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Model study of electron beam charge compensation for positive secondary ion mass spectrometry using a positive primary ion beam
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Applied Surface Science 254 (2008) 2708–2711 www.elsevier.com/locate/apsusc
Model study of electron beam charge compensation for positive secondary ion mass spectrometry using a positive primary ion beam Zhengmao Zhu *, Frederick A. Stevie, Dieter P. Griffis Analytical Instrumentation Facility, North Carolina State University, Raleigh, NC 27695-7531, United States Received 30 April 2007; accepted 5 October 2007 Available online 10 October 2007
Abstract A new modeling approach has been developed to assist in the SIMS analysis of insulating samples. This approach provides information on the charging phenomena occurring when electron and positive primary ion beams impact a low conductivity material held at a high positive potential. The concept of effective leakage resistance aids in the understanding of the dynamic electrical properties of an insulating sample under dynamic analysis conditions. Modeling of steady state electron beam charge compensation involves investigation of electron injection and charge drift. Using a Monte Carlo program to simulate electron injection and dc conduction calculations to predict charge drift, detailed information regarding charging phenomena can be determined. # 2007 Elsevier B.V. All rights reserved. Keywords: Charge compensation; Electron beam; Magnetic sector; SIMS
1. Introduction Secondary ion mass spectrometry (SIMS) analysis requires bombardment of a sample surface with a primary ion beam. When performing positive secondary ion SIMS analysis on insulating samples, the charge buildup resulting from the positive primary ion beam is not significantly reduced by departing secondary particles. Thus, charge can accumulate on the sample surface to a level sufficient to affect SIMS analysis. Possible remedies for this charge buildup include providing a leakage path to mitigate charge buildup and/or providing charge compensation using a charged particle beam of opposite polarity. A metal grid [1] or a continuous conductive coating on the insulator surface can provide a leakage path. In some cases, a negative primary ion beam can be used for positive secondary ion SIMS to reduce charging [2,3], but insufficient compensation and the high negative primary ion beam energy impacting on a positively charged sample render this method less than ideal. Most commonly, SIMS analysis is performed using
* Corresponding author. Current address: Semiconductor Research & Development Center, IBM 2070 Route 52 Hopewell Junction, NY 12533, United States. Tel.: +1 845 892 1365; fax: +1 845 892 6256. E-mail address: [email protected] (Z. Zhu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.10.008
positive primary ions (e.g. O2+) and an electron beam is employed for charge compensation during the analysis of insulating samples [4–6]. Despite wide spread use of electron beam charge compensation under these SIMS conditions, understanding of the mechanisms of charge compensation under these conditions is incomplete. The most widely used theoretical treatment for charging involves consideration of a current balance model, first described by Werner and co-workers [3,7]. The insulator is treated as a resistor R in parallel with a capacitor C. Under steady state conditions, the charging level DU, measured as potential difference between sample surface and sample holder, is determined by the net current I flowing through the resistor: DU ¼ I R
(1)
This simple model provides useful guidelines for charge compensation. Other researchers have identified the limitations of this classic approach and have called for more complex models [8–10]. In particular, Cazaux and Lehuede [10] questioned Werner’s use of ohm’s law and pointed out that this model is restricted to an uncoated surface, does not reveal the field inside the insulator, and does not provide the spatial distribution of surface potential. In this work, the balance model treatment for SIMS analysis of a metal-coated insulator is extended, and the role of the
Z. Zhu et al. / Applied Surface Science 254 (2008) 2708–2711
electron beam during charge compensation is quantitatively characterized. The proposed model accounts for the effects of local parameters on the electrostatic potential distribution both on and below the sample surface. Modeling efforts are focused on magnetic sector SIMS analysis of insulating samples using positive primary ions with positive sample bias. 2. Experimental SIMS analyses were performed on a 20 nm gold-coated sapphire sample (intrinsic resistivity >1E14 V cm) with a CAMECA IMS-6F magnetic sector SIMS using O2+ primary ion beam of 5.5 keV impact energy (10 kV primary high voltage, 4.5 kV sample high voltage, and 418 impact angle). Al2O2+ secondary ions were collected from a 60 mm diameter optically gated area centered on a 180 mm 180 mm rastered area with a narrow pass energy bandwidth of 10 eV. Electron beam charge compensation was performed using the Normal Incidence Electron Gun (NEG) [11], aligned coincident with the primary ion beam raster. The electron gun accelerating voltage is 2 kV and electron impact energy 6.5 keV. Sample charging was evaluated by monitoring the secondary ion energy distribution. Stopping of primary electrons in sapphire was simulated with a Monte Carlo program, Electron Flight Simulator [12]. A finite element method based program, QuickField 5.3 [13], was employed to numerically solve Laplace’s equation to study the electrostatic parameters involved in the electron beam charge compensation process. 3. Results and discussion The electron beam has two roles in SIMS electron beam charge compensation: to provide leakage path for charge buildup and to neutralize surface charge. Materials bombarded by an electron beam can have higher conductivity than the intrinsic materials due to electron beam induced conductivity (EBIC). EBIC has been recognized as an important factor in electron beam charge compensation during SIMS analysis [4,14]. Based on the previous current balance model, a resistance of 1E15 V in the case of sapphire would require a net current flowing through the resistor to be less than 0.01 pA in order to prevent more than 10 V charging. The EBIC effect must be taken into account while deriving the current balance equation for metal-coated insulators: DU ¼ I Re
2709
electron beam currents. Hence we have DU ¼ Re I i Re I e
(3)
where Ii and Ie are the ion beam and the electron beam current impinging onto the ion beam rastered area. A linear relationship between charging level and ion beam current is expected when electron beam is held constant. This relationship has been observed in the analysis of many insulating samples including the sapphire sample used here (Fig. 1). This suggests that the sample under constant electron bombardment behaves as an ohmic material. Both Re and Ie can be extracted from linear fit of the charging level and primary ion beam current data. The EBIC effect is apparent: the measured effective leakage resistance of sapphire under 2.5 A/m2 electron beam bombardment is 7E8 V, which is much lower than the estimated intrinsic value which is on the order of 1E15 V. A correlation between the effective leakage resistance and the electron beam current into the primary ion beam rastered area can be further revealed by measuring Re under different electron gun emission current. Fig. 2 shows effective leakage resistance is inversely proportional to electron beam current. This is consistent with the notion of EBIC as increasingly higher electron beam current induces a larger number of electron-hole pairs and lowers the resistivity of the materials. Another role of the electron beam is to neutralize the positive surface charge brought in by the ion beam. Positive charges cannot capture and be neutralized by the high-energy electrons. The primary electrons penetrate the surface and are slowed and scattered in the material. Also, the concentration of the electrons injected into the material cannot increase indefinitely. The electrostatic potential will increase and drive current through the EBIC region to neutralize surface charge or to be bled off to system ground. A steady state solution of this complex charge compensation process may be possible by treating it as two decoupled processes: electron injection and charge drift.
(2)
where Re is the effective leakage resistance for charges to move through the EBIC region to the surface metal coating and is a comprehensive parameter with a magnitude dependent on both the physical properties of the material being analyzed and the analysis conditions. For simplicity, the contributions from secondary electrons, backscattered electrons, and secondary ions are not discussed as their effect can often be included by using a scaled effective primary current, e.g. the backscattered electron current reduces the effective primary electron current impinging onto the bombarded sample area. Net current can then be expressed as the difference between ion beam and
Fig. 1. A linear relationship between charging level and ion beam current is observed when the electron beam is held constant.
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Z. Zhu et al. / Applied Surface Science 254 (2008) 2708–2711
Fig. 2. Effective leakage resistance is found to be inversely proportional to electron beam current impacting the ion beam rastered area.
Since the injection phase determines the injected charge distribution which is governed largely by electron–matter interaction, Monte Carlo simulations [15] can provide a rather precise prediction of this distribution. The electrostatic potential distribution inside an insulator is expected to play a minor role during this phase for systems in reasonable charge balance, i.e. less than a 100 V charging level, and the space charge effects of the high-energy electrons have negligible impact on the electrostatic field. On the other hand, the charge buildup from the electron and ion beams establishes the electrostatic potential and enables drift currents. A direct mathematical description of charge drift would require solving a set of electrodynamic equations to study the time evolution of the system. The complexity of such calculations limits the solution to a single dimension problem [16]. However, under steady state conditions, the charge drift may be solved with a dc conduction model allowing electrostatic parameters to be calculated with a Laplace equation [17]. Fig. 3a shows a cross sectional view of 3-dimensional dc conduction model of a gold-coated sapphire sample. Cylindrical symmetry is used to reduce computational complexity. A low
Fig. 3. (a) Cross sectional view of 3-dimensional dc conduction model structure of a gold-coated sapphire sample; (b) illustration of calculated electrostatic potential distribution and current density vectors (not to scale).
resistivity layer of 0.5 mm thick (the maximum penetration depth of 6.5 keVelectrons into sapphire calculated using electron flight simulator [12]) is incorporated to account for the EBIC effect. The resistivity of the EBIC region is assumed to be uniform for simplicity and is calculated from the measured effective resistance (1E9 V). The area over which the ion beam was rastered (200 mm in diameter) serves as the source of a positive current of known current density (2.5 A/m2). This area is separated by a gap from the gold coating which is held at constant potential of 4.5 kV. This gap is introduced to account for the erosion of the conductive coating by the low intensity primary ion beam tails. Injected electrons are treated as a negative current source inside the EBIC region of the sample. A single negative current source plane at the mean depth for the injected electrons is used. With the above boundary conditions, the Laplace equation can be numerically solved with QuickField 5.3 [13] to yield the electrostatic potential and current density vector at any point of interest in the insulator (Fig. 3b). Simulation results suggest that matching current densities are required for maintenance of good charge compensation. It is found that a primary ion current density of 2.5 A/m2 requires a matching electron beam having a slightly lower current density of 2.45 A/m2 due to the presence of a 10 mm wide gap. The injected electrons under the uncoated area serve as extra current sink for ion beam current (Fig. 3b). The dimensions of the uncoated area are important. Further calculations with different gap sizes show that the increase of gap size will lead to a decrease in surface potential, shown as increasingly negative charging levels in Fig. 4. The progressive increase in gap size results from the low intensity primary ion beam tail which slowly erodes the gold coating around the crater. This gap is apparent in the optical micrograph in Fig. 5b. Fig. 5a shows an Al2O2+ depth profile of gold-coated sapphire acquired under similar ion and electron beam conditions as used in the simulation. The continuous decrease in surface potential during the profile is reflected by the variation of the secondary ion intensity, as the secondary ion energy distribution shifts into
Fig. 4. Calculated surface potential (charging level) for different gap sizes and ion beam current densities.
Z. Zhu et al. / Applied Surface Science 254 (2008) 2708–2711
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positive ion beam and electron beam bombardment. Effective leakage resistance is introduced to quantify the dynamic electrical properties of a sample under steady state ion beam and electron beam conditions. By treating electron beam charge compensation as two decoupled phases, i.e. electron injection and charge drift, a simple and quantitative dc conduction model can be successfully employed to account for detailed sample and analytical parameters in a 3-dimensional electrostatic potential calculation. Successful prediction of charging levels occurring in the presence of a conductive coating gap demonstrates a useful application of this approach which is based on the understanding and optimization of electron beam charge compensation for positive secondary ion magnetic sector SIMS analysis. This modeling approach can be expanded to study other charging phenomena. Fig. 5. (a) Al2O2+ depth profile of a sapphire sample with charging levels measured at various stages of the profile; (b) optical image of the final crater.
and then out of the 10 eV energy band pass window. This experimental data agrees with the predictions of the models described above. Removal of the gold coating by sputtering with a larger raster (e.g. 220 mm) and then acquiring depth profile data with a smaller raster (e.g.180 mm) eliminates the increase of the sputtered-induced gap width between the analyzed area and the conductive surface coating. With this gap held constant, a constant crater surface potential is achieved which results in a stable secondary ion intensity, confirming the results provided by the model calculations. Under constant electron beam conditions, a larger gap size provides higher electron beam current with respect to a given ion beam rastered area. This condition enables the use of a higher primary ion current which provides a higher sputter rate and improves detection sensitivity for bulk sample analysis. 4. Conclusion The present study provides a practical approach to understanding the charging phenomenon occurring under coincident
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14] [15] [16] [17]
G. Slodzian, Ann. Phys. 9 (1964) 591. C.A. Andersen, H.J. Roden, C.F. Robinson, J. Appl. Phys. 40 (1969) 3419. H.W. Werner, A.E. Morgan, J. Appl. Phys. 47 (1976) 1232. J.M. McKinley, F.A. Stevie, C.N. Granger, J. Vac. Sci. Technol. A 18 (2000) 273. C.W. Magee, W.L. Harrington, Appl. Phys. Lett. 33 (1978) 193. K. Wittmaack, Appl. Phys. 12 (1977) 149. H.W. Werner, N. Warmoltz, J. Vac. Sci. Technol. A 2 (1984) 726. G.J. Leggett, J.C. Vickerman, Appl. Surf. Sci. 84 (1995) 253. J. Cazaux, C. Le Gressus, Scan. Microsc. 5 (1991) 17. J. Cazaux, P. Lehuede, J. Electron Spectrosc. Relat. Phenom. 59 (1992) 49. G. Slodzian, M. Chaintreau, R. Dennebouy, Secondary Ion mass spectrometry, in: A. Benninghoven, et al. (Eds.), Proceeding of the SIMS V, Springer, Berlin, 1986, p. 158. Small World LLC, Electron Flight Simulator, 3.1-E, Vienna, Virginia (2000). Tera Analysis Ltd., QuickField 5.3, Svendborg, Denmark (2005). Y. Homma, Y. Ishii, M. Oshima, Mass Spectrosc. 32 (1984) 345. D.C. Joy, Scan. Microsc. 5 (1991) 329. P.N. Guzdar, A.S. Sharma, S.K. Guharay, Appl. Phys. Lett. 71 (1997) 3302. N. Ida, J. Bastos, Electromagnetics and Calculation of Fields, Springer, Berlin, 1992, p. 458.
Applied Surface Science 254 (2008) 2708–2711 www.elsevier.com/locate/apsusc
Model study of electron beam charge compensation for positive secondary ion mass spectrometry using a positive primary ion beam Zhengmao Zhu *, Frederick A. Stevie, Dieter P. Griffis Analytical Instrumentation Facility, North Carolina State University, Raleigh, NC 27695-7531, United States Received 30 April 2007; accepted 5 October 2007 Available online 10 October 2007
Abstract A new modeling approach has been developed to assist in the SIMS analysis of insulating samples. This approach provides information on the charging phenomena occurring when electron and positive primary ion beams impact a low conductivity material held at a high positive potential. The concept of effective leakage resistance aids in the understanding of the dynamic electrical properties of an insulating sample under dynamic analysis conditions. Modeling of steady state electron beam charge compensation involves investigation of electron injection and charge drift. Using a Monte Carlo program to simulate electron injection and dc conduction calculations to predict charge drift, detailed information regarding charging phenomena can be determined. # 2007 Elsevier B.V. All rights reserved. Keywords: Charge compensation; Electron beam; Magnetic sector; SIMS
1. Introduction Secondary ion mass spectrometry (SIMS) analysis requires bombardment of a sample surface with a primary ion beam. When performing positive secondary ion SIMS analysis on insulating samples, the charge buildup resulting from the positive primary ion beam is not significantly reduced by departing secondary particles. Thus, charge can accumulate on the sample surface to a level sufficient to affect SIMS analysis. Possible remedies for this charge buildup include providing a leakage path to mitigate charge buildup and/or providing charge compensation using a charged particle beam of opposite polarity. A metal grid [1] or a continuous conductive coating on the insulator surface can provide a leakage path. In some cases, a negative primary ion beam can be used for positive secondary ion SIMS to reduce charging [2,3], but insufficient compensation and the high negative primary ion beam energy impacting on a positively charged sample render this method less than ideal. Most commonly, SIMS analysis is performed using
* Corresponding author. Current address: Semiconductor Research & Development Center, IBM 2070 Route 52 Hopewell Junction, NY 12533, United States. Tel.: +1 845 892 1365; fax: +1 845 892 6256. E-mail address: [email protected] (Z. Zhu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.10.008
positive primary ions (e.g. O2+) and an electron beam is employed for charge compensation during the analysis of insulating samples [4–6]. Despite wide spread use of electron beam charge compensation under these SIMS conditions, understanding of the mechanisms of charge compensation under these conditions is incomplete. The most widely used theoretical treatment for charging involves consideration of a current balance model, first described by Werner and co-workers [3,7]. The insulator is treated as a resistor R in parallel with a capacitor C. Under steady state conditions, the charging level DU, measured as potential difference between sample surface and sample holder, is determined by the net current I flowing through the resistor: DU ¼ I R
(1)
This simple model provides useful guidelines for charge compensation. Other researchers have identified the limitations of this classic approach and have called for more complex models [8–10]. In particular, Cazaux and Lehuede [10] questioned Werner’s use of ohm’s law and pointed out that this model is restricted to an uncoated surface, does not reveal the field inside the insulator, and does not provide the spatial distribution of surface potential. In this work, the balance model treatment for SIMS analysis of a metal-coated insulator is extended, and the role of the
Z. Zhu et al. / Applied Surface Science 254 (2008) 2708–2711
electron beam during charge compensation is quantitatively characterized. The proposed model accounts for the effects of local parameters on the electrostatic potential distribution both on and below the sample surface. Modeling efforts are focused on magnetic sector SIMS analysis of insulating samples using positive primary ions with positive sample bias. 2. Experimental SIMS analyses were performed on a 20 nm gold-coated sapphire sample (intrinsic resistivity >1E14 V cm) with a CAMECA IMS-6F magnetic sector SIMS using O2+ primary ion beam of 5.5 keV impact energy (10 kV primary high voltage, 4.5 kV sample high voltage, and 418 impact angle). Al2O2+ secondary ions were collected from a 60 mm diameter optically gated area centered on a 180 mm 180 mm rastered area with a narrow pass energy bandwidth of 10 eV. Electron beam charge compensation was performed using the Normal Incidence Electron Gun (NEG) [11], aligned coincident with the primary ion beam raster. The electron gun accelerating voltage is 2 kV and electron impact energy 6.5 keV. Sample charging was evaluated by monitoring the secondary ion energy distribution. Stopping of primary electrons in sapphire was simulated with a Monte Carlo program, Electron Flight Simulator [12]. A finite element method based program, QuickField 5.3 [13], was employed to numerically solve Laplace’s equation to study the electrostatic parameters involved in the electron beam charge compensation process. 3. Results and discussion The electron beam has two roles in SIMS electron beam charge compensation: to provide leakage path for charge buildup and to neutralize surface charge. Materials bombarded by an electron beam can have higher conductivity than the intrinsic materials due to electron beam induced conductivity (EBIC). EBIC has been recognized as an important factor in electron beam charge compensation during SIMS analysis [4,14]. Based on the previous current balance model, a resistance of 1E15 V in the case of sapphire would require a net current flowing through the resistor to be less than 0.01 pA in order to prevent more than 10 V charging. The EBIC effect must be taken into account while deriving the current balance equation for metal-coated insulators: DU ¼ I Re
2709
electron beam currents. Hence we have DU ¼ Re I i Re I e
(3)
where Ii and Ie are the ion beam and the electron beam current impinging onto the ion beam rastered area. A linear relationship between charging level and ion beam current is expected when electron beam is held constant. This relationship has been observed in the analysis of many insulating samples including the sapphire sample used here (Fig. 1). This suggests that the sample under constant electron bombardment behaves as an ohmic material. Both Re and Ie can be extracted from linear fit of the charging level and primary ion beam current data. The EBIC effect is apparent: the measured effective leakage resistance of sapphire under 2.5 A/m2 electron beam bombardment is 7E8 V, which is much lower than the estimated intrinsic value which is on the order of 1E15 V. A correlation between the effective leakage resistance and the electron beam current into the primary ion beam rastered area can be further revealed by measuring Re under different electron gun emission current. Fig. 2 shows effective leakage resistance is inversely proportional to electron beam current. This is consistent with the notion of EBIC as increasingly higher electron beam current induces a larger number of electron-hole pairs and lowers the resistivity of the materials. Another role of the electron beam is to neutralize the positive surface charge brought in by the ion beam. Positive charges cannot capture and be neutralized by the high-energy electrons. The primary electrons penetrate the surface and are slowed and scattered in the material. Also, the concentration of the electrons injected into the material cannot increase indefinitely. The electrostatic potential will increase and drive current through the EBIC region to neutralize surface charge or to be bled off to system ground. A steady state solution of this complex charge compensation process may be possible by treating it as two decoupled processes: electron injection and charge drift.
(2)
where Re is the effective leakage resistance for charges to move through the EBIC region to the surface metal coating and is a comprehensive parameter with a magnitude dependent on both the physical properties of the material being analyzed and the analysis conditions. For simplicity, the contributions from secondary electrons, backscattered electrons, and secondary ions are not discussed as their effect can often be included by using a scaled effective primary current, e.g. the backscattered electron current reduces the effective primary electron current impinging onto the bombarded sample area. Net current can then be expressed as the difference between ion beam and
Fig. 1. A linear relationship between charging level and ion beam current is observed when the electron beam is held constant.
2710
Z. Zhu et al. / Applied Surface Science 254 (2008) 2708–2711
Fig. 2. Effective leakage resistance is found to be inversely proportional to electron beam current impacting the ion beam rastered area.
Since the injection phase determines the injected charge distribution which is governed largely by electron–matter interaction, Monte Carlo simulations [15] can provide a rather precise prediction of this distribution. The electrostatic potential distribution inside an insulator is expected to play a minor role during this phase for systems in reasonable charge balance, i.e. less than a 100 V charging level, and the space charge effects of the high-energy electrons have negligible impact on the electrostatic field. On the other hand, the charge buildup from the electron and ion beams establishes the electrostatic potential and enables drift currents. A direct mathematical description of charge drift would require solving a set of electrodynamic equations to study the time evolution of the system. The complexity of such calculations limits the solution to a single dimension problem [16]. However, under steady state conditions, the charge drift may be solved with a dc conduction model allowing electrostatic parameters to be calculated with a Laplace equation [17]. Fig. 3a shows a cross sectional view of 3-dimensional dc conduction model of a gold-coated sapphire sample. Cylindrical symmetry is used to reduce computational complexity. A low
Fig. 3. (a) Cross sectional view of 3-dimensional dc conduction model structure of a gold-coated sapphire sample; (b) illustration of calculated electrostatic potential distribution and current density vectors (not to scale).
resistivity layer of 0.5 mm thick (the maximum penetration depth of 6.5 keVelectrons into sapphire calculated using electron flight simulator [12]) is incorporated to account for the EBIC effect. The resistivity of the EBIC region is assumed to be uniform for simplicity and is calculated from the measured effective resistance (1E9 V). The area over which the ion beam was rastered (200 mm in diameter) serves as the source of a positive current of known current density (2.5 A/m2). This area is separated by a gap from the gold coating which is held at constant potential of 4.5 kV. This gap is introduced to account for the erosion of the conductive coating by the low intensity primary ion beam tails. Injected electrons are treated as a negative current source inside the EBIC region of the sample. A single negative current source plane at the mean depth for the injected electrons is used. With the above boundary conditions, the Laplace equation can be numerically solved with QuickField 5.3 [13] to yield the electrostatic potential and current density vector at any point of interest in the insulator (Fig. 3b). Simulation results suggest that matching current densities are required for maintenance of good charge compensation. It is found that a primary ion current density of 2.5 A/m2 requires a matching electron beam having a slightly lower current density of 2.45 A/m2 due to the presence of a 10 mm wide gap. The injected electrons under the uncoated area serve as extra current sink for ion beam current (Fig. 3b). The dimensions of the uncoated area are important. Further calculations with different gap sizes show that the increase of gap size will lead to a decrease in surface potential, shown as increasingly negative charging levels in Fig. 4. The progressive increase in gap size results from the low intensity primary ion beam tail which slowly erodes the gold coating around the crater. This gap is apparent in the optical micrograph in Fig. 5b. Fig. 5a shows an Al2O2+ depth profile of gold-coated sapphire acquired under similar ion and electron beam conditions as used in the simulation. The continuous decrease in surface potential during the profile is reflected by the variation of the secondary ion intensity, as the secondary ion energy distribution shifts into
Fig. 4. Calculated surface potential (charging level) for different gap sizes and ion beam current densities.
Z. Zhu et al. / Applied Surface Science 254 (2008) 2708–2711
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positive ion beam and electron beam bombardment. Effective leakage resistance is introduced to quantify the dynamic electrical properties of a sample under steady state ion beam and electron beam conditions. By treating electron beam charge compensation as two decoupled phases, i.e. electron injection and charge drift, a simple and quantitative dc conduction model can be successfully employed to account for detailed sample and analytical parameters in a 3-dimensional electrostatic potential calculation. Successful prediction of charging levels occurring in the presence of a conductive coating gap demonstrates a useful application of this approach which is based on the understanding and optimization of electron beam charge compensation for positive secondary ion magnetic sector SIMS analysis. This modeling approach can be expanded to study other charging phenomena. Fig. 5. (a) Al2O2+ depth profile of a sapphire sample with charging levels measured at various stages of the profile; (b) optical image of the final crater.
and then out of the 10 eV energy band pass window. This experimental data agrees with the predictions of the models described above. Removal of the gold coating by sputtering with a larger raster (e.g. 220 mm) and then acquiring depth profile data with a smaller raster (e.g.180 mm) eliminates the increase of the sputtered-induced gap width between the analyzed area and the conductive surface coating. With this gap held constant, a constant crater surface potential is achieved which results in a stable secondary ion intensity, confirming the results provided by the model calculations. Under constant electron beam conditions, a larger gap size provides higher electron beam current with respect to a given ion beam rastered area. This condition enables the use of a higher primary ion current which provides a higher sputter rate and improves detection sensitivity for bulk sample analysis. 4. Conclusion The present study provides a practical approach to understanding the charging phenomenon occurring under coincident
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14] [15] [16] [17]
G. Slodzian, Ann. Phys. 9 (1964) 591. C.A. Andersen, H.J. Roden, C.F. Robinson, J. Appl. Phys. 40 (1969) 3419. H.W. Werner, A.E. Morgan, J. Appl. Phys. 47 (1976) 1232. J.M. McKinley, F.A. Stevie, C.N. Granger, J. Vac. Sci. Technol. A 18 (2000) 273. C.W. Magee, W.L. Harrington, Appl. Phys. Lett. 33 (1978) 193. K. Wittmaack, Appl. Phys. 12 (1977) 149. H.W. Werner, N. Warmoltz, J. Vac. Sci. Technol. A 2 (1984) 726. G.J. Leggett, J.C. Vickerman, Appl. Surf. Sci. 84 (1995) 253. J. Cazaux, C. Le Gressus, Scan. Microsc. 5 (1991) 17. J. Cazaux, P. Lehuede, J. Electron Spectrosc. Relat. Phenom. 59 (1992) 49. G. Slodzian, M. Chaintreau, R. Dennebouy, Secondary Ion mass spectrometry, in: A. Benninghoven, et al. (Eds.), Proceeding of the SIMS V, Springer, Berlin, 1986, p. 158. Small World LLC, Electron Flight Simulator, 3.1-E, Vienna, Virginia (2000). Tera Analysis Ltd., QuickField 5.3, Svendborg, Denmark (2005). Y. Homma, Y. Ishii, M. Oshima, Mass Spectrosc. 32 (1984) 345. D.C. Joy, Scan. Microsc. 5 (1991) 329. P.N. Guzdar, A.S. Sharma, S.K. Guharay, Appl. Phys. Lett. 71 (1997) 3302. N. Ida, J. Bastos, Electromagnetics and Calculation of Fields, Springer, Berlin, 1992, p. 458.