Nanofabrication of a two-dimensional array using laser-focused atomic deposition R. Gupta, J.J. McClelland, a )Z. J Jabbour, and R.J.Celotta Electron Physics Group, National Institute of standards and Technology, Gaithersburg, Maryland 20899 Received 16 May 1994; accepted for publication 27 June 1995) Fabrication of a two-dimensional array of nanometer-scale chromium features on a silicon substrate by laser-focused atomic deposition is described. Features 13+1 nm high and having a full-width at half maximum of 80+ 10 nm are fabricated in a square array with lattice constant 212. 78 nm, determined by the laser wavelength. The array covers an area of approximately 100 umX 200 um. Issues associated with laser-focusing of atoms in a two-dimensional standing wave are discussed, and potential applications and improvements of the process are mentioned New methods for of nanometer-scale struc- laser-focused atomic deposition process used in the present tures have been under nvestigation recently be- work is shown in Fig. 1. A laser standing wave is generated cause of the perceived which might arise, such as across the surface of the substrate, and chromium smaller electronic devices, higher-density information stor- collimated to 0.25 mrad in each of two dimensions by laser age, and novel materials. Within the past three years, a new cooling, are directed at the surface, traversing the laser field technique for nanostructure fabrication involving laser- on their way to deposition. The laser field, produced by a focused atomic deposition has been demonstrated I-In this single-frequency CW dye laser, is tuned 500 MHz(100 natu- paper we present a significant enhancement of the original ral line widths)above the atomic resonance line in chromium technique, which was used to fabricate lines on a substrate, at A=425.55 nm(vacuum wavelength). With this tuning, a to demonstrate the first two-dimensional fabrication. With dipole force is exerted on the atoms toward the low intensity these new results. we also discuss some of the considerations regions of the light field. The result is a concentration of associated with generalizing the dimensionality of the pro- atoms at the nodes of the standing wave, which occur at intervals of A/2=212.78 nm In laser-focused atomic deposition, a laser light-field is In one dimension, the process as depicted in Fig. I is used to control the motion of atoms as they deposit onto a relatively straightforward. One aspect of the geometry illus- surface. This approach has a number of potential advantages trated in Fig. I that simplifies the implementation of this in comparison with conventional fabrication techniques, such process is the fact that the positions of the deposited lines on as optical or electron-beam lithography. The advantage over the substrate depend only on the standing wave node posi- optical lithography lies in the potential for much higher reso- tions, which in turn depend primarily on the absolute dis lution. Optical techniques are fundamentally limited by dif- tance along the substrate from the mirror generating the fraction to a minimum feature size of about half the wave- standing wave. To first order, variations in the position or length of the light used. For ultraviolet light, this corresponds direction of the laser beam have no effect on the node pos to about 100 nm. Free-flying thermal atoms, on the other tions, and as long as good stability Is maintained between hand, have De Broglie wavelengths of order 10 pm, so dif- mirror and substrate, the periodicity of the pattern will be fraction effects can in principle be reduced to a negligible level. The actual resolution limits of laser -focused atomic deposition are still a subject of research, though structures Chromium atoms have already been demonstrated at the 65 nm level and theo- retical predictions suggest that 5-10 nm features may be possible This range of feature size is already just electron beam lithography 4 however this pr Laser ently serial, that is, a complex pattern must be fabricated by scanning the electron beam across the surface. For large, complex patterns, the fabrication time and associated drift problems make electron beam lithography less desirable. Laser-focused atomic deposition, on the other hand, does not suffer from these limitations because it can be implemented in a parallel fashion FIG. I. One-dimensional schematic of laser-focused atomic deposition A one-dimensional schematic of the two-dimensional cess, showing chromium atoms being focused by a laser standing wave into its nodes. The trajectories and the deposited peaks represent the results of actual calculations of the focusing process, though the relative vertical ales are highly distor 1378 Appl. Phys. Lett. 67(10), 4 September 1995 Downloaded-v14-may-2008to7222.29.123.220.-redIstributionsubjecttoaip-licenseoncopyright;seehttp:/lapl.aiporglapl/copyrightjspNanofabrication of a two-dimensional array using laser-focused atomic deposition R. Gupta, J. J. McClelland,a) Z. J. Jabbour, and R. J. Celotta Electron Physics Group, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ~Received 16 May 1994; accepted for publication 27 June 1995! Fabrication of a two-dimensional array of nanometer-scale chromium features on a silicon substrate by laser-focused atomic deposition is described. Features 1361 nm high and having a full-width at half maximum of 80610 nm are fabricated in a square array with lattice constant 212.78 nm, determined by the laser wavelength. The array covers an area of approximately 100 mm3200 mm. Issues associated with laser-focusing of atoms in a two-dimensional standing wave are discussed, and potential applications and improvements of the process are mentioned. New methods for fabrication of nanometer-scale struc￾tures have been under intensive investigation recently be￾cause of the perceived benefits which might arise, such as smaller electronic devices, higher-density information stor￾age, and novel materials. Within the past three years, a new technique for nanostructure fabrication involving laser￾focused atomic deposition has been demonstrated.1–3 In this paper we present a significant enhancement of the original technique, which was used to fabricate lines on a substrate, to demonstrate the first two-dimensional fabrication. With these new results, we also discuss some of the considerations associated with generalizing the dimensionality of the pro￾cess. In laser-focused atomic deposition, a laser light-field is used to control the motion of atoms as they deposit onto a surface. This approach has a number of potential advantages in comparison with conventional fabrication techniques, such as optical or electron-beam lithography. The advantage over optical lithography lies in the potential for much higher reso￾lution. Optical techniques are fundamentally limited by dif￾fraction to a minimum feature size of about half the wave￾length of the light used. For ultraviolet light, this corresponds to about 100 nm. Free-flying thermal atoms, on the other hand, have De Broglie wavelengths of order 10 pm, so dif￾fraction effects can in principle be reduced to a negligible level. The actual resolution limits of laser-focused atomic deposition are still a subject of research, though structures have already been demonstrated at the 65 nm level and theo￾retical predictions suggest that 5–10 nm features may be possible.2 This range of feature size is already just attainable with electron beam lithography;4 however this process is inher￾ently serial, that is, a complex pattern must be fabricated by scanning the electron beam across the surface. For large, complex patterns, the fabrication time and associated drift problems make electron beam lithography less desirable. Laser-focused atomic deposition, on the other hand, does not suffer from these limitations because it can be implemented in a parallel fashion. A one-dimensional schematic of the two-dimensional laser-focused atomic deposition process used in the present work is shown in Fig. 1. A laser standing wave is generated across the surface of the substrate, and chromium atoms, collimated to 0.25 mrad in each of two dimensions by laser cooling,5 are directed at the surface, traversing the laser field on their way to deposition. The laser field, produced by a single-frequency CW dye laser, is tuned 500 MHz ~100 natu￾ral line widths! above the atomic resonance line in chromium at l5425.55 nm ~vacuum wavelength!. With this tuning, a dipole force6 is exerted on the atoms toward the low intensity regions of the light field. The result is a concentration of atoms at the nodes of the standing wave, which occur at intervals of l/25212.78 nm. In one dimension, the process as depicted in Fig. 1 is relatively straightforward. One aspect of the geometry illus￾trated in Fig. 1 that simplifies the implementation of this process is the fact that the positions of the deposited lines on the substrate depend only on the standing wave node posi￾tions, which in turn depend primarily on the absolute dis￾tance along the substrate from the mirror generating the standing wave. To first order, variations in the position or direction of the laser beam have no effect on the node posi￾tions, and as long as good stability is maintained between mirror and substrate, the periodicity of the pattern will be stable. a! Electronic mail: jabez@epg.nist.gov FIG. 1. One-dimensional schematic of laser-focused atomic deposition pro￾cess, showing chromium atoms being focused by a laser standing wave into its nodes. The trajectories and the deposited peaks represent the results of actual calculations of the focusing process, though the relative vertical scales are highly distorted for clarity. 1378 Appl. Phys. Lett. 67 (10), 4 September 1995 Downloaded¬14¬May¬2008¬to¬222.29.123.220.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright;¬see¬http://apl.aip.org/apl/copyright.jsp