Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B9/00—Measuring instruments characterised by the use of optical techniques
  • G01B9/02—Interferometers
  • G01B9/02034—Interferometers characterised by particularly shaped beams or wavefronts
  • G01B9/02038—Shaping the wavefront, e.g. generating a spherical wavefront
  • G01B9/02039—Shaping the wavefront, e.g. generating a spherical wavefront by matching the wavefront with a particular object surface shape
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B11/00—Measuring arrangements characterised by the use of optical techniques
  • G01B11/24—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures
  • G01B11/2441—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures using interferometry
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B9/00—Measuring instruments characterised by the use of optical techniques
  • G01B9/02—Interferometers
  • G01B9/02083—Interferometers characterised by particular signal processing and presentation
  • G01B9/02085—Combining two or more images of different regions
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
  • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
  • G01M11/005—Testing of reflective surfaces, e.g. mirrors
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
  • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
  • G01M11/02—Testing optical properties
  • G01M11/0242—Testing optical properties by measuring geometrical properties or aberrations
  • G01M11/0271—Testing optical properties by measuring geometrical properties or aberrations by using interferometric methods
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B2210/00—Aspects not specifically covered by any group under G01B, e.g. of wheel alignment, caliper-like sensors
  • G01B2210/52—Combining or merging partially overlapping images to an overall image

Definitions

• the invention relates to a method and a device for characterizing the surface shape of an optical element, in particular a mirror or a lens of a microlithographic projection exposure apparatus.
• Microlithography is used to fabricate microstructured devices such as integrated circuits or LCDs.
• the microlithography process is carried out in a so-called projection exposure apparatus, which has an illumination device and a projection objective.
• a substrate eg a silicon wafer
• photosensitive layer photoresist
• mirrors are used as optical components for the imaging process because of the lack of availability of suitable light-transmissive refractive materials.
• NA image-side numerical aperture
• NA image-side numerical aperture
• FIG. 2 a shows a schematic illustration for explaining a functional principle of a possible conventional interferometric test arrangement for testing a mirror 201.
• a Fizeau arrangement an interferogram between reference light reflected at a reference surface 210 ("Fizeau plate") (hereinafter referred to as "reference wave”) and a measurement light reflected at the mirror 201 (hereinafter also referred to as "check wave
• the measurement light is shaped by a computer-generated hologram (CGH) 220 into an aspherical wavefront which mathematically exactly matches the "specimen shape” (ie the shape of the relevant mirror 201) at a desired distance equivalent.
• CGH computer-generated hologram
• Collimator 309 a beam splitter plate 308, a diaphragm 307, an eyepiece 306 and a CCD camera 304 and a light source 303 are shown.
• the CCD camera 304 records an interferogram of the respective mirror. men.
• the mirror 201 is moved in a plurality of measuring steps into different positions, wherein only two different positions "A" and "B" are indicated schematically.
• the implementation of a plurality of measuring steps in different positions of the mirror 201 serves, in particular, for the (eg not detectable as a whole in a single interferometric measurement) surface of the mirror 201 from a plurality of overlapping interferometric individual measurements of so-called "subapertures"
• the positions of some of several hundred sub-apertures of a measurement of a larger mirror are shown by way of example in Fig.
• the pass of the mirror or test object ie the deviation
• the pass of the mirror or test object can be determined from the individual subapertures be reconstructed from a predetermined desired shape of the surface), wherein in principle known per se on the one hand, a transformation of the subapertures in a common grid on the specimen in consideration of the respective position of the specimen and on the other a Anpa Said subapertures to each other using so-called compensators or sensitivities (to achieve the best possible match of the individual transformed Subaperturen in their overlapping areas) is made.
• a method for characterizing the surface shape of an optical element comprises the following steps: - performing a plurality of interferometric measurements, in which an interferogram is recorded between a test wave emanating from a respective section of the optical element and a reference wave, wherein between these measurements Position of the optical element is changed relative to the test wave; Calculating the pass of the optical element based on these measurements;
• the invention is based in particular on the concept, in the calculation of the passes of an optical element on the basis of carrying out a plurality of interferometric measurements, the reference wave not neglecting or only one time, insufficient approximation of the above error in the interferometric measurement setup or the optical components therein to consider as constant, but instead to regard the reference wave as a variable parameter, as in an iterative process alternately the Passe the optical element on the one hand and the reference wave on the other hand - and taking into account the information obtained in the previous iteration step - determined.
• the number of iteration steps can be made dependent, for example, on the achievement of a predetermined convergence criterion, or can also be predetermined.
• the above-described alternating execution of respectively one forward calculation for the pass determination and one backward calculation for the determination of the reference wave results in that the reference wave is co-determined pixel-resolved in the course of the iteration and thus a more accurate pass determination can be realized overall.
• the implementation of the above-described iterative method requires in each case a computational "rastering" or transformation between the coordinate system or pixel raster of the test object on the one hand and the coordinate system or pixel raster of the test setup on the other hand. changed.
• the iterative method according to the invention also makes it possible to correctly process or evaluate interferograms recorded in the plurality of interferometric measurements in which only a part of the reference wave contributes to the respective measurement result due to the respective position of the test sample or in the respective interferogram.
• the "stitching" described in the introduction described in more detail below, not only determines the fit of the device under test, but also the reconstruction of the reference wave in the individual iteration steps Application finds.
• the choice of the start reference wave can basically be done in any suitable manner, since the respective choice usually only influences the speed of the convergence of the iterative method.
• the mean value of all measurements with each complete filling of the subapertures a reference wave determined in each case in a preceding measurement or evaluation or also a "zero wave front" can be selected as the starting reference wave.
• the performance of the interferometric measurements comprises a recording of subapertures which in each case do not cover the complete surface of the mirror.
• the respectively adapted reference wave is determined by performing a backward calculation.
• the performance of the backward calculation comprises taking the pass out of the respective measurement data.
• the execution of the forward calculation in each case comprises a rasterization or transformation from a first coordinate system of a measurement setup used in carrying out the interferometric measurements to a second coordinate system of the optical element.
• the performance of the backward calculation in each case comprises a rasterization or transformation from a second coordinate system of the optical element to a first coordinate system of one used in carrying out the interferometric measurements
• the iterative calculation is performed until a predetermined convergence criterion is met.
• the iterative calculation is performed for a predetermined number of iterations.
• the change of the position of the optical element takes place such that a center of curvature of the optical
• the sample when performing the plurality of interferometric measurements, can be rotated about an axis on a rotary bearing and measured in a plurality of rotational positions, wherein these rotational positions can be uniformly distributed or irregularly arranged.
• the position of the specimen can be varied by moving the pivot bearing.
• Connection with iterative stitching can thus be realized as an absolutely calibrating measurement method.
• the optical element is a mirror or a lens.
• the optical element is an optical element of a microlithographic projection exposure apparatus.
• the invention further relates to a device for characterizing the surface shape of an optical element, the device being designed for this purpose is to perform a method with the features described above.
• a device for characterizing the surface shape of an optical element the device being designed for this purpose is to perform a method with the features described above.
• FIG. 1 shows a flow chart for explaining the sequence of a method according to an exemplary embodiment of the invention
• Figure 2-3 are schematic representations for explaining a possible
• FIG. 4 shows a schematic representation of a projection exposure apparatus designed for operation in the EUV.
• FIG. 4 shows a schematic representation of an exemplary projection exposure system designed for operation in the EUV, which has mirrors that can be tested using a method according to the invention.
• a lighting device in a projection exposure apparatus 410 designed for EUV has a field facet mirror 403 and a pupil facet mirror 404.
• the light of a light source unit comprising a plasma light source 401 and a collector mirror 402 is steered.
• a first telescope mirror 405 and a second telescope mirror 406 are arranged in the light path after the pupil facet mirror 404.
• a deflecting mirror 407 is arranged downstream of the light path, which deflects the radiation impinging on it onto an object field in the object plane of a projection objective comprising six mirrors 421-426.
• a reflective structure-carrying mask 431 is arranged on a mask table 430, which is imaged by means of the projection lens into an image plane in which a photosensitive layer (photoresist) coated substrate 441 is located on a wafer table 440.
• the optical element investigated in the context of the invention with respect to its surface shape or passes may be, for example, any desired mirror of the projection exposure apparatus 410, for example the (comparatively large) image plane side last mirror 426 of the projection objective.
• the optical element may also be a lens, for example, a projection exposure apparatus designed for operation in DUV (eg at wavelengths less than 250 nm, in particular less than 200 nm).
• DUV a projection exposure apparatus designed for operation in DUV (eg at wavelengths less than 250 nm, in particular less than 200 nm).
• the method according to the invention will be described with reference to an embodiment with reference to the flowchart shown in FIG. It is assumed that a plurality of (eg fifty) subapertures are recorded in individual interferometric measurements, each with a different positioning of the test object, in order to characterize the passer of a mirror.
• the term "subaperture" is intended in particular to express that the interferometric measurements carried out for recording the subapertures do not cover the entire
• the mirror 201 may also be a substantially planar mirror, which is displaced in a translational manner between the individual interferometric measurements in a direction parallel to the mirror surface.
• a symmetry-breaking arrangement of the individual measuring positions can be selected. This has the advantage that a clear division of the measurement results obtained in the individual sub-aureur measurements into passages and reference wave components can be made.
• the invention proceeds from the "stitching method" known as such, which however - as described below with reference to the flowchart of FIG - is modified.
• a first-time calculation of the pass takes place, specifying a specific reference wave.
• This first-time calculation of the pass also includes, in particular, the already mentioned reshaping or transformation from the coordinate system of the test setup to the coordinate system of the test object or mirror 201.
• the step of "stitching" the passes for a given reference wave) is denoted "S120" in FIG. 1 and is done by solving the following minimization problem (which yields a linear system of equations):
• I 0 is either an externally determined one
• w PRF Fit mask / weight function. , un ⁇ j on e j ne o f ⁇
• a "stitching" of the reference wave takes place, which in particular involves a back transformation of the pass into the coordinate system of the test setup
• Backward calculation taking into account the previously determined pass leads to a more precise or improved information about the reference wave, wherein the corresponding step (ie, a "stitching" of the reference wave for a given passport) in FIG. 1 is denoted by "S140".
• the stitching of the reference wave may also be done in embodiments (e.g., to limit computation time and / or memory requirements) using only a subset of pixels.
• the subaperture masks / weight functions or the passes mask / weight functions may in embodiments of the invention be mask functions (function values 0 or 1) which have valid functions. separate rich from invalid areas in each measurement.
• the subaperture masks / weight functions or the matte mask / weight function may also be "true" weight functions (function value> 0), which are calculated from local measurement errors.
• a relatively smaller weight or even zero weight may be used for image areas having a comparatively large measurement error.
• the weights can be dynamically refined during the iterative process.
• the number of measuring positions can basically be selected as desired, the number being at least two.
• the rasterization function T can be selected (in particular adapted to the specific stitching problem) depending on the specific conditions, in particular with regard to the test object. In this case, it is possible in particular to translocate to a Cartesian grid on the test object.
• the pixel grid spanned on the device under test which is used when stitching the passport, can be varied both with regard to the total number of pixels and with regard to the distortion of the raster.
• this iterative method ends (in step S150) as soon as a predetermined convergence criterion has been reached in accordance with the query in step S130.
• a predefined number of iteration steps can also be defined from the outset, after which the iteration has begun and the last determined pass is output.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Engineering & Computer Science (AREA)
• Signal Processing (AREA)
• Geometry (AREA)
• Exposure And Positioning Against Photoresist Photosensitive Materials (AREA)
• Length Measuring Devices By Optical Means (AREA)
• Testing Of Optical Devices Or Fibers (AREA)

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• 2017
  • 2017-09-29 DE DE102017217371.8A patent/DE102017217371A1/de not_active Ceased
• 2018
  • 2018-09-04 WO PCT/EP2018/073703 patent/WO2019063247A1/de not_active Ceased
  • 2018-09-04 CN CN201880063801.9A patent/CN111148964B/zh active Active
  • 2018-09-04 JP JP2020517826A patent/JP2020535425A/ja active Pending
• 2020
  • 2020-03-30 US US16/834,038 patent/US11118900B2/en active Active

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