Product information EUV Mask Blank Reflectometer · 2007. 10. 24. · Product information: EUV Mask...

Product informationEUV Mask Blank Reflectometer

AIXUV1 GmbH Steinbachstrasse 15 52074 Aachen Germany

1 Pronounce : aEX-U-V

Product information: EUV Mask Blank

Reflectometer

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PRODUCT_MBR.doc

Table of Content

1 Executive Summary 1

1.1 Cleanroom compatible „Tool“-Architecture 21.2 Simplified Operation window for „standard operator“ 31.3 Top level performance data 4

2 Intended use of EUV-Reflectometer 6

3 Discussion of performance 7

3.1 General Features 7

4 Functional Theory 8

4.1 EUV Multilayer Mirrors 84.2 Reflectometry 84.3 Functional Concept 10

4.3.1 Summary on functional concept 10

4.3.2 Shaped illumination of the sample acts as the entrance slit to the spectrograph.12

4.3.3 Comparison of monochromatic and polychromatic reflectometry 13

4.3.4 Discussion on sensitivity to stray light 14

4.3.5 Information obtained in sub-spots 14

4.3.6 Spectrograph is adapted to the requirements 15

4.3.7 Reference mirrors are measured simultaneously with the sample. 15

4.3.8 Commercial long lifetime EUV-discharge Source with window separation 16

4.3.9 Vacuum separation of Source from sample / metrology 16

4.3.10 Precautions are taken to reach long lasting performance integrity 16

4.3.11 Measures to avoid contaminating the sample 17

4.3.12 Mask blank handling 17

4.4 System architecture 184.4.1 Internal architecture of the measuring system 19

4.5 Modular set-up. 214.6 ORIENTATION of EUV Mask blank Reflectometer 21

5 Results on first tests 23

5.1 Data acquisition 235.1.1 Record spectrum of sample point & 2 reference mirrors 23

5.1.2 Extract the three spectra 25

5.1.3 Check this calibration with respect to Krypton absorption line 26

5.2 Data Evaluation 295.3 Amplitude Calibration 29

5.3.1 Tool internal calibration procedure 29

Product information: EUV Mask Blank

Reflectometer

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PRODUCT_MBR.doc

5.4 Checking with “Auto-correlation” 315.5 Dealing with changing illumination of the reference mirrors 31

6 Agreement of evaluations with respect to two reference mirrors 33

6.1 Comparing result obtained to PTB measurements on mirror B2 34

7 Data fitting as a means to get „totally stable results“ 37

8 First Investigations on reproducibility 38

9 Measurements on Absorbers 40

9.1 Absorbing sample 409.1.1 „out of band“ measurement on mirror 41

9.1.2 Conclusion 41

9.2 Quantification of stray light background 41

EUV Mask Blank ReflectometerExecutive Summary

Cleanroom compatible „Tool“-Architecture

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AIXUV GmbH, propietary

1

1 Executive Summary

AIXUV’s EUV-maskblank reflectometer is designed as a stand-alone tool for being used inclean-room environment. This type of EUV-Reflectometer - specially designed for the investiga-tion of maskblank - was ordered by SCHOTT Lithotec AG. The details of the system were jointlydesigned during development, which was funded by the BMBF (German ministry of Educationand Research). The prototype system has been shipped to SCHOTT Lithotec AG in May 2004and is under internal use there since then.

The target specifications of this systems were such, that the conformity of maskblank withSEMI-standards according to SEMI P37-1102 and P38-1102 can be confirmed; and that thesystem can also be applied for measurements on structured EUV masks.

Already the prototype has shown to reach highest level features as e.g.:

• Wavelength accuracy of better than 3 pm total and in the range of. 1 pm 3 σ.

• Times needed for one spot is less than 20 s per spot(measurement in 2000 spectral channels on a spot of 2 mm*100 µm)

• Reproducibility for absolute reflectivity of better than 0.2 %

• Accuracy of absolute reflectivity of better 0.5 %

• Resolution limit on absorbers < 0.1 %

The system is open from top to bottom to laminar vertical flow in order to prevent any contami-nation from the tool itself, although there is no source of contamination contained in the tool it-self. Potential contaminating peripheral components (roughing pumps, coolers) can be placed ingray room.

The EUV-reflectometer itself requires no special clean-room environment, because all samplehandling is accomplished in a ultra-clean vacuum mini-environment in which special care (e.g.in vacuum laminar flow, nano-filtration) is implemented to avoid sample contamination.

Of special danger for contamination is the loading procedure, which has been designed to be aslow erosive as possible. The contact between mask blank and carriers is designed to have onlyminimum contact with the samples and the contact points are only at bevels.

Especially with the semi-manual load-lock it is within customer’s care to provide an ultracleanenvironment and to avoid any contamination. For full industrial use, a fully automatic cassette-to-cassette loader is suggested which may be supplied by AIXUV.


Simplified Operation window for „standard operator“

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1.1 Cleanroom compatible „Tool“-Architecture

The reflectometer system shown has been shipped to SCHOTT Lithotec AG in May 2004; forthe time being only with a semi-manual load-lock. Upgrading with a casette-to-cassete SMIF-compatible loader is feasible, as our supplier has such systems installed already in other sys-tems.

Figure 1: EUV maskblank reflectometer with semi-manualload-lock.

Figure 2: View into EUV maskblank reflectometer displaysfunctional vacuum chamber

Figure 3: Views of internal organization of reflectometer asunder installation (left: functional vacuum chambers; right

compartment with electronic installation.

Figure 4: Views of internal organization of reflectometer asunder installation. On top of the system, to the left, the EUV-lamp looking downwards can be seen. On top, to the right is

the beamline going to the CCD.


Top level performance data

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1.2 Simplified Operation window for „standard operator“

Operation of the EUV maskblank reflectometer is via GUI interface. Password controlled op-erator levels allow interaction with all: the Intranet, the PC controlled system and the basic func-tionality, which is controlled by a real-time, non-PC PLC.

The most simple operation window is for the lowest level operator, who can only perform recipe-type measurements and hence only gets few control elements.

The next higher level of operators can select the location of measurement by hand and changethe exposure. Further levels allow to change basic settings, alignment by hand and diagnosethe system parameters in more detail.

Figure 1: Standard operator window for “low-level” operator. For performing standard measurements (e.g. recipe-driven), theoperator window allows only to use a few push buttons. Higher level-users (password access) may control each device pa-

rameter in similar windows with much higher detail.


Tool Quality System Parameters and Specifications

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1.3 Top level performance data

Within ongoing work to improve the performance of the EUV Reflectometer, which is especially toimprove evaluation software, perform higher quality calibration and remove remaining minor factors onmeasuring performance, highest quality top-level features have been demonstrated:

Parameter AIXUVachieved

Others(PTB, NIST)

Spectral band of measurement 11.9 – 14.8 nm . - 11 – 15 -.nm

Spectral resolution 1.6 pm 10 pm

Reporting data separation 1.6 pm ≥ 10 pm

Time to measure ≤ 20 s per spotwith 2000 spectral channels

1 s per spectralspot

Spot Size < 0.1 × 2 mm

(100 µm * 500 µm) possible

2 × 2 mm

Absolute accuracy Absolute accuracy has notbeen finally checked, yet.Better 0.5 % confirmed

≤ 0.2 %

Reproducibility < 0.2 % ≤ 0.1 %

Wavelength precision Better than ± 2 pm < 5 pm

Contrast of measurement > 11 bit with standard set-up> 12 bit achievable.First allows to measurereflectivities below 0.1 % onabsorbers

Not specified



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1.4 Tool Quality System Parameters and Specifications

Other system specifications, which may be of additional interest for customers are summa-rized in the following table, as specified for the production systems.

EUV Mask Blank ReflectometerTarget Use of EUV-Reflectometer


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2 Target Use of EUV-Reflectometer

AIXUV’s EUV-Reflectometer has been developed to measure the spectral reflectance of planarsamples in the EUV-spectral range.

It is specially designed for measuring the spectral reflectance of EUV mask blanks according toSEMI standards P37-1102 and P38-1102.

12.8

12.9

13.0

13.1

13.2

13.3

13.4

13.5

13.6

13.7

13.8

Wavelength (nm)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Ref

lect

ivity

λ2λ1

Peak reflectivity Full width at halfmaximum (FWHM)

Figure 2: Typical spectral reflectance of EUV multilayer mirror and relevant parameters as defined in SEMI-P38-1102.

With special adapters 6-inch wafers or smaller EUV mirrors can be measured, but part of theautomation may be lost.

EUV Mask Blank ReflectometerDiscussion of performance

General Features

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3 Discussion of performance

3.1 General Features

Most of the general requirements under discussion at SEMATECH Workshops or by potentialcustomers are achieved with the proposed system:

♦ The EUV reflectometer measures all the reflective performances in a selected spectral bandfor the EUV mask blank development activities.

♦ The system is optimized for measuring molybdenum - silicon reflective multilayers with amedian reflectivity centered at around 13.5 nm and reflectivities up to 70 % and for absorb-ers with reflectance values of below 1 %.

♦ The EUV reflectometer provides the multilayer stack peak reflectivity, median reflectivity,the full width half maximum (FWHM), and reflectivity uniformity across the blank that havethe molybdenum - silicon multilayers.

♦ The reflectivity specifications being driven towards are outlined in the SEMI P38-1102“Specification for Absorbing Film Stacks and Multilayers on Extreme Ultraviolet LithographyMask Blanks”.

♦ Trained personnel should be able to operate the EUV reflectometer semi- or fully automatic.User-friendly interfaces for both ease of use and reasonable measurement throughputs areavailable.

♦ The system achieves throughputs of up to 3 blanks per hour with 25 spots per blank and>1000 spectral channels per spot.

♦ The reflectometer records the reflectivity measurements in standard data file structures(CSV) to be exportable to other systems.

♦ Standard computer interfaces and remote access to the system are supplied by standard,state of the art IT-interfaces like Ethernet, via Internet or via telephone line.

♦ The tool is qualified to fulfill cleanroom and work station requirements.

♦ As there are no standards on handling of EUV masks yet, the amount of equipment auto-mation implemented has to be jointly decided between AIXUV and the customer. Upgradingof the system after installation might be one issue.

♦ The first system is set-up with multilayer coated surface “UP” orientation in order to allow forusing it in a pilot line with existing tools. EUV-Mask blank orientation “DOWN”, like sug-gested in drafts of SEMI standards for EUV mask blanks, has been suggested and studiedby AIXUV in the past, which allows for change of orientation for either “beta” phase or forthe “production” tool.

♦ Mask blank or reticle handling is accomplished by only touching the bevels. No chucking isused in order to allow for qualifying blanks without coated backside.

EUV Mask Blank ReflectometerFunctional Theory

Reflectometry

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4 Functional Theory

4.1 EUV Multilayer Mirrors

Multilayer mirrors reflect EUV radiation because of jumps of complex refractive index at the in-terfaces between a transmitting and a absorbing material (e.g. molybdenum and silicon). If thereare many such interfaces coated onto the substrates and all reflected rays are in phase, strongreflection occurs. If the individual reflection is out of phase, absorption occurs. This results in aso called „spectral reflectance“ of the multilayer mirror. This spectral reflectance is described bythe following figure of merit:

♦ maximum reflectivity

♦ wavelength of maximum reflectivity

♦ Bandwidth of maximum reflectivity

♦ Spectral distribution of reflectivityFigure 3:

Example of calculated spec-tral reflectance as function ofwavelength (spectral scan)

of an „ideal“! Multilayermirror optimized for 13.5 nmand 5° angle of incidence.

4.2 Reflectometry

The measurement of the reflectivity of a sample in a spectral scan means to determine the ratiobetween incident and reflected radiation over the whole spectral range of interest.

)()()(

0 λλλ

IIR R=

For EUV reflectometry the spectral range is centered around 13.5 nm and has a typical band-width of 2-3 nm.

Reflectometry can be done in many ways:

1.) Use light, which is made monochromatic in a monochromator. Measure for each spectralchannel separately (MONOCROMATIC λ-scan)

2.) Use monochromatic light from a source (e.g. single line filtered by multilayer) and measureat different angles (MONOCHROMATIC Φ−scan)


Functional Concept

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3.) Use polychromatic light, have the total spectrum reflected from the multilayer and analyzethe reflected spectrum (POLYCHROMATIC λ-scan)

AIXUV has analyzed the task, decided that the POLYCHROMATIC λ-scan is the most efficient andfastest approach, and hence designed our tool based on this concept. This give you the advantage ofa factor of N more efficient use of the source, where N is the number of spectral channels. Using thisconcept enables us to measure – as standard – 2000 spectral channels much faster than others do

with 100 channels. As far as we know, we have invented this approach and are the only ones to applyit.

Figure 4: Schematics of “polychromatic” and “monochromatic” reflectometry in comparison.

Polychromatic (parallel) Monochromatic (sequential)

Slit

Grating

Source

Entrance Slit on sample

Collimating

Slit

Imaging Grating

λ

∆λ

CCD

Sample

Sample

Photodiode

Slit

Source


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4.3 Functional Concept

The features of the basic concept are discussed in detail in the following sections.

Source

Entrance Slit on sample

CollimatingSlit

Imaging Grating

λ

∆λ

CCD

Sample

Figure 5: Basic scheme of polychromatic reflectometry:„White“ radiation from the source is reflected from the sample mirror. The reflected light is spectrally dispersed and

detected with a CCD detector.

It has been shown that the polychromatic aproach is absolutely equivalent to the “mono-chromatic reflectometry” usually been used at synchrotrons, where a monochromatorselects one spectral channel from the incoming radiation and a single detector (e.g.photo-diode) detects the reflected radiation. The main difference is that the measure-ments are faster, because the photons emitted from the source are used more effectivelyand because the mechanical shift of wavelength channels is obsolete.

4.3.1 Summary on functional concept

The basic technical concept of the reflectometer is in summary:

♦ Polychromatic Reflectometry enabling high throughputs.For achieving highest possible throughputs even with small spot area the offered systemuses polychromatic („White Light“) Reflecto-Spectrometry. Polychromatic Reflecto-Spectrometry is the fastest way to obtain spectral reflectance curves with compact labora-tory sources. AIXUV has acquired a license on a patent of Fraunhofer for this scheme.

♦ Detection of the whole reflection curve with one back-illuminated CCD cameraFor measuring each spot on the blank the whole spectrum emitted from the source andbeen able to transmit the beamline window falls onto the sample. The reflected radiation isspectrally dispersed and detected by a multi-channel detector (e.g. back-illuminated slow-scan CCD camera) in up to 2000 channels in parallel. The reference spectrum of the inci-dent radiation is calculated from the parallel measurement of reference mirrors with thesame components (grating, CCD).

♦ The EUV spectrograph is adapted to the CCD camera to make maximum use of the CCD-columns as spectral channels within the spectral band of interest.

♦ Shaped illumination of the sample acts as the entrance slit to the spectrograph.


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♦ Reference mirrors are measured simultaneously with the sample.

♦ Numerical Picture evaluationThe relevant blank parameters are determined numerically from the signals obtained in allthese spectral channels. With reduced statistical evidence it is feasible to measure thewhole reflection curve for one small spot with one single pulse of the source.

♦ Commercial long lifetime EUV-discharge source is used.

♦ Source and sample / metrology vacuum are totally separated by a beamline window whichprevents debris transport.

♦ The beamline window blocks visible, UV and VUV radiation to a great extent from enteringthe sample / metrology vacuum.

♦ Provisions for low level generation of particles and the prevention of transport of particles tothe blank are made.

♦ Interfaces to factory automation are available.


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4.3.2 Shaped illumination of the sample acts as the entrance slit to the spectrograph.

As a special solution of AIXUV is the illuminated area on the sample acts as the entrance slit tothe spectrograph. This solution has significant advantages:

♦ All the radiation falling on the sample is detected, which increases the throughputs.

♦ Radiative load of the measured spot is diminished to the absolute minimum.

♦ With imaging gratings spatial sub-solution can be evaluated within the signal from the smallilluminated spot. With standard grating and/or collimation of radiation, spatially resolved re-sults of measuring spots as small as 500 µm in length (times the width of the slit) can beextracted from the result.

♦ Measuring one spot will take less than 20 seconds. With an upgraded lamp of higher repeti-tion rate around 1 s might be achieved.


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4.3.3 Comparison of monochromatic and polychromatic reflectometry

The advantages of polychromatic reflectometry are:

♦ All EUV emitted from the source into the used solid angle is used for measurement whereasin monochromatic reflectometry only a narrow spectral band is used from every pulse of thesource.Polychromatic reflectometry yields in significantly higher throughputs.It can be shown that the polychromatic reflectometry is a factor of nearly N (N: number ofspectral channels) faster with the same source power than monochromatic reflectometry nottaking into account the time needed to shift spectral channels with monochromatic reflecto-metry. On the other hand this advantage can be used to measure smaller spots and / orwith radiation of lower divergence and / or to measure more spectral channels for one sam-ple spot.

♦ The whole spectral reflectivity curve is measured in parallel, without any changes (e.g. me-chanical) in the system.

♦ This allows e.g. for measuring 2000 spectral channels with about 100 pulses from thesource. Measuring time only depends on the repetition rate of the source and the quality(spectral resolution, shot noise, and spot size) demanded.

♦ All the measuring system can be build very stable.

♦ All fluctuations are measured in parallel on the sample in each spectral channel, so that therelative result is not influenced by fluctuations of the source or any other component.

♦ There is no limitation on the etendué of the source, with the consequence that dischargeplasmas can be effectively been used even with larger source radii.

♦ Spectral detection with a CCD camera allows for detecting secondary effects such as higherorders or stray light distribution.

♦ Additionally, this concept allows realizing a compact system.

♦ The whole measurement system can be build with high mechanical stability (and thus highreproducibility of the measurement) because only the sample has to be moved while all thecomponents which are relevant for the quality of the measurement can stay fixed on oneoptical bench.

There are only few disadvantages of the polychromatic concept when compared to the mono-chromatic concept:

♦ Instead of a simple diode there is need to use a CCD camera as detector which is a largereffort for data evaluation.

♦ The information on the incident radiation I0 has also to be obtained spectrally resolved.We solved this task by using reflection from a known multilayer. Out-coupling of 1st orderfrom transmission grating might be implemented as an additional second reference.

♦ For the time being, the data transfer from the CCD limits the throughputs.


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4.3.4 Discussion on sensitivity to stray light

There is no higher sensitivity to stray light, as may be assumed at first sight. Obviously most ofthe radiation scattered at the grating or the sample is detected by the CCD camera whereaswith monochromatic reflectometry the exit slit is thought to be a good means to suppress scat-tered light. However, one has to take into account that the probability of a stray light photon tobe detected is a factor of N (N is the number of spectral channels to be measured) larger withpolychromatic reflectometry but only 1/N photons are used to obtain the same information.

Additionally stray light background can be numerically accounted for in the measurement, be-cause this information is obtained and can partly be distinguished with the CCD camera.

Stray light traps separate different compartments within the system in order to prevent spread-ing of stray light.

4.3.5 Information obtained in sub-spots

With some imaging capability of the spectrographs grating and/or collimation of radiation, highlyspatially resolved information could be obtained. Realistic is to extract information of measuringspots as small as 20×500 µm². In principle each single line in spectral direction on the CCD canbe evaluated as one single measurement of a spot size of around 13 µm× slit width (≅ 20 µm).However, reproducibility of such evaluations is limited by shot-noise which can be estimated toaround 2 % for a single pixel and to around 0.1 % for the whole convoluted reflectivity curve.Additionally extreme effort would be required for developing a suited grating.


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4.3.6 Spectrograph is adapted to the requirements

The concept implies that the achievable spectral resolution is limited by the spectral widthequivalent of about 2 pixel columns on the CCD detector.

Therefore a trade-off between accessible total bandwidth and spectral resolution has to befound. The usable spectral bandwidth should be as narrow as possible in order to have thespectral width of a single channel as narrow as possible. The spectral resolution and the num-ber of columns on the CCD are connected as

∆λLimit ≅2 ∆Λ /NPixels

where ∆λ: spectral width of a single channel.∆Λ : total spectral bandwidth measuredNPixels: number of columns on the CCD

We are planning to use a CCD camera with 2000 Pixels. This results in∆λlimit ≅ ∆Λ /1000.

This facts demand that the total measured spectral bandwidth has to be matched to the width ofthe CCD detector. Hence, we have designed and optimized a special grating, which solves thistask.

Spreading out of the spectrum over the whole CCD is additionally required to improve photonstatistics for achieving higher reproducibility. The number of photons detected determines shot-noise error. This number is limited by the full-well capacity of the CCD pixels. For a single pixelthe noise can be estimated to (Nfull-well/2)-1/2 which is about 0.7 %. If 100 lines are used (1.3 mmof slit height) the remaining noise is 0.07 %. If we convolute the whole signal in the reflectivityband (about 1000 pixels with ¼ of the signal in average) we reduce the shot noise influence to0.05 % due to the fact that about 4×108 photons contribute to the result.

4.3.7 Reference mirrors are measured simultaneously with the sample.

Simultaneous relative measurement in comparison to calibrated reference mirrors acts as ameasure to determine the incident radiation. This procedure does:

♦ Establish a direct link to absolute values obtained from national standardization labs;

♦ Allows for an easy check of the validity of the system;

♦ Keep the measurement independent from any fluctuations of the source;

♦ Reduces sensitivity with respect to drifts or fluctuations in performance of source, il-luminator, beam forming, reference mirrors or of the detector. Most of the influence ofsuch factors is calibrated out by the relative measurement;

♦ Allows for adjustment of the illuminator / reflectometer in air or / and without the sam-ple chamber in place.


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4.3.8 Commercial long lifetime EUV-discharge Source with window separation

A commercial discharge source operated with Xenon is used. Source lifetime beyond 1000 h ofemission has been demonstrated in tests, where the source has performed more than220.000.000 pulses.

Figure 6 AIXUV’s EUV-Lamp system, as shown here in the stand-alone product configuration, is integrated into the EUV-Reflectometer.

4.3.9 Vacuum separation of Source from sample / metrology

The vacuum chamber of the source is totally separated from the measurement and samplevacuum by a thin foil window in the beamline. This guarantees that no contamination from thesource influences the measurement or contaminates the sample.

4.3.10 Precautions are taken to reach long lasting performance integrity

Vacuum components are designed and treated according to UHV conditions.


System architecture

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On-line monitoring of incident radiation is integrated as a means to check stability of referencemirrors and the total system.

Beamline window can be exchanged in maintenance procedure within one hour.

4.3.11 Measures to avoid contaminating the sample

Precautions are implemented to reduce any chance of particles contaminating the sample:

♦ The system has been installed under clean-room conditions

♦ Generation of particles is avoided as far as possible.In the past we have suggested that the mask blank is handled and measured face-down, so that no particles can fall onto the coated side from above.This precaution has to be given up on the demand of our customers to handle themask blank face-up.

♦ A slight flow of Argon acts as a gas curtain to guide the blank from particles.

Only sample x-y-table, gate-valve and sample transfer will use moving parts. All other compo-nents will stay in position after commissioning.

4.3.12 Mask blank handling

Loading of the mask is through a SEMI standard gate-valve.SEMI standard handling of mask blanks from SMIF-Pots like cluster handler can be upgradedas soon as standards are defined and our customers demand it.

For being measured, the mask blank resides on a carrier attached to the x-y-table just beingcontacted at the bevels. The mask blank handling is very reproducible, due to the fact thatfeedback control adjustment of the sample is implemented.


System architecture

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4.4 System architecture

Figure 8 and 3 show a first schematic drawings of the system.

The whole measuring system of sample chamber, illuminator/spectrograph chamber and CCDare mounted into the space frame with vibration insulation from all other components.

The measuring device is supplied from a control and supply rack, which is located in a separatecompartment of the system. The control rack includes a PLC system, which operates all secu-rity-critical subsystems (e.g. vacuum pumps, gas flow etc) and contains all subsystems neededfor operation. An clean room compatible industrial PC acts as the user interface and performsthe data evaluation, interaction with the external networks and control sample loading, position-ing and measurement.

Figure 7: Schematic drawing of the system concept. Two compartments are containing the functional vacuum installation (left)and the electronic supply and PLC components (right)


System architecture

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4.4.1 Internal architecture of the measuring system

Figure 10 visualizes the internal architecture of the measuring system.

Load-lock

CCD

Sample stage

Illumination, reference

spectrograph

Beamline

EUV-Lamp

Grating

Measure head

Figure 8: Schematic drawing of the system concept. The overall architecture (left) of the vacuum system and the functional corewith beam-paths (right).

Radiation from AIXUV’s EUV-Lamp passes the beamline window. The emitted radiation fromthe source is collimated so that a slit like illumination of the sample is shaped. Part of the radia-tion transmitted through a first collimator slit is out-coupled and is detected over a multilayer mir-ror by a photodiode. The signal obtained is proportional to the inband intensity of the source.This signal is integrated and summed-up over the exposure time for each spot and is used formonitoring potential degradations of reference mirrors and CCD camera.

A reflective grating spectrally disperses the radiation, which is reflected from the sample andfrom the reference mirrors. A back-illuminated CCD camera detects the spectra.

Illuminator, Spectrograph, beam monitor and sample position / tilt sensors are all fixed onto onesingle rigid optical bench.


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Figure 9: Schematic views from various directions onto the functional vacuum chambers and periphery.

Figure 10: Schematic drawing ofthe functional core of the system.The main functional elements are

EUV-Lamp

Illuminator

Beam monitor

Sample position sensors (Z-Position and angular tilt sensor)

Spectrograph grating

CCD camera

Figure 11 and Figure 12 show schematic impressions of the system where Figure 11 is the ac-tual orientation of the system for demonstration of mask blank loading, positioning and meansagainst contamination. The mask blank is handled and investigated with coated surface on top.Sample loading is horizontally from one side through a gate-valve from a load-lock.

The functional elements: illuminator and spectrograph are fixed in adjusted positions on the onerigid optical bench, which is hanging inside the vacuum chamber. Position sensors for sample’sZ-Position and angular tilt are also fixed onto this optical bench. Sample position is adjustedautomatically before measurement in a way that the sample is in the right position with high ac-


ORIENTATION of EUV Mask blank Reflectometer

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curacy. Three capacitive “nulling sensors” are used to measure the distance of the mask fromthe optical bench with accuracy in the range of a few µm.

Figure 11: Schematic drawing of the face down orientation ofthe system for demonstration of the mask blank loading, posi-

tioning and means against contamination.

Figure 12: Schematic of the details of sample and referenceposition. Distance of reference to sample is less than 2 mm.

4.5 Modular set-up.

The whole system is organized in separate modules, which allows to adapt to changing de-mand:

♦ Load-LockFor the time being a semi-manual load-lock is attached. A cassette to cassette loadingsystem is also available.

♦ Sample chamber with X-Y and Z (tilt) stages.

♦ Measuring chamber with optical bench.

♦ EUV-Lamp.

♦ CCD Detector.

4.6 ORIENTATION of EUV Mask blank Reflectometer

AIXUV has originally planned to design the EUV reflectometer for „Multilayer-coated side down“(DOWN) operation. Face down orientation might be advantageous to even more minimize con-tamination, because it avoids aerosol sedimentation on the coated side.

According to customer’s demand, „Multilayer-coated side up“ (UP) has been realized, becausefor the time being, Pilot lines under discussion will use to a large extend existing tools (coaters,inspection tools, etc.), which are operated coated side up.

We at AIXUV have checked both orientations of the system under development. From technicalpoint of view both concepts can be realized without any special drawbacks.

Advantages of the DOWN (corresponding to disadvantages of the UP orientation) are:

♦ Aerosol sedimentation under gravity is “away” from coated surface.

♦ Center of mass of the tool is lying lower, which gives the whole system improved stability orrequires less strong support.


ORIENTATION of EUV Mask blank Reflectometer

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♦ The EUV-Lamp is mounted close to the bottom, which makes handling and replacementeasy.

♦ Aspect ratio of height / width is about 1.

♦ Compatible with discussed SEMI standards.

Advantages of the UP-orientation are:

♦ Compatible with existing tools for Pilot Lines.

♦ Fewer limitations with respect to spectrograph length.

♦ With AIXUV’s concept of EUV Reflectometer the UP version has the small disadvantagethat the EUV-Lamp has to be mounted above the system which requires stable support andimplies some additional tasks to be solved for service handling and alignment.

EUV Mask Blank ReflectometerResults on first tests

Data acquisition

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5 Results on first tests

The following describes some steps of the data acquisition, data evaluation and calibration pro-cedures. Qualification of the prototype system, the development of the evaluation software andimprovements on these procedures is ongoing. Hence, the presented facts should be under-stood as a “snapshot” during development.

5.1 Data acquisition

Data acquisition is to get the raw spectral data from one measurement, which consists of a darkimage and one image with the reflected spectra of three multilayer mirrors are measured.

5.1.1 Record spectrum of sample point & 2 reference mirrors

The typical raw result as recorded by the CCD shows spectrum of the source as reflected from3 multilayer mirrors. These spectra are all taken simultaneously, which is with exactly the samesystem set-up and with the same pulses from the source. With knowledge of the “effective re-flectance” of the two mirrors (left and right), this supplies an on-line calibration of the measure-ment of the sample spot. The spectrum of the sample is recorded in the center between the tworeference mirror's spectra.

Figure 13: Raw picture of data taken from the edge of wafer 1 (in the inhomogeneous regime). The system takes three spectrasimultaneously: at the left is a broadband reference mirror, to the right a narrowband reference mirror. The sample is detected in

the center. With careful inspection, the spatial resolution in the 2 mm slit width allows for detecting inhomogeneities.


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Figure 14: Measurement on wafer 1 at position close to coating edge shown with increased contrast.Spatial inhomogeneous coating is detected within the 2 mm slit length.


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5.1.2 Extract the three spectra

From the raw data image three stripes of columns are selected and integrated to extract the rawspectra from the sample and the two reference mirrors. Typical width of the stripes is 60 – 100columns and are of 2048 spectral channels.

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Sample, rawReference 1. rawreference 2, rawSample, smoothedReference 1, smoothedReference 2, smoothed

Figure 15 Spectra from the reference mirrors and the sample.

For spectral calibration the raw data are evaluated which allows for obtaining sub-pixel resolu-tion for the position of the emitted lines. For the evaluation of spectral reflectance, smootheddata are used. Typical width of integrating is 10 – 30 Pixel.

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Figure 16 Smoothed spectra (narrowband sample)

For determining the wavelength scale of the spectrum, peaks, observed in the spectrum areidentified with published NIST wavelengths:


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Figure 17: Central part of a spectrum with identified peaks and a synthetic spectrum calculated with these peaks. The “valleys inthe spectrum are a sensitive means to adjust the line widths.

Many of the detected peaks can be identified by published NIST lines of Xenon. This identifica-tion results in the dispersion relation, which can with high accuracy be assumed as linear in themeasuring band. The standard deviation of the line positions with respect to the line positioncalculated from the dispersion relation is 0.25 pm, which is about 1/6th of one pixel.

The 1 σ deviation of identified peaks is less than 1 pixel of the CCD camera which correspondsto ∆λ < 1.5 pm or λ/∆λ > 9.000. The dispersion relation taking into account all identified lines is1.58891 pm/pixel. The 1 σ deviation of the dispersion relation is 1.74×10-6 pm/pixel. The 1 σ de-viation of the identified lines is 0.25 pm, which corresponds to 0.2 Pixels of the CCD.

5.1.3 Check this calibration with respect to Krypton absorption line

PTB and NIST use an absorption resonance line of krypton (Kr 3d-5p) as wavelength referenceat 13.5947 nm. The relative accuracy of our wavelength reference has been checked by meas-uring the krypton absorption in the reflectometer. For this purpose, two measurements- one witha few Pa of Krypton in the vacuum chamber and one without were performed.

The ratio of the two spectra is proportional to the absorbency of krypton. In the spectral absorb-ency 3 resonance absorption lines are detected. The line at 13.5947 nm is the largest.

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Figure 18: Transmission of krypton calculated from two measurements on the same sample, where one was with vacuum, theother with laminar Krypton flow.


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Figure 19: Detail of reflected spectra from the same sample for krypton flow and vacuum in comparison. The resulting transmis-sion of krypton is shown in orange. This comparison shows both: the calibration by emission lines and the calibration by ab-

sorbing lines. Important to note is the fact, that the emission lines are much narrower.

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Kr I 3d-5p

Figure 20: Detail of krypton absorption line with the data points of the raw transmission.The position line shows the assumed wavelength of 13.5947 nm as used by national institutes of standard (PTB, NIST).

With the calibration with NIST emission lines, the minimum position is determined to 13.5927 nm giving a deviation of 2 pm.


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Kr I 3d-5p

Figure 21: Finer detail of krypton absorption line with the data points of the raw transmission. The position of this line assumedas 13.5947 nm by national institutes of standard (PTB, NIST) and is in agreement to better than 1.5 Pixel with the detected

maximum absorption.

The expected spectral distance between the xenon XI emission line and the krypton absorptionresonance is 86.9 pm from literature.

The detected distance of the two lines is 54.5 ± 0,5 pixel. This gives a first measurement of thedispersion of the spectra as 1.5945 ± 0.01 pm / Pixel. The value assumed by AIXUV determinedby calibration with NIST lines is 1.59534 pm / Pixel.

As the absorbency line is much broader then the emission line, the relative accuracy is deter-mined by the use of this reference at PTB / NIST. If we assume the accuracy of the national in-stitutes as better than 1 pm, the relative accuracy of the reflectometer can be assumed to bebetter than ± 2 pm (λ/∆λ > 5000).


Amplitude Calibration

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5.2 Data EvaluationThe reflectivity of the sample is determined by

)(*)(

)()( λ

λλ

λ referencereference

samplesample R

SignalSignal

R =

This is equivalent to

)()(

)(λ

λλ

spectrumsourceeffective

samplesample Signal

SignalR =

)()(

)(λ

λλ

reference

referencespectrumsourceeffective R

SignalSignal

With

=

The accuracy of this evaluation gets low for a given wavelength, if the reflection of the referenceis low at that wavelength. In order to avoid this, the EUV-reflectometer uses two reference mir-rors:

♦ A narrowband reference mirror centered at 13.5 nmWhich is also used to guarantee the accurate wavelength calibration and

♦ A broadband reference mirror, which allows to have sufficient signal in the whole spectralrange from 12.3 to 14.3 nm.

The ratios of the spectrum reflected from the sample to the spectra of these reference mirrors isthe raw result of the system. With amplitude calibration of the reference mirrors the final result isobtained.

5.3 Amplitude Calibration

Absolute characteristics and reproducibility of the system is determined by the calibration of thereflection of the reference mirrors. The first assumption used for the reference mirrors is, thattheir reflectivity can be directly determined by measuring at the PTB as close as possible to theedge where it is used in the reflectometer.

In general, it has to be assumed that there is some spectral variation of the sensitivity of thesystem and that – due to not perfect homogeneous slit-width and slit illumination – the ampli-tudes of the three spectra have some apparatus inherent factor.

All these effects are temporally stable and can be attributed to a apparatus function A(λ) so thatthe effective reflectance of the reference mirrors are R(λ) A(λ).

5.3.1 Tool internal calibration procedure

The measurement of a mirror with known reflectance supplies the basis for internally re-calibrating the reference mirrors, called tool internal calibration.

In such a measurement, we get in comparison to B1 the following signals:


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/ PIX

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Samplenarrowband referencebroadband reference

Figure 22: Raw spectra used for the calibration of the reference mirrors. The illumination of the narrowband reference mirror ischosen to be lower than the one of the broadband mirrors in order to have higher dynamics for the measurements on absorbers.

From this we deduced the “effective” spectral reflectance of the reference mirrors. With theknown sample, this allows to internally “re-calibrate” the reference mirrors. This internal calibra-tion procedure of the reference mirrors accounts for all system specific effects like:

♦ The real reflectance of the reference mirrors exactly at the spots close to the edges wherethey are actually used.

♦ For different spectral sensitivities on grating or CCD camera or other components.

♦ For spectral and spatial variations in the illumination of the two reference mirrors.

Trivially, the evaluation and the evaluation procedure itself are independent from the sourceemission in both flux and spectral distribution.

The typical result obtained looks as follows:

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Figure 23: Deduced “effective spectral reflectance ” of the measuring mirrors.

The fact that the narrowband mirror is of low ”effective reflectance” is due to 2 factors beingtaken into account for the calibration:

Being used close to the edge, the reflectance is reduced as seen in former investigations.

The illumination has not been aligned perfectly. The narrowband mirror gets less light.


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5.4 Checking with “Auto-correlation”

Applying the calibrated reference mirrors onto the “Reference sample” is the first check of cali-bration, because this should yield the “known reflectance”. This has been proofed:

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5.5 Dealing with changing illumination of the reference mirrors

During set-up procedure, we have detected that the ratio between the signals from the refer-ence mirrors might change, when the lamp is adjusted to a new position. We use this learningfor the procedure of “best lamp adjustment”. There are two distinct directions for lamp adjust-ment: across the slit and along the slit.

For the direction “across the slit”, the task is simply to adjust for maximum amplitude. The“across the slit” alignment sets the ratio of signals of the two reference mirrors. It has beenfound out, that this alignment means to align the beam-path absolutely parallel to the opticalbench. This – on one hand – slightly influences the angle of incidence by many mrads. On theother hand this angle generates – as we have detected – variations in the signals from the ref-erence mirrors. However, we have found out, that this – in operation variation – can be numeri-cally be accounted for:


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Within one series of measurements on the same spot each time after loading and unloading ofthe maskblank, changes of the signals from the reference mirrors over 1.4 % were detected.These changes were accounted for numerically so that 0.4 % of total variations of the reflectivityremained.

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W6bM1W6bM4W6b M2W6b M3W6 M1W6 M2W6 M3W6 M4

Figure 24: 8 measurements on the same sample on the same spot overlaid. The first 4 measurements were taken before theEUV-lamp was exchanged; 4 measurements after the exchange. Both series were evaluated using a calibration obtained after

the exchange, with the software accounting for different illumination ratios, only.

EUV Mask Blank ReflectometerAgreement of evaluations with respect to two reference mirrors

Comparing result obtained to PTB measurements on mirror B2

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6 Agreement of evaluations with respect to two reference mirrors

6.1 Results with respect to two reference mirrors in parallel

In principle, the EUV reflectometer performs two independent relative measurements:

♦ One comparison between sample and the narrowband reference,

♦ One comparison between sample and broadband reference.

The next figure shows the two evaluations plotted together.

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Figure 25: Result of one specific measurement: We get two reflection curves from the relative measurement of the sample incomparison with each reflectance mirror respectively. These results are well overlapping, if the reflectance of both referencemirrors is high at the wavelength of interest. As larger errors occur in spectral ranges where one of the reference mirrors is of

low reflectance, a weighted average always prefers the mirror of higher reflectance. This is clearly visible at 14.3 nm.



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6.2 Comparing result obtained to PTB measurements on mirror B2

For the check of our calibration, we used the test sample B2 from SCHOTT Lithotec AG, whichhas been measured at the PTB. The sample was on taken out of the process development oneyear ago. We haven’t performed measurements on that sample adapted to the quality of the re-flectometer, yet. Hence, variations of the central wavelength of 20 pm and of the reflectivity inthe order of 0.5 % limited the value to proof the final absolute quality of the system.

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Figure 26: Three reflection curves of B2 as measured by the PTB.

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Figure 27: Raw measurement obtained with AIXUV reflectometer. The “roughness” of the top is attributed to the – for the timebeing – not perfectly managed “Penning background issue”.



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Figure 28: Measurement of the reflectivity on three spots of B2 as obtained by AIXUV’s reflectometer in comparison to the threerepresentatives of the PTB measurements.

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Figure 29: Details of the measurement of the reflectivity on three spots of B2 as obtained by AIXUV’s reflectometer in compari-son to the three representatives of the PTB measurements. For the time being there are smaller remaining discrepancies.

Intermediate results are:

♦ The reflection curve is successfully measured in the spectral range from 12.4 to 14.9 nm.

♦ Some artifacts seem to occur for the wavelength range of 11.8 to 12.4 nm.

♦ An absolute error in the range of 1 % is remaining.

♦ Systematic variations of the reflectivity curves especially near the maxima are observed.

♦ Some slope errors are still detected.

Although – for the time being - the absolute calibration has been performed under not idealconditions, reasonable agreement was achieved, already. The drawbacks were:

♦ Both the sample and the mask blank used for calibration and the one used as “sample”showed variations in central wavelength in the range of 20 pm and in the reflectivity in theorder of 0.5 %, as they were produced during process development. These variations aretoo large to use them for the highest quality calibration of the reflectometer.



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♦ The distance of 50 pm of the data report was quite large compared to the resolution of 1.6pm of our tool.

♦ We did suffer from a background noise from pressure gauges, which is made responsiblefor :

♦ errors in absolute amplitudes and

♦ “Roughness and slope-change in the reflection curves”.

It is assumed, that the “Penning-light” detected in the “Dark-image” is responsible in double re-spect for the remaining discrepancies:

♦ It made us measure the wrong values for the reflectivity of the reference mirrors.

♦ It contributed wrong background to the measurement itself.

The fact that slope errors and roughness are detected leads to the following conclusions: The“Penning background” is neither stable in time nor stable in position nor stable in distribution.

Means to solve the “Penning issue” will be implemented before the system will be set in full op-eration at SCHOTT Lithotec AG.

♦ The evaluation program will handle changing “background intensity”.

♦ The Penning gauge will be placed “light shielded”.

EUV Mask Blank ReflectometerData fitting as a means to get „totally stable results“


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7 Data fitting as a means to get „totally stable results“

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calculated reflectanceFitted data

In order to get even more reproducible results, the high data density allows for fitting the wholeto curve to a theoretical reflection curve. For the time being, we use a “deforming – shifting“model. In the future, we will try to use a physical modeling. This fitting shows us where the re-flection curve still has some weaknesses. The weakest point at 13.5 nm is strongly correlated tothe real maximum of the reference mirror, i.e. to some small wavelength shift of the maximum.

The standard deviation of this fit is 0.06 %.

EUV Mask Blank ReflectometerFirst Investigations on reproducibility


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8 First Investigations on reproducibility

Investigations on showing the reproducibility of the prototype are ongoing. In a first experiment,the same mask blank was measured on the same spot 10 times. Between subsequent meas-urements the blank was unloaded and loaded again.

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123456789100MW

Figure 30: 10 measurements on the same sample on the same spot with loading/unloading between the measurements.

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Reihe1Reihe2Reihe3Reihe4Reihe5Reihe6Reihe7Reihe8Reihe9Reihe10Median

Figure 31: Details of the 10 measurements in the maximum.

EUV Mask Blank ReflectometerFirst Investigations on reproducibility


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ance

Figure 32: Distribution of the results “maximum reflectivity” and “wavelength of maximum”.

The deviations are under investigations in more detail. Much of the deviation is attributed tochanging Penning background, as no systematic source for the deviation has been identified.

In summary, the reproducibility is better 0.3% absolute 3 Sigma in reflectance and 3.7 pm inwavelength of maximum reflectance. We expect that solving the “Penning” problem during sys-tem qualification at SCHOTT Lithotec AG. After this correction, we expect at least a factor of 4better values.

EUV Mask Blank ReflectometerMeasurements on Absorbers

Absorbing sample

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9 Measurements on Absorbers

9.1 Absorbing sample

With a critical absorber we still got sufficient signal to get a spectra from the absorber with stan-dard 1000 Pulses. The maximum signal - < 1 % of the maximum signal of the narrowband ref-erence mirror - was significantly detected. The signal might have gotten close to the scatterlevel, but seems to be very significant, as at maximum 20 counts per pixel are really correlatedto spectral features in the reference mirrors. The origin of the signal at 12.8 nm has to be stud-ied in more detail; especially after we got the results from other measurements on this mirror. Asthe reflection curve shows, we get close to single count / or single photon “steps”, which give a“resolution” of about 0.01%. For such low reflectance mirror a special strategy can be applied toget more accurate results.

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Figure 33: Raw signals from measurement on absorber. Due to the large contrast, different scaling was used for the referencemirrors. The dynamics of the back-illuminated CCD and the averaging algorithms over 100 pixel columns are the basis to get

nearly noise free signals even at sub-1% reflectance.

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EUV Mask Blank ReflectometerMeasurements on Absorbers

Absorbing sample

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9.1.1 „out of band“ measurement on mirror

Analyzing the reflectivity far off the reflecting band of a “standard” mask blank was used to qual-ify measurement ability on absorbers.

W afer 1b

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Figure 34: Reflectance values of less than 1 % are measured close to the silicon absorption edge. Due to the weak data basisof the narrow band reference mirror some discrepancies occur, which the weighted average takes into account by following theresult, obtained from the comparison to the broadband reference. The steps on the curves are due to the fact that the data arestored as “integer counts per pixel” in the evaluation software. As the spectrum results from about 100-pixel line the real data

obtained contains additional information.

9.1.2 Conclusion

For “standard” absorber measurements the standard measuring mode can be applied. Wave-length calibration and validity of the “raw spectra” should be checked critically from time to time.The reproducibility can be expected to be one order of magnitude lower, which is still in therange of 2 %.

Extension of the exposure time as close as possible to the saturation of the brightest spot couldimprove statistics by a factor of two. With much larger efforts an “overlay” evaluation of morethan 10 single measurements could improve signal to noise ratio.

9.1.3 Stray light background

For quantification of the stray light background, a measurement was performed when no samplewas loaded. We evaluated the “spectrum” from the region where the “No-sample” showed somesignal. With this type of experiment, the stray light content in the recorded spectrum of the sam-ple can be quantified.

This experiment showed us the problem with the “PENNING-Light source”, and proofed the factthat this “background signal” usually changes between dark image recording – which is donejust before the measurement – and the time when the measurement is performed.

This was detected especially in this measurement, because the open optical access to thesample stage where the Penning gauge was “burning” is usually blocked by the sample.

When this will be handled, we expect a residual – systematic – background of < 10 counts,which is < 0.1 %. It can well be assumed that this residual will influence the absolute accuracy,but will not contribute to fluctuations in reproducibility.

Product information EUV Mask Blank Reflectometer · 2007. 10. 24. · Product information: EUV Mask...

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Transcript of Product information EUV Mask Blank Reflectometer · 2007. 10. 24. · Product information: EUV Mask...