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Sheath formation and ion flux distribution inside the trench in plasma-based ion implantation
Nuclear Instruments and Methods in Physics Research B 206 (2003) 772–776 www.elsevier.com/locate/nimb
Sheath formation and ion flux distribution inside the trench in plasma-based ion implantation T. Ikehata
a,*
, K. Shioya a, T. Araki a, N.Y. Sato a, H. Mase a, K. Yukimura
b
a
Department of Electrical and Electronic Engineering, Ibaraki University, 4-12-1 Nakanarusawa-cho, Hitachi, Ibaraki 316-8511, Japan b Department of Electrical Engineering, Doshisha University, 1-3 Tatara, Kyotanabe, Kyoto 610-0321, Japan
Abstract Geometrical effects of a three-dimensional workpiece on the plasma-based ion implantation have been studied using trench-shape and L-shape workpieces. Temporal and spatial evolution of the sheath and the ion flux on the workpiece are measured for a negative voltage pulse of )1.8 to )7.0 kV, 40 ls: (1) The trench is occupied by the ion sheath quickly due to the sheath overlapping from both side walls; this effect is not seen in the L-shape workpiece with only one side wall. (2) Electrons are detected inside the trench even after the ion sheath fills the trench, which is attributed to secondary electrons emitted from the surface of the workpiece by ion impact and trapped electrostatically between the side walls. (3) The ion flux incident upon the inner surface of the trench is strongly enhanced because trapped secondary electrons ionize the filling gas as in the hollow-cathode discharge. The enhanced ionization is therefore not seen in Lshape and planar workpieces. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 52.77.Dq; 52.40.Kh; 52.80.Pi; 52.70.Ds Keywords: Plasma-based ion implantation; Three-dimensional workpiece; Ion sheath; Ion flux; Secondary electron emission; Hollowcathode effect
1. Introduction A principal advantage of plasma-based ion implantation (PBII) over the accelerator-based ion implantation is its ability of performing conformal ion implantation into a three-dimensional workpiece. However, conditions of the thin sheath and/ or the high density plasma are necessary to be
*
Corresponding author. Tel.: +81-294-38-5102; fax: +81294-38-5111. E-mail address: [email protected] (T. Ikehata).
established. Although theoretical and numerical investigations on the sheath evolution for a round hole on a plate [1], convex and concave wedges [2], and a trench [3] have been reported so far, there are few experiments on the same subject [4]. We examined temporal and spatial behaviors of the sheath and the ion flux for trench-shape, L-shape and planar workpieces. In particular, electrostatic trapping of secondary electrons, followed by enhanced ionization of the filling gas was clearly identified inside the trench. Our observations are quite similar to those by Cluggish and Munson [5] where the sheath overlapping followed by
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00846-2
T. Ikehata et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 772–776
the secondary electron enhanced discharge was observed among racks of fake pistons arranged in a large vacuum chamber.
2. Experiment 2.1. Arrangement Arrangement of the experiment was described in detail in the literature [6]. The argon plasma is generated in a pressure of 0.1 Pa by the inductively coupled discharge at a net power of 100 W and a frequency of 140 MHz. The vacuum chamber is 20 cm in diameter and 30 cm in length. Model workpieces made of aluminum are connected to a high-voltage feedthrough which is inserted from the bottom end. Schematic diagrams of model workpieces are shown in Fig. 1. The trench of the workpiece has 30 mm in width, 50 mm in depth and 60 mm in length. The L-shape and planar workpieces are made by removing the side walls of the trench-shape workpiece. The plasma density is typically 5 1015 m3 with an electron temperature of 7 eV between z ¼ 10 and 70 mm for the planar workpiece though the density inside the trench (at z ¼ 25 mm) decreases to 40% of that of the planar workpiece. A negative voltage pulse of )1.8 to )7.0 kV and with a duration of 40 ls is applied from a pulse modulator to the workpiece at a rate
773
of up to 50 pps. The sheath dynamics is derived from temporal and spatial characteristics of a movable Langmuir probe biased to +80 V dc. In order to obtain the ion flux distribution along the surface of the workpiece, charge collector plates (0.1 15 50 mm, stainless steel) are pasted on the top, side and bottom surfaces of the trenchshape workpiece with insertion of thin Teflon sheets for electrical insulation. Each collector plate is connected by a lead to the modulator and the current through it is measured by a current probe with an amplifier. Three collector plates are used in trench-shape and L-shape workpieces but only the bottom collector plate is used in the planar workpiece. 2.2. Development and collapse of the ion sheath Fig. 2 shows temporal ion sheath evolution for three types of workpiece at an applied voltage of )7.0 kV. In the case of the planar workpiece, the sheath thickness s increases to 30 mm in 20 ls and evolves quite similar to the theoretical curve of LiebermanÕs model in which a matrix sheath thickness is 12 mm and a stationary sheath thickness is 40 mm. In the case of the trench-shape workpiece, on the other hand, s reaches up to 80 mm in 20 ls; that is, a trench with a depth of 50 mm is filled with the ion sheath almost simultaneously with the voltage application and
Fig. 1. Three types of workpiece tested: (a) trench-shape, (b) L-shape and (c) planar workpieces. Collector plates for ion current measurement are also shown.
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T. Ikehata et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 772–776
Fig. 3. Plasma recovery time as a function of the amplitude of voltage pulse.
Fig. 2. Sheath thickness as a function of time, together with the voltage waveform.
thereafter, the sheath edge further extends beyond the entrance to the trench as if the planar sheath evolves. This phenomenon is attributable to the sheath overlapping effect from both side walls as previously reported by Cluggish and Munson [5] in experiments at the large LANL device. The Lshape workpiece gives an intermediate result between trench-shape and planar workpieces: At x ¼ 0, the sheath behavior is similar to that of the trench-shape workpiece probably due to influence of the side wall, while it is similar to that of the planar workpiece at x ¼ 25 mm because influence of the side wall is negligible there. As the voltage pulse terminates, plasma electrons come back immediately so as to recover the charge neutrality and the sheath collapses. However, the initial plasma density cannot be recovered in a short time because massive ions cannot respond as rapidly as electrons. We define the plasma recovery time as the time period when the plasma density recovers up to 90% of its initial value. In Fig. 3, the plasma recovery time (at z ¼ 5 mm) is compared among trench-shape, L-shape and planar workpieces. For the trench-shape workpiece, the time period exceeding 100 ls is necessary for the replenishment which is five times longer than those of L-shape and planar workpieces. The time dependency for the replenishment
is well correlated to the ion sheath evolution shown in Fig. 2. The plasma recovery time is an important factor determining the maximum repetition rate of PBII. Therefore, this result suggests that the repetition rate of the voltage application in PBII is severely influenced by the geometry of the workpiece such as trench and hole. 2.3. Secondary electrons in the trench Fig. 4(a) shows waveforms of the Langmuir probe (dc bias is +80 V) in the trench-shape workpiece. The pulse voltage is )7.0 kV. A considerable amount of the electron current is detected at z ¼ 4, 20, 28 and 36 mm even inside the trench where plasma electrons are completely excluded. On the other hand, no electron current is detected at z ¼ 60 mm outside the trench (but inside the sheath boundary). Furthermore, it was noticed that the electron current decreased with decreasing the amplitude of the voltage pulse and became negligibly small at V ¼ 1:8 kV. From these results, we conclude that they are secondary electrons emitted from the surface of the workpiece by impact of ions and trapped in an electrostatic potential well formed between the side walls of the trench. This conclusion is supported by the fact that the secondary emission coefficient by ion impact at the ion energy greater than 1 keV increases with the square root of the ion energy [7]. Fig. 4(b) shows the electron current as a function
T. Ikehata et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 772–776
775
Fig. 4. (a) Probe current waveforms showing secondary electrons. (b) Distribution of secondary electron current in three workpieces.
of the distance z for three different workpieces. A remarkably large current is observed only in the trench-shape workpiece which is able to form the potential well for trapping electrons. In addition, the electron current has a peak at the position of z ¼ 12 mm inside the trench and sharply decreases toward the outside of the trench. Therefore, it is clear that the electrostatic trapping of secondary electrons is essential for the probe current increase in the trench. 2.4. Ion flux distribution along the surface of the workpiece The ion flux was measured on the top, side and bottom surfaces of the trench-shape workpiece using collector plates described in Section 2.1. Results are shown in Fig. 5 together with data of the planar workpiece. Fig. 5(a) shows waveforms of the voltage pulse (V ¼ 7:0 kV) and the col-
Fig. 5. (a) Collector current waveforms with and without a plasma shield: they show the displacement current and the ion current (including secondary electrons), respectively. Temporal behavior of each collector current normalized by the top collector current at (b) V ¼ 1:8 kV and (c) V ¼ 7:0 kV.
lector current with and without a plasma shield. The plasma shield means that the workpiece is covered doubly by polymer and aluminum foils to cut the convection current into the workpiece off. Although the displacement current appears at rising and falling portions of the voltage pulse, its contribution is negligible (less than 5% of the total collector current in 5 ls). Figs. 5(b) and (c) show
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T. Ikehata et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 772–776
temporal evolution of the normalized collector current on the side and bottom surfaces of the trench and on the planar workpiece. The normalization is carried out against the top collector current. At an applied voltage of )1.8 kV, the side and bottom currents change similar (Fig. 5(b)). They have a value of 0.5 at 5 ls and decreases to about 1/3 at the time of the voltage termination. Furthermore, these side and bottom currents are less than that of the planar workpiece. This gives an evidence that the three-dimensional geometry of the workpiece influences the sheath evolution and consequently the distribution of the ion current: The top surface of the trench-shape workpiece can be approximated by the outer surface of a cylinder so that the thickness of the sheath on it becomes less than that on the planar workpiece. On the other hand, a deep portion of side and bottom surfaces of the trench can be approximated by the inner surface of the cylinder, so that the sheath thickness on it becomes more than that on the planar workpiece. Fig. 5(c) shows the same data as Fig. 5(b) except the applied voltage of )7.0 kV. We note that the normalized currents on side and bottom surfaces are larger than the case of )1.8 kV though the sheath thickness should become greater at )7.0 kV. This observation may be attributed to enhanced ionization of the filling gas by trapped secondary electrons (in other words, the hollowcathode effect) as discussed in Section 2.3, which is another evidence for geometrical effects of the workpiece.
3. Conclusions Temporal and spatial evolution of the sheath and the ion flux on trench-shape and L-shape workpieces were investigated together with those of the planar workpiece for comparison. Only the trench-shape workpiece shows a rapid expansion of the sheath by the sheath overlapping effect. Electrostatic trapping of secondary electrons was identified in the trench-shape workpiece but not in the L-shape workpiece. These trapped electrons served to increase the ion flux inside the trench by enhanced ionization of the filling gas. To summarize, we reported the formation of the potential well and the associated phenomena inside the trench as one of geometrical effects of the workpiece.
References [1] T.E. Sheridan, Plasma Sources Sci. Technol. 4 (1995) 527. [2] I.J. Donnelly, P.A. Watterson, J. Phys. D: Appl. Phys. 22 (1989) 90; P.A. Watterson, J. Phys. D: Appl. Phys. 22 (1989) 1300. [3] T.E. Sheridan, J. Phys. D: Appl. Phys. 28 (1995) 1094. [4] M. Sano, T. Teramoto, K. Yukimura, T. Maruyama, Surf. Coat. Technol. 136 (2001) 168. [5] B.P. Cluggish, C.P. Munson, J. Appl. Phys. 84 (1998) 5937, 84 (1998) 5945. [6] T. Ikehata, T. Araki, K. Shimatsu, K. Shioya, N.Y. Sato, K. Yukimura, The Papers of Technical Meeting on Plasma Science and Technology, PST-02-41, 2002, p. 87 (in Japanese). [7] M.M. Shamin, J.T. Scheuer, R.P. Fetherton, J.R. Conrad, J. Appl. Phys. 70 (1991) 4756.
Sheath formation and ion flux distribution inside the trench in plasma-based ion implantation T. Ikehata
a,*
, K. Shioya a, T. Araki a, N.Y. Sato a, H. Mase a, K. Yukimura
b
a
Department of Electrical and Electronic Engineering, Ibaraki University, 4-12-1 Nakanarusawa-cho, Hitachi, Ibaraki 316-8511, Japan b Department of Electrical Engineering, Doshisha University, 1-3 Tatara, Kyotanabe, Kyoto 610-0321, Japan
Abstract Geometrical effects of a three-dimensional workpiece on the plasma-based ion implantation have been studied using trench-shape and L-shape workpieces. Temporal and spatial evolution of the sheath and the ion flux on the workpiece are measured for a negative voltage pulse of )1.8 to )7.0 kV, 40 ls: (1) The trench is occupied by the ion sheath quickly due to the sheath overlapping from both side walls; this effect is not seen in the L-shape workpiece with only one side wall. (2) Electrons are detected inside the trench even after the ion sheath fills the trench, which is attributed to secondary electrons emitted from the surface of the workpiece by ion impact and trapped electrostatically between the side walls. (3) The ion flux incident upon the inner surface of the trench is strongly enhanced because trapped secondary electrons ionize the filling gas as in the hollow-cathode discharge. The enhanced ionization is therefore not seen in Lshape and planar workpieces. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 52.77.Dq; 52.40.Kh; 52.80.Pi; 52.70.Ds Keywords: Plasma-based ion implantation; Three-dimensional workpiece; Ion sheath; Ion flux; Secondary electron emission; Hollowcathode effect
1. Introduction A principal advantage of plasma-based ion implantation (PBII) over the accelerator-based ion implantation is its ability of performing conformal ion implantation into a three-dimensional workpiece. However, conditions of the thin sheath and/ or the high density plasma are necessary to be
*
Corresponding author. Tel.: +81-294-38-5102; fax: +81294-38-5111. E-mail address: [email protected] (T. Ikehata).
established. Although theoretical and numerical investigations on the sheath evolution for a round hole on a plate [1], convex and concave wedges [2], and a trench [3] have been reported so far, there are few experiments on the same subject [4]. We examined temporal and spatial behaviors of the sheath and the ion flux for trench-shape, L-shape and planar workpieces. In particular, electrostatic trapping of secondary electrons, followed by enhanced ionization of the filling gas was clearly identified inside the trench. Our observations are quite similar to those by Cluggish and Munson [5] where the sheath overlapping followed by
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00846-2
T. Ikehata et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 772–776
the secondary electron enhanced discharge was observed among racks of fake pistons arranged in a large vacuum chamber.
2. Experiment 2.1. Arrangement Arrangement of the experiment was described in detail in the literature [6]. The argon plasma is generated in a pressure of 0.1 Pa by the inductively coupled discharge at a net power of 100 W and a frequency of 140 MHz. The vacuum chamber is 20 cm in diameter and 30 cm in length. Model workpieces made of aluminum are connected to a high-voltage feedthrough which is inserted from the bottom end. Schematic diagrams of model workpieces are shown in Fig. 1. The trench of the workpiece has 30 mm in width, 50 mm in depth and 60 mm in length. The L-shape and planar workpieces are made by removing the side walls of the trench-shape workpiece. The plasma density is typically 5 1015 m3 with an electron temperature of 7 eV between z ¼ 10 and 70 mm for the planar workpiece though the density inside the trench (at z ¼ 25 mm) decreases to 40% of that of the planar workpiece. A negative voltage pulse of )1.8 to )7.0 kV and with a duration of 40 ls is applied from a pulse modulator to the workpiece at a rate
773
of up to 50 pps. The sheath dynamics is derived from temporal and spatial characteristics of a movable Langmuir probe biased to +80 V dc. In order to obtain the ion flux distribution along the surface of the workpiece, charge collector plates (0.1 15 50 mm, stainless steel) are pasted on the top, side and bottom surfaces of the trenchshape workpiece with insertion of thin Teflon sheets for electrical insulation. Each collector plate is connected by a lead to the modulator and the current through it is measured by a current probe with an amplifier. Three collector plates are used in trench-shape and L-shape workpieces but only the bottom collector plate is used in the planar workpiece. 2.2. Development and collapse of the ion sheath Fig. 2 shows temporal ion sheath evolution for three types of workpiece at an applied voltage of )7.0 kV. In the case of the planar workpiece, the sheath thickness s increases to 30 mm in 20 ls and evolves quite similar to the theoretical curve of LiebermanÕs model in which a matrix sheath thickness is 12 mm and a stationary sheath thickness is 40 mm. In the case of the trench-shape workpiece, on the other hand, s reaches up to 80 mm in 20 ls; that is, a trench with a depth of 50 mm is filled with the ion sheath almost simultaneously with the voltage application and
Fig. 1. Three types of workpiece tested: (a) trench-shape, (b) L-shape and (c) planar workpieces. Collector plates for ion current measurement are also shown.
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T. Ikehata et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 772–776
Fig. 3. Plasma recovery time as a function of the amplitude of voltage pulse.
Fig. 2. Sheath thickness as a function of time, together with the voltage waveform.
thereafter, the sheath edge further extends beyond the entrance to the trench as if the planar sheath evolves. This phenomenon is attributable to the sheath overlapping effect from both side walls as previously reported by Cluggish and Munson [5] in experiments at the large LANL device. The Lshape workpiece gives an intermediate result between trench-shape and planar workpieces: At x ¼ 0, the sheath behavior is similar to that of the trench-shape workpiece probably due to influence of the side wall, while it is similar to that of the planar workpiece at x ¼ 25 mm because influence of the side wall is negligible there. As the voltage pulse terminates, plasma electrons come back immediately so as to recover the charge neutrality and the sheath collapses. However, the initial plasma density cannot be recovered in a short time because massive ions cannot respond as rapidly as electrons. We define the plasma recovery time as the time period when the plasma density recovers up to 90% of its initial value. In Fig. 3, the plasma recovery time (at z ¼ 5 mm) is compared among trench-shape, L-shape and planar workpieces. For the trench-shape workpiece, the time period exceeding 100 ls is necessary for the replenishment which is five times longer than those of L-shape and planar workpieces. The time dependency for the replenishment
is well correlated to the ion sheath evolution shown in Fig. 2. The plasma recovery time is an important factor determining the maximum repetition rate of PBII. Therefore, this result suggests that the repetition rate of the voltage application in PBII is severely influenced by the geometry of the workpiece such as trench and hole. 2.3. Secondary electrons in the trench Fig. 4(a) shows waveforms of the Langmuir probe (dc bias is +80 V) in the trench-shape workpiece. The pulse voltage is )7.0 kV. A considerable amount of the electron current is detected at z ¼ 4, 20, 28 and 36 mm even inside the trench where plasma electrons are completely excluded. On the other hand, no electron current is detected at z ¼ 60 mm outside the trench (but inside the sheath boundary). Furthermore, it was noticed that the electron current decreased with decreasing the amplitude of the voltage pulse and became negligibly small at V ¼ 1:8 kV. From these results, we conclude that they are secondary electrons emitted from the surface of the workpiece by impact of ions and trapped in an electrostatic potential well formed between the side walls of the trench. This conclusion is supported by the fact that the secondary emission coefficient by ion impact at the ion energy greater than 1 keV increases with the square root of the ion energy [7]. Fig. 4(b) shows the electron current as a function
T. Ikehata et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 772–776
775
Fig. 4. (a) Probe current waveforms showing secondary electrons. (b) Distribution of secondary electron current in three workpieces.
of the distance z for three different workpieces. A remarkably large current is observed only in the trench-shape workpiece which is able to form the potential well for trapping electrons. In addition, the electron current has a peak at the position of z ¼ 12 mm inside the trench and sharply decreases toward the outside of the trench. Therefore, it is clear that the electrostatic trapping of secondary electrons is essential for the probe current increase in the trench. 2.4. Ion flux distribution along the surface of the workpiece The ion flux was measured on the top, side and bottom surfaces of the trench-shape workpiece using collector plates described in Section 2.1. Results are shown in Fig. 5 together with data of the planar workpiece. Fig. 5(a) shows waveforms of the voltage pulse (V ¼ 7:0 kV) and the col-
Fig. 5. (a) Collector current waveforms with and without a plasma shield: they show the displacement current and the ion current (including secondary electrons), respectively. Temporal behavior of each collector current normalized by the top collector current at (b) V ¼ 1:8 kV and (c) V ¼ 7:0 kV.
lector current with and without a plasma shield. The plasma shield means that the workpiece is covered doubly by polymer and aluminum foils to cut the convection current into the workpiece off. Although the displacement current appears at rising and falling portions of the voltage pulse, its contribution is negligible (less than 5% of the total collector current in 5 ls). Figs. 5(b) and (c) show
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T. Ikehata et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 772–776
temporal evolution of the normalized collector current on the side and bottom surfaces of the trench and on the planar workpiece. The normalization is carried out against the top collector current. At an applied voltage of )1.8 kV, the side and bottom currents change similar (Fig. 5(b)). They have a value of 0.5 at 5 ls and decreases to about 1/3 at the time of the voltage termination. Furthermore, these side and bottom currents are less than that of the planar workpiece. This gives an evidence that the three-dimensional geometry of the workpiece influences the sheath evolution and consequently the distribution of the ion current: The top surface of the trench-shape workpiece can be approximated by the outer surface of a cylinder so that the thickness of the sheath on it becomes less than that on the planar workpiece. On the other hand, a deep portion of side and bottom surfaces of the trench can be approximated by the inner surface of the cylinder, so that the sheath thickness on it becomes more than that on the planar workpiece. Fig. 5(c) shows the same data as Fig. 5(b) except the applied voltage of )7.0 kV. We note that the normalized currents on side and bottom surfaces are larger than the case of )1.8 kV though the sheath thickness should become greater at )7.0 kV. This observation may be attributed to enhanced ionization of the filling gas by trapped secondary electrons (in other words, the hollowcathode effect) as discussed in Section 2.3, which is another evidence for geometrical effects of the workpiece.
3. Conclusions Temporal and spatial evolution of the sheath and the ion flux on trench-shape and L-shape workpieces were investigated together with those of the planar workpiece for comparison. Only the trench-shape workpiece shows a rapid expansion of the sheath by the sheath overlapping effect. Electrostatic trapping of secondary electrons was identified in the trench-shape workpiece but not in the L-shape workpiece. These trapped electrons served to increase the ion flux inside the trench by enhanced ionization of the filling gas. To summarize, we reported the formation of the potential well and the associated phenomena inside the trench as one of geometrical effects of the workpiece.
References [1] T.E. Sheridan, Plasma Sources Sci. Technol. 4 (1995) 527. [2] I.J. Donnelly, P.A. Watterson, J. Phys. D: Appl. Phys. 22 (1989) 90; P.A. Watterson, J. Phys. D: Appl. Phys. 22 (1989) 1300. [3] T.E. Sheridan, J. Phys. D: Appl. Phys. 28 (1995) 1094. [4] M. Sano, T. Teramoto, K. Yukimura, T. Maruyama, Surf. Coat. Technol. 136 (2001) 168. [5] B.P. Cluggish, C.P. Munson, J. Appl. Phys. 84 (1998) 5937, 84 (1998) 5945. [6] T. Ikehata, T. Araki, K. Shimatsu, K. Shioya, N.Y. Sato, K. Yukimura, The Papers of Technical Meeting on Plasma Science and Technology, PST-02-41, 2002, p. 87 (in Japanese). [7] M.M. Shamin, J.T. Scheuer, R.P. Fetherton, J.R. Conrad, J. Appl. Phys. 70 (1991) 4756.