Applications of Ion Track Lithography in Vacuum Microelectronics
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Applications of Ion Track Lithography in Vacuum Microelectronics Ronald G. Musket1 Materials Science and Technology Division Lawrence Livermore National Laboratory Livermore, CA 94550, USA 1 Present address: 3877 Meadow Wood Drive, El Dorado Hills, CA 95762, USA EXTENDED ABSTRACT When a high-velocity (i.e., typically > 0.1 MeV/amu) ion passes through a material it can change the properties of the material within a cylindrical zone centered on the essentially straight trajectory of the ion. The electronic bonding, phase, and density are among the properties modified in the zone, which is called a latent nuclear, or ion, track. Because the diameters of latent ion tracks are typically less than 20 nm, selective chemical etching is generally employed to improve the detection and assessment of the tracks. Historically, etched nuclear tracks have been used mainly for nuclear particle identification, geochronology, measurement of extremely low-dose radiation levels, and creation of membrane filters. An introduction to the basic concepts involved in the creation and etching of ion tracks to make masks for lithographic processes has been detailed [1]. Ion track lithography provides two main advantages over other lithographic techniques. Ions can be used to generate structures with the largest aspect ratios (i.e., length/width) and the smallest lateral widths. The main limitation of ion track lithography is that the tracks are stochastically distributed over the exposed surface For fabrication of arrays of field emitters the random location of the tracks is not a limitation, because all the field emitters within a given area (e.g., sub-pixel of a video display) will be turned on at the same time. However, the track density should be low enough to provided essentially only isolated tracks after etching the track diameters out to the desired dimensions. As an example, only ~5% of 40-nm diameter tracks will be overlapping at a density of 109/cm2. Such isolated tracks in positive resists (e.g., polycarbonate films) can be etched to form molds for deposition of structures and to form masks for transferring the track pattern to levels below the tracked resist. Specific examples of these two approaches are described here. The first is the creation of arrays of free-standing nano-emitters. Approximately 600-nm thick polycarbonate was spun onto a nickel-covered silicon wafer. Latent ion tracks with a density of ~108/cm2 were created with 13.6 MeV Xe+4 ions incident perpendicular to the surface of the polycarbonate. The tracks were etched near room temperature with 6M KOH to a diameter of ~80 nm, and then nickel was electroplated into the molds created by the etched tracks to nearly fill the cylindrical holes. Removal of the polycarbonate mold by immersion in the KOH solution resulted in the field emitter array shown in Fig. 1. The I-V characteristics of such arrays were found to follow the Fowler-Nordheim predictions, demonstrating that the currents were due to field emission.
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The hole diameter in the polycarbonate has been s
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