This month we feature a metrology article on integrating critical and non critical lithography systems.

Mix & Match of critical and non critical lithography systems for optimum overlay performance

For many years, small fabs and research centers have found themselves with a mix of lithography systems, which at first glance seem incongruous. However, with the right tools, it is possible to characterize and adjust any lithography system, to create a more controllable and predictable lithography system population.

In the case of matched overlay, where does one start? Well, most metrology systems use a "box in box" or "frame in frame" optical contrast based system, which is great for measuring relative differences between 2 systems. These data sets can be taken "off line" to analysis software that can extract the systematic variables used for system correction. One alternative is the ASML self metrology system, which uses the stage grid as a very accurate ruler, calibrated against a worldwide standard that is more of an absolute measurement rather than a relative one. The advantage of this approach is that the user can model each layer against the stage grid and determine which system is generating the error. This is a subtle difference, as in the case of box in box, if both layer 1 and 2 have the same errors the difference will be zero and the actual error remains undetected.

A good example of this effect would be in the measurement of a 5x stepper and a 1x scanner. If layer 1 was the 1x scanner, which had a "scan scew" error, how would you know that this was not a stepper orthogonality error from layer 2? Only be extracting the individual layers from the dataset, and comparing them with the stage grid, is the possible. An example of this approach and methodology is reported in the May 2004, Micromagazine article " Optimizing critical photolithography steps in IC manufacturing processes".

With the correct set of metrology reticles, measurement data, analysis software and methodology, any lithography system population can be optimized. Even contact printer integration to a stepper population is possible; this is particularly interesting when considering the impact of proximity/contact vacuum on overlay performance.

For more discussion, and Simax Lithography engineering support, please This e-mail address is being protected from spambots. You need JavaScript enabled to view it. at Simaxlithography.com