Papers

AVS1997 Session VT+FP+AS-TuA: Pressure and Beam Effects on FPDs and Vacuum Microelectronics

Tuesday, October 21, 1997 2:00 PM in Room N

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		VT+FP+AS-TuA-1 Cold Cathode Field Emitter Array on a Quadrupole Mass Spectrometer: the Route to Miniaturization T.E. Felter (Lawrence Livermore National Laboratory) We have improved the venerable quadrupole mass spectrometer by substituting the conventional hot filament electron source by a field emitter array, FEA. Elimination of the hot filament avoids a number of common problems, including, thermal cracking of delicate molecules, outgassing of the filament itself and nearby components, high power requirements for the filament, large size, stray light, stray magnetic fields, contamination by thoria and tungsten, and long warm-up time. The advantages are clearest for portable applications where power requirements dominate. Here, the power savings are not just in eliminating the filament supply, but more importantly in reducing the largest component of the system, the vacuum pump. This comes about because the filament is the primary gas load and because chemical reactions taking place on it require fast pumping to keep the products from interfering with the spectra. Comparison between hot filament and cold cathode FEA ionization is made using a quadrupole mass spectrometer fitted with both electron sources, independently controlled. The FEA advantage is strongest when the UHV system is throttled to a low pumping speed, mimicking a portable system with a small pump. FEAs also enable miniaturization and a corresponding decrease in pump size. Moreover, with miniaturization, shorter mean free paths and consequently higher working pressures can be tolerated further decreasing pump requirements.
2:20 PM		VT+FP+AS-TuA-2 Silicon Field Emitter Cathodes - Fabrication, Performance and Applications D. Temple, W.D. Palmer (MCNC) Microfabricated field emitters have been investigated as electron sources for flat-panel displays since the devices were first proposed by Buck and Shoulders in 1958. The concept was reduced to practice by Spindt in 1968, who formed the emitter tips by evaporating molybdenum through a shrinking aperture. In 1986, Gray demonstrated similar devices using silicon as the starting material and forming the tips by etching. Since then, the flexibility afforded by the etching techniques has led to expanding interest in using silicon emitters for applications not only in displays, but also as electron sources for microwave power tubes, analytical instruments, and sensors. The switch to silicon brings several advantages. The breadth and depth of silicon processing techniques developed for the integrated circuit industry can be applied directly. The tips are formed early in the process sequence, so the tip shape and sharpness are not dependent on the overall geometry of the device. Thus, devices ranging from simple pyramids to gated tip-on-post emitters with integrated focusing electrodes can be built, tailoring the electrical parameters of the field emitter array (FEA) cathode to the requirements of the application. The shape of the tip can be varied depending on the etching technique employed, affecting the field enhancement at the apex of the tip and, consequently, the operating voltage of the device, and the tips can be sharpened using standard oxidation methods. Further, integration of silicon FEAs with solid-state signal processing devices can be envisioned. This talk will describe the development of the field of vacuum microelectronics, and review the evolution of silicon field emitter cathode fabrication. Electrical characteristics of MCNC silicon FEAs and those of other groups will be presented and compared with data for metal emitters. Potential applications will be discussed, including flat-panel displays, microwave amplifiers, and sensors.
3:00 PM		VT+FP+AS-TuA-4 Emission-Induced Changes in Si FEA I-V Characteristics J.L. Shaw (Naval Research Laboratory) The I-V characteristics of field emitter arrays often change over time, primarily while the arrays are in operation. The current obtained at a given voltage can go either up or down. Typically, the current initially obtained is very low but can be increased by orders of magnitude simply by operating the array. This is often referred to as "conditioning" the array. Following conditioning, the emission can become stable below a maximum voltage, but during conditioning the probability of arc damage is elevated, especially immediately after an increase in current or voltage. Thus the manner in which the array is conditioned typically has great significance in determining the ultimate performance of the arrays. To help quantify the conditioning process, we have measured the I-V characteristics of Si field emitter arrays and extracted the parameters A and B from the empirical relation I=AV²exp(-B/V). The arrays (fabricated at MCNC from 0.02ohm-cm Si) were exercised at constant gate voltage. Both the A and B parameters change with time and current. The initial increase in current is typically due to an increase in A only. Operation at higher current increases B as well as A. The conditioning effect may be caused by changes in surface adsorbates which alter the surface electron density and/or the work function. Arcing could also be caused by adsorbates. Possible mechanisms consistent with these results will be discussed.
3:20 PM		VT+FP+AS-TuA-5 Getters and Gettering in Plasma Display Panels R.M. Caloi, C. Carretti (SAES Getters S.p.A., Italy) Amongst the different types of flat displays, plasma display panels (PDP) are very attractive for large area high definition TV applications, showing wide viewing angle, and simple manufacturing process. In the frame of the PDP technology development, the control of the impurity level inside the discharge gas is an important issue. In fact, even though specific literature on gas contamination in PDPs is not extensive, manufacturers experience showed that some gas species, such as H₂ and H₂O, can increase the discharge voltage and contaminate the MgO layer (when present), whereas Ar atoms can produce significant sputtering effects. Three main contamination sources can be identified inside a PDP: the residual gas after the exhaust process, the impurity content in the filling gas and the internal components outgassing. To reduce these contributions, an efficient exhausting process is necessary; however, the presence of the barrier ribs remarkably reduces the internal conductance of the displays, thus forcing the manufacturers to pump the panels for a long time. In the present paper, starting from a calculation of the internal conductance inside AC and DC PDPs, a simple theoretical model was used to make an estimation of the pressure evolution inside a display during the exhaust process and the life time. All the obtained values were then compared to the results of specific measurements carried out on real PDPs. Based on these data, the role of a getter inside PDPs and the possible getter configurations were discussed: according to the proposed model, the introduction of non evaporable getters (NEG), mounted inside the panel in a suitable arrangement, appears to be able to improve the quality of the devices. NEGs can in fact act as in-situ pump during the production process, reducing the exhausting time, as well as gas purifier during the life of the PDP, sorbing the outgassed species.
3:40 PM		VT+FP+AS-TuA-6 Surface Chemical Reactions Stimulated by an Electron Beam on Flat Panel Display Cathodoluminescent Phosphors P.H. Holloway, T. Trottier, S.J. Jones, J. Sebastian (University of Florida, Gainesville) The correlation between degradation of cathodoluminescent (CL) intensity and spectra and changes in surface chemistry of ZnS and Y₂O₂S based phosphors for field emission flat panel displays has been studied using Auger electron spectroscopy. It was demonstrated that electron beam stimulated formation of non-luminescence surface layers in the presence of oxidizing or reducing gases in the vacuum, and that CL intensity decreased exponentially with electron dose while a fixed degree of degradation depended linearly on gas pressure. The rate of reaction could be characterized by a cross section which increased as the electron energy decreased from 2 to 1keV. In addition, the cross section varied with the type of gas in the vacuum chamber. The degrading surface chemical reactions could be modified by choosing appropriate gases to backfill into the vacuum chamber. In addition, oxygen caused a spectral redistribution in the case of Y₂O₂S:Eu phosphors indicating beam stimulated conversion of Y₂O₂S:Eu to a luminescent Y₂O₃:Eu surface layer. The effects of other surface modification approaches for protection of the CL phosphors will be illustrated and discussed. This work supported by DARPA Grant MDA 972-93-I-0030 through the Phosphor Technology Center of Excellence.
4:00 PM		VT+FP+AS-TuA-7 Investigation of Electron Induced Damaging of Molecular Overlayers by Imaging TOF-SIMS D. Rading, V. Liebing, G. Becker, M. Mört, H. Fuchs, A. Benninghoven (Universität Münster, Germany) Most surface analytical techniques (e.g. AES, EMP, SIMS, XPS, or AFM) are based upon an interaction between a sensing probe (electrons, ions, photons, nanotips) and the respective surface. In particular for molecular surfaces this interaction may result in the destruction or modification of the considered surface layer. Studying this interaction is important for an assessment of structuring and modification capabilities of the respective probe as well as for an assessment of damaging effects in the case of analytical applications. We applied imaging TOF-SIMS under static conditions to investigate the interaction of electrons with a covalently bonded silane SA-layer (FPC) on silicon and a physisorbed LB-film of a phospholipide (DPPC) on gold. For this purpose well defined surface areas of the respective layer system were irradiated with electron beams of different energies (500 eV to 25 keV) and different dose densities (up to 3 x 10¹⁴ cm²). In a second step the prebombarded surface areas were analyzed by imaging static SIMS providing characteristic secondary ion intensities in dependence of the prebombarding parameters (electron energy and dose density). From these results electron induced damage cross sections σ_e(E) could be determined. We found a strong energy dependance of σ_e(E), very similar to that well known for electron induced secondary electron yields. For both investigated layers we found very similar damage cross section σ_e of about 10-15 cm² for 1 keV electrons. This is in contrast to the strong influence of the binding energy of the molecular overlayer to the substrate on the ion induced damage cross sectionσ_i. Our results demonstrate the general capabilities of imaging static SIMS to investigate quantitatively damaging effects in molecular surfaces.
4:20 PM		VT+FP+AS-TuA-8 High-Resolution Electron Energy Loss Spectroscopy Studies of Electron Beam Effects on Trimethylsilane Adsorbed on Si(100) Surfaces. J. Lozano, J.H. Craig, Jr. (University of Texas, El Paso) Previous electron stimulated desorption (ESD) show the development of new hydrogen states as a result of electron beam irradiation of trimethylsilane (TMSi) adsorbed on Si(100) when physisorbed species are present. Under the same circumstances, temperature programmed desorption (TPD) studies show broadening of the high temperature side of the hydrogen ß ₁ peak, due to the desorption of hydrogen from newly deposited carbon on Si(100). This suggest an increase of carbon deposition on Si(100) as a result of electron irradiation of TMSi/Si(100) in the presence of physisorbed species. Changes in the intensity of SiH, CH, and SiC vibrational peaks following electron irradiation for various TMSi dosages and electron beam exposures are discussed and correlated to X-Ray Photoelectron Spectroscopy (XPS). HREELS data suggests that electron irradiation on TMSi/Si(100) at low TMSi dosages (<0.01 L) causes the transfer of hydrogen from silicon to carbon on the surface, whereas at high TMSi exposures (>0.1 L), the electron beam increases the deposition of carbon on Si(100) at 110K.
4:40 PM		VT+FP+AS-TuA-9 Factors Influencing the Reduction of Hexavalent Chromium during X-Ray Photoelectron Spectroscopy C.R. Clayton, G.P. Halada, S.V. Kagwade (State University of New York, Stony Brook) Chromium(VI) is an active constituent of corrosion protective chromate conversion coatings used for aluminum alloys. Due to the current emphasis on elimination of chromium from an ecological and occupational safety perspective, it is important to understand the role of Cr(VI) in chromate conversion coatings in order to aid in the search for suitable and environmentally acceptable replacements. In this study we report the findings of soft x-ray induced reduction of Cr(VI) to Cr(III) during x-ray photoelectron spectroscopy (XPS) as influence by: the presence of organic contaminants from rotary and diffusion pump oils; surface charge compensation; and sample cooling. A time dependent study of the reduction of Cr(VI) to Cr(III) under the combined or solitary influence of the above mentioned sample and vacuum conditions has been studied by x-ray photoelectron spectroscopy. Observations are based on the Cr 2p photoelectron spectra obtained from analytical grade chromium trioxide powder pressed onto indium foils. Data acquisition has been performed at take-off angles of 10 degrees with respect to the sample surface, to detect the onset of reduction taking place within the first few monolayers. Contaminants from the rotary pump and diffusion pump used in pumping down the analysis chamber to 2 X 10^-9 Torr were found to accelerate the reduction of Cr(VI) to Cr(III). In this paper we shall also discuss the influence of Cr(VI) reduction following substitution of the rotary pump with a sorption pump which eliminates the organic contaminants. Similarly, replacing the diffusion pump with an ion pump improves the vacuum by an order of magnitude which in turn lowers the amount of organic contaminants. Surface charge compensation and sample cooling with liquid nitrogen are the other factors influencing the reduction of Cr(VI) to Cr(III).