Date of Award
Master of Engineering (ME)
Materials Science and Engineering
Luo , Jian
Ballato , John
Flexible electronics are conductive circuits, components and lines integrated onto elastic substrates. These systems often consist of metallic thin films deposited onto thin sheets of glass, metals stainless steel or polymeric materials such as polyethylene or polystyrene. These systems are currently being studied for a wide variety of aeronautic and astronautical applications including: electronic skins, solar sails, and antennae structures. For applications include only moderately elevated temperatures (near 200 ¡C) and low strains (insert range), engineers have proposed incorporation metallic lines onto polymeric substrates. Like traditional microelectronic systems with films on hard substrates, the reliability is controlled by the film-substrate interfacial strength and film structure. Currently, there are no reliable models or experimental studies that examine how these if deformation in the substrate takes places. To design these models, experimental evidence of how these systems deform is needed. This project looks at adhesion energy as a function of thickness, rigid interlayer, and substrate stiffness and substrate deformation.
This study will also examine bulge testing as a way to characterize flexible electronic systems and will be modified to be capable of testing with heat. This study has three major aims. The first was to characterize the fracture pathways and morphology from delamination due to compressive and tensile residual stress. The second aim was to differentiate between deformation seen in rigid substrates from that seen in compliant substrates. The last aim was to design a bulge test system that is capable of controlled strain rate and moderate temperature ranges.
This study looked at three distinct behaviors of metallic films on polymer substrates. The first part of this study looked at how film thickness, interlayer chemistry and thin film residual stress influence the fracture along the metal-polymer interface. This was done by observing fracture on two model systems, W/PMMA and W/PS. Tungsten was chosen since this films system can be deposited with a large rage stresses. Compressive and tensile W films (100-600nm) were sputter deposited onto PMMA and also PS substrates. The PS and PMMA systems were characterized using Hutchinson and Sou's models for rigid elastic systems.
For tensile driven fracture of the interface between W/PS it was found that as film thickness increases so does energy release rate (0.002 J/m2 to 0.011 J/m2) since the true adhesion does not change. With the inclusion of an interlayer the energy release rate decreased by an order of magnitude (.001 J/m2 to 0.035 J/m2). The compressive W/PMMA systems showed the same trends; an increase in energy release rate with an increase in film thickness; however, trends of varying buckle sized were observed on the 400 and 600 nm films. The compressive films without an interlayer were up to 3 orders of magnitude larger than the tensile films (0.8 J/m2 to 2.4 J/m2 ) and the interlayer only slightly increase the energy release rate (0.4 J/m2 to 2.8 J/m2 ). A 'ridge' formation in the W/PMMA around the crack tip has been identified, this substrate deformation not seen in metal-ceramic systems. A large buckles corresponds to a large amount of deformation seen in the substrate, film thickness showed no apparent trend.
Bulge testing, is a testing method capable of characterize the composite modulus, residual stress, adhesion and other system properties of these flexible systems. The Clemson University system was verified for system repeatability.
The Clemson University bulge test system measures the deflection of membranes. Using a Polytec MSA 400 laser vibrometer, capable of taking submicron deflection measurements when pressure is applied using a, Druck DP515 pressure controller, which can control the strain rate (0.1 psi/s to 2 psi/s) and a maximum pressure of 30 psi and is accurate to 0.001 psi. The membrane geometry and diameter were either controlled by a Swagelok clamping system for roll-to-roll applications or micromachined membranes, fabricated at the University of Minnesota Nanotechnology Center. The clamping design allowed for use of 4 mm, 6mm and 10 mm diameter circular membranes. Additional considerations for improving the measurements of flexible systems were adjusting the clamping system design to include a microstage, data curve fitting and super gluing the sample to the clamp to provide accurate and reproducible measurements (that range from +/- 500 μm), using 100 nm Au on 25 μm of Kapton as a model system.
A heater was added to measure the effects of temperature on the deflection of flexible systems. Using ceramic heaters (maximium temperature of 300 ¡C) inserted into holes drilled in the side of the clamp, an Omega CNi1/8 temperature controller (accurate to 0.5 ¡C) was used to regulate the temperate with feed back from a J-type thermocouple. The system was verified using a one μm clamped in a six mm circular clamp and found a biaxial modulus of 150 GPa and residual stress of 180 MPa. However, the system showed that was susceptible to and external resonance frequency. This was done by applying a load of three psi and holding for an interval lasting from ten minutes to one hour. The creep could not be calculated due to interference from an outside system, however the strain rates (from 0.1 PSI/s to 2.0 PSI/s) at lower temperature show no effect on the system. When the temperature was increased to 150 ¡C the higher strain rates showed more deflection. This was not expected because thinner polymer membranes have less strain rate sensitivity.
Reid, Julia, "Deformation and Adhesion of Thin Metallic Films on Flexible Substrates" (2012). All Theses. 1499.