On Product Overlay (OPO) is a critical budget for advanced lithography. LithoInSight (LIS), an ASML application product, has proven to improve the ability of advanced process control (APC) for overlay with accurate fingerprint estimation and optimized scanner correction. It is now often used as Process of Record (PoR) for performing chuck/lot based run-to-run (R2R) control in a High Volume Manufacturing (HVM) environment. In order to further improve the on-product performance given the ever-tightening overlay spec. in advanced nodes, the question of how to reduce wafer-to-wafer process-induced variation has been asked frequently. Studies have shown that the wafer-to-wafer overlay variation is driven by certain critical process contexts. Aiming to bring a solution to the HVM phase, the ASML and Micron Data Science teams developed a Wafer Level Grouping Control (WLGC) methodology to perform overlay control given the process context information. This methodology has been implemented in one of the Micron production fabs, and demonstrated both reduced wafer-to-wafer (W2W) overlay variation and improved device yield on a yield-critical layer for a product from Micron 1z DRAM node.
Multi-patterning lithography at the 10-nm and 7-nm nodes is driving the allowed overlay error down to extreme low values. Advanced high order overlay correction schemes are needed to control the process variability. Additionally the increase of the number of split layers results in an exponential increase of metrology complexity of the total overlay and alignment tree. At the same time, the process stack includes more hard-mask steps and becomes more and more complex, with as consequence that the setup and verification of the overlay metrology recipe becomes more critical. All of the above require a holistic approach that addresses total overlay optimization from process design to process setup and control in volume manufacturing. In this paper we will present the holistic overlay control flow designed for 10-nm and 7-nm nodes and illustrate the achievable ultimate overlay performance for a logic and DRAM use case. As figure 1 illustrates we will explain the details of the steps in the holistic flow. Overlay accuracy is the driver for target design and metrology tool optimization like wavelength and polarization. We will show that it is essential to include processing effects like etching and CMP which can result in a physical asymmetry of the bottom grating of diffraction based overlay targets. We will introduce a new method to create a reference overlay map, based on metrology data using multiple wavelengths and polarization settings. A similar approach is developed for the wafer alignment step. The overlay fingerprint correction using linear or high order correction per exposure (CPE) has a large amount of parameters. It is critical to balance the metrology noise with the ultimate correction model and the related metrology sampling scheme. Similar approach is needed for the wafer align step. Both for overlay control as well as alignment we have developed methods which include efficient use of metrology time, available for an in the litho-cluster integrated metrology use. These methods include a novel set models that efficiently describe different process fingerprints. We will explain the methods and show the benefits for logic and DRAM use cases.
For the 28 nm node lithographic production steps, the process window for both overlay and CD are becoming
increasingly tight. The overlay stability of lithography tools must be at a level of 1-2 nm within the product cycle time,
while focus needs to be stable within 5 nm. Well-matched tools are crucial to improve the flexibility of tool usage and
the pressure for higher tool availability is allowing less time for periodic maintenance and tool recovery. Here, we
describe the way of working and results obtained with a long-term stability control application, containing a scanner
performance control system with a correction feedback loop deploying scatterometry. In this study the overlay
performance for immersion scanners was stabilized and the point-to-point difference to a reference is maintained at less
than 4 nm. The capability of tool recovery handling after interventions is demonstrated. Results of overlay matching
between machines are shown. The tool stability for focus was controlled in a range of less than 5 nm while improving the
total focus uniformity.