WO2015053626A2

WO2015053626A2 - Device for placing at least two rigid lenses and/or mirrors coaxially relative to each other, a lens mount assembly comprising such device, and a manufacturing method therefor

Info

Publication number: WO2015053626A2
Application number: PCT/NL2014/050700
Authority: WO
Inventors: Leonard Antonino Cacace
Original assignee: Ac Optomechanix
Priority date: 2013-10-07
Filing date: 2014-10-07
Publication date: 2015-04-16
Also published as: WO2015053626A3

Abstract

The present invention is related to a device for placing at least two rigid lenses and/or mirrors coaxially relative to each other, comprising: - a substantially rigid housing comprising an opening configured for receiving the at least two rigid lenses and/or mirrors therein; - at least two elastic engaging means which are arranged coaxial to each other; - wherein each of the at least two rigid lenses and/or mirrors is engageable by at least one of the two or more elastic engaging means; and - wherein the at least two elastic engaging means each comprise a set of elastic flexure elements which are arranged with a first outer end on the housing and extend therefrom into the opening such that the second outer ends of the flexure elements located opposite the first outer ends describe in the opening a form which in an untensioned rest state is smaller than the circumferential form of the rigid lens and/or mirror situated at that position in an engaging state. The invention is furthermore directed to a self-centering rigid lens mount assembly comprising such a device, and to a method of manufacturing such a device.

Description

DEVICE FOR PLACING AT LEAST TWO RIGID LENSES AND/OR MIRRORS COAXIALLY RELATIVE TO EACH OTHER, A LENS MOUNT ASSEMBLY COMPRISING SUCH DEVICE, AND A MANUFACTURING METHOD THEREFOR The present invention is directed to a device for placing at least two rigid lenses and/or mirrors coaxially relative to each other, and more particularly to a self-centering lens mount. The invention is furthermore directed to a lens mount assembly comprising such a device, and to a method of manufacturing such a device.

High-end, high numerical aperture (NA) microscope objectives comprise a plurality of rigid lens elements that together form the compound lens system that constitutes the objective. These elements may be mounted as single elements, bonded together using optical kit to form doublets or triplets, or mounted together into sub-assemblies to form lens groups within the larger objective.

For the sake of simplicity and readability, in the following text matters are often described by only mentioning a single rigid lens element. It will be clear to a person skilled in the art that in many cases the same principles apply to doublets, triplets, pre-assembled rigid lens groups contained in a mounting barrel, as long as the outside geometry is coaxial with the axis of the functional elements inside.

The alignment of these rigid lens elements relative to each other is extremely critical if aberrations are to be kept within acceptable limits. For this kind of objectives, decentration tolerances are often tighter than tolerances for airspaces. In some cases coaxiality of the optical axis of the different elements to a submicron level is required.

For these high-end, high NA objectives, some features of the rigid lens elements themselves are manufactured to submicron-level accuracy. For high-end objectives, the optical axis of the elements and the outer cylindrical perimeter of the element are usually centered during edging of the lens elements. The accuracy is typically in the micrometer range and sometimes even in the submicron range. Centering of the lens element is usually not the limiting factor for performance with regard to overall decentering of the elements; instead, decentration of the elements is usually determined by the limited tolerances obtained in mounting and assembly.

Although the optic industry producing such elements is a mature and traditional industry, manufacturing accuracy is continuously improving because of market demands for measurement instruments and production machines with ever more stringent requirements.

Several methods exist to mount a system of rigid lenses. In general, the costs of such methods increase with the accuracy obtained by the method. Some of the main methods used for mounting, in order of increasing accuracy and cost, are: "Drop-in" assembly or simple barrel mounting, "Lathe" assembly (German: ein-drehen), subcell assembly (German: justier/centrier- drehen), and "Active alignment".

In "Drop-in" assembly, the single lens elements, doublets, triplets, and/or groups are placed into a barrel together with spacer rings to constrain them in axial direction, and they are fixed into place using adhesives or by means of a retainer ring that is mounted in the barrel.

Common types of retainers are: a burnished edge, a flexure, a snap ring, a threaded retainer, a press fit or interference fit retainer, and an elastomeric retainer. Care is taken during the edging operation to achieve sufficient centering accuracy of the elements and to control the diameter of the elements. Likewise, during manufacturing of the barrel the inside of the barrel is machined to sufficiently high accuracy. An allowance between outer diameter (OD) of the rigid lens and inner diameter (ID) of the barrel is required to allow the rigid lens be placed without jamming, to allow for differences in thermal expansion, and to prevent chipping of the rigid lens elements.

Decentration of the rigid lens elements or mirrors arises from the narrow gap that exists between lens OD and barrel ID because of this allowance.

In "Lathe" assembly or "Ein-drehen" in German, a similar approach is followed as in "Drop-in" assembly mounting, but in this method the rigid lens is measured after the edging step, after which the inner diameter of the barrel is lathed to match the lens geometry. Though this step adds additional work to the manufacturing process, it reduces the space required between barrel and lens element, adding to the overall performance of the objective.

A more accurate method is called "Subcell assembly", or "justierdrehen" /

"centrierdrehen" in German, in which the individual rigid lens elements are mounted in metal subcells before mounting in the objective barrel. The subcell with lens element is placed on an adjustable chuck and the optical axis location of the lens element is measured and the chuck adjusted so that the optical axis and the rotation axis coincide. After these steps the critical surfaces of the metal subcell is lathed so that will be aligned to the lens axis. The barrel is then lathed to match the geometry of the lens and mount, after which the subcell is mounted to the barrel. The metal-metal interface between barrel and subcell allow for thighter clearances to be used resulting in better alignment of the rigid lens elements to each other.

A more accurate prior art method is "active alignment" of all elements, which involves the individual positioning of the rigid lens elements in the objective by measuring and manipulating each individual lens position to reach the desired alignment tolerance. Various methods exist, including individually moving the lens elements in place using actuator tools or applying a specific impulse to the objective structure that moves only a certain element. These methods are invariably labour intensive and require highly skilled resources, however deliver objectives with the highest performance. The above mentioned methods have disadvantages. For example, in order to allow insertion, account for manufacturing tolerances and to accommodate for differences in thermal expansion, some play between inner and outer surfaces of the mating components is required. This play is one of the main causes of decentration. Reducing the play increases the chance of wedging the rigid lens element or subcell in the lens seat or barrel and increases the risk of chipping of glass elements. Reducing play and reducing the manufacturing tolerances also increases the costs and skill involved to manufacture the lens elements and barrel with the lens seat.

In the "Lathe" assembly, the lens geometry and barrel are uniquely matched; therefore it is not possible to interchange or replace lenses in the objective.

All methods that are based on lathing require very accurate lathing operations. In practice, this often limits the material choice to materials with good machinability such a brass and aluminium. This is unfortunate, since these materials have a much higher coefficient of thermal expansion (CTE) than most glasses used.

Thermal stability is an issue in environments with large thermal cycles, as internal stresses arising from the use of materials with non-matching CTE, as is often the case, can cause hysteresis in contact that is based on friction or allows play, leading to misalignment and loss of performance. Adhesives used to fixate the elements may also exceed the stress tolerance and this could lead to cracking, creep or hysteresis, leading to displacements.

In the aforementioned methods, the accuracy increases with each method, but to reach the highest levels of alignment, the amount of manual intervention is also greatly increased. The latter two methods are reliant on highly skilled staff and costly resources to achieve the high accuracy.

The assembly and tuning of a single microscope objective might take more than a full day of work for a highly skilled labourer.

It is an object of the invention to provide a device for placing at least two rigid lenses and/or mirrors coaxially relative to each other, that is improved relative to the prior art and wherein at least one of the above stated problems is obviated. More particularly, the invention aims to provide a self-centering lens mount.

Said object is achieved with the device for placing at least two rigid lenses and/or mirrors coaxially relative to each other according to the present invention, comprising:

- a substantially rigid housing comprising an opening configured for receiving the at least two rigid lenses and/or mirrors therein;

- at least two elastic engaging means which are arranged coaxial to each other;

- wherein each of the at least two rigid lenses and/or mirrors is engageable by at least one of the two or more elastic engaging means; and - wherein the at least two elastic engaging means each comprise a set of elastic flexure elements which are arranged with a first outer end on the housing and extend therefrom into the opening such that the second outer ends of the flexure elements located opposite the first outer ends describe in the opening a form which in an untensioned rest state is smaller than the circumferential form of the rigid lens and/or mirror situated at that position in an engaging state.

The two elastic engaging means are arranged in said opening of said housing at a longitudinal distance from each other. The rigid lenses and/or mirrors are aligned with a high degree of coaxiality since the structure itself is self-centering, provided that the sets of elastic flexure elements of the at least two elastic engaging means are coaxial to each other. The sets of flexure elements determine the position of the rigid lenses and/or mirrors, by exerting forces originating from elastic deflection of the flexures around its circumference. This provides a mount without play between the outer diameter of the rigid lens and/or mirror and the inner diameter of the flexure element array, eliminating a key source of decentration.

According to a preferred embodiment, said device further comprises an abutment which defines the position of the one or more rigid lenses and/or mirrors in a longitudinal direction of the housing and wherein the position transversely of this longitudinal direction is defined by the elastic engaging means which are engageable with said one or more rigid lenses and/or mirrors with the elastic flexure elements. The abutment, i.e. a seat, acts as an axial stop for lenses and/or mirrors, and determines position and orientation in axial direction and additional directions depending on the shape of the lenses and/or mirrors that rests on it.

According to a further preferred embodiment, the at least two elastic engaging means that each comprise a set of at least three elastic flexure elements, define the position of the rigid lens and/or mirror transversely of the longitudinal direction of the housing.

The spring forces of the flexure elements position the rigid lens and/or mirror in the geometric center, thus aligning the optical center of the rigid lens or the center of the mirror with the geometric center of the array of flexure elements.

According to a further preferred embodiment, each elastic engaging means comprises a set of at least 12 elastic flexure elements.

The effect of the flexure array is that the actual displacement of the rigid lens and/or mirror, relative to geometric errors or dirt, scales inversely with the number of elastic elements around the perimeter, namely as d_lens = 1/(2· n- D_p), where d_lens is the displacement of the rigid lens and/or mirror, n is the number of flexure elements and D_p is the size of the geometric irregularity at a single flexure. As a result of this behaviour, higher numbers of elastic elements lead to reduction of the impact that any small geometric deviations will have on the position of the rigid lens and/or mirror. Due to the use of many flexure elements in parallel, the system becomes less sensitive to manufacturing defects because of this so called elastic averaging. The impact of defects in geometry of individual flexures, thus making flexures stiffer or displacing the contact point, will also be attenuated due to elastic averaging. If the defects exist with a normal statistical probability distribution, these effects average out and are attenuated proportional to the square root of the number of flexures.

According to a more preferred embodiment, the number n of flexure elements is at least twenty, more preferably at least thirty and even more preferably at least forty.

According to a further preferred embodiment, the flexure elements are substantially evenly distributed around the circumference of the opening in the housing, e.g. at an angle of 120° relative to each other in the case of three flexure elements, and at an angle of 30° relative to each other in the case of twelve flexure elements.

According to a further preferred embodiment, the arrangement of the flexure elements is substantially rotation-symmetric with respect to the longitudinal direction of the opening in said housing. This is advantageous for the thermal stability. Moreover, such a rotation- symmetric design due to its repetitive characteristics is relatively easy to manufacture and develop.

According to a further preferred embodiment, the second outer ends of the flexure elements describe a form which substantially corresponds to the circumferential form of the rigid lens and/or mirror to be aligned situated at that position in the engaging state, and wherein in an untensioned rest state this form is smaller than the circumferential form of the rigid lens and/or mirror to be aligned such that, when the rigid lens and/or mirror to be aligned is received between the two outer ends, the flexure elements are tensioned outward within their elastic range.

According to a further preferred embodiment, the combined stiffness on each possible half of a set of flexure elements, which is defined by taking any cross-section through the center line, is substantially equal. This provides stability against thermal effects, by ensuring the center line of the part remains aligned with the center line of the structure.

According to a further preferred embodiment, the flexure elements have a tangential stiffness c_t that is smaller than the radial stiffness c_r of said flexure elements. Hysteresis scales with c_t/c_r, and can be reduced to sufficiently small values by choosing c_t/c_r sufficiently small, in practice this means that c_t«c_r must be met. Furthermore, if the flexure elements are designed such that c_t«c_r, the contact points do not slip for occurring lateral disturbance forces or displacements. It is therefore beneficial to dimension the flexure elements such that c_t«c_r. At least, the flexure elements have a tangential stiffness c_t that is smaller than the radial stiffness c_r of said flexure elements.

According to a further preferred embodiment, the flexure elements have a tangential width that is oriented in the tangential direction of the opening, and a radial width that is oriented in the radial direction of the opening, wherein the tangential width of said flexure elements is smaller than the radial width of said flexure elements. For an isotropic material, these relative dimensions satisfy that the flexure elements have a tangential stiffness c_t that is smaller than the radial stiffness c_r of said flexure elements.

According to a further embodiment, the flexure elements are provided with one or more of a notch hinge, a flexural parallelogram, and/or comprises a non-isotropic material. This allows that the criterion c_t«c_r may be satisfied, even if the tangential width of said flexure elements is not per se smaller than the radial width of said flexure elements, which might be a design requirement.

According to a further preferred embodiment, at least the substantially rigid housing and the engaging means together form a monolithic unit. This avoids the risk of hysteresis caused by internal displacements along material boundaries. Next to mechanical advantages, constructing the device as a monolithic structure takes away the requirement to assemble, which has a positive effect on production costs. Using a monolithic structure, the thermal stability of the structure is improved. In contrast to an assembled structure created from multiple substructures, play in contact surfaces is absent when monolithic structures are applied.

According to a further preferred embodiment, said device further comprises an adjusting mechanism configured for adjusting the radial position and/or radial stiffness c_r of one or more elastic flexure elements. A flexure element may be manipulated so that the contact point where it contacts the rigid lens or mirror, is shifted radially inward or outward, the stiffness of a flexure may be affected, a force may be exerted on the flexure that is manipulated or a combination thereof.

The invention is further related to an assembly of a device for placing at least two rigid lenses and/or mirrors coaxially relative to each other and at least two rigid lenses and/or mirrors, wherein the substantially rigid housing comprises an objective barrel structure. The assembly that comprises the device for placing at least two rigid lenses and/or mirrors coaxially relative to each other according to the invention provides a self-centering lens mount.

According to a preferred embodiment of the lens assembly, the objective barrel structure and the elastic flexure elements of the elastic engaging means are formed as a monolithic unit, providing an arrangement that is thermally stable, while also play that would occur between multiple sub parts, is absent.

The invention is further related to a method of manufacturing a device for placing at least two rigid lenses and/or mirrors coaxially relative to each other and at least two rigid lenses and/or mirrors as described above, comprising the step of applying an additive manufacturing method such as 3D printing. Using additive manufacturing methods, labour and cost intensive assembly steps are prevented, furthermore preventing the risk of errors in such assembly steps. The additive manufacturing method also allows complex structures not achievable using traditional machining manufacturing methods based on reduction of material. According to a preferred embodiment of the method, it further comprises the step of finishing the flexure elements using electric discharge machining (EDM), which allows accuracy in the (sub-)micrometer range to be obtained.

In the following description preferred embodiments of the present invention are further elucidated with reference to the drawing, in which:

Figure 1 is a half section view of an embodiment of a lens-positioning device using flexure arrays for holding rigid lenses of equal diameter, said rigid lenses, and spacers, sectioned along the axis of the lens elements.

Figure 2 is a perspective three quarter section view of an embodiment of a lens- positioning device using flexure arrays for holding rigid lens elements of equal diameter, sectioned along its central axis..

Figure 3 is an axial view of an embodiment of a lens-positioning device using flexure arrays for holding rigid lens elements of equal diameter

Figure 4 is a half section view of an embodiment of a lens-positioning device using flexure arrays for holding rigid lenses of different diameters, and said rigid lenses, sectioned along the axis of the lens elements.

Figure 5 is a perspective three quarter section view of an embodiment of a lens- positioning device using flexure arrays for holding rigid lens elements of different diameter, and said rigid lenses, sectioned along the axis of the lens elements.

The invention presented below describes a novel approach to achieving the required specification for coaxial alignment of rigid lens elements 1 in high-end objectives, while significantly reducing the need for complex and labour intensive alignment steps.

The invention comprises a lens mount and assembly method, wherein the rigid lenses 101, 102 are placed in circular arrays of flexure elements 102, 202, and in which a number of these rigid lens and flexure element arrays are combined to form the desired lens system for the objective. Using this technique, the rigid lenses 101, 201 are aligned with a high degree of coaxiality since the structure itself is self-centering, provided that the flexure arrays are coaxial to each other.

Though this method greatly improves the lateral alignment for a given amount of labour, the use of one or more compensator lenses (not shown in figures) may still be required. The device according to the present invention takes advantage of the flexure arrays 101, 201 to accomplish the fine adjustment for such compensator lenses (not shown in figures).

The array of flexure elements 102, 202 determines the position of the rigid lens 101, 201 by exerting forces originating from elastic deflection of the flexures 101, 201 around its circumference. This provides a mount without play between the outer diameter of the rigid lens 101, 201 and the inner diameter of the flexure element 102, 202 array, eliminating a key source of decentration. The axial position can be determined by various methods depending on the application and overall objective design, such as a lens seat 104, 204 fixed to the objective barrel below the flexure array and spacers between the rigid lens elements 105, as depicted in figure 1. Depending on the manufacturing method and/or size and/or shape of the rigid lens elements 101, 201 or subassemblies, different forms and potentially combinations will be more suitable, examples are a retainment ring or using an adhesive.

The flexure element array provides a second advantage, as the spring forces position the rigid lens 101, 201 in the geometric center, thus aligning the optical center of the rigid lens 101, 201 with the geometric center of the flexure array. The effect of the flexure array is that the actual displacement of the rigid lens 101, 201, relative to geometric errors or dirt, scales inversely with the number of elastic elements 102, 202 around the perimeter, namely as d_lens = 1/(2· n- D_p), where d_lens is the displacement of the lens, n is the number of flexure elements and D_p is the size of the geometric irregularity at a single flexure. As a result of this behaviour, higher numbers of elastic elements 102, 202 lead to reduction of the impact that any small geometric deviations will have on the position of the rigid lens 101, 201. Due to the use of many flexure elements 102, 202 in parallel, the system becomes less sensitive to manufacturing defects because of this so called elastic averaging.

The impact of defects in geometry of individual flexures 102, 202, thus making flexures 102, 202 stiffer or displacing the contact point, will also be attenuated due to elastic averaging. If the defects exist with a normal statistical probability distribution, these effects average out and are attenuated proportional to the square root of the number of flexures 102, 202.

The use of an array of elastic flexures 102, 202 instead of more traditional positioning techniques that depend on friction or adhesive, provides advantages when working in environments that require stability to thermal effects. Temperature cycles may cause hysteresis when play or plastic deformation at the interface between lens 101, 201 and mount is present, impacting the position and alignment of the lens elements 101, 201.

In a preferred embodiment, the geometry is such that the combined stiffness on each possible half of the flexure array, which is defined by taking any cross-section through the center line, is equal. This provides stability against thermal effects, by ensuring the center line of the part remains aligned with the center line of the structure 103, 203.

In a further preferred embodiment, the geometry has rotation-symmetry, which provides the mentioned thermal stability, and in addition to this advantage, is straightforward to manufacture and design, because of the repetitive nature of the flexure 102, 202 design.

In a further embodiment, the lens seat 104, 204 acts as an axial stop for lenses 101,

201 at the end of the barrel 103, 104, and determines position and orientation in axial direction and additional directions depending on the shape of the lenses 101, 201 that rests on it. For a convex surface or a concave surface resting along a rotationally symmetric contact, the lens seat 104, 204 determines both lateral positions, while for a flat surface, tip-tilt is determined. In both cases, the lens 101, 201 has its remaining degrees of freedom constrained by the flexure element 101, 201 array, which determines lateral position and so constrains any remaining degrees of freedom in the structure.

Lens elements 101, that have no lens seat 104 that constrains at least 2 degrees of freedom other than the axial position, require two arrays of flexures 102 to constrain the relevant degrees of freedom in the structure.

As mentioned in the preceding text, the use of one or more compensator lenses

(not shown in figures) may still be needed. In high-end systems such a compensator lens is often required to be manipulated with high accuracy. A further embodiment allows fine adjustment of one or more of the flexures, to facilitate such accurate manipulation of a compensator lens. This adjustment mechanism (not shown in figures) functions by manipulating one or more flexures of one or more flexure arrays in such a way that this results in a change of the balance of forces exerted on the rigid lens by the flexures in the array(s) concerned, which, in conjunction with the flexibility of the flexures in the array, results in a small displacement of the compensator lens (not shown in figures). There are various ways in which flexures can be manipulated to achieve this effect: a flexure may be manipulated so that the contact point where it contacts the rigid lens element is shifted radially inward or outward, the stiffness of a flexure can be affected, a force can be exerted on the flexure that is manipulated or a combination thereof.

For the case where the contact point of the flexure is radially shifted, the shift of the rigid lens element can be expressed as d_L=2 d_f m/n, where d_L equals the lateral displacement of the rigid lens element, d_f equals the radial displacement of the contact point of the manipulated flexure(s), m equals the number of manipulated flexures, and n equals the number of flexures in that array. Similar relations can be derived for the cases where force or stiffness of the flexure(s) are manipulated, having in common that the effect of the manipulation of one or more flexures on the position of the rigid lens element is attenuated by the aforementioned elastic averaging effect. This attenuation has advantages since, for a given accuracy to be achieved in rigid lens element positioning, it eases requirements regarding the stability and fineness of adjustability of the adjustment means (which could for example be a simple set screw), this is not shown in the figures.

During insertion of the element or subassembly into the flexure array, slip occurs in axial direction so that tangential friction forces at the contacts between flexure and lens element will be cancelled. Because of this, the rigid lens element can find the exact geometric centroid defined by the flexure array, even if at the onset of insertion of the element it was slightly eccentric to the array, as long as no parasitic lateral forces are present during insertion. Insertion tooling for this purpose can be manufactured.

Since the radial contact force exerted by each flexure is normal to the surface, the contribution of the radial contact force to the constraining of the rigid lens is not influenced by friction effects. In tangential direction however, slip between the contact surface and the rigid lens element is possible, so that friction effects might cause degradation of performance if the flexure array is not dimensioned correctly. Because of this possible slip, there are two effects that can potentially affect positioning of the lens element negatively.

One effect is that if a certain lateral force is exerted on the rigid lens element during insertion or if a certain lateral offset between lens element optical axis and array geometric axis is maintained during insertion, this disturbance will be, at least partly, "frozen" into the contacts because after removal of the lateral force or discontinuing of enforcing the lateral displacement, the flexures will not end up in a state that is stress-free in tangential direction. This leads to hysteresis with respect to insertion disturbances.

The other effect is that a disturbance causes a lateral displacement and if one or more contact points slip in tangential direction this disturbance will be also, at least partly, "frozen" into the one or more contacts concerned because the flexures will not end up in a state that is stress-free in tangential direction.

For both effects, the average position as defined by only the tangential contributions of all the flexures through the tangential frictional force, does not coincide with the average position as defined by only the radial contributions of all the flexures through the normal contact force. The real resulting position will be determined by the combined influence of the tangential and radial contributions, which works out to be the average between the radial contributions and the tangential contributions weighted by the tangential and radial stiffness of the flexures respectively. Hence hysteresis scales with c_t/c_r, and can be reduced to sufficiently small values by choosing c_t/c_r sufficiently small. In practice this means that c_t«c_r must be met.

The second effect can be prevented altogether by dimensioning the flexures such that the contact points do not slip in cases where lateral disturbance forces or displacements occur. For forces, this means that the criterion F_L < 2·η· μ· (c_r ²/c_t+c_r)-d_pt must be met, where F_L is the disturbing lateral force, n is the number of flexure elements, μ is the coefficient of friction between flexure element and rigid lens element, c_t is the tangential stiffness, c_r is the radial stiffness, and d_pt is the radial pretention displacement arising from the difference in diameter between flexure array and rigid lens element. For displacements, this means that the criterion d_L < μ-c/Ct- dpt must be met, where d_L is the disturbing lateral displacement, μ is the coefficient of friction between flexure and rigid lens element, c_t is the tangential stiffness, and c_r is the radial stiffness, and d_pt is the radial pretention displacement arising from the difference in diameter between flexure array and rigid lens element. Both these criteria show also that it is beneficial to dimension the flexure elements such that c_t«c_r.

According to a further preferred embodiment that provides optimal performance, the flexure elements are dimensioned such that abovementioned criteria are met, and so that c_t«c_r. When taking as an example the flexures shaped as depicted in the figures describing the invention, striving for c_t«c_r means that in tangential direction the flexure should be considerably thinner than in radial direction. For the case where the flexure consists of a beam with simple rectangular cross section oriented with its beam length oriented in axial direction, the ratio of the tangential stiffness to the radial stiffness, approximately scales as the square of the ratio of tangential thickness to radial thickness, i.e. c_t/c_r«(t_t/t_r)², where c_t is the tangential stiffness, c_r is the radial stiffness t_t is the tangential thickness, and t_r is the radial thickness.

It should be clear to a person skilled in the art that other means and geometries exist that can serve to achieve that c_t«c_r is satisfied, such as the use of properly oriented notch hinges, flexural parallelograms or non-isotropic materials, these are not shown in the figures.

The abovementioned lens-positioning device aligns the optical axis of the rigid lens to the geometric axis determined by the flexure elements, a multitude of which need to be combined into the coaxial system that comprises the objective. There are various methods to achieve this, and a number of possible methods are described below.

One method to manufacture the outer barrel incorporating the flexure arrays is to manufacture separate lens mounts, each of which contains at least one flexure array, then assembling the mounts into a compound mounting tube in which all flexure arrays are aligned so that their centroids line-up with each other and with the position as defined by any lens seat present in the assembly. The subcells comprising flexure arrays and the assembly structure can be obtained using traditional manufacturing methods. An option would be to manufacture the flexure array subcell by first lathing a ring, then using wire EDM to create the flexure elements at the top, bottom or both top and bottom of the ring. These ring elements are then stacked together, with or without other parts in between them, into an assembled state that is made such that the arrays end up so that their centroids line-up with each other and with the position as defined by any lens seat present in the assembly. This method is not shown in the figures. An advantage of this method would be the use of traditional manufacturing methods, which have reduced in cost and are reliable in the quality of the output.

When the subcells are created, the assembly into the lens objective must be created in such a way that the geometric axes of the subcells are aligned to a high degree. An option would be to design the objective assembly in such a way that rigid lenses used have the same diameter, and "thread" the subcells on a metal rod (not shown in the figures) that has the same diameter as the lenses and is manufactured with high accuracy. Once all subcells are mounted on the rod, by principle the geometric axes align with each other and with the rod. At this stage, an adhesive or other fixation method can be used to attach the subcells and form an aligned structure. Once the rod is removed, the lenses and spacer rings can be mounted in the objective assembly. A similar method can be used to create a stepped barrel for lenses of different diameter by using flexure sub- mounts with different diameter arrays and a matching stepped rod, not shown in figures.

If according to a further preferred embodiment, the objective barrel structure 103, 203 and flexure elements 102, 203 that support and coaxially position the rigid lenses 101, 201 in the objective are formed as a monolithic structure, the thermal stability of the structure is improved. In contrast to an assembled structure created from multiple substructures (not shown in figures), play in contact surfaces is absent when monolithic structures are applied.

According to a further preferred embodiment, the monolithic structure forming the objective barrel structure 103, 203 and flexure elements 102, 202 is manufactured by using an additive manufacturing method, such as 3D printing. Using additive manufacturing methods, no assembly step is required which reduces the workload and cost associated, as well as reduces risk of errors in the assembly step. The additive manufacturing method also allows complex structures not achievable using traditional machining manufacturing methods based on reduction of material.

According to a further preferred embodiment, an additive manufacturing method is used to build up the monolithic structure 103, 203 with the additive manufacturing method as described above, but an additional finishing step is applied to the flexure array elements 102, 202. This step would further dimension the flexure 102, 203 edges that will be in contact with the rigid lens 101, 201 in such a way that the geometric accuracy reaches the single micrometer range, and reduces any manufacturing defects that may arise from the additive manufacturing step.

In a further preferred embodiment, this step is performed using electric discharge machining (EDM) method, where from the center of the device an electric current fed through an electric probe or wire removes material radially outward, allowing (sub-)micrometer accuracy. Two possible methods use this principle are die-sink EDM and wire -cut EDM, where either a probe is lowered in the structure 103, 203, or a wire is fed through the device, both machining radially outward to achieve the desired machining distance.

The methods described can be used depending on geometry of the device 203. When using die-sink EDM, the geometry needs to have a tapered structure as shown in figures 4 and 5 to support decreasing diameters when sinking the probe deeper into the structure 203. In the case of wire-cut EDM, the structure 103 is limited to a single diameter through the structure 103, like depicted in figures 1 , 2 and 3 since the wire that is led through the structure will remove the material at a constant radial distance from the wire along the axis. In this case, the structure 103 would need spacer rings 105 to maintain the desired distance between the rigid lenses 101 and to determine the axial position of the lenses 101. These spacer rings 105 can be held in place by flexure arrays themselves or by the curvature of the lenses. It must be noted that while the spacer rings 105 will be positioned at the outside of the rigid lenses 101, the spacer rings 105 will not require the same positional accuracy as the lenses 101 themselves as misalignment does not create aberrations like lens misalignment.

In a preferred embodiment, material is used that has a high ratio of yield strength to elastic modulus, which gives a large tolerance to stay in the elastic realm, as well as a CTE close to that of the material that is used for the rigid lenses, expanding and contracting at the same rate and avoiding high additional stresses in the structure 103, 203. Examples of such materials are TiAlV and Maraging steel, which exhibit high yield strength to elastic modulus, and have CTE close to glasses commonly used for lenses. In addition to these advantages, TiAlV is a material that is commonly used for additive techniques and allows high resolutions with current state of technology.

In order to place the rigid lens elements 101, 201, tooling (not shown in figures) must be used that inserts the elements 101, 201 in the array of flexures 102, 202 with minimal lateral disturbances. The objective is to insert the lens element 101,201 but let the array of flexures 102, 202 determine the exact position, allowing a certain level of displacement to exist that can be compensated by the flexure array. Since the tangential stiffness is lower than the radial stiffness, by inserting the rigid lens element 101, 201 much of this stress and resulting misalignment will be relieved. This requires a tool that is stiff in axial direction, but has low stiffness in lateral directions, this is not shown in figures.

An alternative embodiment uses one or more flexure arrays as outlined above to function as a straight guide with high accuracy, which will support a rigid lens and/or mirror inserted in the flexure arrays to move axially. The advantage of this embodiment is that the straight guide is self-centering. A further preferred embodiment has a low friction coating applied to the contact surfaces.

Although they show preferred embodiments of the invention, the above described embodiments are intended only to illustrate the invention and not to limit in any way the scope of the invention. It is particularly noted that, although the embodiment shown in the figures comprises a lens mount with solely rigid lenses, one or more mirrors may be coaxially positioned using elastic engaging means according to the invention, e.g. in a lithography device. It is furthermore noted that the skilled person can combine technical measures of the different embodiments. The scope of the invention is therefore defined solely by the following claims.

Claims

1. Device for placing at least two rigid lenses and/or mirrors coaxially relative to each other, comprising:

- at least two elastic engaging means which are arranged coaxial to each other;

- wherein each of the at least two rigid lenses and/or mirrors is engageable by at least one of the two or more elastic engaging means; and

- wherein the at least two elastic engaging means each comprise a set of elastic flexure elements which are arranged with a first outer end on the housing and extend therefrom into the opening such that the second outer ends of the flexure elements located opposite the first outer ends describe in the opening a form which in an untensioned rest state is smaller than the circumferential form of the rigid lens and/or mirror situated at that position in an engaging state.

2. Device according to claim 1 , comprising an abutment which defines the position of the one or more rigid lenses and/or mirrors in a longitudinal direction of the housing and wherein the position transversely of this longitudinal direction is defined by the elastic engaging means which are engageable with said one or more rigid lenses and/or mirrors with the elastic flexure elements.

3. Device according to claim 1 or 2, wherein the at least two elastic engaging means that each comprise a set of at least three elastic flexure elements, define the position of the rigid lens and/or mirror transversely of the longitudinal direction of the housing.

4. Device according to any of the foregoing claims, wherein each elastic engaging means comprises a set of at least 12 elastic flexure elements.

5. Device according to any of the foregoing claims, wherein the flexure elements are substantially evenly distributed around the circumference of the opening in the housing.

6. Device according to any of the foregoing claims, wherein the arrangement of the flexure elements is substantially rotation-symmetric with respect to the longitudinal direction of the opening in said housing.

7. Device according to any of the foregoing claims, wherein the second outer ends of the flexure elements describe a form which substantially corresponds to the circumferential form of the rigid lens and/or mirror to be aligned situated at that position in the engaging state, and wherein in an untensioned rest state this form is smaller than the circumferential form of the rigid lens and/or mirror to be aligned such that, when the rigid lens and/or mirror to be aligned is received between the two outer ends, the flexure elements are tensioned outward within their elastic range.

8. Device according to any of the foregoing claims, wherein the combined stiffness on each possible half of a set of flexure elements, which is defined by taking any cross-section through the center line, is substantially equal.

9. Device according to any of the foregoing claims, wherein the flexure elements have a tangential stiffness c_t that is smaller than the radial stiffness c_r of said flexure elements.

10. Device according to any of the foregoing claims, wherein the flexure elements have a tangential width that is oriented in the tangential direction of the opening, and a radial width that is oriented in the radial direction of the opening, wherein the tangential width of said flexure elements is smaller than the radial width of said flexure elements.

11. Device according to any of the foregoing claims, wherein the flexure elements are provided with one or more of a notch hinge, a flexural parallelogram, and/or comprises a non- isotropic material.

12. Device according to any of the foregoing claims, wherein at least the substantially rigid housing and the engaging means together form a monolithic unit.

13. Device according to any of the foregoing claims, further comprising an adjusting mechanism configured for adjusting the radial position and/or radial stiffness c_r of one or more elastic flexure elements.

14. Assembly of a device for placing at least two rigid lenses and/or mirrors coaxially relative to each other and at least two rigid lenses and/or mirrors, wherein the substantially rigid housing comprises an objective barrel structure.

15. Lens mount assembly according to claim 14, wherein the objective barrel structure and the elastic flexure elements of the elastic engaging means are formed as a monolithic unit.

16. Method of manufacturing a device for placing at least two rigid lenses and/or mirrors coaxially relative to each other and at least two rigid lenses and/or mirrors according to any of claims 1-13, comprising the step of applying an additive manufacturing method such as 3D printing.

17. Method according to claim 16, further comprising the step of finishing flexure elements using electric discharge machining (EDM).