Interferometry for wafer dimensional metrology

Wafer shape and thickness variation are important parameters in the IC manufacturing process. The thickness variation, also called flatness, enters the depth-of-focus budget of microlithography, and also affects film thickness uniformity in the CMP processing. The shape mainly affects wafer handling, and may also require some depth-of-focus if the wafer shape is not perfectly flattened by chucking. In the progression of technology nodes to smaller feature sizes, and hence smaller depth-of-focus of the lithography tool, the requirement for the PV-flatness over stepper exposure sites is becoming progressively tighter, and has reached 45nm for the next technology node of 45nm half pitch. Consequently, in order to be gauge-capable the flatness metrology tool needs to provide a measurement precision of the order of 1nm. Future technology nodes will require wafers with even better flatness and metrology tools with better measurement precision. For the last several years the common capacitive tools for wafer dimensional metrology have been replaced by interferometric tools with higher sensitivity and resolution. In the interferometric tools the front and back surface figure of the wafer is measured simultaneously while the wafer is held vertically in its intrinsic shape. The thickness variation and shape are then calculated from these single-sided maps. The wafer shape, and hence each wafer surface figure, can be tens of microns, necessitating a huge dynamic range of the interferometer when considering the 1nm measurement precision. Furthermore, wafers are very flexible, and hence very prone to vibrations as well as bending. This presentation addresses these special requirements of interferometric wafer measurements, and discusses the system configuration and measurement performance of WaferSight (TM), KLA-Tencor's interferometric dimensional metrology tool for 300mm wafers for current and future technology nodes.