EUV and DUV scatterometry for CD and edge profile metrology on

EUV masks

Bernd Bodermann1a, Matthias Wurma, Alexander Dienera, Frank Scholzeb, Hermann Großb

Physikalisch-Technische Bundesanstalt, a Bundesallee 100, D-38116 Braunschweig, Germany bAbbestraße 2-12, D-10587 Berlin-Charlottenburg, Germany

ABSTRACT

To test the applicability of scatterometry on EUV masks we measured a prototype EUV mask both with an EUV scatterometer and a conventional scatterometer operated at 193 nm and compared the results with AFM and CD-SEM measurements provided to us by the mask supplier. The results of both CD-SEM and EUV- and DUV scatterometry show a quite good agreement in linearity despite constant CD offsets for these different metrology tools. The influences of the multilayer and Si capping layer on top of the multilayer thickness on EUV scatterometry results have been modelled with the help of FEM based simulations. A strong correlation has been found between the thickness of the capping layer and the sidewall angle.

In general these results demonstrate the applicability both of EUV and DUV scatterometry for the characterisation of absorber structures on EUV masks. The application of DUV scatterometry allows to omit any influence from multilayer features and is only sensitive to the absorber structure. In this way EUV and DUV scatterometry complement each other for metrology on EUV masks. For applications in process optimisation and in process control the use of a conventional VIS/DUV-scatterometer may be sufficient in many cases.

Keywords: scatterometry, CD-metrology, mask-metrology, diffraction, lithography, critical dimension, DUV, EUV, inverse problem

1. INTRODUCTION

According to the International Technology Roadmap for Semiconductors (ITRS) Extreme Ultraviolet (EUV) Lithography will be an option for printing circuits at the 22 nm or below [1]. First requirements for EUV mask metrology are at the latest stated for the year 2013. The corresponding metrology is currently under development and different approaches are investigated.

A critical measurand will be the width of the absorbing structures, the “critical dimension” or CD. But also other geometric-dimensional structure features like e. g. line-to-space-ratios, sidewall angles and –profiles or corner rounding are important measurands to characterise the lithographic imaging process. The metrology methods which are usually considered in this context are CD-SEM [2] and EUV scatterometry [3, 5-7].

In scatterometry the radiation diffracted from a periodic structure is evaluated. The diffraction efficiency of one or more diffraction orders is measured as a function either of the angle of incidence or of the wavelength of the incident radiation. The analysis of the measured diffraction efficiency signatures, the scatterograms, is used to characterise the periodic structures, regarding critical dimension (CD) and other profile properties.

For EUV masks, EUV radiation provides direct information on the mask performance comparable to the operating regime in an EUV lithography tool. For the characterisation of the high reflecting MoSi-multilayer EUV reflectometry is the first (and only) choice metrology method. Additionally, first measurements using PTB's EUV reflectometer at the storage ring BESSY II also for scatterometric measurements on periodic absorber structures showed that it is feasible to derive information on the line profile in periodic areas of lines and spaces by means of rigorous numerical modelling. A prototype EUV mask with a matrix of test fields with lines and spaces has been used for the measurements [5-7]. The comparison with CD measurements using a CD SEM showed a good linearity of the results.

1 Bernd.Bodermann@PTB.de; phone +49 531 592 4222; fax 49 531 592 4264; http://www.ptb.de/en/org/4/42/423/_index.htm

25th European Mask and Lithography Conference, edited by Uwe F. W. Behringer, Proc. of SPIEVol. 7470, 74700F · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.835177

Proc. of SPIE Vol. 7470 74700F-1

I a

Detector with slit

Iaperture

mask withL&S structure

However, due to the high sensitivity of the EUV radiation with respect to the MoSi-multilayers, small, unwanted variations or disturbances of the multilayer will also modify the measured EUV scatterograms. Thus also the results for the measurands of the absorber features like CD will be affected.

Here the use of scatterometry at much longer wavelengths, which are mismatched to the multilayer interference structures, can be advantageous. These wavelengths are not that sensitive to local imperfectness and non-homogeneity of the multilayer. Therefore conventional scatterometry can be advantageously used to separate multilayer and absorber structure features.

Recently we investigated the potential, applicability, and advantages of conventional scatterometry in the DUV or VIS spectral range for the dimensional characterisation of the absorber structures on EUV photomasks [8]. By means of numerical investigations on the basis of rigorous diffraction calculations we demonstrated the sensitivity of DUV scatterometry with regard to dimensions and geometry of the absorber structures. On the other hand we showed, that scatterometry in the DUV spectral range is nearly insensitive to perturbations of the reflecting MoSi multilayer. It has further on been verified, that for this reason a quite good reconstruction of geometry and size of the absorber structures from measured data is possible.

At PTB a new type of DUV scatterometer has lately been developed and set up [9, 10]. The concept of the system is very variable and versatile, so that many different types of measurements like e. g. goniometric scatterometry, ellipsometric scatterometry, polarisation dependent reflectometry and ellipsometry can be performed. The main applications are CD, pitch and edge profile characterisation of nano-structured surfaces mainly, but not only, on photomasks. To test the applicability of DUV scatterometry on EUV masks we measured the prototype EUV mask using an illumination wavelength of 193 nm. In this contribution we present the results of this comparison.

In addition we studied in detail the influence of the multilayer parameters on EUV scatterometry on the basis of rigorous simulations and investigated the correlations between multilayer and absorber structure parameters. Some of the results of this study are presented exemplarily and the main outcomes are discussed in this paper, also.

2. MEASUREMENT SYSTEMS

For the scatterometric measurements we used both PTB's EUV reflectometer [11] at the electron storage ring BESSY II and a conventional VIS/DUV scatterometer recently developed and set up at PTB [9, 10].The PTB with its laboratory at

Fig 1a :Mechanics of the EUV-reflectometer. The available motions for sample and detector are indicated.

Fig. 1b: Scheme of scatterometry measurements: The detector angle is scanned at fixed incidence angle.

Proc. of SPIE Vol. 7470 74700F-2

Arm withoptical lift

and analyser

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6-mask holder. xyz-positioning system ona rotary stage

beam preparation(polarising andbeam-shaping elements)

Laser

the electron storage ring BESSY II supports the national and European industry by carrying out high-accuracy at-wavelength measurements in the EUV spectral region [12]. PTB's EUV reflectometer allows mask surface scanning in Cartesian coordinates at 10 μm positioning reproducibility. The probed area (photon beam size) is about 1 mm2. The mechanical set-up of the EUV reflectometer for detector and sample manipulation is shown in Fig. 1. The polarisation vector of the incident radiation is parallel to the X-axis. This results in pure S-polarisation in our normal mode of

operation, i.e. using the Θ and 2Θ axis for the setting of the incidence and exit angle of the radiation. This is shown schematically in Fig. 2. The angular resolution is limited by the photon beam spot size at the sample which can be as

small as 0.5 mm. For the measurements an angle of incidence of Θ = 6° and three different wavelength between 13.4 nm and 13.9 nm have been used.

Fig. 3 shows a technical draft of the conventional VIS/DUV scatterometer system. It is a goniometric scatterometer, that can be operated in a reflectometric, ellipsometric, and diffractometric measurement mode. As radiation source a frequency multiplied Ti:Sa laser provides four wavelength ranges between 840 nm and 193 nm. Since it is operated mainly at 193 nm it is called DUV scatterometer.

The EUV test mask has been measured using goniometric scatterometry. The scatterograms have been measured and evaluated for 6 angles of incidence between

Θ = -60° and Θ = 60° using p-polarised radiation.

Additionally the test mask has been characterised by the mask supplier using a commercial CD SEM and a commercial AFM. The CD SEM has been used to characterise the mean CD, the CD uniformity and the CD range for each die. For the evaluation of the CD SEM data an edge angle of the absorber structures of 90° have been assumed. The AFM has been used to evaluate the top and bottom CD and therewith also the edge angles.

Both commercial systems have not been calibrated for the application on EUV masks, so far. Therefore these reference values have to be seen qualitatively.

3. MATHEMATICAL MODELLING AND SOLUTION METHOD

For the numerical analysis of scatterometry we use the software package DIPOG [13] which solves direct problems for optical gratings and inverse diffraction problems. The finite element method was implemented which allows simulating the diffraction by gratings with arbitrary geometry. For details see [13].

The inverse problem consists in the calculation of an optical grating, whose diffraction properties best approximate the measurement data. In general, the measurement data are a relative small subset of the diffraction efficiency patterns, the phase shifts, the total energy, the reflected and transmitted energy for different angles of incidence, wavelengths, polarisation states and directions (transmission and reflection). For example, for the evaluation of the DUV measurements presented in this paper we chose the efficiency patterns of one wavelength (193 nm) and the TE polarisation in reflection for different angles of incidence. Otherwise, the mask parameters chosen for identification are a subset of all profile and material parameters. Here, the bottom CD, the top CD and height of TaN were selected and compared.

The best approximation is found by the minimization of a nonlinear objective function, in simple cases a least squares or a weighted least squares problem. In certain cases linear terms may be helpful and are optionally incorporated in the

Fig. 2. Technical draft of our new DUV scatterometer. During the construction of the superstructure (tripod with detector arm) great importance was attached to a very precise adjustability of the rotary axis of the detector and on a thermal stability of the system on the whole.

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55nm TaN absorber

10nm 2 buffer

11nm - capping

Quartz 6,35m

60nm Cr backside

multilaye50 Mo/Si - layer350nm

12nm Ta based ARC layer

55nm TaN absorber

10nm 2 buffer

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Quartz 6,35m

60nm Cr backside

multilaye50 Mo/Si - layer350nm

12nm Ta based ARC layer

objective function, too. All five optimization algorithms - available in DIPOG [13] - start with an initial value and operate iteratively, four are gradient based local optimizers [14] and one is a stochastic global optimizer [15]. Unfortunately, as many inverse problems the inverse scatterometry may be ill-posed, that means, at least one of the following conditions is violated: existence, uniqueness or stability of the solution. Often the most serious difficulty in scatterometry consists in the existence of several local minima of the objective function. Nevertheless, there are a lot of supporting techniques to catch the correct solution as regularization methods [16], [17], a convenient selection of measurement data [18], a proper scaling and number of parameters, proper weights within the objective function [18], a limitation of the feasible parameter region.

The DUV data has not yet been evaluated using this inverse evaluation scheme. Here we used a simpler data base approach, where we calculated a sufficiently big data base by performing rigorous forward calculations with varying input parameters for the absorber structures. A search algorithm then looks for the simulation data which fits best to the measured data using the weighted least mean square deviation as criterion.

The EUV data has additionally been evaluated by the ZIB [19] using the software JCMsuite [20], another finite element based Maxwell-solver. These results, which have already been published elsewhere [7], are also compared with our EUV and DUV results.

4. DESIGN AND PARAMETRISATION OF THE EUV TEST MASK

The EUV test mask has been allocated to the PTB by the AMTC in 2006. Fig. 1 shows the design of the stack composition. 50 MoSi bi-layers are applied onto the quartz substrate, followed by a Si capping layer. This multilayer constitutes a high reflecting layer for the EUV radiation around 13,5 nm with a typical maximum reflectance of 64%. On top of this multilayer a SiO2 buffer layer, a TaN absorber layer and a Ta based anti reflection coating layer are placed.

In Fig. 4 the scheme of the cross section profile of the absorber structures on the structured mask is shown. The points pi denotes characteristic geometric points of the absorber structures describing the size and geometry of these structures. The coordinates of these points are adjusted in the optimisation procedure described in the previous chapter.

The optical as well as the geometrical properties of the layers of the EUV-mask used in the numerical calculations in this paper are given in table 1.

The multilayer materials are molybdenum and silicon. Reflectivity measurements by Scholze et al. [7] on unstructured EUV mask areas have shown that it is not sufficient to model a multilayer stack with only these two materials. Instead of that it is necessary to insert additional MoSi2 intersection layers [24]. Thus, in the performed simulations and data evaluations a 4 layer model of the multilayer has been used (cf. Fig. 4 and Table 1). The optical parameters for the DUV spectral range have been evaluated from the reflectometric measurements shown in chapter 6. The MoSi multilayer can be replaced by a single homogeneous layer with average optical constants and a thickness of 355 nm [8].

Fig. 3. Stack composition (design) of the used test mask .

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The test mask is structured with 121 dies “A1” through “K11” as indicated in Fig. 5. Each die is structured into 16 fields containing unstructured areas for reflectance measurements on bright field and dark field areas, respectively. Additionally each die has two different scatterometry test fields with periodic lines and space structures. One is produced with nominally the same CD and pitch for all dies (nominal CD 540 nm and pitch 720 nm) to test the CD uniformity over the mask. The other one is produced with systematically varying line to space (L/S) ratio, whereas the CD values are nominally equal (nominal CD 140 nm) to look for the impact of L/S variations on the CD.

We performed measurements in the CD uniformity test field in the dies D4, D8, F6, H4 and H8. The L/S variation test fields have been measured in the dies H1 to H5, corresponding to a variation of the L/S ratio from 1/20 (H1) to 1/2 (H8).

50x[MoSi - Mo - MoSi - Si]2 2

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12 1) 1) 2.11 [21] 1.27 [21]

Absorber TaN 54.9 1) 1) 2.162 1.771

Buffer SiO2

8 0.97352 [7] 0.01608 [7] 1.63 [21] 0.006 [21]

Oxidation layer SiO2

1.246 0.97352 [7] 0.01608 [7] 2.79 0.315

Capping layer Si 12.536 0.99846[22] 0.00184 [22] 0.885 2.783

ML substitute 355 0.821 2.528

ML1 (mixed) Mo and Si

0.5 0.96675[22] 0.00446 [22]

ML2 Mo 2.259 0.91872[22] 0.00672 [22]

ML1 (mixed) Mo and Si

1.263 0.96675[22] 0.00446 [22]

ML4 Si 3.077 0.99846[22] 0.00184 [22]

substrate SiO2 6.35e06 0.97352[22] 0.01608 [22] 1.561 [23] 0.0 [23]

Fig. 4 Scheme of the cross-section profile of an EUV-mask as it is used for the

numerical simulations. Γ− and Γ+ denotes the substrate and the superstrate, respectively [13]

Table 1 Optical and geometrical properties used for the simulations; height values are derived from EUV reflectometric measurements; ML1 to ML4 are the different sublayers of the MoSi multilayer; if not differently indicated optical parameters for 193 nm are derived from reflectometric measurement (c.f. Chapter 6), for 193 nm the multilayer has been substituted by a single homogenous layer with average optical constants

A B C D E F G H I J K

1 H1

2 H2

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11

Fig. 5 Scheme of the dies at the structured test mask, the measured dies are indicated in red (CD uniformity fields) or blue (L/S variation fields)

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C)SC)>2

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x Param. distribution (cap:2% ML group:0.1 %)

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> -RC)

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82 S81

SWA of TaN layer (dependent parameter)

derived parameter (dependent from p1.p5 & p7)

0.151

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0.1505a :0.29nm

0Dm

0) CDm 150.80 nmmean

0.1504

V0)

E

i 0.1502

0.15 +

+01498

5. SIMULATIONS OF THE INFLUENCE OF THE MULTI- AND CAPPING LAYERS

To test and demonstrate the influence of multilayer and capping layer thickness variations on the reconstruction results of the absorber structures for the evaluation of EUV scatterometry data we performed numerical simulations. For this purpose we used a data set of diffraction efficiencies measured in die H5 and the structure parameters pi derived from this measurement together with the multilayer parameters given in table 1. Then we varied statistically both the multilayer thickness and the capping layer thickness and derived the corresponding reconstruction results for the structure parameters pi. Fig. 6 shows exemplarily a result for 80 data sets with statistically varied input parameters. The height of the TaN absorber layer depends relatively low on the multilayer parameters. However, the derived values for top CD and bottom CD show a significant, relative large dependence on these parameters with a standard deviation of

a)

b)

c)

d)

Fig. 6 a-c) Reconstruction parameter distribution for a

statistical variation of the multilayer thickness and of the capping layer thickness (1% each); results for a) top and bottom CD and for height of the TaN absorber layer, b) mean CD at 50% structure height, c) sidewall angle (SWA) of the TaN absorber layer; red line: median, blue box: 1. and 3. quartil, bars: spread of the results, red crosses: outliers

Fig 6 d) Correlation between sidewall angle SWA and the capping layer thickness; the thickness of the capping layer has been varied with fixed multilayer parameters and the top and bottom CD have been fitted; the resulting SWA is shown versus the capping layer thickness

Proc. of SPIE Vol. 7470 74700F-6

the order of 1%. For the derived structure parameters mean CD (at 50% height of the structure) and sidewall angle we

observe different behaviour: The variation of the mean CD is reduced (standard deviation ≈0.3) as compared with top and bottom CD, which indicates, that the top CD and bottom CD values vary anti-correlated with the variation of the multilayer and capping layer parameters. For the sidewall angle this behaviour of course leads to a relative strong modulation (standard deviation of nearly 2°). Fig. 6d depicts the sidewall angle versus the thickness of the capping layer obtained from these results. This is a more descriptive illustration of this relation demonstrating the strong correlation between these two parameters.

6. MEASUREMENT RESULTS

6.1 Reflectance measurements at 193 nm

To derive optical parameters of the absorber layer and multilayer materials of the EUV for 193 nm we measured the reflectance at this wavelength both in areas with unstructured absorber layer (dark field) and with unstructured multilayer (bright field) using the DUV scatterometer. For the bright field measurement we fitted a simplified model for the multilayer and capping layer: the multilayer was replaced by a single homogeneous layer with a thickness of 355 nm. On top of this layer a 12.536 nm thick Si capping layer was assumed with a silicon oxide layer (natural oxidation) of 1.246 nm thickness. The thickness values have been taken from former corresponding reflectance measurements in the EUV spectral range [7], because EUV reflectometry is more sensitive to the layer thicknesses than the 193 nm radiation. The result of the fit of the n and k values of these layers at 193 nm also shown in Fig. 7 shows an excellent agreement with the measured data.

For the dark field we used the layer model described in Fig. 4 and Table 1. The height values again are taken from [7], the optical constants of the buffer layer and the ARC layer are fixed to feasible values according to [21]. Thus only the complex refractive index of the TaN absorber layer has been fitted. Nevertheless, the agreement with the measured date is even better than for the bright field data fit.

These results and the applied layer model is very well suited for the evaluation of the scatterometry measurements at 193 nm.

0 10 20 30 40 50 60 70 80 900.0

0.2

0.4

0.6

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1.0

Re

fle

cta

nce

angle of incidence θ/°

bright field

dark field

best fits

Fig. 7 Reflectance measurements of the EUV test mask versus the angle of incidence (varied from 3° to 81°) at a wavelength of 193 nm in an unstructured dark field (red dots) and an unstructured bright field (blue dots); the straight lines are fitted values according to the model described in Table 1; further details see text

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I.guFl

6.2 CD measurements at the CD uniformity test fields

Fig. 8 depicts the results obtained with the EUV and DUV scatterometer in comparison with the CD SEM measurements provided by the mask supplier. The data has been evaluated both for the top CD and bottom CD. The EUV data has been evaluated on the one hand by the ZIB using their JCMsuite software and on the other hand by the PTB applying the Dipog software. The ZIB evaluation has optimised the mean CD and the sidewall angles, the results shown in Fig. 8 are calculated from this data. The CD SEM measurements have been evaluated on the assumption of an edge angle of 90°. Therefore top and bottom CD are the same for the SEM data. The relative tendencies of the measured values agree relatively well despite the result in die H4. The relative large spread of the data in this die is ascribed to an insufficient etching. Fig 8 shows additionally a comparison of top down SEM images of the die H4 and exemplarily H8 (the images of the other dies are quite similar to H8). These images confirms this assumption. Therefore the results of die H4 are omitted in the further analysis.

The spread of the scatterometric results is less than 15 nm for the top CD and slightly larger for the bottom CD. The DUV data shows an excellent agreement to the ZIB values for the top CD and our Dipog results for the bottom CD. There is a mean offset -9 nm and +14 nm between the Dipog and ZIB evaluation of the top CD and bottom CD, respectively. This indicates, that the edge angles determined with both methods differ significantly. The mean edge angles derived from the EUV data with Dipog and JCMsuite are 77.5° and 86°, respectively. The DUV measurements give an intermediate result of 79.5° at average, which agrees slightly better with the Dipog evaluation.

To visualise the linearity of the different measurement results, Fig. 9 shows the results after subtraction of the mean offset to the SEM values. The die H4 has been omitted in this evaluation. Both for the top CD and for the bottom CD an excellent linearity is observed for the SEM, DUV and the EUV data evaluated with the Dipog software. In both cases the nonlinearity is well below 5 nm. The linearity of the ZIB results is significantly worse. The anti-correlated offsets for top CD and bottom CD indicate, that the ZIB evaluation probably overestimate the sidewall angle variation. The corresponding plot for the mean CD (not shown), which actually has been fitted by the ZIB, shows an excellent linearity to the corresponding SEM, Dipog and DUV data.

D8 D4 F6 H8 H4520

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Fig. 8 Comparison of measured values for top CD (top left) and bottom CD (top right) both of the EUV and the DUV scatterometer and comparison with CD SEM measurements; the EUV data has been evaluated with two different software tools; right: top down SEM images of the dies H8 and H4 confirms the insufficient etching of die H4

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Fig. 9 Comparison of the linearity of the measured values for top CD (left) and bottom CD (right) of the EUV, DUV and CD SEM data; the mean offset to the SEM data has been subtracted from the scatterometry data; die H4 is omitted

6.3 CD measurements at test fields with varying L/S-ratio

Fig. 10 depicts the results obtained in the dies H1 to H5 with the EUV scatterometer in comparison with CD SEM and AFM measurements provided by the mask supplier. The data again has been evaluated both for the top CD and bottom CD, but only with JCMsuite. A systematic evaluation of these measurements with Dipog is not yet performed. First preliminary results obtained with the DUV scatterometer for the bottom CD are also shown. Whereas the mean difference between the DUV and EUV results for the bottom CD is less than 6 nm, the DUV results show relative bad linearity in comparison with all other measurements. The reason for this might be caused by the evaluation method used. For the DUV data in this first approach we did not allow for an edge angle variation amongst the different dies. For state of the art masks and an established mask manufactoring process this is a very reasonable constrain. However, the manufactoring process of this EUV test mask has of course not been optimised and established in 2006 and the AFM measurements show significant edge angle variation ranging from 86° to 90° for these five dies. Therefore the bad linearity of the DUV results might be attributed at least partly to this constrain for the edge angle variation. For this reason we will repeat the evaluation of the DUV data without this restriction.

The mean edge angles derived from the AFM, EUV and DUV data are 88.6°, 91.5° and 86.3°, respectively, which is a reasonable agreement.

1/20 1/15 1/10 1/5 1/2

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Fig. 10 Comparison of measured values for top CD (left) and bottom CD (right) of the EUV scatterometer and comparison

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Fig. 11 Comparison of the linearity of the measured values for top CD (left) and bottom CD (right) of the EUV, the CD SEM and the AFM data; the mean offset to the SEM data has been subtracted from the scatterometry data

The linearity of the EUV measurements to the AFM and CD SEM results is depicted in Fig. 11. Here again the corresponding mean offset to the SEM values has been subtracted. For the first four dies all measurements show an excellent linearity for the top CD values and also a good agreement between SEM and EUV results for the bottom CD.

For the mean CD, which actually has been evaluated by the ZIB, the corresponding linearity of the ZIB data to the SEM values (not shown) is even better.

The AFM results for the bottom CD show a small but systematic variation in the signature, as compared with the SEM and EUV results, indicating the sidewall angle variations already mentioned.

For the die H5 (L/S ratio = 1/2) the EUV data shows a significant offset to the AFM and SEM values. For this die the evaluation of the corresponding DUV measurements were also quite dissatisfying. A reason for this could be a possible modification of the edge profiles in this die. Here further detailed investigations of this die are required.

7. DISCUSSION AND CONCLUSION

We investigated the potential of the use of EUV and DUV scatterometric measurements for the characterisation of the size and geometry of the absorber structures on EUV masks. The results of both CD-SEM, AFM, EUV- and DUV scatterometry show a quite good agreement in linearity despite constant CD offsets for these different metrology tools. In general these results demonstrate the applicability both of EUV and DUV scatterometry for the accurate and quantitative determination of the size and geometry of the absorber structures on EUV masks. Thus the metrological potential of these methods exceeds current applications of scatterometry in the semiconductor industry like qualitative measurements for process development and process monitoring.

Due to the relative complex assembly of EUV masks, especially their reflection enhancing multilayer composition between the quartz substrate and the absorber structures, the analysis of EUV scatterometry measurements is quite difficult and extensive, because the measured scatterograms exhibit at least as much sensitivity to the multilayer parameters as to the absorber structure parameters. The numerical studies described in chapter 5 have shown the strong correlations between the multilayer parameters, mainly the thicknesses of the different sublayers, and both the top and bottom CD of the absorber structure and, even more pronounced, the sidewall angles. Due to these correlations it is nearly impossible or at least very laborious to determine all these parameters from actinic scatterometric measurements.

A quite obvious solution is the combination with EUV reflectance measurements on unstructured absorber free areas. These measurements can be evaluated for the relevant multilayer parameters [7], which again may be used as fixed input parameters for the evaluation of EUV scatterometry data. However, thickness variations of the multilayer, it's sublayers

Proc. of SPIE Vol. 7470 74700F-10

and also the capping layers over the mask blanks are expected to be even for future high-end mask blanks as big as 0.3% [1]. Therefore the multilayer has to be characterised in the vicinity of the scatterometer fields, ideally by reflectance measurements in the same die. The EUV scatterometry results shown in chapter 6 have been derived with the help of in-die reflectance measurements.

An alternative solution is the application of conventional scatterometry in the visible, UV or DUV spectral range, which is nearly insensitive to modifications or imperfection of the MoSi multilayer. The measurements presented in this paper confirm very well former results based on numerical simulations predicting a high sensitivity of DUV scatterometry regarding geometric-dimensional parameters of the absorbing structures on EUV masks. Therefore it is evident, that the reconstruction of the grating structures, the solution of the inverse diffraction problem, works very well in the DUV case, because the dimensionality of the parameter space needed for the solution of the inverse problem is much reduced as compared with EUV scatterometry. This also leads to a significant reduction of the measurement effort because of the lower number off free parameters which have to be optimised. Moreover, for the analysis of DUV measurements the MoSi multilayer can be replaced in the model by an optically equivalent single layer in the DUV with nearly no loss of accuracy and unambiguity. This could also be confirmed by the measurement results presented in chapter 6. DUV scatterometry allows for an easy separation between absorber structure and multilayer features. For applications not only in process optimisation and in process control but also for absolute dimensional measurements of the absorber structures a DUV-scatterometer may be sufficient in many cases. This statement of course holds also for conventional spectroscopic scatterometers, which are currently already used in mask metrology.

Thus, VIS/DUV scatterometry is an interesting metrology alternative with advantages in measurement time and effort, as compared with EUV scatterometry, and which can be performed using metrology tools, which are commercially available today.

Of course EUV reflectometry is an indispensable tool for the characterisation especially of the multilayer characteristics on EUV masks. Additionally aspects of the printing characteristics of EUV masks can only be analysed and monitored using at-wavelength metrology. Therefore EUV and DUV scatterometry complement each other very well for applications in development and fabrication of EUV masks.

The remaining differences and noticeable problems appearing in the measured DUV and EUV results ask for more in-depth analysis of the reasons. This is especially true for the differences between the two different analysis methods used for the EUV data. Some of these differences may be attributed to imperfections of the structures like e. g. sidewall angle variations, but others will be most likely traced back to possible small differences in the applied structure models and optimisation procedures (e. g. different weighting functions).

Of course (like for CD SEM and AFM) more systematic and accurate studies on scatterometric methods are is still necessary to fulfil the challenging requirements as stated in the ITRS roadmap [1]. These future investigations will benefit from the continuous improvements of the mask processing and of the structure quality on suitable EUV test masks.

Altogether, the presented results are a step forward to establish scatterometry successfully as alternative and supplement to CD SEM and AFM systems to achieve the dimensional metrology needed for future EUV masks.

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