WO2006126133A2

WO2006126133A2 - Cascade arc radiation source with oxygen scavenging means

Info

Publication number: WO2006126133A2
Application number: PCT/IB2006/051499
Authority: WO
Inventors: Joop Geijtenbeek; Petrus A. M. Weerdesteijn; Hendrik A. Van Esveld; Bennie J. De Maagt; Marco Haverlag
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-05-26
Filing date: 2006-05-12
Publication date: 2006-11-30
Also published as: WO2006126133A3

Abstract

The invention relates to a radiation source, comprising: an at least partially radiation-transmissive discharge vessel filled with an ionizable substance, said vessel comprising: at least one anode, at least one cathode comprising tungsten, and one or more plates positioned between said anode and cathode, said plates being electrically substantially insulated, each plate having at least one hole, aligned in such a way that a continuous path is provided between said anode and said cathode over which a discharge can take place. The invention also relates to a system for inspecting an object, comprising such a radiation source.

Description

Radiation source, and system for inspecting an object comprising such a radiation source

The invention relates to a radiation source, comprising: an at least partially radiation-transmissive discharge vessel filled with a ionizable substance, said vessel comprising: at least one anode, at least one cathode comprising tungsten, and one or more plates positioned between said anode and cathode, said plates being electrically substantially insulated, each plate having at least one hole, aligned in such a way that a continuous path is provided between said anode and said cathode over which a discharge can take place. The invention also relates to a system for inspecting an object, comprising such a radiation source.

In the above inspection system, the radiation that comes from the object to be inspected generally contains information on patterns that are present in or on the object, thicknesses of layers on the object and/or compositions of materials on the object. Optical inspection systems are widely used for inspecting objects. For example, in the semiconductor industry use is made of automatic-wafer inspection systems. Such wafer inspection systems are used for the inspection of the quality of wafer processing in order to detect processing defects, layer thicknesses and/or contamination on the wafers. During the processing of a wafer an array of patterns is placed on the wafer and each pattern has to be placed with submicron precision, as line widths and elemental areas are very small. Successive layers are to be built up for each pattern on a wafer and these have to be carefully checked before any further processing can be undertaken. Optical inspection is used to determine whether any defects have been introduced, such as for example misalignment, electrical shortings, and impurities.

In the above system, use is made of one or multiple radiation sources, wherein the plates are in particular placed in a cascade and are electrically substantially insulated from each other, from the cathode(s) and from the anode(s). Such an arrangement of the radiation source is also referred to as a cascade arc radiation source. A cascade arc source per se is known and is, for example, disclosed in US-A-4, 871,580 and in WO 2004048950A1. A cascade arc source generally comprises three major sections; a cathode section, an anode section and a plate section in-between. The plate section, which typically comprises several plates with holes stacked into a cascade, gives the arc its name. Upon operation, the ionizable substance present in the discharge vessel will be ionised at least partially resulting in an electrical current flowing from the anode to the cathode through the holes in the cascade plates, thereby creating a plasma emitting electromagnetic radiation. The ionizable substance present in the discharge vessel for generating said plasma commonly comprises oxygen- containing impurities. These oxygen-containing impurities are commonly formed by trace compounds containing oxygen, or more specifically, which are formed by molecular oxygen. In particular, at operation temperature these oxygen-containing impurities will affect the evaporation of tungsten from a tip of the cathode(s), thereby leading to a considerable transport of tungsten along the cathode(s) with respect to the transport of tungsten under inert conditions. This considerable transport of tungsten commonly results in a detrimental deformation and in particular burn-back of the cathode, which commonly shortens the life expectancy of the cathode(s) significantly. Moreover, this increased transport of tungsten due to oxygen(-containing impurities) commonly also leads to corrosion of a metal inner wall of the discharge vessel, which may also influence the lifetime of the radiation source negatively. It is an object of the invention to provide an improved radiation source with which transport of tungsten can be reduced.

The object of the invention can be achieved by providing a radiation source according to the preamble, characterized in that the vessel further comprises a scavenging means for scavenging oxygen present in the discharge vessel at least partially. By scavenging or insulating and therefore purifying oxygen-containing impurities, or at least a part of these impurities, and in particular molecular oxygen, interaction of these impurities with tungsten of the cathode can be prevented, or at least counteracted. Consequently, by counteracting considerable transport of tungsten on a tip of the cathode both a burn-back of the cathode and corrosion of an inner wall of the discharge vessel can be prevented or at least (also) counteracted. Moreover, reduced transport of tungsten on the cathode leads to a geometric stability of the cathode, thereby preserving the shape of the cathode in a relatively long- lasting manner, as a result of which the life expectancy of the cathode can be increased significantly. By preventing or at least counteracting reactive, and in particular oxidative, oxygen-containing compounds from colliding with the cathode, oxidation of tungsten of the cathode and, subsequently, transport of tungsten along the cathode, can also be prevented.

Scavenging of oxygen can be realized in various manners. However, it is preferred that scavenging of the oxygen-containing impurities by the scavenging means is energetically more favourable than allowing these impurities to interact with tungsten of the cathode. In a preferred embodiment the scavenging means is adapted to reduce the oxidative activity of oxygen. By reducing the oxidative activity of oxygen, reactive (molecular) oxygen can be decomposed and can be bound chemically to a reactant so as to form a relatively inert compound, thereby minimizing a risk of this oxygen to interact with tungsten of the cathode. Preferably, the scavenging means comprises a reducing compound, in particular a reducing gas to take away the oxidative reactivity of (molecular) oxygen in a relatively efficient and effective manner. In a particular preferred embodiment the reducing gas is selected from the group: hydrogen gas and nitrogen gas. However, it may be clear that a person skilled in the art could also apply another kind of reducing substance to chemically scavenge oxygen present in the discharge channel. Besides chemically modifying the oxygen-containing impurities at least partially so as to form a compound which is relatively innocuous with respect to deterioration of the cathode, oxygen may also, and possibly additionally, be scavenged by the scavenging means by binding the oxygen-containing impurity in a chemical and/or physical manner without modifying the molecular structure or ionic structure of these impurities. This kind of binding of oxygen will commonly merely occur at an increased temperature, such as an operating temperature of the cathodes. However, both preferred embodiments impede (undesired) interaction of the oxygen-containing impurities with the cathode(s) by scavenging these impurities by the scavenging means. By binding the oxygen-containing impurities in a substantially unmodified molecular and/or ionic structure physically on a surface (adsorption) and/or chemically to a compound (chemisorption), oxygen can also be prevented, or at least counteracted from interacting with the cathode, at least at increased temperature. In a particular preferred embodiment the scavenging means comprise at least one getter. Commonly, a getter needs to be activated at increased temperature after which oxygen can be bound to said getter. A getter is commonly made of a solid compound, preferably a metal (or alloy), such as barium. Dependent on the kind of metal used to form a getter, oxygen will be bound to the getter in a specific way. For example, if beryllium, magnesium, calcium, zirconium, yttrium and/or strontium or alloys of these materials are used to form the getter, oxygen will be bound to these metals or alloys so as to form an oxide, and a peroxide will commonly be formed in case the getter comprises barium. However, in either situation oxygen is scavenged by the getter, as a result of which interaction of this oxygen with tungsten of the cathode(s) can be prevented, or at least counteracted.

In a preferred embodiment the vessel comprises at least one inlet through which the ionizable substance can be led into said vessel. More preferably, the vessel further has an outlet for ionizable substance contained by the vessel. In this manner, a continuous (gas) flow through the discharge vessel can be generated. A first advantage of a continuous flow is that the discharge vessel can be kept relatively free from impurities from outside the discharge vessel. A second advantage is that the chemical composition of the ionizable substance contained in the discharge vessel can be modified relatively easily thereby increasing the flexibility of the radiation source significantly. Both the inlet and the outlet may be closable to (temporarily) lock up a certain substance in the discharge channel. In an alternative embodiment of the radiation source both the inlet and the outlet of the discharge vessel are omitted, wherein the discharge vessel is substantially medium-tight. In this embodiment the quantity and the chemical composition of the ionizable substance contained by the discharge vessel will be predetermined.

The ionizable substance can be of various composition and nature. Preferably, the ionizable substance comprises at least one noble (or rare) gas selected from the group: argon, neon and krypton. Optimising the chemical composition of the ionizable substance for specific purposes of the radiation source are well known to a person skilled in the art and will therefore not be elucidated more comprehensively.

The invention also relates to a system for inspecting an object, comprising an irradiation system for irradiating the object to be inspected, said irradiation system comprising: at least one radiation source according to the invention as described above, an objective imaging the irradiated object onto an image sensor, and an image sensor for transforming the radiation coming from the object to be inspected into a detectable signal.

Preferably, the inspection system according to the invention provides for a wavelength region that is limited to any band or set of bands of wavelengths, comprising at least radiation at wavelengths of at least 190nm. Radiation at these shorter wavelengths from 190 nm is for example very advantageous when the inspection system is used for inspecting semiconductor devices. Advantageously, the radiation source produces radiation with radiance larger than 10mW/nm/mm²/steradian. Although in principle all kind of objects could be inspected by the system according to the present invention, the system is in particular suitable for inspecting bare, partially or fully processed semiconductor wafers or reticles or masks used in a lithographic process to produce a patterned layer on a semiconductor wafer. Preferably, the irradiation system comprises optical means for homogenizing the spatial distribution of the irradiation in the image plane on the object. In particular, the optical means comprise a homogeniser. Such homogenizer conditions the light coming from the radiation source and homogenizes the spatial distribution of it. The invention can further be illustrated by way of the following non- limitative embodiments, wherein:

Figure 1 shows a cross-section of a first embodiment of a cascade arc radiation source according to the invention, and

Figure 2 shows a cross-section of a second embodiment of a cascade arc radiation source according to the invention.

Figure 1 shows a cross-section of a first embodiment of a cascade arc radiation source 1 according to the invention. The radiation source 1 comprises a discharge vessel 2, said vessel 2 comprising a nozzle-like anode 3 and three cathode tips 4 (of which only one is shown). The three cathode tips 4 are made of an alloy of preferably lanthanum oxide (LaO₃) in tungsten. However, other tungsten-containing alloys and also pure tungsten may be applied to form the cathode tips 4. The cathode tips 4 are preferably arranged rotatively around a central channel 5 and are mounted in hollow holders 6 through which cooling water is fed through a duct 7. The discharge vessel 2 further comprises a stack of cascade plates 8 arranged substantially parallel to each another. The cascade plates 8 are preferably made of copper, and are mutually separated from one another and electrically insulated. The cascade plates 8 are each provided with a central hole 9 to form the central channel 5 from the anode 3 to the cathodes 4. The central channel 5 is filled with an ionizable substance comprising one or more rare gases (not shown). During operation, the ionizable substance flows continuously via an inlet 10 and an outlet 11 respectively through the central channel 5 of the discharge vessel 2 thereby generating a discharge arc. In order to reduce oxygen-containing impurities present in de ionizable substance one or more reducing agents, such as hydrogen gas, are applied to reduce the oxygen-containing impurities substantially before these impurities will deteriorate the cathodes 4, at least at an increased temperature. The vessel 2 is provided with two emission windows 12a, 12b for emitting radiation generated within the central channel 5 e.g. for illuminating a specific object to be inspected. Cooling means 13 are provided to sufficiently cool (by means of water) the anode 3 and the cathode tips 4.

Figure 2 shows a cross-section of a second embodiment of a cascade arc radiation source 14 according to the invention. The radiation source 14 shown in figure 2 is constructively substantially similar to the radiation source 1 as shown in figure 1. However, the radiation source 14 presently shown comprises a discharge vessel 15 which is substantially medium-tight, said discharge vessel 15 comprises an anode 16, multiple cathodes 17, said cathodes 17 comprising tungsten, and a stack of plates 18 enclosing a discharge path 19. The discharge vessel 15 is provided with an ionizable substance (not shown) in a predetermined quantity and with a (substantially) predetermined chemical composition. However, due to oxygen-containing impurities commonly present in the ionizable substance, tungsten of the cathodes 17 may be transported leading to instability of shape of the cathodes 17, which could decrease the life expectancy of these cathodes 17 significantly. To counteract this undesired effect, the discharge vessel 15 further comprises a purifying getter 20 for binding the oxygen-containing impurities to avoid or at least counteract interaction of these impurities with tungsten of the cathodes 17, at least at operation temperature.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. Radiation source, comprising: an at least partially radiation-transmissive discharge vessel filled with a ionizable substance, said vessel comprising: at least one anode, at least one cathode comprising tungsten, and - one or more plates positioned between said anode and cathode, said plates being electrically substantially insulated, each plate having at least one hole, aligned in such a way that a continuous path is provided between said anode and said cathode over which a discharge can take place, characterized in that the vessel further comprises a scavenging means for scavenging oxygen present in the discharge vessel at least partially.

2. Radiation source according to claim 1, characterized in that the scavenging means are adapted for reducing the oxidative activity of oxygen.

3. Radiation source according to claim 2, characterized in that the scavenging means comprises a reducing compound, in particular a reducing gas.

4. Radiation source according to claim 3, characterized in that the reducing gas is selected from the group: hydrogen gas and nitrogen gas.

5. Radiation source according to any one of the preceding claims, characterized in that the scavenging means are adapted for chemisorption of oxygen.

6. Radiation source according to claim 5, characterized in that the scavenging means comprises at least one getter.

7. Radiation source according to any one of the preceding claims, characterized in that the vessel has at least one inlet through which the ionizable substance can be led into said vessel.

8. Radiation source according to any one of the preceding claims, characterized in that the discharge vessel is substantially medium-tight.

9. Radiation source according to any one of the preceding claims, characterized in that the ionizable substance comprises at least one noble gas selected from the group of argon, xenon, neon and krypton.

10. System for inspecting an object, comprising an irradiation system for irradiating the object to be inspected, said irradiation system comprising: at least one radiation source according to any one of claims 1 -9, - an objective imaging the irradiated object onto an image sensor, and an image sensor for transforming the radiation coming from the object to be inspected into a detectable signal.