Metal films on polymer substrates stretched beyond 50%
Wiki entry by : Dongwoo Lee, AP225 Fall 2010.
Paper in this Wiki : Nanshu Lu, Xi Wang, Zhigang Suo and Joost J. Vlassak, Metal films on polymer substrates stretched beyond 50%. Applied Physics Letters 91, 221909 (2007).
The authors demonstrated that metal films on polymer substrates with good adhesion shows long stretching (50%) in a tensile test, which is an beneficial mechanism for the flexible electronics.This mechanism was firstly demonstrated with FEM analysis on the other papers but previous researches have failed to realize such long stretchable metal films. The main reason for this failure was debonding between the materials. The authors solve this problem introducing Cr layer to improve the adhesion and investigated the main mechanism of strain localization.
For one set of specimens, Cu(1um)/Cr(10nm)/Kapton(12.7um) layer was created, while Cu(1um)/Kapton(12.7um) layer was used for the other set of specimens. Then, while stretching the films with a tensile tester, the resistance of the Cu film was measured. Given that there is no volumetric change, the system should be dominated by following equation:
<math>R/R_o = (L/L_o)^2</math> (1)
The left graph in the Fig 1 shows the predicted curve from the eqn(1) and the experimental data for the Cu(1um)/Kapton(12.7um) film. At 30% of elongation, discrepancy between the two values appears. The authors attribute this phenomenon to the debonding between the Cu layer and the Kapton layer. They increased the adhesion between those two layers by introducing 10nm of Cr layer and much elongation was possible as shown in the right plot in the Fig. 2. In the plot, the experimental data shows a good agreement with the predicted line. The elongation of 50% was possible in this case.
To investigate the debonding mechanism, the authors took images showing the strain localization as shown in Fig. 3. The region where has a crack was cut and FIB images were taken. Those figures show debonding between the layers as expected. Further increase in the adhesion may result in even longer elongation of the metal/polymer film.
Soft Matter Discussion
The article shows a brilliant idea for increasing the elongation of the metal thin films. The combination of soft-rigid materials is remarkably beneficial for the design of multi-functional systems as most of previously circuits has the limitation of fabrication on flexible substrates and are fabricated on hard substrates such as Si. In my opinion, even longer elongation of ductile metals may be possible if Cr layer is thicker. The paper reported that the Cr layer was just 10nm, and becomes brittle and debonding occurs on the layer. This make sense because a very thin metal film has a limited number of dislocations which limit the its plasticity. One more possible idea for improving the elongation is that more flexible materials can be used under the metal film. The Young's modulus of Cu is about 120GPa and that of Kapton is approximately 2.5GPa. If one introduces a few more layers whose Young's modulus are less than 2.5GPa between the Kapton and Cu layer, it will be easier to fill out the gap (debonding region) with the flexible material. Then, one can weaken the strain localization.