EUV light source collector erosion mitigation

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patent application Ser. No. 11/174,442, entitled SYSTEMS AND METHODS FOR REDUCING THE INFLUENCE OF PLASMA-GENERATED DEBRIS ON THE INTERNAL COMPONENTS OF AN EUV LIGHT SOURCE, filed on Jun. 29, 2005, and Ser. No. 11/168,190, entitled filed on Jun. 27, 2005, the disclosures of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention related to EUV light source collector erosion mitigation.

BACKGROUND OF THE INVENTION

An EUV LPP light source using a plasma source material, e.g., tin as the source element suffers from erosion of the primary collector mirror. This erosion is due to sputtering by energetic plasma source material ions, e.g., tin ions and neutrals created by the LPP. Applicants according to aspects of embodiments of the present invention seek to address this issue.

SUMMARY OF THE INVENTION

An EUV light source collector erosion mitigation method and apparatus for a collector comprising a multilayered mirror collector comprising a collector outer surface composed of a capping material subject to removal due to a removing interaction with materials created in an EUV light-creating plasma, is disclosed which may comprise including within an EUV plasma source material a replacement material. The replacement material may comprise the same material as the capping material of the multilayered mirror. The replacement material may comprise a material that is essentially transparent to light in a selected band of EUV light, e.g., a spectrum of EUV light generated in a plasma of a plasma source material. The replacement material may comprise a material not susceptible to being etched by an etching material used to remove deposited plasma source material from the collector, e.g., a halogen etchant. The replacement material may comprise a material able to be laid down on the collector outer surface in a uniform smooth coating comprising a surface roughness which does not significantly impact the EUV reflectivity of the multilayer mirror of the collector. The replacement material may comprise a material that is soluble in liquid tin. The solubility of the replacement material in liquid tin may be such that it is sufficient to deliver replacement material to the collector outer surface at a rate sufficient to effectively offset erosion of the collector outer surface, e.g., in the range of 3–5 ppm, or 1–5 ppm, or 3–10 ppm, or 1–10 ppm. The replacement material may comprise a material compatible with the material of the capping layer of the collector multilayer mirror. The replacement material may comprise a material that will not readily diffuse through the material of the capping layer. The replacement material may comprise a material that is selected from the group of materials comprising materials that can withstand exposure to ambient atmosphere, materials that can be easily removed by some means prior to exposure to ambient atmosphere and materials that have reaction products when exposed to ambient atmosphere that are easily removed from the outer surface of the collector multilayer mirror.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically and by way of example an EUV light source according to aspects of embodiments of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As shown schematically and by way of example in FIG. 1, an LPP EUV light source 10 may comprise an EUV light collector 12 having a collector outer surface 14. Plasma source material, e.g., tin or lithium, may be delivered to a plasma initiation site 20 in the form of target droplets 22, delivered from a target droplet generator 24. The target droplets 22, in turn, may be irradiated, e.g., by a laser beam (not shown) to form a plasma which emits EUV light, e.g., at about 13.5 nm wavelength. The collector 12 may comprise an elliptically shaped multilayer mirror for reflecting EUV light with a focus of the ellipse forming the mirror shape at the plasma initiation site 20 and focus a cone of EUV light 30 to an intermediate focus 32 at the second focus of the ellipse, where, e.g., the EUV light may be delivered to a device utilizing the light, e.g., an integrated circuit manufacturing process photolithography apparatus (not shown).

In order to achieve a commercially viable collector 12 lifetime this erosion rate must be greatly reduced or the eroded material must be replaced in-situ.

One method for providing replacement material, according to aspects of an embodiment of the present invention may be to include such replacement material as a small portion in the plasma source material target droplet 22 material. With such an arrangement, mirror replacement material, as well as the tin EUV source element, can be made to become incident on the primary collector surface. Both the tin and the replacement material can then adhere to the collector surface, but the halogen chemistry cleaning, e.g., as proposed in the above referenced co-pending patent applications can be used to remove the tin atoms, thereby leaving only the replacement material.

The replacement material could be selected to be the same as the capping layer of the collector's EUV multilayer mirror outer surface, e.g., molybdenum, but need not be. In the event that the replacement material is selected to be other than the capping layer material the properties for this replacement material include, e.g., that it be (1) highly transparent to the wavelength of the generated EUV light, e.g., at about 13.5 nm; (2) not susceptible to being etched, e.g., by the halogen chemistry used, e.g., to remove deposited tin and/or other debris deposited on the collector; (3) able to be laid down on the collector surface in a uniform, smooth fashion so that, e.g., the surface roughness of the outer surface of the multilayer mirror is not significantly impacted (and thus EUV reflectivity significantly degraded); (4) soluble in liquid tin to the level of, e.g., several parts per million (which is currently expected by applicants to be a concentration of replacement material needed to effectively offset erosion of the outer surface of the collector multilayer mirror); (5) compatible with the materials that make of the EUV reflective multilayer mirror, e.g., its capping layer and, e.g., not readily diffuse through its capping layer and or underlying reflectivity layer; and able to withstand exposure to air during vacuum vessel service or be easily removed by some means prior to opening the vacuum vessel or have reaction products with air that are easily removed once service is complete and the collector is back under vacuum.

Such a material may be, as noted above a capping layer material, e.g., molybdenum, but also may be, e.g., yttrium or ruthenium.

The concentration of the replacement material in the plasma source material, e.g., tin source target droplet must be selected to not be insufficient material, such as can lead to net erosion of the collector and also not be excessive, such as can lead to a loss of mirror reflectivity, e.g., due to a thick layer of the replacement material on the surface of the collector. Those skilled in the art can easily determine empirically without undue experimentation what this amount may be for a particular size of plasma target irradiated by a particular type (e.g., particular wavelength) and intensity of laser light of a particular configuration (e.g., vis-a-vis the cross-section of the target droplet) and other such well understood factors, however, currently for a tin target droplet of around 30 μm in diameter and CO₂ drive laser irradiation at intensity levels of around 100 mJ, applicants believe that around 1–10 ppm will be sufficient and not result in over deposition, however, the ranges may be narrower, e.g., 1–5, 3–5 or 3–10, and may vary with the factors noted above.

According to aspects of an embodiment of the present invention applicants propose an arrangement where the rate of deposition and erosion are essentially equal over essentially the entire surface of the collector. Since parts of the collector are closer to the plasma initiation site than others, a higher rate of influx can exist in some regions of the collector. According to applicants' proposal, however, such different rates of deposit are not necessarily a problem since the replacement material is, e.g., uniformly mixed with the tin and thus higher influx rates of tin will be accompanied by similarly high rates of replacement material and vice-versa. A problem can develop, e.g., if the sputter rate from the tin differs over the collector surface. For example, if a buffer gas is used to sap the energy of the tin ions to limit sputtering by the tin ions, then regions of collector surface further from the LPP can experience lower rates of erosion per incident atom/ion. Thus, the required ratio of replacement material to tin can be different. Such differing rates of erosion and deposition may be avoided, according to aspects of an embodiment of the present invention by, e.g., avoiding such other methods of erosion reduction, e.g., utilization of a buffer gas or buffer plasma, excepting, e.g., those that are distance and collector position independent, such as mechanical debris shields.

Those skilled in the art will understand that an EUV light source collector erosion mitigation method and apparatus for a collector comprising a multilayered mirror collector comprising a collector outer surface composed of a capping material subject to removal due to a removing interaction with materials created in an EUV light-creating plasma, is disclosed which may comprise including within an EUV plasma source material a replacement material. The replacement material may comprise the same material as the capping material of the multilayered mirror, e.g., molybdenum, which will be understood to meet most, if not all of the criteria noted herein. The replacement material may comprise a material that is essentially transparent to light in a selected band of EUV light, e.g. a spectrum of EUV light generated in a plasma of a plasma source material. Molybdenum, as a capping material and other potential capping materials have essentially the transparency required such that while serving the purpose of being a capping layer on an EUV reflective multilayer mirror they are transparent enough that the reduction in reflectivity is acceptable given the other advantages in such a system for generating EUV light as noted above that the material, e.g., molybdenum is acceptable. The replacement material may comprise a material not susceptible to being etched by an etching material used to remove deposited plasma source material from the collector, e.g., a halogen etchant. The replacement material may comprise a material able to be laid down on the collector outer surface in a uniform smooth coating comprising a surface roughness which does not significantly impact the EUV reflectivity of the multilayer mirror of the collector. The replacement material may comprise a material that is soluble in liquid plasma source material, e.g., tin or lithium. The solubility of the replacement material in liquid tin is sufficient to deliver replacement material to the collector outer surface at a rate sufficient to effectively offset erosion of the collector outer surface, e.g., in the range of 3–5 ppm, or 1–5 ppm, or 3–10 ppm, or 1–10 ppm. Depending on erosion rates and the materials used and given solubility for the materials these ranges may vary as will be understood by those skilled in the art and selection of materials and solubility, etc. can be done as will be understood by those skilled in the art, without undue experimentation. The replacement material may comprise a material compatible with the material of the capping layer of the collector multilayer mirror. The replacement material may comprise a material that will not readily diffuse through the material of the capping layer of the collector multilayer mirror of the collector multilayer mirror or the underlying layers or do damage to the reflectivity ability of the combination of the capping layer and the underlying layers or the combination of the underlying layers. The replacement material may comprise a material that is selected from the group of materials comprising materials that can withstand exposure to ambient atmosphere, materials that can be easily removed by some means prior to exposure to ambient atmosphere and materials that have reaction products when exposed to ambient atmosphere that are easily removed from the outer surface of the collector multilayer mirror.

While the particular aspects of embodiment(s) of the EUV LIGHT SOURCE COLLECTOR EROSION MITIGATION described and illustrated in this patent application in the detail required to satisfy 35 U.S.C. §112 is fully capable of attaining any above-described purposes for, problems to be solved by or any other reasons for or objects of the aspects of an embodiment(s) above described, it is to be understood by those skilled in the art that it is the presently described aspects of the described embodiment(s) of the present invention are merely exemplary, illustrative and representative of the subject matter which is broadly contemplated by the present invention. The scope of the presently described and claimed aspects of embodiments fully encompasses other embodiments which may now be or may become obvious to those skilled in the art based on the teachings of the Specification. The scope of the present EUV LIGHT SOURCE COLLECTOR EROSION MITIGATION is solely and completely limited by only the appended claims and nothing beyond the recitations of the appended claims. Reference to an element in such claims in the singular is not intended to mean nor shall it mean in interpreting such claim element “one and only one” unless explicitly so stated, but rather “one or more”. All structural and functional equivalents to any of the elements of the above-described aspects of an embodiment(s) that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Any term used in the specification and/or in the claims and expressly given a meaning in the Specification and/or claims in the present application shall have that meaning, regardless of any dictionary or other commonly used meaning for such a term. It is not intended or necessary for a device or method discussed in the Specification as any aspect of an embodiment to address each and every problem sought to be solved by the aspects of embodiments disclosed in this application, for it to be encompassed by the present claims. No element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element in the appended claims is to be construed under the provisions of 35 U.S.C. § 112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited as a “step” instead of an “act”.

It will be understood by those skilled in the art that the aspects of embodiments of the present invention disclosed above are intended to be preferred embodiments only and not to limit the disclosure of the present invention(s) in any way and particularly not to a specific preferred embodiment alone. Many changes and modification can be made to the disclosed aspects of embodiments of the disclosed invention(s) that will be understood and appreciated by those skilled in the art. The appended claims are intended in scope and meaning to cover not only the disclosed aspects of embodiments of the present invention(s) but also such equivalents and other modifications and changes that would be apparent to those skilled in the art. In additions to changes and modifications to the disclosed and claimed aspects of embodiments of the present invention(s) noted above others could be implemented.