- Email: [email protected]
The features of using of BO2− secondary ions for SIMS depth profiling of shallow boron implantation in silicon
Applied Surface Science 203±204 (2003) 314±317
The features of using of BO2 secondary ions for SIMS depth pro®ling of shallow boron implantation in silicon S.G. Simakin*, V.K. Smirnov Institute of Microelectronics and Informatics RAS, 3, Krasnoborskaya Str., Yaroslavl 150051, Russia
Abstract The strong in¯uence of oxygen to the positive secondary ion yield determines the problems of SIMS depth pro®ling of shallow boron implants in silicon, related to surface oxide and formation of modi®ed layer. High value (4 eV) of electron af®nity of BO2 molecule responds for the high yield of BO2 negative secondary ions, its small sensitivity to the variation of emission properties of the sample surface and relatively weak dependence on oxygen concentration. The features of using of named ions for shallow boron pro®ling was studied with a magnetic sector SIMS instrument in a high mass resolution mode under O2 and NO2 primary ion bombardment. The refuse from oxygen ¯ooding allowed to avoid the initial signi®cant change of sputter rate. The studied approach provides simpli®cation of transient phenomena while still keeping suf®cient sensitivity. # 2002 Elsevier Science B.V. All rights reserved. Keywords: SIMS; Shallow; Depth pro®ling; Glancing; Transient; BO2 ; O2
1. Introduction Glancing angle of incidence of an oxygen primary beam was considered as a possible means to improve depth resolution when depth pro®ling of shallow boron distribution in silicon by SIMS [1]. This way to reduce penetration depth of primary ions and, thus, to get thinner modi®ed layer and smaller transient depth seemed to be fruitful, especially for magnetic sector SIMS instruments. The known limitation of ion microscope, running at the same polarity of primary and secondary ions, is that the primary beam energy cannot be dropped below some value depending on the sample high voltage. In this case the angle of incidence of primary ions trends to the glancing one with the decrease of their energy. * Corresponding author. Tel.: 7-852-350909. E-mail address: [email protected] (S.G. Simakin).
It has become a commonplace that oblique bombardment of silicon by O2 primary beam necessarily requires oxygen ¯ooding. There is strong dependence of the yield of positive secondary ions on the oxygen content. The full oxidation of the sample surface, provided by oxygen ¯ood, eliminates signi®cant loss of sensitivity. Nevertheless, it generally cannot prevent distortion of the measured pro®le caused by the presence of the surface native oxide and the transient phenomena [2]. BO2 secondary ions have come into notice owing to the extremely high (4 eV) [3] value of electron af®nity intrinsic to the BO2 molecule. It should lead to high yield of named negative secondary ions and its low sensitivity to the variation of local emission properties of the surface. This consideration together with certain experience of working with molecular negative primary ions [4] inspired elaboration of some alternative approach to shallow depth pro®ling.
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 6 6 7 - 0
S.G. Simakin, V.K. Smirnov / Applied Surface Science 203±204 (2003) 314±317
It includes the use of magnetic sector SIMS instrument, BO2 secondary ions monitoring and molecular negative primary beam bombardment at oblique angles. Refuse from oxygen ¯ood was expected to exclude initial strong sputter rate variation. 2. Experimental The samples used (#1±#4) were pieces cut from silicon wafers doped with B (2 keV, 5 1014 at./cm3, 100 keV, 1 1015 at./cm3) and BF2 (2 keV, 1 1014 at./cm3). The 2 keV boron pro®le (sample #1) was appropriated to study the response of the emission of BO2 secondary ions to the change of surface oxygenation in the oxygen ¯ooding experiment. Sample #2, with the widest boron distribution, was pre-sputtered by 2 keV O2 ion bombardment to its maximum concentration depth. Encapsulation technique was applied to reveal the in¯uence of transient phenomena to the shape of shallow pro®le measured ``as is'' (sample #3). Amorphous silicon layer of 40 nm thick was deposited onto the surface of the one of BF2 implanted samples (sample #4) by magnetron sputtering. Experiments were performed in a CAMECA IMS4F instrument. O2 and NO2 primary ions were produced in the duoplasmatron. It was ®tted with pure oxygen or oxygen with admixture of about 5±10% volume of nitrogen. Both O2 and NO2 primaries were easily available in the last case by simple readjustment of the mass separator. Effective energy of the primary ions was 2 keV and the angle of incidence was about 598 [5]. Ion current was adjusted down to 15 nA for both primary beams while maximum available values for this energy were 200 nA for O2 and 500 nA for NO2 . Primary beam was scanned over 200 mm 200 mm area whereas beam diameter was about 70 mm for O2 and somewhat smaller for NO2 . O2 primary beam used in oxygen ¯ooding experiment was of 50 nA. Secondary ions (11 B16 O2 and 28 Si16 O2 ) were collected from the area of 62.5 mm in diameter centred in the sputter crater. High mass resolution M=DM 4500 was applied to eliminate 29 Si14 N and 28 Si15 N molecular ions interfering at 43rd atomic mass. Stylus pro®lometry was used to determine crater depths. The ratio of SiO2 to BO2 secondary ion currents was determining during modi®ed layer formation
315
under O ion bombardment of the sample #2. The primary beam energy, angle of incidence and ion current were 10 keV, 258, and 300 nA correspondingly. The boron concentration was considered as changing negligibly near the top of 100 keV boron pro®le. Variation of oxygen concentration in this experiment was started from the stationary level speci®c to the O2 , 598 beam bombardment and upper limited by SiO2 formation. 3. Results and discussion The results of pro®ling of sample #1 under various ¯ooding conditions are shown in Fig. 1. Notably small sensitivity of the yield of BO2 secondary ions to the oxygen content variation can be concluded. The presence of thin oxide layer caused the surface spike of much less magnitude in the case of BO2 secondary ion registration and O2 beam bombardment (curve 1) then common combination (B secondary and O2 primary ions) has given (curve 5). Oxygen pressure (measured in the sample chamber) was raised from
Fig. 1. Sample #1 pro®led with: O2 primary and BO2 secondary ions (1±4); 1Ðno ¯ood; 2Ð3 10 7 Torr; 3Ð2 10 6 Torr; 4Ð 5 10 6 Torr. 5Ð2 keV O2 primary beam, B secondary ions, no ¯ood.
316
S.G. Simakin, V.K. Smirnov / Applied Surface Science 203±204 (2003) 314±317
residual up to 5 10 6 Torr. It led to an increase of BO2 signal and a decrease of the sputter rate. The overall change of the yield of this molecular ion did not exceed the factor 3. It should be noted that maximal yield of BO2 ions is reached at 2 10 6 Torr (curve 3), when the sputter yield decrease is only about 25% from initial (no ¯ood) value. It is quite far from the full silicon oxidation that takes place at 5 10 6 Torr (curve 4) and leads to about 2.5 times drop of sputter yield. The different response to the silicon oxidation degree of matrix (SiO2 ) and boron comprising (BO2 ) secondary ions results from data presented in Fig. 2. The parts of curves in Fig. 2a, limited by time interval 100± 500 s, were used to build dependence of the ratio of BO2 to SiO2 on SiO2 secondary ion intensity (Fig. 2b). This dependence re¯ects the relative sensitivity factor changes: RSF RSF0 K 1 (Iref). It was linearly approximated and taken into account when quantifying. Here Iref is the intensity of 30 Si16 O2 matrix ion normalized to the SiO2 level. The unity
Fig. 2. (a) Initial stage of the bombardment of the sample #2 by O primary beam and (b) dependence of the ratio of BO2 to SiO2 on SiO2 secondary ion intensity derived from data shown in diagram (a).
Fig. 3. Processed depth pro®les of the samples: 1Ð#1; 2Ð#3; 3Ð #4 (see description of the samples in the text). Primary beamÐ O2 , secondary ionsÐBO2 , no oxygen ¯ood.
on Y scale of Fig. 2b corresponds to the stationary value for 2 keV O2 beam bombardment. Boron redistribution/segregation may take place during modi®ed layer formation but it was neglected this time. Boron pro®les shown in Fig. 3 were obtained for samples #1, #3 and #4 under the same conditions (O2 /BO2 /no oxygen ¯ood). Quanti®cation procedure included point-to-point BO2 /SiO2 normalization and using of RSF dependence on reference ion current intensity from Fig. 2b. RSF0 was found to be of 8:34 1020 at./cm3. It was calculated for pro®le 1 via known dose. Pro®le 3 was shifted by 40 nm backwards, so abscissas of points, corresponding amorphous silicon deposit, have become negative. It can be found that surface spike, observed in the raw pro®le of the sample #1 (Fig. 1, pro®le 1), generally smoothed in the processed one (Fig. 3, pro®le 1). Furthermore, pro®le 2 (as implanted) closely coincide the shifted pro®le 3 (encapsulated sample). It means that transient phenomena seemingly do not play a dramatic role under conditions applied in this experiment and no signi®cant sputter rate change takes place during the transition from surface oxide to silicon and during sputter conditions stabilization. The dose as high as 1:2 1014 at./cm3 was found for the pro®le 2, which corresponded closely to the nominal value of 1 1014 at/cm3. Thus, quanti®cation
S.G. Simakin, V.K. Smirnov / Applied Surface Science 203±204 (2003) 314±317
317
same energy. The reason for higher background level, demonstrated by the pro®le 1, is the unresolved interference of 28 Si14 N1 H at 43rd atomic mass. Pro®le 3 in Fig. 4 was measured using both primary beams. O2 was switched on instead of NO2 at the moment when signal intensity has closed in the background level. The ratio of sputter rates for both primaries was determined separately and considered while quantifying. This expedient allowed keeping better depth resolution, speci®c to NO2 beam bombardment, and to get lower detection limit which being an advantage of O2 primary ions. 4. Conclusion
Fig. 4. Processed depth pro®les of the sample #1: 1Ðbackground level for NO2 primary ions; 2ÐO2 primary beam; 3ÐNO2 and O2 primary beams were used sequentially, switching point is marked by the arrow.
procedure works quite well despite the fact that the major part of the pro®le falls to the near surface region where oxygen content changes rapidly from SiO2 to the low stationary level, characteristic to oblique 2 keV O2 bombardment. NO2 primary beam was applied when measuring pro®le 1 and initial part of pro®le 3 shown in Fig. 4. The initial part of pro®le 1 exactly coincided the corresponding part of pro®le 3 and was not shown to lighten the diagram. Pro®le 2, presented for comparison, was obtained with O2 primary beam. Narrower width and steeper trailing edge being characteristic of pro®le 3 testify better depth resolution provided by larger molecular primary ion with the
Despite some loss of sensitivity and the depth resolution still staying far from the physical limit, detection of BO2 secondary ions combined with oblique O2 or (and) NO2 negative primary beam sputtering without oxygen ¯ooding allow to simplify the transient phenomena. Thus, it can provide a convenient way to get more accurate information on shallow boron distribution in silicon at least for users of Cameca IMSÐ3±6F ion microscopes. References [1] P.A. Ronsheim, K.L. Lee, S.B. Patel, M. Schuhmacher, in: Proceedings of the XIth SIMS, Wiley, New York, 1998, p. 301. [2] K. Wittmaack, S.F. Corcoran, J. Vac. Sci. Technol. B 16 (1998) 272. [3] Fizicheskie velichiny, Energoatomizdat, Moscow, 1991, p. 421. [4] S.G. Simakin, in: Proceedings of the XIth SIMS, Wiley, New York, 1998, p. 213. [5] Z.-X. Jiang, P.F.A. Alkemade, Surf. Interf. Anal. 25 (1997) 817.
The features of using of BO2 secondary ions for SIMS depth pro®ling of shallow boron implantation in silicon S.G. Simakin*, V.K. Smirnov Institute of Microelectronics and Informatics RAS, 3, Krasnoborskaya Str., Yaroslavl 150051, Russia
Abstract The strong in¯uence of oxygen to the positive secondary ion yield determines the problems of SIMS depth pro®ling of shallow boron implants in silicon, related to surface oxide and formation of modi®ed layer. High value (4 eV) of electron af®nity of BO2 molecule responds for the high yield of BO2 negative secondary ions, its small sensitivity to the variation of emission properties of the sample surface and relatively weak dependence on oxygen concentration. The features of using of named ions for shallow boron pro®ling was studied with a magnetic sector SIMS instrument in a high mass resolution mode under O2 and NO2 primary ion bombardment. The refuse from oxygen ¯ooding allowed to avoid the initial signi®cant change of sputter rate. The studied approach provides simpli®cation of transient phenomena while still keeping suf®cient sensitivity. # 2002 Elsevier Science B.V. All rights reserved. Keywords: SIMS; Shallow; Depth pro®ling; Glancing; Transient; BO2 ; O2
1. Introduction Glancing angle of incidence of an oxygen primary beam was considered as a possible means to improve depth resolution when depth pro®ling of shallow boron distribution in silicon by SIMS [1]. This way to reduce penetration depth of primary ions and, thus, to get thinner modi®ed layer and smaller transient depth seemed to be fruitful, especially for magnetic sector SIMS instruments. The known limitation of ion microscope, running at the same polarity of primary and secondary ions, is that the primary beam energy cannot be dropped below some value depending on the sample high voltage. In this case the angle of incidence of primary ions trends to the glancing one with the decrease of their energy. * Corresponding author. Tel.: 7-852-350909. E-mail address: [email protected] (S.G. Simakin).
It has become a commonplace that oblique bombardment of silicon by O2 primary beam necessarily requires oxygen ¯ooding. There is strong dependence of the yield of positive secondary ions on the oxygen content. The full oxidation of the sample surface, provided by oxygen ¯ood, eliminates signi®cant loss of sensitivity. Nevertheless, it generally cannot prevent distortion of the measured pro®le caused by the presence of the surface native oxide and the transient phenomena [2]. BO2 secondary ions have come into notice owing to the extremely high (4 eV) [3] value of electron af®nity intrinsic to the BO2 molecule. It should lead to high yield of named negative secondary ions and its low sensitivity to the variation of local emission properties of the surface. This consideration together with certain experience of working with molecular negative primary ions [4] inspired elaboration of some alternative approach to shallow depth pro®ling.
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 6 6 7 - 0
S.G. Simakin, V.K. Smirnov / Applied Surface Science 203±204 (2003) 314±317
It includes the use of magnetic sector SIMS instrument, BO2 secondary ions monitoring and molecular negative primary beam bombardment at oblique angles. Refuse from oxygen ¯ood was expected to exclude initial strong sputter rate variation. 2. Experimental The samples used (#1±#4) were pieces cut from silicon wafers doped with B (2 keV, 5 1014 at./cm3, 100 keV, 1 1015 at./cm3) and BF2 (2 keV, 1 1014 at./cm3). The 2 keV boron pro®le (sample #1) was appropriated to study the response of the emission of BO2 secondary ions to the change of surface oxygenation in the oxygen ¯ooding experiment. Sample #2, with the widest boron distribution, was pre-sputtered by 2 keV O2 ion bombardment to its maximum concentration depth. Encapsulation technique was applied to reveal the in¯uence of transient phenomena to the shape of shallow pro®le measured ``as is'' (sample #3). Amorphous silicon layer of 40 nm thick was deposited onto the surface of the one of BF2 implanted samples (sample #4) by magnetron sputtering. Experiments were performed in a CAMECA IMS4F instrument. O2 and NO2 primary ions were produced in the duoplasmatron. It was ®tted with pure oxygen or oxygen with admixture of about 5±10% volume of nitrogen. Both O2 and NO2 primaries were easily available in the last case by simple readjustment of the mass separator. Effective energy of the primary ions was 2 keV and the angle of incidence was about 598 [5]. Ion current was adjusted down to 15 nA for both primary beams while maximum available values for this energy were 200 nA for O2 and 500 nA for NO2 . Primary beam was scanned over 200 mm 200 mm area whereas beam diameter was about 70 mm for O2 and somewhat smaller for NO2 . O2 primary beam used in oxygen ¯ooding experiment was of 50 nA. Secondary ions (11 B16 O2 and 28 Si16 O2 ) were collected from the area of 62.5 mm in diameter centred in the sputter crater. High mass resolution M=DM 4500 was applied to eliminate 29 Si14 N and 28 Si15 N molecular ions interfering at 43rd atomic mass. Stylus pro®lometry was used to determine crater depths. The ratio of SiO2 to BO2 secondary ion currents was determining during modi®ed layer formation
315
under O ion bombardment of the sample #2. The primary beam energy, angle of incidence and ion current were 10 keV, 258, and 300 nA correspondingly. The boron concentration was considered as changing negligibly near the top of 100 keV boron pro®le. Variation of oxygen concentration in this experiment was started from the stationary level speci®c to the O2 , 598 beam bombardment and upper limited by SiO2 formation. 3. Results and discussion The results of pro®ling of sample #1 under various ¯ooding conditions are shown in Fig. 1. Notably small sensitivity of the yield of BO2 secondary ions to the oxygen content variation can be concluded. The presence of thin oxide layer caused the surface spike of much less magnitude in the case of BO2 secondary ion registration and O2 beam bombardment (curve 1) then common combination (B secondary and O2 primary ions) has given (curve 5). Oxygen pressure (measured in the sample chamber) was raised from
Fig. 1. Sample #1 pro®led with: O2 primary and BO2 secondary ions (1±4); 1Ðno ¯ood; 2Ð3 10 7 Torr; 3Ð2 10 6 Torr; 4Ð 5 10 6 Torr. 5Ð2 keV O2 primary beam, B secondary ions, no ¯ood.
316
S.G. Simakin, V.K. Smirnov / Applied Surface Science 203±204 (2003) 314±317
residual up to 5 10 6 Torr. It led to an increase of BO2 signal and a decrease of the sputter rate. The overall change of the yield of this molecular ion did not exceed the factor 3. It should be noted that maximal yield of BO2 ions is reached at 2 10 6 Torr (curve 3), when the sputter yield decrease is only about 25% from initial (no ¯ood) value. It is quite far from the full silicon oxidation that takes place at 5 10 6 Torr (curve 4) and leads to about 2.5 times drop of sputter yield. The different response to the silicon oxidation degree of matrix (SiO2 ) and boron comprising (BO2 ) secondary ions results from data presented in Fig. 2. The parts of curves in Fig. 2a, limited by time interval 100± 500 s, were used to build dependence of the ratio of BO2 to SiO2 on SiO2 secondary ion intensity (Fig. 2b). This dependence re¯ects the relative sensitivity factor changes: RSF RSF0 K 1 (Iref). It was linearly approximated and taken into account when quantifying. Here Iref is the intensity of 30 Si16 O2 matrix ion normalized to the SiO2 level. The unity
Fig. 2. (a) Initial stage of the bombardment of the sample #2 by O primary beam and (b) dependence of the ratio of BO2 to SiO2 on SiO2 secondary ion intensity derived from data shown in diagram (a).
Fig. 3. Processed depth pro®les of the samples: 1Ð#1; 2Ð#3; 3Ð #4 (see description of the samples in the text). Primary beamÐ O2 , secondary ionsÐBO2 , no oxygen ¯ood.
on Y scale of Fig. 2b corresponds to the stationary value for 2 keV O2 beam bombardment. Boron redistribution/segregation may take place during modi®ed layer formation but it was neglected this time. Boron pro®les shown in Fig. 3 were obtained for samples #1, #3 and #4 under the same conditions (O2 /BO2 /no oxygen ¯ood). Quanti®cation procedure included point-to-point BO2 /SiO2 normalization and using of RSF dependence on reference ion current intensity from Fig. 2b. RSF0 was found to be of 8:34 1020 at./cm3. It was calculated for pro®le 1 via known dose. Pro®le 3 was shifted by 40 nm backwards, so abscissas of points, corresponding amorphous silicon deposit, have become negative. It can be found that surface spike, observed in the raw pro®le of the sample #1 (Fig. 1, pro®le 1), generally smoothed in the processed one (Fig. 3, pro®le 1). Furthermore, pro®le 2 (as implanted) closely coincide the shifted pro®le 3 (encapsulated sample). It means that transient phenomena seemingly do not play a dramatic role under conditions applied in this experiment and no signi®cant sputter rate change takes place during the transition from surface oxide to silicon and during sputter conditions stabilization. The dose as high as 1:2 1014 at./cm3 was found for the pro®le 2, which corresponded closely to the nominal value of 1 1014 at/cm3. Thus, quanti®cation
S.G. Simakin, V.K. Smirnov / Applied Surface Science 203±204 (2003) 314±317
317
same energy. The reason for higher background level, demonstrated by the pro®le 1, is the unresolved interference of 28 Si14 N1 H at 43rd atomic mass. Pro®le 3 in Fig. 4 was measured using both primary beams. O2 was switched on instead of NO2 at the moment when signal intensity has closed in the background level. The ratio of sputter rates for both primaries was determined separately and considered while quantifying. This expedient allowed keeping better depth resolution, speci®c to NO2 beam bombardment, and to get lower detection limit which being an advantage of O2 primary ions. 4. Conclusion
Fig. 4. Processed depth pro®les of the sample #1: 1Ðbackground level for NO2 primary ions; 2ÐO2 primary beam; 3ÐNO2 and O2 primary beams were used sequentially, switching point is marked by the arrow.
procedure works quite well despite the fact that the major part of the pro®le falls to the near surface region where oxygen content changes rapidly from SiO2 to the low stationary level, characteristic to oblique 2 keV O2 bombardment. NO2 primary beam was applied when measuring pro®le 1 and initial part of pro®le 3 shown in Fig. 4. The initial part of pro®le 1 exactly coincided the corresponding part of pro®le 3 and was not shown to lighten the diagram. Pro®le 2, presented for comparison, was obtained with O2 primary beam. Narrower width and steeper trailing edge being characteristic of pro®le 3 testify better depth resolution provided by larger molecular primary ion with the
Despite some loss of sensitivity and the depth resolution still staying far from the physical limit, detection of BO2 secondary ions combined with oblique O2 or (and) NO2 negative primary beam sputtering without oxygen ¯ooding allow to simplify the transient phenomena. Thus, it can provide a convenient way to get more accurate information on shallow boron distribution in silicon at least for users of Cameca IMSÐ3±6F ion microscopes. References [1] P.A. Ronsheim, K.L. Lee, S.B. Patel, M. Schuhmacher, in: Proceedings of the XIth SIMS, Wiley, New York, 1998, p. 301. [2] K. Wittmaack, S.F. Corcoran, J. Vac. Sci. Technol. B 16 (1998) 272. [3] Fizicheskie velichiny, Energoatomizdat, Moscow, 1991, p. 421. [4] S.G. Simakin, in: Proceedings of the XIth SIMS, Wiley, New York, 1998, p. 213. [5] Z.-X. Jiang, P.F.A. Alkemade, Surf. Interf. Anal. 25 (1997) 817.