An extreme high fill-factor microlens array mold insert in photoresist fabrication using a thermal reflow process is presented. The experimental results proved that a square microlens array could be produced without a peripheral gap. A square microlens array with an extreme high fill-factor (almost 100%) was successfully fabricated. In this experiment, square photoresist columns were formed on a silicon substrate using a lithographic process. The square pattern was laid out in an ortho-square on a polyethylene terephthalate (PET) based mask. Precise temperature and time control was used during the thermal reflow process. The square microlens array was formed from the uniformly flowing melted photoresist. The photoresist column surface transforms into a spherical profile due to the surface tension effect. The error was within ±8% between the fabricated microlens characteristics and the theoretical model used to predict the photoresist column thickness and actual thickness. This model is feasible for fabricating various sized high fill-factor square microlens arrays.
A mathematical model for designing and fabricating a hexagonal microlens array using a thermal reflow process was developed in this study. The experimental results proved that a hexagonal microlens array could be produced without a gap at each microlens periphery. A hexagonal microlens array with a higher fill factor was successfully produced. In this experiment, hexagonal photoresist columns were formed onto a silicon substrate made using a lithographic process. The hexagonal pattern was laid out in an ortho-triangle on a PET (polyethylene terephthalate)-based mask. Using precise temperature and time control during the thermal reflow process, a hexagonal microlens array with lateral honeycomb geometry was formed from the melted photoresist flowing outward simultaneously and uniformly. The surface tension effect transformed the photoresist column surface into a spherical profile. The error in the fabricated microlens characteristics was within ±3% between two theoretical models used to predict the photoresist column thickness and actual thickness. This model is feasible for fabricating various sized hexagonal microlens arrays.
This paper presents a mathematical model to design and fabricate micro-ball lens array using thermal reflow in two polymer layers. The experimental results showed that micro-ball lens arrays were fabricated and integrated onto a planar substrate. Two polymer layers were coated onto a silicon substrate. The upper layer was a photoresist. The lower layer was a polyimide material. The polyimide was expected to form a pedestal to sustain the ball lens after the heat reflow process. Once the patterned polymer is heated above its glass transition temperature, the melting polymer surface will change into a spherical profile for minimizing its surface energy. A successful micro-ball array was formed in the photoresist through the different glass transition temperatures between two polymer materials. The interactive force between two material interfaces caused by surface tension causes the upper profile to form a spherical profile. This also forms the polyimide pedestal into a trapezoid with arc sides. The error in the fabricated micro-ball lens characteristics was 8% between the theoretical models used to predict the photoresist pattern thickness. This model is feasible for fabricating various sized micro-ball lens arrays.
The ultra-deep (UD) LIGA strategy for die fabrication that has features up to two millimeters in thickness and 100 microns in linewidth has been developed at SRRC, Taiwan. Here, a successive exposure strategy has been demonstrated to overcome the shortage of hard x-rays generated by a medium energy light source such as the 1.5 GeV Taiwan Light Source. Furthermore, this successive exposure process accumulates much more irradiation dosage in the photoresist that leads to a reduction of the developing time and thus produces a UD microstructure with very high aspect ratio. The present process makes use of a conformal mask technology that permits the sidewalls of the microstructure to be perfectly aligned after multiple exposures and developments. Since the total reflection of x-rays inhibits further dosage deposition on the sidewall after the first exposure, the successive exposure process improves the precision of the microstructure. The main concerns of fabricating a UD microstructure include the low diffusion speed involved in developing a high-aspect-ratio microstructure and the high residual stress between photoresist and substrate. A deeper microstructure can be achieved by choosing proper photoresist material, increasing the diffusion speed of development and designing a suitable structure to balance residual stress. A low temperature process is essential to keep the thermal stress from destroying UD microstructures.
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