70 µm deep "Si Bosch" etch with low ARDE effect

ARDE is "Aspect ratio dependent etch (rate)"
The etch rate lowers at high aspect ratios (> 5 : 1)
due to gas transport limitations.

typical example of ARDE in a
Bosch deep Si etch process

etch rate vs aspect ratio

The Aspect Ratio dependence of the etch rate is a normal phenomenon, which appears in many etch process, especially at high rates, where the rate is often limited by the transport of etching species to the trench bottom and the transport of etch products out of the trenches. There are ways to reduce the effect, typically at the expense of rate, selectivity or profile. It is important to understand the effect well for defining the MEMS design rules !

10 µm deep Si RIE using Cl/ F chemistry
with kind permission of Uni Dortmund (1992)

etch rate vs trench width

110 µm deep Bosch Si etch "without" ARDE effect

80 µm deep Si etch using the Bosch process