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drain for 0.18-μm pMOSFETs
Surface and Coatings Technology 136 Ž2001. 157᎐161
Characteristics of BF3 plasma-doped gatersourcerdrain for 0.18-m pMOSFETs Jung-Min Haa,U , Jung-Woo Park a , Susan Felchb, Kazuyuki Fujiharaa , Ho-Kyu Kang a , Sang-In Lee a a
Semiconductor R & D Center, Samsung Electronics Co. Ltd., San 噛24, Nongseo-Ree, Kiheung-Eup, Yongin-Si, Kyonggi-Do, 449-900, South Korea b Varian Semiconductor Equipment, 35 Dory Road, Gloucester, MA 01930-2297 USA
Abstract A BF3 plasma doping ŽPLAD. process has been utilized in shallow sourcerdrain ŽSrD. extension and sourcerdrainrgate doping for high-performance 0.18-m pMOSFETs. Low-resistance shallow junctions were obtained using a BF3 PLAD system with a high-performance low-energy boron doping technology. The drive current and transconductance of pMOSFETs with plasma-doped SrD extensions are remarkably improved compared to those of BF2q ion-implanted devices, due to the low resistance of the SrD extension at the equivalent short-channel-effect characteristics. The fluorine ions in the BF3 plasma-doped silicon are significantly less than in the BF2q implanted one. In the case of surface channel pMOSFETs with a BF3 plasma-doped gate, the boron penetration and depletion in the gate poly were reduced because of the reduced fluorine incorporation into the gate poly compared to those of a conventional BF2 ion-implanted gate. The improved characteristics of BF3 plasma-doped gate enhance the drive current and gate oxide quality of pMOSFETs compared to conventional BF2q-implanted devices. No plasma damage was identified, and cobalt salicide formation is also very compatible with the plasma-doped pqrn junction. 䊚 2001 Published by Elsevier Science B.V. All rights reserved. Keywords: BF3 doping; Metal-oxide semiconductor field-effect transmitter ŽMOSFET.; Shallow junctions
1. Introduction Recently, plasma-doping technology has been developed to achieve ultra-shallow junctions. It is a promising candidate for realizing ultra-shallow junctions in deep sub-quarter- m devices, because of its low energy, high throughput, small footprint, low cost-ofownership, and room-temperature operation, independent of wafer size. In general, as the device scales down to the subquarter-m regime, it is important to form shallow sourcerdrain extensions with low resistance and to reduce the boron penetration and depletion in the gate poly for surface-channel pMOSFETs.
U
Corresponding author.
A plasma doping method has recently been reported to achieve shallow junctions with high activation efficiency and low process damage w1x. In addition, given the comparable amount of boron doped in silicon using BF2q ion implantation and the BF3 PLAD method, the amount of fluorine incorporated in silicon was significantly greater for the former method w2,3x. Since the presence of fluorine in gate-poly silicon degrades the gate oxide reliability w4x, and degrades the current drivability of surface-channel pMOSFETs, BF3 PLAD can be considered as a preferred method for device reliability and performance enhancement. In this paper, the advantages of BF3 plasma-doped sourcerdrain extension, gate poly, and deep sourcerdrain, are presented in terms of pMOSFET performance, gate oxide quality, and compatibility with cobalt salicide formation.
0257-8972r01r$ - see front matter 䊚 2001 Published by Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 1 0 4 7 - 1
158
J. Ha et al. r Surface and Coatings Technology 136 (2001) 157᎐161
2. Experimental 2.1. PLAD system Plasma doping ŽPLAD. was carried out using undiluted BF3 as the source gas. The silicon wafer to be doped is placed directly in a plasma chamber containing the desired dopant ions. The wafer is then pulse-biased to a negative potential Žy1 to y5kV. to accelerate positive ions into the silicon surface w3x. The other parts of a conventional implanter Žacceleration system, mass-analyzing magnets, ion optics, and scanning systems. are missing. In this system, various ions of BF3q, BF2q, BFq, and q B are created from the pure BF3 source gas in the plasma chamber, but it is hard to produce Fq ions, due to the large electronegativity of fluorine element.
Fig. 2. Threshold voltage roll-off characteristics of pMOSFET with BF3 PLAD and BF2q-implanted SrD extension.
3. Results and discussion 3.1. Characteristics of SrD extension
2.2. Fabrication of pMOSFETs The overall structure of pMOSFETs is shown in Fig. 1. The gate structure consists of 4-nm gate oxide Žnitrided oxide. and 290-nm poly-Si. The sourcerdrain ŽSrD. extensions for pMOSFETs were formed by BF3 PLAD with a 1.5-kV bias to obtain a shallow profile, followed by silicon-nitride sidewall spacer formation. The gate poly and deep SrD were also doped by PLAD with an energy of 5 kV and dose of 5 = 10 15 rcm2 . The reference pMOSFETs were fabricated with conventional BF2q ion implantation Ž10 keV in SrD extension, 25 keV in deep SrD.. Rapid thermal annealing ŽRTA. was performed at 1000⬚C, and cobalt salicide was formed in the gate poly and deep SrD region to reduce the sheet resistance. After the interdielectric layer was deposited, the metal contact was filled with tungsten. Finally, Al metallization was carried out.
Fig. 1. Cross-sectional view of pMOSFET with BF3 plasma-doped GrSrD and extension.
As shown by the threshold voltage roll-off in Fig. 2, the extent of the short-channel-effect of pMOSFETs with a BF3 plasma-doped sourcerdrain extension at 1.5 kV and 1 = 10 15 rcm2 was similar to the BF2q ion-implanted one at 10 keV and 1 = 10 14 rcm2 ; this can be understood in view of the same extension junction depth for both cases ŽFig. 3.. The drain current and transconductance of plasma doped devices, however, were remarkably larger than those of 10-keV BF2q-implanted ones ŽFigs. 4 and 5. for the same short-channel-effect extent, because of the relatively lower sheet resistance of the SrD extension, with a high and abrupt doping profile for the plasma-doped case. 3.2. Characteristics of gate doping Fig. 6a,b shows boron and fluorine secondary ion mass spectroscopy ŽSIMS. profiles for BF3 PLAD and BF2q implantation after rapid thermal annealing ŽRTA.
Fig. 3. Boron SIMS profiles of PLAD and BF2q implantation for SrD extension after RTA at 1000⬚C for 30 s.
J. Ha et al. r Surface and Coatings Technology 136 (2001) 157᎐161
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Fig. 5. Transconductance ŽGm. of pMOSFET with SrD extension formed by BF3 PLAD and BF2q implantation. Fig. 4. Id ᎐Vd characteristics of pMOSFET with extension formed by BF3 PLAD and BF2q implantation.
at 1000⬚C for 30 s. The BF3 PLAD method incorporated significantly less fluorine relative to the BF2q implanted one, given a similar boron-doping profile. This can be understood in view of the fact that it is hard to produce Fq ions, due to the large electronegativity of fluorine element. Moreover, slightly doped fluorine tends to out-diffuse through the silicon surface during subsequent heat treatment. This suggests that the device degradation induced by fluorine will be less
severe in the BF3 PLAD case. The fluorine profile for BF2q implantation shows double peaks; the first one is located at the boron peak, and the second at the amorphousrcrystal transition region. These fluorine double peaks are not shown in the BF3 PLAD case. The possible effects of the presence of fluorine in the gate poly were investigated in terms of the C᎐V characteristics for different gate doping methods. In Fig. 7a the capacitance᎐voltage Ž C᎐V . curve shows that the accumulation mode capacitance of the BF3 plasma-doped pMOS is higher than that of the BF2q ion-implanted one. This result indicates that the gate
Fig. 6. Boron and fluorine SIMS profiles for PLAD and BF2q implantation after RTA at 1000⬚C for 30 s.
Fig. 7. Low frequency C᎐V curve. Accumulation and inversion mode capacitance of a BF3 plasma-doped pMOS is higher than that of a BF2q-implanted one.
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J. Ha et al. r Surface and Coatings Technology 136 (2001) 157᎐161
Fig. 8. Electrical thickness of a gate oxide.
oxide of the BF3 plasma-doped pMOS is thinner than that of the BF2q-implanted one, as shown in Fig. 8. The increased oxide thickness is attributed to the fluorine incorporation into the gate oxide by BF2q implantation. In Fig. 7b the C᎐V curve also shows that the inversion mode capacitance of the BF3 plasma-doped pMOS is higher than that of the BF2q ion-implanted one, indicating the depletion of boron in BF2q-implanted pMOS gate poly. Fig. 9 shows the threshold voltage, current-on Ž Idsat . and current-off relationship for pMOSFET with a plasma-doped or implanted gate poly. BF2q-implanted pMOSFETs exhibit approximately 7% lower drain current than Bq-implanted or BF3 plasma-doped ones at the same threshold voltage and current-off, due to boron depletion in the gate poly and the increased oxide thickness. 3.3. Characteristics of gate oxide and p qr n junction Fig. 10 shows the charge-to-breakdown Ž Q bd . characteristics of the gate oxide of the BF3 plasma-doped pMOS compared to those of the BF2q-, Bq-, or Fqq BF2q-implanted one. BF3 plasma-doped samples showed Q bd characteristics similar to the Bq-implanted samples; BF2q implantation deteriorates the gate oxide
Fig. 9. Vth and Idsat and current-off relationship for pMOSFET with plasma-doped or implanted gate poly.
Fig. 10. Q bd for BF3 PLAD or BF2q-implanted pMOS. BF3 PLAD samples show Q bd characteristics similar to boron-implanted samples. BF2q implantation deteriorates the gate oxide reliability due to fluorine incorporation.
reliability due to fluorine incorporation, and additional fluorine implantation deteriorates the gate oxide quality severely. Fig. 11 shows that the leakage characteristics of the gate oxide in the plasma-doped sample were identical to the implanted samples. This gate oxide quality indicates that degradation by plasma damage was not induced for the PLAD process. Fig. 12 shows a transmission electron microscopy ŽTEM. view of PLAD samples followed by cobalt salicidation. No conspicuous defect was observed by TEM. The pqrn junction leakage characteristics ŽFig. 13. and the sheet resistance ŽFig. 14. of these samples were also not degraded. These results indicate that the plasma-doped pqrn junction is very compatible with the metal salicidation process.
4. Conclusion BF3 plasma doping was applied to a shallow SrD extension and gatersourcerdrain doping for high-performance 0.18-m pMOSFETs. The drive current of
Fig. 11. I᎐V characteristics of gate oxide for PLAD and implanted pMOS.
J. Ha et al. r Surface and Coatings Technology 136 (2001) 157᎐161
q
Fig. 12. TEM of BF3 PLAD p rn junction followed by cobalt salicidation.
161
Fig. 14. Sheet resistance of BF3 PLAD and BF2q-implanted gatersourcerdrain followed by Co-salicidation.
fluorine incorporation into the gate poly for the BF3 PLAD method reduces boron penetration and depletion in the gate poly. Plasma damage or fluorine-induced gate oxide reliability degradation was not found in the plasma-doped pMOS, and cobalt salicide formation was very compatible. References
Fig. 13. pqrn junction leakage characteristics of BF3 PLAD and BF2q-implanted samples followed by cobalt salicidation.
pMOSFETs with a plasma-doped SrD extension was improved due to the lower sheet resistance and abrupt boron depth-profile of the extension compared to that of conventional BF2q-implanted devices. The reduced
w1x M. Takase, K. Yamashita, A. Hori, B. Mizuno, Shallow sourcerdrain extensions for pMOSFETs with high activation and low process damage fabricated by plasma doping, IEDM Technical Digest, Ž1997. 475. w2x A. Sultan, S. Banerjee, Role of silicon surface in the removal of point defects in ultrashallow junctions, Appl. Phys. Lett. 69 Ž15. Ž1996. 2228. w3x T. Sheng, S. Felch, C.B. CooperIII, Characteristics of a plasma doping system for semiconductor device fabrication, J. Vac. Sci. Tech. B 12 Ž2. Ž1994. 969. w4x P.J. Wright, K. Saraswat, The effect of fluorine in silicon dioxide gate dielectrics, IEEE Trans. Electron Devices 36 Ž5. Ž1989. 879.
Characteristics of BF3 plasma-doped gatersourcerdrain for 0.18-m pMOSFETs Jung-Min Haa,U , Jung-Woo Park a , Susan Felchb, Kazuyuki Fujiharaa , Ho-Kyu Kang a , Sang-In Lee a a
Semiconductor R & D Center, Samsung Electronics Co. Ltd., San 噛24, Nongseo-Ree, Kiheung-Eup, Yongin-Si, Kyonggi-Do, 449-900, South Korea b Varian Semiconductor Equipment, 35 Dory Road, Gloucester, MA 01930-2297 USA
Abstract A BF3 plasma doping ŽPLAD. process has been utilized in shallow sourcerdrain ŽSrD. extension and sourcerdrainrgate doping for high-performance 0.18-m pMOSFETs. Low-resistance shallow junctions were obtained using a BF3 PLAD system with a high-performance low-energy boron doping technology. The drive current and transconductance of pMOSFETs with plasma-doped SrD extensions are remarkably improved compared to those of BF2q ion-implanted devices, due to the low resistance of the SrD extension at the equivalent short-channel-effect characteristics. The fluorine ions in the BF3 plasma-doped silicon are significantly less than in the BF2q implanted one. In the case of surface channel pMOSFETs with a BF3 plasma-doped gate, the boron penetration and depletion in the gate poly were reduced because of the reduced fluorine incorporation into the gate poly compared to those of a conventional BF2 ion-implanted gate. The improved characteristics of BF3 plasma-doped gate enhance the drive current and gate oxide quality of pMOSFETs compared to conventional BF2q-implanted devices. No plasma damage was identified, and cobalt salicide formation is also very compatible with the plasma-doped pqrn junction. 䊚 2001 Published by Elsevier Science B.V. All rights reserved. Keywords: BF3 doping; Metal-oxide semiconductor field-effect transmitter ŽMOSFET.; Shallow junctions
1. Introduction Recently, plasma-doping technology has been developed to achieve ultra-shallow junctions. It is a promising candidate for realizing ultra-shallow junctions in deep sub-quarter- m devices, because of its low energy, high throughput, small footprint, low cost-ofownership, and room-temperature operation, independent of wafer size. In general, as the device scales down to the subquarter-m regime, it is important to form shallow sourcerdrain extensions with low resistance and to reduce the boron penetration and depletion in the gate poly for surface-channel pMOSFETs.
U
Corresponding author.
A plasma doping method has recently been reported to achieve shallow junctions with high activation efficiency and low process damage w1x. In addition, given the comparable amount of boron doped in silicon using BF2q ion implantation and the BF3 PLAD method, the amount of fluorine incorporated in silicon was significantly greater for the former method w2,3x. Since the presence of fluorine in gate-poly silicon degrades the gate oxide reliability w4x, and degrades the current drivability of surface-channel pMOSFETs, BF3 PLAD can be considered as a preferred method for device reliability and performance enhancement. In this paper, the advantages of BF3 plasma-doped sourcerdrain extension, gate poly, and deep sourcerdrain, are presented in terms of pMOSFET performance, gate oxide quality, and compatibility with cobalt salicide formation.
0257-8972r01r$ - see front matter 䊚 2001 Published by Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 1 0 4 7 - 1
158
J. Ha et al. r Surface and Coatings Technology 136 (2001) 157᎐161
2. Experimental 2.1. PLAD system Plasma doping ŽPLAD. was carried out using undiluted BF3 as the source gas. The silicon wafer to be doped is placed directly in a plasma chamber containing the desired dopant ions. The wafer is then pulse-biased to a negative potential Žy1 to y5kV. to accelerate positive ions into the silicon surface w3x. The other parts of a conventional implanter Žacceleration system, mass-analyzing magnets, ion optics, and scanning systems. are missing. In this system, various ions of BF3q, BF2q, BFq, and q B are created from the pure BF3 source gas in the plasma chamber, but it is hard to produce Fq ions, due to the large electronegativity of fluorine element.
Fig. 2. Threshold voltage roll-off characteristics of pMOSFET with BF3 PLAD and BF2q-implanted SrD extension.
3. Results and discussion 3.1. Characteristics of SrD extension
2.2. Fabrication of pMOSFETs The overall structure of pMOSFETs is shown in Fig. 1. The gate structure consists of 4-nm gate oxide Žnitrided oxide. and 290-nm poly-Si. The sourcerdrain ŽSrD. extensions for pMOSFETs were formed by BF3 PLAD with a 1.5-kV bias to obtain a shallow profile, followed by silicon-nitride sidewall spacer formation. The gate poly and deep SrD were also doped by PLAD with an energy of 5 kV and dose of 5 = 10 15 rcm2 . The reference pMOSFETs were fabricated with conventional BF2q ion implantation Ž10 keV in SrD extension, 25 keV in deep SrD.. Rapid thermal annealing ŽRTA. was performed at 1000⬚C, and cobalt salicide was formed in the gate poly and deep SrD region to reduce the sheet resistance. After the interdielectric layer was deposited, the metal contact was filled with tungsten. Finally, Al metallization was carried out.
Fig. 1. Cross-sectional view of pMOSFET with BF3 plasma-doped GrSrD and extension.
As shown by the threshold voltage roll-off in Fig. 2, the extent of the short-channel-effect of pMOSFETs with a BF3 plasma-doped sourcerdrain extension at 1.5 kV and 1 = 10 15 rcm2 was similar to the BF2q ion-implanted one at 10 keV and 1 = 10 14 rcm2 ; this can be understood in view of the same extension junction depth for both cases ŽFig. 3.. The drain current and transconductance of plasma doped devices, however, were remarkably larger than those of 10-keV BF2q-implanted ones ŽFigs. 4 and 5. for the same short-channel-effect extent, because of the relatively lower sheet resistance of the SrD extension, with a high and abrupt doping profile for the plasma-doped case. 3.2. Characteristics of gate doping Fig. 6a,b shows boron and fluorine secondary ion mass spectroscopy ŽSIMS. profiles for BF3 PLAD and BF2q implantation after rapid thermal annealing ŽRTA.
Fig. 3. Boron SIMS profiles of PLAD and BF2q implantation for SrD extension after RTA at 1000⬚C for 30 s.
J. Ha et al. r Surface and Coatings Technology 136 (2001) 157᎐161
159
Fig. 5. Transconductance ŽGm. of pMOSFET with SrD extension formed by BF3 PLAD and BF2q implantation. Fig. 4. Id ᎐Vd characteristics of pMOSFET with extension formed by BF3 PLAD and BF2q implantation.
at 1000⬚C for 30 s. The BF3 PLAD method incorporated significantly less fluorine relative to the BF2q implanted one, given a similar boron-doping profile. This can be understood in view of the fact that it is hard to produce Fq ions, due to the large electronegativity of fluorine element. Moreover, slightly doped fluorine tends to out-diffuse through the silicon surface during subsequent heat treatment. This suggests that the device degradation induced by fluorine will be less
severe in the BF3 PLAD case. The fluorine profile for BF2q implantation shows double peaks; the first one is located at the boron peak, and the second at the amorphousrcrystal transition region. These fluorine double peaks are not shown in the BF3 PLAD case. The possible effects of the presence of fluorine in the gate poly were investigated in terms of the C᎐V characteristics for different gate doping methods. In Fig. 7a the capacitance᎐voltage Ž C᎐V . curve shows that the accumulation mode capacitance of the BF3 plasma-doped pMOS is higher than that of the BF2q ion-implanted one. This result indicates that the gate
Fig. 6. Boron and fluorine SIMS profiles for PLAD and BF2q implantation after RTA at 1000⬚C for 30 s.
Fig. 7. Low frequency C᎐V curve. Accumulation and inversion mode capacitance of a BF3 plasma-doped pMOS is higher than that of a BF2q-implanted one.
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J. Ha et al. r Surface and Coatings Technology 136 (2001) 157᎐161
Fig. 8. Electrical thickness of a gate oxide.
oxide of the BF3 plasma-doped pMOS is thinner than that of the BF2q-implanted one, as shown in Fig. 8. The increased oxide thickness is attributed to the fluorine incorporation into the gate oxide by BF2q implantation. In Fig. 7b the C᎐V curve also shows that the inversion mode capacitance of the BF3 plasma-doped pMOS is higher than that of the BF2q ion-implanted one, indicating the depletion of boron in BF2q-implanted pMOS gate poly. Fig. 9 shows the threshold voltage, current-on Ž Idsat . and current-off relationship for pMOSFET with a plasma-doped or implanted gate poly. BF2q-implanted pMOSFETs exhibit approximately 7% lower drain current than Bq-implanted or BF3 plasma-doped ones at the same threshold voltage and current-off, due to boron depletion in the gate poly and the increased oxide thickness. 3.3. Characteristics of gate oxide and p qr n junction Fig. 10 shows the charge-to-breakdown Ž Q bd . characteristics of the gate oxide of the BF3 plasma-doped pMOS compared to those of the BF2q-, Bq-, or Fqq BF2q-implanted one. BF3 plasma-doped samples showed Q bd characteristics similar to the Bq-implanted samples; BF2q implantation deteriorates the gate oxide
Fig. 9. Vth and Idsat and current-off relationship for pMOSFET with plasma-doped or implanted gate poly.
Fig. 10. Q bd for BF3 PLAD or BF2q-implanted pMOS. BF3 PLAD samples show Q bd characteristics similar to boron-implanted samples. BF2q implantation deteriorates the gate oxide reliability due to fluorine incorporation.
reliability due to fluorine incorporation, and additional fluorine implantation deteriorates the gate oxide quality severely. Fig. 11 shows that the leakage characteristics of the gate oxide in the plasma-doped sample were identical to the implanted samples. This gate oxide quality indicates that degradation by plasma damage was not induced for the PLAD process. Fig. 12 shows a transmission electron microscopy ŽTEM. view of PLAD samples followed by cobalt salicidation. No conspicuous defect was observed by TEM. The pqrn junction leakage characteristics ŽFig. 13. and the sheet resistance ŽFig. 14. of these samples were also not degraded. These results indicate that the plasma-doped pqrn junction is very compatible with the metal salicidation process.
4. Conclusion BF3 plasma doping was applied to a shallow SrD extension and gatersourcerdrain doping for high-performance 0.18-m pMOSFETs. The drive current of
Fig. 11. I᎐V characteristics of gate oxide for PLAD and implanted pMOS.
J. Ha et al. r Surface and Coatings Technology 136 (2001) 157᎐161
q
Fig. 12. TEM of BF3 PLAD p rn junction followed by cobalt salicidation.
161
Fig. 14. Sheet resistance of BF3 PLAD and BF2q-implanted gatersourcerdrain followed by Co-salicidation.
fluorine incorporation into the gate poly for the BF3 PLAD method reduces boron penetration and depletion in the gate poly. Plasma damage or fluorine-induced gate oxide reliability degradation was not found in the plasma-doped pMOS, and cobalt salicide formation was very compatible. References
Fig. 13. pqrn junction leakage characteristics of BF3 PLAD and BF2q-implanted samples followed by cobalt salicidation.
pMOSFETs with a plasma-doped SrD extension was improved due to the lower sheet resistance and abrupt boron depth-profile of the extension compared to that of conventional BF2q-implanted devices. The reduced
w1x M. Takase, K. Yamashita, A. Hori, B. Mizuno, Shallow sourcerdrain extensions for pMOSFETs with high activation and low process damage fabricated by plasma doping, IEDM Technical Digest, Ž1997. 475. w2x A. Sultan, S. Banerjee, Role of silicon surface in the removal of point defects in ultrashallow junctions, Appl. Phys. Lett. 69 Ž15. Ž1996. 2228. w3x T. Sheng, S. Felch, C.B. CooperIII, Characteristics of a plasma doping system for semiconductor device fabrication, J. Vac. Sci. Tech. B 12 Ž2. Ž1994. 969. w4x P.J. Wright, K. Saraswat, The effect of fluorine in silicon dioxide gate dielectrics, IEEE Trans. Electron Devices 36 Ž5. Ž1989. 879.