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Metal implant standards for surface analysis by TOF-SIMS and dynamic SIMS: comparison with TRIM simulation
Applied Surface Science 203±204 (2003) 290±293
Metal implant standards for surface analysis by TOF-SIMS and dynamic SIMS: comparison with TRIM simulation A.V. Li-Fatou*, M. Douglas Texas Instruments Inc., P.O. Box 650311, MS 3704, Dallas, TX 75265, USA
Abstract A comprehensive set of ion implanted standards has been prepared and extensively characterized by dynamic and TOF-SIMS. It is shown that the distinct shape of ion implant distribution provides various means to account for measurement artifacts. The results of depth pro®ling have been compared with theoretically modeled distributions. It has been found that measured projected ranges of light elements are 30±40% less than theoretically predicted by Monte Carlo simulations. Chemical segregation is proposed to have played a role in this shift. The experimental data obtained using three different SIMS techniques demonstrates that prepared set of relevant metallic implants provide adequate calibration reference for accurate surface and near-surface trace metal analysis. # 2002 Published by Elsevier Science B.V. Keywords: Trace metals; Standards; Surface analysis; Ion implants; Depth pro®ling; Implant simulation
1. Introduction The measurement of the surface metal contamination on silicon wafers is an essential part of yield enhancement in IC manufacturing. A number of analytical techniques are currently used to monitor metal contamination including TXRF, VPD-ICPMS, surface SIMS and more recently TOF-SIMS. Each of the above techniques has its weaknesses and, therefore, they tend to complement each other. A common problem for these techniques is a lack of appropriate standards that can greatly affect the accuracy of the results. Many currently used standards for surface analysis provide concentration just at the surface and right *
Corresponding author. Tel.: 1-972-995-4771; fax: 1-972-995-2736. E-mail address: [email protected] (A.V. Li-Fatou).
below. The integral depth may vary for various elements in case of TXRF or be determined by the thickness of the native oxide removed in case of VPD techniques. Exposure of the surface to an atmosphere even in a clean room environment will cause a gradual contamination of the surface making these standards less reliable with time. A very elegant solution to use removable layer to preserve the surface until prior to SIMS analysis has been recently published by Stevie et al. [1]. However, the proposed wet chemical oxide removal has to be carefully controlled to archive reliable results particularly for alkali metals. Ion implantation has been widely accepted and used for preparation of accurate standards for SIMS calibration. Several NIST standard reference materials (SRMs) [2] are available to SIMS users for accurate absolute calibration of SIMS depth pro®les. Great reliability of these standards is built upon established independent methods of characterization such as RBS
0169-4332/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 6 6 0 - 8
A.V. Li-Fatou, M. Douglas / Applied Surface Science 203±204 (2003) 290±293
or nuclear reactions analysis (NRA). Historically these standards are prepared with relatively high implantation energy to distance the concentration maximum from the surface to avoid sputtering and ionization artifacts associated with typical SIMS depth pro®ling. Recently, an ultra-low impact energy SIMS has been introduced that greatly reduces surface artifacts to allow characterization of the near-surface region with good accuracy. This paper proposes use of shallow implanted metal standards to determine accurate metal relative sensitivity factor (RSF) values for surface metal characterization by TOF-SIMS and dynamic SIMS. These SIMS-based techniques offer very low metal detection limits in small spatial regimes, enabling analysis of region-speci®c sites as presented by patterned wafers, SIMS surface transient artifacts can affect the accuracy in the ®rst few monolayers.
291
Dynamic SIMS measurements were performed using CAMECA IMS 4f and IMS 6f magnetic sector instruments. All measurements were performed using an O2 primary beam and detecting positive secondary ions. Primary oxygen beam with impact energies of 3 keV (IMS 4f) and 800 eV (IMS 6f) was used. The angles of incidence were 568 and 428, respectively. An oxygen back®ll was applied to ensure the surface is fully oxidized during depth pro®ling. Suf®cient MRP was used to eliminate possible molecular interference during depth pro®ling. TOF-SIMS measurements were made by Physical Electronics TRIFT-3 mass spectrometer using net 700 eV oxygen bombardment and oxygen back®ll.
2. Experimental Single crystal Sih1 0 0i wafers were implanted with 10 metallic species with the parameters summarized in Table 1. The energies and doses were chosen to produce concentration maxima of about 1E19 atoms/cm3 at about 20 nm depth. The implantation was done at 78 with respect to normal to minimize channeling. All implanted species were major isotopes except Fe where the 54 Fe isotope was implanted to avoid Si2 interference during SIMS analysis. Table 1 Ion implantation parameters for metals standards Z
Ion
Energy (keV)
Dose (atoms/cm2)
Angle (8)
23 24 27 39 40 48 52 54 63 66
23
16 16 16 16 18 18 18 22 22 22
3.66E13 3.44E13 3.08E13 2.01E13 2.35E13 2.13E13 2.10E13 2.26E13 2.11E13 2.10E13
7 7 7 7 7 7 7 7 7 7
Na Mg 27 Al 39 K 40 Ca 48 Ti 52 Cr 54 Fe 63 Cu 66 Zn 24
Fig. 1. Dynamic SIMS and TRIM-simulated depth pro®les for 27 Al and 39 K implanted into silicon (a) and 54 Fe and 52 Cr implanted into silicon (b).
292
A.V. Li-Fatou, M. Douglas / Applied Surface Science 203±204 (2003) 290±293
SIMS calibration was done using nominal implanted doses. The nominal doses are being veri®ed by NRA but the results were not available at the time of this publication. Therefore, the absolute accuracy of reported SIMS pro®les is expected to be around 10%. 3. Results and discussion Fig. 1a shows SIMS depth pro®les of Al and K obtained by dynamic SIMS using 800 eV bombardment. The plot also includes TRIM-simulated distributions. SIMS data shows signi®cant channeling that causes the difference in the Cp observed for experimental and
Fig. 2. Correlation diagram for implants' projected ranges of TRIM-calculated and SIMS measured with 3 keV primary impact energy (a) and 800 eV impact energy (b).
theoretical data (TRIM simulation assumes amorphous substrate). This difference is ranging from 10 to 15% for light metals (Na, Mg and Al) to just a few percent for heavier elements (Fig. 1b). Therefore, the use of theoretically predicted values for concentration maxima may introduce a signi®cant error if calibration is done without a proper correction for channeling. All implants show their maxima well below the surface transient region or native oxide. However, measured projected ranges for almost all implants appear closer to the surface than predicted by Monte Carlo simulations (Figs. 1a and b and 2a and b) show correlation diagrams for implants' projected ranges obtained by TRIM simulation and experimentally using dynamic SIMS results obtained at 800 eV and 3 keV and impact energies, respectively. Fig. 3 shows the same correlation with TOF-SIMS data obtained at 700 eV. This Rp shift is found to be independent of the bombardment condition or measured species with the exception of the elements showing heavy segregation [3]. The fact that this shift is independent of the bombardment angle and occurs at such shallow depth allows us to believe that crater bottom roughening is not responsible for this effect. The average shift of 7:8 0:3 nm is also considerably larger than the presence of thin native oxide can explain. The simulation model has been carefully veri®ed using a 5 keV boron implant into an amorphous Si where an agreement with experiment better than 1 nm has been obtained [4].
Fig. 3. Correlation diagram for implants' projected ranges of TRIM-calculated and TOF-SIMS measured with 700 eV primary impact energy.
A.V. Li-Fatou, M. Douglas / Applied Surface Science 203±204 (2003) 290±293
293
One of the possible explanations of this shift is a chemical segregation phenomenon enhanced under oxygen bombardment and surface oxide formation. This effect is well known for Fe and especially Cu but has never been reported for light alkali elements possibly because it is hardly noticeable at a typical implant depth of a few hundred nanometers.
data obtained using three different SIMS techniques demonstrates that the prepared set of relevant metallic implants provide adequate calibration reference for accurate surface and near-surface trace metal analysis.
4. Conclusions
The authors would like to thank Ion Implant Services for helpful suggestions and preparation of the standards and Puneet Kohli of University of Texas at Austin for TRIM simulations.
A comprehensive set of ion implanted standards has been prepared and extensively characterized by dynamic and TOF-SIMS. It is shown that the distinct shape of ion implant distribution provides various means to account for measurement artifacts. The results of depth pro®ling have been compared against theoretically modeled distributions. It has been found that measured projected ranges of light elements are 30±40% less than theoretically predicted by Monte Carlo simulations. Chemical segregation is proposed to have played a role in this shift. The experimental
Acknowledgements
References [1] F.A. Stevie, R. Roberts, J.K. McKinley, M. Decker, C. Granger, in: Proceedings of the USJ Conference, NC, 1999. [2] D.S. Simons, Secondary Ion Mass Spectrometry, SIMS IX, Wiley, Chichester, 1994, p. 140. [3] A.V. Li-Fatou, in preparation. [4] P. Kohli, private communication.
Metal implant standards for surface analysis by TOF-SIMS and dynamic SIMS: comparison with TRIM simulation A.V. Li-Fatou*, M. Douglas Texas Instruments Inc., P.O. Box 650311, MS 3704, Dallas, TX 75265, USA
Abstract A comprehensive set of ion implanted standards has been prepared and extensively characterized by dynamic and TOF-SIMS. It is shown that the distinct shape of ion implant distribution provides various means to account for measurement artifacts. The results of depth pro®ling have been compared with theoretically modeled distributions. It has been found that measured projected ranges of light elements are 30±40% less than theoretically predicted by Monte Carlo simulations. Chemical segregation is proposed to have played a role in this shift. The experimental data obtained using three different SIMS techniques demonstrates that prepared set of relevant metallic implants provide adequate calibration reference for accurate surface and near-surface trace metal analysis. # 2002 Published by Elsevier Science B.V. Keywords: Trace metals; Standards; Surface analysis; Ion implants; Depth pro®ling; Implant simulation
1. Introduction The measurement of the surface metal contamination on silicon wafers is an essential part of yield enhancement in IC manufacturing. A number of analytical techniques are currently used to monitor metal contamination including TXRF, VPD-ICPMS, surface SIMS and more recently TOF-SIMS. Each of the above techniques has its weaknesses and, therefore, they tend to complement each other. A common problem for these techniques is a lack of appropriate standards that can greatly affect the accuracy of the results. Many currently used standards for surface analysis provide concentration just at the surface and right *
Corresponding author. Tel.: 1-972-995-4771; fax: 1-972-995-2736. E-mail address: [email protected] (A.V. Li-Fatou).
below. The integral depth may vary for various elements in case of TXRF or be determined by the thickness of the native oxide removed in case of VPD techniques. Exposure of the surface to an atmosphere even in a clean room environment will cause a gradual contamination of the surface making these standards less reliable with time. A very elegant solution to use removable layer to preserve the surface until prior to SIMS analysis has been recently published by Stevie et al. [1]. However, the proposed wet chemical oxide removal has to be carefully controlled to archive reliable results particularly for alkali metals. Ion implantation has been widely accepted and used for preparation of accurate standards for SIMS calibration. Several NIST standard reference materials (SRMs) [2] are available to SIMS users for accurate absolute calibration of SIMS depth pro®les. Great reliability of these standards is built upon established independent methods of characterization such as RBS
0169-4332/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 6 6 0 - 8
A.V. Li-Fatou, M. Douglas / Applied Surface Science 203±204 (2003) 290±293
or nuclear reactions analysis (NRA). Historically these standards are prepared with relatively high implantation energy to distance the concentration maximum from the surface to avoid sputtering and ionization artifacts associated with typical SIMS depth pro®ling. Recently, an ultra-low impact energy SIMS has been introduced that greatly reduces surface artifacts to allow characterization of the near-surface region with good accuracy. This paper proposes use of shallow implanted metal standards to determine accurate metal relative sensitivity factor (RSF) values for surface metal characterization by TOF-SIMS and dynamic SIMS. These SIMS-based techniques offer very low metal detection limits in small spatial regimes, enabling analysis of region-speci®c sites as presented by patterned wafers, SIMS surface transient artifacts can affect the accuracy in the ®rst few monolayers.
291
Dynamic SIMS measurements were performed using CAMECA IMS 4f and IMS 6f magnetic sector instruments. All measurements were performed using an O2 primary beam and detecting positive secondary ions. Primary oxygen beam with impact energies of 3 keV (IMS 4f) and 800 eV (IMS 6f) was used. The angles of incidence were 568 and 428, respectively. An oxygen back®ll was applied to ensure the surface is fully oxidized during depth pro®ling. Suf®cient MRP was used to eliminate possible molecular interference during depth pro®ling. TOF-SIMS measurements were made by Physical Electronics TRIFT-3 mass spectrometer using net 700 eV oxygen bombardment and oxygen back®ll.
2. Experimental Single crystal Sih1 0 0i wafers were implanted with 10 metallic species with the parameters summarized in Table 1. The energies and doses were chosen to produce concentration maxima of about 1E19 atoms/cm3 at about 20 nm depth. The implantation was done at 78 with respect to normal to minimize channeling. All implanted species were major isotopes except Fe where the 54 Fe isotope was implanted to avoid Si2 interference during SIMS analysis. Table 1 Ion implantation parameters for metals standards Z
Ion
Energy (keV)
Dose (atoms/cm2)
Angle (8)
23 24 27 39 40 48 52 54 63 66
23
16 16 16 16 18 18 18 22 22 22
3.66E13 3.44E13 3.08E13 2.01E13 2.35E13 2.13E13 2.10E13 2.26E13 2.11E13 2.10E13
7 7 7 7 7 7 7 7 7 7
Na Mg 27 Al 39 K 40 Ca 48 Ti 52 Cr 54 Fe 63 Cu 66 Zn 24
Fig. 1. Dynamic SIMS and TRIM-simulated depth pro®les for 27 Al and 39 K implanted into silicon (a) and 54 Fe and 52 Cr implanted into silicon (b).
292
A.V. Li-Fatou, M. Douglas / Applied Surface Science 203±204 (2003) 290±293
SIMS calibration was done using nominal implanted doses. The nominal doses are being veri®ed by NRA but the results were not available at the time of this publication. Therefore, the absolute accuracy of reported SIMS pro®les is expected to be around 10%. 3. Results and discussion Fig. 1a shows SIMS depth pro®les of Al and K obtained by dynamic SIMS using 800 eV bombardment. The plot also includes TRIM-simulated distributions. SIMS data shows signi®cant channeling that causes the difference in the Cp observed for experimental and
Fig. 2. Correlation diagram for implants' projected ranges of TRIM-calculated and SIMS measured with 3 keV primary impact energy (a) and 800 eV impact energy (b).
theoretical data (TRIM simulation assumes amorphous substrate). This difference is ranging from 10 to 15% for light metals (Na, Mg and Al) to just a few percent for heavier elements (Fig. 1b). Therefore, the use of theoretically predicted values for concentration maxima may introduce a signi®cant error if calibration is done without a proper correction for channeling. All implants show their maxima well below the surface transient region or native oxide. However, measured projected ranges for almost all implants appear closer to the surface than predicted by Monte Carlo simulations (Figs. 1a and b and 2a and b) show correlation diagrams for implants' projected ranges obtained by TRIM simulation and experimentally using dynamic SIMS results obtained at 800 eV and 3 keV and impact energies, respectively. Fig. 3 shows the same correlation with TOF-SIMS data obtained at 700 eV. This Rp shift is found to be independent of the bombardment condition or measured species with the exception of the elements showing heavy segregation [3]. The fact that this shift is independent of the bombardment angle and occurs at such shallow depth allows us to believe that crater bottom roughening is not responsible for this effect. The average shift of 7:8 0:3 nm is also considerably larger than the presence of thin native oxide can explain. The simulation model has been carefully veri®ed using a 5 keV boron implant into an amorphous Si where an agreement with experiment better than 1 nm has been obtained [4].
Fig. 3. Correlation diagram for implants' projected ranges of TRIM-calculated and TOF-SIMS measured with 700 eV primary impact energy.
A.V. Li-Fatou, M. Douglas / Applied Surface Science 203±204 (2003) 290±293
293
One of the possible explanations of this shift is a chemical segregation phenomenon enhanced under oxygen bombardment and surface oxide formation. This effect is well known for Fe and especially Cu but has never been reported for light alkali elements possibly because it is hardly noticeable at a typical implant depth of a few hundred nanometers.
data obtained using three different SIMS techniques demonstrates that the prepared set of relevant metallic implants provide adequate calibration reference for accurate surface and near-surface trace metal analysis.
4. Conclusions
The authors would like to thank Ion Implant Services for helpful suggestions and preparation of the standards and Puneet Kohli of University of Texas at Austin for TRIM simulations.
A comprehensive set of ion implanted standards has been prepared and extensively characterized by dynamic and TOF-SIMS. It is shown that the distinct shape of ion implant distribution provides various means to account for measurement artifacts. The results of depth pro®ling have been compared against theoretically modeled distributions. It has been found that measured projected ranges of light elements are 30±40% less than theoretically predicted by Monte Carlo simulations. Chemical segregation is proposed to have played a role in this shift. The experimental
Acknowledgements
References [1] F.A. Stevie, R. Roberts, J.K. McKinley, M. Decker, C. Granger, in: Proceedings of the USJ Conference, NC, 1999. [2] D.S. Simons, Secondary Ion Mass Spectrometry, SIMS IX, Wiley, Chichester, 1994, p. 140. [3] A.V. Li-Fatou, in preparation. [4] P. Kohli, private communication.