KR20020044154A - Microlens arrays having high focusing efficency - Google Patents

Abstract

A microlens array 105 with high focusing efficiency is disclosed. High focusing efficiency is achieved by accurately manufacturing the individual microlenses that make up the array at high fill factors. The array of positive microlenses is made by forming a master having a concave surface-embossed pattern 101 in the positive photoresist 21 using a direct write laser. In this way, the problem associated with the convolution of a finite laser beam with the desired microlens profile is solved. The focusing efficiency of the microlens array according to the invention is at least 75%.

Description

Microlens array with high focusing efficiency {MICROLENS ARRAYS HAVING HIGH FOCUSING EFFICENCY}

Microlenses are required in many applications, such as coupling light from a laser to a fiber, either as a single lens or in the form of an array in which multiple beams are focused onto multiple fibers. Other important applications include light diffusion and screens.

Depending on the application, precise profile microlenses with controlled focusing characteristics are required, and in the case of arrays, high quality is required for most lenses in the array. In order to focus light effectively, the lens profile (or sag function) must be manufactured precisely, for example equal to or greater than 4 / λ. Is the wavelength of the light emitting source.

Also, especially in high density coupling, diffusion or screen applications, microlenses are important because focusing is used on the entire surface. In this way, essentially all incident light can be controlled by the array. When the total effective surface area is used for focusing, the array has a 100% fill factor.

Close packing of the microlenses means that the fill factor is equal to 100%, and that the inner boundaries between neighboring microlenses are in close contact. A simple example of hermetic packing is a hexagonal array. Other arrangements, such as rectangular arrays, can also be packed closely.

In science to literature, microlens arrays with fill factors of up to 100% are found. 1 shows an array in which microlenses 12 are arranged regularly over an effective substrate area 11 with a space between each microlens. In FIG. 1, one of the unit cells of the array is illustrated by a dotted line 13. The fill factor of this array is only 44%.

In order to avoid close contact of the boundaries of the microlenses, there are various methods of manufacturing isolated microlens units or microlens arrays whose edges are well separated. Since there is a finite distance between the inner boundaries of adjacent lenses, the fill factor of the array will inevitably be less than 1 (or 100%).

It is difficult to obtain an effectively closely packed lens array using conventional manufacturing methods because there is no way to accurately maintain the boundaries for microlenses, especially small, strong focal lenses.

As disclosed in US Pat. No. 5,324,623, the method of using thermal deformation is based on volume relaxation, but cannot control the melting of the material at the inner boundaries between the lenses. Melting results in distortion, which reduces focusing capabilities. Thus, the thermal deformation method is simple to implement, but has limitations in removing individual microlens structures.

Another method, such as that disclosed in US Pat. No. 5,300,623, involves creating a mechanical mold that is defined as a reservoir of curable liquids. The curing liquid is injected into the reservoir and the surface tension creates a curved surface that acts as the microlens. The mold with the various reservoirs determines the arrangement of the array. Because of the inherent limitations of this method in controlling the shape of the microlens unit, its efficiency cannot be optimized over typical applications. Other mechanical methods based on direct control of individual microlenses, such as diamond conversion, are more suitable for the manufacture of individual microlenses than for arrays.

A method based on an ion diffusion process that provides a gradient-index array as disclosed in US Pat. No. 5,867,321 shows that regions of two adjacent microlenses typically produce 20% of the microlens repetition spacing. It cannot provide 100% fill factor. Gradient-index arrays are inherently severely limited in mass production due to the slow diffusion process.

Processes for fabricating microlens arrays using laser recording directly to the photoresist are known in the art. PCT Patent Publication WO 99/64929, Gale et al., US Pat. No. 4,464,030 and Micro-optics: Elements, systems and applications , Hans P. Herzig, ed., Taylor & Francis , Bristol, PA, 1997, pp. 53-152. In the above method, the selection of photoresist is positive because photoresist is used more widely than negative photoresist, more thorough research and development efforts by photoresist manufacturers, and generally have higher resolution. do. However, as will be described in detail below, prior to the present invention, it was not possible to produce positive microlens arrays having high focusing efficiency with high fill factor using positive photoresist.

The present invention poses a difficulty in the prior art by providing a method of manufacturing a microlens array having high focusing efficiency through the manufacture of a precision microlens array with a high fill factor. The array may be arranged in any way, square, hexagonal or random. The method can also produce microlenses of varying focusing power (anamorphic lens) as well as in any shape.

The present invention relates to a microlens array having a high focusing efficiency and a method of manufacturing the same.

The invention can be applied to effectively focus laser light on optical fibers, diffuse light, and control the scattering of coherent or incoherent light for projection and transmission displays or other applications.

Justice

Definitions of terms used in the present specification are as follows.

"Microlens array" refers to an array of combined unit cells having a microlens array and one microlens coupled to each unit cell. The microlens array according to the present invention may have a desired structure, for example, in the name of "Structured Screens for Controlled Spreading of Light" in the name of G.Michael Morres and Tasso RMSales 2000 It may be made from a support "piston" of the type disclosed in U.S. Patent Application 60 / 222,033, filed on July 31, 19. This content is incorporated herein by reference in its entirety. The term "microlens" refers to any microstructure capable of focusing light.

The "fill factor" of a microlens array refers to the ratio of the total area in the unit cell occupied by the microlens to the total area of the unit cell.

The "focusing efficiency" of a microlens array is the light that strikes the unit cells of the array against an array that is emitted along its optical axis by a collimated, substantially spatial incoherent light source, for example a collimated white light source. Refers to the sum of the light intensities measured at the focus of the microlenses separated by the sum of the intensities. As will be appreciated by those skilled in the art, this is the "Strehl-type" definition of focusing efficiency.

Since concave microlenses typically have a halo focus (ie, the chromosome chromatographic lens in the air has a negative power, and therefore a halo focus for collimated light), in this case, the intensity is measured so that the intensity can be measured. An auxiliary light system is used to create the focal point. At least to some extent, the auxiliary light system is reduced in intensity at the real focus, which should be taken into account when determining the intensity value for the virtual focus.

In the case of anamorphic microlenses, the light intensity of each focal point of the microlens is included in the sum of the measured light intensities.

1 is a plan view of a lens array with a fill factor of less than 100%.

2 shows a glass substrate with a photosensitive film deposited on its surface.

3 shows the scanning of a laser beam onto a photosensitive film producing a clear chemical property (latent image) region.

4A and 4B illustrate the convolutional effect in the manufacture of convex structures.

5A and 5B illustrate the interaction in hard fabrication for convex and concave arrays, respectively.

6 illustrates a method of measuring focusing efficiency of microlens units of a convex array.

7A and 7B are experimental views of the same microlens profile made of convex and concave shapes, respectively.

8 and 9 illustrate surface-embossed structures with concave cavities formed in positive photoresist with edge boundaries of cavities arranged on top of the photoresist.

10A, 10B, and 10C show replicas of an initial mold with concave cavities to obtain a final array of convex microlenses.

In this regard, the object of the present invention includes at least some or all of the following.

(1) providing a manufacturing method for manufacturing an array of convex microlenses having high focusing efficiency;

(2) providing an array of convex and / or concave microlenses having a focusing efficiency of at least 75%, preferably at least 85% of focusing efficiency, most preferably at least 95% of focusing efficiency;

(3) providing a method for precise fabrication of arrays of convex microlenses with high fill factor; And / or

(4) at least 90% fill factor, preferably at least 95% fill factor, and most preferably nearly 100% fill factor such that the total effective area of the substrate can be used for focusing or, more generally, scattering of the light beam. Providing an array of precisely manufactured convex and / or concave microlenses having a.

In connection with the above object, another object of the present invention is to make the microlenses of the array have any shape (sag function) that can change randomly within the array.

It is yet another object of the present invention to provide an improved method of using positive photoresists to fabricate convex microlens arrays with high fill factors.

To achieve the above objects, the present invention provides a convex microlens in which direct laser recording is used to produce an initial master (initial mold) in a positive photoresist where the surface structure of the initial master is the negative (supplement) of the desired convex microlens array. It provides a manufacturing method for manufacturing an array. That is, the initial master has a concave surface structure instead of convex. As detailed below, this method solves the problem caused by the convolution of the beam with the finite size of the laser beam and the desired profiles of the convex microlenses. By solving this problem, a convex microlens array with high focusing efficiency is achieved.

In general, the high focusing efficiency of the microlens array depends on two factors: (1) high fill factor, and (2) precise reproduction of the desired lens profile. Both arguments are required at the same time and either argument is insufficient.

Thus, a high fill factor can be achieved by the process of changing all parts of the resist film, but if the change does not correspond to the desired lens profile, the focusing efficiency of the array still cannot be improved, which is inaccurate. This is because a part of the resist film having one profile does not properly focus incident light. On the other hand, precise reproduction of the desired lens profile with separate microlenses spaced apart can result in low focusing efficiency, in which case light passes through the spaces between the microlenses.

According to the invention, these two factors can be processed by using the concave shape to initially record the convex lens in the positive resist. According to this method, high focusing efficiency can be achieved through precise manufacture of a desired lens profile with a high fill factor.

According to a preferred embodiment of the present invention, the present invention is made using a substrate, typically made of glass, to support a first medium for producing an initial master (initial mold), which master is later produced at low cost with the desired microlenses Used for precise replication of arrays. More specifically, the photosensitive positive resist film is deposited on the substrate to an appropriate thickness while maintaining the desired thickness to obtain the final microlens array. The positive resist is preferably of a low-contrast type such that a surface-relief profile is created that changes smoothly when exposed to light.

After being deposited on the substrate, the positive resist is exposed to a laser beam having a properly characterized profile. At a predetermined sampling rate, the resist film area is exposed to the laser beam. By changing the beam intensity, the replenishment of the shape of each microlens in the array is encoded in the resist. In particular, laser beam exposure adjusts its physical and chemical properties to produce a latent image in the photosensitive film.

The film is then developed to create a surface-embossed structure. For a positive kind of resist film, the development subtracts the exposed areas leaving the unexposed areas. The combination of surface-embossed structure and photoresist type for the initial master can be achieved through high fill factor only through the combination, and the convolutional effect of the finite laser beam can be minimized in the present invention. Very important.

The convolutional effect of the finite laser beam is generally considered to be essentially irrelevant whether the laser beam exposure produces a convex or concave surface-embossed structure. According to the invention, this is different from the fact, in fact by making an initial master for a convex microlens array as a concave surface-embossed structure, the high fill factor (ie, the fill factor is equal to or substantially equal to 100%). And high focusing efficiency (ie, focusing efficiency is at least 75% or more). Details of the combinations that cause this convolution problem are described below.

Since resist films are generally not suitable for mass replication, an intermediate replication step is generally required to produce a mold suitable for mass replication. For example, concave surface-embossed structures are used to prepare intermediate masters (intermediate molds) that are convex. The intermediate master is then duplicated and concave once to provide the final master (final mold). In order for the final array to be convex and provide high fill factor and high focusing efficiency, mass replication is possible with the final concave master.

The array need not be regularly confined to periodic arrays, such as square or hexagonal arrays, but may be of any general shape as required by the design. Moreover, the lens shape need not be the same, but in fact each microlens in the array can vary widely. For example, the technique according to the invention can be used to fabricate the arrangement and distribution of the microstructures described in the US patent application filed above as "Structured Screens for Controlled Spreading of Light." .

An important point in the present invention is that the top of the concave surface of the concave surface-embossed structure formed in the positive resist film is preferably arranged or gently changes with respect to any adjacent element. If this is not satisfied, a precise profile can only be made in part of the array, and both the fill factor and focusing efficiency of the array will be reduced.

Referring to the drawings, FIG. 2 shows a low contrast photosensitive resist film 21 deposited onto a substrate 22, which is typically made of glass. The thickness of the film should be equal to or greater than the total depth distance defined by the lens array. Depending on the overall thickness of the array, the resist may require a preliminary process such as hardening.

After the initial resist process, as shown in FIG. 3, the laser beam is focused onto the resist film and scanned along the surface to expose the entire resist surface. The intensity of the laser beam varies at each point in such a way that the negative latent image of the desired convex microlens is imprinted in the resist in the form of chemical modification of the resist material.

In order to obtain a surface-embossed structure, a chemically controlled resist film is subjected to a developing process, which consists of exposing the solution, for example a standard alkaline developer, for a time varying the total thickness of the array. Deeper arrays require longer development times. For positive resists, the development process removes the exposed areas, leaving the unexposed areas.

According to the present invention described above, each microlens in the array needs to be manufactured in a concave positive photoresist. Only this method can significantly reduce the rounding effect that appears when microlenses are made in convex form. This is because the manufacturing process itself can introduce features that result in unwanted surface-embossed profiles.

Given the desired surface-embossed structure and the given equation of the recording laser beam, the embossed structure is obtained with the exposure of the resist film, which is generally described as the convolution of the desired surface function with the laser beam function. The operation of the convolution is mathematically represented by Equation 1 below.

Where f represents a mathematical function representing the desired surface relief, g represents the mathematical form of the recording laser beam, S is the surface area to be produced, ( x, y ) is the point on the photosensitive film surface, and F is the final surface It shows form.

The usefulness of Equation 1 lies in the assumption that the interaction of the laser beam and the photosensitive film is linear in that the reaction of the film is directly proportional to the laser exposure intensity, and the overlap of multiple beams has a simple additive effect. If the assumption is correct for a useful approach, one can observe the convex form, ie, the surface-embossed structure made of the structure protruding from the resist surface, as shown in FIGS. 4A and 4B.

The fact that the expected convolutional effect is immediately observed in the convex structure gives rise to the general belief that the same type also occurs for concave shapes. In fact, if using Equation 1, and obtaining a concave shape requires multiplying -1 to a convex shape and adding a constant, the final form is for both concave and convex shapes except for the change of sign. Will be the same.

However, in the end the interaction of the laser beam with the photosensitive film is non-linear, so the convolutional effect can only roughly explain the manufacturing process. In fact, according to the present invention, the laser writing process is similar to the process produced by means of rigid mechanical devices such as diamond tools.

In the manufacturing process, there is still a convolutional effect, but it is different from that of the laser beam in that latent image and overlapping effect are not shown. Producing the surface relief is by contact of a controlled surface with a mechanical mechanism. However, there is essentially asymmetry in the mechanical manufacture of such concave and convex structures. Due to the finite size of the instrument it is not possible to penetrate the narrow area between two adjacent structures, but there is no difficulty in creating the pointed contact points of the two concave structures. This is shown in Figures 5a and 5b.

According to the invention, it can be seen that the laser recording process operates according to similar principles and shows similar asymmetry when considering convex and concave shapes. This surprising result makes it possible to produce a fully packed convex microlens array, unlike conventional methods, which can guarantee a precise profile in part of the array's aperture in a fully packed arrangement.

Importantly, the laser recording process used to produce the concave surface-embossed structure confers an important ability to surpass the advantages of the mechanical control method as well as the advantages of the mechanical control device for the concave structure. For example, the size or shape of the microlenses processed by the laser recording process is practically unlimited. In addition, the size of the mechanical mechanism itself determines the range of the boundary area between adjacent microlenses, but according to laser recording the area can be reduced.

The ability to maintain the concave surface-embossed shape from the apex of the structure to the extreme end of the boundary of the adjacent concave surface is considered for the array structure of convex microlenses with high focusing efficiency. This is because the final convex microlenses have a fully packed arrangement. In contrast, if the array is manufactured directly in convex form, the boundary of two adjacent microlenses cannot be used for focusing, regardless of whether mechanical, laser, or other processes are used, so the array has a reduced focusing efficiency. Will have

The drawbacks of manufacturing the array in convex form, whether mechanical or laser, are shown in FIG. 6. In FIG. 6, the shape of the desired microlens becomes a curve 61 with the focusing area indicated by parameter A. FIG. However, by manufacture, the actual microlens shape becomes a curve 62 with the focusing area shown by parameter B. The rounding effect observed at the microlens boundary transforms incident illumination in addition to the focal point of the microlens, so only area B is useful for focusing. According to the method, the focusing efficiency of the measured microlenses is as follows.

According to the prior art, B is always smaller than A so that the focusing efficiency is 100% or less. According to the present invention, the initial surface-embossed structure is recorded in concave shape such that the sharp edges between the lenses are preferably processed in the final micro array. Once the concave master is replicated, B can essentially get the same convex array as A. Therefore, the focusing efficiency is 100%.

This analysis has been experimentally recognized, especially in convex microlenses of high numerical aperture (fast lens) when light is focused at large angles. FIG. 7A shows the case of a microlens array having a diameter of 50 μm manufactured in the convex mode. The boundaries between the microlenses are clearly rounded and are not efficient for use in focusing. The efficiency for each microlens in this array is 50%.

On the other hand, as shown in Fig. 7B, more preferable results are obtained when the same array is processed in concave shape. The focusing efficiency of the array is 100%. It can also be fully packed without loss of the concave surface-embossed structure efficiency. Direct writing of the convex array cannot reach packing without this loss of efficiency.

Another important element in the present invention is that the concave surface of the concave surface-embossed structure formed in the positive photoresist, as shown in FIG. 8, has distal ends aligned with the surface of the resist. The change from the preferred arrangement should be sufficiently gentle to avoid excessive rounding of the extreme microlenses leading to low transmission efficiency. The similarity between mechanical ruling and laser recording is that adjacent concave surfaces, such as "pistons" in the US patent application named "Structured Screens for Controlled Spreading of Light," exist at relative vertical offsets. When it seems the difference. For some screen application types, reduced efficiency is allowed. However, in other cases, a reduction in focusing efficiency cannot be tolerated.

Referring to Fig. 9, the requirement of the arrangement between the tops of the concave surfaces of the concave surface-embossed structure is compatible with requiring an array in which the focusing characteristics of the individual microlenses vary randomly. In this case, the vertices of the concave surface are not required to be aligned. Similar principles apply to two-dimensional arrays.

The surface-embossed structure obtained by laser exposure after development supplies an initial mold that can be used for replication. If the material constituting the photosensitive film is suitable for replication, the replica of the master can be easily processed into convex form. If concave replication is required, an intermediate replication step is required, which requires a convex mechanism, thereby producing a concave array. Typically, since the photosensitive film is not suitable for mass replication, the mold is preferably made of, for example, a stronger plastic resin.

10A to 10C illustrate the replication process.

10A shows a concave initial surface relief structure 101 with aligned tops. Symbol 102 represents a glass substrate. 10B illustrates another substrate 103 on which the plastic resin 104 is deposited. FIG. 10C is a diagram showing the replication result of the intermediate replication mechanism of FIG. 10B for fabricating the desired array of convex microlenses 105.

The similar results shown in FIG. 10 can be used for the fabrication of high efficiency, high fill factor arrays of concave microlenses again having an original surface relief structure for forming positive resists in concave shapes.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims (23)

In the manufacturing method of a microlens array having a surface structure having a peak and a valley and comprising a plurality of unit cells and a plurality of microlenses, one microlens per unit cell, (a) providing a positive photoresist; (b) exposing the positive photoresist with a laser beam having a finite beam width to form a master having a surface structure where the surface structure of the microlens array is substantially negative; (c) (i) fabricate a microlens array, and / or (ii) fabricate another mask used to form the microlens array, and / or (iii) using the master to produce another first series of masters used to form the microlens array, And the microlens array comprises at least two convex microlenses in adjacent unit cells so that the master includes at least two concave surfaces in adjacent unit cells. The method of claim 1, wherein the microlens array includes only convex microlenses so that the master includes only concave surfaces. 3. The method of claim 2, wherein the master is placed between the first and second planes, the concave surface extending to the master in a direction from the first plane to the second plane, the maximum sag of each concave surface being the first Method for producing a microlens array, characterized in that the plane. 3. The method of claim 2, wherein the master is located between the first plane and the second plane, the concave surface extending to the master in a direction from the first plane to the second plane, the maximum of each concave surface relative to the first plane The sag position varies between at least some adjacent unit cells slowly enough so that the focusing efficiency of the microlens array is not reduced below 75%. The method of claim 1, wherein the master lies between the first plane and the second plane, at least two concave planes extending from the first plane to the master in a direction from the first plane to the top of the at least two concave planes. And a distance between the first plane and the first plane is different from each other. The method of claim 5, wherein the distances are randomly distributed. The method of claim 1, wherein at least one of the at least two concave surfaces is anamorphic. The method of claim 1, wherein the microlens array has a focusing efficiency of at least 75%. The method of claim 1, wherein the microlens array has a focusing efficiency of at least 85%. The method of claim 1, wherein the microlens array has a focusing efficiency of at least 95%. The method of claim 1, wherein the fill factor of the microlens array is at least 90%. The method of claim 1, wherein the fill factor of the microlens array is at least 95%. The method of claim 1, wherein the fill factor of the microlens array is substantially equal to 100%. A microlens array comprising a plurality of unit cells and a plurality of microlenses, one microlens per unit cell, wherein the array has a focusing efficiency of at least 75%. 15. The microlens array of claim 14, wherein said array has a focusing efficiency of at least 85%. 15. The microlens array of claim 14, wherein said array has a focusing efficiency of at least 95%. 15. The microlens array of claim 14, wherein the fill factor of the array is at least 90%. 15. The microlens array of claim 14, wherein the fill factor of said array is at least 95%. 15. The microlens array of claim 14, wherein the fill factor of the array is substantially equal to 100%. 15. The microlens array of claim 14, wherein said microlenses are convex microlenses. 15. The microlens array of claim 14, wherein at least some of the microlenses are anamorphic. 15. The microlens array of claim 14, wherein at least two of the microlenses are randomly different from each other. The microlens array of claim 14, wherein the unit cells are closely packed.