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Cite This: Langmuir 2019, 35, 7735−7743
Temperature-Dependent Mechanochemical Wear of Silicon in Water: The Role of Si−OH Surfacial Groups Zhaohui Liu,† Jian Gong,† Chen Xiao,† Pengfei Shi,† Seong H. Kim,†,‡ Lei Chen,*,† and Linmao Qian*,† †
Tribology Research Institute, State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China Department of Chemical Engineering and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
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ABSTRACT: Mechanochemical wear has attracted much attention due to its critical role in micro/nanodevice applications, reliable microscopy, and ultraprecision manufacturing. As a process of stress-associated chemical reactions, mechanochemical wear strongly depends on temperature; however, the impact mechanism is not fully understood at any length scale. Here, we reported different water-temperature dependence of mechanochemical wear on two typical single crystal silicon (Si) surfaces, involving oxide-covered Si partially terminated with Si−OH groups and oxide-free Si fully terminated with Si−H groups. As the water temperature increased from 10 to 80 °C, the mechanochemical wear of the oxide-covered Si underwent a process from no obvious surface damage to significant material removal but that occurring at all temperatures decreased gradually on the oxide-free Si surface. The opposite temperature-dependence was found to have a strong relation to the growth or degeneration of the Si−OH surfacial groups. The mechanochemical wear on the both Si surfaces decreased with the Si−OH coverage rising, which facilitated the growth of strongly hydrogen-bonded ordered water and then suppressed the chemical reaction between the sliding interfaces. These results can provide new insight into the mechanism of the surrounding temperature affecting the reliable micro/ nanodevices, manufacturing, and microscopy.
1. INTRODUCTION As one of the most common and widespread structural materials of micro/nanodevice, atomic force microscope (AFM) tips, and semiconductor chips, single crystal silicon (Si) is susceptible to mechanochemical wear which strongly limits the reliability and lifetime of micro/nanodevice and AFM tips,1−4 but helps to fabricate atomic-smooth surface in the semiconductor chips’ manufacturing, such as chemical mechanical polishing (CMP).5−7 In mechanochemical wear, Si material removal that occurs under extremely low mechanical interaction is dominated by stress-associated chemical reactions;1,2,8−11 here, temperature should play a significant role in the wear process. Recently, a reaction theory based on Arrhenius kinetics was developed to model the mechanochemical wear at nanoscale and even at atomic level separated from the mechanical wear described by Archard wear law at macroscale.9−13 The Arrhenius-type wear model states that the rate of mechanochemical wear has a monotonously exponential relation with the normal stress and the absolute temperature.14−16 It has been successfully applied to predict the normal stress dependence of atomic wear both in experiments and molecular dynamics (MD) simulations.1,2,17 Nevertheless, there is limited research to detect the role of temperature in the © 2019 American Chemical Society
mechanochemical wear although the complex temperature dependence of the wear originating from the chemical reaction has been experimentally observed.18 For instance, in the CMP process of Si, the volume of material removal increases to a peak value and then decreases with the increase of liquid temperature. A computational simulation for this process indicates that the liquid temperature rising can facilitate the chemical activity of the reaction pairs and enhance the mechanochemical reaction; conversely, the mechanical interaction between the contact interface is reduced due to the solid material softening, lowering the mechanochemical wear.19 Experimentally, mechanochemical wear was found to depend on not only intrinsic properties of materials such as hardness and bond energy, but also extrinsic factors of the sliding system such as environments and surface properties.20−26 Generally, all of these factors are sensitive to surrounding temperature to some extent. However, only little is known about the mechanisms giving rise to the temperature-dependent mechanochemical wear of Si; in particular, the role played by Received: March 17, 2019 Revised: May 12, 2019 Published: May 24, 2019 7735
DOI: 10.1021/acs.langmuir.9b00790 Langmuir 2019, 35, 7735−7743
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Langmuir
room air. Oxidation degrees of Si surfaces in DI water at various temperatures were measured using X-ray photoelectron spectroscopy (XPS; Escalab 250xi, Thermo Fisher Scientific Inc., U.S.A.). Change of the hydrogen bonds coverage on the Si surface and the influenced adsorbed-water structure after the different treatments were characterized using attenuated total reflection-infrared (ATR-IR) spectroscopy.
temperature in this nonmonotonic dependence has not yet been highlighted. In this paper, the role of temperature in the mechanochemical wear of Si in deionized (DI) water was detected by considering the variation of Si−OH surfacial groups. The mechanochemical wear of the two typical Si surfaces, including oxide-covered Si partially terminated by Si−OH and oxide-free Si fully terminated by Si−H, present opposite temperature dependence. The corresponding mechanisms are revealed based on the characterizations of the surface hydrophobicity and the influenced adsorbed-water structure evolving with the water temperature.
3. RESULTS AND DISCUSSION Effects of Water Temperature on the Mechanochemical Wear of Si. Before the temperature-dependent experiments, mechanochemical wear of Si was defined first. Normally, material removal is accompanied by plastic deformation, viscous flow, and fracture of materials, which are denoted as mechanical wear.29−31 Differently, mechanochemical wear is mainly attributed to the stress-assisted bond dissociation or chemical corrosion.32−34 As shown in Figure 1,
2. MATERIAL AND METHODS A p-doped Si(100) wafer (MEMC Electronic Materials, Inc., U.S.A.) with a thickness of 0.5 mm was used as the base substrate. Two kinds of Si samples were used in the nanowear tests. One was Si covered with native oxide layer, and its thickness was measured as ∼0.8 nm by scanning Auger electron spectroscopy.27 This oxide-covered Si surface partially terminated with Si−OH groups presents a relatively hydrophilic property with a water contact angle (θ) of ∼39°. Another was oxide-free Si which was obtained by immersing the oxide-covered Si into 40% aqueous solution of hydrofluoric (HF) acid for 3 min followed by ultrasonic cleaning in DI water and anhydrous ethanol.28 This process is known to remove the native oxide layer and produce the surface terminated with hydrogen groups, resulting in a relatively hydrophobic surface (θ = ∼84°). Using AFM (SPI3800N, Seiko, Japan) with a sharp Si3N4 tip (radius
