Research that uses X-ray technology to study minute flaws inmicrochips has captured an award at Dublin City University. DickAhlstrom reports
Microchips go thorough a remarkably tough time as they are made. They are heated and reheated, blasted with ions and put under tremendous stresses, equivalent to seven tonnes per square inch. It is a marvel they survive at all.
"We are basically pounding the hell out of the silicon," says Dr Patrick McNally, one of Europe's leading researchers in the area of silicon-chip investigation. "The very process of fabricating these integrated circuits damages the silicon on which they are produced."
Dr McNally is director of the Research Institute for Networks and Communications Engineering (RINCE) at Dublin City University. He also heads the institute's microelectronics group. "We are one of the main national centres for research in the information and communications sector."
Last week Dr McNally won the DCU 2001 President's Research Award in science and engineering for his contribution to microchip analysis. A matching humanities and social sciences research award was presented to Dr Michael Cronin.
RINCE involves the work of 19 faculty, five postdoctoral researchers and more than 50 postgraduates in all areas related to generating, transmitting, analysing and presenting electronic content. "It would be one of the largest research centres in the country dedicated to this topic."
The collections of tiny integrated circuits packed into today's microchips are like "miniature cities built of silicon", Dr McNally says. The electrical properties of pure silicon are well known, but as layer upon layer of silicon is added during manufacture, cracks called "dislocations", small holes and pits can arise.
"These have an impact on the electrical properties of the silicon," he says. "It is very important to understand the properties of this to understand what part of the process caused them."
Dr McNally specialises in the use of X-rays to study the imperfections that arise during microchip manufacture. These are no ordinary X-rays, however. Conventional hospital X-ray units could potentially be used to visualise the fine detail inside chips, but they would be anything but practical. They operate at such low power that a single X-ray image would take all day.
Instead, Dr McNally uses synchrotron X-ray radiation, which involves X-ray beams that are a million million times as intense. Fifty images can be taken in just 10 seconds.
"We are using the theory of relativity to look at incredibly microscopic problems," he says, referring to the source of the X-rays. He uses either the European Synchrotron Radiation Facility, in Grenoble, or the HASYLAB-DESY synchrotron, in Hamburg, to take his microchip snaps.
These devices accelerate electrons to relativistic speeds of 99.9999993 per cent of the speed of light. The electrons are forced to race around large rings - Grenoble's is a kilometre long - but lose energy around the curves, giving off synchrotron radiation as a result. This can be had at a variety of wavelengths, from infrared to gamma-ray, but Dr McNally uses it at X-ray wavelengths, which can provide exquisitely detailed images of the inside of microchips.
Specialised film with grain sizes of just 40 billionths of a metre ensure the RINCE team can see the tiny details it is looking for. The method delivers a full three-dimensional image of the inside of the chip, flaws and all, and does so without harming the chip. "The beauty of it is it does so non-destructively," he says.
The goal is to be able to identify what part of the manufacturing process causes the flaws and unwanted stresses. "We can interpret the image to find where the worst strain problems are and how big the strain is."
The "lead bumps" added to allow a Pentium chip to be connected to your computer are produced by applying solder at 350 degrees. This can introduce stresses in the silicon equivalent to a matter of tonnes per square inch, given the very small size of the junctions and circuits involved inside the chip.
Reducing the strains makes the chip more predictable and reliable. "Information is power: in this case, knowing where the strain is and knowing how it is caused." Intel therefore helps fund the research, with support from Enterprise Ireland.
The synchrotron beam is focused to produce a millimetre-square beam; an X-ray image the same size is then taken. Back at DCU, it can be enlarged 100 times, allowing the flaws and stress lines to emerge.