Mesoscopic physical removal of material using sliding nano-diamond contacts

Abstract

Wear mechanisms including fracture and plastic deformation at the nanoscale are central to understand sliding contacts. Recently, the combination of tip-induced material erosion with the sensing capability of secondary imaging modes of AFM, has enabled a slice-and-view tomographic technique named AFM tomography or Scalpel SPM. However, the elusive laws governing nanoscale wear and the large quantity of atoms involved in the tip-sample contact, require a dedicated mesoscale description to understand and model the tip-induced material removal. Here, we study nanosized sliding contacts made of diamond in the regime whereby thousands of nm³ are removed. We explore the fundamentals of high-pressure tip-induced material removal for various materials. Changes in the load force are systematically combined with AFM and SEM to increase the understanding and the process controllability. The nonlinear variation of the removal rate with the load force is interpreted as a combination of two contact regimes each dominating in a particular force range. By using the gradual transition between the two regimes, (1) the experimental rate of material eroded on each tip passage is modeled, (2) a controllable removal rate below 5 nm/scan for all the materials is demonstrated, thus opening to future development of 3D tomographic AFM.

Figure 1

Details of the diamond tips used as sliding counterbody. (a) The apex of one of the diamond tips used in this work, as seen by high-resolution TEM (scale bar 20 nm). (b) The pyramidal shape of the tip is shown at lower magnification. (c) Schematic illustration of an AFM tip used to replicate a single-asperity sliding on a flat surface.

Figure 2

Experimental removal rate as a function of load forces. (a) Trench depths as seen by AFM topographical measurements across seven previously machined regions (size 4 µm × 500 nm) with increasing load force (F₁ < F₂… < F_n). (b) Dependence of the removal rate on the tip load force for Si, SiGe and Ge for a diamond tip scanned in contact with the sample’s surface.

Figure 3

Two contact regimes are acting within the range of load forces investigated. The dependence of the removal rate on the tip load force for SiGe is shown with an added schematic representation of the two distinct contact mechanisms associated with different load forces.

Figure 4

Impact of the material removal conditions on the surface quality of the machined area as studied by AFM and SEM. The same tip is used to scan three locations with the same load force and different line density (a) 128, (d) 256 and (g) 512 lines per image (scale bar 150 nm). The regions highlighted by a dashed box are imaged by SEM and shown in (b), (e) and (h), (scale bar 100 nm). 2D profiles are extracted using tapping mode AFM and are compared with the SEM images in (c), (f) and (i). Note, in the inset of (e) the geometrical considerations to calculate the overlap coefficient (α) are shown; while in the inset of (h) we show the details of the calculation for the deformed contact area.

Figure 5

Modeling of the experimental RR. (a) Very good agreement is observed between the observed groove and the diamond tip, as shown by the over imposition of a 2D profile with a high resolution TEM image of the tip apex, scale bar 5 nm. (b) The RR of SiGe is plotted with the addition of the indentation depth as calculated with the DMT model and the Archard’s law equation derived by fitting the low-force regime. (c) In our model we propose to sum of the two contributions weighted by a coefficient (β) which is used to define the onset of the elastic-to-plastic transition and a coefficient (α) to account for the overlap between adjacent lines. This combination provides an improved approximation of our model with the experimental RR, however a certain underestimation of the high-force regime remains. (d) A complete version of our model requires also a pressure-dependent wear coefficient K(F) which provides a good fit to all our experimental RR in Si, SiGe and Ge.

Figure 6

Impact of the tip-apex contamination and material re-deposition. (a) SEM image of the diamond tip after usage (scale bar 5 µm). (b) Zoom-in image by SEM back-scattered electrons (scale bars 1 µm) and (c) secondary electrons. Thanks to the high secondary-electron emission for diamond, the area of the tip-apex used in contact with the sample is clearly revealed. (d) Apparent decrease in the RR induced by the re-deposition of material in the worn area. (e) This artifact can be removed by a progressive reduction of the scan area during the removal which guarantees a more stable RR.