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 I (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 indepen- dent 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 sub- strate, 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 A (~,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 signi- ficantly. 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 z < A 1.6 1.S I.C l [ ! I 10 TM 1020 2 xl020 3x1020 &x1020 ION C CONCENTRATION(ions/cm3| Figure 3.