JP2008162880A - Optical member comprising od-doped silica glass - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical member which exhibits high transmittance at a short wavelength, especially, at 193 nm or shorter and has other desired characteristics. <P>SOLUTION: The optical member has an optical incident axis and is comprised of OD-doped silica glass having an absorption limit λ (limit) at 165 nm or shorter. In a certain embodiment, the OD-doped silica glass further contains fluorine. In another embodiment, the OD-doped silica glass contains fluorine of less than about 5,000 mass ppm. Furthermore, in another embodiment, the OD-doped silica glass contains Na of less than about 50 mass ppb. In an embodiment, the optical member is a photomask substrate to be used at a wavelength of shorter than 190 nm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical member made of silica glass. The present invention particularly relates to an optical member composed of OD-doped silica glass suitable for use in optical applications operating at wavelengths shorter than about 190 nm. The present invention is useful, for example, in manufacturing photomask substrates and other optical elements for use in devices operating at about 157 nm and about 175 nm.

  Photolithographic technology for use in the manufacture of semiconductor devices meets the requirements that the resolution of those devices required to produce ultra-fine features in modern high speed, high performance, low energy consumption chips is very high. Therefore, it has been extended to the range of vacuum ultraviolet wavelengths. Special optical materials have been researched and developed for use in lenses that manipulate ultraviolet light at such short wavelengths. Synthetic silica has found significant use in such areas.

  Nevertheless, there remains a need for optical components that have short wavelengths, particularly high transmission at 193 nm and below and other desirable characteristics. The present invention satisfies this need.

  Accordingly, in the first aspect, the present invention is an optical member having a light incident axis, and has an absorption limit (edge) λ (limit) of 165 nm or less (in an embodiment, λ (limit) ≦ 163 nm. In other embodiments, λ (limit) ≦ 160 nm, in certain other embodiments, λ (limit) ≦ 158 nm, in certain other embodiments, λ (limit) ≦ 157 nm, certain other embodiments. Λ (limit) ≦ 155 nm), and in a plane perpendicular to its axis, less than about 10 ppm, in some embodiments, less than about 5 ppm, and in some other embodiments, less than about 3 ppm. In another embodiment, an optical member composed of OD doped silica glass having a refractive index change of less than about 1 ppm is provided.

  In an embodiment of the optical member of the present invention, the OD doped silica glass is further doped with fluorine.

  According to certain embodiments of the optical member of the present invention, the OD-doped silica glass in the optical member has a weight of less than about 5000 ppm, in some embodiments, less than about 3000 ppm, and in some embodiments, about Less than 1000 ppm, in some embodiments, less than about 800 ppm, in some embodiments, less than about 500 ppm, in some embodiments, less than about 300 ppm, in some other embodiments, less than about 200 ppm, in some other implementations. In embodiments, less than about 100 ppm, in some other embodiments, less than about 50 ppm, in some embodiments, less than about 10 ppm, and in some embodiments, less than about 5 ppm fluorine.

  According to certain embodiments of the optical member of the present invention, the OD-doped silica glass has a weight of less than about 50 ppb, in some embodiments, less than about 30 ppb, in certain other embodiments, less than about 10 ppb, In certain other embodiments, less than about 5 ppb, and in certain other embodiments, less than about 1 ppb Na.

  According to certain embodiments of the optical member of the present invention, the OD doped silica glass is greater than about 0.98, in certain embodiments, greater than about 0.99, n (OD) / (n (OH ) + N (OD)).

  According to certain embodiments of the optical member of the present invention, the OD-doped silica glass has a mass of [OD] ≦ 200 ppm, in certain embodiments, [OD] ≦ 150 ppm, in certain other embodiments, [OD] ≦ 100 ppm, in certain other embodiments, [OD] ≦ 50 ppm, in certain other embodiments, [OD] ≦ 20 ppm, in certain other embodiments, [OD] ≦ 10 ppm, and others In an embodiment, [OD] ≦ 5 ppm.

According to an embodiment of the optical member of the present invention, the OD-doped silica glass is a glass in which [H ₂ ] (m) and [OD] (m) are each expressed in mol · cm ⁻³ . [H ₂ ] (m) / [OD] (m) ≦ 0.1 when H ₂ and OD concentrations, in one embodiment, [H ₂ ] (m) / [OD] (m) ≦ 0.08, in certain other embodiments, [H ₂ ] (m) / [OD] (m) ≦ 0.05, in certain other embodiments, [H ₂ ] (m) / [OD ] H ₂ is doped at a concentration such that (m) ≦ 0.01.

According to an embodiment of the optical member of the present invention, the OD-doped silica glass is 1 × 10 ¹⁵ molecules / cm ³ ≦ [D ₂ ] ≦ 1 × 10 ¹⁸ molecules / cm ³ . 1 × 10 ¹⁵ molecules / cm ³ ≦ [D ₂ ] ≦ 5 × 10 ¹⁷ molecules / cm ³ , in one embodiment, 1 × 10 ¹⁵ molecules / cm ³ ≦ [D ₂ ] ≦ 1 × 10 ¹⁷ molecules / cm ^{3. In} one embodiment, 1 × 10 ¹⁵ molecules / cm ³ ≦ [D ₂ ] ≦ 5 × 10 ¹⁶ molecules / cm ³ , in one embodiment, 1 × 10 ¹⁶ molecules / cm ³ ≦ [D ₂ ] ≦ 1 × 10 ¹⁸ molecules / cm ³ , in one embodiment, 1 × 10 ¹⁶ molecules / cm ³ ≦ [D ₂ ] ≦ 5 × 10 ¹⁷ molecules / cm ³ , in one embodiment, 1 × 10 ¹⁶ molecules _{^{/ cm 3 ≦ [D 2]}} ≦ 1 × 10 17 molecules / cm ^3, in certain embodiments, 1 × 10 ¹⁶ molecules / cm ³ ≦ [D _2] 5 D ₂ with [D _2] concentration between × 10 ¹⁶ molecules / cm ³ is doped.

  According to certain embodiments of the optical member of the present invention, the OD-doped silica glass has a mass of [Cl] ≦ 10 ppm, in certain embodiments, [Cl] ≦ 5 ppm, in certain other embodiments, Contains chlorine at a [Cl] concentration of [Cl] ≦ 1 ppm.

  According to an embodiment of the optical member of the present invention, the optical member is a photomask substrate for use at a wavelength of less than 190 nm.

  According to one embodiment of the optical member of the present invention, the optical member is a photomask substrate for use in photolithography at about 157 nm.

  According to one embodiment of the optical member of the present invention, the optical member is a lamp envelope for a light source that emits light having a wavelength of less than 190 nm.

  According to one embodiment of the optical member of the present invention, the optical member is a refractive lens element for use with a laser at about 165 nm.

  According to one embodiment of the optical member of the present invention, the optical member is a refractive lens element for use with an excimer laser at about 157 nm.

  According to an embodiment of the optical member of the present invention, the optical member is an optical waveguide for transmitting irradiation light. In certain embodiments, the waveguide is or includes an optical fiber.

  According to one embodiment of the optical member of the present invention, the optical member is a photomask pellicle for use in a lithographic device operating at a wavelength of less than 175 nm.

  According to certain embodiments of the optical member of the present invention, the OD-doped silica glass is at least 50% / cm, in certain embodiments, at least 60% / cm, in certain other embodiments, at least It has an initial transmission at about 157 nm of 70% / cm.

  Additional features and advantages of the invention will be set forth in the detailed description that follows, and in part will be readily apparent to those skilled in the art from the description or described in the claims and claims thereof. , As well as by implementing the invention as illustrated in the accompanying drawings.

  The foregoing general description and the following detailed description are merely exemplary of the invention and are intended to provide an overview or arrangement for understanding the nature and features of the invention as recited in the claims. It will be understood that

  The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification.

  The following description of the present invention is given as a usable teaching of the present invention in the best mode known at present. For this purpose, those skilled in the art will recognize and understand that many changes can be made to the various embodiments of the invention described herein, while still obtaining the beneficial results of the invention. Let's go. It will be apparent that some of the desired advantages of the present invention can be obtained by selecting some of the features of the present invention without using other features. Accordingly, those skilled in the art will recognize that many modifications and adaptations to the present invention are possible and may even be desired in certain circumstances and are part of the present invention. Accordingly, the following description is provided to illustrate the principles of the invention and is not intended to be limiting.

  Unless stated otherwise, all numbers such as weight percentages of components, dimensions, and values of specific physical properties such as potentials used in the specification and claims are modified by the term “about” in all cases. Should be understood as being. It should be understood that the exact numerical values used in the specification and claims constitute additional embodiments of the invention. Efforts have been made to ensure the accuracy of the numerical values disclosed in the examples. However, any measured value may inherently contain some error resulting from the standard deviation found in each measurement technique.

  Ranges can be expressed herein as “about” one particular value and / or “about” another particular value. When such a range is expressed, another embodiment includes up to the one particular value and / or the other particular value. Similarly, when values are expressed as approximations, using the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that each end point of the range is significant in the statistic of the other end points and independent of the other end points.

  As used herein, “% by weight” or “% by weight” of a component is based on the total weight of the composition or article containing the component, unless stated to the contrary. is there.

As used herein, the term “D-containing compound” refers to a basic compound or element comprising a deuterium atom ( ² ₁ D, “D”) and optionally a protium atom ( ¹ ₁ H, “H”). Means a substance, where the ratio of n (D) / (n (D) + n (H)) is greater than the natural isotopic abundance of D, where n (D) is the molecule of the D-containing compound N (H) is the total number of H atoms in the molecule of the D-containing compound. Therefore, examples of D-containing compounds include, but are not limited _{to, D 2, DH, CD 4} , CDH 3, D 2 O, and the like DHO. As used herein, the term “D-containing” refers to elemental materials, compounds where the ratio of n (D) / (n (D) + n (H)) is higher than the natural isotopic abundance of D , Material, or atmosphere.

As used herein, the term “hydroxyl” or OH means a moiety or group of moieties consisting of an oxygen atom and a protium atom (H), respectively. The oxygen atoms may be ¹⁶ O, ¹⁷ O or ¹⁸ O, or a mixture thereof in any ratio. As used herein, n (OH) means the total number of OH moieties in a material.

As used herein, the term “deuteroxyl” or OD means a moiety or group of moieties consisting of an oxygen atom and a deuterium atom (D), respectively. The oxygen atoms may be ¹⁶ O, ¹⁷ O or ¹⁸ O, or a mixture thereof in any ratio. As used herein, n (OD) means the total number of OD portions in a material.

In this application, the two terms “hydroxyl dope” and “OH dope” are used interchangeably. A hydroxyl-doped or OH-doped material is a natural isotope in which the material comprises an OH moiety and optionally an OD moiety, and the ratio of n (OH) / (n (OD) + n (OH)) in the material is H. Means greater than or equal to abundance. To that extent, normal water-derived materials in which all OH moieties contain H ₂ O and D ₂ O with substantially H and D natural isotopic abundance are considered OH doped.

  In this application, the two terms “deuteroxyl dope” and “OD dope” are used interchangeably. A deuteroxyl-doped or OD-doped material is a natural material in which the material comprises an OD portion and optionally an OH portion, and the ratio of n (OH) / (n (OD) + n (OH)) in the material is D Means greater than isotope abundance.

  “F-doped” in this application means that the glass contains at least 1 ppm by weight of fluorine.

  As used herein, OD-doped silica glass has an OD portion in the glass network with an [OD] concentration of 0.1 mass ppm or more of the total mass of the glass, and n (OD ) Is the total number of OD portions in the glass and n (OH) is the total number of OH portions in the glass, the ratio of n (OH) / (n (OD) + n (OH)) in the glass is at least 95 Means%.

  As used herein, “absorption limit” (λ (limit)) means a wavelength of less than 300 nm when the glass has a measured transmission of 50% / cm without surface reflection correction. For the purpose of measuring the absorption limit, only the initial transmission (i.e. the transmission of the glass substantially free of induced absorption by exposure to ultraviolet light) is considered.

  As used herein, “water” in silica glass is meant to include both OH and OD in the glass. OH and OD are known to exist in glass networks as groups directly linked to silicon atoms: ≡Si—OH and ≡Si—OD.

As used herein, unless otherwise specified, “hydrogen atom” in this application includes ¹ ₁ H and ² ₁ D, and mixtures and combinations thereof in any ratio. In this application, unless otherwise stated, “hydrogen gas”, “molecular hydrogen” and “hydrogen molecule” are interchangeable to mean H ₂ , D ₂ , HD, and mixtures and combinations thereof in any ratio Used for.

  As used herein, “refractive index change” or “refractive index change” or “Δn” is approximately 633 nm (He-Ne laser) (as will be described later, tilt and piston). By using interferometry, the maximum change in refractive index measured in a plane perpendicular to the optical axis of the glass material or glass optical member along a given direction is meant. When discussing refractive index changes along a direction, as is typically done by those skilled in the art, the tilt and piston are subtracted. Thus, refractive index changes along a particular direction in the sense of the present application (such as the radial direction in a sample prepared by using the OVD process) do not include tilt or piston. In general, the optical axis of a glass optical member, glass blank, or piece of glass material is perpendicular to the plane (cross section) where the measured refractive index non-uniformity is minimal in order to obtain a glass member with a large aperture area. Selected.

A preferred method for measuring interstitial molecules H ₂ in fused silica, which is also the method used here, is Raman scattering. Raman spectroscopy is obtained using a T64000 spectrometer obtained from Horiba Joban Yvon, equipped with an EEV charge coupled device (CCD) detector. Hydrogen molecule concentration, expressed in molecular / cm ^3, the strength of the silica scattering peak at 800 cm ^-1 in the laser Raman spectrum (I ₈₀₀₎ detected from the hydrogen molecule scattering peak at 4135 cm ^-1 for the intensities of the (I ₄₁₃₅₎ The ratio, ie, I ₄₁₃₅ / I ₈₀₀ (see VSKhotimchenko et al., Prikladnoi Spektroskopii, 46 (6), 987-997 (1986)). More specifically, the intensity of those peaks was determined by integrating the area under the peak using a linear or quadratic fit against the background. The D ₂ and HD concentrations in the glass in this application were similarly measured using Raman spectroscopy. The D ₂ concentration was measured at 2973 cm ⁻¹ and the HD concentration was measured at 3606 cm ⁻¹ .

OH groups, in the fused silica, 2.72μm (3672cm ^-1), have a characteristic absorption band near 2.21μm (4525cm ^-1) and 1.38μm (7246cm ^-1). The OH concentration was measured by FTIR, using the peak height of either the 3672 cm ⁻¹ or 4525 cm ⁻¹ absorption band.

C of OH concentration expressed in mol·liter ⁻¹ is Lambert-Beer's law
A = ε · c · b
Here, the transmittance of the sample at the reference position, which is a specific absorption wavelength such as absorbance A = log (T _ref / T _OH ), T _ref = 4000 cm ⁻¹ , T _OH = OH absorption peak (for silica) the transmittance of the sample at about 3672cm ^-1), ε is the molar extinction coefficient expressed in liter mole ^-1 · cm ^-1, c is concentration, expressed in mole liter ^-1 , B is the path length (sample thickness) in cm:
c (mol·liter ⁻¹ ) = A / (ε · b)

The concentration of OH expressed in ppm by mass (denoted [OH]) was calculated using the density of silica glass (about 2.2 g / cm ³ ) and the molecular weight of OH (about 17 g / mol) as mol liter ^−1. It calculated from c represented by these. The constant ε of high purity silica glass at a specific wavelength can be obtained from the prior art (KMDavis, et al., “Quantitative infrared spectroscopic measurement of hydroxyl concentration in silica glass,” J. Non-Crystalline Solids, 203 (1996). 27-36).

The concentration of OD in the silica glass was obtained in the same way, i.e. by calculation using Lambert-Beer's law, starting from FTIR measurements. To calculate the concentration expressed in ppm, Lambert-Beer's law:
A ′ = ε ′ · c ′ · b ′
Was used. Here, absorbance A ′ = log (T ′ _ref / T _OH ), T ′ _ref = 2780 cm ^{−1 and the} like, the transmittance of the sample at the reference position, which is a specific absorption wavelength, T _OD = OD absorption peak (for silica , the transmittance of the sample at about 2705cm ^-1), ε 'is the molar extinction coefficient (2705cm ^-1, expressed in liter mole ^-1 · cm ^-1, 57.4 liter mole ^-1 · cm ^{- 1} ), c ′ is the concentration of OD expressed in mol·liter ⁻¹ and b ′ is the path length (sample thickness) expressed in cm:
c ′ (mol·liter ⁻¹ ) = A ′ / (ε ′ · b ′)

The concentration of OD expressed in ppm by mass (denoted [OD]) is calculated using the density of silica glass (about 2.2 g / cm ³ ) and the molecular weight of OD (about 18 g / mol) in mol·liter ^−1. It calculated from c 'represented by these. The constant ε ′ of high-purity silica glass at a specific wavelength can be obtained from the prior art (K. Susa, et al., “Perparation of D2O-treated sol-gel preform adn OD absorption bands at IR and near-IR region , "J. Non-Crystalline Solids, 146 (1992) 81-89).

  As used herein, a particle preform refers to an object having a shape and having a plurality of solid particles. Therefore, the particle preform in the present application is, for example, a soot preform consisting essentially of silica soot particles obtained from a flame hydrolysis process, a green body comprising a large number of silica particles obtained from a sol-gel process And so on.

  In co-pending US patent application Ser. No. 11 / 348,956 by the same applicant, it has been described that OD doped high purity synthetic silica has unexpected advantageous properties at deep and vacuum ultraviolet wavelengths. In particular, OD doped silica glass has low induced absorption, low light induced wavefront distortion (LIWFD), and low polarization induced birefringence (PIB), especially when the glass is exposed to linearly polarized laser irradiation at about 193 nm. It has been found that it has an unexpectedly high laser damage resistance. These properties, among other things, make OD doped silica glass particularly advantageous for use in immersion microlithographic devices that generally use a linearly polarized excimer laser at about 193 nm.

  The effect of OH contained in silica glass on the initial transmittance of the glass is very small in the wavelength range from about 193 nm to about 400 nm. For example, synthetic silica glass containing about 1000 ppm by mass of OH with high internal transmission at about 193 nm has been prepared by Corning, Corning, NY, USA. However, below 193 nm, it has been found that when OH is contained in high purity synthetic silica glass, much stronger absorption occurs in the glass. Such absorption is highly undesirable for many applications.

  Recently, the inventors of the present application have found that OD-doped silica glass has become an optical element for use at wavelengths below 190 nm, such as about 172 nm, about 165 nm, and about 157 nm, even for such short wavelengths. It has been found that it can be used particularly advantageously because of its high optical performance.

The inventors of the present application indicated that OD doped glass having an OD concentration expressed by mol · cm ⁻³ of [OD] (m) is expressed by OH at substantially the same concentration expressed by mol · cm ^−3. It has been found that it tends to have an absorption limit at a shorter wavelength than the same silica glass except that it is substituted. For example, silica glass doped with fluorine, OH and OD at concentrations related to moles cm ⁻³ of [F] (m), [OH] (m) and [OD] (m), respectively, is OD in the glass. There same glass (i.e. substituted by OH, relates mol · cm ^-3 of [OD] (m) + [ OH] where (total OH concentration and expressed in mol · cm ^-3 in m) [F] (m) It tends to have an absorption limit at a shorter wavelength compared to glass having a fluorine concentration. Thus, an OD-doped silica glass that is substantially free of OH has an OH concentration with respect to mol · cm ⁻³ that is substantially the same as the OD concentration in OD-doped silica glass. It tends to have a higher initial transmittance than tosilica glass.

  In the wavelength range of deep UV and vacuum UV, both OH and OD contained in the glass affect the absorption limit and initial transmission at any given wavelength shorter than about 193 nm. Discovered by the inventors. In general, if the glass is doped only with OH or OD rather than both OH and OD, the higher the concentration of OH or OD in the glass, the longer the absorption limit wavelength, and the initial glass at a given wavelength. The transmittance is lowered. It was found that dry silica glass without water has an absorption limit of about 156 nm, and silica glass containing about 1000 ppm OH (but substantially free of OD) has an absorption limit of about 170 nm. Thus, certain wavelengths, such as 172 nm, 165 nm and / or 157 nm, which have an absorption limit at short wavelengths and / or are highly desirable for certain applications (such as lithographic devices operating at about 157 nm), especially potentially important applications Silica glass containing low levels of water (OH and OD) is highly desirable to obtain a silica glass with high initial transmission at a certain wavelength. Although not intended or required to be bound by any particular theory, Si—OH and / or Si—OD bonds present in the silica glass network are highly absorbed at short wavelengths such as 157 nm. It is believed that the binding energy is not sufficient to withstand the high energy of photons at such short wavelengths. When these bonds are broken, superabsorbent species are produced in the glass network.

Various general approaches can be applied in the effort to obtain silica glass containing only low levels of water. These include (i) producing silica glass in a substantially hydrogen free environment by using a silicon precursor material that is substantially free of hydrogen, such as SiCl ₄ , (ii) containing H Stripping the H-containing silicon precursor material composed of silica particles, made by using the silicon precursor material and / or in a hydrogen-containing environment, using a desiccant is included.

  Certain dopants in silica glass, including but not limited to Cl, alkali metals, alkaline earth metals, transition metals, etc., are at deep and vacuum ultraviolet wavelengths, including 193 nm and shorter wavelengths. It has been found to be detrimental to the initial transmission of high purity silica glass and several other optical performances. Therefore, it is highly desirable that such dopants not be introduced into the glass during the manufacturing process, or that such dopants be removed by the cleaning agent prior to and / or during the formation of the consolidated glass.

Halogen containing compounds, including but not limited to F 2 and Cl containing compounds such as Cl ₂ , SiF ₆ , CF ₄ , CCl ₄ , alone or in combination with other agents such as CO, or Used as a desiccant and / or purifier in certain processes for mixing to produce high purity synthetic silica glass.

It is impossible to produce a completely dry silica glass that is substantially free of OH and / or OD without leaving a certain amount of residual halogen derived from the desiccant and / or cleaner used in the manufacturing process. It is difficult if not. As mentioned earlier, the inclusion of Cl in the glass is highly undesirable when the glass is used for short wavelength applications, including even about 193 nm. Although not required to be bound by any particular theory, it is believed that Si-Cl bonds present in the glass network are susceptible to cleaving when the glass is exposed to high energy photons. Such cleavage occurs in superabsorbent species (color centers) such as E ′ centers and non-crosslinked oxygen pore centers (NBOHC). Fortunately, contrary to Cl, the presence of fluorine in the silica glass is not detrimental to the transmission properties of the glass at short wavelengths such as 193 nm, 172 nm and even 157 nm. Although not intended or required to be bound by any particular theory, Si-F bonds present in silica glass do not show significant absorption at these wavelengths, and for high energy photons at 157 nm, Si-F bonds Is not expected to be destroyed. F-doped silica glass substantially free of water and Cl has been observed to have a higher initial transmission at 157 nm than non-F-doped silica glass containing OH at equal concentrations expressed in moles cm ^−3. ing. It has been shown by the inventors of the present application that F-doped silica glass can have an initial transmission at about 157 nm, as high as 96% / cm (internal transmission corrected for surface reflection loss).

  Unfortunately, the introduction of F into the glass network to replace otherwise present OH and / or Cl can solve the absorption and initial transmission problems, but undesirable side effects. That is, a high refractive index change occurs in the glass part. While not intending to be bound by any particular theory, it is believed that this is caused by the large influence of F on the refractive index of the glass. It has been found that the presence of F and OH reduces the refractive index of silica glass. However, the influence of fluorine on the refractive index is greater than the influence of OH. This is because a change in refractive index in the glass is greater than OH due to the same level concentration change of F in the glass.

  In many advanced optical applications such as, but not limited to, high precision lithographic devices, including those related to an operating wavelength of about 157 nm, refractive index changes across the aperture of the lens element may be highly undesirable. A refractive index change of about 10 ppm or less is desirable. It has been found that for OH doped silica glass, refractive index changes of less than about 10 ppm, and in certain embodiments, less than about 5 ppm can be achieved. F-doped silica glass with the same level of refractive index change is difficult, if not impossible to achieve, because of the significant influence on F refractive index change. This is especially true for silica glass materials that are doped with a high concentration of fluorine, such as those with [F] greater than about 100 ppm by mass, and even those with [F] greater than about 200 ppm by mass. The higher the total concentration of fluorine in the glass, the greater the refractive index change that can be caused by the same ratio of fluorine concentration changes. Due to the nature of the production of high purity synthetic silica glass, fluorine concentration changes in F doped silica glass are almost inevitable. Therefore, in order to obtain a silica glass that meets the threshold refractive index change requirement, the fluorine concentration in the glass should not exceed a certain amount.

  Fluorine atoms are difficult to remove or move, unlike OH or Cl, once doped into the silica glass material network, and have a higher affinity for the silica glass network than all other stripping agents. high. Therefore, limiting the initial concentration of F in the consolidated glass means that the concentration of F-containing desiccant and / or purifier should not be too high. Such limitations on the concentration of the F-containing desiccant and / or cleaning agent impose constraints on the effectiveness and efficiency of the drying and / or cleaning process. In general, F-doped silica glasses prepared by the soot process with low levels of fluorine are also likely to contain some level of hydrogen. Another problem arising from the high affinity of fluorine for the glass network is that it is inherently difficult to obtain a uniform distribution of the entire glass network. When the silica soot preform is treated in an F-containing atmosphere, the fluorine first bonds with the glass particles closer to the surface of the preform in the process of diffusing from the surface to the center, and in the preform and finally the consolidated glass. Tends to produce a gradient of fluorine concentration. Subsequent homogenization of the glass is generally difficult, expensive, and can introduce undesirable contaminants and / or properties into the glass.

  Moreover, silica glass substantially free of water and fluorine has many undesirable properties. It has been found that dry silica glass that is not doped with hydrogen molecules in a glass network tends to have very high induced absorption once exposed to high energy photons, even at wavelengths as long as 248 nm and 193 nm. It has therefore been proposed to dope hydrogen molecules into the glass network. Hydrogen molecules can function to cure damage caused to the glass network by exposure to high energy photons. However, in a dry silica glass that is substantially free of water and / or fluorine, the network of the glass during its process in which hydrogen molecules are contained in the glass network (a process referred to as “hydrogenation”) It has been observed by the inventors of the present application that they tend to react with hydrogen molecules doped therein. Reduction of the glass network by hydrogen molecules forms an unstable structure in the glass network that absorbs at short wavelengths or has high energy at short wavelengths such as 248 nm, 193 nm, and shorter wavelengths. Forms an absorptive color center when exposed to photons.

  Therefore, in addition to F-doped silica glass that is substantially free of OH, there is a real need for silica glass with a low level of refractive index change and high initial transmission at wavelengths shorter than 193 nm.

  In a totally unexpected manner, the inventors of the present application have found that OD doped synthetic silica glass is less than equivalently OH doped silica at wavelengths shorter than about 193 nm, such as about 172 nm, about 165 nm and about 157 nm. We have found that they tend to have a high initial transmission, and tend to have a lower refractive index change than silica glass substantially free of water but doped with fluorine. Thus, OD doped silica glass, optionally further doped with fluorine, exhibits a good compromise between F-doped silica glass and OH-doped silica glass that is substantially free of water.

  Accordingly, the present invention is an optical member having a light incident axis in the first aspect, wherein the absorption limit λ (limit) ≦ 165 nm (in some embodiments, λ (limit) ≦ 163 nm, In embodiments, λ (limit) ≦ 160 nm, in certain other embodiments, λ (limit) ≦ 158 nm, in certain other embodiments, λ (limit) ≦ 155 nm), and a plane perpendicular to the axis Less than about 10 ppm, in some embodiments, less than about 5 ppm, in some other embodiments, less than about 3 ppm, in some other embodiments, in some other embodiments, An optical member composed of OD-doped silica glass having a refractive index change of less than is provided.

  The short wavelength absorption limit of the silica glass of the optical member of the present invention is caused, inter alia, by the presence of OD intentionally doped in the glass network. According to certain embodiments of the optical member of the present invention, the OD-doped silica glass has a mass of [OD] ≦ 200 ppm, in certain embodiments, [OD] ≦ 150 ppm, in certain other embodiments, [OD] ≦ 100 ppm, in certain other embodiments, [OD] ≦ 50 ppm, in certain other embodiments, [OD] ≦ 20 ppm, in certain other embodiments, [OD] ≦ 10 ppm, and others In an embodiment, [OD] ≦ 5 ppm. According to certain embodiments of the optical member of the present invention, the OD-doped silica glass in the optical member is less than about 500 ppm, in some embodiments, less than about 300 ppm, in certain other embodiments, by weight. Less than about 200 ppm, in some other embodiments, less than about 100 ppm, in some other embodiments, less than about 50 ppm, in some other embodiments, less than about 10 ppm, and in some embodiments, less than about 5 ppm. Contains fluorine.

  The OD-doped silica glass of the present invention preferably contains no fluorine for the purpose of high refractive index uniformity. However, in certain embodiments, the glass may contain a small amount of fluorine in the network, particularly when the fluorine-containing agent is used as a desiccant and / or purifier during glass manufacture. According to certain embodiments of the optical member of the present invention, the OD doped silica glass in the optical member is less than about 500 ppm by weight, in some embodiments, less than about 300 ppm, and in some other embodiments, by weight. Less than 200 ppm, in some other embodiments, less than about 100 ppm, in some other embodiments, less than about 50 ppm, in some embodiments, less than about 10 ppm, and in some embodiments, less than about 5 ppm fluorine. .

  In order to obtain a high transmittance and a stable transmittance during the lifetime of the optical member of the present invention, it is desirable that the glass contains a very low level of metals, especially sodium. Metallic sodium, which has been found to be particularly detrimental to the transmission properties of silica glass for use at about 193 nm, is less harmful at shorter wavelengths, such as 172 nm and 157 nm, as well as It is thought that. Therefore, according to certain embodiments of the optical member of the present invention, the OD-doped silica glass is less than about 50 ppb by weight, in some embodiments less than about 30 ppb, and in some other embodiments by weight. Less than 10 ppb, in certain other embodiments, less than about 5 ppb, and in certain other embodiments, less than about 1 ppb Na.

  Other alkali metals such as K, Rb and Cs, alkaline earth metals such as Mg, Ca, Ba and Sr, transition metals such as Ti, Fe, Ni, Ag and Cu, and Ge, Ga, Sn, Pb and Sb Certain main group metals such as, In, Tl have been found to be detrimental to the transmission and other optical performance of high purity silica glass operating at about 193 nm. These metals are considered equally detrimental to the optical performance of the glass, even at shorter wavelengths such as about 172 nm and 157 nm. Therefore, it is desirable that such a metal is present at a very low level in the silica glass of the optical member of the present invention. In certain embodiments, the silica glass of the optical member of the present invention may comprise each of the above metals in less than about 50 mass ppb, in certain other embodiments, less than about 30 mass ppb, in certain other embodiments. Less than about 10 mass ppb, in some other embodiments, less than about 5 mass ppb, and in some embodiments, less than about 1 mass ppb. In certain embodiments, the silica glass included in the optical member of the present invention comprises a total of all metals, by mass, less than about 100 ppb, and in certain other embodiments, less than about 50 ppb. In embodiments, it is desirable to include a concentration of less than about 20 ppb, and in certain other embodiments, less than about 10 ppb.

  Because of the undesirable OH in the silica glass discussed above, according to some embodiments of the optical member of the present invention, OD doped silica glass, greater than about 0.98, in some embodiments, about 0. A ratio of n (OD) / (n (OH) + n (OD)) greater than .99.

As discussed above, the inclusion of hydrogen molecules in the silica glass glass network of the optical member of the present invention is very advantageous for multiple applications. Therefore, according to an embodiment of the optical member of the present invention, the OD-doped silica glass contained therein is 1 × 10 ¹⁵ molecules / cm ³ ≦ [MH] ≦ 1 × 10 ¹⁸ molecules / cm ³ , In one embodiment, 1 × 10 ¹⁵ molecules / cm ³ ≦ [MH] ≦ 5 × 10 ¹⁷ molecules / cm ³ , and in one embodiment, 1 × 10 ¹⁵ molecules / cm ³ ≦ [MH] ≦ 1 × 10. ¹⁷ molecules / cm ³ , in one embodiment, 1 × 10 ¹⁵ molecules / cm ³ ≦ [MH] ≦ 5 × 10 ¹⁶ molecules / cm ³ , in one embodiment, 1 × 10 ¹⁶ molecules / cm ³ ≦ [ MH] ≦ 1 × 10 ¹⁸ molecules / cm ³ , in one embodiment, 1 × 10 ¹⁶ molecules / cm ³ ≦ [MH] ≦ 5 × 10 ¹⁷ molecules / cm ³ , in one embodiment, 1 × 10 ¹⁶ molecules. Molecule / cm ³ ≦ [MH] ≦ 1 × 10 ¹⁷ molecules / cm ³ , in one embodiment, 1 × 10 Hydrogen molecules are doped at a [MH] concentration of ¹⁶ molecules / cm ³ ≦ [MH] ≦ 5 × 10 ¹⁶ molecules / cm ³ .

The inventors of the present application believe that hydrogen molecules are present in the glass network of silica glass, especially when the glass is heated to high temperatures (such as about 1000 ° C.) or exposed to high energy radiation. It was observed that it can undergo exchange reaction with OD. If the glass is doped with H ₂ and OD but does not contain OH, such exchange reactions will form OH in the glass network. Therefore, according to an embodiment of the optical member of the present invention, the OD-doped silica glass has [H ₂ ] (m) and [OD] (m) expressed in mol · cm ⁻³ , respectively. [H ₂ ] (m) / [OD] (m) ≦ 0.1, in one embodiment, [H ₂ ] (m) / [OD] when the concentration of H ₂ and OD in the glass. (M) ≦ 0.05, in some other embodiments, H ₂ is doped at a concentration such that [H ₂ ] (m) / [OD] (m) ≦ 0.01.

For the reason described above, the OD doped silica glass included in the optical member of the present invention is doped with D ₂ instead of H ₂ . According to an embodiment of the optical member of the present invention, the OD-doped silica glass is substantially free of H ₂ , but 1 × 10 ¹⁵ molecules / cm ³ ≦ [D ₂ ] ≦ 1 × 10 ¹⁸ molecules. / Cm ³ , in one embodiment, 1 × 10 ¹⁵ molecules / cm ³ ≦ [D ₂ ] ≦ 5 × 10 ¹⁷ molecules / cm ³ , in one embodiment, 1 × 10 ¹⁵ molecules / cm ³ ≦ [D ₂ ] ≦ 1 × 10 ¹⁷ molecules / cm ³ , in one embodiment, 1 × 10 ¹⁵ molecules / cm ³ ≦ [D ₂ ] ≦ 5 × 10 ¹⁶ molecules / cm ³ , in one embodiment, 1 × 10 1 ¹⁶ molecules / cm ³ ≦ [D ₂ ] ≦ 1 × 10 ¹⁸ molecules / cm ³ , in one embodiment, 1 × 10 ¹⁶ molecules / cm ³ ≦ [D ₂ ] ≦ 5 × 10 ¹⁷ molecules / cm ³ in embodiments, 1 × 10 ¹⁶ molecules _{^{/ cm 3 ≦ [D 2]}} ≦ 1 × 10 17 molecules / cm ^3, in certain embodiments, 1 × 1 ¹⁶ D ₂ with [D _2] concentration of molecules _{^{/ cm 3 ≦ [D 2]}} ≦ 5 × 10 16 molecules / cm ³ is doped.

  As stated earlier, Cl is highly undesirable due to the adverse effect on the transmission properties of silica glass in the deep and vacuum ultraviolet range. Nevertheless, Cl-containing agents may be advantageously used as desiccants and / or cleaning agents in the process of manufacturing OD doped glass for use in the optical members of the present invention. Therefore, the OD doped silica glass for use in the optical member of the present invention may contain Cl at a low concentration. According to one embodiment of the optical member of the present invention, the OD-doped silica glass has [Cl] ≦ 10 ppm by mass, in one embodiment, [Cl] ≦ 5 ppm, in one other embodiment, Contains chlorine at a [Cl] concentration of Cl] ≦ 1 ppm.

  According to an embodiment of the optical member of the present invention, the optical member is a photomask substrate for use at a wavelength of less than 190 nm.

  According to one embodiment of the optical member of the present invention, the optical member is a photomask substrate for use in photolithography at about 157 nm.

  According to one embodiment of the optical member of the present invention, the optical member is a lamp envelope for a light source that emits light having a wavelength of less than 190 nm.

  According to one embodiment of the optical member of the present invention, the optical member is a refractive lens element for use with ultraviolet radiation at about 165 nm.

  According to one embodiment of the optical member of the present invention, the optical member is a refractive lens element for use with an excimer laser at about 157 nm.

  According to one embodiment of the optical member of the present invention, the optical member is a photomask pellicle for use in a lithographic device operating at a wavelength of less than 175 nm.

  According to certain embodiments of the optical member of the present invention, the OD-doped silica glass is at least 50% / cm, in certain embodiments, at least 60% / cm, in certain other embodiments, at least It has an initial transmission at about 157 nm of 70% / cm.

  Another possible optical member of the present invention is an optical waveguide composed of OD-doped silica glass for transmitting ultraviolet light, particularly ultraviolet light having a wavelength of less than about 248 nm, such as about 193 nm, about 172 nm, and about 157 nm. . In such a waveguide, the OD-doped silica glass is preferably included in a structure through which transmitted light travels. The high transmittance and low loss of the glass contribute to the overall effect of the optical waveguide. The waveguide may be an optical fiber, a planar waveguide, or the like.

  The inventors of the present application have discovered that the absorption cross section of OD is smaller than that of OH in high purity silica glass. Therefore, higher concentrations of OD than OH can be tolerated in silica glass for a given application with a defined absorption target. This means that in some cases where desiccant must be used in the manufacturing process to reach the required low level of OH concentration, for OD-containing silica, no or little desiccant will be required. Means that. If there is no or little residual halogen in the glass, better laser damage resistance and / or better refractive index uniformity will be obtained. In other words, in OD-doped silica, high transmittance can be achieved without the need for using a desiccant and without the problems that occur as a result of the inclusion of halogen in the glass.

  When a desiccant is used and the process yields silica that is not completely dry and contains OY (including OH and OD), at a given [OH] + [OD] The higher the ratio of OD] / ([OD] + [OH]), the higher the glass transmittance. For example, replacing OH with OD in a process that is generally tailored to leave some residual OH in the glass and very little residual level in the glass may leave the OH absorbing. Glasses that transmit well in the known deep and vacuum ultraviolet regions will be obtained.

The inventors of the present application tend to tolerate high temperature hydrogenation better when the glass has high levels of OY (including OH and OD) when the silica glass is doped with hydrogen molecules. I discovered that. Conversely, dry silica glass or silica glass with a very low level of OY can cause absorption centers to form in the glass due to the reaction of hydrogen with the glass network when hydrogen doped at high temperatures. is there. Therefore, hydrogenation of silica glass with low levels of OY generally requires a low addition temperature if such absorption centers should be avoided. Since the hydrogenation process is essentially a diffusion process, its rate is highly temperature dependent. In order to achieve a given hydrogen molecule concentration in the silica glass, the addition at 800 ° C. requires significantly less time than the addition at about 400 ° C. Therefore, for applications where molecular hydrogen is required in the glass, the use of OD doped silica glass will make the hydrogenation process more efficient. Due to the fact that at a given permeability target, the acceptable level of OD is greater than the acceptable level of OH, a silica glass with less hydrogen reactivity should be produced. This means that at a defined transmission level, OD doped silica glass can use higher H ₂ addition temperatures and shorter addition times than OH doped silica glass.

  Other advantages of being able to include higher levels of OD than OH include (1) the lower viscosity of glass at a given temperature and therefore potential if high temperature secondary forming is required. Low secondary molding temperatures, and (2) high resistance to the formation of oxygen-deficient centers (ODC) during high temperature processing.

  Replacing OH in flame hydrolyzed silica glass with OD can be very beneficial. The flame hydrolysis process produces very large and very homogeneous silica pieces in one step. In the past, due to the naturally high OH level of flame hydrolyzed silica, its use has been limited to applications above 193 nm and thin parts for use at 172 nm. When OH is replaced with OD, the UV limit is shifted to shorter wavelengths, thereby extending the usefulness of flame hydrolyzed silica glass to shorter wavelengths and / or thicker parts.

  It has been found that the absorption cross section (σ) (σ (OD)) of deuteroxyl in silica is smaller than that of hydroxyl in silica (σ (OH)) at wavelengths below 193 nm.

High purity silica glasses containing various concentrations of OH and OD were prepared by the OVD soot-to-glass process. The OH or OD concentration of the glass was set by controlling the H ₂ O or D ₂ O vapor partial pressure in the atmosphere during the consolidation process. A sample was cut from the glass preform and polished to 10 × 15 × 20 mm. Transmittance measurements were taken through a 10 mm path length using a Mcpherson S2000 VUV spectrophotometer operated under vacuum conditions. Prior to measuring the VUV transmittance, the sample was cleaned in an O ₂ plasma apparatus to remove surface contamination. FTIR measurements were also made on the samples through a 10 mm path length to determine the OH or OD content of the samples as described above.

FIG. 1 shows about 150-200 nm transmission data for a series of silica glasses containing up to about 1250 ppm by weight OH and up to about 150 ppm by weight OD. To obtain FIG. 1, the raw transmittance data for each sample is such that the transmittance at 220 nm matches the transmittance of the dried F-doped silica sample at 220 nm, ie 89.63 /% T Correction was made by multiplying the data by a factor of (220 nm). Samples A, B, C, D, E, F, G and H have the following compositions as shown in Table 1.

  At these wavelengths, no absorption is introduced by fluorine. The main effect of OH and OD is to shift the UV absorption limit to longer wavelengths. The UV limit of dry silica glass is about 155.5 nm, while the UV limit of glass containing 1235 ppm OH is around 169 nm. In the latter case, OH clearly brings the absorption wavelength closer to 180 nm. OD also shifts the UV absorption limit to shorter wavelengths, but the effect is not as great. This can be seen in FIG. 2 where the ultraviolet limit position is plotted as a function of OH or OD concentration.

Certain effects on the transmittance of a material at any given wavelength are related to the relation:
ABS (λ) = σ (λ) · N
Is defined by the absorption cross section at that wavelength, where ABS (λ) is the absorption expressed in cm ⁻¹ at wavelength λ and N is the specific species expressed in moles / cm ^3. Σ (λ) is the absorption cross section expressed in cm ² at the wavelength λ. The absorption cross section σ (λ) can be obtained by plotting ABS (λ) as a function of N and performing a linear fit to the data. The slope of the straight line fit is σ (λ).

In order to quantify the effect of OH and OD on the transmittance of silica, the absorption cross sections for each were calculated at wavelengths from 155 nm to 172 nm. The transmission data of FIG. 1 is a relational expression that is the transmittance at a wavelength λ where T (λ) is expressed in% / cm:

Was converted to absorption (bottom 10). The concentration of OH and OD is given by the formula:

Was converted from mass ppm to molecules / cm ³ . Here, [OY] is the concentration of OH or OD expressed in ppm by mass, and M is the molecular weight of OH or OD (about 17 for OH and about 18 for OD).

  FIGS. 3 and 4 show the OH-doped silica (filled solid symbol) and OD-doped silica (frame only symbol) as a function of N for λ = 155 nm, 157 nm, 160 nm, and 165 nm, 170 nm, and 172 nm. A plot of ABS (λ) is shown. In FIG. 3, 3.1 (OH), 3.2 (OH), 3.3 (OH), and 3.4 (OH) are curves of the OH doped glass, and 3.1 (OD), 3.2. (OD), 3.3 (OD) and 3.4 (OD) are curves of OD doped glass. In FIG. 4, 4.1 and 4.2 are curves of OH-doped glass, and 4.3 is a curve of OD-doped glass. A linear fit to the data is indicated by solid and dotted lines. In silica, the absorption cross section of OD is shown to be smaller than that of OH at all wavelengths. The absorption cross section of OD is virtually zero at 170 nm, while OH still has a measurable effect on absorption at 172 nm. The results are further shown in FIG. 5 where σ (OH) (λ) and σ (OD) (λ) are plotted against λ = 155-172 nm.

FIG. 6 shows a 150-200 nm transmission curve of a series of OH and / or OD doped silica glasses doped with H ₂ molecules. Hydrogenation was performed by exposing a 10 mm × 15 mm × 20 mm polished sample to a 0.1% H ₂ —N ₂ gas mixture at 350 ° C. and 100 atm for 23 days. In this figure, 6.1 contains 200 ppm by weight F, about 3 ppm by weight OD, about 0.3 ppm by weight OH, and 1 × 10 ¹⁷ molecules · cm ⁻³ H ₂ ; Contains about 58 mass ppm OD and 1 × 10 ¹⁷ molecules · cm ⁻³ H ₂ ; 6.3 contains 142 mass ppm OD and 1 × 10 ¹⁷ molecules · cm ⁻³ H ₂ Including.

  These results show that the same transmittance can be obtained in silica at a higher concentration of OD than OH. Conversely, for the same concentration of OH or OD, silica containing OD has a high transmittance.

  It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. Thus, it is intended that the present invention cover modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Graph showing the transmittance of a series of silica glasses in the wavelength range of about 150 nm to about 200 nm Graph showing the ultraviolet absorption limit of a series of OH and OD doped silica glasses Graph showing absorption as a function of OH or OD concentration in silica at various wavelengths Graph showing absorption as a function of OH or OD concentration in silica at various wavelengths Graph showing OH and OD absorption cross sections in silica glass Graph showing the transmittance of a series of OD-doped silica glasses doped with H2 at wavelengths from about 150 nm to about 200 nm.

Claims (10)

  An optical member having a light incident axis and made of OD-doped silica glass having an absorption limit λ (limit) of 165 nm or less.   The optical member according to claim 1, wherein the OD-doped silica glass further contains fluorine.   The optical member according to claim 1, wherein the OD-doped silica glass contains less than about 5000 ppm by mass of fluorine.   The optical member according to any one of claims 1 to 3, wherein the OD-doped silica glass contains Na of less than about 50 mass ppb.   The optical member according to any one of claims 1 to 4, wherein the OD-doped silica glass has a ratio of n (OD) / (n (OH) + n (OD)) greater than 0.98. .   The optical member according to claim 1, wherein the OD-doped silica glass has an [OD] of 200 ppm by mass or less. 7. The optical device according to claim 1, wherein the OD-doped silica glass is doped with H ₂ at a concentration such that [H ₂ ] / [OD] ≦ 0.1. Element. The OD-doped silica glass is doped with D ₂ at a [D ₂ ] concentration of 1 × 10 ¹⁵ molecules / cm ³ ≦ [D ₂ ] ≦ 1 × 10 ¹⁸ molecules / cm ^3. The optical member according to any one of claims 1 to 7.   9. The optical member according to claim 1, wherein the OD-doped silica glass contains chlorine at a [Cl] concentration satisfying [Cl] ≦ 10 mass ppm.   The optical member according to claim 1, wherein the optical member is a photomask substrate for use at a wavelength of less than 190 nm.

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