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Means of obtaining uniform sputtering in an ion microprobe
SURFACE SCIENCE 32 (1972) 743-747 © North-Holland Publishing Co.
M E A N S OF O B T A I N I N G U N I F O R M S P U T T E R I N G IN AN I O N M I C R O P R O B E * Received 4 February 1972 One of the central problems involved in utilizing ion microprobes to obtain meaningful analysis on thin film samples is the capability to tune the primary beam in such a way as to achieve spatial homogeneity. Because the thickness of thin film samples is usually small compared to the diameter of the microprobe primary beam, it is essential that the sputter crater formed on the sample be uniform in depth so that the sputtered ions comprising the secondary beam will be ejected from a well-defined level in the film. A necessary requirement for the fulfillment of this condition is that the current density in the primary beam be uniform in the plane of the film; deviation from this requirement will result in an enhanced rate of sputtering where the primary beam is most intense, and a crater of uneven depth will be produced. Most work reported to date has involved ion microprobes with primary beams possessing Gaussian current distribution**. To overcome the problems associated with the resultant bowl-shaped sputter craters, mechanical 2) or electronic 3) apertures have been utilized to prevent detection of all ions from the secondary beam except those sputtered from the center portion of the crater base. These techniques result in decreased sensitivity because the number of sputtered ions available for mass analysis in the secondary beam is correspondingly reduced4). In a recent discussion in the literature of the relative capabilities of competing designs, it has been asserted that only the type of ion microprobe utilizing a mechanical aperture is capable of obtaining meaningful composition versus depth analyses on thin filmsS). This article presents evidence that well-defined sputter craters of unifrom depth can be formed on thin samples using an Ion Microprobe Mass Analyzer ( I M M A ) without the disadvantages inherent in rastering or using apertures of the type described above. The technique employed consists of utilizing the proper combination of primary beam-optics focus settings which, in turn, requires both a tuning sample which is optically sensitive to the uniformity of the primary beam 6) and a * This work was supported by the U.S. Atomic Energy Commission. ** For an example of research using a uniform beam, see Evans and Pemslerl). 743
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R.S. BLEWER AND J. W. GUTHRIE
Fig. 1. Typicalsputter crater produced from an IMMA primary beam tuned for spatial uniformity. Its diameter is 70/am and its depth 7500/~. The current density and beam energy used to produce this crater were 0.7 mA/cm~(N~+) and 20 keV, respectively; the sputtering rate was 12.2 A/sec.
means to observe the effect of tuning parameter changes on the sample
during the sputtering process. Though the basic beam profile characteristics are determined principally by the design of the ion source and beam shaping magnets, electrostatic (Einzel) condenser and objective lenses are provided for beam focusing. Normally the lenses are adjusted so that the beam converges to a point focus at the surface of the sample. Even for a uniform beam entering the condenser lens the effects of astigmatism, aberration, field inhomogeneities, and spread in ion energy combine to produce a nonuniformly intense spot at the sample when the beam is tuned in this mode. However, if the lenses are adjusted so that the focal point of the primary beam lies not at the sample surface, but above or below it, edge effects are minimized in relation to the total area of the sample intercepted by the beam. Best results are obtained for a beam
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Fig. 2. Interferometric micrographs (60 x ) of typical sputter craters of uniform depth. These craters were produced without rastering the primary beam but exhibit such definition and uniformity that the use of apertures to eliminate ions sputtered from regions near the crater edge is unnecessary.
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whose focal point lies below the sample surface (virtual focus) rather than above it to minimize space charge effects in the beam. This has been achieved when increasing the objective lens voltage results in a spot of smaller size. Using the beam tuning techniques described, a series of sputter craters have been produced on thin films deposited on polished sapphire substrates. Interferometric analysis and scanning electron microscopy have then been used to determine the characteristics of the craters formed. A typical sputter crater in an erbium thin film sample is exhibited in the scanning electron micrograph in fig. 1. It is well defined, circular in shape, and uniform in depth. The sputter front has proceeded uniformly into the sample while steep crater walls have been maintained at the periphery. A more sensitive measure of the depth profile of a typical sputter crater is obtained by viewing the sample using microscopic interferometry as in fig. 2. The parallel interference fringes are offset (but parallel) in regions of the film onto which the primary beam has been incident. One integral fringe offset would represent a depth of 2730 A in a sputter crater. In fig. 2, the sputter craters pictured represent depressions of 2180 A depth. The steep walls and flat bottoms of the craters are even more evident than in fig. 1. Note also that the bottom surface of the craters is optically flat and remains parallel to the film surface. In experiments to date, we have observed uniform craters for beam diameters ranging from 50-300 I~m, sizes which (for the beam current normally used) result in a sputtering rate that is convenient for analyzing a variety of thin metal films. The size of the spot can be controlled by adjusting the strength of the electrostatic condenser and objective lenses while maintaining the sample-focal point relationships described previously. In summary, it has been shown that achievement of uniform current density in the primary beam of the Ion Microprobe Mass Analyzer is a matter of proper beam tuning and that, for such a beam normally incident to the analysis sample, right circular sputter craters of uniform depth can be routinely attained without rastering or the insertion of apertures between the sample and secondary analysis system. This allows utilization of Ion Microprobe Mass Analyzers for thin film mass analysis without the need to limit the inherent detection sensitivity available in this type of instrument. The assistance of J. K. Maurin in performing the scanning electron microscopy is gratefully acknowledged. R. S. BLEWER and J. W. GUTHRIE
Applied Research Division, Sandia Laboratories, Albuquerque, New Mexico 87115, U.S.A.
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References 1) C. A. Evans, Jr. and J. P. Pemsler, Anal. Chem. 42 (1970) 1060. 2) J. M. Rouberol, J. Guerne, P. Deschamps, J. P. Dagnot and J. M. Guyon de la Berge, in: Proc. 16th Annual A S T M Conf. Mass Spec., Pittsburgh, Pa., 1968, p. 216. 3) T. A. Whatley, C. B. Slack and E. Davidson, Sixth Intern. Conf. X-Ray Optics and Microanalysis, Osaka, Japan, 1971. 4) H. Liebl, J. Mass Spectrom. Ion Phys. 6 (1971) 401. 5) A. J. Socha, Surface Sci. 25 (1971) 158. 6) J. W. Guthrie and R. S. Blewer, Rev. Sci. Instr. 43 (1972) 654.
M E A N S OF O B T A I N I N G U N I F O R M S P U T T E R I N G IN AN I O N M I C R O P R O B E * Received 4 February 1972 One of the central problems involved in utilizing ion microprobes to obtain meaningful analysis on thin film samples is the capability to tune the primary beam in such a way as to achieve spatial homogeneity. Because the thickness of thin film samples is usually small compared to the diameter of the microprobe primary beam, it is essential that the sputter crater formed on the sample be uniform in depth so that the sputtered ions comprising the secondary beam will be ejected from a well-defined level in the film. A necessary requirement for the fulfillment of this condition is that the current density in the primary beam be uniform in the plane of the film; deviation from this requirement will result in an enhanced rate of sputtering where the primary beam is most intense, and a crater of uneven depth will be produced. Most work reported to date has involved ion microprobes with primary beams possessing Gaussian current distribution**. To overcome the problems associated with the resultant bowl-shaped sputter craters, mechanical 2) or electronic 3) apertures have been utilized to prevent detection of all ions from the secondary beam except those sputtered from the center portion of the crater base. These techniques result in decreased sensitivity because the number of sputtered ions available for mass analysis in the secondary beam is correspondingly reduced4). In a recent discussion in the literature of the relative capabilities of competing designs, it has been asserted that only the type of ion microprobe utilizing a mechanical aperture is capable of obtaining meaningful composition versus depth analyses on thin filmsS). This article presents evidence that well-defined sputter craters of unifrom depth can be formed on thin samples using an Ion Microprobe Mass Analyzer ( I M M A ) without the disadvantages inherent in rastering or using apertures of the type described above. The technique employed consists of utilizing the proper combination of primary beam-optics focus settings which, in turn, requires both a tuning sample which is optically sensitive to the uniformity of the primary beam 6) and a * This work was supported by the U.S. Atomic Energy Commission. ** For an example of research using a uniform beam, see Evans and Pemslerl). 743
744
R.S. BLEWER AND J. W. GUTHRIE
Fig. 1. Typicalsputter crater produced from an IMMA primary beam tuned for spatial uniformity. Its diameter is 70/am and its depth 7500/~. The current density and beam energy used to produce this crater were 0.7 mA/cm~(N~+) and 20 keV, respectively; the sputtering rate was 12.2 A/sec.
means to observe the effect of tuning parameter changes on the sample
during the sputtering process. Though the basic beam profile characteristics are determined principally by the design of the ion source and beam shaping magnets, electrostatic (Einzel) condenser and objective lenses are provided for beam focusing. Normally the lenses are adjusted so that the beam converges to a point focus at the surface of the sample. Even for a uniform beam entering the condenser lens the effects of astigmatism, aberration, field inhomogeneities, and spread in ion energy combine to produce a nonuniformly intense spot at the sample when the beam is tuned in this mode. However, if the lenses are adjusted so that the focal point of the primary beam lies not at the sample surface, but above or below it, edge effects are minimized in relation to the total area of the sample intercepted by the beam. Best results are obtained for a beam
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Fig. 2. Interferometric micrographs (60 x ) of typical sputter craters of uniform depth. These craters were produced without rastering the primary beam but exhibit such definition and uniformity that the use of apertures to eliminate ions sputtered from regions near the crater edge is unnecessary.
746
R . S . B L E W E R A N D J. W . G U T H R I E
whose focal point lies below the sample surface (virtual focus) rather than above it to minimize space charge effects in the beam. This has been achieved when increasing the objective lens voltage results in a spot of smaller size. Using the beam tuning techniques described, a series of sputter craters have been produced on thin films deposited on polished sapphire substrates. Interferometric analysis and scanning electron microscopy have then been used to determine the characteristics of the craters formed. A typical sputter crater in an erbium thin film sample is exhibited in the scanning electron micrograph in fig. 1. It is well defined, circular in shape, and uniform in depth. The sputter front has proceeded uniformly into the sample while steep crater walls have been maintained at the periphery. A more sensitive measure of the depth profile of a typical sputter crater is obtained by viewing the sample using microscopic interferometry as in fig. 2. The parallel interference fringes are offset (but parallel) in regions of the film onto which the primary beam has been incident. One integral fringe offset would represent a depth of 2730 A in a sputter crater. In fig. 2, the sputter craters pictured represent depressions of 2180 A depth. The steep walls and flat bottoms of the craters are even more evident than in fig. 1. Note also that the bottom surface of the craters is optically flat and remains parallel to the film surface. In experiments to date, we have observed uniform craters for beam diameters ranging from 50-300 I~m, sizes which (for the beam current normally used) result in a sputtering rate that is convenient for analyzing a variety of thin metal films. The size of the spot can be controlled by adjusting the strength of the electrostatic condenser and objective lenses while maintaining the sample-focal point relationships described previously. In summary, it has been shown that achievement of uniform current density in the primary beam of the Ion Microprobe Mass Analyzer is a matter of proper beam tuning and that, for such a beam normally incident to the analysis sample, right circular sputter craters of uniform depth can be routinely attained without rastering or the insertion of apertures between the sample and secondary analysis system. This allows utilization of Ion Microprobe Mass Analyzers for thin film mass analysis without the need to limit the inherent detection sensitivity available in this type of instrument. The assistance of J. K. Maurin in performing the scanning electron microscopy is gratefully acknowledged. R. S. BLEWER and J. W. GUTHRIE
Applied Research Division, Sandia Laboratories, Albuquerque, New Mexico 87115, U.S.A.
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747
References 1) C. A. Evans, Jr. and J. P. Pemsler, Anal. Chem. 42 (1970) 1060. 2) J. M. Rouberol, J. Guerne, P. Deschamps, J. P. Dagnot and J. M. Guyon de la Berge, in: Proc. 16th Annual A S T M Conf. Mass Spec., Pittsburgh, Pa., 1968, p. 216. 3) T. A. Whatley, C. B. Slack and E. Davidson, Sixth Intern. Conf. X-Ray Optics and Microanalysis, Osaka, Japan, 1971. 4) H. Liebl, J. Mass Spectrom. Ion Phys. 6 (1971) 401. 5) A. J. Socha, Surface Sci. 25 (1971) 158. 6) J. W. Guthrie and R. S. Blewer, Rev. Sci. Instr. 43 (1972) 654.