Metrology as a Process Enabler

Metrology is not inexpensive — its experts and toolsets are not cheap — but if production and yield are the goal, it is a definitive bargain.
 
For years, many in the industry have considered metrology as a non-value-added expense, but its increasing importance in the device manufacturing flow is now quickly metamorphosing it into a key enabler of large revenues. The reasons behind this shift in focus are clearly stated in an impeccably researched seminal paper, which should be considered as a resource of prime importance to anyone producing or using metrology in any of its various inspection, measurement and test incarnations.

In Realizing “Value-Added” Metrology, lead author Benjamin Bunday of the International Sematech Manufacturing Initiative (ISMI, Austin, Texas), together with colleagues from ISMI and Sematech (Austin, Texas), Freescale (Austin, Texas), Advanced Micro Devices (Sunnyvale, Calif.), Intel (Santa Clara, Calif.), Spansion (Sunnyvale, Calif.), Texas Instruments (Dallas) and IBM (East Fishkill, N.Y.), explains how the conventional “wisdom” that viewed metrology as a non-value-added expense has become a misleading and dangerous assertion. As Bunday puts it, “Various key trends in modern state-of-the-art processing, such as optical proximity correction, design for manufacturability, and advanced process control, are based, at their hearts, on the assumption of fine-tuned metrology, in terms of uncertainty and accuracy.”

Bunday and co-authors explain why this is so through examples taken from the areas of critical dimension (CD) metrology, overlay, films and defect metrology. In the case of advanced process control (APC), it is shown how this fundamentally new concept has given metrology a central role in the determination of the final product's quality: “In the past, metrology monitored the outcome of processing and was primarily used to rework through excursions and adjust processing 'after the fact.' APC takes this to a new level, constantly guiding the processing with 'mid-course corrections' to the target.” The paper examines a typical gate-level APC loop, where metrology data is fed back to adjust lithographic setting and forward to adjust the gate etch based on the litho results; rapid thermal anneal implant steps are fine-tuned to further correct for any excursions at the gate etch step.

Flowchart for a typical gate-level APC loop. Note how metrology (yellow) is used everywhere. (Source: ISMI)

In photolithographic process modeling and optical proximity correction (OPC) calibration and verification — all of which are basic to the relentless advance from node to node — metrology has become crucial. “Modern technology development is unthinkable without photolithographic modeling and the use of resolution enhancement technology,” Bunday said. “Knowledge of absolute CDs of printed features is critical for developing photolithographic simulators based on physics [and these] are only as good as the metrology used to verify and calibrate them.”

Also included is a short survey of the capabilities of metrology tooling. This is of interest because it highlights some of the hurdles that need to be overcome, as well as the trends that may result in solutions to present and future metrology problems. Bunday reviews CD metrology platforms, overlay metrology tools and defect metrology systems, concluding that most of these technologies are expected to extend to or beyond the 32 nm node in some form; CD-SEM continues to advance, and OCD tools will take us to around the 22 nm node, although they may have problems beyond that point.

A portion of the paper goes into return on investment (ROI) modeling for metrology tools, showing various methods and cost models to factor into the diverse metrology tools on the factory floor. Bunday concedes that with the various platforms that are used to collect data for OPC, design for manufacturing (DFM), APC, and systematic and random yield improvements, computing the “added value” of these variables demands thorough knowledge of each contribution to the bottom line and ROI.

It is interesting to note that although metrology — as well as process tool — manufacturers have, over the years, exerted great efforts to simplify the operation of their increasingly complex platforms to enable them to be run by techs instead of engineers, the human factor continues to gain in importance. As he puts it, “Metrologists are responsible for improving recipe running quality across the fleet [of toolsets], reducing tool-to-tool mismatches and accelerating the recipe creation process. Exacting maximum value from the metrology toolset requires an understanding of all these [and other] ways the toolset can negatively impact fab productivity.” State-of-the-art equipment is an important part of productivity, but it alone does not ensure the goal.

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