Plasma etching technology has been used in the semiconductor industry since its inception. Etch, due to its very nature is an aggressive and corrosive environment. The MKS Baratron® capacitance manometer was a critical enabling invention for the etch process due to its ability to accurately measure process pressure without significant degradation from the extremely corrosive environment. In the last few years etch processing pressure and stability requirements have increased considerably. The process pressure for critical etch steps has been reduced to very low levels (around 2-3mT) and at the same time there is less tolerance for small pressure instabilities. These instabilities can be caused by process gases impacting the manometer, such as the signal transient seen when manometers are first exposed to fluorine (called the fluorine burn-in effect). Our customers require improved accuracy and stability in the low pressure regime to aid in chamber to chamber and wafer to wafer reproducibility.
Stability and reproducibility are key enablers for success in semiconductor manufacturing since they are intimately associated with productivity and yield. More stable and reproducible processes lead to faster initial qualification times for new fab installations and longer up-time. The ability to accurately match pressures between chambers and from device to device is crucial, and the challenges created by the move to lower pressure set points, along with the fluorine burn-in effect, could lead to lack of reproducibility and stability, thus affecting yield.
MKS worked with semiconductor tool manufacturers and fab end users to understand these problems and their impact. MKS developed a new multi-physics model to predict how the Baratron® manometer would interact, on first exposure, with aggressive etch gases such as fluorine. We generated proposed solutions based on the model and developed a unique live gas test facility to iteratively test the solutions. The final solution is a new, "fluorine friendly" Baratron manometer that provides greater immunity from fluorine burn-in effects. Additionally, we developed a novel calibration procedure that allows the calibrated accuracy of the Baratron manometer to be extended to the lower pressure set points prevalent in some etch applications. This combination of improvements - Baratron manometers with etch sensors, provides greater stability and reproducibility.