Overlay improvement by non-linear error correction and non-linear error control by APC

The continuing downscale of semiconductor fabrication ground rule requires increasingly tighter overlay tolerances, which becomes very challenging at the cutting-edge lithographic node. We need to keep improving overlay performance to admit the requirements of tight overlay budget. The conventional method of overlay control is controlling linear model parameters during alignment and process correction by APC (Advanced Process Control) for linear errors after alignment. Due to this kind of control for linear parameters, this linear error proportion out of total overlay error can be the indicator how well the overlay is being controlled by the conventional overlay control method. After achieving this small proportion of linear error, normally 10 parameters, out of total overlay errors, this conventional method of overlay control face the limitation of improvement and this implies us that it is necessary to work on non-linear overlay error for further improvement. Initial investigation starts from finding out contribution of grid and field for the remained error after 10 parameter linear modeling and the result shows up higher contribution from grid factor. The way to break down grid residual error is by method of control. Nikon provided GCM (Grid Compensation Matching) function which has some options to deal with these non-linear errors, so we tested and simulated a couple of new methods of overlay control to improve the other proportion of total overlay error beside linear overlay error. 1^st approach for further improvement was remaining x,y offset feedback through APC for each field after linear modeling and 2^nd was non-linear alignment and 3^rd is the combination of both methods. This paper will explain which method will improve which part of overlay errors and the test or simulated results of improvement.