The combined effect of surface roughness and internal stresses on nanoindentation tests of polysilicon thin films

Abstract

Flat, low-stress, boron-doped polysilicon thin films were prepared on crystalline silicon substrates by low pressure chemical vapor deposition. The films’ surface morphologies and internal stresses were investigated using scanning probe microscope and AFM-Raman, respectively. The elastic modulus (E) was measured along with the hardness (H) using the nanoindentation technique. It was found that films deposited at the highest temperature had the largest grain size and surface roughness. The internal stresses in the films are tensile stresses, which are not uniform over the surfaces and present the gradient distribution along the thickness. Both the surface roughness and the internal stresses affect the nanoindentation tests. The tensile stresses in the films give extra forces to the diamond indenter, so the depths of penetration should be larger than those obtained from the films with stress free for a given load, which leads to an underestimation of E as well as H. But under the applied conditions, the increasing surface roughness can not only lead to the reductions of depths of penetration but also cause the measurement values of E and H to be more dispersive and uncertain. So for a given load, if the maximum depth of penetration in the rough surface is smaller than that in the smooth surface, the surface roughness is the main factor affecting the nanoindentation tests. On the contrary, the tensile stress is the main factor.