Method for detection of wafer slippage

A non-transitory computer-readable medium having tangibly encoded therein a computer program comprising instructions to cause a computer system to:

• receive, a first sequence of signal values from a first sensor of an in-situ monitoring system and a second sequence of signal values from a second sensor of the in-situ monitoring system, wherein the first sequence of signal values and the second sequence of signal values include a signal strength relative to a thickness of a layer of a substrate, wherein the first sensor and second sensor sweep across and monitor the substrate during polishing when a surface of a layer of the substrate is in contact with a polishing pad, and polishing includes relative motion between the substrate and the polishing pad;
• detect, from the first sequence of signal values from the first sensor of the in-situ monitoring system, a time that the first sensor traverses a leading edge of the substrate and a time that the first sensor traverses a trailing edge of the substrate;
• detect, from the second sequence of signal values from the second sensor of the in-situ monitoring system, a second time that the second sensor traverses the leading edge of the substrate and a second time that the second sensor traverses the trailing edge of the substrate;
• for each signal value of at least some of the first sequence of signal values, determine a position on the substrate for the signal value based on the time that the first sensor traverses the leading edge of the substrate and the time that the first sensor traverses a trailing edge of the substrate;
• for each second signal value of at least some of the second sequence of signal values, determine a position on the substrate for the second signal value based on the time that the second sensor traverses the leading edge of the substrate and the time that the second sensor traverses the trailing edge of the substrate;
• and
• activate a slippage alert if:
• the signal strength varies by 30% or more from the first sequence of signal values to the second sequence of signal values, a position on the substrate for the second signal value cannot be determined, or a combination thereof.