LENS MOUNT FOR, AND METHOD OF MOUNTING HIGH RESOLUTION PROJECTION LENSES

Abstract: A lens mount including a lens cell (10) wherein a lens (20) is mounted through flexures (16). The lens seat (22) in the lens cell has a radius of curvature match-mated to the lens surface (21) which rests upon it. The rotational symmetry axis of the lens seat is coincident with the axis of rotation (19) of the lens cell. The optical axis of the second lens surface is coincident with the axis of rotation (19) of the lens cell.

Description: The present invention relates to a lens mount comprising: a lens having a first spherical optical surface 21 and a second optical surface 23; a lens cell 10 having a top surface 28, a bottom surface 4, an outer diameter 26 and an inner diameter; said outer diameter 26 being concentric about an axis of rotation 19 of said lens cell 10; said top surface 28 and said bottom surface 24 being substantially perpendicular to said axis of rotation 19; a lens seat 22 projecting inwardly from said inner diameter; said lens seat 22 constituting an integral part of said lens cell 10; the invention also relates to a method for mounting a lens to a lens cell. In particular, the present invention relates to aparati and methods for mounting high resolution projection lenses and, more particularly, to such aparati and methods wherein the lens is mounted to the lens cell following the machining of the cell. According to the US patent-A-4 258 982 a lens mount has become known in which the lens cell has a substantially cylindrical wall formed with an angular inwardly extending protrusion providing an angular planar lens element seating surface facing one end of the cell body. The cylindrical wall of the lens cell defines an optical axis for the lens element. The cylindrical wall has at least one slot formed therein and opening through said one end thereof, whereby the lens element having at least one sidewardly extending tab can be inserted into the cell body with one registration face in engagement with the angular seating surface and with its tab registered in said slot. A retainer is secured to said cell body and has an angular seating surface forming a registration surface for the other registration face of the lens element. Furthermore, from the DD-A-226 089 a lens mount had alread become known in which the lens having two spherical optical surfaces is held between two angular rings each having spherical curvature and each being adapted to one side of the lens. Said angular rings are pressed together by two opposite plates which are compressed by screw bolts which extend through corresponding openings in said plates. In order to get optimum performance from high resolution lithographic lens assembles it is imperative that all lenses in a given assembly be centered, within a few microns, to a common optical axis. Further, these lens mounts are expected to maintain an extremely high level of performance under a multitude of environmental conditions during use, storage and shipping. Lenses are commonly affixed within lens cells with mechanical devices. Differences in the coefficient of thermal expansion between the lens and the lens cell can lead to decentration and intolerable stresses in the lens during ambient temperature changes. One approach to the solution of the foregoing problems calls for the lens to be mounted to a lens through flexure means. These flexure means return the lens to a predetermined spatial relation to the lens following temperature excursions away from, then back to, the datum temperature. Following the lens being mounted to the lens cell the outer diameter and upper and lower surfaces of the lens cell are precision machined with respect to the optical axis of the lens. While this approach offered significant advantages over the prior art it does have several drawbacks. There is a chance of damaging the lens mounted in the lens cell during the machining of the lens cell. Also, if an error were made in machining the lens cell it si relatively difficult to remove the lens for installation in another lens cell. This last feature is desirable in that the lenses are quite expensive. The present invention eliminates or ameliorates the foregoing disadvantages of the prior art by providing a lens mount unaffected by large temperature excursions, which can be easily and safely machined without endangering the lens and which can be easily disassembled. In addition, the lens mount provided is very stable and has a high resonant frequency. In order to achieve the desired results the present invention provides a lens mount which is characterized in that said lens seat 22 has a radius of curvature matching the radius of curvature of said first optical surface 21 of said lens; the rotational symmetry axis of said lens seat 22 being coincident with the axis of rotation of said lens cell 19; the first optical surface 21 of said lens being substantially in contact with the said lens seat 22; the optical axis of said second optical surface 23 being substantially coincident with the axis of roation 19 of said lens cell 10, and in that a plurality of flexure means are holding said lens to said lens cell. The desired results are especially achieved in accordance with the invention by a method for mounting a lens to a lens cell comprising: providing a lens cell having a top surface, a bottom surface, an outer diameter, an inner diameter and a lens seat projecting radially inwardly from said inner diameter, said lens seat constituting an integral part of said lens cell; forming said outside diameter concentric about the axis of rotation of said lens cell; forming said top surface and said bottom surface perpendicular to the axis of rotation of said lens cell; providing a lens having a first spherical optical surface and a second optical surface; forming the surface of said lens seat to have a radius of curvature to match the radius of curvature of the first optical surface of said lens and to have a rotational symmetry axis coincident with the axis of rotation of the lens cell; placing the first optical surface of said lens substantially in contact with said lens seat; and aligning the optical axis of said second optical surface of said lens with the axis of rotation of said lens cell, and attaching said lens to said lens cell through a plurality of flexure means. Briefly, the lens cell is machined with a spherical seat for a lens concentric with the axis of rotation. The outer diameter of the lens cell is concentric with the axis of rotation and the top and bottom surfaces of the lens cell are machined perpendicular to the axis of rotation. Three flexure assembles are then affixed to the inner diameter of the lens cells. The lens cell is then placed on an air-bearing table and centered on its axis of rotation by indicating the reference outer surface by using an electronic/air gage. The lens is then placed on its seat in the cell. An electronic/air gage is used to indicate the top surface of the lens and the air-bearing table is rotated. The lens is then moved on its seat, relative to the lens cell, so as to minimize the wobble of the top surface of the lens. At this stage, the axis of the lens is concentric with the axis of the air-bearing table and the axis of the outer diameter of the lens cell. The lens is then bonded to the three flexures. A lens assembly can be made by stacking together a plurality of lens cells fabricated in the manner just described. These cells are stacked and centered on the axis of an air-bearing table by bringing all of their outside diameter into concentricity. The plurality of lens cells, so aligned, are then bonded together to obtain the final lens assembly. Special embodiments of the invention are stated in the subclaims to which particular references are made and which are hereby included in the specification. 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. Fig. 1 is a side sectional view of a lens cell in accordance with the present invention showing the lens attached; Fig. 2 is a plan view of the lens cell of Fig. 1; and Fig. 3 shows a variation of the flexures of Fig. 2. Fig. 1 shows a side sectional view of a lens cell 10 having a lens 20 constrained therein. The lens cell 10, has a top face 28, a bottom face 24, an outside diameter 26 and a spherical surface 22. The lens 20 has an upper surface 23 and a bottom surface 21. When the lens 20 and lens cell 10 are properly assembled together, in a manner described hereinbelow, their constituent elements have the following interrelationships: the outside diameter 26 of the lens cell is concentric about an axis of rotation 19; top face 28 and bottom face 24 are perpendicular to the axis of rotation 19; axis of rotation 19 is the symmetric axis of rotation of spherical seat 22; the spherical surface 21 of lens 20 matches the spherical seat 22 of the lens cell 10 (This relationship ensures that the optical axis of spherical surface 22 is the axis of rotation 19.); and the optical axis of the upper surface 23 of lens 20 is coincident with axis of rotation 19. The above described interrelationships can be established in the manner described hereinbelow. The lens cell is mounted on a lathe, preferably supported by air bearings, and machined, preferably using diamond point tooling. The machining apparatus does not constitute a part of the present invention and is, consequently, not shown. The outside diameter 26 of the lens cell 10 is machined to be concentric with the axis of rotation of the lathe, thus establishing the axis of rotation 19. Top face 28 and the bottom face 24 of the lens cell 10 are machined to be perpendicular to the axis. The spherical seat 22 is machined to match the curvature of the bottom surface 21 of lens 20. In addition, the spherical seat 22 is machined to have a symmetrical axis of rotation coincident with the axis of rotation 19. Following its machining the lens is placed on a precision rotating surface, for example, an air-bearing table. It is then centered on its axis of rotation 19 by indicating the outer diameter 26 using, for example, an electronic/air gage. The lens 20 is then placed on the spherical seat 22 in the lens cell 10. By precisely machining spherical seat 22 and matching spherical seat 22 to spherical surface 21 of lens 20 the optical axis of spherical surface 21 is on the axis of rotation 19. The optical axis of surface 23 of lens 20, however, may not be on the axis of rotation 19 where, for example, surface 23 is aspheric. Accordingly, a gage, for example, an electronic/air gage, is placed on surface 23 and the lens cell 10 is caused to rotate, which causes the lens 20 to likewise rotate. The lens 20 is then rotated relative to the lens cell 10 so as to minimize the run out detected by the gage, not shown. A lubricant, such as Teflon or Moly, can be provided on spherical seat 22 to facilitate this movement process. At this point the axis of rotation 19 is shared by the optical axes of surfaces 21 and 23 of the lens, the rotational axis of symmetry of spherical seat 22 and the axis of rotation of outer diameter 26 of the lens cell 10. In addition, the top face 28 and bottom face 24 of lens cell 10 are perpendicular to the axis of rotation 19. Fig. 2 is a plan view of the lens cell 10 and lens 10 of Fig. 2. In addition, Fig. 2 shows the three flexure assemblies 15-17 used to bond the lens 20 to the cell 10. The flexure assemblies 15-17 are attached to the lens cell through the use of, for example, screws 14. After their attachment by screws 14 the flexure assemblies 15-17 can be rigidly fixed to the lens cell 10 by dabs of epoxy 27. The actual bonding for the lens 20 to the flexure assemblies 15-17 si accomplished by bonding the outer diameter of the lens 20 to the flexure arms 11-13 through an epoxy bondline 18. The epoxy bondline 18 is, for example, an ultra-violet curing epoxy and is applied in any conveniently manner. Fig. 3 shows a variaton of the three flexure assemblies 15-17. In this variation both ends of each flexure arm, e.g., flexure arm 11 of flexure assembly 15 are connected to the main body of the flexure assemblies. In this case, the lens is bonded to the flexure arms at the center protrusions 11a, shown in Fig. 3. Briefly, the flexure arms 11-13 attach the lens 20 to the lens cell 10 through the flexure assemblies 15-17, respectively. As the ambient temperature cycles up or down from a datum temperature the relative diameters of the lens 20 and lens cell change due to differing thermal coefficients of expansion. The flexure assemblies 15-17, through flexure arms 11-13, respectively, and bondlines 18 hold the lens 20. As the relative diameters of the lens 20 and lens cell 10 change the flexure arms 11-13 flex or move relative to the bodies of the flexure assemblies 15-17, hence relative to the lens cell 10. At no time do the flexure arms 11-13, lens 20 or bondline 8 reach the allowable stress limits of the respective materials. When the ambient temperature returns to the datum temperature the lens 20 and lens cell 10 return to their original, as assembled, relationship. The flexure assembles 15-17 thus allow compensation for ambient temperature changes and serve to constrain the lens 20 within the lens cell 10 with high stability and high resonance. That is, at the datum temperature the lens 20 is held in a substantially fixed spatial relationship to the lens cell 10. Since the flexure assembles 15-17 are attached to the lens cell 10 through scews 14 and dabs of epoxy 21, they can be easily removed at any time. Such removal may be necessary if the lens cell 10 were improperly machined and it is desired to salvage the lens 20. In this case the lens 20, with attached flexure assembles 15-17, would be removed from the lens cell 10. A suitable solvent would be used to dissolve the bondline 18 between each of the flexure arms 11-13 and the lens 20. The lens 20, undamaged, is now ready for assembly in another lens cell in the manner hereinabove described. A plurality of lens cells 10 machined in the manner described hereinabove, having substantially equal outside diameters 26 can be assembled in a co-axial assembly. This is done by stacking such a plurality of lens cells 10 in such a way that their outside diameters 26 share a common, imaginary, cylindrical surface. In this way the optical axes of all lenses within such a stack of lens cells are co-axial. By ensuring that the top face 28 and the bottom face 24 of the lens cell 10 are perpendicular to the axis of rotation 19 wedge conditions between lenses 20 in adjacent lens cells 10 can be eliminated or ameliorated. Following stacking and aligning the plurality of lens cells are epoxied together to form a unitary assembly, not shown. There has thus been described a lens mount which is unaffected by large temperature excursions, which can be easily machined without endangering its corresponding optical element or lens and which can be easily disassembled. Further, this lens mount is very stable and has a high resonance frequency and is stiff in the axial direction.

Claims: 1. A lens mount comprising: a lens (20) having a first spherical optical surface (21) and a second optical surface (23); a lens cell (10) having a top surface (28), a bottom surface (24), an outer diameter (26) and an inner diameter; said outer diameter (26) being concentric about an axis of rotation (19) of said lens cell (10); said top surface (28) and said bottom surface (24) being substantially perpendicular to said axis of rotation (19); a lens seat (22) projecting inwardly from said inner diameter; said lens seat (22) constituting an integral part of said lens cell (10); characterized in that said lens seat (22) has a radius of curvature matching the radius of curvature of said first optical surface (21) of said lens (20); the rotational symmetry axis of said lens seat (22) being coincident with the axis of rotation of said lens cell (19); the first optical surface (21) of said lens (20) being substantially in contact with the said lens seat (22); the optical axis of said second optical surface (23) being substantially coincident with the axis of roation (19) of said lens cell (10), and in that a plurality of flexure means are holding said lens (20) to said lens cell. 2. A lens mount as claimed in Claim 1 wherein said flexures (15-17) are removably attached to said lens cell (10). 3. A lens mount as claimed in Claim 2 wherein said flexures comprise: a flexure assembly (15-17) attached to said lens cell (10); a flexure arm (11) attached at one end to said flexure assembly (15-17) and at the other end to said lens (20); and said flexure arm (11) being substantially parallel to, and spaced from said flexure assembly (15-17). 4. A lens mount as claimed in Claim 3 wherein said flexure arm (11, 11a) is attached to said lens (20) with an epoxy. 5. A lens mount as claimed in Claim 2 wherein said flexures comprise: a flexure assembly attached to said lens cell (10); a flexure arm (11) attached at both ends to said flexure assembly (15-17) and at its center (11a) to said lens (20); and said flexure arm (11) being substantially parallel to and spaced from said flexure assembly (15-17). 6. A lens mount as claimed in Claim 5 wherein said flexure arm (11) is attached to said lens with epoxy. 7. A method for mounting a lens (20) to a lens cell (10) comprising: providing a lens cell (10) having a top surface (28), a bottom surface (24), an outer diameter (26), an inner diameter and a lens seat (22) projecting radially inwardly from said inner diameter, said lens seat (22) constituting an integral part of said lens cell (10); forming said outside diameter (26) concentric about the axis of rotation (19) of said lens cell (10); forming said top surface (28) and said bottom surface (24) perpendicular to the axis of rotation (19) of said lens cell (10); providing a lens (20) having a first spherical optical surface (21) and a second optical surface (23); forming the surface of said lens seat (22) to have a radius of curvature to match the radius of curvature of the first optical surface (21) of said lens (20) and to have a rotational symmetry axis coincident with the axis of rotation (19) of the lens cell (10); placing the first optical surface (21) of said lens (20) substantially in contact with said lens seat (22); and aligning the optical axis of said second optical surface of said lens (20) with the axis of rotation (19) of said lens cell (10), and attaching said lens (20) to said cell (10) through a plurality of flexure means.

Inventors: AHMAD, ANEES, HUSE, RICHARD
Applicants: SVG LITHOGRAPHY SYSTEMS, INC.
Application year: 1987
Main classification: G02B7
IPC codes: G02B7, G03F7