WO1984000823A1

WO1984000823A1 - Improvements in or relating to the manufacture of zone plates

Info

Publication number: WO1984000823A1
Application number: PCT/GB1983/000185
Authority: WO
Inventors: Michael Christopher Hutley
Original assignee: Secretary Trade Ind Brit
Priority date: 1982-08-13
Filing date: 1983-07-28
Publication date: 1984-03-01
Also published as: EP0116573A1; JPS60501131A; GB2125987A

Abstract

The use of zone plates has been restricted by low efficiency and marked chromatic aberration. The increasing use of lasers reduces the disadvantage of chromatic aberration; and blazed zone plates have improved efficiency, but have been difficult to make. In the method of the invention, a Fabry Perot interferometer (28), capable of producing a concentric fringe image of high finesse, is made adjustable so that the image can be expanded (or contracted) over, typically, a range one fringe spacing. The image is projected onto a layer (12) of photo resist on a suitable substrate (10). While the image is expanded (say) through one fringe spacing, the light intensity in the image is varied, so that when the photo resist (12) is developed, the radial cross section between any two fringe positions takes on a wedge shape (18); which is that required in a blazed zone plate. Suitable chemical treatment can extend the wedge shape into the substrate partly or wholly. Efficiency of the order of 60% can be achieved.

Description

Title : Improvements in or relating to the manufacture of zone plates.

This invention relates to the manufacture of zone plates.

A zone plate performs a similar function to a lens, the difference in principle being that whereas a lens brings light to a focus by refraction, a zone plate does so by diffraction. In practice zone plates have not been used extensively because they have two serious disadvantages. In the first place, a zone plate produces severe chromatic aberration and secondly the efficiency is low. That is to say for a given amount of incident .energy a smaller proportion will pass into the image with a zone plate than with a lens. The earliest form of zone plate consists of a set of concentric rings, alternatively opaque and transparent. This produces a series of diffracted orders corresponding to both positive and negative focal lengths; and the maximum proportion of incident energy which is directed into a given order is approximately 10%. A subsequent improvement involved making a zone plate in which there was a phase change of 180^o as between the light from adjacent rings; rather than making alternate rings opaque. The maximum efficiency of this kind of zone plate is 40% for the first orders. A further improvement in efficiency has now been achieved by making blazed zone plates, in which the cross section of rings making up the zone plate is triangular; each ring becomes an annular prism, tending to direct light to the focus. The effect is, however, diffractional, and the contributions of light from adjacent rings or zones must be in phase with one another. The manufacture of blazed zone plates has hitherto been difficult; but the present invention sets out to make production easier, and hence cheaper. According to the invention a method of making a blazed zone plate includes the steps of (i) preparing a suitable substrate on which the zone plate is to be formed (ii) arranging on a surface of the substrate a layer of photo sensitive material which is preferentially removable in accordance with quantity of light incident thereon (iii) projecting onto the layer of photo sensitive material a light image in the form of a monochromatic diffraction pattern of concentric rings, and varying the radius of every ring of said pattern simultaneously through a range equal to the radial width for each ring of the respective blazed zone of the zone plate to be made, while also varying the intensity of said light image falling upon the photo sensitive material; whereby the photo sensitive material is exposed to light at a varying degree of intensity in the radial direction and

(iv) treating at least said photo sensitive layer so that a radial cross section of said layer and/or its substrate varies in accordance with the the aforesaid variation in light intensity uniformly around a pattern of concentric zones in the form of a blazed zone plate.

The method may include treating said photo sensitive layer and the substrate so that the cross section produced in the photo sensitive layer is substantially reproduced in the substrate to form a zone plate thereon.

The said diffraction pattern of concentric rings may be produced by an interferometer of the Fabry-Perot kind; in which the radius of each of the rings of said diffraction pattern may be varied by changing the optical separation of the plates of the interferometer.

The optical separation of the plates of the interferometer may be changed by changing the refractive index of the medium between the said plates. Alternatively the optical separation of the plates may be changed by physically moving one plate normally to or from the other. Such movement may be effected, for example, by means of a piezoelectric transducer.

Optionally the radius of each of the rings of said diffraction pattern may be varied by changing the wavelength of the light input to the interferometer. The method may also include the feature whereby the intensity of said light image falling on the photo sensitive material and the radius of every ring of said pattern are varied in discrete steps to approximate to any desired profile. The invention extends to a zone plate made by the aforementioned method.

The invention will be further described by way of example only, with reference to the accompanying drawings in which Fig 1 illustrates diagrammatically the cross section of the profile which may be produced in a photo sensitive layer.

Fig 2 illustrates stages in the method of producing a blazed zone plate.

Fig 3 illustrates diagrammatically means to vary the refractive index of the medium between the plates of a Fabry-Perot interferometer.

Fig 4 illustrates diagrammatically piezo electric means to change the optical separation of the plates of a Fabry-Perot interferometer. The invention employs an interferometer of the Fabry-Perot kind. When the interferometer is illuminated with diffuse monochromatic light from one side thereof, the light emerging from the other side can produce a pattern of concentric rings similar geometrically to the rings of a zone plate. Such an interferometer operates by multiple internal reflection, and as a consequence the rings of the said pattern may be made very narrow. The ratio of the radial thickness of the rings to their radial separation is known as the "finesse". This is indicated in Fig 1, in which d is the radial separation and Δ the radial thickness measured at half-height. The finesse of the pattern is then d/Δ .

The finesse is a function of the reflectance of the internal surfaces of the plates making up the interferometer. A high value of finesse is desirable for the purposes of the invention. Fig 1 also illustrates a stage in producing a zone plate according to the invention. A substrate, which may, for example, be glass, is indicated by 10. On the substrate is arranged a layer 12 of photo sensitive material, for example photo resist. The troughs 14 indicate in cross section where consecutive radial rings of illumination have been projected onto the photo resist from a Fabry-Perot interferometer. The photo resist is modified to an extent dependent on the period of exposure and the intensity of the illumination, and the troughs 14 are produced by subsequent chemical treatment of the photo resist; such treatment being in itself of conventional kind. Further chemical treatment can, if desired, extend the troughs into the substrate as indicated at 16.

Fig 2 illustrates further stages in the method of the invention. As already described, a first exposure produces changes in the photo resist which will result in a profile indicated at 14. However, the desired end result in the production of a blazed cross section, that is to say a triangular cross section as indicated by the chain line 18. Such a triangular cross section is achieved by making further exposures on the photo resist as indicated, for example, at 20 and 22. Three such exposures in all have been indicated but more exposures, more closely spaced could be made, controlled in exposure period and intensity of illumination so that the final profile 18 may be achieved. The successive exposures for 14, 20 and 22 require mutually shifted patterns of concentric rings of light to be produced by the interferometer, which may be carried out in a manner to be described.

In Fig 3 substrate and photo resist are indicated by 10 and 12 respectively, the object being to form on them a blazed zone plate. The source of monochromatic illumination is indicated at 24 and may conveniently be a laser device. Light from the source is passed through a diffusing screen 26 to an interferometer indicated generally by 28. Light emerging from the interferometer (ie to the right as illustrated) is focused by a lens system indicated by 30 onto the layer 12 of photo resist to produce a light image of concentric rings; eg that corresponding to 14 in Fig 2. The light image is changed to patterns corresponding to 20 and 22 by changing the optical separation of the plates 32, 34 of the interferometer. One means of achieving this is indicated in Fig 3. The space 36 between plates 32 and 34 is arranged so as to be subject to a preadjustable gas pressure. As the gas pressure in the space 36 is raised, the refractive index of the gas between the plates 32, 34 is increased, and hence the effective optical separation of the plates. This in turn gives rise to a change in the pattern of concentric rings in the light image formed at 12. For a change in optical separation of one half wave length of the monochromatic light from 24 each ring of the pattern at 12 will move out (or in) until it occupies the original position of the adjacent ring. Thus by changing the gas pressure in the space 36 the successive ring patterns corresponding to 14, 20 and 22 may be produced, or any intermediate or near by positions, an exposure being made for each position. To achieve the profile 18 the exposures may be made successively shorter; or the intensity of illumination may be reduced by the use of diaphragm means, indicated diagrammatically at 38. The gas pressure in the space 36 may be supplied from a pressurised source 40 controlled by reducing valve means 42, and, if necessary, by a controlled leak (not separately indicated) from the space 36 to atmosphere. The required changes in the pattern of concentric circles in the light image formed at 12 (Fig 3) may be achieved alternatively by changing the wave length of the light used. For this the light source 24 may be a laser of the kind in which the wavelength of the output may be adjusted over a small range. Another way of changing the said pattern of circles is by arranging the interferometer so that one of the plates is mechanically adjustable in relation to the other. This is indicated diagrammatically in Fig 4. One plate 34 of the interferometer is fixed in a casing 44 , while the other plate 32 can slide in the casing 44 towards or away from plate 34. Between plate 32 and a ring 46, fixed to casing 44, is arranged a cylinder 48 made of piezo electric material. Throug'n electrodes (not shown) an adjustable d.c voltage is applied to the cylinder 48 so that its axial thickness can be changed, and hence the position of plate 32 relative to plate 34, so varying the optical separation of said plates.

The pattern of concentric circles of light on photo resist 12 may be varied in discrete steps as implied by Fig 2. It is also feasible to vary the pattern in a continuous manner through simultaneous continuous variation of the optical separation of the interferometer plates (for example) and continuous variation of the illumination intensity by control of the diaphragm 38. After exposure to light as described the photo resist is removed by conventional chemical mearts, in the regions irradiated, to produce a blazed profile 18 (Fig 2). Further chemical treatment can transfer the required profile to the substrate, which may be of a more mechanically resistant nature than the photo sensitive layer. If a reflecting zone plate is required, the substrate may be of suitable metal; or a metal coating may be applied, eg by vacuum deposition, to the remaining photo sensitive layer. For a transmission zone plate the substrate may be glass. If required a blazed zone plate made as described may be used as a master for replication of zone plates by casting, or otherwise.

It has been possible to achieve efficiencies of up to 60% with zone plates according to the invention. It is feasible to improve further on the efficiency by employing an interferometer of better quality.

Claims

1. A method of making a blazed zone plate characterised by including the steps of

(i) preparing a suitable substrate (10) on which the zone plate is to be formed.

(ii) arranging on a surface a layer (12) of photo sensitive material which is preferentially removable in accordance with quantity of light incident thereon

(iii) projecting onto the layer of photo sensitive material (12) a light image in the form of a monochromatic diffraction pattern of concentric rings (14) , and varying the radius of every ring of said pattern simultaneously through a range equal to the radial width (18) for each ring of the respective blazed zone of the zone plate to be made, while also varying the intensity of said light image falling upon the photo sensitive material; whereby the photo sensitive material is exposed to light at a varying degree of intensity in the radial direction and

(iv) treating at least said photo sensitive layer (12) so that a radial cross section of said layer and/or its substrate (10) varies in accordance with the aforesaid variation in light intensity, and uniform around a pattern of concentric zones in the form of a blazed zone plate.

2. A method according to claim 1 characterised by including the steρ of treating said photo sensitive layer (12) and the substrate (10) so that the cross section produced in the photo sensitive layer is substantially reproduced in the substrate (10) to form a zone plate thereon.

3. A method according to claim 1 or claim 2 characterised in that the diffraction pattern of concentric rings is produced by an interferometer (28) of the Fabry-Perot kind.

4. A method according to claim 3 characterised in that the radius of each of the rings of said diffraction pattern may be varied by changing the optical separation of the plates (32,34) of the interferometer (28).

5. A method according to claim 4 characterised in that the optical separation of the plates (32,34) is changed by cnanging the refractive index of the medium between the plates.

6. A method according to claim 4 characterised in that the optical separation of the plates (32,34) is changed by physically moving one plate normally in relation to the other.

7. A method according to claim 6 characterised in that the movement is effected by means of a piezo electric transducer(48) .

8. A method according to claim 3 characterised in that the radius of each of the rings of said diffraction pattern is varied by changing the wavelength of the light input (24) to the interferometer.

9. A method according to any one of the preceding claims characterised in that the intensity of the light image falling on the photo sensitive material and the radius of every ring of said pattern are varied in discreet steps, to approximate to any desired profile.

10. A blazed zone plate characterised by having been made by a method according to any one of the preceding claims.

11. Apparatus for making a blazed zone plate, on a suostrate having a layer of photosensitive material thereon, characterised by including an interferometer (28) of the Fabry-Perot kind in wnich the optical separation of the plates (32,34) of the interferometer can be changed, whereby the radius of each of the rings(14) of a diffraction pattern produced by the interferometer (28) on the photo sensitive layer (12) can be varied.

12. Apparatus according to claim 11 characterised in that the optical separation of the plates (32,34) is changed by varying the refractive index of the medium between the plates.

13. Apparatus according to claim 12 characterised in that the medium between the plates (32,34) is a gas and the refractive index is varied by varying the pressure of said gas.

14. Apparatus according to claim 11 characterised in that the optical separation of the plates (32,34) is changed by physically moving one plate in a direction normal to the other.

15. Apparatus according to claim 14 characterised in that the physical movement is caused by varying the dimensions of a ptezo electric transducer (48) acting between the plates (32,34).