Focused Ion And Electron Multi Nanobeams, A Radically New Technology

Single, focused beams of ions and electrons are widely used in nanofabrication both in research and in industry. Electron beam lithography is ubiquitous as a nanofabrication patterning tool. Focused ion beams, so far mainly with gallium ions, are used for various cutting operations. In both cases the output is limited by the fact that the writing is sequential and consists of, in effect, tracing the beam in the desired pattern. This is about to change. Three companies, Mapper in Holland, KLA in California and IMS in Austria, are developing prototype maskless, electron-beam writing systems which are projected to achieve at least two orders of magnitude higher writing speed than single beam systems. The main application driving this multibeam development is maskless lithography, namely the exposure of resist with a beam of electrons (or ions). In low volume integrated circuit manufacturing, these electron multibeam systems are expected to replace optical lithography, thus avoiding the prohibitively high cost of mask sets.

Using the similar concepts in the course of designing and building an electron beam system, IMS has developed a machine for writing with ions and has written 50 nm features with argon ions and 16 nm features with hydrogen ions. This may have a more wide-ranging impact on future technology than just lithography, i.e. resist exposure.

The capability of producing a patterned dose of ions of any element in the periodic table with resolution in the nanometer regime opens many other possibilities. Some of these were recognized and demonstrated about 15 to 20 years ago using single beam systems operating with alloy ion-sources that could produce the dopants of silicon or gallium arsenide. Numerous unique devices were built based on this capability of varying the dose from point to point in the substrate. Including :

1. Bipolar transistors with reduced spreading resistance
2. MOSFETs with almost 0 output impedance
3. Tunable gun diodes from 22 to 42 GHz
4. 1 Ghz flash A / D converters
5. CCDs with 2.5 times increased operating frequency

Many of the integrated circuit manufacturing companies, particularly in Japan, bought focused ion beam systems to exploit this unique implantation capability . However, none of these applications prove to be practical. The writing speed was abysmally slow, and the liquid metal alloy sources proved to be unstable and difficult to operate. With the new multibeam system the total current in the beam is at the least three orders of magnitude higher than with a single focus beam. Moreover, the ion sources are plasma-based and similar to the ones routinely used in ion implanters for many ion species and are quite reliable.

A high current, reliable ion beam nano writer will enabled other applications. For example, certain chemical sensors based on FETs require a light dose of ions implanted in the oxide of a gateless transistor. A sensor chip that senses different chemicals would require several different species to be implanted. Moreover, the area to be implanted on each chip is small, and the use of a multibeam ion implanter would save many conventional fabrication steps : resist coating, resist exposure, resist development, and full wafer implantation.

In addition, the capability of ion bombardment to disrupt the crystal structure can also be exploited. We have recently shown that the identity in RF ID tags could be physically written by implanting a low dose of ions to permanently turn off transistors. Moreover, writing speed with an ion beam would be many times higher than the writing speed of the currently used laser cutting technique.

In situ processing, the concept of fabricating an entire microelectronic type device in one vacuum chamber, was an active research topics some years ago. The success of this concept was limited by the fact that no nano patterning technique other than resist-based lithography was available. Resist-based lithography requires wet chemistry and is not vacuum compatible. Focused ion beams could carry out many of the vacuum nanofabrication steps, but were hopelessly slow and had limited ion species available. But with the multibeam systems this picture will change. Indeed ion beams can be used to remove material, deposit material and implant with minimum dimensions 10 to 50 nm range. As is often the case, a radically new technology will have many applications that are not yet identified.

These are some of the activities this consultant would like to pursue to advance and exploit the multibeam concept :

• Analyze the multibeam system limitations and identify and evaluate applications
• Demonstrate applications, e.g. devices made using ion implantation, material removal or addition or local damage (Continuing single beam research with new motivation.)
• Develop ion sources optimized for multibeam applications
• Design and build a prototype aperture mask (The last two are large projects and could only be pursued in collaboration with the European machine developers or with a large US commitment.)

To see the resume of the expert associated with this case study, see the link below.