Helium Ion Microscope Supplements SEMs

The helium ion microscope (HIM), introduced by ALIS Corp. (Peabody, Mass.), promises to supplement some of the traditional SEM’s shortcomings. In a common SEM, the probe size is limited principally by two conditions. The first is diffraction, attributed to the electron’s non-zero de Broglie wavelength. As electrons stream through an aperture, they diverge, resulting in a larger probe size. A larger aperture moderates the diffraction effect, but results in an increase in the chromatic aberration, the second negative effect.

Another hindrance is that a SEM electron beam tends to scatter under a sample’s surface. This can result in high-energy backscattered electrons, many of which come back out of the surface several nanometers from the incident beam. Thus, if secondary electrons (SEs) are being collected — like most SEMs do — they originate from where the original beam entered (SE₁), but also from every other point where backscattered electrons exit (SE₂). The SE₂ introduces non-local and deeper information, which tends to blur the image.

An HIM works much like a SEM: There is a sample at the bottom, a final lens to focus the beam, a system that scans the beam over the sample, an aperture, and beam alignment stages to get it down the column through the aperture. The difference is at the top, where the field ionization source resides, a technology derived from the venerable field ion microscope. In the HIM, there is an extremely pointed piece of wire that is subjected to a large positive voltage, producing such a massive electric field that it ionizes any neutral atoms near it. It is maintained in high vacuum, and helium gas is introduced to create the ion beam. When helium atoms pass close to it, they are ionized and accelerated away from the tip.

The alignment cross in the SEM reveals topographic information, but relatively little material information (left). The helium ion microscope provides a much stronger contrast mechanism (right).

For the technology to work, the ionization must be produced by only the most protruding atoms at the tip’s apex. To achieve this, the tip is altered to a pyramid shape with three atoms at its apex. The process of reshaping the tip was developed by ALIS. Each of these atoms produces ion beams, but only one is allowed to pass down the column and be focused and scanned. Thus, the virtual source size — the helium ions’ origin — is Ångströms or sub-Ångström in size. The virtual source size, in part, determines how well a charged particle beam can be focused. The HIM is expected to achieve a 0.25 nm probe size, whereas a typical SEM’s is ~2.0 nm.

Aluminum crystal grown in a vacuum. The image illustrates the helium ion microscope’s extensive depth of field.

In a typical SEM, lower beam energies are used to minimize interaction problems. Some SEMs operate at as low as 200 or 500 eV, resulting in a smaller SE₂ contribution, reducing the halo effect. The problem with low landing energies is that they all deposit at the sample surface, potentially damaging it. Higher beam energies are better, because they dissipate energy over a larger volume below the sample surface. A HIM can operate over a range of 1-45 keV, but the energy typically applied is ~20 keV, which does not do much damage.

The SEM and HIM’s contrast mechanisms are different and may vary from material to material, which makes the HIM a complementary technique when the SEM does not provide the necessary resolution or contrast.

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