Fabrication of Flexible Binary Amplitude Masks for Patterning on Highly Curved Surfaces

Abstract

This paper describes soft lithography methods that expand current fabrication capabilities by enabling high‐throughput patterning on nonplanar substrates. These techniques exploit optically dense elastomeric mask elements embedded in a transparent poly(dimethylsiloxane) (PDMS) matrix by vacuum‐assisted microfluidic patterning, UV–ozone‐mediated irreversible sealing, and chemical etching. These protocols provide highly flexible photomasks exhibiting either positive‐ or negative‐image contrasts, which serve as amplitude masks for large‐area photolithographic patterning on a variety of curved (and planar) surfaces. When patterning on cylindrical surfaces, the developed masks do not experience significant pattern distortions. For substrates with 3D curvatures/geometries, however, the PDMS mask must undergo relatively large strains in order to make conformal contact. The new methods described in this report provide planar masks that can be patterned to compliantly compensate for both the displacements and distortions of features that result from stretching the mask to span the 3D geometry. To demonstrate this, a distortion‐corrected grid pattern mask was fabricated and used in conjunction with a homemade inflation device to pattern an electrode mesh on a glass hemisphere with predictable registration and distortion compensation. The showcased mask fabrication processes are compatible with a broad range of substrates, illustrating the potential for development of complex lithographic patterns for a variety of applications in the realm of curved electronics (i.e., synthetic retinal implants and curved LED arrays) and wide field‐of‐view optics.

Programmable Chemical Gradient Patterns by Soft Grayscale Lithography

“…In this paper, we describe a simple (and potentially low‐cost) protocol that can be used to fabricate mechanically flexible grayscale amplitude masks. This work extends patterning capabilities previously reported by us that uses vacuum‐assisted microfluidic filling to create optically dense binary amplitude masks 44. The modified procedures use a carbon‐black‐filled PDMS (Dow Corning, Sylgard 170, herein referred to as bPDMS) mixed with varying proportions of cPDMS (Dow Corning, Sylgard 186), to produce composite materials that provide a wide spectrum of optical absorbance levels.…”

“…Flexible binary amplitude masks are fabricated44 by confining a specifically formulated bPDMS within a cPDMS microfluidic channel pattern generated by means of traditional soft lithography 3, 4, 42, 43, 45. This mask fabrication scheme is directly adaptable to grayscale lithography by replacing an optically dense bPDMS mask element with mixtures that provide a wide range of grayscale levels through optimized dilutions with cPDMS.…”

“…Although we have not done so here, it should be possible to adapt discrete feature filling and printing protocols that have been described in the recent literature. A PDMS stamp containing higher‐ resolution relief features could potentially be selectively filled with the masking material through spin‐coating schemes, and the residual layer removed via etching44 or doctor blading49, 50 to reveal an array of filled discrete features embedded in a clear polymer matrix. Alternatively, we foresee that direct printing methods, specifically direct ink writing,51, 52 could be adapted to realize 3D forms of the polymeric masking material by controlling the deposition conditions (flow rate, drawing time, nozzle size).…”

“…The substrate used in this demonstration is cylindrical, allowing for the mask to be wrapped around the surface without subjecting the pattern to significant distortions. Patterning on spherical surfaces is also possible, but would require careful design of the mask to compensate for the distortions and displacements that occur when the mask is deformed from its planar form to that allowing conformal contact with the curved substrate 44…”

Flexible Plasmonics on Unconventional and Nonplanar Substrates

“…However, the employed surface chemistry procedure (based on mercapto silane) is specifi c to the PDMS substrate and gold. Furthermore, by harnessing both the benefi ts of soft litho graphy and traditional contact photolithography, Bowen and Nuzzo [ 18 ] presented patterning protocols for defi ning photo resist patterns on highly curved substrates. But it is challenging to fabricate features below 0.1 μ m using their method.…”