Abstract:
A method of manufacturing a plurality of elements by replication, includes the steps of: providing a replication tool that includes a plurality of replication sections having structural features defining the shape of the elements, the tool further including a plurality of first spacer portions; providing a substrate; 
       applying a replication material  5  in individual portions, each portion being associated with one of the replication sections  3  and the portion being applied to the replication section  3  and/or to a location on the substrate  7  against which the replication section  3  will be moved in a later step; moving the tool against the substrate, with the replication material in a plastically deformable or viscous or liquid state located between the tool and the substrate; and hardening the replication material to form the elements.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention is in the field of manufacturing optical elements, in particular refractive optical elements and/or diffractive micro-optical elements, by means of a replication process that includes embossing or molding steps.  
         [0003]     2. Description of 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.  
         [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 parts constituting 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 inches (5.08 cm.) and 12 inches (30.48 cm.).  
         [0007]     In wafer-scale replication processes, a single blob of replication material for the replica is disposed on the substrate. However, in such process, depending on properties of the replication material, the aspect ratio of replicated structures in waver-scale replication is limited. If the structures to be replicated are not flat and have a high aspect ratio, it is difficult to make sure that all structures are duly filled by replication material. Also for structures with a limited aspect ratio, one has to dispense a large amount of replication material in order to make sure that also, in peripheral regions, enough replication material remains so that all structures are replicated. Often, it will happen that air gets trapped against the replication surface, i.e. in the mold. This causes defects in the finished replicated elements. In the case of optical elements, defective elements are rejected.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     It is an object of the invention to create a method, and a tool for manufacturing optical elements, which overcome the drawbacks of the prior art and which improve the quality of elements replicated in this manner, and reduce the occurrence of defects.  
         [0009]     According to a first aspect of the invention, a method of manufacturing a plurality of elements by replication is provided, the method comprising the steps of 
        providing a replication tool that comprises one or more replication sections having structural features defining the shape of the elements;     providing a substrate;     applying a replication material in individual portions, each portion being associated with one of the replication sections and the portion being applied to the replication section and/or to a corresponding lateral location on the substrate against which the replication section will be moved in a later step;     moving the tool against the substrate, with the replication material in a plastically deformable or viscous or liquid state located between the tool and the substrate, shaping the replication material according to the shape of the replication sections; and     hardening the replication material to form the elements.        
 
         [0015]     Thus, a predetermined volume of replication liquid is applied locally and individually to at least one of the tool or the substrate prior to pressing the tool against the substrate. This allows provision of a plurality of cavities with an optimal amount of replication liquid, reducing or eliminating the volume of surplus liquid that would have to be removed or diverted from the critical areas of the substrate when a plurality of elements was formed from a single blob of replication liquid.  
         [0016]     After replication, the replication tool is removed, and the substrate with the replication material thereon may be separated (“diced”) into parts each containing an individual element. The invention features the additional advantage that it is possible to confine the replication material on the individual elements, i.e. to have regions on the individual elements where the substrate is not covered by replication material.  
         [0017]     The replication section of the tool defines a replication surface or section with (concave or convex) negative structural features, being a negative of at least some of the structural features of the element to be produced.  
         [0018]     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 and/or diffractive optical element, but also may e.g. also have a micromechanical function at least in regions.  
         [0019]     The tool comprises a plurality of replication sections, i.e. cavities or protrusions, thus allowing for the simultaneous manufacturing of a plurality of elements on a common substrate, which on the substrate are preferably arranged in an array-like manner. The tool comprises a plurality of replication sections, thus allowing for the simultaneous manufacturing of an array of elements on a common substrate. This common substrate may, according to a special embodiment, be 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 parts.  
         [0020]     In a preferred embodiment of the invention, the portion of replication material is applied to the tool, namely to the replication section. Preferably, the replication section is filled, at least to a large part. Especially the critical locations of the replication section corresponding to the highest feature of the future element, which are most sensitive and prone to defects, are filled.  
         [0021]     In a further preferred embodiment of the invention, the flow or dispersion of replication material across the tool is limited by the replication section being a convex part of the lower tool surface, i.e. convex features protruding from a surface of the tool. Preferably, the tool is kept in this orientation, i.e. facing downwards, while being moved to and against the substrate.  
         [0022]     In another preferred embodiment of the invention, the replication material is applied to the convex replication sections by dipping the replication sections into the surface of a volume of replication material. The volume may be a pool in a container, or an amount of replication material spread over a surface. The replication sections are preferably dipped only as far as necessary to wet only the replication sections, leaving the rest of the tool surface free from replication material. Alternatively, the rest of the tool surface may be non-wetting with respect to the replication material, such that, when removing the lower surface of the tool from the volume of replication material, it remains free from replication material. The convex replication sections may be treated chemically or mechanically, or may be made of another material, in order to have a better wetting property, causing a droplet of replication material to adhere to each of the replication sections.  
         [0023]     According to yet another embodiment, a dispensing tool is used for dispensing the replication material on the substrate and/or the replication tool. The dispensing tool, according to this embodiment, is based on the above principle. The dispensing tool, thus, comprises a plurality of protruding replication material loading portions, which are arranged in an array corresponding to the array of replication sections of the replication tool. The replication material loading portions are dipped into the surface of a volume of replication material. The protruding portions are preferably dipped only as far as necessary to wet only portions themselves, leaving the rest of the tool surface free from replication material. Then, the dispensing tool is brought into contact with the surface of the replication tool or the substrate, so that amounts of replication material stick to the replication tool or substrate surface, respectively. Instead of, or in addition to being protruding, replication material loading portions may be made of another material than the surrounding surface of the dispensing tool in order to have a better wetting property, causing a droplet of replication material to adhere to each of the replication material loading portions.  
         [0024]     In this way, dispensing in individual portions is a fully parallel process.  
         [0025]     In another preferred embodiment of the invention, the portion of replication material is applied to the substrate. The replication material forms a, usually convex, droplet isolated from other droplets of replication material.  
         [0026]     Applying the replication material in individual portions may, depending on the material properties of the replication material, even provide an advantage when replicating structures with a high aspect ratio (deep cavities). When the tool with the replication section is moved against the droplet, the convex surface of the droplet reaches into the replication section, and starts displacing the air, at the critical location, before the tool even touches the substrate. This is in contrast to the state of the art, where the entire surface of the substrate is covered with replication material, such that, when spacers surrounding the replication sections reach the replication material, the replication material may block air trapped in the replication section from escaping.  
         [0027]     This approach can be combined with any variant of the previous approach, i.e. the replication material may be applied to both the tool and the substrate.  
         [0028]     In a preferred variant of this embodiment, the flow or dispersion of replication material across the substrate is limited by flow limiting means on the substrate.  
         [0029]     The flow limiting means may be constituted by an edge and/or an area of reduced wetting surrounding a material receiving area of the substrate. Such an area of reduced wetting is created by mechanical and/or chemical treatment of the surface of the substrate. Alternatively or in addition, such an area is created by an inlay of other material arranged in the surface of the substrate. The surface of the material receiving area may be treated as well, in order to increase its wetting capability.  
         [0030]     According to another aspect of the invention, a method of manufacturing a plurality of optical elements is provided, each optical element comprising a refractive lens, the method comprising the steps of: 
        providing a replication tool that comprises a plurality of replication sections having negative structural features defining the shape of the elements, each replication section comprising a dome-shaped portion defining the shape of one of said refractive lenses,     providing a substrate;     dispensing a replication material in a liquid or viscous or plastically deformable state into each one of the dome-shaped portions;     moving the replication tool and the substrate against each other until the replication material is in contact with a surface of the substrate;     hardening the replication material to form the elements;     removing the replication tool; and     separating parts of the substrate each carrying at least one of said refractive lenses from each other.        
 
         [0038]     Further, preferably, the replication tool may comprise spacer portions, for example as disclosed in WO 2004/068198 by the same applicant, herewith incorporated by reference in its entirety. 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.  
         [0039]     The spacer portion is preferably available in a manner that it 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 deformable material layer.  
         [0040]     As an alternative or in addition to spacers abutting the substrate surface, the replication tool may also comprise “floating spacers”, i.e. spacers that remain at a certain distance from the substrate surface during the replication process.  
         [0041]     Floating spacers or contact spacers may, for example, surround a dome-shaped cavity that defines the shape of a refractive lens to be replicated.  
         [0042]     The replica (for example a micro-optical element or micro-optical element component or an optical micro-system) may be made of epoxy, which is cured, for example UV cured, while the replication tool is still in place. UV light curing is a fast process that allows for a good control of the hardening process. Depending on the replication material used, also other hardening processes are possible, for example by cooling, chemical reaction, waiting, etc. For most applications, the replication material is transparent.  
         [0043]     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.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]     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:  
         [0045]      FIG. 1  is a cross section through a replication tool;  
         [0046]      FIG. 2  is an elevated view of a replication tool;  
         [0047]      FIGS. 3-6  are cross sections through further tools and substrates in various production stages;  
         [0048]      FIGS. 7-9  are details of cross sections of substrates and tools;  
         [0049]      FIG. 10  is a cross section of a dispensing tool;  
         [0050]      FIG. 11  is a cross section of a further replication tool; and  
         [0051]      FIG. 12  is a flowchart showing method steps of the invention. 
     
    
       [0052]     In principle, identical or corresponding parts are provided with the same reference symbols in the figures.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0053]      FIG. 1  schematically shows a cross section through a replication tool  9 . The tool  9  comprises a plurality of replication sections  3 , i.e. negative structural features defining the shape of elements  6  to be created with the tool  9 . Each of the replication sections  3  is partially or completely surrounded at its periphery by a first spacer portion (here a local spacer portion or element spacer portion)  1 . The area covered by the replication sections  3  and first spacer portions  1  interspersed in this manner is called the replication area. The replication tool may further comprise a rigid back plate  8  to make it dimensionally stiff on a large scale.  
         [0054]     The first spacer portion  1 , on the one hand, serves to define the shape or the boundary of the element  6  in the region close to a substrate body, henceforth simply referred to as substrate  7 , and on the other hand to define the height of the element  6  with respect to the substrate  7 . That is, the first spacer portion  1  comes to rest against the substrate  7  or at a controllable distance from the substrate  7 . The latter distance, called “element spacer height difference”, is determined by the vertical extension of second spacer portions  2  relative to that of the first spacer portion  1 . The second spacer portions are contact spacer portions protruding further than the first spacer portions and being, during replication, in direct contact with the substrate. In other embodiments of the invention, the local, first spacer portion  1  comes to rest on the substrate  7  without any residual replication material  5  in between, the element spacer height difference being zero, or all spacer portions are at a distance from the substrate, so that the spacer-to-substrate distance is determined by capillary forces and/or surface tension effects or by other means such as by active distance adjusters etc.  
         [0055]     In this text, for the sake of convenience, the dimension perpendicular to the surface of the substrate  7 , 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. The terms “periphery” and “sides” of the element are, thus, understood when looking at the substrate from a direction perpendicular to the essentially flat substrate. The element covers a part of the substrate, and the surrounding other parts of the substrate, i.e. the region of space adjacent to both the substrate and the functional part of the element, in particular under the first spacer portions, may be covered with the replication material, without interfering with the function of the element.  
         [0056]     The replication tool preferably is made of materials with some elasticity, for example PDMS (polydimethylsiloxane) or another elastic material. This results in a conformal thickness control of the element  6  produced, even if the substrate surface, on which the process is executed is not perfectly planar, or if the replication tool is not perfectly planar.  
         [0057]      FIG. 2  shows an elevated view of a replication tool. Individual replication sections  3  are shown surrounded by first spacer portions  1 . The first spacer portions  1  may each surround the replication section  3  in an unbroken circle, or may comprise spill or overflow channels  10  that make it easier for the replication material  5  to flow into areas or spill volumes  4 . A number of separate second, tool-scale, contact spacer portions  2  is arranged around the array of replication sections  3 , at the periphery of the tool  9 .  
         [0058]     The tool  9  is preferably adapted to be used in wafer-scale processing, i.e. the substrate containing the array of replication sections may be disc-shaped. Thus, the diameter of the tool  9  preferably lies in a range from 5 cm to 30 cm. Wafer-scale combination of manufacturing with micro-electronics is possible, as is for example disclosed in WO 2005/083 789 by the same applicant, herewith incorporated by reference.  
         [0059]      FIGS. 3-6  schematically show steps of a replication process. In  FIG. 3 , the replication material  5  is applied as individual portions to the replication sections  3  of the tool  9 . For this purpose, the replication material  5  is e.g. applied by an automated dosage means such as a syringe, the tip of which is located manually or with a robot manipulator at the replication section  3 , and a droplet  19  or drop of the replication material  5  is extruded into the replication section  3 . This may be done with the tool  9  in the position shown in  FIG. 3 , i.e. with the face of the tool facing downwards, or alternatively with the face of the tool facing up. The tool  9  is then positioned face down over the substrate  7  and moved against the substrate  7 , as indicated by a block arrow. Alternatively, if the replication tool is placed with the face facing up, the substrate is placed on top of the replication tool.  
         [0060]     In the alternative step of  FIG. 4 , the replication material  5  is applied in individual portions or droplets  19  to the substrate  7 , at locations corresponding to where the replication sections  3  will meet the substrate  7 , and the tool  9  is positioned over the substrate  7 . The droplets  19  therefore typically are arranged in a grid-like arrangement corresponding to a mirror image of the pattern of replication sections  3 .  
         [0061]     The tool-scale spacer portions  2  are positioned opposite corresponding tool-scale support areas  13  on the substrate  7 . The replication material  5  such as an epoxy is in a plastically deformable or viscous or liquid state. Guiding elements for controlling the relative horizontal displacement and/or the downward movement of the tool  9  may be present, but are not illustrated.  
         [0062]     In another preferred embodiment of the invention, the first spacer portions  1  do not surround every replication section  3 , but are e.g. separate pillars dispersed over the replication area  12 . In this manner, a certain area of the substrate  7  may remain covered with a thicker section of the replication material  5  which is not functional, as compared to the elements  6 .  
         [0063]     Starting out from either the arrangement of  FIG. 3  or that of  FIG. 4 , in  FIG. 5 , the tool  9  has been moved against the substrate  7 . This force driving this movement is preferably only the gravity acting on the tool  9 . Thus, the weight of the tool  9 , including the back plate  8  and optionally an additional mass, defines the force with which the tool  9  is pressed against the substrate  7 . This allows a very precise control of the force, and of any elastic deformation of the tool  9  that may take place. The replication sections  3  are filled with replication material  5 , and the remaining replication material  5  has been displaced into the spill volumes  4 .  
         [0064]     The tool-scale spacer portions  2  touch the substrate  7  without any replication material  5  in between, such that most of the weight of the tool  9  rests on the tool-scale spacer portions  2 . The first spacer portions  1  may be separated from the substrate  7  by the element spacer height difference, the resulting volume being filled with replication material  5 .  
         [0065]     The replication material  5  is then hardened by thermal or UV or chemical curing.  
         [0066]     In  FIG. 6 , the tool  9  has been removed from the substrate  7 , leaving the hardened elements  6  on the substrate  7 . Further processing depends on the nature and the function of the elements  6 , i.e. the elements  6  may be separated from the substrate  7  or remain on the substrate  7  for further steps in a wafer-scale production process and later dicing into separate units.  
         [0067]      FIGS. 7-9  show, in schematic cross sections, details of substrates  7  and tools  9  for confining the replication material  5 . In a preferred embodiment of the invention, for the case in which the replication liquid  5  is applied to the substrate  7 , the substrate  7  comprises a flow stopping or limiting means  11 . The flow limiting means  11  prevents the replication liquid  5  from flowing away, which would cause the droplet  19  to flatten. This, in turn, would jeopardise the desired effect of the droplet  19  filling the replication section  3  while displacing the air in the replication section  3 . Such stopping means may be mechanical means such as ridges on or troughs in the substrate  7 , or a mechanical or etching treatment that reduces the wetting capability of the substrate  7 . Alternatively or in addition, such stopping means may be effected by using a different material for the flow stopping section  12  of the substrate  7 , or by applying a chemical to said section, to reduce the wetting property of the substrate  7 .  
         [0068]      FIG. 7  shows (left part of figure) a edge  14  of a material receiving area  20 , which edge  14  acts as a flow limiting means  11  when a droplet  19  of replication material  5  is applied to the material receiving area  20  (right part of figure). The material receiving area  20  thus forms a hollow or depressed section in the substrate  7 . Seen from the top, the edge  14  may be circular, rectangular or any other shape according to the final shape or circumference of the element  6 .  
         [0069]      FIG. 8  shows (left part of figure) an area of reduced wetting  15  surrounding, on the surface of the substrate  7 , a material receiving area  20 . The area of reduced wetting  15  acts as a flow limiting means  11  when a droplet  19  of replication material  5  is applied to the material receiving area  20  (right part of figure). The area of reduced wetting  15  stops the replication material  5  from flowing away and thus prevents the droplet  19  from flattening.  
         [0070]      FIG. 9  shows a tool  9  and corresponding substrate  7 . The replication section  3  of the tool  9  is embodied as a convex replication section  16 , i.e. it protrudes from the surrounding surface of the tool  9 . Correspondingly, the substrate  7  comprises an opposing concave substrate section  17 , into which the convex replication section  16  is moved, with the droplet  19  hanging from the convex replication section  16 . As in the inverted case, the convex shape of the droplet  19  fills the concave substrate section  17 , replacing the surrounding air. At the outer periphery of the concave substrate section  17 , an overflow volume  18  may be arranged, allowing for excess replication material  5  to spill out of the concave substrate section  17 . Since the substrate  7  is no longer flat in this embodiment, there is no precise distinction between tool  9  and substrate  7  other than that it may be the tool  9  that is moved, while the substrate  7  remains at rest. The substrate  7  and the tool  9  shall thus also both be referred to as “replication body”.  
         [0071]     In a preferred embodiment of the invention, the replication material  5  is applied to a plurality of convex replication sections  16  of the tool  9  simultaneously by dipping the tool  9  into the surface of the replication material  5 . When drawing out the tool  9 , droplets  19  of the replication material  5  will remain hanging from the convex replication section  16 . This offers a significant advantage of speed and simplicity over the individual dosing with a syringe.  
         [0072]     In  FIG. 10 , a section of a dispensing tool  21  for dispensing the replication material on the substrate and/or the replication tool is shown. The dispensing tool comprises a plurality of protruding replication material loading portions  22  (only one portion shown), which are arranged in an array corresponding to the array of replication sections of the replication tool. For dispensing, the replication material loading portions are dipped first into the surface of a volume of replication material. The protruding portions are preferably dipped only as far as necessary to wet only portions themselves, leaving the rest of the tool surface free from replication material. As a consequence, the replication material loading portions  22  are covered by replication material  5 . Then, the dispensing tool is brought into contact with the surface of the replication tool or the substrate, so that amounts of replication material stick to the replication tool or substrate surface, respectively.  
         [0073]      FIG. 11  shows, in section, a replication tool  9  and a substrate  7 . The replication tool shown in  FIG. 11  comprises first spacer portions  1  surrounding the replication sections and further comprises second spacer portions  2  distributed over the tool. In the shown example, the replication material  5  is dispensed on the substrate  7 . It could also be dispensed to the tool, namely into the cavities which constitute the replication sections.  
         [0074]     In  FIG. 12 , a flowchart illustrating steps of an embodiment of the invention is shown.  
         [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 practiced within the scope of the claims.