Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – With magneto-mechanical motive device
Reexamination Certificate
2003-11-12
2004-09-14
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
With magneto-mechanical motive device
C335S223000, C335S282000
Reexamination Certificate
active
06791443
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to actuators, such as Lorentz actuators, and also to velocity transducers.
The invention also relates to a lithographic projection apparatus, and more particularly to a lithographic projection apparatus that has a Lorentz actuator connected to a mask table or a substrate table of the lithographic projection apparatus.
2. Discussion of Related Art
Lorentz actuators comprise a permanent magnet, which produces a magnetic field, and a current element positioned in the magnetic field. They work on the same principle as an electric motor, namely that charge carriers moving through a magnetic field experience a force mutually perpendicular to their velocity and the magnetic field, known as the Lorentz force. The force is given by J×B, where J is the current vector resulting from the velocity of the charge carriers and B is the magnetic field vector. This Lorentz force is used to induce motion or to provide a bias force between the moving parts of the actuator.
Lithographic projection apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatus, each die is irradiated by progressively scanning the projection beam over the reticle pattern, and thus scanning a corresponding image onto the die; such an apparatus is referred to as a step-and-scan apparatus. Both of these types of apparatus require highly accurate relative positioning of the mask and substrate tables, which is generally accomplished with the aid of at least one Lorentz actuator. More information with regard to these devices can be gleaned from International Patent Application WO 97/33204.
Up to very recently, apparatuses of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatuses is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial alignment measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine
In currently available lithographic devices, the employed radiation is generally ultra-violet (UV) light, which can be derived from an excimer laser or mercury lamp, for example; many such devices use UV light having a wavelength of 365 nm or 248 nm. However, the rapidly developing electronics industry continually demands lithographic devices which can achieve ever-higher resolutions, and this is forcing the industry toward even shorter-wavelength radiation, particularly UV light with a wavelength of 193 nm or 157 nm. Beyond this point there are several possible scenarios, including the use of extreme UV light (EUV: wavelength~50 nm and less, e.g. 13.4 nm or 11 nm), X-rays, ion beams or electron beams.
One problem with Lorentz actuators is that, when no current flows, there is no force between the moving parts. When a current is caused to flow to overcome this, it results in dissipation of heat in the device. This is particularly a problem in applications which require the actuator to deliver a bias force, e.g. to support the weight of a component under gravity. With this continuous need to compensate for weight, a base power dissipation is unavoidable, and can cause problems with heat sensitive apparatuses, such as optical devices which require accurate alignment; on the other hand, it necessitates the provision of additional cooling power.
Another problem is that, when such actuators support a load in order to act as isolation bearings, the stiffness of the bearing should be low so as to avoid the transmission of vibrations. Conventionally, it has been difficult to provide such low-stiffness isolation bearings.
Velocity transducers can also operate on the Lorentz principle, by virtue of the fact that the motion of a component through a magnetic field induces a current flow or a resulting EMF which can be measured. In order to measure velocities down to very low frequencies, it is necessary to have a transducer with a very low frequency of resonance, which conventionally has been difficult to achieve. This is because of the problems in producing a transducer with a very low stiffness.
SUMMARY OF THE INVENTION
It is an object of the present invention to alleviate, at least partially, some of the above problems.
Accordingly, the present invention provides a device comprising:
a first member comprising at least one main magnet, and
a second member comprising at least one current element for carrying an electric current, for electromagnetic interaction with said main magnet,
characterized in that said second member further comprises an auxiliary magnetic member which interacts with the magnetic field of said main magnet to produce a bias force between said first and second members.
The invention also relates to a lithographic projection apparatus comprising a radiation system for supplying a projection beam of radiation; a mask table provided with a mask holder for holding a mask; a substrate table provided with a substrate holder for holding a substrate; a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate; and further comprising a Lorentz actuator connected to at least one of the mask table and the substrate table.
The auxiliary magnetic member can be a permanent magnet. Alternatively, it can comprise a ferromagnetic material (e.g. a soft-iron member). In this latter case, as long as the stroke of movement of the current element/auxiliary magnetic member is relatively small (as will generally be the case in applications in short-stroke lithography actuators, for example)—such that the auxiliary magnetic member remains biased to one side of the centerline of the whole assembly—magnetic fluxes going through the ferromagnetic material of the auxiliary magnetic member will produce a bias force component in the desired direction; while less than that produced in the case of a permanent magnetic material, this force will be quite sufficient for particular applications.
The device according to the invention can be substantially planar or cylindrical, and the main magnet can be magnetized perpendicular or parallel to the bias force.
Preferably, the device further comprises a third member, also comprising at least one further main magnet.
The current element may be a coil, and the auxiliary magnetic member is preferably located at a plane substantially centrally between two halves of the coil.
Advantageously, the effective stiffness of the device is 200 N/m or less in magnitude, and ideally close to zero.
The device can be used as an actuator and/or a velocity transducer.
The device can have two second members stiffly connected to each other and arranged such that opposite parasitic torques are generate
ASML Netherlands B.V.
Barrera Ramon M.
Pillsbury & Winthrop LLP
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