Products In Action

Linear Actuators Provide Automation Control in Wet-Etch Process Station at Litton Airtron

The routine of hand-dipping gallium arsenide and silicon wafers in plastic tanks and timing the process with a stopwatch may seem antithetical to the high-tech image of wafermaking and semiconductor manufacturing. Yet some wafer and chip manufacturers continue to use such manual handling techniques in several of their process steps. Amtec Automations (North Billerica, MA) has come up with a fully automated alternative to the old approach--an economical wet-etch process station that uses Rapidtrak linear actuators, SLO-SYN servomotors (with encoders), and an MX2000 controller made by Warner Electric (South Beloit, IL). The three-axis motion control system decreases processing time, increases safety by eliminating exposure to chemicals, and optimizes yield through accurate process control.

"We first used the Rapidtrak when a customer, Litton Airtron (Morris Plains, NJ), required a 15-ft-long station with one acid tank, three rinse tanks, and a hot nitrogen drying station," says Audrey Marsh, vice president and manager of operations for Amtec. "Normally, our stations are 3 to 6 ft long and have just two tanks per robot. For Litton Airtron, we needed a linear actuator that could move long distances and was quick, accurate, reliable, and affordable."

In Litton's wet-etch process, wafer cassettes are moved from bath to bath with a linear actuator mounted to a belt-driven one.

To meet these new needs, Amtec used two linear actuators and a ball-bearing screw actuator. The modular actuator features a self-contained rodless linear actuator with a self-supporting aluminum profile and a compact frame, a design achieved by integrating the saddle and profile. Amtec turned to Warner because it was familiar with the company's products and knew that Danny Grasso, Warner's territory manager, could provide engineering support. He and his company's engineering department calculated all the formulas for loads, transfers, and distances before making a recommendation.

"For instance, the load is about 10 in. off the center of the Rapidtrak, which can cause problems if it isn't handled correctly," says Grasso. "If it were to bend under the load, it would wear out faster, so we added a nondriven rail to provide extra support. When we're talking about thousands of dollars in product, sizing has to be done carefully."

Warner's products also helped Amtec keep the high value that its customers expect. "Besides service, customers are attracted to us because of our low cost," says Marsh. "We keep the cost down by making an effective product without all the added costs. Warner Electric products help us build systems that are right for each customer. Other manufacturers don't customize to their customers' needs and they overbuild, so customers get Cadillacs when all they need is a Chevy — a good solid product that will do the job for less cost."

The wet-etching process at Litton begins when a gripper mechanism, moved by a double-threaded ball-bearing screw, grabs a pair of 6-in. cassettes, which contain 25 to 50 wafers. A 30-in.-long ball-screw-drive linear actuator then lifts up the 25-lb gripper mechanism by 12 in. This entire assembly, mounted to a 141/2-ft-long belt-driven actuator, then moves forward 12 in. to the first processing tank, where the ball-screw actuator lowers the cassettes 24 in. into a sulfuric acid or other chemical bath. After the appropriate time has passed, the arm lifts the cassettes out of the tank and moves them to the next station, typically a rinsewater station. The sequences are repeated for about 20 minutes until the process is complete. Once the start button is pressed, the machine requires no further operator involvement. Amtec chose the belt-driven linear actuator for horizontal motion because of its high-speed operation over long stroke lengths, which is important for the wet-etch process. "A delay in rinsing can destroy a semiconductor, or, instead of having 100 integrated circuits etched into it, it might only have 50," explains Marsh. "Also, excess time in the drying station can damage the wafer. Either way, building a wafer that can pass quality control testing depends on proper speed and timing."

While the linear actuator can move the gripper mechanism load up to 12 in./sec, Amtec programmed the actuator with a speed of 8 to 10 in./sec using the MX2000 controller. The controller also provides a "soft start," accelerating to the set speed in 0.1 seconds. Both programming functions avoid jerking the gripper mechanism.

Customers value the flexibility of a programmable motion control system. Through a touch screen operator interface connected to a master programmable logic controller, customers can choose one of 10 different motion sequences. This enables them to change the chipmaking processes without the delay or extra expense of a hard-wired system. The system facilitates such flexibility because it can provide precise positioning without proximity sensors. "The MX2000 uses simple programming terms, like 'move to point A,' making it easy to work with," adds Marsh.

Ease of programmability is important for the semiconductor processing industry because customers expect quick turnaround times. "Five or 10 years ago, a customer would have given an engineer 2 or 3 weeks to get a system programmed and set up," notes Warner's Grasso. "Today, the machines are expected to start running product almost immediately after they arrive at the plant."

By upgrading from a manual process to an automated one, Litton has saved time, improved accuracy, and increased safety. The wet-etch process station can process twice as many wafers per hour as the manual process could. With the improved handling system, there is less product lost because of human error and less employee exposure to potentially dangerous chemicals. Further, the automated unit frees up employees who once had to stay at the station throughout the process.

"The automated system will help us to cope with future increases in production," says Thomas Litton, manager of technical customer support at Litton Airtron. "It is easier for the operator, less manpower intensive, and the results are more reproducible."

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