Experimental Analysis of Vapor HF Etch Rate and Its Wafer Level Uniformity on a CMOS-MEMS Process

This paper presents the characterization results of the release step with vapor hydrofluoric acid on a Complementary Metal Oxide Semiconductor-Microelectromechanical Systems (CMOS-MEMS) process obtained with a new methodology for controlling the release etch process of CMOS-MEMS devices. The effect of release hole size on etch rate and uniformity was investigated. No appreciable effects were observed for release hole sizes between 0.48 and 1
$\mu \text{m}^{2}$
. With-in-wafer uniformity better than 3% and wafer-to-wafer variability better than 2.5% were found, while achieving constant etch rates of
$\sim 0.25~\mu $
mmin
$^{-1}$
and maintaining the required selectivity. The new characterization methodology is based on monitoring capacitance values of a set of test structures distributed across the analyzed wafers rather than monitoring oxide undercuts. A new parameter defined as etch ratio greatly improved the accuracy of this methodology by removing undesirable contributions to the capacitance coming from sources not related to the release process itself. Results showed that this methodology provides a characterization accuracy one order of magnitude better than what was achieved with a method based on optical measurements of oxide lateral undercuts. [2015-0290]