IL94456A - Method for manufacturing microlenses and other microstructures - Google Patents

Description

94456/2 METHOD FOR MANUFACTURING MICROLENSES AND OTHER MICROSTRUCTURES - 2 - 94456/2 The present Invention relates to a method for manufacturing a microlens or an array of mlcrolenses, or other microstructures.

Microlenses, i.e., lenses of a diameter from 1 m to 1 mm (and cylindrical lenses of corresponding dimensions) are widely used today, with even wider applications in sight. Present applications include use in CCD-cameras, facsimile systems, in CCD Image sensors where they serve for enhancement of quantum yield, in solar energy collection, and in optical computing, imaging and communication.

Therefore, specifications for microlenses, such as focal length, F-number and geometrical size are diverse to such an extent that existing techniques for producing such microlenses are generally unable to cover the whole range. Moreover, the trend today is towards fabrication techniques similar to those used in the production of integrated circuits, 1n order to Integrate microlenses and other microstructures and microelectronic devices.

Known methods for microlens fabrication are: 1) The mechanical method The classical methods for generating optical surfaces by mechanical means: turning, grinding, polishing or molding are of restricted applicability in the microlens field, mainly because of their limitations as to size. - 3 - 94456/2 2) Distributed-index method This method has been used to make a microlens array monolith-ically and is a great improvement compared to mechanical methods. By selectively diffusing a dopant into a planar substrate, a distributed-: index planar microlens is obtained. This type of planar microlens could be used 1n 2-D arrayed "Selfoc" or in a multichannel coupler in a single-mode fiber system.

The problems inherent in this technique are: a) Difficulty of dopant distribution control; b) Limitations of lens diameter to a size greater than 0.5 mm, and c) Relatively high cost of fabrication. 3) Resin thermal flow method In this method the microlens is produced by heating a resin which, by microlithographic methods, is deposited onto a substrate in the form of arrays of upright cylinders. Subsequently heating the cylinders, the material melts and flows to form the array of spherical microlenses. This method is fully compatible with IC fabrication and has a low cost level for high volume production.

Its limitations, however, are: a) Due to spreading of material during the heat treatment the ratio: diameter/thickness 1s limited to 3. This 1s a serious limitation on the range of microlens specifications which can be produced by this process; - 4 - 944562 b) Control of uniformity of the geometrical parameters of the microlens is very difficult. 4) 2-D Photoresist-developed relief vs. exposure technique In this process, similar to the resin thermal flow technique, microlenses are formed directly in photoresist material. This is done by exposing a positive or negative photoresist layer to a so-called tailored l ght-intensity distribution. After development, the profile of the photoresist is nearly identical to the spatial ligh intensity distribution. The final product -the shaped photoresist with a refractive index of n =1.6 - is the desired microlens.

This method is potentially compatible with IC fabrication technique, provided that an appropriate technique to achieve the tailored light distribution 1s used.

Two known techniques for tailoring the light intensity distribution are: Line raster mode exposure by an intensity-modulated laser beam, in which light distribution is controlled by an acousto-optical modulator; Holographic recording, in which a 2-dimensional recording is achieved by simultaneous exposure to three coherent laser beams.

The need for a very highly precise scan movement is an inherent limitation of the two techniques. Moreover, in the first method (light raster), an exposure time of about 15 hours is needed to create - 5 - 94456/2 an array (15x15 mm) of mlcrolenses with 45 microns diameter. The second method (holographic recording) 1s limited to the creation of microlenses with a maximum diameter of 10-15 microns. Because of these limitations, the compatibility of the IC fabrication technique with this process is very problematic.

A serious disadvantage common to the two latter methods resides in the fact that the developed photoresist which, as already explained constitutes the microlenses, is very sensitive to the corrosive effects of various chemical substances in either vapor at liquid form. Also, the use of these microlenses is limited to wavelengths to which the photoresist material is transparent. This would exclude several important ranges, particularly in the far infrared.

It is therefore one of the objects of the present invention to overcome the drawbacks and disadvantages of the prior art methods and provide a method that uses a novel tailored light-intensity distribution technique which, in combination with contact-printing photolithography or projection lithography, avoids the above-mentioned limitations, that is capable of covering the entire range of parameters of microlenses, microlens arrays and other mlcrostructures, 1s compatible with IC-production techniques, facilitates high output rates at substantially lower costs, and permits the use of any lens material, such as glass, quartz, sapphire, germanium, etc.

According to the Invention, this is achieved by providing a method for manufacturing a microlens or an array of microlenses, or - 6 - 94456/2 other microstructures, comprising the steps of providing a substrate made of a material transparent to the light to be refracted by said microlens or array of microlenses, or other microstructures, depositing on said substrate a layer of photoresist, covering said photoresist on said substrate with a mask exhibiting a pattern of varying light transmittance, exposing said photoresist through said mask, which mask produces a tailored light-intensity distribution, developing said exposed photoresist, thereby obtaining a surface configuration corresponding to the surface configuration of the desired microlens or array of microlenses or other microstructures.

The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.

With specific reference now to the figures 1n detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the Invention in more detail than 1s necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled 1n the art how the several forms of the Invention may be embodied 1n practice.

In the drawings: F1gs. 1a-1f Illustrate the various stages of preparation of a micro-lens array; F1g. 2 1s a graph Indicating the characteristics of a typical photoresist (Shipley AZ-1300); Fig. 3 shows a commercially available sine pattern mask; Fig. 4 represents the Intensity distribution of the light transmitted by a sine pattern mask; Fig. 5 1s a schematic drawing of a lens, illustrating the notation of Its physical parameters; F1g. 6 1s a schematic representation of a contact printing setup for the manufacturing of cylindrical mlcrolens arrays; F1g. 7 1s a similar llustration of a set-up for spherical mlcrolens arrays, and Fig. 8 1s a graph representing the Intensity distribution at the focus of such a lens.

Referring now to the drawings, there are seen 1n Figs, la-lf schematic representations of the steps of the method according to the Invention. The substrate 2 of Fig. 1a can be any material transparent to the wavelength the mlcrolens array to be produced 1s designed to handle. Some of the possible materials are glass, quartz, sapphire and germanium.

In Fig. lb, a photoresist layer 4 has been deposited on the substrate surface. The most suitable photoresist for the present purpose 1s the positive photoresist commonly used 1n semiconductor lithography. To produce a given relief pattern (I.e., the surface of the mlcrolens array), an exact knowledge 1s required of the photoresist characteristics (exposure as a function of the depth of layer to be removed by developing). A typical positive photoresist characteristic (Shipley AZ-1300) 1s shown 1n Fig. 2.

In Fig. 1c, a mask 6 has been placed on the photoresist applied 1n the previous step. Such masks, selection of which, having different frequencies 1s shown 1n Fig. 3, are strips, the light transmission of which periodically varies according to a sine law and the light passing which has consequently a sinusoidal Intensity distribution. These masks or sine patterns are commercially available, being routinely used for an entirely different purpose, namely the determination of the so-called modulation transfer function (MTF) which 1s an Important criterion 1n the assessment of the performance of optical systems. A graph Illustrating the Intensity distribution of the light transmitted by one of these masks 1s shown 1n Fig. 4, with the horlzonal scale representing distances of 780 micron/ division.

As they appear 1n Fig. 3, the sine patterns will produce arrays of cylindrical mlcrolenses. To produce arrays of spherical lenses, two Identical sine patterns are crossed. I.e., mutually offset angularly by 90°.

In Fig. Id, the photoresist 1s exposed, through the mask 6, to Irradiation, 1n this case, from a UV-source.

Schematized contact-printing setups for producing cylindrical and spherical arrays of mlcrolenses are shown 1n Figs. 6 and 7, respectively.

There 1s seen 1n Fig. 6 a base plate 10 to one edge of which 1s hlngedly articulated a (transparent) Hd 12. The photoresist-coated substrate 2 1s mounted on the base plate 10 and covered with the sine-pattern mask 6. The I1d 12 1s then closed, pressing the mask 6 tightly against the substrate 2. In this state, the setup 1s exposed to whatever light source 1s being used.

In F1g. 7 which represents the setup used to produce spherical lenses, second sets of sine patterns, 6', are superposed on the basic mask 6. The mutually crosswise orientation of the sine patterns 1s clearly seen.

The Importance of tight control of exposure has been pointed out earlier. The two optical characteristics of the lenses, namely the focal length f and the F-number F^ depend on the physical parameter R, the radius of curvature (apart, of course, from the Index of refraction n), and R being determined by the parameters r and h (see F1g.5). While r 1s defined by the frequency ("pitch") of the sine pattern, h 1s determined by the characteristics of the photoresist (e.g., type, thickness), which, once selected, become the Independent variables, and by exposure and, to some degree, also by development, which constitute the dependent variables.

Below are the expressions Unking the optical and physical parameters of the microlenses: R r2 + h2 (n-1) 2h(n-l) f r2 + h2 F 2r 4rh(n-1) For h«r r2 f 2h(n-l) r 4h(n-1) Having been exposed, the photoresist 4 1s developed and Its surface 8 1s now configured as an array of mlcrolenses, as shown in F1g. 1e* The mlcrolenses produced by the method according to the Invention have been tested and found of consistently high quality, as can be seen from Fig. 8, 1n which the horizontal scale represents 117 micron/ division.

While the Intensity distribution at the focus of one of the microlenses, shown 1n Fig. 8 (horizontal scale representing 117 m1cron/d1v1s1on) 1s Indicative of a resolving power close to the diffraction limit.

While for many practical purposes the thus developed photoresist 4 on Its substrate 2 constitutes a useful optical tool and the development stage can thus be regarded as the final method step, the arrays of microlenses consisting of the photoresist material Itself still suffer from the above-mentioned disadvantages, namely their vulnerability to chemical effects. Also, as mentioned earlier their use 1s limited to wavelengths to which the photoresist material 1s transparent, and that excludes several Important ranges, particularly 1n the far Infrared.

The method according to the Invention therefore provides another step, schematically Illustrated 1n Fig. If. There, the array, emerging from steps la-le 1s subjected to bombardment by an ion beam, which not only erodes or dry-etches away the photoresist array of microlenses, but "copies" 1t right Into the substrate 2, providing a true replica 8' of the original photoresist array surface 8 1n whatever material was chosen for the substrate. The time required for completion of the etching process can be substantially reduced by using the so-called reactive 1on beam process, 1n which a gas, reactive with the substrate material, participates 1n the etching process.

It will be evident to those skilled 1n the art that the Invention 1s not limited to the details of the foregoing Illustrative embodiments and that the present Invention may be embodied 1n other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered 1n all respects as Illustrative and not restrictive, the scope of the Invention being Indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore Intended to be embraced therein.

Claims (8)

- 13 - 94456/2 WHAT IS CLAIMED IS:

1. A method for manufacturing a microlens or an array of micro-lenses or other microstructures, comprising the steps of: providing a substrate made of a material transparent to the light to be refracted by said microlens or array of microlenses or other microstructures; depositing on said substrate a layer of photoresist; covering said photoresist on said substrate with a mask exhibiting a pattern of varying light transmittance; exposing said photoresist through said mask, which mask produces a tailored light-intensity distribution; developing said exposed photoresist, thereby obtaining a surface configuration corresponding to the surface configuration of the desired microlens or array of microlenses or other microstructures.

2. The method as claimed in claim 1, further comprising the step of copying said surface configuration onto said substrate by a process of etching.

3. The method as claimed in claim 1, wherein the pattern of said mask is a sinusoidal pattern, producing a sinusoidal Hght-intenslty distribution.

4. The method as claimed in claim 1, wherein said photoresist is exposed to said tailored light-intensity distribution produced by said mask in a contact-printing arrangement. - 14 - 94456/2

5. The method as claimed in claim 2, wherein said etching process 1s a dry-etching process.

6. The method as claimed 1n claim 5, wherein dry-etching is carried out by means of bombardment by an ion beam or by plasma etching.

7. The method as claimed 1n claim 5, wherein dry-etching is carried out by means of bombardment with a reactive ion beam or by reactive plasma etching.

8. A method for manufacturing a microlens or an array of micro-lenses or other microstructures as claimed in claim 1, substantially as hereinbefore described and with reference to the accompanying drawings. FOR THE APPLICANT WOLFF, BREGMAN AND GOLLER