Defect inspection metrology is an integral part of the yield ramp and process monitoring phases of semiconductor manufacturing. High aspect ratio structures have been identified in the ITRS as critical structures where there are no known manufacturable solutions for defect detection. We present case studies of a new inspection technology based on digital holography that addresses this need. Digital holography records the amplitude and phase of the wavefront from the target object directly to a single image acquired by a CCD camera. Using deep ultraviolet laser illumination, digital holography is capable of resolving phase differences corresponding to height differences as small as several nanometers. Thus, the technology is well suited to the task of finding defects on semiconductor wafers. We present a study of several defect detection benchmark wafers, and compare the results of digital holographic inspection to other wafer inspection technologies. Specifically, digital holography allows improved defect detection on high aspect ratio features, such as improperly etched contacts. In addition, the phase information provided by digital holography allows us to visualize the topology of defects, and even generate three-dimensional images of the wafer surface comparable to scanning electron microscope (SEM) images. These results demonstrate the unique defect detection capabilities of digital holography.
A method for recording true holograms directly to a digital video medium in a single image has been invented. This technology makes the amplitude and phase for every pixel of the target object wave available. Since phase is proportional wavelength, this makes high-resolution metrology an implicit part of the holographic recording. Measurements of phase can be made to one hundredth or even one thousandth of a wavelength, so the technology is attractive for dining defects on semiconductor wafers, where feature sizes are now smaller than the wavelength of even deep UV light.