Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1999-06-21
2002-04-16
Berman, Jack (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C356S399000
Reexamination Certificate
active
06373072
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a lithographic projection apparatus and more particularly a lithographic projection apparatus with reduced effects of disturbances on the motions of substrate and mask tables.
2. Description of Related Art
An apparatus of the above-noted type 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. 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, thereby transferring an image of the irradiated reticle pattern onto the die. Since, in general, the projection system will have a magnification factor M (generally M<1), the speed at which the wafer table is scanned will be a factor M lower than that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
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 apparatuses 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.
Lithographic apparatus may employ various types of projection radiation, such as ultra-violet light (UV), extreme. V, X-rays, ion beams or electron beams, for example. Depending on the type of radiation used and the particular design requirements of the apparatus, the projection system may be refractive, reflective or catadioptric, for example, and may comprise vitreous components, grazing-incidence mirrors, selective multi-layer coatings, magnetic and/or electrostatic field lenses, etc. The apparatus may comprise components which are operated in vacuum, and are correspondingly vacuum-compatible. As mentioned in the previous paragraph, the apparatus may have more than one wafer table and/or mask table.
The manufacture of ICs and other devices with such an apparatus generally involves the replication of extremely fine sub-micron patterns, with an exceptionally high degree of positional accuracy. For this reason, it is essential to properly isolate various critical parts of the apparatus (such as the substrate table and mask table, for example) from spurious motion, vibration, mechanical shocks, etc. In general, this is achieved using such measures as carefully designed metrology frames, air-mounts, motional counterweights and dampers, which serve to isolate the apparatus' critical parts from most unwanted mechanical influences. However, such measures are not completely effective in eliminating a number of unwanted influences, such as, for example:
1. vibrations caused by mask handling devices (robots);
2. vibrations caused by motion of reticle masking blades;
3. resonance effects caused by the presence of air showers;
4. vibrations in the substrate table caused by motion of the mask table, and vice versa.
Although these effects are relatively small, they become increasingly important as the need to produce ever-higher device resolutions increases, and they now form a substantial barrier to the viable realization of large-area ICs having critical dimensions of the order of 0.15 &mgr;m and less.
SUMMARY OF THE INVENTION
It is an object of the invention to alleviate this problem. More specifically, it is an object of the invention to provide a step-and-scan lithographic projection apparatus in which the effects of the above-mentioned disturbances are reduced. In particular, it is an object of the invention that the said apparatus should contain means to counteract motional a synchronism between the substrate table and mask table arising as a result of such disturbances.
These and other objects are achieved in 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;
first driving means, for moving the mask table in a given reference direction substantially parallel to the plane of the table;
second driving means, for moving the substrate table parallel to the reference direction so as to be synchronous with the motion of the mask table.
The apparatus further comprises:
first measuring means, for determining the momentary position of the mask table with respect to a fixed reference point;
second measuring means, for determining the momentary position of the substrate table with respect to a fixed reference point, and
means for comparing the measured momentary position of the substrate table with a desired momentary position (setpoint) of the substrate table, for generating a position error signal in accordance with a difference between the said two positions, and for passing that signal to correction means which serve to adjust the momentary position of the mask table so as to compensate for such difference.
The invention is founded upon the insight that the disturbances referred to above are substantially impossible to remove using solely structural alterations to the lithographic apparatus, and that the only practical solution is to employ an approach based on Control Theory. In experiments leading to the invention, the inventors attempted to correct servo errors in the substrate table by using the second measuring means to quantify such errors (by comparing the substrate table's measured and intended positions), and employing a feedback loop to the second driving means to iteratively remove such errors. However, although this approach produced a certain improvement, it failed to remove all unwanted disturbances; further analysis showed that disturbances with characteristic frequencies of the order of about 200 Hz were not being satisfactorily attenuated using this plan of attack, although lower-frequency disturbances (e.g. 20-80 Hz) did demonstrate a reduced amplitude.
In a fresh approach to the problem, the inventors abandoned the idea of attempting to completely remove the effects of unwanted motional disturbances in the substrate table, and turned their attention instead to the mask table. Because the mask table generally has a less complex and more lightweight construction than the substrate table, the inventors expected that the former would have a less complex resonance signature than the latter. Experiments verified that this was indeed the case, but, at frequencies of the order of about 200 Hz, the amplitude of the disturbances in the mask table was substantially equal to that of those in the substrate table.
However, the inventors next arrived at th
Butler Hans
Warmerdam Thomas Petrus Hendricus
ASM Lithography B.V.
Berman Jack
Fernandez K.
Pillsbury & Winthrop LLP
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