Thin polysilicon layers are used in microelectronics as a first metal level. With careful annealing, a doped polysilicon layer can also act as a diffusion source for the underlying Si substrate. For such a case, the quality of the polysilicon to substrate Si interface plays an important role in the electrical and migratory properties of the final structure. Therefore, the interface must be observed very closely.

Arsenic doped polysilicon was deposited on a <001> oriented Si wafer. Cross-section TEM specimen preparation was performed by first cutting dices centered on the prespecified region of interest. The dices are bounded face to face, and then a parallel double face mechanical polishing of the section reduces the thickness of the cross-section to about 30 microns. An ion milling operation is used to finish the preparation. Localisation precision by this method is not better than 100 microns, but this can be improved to 10 microns by replacing one of the dices with a piece of glass to allow visualization of the surface during polishing.

The advantage of this method is that a large region can be observed, 1.5 mm long on each piece, with rather good localisation.

An application is the HREM imaging of a typical interfacial region. We could see an ultrathin (1nm) amorphous layer running along the interface between the Si and the polysilicon. Physical characterization (EDXS, EELS) was obtainable from this ultrathin layer.