A more Rigorous Coupled-Wave Analysis

M.G.M.M van Kraaij

Project description:

ASML is a world leader in the manufacture of advanced technology systems for the semiconductor industry. The company offers lithography systems, mainly for manufacturing complex integrated circuits (also called ICs or chips) onto wafers. With every generation, the complexity of producing integrated circuits with more functionality increases. Therefore good models and simulation tools are needed that describe this entire process.

 Lithography system    

During my Master's thesis I focused on a small part of the chip making process that dealt with positioning a wafer. From the pictures it can be seen that ICs consist of all sorts of different layers. Because each new layer with its own tiny structures is created on top of the previous layer, it is very important that these new structures make contact with the underlying structures. Therefore knowing the position of the wafer during the entire process is crucial. ASML uses diffraction gratings to determine the position of a wafer. Gratings are periodic structures printed on the wafer and are even smaller than ICs. These gratings are then illuminated with a laser beam and from the diffracted light one can get information on the position of a wafer. In order to model this grating diffraction problem Maxwell's equations are the starting point. An algorithm known as RCWA (Rigorous Coupled-Wave Analysis) is then used to solve these equations in a stable and accurate way.

                Wafer with ICs                                   Close up of an IC                          Close up of a grating    

Because my Master's thesis suggested that there was room for improvement in the current model, we are now trying to modify the RCWA algorithm so that it will be faster and more accurate. Moreover in this project we are also working on a new sensor concept that should further improve the quality of the wafer. Finally also new software tools will be developed for this sensor that have the improved RCWA algorithm incorporated.