Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-02-16
2003-11-11
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S491100, C250S492100, C250S492210, C250S492220, C250S492300, C355S069000, C355S067000
Reexamination Certificate
active
06646274
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation of a first type;
a mask table for holding a mask;
a substrate table for holding a substrate;
a projection system for imaging a portion of the mask, irradiated by the projection beam, onto a target portion of the substrate.
In particular, the invention relates to such a device in which the radiation of the first type comprises particulate radiation (e.g. electrons or ions), X-rays or extreme ultra-violet radiation (EUV).
BACKGROUND OF THE INVENTION
An apparatus of this 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 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 at one time, such an apparatus is commonly referred to as a wafer stepper. 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 (usually, M<1), the speed &ngr; 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, for example.
Until 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 and/or leveling 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 reduces the cost of ownership of the machine.
When radiation of the first type impinges on the substrate, energy from the radiation will generally be absorbed by the substrate, causing localized heating of the target area (die) which is being irradiated at that moment. In contrast, the substrate area outside the die, which is not being irradiated at that moment, will not undergo localized heating in this manner. Substrate heating is thus highly differential in nature, and can consequently cause significant differential stress in the substrate, with attendant mechanical deformation (expansion/contraction). This deformation can have a highly detrimental effect inter alia on the so-called overlay performance of the lithographic apparatus, which term refers to the accuracy with which a second patterned layer (new layer) can be juxtaposed upon a first patterned layer (old layer) already present on the substrate.
Although this problem is, in principle, present to some extent in all currently produced lithographic projection apparatus (in which the radiation of the first type is, for example, ultra-violet (UV) radiation with a wavelength of 365 or 248 nm), its magnitude is usually not so great as to cause substantial under-performance of the apparatus. However, a very different situation applies in the case of next-generation lithography systems, in which the radiation of the first type comprises, for example, electrons, ions, X-rays or EUV radiation (i.e. UV with a wavelength in the range 10-25 nm, e.g. 13.6 nm); in such apparatus, localized heating of the substrate can be quite intense.
It is an object of the invention to alleviate this problem. In particular, it is an object of the invention to provide a lithographic projection apparatus in which the effects of differential heating of a substrate during exposure are reduced.
These and other objects are achieved in an apparatus as specified in the opening paragraph, characterized in that the apparatus further comprises:
a secondary source for supplying radiation of a second type, which can be directed onto the substrate;
control means for patterning the radiation of the second type so that it impinges on the substrate according to a certain pattern.
By suitable embodiment of the control means, the sum of the fluxes of the radiations of the first and second type at substrate level will cause an elevation of the substrate temperature which is substantially constant across at least a given area of the substrate.
In essence, the invention ensures that those parts of the substrate which fall in the shadow of the radiation of the first type are illuminated (and consequently heated) by radiation of the second type, and vice versa; however, appropriate choice of the photosensitive material on the substrate, and of the type (e.g. wavelength) of the radiation of the second type, will ensure that the photosensitive material will only be exposed by the first-type radiation, and not by the second-type radiation. Proper adjustment of the second-type radiation dose at substrate level ensures that the substrate surface is heated to a substantially uniform temperature over at least that area which is to be covered with whole dies, thereby combating differential heating effects. Use of the word “substantially” in this context does not require exact uniformity of the substrate's surfacial temperature (although this is, of course, generally preferred); rather, the invention strives to produce at least some smoothing (and preferably a significant smoothing) of the differential heating effects which would occur in the absence of the invention.
The current invention can be envisaged at different levels, thus determining the size of the “given area” referred to in the penultimate paragraph above. For example, at a basic level, when one die (target area) on the substrate is being exposed to first-type radiation, the (whole) substrate area outside that die can be concurrently exposed to second-type radiation; this will be referred to hereunder as a “coarse-level correction”. On a deeper level, the invention can be applied within a given die: the shadowed (masked) areas within that die are then irradiated with second-type radiation while the rest of the area within the die is exposed to first-type radiation; this will be referred to hereunder as a “fine-level correction”. It is also possible to combine a coarse-level and fine-level correction.
In a first embodiment of the apparatus according to the invention, the radiation of the first type is selected from the group consisting of electrons, ions, X-rays and EUV radiation. Lithographic systems employing such post-optical radiation are presently undergoing development and preliminary testing by several companies, in reply to the semiconductor industry's continuing drive toward smaller feature sizes, and the consequent demand for greater lithographic image resolution. Preliminary investigations have shown that the use of such radiation types can lead to very substantial substrate heating,
ASML Netherlands B.V.
Lee John R.
Pillsbury & Winthrop LLP
Vanore David A.
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