Nanostructured surfaces

The effect of helium irradiation on the onset of plasticity in nanodeformation of silicon 
S.N. Dub1, G.N. Tolmachova2, A.S. Kuprin2, O. Morozov2, V.A. Belous2
1 Institute for Superhard Materials, NAS of Ukraine, 04074 Kiev, Ukraine. 
2 NSC-Kharkov Institute of Physics and Technology, 61108, Kharkov, Ukraine
E-mail: kuprin@kipt.kharkov.ua 

Implantation of hydrogen into the silicon and subsequent annealing are applied (Smart-cut technology) to produce microelectronic system (MEMS). Improving the characteristics of the resulting structures was achieved by implantation of deuterium and helium instead of hydrogen [1]. Nanomechanical properties of silicon studied in detail [2], but the depth profile of mechanical properties modified by ion-beam irradiation was rarely reported. 
In this work, the mechanical properties were investigated on crystalline silicon (111) samples irradiated by He+ ion beam (energy of 24 keV, fluencies from 1×1016 to 1×1018 ion/cm2 at room temperature ~ 293 К). Nanoindentation have been performed on Nano Indenter G200 device by Berkovich indenter. The real Berkovich indenter is not ideally sharp. The shape of the indenter tip is close to sphere with radius about 340 nm. Therefore, elastic – plastic transition occurs at the initial stage of indenter penetration into the sample surface. Nanoindentation in dynamic regime (Continuous stiffness measurement – CSM) was used to observe the elastic – plastic transition in the contact and to determination of yield strength of silicon single crystal at the nanoscale. Maximum indentation depth was 1000 nm. 
It was found that yield strength of initial silicon is about 4.5 GPa. Yield strength of silicon increased to 5.2 GPa after irradiation by helium at doses up to 5×1016 ion/cm2. However, irradiation at doses 5×1017 ion/cm2 and 1×1018 ion/cm2 leads to a drastic decrease of silicon yield strength to 1.86 and 1.4 GPa, respectively. Nanoindentation results point out on formation of “buried” layer at the depth about 100 nm.

1.	Moutanabbir O., Terreault B., Chicoine M., Schiettekatte F., Simpson P. Influence of isotopic substitution and He coimplantation on defect complexes and voids induced by H ions in Si // Phys. Rev. B, 75 (2007), art. 075201.
2.	Domnich V., Gogotsi Y., Dub S. N. Effect of phase transformations on the shape of unloading curve in the nanoindentation of silicon // Appl. Phys. Lett. 76 (2000), P. 2214-2216.