GB2038027A - Image reduction or enlargement system for radiation such as X-rays and neutron beams - Google Patents

Abstract

A Bragg reflector is adapted for reducing or enlarging a pattern. The reflector comprises a crystal 13, e.g. a silicon crystal, having a surface 14 formed partly by the crystal material itself and partly by co-planar areas of a mask 17 of radiation-opaque material embedded in recesses 20 of the crystal material. A beam X1 of X- radiation incident on the crystal surface 14 is Bragg-reflected by lattice planes 21 of the crystal except where absorbed by the opaque material. The reflected beam X2 therefore contains an image of the pattern defined by the unmasked areas of the crystal surface. The crystal surface is at an angle theta to the lattice planes so that incidence and reflection are asymmetric in respect to that surface and effect a reduction or enlargement according to whether the incident beam has the larger or the smaller of the angles formed between the surface and the incident and reflected beams. The co-planar arrangement of the Bragg-reflecting and opaque materials ensures that the opaque material does not obstruct the reflected rays. The reduction or enlargement is one-dimensional and a second reflector is provided for reducing or enlarging the pattern in the second dimension <IMAGE>

Description

SPECIFICATION Image reduction system This invention relates to an image reduction system and is based on the consideration that if one were to use radiation of substantially smaller wave length than that of light, the limitations imposed by the unavoidable diff.raction of the radiation on the resolution of the system could be correspondingly reduced. However, radiation of substantially smaller wave length than that of light leads one into the range of penetrative radiation e.g. X-radiation or neutron radiation, where optical systems pertaining to light cannot be used.

However, such short wave length, penetrative radiation can be reflected at lattice planes in the interior of a crystal. This is known as Bragg reflection and a crystal for this purpose is herein referred to as a Bragg reflector. It is an object of this invention to provide an image reduction system using penetrative radiation and Bragg reflection.

According to this invention an image reduction system comprises a source of collimated penetrative radiation, a crystal having a surface exposed to said radiation and having lattice planes at which said radiation is Bragg-reflectable, the system including a mask of material opaque to said radiation provided at or adjacent said surface and defining a pattern to be reduced, the angle of incidence of said radiation in respect of said lattice planes being so chosen that radiation reflected at said lattic planes leaves said surface at an acute angle thereto, whereby an image in a plane perpendicular to the said reflected radiation is a one-dimensional reduction of said pattern, the reduction factor being the greater the smaller said acute angle.

Also according to this invention there is provided, for use in an image reduction system, a Bragg reflector comprising a crystal having lattice planes capable of reflecting penetrative radiation, and a mask of material opaque to said radiation provided at a surface of said crystal.

Any crystal having the appropriate Bragg reflection characteristics may be used but preferably it should be as nearly as possible a perfect crystal. A silicon crystal is suitable. Any appropriate opaque material may be used. Gold is an example. The penetrative radiation may be Xradiation or neutron radiation. X-radiation, for example, has a wave length of an order of magnitude of 1/1000th of that of light and provides a correspondingly significant improvement in resolution.

One application of the invention is in the reduction of a pattern to a sub-micrometre scale as part of a method of producing a correspondingly miniaturized integrated circuit.

An example of a system and a Bragg reflector according to this invention will now be described with reference to the accompanying drawings wherein: Fig. 1 is a perspective view of the system, Fig. 2 is an enlarged section on the line Il-Il in Fig. 1 showing details of the reflector, and Fig. 3 is a view similar to Fig. 2 but showing a modification.

Referring to Fig. 1, a beam X1 of X-radiation is directed by a source 11 on to a reflector 1 2 comprising a crystal 13 e.g. a silicon crystal. The reflector has a flat surface 14 constituted partly by surface areas 1 5 of the crystal itself and partly by surface areas 1 6 of a mask 1 7 of a radiation opaque material such as gold, the crystal itself being relatively transparent to said X-radiation.

The areas 15 define a pattern P1 to be reduced.

The configuration of the pattern is chosen arbitrarily for the purpose of illustration. The pattern P1 has overall dimensions M,N.The reflector 12 reflects the beam X1 in the form of a beam X2 containing an image 12 representing the pattern P 1 except that the dimension M is reduced as shown at M2. A second reflector 19 has a pattern P2 congruent with the image 12 and is orientated to reflect the image 12 in the form of a beam X3 containing an image 13 in which also the N dimension of the pattern P 1 is reduced as shown at N3. The image 13 is recorded photographically as a pattern P3 on a receptor 22, being a suitably coated crystal, with a view to producing a miniaturized integrated circuit based on the configuration of the pattern P 1.

Referring to Fig. 2 the beam X1 is reflected by the crystal 1 3 in accordance with the principles of Bragg reflection, that is, the X radiation penetrates the crystal 13 and is reflected by certain lattice planes 21 of the crystal structure in accordance Bragg's law which states that A = 2d sin 0 where :1 = the wave length of the radiation, d = the spacing of the planes 21, and O = the angle of reflection of the rays at the planes 21.

In the context of the present invention the surface 14 is cut to have an angle + with the planes 21 so related to the angle of reflection 0 that incident rays R1 have with the outside of the surface 14 an angle a greater than an angle P formed between the outside of that surface and reflected rays R2. As will be seen the smaller the angle 5 the greater is the image reduction factor.

The direction of the rays R1 must of course lie, at least substantially, in planes perpendicular to the planes 21, i.e. in the plane of the drawing.

The opaque mask 1 7 is embedded in recesses 20 of the surface 14, the thickness T of the mask being sufficient to absorb the incident rays R 1. The geometry of reflection is as follows. Rays R1A, R1 B of the beam X1, being rays which fall on points A, B at the junction of the surfaces 15, 16 are reflected as ray R2A, R2B. Rays R1 penetrating the body of the crystal are reflected into the bundle bound by the rays R2A, R2B only insofar as reflection occurs within a triangle A, B, Q where 0 is the point of intersection of the lines defining the direction of the rays R1 A, R2B.Any ray R1 C penetranting the crystal beyond the line QB gives rise to reflected rays R2C which are absorbed by the layer 1 7. It follows that the bundle of rays bound by the rays R2A, R2B has a width W2 related to the width W1 of the area 1 5 by the expression W2 = W1 sin /3. In other words the reduction factor is 1/sin p.

The reason for arranging the mask 1 7 in the recesses 20 is as follows. If the mask 1 7 were to stand proud of the surface 14 it would absorb at least a part of the radiation reflected from within the triangle A, B, Q, and the process would not be appropriate to cases where image reduction is to be attained from areas 1 5 whose width W1 is small in relation to the thickness T. However, by locating the mask 1 7 in the recesses 20, the whole of the width W1 is reflected. The width W1 is therefore independent of the thickness T. In this way reduction to sub-micrometer dimensions, i.e.

dimensions less than 1/1000 mm, is achievable.

Experiments have been shown that image reduction factors of 9.5 and 22.6 are possible.

Thus for a factor of 9.5 a width W2 of less than 1 ym can be achieved for widths W1 of less than 9.5 ,um. Similarly, with a factor of 22.6 a width W1 of, say 10 to 15 ,um will show a reduction of 0.5 to 0.75 ym. The mask 17 has to have a thickness T of, say not less than 3ym. It will be clear that such a thickness, if protruding significantly above the surface 14, would make impossible any worthwhile reductions.

The reflector 1 2 is made by the steps of coating the crystal 1 3 with a photo-resist solution, photographically exposing the coating to an image of the pattern, dissolving the photographically exposed part of the coating thereby to physically expose the surface 14, etching the surface 1 4 with a suitable etchant to produce the recesses 20, and filling the grooves with the mask material.

The surface may be finally polished and lightly etched with a finishing etchant to remove polish marks and leave a surface satisfactory for present purposes.

The width W1 is therefore to some extent dependent on the quality of the photo-resist process and subsequent etching of the recesses 20. A relatively large width W1 is desirable from the point of view of allowing for tolerances in the preparation of the mask, while a small width W1 is desirable from the point of view of being able to use a smaller reduction factor or accommodating a wider tolerance in the coherence of the incident or reflected rays.

It will be clear that if the direction of the radiation is reversed, i.e. if the receptor 22 is constructed as a reflector such as the reflector 12.

and is appropriately illuminated, the reflectors 19, 12 create an enlargement of the pattern P3. The enlargement is then realizable on a suitable screen (not shown).

In a first modification the mask, instead of being provided at the surface 1 4 in the sense of being embedded therein the mask is provided in a position adjacent to and spaced from that surface.

This is shown in chain dotted lines in Fig. 2 where a radiation-transparent plate 25 supports a mask being layers 1 7A of opaque material deposited on the plate. Preferably the plate lies in a plane at right angies to the incident beam X1. A representative portion of the pattern to be reduced is indicated by a width WX between the rays R1A, R1B. In this case the widths W2, WX are related by the expression W2 = WX sin plsin a, and the reduction factor is sin a/sin p. The plate 25 is arranged to be as close as is practicable to the crystal 13 but of course clear of the path of the reflected beam X2.

In a second modification, Fig. 3, a beam X10 of X-radiation is directed on to a reflector 112 comprising a crystal 11 3 e.g. a silicon crystal. The reflector has a flat surface 114 constituted partly by surface areas 115 of the crystal itself and partly by surface areas 1 6 of a mask 117 of radiationopaque material embedded in recesses 120 of the surface 1 14, all the same as in respect of the crystal 1 2 shown in Fig. 2. However, in the present example the crystal has lattice planes 121 parallel to the surface 114 and the beam X10 is directed on to the surface 114 at an angle p 10 similar to the angle P of Fig. 2. The beam is therefore reflected at an angle ,B20 the same as the angle p10 and the reduction factor is the same as that of Fig. 2, namely 1/sin s.

Claims (9)

1. A Bragg reflector comprising a crystal having lattice planes capable of reflecting penetrative radiation, and a mask of material opaque to said radiation provided at or adjacent a surface of said crystal.

2. A reflector according to Claim 1 wherein said surface of the crystal has recesses containing said mask material and said material extends primarily below said surface.

3. A reflector according to Claim 1 wherein the mask is arranged in spaced apart relationship to said surface.

4. A Bragg reflector according to Claim 1 or Claim 2 in combination with a source of collimated penetrative radiation directed on to said reflector at an angle of incidence with said lattice planes so chosen that radiation reflected at said lattice planes leaves said surface at an acute angle thereto and an image in a plane perpendicular to the said reflected radiation is a one-dimensional reduction of said pattern, the reduction factor being the greater the smaller said acute angle.

5. The combination according to Claim 4 comprising a further said reflector having a surface exposed to said reflected radiation and situated at such an angle to said reflected radiation that the image is reduced in a second dimension.

6. The combination according to Claim 4 wherein said lattice planes lie at an angle to said surface and the angle between the direction of said incident radiation and said surface is correspondingly greater than the angle between said surface and said reflected radiation.

7. The combination according to Claim 4 wherein said lattice planes are parallel to said surface and the angle betweeri the direction of said incident radiation and said surface is correspondingly equal to the angle between said surface and said reflected radiation.

8. A Bragg reflector substantially as described herein with reference to the accompanying drawings.

9. A Bragg reflector in combination with a source of collimated penetrative radiation substantially as described herein with reference to the accompanying drawings.