Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1999-12-15
2002-07-16
Berman, Jack (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
Reexamination Certificate
active
06420716
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to servo control methods and apparatus. More particularly, the invention relates to the application of such methods and apparatus in a lithographic projection apparatus.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. In addition, the first and second object tables may be referred to as the “mask table” and the “substrate table”, respectively.
Lithographic projection apparatus 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 that 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—which is commonly referred to as a step-and-scan apparatus—each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally<1), the speed at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Applications WO 97/33205 and WO 96/38764, for example.
Up to very recently, apparatus 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 apparatus 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.
The projection radiation in current lithographic devices is generally UV (ultra-violet) light with a wavelength of 365 nm, 248 nm or 193 nm. However, the continual shrinkage of design rules in the semiconductor industry is leading to an increasing demand for new radiation types. Current candidates for the near future include UV light with wavelengths of 157 nm or 126 nm, as well as extreme UV light (EUV) and particle beams (e.g. electron or ion beams).
In an apparatus as described above, it is necessary to control the relative position of the object tables and the lens to a very high degree of accuracy. Transient inaccuracies in this relative position, which may be caused by vibrations, are therefore particularly problematic. Whilst it may be relatively easy to detect the existence of such vibrations, it requires considerable work to identify and eliminate their sources. Lens vibrations may, for example, be caused by floor vibrations, indirect scanning forces (in the case of step-and-scan devices), noise in vibration isolation systems (originating in pneumatic suspension devices in the apparatus) or acoustic noise, among other things. Since the lens is generally quite large and heavy (e.g. with a mass of the order of about 50-250 kg), it is particularly sensitive to vibrations with a relatively low frequency.
A lithographic projection process may require the positional error of the substrate holder and/or mask holder relative to the lens to be of the order of 2 nm or less. In addition, practical considerations in servo system design can demand that the positional stability of the lens be within tolerances of the order of 1 nm. In tests, the inventors have observed that positional errors of this magnitude can, under certain conditions, be produced by disturbance forces of the order of as little as 1N (acting on a machine that may have a mass of several hundred to several thousand kg). The desired degree of stability can therefore be very difficult to achieve.
SUMMARY OF THE INVENTION
It is an object of the present invention to alleviate this problem. More specifically, it is an object of the invention to provide a lithographic projection apparatus in which effective measures are taken to reduce the detrimental effect of lens vibrations on the accuracy with which the substrate and/or reticle tables can be positioned relative to the lens.
According to the present invention, these and other objects are achieved in a lithographic projection which includes
a detection mechanism for detecting accelerations of the projection system, and generating at least one acceleration signal representative thereof, and
a control mechanism responsive to the acceleration signal, for generating at least one control signal to control at least one of the positioning mechanisms so as to move the corresponding object table, thereby to compensate for movements of the projection system.
The present invention also provides a method of controlling the relative position of at least one of the object tables and the projection system in such a lithographic projection apparatus, the method comprising the steps of:
measuring accelerations of the projection system;
determining a force to be applied to at least one of the object tables to cause movement thereof so as to compensate for movements of the projection system;
applying the determined force to that object table.
The feedfoward control provided by the present invention can substantially reduce the effect of vibrations (e.g. in the main frame or base plate of the lithography device) on the relative positions of the lens and object table (wafer table and/or reticle table). This feedforward control can be specifically tuned to provide maximum compensation within particular frequency bands, e.g. around the eigenfrequency of the lens.
The invention is applicable to one or more of the 6 degrees of freedom of the lens, substrate table and/or mask table. For the sake of simplicity, the following discussion will concentrate on a situation whereby correction occurs in only one degree of freedom; however, the presented considerations are equally valid for more degrees of freedom. In this latter case, it will be usual to have a set of detection mechanisms (e.g. one per controlled table per degree of freedom) and to generate several control signals (e.g. one per detection mechanism in the set).
In a preferential embodiment of the invention, the detection mechanisms mounted on the projection system in relatively close proximity (and preferably as close as possible) to the object table/tables whose position is/are to be controlled in response to the control signal. In such a case, a lens acceleration measured by the detection mechanism can be translated with relatively high accuracy into a force to be applied to the object tables(s). On the other hand, the accuracy of the extrapolated required movement of the table(s) is reduced when the detection mechanism is re
Cox Harry H. H. M.
Plug Reinder T.
ASML Netherlands B.V.
Berman Jack
Pillsbury & Winthrop LLP
Smith II Johnnie L
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