Photocrosslinkable polymers for temporally stable organic nonlinear optical materials and dual-tone photolithography
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The achievement of thermally stable second-order nonlinear optical (NLO) coefficients in polymers is an important but elusive goal in obtaining useful materials for commercial devices. In order to demonstrate the effect of photocrosslinkable groups on enhancement of stability of NLO properties, a series of NLO polymers were investigated, which contain side-chain NLO chromophores with a variable number of crosslinkable methacrylate groups attached to the chromophore opposite the main chain attachment point. Following electric field poling, polymers were crosslinked by irradiating polymer samples containing a photoinitiator. Decay of second-order NLO properties was measured by second harmonic generation (SHG), and stability of SHG coefficients was significantly enhanced after photocrosslinking for polymers containing multiple crosslinkable groups on the chromophore. Addition of reactive diluents further enhanced stability. The enhancement of thermal stability demonstrates the efficacy of photocrosslinking and suggests that the stability improvements may carry over to systems with more efficient chromophores and more stable polymer backbones. The addition of photocrosslinkable materials is also exploited in photoresists to mitigate alignment limitations on flexible substrates during photolithographic patterning by enabling imaging of two device layers in a single lithographic exposure. To facilitate the simultaneous patterning of two device layers, a new photoresist system was developed which is able to store and transfer two images concurrently. The dual-tone photoresist system has the ability to store two independent latent images, distinguished by the incident exposure light wavelength, while remaining compatible with reactive ion etch image transfer processes. A modified chemically amplified photoresist for 193 nm lithography containing an added photobase generator (PBG) leads to a dual-threshold behavior of the dissolution vs. dose contrast curve that enables pitch division of patterned images. To improve these resists, a study of resist components’ effects on line edge roughness and pitch division process windows was performed. The kinetics of two-stage PBGs are compared to single-stage PBGs with a view toward achieving higher acid latent image gradients at the line edge. Application of nuclear magnetic resonance spectroscopy to determine the rate constants for resist components in thin films is demonstrated and discussed as a tool for formulating dual-threshold resists.