Optical Fourier Transform Scatterometry for LER and LWR metrology

P. Boher*a, J. Petita, T. Lerouxa, J. Foucherb, Y. Desièresb, J. Hazartb, P. Chatonb, aELDIM, 1185 rue d’Epron, 14200 Herouville St Clair, FRANCE,

bCEA Grenoble, LETI, 17 rue des martyrs, 38054 Grenoble cedex 9 FRANCE

ABSTRACT We present an innovating instrument based on optical Fourier transform (OFT) capable to measure simultaneously the specular and non specular diffraction pattern of sub-micronic periodic structures. The sample is illuminated at fixed wavelength (green laser) versus a large angular aperture both in incidence (0 to 80°) and azimuth (0 to 180°). In the present paper we focus on the possibility to measure line edge roughness (LER) and line width roughness (LWR) using this new technique. To understand the problem, different gratings with artificial periodic LER and LWR roughness have been fabricated and characterized precisely by atomic force microscopy (AFM). Different light scattering measurements have been performed using the OFT instrument with different illuminations in order to understand precisely the optical behavior of these systems. We show that we can distinguish LER and LWR by measuring simultaneously the diffracted contributions coming from the grating and from the periodic roughness. In phase LER with small LWR does not give first order diffraction contribution for the periodic roughness. In contrast, LER in opposite phase with large LWR gives a strong contribution for the first order of diffraction of the periodic roughness. In any case, the sensitivity to LER and LWR is better than 5nm for 500nm period gratings measured at 532nm. This result can be extended to samples with real LER and LWR. It shows without ambiguity that simultaneous measurement of the specular and diffracted light diffraction patterns is necessary to extract separately the two parameters. Keywords: Scatterometry, OFT, Fourier optics, AFM, Line edge roughness, LER, Line width roughness, LWR

1. INTRODUCTION The continuous reduction of the critical size in the lithography process is one of the key problems for the next generation of IC’s. ITRS roadmap (1) has pointed out the most difficult challenges for the next technology nodes not only for the critical dimension (CD) reduction but also in terms of line edge roughness (LER) and line width roughness (LWR). ITRS defines LWR as 3 sigma of the line width over a range of spatial frequencies (cf. figure 1). It is as low as 3nm for 90nm node and will become less than 2nm for the next generations. For the first time these small dimensions are approaching the dimensions of polymer molecular size used for the resists putting a great pressure on resist makers (2). The impact of LER/LWR to device performance has been experimentally evaluated (3). It is generally pointed out that LWR is more critical than LER for threshold voltage failure or drain current failure and large LER do not necessarily mean large LWR in particular for winding shape poly gates. Therefore separated characterization of the LER and LWR in addition to CD profile metrology is needed for a good control of this critical lithography step. Up to now CD-SEM is the technique of choice to measure all these parameters (4). Other 3D metrology tools like X-SEM and CD-AFM have also been used to extract LER and LWR in addition to CD values (5). All these techniques are quite long and difficult to systematically apply in a production environment even if many progresses have been made in particular in the automation. In this respect, optical techniques always have big advantages and the development of scatterometry or Optical Digital Profilometry (ODP) has been proven by most suppliers to be an efficient solution for CD metrology at the 90nm node (6). ODP is the technique of choice for industry almost for the next nodes (7). The present ODP technique is based on standard optical techniques already validated for years for film thickness measurements. Basically two techniques are presently used, spectroscopic ellipsometry and polarized reflectometry. If a lot of efforts have been devoted to the simulation and regression of this kind of measurement, it seems that development of specific hardware was not necessary up to now. Spectroscopic ellipsometers are proposed by different suppliers for this new application (8-10). The wavelength dependence provides the structural information through simulation software generally based on Rigorous Coupled Wave Theory (11). Nevertheless, some drawbacks are well known. The validity of the

* pboher@eldim.fr; phone 33 2 31 94 76 00, fax 33 2 31 94 09 50; www.eldim. com

solution or the “inverse problem” is one of them, especially when the model used for the simulation is complicated with undercut effects for example. The other important practical problem of all these optical techniques is the lack of information on LER and LWR provided by standard SEM technique. In the previous edition of this conference, we have presented a new way to address variable angle scatterometry using OFT system. The basic idea was to apply the experience of ELDIM in Fourier optics to this new problem. Indeed, since 1991 ELDIM has developed a conoscopic system based on Fourier optics and CCD sensor capable to measure very rapidly and precisely the emission pattern of LCD displays with a very large angular aperture (up to ±88°)(12-13). This instrument called EZContrast can also include a controlled illumination to perform reflection properties with a good control of the polarization. In the previous papers (14-15) we have shown that simultaneous measurement of the specular contribution and the diffracted orders due to the grating is a powerful tool. Using the specular contribution, it is possible to deduce CD profiles with improved sensitivity compared to standard OCD techniques. Indeed, the best measurement conditions in terms of CD profile are always for incidence planes at an azimuth not along or perpendicular to the grooves of the grating. In addition we have shown experimentally that the diffracted orders are much more sensitive to the grating imperfections than the specular contribution. This is not surprising since all the light in the diffracted orders is due to the grating constructive interferences which are directly affected by the imperfections. This light is removed from the specular contribution and appears then as a second order effect. In the present paper, the idea is to understand more precisely how the light is diffracted by a grating with LER and LWR imperfections. In this respect OFT is a powerful tool because it allows us to directly measure the whole pattern of light diffracted by the structure. As we will see in the following, any real LER and LWR can be decomposed in Fourier series of periodic roughness. A good approach for better understanding is then to make artificial periodic roughness that can produce well defined diffracted orders due to the roughness in addition to the normal diffracted orders due to the grating itself. This type of approach has already been proposed to take into account innovative critical dimension small angle x-ray scattering measurements (16-17) and additional diffraction peaks due to the correlated roughness have been evidenced in some cases using this technique. In the present study, we have fabricated different grating samples with controlled periodic LER and LWR. These samples have been characterized by CD-AFM to precisely measure their morphology and the different structural parameters. They have then been measured by OFT under different illumination conditions. These experimental results allow us to achive quite general conclusions on the possibility to detect LER and LWR by optical scattterometry.

LER = 3σdeviation ofedge to line

LWR = 3σdeviation of

width

Period

CD

LER = 3σdeviation ofedge to line

LWR = 3σdeviation of

width

Period

CD Figure 1: schematic drawing of a grating with LER and LWR definitions

2. SAMPLES & EXPERIMENTAL DETAILS 2.1 Description of the samples A series of gratings has been realized by electron beam lithography on a 200 nm thick resist layer on silicon. The periodicity is fixed at 500nm and the mean CD value is 125nm. A controlled roughness is added on one side or both sides

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of the grooves with variable amplitude and variable periodicity. The periodicity is 50, 100, 250, 500 or 1000nm and the amplitude is about 10, 25 or 50nm. In addition, the roughness can be perfectly correlated on both sides of the grooves (samples PHA) leading to very small LWR values even for large LER values or in contrast perfectly in opposite phase (samples DEP). The last case gives simultaneously large LER and large LWR values. A top CD-SEM photograph of the three types of samples is shown in figure 2. The samples can be considered as extreme situations. They are nevertheless representative of more realistic situations.

PHA DEP UNI

Figure 2: Top CD-SEM photograph of three samples with artificial roughness. The grating period is always 500nm. The mean CD value is always 125nm. The roughness periodicity is 500 or 1000nm and its amplitude is 10, 25 or 50nm. It is applied only on one side (UNI samples), on both sides in phase (PHA samples) or in opposite phase (DEP samples). 2.1 Description of the OFT instrument Fourier optics has the capacity to transform the angular response of a sample in spatial information that can be imaged by a 2D sensor (cf. figure 3.a). Each light beam emitted from the sample surface with an angle θ with regards to the normal of the surface is focused on the Fourier plane at the same azimuth and at a position x = F tan(θ). The angular emission of the sample is then measured simply and quickly without any mechanical movement. In an Optical Fourier Transform (OFT) instrument the Fourier optics is generally an achromatic combination of different lenses (6 to 9) that enable the analysis of a given spot size (up to 2mm for the standard instrument of ELDIM) with a reasonable working distance (~2mm). The size of the measuring spot can be easily adapted by an iris. The size is independent of the angle which enables accurate measurement over the whole cone (from -80 to +80° of incidence angle with all the azimuths). Results are generally reported in the Fourier plane (cf. figure 3.b) where each point corresponds to one incidence angle θ and one azimuth angle φ. The angular calibration on the CCD sensor is achieved using an illuminated circular target with accuracy better than 0.1° for the whole angular range of the instrument. The optical resolution mainly depends on the optical setup, the measuring spot size and the pixel size on the CCD detector. It is less than 0.5° in our configuration. The sensor is a Peltier cooled 14bits CCD with 1536x1024 pixels. The measurement stability is better than 0.5% for the whole angular range. As reported in figure 4, a beam splitter is included in the optical path for reflection measurements and an optical relay system produces a secondary Fourier plane which can be used to illuminate the sample. A mask placed on this secondary plane allows a precise control of the illumination from complete diffused one (no mask) to collimated beam with a good angular aperture (< 0.5°). The source used in the following experiments is a NdYag laser emitting at 532nm injected inside an integration sphere. The illumination Fourier plane is located at the exit of the integration sphere. An additional element is used to destroy the source coherency and avoid speckle problems. The polarization is selected before the detector using a fixed polarizer sheet with high extinction ratio (> 10000). In a previous paper (14), we have shown that such a multi-angle polarized reflectometer is capable of giving thickness and optical indices of reference homogeneous SiO2/Si samples with accuracy comparable to spectroscopic ellipsometry.

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Figure 3: concept of Fourier optics and definition of the Fourier plane

Optical Relay Systems

Second Fourier illumination plane

Fourier Plane

Fourier Optics

Sample

Cooled CCD Sensor

Beam splitter Polarizer

Figure 4: Optical setup for reflection properties

3. EXPERIMENTAL RESULTS

3.1 AFM characterization All the samples have been measured by CD-AFM using a Veeco AFM (dimension X3D) instrument. As shown in Figure 5, CD-AFM scanning directly provides a high resolution 2D shape of a part of the grating. 3D reconstructions are helpful to put in evidence the artificial roughness on one side and the residual roughness on the other side. Quantitative measurements can be derived using the vertical profile variation along the line on the left and right side (cf. figure 6.a). The in-phase or out-phase character of the roughness can also be verified. The artificial frequency of the roughness can be checked using a simple Fourier transform of the profile variation as shown in figure 6.b.

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LEFT SIDE

RIGHT SIDE

One side UNI A = 25nm F = 100nm

3D VIEW

3D VIEW

Figure 5: Top view AFM scanning of sample UNI 25-100. The 3D view shows the artificial periodic roughness on left side and the residual roughness on right side. (a) (b)

Figure 6: Side profile from AFM measurement (a) and Fourier transform for frequency distribution (b). This example is for the 1000nm period with about 50nm roughness amplitude on both sides. The profiles are in opposite phase as show in figure 6.a. A summary of the different AFM results is reported in Table I. The roughness periodicity and the phase character have been verified in all cases. The periodicity is very closed to the target (within ±2%). The roughness amplitude slightly changes for one sample to the other depending on the targeted value. It is generally always higher than the targeted value (about 65nm for a target of 50nm for example). The intrinsic roughness value can be evaluated from the right side of the UNI samples. It is about 13±3nm. The CD deviation versus the roughness amplitude is a good indication of the regularity of the samples (cf. figure 7). As expected, it is quasi almost times the amplitude for the DEP samples and close to the noise level for PHA samples.

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Sample Type Periodicity Roughness amplitude (nm) CD deviation (nm) Left side Right side (nm)

DEP 10-500 Opposite phase 500 15.7 17.2 28.6 DEP 25-500 Opposite phase 500 39.0 34.7 71.3 DEP 50-500 Opposite phase 500 62.7 62.3 121.2 DEP 10-1000 Opposite phase 1000 20.0 22.5 28.4 DEP 25-1000 Opposite phase 1000 43.9 39.6 78.8 DEP 50-1000 Opposite phase 1000 66.0 66.5 131.4 PHA 10-500 In phase 500 16.6 17.8 9.7 PHA 25-500 In phase 500 40.1 36.9 15.0 PHA 50-500 In phase 500 59.2 59.7 20.6 PHA 10-1000 In phase 1000 17.9 19.5 11.7 PHA 25-1000 In phase 1000 41.4 46.3 17.2 PHA 50-1000 In phase 1000 66.5 66.0 18.0 UNI 10-500 / 500 17.0 12.4 15.7 UNI 25-500 / 500 40.1 14.2 38.6 UNI 50-500 / 500 68.2 11.4 63.3

UNI 10-1000 / 1000 16.2 13.3 16.7 UNI 25-1000 / 1000 44.9 16.5 43.9 UNI 50-1000 / 1000 64.0 15.0 65.8

Table I: Summary of the AFM results.

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60 70 80

Roughness am plitude (nm )

CD

var

iatio

n (n

m)

DEP

PHA

UNI

Figure 7: CD variation versus roughness amplitude measured by AFM.

3.2 Diffraction on a grating with periodic roughness The electric field amplitude diffracted by the grating can be decomposed in plane wave spectrum by means of a Fourier transformation following:

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qx, qy and qz are the wave vector components of each plane wave (cf. figure 8). There are related to the Fourier plane angular coordinates by:

ϕθ cossin=xq ϕθ sinsin=yq θcos=zq

x

z

rq

gq

ik

dk

y

φi

θi

D

d

x

z

rq

gq

ik

dk

y

φi

θi

D

d

Figure 8: The grating can be characterized by two wave vectors gq and rq characteristic of the periodicity of the grating

and of the roughness respectively. We obtain constructive interferences when a combination of the wave vectors representative of the grating periodicities match the wave vector of the plane waves. The “normal” grating wave vector gq has only a component versus x (qx, =

2π/D, qy =0, qz = 0) and the wave vector of the periodic roughness has only a component versus y (qrq x, = 0 , qy =2π/d, qz = 0)(cf. figure 8). This gives general diffraction conditions along the direction (θn,m, φn,m) following:

Dnqq x

dx

λ+= and

dmqq y

dy

λ+=

D and d are the grating and roughness periodicity respectively. The angular position of the (n,m) diffraction order is then provided by:

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

+

+= −

Dn

dm

mn λϕθ

λϕθϕ

cossin

sinsintan 1

,

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

+

+++= −

mnmnmn

dm

Dn

,,

1, sincos

sinsincossinsin

ϕϕ

λϕθλϕθθ [1]

The equations [1] are symmetric in n and m. The double periodicity plays the same role and in both cases the diffraction orders are not in incidence plane except for special configurations. Therefore, the capability to measure all the azimuths at once is mandatory for this kind of structure otherwise we must restrict to “standard” perpendicular configuration. An important difference compared to simple grating situation is the possibility to get diffraction orders due to the grating and roughness periodicities simultaneously when n and m are >= 1. 3.3 OFT measurements at fixed incidence angle We have first measured all the samples using a fixed incidence angle configuration for the illumination. The mask positioned on the illumination Fourier plane fix the incidence angle to 70° for any azimuth between +90° and -90° (cf. figure 9.a). The grating is always orientated along the φ = 90° direction. With this configuration and using equations [1], it is easy to compute the position of the different diffraction orders in the Fourier plane for a given grating. The example of a grating with D = 500nm and d = 1000nm is reported in figure 9.b. The simulation predicts different diffracted orders

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related to the roughness alone and the combination of the grating and roughness periodicity in addition to the normal (-1,0) diffraction order of the grating. On the measurement of samples PHA-50-1000 and DEP-50-1000 reported in figure 10, we can easily see the (0, -1) orders of the grating and the roughness and the (-1,-1) combined order. The (0,-2) and (-1,-2) orders cannot be detected due to their low intensity.

Fixed incidence 70°Variable azimuth Order 0,0Order -1,0 Order 0,-1

Order -1,-1

Figure 9: OFT measurement at fixed incidence angle. The mask used for the illumination (a) fixes the incidence angle to 70°. The azimuth varies from -90 to +90°. Simulation (b) is made for a roughness periodicity of 1000nm.

-1 roughness diffraction order

Figure 10: OFT measurements on DEP and PHA samples (roughness amplitude 50, roughness periodicity 1000nm). The boxes integrate the light diffracted by the periodic roughness (order 0, -1). No important difference between samples DEP and PHA can be detected. We had two concerns performing these measurements, first to evaluate the sensitivity of the OFT measurements to the artificial roughness, and second to compare the PHA and DEP behaviors. This explains why we have integrated the light

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coming on the (0,-1) orders due to the periodic roughness (boxes in figure 10). Results are summarized in Figure 11 for all type of samples. We can notice that the intensity of the 0,-1diffracted order is proportional to the amplitude of the roughness for all the samples. For the UNI samples it is about two times lower than for the PHA or DEP samples. The DEP and PHA samples with same roughness amplitude give nearly the same signature on the diffracted orders. We can conclude then that in this configuration the right and left sides of the grooves diffract independently without any discrimination between the DEP and PHA samples. The sensitivity of the technique is also sufficient to detect the roughness even for small amplitude samples.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 10 20 30 40 50 60

roughness amplitude (nm)

Inte

nsity

(u.a

)

PHA

DEP

UNI

Noise level

Figure 11: Intensity of the 0,-1 diffracted order in the region 45°<φ<90° for the different type of samples.

3.3 OFT measurements at fixed azimuth angle To emphasize the differences between DEP and PHA samples we need to select a particular azimuth where the phase coherence between light diffracted by the right and left sides of the grooves can be constructive or destructive. This particular configuration is represented in figure 12. The azimuth is fixed at 0° perpendicular to the grooves of the grating and all the incidence angles are illuminated. In this case, the previous formulas [1] predict that in addition to the ordinary diffracted order in the same azimuth plane, one contribution 0,-1 due to the periodic roughness will appear on both sides of the specular contribution (cf. figure 13.b). A composite contribution -1,-1 is also expected on both sides of the diffracted contribution (-1,0) (cf. figure 13.b). In practice all the diffracted orders can be measured but not on all samples. As shown in figure 14 in the case of samples DEP and PHA 50-1000, the diffraction order (0,-1) is very strong for the DEP sample but cannot be detected on the PHA sample. This phenomenon is particular interesting because it demonstrates that an experimental discrimination can be made using OFT between samples with correlated or uncorrelated right side and left side roughness, or in other words that an independent evaluation of the LER and LWR of the gratings is possible. The intensities of the diffracted orders measured on the different samples are summarized in figure 15. It confirms our first observations. The (0,-1) diffraction order has a very different behavior for samples DEP and PHA. The UNI samples gives a medium (0,-1) contribution characteristic of the diffraction on one side of the grating without any correlation.

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Fixe azimuth 180°Variable incidence Order 0,0Order -1,0

Order 0,-1

Order -1,-1

Figure 12: Fixed azimuth angle OFT: the orientations of the mask and of the grating are indicated on the left. The theoretical positions of the different diffraction order are reported on the right.

Order 0,-1

Order -1,0

Order -1,-1

Figure 13: Comparison of two OFT measurements on DEP and PHA 50-1000 samples. The diffraction order 0,-1 is not detected on the PHA sample. One can understand this effect quite easily. The two sides of each groove diffract the light separately (as shown in the previous paragraph), but they can interfere when the phase shift is a multiple of the roughness periodicity. When both side of the grooves are coherent (sample PHA) the light beam diffracted by one side of the groove has exactly a phase shift of d/2 compare to the one diffracted on the other side with regards to the symmetry and the direction of the index variation. These two light beams interfere destructively and the (0,-1) order is completely suppressed. This effect is only valid when the wave vector is perfectly aligned along x. It is exactly the opposite for DEP samples where the light diffracted by each side of the grove is in phase and so interfere in a constructive way.

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0.00E+00

1.00E-04

2.00E-04

3.00E-04

4.00E-04

5.00E-04

6.00E-04

7.00E-04

8.00E-04

9.00E-04

1.00E-03

0 10 20 30 40 50 60

Roughness amplitude (nm)

Inte

nsity

(u.a

)

PHA order 0,-1

PHA order -1,-1

DEP order 0,-1

DEP order -1,-1

UNI order 0,-1

UNI order -1,-1

Noise level

Figure 14: Summary of OFT measurement on 1000nm periodic roughness samples. The diffraction order 0,-1 of the roughness is very strong only for DEP samples. It is completely suppress for PHA samples.

3.4 Consequence for LER and LWR measurements on “real” samples For a realistic sample which exhibits arbitrary LER and LWR, the distribution amplitude of each side of the groove can be decomposed in a series of sinusoidal contributions in phase or in opposite phase with a given distribution of frequencies:

[ ]∫ +=f

dfyffbyffayA )2cos()()2sin()()( ππ

So the scattering pattern can be considered as a combination of elementary scattering partterns of gratings with perfectly periodic roughness (PHA and DEP structures as described above). The rules calculated for the perfectly periodic roughness and verified experimentally by OFT can be extended to real samples. The main difference remains that the light diffracted by the roughness does not produce unique diffraction orders but distributed light in the Fourier plane due to the frequency distribution. With OFT we can measure the specular contribution, the diffracted orders of the grating and also the residual light all along the Fourier plane. Two successive measurements versus azimuth and incidence angle as reported above can be used to discriminate LER and LWR effects. If we only access to the specular contribution as with spectroscopic ellipsometry, the discrimination between LER and LWR cannot be made. The measurement is only sensitive to “loss of light” that can be attributed to roughness diffusion but without any more precision.

4. CONCLUSION A new metrology system for characterization of submicron periodic structures has been used to precisely examine the influence of LER and LWR on the light diffraction on sub-micronic gratings. Different gratings with artificial periodic LER and LWR roughness have been fabricated and accurately characterized by atomic force microscopy (AFM). Different light scattering measurements have then been performed using the OFT instrument with different illuminations in order to precisely understand the optical behavior of these systems. We show for the first time that we can distinguish

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LER and LWR by an optical technique simultaneously measuring the diffracted contributions coming from the grating and from the periodic roughness. In phase LER with small LWR does not give first order diffraction contribution for the periodic roughness. In contrast, LER in opposite phase with large LWR gives a strong contribution for the first order of diffraction of the periodic roughness. In any case, the sensitivity to LER and LWR is better than 5nm for 500nm period gratings measured at 532nm. This result can be extended to samples with real LER and LWR. It shows without ambiguity that simultaneous measurement of the specular and diffracted light diffraction patterns is necessary to separately extract the two parameters. Another paper of the same conference (18) examines the possibility to use this new technique for overlay measurement.

5. ACNOWLEDGEMENTS This work is part of the European projects MEDEA+ T304, T406 and 2T102. The authors thank the French Ministry of Industry for its financial support. P Thony and D Henry from ST-Crolles are also thank for their interest and their support.

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