(Abstract) Recently, functional films are attracting attention in electronic devise industry. Typically, a functional film is a thin film deposited on brittle substrate, such as silicon and quartz glass which is chemically stable. It is important to prevent interfacial delamination between film and substrate. Thus, quantitative evaluation of adhesion strength for such films is strongly demanded. This study newly developed a laser shock-wave adhesion test (called LaSAT) for thin films deposited on brittle substrates. This enables us to quantitatively evaluate the adhesion strength in non-contact manner. It is noted that the brittleness of such a substrate is very weak for impact loading, and it is difficult to conduct general LaSAT experiment. Therefore, this study improved the specimen structure using multi-layer material system. We successfully conducted LaSAT for delamination of a nano film on fused silica substrate. The critical laser energy at which delamination occurred was obtained for various films and evaluate adhesion strength.(Method) In LaSAT experiment, pulsed Nd:YAG laser is irradiated to the grease layer, which forms on the back surface of substrate to generate laser ablation. Due to the laser ablation, a strong compressive stress wave is excited and propagates in the specimen. When the compressive stress wave reaches opposite free surface, the free end reflection occurs. Due to this wave reflection, tensile stress wave generates, traveling to the interface. At the critical stress level, interfacial fracture occurs. Thus, we can quantitatively measure the tensile adhesion strength under impact loading in a non-contact manner. This study newly developed the sample structure using multi-layer system, which is applicable to nano films delamination. Indeed, the present thin film is metallic film deposited on a brittle substrate of fused silica. The thickness is about 200 nm. Due to the brittleness of substrate, it is needed to prevent brittle fracture of substrate. In this study, a dummy substrate (bonded with metallic thick substrate) was employed. Two types of specimens were prepared: one is Cu thin film and the other includes Ti film as an intermediate layer.(Results) As a result of the experiment, delamination occurred at 260 mJ laser energy for Cu thin film, while delamination occurred at 360 mJ when the Ti interlayer was added. This result indicates that the adhesion strength with Ti layer increased. This result is similar to previous papers. In order to confirm that the delamination occurred at the interface, other experiments were conducted for no film material (only dummy substrate bonding). As a result, bonding delamination occurred at 400 mJ, but there is no damage on the substrate. In other words, it was confirmed that the delamination does not occur inside multi-layer specimen, and delamination occurs at just the interface. Therefore, the adhesion strength of nano film under impact loading was successfully evaluated by using LaSAT in this study. It was also found that the adhesion strength was improved by the Ti intermediate layer.(Conclusion) In this study, we newly developed the LaSAT which can be applicable to thin films on brittle substrates and evaluated the adhesion strength of the thin films under impact loading. As a result, it was found that delamination at the target interface could successfully occurred without any damage of other layers. It was also found that the adhesion strength was increased by adding an intermediate layer.

Development of Laser Shock-Wave Adhesion Test for Nano Films Deposited on Fused Silica

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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