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A UHV-compatible ΔE−E gas telescope for depth profiling and surface analysis of light elements
108
Nuclear Instruments and Methods in Physics Research B28 (1987) 108-112 North-Holland, Amsterdam
A UHY-COMPATIBLE BE-E GAS TELESCOPE FOR DEPTH PROFILING AND SURFACE ANALYSIS OF LIGHT ELEMENTS A.M. BEHROOZ, R.L. HEADRICK University of Pennsylvania,
Department
*, L.E. SEIBERLING
of Physics, Philadelphia,
and R.W. ZURMOHLE
PA 19104, USA
Received 19 January 1987 and in revised form 10 March 1987
We have developed a compact AE-E gas telescope that can easily be incorporated into a standard ultrahigh vacuum ion scattering chamber. If energetic, heavy ions are available as a primary beam, the gas telescope can be used to detect light elastic recoils scattered from the surface region of a sample. The detector consists of a gas ionization chamber in front of a surface-barrier detector. The energy loss signal in the gas (AL?) is taken in coincidence with the energy signal in the surface-barrier detector (E) to identify both the atomic number and the total energy of the elastic recoil. The design parameters of the detector have been chosen to allow detection of elements from H to 0 with good depth resolution ( < 100 A) and submonolayer sensitivity. The mass resolution is adequate to easily separate all elements from H to 0.
1. Introduction Rutherford backscattering spectrometry (RBS) is a well-established technique for depth profiling heavy impurities in a lighter substrate, or measuring the absolute concentration of a heavy impurity with submonolayer sensitivity [l]. Ten years ago, L’Ecuyer et al. [2] introduced a novel approach to ion scattering that made depth profiling of light elements possible. The technique, known as elastic recoil detection analysis (ERDA), is based on the fact that the atom struck in a nuclear collision receives a substantial fraction of the energy of the incident ion if the atom recoils at a small forward angle. The elastic recoil cross sections are given by the well-known Rutherford scattering formulae, and depend weakly on the mass of the recoil [3]. Typically, energetic heavy ions are used as a primary beam, and the recoiling light atoms pass through an absorber foil used to filter out the (far more abundant) scattered incident ions. The recoils are then identified by their energies. Energy detection alone can impose a severe restriction on the technique, however, because often a very light and a heavier recoil will have almost the same energy after passing through the absorber foil. This situation arises because of the combined effects of kinematics and energy loss in the absorber. Thus, if such atoms have a distribution in depth, their energies will overlap, making depth profiling impossible. This problem is eliminated by simultaneously determining the atomic number as welt as the energy of the detected * University of Pennsylvania, Department of Materials Science and Engineering, Philadelphia, PA 19104, USA. 0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
particle. Time-of-flight techniques [4] as well as gas telescope recoil detectors [5,6] have been recently used for this purpose. The primary advantages of the gas telescope detector described in this paper are its compactness and simplicity, and its ultra-high vacuum (UHV) compatibility, a necessary requirement for use in surface studies.
2. Scattering chamber and experimental setup Fig. 1 shows a schematic diagram of the scattering chamber and experimental geometry. The sample is attached to a goniometer with two rotational degrees of freedom (about an axis perpendicular to the plane of the figure, and about an axis perpendicular to the sample face) and three translational degrees of freedom. Heavy-ion beams are produced by the Penn FN tandem Van de Graaff accelerator. The choice of primary ion depends, of course, on the requirements of the experiment. A heavier ion will have a larger cross section, but will impart a smaller fraction of its energy to the recoil. A higher energy beam will have a lower cross section, but will produce a proportionately higher energy recoil. Thus, if a deep distribution of a heavier recoil, such as 0, is to be measured, a high energy, less massive primary ion would be appropriate. On the other hand, a greater sensitivity to light elements on a surface could be realized by employing a very heavy, lower energy primary ion. In either case, the thickness of the absorber foil should be just great enough to stop all of the scattered primary ions. Slits adjustable to a minimum size of 0.13 mm,
A.M. Behrooz et al. / UHV-compatible
A E - E gas telescope CATHODE
109 ANODE
Fig. 1. A schematic diagram of the scattering chamber and experimental geometry.
6 I pm
HAVAR
FOIL
SOLID
placed 2 m apart, are used to define the beam spot size. The beam spot size can be verified by measuring the transmission through an aperture located adjacent to the sample. The total beam dose is measured using a current integrator located between the second adjustable slit and the sample. The current integrator consists of a beam stop mounted on a magnetically actuated feedthrough. The beam stop is made of Al, with a thin (0.1 pm) Au overlayer. The face of the beam stop is oriented at 45“ to the incident beam, so that the Au layer scatters ions into an auxiliary surface-barrier detector. The beam stop is controlled by an electronic timer, typically set at a duty cycle of 20% The number of counts in the auxiliary detector can be calibrated against the beam current measured in the Faraday cup located at the beam-exit side of the chamber. This procedure routinely yields current integration to better than 5% accuracy. Our recoil detector is at a fixed angle of 18O to the incident beam direction. At 18 O, the detector just clears the path of the incident beam when it is put through to the Faraday cup.
3. Design and performance of the detector Fig. 2 shows a cross sectional drawing of the detector. The distance from the entrance window (Havar foil) to the base of the solid state detector is 7 cm. The front part of the telescope (cathode) is electrically insulated from the back section (anode) by a thin, annular Teflon spacer. The two sections are joined by a Viton O-ring seal, and the bolts are isolated from the cathode by Teflon bushings. The window of the ionization chamber consists of a 6.1 pm thick Havar foil [7] that is attached to the cathode with a metal flange. A Viton O-ring provides the vacuum seal. This pressure window can easily withstand the pressure difference of 1 atm that occurs during pumpdown of the scattering chamber. In fact, a single piece of Havar foil has been in place for over two years and has been cycled between vacuum and atmospheric pressure more than fifty times without detectable leakage. The ultimate pressure in the scattering chamber with atmospheric pressure in the ionization chamber is less than 1 X 10-r’ Torr. This pressure is achieved after baking the chamber for several days at
TEFLON AND
BUSHINGS
STATE
TUBES
FOR
GAS FLOW
SPACER
Fig. 2. A cross sectional drawing of the AE-E gas telescope. The distance from the entrance window (Havar foil) to the base of the solid state detector is 7 cm.
120 o C. The operating pressure of the ionization chamber is typically several hundred Torr. The Havar foil also serves to stop the elastically scattered primary ions. The gas used in the ionization chamber is introduced through the gas flow tubes shown in fig. 2. We selected a gas mixture of 90% Ar and 10% CH, because it offers a high electron mobility at modest values of the electric field strength. The gas flow tubes are attached to a gas manifold with a manostat, so that the ionization chamber can be held at a constant pressure while allowing a constant flow of gas through the detector. This is necessary to avoid contamination of the gas by outgassing from the inside walls of the detector. When a static charge of gas is used in the ionization chamber, a slow degradation of the AE resolution is observed over several hours. The surface-barrier detector that generates the E signal has an active area of 100 mm2 and a depletion depth of 300 pm (Ortec model BA-16-100300). In order to reduce the leakage current, the detector is cooled by attaching a copper braid to the base of the anode inside the UHV chamber. The other end of the braid is attached to a chilled water feedthrough. Our telescope is presently configured so that its entire body forward of the base is inside the UHV chamber. This geometry requires that the surface-barrier detector be cooled during bake out of the vacuum chamber. Standard surface-barrier detectors are very sensitive to temperatures above about 30” C. in the past, we have provided cooling to the surface-barrier detector during bake out by flowing chilled N, gas through the gas flow tubes. However, ion-implanted passivated Si detectors having excellent high temperature properties are now available, and the installation of
110
A.M. Behrooz et al. / UHV-compatible
such a device would allow bake out temperatures up to 200 o C, a limit which is imposed by the Viton seals. Our gas telescope detector was modeled after one described by Zurmtihle and Csihas [EL],in which the counter window serves as the anode of the ionization chamber, i.e., it is positively biased and collects the electrons that are generated by ionization from the particle that is being detected. In our setup we found that the positive bias resulted in a substantial flow of secondary electrons from the sample to the anode. For that reason we operated the ionization chamber at a reverse bias of - 150 V. This prevented most secondary electrons from reaching the counter. The front electrode then effectively serves as cathode of the ionization chamber. The energy loss signal (AE) is produced by the image charge of the electrons generated in the ionization chamber. The resolution in A E is determined by the uniformity of the electric field through which the recoils pass [8], and the energy straggling of the recoils in the gas. The geometry shown in fig. 2 was chosen because it produces a highly uniform electric field along the flight path of the recoils [8]. Thus, for this detector, the resolution in AE is dominated by energy straggling in the gas. Fig. 3 shows a plot of A E versus energy deposited in the surface-barrier detector (E), for a 58 MeV Cu ion beam on a Ta sample that has been chemically cleaned then exposed to air for several days. The AE and E signals are taken in coincidence, and the number of counts with a particular value of A E and E is indicated
by the size of the dot at that point. The recoils with the largest atomic number suffer the largest energy loss in the gas, as indicated in the figure. The energy distribution for each recoil is well-separated from the others, allowing a clean depth profile to be extracted for each atom. This Ta sample shows clear signals for H, C and 0, and a smaller number of N counts. The ionization gas pressure in this case (125 Torr) was made high enough to completely separate the C, N and 0 signals. A higher pressure would increase the separation further, but would reduce the maximum depth from which these elements could be detected. In order to display the mass resolution for C, N and 0 more clearly, fig. 4 shows a plot of counts versus AE, where all energies above approximately 5 MeV have been summed for each value of AE. Clearly, the resolution in A E is adequate to completely separate these elements. The ionization gas pressure can easily be adjusted to accommodate the needs of each particular experiment. For example, if a 3 MeV 4He beam were being used to profile hydrogen and deuterium in a metal, a gas pressure of 250 Torr would produce a separation in the A E signals for hydrogen and deuterium of 95 keV, which is about three times the resolution in AE for these isotopes. The energy deposited in the surface-barrier detector would be 900 keV for hydrogen and 1.36 MeV for
AE - E gas telescope
0
2
4
6
8
10
12
14
16
( YeV) Fig. 3. A plot of energy loss in the gas (AE) versus energy deposited in the surface-barrier detector (E), for a 58 MeV Cu ion beam on a Ta sample that had been chemically cleaned then exposed to air for several days. The AE and E signals are taken in coincidence, and the number of counts with a particular value of AE and E is indicated by the size of the dot at that point. The recoils having the largest atomic number suffer the largest energy loss in the gas, as indicated in the figure. ENERGY
deuterium. These calculated values assume a recoil angle of 18O and a Havar foil thickness of 6.1 pm (just thick enough to stop the scattered 4He ions). The spread in the energy signal for monoenergetic recoils limits depth resolution. Energy resolution is determined primarily by three things; the angular acceptance of the recoil detector and finite beam spot size (because the recoil energy varies with angle), energy straggling in the Havar foil and gas, and the intrinsic energy resolution of the surface barrier detector. In order to limit the angular acceptance of the recoil detector, a vertical slit, 1.5 mm wide was attached to the entrance window of the detector cathode. The beam was also collimated to a width of 1.5 mm using adjustable beam line slits. These factors produce a calculated energy spread of 540 keV for 58 MeV Cu ions on C. This contribution to the energy resolution is much greater
ENERGY > 5 3MeV
OXIDIZED Ta SAMPLE
ENERGY
LOSS
(MeV)
Fig. 4. A plot of counts versus energy loss in the gas, for the sample of fig. 3. All energies above roughly 5 MeV have been summed for each value of A E.
A.M. Behrooz et al. / UHV-compatible
A E - E gas telescope
111
ple is approximately 70 A. In applications for which sensitivity is important and resolution is not a factor, .:.
:
OS ._,’ ....fg..~~
0
2
4
6
6
10
12
14
16
ENERGY ( MeV) Fig. 5. A plot of AE versus E for a 58 MeV Cu ion beam on a chemically cleaned Si wafer with a 220 A Ag layer evaporated on the surface. For this sample, C and 0 are found on the Ag
surface (these peaks are labeledC, and O,), and at the Ag/Si interface(labeledCi and Oi).
than the intrinsic energy resolution of the surface-barrier detector (roughly 100 keV for 0). Fig. 5 shows a plot of AE versus E for a 58 MeV Cu ion 0beam on a chemically cleaned Si wafer with a 220 A Ag layer evaporated on the surface. For this sample, C and 0 are found on the Ag surface (these peaks are labeled C, and O,), and at the Ag/Si interface (labeled Ci and Oi). A map can be drawn around any desired region on the A E versus E plot, and the events inside that region can then be plotted versus E. Such a plot for the C and 0 data is displayed in fig. 6, where the peaks for C and 0 recoils originating from the Ag surface (subscript s) and the Ag/Si interface (subscript i) are marked on the plot. It can be seen from the C surface peak that the energy resolution of the detector for C is roughly 700 keV. Thus, in this case, energy straggling of the recoil C ions contributes roughly the same to the energy resolution as do the slit and beam spot widths (the contributions add in quadrature). The depth resolution for C in this sam-
140/.,, i20100
‘0
-
CARBON OXYGEN
A 2
,c
4
such as detection of light adsorbates on a surface, the vertical detector slits can be removed, thereby increasing the count rate by roughly a factor of three. By choosing a recoil angle of 18 O, we have sacrificed some sensitivity, because the recoil cross section increases with angle. However, we have gained in the ability to depth profile heavier recoils, such as 0 or F, because of their higher energy at small angles. The sensitivity of a detector can be limited by background, count rate (statistical uncertainties), and by damage to the sample by the incident ion beam. It is clear from figs. 5 and 6 that background counts are not a limiting factor for this detector. The cross section for C recoils at 18O with 58 MeV Cu ions is 2.1 barn/sr. This results in a count rate of 0.019 counts/s per monolayer for our detector geometry and beam current (2 particle nA, with a beam spot size of 0.039 cm2). Thus, a monolayer of C may be measured with a 10% uncertainty in 90 min. In order to investigate possible changes in the sample produced by ion bombardment, the sample of figs. 5 and 6 was bombarded with consecutive doses of 1.75 X 1013 ions/cm2. The AE versus E spectra collected during each dose were analyzed to determine the concentration of H, C and 0 at the Ag/Si interface at that dose. The interfacial concentrations versus ion dose are displayed in fig. 7. Clearly, the sensitivity to H is limited by beam damage, whereas for C and 0 it is limited by counting statistics. No changes in the peak shapes were detected, indicating that C and 0 atoms at the interface were not being moved perpendicular to the interface by more than a few tens of A during ion bombardment. A sputter/Auger depth profile of another Si wafer with a 220 A Ag overlayer (prepared in exactly the same way as the one discussed above) was measured using a PHI model 600 Scanning Auger Microprobe. The results of this measurement are displayed in fig. 8. This is
AND MAP
6
ENERGY
8
10
12
14
16
(MeV)
Fig. 6. A plot of counts versus E for the C and 0 data of fig. 5. The peaks for C and 0 recoils originatingfrom the Ag surface (subscript s) and the Ag/Si interface (subscript i) are marked on the plot.
DOSE
(10’4hn2)
Fig. 7. The concentrationof H, C and 0 at the Ag/Si interface of the sample of fig. 5 versus Cu ion dose. The beam current density was 50 particle nA/cm*.
112
A.M. Behrooz et al. / UHV-compatible
UHV scattering chamber. If energetic heavy ions are available as a primary beam, this detector can significantly enhance the versatility of an ion scattering facility by making depth profiling of light elements possible. Test results show that elements from H to 0 can be detected with good depth resolution (< 100 A) and submonolayer sensitivity. The mass resolution is adequate to easily separate all elements from H to 0.
u 60
r p a
A E - E gas ielescope
40 20
0
0
2 SPUTTER
4
3 TIME
5
(min)
Fig. 8. A sputter/Auger depth profile of a chemically cleaned Si wafer with a 220 A Ag layer evaporated on the surface. The Auger technique is not sensitive to H, and the C concentration (5 X 1015 atoms/cm2) is below the limit of detectability. This sample was prepared in the same way as that in fig. 5.
a standard technique for measuring depth profiles in the near-surface region of a sample, and consists of measuring Auger electron peak heights while continuously sputtering to produce a depth profile. Auger electron spectroscopy is not sensitive to H, and for this sample, the C concentration (5 X 1015 atoms/cm2) is below the limit of detectability. The 0 concentration (1 X 1015 atoms/cm2) is clearly just at the limit of detectability. The sputter/Auger technique is not only less sensitive to light elements, but can only determine relative concentrations. The ERDA technique, on the other hand, gives absolute concentrations, with excellent sensitivity.
4. Conclusions We have presented a compact AE-E gas telescope detector that can easily be incorporated into a standard
The contributions of D.P. Balamuth during the early design stages of the detector, and, in particular, the original suggestion of using a gas telescope for particle identification in ERDA are gratefully acknowledged. The authors wish to acknowledge the technical expertise and helpful suggestions of J. Klein and SF. Pate, and the assistance of D.M. Shadovitz in the construction of the gas manifold. This work was supported by the NSF and the IBM Corp.
References [l] W.K. Chu, J.W. Mayer and M.-A. Nicolet, Backscattering Spectrometry (Academic Press, New York, 1978). [2] J. L’Ecuyer, C. Brassard, C. Cardinal, J. ChabbaI, L. Deschenes and J.P. Labrie, J. Appl. Phys. 47 (1976) 381. [3] C. No&her, K. Brenner, R. Knauf and W. Schmidt, Nucl. Instr. and Meth. 218 (1983) 116. [4] R. Groleau, SC. Gujrathi and J.P. Martin, Nucl. Instr. and Meth. 218 (1983) 11. [5] M. Petrascu, I. Berceanu, I. Brancus, A. Buta, M. Duma, C. Grama, I. Lazar, M. Mihaila and I. Ghita, Nucl. Instr. and Meth. B4 (1984) 396. [6] R.Q. Yu and T. Gustafsson, to be published. [7] Havar is a product of Hamilton Technology, Inc., Lancaster, PA, USA. [S] R.W. Zurmtihle and L. Csihas, Nucl. Instr. and Meth. 203 (1982) 261.
Nuclear Instruments and Methods in Physics Research B28 (1987) 108-112 North-Holland, Amsterdam
A UHY-COMPATIBLE BE-E GAS TELESCOPE FOR DEPTH PROFILING AND SURFACE ANALYSIS OF LIGHT ELEMENTS A.M. BEHROOZ, R.L. HEADRICK University of Pennsylvania,
Department
*, L.E. SEIBERLING
of Physics, Philadelphia,
and R.W. ZURMOHLE
PA 19104, USA
Received 19 January 1987 and in revised form 10 March 1987
We have developed a compact AE-E gas telescope that can easily be incorporated into a standard ultrahigh vacuum ion scattering chamber. If energetic, heavy ions are available as a primary beam, the gas telescope can be used to detect light elastic recoils scattered from the surface region of a sample. The detector consists of a gas ionization chamber in front of a surface-barrier detector. The energy loss signal in the gas (AL?) is taken in coincidence with the energy signal in the surface-barrier detector (E) to identify both the atomic number and the total energy of the elastic recoil. The design parameters of the detector have been chosen to allow detection of elements from H to 0 with good depth resolution ( < 100 A) and submonolayer sensitivity. The mass resolution is adequate to easily separate all elements from H to 0.
1. Introduction Rutherford backscattering spectrometry (RBS) is a well-established technique for depth profiling heavy impurities in a lighter substrate, or measuring the absolute concentration of a heavy impurity with submonolayer sensitivity [l]. Ten years ago, L’Ecuyer et al. [2] introduced a novel approach to ion scattering that made depth profiling of light elements possible. The technique, known as elastic recoil detection analysis (ERDA), is based on the fact that the atom struck in a nuclear collision receives a substantial fraction of the energy of the incident ion if the atom recoils at a small forward angle. The elastic recoil cross sections are given by the well-known Rutherford scattering formulae, and depend weakly on the mass of the recoil [3]. Typically, energetic heavy ions are used as a primary beam, and the recoiling light atoms pass through an absorber foil used to filter out the (far more abundant) scattered incident ions. The recoils are then identified by their energies. Energy detection alone can impose a severe restriction on the technique, however, because often a very light and a heavier recoil will have almost the same energy after passing through the absorber foil. This situation arises because of the combined effects of kinematics and energy loss in the absorber. Thus, if such atoms have a distribution in depth, their energies will overlap, making depth profiling impossible. This problem is eliminated by simultaneously determining the atomic number as welt as the energy of the detected * University of Pennsylvania, Department of Materials Science and Engineering, Philadelphia, PA 19104, USA. 0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
particle. Time-of-flight techniques [4] as well as gas telescope recoil detectors [5,6] have been recently used for this purpose. The primary advantages of the gas telescope detector described in this paper are its compactness and simplicity, and its ultra-high vacuum (UHV) compatibility, a necessary requirement for use in surface studies.
2. Scattering chamber and experimental setup Fig. 1 shows a schematic diagram of the scattering chamber and experimental geometry. The sample is attached to a goniometer with two rotational degrees of freedom (about an axis perpendicular to the plane of the figure, and about an axis perpendicular to the sample face) and three translational degrees of freedom. Heavy-ion beams are produced by the Penn FN tandem Van de Graaff accelerator. The choice of primary ion depends, of course, on the requirements of the experiment. A heavier ion will have a larger cross section, but will impart a smaller fraction of its energy to the recoil. A higher energy beam will have a lower cross section, but will produce a proportionately higher energy recoil. Thus, if a deep distribution of a heavier recoil, such as 0, is to be measured, a high energy, less massive primary ion would be appropriate. On the other hand, a greater sensitivity to light elements on a surface could be realized by employing a very heavy, lower energy primary ion. In either case, the thickness of the absorber foil should be just great enough to stop all of the scattered primary ions. Slits adjustable to a minimum size of 0.13 mm,
A.M. Behrooz et al. / UHV-compatible
A E - E gas telescope CATHODE
109 ANODE
Fig. 1. A schematic diagram of the scattering chamber and experimental geometry.
6 I pm
HAVAR
FOIL
SOLID
placed 2 m apart, are used to define the beam spot size. The beam spot size can be verified by measuring the transmission through an aperture located adjacent to the sample. The total beam dose is measured using a current integrator located between the second adjustable slit and the sample. The current integrator consists of a beam stop mounted on a magnetically actuated feedthrough. The beam stop is made of Al, with a thin (0.1 pm) Au overlayer. The face of the beam stop is oriented at 45“ to the incident beam, so that the Au layer scatters ions into an auxiliary surface-barrier detector. The beam stop is controlled by an electronic timer, typically set at a duty cycle of 20% The number of counts in the auxiliary detector can be calibrated against the beam current measured in the Faraday cup located at the beam-exit side of the chamber. This procedure routinely yields current integration to better than 5% accuracy. Our recoil detector is at a fixed angle of 18O to the incident beam direction. At 18 O, the detector just clears the path of the incident beam when it is put through to the Faraday cup.
3. Design and performance of the detector Fig. 2 shows a cross sectional drawing of the detector. The distance from the entrance window (Havar foil) to the base of the solid state detector is 7 cm. The front part of the telescope (cathode) is electrically insulated from the back section (anode) by a thin, annular Teflon spacer. The two sections are joined by a Viton O-ring seal, and the bolts are isolated from the cathode by Teflon bushings. The window of the ionization chamber consists of a 6.1 pm thick Havar foil [7] that is attached to the cathode with a metal flange. A Viton O-ring provides the vacuum seal. This pressure window can easily withstand the pressure difference of 1 atm that occurs during pumpdown of the scattering chamber. In fact, a single piece of Havar foil has been in place for over two years and has been cycled between vacuum and atmospheric pressure more than fifty times without detectable leakage. The ultimate pressure in the scattering chamber with atmospheric pressure in the ionization chamber is less than 1 X 10-r’ Torr. This pressure is achieved after baking the chamber for several days at
TEFLON AND
BUSHINGS
STATE
TUBES
FOR
GAS FLOW
SPACER
Fig. 2. A cross sectional drawing of the AE-E gas telescope. The distance from the entrance window (Havar foil) to the base of the solid state detector is 7 cm.
120 o C. The operating pressure of the ionization chamber is typically several hundred Torr. The Havar foil also serves to stop the elastically scattered primary ions. The gas used in the ionization chamber is introduced through the gas flow tubes shown in fig. 2. We selected a gas mixture of 90% Ar and 10% CH, because it offers a high electron mobility at modest values of the electric field strength. The gas flow tubes are attached to a gas manifold with a manostat, so that the ionization chamber can be held at a constant pressure while allowing a constant flow of gas through the detector. This is necessary to avoid contamination of the gas by outgassing from the inside walls of the detector. When a static charge of gas is used in the ionization chamber, a slow degradation of the AE resolution is observed over several hours. The surface-barrier detector that generates the E signal has an active area of 100 mm2 and a depletion depth of 300 pm (Ortec model BA-16-100300). In order to reduce the leakage current, the detector is cooled by attaching a copper braid to the base of the anode inside the UHV chamber. The other end of the braid is attached to a chilled water feedthrough. Our telescope is presently configured so that its entire body forward of the base is inside the UHV chamber. This geometry requires that the surface-barrier detector be cooled during bake out of the vacuum chamber. Standard surface-barrier detectors are very sensitive to temperatures above about 30” C. in the past, we have provided cooling to the surface-barrier detector during bake out by flowing chilled N, gas through the gas flow tubes. However, ion-implanted passivated Si detectors having excellent high temperature properties are now available, and the installation of
110
A.M. Behrooz et al. / UHV-compatible
such a device would allow bake out temperatures up to 200 o C, a limit which is imposed by the Viton seals. Our gas telescope detector was modeled after one described by Zurmtihle and Csihas [EL],in which the counter window serves as the anode of the ionization chamber, i.e., it is positively biased and collects the electrons that are generated by ionization from the particle that is being detected. In our setup we found that the positive bias resulted in a substantial flow of secondary electrons from the sample to the anode. For that reason we operated the ionization chamber at a reverse bias of - 150 V. This prevented most secondary electrons from reaching the counter. The front electrode then effectively serves as cathode of the ionization chamber. The energy loss signal (AE) is produced by the image charge of the electrons generated in the ionization chamber. The resolution in A E is determined by the uniformity of the electric field through which the recoils pass [8], and the energy straggling of the recoils in the gas. The geometry shown in fig. 2 was chosen because it produces a highly uniform electric field along the flight path of the recoils [8]. Thus, for this detector, the resolution in AE is dominated by energy straggling in the gas. Fig. 3 shows a plot of A E versus energy deposited in the surface-barrier detector (E), for a 58 MeV Cu ion beam on a Ta sample that has been chemically cleaned then exposed to air for several days. The AE and E signals are taken in coincidence, and the number of counts with a particular value of A E and E is indicated
by the size of the dot at that point. The recoils with the largest atomic number suffer the largest energy loss in the gas, as indicated in the figure. The energy distribution for each recoil is well-separated from the others, allowing a clean depth profile to be extracted for each atom. This Ta sample shows clear signals for H, C and 0, and a smaller number of N counts. The ionization gas pressure in this case (125 Torr) was made high enough to completely separate the C, N and 0 signals. A higher pressure would increase the separation further, but would reduce the maximum depth from which these elements could be detected. In order to display the mass resolution for C, N and 0 more clearly, fig. 4 shows a plot of counts versus AE, where all energies above approximately 5 MeV have been summed for each value of AE. Clearly, the resolution in A E is adequate to completely separate these elements. The ionization gas pressure can easily be adjusted to accommodate the needs of each particular experiment. For example, if a 3 MeV 4He beam were being used to profile hydrogen and deuterium in a metal, a gas pressure of 250 Torr would produce a separation in the A E signals for hydrogen and deuterium of 95 keV, which is about three times the resolution in AE for these isotopes. The energy deposited in the surface-barrier detector would be 900 keV for hydrogen and 1.36 MeV for
AE - E gas telescope
0
2
4
6
8
10
12
14
16
( YeV) Fig. 3. A plot of energy loss in the gas (AE) versus energy deposited in the surface-barrier detector (E), for a 58 MeV Cu ion beam on a Ta sample that had been chemically cleaned then exposed to air for several days. The AE and E signals are taken in coincidence, and the number of counts with a particular value of AE and E is indicated by the size of the dot at that point. The recoils having the largest atomic number suffer the largest energy loss in the gas, as indicated in the figure. ENERGY
deuterium. These calculated values assume a recoil angle of 18O and a Havar foil thickness of 6.1 pm (just thick enough to stop the scattered 4He ions). The spread in the energy signal for monoenergetic recoils limits depth resolution. Energy resolution is determined primarily by three things; the angular acceptance of the recoil detector and finite beam spot size (because the recoil energy varies with angle), energy straggling in the Havar foil and gas, and the intrinsic energy resolution of the surface barrier detector. In order to limit the angular acceptance of the recoil detector, a vertical slit, 1.5 mm wide was attached to the entrance window of the detector cathode. The beam was also collimated to a width of 1.5 mm using adjustable beam line slits. These factors produce a calculated energy spread of 540 keV for 58 MeV Cu ions on C. This contribution to the energy resolution is much greater
ENERGY > 5 3MeV
OXIDIZED Ta SAMPLE
ENERGY
LOSS
(MeV)
Fig. 4. A plot of counts versus energy loss in the gas, for the sample of fig. 3. All energies above roughly 5 MeV have been summed for each value of A E.
A.M. Behrooz et al. / UHV-compatible
A E - E gas telescope
111
ple is approximately 70 A. In applications for which sensitivity is important and resolution is not a factor, .:.
:
OS ._,’ ....fg..~~
0
2
4
6
6
10
12
14
16
ENERGY ( MeV) Fig. 5. A plot of AE versus E for a 58 MeV Cu ion beam on a chemically cleaned Si wafer with a 220 A Ag layer evaporated on the surface. For this sample, C and 0 are found on the Ag
surface (these peaks are labeledC, and O,), and at the Ag/Si interface(labeledCi and Oi).
than the intrinsic energy resolution of the surface-barrier detector (roughly 100 keV for 0). Fig. 5 shows a plot of AE versus E for a 58 MeV Cu ion 0beam on a chemically cleaned Si wafer with a 220 A Ag layer evaporated on the surface. For this sample, C and 0 are found on the Ag surface (these peaks are labeled C, and O,), and at the Ag/Si interface (labeled Ci and Oi). A map can be drawn around any desired region on the A E versus E plot, and the events inside that region can then be plotted versus E. Such a plot for the C and 0 data is displayed in fig. 6, where the peaks for C and 0 recoils originating from the Ag surface (subscript s) and the Ag/Si interface (subscript i) are marked on the plot. It can be seen from the C surface peak that the energy resolution of the detector for C is roughly 700 keV. Thus, in this case, energy straggling of the recoil C ions contributes roughly the same to the energy resolution as do the slit and beam spot widths (the contributions add in quadrature). The depth resolution for C in this sam-
140/.,, i20100
‘0
-
CARBON OXYGEN
A 2
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4
such as detection of light adsorbates on a surface, the vertical detector slits can be removed, thereby increasing the count rate by roughly a factor of three. By choosing a recoil angle of 18 O, we have sacrificed some sensitivity, because the recoil cross section increases with angle. However, we have gained in the ability to depth profile heavier recoils, such as 0 or F, because of their higher energy at small angles. The sensitivity of a detector can be limited by background, count rate (statistical uncertainties), and by damage to the sample by the incident ion beam. It is clear from figs. 5 and 6 that background counts are not a limiting factor for this detector. The cross section for C recoils at 18O with 58 MeV Cu ions is 2.1 barn/sr. This results in a count rate of 0.019 counts/s per monolayer for our detector geometry and beam current (2 particle nA, with a beam spot size of 0.039 cm2). Thus, a monolayer of C may be measured with a 10% uncertainty in 90 min. In order to investigate possible changes in the sample produced by ion bombardment, the sample of figs. 5 and 6 was bombarded with consecutive doses of 1.75 X 1013 ions/cm2. The AE versus E spectra collected during each dose were analyzed to determine the concentration of H, C and 0 at the Ag/Si interface at that dose. The interfacial concentrations versus ion dose are displayed in fig. 7. Clearly, the sensitivity to H is limited by beam damage, whereas for C and 0 it is limited by counting statistics. No changes in the peak shapes were detected, indicating that C and 0 atoms at the interface were not being moved perpendicular to the interface by more than a few tens of A during ion bombardment. A sputter/Auger depth profile of another Si wafer with a 220 A Ag overlayer (prepared in exactly the same way as the one discussed above) was measured using a PHI model 600 Scanning Auger Microprobe. The results of this measurement are displayed in fig. 8. This is
AND MAP
6
ENERGY
8
10
12
14
16
(MeV)
Fig. 6. A plot of counts versus E for the C and 0 data of fig. 5. The peaks for C and 0 recoils originatingfrom the Ag surface (subscript s) and the Ag/Si interface (subscript i) are marked on the plot.
DOSE
(10’4hn2)
Fig. 7. The concentrationof H, C and 0 at the Ag/Si interface of the sample of fig. 5 versus Cu ion dose. The beam current density was 50 particle nA/cm*.
112
A.M. Behrooz et al. / UHV-compatible
UHV scattering chamber. If energetic heavy ions are available as a primary beam, this detector can significantly enhance the versatility of an ion scattering facility by making depth profiling of light elements possible. Test results show that elements from H to 0 can be detected with good depth resolution (< 100 A) and submonolayer sensitivity. The mass resolution is adequate to easily separate all elements from H to 0.
u 60
r p a
A E - E gas ielescope
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0
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Fig. 8. A sputter/Auger depth profile of a chemically cleaned Si wafer with a 220 A Ag layer evaporated on the surface. The Auger technique is not sensitive to H, and the C concentration (5 X 1015 atoms/cm2) is below the limit of detectability. This sample was prepared in the same way as that in fig. 5.
a standard technique for measuring depth profiles in the near-surface region of a sample, and consists of measuring Auger electron peak heights while continuously sputtering to produce a depth profile. Auger electron spectroscopy is not sensitive to H, and for this sample, the C concentration (5 X 1015 atoms/cm2) is below the limit of detectability. The 0 concentration (1 X 1015 atoms/cm2) is clearly just at the limit of detectability. The sputter/Auger technique is not only less sensitive to light elements, but can only determine relative concentrations. The ERDA technique, on the other hand, gives absolute concentrations, with excellent sensitivity.
4. Conclusions We have presented a compact AE-E gas telescope detector that can easily be incorporated into a standard
The contributions of D.P. Balamuth during the early design stages of the detector, and, in particular, the original suggestion of using a gas telescope for particle identification in ERDA are gratefully acknowledged. The authors wish to acknowledge the technical expertise and helpful suggestions of J. Klein and SF. Pate, and the assistance of D.M. Shadovitz in the construction of the gas manifold. This work was supported by the NSF and the IBM Corp.
References [l] W.K. Chu, J.W. Mayer and M.-A. Nicolet, Backscattering Spectrometry (Academic Press, New York, 1978). [2] J. L’Ecuyer, C. Brassard, C. Cardinal, J. ChabbaI, L. Deschenes and J.P. Labrie, J. Appl. Phys. 47 (1976) 381. [3] C. No&her, K. Brenner, R. Knauf and W. Schmidt, Nucl. Instr. and Meth. 218 (1983) 116. [4] R. Groleau, SC. Gujrathi and J.P. Martin, Nucl. Instr. and Meth. 218 (1983) 11. [5] M. Petrascu, I. Berceanu, I. Brancus, A. Buta, M. Duma, C. Grama, I. Lazar, M. Mihaila and I. Ghita, Nucl. Instr. and Meth. B4 (1984) 396. [6] R.Q. Yu and T. Gustafsson, to be published. [7] Havar is a product of Hamilton Technology, Inc., Lancaster, PA, USA. [S] R.W. Zurmtihle and L. Csihas, Nucl. Instr. and Meth. 203 (1982) 261.