PSI - Issue 43

Kevin Blixt et al. / Procedia Structural Integrity 43 (2023) 9–14

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Author name / Structural Integrity Procedia 00 (2022) 000 – 000

(MEMS) needed is by methods resting on material removal. Here the focus is on ultra-precision tip-based cutting using the nano-indentation technique. This technique ensures high reproducibility at low manufacturing costs. Like at the macroscopic scale, the parameters controlling the cutting process such as applied force at the tool, tool velocity, and cutting depth, must be tuned to desired end-product specifications. But at the nanoscale, additional considerations must be taken. At this scale the properties of a structure are determined at the atomic level and, i.e., crystallographic lattice orientation must be considered as well as the surface close relocation of electron clouds, impacting on final mechanical, optical, and electrical properties. The possibility to predict the machine parameters at the nanoscale would result in the possibility to swiftly and precisely design and manufacture MEMS with high reproducibility, guaranteeing a minimum of waste both during the design process and in the production phase, at low cost. Here we employ molecular dynamic (MD) simulations using the freeware LAMMPS (https://www.lammps.org), developed by Plimpton (1995), to investigate the chip formation process and subsurface development on a workpiece of single crystal copper of different orientations cut by a cubic diamond tool. 2. Problem formulation A schematic of the MD cutting model is seen in Fig. 1 where a coordinate system ( x,y,z ) is introduced. The copper atoms in the workpiece of length l = 160nm and hight h = 20nm are grouped into boundary atoms (blue), thermostatic atoms (red), and Newtonian atoms (grey). The thickness of the boundary layer is 6.5Å and of the thermostatic layer 13Å. The boundary atoms are held fix and the thermostatic atoms are kept at room temperature whereas the Newtonian atoms follow Newtonian dynamics. The work piece has periodic boundary conditions applied in the z -direction thus mimicking infinite width.

Fig 1. Schematic of the cutting process geometry with different atomic regions.

The tool (green) is built from cubic diamond where the atoms are mutually immobile but interacts with the copper atoms in the workpiece. The geometry of the tool is a cylinder arc segment with radius R and thickness 10Å. The tool width equals that of the work piece and holds periodic boundary conditions in the z -direction. The tool attacks the workpiece with cutting velocity v c at a cutting depth d = 2nm in the cutting direction x . Three different crystallographic orientations for the workpiece were investigated. Referring the coordinate system ( x , y , z ) these are: ( x , y , z ), = ([100], [010], [001]), ([110], [-110], [001]), and ([111], [-110], [-1-12]). From here on they are denoted according to their x -coordinate. The crystallographic orientation for the carbon atoms in the tool was not altered. 3. MD simulations The MD simulations are performed using the freeware LAMMPS and all atomic illustrations are produced with OVITO, developed by Stukowski (2010). In LAMMPS proper potentials needed to for the present problem are provided. For the Cu-Cu interactions an embedded atom method (EAM) potential, developed by Foils et al. (1986) in accordance with Daw and Baskes (1984), and for the Cu-C interactions a Morse potential in accordance with Zhang and Tanaka (1997), with coefficients D 0 = 0.087eV, α = 5.14Å−1 , r 0 = 2.05 Å and cut-off radius, r c = 6.5Å . The lattice