Spatial heterodyne interferometry techniques and applications in semiconductor wafer manufacturing

Spatial heterodyning is an interferometric technique that allows a full complex optical wavefront to be recorded and quickly reconstructed with a single image capture. Oak Ridge National Laboratory (ORNL) has combined a highspeed, image capture technique with a Fourier reconstruction algorithm to produce a method for recovery of both the phase and magnitude of the optical wavefront. Single frame spatial heterodyne interferometry (SHI) enables high-speed inspection applications such as those needed in the semiconductor industry. While the wide range of materials on wafers make literal interpretation of surface topology difficult, the wafers contain multiple copies of the same die and die-to-die comparisons are used to locate defects in high-aspect-ratio structures such as contacts, vias, and trenches that are difficult to detect with other optical techniques. Metrology with SHI has also been investigated by ORNL, in particular the use of SHI to perform metrology of line widths and heights on photolithographic masks for semiconductor wafer production. Several types of masks are currently in use with phase shifting techniques being employed to extend the wafer printing resolution. With the ability to measure the phase of the wavefront, SHI allows a more complete inspection and measurement of the phase shifting regions.