1. Introduction

The microlens is an essential element for many optical devices that are applied in a wide range of fields such as photoelectric devices [1,2], light beam shaping [3], high-definition displays [4], micro/nanofabrication [5], integrated micro-optics [6], and bionic structures [7]. To date, a variety of methods have been proposed to fabricate microlenses including thermal reflow [8], droplet method [9], femtosecond laser machining [10] and electrochemical technology [11]. Most of these methods are used to fabricate microlenses with polymer materials, which could easily deform and fail under harsh conditions. For these reasons, considerable attention has been given to fabricate microlenses with hard materials such as GaN [12], sapphire [13], fused silica [14,15], diamond [16,17], and others, which have many potential applications under harsh and special conditions. Among these materials, diamond has prominent optical properties with a wide optical transmission bandwidth, large refractive index, and high Raman gain coefficient [18], making it an ideal candidate for micro-optics such as Raman laser [19] and optical nanocavity [20]. In addition, the high thermal conductivity, high hardness, and stable chemical inertness of diamond [18] greatly support its optical applications. This makes diamond optics very stable and resistant. Therefore, many scientists have conducted research on methods for fabricating diamond microlenses for high and stable performance. Nevertheless, diamond’s hardness and chemical inertness result in processing difficulties. Until now, the most common method to fabricate diamond microlens is thermal reflow followed by dry etching [21–23]. However, concave microlenses, which have extensive applications such as in diffusers [24] and laser beam shaping [25], cannot be fabricated using this well-established technique.

In this study, we proposed a simple approach to fabricate concave microlenses on a diamond. This approach includes spin coating process to form photoresist concave structures and dry etching technique to transfer the concave patterns onto diamond surface. Qiao Xu et al. presented a method for realizing polymer concave microlenses by spinning the acrylate resin over the modified and solidified polydimethylsiloxane holes [26]. However, acrylate resin is not resistant to etch. In our work, we used photoresist to form concave structures by the effect of interfacial tension and centrifugal force. After fabrication, the morphology and the optical performance of diamond concave microlenses were characterized. The fabricated microlens showed a spherical profile with a low surface roughness and performed well in basic imaging and focusing tests.

2. Experiment

The sample used in this study was a high-pressure and high-temperature (001) Ib single crystal diamond substrate with a dimension of 3 $\times$ 3 $\times$ 0.4 mm$^3$. The fabrication of microlenses on a diamond substrate is graphically illustrated in Fig. 1. First, a thick layer of SPR 220-7.0 photoresist was twice coated on the diamond substrate with a spin speed of 2000 rpm, resulting in a photoresist thickness of approximately 17 µm. Second, cylindrical holes were generated by photolithography in the photoresist layer with diameters of 100 µm and a center distance between adjacent holes of 140 µm. Some photoresist remained on the diamond substrate in the hole areas after photolithography, resulting in holes with depths of 14 µm. Third, the hole structures were filled with AZ 5214 photoresist and spun at a speed of 5000 rpm. Through the effects of centrifugal force and interfacial tension, concave structures with meniscus shapes were obtained. Finally, through an inductively coupled plasma (ICP) etching process, the photoresist concave structures were transferred onto the diamond substrate. The ICP processing conditions were: 450 W ICP coil power and 23 W bias power. The etching gas was a mixture of O$_2$ and Ar with flow rates of 40 and 15 sccm, respectively.

 

Fig. 1. Schematic of diamond concave microlenses fabrication process: (a) spin coating of SPR 220-7.0 photoresist on the diamond substrate; (b) photolithography to form cylindrical holes; (c) coating of AZ 5214 photoresist on the holes; (d) formation of concave meniscus structures by a spinning process; (e) transfer of the concave meniscus structures onto the diamond by dry etching.

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3. Results and discussion

The geometries and properties of the photoresist and diamond concave microstructures were characterized using various methods. Figure 2 shows a 2D image from an optical microscope (OM) (Olympus, BX-51) and a cross-section profile from a stylus profiler (DEKTAK-XT) of the photoresist concave structures obtained after the second spin coating process. The accuracy of OM is greater than or equal to 1 µm, and the vertical resolution of stylus profiler is 0.1 nm. During the second spin coating process, the AZ 5214 photoresist was spin coated and some of it reflowed to the thick regions between the holes due to the effect of interfacial tension between the two different photoresist layers. Therefore, it appears in Fig. 2(a) that the concave structures with meniscus shapes are closely arranged. Nevertheless, the surfaces in the regions between the holes do not fit the spherical surfaces. Therefore, it is considered that the hole areas within the white dot circle line in Fig. 2(a) are the regions of concave spherical structures. The cross-sectional profile was measured at the center of the concave structure with a length of approximately 100 µm (along the black dot-dashed line in Fig. 2(a)), which conformed well to a spherical curve with a depth of 2.21 µm and a radius of curvature (ROC) of 565 µm (as shown by the fitted red line). The depth is related to the depth of hole in the first photoresist layer, which would be affected by the thickness of the first photoresist layer and thickness of residual photoresist. Different concentration and viscosity of photoresist cause different interfacial tension between layers. The spin coating speed will affect the centrifugal force. Thus, concave structures with different depths and ROCs can be fabricated by changing the depth of first photoresist layer, concentration and viscosity of photoresist and spin coating speed during the second spin coating process.

 

Fig. 2. (a) OM image of PR concave structures by objective lens with a magnification of $\times$ 20. (b) Cross-sectional profile along the black dot-dashed line in Fig. 2(a).

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To obtain concave microlenses on a diamond, ICP etching was used to transfer photoresist concave structures onto the diamond substrate. The diamond substrate was loaded on a holder cooled with circulating purified water to maintain the sample temperature at approximately 20 $^{\circ }$C, thereby avoiding deformation of the photoresist concave structure mask, which might otherwise be caused by excess heat accumulation. Figure 3 shows 3D images and the cross-sectional profile by a white light interferometer (CCI 6000, Talysurf) with vertical resolution of 0.01 nm. As Fig. 3(c) shows, the diameter and depth of the diamond concave microlens were 100 µm and 198 nm, respectively, indicating a low etch selectivity (defined as the ratio of diamond etch rate to that of photoresist) of 0.090. This low etch selectivity manifests in difficulties in processing diamond because of its hardness and chemical inertness. The cross-sectional profile was circle fitted with an ROC of 6000 µm and is indicated in the red line plot as well. According to the theory of geometrical optics, the focal length (f) and numerical aperture (NA) of a spherical concave microlens can be calculated, where f and NA are determined by Eqs. (1) and (2), respectively, when the latter is small [27].

 

Fig. 3. 3D images of (a) four close-packed diamond concave microlenses and (b) a single diamond concave microlens using a white light interferometer measurement. (c) Cross-sectional profiles along the black dot-dashed line in Fig. 3(b).

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The low surface roughness is an essential standard for optical devices. Therefore, a lower bias power of 23 W was chosen to reduce the defects caused by the ion bombardment in the ICP etching process [29]. The root-mean-square values of surface roughness of three randomly selected $10\times 10$ µm$^2$ areas were all less than 1.5 nm measured by atomic force microscopy (Innova), which satisfies the requirement of optical applications. To eliminate the influence of the surface curvature, the results were obtained after a flattening operation (where the curved surface as the reference background was subtracted).

To demonstrate the optical performance of the diamond concave microlens, an optical microscopy system was conducted, as depicted in Fig. 4. The diamond microlens was fixed on the movable sample stage of an OM and illuminated with white light through a projection mask from below. The distance between the sample stage and the mask was set to 51 mm. The mask was a black glass plate with a transparent letter "F" with a diagonal length of 1.2 mm. According to geometrical optics, no real focal point exists when a light beam propagates through a concave microlens for its divergence property. However, the virtual image of letter "F" and virtual focal point can be obtained by charge-coupled device (CCD) camera at the crossing point of the light beam propagating in the negative direction. Consequently, the image of "F" on the false plane of the microlens was clearly observed through the objective lens and CCD camera in Fig. 4(c). The projected image had a diagonal length of 86 µm, and the magnification was 0.0717. We considered that the part of light beam irradiating diamond concave microlenses could be regarded as collimated beam for the size was small enough relative to the distance between mask and sample stage. Based on the geometrical optics, the focal length was calculated as 3.94 mm, which agreed well with the theoretical calculation by Eq. (1).

 

Fig. 4. Simplified setups for the (a) imaging and (b) focusing performance measurements. (c) Image projected by diamond concave microlenses through the objective lens with a magnification of $\times$ 10. 3D light intensity distributions of (d) the focusing image by objective lens with a magnification of $\times$ 50 in the insert graph and (e) simulated focal spot in the insert graph (Scale bar = 20 µm).

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Focusing performance was also tested, and the measurement setup was schematically shown in Fig. 4(b). The projection mask was removed, and as a result, a bright spot was observed on the false plane from the OM shown in Fig. 4(d). According to the 3D light intensity distribution of the focusing image, the full width at half maximum (FWHM) of light intensity is approximately 25 µm. The focal spot size is related to the NA of the diamond microlens [30]. The focal spot was also calculated by ray tracing simulation in Fig. 4(e). The light source was set to be a point-source of light with a wavelength of 600 nm. Initial parameters were set consistent with the experiment. The concave microlens was set with the ROC of 6000 µm, thickness of 0.4 mm, diameter of 100 µm and refractive index of 2.42 at wavelength of 600 nm. The light source was 51 mm away from the concave microlens. We added a convex lens with a focal length of 5 mm to trace the virtual focus spot of the light passing through the microlens. Then the spot was projected on the detector (10 mm away from the convex lens). The FWHM of intensity distribution of the simulated focal spot is 11 µm. The experiment result did not perfectly agree with the simulation, which might be caused by optical aberrations of OM and the achromatic light source.

4. Conclusion

A concave microlens with an ultralow NA was successfully fabricated on a diamond substrate through a spin coating process, followed by dry etching. The fabricated diamond concave microlens exhibited a low surface roughness. The optical performances were characterized through an optical microscopy system, revealing good imaging and focusing performances. We believe that the diamond concave microlens could have a wider range of applications, as it is more stable under harsh conditions and is expected to have considerably longer life as compared with microlenses made of other materials.