Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
2000-06-07
2002-06-18
Duda, Kathleen (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S322000, C430S394000, C430S396000
Reexamination Certificate
active
06406834
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of projecting an image onto a plurality of target areas on a substrate whereby use is made of 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 area of the substrate,
whereby the substrate is to be irradiated with images from at least two different masks.
2. Discussion of the Related Art
A lithographic projection apparatus as described in the opening paragraph 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 (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target areas that are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each target area is irradiated by exposing the entire reticle pattern onto the target area at once; 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 target area 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 v 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 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 metrology steps 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 UV, 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; for simplicity, such components may be loosely referred to in this text, either singly or collectively, as a “lens”. 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 substrate table and/or mask table.
In many applications of a lithographic projection apparatus, each target area on a given substrate is exposed using a single mask (per layer). However, in certain applications, it is desirable to expose each target area to a patterned image from two or more different masks (both images being projected onto the same layer of radiation-sensitive material on the target area); within each target area, these distinct mask images may, for example, be projected in proximity to one another, in juxtaposition with one another, or in overlap (to a lesser or greater degree). Alternatively, one can expose a layer of radiation sensitive material on a substrate using two or more different masks in such a manner that different mask images are projected onto different target areas, such that each target area will be exposed with one mask image. However, a problem with such multiple-mask exposure is that it incurs a substantial time penalty per substrate. This is because, after exposure of the (relevant) target areas on a substrate using the first mask, the first mask has to be removed from the mask table, has to be replaced by a second mask, and then this second mask has to be aligned with the substrate. Even when employing the fastest and most sophisticated mask handling apparatus, this interchange procedure can be very time consuming. Another problem is that, after each mask change, the optics (e.g. masking shutters) in the radiation system (illuminator) will generally have to be adjusted. Not only does this incur a further time penalty, but it also necessitates much more frequent use of the actuators used to adjust the optics, leading to problems of early wear.
SUMMARY OF THE INVENTION
It is an object of the invention to alleviate these problems. In particular, it is an object of the invention to provide a multiple-mask exposure method that allows a significantly greater machine throughput than known methods.
These and other objects are achieved in a method as specified in the opening paragraph, characterized by the following steps:
(a) providing a batch of substrates, each at least partially coated with a layer of radiation-sensitive material;
(b) providing storage means for temporary storage of the batch;
(c) providing a first mask on the mask table;
(d) irradiating a first set of target areas of a first substrate with an image from the first mask, and then placing that substrate in the storage means;
(e) repeating step (d) for each of the other substrates in the batch;
(f) replacing the first mask by a second mask;
(g) providing a primary substrate from the storage means on the substrate table and irradiating a second set of target areas of that substrate with an image from the second mask;
(h) repeating step (g) for each of the other substrates stored in the storage means.
The term “batch” as used in the context of the present invention should be interpreted as referring to a set of substrates that are to be processed according to a given “recipe”, e.g. in relation to a particular layout or combination of target areas or die types on the available substrate area. Such a “batch” of substrates is offered to the lithographic projection apparatus in one go. In the event that the total number of substrates to be processed according to the said recipe is greater than the size of the said batch, then many such batches may be consecutively offered to the apparatus, until the relevant recipe set is exhausted.
The method according to the invention has the advantage that a mask interchange only has to occur once per batch instead of one per substrate, so that there is less time-overhead per substrate for each such interchange, and so that there is less wear of adjustable optical components in the radiation system. The wear of the masks and the mask handling system will also generally decrease when the method according to the invention is used. The inventors arrived at the invention only after they had discovered that, contrary to what one might think, the radiation-sensitive layer on a given substrate does not degene
Kuit Jan Jaap
Smits Jan A. M.
Stoeldraijer Judocus M. D.
ASML Netherlands B.V.
Duda Kathleen
Pillsbury & Winthrop LLP
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