Scanning Helium Ion Microscope

A helium -ion microscope (english scanning helium ion microscope, SHIM ) is an imaging method that is based on that a helium ion beam scans the object to be examined. The process is similar to a scanning electron microscope. Developed Helium Ion Microscope by Billy Ward among others, founded by them U.S. company ALIS, which was acquired by Carl Zeiss SMT 2006. This is currently (2013 ) is the only manufacturer of such a device.

Structure and beam generation

In the frozen, under high vacuum apparatus, a high voltage in the strong electric field generated near a sharp tungsten needle helium ions: helium atoms enter ( through the tunnel effect) electrons to the tungsten needle off and then wegbeschleunigt of the needle. The helium ion beam is prepared, i.e., bundled and directed and guided to the material to be examined. Are measured, the intensities of the transmitted through the sample beam and the reflected beam from the sample, and the number of generated secondary electrons.

Comparison with a scanning electron microscope

The technology of SHIM has several advantages compared with the scanning electron microscope:

• The resolving power of microscopes has a theoretical limit, the Abbe resolution limit, which is given mainly by the wavelength of light, or by the de Broglie wavelength of the particle. A helium ion beam due to the larger mass of particles has a shorter wavelength than a comparable electron beam, and thus the corresponding theoretical achievable only in the ideal case of a SHIM resolution is better than an electron microscope. In practice, this resolution limit hardly plays for both types of microscope but a role: The wavelength of an electron located at relatively small acceleration voltages below one nanometer, and is for 10 kV electrons at 12.3 pm far from the actual achievable resolution.
• The penetration of the ion beam into the sample, the beam is expanded less than an electron beam having the same energy. This information field is limited to a smaller area. In Rasterlektronenmikroskopen formed by the wide spread of the electrons, a so-called excitation bulb with a diameter of more than 100 nm to a few micrometers.
• The helium ions are taken in the highest resolution mode from a range in the vicinity of a single atom, that is, from an almost point-like source.
• As compared to an electron beam, the secondary electron yield is relatively high. Since these are detected, their number determines the gray value of each pixel; the SHIM can therefore provide high-contrast images with the same Primärstralintensität. The detectors provide information-rich images that reflect topographic, material, crystallographic and electrical properties of the sample.
• Studies show that when the proportion of the detected electrons SHIM, coming from the bottom of the sample, is considerably smaller, i.e., the secondary electrons produced in the depth of reach of the harder surface and the detector. The recordings are therefore sensitive surface. Modern scanning electron microscopes with improved imaging performance with an acceleration voltage of less than 1 keV but can achieve similar improvements.
• The depth of SHIM is very good, even better than that of a scanning electron microscope.
• The ion beam damage polymers less than an electron beam.

Compared to other focused-ion -beam microscopes

Focused ion beam devices usually work with gallium ions to edit surfaces in the micrometer range. Due to the low mass of the helium ions of the sputtering effect is much smaller, but still exists and can be also used. Systematic studies on the substrate damage are not before.

The microscopes on the market since 2007, an increase of up to a million times and a minimum resolution of 0.75 nm is given. A resolution record of 0.24 nm was announced on 21 November 2008. The value is, however, difficult to compare, since the width of the edge is measured and not ( as in the normal default), the minimum distance between two objects.