Say one thing for next-generation semiconductor manufacturing: Savvy methods of motion control are becoming a key factor in designing equipment able to produce defect-free integrated circuits. Considerations that are common in motion systems — gear train backlash, bearing performance, and vibration control, for instance — can be orders of magnitude more difficult when dealing with the perfection levels demanded of this modern equipment.
Motion-control strategies must reflect these realities. In particular, examples of such approaches can be drawn from newly designed manufacturing equipment that produces ingots of single- crystal silicon. The new standard diameter for next-generation semiconductor wafers, 300 mm, presents special technical challenges for designers of this equipment.
Crystal-growing systems convert raw polysilicon material into monocrystalline ingots from which wafers are sliced. A variety of factors make it impossible to simply scale up systems growing 200-mm-diameter ingots (the previous standard) to handle 300 mm. There is a dramatic increase in ingot weight, for example: 300 Kg for a 300-mm ingot compared to 150 Kg for 200-mm versions. The heavier ingots need a massive support structure able to minimize vibrations which can harm growing crystals.
Moreover, commercial realities dictate that such systems have several structurally stable mounting options. The reason: the first few systems will double as R&D test platforms for tweaking recipes before being cranked up in a production setting. Configurations for R&D purposes tend to be much different than for production.
Smooth, high-accuracy servodrives are critical components in much of today’s state-of-the-art semiconductor manufacturing equipment. In the case of the 300-mm crystal puller, there are four main servos. One slowly rotates a crucible of molten silicon while a second moves the crucible up and down incrementally as silicon is used up. Metal bellows seal the linear motion, and a 140-mm Ferrofluidic rotary seal maintains a vacuum at the base of the crucible lift assembly.
A third servodrive is part of what is called the pull head, which draws a seed crystal upward, out of a molten crucible of silicon, via a lifting cable (made of tungsten strands to withstand high temperatures). The cable winds around a spool in the pull head. Finally, a fourth servo slowly rotates the whole cable and spool assembly.
The system operates this way: As the crystal rises upward and out of the crucible, a freeze zone forms at the liquid/solid interface, thus creating the single-crystal material. The crystal rotates slowly as it rises. As the ingot forms, meanwhile, the crucible rotates in the opposite direction. Precise rotational speeds keep temperature uniform, and the upward draw rate sets the diameter of the growing crystal.
As more of the molten silicon solidifies to form the ingot, the crucible rises to maintain the level of the ingot/liquid interface. This ensures there is enough material to continue forming the crystal.
The process ends when either the ingot grows to the desired length, or the molten silicon is depleted. The pull head draws the ingot up out of the growing chamber and into a receiving chamber. An 18-in. ID isolation valve then closes to seal the grow chamber from the atmosphere.
A special cam assembly moves the valve plate horizontally and vertically in one smooth motion. This patent-pending mechanism has fewer moving parts than older schemes where the valve plate first swings over and then comes down to form a seal. Moreover, there are no internal bearings or sliding metal parts which could produce contaminating particles that might drop into the molten silicon.
LIFT AND ROTATE
Modern crystal-growing systems need extraordinary measures to guard against minute vibrations or transients that could otherwise cause ingot defects. One typical source of such troubles in servosystems is gear train backlash. For this reason, the four main servodrives on the crystal puller each operate through harmonic gearboxes, which have the qualities of zero backlash and no slip. These drivetrain components also provide a high degree of mechanical advantage which is important for precision motion.
For example, the servo driving the cable drum has a speed range of about 0 to 5,000 rpm. Gearing brings this down to a point where the drum raises or lowers the cable anywhere from a fraction of a millimeter per minute to about 8 mm/min. High mechanical advantage also permits using only fractional horsepower drives, despite the large inertia of the heavy rotating crucible.
Similar measures are needed in handling the ingot. For example, a lift-cable damping system eliminates the possibility of a pendulumlike oscillatory motion arising as the heavy ingot hangs at the end of a 4-meterlong cable. Designers planning the system dynamics must take into consideration the natural frequencies corresponding to the dimensions of components within the system. One obvious place for such concerns is the wavelength matching the length of cable between the ingot and the drum in the pull head. To ensure a stable system, frequencies of motion within the system must not be integral multiples of this frequency, or even close.
Zhixin Li is director and chief design engineer of the 300-mm program at Ferrofluidics. He received his Ph.D. in mechanical engineering from M.I.T.
Jurek Koziol is manager of process electrical and software engineering at Ferrofluidics and holds several U.S. patents in crystal growing. He received his Ph.D. in material science and engineering from M.I.T.
THE CHALLENGE OF VACUUM SEALING
Crucible lift rotation, pull head, pull-head cable rotation, feeder isolation valve rotation, argon gas feed joints, and the crystal support pin leadscrew rotation all employed magnetic fluid seals. These seals take advantage of how magnetic fluid responds to an applied magnetic field. The basic seal components consist of a ferrofluid, a permanent magnet, two pole pieces, and a magnetically permeable shaft. The magnetic structure, completed by the stationary pole pieces and the rotating shaft, concentrate magnetic flux in the radial gap under each pole. A ferrofluid in the radial gap assumes the shape of a liquid O-ring and produces a hermetic seal.
All Ferrofluidic seals are noncontact devices that don’t wear. They work over a wide speed range, have a long life, and transmit 100% of all applied torque.
COMPUTER FAULTS DON’T DERAIL NEW CRYSTAL GROWER
Redundant controls found on the Ferrofluidics 300-mm crystal grower are a case in point. A highspeed Profibus provides connections to 400 remote I/O lines. An open architecture permits customer modifications to tweak process recipes.
A special backup PLC (model S7-300) takes over automatically if the main Simatic TI-555 PLC has a major fault. This PLC, with its own I/O modules and independent power supply, monitors the health of the main unit by checking its status bits and by toggling dedicated handshaking lines and looking for the proper response. It also uses a process parameter tracking scheme to maintain the same I/O status as the main unit.
A major fault forces the backup computer to take over control of critical operations such as crystal rotation and pull system pressure, temperature control, and gas pressure. Control transfer is seamless — the backup PLC gets the same critical I/O data as the master — and lets the operator either continue growing or terminate the process safely.
Backup facilities such as a redundant PLC guard not only against faults in the main computer, but also against difficulties that end users can accidentally induce themselves. Use of an open architecture gives ingot manufacturers flexibility to handle a variety of scenarios — they can modify the source code driving the system, for example. Thus redundant controls can serve as a safety net for such mishaps.