GB1605160A

GB1605160A - Annular field optical system

Info

Publication number: GB1605160A
Application number: GB5682/78A
Authority: GB
Original assignee: Perkin Elmer Corp
Current assignee: Applied Biosystems Inc
Priority date: 1977-02-11
Filing date: 1978-02-13
Publication date: 1982-08-11
Also published as: IT7848011A0; JPH0130125B2; IT1101789B; DE2801882C2; FR2380563A1; CA1103498A; CH625055A5; FR2380563B1; DE2801882A1; JPS53100230A; GB1605161A

Abstract

Annular-field optical device which can be used for photographically exposing an image-receiving surface to a light image of an object, including at least one convex mirror (12) and one concave mirror (14) which are placed concentrically along the optical axis (SA). It is arranged so as to form conjugate planes perpendicular to the axis for which the system has a specific (unitary) power. A pair of concentric meniscal elements (16) are placed symmetrically, the convex radii of which are greater than the concave radii and the thicknesses of which are greater than the difference between the convex and concave radii. The optical device includes a first half and second half (10), each comprising an optical device having an optical axis (SA) and conjugate planes perpendicular to the axis for which the power of the system is specific (unitary), the two halves being placed coaxially, back to back, so that the conjugate planes are superposed on at least one side of the device so as to form an intermediate image position, and are arranged in order to produce separate object (O) and image (I) positions on the other side of the optical device, each of the halves including a meniscal element (16) placed symmetrically and concentrically, the convex radius of which is greater than the concave radius and the thickness of which is greater than the difference between the convex and concave radii. <IMAGE>

Description

(54) ANNULAR FIELD OPTICAL SYSTEM (71) We, THE PERKIN-ELMER CORPOR ATION, a Body Corporate organized and existing under the laws of the State of New York, United States of America, having a principal place of business at Main Avenue, Norwalk, Connecticut 06856, United States of America do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to optical projection systems for projecting an image of an object at unit magnification and with high resolution, such as used for exposing a semi-conductor wafer to the image of a master in microcircuit fabrication.

The optical system of the present invention is closely related to annular projection systems of the type disclosed in U S patent no: 3 748 015. While this optical system has many advantaxes, the present invention enables additional advantages to be realised within the said system and systems of the same general type, as will become apparent from the description that follows.

An optical projection system for wafer exposure must ensure the very best image fidelity to the master that the optical art can provide. High resolution and flatness of field are of course of paramount importance since it is quite common in present day microcircuit design to allow for spacing between contiguous elements of the order of a micron. In order to alleviate the optical design problem in achieving such a high peak of performance, the useful field has had to be severely restricted and the projection system designed for the scanning mode of exposure. In this considerable advancement in the art was brought about by the recognition that off-axis fields could be used to best advantage whereas traditionally designers had concentrated on paraxial fields.

In fact, the system disclosed in the U.S. patent referred tu above provides a narrow annular field highly corrected against optical aberrations, including of course field curvature.

It is well known that the monochromatic aberrations that may afflict an optical system are of five different kinds and that no system can ever be perfectly corrected for all of them simultaneously. The design problem is further compounded if the light to be used for image projection is not monochromatic and the system includes refracting elements. In such case, it becomes necessary to take into account the chromatic aberration arising from the fact that the index of refraction of optical material such as glass changes with the wavelength of the light traversing it. Since a common factor of the five monochromatic aberrations is the index of refraction, it follows that chromatic aberration modifies each of the monochromatic aberrations. In other words, chromatic aberration is always something to be contended with in optical design, unless the imaging light is monochromatic or the imaging optics is all-reflecting.

The use of monochromatic light is of course a severe limitation, since suitable high intensity light sources are either unavailable or very inconvenient; and all reflecting optical systems such as the one described in the aforesaid U.S.

patent, have their limitations, which, as we shall presently disclose, may be overcome in accordance with the present invention by introducing refracting elements, albeit at the cost ofbringing back and then defeating the chromatic aberration problem.

There are occasions in optical design when in an attempt to refine the performance of a high resolution optical system a change is introduced in the effect of the chromatic aberration present which actually results in variation of focus or wavelength being introduced or increased. In this situation, the present invention recognises that the problem can be overcome by generating a counteracting chromatic aberration the effect of which is used substantially to neutralise the effect of the disturbing chromatic aberration that gives rise to chromatic variation of focus with field position.

According to the present invention, an optical projection systems adapted to provide a discrete off-axis field zone which, as generated, is annular in shape, said field zone having a substantially flat focal surface within a range of projection light wavelengths, comprises optical elements including reflecting means, the system being arranged to form conjugate planes normal to its optical axis for which the system is of substantially unit power, and refracting means mounted between the reflecting means and the conjugate planes and comprising at least one pair of symmetrically disposed, nearly concentric meniscus elements whose thickness is greater than the difference between their convex and concave radii, the optical system being constructed and arranged so that the Petzval sum is substantially zero, and the refracting means including provision for balancing the effects of variation in said Petzval sum due to variation in colour by introducing axial chromatic aberration of the opposite sense so that the positions of focus at the annular field portions of the conjugate planes remain substantially constant.

The term "Petzval sum" is a well known one meaning the algebraic sum of the quantities obtained by dividing the power of each surface by its index of refraction, the index of refraction of a reflecting surface being defined as negative one for this computation.

Preferably the relationship between the annular radius of the system and the characteristics of each nearly concentric meniscus element is defined by the formula: R2 > R1 and t > R2 -R1 + (H2/2N2) (l/RI - lIR2) where: H = the annular radius of the system Rl = the concave radius of the men iscus lens R2 = the convex radius of the meniscus t = the index of refraction of the meniscus lens.

In one embodiment the optical system includes a concave spherical mirror and a convex spherical mirror facing the concave mirror, said mirrors being supported with their centres of curvature substantially coincident. Means are provided to define a location for an object the image of which is a real image at a second location, with said convex mirror being positioned to reflect to the concave mirror light from the object location initially reflected to the convex mirror from the concave mirror, whereby light from the object location will be reflected at least twice at the concave mirror and at least once at the convex mirror before being focussed at the second location. The small aberrations arising from the departure of the meniscus from concentricity are compensated for by introducing a small departure from concentricity in the mirror pair. That is, in each half of the optical system, the concave and convex mirrors are supported with a distance between their centers of curvature of less than about two percent of the length of the shorter radius.

Also the system preferably includes a substantially plane parallel colour correction plate mounted normal to the optical axis of the mirrors.

There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. Those skilled in the art will appreciate that the conception upon which the disclosure is based may readily be utilised as a basis for the designing of other systems for carrying out the several purposes of the invention. It is important, therefore, that the claims be regarded as including such equivalent systems as do not depart from the spirit and scope of the invention.

Specific embodiments of the invention have been chosen for purposes of illustration and description, and are shown in the accompanying drawings, forming a part of the specification.

Different forms of optical system in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of an optical system constructed in accordance with the concepts of the present invention; Figure 2 is a schematic representation of an optical system similar to Figure 1 but showing another embodiment of the invention; and Figure 3 is a graphic representation showing the variation of the focal position as a function of the distance from the axis and the wavelength of the image forming light.

For a proper appreciation of the present embodiment as a practical application of the inventive concept disclosed herein, reference should be made to the optical system illustrated in Figure 2 of U.S. specification no: 3 748 015 wherein the concave mirror shown forms an image of the object point at I and the convex mirror forms a virtual image of I at 0, the latter being re-imaged by the concave mirror at I.

A limitation of that prior art system imposed by the fifth order astignatism inheretent in the design, is the comparatively narrow width of the corrected annular field that can be achieved.

The cause of the astigmatism is the spherical aberration suffered by the principal rays parallel to the optical axis, which may be minimised in known manner by introducing (at a location presently to be indicated) a pair of meniscus elements concentric with the mirrors. Unfortunately, when so arranged the elements introduce third order field curvature, which is, or course, a severe penalty but is not accompanied by any other third order aberration.

The reason why the meniscus elements introduce different field curvatures for different wavelengths is that the index of refraction of the material used in their construction, along with that of any transparent media, changes with the wavelength of the light transversing it, and field curvature is related to said index. In fact, the necessary condition for the absence of field curvature, i.e. for a flat image surface, in an optical system is that the algebraic sum of te quantities obtained by dividing the power (in diopters) of each surface encountered from an object point by the index of refraction of the surface be zero, the index of refraction of a reflecting surface being defined as negative one for this computation. The said sum is known in the art as the Petzval sum. It follows that if the index of refraction for one particular wavelength causes the Petzval sum to be zero, and therefore the field to be flat, that for a different wavelength will give a value different from zero and thus introduce field curvature.

Naturally, if a monochromatic projection light were used, the problem would not arise, since the optical system would be designed to meet the Petzval condition at the single wavelength chosen. However, in terms of the energy that can be passed through it, the system is far more efficient in practice if it can be made to accept a spectral band that is reasonably broad compared with a monochromatic line.

It is thus seen that, in attempting to increase the width of the corrected annular field in the prior art system referred to by resorting to a pair of concentric meniscus elements for removing the spherical aberration that limited the width in the first place, a disturbing chromatic aberration is introduced where none existed. However, since no other third order aberration is introduced at the same time, the attainment of the wider annular field becomes possible if some means is found of combating the adverse effect of chromatic aberration on field flatness.

The disturbing chromatic aberration will not effect the position of a focal point in the lateral direction, i.e. perpendicular to the optical axis in the image space, because of the symmetry of the system.

The effect in the longitudinal direction, i.e.

parallel to said optical axis, is another matter entirely. Taking as a datum the position of a focal point for a given wavelength, that position will shift in one direction along a path parallel to the optical axis for a longer wavelength, and in the opposite direction for a shorter wavelength. In other words, a longitudinal spread of the focal position will take place, and this is where the application of the inventive concept disclosed herein comes into play. This concept, as already stated, involves the generation of a counteracting chromatic aberration, the effect of the disturbing chromatic aberration. In the application under review, the disturbing aberration we are concerned about is the component thereof operating in the longitudinal or axial direction and therefore the counteracting aberration will be made to have a longitudinal effect.

For all iiitroductory appreciation of how the present invention is applied to the refinement of an optical system as depicted in Figure 2 of the U S patent hereinbefore referred to, we need to emphasise the well known fact that the prior art solution of introducing a concentric meniscus with a conjugate at the centre of curvature causes no longitudinal chromatic aberration (which means that the focal point along the optical axis of the system is the same for all wavelengths within the spectrum of the projection light) but does cause an increasing spreading out of the focal positions as between long and short wavelengths for discrete off-axis field zones of increasing separation from the optical axis.

If we assume that the whole system, including the concentric meniscus, has been so designed that the Petzval sum is zero for a chosen datum wavelength, which for the purposes of this explanation we shall take as the middle wavelength in the spectral range of the projection light, the focal surface at the discrete off-axis zone will naturally be substantially flat. For the purposes of this explanation, we shall call that surface the datum focal surface.

Now the concentric meniscus has a slight negative power and consequently a slight negative contribution to the Petzval sum of the system. The Petzval contribution is more negative for wavelengths shorter than the chosen datum wavelength because the index of refraction of the refracting material of which the meniscus is made increases with decreasing wavelength. As a result of this, the focus for short wavelengths will be downstream of the datum focal surface and the long wavelengths upstream. This clearly cannot be tolerated in a high resolution system.

The situation may be remedied by applying the concept of the present invention. In fact, by giving the convex surface of the meniscus a controlled amount of positive power, and ending up therefore with a nonconcentric meniscus (the negative power of which is, incidentally, reduced as compared to a concentric meniscus of the same thickness) longitudinal chromatic aberration is introduced by virtue of which the focal position separation between short and long wavelengths is at its maximum along the optical axis and decreases in an offaxis direction. Positive power naturally means that the shorter wavelengths focus upstream of the datum focal surface and lower downstream.

We thus have an effect which is opposite to that of the concentric meniscus of the prior art.

By selecting the right amount of positive power in relation to the chosen discrete off-axis zone, a balance is established whereby all the wavelengths within the spectral range of the projection light may be brought to a substantially common focus at the datum focal surface.

In the embodiment of the invention shown in Figure 1, the new and improved optical system, indicated generally at 10, comprises two spherical mirrors, a convex mirror 12 and a concave mirror 14, arranged to provide three reflections within the system. The mirrors are arranged with their centres of curvature along the system axis SA and to have off axis conjugate areas centered at points 0 and I. The points 0 and I are each a distance H from the reference axis SA at opposite sides thereof.

A pair of symmetrically disposed meniscus elements 16 are provided for reducing the spherical aberration of the principal rays. It is noted that the meniscus elements would also be effective for reducing the spherical aberration of the principal rays if they were mounted directly adjacent the convex mirror 12 so that the surface of the mirror 12 and the convex surface of the meniscus elements 16 are parts of the same spherical surface. It will be appreciated that the high order astigmatism has been greatly reduced with the result that the width of the corrected annalus is increased by an order of magnitude.

The small aberrations arising from the departure of the meniscus elements 16 from concentricity are compensated for by introducing a small departure from concentricity in the mirror pair 12 and 14. That is, the concave mirror 14 and the convex mirror 12 are supported with a distance between their centres of curvature of less than about two percent of the length of the shorter radius. Colour correcting plates 18 operate in the manner subsequently described.

Other suitable arrangements of these mirrors are shown and described in the prior U.S. patent no: 3748015.

A necessary condition for the absence of field curvature, i.e. for a flat image surface, in the resultant system is that the algebraic sum of the quantities obtained by dividing the power of each surface by its index of refraction be substantially zero, the index of refraction of a reflecting surface being defined as negative one for this computation. Since the index of refraction of the meniscus elements varies with the wavelength of the image forming light, it will be appreciated that the incorporation of these elements in the optical system results in a variation of field curvature with wavelength. This results in a variation of the focal position as a function of the distance from the axis and as a function of the wavelength of the image forming light as shown in Figure 3. In an annular field optical system, the variation with distance from the axis is effectively removed by restricting the field to an annulus whose distance from the axis is constant. The variation of field curvature with wavelength and it can be balanced by the introduction of color aberration of the opposite sense. To accomplish this in accordance with the invention, the refracting meniscus departs from exact concentricity by having its convex radius of curvature shorter than the sum of its concave radius and its thickness. That is, its thickness is greater than the difference between the radii of its convex and concave surfaces. The way in which this works can be explained as follows: The variation of field curvature with wavelength introduced by a nearly concentric meniscus whose power is negative is such that the back focus is greater for short wavelength than for long wavelengths. A concentric meniscus with conjugate at its center of curvature does not introduce any longitudinal color aberration The same is substantially true of such a meniscus with a conjugate near its center of curvature.

The addition of a positive lens to such a meniscus introduces longitudinal color of the sense required to balance the variation in focus with wavelength resulting from the variation of the field curvature (contributed by the meniscus) with wavelength. This can be accomplished by macing the convex radius of the meniscus shorter than the sum of its concave radius and its thickness. The meniscus is then equivalent to two lenses, one being a fictitious concentric meniscus with convex radius equal to the sum of the concave radius and the thickness, while the second is a zero thickness positive meniscus whose concave radius is the conves radius of the fictitious meniscus and whose convex radius is the convex radius of the actual meniscus. For a nearly concentric meniscus with concave radius R1 convex radius R2, thickness t, and refractive index N, the longitudinal color compensates for the change in focus due to the variation of field curvature with wavelength in an annulus of radius H when R2 > R1 and t ~ R2 - R1 + (H2/2N2) (11R1 - lIR2) (1) It has been found that the introduction of a pair of menisci whose parameters substantially satisfy equation (1) into an optical system of the type disclosed in the aforementioned U.S.

Patent No. 3 748 015, together with accompanying modifications which will be discussed more fully hereinafter, results in a reduction in the high order astigmatism over a wide spectral band.

The resultant system can be improved considerably by modifying the menisci so that their thicknesses are greater than the values given by equation (1). This results in a variation of focus of an annular field system whose sense is such that it can be compensated for by the introduction of a plane parallel plate of appropriate thickness, as indicated at 18 in Fig. 1. The extra degrees of freedom provided by the additional element makes possible a much greater degree of correction.

Further improvement can be obtained by modifying the plane parallel plates in one of two ways: (1) One of the faces of the plane parallel plate may be made aspheric.

(2) The plane parallel plate may be "bent" resulting in a meniscus element.

The highest degree of correction has been obtained with a system in which the thickness of the menisci is greater than the value given by equation (1) and in which colour compensation is obtained by adding plane parallel plates modified in accordance with one of the two ways described above.

Table 1 is an example, indicating the construction data, of the annular field optical system of Figure 1. As is well known in the art, a plus sign is used to denote that a surface is TABLE 1 RADIUS OFANNULUS = 100 mm.

SURFACE NO. RADIUS(mm) DISTANCE TO MATERIAL NOTE FROM OBJECT NEXT SURFA CE TO IMA GE (mm) O (PLANE) 144.92 AIR OBJECT - 144.96 11.03 FUSED SILICA 2 -151.75 88.70 AIR 3 - 957.30 16.75 FUSED SILICA 4 - 967.84 295.25 AIR 5 - 551.15 -279.07 AIR MIRROR 6 - 267.18 279.07 AIR MIRROR 7 - 551.15 -295.25 AIR MIRROR 8 - 967.84 - 16.75 FUSED SILICA 9 -957.30 -88.70 AIR 10 - 151.75 -11.03 FUSED SILICA 11 -144S6 - 144.92 AIR 12 (PLANE) IMAGE TABLE II N.A. = 0.l7ATOBJECTANDIMAGE RADIUS OF RMS WA VE ABERRATION (WAVELENGTH UNITS) ANNUL US (mm) WA Vk'Lk'NGTH( NGSTROM UNITS) 2800 3200 3650 4000 4358 5461 105 .09 .12 .13 .13 .13 .12 104 .05 .08 .09 .09 .09 .08 103 .02 .04 .05 .06 .06 .05 100 .02 .01 .01 .01 .01 .01 97 .04 .01 .02 .02 .02 .03 96 .06 .02 .02 .03 .03 .03 95 .08 .04 .04 .04 .04 .04 94 .11 .06 .05 .05 .05 .05 93 .13 .08 .07 .07 .06 .06 convex to the object and that distance is measured from left to right whereas a minus sign is used to denote that a surface is concave to the object and that a distance is measured from right to left.

Table II is a table of the computed performance of the annular field optical system of Table 1 over an extended spectral range (2800 A to 5461 A) in terms of the rms wave aberration at various annular radii. The width of the usable annulus is the difference between the values of the upper and lower radii for which the performance is adequate for the application.

It is noted that a system is usually called 'diffraction limited", or more precisely "aperture limited" when the rms wave aberration is less than 0.07. For a scanning system, the rms wave aberration may be as high as 0.09 or 0.1 at the edges of the annulus.

Figure 2 of the present specification shows an arrangement which includes a convex mirror 1 2e and a concave mirror 14e arranged substantially concentrically along an optical axis SA in a manner utilizing a total of five reflections within the system, there being three reflections from the concave mirror 14e and two from the convex mirror 12e. In this embodiment the algebraic sum of the powers of the reflecting surfaces utilized is zero when the radius of the convex mirror 12e is two-thirds that of the concave mirror 14e. In a manner similar to that described in connection with the embodiment of Figure 1, the system of Figure 2 includes meniscus elements 1 6e and a colour correcting plate 1 8e that function in the aforesaid manner.

It is noted that annular field optical systems of the type described are usually used in a scanning mode and, for this purpose, it is highly desirable that the orientation of the object and image be the same so that their physical supports can be maintained in fixed relation to each other while being moved relative to the optical system for scanning and so that the accuracy requirements of the scanning motion are minimised. An arrangement that achieves this by incorporating three flat mirrors in the optical system was shown in the U.S. patent no: 3 951 546.

WHAT WE CLAIM IS: 1. An optical projection system adapted to provide a discrete off-axis field zone which, as generated, is annular in shape, said field zone having a substantially flat focal surface within a range of projection light wavelengths, wherein the system comprises optical elements including reflecting means, the system being arranged to form conjugate planes normal to its optical axis for which the system is of substantially unit power, and refracting means mounted between the reflecting means and the conjugate planes and comprising at least one pair of symmetrically disposed, nearly concentric meniscus elements whose thickness is greater than the difference between their convex and concave radii, the optical system being constructed and arranged so that the Petzval sum is substantially zero, and the refracting means including provision for balancing the effects of variation in said Petzval sum due to variation in colour by introducing axial chromatic aberration of the opposite sense so that the positions of focus at the annular field portions of the conjugate planes remain substantially constant.

2. An optical system according to Claim 1 wherein the relationship between the annular radius of the system and the characteristics of each nearly concentric meniscus element is defined by the formula: R2 > R1 and t > R2 - R1 + (H2/2N2) (l/RI - 11R2) where: H = the annular radius of the system Rl = the concave radius of the meniscus lens R2 = the convex radius of the meniscus lens t = the thickness of the meniscus lens and N = the index of refraction of the meniscus lens.

3. An optical system according to Claim 1 or Claim 2 in which the reflecting means comprise at least one convex and one concave mirror substantially concentrically arranged along the optical axis.

4. An optical system according to Claim 3 wherein the conjugate planes including means to define a location for an object the image of which is a real image at a second location, the convex mirror being positioned about the centre of curvature to reflect to the cocave mirror light from the object location initially reflected to the convex mirror from the concave mirror, whereby light from the object location will be reflected at least twice at the concave mirror and at least once at the convex mirror before being focussed at the second location.

5. An optical system according to Claim 3 or Claim 4 and also including a substantially plane parallel colour correction plate mounted normal to the optical axis of the mirrors.

6. A modification of an optical system according to Claim 5 wherein one of the faces of the plane parallel plate is aspheric.

7. An optical system according to any one of Claims 4 to 7, wherein the concave and convex mirrors are supported with a distance between their centres of curvature of less than about two percent of the length of the radius of the convex mirror.

8. An optical system according to any one of Claims 3 to 6 wherein the mirrors are arranged so that there are three reflections from the concave mirror and two reflections from the convex mirror.

9. An optical system according to Claim 4 wherein the mirrors are supported with a distance between their centres of curvature of less than about two percent of the length of the shorter radius and the system also includes a substantially plane parallel colour correction plate interposed between the mirrors and the object and image locations, the plate being mounted normal to the optical axis of the mirrors whereby the algebraic sum of the quantities obtained by dividing the power of each surface in the system by its index of refraction is substantially zero with the index of refraction of a reflecting surface being defined as minus one.

10. An optical system according to Claim 9 wherein the meniscus elements are mounted a

Claims

**WARNING** start of CLMS field may overlap end of DESC **. accuracy requirements of the scanning motion are minimised. An arrangement that achieves this by incorporating three flat mirrors in the optical system was shown in the U.S. patent no: 3 951 546. WHAT WE CLAIM IS:

1. An optical projection system adapted to provide a discrete off-axis field zone which, as generated, is annular in shape, said field zone having a substantially flat focal surface within a range of projection light wavelengths, wherein the system comprises optical elements including reflecting means, the system being arranged to form conjugate planes normal to its optical axis for which the system is of substantially unit power, and refracting means mounted between the reflecting means and the conjugate planes and comprising at least one pair of symmetrically disposed, nearly concentric meniscus elements whose thickness is greater than the difference between their convex and concave radii, the optical system being constructed and arranged so that the Petzval sum is substantially zero, and the refracting means including provision for balancing the effects of variation in said Petzval sum due to variation in colour by introducing axial chromatic aberration of the opposite sense so that the positions of focus at the annular field portions of the conjugate planes remain substantially constant.

10. An optical system according to Claim 9 wherein the meniscus elements are mounted adjacent the convex mirror and the substantially plane parallel colour correction plate is interposed between the meniscus elements and the object and image locations respectively.

11. An optical system according to Claim 9 wherein the meniscus elements are mounted at conjugates selected so as to minimise their contribution to astigmatism.

12. An annular field optical system according to Claim 9 and having construction data as set out in Table -1 herein.