We demonstrate the use of Beam Profile Reflectometry (BPR) to measure coating thicknesses on small, highly curved
devices such as cardiac stents. BPR has long been used in the semiconductor industry as a powerful technique for
measuring the thickness and refractive index of transparent films. The method uses a diffraction-limited focused laser
beam to provide light at multiple angles-of-incidence simultaneously within a sub-micron measurement area. By
analyzing the reflected light as a function of angle-of-incidence and polarization, robust and deterministic measurements
of film-thickness and refractive index can be obtained taking proper account of birefringence.
For the current work, the technique has been implemented in a compact desktop configuration suitable, for example, for
the in-line monitoring of coating thickness and composition as part of a stent manufacturing process. Provision is made
for the alignment and manipulation of the small and fragile samples, and for the location of the measurement spot at the
appropriate site on the stent's surface.
Validation measurements on stent-like reference samples, comparing results from the technique with destructive
measurements on the same samples, show correlation of better than 99% over a range of coating thicknesses and sample
morphologies down to curvature radii of ~50μm.
A selection of thin Si layers grown epitaxially upon thick relaxed SiGe films were measured using the combination of optical metrology techniques available on the Opti-Probe 7341 system. The techniques used included in particular (i) angle resolved laser Beam Profile Reflectometry (BPR) with S and P polarization, (ii) Broad-band visible-DUV spectrophotometry (BB), and (iii) spectroscopic ellipsometry (SE). The measured parameters included the Ge-content of the relaxed SiGe layer, the thickness and optical dispersion of the thin Si layer, and the thickness of the native oxide layer on the strained Si. Strain in the Si layer can be recognized by a significant downwards shift in the energy of the E1 peak and in the magnitude of the E2 peak in the ε<sub>2</sub> dispersion curve, which is consistent with theoretical predictions when the strain in the layer is tensile.
The thickness measurements of the Si layer made by the Opti-Probe were found to be in agreement with subsequent SIMS analysis to within 5Å for the strained-Si layer. Measurement precision for thickness was <1.5Å (3σ). for the strained-Si layer. Overall, the results show that a reliable and stable measurement of Strained-Si is possible using optical metrology.