a cmp heterodyne in - situ sensor system utilizes optical heterodyne interferometry . a wafer undergoing cmp is illuminated through the wafer thickness using an infrared laser beam at a wavelength of 1 . 1 μm or greater . the beam is transmitted through the wafer and is reflected from the front wafer surface . as the wafer is polished , the optical beam path through the wafer is shortened , causing the reflected optical frequency to undergo a doppler shift . by measuring this shift , the change in wafer thickness is determined . the frequency shift generates a signal , which enables dynamic process control . in embodiments where the wafer includes a planarization film , the frequency shift provides a measurement of changing film thickness . embodiments of the invention utilize phase detection independent of intensity , and hence do not suffer from intensity fluctuations . some embodiments detect thickness changes less than 2 . 5 nm . c - his sensors operate in both polished - to - thickness and polished - to - stop scenarios .