Description of the Related Art Ferrofluids are dispersions of finely divided mag- netid or magnetizable particles, generally ranging be- tween about 30 and 150 Angstroms in size, dispersed in a liquid carrier. The magnetic particles are typically covered with surfactants or a dispersing agent. The sur- factants assure a permanent distance between the magnetic particles to overcome the forces of attraction caused by Van der Waal forces and magnetic interaction, and also provide a chemical composition on the outer layer of the covered particles which is compatible with the liquid carrier and the chemicals in the surrounding environment.

Ferrites and ferric oxides employed as magnet particles offer a number of physical and chemical properties to the ferrofluid, including saturation magnetization, viscos- ity, magnetic stability and chemical stability. Several types of ferrofluids are provided by Ferrotec (USA) Cor- poration of Nashua, New Hampshire. A summary of patents related to the preparation of ferrofluids is provided in

Patent No. 6,056, 889, while the use of ferrofluid bear- ings in a moving magnet electrical generator is discussed in copending Patent Application Serial No. 10/078, 724 <BR> <BR> <BR> enwtled"Electrical'Generator With Ferrofluid Bearings", filed on the same day as the present invention by Jef- frey T. Cheung and Hao Xin, and also assigned to Innova- tive Technology Licensing, LLC, the assignee of the pre- sent invention. The contents of this copending applica- tion are hereby incorporated herein by reference.

A ferrofluid's'frictional coefficient is roughly re- lated to its viscosity (measured in centipoise (cp) ), but not directly., For example, a ferrofluid with a viscosity of 300. cp has been found to have a static friction coef- ficient of about 0.015, the EFH1 ferrofluid from Ferrotec (USA.) Corporation has a viscosity on the order of 6 cp and a static friction coefficient of about 0.002, but a water based ferrofluid with a viscosity of 5 cp has been found to have a static friction coefficient of about 0.01. The higher friction coefficient for the somewhat lower viscosity composition has been attributed to sur- face tension associated with a water based solvent.

Low friction systems can have other problems. For example, depositing a thin film over a substrate by vapor deposition is performed in a vacuum chamber by rotating and translating the substrate inside the chamber to achieve uniform deposition over a large area. The mecha- nism that controls this motion is cumbersome, consisting of gears, chains, and both rotary and translational mo- tion vacuum feedthroughs. A simpler less expensive mo- tion control for the substrates would be highly desir- able.

Traditional lubricants employed to assist movement of a load bearing mechanism on a substrate, while consid- erably reducing the frictional forces resisting motion, scill involve a considerable amount of friction. For ex- ample, whereas the static coefficient of friction between unlubricated hard steel surfaces is. typically about 0. 6, the corresponding coefficient is about 0.08-0. 1 for vege- table and animal oil lubricants, 0. 14-0.2 for mineral oils, 0.12 for graphitised oils., 0.1 for molybdenum di- sulfide, 0.08 for oleic acid, 0.4 for alcohol and ben- zene, and 0.2 for glycerine. Even lower frictional coef- ficients, making it easier to translate a load, would be desirable.

SUMMARY OF THE INVENTION The present invention provides a mechanical transla- tor system with an ultra low degree of friction, and a novel ferrofluid composition, that can be used as a lubri- cant for the translator. The term"translation"as used herein includes both changes of position and/or rotation.

In a preferred embodiment the translator includes a move- able magnet that is arranged to carry a load and is dis- posed for movement on a substrate. Its magnetic axis is generally transverse to the substrate, such that its mag- netic field has a maximum density adjacent the substrate.

A ferrofluid lubricant is preferably provided between the magnet and substrate. A ferrofluid composition that can be used to achieve a particularly low friction has a vis- cosity substantially less than 5 cp. It. consists of a light mineral oil medium mixed with isoparaffinic acid, with the ratio of isoparaffinic acid to light mineral oil

preferably in the range of about 2: 1 to about 4: 1. The resulting coefficient of static friction has been found to be in the approximate range of 0. 0008-0. 0012.

A plurality of moveable magnets, each with a mag- netic axis generally transverse to the substrate, can be used to support a platform upon which a load can be placed, or which itself can comprise a load. The magnets are preferably connected to respective fixed locations on the platform, which can be formed from a magnetic mate- rial, or a nonmagnetic material which might include an alloy that shields magnetic fields. A ferrofluid lubri- cant is provided between the magnets and substrate for ultra low frictional movement. A ferrofluid with a low vapor pressure should be selected for environments in which other ferrofluids tend to dry out.

When applied to vapor deposition apparatus, the moveable'magnets are disposed on the floor of a vacuum chamber which includes a vapor source for depositing a thin film on a substrate carried by the platform. A con- troller formed from a magnetic material outside the vac- uum chamber, on the opposite side of the floor from the moveable magnets, controls the movement of the magnets and platform to achieve uniform deposition, eliminating the need for the cumbersome control equipment and its me- chanical communication through the vacuum chamber that were previously required.

These and other features and advantages of the in- vention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating the magnetic field lines associated with a bar magnet ; FIG. 2 is a diagram illustrating the ultra low fric- tion achieved with a ferrofluid lubricant in accordance with one aspect of the invention; FIGs. 3a and 3b are respectively frontal and side elevation views of a low friction, load bearing mechani- cal translator in accordance with the invention; and FIG. 4 is a diagram of a vapor deposition chamber using one embodiment of a mechanical translator in accor- dance with the invention.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates a bar magnet 2 and its associated magnetic field lines 4. As is well known, the field lines radiate out mostly from the magnet's north pole, and loop around to return to the magnet's south pole.

The greatest magnetic field concentrations external to the magnet are at its opposite poles, and it is in these regions that a ferrofluid will tend to accumulate when the ferrofluid is presented to the magnet. The ferro- fluid concentration formed at the opposite ends of the magnet, indicated by dashed ovals 6 and 8 around the north and south magnet poles, respectively, can be used as bearings to provide a lubricated movement of the mag- net along a surface. Ferrofluid bearings would also tend to form at the opposite ends of an electro-magnet as well as a permanent magnet.

Referring now to FIG. 2, a magnet 10 is shown sup- ported on a substrate 12, which in turn is on a horizon-

tal support surface 14. A ferrofluid bearing 16 provides an ultra low friction interface between the magnet 10 and substrate 12. The magnet is oriented with its magnetic axis 18 generally transverse to the substrate 12. Thus, the magnet's magnetic field has it maximum external den- sity adjacent the substrate. Although for purposes of this invention the magnet's magnetic axis will normally be orthogonal to substrate 12, it can also be tilted at an angle up to about 45'from a, perpendicular to the sub- strate, preferably with a corresponding chamfer at the. end of the magnet adjacent the substrate.

With the use of an appropriate ferrofluid 16 between the substrate and magnet, ultra low degrees of friction can be achieved between the magnet and substrate, making the (magnet highly responsive to a tilting of the sub- strate or a translational force applied to the magnet.

Presently available ferrofluids from Ferrotec (USA) Cor- poration are among those that can be used as low friction bearings. For example, a water based ferrofluid desig- nated EMG805 has a coefficient of static friction of about 0.01 and a viscosity of about 5 cp, while a light mineral oil ferrofluid composition designated EFH1 has a coefficient of static friction of about 0.002 and a vis- cosity slightly on the order of 6 cp.

Even lower levels of friction can be achieved with a new formulation in which the light mineral oil EFH1 fer- rofluid is mixed (preferably for about 24 hours) with isoparaffinic acid. Two sources of isoparaffinic acid at present are Isopar M and Isopar G hydrocarbon fluids, both from ExxonMobil Chemical Corporation ; both appear to work equally well. With a ratio of isoparaffinic acid to

EFH1 light mineral oil ferrofluid in the range of ap- proximately 2 : 1 to 4: 1, ultra low static coefficients of friction in the range of 0.0008-0. 0012 were achieved.

The viscosity of the mixture was significantly less than 2 cp, on the order of 1 cp. While the mixture's static friction coefficient was even lower than for the EFH1 ferrofluid by itself, the EFH1 composition has a somewhat greater load bearing capability.

The static friction coefficient was measured by raising one end of the substrate 12 off the horizontal surface 14 until a mass supported by the ferrofluid began to slide along the substrate, determining the substrate's critical off-horizontal angle of displacement at which sliding movement began, returning the substrate to hori- zontal, lifting its other end until the mass started to slide in the opposite direction, determining the critical angle of displacement from horizontal for sliding to be- gin in that direction, and averaging the two angles.

With the preferred EFH1/isoparaffinic acid mixture, the mass began to slide at an average angle of much less than 1 degree, and even considerably less than 10 minutes. In fact, the critical angle for displacement from a horizon- tal static position was found to be approximately 0.07 degree.

FIGs. 3a and 3b illustrate the invention applied to an ultra low friction translatable load bearing platform.

A set of magnets 20 are disposed with their magnetic axes generally transverse to a substrate 22, with one end of the magnets provided with ferrofluid bearings 24 for ul- tra low friction movement over the substrate, and the other end of the magnets attached to a load bearing plat-

form 26. A load to be carried by the platform is indi- cased by dashed lines 28. The shape and size of the platform, as well as the number of magnetic posts, arbitrary and can be chosen depending upon the load to be placed on the platform. The platform can be formed from a magnetic material so that it'diffuses the magnetic field, leaving the strongest field location at the oppo- site end of the magnet adjacent the substrate. This tends to concentrate the ferrofluid away from the plat- form and towards the bottom of the magnet, where it func- tions as a lubricant for magnet movement over the sub- strate. The platform can also be formed from a non- magnetic material. Another option is to form the plat- form from a nonmagnetic alloy that provides effective shielding of the magnetic field, thereby creating a zone above the platform which is free of magnetic flux. Such a material is available from Spang & Company Corp. under the trademark MUMETAL. The. magnet posts can be attached to the platform by'mechanical devices such as clamps, bolts or adhesives.

The ferrofluid solution is applied to the bottom of . the posts where, because of their magnetic property, they bond strongly to engulf the post bottoms. The platform thus rests on ferrofluid"cushions", without a direct contact between the magnets and substrate. Because of the low friction achievable with ferrofluid bearings, the platform can-be pushed to move freely with a slight force. The movement can be controlled by either applying it directly to the platform and/or magnets, or by moving a magnetic control object (formed from either a magnet- ized or a magnetizable material) on the other side of the

substrate. The movement of the platform can be con- trolled remotely by moving the external controller on the opposite side of the substrate, without making any direct contact to the platform assembly itself.

An application for this type of motion control is a vapor deposition chamber, illustrated in FIG. 4. The chamber consists of a vacuum enclosure 30, the floor 32 of which comprises a substrate upon which a load bearing platform 34 moves via attached magnets 36 and ferrofluid bearings 38. As illustrated, the magnets 36 in FIG. 4 have a magnetic polarity opposite to that illustrated in FIGs. 3a and 3b ; the choice is arbitrary. A substrate 40 upon which a thin film is to be deposited is placed on the platform 34.

The chamber is evacuated a vacuum pump 42, while a vapor source generically indicated by reference number 44 provides a material to be deposited on the substrate. In practice, the vapor deposition process can take several forms, such as thermal evaporation, e-beam evaporation. or different forms of sputtering. These all require that the substrate holder be able to rotate and translate in- side the vacuum chamber to achieve a uniform deposition over a large substrate area. The prior need for gears, chains, and both rotary and translational motion vacuum feedthroughs are eliminated with the use of an external magnetized or magnetizable control mass 46, placed on the opposite side of substrate 32 from the platform 34 and aligned with its support magnets. The external control- ler 46 can be translated in an x-y plane, as indicated by arrows 48 and 50, or rotated as indicated by circular ar- row 52, imparting a corresponding movement to the inte-

rior magnets and the platform they support. The external controller 46 is preferably a single mass which encom- passes the area subtended by the interior magnets 38, or can equivalently be an array of separate controllers aligned with the individual magnets 36 and moved to'- gether. The platform and the substrate which it carries can be rotated and'translated over a large area simply by imparting the same movements to the easily accessible ex- ternal controller. If the controller is magnetized, its polarity should be oriented in the same direction as the internal magnets for mutual attraction.

APG S10 grade ferrofluid from Ferrotec (USA) Corpo- ration was used for the vapor deposition application be- cause of its low vapor pressure, which allows it to be used. in a vacuum or ambient atmosphere with a long opera- tional lifetime. Some other ferrofluids have a tendency to dry out in this environment. A 7.6 cm diameter plat- form with four 0.95 cm diameter, 6.4 cm length, Grade 30 NdFeB magnets, cushioned by the APG S10 grade ferrofluid, was found to be capable of supporting a 250 gram load, while maintaining a low static coefficient of friction in the range 0.01-0. 02. While not as low as the other fer- rofluid compositions discussed above, this value was still considerably lower than traditional lubricants.

In addition to providing a lower level of friction, the use of a ferrofluid allows the movement of the plat- form to be controlled externally by a moving magnet body.

With traditional lubricants, placing a magnet on the op- posite side of the wall from the platform can attract the platform's magnet posts strongly enough that the lubri- cant is squeezed out, leaving the posts in a direct high

friction contact with the wall. However, due to the at- traction between the magnet posts and the magnetic nanoparticles in a ferrofluid, a cushion of lubricant will remain between the magnet posts and the chamber wall.

While various embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Ac- cordingly, it is intended that the invention be limited only in terms of the appended claims.