Literature review of refractive microlenses fabrication

The study of microlenses has been an area of activity for many years. Hooke studied the effect of melting the ends of Venetian glass rods to produce microscope objectives in the 17th century [34]. One of the more recent methods used to produce arrays of microlens arrays is the photoresist reflow method, suggested by Popovic in 1988 [35].

Therefore, some of the special methods used to produce refractive microlens shall now be discussed in more detail.

(A) The use of surface tension for microlens fabrication

The use of surface tension is a common method to fabricate refractive lenses. Based on the use of surface tension, the developed methods for microlens fabrication are listed below.

․Single-layer photoresist reflow method [36]

The simplest and most ingenious way to create microlenses is to make a resist pattern in the shape of disks. Fig.2.4 shows the microlens fabrication flowchart by single-layer photoresist reflow method. In this case one exposes the resist of thickness L through a mask made up of clear circles with diameter D. Upon development, this turns into the disk shape. One now merely heats the substrate to melt the photoresist. The surface force controls the shape and a segment of a sphere represents the minimum energy surface.

․ Double-layer photoresist reflow method [37]

Fig.2.4 shows the microlens fabrication flowchart by double-layer photoresist reflow method. Two polymer layers were coated onto a silicon substrate. The upper layer was a photoresist. The lower layer was a polyimide material. The polyimide was expected to form a pedestal to sustain the ball lens after the heat reflow process. Once the patterned polymer is heated above its glass transition temperature, the melting polymer surface will change into a spherical profile for minimizing its surface energy. A successful micro-ball array was formed in the photoresist through the different glass transition temperatures between two polymer materials. The interactive force between two material interfaces caused by surface tension causes the upper profile to form a spherical profile. This also forms the polyimide pedestal into a trapezoid with arc sides.

․Vertical photoresist reflow method [38]

Vertical reflow microlenses (VRM) are fabricated by combining the thermal reflow

method with succeeding silylative treatment on the lens surface (shown in Fig.2.6). By virtue of the strong adhesive force and cohesion, the VRM is formed by hanging the photoresist on a standingwall and then reflowing the photoresist. In order to enhance the thermal stability and reliability, the reflowed VRM is exposed to a silylative reagent to form a hard shell.

․Liquid microjet method[39]

Fig.2.7 shows the microlens fabrication flowchart by liquid microjet method. The use of the microjet technology, developed primarily for printing, has also been adapted to making microlenses. The analogous microjet system used a piezoelectric ceramic with a microchannel machined in it with a nozzle on one end connected to a reservoir on the other. An electrical pulse bends the channel and forces a droplet through the aperture. The droplet is directed to a substrate which is mounted on an XYZ micropositioner; the liquid drop solidifies on contact with the substrate and surface tension causes a spherical surface to form.

․Liquid dipping method [40]

Fig.2.8 shows the microlens fabrication flowchart by liquid dipping method. Polymeric vertical microlenses (PVMs) using photoresist material SU-8 are fabricated with a simple and low-cost process. By virtue of the strong adhesive force and liquid cohesion, the PVMs are formed by hanging the liquid SU-8 on walls by a dip method. Then, in order to enhance the thermal stability and reliability of the PVMs, the lenses are baked and exposed in the ultraviolet light to crosslink the SU-8.

․Hydrophobic effect [41]

Fig.2.9 shows the microlens fabrication flowchart by hydrophobic effect. In the first step, an adhesive hydrophobic layer is mechanically applied to the substrate. Regardless of what material is used, the substrate can then be lithographically patterned and the hydrophobic layer selectively etched from the exposed regions. The substrate is then dipped into and withdrawn from a UV-curable-monomer solution, the monomer will self-assemble into lenses on the hydrophilic domains. After a UV cure the lenses become hard and stable.

․Laser heating [42]

such as to produce local melting at the focal spot. Fig.2.10 shows the microlens fabrication flowchart by laser heating method. The explanation put forward for the formation of the bump is that the glass is initially melted and then resolidified. Since the soft glass with the higher temperature has a lower density, the excess volume wells up out of the colder solid substrate glass and reforms upon solidification into a lenslike shape.

(B) The application of Diffusion for microlens fabrication

․Bulk micromachining technology [43]

Fig.2.11 shows the microlens fabrication flowchart by bulk micromachining technology. Boron diffusion and etching selectivity with respect to boron density in an ethylenediamine pyrocatechol (EDP) etchant are utilized. When the EDP etchant meets the heavily boron doped p++ layer (above 2.5*10¹⁹ atoms/cm3), the etching is nearly self-stopped. As a result, Si microlens is formed.

․One-Step wet etching technology [44]

Refractive semiconductor microlenses were fabricated by using a diffusion-limited chemical etching technique based on Br2 solution. Fig.2.12 shows the microlens fabrication flowchart by One-Step wet etching technology. In this etching process, the etched profile is heavily dependent on the diffusion dynamics of the etching species, especially around etch-mask boundaries where the differential change in diffusion (and thus in etching rate) is rapid. The process for microlens fabrication also utilizes the nature of the diffusion-limited etching.

․Isotropic etch of ICP technology [45]

Fig.2.13 shows the microlens fabrication flowchart by isotropic etch of ICP technology.

The isotropic etching properties of an inductively coupled plasma (ICP) etcher for masked and maskless etching steps in reference to fabrication of a silicon microlens mold.

The silicon etching is performed with a continuous SF6 based ICP. For the masked etching step a consistent picture of the profile evolution is obtained, including a relation between the etching depth, the radius of curvature of the profile, the etching time and the size of the mask opening. For the maskless etching step, the optimal etch is purely isotropic.

(C) Photolithgraphy for microlens fabrication

․Proximity printing method [46]

Fig.2.14 shows the microlens fabrication flowchart by proximity printing method. This way to fabricate microlens arrays is presented by controlling the printing gap in the UV lithography process. The proximity printing bends the UV light away from the aperture edges and produces a certain exposure in the photoresist outside the aperture edges due to the diffraction effects. This causes the photoresist bottom in two adjacent patterns to link together after development. The fabricated microlens diameter has the same size as the pitch distance between the two apertures.

․Diffusion photolithography technology [47]

Fig.2.15 shows the microlens fabrication flowchart by diffusion photolithography technology. This simple and effective method to fabricate a plastic microlens array with controllable shape and high fill-factor, which utilizes the conventional lithography and plastic replication, is presented. The only difference from conventional lithography is the insertion of a diffuser that randomizes paths of the incident ultraviolet (UV) light to form lens-like 3D latent image in a thick positive photoresist. After replication of the developed concave microlens mold onto the polydimethylsiloxane (PDMS), PDMS microlens arrays are achieved.

․Gray-tone photolithography technology [48]

Fig.2.16 shows the microlens fabrication flowchart by gray-tone photolithography technology. The principle of a gray-tone mask for fabricating the microlenses involves modulation of the incident light by a different transparent area on the mask and by the number of laser pulses. Conventionally, there are three methods used to encode a 3D profile into the gray-tone mask layout: (1) pulse width modulation (PWM), (2) pulse density modulation (PDM) and (3) a combination of PWM and PDM. Fig. 2.16 illustrates the different layouts between PWM and PDM and modulation of transmissions of thus designed gray-tone masks. When the transmission of the mask changes from 25% to 50%, the transparent holes in PWM broaden, while the area of the transparent holes remains constant but the density increases in PDM. Base on the energy distribution through gray-tone mask in the UV lithography process, the microlens structures can be directly fabricated.

․Deep lithography with protons [49]

Fig.2.17 shows the microlens fabrication flowchart by deep lithography with protons.

In the first step of the microlens fabrication process, the PMMA sample is bombarded with an 8.3 MeV proton beam which is generated by a cyclotron and is shaped by a highaspect-ratio noncontact metal mask, featuring a high precision circular aperture. The shape of this aperture is directly projected onto the PMMA sample where the impinging high-energy protons split the polymer chains and create free radicals, hence reducing the molecular weight of the polymeric material. In the second step, the in-diffusion of the monomer causes a volume expansion which results in hemispherical surfaces because of the circular footprint of the irradiated zones. Because the degraded PMMA chains are incapable, because of their length, to diffuse towards the plates in the monomer vapor, the surface evolution of the PMMA plates is only determined by the diffusion of the monomer vapor in the bombarded zones and its accompanying volume expansion. The final shape of the swollen regions is determined by a balance between the internal stresses caused by the diffused monomers in the outwards direction, the surface tension and the gravitation force acting on the swollen layer.

․Ultraviolet-cured polymer [50]

The selected lens materials used for the UV-curing method should have a large change in volume without any physical damage during photopolymerization. Fig.2.18 shows the microlens fabrication flowchart by ultraviolet-cured polymer. First, the selected materials are poured into a cell that consists of a flat glassbottomed plate. A chromium mask with periodically arranged circular apertures deposited on a glass substrate is then placed on top of the cell. Next, the liquid monomer is irradiated from the upper side for 1 min with a metal-halide lamp that has a radiation spectrum ranging from 200 to 400 nm. The UV light induces photopolymerization, and the medium increases in density as it changes from the liquid to the solid phase. Because of this contraction effect, the molecules in the shaded regions of the medium moved into the irradiated regions to join the polymerization process, forming protrusions on both surfaces of the polymer at positions corresponding to the circular apertures.

․Photothermal process [51]

A photochemical modification to fabricate microlenses is presented by the use of a

special photosensitive glass. The basis of the effect is generated by a photonucleation of phase crystallization within the glass which produces a physical change in density. The total crystal content is of the order of 10-20% volume. Under the appropriate exposure pattern this density change can be used to produce surface features that ultimately act as lenses. A schematic of the process is shown in Fig.2.19. The photopatterning is done by conventional photolithographic technologies. After the glass is exposed, it is heated to about 600℃ to effect the crystallization. The results are circular regions where the light is blocked, surrounded by crystallization regions of higher density. The effect of this is to squeeze the soft unexposed glass beyond the surface that then forms a minimum energy surface. The surface bumps are formed on both.

․Vacuum-ultraviolet F2 laser [52]

A photochemical modification to fabricate microlenses is presented. Fig.2.20 shows the microlens fabrication flowchart by vacuum-ultraviolet F2 laser. The silicone surface irradiated by an F2 laser beam swells and is modified to SiO2 by means of photochemical reaction. When the surface is irradiated under a suitable condition of short laser pulses, it becomes smooth and spherical.

(E) The use of PDMS for microlens fabrication

․ Micro-transfer molding with soft mold[53]

A novel technique for fabricating polymeric microlens arrays based on micro-transfer molding with soft mold is presented. Fig.2.21 shows the microlens fabrication flowchart by micro-transfer molding with soft mold. The soft mold with a micro-holes array is made by casting a pre-polymer of PDMS against a silicon master. The silicon master of the micro-cylinders array is prepared using photolithography and deep reactive ion etching. During the micro-transfer molding operation, the surface of the soft mold of the micro-holes array is filled with liquid UV curable photopolymer, and the soft mold is then pressed against the flat substrate with a slight pressure for a period of time. After the soft mold is removed from the substrate, surface tension causes the liquid photopolymer cylinders to assume a spherical shape. Finally, the liquid photopolymer is cured by UV irradiation at room temperature. A substrate with a microlens array pattern can be successfully fabricated.

․Organic selective-area patterning method [54]

A novel method to fabricate a polymer microlens array based on a selective-area patterning method is presented. Fig.2.22 shows the microlens fabrication flowchart by organic selective-area patterning method. The surfaces of glass substrates were defined as either hydrophilic or hydrophobic regions by microcontact printing of self-assembled monolayers (SAMs). After spin coating of the prepolymer on the substrate, the microlenses were self-organized on the defined regions. Finally, the microlenses were completed by UV curing.

․Soft roller stamping process [55]

An innovative technique for rapid fabrication of ultraviolet-cured polymer microlens arrays based on soft roller stamping process is presented. Fig.2.23 shows the microlens fabrication flowchart by soft roller stamping process. In this method, a soft roller with microlens array cavity is made by casting a pre-polymer of polydimethylsiloxane (PDMS) in a plastic master of microlens array. The plastic master is prepared using gas-assisted hot embossing of polycarbonate (PC) film over a silicon mold with micro-holes array.

The microlens array cavity on the soft roller is filled with liquid UV curable polymer first.

The roller rolls and stamps over the traveling transparent substrate. The microlens array pattern is formed. At the same time, the pattern on the substrate is cured by the UV light radiation while traveling through the rolling zone.

(F) The use of microspheres for microlens fabrication

․ Microballs severed directly as microlens[56]

Self-assembled 2-D arrays of microspheres act as dense-packed arrays of microlenses and generate hexagonal dense arrays of micropatterns. Fig.2.24 shows the microlens fabrication flowchart by microballs severed directly as microlens. Use of 2-D microballs arrays, which can cause directly incident illumination to converge or diverge and can generate 2-D periodic optical patterns. These 2-D microballs arrays for photolithography and demonstrated that the lens arrays can generate 2-D arrays of uniform micropatterns over areas of several square centimeters.

․Microballs severed directly as molds [57]

A facile, reproducible soft-lithography-based method for fabricating hexagonally

close-packed microlens arrays by templating the surface of a colloidal monolayer, which is formed by spin-casting monodisperse polystyrene microsphere, is described. Fig.2.25 shows the microlens fabrication flowchart by microballs severed directly as molds. The relief structure of colloidal monolayers has successfully generated PDMS elastomers with hexagonal arrays of hemispherical air voids. Closely packed hemispherical microlens arrays were imprinted on ultraviolet-curable photopolymers which are bound on glass substrates.

․Replication by the use of microball [58]

Fig.2.26 shows the microlens fabrication flowchart by replication form microballs.

Microporous polymer films with a hexagonal arrangement of pores were prepared by simple casting of various polymer solutions under humid conditions. Hexagonally packed micropores were prepared by using condensed water droplets as templates on the surface of polymer solutions. Spherical micro lens arrays (MLAs) were fabricated simply by molding from the resulting honeycomb structures. By peeling off the upper layer with adhesive tape, the pillars were severed, forming pins on each layer, and a hexagonal array of pincushion structures was generated by this procedure. Hemispherical MLAs were also fabricated by molding the pincushion structures.

․ Suspension method [59]

A technique for producing ordered microlens arrays which is based entirely on self-assembly of charged latex particles spread at the oil–water interface is developed.

Fig.2.27 shows the microlens fabrication flowchart by suspension method. The method includes using of a gel trapping technique and replication of the ordered particle monolayers by casting with curable PDMS. Microlens arrays have been fabricated by taking an inverse replica of the PDMS template with a photopolymer. This method allows immediate control of the microlens array lattice constant by using different amounts of particles spread at the original oil–water interface.