Sequential shrink photolithography for plastic microlens arrays

Abstract

Endeavoring to push the boundaries of microfabrication with shrinkable polymers, we have developed a sequential shrink photolithography process. We demonstrate the utility of this approach by rapidly fabricating plastic microlens arrays. First, we create a mask out of the children's toy Shrinky Dinks by simply printing dots using a standard desktop printer. Upon retraction of this pre-stressed thermoplastic sheet, the dots shrink to a fraction of their original size, which we then lithographically transfer onto photoresist-coated commodity shrink wrap film. This shrink film reduces in area by 95% when briefly heated, creating smooth convex photoresist bumps down to 30 lm. Taken together, this sequential shrink process provides a complete process to create microlenses, with an almost 99% reduction in area from the original pattern size. Finally, with a lithography molding step, we emboss these bumps into optical grade plastics such as cyclic olefin copolymer for functional microlens arrays.

Progress of shrink polymer micro- and nanomanufacturing

“…Overall, these works have paved the way to further eliminate the technical limitations of micro/nanofabrication involving shrink polymers. Microlenses: David Dyer et al 91 reported a microlens array with a focal length of 74 µm obtained with the shrinking technique. They printed dots onto PS films with a laser-jet printer and shrank the patterned films by 66% to create microlens masks.…”

Multi-step proportional miniaturization to sub-micron dimensions using pre-stressed polymer films

“…This effect is particularly important in miniaturization of patterns using pre-stressed lms and has led to previous attempts being restricted to only two steps or to large feature sizes. 11,13 In the current fabrication process, the pre-stressed PS lm is etched by RIE which leaves behind a rough surface in the exposed and etched regions. During the shrinking process the lateral dimensions of the lm reduces by 60-65% while the vertical dimension of the features in it increase by a factor of $6.25 due to volume conservation.…”

UV‐Micropatterned Miniaturization: Rapid In Situ Photopatterning and Miniaturization of Microscale Features on Shrinkable Thermoplastics

“…Among these techniques, micropatterning based on temporary deformation of thermoplastic surface (e.g., via embossing or imprinting) may not work well with shrink‐induced pattern miniaturization because the patterns can easily deform or completely vanish due to the material reflow during the thermal shrinking process. [ 13 ] Subtractive patterning by removal of parts from the bulk material [ 5,11,12,14 ] (e.g., via physical/chemical etching) or additive patterning by material deposition [ 15 ] (e.g., via contact printing) may be utilized for shrink‐intended micropatterning such that the patterned features remain intact in miniaturized form after substrate shrinkage. However, the reliance on special equipment and chemicals still makes most of these prior patterning methods inefficient and unsuitable to allow rapid microfabrication and prototyping in a standard laboratory where budget, resource, or expertise might be limited.…”

“…Therefore, we conjecture that the planar shrinkage of PS film (≈84% reduction in pattern surface area) may have simultaneously resulted in a significant increase in the micropatterns' feature height, leading to the clearly visible micropatterns with pronounced depth profiles after the biaxial miniaturization process. Our efficient UV‐micropatterned miniaturization technique also worked effectively with PO shrink films [ 15,17 ] (Sealed Air Cryovac D955, 100 gauge), generating miniaturized micropatterns with a remarkable ≈95% reduction in pattern surface area (Figure 2c and Figure S3, Supporting Information). The lateral shrinkage ratios of both materials showed a small directional bias (<3%), yet it was consistent in multiple shrink tests and was likely due to slightly uneven pre‐stretch on the films during the polymer extrusion process in manufacturing.…”