Soft Matter Beats Hard Matter: Rupturing of Thin Metallic Films Induced by Mass Transport in Photosensitive Polymer Films

Abstract

The interface between thin films of metal and polymer materials play a significant role in modern flexible microelectronics viz., metal contacts on polymer substrates, printed electronics and prosthetic devices. The major emphasis in metal-polymer interface is on studying how the externally applied stress in the polymer substrate leads to the deformation and cracks in metal film and vice versa. Usually, the deformation process involves strains varying over large lateral dimensions because of excessive stress at local imperfections. Here we show that the seemingly random phenomena at macroscopic scales can be rendered rather controllable at submicrometer length scales. Recently, we have created a metal-polymer interface system with strains varying over periods of several hundred nanometers. This was achieved by exploiting the formation of surface relief grating (SRG) within the azobenzene containing photosensitive polymer film upon irradiation with light interference pattern. Up to a thickness of 60 nm, the adsorbed metal film adapts neatly to the forming relief, until it ultimately ruptures into an array of stripes by formation of highly regular and uniform cracks along the maxima and minima of the polymer topography. This surprising phenomenon has far-reaching implications. This is the first time a direct probe is available to estimate the forces emerging in SRG formation in glassy polymers. Furthermore, crack formation in thin metal films can be studied literally in slow motion, which could lead to substantial improvements in the design process of flexible electronics. Finally, cracks are produced uniformly and at high density, contrary to common sense. This could offer new strategies for precise nanofabrication procedures mechanical in character.

Solving an old puzzle: fine structure of diffraction spots from an azo-polymer surface relief grating

“…Home-made set-up for studying SRG formation in-situ. The set-up consists of three parts: (1) two beam interference lithography, (2) atomic force microscopy (AFM) and (3) diffraction efficiency (DE) measurement (Scheme 1). The two beam interference lithography with a continuous wave diode pumped solid state laser (Cobolt Calypso, =491nm) allows generating well-defined spatiotemporal interference patterns by changing the polarization of two interfering beams in a controlled way.…”

Confocal Raman Microscopy and AFM Study of the Interface Between the Photosensitive Polymer Layer and Multilayer Graphene

“…In order to follow the kinetic of the SRG inscription in-situ, a home-made set up combining optical part for generation of interference pattern and an AFM was applied. 26,27,35 The The experimental details of the Raman microscope and measurements of polymer surface relief gratings have been described elsewhere. 28 Here we describe in details experimental conditions related to the multilayer-graphene flakes (ML-graphene).…”

“…The opto-mechanical forces generated within the polymer film are fairly large: 32,33,34 metal films deposited on top can be ruptured up to thicknesses of several tens of nm. 35 Also adsorbed graphene layers can be stretched during photo-induced polymer mass transport this way. 36 Raman spectroscopy has previously been employed to characterize the deformation of graphene adsorbed on a polymer film put under tensile stress.…”

Light induced reversible structuring of photosensitive polymer films

“…Since the polymer material is in a glassy state, the opto-mechanical stresses needed for the deformation of mechanically stable polymer lms (Young modulus of several GPa) must be quite high of 100 MPa up to $1 GPa, as was recently shown. [37][38][39][40] The process can be viewed as follows: under irradiation inducing cyclic trans-cis-trans isomerization, 41 the azobenzene molecules rotate and re-orient perpendicularly to the electrical eld vector (formation of bulk birefringence grating) causing a re-orientation of the polymer backbones to which they are attached. This creates local gradients in internal stresses within the polymer material, varying with the distribution of the interference pattern and thus induces a viscoplastic deformations of the solid material (formation of topographical grating).…”