A new in-situ warpage measurement of a wafer with speckle -free digital image correlation (DIC) method

Abstract

During microelectronic manufacturing, a wafer undergoes many fabrication processes. Such processes include exposing a wafer to high temperature. Significant residual stresses are built up in a wafer and they are manifested as warpage of a wafer. Understanding the wafer behavior, especially warpage, during reflow process becomes one of the most important things in developing assembly process. Three Dimensional (3-D) digital image correlation (DIC), as a non-contact optical deformation measurement method, documents both in-plane and out-of plane deformation. The in-situ measurement capability allows to understand a wafer (or a wafer with multiple chips) behavior during reflow (or assembly) process. However, an object such as a monotonic surface or a mirror like wafer needs artificial speckles on the sample surface to be discernable and measured by DIC technique. To avoid contamination on wafer for surface treatment, a speckle-free 3D DIC method is proposed and its effectiveness is investigated. This method introduces an optical pattern projection method and subsequent processes in documenting topography of a wafer placed in an environmental chamber to mimic a reflow process. To validate the new method, warpage measurement result is compared with the measurement from optical profiler, a white light interferometer, and its accuracy and sensitivity are assessed quantitatively.