Cold Laser Beam Cuts Clean
Brent D. Johnson
As silicon wafers continue to become more densely populated, the technology for dicing the semiconductors has reached an impasse. The standard diamond saw cannot slice the material cleanly enough to support further reductions in die geometries without chipping the material and cannot accommodate the demand for round dies needed in high-voltage thyristors and diodes.
A cold laser beam enables free-shape cuts on a 250-µm-thick silicon wafer with a kerf width of 75 µm. Magnification is 75×.
Lasers, the only apparent alternative, are impractical for die cutting because thermal loading cracks and deforms the silicon. Likewise, the kinetic effects of water-jet processing are too intense for silicon wafers.
Synova SA of Lausanne, Switzerland, has developed a solution. It combined a water jet with a laser beam to produce what sounds like an oxymoron -- a cold laser beam. The laser beam is focused through a nozzle along with a fine stream of water. The reflection at the transition between the water and air produces a fluid optical waveguide. Once the beam is confined in the water jet, all of the problems associated with beam focusing, such as small focal depth, are eliminated.
For this cut, the laser beam was focused through a 50-µm nozzle onto a 100-µm-thick GaAs wafer. Magnification is 70×.
The system, called the Laser-MicroJet, can cut 25- to 5000-µm silicon wafers at speeds of 120 mm/s with a cutting width of 50 µm in any shape desired.
Natalia Dushkina, optics laboratory director for Synova's American counterpart, Gem City Engineering, said that the concept works because the water is transparent at the 1064-nm wavelength of the Nd:YAG laser. If the laser encounters a surface that absorbs at that wavelength, it will produce a plasma. "In principle, the device can be used for any material that absorbs at that wavelength," she said, including any kind of semiconductor material.
The key to the Laser-MicroJet's success is its cooling effect, which is the result of a pulsed laser in combination with a water jet. The plasma is generated only as long as the laser beam is activated and interacts with matter. When the beam is turned off, the water jet immediately cools the cut. The current system uses the Trumpf HL 102 P high-peak-power pulsed Nd:YAG. The duration of the laser pulse can be continuously adapted between 80 µs and 1 ms, and the pulse performance is adjustable up to 18 kW.
Four of these machines have been installed in the US and two more will be deployed in December. They have also been used in the semiconductor markets in Japan and Taiwan. The systems are drawing interest from the medical community for certain applications, such as cutting endoscopes.
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