Study On The Formation, Mechanism And Application Of Friction-Induced Hillock On Monocrystalline Silicon

Nanotechnology dominates the development of science and technology in21st century. As a research area preferentially supported by a great number of countries, nanotechnology is changing the production and lifestyle of the human. The nanofabrication provides an approach for the application of nanoscience and nanotechnology. However, achievement of ultra tiny structures poses a significant challenge in current fabrication techniques as the scaling down of device dimensions continues. It remains essential to explore new nanofabrication methods. Therefore, the present study on nanofabrication is of great significance in both developing nanotechnology and promoting the competitiveness in science and technology of our country.It was found that protrusive nanostructures can be produced on monocrystalline silicon surface during nanoscratching under an ultra low normal load. In the present study, the effect of experimental parameters, such as ambience, normal load, number of scratching cycles, sliding velocity, and so on, on the hillock formation was investigated systemically by an atomic force microscope. With a scanning X-ray microprobe and a scanning Auger nanoprobe, the chemical composition of the hillock surface was carefully detected. Through the analysis of the microstructure of the cross-section of the hillock by a high-resolution transmission electron microscope (HRTEM), the formation mechanism of the hillock was further established. Finally, the mechanical performance of the hillock was studied by a nanomechanical test instrument and the friction-induced nanofabrication method was proposed. The main experimental results and conclusions can be summarized as below.(1) The formation of friction-induced hillocks was observed in vacuum in the first time. The formation of friction-induced hillocks was observed in vacuum (<5.0×x10-6Torr) by nanoscratching and in-situ scanning with a diamond tip on an atomic force microscope. It was also noted that the sliding in vacuum will facilitate hillock formation compared to that in air.(2) The formation rules of friction-induced hillocks were clarified. The friction-induced hillock will be produced before the yield of monocrystalline silicon. An increase in the applied normal load will lead to the formation of surface depression. The height and volume of the hillock increase with the applied normal load or number of scratching cycles and then trend to level off, but decrease with the sliding velocity (10～1000μm/s) either in air or in vacuum. The crystal plane revealed an obvious effect on the hillock formation, and higher hillock will be produced on the crystal plane with low elastic modulus. Compared to original silicon surface with native oxides, lower hillocks can be produced on the hydrophobic, hydrophilic or self-assembled film coated silicon surface, and hence silicon surface with adsorbed water film or chemically grafted film can to some extend limits the hillock formation.(3) The formation mechanism of the friction-induced hillocks was explored. Detection of the chemical composition by a scanning X-ray microprobe and a scanning Auger nanoprobe showed that even though the formation of friction-hillock was the coupled results of oxidation and mechanical interaction, the latter was the dominative one. Based on the detection on the cross-sectional microstructure by the HRTEM, the friction-induced hillock was structured by mainly an amorphous silicon layer towards the top and plenty of deformed silicon matrixes beneath. Further analysis showed that the silicon amorphization contributed chiefly to the hillock height.(4) The friction-induced nanofabrication method was initially established. The friction-induced hillocks on silicon and quartz substrates reveal the similar mechanical performance as their substrates. Nanoscratch results show that the friction-induced hillocks on silicon and quartz surfaces can withstand the typical contact and sliding in dynamic devices. With the friction-induced process, surface patterned structures, such as nanodots, nanolines, surface mesas, can be produced by scratching along the designed tracks under a given normal load, and the friction-induced nanofabrication method was initially established.To sum up, based on the investigation on the formation and mechanism of the friction-induced hillocks, a new nanofabrication method was proposed. Without applying voltage and any template, the nanofabrication can be performed directly on silicon, quartz, glass surfaces, and so on, and the fabrication process brings hardly any pollution to environment. Moreover, a selective post-etching can optimize the aspect ratio of the friction-induced hillock, which provides a new approach to produce the template for nanoimprint lithography. Therefore, the present study will not only enriches the knowledge in nanotribology, but also plays a positive role in promoting the SPM-based nanofabrication.