Patent Application
20110255068
The present disclosure relates generally to mirrors, and in particular to a an extreme ultraviolet (EUV) mirror module formed from an electroformed shell, a carrier substrate and a conductive bonding material for use in a EUV lithography system
EUV lithography is presently being pursued as the next generation lithography process for fabricating semiconductor circuits with linewidths below 50 nm. The EUV electromagnetic radiation has a wavelength typically in the range from 10 nm to 15 nm (e.g., 13.5 nm). While EUV lithography enables printing extremely fine features without diffraction-related distortions, it requires a set of high-precision-mirrors that combine high shape accuracy with extremely low surface roughness at the atomic level.
In addition to these stringent optical requirements, the high-precision mirrors must be compatible with operation in a high-vacuum environment. Where the high-precision mirrors are used as EUV collectors in an EUV collector system, they need to withstand an aggressive plasma environment where high thermal fluxes are present. As a consequence, the EUV mirrors must be fabricated using (ultra) high vacuum compatible materials and techniques, while also having thermal management capability for certain applications.
One common method of forming a high-precision EUV mirror is to machine and polish a relatively thick substrate. For example, a typical polished silicon, ZERODUR or silicon carbide mirror has a substrate thickness of about 5 cm to 10 cm. Another method of forming a high-precision mirror is to form a thin shell-type mirror, such as by electroforming. An electroformed mirror shell has a thickness of about 1 mm to 2 mm. These two mirror types have significantly different thermal properties, such as thermal mass, thermal conductance and coefficient of thermal expansion, as well as different mechanical properties, with the most notable being structural rigidity.
An advantage of forming a high-precision mirror using a relatively thick substrate is that its thermal and mechanical properties are generally more desirable than those of an electroformed mirror shell, which typically requires a web-like mechanical support structure. On the other hand, an advantage of forming a high-precision electroformed mirror is that the surface shape can easily be replicated using a mandrel, and the surface can be made to the high precision needed to meet stringent optical performance requirements.
Accordingly, there is a need for EUV mirror systems and methods whereby a relatively thin electroformed mirror can be used in an optical system, such as a EUV lithography system, designed to employ relatively thick conventional mirrors.
Additional features and advantages of the disclosure will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the disclosure as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
An aspect of the disclosure is an EUV mirror module that includes a substrate having a body with a curved upper surface and a electroformed mirror having front and rear curved surfaces. A self-adjusting bonding material is disposed between the substrate upper surface and the electroformed mirror rear surface. The bonding material is flowable at a melting temperature and serves to bond the substrate to the electroformed mirror at a temperature below the melting temperature.
Another aspect of the disclosure is EUV lithography system for illuminating a reflective mask. The system includes a source of EUV radiation and an EUV collector system that includes the EUV mirror module of the present disclosure. The EUV collector system is configured to receive the EUV radiation and form collected EUV radiation. The system also includes an illuminator configured to receive the collected EUV radiation and form condensed EUV radiation for illuminating the reflective reticle.
Another aspect of the disclosure is a method of forming an EUV mirror module. The method includes providing a substrate having a body with a curved upper surface, and providing a electroformed mirror having front and rear curved surfaces. The method also includes disposing a self-adjusting bonding material between the substrate upper surface and the electroformed mirror rear surface, the bonding material having a melting point. The method also includes heating the bonding material to its melting point, thereby causing the bonding material to flow between the substrate upper surface and the electroformed mirror rear surface. The method additionally includes cooling the bonding material to below its melting point, thereby forming a thermally conductive bond between the electroformed mirror and the substrate.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the disclosure, and are intended to provide an overview or framework for understanding the nature and character of the disclosure as it is claimed. The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the disclosure and together with the description serve to explain the principles and operations of the disclosure.
The various elements depicted in the drawing are merely representational and are not necessarily drawn to scale. Certain sections thereof may be exaggerated, while others may be minimized. The drawing is intended to illustrate an example embodiment of the disclosure that can be understood and appropriately carried out by those of ordinary skill in the art.
Mirror module 10 has a central axis A1 and includes a relatively thick carrier substrate (“substrate”) 20 having a substrate body 21 with a curved upper surface 22 and an outer edge 26. Mirror module 10 also includes a relatively thin electroformed mirror 30 having a front surface 32, a rear surface 34 and an outer edge 36. Mirror module 10 also includes a self-adjusting bonding material 40 disposed between substrate upper surface 22 and electroformed mirror rear surface 34 and that bonds the substrate to the electroformed mirror.
In an example embodiment, bonding material 40 has good thermal conductivity that provides for thermal conduction between substrate upper surface 22 and electroformed mirror rear surface 34. A region R in which bonding material 40 resides is shown between substrate upper surface 22 and mirror surface 34. This region R in practice is relatively small, and bonding material 40 ends up self-adjusting to conformally fill the region when the mirror module is assembled, as described below in connection with
In an example embodiment, mirror module 10 has substantially the same mechanical dimensions, relevant physical properties and optical function as a single, thick monolithic mirror fabricated using conventional mirror fabrication methods. An example thickness of substrate 20 is between 5 cm and 10 cm. An example thickness of electroformed mirror 30 is between 1 mm and 2 mm. An example thickness of bonding material 40 is between 10 microns and 1000 microns.
Substrate upper surface 22 need not have the exact surface figure as electroformed mirror 30 since bonding material 40 serves as an initially deformable interface layer between the substrate and the electroformed mirror, as described below. In an example, substrate upper surface 22 has an intermediate surface accuracy, e.g., 10 microns. Such a surface accuracy can be achieved with conventional machining methods at a reasonable cost. Experiments on an example mirror module 10 show that the shape accuracy of the electroformed mirror front surface 32 can be maintained much better than the surface accuracy of a machined substrate surface 22, i.e., to about 1 micron root-mean square (RMS). Thus, in an example, electroformed mirror front and rear surfaces 32 and 34 have a curvature (surface figure) that only approximately or substantially matches that of substrate surface 22.
In an example embodiment, substrate 20 and electroformed mirror 30 are made of the same material so that at least these two components of the mirror module have a substantially uniform coefficient of thermal expansion (CTE). Example materials for substrate 20 include nickel, nickel alloys, copper and copper alloys, silicon, silicon carbide and ZERODUR. Example materials for electroformed mirror 30 include nickel, nickel alloys, copper and copper alloys.
An example bonding material 40 is a low-temperature bonding solder, such as those that include indium (e.g., Indalloy 3, which is 90% In and 10% Ag). In one example, bonding material 40 is in the form of a metallic foil (e.g., an indium or an indium-based foil), e.g., 200 microns thick. Bonding material 40 preferably has a relatively low melting temperature T40, e.g., in the range from 140° C. to 250° C. A typical melting temperature T40 for an indium-based solder is about 150° C. A relatively low melting temperature T40 allows for bonding material 40 to be disposed in its solid form in a region R between substrate 20 and electroformed mirror 30 at room temperature. An example of this arrangement is illustrated in the schematic cross-sectional view of
Bonding materials 40 in the form of a metallic conductive material such as Indium have the advantage of being compatible with operation in a ultra-high vacuum, or an aggressive plasma environment. Unlike organic epoxies, Indium does not degrade or outgas, even upon exposure to a plasma.
Once bonding material 40 is so arranged, then with reference to
Note that bonding material 40 when melted is capable of flowing and filling in any surface irregularities in substrate upper surface 22 and to generally make up for differences in curvature between the substrate upper surface and the curvature of electroformed mirror rear surface 34. In the embodiment where bonding material 40 has good thermal conductivity, it also serves to provide efficient thermal contact between substrate 20 and electroformed mirror 30.
During this self-adjusting step, there is no need for rigid mechanical constraints on mirror module 10 because the flowing bonding material has no stiffness. Therefore, electroformed mirror 30 will tend to adjust to its intrinsic, free-standing shape, with some predictable and systematic long-range adjustments for capillary forces and thermal expansion. The flowing bonding material 40 self-adjusts to short-range fluctuations in region R between substrate upper surface 22 and mirror surface 34.
In an example, one or both of substrate surface 22 and electroformed mirror rear surface 34 is/are treated to facilitate the bonding process, such as by depositing an adhesion or wetting layer to one or both of these surfaces.
There are many optical system environments, such as those associated with EUV lithography system, laser fusion systems or synchrotron mirrors, where heating could raise the temperature of mirror module 10 to beyond the bonding material melting temperature T40, or deform the mirror surface. Thus, with reference to
The relatively thin layer of bonding material 40 provides a sufficient thermal contact between substrate 20 and electroformed mirror 30 in the axial direction to allow for placing cooling channels 100 relatively close to substrate surface 22 while avoiding print-through. In an example, channels 100 are located a distance D1 from substrate upper surface 22, where distance D1 is in the range from 5 mm to 20 mm In addition, substrate 20 provides for efficient lateral heat diffusion that serves to mitigate print-through effects by smearing their signature. In sum, cooling channels 100 can be configured to provide for short and efficient thermal diffusion paths from electroformed mirror 30 to cooling channels 100.
Mirror module 10 of
For example, consider an application where mirror module 10 is used to reflect a collimated and substantially uniform light beam of high intensity that subjects the mirror module to a substantially uniform heat load. In this case, cooling lines 100 are preferably configured to provide uniform cooling over most of if not the entire area of electroformed mirror front surface 32. On the other hand, consider an application where mirror module 10 is used to reflect a substantially non-uniform light beam of high intensity that subjects the mirror module to a substantially non-uniform heat load. In this case, cooling lines 100 are preferably configured to provide non-uniform cooling over most if not all of the area of electroformed mirror front surface 32 in a manner that corresponds to the anticipated non-uniform thermal load.
System 200 includes a system axis AS and EUV light source LS, such as a hot plasma source, that emits working EUV radiation 302 at λ=13.5 nm. EUV radiation 202 is generated, for example, by an electrical discharge source (e.g., a discharged produced plasma, or DPP source), or by a laser beam (laser-produced plasma, or LPP source) on a target of Lithium, Xenon or Tin. EUV radiation 202 emitted from such a LPP source may be roughly isotropic and, in current DPP sources, is limited by the discharge electrodes to a source emission angle of about 60° or more from optical axis AS. It is noted that the isotropy of the LPP source that uses target pellets will depend on the mass of the target pellet. For relatively high-mass targets, the emission is anisotropic, with most of the emerging radiation headed back toward the laser beam due to absorption in the forward direction by the target mass. For a low mass LPP target, which is almost entirely ionized by the laser, the emission is much closer to isotropic.
System 200 includes a mirror module 10 as described above that serves as an EUV collector system 210. Collector system 210 is arranged adjacent and downstream of EUV light source LS, with collector axis AC lying along system axis AS. EUV collector system 210 collects EUV working radiation 202 from EUV light source LS located at source focus SF and forms intermediate source image SI at intermediate focus IF.
An illumination system 216 with an input end 217 and an output end 218 is arranged along system axis AS and adjacent and downstream of EUV collector system 210, with the input end adjacent the EUV collector system. Illumination system 216 receives at input end 217 EUV radiation 202 from source image IS and outputs at output end 218 a substantially uniform EUV radiation beam 220 (i.e., condensed EUV radiation). Where system 200 is a scanning type system, EUV radiation beam 220 is typically formed as a substantially uniform line of EUV radiation at reflective reticle 236 that scans over the reticle.
A projection optical system 226 is arranged along (folded) system axis AS downstream of illumination system 216. Projection optical system 226 has an input end 227 facing illumination system output end 218, and an opposite output end 228. A reflective reticle 236 is arranged adjacent the projection optical system input end 227 and a semiconductor wafer 240 is arranged adjacent projection optical system output end 228. Reticle 236 includes a pattern (not shown) to be transferred to wafer 240, which includes a photosensitive coating (e.g., photoresist layer) 242.
In operation, the uniformized EUV radiation beam 220 irradiates reticle 236 and reflects therefrom, and the pattern thereon is imaged onto photosensitive surface 242 of wafer 240 by projection optical system 226. In a scanning system 200, the reticle image scans over the photosensitive surface to form the pattern over the exposure field. Scanning is typically achieved by moving reticle 236 and wafer 240 in synchrony.
Once the reticle pattern is imaged and recorded on wafer 240, the patterned wafer 240 is then processed using standard photolithographic and semiconductor processing techniques to form integrated circuit (IC) chips.
Note that in general the components of system 200 are shown lying along a common folded axis AS in
In the past, replacing a conventional mirror with an electroformed mirror in a high-performance optical system required substantial system-level changes. At a minimum, the mechanical mounting structure for the mirror needed to be changed. If an active thermal control system was also used, this control system needed to be modified to take into account the considerably different thermal mass and backside infrared reflectivity coefficient. As a consequence, it was highly likely that other system components, such as the mounting stage, the heater assembly and the control electronics would also need to be substantially modified or changed altogether.
Mirror module 10 greatly simplifies the proposition of changing out the conventional mirror with an electroformed mirror in an optical system. Mirror module 10 can be formed to have dimensions, a thermal mass, a backside reflectivity coefficient and other properties that are very close to those of a particular conventional mirror. For example, in the case of a mirror collector for a laser-produced plasma (LPP) light source, 96% to 99% of the volume of the mirror module can be made very close to or identical to that that of the counterpart conventional mirror. Further, the mirror module thermal properties can be made sufficiently close to that of the counterpart conventional mirror so that the thermal performance of the two mirrors in the optical system would be substantially the same. Thus, from an optical-system-level point of view, a mirror module of the present disclosure can be used in an optical system with very little difference and with minimal adjustment or modification of the optical system and its components.
Also, because precision machining and super-polishing are dominant factors in the cost of conventional mirrors, use of a mirror module can be cost effective for many optical system applications that presently require conventional mirrors.
Accordingly, cost-effective electroformed mirror-based modules can be used as spare parts of optical systems that were originally designed to use a conventional mirror.
Another advantage of mirror module 10 is that the use of a low-temperature bonding material 40 allows one to quickly disassemble the mirror module by de-bonding substrate 20 from electroformed mirror 30 by heating the mirror module to a moderate temperature. This disassembly may be performed, for example, at the end of the lifetime of the electroformed mirror 30 or its reflective coating so that a new electroformed mirror may be bonded to substrate 20 to form a refurbished mirror module.
It will be apparent to those skilled in the art that various modifications and variations can be made to the present disclosure without departing from the spirit and scope of the disclosure. Thus it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.
