Controls help harmonic spray do OK removing residues

July 10, 2002
Innovative wafer-cleaning equipment hit the market in a timely fashion thanks in part to controls maker Rockwell Automation.

Clean-room technicians utilize graphic user interface screens on Planar Semiconductor Inc.'s RPC200 wafer-cleaning machine. An Allen-Bradley RAC6181 industrial computer equipped with Rockwell Software RSView32 human-machine interface software lets operators track machine alarm status, wafer mapping, timing and sensor functions for the Rapid Pulse Clean Series.

Inside Planar Semiconductor Inc.'s RPC-200 access doors, a six-axis robot arm transports wafers from input cassette to the individual wash/dry modules and back to an output cassette. Wafers are oriented vertically in the cleaning process. Allen-Bradley 1398 Ultra 100 and 200 servodrives and motors accurately rotate 25 silicon wafers in the clean and spin-dry modules.

Innovative wafer-cleaning equipment hit the market in a timely fashion thanks in part to controls maker Rockwell Automation. Planar Semiconductor Inc., Kingston, N.Y., used Rockwell to design and build the control system for its new Rapid Pulse Clean Series of wafer-cleaning machines. The machines employ a noncontact harmonic spray process that uses only water instead of harmful chemicals.

PSI figures it saved on nonrecurring engineering costs and carved nine months off development time by going with an Allen-Bradley RAC6181 industrial computer carrying RSView32 human-machine interface software. The industrial computer lets operators track machine alarm status, do wafer mapping, and handle timing and sensor functions. The process-control system also lets users track all actions and when they took place on each wafer. RSView32 GEMTool software presents process data to the fab host via SECS-II communication.

According to PSI Executive Vice President Jerry Thietje, Rapid Pulse machines reduce wafer-cleaning time by 80% compared to conventional processes. Particle removal counts are better than 95% and process time is 20 sec or less, while competing devices clean in about 96 sec. Moreover, the payback period is four months rather than the 1.5 years that is more typical for conventional cleaning equipment.

Older processes use chemicals that include hydrofluoric, hydrochloric, and ammonium acetate acid, and sulfuric ammonium hydroxide as cleaning agents to rid wafers of dust and contaminants. Increasingly strict OSHA standards have made it costly to transport, neutralize, and remove these chemicals.

In contrast, the Rapid Pulse Cleaning system uses only water. It sets up a harmonic pulse that creates vibrations to loosen the bonds of unwanted particles on wafers. "Water cleans without the residue, cost and hassle of chemicals," says PSI's Thietje. The harmonic pulse process is also brushless, unlike conventional cleaning methods that use expensive rotating brushes. These brushes can scratch wafers or collect and redistribute contaminants.

The new PSI machine uses an Allen-Bradley ControlLogix automation control platform to handle sequential, process, and motion control, as well as I/O and communication functions. The control system includes two AllenBradley 1336 Plus II variable frequency ac drives to control pumps bringing water into the machine. Allen-Bradley 1398 Ultra 100 and 200 servodrives and motors rotate 25 silicon wafers in the clean and spin-dry modules. AllenBradley photoelectric sensors note wafer position and communicate this information via DeviceNet to the ControlLogix controller.

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