Lithography with Zeiss Orion Helium-ion Microscope. Installation tests

Milling
Several attempts to mill a silicon substrate at different currents and exposure times (dwell time) in the range of 0.5-10 C/cm² were undertaken. Unfortunately, we could not observe any milling of the silicon surface. Instead, at long exposures contamination deposition was observed. A similar negative result was also observed when attempting to mill a thick gold film (thickness about 100 nm). Milling of thin gold films on carbon (gold resolution standard sample) was achieved at good beam focusing, i.e. at creating a sufficiently high current density. An example of patterning on a gold film by milling is shown in Fig. 1.
The etching was performed with the current of 1 pA in little less than 3 minutes. Distortion of the last letter and of the emphasizing line observed on the right side is due to curvature of the gold particle surface. Evaluation of the sensitivity to milling for this case gives ~ 10⁶ C/cm³.

PMMA resist exposure
Silicon substrates coated with positive PMMA 950 K resist of 150 nm thickness were used for exposure. at accelerating voltage of about 17 kV and ion beam current of 0.5 pA. Exposure steps (point-to-point distance) have been set to 4 nm.

Two structures of dose wedge with different exposure times (dwell time) per point - 1 us and 2 us in order to assess the sensitivity of resists to ion beam irradiation had been drawn.

The experiment showed significantly higher efficiency of the resist exposure in comparison with electron beam lithography (more than 50 times).

For inspection in the ion microscope a gold-palladium film of 5 nm thickness was deposited above the resist.
It was found that all lines of 100 nm, 200 nm, 400 nm and 700 nm width are not developed to the bottom in the structure with 1.5 us exposure time, but lateral dimensions are sustained. Pictures on Fig. 4 and Fig. 5 demonstrate fragments of the structure. Fig. 4 presents central part of the structure and Fig. 5 shows 100 nm lines.
For another structure where the dwell time per point was 1.7 us the lines of 700 nm and 400 nm width are developed to the bottom. Whereas lines of 200 nm and of 100 nm width look underdeveloped. In addition, there is a degradation of the lateral size by 16-18 nm demonstrated in photos of Fig. 6 - central part of the structure, of Fig. 7 - 100 nm lines, of Fig. 8 - 200 nm lines, of Fig. 9 - 400 nm lines.

As follows from the results of the dose wedge exposure, the ion structuring method used has a very high sensitivity to variations in dose. We have tried to use this to create 3D structure in 150 nm resist. As initial drawing the emblem of the University was chosen. Using functions implemented in NanoMaker a bitmap picture was imported into NanoMaker Editor as "DoseMap" structure. The dose range was adjusted by using the Transform by Formula option so that an exposure dose at a point is proportional to grayscale level of the picture. Structure size was chosen about 66 um x 66 um, so that the smallest details of the image was about 100 nm. A few structures were exposed with a small variation of dwell time. Fig 10 and Fig. 11 shows photos of one of the resulting structures, observed in the optical and helium ion microscopes, respectively. SIM image is acquired with the tilt of the sample by 45 deg.