Scanning helium microscopy

Microscopes can be divided into two general classes: those that illuminate the sample with a beam, and those that use a physical scanning probe.

Since the denominator of the above equation for the Abbe diffraction limit will be approximately two at best, the wavelength of the probe is the main factor in determining the minimum resolvable feature, which is typically about 1 μm for optical microscopy.

To overcome the diffraction limit, a probe that has a smaller wavelength is needed, which can be achieved using either light with a higher energy, or through using a matter wave.

X-rays have a much smaller wavelength than visible light, and therefore can achieve superior resolutions when compared to optical techniques.

By focussing the X-rays to a small point and rastering across a sample, a very high resolution can be obtained with light.

Since electrons are charged, they can be manipulated using electromagnetic optics to form extremely small spot sizes on a surface.

Due to the wavelength of an electron beam being low, the Abbe diffraction limit can be pushed below atomic resolution and electromagnetic lenses can be used to form very intense spots on the surface of a material.

While the spot size on the surface can be extremely low, the electrons will travel into the bulk and continue interacting with the sample.

One method to mitigate the issue when imaging insulating surfaces is to use an environmental scanning electron microscope (ESEM).

Thus, if the objective were to study the surface of a material at a resolution that is below that which can be achieved with optical microscopy, it may be appropriate to use atoms as a probe instead.

Using particles of a higher mass than that of an electron means that it is possible to obtain a beam with a wavelength suitable to probe length scales down to the atomic level with a much lower energy.

[3] The turning point is well above the surface atom cores, meaning that the beam will only interact with the outermost electrons.

King and Bigas suggest it could be possible to form an image by scattering atoms from the surface, though it was some time before it was demonstrated.

The first two-dimensional neutral helium images were obtained using a conventional Fresnel zone plate[9] by Koch et al.[14][non-primary source needed][when?]

Helium will not pass through a solid material, therefore a large change in the measured signal is obtained when a sample is placed between the source and the detector.

Nevertheless, the focussing obtained with a zone plate offers the potential for improved resolution due to the small beam spot size in the future.

Research into neutral helium microscopes that use a Fresnel zone plate is an active area in Holst’s group at the University of Bergen.

Since using a zone plate proved difficult due to the low focussing efficiency, alternative methods for forming a helium beam to produce images with atoms were explored.

The lack of atom optics means that the beam width will be significantly larger than in an electron microscope.

[15][non-primary source needed] A small pinhole is placed very close to a sample and the helium scattered into a large solid angle is fed to a detector.

Images are collected by moving the sample around underneath the beam and monitoring how the scattered helium flux changes.

In parallel to the work by Witham and Sánchez, a proof of concept machine named the scanning helium microscope (SHeM) was being developed in Cambridge in collaboration with Dastoor's group from the University of Newcastle.

The use of a focusing element (such as a zone plate) allows beam spot sizes below 1 μm to be achieved, but currently still comes with low signal intensity.

The inertness of helium that makes it a gentle probe means that it is difficult to ionise and therefore reasonably aggressive electron bombardment is typically used to create the ions.

Once the flux from a specific part of the surface is collected, the sample is moved underneath the beam to generate an image.

[21][22] When designing a scanning helium microscope, scientists strive to maximise the intensity of the imaging beam while minimising its width.

The reason behind this is that the beam's width gives the resolution of the microscope while its intensity is proportional to its signal to noise ratio.

The derivative of the intensity with respect to the zone plate radius can be reduced the following cubic equation (once it has been set equal to zero): Here some groupings are used:

is a constant that gives the relative size of the smallest aperture of the zone plate compared with the average wavelength of the beam and

This goes in line with the results obtained for the pinhole configuration, and has as its practical consequence the design of smaller scanning helium microscopes.

A diagram showing how a scanning helium microscope works. A beam is formed by a gas expansion and collimation through a skimmer and pinhole. The beam is then incident on the sample, where the gas is scattered and collected through a detector aperture. The scattered gas is then detected using a mass spectrometer. By then rastering the sample, an image of the sample can be formed.
A helium atom image of a fly's eye
SHeM Contrast Mechanism Tree
Geometry of a scanning helium microscope in its pinhole configuration showing the variables used in this article. Image taken from [ 22 ] (uploaded by the author).
Geometry of a scanning helium microscope in its zone plate configuration showing the variables used in this article. Image taken from [ 27 ] (uploaded by the author).