# US 2014/0192344 A1 · Method and apparatus for determining the absorption in a blank > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2014/0192344 A1 - Document ID: US20140192344A1 - Status: Published application - Kind: A1 - Application number: 14/150,552 - Filed: 2014-01-08 - Published: 2014-07-10 - Earliest claimed: 2011-07-08 - Assignees: Carl Zeiss SMT GmbH - Applicants: CARL ZEISS SMT GmbH - Inventors: Eric EVA - Web page: https://patentgrep.com/patent/US20140192344A1 ## CPC - G01N 21/17: Systems in which incident light is modified in accordance with the properties of the material investigated (where the material investigated is optically excited causing a change in wavelength of the incident light G01N21/63) - G01M 11/0257: {by analyzing the image formed by the object to be tested} - G01M 11/0285: {by measuring material or chromatic transmission properties (G01M11/0292 takes precedence)} - G01N 21/171: {with calorimetric detection, e.g. with thermal lens detection} - G02B 13/143: {for use with ultraviolet radiation} - G01N 2021/1712: {Thermal lens, mirage effect} - G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q) - G01M: TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR - G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS ## Abstract A method for determining the absorption of a blank (2) for producing an optical element (3), including: radiating a heating light ray (8) through the blank (2) for the purpose of heating the blank (2), and determining the absorption in the blank (2) by measuring at least one property of a measurement light ray (10) influenced by the heating of the blank (2). In the method, either the heating light ray (8) and the measurement light ray (10) or the heating light ray and a further heating light ray are oriented to enter into the blank (2) through a first polished surface (2a) or a second polished surface (2b), situated opposite the first surface, and meet one another exclusively in the interior of the blank (2), preferably in a volume (12) used for the production of the optical element (3). An associated measuring apparatus (1), optical element (3), and optical arrangement are also disclosed. ## Claims ### Claim 1 Method for determining absorption of a blank provided for producing an optical element, comprising: radiating a heating light ray through the blank for heating the blank, and determining the absorption in the blank by measuring at least one property of a measurement light ray influenced by the heating of the blank, wherein either the heating light ray and the measurement light ray or the heating light ray and a further heating light ray are oriented to enter into the blank through a first polished surface or a second polished surface situated opposite the first surface, and to meet one another exclusively in an interior of the blank. ### Claim 2 (depends on 1) Method according to claim 1, wherein the heating light ray and the measurement light ray or the two heating light rays meet one another in the interior of the blank at an angle (β) of less than 90°. ### Claim 3 (depends on 1) Method according to claim 1, wherein, as a property of the measurement light ray, a wavefront deformation (ΔW) of the measurement light ray brought about by the heating light ray is measured in a spatially resolved manner. ### Claim 4 (depends on 3) Method according to claim 3, wherein the wavefront deformation is measured with a Shack-Hartmann sensor. ### Claim 5 (depends on 1) Method according to claim 1, wherein the position of the overlap region at which the measurement light ray and the heating light ray or the two heating light rays meet one another is varied in the volume used for the production of the optical element in at least two dimensions (X, Y), (X, Y, Z), in order to determine the spatially resolved absorption behavior in the blank). ### Claim 6 (depends on 5) Method according to claim 5, further comprising creating a model for the temperature-dependent refractive index variation (Δn) of the optical element from the blank 2′ from the spatially resolved absorption behavior in the blank. ### Claim 7 (depends on 1) Method according to claim 1, wherein the measurement light ray has a rectangular or an elliptical cross section. ### Claim 8 (depends on 1) Method according to claim 1, wherein the measurement light ray and the heating light ray are at a distance (a) of at least 5 mm from one another on the first and the second polished surfaces. ### Claim 9 (depends on 1) Method according to claim 1, wherein at least one property of the measurement light ray deflected by the polished surface in a region in between the two heating light rays is measured. ### Claim 10 (depends on 1) Method according to claim 1, wherein the wavelength (λh) of the heating light ray deviates from the used wavelength (λu) of the optical element by less than 5 nm, wherein the used wavelength (λu) is 250 nm or less. ### Claim 11 (depends on 1) Method according to claim 1, wherein the wavelength (λm) of the measurement light ray deviates from the wavelength (λh) of the heating light ray by more than 250 nm. ### Claim 12 (depends on 1) Method according to claim 1, further comprising: irradiating the blank with a predefined radiation dose before determining the absorption of the blank. ### Claim 13 (depends on 1) Method according to claim 1, wherein an optical element is produced from blanks in which, in the entire volume used for the production of the optical element, at a wavelength of 193 nm, an absorption coefficient k0 of 2×10−4/cm or less is measured. ### Claim 14 Apparatus for determining the absorption of a blank for producing an optical element, comprising: a holding device for the blank, at least one heating light source to generate at least one heating light ray for heating the blank, a measurement light source to generate a measurement light ray, and a detector unit to measure at least one property of the measurement light ray influenced by the heating of the blank, wherein either the heating light ray and the measurement light ray or the heating light ray and a further heating light ray are oriented to enter into the blank through a first polished surface or a second polished surface situated opposite the first surface, and to meet one another exclusively in an interior of the blank. ### Claim 15 (depends on 14) Apparatus according to claim 14, wherein the heating light ray and the measurement light ray or the two heating light rays meet one another in the interior of the blank at an angle (β) of less than 90°. ### Claim 16 (depends on 14) Apparatus according to claim 14, wherein the holding device for the blank is displaceable linearly in at least two directions (X, Y). ### Claim 17 (depends on 14) Apparatus according to claim 14, wherein the detector unit comprises a Shack-Hartmann sensor configured to measure wavefront deformations. ### Claim 18 (depends on 1) Optical element which, in an entire volume thereof, at a wavelength of 193 nm, has an absorption coefficient k0 of 2×10−4/cm or less, wherein the absorption coefficient k0 is determined by a method according to claim 1. ### Claim 19 (depends on 18) Optical arrangement, comprising at least one optical element according to claim 18. ### Claim 20 (depends on 19) Optical arrangement according to claim 19, wherein at least one optical element having a subaperture ratio of greater than 50% at a wavelength of 193 nm, has an absorption coefficient k0 of 1×10−4/cm or less. ### Claim 21 (depends on 19) Optical arrangement according to claim 19, wherein at least one optical element arranged upstream of a first pupil plane or downstream of a second pupil plane, at a wavelength of 193 nm, has an absorption coefficient k0 of 1×10−4/cm or less. ### Claim 22 (depends on 20) Optical arrangement according to claim 20, configured as a projection lens for microlithography which, given a throughput of at least 200 wafers/hour having a wafer diameter of 300 mm and a resist sensitivity of 33 mJ/cm2, has an uncorrected dynamic wavefront deformation of less than 80 nm PV. ### Claim 23 (depends on 1) Method according to claim 1, wherein the interior of the blank is located in a volume of the blank used for production of the optical element. ### Claim 24 (depends on 2) Method according to claim 2, wherein the angle (β) is less than 30°. ### Claim 25 (depends on 14) Apparatus according to claim 14, wherein the interior of the blank is located in a volume used for production of the optical element. ### Claim 26 (depends on 14) Apparatus according to claim 14, wherein the angle (β) is less than 30°. ### Claim 27 (depends on 19) Optical arrangement according to claim 19, wherein the absorption coefficient k0 is 0.5×10−4/cm or less at a subapertue ratio is greater than 80% at a wavelength of 193 nm. ### Claim 28 (depends on 20) Optical arrangement according to claim 20, wherein the projection lens has an uncorrected dynamic wavefront deformation of less than 20 nm PV.