JST station processes silicon to purified form

JST station processes silicon to purified form
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November 2015

 

A 42-meters-long wet bench silicon cleaning station built by JST Manufacturing makes silicon ready for semiconductor manufacturing – and makes it as clean as possible.

Silicon is the key ingredient for the semiconductors used in smartphones and other electronic devices, but only a purified form can be utilized to make semiconductor chips. The required 99.99 percent purity of the polysilicon chunks is achieved by the wet bench cleaning stations produced by JST Manufacturing. These stations are typically no more than nine meterst long. “But recently for an overseas customer, we built a 42-meter-long line that employs multiple gantry robots,” says Tony Bertagnolli, chief operations officer at JST in Boise, Idaho, USA. Handling the robot travel requirements for that distance took a combination of precise and robust linear motion and control technologies.

Four tons of silicon in 22 hours

Wet bench cleaning stations are deployed at the beginning of the silicon manufacturing process. At that stage, long cylindrical polysilicon rods, known as ingots, are broken into chunks – no bigger than a fist – and placed into special baskets. They then pass through a succession of treatments. First, the baskets enter tanks of chemical etch to remove silicon oxides and other contaminants. The chunks are then soaked, rinsed and dried. In a separate process, the chunks are melted in a furnace and transformed into pure monocrystalline silicon cylinders that are eventually developed into computer chips. JST’s extra-long wet bench station was designed to process four tons of silicon chunks in a 22-hour shift. To maintain that throughput, one basket holding 20 kilograms of silicon chunks has to be cleaned every 6.6 minutes.

“That requires a unique wet bench cleaning station design,” says Bertagnolli. “We engineered and built a station comprised of two units – the A1 and A2.” In the seven-meter-long A2 unit, baskets of chunks are manually loaded before two-axis robots cycle the baskets through five acid etch baths and two rinse baths. As the baskets move down the X-axis, they are lowered into each chemical and rinse bath. Then the baskets are transferred through an airlock into the 26-meters-long A1 section of the line, which uses three-axis gantry robots to cycle the baskets through additional soak and dry baths. At the end of the process, they are automatically unloaded onto customer-supplied conveyor belts.

Rexroth Cartesian Robot positions a basket of polysilicon chunks into a wet processing bath. Magnifier

Rexroth Cartesian Robot positions a basket of polysilicon chunks into a wet processing bath.

 
The frames, made of aluminum sections, and the rail guides had to match exactly in order to make possible uniform motion of the linear units. Magnifier

The frames, made of aluminum sections, and the rail guides had to match exactly in order to make possible uniform motion of the linear units.

Clean components

“What’s unusual is that the A1 unit employs overhead gantry or Cartesian robots, which are not often used in semiconductor process systems in order to avoid generating particles overtop the product. However, after extensive particle testing in clean room conditions, we found that the Rexroth profiled linear guide rails and linear motion modules used in the system generated far fewer particles than the specifications allowed,” says Bertagnolli. “It’s a real credit that the Rexroth Cartesian motion components can handle that level of cleanliness.” An additional challenge was posed by the 26-meter length of the A1 unit along which the gantry robots travel. To achieve the required accuracy over the entire distance, the engineers used six three-meter sections and three 2.7-meter sections of Rexroth profiled linear guide rails mounted on high-strength aluminum structural framing. Due to the length, the framing and profiled rails needed to fit precisely at the joints to ensure smooth motion of the robots.

Reliable drives

JST also uses Rexroth technology to move the baskets laterally on the Y axis. Each gantry robot uses a CKR compact belt-driven module powered by an IndraDyn MSK servo motor. To lower the baskets with the polysilicon chunks into the baths, the gantry robots utilize compact Rexroth CKK ball screw modules. The ball screws have to reliably withstand both the required torque and the axial loads that arise. “The compact Rexroth CKK linear actuators proved to be more reliable and robust than ball screws from other suppliers. They lift the baskets from the baths smooth and quickly,” says Bertagnolli. The build of the A2 unit is similar to that of the A1 unit, but no lateral Y-axis motion is required because there is only a single row of baths. “The Rexroth Cartesian motion system demonstrates that we go to great lengths to meet our customers’ exact specifications,” comments Bertagnolli.

 
Wet bench cleaning station made by JST Manufacturing treats silicon chunks so that they attain 99.99 percent purity. Magnifier

Wet bench cleaning station made by JST Manufacturing treats silicon chunks so that they attain 99.99 percent purity.