WO1982003924A1

WO1982003924A1 - Afocal apparatus for the optical-mechanical scanning of an object

Info

Publication number: WO1982003924A1
Application number: PCT/SE1982/000156
Authority: WO
Inventors: Stig Leopold Berglund
Original assignee: Stig Leopold Berglund
Priority date: 1981-05-07
Filing date: 1982-05-06
Publication date: 1982-11-11
Also published as: EP0078297A1

Abstract

An afocal apparatus for the optical-mechanical scanning of an object in two dimensions includes a collimation objective (O), a first scanning means (1) for scanning the object in a first dimension via the objective (O), a second scanning means (2) for scanning the object in a second dimension, intermediate optics (M) between the first and the second scanning means, a detector (D) and a relay optical system (OR) between the detector (D) and the second scanning means (2). The intermediate optics is formed by a primary mirror (12) and a secondary mirror (11) which are substantially concentric and face-to-face with a substantially common centre (3) of curvature and a mutual spacing sustantially corresponding to the radius of curvature of the secondary mirror. A pupil position (P') can be arranged at one of the scannig means, the other scanning means being situated at a conjugate point (P'') to the pupil position in relation to the intermediate optics (M). The width ( DELTA ) of the secondary mirror (11) is adjusted to the width of the detector image (D) so that the longitudinal edges of the secondary mirror (11) constitute a field line aperture (F). The bundles of rays incident on, and coming from the intermediate optics can be limited and situated as close to the secondary mirror as posible. The primary mirror can be parted in the symmetrical plane of the optics, the mirror halves being outwardly turned a small angle. The mirrors of the intermediate optics may consist of ellipsoid cutouts. Of the possible combinations of ellipsoid cutouts, there are some which are significantly better than the others.

Description

AFOCA APPARATUS FOR THE OPTICAL-MECHANICAL SCANNING OF AN OBJECT:

Technical Field The invention relates to an afocal apparatus for the optical-mechanical scanning_, of an object in two dimensions, including a collimation objective, a first scanning means for scanning the object in a first dimension via the^' objective, a second scanning means for scanning the object in a second dimension, and an intermediate optical system between the first and the second scanning means, a detector and a relay lens system between the detector and the second scanning means.

Background Art The British patent GB 2,023,964 illustrates a so- called scanner, i.e. an apparatus of the kind described above,.and on hich the invention is based.

In the known afocal scanner mentioned, two lenses are utilized between both scanning mirrors as intermediate optics. Particularly in the case where the scanner is to be utilized in conjunction with heat radiation, the lenses must be made of special and expensive material,and they must also be made with very great accuracy, due to the demands present for small aberrations and large physical dimensions caused by desired field angle. Furthermore, a separate field aperture is required in the known scanner with respect to the detector.

Object of the Invention

One aim of the invention is to improve a scanner structure of the kind disclosed in the British Patent No. 2,023,964 A in respect of the aberrations, production cost and space requirement of the intermediate optical system, at the same time providing in a favourable mode a field aperture for a detector.

OMPI Summary of the Invention

For an afocal apparatus for optical-mechanical scanning of an object in two dimensions, including a colli- mation objective, a first scanning means for scanning the object in a first dimension via the objective, a second scanning means for scanning the object in a second di¬ mension, an intermediate optical system between the first and the second scanning means, a detector and relay optical system between the detector and the second scanning means, said aim is achieved by having the intermediate optics com¬ posed of a primary mirror and a secondary mirror, the mirτors being substantially concentric and face-to-face with a substantially common centre of curvature and a mutu¬ al spacing, substantially corresponding to the radius of curvature of the secondary miτror, the width Δ of the secondary mirror being adjusted to the width of t-he detect¬ or image so that the elongate edges of the secondary mirror constitute a field aperture with respect to the detector image. A pupil position can be arranged at, or adjacent, one of the scanning means, the other one being situated at a conjugate point with the pupil position in relation to the intermediate optics.

To advantage, the scanning means can be constructed as mirrors. Also to advantage, the relay optical system may be a catoptrical system, the pupil of which, to advantage, coincides with that of the second scanning means.

Catoptrical imaging systems similar to the inter¬ mediate optics utilized in the present invention are alread known per se, e.g. from US Patents 3,748,015 and 4,097,125, which by utilizing slits arranged diametrically opposing and substantially in a normal plane to the axis through the centTe curvature of the mirrors, with respect to the optical axis, afford aberration-free images which can be utilized, e.g. in the production of integrated circuits. The invention utilizes such a catoptrical system as intermediate optics in an afocal scanner, however, so that

OMPI substantially colli ated light is incident on, and departs from the intermediate optics. The secondary mirror width can in such a case substantially correspond to the size of the detector image, so that the primary mirror width can be reduced to a dimension corresponding to twice the object¬ ive aperture plus the secondary mirror width. The elongate edges of the secondary mirror constitute a line or band field aperture for the detector image.

The respective length of the primary and secondary mirrors can further be adjusted to the sweep angle in the second scanning dimension. There is thus obtained a.sub¬ stantially aberration-free and very compact intermediate optics which reduces the need of a separate field aperture for the detector image. With intermediate optics of the kind presently under consideration, axial coma is the most significant aberrat¬ ion. One or more of the following measures may be adopted for reducing primarily this aberration.

Firstly, the pupil can be limited so that the ray bundle towards - nd away from the intermediate optics is cut off parallel to the symmetry plane of the intermediate optics, At least adjacent the secondary mirror, the ray bundle being disposed with its limited edge as close to the edges of the secondary mirror as possible. The bundle of rays can be masked on the opposite side also.

In both cases the limitation should be less than half the radius of the bundle of rays. The limitation itself re¬ duces coma. By moving the bundle of rays closer to the plane of symmetry of the intermediate optics, further re- duction in aberration is achieved.

Secondly, the primary mirror can be divided into two parts by a section along the symmetry plane of the inter¬ mediate optics to further provide aberration reduction, both parts being turned outwards about a line lying in the plane of symmetry and at right angles to the axis of the intermediate optics, and possibly translated axially.

OMPI Optimally, the turning-out angle is less than 1.5°, if translation is zero.

The translation between the parts along the axis of the intermediate optics, relative to the nominal distance, i.e. the mean distance between primary and secondary mirror is at most 0.2, for which the turn-out angle should be less than 5°.

Optimally, said nominal distance is less than half the radius of curvature of the primary mirror, and greater than or equal to 0.9 times said one-half of the radius of curvature.

Thirdly, different cutouts from rotation ellipsoids may be used instead of spherical surfaces. A rotation ellipsoid may be described by: Z - ^(P² + Ce² + 1)Z²) = o CD where p = x 2 + y2 e = eccentricity of the ellipse generating the ellipsoid.

There are six possible types of such cutouts of gene- rations having the desired symmetrical properties. This gives 36 different combinations. It may be assumed that an ellipse rotates about its longitudinal axis or its short axis. The rotational axis can then be designated as a Z axis, and the Z axis can be oriented in different ways, as will be apparent from the appended drawings, Figs. 7A, 7B and 7C. In total, there are 36 different combinations of ellipsoid mirrors for forming intermediate optics in accordance with the invention.

The nine best combinations are set forth in the table below. Five combinations in the table are marked with a ring as an indication that these consitute the best combi¬ nations among the nine selected. Characteristic for the primary mirror is that it is in all cases generated by an ellipse, the eccentricity of which is approximately equal to the quotient of the distance between the mirror "axis" and the centre of the bundle of rays on the one hand, and on the other hand the distance between the mirror and its

OMPI IPO centre point. Optimally, the eccentricity is less than three times said eccentricity. The distance between the mirror and its "centre point" is equal to R, as will be seen from formula (1). In the case of ellipsoid mirrors, by "axis" is intended the normal to the vertex for the mirror.

A possible embodiment, of the intermediate optics includes an ellipsoid-shaped primary mirror and a sϋheri- cal secondary mirror, since as the secondary mirror is at a focal plane its form is relatively non-critical.

TABLE

by "axis orientation" is intended orientation of the axis about which the ellipse is rotated to form an ellipsoid, i.e. the Z axis. The Z axis can be oriented according to any of the Figures 7A - 7C on the appended drawing.

The Z axis is either the long or the short axis in the ellipse.

OMPI ^■ 1PO Since the secondary mirror is at a focal plane, its form is relatively non-critical, but it can be decentred and turned to give optimum aberration balancing.

The rating given in the table is a calculated indi- cation of the suitability of the different combinations, and from the rating it will be seen that one can assume that combination 1 is the most favourable, with relation to use of the intermediate optics in a scanning apparatus in accordance with the invention. The invention will now be described in detail in the following with reference to the appended drawing.

Drawing

Fig. 1 schematically illustrates a scanning apparatus in accordance with the invention. Fig. 2 is a view along the line II-II in Fig. 1. Fig. 3 is a partial view taken along the line III.-III in Fig. 2. Fig. 4 illustrates a relay optical system whic can be used in the apparatus according to Fig. ;1. Fig. 5 illustrates a preferred embodi¬ ment of the intermediate optical system. Fig. 6 is a odi- fication of the intermediate optics. Figs.7 A-C illustrate ellipsoidal generation for surfaces in the intermediate optics in accordance with the invention.

Embodiment Example

An objective 0 is illustrated in Fig. 1, and it can be focused towards the object which is to be scanned and which emits substantially collimated light to a rocking mirror 1. The mirror 1 scans the object in a first dimens¬ ion and directs the light towards a second scanning means 2 in the form of-a rotating mirroring means via inter- mediate optics M. The light is directed from the second scanning means 2 via a relay optical system OR towards a detector D.

The intermediate optics M is formed by two concent¬ ric mirrors 11, 12, which are face-to-face and have a

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OMPI WPO substantially common centre of curvature 3, the spacing between the mirrors 11, 12 substantially corresponding to the radius of curvature of the secondary mirror 11.

The pupil positions of the first and second deflect- ing means 1 are arranged in conjugate points of the inter¬ mediate optics M, and the entry pupil of the relay optical system OR coincides with the pupil position P* of the second scanning means. Further, the intermediate optics is constructed such that an image D' of the detector D falls on the secondary mirror 11, the elongate edges thereof constituting a line or band array aperture"'F for the de¬ tector image.

In the illustrated embodiment, the pupil positions P' and P" are in the normal plane to the axis of the inter- mediate optics M through the centre of curvature 3.

It will also be apparent from Fig. 1 that the width of the primary mirror 12 can be minimized to a dimension corresponding to twice the width of the bundle of rays plus the width Δ of the secondary mirror 11. As is apparent from Fig. 2, the arcuate length of the primary mirror 12 and secondary mirror 11 may be ad¬ justed, to the sweep angle of the mirroring means 2. This means that the physical dimensions of the intermediate optics can be reduced to those apparent from Fig. 3, where α = 2λ + _R-^p ■, wherein α denotes the angular dimension of the primary mirror in relation to the centre 3 and λ denotes half the sweep angle of the mirroring means.

It should be noted that the radius of curvature R₂ of the secondary mirror 11 can to advantage be allowed to be somewhat larger than half the radius of curvature Rι+R_>=R of the primary mirror 12, although at most 1.1•- R>■■■.

A relay optical system OR affording a pupil position

P' on the mirroring means 2 is shown in Fig. 4. The relay optics OR according to Fig. 4 comprises a catoptric unit comprising a primary mirror 22 and a secondary mirror 21 having a substantially common centre 23 of curvature. An aperture F is arranged on the secondary mirror 21. The pupil position P' of the relay optics is disposed on the mirroring means 2 (see Fig. 1). Further relay optics 24, e.g. in the form of a focusing lens, can be disposed 5 between the relay optics OR and the detector D to focus towards the detector. With a scanning apparatus in accordance with the invention, the number of necessary true lenses is reduced, mirror elements being utilized instead, the apparatus thus being more sensitive to the

10 radiation wavelength.

The objective 0 can be a zero objective, i.e. it may be dispensed with, if the object which is to be scanned in practice is at infinity.

Fig. 5 illustrates how the bundles of rays to and

]5 from the intermediate optics can be limited and caused to have their limited edges parallel to, and adjacent the elongate edges of the secondary mirror. The bundles can be limited or vignetted on two opposing sides, as illustrated. The limitations are denoted 17 and 18. The bundle of rays

20 is assumed to originally have had a circular cross section. The distance d between the centre of the bundles of rays and the axis of the optical system is suitably equal to the distance between the centre and one pole in the ellipse from which the mirror surface is generated.

25 Fig. 6 illustrates an embodiment in which the primary mirror is parted along a plane S in Fig. 5, which is at right angles to the plane of the figure and contains the axis of the intermediate optics. The numeral 3 denotes the substantially common centre of curvature,or the centre

30 points for the mirrors,or the ellipsoids from which the mirrors have been cut. The parts 12A, 12B of the primary mirror are turned out an angle β about a line lying in the centre plane and which is normal to the axis of the optics i.e. a normal to the plane of Fig. 6. The distance 5. 35 between the mirrors 11, 12 is O.θ R < I < p where

R is the same as in formula (1) R₂ is substantially equal to • R» or < 1.1£.

OMPI The parts 12A, 12B of the primary mirror are trans¬ lated a distance σ , wherein is the mean distance to the mirror parts from the secondary mirror.

If σ » 0, β is less than 1.5° as an optimum, β in¬ creases for increasing σ, and β is less than 5° for σ/Jl '< 0.2.

According to the invention, the mirrors 11, 12 can to advantage have the form of cutouts from ellipsoids. Characteristic for the mirror 12 is that it is generated by an ellipse with the eccentricity e "^ •_*.

The ellipsoid is generated by rotation of an ellipse about its short axis or long axis. On an ellipsoid there are six different cutouts which are usable as surfaces for a mirror in the intermediate optics. These six cutouts are formed by having an ellipse which is dimensioned for the mirror rotated about its rotational axis with the latter oriented according to Figs. 7 A-C. This signifies that -there are three possible rotational axis orientations and two possible selections of the axis about which the ellipse is rotated.

The same thing applies to both mirrors 11, 12, so that 36 different combinations may be found for an inter¬ mediate optical system.

In the table given above, the nine best combinations are given, of which the five ringed combinations are re¬ garded as being the best. In the right-hand column of the table there is given a rating which has been calculated to indicate considered drawbacks, inter alia aberration. A low rating signifies low aberration. Combination 1 thus appears to be preferable.

The advantages of the inventive kind of intermediate optical system is that the detector turn with relation to scanned line is negligible, that the line curvature is negligible and that the detector unit or array covers at least 101 of the scanned image.

In an apparatus in accordance with the invention,

OMPI with the distance 25 mm between the mirrors 11, 12, a geometric resolution of 320 lines, a line field angle of

2 λ = 40 and a mirroring means 2 with a described circle of 50 mm diameter and 9 edge surfaces, a rating of G < 0.4 is obtained

G = l/Ctanλt η') where λ = one-half of the line field angle φ - area of aperture or ray bundle η - line scanning efficiency (the part of the mirroring means periphery which is usable for line scanning) . λ is practically limited by the input objective, λ is optimally about 20° for reasonably large objectives.

The lower the rating G is, the better is the tempe- rature resolution of the apparatus.

If the inventive improvements are now applied to the intermediate optical system, even better ratings can be obtained, or the aberrations reduced.

Claims

1. Apparatus for optical-mechanical scanning of an object in two dimensions, including a collimation objective (0), a first scanning means (1) for scanning the object in a first dimension via the objective (0) , a second scanning means (2) for scanning the object in a second dimension, and intermediate optical system (M) between the first and the second scanning means, a detector (D) and a relay opti¬ cal system (OR) between.the detector (D) and the second scanning means (2) , the intermediate optics being formed by a primary mirror (12) and a secondary mirror (11), which are substantially concentric and.face-to-face with a sub¬ stantially common centre (3) of curvature and a mutual spacing substantially corresponding to the radius of cur¬ vature of the secondary mirror, characterized in that the width (Δ) of the secondary mirror (11) is adjusted to the width (D) of the detector image such that the elongate edges of the secondary mirror (11) constitute a field line aperture (F) in relation to the detector image.

2. Apparatus as claimed in claim 1, characterized in that a pupil position (P^f) is arranged at, or adjacent one of the scanning means, and that the other scanning means is situated in a conjugate point (P") to the pupil position in relation to the intermediate optical system (M)

3. Apparatus as claimed in claim 1, characterized in that the first and second scanning means are formed by mirrors.

4. Apparatus as claimed in claim 1 or 2, character¬ ized in that the relay optical system (OR) is formed by reflecting surfaces.

5. Apparatus as claimed in claim 1, characterized in that the relay optical system (OR) has a pupil which substantially coincides with the second scanning means (2) .

6. Apparatus as claimed in any of claims 1-5, characterized in that the bundles of rays which are inci¬ dent on, and depart from the intermediate optics are

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OMPI limited nearest the secondary mirror, that the limited edges of the bundles of rays are arranged to extend parallel to, and adjacent the elongate edges of the secondary mirror and that the limitation attains at most half the radius of the ray bundle.

7. Apparatus as claimed in claim 6, characterized in that the ray bundle is also limited at a distance from the secondary mirror parallel to the first limitation.

8. Apparatus as claimed in any of claims 1-7, characterized in that the primary mirror (12) is parted along the plane of symmetry of the intermediate optics between the scanning means, and that at least one of the parts (12A, 12B) of the primary mirror is turned outwards relative the plane of symmetry by an angle (β) which is less than 5°, and preferably attaining substantially 1.5 , if the mirror halves are not relatively translated in the axial direction of the intermediate optics.

9. Apparatus as claimed in any of claims 1-8, characterized in that the primary mirror (11) of the inter¬ mediate optical system is generated by rotation of an ellipse about its longitudinal axis, which is oriented as a normal to the symmetrical plane of the intermediate optics between the scanning means, the secondary mirror of the intermediate optics being generated by rotation of an ellipse about its longitudinal axis, said axis being oriented as a normal to the plane of symmetry of the inter¬ mediate optics or is in the plane of symmetry at right angles to the intermediate optics axis, or the secondary mirror being generated by rotation of the ellipse about its short axis coinciding with the intermediate optics axis, alternatively that the intermediate optics primary mirror is generated by rotation of an ellipse around its short axis lying in the plane of symmetry of the inter¬ mediate optics at right angles to the axis thereof, the secondary mirror of the intermediate optics being generat¬ ed by rotation of an ellipse about its short axis oriented

- REX;

OMPI to coincide with the axis of the intermediate optics, so as to lie in the normal to the plane of symmetry of the intermediate optics, or lie in the plane of symmetry of the intermediate optics at right angles to the axis thereof.

10. Apparatus as claimed in any of claims 1-9,

R R characterized in that R₂ - '1.1£, and 0 .9 < I < where I is the axial distance between the primary and secondary mirrors, R2 is the distance of the secondary mirror to its central point of generation and R is the distance of the primary mirror to its central point of generation.

OMPI