Getting Most of the Spectral Interferometry

In the last decade, fiber optic low coherence spectral interferometers (FOLCSIs) have been employed in the backend quality control for MEMS and semiconductor structures [1]. Recently we carefully studied how to optimize the depth resolution (DR) of the FOLCSI system by adjusting the relative lengths of the sample and reference arm of the interferometer. We are using our patent-pending Optoprofiler 200 system as an example of FOLSCI and we checked agreement between theoretical predictions and actual tool performance.

Results of this study have been submitted to Photonics West 2022 conference (which has its abstract submission deadline today 8/11/21).

We found that when both arms have similar lengths then the spacing between observed spectral fringes becomes comparable or larger than the size of the detector, and measurement becomes not very accurate. The approximate analytical results show that in this case, the DR of the system in the presence of the detector noise depends on the phase difference (PD) of the light propagating in the reference and probe arms, and depends on the sample material. ???

In the second case of FOLCSIs with very different arms lengths, the DR is independent of PD and is proportional to RMS single-pixel noise.?The finite spectral resolution of the spectrograph used in the system gives a practical limit for spacing of the fringes and DR.

References:

Aiyer, Arun A., Nikolai Maltsev, and Jae Ryu. "Optical technologies for TSV inspection."?25th Annual SEMI Advanced Semiconductor Manufacturing Conference (ASMC 2014). IEEE, 2014.??