Fabrication of microlens array with graduated sags using UV proximity printing method
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- Yang, H., Chao, C., Lin, T. et al. Microsyst Technol (2005) 12: 82. doi:10.1007/s00542-005-0025-7
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A graduated microlens array is presented in this paper. The proposed device has the same aperture microlens with a gradually increasing sag in the substrate. The design produces gradual decrease in the focal length and intensity when the light passes through the graduated microlens array. This paper presents a new graduated microlens array fabrication method that uses a variable printing gap in the UV lithography process. This method can precisely control the geometric profile of each microlens array without using the thermal reflow process. The angles between the mask and photoresist were placed at 5°, 8°, 10°, 15°, and 20° using a fixture designed in this study. The mask patterns were ellipses with an isosceles triangle arrangement to compensate for the partial geometry.
KeywordsMicrolens arrayUV proximity printingGradual sag heightMicro-optics
Liquid crystal displays, mobile phone panels, and personal digital accessories require light sources. These light sources must be made brighter and longer lasting while reducing the device power consumption, size, and weight. While both inorganic and organic light emitting diodes (referred to as LEDs and OLEDs, respectively) are used in device displays, only a small fraction of the light generated in these devices can escape due to the total internal reflection in the high refractive index substrate. Enhancement of the outcoming light efficiency was achieved using an ordered array of microlenses (Moller and Forrest 2002). They showed that a microlens array increased the external quantum efficiency of OLEDs by a factor of at least 1.5, resulting in a considerable decrease in power consumption for a particular emission intensity. The integrated microlens array also provides interesting applications for various fields such as enhancing the illumination brightness and simplifying light-guide module construction. In a laptop display, a 25% increase in light output was reported when using this microlens technology (Ezell 2001). Other micro-optical functions and devices, such as focal plane optical concentration, optical efficiency enhancements, color separation, beam shaping, and miniature optical scanning, have shown potential for this technology. Micro-manufacturing technology allows compact and mini-features to be fabricated. Micro-electro-mechanical system (MEMS) technology offers a wide range of applications for the military, industrial, and consumer markets. Numerous academic and research institutions have been involved in the development of MEMS technology and commercial products. Component miniaturization is a common objective in electro-optical systems. Miniaturizing devices using micro-optics promise to revolutionize many electro-optical systems—from video cameras, video phones, compact disk data storage, robotic vision, optical scanners, and high-definition projection displays (Motamedi 1994). Higher accuracy and lower microlens fabrication costs are needed to meet the rapid growth of these commercial devices.
Micro-scale refractive lenses offer several important features: significantly reduced wavelength sensitivity compared to diffractive optics (necessary for broadband applications), the possibility of very large numerical apertures, and high light efficiency (Sinzinger and Jahns 1999). Several fabrication techniques have been applied to refractive microlens fabrication processes. One way to fabricate refractive microlenses is by melting cylindrical photoresist posts (Hutley 1990). This is known as microlens reflow processing. The photoresist cylinders are formed first using a lithographic process and heated above the photoresist glass temperature. Surface tension causes the photoresist cylinders to assume a spherical shape. The focal lengths of the resulting microlenses depend on the surface on which the resist is reflowed and the contact angle between the resist and the surface. By using a resist base layer, it is possible to obtain small contact angles and thereby relatively long focal lengths (Schilling et al. 2000). The reflow process produces large microlens arrays. This process is extraordinary compared to conventional macro-optic fabrication methods. In very large-scale integration-based processing techniques, coherent refractive microlens arrays are made on a silicon surface using a combination of lithography and reactive ion etching (RIE) techniques (Matamedi et al. 1991). Multi-level photoresist mask patterning and sequential RIE are used to form binary optic-microlens arrays.
A laser writing system for continuous-relief micro-optical element fabrication in photoresist was described by Gale et al. (1994). The photoresist-coated substrate was exposed using x–y raster scanning under a focused HeCd laser beam (λ = 442 nm), synchronously programmable controlled in intensity to write two-dimensional exposed patterns. Further development of three-dimensional (3-D) micro-structures with analogous topology using excimer laser (λ = 248 nm) ablation produced versatile micro-optic applications (Zimmer et al. 1996). Microlens arrays with lateral dimensions from 10 to 1,000 μm and profile heights up to 10 μm were fabricated using this technique. An optimal gray scale mask is necessary to produce fine roughness. Micro-optics printing technology offers printing a number of droplets onto a substrate and forming circular microlens arrays (Cox et al. 2001). Microlenses ranging in diameter from 20 μm to 5 mm have been fabricated. The piezoelectric actuator-based and drop-on-demand mode of ink-jet printing was developed to control different fluid volumes. Liquid droplets were dispensed onto a substrate to form refractive microlens arrays.
Using deep X-ray lithography to fabricate micro-optical components shows great mass production potential (Gottert and Mohr 1991). Lee et al. (2002) used the modified LIGA process to fabricate microlenses by melting the deep X-ray irradiated pattern in a PMMA substrate. Micro-optical components of any desired shape with smooth and vertical sidewalls, lateral dimensions in the micrometer range, and heights up to several hundred micrometers can be achieved. Following a molding process (either injection molding or hot embossing), optical component mass production can be achieved (Yang et al. 2001). The microlens array mold or mold inserts play an important role in the mass production molding process. This replication process promises the desired profile as final products.
A considerably less expensive option is to use an UV exposure tool. The UV radiation is limited by aerial image degradation as the image propagates through the thick photoresist layer. In addition to diffraction, run-out from uncollimated light or absorption in the photoresist easily dominates the aerial image problems in thick films (Dentinger et al. 2002). The proximity printing resolution is influenced mainly by the exposure geometry, resist properties, and light diffraction. Diffraction-induced exposure variations reduce the imaging capabilities in proximity printing. However, it is possible to construct a thick resist with a 3-D structure (Henke et al. 1990). The thermal reflow process can be eliminated in microlens array fabrication. The cycle time and thermal budget in the fabrication process can therefore be reduced. This new microlens array fabrication method using UV proximity printing generates uniform hemispherical microlens arrays (Lin et al. 2003). Variable sag is required in light-guide plates with microlens arrays. A similar geometry involves varied prisms for the light-guide plate resulting in the luminance rising 20% (Tanaka 2003). Thus, a research target to provide variable microlens sags for the light-guide plate fabrication will be useful.
2 Experimental method
2.1 Exposure characteristics
Lithographic exposure operations using the proximity mode are in the near field or Fresnel diffraction regime. The diffraction pattern resulting from the light passing through the mask directly impacts onto the photoresist surface because there is no lens between the photoresist and mask. The created aerial image therefore depends on the near field diffraction pattern. Because of the diffraction effects, the light bends away from the aperture edges and produces partial exposure outside the aperture edges. Although the contact mode can minimize these effects by reducing the gap to zero, the gap is not strictly zero in practice because the top surface of the photoresist is not perfectly flat.
2.2 Experiment design
Microlens array fabrication using proximity printing was reported in our previous study (Lin et al. 2003). The printing gap between the mask and photoresist substrate should be twice the designed pattern or greater. This will produce the desired microlens curvature profile. The microlens sag after development was less than the initial photoresist thickness because of the diffractive exposure effect. As the printing gap between the photoresist and mask gradually increased, the height and sternness of the microlens at the lateral sideline became thinner and flatter. A small printing gap is not suitable for microlens array fabrication because the light intensity distribution in the photoresist will have insufficient diffraction to produce spherical structures. Printing gaps ranging from 240 to 840 μm using the same pattern on the mask can generate microlens arrays in photoresist with various curvature radii, focal lengths, and numerical apertures.
3 Fabrication process
Related data for various tilt angles for aperture diameter 80 μm microlens array fabrication
4 Results and discussion
4.1 Proximity printing gap size effect
4.2 Parameters of microlens array with gradual sags
Microlens sags and their focal lengths when using various mask tilt angles
Microlens arrays with gradual sags using the UV proximity printing method were successfully realized. A large mask tilt angle results in high varied sags and shorter microlens array focal lengths. This result may provide an alternative technique for advanced light-guide plate fabrication and other potential micro-optical applications in the future.
This work was supported by the National Science Council (series no. NSC92-2212-E-005-005) of Taiwan, R.O.C. Thanks are due to G. Marso Electronics Inc. (GME) for their cell phone panel module knowledge input to generate research interests.