- Email: [email protected]
Ion-beam assisted coating of tube inner walls by plasma immersion ion implantation
Surface and Coatings Technology 128᎐129 Ž2000. 270᎐273
Ion-beam assisted coating of tube inner walls by plasma immersion ion implantation W. Ensinger U , K. Volz Philipps-Uni¨ ersity of Marburg, Department of Chemistry and Material Science Center, 35032 Marburg, Germany
Abstract Tubes are often required to exhibit better performance in corrosion and wear behaviour than the material the tube is made of can offer. The situation can be improved when the tube is coated with a protective film. This is possible when sputter coating with ions is performed. A sputter target is located inside the tube. Energetic ions extracted from a plasma in which the tube is immersed Žplasma immersion ion implantation PIII. are accelerated towards the tube, enter it and impinge onto the target. Thus, material is sputtered from the target onto the inner walls of the tube. Tubes made of stainless steel and of tantalum were used. A part of the stainless steel tube was cut away and replaced by a segment of a silicon wafer. A zirconium sputter target was inserted into the tube. Ions from a nitrogen plasma were accelerated into the tube. Thus, zirconium oxynitride films were formed. Tantalum tubes with a platinum sputter target inside were treated in an argon plasma. The deposited films were analysed by Rutherford backscattering spectrometry. The results show that PIII treatment of tube inner walls is an effective technique for coating inner walls of tubes with protective films. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Plasma immersion ion implantation; Tube coating; Zirconia oxynitride
1. Introduction In analogy to flat objects, the inner walls of hollow objects such as tubes can be protected against mechanical and chemical attack when they are coated with an appropriate material. In principle, for this purpose it is possible to use physical vapour deposition techniques. However, the line-of-sight character of a number of PVD techniques renders the process difficult because the material to be deposited has to enter the tube under very oblique angles to the wall normal, depending on the ratio of tube diameter to length. This problem can be solved by introducing the source of the material to be deposited into the tube. A sputter target is inserted into the tube. Energetic ions enter the tube, impinge onto the target and sputter material onto the
U
Corresponding author. Tel.: q49-6421-2825727; fax: q49-64212822124. E-mail address: [email protected] ŽW. Ensinger..
tube inner wall. We have already reported two apparatuses which apply a beam of ions from an accelerator beam-line w1,2x. In the present paper, a technique based on plasma immersion ion implantation w3,4x is described.
2. The set-up of the apparatus, experimental details The apparatus consists of a cylindrical process chamber with an RF matching box and an antenna on top. It is connected to an RF generator with an excitation frequency of 13.56 MHz at a maximum electrical power of 1250 W. The sample holder is connected to a high voltage power supply with a pulse generator. The apparatus has been described in detail elsewhere w5x. Tubes of stainless steel and tantalum with inner diameters of 15 mm and a length of 150 mm have been used. They have been mounted on the sample holder standing upright with their aperture under the antenna. Fig. 1 shows the set-up schematically. Cones and sheets
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 6 0 3 - 4
W. Ensinger, K. Volz r Surface and Coatings Technology 128᎐129 (2000) 270᎐273
271
Fig. 2. RBS spectra of Pt film in a tantalum tube at three different positions in the tube.
Fig. 1. Schematic presentation of experimental set-up.
When a reactive metal as a sputter target and a reactive gas as a plasma-forming species is used, the film can be deposited as a compound coating. Thus, reactive transition metals such as titanium or ziconium can be converted to nitride or oxide films. Fig. 3 shows
of zirconium and platinum were placed into the tubes. For the experiments with zirconium, a part of the steel tubes of 5 mm thickness was cut away and replaced by a 150-mm long segment of a Ž100. silicon wafer. For forming the plasma, nitrogen or argon was backfilled into the chamber up to a pressure of 0.1 Pa. The samples were subjected to high voltage pulses of y35 kV at 400 Hz repetition rate for 3600 s in the case of Zr and at ᎐40 kV and 200 Hz for 3600 s in the case of Pt. The composition of the deposited films was determined by Rutherford backscattering spectrometry with 3.7 MeV He ions. The obtained spectra were quantified by means of simulation by the RUMP code w6x. 3. Results Fig. 2 shows a series of RBS spectra of a Pt film deposited on the inner walls of a tantalum tube by argon PIII treatment with a conical Pt sputter target inside. At the lower part of the tube Ž1 mm from tube bottom. a typical RBS spectrum of a Pt film on Ta is found. Going from the tube bottom away from the sputter target towards the tube aperture the spectrum changes. An intermixed interface between Pt and Ta is found which is formed by ion beam mixing as a result of ion irradiation. At positions further away, the film becomes thinner and is even more mixed. Such a gradient can be expected when the emission characteristics of a sputter target are considered, and when a certain degree of ion bombardment is taken into account. This demonstrates that it is possible to use PIII for coating the inner walls of tubes.
Fig. 3. RBS spectra of a ZrŽN,O. film on silicon. Ža. Si substrate, Žb. Zr peaks at different position in the tube.
W. Ensinger, K. Volz r Surface and Coatings Technology 128᎐129 (2000) 270᎐273
272
RBS spectra of a film deposited by sputtering Zr in a nitrogen plasma onto silicon. In Fig. 3a the substrate continuum with the peaks of oxygen and nitrogen on top is depicted. In Fig. 3b Zr peaks from measurements at different positions in the tube are shown. Close to the sputter target, the amount of deposited Zr is rather large. Going along the tube away from the sputter target towards the tube aperture, a decreasing amount of Zr is found. As the Zr film was deposited in a nitrogen plasma, nitrogen is incorporated into the film. Fig. 4 shows the number of deposited Zr atoms and incorporated nitrogen and oxygen atoms as a function of the position in the tube. Large amounts of nitrogen and oxygen, the latter from the residual gas, are incorporated. Up to a distance of 20 mm the film is comparatively uniform, then the thickness decreases rapidly.
4. Discussion One of the main features of PIII is that ions are accelerated from the cathode sheath and impinge onto the target from all sides simultaneously. Exactly for this purpose, particularly to circumvent the line-of-sight restrictions of conventional beam-line ion implantation, PIII has been developed w3,4x. In the literature it has been reported that the inner walls of trenches are homogeneously treated when these are microtrenches w7,8x, but a quite different behaviour has been observed when the trench widths are in the range of millimeters or centimeters w9x. While the side walls are hardly affected by the ions a large amount impinges onto the trench bottom. This
has been attributed to the development of the cathode sheath from which the ions are accelerated and to the resulting ion trajectories. During the high voltage pulse the cathode sheath rapidly withdraws from the trench. When the ions are accelerated from the sheath edge outside the trench their majority will not be implanted into the trench side-walls but rather into the bottom. This phenomenon can be used to coat tube inner walls by sputtering. Fig. 5 shows a schematic presentation of the process. In the initial phase of the high voltage pulse at the voltage rise-time, ions are accelerated from their positions inside the tube towards the tube walls and towards the sputter target ŽFig. 5a.. These ions do not have full kinetic energy. They are implanted into the walls, however, their implantation profile remains shallow. Additionally to the implantation, they sputter erode the wall surface and clean it to a certain extent. When the high voltage during the pulse rises, the cathode sheath leaves the tube. At the end of the 20-s pulse under the given conditions, the plasma sheath is at a distance of 100 mm from the sample. The shape of the cathode sheath has become cylindrical with the upper end at the tube aperture being spherical. The ions which are accelerated from the region above the tube aperture gain large kinetic energy and momentum when they travel towards the tube ŽFig. 5b.. When they enter it, due to their inertia they are not able to follow the electrical field lines. Instead, a number of them will travel parallel to the tube axis and impinge onto the sputter target. There, most of them will sputter etch the target. Thus, a film is formed on the tube wall. A part of the ions will be
Fig. 4. Concentration of deposited Zr atoms and incorporated nitrogen and oxygen atoms at different positions in the tube.
W. Ensinger, K. Volz r Surface and Coatings Technology 128᎐129 (2000) 270᎐273
273
5. Summary When short or medium-sized tubes with a sputtertarget inside are subjected to plasma immersion ion beam treatment, the major part of the ions which enter the tube impinges onto the sputter target and causes deposition of a coating onto the inner walls of the tube. This is due to the trajectories of the ions. Most of them are accelerated from the cathode sheath outside the tube at a distance from the tube. Due to their large kinetic energy and inertia they enter the tube parallel to its axis rather than impinging onto the tube walls. This technique allows the deposition of thin films onto the inner walls of tubes. When reactive sputtering materials and ions are used, compound films such as nitride or oxides can be formed. For coating the tube interior completely, the sputter target has to be moved. Further work in this direction is in progress. Fig. 5. Schematic presentation of the PIII coating process: Ža. initial phase of high voltage pulse, pulse rise time: ions are accelerated from the cathode sheath to low energies inside the tube; Žb. cathode sheath expands, ions are accelerated to higher energies, enter the tube parallel to its axis and impinge on the sputter target.
reflected and be implanted into the tube wall or the film. In the beginning of thin film growth, the implanted ions will penetrate through the film and through the interface filmrsubstrate and cause ion beam mixing. Later when the film has grown thicker, the ions will be implanted into the film. Thus, a film of reactive transition metals will be converted into the nitride by different processes: reaction of the sputtered atoms with nitrogen gas from the ambient under ion irradiation, direct nitrogen ion implantation, and sputter deposition of those nitrogen atoms which have been implanted before into the sputter target.
Acknowledgement This work has been supported by Forschungsgemeinschaft.
Deutsche
References w1x W. Ensinger, Rev. Sci. Instr. 67 Ž1996. 318. w2x T. Kraus, R. Kern, B. Stritzker, W. Ensinger, Nucl. Instr. Meth. Phys. Res. B148 Ž1999. 912. w3x J.R. Conrad, T. Castagna, Bull. Am. Phys. Soc. 31 Ž1986. 1479. w4x J. Tendys, I.J. Donnelly, M.J. Kenny, J.T.A. Pollock, Appl. Phys. Lett. 53 Ž1988. 2143. w5x W. Ensinger, K. Volz, B. Enders, Surf. Coat. Technol., Surf. Coat. Technol. 120᎐121 Ž1999. 343. w6x L.R. Doolittle, Nucl. Instr. Meth. Phys. Res. B9 Ž1985. 344. w7x B. Mizuno, I. Nakayama, N. Aoi, M. Kubota, T. Komeda, Appl. Phys. Lett. 53 Ž1988. 2059. w8x X.Y. Qian, N.W. Cheung, M.A. Lieberman, R. Brennan, M.I. Current, N. Jha, Nucl. Instr. Meth. Phys. Res. B55 Ž1991. 898. w9x W. Ensinger, K. Volz, T. Hochbauer, Surf. Coat. Technol., in ¨ press.
Ion-beam assisted coating of tube inner walls by plasma immersion ion implantation W. Ensinger U , K. Volz Philipps-Uni¨ ersity of Marburg, Department of Chemistry and Material Science Center, 35032 Marburg, Germany
Abstract Tubes are often required to exhibit better performance in corrosion and wear behaviour than the material the tube is made of can offer. The situation can be improved when the tube is coated with a protective film. This is possible when sputter coating with ions is performed. A sputter target is located inside the tube. Energetic ions extracted from a plasma in which the tube is immersed Žplasma immersion ion implantation PIII. are accelerated towards the tube, enter it and impinge onto the target. Thus, material is sputtered from the target onto the inner walls of the tube. Tubes made of stainless steel and of tantalum were used. A part of the stainless steel tube was cut away and replaced by a segment of a silicon wafer. A zirconium sputter target was inserted into the tube. Ions from a nitrogen plasma were accelerated into the tube. Thus, zirconium oxynitride films were formed. Tantalum tubes with a platinum sputter target inside were treated in an argon plasma. The deposited films were analysed by Rutherford backscattering spectrometry. The results show that PIII treatment of tube inner walls is an effective technique for coating inner walls of tubes with protective films. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Plasma immersion ion implantation; Tube coating; Zirconia oxynitride
1. Introduction In analogy to flat objects, the inner walls of hollow objects such as tubes can be protected against mechanical and chemical attack when they are coated with an appropriate material. In principle, for this purpose it is possible to use physical vapour deposition techniques. However, the line-of-sight character of a number of PVD techniques renders the process difficult because the material to be deposited has to enter the tube under very oblique angles to the wall normal, depending on the ratio of tube diameter to length. This problem can be solved by introducing the source of the material to be deposited into the tube. A sputter target is inserted into the tube. Energetic ions enter the tube, impinge onto the target and sputter material onto the
U
Corresponding author. Tel.: q49-6421-2825727; fax: q49-64212822124. E-mail address: [email protected] ŽW. Ensinger..
tube inner wall. We have already reported two apparatuses which apply a beam of ions from an accelerator beam-line w1,2x. In the present paper, a technique based on plasma immersion ion implantation w3,4x is described.
2. The set-up of the apparatus, experimental details The apparatus consists of a cylindrical process chamber with an RF matching box and an antenna on top. It is connected to an RF generator with an excitation frequency of 13.56 MHz at a maximum electrical power of 1250 W. The sample holder is connected to a high voltage power supply with a pulse generator. The apparatus has been described in detail elsewhere w5x. Tubes of stainless steel and tantalum with inner diameters of 15 mm and a length of 150 mm have been used. They have been mounted on the sample holder standing upright with their aperture under the antenna. Fig. 1 shows the set-up schematically. Cones and sheets
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 6 0 3 - 4
W. Ensinger, K. Volz r Surface and Coatings Technology 128᎐129 (2000) 270᎐273
271
Fig. 2. RBS spectra of Pt film in a tantalum tube at three different positions in the tube.
Fig. 1. Schematic presentation of experimental set-up.
When a reactive metal as a sputter target and a reactive gas as a plasma-forming species is used, the film can be deposited as a compound coating. Thus, reactive transition metals such as titanium or ziconium can be converted to nitride or oxide films. Fig. 3 shows
of zirconium and platinum were placed into the tubes. For the experiments with zirconium, a part of the steel tubes of 5 mm thickness was cut away and replaced by a 150-mm long segment of a Ž100. silicon wafer. For forming the plasma, nitrogen or argon was backfilled into the chamber up to a pressure of 0.1 Pa. The samples were subjected to high voltage pulses of y35 kV at 400 Hz repetition rate for 3600 s in the case of Zr and at ᎐40 kV and 200 Hz for 3600 s in the case of Pt. The composition of the deposited films was determined by Rutherford backscattering spectrometry with 3.7 MeV He ions. The obtained spectra were quantified by means of simulation by the RUMP code w6x. 3. Results Fig. 2 shows a series of RBS spectra of a Pt film deposited on the inner walls of a tantalum tube by argon PIII treatment with a conical Pt sputter target inside. At the lower part of the tube Ž1 mm from tube bottom. a typical RBS spectrum of a Pt film on Ta is found. Going from the tube bottom away from the sputter target towards the tube aperture the spectrum changes. An intermixed interface between Pt and Ta is found which is formed by ion beam mixing as a result of ion irradiation. At positions further away, the film becomes thinner and is even more mixed. Such a gradient can be expected when the emission characteristics of a sputter target are considered, and when a certain degree of ion bombardment is taken into account. This demonstrates that it is possible to use PIII for coating the inner walls of tubes.
Fig. 3. RBS spectra of a ZrŽN,O. film on silicon. Ža. Si substrate, Žb. Zr peaks at different position in the tube.
W. Ensinger, K. Volz r Surface and Coatings Technology 128᎐129 (2000) 270᎐273
272
RBS spectra of a film deposited by sputtering Zr in a nitrogen plasma onto silicon. In Fig. 3a the substrate continuum with the peaks of oxygen and nitrogen on top is depicted. In Fig. 3b Zr peaks from measurements at different positions in the tube are shown. Close to the sputter target, the amount of deposited Zr is rather large. Going along the tube away from the sputter target towards the tube aperture, a decreasing amount of Zr is found. As the Zr film was deposited in a nitrogen plasma, nitrogen is incorporated into the film. Fig. 4 shows the number of deposited Zr atoms and incorporated nitrogen and oxygen atoms as a function of the position in the tube. Large amounts of nitrogen and oxygen, the latter from the residual gas, are incorporated. Up to a distance of 20 mm the film is comparatively uniform, then the thickness decreases rapidly.
4. Discussion One of the main features of PIII is that ions are accelerated from the cathode sheath and impinge onto the target from all sides simultaneously. Exactly for this purpose, particularly to circumvent the line-of-sight restrictions of conventional beam-line ion implantation, PIII has been developed w3,4x. In the literature it has been reported that the inner walls of trenches are homogeneously treated when these are microtrenches w7,8x, but a quite different behaviour has been observed when the trench widths are in the range of millimeters or centimeters w9x. While the side walls are hardly affected by the ions a large amount impinges onto the trench bottom. This
has been attributed to the development of the cathode sheath from which the ions are accelerated and to the resulting ion trajectories. During the high voltage pulse the cathode sheath rapidly withdraws from the trench. When the ions are accelerated from the sheath edge outside the trench their majority will not be implanted into the trench side-walls but rather into the bottom. This phenomenon can be used to coat tube inner walls by sputtering. Fig. 5 shows a schematic presentation of the process. In the initial phase of the high voltage pulse at the voltage rise-time, ions are accelerated from their positions inside the tube towards the tube walls and towards the sputter target ŽFig. 5a.. These ions do not have full kinetic energy. They are implanted into the walls, however, their implantation profile remains shallow. Additionally to the implantation, they sputter erode the wall surface and clean it to a certain extent. When the high voltage during the pulse rises, the cathode sheath leaves the tube. At the end of the 20-s pulse under the given conditions, the plasma sheath is at a distance of 100 mm from the sample. The shape of the cathode sheath has become cylindrical with the upper end at the tube aperture being spherical. The ions which are accelerated from the region above the tube aperture gain large kinetic energy and momentum when they travel towards the tube ŽFig. 5b.. When they enter it, due to their inertia they are not able to follow the electrical field lines. Instead, a number of them will travel parallel to the tube axis and impinge onto the sputter target. There, most of them will sputter etch the target. Thus, a film is formed on the tube wall. A part of the ions will be
Fig. 4. Concentration of deposited Zr atoms and incorporated nitrogen and oxygen atoms at different positions in the tube.
W. Ensinger, K. Volz r Surface and Coatings Technology 128᎐129 (2000) 270᎐273
273
5. Summary When short or medium-sized tubes with a sputtertarget inside are subjected to plasma immersion ion beam treatment, the major part of the ions which enter the tube impinges onto the sputter target and causes deposition of a coating onto the inner walls of the tube. This is due to the trajectories of the ions. Most of them are accelerated from the cathode sheath outside the tube at a distance from the tube. Due to their large kinetic energy and inertia they enter the tube parallel to its axis rather than impinging onto the tube walls. This technique allows the deposition of thin films onto the inner walls of tubes. When reactive sputtering materials and ions are used, compound films such as nitride or oxides can be formed. For coating the tube interior completely, the sputter target has to be moved. Further work in this direction is in progress. Fig. 5. Schematic presentation of the PIII coating process: Ža. initial phase of high voltage pulse, pulse rise time: ions are accelerated from the cathode sheath to low energies inside the tube; Žb. cathode sheath expands, ions are accelerated to higher energies, enter the tube parallel to its axis and impinge on the sputter target.
reflected and be implanted into the tube wall or the film. In the beginning of thin film growth, the implanted ions will penetrate through the film and through the interface filmrsubstrate and cause ion beam mixing. Later when the film has grown thicker, the ions will be implanted into the film. Thus, a film of reactive transition metals will be converted into the nitride by different processes: reaction of the sputtered atoms with nitrogen gas from the ambient under ion irradiation, direct nitrogen ion implantation, and sputter deposition of those nitrogen atoms which have been implanted before into the sputter target.
Acknowledgement This work has been supported by Forschungsgemeinschaft.
Deutsche
References w1x W. Ensinger, Rev. Sci. Instr. 67 Ž1996. 318. w2x T. Kraus, R. Kern, B. Stritzker, W. Ensinger, Nucl. Instr. Meth. Phys. Res. B148 Ž1999. 912. w3x J.R. Conrad, T. Castagna, Bull. Am. Phys. Soc. 31 Ž1986. 1479. w4x J. Tendys, I.J. Donnelly, M.J. Kenny, J.T.A. Pollock, Appl. Phys. Lett. 53 Ž1988. 2143. w5x W. Ensinger, K. Volz, B. Enders, Surf. Coat. Technol., Surf. Coat. Technol. 120᎐121 Ž1999. 343. w6x L.R. Doolittle, Nucl. Instr. Meth. Phys. Res. B9 Ž1985. 344. w7x B. Mizuno, I. Nakayama, N. Aoi, M. Kubota, T. Komeda, Appl. Phys. Lett. 53 Ž1988. 2059. w8x X.Y. Qian, N.W. Cheung, M.A. Lieberman, R. Brennan, M.I. Current, N. Jha, Nucl. Instr. Meth. Phys. Res. B55 Ž1991. 898. w9x W. Ensinger, K. Volz, T. Hochbauer, Surf. Coat. Technol., in ¨ press.