Microsystem Technologies
MIPE2022. 2022 JSME-IIP/ASME-ISPS Joint Conference 
MIPE2022 special issue

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


Takeru Tomita1, Kenta Nakazawa1, Takahiro Hiraoka2, Yuichi Otsuka2, Kensuke Nakamura2 & Futoshi Iwata3

Graduate School of Integrated Science and Technology, Shizuoka University

2Ushio Inc

3Research Institute of Electronics, Shizuoka University


We have developed a confocal laser displacement sensor that enables in-process monitoring of the etching depth of the area where the etching is progressing by irradiating an atmospheric pressure plasma jet. Furthermore, using the developed confocal laser displacement sensor, we succeeded in monitoring the etching progress and the increase in the depth of the processing point and improved the etching accuracy by stopping the atmospheric pressure plasma jet irradiation when the desired etching depth was reached. For microfabrication, the atmospheric pressure plasma jet used a nozzle with a 10 µm diameter aperture, which was made by heat-drawing a glass tube. A wire-type high-voltage electrode was inserted into the glass nozzle, argon gas was introduced to discharge, and an atmospheric pressure plasma jet was irradiated from the tip of the nozzle. The confocal laser displacement sensor uses a narrow band filter corresponding to the wavelength of the light source used for measurement to block the plasma emission, thereby reducing noise due to the plasma emission. The spot position is modulated at 90 Hz to facilitate filtering out signal noise caused by electromagnetic noise. 

The response time of the developed confocal laser displacement sensor was measured by a step displacement test, and the response time was 0.11 s when a 0.2 µm displacement was applied to the measured sample. Considering that the etching rate is about 0.4 µm/s, this response time is sufficient to monitor etching. When the step displacement was varied from 0.2 µm, 0.15 µm, 0.10 µm, 0.05 µm, and 0.005 µm, the step displacement of 0.01 µm could be sufficiently discriminated and the resolution was 0.01 µm. An etching test was performed, and the shape became an ellipse in the parallel direction to the substrate, and the full width at half maximum was 44 µm in the short axis and 83 µm in the long axis when the etching depth was 10 µm. When the etching depth was monitored and etching was stopped when the target etching depth was reached, the average, maximum and minimum etching depths were 10.0 µm, 10.7 µm, and 9.2 µm, respectively. Without monitoring, the average, maximum and minimum values were 9.6 µm, 10.7 µm, and 7.2 µm, respectively. With monitoring, the standard deviation of etching depth could be reduced by 50%, indicating that in-process monitoring improves etching precision. 

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