Patent Document

FIELD OF THE INVENTION 
     This disclosure is directed to the cooling of linear motors. More particularly the disclosure relates to a structure for cooling the coil assembly of a linear motor and preventing heating of the surrounding environment. 
     BACKGROUND 
     Excessive heating of the coils of a linear motor causes an increase in the resistance of the coils, exacerbating the heat problem and reducing the performance of the motor. In addition, this heat is carried away to the outside air and often to the rest of the machine in which the motor is utilized. Heat changes the index of refraction of air and consequently reduces the accuracy of laser interferometers and other optical systems. In addition, the heat causes thermal expansion of machine components, resulting in inaccuracy of precision mechanical systems. 
     Most commercially available linear motors are not actively cooled. Typically the coils are potted in a moderately conductive epoxy and the motor is cooled through convection into the surrounding air. Trilogy Systems provides an option to their motor where cooling fluid is circulated through a metal mounting bracket of the coil assembly. Because this bracket is mounted only to the top of the motor, not all of the heat is carried away from the motor and a significant portion of it is still convected into the surrounding environment. 
     U.S. Pat. No. 4,749,921 issued to Anwar Chitayat describes a linear motor. Included is a concept for cooling the linear motor coils. FIG. 8 in this patent shows a system of hollow tubes that are potted with the coil assembly. Coolant can flow through these tubes providing cooling. In U.S. Pat. No. 4,625,132 also issued to Anwar Chitayat, a controlled flow of cooling gas is directed between the motor stator and the moving element with flexible seals on each arm of a U-shaped channel mount a wound stator. In another Chitayat U.S. Pat. No. 4,839,545 an armature of a linear motor is cooled by coolant flowing through a lower serpentine channel in thermal contact with laminations of the motor armature. 
     U.S. Pat. No. 4,906,878 discloses a linear motor with cross-flow passageways or tubes connecting between inlet and outlet manifolds to remove heat from the motor coils. U.S. Pat. No. 4,916,340 utilizes heat insulating materials with a cooling medium (water) flowing through passageways on coil supporting members. U.S. Pat. No. 5,073,734 discloses a coolant for cooling between linear motor spacers and a screen support having cooling fins. 
     Yaskawa Japan Laid Open Application Heisei 8-168229 provides a linear motor that is enclosed in a stainless steel can (housing). This can has a small gap along the outside of the coils, which enables (not disclosed) coolant to be forced along the gap between the can and the coils to provide cooling of the motor. Yaskawa Utility Model Application Heisei 5-45102 includes a coil bobbin with a cooling path inside the bobbin. 
     Typical linear motors that are not cooled have inefficient motor operation due to increased coil resistance with temperature, heating of surrounding air, and heating of surrounding machine elements as discussed above. Motors that are only cooled through the mounting bracket do not provide direct cooling of the coils and suffer from the same disadvantages. The cooled motor of U.S. Pat. No. 4,749,921 and others of the above patents require cooling passages to be created within the coil assembly. This is difficult and can typically only be done by wrapping the coils with tubing and encapsulating the assembly in an epoxy. It also does not completely isolate the motor from the outside air because the cooling tubes do not completely enclose the coils. The Yaskawa disclosures include cooling arrangements which cannot be completely adapted to all motor configurations. In addition, both rely on an exterior thermal insulation or an exterior can (13 and 29, respectively) that may be difficult to fabricate. In both Yaskawa disclosures the cooling is on the inside of the bobbin or inside the can; coil heat may be transferred directly from the coil outer surfaces to the outside environment resulting in detriment to the machine in which the motor is being utilized. 
     SUMMARY 
     This disclosure is directed to novel cooling structures for linear motors. In accordance with some embodiments, no extra cooling tubes or components are needed within the coil assembly itself and cooling is accomplished by flowing coolant in a passage or a space between the surfaces of the coils and the coil enclosures. This prevents heat from the coils from reaching, for example, nearby interferometer or other optical systems, where the heat can change the index of refraction of air and reduce the accuracy of such systems or cause thermal expansion of machine components with resultant inaccuracies of the precision mechanical systems. 
     Typically in a lithographic (e.g. stepper) machine used in the processing of semiconductors wafers and the like, as many as eight linear motors are used to drive positioning elements (such as the reticle stage and wafer stage) of the stepper. This multiplicity of motors obviously compounds the problem of detrimental heat from the individual linear motors. In most applications, motors are cooled to prevent the motor from overheating and the coolant transfers the motor heat to the environment. 
     In the case of lithographic machines, the problem as recognized by the present inventors is not motor overheating but preventing the motor heat from reaching the environment and thereby adversely affecting the machine&#39;s interferometry systems. Hence here the motor heat is confined to the coil and coolant so that it is not transferred to the motor coil housing. Thus direct thermal contact between the motor coils and their housing is minimized. 
     One embodiment solves these problems in a band coil arrangement by providing integrally cast recesses forming cooling channels in a cast encapsulant block partially surrounding the coil assembly, along with closure members affixed over the cooling channels. An overall linear armature of a required substantial length with a minimal transverse thickness results. The structure allows coolant flow parallel to the length of the coil assembly along the height and length of the exterior surfaces of the coil assembly, the flow being between the coil assembly and the conforming coil enclosure, thus preventing heat from the coils escaping into the surrounding air. 
     While in one embodiment the coil assembly is partially encapsulated in e.g. a cast rigid epoxy which has a relatively low thermal conductance and a minimal thickness to provide a short heat path to the flowing coolant, preferably no such epoxy is present between most of the actual coil surfaces and the coolant. In order to prevent short-circuiting of the flowing coolant in the cast recesses, in one embodiment an integrally cast longitudinal spacer is provided extending over part of the length of the recesses in the cast encapsulant forming the coolant channels. The spacers terminate short of the ends of the recesses so that a cast divided annular channel is formed on each of opposite surfaces of the encapsulated coil assembly. Sealing of the channel is provided by a sheet metal or plastic closure member which with the ends of the assembly form an enclosure around the assembly of coils. The member may be adhered to or otherwise connected to the encapsulation block surrounding the respective recesses. The insulation block also contains a coolant inlet and plenum with coolant bores directing coolant to one end of the sealed recesses and a coolant outlet and plenum with coolant bores directing pumped coolant from the other end of the recesses to the coolant outlet. 
     In another embodiment called a centerpole cooling arrangement, physical insulating spacers are placed between each coil and extending between the outside enclosure (can or housing), and an inside enclosure, with a gap therebetween. Cooling fluid flows along the outside of the coils and through a gap between the coils and the inner enclosure and between the coils and inner surface of the outer enclosure. 
     The centerpole motor cooling arrangement includes a coil assembly with a thin e.g. metal enclosure on both the inside and the outside. The coils are separated slightly by insulating spacers along the axis of travel. The insulating spacers are slightly larger than the coils and create a small gap between the coil and the outside and inside enclosures. Cooling fluid can then be pumped or forced into an inlet plenum in the top of the enclosure and along the top of the coils. As the fluid flows along this channel the fluid runs along the coils and down the small channels along the sides of the coils. The fluid collects in an outlet plenum and flows out of the enclosure. This cooling approach is advantageous because the outside environment is affected only by the heat of the cooling fluid as transmitted through the enclosure. The much hotter coils are in all locations insulated from the outside by the cooling fluid. Thus, provided the flow rate of the cooling fluid can carry away the generated heat without excessive temperature rise, the heat transfer rate from the coils is unimportant, provided that the temperature of any coil does not rise to the point of causing damage. 
     A double layer coil cooling arrangement uses an array of coils that are stacked such that internal cooling passages exist between the coils. In this arrangement, pockets exist in the center of each coil. The coils are configured to allow for small passages between adjacent pockets and the conforming enclosure, thus coolant can flow from one pocket to the next along the length of the coil assembly. The coil assembly is sandwiched between thin e.g. metal or plastic sheets, creating an enclosure around the coils. Thus, coolant can be introduced at one end of the coil assembly into an inlet plenum, carry heat away from each of the coils along the length of the assembly, and exit at the other end of the assembly from an outlet plenum. 
     In a dogbone motor coil cooling arrangement an enclosure for the coil has the shape of a conforming “dogbone”. A small gap is defined between the coils and the enclosure inside walls, along the sides, top, and bottom, using insulating spacers. Cooling fluid can then be introduced at the inlet plenum end of the coil assembly inside the enclosure, and will flow along the outside of the coils in the gap created by the spacers to an outlet plenum. This fluid will carry away the heat generated by the coils. As in other of the embodiments the coolant flows between the coils and the surrounding environment, thus preventing heat from the coils from escaping. 
     Hence in one embodiment a cooling structure and method for a coil of a linear motor include an enclosure member conforming to at least one side wall of the motor coil. Coolant passages are provided between the enclosure member and the motor coil sidewall. Inlet and outlet connections are provided to flow a fluid coolant through the coolant passages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic top view of a stage for a stepper (lithographic) machine with the linear motor driving the stage. 
     FIG. 1B is a side view of an entire lithographic apparatus which may use the FIG. 1A structure. 
     FIG. 2 is a schematic end view of the FIG. 1A structure. 
     FIG. 3 is a cross-sectional view of the band coil embodiment of the linear motor. 
     FIG. 4 is a perspective view of the encapsulated coil assembly thereof. 
     FIG. 5 is a perspective partial view of the coolant inlet and outlet and coolant bores of the coolant loop broken away to show the encapsulated coils. 
     FIG. 6A is a schematic partially cutaway perspective view of the centerpole embodiment; 
     FIG. 6B is a side cutaway view of the same structure. 
     FIG. 7 is a cross-sectional side view thereof taken on the line  7 — 7  of FIG.  6 A. 
     FIG. 8 is a cross-sectional view taken on the line  8 — 8  of FIG.  7 . 
     FIG. 9 is a schematic perspective view of the double layer of coils embodiment less an enclosure therearound. 
     FIG. 10 is a top view thereof. 
     FIG. 11 is a longitudinal cross-sectional view of the coil assembly taken on the line  11 — 11  of FIG.  10 . 
     FIG. 12 is a magnified cross-sectional view of the circled portion A of FIG.  11 . 
     FIG. 13 is a magnified plan view of the gap in the circled portion B of FIG.  12 . 
     FIG. 14 is a schematic side view of the overall double layer embodiment including the exterior outer enclosure. 
     FIG. 15 is a schematic perspective of the dog-bone-shaped coil embodiment less the outer enclosure. 
     FIG. 16 is a cross-sectional view taken on the line  16 — 16  of FIG. 15 also showing in cross-section the outer enclosure and the motor magnet track. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1A schematically shows a top view of a stepper chamber  14  in a lithographic measuring and processing system  10  (e.g., a stepper machine). The lithographic system  10  includes a stage  12  (e.g., a wafer stage) and an apparatus floor support surface  21 . A yoke  18  and two linear guideways  19  are mounted on the floor support surface  21 . The stage  12  has a mirror system  9  and a wafer table  22  mounting a wafer on the upper surface thereon, and has a linear motor coil assembly  11  driving the stage  12  on the under surface. In order to measure the position of the stage  12 , the interferometry measurement system  26  emits a laser beam  20  incident on the mirror system  9 . As is known in the interferometry measurement art, the beam is split through a 45° beam splitter in system  26 . One beam is reflected off a fixed reference mirror in system  26  and the other beam is reflected off mirror system  9  attached to the wafer stage. These beams are then recombined and a sensor in system  26  detects changes in the position of the wafer stage mirror  9 . While FIG. 1A illustrates a wafer stage, the present invention is also applicable to reticle stages and other applications of linear motors. 
     FIG. 1B is taken from Nishi U.S. Pat. No. 5,473,410 FIG. 3, issued Dec. 5, 1995, incorporated herein by reference in its entirety. This illustrates a projection exposure (lithographic) apparatus in which the FIG. 1A structure may be used as an x-y wafer stage or as the reticle stage (with suitable design adaptations apparent to one skilled in the art). 
     The follow description of present FIG. 1B follows that of FIG. 3 of U.S. Pat. No. 5,473,410, except that the reference numbers, instead of being identical to those of U.S. Pat. No. 5,473,410 FIG. 3, each have the letter “a” appended thereto for convenience of reference. 
     In FIG. 1B a projection lens PL is a projection optical system. Exposure illumination light emitted from a mercury lamp  2   a  is condensed at a second focal point through an elliptical mirror  4   a . Disposed at the second focal point is a rotary shutter  6   a . This operates with the aid of a motor  8   a . The exposure illumination light passing through shutter  6   a  is reflected by a mirror  10   a . The illumination light beam is incident on a fly eye lens system  14   a  via an input lens  12   a . The illumination light falls on a lens system (condenser lens)  18   a  via a beam splitter  16   a . Removable blades BL 1 , BL 2 , etc. at the blind mechanism  20   a  are individually independently moved by a driving system  22   a . (This blind system is not required in such lithographic machines, however.) A reticle R is illuminated with the illumination light via a lens system  24   a , a mirror  26   a  and a main condenser lens  28   a . 
     The reticle R undergoes the illumination light defined by the aperture, defined by the blades and is held on the reticle stage  30   a  moving at least in the x direction on a column  32   a . The reticle stage  30   a  is moved by driving system  34   a . A movable mirror  36   a  reflects a length measuring beam emitted from a laser interferometer  38   a  is fixed to one end of the reticle stage  30   a . A fixed mirror  40   a  for the laser interferometer  38   a  which is fixed to an upper edge of the lens barrel of the projection lens PL. Wafer W is held, with a fiducial mark FM, by a wafer holder  44   a , capable of making microscopic rotation. The holder  44   a  is installed on a z stage  46   a  capable of effecting a micromotion in the z (vertical) direction. 
     The z stage  46   a  is installed on an x-y stage  48   a  moving two dimensionally in the x and y directions. Stage  48   a  is driven by a driving system  54   a . Yawing and coordinates of x-y stage  48   a  are measured by a laser interferometer  50   a . A fixed mirror  42   a  for laser interferometer  50   a  is fixed to a lower edge of the lens barrel of projection lens PL. A movable mirror  52   a  is fixed to one edge of the z stage  46   a . There is an alignment system  60   a , using through the reticle alignment, for detecting the alignment mark (or fiducial mark fm) on wafer W. There is also an alignment system  62   a , using through the lens alignment, for detecting the alignment mark or fiducial mark FM on the wafer W through the projection lens. 
     Photoelectric sensor  64   a  receives light from a luminescent mark (when the fiducial mark FM is luminescent) via the projection lens PL, the reticle R, the condenser lens  28   a , the lens systems  24   a ,  18   a  and the beam splitter  16   a . This determines the position of the reticle R. Sequence and controlling this system is performed by main control unit  100   a . This controls the structures shown connected thereto by the depicted lines terminating in arrows. 
     FIG. 2 shows the side-face of the stage  12  of FIG.  1 A. The stage  12  is supported by e.g. air bearings  15  on the linear guide  19 , typically a smooth flat granite surface. Roller bearings or magnetic bearings may also be utilized. Coil magnets or permanent magnets  17  are mounted on inwardly-facing surfaces  16  of the yoke  18 . The magnets  17  on one side of the coil assembly  11  and those on the other side of coil  11  are aligned so the magnets  17  produce a strong magnetic field between them. Magnets  17  and yoke  18  collectively are a magnet track. A housing block  28  contains a coolant inlet and outlet. 
     Referring to FIGS. 3-5 which show the band coil embodiment, the linear motor  60  includes a yoke  18  which mounts a pair of spaced permanent magnets  17 . A coil assembly  11  passes linearly through the magnets  17  with a gap  90  therebetween. Magnets  17  typically are neodymium iron boron (NdFeB) magnets. The overall armature  61  (FIG. 4) includes coils  62 , such as band coils which are coils formed e.g. by insulated wire and which are supported by a cast structure  63 . In one embodiment, eight coils are so supported, the coils being electrically connected in series. Typically, the cast support is a cast epoxy resin such as CB-1054A available from Dolph Co. of Monmouth Junction, N.J. Other casting resins may be employed. The casting resin may contain heat transmitting metal powders such as Alumina (assuming the coils are electrically insulated). 
     The resultant cast support  63  thus forms an elongated parallelogrammatic block  65 . The block  65  is preferably such that a maximum amount of the coil&#39;s outer surface is not covered by the epoxy, but is in direct contact with the coolant. Cast in the block  65  are elongated recesses or slots  66  and  68  which extend along the outwardly facing opposite sides  67  and  69 , respectively, of the coils  62 . The recesses have a width and length substantially the same as the width and length of the banded coils  62 . An integrally cast spacer  70  extends longitudinally of the recesses with the ends  71 ,  72  of the spacer terminating short of the ends  73  of the recesses, thus forming a divided pathway  74  having a first channel portion  75  connected to a parallel second channel portion  76 . The spacer also functions as a central structural support since it is abutted and bonded to by a closure member  48 . The spacer prevents “ballooning out” of the closure member which can affect the clearance provided by gap  90 . Closure members  48  are in the form of rectangular thin sheets which with the assembly ends  65   a  and  65   b  form an outside enclosure (can) around the embedded coils. The thickness of the sheets may be from about 0.4 mm to about 2 mm. 
     The sheets may be metal, such as titanium or non-magnetic stainless steel, plastic such as nylon, or ceramic such as alumina. The sheets  48  are bonded to block  65  by an adhesive such as an epoxy adhesive (not shown) available from 3M Co. of Minneapolis, Minn., or otherwise connected to the block  65  so that the recesses are sealed at their peripheries. The interior surfaces of the closure members face the gap  90  between the overall armature  61  and the magnets  17 . The block  65  has a T-configuration including a vertical portion  80  and a horizontal top cross-piece  81 . A coolant inlet port (not shown) is provided in cross-piece  81  forming part of an inlet plenum  47  and a coolant outlet port (not shown) forming part of an outlet plenum  45  is provided in vertical portion  80  with a connecting outlet port in cross piece  81 . Four electrical terminals  78  provide connections to the coils. 
     In one embodiment, the recesses have a depth  8  of from about 0.4 mm to about 2 mm. The coolant is e.g. Fluorinert coolant FC-77 available from the 3M Co. Typically a pumped coolant flow rate of about 3 liters/minute is employed with a 1° C. temperature rise being able to carry away about 90 watts of heat from the coils. A thermoelectric cooler in the coolant circulation system removes heat from the coolant. 
     FIGS. 6A-8 illustrate the centerpole motor cooling arrangement which includes a coil assembly  30  with a thin metal enclosure (can)  33  on the inside of the assembly and a second enclosure structure  34  on the outside thereof. The thin inside enclosure  33  is typically of non-magnetic stainless steel and has an oval configuration as seen in FIG.  7 . FIG. 6A shows a cutaway perspective view; FIG. 6B shows a corresponding side view, but showing the full length of the structure FIG. 7 is a cross-sectional view along line  7 — 7  of FIG.  6 A and FIG. 8 is a cross-sectional view along line  8 — 8  of FIG.  7 . The outer enclosure  34  includes the side edges of block  35  and thin e.g. metal plates  34   a , typically non-magnetic stainless steel sheets abutting and welded to the side edges of block  35  and extending laterally and vertically spaced from the exterior side walls of a series of coils  31  aligned side-by-side. Structures  33 ,  34 ,  35  are e.g. one piece or a welded assembly. 
     The individual coils  31   a  through  31   e  are separated by a series of insulating spacers  32   a  through  32   f  which have inner ends abutting the inner enclosure  33  and outer ends abutting the metal side plates  34 . The spacers are constructed of a phenolic or other thermally insulating material and have dimensions so as to create a series of gaps  39   a  between the exterior of the inner enclosure  33  and the interior walls of the coils  31  and gaps  39   b  between the interior wall of outer enclosure structure  34  and the exterior walls of the coils  31 . The arrows represent the flow of pumped coolant fluid from a plenum inlet  38  in a top-piece  36  to an outlet  37  of the assembly  30 . Both elements  37 ,  38  are e.g. tapped holes into which respective screw-in pipe fittings  37   b ,  38   b  (FIG. 6B) fit. The coolant fluid flows outwardly and downwardly along the top of coils  31  through both the gaps  39   a  and  39   b  to draw off heat generated by the operation of the coils. Spacing between the coils and the cans is e.g. 1 to 2 mm. The coolant fluid with its acquired heat out flows through outlet  37 . Thus the environment outside the outer enclosure structure is only exposed to the heat of the cooling fluid as transmitted through the outer enclosure structure. The much hotter coils  31  are at all locations insulated from the outside environment by the cooling fluid. The flow rate of the cooling fluid is chosen so that the cooling fluid carries away the coils-generated heat without an excessive, i.e. not more than about 1° C., temperature rise in the coolant fluid. 
     FIGS. 9-14 illustrate the double layer coil cooling arrangement which includes a coil assembly  40  having layers  41  and  42  of coils,  41   a  through  41   f  and  42   a  through  42   f , respectively, the layer  42  being staggered with respect to layer  41 . The respective coils are stacked such that internal cooling passages exist between the coils. Pockets (gaps) exist in the center of each coil so there is a serpentine coolant flow  48  under a coil  41  and over a coil  42  as seen in FIG.  12 . The coils are configured to allow for a small gap, e.g. about 1 mm, between adjacent coils so that coolant fluid can flow from one pocket to the next pocket along the length of the coil assembly  40 . The coil assembly is sandwiched between thin e.g. metal or plastic sheets  47  creating an outer enclosure  49 . FIG. 13 illustrates chamfered edges  46   a  and  46   b  of the coils, which together form one of the gaps  46 , namely between two of the stacked and staggered coils  42   b  and  41   c . At each end of the assembly  40  are generally rectangular spacers  43  and  44  which also serve as an inlet plenum and an outlet plenum, respectively, by defining suitable passages. 
     FIGS. 15 and 16 illustrate the dog-bone shaped motor coil embodiment where a coil assembly  100  includes bent-ended coils  91  and  92 , which when placed in an abutted side-by-side relationship, together form coils having, in one section of the coil, a profile approximating a dog-bone shape. Elongated longitudinal spacers  93  are adhesively affixed to transverse parallel portions of the exterior of both sides of the coil assembly. A conforming dog-bone shaped enclosure  94  surrounds the coil assembly with the spacers forming coolant channels  95  between the exterior surfaces of the coils  91 ,  92  and the inner surfaces of enclosure  94 . The width of the channels  95  is determined by the thickness of the spacers  93 . 
     The dog-bone enclosure is welded to the inner edges of a top-piece  96  having cooling fluid inlet  97  and an outlet  98  which permits coolant flow and a thickness of coolant liquid between the exterior of the coils and the interior of the enclosure  94 . The environment outside the enclosure  94  is only exposed to the heat from the flowing coolant. FIG. 16 also shows the associated magnet track  98  (similar to that shown in FIG. 3) The arrows  99  in FIG. 15 show the flow of coolant across the exterior surfaces of one side of the coils  91 ,  92 . Flow of coolant is also across the exterior surface of the coils on the opposite side of the assembly. 
     The above is intended to be illustrative and not limiting. Other embodiments and modifications will be obvious to those skilled in the art in view of the above disclosure and are intended to fall within the scope of the appended claims.

Technology Category: y