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Pneumatics keep clean rooms clean

Pneumatics keep clean rooms clean

Elmhurst, IL--In the ultra-clean world of microelectronics manufacturing, a speck of dust is like a boulder. The tiniest particle can short circuit a chip. That's why makers of integrated circuits go to extremes to maintain clean manufacturing facilities.

It's also why engineers must be careful about the linear actuators they choose for clean-room equipment. Devices such as air cylinders, though they appear benign to the naked eye, can be sources of contamination. They blow dust, spray bits of lubricant, and discharge tiny chunks of abraded material.

Now, thanks to a new design, these cylinders are serving in the fabrication of computer chips, and in the construction of disk drives. They've even been cleared for use in Class 10 clean rooms--locales containing only ten particles larger than half a micron in each cubic foot of air. That's an extraordinary step, especially when considering that the average non-smoking office has between 200,000 and 300,000 such particles in every cubic foot of air.

A few years ago, the use of pneumatics in a Class 10 clean room would have been unthinkable. That's why automated semiconductor manufacturing machinery typically used electric motors. In motors, all of the moving parts were enclosed, the particles were contained, and air turbulence minimized.

But the new cylinder, designed by engineers at Hoerbiger-Origa in Stuttgart, Germany, offers those same capabilities. Engineers have also endowed subsequent versions of it with position feedback, resulting in positioning accuracy of 0.004 inch. As a result, it's now an even more viable alternative to motors in semiconductor manufacturing, where positioning accuracy can be critical. Combined with pneumatics' traditional advantages--high force, high speed, and low cost--the new breed of air cylinders could open untapped markets.

Vacuum solution. Hoerbiger-Origa engineers first addressed cylinder emission problems in May 1991. At that time, a German electronics manufacturer asked the company to custom design a cleaner pneumatic cylinder. "Actuator contamination was not acceptable for them anymore," recalls Joachim Kipke, president of Hoerbiger-Origa Germany. "Their products were changing and the new product line was more sensitive to contamination."

After considering the problem, the company's engineering team decided to use a rodless cylinder design. Rodless cylinders typically contain a slot in one side of the aluminum barrel, through which the piston yoke travels. The slot is sealed by inner and outer stainless-steel bands.

In the past, the slot had been a source of particle emissions. Dust, dirt, and lubricants were stirred up by movement of the piston. Friction between the tube and piston seal also produced abraded material that could pass through the slot into the surrounding atmosphere.

Hoerbiger-Origa engineers understood that for microelectronic manufacturers, such particle emissions could be catastrophic. A particle measuring only a millionth of a meter in diameter is big enough to bridge two electrical lines on a typical chip, causing a short circuit. A room filled with such particles can ruin yields, resulting in losses of tens of thousands of dollars.

The engineering team solved those problems by designing a system that draws a partial vacuum between the inner and outer sealing bands of the cylinder. They added special vacuum ports at each end of the barrel for attachment to conventional vacuum lines. The vacuum lines create a suction flow out of the cylinder. Using a flow rate of about 1.8 cubic feet per minute, they found that contaminants could be extracted and filtered outside the clean room environment.

"Contaminants from the bearing surfaces and other areas, which could have migrated out of the cylinder, can now be safely exhausted," notes Ray Barnes, marketing manager for Hoerbiger-Origa in Elmhurst, IL. "And most clean rooms have a vacuum system readily available."

Ensuring that the new concept worked, however, was a more difficult matter. Origa and its customer cooperated with engineers at the Institute for Production Technology and Automation in Stuttgart to determine the cylinder's level of particle emissions. They tested cylinders with an average bore size of 25 mm and strokes of 1,000 mm using optical airborne particle counters and surface scanners. After nearly nine months of testing, they found that the design passed Class 1 specifications.

Electric motor alternative. The company's engineers quickly learned, however, that their initial customer wasn't the only one who wanted to use pneumatics in the clean room. They were later contacted by a maker of semiconductor manufacturing machinery, then by a disk drive maker in the Far East.

In subsequent applications, Hoerbiger-Origa has learned that the technology offers a multitude of advantages in the clean room. FlexLink Systems Co., a unit of SKF USA, for example, used the commercial version of the cylinder, known as the Origa C-Series, in a conveyor system for a Class 10 clean room. FlexLink's customer employs the conveyor to lift computer hard disks a vertical distance of four feet and to rotate the vertical motion system in 90-degree increments.

Engineers say the cylinder meets the application's needs in three critical ways: It's clean; it's rigid; and it offers precise positioning capabilities.

The design meets the room's cleanliness needs not only by reducing particle emissions from the cylinder's slot, but by slowing movement of the loads. Since it can move at speeds of one inch per second, it doesn't stir up existing particles because it doesn't disturb the flow of air.

Hoerbiger-Origa worked with Flex-Link on the slower speeds as a way of reducing air turbulence. Up to now, says Barnes, engineers typically have designed pneumatic cylinders for speeds of four inches per second or faster. "In the past, the industry has avoided slower speeds, because they can lead to a stick-slip effect," says Barnes. He notes that they avoided the stick-slip phenomena by designing for slower speeds from the outset.

Engineers also feared that the long stroke, substantial loads, and rotational motion would necessitate use of structural support. But using an extrusion with two 32 mm bores, they doubled the load-carrying capacity, as well the stiffness.

FlexLink engineers say, however, that the cylinder's position feedback capabilities proved as important as its cleanliness. During operation, the cylinder stops at positions along the length of the stroke with a precision of 0.040 inches. It accomplishes that by employing a reflective strip on its side, and using it in conjunction with an LED sensor.

Together, the LED sensor and reflective strip form a linear encoder. As the LED passes over black and white marks on the strip, it sends an output signal to a programmable logic controller, which computes the cylinder's location. By operating in this way, it provides a level of accuracy that's often needed in microelectronic manufacturing.

The ability to provide such precise mid-stroke positioning is part of an important new trend in pneumatics. For years, pneumatics offered on-off, A-to-B-type positioning. By offering these new levels of accuracy, however, pneumatics become a viable alternative to stepper motors.

Timeline for design MAY 1991
German electronics company asks Hoerbiger-Origa to design a clean pneumatic cylinder
SEPTEMBER 1991
Cleanliness testing begins
JUNE 1992
Cleanliness testing completed. Hoerbiger-Origa introduces cylinder to European market
1995
Hoerbiger-Origa introduces clean room cylinder to U.S. market

FlexLink engineers say that, in this application, a stepper-motor system would have cost about three times as much as the pneumatic version that they selected. "The precision of this system is not the same as the stepper, but not every application requires accuracy levels of one ten-thousandths of an inch," notes Albert Bruder, a sales application engineer for FlexLink. By offering the best combination of cleanliness, cost, and precision, Bruder says, the pneumatic solution best fulfilled the needs of the application.

Untapped markets. Hoerbiger-Origa's new technology may also have uses outside the clean room, say the company's engineers. The vacuum area between the inner and outer bands can be pressurized (instead of depressurized), they say, opening up a new class of applications. The reason: By pressurizing, the cylinder can prevent ingression of particles, as well as egression. In machine-tool shops, near grinders or mills, the outward air flow would push metal scraps and dust away from the slot.

The company also foresees continued growth of the concept in biotechnology, medical, and pharmaceutical processing, where sterility is critical, and in defense and aerospace, where it could be used for fabrication of electronic devices. It also could serve in the manufacture of compact disks.

The key, say users, is that it provides one more tool for engineers, particularly in clean-room applications. "In the past, we've tried to reduce the number of pneumatic cylinders in the clean room," says Bruder. "But these new levels of cleaniness and positioning accuracy open up a whole new design option."

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