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Abstract
In this study, a biomimetic compound eye (BCE) was realized on diamond by combining thermal reflow with dry etching techniques. Firstly, photoresist pillars were developed on diamond surface by standard photolithography. Then, these pillars were reflowed on a hotplate to form spherical segment patterns. Furthermore, dry etching technique was used to transfer these patterns into diamond surface to form the convex curve surface with diameter of 300 μm, on which, ommatidia with diameter of 18 μm and space of 35 μm were fabricated with the same processes to obtain BCE. Finally, the as-fabricated diamond BCE was characterized, indicating a well-uniformity according to the point spread function and exhibiting clear images of the testing pattern in projection experiment, which is expected to work under harsh conditions such as high intensity irradiation and strong acid.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nature compound eyes of insects are commonly comprised of tens to thousands of spherical-distributed and hexagonal-shaped ommatidia, and have many advantages of wide field of view (FOV), fast temporal resolution, high sensitivity for detecting moving targets and high energy utilization ratio [1–4]. Inspired by the nature compound eyes, researchers hope to develop BCE structures and apply them to medical endoscopes, panoramic imaging, micro aircraft navigation, wearable devices, high-speed target capture and accurate guidance [1, 5–13]. To date, a great number of methods have been proposed to fabricate BCEs with polymer materials such as photoresist [14,15] and PMMA [5,6,12]. Nevertheless, those polymer BCE structures are easily deformed under harsh conditions (such as high temperature and high pressure) and their life time is limited. Therefore, scientists have to search for other BCE materials to meet the special requirement. Due to its outstanding properties, such as high hardness, high thermal conductivity, stable chemical inertness and wide optical transmission bandwidth, diamond is an ideal material for fabricating optical devices which could be used in harsh conditions. As a consequence, much attention has been attracted to fabricate diamond microlenses with specific parameters and applied to various fields, such as lasers, homogenizers and photodetectors [16–22]. However, few are focused on BCEs on curved surface of diamond because it is hard to process.
In this work, the diamond BCE structure was fabricated by two-step approach via thermal reflow and dry etching techniques and characterized by optical experiments. The morphology and the optical performance of as-fabricated diamond BCE were characterized, which indicates that this simple method is practical to fabricate such 3D microstructures for optical application.
2. Experiment
The sample used in this work was a high pressure and high temperature (HPHT) (001) Ib single crystal diamond with area of 3×3 mm2. The fabrication process of diamond BCE is illustrated in Fig. 1. First, thick layer of SPR 220-7.0 photoresist was coated for two times onto the diamond substrate with a speed of 4000 rpm, resulting in a PR thickness of approximately 10 μm. Second, the photolithography process was employed to pattern the PR layer film in order to form PR pillars with a diameter of 300 μm. Third, the sample was baked on a hotplate under the temperature of 160 °C for 20 s to reflow the pillars to spherical segments. Finally, by using an inductively coupled plasma (ICP) etch process, the PR spherical segments were transferred into diamond surface.
Via the process illustrated above, the diamond microlens was fabricated, serving as the curved surface of the BCE. The similar process was adopted to fabricate the ommatidia on the curved surface. During the second fabrication process, SPR 220-3.0 PR was utilized and the diameter of the PR ommatidia was 20 μm. Thus, the diamond BCE with an overall size on a micrometer scale was obtained. The ICP processing conditions were as follows: 450 W ICP coil power and 25 W bias power. The etching gas was a mixture of O2 and Ar with flow rates of 40 and 15 sccm, respectively.
3. Result and discussion
3.1. Geometrical tests
Geometry of the diamond BCE was characterized with various methods. An optical microscope (OM) (Olympus, BX-51) was utilized to obtain a general 2D picture. Figure 2 shows the OM images of the PR spherical segment, the diamond spherical substrate, the PR ommatidia on the curved diamond surface and the fabricated diamond BCE. The PR spherical segment was obtained by reflowing the initial pillar with diameter of 300 μm. As demonstrated in Fig. 2, the diameters of the PR spherical segment in Fig. 2(a) and the diamond spherical substrate of BCE in Fig. 2(b) are 296 μm and 298 μm, respectively, which are almost the same as that of the designed PR pillar.
Fig. 2 OM images of (a) the PR spherical segment, (b) the diamond spherical substrate, (c) the PR ommatidia on the curved surface and (d) the fabricated diamond BCE.
Figure 3 shows the SEM images of the fabricated BCE with a tilted angle of 45°. As can be seen from the overall view of a BCE in Fig. 3(a), the ommatidia align orderly on the surface of the bottom spherical segment with a low aspect ratio. Thus the spherical substrate of the BCE is not apparent according to the image. To have a clear sight of ommatidia, the close-up image of the ommatidia were obtained and are shown in Fig. 3(b). The surface smoothness is important for optical devices. However, etch process will induce defects of the sample mainly due to the ion bombardment. To minimize the effect, a low bias power of 25W was used in our experiment to reduce the energy of the ion bombardment. AFM measurement was taken at different areas on the diamond BCE sample. The AFM images are shown in Fig. 4. The root-mean-square (RMS) values of the surface roughness of the planar area, curved area between ommatidia and ommatidium area of 10 × 10 μm2 on the diamond sample are 0.529 nm, 0.517 nm and 1.05 nm, respectively, indicating a low value of roughness. The nanometers roughness value measured at the top of the ommatidium is mainly due to pits on the ommatidium surface caused by the air bubbles existing between interface of PR and substrate.
Fig. 3 SEM images (tilt 45°) of the fabricated BCE: (a) overall morphology; (b) zoom-in picture of ommatidia.
Fig. 4 AFM images of different areas on fabricated diamond BCE sample with area of 10 × 10 μm2: (a) planar area; (b) curved area between ommatidia after flattening (the curved surface as the reference background has been substrated); (c) ommatidium area; (d) ommatidium area after flattening (the curved surface as the reference background has been substrated).
To investigate the 3D profile of the diamond BCE, a laser scanning confocal microscope (LSCM) (OLS4000, Olympus) and stylus profiler (DEKTAK-XT) were used and the results are shown in Fig. 5. Figure 5(a) shows the overall mophology of the BCE. As we can see, the sample exhibits a good BCE shape that ommatidia align periodically on the curved surface. To obtain the sizes of the as-fabricated BCE, cross-section profiles of the spherical substrate and ommatidia were measured as shown in Figs. 5(b) – 5(d). These cross-section profiles were circle fitted and indicated in the red line plot. The heights of the PR spherical segment and the diamond spherical substrate before fabricating ommatidia are 11.8 μm and 853 nm, respectively, shown in Fig. 5(b), which indicates a low etch selectivity (defined as the ratio of diamond etch rate to that of PR) of 0.072. This low etch selectivity suggests the difficulties to process diamond due to its hardness and chemical inertness. Figure 5(c) is the center cross section profile of the as-fabricated diamond BCE. Furthermore, the surface profile of a single ommatidium was sample out in the center of the BCE and is shown in Fig. 5(d). As shown, the profile of the center ommatidium matches well with the red circle fitted profile, and the diameter and height of the center ommatidium are 17.58 μm and 139.3 nm, respectively.
Fig. 5 (a) Image of the fabricated diamond BCE with laser scanning confocal microscope measurement. (b) Cross-sectional profiles in the center of the PR spherical segment and the diamond spherical substrate. (c) Cross-sectional profiles in the center of the diamond BCE micro-structure. (d) Cross-sectional profiles of a single ommatidium of BCE in the box in Fig. 5 (c).
The radii of curvature (ROC) of the diamond spherical substrate and ommatidium are calculated by Eq. (1)
where ROC is radius of curvature, h is spherical segment height and r is half of the spherical segment diameter. Therefore, the ROCs of the diamond spherical substrate and ommatidium were calculated to be 13 mm and 277 μm, respectively. Based on the geometry and optical theory, focal length is determined by Eq. (2) where f is focal length, n is refractive index. Here, refractive index of diamond is 2.42 [23], and the focal length was calculated to be 195 μm.3.2. Optical tests
To characterize the optical properties of the diamond BCE, point spread function (PSF) and projection performances were tested. First, the PSF was tested by an optical measurement system depicted in Fig. 6(a). In the system, a He-Ne laser beam with a wavelength of 632.8 nm was restricted by an aperture. Next, the beam was reflected by a 45° tilted mirror and transmitted perpendicularly through the diamond BCE. The beam passing through BCE sample was then captured by microscope and detected by CCD detector. The focal point image was captured by focusing the microscope at the focal plane of the BCE. Figure 6(b) shows the lateral intensity distribution of measured 2D PSFs of ommatidia of the BCE by illustrating random selected light spots in the focal point image. The measured PSFs of ommatidia were relatively consistent, which indicates a good uniformity of the optical performance. It’s also indicated that ommatidia of the BCE can form a sharp focusing point on the focal plane due to the uniformity and excellent profiles of ommatidia of BCE. Furthermore, the sample was laser irradiated for 4 hours during the experiment, the focusing point of each ommatidium can be clearly observed with the irradiation time increasing. This manifests the long-term performance stability of the diamond BCE.
Fig. 6 PSF test of fabricated diamond BCE: (a) configuration of the test setup; (b) 2D PSFs of the ommatidia.
Second, an optical microscopy system was carried out to demonstrate the imaging properties of as-fabricated diamond BCE shown in Fig. 7(a). Firstly, the diamond BCE was fixed on the movable sample stage of an optical microscope and illuminated with white light from below. Between white light source and BCE, a photomask with a transparent letter “Y” was positioned for projection. Then, the light passing through the photomask was focused by each diamond ommatidium and projected on the CCD of the optical microscope.
Fig. 7 (a) Simplified setup for the projection measurements. (b) Images are projected by the ommatidia of BCE through the objective lens with a magnification of ×20 and ×50.
As a consequence, an array of “Y” on the false plane of the ommatidia is clearly observed through the objective lens in Fig. 7(b). As can be seen, most of the obtained images are clear without obvious deformation and uniform in size and shape, which indicates a good imaging property and uniformity. While, the brightness of the images is a little bit different, which may be caused by the stains on the back side of the sample.
4. Conclusion
In conclusion, we have demonstrated a simple method to fabricate monolithic diamond BCEs by thermal reflow process and ICP etch technique. Characterized by two optical measurements, the focal spots of ommatidia of BCE are uniform in size and intensity, and the BCE can form clear images of the testing pattern, which indicate that ommatidia of the fabricated diamond BCE have a good uniformity and excellent profiles. Besides, compared with BCEs made of other materials, diamond BCEs are more stable and expected to have a quite longer life time.
Funding
National Key R&D Program of China (2017YFB0402800); National Natural Science Foundation of China (NSFC) (61705176, 61627812); China Postdoctoral Science Foundation (2017M620449, 2018T111057).
Acknowledgments
This work was supported by Dongguan Introduction Program of Leading Innovative and Entrepreneurial Talents. The authors are thankful to Ms. Nan Zhu from State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University for her help in AFM measurement.
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