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Reflectance anisotropy spectroscopy (RAS), which was originally invented to monitor
epitaxial growth, can—as we have previously shown—also be used to monitor the reactive ion
etching of III/V semiconductor samples in situ and in real time, as long as the etching rate is not
too high and the abrasion at the etch front is not totally chaotic. Moreover, we have proven that—
using RAS equipment and optical Fabry-Perot oscillations due to the ever-shrinking thickness of the
uppermost etched layer—the in situ etch-depth resolution can be as good as +/-0.8 nm, employing a
Vernier-scale type measurement and evaluation procedure. Nominally, this amounts to +/-1.3 lattice
constants in our exemplary material system, AlGaAsSb, on a GaAs or GaSb substrate. In this
contribution, we show that resolutions of about +/-5.6 nm can be reliably achieved without a Vernier
scale protocol by employing thin doped layers or sharp interfaces between differently doped layers
or quantum-dot (QD) layers as etch-stop indicators. These indicator layers can either be added
to the device layer design on purpose or be part of it incidentally due to the functionality of the
device. For typical etch rates in the range of 0.7 to 1.3 nm/s (that is, about 40 to 80 nm/min), the RAS
spectrum will show a distinct change even for very thin indicator layers, which allows for the precise
termination of the etch run.
Semiconductor multilayer and device fabrication is a complex task in electronics and opto-electronics. Layer dry etching is one of the process steps to achieve a specific lateral device design. In situ and real-time monitoring of etch depth will be necessary if high precision in etch depth is required. Nondestructive optical techniques are the methods of choice. Reflectance anisotropy spectroscopy equipment has been used to monitor the accurate etch depth during reactive ion etching of III/V semiconductor samples in situ and real time. For this purpose, temporal Fabry–Perot oscillations due to the etch-related shrinking thickness of the uppermost layer have been exploited. Earlier, we have already reported an etch-depth resolution of ±16.0 nm. By the use of a quadruple-Vernier-scale measurement and an evaluation protocol, now we even improve the in situ real-time etch-depth resolution by a factor of 20, i.e., nominally down to ±0.8 nm.