GB2173608A - Imaging system having three curved and one aspherical mirror - Google Patents

Abstract

An imaging system comprises three or more optical components, referred to as the orthogonal components, each containing a curved mirror 24, 29 and 31 and an additional component which is an aspherical mirror 22. Obstruction of the beam by successive mirrors is prevented by techniques such as beam splitters, perforated mirrors, and the production of an internal focus 25 where the beam can be deflected by a small flat mirror 26. One application lies in the production of microcircuits. <IMAGE>

Description

SPECIFICATION Imaging system This invention relates to the field of optics. It provides a new imaging system or relay which gives fine spatial resolution and a large number of resolved picture elements. The numerical aperture is sufficient to give small diffraction images. The system typically includes: a source of light; an illuminated object; components which give a focussed image (the "imaging components"); and means for using the image produced.

The novel features lie mainly in the arrangement of the imaging components. The various parts of the system can be designed and positioned relative to one another in accordance with this invention with the effect that optical aberrations are neg ligibly small. Therefore accurately defined object structures can be suitably reproduced in the image. A noteworthy result is that the good image quality can be maintained over a usefuily large image area when the imaging system is not simply a one-to-one relay but is arranged to provide magnification or its reverse (scale reduction).

One application of this invention would lie in the production of microcircuits. As they contain structures of a literally microscopic size, it is convenient first to prepare a mask which resembles the required design of the circuit but is larger. The mask is then used as the object of the imaging system. The imaging system is set up to reduce the scale so that the size of the image is as required for the product. Owing to the qualities of the imaging system, the image can be recorded on a suitable medium such as photoresist and retain the fine features and accurate geometry of the mask in spite of the reduction in size.

The imaging system can be described as follows. The terms used are mainly as defined by W. T. Welford in "Aberrations of the Symmetrical Optical System", Academic Press, 1974. A "component" is any subsystem of one or more adjacent optical surfaces or pieces of material.

The imaging components can include a number of components referred to here as the "orthogonal components" or "orthogonal surfaces". These have the property of reversing the direction of principal (chief) rays. The simplest type is a spherical mirror whose surface is orthogonal to principal rays; therefore the centre of the sphere is at the axial point in the pupil (or real or virtual pupil image). This relationship to a pupil eliminates some of those aberration contributions which depend on the distance of the image from the axis. However, an optical system composed of an arbitrary set of such surfaces suffers from a number of problems even if it has the correct paraxial properties such as focal length.In particular, curvature of the field is usually present, as is spherical aberration, and some aberrations of off-axis images can be related to the obliquity of the pupil or to pupil aberrations. Large obstructions of the beam may occur when reflecting components are used. The present invention combines a number of features which moderate these problems. (Having designed a system which embodies these orthogonal components, it may then be modified by moving the practical pupil and hence the pupil images, and rebalancing the aberrations in the usual way: this may be done to change vignetting characteristics and may reduce the size and manufacturing costs of some surfaces.However, the system is qualitatively similar after moving the pupil, and it is possible to determine the alternative pupil and pupil image positions which enable a group of orthogonal components to be identified.) First, curvature of the field is reduced or alternatively tolerated. Reduction is achieved while still using the orthogonal components by employing three or more of those components in the following way. For two of these components which contribute to the Petzval sum in the same sense as a concave mirror, a third contributes in the sense of a convex mirror, and therefore the sum can be much less than any of the individual contributions. Although some embodiments of the system thus provide a flat surface containing well-focussed images, systems can also be designed according to the invention but having residual field curvature.Customary optical design practice is sometimes to add a lens element known as a field flattener which is close to the image or object; although that can be done, field flatteners may introduce undesirable stray light effects such as ghost images and are not ideal in some applications. In that case the following modification is also included: either the object or the detector surface or both are manufactured with calculated departures from flatness so that a tolerable fit is obtained between the surface of best images and the surface of the detector. Any means (for using the image) which can be correspondingly modified for a curved field is appropriate.

Second, if an intolerable total of spherical aberration occurs in these three or more orthogonal components, it is corrected by providing an aberration of the opposite sign by means of components referred to as the "additional components". These additional components are positioned to operate with a pupil consistent with that of the orthogonal components. These additional components also have, in total, the property of producing only small additional aberrations for off-axis field points compared with the axial field point. This can be achieved for example by the use of an appropriately shaped aspherical surface positioned at a pupil or by surfaces which are orthogonal to a principal ray: these can include further orthogonal components.

Third, the beam obstruction which frequently occurs in on-axis reflecting systems is dealt with by one or more methods such as those shown in Figures 1.1 to 1.5. (The profiles of optical components are shown by cross-sections.) In Figure 1.1, a small oblique mirror 10 is placed near an intermediate focus to direct the rays to a new space 11 for subsequent components. In Figure 1.2, a non-axisymmetric system serves a similar purpose, using a tilted mirror 12. In Figure 1.3, a beamsplitter 13 is used to allow transmission ofthe beam 14 returning from a mirror 15. The aberrations introduced by the beamsplitter may be reduced if the the mirror 15 (with any preceeding parts of the system) gives a beam 14which is collimated.Alternatively the beamsplitter aberrations may be corrected by the insertion of a compensator 16 of one of various types or by suitable design of the beamsplitter itself. In Figure 1.4, a perforated mirror 17 is used. In Figure 1.5, a mirror 18 is used which is both tilted and perforated.

There are severai useful variations. The orthogonal components are typically front-surface mirrors, but other types may be substituted. A rear-surface mirror (Figure 2.1 ) can both protect a metallised coating and provide different aberration contributions; the transmitting element in front of the mirror may be separated as a simple or compound lens (Figures 2.2 and 2.3 are examples). However, the characteristic of reversing the path of a principal ray is maintained. The shapes of the orthogonal components may be modified to give a further improvement in aberration correction and this can include the use of an aspherical profile as well as changes in vertex curvature; in this case principal rays are not exactly reversed, but the axial intersections of the incident and reversed principal rays are at the same distance from the component within 30 per cent of the larger (Figure 2.4).The object and image fields and other optical apertures may be masked to shapes other than axially centered circles; this may, for example, reduce stray light. Distortion is small in many embodiments but it is also possible to manufacture an object with calculated distortion which counteracts any introduced in the imaging system.

The additional components which provide spherical aberration contributions of the opposite sign to that of the orthogonal components can take various forms including that of a single mirror, the rim of which may conveniently form the practical pupil. Favoured embodiments use this mirror in the forms 12 or 15 in Figures 1.2 and 1.3. Useful aberration correction can be obtained from that surface if it is paraxiaily flat but modified by a radial aspheric (polynomial function). A refinement is now to change the base curvature of that surface together with modifications to the shapes of the orthogonal components as described in the previous paragraph.The effect required here isthatthis additional component should provide an aberration contribution having the form of astigmatism, as well as its major spherical aberration property, to give an enhanced overall performance together with the modified orthogonal components. The additional components may take other forms including that of an aspherical lens or doublet lens or catadioptric or other simple or compound systems. These are further characterised in Figures 3.1 and 3.2. Figure 3.1 shows an aspherical corrector lens of the generic form of the corrector plate in the system known as the Schmidt camera. (The height of the lens profile in cross-section is exaggerated for clarity.) Figure 3.2 shows a meniscus lens element which resembles an element used in the wide-angle cameras known as the Bouwers or Maksutov cameras.The meniscus lens may be split to form a doublet. (No beamsplitter need be used if the additional components take dioptric forms, and the meniscus lens has the further advantage that the departure from sphericity is eliminated or greatly reduced. The reflecting form of the additional components is, however, preferred when various wavelengths are to be used, for testing, operation or adjustment.) Automatic control of focus, alignment and magnification may require movements of the components; in that case it may be convenientto move a component which has relativelyweakfocussing power, and typically that component is among the additional components.

Within the principles of the system, some embodiments are telecentric. This has the effect that magnification changes which may be consequent on tolerable errors of focus are small: they move features of the image by linear distances which are much less than the size of the resolved image element. This is a useful property for applications such as the production of microcircuits when several successively formed images must be in accurate registration over the whole image field.

The invention may be applied using ultraviolet light or indeed many types of radiant or wave energy. The object may be illuminated by lasers or other sources or be self-luminous, as with cathode ray tubes or light-emitting diodes: a different but applicable type of object is produced by scanning a spot of light with modulated intensity over the input focal surface. The field of view may be extended indefinitely as a composite of more than one system or more than one image or as in a panoramic camera. The means for using the image may include any means for detecting, recording, further transmitting the image through another optical system, or image processing.

Turning now to a particular embodiment of the invention, an imaging system is shown in Figure 4.

Dimensions are shown in Table I. This is designed to give a scale reduction by a factor often, and an output numerical aperture equal to 0.24 (f/2). Apart from a beamsplitter, the imaging components are mirrors so that ultraviolet light can be used. The image surface is flat. In Figure 4, an object 19 has the form of a perforated mask which is illuminated 20 from behind. Aflat beamsplitter 21 introduces the beam into a reflecting system. The "additional components" in this embodiment are placed preceeding the "orthogonal components". The additional components take the form of a single mirror 22. The surface profile of mirror 22 is a polynomial aspheric on a concave base sphere. The mirror 22 has effects including the correction of spherical aberration, an aberration contribution in the form of astigmatism and the reflected beam 23 is more nearly collimated to reduce aberrations in the subsequent passage through the beamsplitter. The rim of this mirror effectively forms the practical pupil. The beam 23 returning from the mirror 22 re-encounters the beamsplitter 21 and the transmitted light is incident on the concave mirror 24 which is the first of the orthogonal components. Mirror 24 is close to a sphere centered on the axial point of mirror 22; aspherical terms are used in this embodiment but the departure from a sphere is much less than for mirror 22. Because of the definition of these orthogonal components, and the fact that the beam 23 tends to be collimated, an intermediate reai image 25 is formed.This allows a small flat mirror 26to divert the beam into an available space 27. Also because of the definition of the orthogonal components, a pupil image is formed at a position 28. The beam is next incident on a concave mirror 29 which is the second orthogonal component. This mirror 29 is therefore close to a sphere centered on the axial point of the pupil image at 28; small aspherical terms are used of a similar magnitude to those on mirror 24. Mirror 29 has a perforation 30. The next surface is the third orthogonal component, convex mirror 31. Mirror 31 is close to a sphere centered on the axial point of the pupil image at 28, or its re-image in mirror 29: again small aspherical terms are used. The beam 32 refelected from the mirror 31 passes through the perforation 30 to the flat image focal surface 33.

The contents of Table I are further explained here. The table gives a list of the curved optical surfaces together with their positions relative to the object and image surfaces as shown in Figure 4. The small flat mirror 25 and the flat beamsplitter 21 are not listed, but the table gives the total separations of curved surfaces measured along the reflected axes. The separations are all shown as positive and the direction of separation is available by reference to Figure 4. The focal surface is 0.0002 centimetres in front of the paraxial focus.

TABLE I Dimensions in units of centimetres (For detailed explanation of this table, see text.) Radius of Separation Notes Curvature at vertex (flat) Object 213.8448 280.0713 Concave mirror 22 69.7804 66.4580 Concave mirror 24 86.2493 20.2611 Concave mirror 29 7.7114 14.9490 Convex mirror 31 12.3474 (flat) Image Aspheric coefficients are defined by the following equation: 10 z = a4r4 +a6r6 +a8r5 +aOr where z is the height above the vertex sphere, as a function of the distance from the axis.

Positive z indicates added material.

Mirror a4 a6 a8 a10 22 -.13877E-04 -.35346E-07 -.60029E-11 -.67270E-12 24 -.60483E-07 0.87161E-10 -.65254E-11 0.29441E-13 29 0.19415E-07 -.27197E-09 -.80021E-11 -.83037E-14 31 -.21913E-06 0.11322E-06 0.14112E-07 -.89642E-10

Claims (13)

1. An imaging system of a number of components which is essentially a system with an optical axis of symmetry and in which four or more components have optical power and are characterised in that: three or more of the said components (which shall be denoted the "orthogonal" components) each contain a curved mirror whose centre of curvature is coincident with the axial point of another component of the system (the "additional" component) or equivalently with an image of that point where such an image is formed by reflection in one or more of the orthogonal components; the additional component is an aspherical mirror.

2. An imaging system as in claim 1 where the orthogonal components each contain or consist of an aspherical mirror surface and the centre of curvature of each such surface as required in claim 1 is defined with respect to the local curvature at the axial point of each such surface.

3. An imaging system as in any other claim where the centre of curvature of one or more orthogonal components departs from the axial point of the additional component or the images of that point as mentioned by an amount not exceeding thirty per cent of the radius of curvature of such orthogonal surface.

4. An imaging system as in any other claim where the orthogonal component which is the last in order of light incidence before the final image is a convex mirror.

5. An imaging system as in other claims were each component is a single mirror and the radius of curvature at the vertex of each successive surface in order of light incidence in a demagnifying system is less than the radius of curvature of the preceeding surface.

6. An imaging system as in claim 5 where the mirrors in the order stated are concave, concave, concave, convex and the third concave mirror is perforated.

7. An imaging system as in any claim in which the optical path is folded by the addition of one or more flat mirrors.

8. An imaging system as in any claim were the additional component is replaced by an aspherical lens of a type resembling a Schmidt corrector plate.

9. An imaging system as in any claim where the additional component is replaced by a lens resembling a Maksutov corrector plate or the same split into a doublet lens.

10. An imaging system as in claims 1 to 7 where a partially transmitting flat plate (a beamsplitter) is used to permit the additional component to be an on-axis mirror.

11. An imaging system as in claims 1 to 7 where the additional component being a mirror is slightly tilted from the axially symmetrical condition.

12. An imaging system as in any claim where any orthogonal additional component being a mirror is formed as a rear surface rather than a front surface mirror.

13. An imaging system as in any claim where the shape of the object of the system is adjusted by distortion or surface curvature to produce any required effect.