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Accelerator based mass spectrometry of semiconductor materials
Nuclear Instruments and Methods in Physics Research 218 (1983) 463 467 North-Holland, Amsterdam
ACCELERATOR
BASED
MASS
SPECTROMETRY
463
OF SEMICONDUCTOR
MATERIALS
J.M. ANTHONY Materials Science Laboratory, Texas Instruments Incorporated. P.O. Box 225936, MS 147, Dallas, TX 75265, USA
J. T H O M A S Kellogg Radiation Laboratory 106-38, California Institute of Technology, Pasadena, CA 91125, USA
We have investigated the possibility of using accelerator based mass spectrometry for the study of semiconductor materials. especially Si and GaAs. Several impurities have been examined, including B, Be, Si, Nb, Sb and Te. For most of these impurities, the count rates from high background levels in the ion source totally dominated the signals from the samples. For Nb. however, the background levels were quite low, and detection limits of - 0.1 parts-per-billion have been demonstrated. The results discussed here suggest that the technique is applicable to most impurity substrate combinations, but the development of a clean ion source is necessary for routine use of this method.
1. Introduction One of the most i m p o r t a n t techniques for impurity analysis in semiconductors is Secondary Ion Mass Spectrometry (SIMS) [1]. In normal SIMS measurements, energetic ions ( 1 0 - 2 0 keV) are used to sputter material from a sample of interest. During the sputtering process some of the surface atoms become ionized, a n d these ions are collected a n d mass analyzed. There are several b a c k g r o u n d problems which limit the sensitivity of normal SIMS instruments, however, including: (1) mass interferences between impurities a n d molecules, such as 31p and 3°Sill, (2) particles with kinematic properties similar to those of the impurity of interest, such as 2SSi2+ and 14N+, etc. The sensitivity of the technique is - 1 part-per-million (ppm) atomic for most elements of interest, although lower values have been quoted in some cases [2,3]. As the sizes of semiconductor devices decrease, material purity becomes increasingly important, and there is a strong need for analysis at the part-per-billion ( p p b ) level a n d below. One possible candidate for measurements of this type is accelerator based mass spectroscopy. In this technique, negative ions are produced in a sputter source as in regular SIMS and injected into a t a n d e m accelerator. The ions are accelerated by a high positive potential a n d stripped to a positive charge state by a gas or foil stripper. The resultant ions are again accelerated by the positive potential, which results in high ion energies (several MeV). After leaving the accelerator, the particles are analyzed and detected using conventional nuclear physics techniques (fig. 1) [4-8]. The advantage of accelerator based mass spectrometry is that is is not subject to the b a c k g r o u n d problems 0 1 6 7 - 5 0 8 7 / 8 3 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
m e n t i o n e d in connection with s t a n d a r d SIMS. For example, although molecules will still be produced in the sputter source, electron stripping in the gas canal produces a " C o u l o m b explosion" due to the electrostatic repulsion between the atoms in the molecule, which dissociates the molecule a n d thereby removes the interference. Also, the high energy of the particles allows the use of nuclear detection techniques which overdetermine the charge, mass a n d energy of these particles. This effectively separates the b a c k g r o u n d c o u n t s from the c o n t a m i n a n t counts, which greatly increases the sensitivity. GRIDDED LENS
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Fig. 1. Schematic representation of the experimental apparatus. Negative ions produced in the Cs source are deflected by the ion source magnet and injected into the accelerator with an energy of 200 keV. After acceleration, the ions pass through the analyzing magnet, the switching magnet and the electrostatic separator. A removable Faraday cup and the ,:1E - E ionization chamber located after the separator allow detection of a broad range of counting rates.
464
J.M. Anthony. J. Thomas / Accelerator based mass spectromett T
To date, accelerator based mass spectrometry has been used primarily for geologic dating, in which ratios of rare radioactive isotopes to a b u n d a n t stable isotopes (such a s 14C/12C or m B e / g B e ) are measured [9,10], and concentrations of the rare isotopes as low a s 10 7 a t / c c have been detected. Extending this technique to stable isotope impurity m e a s u r e m e n t s requires some care, however. The impurities of interest in most semic o n d u c t o r materials are not necessarily similar in mass to the bulk material, and the transmission properties of the impurity and bulk ions may be quite different. Also, most ion sources on existing t a n d e m accelerators are not designed for impurity measurements, and thus the source itself may produce high b a c k g r o u n d counts for m a n y elements, With these constraints in mind, we have used the Yale T a n d e m Van de G r a a f f accelerator to study the feasibility of performing ultrasensitive mass spectrometry on semiconductor materials. Both Si and G a A s were used as bulk materials, and several elements of interest were examined in each substrate. The results suggest that the major hurdle to routine use of this technique for impurity m e a s u r e m e n t s is the design of an ultra-clean ion source.
impurity and the substrate have the same magnetic rigidity after leaving the accelerator. A change of beams then requires only a change in the terminal voltage, the electrostatic analyzer and the ion source magnet, with no tuning between the accelerator and the electrostatic analyzer. This method has the added advantage that the major accelerator tuning can be done with the substrate beam, which is quite easy due to the large beam currents involved. M o n i t o r i n g the substrate current before and after each impurity m e a s u r e m e n t then allows a correction for any accelerator drift. Two sets of samples were used for this experiment. The first group consisted of bulk doped G a A s crystals, with B, Si and Te impurities at the 10 Iv 10 is a t / c c level. In the second set, 100 keV Be, N b and Sb ions were implanted at three different doses into both Si and G a A s slices. The i m p l a n t e d area was then removed and weighed, and the samples were crushed and mixed to give an approximately constant concentration. The samples were then m o u n t e d in the ion source, along with high purity control samples of Si and GaAs. The impurity concentrations were determined from the implanted dose and the weight of each sample. The concentrations ranged from 2 × 10 ~ to 6 × 10 s impurity a t o m s / s u b strate atom.
2. Experimental procedure 3. Analysis Fig. 1 shows a schematic diagram of the experimental a r r a n g e m e n t for these measurements. A reflected b e a m geometry was used in the Yale ion source to direct 20 keV Cs + ions onto the sample material, and sputtered negative ions were extracted and injected into the accelerator [6]. After acceleration and electron stripping, the ions (now positive) pass through a 90 ° bending magnet, which defines a unique value of M E / Q 2. A switching magnet then redirects the ions into a 45 ° b e a m line where they traverse an 11 ° electrostatic analyzer, which determines E / Q . After leaving the analyzer the ions pass through a thin (2 m g / c m 2) H a v a r window into a A E - E ionization c h a m b e r filled with 10 Torr of isobutane. Since the energy loss of heavy ions at these energies is approximately proportional to the square of the charge [11], a unique identification of E, M and Q can be made with this a r r a n g e m e n t [8,12]. It is not possible to determine the absolute impurity c o n c e n t r a t i o n in a sample solely from the impurity c o u n t rate, since source emission, accelerator tuning, etc. can all effect these rates. However. a ratio of impurity counts to substrate counts will avoid most of these problems and therefore we have monitored both these quantities. Since the substrate beam currents are much too large for the ions to be counted individually, these beams were collected and measured in a removable Faraday cup located in front of the A E - E detector. We have chosen accelerator settings such that the
Initial measurements focussed on the bulk doped G a A s crystals, and As ions were used as the substrate current, in each case, the impurity b e a m current out of the source was maximized using the doped sample. A digital gaussmeter monitored the magnetic field produced by the ion source magnet, a n d these results were recorded. This allowed the source magnet to be returned to a s t a n d a r d setting as we switched from substrate to impurity c o n c e n t r a t i o n samples. Count rates for each impurity were measured on three samples, including (1) bulk doped, (2) pure GaAs, and (3) bare sample mount. The results of these m e a s u r e m e n t s confirm that the switching time is well within the limits of stability of the machine. U n f o r t u n a t e l y the background signals from b o r o n and silicon c o n t a m i n a n t s in the ion source were higher than the signal from the G a A s samples, and therefore no detection limits could be determined. Visual inspection of the sample m o u n t after the experiment showed that the Cs beam spot had a large (1 cm diameter) halo, which was sputtering the copper m o u n t a n d the impurities in it. We also detected a fairly large tellurium b a c k g r o u n d on several samples. Other materials were searched for and identified at low levels in the ion source, including CI, Br and I. Since the Yale group makes measurements o f 26AI/X7AI ratios [13], we looked for both these isotopes in the A E - E detector. Slight a m o u n t s of 27A1 w e r e detected, but n o 26A1 was seen.
465
J.M. Anthony, J. Thomas / Accelerator based mass spectrometry
fortunately, the Be b a c k g r o u n d concentration in the source was quite high, and there was essentially no difference between our Be signal on and off the prep a r e d samples. This result is p r o b a b l y due to cont a m i n a t i o n in the source from the Be dating program at Yale. The sample m o u n t analyzed with SIMS had never been introduced into the ion source, which explains the lack of a Be signal in the mass scan. The N b implants produced encouraging results. Fig. 3 shows the N b concentration in G a A s versus the measured counting ratio for these implanted samples. A straight line has been drawn through the data. The horizontal error bars on these measurements come primarily from uncertainties in the b e a m current during data acquisition and statistical fluctuations in the A E - E counts. Vertical error bars reflect uncertainties in measuring the implanted dose during ion implantation. The impurity to substrate ratio on the u n i m p l a n t e d control sample was also measured with a value of 1.0 × 10 - l ° for N b in GaAs. Based on the straight line in fig. 3 this ratio represents a b a c k g r o u n d concentration of - 10 ppb, which is p r o b a b l y due to residual N b in the source. A subtraction of this b a c k g r o u n d level from the raw data would bring the slope of the line close to a value of unity, as expected for low concentrations.
Due to the high b a c k g r o u n d levels, the simplest way to d e m o n s t r a t e the feasibility of this technique for cont a m i n a n t measurements was to choose an element not normally present in the source and make it an artificial impurity in GaAs. The central Cs b e a m spot is - 2 m m in diameter but a cesium halo extends out much farther, so any impurities present in either the sample m o u n t or the extraction electrodes will also be seen at low levels in the A E - E detector. Therefore a piece of the ion source sample m o u n t material (Cu holder) was analyzed using conventional SIMS techniques in an effort to identify the impurities in the Cu. Fig. 2 shows the results of these measurements, which d e m o n s t r a t e quite clearly the origin of the B and Si b a c k g r o u n d measured in our experiments. This mass scan also shows many o t h e r impurities which are present in the ion source at low levels. Based on these results, and the fact that the constituents of stainless steel will also generate a backg r o u n d level, we chose Be, N b and Sb as materials that were not present in the mass scan and could easily be introduced as an impurity through ion implantation. These ions were then implanted in Si and G a A s and m o u n t e d in the ion source. Once again, the stability and reproducibility of the accelerator during the measurements was excellent. Un-
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466
J.M. Anthony, J. Thomas / Accelerator based mass spectrometry
The counting rates o b t a i n e d allow extrapolation d o w n to a m i n i m u m detection limit. C o u n t rates as low as 2 × 10 3 Hz have been measured in the geologic dating experiments at Yale, while the rates for the control sample in these experiments were always greater than - 0.15 Hz. This suggests that for N b in GaAs, the detection limits are - 0 . 1 p p b for the energies and charge states discussed here. Similarly to the N b in G a A s data, the N b / S i ratio on the Si slices showed a decrease with N b concentration. However, the high dose N b in Si sample showed a large variation in the N b / S i ratio on successive measurements. The N b signal on this cone varied greatly with position, while the Si signal stayed constant, which suggests that the sample p r e p a r a t i o n did not produce a truly h o m o g e n e o u s concentration. The Sb signal was not d o m i n a t e d by a source background. However, there was no consistent correlation between the p r e p a r e d standards a n d the observed count rates. A l t h o u g h the cause of this problem was not determined, it may have been partly due to an interference from Cs in the source, which has similar kinematic properties. A t h i n n e r window at the entrance to
the detector would allow' lower energy Sb to be run, which would better separate these signals. In the analysis of the implanted standards, the accelerator parameters (terminal voltage, beam energy, charge state, etc.) were chosen to give the best yield for each i m p u r i t y - s u b s t r a t e combination. It is interesting to note that the results for N b in GaAs, for example, were o b t a i n e d using a 61 MeV N b b e a m (charge state 8). High N b energies were necessary in order to have reasonably large signals in the J E - E detector after passing through the Havar foil. In this case only - 4% of the initial N b b e a m emerged from the gas stripper with charge state 8 [15,16]. A thinner entrance window to the ionization c h a m b e r (or a differentially p u m p e d gas cell) would thus offer several advantages, since the m i n i m u m N b energy would be greatly reduced. These lower energies would (a) al[ow the experiments to be run at much lower terminal voltages, and (b) produce higher yields of the impurity beam through the gas stripper, since lower, more p r o b a b l e charge states could be monitored. This would directly increase the sensitivity of the technique.
4. Conclusions 1019
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This study was designed to investigate the problems a n d benefits of applying accelerator based mass spectrometry to semiconductor materials. The measurements reported here suggest that this technique can be quite useful for detecting low level impurity c o n c e n t r a t i o n s in materials, and detection limits of 0.1 p p b have been demonstrated. These conclusions are not surprising, based on the results available from geologic dating experiments with Van de G r a a f f accelerators. However, it is very satisfying to d e m o n s t r a t e that the large mass differences between impurity and matrix atoms present no problems for the technique. The main requirement at this time for routine use of such a system is a " c l e a n " ion source. Sources on existing accelerators are designed to maximize ion yield, with little concern for the introduction of low' level impurity c o n c e n t r a t i o n s in the source itself. Thus there are many i m p r o v e m e n t s that would decrease source b a c k g r o u n d levels by several orders of magnitude. Once this problem is overcome, accelerator based mass spectrometry will represent a unique capability for impurity detection in materials.
GaA
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Nb/As COUNT RATIO
Fig. 3. Data for the Nb implants in GaAs. The ordinate shows the average Nb concentration in a given sample. A straight line has been drawn through the data.
This work was supported in part by the US Dept. of Energy under C o n t r a c t No. DE-AC02-76ER03074. The authors wish to t h a n k S. Matteson and W. D u n c a n for providing the sample materials, and R. D ' A l e x a n d e r for technical assistance with the accelerator operation.
J.M. Anthony, J. Thomas / Accelerator based mass spectrometry
References [1] H. Liebl, J. Phys. E: Sci. Instr. 8 (1975). [2] J. B. Clegg, G.B. Scott, J. Hallais and A. Mircea-Roussel, J. Appl. Phys. 52 (1981) 1110. [3] H.M. Hobgood, G.W. Eldridge, D.L. Barrett and R.N. Thomas, IEEE Trans. Electron. Dev. Ed-28 (1981) 140. [4] W.A. Lanford, P.D. Parker, K. Bauer, K.K. Turekian, J.K. Cochran and S. KrishnaswamL Nucl. Instr. and Meth. 168 (1980) 505. [5] A.E. Litherland, R.P. Beukens, L.R. Kilius, J.C. Rucklidge, H.E. Gove, D. Elmore and K.H. Purser, Nucl. Instr. and Meth. 186 (1981) 463. [6] J. Thomas, A. Mangini, and P. Parker, IEEE Trans. Nucl. Sci. NS-28 (1981) 1478. [7] A.E. Litherland, R.P. Beukens, L.R. Kilius, J.C. Rucklidge, H.E. Gove, D. Elmore, K.H. Purser and C.J. Russo, IEEE Trans. Nucl. Sci. NS-28 (1981) 1469.
467
[8] J. Thomas, Ph. D. Thesis, Yale University (1982). [9] D.E. Nelson and R.G. Korteling, Science 198 (1977) 507. [10] K.H. Turekian, J.K. Cochran, S. Krishnaswami, W.A. Lanford, P.D. Parker and K.A. Bauer, Geophys. Res. Lett. 6 (1979) 417. [11] J.M. Anthony and W.A. Lanford, Phys. Rev. A25 (1982) 1868. [12] K.H. Puser, A.E. Litherland and H.E. Gove, Nucl. Instr. and Meth. 162 (1979) 637. [13] J. Thomas, P.D. Parker, S. Krishnaswami, G. Herzog and D. Pal, to be published. [14] H. Liebl, J. Appl. Phys. 38 (1967) 5277. [15] H.D. Betz, Rev. Mod. Phys. 44 (1972) 465. [16] S. Datz, C.D. Moak, H.O. Lutz, L.C. Northcliffe and L.B. Bridwell, At. Data 2 (1971) 273.
ACCELERATOR
BASED
MASS
SPECTROMETRY
463
OF SEMICONDUCTOR
MATERIALS
J.M. ANTHONY Materials Science Laboratory, Texas Instruments Incorporated. P.O. Box 225936, MS 147, Dallas, TX 75265, USA
J. T H O M A S Kellogg Radiation Laboratory 106-38, California Institute of Technology, Pasadena, CA 91125, USA
We have investigated the possibility of using accelerator based mass spectrometry for the study of semiconductor materials. especially Si and GaAs. Several impurities have been examined, including B, Be, Si, Nb, Sb and Te. For most of these impurities, the count rates from high background levels in the ion source totally dominated the signals from the samples. For Nb. however, the background levels were quite low, and detection limits of - 0.1 parts-per-billion have been demonstrated. The results discussed here suggest that the technique is applicable to most impurity substrate combinations, but the development of a clean ion source is necessary for routine use of this method.
1. Introduction One of the most i m p o r t a n t techniques for impurity analysis in semiconductors is Secondary Ion Mass Spectrometry (SIMS) [1]. In normal SIMS measurements, energetic ions ( 1 0 - 2 0 keV) are used to sputter material from a sample of interest. During the sputtering process some of the surface atoms become ionized, a n d these ions are collected a n d mass analyzed. There are several b a c k g r o u n d problems which limit the sensitivity of normal SIMS instruments, however, including: (1) mass interferences between impurities a n d molecules, such as 31p and 3°Sill, (2) particles with kinematic properties similar to those of the impurity of interest, such as 2SSi2+ and 14N+, etc. The sensitivity of the technique is - 1 part-per-million (ppm) atomic for most elements of interest, although lower values have been quoted in some cases [2,3]. As the sizes of semiconductor devices decrease, material purity becomes increasingly important, and there is a strong need for analysis at the part-per-billion ( p p b ) level a n d below. One possible candidate for measurements of this type is accelerator based mass spectroscopy. In this technique, negative ions are produced in a sputter source as in regular SIMS and injected into a t a n d e m accelerator. The ions are accelerated by a high positive potential a n d stripped to a positive charge state by a gas or foil stripper. The resultant ions are again accelerated by the positive potential, which results in high ion energies (several MeV). After leaving the accelerator, the particles are analyzed and detected using conventional nuclear physics techniques (fig. 1) [4-8]. The advantage of accelerator based mass spectrometry is that is is not subject to the b a c k g r o u n d problems 0 1 6 7 - 5 0 8 7 / 8 3 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
m e n t i o n e d in connection with s t a n d a r d SIMS. For example, although molecules will still be produced in the sputter source, electron stripping in the gas canal produces a " C o u l o m b explosion" due to the electrostatic repulsion between the atoms in the molecule, which dissociates the molecule a n d thereby removes the interference. Also, the high energy of the particles allows the use of nuclear detection techniques which overdetermine the charge, mass a n d energy of these particles. This effectively separates the b a c k g r o u n d c o u n t s from the c o n t a m i n a n t counts, which greatly increases the sensitivity. GRIDDED LENS
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,
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Fig. 1. Schematic representation of the experimental apparatus. Negative ions produced in the Cs source are deflected by the ion source magnet and injected into the accelerator with an energy of 200 keV. After acceleration, the ions pass through the analyzing magnet, the switching magnet and the electrostatic separator. A removable Faraday cup and the ,:1E - E ionization chamber located after the separator allow detection of a broad range of counting rates.
464
J.M. Anthony. J. Thomas / Accelerator based mass spectromett T
To date, accelerator based mass spectrometry has been used primarily for geologic dating, in which ratios of rare radioactive isotopes to a b u n d a n t stable isotopes (such a s 14C/12C or m B e / g B e ) are measured [9,10], and concentrations of the rare isotopes as low a s 10 7 a t / c c have been detected. Extending this technique to stable isotope impurity m e a s u r e m e n t s requires some care, however. The impurities of interest in most semic o n d u c t o r materials are not necessarily similar in mass to the bulk material, and the transmission properties of the impurity and bulk ions may be quite different. Also, most ion sources on existing t a n d e m accelerators are not designed for impurity measurements, and thus the source itself may produce high b a c k g r o u n d counts for m a n y elements, With these constraints in mind, we have used the Yale T a n d e m Van de G r a a f f accelerator to study the feasibility of performing ultrasensitive mass spectrometry on semiconductor materials. Both Si and G a A s were used as bulk materials, and several elements of interest were examined in each substrate. The results suggest that the major hurdle to routine use of this technique for impurity m e a s u r e m e n t s is the design of an ultra-clean ion source.
impurity and the substrate have the same magnetic rigidity after leaving the accelerator. A change of beams then requires only a change in the terminal voltage, the electrostatic analyzer and the ion source magnet, with no tuning between the accelerator and the electrostatic analyzer. This method has the added advantage that the major accelerator tuning can be done with the substrate beam, which is quite easy due to the large beam currents involved. M o n i t o r i n g the substrate current before and after each impurity m e a s u r e m e n t then allows a correction for any accelerator drift. Two sets of samples were used for this experiment. The first group consisted of bulk doped G a A s crystals, with B, Si and Te impurities at the 10 Iv 10 is a t / c c level. In the second set, 100 keV Be, N b and Sb ions were implanted at three different doses into both Si and G a A s slices. The i m p l a n t e d area was then removed and weighed, and the samples were crushed and mixed to give an approximately constant concentration. The samples were then m o u n t e d in the ion source, along with high purity control samples of Si and GaAs. The impurity concentrations were determined from the implanted dose and the weight of each sample. The concentrations ranged from 2 × 10 ~ to 6 × 10 s impurity a t o m s / s u b strate atom.
2. Experimental procedure 3. Analysis Fig. 1 shows a schematic diagram of the experimental a r r a n g e m e n t for these measurements. A reflected b e a m geometry was used in the Yale ion source to direct 20 keV Cs + ions onto the sample material, and sputtered negative ions were extracted and injected into the accelerator [6]. After acceleration and electron stripping, the ions (now positive) pass through a 90 ° bending magnet, which defines a unique value of M E / Q 2. A switching magnet then redirects the ions into a 45 ° b e a m line where they traverse an 11 ° electrostatic analyzer, which determines E / Q . After leaving the analyzer the ions pass through a thin (2 m g / c m 2) H a v a r window into a A E - E ionization c h a m b e r filled with 10 Torr of isobutane. Since the energy loss of heavy ions at these energies is approximately proportional to the square of the charge [11], a unique identification of E, M and Q can be made with this a r r a n g e m e n t [8,12]. It is not possible to determine the absolute impurity c o n c e n t r a t i o n in a sample solely from the impurity c o u n t rate, since source emission, accelerator tuning, etc. can all effect these rates. However. a ratio of impurity counts to substrate counts will avoid most of these problems and therefore we have monitored both these quantities. Since the substrate beam currents are much too large for the ions to be counted individually, these beams were collected and measured in a removable Faraday cup located in front of the A E - E detector. We have chosen accelerator settings such that the
Initial measurements focussed on the bulk doped G a A s crystals, and As ions were used as the substrate current, in each case, the impurity b e a m current out of the source was maximized using the doped sample. A digital gaussmeter monitored the magnetic field produced by the ion source magnet, a n d these results were recorded. This allowed the source magnet to be returned to a s t a n d a r d setting as we switched from substrate to impurity c o n c e n t r a t i o n samples. Count rates for each impurity were measured on three samples, including (1) bulk doped, (2) pure GaAs, and (3) bare sample mount. The results of these m e a s u r e m e n t s confirm that the switching time is well within the limits of stability of the machine. U n f o r t u n a t e l y the background signals from b o r o n and silicon c o n t a m i n a n t s in the ion source were higher than the signal from the G a A s samples, and therefore no detection limits could be determined. Visual inspection of the sample m o u n t after the experiment showed that the Cs beam spot had a large (1 cm diameter) halo, which was sputtering the copper m o u n t a n d the impurities in it. We also detected a fairly large tellurium b a c k g r o u n d on several samples. Other materials were searched for and identified at low levels in the ion source, including CI, Br and I. Since the Yale group makes measurements o f 26AI/X7AI ratios [13], we looked for both these isotopes in the A E - E detector. Slight a m o u n t s of 27A1 w e r e detected, but n o 26A1 was seen.
465
J.M. Anthony, J. Thomas / Accelerator based mass spectrometry
fortunately, the Be b a c k g r o u n d concentration in the source was quite high, and there was essentially no difference between our Be signal on and off the prep a r e d samples. This result is p r o b a b l y due to cont a m i n a t i o n in the source from the Be dating program at Yale. The sample m o u n t analyzed with SIMS had never been introduced into the ion source, which explains the lack of a Be signal in the mass scan. The N b implants produced encouraging results. Fig. 3 shows the N b concentration in G a A s versus the measured counting ratio for these implanted samples. A straight line has been drawn through the data. The horizontal error bars on these measurements come primarily from uncertainties in the b e a m current during data acquisition and statistical fluctuations in the A E - E counts. Vertical error bars reflect uncertainties in measuring the implanted dose during ion implantation. The impurity to substrate ratio on the u n i m p l a n t e d control sample was also measured with a value of 1.0 × 10 - l ° for N b in GaAs. Based on the straight line in fig. 3 this ratio represents a b a c k g r o u n d concentration of - 10 ppb, which is p r o b a b l y due to residual N b in the source. A subtraction of this b a c k g r o u n d level from the raw data would bring the slope of the line close to a value of unity, as expected for low concentrations.
Due to the high b a c k g r o u n d levels, the simplest way to d e m o n s t r a t e the feasibility of this technique for cont a m i n a n t measurements was to choose an element not normally present in the source and make it an artificial impurity in GaAs. The central Cs b e a m spot is - 2 m m in diameter but a cesium halo extends out much farther, so any impurities present in either the sample m o u n t or the extraction electrodes will also be seen at low levels in the A E - E detector. Therefore a piece of the ion source sample m o u n t material (Cu holder) was analyzed using conventional SIMS techniques in an effort to identify the impurities in the Cu. Fig. 2 shows the results of these measurements, which d e m o n s t r a t e quite clearly the origin of the B and Si b a c k g r o u n d measured in our experiments. This mass scan also shows many o t h e r impurities which are present in the ion source at low levels. Based on these results, and the fact that the constituents of stainless steel will also generate a backg r o u n d level, we chose Be, N b and Sb as materials that were not present in the mass scan and could easily be introduced as an impurity through ion implantation. These ions were then implanted in Si and G a A s and m o u n t e d in the ion source. Once again, the stability and reproducibility of the accelerator during the measurements was excellent. Un-
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Fig. 2. Secondary ion mass scan of the Cu sample holder. 20 keV O~" ions were used to sputter the surface, and positive secondary ions were collected. The ion intensity is a function of both the actual concentration of a given impurity, and the probability of positive ion formation by that element.
466
J.M. Anthony, J. Thomas / Accelerator based mass spectrometry
The counting rates o b t a i n e d allow extrapolation d o w n to a m i n i m u m detection limit. C o u n t rates as low as 2 × 10 3 Hz have been measured in the geologic dating experiments at Yale, while the rates for the control sample in these experiments were always greater than - 0.15 Hz. This suggests that for N b in GaAs, the detection limits are - 0 . 1 p p b for the energies and charge states discussed here. Similarly to the N b in G a A s data, the N b / S i ratio on the Si slices showed a decrease with N b concentration. However, the high dose N b in Si sample showed a large variation in the N b / S i ratio on successive measurements. The N b signal on this cone varied greatly with position, while the Si signal stayed constant, which suggests that the sample p r e p a r a t i o n did not produce a truly h o m o g e n e o u s concentration. The Sb signal was not d o m i n a t e d by a source background. However, there was no consistent correlation between the p r e p a r e d standards a n d the observed count rates. A l t h o u g h the cause of this problem was not determined, it may have been partly due to an interference from Cs in the source, which has similar kinematic properties. A t h i n n e r window at the entrance to
the detector would allow' lower energy Sb to be run, which would better separate these signals. In the analysis of the implanted standards, the accelerator parameters (terminal voltage, beam energy, charge state, etc.) were chosen to give the best yield for each i m p u r i t y - s u b s t r a t e combination. It is interesting to note that the results for N b in GaAs, for example, were o b t a i n e d using a 61 MeV N b b e a m (charge state 8). High N b energies were necessary in order to have reasonably large signals in the J E - E detector after passing through the Havar foil. In this case only - 4% of the initial N b b e a m emerged from the gas stripper with charge state 8 [15,16]. A thinner entrance window to the ionization c h a m b e r (or a differentially p u m p e d gas cell) would thus offer several advantages, since the m i n i m u m N b energy would be greatly reduced. These lower energies would (a) al[ow the experiments to be run at much lower terminal voltages, and (b) produce higher yields of the impurity beam through the gas stripper, since lower, more p r o b a b l e charge states could be monitored. This would directly increase the sensitivity of the technique.
4. Conclusions 1019
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This study was designed to investigate the problems a n d benefits of applying accelerator based mass spectrometry to semiconductor materials. The measurements reported here suggest that this technique can be quite useful for detecting low level impurity c o n c e n t r a t i o n s in materials, and detection limits of 0.1 p p b have been demonstrated. These conclusions are not surprising, based on the results available from geologic dating experiments with Van de G r a a f f accelerators. However, it is very satisfying to d e m o n s t r a t e that the large mass differences between impurity and matrix atoms present no problems for the technique. The main requirement at this time for routine use of such a system is a " c l e a n " ion source. Sources on existing accelerators are designed to maximize ion yield, with little concern for the introduction of low' level impurity c o n c e n t r a t i o n s in the source itself. Thus there are many i m p r o v e m e n t s that would decrease source b a c k g r o u n d levels by several orders of magnitude. Once this problem is overcome, accelerator based mass spectrometry will represent a unique capability for impurity detection in materials.
GaA
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Nb/As COUNT RATIO
Fig. 3. Data for the Nb implants in GaAs. The ordinate shows the average Nb concentration in a given sample. A straight line has been drawn through the data.
This work was supported in part by the US Dept. of Energy under C o n t r a c t No. DE-AC02-76ER03074. The authors wish to t h a n k S. Matteson and W. D u n c a n for providing the sample materials, and R. D ' A l e x a n d e r for technical assistance with the accelerator operation.
J.M. Anthony, J. Thomas / Accelerator based mass spectrometry
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[8] J. Thomas, Ph. D. Thesis, Yale University (1982). [9] D.E. Nelson and R.G. Korteling, Science 198 (1977) 507. [10] K.H. Turekian, J.K. Cochran, S. Krishnaswami, W.A. Lanford, P.D. Parker and K.A. Bauer, Geophys. Res. Lett. 6 (1979) 417. [11] J.M. Anthony and W.A. Lanford, Phys. Rev. A25 (1982) 1868. [12] K.H. Puser, A.E. Litherland and H.E. Gove, Nucl. Instr. and Meth. 162 (1979) 637. [13] J. Thomas, P.D. Parker, S. Krishnaswami, G. Herzog and D. Pal, to be published. [14] H. Liebl, J. Appl. Phys. 38 (1967) 5277. [15] H.D. Betz, Rev. Mod. Phys. 44 (1972) 465. [16] S. Datz, C.D. Moak, H.O. Lutz, L.C. Northcliffe and L.B. Bridwell, At. Data 2 (1971) 273.