Time-domain Optical Coherence is the most robust solution for TSV depth, bow & warp and individual layer TTV of a stack measurement as each interface is detected in the right order over a larger range than spectral interferometry. The only limitation is the minimum measurable thickness due to the high coherence length of the IR LED source.
A new Near-Infrared Interferometer was recently developed at Unity Laboratories. The spectral bandwidth was increased to 110 nm, i.e. the minimum detectable optical thickness was improved by a factor of nearly three. This technology has been implemented into the Unity SC T-MAP series.
The NST 300 has been qualified by a major customer for CMP process control performed prior to 3D hybrid bonding process used in BSI (Back Side Imager) manufacturing. The tool is able to measure nanoscale topography at die level and at the extreme edge of the wafer with high lateral resolution and from several hundred of nm to few nm level of amplitude. Mechanical profilometry comparisons have shown good potential to perform measurement on wafers without metal layer coating. This represents a key advantage for inline control, as the technology will allow higher throughput than mechanical profilometry and the ability to give much more information to tune and control the CMP process.