Focused nanoparticle-beam deposition of patterned microstructures
APPLIED PHYSICS LETTERS
VOLUME 77, NUMBER 6
7 AUGUST 2000
Focused nanoparticle-beam deposition of patterned microstructures F. Di Fonzo, A. Gidwani, M. H. Fan, D. Neumann, D. I. Iordanoglou, J. V. R. Heberlein, P. H. McMurry, and S. L. Girshicka) Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota 55455
N. Tymiak and W. W. Gerberich Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
N. P. Rao Microtherm, LLC, Minneapolis, Minnesota 55414
共Received 5 May 2000; accepted for publication 8 June 2000兲 A method was developed for fabricating nanocrystalline microstructures. This method involves synthesizing nanoparticles in a thermal plasma expanded through a nozzle, and then focusing the nanoparticles to a collimated beam by means of aerodynamic lenses. High-aspect-ratio structures of silicon carbide and titanium were deposited on stationary substrates, and lines and two-dimensional patterns were deposited on translated substrates. Linewidths equalled approximately 50 m. This approach allows the use of much larger nozzles than in previously developed micronozzle methods, and also allows size selection of the particles that are deposited. © 2000 American Institute of Physics. 关S0003-6951共00兲00432-0兴
could confer important advantages with regard to mechanical, optoelectronic, or other properties. Particle beams have been used extensively in aerosol measurement instruments; but to our knowledge the only previous work in which collimated beams of nanoparticles were used deliberately for materials deposition involved a process developed by Hayashi and co-workers,9,10 which these authors termed ‘‘gas jet deposition.’’ In this method, condensible vapor is generated above a heated crucible, and particles nucleate in an inert carrier gas. The particle-laden flow then expands supersonically through a micronozzle, producing a particle beam whose dimensions are approximately the same as those of the nozzle, or perhaps somewhat smaller, although these authors were apparently unaware of a possible aerodynamic focusing effect. Nozzles with inside diameters of 100 m were used to deposit gold particles, producing tapered needle-shaped structures. Akedo and co-workers11,12 used this method to fabricate miniature piezoelectric actuators composed of lead zirconate titanate, and further extended the process to demonstrate threedimensional microfabrication through a variety of techniques including free forming, insert molding, and substrate masking.13 To suppress clogging, the nozzles were heated so as to drive particles away from the nozzle walls by thermophoretic forces. The essential feature of the present work is that we deposit nanoparticles using an aerodynamic focusing system. This allows us to generate particle beams whose dimensions are much smaller than the dimensions of the nozzles or orifices utilized, so that fine micronozzles are not required. The principles of aerodynamic focusing are discussed in detail by Liu et al.7 The focusing assembly consists of a series of thin plates mounted in a cylindrical barrel. Each plate has an orifice, or ‘‘aerodynamic lens,’’ located at its center, and the assembly terminates in an exit nozzle which is a sonic orifice. In passing through each lens the gas contracts and then
The current interest in nanostructured materials has led to the search for methods to synthesize such materials in the form of bulk solids or films. We previously reported a method for depositing nanostructured films, known as ‘‘hypersonic plasma particle deposition’’ 共HPPD兲.1–5 In this process, vapor-phase reactants are injected into a thermal plasma, which is then expanded to low pressure through a nozzle. Rapid cooling in the nozzle expansion drives the nucleation of nanoparticles, which are then accelerated in the hypersonic free jet issuing from the nozzle. A substrate is positioned normal to the flow, and particles as small as a few nanometers in diameter deposit by inertial impaction.6 Ballistic compaction forms a dense, nanostructured coating. In experiments involving silicon-carbide deposition, the grain size observed by scanning electron microscopy 共SEM兲, typically, around 20 nm, corresponded closely to measurements by scanning electrical mobility spectrometry of the aerosol sampled in-flight downstream of the nozzle, indicating that the film retained the grain size of the impacting particles.2 We report a modification of the HPPD process, in which the deposition substrate is replaced by a set of ‘‘aerodynamic lenses’’ 7,8 that collimates the nanoparticles to form a beam. A substrate is placed downstream of the lens assembly, and particles in the beam deposit, again by inertial impaction. If the substrate is stationary, a high-aspect-ratio ‘‘tower’’ of nanoparticles is formed. By translating the substrate it is possible to ‘‘write’’ lines and two-dimensional patterns consisting of nanoparticles, with linewidths on the order of tens of microns. With computer-controlled rastering of the substrate, this process could potentially be used for microfabrication of three-dimensional structures in microelectromechanical systems. Depending on the material and on the application, the inherent nanocrystalline nature of these microstructures a兲
Electronic mail: [email protected]
0003-6951/2000/77(6)/910/3/$17.00
910
© 2000 American Institute of Physics
Appl. Phys. Lett., Vol. 77, No. 6, 7 August 2000
Di Fonzo et al.
911
FIG. 1. Predicted flow streamlines 共top兲 and particle trajectories 共bottom兲 in the aerodynamic lens assembly.
FIG. 2. Scanning electron micrograph of a silicon-carbide ‘‘tower’’ deposited on a stationary substrate.
re-expands. Very small particles follow the gas streamlines. Very large particles accelerate radially towards the axis as the flow approaches the orifice. Due to their high inertia, these particles are projected across the centerline and impact on the opposite wall. Particles of an intermediate size are also accelerated towards the axis, but due to their shorter aerodynamic stopping distance they terminate their radial motion on a flow streamline that is closer to the axis than the one on which they originated. This leads to a concentration of such particles along the axis. By careful design of the lens assembly it is possible to collimate particles having a specified range of sizes. Experiments were conducted in which either SiC or titanium nanoparticles were generated using the same apparatus as previously described for HPPD experiments.1–5 Briefly, an argon–hydrogen plasma is generated by a direct-current arc. Reactants are injected into the plasma at the upstream end of the expansion nozzle. The reactants consisted either of silicon tetrachloride and methane, for silicon-carbide synthesis, or titanium tetrachloride, for titanium synthesis. The pressure was approximately 50 kPa at the nozzle inlet and 345 Pa in the expansion chamber. The plasma is hot 共approximately 2000 K兲 at the nozzle exit and then expands supersonically into the large expansion chamber. The inlet tube to the aerodynamic lens assembly was coaxially located 75 cm downstream of the plasma expansion nozzle. The flow in the expansion chamber experiences a series of expansion and compression waves, and is expected to have relaxed to close to room temperature at the inlet of the lens assembly. The lens assembly consisted of a series of five lenses, each with an orifice diameter of 2.26 mm. The inner diameter of the exit nozzle was 1.85 mm. Each lens was 0.3 mm thick, and the lenses were spaced 47 mm apart. The entire unit was constructed of stainless steel. We conducted numerical simulations to predict the flow of carrier gas and the trajectories of 共assumed spherical兲 silicon-carbide particles of various sizes through a lens system with the same geometry and conditions as in the experiments. These simulations solved the conservation equations for steady, laminar, com-
pressible flow, and calculated particle trajectories accounting for viscous drag but not Brownian diffusion. Brownian diffusion would be expected to broaden the width of the focused beam, especially for particles smaller than about 10 nm in diameter. Figure 1 shows predicted gas streamlines and trajectories of 20-nm-diam particles that enter the lens assembly along various streamlines. The particles are predicted to be well collimated along the flow axis by the exit of the final lens. The particle beam exiting the lens assembly issued into a chamber that was maintained at a pressure of 1.0 Pa. Substrates were mounted in this chamber, typically, 3 mm downstream of the exit nozzle. We estimate the particle impact velocity to be in the range 200–300 m/s. In all of these experiments substrates were at room temperature. Various substrate materials were used, including stainless steel, aluminum, brass, and glass. Adherent deposits formed on all of the metal substrates but not on glass.
FIG. 3. Scanning electron micrograph of a pattern composed of siliconcarbide particles, deposited on a manually translated substrate.
912
Di Fonzo et al.
Appl. Phys. Lett., Vol. 77, No. 6, 7 August 2000
Figure 2 shows a SEM image of a needle-shaped siliconcarbide tower deposited on a stationary substrate. The height of this structure is 1.3 mm. The compact, tapered appearance is typical of our deposits, for both SiC and titanium. Highresolution SEM images obtained from cross sections of the titanium deposits show grain sizes of about 20 nm, similar to those previously reported for HPPD of silicon carbide.2 In general, the height, half width, and base diameters of these towers increase linearly with deposition time, and the dimensions of our deposits are similar to those previously reported using micronozzles.9–13 However, our results show that aerodynamic focusing allows one to use much larger nozzles. As a tenfold increase in nozzle diameter corresponds to a 100fold increase in flow rate, aerodynamic focusing allows much higher throughputs. In addition, the use of larger nozzles should alleviate the problem of nozzle clogging, although our experiments have indicated that clogging may still be an issue. In preliminary experiments we have demonstrated the feasibility of depositing lines and two-dimensional patterns by translating the substrate. Figure 3 shows a pattern formed by SiC particles. The substrate was translated manually in a rapid zig-zag motion. The minimum line width is about 50 m. As can be seen in Fig. 3, towers began to grow at several points when the translation momentarily paused. We have recently implemented an automated x – y translation system, and have used this to deposit titanium lines, again with a width of about 50 m.
This work was partially supported by the National Science Foundation 共Grant DMI-9871863兲 and by the Minnesota Supercomputing Institute.
1
N. P. Rao, H. J. Lee, M. Kelkar, D. J. Hansen, J. V. R. Heberlein, P. H. McMurry, and S. L. Girshick, Nanostruct. Mater. 9, 129 共1997兲. 2 N. P. Rao, N. Tymiak, J. Blum, A. Neuman, H. J. Lee, S. L. Girshick, P. H. McMurry, and J. Heberlein, J. Aerosol Sci. 29, 707 共1998兲. 3 N. P. Rao, S. L. Girshick, P. H. McMurry, and J. V. R. Heberlein, U.S. Patent No. 5,874,134, February 23, 1999. 4 A. Neuman, J. Blum, N. Tymiak, Z. Wong, N. P. Rao, W. Gerberich, P. H. McMurry, J. V. R. Heberlein, and S. L. Girshick, IEEE Trans. Plasma Sci. 27, 46 共1999兲. 5 J. Blum, N. Tymiak, A. Neuman, Z. Wong, N. P. Rao, S. L. Girshick, W. W. Gerberich, P. H. McMurry, and J. V. R. Heberlein, J. Nanoparticle Res. 1, 31 共1999兲. 6 J. Fernandez de la Mora, S. V. Hering, N. Rao, and P. H. McMurry, J. Aerosol Sci. 21, 169 共1990兲. 7 P. Liu, P. J. Ziemann, D. B. Kittelson, and P. H. McMurry, Aerosol. Sci. Technol. 22, 293 共1995兲. 8 P. Liu, P. J. Ziemann, D. B. Kittelson, and P. H. McMurry, Aerosol. Sci. Technol. 22, 314 共1995兲. 9 S. Kashu, E. Fuchita, T. Manabe, and C. Hayashi, Jpn. J. Appl. Phys., Part 2 23, L910 共1984兲. 10 M. Oda, I. Katsu, M. Tsuneizumi, E. Fuchita, S. Kashu, and C. Hayashi, Mater. Res. Soc. Symp. Proc. 286, 121 共1993兲. 11 A. Schroth, R. Maeda, J. Akedo, and M. Ichiki, Jpn. J. Appl. Phys., Part 1 37, 5342 共1998兲. 12 J. Akedo and M. Lebedev, Jpn. J. Appl. Phys., Part 1 38, 5397 共1999兲. 13 J. Akedo, M. Ichiki, K. Kikuchi, and R. Maeda, Sens. Actuators A 69, 106 共1998兲.
VOLUME 77, NUMBER 6
7 AUGUST 2000
Focused nanoparticle-beam deposition of patterned microstructures F. Di Fonzo, A. Gidwani, M. H. Fan, D. Neumann, D. I. Iordanoglou, J. V. R. Heberlein, P. H. McMurry, and S. L. Girshicka) Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota 55455
N. Tymiak and W. W. Gerberich Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
N. P. Rao Microtherm, LLC, Minneapolis, Minnesota 55414
共Received 5 May 2000; accepted for publication 8 June 2000兲 A method was developed for fabricating nanocrystalline microstructures. This method involves synthesizing nanoparticles in a thermal plasma expanded through a nozzle, and then focusing the nanoparticles to a collimated beam by means of aerodynamic lenses. High-aspect-ratio structures of silicon carbide and titanium were deposited on stationary substrates, and lines and two-dimensional patterns were deposited on translated substrates. Linewidths equalled approximately 50 m. This approach allows the use of much larger nozzles than in previously developed micronozzle methods, and also allows size selection of the particles that are deposited. © 2000 American Institute of Physics. 关S0003-6951共00兲00432-0兴
could confer important advantages with regard to mechanical, optoelectronic, or other properties. Particle beams have been used extensively in aerosol measurement instruments; but to our knowledge the only previous work in which collimated beams of nanoparticles were used deliberately for materials deposition involved a process developed by Hayashi and co-workers,9,10 which these authors termed ‘‘gas jet deposition.’’ In this method, condensible vapor is generated above a heated crucible, and particles nucleate in an inert carrier gas. The particle-laden flow then expands supersonically through a micronozzle, producing a particle beam whose dimensions are approximately the same as those of the nozzle, or perhaps somewhat smaller, although these authors were apparently unaware of a possible aerodynamic focusing effect. Nozzles with inside diameters of 100 m were used to deposit gold particles, producing tapered needle-shaped structures. Akedo and co-workers11,12 used this method to fabricate miniature piezoelectric actuators composed of lead zirconate titanate, and further extended the process to demonstrate threedimensional microfabrication through a variety of techniques including free forming, insert molding, and substrate masking.13 To suppress clogging, the nozzles were heated so as to drive particles away from the nozzle walls by thermophoretic forces. The essential feature of the present work is that we deposit nanoparticles using an aerodynamic focusing system. This allows us to generate particle beams whose dimensions are much smaller than the dimensions of the nozzles or orifices utilized, so that fine micronozzles are not required. The principles of aerodynamic focusing are discussed in detail by Liu et al.7 The focusing assembly consists of a series of thin plates mounted in a cylindrical barrel. Each plate has an orifice, or ‘‘aerodynamic lens,’’ located at its center, and the assembly terminates in an exit nozzle which is a sonic orifice. In passing through each lens the gas contracts and then
The current interest in nanostructured materials has led to the search for methods to synthesize such materials in the form of bulk solids or films. We previously reported a method for depositing nanostructured films, known as ‘‘hypersonic plasma particle deposition’’ 共HPPD兲.1–5 In this process, vapor-phase reactants are injected into a thermal plasma, which is then expanded to low pressure through a nozzle. Rapid cooling in the nozzle expansion drives the nucleation of nanoparticles, which are then accelerated in the hypersonic free jet issuing from the nozzle. A substrate is positioned normal to the flow, and particles as small as a few nanometers in diameter deposit by inertial impaction.6 Ballistic compaction forms a dense, nanostructured coating. In experiments involving silicon-carbide deposition, the grain size observed by scanning electron microscopy 共SEM兲, typically, around 20 nm, corresponded closely to measurements by scanning electrical mobility spectrometry of the aerosol sampled in-flight downstream of the nozzle, indicating that the film retained the grain size of the impacting particles.2 We report a modification of the HPPD process, in which the deposition substrate is replaced by a set of ‘‘aerodynamic lenses’’ 7,8 that collimates the nanoparticles to form a beam. A substrate is placed downstream of the lens assembly, and particles in the beam deposit, again by inertial impaction. If the substrate is stationary, a high-aspect-ratio ‘‘tower’’ of nanoparticles is formed. By translating the substrate it is possible to ‘‘write’’ lines and two-dimensional patterns consisting of nanoparticles, with linewidths on the order of tens of microns. With computer-controlled rastering of the substrate, this process could potentially be used for microfabrication of three-dimensional structures in microelectromechanical systems. Depending on the material and on the application, the inherent nanocrystalline nature of these microstructures a兲
Electronic mail: [email protected]
0003-6951/2000/77(6)/910/3/$17.00
910
© 2000 American Institute of Physics
Appl. Phys. Lett., Vol. 77, No. 6, 7 August 2000
Di Fonzo et al.
911
FIG. 1. Predicted flow streamlines 共top兲 and particle trajectories 共bottom兲 in the aerodynamic lens assembly.
FIG. 2. Scanning electron micrograph of a silicon-carbide ‘‘tower’’ deposited on a stationary substrate.
re-expands. Very small particles follow the gas streamlines. Very large particles accelerate radially towards the axis as the flow approaches the orifice. Due to their high inertia, these particles are projected across the centerline and impact on the opposite wall. Particles of an intermediate size are also accelerated towards the axis, but due to their shorter aerodynamic stopping distance they terminate their radial motion on a flow streamline that is closer to the axis than the one on which they originated. This leads to a concentration of such particles along the axis. By careful design of the lens assembly it is possible to collimate particles having a specified range of sizes. Experiments were conducted in which either SiC or titanium nanoparticles were generated using the same apparatus as previously described for HPPD experiments.1–5 Briefly, an argon–hydrogen plasma is generated by a direct-current arc. Reactants are injected into the plasma at the upstream end of the expansion nozzle. The reactants consisted either of silicon tetrachloride and methane, for silicon-carbide synthesis, or titanium tetrachloride, for titanium synthesis. The pressure was approximately 50 kPa at the nozzle inlet and 345 Pa in the expansion chamber. The plasma is hot 共approximately 2000 K兲 at the nozzle exit and then expands supersonically into the large expansion chamber. The inlet tube to the aerodynamic lens assembly was coaxially located 75 cm downstream of the plasma expansion nozzle. The flow in the expansion chamber experiences a series of expansion and compression waves, and is expected to have relaxed to close to room temperature at the inlet of the lens assembly. The lens assembly consisted of a series of five lenses, each with an orifice diameter of 2.26 mm. The inner diameter of the exit nozzle was 1.85 mm. Each lens was 0.3 mm thick, and the lenses were spaced 47 mm apart. The entire unit was constructed of stainless steel. We conducted numerical simulations to predict the flow of carrier gas and the trajectories of 共assumed spherical兲 silicon-carbide particles of various sizes through a lens system with the same geometry and conditions as in the experiments. These simulations solved the conservation equations for steady, laminar, com-
pressible flow, and calculated particle trajectories accounting for viscous drag but not Brownian diffusion. Brownian diffusion would be expected to broaden the width of the focused beam, especially for particles smaller than about 10 nm in diameter. Figure 1 shows predicted gas streamlines and trajectories of 20-nm-diam particles that enter the lens assembly along various streamlines. The particles are predicted to be well collimated along the flow axis by the exit of the final lens. The particle beam exiting the lens assembly issued into a chamber that was maintained at a pressure of 1.0 Pa. Substrates were mounted in this chamber, typically, 3 mm downstream of the exit nozzle. We estimate the particle impact velocity to be in the range 200–300 m/s. In all of these experiments substrates were at room temperature. Various substrate materials were used, including stainless steel, aluminum, brass, and glass. Adherent deposits formed on all of the metal substrates but not on glass.
FIG. 3. Scanning electron micrograph of a pattern composed of siliconcarbide particles, deposited on a manually translated substrate.
912
Di Fonzo et al.
Appl. Phys. Lett., Vol. 77, No. 6, 7 August 2000
Figure 2 shows a SEM image of a needle-shaped siliconcarbide tower deposited on a stationary substrate. The height of this structure is 1.3 mm. The compact, tapered appearance is typical of our deposits, for both SiC and titanium. Highresolution SEM images obtained from cross sections of the titanium deposits show grain sizes of about 20 nm, similar to those previously reported for HPPD of silicon carbide.2 In general, the height, half width, and base diameters of these towers increase linearly with deposition time, and the dimensions of our deposits are similar to those previously reported using micronozzles.9–13 However, our results show that aerodynamic focusing allows one to use much larger nozzles. As a tenfold increase in nozzle diameter corresponds to a 100fold increase in flow rate, aerodynamic focusing allows much higher throughputs. In addition, the use of larger nozzles should alleviate the problem of nozzle clogging, although our experiments have indicated that clogging may still be an issue. In preliminary experiments we have demonstrated the feasibility of depositing lines and two-dimensional patterns by translating the substrate. Figure 3 shows a pattern formed by SiC particles. The substrate was translated manually in a rapid zig-zag motion. The minimum line width is about 50 m. As can be seen in Fig. 3, towers began to grow at several points when the translation momentarily paused. We have recently implemented an automated x – y translation system, and have used this to deposit titanium lines, again with a width of about 50 m.
This work was partially supported by the National Science Foundation 共Grant DMI-9871863兲 and by the Minnesota Supercomputing Institute.
1
N. P. Rao, H. J. Lee, M. Kelkar, D. J. Hansen, J. V. R. Heberlein, P. H. McMurry, and S. L. Girshick, Nanostruct. Mater. 9, 129 共1997兲. 2 N. P. Rao, N. Tymiak, J. Blum, A. Neuman, H. J. Lee, S. L. Girshick, P. H. McMurry, and J. Heberlein, J. Aerosol Sci. 29, 707 共1998兲. 3 N. P. Rao, S. L. Girshick, P. H. McMurry, and J. V. R. Heberlein, U.S. Patent No. 5,874,134, February 23, 1999. 4 A. Neuman, J. Blum, N. Tymiak, Z. Wong, N. P. Rao, W. Gerberich, P. H. McMurry, J. V. R. Heberlein, and S. L. Girshick, IEEE Trans. Plasma Sci. 27, 46 共1999兲. 5 J. Blum, N. Tymiak, A. Neuman, Z. Wong, N. P. Rao, S. L. Girshick, W. W. Gerberich, P. H. McMurry, and J. V. R. Heberlein, J. Nanoparticle Res. 1, 31 共1999兲. 6 J. Fernandez de la Mora, S. V. Hering, N. Rao, and P. H. McMurry, J. Aerosol Sci. 21, 169 共1990兲. 7 P. Liu, P. J. Ziemann, D. B. Kittelson, and P. H. McMurry, Aerosol. Sci. Technol. 22, 293 共1995兲. 8 P. Liu, P. J. Ziemann, D. B. Kittelson, and P. H. McMurry, Aerosol. Sci. Technol. 22, 314 共1995兲. 9 S. Kashu, E. Fuchita, T. Manabe, and C. Hayashi, Jpn. J. Appl. Phys., Part 2 23, L910 共1984兲. 10 M. Oda, I. Katsu, M. Tsuneizumi, E. Fuchita, S. Kashu, and C. Hayashi, Mater. Res. Soc. Symp. Proc. 286, 121 共1993兲. 11 A. Schroth, R. Maeda, J. Akedo, and M. Ichiki, Jpn. J. Appl. Phys., Part 1 37, 5342 共1998兲. 12 J. Akedo and M. Lebedev, Jpn. J. Appl. Phys., Part 1 38, 5397 共1999兲. 13 J. Akedo, M. Ichiki, K. Kikuchi, and R. Maeda, Sens. Actuators A 69, 106 共1998兲.
