Sidewall structure estimation from CD-SEM for lithographic process control

In semiconductor device manufacturing, critical dimension (CD) metrology provides a measurement for precise line-width control during the lithographic process. Currently scanning electron microscope (SEM) tools are typically used for this measurement, because the resolution requirements for the CD measurements are outside the range of optical microscopes. While CD has been a good feedback control for the lithographic process, line-widths continue to shrink and a more precise measurement of the printed lines is needed. With decreasing line widths, the entire sidewall structure must be monitored for precise process control. Sidewall structure is typically acquired by performing a destructive cross sectioning of the device, which is then imaged with a SEM tool. Since cross sectioning is destructive and slow, this is an undesirable method for testing product wafers and only a small sampling of the wafers can be tested. We have developed a technique in which historical cross section/top down image pairs are used to predict sidewall shape from top down SEM images. Features extracted from a new top down SEM image are used to locate similar top downs within the historical database and the corresponding cross sections in the database are combined to create a sidewall estimate for the new top down. Testing with field test data has shown the feasibility of this approach and that it will allow CD SEM tools to provide cross section estimates with no change in hardware or complex modeling.