Conclusions

A transparent and self-assembled MLAs fabricated by use of the hydrophilic effect under UV/ozone treatment was presented and demonstrated experimentally. This method provides a fast, low cost, no etch-transfer, no lithography fabrication processes.

The MLA was made of negative photoresist SU-8 (n=1.63 @ 530 nm) on a glass substrate. The focal length of MLA is controllable by changing UV/ozone treatment time. A larger shadow mask opening and a longer UV/ozone treatment time produced MLA with longer focal length. MLAs of 50 μm, 100 μm, 200 μm diameters for one, two, three, and four minutes UV/ozone treatment time has been fabricated successfully. The numerical apertures of microlens were from 0.06 to 0.19. The focal lengths were from 0.06 mm to 2.78 mm. The surface roughness was less than 0.07 μm and the shape was nearly spherical. The yield rate was higher than 96 %. The corresponding PV and RMS values of wavefront were lower than 0.949 λ and 0.143 λ.

References

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Chapter 3 Fabrication of Transparent and Self-Assembled Microlens Array Using Hydrophilic Effect and Electric Fielding Pulling

In this work, we present a variable and self-assembled spherical microlens array (MLA) fabricated by the use of the hydrophilic effect under ultra-violet (UV)/ozone treatment. The optical power and surface roughness of MLA was further enhanced by applying external electric field. This method provides a fast, simple, and low cost process, because it does not require heating, or etch-transfer processes. The MLA was made of negative photoresist SU-8 (n = 1.63) on a glass substrate. Microlenses from 50 μm to 200 μm diameters with one, two, three, and four minutes UV/ozone treatment time were successfully fabricated. The optical focusing power of 100-μm diameter MLA was also improved by using electric field of 1.7 V/μm, and 3.4 V/μm. A 10-μm thick shadow mask was used to define the UV/ozone treatment area to create more 2.91 mm of spherical MLA were experimentally demonstrated and investigated.

3.1 Introduction

Micro-lens array (MLA) is an important component which is used widely in many applications, such as increasing the light extraction efficiency [1], Shack Hartmann wavefront sensor [2], beam shaping for illumination [3], light gathering for solar collectors [4], and information displays [5]. There have been many techniques to fabricate micro-lens array, including thermal reflow [6], UV resin stamping [7], laser ablation [8], ink-jet printing [9], direct laser writing [10], and gray-scale mask photolithography [11] methods. Most of these mentioned methods have the drawbacks of high facility cost, time consumption, alignment inaccuracy, or high temperature process. The main problem of the high temperature process in some of these methods is inducing device defects in several applications, such as backlight module of liquid crystal display (LCD) and the encapsulation of light emitting diode (LED). There is another approach to fabricate MLAs by the use of hydrophobic effect [12, 13]. This approach allows accurate and direct fabrication of polymer microlens array without heating. Thus, using the hydrophobic effect to fabricate MLAs is a simpler way.

However, it still requires either an etch-transfer process or lithography to define hydrophobic boundaries, which increases the cost and process complexity. Therefore, a cost effective and low temperature process is still desirable to fabricate MLAs.

SU-8 photoresist (MicroChem, MA) has high optical transmittance from visible to near-infrared wavelength and high refractive index (~1.63) [14]. Furthermore, it has better chemical resistance and mechanical strength than other popular polymers, such as polycarbonate or poly-methyl-methacrylate (PMMA). As a result, SU-8 photoresist is

footprint of SU-8 photoresist liquid droplet was fixed on a substrate and the center part, or sag height, was pulled up. This made a higher sag height and a corresponding larger optical power [16]. tension energy on the SU-8 photoresist base layer of more-hydrophilic circular zones. In some applications, a larger optical power MLA is desirable. Therefore, an external electric field was applied to increase the optical power and to smooth the surface roughness before UV curing of SU-8 photoresist. The presented method provides a fast, low cost, and no etch-transfer process. The entire fabrication was done in room temperature to prevent it from thermal residual stress and material issues. The radius of curvature of the microlens can be easily controlled by the design of shadow mask opening, the UV/ozone treatment time, and the external electric field. In the following sections, the working principle and fabrication process are discussed. This work investigates the focal length of MLA under different lens diameters, the UV/ozone treatment time and the electric field strength. Experimental results of optical microscopy images, focus spot diagrams, lens profiles, surface roughness, and scanning electric microscope (SEM) images of the fabricated MLA were presented in this work.