GB2076589A - Electron-beam Shaping Apparatus - Google Patents

Abstract

Electron-beam shaping apparatus has first aperture 28 and second aperture 30 through which electron- beam 20 is focused, double deflection apparatus 38, 40, 42, 44, of either electro-magnetic or electrostatic character, being positioned between the aperture plates so that the image of both apertures can be focused on a target. The double deflection ensures that the incident and exit beam paths are parallel, and that target current density does not vary with spot size. <IMAGE>

Description

SPECIFICATION Electron-Beam Shaping Apparatus Background of the Invention This invention is directed to an apparatus for shaping an electron beam, particularly for shaping the cross section of a focused electron beam for lithographic exposure and the like. The apparatus provides a selection of sizes and shapes of electron beam on the target.

Electron beams are used to expose photolithographic sensitive resist material on a semi-conductor wafer so that after removal of the non-masking portions of the resist, the wafer can be doped, usually by ion implantation. As resolution improves, finer lines can be drawn with closer spacing but such small spots require considerable time to expose large areas. The writing speed of conventional electron beam lithography systems, in which the demagnified image of a thermionic electron source serves as the writing spot, is inadequate for sub-micron circuit fabrication purposes. It is estimated that the throughput of such a prior art system is limited to 0.1-0.2 wafer-levels per hour for four inch diameter wafers with 0.5 micron minimum feature size, and with a resist senstivity of 10-5 coulomb/square centimeters.This limit derives from the finite brightness of thermionic emitters, from the circular and diffused shape of Gaussian spots and from spot increases due to spherical aberrations in the final lens.

Several alternative column configurations offer higher submicron performance potential.

However, each of these configurations also has a problem. One approach is based on the exceptionally high brightness of field emitters, as outlined in the following papers: D. Kern, D. Kurz, and R. Speidel, Optik 52, 61 (1978); L.

Venerklasen and J. Wiesner 8th International Conf. on Electron and lon Beam Science and Technology, Seattle (1978); H. P. Kuo and B.

Siegel, 8th International Conf. on Electron and lon Beam Science and Technology, Seattle (1978); and J. E. Wolfe, 1 5th Symposium on Electron, lon and Photon Beam Technology, Boston (1979).

These configurations generate a spot of very high current density by imaging the virtual source of field emitter onto the target plane. In concept, this approach is very attractive since it requires only one or two lenses. However, several physical and technological constraints make difficult the accomplishments of the goals. First, the spot current density must reach values of several thousand amperes per square centimeters in order that fabrication rates of several wafer-levels per hour (of the wafer conditions described above) can be achieved. At such high current densities energy broadening in conjunction with chromatic lens aberations restrict the working distance to about 3-5 centimeters. As a result, the size of the scan-field is severely limited so that it cannot be employed on a four inch wafer.Second, the positional stability of the emitter tip must be about equal to the virtual source diameter, which ranges from tens to hundreds of Angstroms.

Otherwise, the writing spot wanders over distances which exceeds its own diameter. To keep an emitter tip within such close tolerancess for the duration of a wafer writing period (in the order of tens of minutes) is a challenge. Third, exposing resist with a sensitivity of 10-5 Coulombs per square centimeter at current densities of several thousand amperes per square centimeter leads to spot dwell times in the nanosecond range. Therefore, the deflection system must be able to provide stepping rates of nearly one GHz. To achieve such speeds with the required high precision is a formidable task.

A second approach to high-speed, sub-micron electron beam lithography is to use shaped spots.

Several investigators have shown that variable shaped spots with good edge resolution can increase the writing speed potential dramatically as outlined in the following papers: M. G. R.

Thomson, R. J. Collier, and D. R. Herriott, Journal of Vacuum Science Technology 15,891, (1978); and H. C. Pfeiffer, Journal of Vacuum Science Technology 15, 887 (1978). In fact, for minimum feature sizes of 1-2 microns, this approach has been proven in an actual production environment, see: G. J. Giuffre, J. F. Marquis, H. C. Pfeiffer, W.

Stickel, 1 5th Symposium on Electron, lon and Photon Beam Technology, Boston, (1979).

Extension of the spot-shaping technique to sub-micron levels must be accompanied by a reduction in beam aperture, in order that chromatic and spherical aberrations be prevented from blurring the spot edge. For 0.5 micron minimum feature size, this constraint is expected to limit throughput rates to less than ten waferlevels per hour (with the wafer specified above) with sources of about 3x 105 amperes per square centimeter brightness.

A drawback of the spot-shaping method is the complexity of its electron-optics. As described by M. G. R. Thomson et al and H. C. Pfeiffer, supra, spot shaping columns consist of 5-6 lenses and 2-4 sets of beam alignment coils. Two of the lenses provide variable aperture imaging, while the remaining lenses serve to demagnify and project the shaped spot onto the target plane.

Alignment coils are required since misalignment results in loss of the beam, as compared to Gaussian systems where beam misalignment only results in spot displacement.

Therefore, there is need for an electron beam system which provides for shaped aperture exposure of resist, with the apparatus being simple in configuration so that it can be readily and economically mechanized and controlled.

Summary of the Invention In order to aid the understanding of this invention it can be stated in essentially summary form that it is directed to an electron-beam shaping apparatus which employs a hot filament emitter and a first lens which serves as a condenser and illuminates first and second serial apertures. The focal length of the condenser projects a magnified image of the virtual source onto the aperture plane of the final lens. The beam leaving the first aperture is double deflected by a double deflector between the apertures so that a selected image shape can be projected onto the target.

It is thus an object of this invention to provide an electron-beam shaping apparatus which employs a hot filament source and through a simple electron optical system can project a spot of selected size and shape onto a target. It is another object to provide an electron-beam shaping apparatus which provides spot shaping capability with only three lenses.

It is a further object to provide such an apparatus wherein spot shape is varied by double deflection through two apertures which are made to optically overlap along the optical path. It is another object to provide such an apparatus wherein the two apertures are close together so that the first shadows the other and a compact beam deflection system is mounted between them for spot shape control.

It is another object to provide an electronbeam shaping apparatus to increase the writing speed of electron-beam resist exposure systems by writing small pattern elements with small spots and coarse pattern elements with larger spots. It is a further object to provide an electronbeam shaping apparatus wherein the beam is deflected laterally between two closely spaced apertures so that the first aperture directly shadows the second aperture.

It is another object of this invention to provide an electron-beam column with a variable shaped spot which is significantly less complex and provides a high rate of exposure. It is a further object to provide such an apparatus wherein the apertures are positioned so close that without a lens between them both can be imaged with sufficient sharpness onto the target plane. It is another object to provide such an apparatus where the beam deflectors are positioned between the apertures and provide for double deflection so that the illumination angie of the second aperture can remain constant while the beam is deflected so that the target current density does not change while the spot size is varied.

Other objects and advantages of this invention will become apparent from a study of the following portion of this specification, the ciaims and the attached drawings.

Brief Description of the Drawings Fig. 1 is a generally central longitudinal section through an electron-beam exposure column having the beam shaping apparatus in accordance with this invention.

Fig. 2 is an enlarged perspective view of the apertures and deflection yokes of the apparatus of Fig. 1.

Fig. 3 is an enlarged detailed view of the beam passing through the apertures, shown as a centerline section.

Fig. 4 is a raytrace of the electron optical structure of the apparatus.

Fig. 5 shows a selection of some of the spot shapes available with square apertures.

Fig. 6 is an enlarged detailed view similar to Fig. 3 of another preferred embodiment, shown as a centerline section with parts broken away.

Fig. 7 is a perspective view of another preferred embodiment of the electron-beam shaping apparatus wherein electro-static deflection is employed for beam double deflection and a spot is selectable from a plurality of nonrectangular pattern shapes.

Description of the Preferred Embodiments The electron-beam apparatus of this invention is generally indicated at 10 in Fig. 1. The apparatus includes a housing 12 within which a sufficient vacuum is drawn to permit beam management. Hot filament electron source -14 is a tungsten hairpin filament which is biased with respect to target wafer 1 6 on target stage 18 at 20 kilovolts so that the tungsten hairpin filament source generates a beam with a brightness of about 105 amperes per square centimeter steradian, and with a virtual source diameter at crossover of approximately 50 microns. The electron-beam is indicated at 20 in Figs. 3 and 4.

Magnetic beam alignment coils 22 are on the beam path, which is central down the axis of Fig.

1, as it passes to first lens 24. The first lens 24 is a magnetic lens with a magnetic gap 26. The first lens serves as a condenser which illuminates the two square apertures 28 and 30 respectively in aperture plates 32 and 34. The distance between the source and the apertures and the size of the apertures is chosen such that only the uniformly bright core of the beam at about 10-3 rad can pass through both apertures. The focal length of the condenser lens 24 is adjusted to project a magnified image of the virtual source onto the aperture plane 36. A round final lens aperture 36 serves to limit aberations to well below 0.1 micron and to select spot forming rays from a well defined center region of the crossover at the aperture plane. This assures that small alignment changes do not affect the uniformity of spot illumination.

The square beam passing the first square aperture 28 is deflected by a ferrite core double deflector, see Figs. 2 and 3. Ferrite cores 38 and 40 serve to deflect the beam laterally within the plane of the paper in Fig. 3 with the first core 38 providing lateral beam deflection and the second core 40 returning the beam path to a direction essentially parallel to the center line. Similarly, ferrite cores 42 and 44 serve as an electron-beam double deflection pair to deflect the beam in a direction normal to the paper in Fig. 3, with the resultant beam being essentially parallel to and displaced laterally from the original beam path.

Depending upon the degree of deflection, a larger or smaller portion of the square beam from the first aperture passes through the second aperture.

The resulting variable shaped beam is demagnified by the second and third lenses 46 and 48 and imaged onto target 16 on the target plane. Since the two shaping apertures 28 and 30 are located at different distances from the target plane, both cannot be imaged sharply at the same time. An immediate focussing plane must be chosen, which leads to some edge blurring. The aperture edges are imaged as if they were difussed lines of about 2 micron width. In the target plane, where a 100:1 demagnified images is created, edge blurring amounts to about 100 Angstroms. This is well below the edge resolution in most applications. Deflection aberations associated with the dual deflection are even smaller since beam aperture and beam deflection angles are small.

Fig. 5 shows spot 50 which is achieved by only lateral deflection by cores 38 and 40 while spot 52 shows spot configuration obtainable by double deflection by cores 42 and 44. Spots 50 and 52 are rectangular. Spots 54 and 56 are different size squares obtainable by different amounts of deflection by both sets of ferrite cores. Of course a full size square is also available as a selected spot, and is accomplished without deflection. Small rectangles are also available by selective deflection.

Fig. 6 shows a structure similar to the first preferred embodiment of Figs. 1-5. In the electron beam apparatus of Fig. 6 the two apertures 28' and 30' are placed between the deflection yolses. This permits a closer focus but the larger deflection angle brings other aberations which prevent a better image definition. The parts in Fig. 6 which are similar to Fig. 3 are shown with a prime.

Fig 7 illustrates another preferred embodiment of aperture plates and double deflection structures which can be directly substituted for the aperture plates and ferrite core deflectors thus far described. Aperture plate 58 has a square aperture 60, the same as aperture plate 32.

Lower aperture plate-62 has a square aperture 64, the same as aperture plate 34, but also has other shaped apertures adjacent the central aperture 64. The other shaped apertures are chevron shaped apertures 66 and 68 and circular aperture 70. Double deflection of the beam is accomplished between the aperture plates by electro-static deflection plates in deflection plate holders 72 and 74. Each of the deflection plate holders has four orthogonally positioned electrostatic deflection plates which arrange to double deflect the beam in either of the two orthogonal plates. When a small amount of deflection is provided, the spot shapes available are the same as in Fig. 5. When the larger deflection is employed, then one of the chevrons or the circle acts as the lower aperture so that whole or partial chevrons or chords of the circular aperture 70 are available as the exposure spot.

In view of the fact that all of the electron optical components used in shaping and varying the the shape of the spot are combined in a single compact unit, this structure provides for a compact electron beam apparatus suited for high speed submicron lithography.

This invention has been described in its presently contemplated best mode and it is clear that it is susceptible to numerous modifications, modes and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of this invention is defined by the scope of the following claims.

Claims (11)

Claims

1.An electron-beam shaping apparatus comprising: means for producing a substantially collimated electron-beam substantially along a center line; first and second axially spaced apertures positioned along the center line; means for double deflection of the beam positioned between said apertures and deflecting the beam so that the beam passing out of the second aperture is on a path substantially parallel to the beam entering the first aperture and the beam leaving the second aperture is shaped by the edges of the first and second apertures.

2. The apparatus of Claim 1 wherein spot means for producing an electron-beam comprises a hot filament electron source.

3. The apparatus of Claim 2 wherein an electron-beam condenser lens is positioned between said hot filament and said first aperture.

4. The apparatus of Claim 1 wherein said means for producing an electron beam includes a condenser lens positioned upstream of said first aperture.

5. The apparatus of Claim 4 wherein second and third lens are positioned downstream from said second aperture to demagnify and focus the beam which passed through said first and second apertures onto a target.

6. The apparatus of Claim 1 wherein two lenses are serially positioned downstream from said second aperture to demagnify and focus the beam which passed through said first and second apertures onto a target.

7. The apparatus of Claim 1 wherein said means for double deflection is the only electron optical element positioned between said first and second apertures.

8. The apparatus of Claim 7 wherein said electron-beam double deflection means comprises electro magnetic deflection cores.

9. The apparatus of Claim 7 wherein said eiectron-beam double deflection means comprises electro static double deflection plates for othogonal deflection of the electron-beam.

10. The apparatus of Claim 7 wherein there is a plurality of adjacent apertures in one of said aperture plates and said deflection means can be operated to cause the beam to at least partially pass through any selected one of said plurality of apertures.

11. The appparatus of Claim 1 wherein there is a plurality of adjacent apertures in one of said aperture plates and said deflection means can be operated to cause the beam to at least partially pass through any selected one of said plurality of apertures.

1 2. Electron beam shaping apparatus substantially as herein described with reference to and as illustrated by the accompanying drawings.