Photocopying – Projection printing and copying cameras – Distortion introducing or rectifying
Reexamination Certificate
2002-03-15
2003-04-01
Mathews, Alan A. (Department: 2851)
Photocopying
Projection printing and copying cameras
Distortion introducing or rectifying
C355S053000, C355S077000, C250S399000, C250S400000, C250S400000, C065S061000, C359S506000
Reexamination Certificate
active
06542219
ABSTRACT:
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 through 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 v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. In both types of apparatus, after each die has been imaged onto the wafer, the wafer table can be “stepped” to a new position so as to allow imaging of a subsequent die. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
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 projection lithography, a very high-quality projection system is used to project a reduced image of the mask pattern onto the silicon wafer. As compared to other types of projection systems, lithographic projection systems have to satisfy very stringent requirements with respect to inter alia aberration correction, flatness of field and absence of distortion. This latter aberration is harmful regarding the aspect of “overlay precision”: different patterns, projected onto the wafer in subsequent process steps, should superimpose upon one another with an accuracy of the order of about 30 nm (conventional figure) over the full image field, which has typical dimensions of the order of about 25×25 mm
2
. For this reason, the residual distortion of a high-quality lithographic objective should be extremely low. While the as-designed value of the residual distortion can be very low (only a few nm), the value of a manufactured objective may show larger values. Possible reasons for this residual distortion are small mounting errors of the (typically) large number of optical elements in a given projection system, but also small index variations in refractive material and/or reflective coatings used in such elements. In some cases, a highly expensive objective that satisfies all other specifications (e.g. aberration correction level, field flatness) may have to be rejected because of its residual distortion.
SUMMARY OF THE INVENTION
It is an object of the invention to address this problem. In particular, it is an object of the invention to provide a mechanism for reducing the residual distortion of a projection system to below a specified level, so as to avoid having to quality-reject such a system. Moreover, it is an object of the invention to provide a mechanism of correcting a potential drift in distortion during the installed “lifetime” of a projection system in a lithographic projection apparatus.
These and other objects are achieved in a lithographic projection apparatus in which a correction mechanism is provided at a point outside the projection system but along its optical axis, the correction mechanism comprising a plate of material which is substantially transparent to the radiation supplied by the radiation system, the plate having a substantially uniform thickness and an aspherical surface profile, which surface profile is calculated so as to have a counteractive effect on a measured optical distortion of the projection system.
It should be explicitly noted that the term “projection system” as employed in this text encompasses not only lenses made of refractive material, but also projection mirror systems and catadioptric systems, for example.
According to the invention, an aspherically deformed plate with substantially constant thickness is positioned at some point along the radiation path through the lithographic projection apparatus, e.g. between the mask table and the projection system. By means of the locally varying inclination of the aspherical plate, the apparent position of an object point on the mask suffers a lateral shift. The lateral shift &dgr;
x
on the mask from A to A′ (in the X-direction: see
FIG. 2
) is given by
δ
x
=
(
n
-
1
)
n
ϑ
x
·
t
with a comparable expression for the shift &dgr;
y
in the Y-direction.
Thanks to the constant thickness t of the correction plate, the field flatness of the projected image is not affected by the presence of the plate. Moreover, the plate's constant thickness t ensures that any absorption of the projection radiation which occurs in the plate will be substantially homogenous across the extent of the plate, thus preventing the occurrence of substantial dose and uniformity errors at substrate level as a result of the presence of the plate.
To compute the shape of the correction plate required in a given situation, distortion data are measured at a certain number of sample points (e.g. 100 points) in the image field of the projection system. This can, for example, be done by exposing a test substrate with an image of a test mask (e.g. a special distortion measurement mask), then selecting a certain number of sample (object) points on the mask and measuring the corresponding (image) points on the substrate. The theoretical position of the image points on the substrate in the absence of distortion can be calculated by correcting for the magnification of the projection system. By comparing the positions of the calculated image points and those of the actual measured image points on the substrate, one can calculate the distortion (&dgr;
x
, &dgr;
y
)
k
at a particular point. These data yield a set of values (&dgr;
x
, &dgr;
y
)
k
which are translated into local inclination angles (&thgr;
x
, &thgr;
y
)
k
. The aspherical shape of the plate (with substantially constant thickness) is obtained by finding a least-squares solution of the resulting set of linear equations, with the required slopes as variables and with the continuity of t
Braat Josephus J. M.
van der Laan Cornelis J.
ASML Netherlands B.V.
Brown Khaled
Mathews Alan A.
Pillsbury & Winthrop LLP
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