Electric lamp and discharge devices – Cathode ray tube
Reexamination Certificate
1998-10-23
2001-05-15
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
C313S413000, C313S444000
Reexamination Certificate
active
06232709
ABSTRACT:
An Appendix is included in this application that contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the Appendix, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.
BACKGROUND OF THE INVENTION
In applications of electron optics, it is often desirable to create small, bright focused spots. There are many factors that can limit the ability to finely focus an electron beam. Some of the more common ones include spherical and chromatic aberrations, variations in the mass or charge of the beam particles, magnified source size, misalignments of key components, mutual coulomb repulsion of the charged particles in the beam, inadequate magnetic and electrostatic shielding, mechanical vibrations, and deflection aberrations. This invention is primarily directed to the correction of deflection aberrations although some of the other aberrations will come into consideration since they are often linked in practical designs.
Electron beam probes having a diameter of a few Angstroms are possible, but only within a very small scanned field of a few hundred Angstroms. Most applications of electron beams, however, require moving the beam around appreciably more. When a beam is deflected, aberrations of deflection are induced. These deflection aberrations are usually significant and often much larger than the undeflected focused spot size. As those skilled in the art will appreciate, the above holds equally true for electron beams as well as for other charged particle streams.
There are two types of image defects that result from deflection. The first is a field defect that distorts rectangles into pincushion or barrel shapes. The second type of defect causes the focused spot to increase in size. The aberrations that cause the focused beam size to increase are of more concern. Depending on the application, an electron beam designer will usually want perimeter or corner resolution to be the same or at least not significantly worse than resolution at the center of focus. Because deflection aberrations are approximately proportional to the beam diameter and the square of the deflection angle, the designer will often compromise center brightness and resolution in order to make corner resolution and brightness acceptable. This is typically achieved by reducing the diameter of the beam in the deflection region. As a demonstration of the interdependence among the aberrations, this tends to increase space charge repulsion.
There are two ways to deflect an electron beam—transverse magnetic fields and transverse electric fields. One aberration of concern results from different parts of the beam experiencing different deflections due to inherent non-uniformities of magnetic or electric fields in a vacuum. For the same amount of deflection, the coefficient of deflection aberration is larger for electric field deflection as compared to magnetic field deflection. Electrostatic deflection can be modulated at a very rapid rate and requires low power, but electrostatic deflection aberrations can be several times worse than magnetic deflection aberrations. Researchers have tried to reduce electrostatic deflection aberrations since cathode ray tubes first became useful devices early in this century. (Electric field deflection is often called electrostatic deflection whether the activity is static or dynamic.) While some improvements have been demonstrated over the years, a major solution has yet to be satisfactorily identified.
For some applications, charged particle beams are scanned or dynamically deflected. For other applications the beams are deflected statically. Yet other applications include both static deflections and dynamic deflections. The above concerns apply equally to stable deflection, dynamic deflection and combinations of both.
The standard approach to studying deflection aberrations is to approximate solutions to complex integral equations using polynomial expansions of displacement or angle to third and higher order terms. As those skilled in the art will appreciate, these higher order calculations are enormously complicated and typically require equations that fill entire pages. The results of these calculations, however, show that deflection aberrations partially represent quadrupoles, and thus the net effect of deflection aberrations can be reduced by suitable introduction of another quadrupole of opposite polarity. Correcting quadrupoles are well known and are often called stigmators.
A quadrupole produces astigmatism. Astigmatism, unlike most other electron optical aberrations can be either negative or positive. Pure astigmatism can be exactly canceled by another quadrupole of opposite sign disposed elsewhere in the optical stream. The correcting quadrupole could be either magnetic or electrostatic, but should be adjustable in magnitude and orientation. Suitable quadrupole devices are generally known in the art. Quadrupoles can therefore reduce deflection aberrations for both magnetic deflection as well as electrostatic deflection.
In the case of electrostatic deflection, nonuniformities in the deflecting field appear most pronounced near the plate surfaces, and deflection aberrations are considered to be smallest in the exact center between two oppositely charged deflection plates. Because there is no preferred direction to scan the beam, the scan is typically performed equally toward either plate. Thus, in the art of electrostatic deflection, the scan is usually symmetrical and beams are centered between the plates. The distance from the beam to the end of the plates in the direction transverse to deflection is sufficient to prevent edge effect fields from perturbing the deflection. Typically, 0.25 to 0.5 inches is used in the art where beams are centered between the plates and scanned symmetrically using electrostatic deflection. In general, prior known asymmetrical scanning systems have not addressed the correction of deflection aberrations. One solution is disclosed in co-pending application Ser. No. 08/623,918, the contents of which are hereby expressly incorporated herein by reference.
Assuming that electrostatic deflection aberrations could be totally eliminated, as already indicated, that may not necessarily mean that deflected beams could be focused to an infinitesimally small spot. There are other contributing factors. Of special interest, among these other terms are chromatic aberrations and variations if any in the ratio of charge to mass of the particles. Electrostatic components are particularly known to have relatively high chromatic aberrations. Chromatic aberrations can limit performance in some important applications of electron optical systems and is often a consideration in practical product designs. These aberrations stem from thermal energy differences that exist among the electrons in a beam. These energy differences are on the order of kT, which is Boltzman's constant times the absolute temperature of the material from which the electrons are emitted. For room temperature of ≈300 degrees Kelvin, kT is approximately 0.02 electron volts. For indirectly heated oxide coated cathodes (≈1100 degrees Kelvin), which are normally used in television tubes, the energy spread has a mean value of 0.1 electron volt. For directly heated tungsten filaments (≈2500 degrees Kelvin), the energy spread mean is 0.2 volts. This energy spread can result in an appreciable chromatic aberration that limits performance for many applications.
It is not difficult to estimate the importance of these thermal energies. The thermal energy is randomly oriented and necessarily has transverse and longitudinal components. The transverse component is most troublesome. For example, a 20 kv beam is deflected 10 inches over a throw of 12 inches. Ignoring small relativistic effects at this voltage, the velocity of an electron is proportional to the square root of the energy of the electron. The angular spread of the 20,000 volt bea
Brinks Hofer Gilson & Lione
Haynes Mack
Patel Nimeshkumar D.
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