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Interplay of surface adsorption and preferential sputtering in metal plasma immersion ion implantation and deposition
Nuclear Instruments and Methods in Physics Research B 206 (2003) 663–667 www.elsevier.com/locate/nimb
Interplay of surface adsorption and preferential sputtering in metal plasma immersion ion implantation and deposition S. M€ andl a
a,*
, D. Manova
a,b
, B. Rauschenbach
a
Institut f€ur Oberfl€achenmodifizierung (IOM Leipzig), Permoserstr. 15, Leipzig 04303, Germany b Institut f€ur Experimentalphysik IV, Universit€at Augsburg, Augsburg 86135, Germany
Abstract Formation of hard ceramic surface layer by metal plasma immersion ion implantation and deposition (MePIIID) can be a quite complex process as quite a number of different species, including condensable metallic ions, electrons and neutral gas, which can be partially ionised, are interacting in the vacuum chamber and impinging on the surface. The resulting growth rate and chemical composition will be determined by the complex interplay of surface adsorption, ion implantation and preferential sputtering. Here, an overview of these effects is given for the important systems ZnO, TiO2 , TiN and AlN prepared by MePIIID. In ZnO, the predominant mechanism is preferential oxygen sputtering. However, the oxygen adsorption from the background gas is stronger in the case of TiO2 , leading to a constant Ti/O ratio beyond a threshold in the oxygen gas flow for this compound. This effect is less pronounced for TiN, where a continuously varying Ti/N ratio was found only for a varying gas flow and independent of the pulse voltage. In contrast, a constant Al/N ratio over a broad range of nitrogen gas flows was observed for AlN. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 52.77.Dq; 82.80.Yc; 79.20.Rf; 68.43.)h Keywords: Plasma immersion ion implantation; Sputtering; Adsorption; Rutherford backscattering spectroscopy
1. Introduction Plasma immersion ion implantation (PIII) was developed as a fast and cost-effective alternative to beamline implantations for complex-shaped three dimensional objects [1,2]. Here, an isotropic implantation into convex targets is possible for the quasi-static ions [3,4]. Extending PIII by using a metal ion plasma source in conjunction with the
*
Corresponding author. Tel.: +49-341-235-2944; fax: +49341-235-2313. E-mail address: [email protected] (S. M€andl).
negative high voltage pulses, predominantly a cathodic arc, results in metal plasma immersion ion implantation and deposition (MePIIID) [5,6]. Here, the plasmas can condense on substrates, i.e. film deposition occurs between the pulses, while a combination of implantation and deposition takes place during the pulses [7]. Regarding the process characteristics, we found MePIIID to be is closely related to ion beam assisted deposition (IBAD). It is possible to adjust the film texture by variation of the ion momentum [8,9] and the stress state by adjustment of the energy deposition [10]. Additionally, sputter saturation, often encountered in IBAD at high ion fluxes
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00816-4
S. M€andl et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 663–667
and energies of some 1 keV and above [11], is no major factor in MePIIID, due to a pulsed operation mode. However, additional parameters are present for MePIIID, which may influence the film deposition process and have to be accounted for. Preferential sputtering, well known from ion sputtering for elemental depth profiles in transition metal oxides [12], can be expected for an augmented effect in the ion energy range of 1–10 keV. Electrons impinging between the negative voltage pulses may lead to a surface activation [13] and, finally, neutral gas atoms, present at a higher density due to the increased working pressure compared to IBAD, interact at a higher rate with the surface and can be adsorbed in a considerable amount. Here, a qualitative overview, indicating the dominance of one or more of these effects, is given for the systems ZnO, AlN, TiO2 and TiN, which are important semiconductor materials [14,15] or surface layers for tribological and wear applications [16,17]. All materials were prepared by MePIIID in the same vacuum system at varying pulse voltages and gas flows.
2. Experiment Small silicon samples (diameter <60 mm) were treated in a 250 l cylindrical vacuum chamber, diameter 700 mm, height 650 mm and a base pressure better than 104 Pa, equipped with a vacuum cathodic arc [18]. No cleaning of the silicon to remove the native oxide layer prior to the treatment was performed. Employed cathode materials were titanium, zinc or aluminum (purity 99.99%), with the arc current between 70 and 100 A. Additionally, nitrogen or oxygen was introduced into the chamber at variable gas flows up to 50 sccm. The distance between the cathode surface and the sample was 39 cm. A simple shield was used for macroparticle filtering [19] (diameter 100 mm, mounted at a distance of 100 mm from the cathode) in the case of AlN and ZnO. No such device was necessary for TiN and TiO2 , where a macroparticle density of 0.1 mm2 (cut-off diameter 1 lm) or less were found using a scanning electron microscope [20].
High voltage pulses between 0 and )10 kV were applied with a length of 30 ls at a constant repetition rate of 3 kHz, yielding a duty cycle of 9%. Hence an average ion energy between 50–75 eV (without bias) and 1000–1500 eV (at )10 kV), depending on the average charge state in the plasma was obtained. The total deposition time was about 5 min in all cases. The elemental depth profiles were obtained either by elastic recoil detection analysis (ERDA) or Rutherford backscattering spectroscopy (RBS). The phase composition and texture were analysed using an X-ray facility. Spectroscopic ellipsometry (SE) was employed to determine the optical properties over the spectral range from 250 to 820 nm using a system equipped with an Xe arc lamp, rotating analyser and a diode array.
3. Results and discussion Textured films with the c-axis normal to the surface were obtained for ZnO, deposited at 100 A arc current and 50 sccm oxygen gas flow, as texture was independent of the bias voltage for the range between 0 and )10 kV. However, as the bias voltage is increased, the oxygen content of the films decreases from 49 to 47 at.% (ZnO0:96 to ZnO0:88 ), as shown in Fig. 1. This indicates a preferential oxygen sputtering, similar to the nuisance effect during sputter depth profiling of TiO2 with Ar, where Ti4þ is converted into Ti3þ [12]. At the same time, the optical adsorption increases
1.00
0.95
O/Zn Ratio
664
0.90
0.85
0.80 0
1
2
3
4
5
Pulse Voltage (kV)
Fig. 1. Oxygen/zinc ratio measured for ZnO films deposited with shield as a function of pulse voltage. The dotted line is a guide for the eye.
S. M€andl et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 663–667
approximately by a factor of 10 compared to the unbiased sample. This could be assigned to oxygen vacancies and additional point defects produced by the ion bombardment which are not annealed near room temperature. Albeit, no influence of the pulse voltage was observed on the stoichiometry for titania films produced by MePIIID in the same voltage range from 0 to )10 kV. The reason seems to be a very strong oxygen affinity of Ti, which leads to a strongly increased oxygen absorption rate from the gas phase. A comparison of the plasma density and the growth rate indicates that about 25% of the total atoms in the film, i.e. about 40% of the oxygen atoms, are not arriving as ions but as atoms [18]. The upper panel of Fig. 2 shows the oxygen-to-titanium ratio together with the working pressure at different flow/current ratios. The arc current was varied between 70 and 100 A, while the gas flow stayed between 40 and 50 sccm. The corresponding film thickness is presented in the lower panel. At low flow/current ratios the gettering efficiency of the freshly deposited titanium film is high enough to remove nearly all oxygen and reduce the working pressure to 0.01 Pa. For comparison, the working pressure for ZnO at 50 sccm was 0.03 Pa. The oxygen content of these films is below 60 at.%. With increasing oxygen flow, a saturation of the oxygen content is observed at a level of
1.4
0.05
-2 18
1.0
0.00
6 4 2 0 0.5
0.6
0.7
Gas Flow/Arc Current Ratio (sccm/A)
4.5
Plasmon Energy (eV)
0.10
1.6
1.2
Thickness (10 cm )
63 at.%, beyond which no further oxygen is adsorbed, leading to a higher working pressure, shorter mean free path and, hence, lower deposition rates. The varying deposition rates at a ratio of 0.5 sccm/A result from different absolute ion currents. For TiN, a similar effect was observed (see Fig. 3). Here, a wider range of gas flows between 20 and 50 sccm was used. As the change in the composition is less pronounced than for TiO2 , spectroscopic ellipsometry was employed to determine the screened plasmon energy, closely correlated with the nitrogen content [21]. These values are more accurate than results from RBS measurements. However, no linear relation between the plasmon energy and the composition exists over the whole range, so that only the plasmon energy is presented. Here, the saturation for high gas flows occurs at a plasmon energy of 2.95 eV, corresponding to a N/Ti ratio of 0.95, whereas the highest value of 4.15 eV corresponds to a N/Ti 0.5. While varying the pulse voltage, again no strong influence on the stoichiometry is found. At the same time, the texture evolves with pulse voltage, starting with (1 1 0) at 1 kV and changing to (1 0 0) for 3 kV and above [22]. As a last example, the behavior of AlN, deposited with a shield for droplet reduction, is presented in Fig. 4. Here, a fixed arc current of 100 A was used, while the gas flow was decreased down to 15 sccm. All films exhibit a strong c-axis preferential
0.15
1.8
Pressure (Pa)
O/Ti Ratio
2.0
665
0 kV 3 kV 7.5 kV
4.0
4 3.5
3.0
Stoichiometric TiN 2.5 0.2
Fig. 2. Oxygen/titanium ratio, film thickness and working pressure as a function of the gas flow/arc current ratio for MePIIID without shield. The deposition time was five minutes for each sample. The dotted and dashed lines are guides for the eye.
1 kV 5 kV 10 kV
0.3
0.4
0.5
0.6
0.7
Gas Flow/Arc Current Ratio (sccm/A)
Fig. 3. Plasmon energy obtained from spectroscopic ellipsometry measurements for TiN deposited without shield. A plasmon energy of 2.6 eV corresponds to stoichiometric TiN.
S. M€andl et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 663–667 2.50 -2
0.006
4 3
0.004
2 0.002 1 0 0
10
20
30
40
0.000 50
Gas Flow (sccm)
1.2
17
Pressure (Pa)
0.008 5
17
-2
Thickness (10 cm )
6
2.25 1.1
1.0
2.00
N/Ti Ratio
0.010
7
Atomic Layer Thickness (10 cm )
666
0.9 1.75
0.8 1.50 0
Fig. 4. AlN film thickness and working pressure for MePIIID with shield. The deposition time was always five minutes. The dotted line is a guide for the eye, while the full line indicates a linear relation between the gas flow and the working pressure.
5
10
15
20
25
30
Distance from Edge [mm]
Fig. 5. Lateral thickness and composition variation for TiN with 10 kV pulses, as determined with RBS.
4. Summary and conclusions orientation [23]. AlN absorbs nitrogen from the gas phase until a ratio of N =A ¼ 1 is reached. Nevertheless, the working pressure is approximately proportional to the gas flow, albeit with a strongly reduced absolute value (cf. pressure scale in Figs. 2 and 4), thus having rather no influence on the mean free path. So, the deposition rate is nearly independent of the gas flow. However, minor variations in conjunction with the plasma flow around the shield, caused by a change in the mean free path with increasing pressure, cannot be dismissed [22]. Again, no significant influence of the high voltage on the stoichiometry was observed. Until now, only the variation of the film properties in the center of the sample was discussed. Similar effects to the edge effect in PIII [24] can be expected during MePIIID, even further complicating the process control. As the ion incidence angle changes from normal to very shallow, it can be expected that the sputter rate and subsequent parameters will change from the center towards the edge. Fig. 5 shows the thickness variation and evolution of the N/Ti ratio across a 60 mm sample. The thickness is clearly decreasing towards the edge by about 20%, caused by the increased sputter yield from the obliquely impinging ions. At the same time, the composition is barely affected, with a slight trend towards a lower nitrogen content near the edge, which could be explained by a preferential sputtering dominating near the edge over the adsorption.
For ZnO, a strong influence of the pulse voltage on the oxygen content was found, indicating that preferential sputtering is the dominant process in this system. In contrast, the behavior of titania is mainly ruled by the oxygen adsorption from the gas phase, leading to a saturation near a O/Ti ratio of 1.7. A more gradual process was found for TiN, where a continuous variation of the N/Ti ratio was found, albeit again independent of the pulse voltage. Stoichiometric AlN was formed for all gas flow/arc current ratios, while the process pressure and the deposition rate increased with increasing gas flow. The effects of surface adsorption, chemical activation and preferential sputtering are present in all investigated systems, however with their relative contributions which strongly vary across them. It is necessary to adjust the process parameters accordingly to obtain the desired film structure. However, no general rules can be given at the moment for this procedure. At the same time, additional factors as the sample geometry can determine the lateral distribution of film properties.
References [1] J. Conrad, J.L. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4519. [2] J. Tendys, I.J. Donnelly, M.J. Kenny, J.T.A. Pollock, Appl. Phys. Lett. 53 (1988) 2143.
S. M€andl et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 663–667 [3] M.A. Lieberman, J. Appl. Phys. 66 (1989) 2926. [4] S. M€ andl, R. G€ unzel, W. M€ oller, J. Phys. D 31 (1998) 1109. [5] I.G. Brown, Appl. Phys. Lett. 58 (1991) 1392. [6] A. Anders, S. Anders, I.G. Brown, M.R. Dickinson, R.A. MacGill, J. Vac. Sci. Technol. B 12 (1994) 815. [7] A. Anders (Ed.), Handbook of Plasma Immersion Ion Implantation and Deposition, Wiley, New York, 2000, p. 282. [8] B. Rauschenbach, J.W. Gerlach, Cryst. Res. Technol. 35 (2000) 7. [9] P. Huber, D. Manova, S. M€andl, B. Rauschenbach, Surf. Coat. Technol. 156 (2002) 136. [10] M.M.M. Bilek, D.R. McKenzie, R.N. Tarrant, S.H.N. Lim, D.G. McCulloch, Surf. Coat. Technol., in press. [11] M. Plass, W. Fukarek, A. Kolitsch, U. Kreißig, Surf. Coat. Technol. 84 (1996) 383. [12] T.E. Parker, R. Kelly, J. Phys. Chem. Solids 36 (1975) 377. [13] G. Palasantzas, D.T.L. van Agterveld, J.Th.M. De Hosson, Appl. Surf. Sci. 191 (2002) 266.
667
[14] G.E. Jellison Jr., L.A. Boatner Jr., Phys. Rev. B 58 (1998) 3586. [15] J.H. Edgar, Properties of Group III Nitrides (INSPEC, 1994), p. 22. [16] G.V. Samaonov (Ed.), The Oxide Handbook, IFI/Plenum, New York, 1973, p. 316. [17] J.-E. Sundgren, Thin Solid Films 128 (1985) 21. [18] G. Thorwarth, S. M€andl, B. Rauschenbach, Surf. Coat. Technol. 128/129 (2000) 116. [19] H. Brandolf, US Patent 1985: 4-511-593. [20] P. Huber, D. Manova, S. M€andl, W. Assmann, B. Rauschenbach, in: H. Ryssel (Ed.), Ion Implantation Technology, IEEE Press, Piscataway, NJ, 2000, p. 508. [21] P. Huber, D. Manova, S. M€andl, B. Rauschenbach, Surf. Coat. Technol. 142–144 (2001) 418. [22] P. Huber, D. Manova, S. M€andl, B. Rauschenbach, Vacuum 69 (2003) 133. [23] D. Manova, P. Huber, S. M€andl, B. Rauschenbach, Surf. Coat. Technol. 142–144 (2001) 61. [24] S. M€andl, G. Thorwarth, P. Huber, S. Schoser, B. Rauschenbach, Surf. Coat. Technol. 139 (2001) 81.
Interplay of surface adsorption and preferential sputtering in metal plasma immersion ion implantation and deposition S. M€ andl a
a,*
, D. Manova
a,b
, B. Rauschenbach
a
Institut f€ur Oberfl€achenmodifizierung (IOM Leipzig), Permoserstr. 15, Leipzig 04303, Germany b Institut f€ur Experimentalphysik IV, Universit€at Augsburg, Augsburg 86135, Germany
Abstract Formation of hard ceramic surface layer by metal plasma immersion ion implantation and deposition (MePIIID) can be a quite complex process as quite a number of different species, including condensable metallic ions, electrons and neutral gas, which can be partially ionised, are interacting in the vacuum chamber and impinging on the surface. The resulting growth rate and chemical composition will be determined by the complex interplay of surface adsorption, ion implantation and preferential sputtering. Here, an overview of these effects is given for the important systems ZnO, TiO2 , TiN and AlN prepared by MePIIID. In ZnO, the predominant mechanism is preferential oxygen sputtering. However, the oxygen adsorption from the background gas is stronger in the case of TiO2 , leading to a constant Ti/O ratio beyond a threshold in the oxygen gas flow for this compound. This effect is less pronounced for TiN, where a continuously varying Ti/N ratio was found only for a varying gas flow and independent of the pulse voltage. In contrast, a constant Al/N ratio over a broad range of nitrogen gas flows was observed for AlN. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 52.77.Dq; 82.80.Yc; 79.20.Rf; 68.43.)h Keywords: Plasma immersion ion implantation; Sputtering; Adsorption; Rutherford backscattering spectroscopy
1. Introduction Plasma immersion ion implantation (PIII) was developed as a fast and cost-effective alternative to beamline implantations for complex-shaped three dimensional objects [1,2]. Here, an isotropic implantation into convex targets is possible for the quasi-static ions [3,4]. Extending PIII by using a metal ion plasma source in conjunction with the
*
Corresponding author. Tel.: +49-341-235-2944; fax: +49341-235-2313. E-mail address: [email protected] (S. M€andl).
negative high voltage pulses, predominantly a cathodic arc, results in metal plasma immersion ion implantation and deposition (MePIIID) [5,6]. Here, the plasmas can condense on substrates, i.e. film deposition occurs between the pulses, while a combination of implantation and deposition takes place during the pulses [7]. Regarding the process characteristics, we found MePIIID to be is closely related to ion beam assisted deposition (IBAD). It is possible to adjust the film texture by variation of the ion momentum [8,9] and the stress state by adjustment of the energy deposition [10]. Additionally, sputter saturation, often encountered in IBAD at high ion fluxes
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00816-4
S. M€andl et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 663–667
and energies of some 1 keV and above [11], is no major factor in MePIIID, due to a pulsed operation mode. However, additional parameters are present for MePIIID, which may influence the film deposition process and have to be accounted for. Preferential sputtering, well known from ion sputtering for elemental depth profiles in transition metal oxides [12], can be expected for an augmented effect in the ion energy range of 1–10 keV. Electrons impinging between the negative voltage pulses may lead to a surface activation [13] and, finally, neutral gas atoms, present at a higher density due to the increased working pressure compared to IBAD, interact at a higher rate with the surface and can be adsorbed in a considerable amount. Here, a qualitative overview, indicating the dominance of one or more of these effects, is given for the systems ZnO, AlN, TiO2 and TiN, which are important semiconductor materials [14,15] or surface layers for tribological and wear applications [16,17]. All materials were prepared by MePIIID in the same vacuum system at varying pulse voltages and gas flows.
2. Experiment Small silicon samples (diameter <60 mm) were treated in a 250 l cylindrical vacuum chamber, diameter 700 mm, height 650 mm and a base pressure better than 104 Pa, equipped with a vacuum cathodic arc [18]. No cleaning of the silicon to remove the native oxide layer prior to the treatment was performed. Employed cathode materials were titanium, zinc or aluminum (purity 99.99%), with the arc current between 70 and 100 A. Additionally, nitrogen or oxygen was introduced into the chamber at variable gas flows up to 50 sccm. The distance between the cathode surface and the sample was 39 cm. A simple shield was used for macroparticle filtering [19] (diameter 100 mm, mounted at a distance of 100 mm from the cathode) in the case of AlN and ZnO. No such device was necessary for TiN and TiO2 , where a macroparticle density of 0.1 mm2 (cut-off diameter 1 lm) or less were found using a scanning electron microscope [20].
High voltage pulses between 0 and )10 kV were applied with a length of 30 ls at a constant repetition rate of 3 kHz, yielding a duty cycle of 9%. Hence an average ion energy between 50–75 eV (without bias) and 1000–1500 eV (at )10 kV), depending on the average charge state in the plasma was obtained. The total deposition time was about 5 min in all cases. The elemental depth profiles were obtained either by elastic recoil detection analysis (ERDA) or Rutherford backscattering spectroscopy (RBS). The phase composition and texture were analysed using an X-ray facility. Spectroscopic ellipsometry (SE) was employed to determine the optical properties over the spectral range from 250 to 820 nm using a system equipped with an Xe arc lamp, rotating analyser and a diode array.
3. Results and discussion Textured films with the c-axis normal to the surface were obtained for ZnO, deposited at 100 A arc current and 50 sccm oxygen gas flow, as texture was independent of the bias voltage for the range between 0 and )10 kV. However, as the bias voltage is increased, the oxygen content of the films decreases from 49 to 47 at.% (ZnO0:96 to ZnO0:88 ), as shown in Fig. 1. This indicates a preferential oxygen sputtering, similar to the nuisance effect during sputter depth profiling of TiO2 with Ar, where Ti4þ is converted into Ti3þ [12]. At the same time, the optical adsorption increases
1.00
0.95
O/Zn Ratio
664
0.90
0.85
0.80 0
1
2
3
4
5
Pulse Voltage (kV)
Fig. 1. Oxygen/zinc ratio measured for ZnO films deposited with shield as a function of pulse voltage. The dotted line is a guide for the eye.
S. M€andl et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 663–667
approximately by a factor of 10 compared to the unbiased sample. This could be assigned to oxygen vacancies and additional point defects produced by the ion bombardment which are not annealed near room temperature. Albeit, no influence of the pulse voltage was observed on the stoichiometry for titania films produced by MePIIID in the same voltage range from 0 to )10 kV. The reason seems to be a very strong oxygen affinity of Ti, which leads to a strongly increased oxygen absorption rate from the gas phase. A comparison of the plasma density and the growth rate indicates that about 25% of the total atoms in the film, i.e. about 40% of the oxygen atoms, are not arriving as ions but as atoms [18]. The upper panel of Fig. 2 shows the oxygen-to-titanium ratio together with the working pressure at different flow/current ratios. The arc current was varied between 70 and 100 A, while the gas flow stayed between 40 and 50 sccm. The corresponding film thickness is presented in the lower panel. At low flow/current ratios the gettering efficiency of the freshly deposited titanium film is high enough to remove nearly all oxygen and reduce the working pressure to 0.01 Pa. For comparison, the working pressure for ZnO at 50 sccm was 0.03 Pa. The oxygen content of these films is below 60 at.%. With increasing oxygen flow, a saturation of the oxygen content is observed at a level of
1.4
0.05
-2 18
1.0
0.00
6 4 2 0 0.5
0.6
0.7
Gas Flow/Arc Current Ratio (sccm/A)
4.5
Plasmon Energy (eV)
0.10
1.6
1.2
Thickness (10 cm )
63 at.%, beyond which no further oxygen is adsorbed, leading to a higher working pressure, shorter mean free path and, hence, lower deposition rates. The varying deposition rates at a ratio of 0.5 sccm/A result from different absolute ion currents. For TiN, a similar effect was observed (see Fig. 3). Here, a wider range of gas flows between 20 and 50 sccm was used. As the change in the composition is less pronounced than for TiO2 , spectroscopic ellipsometry was employed to determine the screened plasmon energy, closely correlated with the nitrogen content [21]. These values are more accurate than results from RBS measurements. However, no linear relation between the plasmon energy and the composition exists over the whole range, so that only the plasmon energy is presented. Here, the saturation for high gas flows occurs at a plasmon energy of 2.95 eV, corresponding to a N/Ti ratio of 0.95, whereas the highest value of 4.15 eV corresponds to a N/Ti 0.5. While varying the pulse voltage, again no strong influence on the stoichiometry is found. At the same time, the texture evolves with pulse voltage, starting with (1 1 0) at 1 kV and changing to (1 0 0) for 3 kV and above [22]. As a last example, the behavior of AlN, deposited with a shield for droplet reduction, is presented in Fig. 4. Here, a fixed arc current of 100 A was used, while the gas flow was decreased down to 15 sccm. All films exhibit a strong c-axis preferential
0.15
1.8
Pressure (Pa)
O/Ti Ratio
2.0
665
0 kV 3 kV 7.5 kV
4.0
4 3.5
3.0
Stoichiometric TiN 2.5 0.2
Fig. 2. Oxygen/titanium ratio, film thickness and working pressure as a function of the gas flow/arc current ratio for MePIIID without shield. The deposition time was five minutes for each sample. The dotted and dashed lines are guides for the eye.
1 kV 5 kV 10 kV
0.3
0.4
0.5
0.6
0.7
Gas Flow/Arc Current Ratio (sccm/A)
Fig. 3. Plasmon energy obtained from spectroscopic ellipsometry measurements for TiN deposited without shield. A plasmon energy of 2.6 eV corresponds to stoichiometric TiN.
S. M€andl et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 663–667 2.50 -2
0.006
4 3
0.004
2 0.002 1 0 0
10
20
30
40
0.000 50
Gas Flow (sccm)
1.2
17
Pressure (Pa)
0.008 5
17
-2
Thickness (10 cm )
6
2.25 1.1
1.0
2.00
N/Ti Ratio
0.010
7
Atomic Layer Thickness (10 cm )
666
0.9 1.75
0.8 1.50 0
Fig. 4. AlN film thickness and working pressure for MePIIID with shield. The deposition time was always five minutes. The dotted line is a guide for the eye, while the full line indicates a linear relation between the gas flow and the working pressure.
5
10
15
20
25
30
Distance from Edge [mm]
Fig. 5. Lateral thickness and composition variation for TiN with 10 kV pulses, as determined with RBS.
4. Summary and conclusions orientation [23]. AlN absorbs nitrogen from the gas phase until a ratio of N =A ¼ 1 is reached. Nevertheless, the working pressure is approximately proportional to the gas flow, albeit with a strongly reduced absolute value (cf. pressure scale in Figs. 2 and 4), thus having rather no influence on the mean free path. So, the deposition rate is nearly independent of the gas flow. However, minor variations in conjunction with the plasma flow around the shield, caused by a change in the mean free path with increasing pressure, cannot be dismissed [22]. Again, no significant influence of the high voltage on the stoichiometry was observed. Until now, only the variation of the film properties in the center of the sample was discussed. Similar effects to the edge effect in PIII [24] can be expected during MePIIID, even further complicating the process control. As the ion incidence angle changes from normal to very shallow, it can be expected that the sputter rate and subsequent parameters will change from the center towards the edge. Fig. 5 shows the thickness variation and evolution of the N/Ti ratio across a 60 mm sample. The thickness is clearly decreasing towards the edge by about 20%, caused by the increased sputter yield from the obliquely impinging ions. At the same time, the composition is barely affected, with a slight trend towards a lower nitrogen content near the edge, which could be explained by a preferential sputtering dominating near the edge over the adsorption.
For ZnO, a strong influence of the pulse voltage on the oxygen content was found, indicating that preferential sputtering is the dominant process in this system. In contrast, the behavior of titania is mainly ruled by the oxygen adsorption from the gas phase, leading to a saturation near a O/Ti ratio of 1.7. A more gradual process was found for TiN, where a continuous variation of the N/Ti ratio was found, albeit again independent of the pulse voltage. Stoichiometric AlN was formed for all gas flow/arc current ratios, while the process pressure and the deposition rate increased with increasing gas flow. The effects of surface adsorption, chemical activation and preferential sputtering are present in all investigated systems, however with their relative contributions which strongly vary across them. It is necessary to adjust the process parameters accordingly to obtain the desired film structure. However, no general rules can be given at the moment for this procedure. At the same time, additional factors as the sample geometry can determine the lateral distribution of film properties.
References [1] J. Conrad, J.L. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4519. [2] J. Tendys, I.J. Donnelly, M.J. Kenny, J.T.A. Pollock, Appl. Phys. Lett. 53 (1988) 2143.
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