Field ion image acquisition using deflector for synchronization with atom probe analysis

抄録

The authors have been working on integrating atom probe (AP) mass analysis and structural observation based on field ion microscopy (FIM) [1]. FIM has an advantage in image quality against ion mapping by 3D-AP data. To combine the FIM and the AP, the image gas ion must be blocked to go into the ion detector of the mass spectrometer. In the conventional FIM-AP system, the toroidal lens tuned for the energy of the field evaporated ion served as the energy filter. Though FIM observation and AP analysis can be done in an identical system in this way, there is one disadvantage the probed region itself is not visible. The authors are trying to eliminate the difficulty by introducing an image deflector driven by an AP analysis system.

The deflector installed between the specimen and the phosphor screen with a probe hole shifts the FIM image projected on the screen to show the probed region image not visible during AP analysis. During the AP triggering sequence, the deflectors are not biased to make no influence on the trajectory of image gas ions. When a field-evaporated ion is detected, the acquisition system stops the trigger. Then the deflectors are biased at +/-1 kV and the CCD camera captures the shifted FIM image. After the image acquisition, the system reverts to AP analysis. Thus, the image data of probed region and the mass analysis data are synchronized in the analysis.

For the improvement of the preliminary system reported elsewhere [1], the authors have been working on the following points; (1) the boost of deflector bias voltage to obtain the efficient shift of the FIM image, (2) the reduction of back pressure during FIM-AP analysis. The bias voltages of +/-1 kV were achieved. As the image shift is almost proportional to the amplitude of applied voltage on the deflector plates, the shift stroke of the image is 1.5 times of probe hole diameter as shown in Figure 1. There is no overlapping of the probed region and the probe hole.

The reduction of back pressure in the time of flight mass spectrometer is required for the improvement of the detection efficiency of the AP system. The image gas (helium, typically) of the 10^-3 Pa region is introduced for the FIM image observation. The ionized gas emitted by the specimen is eliminated by the energy filter (toroidal lens) in front of the ion detector but the neutral species also disturb the flight of the field evaporated sample ions. To minimize the dose gas amount, the dose tube of 0.8 mm I.D. with pre-cooling was mounted at a distance of 5 mm from the sample tip. The evacuation pump layout was also arranged to reduce the back pressure in the spectrometer. By these modifications, the back pressure measured in the toroidal lens was reduced to a 10^-4 Pa order. Other results will be also shown in the presentation.

[1] M. Taniguchi, Y. Yamauchi, K. Yoshikawa, J. of Vac. Sci. Technol. B, 41, 042806 (2023). https://doi.org/10.1116/6.0002607