Patent Number: 
Section: description

It is evident from FIG. 1 that two drive units 1, 2 are connected by a ceramic profile that is part of a linear guidance system 3. The ceramic profile of linear guidance system 3 carries a nonmagnetic retention system 4 for a substrate, in this case e.g. for a wafer. Retention system 4 is arranged displaceably on linear guidance system 3. Linear guidance system 3, inclusive of retention system 4, is to be displaced by means of the two drive units 1, 2 in a direction parallel to the substrate surface, and guided and driven in all directions in zero-backlash fashion with little elastic resilience, as will be shown below. What is achieved according to the present invention is that retention system 4, by way of the two drive units 1, 2 that are spaced apart from one another by the length of linear guidance system 3 and are also magnetically shielded separately from one another, can be positioned in all six spatial degrees of freedom with high precision and dynamics in the vacuum chamber of a device that serves for exposure of the wafer and/or for measurement on the wafer by means of radiation. In the installed state, drive units 1, 2 can of course assume any position in the chamber, although a horizontal orientation is preferred. The direction of motion of retaining system 4 is in that case oriented vertically, i.e. in the direction of gravity, along linear guidance system 3. The ceramic profile of linear guidance system 3 serves as a guide element and can simultaneously receive drive elements that are necessary for triggering the displacement motion of retention system 4 (not depicted in the drawing). FIG. 2 indicates that the two drive units 1, 2 are embodied as linear motors, the air gap between stator 5, 6 and rotor 7, 8 being modifiable in each case. Located on rotors 7, 8 are magnetic bridges having permanent magnets, to compensate for the weight of the guided unit. They are configured in such a way that the electromagnets integrated into rotors 7, 8 must generate comparatively small forces for positional stabilization, and thermal loads are thus reduced. The two drive units 1, 2 are magnetically guided in each of four degrees of freedom. In the remaining two degrees of freedom, guidance is accomplished by the linear motors, in which no mechanical contact points exist between stator 5, 6 and rotor 7, 8. The highly dynamic fine adjustment motion is implemented within the range of motion by controlling the air gap of the magnetic guides in degrees of freedom Y, Z, RX, and RY, and by positioning the linear motors in degrees of freedom X, RZ. Measurement of the position of rotors 7, 8 is accomplished by means of two plane mirror interferometers 9, 10 operating independently of one another. Capacitative sensors (not depicted in the drawing), which are used together with plane mirror interferometers 9, 10 to measure the position of retention system 4, are additionally provided. Also provided for each drive unit are magnetic shielding walls 11 to protect the particle beam region from interfering magnetic fields; these are each embodied in multiple layers, the slots necessary for motion transfer being offset laterally from one another in the individual layers, thus creating a meander-shaped magnetic seal which makes possible a rigid connection between rotors 7, 8 and the zero-magnetic-field retention system 4. For the sake of clarity, in FIG. 2 magnetic shielding walls 11 are shown only on drive unit 2. In order to eliminate disruptive thermal expansion of the subassemblies of drive units 1, 2, and in particular also of the subassemblies of retention system 4, the frame-mounted coils of the linear motors, with their mount, are water-cooled. This mount and also the surfaces of rotors 7, 8 are moreover equipped with a suitable surface coating so that effective radiative cooling is implemented in order to dissipate the heat of the magnetic bearings (not depicted in the drawing). The linear motors and magnetic bearings are advantageously arranged outside the particle beam region, i.e. outside the region in which the radiation used for exposure and/or measurement travels, and are not mounted directly on retention system 4 for the substrate. They are arranged around this particle beam region, a symmetrical arrangement being preferred. It is also evident from FIG. 2 that retention system 4 is equipped with a wafer chuck 12 for receiving the wafer with the wafer surface oriented vertically. A stepping motor drive 13 allows (coarse) positioning over an adjustment range of approximately 320 mm in the vertical Y axis. Wafer chuck 12, on which the wafer is retained electrostatically, is fabricated with high precision from temperature-stable Zerodur. Machined laterally onto wafer chuck 12 are mirror surfaces that are used for ascertaining and monitoring the chuck position with a six-beam laser interferometer arrangement 14 (resolution: 0.6 nm) in all spatial degrees of freedom except the Z coordinate. The position of the wafer with respect to the Z coordinate is ascertained directly on the wafer surface with the aid of three highly accurate capacitative sensors (not depicted). The measured signals thereby obtained are also referred to hereinafter as xe2x80x9cglobalxe2x80x9d signals, since they represent the immediate position of the wafer to be exposed. Wafer chuck 12 is coupled in stress-free fashion onto a frame made of titanium profiles which is guided vertically with the aid of lubrication-free ceramic ball bearings along the ceramic profile of linear guidance system 3. The vertical motion for retention system 4 with wafer chuck 12 is coupled in, as already explained, via a Bowden cable driven by a fast stepping motor 13. With the aid of piezoactuators (not depicted), the frame can be clamped in any desired vertical Y coordinate in a range of xc2x1160 mm with an accuracy of approx. xc2x110 xcexcm. This yields repeatabilities in the range of a few xcexcm/xcexcrad in all other coordinates. Once the desired vertical position of the wafer is reached, the wire of the Bowden cable is detensioned to minimize its influence on drive units 1, 2. The electrodynamic direct drives or linear motors provided in drive units 1, 2 are magnetically guided and triply shielded (shielding walls 11). They make possible a highly accurate horizontal X motion of xc2x1160 mm, which is measured with the aid of the two plane mirror interferometers 9, 10, with a resolution of 5 nm, on the upper and lower linear motors. A controlled asynchronous movement of the two linear motors results in the RZ rotation. Each drive unit 1, 2 is equipped with a total of five electromagnetic actuators 1.1, 1.2, 1.3, 1.4, 1.5 and 2.1, 2.2, 2.3, 2.4, 2.5, of which four in each case (actuators 1.1, 1.2, 1.3, 1.4 and 2.1, 2.2, 2.3, 2.4) serve to implement adjustment motions in the Z direction, and one in each case (actuators 1.5 and 2.5) to implement adjustment motions in the Y direction. FIG. 3 depicts the arrangement of actuators 1.1, 1.2, 1.3, 1.4, and 1.5 in drive unit 1. Each of these actuators 1.1, 1.2, 1.3, 1.4, 1.5 and 2.1, 2.2, 2.3, 2.4, 2.5 possesses its own xe2x80x9clocalxe2x80x9d capacitative measurement system for highly accurate measurement of the air gap between stator and rotor, or between the working surface of the actuator and the guide surface on the stator, in a range of xc2x10.5 mm at a resolution of 20 nm. Motion in coordinates Y, Z, RY, and RX is made possible by influencing the width of the air gap in controlled fashion. Actuators 1.5 and 2.5 are in this case of hybrid design, i.e. they possess built-in permanent magnets that compensate without power dissipation for the predominant portion of the weight (approx. 50 kg) of the mass to be moved. If, in other embodiments of the invention, the Z coordinate rather than the Y coordinate should point in the direction of gravity, actuators 1.1, 1.2, 1.3, 1.4 and 2.1, 2.2, 2.3, 2.4 are designed accordingly. Taking into consideration the geometric data of the arrangement as indicated in FIG. 3 and the resolution of the measurement systems of the individual xe2x80x9clocalxe2x80x9d actuators 1.1, 1.2, 1.3, 1.4, 1.5 and 2.1, 2.2, 2.3, 2.4, 2.5, the following (theoretical) displacement ranges and positional resolutions are obtained in the individual coordinates: Xxc2x1160 mm (5 mn); Yxc2x1160 mm as total displacement range and xc2x10.5 mm as parallel shift (20 nm); Zxc2x10.5 mm (5 nm); RXxc2x10.4 mrad (16 nrad); RYxc2x14 mrad (160 nrad); RZxc2x15 mrad (6 nrad). The manner in which the adjustment motions are achieved in the degrees of freedom X, Y, Z, RX, RY, and RZ will be explained once again with reference to FIG. 4. The symbolically depicted linear guidance system 3; drive unit 1 having stator 5, rotor 7, and actuators 1.1, 1.2, 1.3, 1.4, 1.5; and drive unit 2 having stator 6, rotor 8, and actuators 2.1, 2.2, 2.3, 2.4, 2.5, are evident. Actuators 1.1, 1.2, 1.3, 1.4 are provided to modify the width measured in direction Z of the air gap between stator 5 and rotor 7 on drive unit 1, and actuators 2.1, 2.2, 2.3, 2.4 to modify the width measured in direction Z of the air gap between stator 6 and rotor 8 on drive unit 2. Actuator 1.5 on drive unit 1 and actuator 2.5 on drive unit 2 serve to modify the width of the air gap measured in direction Y. The adjustment motions are implemented as follows: Parallel displacement in coordinate X in one or the other direction by synchronous activation of the linear motors (rotor 7 in drive unit 1 and rotor 8 in drive unit 2); Parallel displacement in coordinate Y in one or the other direction by synchronous activation of actuators 1.5 and 2.5; Parallel displacement in coordinate Z in one or the other direction by synchronous activation of actuator pairs 1.1/1.2 and 1.3/1.4 and actuator pairs 2.1/2.2 and 2.3/2.4; Rotation RX about coordinate X by activation of actuator pairs 1.1/1.2 and 1.3/1.4 asynchronously with activation of actuator pairs 2.1/2.2 and 2.3/2.4 (and thus modification in opposite directions of the air gaps on the linear motors); Rotation RY about coordinate Y by activation of actuator pairs 1.1/1.2 and 2.1/2.2 asynchronously with activation of actuator pairs 1.3/1.4 and 2.3/2.4 (and thus modification in opposite directions of the air gaps within the two linear motors); Rotation RZ about coordinate Z by activation of the linear motor in drive units 1 asynchronously with activation of the linear motor in drive units 2. The arrangement selected has the following advantages: A self-contained unit can be moved with high precision in all six spatial degrees of freedom. It is magnetically guided and xe2x80x9cfloatsxe2x80x9d in three dimensions in almost noncontact fashion (aside from electrical supply lines and the influence of the Bowden cable), i.e. is largely free of friction and wear. The drives, representing potential interference field sources, are comparatively far away (more than half a meter) from the exposure location. The field proceeding from the drives can be further drastically reduced by suitable (in the present case, triple) shielding. A further region around the ion beam used for exposure is iron-free, thereby minimizing distortion of the exposure. As a result of the permanent-magnet-based weight compensation in the magnetic guidance system, the electromagnetic actuators of the guidance system can be operated with almost zero static current, resulting in low power conversion and thus little heating of the drives in vacuum. The coils in the direct drives for rapid and accurate horizontal positioning are statically mounted and therefore easy to cool. Large working air gaps in the magnetic guidance system are needed in order to ensure a sufficient movement range in the rotation axes, in particular RX. This results in lower resolution for the rotation axes with a smaller base spacing, in this case RY. Advantageously, drive units 1, 2 are each located in a housing made of steel. This steel housing is at the same time the first layer of the magnetic shielding; two further layers of mu metal are applied once drive units 1, 2 are completely installed and aligned. Each shield is equipped with a labyrinth seal for the magnetic interference field proceeding from drive units 1, 2 through which motion passes outward. Experiments in a shielding chamber have shown that with the three-layer shielding, it is possible to reduce the magnetic field proceeding from a drive to 10 nT (static) and 5 pT (dynamic) at the exposure location. The problem of outgassing and heating of the drive elements was also investigated. Aluminum foil equipped on all sides with an oxide coating is used as the coil material. The coil cores, also equipped with an oxide coating, largely prevent the formation of eddy currents and thus result in less heating and a short time constant for the coils. The heat created in the coils of the electrodynamic direct drives is dissipated at the ends of the coil cores through copper blocks having channels for a cooling fluid. These additionally impart a stable T-shape to the stator of the direct drive. A different approach was used for the electromagnets and their coils that are present in the actuators of the direct drives. To minimize the number of supply lines to the moving part, cooling lines were dispensed with here. The electromagnets were instead optimized for a low current/force ratio and a high force/mass ratio. As a result, the electromagnets of the Z guidance system achieve, at a force of 100 N and a 1-mm air gap, a power dissipation of only 3 W at a weight of 0.6 kg each, while the figures for the Y electromagnetsxe2x80x94more heavily loaded because their weight compensation is not quite completexe2x80x94are 1.4 kg and 1.3 W at 100 N and a 1-mm air gap. The aforesaid forces are needed, however, only with strong accelerations and usually at smaller air gaps (approx. 0.5 mm). Since the electromagnets are operated with almost zero static current (aside from small forces that must always be applied to compensate for torques and residual weight), the average power consumption is considerably lower, being in total approximately 0.5 W in the entire magnetic guidance system of a direct drive. The overtemperature in the immediate vicinity of the electromagnet coils that can be estimated therefrom is 3 K, decreasing to  less than 1 K in the immediate vicinity of the coils. Since both the actuator and the stator in the direct drive are equipped with a black aluminum oxide coating, the power consumed in the guidance systems is at least partially emitted as thermal radiation to the cooled stator. In summary, this example of a positioning system describes a magnetically guided, electromagnetically driven, high-precision vertical wafer stage that emits very low magnetic interference fields and is suitable for use in high vacuum. With this stage, despite a difficult system environment, positioning smoothness and accuracy values in the sub-micrometer or -xcexcrad range, and moreover particularly good synchronization of the wafer stage, are achieved.