Multiple Condensing Lens Array For Concentrating Energy On A Plurality Of Electron Beam Sources Of A Photocathode

Abstract

The invention is an array of condensing lense for use in image tube pattern fabricating applications to provide selective and intensive illuminations of individual electron emitting sources of a multiple electron beam photocathode source.

Description

GOVERNMENT CONTRACT

The invention herein described was made in the course of or under a contract with the United States Air Force.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a lens system for directing light onto a photocathode and more particularly to an array of lenses for use in conjunction with a multiple electron beam emitting source.

2. Description of the Prior Art

The use of a multiple electron beam source in an image tube application is the subject of a copending application Ser. No. 784,551 of T. W. O' Keeffe filed Dec. 18, 1968, entitled, A MULTIPLE BEAM ELECTRON SOURCE FOR PATTERN GENERATION, and assigned to the assignee of this invention. The multiple electron beam source is utilized to define an array of multiple images by simultaneously scanning the multiple beams across the surface of a target coated with an organic etch resist. In certain applications such as rapid exposure of high resolution patterns, the current emitted by each of the electron beam sources must be quite high.

An obvious method of increasing the output of a photoelectron source is by increasing the illumination of the photocathode. If the photocathode is inefficient and a well matched source of illumination if not available, a great deal of nonuseful energy must be directed onto the photocathode. This excess energy can result in detrimental performance of the photocathode due to heating.

For a photocathode containing only a small relative area of emitting sources as in the case of the multiple electron beam source, much useful radiation is wasted if the photocathode is totally illuminated, i.e., most of the useful radiation falls on areas of the photocathode which do not emit electrons.

SUMMARY

The invention consist of a multiple condensing lens which, when positioned in front of the photocathode between a light source and the photocathode, concentrates the radiation from the light source onto the electron emitting areas of the photocathode.

A zone plate is a system of concentric circles of diameters increasing as the square root of the natural numbers such that the areas of the successive circles are equal. The circles are treated such that the circles are alternatively transparent and opaque. The zone plate acts somewhat like a convergent lens in the sense that an intense bright image will be formed at a point on the side of the photocathode opposite to the light source. The zone plate has a primary focal length which is a function of the wavelength and the radius of the smallest circle.

In a normal zone plate alternate zones are opaque so that half of the incoming light is immediately lost. The opaque material removes that portion of the wavefront that would produce secondary waves at the focus point which would destructively interfere with the waves from the zones adjacent to it. If the alternate zones instead of removing the light completely cause a 180.degree. phase shift, then all zones would constructively interfere at the focus point. Such a zone plate is referred to as a "half wave (or phase reversal) zone plate" and is more efficient since no incoming light is removed by the opaque zones, and the illumination at the focus point is therefore significantly greater.

The invention comprises the novel application of the zone plate in a multiple condensing lens device positioned in alignment with the electron sources of a multiple electron beam source to individually illuminate each electron source.

State of the art resist technique permit the fabrication of an array of zone plates of the dimension and arrangement required to match the array of multiple electron beam sources described in the above identified copending application of T. W. O' Keeffe.

It is an object of this invention to provide selective and intense illumination of the electron emitting sources of a multiple electron beam photocathode and thereby provide improved image tube pattern fabrication efficiency and improved pattern exposure speed.

DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of an embodiment of apparatus in accordance with the present invention;

FIG. 2 is a partial detailed view of an embodiment of the present invention .

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 there is shown an image tube 10 having a cathode plate 12 and a target plate 14 that are essentially planar and parallel.

On the cathode plate 12 is a photocathode 11 comprising multiple electron beam sources 13 defined by a mask 21 and a multiple lens array 30 here shown to be in axial alignment with the array electron beam sources 13 such that the incident light from a light source 20 illuminating the multiple lens array 30 results in concentrated illumination of the individual electron beam sources 13 thereby providing electron response from the electron sources 13. The target plate 14 contains one or more workpieces 15 that have a layer of organic etch resist material 16 thereon.

The image tube approach to resist exposure has several distinct advantages over both optical and scanning electron beam method. As a specific case a one to one magnetically focused image tube of about 1 centimeter cathode to anode target spacing, operating at about 10 KV accelerating potential and focus field of about 1 KG established by a coil (not shown) is capable of resolving images of less than 1 micron over a working area greater than 2 centimeters in width.

Deflection coils 18 provide means for simultaneously deflecting the multiple electron beams to draw a plurality of identical patterns on the target plate 14.

The concept of image tube resist exposure is described in detail in the copending patent application Ser. No. 753,373 by T. W. O' Keeffe and R. M. Handy filed Aug. 19, 1968, "Pattern Definition in Resist Layers by Application of an Electron Image " and assigned to the assignee of the present invention.

The fabrication of the multiple electron beam source is described in detail in the copending application Ser. No. 784,801, by T. W. O ' Keeffe and J. R. Morris filed Dec. 18, 1968, entitled, "Mask Photocathode and Method of Making Same, " and assigned to the assignee of the present invention.

The multiple lens array 30 is more clearly illustrated in the partial view of one surface of the cathode plate 12 illustrated in FIG. 2

Utilizing state of the art resist techniques the multiple lens plate 30 consisting of a plurality of individual zone plate lens elements 32 can be etched on a suitable material such as a thin layer of metal, i.e. aluminum in the case of a normal zone plate or can be etched directly on a quartz plate to form a half wave zone plate. In the etching of the quartz half wave zone plate the alternate circles are etched to different depths to produce the desired constructive interference of adjacent zones 34 and 36. The area of each circle is identical and the diameter of the circles increase as the square root of the natural number. A complete discussion of zone plates is presented "Concepts Of Classical Optics " by J. Strong, published by W. H. Freeman and Company, 1958.

The focal length of a zone plate is dependent on both the radius of the smallest circle and the wavelength .lambda./N of the light in the cathode plate 12, where N is the index of refraction. This relationship is expressed as f=a.sup.2 N/.lambda., where a is the radius of the smallest circle. If a useful wavelength of 2537A. is to be focused on the electron sources 13 located on the inner surface of a 1/8 inch thick quartz cathode plate 12, the index of refraction of which is 1.50, the diameter of the smallest circle is approximately 2 mils.

The effective concentration of light energy on the respective electron emitting sources 13 by the zone plate lenses 32 increases the intensity of the electron beams generated by the sources 13 in response to radiant energy from the light source 20. Due to the efficient utilization of the light source light energy the intensity of the light source may be less than that required in less efficient systems. This reduction in light source intensity minimizes adverse effects of heating which occur in less efficient systems. Also since the zone plate is wavelength dependent only the required wavelength would be focused. The remainder of the spectrum would be ineffectively focused so that intense "hot spots" would not be produced at the photoemissive surface of cathode plate 12.

The multiple electron beam photocathode as described in the copending application of T. W. O' Keeffe, Ser. No. 784,551, can be comprised of an array of elements each including a uniformly arranged plurality of various size electron emitting sources. These micron size electron sources of each photocathode element are so arranged that a single zone plate can be utilized to illuminate any of the electron sources within an element. The selection of the source to be illuminated in each photocathode element can be controlled by radially displacing the zone plate array 30 with respect to the light source 20 thereby changing the photocathode areas illuminated. The relative displacement of the light source 20 and the lens array 30 can be accomplished by radially shifting the array 30 or by radially displacing the light source 20 as indicated by the arrows. This radial displacement changes the angle between the light source 20 and each zone plate identically thereby resulting in illumination of identical electron sources in each of the uniformly arranged photocathode elements.

In the event the photocathode consists of a complete pattern or plurality of complete patterns which are to be exposed directly on a workpiece, a pair of zone plates can be employed to excite electron emissive alignment marks to generate high intensity electron beams for impingement on the workpiece surface. The workpiece will be fabricated to include a pair of electron beam induced conductivity devices which will respond to the alignment electron beam to indicate proper registration between the photoemissive pattern and the workpiece.

Various modification may be made within the scope of the invention.

Claims

We claim:

1. In an apparatus for generating a plurality of substantially identical electron beams comprising, a multiple electron beam photocathode source, said photocathode source comprising a plurality of substantially identical, individual electron emitting sources, a source of radiation energy for exciting said electron emitting sources, and a condensing lens array disposed between said radiation source, and said multiple electron beam photocathode, said condensing lens array including a plurality of individual lenses, each of said lenses being operatively associated with one of said electron emitting sources to effectively concentrate energy from said radiation source onto the respective electron emitting sources.

2. A device as claimed in claim 1, wherein said multiple element lens array is comprised of a plurality of zone plates.

3. A device as claimed in claim 2, wherein said zone plates are half wave zone plates.

4. A system for exposing high resolution patterns on an electron sensitive anode comprising:

a multiple electron beam photocathode source, said photocathode source comprising a plurality of substantially identical electron emitting sources,

a radiation source for illuminating said photocathode, said photocathode illumination producing electron emission from said electron emitting sources,

electron beam control means for directing said electron beams to said anode for exposing a plurality of substantially identical patterns thereon, and

a condensing lens array disposed between said radiation source and said multiple electron beam photocathode, said condensing lens array including a plurality of individual lenses, each of said lenses operatively associated with one of said electron emitting sources to effectively concentrate illumination from said radiation source onto the respective electron emitting sources of said photocathode.

5. A system as claimed in claim 4, wherein said multiple lens array is comprised of a plurality of zone plates.

6. A system as claimed in claim 5, wherein said zone plates are half wave zone plates.

7. In an apparatus as claimed in claim 1 wherein each of said individual electron emitting sources is comprised of an array of discrete electron emitting sources.

8. In an apparatus as claimed in claim 1 wherein each of said individual electron emitting sources is comprised of a circuit pattern in the form of a single electron emitting source.