Detection of Native Oxide on Silicon Nitride Wafers
The zeta potential is a very sensitive parameter to detect traces of impurities on solid surfaces and their removal during surface cleaning.
Silicon nitride is commonly used as a top layer on semiconductor wafers. Besides its application as an etch mask, the dielectric properties favor its use as a passivation layer.
The surface of silicon nitride wafers is extremely sensitive to oxidation when stored in air as well as in aqueous solution. The zeta potential of differently treated silicon nitride wafers reveals this sensitivity and the presence of Si–O groups on the surface.
The pH titration of an untreated silicon nitride wafer, i.e., after storage in air and contact with an aqueous solution, gives an isoelectric point (IEP) at pH 4. This is clearly shifted to higher pH compared to the IEP of a silicon oxide wafer. However, after etching with 5 % HF for several seconds, the IEP increases to pH 5.3 indicating the efficient removal of native oxide.
Pretreatment of the silicon nitride wafer with Piranha solution prior to the HF etching even enhances the removal of oxide.
The effect of Piranha cleaning is surprising since Piranha treatment is known to introduce Si–O groups. On the other hand Piranha solution removes organic contaminants. We conclude that such contaminants on the untreated silicon nitride surface inhibit the degradation of native silicon oxide by HF.
T. Luxbacher, SEMATECH Surface Preparation and Cleaning Conference 2009