Glass composite for use in extreme ultra violet lithography

A glass composite for use in Extreme Ultra-Violet Lithography (EUVL) is provided. The glass composite includes a first silica-titania glass section. The glass composite further includes a second doped silica-titania glass section mechanically bonded to a surface of the first silica-titania glass section, wherein the second doped silica-titania glass section has a thickness of greater than about 1.0 inch.

FIELD

The present disclosure relates to a bonded silica-titania composite. More particularly, the present disclosure relates to a glass composite having a doped silica-titania glass section mechanically bonded to a silica-titania glass section.

BACKGROUND

Extreme Ultra-Violet Lithography (EUVL) is a leading emerging technology for 13 nm mode and beyond for the production of Micro Processing Unit and Dynamic Random Access Memory (MPU/DRAM) integrated chips. Presently, EUVL scanners which produce these Integrated Chips (ICs) are used on a small scale for purposes of evaluating and demonstrating this new technology. The optics systems, which include reflective optical elements, are an important part of these scanners. As EUVL development proceeds towards high volume manufacture, the specifications are expected to continue to become more stringent for the optics system parts.

In EUVL scanners, the optical elements are exposed to an intense extreme ultraviolet (EUV) radiation having a wavelength of 13.5 nm. Some portion of this EUV radiation is absorbed by the reflective coatings on the optical elements of the systems, which results in the heating of the top surface of the optical element. This causes the surface of the optical element to be hotter than the bulk of the optical element and results in a temperature gradient through the optical element. In addition, in order to image a pattern on semiconductor wafers, the surface of the optical element is not uniformly exposed to the EUV radiation, which leads to a temperature gradient on the optical surface. This results in a complex three dimensional temperature gradient through the thickness of the optical element, as well as along the optical element surface receiving the radiation. These temperature gradients lead to a distortion of the optical element, which in turn leads to smearing of the image being formed on the wafers. It is expected that the difficulties of heat dissipation will be exacerbated by the increased optical element sizes and the increased power levels that are anticipated to meet the demands of future EUVL developments.

SUMMARY

According to an embodiment of the present disclosure, a glass composite for use in Extreme Ultra-Violet Lithography (EUVL) is provided. The glass composite includes a first silica-titania glass section. The glass composite further includes a second doped silica-titania glass section mechanically bonded to a surface of the first silica-titania glass section, wherein the second doped silica-titania glass section has a thickness of greater than about 1.0 inch.

According to another embodiment of the present disclosure, a method for forming a glass composite for use in Extreme Ultra-Violet Lithography (EUVL) is provided. The method includes forming a first silica-titania glass section, forming a second silica-titania glass section, and doping the second silica-titania glass section to form a doped silica-titania glass section. The method further includes mechanically bonding the doped silica-titania glass section to a surface of the first silica-titania glass section. The doped silica-titania glass section has a thickness of greater than about 1.0 inch.

DETAILED DESCRIPTION

Reference will now be made in detail to the present embodiment(s), an example(s) of which is/are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.

The singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. The endpoints of all ranges reciting the same characteristic are independently combinable and inclusive of the recited endpoint. All references are incorporated herein by reference.

Glass composites capable of use for photolithography, and methods of forming such glass composites, are provided herein. As used herein, the term “glass composite” refers to a glass article having at least two glass sections that differ in chemical composition, where the individual glass materials remain separate and distinct in the finished structure of the glass article.FIG. 1illustrates a bonded silica-titania composite100in accordance with embodiments of the present disclosure. As shown, the bonded silica-titania composite100has a length Lc, a width Wcand a thickness Tc. The bonded silica-titania composite100includes a first glass section110and a second glass section120bonded to the first glass section110. The second glass section120may be mechanically bonded to the first glass section110using fusion bonding, frit bonding or low temperature sol-gel bonding. The first glass section110and the second glass section120have substantially equal lengths and widths that are substantially equal to the length Lcand the width Wcof the silica-titania composite100. The first glass section110has a thickness T1of between about 1.0 inch and about 12 inches. For example, the first glass section110may have a thickness T1of between about 3.0 inches and about 10 inches. The second glass section120has a thickness T2of at least about 1.0 inch. For example, the second glass section120has a thickness T2between about 1.0 inch and about 2.0 inches, or even between about 1.25 inches and about 2.0 inches.

The bonded silica-titania composite100has a thickness Tcof between about 2.0 inches and about 14 inches. For example, the silica-titania composite100may have a thickness Tcbetween about 4.0 inches and about 12 inches. According to embodiments of the present disclosure, the first glass section110may comprise greater than about 50% of the thickness Tcof the silica-titania composite100. For example, the first glass section110may comprise between about 50% and about 95%, or between about 70% and about 90% of the thickness Tcof the silica-titania composite100. The bonded silica-titania composite100has a mass of greater than about 20 kg. For example, the silica-titania composite100may have a mass of greater than about 30 kg, or even greater than about 120 kg or more.

The first glass section110is a silica-titania glass containing between about 1.0 wt. % and about 16 wt. % titania and having a near-zero coefficient of thermal expansion (CTE) in a temperature range of about 25° C. and about 35° C. A material that has near-zero thermal CTE is one that undergoes little or no dimensional change in response to changing temperature. The slope of CTE versus temperature at 20° C. of the first glass section110is less than about 1.6 ppb/K2at 20° C., for example, less than about 1.3 ppb/K2, or less than about 1.0 ppb/K2. As described in further detail below, the CTE properties of the first glass section110are tailored to substantially match the CTE properties of the second glass section120. The first glass section110may have, for example, between about 5.0 wt. % and about 16 wt. % titania, or between about 10 wt. % and about 16 wt. % titania. At least a portion of the titania in the first glass section110is in crystalline form with the crystalline form being predominantly anatase.

The second glass section120is a doped silica-titania glass containing between about 1.0 wt. % and about 14 wt. % titania. The second glass section120may have, for example, between about 5.0 wt. % and about 13 wt. %. titania, or between about 8.0 wt. % and about 12 wt. % titania. The second glass section120may also include between about 1.0 wt. % and about 10 wt. % of at least one dopant, or between about 1.0 wt. % and about 6.0 wt. % of at least one dopant. The at least one dopant may include, but is not limited to, boron, fluorine, chlorine, phosphorous, hydroxyl groups and mixtures thereof. The doped silica-titania glass has a uniform composition in which the variation of the titania concentration through the thickness of the glass is less than about 0.20 wt. %, and the variation of dopant concentration through the thickness of the glass is less than about 0.20 wt. %. The variations of the concentration of titania and of the at least one dopant in the second glass section120provides a uniformity that renders the second glass section120highly polishable.

The second glass section120has a near-zero CTE in a temperature range of about 25° C. and about 35° C., and a slope of CTE versus temperature at 20° C. of less than about 1.6 ppb/K2. For example, the slope of CTE versus temperature at 20° C. may be less than about 1.3 ppb/K2, or even less than about 1.0 ppb/K2. The second glass section120may also have a CTE uniformity having a variation that is less than about 10 ppb/K. For example, the CTE variation may be less than about 5.0 ppb/K, or even less than about 1.0 ppb/K.

In accordance with embodiments of the present disclosure, the second glass section120may include a curved reflective surface which is intended to receive impinging radiation. For example, the curved reflective surface may be an aspherical surface formed by machining and/or grinding a surface of the second glass section120. The uniform composition of the second glass section120renders the surface that is intended to receive impinging radiation capable of being finished to a surface roughness that meets EUVL specifications, such as a Mid-Spatial Frequency Roughness (“MSFR”) of less than about 0.2 nm rms. For example, the surface of the second glass section120that is intended to receive impinging radiation may have an MSFR of less than about 0.15 nm rms, less than about 0.12 nm rms, or even less than about 0.10 nm rms.

A method for forming a bonded silica-titania composite100is provided herein. The method includes forming a first glass section110of silica-titania glass. The silica-titania glass of the first glass section110may be formed in accordance with a one-step process (the direct process) or a two-step process (the indirect process). Each of the one-step and the two step processes include transporting vaporous reactants to a reaction site, such as a burner, via a carrier gas, wherein the vaporous gas streams are combusted in a burner flame fueled with natural gas and oxygen. The presence of oxygen serves to convert the vaporous reactants to their respective oxides upon exiting the burner orifice to form a stream of volatile gases and finely-divided, spherical particles of soot that are deposited onto a substrate, forming a porous blank, or preform, of soot. In the one-step process, the temperature is maintained at a consolidation temperature so that the particles are formed and consolidated into glass substantially simultaneously. In the two-step process, the blank, or preform, is formed in a first step as described above and heat treated in a second step in a helium/chlorine atmosphere to full consolidation. The direct process is described in U.S. Pat. Nos. 8,541,325, RE41220 and 7,589,040, and the indirect process is described in U.S. Pat. No. 6,487,879, the specifications of which are hereby incorporated by reference in their entirety. Apparatuses which may be used in the direct and indirect processes are described in U.S. Pat. No. RE40,586 and U.S. Patent Application No. 2011-0207593, the specifications of which are incorporated by reference in their entirety.

As formed, the silica-titania glass of the first glass section110has a negative CTE which may be modified by heat treatment. As such, the method further includes heat treating the silica-titania glass of the first glass section110to form a crystalline form of titania with the crystalline form being predominantly anatase. Heat treating includes heating the silica-titania glass of the first glass section110to a temperature of between about 700° C. and about 1000° C. for a period of between about 1.0 hour and about 10 hours. The formation of the anatase crystals, which are known to have a positive CTE, neutralizes the CTE of the silica-titania glass as formed. Thus, the heat treating step provides the first glass section110with a near-zero CTE.

The temperature and time period of the heat treating step may be selected to crystalize a predetermined amount of titania crystals in order to substantially match the CTE of the first glass section110with the CTE of the second glass section120. For example, where the second glass section120has a near-zero CTE, heat treating the silica-titania glass of the first glass section110may provide the first glass section110with a near-zero CTE as well. The difference between the CTE of the first glass section110and the CTE of the second glass section120may be less than about 5.0 ppb/K, or less than about 3.0 ppb/K. Matching the CTE of the first glass section110and the second glass section120prevents stresses during bonding of the two sections that result from CTE differences. Furthermore, matching the CTE of the first glass section110and the second glass section120provides uniform thermal expansion properties through the thickness Tcof the bonded silica-titania composite100, which in turn minimizes temperature-related distortions and enables use of the bonded silica-titania composite100in EUVL applications.

The method may optionally further include polishing a surface of the first glass section HO to prepare the first glass section110for bonding to a surface of the second glass section120.

The method further includes forming a second glass section120of doped silica-titania glass. The doped silica-titania glass of the second glass section120may be formed using “wet” processes, such as sol-gel processes in which silica and titania particles, or silica-titania soot particles having a predetermined concentration of titania and at least one dopant precursor, are dispersed in water and/or other solvents to form a sol that is aged, or mixed with a chemical composition, to induce gelation thereof. The gel is then dried and subjected to a thermal treatment in order to remove organic substances from the gel. The resultant gel may then be subjected to an additional thermal treatment to remove hydroxyl groups and to sinter the gel to form a glass article. Alternatively, the doped silica-titania glass of the second glass section120may be formed using silica-titania soot particles having a predetermined concentration of titania and at least one dopant precursor by dissolving the at least one dopant precursor in water or other solvents and mixing the silica-titania soot particles to form a slurry. The slurry may then be spray dried to form doped silica-titania powder which may then be dry pressed and consolidated to form a glass article. The doped silica-titania glass of the second glass section120may also be formed using chemical vapor deposition (CVD) processes, such as outside vapor deposition (OVD), in which vaporous reactants are transported to a reaction site, such as a burner, via a carrier gas, wherein the vaporous gas streams are combusted in a burner flame fueled with natural gas and oxygen. The presence of oxygen serves to convert the vaporous reactants to their respective oxides upon exiting the burner orifice to form a stream of volatile gases and finely-divided, spherical particles of soot that are deposited onto the surface of a rotating mandrel to form a preform of soot. The preform is then dried and sintered to form a glass article. The doped silica-titania glass of the second glass section120may also be formed using soot pressing processes in which soot particles containing silica, titania and at least one dopant are formed using a CVD process. The soot particles are collected in a mold and pressure is applied to form a soot compact. The soot compact is then sintered to form a doped glass article.

In any of the processes discussed above, the silica and titanic particles and/or soot may be doped by adding particles of the at least one dopant prior to the formation of the doped silica-titania glass. Similarly, where a soot compact is formed using a soot pressing process, the soot compact may be doped by being contacted with at least one dopant gas after formation of the soot compact and/or during a pre-sintering step where the soot compact is placed in a closed system and heated to a temperature below a sintering temperature. For example, the soot compact may be heated to a temperature of less than about 1200° C. The at least one dopant may include, but are not limited to, boron, fluorine, chlorine, phosphorous, hydroxyl groups and mixtures thereof. Without wishing to be limited by any particular theory, it is believed that the dopant can reduce glass viscosity, which reduces fictive temperature, and, in turn, reduces the slope of CTE versus temperature.

The method may further include annealing the doped silica-titanic glass of the second glass section120. Following consolidation, the glass article may be cooled to a holding temperature of less than about 1000° C., for example, between about 600° C. and about 1000° C., for a holding period of at least about 30 minutes, for example, between about 30 minutes and about 2.0 hours. After completion of the holding period, the temperature may be decreased to a predetermined temperature, between about 600° C. and about 850° C., at a rate of less than about 10° C. per hour. For example, the rate may be between about 0.10° C. per hour and about 10° C. per hour, or between about 0.10° C. per hour and about 5.0° C. per hour, or even between about 0.10° C. per hour and about 3.0° C. per hour. Once the predetermined temperature is reached, heat from a heat source may be removed, and the temperature may be allowed to cool to ambient temperature. Annealing the glass article as disclosed herein provides control of the CTE and the slope of CTE versus temperature of the doped silica-titania glass and achieves high spatial uniformity of thermal expansion properties. Annealing the doped silica-titania glass of the second glass section120in accordance with the method described herein further reduces the slope of CTE versus temperature of the doped silica-titania glass. In combination with the effect of the at least one dopant, annealing the doped silica-titania glass of the second glass section120can reduce the slope of CTE versus temperature by greater than about 40% as compared to undoped silica-titania glass. For example, annealing the doped silica-titania glass of the second glass section120can reduce the slope of CTE versus temperature by greater than about 50% as compared to undoped silica-titania glass.

The method may optionally further include polishing a surface of the second glass section120to prepare the second glass section120for bonding to a surface of the first glass section110.

The method further includes mechanically bonding a surface of the second glass section120to a surface of the first glass section110. Bonding may be performed using fusion bonding in which a welded interface is formed between the two glass sections by bringing the surfaces of the glass sections into contact, and heating the surfaces to a temperature near the softening point of the glass sections. Bonding may also be performed using frit bonding in which a frit, or a glass material having a melting temperature below the melting temperature of the glass sections, is placed between the surfaces of the glass sections. The frit is then melted by being heated by an irradiation source, such as a laser or infrared light, and then is cooled to form a hermetic seal that connects the second glass section120to the first glass section110. The frit may be doped with a material that lowers the CTE of the frit so that the frit softens and forms a bond at lower temperatures and avoids thermal damage to the glass sections. Where frit bonding is used, the frit may be selected to have a CTE that substantially matches the CTE of the first glass section110and the second glass section120. Optionally, the frit may include CTE modifying additives which further allow the CTE of the frit match the CTE of the first glass section110and the second glass section120. Bonding may also be performed using low temperature sol-gel bonding where a sol-gel resin is produced and, after being subjected to a curing process, forms a final glass-like film. At least one of the surfaces to be bonded is coated with the glass-like film and the sol-gel layer is cured (either thermally or using UV irradiation) to form a bond between the first glass section110and the second glass section120.

Once the second glass section120is bonded to a surface of the first glass section110, the surface of the second glass section120opposite the surface bonded to the first glass section110may be subjected to machining and/or grinding to form a curved reflective surface. For example, the curved reflective surface may be an aspherical surface formed by machining and/or grinding a surface of the second glass section120.

Bonded silica-titania glass composites according to the present disclosure may be formed to have dimensions large enough to be used in EUVL applications. Such large composites include a first section of glass having a thickness comprising greater than about 50% of the thickness of the composite which can be formed using cost effective and time efficient processes. The large composites also include a second section of glass having a thickness comprising less than about 50% of the thickness of the composite which has a uniform composition that renders the section of glass highly polishable and capable of being finished to a surface roughness that meets EUVL specifications. Additionally, the bonded silica-titania glass composites also have thermal expansion properties that allow for their use in EUVL applications.