ICMCTF2009 Session E2-1: Mechanical Properties and Adhesion

The impact of adhesion on the reliability of small systems that utilize metals on polymers, both on the micro- and meso-scale, can directly determine the reliability of these devices in service. In this paper the toughness of the gold – polyimide and gold – polymethyl methacrylate interface will be measured experimentally, and the results of the toughness will be correlated to sample preparation, the presence of an adhesion promoting layer, and the results of reliability testing. These systems are applicable to many modern electronic technologies incorporating flexible substrates in order to increase effectiveness and reliability over a wide range of conditions. Microelectronic devices which utilize polymers coated with patterned thin metal films are already being developed for such uses as electronic paper, computer displays, and smart textiles. The properties of flexible electronics also make them a viable option for small, implantable electrodes for recording bra in signals. They are also possible candidates for power generation in vibration harvesting applications. The need for quantitative methods arose from the desire to predict film adhesion under a variety of externally applied stresses and also directly compare different film systems. These quantitative measuring techniques can be subdivided into two different groups. The first group uses compressive stresses to induce fracture along an interface and then estimates the adhesion energy by modeling the delamination morphology the stressed overlayer technique; while the second group measures the crack growth rate as a function of applied load. This paper will demonstrate the toughness of polymer – metal system interfaces is related to both the type and presence of adhesion promoting layers (Cr, Ti-W, SiO₂, and Ta) and alterations in surface chemistry through plasma treatments. Both methods show improvements, with plasma treating causing an improvement of the toughness of the gold – Kapton interface by 50%, and adhesion promoting layers improving the toughness of the gold – PMMA system a similar amount. Initial results on a piezoelectric polymer (PVDF) will also be presented.

The authors wish to acknowledge the financial support of the National Institute of Mental Health (NIMH 60263 & NIMH 71830) and the Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

The essential elements in different technological devices, range nowadays at a submicron scale. At this scale the mechanical properties of thin films, as their response to external solicitations, differ from their bulk counterpart, essentially because their microstructure. As a consequence, there is a permanent lack of information on films behaviour, because microstructure derives from obtention conditions, and these conditions may differ from laboratory, to another.

We present and discuss results coming out from in-situ micro tensile experiments, performed on two sort of “thin” samples: electroplated Copper deposited on a polymer substrate, and self standing thin films of gold and aluminum.

The insitu experiments are performed with a dedicated device and the samples are obtained by lithography as wet and/or dry etching.For the self standing samples (thinner than oneµm) the results are discussed in terms of deduced mechanical parameters, and related to the microstructure.

For the electroplated Copper on polymers, different patterns of the thin Copper electroplated depositions were studied. In this case, we point out the degradation of the film when applying to the system cyclic solicitations; as the effect of singularities, associated to the geometry of the Copper patterns on the substrate.

In the presentation procedures and formulae for a new and more general surface tester concept will be given and discussed. The concept is based on the idea that the next generation of surface testers will provide the means to use all degrees of freedom of movement a probe on a sample surface could perform. Thus, in addition to the ordinary normal stiffness also lateral, tilting and twisting stiffness will be measured and used in the subsequent parameter determination of the investigated materials. It will be demonstrated that such a concept would not only completely solve classical problems like "pile-up" and "sink-in" it would also supersede the need of area function calibration for the indenter tips and allow direct measurement of local intrinsic and residual stresses, anisotropy and many other things, too.

¹G. M. Pharr, A. Bolshakov: J. Mater. Res., Vol. 17, No. 10, Oct 2002

Deformation and fracture of thin films on compliant substrates are key factors constraining the performance of emerging flexible substrate devices. However, the effects of substrate compliance on interfacial film fracture are not well defined. We are therefore studying these effects for sputter deposited thin hard tungsten films on substrates that span two orders of magnitude in compliance. Following film deposition, high compressive film stresses triggered spontaneous delamination and buckling in films on the PMMA substrates with buckle height-to-width ratios much greater than elastic theory predicts due to substrate yielding. As a consequence, interfacial fracture energies calculated using elastic buckle theory are not an accurate representation of the energy for crack growth. We therefore used finite element analysis with a cohesive zone model to simulate interfacial crack growth. When substrate yielding was included in the analysis, calculated energy release rates w ere substantially higher than those calculated using an elastic film-rigid substrate approach. Moreover, calculated height-to-width ratios matched observations. In this presentation we will show that combining experimental measurements for film failure and simulations with cohesive zone models provides a means to accurately describe and predict device performance.

This work is supported by Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.