Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1999-12-14
2002-09-24
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C355S053000
Reexamination Certificate
active
06455862
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a lithographic projection apparatus having a radiation system for supplying a projection beam of electromagnetic 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.
2. Description of Related Art
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 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 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, 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 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 lithographic projection equipment most commonly used today operates at an exposure wavelength of 365 nm (so-called i-line apparatus) or 248 nm (so-called DUV apparatus). However, the ever-decreasing design rules in integrated circuitry have created a demand for even smaller exposure wavelengths &lgr;, since the resolution that can be attained with lithographic equipment scales inversely with &lgr;. Consequently, much research has been devoted to finding new light sources operating at wavelengths shorter than 248 nm. Currently, attention is being focused on new wavelengths that can be produced by excimer lasers, such as 193 nm, 157 nm and 126 nm, and researchers hope that such lasers can be refined so as to produce sufficient intensity for lithography purposes (so as to guarantee adequate throughput). In this context, it should be noted that currently available i-line equipment generally employs a mercury lamp with a power of the order of about 3-5 kW, whereas DUV apparatus typically uses excimer lasers with a power of the order of about 5-10 W, or even higher. The intensity demands on the new-wavelength excimer lasers are therefore very high.
The assignee of the current patent application recently announced the successful development of the world's first fully functional, wide-field, production-level lithographic projection apparatus operating at 193 nm; up to that point, only relatively primitive test tools operating at 193 nm had been available. The introduction of this apparatus was preceded by intense research efforts into source development, illuminator design, and lens materials. During this research, an important difference was observed between the new 193-nm machine and existing 248-nm devices, as will now be discussed.
In experiments leading to the invention, the inventors observed that intense radiative fluxes of 193-nm light caused transient changes in the characteristics of refractive materials placed in their paths (for example, quartz or CaF
2
lens elements). Moreover, the same effect was observed by the inventors to occur in various optical coatings present on lenses or mirrors located in the optical path. These changes were observed to affect, for example, the transmissivity of the projection system, thus altering the radiation intensity received at the substrate, even if the intensity delivered by the radiation system (excimer laser) was kept constant; consequently, such effects could cause serious exposure errors on the substrate (e.g. under-exposure of a resist layer). To make matters worse, the inventors observed that these transmissivity changes demonstrated a complex temporal dependence.
Typically, an apparatus as described in the opening paragraph will additionally comprise one or more intensity (energy) sensors. For example, at a test position prior to the mask, it is possible to divert a small portion of the radiation in the projection beam out of the main path of the beam and onto an intensity sensor, thus allowing continual monitoring of the intensity produced by the radiation system. Similarly, it is possible to provide the upper surface of the substrate table with an intensity sensor, located outside the perimeter of the substrate; such a sensor can then be used to calibrate the apparatus on a regular basis, by allowing periodic comparisons of the intensity produced by the radiation system and the actual intensity I
s
received at the substrate. In analogy to the effects described in the previous paragraph, the inventors discovered that the sensitivity of such sensors could demonstrate a significant temporal drift as a result of irradiation with 193-nm radiation, resulting in intrinsic errors in the intensity measured at substrate level. Needless to say, if there is a (variable) intrinsic error in I
s
as a result of such sensitivity drift, this will result in a miscalibration of the apparatus, with the attendant risk of exposure errors.
In the case of radiation wavelengths at or above 248 nm, the effects described in the previous two paragraphs have hitherto not been observed. However, in the case of machines operating at 193 nm, these effects can be very serious. For example, in investigative experiments, the inventors observed that, in the case of a step-and-scan test apparatus employing a 5W ArF laser (193 nm) and various optical components comprising quartz and/or CaF
2
elements (inter alia a fly-eye lens or light mixing rod, lenses near the reticle masking blades, the main projection lens, etc.) the transmission T along the path of the radiation (between the laser and the substrate table) decreased by as much as 5-7% within 2-3 minutes of initiating irradiation, and then slowly relaxed upward once more (within a time of the order of about 5 minutes) when irradiation was interrupted (or set to another level). Moreover, differences in amplitude and temporal behavior were observed for different optical materials and material combinations. Such large transmission changes can cause serious dose errors at substrate level, with the possibility of large numbers of substrate rejects (particularly in IC manufacture).
SUMMARY OF THE INVENTION
It is an ob
Noordman Oscar F. J.
van der Veen Paul
ASML Netherlands B.V.
Lee John R.
Smith II Johnnie L
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