Effect of oxide and nitride films on strength of silicon: A study using controlled small-scale flaws
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Strength properties of silicon substrates containing dense oxide and nitride surface films are investigated using nanoindentations to introduce small flaws of predetermined scale. The indentation flaws provide favored sites for failure in subsequent flexure loading, even in the subthreshold region for indentations without visible corner cracking, confirming that microflaws generated within the indentation zone act as effective crack sources in the substrate. Deposition of the oxide films increases the strength while the nitride films diminish it at any given indentation load. The strength shifts are attributed primarily to the presence of residual compressive stress in the oxide, tensile stress in the nitride. A fracture mechanics formulation based on a previous analysis for monolithic substrates is here adapted to allow for a superposed crack closing or opening stress-intensity factor term associated with the residual stresses. Allowance is also made in the mechanics for the influence of the film on effective hardness and modulus of the substrate. The formulation accounts for the basic strength shifts and enables evaluation of the magnitude of the residual stresses. The results quantify the susceptibility of basic device materials to damage from small-scale contacts and impacts.
I. INTRODUCTION
The presence of thin films is of practical importance in semiconductor and microelectromechanical system (MEMS) technology. Such films can profoundly influence the properties of a device material. For thin film systems in general, the mechanical properties that have been most widely studied are elastic modulus E and hardness H using nanoindentation probe techniques.1–9 A recent study of silicon deposited with oxide and nitride films reveals distinctive trends in these two properties:10 nitride films tend to harden and stiffen the silicon substrate; oxide films tend the opposite way. One mechanical property that has received much less attention is strength, especially in the context of susceptibility to damage from small-scale contacts. Strength properties of pristine silicon without any films (other than ≈1 nm native oxide) have been recently measured.11,12 However, the effect of thin films on strength properties is yet to be studied. It is well documented that residual stresses in the film layers can seriously deform substrates in small-scale devices,13 but how do these residual film stresses influence the critical stresses to cause
failure? There is a need to understand the basic strengthdetermining factors that can limit the prospective lifetime of device materials. In this paper, we examine the role of surface films on the susceptibility of silicon to extraneous damage. Silicon is an ideal substrate material, not only because it is the industry standard for microelectronic and MEMS systems, but also because its basic strength properties are well understood. Dense thermally grown oxides and lowpressure chemical-vapor deposited (LPCVD) nitride films of prescribed thicknesses are adopted as coating systems. Nanoindentat
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