Photoresist Microlens Array Research Articles

Photoresist Microlens Arrays Fabricated by Nanoimprint Technique to Improve the Photoelectric Conversion Efficiency of Solar Cells

Silicon solar cells have the advantages of non-toxicity, reliability, low price, and stability. Microlens arrays (MLAs) are widely used in solar cells to improve photoelectrical conversion efficiency (PCE). In this research, different MLAs mold was designed by a method of thermal reflow. Then the photoresist film MLAs structure was replicated on the surface of silicon solar cells through UV nanoimprint technology. The optical transmission and surface morphology of these photoresist films were respectively measured by using a UV spectrometer and an atomic force microscope. The surface morphology and imaging capabilities of photoresist film MLAs were respectively measured by using a scanning electron microscope and optical microscope. Finally, the photovoltaic performance of the silicon solar cell with the photoresist film MLAs was investigated, and the PCE value of the silicon solar cell improved from 11.53% for the sample without MLAs to 13.19% for the sample with the square MLAs and the PCE improvement is about 14.40%. All these results above show that the photoresist film MLAs can significantly improve the efficiency of silicon solar cells and have great application potential in the field of solar cells.

Grating/microlens arrays fabricated by hot-melting, self-assembly and replication

This paper reports a convenient method to fabricate hybrid optical element of grating/microlens array (GMA) by hot-melting, self-assembly and replication on the same surface. The polydimethylsiloxane (PDMS) microlens array with lens diameters of 700 μm and effective focal length of 3.7 mm is replicated twice from the photoresist microlens array by hot-melting. And the self-assembly gratings with groove density of 886 lines/mm are fabricated on the surface of stretching microlens array by oxygen plasma. A single part of GMA operates across a wavelength spectrum from 450 nm to 650 nm with a maximum resolution of 6.9 nm at 450 nm.

Fabrication of refractive silicon microlens array with a large focal number and accurate lens profile

In this paper, we demonstrate the fabrication of refractive silicon microlens array with a large focal number and almost perfect spherical lens shape. Through a modified thermal reflow, photoresist microlens array of a large focal number is fabricated, which is then transferred into the silicon substrate by ion beam milling. To reach an accurate spherical lens profile, we both theoretically and experimentally study the practical factors that harm the pattern transfer fidelity, which mainly include the etching selectivity and faceting effect. Other secondary etching effects, such as the trenching effect and re-deposition effect, are also discussed. Based on these studies, a silicon microlens array with a focal number of 1.35 has been successfully obtained, with the profile error controlled well-below 0.121 μm, less than λ/6 within the whole infrared wavelength band. Besides, the fabricated microlens array exhibits a good uniformity and fine surface smoothness. The fabricated silicon microlens arrays can be applied in minatured infrared and terahertz imaging devices, or used as the master mould for soft lithography.

Fabrication of microlens array with well-defined shape by spatially constrained thermal reflow

We demonstrate a novel method for fabricating a well-defined and large-area microlens array combining digital micro-mirror device (DMD)-based lithography and spatially constrained thermal reflow. Using the flexible processing ability of DMD-based lithography, a micro pillar array is easily prepared on a glass substrate and then the microlens array is formed after a thermal reflow process. Differing from conventional approaches, the proposed thermal reflow process is conducted in polydimethylsiloxane (PDMS) solution instead of in the air. During the process, the partly cured PDMS can effectively prevent the photoresist crawling in the diameter direction, which is very convenient for controlling the precise shape of the microlens. In addition, the photoresist microlens array and PDMS soft mold with a concave cavity array can be achieved in one step. The experimental results showed that the photoresist microlens was good in both shape and surface quality. The presented method may offer great promise for fabricating a high-quality microlens array with different requirements in optical applications.

Fabrication of a 100% fill-factor silicon microlens array

A simple method has been developed for the fabrication of a silicon microlens array with a 100% fill factor and a smooth configuration. The microlens array is fabricated by using the processes of photoresist (SU8-2005) spin coating, thermal reflow, thermal treatment and reactive ion etching (RIE). First, a photoresist microlens array on a single-polished silicon substrate is fabricated by both thermal reflow and thermal treatment technologies. A typical microlens has a square bottom with size of 25 μm, and the distance between every two adjacent microlenses is 5 μm. Secondly, the photoresist microlens array is transferred to the silicon substrate by RIE to fabricate the silicon microlens array. Experimental results reveal that the silicon microlens array could be formed by adjusting the quantities of the reactive ion gases of SF6 and O2 to proper values. In this paper, the quantities of SF6 and O2 are 60 sccm and 50 sccm, respectively, the corresponding etch ratio of the photoresist and the silicon substrate is 1 to1.44. The bottom size and height of a typical silicon microlens are 30.1 μm and 3 μm, respectively. The focal lengths of the microlenses ranged from 15.4 to 16.6 μm.

New high fill-factor dual-curvature microlens array fabrication using UV proximity printing

A new method to fabricate a high fill-factor dual-curvature microlens array using the proximity printing in lithography process is reported. The technology utilized the UV proximity printing by controlling a printing gap between the mask and substrate. The designed high density microlens array pattern can produce high fill-factor dual-curvature microlens array in photoresist. It is due to the UV light diffraction to deflect away from the aperture edges and produce a certain exposure in photoresist material outside the aperture edges. This method can precisely control the geometric profile of high fill factor dual-curvature microlens array. The experimental results showed that the dual-curvature micro-lens array in photoresist could be formed automatically when the printing gap ranged from 360 to 600 µm. The gapless dual-curvature microlens array will be used to enhance of luminance and uniformity for light-emitting diodes.

New high fill-factor triangular microlens array fabrication method using UV proximity printing

A simple and effective method for fabricating a high fill-factor triangular microlens array using the proximity printing in lithography process is reported. The technology utilizes the UV proximity printing by controlling the printing gap between the mask and substrate. The designed approximate triangle microlens array pattern can be fabricated in photoresist. This is because to the UV light diffraction deflects away from the aperture edges and produces a certain exposure in photoresist material outside the aperture edges. This method can precisely control the geometric profile of a high fill-factor triangular microlens array. The experimental results showed that the triangular photoresist microlens array could be formed automatically when the printing gap ranged from 240 to 840 μm. The gapless triangular microlens array will be used to increase the luminance for the backlight module of liquid crystal displays.

Microlens array fabrication by enhanced thermal reflow process: Towards efficient collection of fluorescence light from microarrays

This paper describes two enhanced resist reflow methods for the fabrication of microlens arrays and demonstrates their use for integrated biomolecular fluorescence detection on printed microarrays. A PDMS (polydimethylsiloxane) microlens array was fabricated by a double soft lithography approach using a photoresist microlens array as master mold. Additionally, by using both a careful control of the surface wettability and thermal treatments, we demonstrate the possibility to extend the resist reflow process in order to tune the diameters of microlens array over a large range by using a unique photomask pattern. We introduce an enhanced reflow on hydrophobic surfaces obtained by fluorosilane treatment and identify a threshold shrinkage temperature (T shrinkage) of 140 °C, above which the diameter of microlenses can be then reduced down to 40% compared with the initial pattern on the photomask. Furthermore, on hydrophilic substrates, achieved by an accurate incomplete development of the photoresist, we demonstrate a nearly perfect linear dependency (1.4 μm/°C) of microlens diameter spreading up to 70% the initial diameter inside a temperature reflow window of 110–140 °C. For both approaches, above a freezing temperature (T freezing) of 170 °C, the microlens profile characteristics are temperature independent. By using high numerical aperture microlens array, we provide a proof of concept for the integration and enhanced light collection of the fluorescent signals collected form a microarray of fluorescent spots thus showing the potential of the concept for biophotonic integration.

New high fill-factor triangular microlens array fabrication method using UV proximity printing

A simple and effective method to fabricate a high fill-factor triangular microlens array using the proximity printing in lithography process is reported. The technology utilizes the UV proximity printing by controlling a printing gap between the mask and substrate. The designed approximate triangle microlens array pattern can be fabricated the high fill-factor triangular microlens array in photoresist. It is due to the UV light diffraction to deflect away from the aperture edges and produce a certain exposure in photoresist material outside the aperture edges. This method can precisely control the geometric profile of high fill factor triangular microlens array. The experimental results showed that the triangular micro-lens array in photoresist could be formed automatically when the printing gap ranged from 240 mum to 840 mum. The gapless triangular microlens array will be used to increases of luminance for backlight module of liquid crystal displays.

Soft-lithography-enabled fabrication of large numerical aperture refractive microlens array in hybrid SiO2–TiO2 sol-gel glass

This paper describes a low-cost soft-lithography method for building large-numerical-aperture microlenses in hybrid sol-gel glass. The fabrication processes comprise three steps, namely fabrication of large numerical aperture microlens array in photoresist as a master, replication of the master in poly-dimethylsiloxane (PDMS) as elastomeric molds and reproduction of the PDMS replica onto the hybrid sol-gel glass as an end structure. Comparing with the direct UV fabrication in the photosensitive sol-gel glass, the proposed method provides a unique advantage in terms of fabrication freedom in shape and thickness of micro-optical elements in the sol-gel glass. This eliminates the difficulties in direct photolithography in sol-gel glass for arbitrary shapes and great thickness due to ever increasing demands.

A new microlens array fabrication method using UV proximity printing

This paper presents a new microlens array fabrication method that controls the printing gap in the UV lithography process. This method can precisely control the microlens array geometric profile array in the fabrication process without thermal reflow. The proximity printing bends the UV light away from the aperture edges and produces a certain exposure in the photoresist outside the aperture edges due to the diffraction effects. This causes the photoresist bottom in two adjacent patterns to link together after development. The fabricated microlens diameter has the same size as the pitch distance between the two apertures. For example, a 120 µm aperture pitch distance will generate a 120 µm diameter microlens. The proximity gap between the mask and photoresist should be two times the aperture pitch distance or greater to generate an accurate microlens array. The experimental results showed that photoresist microlens arrays could be formed automatically without using the thermal reflow process when the printing gap ranged from 240 µm to 840 µm. A hemispherical microlens array with a 120 µm diameter and various geometric profiles can be fabricated using this method.

Extension of the useful focal length range of microlenses by oil immersion

A technique is investigated for increasing the focal length of an existing microlens or microlens array to realize long focal length microlenses. Photoresist microlens arrays have been fabricated and immersed in oils of various refractive indices (index-matching oil), extending the range of easily achievable numerical apertures towards lower values. The focal length can be altered by exchanging the oil for another with a different refractive index. We derive simple design relations which predict the changes in focal length, spherical aberration and astigmatism due to the oil immersion of the lens. Using these relations and oils of various refractive indices we have calculated the photoresist refractive index to be . The effects of altering the focal length on the on- and off-axis aberrations have also been measured and agreement with our design equations has been demonstrated.