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  • Coming Clean with a Shock

Photonics Spectra
Sep 2007
Laser-induced plasma technique has been refined to remove smaller particles from semiconductor chips.

Hank Hogan

Semiconductors must be extremely clean during processing. Cleaning issues account for a majority of chip yield problems in factories, according to figures presented at an April conference in Austin, Texas, hosted by Sematech Inc., a semiconductor research consortium based in Austin. Traditionally, surface contaminants have been removed by wet chemistries, but these methods may have an increasingly difficult time overcoming the attraction between a surface and small particles.

Removing surface contaminants is becoming more challenging as chip feature sizes shrink below 65 nm, but researchers at Clarkson University in Potsdam, N.Y., have shown that they can safely blow particles away using shock waves.

By firing a laser at a focal point in either air or water, investigators can produce plasmas that, in turn, create an expanding shock wave that removes particles from a wafer. The greater the pressure, the smaller the particle that can be removed. Getting the pressure high enough to remove particles smaller than 100 nm typically brings the plasma too close to the surface, thereby risking heat damage.

“We can remove polystyrene latex particles, on silicon, 45 nm and smaller with the pressure levels measured,” said Cetin Cetinkaya, an associate professor of mechanical and aeronautical engineering.

The Clarkson researchers created a shock wave using laser-induced plasma but contained it within a shock tube and directed the wave at a surface. This technique enabled them to increase the pressure amplitude almost an order of magnitude without damaging the substrate. The use of a shock tube also allows the laser-induced plasma to be farther away from the surface, minimizing heating.

In their work, they fired a pulsed 1064-nm Nd:YAG laser from Quantel SA of Les Ulis, France, through an opening in a brass tube with an inside diameter of 3.5 mm that was closed at one end and open at the other. After submerging the tube in water, they focused the laser beam onto a point inside the tube, creating a plasma. They measured the resulting pressure using a transducer, evaluating the effect of various shock tube designs and stand-off gaps. They found that one tube produced a peak transient pressure of 6.48 MPa and a pressure amplification factor of 8.95.

Cetinkaya noted that the technique has promise, and research in this area continues. The ultimate objective is the development of a compact laser-induced plasma system for selective removal of nanoparticles.

“At the moment, we do not have any commercialization plans, but many parties have expressed interest,” he said.

Applied Physics Letters, July 30, 2007, Vol. 91, 051912.

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