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Impurity incorporation during beam assisted processing analyzed using nuclear microprobe
Nuclear Instruments and Methods in Physics Research B 158 (1999) 487±492
www.elsevier.nl/locate/nimb
Impurity incorporation during beam assisted processing analyzed using nuclear microprobe Y.K. Park a
a,*
, M. Takai a, C. Lehrer b, L. Frey b, H. Ryssel
b
Research Center for Materials Science at Extreme Conditions, Osaka University, Toyonaka, Osaka 560-8531, Japan b Fraunhofer Institut fuer Integrierte Schaltungen, Schottkystrasse 10, 91058 Erlangen, Germany
Abstract Impurity incorporation due to localized beam processing by focused ion beams (FIBs) and electron beams (EBs) such as physical sputtering, gas-assisted etching (GAE) using iodine gas and beam-assisted deposition using (CH3 )3 CH3 C5 H9 Pt precursor gas has been investigated by a medium energy nuclear microprobe. RBS mapping on incorporated impurities has been performed using 300 keV Be2 beams with a beam spot size of 80 nm. RBS mapping images for physically sputtered and GAE areas indicate the localized Ga and I atoms distributions from the Ga FIB and from the etchant gas, respectively. The amount of Ga in the GAE area was estimated to be 2.7 times smaller than that in the physically sputtered area. RBS mapping images for FIB assisted Pt deposited areas revealed that Pt and Ga atoms were distributed in the processed areas, while C atoms resided beyond the processed areas. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.85; 81.15.Jj Keywords: Nuclear microprobe; Impurity incorporation; Focused ion beam (FIB); Electron beam (EB); Micro-RBS; RBS mapping image
1. Introduction Nuclear microprobe techniques using Rutherford backscattering (RBS) have been applied to semiconductor process development, in which minimum feature sizes of several micrometers down to submicrometer and multilayered structures were used [1±3]. Micro-RBS analysis can provide information on the atomic composition
* Corresponding author. Tel.: +81-6-850-6304; fax: +81-6850-6341; e-mail: [email protected]
and distribution of both the material and impurity atoms beneath sample surfaces. Materials modi®cation such as etching and deposition using FIB or EB has been used for repairs of photo- [4] or X-ray masks [5,6] and integrated circuits [7±9]. Maskless physical sputtering or GAE for material removal, and beam assisted deposition for material deposition have been used in such applications. However, the impurity incorporation from etchant gas species, precursor gas or ion beam itself during beam-assisted processing has not been clari®ed yet, though nuclear microprobe is a powerful tool to facilitate the
0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 7 1 - 7
488
Y.K. Park et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 487±492
localized impurity analysis of beam process-induced structures [10,11]. In this study, the impurity incorporation due to the localized beam processing such as physical sputtering, GAE using iodine gas by FIBs, and beam assisted deposition using (CH3 )3 CH3 C5 H9 Pt precursor gas by FIBs or EBs has been investigated by a medium energy nuclear microprobe. RBS mapping images on incorporated impurities during beam processing have been obtained using 300 keV Be2 beams with a beam spot size of 80 nm. 2. Experimental procedures 30 keV Ga ion or 10 keV electron beams were raster-scanned by 10 ´ 10 lm2 for etching and deposition with a spot size of 100 nm full width at half maximum (FWHM) in o-axis (1 0 0) Si. The ion or electron dose was varied from 1 ´ 1015 to 1.2 ´ 1019 ions/cm2 according to the irradiation time. The beam current for etching and deposition was set to 1000 and 200 pA for FIB processing and 100 pA for EB processing. For physical sputtering, the beam dwell time and the overlap were set to 1.0 ls and 50%, respectively. In the case of GAE or deposition the beam dwell time and the overlap were set to 0.4 ls and 0%, respectively. For GAE, iodine was used as an etchant and heated to 29°C resulting in a gas ¯ux of 1015 molecules/cm2 s. For beam assisted deposition, metalorganic gas molecules, i.e. (CH3 )3 CH3 C5 H9 Pt were used as a precursor and heated up to 40°C resulting in a gas ¯ux of 1018 molecules/cm2 s. The gas was fed to the sample surface through an injector needle positioned to within 300 lm of the sample. The detailed experimental conditions are published elsewhere [12]. The samples were measured using a 300 keV Be2 nuclear microprobe with a beam spot size of 80 nm [13,14]. The samples were mounted on a sample holder with a six-axis goniometer in a UHV chamber with a vacuum higher than 5 ´ 10ÿ9 Torr during measurement. The beam current was 30 or 40 pA during measurement. Ions backscattered from the sample into a scattering angle of 97° were measured by a surface barrier detector.
The data acquisition time and dose for the RBS mapping were 80 min and 2.1 ´ 1017 Be2 /cm2 , respectively. Beam induced crystal damage at a dose of 2.1 ´ 1017 Be2 /cm2 was not observed by micro-channeling measurements as reported elsewhere [3]. 3. Results and discussion Fig. 1 shows the scanning electron microscope (SEM) view of beam processed area by physical sputtering (a), iodine gas assisted etching (b) with a dose of 4 ´ 1017 Ga /cm2 , FIB assisted Pt deposition with a dose of 2.38 ´ 1016 Ga /cm2 (c) and EB assisted Pt deposition with a dose of 2.4 ´ 1018 / cm2 (d), respectively. In the case of physical sputtering the rounded edge of milled areas was considered to be due to the pro®le of the ion beam that has a Gaussian distribution with a long tail [15]. Contrast change outside the etched area was observed in Fig. 1(b). This was due to the impurity incorporation of I and Ga, as observed in Fig. 3. The well-de®ned rectangular parts correspond to the deposited Pt layers by FIB and EB in Fig. 1(c) and (d).
Fig. 1. SEM view of beam processed area by physical sputtering (a), iodine gas assisted etching with a dose of 4 ´ 1017 Ga /cm2 (b), FIB assisted Pt deposition with a dose of 2.38 ´ 1016 Ga /cm2 (c) and EB assisted Pt deposition with a dose of 2.4 ´ 1018 /cm2 (d), respectively.
Y.K. Park et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 487±492
Fig. 2 shows the micro-RBS spectra for physically sputtered area (a) and gas assisted etched area (b) with a dose of 8.0 ´ 1016 Ga /cm2 . The leading edges for I, Ga and Si are indicated with arrows. The micro-RBS spectra were extracted from a micro-RBS data set with a size of 5 ´ 5 lm2 . To extract quantitative information about layer structures the micro-RBS spectra were compared with those by simulation using a RBX code [16] as indicated with solid lines. In the case of physical sputtering a large Ga peak appeared in the channels (61±90), whereas another signal in the channels (95±110) in addition to the Ga signal was observed in GAE. This new signal started at the I leading edge, i.e., iodine is present in the GAE process. The calculated residual Ga and I concentrations were estimated to be 2.17 ´ 1016 Ga / cm2 for physical sputtering, and 8.1 ´ 1015 Ga / cm2 and 4.1 ´ 1014 I /cm2 for gas assisted etching in case of 8.0 ´ 1016 Ga /cm2 irradiation, respectively. Thus the amount of Ga in the GAE area was estimated to be 2.7 times smaller than that of physically sputtered area. It should be noted that the GAE with an even higher material removal rate was more eective to reduce the incorporation of Ga than physical etching. Fig. 3 shows the secondary electron and RBS mapping images with an energy window of Ga atoms (61±90) for physically sputtered area (a), and with an energy window of Ga atoms and I atoms (95±110) for gas assisted etched area (b) with doses of 1.0 ´ 1017 Ga /cm2 , respectively. The area etched by physical sputtering in Fig. 3(a) was assumed to be amorphous and have a uniform
Fig. 2. Micro-RBS spectra for physically sputtered area (a) and gas assisted etched area (b) with a dose of 8.0 ´ 1016 Ga /cm2 . The leading edges for I, Ga and Si are indicated with arrows.
489
distribution of Ga atoms. The Ga atoms detected were considered to be implanted atoms at the bottom of the etched area or redeposited atoms from nearby process areas. An uniform distribution of Ga atoms was observed after GAE with iodine gas. The GAE processed areas showed iodine distribution at and nearby the etched area due to adsorbed iodine etchant gas. Fig. 4 shows the etched depth (a) and residual Ga concentration (b) as a function of ion dose from the simulated data using the TRIDYN code and the measured data with and without iodine gas. The physical sputtering of Si layers started above 1016 ions/cm2 . The simulated etched depth of Si using Ga ions drastically increased from 18 A up to 0.73 lm. The simulated depth was in good agreement with the measured depth for physical sputtering. GAE with iodine gas results in a higher material removal rate by a factor of 10. Thus, the amount of residual Ga due to implantation would be reduced almost completely by GAE if the redeposition of Ga atoms is disregarded. The simulated data using TRIDYN code (solid line in Fig. 4(b)) indicated that the residual Ga concentration due to the implantation process increased with ion dose at and below 1 ´ 1017 Ga / cm2 and that the residual Ga concentration above 1 ´ 1017 Ga /cm2 gradually saturated due to the sputtering. The residual Ga concentration indicates that the sputtering yield is much higher than that of simulated data obtained by the TRIDYN code in Fig. 4(a). The data from GAE with iodine gas indicated that the Ga concentration did not increase with increase in ion dose from 6 ´ 1016 to 1 ´ 1018 Ga /cm2 . Thus the GAE can reduce the Ga incorporation at these doses. Fig. 5 shows the micro-RBS spectra for FIB assisted deposited areas with a dose of 1.13 ´ 1016 Ga /cm2 (a), and for EB assisted deposited areas with a dose of 2.4 ´ 1018 /cm2 (b), respectively. The micro-RBS spectra were extracted from a microRBS data set with a size of 5 ´ 5 lm2 . In case of FIB assisted deposition, the micro-RBS spectra indicate the Pt signal of the deposited Pt and Ga signal (shoulder of the Pt-peak) of the residual Ga atoms from the FIB itself, the implantation and the redeposition. In case of EB assisted deposition the micro-RBS spectrum shows the Pt signal of the
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Fig. 3. Secondary electron and RBS mapping images with an energy window of Ga atoms (61±90) for physically sputtered area (a), and with an energy window of Ga atoms and I atoms (95±110) for gas assisted etched area (b) with doses of 1.0 ´ 1017 Ga /cm2 , respectively. The left-hand side indicates a secondary electron image and the right-hand side indicates an RBS mapping image.
deposited Pt atoms with an energy window from 95 to 125. Fig. 6 shows the secondary electron and RBS mapping images with an energy window of Pt atoms (95±150), Ga atoms (61±90) and low-Z elements (15±40) such as C for Pt deposited areas by FIB with a dose of 2.38 ´ 1016 Ga /cm2 (a), and the secondary electron and RBS mapping images with an energy window of Pt atoms (95±150) for Pt deposited areas by EB with a dose of 2.4 ´ 1018 /cm2 (b), respectively. The secondary electron image for FIB processed area in Fig. 6(a) shows the contrast change beyond the deposited layers probably due
to the incorporation of carbon as shown in RBS mapping image (15±40 ch). The beam-processed areas by FIB or EB showed a Pt distribution at and nearby the processed area. The excess distribution of Pt and Ga atoms at the left-hand and right-hand sides nearby the FIB processed area within 3 lm in Fig. 6(a) was considered due to the pro®le of the ion beam that has a Gaussian distribution with a long tail [15]. The deposition rate depends on the beam current density [17], so that the boundary region has lower deposition rate due to the lower current density pro®le of Ga FIBs.
Fig. 4. Etched depth (a) and residual Ga concentration (b) as a function of ion dose from the simulated data using the TRIDYN code and the measured data with and without iodine gas.
Fig. 5. Micro-RBS spectra for FIB assisted deposited areas with a dose of 1.13 ´ 1016 Ga /cm2 (a), and for EB assisted deposited areas with a dose of 2.4 ´ 1018 /cm2 (b), respectively. The leading edges for Pt, Ga and Si are indicated with arrows.
Y.K. Park et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 487±492
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Fig. 6. Secondary electron and RBS mapping images with an energy window of Pt atoms (channels: 95±150), Ga atoms (61±90) and low-Z elements (15±40) such as C for Pt deposited areas by FIB with a dose of 2.38 ´ 1016 Ga /cm2 (a), and the secondary electron and RBS mapping images with an energy window of Pt atoms (95±150) for Pt deposited areas by EB with a dose of 2.4 ´ 1018 /cm2 (b), respectively. The left-hand side indicates a secondary electron image and the right-hand side indicates an RBS mapping image.
The lateral distribution, obtained by an energy window of 15-40 ch in Fig. 6(a), indicates the lateral low-Z element distribution, since the RBS yield of bulk Si signals from the deposited layers increases due to the lowering of the incident probe beam energy. A separate Auger electron spectroscopy (AES) measurement indicates that the composition of the deposited layers is Pt:Ga:C 3:2:5. Therefore this low-Z element would be carbon. Carbon may come from the insucient decomposition of source metalorganic gas. The size of lateral distribution for carbon in Fig. 6(a) broadens up to 10 lm beyond processed area. The sputtering at the side-wall of the deposited layer would be enhanced by the glancing angle incidence of the Ga beam for prolonged FIB irradiation. The low Z elements such as C and O would easily be sputtered away from the side-wall of the deposited layer due to the enhanced sputtering rate and redeposited around the initial area by about 10 lm.
4. Conclusions The incorporated impurities after localized beam processing, such as physical sputtering, GAE using iodine gas by FIBs and beam assisted deposition using (CH3 )3 CH3 C5 H9 Pt precursor gas by FIBs and EBs, have been investigated using RBS mapping by a 300 keV Be2 microprobe with a beam spot size of 80 nm. It was revealed that the residual Ga atoms ranging from 1 ´ 1016 to 5 ´ 1016 Ga /cm2 distributed at and nearby the bottom of the etched area due to implantation and redeposition from the Ga FIB. GAE with iodine gas can reduce the incorporation of Ga atoms, although additional iodine incorporation at a level of 1014 /cm2 occurs. The quantitative analysis on the residual impurity by micro-RBS spectra indicated that the amount of Ga atoms in the GAE processed areas was estimated to be 2.7 times smaller than that of physical etching.
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The beam-deposited areas by FIB or EB deposition showed a Pt distribution at and nearby the processed area, whereas the incorporated impurity of low-Z element such as C distributed at and around the processed areas within 10 lm. The amount of Pt atoms after EB induced deposition was much less than that by the FIB induced deposition and Pt was con®ned almost within the processed area presumably because of the nonsputtering feature during electron irradiation and the narrower electron beam pro®le. Acknowledgements Y.K. Park would like to express his thanks to the RCMSEC for ®nancial support. Authors thank to Mr. T. Nagai and T. Tsurumoto for helping the micro-RBS measurements. References [1] M. Takai, Scanning Microsc. 6 (1991) 147. [2] M. Takai, Nucl. Instr. and Meth. B 85 (1994) 664.
[3] M. Takai, T. Kishimoto, R. Mimura, H. Sawaragi, R. Aihara, Nucl. Instr. and Meth. B 118 (1996) 418. [4] J.R.A. Cleaver, H. Ahmed, P. Heard, P. Prewitt, G. Dunn, H. Kaufman, Microelectron. Eng. 3 (1985) 253. [5] A. Wagner, J.P. Levin, J.L. Mauer, P.G. Blauner, S.J. Kirch, P. Longo, J. Vac. Sci. Technol. B 8 (1990) 1557. [6] D.K. Stewart, J. Morgan, B. Ward, J. Vac. Sci. Technol. B 9 (1991) 2670. [7] T. Ishitani, Y. Kawanami, H. Todokora, Jpn. J. Appl. Phys. 24 (1985) L114. [8] T. Tao, W. Wilkinson, J. Melngailis, J. Vac. Sci. Technol. B 9 (1991) 162. [9] M. Takai, T. Kishimoto, M. Yamashita, H. Morimoto, S. Yura, A. Hosono, S. Okuda, S. Lipp, L. Frey, H. Ryssel, J. Vac. Sci. Technol. B 14 (3) (1996) 1973. [10] M. Takai, Nucl. Instr. and Meth. B 113 (1996) 330. [11] D.N. Jamieson, Nucl. Instr. and Meth. B 136±138 (1998) 1. [12] S. Lipp, L. Frey, C. Lehrer, B. Frank, E. Demm, H. Ryssel, J. Vac. Sci. Technol. B 14 (6) (1996) 3996. [13] M. Takai, R. Mimura, H. Sawaragi, R. Aihara, Scanning Microsc. 7 (1993) 815. [14] Y.K. Park, T. Kishimoto, M. Takai, Nucl. Instr. and Meth. B 130 (1997) 534. [15] K.D. Cummings, L.R. Harriott, G.C. Chi, F.W. Ostermayer Jr., Proc. SPIE 632 (1986) 93. [16] E. K otai, Nucl. Instr. and Meth. B 85 (1994) 588. [17] H.C. Petzold, P.J. Heard, J. Vac. Sci. Technol. B 9 (1991) 1826.
www.elsevier.nl/locate/nimb
Impurity incorporation during beam assisted processing analyzed using nuclear microprobe Y.K. Park a
a,*
, M. Takai a, C. Lehrer b, L. Frey b, H. Ryssel
b
Research Center for Materials Science at Extreme Conditions, Osaka University, Toyonaka, Osaka 560-8531, Japan b Fraunhofer Institut fuer Integrierte Schaltungen, Schottkystrasse 10, 91058 Erlangen, Germany
Abstract Impurity incorporation due to localized beam processing by focused ion beams (FIBs) and electron beams (EBs) such as physical sputtering, gas-assisted etching (GAE) using iodine gas and beam-assisted deposition using (CH3 )3 CH3 C5 H9 Pt precursor gas has been investigated by a medium energy nuclear microprobe. RBS mapping on incorporated impurities has been performed using 300 keV Be2 beams with a beam spot size of 80 nm. RBS mapping images for physically sputtered and GAE areas indicate the localized Ga and I atoms distributions from the Ga FIB and from the etchant gas, respectively. The amount of Ga in the GAE area was estimated to be 2.7 times smaller than that in the physically sputtered area. RBS mapping images for FIB assisted Pt deposited areas revealed that Pt and Ga atoms were distributed in the processed areas, while C atoms resided beyond the processed areas. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.85; 81.15.Jj Keywords: Nuclear microprobe; Impurity incorporation; Focused ion beam (FIB); Electron beam (EB); Micro-RBS; RBS mapping image
1. Introduction Nuclear microprobe techniques using Rutherford backscattering (RBS) have been applied to semiconductor process development, in which minimum feature sizes of several micrometers down to submicrometer and multilayered structures were used [1±3]. Micro-RBS analysis can provide information on the atomic composition
* Corresponding author. Tel.: +81-6-850-6304; fax: +81-6850-6341; e-mail: [email protected]
and distribution of both the material and impurity atoms beneath sample surfaces. Materials modi®cation such as etching and deposition using FIB or EB has been used for repairs of photo- [4] or X-ray masks [5,6] and integrated circuits [7±9]. Maskless physical sputtering or GAE for material removal, and beam assisted deposition for material deposition have been used in such applications. However, the impurity incorporation from etchant gas species, precursor gas or ion beam itself during beam-assisted processing has not been clari®ed yet, though nuclear microprobe is a powerful tool to facilitate the
0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 7 1 - 7
488
Y.K. Park et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 487±492
localized impurity analysis of beam process-induced structures [10,11]. In this study, the impurity incorporation due to the localized beam processing such as physical sputtering, GAE using iodine gas by FIBs, and beam assisted deposition using (CH3 )3 CH3 C5 H9 Pt precursor gas by FIBs or EBs has been investigated by a medium energy nuclear microprobe. RBS mapping images on incorporated impurities during beam processing have been obtained using 300 keV Be2 beams with a beam spot size of 80 nm. 2. Experimental procedures 30 keV Ga ion or 10 keV electron beams were raster-scanned by 10 ´ 10 lm2 for etching and deposition with a spot size of 100 nm full width at half maximum (FWHM) in o-axis (1 0 0) Si. The ion or electron dose was varied from 1 ´ 1015 to 1.2 ´ 1019 ions/cm2 according to the irradiation time. The beam current for etching and deposition was set to 1000 and 200 pA for FIB processing and 100 pA for EB processing. For physical sputtering, the beam dwell time and the overlap were set to 1.0 ls and 50%, respectively. In the case of GAE or deposition the beam dwell time and the overlap were set to 0.4 ls and 0%, respectively. For GAE, iodine was used as an etchant and heated to 29°C resulting in a gas ¯ux of 1015 molecules/cm2 s. For beam assisted deposition, metalorganic gas molecules, i.e. (CH3 )3 CH3 C5 H9 Pt were used as a precursor and heated up to 40°C resulting in a gas ¯ux of 1018 molecules/cm2 s. The gas was fed to the sample surface through an injector needle positioned to within 300 lm of the sample. The detailed experimental conditions are published elsewhere [12]. The samples were measured using a 300 keV Be2 nuclear microprobe with a beam spot size of 80 nm [13,14]. The samples were mounted on a sample holder with a six-axis goniometer in a UHV chamber with a vacuum higher than 5 ´ 10ÿ9 Torr during measurement. The beam current was 30 or 40 pA during measurement. Ions backscattered from the sample into a scattering angle of 97° were measured by a surface barrier detector.
The data acquisition time and dose for the RBS mapping were 80 min and 2.1 ´ 1017 Be2 /cm2 , respectively. Beam induced crystal damage at a dose of 2.1 ´ 1017 Be2 /cm2 was not observed by micro-channeling measurements as reported elsewhere [3]. 3. Results and discussion Fig. 1 shows the scanning electron microscope (SEM) view of beam processed area by physical sputtering (a), iodine gas assisted etching (b) with a dose of 4 ´ 1017 Ga /cm2 , FIB assisted Pt deposition with a dose of 2.38 ´ 1016 Ga /cm2 (c) and EB assisted Pt deposition with a dose of 2.4 ´ 1018 / cm2 (d), respectively. In the case of physical sputtering the rounded edge of milled areas was considered to be due to the pro®le of the ion beam that has a Gaussian distribution with a long tail [15]. Contrast change outside the etched area was observed in Fig. 1(b). This was due to the impurity incorporation of I and Ga, as observed in Fig. 3. The well-de®ned rectangular parts correspond to the deposited Pt layers by FIB and EB in Fig. 1(c) and (d).
Fig. 1. SEM view of beam processed area by physical sputtering (a), iodine gas assisted etching with a dose of 4 ´ 1017 Ga /cm2 (b), FIB assisted Pt deposition with a dose of 2.38 ´ 1016 Ga /cm2 (c) and EB assisted Pt deposition with a dose of 2.4 ´ 1018 /cm2 (d), respectively.
Y.K. Park et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 487±492
Fig. 2 shows the micro-RBS spectra for physically sputtered area (a) and gas assisted etched area (b) with a dose of 8.0 ´ 1016 Ga /cm2 . The leading edges for I, Ga and Si are indicated with arrows. The micro-RBS spectra were extracted from a micro-RBS data set with a size of 5 ´ 5 lm2 . To extract quantitative information about layer structures the micro-RBS spectra were compared with those by simulation using a RBX code [16] as indicated with solid lines. In the case of physical sputtering a large Ga peak appeared in the channels (61±90), whereas another signal in the channels (95±110) in addition to the Ga signal was observed in GAE. This new signal started at the I leading edge, i.e., iodine is present in the GAE process. The calculated residual Ga and I concentrations were estimated to be 2.17 ´ 1016 Ga / cm2 for physical sputtering, and 8.1 ´ 1015 Ga / cm2 and 4.1 ´ 1014 I /cm2 for gas assisted etching in case of 8.0 ´ 1016 Ga /cm2 irradiation, respectively. Thus the amount of Ga in the GAE area was estimated to be 2.7 times smaller than that of physically sputtered area. It should be noted that the GAE with an even higher material removal rate was more eective to reduce the incorporation of Ga than physical etching. Fig. 3 shows the secondary electron and RBS mapping images with an energy window of Ga atoms (61±90) for physically sputtered area (a), and with an energy window of Ga atoms and I atoms (95±110) for gas assisted etched area (b) with doses of 1.0 ´ 1017 Ga /cm2 , respectively. The area etched by physical sputtering in Fig. 3(a) was assumed to be amorphous and have a uniform
Fig. 2. Micro-RBS spectra for physically sputtered area (a) and gas assisted etched area (b) with a dose of 8.0 ´ 1016 Ga /cm2 . The leading edges for I, Ga and Si are indicated with arrows.
489
distribution of Ga atoms. The Ga atoms detected were considered to be implanted atoms at the bottom of the etched area or redeposited atoms from nearby process areas. An uniform distribution of Ga atoms was observed after GAE with iodine gas. The GAE processed areas showed iodine distribution at and nearby the etched area due to adsorbed iodine etchant gas. Fig. 4 shows the etched depth (a) and residual Ga concentration (b) as a function of ion dose from the simulated data using the TRIDYN code and the measured data with and without iodine gas. The physical sputtering of Si layers started above 1016 ions/cm2 . The simulated etched depth of Si using Ga ions drastically increased from 18 A up to 0.73 lm. The simulated depth was in good agreement with the measured depth for physical sputtering. GAE with iodine gas results in a higher material removal rate by a factor of 10. Thus, the amount of residual Ga due to implantation would be reduced almost completely by GAE if the redeposition of Ga atoms is disregarded. The simulated data using TRIDYN code (solid line in Fig. 4(b)) indicated that the residual Ga concentration due to the implantation process increased with ion dose at and below 1 ´ 1017 Ga / cm2 and that the residual Ga concentration above 1 ´ 1017 Ga /cm2 gradually saturated due to the sputtering. The residual Ga concentration indicates that the sputtering yield is much higher than that of simulated data obtained by the TRIDYN code in Fig. 4(a). The data from GAE with iodine gas indicated that the Ga concentration did not increase with increase in ion dose from 6 ´ 1016 to 1 ´ 1018 Ga /cm2 . Thus the GAE can reduce the Ga incorporation at these doses. Fig. 5 shows the micro-RBS spectra for FIB assisted deposited areas with a dose of 1.13 ´ 1016 Ga /cm2 (a), and for EB assisted deposited areas with a dose of 2.4 ´ 1018 /cm2 (b), respectively. The micro-RBS spectra were extracted from a microRBS data set with a size of 5 ´ 5 lm2 . In case of FIB assisted deposition, the micro-RBS spectra indicate the Pt signal of the deposited Pt and Ga signal (shoulder of the Pt-peak) of the residual Ga atoms from the FIB itself, the implantation and the redeposition. In case of EB assisted deposition the micro-RBS spectrum shows the Pt signal of the
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Fig. 3. Secondary electron and RBS mapping images with an energy window of Ga atoms (61±90) for physically sputtered area (a), and with an energy window of Ga atoms and I atoms (95±110) for gas assisted etched area (b) with doses of 1.0 ´ 1017 Ga /cm2 , respectively. The left-hand side indicates a secondary electron image and the right-hand side indicates an RBS mapping image.
deposited Pt atoms with an energy window from 95 to 125. Fig. 6 shows the secondary electron and RBS mapping images with an energy window of Pt atoms (95±150), Ga atoms (61±90) and low-Z elements (15±40) such as C for Pt deposited areas by FIB with a dose of 2.38 ´ 1016 Ga /cm2 (a), and the secondary electron and RBS mapping images with an energy window of Pt atoms (95±150) for Pt deposited areas by EB with a dose of 2.4 ´ 1018 /cm2 (b), respectively. The secondary electron image for FIB processed area in Fig. 6(a) shows the contrast change beyond the deposited layers probably due
to the incorporation of carbon as shown in RBS mapping image (15±40 ch). The beam-processed areas by FIB or EB showed a Pt distribution at and nearby the processed area. The excess distribution of Pt and Ga atoms at the left-hand and right-hand sides nearby the FIB processed area within 3 lm in Fig. 6(a) was considered due to the pro®le of the ion beam that has a Gaussian distribution with a long tail [15]. The deposition rate depends on the beam current density [17], so that the boundary region has lower deposition rate due to the lower current density pro®le of Ga FIBs.
Fig. 4. Etched depth (a) and residual Ga concentration (b) as a function of ion dose from the simulated data using the TRIDYN code and the measured data with and without iodine gas.
Fig. 5. Micro-RBS spectra for FIB assisted deposited areas with a dose of 1.13 ´ 1016 Ga /cm2 (a), and for EB assisted deposited areas with a dose of 2.4 ´ 1018 /cm2 (b), respectively. The leading edges for Pt, Ga and Si are indicated with arrows.
Y.K. Park et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 487±492
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Fig. 6. Secondary electron and RBS mapping images with an energy window of Pt atoms (channels: 95±150), Ga atoms (61±90) and low-Z elements (15±40) such as C for Pt deposited areas by FIB with a dose of 2.38 ´ 1016 Ga /cm2 (a), and the secondary electron and RBS mapping images with an energy window of Pt atoms (95±150) for Pt deposited areas by EB with a dose of 2.4 ´ 1018 /cm2 (b), respectively. The left-hand side indicates a secondary electron image and the right-hand side indicates an RBS mapping image.
The lateral distribution, obtained by an energy window of 15-40 ch in Fig. 6(a), indicates the lateral low-Z element distribution, since the RBS yield of bulk Si signals from the deposited layers increases due to the lowering of the incident probe beam energy. A separate Auger electron spectroscopy (AES) measurement indicates that the composition of the deposited layers is Pt:Ga:C 3:2:5. Therefore this low-Z element would be carbon. Carbon may come from the insucient decomposition of source metalorganic gas. The size of lateral distribution for carbon in Fig. 6(a) broadens up to 10 lm beyond processed area. The sputtering at the side-wall of the deposited layer would be enhanced by the glancing angle incidence of the Ga beam for prolonged FIB irradiation. The low Z elements such as C and O would easily be sputtered away from the side-wall of the deposited layer due to the enhanced sputtering rate and redeposited around the initial area by about 10 lm.
4. Conclusions The incorporated impurities after localized beam processing, such as physical sputtering, GAE using iodine gas by FIBs and beam assisted deposition using (CH3 )3 CH3 C5 H9 Pt precursor gas by FIBs and EBs, have been investigated using RBS mapping by a 300 keV Be2 microprobe with a beam spot size of 80 nm. It was revealed that the residual Ga atoms ranging from 1 ´ 1016 to 5 ´ 1016 Ga /cm2 distributed at and nearby the bottom of the etched area due to implantation and redeposition from the Ga FIB. GAE with iodine gas can reduce the incorporation of Ga atoms, although additional iodine incorporation at a level of 1014 /cm2 occurs. The quantitative analysis on the residual impurity by micro-RBS spectra indicated that the amount of Ga atoms in the GAE processed areas was estimated to be 2.7 times smaller than that of physical etching.
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The beam-deposited areas by FIB or EB deposition showed a Pt distribution at and nearby the processed area, whereas the incorporated impurity of low-Z element such as C distributed at and around the processed areas within 10 lm. The amount of Pt atoms after EB induced deposition was much less than that by the FIB induced deposition and Pt was con®ned almost within the processed area presumably because of the nonsputtering feature during electron irradiation and the narrower electron beam pro®le. Acknowledgements Y.K. Park would like to express his thanks to the RCMSEC for ®nancial support. Authors thank to Mr. T. Nagai and T. Tsurumoto for helping the micro-RBS measurements. References [1] M. Takai, Scanning Microsc. 6 (1991) 147. [2] M. Takai, Nucl. Instr. and Meth. B 85 (1994) 664.
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