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TiN deposition and ion current distribution for trench target by plasma-based ion implantation and deposition
Surface & Coatings Technology 193 (2005) 17 – 21 www.elsevier.com/locate/surfcoat
TiN deposition and ion current distribution for trench target by plasma-based ion implantation and deposition Ken Yukimuraa,*, Xinxin Mab, Takashi Ikehatac a
Doshisha University, Department of Electrical Engineering, 1-3 Tatara Miykodani, Kyotanabe, Kyoto 610-0321, Japan b Harbin Institute of Technology, School of Material Science and Engineering, Harbin, 150001, PR China c Ibaraki University, Department of Electrical and Electronic Engineering, Hitachi, Ibaraki 316-8511, Japan Available online 23 September 2004
Abstract Titanium nitride (TiN) films are prepared on a trench by plasma-based ion implantation and deposition (PBII & D). The distributions of film thickness and implanted dose are studied, considering the current distribution inside the trench and the ion sheath evolution. The trench as a target is immersed in a titanium cathodic arc discharge with a current of 80 A dc generated at a nitrogen pressure of 10 Pa. The opening of the trench faces the cathodic arc discharge at a distance of 400 mm. A pulsed voltage is applied to the trench to make a transient ion sheath around the trench. At the inside wall of the trench, the implanted dose is strongly governed by the ion sheath evolution because the ion sheath edge leaves from the inside region of the trench in a short time less than 1 As. The fast sheath evolution brings a shortage of ions arriving at the deeper section of the trench. As a result, ion current flowing into the inner wall notably decreases in a deeper section. From the ion sheath evolution, it is also found that the sheath structure changes with time. Ion sheath edge, which departed from each wall, merges at the area inside the trench. The merging of the ion sheath results that the sheath edge becomes obscure. This may change the implantation characteristics. At the bottom of the trench, the large deposition rate causes the small dose near at the substrate surface. At the inside wall, the implanted dose is largest at the top and markedly decreases toward the bottom. These are certified due to a rapid evolution of the ion sheath, which is firstly generated at the wall inside the chamber by applying a pulse voltage to the trench, and the sheath edge is out from trench inside in enough short time. As a result, no ions reach to the bottom of the trench. D 2004 Elsevier B.V. All rights reserved. Keywords: TiN; Plasma-based ion implantation; Ion sheath; Deposition
1. Introduction When a pulse voltage is applied to a substrate immersed in a plasma, ions are uniformly implanted into the substrate, even with a three-dimensional shape. The implanted dose is closely related to the configuration of the ion sheath produced around the substrate. Recently, a hybrid film preparation system, in which a thin film is deposited simultaneously with ion implantation, has been extensively studied for actualizing a fast and efficient process system [1–3]. We have researched PBII & D using cathodic arc * Corresponding author. Tel.: +81 774 65 6266; fax: +81 774 65 6816. E-mail address: [email protected] (K. Yukimura). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.133
plasma in a gas circumstance to a three dimensional materials. As fundamental studies, titanium ions are implanted into a sphere [4,5], a trench [4,5], a pipe [6,7] and a slanted surface [8]. For homogenous treatment to three-dimensional substrates, it is important that the ion sheath evolution and its induced current distribution should be studied with characterization of the prepared films. In particular, when a cathodic arc is utilized as a source for metallic species and ions, strongly directed stream of the plasma species influences the deposition and implantation characteristics [5]. The distribution of the ion-implanted current and the ion sheath evolution are experimentally observed to find a relationship to prepared film characteristics.
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K. Yukimura et al. / Surface & Coatings Technology 193 (2005) 17–21
2. Experimental Experimental facilities are described in detail elsewhere [4]. The inner dimensions of the width, depth and height of the vacuum chamber were 340, 550 and 380 mm, respectively. A water-cooled titanium cathode was mounted with a trigger electrode at the other end of the chamber. The vacuum chamber was grounded to make an anode. Both cathode and trigger electrodes were connected to a dc power source. The dc arc current and voltage were 80 A and 20 V, respectively. A trigger discharge was ignited to generate a dc metal-arc between the cathode and the grounded chamber by disconnecting the trigger electrode from the cathode. The pulse modulator was used for applying a pulse voltage of 40 kV with a width of 20 As. The repetition rate was 400 Hz. The amplitude of pulsed voltage is varied from 0.5 to 5 kV for measuring ion sheath evolution and ion current through a strip type current monitoring plate. A voltage of 30 kV is applied for ion implantation into the substrate. The ion implantation during the pulses alternates with deposition between them. The process time was 2 min. The pumping system consisted of a turbo-molecular pump and two rotary pumps, which were positioned in a side-wall of the discharge chamber. The nitrogen flow rate was approximately 250 sccm, which corresponds to a pressure of about 10 Pa. A silicon substrate (p-type, (111), 0.626 mm in thickness) was used for the deposition of titanium nitride film. The implanted dose was estimated using X-ray photoelectron spectroscopy (XPS) as an arbitrary value. The thickness of prepared film is measured using a thickness meter. The substrates were positioned at about 400 mm from the arc source. The trench is schematically shown in Fig. 1, which is used for deposition and ion implantation. The width and depth of the trench were 16 mm, and the seven substrates were placed on the bottom, inner side, outer side and back wall of the trench. The substrates, bHighQ, bMiddleQ and bLowQ, are positioned at distances of 13, 8 and 3 mm from the bottom surface, respectively. In addition, the two substrates were placed at the center and corner on the bottom wall. The substrates, bCenterQ and bCornerQ, were positioned at distances of 8 and 3 mm from the tip of the corner, respectively. A trench shown with a width of 30 mm and a height (depth) of 20 mm is used for measuring the propagation of the sheath edge inside of the trench. In order to measure the propagation of the sheath edge, the probe is biased at +20 V and the trace is monitored on an oscilloscope. The similar method was carried out [9]. During the probe being in the plasma, the electron current flows through the probe. When a pulsed negative voltage is applied to the trench, the electrons are expelled so as to make an ion sheath. As a result, a change in the probe current is observed because the electron current disappears. From change in current,
Fig. 1. A schematic diagram of the trench for deposition and ion implantation.
arrival of the ion sheath edge can be known. The sheath evolution was measured by varying the probe position inside the trench. The pitch is 1–5 mm. In each section inside the trench, strip-type current monitoring plates with a width of 4 mm and a length of 100 mm are pasted on the trench surface. The plates are insulated from the trench using plastic sheets with a thickness of 1 mm and a pulsed voltage is applied to them simultaneously with the trench. To avoid the electrical breakdown around the plates, the applied voltage is reduced. Current observation by the similar method was carried out for a pipe [10] in a cathodic arc and a trench [11] in argon gas plasma.
3. Results and discussion 3.1. Deposition and ion implantation characteristics of trench Fig. 2a, b and c shows deposition rates and implanted doses (arbitrary unit) for whole area of the trench, inner wall and bottom of the trench, respectively. For all of the cases, the film thickness is distributed due to strongly directed stream of the plasma species, which is peculiar to cathodic arcs. The strong directivity is originated from violent partial vaporization of the cathode materials. The implanted doses are also distributed.
K. Yukimura et al. / Surface & Coatings Technology 193 (2005) 17–21
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applying a pulsed voltage. It is also noticed that the implanted dose is extremely small at the inner side wall. Compared to the outer side wall, the thickness of the deposited layer at the inner side wall is smaller by a factor of 0.75, while the dose is only 3%. Fig. 2b shows the deposition rate and implanted dose at the inner wall of the trench. Both values decrease toward the deeper section. However, the difference in the deposition rate is in a range of F10%, while the titanium dose at the deepest section bdQ is only a dose of 5% to the top region shown as bbQ. Invasion of the plasma species into the trench due to a strong flow mentioned above influences the small scatter of the deposited thickness. Concerning the large difference in the implanted dose, the ion sheath evolution is suggested to be strongly involved in the implanted dose. No difference in deposition thickness and the implanted dose for the corner and center at the bottom is seen in Fig. 2c. 3.2. Ion sheath evolution Fig. 3 shows ion sheath evolutions in and near the trench at 0.5 kV applied to the target. The trench width is shown in a range of 15 mm to +15 mm at a horizontal position. A matrix sheath is simultaneously formed along the trench surface, followed by evolving the ion sheath edge toward the center of the trench. Finally, the sheath edge merges and the entire trench surface is surrounded by the ion sheath. This behavior influences the extracted ion current characteristics. The sheath edge is positioned at about 8 mm from the bottom at 1 As. The sheath edges departed from each wall merge near at the center of trench. As a result, the ion sheath edge at the center seems to propagate to the vertical direction. At 10 As, the sheath edge is out of the trench. Thus, the sheath edge loses a configuration along the trench surface. As a result, conformal ion implantation may not be expected. When an applied voltage is varied from 0.5 to 5 kV with a negative polarity, the sheath edge generated at each wall propagates toward the center of the trench (X=0)
Fig. 2. Deposition rate and titanium ion dose of TiN film on the trench. (a) Whole area, (b) Inner wall, (c) Bottom.
The film thickness is the largest at the bottom and differs by the positions on the trench by a factor of around 10. In contrast, the implanted dose near at the interface between the deposited layer and substrate is the highest at the outer side wall. Compared between the bottom and outer side wall, it is noticed that the low implanted dose at the bottom is caused by the large thickness of the deposited layer. The implanted dose is also recognized at the back wall, which is shaded from the arc source. The ions in the cathodic arc discharge are turned around to the back wall by
Fig. 3. Ion sheath edge position as a function of the horizontal position at an applied voltage of 0.5 kV.
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Fig. 4. Waveforms of Langmuir probe current showing arrival and depletion of the ion sheath.
and merges. Even at 1 As, the sheath edge is out of the trench at 3 kV. Fig. 4 shows waveforms of the current through the positively biased Langmuir probe at the center of the trench shown at X=0, when a pulse voltage of 1 kV with a width of 20 As is applied to the trench at a time t=0. Due to a positive bias of the Langmuir probe immersed in the plasma, electron current appears with an amplitude of about 0.2 A before applying a pulse voltage. By applying a negative pulse voltage, electrons are expelled to make an ion sheath, which causes the decrease of the electron current. However, it is seen in Fig. 4 that the current waveform is different by the vertical position of the trench. Near at the bottom of the trench, Y= 15 mm, the current quickly decrease to zero in a short time less than 1 As because the probe is immersed in an ion sheath with a clear edge. For approaching the top of the trench, the change in current becomes uncertain. This shows that the ion sheath departs from the both sides of the trench and then merges, during which the clear sheath edge disappears. The obscure ion sheath edge suggests that
Fig. 6. Stationary ion current as a function of the depth of trench at an applied voltage of 1 kV.
electrons also remain around the probe. After the pulse ends, the plasma is replenished with a designated time constant, which is dependent on the plasma density [7]. 3.3. Ion current characteristics Fig. 5 shows waveforms of the current through the strip probes at distances of Y= 10 and Y= 30 mm for a trench with a width of 30 mm and a depth of 80 mm, where the voltage applied to the strip probe is 1 kV with a pulse width of 20 As. The current has a sharp peak at the initial stage as seen in the waveform in a reduced scale shown in Fig. 5, followed by a stationary state. It is seen that the stationary current becomes smaller for a deeper position of the trench. Fig. 6 shows a stationary current through the strip probe at the pulse end as a function of the depth of the trench for an applied voltage of 1 kV with a pulse width of 20 As. It is seen that the current markedly decreases with increasing the depth of the trench, and in particular, for depths larger than 30 mm, no ion current is observed. This is caused by the sheath propagation as shown in Fig. 3 and non-conformal ion implantation occurs inside the trench. In this case, ions can penetrate only in a depth less than 30 mm at the side wall. The decrease in ion current inside the trench reflects the implanted dose shown in Fig. 2. In summary, in order to obtain a conformal ion implantation into a trench using a cathodic arc discharge, the ion sheath edge should be along the trench wall. From this view point for suppressing the ion sheath evolution, short pulse-width of the target voltage is desirable. In order to compensate the shortage of the implanted dose, high frequency operation is necessary.
4. Summary Fig. 5. Waveforms of ion current through the strip current monitor set inside trench at depths of 10 and 30 mm at an applied voltage of 1 kV.
Ion implantation and film deposition characteristics of the trench-shaped substrate are discussed for a cathodic arc
K. Yukimura et al. / Surface & Coatings Technology 193 (2005) 17–21
discharge source. The deposited film is titanium nitride by titanium cathodic arc produced in nitrogen circumstance. The opening of the trench faces the cathodic arc source with a dc current of 80 A. The implanted dose and the deposition rate at the bottom of the trench are identical at the bottom section of the trench. At the outside wall of the trench, the implanted dose at the interface between the substrate and the deposited film decreases due to a large thickness of the prepared film. Even at the backside of the trench, which is shaded by the trench itself from the arc source, the ions are implanted. This means that the ions are turned around from the cathodic arc discharge with a strong stream. At the inside wall of the trench, the deposition rate is large near at the top of the trench and decreases with increasing the depth of the trench. The marked decrease of the implanted dose has a close relationship to the time propagation of the ion sheath. That is, the ion sheath edge is out of the trench opening in a short time of 1 As after the pulse voltage application. As a result, no conformal ion implantation is attained. The strong current distribution is certified by the experimentally observed ion current distribution on a side wall inside the trench. It is commonly seen in the observed ion current that a sharp peak at the initial stage appears, followed by a stationary state. The larger current flows nearer at the top of the trench. In the deeper position of the inner wall of the trench, the current becomes smaller. The close relationship between the implanted dose and time evolution of the ion sheath is shown. As a proposal, in order to realize a conformal ion implantation, the size of the ion sheath should be suppressed so as to be along the trench
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surface. The PBII & D operation of the short pulse with a high frequency is desirable to realize the conformal ion implantation. References [1] P. Huber, D. Manova, S. M7ndl, B. Rauschenbach, Vacuum 69 (2003) 133. [2] S.A. Nikiforov, K.W. Urm, G.H. Kim, G.H. Rim, S.H. Lee, Surface and Coatings Technology 171 (2003) 106. [3] B. Liu, G. Zhang, D. Cheng, C. Liu, R. He, S.-Z. Yang, Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films 19 (2001) 2958. [4] M. Sano, T. Teramoto, K. Yukimura, T. Maruyama, Surface and Coatings Technology 128–129 (2000) 245. [5] M. Sano, T. Teramoto, K. Yukimura, T. Maruyama, Surface and Coatings Technology 136 (2001) 168. [6] E. Kuze, T. Teramoto, K. Yukimura, T. Maruyama, Surface and Coatings Technology 158–159 (2002) 577. [7] X.X. Ma, K. Yukimura, T. Muraho, Nuclear Instruments and Methods in Physics Research. B, Beam Interactions with Materials and Atoms 206 (2003) 787. [8] M. Sano, T. Teramoto, K. Yukimura, T. Maruyama, S. Kurooka, Y. Suzuki, A. Chayahara, A. Kinomura, Y. Horino, Nuclear Instruments and Methods in Physics Research. B, Beam Interactions with Materials and Atoms 148 (1999) 37. [9] S.M. Malik, K. Sridharan, R.P. Fetherston, A. Chen, J.R. Conrad, Journal of Vacuum Science & Technology, B 12 (1994) 843. [10] X.X. Ma, K. Yukimura, T. Ikehata, Y. Miyagawa, Nuclear Instruments and Methods in Physics Research. B, Beam Interactions with Materials and Atoms 206 (2003) 813. [11] T. Ikehata, K. Shioya, T. Araki, N.Y. Sato, H. Mase, K. Yukimura, Nuclear Instruments and Methods in Physics Research. B, Beam Interactions with Materials and Atoms 206 (2003) 772.
TiN deposition and ion current distribution for trench target by plasma-based ion implantation and deposition Ken Yukimuraa,*, Xinxin Mab, Takashi Ikehatac a
Doshisha University, Department of Electrical Engineering, 1-3 Tatara Miykodani, Kyotanabe, Kyoto 610-0321, Japan b Harbin Institute of Technology, School of Material Science and Engineering, Harbin, 150001, PR China c Ibaraki University, Department of Electrical and Electronic Engineering, Hitachi, Ibaraki 316-8511, Japan Available online 23 September 2004
Abstract Titanium nitride (TiN) films are prepared on a trench by plasma-based ion implantation and deposition (PBII & D). The distributions of film thickness and implanted dose are studied, considering the current distribution inside the trench and the ion sheath evolution. The trench as a target is immersed in a titanium cathodic arc discharge with a current of 80 A dc generated at a nitrogen pressure of 10 Pa. The opening of the trench faces the cathodic arc discharge at a distance of 400 mm. A pulsed voltage is applied to the trench to make a transient ion sheath around the trench. At the inside wall of the trench, the implanted dose is strongly governed by the ion sheath evolution because the ion sheath edge leaves from the inside region of the trench in a short time less than 1 As. The fast sheath evolution brings a shortage of ions arriving at the deeper section of the trench. As a result, ion current flowing into the inner wall notably decreases in a deeper section. From the ion sheath evolution, it is also found that the sheath structure changes with time. Ion sheath edge, which departed from each wall, merges at the area inside the trench. The merging of the ion sheath results that the sheath edge becomes obscure. This may change the implantation characteristics. At the bottom of the trench, the large deposition rate causes the small dose near at the substrate surface. At the inside wall, the implanted dose is largest at the top and markedly decreases toward the bottom. These are certified due to a rapid evolution of the ion sheath, which is firstly generated at the wall inside the chamber by applying a pulse voltage to the trench, and the sheath edge is out from trench inside in enough short time. As a result, no ions reach to the bottom of the trench. D 2004 Elsevier B.V. All rights reserved. Keywords: TiN; Plasma-based ion implantation; Ion sheath; Deposition
1. Introduction When a pulse voltage is applied to a substrate immersed in a plasma, ions are uniformly implanted into the substrate, even with a three-dimensional shape. The implanted dose is closely related to the configuration of the ion sheath produced around the substrate. Recently, a hybrid film preparation system, in which a thin film is deposited simultaneously with ion implantation, has been extensively studied for actualizing a fast and efficient process system [1–3]. We have researched PBII & D using cathodic arc * Corresponding author. Tel.: +81 774 65 6266; fax: +81 774 65 6816. E-mail address: [email protected] (K. Yukimura). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.133
plasma in a gas circumstance to a three dimensional materials. As fundamental studies, titanium ions are implanted into a sphere [4,5], a trench [4,5], a pipe [6,7] and a slanted surface [8]. For homogenous treatment to three-dimensional substrates, it is important that the ion sheath evolution and its induced current distribution should be studied with characterization of the prepared films. In particular, when a cathodic arc is utilized as a source for metallic species and ions, strongly directed stream of the plasma species influences the deposition and implantation characteristics [5]. The distribution of the ion-implanted current and the ion sheath evolution are experimentally observed to find a relationship to prepared film characteristics.
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K. Yukimura et al. / Surface & Coatings Technology 193 (2005) 17–21
2. Experimental Experimental facilities are described in detail elsewhere [4]. The inner dimensions of the width, depth and height of the vacuum chamber were 340, 550 and 380 mm, respectively. A water-cooled titanium cathode was mounted with a trigger electrode at the other end of the chamber. The vacuum chamber was grounded to make an anode. Both cathode and trigger electrodes were connected to a dc power source. The dc arc current and voltage were 80 A and 20 V, respectively. A trigger discharge was ignited to generate a dc metal-arc between the cathode and the grounded chamber by disconnecting the trigger electrode from the cathode. The pulse modulator was used for applying a pulse voltage of 40 kV with a width of 20 As. The repetition rate was 400 Hz. The amplitude of pulsed voltage is varied from 0.5 to 5 kV for measuring ion sheath evolution and ion current through a strip type current monitoring plate. A voltage of 30 kV is applied for ion implantation into the substrate. The ion implantation during the pulses alternates with deposition between them. The process time was 2 min. The pumping system consisted of a turbo-molecular pump and two rotary pumps, which were positioned in a side-wall of the discharge chamber. The nitrogen flow rate was approximately 250 sccm, which corresponds to a pressure of about 10 Pa. A silicon substrate (p-type, (111), 0.626 mm in thickness) was used for the deposition of titanium nitride film. The implanted dose was estimated using X-ray photoelectron spectroscopy (XPS) as an arbitrary value. The thickness of prepared film is measured using a thickness meter. The substrates were positioned at about 400 mm from the arc source. The trench is schematically shown in Fig. 1, which is used for deposition and ion implantation. The width and depth of the trench were 16 mm, and the seven substrates were placed on the bottom, inner side, outer side and back wall of the trench. The substrates, bHighQ, bMiddleQ and bLowQ, are positioned at distances of 13, 8 and 3 mm from the bottom surface, respectively. In addition, the two substrates were placed at the center and corner on the bottom wall. The substrates, bCenterQ and bCornerQ, were positioned at distances of 8 and 3 mm from the tip of the corner, respectively. A trench shown with a width of 30 mm and a height (depth) of 20 mm is used for measuring the propagation of the sheath edge inside of the trench. In order to measure the propagation of the sheath edge, the probe is biased at +20 V and the trace is monitored on an oscilloscope. The similar method was carried out [9]. During the probe being in the plasma, the electron current flows through the probe. When a pulsed negative voltage is applied to the trench, the electrons are expelled so as to make an ion sheath. As a result, a change in the probe current is observed because the electron current disappears. From change in current,
Fig. 1. A schematic diagram of the trench for deposition and ion implantation.
arrival of the ion sheath edge can be known. The sheath evolution was measured by varying the probe position inside the trench. The pitch is 1–5 mm. In each section inside the trench, strip-type current monitoring plates with a width of 4 mm and a length of 100 mm are pasted on the trench surface. The plates are insulated from the trench using plastic sheets with a thickness of 1 mm and a pulsed voltage is applied to them simultaneously with the trench. To avoid the electrical breakdown around the plates, the applied voltage is reduced. Current observation by the similar method was carried out for a pipe [10] in a cathodic arc and a trench [11] in argon gas plasma.
3. Results and discussion 3.1. Deposition and ion implantation characteristics of trench Fig. 2a, b and c shows deposition rates and implanted doses (arbitrary unit) for whole area of the trench, inner wall and bottom of the trench, respectively. For all of the cases, the film thickness is distributed due to strongly directed stream of the plasma species, which is peculiar to cathodic arcs. The strong directivity is originated from violent partial vaporization of the cathode materials. The implanted doses are also distributed.
K. Yukimura et al. / Surface & Coatings Technology 193 (2005) 17–21
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applying a pulsed voltage. It is also noticed that the implanted dose is extremely small at the inner side wall. Compared to the outer side wall, the thickness of the deposited layer at the inner side wall is smaller by a factor of 0.75, while the dose is only 3%. Fig. 2b shows the deposition rate and implanted dose at the inner wall of the trench. Both values decrease toward the deeper section. However, the difference in the deposition rate is in a range of F10%, while the titanium dose at the deepest section bdQ is only a dose of 5% to the top region shown as bbQ. Invasion of the plasma species into the trench due to a strong flow mentioned above influences the small scatter of the deposited thickness. Concerning the large difference in the implanted dose, the ion sheath evolution is suggested to be strongly involved in the implanted dose. No difference in deposition thickness and the implanted dose for the corner and center at the bottom is seen in Fig. 2c. 3.2. Ion sheath evolution Fig. 3 shows ion sheath evolutions in and near the trench at 0.5 kV applied to the target. The trench width is shown in a range of 15 mm to +15 mm at a horizontal position. A matrix sheath is simultaneously formed along the trench surface, followed by evolving the ion sheath edge toward the center of the trench. Finally, the sheath edge merges and the entire trench surface is surrounded by the ion sheath. This behavior influences the extracted ion current characteristics. The sheath edge is positioned at about 8 mm from the bottom at 1 As. The sheath edges departed from each wall merge near at the center of trench. As a result, the ion sheath edge at the center seems to propagate to the vertical direction. At 10 As, the sheath edge is out of the trench. Thus, the sheath edge loses a configuration along the trench surface. As a result, conformal ion implantation may not be expected. When an applied voltage is varied from 0.5 to 5 kV with a negative polarity, the sheath edge generated at each wall propagates toward the center of the trench (X=0)
Fig. 2. Deposition rate and titanium ion dose of TiN film on the trench. (a) Whole area, (b) Inner wall, (c) Bottom.
The film thickness is the largest at the bottom and differs by the positions on the trench by a factor of around 10. In contrast, the implanted dose near at the interface between the deposited layer and substrate is the highest at the outer side wall. Compared between the bottom and outer side wall, it is noticed that the low implanted dose at the bottom is caused by the large thickness of the deposited layer. The implanted dose is also recognized at the back wall, which is shaded from the arc source. The ions in the cathodic arc discharge are turned around to the back wall by
Fig. 3. Ion sheath edge position as a function of the horizontal position at an applied voltage of 0.5 kV.
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Fig. 4. Waveforms of Langmuir probe current showing arrival and depletion of the ion sheath.
and merges. Even at 1 As, the sheath edge is out of the trench at 3 kV. Fig. 4 shows waveforms of the current through the positively biased Langmuir probe at the center of the trench shown at X=0, when a pulse voltage of 1 kV with a width of 20 As is applied to the trench at a time t=0. Due to a positive bias of the Langmuir probe immersed in the plasma, electron current appears with an amplitude of about 0.2 A before applying a pulse voltage. By applying a negative pulse voltage, electrons are expelled to make an ion sheath, which causes the decrease of the electron current. However, it is seen in Fig. 4 that the current waveform is different by the vertical position of the trench. Near at the bottom of the trench, Y= 15 mm, the current quickly decrease to zero in a short time less than 1 As because the probe is immersed in an ion sheath with a clear edge. For approaching the top of the trench, the change in current becomes uncertain. This shows that the ion sheath departs from the both sides of the trench and then merges, during which the clear sheath edge disappears. The obscure ion sheath edge suggests that
Fig. 6. Stationary ion current as a function of the depth of trench at an applied voltage of 1 kV.
electrons also remain around the probe. After the pulse ends, the plasma is replenished with a designated time constant, which is dependent on the plasma density [7]. 3.3. Ion current characteristics Fig. 5 shows waveforms of the current through the strip probes at distances of Y= 10 and Y= 30 mm for a trench with a width of 30 mm and a depth of 80 mm, where the voltage applied to the strip probe is 1 kV with a pulse width of 20 As. The current has a sharp peak at the initial stage as seen in the waveform in a reduced scale shown in Fig. 5, followed by a stationary state. It is seen that the stationary current becomes smaller for a deeper position of the trench. Fig. 6 shows a stationary current through the strip probe at the pulse end as a function of the depth of the trench for an applied voltage of 1 kV with a pulse width of 20 As. It is seen that the current markedly decreases with increasing the depth of the trench, and in particular, for depths larger than 30 mm, no ion current is observed. This is caused by the sheath propagation as shown in Fig. 3 and non-conformal ion implantation occurs inside the trench. In this case, ions can penetrate only in a depth less than 30 mm at the side wall. The decrease in ion current inside the trench reflects the implanted dose shown in Fig. 2. In summary, in order to obtain a conformal ion implantation into a trench using a cathodic arc discharge, the ion sheath edge should be along the trench wall. From this view point for suppressing the ion sheath evolution, short pulse-width of the target voltage is desirable. In order to compensate the shortage of the implanted dose, high frequency operation is necessary.
4. Summary Fig. 5. Waveforms of ion current through the strip current monitor set inside trench at depths of 10 and 30 mm at an applied voltage of 1 kV.
Ion implantation and film deposition characteristics of the trench-shaped substrate are discussed for a cathodic arc
K. Yukimura et al. / Surface & Coatings Technology 193 (2005) 17–21
discharge source. The deposited film is titanium nitride by titanium cathodic arc produced in nitrogen circumstance. The opening of the trench faces the cathodic arc source with a dc current of 80 A. The implanted dose and the deposition rate at the bottom of the trench are identical at the bottom section of the trench. At the outside wall of the trench, the implanted dose at the interface between the substrate and the deposited film decreases due to a large thickness of the prepared film. Even at the backside of the trench, which is shaded by the trench itself from the arc source, the ions are implanted. This means that the ions are turned around from the cathodic arc discharge with a strong stream. At the inside wall of the trench, the deposition rate is large near at the top of the trench and decreases with increasing the depth of the trench. The marked decrease of the implanted dose has a close relationship to the time propagation of the ion sheath. That is, the ion sheath edge is out of the trench opening in a short time of 1 As after the pulse voltage application. As a result, no conformal ion implantation is attained. The strong current distribution is certified by the experimentally observed ion current distribution on a side wall inside the trench. It is commonly seen in the observed ion current that a sharp peak at the initial stage appears, followed by a stationary state. The larger current flows nearer at the top of the trench. In the deeper position of the inner wall of the trench, the current becomes smaller. The close relationship between the implanted dose and time evolution of the ion sheath is shown. As a proposal, in order to realize a conformal ion implantation, the size of the ion sheath should be suppressed so as to be along the trench
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surface. The PBII & D operation of the short pulse with a high frequency is desirable to realize the conformal ion implantation. References [1] P. Huber, D. Manova, S. M7ndl, B. Rauschenbach, Vacuum 69 (2003) 133. [2] S.A. Nikiforov, K.W. Urm, G.H. Kim, G.H. Rim, S.H. Lee, Surface and Coatings Technology 171 (2003) 106. [3] B. Liu, G. Zhang, D. Cheng, C. Liu, R. He, S.-Z. Yang, Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films 19 (2001) 2958. [4] M. Sano, T. Teramoto, K. Yukimura, T. Maruyama, Surface and Coatings Technology 128–129 (2000) 245. [5] M. Sano, T. Teramoto, K. Yukimura, T. Maruyama, Surface and Coatings Technology 136 (2001) 168. [6] E. Kuze, T. Teramoto, K. Yukimura, T. Maruyama, Surface and Coatings Technology 158–159 (2002) 577. [7] X.X. Ma, K. Yukimura, T. Muraho, Nuclear Instruments and Methods in Physics Research. B, Beam Interactions with Materials and Atoms 206 (2003) 787. [8] M. Sano, T. Teramoto, K. Yukimura, T. Maruyama, S. Kurooka, Y. Suzuki, A. Chayahara, A. Kinomura, Y. Horino, Nuclear Instruments and Methods in Physics Research. B, Beam Interactions with Materials and Atoms 148 (1999) 37. [9] S.M. Malik, K. Sridharan, R.P. Fetherston, A. Chen, J.R. Conrad, Journal of Vacuum Science & Technology, B 12 (1994) 843. [10] X.X. Ma, K. Yukimura, T. Ikehata, Y. Miyagawa, Nuclear Instruments and Methods in Physics Research. B, Beam Interactions with Materials and Atoms 206 (2003) 813. [11] T. Ikehata, K. Shioya, T. Araki, N.Y. Sato, H. Mase, K. Yukimura, Nuclear Instruments and Methods in Physics Research. B, Beam Interactions with Materials and Atoms 206 (2003) 772.