With the trend toward ever higher integration of integrated circuits, photolithography patterns formed on a semiconductor substrate are continually becoming increasingly fine and precise. Linewidth control of micro-lithographic processes is negatively impacted by numerous effects, such as resist thickness variations, bake non-uniformities, batch-to-batch resist sensitivity changes, thin film interference effects (e.g., swing curve), non-flat wafers, lens field non-flatness, etc. Many of these issues can be classified as producing an effective focus change. Therefore, to improve linewidth control one must either improve the focus window of the process or reduce the focus variations. A key problem in accomplishing either approach is to accurately measure the focus variations present in practical processes.
Determination of optical focus in photolithography has always been a time-consuming and relatively uncertain process. Traditionally, focus is determined by exposing a matrix of fields through a range of focus settings, then inspecting the resultant patterns for the best looking images. Experienced operators can be quite good at this, but the process is necessarily slow and always inherently subjective. A variety of automated schemes for determining tool focus have also been developed. Most of these methods use an aerial image monitor of some sort to determine the spatial location of best focus. The wafers being exposed are then placed at this best focus location (or at a predetermined offset from this position) by high-precision mechanical means. Although in the abstract such a technique is very accurate, in practice it is susceptible to slight drifts in the wafer positioning mechanism or changes in the required focus offset induced by changes in the film stack from one batch of wafers to the next.
Both of the above-noted focusing techniques are used to set up lithographic equipment to expose the best possible image. But after a wafer is exposed, developed, and inspected, there is no good method for confirming that focus was good while the wafer was being exposed. Loss of image quality can be seen by subjective inspection of a wafer, and loss of linewidth control can be confirmed by scanning electron microscope (SEM) linewidth measurement, but image quality and linewidth control are affected by many variables besides focus. Even if an image appears obviously out of focus to an experienced operator, it is impossible to determine quantitatively how many microns the focus offset has shifted, or even whether the shift has been in a positive or negative direction.
Therefore, a means of unambiguously determining focus accuracy (e.g., from a structure on the kerf of a product wafer) would be a valuable asset to the effort to control lithographic tools and enhance the lithographic process. Even greater value in terms of time savings would be achieved if the monitor structure/process could quantitatively measure the magnitude and sign of the defocus. If measurable, this information could be used to adjust the exposure tool without the need of a focus check using one of the conventional focus evaluation techniques.