Fabrication of diamond microlens arrays for monolithic imaging homogenizer

Accepted Manuscript Fabrication of diamond microlens arrays for monolithic imaging homogenizer

Tian-Fei Zhu, Jiao Fu, Fang Lin, Minghui Zhang, Wei Wang, Feng Wen, Xiaofan Zhang, Renan Bu, Jingwen Zhang, Jingping Zhu, Jingjing Wang, Hong-Xing Wang, Xun Hou PII: DOI: Reference:

S0925-9635(17)30358-8 doi:10.1016/j.diamond.2017.09.017 DIAMAT 6945

To appear in:

Diamond & Related Materials

Received date: Revised date: Accepted date:

30 June 2017 12 September 2017 26 September 2017

Please cite this article as: Tian-Fei Zhu, Jiao Fu, Fang Lin, Minghui Zhang, Wei Wang, Feng Wen, Xiaofan Zhang, Renan Bu, Jingwen Zhang, Jingping Zhu, Jingjing Wang, Hong-Xing Wang, Xun Hou , Fabrication of diamond microlens arrays for monolithic imaging homogenizer. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Diamat(2017), doi:10.1016/ j.diamond.2017.09.017

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ACCEPTED MANUSCRIPT Fabrication of diamond microlens arrays for monolithic imaging homogenizer Tian-Fei Zhu1, Jiao Fu1, Fang Lin1, Minghui Zhang1, Wei Wang1, Feng Wen1, Xiaofan Zhang1, Renan Bu1, Jingwen Zhang1, Jingping Zhu2, Jingjing Wang3**, Hong-Xing Wang1* and Xun Hou1

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1 Institute of Wide Band Gap Semiconductors, Xi’an Jiaotong University, Xi’an, 710049, China

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2 Key Laboratory for Physical Electronics and Devices of the Ministry of

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Education and Shaanxi Key Lab of Informantion Photonic Technique, Xi’an Jiaotong University, Xi’an, 710049. China

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3 Nation Key Laboratory of ASIC, Hebei Semiconductor Research Institute, Shijiazhuang, 050051, China

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*[email protected] (H.X. Wang) **[email protected] (J.J. Wang)

Keywords: microlens arrays; single crystal diamond; homogenizer; beam shaping

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Abstract

Compact microlens arrays (MLAs) have been fabricated on diamond substrates using thermal reflow and dry etching techniques for laser beam homogenization. Firstly,

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close-packed hexagonal photoresist pillars developed by photolithography were reflowed upon heat plate to form MLAs mask on one side of diamond substrate.

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Secondly, the mask pattern was transferred into substrate with inductively coupled plasma etching to form closely arranged diamond MLAs. Then, the same processes were utilized to fabricate MLAs on the other side of diamond substrate. The MLAs on both sides of substrate are aligned. Thirdly, the obtained diamond MLAs demonstrate compact arrangement, well-uniformity, and good imaging performance with projection experiment. Eventually, the double-sides MLAs-patterned diamond substrate was utilized as monolithic imaging homogenizer, exhibiting a good homogenizing performance.

1. Introduction

ACCEPTED MANUSCRIPT Microlens arrays (MLAs) are preferable choice for light homogenization because of their independence from entrance intensity profile and wide spectrum of wavelength [1]. There are two common types of homogenizer: the non-imaging and the imaging homogenizers [2]. Non-imaging homogenizer includes single MLAs and a Fourier lens, while the imaging type includes two MLAs and a Fourier lens. In the imaging

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homogenizer, two identical MLAs are placed in series and aligned along the light direction, and the spacing of two MLAs is the same as focal length of first MLAs. In

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this situation, each lens in the second MLAs acts as a field lens and stands the irradiation focused by first MLAs. Nevertheless, in practical application, the second

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MLAs are not positioned at the focal points of first MLAs due to the disruptive power of focal point [3] and lower damage threshold of MLAs materials.

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Important to many potential applications under harsh conditions, diamond is a suitable alternative material for fabrication of MLAs to achieve good performance imaging

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homogenizer. It is because diamond has high thermal conductivity and stable chemical inertness, resulting the ability to stand high energy irradiation from the focal

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point by the first MLAs.

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However, diamond’s hardness and chemical inertness lead to difficulties in patterning microlens structure on its surface. Generally, main ideas to fabricate diamond microlens including two steps: firstly, preparation of photoresist (PR) pattern;

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secondly, transfer of PR pattern into diamond substrate. To date, popular approaches to fabricate diamond microlens are based on combination of thermal reflow and dry

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etching techniques [4, 5, 6]. Although with this approach, spherical shape microlenses with different sizes are obtained, contact line of PR pattern is fixed which limits fabrication of MLAs with high aspect ratio. There are few reports on fabrication of compact MLAs on diamond via enlarging contact line of PR pattern by increasing the PR fluidity. Also, there are few reports on fabrication of compact MLAs on both sides of one diamond substrate with alignment for monolithic imaging homogenizer. In this paper, compact diamond MLAs were firstly fabricated on diamond surface for laser homogenizer. The morphology of as-fabricated diamond MLAs was characterized. The projection experiment was carried out to evaluate optical properties

ACCEPTED MANUSCRIPT of diamond MLAs. Homogenizing performance of imaging homogenizer using double-sides MLAs diamond substrate was investigated in a home-built operating platform with 308 nm wavelength laser.

2. Experimental Procedure 2.1. Fabrication

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The substrates used in the present work were double-sides polished chemical vapor deposition (CVD) (001) single crystal diamonds with area of 3×3 mm2 and thickness

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of about 0.5 mm. Diamond MLAs fabrication processes are schematically illustrated

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in Figure 1. The SPR220 PR was spun on diamond substrate with a speed of 6000 rpm, resulting in a PR thickness of about 5.5 μm. Then the standard photolithography

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process was used to form hexagonal PR pillars with circumcircle diameter of about 80 μm. Distance between two neighbor pillars was 5 μm. By holding the sample on a hot

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plate for different time at 160 ℃, the pillars melted and formed spherical segments. Finally, the PR patterns were transferred into diamond surface using an inductively coupled plasma (ICP) etch process with O2 and Ar as the etch gas. Flow rates of O2

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and Ar were 40 and 15 sccm, respectively. Chamber pressure, coil power, bias voltage

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were 10 mTorr, 450 W, and -135 V, respectively. Once MLAs had been fabricated on one side of diamond substrate, same MLAs were

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fabricated on the other side of substrate with identical processes to achieve monolithic imaging homogenizer. During the second photolithography, PR pillars were aligned with these fabricated microlenses on the first side of diamond substrate with an

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alignment offset of less than 2 μm by carefully tuning the exposure position with focusing microscope. Then, via thermal reflow and ICP etch processes, a monolithic double-sides MLAs homogenizer was achieved.

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Figure 1 Schematic of MLAs fabrication process.

2.2. Characterizations

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PR mask patterns were investigated by optical microscope (OM). The morphologies of the fabricated diamond MLAs were characterized by OM, scanning electron

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microscope (SEM), atomic force microscope (AFM) and step profiler. The optical performance of the diamond MLAs was investigated using a modified optical

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microscopy system. The homogenizing performance of fabricated monolithic imaging

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homogenizer was evaluated using a home-built optical system setup.

3. Results and discussion

3.1. Fabrication of PR mask and diamond MLAs

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To achieve compact MLAs, closely arranged PR spherical segment arrays were fabricated on diamond substrate as the mask. Figure 2(a) shows PR pillars fabricated

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by standard photolithography. Figure 2(b)-2(d) display PR spherical segment arrays reflowed at 160 ℃ for 15 s, 30 s, and 60 s, respectively. When the reflow time is less than 30 s, separated spherical segments were achieved. Usually, during thermal reflow treatment, PR pillars turned to be fluid state and form spherical segments due to surface tension with a fixed contact line. The evolution of PR is considered as an edge bulge forms first at the borders of the pillar and then propagates inward gradually [7]. In present experiment, reflow temperature of 160 ℃ (much higher than PR glass transition point) was chosen to achieve PR with high fluidity during thermal reflow process. Combined with gravity, a slightly enlargement of PR pattern contact line is

ACCEPTED MANUSCRIPT obtained which result in more compact PR spherical segments arrays. Hence, with increasing reflow time, PR spherical segment diameter is slightly increased. However, due to the narrow distance between two neighbor pillars, the junctions between PR spherical segments were observed when the reflow time is longer than 60 s. These results demonstrate that a small distance between two spherical segments can be

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obtained by optimizing thermal reflow time. It is worth to point out that when the reflow time is longer than 120 s, PR shape is independent of reflow temperature. That

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is because solvent in PR effected fluidity during thermal treatment and evaporated out

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eventually.

30 s and 60 s.

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Figure 2. Optical images of PR (a) pillars and (b-d) spherical segment arrays thermal reflowed at 160 ℃ for 15 s,

Based on above results, PR spherical segment arrays with neighbor distance of less

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than 2 μm were prepared as a mask. Via ICP etch, the mask shape was successfully transferred into diamond surface as shown in Figure 3. Figure 3(a) shows the as-fabricated diamond MLAs, indicating a uniform microlens. Figure 3(b) exhibits the enlarged diamond microlens which has smooth surface and compact arrangement. AFM measurement was carried out to evaluate surface condition of diamond microlens shown in Figure 3(c), indicating a roughness of 1.2 nm. Cross section surface profile of as-fabricated diamond microlens array measured by step profiler is shown in Figure 3(d), suggesting that a compact diamond MLAs was

ACCEPTED MANUSCRIPT obtained. Figure 3(e) exhibits the enlarged view of the microlens profile in the box in Figure 3(d), and also the PR mask profile before ICP etching process. These measured data are circle fitted and indicated in the red line plot. Experimental and fitted data are close, implying a spherical profile of as-fabricated microlens. Figure 3(e) indicates a low etch selectivity (defined as the ratio of diamond etch rate to that of PR) of 0.105

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since diamond is extreme hard and chemical inertness. Based on the geometry and optical theory, focal length is determined by radius and height of microlens [8].

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(h 2  r 2 ) ROC ROC  , and f  2h n 1

(1, 2)

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Where f is focal length, ROC is radius of curvature, n is refractive index, h is lens height and r is lens radius. Here, refractive index of diamond is determined by the

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wavelength of incident light. In this work, n is 2.55 for the 308 wavelength light [9] in the homogenization experiment. Thus, the focal length was calculated to be 544 μm.

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The Fresnel number, an important parameter to characterize the influence of diffraction effects onto the single lens of the MLAs [10], is calculated to be about 8.5

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homogenization [1, 2].

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based on the formula below [1]. The value is within the acceptable range for imaging

r2 FN   f

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Where FN is Fresnel number,  is wavelength of incident light.

(3)

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Figure 3 (a-b) SEM and (c) AFM images of fabricated diamond microlenses with different magnifications. (d) Cross sectional surface profile measured with step profiler. (e) Cross sectional surface profiles of single diamond

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3.2. Optical Properties

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micolens and its PR mask and their profile fitting.

Optical properties of these diamond MLAs are evaluated using projection and homogenization experiments.

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A single-side MLAs diamond substrate was utilized for projection experiment, which was carried out with an optical microscopy system as depicted in Figure 4(a). The

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diamond microlens sample was fixed on the sample stage and illuminated under white light through a projection photomask, and distance between the sample to photomask was set about 10 cm. The light passing through the photomask was focused by each diamond microlens and projected on the phase plane to exhibit miniaturized photomask pattern images. The projected images were captured through the camera fixed on the objective lens of the microscope. “A” photomask with line width of 2 mm was utilized. The images of “A” after projection through diamond MLAs are clearly and uniformly hexagonal arranged as shown in Figure 4(b), which suggests that the fabricated diamond MLAs have good optical performance and uniformity.

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Figure 4 (a) Simplified setups of the optical system for the projection measurements. (b) Images projected by the microlenses with ‘A’ photomask, flipped by 180° relative to the object.

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The homogenizing performance of a double-sides MLAs diamond substrate was also

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tested. The optical system and schematic illustration of the MLAs homogenizer are shown in Figure 5(a) and 5(b), respectively. In this experiment, microlens with diameter of about 80 μm and focal length of 495.5 μm was chosen for diamond

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homogenizer. Due to large depth of field (DOF) of microscope with low power objective, MLAs on both sides of diamond substrate can be observed on one focal

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plane in microscope. In Figure 5(c), black and red circles indicate the microlens arrays on back and front sides of diamond substrate, respectively. Almost the same positions of each sub-microlens on two sides suggest their alignment. Thickness of the diamond substrate is 498 μm, which is close to the lens focal length. It enables feasibility of monolithic imaging homogenizer, whose configuration is illustrated in Figure 5(b). In the monolithic configuration, since each microlens in the first MLAs is aligned with a corresponding microlens in the second MLAs with the distance of their focal length, imaging condition for the second MLAs is better met to achieve high quality beam compared with practical case. A 308 nm wavelength laser was utilized in

ACCEPTED MANUSCRIPT the homogenization experiment. The homogenization pattern captured by laser analyzer is hexagonal due to the same shape of microlens. Raw beam is shown in inset in Figure 5(d) and exhibits irregular intensity distribution. Beam passed though the diamond homogenizer is improved and shows a relative uniform intensity distribution in comparison with raw beam, demonstrating the homogenizing

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performance of the monolithic diamond imaging homogenizer. In our results, there are lot of speckles in the homogenization pattern, which could be stemmed from the

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coherence of incident laser. Hence, optimization of homogenization system is required

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to obtain better results in future work.

Figure 5 (a) Photograph of the optical process platform. (b) Scheme of homogenization experiment for

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double-sides MLAs homogenizer. fLA1 is focal length of diamond microlens on the beam incident side of diamond, fL is focal length of lens. (c) optical image of diamond homogenizer using double-sides MLAs diamond substrate.

beam.

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(d) Homogenizing result of monolithic diamond imaging homogenizer, inset: intensity distribution of raw incident

This investigation demonstrates that enlargement of PR pattern contact line by increasing PR fluidity is feasible to achieve MLAs with high aspect on diamond in thermal reflow method. The proposed technique could be further improved by optimizing fabrication process, such as pre-preparation of pattern boundary. Feasibility of monolithic imaging homogenizer utilizing double-sides MLAs diamond is also demonstrated in homogenization experiment, illustrating a homogenizing ability. Along with diamond’s unique properties including high thermal conductivity,

ACCEPTED MANUSCRIPT stable chemical inertness and wide transmittance window, diamond monolithic homogenizer has a wide range of potential applications such as materials machining, photolithography, laser measurement and laser analysis under high energy irradiation [1, 11, 12].

4. Conclusions

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In this work, monolithic diamond imaging homogenizer using two sides MLAs structure has been fabricated by thermal reflow process and ICP etch technique.

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Aspect ratio of microlenses on diamond is dependent on reflow time during thermal

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reflow process of PR at 160 ℃. These MLAs are compactly and uniformly arranged on diamond and exhibit good optical properties. Monolithic imaging homogenizer

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with double-sides MLAs shows a good homogenizing performance.

Acknowledgements

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We thank associate researcher Yongsheng Zhang from Northwest Institute of Nuclear Technology for his helpful discussions. We also are thankful to Jicheng Li from State

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Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University

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for his help in ICP etching experiment and Yanzhu Dai and Chuansheng Ma from International Center for Dielectric Research (ICDR), Xi’an Jiaotong University for

Funding

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their help in SEM measurements.

This work was supported by National Natural Science Foundation of China (NSFC)

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(grant number 61627812, 61605155), Technology Coordinate and Innovative Engineering Program of Shaanxi (grant number 2016KTZDGY02-03) and Postdoctoral Science Foundation of China (PSFC) (grant number 2015M580850).

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights: Compact and uniform arranged diamond microlens arrays have been fabricated with combination of chemical reflow and ICP techniques.

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Monolithic imaging homogenizer is achieved using double-side microlens arrays patterned diamond substrate.

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The monolithic diamond imaging homogenizer exhibits a good homogenizing performance in homogenization experiment system.

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