Robustness test of transmission electron microscope evacuation system to gas leak and toughness test of window for environmental cell sample holder for in-situ gas environmental experiments
There are two methods of in-situ observation using a TEM, those are, an environmental cell type using a special sample holder and a differential pumping type. The differential pumping type requires a dedicated electron microscope. On the other hand, in the environmental cell type, the in-situ observation can be easily performed with the existing electron microscope. Recently, it is more preferred to use the environmental cell type in-situ observation. That type, a miniature reaction chamber is built at a tip on a TEM holder. This chamber has a window made of SiN film, which allows electrons to transmit, and separates the environment inside the reaction chamber from the vacuum. With an electron beam intense enough to break the window, gas or liquid will leak into and spread out the TEM column. Therefore, it is important to test how robust the window is to perform a secure in-situ experiment and we report on the issue.
For the experiments in this report, we used an aberration corrected microscope (JEM-ARM300F, JEOL) with an evacuation system of differential pumping type. In the differential pumping system, each chamber is evacuated with dedicated pump, and each chamber is separated by orifices. Thus, the specimen chamber is evacuated by independent pump to minimize the affection of the vacuum level of the specimen chamber to the other chambers. In the microscope for the experiment, the vacuum pumps to evacuate specimen chamber are switchable with turbo molecular pump or sputter ion pump and two evacuation modes are prepared one is normal observation mode and the other is gas environment observation mode. When the in-situ observation mode was selected, the ion pump around the sample holder was stopped and the turbo pump was activated. We measured vacuum level at each ion pump of the electron gun, while we operated the microscope in the in-situ observation mode. As a result, it was found that even if the vacuum level around the sample holder deteriorated to about 10-3 Pa, there was no problem on the emitter area, indicating that the turbo pump evacuating the specimen chamber was working effectively.
In order to investigate the toughness of the MEMS chip, the electron beam was concentrated on the window of the chip and measured the dependence of the thickness of the SiN film. As a result, it was found that under normal TEM observation irradiation conditions, the durability of the window of 30 nm thick was 30 minutes to 1 hour.