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Lithography with Zeiss Orion Helium-ion Microscope. Installation tests

Lithography with Zeiss Orion Helium-ion Microscope. Installation tests


At the end of 2010, St. Petersburg State University purchased our product NanoMaker to be hooked on Carl Zeiss SMT ORION® Helium-ion Microscope. During installation several tests (together with the University staff, Prof. O. Vyvenko and Dr. Yu. Petrov) were carried out to assess possibilities of the instrument-driven milling and exposure of a positive PMMA 950K resist using a focused helium ions beam. All tests were performed at accelerating voltage of about 17 kV. Ion beam current range was limited to an upper limit of about 1 pA at 50 um aperture to achieve high resolution.
Orion SIM tests
An example of milling of a gold film Orion SIM 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.

Fig. 1. An example of patterning on a gold film,
resolution better than 10 nm was easily achieved
Orion SIM tests Orion SIM tests

Fig. 2 shows an optical microscope image of the dose wedge exposed at 1 us per point after 5 seconds of development. Waviness of the wedge surface is related to oscillations of the probe current (due to helium in bottle was nearing to the end and the system was trying to regulate gas flow with a period of approximately 15 seconds).

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

Orion SIM tests An optical microscope 
      image of the dose wedge
Fig. 2. An optical microscope image of the dose wedge
Orion SIM tests Orion SIM tests Orion SIM tests
An optical microscope 
      image of a resolution test structure To estimate resolution of the ion lithography method for given parameters of the microscope and the exposed substrate (150 nm PMMA resist on Si) a structure containing a set of long, repetitive lines of 700 nm, 400 , 200 nm and 100 nm width was exposed. Dose of all elements in the structure was equal to 100 %.
The structure was exposed twice with different exposure times (dwell time) per point - 1.5 us and 1.7 us. The sample was developed for 15 seconds. At inspection in an optical microscope with 1000x zoom the lines of 700 nm, 400 nm and 200 nm width were distinctly observed in both structures. Fig. 3 shows an optical microscope photo of the structure with 1.7 us exposure time per point.

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.

Fig. 3. An optical microscope image of a resolution test structure.
Dwell time 1.7 us

All photos obtained with the 30 deg tilt of the sample. Note that the waviness clearly visible for exposed structure (Fig. 2) is hardly noticeable in diagnostic (observation) mode.
Orion SIM tests Orion SIM tests
SIM image of 100 nm lines SIM image of exposed structure
Orion SIM tests Fig. 4. SIM image of structure central part.
Dwell time 1.5 us
Orion SIM tests Orion SIM tests
Fig. 5. SIM image of 100 nm lines.
Dwell time 1.5 us
Orion SIM tests
SIM image of 100 nm lines SIM image of structure central part
Orion SIM tests Fig. 6. SIM image of structure central part.
Dwell time 1.7 us
Orion SIM tests Orion SIM tests
Fig. 7. SIM image of 100 nm lines.
Dwell time 1.7 us
Orion SIM tests
SIM image of 200 nm lines SIM image of 400 nm lines
Orion SIM tests Fig. 9. SIM image of 400 nm lines.
Dwell time 1.7 us
We made a simulation of the helium ions beam scattering in the PMMA resist using TRIM program by James F. Ziegler, (http://www.srim.org/). At the accelerating voltage of 17 kV infinitely thin beam of helium ions in 150 nm PMMA resist
Fig. 8. SIM image of 200 nm lines.
Dwell time 1.7 us
at the interface with silicon dissipates to a diameter of 100 nm. This, in principle, agrees with our experimental data and limits of the method resolution for given parameters of the microscope-sample system.

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.

An optical microscope image of 3D structure Orion SIM tests One can see clearly the influence of the probe current oscillations mentioned.
An optical microscope image of 3D structure SIM image of the 3D structure
Fig. 10. An optical microscope image of 3D structure in PMMA resist of 150 nm thickness Orion SIM tests
Conclusions
High resolution in milling mode on thin gold film has been demonstrated. Evaluation of the sensitivity to milling gives ~ 106 C/cm3. Exposure of positive resist by ion beam has been investigated, sensitivity of the resist has been determined. It was found significantly higher efficiency of the resist exposure, however, for 17 kV acceleration voltage the resolution was lower than in electron-beam lithography, which is due to high lateral scattering of ions at resist thickness of 150 nm.
Fig. 11. SIM image of the same structure
Date Added: Feb 17, 2011
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
 
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