When moving seals are needed, ferrofluid seals often offer performance and life benefits that surpass alternatives. This is most true when rotary motion must be transmitted into an isolated environment. Examples involve vacuum or delicate chemical atmospheres: Motion in vacuum wafer-making chambers, rotating gas unions in manufacturing, and conveyor drives in large coating chambers for flat-panel displays, architectural glass, and metalized food packaging rolls. In these situations where it's imperative that the isolated environment not leak, ferrofluid seals see force combinations including applied pressure, fluid dynamics, gravity, vibrations, and magnetic fields. Despite this, ferrofluids remain stable, retained in the seal by the permanent magnetic field.

There is another reason to use ferrofluid seals. Motors that power motion systems don't often operate in vacuums. First, without air there's no convection to cool motors to acceptable operating temperatures. Also, motors can be dirty when exposed in the vacuum chamber. Even worse though, they can cause electrical field and arcing problems. Finally, in vacuum coating applications, stray materials can deposit on motor parts.

Their basic makeup

Ferrofluids are colloidal suspensions — fluid infused with magnetic particles. The particles are suspended in the carrier liquid by a surface active layer. How exactly does this layer hold metal particles? (In other words, why don't the metal particles get pulled out of the carrier fluid by the applied magnetic force?) The bonds between the magnetic particles and the surface layer — and the resulting forces keeping the particles separate — are greater than both the force of gravity and the magnetic force acting on the tiny particles.

The inside of the surface active layer bonds to the magnetic particle, and the outside repels the surface layer on all the other magnetic particles because of like charge. This repulsion between the surface layers of all the magnetic particles keeps the particles separated, unable to agglomerate, and therefore stably suspended. Depending on the application, the carrier fluid can be a hydrocarbon, ester, or perfluoropolyether (PFPE) — basically inert and stable, low-vapor-pressure fluids.

The magnetic particles — just a few nanometers across in some cases — are single-domain grains. A single-domain grain is a permanent-magnet particle just large enough to have in itself a “north” and a “south” magnetic pole. However, because they're effectively separated by the surface active layer, the particles don't permanently align, but acquire a residual magnetic charge when exposed to an applied magnetic field.

In the absence of a magnetic field, the energy of a single-domain particle is minimized when its magnetic moment aligns with the anisotropy axis of the particle with the two opposite orientations being equivalent. These two orientations are separated by an energy barrier U =KV, where V is the volume of the particle and K the magnetic anisotropy constant characteristic of the material.

Besides the fluid just described, a ferrofluid seal also consists of a permanent magnet, two pole pieces, and a magnetically permeable shaft. It works like this: The magnetic circuit, completed by the stationary pole pieces and a rotating shaft, concentrates magnetic flux in the radial gap between these two components. When the ferrofluid enters this gap, it sticks magnetically, assuming the shape of an O-ring. The magnet holds the fluid firmly in a closed ring shape, sealing off the round gap between pole and shaft. Applied pressure cannot exert enough force to displace or burst the fluid, and the fluid itself is impermeable to atmospheric materials.

As we'll soon explore further, an individual stage can hold a fraction of an atmosphere. In multiple-stage seals, the ferrofluid stages work together to hold a much higher overall pressure. Total pressure is shared by the individual stages when one yields briefly and allows some gas through, creating a differential pressure on the next stage. After several stages are exercised in this fashion, the larger differential pressure is ultimately held back by the combination of the stages, each supporting its individual part.

Vacuum rotary feedthroughs — the connectors that surround outer power-transmission shafts to inside a vacuum's wall — are the most common use for ferrofluid seals. These feedthroughs employ multiple rings of ferrofluid contained in stages; these stages are formed by grooves machined into either the shaft or pole pieces. Typically, a single stage can sustain a pressure differential of 0.2 atmospheres, or 200 mbar. The pressure capacity of the entire feedthrough is approximately equal to the sum of the pressure capacities of the individual stages.

Leakage rates are around 10-11 [He]mbar l/sec. Because the sealing medium is a fluid, there is virtually no friction between the rotating and stationary components, so the seal does not wear. What's more, because there is no mechanical friction within the ferrofluid or between the ferrofluid and the static seal components or shaft, no particles are produced to contaminate the system. Low vapor pressure ferrofluids maintain seal integrity even in vacuums better than 10-9 mbar. In addition, ferrofluid seals maintain hermetic sealing at high rotational speeds. Current technology has produced configurations that perform to d × N values of 500,000, where d = shaft diameter (in mm) and N = rotational speed (in rpm). For example, with a 1-in. feedthrough this equates to a rotational speed of 20,000 rpm.

Optimum torque transmission

Through-shaft construction permits 100% of the engineered torque transmission and provides in-phase rotation without backlash or slip errors. Ferrofluid seals also provide leak-free performance during intermittent and static conditions. Unlike elastomeric seals, they are not subject to plasticizing and relaxation effects during idle periods. The low-viscosity drag torque of the ferrofluid, which is the resistance of the shaft to turn (resulting from ferrofluid's viscosity) is not affected by differential pressure across the seal. This means that applied pressure has no impact on fluid viscosity — and therefore, has no effect on the torque needed to rotate the seal within the ferrofluid seal. For this reason, operation is very smooth.

Precise fluid makeup

The standard ferrofluid base is synthetic hydrocarbon with low volatility — for low outgassing and long life. It offers medium drag torque and excellent reactive gas and temperature resistance. However, for certain applications other fluid may be more appropriate.

Synthetic ester-based ferrofluids are used where low torque is required. Though they have higher volatility, esters are most often used in outdoor and low-vacuum applications where this doesn't compromise design life. Fluorocarbon-based ferrofluids are used in applications involving the most reactive gases and highest temperatures. They have the lowest outgassing rates and offer the longest life. But again, every setup has its tradeoffs: Because fluorocarbons have a higher viscosity, they do increase starting and running torque. Ferrofluids for ultra-high vacuum rotary sealing applications usually have low oil vapor pressures: less than 10-8 mbar for hydrocarbon based sealing fluids and 10-10 mbar for PFPE based sealing fluids.

The fluid's magnetic particles can be one of a variety of ferrites or transition metals, such as iron and cobalt. The mean particle diameter (5 to 13 nm) can be tailored to requirements, as can the concentration of particles and the ferrofluid viscosity. Overall magnetic content of the fluid and overall magnetic strength can also be adjusted.

Dealing with heat

Three things heat up ferrofluid: hot elements in the process chamber, the viscous shear in the fluid, and dynamic effects in the ball bearings. Certain construction materials and either electrical, magnetic, or high-frequency fields near the seal can further contribute heat. For this reason, cooling systems are sometimes necessary when a high-temperature application is critical.

Most seals can be water-cooled, which allows operation at higher temperatures. This is usually achieved by passing a cooling liquid into the pole pieces through channels in the feedthrough housing. Sometimes for higher temperature applications it's useful to cool the shaft as well. In this case, coolant is supplied to the rotating shaft through a rotary water union. Additional features that can be incorporated include:

  • Custom magnets, for resistance to demagnetization (or for processes sensitive to magnetic fields)

  • Heat-treated shafts, for higher torque capacity

  • Electrically or thermally insulating sleeves and flanges.

Custom designs

There are many feedthrough parameters that can be modified to match applications. These include the mounting configuration, bearing type and position, and placement of sealing stages on either the stator or rotor.

  • Rotary gas unions are suitable for chemical vapor deposition (CVD) systems and gas handling modules. Ferrofluid rotary gas unions offer manufacturers of CVD and other deposition systems an efficient and flexible method of introducing gases into a process. A static gas feed runs into the rotating shaft that supports the wafer to ensure that the gas outlet is in the center of the wafer for uniform coating characteristics.

  • Multi-axial feedthroughs are appropriate for wafer handling rotation with a stationery inner shaft. Multi-axial feedthroughs offer extremely high repeatability with zero backlash. Linear motion can be incorporated and a cantilevered seal design is optional for UHV optimization. (UHV stands for ultra-high vacuum, generally defined as any pressure lower than 1 × 10-8 torr.) Since the feedthroughs employ a single shaft per axis to transmit torque to the load, torsional stiffness is maximized — for the highest torque transmission of any UHV wafer handling rotary sealing technology.

  • In-line drive motorized feedthroughs can be used where motors are needed, particularly where servo control is necessary. An in-line motorized feedthrough is compact in comparison to an offset or shaft-coupled drive. These direct-drive motorized feedthroughs include a brushless servomotor and matching amplifier with power supply and sinusoidal commutation, as well as application-appropriate feedback devices and command loops. As this type of feedthrough has a motor fully integrated around the shaft (rather than coupled to it) it provides optimum drive-to-load torque efficiency.

  • High-speed, large-diameter feedthroughs are suitable for optical coating applications and wafer rotation mechanisms. Hollow-shaft feedthroughs are ideal for coating applications — in fiber-optic filter manufacturing, for example. They are configured with a double ferrofluid seal to enable static access to the rear of the seal. Driving force is supplied via a toothed belt through the side of the housing or an integral brushless motor within the feedthrough housing. The product being manufactured can then be accurately observed or measured through the large diameter hollow shaft without the difficulties associated with a rotating window. As a generalized example, a seal with an 8-in. internal diameter might be used at speeds to 1,000 rpm.

  • High-precision spindle designs are especially appropriate for wafer handling and ion implantation. Ferrofluid spindles can be engineered to rotate with as little as 0.0001 in. of runout. These are ideal for high-precision wafer handling/aligning applications and wafer/substrate rotation applications where wobble requirements are very stringent. Axial, radial, and torsional spindle stiffness is achieved by custom engineering the bearing and shaft designs.

  • Rotary/linear feedthroughs are particularly useful for z-motion alignment. Rotary/linear feedthroughs integrate a ferrofluid rotary seal with an edge-welded metal bellows linear seal. With preloaded angular-contact rotary bearings and sleeve or ball-type linear bearings, these feedthroughs are suitable for aligning applications where indexing and translation are involved. In addition, servo-controlled rotary feedthrough combines servo control and high-resolution encoder feedback with a non-wearing high-vacuum ferrofluid rotary feedthrough. Linear speed accuracy can reach ±0.1% combined with smooth rotation.

As an example: In one particular design, a wafer-handling arm subassembly was integrated into a cluster tool where it positions 200 and 300-mm wafers, picking and placing them at different locations for processing. The sub-assembly features a ferrofluid coaxial feedthrough powered by a brushed dc motor and a harmonic drive, integrated with in-vacuum components and other hardware. The servo control with high-resolution encoder feedback offers indexing control and torsional stiffness.

For more information, visit www.ferrotec.com.