Motion Control Builds Semiconductors, Part I of II
| By: Kristin Lewotsky, Contributing Editor
When you think of motion control in semiconductor fabrication, you probably envision robotic arms placing wafers on vacuum chucks or servo motors shifting the exposure area in step-and-scan lithography machines. The reality goes far beyond actual process tools, however, to encompass everything from moving pods of wafers around the fab to assembling circuit boards with pick-and-place operations.
In semiconductor fabrication as nowhere else, time is money and yield is king. “The direction everything is headed is to gain more precision, to get there faster and to accelerate harder,” says Ty Safreno, CEO/CTO of Trust Automation Inc. (San Luis Obispo, California). New trends in motion control are providing the tools to do just that.
Given today’s ever-shrinking feature sizes, contamination control is paramount in ensuring yield. Although the fab floor is typically maintained at U.S. federal standard Class 100 to Class 1000, within the process tools the standard is considerably tighter. To protect 300 mm silicon wafers from contamination, they are whisked from process tool to process tool in front-opening unified pods (FOUPs). With a capacity of 13 to 25 wafers, a loaded FOUP weighs in excess of 19.8 lbs (9 kg) and can hold as much as $500,000 of product. At the equipment front end module (EFEM) load ports to the process tools, the wafers pass from the FOUP into the tool interior; after the process is complete, they are reloaded into the FOUP and taken on to the next step.
The FOUPs typically move around on an automated overhead transport system involving as many as a thousand cars, each of which can travel as fast as 10 mph over a varying route within the fab before stopping at the load port of a given process tool. According to Anthony Bonora, vice president and CTO of Asyst Technologies Inc. (Fremont, CA), the cars contain multiple axes of servo drive systems. Belts or cables lower the container to the port in a matter of seconds, to a positional accuracy of a few millimeters over some 2 to 3 m of drop.”
Here’s where one challenge emerges -- stopping a nearly 20 lb FOUP without ringing or overshoot. Highly-refined drive hardware and control algorithms provide the solution. The use of belts for vertical positioning would seem to fly in the face of conventional wisdom. After all, if a belt breaks, you run the risk of having a $500,000 load in freefall. On the other hand, belts are the only means of achieving sufficiently fast motion. Asyst uses belts which contain embedded conductors that monitor belt integrity and enable power and communication to the mechanical gripping element.
EFEMs on the Move
Motion control and contamination control both come into play at the EFEM load ports. The combination of docked FOUP and load port uses a system of internal doors and locks to prevent particles from the fab floor from entering the tool. When the FOUP docks against the load port, back pressure on the process-tool side whisks away particles. The load ports themselves thus typically contain three or four axes of servo drive mechanisms with encoders to unlatch, retract and lower the doors, and correctly position the FOUP.
EFEMs not only include wafer-handling robots, they also integrate machine vision capabilities to allow the system to identify and align the wafers. The EFEM is motion control intensive, Bonora says. An Asyst EFEM product fitted with four load ports, for example, has a total of 22 servo axes: four for each load port, four more for the wafer engine that extracts and moves the wafers, and two more for prealignment.
How do they handle all those axes? “We use a centralized controller but we have the servo driver boards distributed,” says Bonora, describing the SynqNet-based system. “With one central processing unit, we can coordinate the activities of all of the load ports, if required, and all of the activities of the wafer engine. It’s distributed drivers but centralized high level control.”
Distributed control allows them to minimize cabling, and thus particulation, always important in a cleanroom environment. “Counting all the limit switches and other types of sensors you might have - that can be dozens of wires,” Bonora says. “Those are the things we eliminate with this control architecture. It also allows us to utilize very powerful teaching and diagnostic tools.”
That’s a commonly cited benefit of networking and smart components, but Bonora emphasizes that their control architecture also imparts a necessary flexibility. “A maintenance person can access all the different kinds of servo motors being used with a common platform for diagnostics and teaching, and yet it allows us to use a diverse set of motors, whether they’re brushless motors, linear motors or brush type as required at the point of use.”
Motion for Metrology
It’s not just synchronization to load ports that requires fast settling times. The nanometer-scale metrology used in today’s processes requires even better accuracy. At Trust Automation, Safreno and his group achieve that performance using class AB linear servo drives or amplifiers instead of more conventional digital servo drives with pulse-width modulation (PWM).
Digital servo drives with PWM are driven by square waves -- to go 10 m in 10 minutes, for example, the system travels a meter in one jump, waits until the system clock counts off the next minute and goes another meter, and so on. With the use of filtering techniques and kilohertz or higher modulation rates, the fundamentally step-wise motion can be smoothed to resemble analog output, but only to a point.
“In metrology applications, they can’t tolerate vibration,” Safreno says. “On a PWM drive, you may not see your encoder counts ticking by because you've settled to less than a count -- you're at zero or bouncing between zero and one very subtly, you’re not vibrating enough to take you over to the next count.”
PWM drives also suffer from noise generated in the encoder signal interpolation process. “You have a vibration because of the mechanical, electro-mechanical things we all think of,” he notes, “but you also have vibration because the sensitive electronics in the encoder think that things are moving when they're not.”
Piezoelectric actuators offer one alternative for fine positioning. They have a number of advantages -- they don’t generate heat, for example, and they don’t generate magnetic fields. As with all things in real-world engineering, however, piezoelectric actuators involve tradeoffs.
“Piezos are not the world's answer to fine motion control,” Safreno says. “They have some cool applications but they typically don’t have a whole lot of holding force. They don't really have a huge lifetime in the grand scheme of things. There is particulate generation because the piezo motors perform a rubbing drag type action of motion, and if you need high forces or high accelerations, you won’t get it.”
Enter class AB linear servo drives. Unlike pulse-width modulated digital servo drives, AB linear servo drives output variable voltage to generate motion at a constant speed rather than motion in discrete steps. Such smooth motion can provide the levels of accuracy and the settling times that nanoscale metrology applications require.
Once again, though, there are tradeoffs, Safreno notes. “The reason you get a steady DC voltage out is because you figure out how much voltage [you need] and that equates to a current into the motor. The remaining power, you throw away as heat. It makes it a very accurate, precise drive, but it also makes it an inefficient drive.” In certain applications, though, the need for performance outweighs efficiency concerns.
Part II of this article will discuss the use of motion control in pick-and-place electronics assembly. Don’t miss it!