摘要:

Discussions of energy dissipation during friction processes have captured the attention of engineers and scientists for over 300 years. Why then do we know so little about either dissipation or friction processes? A simple answer is that we cannot see what is taking place at the interface during sliding. Recently, however, devices such as the atomic force microscope have been used to perform friction measurements, characterize contact conditions, and even describe the ‘‘worn surface.’’ Following these and other experimental developments, friction modeling at the atomic level—particularly molecular dynamics (MD) simulations—has brought scientists a step closer to ‘‘seeing’’ what takes place during sliding contact. With these investigations have come some answers and new questions about the modes and mechanisms of energy dissipation at the sliding interface. This article will review recent theoretical and experimental studies of friction processes at the atomic scale. Theoretical treatments range from simple, analytical models of two‐dimensional, coupled ball‐spring systems at 0 K, to more complex MD simulations of three‐dimensional arrays of hydrogen‐ and hydrocarbon‐terminated surfaces at finite temperatures.Results are presented for the simplest yet most practical cases of sliding contact: sliding without wear. Sliding without friction is seen in weakly interacting systems. Simple models can easily explain the energetics of such friction processes, but MD studies are needed to explore the dynamics (excitation modes, energy pathways,...) of thermally excited atoms interacting in three‐dimensional fields. These studies provide the first atomic‐scale models for anisotropic friction and boundary lubrication. Friction forces at atomic interfaces must ultimately be measured at the macroscopic level; these measurements, which depend on the mechanical properties of the measuring system, are discussed. Two rather unique experimental studies of friction are also reviewed. The first employs a ‘‘surface force apparatus’’ to measure adhesion and friction between surfactant monolayers. The correlation of adhesionhysteresisand friction provides a new mechanism of friction; moreover, the interpretation for the effect—hysteresis from entanglement of the molecular chains during a phase transformation—implies that the dynamics are taking place at an accessible time scale (seconds to minutes). The second study extends the time domain at which friction can be measured to the nanosecond scale. A quartz‐crystal oscillator is used to monitor the viscosity of monolayer liquids and solids against solid surfaces. Interfaces slip angstroms in nanoseconds. Modelers have suggested a variety of mechanisms for this atomic‐scale friction process, from defect‐mediated sliding to electron drag effects. The article ends by identifying the vast, barely charted time‐space domain (micro‐to‐pico time and length scales) in which experiments are needed to further understand the dynamic aspects of friction processes.

ISSN:0734-2101    

DOI:10.1116/1.579079

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

The potential dependence in the surface second harmonic response from hydrogen terminatedn‐Si(111) and oxidizedn‐Si(111) surfaces has been examined in aqueous NH4F and H2SO4solutions. The relative phase of the nonlinear response as measured by rotational anisotropy experiments is found to be highly sensitive to the presence of the oxide and the field applied across the Si(111)/oxide/electrolyte interface. These observations are attributed to field effects within the space–charge region of the semiconductor which vary with the presence and thickness of the insulating oxide layer on the Si(111) surface.  

ISSN:0734-2101    

DOI:10.1116/1.579080

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

The dependence of the Si/SiO2interface characteristics on the thickness and oxidation temperature for SiO2films grown on different Si orientations was studied by spectroscopic immersion ellipsometry (SIE) and atomic force microscopy (AFM). Essentially, SIE uses liquids that match the refractive index of the films, thereby optically removing the films and consequently increasing the sensitivity to the interface. We show that as the thickness of the thermally grown SiO2overlayer increases, the thickness of the suboxide layer at the interface also increases, and the average radius of the crystalline silicon protrusions (roughness) at the interface decreases for the three different Si orientations (100), (110), and (111), and two different oxidation temperatures (800 and 1000 °C) studied. The dependence of the interface roughness on the thickness of the SiO2overlayer was confirmed by AFM. The results include unintentionally and intentionally roughened Si samples and are shown to be consistent with the commonly accepted Si oxidation model.

ISSN:0734-2101    

DOI:10.1116/1.579081

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

We have used laser‐induced thermal desorption, combined with laser‐induced fluorescence of SiCl(g)to study, in real time, the Si‐chloride (SiClx(ads)) layer that is present on the surface during Si etching in a high‐plasma density, low pressure Cl2helical resonator plasma. The SiClx(ads)layer that builds up during etching contains about twice as much Cl as the saturated layer that forms when Si is exposed to Cl2gas. By varying the laser repetition rate we determined that the surface is chlorinated with an apparent first‐order time constant of ∼6 ms at 1.0 mTorr, and 20 ms at 0.3 mTorr. Therefore in the plasma at pressures above ∼0.5 mTorr, the SiClx(ads)layer reaches saturated coverage on a time scale that is short compared to the time required to etch one monolayer (40 ms). From the weak dependence of the SiClx(ads)layer coverage on discharge power (0.2–1 W/cm3), substrate bias voltage (from 0 to −50 V dc), and pressure (0.5–10 mTorr), we conclude that ion flux, and not neutral etchant flux (i.e., Cl and Cl2), limits the etch rate, even in a low pressure, high‐charge‐density plasma. The chemically enhanced Cl+2sputtering yield is 0.38 at an ion energy of 50 eV and 0.60 at 125 eV. Because of the relatively low neutral‐to‐ion flux ratios (∼2:1 at the lowest pressures) compared to reactive ion etching conditions, a substantial portion of the chlorine needed to form volatile products can be provided by the impinging ions. The SiClx(ads)layer does not change appreciably (<10% decrease in Cl coverage) after the plasma is extinguished and the gas is pumped away. Consequently, post‐etching surface analysis measurements on samples that are transferred under ultrahigh vacuum to an analysis chamber provide information on the surface as it was during etching. The SiClx(ads)coverage and etch rate decreases with increasing addition of O2to Cl2, due to the competition for adsorption sites by O.

ISSN:0734-2101    

DOI:10.1116/1.579082

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

The dependence of the formation of surface roughness in Si on the incident angle of the Cs+primary ion in secondary ion mass spectrometry is reported. No ripples formed in the analytical crater bottom when the primary ion incident angle was from 0° to 30° for sputtered depths of less than 4 μm, but ripples were observed when the incident angle was from 45° to 75°. The depth of ripple formation became shallower with the incident angle increasing. Cross sections of ripples observed by a scanning electron microscope and a transmission electron microscope suggest that ripples grow by forming facets from those plane faces with the largest sputtering rates so that the shapes of the facets remain constant during sputtering.

ISSN:0734-2101    

DOI:10.1116/1.579083

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

A two‐dimensional variation method was used to examine the fractal nature and extract the fractal dimension of rough silicon surfaces prepared by chemical etching and rapid thermal chemical vapor deposition. The measurement of the topography was made with an atomic force microscope and the analysis included traditional characterization parameters as well as fractals. Our results show that the surfaces under investigation exhibited fractal behavior and that the variation method is well suited to extract the fractal dimension from atomic force microscopy data.

ISSN:0734-2101    

DOI:10.1116/1.579084

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

Fractal analysis was applied to images of rough silicon surfaces which were acquired with an atomic force microscope. Spectroscopic ellipsometry was also used to extract roughness information using an optical model and the Bruggeman effective‐medium approximation. Different rough silicon surfaces were examined from three microelectronics processes; rapid thermal chemical vapor deposition, chemical etching, and thermal oxidation. The fractal nature of the surfaces and the correlation between fractal, optical, and topographic parameters were explored.  

ISSN:0734-2101    

DOI:10.1116/1.579085

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

Amorphous fluoropolymers (AF) have recently been developed to provide several favorable characteristics not present in the more crystalline materials. However, these polymers have not been fully characterized and reports of surface modification are not extensive. This work presents results of surface analysis and modification with vacuum ultraviolet (VUV) irradiation of Teflon‐AF‐2400 using contact angle measurements, x‐ray photoelectron spectroscopy (XPS), and laser‐assisted Fourier transform mass spectroscopy (FTMS). Results are compared to those for other, more crystalline, fluoropolymers, namely, poly(tetrafluoroethylene) (PTFE) and the copolymer of tetrafluoroethylene and perfluoroalkoxyvinyl ether (PFA). These three fluoropolymers differ in structure and amount of oxygen that they contain. Samples were irradiated for sixty minutes downstream from a helium microwave plasma, in some instances through a LiF crystal filter. In the latter case, modification is due solely to photons with wavelengths greater than 104 nm. For films treated without filters, reduction in the advancing contact angle was greatest for AF‐2400 and least for PTFE. XPS indicated radiation‐induced defluorination for all polymers treated with or without filters. Concurrently, oxygen incorporation occurs upon exposure to air during sample transfer from the plasma reactor to the XPS instrument. Wetting for treated films improved as the ratio of atomic concentrations for oxygen and fluorine, [O]/[F], increased at the surface. As expected, greater modification was achieved with no filter, possibly due to the effect of higher energy photons and/or helium metastable atoms. Small amounts of nitrogen were also incorporated following treatment, upon exposure to nitrogen gas. Laser‐assisted FTMS experiments on VUV‐treated PTFE and PFA samples resulted in a series of high molecular weight fragments (in increments of 50 amu). This was not observed for untreated films analyzed under similar conditions. It is proposed that VUV irradiation tends to crosslink PTFE and PFA films through defluorination of the –CF2– repeating units, while the AF‐2400 is prone to ring cleavage and does not exhibit this tendency toward crosslinking.

ISSN:0734-2101    

DOI:10.1116/1.579086

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

Co/Pd multilayered films with ultrathin layers of Co were prepared by alternating deposition in an ultrahigh‐vacuum physical‐vapor‐deposition system. The individual layer thicknesses of the samples were estimated making use of the angular positions of x‐ray diffraction peaks. The magnetic and magneto‐optical properties were found to vary depending on Pd buffer layer and Pd sublayer thicknesses, as well as Co sublayer thickness. Some of the films showed good properties for magneto‐optical medium, such as large perpendicular magnetic anisotropy and Kerr rotation, and perfect squareness of the magnetic hysteresis loop. The Pd‐predeposited films were found to have a remarkably high coercivity of 4723 Oe and a greatly enhanced interfacial magnetic anisotropy of 0.72 mJ/m2. The transition from in‐plane to perpendicular magnetic anisotropy took place at a Co sublayer thickness of 8.2 Å for the multilayer with Pd predeposition. Kerr rotation was inversely proportional to the Pd/Co sublayer thickness ratio, showing the maximum value of 0.12°. The magnetic and magneto‐optical properties were correlated systematically to the structural parameters of the multilayered films.

ISSN:0734-2101    

DOI:10.1116/1.579087

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

The possible extraction of inelastic mean free path (IMFP or λi) data from elastic backscattering experiments is analyzed within the transport approximation. It is found that the characteristic length obtained by this method is not the IMFP, λi, as previously thought, but a length Λ=[λ−1i(λ−1i+λ−1tr)]−1/2, where λtr, the transport mean free path, is the length associated with large‐angle deflection. It is significant that this length Λ is also the length which characterizes the overlayer experiment for an ultrathin overlayer (thickness≪IMFP) and at an exit angle of ∼45°, and is also the length obtained using two‐stream methods [V. M. Dwyer, Surf. Sci.291, 261 (1993)].

ISSN:0734-2101    

DOI:10.1116/1.579088

出版商:American Vacuum Society    

年代:1994

数据来源: AIP