AVS 50th International Symposium
    Applied Surface Science Friday Sessions
       Session AS-FrM

Paper AS-FrM8
SIMS Backside Depth Profiling of Test Pads on PMOS Patterned Wafers

Friday, November 7, 2003, 10:40 am, Room 324/325

Session: SIMS
Presenter: E.S. Windsor, National Institute of Standards and Technology
Authors: E.S. Windsor, National Institute of Standards and Technology
J.G. Gillen, National Institute of Standards and Technology
P.H. Chi, National Institute of Standards and Technology
J.A. Bennett, International Sematech
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In semiconductor electronics, the diffusion of elements from one layer to another can lead to poor performance or device failure. Interlayer diffusion is of particular interest in process design where experimentation with films of varying composition and thickness is common. In this study, we investigate a patterned PMOS wafer containing hafnium oxide as an experimental gate dielectric. The question arises as to whether boron from the overlying polysilicon layer has diffused through the dielectric down into the silicon substrate below. We use Secondary Ion Mass Spectrometry (SIMS) to investigate possible boron diffusion because of the high analytical sensitivity and excellent depth resolution of the SIMS technique. Front-side SIMS analyses of these patterned wafers can be limited by: (1) difficulty sputtering patterned (non-planar) surfaces, (2) initial or sputter induced topography of the sample surface and (3) degraded depth resolution caused by ion mixing when sputtering from high to low concentrations of the element(s) of interest. To minimize these effects, it is often desirable to sputter (analyze) from the backside of the wafer. Backside analysis requires the removal of the majority of the silicon substrate (final substrate thickness less than 1 micron). This is accomplished by mechanical grinding and polishing. Since the features of interest (test pads in this study) can not be observed from the backside, they must be marked from the front side prior to mechanical preparation. Marking is accomplished by ion sputtering, and we sputter both holes and raster craters. Holes mark the features of interest while the larger raster craters are used to aid thickness determinations and adjust planarity during mechanical preparation. Also investigated are factors that limit the usefulness of mechanical backthinning preparation such as polished surface roughness and sample polishing planarity.