In-process monitoring of atmospheric pressure plasma jet etching using a confocal laser displacement sensor

In this study, an in-process depth monitoring method was developed to improve the atmospheric-pressure plasma jet (APPJ) etching process. Although the APPJ etching process has attracted significant attention as a micromachining technology, it has low processing precision. Therefore, we propose an etching time control method based on in-process monitoring to improve the precision and accuracy of the APPJ etching process. In the conventional method, the APPJ etching time is set to achieve the desired etch depth based on the etching rate investigated in advance. In the proposed method, the etch depth is measured in real time using a confocal laser displacement sensor, and APPJ irradiation is stopped when the etch depth is reached. Therefore, under- and over-etching can be prevented, thus improving the etching accuracy and precision of APPJ etching. A special confocal laser displacement sensor that can be operated under APPJ etching was developed for in-process monitoring. The response time and the depth resolution of the sensor were 0.11 s and 0.10 μm, respectively, under the APPJ irradiation condition. Using the developed sensor, the etch depth was successfully monitored during the APPJ etching process. The standard deviation of the etch depth, corresponding to the etching precision, was reduced by 50% compared with that without in-process monitoring. The average error of the etch depth, corresponding to etching accuracy, was eliminated, demonstrating an improvement in etching precision and accuracy by the in-process monitoring system. The study findings provide insights for the development of APPJ etching systems for post-fabrication processes of microdevices.