1. Field of the Invention
The present invention relates to a lithographic apparatus, a projection assembly and a combination of a structure and an active damping assembly.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The high accuracy and high resolution currently aimed at in lithography require an accurate positioning of parts of the lithographic apparatus such as the reticle stage to hold the mask, the projection system and the substrate table to hold the substrate, with respect to each other. Apart from the positioning of e.g. the reticle stage and the substrate table, this also poses requirements on the projection system. The projection system in current implementations may consist of a carrying structure, such as a lens mount (in case of transmissive optics) or a mirror frame (in case of reflective optics) and a plurality of optical elements such as lens elements, mirrors, etc. In operation, the projection system may be subject to vibrations due to a plurality of causes. As an example, movements of parts in the lithographic apparatus may result in vibrations of a frame to which the projection system is attached, a movement of a stage such as the substrate stage or the reticle stage, or accelerations/decelerations thereof, which may result in a gas stream and/or turbulence and/or acoustic waves affecting the projection system. Such disturbances may result in vibrations of the projection system as a whole or of parts thereof. By such vibrations, displacements of lens elements or mirrors may be caused, which may in turn result in an imaging error, i.e. an error in the projection of the pattern on the substrate.
Commonly, a damping system is provided to dampen vibrations of the projection system or parts thereof. Thereto, a passive damping system may be provided as known in many forms, or a active damping system, or a combination of a passive and a active damping system. In this document, the term active damping system is to be understood as a damping system which comprises a sensor to detect an effect of a vibration (e.g. a position sensor, velocity sensor, acceleration sensor, etc) and a actuator to act on the structure to be damped or a part thereof, the actuator being driven by e.g. a controller in dependency of a signal provided by the sensor. By driving the actuator in dependency of the actuator, i.e. in response to a signal provided by the sensor, an effect of the vibration on the projection system or a part thereof, may be reduced or cancelled to a certain extent. An example of such active damping system may be provided by a feedback loop: the sensor to provide a position quantity (such as a position, speed, acceleration, jerk, etc of the projection system or a part thereof), the controller being provided with the position quantity and generating a controller output signal to drive the actuator, the actuator in turn acting on the projecting system or the part thereof so that a feedback loop is provided. The controller may be formed by any type of controller and may be implemented in the software to be executed by a microprocessor, microcontroller, or any other programmable device, or may be implemented by dedicated hardware.
A problem needing to be solved relates to the stability of the feedback loop, i.e. it is desirable to achieve a frequency behavior of the feedback loop wherein ringing and/or oscillation is prevented. At the same time, a high bandwidth of the active damping system is required, as a high bandwidth of the active damping system will allow to suppress vibrations within such high bandwidth. Due to the ever increasing demands on speed of the lithographic apparatus, movements in the lithographic apparatus tend to take place at a higher speed and consequently involving faster transients, which may result in a generation of vibrations at increasingly higher frequencies. Thus, as speed increases, it is desirable to achieve a higher bandwidth of the active damping system.
A projection system is commonly built up from a variety of parts, comprising e.g. lenses, mirrors and/or other optical elements, lens mountings and/or mirror mountings, a housing of the projection system such as a lens body, etc. As a consequence, a frequency behavior of the projection system starts, at a low frequency extreme, as a rigid body mass, thereby providing a transfer function which is inversely proportional to frequency, as illustrated in FIG. 2, where on a vertical axis a transfer function has been plotted (on a logarithmic scale), while in a horizontal axis a frequency has been plotted (on a logarithmic scale), and where RBM indicates a frequency behavior substantially corresponding to a rigid body mass. In a resonance frequency range RES, a resonance of the projection system is observed, which may be followed by a plurality of further resonance frequencies with increasing frequency, thereby overall resulting in an increase of the magnitude of the transfer function. Effectively, as from the resonance area, the projection system does not behave as a single object anymore, however instead shows a variety of resonance phenomena each corresponding to resonance of an element of the projection system. As a result thereof, the higher the frequency, the lower the remaining mass which “contributes” to the transfer function, which may be considered an explanation for the fact that the magnitude of the transfer function increases with increasing frequency, in the frequency range above the resonance frequency range. As will be understood by a skilled person, the frequency behavior of the projection system as outlined above, may result in stability problems when attempting to achieve a bandwidth of the active damping system which reaches or exceeds the resonance of the projection system. The transfer function may be expressed in terms of e.g. velocity of the projection system as a function of a force on the projection system. It is noted that the transfer function may also be expressed in any other suitable quantity, such as acceleration of the projection system as a result of force on projection system. In that case, a low frequency behavior of the transfer function will show to be frequency independent, followed by a resonance frequency range and an increase of the transfer function (showing multiple resonance peaks) above the resonance frequency range.