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Microlens array for shortwave infrared detectors
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Abstract
A microlens with a specific size is fabricated by using melting photoresist, and the microlens can be applied to a shortwave 1 μm ~3 μm infrared detector, which can effectively improve the photoelectric performance of the detector. Using AZP4620 thick photoresist and UV lithography technology, the lens production in the soft bake, exposure and development, hardening, hot melt and other processes were carried out in-depth and detailed experimental study was done to determine the optimal process parameters. The micro-lens with a crown diameter of (5.5 ± 0.5) μm and a radius of curvature of 3 μm was realized. The lens has good uniformity and consistency to meet the requirements of near infrared detection device.-
Keywords:
- microlens array /
- melting photoresist /
- pre-baking /
- hot melt
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References
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Overview
Abstract: Most of our domestic infrared detector’s photosensitive surface is less than the pixel surface area. A part ofthe incident light irradiates to the photosensitive area between the dead zone, and this part is not used but reflectedand scattered. The microlens array with specific size was fabricated by photoresist fusion method, then a microlensarray was used to converge to a 1 μm ~3 μm infrared detector, the surface area of the infrared detector can be expanded to reduce noise of the infrared detector and prevent incident light from entering the dead zone. The mainprocess steps include: substrate cleaning, coating and glue, soft baking, exposure and development, baking, hot melt,ion beam etching and so on. The specific process steps: first, in order to obtain the photoresist pattern required for themicromirror curvature, we chose the AZP4620 positive photoresist for the thick film, and the refractive index of thephotoresist was 1.64. Second, the substrate treatment, removing the substrate surface grease and other impurities toensure that the substrate and the photoresist had good adhesion. Third, the substrate was coated with a uniform photoresist, and the photoresist was placed under the mask plate which had been set in advance and subjected to UV exposure, the corresponding cylindrical colloid was formed by the development of the image; Fourth, the substrate putinto the rapid annealing furnace for hot-melt, the photoresist was heated in the rapid annealing furnace, the surfacearea of the melted photoresist would shrink to a minimum and the surface energy was the lowest due to the combinedaction of the surface tension and the substrate adhesion. After the hot melt getting a stable spherical crown microlensmust require photoresist cylinder that reduced the amount of gravity is equal to the increase in the amount of potential energy. In the experiment, not any size and the thickness of the cylindrical colloid can form good spherical surfaceshape after hot melt to meet the design requirements of spherical shape by the photoresist cylinder diameter size,height, and the infiltration degree of glue and basal decision. Photoresist as an amorphous polymer is composed of avariety of chemical composition. The melting point of the photoresist is not an accurate temperature, but a temperature range in which the state of the photoresist exhibits a liquid state. Because of the different types of photoresist, themelting point range is different. Finally, the uniform microlens array was obtained with an ion beam etch machine. Byoptimizing the temperature and time parameters of each step process, the microlens with a crown diameter of (5.5 ±0.5) μm and a radius of curvature of 3 μm was realized, the microlens had good uniformity and consistency, and theinfrared detection was carried out in the 1 μm ~3 μm band device requirements.
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Figure 1.
Rotary-type photoresist on InP substrate.
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Figure 2.
Exposure.
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Figure 3.
Development.
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Figure 4.
Hot melt.
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Figure 5.
Pre-baking temperature and time. (a) Pre-baking temperature 60℃, time 45 min. (b) Pre-bakingtemperature 85℃, time 20 min.(c) Pre-baking temperature 100℃, time 8 min.
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Figure 6.
Microlens array.
