3D Align overlay verification using glass wafers

In the MEMS world, increasing attention is being given to 3D devices requiring dual-sided processing. This requires lithography tools that are able to align a wafer to both its back side as front side. Overlay describes how well front and back side layers are positioned with respect to each other. Currently there is no simple and fast method to qualify the overlay. This paper covers a method of measuring the overlay between front- and back side patterns using a glass substrate. We describe the methods used, special process requirements and measurement data. The main advantages of the presented method are the simplicity of the concept and the need for only basic fab processing equipment. The substrate employed is re-usable and low cost. The results are as follows: 1. Glass wafers can be used to measure front to back side overlay. The accuracy of the proposed method is better than 100 nm (3?) on ASML PAS 5000/5200 machines. On ASML PAS 5500 steppers, the expected accuracy is better than 80 nm (3?). 2. The proposed method of measuring the absolute glass shift, from a glass-on-silicon stack, yields unreliable information. This is due to deformation of the glass. An alternative method is described which builds on result 1 (above). 3. Processing of glass wafers has been established, and a glass overlay measurement wafer has been defined. 4. The benefit of Anti Reflective (AR) coatings is suspected, but not yet proven. Minimizing bi-refringency does not play a role in the measurement accuracy of glass wafers for overlay measurements