Scanning probe lithography tips with spring-on-tip ... - Semantic Scholar
APPLIED PHYSICS LETTERS 87, 054102 共2005兲
Scanning probe lithography tips with spring-on-tip designs: Analysis, fabrication, and testing Xuefeng Wang, Loren Vincent, David Bullen, Jun Zou, and Chang Liua兲 Micro and Nanotechnology Laboratory, University of Illinois at Urbana-Champaign, 208 North Wright Street, Urbana, Illinois 61801
共Received 7 December 2004; accepted 28 June 2005; published online 27 July 2005兲 This letter reports a special tip design for probes used in scanning probe lithography applications. The sidewalls of the pyramidal tip located at the distal end of a cantilever probe are modified to contain folded spring structures to reduce the overall force constant of the scanning probe. The spring structure is generated using focused ion beam milling method. We have conducted finite element simulation of the force constants of such folded springs under various geometries. We also demonstrated sub-100 nm scanning probe lithography using a modified spring tip in the dip pen nanolithography writing mode. © 2005 American Institute of Physics. 关DOI: 10.1063/1.2006210兴 Scanning probe microscopy 共SPM兲 has been widely used for surface characterization and modification.1–4 It is well known that the scanning probes used in SPM instruments can modify substrate surfaces through mechanical,5–7 electrical,8,9 or optical10,11 approaches. This has led to a rapidly emerging research field—scanning probe lithography 共SPL兲. For example, dip pen nanolithography 共DPN兲12,13 is an additive SPL method. It uses a pre-coated cantilever tip to transfer “ink” molecules onto a substrate. It is uniquely capable of directly patterning many materials 共e.g., organic molecules, biomolecules, metal salts, polymers, and nanoparticles13兲 at molecular level. In all cantilever-probe-based SPL methods, the probe consists of a cantilever with a sharp tip at its distal end, with the cantilever serving as a spring. The mechanical structure of the cantilever determines the stiffness of the probe and consequently the contact force between the tip and the substrate surface. The force constant of the cantilever, k, is
k=
Ewt3 , 4ᐉ3
共1兲
where E is the modulus of elasticity of the cantilever material, and w, t, l are the width, thickness, and length of the rectangular cantilever, respectively. A desired force constant of a cantilever can be reached by trading off the width, length, and thickness parameters of the cantilever. As an example, DPN operation has been conducted using commercial silicon 共Si兲 or silicon nitride 共Si3N4兲 atomic force microscope atomic force microscope 共AFM兲 probes, with their force constants usually in the range of 0.01– 5 N / m. An overly soft probe tip can adhere to the writing surface, whereas an overly stiff one may result in mechanical scratches in the contact mode. For force constant in this range, if the thickness and width of a Si3N4 cantilever are fixed 共e.g., t = 1 m, w = 30 m兲, the length of the cantilever must range from 540 to 68 m. The motivation of this work stems from the need to produce high-density, arrayed SPL probes, especially twodimensional 共2D兲 arrays. The length of the cantilever dictates a兲
the minimal tip-to-tip spacings. The finite length of individual cantilevers will limit the achievable density of tip arrays. To solve this problem, we present a type of probe tip with integrated spring structures. Figure 1 shows two tips with the spring-on-tip 共SOT兲 designs. The tip may consist of 1–4 springs, each occupying one facet of the tip. Selected faces can be completely removed to reduce the number of
Author to whom correspondence should be addressed; electronic mail: [email protected]
FIG. 1. Scanning electron microscopy 共SEM兲 images of Si3N4 SOT tips: 共a兲 a tip with all four sides etched to identical profile, which can be considered as a tip supported by four identical springs connected in parallel; 共b兲 a different profile of a spring tip where the tip is effectively supported by one spring connected to the probe cantilever.
0003-6951/2005/87共5兲/054102/3/$22.50 87, 054102-1 © 2005 American Institute of Physics Downloaded 27 Jul 2005 to 128.174.190.93. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
054102-2
Appl. Phys. Lett. 87, 054102 共2005兲
Wang et al.
FIG. 2. A SOT tip with springs on two sidewalls. Two of four sidewalls have been completely removed.
springs 共see an example in Fig. 2兲. Alternatively, spiral springs may be made 关Fig. 1共b兲兴. The fabrication process for the tips is discussed here. First, pyramidal tips with continuous surfaces were made using Si3N4 film.14 Many practical methods for realizing pyramid tips have been demonstrated in the past. This current method can be used on tips made using a variety of thin film materials. The spring structures are created afterwards on the pyramidal tip using focused ion beam 共FIB兲 milling 共etching兲 method. The FIB milling width can be varied from nanometer to micrometer scale by adjusting FIB parameters. In order to optimize the spring design at the tip, an analytical model combined with finite element analysis is used to evaluate the force constant. It is assumed that the pyramidal tip has four sidewalls with identical spring structures. In this case, the tip can be considered as supported by four springs with equal spring constants 共ks兲 connected in parallel. The force constant of the SOT tip 共kt兲 is the sum of the four spring constants 共2兲
kt = 4ks .
A SPL probe 共or even a SPM probe for this matter兲 can use both traditional cantilever spring and the SOT springs simultaneously. The overall force constant of the tip is distributed to two independent parts: the probe cantilever and the folded springs on tip. In this case the entire probe can be considered as these two spring systems connected in series, as shown in Fig. 3. The design of the cantilever and tip can be conducted separately. The overall force constant of the probe 共k p兲 can be determined by the spring constants of the cantilever 共kc兲 and the tip 共kt兲
kp =
1 1 1 + kt kc
=
4kskc k tk c = . kt + kc 4ks + kc
共3兲
FIG. 4. ANSYS simulation result for a sidewall of a SOT tip. The thickness of the film is 0.9 m. The result shows a 3.47 m deflection under 1 N force applied at the tip.
The spring constant of the SOT tip 共kt兲 is difficult to evaluate using analytical method due to its complex shape and loading conditions. The best way to obtain this spring constant is through finite element simulation. Three-dimensional 共3D兲 models were made based on actual dimensions of microfabricated tips for simulation. In 3D modeling, one facet of the tip is extracted and modified into a spring shape. Then the deformation of the tip under a concentrated force is simulated using the ANSYS finite element simulation software to determine its force constant. Among many possible spring designs, one representative result is discussed here. As shown in Fig. 4, a Si3N4 tip facet is cut into a spring shape by 12 0.5-m-wide FIB strokes. The film thickness is 0.9 m. In the simulation, the two crosssectional sidewalls of the facet are constrained to deform only in the vertical directions. The bottom plane of the facet is fixed in all directions. The dimensions and the simulation result of the facet are shown in Table I. When a 1 N force is applied at the tip vertically 共pointing down兲, the ANSYS simulation yields a vertical tip displacement of 3.47 m, giving a spring constant of 0.288 N / m. The force constant of a tip with four sidewalls is 1.15 N / m according to Eq. 共2兲. By adjusting the Si3N4 film thickness and the FIB milling size and number, smaller force constants can be achieved. For example, if the film thickness in the tip design of Fig. 4 reduces to 0.3 m, an ANSYS simulation has yielded a vertical displacement of 12.26 m under 1 N force applied at the tip 共Table I兲, corresponding to a force constant of 0.082 N / m for one sidewall of the SOT tip. A tip consisting of four identical spring sidewalls thus has a force constant of 0.32 N / m. The simulation results show that the folded springs alone can provide proper spring constants for selected SPL applications 共e.g., DPN兲. TABLE I.
ANSYS
simulation results for SOT tip designs.
Input and output parameters
1st tip design
2nd tip design
Si3N4a film thickness 共m兲 Tip height 共m兲 Width of tip at bottom 共m兲 FIB milling number FIB milling width 共m兲 Applied force at tip 共N兲 Vertical displacement of tip 共m兲
0.9 7 10 12 0.5 1 3.47
0.3 7 10 12 0.5 1 12.26
a
An elastic modulus of the 225 GPa and a Poisson’s ratio of 0.23 are used FIG. 3. The spring system used to analyze the SOT probe. for the Si3N4 film material in the simulation 共see Ref. 14兲. Downloaded 27 Jul 2005 to 128.174.190.93. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
054102-3
Appl. Phys. Lett. 87, 054102 共2005兲
Wang et al.
The tip is inked with 1-octadecanethiol 共ODT兲 ink, using techniques discussed elsewhere.15 After inking, the tip is brought into contact with a gold-coated silicon substrate for DPN writing. ODT line patterns have been successfully generated by moving the tip on the substrate at different speeds. Figure 5共a兲 shows a lateral force microscopy 共LFM兲 image of three ODT lines generated on a gold surface with writing speed of 0.1, 0.2, and 0.3 m / s. The linewidth is 175, 120, and 95 nm, respectively. Increased writing speed results in decreased linewidth 关Fig. 5共b兲兴, as expected. The linewidth tends to saturate at 43 nm 共at writing speeds of 0.9 and 1 m / s兲, probably due to the relatively big curvature radius of the tip 共⬃0.9 m兲. In conclusion, this letter introduces a spring-on-tip structure for scanning probe lithography. Conventional cantilever tip can be converted to SOT tip using focused ion beam milling. An analytical method combined with finite element analysis has been used to model the force constant of the SOT tip, or a cantilever with an SOT tip located at its distal end. The SOT tip has been successfully used for demonstrating DPN nanolithography. This work was supported by DARPA Advanced Lithography Program and NSF EEC-0118025. The FIB milling experiments were conducted in the Center for Microanalysis of Materials, University of Illinois, which is partially supported by the DOE DEFG02-91-ER45439. FIG. 5. 共a兲 A LFM image of ODT lines generated by a SOT tip on a gold surface. The writing speeds for the lines are 0.1, 0.2, 0.3 m / s and the linewidths are 175, 120 and 95 nm, respectively. 共b兲 ODT linewidths generated by a SOT tip at different writing speeds.
The effective reduction of the SPL tip force constant by FIB milling leads to the possibility of utilizing cantilever-less tip arrays for high-density one-dimensional or 2D SPL patterning. In theory, arrays of SPL tips can be made closer to each other without supporting cantilevers. The linewidth of FIB milling is a function of materials, ion beam current, dwell time, and operating voltage. Using an FEI Dual-Beam DB-235 FIB/scanning electron microscope 共SEM兲, we have achieved minimal linewidth of 20 nm using 1 pA ion current operated at 30 keV. A large milling linewidth can be achieved with high ion current. The SOT tip shown in Fig. 1共a兲 required about 10 min to etch all sides. It is envisioned that the FIB etch time can be reduced in the future with increased experiences and through software automation of FIB milling tools. To verify the functionality of the tips after FIB milling, SOT tips were used for DPN writing. A Si3N4 cantilever SOT tip similar to the one shown in Fig. 1共a兲 is used. The tip is mounted on a Thermomicroscopes AutoProbe M5 AFM.
R. M. Nyffenegger and R. M. Penner, Chem. Rev. 共Washington, D.C.兲 97, 1195 共1997兲. 2 G. S. McCarty and P. S. Weiss, Chem. Rev. 共Washington, D.C.兲 99, 1983 共1999兲. 3 D. A. Bonnell, Scanning Probe Microscopy and Spectroscopy: Theory, Techniques, and Applications 共Wiley, New York, 2000兲. 4 S. Kramer, R. R. Fuierer, and C. B. Gorman, Chem. Rev. 共Washington, D.C.兲 103, 4367 共2003兲. 5 D. M. Eigler and E. K. Schweizer, Nature 共London兲 344, 524 共1990兲. 6 T. Sumomogi, T. Endo, K. Kuwahara, and R. Kaneko, J. Vac. Sci. Technol. B 13, 1257 共1995兲. 7 C. F. Quate, Surf. Sci. 386, 259 共1997兲. 8 J. K. Schoer, F. P. Zamborini, and R. M. Crooks, J. Chem. Phys. 100, 11086 共1996兲. 9 J. K. Schoer and R. M. Crooks, Langmuir 13, 2323 共1997兲. 10 S. Wegscheider, A. Kirsch, J. Mlynek, and G. Krausch, Thin Solid Films 264, 264 共1995兲. 11 P. J. Moyer, K. Walzer, and M. Hietschold, Appl. Phys. Lett. 67, 2129 共1995兲. 12 R. D. Piner, J. Zhu, F. Xu, S. Hong, and C. A. Mirkin, Science 283, 661 共1999兲. 13 D. S. Ginger, H. Zhang, and C. A. Mirkin, Angew. Chem., Int. Ed. 43, 30 共2004兲. 14 D. Bullen, X. Wang, J. Zou, S.-W. Chung, C. A. Mirkin, and C. Liu, J. Microelectromech. Syst. 13, 594 共2004兲. 15 X. Wang, K. S. Ryu, D. A. Bullen, J. Zou, H. Zhang, C. A. Mirkin, and C. Liu, Langmuir 19, 8951 共2003兲. 1
Downloaded 27 Jul 2005 to 128.174.190.93. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
Scanning probe lithography tips with spring-on-tip designs: Analysis, fabrication, and testing Xuefeng Wang, Loren Vincent, David Bullen, Jun Zou, and Chang Liua兲 Micro and Nanotechnology Laboratory, University of Illinois at Urbana-Champaign, 208 North Wright Street, Urbana, Illinois 61801
共Received 7 December 2004; accepted 28 June 2005; published online 27 July 2005兲 This letter reports a special tip design for probes used in scanning probe lithography applications. The sidewalls of the pyramidal tip located at the distal end of a cantilever probe are modified to contain folded spring structures to reduce the overall force constant of the scanning probe. The spring structure is generated using focused ion beam milling method. We have conducted finite element simulation of the force constants of such folded springs under various geometries. We also demonstrated sub-100 nm scanning probe lithography using a modified spring tip in the dip pen nanolithography writing mode. © 2005 American Institute of Physics. 关DOI: 10.1063/1.2006210兴 Scanning probe microscopy 共SPM兲 has been widely used for surface characterization and modification.1–4 It is well known that the scanning probes used in SPM instruments can modify substrate surfaces through mechanical,5–7 electrical,8,9 or optical10,11 approaches. This has led to a rapidly emerging research field—scanning probe lithography 共SPL兲. For example, dip pen nanolithography 共DPN兲12,13 is an additive SPL method. It uses a pre-coated cantilever tip to transfer “ink” molecules onto a substrate. It is uniquely capable of directly patterning many materials 共e.g., organic molecules, biomolecules, metal salts, polymers, and nanoparticles13兲 at molecular level. In all cantilever-probe-based SPL methods, the probe consists of a cantilever with a sharp tip at its distal end, with the cantilever serving as a spring. The mechanical structure of the cantilever determines the stiffness of the probe and consequently the contact force between the tip and the substrate surface. The force constant of the cantilever, k, is
k=
Ewt3 , 4ᐉ3
共1兲
where E is the modulus of elasticity of the cantilever material, and w, t, l are the width, thickness, and length of the rectangular cantilever, respectively. A desired force constant of a cantilever can be reached by trading off the width, length, and thickness parameters of the cantilever. As an example, DPN operation has been conducted using commercial silicon 共Si兲 or silicon nitride 共Si3N4兲 atomic force microscope atomic force microscope 共AFM兲 probes, with their force constants usually in the range of 0.01– 5 N / m. An overly soft probe tip can adhere to the writing surface, whereas an overly stiff one may result in mechanical scratches in the contact mode. For force constant in this range, if the thickness and width of a Si3N4 cantilever are fixed 共e.g., t = 1 m, w = 30 m兲, the length of the cantilever must range from 540 to 68 m. The motivation of this work stems from the need to produce high-density, arrayed SPL probes, especially twodimensional 共2D兲 arrays. The length of the cantilever dictates a兲
the minimal tip-to-tip spacings. The finite length of individual cantilevers will limit the achievable density of tip arrays. To solve this problem, we present a type of probe tip with integrated spring structures. Figure 1 shows two tips with the spring-on-tip 共SOT兲 designs. The tip may consist of 1–4 springs, each occupying one facet of the tip. Selected faces can be completely removed to reduce the number of
Author to whom correspondence should be addressed; electronic mail: [email protected]
FIG. 1. Scanning electron microscopy 共SEM兲 images of Si3N4 SOT tips: 共a兲 a tip with all four sides etched to identical profile, which can be considered as a tip supported by four identical springs connected in parallel; 共b兲 a different profile of a spring tip where the tip is effectively supported by one spring connected to the probe cantilever.
0003-6951/2005/87共5兲/054102/3/$22.50 87, 054102-1 © 2005 American Institute of Physics Downloaded 27 Jul 2005 to 128.174.190.93. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
054102-2
Appl. Phys. Lett. 87, 054102 共2005兲
Wang et al.
FIG. 2. A SOT tip with springs on two sidewalls. Two of four sidewalls have been completely removed.
springs 共see an example in Fig. 2兲. Alternatively, spiral springs may be made 关Fig. 1共b兲兴. The fabrication process for the tips is discussed here. First, pyramidal tips with continuous surfaces were made using Si3N4 film.14 Many practical methods for realizing pyramid tips have been demonstrated in the past. This current method can be used on tips made using a variety of thin film materials. The spring structures are created afterwards on the pyramidal tip using focused ion beam 共FIB兲 milling 共etching兲 method. The FIB milling width can be varied from nanometer to micrometer scale by adjusting FIB parameters. In order to optimize the spring design at the tip, an analytical model combined with finite element analysis is used to evaluate the force constant. It is assumed that the pyramidal tip has four sidewalls with identical spring structures. In this case, the tip can be considered as supported by four springs with equal spring constants 共ks兲 connected in parallel. The force constant of the SOT tip 共kt兲 is the sum of the four spring constants 共2兲
kt = 4ks .
A SPL probe 共or even a SPM probe for this matter兲 can use both traditional cantilever spring and the SOT springs simultaneously. The overall force constant of the tip is distributed to two independent parts: the probe cantilever and the folded springs on tip. In this case the entire probe can be considered as these two spring systems connected in series, as shown in Fig. 3. The design of the cantilever and tip can be conducted separately. The overall force constant of the probe 共k p兲 can be determined by the spring constants of the cantilever 共kc兲 and the tip 共kt兲
kp =
1 1 1 + kt kc
=
4kskc k tk c = . kt + kc 4ks + kc
共3兲
FIG. 4. ANSYS simulation result for a sidewall of a SOT tip. The thickness of the film is 0.9 m. The result shows a 3.47 m deflection under 1 N force applied at the tip.
The spring constant of the SOT tip 共kt兲 is difficult to evaluate using analytical method due to its complex shape and loading conditions. The best way to obtain this spring constant is through finite element simulation. Three-dimensional 共3D兲 models were made based on actual dimensions of microfabricated tips for simulation. In 3D modeling, one facet of the tip is extracted and modified into a spring shape. Then the deformation of the tip under a concentrated force is simulated using the ANSYS finite element simulation software to determine its force constant. Among many possible spring designs, one representative result is discussed here. As shown in Fig. 4, a Si3N4 tip facet is cut into a spring shape by 12 0.5-m-wide FIB strokes. The film thickness is 0.9 m. In the simulation, the two crosssectional sidewalls of the facet are constrained to deform only in the vertical directions. The bottom plane of the facet is fixed in all directions. The dimensions and the simulation result of the facet are shown in Table I. When a 1 N force is applied at the tip vertically 共pointing down兲, the ANSYS simulation yields a vertical tip displacement of 3.47 m, giving a spring constant of 0.288 N / m. The force constant of a tip with four sidewalls is 1.15 N / m according to Eq. 共2兲. By adjusting the Si3N4 film thickness and the FIB milling size and number, smaller force constants can be achieved. For example, if the film thickness in the tip design of Fig. 4 reduces to 0.3 m, an ANSYS simulation has yielded a vertical displacement of 12.26 m under 1 N force applied at the tip 共Table I兲, corresponding to a force constant of 0.082 N / m for one sidewall of the SOT tip. A tip consisting of four identical spring sidewalls thus has a force constant of 0.32 N / m. The simulation results show that the folded springs alone can provide proper spring constants for selected SPL applications 共e.g., DPN兲. TABLE I.
ANSYS
simulation results for SOT tip designs.
Input and output parameters
1st tip design
2nd tip design
Si3N4a film thickness 共m兲 Tip height 共m兲 Width of tip at bottom 共m兲 FIB milling number FIB milling width 共m兲 Applied force at tip 共N兲 Vertical displacement of tip 共m兲
0.9 7 10 12 0.5 1 3.47
0.3 7 10 12 0.5 1 12.26
a
An elastic modulus of the 225 GPa and a Poisson’s ratio of 0.23 are used FIG. 3. The spring system used to analyze the SOT probe. for the Si3N4 film material in the simulation 共see Ref. 14兲. Downloaded 27 Jul 2005 to 128.174.190.93. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
054102-3
Appl. Phys. Lett. 87, 054102 共2005兲
Wang et al.
The tip is inked with 1-octadecanethiol 共ODT兲 ink, using techniques discussed elsewhere.15 After inking, the tip is brought into contact with a gold-coated silicon substrate for DPN writing. ODT line patterns have been successfully generated by moving the tip on the substrate at different speeds. Figure 5共a兲 shows a lateral force microscopy 共LFM兲 image of three ODT lines generated on a gold surface with writing speed of 0.1, 0.2, and 0.3 m / s. The linewidth is 175, 120, and 95 nm, respectively. Increased writing speed results in decreased linewidth 关Fig. 5共b兲兴, as expected. The linewidth tends to saturate at 43 nm 共at writing speeds of 0.9 and 1 m / s兲, probably due to the relatively big curvature radius of the tip 共⬃0.9 m兲. In conclusion, this letter introduces a spring-on-tip structure for scanning probe lithography. Conventional cantilever tip can be converted to SOT tip using focused ion beam milling. An analytical method combined with finite element analysis has been used to model the force constant of the SOT tip, or a cantilever with an SOT tip located at its distal end. The SOT tip has been successfully used for demonstrating DPN nanolithography. This work was supported by DARPA Advanced Lithography Program and NSF EEC-0118025. The FIB milling experiments were conducted in the Center for Microanalysis of Materials, University of Illinois, which is partially supported by the DOE DEFG02-91-ER45439. FIG. 5. 共a兲 A LFM image of ODT lines generated by a SOT tip on a gold surface. The writing speeds for the lines are 0.1, 0.2, 0.3 m / s and the linewidths are 175, 120 and 95 nm, respectively. 共b兲 ODT linewidths generated by a SOT tip at different writing speeds.
The effective reduction of the SPL tip force constant by FIB milling leads to the possibility of utilizing cantilever-less tip arrays for high-density one-dimensional or 2D SPL patterning. In theory, arrays of SPL tips can be made closer to each other without supporting cantilevers. The linewidth of FIB milling is a function of materials, ion beam current, dwell time, and operating voltage. Using an FEI Dual-Beam DB-235 FIB/scanning electron microscope 共SEM兲, we have achieved minimal linewidth of 20 nm using 1 pA ion current operated at 30 keV. A large milling linewidth can be achieved with high ion current. The SOT tip shown in Fig. 1共a兲 required about 10 min to etch all sides. It is envisioned that the FIB etch time can be reduced in the future with increased experiences and through software automation of FIB milling tools. To verify the functionality of the tips after FIB milling, SOT tips were used for DPN writing. A Si3N4 cantilever SOT tip similar to the one shown in Fig. 1共a兲 is used. The tip is mounted on a Thermomicroscopes AutoProbe M5 AFM.
R. M. Nyffenegger and R. M. Penner, Chem. Rev. 共Washington, D.C.兲 97, 1195 共1997兲. 2 G. S. McCarty and P. S. Weiss, Chem. Rev. 共Washington, D.C.兲 99, 1983 共1999兲. 3 D. A. Bonnell, Scanning Probe Microscopy and Spectroscopy: Theory, Techniques, and Applications 共Wiley, New York, 2000兲. 4 S. Kramer, R. R. Fuierer, and C. B. Gorman, Chem. Rev. 共Washington, D.C.兲 103, 4367 共2003兲. 5 D. M. Eigler and E. K. Schweizer, Nature 共London兲 344, 524 共1990兲. 6 T. Sumomogi, T. Endo, K. Kuwahara, and R. Kaneko, J. Vac. Sci. Technol. B 13, 1257 共1995兲. 7 C. F. Quate, Surf. Sci. 386, 259 共1997兲. 8 J. K. Schoer, F. P. Zamborini, and R. M. Crooks, J. Chem. Phys. 100, 11086 共1996兲. 9 J. K. Schoer and R. M. Crooks, Langmuir 13, 2323 共1997兲. 10 S. Wegscheider, A. Kirsch, J. Mlynek, and G. Krausch, Thin Solid Films 264, 264 共1995兲. 11 P. J. Moyer, K. Walzer, and M. Hietschold, Appl. Phys. Lett. 67, 2129 共1995兲. 12 R. D. Piner, J. Zhu, F. Xu, S. Hong, and C. A. Mirkin, Science 283, 661 共1999兲. 13 D. S. Ginger, H. Zhang, and C. A. Mirkin, Angew. Chem., Int. Ed. 43, 30 共2004兲. 14 D. Bullen, X. Wang, J. Zou, S.-W. Chung, C. A. Mirkin, and C. Liu, J. Microelectromech. Syst. 13, 594 共2004兲. 15 X. Wang, K. S. Ryu, D. A. Bullen, J. Zou, H. Zhang, C. A. Mirkin, and C. Liu, Langmuir 19, 8951 共2003兲. 1
Downloaded 27 Jul 2005 to 128.174.190.93. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
