Raman imaging as a structural probe for silicon structures

The Microline Focus Spectrometer (MiFS) instrument is capable of obtaining Raman spectra from a surface with sub-micron resolution and can generate profiles and images representing intensity (species concentration), frequency (stress) and bandwidth (crystallinity). This approach is an objective compromise between the time consuming and high positional stability demands of point by point sampling of the surface and the spectroscopically limited global illumination method. In addition the method does not rely upon moving optical components and thus is inherently stable. An essential element of Raman imaging is the ability to achieve a direct registration of the Raman image with a reflected light image. This is easily achieved using the MiFS approach, either by generating an image in reflected laser light from the surface as a separate experiment or by collecting reflected laser plasma line intensity simultaneously with the Raman image data. Raman images are shown from an experimental silicon structure with groups of alternate stripes of oxide and nitride on the surface. The images clearly reveal a marked change from tensile to compressive stresses as the stripe spacing is reduced to 0.5 μm. Closer analysis shows more complex variations in shift and bandwidth across and between the stripes. Silicon etching to produce complex microstructures is controlled by etch stop boundaries of boron doped silicon. Diffusion of boron into the silicon matrix results in Raman shift and bandwidth changes as the silicon lattice is adjusted to accommodate the B atoms. Raman mapping of the cross sectional edge of a silicon wafer, which has been subjected to 2 hours of B diffusion, is shown. Both the Raman shift and bandwidth images show the B doped region extending 4.5 μm into the silicon surface