In a chemical-mechanical 
polishing (CMP) process, 
semiconductor substrates are rotated against a 
polishing pad in order to planarize substrate 
layers. The condition of the 
polishing pad directly affects the polishing rate of 
material removal and uniformity of removal from the 
semiconductor wafer. Conditioning of the polishing pad surface with an 
abrasive improves polishing uniformity and rates, however, it has the detrimental affect of removing a quantity of pad material. A method and apparatus for monitoring polishing pad wear during 
processing is developed to extend the pad's useful life, and maintain pad uniformity. This is accomplished in the present invention by measuring and monitoring the diminished pad thickness using a non-intrusive measurement 
system, and creating a closed-loop 
system for adjusting the chemical-mechanical polishing tool process parameters. The non-intrusive measurement 
system consists of an interferometer measurement technique utilizing 
ultrasound or 
electromagnetic radiation transmitters and receivers aligned to cover any portion of the radial length of a polishing pad surface. The measurement system is sensitive to relative changes in pad thickness for uniformity, and to abrupt changes such as detecting 
wafer detachment from the CMP 
wafer carrier.