Helium Ion Microscope is the latest additions to the microscopy toolbox. As the name implies, helium ion microscope (HIM) utilizes ~ 35kV He ions focused on a small spot (a few angstroms in diameter). Upon interactions with the sample, secondary electrons (SEs) are emitted and counted by a detector. By rastering the ion beam across the sample, we can obtain an image of the surface. Significantly smaller beam spot size and high SE yield gives important advantages over a Scanning Electron Microscope (SEM). The main superiority of HIM is the much larger field of view and unique capability to image non-conducting samples without a deposited metal overlayer.
The key components of the instrument are the ion source, the extractor electrode, the ion optics, and the secondary electron detector.
The ion source in HIM consists of a very sharp monocrystalline metal needle (the exact composition is a closely guarded trade secret but is widely believed to be W or a W alloy) with the tip of ~ 100 nm in diameter ending with only 3 atoms (referred to as a trimer). The three-atom configuration allows greater stability and consequently longer operation times compared to a single atom. The needle is cooled with liquid nitrogen to ~75 K. Cooling the ion source serves several purposes: it reduces vibrations in the needle, thus further increasing stability of the trimer as vibrations in the ion source may result in blurring of the image, and also reduces background gases. The needle is connected to a positive high voltage power supply (tunable from +5 keV to +30 keV). This generates a very high electric field (~4.4V/ Å) in the area of the trimer resulting in field ionization of the He atoms. The field ionization potential of He is the highest of any element, resulting in background gases being field ionized further away from the tip and then accelerated away. This means that the tip is being protected from contaminants, resulting in a longer life time.
Right below the tip is the grounded metal electrode – extractor. Upon ionization, the He ion is instantly accelerated away from the tip through the axially aligned hole in the extractor electrode. Further below He beam is aligned, focused and rastered on the sample.
Once reaching the sample, the incident He ions interact mainly with electrons. Since the He mass is much larger than that of the electron, the ions do not deviate much from the initial trajectory creating significantly small interaction volume than for incident electrons. Some electrons are ejected from the sample (secondary electron generation) and attracted and counted by the detector. The SE have energies sharply peaked at just a few electron volts and the SE emission probability is larger than in a SEM.
Amongst other factors, number of secondary electrons reaching detector depends on the angle of incidence. This particular phenomenon allows HIM to generate exceptionally clear images of complex 3D structures which resemble familiar macroscopic light and shadow regions, but with much better resolution.
Upon interaction with the sample, He ions provide positive charge which is normally dissipated in the conductive sample, yet when dealing with insulating materials, the problem of charging arises. Large concentration of positive charges on the surface constitutes electron deficiency thus diminishing the secondary electron yield. Therefore, insulating areas appear black during imaging.
In SEM, insulating materials are sometimes coated with a thin conductive film such as gold to negate the charging effect. However, even a gold monolayer can hide important functional features and in some cases may completely conceal small objects. In HIM the charging problem is mitigated with the addition of flood gun (a source of low energy electrons), rastered slightly behind the He beam, effectively neutralizing positive charge build up and making it possible to obtain clear high resolution images of polymers and biological samples without surface modification with gold. While imaging with HIM, it is important to consider also He interactions with the atomic nuclei in the sample. Even though the probability of He transferring enough energy to eject atom (sputter) from the samples is low, damage to the sample can be observed at higher doses. Hence, it is important to keep in mind the delicate balance between clear high-resolution images (increased He ion dose) and minimizing sputtering, as well as He implantation effects.
However, in some cases He beam damage can be harnessed to the benefit of the researcher. For instance, by delivering beam damage to the predetermined areas we can generate patters like words, images or even devices. The resolution of the patters is limited by the resolution of the microscope and software controlling the beam.