Lasers are key to future chip-processing detective work

Engineers at Purdue University have demonstrated concepts that could save computer chip manufacturers millions of dollars in downtime by using lasers to identify the sources of contamination and other defects.

Engineers at Purdue University have demonstrated concepts that could eventually save computer chip manufacturers millions of dollars in downtime every year by using lasers to quickly identify and pinpoint the sources of contaminating dust and other defects.

Semiconductor manufacturers already use lasers to detect dust particles on expensive silicon wafers, which contain hundreds of chips. But then the manufacturing operation must be shut down while workers try to determine what the dust particles are made of and where they came from, especially when large quantities are found. If an electron microscope is needed to trace the dust, the contaminated wafer is essentially destroyed in the process, adding to the loss.

Each 300 millimetre-wafer, roughly 12 inches in diameter, can yield 200 chips, which might eventually be worth nearly $1 million. If a new wafer rolls off the line every minute, one hour of downtime can cost tens of millions of dollars.

‘As the features in circuits are getting smaller every 18 months or so, the size of a killer defect is getting smaller and smaller. One way to rapidly detect and identify smaller defects is to use laser beams of shorter wavelengths, such as in the deep ultraviolet range,’ said E. Dan Hirleman, a professor and head of the Purdue School of Mechanical Engineering.

The same lasers used to detect dust might also be used to identify the particles and begin tracing the contamination source within seconds.

Because circuits in new computer chips are only slightly wider than the particles, the contaminants are large enough to ruin or short-circuit the tiny ‘wires’ in the chips. Although wafers are produced in clean environments that are isolated from the outside world, all dust cannot be eliminated.

Purdue researchers have developed mathematical models that reveal a dust particle’s fingerprint hidden in the precise way in which it ‘scatters’ laser light. The laser reflects off of the silicon wafer’s near-perfect mirror surface, but the light does not reflect well from dust particles. Some of the light is said to be scattered as it bounces off in different directions after hitting a dust particle. Different types of particles have specific scattering signatures, which can be modelled mathematically; creating fingerprints that can help identify certain types of particles.

Data from the models might be used by industry to design new types of instruments that reduce downtime. Future technologies might even use lasers to remove contamination, a sort of ‘laser dry-cleaning’ system that would shake off the particle, Hirleman said.

Such a dry-cleaning technique would be needed because wet-cleaning methods often deposit more particles than they remove, actually increasing the contamination. The models developed at Purdue also will be important for designing such instruments.

The research engineers precisely measure how laser light bounces off dust using a new instrument called a scatterometer. Then they use those data to validate complex mathematical modelling software that identifies a particle’s composition based on its light-scattering fingerprint.

Purdue researchers are also developing a new, more precision deep ultraviolet scatterometer that can detect smaller particles, which is essential to keep up with the constantly shrinking dimensions of computer chip circuitry.