According to a recent NewScientist story, the UK company NFAB is now developing a microscope on a chip four times more powerful than the best scanning electron microscopes (SEMs) available today. The best SEMs currently have a resolution of 0.05 nm. This new one, which will be small enough to fit onto a fingertip, should achieve a resolution of 0.01 nm. The prototype should be ready by the end of the year. If successful, it could be used for a variety of applications, such as making holograms of large single molecules.
This project has been led by Derek Eastham of NFAB, a condensed matter physicist who was previously the technical coordinator of the EU-funded Monarch project, whose aim was to develop a “ultra-bright nanoscale SEM-on-a-chip.”
NFAB has picked up the sub-miniature scanning electron technology, which New Scientist says should be completed in product form in three to six months. According to Sky News, the microscope is expected to be available for trials within two years for a cost of about US$195,000. Current SEMs with a resolution of 0.05 nm fetch price tags in the multi-millions.
Eastham’s design uses a much lower energy beam in a device with just a few millimeters between the electron generator and the object being studied. That distance is more usually a few feet. Instead of firing electrons from a tungsten filament, it will shoot them from a single atom at the peak of a tiny gold pyramid with a height of around 100 nm. The beam will be focused as it passes through a 2 μm hole in a silicon chip before it hits the target below. The electron beam itself is just 10 μm long in Eastham’s new microscope—the beam of a standard SEM is around 60 cm long.
The electrostatic lens used in the new SEM still contains imperfections that will limit the microscope's resolution, says Eastham, but the effect should be much smaller.
Eastham's approach produces a beam with around 100 times less energy than usual in an SEM. Cutting power consumption addresses one of the greatest costs of SEM technology, he claims. Delicate biological structures, for example, must be carefully prepared before use by an electron microscope, which can easily destroy it.
Other microscopy experts are more conservative in their expectations of the microscope's performance. "The basic concept is certainly correct," says David Joy at the Univ. of Tennessee in Knoxville, Ky. It has been noted before that a shorter electron column would give superior performance, he says, although the idea has proven hard to put into practice.
But Joy thinks resolutions of 0.01 nm are unlikely. "It is now possible to 'aberration correct' electron optical lenses," he says, which allows the best, multi-million dollar SEMs to reach about 0.04 nm resolution.
Tiny fluctuations in the energy of the beam that limits resolution, an effect Joy expects to hamper Eastham's "nano-SEM", too.
Edward Boyes at the Univ.y of York, UK, is also interested to see if the microscope can deliver. "These devices are potentially useful as a route to lower costs," he says.
But he adds that the new design will lack the large depth of field that lets SEMs produce images with a 3-D appearance, because of the design of its aperture.
Instrument specifications from NFAB
NFAB says its sub-miniature scanning electron microscope is a completely new concept in electron microscopy. The microscope is 5 μm long and has atomic resolution (2 Å) at 500 eV energy and 10 nA of current. The low energy means it can identify single atoms on a surface as well as being able to make holograms of large molecules.
All of the component parts can be manufactured by existing commercial technology. For example, the electron source is a gold nanopyramid. These can be routinely manufactured in nanotechnology laboratories. The microscope body is manufactured by standard MEMS techniques.
The final packaged instrument, says NFAB, will be a microtip with the microscope on the end rather like a scanning tunneling microscope with an electron beam focused to atomic sizes. Because the depth of field is large and the beam current is high, it will scan much more quickly than an STM, can study practical surfaces and can identify atoms directly from the elastic scattering.
The company has patented an enabling technology for a generation of miniature scanning electron microscopes, multiple-beam lithography machines and focused ion beam millers to be manufactured. These are currently under development and testing at Salford Univ., UK, and various other European universities and nanotechnology companies under an EU-funded collaborative research grant (CRAFT).
