Abstract:
A method of manufacturing an element using a replication tool, including the steps of providing a replication tool that defines the shape of the element; providing a substrate; pressing the tool against the substrate, with a replication material located between the tool and the substrate; confining the replication material to a predetermined area of the substrate, which predetermined area exceeds the desired area of the element on covering the substrate, in at least one direction along the surface of the substrate by less than a predetermined distance.; hardening (e.g. curing) the replication material to form the element.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     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 molding steps. More concretely, it deals with a method of replicating an optical element and a replication tool therefore.  
         [0003]     2. Description of the Related Art  
         [0004]     Replicated optical elements include diffractive and/or refractive micro-optical elements for influencing an optical beam in any pre-defined manner, refractive elements such as lenses, potentially at least partially reflecting elements etc.  
         [0005]     When optical elements are produced by replication, there is often a basic configuration involving a substrate and replication material on a surface thereof, which replication material is 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 must be 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, the volume of the replicated element is alsowell 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.  
         [0006]     Of special interest are the wafer-scale fabrication processes, where an array of optical elements is fabricated on a disk-like (“wafer-”) structure, which subsequently to replication is separated (“diced”) into the individual elements or stacked on other wafer-like elements, and after stacking separated into the individual elements, as for example described in WO 2005/083 789. ‘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. (5.08 cm.) and 12 in. (30.48 cm.) 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 a co-pending application “Method and Tool for Manufacturing Optical Elements” by the same inventors and filed on the same day as the present application, an array replication method is disclosed according to which for every optical element or sub-group of optical elements to be created, a blob of replication material is dispensed in an array like manner, either on the substrate or on the tool.  
         [0007]     In such an array replication process, excess material will ooze out sideward from the element volume. For example, miniature optical lenses may 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, may interfere with further processing steps of the stack comprising the semiconductor wafer and the lenses, e.g. bonding.  
         [0008]     WO 2004/068198, by the same applicant, herewith incorporated by reference in its entirety, describes a replication process for creating micro-optical elements. A structured (or micro-structured) element is manufactured by replicating/shaping (molding 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.  
         [0009]     The spacer portions allow for an automated and accurate thickness control of the deformable material on the substrate. They may comprise “leg like” structures built into the tool. In addition, the spacers prevent the deformation of the micro optical topography since the spacers protrude further than the highest structural features on a tool.  
         [0010]     The spacer portion is preferably available in a manner that is ‘distributed’ over at least an essential fraction 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 consists of a plurality of spacers distributed over the replication surface of the replication tool. The spacers allow for an automated and accurate thickness control of the replication material layer.  
         [0011]     The replication process may 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. This surface, thus, serves as a stop face for the embossing.  
         [0012]     For these reasons, the replication process described in WO 2004/068198 is one particularly advantageous possibility of controlling 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.  
         [0013]     For the reasons stated above, the embossing step causes residual material to remain in the areas between the elements, and, for example, also around the periphery of each of the elements. If the replication tool comprises a spacer portion, this may also be true for the spacer area surrounding an element.  
       BRIEF SUMMARY OF THE INVENTION  
       [0014]     It is therefore an object of the invention to create a method of replicating an element and a replication tool of the type mentioned initially, which overcomes the disadvantages mentioned above.  
         [0015]     According to a first aspect of the invention, a method of manufacturing an element by means of a replication tool is provided, the method comprising the steps of: 
        providing a replication tool that defines the shape of the element;     providing a substrate;     pressing the tool and the substrate against each other, with a replication material in a liquid or viscous or plastically deformable state located between the tool and the substrate;     confining the replication material to a predetermined area of the substrate, which predetermined area exceeds the desired area of the element on the substrate, in at least one direction along the surface of the substrate by less than a predetermined distance;     hardening the replication material to form the element.        
 
         [0021]     The replication material is 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 will, after hardening by e.g. curing, only cover part of the substrate. The element will not extend to cover the substrate in predetermined areas, leaving them free for e.g. bonding.  
         [0022]     The replication tool may comprise 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 may additionally comprise at least one buffer and/or overflow volume. The spacer or spacer portions protrude from the replication surface. In the embossing process, the spacer or spacer portions abut against the substrate and/or float on a thin basis layer of replication material.  
         [0023]     The force by which the tool and the substrate are pressed against each other may be chosen based on specific requirements. For example, the force may 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 may lie on the replication tool. The force may, according to yet another alternative, be higher or lower than the weight and may, 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.  
         [0024]     Before the replication tool and the substrate are brought together for the embossing 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 may comprise a plurality of sections each defining an element to be replicated. Then, preferably the method comprises applying a (possibly pre-defined) volume of replication material 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 must be 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.  
         [0025]     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 may be hardened by curing, for example UV curing. As an alternative, it may be hardened by cooling. Depending on the replication material chosen, other hardening methods are possible. Subsequently, the replication tool and the replication material are 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 may e.g. have a micromechanical function, at least in regions.  
         [0026]     The element volume covers a part of the substrate and constitutes the functional part of the element. The remainder of the cured replication material may 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. The invention allows for controlling how far the replication material may move along the substrate at each side of the element volume.  
         [0027]     In a preferred embodiment of the invention, 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.  
         [0028]     As an example, the replication tool may 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. 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 a 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.  
         [0029]     The cavity, in this example, may for example consist of the element volume only. It may 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.  
         [0030]     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, may 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.  
         [0031]     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 may 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” relate to a direction perpendicular to the z-direction.  
         [0032]     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.  
         [0033]     Especially, the predetermined volume of replication material may 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 will then be stopped by fluid forces acting at the inner edge from flowing into the buffer volume.  
         [0034]     In yet another preferred embodiment of the invention, 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 molding, 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.  
         [0035]     In a preferred embodiment of the invention, the method comprises the further steps of: 
        confining the flow of the replication material towards at least one side of the tool by a contact spacer that touches the substrate; and     enabling the flow of the replication material towards another side of the tool by an overflow channel.        
 
         [0038]     This allows for the diverting of the replication material away from the critical areas and guiding it to an overflow volume located in a noncritical area.  
         [0039]     Also according to the invention, a replication tool for replicating an element from a replication material is provided, the replication tool comprising a replication side, a plurality of cavities on the replication side, each defining the shape of one element or a group of elements, the replication tool further comprising at least one spacer portion, protruding, on the replication side, from the cavities, the replication tool further comprising means for confining the replication material to a predetermined area of the tool, when the tool is pressed against a substrate, whose 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.  
         [0040]     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 may 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 control of the flow despite inaccuracies when dispensing the replication material to individual cavities or onto corresponding individual locations on the substrate.  
         [0041]     In other words, the cavity comprises 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.  
         [0042]     In a further preferred embodiment, the replication tool comprises 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.  
         [0043]     In a further preferred embodiment, the replication tool comprises 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.  
         [0044]     In a further preferred embodiment, the replication tool comprises 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.  
         [0045]     The tool comprises a plurality of cavities, thus preferably allowing for the simultaneous manufacture 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.  
         [0046]     Features of the method claims may be combined with features of the device claims and vice versa.  
         [0047]     The replica (for example a micro-optical element or micro-optical element component or an optical micro-system) may be made of epoxy. The hardening step, which is done while the replication tool is still in place may then be an 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.  
         [0048]     “Optical” elements include elements that are capable of influencing electromagnetic radiation not only in the visible part of the spectrum. Especially, optical elements include elements for influencing visible light, Infrared radiation, and potentially also UV radiation. The word “wafer” in this text does not mean any restriction as to the shape of the substrate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0049]     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:  
         [0050]      FIGS. 1 and 2  cross sections through tools placed on a substrate;  
         [0051]      FIG. 3  an elevated view of the arrangement of  FIG. 2 ;  
         [0052]      FIG. 4  an example of an alternative geometrical shape of a transition between a buffer volume and an overflow volume;  
         [0053]      FIGS. 5-9  cross sections through further tools;  
         [0054]      FIG. 10  an elevated view of the arrangement of  FIG. 9 ; and  
         [0055]      FIG. 11 a  flow diagram of the method according to the invention.  
         [0056]     The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0057]      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 a 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 .  
         [0058]     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 the element volume  1  from being 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.  
         [0059]     The inner edge  2  constitutes a first discontinuity, stopping the flow at 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 .  
         [0060]      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  18  may, 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 confinement by the replication material especially well in one desired direction, corresponding to the lower left corner in the sketched configuration, as may be especially desired in configurations with an off-center optical element.  
         [0061]     The tool preferably comprises, as it may in the embodiment of  FIG. 1  and in all of the hereafter-described embodiments, multiple sections each corresponding to an element to be replicated. The sections are 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 to.  
         [0062]     As shown in  FIGS. 2 and 3 , an asymmetry of material flow between different directions can be implemented in a way that is based on different distances. 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 liquid. 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.  
         [0063]      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.  
         [0064]     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  may, for example, be of the kind described in WO 2004/068198.  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 .  
         [0065]     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 means of surface tension and/or capillary forces, thus, does not rely on there being a contact spacer surrounding the cavity. This is illustrated in  FIG. 6 .  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 another, for example, peripheral lateral position, by active distance adjusters and/or controllers, or other means.  
         [0066]      FIG. 7  shows a cross section of a tool  10  with further edges  17  formed at the surface of the buffer volume  3 . These further edges  17  confine the flow of the replication material  13 , and come into action depending on the total volume of the replication material  13 , which may 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.  
         [0067]      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.  
         [0068]      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 .  
         [0069]     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 .  
         [0070]     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 ′ may be sharper at the transition to the free volume  6  and rounder at the transition to the overflow volume  5 .  
         [0071]     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 may be stopped by the contact spacer itself. In that case, the contact spacer may be immediately adjacent to the element volume  1 , without there being a need for the buffer volume and the free volume  6 .  
         [0072]     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 may also be used upside down.). The overflow volume  5  will simply be filled in accordance with the surplus replication material  13  volume.  
         [0073]     In an exemplary embodiment of the invention, a diameter of the element volume  1  is between 1 and 2 millimeters 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 ca. 10 micrometers, the length of the buffer volume  3 , i.e. the distance from the inner edge  2  to the outer edge  4  is ca. 50 to 200 micrometers.  
         [0074]      FIG. 11  shows a flow diagram of the method described.  
         [0075]     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.