Polarization-selective polymerization in a photo-crosslinking monomer film

Abstract

The molecular order in a smectic monomer is utilized to create a photoreactive material that is highly polarization-selective during its photo-crosslinking polymerization. A dichroic photoinitiator adapts the molecular orientation of its aligned smectic B host monomer which reaches a degree of orientation larger than 0.9. This value correlates to a dichroic ratio, defined as the absorbance ratio of light with its polarization respectively parallel and perpendicular to the orientation axis, of 32. This is substantially higher than the dichroic ratio of similar dichroic initiators previously studied in nematic host monomers which considerably improves the polymerization contrast. The presence of an inhibitor, such as a hydroquinone, in the monomeric mixture enhances the polymerization contrast further. The effect of the inhibitor is due to the close to zero monomer to polymer conversion for the polarization perpendicular to the alignment in the initial stages of polymerization. It is argued that the high polarization-selectivity of the above-described system is potentially useful in nano and/or microstructuring techniques without the use of masks such as polarization holography.

Polarization‐Selective Patterning in an Anisotropic Smectic B Film

“…[ 7 ] The polarization-selectivity of the LC photoreactive material arises from a rod-shaped dichroic photoinitiator (DPI) which mainly absorbs the polarization state of light that is along the long axis of this molecule. [ 8,9 ] An example of a DPI is shown in Figure 1 a. This material photodissociates into free-radical fragments when addressed with UV light with its electrical fi eld vector parallel to the long axis of the molecule and is largely insensitive for light with its vector perpendicular to that.…”

“…Previous studies have shown that the polarization-selectivity of the LC photoreactive mixture largely depends on the degree of order of the LC material. [ 8,9 ] A preferred liquid crystal phase for our purpose is the smectic B where the rod-shaped monomer molecules and its rod-shaped DPI are densely hexagonally packed in a layered organization. The acrylate molecules we used to create a polymerizable liquid crystal mixture (LCM) are shown in Figure 1 a.…”

Enhanced lithographic resolution using longitudinal polarization state of light

“…The threshold is caused by the presence of the inhibitor, which delays the polymerization process. 31 The threshold is approximately 100 and 250 nJ for radially and linearly polarized light, respectively, which means the LC photoresist shows selectivity toward absorbing the longitudinal component. This confirms the homeotropic alignment of the resist, which is needed to establish the polarization-selective polymerization.…”

“…4(a) ]. 31 To promote absorption of the longitudinal polarization component, a homeotropic alignment of the photoresist is required where the long axis is orthogonal to the substrate's surface [ Fig. 4(b)].…”

“…Recently, a polarization-selective photoresist was developed by coaligning a dichroic photoinitiator, which is the polarization-selective component, in a reactive liquid crystalline (LC) monomer host. 31 Uniaxial alignment of the mixture enables the selectivity of the photoresist toward a single polarization state of light that is parallel to the alignment. Upon illumination of this resist, an insoluble polymer network forms, i.e., it is a negative photoresist.…”