Papers

AVS2001 Session PS2-TuM: Emerging Applications of Plasmas

Tuesday, October 30, 2001 8:20 AM in Room 104

Tuesday Morning

Start	Invited?	Item
8:20 AM		PS2-TuM-1 Plasma Applications for Layer Transfer Technology N.W. Cheung (University of California, Berkeley) Recent progress in bonding and thin-layer splitting enables a new approach to integrate dissimilar thin-film electronic materials. The transfer process incorporates the bonding of two substrates and the use of an ion-cut technique to separate thin films of semiconductors onto various receptor substrates. This paste-and-cut method is an appealing alternative to heterogeneous epitaxial growth approaches because each material layer for a given function can be grown/fabricated on an ideally suited substrate and then combined with a dissimilar receptor substrate. Plasma activated direct bonding and plasma implantation are two key process modules for the success of the layer transfer approach. We will present both low-temperature bonding results using plasma surface activation and ion-cut results using plasma implantation. Material systems such as silicon-on-insulator, III-V semiconductors, and MEMS will be used as examples to illustrate versatility of this approach. Transfer of patterned materials and prefabricated devices have also been proven successful using this technique. * Supported in part by the California SMART Program and National Science Foundation XYZ-on-a-Chip Program.
9:00 AM		PS2-TuM-3 High Density Discharges in Magnetic Fields: Examples of Plasma Thrusters and RF Ion Sources A.B. Bouchoule, M. Irzyk, M. Prioul (GREMI Laboratory, Orléans University, France) Magnetized plasmas are involved in various fields of research and applications for their ability to improve power deposition and reduce transport phenomena in gaseous discharges. Two illustrations are described in this contribution. The first one concerns plasma thrusters for space applications, based on the so-called "closed electron drift" discharges. The idea to use Eï‚´B situations in order to achieve simultaneously a high ionization efficiency and an acceleration of the produced ions, at a level close to the discharge voltage, was initiated in the sixties.The improvements of these devices, and the present requirements for satellites control in space, lead to active R&D programs devoted to these thrusters. Their proved efficiency is already very high, but insights on their complex physics remains required, in order to improve modeling and simulations able to predict optimized designs. Obtained within the frame of a national program, results on time averaged or transient phenomena occurring in these thrusters will be described. From the experimental point of view, specific electrical and optical diagnostics have been developed and will be reviewed. In connection with modeling studies, these data improved our understanding of physical properties of these thrusters, both for their transient and time averaged behavior. The second one concerns high density ion sources, derived from RF inductive discharges in atomic or molecular gases, and using conventional gridded extraction system. Discharges have been obtained in various magnetic field configurations by using a "Nagoya type" external antenna. Results show the impact of the magnetic topography on efficiency and other characteristics of such ion sources. High atomic ion current densities have been obtained both for oxygen and nitrogen discharges. An extensive PIC simulation code for such high current density sources has been developed and will be presented,with its experimental validation.
9:40 AM		PS2-TuM-5 Characterization of Toroidal Plasma Sources for Semiconductor Processing Applications T. Tanaka, C. Lai, M. Cox, T. Nowak, S. Wolff, S. Shamouilian, D. Silvetti, H. Hanawa (Applied Materials Inc.) Toroidal plasma sources have been widely studied for nuclear fusion applications for several decades. The closed topography of the magnetic field provides a superior plasma confinement capability compared to other configurations. This is exemplified by the success of devices such as Tokamak. Over the last few years, there has been growing interest in the toroidal configuration for semiconductor processing applications. Because of the shape of the plasma, the source was primarily considered for downstream remote processing. In this context, plasma confinement is not as important due to the relatively high operating pressures typically used (100mTorr and above). The emphasis rather is placed on the ability to couple the inductive field much more effectively than with a conventional inductively coupled plasma (ICP) source while all but eliminating electrostatic coupling of RF power. Furthermore, by using a magnetic core to couple the driving inductive field, it is possible to operated these source at frequencies less than 400kHz. This is advantageous from the commercial stand point, because of the availability of inexpensive components for the power generator designed to operate at these lower frequencies. At Applied Materials, we have studied electrical properties of toroidal plasma sources. Experimental data is compared to a simple transformer model. We show that power coupling efficiency with these sources is extremely high. Coupling can exceed 98%, which is much higher than with a regular ICP. We also discuss some of the applications of these sources including CVD chamber cleaning
10:00 AM		PS2-TuM-6 Syntheses of Carbon Nanotubes and Nanocapsules by Plasma Chemical Vapor Deposition Y. Hayashi, M. Kawana, S. Nishino (Kyoto Institute of Technology, Japan) Carbon nano-materials such as nanotubes, fullerenes, or nanocapsules have been synthesized by the methods of arc discharge, laser evaporation, and thermal chemical-vapor-deposition (CVD). Recently it was reported that multi-wall carbon nanotubes were grown being well aligned perpendicularly to substrates of catalytic metals by plasma CVD, and they are expected to be used for a field emission display. From these results, we came to think that fine-particles of carbon nano-materials can be synthesized by glow-discharge plasma CVD. Fine-particles are suspended in a glow-discharge plasma for a long period because they are negatively charged in it. Therefore larger fine-particles composed of nano-materials can grow in the plasma. And also the modification and control of the materials are expected being suspended in a plasma. A surface-wave-excited 2.45GHz-microwave plasma was used for the syntheses. It was generated in a vacuum chamber by a microwave propagating through a slot antenna. A fine-particle collector, whose electric potential can be controlled from the outside, was put below the plasma. The gas of 10-40 % methane diluted in hydrogen was introduced into the chamber under a gas pressure of 100-700 Pa. Collected fine-particles were analyzed by scanning electron microscopy. Materials of tubular structure of 50-100 nm in diameter and cavity-like spherical and cubic structures of 100-200 nm in size were observed. They should be carbon nanotubes and nanocapsules. Detailed analyses by transmission electron microscopy are going to be performed.
10:40 AM		PS2-TuM-8 The Development of the VASIMR Engine for Space Propulsion J.P. Squire, F.R. Chang-Diaz (NASA, Johnson Space Center) The development of advanced propulsion technologies represents a cornerstone in the successful realization of long-term human space travel. Unlike their robotic precursors, human interplanetary spacecraft must be fast, reliable, "power rich," and be capable of reasonable abort options --essential features for the preservation of human life. The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is a new approach to plasma propulsion, which addresses these issues and provides an evolutionary path to fusion rockets, but with immediate and exciting non-fusion applications along the way. A NASA-led research effort, involving government, academia and private industry teams in the United States, is exploring the foundations of this concept, and pursuing its rapid development and test. This presentation will cover the basic principles of VASIMR operation, the latest experimental and theoretical results, as well as the most important technological developments and challenges for the future. Light gas (hydrogen, deuterium, helium and mixtures) helicon plasma production and subsequent Ion Cyclotron Resonant Acceleration (ICRA) are key experimental efforts. High density (~ 10¹⁹ m^-3) hydrogen, deuterium and helium plasma discharges have been achieved, with nearly 50% of the injected gas being accounted for in the plasma flow. Recent experiments with a strong magnetic choke (~ 1 tesla) downstream of the helicon source have demonstrated high Mach number (> 1) plasma flows. In certian conditions, high energy (> 50 eV) ion tails have been observed from the helicon source alone. Parametric (e.g. power, gas flow, and magnetic field) studies of the helicon source will be presented. ICRA experiments are in progress, so the configuration and most recent results will be discussed. The conceptual application of the VASIMR to fast human Mars missions, as well as plans for near-term flight demonstrations will also be highlighted.
11:40 AM		PS2-TuM-11 Plasma Etching of Cesium Iodide¹ X. Yang, J.A. Hopwood (Northeastern University); S. Tipnis, V. Nagarkar, V. Gaysinskiy (Radiation Monitoring Devices, Inc.) Scintillator films that convert an incident x-ray image into visible light play an important role in many imaging applications. Because of its superior light output (59,000 photons/MeV), high density (4.54 g/cc), high effective atomic number (52) and rugged nature, CsI(Tl) is often the material of choice for scintillator films. For adequate detection of 8-70 keV x-rays, 30-200 µm thick CsI films are needed. Spreading of light in the scintillator volume, however, limits the resolution of the resulting images. To address this problem we are micromachining CsI screens to form a finely pixelated structure. When coated with a low refractive index material, each micro-pixel acts as an optical waveguide that minimizes the spread of scintillation light in the screen. The micromachining process uses a high-density inductively coupled plasma to etch CsI samples held by a heated, rf-biased chuck. Fluorine-containing gases such as CF₄ are found to enhance the etch rate by an order of magnitude compared to Ar⁺ sputtering alone. Without inert-gas ion bombardment, however, the fluorine-based etch becomes self-limited within a few microns of depth. The formation of a thick passivation layer on the sidewalls of etched features is confirmed by SEM. EDS indicates the passivation layer has a high ratio of Cs to I. Etching exhibits an Arrhenius-type behavior in which the etch rate increases from ~40 nm/min at 40 C to 380 nm/min at 330 C. This temperature dependence corresponds to an activation energy of 0.13 eV. Similar activation energies have been reported for the electronic sputtering of other alkali halides. This suggests that this CsI etch process, similar to alkali halide sputtering, is rate-limited by the thermal migration of ion-induced defects to the CsI surface. Additional results will support a more complete picture of the etching mechanisms. ¹This work is supported by the NIH contract No. 2R44 CA76758-02.