Papers

AVS2002 Session PN-ThA: Charged Particle Patterning and Emission

Thursday, November 7, 2002 2:00 PM in Room C-109

Thursday Afternoon

Start	Invited?	Item
2:00 PM		PN-ThA-1 Nanoscale Electron Beam Stimulated Processing P.D. Rack, R.C. Burns, Y. Deng, Y. Choi, D.C. Joy (University of Tennessee) The ability to manipulate materials at the nanoscale is critical for the nanotechnology revolution that is occurring. To intelligently design and or repair nanoscale devices requires techniques to selectively and nanoscopically deposit and remove material in a controllable fashion. Current techniques to selectively deposit or etch microscopic features utilize ion beam deposition and etching, laser ablative etching using far field and near field optics, and mechanical abrasion using a fine microtip. In this paper we will discuss electron stimulated nanoscale processing. The experimental set-up for the process will be presented and experimental results for electron stimulated nanoscale processing will be shown. The relevant electron-solid, electron-gas, and gas-solid interactions that are critical for nanoscale electron beam processing will be discussed. The effect that the beam energy has on the reaction rate for stimulated deposition and etching will be illustrated and the growth kinetics will be discussed. To minimize the effective spot size, strategies to control surface diffusion as well as electron-gas scattering will also be discussed.
2:20 PM		PN-ThA-2 Electron Beam Patterning with Carboneous Contamination Rezists below 10 nm Linewidth M. Malac (University of Alberta, Canada); J. Lau (Brookhaven National Laboratory); R. Egerton, M. Freeman (University of Alberta, Canada); Y. Zhu (Brookhaven National Laboratory) We have studied the processes determining the resolution limits of contamination patterning in a transmission electron microscope (TEM) as a means to produce magnetic nanostructures. By optimizing the exposure parameters we achieved linewidth of 7 nm. Sputter etch was used to transfer the pattern to cobalt, permalloy and bismuth films on silicon nitride membranes. Previously the resolution of the technique was determined by the crystallite size of the patterned film.¹ We have investigated patterning of our structures within one large crystallite and we have studied the limitations dictated by the kinetics of the surface diffusion processes. A comparison of pattern writing at conditions when importance of surface diffusion is suppressed (high dose rate and high writing speed) with diffusion controlled writing (slow writing at low dose rate) was made. We have explored the reproducibility and suitability of different types of materials for precontaminating the samples. Transmission electron microscopy, electron holography, electron energy loss spectroscopy and high resolution TEM was used to characterize structure and to study properties of such structures. ¹Broers et al, Appl. Phys. Letters 29 (9), 1976 page 596.
2:40 PM		PN-ThA-3 A Study of PMMA EBL Cross-sections Prepared by a Novel Process W. Hu, T. Orlova, G.H. Bernstein (University of Notre Dame) We have developed a novel technique of silicon cross-section preparation with high breaking precision with which we have studied cross-section profiles of sub-50 nm electron beam lithography (EBL) patterns in PMMA resist, including post-exposure aging effects. This technique enables us to break our samples along a desired axis with about 1 µm precision, and gives cross-sections allowing imaging with scanning electron microscope (SEM) at any angle. This is a complementary method to cleaving, wedge-polishing and focused ion beam, the last two of which create damage to the surface of the exposed edges. The precise breaking of the wafer or small sample was accomplished by high-aspect-ratio plasma etching of a 2~10 µm wide line in an inductively coupled plasma etcher using the Bosch process (Alcatel 601E) with small sample areas in the line unetched for EBL. After EBL, wafers were broken along the etched line using a simple mechanical. Cross-section studies can provide improved information on the nanometer scale about PMMA EBL profiles, development contrast, and the metal or molecule liftoff process. However, damage to PMMA by the SEM imaging, cross-section preparation and sample metal coating interfere with data analysis. We have developed an optimized process which enables us to obtain nearly original PMMA EBL profiles by coating samples with AuPd alloy using thermal evaporation first and followed by a plasma sputter coating. The fully-covered PMMA trenches are broken along the desired axis and imaged with our Hitachi S-4500 cold cathode field emission SEM. Using these techniques, we studied the aging effect of PMMA resist before development of EBL, which is seldom taken into consideration in fabrication processes. Our cross-section results of PMMA EBL trenches indicate a loss of contrast due to one week aging prior to development, and will result in unsuccessful liftoff. Other cross-sectional views of EBL in a variety of resists will be discussed.
3:00 PM		PN-ThA-4 Nanometer Scale Patterning of Dielectric Surfaces with Combined Electron/laser Beams and Chemical Exposure K.H. Nwe, J.T. Dickinson (Washington State University) We examine the nanometer scale patterning of insulating surfaces exposed to simultaneous beams of radiation and reactive chemicals. Specifically, we show that ionic crystals in the presence of low pressure water exposure leads to rapid material etching in self-organized patterns under UV laser or keV electron irradation. Radiation effects on reactive layers on single crystal inorganics are poorly understood. We find that dense ordered arrays of highly oriented nanoscale conical structures, with aspect ratios greater than 200, are produced under such exposure. We show that the nanostructure formation mechanism involves photo/electron stimulated decomposition of the matrix via defect mediated interactions. Using time resolved mass spectroscopy we are able to provide detailed kinetics for these electronic etching processes.
3:20 PM		PN-ThA-5 Field Emitter Arrays with Sharp Tips for THz Microwave Source C. Peng, S.W. Pang (The University of Michigan) Ballistic tunneling transit time device consists of a field emitter cathode, a drift region, and a collecting anode. These new transit time devices can produce mW level power in THz range with optimized operating voltage and anode to cathode separation. The field emission devices also need to have sharp tips, high emission current, low threshold voltage, and small dimensions for high packing density. In this work, nanofabrication technology to generate large arrays of sharp Si field emitters is developed. Sharp Si tips with tip radius of 20 nm are fabricated by dry etching with Cl₂ in an inductively coupled plasma system. The tip sharpness, height, and profile are controlled by balancing the lateral etch rate of the SiO₂ mask and the vertical etch rate of the Si substrate. It is found that tip height and tip radius vary with rf power applied to the plasma source and the etch time. Tip height decreases with rf power and increases with etch time, while tip radius increases with source power and decreases with etch time. Uniform, large arrays of Si field emitters are formed with tip density of 1.1x10⁷ tips/cm² and tip height varying from 2 to 4 µm. For these field emitters to function as THz microwave source, close tip to anode spacing in the range of 0.5 to 2 µm is needed. Etch time, spacer, and bonding technology are used to provide flexible and precise control of the tip to anode distance. In addition, emitter tip sharpening by oxidation and coating the tips with low work function materials will be studied to improve the emission current characteristics.
3:40 PM		PN-ThA-6 Structure and Field Emission Properties of SiC Nanotip Arrays Fabricated by Electron-cyclotron-resonance Plasma Process of Monolithic Si Wafer L.C. Chen (National Taiwan University); H.C. Lo, J.S. Hwang, J.S. Hsu (Institute of Atomic and Molecular Sciences); C.F. Chen (National Chiao-Tung University, Taiwan); D. Das, K.H. Chen (Institute of Atomic and Molecular Sciences) Fabrication of SiC nanotip arrays of a few microns in height and around 1 nm in diameter at the tip and of density as high as 10¹²cm^-2 is reported. The mechanism of the nanotip formation by electron cyclotron resonance (ECR) plasma process and detailed structure analyses using high resolution TEM will be presented. It is concluded that the tips are primarily composed of SiC. The nanotip arrays showed magnificent field emission property with typical field emission current of 0.5 mAcm^-2 at an applied field as low as 0.8 V/µm. Furthermore, the nanotip arrays also exhibited excellent stability, as evident by temporal evolution of the emission current at a constant applied voltage measurement which showed less than 3% fluctuation in one hour. The SiC nanotip array produced by ECR-plasma process of monolithic Si wafer offers a reliable, economic field emission electron source alternative to carbon nanotubes. Formation of the SiC nanotip on top of a Si cantilever has also been demonstrated. Such geometry provides potential applications in ultrahigh resolution SPM and field emission microscopy.
4:00 PM		PN-ThA-7 Field Emission from Well-Aligned Heterojunctions of Carbon Nanotubes and Silicon Nanowires M. Lu, M.-K. Li, H.-L. Li (Lanzhou University, P.R. China) Silicon nanowires (SiNWs) has recently attracted much attention due to its unusual properties. However, it is easily oxidized in air, which can hurt the uniformity and the efficiency of electron emission. It is our motivation to design nanowire/tube growth process and fabricate well-aligned CNTs/SiNWs heterojunctions by chemical vapor deposition (CVD) based on AAO template-synthesis method.¹ It is clearly observed from SEM and TEM images the junctions consist of a core-sheath structure and have a highly-orientation. The filling material is proved to be a polycrystalline Si from SAED pattern. Field emission measurements show the turn-on field of the junctions decreases to ~7 V/µm from ~14 V/µm (pure SiNWs) and the maximum emission current density increases to ~35 mA/cm² from ~3 mA/cm² (pure SiNWs). A significant improvement in the turn-on field, the total emission current and the emission stability is apparent from the junctions compared with pure SiNWs. We also notice that the threshold field is in the range of 22-25V/µm for the junctions. The Fowler-Nordheim plot almost follows a linear relationship that indicates the field emission from the junctions is a barrier tunneling, quantum mechanical process. Moreover, the field emission increased with decreasing diameter of the junctions. During 24 h of continuous operation at 5 mA/cm², the current fluctuation was as low as ±4% and the average current did not decrease over this period. When a sample was found to air for weeks after it was grown, it exhibited essentially the same emission behavior as the freshly grown surface. This suggests that the sheath of CNTs is indeed stable and chemically inert. ¹, C.W. Wang, M.K. Li, and H.L. Li, Science in China 44(2), 234 (2001)
4:20 PM		PN-ThA-8 Evolution of Surface Morphology and Microstructure during Ion Sputtering of Diamond D.P. Adams, T.M. Mayer, M.J. Vasile (Sandia National Laboratories) This work investigates the morphology and structure of diamond surfaces that evolve during focused ion beam (FIB) sputtering and chemically amplified processes. We include a detailed study of morphology for a broad range of ion incidence angles (0-88 deg) and dose. A single ion (Ga) and incident energy (20keV) are used. For incidence angles between 30-75 degrees referenced from substrate normal, we observe the formation of ripple patterns. Ripples form with a wavevector aligned to the incident beam direction over this range of angles, similar to that predicted by Bradley-Harper for small theta. In order to develop an understanding of the mechanisms underlying a rippled topography, we analyze the evolution of morphology for increasing dose. AFM is used to quantify changes in ripple wavelength and amplitude, while TEM and Raman spectroscopy probe the structure in the near surface region. Beginning at 75 and continuing to 87 degrees, a step/terrace morphology forms during sputtering. Microfacets develop at the earliest stages (lowest dose) indicating a role of initial surface topography. Obvious changes in terrace length over this range of angles suggest that morphology is affected by shadowing between neighboring features. In general, a decrease in yield is discovered with increasing dose over this entire range of angles (30-87 deg). The presence of ripples/microfacets and, more specifically, their evolving shape influence the number of sputtered atoms per ion. Dramatic effects on yield (200% decrease) with increasing dose are observed at angles >80 degrees. Additional research investigates the effects of chemical environment on morphology. Experiments involve ion bombardment in the presence of H2O, methanol and H2O2. When sputtering in the presence of these gases, dramatic changes in morphology result as a function of dose and incidence angle. Changes in sputter yield due to the presence of a gas are also quantified as a function of incidence angle from 0-88 degrees.
4:40 PM		PN-ThA-9 Nanoscale Morphology Control Using Ion Beams M.J. Aziz (Harvard University) Low energy ion irradiation of a solid surface can be used to control surface morphology on length scales from 1 micron to 1 nanometer. Focused or unfocused ion irradiation induces a spontaneous self-organization of the surface into nanometer-sized ripples, dots, or holes; it also induces diameter increases and decreases in a pre-existing nanopore by a tradeoff between sputter removal of material and stimulated surface mass transport. Here we report experiments that illuminate the kinetics of evolution of the surface morphological instability; the influence of initial and boundary conditions on guiding the self-organization; and the kinetics governing the fabrication of nanopores for single-molecule detectors.