Patent Application
20080193061
The invention relates to an air bearing.
Air bearings for vibration isolation are known. Thus, the European patent Application “Isolator Geometry of a vibration isolation system” with the disclosure number EP 1 803 965 (Application number) 06 026 425.1 shows an air bearing with increased bearing force. The disclosure content of this document is integrated by reference.
Air bearings of this type have a mechanical coupling element between a vertical piston and y piston plate or a load respectively. By doing so, it is achieved that indeed vertical forces are transferred to the piston plate, but horizontal forces essentially not. Also, changes of the parallelism of the piston plate with respect to the base plate are not transferred, which avoids the dumping of the isolator pistons into each other.
Without taking into consideration isolator-internal structural resonances, known vibration isolation systems according to the principle of the air bearing are designed for a frequency range up to approximately 100 Hz (max. 300 Hz). In the course of the further increasing resolution and precision of lithography devices, steppers and immersion systems, however, the frequency characteristics above 300 Hz become more and more important. Therefore, it is important to optimize the frequency behaviour of the air bearing itself to keep the isolation effect also above 300 Hz. Besides other effects, which are relevant with air bearings according to the state of the art, the mechanical coupling element is of decisive importance. On the one hand, the weight to be supported bears on it, on the other hand, it is the only element between the load to be supported and the isolator. The element being screwed in the piston plate (above) may be neglected in comparison to the lower element being screwed in the vertical piston of the air bearing, because it is considerably smaller. The lower element (called pin) offers a mechanical stiffness, which depends on the material in use, the length of the pin, the thread thickness of the screw connection and the supporting area of the screw connection. In case of known air bearings, it is designed in such a manner that it has a stability being sufficient for the case of static load, but is particularly not designed for dynamic stiffnesses and resonances.
With known air bearings, a deflection of the pin may be effected, which, together with the piston and the air cushion to the horizontal bearing, may set up a vibratory system, whose eigenfrequencies are mostly above 300 Hz. The result is a worsened isolation effect in frequency ranges above 300 Hz.
It became clear that this effect may only conditionally be reduced by changing the system parameter “mass bearing on” and “air cushion to the horizontal bearing”.
In contrast, it is an object of the invention to at least reduce the aforementioned disadvantages.
Particularly, it is an object of the invention to design a pin in such a manner that it achieves the mechanical stiffness for the static load and at the same time has a dynamic stiffness such that the eigenresonances of the system “pin+piston” are minimized.
The object of the invention is already achieved by the air bearing according to one the independent claims.
Accordingly, a vertically and horizontally effective air bearing is provided, which has a base plate, an interface part, which is movably arranged on the base plate, a piston, which is movably arranged inside the interface part and supports a load, which is vibration isolated supported in horizontal and vertical directions with respect to the base plate, and a coupling element, which connects the piston with the load, wherein the coupling element preferably comprises a pin, which is connected with the piston, particularly screwed into the piston.
According to the invention, the coupling element, which connects the piston with the load, minimally has a distance to the load of one fourth, preferably one half of the piston length.
Therefore, this embodiment of the invention is another approach, with which not the resonance is shifted into frequency ranges, which are irrelevant for the whole system, but with which the forces transferred at the coupling element are shifted in their plane of effect in such a manner that the piston does not longer tend to dumping and therefore does not longer forms a vibratory mechanical impedance.
This succeeds in a surprisingly easy manner thus that the coupling element is designed such that the connection of piston and load has a distance to the load of at least one fourth of the piston length.
Therefore, the connection preferably established via a pin or a screw connection is rearranged downwards in the piston, so that the tendency of the piston to dumb is removed.
With a further embodiment of the invention, the coupling element provides an extension ranging from above through the piston and is preferably essentially pyramid-shaped or cone-shaped formed, which coupling element is connected with the piston. Preferably, the diameter of the extension is reduced in the direction to the base plate.
Under “preferably pyramid-shaped or cone-shaped”, also extensions are understood, which do not form a complete cone, but for example consist of single segments being at a distance from each other.
The diameter of the extension, which diameter preferably extends in the direction to the base, does not mandatory refer to an extension with circular cross section, but the extension may also have a polygon-shaped, preferably a quadratic cross section.
With a further preferred embodiment of the invention, a further extension is attached at the load, which extension is connected with the coupling element.
This further extension preferably runs inside the first extension and is reduced also in the direction to the base. An alternative embodiment of the invention also relates to vertically and horizontally effective air bearing with a base plate, an interface part, which is movably arranged on the base plate, and a piston, which is movably arranged inside the interface part and supports a load, which is vibration isolated supported in horizontal and vertical directions with respect to the base plate. Thus, the coupling element connects the piston with the load, wherein the coupling element has a pin, connected to the piston, preferably screwed into the piston.
The inventors have found that by selecting a pin with a length to diameter ratio below 0.7, preferably below 0.5 and in particular preferably below 0.2, resonance-caused decreases of the isolation effect are extensively avoided in frequency ranges below 300 Hz.
In this context, the ratio “mass bearing on” to the stiffness of the pin seems to be decisive for the expected structural resonance of the whole system. The smaller this ratio is, the less structural resonances are to be expected and so that the better assembled is the isolator in view of future requirements.
Alternatively or in combination, a pin is used, which has a diameter of minimally 30 mm, preferably minimally 35 mm and in particular preferably 40 mm.
The pin has a length of below 18 mm, preferably of below 10 mm and in particular preferably of below 6 mm.
Furthermore, the pin preferably has a thread, wherein the ratio of the diameter of the thread to the length of the piston is larger than 0.8, preferably greater than 1.5 and in particular preferably greater than 3. Particularly, the thread has a diameter of above 10 mm, preferably of above 15 mm and in particular preferably of above 20 mm.
For increasing the stiffness, the pin is preferably made of steel, titanium or a corresponding alloy.
In the following, the invention shall be described in more detail by means of the drawings
Furthermore, the pin 6 has a thread 9 for screwing the pin in the piston (3 in
For that purpose, the air bearing has a first extension 10, which is connected, here screwed, with the piston 3 at the upper side and essentially extends till the base 12 of the air bearing 1.
In this embodiment, the first extension 10 is essentially formed as a hollow cylinder.
A further extension 11 is attached to the load, which extension extends downwards inside the first extension 10.
Thus, the touching point between the first 10 and the second extension 11 and therefore between the load 4 and the piston 3, which point is finally defined by the coupling element formed between lower the lower pin 6 and the upper pin 7, is shifted downwards in the direction to the base part 12.
At the top, the further extension 11 is fixedly screwed with the load.
By shifting the touching point between the lower pin 6 and the upper pin 7, the tendency to dumping of the piston 3 is reduced in such a manner that disturbing eigenfrequencies are extensively eliminated.
In this embodiment, the coupling element 5 is made up of a lower 6 and an upper pin 7, which are screwed with the first extension 10 and with the second extension 11 by means of a thread (not shown) and a screw nut 13.
The first and/or the second pin 5,6 may be made of elastic material to reduce the passage of vibrations.
It shall be understood that the invention is not limited to a combination of the feature described above, but that the person skilled in the art will combine all features with each other as far as it is meaningful.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 007 909.7 | Feb 2007 | DE | national |
