Development of a wafer geometry measuring system : a double sided stitching interferometer

Abstract

The drive for miniaturization of electrical devices and the increased production size of chips has forced lithographic production techniques to improve continuously. With this the requirements for silicon wafers, which form the basis of chips, have increased continuously as well. To ??nd a cost e??ective solution for the characterization of large diameter double side polished silicon wafers, the development of a measurement machine has been started. The measurement machine should measure the free form ??atness and thickness variations of wafers with a diameter of up to, and possibly beyond, 300 mm. The proposed measurement concept should have the potential to achieve a high throughput and a low measurement uncertainty, while reducing the cost of ownership signi??cantly compared to currently available systems on the market. The designed measurement setup which is described in this thesis is intended to demonstrate the potential of a chosen measurement concept for measuring double side polished silicon wafers. In the innovative measurement concept a double sided stitching Fizeau type interferometer has been adopted. A surface interferometer o??ers the required high accuracy and the scanning principle of a small aperture stitching interferometer allows the use of relatively small and low cost optical components which can measure with a high spatial resolution. The self referencing capability of a double sided Fizeau interferometer is important for achieving high accuracy when measuring thickness variations. In the proposed measurement concept the aperture of a single interferometer is split to measure the frontside and backside ??atness of a wafer simultaneously. The thickness variations can be derived from the measured ??atness measurements. A prototype measurement setup has been designed, built and tested. All major mechanical and optical error sources have been eliminated by using advanced calibration techniques. By using proper measurement principles and advanced software a robust and traceable wafer thickness and ??atness measurement instrument is created for measuring nominally ??at objects. The developed calibration techniques enable low uncertainty measurements to be taken while using relatively low quality optical and mechanical components. Several measurement method have been applied to derive accurate geometrical parameters from the recorded interferograms. Besides the processing of interferograms and development of calibration techniques a surface stitching software package has been developed which combines many subaperture ??atness maps into a large scale ??atness map of the entire wafer