The phase shift focus monitor is a relatively new technique which can be used to determine the optimal imaging plane for a photo-stepper. This determination ofbest focus is accomplished through phase shifting technology developed and patented by IBM. Adapting this concept to an overlay measurement target has advantages of simultaneous x and y measurements, high precision, and automated data collection. Printed on the reticle are phase shifting structures using the box-in-box pattern. These phase shifting structures produce lateral shifting ofimages at the wafer plane when they are imaged out offocus. This is a direct result from utilizing non- 1 80 degree phase shifting material. When the patterns are imaged in focus, no lateral shifting, or overlay error is exhibited by the box-in-box patterns. It has been found that the overlay error of the printed structures increases with the magnitude of defocus. If overlay data is collected from several different levels of defocus, a direct linear function can be found to relate best focus to the overlay error of the printed box-in-box patterns. Additionally, it can be shown that this relation is dependent upon the NA (numerical aperture) and PC (partial coherence) of the imaging system. This technique can be used as a very powerful analysis tool for characterizing photo-steppers, as it provides for the collection ofvast amounts ofdata about a stepper's condition rather quickly. It is laid out in an arrayed pattern with 121 modules in an 1 1 x 11 matrix. The complete matrix is contained in a 22 x 22 mm field (at wafer level on a 5x reduction stepper).Each module contains the phase-shifted box-in box patterns. Several potential applications have been suggested already. One wafer can be used to collect data on current best focus and field tilts in a few minutes. More static characteristics specific to the lens can be determined as well, i.e. astigmatism and field curvature. All of this information can be collected by measuring the overlay error from several of the 121 module positions in the field. This paper will discuss techniques to use the focus monitor to characterize steppers equipped with variable NA and PC. In addition, it can be shown that the monitor can provide quick and accurate measurements to verify and track the stepper's own internal diagnostics for statistical process control.
Keywords: phase shift, focus, photostepper, reticle, numerical aperture, coherence, overlay