Full Length ArticleNanoscratch of aluminum in dry, water and aqueous H2O2 conditions
Graphical abstract
Introduction
Deformation behavior at sliding interface has been considered for a long time as a significant subject in a wide spectrum of sciences, ranging from industrial engineering to atomistic simulations of nanometer-scale systems [1], [2]. Macroscopic understanding of the material deformation with asperity contacts at sliding interface has been provided by continuum approaches, which is applied to design a tribosystem [3]. Lately, a new challenge has been to extend the validity of these continuum contact laws down to the nanoscale [4], [5]. Deformation behavior in association with physical and chemical interactions at sliding interface has not yet been integrally elucidated at the nanoscale length. Fundamental insights from individual nanoscratch at sliding interface could contribute to the understanding of wear and atomistic material removal at the nanoscale, and to being benefit for reliable miniaturized devices with optimal mechanical performance.
As well-known [6], [7], the investigation of sliding between a single nano-particle with high hardness and substrate surface is crucial for exploring the material removal mechanism in nano-manufacturing process, such as chemical mechanical planarization (CMP). This technology is a stand and ubiquitous method to achieve global planarization on wafers in IC industry [8], [9], [10]. Variable strategies have been employed to reveal scratch mechanism of a single abrasive particle with substrate during CMP at the atomic scale [10], [11]. One of the most promising methodologies is to study the scratch process at sliding interface by molecular dynamics simulation (MD), as a powerful tool to capture the material deformation behavior at the nanoscale. Cho et al. [12] simulated the scratching of copper for a number of contact geometries at the atomic scale with a rigid Ni tip. It was found that the stick-slip phenomena is accompanied with the elastic and plastic deformation. In addition, the research results of Mo et al. [5] suggested that the real atomic contact area at sliding interface leads to the deformation behavior instead of asperities contact at the nanoscale. However, the above reasonable approaches could not reveal insights of the chemical interactions at sliding interface, which plays an essential role in several nano-manufacturing processes. Most recently, principles molecular dynamics (FPMD) [13] and ReaxFF reactive molecular dynamics (RRMD) [14] were subsequently proposed to evaluate the abrasive scratch at sliding interface in aqueous H2O2 condition. These findings advanced the understanding of chemical bond interactions at sliding interface. However, an understanding of how these laws, i.e., contact elasticity and contact friction, are modified in real nano-sliding system with a high number of interacting objects and imperfect surface states remains a challenging obstacle, since MD deals with ideal systems with well-controlled surface states.
Another alternative means to describe the scratch process at sliding interface is carried out by atomic force microscope (AFM) at the nanoscale [15]. Generally, the AFM tip is considered as a nanoparticle to mimic the friction behavior of a single abrasive on various surfaces. Bhushan et al. [16] reported that the material movement along the sliding tip is believed to occur as a result of plastic deformation of the tape surface in dry nanoscale contact. As an extension of AFM simulation, Xu et al. [17] established a novel method to describe the deformation behavior at sliding interface based on the modeling of evolution of frictional coefficient in the nanoscratching process of copper. However, the depending of deformation behavior on chemical effect in the nanoscratching process remains elusive. Recently, Chen et al. [18] firstly demonstrated a single atomic layer removal by AFM experiment at the sliding interface of Si wafer with SiO2 tip in water condition. These works disclose invaluable information about material deformation behavior at sliding interface.
However, the general CMP process is remarkably affected by the chemical reactions of aqueous slurries [19], [20], [21]. Therefore, it is of importance to explore the deformation behavior at sliding interface with aqueous chemical solutions. Concurrently, a specific application of Al-CMP is implemented in the “gate-last” integration scheme for high-K/metal gate transistors with the <28 nm technology node [22], [23], [24]. Normally, H2O2 as an oxidizer has been widely used in Al-CMP process, which does not cause metal ion contamination of the wafer [25]. Therefore, in present study, to elucidate the CMP process in an aqueous chemical solution, including chemical reaction and mechanical friction, the deformation behavior of Al substrate at the nanoscale was investigated by a nanoscratch approach in variable conditions involving dry, water and aqueous H2O2.
Section snippets
Sample preparation
Physical vapor deposition (PVD) technique was used to deposit Al films on commercial blank silicon (1 0 0) substrate. Before scratching, Al surface was cleaned in low pH solutions prepared by HNO3 (0.5 wt%) addition to remove any contaminated surface oxides and thoroughly rinsed with deionized water and dried by nitrogen gas. All samples were kept in clean petri dishes in a desiccator to avoid the formation of surface oxides. The surface morphology of Al film was shown in Fig. 1. It can be seen
Variation of scratch depths in different lubrication conditions
In present nanoscratch test, the normal force was programed to increase from 0 to 500 µN with the sliding distance increasing from 0 to 10 µm. Scratching of a surface involves indentation and lateral movement of the tip. Fig. 2 shows the variation of scratch depth as a function of lateral displacement in different lubrication conditions. The maximum scratch depth of ∼200 nm is observed in dry condition at load of 450 µN in Fig. 2(a). With the adding of water in Fig. 2(b), the scratch depth
Deformation mechanism under variable lubrication conditions
The deformation of substrate in nanoscratch process is one of the major challenges for understanding the material removal mechanism at atomistic level in frictional process. Mo et al. [5] concluded that the friction force (Ff) is proportional to the real contact area (Areal), which is formed by atoms interacting across the interface.where is an effective shear strength of contacting bodies. , where N is the number of atoms forming interaction across the interface, and
Summary
This paper investigated the nanoscale deformation behavior at sliding interface utilizing the nanoscratch experiment of Al with ramp load model in variable conditions including dry, water and H2O2 conditions, which has not been discussed previously. The results showed that deformation of aluminum in the nanoscratch process could be divided into two regimes based on the evolution of frictional coefficient. On one hand, as the load is less than 100 µN, the elastic deformation dominates the
Acknowledgements
This research work was financially supported by National Natural Science Foundation of China (Grant numbers: 51775360, 51501121, U1533101), China Postdoctoral Science Foundation (Grant number: 2015M571800), Jiangsu Postdoctoral Science Foundation (Grant number: 1402121C).
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