N SARMA and A G WAGH Nuclear Physics Division, Bhabha Atomic Research Centre, Bombay 400085 MS received 8 October 1975 Abstract. Stnoies on the changes of the index of refraction in glass due to ion implantation provide an insight into the structure of amorphous ~ubstanees, besides being important for the development of techniques for the production of optical integrated circuits. Using a hea,'y ion accelerator, optically ~at samples of Pyrex and Coming borosilicate glas~ were implanted with ions of gallium and argon at various incident energies and doses. The refractive index was then measured and found to be between i-5 and 1-8 at a wavelength of 5893 A.U. The change in the refractive index was found to vary linear;y with the incident dose irrespective of the ion species. This suggests that bombardment damage is mainly responsible for the effect. The dependence of the refractive index change on the incident dose however depends strongly oa the chemical composition of the substrate glass. Keywords, Glass; refractive index; ion implantation.

1. Introduction The use of integrated optical circuits in optical communication systems (Barnoski 1973) has become a matter of active investigation in the last few years. The crucial requirement for the fabrication of optical circuits is the ability to produce narrow regions of significantly higher refractive index on the glass substrate. Melt techniques are not suitable when precise definition of the regions is required but thin film deposition has been used with some success (Tien 197t). A powerful technique that can be used to induce accurately controllable changes in the optical properties of dielectrics over depths comparable with optical or near infrared wavelengths is ion implantation (Goell etal 1972, Townsend etal 1973). In addition ion implantation appears to have a distinct advantage over thin film deposition insofar as economy, reliability and ease of production are concerned. It would therefore be interesting to examine this technique as applied to common glass and to measure the change in refractive index as a function of the various parameters of the implantation process.

2. Theory of the measurement The measurement technique employed (Abel,s 1950) gives the index of refraction of the implanted film directly by the observation of the reflectance at the Brew- ster angle (figure 1). Consider parallel unpolarised light incident at an angle, 4' which is refracted by an angle, r in passing from a medium of refractive index, no into another index, nl. The reflectance for light polarised with electric vector perpendicular to the plane of incidence is given by

102 Effect of ion implantation on the refractive &dex oJ glass 103

(a) O

1.0

LuZ BREWSTER u.U"~ ANGLE= 56.3~

o= 0.5 (b) (J Z 0

(J UJ .J U.I = O.C 0 30 60 90 ANGLE OF INCIDENCE ~(DEGREES)

Figure 1 (a). Reflectance vs. angle of incidence when light polarised in planes parallel and perpendicular to the plane of incidence is incident on glass. Figure I (b). Reflection ceefficient of a single surface of a dielectric in vacuum (nl = 1"50).

p.~ -- sin 2 ($ --r)/sin 2 (4 + r) whereas for that parallel to the plane of incidence, p, --- tan 2 (~ --r)/tan 2 (~, + r) Obviously p~j drops to zero when ~ is such that, at the Brewster angle, ~, = 4o the value of tan~($+r) ~-c~ or when tan$o=nx/no. In the case of a film of index, nl on a transparent substrate of index, n2 and if $0, $J and ~2 are the angles of incidence of light of wavelength, A polarised with electric vector in the plane of incidence in the media, no, n~ and n~ respectively, the Fresnel reflection coefficient r 1 for propagation from no to n~ is rl = tan (~o --~l)/tan (~,o + ~1). 104 P K Bhatmcharya et al

The change in the phase of the wave in traversing the thickness, dj of the film is 8 = (2rr/h) nldlcos4,1 and the Fresnel coefficient, r2for propagation from n~ to n2 may be written as r2 = tan (41 --~)/tan (~1 + ~2). Consequently the total reflectance, R is given by

R = 'rl~ + 2rlrz cos 281 + r2 ~ 1 + 2rlr 2 cos 281 + rl 2 rz ~ " When ~0 = the Brewster angle of the film, ~o + q~l ---- ~r/2 tan (~o + q~l) = leads to r 1 = 0. Then the value of r~ = tan (~0 --4,.,)/tan (4~o + ~.,) leads to a total reflectance of the film-on-substrate to r22 = tan 2 (('2 --4>0)/tan2 (,~o + 4~2).

Thus at the Brewster angle, r 1 = 0 and therefore R = r~~'. In other words, the reflectance reduces to that of the substrate at the Brewster angle, and the inhomo- geneity of the substrate does not cause any difference to the measurement (Heavens 1955). In the measurement, the angle of incidence of the light is varied till the reflected intensity is the same as that due to the substrate alone. This procedure does not require the absolute evaluation of reflection coefficients and is independent of the thickness of the film and the index of the substrate. Provided that the refractive index of the film does not deviate too much from that of the substrate, an accuracy of ± 0.5 per cent is obtained for the film refractive index for an angular accuracy of 0.5 minutes of arc in the measurement.

3. Experimental arrangement A schematic diagram of the implantation equipment is shown in figure 2. The ions are produced in a hollow cathode electron bombardment source (Bhatta- charya et al 1973)within a 400 keV Van de Graaffaccelerator. The mass selection of the dopant species and the energy analysis was obtained with a magnet with

(~,02 ( D

Figure 2. Schematicdiagram of the ion implantation equipment : A, B, C--represent diffusion pumps with liquid nitrogen trap, Q1, Q2--the quadrupoles forming a doublet, M--mass analyzing 90 ° sector magnet, X and Y are X, Y scanners, T--the target chamber and D---on line liquid nitrogen trap. Effect of ion implantation on the refractive index of glass 105

field stability better than 0" 1 per cent. A magnetic quadrupole lens system then focussed the beam to a spot size of 1 ram. diameter; this spot was then swept over the target uniformly by an elctrostatic X-Y beam deflector. The beam line and magnet chamber were evacuated by oil diffusion pumps equipped with liquid nitrogen traps to avoid backstreaming of hydrocarbon vapours. Ultraclean vacuum in the implantation chamber was achieved by pumping with a mercury diffusion pump through a liquid nitrogen trap. In addition the target was sur- rounded by a cold surface to trap all condensible vapours and contaminants. Surface smoothness and volume homogeneity of the glass before implantation are essential if the measurements of the optical properties near the surface are to be consistent. The optically fiat glass samples were examined under a microscope and only those free of strains, crazings (fine cracks) or bubbles were selected. The substrates of size 20ram × 20 mm were rubbed with lint free tissue and Teepol-300 dissolved in hot water. They were then washed in deionised water, ethyl alcohol, acetone and isopropyl alcohol respectively and dried in an upright position with hot compressed air at 60 ° C. The prepared glass samples were irradiated by gallium and argon ion beams in the energy range of 75 to I00 keV to doses ranging between 10TM and 3 × 1020 per cm ~. The dose rate was regulated by controlIing the current extracted from the ion source so that the surface of the irradiated area was not damaged significantly. The overall ion current on target during these experiments was of the order of 0.5 microamperes over a swept area of 3 cm square. Visual inspection of the implanted samples showed a region of slightly dark contrast and randomly distributed scattering centres throughout the irradiated area for doses in excess of 2 × 1013 per cm 2. The samples were mounted on the prism table of an optical spectrometer for reflectivity measurements and the reflected light intensity was measured with a photomultiplier attached to the telescope arm after removing the eyepiece. Reflectivity was measured as a function of the angle of incidence with sodiumlight (5893 A.U.) polarised in the plane of incidence of both the implanted and unimplanted samples. Assuming a uni- form lossless film, the reflectivities for the film-substrate combination and the substrate alone become equal when the angle of incidence equals the Brewster angle for the film. The uncertainty in the measurement of the refractive index was minimised by choosing the maximum projected range of the ions to be about 0" 1 micron, thus adjusting the optical thickness, i.e., the product of the refractive index and the thickness of the film, to be about one quarter of the wavelength. Semiangular width of the incident light beam was restricted to about one minute of arc and the maximum error in the measurement of the refractive index due to the least count of the measuring equipment was estimated to be less than 0.002.

4. Results

The refractive index for pyrex and coming borosilicate glass substrates, measured as a function of the volume concentration of the implanted ions is shown in figure 3. The measured changes in the refractive index exceed considerably the value of 1" 5 per cent (Primal< 1958) caused by neutron irradiation. In that work a rapid increase of refractive index was found for fused silica whic saturated at 1 "47 at 106 P K Bhattacharya et al

1.9

1.8

1.7

< A 1.6

1.S

I.C l [ ! I 10 TM 1020 2 xl020 3x1020 &x1020 ION C CONCENTRATION(ions/cm3| Figure 3. Refractive index of the implanted film vs. ionic concentration: Curve A gives no= 1"481 for unimplanted corning--borosilicate glass, and curve B gives no = t-484 for unimplanted pyrex.

a fluence of 5 x 10a9 neutrons per cm 2. In our measurements on glass, the refractive index shows no sign of saturation up to a fluence of 3 x 1015 per cm 2. A similar observation has been made in the ion bombardment of silica (Dearnaley et al 1973) up to a fluence of 1015 per cm ~. The refractive index of the implanted film increases linearly obeying a relation- ship

An=C. N where An is the change in refractive index and N is the ionic concentration. The constant of proportionality, C assumes values, 0.6 x 10-32 and 0-8 x 10-21 cm 8 for pyrex and corrting borosilicate glass respectively. It is also independent of the implanted ion species, thereby implying that chemical doping is of minor importance in the process. Effect of ion implantation on the refractive index of glass 107

5. Discussion The implanted gallium or argon ions in the energy range utilised lose their energy predominantly by nuclear collisions (Lindhard etal 1963). Consequently the displacements of the struck atoms of the substrate create a damage region surround- ing the track of the incident ion due to this collision cascade. Comparison with neutron induced effects is therefore not straightforward since the damage effects due to the neutrons are 10-v times less dense than for charged particles (Thompson 1969). The nature of the damage is also somewhat different in nature. The chemical composition of the glass used in this work were obtained from suppliers' specifications. The corning borosilicate come under the general class of soda-alumino-silicate glass and the chemical composition was 55% SiO~, 2070 A1203, 5% B203, 1% Na20, 10% MgO and other oxides; the total metallic impurity is as much as 36%. On the other hand, Pyrex glass has 80% SiO2, 2% AI.:O3, 14% B203 and 4% Na~O with about 20 per cent of metallic impurity. All silicate glasses are thought to be made up of silicon oxide tetrahedra which form a network together with other components. These basic units or micro- blocks combine only at the corners through the bridging oxygen atoms. During implantation, the incident ions displace the non-bridging oxygen ions and the consequent change in polarisability of these ions by the silicon ions contributes to the change in the refractive index to a major extent. In borosilicate glass, lhe boron, aluminium, magnesium and sodium atoms change this picture. Here the sodium atom bond energy is much less than the Si-bond and so the sodium atom is almost unrestricted in its movement in a binary glass system. Conse- quently the energy transfer of the incident ion is to sodium rather than silicon and there is a definite tendency for the sodium to leach out of the system (Carter et al 1966) and change its reflectance. In the samples implanted in this work, the sodium concentration is low and such a process may be ruled out. It is more probable that the radiation damage causes the SiO~ tetrahedra to form rings of five, seven or eight members as well as broken rings with non-bridging oxygen atoms instead of the common six member rings. Eventually the long range order of domains is completely altered. In eorning borosilicate glass, the addition of alkali atoms causes a change from a four-fold coordination (Grjotheim and Krogh-Moe 1954) of boron with oxygen to a three-fold coordination and as a result more non bridging oxygen ions are released. In pyrex glass, though the percentage of boron oxide is high, the lower concentration of alkali oxides reduces the number of non-bridging oxygen atoms for each network unit. It would therefore be reasonable to expect that the change in the refractive index specified by the constant, C is smaller in pyrex glass.

6. Conclusions

The displacement of substrate atoms appear to be the predominant cause for the change in the refractive index caused by the implantation of ions of gallium and argon up to fluences of 3 × 1020 per cm 2. No saturation of the refractive index was observed up to a concetration of 4 × 1022 per cm ~ and the effect is independent of the ion species. The variation of refractive index with dose differs by an order of magnitude between pyrex and corning glass (0.6 × 10-2~ for pyrex and 108 P K Bhattacharya et al

0' 8 × I0-2x for coming) and it is probable that the alkali atoms are responsible for structural changes that cause the variation in the index of refraction.

References Abel6s F A 1950 J. Phys. Rad. 11 310 Barnoski M K 1973 Introduction to intergrated optics (Plenum Press) 127 Bhattacharya P K, Wagh A G, Thampi N S and Sarma N 1973 Nuclear physics and solid state physics (India) I6 B 254 Carter G and Grant W A 1966 Phys. Chem. Glasses 7 94 Dearnaley G, Freeman J H, Nelson R S and Stephen J 1973 Ion implantation, Ser. defect in crystalline solid~, eds S Amelinckx, R Grevers and J Nihoul (North-Holland Pub, Amster- dam) Chapter 6, 727 Grjotheim K and Krogh-Moe J 1954 Kg. Norske Videnskab Selsk Forhandel 27 (18) 1 GoeU J E, Standley R D, Gibson W M and Rodgers J W 1972 Appl. Phys. Lett. 21 72 Heavens O S 1955 Optical properties of thin fihns (Academic Press, New York) 73 Lindhard J, Scharff M and Schiott H E 1963 Mat. Fys. Medd. 33 14 Primak W 1958 Phys. Rev. 110 1240 Thompson M W 1969 Defects and radiation damage in metals (University Press Cambridge) 143 Tien P K 1971 Applied Optics 10 2395 Townsend P D and Bayly A R 1973 .I. Phys. D 6 1115