© 2004 Robert A. Freitas Jr. and Ralph C. Merkle. All Rights Reserved.

Robert A. Freitas Jr., Ralph C. Merkle, Kinematic Self-Replicating Machines, Landes Bioscience, Georgetown, TX, 2004.

4.7 Shoulders Electronic Micromachining Replicator (1960-1965)

In 1960-1965, the controversial inventor K.R. Shoulders [2253-2256] speculated on a completely electronic-mediated mode of replication that envisioned manipulation of physical materials:

In the simplest case, the machine should be an iterative array of components that are isolated from each other, but have the ability to intercommunicate via sinuous electron paths. The components must be able to launch, adsorb, receive and steer electrons or groups of electrons. Such an organization would allow the creation of order within the machine at the dictate of other organized areas. This newly-made order could be propagated physically through the machine and then be destroyed if found wanting. The components could then be used again for higher levels of organization. This arena concept (using fundamental elements at fixed or movable loci but having flexible connectivity and a reversible change of state without leaving a residue) would greatly modify most present concepts of machine organization…. We will claim that field-emission devices could be used to fabricate a ‘wireless’ machine using any interesting organization…and that a complete machine could be built on this principle.

We propose a component based upon the quantum-mechanical tunneling of electrons into the vacuum. Ultimately, we would use a vacuum-tunnel effect cathode array for our electron source. The emission from discrete areas would be controlled by local grids. Thus we have components made by electronic micromachining responsible for the building of new systems by the same method. In the end, self-reproduction would be a distinct possibility without the use of a lens system, because all copies would be made on a one-to-one size basis. [1012]

In 1961, Shoulders wrote extensively [2254] about using electron-beam-activated machining techniques to produce microelectronics components. The above passage clearly suggests that he envisioned such systems could ultimately be capable of self-replication. Shoulders appears to have been the first to discuss beam-based machine replication, a concept subsequently briefly explored by Taylor in 1978 (Section 3.10) and Freitas in 1981 (Section 3.14).

Four decades later, much progress has been made in this field, though as yet no replication has been achieved – although e-beam text-writing garnered Newman the second Feynman challenge prize in 1985 (Section 4.6). Electron beam induced deposition (EBID) is the process of using a high-intensity electron beam within an electron microscope to induce the formation of structures on the scanned surface [2257, 2270]. EBID is a slow but versatile direct-write additive lithography process for the fabrication of mesoscale and nanoscale structures with high lateral accuracies. In this process, the microscope interior is filled with a low pressure precursor gas which is readily broken down under intensive e-beam illumination. For example [2258], W(CO)₆ carbonyl gas [2259-2261] or WF₆ gas [2262] is used to grow tungsten carbide or tungsten wires, Cr(CO)₆ and Re₂(CO)₁₀ are used to grow chromium and rhenium nanorods [2261], Al(CH₃) ₃ is used to grow aluminum, C₇H₇O₂F₆Au to grow gold, and (methylcyclopentadientyl)trimethylplatinum is used to grow platinum. In the EBID process, the electron beam is trained on a spot and held there, whereupon the precursor gas breaks down locally, depositing metal atoms only on the spot. As the beam continues to hit the spot, more atoms are deposited and a cylindrical column of metal grows, typically at a rate of ~3-300 nm/minute [2263]. By slowly translating the beam position in the XY plane and controlling the substrate angle and the growth conditions, 3-dimensional nanostructures such as atomic force microscopy tips [2264, 2265], interconnects [2277, 2278], walls [2277], and more complex structures [2266], including a world map printed on a spherical 60-micron resin microsphere with 10 nm features [2267], have been made via “electron beam stereolithography.” (See also Section 3.20.)

Koops notes that objects are readily constructed in vertically free-standing geometric shapes such as Y’s, X’s, and more complex structures containing internal voids such as arches or closed loops atop columns (i.e., round or square tennis-racquet shapes) [2268], including in one case a “three-dimensional build-up with conducting material resembling a field emission electron source, having an emitter tip and an extractor ring around it...[with] a capacitance of 24 aF between the tip and the ring” and in another case an artistic “Micro Coliseum” structure made by “FIB [focused ion beam] assisted deposition using a hydrocarbon precursor gas. The diameter of the structure is 1.8 µm, its height is 0.5 µm, the employed linewidth of the arches constructing the building is 0.08 µm.” EBID has also been used to deposit carbonaceous structures [2269-2274], ruthenium nanoparticle chains [2275], and a variety of conducting and nonconducting materials [2276-2279]. Conductive nanostructured devices such as field emission sources have been fabricated [2280, 2281] – single-atom STM tips can emit up to 10 microamp in field emission mode [2282], although 0.1-1 microamp is more common for field emission from carbon atomic wires [2283]. Electron emission from EBID-grown columns or tips has been widely reported in the literature [2284-2290].

EBID resolution is somewhat restricted [2291] because electrons scatter quickly in solids, beam electrons are mutually repulsive, and electron wavelengths are typically 20-50 nm. This often limits practical e-beam lithographic resolution to dimensions >10 nm, although resolutions as small as 2 nm have been obtained in a few materials [2258], wires 80 nm in diameter have been fabricated with surface roughness below 2-3 nm [2292, 2268], tips for AFM/STM work can be made with radii of ~5 nm [2270, 2272], and EBID-fabricated carbon supertips have “customizable shapes with features down to below 10 nm” [2274]. Unlike CVD (chemical vapor deposition), EBID is not a thermal decomposition process, so the purity of the deposit is often much lower unless the substrate is heated [2280, 2293]. However, a few high-purity free-standing metal nanorods have been made by EBID [2261], and a rapid fabrication process could be achieved by controlling the e-beam spot position with a computer, as demonstrated by Koops et al [2280], or by employing multiple beams to achieve parallel operation.

Related opportunities for crude atomic or molecular positional assembly may exist with other emerging technologies, as, for example, the beam lithography/nanostencil technique under investigation by IBM Zurich in their “ATOMS” project [2294], neutral molecular beam deposition [2295], 3-D proton beam nanolithography [2296], the manipulation of Bose-Einstein condensates along an “atomic conveyor belt” above a silicon chip [2297], light-driven molecular assembly [2298], atomic holography [2299], and various proposals for quantum mechanical automata [2300, 2301]. Older techniques of automated electrodeposition microfabrication [1122, 1123] employ a fine-tipped probe in an electroplating solution to locally electrodeposit metals out of solution to form microstructures, such as regularly-spaced arrays of 100-nm tall nickel columns [1124] or >100-layer 3-D patterns using layers a few microns thick [1123].

Last updated on 1 August 2005