GB2199156A - Lens manufacture - Google Patents

Abstract

A high quality lens is made quickly, simply and inexpensively by depositing on a substrate droplets of a substance in a liquid state, which is solidifiable such as to be optically transmissive; allowing the substance to assume, under the influence of surface tension, a body substantially lenticular in shape; and then solidifying the body.

Description

LENS MANUFACTURE The present invention relates to a method of manufacturing a lens, and to a lens produced by that method.

It has long been a problem in the art to produce, by simple methods, high resolution microlenses with good spot profiles, Existing methods of production are complex and, consequently, entail relatively high production costs. In addition, the lenses produced by such methods are not of as high a quality as would be desirable for their many applications.

An object of this invention is to produce good quality microlenses by a quick, simple and relatively inexpensive method which lends itself well to multiple production.

The present invention provides a method of manufacturing a lens, the method comprising: applying to a substrate a predetermined quantity of a substance when in a liquid state, the substance being solidifiable such as to be optically transmissive, the applying step including depositing a droplet of the substance onto the substrate; allowing the said substance to assume, under the influence of surface tension, a body substantially lenticular in shape, and then solidifying the said body.

Preferably, the body is allowed to assume a plano-convex shape.

Preferably, the solififying step includes curing of the body with ultra-violet radiation.

Preferably, the substrate used in the method has a structure comprising of a flat substrate; a first masking layer and a second layer of positive photoresist; the first and second layers being provided with a plurality of windows through tc the substrate, to act as recepticals for droplets of the substance.

This structure can be obtained using conventional photolithographic techniques.

According to another aspect of the invention there is provided apparatus for the production of microlenses the apparatus comprising: a masked substrate provided with a plurality of windows to act as recepticals for a quantity of a substance in liquid form placed therein; means to deposit a droplet of the substance into a re receptical to form a body substantially lenticular in shape; and means to solidify the body so formed.

In order that the invention may be more readily understood, a description is now given, by way of example only, reference being made to the accompanying drawings in which: Figure 1 shows a schematic vertical cross-section of a masked substrate with recepticals for use in the method embodying the present invention; Figures 2 and 3 show schematically a perspective view of an early stage in the manufacture of the masked substrate in Figure 1; Figure 4 shows a schematic vertical section of the substrate in a further stage of manufacture; Figures 5a and 5b show a schematic perspective and vertically cross-sectional view respectively of the substrate in a still further stage of manufacture; Figure 6 shows a schematic vertical cross-sectional view of the substrate in a yet further stage of manufacture.

In the method of manufacture herein described the masked substrate shown in Figure 1 acts as a receptical for the lenses during production and comprises; a flat glass substrate 1; a first layer of aluminuim 2 and a second layer of positive photoresist 3, the first and second layers 2 and 3 being provided with circular windows 4 through to the glass substrate 1.

A droplet of urethane acrylic resin in liquid form, refractive index 1.47, is transferred and deposited in the circular recepticals 4 in Figure 1, using a piece of stainless steel tubing. On making contact with the flat glass substrate 1 the liquid droplet assumes a circular shape similar to a plano-convex lens. The resin is then cured under ultraviolet light in a nitrogen atmosphere to produce a solid lens.

Stainless steel tubing of 150pm diameter is used to obtain lenses ranging from 54pom to 200pm in size, although a variety of tube diameters may be used depending on requirements. The transfer time and the tube-to-substrate distance are critical in determining the exact size and shape of the lenses.

Advantageously, the applicator tube is brought down at a constant speed (0.25 mm/s), until the meniscus of liquid that is suspended from the bottom of the tube just makes contact with the substrate. The downward motion if halted within 1 r of this position and the applicator is then held stationery ror a delay of 2ms, during which time a small quantity of liquid is transferred to the substrate by surface tension forces. At the end of this period the applicator is withdrawn at 0.6mm/s.

Larger diameter lenses can be formed by bringing the applicator closer to the substrate than the minimum distance of 1 m, or by using a longer delay time but this may produce a decrease in the optical performance of these larger lenses.

Although a urethane acrylic resin is used in the method as described above, any suitable liquid which can be solidified by either chemical or physical means may equally well be used.

The masked substrate shown in Figure 1 is prepared in a number of steps, some of which are shown schematically in Figures 2 to 6, and comprising: deposition a layer of positive photoresist 5 onto a flat glass substrate 6 (Figure 2); placing a standard mask 7 over the substrate 6, the mask being transparent to ultraviolet light, except for a plurality of chrome discs 8, which define the positions of the lenses; exposing the substrate 6, with mask 7 in place, to ultraviolet light shone through the mask 7; removing the mask 7 from the substrate 6 and treating the substrate 6 with a suitable solvent which removes that part of the positive photoresist layer 5 which has become soluble after exposure to ultraviolet light, while leaving in place, on the bare glass substrate 6, those parts 9 (Figure 3) which have been sheltered from the ultraviolet light by the chrome discs 8 in the mask 7;; vacuum depositing a layer of aluminuim 10 onto the substrate 6 and photoresist parts 9 (Figure 4); treating the layered substrate as shown in Figure 4, with a solvent which lifts off the positive photoresist parts 9 along with parts 11 of the aluminuim layer 10 to leave windows 12 as shown in Figures 5a and 5b; spinning a layer of positive photoresists 13 onto the remaining aluminuim layer 10 and the exposed part of substrate 6 (Figure 6); exposing the resulting layered substrate to ultraviolet light from the direction shown by arrows 14, so that the photoresit layer 13 is only exposed where there are holes 15 in the aluminuim layer 10; and then treating the layered substrate, as shown in Figure 6, with a suitable solvent to remove those parts of the photoresist layer 13 which lie in the holes 15 of the aluminuim layer 10.

It should be noted that chromium can equally well be used in place of aluminuim in the masked substrate of Figure 1.

Further to the above, it is found that the lenses formed by the method herein described are more regular in shape if the glass substrate 1 is coated with a suitable coating.

Particularly successful coatings are silicon monoxide (SiO) or magnesium fluoride (MgF2), which are applied before the photolithographic process described above or, alternatively, a coating of hexamethyldisilazone is applied after the photolithographic process. It is believed that such coatings act to eliminate, or reduce, random distribution of surface charge on the substrate.

The quality of the lenses produced by the method of the presentation can be assessed by measuring the focussed spot size obtained using a particular size of lens and comparing it with the theoretical value given by the well-known formula: Spot size = 1.22S N.A.

Where X is the wavelength of the light used, 1.22 is a mathematically calculated value well-known in the art and N.A.

is the numerical aperture, which is calculated from the diameter and focal length of the lens. The term "spot size" refers to the diameter of the first dark band of the spot that is the image obtained when light is shone through a lens.

The spot size is measured by expanding it and recording it photographically. Typical results for lenses produced by this method are: for a 16 cm diameter lens with a numerical aperture of 0.155 the measured spot size is 6.25cm compared with a theoretical value of 5Fm, that is 252 higher than the expected diameter.

If these results are compared with results for similar sized lenses produced by conventional methods, it is found that the results for the lenses produced by this method are much better. For example, a 180cm diameter lens with a numerical aperture of 0.19 made by conventional techniques has a measured spot size of 6.nn compared with a theoretical value of 4from, that is 40% higher than theory and therefore much worse than the results obtained for our lenses.

Thus, the method herein described provides a simple method of manufacturing a miorolens which lends itself well to multiple production and, in addition, produces a lens of better quality, with a superior spot profile, than that produced by conventional techniques.

In a modification, the droplets of resin can be deposited onto parts of the substrate which are not flat, but rather arcuate such as to produce a lens which is not plano-convex in shape but, for example, arc bi-convex.

The diameter and shape of a lens produced by the method of the present invention can be regulated and determined by controlling any one or more of the following: the volume of substance constituting a droplet for deposition, the viscosity of the substance, the surface energy between the substrate and the substance, , the application of an external force (e.g.

gravitational centripetal or electrical field) to the liquid.

The focal length of the lens produced by the method can be varied by changing the refractive index of the substance or by choosing another substance with an appropriate refractive index.

Claims (10)

1. A method of manufacturing a lens, the method comprising: applying to a substrate a predetermined quantity of a substance when in a liquid state, the substance being solidifiable such as to be optically transmissive, the applying step including depositing a droplet of the substance onto the substrate; allowing the said substance to assume, under the influence of surface tension, a body substantially lenticular in shape; and then solidifying the said body.

2. A method according to Claim 1, wherein the body is allowed to assume a plano-convex shape.

3. A method according to Claim 1 or Claim 2, wherein the solidifying step includes curing of the body with ultra-violet radiation.

4. A method according to Claim 3, wherein the solifification step includes curing under ultraviolet light in a nitrogen atmosphere.

5. A method according to any one of the preceding claims, wherein the substance incorporates a urethane acrylic resin.

6. A method according to any one of the preceding claims, wherein the substrate has a structure comprising of: a flat substrate; a first masking layer and a second layer of positive photoresist material, the first and second layers being provided with windows through to the substrate to act as recepticals for droplets of the substance.

7. A method according to any one of the preceding claims, wherein the substrate has a coating to inhibit random distribution of surface charge on the substrate.

8. A method according to Claim 8, wherein the coating is silicon monoxide, or magnesium fluoride or hexamethyldisilazone.

9. A method of manufacturing a lens, the method being substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

10. A lens made by the method of any one of Claims 1 to 9.