On the nanoindentation characterization of Al2O3 thin films grown on Si-wafer by atomic layer deposition

Atomic layer deposition (ALD) is one of the most promising technologies in producing highly conformal coatings with a strict tolerance in thickness. In the study, ALD Al2O3 coatings of thickness varying from 10 to 600 nm are produced on single-side polished silicon (SSP-Si) wafer at the deposition temperature of 300 °C. This study aims in developing a convenient, efficient and inexpensive method of obtaining reliable coating properties via instrumented nanoindentation to meet the demand of silicon-based microelectronics industry. Hardness and elastic modulus are the most common mechanical properties that are directly derived from nanoindentation data. From Oliver and Pharr's analysis, the unloading data can be well described by a power-law relation: P = a(h-hf)m, where P is the indent load, hf is the residual depth after complete unloading and a and m are material constants. The power-law exponent, m, is found only slightly material dependent for a variety of materials, its value falling between 1.2 and 1.6. However, large m values up to 4 are obtained in certain materials in a consistent basis so that instrument/environment origin can be ruled out. According to the elastic assumption and the extended Hertzian treatment on unloading, m>2 has no physical meaning. The large m is often ascribed to inelastic or reverse plasticity in unloading. Silicon is one of the materials that can demonstrate reverse plasticity through phase transformation during unloading. A careful investigation of the unloading phenomena of the silicon wafer established that reverse plasticity occurred when contact depth was larger than about 8 nm, indicated by large m values at about 3.5. Meanwhile, the characteristics of the loading curve indicate that this depth marked the moment at which transition from elastic to elastic-plastic deformation on loading occurred. When the ALD Al2O3 coatings on the Si-wafer were indented, m dropped and varied narrowly around 1.5 over a large range of indent depths. A conclusion was then reached that the plot of m vs. hC (indent depth) could be used to monitor whether the Si substrate was in elastic or elastic-plastic state when the whole system was under external loading. This provided a guideline for picking up correct values of coating property under the conditions that the substrate only provided elastic support of a small fraction of the indenting load. As a result, it was found that the 10% rule commonly used in extracting coating property from a layered structure could be relaxed and the subtle property differences between the Al2O3 coatings and the Si substrate were revealed with good confidence.