Today, with the accelerating complexity of nanoelectronics for memory applications, in-die overlay metrology has required much tighter control. A typical in-device overlay control strategy utilizes high-voltage SEM metrology across several key layers, but lot and wafer sampling is limited due to low system throughput. Our objective is to find a faster, more robust, and more efficient optical metrology solution that can produce the same in-die overlay results vs. SEM. In this work, we create a novel solution using the KLA SpectraShape 11k dimensional metrology system to demonstrate improved nonzero overlay (NZO) control that meets the tighter overlay budget requirement. We combined the spectroscopic Mueller matrix of SpectraShape 11k and the machine learning algorithm of TurboShape modeling software. Both real spectra collected by SpectraShape 11k and theoretical spectra generated from the scatterometry model are trained against their corresponding SEM reference and synthetic reference data respectively to predict the overlay value. Accurate and robust optical in-device overlay results are proven with a high correlation to the HV-SEM data. In addition, the SpectraShape 11k in-device overlay is equipped with a few key performance indicators (KPIs) including CIndex and CD profile, which are designed to flag process excursions in an HVM environment. Good agreement is observed between the KPIs and overlay delta to HV-SEM. Finally, the 4x-8x throughput advantage of optical metrology in-device overlay vs. SEM in-device overlay allows users to set more dense wafer measurements by lot or dense site measurements by wafer, enabling better lot-to-lot or wafer-to-wafer NZO control.
When a novel dry-etch tool was introduced for the shallow trench isolation (STI) process, it resulted in poorer overlay performance downstream. The patterned wafer geometry (PWG™) tool was utilized to investigate the observed difference in results between the new tool and the POR tool. By using metrics representing process-induced local shape and/or stress, a post-STI oxide fill rapid thermal process (RTP) was identified as the process step where the difference between the dry-etch tools was magnified. An experiment based on RTP temperature and ramp rate was conducted and the effect was evaluated using both GEN4–a predicted shape overlay metric derived from PWG shape measurement–and overlay measurements. Both GEN4 and overlay results indicated that high RTP temperature and low ramp rate could compensate for the process effects introduced by the new etch tool. The strong correlation between GEN4 and overlay also suggested that GEN4 may be a suitable upstream predictor of process-induced overlay excursion in such a case.
In leading edge lithography, overlay is usually controlled by feedback based on measurements on overlay targets, which are located between the dies. These measurements are done directly after developing the wafer. However, it is well-known that the measurement on the overlay marks does not always represent the actual device overlay correctly. This can be due to different factors, including mask writing errors, target-to-device differences and non-litho processing effects, for instance by the etch process.1
In order to verify these differences, overlay measurements are regularly done after the final etch process. These post-etch overlay measurements can be performed by using the same overlay targets used in post-litho overlay measurement or other targets. Alternatively, they can be in-device measurements using electron beam measurement tools (for instance CD-SEM). The difference is calculated between the standard post-litho measurement and the post-etch measurement. The calculation result is known as litho-etch overlay bias.
This study focuses on the feasibility of post-etch overlay measurement run-to-run (R2R) feedback instead of post-lithography R2R feedback correction. It is known that the post-litho processes have strong non-linear influences on the in-device overlay signature and, hence, on the final overlay budget. A post-etch based R2R correction is able to mitigate such influences.2
This paper addresses several questions and challenges related to post-etch overlay measurement with respect to R2R feedback control. The behavior of the overlay targets in the scribe-line is compared to the overlay behavior of device structures. The influence of different measurement methodologies (optical image-based overlay vs. electron microscope overlay measurement) was evaluated. Scribe-line standard overlay targets will be measured with electron microscope measurement. In addition, the influence of the intra-field location of the targets on device-to-target shifts was evaluated.
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