A supporting device and a lithographic device of the kinds mentioned in the opening paragraphs are known from WO-A-96/38766. The known lithographic device is used in the manufacture of integrated semiconductor circuits by means of an optical lithographic process. The radiation source in the known lithographic device is a light source, while the focusing unit is an optical lens system by means of which a partial pattern of an integrated semiconductor circuit, which is present on a mask which can be placed on the mask holder, is imaged on a reduced scale on a semiconductor substrate which can be placed on the substrate holder. Such a semiconductor substrate comprises a large number of fields on which identical semiconductor circuits are provided. The individual fields are for this purpose exposed consecutively through the mask, a next field of the semiconductor substrate being brought into position with respect to the focusing unit by means of the positioning device each time between two consecutive exposure steps. This process is repeated a number of times, with a different mask comprising a different partial pattern each time, so that integrated semiconductor circuits with comparatively complicated structures can be manufactured. Since these structures have detail dimensions which lie in the sub-micron range, the partial patterns present on the consecutive masks should be imaged on the fields of the semiconductor substrate with an accuracy which lies in the sub-micron range. The substrate holder and the mask holder should accordingly occupy accurate positions relative to the focusing unit during operation.
The frame of the known lithographic device is supported in a vertical direction by means of three supporting devices of the known kind, the first parts of the supporting devices being fastened to the base of the lithographic device, and the second parts of the supporting devices being fastened to the frame. The frame of the known lithographic device supports not only the focusing unit but also the mask holder and the substrate holder in the vertical direction, while the base is placeable on a floor. Since the substrate holder and the mask holder must occupy accurate positions relative to the focusing unit during operation, vibrations of the frame must be prevented as much as possible during operation. Such vibrations of the frame may be caused by vibrations in the base which are caused, for example, by floor vibrations. In the known lithographic device, vibrations in the base are in addition caused by reaction forces of the positioning device of the substrate holder and reaction forces of a further positioning device by means of which the mask holder is displaceable relative to the focusing unit. The three supporting devices accordingly serve not only for supporting the frame in the vertical direction, but also for preventing a transmission of vibrations from the base to the frame.
The pressure chamber of the gas spring of the known supporting device is bounded by a cylindrical vessel and by an annular membrane by means of which said vessel is suspended in the pressure chamber of the gas spring. The vessel is fastened to the second part by means of three comparatively thin tension rods which are positioned in the vessel and which extend parallel to the support direction. To prevent a transmission of vibrations from the base to the frame as much as possible, a mass spring system formed by the supporting devices and the frame together with the components of the lithographic device supported thereby must have natural frequencies which are as low as possible in directions parallel to the support direction and perpendicular to the support direction. The pressure chamber of the gas spring of the known supporting device has a comparatively large volume so as to provide the lowest possible natural frequency for the mass spring system mentioned above parallel to the support direction. The tension rods mentioned above have a comparatively great length so as to provide the lowest possible natural frequency of said mass spring system perpendicular to the support direction.
A disadvantage of the known supporting device is that the membrane used therein and bounding the pressure chamber of the gas spring has a comparatively great stiffness, whereby the natural frequency of said mass spring system parallel to the support direction is raised, and vibrations of the first part directed parallel to the support direction are partly transmitted to the second part. In addition, the membrane exerts a damping force on the second part during operation, whereby the transmission of vibrations having a comparatively high frequency from the first part to the second part is enhanced. Furthermore, an admissible length of the tension rods used in the known supporting device is limited by an admissible constructional height which the supporting device has parallel to the support direction. A minimum achievable value of the natural frequency of said mass spring system perpendicular to the support direction is also limited thereby, so that also vibrations of the first part directed perpendicular to the support direction are partly transmitted to the second part.
USPTO Backgrounds
USPTO Backgrounds