Micro-raman Spectroscopy Stress Measurement Method For Porous Silicon Film

Porous silicon (PS) film is a kind of nanometer semiconductor material withexcellent mechanical, thermal, optical and electrical performance. Nevertheless,residual stresses always introduced into porous silicon when the film produced. Suchresidual stresses may induce warp, collapses and cracks inside the film and on theinterface of porous silicon structure and devices. Thus, it is necessary to research onthe effective and nondestructive residual stress measurement technique for poroussilicon structure or devices. Micro-Raman Spectroscopy (MRS) is an effective stressmeasurement method with special advantages including nondestructive, noncontact,high spatial resolution. Raman measurement is hardly applicable to thequantificational investigation on the mechanical problem of porous silicon since thereis a lack of the quantitative relationship between Raman shift and stress for poroussilicon material, which is the essential requirement for the application of MRS inporous silicon material. In this paper, a theoretical investigation of Raman stressmeasurement for porous silicon material and from which a determination of thequantitative relationship between Raman shift and stress were presented.In this paper, a transversely-isotropic mechanical model of porous siliconmaterial was set up, a Raman stress measurement theory suitable fortransversely-isotropic material was investigated, and a mathematical expressions forthe relationship between the Raman shift and the stress in transversely-isotropicmaterial was deduced. Meanwhile, nano-indentation experiment and DSCM wereintroduced to detect the elastic modulus and Poisson’s ratio of porous silicon material,respectively. The theory along with material parameters obtained in experiment wasapplied to determine the Raman Shift to Stress coefficients of porous-silicon materialas well as the variation regularity with material porosity. The analyses based onexperimental results show that spatial orientation and parameters of measuredmaterial, especially the elastic modulus, are primary factors affecting the Raman shiftto stress coefficient of transversely-isotropic material. Furthermore, the Raman shiftto stress coefficient decreased rapidly following increased porosity, and the coefficientof60%-porosity porous silicon material was one order lower than that ofsingle-crystal silicon.By using the quantitative relationship between Raman shift and stress for porous silicon material, the distribution of intrinsic residual stress of porous-silicon film withsingle-crystal silicon substrate structure along the thickness direction was measuredby Raman scanning the cross section of the structure.The results show that there existsa considerable residual stress inside PS film, which is the major cause of the fractureof porous-silicon film. Particularly, the residual stress changed sharply from tensileinto compressive stress around the interface. Besides, the stress in the silicon substratewas distributed linearly, which corresponded to the structure warpage.