The drive to increasingly higher density ultra-large-scale-integration (ULSI) (of electronic circuits) is fuelled primarily by cost; on-chip interconnects are far cheaper than the less dense offchip interconnects. At the same time the escalating cost of an IC factory (‘fab’) is making headlines as it goes through $1B and a large part of this escalation is the cost of high performance lithography tools. The lithographic technology to go below 0.1μm will almost certainly be very different from an extension of today's optical projection and the cost of replacing today's technology will be enormous. A second drawback to higher density is the resistance of narrow interconnects. As a result some people have suggested that this situation is analogous to that of airliner speed which increased over a period of thirty years from about 100 mph to close to 600 mph but has not increased in the last 35 years. Still faster speed was technically possible, and hence was pursued by the military, but is uneconomical for most commercial use. Current technology might take us to 0.1μm which will probably be state of the art 10 years hence so technologies for replacing optical lithography e.g. scanned arrays of proximal probes should be researched now. Other challenges include how to achieve useful interconnect networks employing 50 nm features.