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Ceramic surface modification by a keV ion irradiation
Nuclear Instruments and Methods in Physics Research B 148 (1999) 740±744
Ceramic surface modi®cation by a keV ion irradiation W.K. Choi *, S.C. Choi, H.-J. Jung, S.K. Koh Thin Film Technology Research Center, Korea Institute of Science and Technology, Cheongryang, P.O. Box 131, Seoul 130-650, South Korea
Abstract New functional complexes such as AlON and SiON were successfully formed by keV Ar ion irradiation with concurrent reactive gas blowing near the irradiated AlN and Si3 N4 ceramic surfaces. Single crystal Al2 O3 was also treated by only N 2 bombardment. At energies higher than 500 eV, AlON bonding arose, while AlN bonds were formed at 1 keV irradiation. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 81.65; 82.65.J; 68.35.R Keywords: keV ion beam; Metal ®lm adhesion; Functional layer; Ceramic surface modi®cation
1. Introduction Since a keV low energy ion assisted reaction (IAR) was introduced at MRS 1995 Fall meeting as a prospective surface modi®cation technique of polymer without damage, there have been reported experimental results of various kinds of polymer surfaces and their applications [1±5]. As an extension of the IAR method, in this study a keV ion beam irradiation in reactive gas environments or reactive ion irradiation itself is adopted in ceramic surface modi®cation. Since the keV energy is also quite enough to break any ionic and covalent bonding in ceramic materials, it can be easily expected to change the surface chemical composition and form a new layer using keV ion irradiation.
* Corresponding author. Fax: +82-2-958-5509; e-mail: [email protected]
Once the new functional layer is successfully created, it can eectively aect the features of the ceramic surface such as its mechanical property, chemical stability, and other functionality. In this experiment, two kinds of nitrides AlN and Si3 N4 , and one of the most popular oxides, Al2 O3 , are selected. Although AlN has good thermal conductivity and electrical insulation property [6], poor adhesion to a metal ®lm should be improved for good metallization when AlN is used for the electrically insulating substrate. In case of Si3 N4 , poor adhesion between electrode and silicon nitride has been investigated for PZT memory devices and MEMS applications [7]. Therefore, if functional groups like oxynitride could be introduced, it will help to increase the adhesion between the metal and modi®ed AlN and Si3 N4 surfaces. Al2 O3 is chosen to check whether an intermediate layer like AlON and AlN can be formed and further be used as a thin buer layer for the growth of GaN,
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 8 4 8 - 9
W.K. Choi et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 740±744
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instead of present an used AlN or a GaN buer thin ®lm grown at low temperature [8]. 2. Experimental Polycrystalline AlN blocks were cut into 1 cm ´ 1 cm ´ 0.5 mm samples and irradiated by 1 keV Ar with varying ion dose from 1 ´ 1015 to 5 ´ 1016 ions/cm2 at a constant oxygen ¯ow rate of 4 ml/min and an ion current density of 28 lA/cm2 . The irradiation was conducted with a cold hollow cathode type ion source. More details on the experimental procedure of IAR has been described or 50 A were previously [9]. Cu ®lms with 1000 A deposited on the unirradiated/irradiated AlN substrates by ion beam sputtering, to be used for a scratch test, and for an XPS study. Bond strengths were measured by a scratch test equipped with a Knoop hardness tester and ultrasonic sensor. The normal load was gradually increased from 0 to 40 N. Si-rich Si3 N4 on Si substrate grown by LPCVD was treated with a 200±1000 eV Ar ion beam with simultaneous O2 ¯ow. a-Al2 O3 (0 0 0 1) surfaces were modi®ed by direct N 2 ion irradiation with a constant dose of 1 ´ 1016 /cm2 and an ion-beam energy of 100±1000 eV at ®xed ion ¯uence. The morphology of a sapphire surface treated at various conditions was observed by AFM (Park Scienti®c Instrument STM-SU-210). Creations of new chemical bonding on AlN, Si3 N4 , and sapphire surfaces were identi®ed by X-ray photoelectron spectroscopic (Surface Scienti®c Instrument model 2803-S) analysis. 3. Results and discussion The AlN surface was irradiated in an oxygen Cu environment in the IAR system, and 1000 A was deposited ex-situ for the bond strength measurement. The result of the scratch test is shown in thick on non Fig. 1. Cu ®lms deposited with 1000 A irradiated AlN substrate showed poor bond strength and slightly increasing acoustic signal from the beginning stage. On the other hand, Cu ®lms sputtered on the Ar irradiated surface with 1 ´ 1015 /cm2 at the oxygen ¯ow rate of 4 ml/min
Fig. 1. The variation of acoustic emission as an increase of normal load during scratch test of Cu ®lm on AlN substrate.
showed an increased ultrasonic emission at a normal load of 30 N. These results of the scratch test indicate that the bond strength between the Cu ®lm and the AlN substrate was signi®cantly increased by the Ar irradiation in an O2 environment. Fig. 2 represents the high resolution Cu3p and Al2p near core levels spectra of Cu(50 A)/AlN. As shown in Fig. 2, two Cu metal peaks (spectrum Fig. 2(a)) of spin±orbit splitting (Cu3p3=2 , Cu3p1=2 ) appeared at binding energies of 75.5 and 77.5 eV after 20 s of sputtering, respectively. The spectrum after 10 min sputtering (Fig. 2(b)) shifted to a little higher binding energies and became wider in full width at half maximum (FWHM). The results indicate that copper oxide has formed in this layer. In spectrum Fig. 2(b), broadening of Cu3p3=2 to lower binding energy indicated that some complex phases of copper oxide arose and these would be related to a newly formed layer on modi®ed AlN. After 25 min sputtering, the A12p near core level peak (spectrum Fig. 2(c)) appeared at 72.2 eV. This peak position was shifted to higher binding
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W.K. Choi et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 740±744
Fig. 2. Cu3p and Al2p XPs core level spectra of Cu (50 A)/AlN.
energy compared with that (71.8 eV) of spectrum Fig. 2(d) obtained after 45 min sputtering. Since no change of peak position was observed after sputtering longer than 45 min, this peak was believed to be A12p from AlN. Therefore the shift of spectrum Fig. 2(c) to higher binding energy than the peak in spectrum Fig. 2(d) was considered to result from formation of aluminum oxynitride due to chemical reaction with blown oxygen during the Ar irradiation. The high resolution Ols and Nls core level XPS spectra are shown in Fig. 3(a) and (b). In Fig. 3(a), the upper spectrum corresponds to the Ols core level obtained after 10 min of sputtering. The peaks at 531.5 and 529.5 eV probably results from copper oxide and aluminum oxynitride, respectively [10]. The lower spectrum showing only one peak might be related with aluminium oxynitride formation. Moreover, two distinct peaks at 393.8 eV and 400.4 eV occurred in the Nls spectrum (Fig. 3(b)) taken after 25 min of sputtering. The peak at 393.8 eV is related with AlN and that at
Fig. 3. Ols (a) and Nls (b) XPS core level spectra of Cu (50 A)/ AlN.
W.K. Choi et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 740±744
the higher binding energy of 400.4 eV originate from aluminum oxynitride [11]. These results con®rmed that intermediate layers such as CuO and AlON were formed between the deposited Cu and the new surface layer due to chemical reaction between the irradiated AlN surface, blown oxygen, and the energetic Ar ion. The presence of intermediate layer should contribute directly to improve the adhesion force between Cu ®lms and the AlN substrate. Fig. 4 shows N1s and Si2p core-level XPS spectra for Si-rich Si3 N4 modi®ed by 1 keV Ar ion irradiation with the blowing of O2 (4 ml/min) near the silicon nitride substrate. As shown in Fig. 4, two well resolved peaks of N1s spectra were found at 397.15 and 399.52 eV in the irradiated sample and which correspond to Si3 N4 and SiON, respectively. Also, the formation of a new functional group could be supported by the fact that the peak position of Si2p shifts to high binding energy than that of nonirradiated Si2p for Si-rich Si3 N4 . From this result, IAR can produce new functional groups in silicon nitride surface like silicon oxynitride (SiON) layer. The high resolution N1s core level XPS spectra for sapphire are shown in Fig. 5. No peak related to nitrogen binding can be found in the modi®ed sapphire surface by N 2 irradiation below 500 eV ion energy. As the N 2 ion energy was increased, two peaks at binding energies of 397.6 eV and 402.5 eV arise. These peaks could be attributed to AlON and absorbed or implanted nitrogen (N1s), re-
743
Fig. 5. N1s core-level XPS spectra of a-Al2 O3 (0 0 0 1) surfaces 16 modi®ed by N 2 ion irradiation at the constant dose of 1 ´ 10 / cm2 and at ion energy ranging from 500 to 1000 eV.
spectively [11]. With the increase of the dose from 5 ´ 1016 to 1 ´ 1017 /cm2 , the relative AlON peak intensity, not shown here, generally becomes larger and the N1s peak FWHMÕs also broadens. Until 900 eV N 2 ion irradiation only two peaks were observed, but a dramatic change occurred in the N1s spectrum irradiated by 1 keV N 2 ion. A new peak shifted by 6.37 eV on the low binding energy side of the AlON is clearly observed. From Ref. [11], the binding energy dierence between AlON and AlN is accurately known to be as much as about 6.5 eV. The peak found at 396 eV binding energy can be undoubtedly assigned to AlN. Apparently, a new functional layer can be created on the alumina ionic crystal surface by only N 2 ion irradiation this eect is extremely dependent on the ion energy. 4. Conclusions
Fig. 4. N1s and Si2p core level XPS spectra of Si-rich Si3 N4 modi®ed by 1 keV Ar ion irradiation with simultaneous O2 (4 ml/min) blowing.
Adhesion of Cu ®lms on AlN substrates was signi®cantly improved by the formation of an
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W.K. Choi et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 740±744
intermediate layer such as copper oxide and aluminium oxynitride, due to a chemical reaction between the Ar irradiated AlN surface and the blown oxygen. In a similar way, SiON bending could be created by 1 keV Ar irradiation with concurrent oxygen blowing. An AlN bond was found on sapphire surface modi®ed with 1000 eV N 2 ion irradiation, and it coexisted with AlON. Consequently, IAR is also proved very useful technique in ceramic surface modi®cation to generate function group without much damage. References [1] J.S. Cho, W.K. Choi, H.-J. Jung, S.K. Koh, J. Mater. Res. 12 (1997) 277.
[2] S.K. Koh, S.C. Park, S.R. Kim, W.K. Choi, H.-J. Jung, K.D. Pae, J. Appl. Poly. Sci. 64 (1997) 1913. [3] S.K. Koh, S.C. Park, W.K. Choi, S.K. Song, H.-J. Jung, K.D. Pae, Mater. Res. Soc. Symp. Proc. 396 (1996) 335. [4] S.K. Koh, S.K. Song, W.K. Choi, S.N. Han, H.-J. Jung, J. Mater. Res. 10 (1995) 2390. [5] S.K. Koh, W.K. Choi, J.S. Choi, S.K. Song, H.-J. Jung, Mater. Res. Soc. Symp. Proc. 354 (1995). [6] J.N. Kuznia, M.A. Khan, D.T. Olson, R. Kaplan, J. Freitas, J. Appl. Phys. 73 (1993) 73. [7] J.H. Kim, Y.S. Yoo, D.L. Polla, Jpn. J. Appl. Phys. 37 (1998) 948. [8] A. Nakamura, M. Singh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L797. [9] W.K. Choi, S.K. Koh, H.-J. Jung, J. Vac. Sci. Technol. 14 (1996) 2366. [10] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Mullenberg, Handbook of X-ray photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, 1979. [11] A.D. Katnani, K.I. Papathomas, J. Vac. Sci. Technol. A 5 (1987) 1355.
Ceramic surface modi®cation by a keV ion irradiation W.K. Choi *, S.C. Choi, H.-J. Jung, S.K. Koh Thin Film Technology Research Center, Korea Institute of Science and Technology, Cheongryang, P.O. Box 131, Seoul 130-650, South Korea
Abstract New functional complexes such as AlON and SiON were successfully formed by keV Ar ion irradiation with concurrent reactive gas blowing near the irradiated AlN and Si3 N4 ceramic surfaces. Single crystal Al2 O3 was also treated by only N 2 bombardment. At energies higher than 500 eV, AlON bonding arose, while AlN bonds were formed at 1 keV irradiation. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 81.65; 82.65.J; 68.35.R Keywords: keV ion beam; Metal ®lm adhesion; Functional layer; Ceramic surface modi®cation
1. Introduction Since a keV low energy ion assisted reaction (IAR) was introduced at MRS 1995 Fall meeting as a prospective surface modi®cation technique of polymer without damage, there have been reported experimental results of various kinds of polymer surfaces and their applications [1±5]. As an extension of the IAR method, in this study a keV ion beam irradiation in reactive gas environments or reactive ion irradiation itself is adopted in ceramic surface modi®cation. Since the keV energy is also quite enough to break any ionic and covalent bonding in ceramic materials, it can be easily expected to change the surface chemical composition and form a new layer using keV ion irradiation.
* Corresponding author. Fax: +82-2-958-5509; e-mail: [email protected]
Once the new functional layer is successfully created, it can eectively aect the features of the ceramic surface such as its mechanical property, chemical stability, and other functionality. In this experiment, two kinds of nitrides AlN and Si3 N4 , and one of the most popular oxides, Al2 O3 , are selected. Although AlN has good thermal conductivity and electrical insulation property [6], poor adhesion to a metal ®lm should be improved for good metallization when AlN is used for the electrically insulating substrate. In case of Si3 N4 , poor adhesion between electrode and silicon nitride has been investigated for PZT memory devices and MEMS applications [7]. Therefore, if functional groups like oxynitride could be introduced, it will help to increase the adhesion between the metal and modi®ed AlN and Si3 N4 surfaces. Al2 O3 is chosen to check whether an intermediate layer like AlON and AlN can be formed and further be used as a thin buer layer for the growth of GaN,
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 8 4 8 - 9
W.K. Choi et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 740±744
741
instead of present an used AlN or a GaN buer thin ®lm grown at low temperature [8]. 2. Experimental Polycrystalline AlN blocks were cut into 1 cm ´ 1 cm ´ 0.5 mm samples and irradiated by 1 keV Ar with varying ion dose from 1 ´ 1015 to 5 ´ 1016 ions/cm2 at a constant oxygen ¯ow rate of 4 ml/min and an ion current density of 28 lA/cm2 . The irradiation was conducted with a cold hollow cathode type ion source. More details on the experimental procedure of IAR has been described or 50 A were previously [9]. Cu ®lms with 1000 A deposited on the unirradiated/irradiated AlN substrates by ion beam sputtering, to be used for a scratch test, and for an XPS study. Bond strengths were measured by a scratch test equipped with a Knoop hardness tester and ultrasonic sensor. The normal load was gradually increased from 0 to 40 N. Si-rich Si3 N4 on Si substrate grown by LPCVD was treated with a 200±1000 eV Ar ion beam with simultaneous O2 ¯ow. a-Al2 O3 (0 0 0 1) surfaces were modi®ed by direct N 2 ion irradiation with a constant dose of 1 ´ 1016 /cm2 and an ion-beam energy of 100±1000 eV at ®xed ion ¯uence. The morphology of a sapphire surface treated at various conditions was observed by AFM (Park Scienti®c Instrument STM-SU-210). Creations of new chemical bonding on AlN, Si3 N4 , and sapphire surfaces were identi®ed by X-ray photoelectron spectroscopic (Surface Scienti®c Instrument model 2803-S) analysis. 3. Results and discussion The AlN surface was irradiated in an oxygen Cu environment in the IAR system, and 1000 A was deposited ex-situ for the bond strength measurement. The result of the scratch test is shown in thick on non Fig. 1. Cu ®lms deposited with 1000 A irradiated AlN substrate showed poor bond strength and slightly increasing acoustic signal from the beginning stage. On the other hand, Cu ®lms sputtered on the Ar irradiated surface with 1 ´ 1015 /cm2 at the oxygen ¯ow rate of 4 ml/min
Fig. 1. The variation of acoustic emission as an increase of normal load during scratch test of Cu ®lm on AlN substrate.
showed an increased ultrasonic emission at a normal load of 30 N. These results of the scratch test indicate that the bond strength between the Cu ®lm and the AlN substrate was signi®cantly increased by the Ar irradiation in an O2 environment. Fig. 2 represents the high resolution Cu3p and Al2p near core levels spectra of Cu(50 A)/AlN. As shown in Fig. 2, two Cu metal peaks (spectrum Fig. 2(a)) of spin±orbit splitting (Cu3p3=2 , Cu3p1=2 ) appeared at binding energies of 75.5 and 77.5 eV after 20 s of sputtering, respectively. The spectrum after 10 min sputtering (Fig. 2(b)) shifted to a little higher binding energies and became wider in full width at half maximum (FWHM). The results indicate that copper oxide has formed in this layer. In spectrum Fig. 2(b), broadening of Cu3p3=2 to lower binding energy indicated that some complex phases of copper oxide arose and these would be related to a newly formed layer on modi®ed AlN. After 25 min sputtering, the A12p near core level peak (spectrum Fig. 2(c)) appeared at 72.2 eV. This peak position was shifted to higher binding
742
W.K. Choi et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 740±744
Fig. 2. Cu3p and Al2p XPs core level spectra of Cu (50 A)/AlN.
energy compared with that (71.8 eV) of spectrum Fig. 2(d) obtained after 45 min sputtering. Since no change of peak position was observed after sputtering longer than 45 min, this peak was believed to be A12p from AlN. Therefore the shift of spectrum Fig. 2(c) to higher binding energy than the peak in spectrum Fig. 2(d) was considered to result from formation of aluminum oxynitride due to chemical reaction with blown oxygen during the Ar irradiation. The high resolution Ols and Nls core level XPS spectra are shown in Fig. 3(a) and (b). In Fig. 3(a), the upper spectrum corresponds to the Ols core level obtained after 10 min of sputtering. The peaks at 531.5 and 529.5 eV probably results from copper oxide and aluminum oxynitride, respectively [10]. The lower spectrum showing only one peak might be related with aluminium oxynitride formation. Moreover, two distinct peaks at 393.8 eV and 400.4 eV occurred in the Nls spectrum (Fig. 3(b)) taken after 25 min of sputtering. The peak at 393.8 eV is related with AlN and that at
Fig. 3. Ols (a) and Nls (b) XPS core level spectra of Cu (50 A)/ AlN.
W.K. Choi et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 740±744
the higher binding energy of 400.4 eV originate from aluminum oxynitride [11]. These results con®rmed that intermediate layers such as CuO and AlON were formed between the deposited Cu and the new surface layer due to chemical reaction between the irradiated AlN surface, blown oxygen, and the energetic Ar ion. The presence of intermediate layer should contribute directly to improve the adhesion force between Cu ®lms and the AlN substrate. Fig. 4 shows N1s and Si2p core-level XPS spectra for Si-rich Si3 N4 modi®ed by 1 keV Ar ion irradiation with the blowing of O2 (4 ml/min) near the silicon nitride substrate. As shown in Fig. 4, two well resolved peaks of N1s spectra were found at 397.15 and 399.52 eV in the irradiated sample and which correspond to Si3 N4 and SiON, respectively. Also, the formation of a new functional group could be supported by the fact that the peak position of Si2p shifts to high binding energy than that of nonirradiated Si2p for Si-rich Si3 N4 . From this result, IAR can produce new functional groups in silicon nitride surface like silicon oxynitride (SiON) layer. The high resolution N1s core level XPS spectra for sapphire are shown in Fig. 5. No peak related to nitrogen binding can be found in the modi®ed sapphire surface by N 2 irradiation below 500 eV ion energy. As the N 2 ion energy was increased, two peaks at binding energies of 397.6 eV and 402.5 eV arise. These peaks could be attributed to AlON and absorbed or implanted nitrogen (N1s), re-
743
Fig. 5. N1s core-level XPS spectra of a-Al2 O3 (0 0 0 1) surfaces 16 modi®ed by N 2 ion irradiation at the constant dose of 1 ´ 10 / cm2 and at ion energy ranging from 500 to 1000 eV.
spectively [11]. With the increase of the dose from 5 ´ 1016 to 1 ´ 1017 /cm2 , the relative AlON peak intensity, not shown here, generally becomes larger and the N1s peak FWHMÕs also broadens. Until 900 eV N 2 ion irradiation only two peaks were observed, but a dramatic change occurred in the N1s spectrum irradiated by 1 keV N 2 ion. A new peak shifted by 6.37 eV on the low binding energy side of the AlON is clearly observed. From Ref. [11], the binding energy dierence between AlON and AlN is accurately known to be as much as about 6.5 eV. The peak found at 396 eV binding energy can be undoubtedly assigned to AlN. Apparently, a new functional layer can be created on the alumina ionic crystal surface by only N 2 ion irradiation this eect is extremely dependent on the ion energy. 4. Conclusions
Fig. 4. N1s and Si2p core level XPS spectra of Si-rich Si3 N4 modi®ed by 1 keV Ar ion irradiation with simultaneous O2 (4 ml/min) blowing.
Adhesion of Cu ®lms on AlN substrates was signi®cantly improved by the formation of an
744
W.K. Choi et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 740±744
intermediate layer such as copper oxide and aluminium oxynitride, due to a chemical reaction between the Ar irradiated AlN surface and the blown oxygen. In a similar way, SiON bending could be created by 1 keV Ar irradiation with concurrent oxygen blowing. An AlN bond was found on sapphire surface modi®ed with 1000 eV N 2 ion irradiation, and it coexisted with AlON. Consequently, IAR is also proved very useful technique in ceramic surface modi®cation to generate function group without much damage. References [1] J.S. Cho, W.K. Choi, H.-J. Jung, S.K. Koh, J. Mater. Res. 12 (1997) 277.
[2] S.K. Koh, S.C. Park, S.R. Kim, W.K. Choi, H.-J. Jung, K.D. Pae, J. Appl. Poly. Sci. 64 (1997) 1913. [3] S.K. Koh, S.C. Park, W.K. Choi, S.K. Song, H.-J. Jung, K.D. Pae, Mater. Res. Soc. Symp. Proc. 396 (1996) 335. [4] S.K. Koh, S.K. Song, W.K. Choi, S.N. Han, H.-J. Jung, J. Mater. Res. 10 (1995) 2390. [5] S.K. Koh, W.K. Choi, J.S. Choi, S.K. Song, H.-J. Jung, Mater. Res. Soc. Symp. Proc. 354 (1995). [6] J.N. Kuznia, M.A. Khan, D.T. Olson, R. Kaplan, J. Freitas, J. Appl. Phys. 73 (1993) 73. [7] J.H. Kim, Y.S. Yoo, D.L. Polla, Jpn. J. Appl. Phys. 37 (1998) 948. [8] A. Nakamura, M. Singh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L797. [9] W.K. Choi, S.K. Koh, H.-J. Jung, J. Vac. Sci. Technol. 14 (1996) 2366. [10] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Mullenberg, Handbook of X-ray photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, 1979. [11] A.D. Katnani, K.I. Papathomas, J. Vac. Sci. Technol. A 5 (1987) 1355.