An improved method for leveling and focus control

Lithography steppers need a level sensor to measure the vertical position of wafer surface relative to the lens system. The problem is photoresist is transparent to UV light. Thus the layer below the resist will be seen by the level sensor. We investigated varying spectral width of white light interferometry as a solution, with the aim to prove accuracy. A major difficulty is distance between wafer and interferometer cannot be too small otherwise it will damage the wafer measured. Several types of OCT are investigated. This has been done experimentally on a breadboard setup, and also by simulation. As a result, we find that double Fizeau interferometer OCT is the most suitable option. It is shown that an accuracy of 5 to 10 nanometer for the vertical position measurement is achievable.