EUV reticle substrates with high thermal conductivity

A reflective reticle substantially reduces or eliminates pattern distortion that results from the absorption of EUV radiation while maintaining a reticle thickness consistent with industry standards. The reflective reticle includes a layer of ultra-low expansion (ULE) glass and a substrate of Cordierite having a thermal conductivity substantially larger than that of ULE glass. An aluminum layer is disposed onto a first surface of the ULE glass and a second surface of the ULE glass is polished to be substantially flat and defect-free. The Cordierite substrate can be directly bonded to the aluminum layer using anodic bonding to form the reflective reticle. Alternatively, a first surface of an intermediate Zerodur layer can be bonded to the aluminum layer, and a second aluminum layer can be used to anodically bond the Cordierite substrate to a second surface of the Zerodur layer, thereby forming the reflective reticle.

BACKGROUND

1. Field of the Invention

The present invention relates to patterning devices for use in lithographic apparatus.

2. Related Art

Lithography is widely recognized as a key process in manufacturing integrated circuits (ICs) as well as other devices and/or structures. A lithographic apparatus is a machine, used during lithography, which applies a desired pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device (which is alternatively referred to as a mask or a reticle) generates a circuit pattern to be formed on an individual layer in an IC. This pattern may be transferred onto the target portion (e.g., including part of, one, or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate contains a network of adjacent target portions that are successively patterned. Manufacturing different layers of the IC often requires imaging different patterns on different layers with different reticles. Therefore, reticles must be changed during the lithographic process.

Existing extreme-ultraviolet (EUV) lithographic apparatus incorporate reflective reticles having substrates formed from ultra-low expansion (ULE) glass, a glass-ceramic material having a coefficient of thermal expansion that is substantially zero across a wide range of operating temperatures. The selection of ULE glass as a substrate is based on the coefficient of thermal expansion of the ULE glass and on an ability to polish a surface of the ULE glass to the fine surface requirements necessary for EUV lithographic applications (i.e., surfaces that exhibit very low roughness, that are substantially free of defects, and that are substantially flat).

In general, existing reflective reticles for EUV lithographic apparatus exhibit a reflectance of approximately 70%. Therefore, depending on a pattern to be printed, an existing reflective reticle can absorb between approximately 30% and 100% of the energy of an incident EUV radiation beam. Such absorption can lead to significant heating of the reticle, which can distort the reticle surface, in spite of the relatively-low coefficient of thermal expansion of the ULE glass substrate, and introduce errors in the projected image.

Further, even if a back side of such a reflective reticle were optimally cooled, the absorption of EUV radiation could result in an excessively large thermal gradient across a thickness of a reticle having a ULE glass substrate. Such excessively-large thermal gradients can result from the relatively-low thermal conductivity of the ULE glass substrate, which promotes a relatively high thermal resistance within the ULE glass substrate and hence, within the reticle. One modification to existing reticle designs that would reduces the thermal resistance of the reticle is to thin the ULE glass substrate, and hence thin the reticle. However, this modification can produce extreme, and potentially insurmountable, difficulties in keeping the patterned surface flat. In addition, such a reticle would deviate from accepted industry thickness for EUV reflective reticles (e.g., approximately 6.35 mm±0.10 mm).

SUMMARY

Therefore, what is needed is a reflective reticle for use in EUV lithographic applications that substantially reduces or eliminates pattern distortion due to the absorption of EUV radiation, while maintaining a reticle thickness consistent with industry standards, thereby substantially obviating the drawbacks of the conventional systems.

In one embodiment, a reticle includes an optical layer having a first surface and a second surface. The reticle also includes a substrate having a thermal conductivity substantially larger than a thermal conductivity of the optical layer. A conductive layer is disposed between the optical layer and the substrate. The conductive layer is bonded to one or more of (i) a surface of the substrate and (ii) the first surface of the optical layer. For example, the optical layer can be a material having a substantially-zero coefficient of thermal expansion, the substrate can be a material having a substantially-zero coefficient of thermal expansion, and the conductive layer can be aluminum.

In an further embodiment, a lithographic apparatus includes an illumination system configured to produce a beam of radiation, a reticle configured to pattern the beam of radiation, and a projection system configured to project the patterned beam onto a target portion of a substrate. The reticle includes an a optical layer having a first surface and a second surface. The reticle also includes a substrate having a thermal conductivity substantially larger than a thermal conductivity of the optical layer. A conductive layer is disposed between the optical layer and the substrate, and the conductive layer is bonded to one or more of (i) a surface of the substrate and (ii) the first surface of the optical layer. For example, the optical layer can be a material having a substantially-zero coefficient of thermal expansion, the substrate can be a material having a substantially-zero coefficient of thermal expansion, and the conductive layer can be aluminum.

In a further embodiment, there is provided a method for making a reticle that disposes a layer of a conductive material onto a first surface of an optical layer. The layer of conductive material is then bonded to one of (i) a first surface of an intermediate layer or (ii) a surface of a substrate having a thermal conductivity substantially larger than a thermal conductivity of the optical layer.

In a further embodiment, there is provided a method for fabricating reticles for use in an extreme ultraviolet lithography (EUVL) system. A thick substrate is bonded to a thin film multilayer coating to provide an EUVL reticle having a first thermal conductivity that is relatively higher than a second thermal conductivity of a reflective lithography reticle formed with a single material layer.

One or more embodiments of the present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers can indicate identical or functionally similar elements.

DETAILED DESCRIPTION

The present invention is directed to reticles that include substrates having high thermal conductivities, and in particular, to substrates for EUV reflective reticles having high thermal conductivities. This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the invention. The scope of the invention is not limited to the disclosed embodiment(s). The invention is defined by the claims appended hereto.

In various embodiments, a reflective reticle suitable for use in an EUV lithographic apparatus includes an optical layer having a coefficient of thermal expansion that is substantially zero across a range of temperatures to which the reticle is exposed in the EUV lithographic apparatus. The optical layer includes a first surface, onto which a conductive layer is disposed, and a second surface, which can be polished to be substantially flat and substantially free of defects. For example, the optical layer can be formed from ultra-low expansion (ULE) titanium-silicate glass and the conductive layer can be aluminum.

In an embodiment, the conductive layer is bonded directly to a first surface of a substrate having a coefficient of thermal expansion that is substantially zero across a range of operating temperatures and a thermal conductivity substantially higher than that of the optical layer. In such an embodiment, the substrate can be formed from Cordierite, which has a thermal conductivity approximately three times larger than that of ULE glass. The bonded substrate and optical layer form a reticle suitable for use in EUV lithographic applications.

In an additional embodiment, a second conductive layer can be disposed onto a first surface of the substrate. Further, the first layer of conductive material can then be bonded to a first surface of an intermediate layer, and the second conductive layer can be bonded to a second surface of the intermediate layer. For example, the substrate can be formed from Cordierite, as described above, the intermediate layer can be formed from Zerodur, a non-porous, inorganic glass ceramic material, and the second conductive layer can be aluminum. In such an embodiment, the bonded optical layer, intermediate layer, and substrate form a reticle suitable for use in EUV lithographic applications.

These reflective reticles, as described below in their various embodiments, substantially reduce or eliminate pattern distortion that results from the absorption of EUV radiation, while maintaining a reticle thickness consistent with industry standards. As such, these reflective reticles substantially obviating the drawbacks of the existing EUV reticle technologies.

Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

II. An Example Lithographic Environment

A. Example Reflective and Transmissive Lithographic Systems

FIGS. 1A and 1Bschematically depict lithographic apparatus100and lithographic apparatus100′, respectively. Lithographic apparatus100and lithographic apparatus100′ each include: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., DUV or EUV radiation); a support structure (e.g., a mask table) MT configured to support a patterning device (e.g., a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position the patterning device MA; and a substrate table (e.g., a wafer table) WT configured to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatuses100and100′ also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (e.g., comprising one or more dies) C of the substrate W. In lithographic apparatus100the patterning device MA and the projection system PS is reflective, and in lithographic apparatus100′ the patterning device MA and the projection system PS is transmissive.

The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA, the design of the lithographic apparatuses100and100′, and other conditions, such as for example whether or not the patterning device MA is held in a vacuum environment. The support structure MT may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which may be fixed or movable, as required. The support structure MT may ensure that the patterning device is at a desired position, for example with respect to the projection system PS.

The term “patterning device” MA should be broadly interpreted as referring to any device that may be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B may correspond to a particular functional layer in a device being created in the target portion C, such as an integrated circuit.

The term “projection system” PS may encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid or the use of a vacuum. A vacuum environment may be used for EUV or electron beam radiation since other gases may absorb too much radiation or electrons. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

Lithographic apparatus100and/or lithographic apparatus100′ may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables) WT. In such “multiple stage” machines the additional substrate tables WT may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other substrate tables WT are being used for exposure.

Referring toFIGS. 1A and 1B, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatuses100,100′ may be separate entities, for example when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatuses100or100′, and the radiation beam B passes from the source SO to the illuminator IL with the aid of a beam delivery system BD (FIG. 1B) comprising, for example, suitable directing mirrors and/or a beam expander. In other cases, the source SO may be an integral part of the lithographic apparatuses100,100′—for example when the source SO is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD, if required, may be referred to as a radiation system.

Referring toFIG. 1A, the radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus100, the radiation beam B is reflected from the patterning device (e.g., mask) MA. After being reflected from the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2(e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT may be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor IF1may be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B. Patterning device (e.g., mask) MA and substrate W may be aligned using mask alignment marks M1, M2and substrate alignment marks P1, P2.

Referring toFIG. 1B, the radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support structure (e.g., mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted inFIG. 1B) can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g., after mechanical retrieval from a mask library, or during a scan.

In general, movement of the mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies.

The lithographic apparatuses100and100′ may be used in at least one of the following modes:

2. In scan mode, the support structure (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g., mask table) MT may be determined by the magnification (or de-magnification) and image reversal characteristics of the projection system PS.

3. In another mode, the support structure (e.g., mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiation source SO may be employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation may be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to herein.

Combinations and/or variations on the described modes of use or entirely different modes of use may also be employed.

In a further embodiment, lithographic apparatus100includes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV source is configured in a radiation system (see below), and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

FIG. 2schematically depicts an exemplary EUV lithographic apparatus200according to an embodiment of the present invention. InFIG. 2, EUV lithographic apparatus200includes a radiation system42, an illumination optics unit44, and a projection system PS. The radiation system42includes a radiation source SO, in which a beam of radiation may be formed by a discharge plasma. In an embodiment, EUV radiation may be produced by a gas or vapor, for example, from Xe gas, Li vapor, or Sn vapor, in which a very hot plasma is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma can be created by generating at least partially ionized plasma by, for example, an electrical discharge. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. The radiation emitted by radiation source SO is passed from a source chamber47into a collector chamber48via a gas barrier or contaminant trap49positioned in or behind an opening in source chamber47. In an embodiment, gas barrier49may include a channel structure.

Collector chamber48includes a radiation collector50(which may also be called collector mirror or collector) that may be formed from a grazing incidence collector. Radiation collector50has an upstream radiation collector side50aand a downstream radiation collector side50b, and radiation passed by collector50can be reflected off a grating spectral filter51to be focused at a virtual source point52at an aperture in the collector chamber48. Radiation collectors50are known to skilled artisans.

From collector chamber48, a beam of radiation56is reflected in illumination optics unit44via normal incidence reflectors53and54onto a reticle or mask (not shown) positioned on reticle or mask table MT. A patterned beam57is formed, which is imaged in projection system PS via reflective elements58and59onto a substrate (not shown) supported on wafer stage or substrate table WT. In various embodiments, illumination optics unit44and projection system PS may include more (or fewer) elements than depicted inFIG. 2. For example, grating spectral filter51may optionally be present, depending upon the type of lithographic apparatus. Further, in an embodiment, illumination optics unit44and projection system PS may include more mirrors than those depicted inFIG. 2. For example, projection system PS may incorporate one to four reflective elements in addition to reflective elements58and59. InFIG. 2, reference number180indicates a space between two reflectors, e.g., a space between reflectors142and143.

In an embodiment, collector mirror50may also include a normal incidence collector in place of or in addition to a grazing incidence mirror. Further, collector mirror50, although described in reference to a nested collector with reflectors142,143, and146, is herein further used as example of a collector.

Further, instead of a grating51, as schematically depicted inFIG. 2, a transmissive optical filter may also be applied. Optical filters transmissive for EUV, as well as optical filters less transmissive for or even substantially absorbing UV radiation, are known to skilled artisans. Hence, the use of “grating spectral purity filter” is herein further indicated interchangeably as a “spectral purity filter,” which includes gratings or transmissive filters. Although not depicted inFIG. 2, EUV transmissive optical filters may be included as additional optical elements, for example, configured upstream of collector mirror50or optical EUV transmissive filters in illumination unit44and/or projection system PS.

The terms “upstream” and “downstream,” with respect to optical elements, indicate positions of one or more optical elements “optically upstream” and “optically downstream,” respectively, of one or more additional optical elements. Following the light path that a beam of radiation traverses through lithographic apparatus200, a first optical elements closer to source SO than a second optical element is configured upstream of the second optical element; the second optical element is configured downstream of the first optical element. For example, collector mirror50is configured upstream of spectral filter51, whereas optical element53is configured downstream of spectral filter51.

All optical elements depicted inFIG. 2(and additional optical elements not shown in the schematic drawing of this embodiment) may be vulnerable to deposition of contaminants produced by source SO, for example, Sn. Such may be the case for the radiation collector50and, if present, the spectral purity filter51. Hence, a cleaning device may be employed to clean one or more of these optical elements, as well as a cleaning method may be applied to those optical elements, but also to normal incidence reflectors53and54and reflective elements58and59or other optical elements, for example additional mirrors, gratings, etc.

Radiation collector50can be a grazing incidence collector, and in such an embodiment, collector50is aligned along an optical axis O. The source SO, or an image thereof, may also be located along optical axis O. The radiation collector50may comprise reflectors142,143, and146(also known as a “shell” or a Wolter-type reflector including several Wolter-type reflectors). Reflectors142,143, and146may be nested and rotationally symmetric about optical axis O. InFIG. 2, an inner reflector is indicated by reference number142, an intermediate reflector is indicated by reference number143, and an outer reflector is indicated by reference number146. The radiation collector50encloses a certain volume, i.e., a volume within the outer reflector(s)146. Usually, the volume within outer reflector(s)146is circumferentially closed, although small openings may be present.

Reflectors142,143, and146respectively may include surfaces of which at least portion represents a reflective layer or a number of reflective layers. Hence, reflectors142,143, and146(or additional reflectors in the embodiments of radiation collectors having more than three reflectors or shells) are at least partly designed for reflecting and collecting EUV radiation from source SO, and at least part of reflectors142,143, and146may not be designed to reflect and collect EUV radiation. For example, at least part of the back side of the reflectors may not be designed to reflect and collect EUV radiation. On the surface of these reflective layers, there may in addition be a cap layer for protection or as optical filter provided on at least part of the surface of the reflective layers.

The radiation collector50may be placed in the vicinity of the source SO or an image of the source SO. Each reflector142,143, and146may comprise at least two adjacent reflecting surfaces, the reflecting surfaces further from the source SO being placed at smaller angles to the optical axis O than the reflecting surface that is closer to the source SO. In this way, a grazing incidence collector50is configured to generate a beam of (E)UV radiation propagating along the optical axis O. At least two reflectors may be placed substantially coaxially and extend substantially rotationally symmetric about the optical axis O. It should be appreciated that radiation collector50may have further features on the external surface of outer reflector146or further features around outer reflector146, for example a protective holder, a heater, etc.

In the embodiments described herein, the terms “lens” and “lens element,” where the context allows, may refer to any one or combination of various types of optical components, comprising refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

Further, the terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, comprising ultraviolet (UV) radiation (e.g., having a wavelength λ of 365, 248, 193, 157 or 126 nm), extreme ultra-violet (EUV or soft X-ray) radiation (e.g., having a wavelength in the range of 5-20 nm, e.g., 13.5 nm), or hard X-ray working at less than 5 nm, as well as particle beams, such as ion beams or electron beams. Generally, radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, it is usually also applied to the wavelengths, which can be produced by a mercury discharge lamp: G-line 436 nm; H-line 405 nm; and/or I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by air), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in an embodiment, an excimer laser can generate DUV radiation used within lithographic apparatus. It should be appreciated that radiation having a wavelength in the range of, for example, 5-20 nm relates to radiation with a certain wavelength band, of which at least part is in the range of 5-20 nm.

III. Exemplary Substrates for EUV Reticles Having High Thermal Conductivities

FIG. 3schematically depicts an example of an existing EUV reflective reticle300that is formed from a single layer of substrate material. InFIG. 3, a reticle300includes a substrate302onto which one or more layers of highly-reflective material have been disposed to form a reflective layer304. A pattern (not shown) can be formed onto a reflective surface304aof layer304through chemical etching of a patterned layer of resist or, additionally or alternatively, through any other technique apparent to one skilled in the art.

Substrates of existing EUV reflective reticles, such as substrate302, are often constructed from optical-grade glasses, ceramics, glass-ceramic materials, and other materials that are characterized by a relatively-low coefficient of thermal expansion and a high modulus of elasticity over a wide range of temperatures, e.g., including a range of temperatures to which the substrate is exposed in the EUV lithographic apparatus. For example, substrate302can be formed from ultra-low expansion titanium-silicate glass, such as that manufactured by Corning, Incorporated, of Corning, N.Y., and sold under the trademark ULE.

However, substrates constructed from these material also exhibit relatively-low values of thermal conductivity across the range of temperatures to which they are exposed in the EUV lithographic apparatus. For example, a mean thermal conductivity of ULE glass is approximately 1.31 W/(m-° C.) at 25° C., while that of aluminum is approximately 250 W/(m-° C.) at 25° C. Such values of thermal conductivity can lead to a relatively large thermal resistance across a thickness of the substrate (i.e., a quotient of the thickness of the substrate and its thermal conductivity), thereby inhibiting a uniform distribution of heat throughout the substrate and conduction of heat from the substrate to surrounding portions of the EUV lithographic apparatus, including, but not limited to one or more devices supporting the reticle.

As described above, existing EUV reticles an absorb between approximately 30% and 100% of the energy of an incident EUV radiation beam. Such absorption of EUV radiation by the reticle can result in localized heating of the substrate that, due to the mechanical properties of the substrate, is often unable to diffuse throughout the substrate or be conducted away from the substrate. In such instances, this heating can locally deform the substrate and therefore, a patterned surface of the corresponding reflective layer (e.g., surface308aof reflective layer308inFIG. 3). Further, thermally-driven distortion of the pattern surface can distort the pattern imparted onto an incident radiation beam and introduce errors into an image projected by the EUV lithographic apparatus onto a substrate.

In general, existing EUV lithographic apparatus are unable to compensate for more than a small fraction of the errors introduced into the patterned image by the thermal distortion of the patterning surface. As such, the thermal distortion of both the patterned image and the patterning surface is a factor that limits imaging performance in existing EUV lithographic apparatus. Further, the problem of pattern distortion due to reticle heating will likely be exacerbated as more energy is delivered to reflective reticles to meet the increased throughput demands of volume manufacturing in EUV lithographic apparatus.

In an embodiment, the effects of localized reticle heating due to radiation absorption can be mitigated by increasing a thermal conductivity of a substrate in a reflective reticle. By increasing the thermal conductivity of the substrate, and thereby lowering a thermal resistance of the substrate (at a constant thickness), localized heating due to absorbed radiation can be more uniformly distributed throughout the substrate and can be more effectively conducted away from the substrate to surrounding support devices, including, but not limited to a reticle chuck or mask table. Therefore, an increase in the thermal conductivity of the reticle substrate can substantially reduce or eliminate any induced distortion of the patterning surface, and thus, any induced errors in the patterned image.

One suitable substrate material for inclusion into a reflective reticle for EUV lithographic apparatus is Cordierite, a ceramic material available from a number of suppliers, including, but not limited to, Hitachi Metals America Ltd. of Tarrytown, N.Y. Cordierite, while having a coefficient of thermal expansion that is substantially zero over the range of operating temperature experienced by EUV reticles, also has a thermal conductivity that is approximately three times larger than existing reticle substrate materials. For example, while ULE glass has a thermal conductivity of approximately 1.31 W/(m-° C.) at 25° C., a thermal conductivity of Cordierite at 25° C. is approximately 3.0 W/(m-° C.).

However, the microstructure of solid Cordierite renders the material unsuitable for use as a substrate in existing reflective reticles. Solid Cordierite incorporates microscopic voids that, upon polishing, form holes and other defects in a polished Cordierite surface. The presence of these surface defects renders the polished Cordierite surface unsuitable for the application of reflective materials to form a reflective layer of the reticle, e.g., reflective layer308of reticle300inFIG. 3.

In an embodiment, the microstructural defects that render Cordierite unsuitable for use as a substrate in an existing reflective reticle can be remedied by binding a relatively-thin optical layer of material to a surface of the Cordierite substrate. In such an embodiment, and as depicted inFIGS. 4A and 4B, the thin optical layer can be processed (e.g., polished) to generate a substantially-flat surface that is free of those defects characteristic of polished Cordierite surfaces and is suitable for the application of reflective materials. In such an embodiment, the optical layer is sufficiently thin to provide a small thermal resistance, while providing a conventional glass surface on which to perform surface polishing, film application, and patterning.

FIG. 4Ais an exploded, schematic view of an exemplary reflective reticle400suitable for use in a lithographic apparatus, according to an embodiment of the present invention. In contrast to existing reflective reticle technologies for EUV lithographic apparatus, such as that depicted inFIG. 3, reticle400includes a substrate402, an optical layer404, and conductive layer406disposed between substrate402and optical layer404. In such an embodiment, the composite nature of reticle400substantially reduces or eliminates the distortion of the reticle surface and the introduction of errors in the patterned image due to the absorption of radiation that is characteristic of existing EUV reflective reticles.

InFIG. 4A, optical layer404has a first surface404aand a second surface404b, and substrate402has a first surface402aand a second surface402b. In such an embodiment, conductive layer406is disposed between first surface404aof optical layer404and first surface402aof substrate402. Further, in an embodiment, conductive layer406can be disposed onto first surface404aof optical layer404.

In an embodiment, conductive layer406can be a metallic layer, including, but not limited to aluminum, a non-metallic conducting material, such as graphite, or any combination thereof. Further, in an embodiment, conductive layer406can be deposited onto first surface404aof optical layer404through any of a number of deposition techniques apparent to one skilled in the art and appropriate to the materials. For example, conductive layer406can be deposited on first surface404aof optical layer404using sputter deposition or spray deposition. In additional embodiments, conductive layer406can be pre-fabricated and bonded to first surface404aof optical layer404using any additional or alternate technique appropriate to conductive layer406and optical layer404.

FIG. 4Bschematically depicts additional features of exemplary reticle400depicted inFIG. 4A. InFIG. 4B, reticle400has been formed by bonding conductive layer406, which is disposed onto surface404a, to a first surface402aof substrate402to form reticle400. In an embodiment, conductive layer406can be bonded to first surface402ausing any number of techniques apparent to one skilled in the art, including, but not limited to, anodic bonding.

In an embodiment, a second surface402bof reticle400(and substrate402) can be supported by a reticle chuck, mask table, or any other device within the EUV lithography apparatus. In such an embodiment, the reticle check, mask table, or other device can act as a heat sink for reticle400, thereby facilitating the transfer of heat from reticle400to various components of the EUV lithographic apparatus.

In an embodiment, and as depicted inFIG. 4B, optical layer404can be bonded, or otherwise attached, to substrate402such that first layer404aof optical layer404is substantially parallel with first surface402aof substrate402. Further, second surface404bof optical layer404is also substantially parallel with first surface404a, and similarly, second surface402bof substrate402is substantially parallel with first surface402a. However, the present invention is not limited to substrates and optical layers bonded or otherwise attached in such configurations. In an additional embodiments, respective first and second surfaces of substrate402and optical layer404can be oriented, respectively in any configuration, including, but not limited to, arranged at any angle with respect to each other, without departing from the spirit or scope of the present invention.

In an embodiment, and as described above, optical layer404can be formed from a material having a coefficient of thermal expansion that is substantially zero over a range of temperatures experienced by reticle400. For example, optical layer404can be formed from ultra-low expansion (ULE) titanium silicate glass, which is manufactured by Corning, Incorporated, of Corning, N.Y. In such an embodiment, a thickness of optical layer404can be selected to maintain a relatively low thermal resistance, while providing a surface of sufficient integrity to support polishing and an application of reflective films. For example, the thickness of optical layer404could range from approximately 0.1 mm to approximately 0.5 mm, although thicknesses as low as approximately 0.025 mm may be possible.

Further, and as described above, substrate402can be formed from Cordierite, which has a substantially-zero coefficient of thermal expansion over the range of temperature and which also has a thermal conductivity approximately three times larger than that of the optical layer (e.g., Cordierite has a thermal conductivity of approximately 3.0 W/(m-° C.) at 25° C., and ULE glass has a thermal conductivity of approximately 1.31 W/(m-° C.) at 25° C.). In an embodiment, a thickness of substrate402can range from approximately 5.25 mm to approximately 6.25 mm.

As described above, reticle400, when incorporated into an EUV lithographic apparatus, can absorb between approximately 30% and 100% of an incident EUV radiation beam. However, in the embodiment ofFIGS. 4A and 4B, localized heating of optical layer404, which can result from absorption of EUV radiation, is rapidly diffused or conducted through optical layer404due to its low thermal resistance and through conductive layer406into substrate402. Further, as the thermal conductivity of substrate402is substantially higher than a thermal conductivity of substrates of existing EUV reticles, localized heating due to absorption of EUV radiation uniformly diffuses throughout the substrate and is rapidly dissipated through the substrate into a reticle check, mask table, or other structure that supports reticle400within the EUV lithographic apparatus. Therefore, and in contrast to the existing EUV reticle depicted inFIG. 3, reticle400substantially reduces or eliminates any distortion of the reticle surface, and hence, any induced pattern errors, due to localized heating from absorbed EUV radiation.

In the embodiments ofFIGS. 4A and 4B, conductive layer406is disposed on first surface404aof optical layer404, and conductive layer406is subsequently bonded to first surface402aof substrate402. However, in additional embodiments, an intermediate layer of material may further insulate optical layer404from substrate402. For example, a substrate formed from Cordierite may not be sufficiently electrically conductive to be anodically bonded to conductive layer406. In such an embodiment, the intermediate layer can be positioned between the substrate402and optical layer404to facilitate such anodic bonding.

FIG. 5Ais an exploded, schematic view of an exemplary reflective reticle500for use in an EUV lithographic system, according to an additional embodiment of the present invention. In contrast to the embodiment ofFIG. 4, reticle500includes an intermediate layer530that separates substrate502from an optical layer504. In such an embodiment, intermediate layer530facilitates bonding between optical layer504and substrate502.

Similar to the embodiment ofFIGS. 4A and 4B, an optical layer504has a first surface504aand a second surface504b. In an embodiment, second surface504bcan be processed to be substantially flat and free of defects through any of a number of techniques apparent to one skilled in the art, including, but not limited to polishing using various abrasive compounds.

A conductive layer506is disposed between first surface504aof optical layer504and a first intermediate surface530aof intermediate layer530. In the embodiment ofFIG. 5A, conductive layer506is disposed onto first surface504a. However, the present invention is not limited to such configurations, and in additional embodiments conductive layer506may be disposed onto a first intermediate surface530aof intermediate layer530without departing from the spirit or scope of the present invention.

In such embodiments, conductive layer506can be a metallic layer, including, but not limited to aluminum, a non-metallic conducting material, such as graphite, or any combination thereof. Further, in an embodiment, conductive layer506can be deposited onto first surface504aof optical layer504(or alternatively, onto first intermediate surface530aof intermediate layer530) through any of a number of deposition techniques apparent to one skilled in the art and appropriate to the materials. For example, conductive layer506can be deposited on first surface504aor first intermediate surface530ausing any sputter deposition, spray deposition, or physical vapor deposition technique.

In additional embodiments, conductive layer506can a pre-fabricated layer of conductive material. In such embodiments, pre-fabricated conductive layer506can be bonded to either first surface504aof optical layer504or first intermediate surface530aof intermediate layer530apparent to one skilled in the art and appropriate to the materials.

In further contrast to the embodiments ofFIGS. 4A and 4B, a second conductive layer516is disposed between intermediate layer530and substrate502. In the embodiment ofFIG. 5A, second conductive layer516is disposed onto first surface502aof substrate502. However, the present invention is not limited to such a configuration, and in an additional embodiment, conductive layer516can be disposed onto a second intermediate surface530bof intermediate layer530without departing from the spirit or scope of the present invention.

In such embodiments, and similar to those described above, conductive layer516can be a metallic layer, including, but not limited to aluminum, a non-metallic conducting material, such as graphite, or any combination thereof. Further, in an embodiment, conductive layer516can be deposited onto first surface502aof substrate502(or alternatively, onto second intermediate surface530bof intermediate layer530) through any of a number of deposition techniques apparent to one skilled in the art and appropriate to the materials. For example, conductive layer516can be deposited on first surface502aor second intermediate surface530busing any sputter deposition, spray deposition, or physical vapor deposition technique.

FIG. 5Bschematically depicts additional features of reticle500depicted inFIG. 5A. InFIG. 5B, reticle500has been formed by first bonding conductive layer506, as disposed onto first surface504aof optical layer504, to first intermediate surface530aof intermediate layer530, and by then bonding conducting layer516, as disposed on first surface502aof substrate502, to second intermediate surface530bof intermediate layer530. In an embodiment, conductive layers506and513can be anodically bonded, respectively, to first and second intermediate surfaces530aand530b. However, the present invention is not limited to anodic bonding, and in additional embodiments, one or more of conductive layers506and516can be bonded or otherwise attached, respectively, to corresponding first and second intermediate surfaces530aand530busing any of a number of techniques apparent to one skilled in the art and appropriate to intermediate layer530and conductive layers506and516.

In an embodiment, a second surface502bof substrate502(and hence, reticle500) can be supported by a reticle chuck, mask table, or any other device configured to support reticle500within the EUV lithography apparatus. In such an embodiment, the reticle check, mask table, or other device can act as a heat sink for reticle500, thereby facilitating the transfer of heat from reticle500to various components of the EUV lithographic apparatus.

In the embodiment ofFIGS. 5A and 5B, first and second surface504aand504bof optical layer504, first and second surfaces502aand502bof substrate502, and first and second intermediate surfaces530aand530bof intermediate layer530are all respectively, substantially mutually parallel. However, the present invention is not limited to substrates and optical layers bonded or otherwise attached in such configurations. In an additional embodiments, one or more of first and second surface504aand504bof optical layer504, first and second surfaces502aand502bof substrate502, and first and second intermediate surfaces530aand530bof intermediate layer530can be disposed at any angle with respect to any other surface that would be apparent to one skilled in the art and appropriate to reticle500without departing from the spirit or scope of the present invention.

In an embodiment, and as described above in reference toFIGS. 4A and 4B, optical layer504can be formed from a material having a coefficient of thermal expansion that is substantially zero over a range of temperatures experienced by reticle500, including, but not limited to ultra-low expansion (ULE) titanium silicate glass. In such an embodiment, a thickness of optical layer504can be selected to maintain a relatively low thermal resistance and to provide a surface of sufficient integrity to support polishing. For example, the thickness of optical layer504could range from approximately 0.1 mm to approximately 0.5 mm, although thicknesses as low as approximately 0.025 mm may be possible.

Further, and as described above, substrate502can be formed from Cordierite, which has a substantially-zero coefficient of thermal expansion over the range of operating temperature and which also has a thermal conductivity approximately three times larger than that of the optical layer (e.g., Cordierite has a thermal conductivity of approximately 3.0 W/(m-° C.) at 25° C., and ULE glass has a thermal conductivity of approximately 1.31 3.0 W/(m-° C.) at 25° C.). In an embodiment, a thickness of substrate502can range from approximately 5.25 mm to approximately 6.25 mm.

Further, in the embodiment ofFIGS. 5A and 5B, intermediate layer530can be formed from a glass material, a ceramic material, or a glass-ceramic material having a coefficient of thermal expansion that is substantially zero. For example, intermediate layer530may be formed from Zerodur, a non-porous, inorganic glass-ceramic material manufactured by Schott North America, Inc. of Elmsford, N.Y. Further, in an embodiment, a thickness of intermediate layer530may be substantially smaller than that of substrate502and selected such that a thermal resistance of the intermediate layer is substantially equivalent to or less than that of optical layer504. For example, the thickness of an intermediate layer formed from Zerodur could range from approximately 0.1 mm to approximately 0.5 mm, although thicknesses as low as approximately 0.025 mm may be possible.

However, the present invention is not limited to intermediate layer formed from Zerodur, and in additional embodiments, intermediate layer530can be formed from any of a number of materials that have appropriate mechanical properties (e.g., substantially-zero coefficient of thermal expansion over a range of operational temperatures) and that are capable of facilitating anodic bonding with optical layer504and substrate502.

As described above, reticle500, when incorporated into an EUV lithographic apparatus, can absorb between approximately 30% and 100% of an incident EUV radiation beam. However, and similar to the embodiment ofFIGS. 4A and 4B, any localized heating of optical layer504, which can result from absorption of EUV radiation, is rapidly diffused (e.g., conducted) through the optical layer due to its low thermal resistance, through conductive layer506into intermediate layer530, and subsequently through second conductive layer516and into substrate502. Further, as the thermal conductivity of substrate502is substantially higher than the thermal conductivity in substrates of existing EUV reticles, localized heating of the substrate due to absorption of EUV radiation diffuses throughout the substrate and is dissipated through a reticle check, mask table, or other structure that supports reticle500within the EUV lithographic apparatus. Therefore, and in contrast to the existing EUV reticle depicted inFIG. 3, reticle500also substantially reduces or eliminates any distortion of the reticle surface, and hence, any induced pattern errors, due to localized heating from absorbed EUV radiation.

FIG. 6schematically depicts a portion of an exemplary reticle600(such as those depicted inFIGS. 4A-4BandFIGS. 5A-5B), after additional processing and patterning, according to an embodiment of the present invention. InFIG. 6, an optical layer604of a reticle600has a first surface604a, onto which a conductive coating606is disposed, and a second surface604b. In the embodiment ofFIG. 6, second surface604bis processed using any of a number of techniques apparent to one skilled in the art to generate a substantially-flat surface that is substantially free of any defects. In such an embodiment, a layer608of material that is highly-reflective to EUV radiation can be applied to polished surface604b, and a pattern can be formed within the reflective layer. For an example, a layer of resist can be applied to layer608, that resist layer can be exposed to radiation of an appropriate wavelength, and the exposed resist layer can be etched using any technique apparent to one skilled in the art to form a pattern on layer608.

In an embodiment, a manufacturer of the reticle can apply highly-reflective layer608to surface604bof optical layer604. However, in additional embodiments, an end-user of reticle600can apply highly-reflective layer608to second surface604bof optical layer604after reticle600is delivered to the user. Further, in an embodiment, the end-user of the reticle may also pattern highly-reflective layer608after delivery, as described above.

FIG. 7is a flowchart of an exemplary method700for making a reticle, such as reticle400ofFIGS. 4A and 4B, suitable for use in an EUV lithographic apparatus, according to an embodiment of the present invention. In step702, a layer of a conductive material, including, but not limited to, aluminum, is disposed onto a first surface of a layer of ultra-low expansion (ULE) titanium-silicate glass. In such an embodiment, a thickness of the layer of ULE glass is selected such that a thermal resistance of the layer of ULE glass is relatively low across a range of temperatures.

In an embodiment, the conductive layer can be disposed onto the first surface of the layer of ULE glass using any of a number of deposition techniques apparent to one skilled in the art and appropriate to the materials. For example, step702can deposit the conductive layer onto the first surfaces of layer of ULE glass using any sputter deposition, spray deposition, or physical vapor deposition technique.

The conductive layer disposed on the layer of ULE glass is subsequently bonded in step704to a first surface of a Cordierite substrate to form the reticle. In an embodiment, step704anodically bonds the first surface of the Cordierite substrate to the conductive layer. However, in additional embodiments, the first surface of the Cordierite substrate can be bonded or otherwise attached to the conductive layer using any number of techniques apparent to one skilled in the art and appropriate to the Cordierite substrate and conductive layer without departing from the spirit or scope of the present invention.

A second surface of the layer of ULE glass is then processed in step706to form a substantially-flat surface that is substantially free from defects. In an embodiment, the second surface of the layer of ULE glass can be polished in step706to yield the substantially-flat and substantially-defectless surface. However, in an additional or alternate embodiment, the second surface may be processed in step706using any technique apparent to one skilled in the art without departing from the spirit or scope of the present invention.

FIG. 8is a flowchart of an exemplary method800for making a reticle, such as reticle500ofFIGS. 5A and 5B, suitable for use in an EUV lithographic apparatus, according to an embodiment of the present invention. In step802, a layer of a conductive material, including, but not limited to, aluminum, is disposed onto (i) a first surface of a layer of ultra-low expansion (ULE) titanium-silicate glass and (ii) onto a first surface of a Cordierite substrate. In an embodiment, a thickness of the ULE glass (or other optical layer) is selected such that a thermal resistance of the ULE glass (or other optical layer) is relatively low across an applicable range of temperatures.

In an embodiment, the conductive layer can be disposed onto the first surface of the layer of ULE glass and the first surface of the Cordierite substrate using any of a number of deposition techniques apparent to one skilled in the art and appropriate to the materials. For example, step802can deposit the conductive layer onto the first surfaces of layer of ULE glass and the Cordierite substrate using any sputter deposition, spray deposition, or physical vapor deposition technique.

The conductive layer disposed on the Cordierite substrate is subsequently bonded in step804to a first surface of a intermediate layer of Zerodur (e.g., layer530ofFIGS. 5A and 5B). Further, in step806, the conductive layer disposed on the layer of ULE glass is then bonded to a second surface of the Zerodur layer, thereby forming the reticle.

In an embodiment, one or more of the conductive layers can be anodically bonded to respective surfaces of the Zerodur layer in steps804and806. However, in additional embodiments, the conductive layers can be bonded or otherwise attached to the respective surface of the Zerodur layer in steps804and806using any number of techniques apparent to one skilled in the art and appropriate to the Cordierite substrate and Zerodur layer without departing from the spirit or scope of the present invention.

A second surface of the layer of ULE glass is then processed in step808to form a substantially-flat surface that is substantially free from defect. In an embodiment, the second surface of the layer of ULE glass can be polished in step808to yield the substantially-flat and substantially-defectless surface. However, in an additional or alternate embodiment, the second surface may be processed in step808using any other technique apparent to one skilled in the art without departing from the spirit or scope of the present invention.

In an additional embodiment (not shown), a layer of material that is highly-reflective to EUV radiation can be applied to the polished surfaces of the reticles produced by the exemplary methods ofFIGS. 7 and 8. Further, a pattern can be formed on the applied reflective layer using any of a number of techniques appropriate to the reflective material and the EUV lithography process, as would be apparent to one skilled in the art. For example, a layer of resist can be applied to polished surface, that resist layer can be exposed to radiation of an appropriate wavelength, and the exposed resist layer can be etched to form a pattern on polished surface. Further, in additional embodiments, such additional application and patterning steps can be performed by a manufacturer of the reticles prior to delivery to an end-user. Alternatively, the reticle can be manufactured using the methods ofFIGS. 7 and 8, and the end-user can apply and pattern the reflective coating.

As described above with reference toFIGS. 5A and 5B, the present invention is not limited to intermediate layer formed from Zerodur. In additional embodiments, an intermediate layer can be formed from any of a number of materials that have appropriate mechanical properties (e.g., substantially-zero coefficient of thermal expansion over a range of operational temperatures) and that are capable of facilitating anodic bonding with the layer of ULE glass and the Cordierite substrate.

In the embodiments described above, reflective reticles are described in terms of an optical layer formed from ultra-low expansion (ULE) titanium-silicate glass. However, the optical layers of the present invention are not limited to such materials. In additional embodiments, the reflective reticles described herein can include an optical layer formed from any material (i) having a coefficient of thermal expansion that is substantially zero across a range of operating temperatures characteristic of the EUV lithography apparatus and (ii) capable of being processed to yield a substantially-flat surface that is substantially free of defects and amenable to the application of one or more layers of reflective material.

Further, in the embodiments described above, reticle substrates are described in terms of a Cordierite ceramic material. However, the reticle substrates of the present invention are not limited to such materials. In additional embodiments, the reticles described herein can include a substrate formed from any material (i) having a coefficient of thermal expansion that is substantially zero across a range of operating temperatures characteristic of the EUV lithography apparatus; (ii) a modulus of elasticity that is relatively high over that range of temperatures, and (iii) a thermal conductivity that is substantially higher than a thermal conductivity of the optical layer over that range of temperatures.

The reflective reticles of the present invention, as described herein through their various embodiments, substantially reduce or eliminate any distortion of the reticle surface, and hence, any induced pattern errors, due to localized heating from absorbed EUV radiation. Any localized heating of an optical layer is rapidly diffused through the optical layer due to its low thermal resistance and into the substrate. Further, as the thermal conductivity of the substrate is substantially higher than the thermal conductivity in substrates of existing EUV reticles, any localized heat flux received at the substrate due to absorption of EUV radiation diffuses throughout the substrate and is dissipated through the substrate and into a reticle check, mask table, or other structure that supports the reticle within the EUV lithographic apparatus. Therefore, and in contrast to the existing EUV reticle depicted inFIG. 3, the reticles of the present invention substantially reduce or eliminate thermal distortion of the patterning surface due to absorbed EUV radiation while maintaining a reticle thickness consistent with industry standards.

CONCLUSION