Overlay and alignment target recovery process is often required in recording head fabrication where targets are covered by opaque materials. In this study, we have investigated the recovery process impact on the image based overlay (IBO) measurement performance in critical stages of the recording head fabrication. The trench topography created by the target recovery process can result in the asymmetric resist coating uniformity across the wafer and result in errors in the measured overlay values and modeled correctable wafer terms such as the scale and rotation. These errors become significant at critical pattering layers when there is a large z-spacing between the current resist overlay mark and the previous overlay reference mark layer. The recovery pattern size and the recovery depth impact on the measured overlay performance are evaluated. The overlay mark needs to be optimized to reduce the overlay measurement variation. Overlay mark designs, including box-in-box, AIMid and multi AIMid overlay marks, are investigated. Self-referencing marks (SRM) are used to evaluate recovery process and overlay mark impacts on overlay measurement accuracy.
Overlay control for recording head manufacturing is becoming increasingly challenging as design geometry specifications tighten in the transition to advanced Heated Assisted Magnetic Recording (HAMR) devices. As overlay control requirements for critical patterning layers tighten below 5nm, the need for accurate and robust overlay metrology is key to enabling the patterning roadmap and improving yields. This work focuses on improving Imaged Based Overlay (IBO) metrology to ensure accurate overlay measurements for critical lithography steps in the recording head fabrication process. Selection of the optimal settings for color filter and Numeric Aperture (NA) parameters can have a significant impact on overlay measurement accuracy and Total Measurement Uncertainty (TMU) when setting up a new overlay metrology recipe1-2. These overlay metrology recipe parameters can be selected to minimize the influence of process induced overlay target imperfections on the measured overlay 3. This paper reports our revised workflow for selecting IBO measurement conditions and evaluating pre-existing recipes for robustness. We utilize metrics such as the overlay model residuals, Tool Induced Shift (TIS), and Qmerit error in identifying recipes requiring improvement. The new “Train Log” feature available on KLA’s Archer™ IBO metrology tool platform can be used to compare the through focus contrast precision of different illumination conditions at the time of recipe creation. In this study, we show that an optimization workflow that utilizes the “Train Log” collection of the contrast precision metric and imageless recipes in selecting color filter and numeric aperture settings can significantly improve the overlay measurement repeatability and tool to tool matching.
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