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Lateral implantation dose measurements of plasma immersion ion implanted non-planar samples
Nuclear Instruments and Methods in Physics Research B 112 (1996) 255-258
Beam Intwaotions wlth Materials 8 Atoms
ELSEVIER
Lateral implantation dose measurements of plasma immersion ion implanted non-planar samples
*,
J. Hartmann a* W. Ensinger a, R.W. Thomae b, H. Bender b, A. Kiiniger a, B. Stritzker a, B. Rauschenbach a a Uniuersiriir Augsburg, Institurfir Physik, 86159 Augsburg, Germuny b Uniuersiriif Frankfurt, Instirurfir Angewandre Physik, 60054 Frankfurt. Germany Abstract The homogeneity of plasma immersion ion implantation of non-planar samples was determined by Rutherford backscattering analysis. Half-cylinders made of carbon treated by argon ions served as a model system. The distribution of the implanted argon dose on the top plane and on the side plane of the samples was analysed. The results show a maximum dose at the centre of the top plane and a decrease towards the edges. On the side plane the dose increases from top to bottom. These variations were up to fifty percent. Additionally, a variation in implantation depth depending on the energy of the implanted ions and on the angle of incidence was observed. These effects are due to the sample size and the process parameters determining the dimensions of the plasma sheath which wraps the sample. A possible explanation for these phenomena is based on the assumption that on their way from the plasma to the sample the ions do not follow bent electrical field lines which occur near the edges of a sample.
1. Introduction In recent years plasma immersion ion implantation (PHI) or plasma source ion implantation has rapidly developed and attracted the attention both of the scientific community and potential industrial users [l-5]. The reason for this lies in the capability of the PI11 technique to treat complex-shaped samples. In contrast to conventional beam-line ion implantation where non-planar samples and/or the ion beam have to be manipulated for achieving uniform treatment, in the case of plasma immersion ion implantation the sample is inserted into the plasma and irradiated with ions from all directions simultaneously. This is achieved by biasing the sample with negative high voltage pulses. Ions from the plasma are accelerated in the electrical field and impinge onto the sample surface. An advantage of plasma immersion ion implantation is that it leads to an acceptable uniformity of implanted ions in non-planar samples. Therefore, the difference in implantation dose between the centre of a plane of a three-dimensional polygone sample and the places near edges is of particular interest. The effect of the shape of the sample and of the structure of the plasma sheath on the spatial distribution of the ions that impinge onto the surface has been studied
* Corresponding author. Fax +49 821 5983 425.
theoretically in previous papers [6- 101. Conrad et al. [ 111 measured the uniformity of implanted nitrogen into spherical Ti-6Al-4V samples and found that the implantation dose is within a tolerable range for non-semiconductor applications. Recently, Malik et al. [ 121 have analysed the nitrogen concentration in the vicinity of the edges of wedge-shaped samples following PI11 treatment. The smaller implantation doses at acute angle edges compared to rectangular edges are consistent with the theoretical predictions by Watterson [6]. In the present paper, a model system has been chosen in order to study the uniformity of the implantation dose depending on the distance from edges after PIII. Truncated carbon cylinders were treated by plasma immersion argon ion implantation. The homogeneity of the implantation dose was determined by scanning Rutherford backscattering analysis (RBS). The combination of argon ions and a carbon target has several advantages. First, there is a great difference between the masses of the ions implanted and the host material which allows a RBS analysis without a disturbing overlap of the two species in the spectrum. Another advantage is that argon forms only one single ionic species, in contrast to molecular gaseous species (e.g. nitrogen which forms both Nf and NT). Finally there is no chemical interaction between argon and carbon. Disturbing reactions with residual gas that may occur between reactive species are therefore eliminated.
0168-583X/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDf 0 168-583X(95)01 248-6
IX. ENGINEERING
256
J. Hartmnnn et al./Nucl.
Instr. and Meth. in Phys. Res. B 112 (1996) 255-258
2. Experimental
Energy (MeV) 0.62
Cylindrical rods of graphite with diameters of 10 mm and 50 mm, respectively, were cut to a length of 14 mm. Then the cylinders were cut in halves in order to obtain a flat plane. Top face and side plane were polished. The samples were treated in an argon plasma in the PI11 apparatus of Frankfurt University. The plasma is generated by means of a 13.56 MHz RF plasma source. The sample holder is located 20 cm from the aperture of the plasma generator and is connected to a high-voltage pulse generator which consists of a high voltage power supply and a pulse circuit with resistor, capacitor and spark gap. Details of the facility ate described elsewhere [ 131. The charge carrier density under the present conditions was 5 X lo8 cmL3. The base pressure of the PI11 chamber was 10e4 Pa, the process pressure was 3.5 X 10e2 Pa argon. The ions consisted mainly of singly charged argon; the number of multiply charged species was negligible. The sample was charged with high voltage pulses of 30 kV with a repetition rate of 5 Hz and a duration of about 5 ps. According to the discharge current of the capacitor and the process time, a charge of 6.8 X 10m2 C/cm2 passed through the samples. This charge consists of ions and secondary electrons. For the present study a PI11 process time of 1 h was chosen under the given parameters. The concentration of implanted argon was analysed at different positions by RBS with 1 MeV He+ ions. The detector angle was 170” and the beam spot diameter was 2 mm. The ions impinged onto the surface parallel to the surface normal. For quantification, the obtained spectra were simulated by RUMP code [I 41 and the measured concentrations of argon were normalised by the maximum implantation dose in the centre of the top plane.
3. Results Applying RBS with a spot diameter of 2 mm mapping of the implantation dose was possible. This made significant and reproducible gradients from the centre to the edges evident. In Fig. 1 typical RBS spectra of implanted argon are shown. The spectra were taken from three different positions along a line drawn from the centre to the edge of the top plane of the sample with 50 mm diameter. The amount of incorporated argon corresponding to the peak area decreases from the centre to the edge; the maximum dose in the center being 1.1 X lOI Ar/cm’. At the same time the profiles of the implanted argon ions shift to the surface. Fig. 2 shows schematically the top plane of the 10 mm sample with the positions of the spots analysed. The measured doses are included as percentages of the maximum dose. The maximum of the incorporated argon is located in the centre of the plane. In Fig. 3 the inhomogeneity of the implantation dose of the side plane can be
600
620
0.64
640
0.66
0.68
660
680
700
Channel Fig. 1. Argon peaks obtained from RBS spectra taken along a line from centre to edge of the top plane of the sample with 50 mm
diameter. seen. The concentration of argon increases from top to bottom and from the edges to the centre. Fig. 4 shows the top plane of the sample with 50 mm diameter. It can be assumed that the implantation process has occurred symmetrically. Therefore analysing just one half is sufficient to obtain full information on the whole sample. The amount of implanted argon is indicated by lines which reflect regions with the same argon concentration. The result resembles the one of the smaller sample (10 mm diameter), however, with the gradient in concentration of argon decreasing to a minimum of 50%. The region near the half-cylindrical edge received only half the argon concentration of the position near the straight edge which gained the maximum amount. Fig. 5 shows the percentage of the dose of incorporated argon along the third line from the bottom in Fig. 4 (see arrow) as a function of the distance from the centre of the sample. The concentration decreases from 90% down to 60% of the maximum implantation concentration. Along with the reduction of argon dose goes a decrease in the implantation depth of the ions. The dependence of the depth of the concentration profile maximum on the distance from the centre along the same line (arrow in Fig.
/
=_
-W
* 10 mm
Fig. 2. Top plane of sample with 10 mm diameter with different positions of RBS analysis. The implantation dose is given as percentage of the maximum incorporated argon concentration (in the centre of the sample).
J. Hartmann
et al./
251
Nucl. Instr. and Meth. in Phys. Res. B 112 (1996) 2.X5-258 100
I__~___._
8
*O-
;
‘O-
;
60-
E @
50-
10 mm Fig. 3. Side plane of sample with different positions of RBS analysis. The implantation dose given in percent is normalised by the maximum incorporated argon concentration (in the centre of the top plane of the sample).
4) is shown in Fig. 6. It turns out that the ions near the edge are implanted into shallower depths than the ones in the centre. The maximum implantation depth assigned to argon ions which have gained the full acceleration energy corresponds well with the data calculated by TRIM code [ 151for 30 keV argon ions in carbon. Ibis can be expected in case of a collisionless transport of the ions from the sheath to the sample. This is apparently possible when the background pressure is low enough, as in the present case. Hence, from these experimental results the following conclusions may be drawn: 1. Near the edges of the samples lower implantation doses are observed than at the center. 2. The implantation depth in the vicinity of edges is reduced (shallower implantation) compared to the centre part of the planes.
0
10 5 15 distance fromcentre [mm)
20
25
Fig. 5. Normali& dose as a function of the distance from the centre of the top plane of the sample.
treated by plasma immersion ion implantation can be understood qualitatively in terms of a misalignment of the electrical field lines and the ion trajectories. In this process an electrical field is built up between the plasma and the sample. This field can be characterised by field lines which enter the sample surface at a right angle. When accelerated in the field the ions travel along the field lines from the plasma sheath to the sample. Due to their inertia they are not able to follow the field lines when these are bent. This is the case near the edges of the sample. As a result, less
4. Discussion Recently, Malik et al. [12] have also measured a lower implantation dose of nitrogen in silicon in the vicinity of acute angle egdes compared to the planar faces after PI11 treatment. Faehl et al. 191,however, detected an increased concentration near the edges of long cylindrical objects. The observed gradient both in implantation dose and implantation depth on the surface of a non-planar sample
25 mm 80% 90% 100% 90% 0
50mm
Fig. 4. Top plane of sample with 50 mm diameter with different positions of RBS analysis indicated as spots. Lines along a constant argon concentration are included (iso-intensity lines).
5
10
15
20
25
distance from center [mm]
Fig. 6. Depth of maximum of the argon concentration peak as a function of distance from the centre. The result of a TRIM calculation [ 151is included.
IX. ENGINEERING
258
J. Hamnann et al./Nucl.
Instr. and Meth. in Phys. Res. B 112 (1996) 255-258
ions impinge onto the region near the edges. This explains the increasing depletion of ions along the sample surface from the centre to the edges. The reduced penetration depth into the solid can be explained by three effects. One is that due to the above mentioned effects fewer high energy ions reach the outer area. Another effect is that with increasing deviation of the angle of ion incidence from the right angle the sputter coefficient increases. Enhanced sputter etching shifts the place where the ions come to rest in the solid towards the surface. The same result is obtained geometrically when the angle of ion incidence is changed. Angles smaller than the right angle lead geometrically to a more shallow position of the implanted ions. To date it is not yet clear which of these effects is most decisive for the observed shift in the implantation profile. The above discussed explanation is supported by the theoretical work of Sheridan and Alport [16]. They have calculated the structure of the plasma sheath around a square bar. They found that the electrical field lines are nearly radial in the outer regions of the sheath. When the ions are accelerated towards the target, they will not reach the comer of the bar when they have traversed the sheath because their momentum will carry them further in direction of the centre of the bar instead along the bent field lines. These conditions are valid in the present experiment where the sheath width, calculated according to the theory of Sheridan and ‘Alport, is around 18 cm which is far beyond the sample dimensions.
5. Summarising remarks The above described results have shown that plasma immersion ion implantation at fairly low pressure and medium ion density does not allow a homogenous treatment of three-dimensional samples. Size and shape both of sample and plasma sheath determine how the ions are implanted into the material. The treatment of the centre parts of the samples corresponds best to conventional beam-line ion implantation with ion incidence at a right angle. Closer to the edges, however, both implantation dose and depth differ from the values in the centre. Although to date the reasons for this observation are not evident, the assumption can be raised that the shape of the electrical field in which the ions are accelerated is of
special importance. Further studies on the dependence of the inhomogeneity on the process conditions and sample dimensions are required. Whether or not the gradients in dose are tolerable depends on the particular application. For tribological applications deviations up to 50% might still be acceptable; for doping in the semiconductor field, if non-planar specimens have to be treated, the deviations might be intolerable.
Acknowledgements The present work was supported by the Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie under contract number 13N6441.
References [II J.R. Conrad, J. Appl. Phys. 62 (1987) 777. Dl I. Tendys, I.J. DonneIIy, M.J. Kenny and J.T.A. Pollock, Appl. Phys. Lett. 53 (1988) 2143. (31 N.W. Cheung, Nucl. Instr. and Meth. B 55 (1991) 811. [41 J.N. Matossian, J. Vat. Sci. Technol. B 12 (1994) 850. [51 W. Ensinger, J. Hartmann, R.W. Thomae, A. KBniger, B. Snitzker and B. Rauschenbach, Surf. Coat. Technol., in press. h51P.A. Watterson, J. Phys. D 22 (1989) 1300. [71 J.T. Scheuer, M. Shamin and J.R. Conrad, J. Appl. Phys. 67 (1990) 1241. [81 T.E. Sheridan and M.J. Alport, Appl. Phys. Lett. 64 (1994) 1783. [91 R. Faehl, B. De Volder and B. Wood, J. Vat. Sci. Technol. B 12 (1994) 884. [lOI D. Wang, T. Ma and X. Deng, I. Vat. Sci. Technol. B 12 (1994) 905. [I 11 J.R. Conrad, S. Baumann, R. Fleming and G.P. Meeker, J. Appl. Phys. 65 (1989) 1707. 1121S.M. Malik, D.E. Muller, K. Sridharan, R.P. Fetberston, N. Tran and I. Conrad, J. Appl. Phys. 77 (1995) 1015. [131 R.W. Thomae, B. Seiler, H. Bender, J. Brutscher, R. Giinzel, J. Halder, H. Klein, J. Mtlller and M. Sarstedt, Nucl. Instr. and Meth. B 99 (1995) 569. 1141L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. D51 J.F. Ziegler, J.B. Biersack and U. Littmark, The stopping and range of ions in solids (Pergamon, New York, 1985). T.E. Sheridan and M.J. Alport, J. Vat. Sci. Technol. B 12 [I61 (1994) 897.
Beam Intwaotions wlth Materials 8 Atoms
ELSEVIER
Lateral implantation dose measurements of plasma immersion ion implanted non-planar samples
*,
J. Hartmann a* W. Ensinger a, R.W. Thomae b, H. Bender b, A. Kiiniger a, B. Stritzker a, B. Rauschenbach a a Uniuersiriir Augsburg, Institurfir Physik, 86159 Augsburg, Germuny b Uniuersiriif Frankfurt, Instirurfir Angewandre Physik, 60054 Frankfurt. Germany Abstract The homogeneity of plasma immersion ion implantation of non-planar samples was determined by Rutherford backscattering analysis. Half-cylinders made of carbon treated by argon ions served as a model system. The distribution of the implanted argon dose on the top plane and on the side plane of the samples was analysed. The results show a maximum dose at the centre of the top plane and a decrease towards the edges. On the side plane the dose increases from top to bottom. These variations were up to fifty percent. Additionally, a variation in implantation depth depending on the energy of the implanted ions and on the angle of incidence was observed. These effects are due to the sample size and the process parameters determining the dimensions of the plasma sheath which wraps the sample. A possible explanation for these phenomena is based on the assumption that on their way from the plasma to the sample the ions do not follow bent electrical field lines which occur near the edges of a sample.
1. Introduction In recent years plasma immersion ion implantation (PHI) or plasma source ion implantation has rapidly developed and attracted the attention both of the scientific community and potential industrial users [l-5]. The reason for this lies in the capability of the PI11 technique to treat complex-shaped samples. In contrast to conventional beam-line ion implantation where non-planar samples and/or the ion beam have to be manipulated for achieving uniform treatment, in the case of plasma immersion ion implantation the sample is inserted into the plasma and irradiated with ions from all directions simultaneously. This is achieved by biasing the sample with negative high voltage pulses. Ions from the plasma are accelerated in the electrical field and impinge onto the sample surface. An advantage of plasma immersion ion implantation is that it leads to an acceptable uniformity of implanted ions in non-planar samples. Therefore, the difference in implantation dose between the centre of a plane of a three-dimensional polygone sample and the places near edges is of particular interest. The effect of the shape of the sample and of the structure of the plasma sheath on the spatial distribution of the ions that impinge onto the surface has been studied
* Corresponding author. Fax +49 821 5983 425.
theoretically in previous papers [6- 101. Conrad et al. [ 111 measured the uniformity of implanted nitrogen into spherical Ti-6Al-4V samples and found that the implantation dose is within a tolerable range for non-semiconductor applications. Recently, Malik et al. [ 121 have analysed the nitrogen concentration in the vicinity of the edges of wedge-shaped samples following PI11 treatment. The smaller implantation doses at acute angle edges compared to rectangular edges are consistent with the theoretical predictions by Watterson [6]. In the present paper, a model system has been chosen in order to study the uniformity of the implantation dose depending on the distance from edges after PIII. Truncated carbon cylinders were treated by plasma immersion argon ion implantation. The homogeneity of the implantation dose was determined by scanning Rutherford backscattering analysis (RBS). The combination of argon ions and a carbon target has several advantages. First, there is a great difference between the masses of the ions implanted and the host material which allows a RBS analysis without a disturbing overlap of the two species in the spectrum. Another advantage is that argon forms only one single ionic species, in contrast to molecular gaseous species (e.g. nitrogen which forms both Nf and NT). Finally there is no chemical interaction between argon and carbon. Disturbing reactions with residual gas that may occur between reactive species are therefore eliminated.
0168-583X/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDf 0 168-583X(95)01 248-6
IX. ENGINEERING
256
J. Hartmnnn et al./Nucl.
Instr. and Meth. in Phys. Res. B 112 (1996) 255-258
2. Experimental
Energy (MeV) 0.62
Cylindrical rods of graphite with diameters of 10 mm and 50 mm, respectively, were cut to a length of 14 mm. Then the cylinders were cut in halves in order to obtain a flat plane. Top face and side plane were polished. The samples were treated in an argon plasma in the PI11 apparatus of Frankfurt University. The plasma is generated by means of a 13.56 MHz RF plasma source. The sample holder is located 20 cm from the aperture of the plasma generator and is connected to a high-voltage pulse generator which consists of a high voltage power supply and a pulse circuit with resistor, capacitor and spark gap. Details of the facility ate described elsewhere [ 131. The charge carrier density under the present conditions was 5 X lo8 cmL3. The base pressure of the PI11 chamber was 10e4 Pa, the process pressure was 3.5 X 10e2 Pa argon. The ions consisted mainly of singly charged argon; the number of multiply charged species was negligible. The sample was charged with high voltage pulses of 30 kV with a repetition rate of 5 Hz and a duration of about 5 ps. According to the discharge current of the capacitor and the process time, a charge of 6.8 X 10m2 C/cm2 passed through the samples. This charge consists of ions and secondary electrons. For the present study a PI11 process time of 1 h was chosen under the given parameters. The concentration of implanted argon was analysed at different positions by RBS with 1 MeV He+ ions. The detector angle was 170” and the beam spot diameter was 2 mm. The ions impinged onto the surface parallel to the surface normal. For quantification, the obtained spectra were simulated by RUMP code [I 41 and the measured concentrations of argon were normalised by the maximum implantation dose in the centre of the top plane.
3. Results Applying RBS with a spot diameter of 2 mm mapping of the implantation dose was possible. This made significant and reproducible gradients from the centre to the edges evident. In Fig. 1 typical RBS spectra of implanted argon are shown. The spectra were taken from three different positions along a line drawn from the centre to the edge of the top plane of the sample with 50 mm diameter. The amount of incorporated argon corresponding to the peak area decreases from the centre to the edge; the maximum dose in the center being 1.1 X lOI Ar/cm’. At the same time the profiles of the implanted argon ions shift to the surface. Fig. 2 shows schematically the top plane of the 10 mm sample with the positions of the spots analysed. The measured doses are included as percentages of the maximum dose. The maximum of the incorporated argon is located in the centre of the plane. In Fig. 3 the inhomogeneity of the implantation dose of the side plane can be
600
620
0.64
640
0.66
0.68
660
680
700
Channel Fig. 1. Argon peaks obtained from RBS spectra taken along a line from centre to edge of the top plane of the sample with 50 mm
diameter. seen. The concentration of argon increases from top to bottom and from the edges to the centre. Fig. 4 shows the top plane of the sample with 50 mm diameter. It can be assumed that the implantation process has occurred symmetrically. Therefore analysing just one half is sufficient to obtain full information on the whole sample. The amount of implanted argon is indicated by lines which reflect regions with the same argon concentration. The result resembles the one of the smaller sample (10 mm diameter), however, with the gradient in concentration of argon decreasing to a minimum of 50%. The region near the half-cylindrical edge received only half the argon concentration of the position near the straight edge which gained the maximum amount. Fig. 5 shows the percentage of the dose of incorporated argon along the third line from the bottom in Fig. 4 (see arrow) as a function of the distance from the centre of the sample. The concentration decreases from 90% down to 60% of the maximum implantation concentration. Along with the reduction of argon dose goes a decrease in the implantation depth of the ions. The dependence of the depth of the concentration profile maximum on the distance from the centre along the same line (arrow in Fig.
/
=_
-W
* 10 mm
Fig. 2. Top plane of sample with 10 mm diameter with different positions of RBS analysis. The implantation dose is given as percentage of the maximum incorporated argon concentration (in the centre of the sample).
J. Hartmann
et al./
251
Nucl. Instr. and Meth. in Phys. Res. B 112 (1996) 2.X5-258 100
I__~___._
8
*O-
;
‘O-
;
60-
E @
50-
10 mm Fig. 3. Side plane of sample with different positions of RBS analysis. The implantation dose given in percent is normalised by the maximum incorporated argon concentration (in the centre of the top plane of the sample).
4) is shown in Fig. 6. It turns out that the ions near the edge are implanted into shallower depths than the ones in the centre. The maximum implantation depth assigned to argon ions which have gained the full acceleration energy corresponds well with the data calculated by TRIM code [ 151for 30 keV argon ions in carbon. Ibis can be expected in case of a collisionless transport of the ions from the sheath to the sample. This is apparently possible when the background pressure is low enough, as in the present case. Hence, from these experimental results the following conclusions may be drawn: 1. Near the edges of the samples lower implantation doses are observed than at the center. 2. The implantation depth in the vicinity of edges is reduced (shallower implantation) compared to the centre part of the planes.
0
10 5 15 distance fromcentre [mm)
20
25
Fig. 5. Normali& dose as a function of the distance from the centre of the top plane of the sample.
treated by plasma immersion ion implantation can be understood qualitatively in terms of a misalignment of the electrical field lines and the ion trajectories. In this process an electrical field is built up between the plasma and the sample. This field can be characterised by field lines which enter the sample surface at a right angle. When accelerated in the field the ions travel along the field lines from the plasma sheath to the sample. Due to their inertia they are not able to follow the field lines when these are bent. This is the case near the edges of the sample. As a result, less
4. Discussion Recently, Malik et al. [12] have also measured a lower implantation dose of nitrogen in silicon in the vicinity of acute angle egdes compared to the planar faces after PI11 treatment. Faehl et al. 191,however, detected an increased concentration near the edges of long cylindrical objects. The observed gradient both in implantation dose and implantation depth on the surface of a non-planar sample
25 mm 80% 90% 100% 90% 0
50mm
Fig. 4. Top plane of sample with 50 mm diameter with different positions of RBS analysis indicated as spots. Lines along a constant argon concentration are included (iso-intensity lines).
5
10
15
20
25
distance from center [mm]
Fig. 6. Depth of maximum of the argon concentration peak as a function of distance from the centre. The result of a TRIM calculation [ 151is included.
IX. ENGINEERING
258
J. Hamnann et al./Nucl.
Instr. and Meth. in Phys. Res. B 112 (1996) 255-258
ions impinge onto the region near the edges. This explains the increasing depletion of ions along the sample surface from the centre to the edges. The reduced penetration depth into the solid can be explained by three effects. One is that due to the above mentioned effects fewer high energy ions reach the outer area. Another effect is that with increasing deviation of the angle of ion incidence from the right angle the sputter coefficient increases. Enhanced sputter etching shifts the place where the ions come to rest in the solid towards the surface. The same result is obtained geometrically when the angle of ion incidence is changed. Angles smaller than the right angle lead geometrically to a more shallow position of the implanted ions. To date it is not yet clear which of these effects is most decisive for the observed shift in the implantation profile. The above discussed explanation is supported by the theoretical work of Sheridan and Alport [16]. They have calculated the structure of the plasma sheath around a square bar. They found that the electrical field lines are nearly radial in the outer regions of the sheath. When the ions are accelerated towards the target, they will not reach the comer of the bar when they have traversed the sheath because their momentum will carry them further in direction of the centre of the bar instead along the bent field lines. These conditions are valid in the present experiment where the sheath width, calculated according to the theory of Sheridan and ‘Alport, is around 18 cm which is far beyond the sample dimensions.
5. Summarising remarks The above described results have shown that plasma immersion ion implantation at fairly low pressure and medium ion density does not allow a homogenous treatment of three-dimensional samples. Size and shape both of sample and plasma sheath determine how the ions are implanted into the material. The treatment of the centre parts of the samples corresponds best to conventional beam-line ion implantation with ion incidence at a right angle. Closer to the edges, however, both implantation dose and depth differ from the values in the centre. Although to date the reasons for this observation are not evident, the assumption can be raised that the shape of the electrical field in which the ions are accelerated is of
special importance. Further studies on the dependence of the inhomogeneity on the process conditions and sample dimensions are required. Whether or not the gradients in dose are tolerable depends on the particular application. For tribological applications deviations up to 50% might still be acceptable; for doping in the semiconductor field, if non-planar specimens have to be treated, the deviations might be intolerable.
Acknowledgements The present work was supported by the Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie under contract number 13N6441.
References [II J.R. Conrad, J. Appl. Phys. 62 (1987) 777. Dl I. Tendys, I.J. DonneIIy, M.J. Kenny and J.T.A. Pollock, Appl. Phys. Lett. 53 (1988) 2143. (31 N.W. Cheung, Nucl. Instr. and Meth. B 55 (1991) 811. [41 J.N. Matossian, J. Vat. Sci. Technol. B 12 (1994) 850. [51 W. Ensinger, J. Hartmann, R.W. Thomae, A. KBniger, B. Snitzker and B. Rauschenbach, Surf. Coat. Technol., in press. h51P.A. Watterson, J. Phys. D 22 (1989) 1300. [71 J.T. Scheuer, M. Shamin and J.R. Conrad, J. Appl. Phys. 67 (1990) 1241. [81 T.E. Sheridan and M.J. Alport, Appl. Phys. Lett. 64 (1994) 1783. [91 R. Faehl, B. De Volder and B. Wood, J. Vat. Sci. Technol. B 12 (1994) 884. [lOI D. Wang, T. Ma and X. Deng, I. Vat. Sci. Technol. B 12 (1994) 905. [I 11 J.R. Conrad, S. Baumann, R. Fleming and G.P. Meeker, J. Appl. Phys. 65 (1989) 1707. 1121S.M. Malik, D.E. Muller, K. Sridharan, R.P. Fetberston, N. Tran and I. Conrad, J. Appl. Phys. 77 (1995) 1015. [131 R.W. Thomae, B. Seiler, H. Bender, J. Brutscher, R. Giinzel, J. Halder, H. Klein, J. Mtlller and M. Sarstedt, Nucl. Instr. and Meth. B 99 (1995) 569. 1141L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. D51 J.F. Ziegler, J.B. Biersack and U. Littmark, The stopping and range of ions in solids (Pergamon, New York, 1985). T.E. Sheridan and M.J. Alport, J. Vat. Sci. Technol. B 12 [I61 (1994) 897.