The fabrication 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 wavelength ranged between 200nm and 1100nm

Fig4.19 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

Fig.4.20 Focused spot measurement (a) Schematic showing the experimental setup (b) focused spot image

Table 4.1 Fabrication parameter for film thickness and microlens height The height of a microlens

The thickness of a

PMMA films (μm) Φ=50μm Φ=100μm Φ=150μm

22 15.4 24.8 30.3

20 14.7 22.9 28.5

15 14.0 18.6 25.6

12 12.3 16.2 24.5

Table 4.2 The refractive index and color of various powders

Type of powderes

TiO₂ ZnO ZrO CaCO₃ Al₂O₃ SiO₂ Refractive

index of powders

2.76 2.02 2.17 1.58 1.7 1.55

Color of powders

White Light yellow

White Off-white White Translucent