Applied Surface Science 203±204 (2003) 264±267
Accurate SIMS depth pro®ling for ultra-shallow implants using backside SIMS C. Hongoa,*, M. Tomitaa, M. Takenakab, A. Murakoshic a
Corporate Research & Development Center, Toshiba Corporation, 8 Shinsugita-cho, Isogo-ku, Yokohama 235-8522, Japan b Corporate Research & Development Center, Toshiba Corporation, 1 Komukai Toshiba-cho, Saiwai-ku, Kawasaki, Japan c Process & Manufacturing Engineering Center, Semiconductor Company, Toshiba Corporation, 8 Shinsugita-cho, Isogo-ku, Yokohama, Japan
Abstract We studied accurate depth pro®ling for ultra-shallow implants using backside SIMS. In the case of measuring ultra-shallow pro®les, the effects of surface transient and knock-on are not negligible. Therefore, we applied backside SIMS to analyze ultrashallow doping to exclude these effects. Comparing the SIMS pro®les of surface-side and those of backside, backside pro®les show a sharper ion implantation tail than surface-side pro®les. Furthermore, backside SIMS pro®les show almost no dependence on primary ion energy. This indicates that backside SIMS provides sharp B pro®les suitable for analyzing ultra-shallow implants, using higher primary ion energy in comparison with implantation energy. The backside SIMS technique has a good potential to be used for next generation devices. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Backside SIMS; Ultra-shallow doping; Knock-on effect; Surface transient
1. Introduction Ultra-shallow doping techniques using sub-keV dopant implantation have been developed to enhance the packing densities and switching speed of electronic devices in ULSI technology. The accurate measurement and quanti®cation of ultra-shallow dopant pro®les are necessary for developing new implantation technologies. However, previous works pointed out several problems associated with low energy SIMS depth pro®ling [1,2], such as surface transients and surface roughening. Furthermore, the knock-on effect is not negligible since the implantation energy for next generation devices is equal to or less than the *
Corresponding author. Tel.: 81-45-770-3521; fax: 81-45-770-3569. E-mail address:
[email protected] (C. Hongo).
minimum primary ion energy in an advanced SIMS instrument. In this study, we applied backside SIMS to analyze samples having ultra-shallow doping (B implanted, 0.2 keV, 1 1015 cm 2) in order to exclude the effects due to surface transients and knock-on. Comparing the SIMS pro®les of surface-side and those of backside, we con®rmed that backside SIMS was advantageous for analyzing samples having ultra-shallow doping. In addition, the backside SIMS technique showed good possibility to be used for the analysis of next generation devices. 2. Experimental SIMS experiments were performed by using a Physical Electronics ADEPT1010 quadrupole SIMS
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 8 7 6 - 0
C. Hongo et al. / Applied Surface Science 203±204 (2003) 264±267
instrument. Oxygen ion was used for the primary beam with oxygen ¯ooding. The impact energies of the O2 primary ions were 0.35, 0.5 and 1 keV. The incident angle was set at 458. The pressure of the O2 ¯ooding was approximately 8 10 7 Torr. Large raster ranging from 500 mm 500 mm to 600 mm 600 mm were used to reduce the effect of crater edges. Electronic gating was used to limit the collection area to the central 4%. Four equally spaced boron delta layers were used to calibrate the sputtered depth. The spacing between the delta layers was 5 nm. Sputtering rates were calculated from the distance between the ®rst and fourth delta layers. With oxygen ¯ooding, the sputtering rate of the transient region becomes higher than that of deep region [3]. Therefore, the ®rst delta layer shifted towards the surface. The distance of this shift was calculated and taken into account for evaluating surface-side SIMS pro®les throughout this study. Concentration values were calibrated using B bulk-doped Si samples. Samples with ultra-shallow doping (B implanted, 0.2 keV, 1 1015 cm 2) were used in this study. Silicon wafers were pre-amorphized with 5 keV Ge before B implantation in order to reduce channeling effect. Furthermore, amorphous silicon (a-Si) or silicon nitride (SiN) deposition was performed for backside SIMS samples. The deposition temperature was 85 8C for a-Si and 450 8C for SiN. These capped samples were glued upside down onto a glass substrate with epoxy bond and carefully polished from the backside. In order to de®ne the location of the original surface in the depth pro®le of the capped sample, 10 B or 40 Ca was used as the marker to ®nd the location of the interface.
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Fig. 1. Primary energy dependence of B pro®le of backside SIMS.
tion. Boron pro®les of the SiN/Si interface region show a weak dependence on primary ion energy. Reduction in primary ion energy reduces the ion mixing effects and provides a higher B concentration peak. Furthermore, lower primary ion energy reduces the B mixing to the SiN ®lm. In Fig. 2, the B pro®les for higher primary ion energy levels show reduced concentration peaks and deeper implant tails comparing with the B pro®le for
3. Results and discussion Fig. 1 shows the primary ion energy dependence of backside SIMS pro®les and Fig. 2 shows that of surface-side. Although B pro®les of backside SIMS using SiN capped samples show almost no dependence on primary ion energy, those of surface-side SIMS become broader when primary ion energy is increased. This indicates that the knock-on effect on B pro®les of backside SIMS is small. Using higher primary ion energy in comparison with the ion implantation energy, backside SIMS is considered to be able to obtain a sharper pro®le for ultra-shallow implanta-
Fig. 2. Primary energy dependence of B pro®le of surface-side SIMS.
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C. Hongo et al. / Applied Surface Science 203±204 (2003) 264±267
Fig. 3. Primary energy dependence on total B dose of surface-side and backside SIMS.
350 eV. Increasing the primary ion beam energy, the knock-on effects are enhanced and the shape of the B pro®les becomes broader. Fig. 3 shows the total B dose measured by backside SIMS and surface-side SIMS. When the primary ion energy is increased, the total dose measured by surface-side SIMS decreases because of the decrease of ionization ef®ciency at the transient region [4]. Contrary to surface-side SIMS, the total dose measured by backside SIMS is almost constant. In other words, the ionization ef®ciency does not vary so much at the location of the original surface when backside SIMS is used. The total B dose values measured by backside SIMS were about
7 8 1014 cm 2. However, the total B dose value measured by nuclear radiation analysis (NRA) for backside SIMS sample was 9 1014 cm 2. This indicates that the B dose data obtained by backside SIMS is slightly smaller than the actual value. This is probably due to the charge-up effect, because the substrate was glass. Fig. 4 shows the comparison of the SIMS pro®les of surface-side and those of backside. In this experiment, an a-Si deposited sample was used for the backside SIMS sample in order to avoid B diffusion due to the high deposition temperature for SiN. The difference between backside SIMS and surface-side SIMS is considerably large at the original surface region. This difference cannot be explained only by the charge-up effect of backside SIMS. This phenomenon is probably due to the sputtering rate difference existing at the transient region. The backside pro®le shows a sharper ion implantation pro®le than the surface-side
Fig. 4. Boron pro®le of backside and surface-side SIMS measured with 350 eV O2 at 458 incidence with O2 ¯ooding.
pro®le at depths ranging from 5 to 8 nm, because the knock-on effect of backside SIMS is smaller than that of surface-side SIMS. Furthermore, a large difference is seen between them especially at the tail region (from 8 to 15 nm). This difference is not caused either by the crater edge effect or crater wall contribution [5,6], since the same result was obtained after mesa scanning by which the outside of the analyzing area ranging from 0.5 to 1 mm was sputtered in order to remove the high B concentration region. It may be the memory effect. The B concentration of this region is very important because this is related to the p±n junction depth. Since the B concentration for a p±n junction depends on the device design, the estimated depth for a p±n junction also differs. The concentration for a p±n junction becomes smaller, if the difference of junction depth for both methods becomes larger. 4. Conclusions Comparing the SIMS pro®les of surface-side and those of backside, backside SIMS pro®les show a sharper ion implantation tail than surface-side pro®les. In addition, the primary ion energy dependence becomes weaker when backside SIMS is used. Therefore backside SIMS suppresses the knock-on effect and provides accurate depth pro®les for ultra-shallow implantation. Furthermore, backside SIMS can measure lower B concentration since the memory effect does not
C. Hongo et al. / Applied Surface Science 203±204 (2003) 264±267
in¯uence on backside SIMS pro®le. Thus the measurement by using backside SIMS provides a sharp tail of boron pro®les, and enables an accurate estimation of p±n junction depth. In summary, we experimentally con®rmed that the backside SIMS analysis enables accurate depth pro®ling expected for analyzing ultra-shallow implants for next generation devices. Acknowledgements The authors would like to thank Dr. T. Hoshi and Dr. Z. Li of ULVAC-PHI Inc. for SIMS measurements.
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