Precision Milling of Carbon Nanotube Forests Using Low Pressure Scanning Electron Microscopy

The overall goal of this scalable technique is to mill carbon nano materials, such as carbon nanotubes with sufficient precision to selectively machine an individual nanotube. This method can help reveal inner structural morphology of carbon nanotube forests, and introduce complex 3D features into carbon nanotube microstructures. The main advantage of this technique is that it can be used to mill both individual carbon nanotubes, or tins of cubic micron material with less material re-deposition than related techniques.

Though this method can provide insight into carbon nanotube systems, it can also be applied to other carbon-based materials, such as graphene, diamond, and biological cells. Visual demonstration of this method is critical because the carbon nanotube milling steps are quite different from conventional SEM-based imaging techniques. Prior to beginning this procedure, grow nanotube forests in thermally oxidized silicon wafers coated with alumina and iron.

To begin sample preparation, secure a CNT forest sample on a standard 5 inch SEM stub using carbon tape. Ensure the sample is over the SEM stub edge. Alternatively, attach the sample to an electron beam lithography mount.

For samples where the CNT cross section will be milled, mount the stub in a 45 degree stub holder. Then, vent the ESEM, open the SEM chamber, and secure the stub to the sample stage. Close the chamber and start evacuation of the ESEM.

While the pressure is decreasing, set the electron beam acceleration voltage to five kilovolts, and spot size to 3.0. Select the secondary electron detector. Once the SEM chamber pressure is below 1.5 times 10 to the minus two pascal, activate the electron beam.

Focus the sample image with the manual SEM focus control knobs, then, tilt the sample to 45 degrees. Focus the image on the highest sample. Link the focal distance to the working distance, and set the z-coordinate to seven millimeters.

Refine the image using focus, stigmation, brightness, and contrast knobs. Next, select a milling region using the manual control knobs, or the instrument's software. Then, navigate to a location approximately 100 microns from the milling region.

Estimate the material removal rate from previously obtained data for the intended pressure acceleration voltage, dwell time per pixel, and beam current. If milling will be on a micrometer scale, increase the electron beam acceleration voltage to 30 kilovolts, and spot size to 5.0. Use the manual control knobs to adjust the image focus, brightness, and contrast.

Manually set the aperture to one millimeter, and resolve the image again. Then, decrease magnification to less than one thousand x. Set a pressure of 10 pascals in the instrument software.

Select low pressure mode to introduce water vapor into the sample chamber. The most critical step is introducing the water vapor, which will generate a reactive species that will etch the CNT's in the defined regions. Once the pressure has stabilized, reactivate the electron beam.

Set the dwell time to less than 10 microseconds, and the resolution to 1024x884 pixels. Adjust the image focus, stigmation, brightness, and contrast as needed. Navigate to the milling region, and if necessary, rotate the image to align with the native vertical and horizontal scan orientation of the SEM.

Set the magnification to 40 thousand x for milling features on the order of one micron, or 20 thousand x for milling features on the order of 5 microns. Pause the electron beam and identify the desired reduced area milling regions. To begin the milling procedure for our rectangular area, select the reduced area tool and extend the reduced area rectangle over the desired milling region.

Increase the dwell time to two milliseconds. Set the image resolution to 2048x1768. Unpause the electron beam and immediately repause the beam so that the beam only rasters the selected area once.

Once rastering has finished, decrease the magnification to less than one thousand x. To mill a pre-designed pattern via lithography software, import the pattern and assign control of the ESEM instrument to the lithography software. Select the pattern file and start the milling process.

Once milling has finished, return the ESEM to SEM mode. After milling by either method, set the ESEM back to the high vacuum state, and if necessary, return the beam parameters to five kilovolts and 3.0 spot size. Activate the electron beam and obtain an image of the milled sample.

When finished, vent the ESEM chamber, and remove the sample and stub, or mount. Close and evacuate the chamber. Both large and small areas of the CNT forests were milled with this technique.

Here, the top of a 10 micrometer wide CNT forest micropillar was selected with a reduced area box. Upon rastering the beam, the top of the pillar was precisely milled. On a smaller scale, individual CNT's were selected with a reduced area box and an aperture of less than 50 micrometers in diameter was used.

Upon rastering the beam, the individual CNT's were milled from within the forest. By using software controlled electron beam rastering, non-rectangular patterns can be milled into a CNT forest. Here, the forest was milled parallel to the growth direction, fully to the underlying silicon substrate.

While attempting this procedure, it's important to remember that different carbon-based materials will mill at different rates, and experimentation may be necessary to determine the optimal milling rate.