and uses it to determine the polishing endpoint e g by detecting a sudden change in the reflectivity of the substrate that indicates the exposure of a new layer by calculating the thickness removed from of the outer layer such as a transparent oxide layer using interferometric principles or by monitoring the signal for predetermined endpoint criteria one problem with placement of a normal large rectangular window e g a 2 25 by 0 75 inch window into a very thin polishing layer is delamination during polishing in particular the lateral frictional force from the substrate during polishing can be greater than the adhesive force of the molding of the window to the sidewall of the pad returning to fig2 the window 40 is thin along the direction of the frictional force applied by the substrate during polishing tangential to a radius in the case of a rotating a polishing pad and wide in the direction perpendicular direction along a radius in the case of a rotating a polishing pad for example the window 40 can use an area about 4 mm wide and 9 5 mm long centered a distance d of about 7 5 inches 190 50 mm from the center of the polishing pad 18 the window 40 can have an approximately rectangular shape with its longer dimension substantially parallel to the radius of the polishing pad that passes through the center of the window however the window 40 can have a ragged perimeter 42 e g the perimeter can be longer than a perimeter of a similarly shaped rectangle this increases the surface area for contact of the window to the sidewall of the polishing pad and can thereby improve adhesion of the.