Radiant energy – Radiant energy generation and sources
Reexamination Certificate
2000-05-11
2003-05-27
Lee, John R. (Department: 2881)
Radiant energy
Radiant energy generation and sources
C250S492300
Reexamination Certificate
active
06570172
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to ion beam sources, and more particularly, to a negative ion beam source that evidences a large area having either a circular or rectangular geometry.
BACKGROUND OF THE INVENTION
The prior art includes a number of procedures for depositing thin films of materials on both conductive and nonconductive substrates. Ion beam deposition is beneficial for many types of coating techniques. For deposition of diamond-like carbon coatings, chemical vapor deposition and vapor phase deposition techniques are predominant. Chemical vapor deposition procedures include hot filament emission systems, plasma assisted deposition systems, plasma jet and DC arc jet systems. Each of the deposition-procedures requires that the substrate have a high temperature. The processes further employ a gas mixture with a large percentage of hydrogen. High concentrations of hydrogen often result in polymer-like hydrocarbon impurities in the films and require an addition of gases like oxygen to burn off the polymer. Vapor phase deposition procedures include carbon arc systems, sputtering and laser ablation systems. While such systems maintain the temperature of the substrate relatively low, substantial effort is required to control the properties of deposited films. Further, scale-up of vapor phase deposition processes to enable continuous deposition, requires substantial investment.
Ion beam deposition has many benefits when compared with chemical vapor deposition. Ion beam deposition procedures enable room temperature deposition. The deposition rate of metal negative ions is not effected by temperature. Little or no sample surface preparation is required and the procedure employs no hydrogen in the ion beam atmosphere. The deposition rate from a focused negative ion beam can be as great as 100 microns per hour. In general, vapor phase prior art deposition procedures have not enabled metal ions to be deposited over large deposition areas.
Ion beam deposition systems employing a plasma or gas discharge to sputter an electrode, generating an ion beam for film deposition, are widely used. Sputtering is a phenomenon where, when an electrical discharge occurs between two electrodes in a low gas pressure environment, molecules of the gas are ionized, and material from the more negative electrode (−), or target, is slowly disintegrated by the bombardment of the ionized gas molecules. The material is disintegrated from the target in the form of free atoms, or in chemical combination with the residual gas molecules. Some of the liberated atoms may be condensed on surfaces surrounding the target while the remainder may be returned to the target by collision with gas molecules.
Reference in this regard may be had to Vacuum Deposition of Thin Films, L. Holland (Chapman and Hall, London, 1966).
This process is also referred to as cathodic sputtering and may be compared to a fine sand blasting in which the momentum of the bombarding particles is more important than their energy. Argon is typically utilized as the sputtering gas because it is inert, plentiful, a heavy gas and has a low ionization potential. The inert nature of argon inhibits compounds from being formed on the target surface.
Once disintegrated, or sputtered, from the target, the atoms travel until they reach a nearby surface, for example, a substrate. The deposited layer forms or grows on the substrate structure and is influenced by such things as material, temperature and gas structure.
When the ions strike the target, they gain back their lost electron, their primary electrical charge is then neutralized, and they return to the process as atoms. Direct current sources generally prevail as the electrical energy source. If the target is an insulator, the neutralization process results in a positive charge on the target surface. This charge may grow to the point that the bombarding ions (+) are repelled and the sputtering process stops. In order to make the process continue, the polarity of the target must be reversed to attract enough electrons from the discharge to eliminate the surface charge. In order to attract the electrons and not repel the ions, the frequency must be high enough to reverse the polarity before the direction of the ions are affected. A typical industrial frequency for such purposes is 13.56 Mhz. Since this is a radio frequency, this process is commonly referred to as RF sputtering. RF sputtering makes it possible to sputter insulators.
FIG. 1
shows an example of how ions are generated and deposition occurs as a result of RF sputtering.
It is known to utilize magnetic fields for concentrating the electrical discharge. In a configuration referred to as a planar magnetron configuration, a magnetic field is imposed in such a way that the electrons generated by the ionization process are trapped in a region near the target surface. These electrons are held much closer together than in the case of a non magnetic configuration and are forced to move in a path within the magnetic field. If the magnetic path is closed, then the electrons will circulate freely around the enclosed magnetic field. The magnetic field lines are typically circular, causing the electron path to also be circular. Because the electrons are concentrated within this region, a gas atom entering this electron cloud has a greater probability of losing an electron and thus becoming ionized. This increased efficiency means that a lower gas pressure is required to maintain the sputtering process. This so called magnetron action only occurs when the magnetic field and the electric field are normal, that is, at 90° to one another. Therefore, the majority of the ionization occurs in the center of the electron cloud. The ions thus formed are instantly attracted to the negatively biased target where, as described above, they collide with its surface, causing atoms of material from its surface to be ejected (sputtered). An example of the operation of a planar magnetron sputtering configuration is shown in FIG.
2
.
In the planar magnetron configuration, the rate of removal of material from the target may be approximately ten times faster than in the case of non magnetic configurations. Because the electrons are confined by the magnetic field, and are not allowed to move about freely, very few of them reach the substrate, which is normally at ground potential. This eliminates much of the substrate heating experienced when the electrodes are not so confined. The target, on the other hand, comes under fierce ion bombardment, particularly in the area where most of the ions are produced, causing a very localized heating of the target material in the area defined by the magnetic field.
The sputtering systems described thus far utilize ion bombardment to sputter neutral particles, that is, atoms, from an electrode. The neutral particles travel until they are deposited on nearby surfaces. It is also known to bombard a target with a first ion beam in such a manner so as to cause a sputtering of oppositely charges ions from an electrode, causing the production of a second ion beam, whose ions are then deposited on the substrate. It has been determined that the use of cesium ions as the bombarding first ion source produces a high yield of oppositely charged ions from metal and refractory metal targets. This is because a cesium coating on the sputtering surface lowers the work function required to sputter negative ions from the target material. Other materials with a low electron affinity, such as rubidium or potassium, will also lower the work function, however, cesium, having the lowest electron affinity of any non radio-active element, is the most efficient in this regard.
Solid state cesium sources have been developed for ion beam deposition systems. Kim and Seidl describe a solid source of Cs+ ions in “Cesium Ion Transport Across A Solid Electrolyte-Porous Tungsten Interface”, Journal of Vacuum Science Technology A7(3), May/June 1989, pages 1806-1809, and “A New Solid State Cesium Ion Source”, General Applied Physics 67(6), Mar.
Kim Steven
Sohn Minho
Lee John R.
Morgan & Lewis & Bockius, LLP
Plasmion Corporation
Quash Anthony G.
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