Fabrication of Nanoscale Silicon Fracture Test Specimens and Calculation of Ideal Strength of Silicon

This work is an extension of that performed by Alan, et al. [1, 2], investigating the effect of surface characteristics on the strength of nanoscale silicon (Si) structures. Progress, utilizing CNF, during the 2011/2012 year has been focused in two main areas. We have replicated the process of Alan, et al., to fabricate nanoscale Si beams with current equipment and techniques, and have made progress in modeling Si fracture at the atomic scale using molecular dynamics. The molecular dynamics calculations have been completed using CNF’s Nanolab computing cluster. Summary of Research, Test Specimen Fabrication: Alan, et al., developed a method to fabricate nanoscale Si beam structures based on work by Wang, et al. [3]. We spent a large amount of effort during 2011/2012 to update Alan’s procedure with new equipment and techniques. Samples are fabricated from <111>-oriented Si wafers. The fabrication involves two sets of photolithography and reactive ion etching (RIE) steps. The first etch depth is done to a depth of about 200 nm and roughly controls the thickness of the final beam structure. Before the next round of lithography and etching, a 100 nm thermal oxide layer is grown. The second RIE step is done to a much greater depth of about 10 μm and forms the trench above which the final beam structure will be suspended. At this point the beam is still connected to the substrate along its entire bottom side. Before removing the Si from under the beam, the devices are cleaned with a modified RCA process [4]. The beams are then anisotropically etched using KOH and tetramethyl ammonium hydroxide (TMAH). This etch terminates on Si {111} surfaces. Because the bottom side of the beams are <111>-oriented, and the top and sides are protected by the thermal oxide, the beams themselves are not etched away. Finally, the thermal oxide is removed with a buffered oxide solution. The resulting beam structure is shown in Figure 1. Failure on Si {111} planes under mixed loading using molecular dynamics: Work has also been done on calculating the strength of Si {111} planes under mixed mode loading conditions. This is being done using the molecular dynamics code Large Atomistic and Molecular Massively Parallel Simulator (LAMMPS) and the modified embedded atom method (MEAM). Roundy and Cohen have calculate the theoretical strength of Si {111} planes under pure tension and pure shear in a <112> direction using density functional theory [5]. Because the top and bottom of the nanobeam shown in Figure 1 are <111>-oriented, the {111} fracture planes are not orthogonal to the long axis of the beam. Even though the middle of the beam is under approximately pure tensile loading at fracture, the {111} planes are under a mix of tensile loading as well as shear in both directions. For this reason, the Roundy and Cohen results cannot be directly compared to fracture data obtained from such beams. Figure 1: Nanoscale Si fracture test specimen fabricated at CNF.