- Email: [email protected]
Ultra-high resolution mass spectroscopy of boron cluster ions
Nuclear Instruments and Methods in Physics Research B 237 (2005) 406–410 www.elsevier.com/locate/nimb
Ultra-high resolution mass spectroscopy of boron cluster ions Dale Jacobson *, Thomas Horsky, Wade Krull, Bob Milgate SemEquip, Inc., 34 Sullivan Road, Suite 21, Billerica, MA 01862, USA Available online 1 August 2005
Abstract Boron clusters have recently received considerable attention as a possible solution to the throughput dilemma associated with ultra-low energy (sub keV) p-type source drain extension implants required by cutting edge complimentary metal–oxide semiconductor (CMOS) technology. Boron cluster ion beams contain many masses due to the binomial distribution of the two naturally occurring isotopes (masses 10 and 11) of boron. The broadness of the mass distribution peak in the dispersive plane is further complicated by a plurality of ion states, due to the varying number of hydrogen atoms remaining attached to the borohydride molecule when it is ionized. The B18 Hþ x cluster ion mass spectrum from an electron impact ionization source will be analyzed in detail. An ultrahigh resolution mass spectrum, exhibiting 1 AMU resolution of a mass 220 cluster ion will be shown. It will be compared to high-resolution spectra of decaborane (B10H14) cluster ions obtained from natural abundance decaborane and from isotopically enriched material. The deconvolution of the binominal distribution from ion states present in the cluster ion beam reveals the hydrogen distribution function. The hydrogen distribution functions as well as the binomial distributions will be presented and discussed. Physical models will be presented that explain the origin of hydrogen distribution function for these high mass borohydride cluster ions. This ultra-high mass resolution is usually unavailable to the ion implant community, however our 120° mass analyzing magnet and the extremely low emittance of the ion beam extracted from the ClusterIonÒ source coupled with a variable width beam defining aperture and variable width mass defining slits allow for superior mass resolution. Ó 2005 Published by Elsevier B.V. PACS: 36.40.Wa; 41.8555.Ar; 41.85.Ew; 61.72.Tt Keywords: Ion implantation; Ultra-low energy ion implantation; Cluster ion implantation; Octadecaborane; ClusterBoron; Decaborane; Molecular ion implantation; High resolution mass spectroscopy
1. Introduction *
Corresponding author. Tel.: +978 262 0911; fax: +978 262 0950. E-mail address: [email protected] (D. Jacobson). 0168-583X/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.nimb.2005.05.025
As CMOS devices scale the energies necessary to fabricate the source/drain extensions (SDE)
D. Jacobson et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 406–410
are being driven to extremely low levels. In fact many manufactures are exploring 100 eV B implants to form the p-type SDE at the 45 nm node. The Child–Langmuir law [1] places a upper limit on the beam current that can be extracted from an ion source. As shown by Eq. (1) it imposes a V3/2 relationship on the maximum extractable beam current density. This becomes a limiting factor at energies below about 10 keV depending on the design of the source and beam line. Clearly there are several approaches to alleviate this problem, which becomes more severe as the beam energy decreases. One can extract a large beam to reduce the current density, or one can extract at higher voltages and then decelerate the beam just before it impacts the wafer, thus avoiding the Child–Langmuir limitations. These have been used by various implanter Original Equipment Manufactures (OEMs) over the past decade. Each has its own set of new problems. For example performing low energy implantation in the deceleration mode causes high energy components to appear in the beam due to neutral particles in the beam not being decelerated by the retarding electric field applied in the deceleration lens. This results in high energy tails in the B distribution, thus increasing the junction depth, the very thing that one is attempting to make shallower. Ion beams that are large in the scan direction require extended over scan thus mollifying any advantage gained at extraction. If an unscanned large beam is used the implant uniformity on the wafer is determined by the beam density uniformity across a beam that exceeds 300 mm in height. Controlling the beam density to less than 1% on such a large beam is a daunting task which often requires a long and complicated beam line which in itself wastes beam current, the very commodity we are struggling to enhance. Cluster ion implantation, primarily with decaborane (B10H14), has been investigated for more than a decade [2–5]. However not until very recently was this option been viable due low beam currents and an unacceptably short lifetime for sources and vapor delivery systems. These problems have recently been solved by the genesis of the SemEquip ClusterIonÒ source and the discovery of a large and stable borohydride molecule,
407
octadecaborane (B18H22). This new material to the ion implantation arena, know as ClusterBoronTM allows extraction from the ion source at 20X the desired implant energy and delivers 18 dopant atoms per single electrical charge. These advantages virtually reduce the Child–Langmuir limitations. For example a 20 keV, 1 mA ClusterBoronTM ion beam is process equivalent to a 1 keV, 18 mA implant, a condition not allowed by the Child–Langmuir law. However, a 20 keV, 1 mA ion beam is not impacted by the Child– Langmuir limit and is easily transported through almost any commercial implanter beam line.
2. Experimental Decaborane and octadecaborane ion beam extracted at 20 kV from a SemEquip ClusterIonÒ source have been magnetically mass analyzed in a test stand shown in Fig. 1. This test stand utilizes a 120° magnet manufactured by Buckley Systems. It has a variable beam forming slit at the entrance to the magnet and a variable mass defining slit a the focal plan of the system. Approximately 1 m down steam from the mass defining slit is a fully suppressed Faraday cup, in which the beam current is measured. The linear output from a picoampmeter attached to the Faraday is used to measure the beam current. The magnetic field in the analyzing magnet is measured with a model DTM-151 GMW Gauss Meter with its associated Hall probe inserted between the pole pieces in a
Fig. 1. Line drawing of the test stand on which the data were collected.
D. Jacobson et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 406–410
aluminum well that penetrates the evacuated region of the magnet. The analog linear output from the Gauss meter is used to drive the X-axis on a Houston OmniGraphic 2000 X–Y recorder. The Y-axis is driven by the analogue linear out put of the pico-ampmeter. Plots of beam intensity versus magnetic field are generated as the magnet coil current is ramped. The resulting plots are scanned and then digitized using DigiPro software. The B Field (Square root of Mass) of the X-axis is converted to AMU.
20 keV B18H22 100
Normalized Current
408
80 60 40 20 0 180
185
190
195
200 205 210 Mass (AMU)
215
220
225
Fig. 3. High resolution mass spectrum of a 20 keV B18 Hþ 22 ion beam.
3. Results Fig. 2 is a low resolution mass spectrum of B18H22 extracted at 20 keV. The mass defining aperture located at the focal plane is width adjusted in the dispersive plane to pass 15 AMU. This is the width that is typically used during ion implantation. This setting allows nearly 90% of the B18Hx beam to pass but eliminates all of B17Hx component of the beam. Fig. 3 is a high-resolution spectrum obtained in a similar way as the spectrum in Fig. 2 except the beam defining aperture at the entry to the magnet is set to 5 mm and the mass defining aperture to 1 mm. These settings severely reduce the transmitted beam current but allow 1 AMU mass resolution. The peak at mass 220 is from 18 11B atoms plus all 22 H atoms. The rest of the peaks are composites of combinations of 11B and 10B atoms and dif-
ferent numbers of H atoms. The distribution of B atoms described by a binomial distribution of 18 atoms with 20% being mass 10 and 80% being mass 10 is shown in Fig. 4. Fig. 5 is an overlay of the binomial distribution and the peak heights form Fig. 4. Clearly the binomial distribution is too narrow to adequately describe the actual mass distribution. This is because not all of the ions have 22 hydrogen atoms remaining attached. During ionization some to the hydrogen–boron bonds are broken. Fig. 6 is a hydrogen distribution func-
Binomial Distribution 1.2
1000 900 800 700 600 500 400 300 200 100 0
B18H22 Current (µA)
15.4 mA B @ 1keV
Relative Abundance
1.0 20 keV B18Hx
0.8 0.6 0.4 0.2 0.0
0
20 40 60 80 100 120 140 160 180 200 220 240 260 Mass (AMU)
Fig. 2. Low resolution mass spectrum of a 20 keV B18 Hþ 22 ion beam.
0
2
4
6
8
10
12
14
16
18
Number of mass 11 atoms
Fig. 4. Binomial distribution of 18 boron atoms 20% mass 10 and 80% mass 11.
D. Jacobson et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 406–410
409
4. Discussion
Binomial Distribution 120 Binomial Measured
Normalized Intensity
100 80 60 40 20 0 190
195
200
205
210
215
220
225
Mass (AMU)
Fig. 5. Overlay of peak heights of Fig. 3 with the binomial distribution Fig. 4.
Hydrogen Distribution Function 0.35
Probability
0.3 0.25 0.2 0.15 0.1 0.05 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Number of Bonded Hydrogen Atoms
Fig. 6. The hydrogen distribution function necessary to expand the binomial distribution to fit the data.
It is noteworthy that the hydrogen distribution function contains no odd number of hydrogen atoms. The driving force for this symmetry is not understood, but is clearly required to lower the free energy of the system. The bimodal nature of the hydrogen distribution function is a result of the molecular structure of the B18H22 molecule. There are six bridge hydrogen atoms in this molecule. These bonds are unique to borohydride chemistry. One hydrogen atoms is bonded to two boron atoms. Clearly these are very weak covalent bonds with just a single electron being shared by three atoms. These weak bonds will be the first to be broken. The mass 220 peak is due to all 22 hydrogenÕs being bonded. The second large peak at mass 210 is from having 16 hydrogen atoms bonded to the molecule. Clearly it is possible to choose an appropriate mass selection aperture to exclude the entire B17Hx beam yet allow the maximum transmission of the B18Hx beam. An aperture that passes 15 AMU will accomplish this task. Fig. 8 is a mass spectrum obtained with mass 11 isotopically enriched B18H22. With this material it is not necessary to deconvolute the effect of the binominal distribution. The mass spectrum is the hydrogen distribution function. It is clear that
20 keV
B18H22
120
B18H22 Mass Distribution 25
B18H16
100 20 15
Fit Data
10 5 0 185
190
195
200 205 210 Mass (AMU)
215
220
225
Fig. 7. The data, the XÕs, and the fit, the boxes, overlaid to illustrate the quality of the fit.
Beam Current (a.u.)
Normalized Current
11
80 60
B18H22
B18H14 B18H12
B18H18
40 B18H10 B18H20
20
0 200 202 204 206 208 210 212 214 216 218 220 222 224 Mass (AMU)
tion that when multiplied by the binomial distribution fits the data. Fig. 7 shows the fit of the data.
Fig. 8. Mass spectrum of a 20 keV mass 11 enriched B18 Hþ 22 ion beam.
410
D. Jacobson et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 406–410
the deconvoluted spectrum is very similar to the directly measured hydrogen distribution function obtained from the enriched material.
bution function is a direct result of the molecular structure of octadecaborane.
5. Conclusions
References
It has been shown that the mass spectrum of B18H22 is comprised of many masses that result from the binomial distribution of the two naturally occurring isotopes of boron and a hydrogen distribution function which describes the probability associated with each possibility of retained H atoms (0–22 H atoms). Furthermore only even numbers of H are allowed on the B18 Hþ x ion. Lastly it has been shown that the hydrogen distri-
[1] I. Brodie, J.J. Muray, The Physics of Microfabrication, Plenum Press, New York, 1982, p. 18. [2] K. Goto, J. Matduo, T. Sugii, I. Yamada, T. Hisatsugu, IEDM-96, IEEE (1997) 768. [3] D.C. Jacobson, K. Bourdelle, H.-J. Gossmann, M.A. Albano, V. Babaram, J.M. Poate, A. Agarwal, A. Perel, T. Horsky, in: IIT-2000 Proceedings, p. 300. [4] A.S. Perel, W. Krull, D. Hoglund, K. Jackson, T. Horsky, in: IIT-2000 Proceedings, p. 304. [5] S. Umisedo, N. Hamamoto, S Sakai, M. Tanjyo, N. Nagai, M. Naito, in: IWJT-2004 Proceedings, p. 27.
Ultra-high resolution mass spectroscopy of boron cluster ions Dale Jacobson *, Thomas Horsky, Wade Krull, Bob Milgate SemEquip, Inc., 34 Sullivan Road, Suite 21, Billerica, MA 01862, USA Available online 1 August 2005
Abstract Boron clusters have recently received considerable attention as a possible solution to the throughput dilemma associated with ultra-low energy (sub keV) p-type source drain extension implants required by cutting edge complimentary metal–oxide semiconductor (CMOS) technology. Boron cluster ion beams contain many masses due to the binomial distribution of the two naturally occurring isotopes (masses 10 and 11) of boron. The broadness of the mass distribution peak in the dispersive plane is further complicated by a plurality of ion states, due to the varying number of hydrogen atoms remaining attached to the borohydride molecule when it is ionized. The B18 Hþ x cluster ion mass spectrum from an electron impact ionization source will be analyzed in detail. An ultrahigh resolution mass spectrum, exhibiting 1 AMU resolution of a mass 220 cluster ion will be shown. It will be compared to high-resolution spectra of decaborane (B10H14) cluster ions obtained from natural abundance decaborane and from isotopically enriched material. The deconvolution of the binominal distribution from ion states present in the cluster ion beam reveals the hydrogen distribution function. The hydrogen distribution functions as well as the binomial distributions will be presented and discussed. Physical models will be presented that explain the origin of hydrogen distribution function for these high mass borohydride cluster ions. This ultra-high mass resolution is usually unavailable to the ion implant community, however our 120° mass analyzing magnet and the extremely low emittance of the ion beam extracted from the ClusterIonÒ source coupled with a variable width beam defining aperture and variable width mass defining slits allow for superior mass resolution. Ó 2005 Published by Elsevier B.V. PACS: 36.40.Wa; 41.8555.Ar; 41.85.Ew; 61.72.Tt Keywords: Ion implantation; Ultra-low energy ion implantation; Cluster ion implantation; Octadecaborane; ClusterBoron; Decaborane; Molecular ion implantation; High resolution mass spectroscopy
1. Introduction *
Corresponding author. Tel.: +978 262 0911; fax: +978 262 0950. E-mail address: [email protected] (D. Jacobson). 0168-583X/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.nimb.2005.05.025
As CMOS devices scale the energies necessary to fabricate the source/drain extensions (SDE)
D. Jacobson et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 406–410
are being driven to extremely low levels. In fact many manufactures are exploring 100 eV B implants to form the p-type SDE at the 45 nm node. The Child–Langmuir law [1] places a upper limit on the beam current that can be extracted from an ion source. As shown by Eq. (1) it imposes a V3/2 relationship on the maximum extractable beam current density. This becomes a limiting factor at energies below about 10 keV depending on the design of the source and beam line. Clearly there are several approaches to alleviate this problem, which becomes more severe as the beam energy decreases. One can extract a large beam to reduce the current density, or one can extract at higher voltages and then decelerate the beam just before it impacts the wafer, thus avoiding the Child–Langmuir limitations. These have been used by various implanter Original Equipment Manufactures (OEMs) over the past decade. Each has its own set of new problems. For example performing low energy implantation in the deceleration mode causes high energy components to appear in the beam due to neutral particles in the beam not being decelerated by the retarding electric field applied in the deceleration lens. This results in high energy tails in the B distribution, thus increasing the junction depth, the very thing that one is attempting to make shallower. Ion beams that are large in the scan direction require extended over scan thus mollifying any advantage gained at extraction. If an unscanned large beam is used the implant uniformity on the wafer is determined by the beam density uniformity across a beam that exceeds 300 mm in height. Controlling the beam density to less than 1% on such a large beam is a daunting task which often requires a long and complicated beam line which in itself wastes beam current, the very commodity we are struggling to enhance. Cluster ion implantation, primarily with decaborane (B10H14), has been investigated for more than a decade [2–5]. However not until very recently was this option been viable due low beam currents and an unacceptably short lifetime for sources and vapor delivery systems. These problems have recently been solved by the genesis of the SemEquip ClusterIonÒ source and the discovery of a large and stable borohydride molecule,
407
octadecaborane (B18H22). This new material to the ion implantation arena, know as ClusterBoronTM allows extraction from the ion source at 20X the desired implant energy and delivers 18 dopant atoms per single electrical charge. These advantages virtually reduce the Child–Langmuir limitations. For example a 20 keV, 1 mA ClusterBoronTM ion beam is process equivalent to a 1 keV, 18 mA implant, a condition not allowed by the Child–Langmuir law. However, a 20 keV, 1 mA ion beam is not impacted by the Child– Langmuir limit and is easily transported through almost any commercial implanter beam line.
2. Experimental Decaborane and octadecaborane ion beam extracted at 20 kV from a SemEquip ClusterIonÒ source have been magnetically mass analyzed in a test stand shown in Fig. 1. This test stand utilizes a 120° magnet manufactured by Buckley Systems. It has a variable beam forming slit at the entrance to the magnet and a variable mass defining slit a the focal plan of the system. Approximately 1 m down steam from the mass defining slit is a fully suppressed Faraday cup, in which the beam current is measured. The linear output from a picoampmeter attached to the Faraday is used to measure the beam current. The magnetic field in the analyzing magnet is measured with a model DTM-151 GMW Gauss Meter with its associated Hall probe inserted between the pole pieces in a
Fig. 1. Line drawing of the test stand on which the data were collected.
D. Jacobson et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 406–410
aluminum well that penetrates the evacuated region of the magnet. The analog linear output from the Gauss meter is used to drive the X-axis on a Houston OmniGraphic 2000 X–Y recorder. The Y-axis is driven by the analogue linear out put of the pico-ampmeter. Plots of beam intensity versus magnetic field are generated as the magnet coil current is ramped. The resulting plots are scanned and then digitized using DigiPro software. The B Field (Square root of Mass) of the X-axis is converted to AMU.
20 keV B18H22 100
Normalized Current
408
80 60 40 20 0 180
185
190
195
200 205 210 Mass (AMU)
215
220
225
Fig. 3. High resolution mass spectrum of a 20 keV B18 Hþ 22 ion beam.
3. Results Fig. 2 is a low resolution mass spectrum of B18H22 extracted at 20 keV. The mass defining aperture located at the focal plane is width adjusted in the dispersive plane to pass 15 AMU. This is the width that is typically used during ion implantation. This setting allows nearly 90% of the B18Hx beam to pass but eliminates all of B17Hx component of the beam. Fig. 3 is a high-resolution spectrum obtained in a similar way as the spectrum in Fig. 2 except the beam defining aperture at the entry to the magnet is set to 5 mm and the mass defining aperture to 1 mm. These settings severely reduce the transmitted beam current but allow 1 AMU mass resolution. The peak at mass 220 is from 18 11B atoms plus all 22 H atoms. The rest of the peaks are composites of combinations of 11B and 10B atoms and dif-
ferent numbers of H atoms. The distribution of B atoms described by a binomial distribution of 18 atoms with 20% being mass 10 and 80% being mass 10 is shown in Fig. 4. Fig. 5 is an overlay of the binomial distribution and the peak heights form Fig. 4. Clearly the binomial distribution is too narrow to adequately describe the actual mass distribution. This is because not all of the ions have 22 hydrogen atoms remaining attached. During ionization some to the hydrogen–boron bonds are broken. Fig. 6 is a hydrogen distribution func-
Binomial Distribution 1.2
1000 900 800 700 600 500 400 300 200 100 0
B18H22 Current (µA)
15.4 mA B @ 1keV
Relative Abundance
1.0 20 keV B18Hx
0.8 0.6 0.4 0.2 0.0
0
20 40 60 80 100 120 140 160 180 200 220 240 260 Mass (AMU)
Fig. 2. Low resolution mass spectrum of a 20 keV B18 Hþ 22 ion beam.
0
2
4
6
8
10
12
14
16
18
Number of mass 11 atoms
Fig. 4. Binomial distribution of 18 boron atoms 20% mass 10 and 80% mass 11.
D. Jacobson et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 406–410
409
4. Discussion
Binomial Distribution 120 Binomial Measured
Normalized Intensity
100 80 60 40 20 0 190
195
200
205
210
215
220
225
Mass (AMU)
Fig. 5. Overlay of peak heights of Fig. 3 with the binomial distribution Fig. 4.
Hydrogen Distribution Function 0.35
Probability
0.3 0.25 0.2 0.15 0.1 0.05 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Number of Bonded Hydrogen Atoms
Fig. 6. The hydrogen distribution function necessary to expand the binomial distribution to fit the data.
It is noteworthy that the hydrogen distribution function contains no odd number of hydrogen atoms. The driving force for this symmetry is not understood, but is clearly required to lower the free energy of the system. The bimodal nature of the hydrogen distribution function is a result of the molecular structure of the B18H22 molecule. There are six bridge hydrogen atoms in this molecule. These bonds are unique to borohydride chemistry. One hydrogen atoms is bonded to two boron atoms. Clearly these are very weak covalent bonds with just a single electron being shared by three atoms. These weak bonds will be the first to be broken. The mass 220 peak is due to all 22 hydrogenÕs being bonded. The second large peak at mass 210 is from having 16 hydrogen atoms bonded to the molecule. Clearly it is possible to choose an appropriate mass selection aperture to exclude the entire B17Hx beam yet allow the maximum transmission of the B18Hx beam. An aperture that passes 15 AMU will accomplish this task. Fig. 8 is a mass spectrum obtained with mass 11 isotopically enriched B18H22. With this material it is not necessary to deconvolute the effect of the binominal distribution. The mass spectrum is the hydrogen distribution function. It is clear that
20 keV
B18H22
120
B18H22 Mass Distribution 25
B18H16
100 20 15
Fit Data
10 5 0 185
190
195
200 205 210 Mass (AMU)
215
220
225
Fig. 7. The data, the XÕs, and the fit, the boxes, overlaid to illustrate the quality of the fit.
Beam Current (a.u.)
Normalized Current
11
80 60
B18H22
B18H14 B18H12
B18H18
40 B18H10 B18H20
20
0 200 202 204 206 208 210 212 214 216 218 220 222 224 Mass (AMU)
tion that when multiplied by the binomial distribution fits the data. Fig. 7 shows the fit of the data.
Fig. 8. Mass spectrum of a 20 keV mass 11 enriched B18 Hþ 22 ion beam.
410
D. Jacobson et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 406–410
the deconvoluted spectrum is very similar to the directly measured hydrogen distribution function obtained from the enriched material.
bution function is a direct result of the molecular structure of octadecaborane.
5. Conclusions
References
It has been shown that the mass spectrum of B18H22 is comprised of many masses that result from the binomial distribution of the two naturally occurring isotopes of boron and a hydrogen distribution function which describes the probability associated with each possibility of retained H atoms (0–22 H atoms). Furthermore only even numbers of H are allowed on the B18 Hþ x ion. Lastly it has been shown that the hydrogen distri-
[1] I. Brodie, J.J. Muray, The Physics of Microfabrication, Plenum Press, New York, 1982, p. 18. [2] K. Goto, J. Matduo, T. Sugii, I. Yamada, T. Hisatsugu, IEDM-96, IEEE (1997) 768. [3] D.C. Jacobson, K. Bourdelle, H.-J. Gossmann, M.A. Albano, V. Babaram, J.M. Poate, A. Agarwal, A. Perel, T. Horsky, in: IIT-2000 Proceedings, p. 300. [4] A.S. Perel, W. Krull, D. Hoglund, K. Jackson, T. Horsky, in: IIT-2000 Proceedings, p. 304. [5] S. Umisedo, N. Hamamoto, S Sakai, M. Tanjyo, N. Nagai, M. Naito, in: IWJT-2004 Proceedings, p. 27.