New Wafer Cleaning Method Eliminates Water/Chemical Requirements
By Susan english
Bethesda, MD–Industry Week magazine has chosen as a finalist for its “Technology of the Year” award, Radiance Services Co., marketer of the Radiance Process, an internationally patented new way of cleaning contamination from wafer surfaces, based on an application of quantum physics, using a pulsed laser and a flowing inert gas such as argon or nitrogen.
The new cleaning tool will eliminate the water and chemical usage and toxic wastes of conventional methods. Based on the use of a deep-ultraviolet (DUV) excimer laser, the production version of the tool will be capable of processing up to 60 wafers per hour at a cost of about $2 per wafer per clean, removing particles to 0.1 mm, according to the magazine. (The cost of the current cleaning standard is about $7 per wafer per clean.) The process exposes a chip to a 20 mm wide laser beam for 1/50 of a second, exciting particles on its surface and breaking their bond with the wafer`s surface. The particle is then levitated above the chip`s surface and entrained in a flow of inert gas across the surface. Radiance`s president and CEO, Donna R. Fitzpatrick, compares the process to placing a tennis ball on a trampoline, smacking the tramp`s surface with your hand, and watching the ball jump skyward. “The laser beam does the same thing. You shine a light on it and flow some gas across the surface,” she says.
The process was discovered by accident in 1987 by Dr. Audrey Engelsberg, while she was experimenting with surface modeling of laser-assisted chemical vapor deposition for work on her doctoral dissertation at New Jersey`s Rensselaer Polytechnic Institute. Engelsberg, now vice president and chief technical officer of Radiance, explains that she was attempting to deposit aluminum lines, when she found the laser was removing the aluminum as well as other surface particles, while leaving the aluminum-silicate structure that was bonded to the silicon wafer intact. A colleague, Dr. Joseph Dehais–now chairman of Radiance–realized she had stumbled on a dry-cleaning method with commercial potential. Since then, the process has been characterized and developed for commercial applications using a krypton fluoride (KrF) excimer laser operating at 248 nm wavelength. At 248 nm, the energy per photon is 5.01 eV, which is sufficient energy to break many covalent bonds–the atomic bonds that attach surface particles to a wafer. Research on semiconductors, quartz, metals, and polymers with a KrF excimer laser has demonstrated that the process can remove particles and flakes from 0.1 to 80 mm in size. Fingerprints and other organic films, hazes, as well as metallic ions can be removed from wafers, photomasks, and microstructures without micro-roughening of surfaces. Also, analysis of the reflectivity thin film characteristics of bright and dark chrome on quartz photomasks and ITO (indium tin oxide) on quartz and thin glass have indicated no thin film degradation, making the process an important tool for surface adhesion preparation as well as planarization.
The Radiance Process removes contaminants in a single step rather than requiring a chemistry-specific step for each contaminant as in conventional wet cleaning methods, and with minimal microroughening of silicon or gallium arsenide surfaces, and because it is a dry clean method, Radiance chairman Dehais says the process has the potential to save chipmakers 10 percent on the cost of building a new fab and savings of up to 20 percent on fab operating costs through reductions in the millions of gallons of ultrapure water used annually to clean wafers. (His calculations are based on figures supplied by Sematech on the current average cost of cleaning a wafer, which it estimates is $7.35 for each 150 mm wafer.) n