- Email: [email protected]
Depth profiling of ultrathin films using medium energy ion scattering
Current Applied Physics 3 (2003) 75–82 www.elsevier.com/locate/cap
Depth profiling of ultrathin films using medium energy ion scattering q Joonkon Kim a,*, W.N. Lennard b, C.P. McNorgan b, J. Hendriks b, I.V. Mitchell b, D. Landheer c, J. Gredley c a
Korea Institute of Geoscience and Mineral Resources, Yusung-ku, Daejon 305-350, South Korea Department of Physics and Astronomy, The University of Western Ontario, London, Ont., Canada Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Ont., Canada
b c
Received 30 September 2002; accepted 18 November 2002
Abstract The medium energy ion scattering (MEIS) system at the University of Western Ontario has been modified by replacing the original one-dimensional position sensitive detector with a 2-D array. Calibration and analysis procedures for quantitative depth profiling are devised and established in this work: distortion correction, image tiling, charge state distribution of the scattered hydrogen ions, etc. The software to simultaneously control the sample manipulator (three orthogonal rotations), toroidal electrostatic analyzer and spectrum acquisition has been developed using LabViewR . This development makes for easy sample alignment to the incident ion beam and automatically collects the step images. Additionally, the tiling procedure using corrected step images is accomplished within LabViewR to produce a final energy–angle spectra. Our QUARK (quantitative analysis of Rutherford kinematics) spectrum simulation package has been modified to provide for non-linear least squares fitting to a measured MEIS energy spectrum. As a reference for quantitative analysis, a shallow Sb-implanted graphite sample was used with normalization to the height of the thick target carbon region by applying 1 H stopping power values from Konac et al. [Nucl. Instr. Meth. Phys. Res. B 136–138 (1998) 159]. To determine the system suitability for compositional analysis, Zr silicate films of thickness 2–7 nm on Si (1 0 0) substrates have been characterized by MEIS, RBS and NRA. The absolute areal densities of constituent elements are in good agreement (within 15%) among the three methods. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 81.70.)q Keywords: MEIS; Depth profiling; QUARK; Zr silicate
1. Introduction Tromp et al. [1] have described a two-dimensional detector for the detection of ions scattered from a solid target and analyzed in both energy and scattering angle by a toroidal electrostatic analyzer (TEA). The specific TEA detector system described in that work is manufactured commercially by HVEE and such facilities have been installed in UHV target chambers of low 109 Torr q Original version presented at the 2nd International Workshop on Ion Beam Techniques for the Analysis of Composition and Structure with Atomic Layer Resolution, 24–27 September 2002, Kyongju, Korea. * Corresponding author. Tel.: +82-42-868-3663. E-mail address: [email protected] (J. Kim).
pressure which are coupled to both single-ended and Tandetron-type particle accelerators. Here we report on the installation of an identical detector in the medium energy ion scattering (MEIS) facility located at the University of Western Ontario. We will describe the necessary procedural details for using such a detector to analyze the composition of condensed media using incident 100 keV 1 H projectiles. The experimental summary addresses the following issues: range of linearity, distortion correction along the energy direction, charge state distribution of scattered ions, absolute areal density quantification and simulation of measured spectra including a non-linear least squares fitting procedure. We will also describe the software that has been developed for TEA control, spectrum acquisition and analysis. Since the main focus of research relates to
1567-1739/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1567-1739(02)00240-7
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J. Kim et al. / Current Applied Physics 3 (2003) 75–82
compositional as distinct from surface structure information, we will concentrate this report on the measured energy spectra. Other authors have reported surface structure investigations using similar facilities: for example, the reader is referred to annual reports from CLRC Daresbury [2].
software. The National Instruments PCI-DIO-32HS is a high speed 32-bit parallel digital interface installed in the computer. It is used to transfer the 2D data histogram into the computer at a speed of 0.667 MHz and send commands to the Quantar 2401B unit. The PCI-DIO32HS to Quantar Technology Interface interconnects the PCI-DIO-32HS to the Quantar Technology 2401B positional analyzer.
2. Experimental Since the operation of a MEIS facility has been described previously [1], only a brief description is reported here. Hundred-kilo-electron-volts 1 H ions are produced by a 1.7 MV high current Tandetron accelerator and impinge on a target through a set of slits. The final slits have a size of 1 mm in the horizontal direction and 0.25 mm in the vertical direction. It is the vertical beam spot size that is important when considering the energy spread across the exit slit of the TEA. The UHV target chamber features a load-lock pre-chamber that obviates the need to vent the main chamber during sample insertion. The target is mounted on a six-axis goniometer with three orthogonal angular rotations that are stepping motor controlled. The three orthogonal translational motions are performed manually. The incident beam current is integrated directly from the biased target, although the integrity of this measurement is not guaranteed. At the present time, we do not rely on the current integration for absolute measurements, for obvious reasons, but use conventional MeV ion beam analysis techniques for quantification. A surface barrier detector (SBD) is located at a scattering angle of 170°, which provides a means for rapid target alignment (i.e. channeling in and/or blocking out) in the case of crystalline targets especially for aligning the incident beam with a major crystallographic axial direction.
4. Calibration and quantification 4.1. Distortion correction and tiling Because two-dimensional raw data from the analyzer is distorted, image correction is critical for further data processing and is particularly important along the energy direction. We have also carefully investigated the distortion (expansion and contraction) along the angular direction using blocking dips; however, we have not observed any evidence of image distortion within experimental uncertainty. We have used the h2 1 1i axis of a Si(1 0 0) crystal, which is directed at a 35.2644° angle from the wafer normal, for angular measurements. By aligning the normal to the wafer surface with the incident beam using the h1 0 0i channeling dip, we define this position as a zero tilt. Rotating the sample manipulator by 30°, we obtain a reliable angle, i.e. 65.2644°. Fig. 1(a) shows the (2 1 1) blocking dips for two different TEA positions. At five TEA positions, Fig. 1(b), we have confirmed and evaluated the degree of angular distortion and a dispersion relation between angular channel ðCÞ and corresponding scattering angle ðhÞ at an arbitrary TEA position. The dispersion is fitted with a linear equation h ½° ¼ TEA position þ
3. Control and acquisition The MEIS two-dimensional data acquisition system is comprised of five main components: a desktop computer, National Instruments PCI DIO-32HS high speed digital I/O board, PCI-DIO-32HS to Quantar Technology Interface, Quantar 2401B Positional Analyzer with 2415B histogramming buffer memory and Quantar Technology 24012 detector pre-amplifier. The computer is a 0.8 GHz PIII running the Windows 2000 operating system and a customized MEIS data acquisition program (written in LabViewR 5.1). The 24012 pre-amplifier amplifies the four MEIS 2D-detector signals before they are fed into the 2401B positional analyzer. The Quantar Technology 2401B positional analyzer with 2415B histogramming buffer memory converts the MEIS signals from the pre-amplifier into a 1024 1024, 16-bit histogram which can be read into the MEIS acquisition
C p0 : p1 ½ch:=°
For the raw image, 26.38 angular channels ðp1 Þ covers a 1° scattering angle and the nominal TEA angular position corresponds to p0 ¼ 370:29. The best fit polynomial is so linear that each point deviates from this line by less than 0.1% for a region 10° around the TEA center. At this point, the two-dimensional raw image is compressed by factor of 4 in angle, since more than six channels ( ¼ 26.38/4) after compression is enough to contain 1° in the scattering angle. The distorted raw image along the energy direction was corrected using the Ta leading edge from a thick 173.7 nm Ta2 O5 (on Ta substrate) target for each angular channel. We compared the position of the Ta leading edges produced by incident hydrogen ions for a 1 keV energy difference in incident energy at a fixed TEA pass energy ðEpass Þ. The entire TEA angular range was divided into two parts, i.e. those covering 15° lower and higher scattering angles, respectively, and edge pairs
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Fig. 2. Leading edges of Ta2 O5 . Left data set is for the TEA at 115° and right data set is for 100°. The upper and lower lines are edges for hydrogen incident energies differing by 1 keV.
Fig. 1. Calibration for angular channel: (a) blocking dips for h2 1 1i axis at different TEA positions; (b) dispersion relation between angular channel and corresponding scattering angle relative to the TEA position.
were collected separately because we already know that only a restricted fraction of the pass energy window will be linear enough to use. Although the pass energy window ðDEÞ of the TEA is known to be 2% of the pass energy, both ends of the energy window could be somewhat distorted (e.g. contracted or expanded when compared with the middle part of DE). The energies of scattered hydrogen ions differ by 0.18% for a scattering angle change from 105° to 115°. By dividing the TEA angular span into two parts, we can use the central part of the energy window that is less than 1.2% Epass . In Fig. 2, the upper and lower lines are Ta edges corresponding to incident energies of 100 and 99 keV, respectively. Using the previous angle calibration, we can calculate the corresponding energy difference between both edge lines for each angular channel, DE ¼ KTa ðhÞDEincidence . The open circles of Fig. 3(b) are the energy width per channel obtained using this relation. We have chosen to fit the measured values with a seven-parameter polynomial instead of using the measured values directly. Besides the energy width per channel, one more parameter is necessary to complete the image correction. There are two types of image distortion. The first is a saw-tooth shaped oscillation along the angular direction due to the discrete nature of two electrodes in the po-
Fig. 3. Calibration parameters for the distortion correction along the energy direction for each angular channel. (a) Open circles show the ETEA pass line and the solid line is the mid-point of the step image. (b) Energy width per energy channel. Solid line results from a sevenparameter polynomial fit.
sition sensitive array. The second gross distortion arises from a combination of many sources: imperfection of the TEA electrode spacing, mechanical misalignment of the exit slits and the inevitable asymmetric nature of the four charge sensitive amplifiers connected to each
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corner. Fig. 3(a) shows the mismatch of the Epass line with the mid-points of the step image as a result of the total gross distortion. With these two calibration parameters, energy width per channel and TEA pass energy contour, all the counts were redistributed into the new image bin. In spite of the correction process for each step image, we cannot use the entire energy window because of a residual distortion near both edges of the energy window and asymmetric distances from the TEA pass energy to the edges at different angles. Thus, only part of the energy window is useful for an image mosaic, where each corrected step image is positioned side by side in a process denoted by the expression image ‘‘tiling’’. As shown in Fig. 4(b), the energy window of a single step image covers a region almost 2.4% of the TEA pass energy, which is defined by the 3.5 mm exit slit between the TEA and the MCP. To show explicitly what happens if the selected energy window is too wide, two energy spectra are compared in Fig. 4(a) which have been tiled with different step energy widths. The open circles show a 1 keV TEA step energy interval and the open triangles show the result after tiling with a 2 keV step. In Fig. 4(b), almost 1.8% of the effective energy window is observed to be acceptable for tiling of this step image. But
Fig. 4. (a) Tiled spectrum with step energy of 1 keV (circles) and 2 keV (triangles). The open squares show the difference of two tiled energy spectra. (b) Energy spectrum of single step image at 125°. The horizontal axis is normalized to the TEA pass energy.
considering the mismatch between the image center and the constant energy contour shown in Fig. 3(a), only the narrower energy window is acceptable for the entire angular region. Thus we have collected images using a step size of 1% of the TEA pass energy. 4.2. Proton charge state distribution The TEA in MEIS experiments only detects charged particles, in contrast to silicon SBDs used in conventional MeV elastic scattering (RBS) experiments. Hence it is necessary to know the fraction of charged particles scattered into the TEA in order to determine absolute elemental compositions. A few previous works [3–8] have reported that the fraction of charged particles depends on the surface species and is also a function of exit energy. However, early experiments [5,6] were performed in poor vacuum conditions (105 to 106 Torr) and even the latest data of Ross [4] has been measured for rather modest vacuum conditions, P 107 Torr. Since the charge state distribution of scattered ions is strongly dependent on the composition of the last several Angstroms of the target [6,7], an UHV environment is essential to avoid effects caused by adventitious surface contamination during the measurements. As a reference, the base pressure in most MEIS target chambers is in the region P 6 109 Torr. The charge state fractions for scattered hydrogen ions were measured using a Au monolayer film (thickness of 0.32 nm measured by RBS) deposited on a thick DLC substrate for the energy range of 60–100 keV incident 1 H ions. The positively charged fraction, f þ , was determined by comparing the yields with the TEA positioned at 130° scattering angle compared to the counts recorded by a Si SBD positioned near 170° scattering angle. Since the exact angle position and solid angle of the SSBD were unknown, the charged particle fraction measurements were then relative. We have normalized our data at 96.2 keV for the easy comparison with the previous works. However, the energy dependence of relative f þ values is sufficient for a quantitative analysis: f þ ¼ pþ =½p0 þ pþ , where pþ ¼ counts by TEA and p0 þ pþ ¼ counts for SBD. Because the resolution of the SSBD was not sufficient to separate the Au peak completely from the carbon substrate signal, the total Au yields were extracted via a Gaussian fitting procedure as shown in Fig. 5. Although both detectors were positioned at different angles ( 170° for SBD and 130° for TEA), the angle dependence of the screening effect can be ignored for both scattering geometries [9] since the collisions can be defined as ÔcloseÕ enough in both cases. The charge fraction resulting from scattering from Au is compared with previous data in Fig. 6. Our data are observed to approach RossÕs values [4]. Further measurements for different surface films are not warranted, we believe, based on the present results. In contrast to
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ple scattering is not. The latter shortcoming should not be significant for thin film and surface analysis, particularly using incident protons. A newer version of this program has now been developed that allows for spectrum fitting using a Levenberg–Marquardt fitting algorithm, wherein both the thickness and composition of the layers can be varied to determine a best fit. These simulation packages are part of the QUARK (quantitative analysis of Rutherford kinematics) series available for analytical ion beam applications (see UWO website of WNL for downloads of other packages, http://www. uwo.ca/isw/people/wlenna/index.html). 4.4. Energy resolution and quantification Fig. 5. Scattered hydrogen spectrum from 0.3 nm Au film on a DLC substrate recorded by a Si SBD for an incident energy of 100 keV.
Fig. 6. Charged fraction of scattered hydrogen ions from 0.3 nm Au layer. The Si SBD is positioned at 170° and the TEA is positioned at 125° scattering angle. Previous data are shown for comparison: Marion and Young (dash-dot), Busch (short-dash) and Ross (solid line).
Energy resolution is one of the key parameters in ion beam analysis especially in backscattering spectra. For MEIS, the total energy resolution in the energy spectrum is comprised of major contributions from: the beam energy spread, the divergence of the incident beam at the target and the vertical velocity component of scattered ions when entering the TEA. The electronic noise during signal processing also composes one of the unavoidable noise terms that deteriorates the system resolution. A value r 215 eV resolution was obtained by fitting the leading edge of the Ta portion of the thick Ta2 O5 spectrum (by integrating a Gaussian function at 125° scattering angle, see Fig. 7). A graphite sample implanted with 50 keV 121 Sb ions to a fluence of 1 1015 ions/cm2 was analyzed with the UWO MEIS system. Since the MEIS facility does not have a reliable fluence measuring device for integrating the incident proton flux, the spectrum height for the carbon substrate was used for normalization. As shown in Fig. 8, the measured Sb dose is 2% higher than the nominal value. When considering the uncertainty embedded in the stopping powers, a 2% difference is a most encouraging result.
previous data, our experiments were performed inside the MEIS target chamber with a pressure in the low 109 Torr range. Thus, these data should be free from any adventitious contamination problem arising during irradiation. In comparing these rudimentary data with existing formulae [3,4,8], we then selected a data set–– specifically, the Marion and Young results––to compensate for the charge fraction of scattered hydrogen ions. 4.3. Spectrum fitting A Windows-based simulation program has been developed to calculate the elastic scattering spectrum for multilayer targets. This program is available from the authors and incorporates, in our opinion, the most reliable electronic stopping power values to date [10]. Electronic energy loss straggling is included, but multi-
Fig. 7. The total MEIS energy resolution was obtained by fitting the Ta leading edge at 125° scattering angle using Gaussian integration.
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Fig. 8. MEIS spectrum of 121 Sb-implanted sample. The measured value is 2% higher compared to the nominal value of 1:0 1015 Sb ions/cm2 .
5. Analysis of Zr silicate As a first application for compositional analysis with the modified UWO MEIS, four Zr silicate films of thickness 2–7 nm were deposited on c-Si (1 0 0) samples. The sample structure is shown schematically in Fig. 9. The three major constituents––Zr, Si and O––were measured by MEIS as well as using other methods; e.g. RBS for zirconium, NRA for oxygen and XPS for the Si:Zr ratio. The double alignment technique was used to provide for the low-Z oxygen measurement in MEIS. The samples were tilted by 45° to align the h1 0 1i axis to the incident beam direction, then the TEA was positioned to include the h1 0 1i axis for the scattered particles, as in Fig. 10. With the incident beam ‘‘channeling in’’ along a major crystallographic axis and the scattered particles emerging in a ‘‘blocking out’’ direction parallel to another major crystallographic axis, then the scattering yield from the crystalline Si substrate was drastically reduced. The heaviest and lightest elements were zirconium and carbon. In general, some adventitious carbon contamination may be unavoidable during sample transportation and/or ion beam irradiation; for the present samples, some monolayer carbon on the surface may be present as a residual of an organic precursor during the growth process. The TEA scan inter-
Fig. 9. Schematic structure of Zr silicate. Surface carbon has several possible sources.
Fig. 10. Double alignment configuration in order to reduce the scattering background arising from the crystalline Si substrate.
val was chosen to include energies corresponding to these elements, i.e. from a starting energyKZr E0 . A TEA step energy size of 1 keV was adequate considering the 1% effective energy width when using 100 keV incident (negative) hydrogen ions. The tiled angle–energy plot was constructed to accommodate all the elements of interest and spanned an angular region 30° around the TEA center angle. As shown in Fig. 11, the energy spectrum was taken along one of the major blocking dips, for example at a 135° scattering angle, i.e. corresponding to the sample normal. Although the energy spectra are obtained for a channeling/blocking geometry, there still remains the Si surface peak yield and background counts from the Si
Fig. 11. Top view of tiled image. The energy spectrum is obtained by a cut along one of the major blocking dips. The incident beam is already aligned with another crystal axis.
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Table 1 MEIS results for zirconium silicate compared to RBS and NRA values sample
#1132 #1135 #1137 #1138
Zr (1E15/cm2 )
Si (1E15/cm2 )
O (1E15/cm2 )
MEIS
RBS
MEIS
MEIS
NRA
MEIS
RBS and NRA
MEIS
XPS
2.5 5.2 12.4 0.3
2.9 5.7 12.0 0.4
13.4 9.4 9.5 8.4
25.1 23.3 40.4 14.2
30.0 27.6 41.9 17.3
10.24 4.52 3.26 48.14
10.31 4.85 3.49 40.14
5.5 1.8 0.8 28.3
5.23 1.33 0.5 23.5
substrate crystal; the latter contribution amounted to a few % of the random height. This background obviously is dependent on the film thickness due to dechanneling in the (amorphous) overlayer which arises from multiple scattering along both inward and outward paths. Background subtraction is necessary before doing spectrum simulation. With this energy spectrum, quantification was performed by simulation as shown in Fig. 12 where we have relied on the measured current integral. The resulting absolute areal density for the four samples are summarized in Table 1. In the table, results obtained by other analysis methods are shown in order to assess the quantification accuracy of the MEIS system. Analysis results from independent methods were in agreement at a level of 15%. We expect that the current integration technique may be in error by as much as 10%. 6. Summary and conclusion The UWO MEIS system has been modified by replacing the previous one-dimensional position detector with a 2D position sensitive array. The calibration procedure, operation scheme and the spectrum simulation
O/Zr
Si/Zr
Table 2 Analytical capability of the UWO MEIS facility compared with those for the conventional RBS technique using MeV ions
Mass resolution (amu) Depth resolution (nm) Accessible depth (nm) Surface sensitivity (ML)
MEIS
RBS
<1 within M2 < 20 <1 <10 <103
<1 within M2 < 40 <10 1000 <102
program are new. The angular calibration was performed using blocking dips from crystalline silicon; the energy calibration was accomplished using the leading edge of the scattered particle energy spectrum from a Ta target, since Ta is a virtually monoisotopic element. We also determined the effective energy window that is sufficiently linear for serial image tiling purposes without distortion. A rudimentary experiment has been performed to determine the energy dependent charge fraction using a thin Au film evaporated onto a (thick) DLC substrate. All the system control, data acquisition and data processing to obtain energy spectra are performed by a customized software package written in LabViewR . A non-linear least squares fitting simulation code has been developed for extracting compositional information. To confirm the quantitative accuracy of the setup, a known amount of Sb implanted into a graphite substrate was analyzed as a reference sample: the nominal and measured doses were in excellent agreement. The total energy resolution of the UWO system was measured to be 215 eV. The depth and mass resolutions derived from this r-value are summarized and compared with those of RBS in Table 2. MEIS has an enhanced depth resolution of an order of magnitude against RBS and is more sensitive for light elements in the near-surface region. However, MEIS is only suitable for relatively shallow depths and has relatively poor mass resolution compared to MeV ion scattering.
Acknowledgements Fig. 12. Spectrum simulation. Open circles show the measured energy spectrum. The solid line overlapping the measured data shows the results from fitting the spectrum. The dotted line is the expected spectrum without the MEIS double alignment geometry.
One of the authors, J. Kim, is grateful for the financial support from the Center for Chemical Physics during stay at the University of Western Ontario.
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References [1] R.M. Tromp, M. Copel, M.C. Reuter, M. HornvanHoegen, J. Speidell, R. Koudijs, Rev. Sci. Instr. 62 (1991) 2679. [2] MEIS, RUSTI, 1998–1999 Annual Reports, Daresbury Laboratory. [3] J.B. Marion, F.C. Young, Nuclear Reaction Analysis: Graphs and Tables, North-Holland, Amsterdam, 1968. [4] G.G. Ross, B. Terreault, Nucl. Instr. Meth. Phys. Res. B 15 (1986) 146.
[5] [6] [7] [8]
T. Hall, Phys. Rev. 79 (1950) 504. J.A. Phillips, Phys. Rev. 97 (1954) 404. M. Copel, IBM, private communication. B.W. Busch, Doctoral thesis, Rutgers, The State University of New Jersey, 2000. [9] J. LÕEcuyer, J.A. Davies, N. Matsunami, Nucl. Instr. Meth. 160 (1979) 160. [10] W.N. Lennard, G..R. Massoumi, T.W. Simpson, I.V. Mitchell, Nucl. Instr. Meth. Phys. Res. B 152 (1999) 370.
Depth profiling of ultrathin films using medium energy ion scattering q Joonkon Kim a,*, W.N. Lennard b, C.P. McNorgan b, J. Hendriks b, I.V. Mitchell b, D. Landheer c, J. Gredley c a
Korea Institute of Geoscience and Mineral Resources, Yusung-ku, Daejon 305-350, South Korea Department of Physics and Astronomy, The University of Western Ontario, London, Ont., Canada Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Ont., Canada
b c
Received 30 September 2002; accepted 18 November 2002
Abstract The medium energy ion scattering (MEIS) system at the University of Western Ontario has been modified by replacing the original one-dimensional position sensitive detector with a 2-D array. Calibration and analysis procedures for quantitative depth profiling are devised and established in this work: distortion correction, image tiling, charge state distribution of the scattered hydrogen ions, etc. The software to simultaneously control the sample manipulator (three orthogonal rotations), toroidal electrostatic analyzer and spectrum acquisition has been developed using LabViewR . This development makes for easy sample alignment to the incident ion beam and automatically collects the step images. Additionally, the tiling procedure using corrected step images is accomplished within LabViewR to produce a final energy–angle spectra. Our QUARK (quantitative analysis of Rutherford kinematics) spectrum simulation package has been modified to provide for non-linear least squares fitting to a measured MEIS energy spectrum. As a reference for quantitative analysis, a shallow Sb-implanted graphite sample was used with normalization to the height of the thick target carbon region by applying 1 H stopping power values from Konac et al. [Nucl. Instr. Meth. Phys. Res. B 136–138 (1998) 159]. To determine the system suitability for compositional analysis, Zr silicate films of thickness 2–7 nm on Si (1 0 0) substrates have been characterized by MEIS, RBS and NRA. The absolute areal densities of constituent elements are in good agreement (within 15%) among the three methods. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 81.70.)q Keywords: MEIS; Depth profiling; QUARK; Zr silicate
1. Introduction Tromp et al. [1] have described a two-dimensional detector for the detection of ions scattered from a solid target and analyzed in both energy and scattering angle by a toroidal electrostatic analyzer (TEA). The specific TEA detector system described in that work is manufactured commercially by HVEE and such facilities have been installed in UHV target chambers of low 109 Torr q Original version presented at the 2nd International Workshop on Ion Beam Techniques for the Analysis of Composition and Structure with Atomic Layer Resolution, 24–27 September 2002, Kyongju, Korea. * Corresponding author. Tel.: +82-42-868-3663. E-mail address: [email protected] (J. Kim).
pressure which are coupled to both single-ended and Tandetron-type particle accelerators. Here we report on the installation of an identical detector in the medium energy ion scattering (MEIS) facility located at the University of Western Ontario. We will describe the necessary procedural details for using such a detector to analyze the composition of condensed media using incident 100 keV 1 H projectiles. The experimental summary addresses the following issues: range of linearity, distortion correction along the energy direction, charge state distribution of scattered ions, absolute areal density quantification and simulation of measured spectra including a non-linear least squares fitting procedure. We will also describe the software that has been developed for TEA control, spectrum acquisition and analysis. Since the main focus of research relates to
1567-1739/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1567-1739(02)00240-7
76
J. Kim et al. / Current Applied Physics 3 (2003) 75–82
compositional as distinct from surface structure information, we will concentrate this report on the measured energy spectra. Other authors have reported surface structure investigations using similar facilities: for example, the reader is referred to annual reports from CLRC Daresbury [2].
software. The National Instruments PCI-DIO-32HS is a high speed 32-bit parallel digital interface installed in the computer. It is used to transfer the 2D data histogram into the computer at a speed of 0.667 MHz and send commands to the Quantar 2401B unit. The PCI-DIO32HS to Quantar Technology Interface interconnects the PCI-DIO-32HS to the Quantar Technology 2401B positional analyzer.
2. Experimental Since the operation of a MEIS facility has been described previously [1], only a brief description is reported here. Hundred-kilo-electron-volts 1 H ions are produced by a 1.7 MV high current Tandetron accelerator and impinge on a target through a set of slits. The final slits have a size of 1 mm in the horizontal direction and 0.25 mm in the vertical direction. It is the vertical beam spot size that is important when considering the energy spread across the exit slit of the TEA. The UHV target chamber features a load-lock pre-chamber that obviates the need to vent the main chamber during sample insertion. The target is mounted on a six-axis goniometer with three orthogonal angular rotations that are stepping motor controlled. The three orthogonal translational motions are performed manually. The incident beam current is integrated directly from the biased target, although the integrity of this measurement is not guaranteed. At the present time, we do not rely on the current integration for absolute measurements, for obvious reasons, but use conventional MeV ion beam analysis techniques for quantification. A surface barrier detector (SBD) is located at a scattering angle of 170°, which provides a means for rapid target alignment (i.e. channeling in and/or blocking out) in the case of crystalline targets especially for aligning the incident beam with a major crystallographic axial direction.
4. Calibration and quantification 4.1. Distortion correction and tiling Because two-dimensional raw data from the analyzer is distorted, image correction is critical for further data processing and is particularly important along the energy direction. We have also carefully investigated the distortion (expansion and contraction) along the angular direction using blocking dips; however, we have not observed any evidence of image distortion within experimental uncertainty. We have used the h2 1 1i axis of a Si(1 0 0) crystal, which is directed at a 35.2644° angle from the wafer normal, for angular measurements. By aligning the normal to the wafer surface with the incident beam using the h1 0 0i channeling dip, we define this position as a zero tilt. Rotating the sample manipulator by 30°, we obtain a reliable angle, i.e. 65.2644°. Fig. 1(a) shows the (2 1 1) blocking dips for two different TEA positions. At five TEA positions, Fig. 1(b), we have confirmed and evaluated the degree of angular distortion and a dispersion relation between angular channel ðCÞ and corresponding scattering angle ðhÞ at an arbitrary TEA position. The dispersion is fitted with a linear equation h ½° ¼ TEA position þ
3. Control and acquisition The MEIS two-dimensional data acquisition system is comprised of five main components: a desktop computer, National Instruments PCI DIO-32HS high speed digital I/O board, PCI-DIO-32HS to Quantar Technology Interface, Quantar 2401B Positional Analyzer with 2415B histogramming buffer memory and Quantar Technology 24012 detector pre-amplifier. The computer is a 0.8 GHz PIII running the Windows 2000 operating system and a customized MEIS data acquisition program (written in LabViewR 5.1). The 24012 pre-amplifier amplifies the four MEIS 2D-detector signals before they are fed into the 2401B positional analyzer. The Quantar Technology 2401B positional analyzer with 2415B histogramming buffer memory converts the MEIS signals from the pre-amplifier into a 1024 1024, 16-bit histogram which can be read into the MEIS acquisition
C p0 : p1 ½ch:=°
For the raw image, 26.38 angular channels ðp1 Þ covers a 1° scattering angle and the nominal TEA angular position corresponds to p0 ¼ 370:29. The best fit polynomial is so linear that each point deviates from this line by less than 0.1% for a region 10° around the TEA center. At this point, the two-dimensional raw image is compressed by factor of 4 in angle, since more than six channels ( ¼ 26.38/4) after compression is enough to contain 1° in the scattering angle. The distorted raw image along the energy direction was corrected using the Ta leading edge from a thick 173.7 nm Ta2 O5 (on Ta substrate) target for each angular channel. We compared the position of the Ta leading edges produced by incident hydrogen ions for a 1 keV energy difference in incident energy at a fixed TEA pass energy ðEpass Þ. The entire TEA angular range was divided into two parts, i.e. those covering 15° lower and higher scattering angles, respectively, and edge pairs
J. Kim et al. / Current Applied Physics 3 (2003) 75–82
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Fig. 2. Leading edges of Ta2 O5 . Left data set is for the TEA at 115° and right data set is for 100°. The upper and lower lines are edges for hydrogen incident energies differing by 1 keV.
Fig. 1. Calibration for angular channel: (a) blocking dips for h2 1 1i axis at different TEA positions; (b) dispersion relation between angular channel and corresponding scattering angle relative to the TEA position.
were collected separately because we already know that only a restricted fraction of the pass energy window will be linear enough to use. Although the pass energy window ðDEÞ of the TEA is known to be 2% of the pass energy, both ends of the energy window could be somewhat distorted (e.g. contracted or expanded when compared with the middle part of DE). The energies of scattered hydrogen ions differ by 0.18% for a scattering angle change from 105° to 115°. By dividing the TEA angular span into two parts, we can use the central part of the energy window that is less than 1.2% Epass . In Fig. 2, the upper and lower lines are Ta edges corresponding to incident energies of 100 and 99 keV, respectively. Using the previous angle calibration, we can calculate the corresponding energy difference between both edge lines for each angular channel, DE ¼ KTa ðhÞDEincidence . The open circles of Fig. 3(b) are the energy width per channel obtained using this relation. We have chosen to fit the measured values with a seven-parameter polynomial instead of using the measured values directly. Besides the energy width per channel, one more parameter is necessary to complete the image correction. There are two types of image distortion. The first is a saw-tooth shaped oscillation along the angular direction due to the discrete nature of two electrodes in the po-
Fig. 3. Calibration parameters for the distortion correction along the energy direction for each angular channel. (a) Open circles show the ETEA pass line and the solid line is the mid-point of the step image. (b) Energy width per energy channel. Solid line results from a sevenparameter polynomial fit.
sition sensitive array. The second gross distortion arises from a combination of many sources: imperfection of the TEA electrode spacing, mechanical misalignment of the exit slits and the inevitable asymmetric nature of the four charge sensitive amplifiers connected to each
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corner. Fig. 3(a) shows the mismatch of the Epass line with the mid-points of the step image as a result of the total gross distortion. With these two calibration parameters, energy width per channel and TEA pass energy contour, all the counts were redistributed into the new image bin. In spite of the correction process for each step image, we cannot use the entire energy window because of a residual distortion near both edges of the energy window and asymmetric distances from the TEA pass energy to the edges at different angles. Thus, only part of the energy window is useful for an image mosaic, where each corrected step image is positioned side by side in a process denoted by the expression image ‘‘tiling’’. As shown in Fig. 4(b), the energy window of a single step image covers a region almost 2.4% of the TEA pass energy, which is defined by the 3.5 mm exit slit between the TEA and the MCP. To show explicitly what happens if the selected energy window is too wide, two energy spectra are compared in Fig. 4(a) which have been tiled with different step energy widths. The open circles show a 1 keV TEA step energy interval and the open triangles show the result after tiling with a 2 keV step. In Fig. 4(b), almost 1.8% of the effective energy window is observed to be acceptable for tiling of this step image. But
Fig. 4. (a) Tiled spectrum with step energy of 1 keV (circles) and 2 keV (triangles). The open squares show the difference of two tiled energy spectra. (b) Energy spectrum of single step image at 125°. The horizontal axis is normalized to the TEA pass energy.
considering the mismatch between the image center and the constant energy contour shown in Fig. 3(a), only the narrower energy window is acceptable for the entire angular region. Thus we have collected images using a step size of 1% of the TEA pass energy. 4.2. Proton charge state distribution The TEA in MEIS experiments only detects charged particles, in contrast to silicon SBDs used in conventional MeV elastic scattering (RBS) experiments. Hence it is necessary to know the fraction of charged particles scattered into the TEA in order to determine absolute elemental compositions. A few previous works [3–8] have reported that the fraction of charged particles depends on the surface species and is also a function of exit energy. However, early experiments [5,6] were performed in poor vacuum conditions (105 to 106 Torr) and even the latest data of Ross [4] has been measured for rather modest vacuum conditions, P 107 Torr. Since the charge state distribution of scattered ions is strongly dependent on the composition of the last several Angstroms of the target [6,7], an UHV environment is essential to avoid effects caused by adventitious surface contamination during the measurements. As a reference, the base pressure in most MEIS target chambers is in the region P 6 109 Torr. The charge state fractions for scattered hydrogen ions were measured using a Au monolayer film (thickness of 0.32 nm measured by RBS) deposited on a thick DLC substrate for the energy range of 60–100 keV incident 1 H ions. The positively charged fraction, f þ , was determined by comparing the yields with the TEA positioned at 130° scattering angle compared to the counts recorded by a Si SBD positioned near 170° scattering angle. Since the exact angle position and solid angle of the SSBD were unknown, the charged particle fraction measurements were then relative. We have normalized our data at 96.2 keV for the easy comparison with the previous works. However, the energy dependence of relative f þ values is sufficient for a quantitative analysis: f þ ¼ pþ =½p0 þ pþ , where pþ ¼ counts by TEA and p0 þ pþ ¼ counts for SBD. Because the resolution of the SSBD was not sufficient to separate the Au peak completely from the carbon substrate signal, the total Au yields were extracted via a Gaussian fitting procedure as shown in Fig. 5. Although both detectors were positioned at different angles ( 170° for SBD and 130° for TEA), the angle dependence of the screening effect can be ignored for both scattering geometries [9] since the collisions can be defined as ÔcloseÕ enough in both cases. The charge fraction resulting from scattering from Au is compared with previous data in Fig. 6. Our data are observed to approach RossÕs values [4]. Further measurements for different surface films are not warranted, we believe, based on the present results. In contrast to
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ple scattering is not. The latter shortcoming should not be significant for thin film and surface analysis, particularly using incident protons. A newer version of this program has now been developed that allows for spectrum fitting using a Levenberg–Marquardt fitting algorithm, wherein both the thickness and composition of the layers can be varied to determine a best fit. These simulation packages are part of the QUARK (quantitative analysis of Rutherford kinematics) series available for analytical ion beam applications (see UWO website of WNL for downloads of other packages, http://www. uwo.ca/isw/people/wlenna/index.html). 4.4. Energy resolution and quantification Fig. 5. Scattered hydrogen spectrum from 0.3 nm Au film on a DLC substrate recorded by a Si SBD for an incident energy of 100 keV.
Fig. 6. Charged fraction of scattered hydrogen ions from 0.3 nm Au layer. The Si SBD is positioned at 170° and the TEA is positioned at 125° scattering angle. Previous data are shown for comparison: Marion and Young (dash-dot), Busch (short-dash) and Ross (solid line).
Energy resolution is one of the key parameters in ion beam analysis especially in backscattering spectra. For MEIS, the total energy resolution in the energy spectrum is comprised of major contributions from: the beam energy spread, the divergence of the incident beam at the target and the vertical velocity component of scattered ions when entering the TEA. The electronic noise during signal processing also composes one of the unavoidable noise terms that deteriorates the system resolution. A value r 215 eV resolution was obtained by fitting the leading edge of the Ta portion of the thick Ta2 O5 spectrum (by integrating a Gaussian function at 125° scattering angle, see Fig. 7). A graphite sample implanted with 50 keV 121 Sb ions to a fluence of 1 1015 ions/cm2 was analyzed with the UWO MEIS system. Since the MEIS facility does not have a reliable fluence measuring device for integrating the incident proton flux, the spectrum height for the carbon substrate was used for normalization. As shown in Fig. 8, the measured Sb dose is 2% higher than the nominal value. When considering the uncertainty embedded in the stopping powers, a 2% difference is a most encouraging result.
previous data, our experiments were performed inside the MEIS target chamber with a pressure in the low 109 Torr range. Thus, these data should be free from any adventitious contamination problem arising during irradiation. In comparing these rudimentary data with existing formulae [3,4,8], we then selected a data set–– specifically, the Marion and Young results––to compensate for the charge fraction of scattered hydrogen ions. 4.3. Spectrum fitting A Windows-based simulation program has been developed to calculate the elastic scattering spectrum for multilayer targets. This program is available from the authors and incorporates, in our opinion, the most reliable electronic stopping power values to date [10]. Electronic energy loss straggling is included, but multi-
Fig. 7. The total MEIS energy resolution was obtained by fitting the Ta leading edge at 125° scattering angle using Gaussian integration.
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Fig. 8. MEIS spectrum of 121 Sb-implanted sample. The measured value is 2% higher compared to the nominal value of 1:0 1015 Sb ions/cm2 .
5. Analysis of Zr silicate As a first application for compositional analysis with the modified UWO MEIS, four Zr silicate films of thickness 2–7 nm were deposited on c-Si (1 0 0) samples. The sample structure is shown schematically in Fig. 9. The three major constituents––Zr, Si and O––were measured by MEIS as well as using other methods; e.g. RBS for zirconium, NRA for oxygen and XPS for the Si:Zr ratio. The double alignment technique was used to provide for the low-Z oxygen measurement in MEIS. The samples were tilted by 45° to align the h1 0 1i axis to the incident beam direction, then the TEA was positioned to include the h1 0 1i axis for the scattered particles, as in Fig. 10. With the incident beam ‘‘channeling in’’ along a major crystallographic axis and the scattered particles emerging in a ‘‘blocking out’’ direction parallel to another major crystallographic axis, then the scattering yield from the crystalline Si substrate was drastically reduced. The heaviest and lightest elements were zirconium and carbon. In general, some adventitious carbon contamination may be unavoidable during sample transportation and/or ion beam irradiation; for the present samples, some monolayer carbon on the surface may be present as a residual of an organic precursor during the growth process. The TEA scan inter-
Fig. 9. Schematic structure of Zr silicate. Surface carbon has several possible sources.
Fig. 10. Double alignment configuration in order to reduce the scattering background arising from the crystalline Si substrate.
val was chosen to include energies corresponding to these elements, i.e. from a starting energy
Fig. 11. Top view of tiled image. The energy spectrum is obtained by a cut along one of the major blocking dips. The incident beam is already aligned with another crystal axis.
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Table 1 MEIS results for zirconium silicate compared to RBS and NRA values sample
#1132 #1135 #1137 #1138
Zr (1E15/cm2 )
Si (1E15/cm2 )
O (1E15/cm2 )
MEIS
RBS
MEIS
MEIS
NRA
MEIS
RBS and NRA
MEIS
XPS
2.5 5.2 12.4 0.3
2.9 5.7 12.0 0.4
13.4 9.4 9.5 8.4
25.1 23.3 40.4 14.2
30.0 27.6 41.9 17.3
10.24 4.52 3.26 48.14
10.31 4.85 3.49 40.14
5.5 1.8 0.8 28.3
5.23 1.33 0.5 23.5
substrate crystal; the latter contribution amounted to a few % of the random height. This background obviously is dependent on the film thickness due to dechanneling in the (amorphous) overlayer which arises from multiple scattering along both inward and outward paths. Background subtraction is necessary before doing spectrum simulation. With this energy spectrum, quantification was performed by simulation as shown in Fig. 12 where we have relied on the measured current integral. The resulting absolute areal density for the four samples are summarized in Table 1. In the table, results obtained by other analysis methods are shown in order to assess the quantification accuracy of the MEIS system. Analysis results from independent methods were in agreement at a level of 15%. We expect that the current integration technique may be in error by as much as 10%. 6. Summary and conclusion The UWO MEIS system has been modified by replacing the previous one-dimensional position detector with a 2D position sensitive array. The calibration procedure, operation scheme and the spectrum simulation
O/Zr
Si/Zr
Table 2 Analytical capability of the UWO MEIS facility compared with those for the conventional RBS technique using MeV ions
Mass resolution (amu) Depth resolution (nm) Accessible depth (nm) Surface sensitivity (ML)
MEIS
RBS
<1 within M2 < 20 <1 <10 <103
<1 within M2 < 40 <10 1000 <102
program are new. The angular calibration was performed using blocking dips from crystalline silicon; the energy calibration was accomplished using the leading edge of the scattered particle energy spectrum from a Ta target, since Ta is a virtually monoisotopic element. We also determined the effective energy window that is sufficiently linear for serial image tiling purposes without distortion. A rudimentary experiment has been performed to determine the energy dependent charge fraction using a thin Au film evaporated onto a (thick) DLC substrate. All the system control, data acquisition and data processing to obtain energy spectra are performed by a customized software package written in LabViewR . A non-linear least squares fitting simulation code has been developed for extracting compositional information. To confirm the quantitative accuracy of the setup, a known amount of Sb implanted into a graphite substrate was analyzed as a reference sample: the nominal and measured doses were in excellent agreement. The total energy resolution of the UWO system was measured to be 215 eV. The depth and mass resolutions derived from this r-value are summarized and compared with those of RBS in Table 2. MEIS has an enhanced depth resolution of an order of magnitude against RBS and is more sensitive for light elements in the near-surface region. However, MEIS is only suitable for relatively shallow depths and has relatively poor mass resolution compared to MeV ion scattering.
Acknowledgements Fig. 12. Spectrum simulation. Open circles show the measured energy spectrum. The solid line overlapping the measured data shows the results from fitting the spectrum. The dotted line is the expected spectrum without the MEIS double alignment geometry.
One of the authors, J. Kim, is grateful for the financial support from the Center for Chemical Physics during stay at the University of Western Ontario.
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T. Hall, Phys. Rev. 79 (1950) 504. J.A. Phillips, Phys. Rev. 97 (1954) 404. M. Copel, IBM, private communication. B.W. Busch, Doctoral thesis, Rutgers, The State University of New Jersey, 2000. [9] J. LÕEcuyer, J.A. Davies, N. Matsunami, Nucl. Instr. Meth. 160 (1979) 160. [10] W.N. Lennard, G..R. Massoumi, T.W. Simpson, I.V. Mitchell, Nucl. Instr. Meth. Phys. Res. B 152 (1999) 370.