Evaluation of the Adhesion Strength of Ultrathin Gold Coatings on Substrates of Soda-Lime Glass and Cyclo-Olefin-Polymer by Cross-Cut and Scratch Tests under the Influence of a Thermal Shock Test for Use in Biosensors

The current demand for highly sensitive, optical sensors in biodiagnostics has prompted the development of ultrathin metal coatings on a range of substrates. Given the potential attenuation of the signal from a plasmonic sensor for the detection of fluorescent molecules when an adhesion layer between the substrate and coating is employed, this study examines the impact of various factors on the adhesion strength between gold coatings and substrates comprising glass and cyclo-olefin-polymer (COP). The objective is to identify potential configurations for high adhesion strength, thereby eliminating the need for an adhesion layer in the fabrication of optical sensors with gold coatings for diagnostic applications or to utilize a minimal adhesion layer thickness. The influence of pretreatments prior to coating deposition with oxygen plasma, ultrathin adhesion layers of titanium as well as the impact of thermal shock tests with various cycles on the coating's integrity are investigated. Two methods of adhesion strength testing were applied to ultrathin coatings on glass and polymer substrates. Cross-cut and nano scratch tests were used to assess the adhesion of the ultrathin metal layers, which provided qualitative and quantitative results. The results of the two test methods on glass substrates are in good accordance, demonstrating the highest adhesion strength for gold coatings on glass with an adhesion layer, in combination with or without plasma treatment. The cross-cut testing on COP demonstrates a constant high adhesion strength when using an adhesion layer alone or without any pretreatment, while a low adhesion strength is observed in combination with plasma treatment. Following the thermal shock test, alterations in adhesion strength were observed for the remaining configurations. The rise in the coefficient of thermal expansion of COP between +30 and 85 °C in comparison to lower temperatures, as evidenced by thermomechanical analysis, may account for the disparate adhesion strengths of COP following thermal shock tests.