- Email: [email protected]
Optimization of SIMS analysis conditions for Na, S, P and N in Cu films
Applied Surface Science 231–232 (2004) 796–799
Optimization of SIMS analysis conditions for Na, S, P and N in Cu films Yupu Li Materials Analytical Services, Sunnyvale Lab, 285 N. Wolfe Road, Sunnyvale, CA 94085, USA Available online 21 April 2004
Abstract Using a Csþ primary beam on a Cameca IMS SIMS instrument and ion implanted standards in Cu, it has been found that H, C, O, Cl in Cu films can be measured using negative secondary ions and a low to medium mass resolution (M/DM < 3000), however, S and P analysis require a higher mass resolution to resolve the mass interferences present in the barrier layer (TiSiN and TaN). Analysis of nitrogen required monitoring of the (Cs2N)þ secondary ions. Sodium and potassium secondary ions need to be measured as positive secondary ions. The analysis energy should be chosen based on the thickness of the Cu films, where thinner films (20 nm) require a very low profiling energy and this low profiling energy can be achieved by monitoring of the (Cs2M)þ molecular ions (M ¼ H, C, etc.). # 2004 Elsevier B.V. All rights reserved. Keywords: Contamination; Cu film; SIMS
1. Introduction New manufacturing processes that use copper interconnects in place of aluminum are well documented [1]. SIMS applications for the characterization of Cu films can be summarized as follows: (1) to aid in the selection of barrier materials that prevent the diffusion of Cu into the barrier layer and underlying oxide. Depth resolution loss due to the sputter induced topography and the memory effect require either analysis after backside polishing or stripping the Cu layer prior to SIMS analysis [2,3]. (2) Measurement of contamination or doping levels in the Cu films since particular contaminants can have an affect on the conductivity of the Cu films and therefore there presence needs to be monitored. Additionally, doping and contamination levels also affect the adhesion E-mail address: [email protected] (Y. Li).
of Cu layer to other layers [3]. This paper focuses on the measurement of contamination levels in Cu films.
2. Experimental The Cu films used in this work have a stack structure as follows: 800–1000 nm thick Cu on a very thin (10–30 nm) barrier layer over a 200 nm oxide layer. Implanted standards in Cu films were prepared with a device grade ion implanter. Low abundance isotopes (i.e. 13 C or 18 O) were chosen for implantation to achieve lower SIMS backgrounds. SIMS measurements were performed on a Cameca IMS-4f SIMS instrument. For negative secondary ion detection a Csþ primary ion beam was accelerated to 6 keV and secondary ions were extracted at 4.5 keV, resulting in a net profiling energy of 10.5 keVat an impact angle of
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.074
Y. Li / Applied Surface Science 231–232 (2004) 796–799
1E+19 CONCENTRATION (atoms/cc)
22.18. Crater depth measurements were performed on a KLA-Tencor Alphastep 500 surface profilometer.
3. Results and discussion Fig. 1 shows SIMS depth profiles of 13 C; 19 F; 18 O; and 35 Cl in the Cu implanted reference material. Negative secondary ions for 13 C, etc. and 65 Cu (as matrix reference) were followed. It should be noted that 13 C in the implanted standard needs to be measured using high mass resolution to resolve the mass interference with ð1 H þ 12 CÞ , while 12 C along with 1 H; 16 O; 19 F; and 35 Cl can be measured in the unknown Cu samples with low to medium mass resolution (M/DM 3000). Table 1 shows a comparison of the experimental and calculated range data by SRIM [4], showing good agreement between the experimental and modeling data for Rp. Maximum concentration (Cmax) (SIMS) and elemental relative sensitivity factors (RSFs) are also given in Table 1. Fig. 2 shows SIMS depth profiles for 31 P (200 keV 1:0 1014 atoms/cm2) versus a SRIM simulation in the Cu implanted standard. It is clear that the measured profile is somewhat wider than the simulated profile, and the implanted peak is at a smaller depth in the measured profile. Additionally, we measured profiles for 32 S (200 keV 1:0 1014 atoms/cm2) and the same trend as was observed for phosphorus was observed. For 31 P and 32 S, it is observed that the simulated profiles by SRIM [4] do not predict the long implanted tails. Since the total implanted dose is used to quantify the P or S concentration in the bulk Cu films the profile shape in the implanted standards have less effect on the quantification. It should be noted that both 31 P and 32 S ions need to be measured under high mass
797
1: 13C 2: 18O 3: Cl
1E+18
4: F
1E+17
1E+16 0
2000
4000
6000
8000
DEPTH (Angstroms) Fig. 1. SIMS depth profiles of implanted 13 C (90 keV), 19 F (78.7 keV), 16 O (110 keV), and 35 Cl (200 keV) in the Cu implanted standard. The dose is 1:0 1014 atoms/cm2 for each item except for 19 F, 6:7 1013 atoms/cm2 as dose.
resolution conditions (M/DM at 4000) to resolve mass interferences from (16 O þ 16 O) or (30 Si þ H). For example, see Fig. 3, the P SIMS profile was obtained using high mass resolution for a Cu film fabricated by electrochemical plating. In this case, there is no obvious P in the bulk film and at the interface between the Cu and barrier layer. It should be noted that in the barrier layer, oxygen, hydrogen, and Si content may be high and without the use of high mass resolution, P and S will show high intensity peaks in the surface and barrier layer due solely to the mass interference. Finally, it is important to mention that P doping is an important issue in Cu technology since P doping in the barrier layer may improve its barrier properties against copper diffusion [5].
Table 1 Description of the implanted standard in Cu: ion specie, ion energy, ion dose, experimental and theoretical projected range (Rp) in Cu Element
Energy (keV)
Dose (ions/cm2)
13
90 110 77.6 200
1.0 1.0 6.7 1.0
C O 19 F 35 Cl 18
1014 1014 1013 1014
˚) Rp (SIMS) (A
˚) Rp (SRIM) (A
Cmax (SIMS)
1410 1050 590 870
1165 1119 707 909
5.1 7.5 8.1 6.8
1018 1018 1018 1018
El RSF (cm3) 2.0 2.4 9.8 1.6
1021 1020 1018 1019
Cmax(Rp) is the peak concentration measured by SIMS. Based on the implanted standard in Cu, in the last column, the elemental (not isotopic) relative sensitivity factors are also tabulated. Measurement conditions: 10.5 keV (i.e. 6 þ 4:5 keV) profiling energy for negative 13 C , etc., with Cameca 4f). The matrix signal is 65 Cu .
798
Y. Li / Applied Surface Science 231–232 (2004) 796–799
CONCENTRATION (atoms/cc)
1E+19
Nitrogen has a very high ionization potential and a negative electron affinity resulting in poor or nonexistent ion yields for detection of nitrogen. Indeed, we have found that neither the Nþ nor N signal can provide information for an implanted nitrogen peak (i.e. 5:7 1018 atoms/cm3 after 100 keV 14 N implantation to a dose of 1:0 1014 atoms/cm2) in Cu. The (SiN) ion is typically monitored for analysis of N in silicon, however, there are strong mass interference between ð63 Cu þ NÞ or ð65 Cu þ NÞ and ð65 Cu þ 12 CÞ or ð63 Cu þ 16 OÞ. In fact, we have found that following the negative cluster ions at mass 77½ð14 N63 CuÞ and ð12 C65 CuÞ , resulted in no obvious nitrogen peak the implanted standard. Fig. 4 shows 14 N concentration profiles in the implanted standard obtained from the (Cs2N)þ and (CsN)þ cluster ions. It is clear that following (Cs2N)þ is a better choice for nitrogen profiling, since it results in a better detection limit for nitrogen and less interference in the surface region. As shown in Fig. 4, the measured 14 N profile is found to be in very good agreement with the simulated 14 N profile by SRIM [4]. Using the (Cs2M)þ detection mode, we have found that the yields of (Cs2M)þ (M ¼ H, C, etc.) are high enough to monitor the relative concentrations in the Cu films, and so it can be applied to profiling very thin Cu films since one needs very low profiling energy to obtain acceptable results.
P (SRIM) P (SIMS
1E+18
1E+17
1E+16
0
2000
4000
6000
8000
DEPTH (Angstroms)
Fig. 2. SIMS depth profile of implanted 31 P (open triangle line) and simulated 32 P profile (open circle line) by SRIM code [4].
Under Csþ profiling, Na and K negative ion yields are poor, and positive ion yields of Naþ and Kþ are sufficiently high so that they may be measured to monitor Na and K concentrations. In Fig. 3, a practical Na profile in the Cu film is shown. Positive cluster ions of (Cs2Na)þ and (Cs2K)þ also have high yields. Given the good positive ion yields for K and Na under Csþ bombardment it is not necessary to use an O2 primary beam for profiling of Na and K in thick Cu films. 1E+23
1000000
CONCENTRATION (atoms/cc
1E+22 100000 1E+21 1E+20
10000
1E+19 1000
P H C Na Cu(intensity)>
1E+18 1E+17
100
1E+16 10 1E+15 1E+14
1 0
2000
4000
6000
8000
10000
12000
DEPTH (Angstroms)
Fig. 3. SIMS depth profiles of P, H, C, Na in a Cu film, with 65 Cu as raw intensity. It should be noted that the measurements were terminated when they reached to the oxide layer. The quantifications are valid for Cu only.
Y. Li / Applied Surface Science 231–232 (2004) 796–799
799
CONCENTRATION (atoms/cc)
1E+19
N from Cs+Cs+N N from Cs+N N-SRIM code 1E+18
1E+17 0
2000
4000
6000
8000
DEPTH (Angstroms)
Fig. 4. SIMS depth profiles of implanted 14 N (100 keV 1:0 1014 atoms/cm2) in Cu. Open square line: quantified from ðCs þ Cs þ NÞþ signal; triangle line: quantified from ðCs þ NÞþ signal; solid line: simulated 14 N profile by SRIM code [4].
4. Conclusion
References
In summary, using a Csþ primary beam on a Cameca IMS SIMS instrument and ion implanted standards in Cu, it has been found that H, C, O, Cl in the thick Cu films can be measured using negative secondary ions and a low to medium mass resolution, while S and P analysis require a higher mass resolution to resolve the mass interferences. Analysis of nitrogen required monitoring of the (Cs2N)þ secondary ions. Sodium and potassium secondary ions need to be measured as Naþ and Kþ. The analysis energy should be chosen based on the thickness of the Cu films, where thinner films require a very low profiling energy and this low profiling energy can be achieved by monitoring of the (Cs2M)þ molecular ions (M ¼ H, C, etc.).
[1] Does Advanced Technology Really Matter, Intel 2002 Annual Report. http://www.intel.com. [2] C. Gu, A. Pivovarov, R. Garcia, F. Stevie, D. Griffis, J. Moran, L. Kulig, J.F. Richards, Joint Presentation on Secondary Ion Mass Spectrometry Back Side Analysis of Barrier Layers for Copper Diffusion, North Carolina State University/Analytical Instrumentation Facility and Intel/Materials Technology, 2003. [3] Y. Li, SIMS study of copper diffusion into the oxide layers: backside polished samples and samples with Cu layer stripped, Internal Report, Materials Analytical Services, SIMS Lab. [4] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in Solids, New Version as SRIM-2003, Pergamon Press, New York, 1985. [5] A. Kohn, M. Eizenberg, Y. Shacham-Diamand, J. Appl. Phys. 92 (2002) 5508.
Optimization of SIMS analysis conditions for Na, S, P and N in Cu films Yupu Li Materials Analytical Services, Sunnyvale Lab, 285 N. Wolfe Road, Sunnyvale, CA 94085, USA Available online 21 April 2004
Abstract Using a Csþ primary beam on a Cameca IMS SIMS instrument and ion implanted standards in Cu, it has been found that H, C, O, Cl in Cu films can be measured using negative secondary ions and a low to medium mass resolution (M/DM < 3000), however, S and P analysis require a higher mass resolution to resolve the mass interferences present in the barrier layer (TiSiN and TaN). Analysis of nitrogen required monitoring of the (Cs2N)þ secondary ions. Sodium and potassium secondary ions need to be measured as positive secondary ions. The analysis energy should be chosen based on the thickness of the Cu films, where thinner films (20 nm) require a very low profiling energy and this low profiling energy can be achieved by monitoring of the (Cs2M)þ molecular ions (M ¼ H, C, etc.). # 2004 Elsevier B.V. All rights reserved. Keywords: Contamination; Cu film; SIMS
1. Introduction New manufacturing processes that use copper interconnects in place of aluminum are well documented [1]. SIMS applications for the characterization of Cu films can be summarized as follows: (1) to aid in the selection of barrier materials that prevent the diffusion of Cu into the barrier layer and underlying oxide. Depth resolution loss due to the sputter induced topography and the memory effect require either analysis after backside polishing or stripping the Cu layer prior to SIMS analysis [2,3]. (2) Measurement of contamination or doping levels in the Cu films since particular contaminants can have an affect on the conductivity of the Cu films and therefore there presence needs to be monitored. Additionally, doping and contamination levels also affect the adhesion E-mail address: [email protected] (Y. Li).
of Cu layer to other layers [3]. This paper focuses on the measurement of contamination levels in Cu films.
2. Experimental The Cu films used in this work have a stack structure as follows: 800–1000 nm thick Cu on a very thin (10–30 nm) barrier layer over a 200 nm oxide layer. Implanted standards in Cu films were prepared with a device grade ion implanter. Low abundance isotopes (i.e. 13 C or 18 O) were chosen for implantation to achieve lower SIMS backgrounds. SIMS measurements were performed on a Cameca IMS-4f SIMS instrument. For negative secondary ion detection a Csþ primary ion beam was accelerated to 6 keV and secondary ions were extracted at 4.5 keV, resulting in a net profiling energy of 10.5 keVat an impact angle of
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.074
Y. Li / Applied Surface Science 231–232 (2004) 796–799
1E+19 CONCENTRATION (atoms/cc)
22.18. Crater depth measurements were performed on a KLA-Tencor Alphastep 500 surface profilometer.
3. Results and discussion Fig. 1 shows SIMS depth profiles of 13 C; 19 F; 18 O; and 35 Cl in the Cu implanted reference material. Negative secondary ions for 13 C, etc. and 65 Cu (as matrix reference) were followed. It should be noted that 13 C in the implanted standard needs to be measured using high mass resolution to resolve the mass interference with ð1 H þ 12 CÞ , while 12 C along with 1 H; 16 O; 19 F; and 35 Cl can be measured in the unknown Cu samples with low to medium mass resolution (M/DM 3000). Table 1 shows a comparison of the experimental and calculated range data by SRIM [4], showing good agreement between the experimental and modeling data for Rp. Maximum concentration (Cmax) (SIMS) and elemental relative sensitivity factors (RSFs) are also given in Table 1. Fig. 2 shows SIMS depth profiles for 31 P (200 keV 1:0 1014 atoms/cm2) versus a SRIM simulation in the Cu implanted standard. It is clear that the measured profile is somewhat wider than the simulated profile, and the implanted peak is at a smaller depth in the measured profile. Additionally, we measured profiles for 32 S (200 keV 1:0 1014 atoms/cm2) and the same trend as was observed for phosphorus was observed. For 31 P and 32 S, it is observed that the simulated profiles by SRIM [4] do not predict the long implanted tails. Since the total implanted dose is used to quantify the P or S concentration in the bulk Cu films the profile shape in the implanted standards have less effect on the quantification. It should be noted that both 31 P and 32 S ions need to be measured under high mass
797
1: 13C 2: 18O 3: Cl
1E+18
4: F
1E+17
1E+16 0
2000
4000
6000
8000
DEPTH (Angstroms) Fig. 1. SIMS depth profiles of implanted 13 C (90 keV), 19 F (78.7 keV), 16 O (110 keV), and 35 Cl (200 keV) in the Cu implanted standard. The dose is 1:0 1014 atoms/cm2 for each item except for 19 F, 6:7 1013 atoms/cm2 as dose.
resolution conditions (M/DM at 4000) to resolve mass interferences from (16 O þ 16 O) or (30 Si þ H). For example, see Fig. 3, the P SIMS profile was obtained using high mass resolution for a Cu film fabricated by electrochemical plating. In this case, there is no obvious P in the bulk film and at the interface between the Cu and barrier layer. It should be noted that in the barrier layer, oxygen, hydrogen, and Si content may be high and without the use of high mass resolution, P and S will show high intensity peaks in the surface and barrier layer due solely to the mass interference. Finally, it is important to mention that P doping is an important issue in Cu technology since P doping in the barrier layer may improve its barrier properties against copper diffusion [5].
Table 1 Description of the implanted standard in Cu: ion specie, ion energy, ion dose, experimental and theoretical projected range (Rp) in Cu Element
Energy (keV)
Dose (ions/cm2)
13
90 110 77.6 200
1.0 1.0 6.7 1.0
C O 19 F 35 Cl 18
1014 1014 1013 1014
˚) Rp (SIMS) (A
˚) Rp (SRIM) (A
Cmax (SIMS)
1410 1050 590 870
1165 1119 707 909
5.1 7.5 8.1 6.8
1018 1018 1018 1018
El RSF (cm3) 2.0 2.4 9.8 1.6
1021 1020 1018 1019
Cmax(Rp) is the peak concentration measured by SIMS. Based on the implanted standard in Cu, in the last column, the elemental (not isotopic) relative sensitivity factors are also tabulated. Measurement conditions: 10.5 keV (i.e. 6 þ 4:5 keV) profiling energy for negative 13 C , etc., with Cameca 4f). The matrix signal is 65 Cu .
798
Y. Li / Applied Surface Science 231–232 (2004) 796–799
CONCENTRATION (atoms/cc)
1E+19
Nitrogen has a very high ionization potential and a negative electron affinity resulting in poor or nonexistent ion yields for detection of nitrogen. Indeed, we have found that neither the Nþ nor N signal can provide information for an implanted nitrogen peak (i.e. 5:7 1018 atoms/cm3 after 100 keV 14 N implantation to a dose of 1:0 1014 atoms/cm2) in Cu. The (SiN) ion is typically monitored for analysis of N in silicon, however, there are strong mass interference between ð63 Cu þ NÞ or ð65 Cu þ NÞ and ð65 Cu þ 12 CÞ or ð63 Cu þ 16 OÞ. In fact, we have found that following the negative cluster ions at mass 77½ð14 N63 CuÞ and ð12 C65 CuÞ , resulted in no obvious nitrogen peak the implanted standard. Fig. 4 shows 14 N concentration profiles in the implanted standard obtained from the (Cs2N)þ and (CsN)þ cluster ions. It is clear that following (Cs2N)þ is a better choice for nitrogen profiling, since it results in a better detection limit for nitrogen and less interference in the surface region. As shown in Fig. 4, the measured 14 N profile is found to be in very good agreement with the simulated 14 N profile by SRIM [4]. Using the (Cs2M)þ detection mode, we have found that the yields of (Cs2M)þ (M ¼ H, C, etc.) are high enough to monitor the relative concentrations in the Cu films, and so it can be applied to profiling very thin Cu films since one needs very low profiling energy to obtain acceptable results.
P (SRIM) P (SIMS
1E+18
1E+17
1E+16
0
2000
4000
6000
8000
DEPTH (Angstroms)
Fig. 2. SIMS depth profile of implanted 31 P (open triangle line) and simulated 32 P profile (open circle line) by SRIM code [4].
Under Csþ profiling, Na and K negative ion yields are poor, and positive ion yields of Naþ and Kþ are sufficiently high so that they may be measured to monitor Na and K concentrations. In Fig. 3, a practical Na profile in the Cu film is shown. Positive cluster ions of (Cs2Na)þ and (Cs2K)þ also have high yields. Given the good positive ion yields for K and Na under Csþ bombardment it is not necessary to use an O2 primary beam for profiling of Na and K in thick Cu films. 1E+23
1000000
CONCENTRATION (atoms/cc
1E+22 100000 1E+21 1E+20
10000
1E+19 1000
P H C Na Cu(intensity)>
1E+18 1E+17
100
1E+16 10 1E+15 1E+14
1 0
2000
4000
6000
8000
10000
12000
DEPTH (Angstroms)
Fig. 3. SIMS depth profiles of P, H, C, Na in a Cu film, with 65 Cu as raw intensity. It should be noted that the measurements were terminated when they reached to the oxide layer. The quantifications are valid for Cu only.
Y. Li / Applied Surface Science 231–232 (2004) 796–799
799
CONCENTRATION (atoms/cc)
1E+19
N from Cs+Cs+N N from Cs+N N-SRIM code 1E+18
1E+17 0
2000
4000
6000
8000
DEPTH (Angstroms)
Fig. 4. SIMS depth profiles of implanted 14 N (100 keV 1:0 1014 atoms/cm2) in Cu. Open square line: quantified from ðCs þ Cs þ NÞþ signal; triangle line: quantified from ðCs þ NÞþ signal; solid line: simulated 14 N profile by SRIM code [4].
4. Conclusion
References
In summary, using a Csþ primary beam on a Cameca IMS SIMS instrument and ion implanted standards in Cu, it has been found that H, C, O, Cl in the thick Cu films can be measured using negative secondary ions and a low to medium mass resolution, while S and P analysis require a higher mass resolution to resolve the mass interferences. Analysis of nitrogen required monitoring of the (Cs2N)þ secondary ions. Sodium and potassium secondary ions need to be measured as Naþ and Kþ. The analysis energy should be chosen based on the thickness of the Cu films, where thinner films require a very low profiling energy and this low profiling energy can be achieved by monitoring of the (Cs2M)þ molecular ions (M ¼ H, C, etc.).
[1] Does Advanced Technology Really Matter, Intel 2002 Annual Report. http://www.intel.com. [2] C. Gu, A. Pivovarov, R. Garcia, F. Stevie, D. Griffis, J. Moran, L. Kulig, J.F. Richards, Joint Presentation on Secondary Ion Mass Spectrometry Back Side Analysis of Barrier Layers for Copper Diffusion, North Carolina State University/Analytical Instrumentation Facility and Intel/Materials Technology, 2003. [3] Y. Li, SIMS study of copper diffusion into the oxide layers: backside polished samples and samples with Cu layer stripped, Internal Report, Materials Analytical Services, SIMS Lab. [4] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in Solids, New Version as SRIM-2003, Pergamon Press, New York, 1985. [5] A. Kohn, M. Eizenberg, Y. Shacham-Diamand, J. Appl. Phys. 92 (2002) 5508.