Mechanical failure in thin‐film nonlinear optical polymers: Structure and processing issues

Stress developed during the spin‐coating process often initiates cracks leading to failure in nonlinear optical devices. The primary intrinsic reasons for this were shown to be a lack of a secondary relaxation process below the glass transition as well as an insufficiently large molecular weight to enhance toughness by chain entanglement, as detected by dielectric measurements and viscoelastic shear moduli, respectively. A detailed systematic synthetic effort was undertaken to increase the chain length through improved free‐radical methods. Results showed that as the molecular weight was raised chain entanglements were observed and film toughness was improved as evidenced by the thicker, crack‐free film attained. As well as thermal stresses, solvent stress cracking was identified as a major source of fabrication defects in the multilayer spin‐coating process. A plasma polymer layer deposited on the film surface was successful as a barrier layer preventing solvent penetration and crack formation. © 1994 John Wiley & Sons, Inc.