Although silicon is the key ingredient for semiconductors used in smartphones and other electronic devices, only a purified form can be used to make semiconductor chips. Producing semiconductor-grade silicon involves upstream processing of polysilicon chunks to a purity of 11N, which translates to 99.999999999%. That’s a challenge that JST Manufacturing of Boise, Idaho, helps customers to meet by building advanced wet bench cleaning stations using linear motion and electric drive and control technology from Bosch Rexroth.
JST builds cleaning and processing equipment for cleanroom applications, from single wafer etching to automated wet process equipment. That includes wet bench cleaning stations of all sizes, but typically no more than 30 ft long. But recently for an overseas customer, we built a 138-ft-long line that employs multiple gantry robots (Figure 1). Handling the robot travel requirements for that distance took a combination of linear motion and control technologies with precision, robustness and cleanliness. The new system was developed solely as a custom solution for the overseas customer. The selling cycle took a long time. But, once it was sold, the project took about a year.
JST’s wet bench cleaning station is deployed at the beginning of the silicon manufacturing process. At that stage, long cylindrical polysilicon rods, known as ingots, are broken into 1-to-3 in chunks — no bigger than a fist. The chunks are placed into special baskets that enter tanks of chemical etch in one station 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 cooled, etched and eventually developed into computer chips.
For the front-end chunk-cleaning process, JST’s wet bench station was designed to process approximately 4 tons of silicon chunks in a 22-hr shift. To maintain that throughput, one basket holding 44 lb of silicon chunks has to be cleaned every 6.6 min.
That requires a unique wet bench cleaning station design. We engineered and built a station comprised of two units, the A1 and A2. In the 24-ft-long A2 unit, baskets of chunks are manually loaded through an auto-door. Then two-axis robots cycle the baskets through five acid etch baths and two rinse baths arranged in a single row down the length of the x-axis (Figure 2). As the baskets move down the x-axis, they are lowered into each chemical and rinse bath. (z-axis).
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After completing the acid etch and rinse steps in the A2 unit, the baskets are transferred automatically through an airlock into the 88-ft-long A1 section of the line, which uses three-axis gantry robots to cycle the baskets through two x-axis rows containing soak and dry baths. At the end of the process, the empty baskets are automatically unloaded using customer-supplied conveyor belts.
What’s unusual is that the A1 unit employs overhead gantry, or Cartesian robots. Overhead Cartesian robots are not often used in semiconductor process systems to avoid generating particles over the product. However, after extensive particle testing in cleanroom conditions, we found that the Rexroth profiled linear guiderails and linear motion modules used in the system generated far fewer particles than the specifications allowed.
Throughout the process, protecting the components against contamination and pitting is critical. The Rexroth seals keep the linear motion rolling strips clean. And they keep the chemical etch from pitting the linear modules. Bosch Rexroth incorporated other features to ensure the system met Class 10 cleanroom standards or better. The motion system components use thin dense chrome plating for corrosion resistance. They were also lubricated with cleanroom grease.