2 Considerations for Accurate Optical Monitoring

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Throughout the monitoring process, the system manufacturer and thin film engineer have several process choices to make regarding a number of factors. How do the different monitoring options affect the level of precision?

Quartz Crystal vs. Optical Monitors

Quartz crystal monitoring (QCM) is used to measure physical thickness while optical monitoring systems (OMS) measure optical thickness. Both methods offer similar levels of accuracy, but OMS provides better precision for optical films. The advent of computers made the automation of optical monitoring possible, allowing manufacturers and engineers to control many factors at once, giving higher precision and yield due to fewer errors made.

Quartz crystal monitors have to be used indirectly; optical monitors, on the other hand, can be direct or indirect. Direct monitoring focuses on the parts, while the indirect method involves monitoring a separate chip or chips. QCM provides a linear change in crystal frequency to signal the film mass accumulation, or thickness, on the crystal. Because QCM requires the indirect method it will also require calibration based on its position within the tool.

Accuracy and Precision in Optical Monitoring

Control of film optical thickness is the primary objective of optical monitoring, and accuracy and precision in this monitoring becomes all-important. Without an accurate, precise reading, it becomes extremely difficult to control critical outcomes such as yield and device performance. With accurate optical monitoring, however, manufacturers and engineers can have confidence in a tightly-controlled process.

Achieving this accuracy can be difficult because the effect of the substrate needs to be accounted for. H.A. Macleod discusses this premise in “The Monitoring of Thin Films for Optical Purposes,” stating that “accuracy and tolerancing calculations usually assume thin film behavior and it is only by considering their real behavior that it is possible to begin to understand the exasperating difficulties which occur when attempting to coat a substrate which is a little different than normal.”

The higher the accuracy and precision of the monitoring, the higher the yield for single coating runs. An accurate, precise monitor enables higher device performance and higher yields.

To further explore the accuracy and precision of optical monitoring, we performed a small study on the variation in precision using a fully automated optical monitor system. To see our results, download the technical paper, “Considerations and Examples for Determining Precision of Indirect Optical Monitoring“.