Development of accurate linewidth measurement techniques for in-process wafers
Measurements of the device geometries on “in-process” wafers present some interesting challenges. Sample coating is not possible and the measurement probe must not damage the devices. The Scanning Electron Microscope (SEM) is the standard tool for submicrometer measurements. However, operating at the low electron beam accelerating voltage required to avoid damage to integrated circuits introduces problems in resolution. Also, the measurement accuracy may be limited by the effects of surface charging and topography. Further, SEM linewidth standards do not exist at the present time. Optical measurements are attractive because, in general, they display greater precision, are typically less expensive to implement and have a higher throughput. However, diffraction effects associated with the complex three-dimensional geometries of integrated circuit structures make accurate measurements very difficult. The "blur" regions that occur in the optical image at each edge must be interpreted to predict the actual location of the structure edges.