Adhesion characterization of SiO2 thin films evaporated onto a polymeric substrate
Abstract
To ensure good adhesion between a 200 nm thick silicon dioxide layer and a 4.5 μm thick hardcoat polymeric coating, a better understanding of mechanisms of adhesion at this interface is needed. To reach this purpose, focus is placed on two axes: characterizing mechanical properties of materials composing the system and in parallel, finding an applicable and effective method to quantify adhesion. Small dimension of SiO2 thin film makes it challenging to accurately characterize it. Hence the use of both nano-indentation and AFM to attempt assessment of SiO2 thin film elastic modulus Ef; taking into account limitations and uncertainty associated with each technique. Elastic modulus of SiO2 thin film determined by nanoindentation is roughly 50 GPa on a wafer substrate and 15 GPa on a lens substrate. As for AFM, modulus measured is approximately 56 GPa on a wafer substrate and 22 GPa on a lens substrate. This highlights significant influence of substrate for both techniques. Impact on mechanical properties between SiO2 thin films under different intrinsic stresses was also investigated. Results suggest that higher density of SiO2 thin film leads to higher elastic modulus.
To quantify adhesion, micro-tensile and micro-compression tests were performed. Micro-tensile experiments give ultimate shear strengths of hardcoat-substrate interface ranging from 9 to 14 MPa. Values of energy release rates of SiO2 / Hardcoat, range from 0.1 J/m² to 0.5 J/m², depending on moduli values found on wafer or lens substrate.
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Introduction
Ophthalmic lenses are made of plastic polymeric substrates usually coated with functional treatments composed of 5 to 15 layers, ranging from micrometers to nanometers. The first treatment consists of a primer, conferring impact resistance properties to the lens. A hardcoat with nanoparticles, is then deposited on top of this primer, bringing anti-scratch properties to the system. Both primer and hardcoat are within the micrometer scale and are deposited by wet chemical methods. Nanometric anti-reflective stacks are then evaporated onto the hardcoat by vapor deposition technology, to enhance wearers’ comfort. Interface quality is essential to ensure stability and durability of ophthalmic multi-layer. In fact, insufficient adhesion between layers causes higher susceptibility to emergence of defects or delamination. Occurrence of these phenomena affects wearers’ comfort and has tremendous impact on products’ lifetime. Therefore, it is essential to develop a method enabling quantitative assessment of interface quality, which will ultimately enable to ensure high product reliability.
General behavior of whole system must be fully characterized. To reduce the complexity of this characterization, focus is first placed on studying the interface between the anti-reflective stack and the hardcoat. More specifically, this paper centers around the SiO2 thin film / hardcoat interface, which is particularly sensitive because of mechanical and dimensional contrast between SiO2 thin film and hardcoat.
This entails studying both materials composing each layer and interface. Interface quality can be evaluated through quantification of adhesion energy. Over 300 adhesion tests are referenced in the literature [1]. Choice of appropriate method to access adhesion is made according to compatibility with system under study, repeatability, ease of implementation, cost effectiveness and representativeness of defects observed in real life versus defects generated by mechanical tests. Techniques commonly used on similar structure - rigid thin film of 200 nm on soft substrate - include Superlayer, Laser adherence test (Lasat), Bulge test, Pull-off test, three-point bending, micro-tensile and micro-compression tests. Superlayer adhesion test [2], consisting in depositing a highly stressed layer on top of interface of interest, is examined. Lau [3] determines interfacial fracture toughness ranging from 12 J/m² to 24.5 J/m² for dry samples of 0.9 μm silica on 20 μm epoxy, using stressed Chrome Superlayer. Several attempts to generate spontaneous delamination of studied SiO2 thin film using a Zirconium Superlayer were ineffective. This attests a strong adhesion at SiO2 / Hardcoat interface. Mechanical test generated by acoustic shock wave, Lasat [4], Bulge test [5] were considered but have not been implemented. Indeed, Lasat test involves laser shock wave that is likely to alter soft polymeric substrate and therefore, is inadequate to structure being studied. Bulge test requires etching step which is delicate considering small dimensions at stake.
Conclusion
Elastic moduli of SiO2 Type A and B were determined by nano-indentation, giving results ranging between 46 and 58 GPa for Si wafer substrate and 14 and 17 GPa for polymeric lens substrate. Same measurements were carried out using AFM. Moduli found are 48 and 64 GPa for Si wafer substrate and 22 and 23 GPa for polymeric lens substrate. Modulus of SiO2 Type B was found to be roughly 20% higher than SiO2 Type A, by nano-indentation. Regarding the high standard deviation of moduli measured by AFM, no significant difference between elastic moduli of SiO2 Type A and B was observed by AFM on both substrates. However, an important difference between moduli of SiO2 on lens and on Si wafer was observed. This exposes unexpected influence of substrate on mechanical measurements using AFM, which has been hypothetically attributed to impact of Peak Force high frequency oscillations on viscoelastic substrates. Adhesion characterization by microtensile experiments gives ultimate shear strengths of interface hardcoat-polycarbonate substrate ranging from 9 to 14 MPa. Detection of cracking of SiO2 on hardcoat is ongoing. EDX profiles over delaminated area obtained by micro-compressive tests, strengthen the hypothesis that delamination occurred at the interface of interest. This gives values of energy release rates ranging from 0.1 J/m² to 0.5 J/m², depending on moduli values found on wafer or lens substrate. Repeatability and reproducibility studies are undergoing to fully validate this adhesion test. Future immediate perspectives include comparison of energy release rates G for different configurations of SiO2 thin film. Broader perspectives consist of simulating the microcompressive adhesion test. Simulation is indeed the only method known to access information such as plastic dissipation or deformation, allowing better analysis of adhesion test