Imaging of contamination on wafers using a scanning surface potential difference measurement technique

The sensitivity of the ChemetriQ® method toward various contaminants was evaluated. Offering high resolution mapping without edge exclusion, the method based on surface work function lateral non-uniformities, i.e. the so called surface potential difference imaging (SPDI), appears to be very sensitive to traces of metals on Si wafers with native oxide. Using intentional contamination protocols and comparative methods such as total-reflection X-ray fluorescence (TXRF), different dynamics have been seen element to element. As an example, the signal for Fe and Cu contamination on Si is very strong, offering the capability to detect levels in the 1010 at/cm2 range, while it is in the 1012 at/cm2 range for Al. This does not obviously agree with work function differences between Si and the metals; and the exact reason for signal variations must be further investigated. Practical results show that the method can be applied indifferently to CZ wafer front side or backside, whatever the polish finish; can be applied to silicon on insulator (SOI) wafers; and can be very useful for the control of wafer edge contamination left after deposition and further bevel and edge etch steps.