激光增强湿法蚀刻制造的大规模高质量玻璃微透镜阵列

Abstract: Large-scale high quality microlens arrays (MLAs) play an  important role in enhancing the imaging quality of CCD and CMOS as well  as the light extraction efficiency of LEDs and OLEDs. To meet the  requirement in MLAs’ wide application areas, a rapid fabrication method to  fabricate large-scale MLAs with high quality, high fill factor and high  uniformity is needed, especially on the glass substrate. In this paper, we  present a simple and cost-efficient approach to the development of both  concave and convex large-scale microlens arrays (MLAs) by using  femtosecond laser wet etching method and replication technique. A largescale high quality square-shaped microlens array with 512 × 512 units was  fabricated.The unit size is 20 × 20 μm2 on the whole scale of 1 × 1 cm2 . Its  perfect uniformity and optical performance are demonstrated.

Microlens arrays (MLAs) are crucial optical devices because of its broad applications in  micro-optical systems, optical fiber coupling, artificial compound eye structures, light  diffusers and optical sensing technology . However, the scale of MLAs remains a big  obstacle, which restricts its further practical applications. Large-scale MLAs play an  irreplaceable role in high resolution imaging and enhancement of light extraction efficiency  of light emitting diodes (LEDs) and organic light-emitting diodes (OLEDs) . Notably,  most of the image sensors are packed with square unit cells. In order to match with them, the  MLAs should be fabricated square-shaped with high fill factor. Image sensors used in the  high definition imaging field nowadays, like charge-coupled device (CCD) and  complementary metal oxide semiconductor (CMOS), can hardly achieve a 100% pixel  aperture ratios. The square-shaped MLAs can help to improve the ratio by focusing light on  the photosensitive areas . That will enhance the CCD and CMOS devices’ signal to noise  ratio (SNR), and improve its imaging capability. Furthermore, MLAs help to lower image  sensors’ sensitivity threshold, achieving the approximate imaging effect of starlight CCD by  using ordinary CCD. In solid state light market, low light out-coupling efficiency restricts the  wide application of OLEDs and LEDs. In order to improve the output efficiency, microlens  array is combined with OLEDs and LEDs. The output power of OLEDs and LEDs covered  with microlens array could be enhanced by 2 times .

In this paper, we present a femtosecond laser wet etching technique , to fabricate  high quality large-scale concave MLAs on glass. By improving the crafts and parameters, we  solve the problems of efficiency, uniformity and quality. A large-scale concave MLAs with  more than 260,000 units was fabricated.

Theoretically, the fabrication procedure of the large-scale square-shaped MLAs is basically  the same with the small-scale ones. However, in the actual fabrication process there are some  factors, including the flatness of the sample, continuous operation ability of the mechanical  shutter and the irradiance uniformity of fs-laser and so on, which restrict the feasibility to  fabricate large-scale MLAs. Therefore, the crafts and parameters are improved to fabricate it.  The optimizational crafts and parameters will be discussed in Section 4.

Here we show the process of fabricating MLAs by fs-laser wet etching method. As shown  in Fig. 1, the fabrication process can be divided into a four-step process, including fs-laser  exposure, wet etching, replica molding and cleaning . Firstly, fs-laser pulses (800 nm, 50  fs, 1 KHz) with laser power (P) of 5mW were focused onto the surface of silica glass by an  objective lens (NA = 0.5), generating a square-arranged laser-modified spots array [Fig. 1(a)].  The laser exposure craters were generated point-by-point. For each point, the exposure time  could be controlled by a fast mechanical shutter and the laser power could be tuned through a  variable density filter. Subsequently, the post-preparative sample was treated with 3%  hydrofluoric (HF) acid solution at room temperature. The square-arranged craters were  transformed into square-shaped concave structures after the wet etching process [Fig. 1(b)].  During the etching process, it is worth to point out that an ultrasonic bath is used to guarantee  the conformity of the microlenses and high speed of the etching process. The ultrasonic bath  is conductive to removing the products and bubbles produced during the etching process.  Then, the convex MLAs could be replicated by pouring liquid PDMS onto the square-shaped  concave structures and keeping it at the temperature of 80 °C for about 60 minutes [Fig. 1(c)].  Finally, we could get square-shaped convex MLAs on the surface of solidified PDMS by  removing the mold [Fig. 1(d)]. Additionally, owing to thermoplastic effect of PMMA, convex  MLAs could be also fabricated by pressing PMMA chip onto the mold at the temperature of  120 °C for several minutes.

Fig1

Figures 3(a) and 3(b) show the SEM images of the square-shaped MLAs captured from  different angles and different magnifications, which excellently reveal the MLAs’ perfect  surface quality and uniformity. Figures 3(c) and 3(d) present the three-dimensional (3D)  morphologies and cross-sectional profiles of the concave structures which were observed by a  CLSM. The inserted figure in Fig. 3(d) shows the SEM cross-sectional profiles of the concave  structures. The side length of the square-shaped concave microlens is 19.967 μm, which is  very close to the interspaces (20 μm) that we set at the beginning, and the depth of the  microlens is 2.352 μm. The curvature radius, R, of the square-shaped concave MLAs can be  figured out by a simple equation: R = (h2  + r2 )/2h (1), where h is the depth of the microlens, r  is a half of the side length of the squared-shaped microlens. After the calculation, the result of  the curvature radius of the concave microlens is 43.56 μm. The focal length of the microlens  array, f, can be calculated by the equation: f = R/(n-1) (2), where n is the value of the  refractive index of the silica glass at wavelength of 633 nm. Considering the approximation of  n, 1.45, the focal length of microlens array equals 96.8 μm.