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In chip fabs, but also research labs, floor vibrations can completely ruin the performance of high-tech machinery. Mecal has designed a simple, flexible solution that suppresses those disturbing external vibrations. Its Equalizer systems have already been installed at several Asian customers.
It’s something developers often won’t put on top of their agenda when they’re listing the design requirements for a new machine. But if the performance of their super-accurate system can be affected by outside disturbances and vibrations, there’s simply no getting around – then they have to invest in proper vibration suppression.
To prevent external vibrations from affecting the processes in a machine, several options are available. The first may be a bit lame, but that doesn’t make it any less useful: remove the source of the vibration. Pumps in the building or a shaky machine nearby can easily throw a spanner in the works. “It’s often easier said than done to eliminate those sources,” says Bernhard Bakker, CTO of Mecal. “It’s impressive to see how much equipment and how many cables and pipes are installed in the service area of a chip factory. Good luck trying to pinpoint where in that maze the vibrations are coming from that are disturbing your inspection tool.”
Of course, there are tricks and techniques to map vibrations and discover where they originate. But obviously, all those machines and systems are installed for a reason. Often you simply can’t remove them because they’re an indispensable link in the production process. “If it’s possible to remove the disruptive vibration source, that’s always the best solution. However, it’s often impossible,” according to Bakker.
A second remedy for disturbing vibrations is vibration isolation. In short, this means making sure that the vibrations in the floor don’t reach the machine platform. “You do that by putting a weak suspension in between,” explains Bakker. “Such a solution works very well. Above a certain frequency, you can filter out all vibrations. The disadvantage is that a machine often wants to transfer its internal forces to the environment, but that’s not possible through a weak suspension.” Vibration isolation is therefore often not the right solution.
Mecal has devised a third method, comparable to noise-canceling headphones. “You can apply the principle of active noise control to floor vibrations,” Bakker points out. “With sensors, you measure the vibrations, you calculate the frequency and the amplitude of the correction that’s required, and you generate a counterforce.” That’s basically how Mecal’s Equalizer system works.
When a chip manufacturer wants to install a new system, he has to comply with the floor specifications given by the machine manufacturer. The requirements aren’t very high for a lot of equipment, so a solid concrete floor can be sufficient. But especially in the semiconductor industry, where every micrometer counts, normal building constructions are generally inadequate for the highly sensitive machinery they house.
The bar is set significantly higher in the construction of chip factories. “They’re built based on standardized specs for floor vibrations. These are usually at VC-C level, which means that vibrations in the frequency range between 1 and 100 Hz have a maximum speed of 13 micrometers per second,” Bakker indicates.
That may sound very accurate, but electron microscopes or inspection tools for EUV masks, for example, have even higher demands. “VC-D – 6 µm/s – or better is required. That step isn’t obvious," observes Bakker.
It was a supplier of mask inspection tools that set Mecal on the path leading to the Equalizer. “The system the company had developed couldn’t be installed at potential customers because the requirements for the floor vibrations were too strict,” Bakker says. “It had its own internal vibration isolation system. For fear of interference, the company didn’t allow users to place a second active isolation system underneath.” The customer was stuck.
Mecal engineers first tried to solve the problem with a so-called tuned mass damper. This technique is also used to counterbalance vibrations in skyscrapers. At the top of such a building hangs a heavy ball. When the skyscraper threatens to sway due to the wind, the massive ball moves in the opposite direction, making the net force zero.
“It works great for skyscrapers, but there are some drawbacks if you want to use the principle with machine vibrations,” clarifies Bakker. “Such a mass-spring system only works on a single frequency, which is of course tuned to the eigenfrequency of the skyscraper. In addition, you need quite a large mass, even if you’d like to compensate for the vibrations of a much lighter machine.”
The idea of a damping counterbalance did put Mecal on the right track. With its knowledge of dynamics and simulation, it devised a concept based on vibration sensors, a relatively small mass and an actuator driven by a controller. “Our calculations showed that it worked surprisingly well to create a counterforce for floor vibrations without unwanted interference with the process of the customer,” Bakker looks back. “The machine builder also examined the system and concluded that the interference with the internal systems would be extremely small. Their controller measures the vibrations of an isolated mass in the machine and reacts by applying forces to that mass. Those reaction forces are passed on to the floor, but that hardly affects the existing floor vibrations, so they don’t influence the controller of the Equalizer.”
The Equalizer sensor is connected to the floor so that it registers vibrations directly. “You don’t even need a super-sensitive sensor because vibrations in this frequency range are relatively easy to detect,” says Bakker.
While the Equalizer works equally well horizontally, it’s more often about resolving vertical vibrations. A factory floor basically consists of a concrete slab resting on pillars. Such a construction is much more sensitive to vibrations in the vertical direction. “Things are often very rigidly connected horizontally and much more mass has to move with it, so that the disturbances are much smaller,” Bakker explains. “The amplitude of the vertical vibration is often greatest in the middle between the pillars, but you can suppress that perfectly with one or more Equalizers.”
Mecal works with a balance mass in the order of ten kilograms. That suffices because the vibrations are small. After all, it’s only a few micrometers per second. “Of course, they can be annoying, but for a good motion control system, it’s actually not that difficult. Especially, because it only concerns a limited frequency band,” says Bakker. “You don’t need that much energy to absorb such vibrations. That’s to say, if you do it smartly.”
It’s precisely in that last remark where the added value of Mecal lies. To be able to set up everything properly, the company’s engineers first map out the situation in detail. “Generally speaking, a customer knows quite well what’s going on in his fab and he has already checked whether he can remove the vibration source. So he comes to us with a clear problem. Our engineers then go to the factory to determine its geometry, to see where the machine will be placed and to measure the vibration levels and dynamics of the building. It’s crucial to do this ourselves so that we avoid errors or ambiguities. We never dive in blindly.”
From such a thorough analysis, Mecal deduces which vibration suppression technique provides the best result. If Equalizers are the way to go, the proprietary software calculates how many boxes are needed and what the ideal locations are. “It often turns out that a few frequencies stand out,” Bakker has experienced. “Usually, you’ll see a peak around 50 Hz, because that’s the frequency at which most pumps operate. Often a few more frequencies pop up. We can pinpoint the controller in the Equalizer with filters on those peaks and suppress them by a factor of 5-6. This way, we can create a machine surface conforming to the VC-D vibration standard.”
This article was written in close collaboration with Mecal.