The character of PDMS microlenses

Because the coefficient of thermal expansion of PDMS is quite small, replication structures cast against a suspended PMMA mold is faithfully replicated in Step 4. Fig.4.4 reveals that surface relief of PDMS microlenses with different diameters have different heights at spinning coating rotating speed of 5000 rpm. Then, the optical character of PDMS was measured by UV-visible spectrophotometer. Fig. 4.5 shows a transmittance curve of PDMS as a function of incident wavelength. The PDMS transmittance is of approximately 85% at the wavelength range of 290 nm to 1100nm. This wavelength range contains ultraviolet rays, visible light, and a part of infrared rays. To compared with

poly(methyl methacrylate) (PMMA)(~90%), the transmittance of PDMS seems to be adopted. Hence, PDMS is surely a suitable material for lenses. Next, focusing properties of a 3 by 3 PDMS microlens arrays, of which each has diameters of 100 µm were examined through a optical setup consisted of a He–Ne laser diode as a light source, beam focusing and expanding lenses, filter, microscope objective, CCD camera, and imagine display and recording system. A schematic of the experimental setup is shown in Fig. 4.6. Fig. 4.7 shows a portion of the spot pattern produced by a 3 by 3 microlens array.

The imagine reveals strong focusing function of the microlens arrays. The theoretical diffraction-limited spot size of these microlenses is 2.68μm. Measurement shows the focused spots have the size of 2.88 μm. Finally, Fig. 4.8 shows that the root mean square surface roughness was about 1 nm in areas of 5*5μm² by AFM. The RMS values of the scanned surfaces were all within 8 nm, corresponding to a total integrated scattering within 5% for visible light. This is because the spin-coated PMMA film should be stayed in the liquid state for a reasonable time and with a certain film thickness so that the effect of the surface tension can modify the surface uniformity. Hence, PDMS microlens arrays with a high-quality surface could be obtained.

Parameters such as heighth , radius of curvatureR, focal length f , and F numbers are all usually employed to characterize the microlenses. For an axial symmetrical planoconvex lens, focal length, height, radius of curvature, diameter and the number are related through the following formulas expressed as [66]:

h refractive index of the material of the microlens. To estimate characteristics of the crolenses, we approximate the profile as parabolic the parabolic fit in Fig. 4.3, and thus, mi

K take to be 1. The corresponding focal lengths of the microlenses as a function of the thickness of the PMMA film is presented in Fig. 4.9. In these calculations, the refractive index is taken to be 1.41 for the PDMS mircolenses. These results demonstrate that the geometrical configuration of the microlenses can be regulated by suitable arrangement of

fabricated microlens is between 0.8 to 1.5.

4.

a. Hence, the fabricated diffuser can effectively expand a igh-intensity laser beam.

4.

2.4 The diffusers plate using PDMS microlenses

With the fast development of flexible display, PDMS microlens arrays could be attached to a flexible plastic substrate and be used in the flexible display. Due to the soft nature of PDMS, these microlenses would be not damaged by stresses which existed in deformable devices. However, it would be difficult for general microlenses to bend the underlay substrate, due to stress destruction. In addition, In order to increase this fill factor, adding another microlens array with the original microlens shifted halfway along the diagonal. This process can not only enhance the fill-factor in an underlay substrate but also be used as diffusers. A schematic diagram of diffusers plate using PDMS microlenses is shown in Fig.4.10. To understand how efficiency for the made diffuser is, a schematic diagram of the experimental setup for measuring efficiency of components is shown in Fig. 4.11 (a). The setup consisted of a laser diode as a light source, samples, screen and CCD camera, imagine display and recording system. As a laser beam propagated through the sample, an expended beam would be projected finally on the screen. A CCD camera can be acted as a detector and capture the image of intensity distribution of a laser beam. Therefore, optical intensity distributions can be clearly analyzed through commercial optical software called beam profile. Three kinds of cases, including a laser beam without any elements, with microlens arrays attached to one side of an underlay substrate, and with the made diffuser, were respectively examined. Three kinds of resulting images are shown in Fig. 4.11 (b)-(d), respectively. The intensity distribution for a high-intensity laser without any component is shown in Fig. 4.11 (b). In Fig. 4.11 (c), the original shape of the laser beam started to expand through the underlay substrate with microlens arrays, but the center of the expanded beam still kept a high intensity. However, the laser beam passed through the made diffuser, a uniform intensity could be clearly observed in Fig. 4.11 (d) and the center of the expanded beam was almost similar to other are

h

2.5 The fabrication of polymeric microlenses using soft replica molding

In most photoelectric systems, microlenses are made of inorganic material, such as

silica, diamond and sol–gel, due to its environment stability and a long life cycle. A simple approach to transfer polymeric microlens into inorganic ones is the technology of reactive ion etching technology. After etching, PDMS has a rugged and rough surface.

Hence, PDMS is not a good material for reactive ion etching process. In order to improve this problem, the best way is the use of commercial polymer. Therefore, PDMS is often served as materials for molds, due to low surface energy. Here we developed a novel technology to generate positive and negative PDMS molds which can be used to replica po

molding. Fig 4.12 shows the fabrication of polymeric icrolens by soft replica molding.

4.

lymeric structures by following our earlier works.

Based on the excimer laser microdrilling and the spin coating scheme, polymeric molds on a thin plastic pedestal sheet was proposed. A soft PDMS mold with microlens patterns is formed by a casting process. Specifically, a liquid PDMS mixture was cast onto the PMMA film obtained in Step 2. After baking at 70℃ for 30 min, the solidified PDMS mold can be easily stripped from the PMMA film. Then, a concave soft mold can be fabricated by a second replica molding using the convex PDMS mold. To make sure both the concave and convex PDMS molds can be separated after baking, the weight ratio of the silicone elastomer and the curing agent for the concave mold should be different from that for the convex mold. Here, the ratio for the concave mold is set at 5 to 1.

Finally, both concave and convex polymer microlens arrays can be fabricated by another replica molding by casting the desired polymeric liquid, for example PMMA, SU-8 and other photoresists, in the corresponding convex and concave molds, respectively. For the replica process of PDMS molds which are reproduce from another PDMS molds, we called this technology soft replica

m

2.6 The fabrication of various surface relief structures

Compared to photography lithography, pulsed eximer laser through metallic mask is flexible to create a corresponding microhole severed as a plastic pedestal and the depth of the pedestal also can be controlled by the number of laser shots or the power intensity. As the depth of a pedestal is deeper than the drooping depth of a liquid film, a parabolic shape is not restricted by the bottom of PC plate. Hence, microlenses can be fabricated.

However, the depth of the pedestal is shallower than the drooping depth of a liquid film.

The bottom of the spin-coated liquid film is able to touch the bottom of the pedestal during the spin-coating step. Instead of shaping a curved film, a flat bottom curved film is formed. After the process of soft replica molding, microlens array with flat top surface was fabricated. Fig. 4.13(a) shows a 4 by 4 microlens array with flat top surface. The height of this structure is about 8 lm and the width of a platform is about 100 lm that is shown in Fig. 4.13(b). With decreasing the depth of a pedestal, the width of a flat-top structure increases gradually. Hence, the size of a focused spot can be easily determined by changing the width of the platform. Such structures have been used in improving the ex

te planes on dual-layer disks or fo

ternal quantum efficiency of organic light emitting diodes [67].

Besides, a lens-on-lens structure can be also created by this technology. A concentric pedestal composed of an outer square and an inter square on the PC plate is considered.

Under our fabricated processes, the lens-on-lens structure is, for example, consisted of a smaller lens with a width of 200µm and the bigger lens with a width of 400µm and the smaller lens is stacked on the bigger one. Fig.4.14 shows a stacked structure of smaller lens on bigger lens. Based on this fact that the size of microholes can determine the curvature of a liquid film, both lenses have different focal lengths. Hence, the element can generate two focus spots but the spots have different focal length. Such an element has been applied in focusing laser light onto two separa

r multiple-layer optical tweezers configurations [68].

Although an eximer laser can easily create desired microholes through desired metallic masks, whereas the sidewall of microholes is not vertical under a high aspect ratio condition. The depth of microholes is more and more deep, leading the focused plane of the laser to go away from a suitable position. Hence, a cone-like microhole is fabricated, due to energy scatter. Therefore, the slanting angle for cone-like microholes is smaller than the curved face of a liquid film, leading to have no influences on the formation of the curved face. However, a pedestal combined two adjacent microholes, fabricated by laser microdrilling, have obviously influences. The middle of this pedestal has a support, due to a cone-like shape of microholes. A schematic diagram of the pedestal structure with the support formed through laser microdrilling is shown in Fig. 4.15(a). After coating a liquid film and soft replica molding, a double-peak structure is fabricated (shown in Fig. 4.15(b)). The double-peak structure consists of two single microlenses, of

which each has the diameter of 100µm. Due to the meddle support, two microlenses are in

mmetric structure is very difficult to fabricate by e use of traditional photolithography.

rolens 4.

r a reflective dependent.

For the function of the middle support, it can support a liquid film to form a complicated surface. For example, the special pedestal was created through a mask with a circular pattern. First, a circular hole was randomly fabricated on the PC plate. Then, moved the distance within a quarter of the diameter of a circular pattern from the original location, laser was again bombarded the PC plate until a suitable depth was achieved.

After coating a liquid film, an asymmetry structure can be fabricated. Fig.4.16 shows SEM image for the asymmetry structure. This structure consists of a lens-like pattern and a meniscus pattern. The lens-like pattern has the diameter of 100µm and a meniscus pattern has the width of 25µm. Such asy

th

4.3 The fabrication of reflective mic 3.1 Introduction of reflective microlens

Micro-optical elements with their own scale of a few to several hundred micrometers are extensively applied to optical devices using MEMS technology to collect, distribute and modify optical radiation. In general, micro-optical elements can be roughly grouped into three generic areas; one is refractive, another is diffractive and the other is reflective.

Refractive elements used in photoelectric systems are usually more popular than reflective ones. But reflective optics is useful for packaging an optical system into a smaller space than that which can be currently used for refractive optical elements [69].

Therefore, the optical characteristics of refractive elements depend mainly on the spectroscopic characteristics of photopolymerized resin used for its fabrication [69]. In order to eliminate this dependency, a reflective element could substitute fo

one and its optical performance was as good as that of a refractive element.

Generally speaking, reflective elements are essentially consisted of surface relief structures and metallic reflective coating. The focused ability of reflective elements mainly depends on surface relief structures and surface metal coatings. Therefore, various metal coatings could reflect light with a special region of wavelength. For example, Silver films have a good reflectance in the whole visible spectrum and in the

near-ultraviolet; Gold films have a good reflectance in the infrared region [70] Therefore, we hope to develop additionally a reflective function under analogous fabrication processes. Although one of most direct approaches is that a metallic film deposited on the PDMS surface, a PDMS layer covered a metallic film would form spontaneously wavy structures on the PDMS surface due to the different contraction rates of cooling between the metal film and the PDMS layer. Fig.4.17 shows the SEM image of wavy structures on the patterned surface of PDMS covered a metallic film with 100 nm thickness. These wavy structures on PDMS surface would cause the diffusion of light as light irradiated this surface. Therefore, metal films would easily crack under a small physical

eformation due to the different ductility between the metal film and the PDMS layer.

4.

e, the PDMS/TiO2 mixtures could su

d

3.2 The preparation of soft reflective materials

Because most of photoelectric systems have a light source with a visible-light wavelength ranged between 350 and 700 nm, we hoped to get a high-reflectance material in the visible-light region. PDMS mixtures mixed with various kinds of nanoscale powder, including silica (SiO2), titania (TiO2), calcium carbonate (CaCO3), zirconia (ZrO), zinc oxide (ZnO), and alumina (Al2O3)were respectively characterized by UV-VIS spectrophotometer. Fig. 4.18 shows a reflectance curve of PDMS mixture polymers as a function of incident wavelength ranged between 200nm and 1100nm. Consequently, we found that the reflectance of TiO2/ PDMS, which achieved 98%, was the most suitable candidate in these PDMS mixtures. The TiO2/ PDMS mixtures could reflect most of visible light and its reflectance value was better than the value of traditional aluminum coatings (90%) using as reflective films. Henc

bstitute for metal coatings to reflect visible light.

It has been known that the processes of absorption, refraction, scattering and other interactions would occur as light illuminated on any matter. In this study, light can be reflected by PDMS mixtures, which is due to light scattering phenomenon. As PDMS mixtures in which nanoscale particles distributed uniformly were illuminated by light, particles would absorb light with a special energy and then excited electrons in particles released other types of radiation such as ultraviolet rays, visible light and infrared rays in all directions when excited electrons returned ground state. Hence, scattering processes

can be simply expressed as: Scattering = excitation + reradiation [71]. In fact, light scattering processes involved many factors, such as the size and shape of particles, the type and property of materials and other corresponding factors etc. Additionally, the surface color of powders was also an important factor. For example, as the color of powders is close to white, it means that most of visible light is reflected; On the contrary, as

scattering formula, the intensity of light scattering can be roughly expressed as [71]:

the color of powders is black, it means most of light is absorbed.

Here, we adopted a simple model to explain experimental results. According to Rayleigh incident angle of light. The intensity of light scattering depended strongly on the size and refractive index of powders. Table 4.2 shows the refractive index and color of various powders. White TiO2 powders have a larger refractive index than other materials in Table 4.2, causing to have the best reflectance for TiO2/PDMS mixtures in Fig. 4.18. Then, the size influence of TiO2 powders was also considered. Fig4.19 shows a reflectance curve of TiO2 with three sizes, r =40nm, 400nm, and 45µm, /PDMS mixtures as a function of incident wavelength ranged between 200nm to 1100nm. We found that the intensity of light scattering decreased slightly with the increase of particle size. Experimental results were contradicted with Rayleigh formula in which the intensity of light scattering was proportional to the six power of particle size. We observed that bigger particles in high-viscosity polymers would aggregate easily to form a bulk and such bulk would deposit on the bottom of PDMS. This aggregate phenomenon led the non-uniform distribution of particles and lowered the intensity of light scattering. Hence, the best

llers should be nanosize powders with a high refractive index.

4.

fi

3.3 The fabrication and optical performance of soft reflective microlenses

Instead of PDMS without filler, PDMS with TiO2 powders was used as the mold’s material. It has been demonstrated that both of positive and negative molds can be

reflective microlenses without a metal coating, avoided the problem of wavy structures ge

ults demonstrated fabricated concave elements are capable of generating spot arrays.

neration on PDMS surface.

Then, the focusing property of the fabricated concave elements was examined experimentally. A schematic diagram of the experimental setup is shown in Fig. 4.20 (a).

This setup consisted of a lamp as a white light source, microscope, CCD camera, image display and a micrometer scale resolution Z-stage. A 2*2 concave element with 100μm diameter array was placed on the Z-stage and a lamp with a small inclined angle illuminated the surface of the concave element. The light would be focused on a spot by the reflection of concave surface structures. When the bright spot was arrived a minimal scale along the move of Z direction, this minimal spot was the focused spot of the concave element. The focused spots shown in Fig.4.20 (b) were about 4.2μm. The other spots in Fig.4.20 (b) were due to the light reflected form the edge of the concave element.

Hence, experiment res

Fig.4.1 Schematic for fabrication of PDMS arrays by replica molding

Deth(μm)

Fig.4.2 Surface profile of the PMMA film scanned by the α-step Distance (μm)

Fig.4.3 The scanned profile of a suspended PMMA film through the measure of α stepper

Fig.4.4 3-D images of microlens arrays by confocal microscope

Fig.4.5 A transmittance curve of PDMS as a function of incident wavelength

Fig.4.6 Schematic showing the experimental setup for measuring focused spots

Fig.4.7 The optical image of focused spots of microlens

Fig.4.8 Images of surface roughness by AFM

Focal length(μm)

Fig.4.9 Focal length of microlens as a function of film thickness Film thickness (μm)

Fig.4.10 A schematic diagram of diffusers plate using PDMS microlenses

Fig.4.11 Diffuser spot measurement: (a) schematic showing the experimental setup, (b) optical intensity of a laser beam without any component, (c) optical intensity of a laser beam with microlens arrays, and (d) optical intensity of a laser beam with diffusers

Fig.4.12 Schematic for fabrication of polymeric microlens by soft replica molding

Fig.4.13 SEM images of the microlens with flat top surface (a) a patterned array and (b) a single pattern

Fig.4.14 The SEM image for a bifocal structure of smaller lens stacked on bigger lens

Fig.4.15 (a) Schematic diagram of the pedestal structure with the support formed through laser microdrilling (b) the SEM image for a double-peak structure

Fig.4.16 shows SEM image for the asymmetry structure consisted of a lens-like pattern and a meniscus pattern

Fig4.17 The SEM image of wavy structures on the patterned surface of PDMS covered a metallic film with 100 nm thickness.

Fig 4.18 A reflectance curve of PDMS mixture polymers as a function of incident

Fig 4.18 A reflectance curve of PDMS mixture polymers as a function of incident