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We introduce a chemical reflow method to fabricate diamond microlenses. First, photoresist pillars developed by photolithography are reflowed in organic solvent vapor atmosphere at 20 °C to form spherical segment patterns on diamond substrate. The effects of chemical solvent type and reflow time on photoresist pattern profiles are investigated. Second, via dry etching, diamond microlenses are fabricated by transferring the spherical segment pattern into substrate. Furthermore, these diamond microlenses demonstrate low numerical aperture, well-controllable curvature, and good imaging performance with projecting experiment.
© 2017 Optical Society of America
1. Introduction
Microlens-based optical devices have diverse applications in a wide range of scientific fields such as optics, material science, microelectronics, medicine and so on [1–5]. With progress of fabrication technology, many materials such as GaN [6], SiC [7], sapphire [8] has been applied to make microlenses which indicate higher performances. In contrast with such materials, diamond has outstanding properties including wide transmittance range, large refractive index and high Raman gain coefficient, making it to be an ideal material for micro-optical applications such as Raman laser and vertical cavity surface-emitting laser [4, 9]. Additionally, due to its high thermal conductivity, high breakdown voltage and high carrier mobility, diamond is also a promising semiconductor material for high power electronic and optoelectronic devices. Based on geometry and optical theory, the optical properties of microlens are determined by its physical parameters such as radius and height [10]. Shallow microlenses, which mean low numerical aperture (NA) or large radius of curvature (ROC), have advantages in accuracy of wave front detection and chromatic aberration [11–13], as well as non-damage converged beam energy in solid body. Realization of such microlens in diamond would make relevant devices more stable under harsh conditions due to diamond’s hardness, chemical inertness and other properties mentioned above. Recently, it is reported that shallow microlens in diamond can considerably improve conversion efficiencies in Raman laser [4]. Besides, due to its smooth surface, microlens structures in diamond are also good molds for replication of plastic microlenses [14]. Thus more researchers are attracted to develop methods for fabrication of diamond microlenses for higher performance and stable relevant devices.
On the other hand, its hardness and chemical inertness lead to difficulties in patterning microlens structure in diamond. Generally, main ideas to fabricate diamond microlens contain two steps: first, preparation of photoresist (PR) pattern on diamond surface; second, transfer of PR pattern into diamond substrate. The first step is crucial to final diamond microlens since the shape of PR pattern directly determines the profile of microlens in substrate. Up to date, diamond microlenses have been fabricated with thermal reflow method [15–17]. Although, it allows a very accurate shaping of the lens profile, this technique is faced with difficulty in fabricating microlens with low NA or small lens height. This is mainly due to threshold of PR thickness below which a spherical shape cannot be formed during thermal reflow process [18]. Meanwhile, PR will swell but the diameter of pattern remains unchanged. Thus, it leads to an increase of PR segment height, which is about twice the initial PR thickness [16, 19], and this could hinder the fabrication of shallow diamond microlens. Furthermore, in thermal reflow method, curvature of microlens is mainly adjusted by controlling PR thickness as the diameter is almost fixed. Although the problem is improved with multiple-layer PR coating and lower etch selectivity [16, 18], the microlens size is increased and the process become complicated.
In this paper, a chemical reflow method with different kinds of solvent vapors was adopted to fabricate diamond microlenses. Relationship between profile of PR spherical segment pattern and reflow time was studied. Then, an inductively coupled plasma (ICP) etching process with low-cost Ar/O2 etch gas was applied to transfer PR pattern into diamond substrate. Our results indicate that diamond microlenses with ultralow NA and different curvatures can be obtained.
2. Expreimental details
2.1. Fabrication
The substrates used in present work are HPHT (001) Ib single crystal diamond with the dimension of 3 × 3 × 0.3 mm3. Diamond microlenses fabrication process is schematically illustrated in Fig. 1(a). The SPR220 PR was spun on diamond substrate with a speed of 4,000 rpm, resulting in a PR thickness of 6.5 μm. Then, the standard photolithography process was used to form PR pillars with a diameter of 100 μm. By holding the samples in organic solvent vapor atmosphere for 15-120 s under saturated vapor pressure at 20 °C, the pillars diffused and formed almost perfect spherical segments. Three types of organic solvents were chosen to form vapor atmospheres for chemical reflow treatment. Finally, to transfer PR patterns into diamond surface, an ICP etching (Oxford, ICP-180) process was utilized with low-cost O2 and Ar as the etch gas. Flow rates of O2 and Ar were 40 and 15 sccm, respectively. Chamber pressure, coil power, bias voltage were 10 mTorr, 450 W, and −145 V, respectively.
Fig. 1 (a) Schematic of the diamond microlenses fabrication process. (b) Optical images of PR pillars and spherical segment patterns after chemical reflow treatment, respectively.
2.2. Characterization
PR patterns were investigaed by optical microscope (OM) (Olympus, BX-51) and laser scanning confocal microscopy (LSCM) (Olympus, LEXT-OLS4000). The morphologies of the fabricated diamond microlenses were characterized by scanning electron microscopy (SEM) (Quanta, F-250), white-light interferometer (Talysurf, CCI-2000), atomic force microscopy (AFM) (Veeco, Innova) and the step profiler (AlphaStep, D-100). The optical performance of the diamond microlenses was investigated by a modified optical microscope system.
3. Results and discussion
3.1. Morphology of PR spherical segment mask
Figure 1(b) shows the typical OM images of PR patterns on diamond substrates before and after chemical reflow treatment in ethanol atmosphere for 15 s. The initial height of PR pillar is 6.5 μm. Each pillar has a diameter of 100 μm and the distance between two adjacent pillars is about 450 μm. After chemical reflow treatment, PR pillar is changed to spherical structure due to surface tension and gravity, whose diameter is expanded a little larger.
Figure 2 shows the typical LSCM images of reflowed PR patterns with different reflow time in ethanol atmosphere. When the reflow time is less than 5 s, there is an edge bulge in the PR pattern, as shown in Figs. 2(a) and 3. No spherical shape is obtained mainly because PR has not been sufficiently diffused by organic vapor with 5 s reflow treatment. So the height of pattern is near 6.5 μm, which is comparable to initial height. When the reflow time is longer than 15 s, the images depict a perfect shape of spherical segment, as shown in Figs. 2(b)–2(e). All these results indicate how the PR pattern profile is influenced by the reflow time ranging from 0 to 120 s in ethanol atmosphere for the same PR thickness. As reflow time is extended to 15, 30, 60 and 120 s, the measured heights of spherical segment patterns are 10.5, 8.9, 7.7, and 5.9 μm and the corresponding radii are 55.3, 60.0, 71.4 and 80.0 μm, respectively. Since the PR pattern with 5 s reflow treatment is not a spherical segment, the subsequent discussions will focus on other treatment results.
Fig. 2 Images with laser scanning confocal microscope measurement in ethanol atmosphere at 20 °C for various reflow time. (a–e) 2D image for 5 s, 15 s, 30 s, 60 s, 120 s, sequentially; (f–j) 3D image for 5 s, 15 s, 30 s, 60 s, 120 s, sequentially.
Fig. 3 The LSCM measured and fitted surface profiles of PR spherical segment patterns on diamond substrate formed in ethanol atmosphere for various time.
Figure 3 shows the cross sectional profiles of PR spherical segment patterns sampled in LSCM images, whose theoretical curvatures were circle fitted and indicated in red dot lines. Compared with the theoretical profile, the experiment profile exhibits only a little deviation, indicating that the PR spherical segment pattern profile is very close to a spherical shape. The height (10.5 μm) of PR spherical segment pattern, which is treated for 15 s reflow time, is larger than that of initial PR pillar, indicating that the PR pattern volume becomes large. The swelling is common phenomenon in polymer materials and stemmed from solvent induced molecular relaxation. During swelling, binding force of networks in PR becomes repulsive by interaction between networks and permeated solvent, and chains between network junctions are required to assume elongated configurations [20]. It could also be seen that the PR spherical segment pattern shape is strongly dependent on the reflow time. When the reflow time is extended longer than 15 s, the PR spherical segment pattern indicates a decrease in height and an increase in diameter.
To investigate the effect of solvent types on PR spherical segment pattern formation, the same experiments in acetone and isopropanol atmospheres were carried out. Figure 4 shows the influence of the solvent types and reflow time on the parameters of PR spherical segment patterns. Three group plots represent the parameters of spherical segment patterns treated in acetone, isopropanol and ethanol. The effect of reflow time on spherical segment pattern parameters indicates the same trend in these solvent vapors. It can be easily seen that radius is increased while height is decreased in Figs. 4(a) and 4(b). It is worth to mention that radius deviation could be controlled within 2% under the same process condition according to our experiment results. In order to precisely control the microlens size, the process should be optimized, and detail results will be presented in the future. Figure 4(c) shows the ratio of radius to height of spherical segment pattern with reflow time, implying that NA can be adjusted quantitatively. Figure 4(d) shows that ROC of PR spherical segment pattern increases with extending reflow time. ROC increasing rate is evaluated by fitting experimental data and indicated in red lines. The rates (represented by red line slopes) in ethanol and isopropanol are 3.1 and 5.7, respectively, and their values are far lower than that of acetone (38.7). This could be ascribed to the difference of saturated vapor pressure (acetone: 24.0 kPa, ethanol: 5.6 kPa, isopropanol: 4.4 kPa at 20 °C and standard atmospheric pressure), implying that the change rate of ROC also depends on the vapor pressure.
Fig. 4 Optical parameters of PR spherical segment patterns reflowed for different reflow time in different solvent types, acetone, isopropanol and ethanol. (a) Radius, (b) height of PR spherical segment pattern, (c) ratio of radius to height, and (d) radius of curvature obtained by fitting, versus reflow time.
3.2. Morphology of the diamond microlenses
Based on the results mentioned above, three typical PR spherical segment patterns were transferred into diamond substrates to form diamond microlenses by ICP etching process. Lower power in ICP etching process is chosen to avoid PR pattern mask deformation which may be caused by excess heat accumulation. In order to maintain the sample temperature at about 20°C, the diamond sample is loaded on a holder cooled with circulating purified water.
Figures 5(a)–5(c) show the SEM images of the diamond microlenses, Figs. 5(d)–5(f) display the 3D profiles of diamond microlenses measured by interferometer, and Figs. 5(g)-5(i) exhibit the cross sectional profiles measured by step profiler, which indicate a good transfer from PR spherical segment pattern to diamond substrate. All these results show that diamond microlens has been fabricated in substrate. The interferometer results indicate that these diamond microlenses have roughness values of 2.86, 4.33, 4.95 nm, respectively, which are better than that reported by other group [19]. Several nanometers roughness value measured by white interferometer is mainly due to pits on the microlens surface, which are thought to be caused by the air bubbles existing between interface of PR and substrate. AFM was used to evaluate the local roughness in three different scan areas of 5 × 5 μm2, and the results are all below 1 nm. These diamond microlenses have diameters of 138, 128, 123 μm, and their corresponding heights are 584, 708, 833 nm, respectively. On the basis of geometry and optical theory, NA gives a comprehensive description to microlenses with different sizes and focal lengths. It can be related with focal length as
where r is radius of microlens, f is the focal length determined by radius (r) and height (h) of microlens. where ROC is radius of curvature, n is refractive index. Here, refractive index of diamond is 2.42, and the parameters can be obtained using Eqs. (2) and (3) and shown in Table 1. It can be seen in Table 1 that NA values of diamond microlenses are small, indicating that microlenses have large focal length with small radii. Meanwhile, the maximum values of ROC and focal length are 4.01 mm and 2.87 mm, respectively.Fig. 5 (a–c) SEM images of fabricated microlenses with different sizes. (d–i) Interferometer pictures and step profiler measured cross-sections of microlenses in (a–c), sequentially.
Table 1. Optical parameters of diamond microlenses calculated with radius, height and n.
The data measured from microlens cross-sections are also circle fitted and indicated in red dot plots of Figs. 5(d)–5(f), the ROC of these fitted circles are 3.72, 2.64, 2.18 mm, respectively, exhibiting a comparable values with calculation results.
3.3. Optical property of the diamond microlenses
The optical properties of the microlenses are mainly determined by its size and curvatures. To demonstrate diamond microlens optical performance, microlenses shown in Fig. 5 were used in a projection experiment, which is carried out with an optical microscope system [21] as depicted in Fig. 6(a). The diamond microlens sample was fixed on the sample stage and illuminated by white light through a projection photomask. The distance between the stage and the photomask was set about 11 cm. The light passing through the photomask is focused by diamond microlenses and projected on the phase plane to exhibit miniaturized photomask pattern image. The projected image is captured through the camera fixed on the objective lens of the microscope. A photomask with line width of 2.16 mm was used. Figure 6(b) shows the images of “A” after projection through diamond microlenses with different sizes. The image line width is increased with the increase of microlens diameter, which is attributed to the smaller curvature and larger radius of microlenses [22].
Fig. 6 (a) Simplified setups of the optical system for the projection measurements. (b) Images projected by the microlenses with diameter of 123, 128, 138 μm with ‘A’ photomask, objective: 20 × .
The line widths of projected images are 41.8, 60.7, 95.7 μm for microlenses with diameters of 123, 128 and 138 μm, respectively. The images indicate that diamond microlenses fabricated with chemical reflow method have a fine imaging property and are easy to be adjusted by experimental parameters.
Furthermore, the chemical reflow method is also applicable to microlens fabrication on other materials.
4. Conclusions
In summary, diamond microlenses with utralow NA and controllable curvature are successfully fabricated by chemical reflow method. Our results imply that the key point to affect profile of PR spherical segment pattern on diamond is the reflow time and the saturated solvent vapor pressure. By transferring the pattern into diamond substrate with ICP etching process, a diamond microlens with ultralow NA of 0.024 and large focal length of 2.87 mm has been fabricated. Projection experiment shows that diamond microlenses have good optical properties. This method is also applicable to microlens fabrication on other materials.
Funding
National Natural Science Foundation of China (NSFC) (61627812, 61605155), Technology Coordinate and Innovative Engineering Program of Shaanxi (2016KTZDGY02-03), Postdoctoral Science Foundation of China (PSFC) (2015M580850).
Acknowledgments
The authors are thankful to Jicheng Li from State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University for his help in ICP etching experiment and Yanzhu Dai from School of Electronics and Information Engineering, Xi’an Jiaotong University for her help in SEM measurement.
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