局部湿蚀刻法制备硅玻璃凹微透镜阵列

Abstract: A simple and efficient technique for large-area manufacturing of  concave microlens arrays (MLAs) on silica glasses with femtosecond (fs)- laser-enhanced chemical wet etching is demonstrated. By means of fs laser  in situ irradiations followed by the hydrofluoric acid etching process, large  area close-packed rectangular and hexagonal concave MLAs with diameters  less than a hundred of micrometers are fabricated within a few hours. The  fabricated MLAs exhibit excellent surface quality and uniformity. In  contrast to the classic thermal reflow process, the presented technique is a  maskless process and allows the flexible control of the size, shape and the  packing pattern of the MLAs by adjusting the parameters such as the pulse  energy, the number of shots and etching time.

During the last decade, some techniques for the fabrication of concave MLAs have been  proposed. For example, a concave refractive microlens array was fabricated in solgel glasses  by a proximity-effect-assisted reflow technique, which was proposed in 2004 [14]. Later on,  Ruffieux and associates developed a two-step process for fabrication of diffraction limited  concave microlens arrays [11]. First, cylindrical holes were produced by the photolithograph  method followed by a melting step preformed at 150°C for half an hour; the melted structures  were then filled by a second spinning step of photoresist. Other techniques such as 3D diffuser  lithograph and “breath figures” method [15,16], which were employed to fabricate concave  molds, can also fabricate the concave MLAs. Moreover, through the reversal replication  technique, the concave MLAs could be produced by the convex MLAs. We notice that most  approaches to the fabrication of concave MLAs are based on the process using the  photomasks, which are very expensive. The maskless processes such as LDW are complex  and inefficient, which are not suitable for the large area fabrication of concave MLAs. Herein,  a simple, high-efficient maskless technique for the concave MLAs is developed using a  femtosecond (fs) laser-induced crater arrays followed by a chemical etching process. This  method simplifies the classic laser etching process [17,18], improving the fabrication  efficiency significantly. In addition, it allows direct manufacturing of various concave MLAs  on glasses, which have better physical and chemical properties than the photoresist,  polymethylmethacrylate (PMMA) or polydimethylsiloxane (PDMS), and more importantly,  the reflection loss of lights caused by the interfaces between the polymer layers and substrates  is not exist. Other advantages, such as simple process, facile processing environment and  flexible control of the size, the packing pattern and the shape of the MLAs by adjusting some  parameters, are also demonstrated.

The rectangular and hexagonal-packed concave MLAs are fabricated by a three-step process,  as depicted in Fig. 1. Initially, ablation-induced craters with diameters of a few micrometers  are induced on polished silica glass chips (10 × 10 × 1 mm3 , China Daheng Group Inc., GCL- 1202) using a 30-fs and 800-nm laser pulses at a repetition of 1 kHz (the laser source is a Ti:  sapphire pulsed laser oscillator-amplifier system). The femtosecond laser, owing to its  advantages of negligible thermal and shockwave-induced damages [19], when focused by an  objective lens (NA = 0.5), can easily induce craters on transparent materials such as silica  glasses without melting-ejections and cracks which will impact on the morphology of the  fabricated microlenses. The diameter of the focal spot is about 1.4 µm (1/e). The pulse energy  can be varied by a variable neutral density filter and the number of shots is controlled by a  shutter. More details of the setup used here can be found in Ref [20]. Subsequently, the  samples with craters are treated in 5% hydrofluoric (HF) acid solution assisted by an  ultrasonic bath at 23°C. During this process, the chemical etching velocity is accelerated in  the laser-induced craters and the concave spherical surfaces begin to form; the MLAs are  consequently fabricated in tens of minutes. Finally, the samples are cleaned by the ultrasonic  bath in acetone, alcohol and deionized water for 15 minutes, respectively, and dried in  ambient air.

Fig1

In the experiment, the whole fabrication process is monitored by an optical microscope (OM)  equipped with a CCD camera. Figure 2 shows the evolution of the rectangular and hexagonal  MLAs changing from the laser-induced crater arrays. In the beginning, the isotropic chemical  etching occurs in the laser treated points [black dots shown in Figs. 1(a) and 1(d)], producing  circular-shaped concave structures in the rectangular [Fig. 1(b)] or hexagonal [Fig. 1(e)]  patterns, which can serve as the circular MLAs. Then the aperture diameter of the circular  microlenses expands gradually with the chemical etching, and eventually, the adjacent ones  “overlapped” with each other, resulting in the formation of the tetragonal and hexagonal  shaped microlenses, as shown in Figs. 1(c) and 1(f). It demonstrates that the packing pattern  and the shape of the microlenses can be easily controlled by the arrangement of the laser  irradiated points and the chemical etching time. The areas of the fabricated rectangular and  hexagonal MLAs are about 3 × 3 mm2  and 1.5 × 1.5 mm2 , respectively, and the whole  processing time is about 3 hours, which is more efficient than the LDW process [20].  Figures 3(a) and 3(b) show the SEM images of the rectangular and hexagonal MLAs,  respectively. They visually express the excellent surface quality and uniformity of the MLAs.

To evaluate the focal length of the MLAs, fexp, an optical system equipped with a He-He  laser (633 nm), a computer-controlled stage, a lens and a CCD camera, is built up, as shown in  Fig. 5. Moving the computer-controlled stage along the direction parallel to the laser beam (zaxis), the position of the top surface and the focal point can determined by the images  captured by the CCD camera. The values of fexp are obtained by equation, fexp = L - h, where L is the distance between the focal point and the top surface of the MLAs. The focal length of  the rectangular and the hexagonal MLAs is 125.0 ± 5 µm and 84.0 ± 5 µm, respectively.  These values match well with the calculated focal length, fcal, which are obtained by  equations: fcal = R/[n-1] (2), where n denotes for the refractive index of the silica glass at  wavelength of 633 nm. Considering n = 1.45, the values of fcal are 128.80 µm and 81.06 µm  for the rectangular and hexagonal MLAs. Furthermore, the values of numerical aperture, NA,  for both MLAs are calculated by equation: NA = D/2f (3), and the results are 0.26 and 0.19,  respectively. The experimental results are summarized in Table 2.