T-ray Wavelength Decoupled Imaging and Profile Mapping of a Whole Wafer for Die Sorting and Analysis

This paper describes a terahertz (T-ray) cameraless imaging and profile mapping technique for accomplishing the imaging and/or mapping of a whole wafer with fabricated dies for devising a criterion to sort out good dies. A stratagem for decoupling the wavelength's dependence on image formation is described, whereby the Abbe diffraction limit is overcome, and a high-resolution image is generated by a larger wavelength T-ray. The mechanics of cameraless image formation is discussed. A 200 mm diameter patterned wafer's image details have been presented from which die-to-die inconsistencies were investigated. A profile of a row of dies was formed from the scanned intensity and compared with the profiles obtained from the graphical analysis of the image of the same dies. It is demonstrated that a criterion could be established either from the scanned profile or from the profile generated from the graphical analysis of the image. A known good die's profile could be used as a reference to compare with the other dies' profiles on the same wafer. Such a criterion could be used to sort the good from bad dies. The technique is extended to a whole wafer populated with die patterns via the profile mapping of the entire wafer. The profile mapping of the whole wafer could be used to compare and sort all wafers from the same batch. The Fab yield is improved by maximizing the count of good dies by applying the efficient sorting criterion.