GB2133757A - A frictionless transport system - Google Patents

Abstract

The system is particularly intended for transporting semiconductor wafers (12) in a clean room environment and comprises a load carrier (14) supported on a cushion of air formed between track (10) and the carrier and laterally guided on the track by magnetic means, e.g. the interaction of magnetic track guide rails (24,26) with carrier magnets (23). In the Fig. 1 system the carrier (14) is propelled by sequentially engergized track magnets (e.g. 16-18), and curved track and turntable arrangements for this form of drive are disclosed. Alternatively mechanical displacement of a member located within the track and magnetically coupled to the carrier may propel the latter, overhead and vertical track arrangements being disclosed in this instance. The track may be inclined whereby the carrier is propelled or retarded by gravity. The air cushion may be created from the track or the carrier. <IMAGE>

Description

SPECIFICATION Africtionless transport system The present invention is directed to a system for providing friction less transport. A preferred embodiment is suitablefortransporting semiconductor wafers between processing stations in a clean room environment.

The manufacturing of semiconductor integrated circuits usually involves a series of processing steps performed on a semiconductor wafer, such as a silicon wafer. Such processing steps must take place in a clean room environmentto avoid particulate contamination of the wafer. The transport of the wafers in and out of such clean room, and between processing stations within the clean room, must introduce no particulate contamination, must meet critical proces singtimecontraintswhileatthesametime provide gentle motion ofthe wafers to minimize wafer jostling.

One known technique for handling and conveying semiconductor wafers is described in U.S. Patent 3,721,472 issued on March 1973 to W. K. Mammel.

Such a known technique makes use of the Coanda effect for transporting plate-like workpieces on a layer offluid. Although this known method facilitates handling and transporting of semiconductor wafers and operates satisfactorily for its intended purpose, some physical contact between the wafers and a retaining member is needed in orderto limit their movements. Contamination and particle generation may resultfrom such physical contact thereby rendering such known arrangement inadvisable for a clean room application. Moreover, such known technique is capable of transporting wafers substantially only along one direction due to the physical configuration ofthe apparatus generating the Coanda effect.

Another known method and apparatusfortransporting articles along a track-like surface is described in U.S. Patent 4,010,981 to T. A. Hodge. This apparatus comprises an air conveyor including a plenum and a perforated deck plate for communicating airtothe surface ofthe deck plate. The air perforations of the plate create directional air jets having a major component along the plate surface, and articles are moved underthe influence of the conveying air jets. In otherwords, such pneumatic conveying system is undirectional and is suitable for moving objects between two predetermined locations. Such known arrangement cannot selectively transport several articles at a time on the same deck plate, nor can it transport one article while the other articles remain on the plate.In essence, the lack of selectivity and control on the articles being transported renders such arrangement unsuitable for semiconductor manufacturing facilities.

A commercial wafertransport system directed at reducing contamination in semiconductor manufacturing facilities is known as the NAMTRAKsystem available from Nacom Industries Inc., Tustin, Califor nia. In such a known system, a wafer cassette is transported on a carriage which moves on a pathway by means of drive wheels. The carriage is moved using magnetic coupling with a drive mechanism moving on a track positioned underneath the pathway. The entire drive mechanism is enclosed and sealed to contain contaminants generated by its movement.Although enclosure of the carriage and pathway within a nitrogen atmosphere may reduce contamination, the physical contact and friction existing between the carriage drive wheels and the pathway cooperate to generate contaminating parti clesthatdeleteriouslyaffecttheyield ofthesemicon- ductorcomponents ultimately produced.

Therefore, there exists a need for a frictionless wafer transport system exhibiting selectivity in the control of, and flexibility in the moving and directing of, wafer load carriers while substantially eliminating particulate contamination.

Thepresentinventionsolvestheforegoing prob- lems with a frictionless transport system comprising an elongated track defining a tracking surface, means for moving a load carrier in a first direction along the tracking surface, meansforforming a fluid cushion between the tracking surface and the load carrier, and magnetic guidance means coupled to the load carrier for providing lateral guidance to the load carrier in a second direction substantially perpendicularto the first direction.

In accordance with one illustrative embodiment of the invention, the means for moving the load carrier is a magnetic propulsion arrangement comprising a plurality of selectively energizable electromagnetic pole members positioned within the elongated track in a direction substantially perpendicualrtothe tracking surface, and a plurality offerromagnetic bodies positioned within the load carrier substantially along the second direction for providing a plurality of transversalfluxreturn paths to the pole members. In such one illustrative embodiment, pressurized fluid generating means are coupled to the elongated track and comprise a plurality of air slots located in the tracking surface along the periphery of each one ofthe plurality of electromagnetic pole members.The magnetic guidance means comprise at least one pair of guidance magnets embedded within the load carrier and adapted to be magnetically coupled to a pair of guidance bars positioned within the elongated trackalong thefirstdirection thereof.

In accordance with another illustrative embodiment ofthe invention,the meansfor moving the load carrier comprise magnetic means positioned proximate to the tracking surface and selectively movable along the first direction within the elongated track, and ferromagnetic elements embedded within the load carrier and adapted to be magnetically coupled to the magnetic means. The magnetic guidance means include at leat one pair guidance magnetsselective- ly movable along the first direction within the elongated track. In such other illustrative embodiment, the pressurized fluid generating means are preferably coupled to the load carrier.

In a further illustrative embodiment of the invention, the means for moving the load carrier comprise a track support structure for positioning a first section of the elongated track at a predetermined angle with respect to a reference horizontal plane. A second section of the elongated track comprises a magnetic propulsion arrangement ofthetype described above. The angle ofthefirstsection ofthe elongated track is selected to impart to the load carrier either a gravity accelerating vector or a gravity deceleration vector.

One advantage ofthe present invention is the substantial reduction of particulate generation and contamination, the latter being particularly undesirable in a semiconductor processing clean room environment.

Another advantage of the present invention is the possibility of simultaneously transporting and controlling several wafer cassettes on the same track.

A further advantage of the present invention is the abilityto achieve an air cushioned bidirectional frictionless transport system.

A still further advantage of the present invention is the ability to accurately control the gentle motion of the wafer without resulting in any vibrations thereof.

Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings.

FIG. 1 is a perspective view of a frictionless transport system in accordance with one embodiment of the invention; FIG. 2 is a partial cross-sectional end view of the embodiment of FIG. 1 illustrating details of a magnetic guidance arrangement in accordance with an embodiment ofthe invention; FIG. 3 is a partial cross-sectional end view of FIG. 1 illustrating details of an air suspension arrangement in accordance with an embodiment of the invention; FIG. 4 is a partial cross-sectional side view of FIG. 1 illustrating details of a magnetic propulsion arrange ment in accordance with an embodiment of the invention; FIGS. 5A, 5B, and 5C illustrate a frictionless vertical shuttle transport system in accordance with a further illustrative embodiment of the present invention;; FIG. 6 is a perspective view of a friction less horizontal shuttle transport system in accordance with a still further illustrative embodiment ofthe present invention; FIG. 7 is a perspective view of a frictionless transport system in accordance with another embodiment of the invention; FIG. 8 illustrates a curve track assembly in accordancewith an embodimentofthe invention; and FIG. 9 is a turntable assembly in accordance with an embodiment of the invention.

Shown in FIG. 1 is a perspective view of a frictionless transport system 1 in accordance with an embodiment ofthe invention. An elongated track in the form of a plenum chamber 10 of substantially a parallelepiped shape has one of its outer walls defining a tracking surface 11. Semiconductorwafers 12to be trans ported by the presentfrictionlesssystem are held in a cassette 13which in turn sits on a load carrier or pallet 14. The latter is typically in the form of a flat-bottomed sled adapted to move in spaced relation to the tracking surface 11 with an air cushion formed therebetween.

In one embodiment of the invention, the movement of the load carrier 14 along the tracking surface 11 is achieved by means of a magnetic propulsion arrange ment comprising a plurality of pairs of selectively energizable electromagnetic pole members, e.g., 16 and 17, positioned within the plenum chamber 10 in a direction substantially perpendicularto the tracking surface 11. Each pole member 16 and 17 comprises a pole cartridge 18 surrounded by a coil 19. A circuit bar 21 is positioned underneath the pole members 16 and 17 to close the magnetic circuit thereof.Several ferromagnetic bodies, of which only one body22 is shown in FIG. 1, are positioned within the load carrier 14 for providing tra nsversa I flux return paths to tIle plurality of pairs of pole members asthecarriertravels along the tracking surface 11.

Guidance ofthe load carrier 14 in a lateral direction,, i.e., in a direction perpendiculartothe longitudinal direction of movementofthe carrier, is achieved in accordance with an embodiment of the invention by meansofatleastone pairofmagneticguidance members positianedwithinthe load carrier 14 at a distance from eadr. other ofaboutthe width of the tracking surface TT::#PreferaIbly, the load carrier 14 has four guidance magnets, of the same type as guidance magnet 23 shown ireFIG. 1, positioned substantially at the fourcornersthereof. The guidance magnet 23 is adapted to be magneticalI#coupled to a guidance bar 24 positioned within plenum chamber 10 along the longitudinal direction thereof. Similarly, a second guidance magnet (not shown) positioned within a load carrier opposite to magnet 23 is adapted to be magnetically coupled to guidance bar 26. A pair of safety guidance legs 27 and 28 are coupled to each side ofthe load carrier 14to limitanysubstantial movement of the carrier in the lateral direction.The safety guidance legs are spaced from the sides of the load carrier 14 as well as from the sides of the plenum chamber 10.

Also shown in FIG. 1 is a pair of elongated magnetic strips 15 and 20 positioned at both ends ofthe load carrier 14. The magnetic strips 15 and 20 are poled to cause a repulsive force along the longitudinal direction between two adjacent and proximate load carriers 14 on the track 10. In essence, the magnetic strips 15 and 20 act as a pair of contactless magnetic bumpers which prevent any physical contact or friction between successive load carriers simultaneously present on the tracking surface 11.

MAGNETIC GUIDANCEARRMGEMENT FIG. 2 illustrates an endview, partially in crosssection, specifically intended to clarifythe magnetic guidance arrangementofthe illustrative embodiment of FIG. 1. Preferablyrtheloadcarrier 14 has two pairs of guidance magnets positioned therein, of which only one pair of magnets23 and 31 is shown. The magnets 23 and 31, which are preferably U-shaped, have respective widths comparable to the width of their corresponding guidance bars 24 and 26. As shown in the enlarged portion of FIG. 2, a magnetic field exists between the guidance magnet31 and the guidance bar 26 as illustrated by the magneticflux lines 32.

As long asthe load carrier 14travels along its intended longitudinal course on the tracking surface 11, the guidance magnets 23 and 31 remain substantially aligned, and in magnetic coupling, withtheir corresponding guidance bars 24 and 26, respectively.

In the eventthatthe load carrier 14 deviates inthe lateral direction from its intended course, the magnetic coupling existing between the guidance magnets 23 and 31 and their corresponding guidance bars 24 and 26 causes a restoring force to bring the load carrier backon-course on the tracking surface 11. The safety guidance legs 28 and 29 are designed and adapted to prevent any deviation ofthe load carrier 14 beyond a point where the foregoing restoring force becomes ineffective in bringing the load carrier back to its on-course position.

AIR SUSPENSION ARRANGEMENT FIG. 3 illustrates an end view, partially in crosssection, specifically intended to clarify the air suspension arrangement of the illustrative embodiment of FIG. The plenum chamber 10 comprises a plurality of pairs of selectively energizable electromagnetic pole members of which a single pair 25 is shown in FIG. 3. Each pole member comprises a pole cartridge 33,34surrounded by an energizing coil 36,37.The pole members are mounted in the plenum chamber 10 such that the top portion of their respective pole cartridges 33,34 is flush with the tracking surface 11.

In accordance with an embodiment of the invention, pressurized fluid, e.g., airfrom a source not shown, is coupled to the plenum chamber 10 and is adaptedto flow out of such chamber in a direction substantially perpendicularto the tracking surface 11.

However, other sources and types of pressurized gas, usually available in semiconductor clean rooms, may well be used in the suspension arrangement of the present invention. As shown in the enlarged portion of FIG. 3, small slots or gaps 38 exist between the top portion of pole cartridge 34 and the adjacent regions ofthetracking surface 11. Such small slots or gaps 38 are preferably distributed along the periphery of the pole cartridge 34thereby causing an out flow of pressurized air in a direction substantially perpendicularto the tracking surface 11. The presence of the load carrier 14 on top of the tracking surface 11 causestheformation of an aircushiontherebetween.

Although the air slots 38 may be positioned anywhere on the tracking surface 11, they are preferably located around the periphery of the pole cartridges 33,34 thereby eliminating separate drilling operations for the pole members and the air slots. Such a preferred arrangement may be achieved by surrounding the top portion of a pole cartridge, e.g., 34, with a non-ferrous ring 39 having an outer diameter larger than the outer diameter of the pole cartridge 34, an inner diameter large enough to enable the formation of several peripheral adjacent air slots or gaps 38 between the pole cartridge 34 and the ring 39, and a thickness of the order of that of the tracking surface.

The source of pressurized airsupplied to the plenum chamber 10may be, for example, a conventional commercially available air pump.

MAGNETIC PROPULSIONARRANGEMENT FIG. 4 illustrates a side view, partially in cross- section, specifically intended to clarifythe magnetic propulsion arrangement ofthe illustrative embodi ment of FIG.1. As shown in FIGS. 1 to 3, the propulsion arrangement comprises a plurality of pairs of selectively energizable electromagnetic pole members positioned within the elongated track 10.

Each pair of pole members comprises two pole cartridgeswith corresponding energizing coils, such pole cartridges being positioned side-by-side in the lateral direction ofthe elongated tracking surface 11.

Forthe purpose of explaining the operation ofthe magnetic propulsion arrangement, only one pole member (40 to 46) of each pair of pole members of a portion of track 10 is shown in FIG. 4. Each electromagnetic pole member, 40 to 46, comprises a pole cartridge, 47 to 53, surrounded by its corresponding energizing coil, 54 to 60. The load carrier 14with its two magnetic end bumpers 15 and 20, and the wafer cassette 13 are shown positioned on top of the tracking surface 11 of track 10. Several magnetic bodies, 61 to 65, are positioned within the load carrier 14, substantially along the lateral direction thereof, for providing a plurality of transversal flux return pathsforthe magneticfields generated by each pair of laterally adjacent pole members.In essence, the propulsion arrangement is of a variable reluctance linear stepping motor type having a transverse flux configuration with two pole members on the track, a transverse circuit bar 21 positioned underneath the pole member pairs to close the magnetic circuit thereof, and a transverse pole piece including the transverse flux magnetic body 61 to 65 on the load carrier 14. For purpose of illustration only, a fourphase motor having a five transverse flux magnetic bodies 61-65 is shown. However, the number of magnetic bodies such as 61-65, i.e., the total number of flux return paths, may be varied depending upon the size ofthe load carrier 14, and the energizing sequence ofthe electromagnetic pole members.

In operation of the magnetic propulsion arrangement, first pole member 48 is energized by coupling a pulseto its corresponding energizing coil 55. The transverse flux magnetic body 61 of load carrier 14 lines up with pole member 48 (as shown) in a position corresponding to the minimum reluctance position.

In other words, pole member41 and magnetic body 61 are in mechanical alignment which corresponds to the minimum reluctance ofthe magnetic circuit therebetween. Next, pole member 41 is de-energized and pole member42 is energized. This results in load carrier 14 moving to the right direction (in FIG. 4) becausethetransverse pole piece 62 will line up with pole member42 in the minimum reluctance position, i.e., in the mechanical alignment between 42 and 62.

Next, pole member 42 is de-energized and pole member 43 is energized. Again, the load carrier 14 will move to the right in FIG. since the transverse pole piece 63 will tend to line up with energized pole member 43. Continuing the sequential organization process for pole member 44, load-carrier 14 moves further in the right direction a predetermined distance since energizing pole member 42 and transverse pole piece 64 become lined up. Thus, the sequential energizing of the pole members 41 to 44 causes the load carrier 14to travel a total distance equal to four times the difference, AS, in pitch between the transverse magnetic bodies and the pole members.

As the foregoing sequential energization continues, pole member45 becomes energized, resulting in the load carrier 1 4further moving in the right direction because transverse pole piece 65 lines up with pole member 45.

If PT is the pitch between consecutive pairs of pole members along the track, and if Pc is the pitch between consecutive transverse pole pieces in the carrier, then in orderto have movement of the carrier relative to the track, the following conditions must be met: PT#Pc (1) and N(P1-Pc)=Pc (2) where N is the number of phases of the motor.

Equation (2) can be written as: N Pc=PT ( ) (3) N+1 Thus, in the above described embodiment where N = 4, we derive: 4 Pc - PT (4) 5 and the distance, AS, moved per phase switch becomes: PT AS=PT-Pc= = 5 The energizing signals coupled to the respective energizing coils 54 to 60 ofthe electromagnetic pole members40to 46 may be generated bya pulse sequence generator 66 capable of delivering at one or at a plurality of outputs a sequence of pulses of predetermined characteristics. The repetition rate of these pulseswill ultimatelydeterminethe speed of movementofthe load carrier 14.Moreover, the direction of movement of the load carrier 14 on the track is determined by the order in which the consecutive energizing coils are energized. In the above description, the load carrier 14 moves to the right when the time sequence of energization ofthe respective coils is in the order: 55,56,57,58. The load carrier 14would move to the left ifthe energization sequence ofthe coils is 58,57,56,55.

VERTICAL SHUTTLE TRANSPORTSYSTEM Referring nowto FIG. SAwherein a frictionless elevator system S is shown,the elevator combining the features of magnetic propulsion, air suspension and magnetic guidance. The elevator comprises an elongated tubulartrack70 of substantially rectangular cross section with one of its outer walls 71 defining the tracking surface of the elevator. The track 70 may comprise, for example, an elongated aluminum tubing. A carriage 72, located within the track 70, is capable of moving in the longitudinal direction of the track by means,for example, of a positioning lead screw 73 coupled thereto.However, the carriage 72 may be moved in within the track 70 using other mechanisms, such as pneumatic means, coupled thereto and adapted to selectively move the carriage in the up or in the down direction of thetrack.

A load carrier74 has a sled or pallet portion 75 capable of moving along the tracking surface 71.

Preferably, the load carrier 74 has a pair of support members 76 and 77 attached to the sled portion 75 in a direction substantially perpendicularthereto. The support members 76 and 77 have each a pairof cavities 78,79 capable of receiving the safety guidance legs (e.g., 27,28 in FIG. 1 and 28,29 in FIG. 2) of a wafer cassette pallet to be transported by the elevator. Alternatively, the load carrier 74 may comprise a platform attached to the sled portion 75 by means, for example, of bracket-type supports. A wafer cassette pal let to be transported by the elevator would then be placed directly on the platform.

In accordance with an embodiment of the invention,the carriage 72, as shown in FIG. SB, has a plurality of magnetic elements 80 to 85 attached thereto and positioned proximate to the tracking surface 71. These magnetic elements 80 to 85, of which onlythree elements 80,81 and 82 are shown in FIG. 5A, are adapted to be magnetically coupled to a plurality of corresponding ferromagnetic bodies attached to the sled portion 75 of the load carrier74.

FIG. SIC showsthe surface 86 of the sled portion 75 which faces the tracking surface 71. The ferromagnetic bodies 87 to 92 are embedded in the sled surface 86 and adapted to be magnetically coupled to the magnetic elements 80 to 85 ofthe carriage72, respectively.

In a preferred embodiment, the magnetic elements 80 to 85 are permanent magnets and the ferromagnetic bodies 87 to 92 are made of ferrous material.

Shown in FIGS. 5A and 5C is a flexible conduit 93 coupled to the sled portion 75 for supplying pressurized airthereto. The sled portion 75 has a plurality of apertures 94 in its su rface 86 facing the tracking surface 71. These apertures 94 communicate with the air supply conduit 93 such thatthe flow of pressurized air out of such apertures causes an air cushion to be formed between thetracking surface 71 and the surface 86 of the sled portion75 of the load carrier 74.

In other words, the sled surface travels along the tracking surface 71 with an air cushion therebetween.

The movementofthe load ca rrier 74 u p and down thetracking surface71 is achieved by moving the carriage 72 up and down within the track 70 by means of the positioning lead screw 73. The permanent magnets 82 and 85 and their respective corresponding ferromagnetic bodies 89 and 92 attached to the sled portion 75 achieved the magnetic guidance of the load carried 74 in the lateral direction ofthe track, i.e., the direction along the width of the tracking surface 71. The magnetic coupling between the permanent magnets80,81,83 and 84 attached to the carriage 72 and their respective corresponding ferromagnetic bodies 87,88,90 and 91 attached to the sled portion 75 causes the sled portion 75 of the load ca rrier 74 to follow the movi ng carriage 72.Although this embodimentwas described with permanent magnets 80 to 85 attached to the carriage 72, permanent magnets may be substituted forthe ferromagnetic bodies 87 to 92, and ferromagnetic bodies attached to the carriage 72.

Alternatively, the carriage 72 as well asthe load carrier74 may each comprise permanentmagnetsor electromagnets poled to attract each other.

HORIZONTAL SHUTTLE TRANSPORTSYSTEM FIG. 6 shows a horizontal frictionlessshuttle transport system 6 similarto the frictionless elevator system 5 described in FIGS. 5A to 5with most of its structural elements having the same reference numerals asthealready described elements in FIGS.

5A to 5C. The horizontal shuttle system shown in FIG.

6 has a load carrier96with a sled portion 75 capable of moving along the tracking surface 71 using the foregoing magnetic propulsion, magnetic guidance and air suspension principles. The load carrier 96 further comprises a loading platform 97 coupledto the sled portion 75 by means of a coupling arrangement 98. The latter may be of an extendibletype having a pneumatic or a lead screw configuration.

Such an extendible coupling would enable an up and down movementofthe loading platform 97,whilethe combination of magnetic propulsion/guidance and air suspension achieves side-to-side or longitudinal motion of the load carrier 96. The foregoing combination would facilitate, for example, the raising and lowering ofsemiconductorwafer cassettes into chemical processing baths, as well as the frictionless transport of such wafer cassettes between different chemical processing baths. Other uses ofthis horizontal shuttle system are well within the scope of the present teachings.

Although the above magnetic guidance and air suspension arrangements were described in connectionwith a transport system propelled by selectively energizable electromagnetic means or by selectively movable magnetic elementswithin the track, it is possibleto achieve frictionlesstransportsystems using other propulsion meansfor moving the load carrier along the tracking surface. One of such other frictionlesstransport systems TOO is shown in FIG. 7 wherein atrack 101 defining atracking surface 102 is in the form of an elongated plenum chamber 105 coupled to a source of pressurized air.Semicinductor wafers 1 03to be transported by the frictionless transport system 100 are held ina cassette 1 which in turn sits on a load carrier or pallet 106. As mentioned above with reference to FIGS. 1-4, the load carrier 1 OSistypically in the form of a flat-bottomed sled adaptedto ride on an air cushion on top of the tracking surface 102.Such an air cushion is formed in the embodiment of FIG. 7 by means of a pluralityofair slots orapertures 107 formed in the tracking surface 1û2 and adapted to cause a flow of pressurizedairto flowoutofthe elongated plenum chamber 105 in a direction substantially perpendicularto thetracking surface 102.

Guidance of the load carrier 106 in a lateral direction, i.e., in the X-axis direction in FIG. 7, is achieved by means of at least one pair of guidance magnets embedded in the load carrier 106. One guidance magnet 108 of such pair of guidance magnets is shown in FIG. 7. The guidance magnet 108 is magnetically coupled to a guidance bar or rail 109 positioned within the plenum chamber 105 along the longitudinal direction LA A second guidance bar 111 is also magnetically coupled to the second guidance magnet (not shown) of the pair of guidance magnets.

The structure and operational features ofthis magnetic guidance arrangement are similarto those described above in connection with the embodiment shown in FIGS. 1-4.

In the frictionless transport system 100 of FIG. 7, the load carrier 106 is moved along the tracking surface 102 by positioning the track 101 on a support structure 112 ata small angle 6 (ofthe order of a few degrees) with respecttoa horizontal plane XY such thatthe load carrier 106 is subjected to a gravity propulsion vector. In the embodiment shown, the support structure 112 and the value and sign ofthe angle 6 subjectthe load carrier 106 to a gravity acceleration vector resulting in the movement of the load carrier in the direction shown by arrow 110.

Alternatively,thesupportstructure 112 maybe designed to support the track 101, and thus the tracking surface 102, at another angle that would result in a gravity deceleration vector being imposed on the load carrier 106.

In a semicinductorclean room environment, the gravity-propelled track 101 may be located proximate to a loading/unloading platform orworkstation. Also, such gravity-propelled track may be positioned adjacent to a magnetically-propelled horizontal track section of the type described in FIG. 1, or between two of such horizontal track sections. Furthermore, such a gravity-propelled track may be located proximate to a frictionless elevator system 5 of the type described in FIGS. SAto 5C, which elevator may load or unload the carrier 106 on or offthe track 101. In accordance with this embodiment, gravity propulsion ofthe load carrier is implemented by tilting the track section.The coupling of a magnetically-propelled horizontal track section subsequent to a gravity-propelled tilted section enables a controlled deceleration ofthe load carrier once it enters the horizontal track section.

Similarly, the coupling of a magnetically-propelled horizontal track section prior to a gravity-propelled tilted section enables a controlled acceleration of the load carrier as it leaves the horizontal track section. In essence, the electromagnetic propulsion in the horizontal track section can "lock on" to the load carrier 106 thereby decelerating it or accelerating it in a well controlled fashion over a predetermined travel distance.

Referring nowto FIG. wherein a curved section of a track 120 is shown coupled to two lineartrack sections 121 and 122. The latter may be, for example, ofthetype described above in connection with FIGS.

1 to 4 or of the type described in FIG. 7. The design and dimensioning of the curvedtracksection 120 must be such that a load carrier or pallet 123 travelling on the linear section 121 enters the curved section 120 and remains thereon until it couples to the linear section 122. The lineartrack sections 121 and 122, as shown, have each a plurality of pairs of selectively energiz able pole members, of which few pairs 124,125,126 and 127 are shown.Each one of the pole members of pairs 124,125,126 and 127 comprises a pole cartridge 128,129,131 and 132, respectively, with a corres ponding energizing coil (notshown).The pole cartridges 128,129, and 131,132 are mounted in the linear track sections 121 and 122 such that their respective top portions are flush with linear tracking surfaces 133 and 1 34 of the sections 121 and 122, respectively. Each one of the pole cartridges 128, 129, 131 and 132 is surrounded bya non-ferrous ring 136, 137,138 and 139 with air slots formed therebetween as described above in connection with the embodi ment of FIGS. 1 two 4.

The curved track section 120, comprises a plurality of pairs of selectively energizable pole members such as 141,142 and 143 capable of generating a plurality of mag netic ci rcu its. Since the load carrier 123 comprises a plurality offerromagnetic bodies 144 positioned substantially along its lateral direction, these ferromagnetic bodies 144 provide a plurality of transversal flux return paths forthe foregoing magnetic circuits. The first requirement imposed on the design of the curved track section 120 is that its radius of curvature, r, be selected such that the ferromagnetic bodies 144 ofthe load carrier 123 can still act as transversal flux paths for the pole members such as 141,142 and 143.A second requirement imposed on the design of the curved track section 120 isthatthe magneticfields generated bythe pole members 141 to 143 be sufficiently strong to prevent the load carrier123from stopping inthe curve portion ofthe track.

Moreover, the distance between consecutive pairs of pole members (e.g., 141 and 142142 and 143) along arcuate axis 146 ofthe curved section 120 must besuchthatmagneticcoupling exists between the pole members and the ferromagnetic bodies 144. The track sections 120 to 122 have a width along the lateral direction of the order of 4 inches for example.

In this case the radius of curvature r, of the 90 curved tracksection 120 is preferably of the order of at least two feetthus allowing the load carrier 123 to travel continuously along the curve portion ofthetrack.

Also, the pitch, PT, ofthe pairs of pole members 141 to 143 is ofthe orderofthe pitch ofthe pairs of pole members 124,125 and 126,127. In the example, PT is of the order of 1.5 inches.

In orderto generate magneticfields of sufficient magnitude to insure continuous movementofthe load carrier 123 on the track section 120, each one of the pole members pairs (e.g., comprisesa smaller size pole cartridge 147 surrounded by a non-ferrous ring 1 48. The outer diameter of the non-ferrous ring 148 in the curved track section 120 is of the same magnitude as that of the non-ferrous rings 136 to 139 in the lineartrack sections 121 and 122. However,the pole cartridge 147 has a diameter of the order of one halfthe diameter of the pole cartridges 128,129,131 and 132 of the lineartrack sections 121 and 122.The smaller size pole cartridge 147 results in an increased flux density in the gap between the tracking surface of the section 120 and the load carrier 123 by about a factor offour. This increased flux density is to insure the movement of the carrier 123 in lightofthesomewhatdistorted geometryofthe resultantvariable reluctance "curved" stepping motor configuration.

Shown in FIG. 9 is a turntabletrack section 150 capable of selectively switching a load carrier 151 between various track sections 152,153 and 154 of a frictionless transport system. Each one of the track sections 152 to 154 comprises a plurality of pairs of selectively energizable pole members 156 positioned at a pitch distance PTfrom each other as described above in connection with FIGS. 4 and 8. The load carrier 151, which is adapted to support a wafer cassette 157 capable of holding a plurality of semi conductorwafers 158, comprises a pair of elongated magnetic bumpers 159 and 161 positioned at both endsthereof.Two pairs of magnetic guidance mag nets 162 and 163 positioned in the load carrier 151 substantially atthefour corners thereof, are capable of magnetically coupling with corresponding ferromagnetic rails (notshown) positioned within the track sections 152, 153 and 154. The load carrier 151 further comprisessafetyguidance legs 164,166,167and 168 attached to its two sides, these safety guidance legs being ofthe same type and shape as legs 27,28 of FIG.1 and 28,29 of FIGS.2 and 3.

The turntable 150 positioned atthe intersection of the track sections 152,153 and 154 comprises a circular cylindrical platform 171 capable of rotating in a clockwise or in a counterclockwise direction as illustrated by bidirectional arrow 172. The rotation of the platform 171 may be accomplished by means of any well known rotary actuator mechanism. The cylindrical platform 171 comprises a plurality of pairs of selectively energizable pole members 173 positioned therein in a direction substantiallyperpendiculay to its top surface. The top surface of the platform 171 is leveled with the respective tracking surfaces of track sections 152,153 and 154 in order to achieve load carrierswitching therebetween.Also, in order to selectively control the position ofthe load carrier 151 on the platform 171, the pairs of pole members 173 therein are positioned at the same distance, PT, from each otherthan the pole members 156 ofthetrack sections 152 to 154. Similarly, a source of pressurized air(notshown) is coupledtothe cylindrical platform 171 for generating an airflow around each one of the pole members 173. Therefore, the turntable 150 makes use of the above-described principles of magnetic propulsion and air suspension. Moreover, lateral guidance ofthe load carrier 151 on the top surface of the platform 171 is achieved by a pair of ferromagneticrailsl74and 176 positionedwithin the platform 171.The ferromagnetic rails 174and 176 are adapted to be magnetically coupled to their corresponding guidance magnets 162 and 163 of the load carrier 151. Thus,the above-described magnetic guidance principles are also applied to the platform 171.

The top surface of platform 171 being at the same level as the tracking surfaces ofthe track sections 152, 1153 and 154, care should be taken thatthe safety guidance legs 164,166,167 and 168 ofthe load carrier 151 do not collide with the platform 171. In orderto avoid such a collision, two substantially parallel grooves 177 and 178 are formed in the top surface of the platform 171 for respectively receiving and guiding the safety guidance legs 164,166 and 167, 1 while the load carrier 171 is on the turntable 150.

Thewidth and depth of each one of the grooves 1 77 and 178 are selected to be somewhat largerthan the Thickness and height of the safety guidance legs, respectively, in order normally to avoid any contact between the guidance legs 164,166,167,168and the platform 171.

The operation of the turntable track section of FIG. 9 is as follows: if the load carrier 151 is to travel from the track section 152 to the track section 153 through the turntable ISO, the latter is positioned as shown in the drawing and its pairs of energizable pole mem bers 173 are sequentially energized as if they were part of the pole members 156 of the track sections 152 and 153. In other words, the platform 171 of the turntable 150 is comparable to a linearfrictionless track portion interposed between the track sections 152 and 153.

On the other hand, if the load carrier 151 is to be transported from the track section 152 to the track section 154 via the turntable 150, the load carrier 151 is first moved from thetracksection 152 to the platform 171. Next, the load carrier 151 is stopped on the platform 171 by deenergizing all its pole members 173. At this point in time, the load carrier 151 is supported on an aircushionformed betweenthetop surface of the platform 171 and the bottom surface of the carrier 151. Then, the cylindrical platform 171, with the load carrier 151 on its top surface, is rotated bya predetermined angle until its pole members 173 become aligned with the pole members 156 ofthe track section 154.The pole members 173 are then sequentially energized to move the load carrier 151 out ofthe turntable section 150 onto the track section 154. As mentioned above in connection with the embodiment of FIG. 8, track sections 152,153 and 154 would typically have a width of the order of four inches, and a pole member pitch PTof the order of 1 .5 inches. The turntable 150, as shown, has six pairs of energizable pole members 173 also positioned at a pitch distance PT ofthe order of 1.5 inches, and has an outer diameter of the orderof 9 inches.

In accordance with a preferred operation ofthe turntable, priorto rotating the cylindrical platform 171 with the load carrier 151 thereon, the pressurized air supplied to the turntable 150 is shut-offthereby placing the load carrier 151 in direct contact with the top surface of the platform 171. The foregoing prevents any oscillations or movements ofthe load carrier 151 while the platform 171 is being rotated, and in turn minimizes wafer jostling during the platform rotation step.

Claims (47)

1. Africtionless transport system comprising: an elongated track defining atracking surface; means for moving a load carrier in a first direction along said tracking surface; meansforforming a fluid cushion between said tracking surface and said load carrier; and magnetic guidance means coupled to said load carrierfor providing lateral guidance to said load carrier in a second direction substantially perpendicularto said first direction.

2. Africtionless transport system according to claim 1, wherein the means for moving the load carrier is a magnetic propulsion arrangement comprising: a pluralityofselectively energizableelectromagnetic pole members positioned within said elongated track on a direction substantially perpendicularto said tracking surface; and a plurality of ferromagnetic bodies positioned within said load carrier substantially along said second direction for providing a plurality oftransver- sal flux return paths to said pole members.

3. Africtionless transport system according to claim 2, wherein said meansforforming the fluid cushion comprise means for generating a flow of pressurized fluid in a direction substantially perpendicularto said tracking surface.

4. A frictionless transport system according to claim 3, wherein said pressurized fluid generating meansarecoupledtosaid elongated track and comprise a plurality of air slots located in the tracking surface.

5. Africtionless transport system according to claim 4, wherein the air slots are located along the periphery of each one of said plurality of electromagnetic pole members.

6. Africtionlesstransportsystem according to claim 1, wherein said magnetic guidance means comprise at least one pair of guidance magnets embeddedwithinsaid loadcarrierand positioned along said second direction at a distance ofthe order of the width of said track.

7. Africtionless transport system according to claim 6, wherein the magnetic guidance means further comprise a pair of guidance bars positioned within said elongated track along said first direction and magnetically coupled to said at least one pair of guidance magnets.

8. Africtionlesstransportsystem according to claim 1, wherein the means for moving the load carrier comprise: magnetic means positioned proximate to said tracking surface and selectively movable along said first direction within said elongated track; and ferromagnetic members embedded within said load carrier and adapted to be magnetically coupled to said magnetic means.

9. A friction less transport system according to claim 8, wherein said magnetic means comprise at least one pair of permanent magnets positioned at a distance along said second direction substantially matching the width of said track.

10. Africtionless transport system according to claim 8, wherein the magnetic guidance means include a pair of guidance magnets selectively movable along said first direction within said elongated track.

11. A frictionless transport system according to claim 10, wherein the magnetic guidance means further include ferromagnetic members centrally located within the load carrier along said first direction and magnetically coupled to said pair of guidance magnets.

12. Africtionless transport system according to claim 8, wherein said pressurized fluid generating means are coupled to said load carrier.

13. Africtionless transport system according to claim 1, wherein the means for moving the load carrier comprise a track support structure for posi tioning a first section of the elongated track at a predeterm angle with respect to a reference horizontal plane.

14. Africtionless transport system according to claim 13, wherein the means for moving the load carrierfurthercomprisea magnetic propulsion arrangement including: a plurality of pairs of selectively energizable electromagnetic pole members positioned within a second section ofthe elongated track in a direction substantially perpendicularto said tracking surface; and a plurality offerromagnetic bodies positioned within said load carrier substantially along said second direction for providing a plurality oftransver- sal flux return paths to said pole members.

15. Africtionless transport system according to claim 13, wherein the predetermined angle ofthe first section ofthe elongated track is selected to impart a gravity acceleration vector to the load carrier.

16. Africtionless transport system according to claim 13, wherein the predetermined angle of the first section ofthe elongated track is selected to impart a gravity deceleration vector to the load carrier.

17. Africtionless transport system substantially as any of the embodiments hereinbefore described with reference to the accompanying drawings.

18. A frictionless wafertransport system comprising: an elongated plenum chamber having an outerwall defining a tracking surface; magnetic propulsion means located within said chamberfor moving a wafer cassette carrier in a longitudinal direction along said tracking surface; means for generating a flow of pressurized fluid in a direction substantially perpendicular to said outer wall thereby forming a fluid cushion between the tracking surface and said cassette carrier; and magnetic guidance means located within said plenum chamber and within said cassette carrier for providing guidance to said cassette carrier in a lateral direction substantially perpendicularto said longitudinal direction.

19. Africtionless wafertransport system according to claim 17, wherein the magnetic propulsion means comprise: a plurality of pairs of selectively energizable electromagnetic pole members positioned within said elongated plenum chamber in a direction substantially perpendicularto said outer wall; and a plurality of ferromagnetic bodies positioned within said wafer cassette carrier substantially along said lateral direction for providing a plurality of transversal flux return paths to said pole members.

20. Africtionless wafer transport system according to claim 18, wherein said pressurized fluid generating means are coupled to said elongated plenum chamber and comprise a plurality of air slots located in the outerwall thereof.

21. Africtionless wafer transport system according to claim 19, wherein the air slots are located along the periphery of each one of said plurality of electromagnetic pole members.

22. Africtionless wafertransport system according to claim 17, wherein said magnetic guidance means comprise at least one pair of guidance magnets embedded within said wafer cassette carrier and positioned along said lateral direction at a distance of the order of the width of said tracking surface.

23. Africtionless wafer transport system according to claim 21,wherein the magnetic guidance means further comprise a pair of guidance bars positioned within said elongated plenum chamber along said longitudinal direction and magnetically coupled to said at least one pair of guidance magnets.

24. Africtionless wafertransport system accord ing to claim 17, wherein the magnetic propulsion means comprise: magnetic means positioned proximate to said outerwall and selectively movable along said longitudinal direction within said elongated plenum chamber; and ferromagnetic members embedded within said wafer cassette carrier and adapted to be magnetically coupled to said magnetic means

25. Africtionless wafer transport system according to claim 23, wherein said magnetic means comprise at least one pair of permanent magnets positioned along said lateral direction at a distance of the order of the width of said tracking surface.

26. A frictionless wafertransport system according to claim 23, wherein the magnetic guidance means include a pair of guidance magnets-selectively movable along said longitudinal direction within said elongated plenum chamber.

27. Africtionless wafertransport system according to claim 25, wherein the magnetic guidance meanscomprisea pair of ferromagnetic strips centrally located within the wafer cassette carrier along said longitudinal direction and magnetically coupled to said pair of guidance magnets.

28. Africtionless wafertransport system according to claim 23, wherein said pressurized fluid generating means are coupled to said wafer cassette carrier.

29. Atrackforafrictionlesswafertransport system comprising an elongated plenum chamber hav;ng a bottom wall, a pair of substantially parallel sidewalls and atopwall defining atrackingsurface, said top and bottom walls having a plurality of pairs of adjacent apertures, each aperture being dimensioned to hold a selectively energizable electromagnetic pole memberwithin the plenum chamber in a direction substantially perpendicularto said top and bottom walls.

30. Atrackfor a frictionless wafertransport system according to claim 28, wherein each aperture of said top wall comprises a plurality of air slots formed around the periphery ofthe selectively energizable electromagnetic pole member.

31. Atrackfor a friction less wafertransport system according to claim 29 further comprising a pair of ferromagnetic guiding rails positioned within the plenum chamber and extending along the entire length thereof, said guiding rails being located at the corner intersections of the top wall and the two sidewalls.

32. Atrackfor a frictionless wafertransport system according to claim 28, wherein the elongated plenum chamber comprises: a lineartrack section including a plurality of pairs of first electromagnetic pole members havinca first diameter and located within the lirteartrack section at a predetermined pitch distance, PT,from each other; and a curve track section, coupled to said Fineartrack section, including a plurality of pairs ofseconcl electromagnetic pole members having diameter smallerthan said first diameter, said pairs of second pole members being locateciwithinthe curve track section at said pitch distance Prfrom each other.

33. Atu rntabletrack section for africtionless wafer transport system cam prising: a rotatable cylindrical platform having a plurality of pairs of selectively energizable electromagnetic pole members positioned therein in a direction substan tiallyperpendicularto itstopsurface; first and second elongatedferromagneticguidance rails positioned within said cylindrical platform at a distance from each other of the order of a track width; and first and second guidance grooves located in the top surface of the platform at a distance from each otherlargerthanthetrackwidth.

34. Aturntable track section according to claim 32, furthercomprising means coupled to said rotat able cylindrical platform for generating a flow of pressurized air in a direction substantially perpen diculartoftstopsurface.

35. A load carrier for a frictionless wafertransport system comprising: a sled having two parallel longitudinalsides,two parallel lateral sides, a top surface adapted to receive a wafer cassette, anda a flat-bottomed tracking surface adapted totravelonatrack; a plurality offerromagnetic bodies embedded within the sled in a direction substantially parallel to its lateral sides; magnetic guidance means embedded withinthe sled and positioned proximate to the tracking surface ofthe sled substantially at the intersections between the longitudinal and the lateral sides thereof; elongated magnetic means attached to each one of the lateral sides of the sled; and a pair of safety guidance means coupled to each one of the longitudinal sides of the sled.

36. A load carrier accordingto claim 34, wherein the ferromagnetic bodies are spaced from each other a predetermined pitch distance, Pc, given by: N Pc=PT (--r N+1 where PT is the pitch between consecutive pairs of electromagnetic pole members of a linearstepping motor positioned within the track; and N isthe number of phases of the motor.

37. A load carrier according to claim 34, wherein the magnetic guidance means comprise a U-shaped permanent magnet substantially located at the four corners of the sled.

38. A load carrier according to claim 34, wherein the safety guidance means include L-shaped safety legs attached to the longitudinal sides ofthe sled and extending outwardly therefrom.

39. A load carrier according to claim 34, wherein the dimension ofthe lateral sides of the sled is ofthe order of the width of the track.

40. Amethodforfrictionlesslytransporting an article along a track comprising the steps of: forming a fluid cushion between a tracking surface ofsaidtrackand said article; moving the article in a first direction along said track; and magnetically guiding the article in a second direction substantially perpendicularto said first direction.

41. A method according to claim 39, wherein the moving step comprises the steps of: positioning a plurality of pairs of electromagnetic polememberswithinsaidtrackina direction substantially perpendicularto said tracking surface; positioning a plurality offerromagnetic bodies within an article carrier substantially along said second direction for providing a plurality of transversal flux return paths to said pole members; and sequentially energizing each pair of said plurality of pairs of electromagnetic pole members.

42. A method according to claim 40, wherein said forming step comprises the step of generating a flow of pressurized fluid in a direction substantially perpendicularto said tracking surface.

43. A method according to claim 41, wherein said pressurized fluid generating step comprises forming a plurality of air jets in the tracking surface along the periphery of each one of said plurality of electromagnetic pole members.

44. A method according to claim 39, wherein said magnetically guiding step comprisesthe step of positioning at least one pair of guidance magnets in said article carrier at a distance along said second direction ofthe order of the width of said track.

45. A method according to claim 43, further comprising: locating a pairofguidance bars within said track along said first direction; and magnetically coupling said bars to said at least one pair of guidance magnets.

46. A method according to claim 39, wherein the moving step comprises the steps of: selectively moving along said first direction within saidtracka magnetic carriage; and magnetically coupling an article carrierto said magneticcarriage.

47. Amethod according to claim 39,wherein the moving step comprises supporting the track at a predetermined angle with respect to a reference horizontal planethereby imparting a gravityvectorto the article.