Post-CMP Cleaning Affected by Laser Process
Maria Lester, Associate Editor -- Semiconductor International, 10/1/1998
C ontamination control continues to challenge the industry as geometries shrink and structures become more complex. This is driving new technologies such as ozone-saturated baths, isoproponal spray baths, high dilution rate chemical baths and the use of lasers. In particular, laser cleaning is a technology that can be used in place of conventional wet clean processes. Motorola Advanced Process Characterization Laboratories (Tempe, Ariz.) studied removal of surface contamination from flat panel display material, photoresist from silicon wafers and slurry from planarized wafers using excimer lasers. Results indicate equal or slightly better reduction in contamination for flat panel material compared to non-standard wet chemical processes. Photoresist residues were unaffected by the laser process, but reduction of post-CMP contamination showed significantly better results.
Laser cleaning was performed using the Radiance Process. This is a patented dry process using a laser cleaning station for removal of surface contaminants. The process uses a deep UV photon flux from excimer or Nd:YAG lasers. An inert gas such as nitrogen is used to prevent redeposition on the surface. Cleaning is achieved when the laser light breaks contaminants bonded to the surface. Following the cleaning process, the gas can be filtered and reused or the particles removed and the nitrogen carrier gas exhausted. The process is performed at room temperature and atmospheric pressure. Because of this, the wafer experiences no melting or annealing and minimal microroughening.
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| Fig. 1. Laser cleans indicate better particle removal when compared to traditional wet cleans. |
Although not all contaminants were removed from all surfaces, the
Radiance Process takes a step closer to optimizing contamination
reduction. The technology holds considerable potential, but more
research is needed to integrate laser cleaning on a larger scale. 
Resist Removal Process
A residual-free, damage-free photoresist removal process has been developed at GaSonics International (San Jose, Calif.) for implanted resist. Because the process prevents residues, the need for a wet process step can be eliminated, which can reduce total process time and lower cost of ownership. The process is targeted to advanced IC fabrication.
The standard downstream oxygen/nitrogen process is used commonly for photoresist removal. Although it is effective at high temperatures, it is inefficient at low temperatures. The implanted resist has a skin layer with solvents underneath. High temperatures can cause the skin to "pop." Typically, the skin resist is a layer of carbonized crust formed during the implant process, leaving behind residues.
GaSonics' removal process consists of downstream directional plasma. The residue-free quality can omit the subsequent wet processing step. Research showed the skin breakthrough time exhibited a reduction from 150 to 30 sec as well as orders of magnitude cleaner.
A leading European IC manufacturer is beta testing this cleaning
process. Preliminary tests indicate that the process reduces implant
resist skin breakthrough time by up to 75% with no photoresist
"popping." "It can potentially allow customers to follow their residue
removal process with a DI rinse only, requiring no wet processing," said
Andy Kirkpatrick, GaSonics director of marketing. The technology may be
retrofitted on existing Performance Enhancement Platforms (PEPs).
