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Ion beams in optics—an introduction
Ion beams in optics - an introduction P. D. TOWNSEND Physics Dept, University of Sussex, Brighton
This article discusses effects, uses and accuracy of ion beams in glass polishing. Totally new horizons in lens design have been opened by ion bombardment. The technique has the inherent possibility of producing variations in refractive index across a surface and a highly polished surface in the same specimen. Controlled manufacture of aspheric lenses is possible and is cheaper and faster than conventional methods. Removal of residual sleeks from conventional polishing are reported. As beam energy increases, surface penetration increases and ion implantation is possible. Effects of ion beams on insulators are discussed and control of optical absorption and luminescence, as well as chemical stability and conductivity, is possible. The author concludes by pointing out the great scope for applications of irradiation damage in glass.
IT MAY BE MUCH SIMPLER to design an Optical SYStern than to make it. Glass making and polishing have developed over the centuries as highly skilled techniques demanding materials that will polish and remain stable. Totally new horizons in lens design have now been opened by a major new technique of ion bombardment. This has the inherent possibilities of producing variations in refractive index across a surface, shaping the glass into any chosen curve and producing a highly polished surface-all on the same specimen. The ‘new’ skill is not in fact so new, as many of the details have been described in the literature of radiation damage, but the application and relevance to glasses has only recently been appreciated. ION BEAM EFFECTS
The changes that can be produced are of two types. These are displacement of atoms from their normal environment and changes related to electronic energy levels. Because much more energy is required in the former, there are fewer examples of such effects in common use. One outstanding is the photographic process in which excitation of particular atoms leads to an instability, which allows atoms to migrate. More energetic changes with X or gamma-rays were used as early as 1923 to change the quality of sapphires and diamonds. This only involved rearrangement of the electrons so the effects were reversible with daylight. Permanent colour changes and refractive index changes became commonplace with the development of reactors; for example in a reactor glass turns brown. This may not be as exciting as the production of blue or green diamonds but these have been made and involve the same physical principles. For most purposes reactors are too crude as only bulk properties can be altered and there are troubles
with induced radioactivity. Both these effects can be avoided with ion or electron accelerators whilst still making the desired property change. The operating conditions of accelerators are sufficiently flexible that both the spatial position of the beam and the irradiation time can be varied to achieve any desired result. The energy transfer from the incoming ion beam to the glass atoms can be visualised as a series of billiard ball type collisions. Maximum energy transfer in such a head on collision is given by E = 4EinM,Mz/(M,
+ Mz)’
where M, and Ma are the masses of the ion and lattice atom. For example a 1OkeV proton on striking a silicon atom transfers 1.33keV. The process becomes much more efficient with a closer match of the masses and a 1OkeV argon ion would transfer 9. ‘7keV. This expression is appropriate in the low energy range of ion bombardment where the hard sphere model is relevant. Because this is only the upper limit of the energy transferred most atoms receive far less energy individually, but the number receiving more than the displacement energy -25keV may be large. The net result is that very many atoms become excited and eventually relax back into their original site, some atoms change their charge state and an important few are actually moved from their original position. Considerable momentum will travel back towards the surface and this will produce sputtering of atoms out of the material. The number of atoms ejected per bombarding ion can be as high as ten. For normal incidence of 15keV argon ions on glass it is about three. Several factors influence yield. Irradiation at oblique incidence obviously deposits far more energy near the surface and greatly increases the number of ejected atoms. With simple accelerators operating at say
Optics
Technology
May 1970
65
15keV beam currents ranging from nA to mA per square centimetre are typical. This corresponds to glass removal rates as low as 5 X 10-s A/set and as high as 5A/set. (The very low rate would unfortunately require an excellent vacuum system, less than lo-lo torr to remove atoms faster than the surface is contaminated by the stray gas). Beam focusing and accelerator refinements could greatly increase this rate over a large area and certainly very high current densities are obtainable in limited regions.
GLASS POLISHING For glass polishing or contouring this removal of atoms from the surface is the desired property. Because we can alter the beam current and focus to spots as small as a micron, this gives us a very selective tool for removing glass on a fine scale. Whether we choose to call this glass polishing or machining is a semantic problem which depends on the scale of the operation. Although the process is a simple function of the beam energy and type of ion used we do not necessarily increase the removal rate by increasing the energy for two reasons. Firstly the range increases so that only a small fraction of the energy is deposited in the surface region, and secondly the mechanism of energy loss changes from the hard sphere type collisions to electronic excitation at high (MeV) energies. Within a simple apparatus it is thus possible to switch from coarse surface shaping to fine-grade polishing merely by altering the beam conditions. Without disturbing the sample it should also be feasible to monitor the sample and hence automate the process by suitable feedback. A step towards this has been made by Schroeder, Bashkin & Nester1 in an experiment where they arranged that the specimen could be irradiated and also form one arm of a Twyman Green interferometer, to monitor the surface. At high beam currents, the specimen heated sufficiently to distort the interference pattern, but at lower currents or by turning the beam off reliable patterns could be obtained throughout irradiation. The most exciting prospect this offers is the controlled manufacture of aspheric surfaces. Narodny 8~ Tarasevich2 started with a spherical surface and figured on it a f/6,lOcm paraboloid by simply changing the time of irradiation at different parts of the surface. The greatest depth of material removed was about five wavelengths for the parabolid chosen. The irradiation time involved was 11 hours. This could, they claim, have been reduced to 1.5 hours. Even at this early stage of development these times are certainly competitive with conventional systems of polishing. An important consideration in this work is the final polish on the sample. Both the preceding papers and that of Meinel et al3 mention that the interferogram suggested a surface as smooth as, or better than, the original. However in the final stages of polishing an interferogram is too crude to show the fine polishing sleeks and marks which cause surface scatter. With the more sensitive techniques of electron microscopy or phase contrast microscopy one finds that ion beam irradiation can produce surfaces which are comparable with glass polished in the normal way. Our 66
Optics
Technology
May 1970
own work has been even more encouraging and shown that the residual sleeks, from conventional polishing, have been removed with an ion beam.
ION IMPLANTATION When we increase the energy of the ion beam a complete new range of phenomena appear. The sputtering rate drops at high energies (because the energy loss mechanism has changed), but penetration depth is increased. For example in silica a 50keV xenon ion might travel 0.03~ but a 2MeV proton could travel 56~. These may be extreme cases but they illustrate that, with conditions readily obtainable with simple ‘off the shelf’ type accelerators, we can influence a large depth of surface. The uses of the ion beam now become enormous. A change in the refractive index will be produced in the irradiated material which will be greatest where most damage is done. Because we can control the range, and most of the damage is done near the end of the range, we can induce any chosen refractive index profile with depth. Additional control is possible because we also control the type of ion which is implanted. In this sense the literature of ion irradiations may be misleading because most people commence with the inert gases, but ion sources from a large number of elements have been used and no element need be ignored. Similarly if we do not wish to implant any new ions we can use a mixture of ions which already occur in the glass. It may also be apparent that because we are not making the glass under the normal thermal conditions we are not restricted by the conventional solubility and stoichiometry problems. We can therefore make a very exotic glass in the surface region which is quite different from the bulk material. The effects of this could be that we can use a reactive bulk material which is then coated with a stable glass. Alternatively we could induce a compressive stress in the surface to improve the strength. Spatial control of the refractive index changes produced by ion beams has been used very skilfully by Schineller, Flam & Wilmot? to produce optical waveguides. A discontinuity in the refractive index causes reflection at the interface and if there are two such boundaries then the light will be trapped between them. Schineller et al irradiated silica with Hs and Hf ions. Since there were two types of ions, and thus two ranges, they each produced two boundary regions which trapped light between them. By irradiation of selected parts of the silica they were able to write the path of the waveguide and even to make a successful waveguide coupler. Optical waveguides.
It is clear that control of the refractive index over a surface in all dimensions, offers a further degree of freedom in lens design which is not obtainable with simple bloomings. ELECTRICAL
CHANGES
The second effect of ion bombardment of insulators is to change the electrical properties by adding levels to the valence-conduction band energy gap, or by distorting the shapes of the bands. This possibility was soon realised in semiconductors and ion implantation to form various devices is now a standard practice. The direct effects of controlling the extra energy levels
between the bands are that we can control the optical absorption, luminescence, chemical stability and electrical conductivity. The quantity of material implanted must depend on the ion beam energy and the way in which it is accepted by the host. If the ion is not implanted in just the way we want, we can produce too many absorption bands, then the whole range of solid-state tricks can be used. These include thermal annealing, optical bleaching, additional ionisation or electrically controlled dif fusion as means of readjusting either the atoms or the population of the energy levels. This application of ion beams is going to be of most use when dealing with very specific problems. In producing the effects one is limited by the total number of atoms which can be implanted in the near surface region. This is a limitation if one is intending to make a coloured glass as the colour will depend on the total number of absorbers in the direction of the light beam. The fine detail involved in preparing a glass with a particular energy band structure is going to be difficult but the success of similar work with semiconducting materials and alkali halides suggests that control is possible as there are sufficient techniques available to overcome the problems. This feature of irradiation of glass does not have the simplicity and direct applicability of sputtering and there is still great scope for imaginative applications of irradiation damage in glass. To illustrate this,one might consider making a three-dimensional memory with enormous storage capability by forming a network of colour centres in the glass. The presence or absence of an electron in the centre would give the requisite binary type coding. This might then be viewed by changes in the concentration of colour centres. Access to such a store would probably be the greatest problem!
CONC LUSION This introduction shows that there is a clear case for considering ion beams as a machining or polishing tool, and in the case of aspheric surfaces this can be cheaper and faster than conventional methods. Once the technique makes its full impact the demand for the method will soar. Of the second type of application little can be said in a general article but specific problems do exist which could be solved with ion beams. This art is still in its infancy but the successes so far suggest that it could soon form an integral part of optics technology.
REFERENCES Narodny, L. H.& Tarasevich, M. Parabaloid figured by ion bombardment. App. Opt. vol. 6. 1967. p. 2010 Schroeder, J. B., Bashkin, S.& Nester, J. F. Ionic polishing of optical surfaces. Appl. Opt. vol. 5. 1966. p. 1031 Meinel, A. B. Bashkin S.& Loomis, D. A. Controlled figuring of optical surfaces by energetic ion beams. Appl. Opt.vol. 4.1965. p. 1674 Schineller, E. R., Flam, R. P.& Wilmot, D. W. Optical waveguides formed by proton irradiation of fused silica. J. Opt. Sot. Am. vol. 58.1968. p. 1171 A review of ionic polishing of quartz and fused silica by R. G. Wilson was published in Optics Technology,vol 2.197O.pp. 19-26.
Optics
Technology
May 1970
67
This article discusses effects, uses and accuracy of ion beams in glass polishing. Totally new horizons in lens design have been opened by ion bombardment. The technique has the inherent possibility of producing variations in refractive index across a surface and a highly polished surface in the same specimen. Controlled manufacture of aspheric lenses is possible and is cheaper and faster than conventional methods. Removal of residual sleeks from conventional polishing are reported. As beam energy increases, surface penetration increases and ion implantation is possible. Effects of ion beams on insulators are discussed and control of optical absorption and luminescence, as well as chemical stability and conductivity, is possible. The author concludes by pointing out the great scope for applications of irradiation damage in glass.
IT MAY BE MUCH SIMPLER to design an Optical SYStern than to make it. Glass making and polishing have developed over the centuries as highly skilled techniques demanding materials that will polish and remain stable. Totally new horizons in lens design have now been opened by a major new technique of ion bombardment. This has the inherent possibilities of producing variations in refractive index across a surface, shaping the glass into any chosen curve and producing a highly polished surface-all on the same specimen. The ‘new’ skill is not in fact so new, as many of the details have been described in the literature of radiation damage, but the application and relevance to glasses has only recently been appreciated. ION BEAM EFFECTS
The changes that can be produced are of two types. These are displacement of atoms from their normal environment and changes related to electronic energy levels. Because much more energy is required in the former, there are fewer examples of such effects in common use. One outstanding is the photographic process in which excitation of particular atoms leads to an instability, which allows atoms to migrate. More energetic changes with X or gamma-rays were used as early as 1923 to change the quality of sapphires and diamonds. This only involved rearrangement of the electrons so the effects were reversible with daylight. Permanent colour changes and refractive index changes became commonplace with the development of reactors; for example in a reactor glass turns brown. This may not be as exciting as the production of blue or green diamonds but these have been made and involve the same physical principles. For most purposes reactors are too crude as only bulk properties can be altered and there are troubles
with induced radioactivity. Both these effects can be avoided with ion or electron accelerators whilst still making the desired property change. The operating conditions of accelerators are sufficiently flexible that both the spatial position of the beam and the irradiation time can be varied to achieve any desired result. The energy transfer from the incoming ion beam to the glass atoms can be visualised as a series of billiard ball type collisions. Maximum energy transfer in such a head on collision is given by E = 4EinM,Mz/(M,
+ Mz)’
where M, and Ma are the masses of the ion and lattice atom. For example a 1OkeV proton on striking a silicon atom transfers 1.33keV. The process becomes much more efficient with a closer match of the masses and a 1OkeV argon ion would transfer 9. ‘7keV. This expression is appropriate in the low energy range of ion bombardment where the hard sphere model is relevant. Because this is only the upper limit of the energy transferred most atoms receive far less energy individually, but the number receiving more than the displacement energy -25keV may be large. The net result is that very many atoms become excited and eventually relax back into their original site, some atoms change their charge state and an important few are actually moved from their original position. Considerable momentum will travel back towards the surface and this will produce sputtering of atoms out of the material. The number of atoms ejected per bombarding ion can be as high as ten. For normal incidence of 15keV argon ions on glass it is about three. Several factors influence yield. Irradiation at oblique incidence obviously deposits far more energy near the surface and greatly increases the number of ejected atoms. With simple accelerators operating at say
Optics
Technology
May 1970
65
15keV beam currents ranging from nA to mA per square centimetre are typical. This corresponds to glass removal rates as low as 5 X 10-s A/set and as high as 5A/set. (The very low rate would unfortunately require an excellent vacuum system, less than lo-lo torr to remove atoms faster than the surface is contaminated by the stray gas). Beam focusing and accelerator refinements could greatly increase this rate over a large area and certainly very high current densities are obtainable in limited regions.
GLASS POLISHING For glass polishing or contouring this removal of atoms from the surface is the desired property. Because we can alter the beam current and focus to spots as small as a micron, this gives us a very selective tool for removing glass on a fine scale. Whether we choose to call this glass polishing or machining is a semantic problem which depends on the scale of the operation. Although the process is a simple function of the beam energy and type of ion used we do not necessarily increase the removal rate by increasing the energy for two reasons. Firstly the range increases so that only a small fraction of the energy is deposited in the surface region, and secondly the mechanism of energy loss changes from the hard sphere type collisions to electronic excitation at high (MeV) energies. Within a simple apparatus it is thus possible to switch from coarse surface shaping to fine-grade polishing merely by altering the beam conditions. Without disturbing the sample it should also be feasible to monitor the sample and hence automate the process by suitable feedback. A step towards this has been made by Schroeder, Bashkin & Nester1 in an experiment where they arranged that the specimen could be irradiated and also form one arm of a Twyman Green interferometer, to monitor the surface. At high beam currents, the specimen heated sufficiently to distort the interference pattern, but at lower currents or by turning the beam off reliable patterns could be obtained throughout irradiation. The most exciting prospect this offers is the controlled manufacture of aspheric surfaces. Narodny 8~ Tarasevich2 started with a spherical surface and figured on it a f/6,lOcm paraboloid by simply changing the time of irradiation at different parts of the surface. The greatest depth of material removed was about five wavelengths for the parabolid chosen. The irradiation time involved was 11 hours. This could, they claim, have been reduced to 1.5 hours. Even at this early stage of development these times are certainly competitive with conventional systems of polishing. An important consideration in this work is the final polish on the sample. Both the preceding papers and that of Meinel et al3 mention that the interferogram suggested a surface as smooth as, or better than, the original. However in the final stages of polishing an interferogram is too crude to show the fine polishing sleeks and marks which cause surface scatter. With the more sensitive techniques of electron microscopy or phase contrast microscopy one finds that ion beam irradiation can produce surfaces which are comparable with glass polished in the normal way. Our 66
Optics
Technology
May 1970
own work has been even more encouraging and shown that the residual sleeks, from conventional polishing, have been removed with an ion beam.
ION IMPLANTATION When we increase the energy of the ion beam a complete new range of phenomena appear. The sputtering rate drops at high energies (because the energy loss mechanism has changed), but penetration depth is increased. For example in silica a 50keV xenon ion might travel 0.03~ but a 2MeV proton could travel 56~. These may be extreme cases but they illustrate that, with conditions readily obtainable with simple ‘off the shelf’ type accelerators, we can influence a large depth of surface. The uses of the ion beam now become enormous. A change in the refractive index will be produced in the irradiated material which will be greatest where most damage is done. Because we can control the range, and most of the damage is done near the end of the range, we can induce any chosen refractive index profile with depth. Additional control is possible because we also control the type of ion which is implanted. In this sense the literature of ion irradiations may be misleading because most people commence with the inert gases, but ion sources from a large number of elements have been used and no element need be ignored. Similarly if we do not wish to implant any new ions we can use a mixture of ions which already occur in the glass. It may also be apparent that because we are not making the glass under the normal thermal conditions we are not restricted by the conventional solubility and stoichiometry problems. We can therefore make a very exotic glass in the surface region which is quite different from the bulk material. The effects of this could be that we can use a reactive bulk material which is then coated with a stable glass. Alternatively we could induce a compressive stress in the surface to improve the strength. Spatial control of the refractive index changes produced by ion beams has been used very skilfully by Schineller, Flam & Wilmot? to produce optical waveguides. A discontinuity in the refractive index causes reflection at the interface and if there are two such boundaries then the light will be trapped between them. Schineller et al irradiated silica with Hs and Hf ions. Since there were two types of ions, and thus two ranges, they each produced two boundary regions which trapped light between them. By irradiation of selected parts of the silica they were able to write the path of the waveguide and even to make a successful waveguide coupler. Optical waveguides.
It is clear that control of the refractive index over a surface in all dimensions, offers a further degree of freedom in lens design which is not obtainable with simple bloomings. ELECTRICAL
CHANGES
The second effect of ion bombardment of insulators is to change the electrical properties by adding levels to the valence-conduction band energy gap, or by distorting the shapes of the bands. This possibility was soon realised in semiconductors and ion implantation to form various devices is now a standard practice. The direct effects of controlling the extra energy levels
between the bands are that we can control the optical absorption, luminescence, chemical stability and electrical conductivity. The quantity of material implanted must depend on the ion beam energy and the way in which it is accepted by the host. If the ion is not implanted in just the way we want, we can produce too many absorption bands, then the whole range of solid-state tricks can be used. These include thermal annealing, optical bleaching, additional ionisation or electrically controlled dif fusion as means of readjusting either the atoms or the population of the energy levels. This application of ion beams is going to be of most use when dealing with very specific problems. In producing the effects one is limited by the total number of atoms which can be implanted in the near surface region. This is a limitation if one is intending to make a coloured glass as the colour will depend on the total number of absorbers in the direction of the light beam. The fine detail involved in preparing a glass with a particular energy band structure is going to be difficult but the success of similar work with semiconducting materials and alkali halides suggests that control is possible as there are sufficient techniques available to overcome the problems. This feature of irradiation of glass does not have the simplicity and direct applicability of sputtering and there is still great scope for imaginative applications of irradiation damage in glass. To illustrate this,one might consider making a three-dimensional memory with enormous storage capability by forming a network of colour centres in the glass. The presence or absence of an electron in the centre would give the requisite binary type coding. This might then be viewed by changes in the concentration of colour centres. Access to such a store would probably be the greatest problem!
CONC LUSION This introduction shows that there is a clear case for considering ion beams as a machining or polishing tool, and in the case of aspheric surfaces this can be cheaper and faster than conventional methods. Once the technique makes its full impact the demand for the method will soar. Of the second type of application little can be said in a general article but specific problems do exist which could be solved with ion beams. This art is still in its infancy but the successes so far suggest that it could soon form an integral part of optics technology.
REFERENCES Narodny, L. H.& Tarasevich, M. Parabaloid figured by ion bombardment. App. Opt. vol. 6. 1967. p. 2010 Schroeder, J. B., Bashkin, S.& Nester, J. F. Ionic polishing of optical surfaces. Appl. Opt. vol. 5. 1966. p. 1031 Meinel, A. B. Bashkin S.& Loomis, D. A. Controlled figuring of optical surfaces by energetic ion beams. Appl. Opt.vol. 4.1965. p. 1674 Schineller, E. R., Flam, R. P.& Wilmot, D. W. Optical waveguides formed by proton irradiation of fused silica. J. Opt. Sot. Am. vol. 58.1968. p. 1171 A review of ionic polishing of quartz and fused silica by R. G. Wilson was published in Optics Technology,vol 2.197O.pp. 19-26.
Optics
Technology
May 1970
67