Changing the surface mechanical properties of silicon and α-Al₂O₃ by ion implantation

Abstract

Microhardness indentation testing has been used as a means of introducing controlled localized deformation and fracture in both ion-implanted and unimplanted {1 1 1} silicon and {1 0 ¯1 2} sapphire single crystal surfaces. The microstructural alterations due to implantation with N ⁺₂ and Al⁺ into silicon and Y⁺ into sapphire have been characterized using channelled Rutherford backscattering, transmission electron microscopy and electron channelling in the scanning electron microscope. It was found that sapphire only became amorphous at doses ⪞3×10¹⁶ Y⁺cm⁻² which corresponds to a total energy deposition of ∼3×10²³ keV cm⁻³ (∼44 kJ mm⁻³). The low-load microhardness (<50 gf) was found to be sensitive to the thickness of the amorphous layer produced by implantation into both silicon and sapphire. Compared with the parent crystal, this layer was found both to be softer and to behave in a relatively plastic manner with considerable plastic pile-up occurring around indentations in the higher dose specimens. The indentation fracture behaviour was found to be dominated by the presence of implantation-induced compressive stresses. The resulting effects were: (a) a decrease in the size of the radial crack traces (henceK _IC is apparently increased when evaluated using indentation fracture mechanics), (b) a decrease in the frequency of occurrence of lateral break-out in silicon and subsurface lateral cracking in sapphire, (c) initiation of lateral cracks further below the surface in both silicon and sapphire. Thus in general, it is concluded that hardness and surface plasticity are associated with softer amorphous layers whilst indentation fracture modifications are principally stress related.