Abstract:
An optical element is manufactured using a replication tool comprising a negative structural feature defined in a replication side of the replication tool, and a peripheral feature formed in the replication side of the replication tool adjacent the negative structural feature. The negative structural feature defines the shape of the optical element. A replication material is disposed between a substrate and the replication tool, which are moved toward each other. The peripheral feature confines the replication material to a predetermined area of the substrate. The replication material can be hardened to form the optical element from the replication material attached to the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation-in-part of U.S. patent application Ser. No. 11/384,562, filed Mar. 20, 2006, the disclosure of which is herein incorporated by reference in its entirety. 

   FIELD OF THE INVENTION 
   The invention is in the field of manufacturing miniature optical or mechanical elements, in particular refractive optical elements or diffractive micro-optical elements, by means of a replication process that includes embossing or moulding steps. 
   BACKGROUND OF THE INVENTION 
   Replicated optical elements include refractive elements such as lenses and diffractive and/or refractive micro-optical elements for influencing an optical beam in a pre-defined manner. 
   When optical elements are produced by replication, there is often a basic configuration involving a substrate and replication material on a surface thereof. The replication material can be shaped and hardened in the course of a replication process. Often, the dimension perpendicular to the named substrate surface—the thickness or height of the replicated structures, also termed z-dimension—is important and is well-defined and controlled. Since the other dimensions of the element are defined by the replication tool—this being the nature of the replication process—also the volume of the replicated element is well defined. However, small volumes of dispensed liquid or viscous material are generally difficult and costly to control. Since elements that are only partially filled are defective and lost, it is therefore advantageous to dispense excess replication material. By this, one makes sure that also for replication material volumes that fluctuate between different elements, no or only few elements are lost. 
   Of special interest are the wafer-scale fabrication processes, where an array of optical elements is fabricated on a disk-like (“wafer-”) structure, which subsequent to replication is separated (“diced”) into the individual elements or stacked on other wafer-like elements and after stacking separated into the individual elements. Wafer scale refers to the size of disk like or plate like substrates of sizes comparable to semiconductor wafers, such as disks having diameters between 2 in and 12 in. In conventional wafer-scale replication processes, replication material for the entire, wafer-scale replica is disposed on the substrate in a single blob. However, there might be areas sideward of the element where replication material is not wanted in later replication steps. In certain applications, the fabricated elements must for example be used in combination with other elements, and the residual material will impair the function of the combined structure. 
   In such an array replication process, excess material can ooze out sideward from the element volume. For example, miniature optical lenses can be replicated above the surface of a wafer carrying semiconductor chips each embodying a CCD or CMOS-camera sensor array. The residual material, if it covers critical areas, can interfere with further processing steps of the stack comprising the semiconductor wafer and the lenses, e.g., bonding. 
   A structured (or micro-structured) element can be manufactured by replicating/shaping (moulding or embossing or the like) a 3D-structure in a preliminary product using a replication tool. The replication tool comprises a spacer portion protruding from a replication surface. A replicated micro-optical element is referred to as replica. 
   The spacer portions can allow for an automated and accurate thickness control of the deformable material on the substrate. They can include “leg like” structures built into the tool. In addition, the spacers can prevent the deformation of the micro optical topography since the spacers protrude further than the highest structural features on a tool. 
   The spacer portion is preferably available in a manner that it is distributed over at least a portion of the replication tool, for example, over the entire replication tool or at the edge. This means that features of the spacer portion are present in an essential fraction of the replication tool, for example, the spacer portion includes a plurality of spacers distributed over the replication surface of the replication tool. The spacers can allow for an automated and accurate thickness control of the replication material layer. 
   The replication process can be an embossing process, where the plastically deformable or viscous or liquid replication material for the product to be shaped is placed on a surface of a substrate, which can have any size. In the embossing step, the spacer portions abut against the top surface of the substrate. The surface thus serves as a stop face for the embossing, which can control the thickness (height, z-dimension) of the replicated elements. Other ways of controlling the z-dimension include measuring the distance between a tool plane and a substrate plane, and actively adjusting this distance at different places by a robot. The embossing step and/or the spacer portion, however, can cause residual material to remain in the areas between the elements, and for example, also around the periphery of each of the elements. 
   SUMMARY OF THE INVENTION 
   The invention, in one embodiment, features a method of replicating an optical element and a replication tool for forming the optical element. The replication material can be confined between the tool and the surface of the substrate. By confining the replication material to only part of the substrate surface, the resulting element can, after hardening, by e.g., curing, cover part of the substrate. The element need not extend to cover the substrate in predetermined areas, leaving them free for bonding. 
   The replication tool can include one or more sections, each defining a negative structural feature that defines the shape of a component (such as an optical element or a lens). The sections can be separated from the other sections by dicing the substrate or an assembly including the substrate, to form individual elements. The confining of replication material to a predetermined area of the substrate can include confining the replication material to a plurality of regions, each region around a replication section, the regions preferably not overlapping. For example, the replication sections can be present as an array of identical replication sections, where around each of the replication sections the replication material is confined to a region. 
   In one aspect, the invention features a method of manufacturing an optical element. The method uses a replication tool including a negative structural feature defined in a replication side of the replication tool and a peripheral feature formed in the replication side of the replication tool adjacent the negative structural feature. The negative structural feature defines the shape of the optical element. A replication material is disposed between a substrate and the replication tool, which are moved toward each other. The peripheral feature confines the replication material to a predetermined area of the substrate. The replication material is hardened to form the optical element from the replication material attached to the substrate. 
   In another aspect, the invention features a replication tool for replicating an optical element from a replication material. The replication tool includes a replication side and a negative structural feature defined in the replication side. The negative structural feature defines the shape of the optical element. The replication tool also includes a ridge formed in the replication side adjacent the negative structural feature and a recess defined in the replication side adjacent the ridge. An edge of the ridge confines an outward flow of the replication material to a predetermined area of the replication tool, when the replication tool is pressed against a substrate. 
   In yet another aspect, the invention features a replication tool for forming an optical element. The replication tool includes a replication side and a negative structural feature defined in the replication side. The negative structural feature defines the shape of the optical element. The replication tool also includes means for confining an outward flow of the replication material to a predetermined area of the replication tool when the replication tool is pressed against a substrate forming the optical element attached to the substrate. In some embodiments, the means for confining the outward flow of the replication material does so by at least one of capillary force and surface tension acting at the means. 
   In other examples, any of the aspects above, or any apparatus or method described herein, can include one or more of the following features. The replication material can be in at least one of a plastically deformable, viscous, or liquid state. Each optical element can be a refractive lens. The substrate can be optically transparent. 
   In certain embodiments, the predetermined area exceeds the desired area of the optical element on the substrate, in at least one direction along the surface of the substrate by less than the predetermined distance. The flow of the replication material can be controlled, while moving the replication tool against the substrate, by at least one of capillary forces and surface tension acting at the peripheral feature of the replication tool. An edge of the peripheral feature can control the flow. 
   In some embodiments, a pre-determined volume of the replication material can be applied to the substrate, and the flow of the replication material can be controlled, while moving the replication tool against the substrate, by at least one of capillary forces and surface tension acting at the peripheral feature of the replication tool. 
   The peripheral feature can include a ridge formed in the replication tool adjacent the negative structural feature. In certain embodiments, a dimension of the peripheral feature can be predetermined in accordance with a selected volume of the replication material to be applied. 
   In some embodiments, the negative structural feature is limited, at least in one lateral direction, by a flat section, an inner edge being formed between the negative structural feature and the flat section. The replication side of the replication tool can include at least one overflow volume. An outer edge can be formed between the flat section and the overflow volume, and the volume of the replication material can be larger than the volume of the negative structural feature so that the outer edge acts as the edge confining the replication material. The flat section can be asymmetrical with respect to a central axis of the negative structural feature. 
   The replication tool can define a plurality of negative structural features, each defining the shape of one optical element of a plurality of optical elements. In some embodiment, the replication tool is removed after the replication material is hardened, and the substrate is separated into discrete sections. The substrate can be separated along dicing lines defined by lateral positions of the substrate free of any replication material. 
   The peripheral feature can include a plurality of step features defined in the tool. At least one of the step features can confine the replication material to the predetermined area of the substrate. 
   The replication tool can include an overflow channel enabling flow of the replication material away from a central portion of the negative structural feature. In some embodiments, an edge of the ridge of the replication tool is adapted to selectively control flow of the replication material by at least one of capillary force and surface tension acting at the edge. 
   The replication tool can include a spacer portion. In such a tool, at least one cavity of the tool defines a replication surface with negative structural features, being a negative of at least some of the structural features of the element to be produced. The cavity contains the element volume and can additionally comprise at least one buffer and/or overflow volume. The spacer or spacer portions protrude from the replication surface. In the replication process, the spacer or spacer portions abut against the substrate and/or float on a thin basis layer of replication material. 
   The force by which the tool and the substrate are pressed against each other can be chosen based on specific requirements. For example, the force can be just the weight of the replication tool lying, by way of spacer portions abutting the substrate surface and/or floating on a thin basis layer of replication material, on the substrate. Alternatively, the substrate can lie on the replication tool. The force can, according to yet another alternative, be higher or lower than the weight and can for example be applied by a mask aligner or similar device, which controls the distance of the substrate and the replication tool during the replication process. 
   Before the replication tool and the substrate are brought together for the replication process, replication material in a liquid or viscous or plastically deformable state is placed on the replication tool and/or the substrate. The replication tool can, as mentioned above, can include a plurality of sections each defining an element to be replicated. Then, a preselected volume of replication material is applied locally and individually, at laterally displaced positions, each position corresponding to one section, to at least one of the tool and the substrate prior to pressing the tool against the substrate. This allows providing a plurality of cavities, each corresponding to an optical element, with an optimal amount of replication material. By this, the volume of surplus replication material that is removed or diverted from the critical areas is reduced or eliminated, as compared to the case where a plurality of elements would be formed from a single blob of replication material. 
   While the replication tool and the substrate are in the replication position—in which the replication tool and the substrate are brought together, for example the replication tool is placed on the substrate—the replication material is hardened. Depending on the replication material chosen, it can be hardened by curing, e.g., UV curing. As an alternative, it can be hardened by cooling. Depending on the replication material chosen, other hardening methods are possible. Subsequently, the replication tool and the replication material can be separated from each other. For most applications, the replication material remains on the substrate. The optical element typically is a refractive or diffractive optical element, but also can have a micromechanical function. 
   The element volume covers a part of the substrate and constitutes the functional part of the element. The remainder of the cured replication material can fill a volume at the sides of the element, i.e., the region of space adjacent to both the substrate and the functional part of the element, and does not interfere with the function of the element. How far the replication material can move along the substrate can be controlled at each side of the element volume. 
   In some embodiments, the flow of the replication material is controlled and/or limited by capillary forces and/or surface tension. This exploits the property of geometric features to further or to hinder the flow of the replication material between the tool and the substrate. 
   As an example, the replication tool can be chosen to comprise a plurality of cavities each defining the shape of one element or a group of elements, each cavity being limited, at least in one lateral direction, by a flat section. An inner edge is formed between the cavity and the flat section. The replication tool further comprises a plurality of overflow volumes or one contiguous overflow volume between the cavities. And an outer edge is formed between the flat section and the overflow volume. The dispensed replication material (per cavity) is chosen to be larger than the volume of the cavity. The flat section then serves as floating (non-contact) spacer, which preferably surrounds the cavity. The outer edge constitutes a discontinuity stopping a flow the replication material. Without such discontinuities, capillary forces would cause the replication material to eventually drain the replication material from the element volume. 
   The cavity, in this example, can include, for example, the element volume only. It can be dome-shaped so that the element is a convex refractive lens adjacent to which a thin base layer is formed, the base layer being what replication material remains underneath the floating spacer. 
   Even in the case of a cylinder symmetric optical element, the shape of the flat section, when seen in the direction perpendicular to the substrate surface, e.g. along a central axis of the element, can be asymmetrical so that a bulge of replication material forming along the outer edge in the overflow volume is farther away from the replication element towards one side of the element than towards an other side. 
   Here and in the following, for the sake of convenience, the dimension perpendicular to the surface of the substrate, which comprises an essentially flat surface, is denoted as height. In actual practice, the entire arrangement can also be used in an upside down configuration or also in a configuration where the substrate surface is vertical or at an angle to the horizontal. The according direction perpendicular to the surface is denoted z-direction. The terms periphery, lateral and sides typically relate to a direction perpendicular to the z-direction. 
   In another example, control of the flow is done by a cavity in the tool defining the shape of the element, and the cavity including a buffer volume along at least one side of the element, which buffer volume is separated from the element volume by an inner edge. Furthermore, the predetermined volume of replication material applied individually to the element volume of the cavity is smaller than the volume of the cavity. This causes the inner edge to limit the flow of the replication material into the buffer volume by capillary forces acting at the inner edge and by surface tension. 
   The predetermined volume of replication material can be about the volume of the element volume (or slightly smaller or slightly larger). The element volume is the volume of the functional element, extending from the outer shape of the element defined by the tool on one side to the substrate on the other side. The replication material can be stopped by fluid forces acting at the inner edge from flowing into the buffer volume. 
   In certain embodiments, when pressing the tool against the substrate, an inclined spacer displaces the replication material towards the element volume, and in particular a buffer volume adjacent to the element volume. The inclined spacer has an inclined surface that is to be brought into contact with the surface of the substrate. The inclined surface, when no pressure is applied, touches the substrate at an outer periphery, and in regions closer to the element volume, gradually moves away from the substrate. When, during embossing or moulding, pressure is applied to the tool, the tool, being slightly elastic, is deformed, and the inclined surface causes replication material to be displaced from under the inclined spacer. 
   In certain embodiments, the flow of the replication material can be confined toward at least one side of the tool by a contact spacer that touches the substrate. The flow of the replication material can be enabled toward another side of the tool by an overflow channel. This can allow diverting the replication material away from the critical areas and guiding it to an overflow volume located in a noncritical area. 
   The replication tool can include at least one spacer portion, protruding, on the replication side, from the cavities. The replication tool can include means for confining the replication material to a predetermined area of the tool, when the tool is pressed against a substrate, which predetermined area exceeds the desired volume of the element in at least one direction along the surface of the substrate by less than a predetermined distance. 
   Such means for confining the replication material, or flow confining features are constituted by the inner edge, the buffer volume, the outer edge, the spacer and the inclined spacer; each of them alone, or several of them in combination. They can be combined to form a multi-tiered flow confinement, which, according to the amount of replication material actually present, stops the flow at an earlier or a later limit. This allows controlling the flow despite inaccuracies when dispensing the replication material to individual cavities or onto corresponding individual locations on the substrate. 
   The cavity can include an element volume and a further volume, at a periphery of the element volume, the boundaries of the further volume comprising discontinuities for selectively inhibiting and/or enabling capillary flow of the replication material when pressing the tool against the substrate, with the replication material in between. 
   The discontinuities can be for circular optical elements, also circular in shape and concentric. For other shapes of optical elements, e.g. rectangles or rounded rectangles, the consecutive discontinuities can follow the shape of the optical element at increasing distances. 
   The discontinuity can be a ridge or a recess. In some embodiments, the discontinuity lies between ridges and recesses formed in the replication tool. Thus, the discontinuities can be the edge between circular or rectangular ridges and/or channels formed around the section of the replication tool that defines the optical element. A series of consecutive ridges and channels can, therefore, define a quantisation of the extension of the surplus replication material, since the outward flow of the replication material is inhibited or stopped at each edge or discontinuity, and continues only if the volume of the replication material in relation to the volume of the element volume exceeds a certain limit. 
   To minimize the area surrounding the optical element that remains covered by nonfunctional replication material, a floating spacer surrounding the element volume and defining its outer border is, for example, made as thin as possible while still providing the function of a spacer, e.g., providing sufficient support for the tool. Furthermore, the recess or several recesses outside the floating spacer, or outside the outer edge of the element volume (when there is no floating spacer), is preferably made as deep as possible, e.g. up to the depth of the element volume. In consequence, the volume defined by the recess is increased, and the volume of replication material it can absorb before the material spills over into the next recess is also increased. 
   The volume of each circumferential recess or channel is preferably correlated with the precision with which the volume of the replication material deposited dropwise can be controlled. For example, if the latter volume can be controlled to a high degree, then it is known that the size or volume of the surplus material varies within narrow bounds. In consequence, a recess is preferable located and sized such as to cover this variation in surplus size or volume. That is, with the minimum expected surplus size, according to the deposition accuracy, the recess is not filled, and for the maximum surplus size, the recess is filled just up to its limit. In other words, the dimensions of means for limiting the flow of the replication material (e.g., the dimensions of discontinuities or edges and of the intervening recesses and their volumes) are designed in accordance with expected values of the volume of replication material being applied. 
   Depending on other constraints, the volume of the recess is adjusted by selecting the depth and the width of the recess. The depth is e.g. limited by the process for creating the replication tool, and the width of the recess is limited by design constraints limiting the overall size of the optical element plus surplus material. So the overall design of the recess and drop deposition constitutes an optimal selection of the interrelated features of drop deposition precision and recess geometry, according to optimisation criteria that depend on the individual product. 
   In one embodiment, the replication tool includes a spacer dimensioned to stop the flow of the replication material by touching the substrate at one side of the cavity; and an overflow channel enabling the flow of the replication material towards another side of the cavity. 
   In one embodiment, the replication tool includes a buffer volume at at least one side of the element volume defined by the cavity, the buffer volume and the element volume defining, at their common boundary, an inner edge for inhibiting the flow of the replication material into the buffer volume. 
   In certain embodiments, the replication tool includes further edges in the surface of the buffer volume for inhibiting the flow of the replication material into the buffer volume. The further edges follow the shape of the inner edge at least roughly in parallel curves. 
   The tool can include a plurality of cavities, thus preferably allowing for the simultaneous manufacturing of an array of elements on a common substrate. This common substrate preferably is part of an opto-electronic or micro-opto-electronic assembly comprising optical and electronic elements produced on a wafer scale and later diced into separate units. 
   Further preferred embodiments are evident from the dependent patent claims. Features of the method claims may be combined with features of the device claims and vice versa. 
   The replica (for example, a micro-optical element, micro-optical element component or an optical micro-system) can be made of epoxy. The hardening step, which is done while the replication tool is still in place—can be a UV curing step. UV light curing is a fast process that allows for good control of the hardening process. The skilled person will know other materials and other hardening processes can be used. 
   An optical element can be an element that is capable of influencing electromagnetic radiation in, for example, the visible, infrared, or UV part of the electromagnetic spectrum. A wafer is not limited by the shape of the substrate. 
   Further preferred embodiments are evident from the dependent patent claims. Features of the method claims can be combined with features of the device claims and vice versa. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments, which are illustrated in the attached drawings, which schematically show: 
       FIGS. 1 and 2 : cross sections through a tools placed on a substrate; 
       FIG. 3 : an elevated view of the arrangement of  FIG. 2 ; 
       FIG. 4 : an example of an alternative geometrical shape of a transition between a buffer volume and an overflow volume; 
       FIGS. 5-9 : cross sections through further tools; 
       FIG. 10 : an elevated view of the arrangement of  FIG. 9 ; 
       FIGS. 11-14 : cross sections through further tools; and 
       FIG. 15 : a flow diagram of the method according to the invention. 
   

   DESCRIPTION OF THE INVENTION 
     FIG. 1  schematically shows a cross section through a tool  10  placed on a substrate  12 . The tool  10  forms a cavity  8  that defines the shape of the element to be formed by an element volume  1 . In the shown case, the optical element is simply a refractive lens. The element volume  1  lies between the tool  10  and the substrate  12 . It is surrounded by a protruding element of the tool  10 , which here is denoted as floating spacer  14 . A flat surface  17  of the spacer runs approximately parallel to the surface of the substrate  12  and here is at a distance of about 5 μm to 15 μm therefrom. Underneath the floating spacer  14 , between the flat surface  17  and the substrate  12 , a small buffer volume  3  forms. Between the element volume  1  and the buffer volume  3 , the tool  10  comprises an inner edge  2 . Between the buffer volume  3  and an overflow volume  5 , the tool  10  comprises an outer edge  4 . 
   The main function of the floating spacer  14  is to pull out excess material by capillary forces. The flow stops at the outer edge  4  and forms a bulge  18 , and therefore prevents that the element volume  1  is emptied by the capillary forces. In this way, the width of the floating spacer  14  and the shape and size of the overflow volume  5  define where excess material is to go. Therefore, by keeping the replication material volume below a certain maximum volume, the replication material is confined. 
   The inner edge  2  constitutes a first discontinuity, stopping the flow an outer boundary of the replication material  13 , as is also shown in following Figures. The outer edge  4  constitutes a second discontinuity, stopping the replication material  13  from flowing to the buffer volume  5  adjacent to the buffer volume  3 . Without such discontinuities, capillary forces would cause the replication material  13  to continuously flow along the channel formed by the buffer volume  3 , eventually draining the replication material  13  from the element volume  1 . 
     FIG. 2  shows a variation of the above principle. In this variation, the floating spacer  14  surrounding the element volume  1  is asymmetric. By this, the excess material can be transported to areas where it is not disturbing other processes. A top view of the configuration of  FIG. 2  is shown in  FIG. 3 . The bulge of replication material (shown in  FIGS. 1 and 2 , but not in  FIG. 3 , since the latter only shows the tool  10  without the replication material) extending around the outer edge  4  can, for example, be approximately constant in its cross section. By the asymmetric shape of the floating spacer, the length of the outer edge  4  is increased. For these reasons, the asymmetric solution allows to confine by the replication material especially well in one desired direction, corresponding to the lower left corner in the sketched configuration, as can be especially desired in configurations with an off-center optical element. 
   The tool can include multiple sections each corresponding to an element to be replicated. The sections can be arranged array-like, for instance in a grid with grid  11  lines corresponding to cutting or dicing lines for later separation of the substrate  12  carrying the manufactured optical elements or corresponding to bonding areas where other elements are later to be bonded. 
   As shown in  FIGS. 2 and 3 , an asymmetry of material flow between different directions can be implemented. However, it is also possible to influence the replication material flow by other means such as different surface properties at different locations or by geometrical shape. The outside portions of the spacers  14  can be formed in a way so that differing surface tensions can be used to control the excess material. An example is shown in  FIG. 4 . The spacer  14  at one side comprises a geometrical feature  20  that causes the flow towards this side to be different from the flow towards the other side. 
     FIG. 5  shows a cross section of a tool  10  with replication material  13  just filling the element volume  1  and being contained by the discontinuity of the inner edge  2  between the element volume  1  and the buffer volume  3 . The length of the buffer volume  3  preferably lies in the range of 100 to 300 or 500 or 800 micrometers. 
   In  FIG. 5 , the buffer volume  3  is within the cavity  8 . Also, the z-dimension and thus the element height and ultimately the element volume are fixed by a contact spacer  9  surrounding the cavity  8 . The contact spacer  9  can, for example, be of the kind described in WO 2004/068198, herein incorporated by reference in its entirety.  FIG. 5  thus shows an example where the replication material is confined by a combination of an exact dispensing of the replication material volume corresponding to the element volume  1  (or to a slightly smaller or larger volume) and the effect of surface tension in combination with the impact of an edge  2 . 
   The embodiment relying on a more or less exact dispensing of the replication material and a geometrical element (such as an edge) limiting the replication material flow in at least one direction by surface tension and/or capillary forces does not rely on there being a contact spacer surrounding the cavity, although there can be.  FIG. 6  shows part of a cross section of a tool  10  in which on one side, an (optional) elevated spacer section  14  is shown. In such an embodiment, the z-dimension is defined in another way, for example by contact spacers on an other side (not shown) or at an other, for example peripheral lateral position, by active distance adjusters and/or controllers, or other means. 
     FIG. 7  shows a cross section of a tool  10  with further edges  21  formed at the surface of the buffer volume  3 . These further edges  21  confine the flow of the replication material  13 , and come into action depending on the total volume of the replication material  13 , which can vary when applying the replication material  13  individually with a doser, such as a dosing syringe, to the cavity  8 , to the substrate  12  at locations opposite to the cavities  8 , or generally, if no spacers and thus no cavities are present, on the lateral positions of the elements to be replicated, either to the substrate or to the replication tool or to both. 
     FIG. 8  shows part of a cross section of a tool  10  that has an inclined spacer  15  prior to being pressed against the substrate  12 . The arrow shows the direction of flow of the replication material  13  under the inclined spacer  15 , as it is being compressed. Usually, the weight of the replication tool, with optional additional weights, is sufficient to generate the required pressure. The buffer volume  3  takes up the replication material  13  displaced from under the inclined spacer  15 . In this embodiment, it is the inclined spacer that limits the flow. 
     FIG. 9  schematically shows a cross section through a tool  10  placed on a substrate  12 .  FIG. 10  shows a corresponding elevated view. The tool  10  comprises a cavity  8  that defines the shape of the element to be formed by an element volume  1 . The element volume  1  lies between the tool  10  and the substrate  12 , and is surrounded by a buffer volume  3 . Between the element volume  1  and the buffer volume  3 , the tool  10  comprises an inner edge  2 . Between the buffer volume  3  and an overflow volume  5 , and between the buffer volume  3  and a free volume  6 , the tool  10  comprises an outer edge  4 ,  4 ′. The buffer volume  3  constitutes an outlet or overflow channel  16  for surplus material, in the case that the amount of replication material  13  exceeds the volume of the element volume  1 . 
   For cases in which a large volume tolerance is required, the cavity  8  comprises an overflow volume  5  on one side of the element volume  1 . On the other side, the outer edge  4 , or the free volume  6  or the spacer  9  defines the limit of flow for the replication material  13 , keeping the replication material  13  away from critical areas of the substrate. This outer edge  4 , together with the outer limit of the overflow volume  5 , defines a predetermined area  7  that gives the maximum area of substrate  12  that can be covered by the replication material  13 . 
   The outer edge  4 ,  4 ′ is shaped differently between the transition  4  from the buffer volume  3  to the free volume  6  on the one hand and the transition  4 ′ from the buffer volume  3  to the overflow volume  5  on the other hand, so that surface tension and/or capillary forces cause excess replication material to flow into the overflow volume  5  but not to the free volume  6 . For example, the outer edge  4 ,  4 ′ can be sharper at the transition  4  to the free volume  6  and rounder at the transition  4 ′ to the overflow volume  5 . 
   The tool  10  here rests on (optional) contact spacers  9  placed against the substrate  12 . The function of the free volume  6 , which is not to be filled by replication material, is, in combination with the outer edge  4 , to stop the flow of the replication material and also to thereby prevent it from flowing underneath the contact spacer  9 . Depending on the viscosity of the replication material, surface tension and capillary forces, this may not be necessary, and the flow can be stopped by the contact spacer itself. In that case, the contact spacer can be immediately adjacent to the element volume  1 , without there being a need for the buffer volume and the free volume  6 . 
   Since the overflow volume  5  is higher than the buffer volume  3 , following a discontinuity or step in height at the outer edge  4 , capillary forces are no longer relevant. For the sake of convenience, the dimension perpendicular to the surface of the substrate  12  is denoted as height. In actual practice, the entire arrangement can also be used upside down. The overflow volume  5  can be filled in accordance with the surplus replication material  13  volume. 
   In an exemplary embodiment of the invention, a diameter of the element volume  1  is between 1 and 2 millimetres and has a height around 250 micrometers, the height of the buffer volume  3 , i.e. the distance between the cavity  8  and the substrate  12  in the region of the buffer volume  3  is about 10 micrometers, the length of the buffer volume  3 , i.e. the distance from the inner edge  2  to the outer edge  4  is about 50 to 200 micrometers. 
     FIGS. 11 through 13  show cross sections through further tools which comprise buffer volumes with recesses adapted to the expected size or volume of individual drops of replication material.  FIG. 11  shows a tool  10  similar to that of  FIG. 7 , e.g., without a floating spacer, in which the further edges  21  constitute the boundaries between ridges  23  and recesses  19 ′,  19 ″. The ridges  23  and recesses  19 ′,  19 ″, as in the other figures, run around the element volume  1 , e.g., as concentric circles or following the contour of a noncircular optical element. In the latter case, the width and depth of each circumferential ridge or channel formed in this manner preferably remains constant around its circumference. In  FIG. 11 , a first, inner recess  19 ′ has a larger volume, since its width and/or depth is larger than that of a second, outer recess  19 ″. The inner recess  19 ′ can accept a relatively large volume of surplus replication material and is preferably located and sized such that
         the volume of replication material required to reach a first, inner edge  21 ′ of the inner recess  19 ′ corresponds to an expected minimum volume deposited by a drop deposition device (with a given probability); and   the volume of replication material required to reach a second, outer edge  21 ″ of the inner recess  19 ′ corresponds to an expected maximum volume deposited by a drop deposition device (with a given probability).       
   For cases in which the replication material exceeds the expected maximum volume (with low probability, but not to be ruled out completely), the second, outer recess  19 ″ can be arranged to create a limit according to its edges. This arrangement of ridges can be combined with an overflow volume  5 , indicated by dashed lines, or not. 
     FIG. 12  shows a tool  10  with recesses  19 ′,  19 ″,  19 ′″ and ridges  23  arranged and dimensioned in an analogue fashion as in  FIG. 11 , but separated from the element volume  1  by a elevated (floating) spacer  14 , as in  FIG. 1 . As a variant of the recess/ridge arrangement of  FIG. 11 , a further recess  19 ′″ is arranged inside the larger recess  19 ′, in order to account for the low probability cases in which the surplus material is less than the expected minimum volume of deposited material, and to provide for a defined contour of the replication material. Again, this arrangement of ridges can be combined with an overflow volume  5 , indicated by dotted lines, or not. 
     FIG. 13  shows a tool  10  with an inclined or sloped surface  22  extending outwards from an elevated spacer  14  with increasing height. As a result, the relation between the distance from the element volume  1  and the volume of replication material required to fill the cavity under the inclined surface  22  up to this distance is non-linear. This nonlinearity is not only caused by the area covered increasing with the square of the radius, but further also because of the height of the sloped surface increasing with the radius. Depending on the viscosity and other flow properties (in particular adhesion vs. cohesion) of the replication material, this geometry can be of advantage. This geometry can be combined with regular or irregularly sized recesses and ridges as in  FIGS. 11 and 12 , and with or without an overflow volume  5 . A geometry with an inclined surface can also be used in set-ups without the contact spacers  9 . For example, it can be used in configurations like the one in  FIG. 1  as a surface of the surplus volume  5 . 
     FIG. 14  shows another embodiment of a tool  10  including a peripheral feature  30  for confining the replication material  13  to a predetermined area of the substrate  12 . The peripheral feature  30  is positioned adjacent the cavity  8 . The peripheral feature  30  can be a stepped region defined in the tool  10 . As shown in  FIG. 14 , the stepped region can include a plurality of step features defined in the tool  10 . At least one of the step features can confine the replication material  13 . 
   For example, the stepped region can include one or more lateral positions  32   x  at which the replication material is confined. A lateral position  32   a  can be at an edge of a step, or a lateral position  32   b  can be at an intermediate region of a step. The lateral positions can be determined by the thickness of the optical element being formed and the resulting distance between the tool  10  and substrate  12  after they are moved toward each other. For example, the replication tool  10  can rest on a film of replication material  13 . The amount of replication material  13  can be pre-determined so that the replication tool  10  rests a predetermined distance from the substrate  12 . 
     FIG. 15  shows a flow diagram of the method described. 
   While the invention has been described in present preferred embodiments of the invention, it is distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practised within the scope of the claims.