Positioning device

Abstract: A positioning device comprising a table (5, 69), which is displaceable in two coordinate directions (X, Y) by means of Lorentz forces of electric linear motors over a base (3) and is guided over the base (3) by means of a static gas bearing, which has an air gap thickness independent of the coil systems and magnet systems (15-21, 37-43) in the motors. The positioning device is particularly suitable for the manufacture of masks with patterns for integrated circuits.

Description: The invention relates to a positioning device comprising a table, which is displaceable in at least two coordinate directions (X, Y) by means of at least one electric linear X-motor and at least one electric linear Y-motor and is supported without contact by a static gas bearing on a base, coil systems and magnet systems corresponding to the two motors being connected to the base and to the table, the static gas bearing being arranged so as to have an air gap thickness which is substantially independent of temperature variations of the coil systems. A positioning device of the kind mentioned in the opening paragraph, which is known from US-A-4,507,597, comprises two C-shaped magnetic circuits disposed in spaced relationship with respect to each other. A structural component carrying an object table is arranged so as to be movable between the parallel legs of each magnetic circuit in X- and Y-directions parallel to said legs, one element of each magnetic circuit being fixedly attached to the structural component, whereby the structural component is movable under the influence of the interactive forces between the magnetic circuits and said elements. In a preferred embodiment of the known positioning device, said elements are guided by air bearings mounted between said elements and the lower plates of the C-shaped magnetic circuits, electric coil systems being provided on the upper plates of the C-shaped magnetic circuits. In this way the structural component is movable in the X- and Y-directions substantially without friction between the parallel legs of the magnetic circuits, the gas bearings having an air gap thickness which is substantially independent of fluctuations of the dimensions of the coil systems caused by temperature variations of the coil systems during operation of the device. A disadvantage of the known positioning device is that the forces acting on the structural component in the X- and Y-directions are not only the result of the interaction between the magnetic fluxes in the magnetic circuits and the electric currents to be supplied to the coils, but are also the result of the magnetic attraction between said elements of the structural component and the transverse legs of the magnetic circuits. Consequently, during operation a current has to be supplied to the coils for equilibrating the forces due to said magnetic attraction when the structural component is to be maintained in a steady position, and an additional current has to be supplied to the coils when the structural component is to be moved in the X- and/or Y-direction, so that high electrical resistance losses are caused in the coils. The invention has for its object to provide a positioning device, in which the disadvantages described above are avoided. The positioning device of the afore-mentioned type is for this purpose characterized in that during operation the coil systems and magnet systems are guided with respect to each other and are coupled to each other in the two coordinate directions (X, Y) solely by Lorentz forces produced by the coil systems and magnet systems. It should be noted that the term "Lorentz force" is to be understood to mean the force which is the vectorial product of the magnetic induction and the current strength Since the coil systems and magnet systems are coupled solely by Lorentz forces in the two coordinate directions, no forces are exerted on the table in the absence of a current in the coils, so that a steady position of the table can be maintained without a current in the coil and, therefore, without electrical resistance losses in the coil. A special embodiment of the positioning device is characterized in that each of the magnet systems comprises two permanent magnets magnetized in opposite senses parallel to a Z-coordinate direction which extends perpendicular to the base, and that each of the coil systems comprises two coils wound in opposite senses and situated opposite to the magnets of the magnet systems. In this embodiment, a comparatively short magnetic circuit of each of the linear motors is obtained constituted by the iron-cobalt extending between the magnets of a pair of magnets, the permanent magnets and the cores of the coil systems. A particular embodiment of the positioning device comprising a very stiff static gas bearing is further characterized in that at least a part of the base is magnetically conducting and forms a bridge between the permanent magnets of each of the magnet systems mounted to an air foot of the static gas bearing, a pre-stress in the static gas bearing being also determined by magnetic attraction between the permanent magnets in the air foot and the magnetically conducting part of the base. A further embodiment of the positioning device having a comparatively high tilting stiffness of the table is characterized in that a pair of coil systems and magnet systems is associated with each of the two coordinate directions. A still further embodiment of the positioning device is characterized in that the coil systems are in thermal contact with a cooler common to all coil systems on their sides remote from the magnet systems. The cooler common to all coil systems permits of obtaining a simple construction and a uniform temperature distribution of the positioning device. Another embodiment of the positioning device is characterized in that the cooler comprises cooling-pipe turns located in a plane parallel to an X-Y plane and having a rectangular cross-section. The comparatively large contact surface of the cooler with the coil systems obtained due to a flat shape of the cooler results in an efficient cooling of the positioning device. In should be noted that SPIE Vol. 1088 "Optical Laser Microlithography" II (1989) p. 428 and US-A-4,506,204 disclose a positioning device, in which the table is journalled on the base by means of four air feet having a particular construction. In each of the four air feet, a so-called "voice-coil-type" actuator or electrodynamic actuator is integrated for tilting the table. The said electrodynamic actuator in each air foot results not only in that the journalling becomes comparatively soft so that the air gap thickness can be kept only with difficulty within the range of optimal bearing stiffness. An embodiment of the invention will be described more fully with reference to the drawing, in which: Fig. 1 is a perspective view of a part of the positioning device according to the invention, Fig. 2 is a plan view of the positioning device embodying the invention, Fig. 3 is a sectional view taken on the line III-III in Fig.2, Fig. 4 is a side elevation of the positioning device shown in Fig. 1. The positioning device 1 shown in Figures 1 to 4 comprises a horizontally arranged flat base 3 of a magnetically conducting iron-cobalt alloy, over which a round foot 5 is guided. In operation, the foot 5 is supported on the base 3 by means of a static gas bearing (aerostatic bearing) of a usual kind known from European Patent Application EP-A1-0244012. As appears from Figures 1 and 2, four corner stays 7, 9, 11 and 13 are secured on the base 3, of which the last-mentioned stay is not shown in Fig. 1. In the corner stays engagenment surfaces are formed for four coil systems 15, 17, 19 and 21, of which in Fig. 1 the coil system 15 is entirely visible and the coil system 19 is visible by half, while in Fig. 2 only the coil systems 15, 17, 19 and 21 are indicated by dotted lines. The coil systems 15-21 are supported via their coil cores on the said engagement surfaces. Of these engagement surfaces, Fig. 1 only shows the engagement surfaces 23 and 25 of the coil systems 17 and 21 as well as the engagement surface 27 of the coil system 19. In the coil system 19, a coil core 29 of the coil system 19 supported on the engagement surface 27 is only just visible. Each of the coil systems 15-21 comprises two coils wound in opposite senses, such as the coils 31 and 33 of the coil system 15. Of the coil system 19, only a coil 35 is visible in Fig. 1. Opposite to and below each coil system, a magnet system is situated, which is secured to the air foot 5 and forms together with the relevant coil system an electric linear motor for the displacement in the X- and Y coordinate directions. As appears from Fig. 2, magnet systems 37, 39, 41 and 43 are arranged opposite to the coil systems 15, 17, 19 and 21, respectively, in the air foot 5. Each of the magnet systems 37-43 comprises two permanent magnets magnetized parallel to the Z-coordinate direction in opposite senses. The pairs of permanent magnets (45, 47), (49, 51), and (53, 55) and (57, 59) are associated with the magnet systems 37, 39, 41 and 43, respectively. In Fig. 1, of these pairs of permanent magnets the magnet 59 of the coil system 21, the magnets 49 and 51 of the magnet system 39 and the magnet 53 of the magnet system 41 are visible. The magnetic circuit of each of the linear motors described is constituted by the iron-cobalt extending between the magnets of a pair of magnets, the permanent magnets and the cores of the coil systems. The thickness of the air gap in the magnetic circuit between the coil systems and the permanent magnets is fully determined by the thickness of the air gap between the base 3 and the air foot 5 in the aerostatic bearing. Due to the fact that the magnetic shortcircuit between the permanent magnets in a magnet system is located in the iron-cobalt of the base 3, a magnetic pre-stress of the aerostatic bearing is obtained. This pre-stress is chosen so that the stiffness of the aerostatic bearing is optimal. On behalf of the static gas bearing, the air foot 5 is provided with ducts, which extend in radial direction. In Figures 1 and 3, a duct 61 and a duct 63 are visible. Via transverse ducts 65 and 67 connected to the ducts 61 and 63 and merging into the air gap between the air foot 5 and the base 3, the air in the aerostatic bearing reaches the bearing areas at which an accumulation of pressure is built up. The pre-stress of the aerostatic bearing is also determined by the weight of the air foot 5. Further, by means of a vacuum the central part of the air foot 5 can supply an additional pre-stress. The central part of the air foot constituted by a cylinder 69 must then be sealed from the outer part of the air foot, at which the bearing is situated. The cylinder 69 serves as a table in the positioning device. Via the cylinder 69, the ducts 61 and 63 are also connected to an air source. On the sides remote from the magnet systems, the coil systems 15, 17, 19 and 21 are in thermal contact with a cooler 71, which is common to all coil systems (see Figures 2 and 3). The cooler 71 is constituted by cooling-pipe turns having a rectangular cross-section. The turns are located in a plane parallel to the X-Y plane. Thus, a largest possible thermally conducting surface is obtained. The collection of turns may be replaced by a single flat box, in which liquid flow conductors are provided. The required power of the liquid pump is higher in the case of the cooler having a helically wound cooling-pipe because of the higher flow resistance, but the direction of the flow is more accurately defined under given operating conditions than in the case of a single flat box. Since the coil systems are fixedly arranged, the liquid connections of the cooler can also occupy a fixed position and are protected from a varying load occurring in liquid-cooled moving coil systems. When energizing the coil systems 17 and 19 of the linear electric motors for the displacement of the air foot 5 and the cylinder 69 parallel to the X-Y coordinate direction with currents of the same polarity and value, a pure translation of the air foot 5 is obtained. In an analogous manner, this is also the case for the Y-coordinate direction. In the neutral position of the table 69 shown in Figures 1 and 2, a centre M of the table lies on the Z-axis of the coordinate system. A rotation  z of the table 69 about the Z-axis without translations of the centre M can be obtained in that either the two linear electric motors of the X coordinate direction are energized with currents of equal value and of opposite polarity or in that the linear motors of the Y-coordinate direction are energized in an analogous manner. For a rotation  z also the four linear motors can be energized. Translations and rotations of the centre M may also be performed simultaneously. The positioning device described is particularly suitable for the manufacture of masks used in the production of integrated circuits. These masks must be subjected during the manufacture to one or more exposures according to a very accurate pattern. These masks are then used in an opto-lithographic device for exposing semiconductor wafers. Also in such an opto-lithographic device (a so-called wafer stepper), use may be made of the positioning device described above. Further, the positioning device may be used inter alia for the manufacture of holographic rasters, for writing patterns into integrated optical components and for examining patterns already manufactured.

Claims: A positioning device comprising a table (69), which is displaceable in at least two coordinate directions (X, Y) by means of at least one electric linear X-motor (15, 37) and at least one electric linear Y-motor (17, 39) and is supported without contact by a static gas bearing (5, 61, 63, 65, 67) on a base (3), coil systems (15, 17) and magnet systems (37, 39) corresponding to the two motors (15, 37; 17, 39) being connected to the base (3) and to the table (69), the static gas bearing (5, 61, 63, 65, 67) being arranged so as to have an air gap thickness which is substantially independent of temperature variations of the coil systems (15, 17), characterized in that during operation the coil systems (15, 17) and magnet systems (37, 39) are guided with respect to each other and are coupled to each other in the two coordinate directions (X, Y) solely by Lorentz forces produced by the coil systems (15, 17) and magnet systems (37, 39). A positioning device as claimed in Claim 1, characterized in that each of the magnet systems (37, 39) comprises two permanent magnets (45, 47; 49, 51) magnetized in opposite senses parallel to a Z-coordinate direction which extends perpendicular to the base (3), and that each of the coil systems (15, 17) comprises two coils (31, 33) wound in opposite senses and situated opposite to the magnets (45, 47; 49, 51) of the magnet systems (37, 39). A positioning device as claimed in Claim 2, characterized in that at least a part of the base (3) is magnetically conducting and forms a bridge between the permanent magnets (45, 47; 49, 51) of each of the magnet systems (37, 39) mounted to an air foot (5) of the static gas bearing (5, 61, 63, 65, 67), a pre-stress in the static gas bearing (5, 61, 63, 65, 67) being also determined by magnetic attraction between the permanent magnets (45, 47; 49, 51) in the air foot (5) and the magnetically conducting part of the base (3). A positioning device as claimed in Claim 1, 2 or 3, characterized in that a pair of coil systems (15, 19; 17, 21) and magnet systems (37, 41; 39, 43) is associated with each of the two coordinate directions. A positioning device as claimed in Claim 4, characterized in that the coil systems (15, 17, 19, 21) are in thermal contact with a cooler (71) common to all coil systems (15, 17, 19, 21) on their sides remote from the magnet systems (37, 39, 41, 43). A positioning device as claimed in Claim 5, characterized in that the cooler (71) comprises cooling-pipe turns located in a plane parallel to an X-Y plane and having a rectangular cross-section.

Inventors: BOUWER ADRIANUS GERARDUS, VAN ENGELEN GERARD, BOUWER, ADRIANUS GERARDUS, VAN ENGELEN, GERARD
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1990
Main classification: B23Q1
IPC codes: H02K41, B23Q1, H02K5, F16C32, G05D3, F16C37