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High accuracy EUV reflectometry at large optical components and oblique incidence Christian Laubis*, Frank Scholze, Christian Buchholz, Andreas Fischer, Steven Hesse, Annett Kampe, Jana Puls, Christian Stadelhoff and Gerhard Ulm Physikalisch-Technische Bundesanstalt, Abbestraße 2-12, D-10587 Berlin, Germany

ABSTRACT The development of EUV lithography is critically based on the availability of suitable metrology equipment. To meet industry's requirements, the Physikalisch-Technische Bundesanstalt (PTB) operates an EUV reflectometry facility at the electron storage ring BESSY II. It is designed for at-wavelength metrology of full-sized EUVL optics with a maximum weight of 50 kg and a linear dimension of up to 1 m. With the development of EUV lithography tools, the requirements for lower measurement uncertainty are steadily increasing. For small test samples at near normal incidence, a total uncertainty of 0.10 % for peak reflectance is achieved with a reproducibility of 0.05 % and the uncertainty in the center wavelength of 2 pm is mainly given by the uncertainty for the reference wavelength of the Kr 3d5/2-5p resonance. For real optical elements like PO-box mirrors and collectors for EUV pulsed plasma sources it is also essential to measure at the exact location on the mirror because of gradients in the layer thickness and also to measure at the correct local angle of incidence (LAOI) which may deviate significantly from normal. Thus alignment becomes critical for achieving low measurement uncertainties. Here we present PTB's experience in measuring large EUV optical components.

Keywords: EUV, metrology, at-wavelength, reflectometry, lithography, synchrotron radiation, oblique incidence

1. INTRODUCTION Extreme Ultraviolet Lithography1,2 (EUVL) is the only technology with a demonstrated ability for high volume manufacturing for the 22 nm node and beyond3,4. At present EUVL tools for high volume manufacturing are being developed5. In this development, the refinement of the optical components6,7,8 is critically dependent on the availability of suitable metrology equipment9,10,11. To meet these requirements, the Physikalisch-Technische Bundesanstalt (PTB) operates an EUV reflectometry facility, designed for at-wavelength metrology of full-sized curved mirrors12. Recently for small test samples at near normal incidence, we have shown a total uncertainty of 0.10 % for peak reflectance with a reproducibility of 0.05 % and an uncertainty in the center wavelength of 2 pm10,11. Here the main contribution to the uncertainty of wavelength measurements is the uncertainty of the reference wavelength of the Kr 3d5/2-5p resonance13. Full-sized EUVL optics like PO-box mirrors and collectors for EUV pulsed plasma sources introduce two additional challenges for accurate wavelength measurements: They usually have a gradient in the layer thickness and they are often operated at significantly higher angles of incidence (AOI) which we call 'oblique incidence' (see figure 1). Both of these points require accurate alignment of the measurement position on the mirror to achieve low measurement uncertainties.

*

Corresponding author: [email protected], phone +49 30 6392 5097, fax +49 30 6392 5082 Alternative Lithographic Technologies, edited by Frank M. Schellenberg, Bruno M. La Fontaine Proc. of SPIE Vol. 7271, 72713Y · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.813697

Proc. of SPIE Vol. 7271 72713Y-1

surface near normal normal incidence oblique incidence

figure 1

grazing incidence

Sketch depicting different regions of angle of incidence relative to the surface normal to define the terminology used. Here the AOI always is the angle between incident or reflected beam and the surface normal.

Figure 1 shows a sketch depicting different regions of angle of incidence relative to the surface normal to define the terminology used: 'Near normal incidence' is the small angular area around the surface normal where the effect of illuminating the multilayer at an angle to the normal can be dismissed, typically below 5° AOI. 'Grazing incidence' is the angular region where total reflection occurs and the beam is almost parallel to the surface, typically with an AOI greater than 80°. 'Oblique incidence' is the region between 'normal' and 'grazing'.

2. MEASUREMENT EQUIPMENT Taking advantage of the stable conditions the BESSY II synchrotron facility provides, notably the stability of the source point of the beam, PTB has set up a beamline to cover the soft X-ray region from 0.65 nm to 34 nm. At this beamline, PTB operates a reflectometer which can accommodate full-sized EUVL optics with a maximum weight of 50 kg and a linear dimension of up to 1000 mm. 2.1.

Beamline

The soft X-ray radiometry beamline was designed first of all for a parallel beam with a reasonably low spot size. This was driven by the demand for a high angular resolution in the reflected beam for scatterometry applications. For the standard measurement beam, the divergence is below 1 mrad and the typical spot size on the mirror surface is a box shape of 1 mm x 1 mm. figure 2 Graphics showing the setup of the PTB soft X-ray beamline at BESSY II

top view 176° entrance aperture

exi t sl it bending magnet

cooled apertures

toroidal mirror

plane mirror

plane grating 178° filter focusing mirror

side view

17 m

23 m

31 m

33 m


A prerequisite to accurate measurements of wavelength - be it central wavelength at 50 % relative reflectance (CTW50) or peak wavelength (CTW100) - is a well-referenced energy of the incident beam. Therefore, after every injection we check our beamline wavelength calibration against the K-edge of the Be filter in our beamline. Less frequently, we also measure the Kr 3d5/2-5p resonance. We realign our monochromator if we find a discrepancy of more than 1.8 pm. An overview of our energy calibrations over the last 8 years is given in figure 3.

figure 3

Wavelength of the Be K absorption edge measured as a wavelength reference during the operation of the EUV beamline starting from the commissioning in mid 2000. The dashed lines indicate the tolerance limits and the solid line the reference value. A total number of more than 1000 data points is shown in the figure.

Long term stability By measuring a set of mirrors regularly every other month, we have gathered data to check the long term stability of our measurement setup, sample alignment procedures and measurement method. In figure 4, three reflectance curves of a mirror measured over seven years are shown.

figure 4

Reflectance of a mirror measured in March 2002 (blue), August 2004 (red) and July 2008 (green). The diamonds are the spectral reflectance, left scale. The circles are the photocurrent signal, right scale. The reflectance curves are almost identical, the phase shift of the photocurrent curve indicates an increase in top layer thickness by about 0.2 nm between 2002 and 2004 and another 0.5 nm between 2004 and 2008.

The photocurrent curves shown in figure 4 indicate an increase in top layer thickness due to contamination over the years. To better show trends or respectively their absence, all measured data for maximum reflectance and center wavelength over the years is given below.


figure 5

Maximum reflectance for the same mirror as in figure 4. Data for all measurements performed is given. Please note the small decline in reflectance due to contamination.

figure 6

Center wavelength at 50 % relative reflectance (CTW(50)) for the same mirror as in figure 4. Data for all measurements performed is given.

Even over the extended period of time we have been performing these stability measurements, the mirror reflectance (figure 5) and CTW(50) (figure 6) have stayed within our error budget. As we see the combined effect of our measurement uncertainty and the stability of the mirror in the results given above, an exceptional stability for the mirror used is proven. The slight decline of reflectance (figure 5) over the years is due to contamination which we have quantified using photocurrent measurements as in figure 4. 2.2.

Reflectometry

To use the available high quality beam for accurate measurements11, the mirror under test has to be properly aligned. To achieve that, PTB has a large reflectometer with the capability to characterize samples with a linear dimension of up to 1000 mm, a span in height of 150 mm from the highest to the lowest measurement position and a weight of 50 kg. To illustrate this, figure 7 shows the mechanical design of the goniometer with the movements indicated. The sample can be moved in all three orthogonal linear directions and be tilted around all of them (red arrows). The detector can be turned to face either the incoming beam or the reflected beam, facilitating reference scans. Also the detector's angle to the beam, the position of the measurement plane and the distance from the sample-surface are adjustable (blue arrows). The concept of a reflectometry measurement is simple: It is the quotient of the reflected and incoming beam power. We already aim for accuracy at this stage by using the same diode to measure the incoming and reflected beam to avoid detrimental effects from differently sensitive diodes. For higher angles of incidence, the alignment of plane mirrors already requires accurate scales in Θ and 2Θ (see reference 14) and the surface of the sample to coincide with the main axis (Θ) of the goniometer within a few tenths of a millimeter. For figured mirrors positional alignment becomes important because of gradients in the multilayer and an AOI which depends on the position on the mirror.


figure 7 N

PTB reflectometer with movements indicated. Red: Sample movements Blue: Detector movements

Alignment of AOI To align the angle of incidence, the detector is rotated (in 2Θ) to 2*AOI and the mirror under test moved (in Θ) so that the incident beam is reflected onto the detector thus realizing the desired AOI on the mirror. Of course this method only works flawlessly if the mirror surface coincides with the axis of rotation (Θ). The alignment methods employed to achieve this coincidence are given elsewhere15. Recently we found another albeit small influence on the accuracy of the AOI alignment. Consider what happens if the Θ and 2Θ axes are not coincident in space but are a small distance behind each other as seen from the incoming beam. In figure 8 details are shown, with parts pertaining to detector alignment in blue and the incoming and reflected beam in red. Centers of rotation for mirror and detector are indicated.

center of detector rotation (21 )

detector

beam

center of mirror rotation (1)

2 AOI

21

figure 8 Geometrical details for detector and sample main rotational axes (2Θ and Θ axes respectively) relative to the beam. Detector details are given in blue, sample details are black, beam in and out is red.

d offset between centers of rotation

r o t c e t e d

The offset along the beam axis between the centers of rotation of the mirror and the detector is given by d. Between the incident and reflected beam (in red) the desired 2*AOI angle is indicated. To put the detector in the right position it can be seen that for a center of rotation of the detector behind the mirror, the angle for the detector must be a bit smaller than 2*AOI. With Rdetector (Det.-R in figure 7) being the distance from the center of rotation of the detector to the detector diode and d known from goniometer calibrations (see below), the correcting offset for the detector angle (∆2Θ) is given by: d sin( Δ 2Θ) = sin( 2 AOI ) R

We accomplished the measurement of the actual offset, so bearing it in mind, we can achieve AOI alignments with an uncertainty below 0.02°.


Calibration of goniometer axis offsets

When first commissioning our instrument, effort was put into the measurement of the axes of the goniometer and the proper working of all axes was ensured. Now we find that to enhance our alignment accuracy, we have to include higher order imperfections - like offsets between axes - into our considerations. Fortunately in our alignment scans, we see effects of an order below our total uncertainties. By analyzing those for systematical components we can find and calibrate these higher order imperfections like the offsets between goniometer axes. Iteratively we advanced our understanding of our goniometer's alignments and so lowered our uncertainties in sample positioning. To derive the distance between the axis of rotation of the sample (Θ) and the axis of rotation of the detector (2Θ) - see figure 7 to identify the axes and figure 8 for details - we employed a novel method to initially align the AOI: We installed an additional diode with a slit which is only used for angular alignment but cannot be used for measurements. Threading the incident beam through this split diode and having this diode look at the mirror, we can align the mirror so that the beam is reflected back into itself, defining the AOI = 0° position. By comparison to the established angular alignment where we set the measurement diode to 2*AOI, the offset between the axes under consideration can be calculated. To measure a mirror in polar coordinates (Ф,r) and pre-calculate the positions of the measurement points from a single alignment, we need to know the offset of the rotational axis Ф to the point of origin of the Cartesian coordinates (x,y).

y

circular test mirror center of Φ-rotation x

figure 9

Using a circular plane mirror to derive the distance (orange) between the rotational axis for positioning in polar geometry (Ф, see figure 7) and the origin of (x/y). The circular test mirror (pink) is rotated to different angles in Ф and centered in x/y. From opposing angles, the center of Ф rotation can be calculated.

To derive the distance (orange) between the rotational axis for positioning in polar geometry and the origin of (x/y), we used a circular plane mirror. This test mirror (pink) is rotated to different angles in Ф and centered in x/y. From opposing angles, the center of Ф rotation can be calculated. Alignment and checking

We use the reflected beam from the mirror surface to align the mirror in angle. The position of the mirror is defined either by searching for the edges of the reflecting surface or using additional alignment mirrors at known positions (see reference 15). The alignment of the mirror can be proven by aligning further points at different positions and comparing the coordinates in angle and position with the known figure of the mirror or by measuring the same point close to the origin of the polar coordinates system with different Ф. On mirrors with gradients it is obviously important to achieve high positional accuracy. Here we always check our positional alignment by measuring one central point twice and rotating the mirror by 180° around the Ф axis in between, taking care that the Ф positions we use are aligned along the gradient to give the highest sensitivity. 2.3.

Accuracy considerations

With the mirror aligned for the beam to hit the right spot under the right AOI, the geometry of the reflection is correct. Now we have to ensure that we measure the reflected beam with high accuracy. Since no diode is perfectly homogeneous, we always place the reflected beam onto the center of the diode to minimize the homogeneity issues.


Beam divergence

The length of the beam path between the measurement of the direct beam and the measurement of the reflected beam is 0.5 m for standard measurements. Even with a beam divergence of below 1 mrad for standard settings of the beamline, we see a slight increase in spot size. As every measurement diode is not perfectly homogeneous, changing the size of the beam spot can lead to additional uncertainties. Since we are able to place our measurement diode at a position where the beam path length is equal to the reflected beam but without a mirror, we are able to measure the actual signal difference between the reference position and the measurement position of the diode. We perform this measurement once every week and are therefore able to make corrections for this effect. For figured mirrors, however, there is the focusing/defocusing of the beam which yields another spot size on the detector diode thus leading to an error dependent on the homogeneity of the measurement diode. To minimize the effect of the focusing of a concave mirror, we can adjust the distance between the mirror surface and the diode to largely measure the same spot size in the reflected beam as we have in the incident beam. For concave mirrors, we can only move to our minimal distance. Therefore, it is paramount for accurate measurements of curved mirrors - the necessity increasing with increasing curvature of the mirror - to employ a measurement diode which is as homogeneous as possible. figure 10

Homogeneity scan of a new measurement diode (Type G1127-04 GaAsP from Hamamatsu). Average sensitivity in the center area (indicated by the crosses) is 0.112 A/W. Each colorstep is 0.001A/W.

A diode homogeneity scan is shown in figure 10, the center area is indicated by crosses. We qualify every measurement diode before we use it. Between measurements, we monitor the diode homogeneity and we replace the diode as soon as its center area becomes more than 0.5 % inhomogeneous.


3. STATUS OF MEASUREMENT UNCERTAINTY Two contributions to our measurement uncertainty are directly influenced by changing the AOI. Firstly, polarization which causes uncertainties in reflectance because S- and P-polarized radiation is reflected differently depending on the AOI. Secondly, the CTWs of multilayer mirrors depend on the AOI. For normal incidence, the measured CTW is largest and decreases with cos(AOI). 3.1.

Near normal incidence (NI)

At AOIs near normal incidence, because of the small angular difference to the surface normal, the cos dependence of the peak wavelength of the reflectivity of the multilayer leads to measurements which are quite insensitive to a misalignment of the AOI. Also, at normal incidence, reflectance in S-polarization or P-polarization is barely different. Therefore, near NI measurements yield results with the smallest total uncertainties available for reflectometry of multilayer mirrors in the EUV. 3.2.

Oblique incidence (OI)

The same considerations as for NI also apply to OI. The difference is one of degree and comes from the higher AOI. With a higher AOI, the cos function becomes much more sensitive to small angular deviations and, therefore, the uncertainty contribution at OI becomes sensitively dependent on the AOI. Also the difference in reflectivity for S- or respectively P-polarized radiation increases significantly, becoming most pronounced at the Brewster angle, where P-polarized radiation does not reflect at all. 3.3.

Grazing incidence (GI)

Since at smaller angles between mirror surface and incident beam, the footprint of the beam becomes longer, one critical alignment necessity for grazing incidence measurements is the need to ensure that the beam footprint does not exceed the mirror and that the AOI is well aligned. With grazing incidence measurements in the region of total reflection, there is no reflection peak to be found, so the methodology is different. For this reason, the GI angular region is not included in the table below.

Peak reflectance

Here considered for an AOI of:

Uncertainty contribution u /%

Peak wavelength

Uncertainty contribution u /pm

NI

OI

NI

OI

1.5°

20°

1.5°

20°

Stability of normalized intensity

0.02

Repeatability of wavelength

0.06

Inhomogeneity of the detector

0.04

Reproducibility of wavelength (reference to Be K-edge)

1.1

Higher diffraction orders

0.02

Kr resonance wavelength

1.6

Diffuse scattered light

0.08

Sample temperature (∆T = 5 K)

0.6

Polarization Total uncertainty of peak reflectance Table 1

0.0015

0.30

Incidence angle (∆Θ = 0.02°)

0.1

1.6

0.10

0.31

Total uncertainty of peak wavelength

2.0

2.6

Compilation of measurement uncertainty contributions for peak reflectance and peak wavelength differentiated by color for the regions of angle of incidence as given in figure 1.


4. CONCLUSION PTB operates a high accuracy reflectometer with the capability to characterize samples with a linear dimension of up to 1000 mm, a span in height of 150 mm from the highest to the lowest measurement position and a weight of 50 kg. For the measurement of EUV lithography tool optics, increasing requirements for measurement accuracy were met. Especially the alignment of the AOI for oblique incidence was improved. We are able to achieve a total uncertainty of 0.10 % for peak reflectance with a reproducibility of 0.05 % and an uncertainty in the center wavelength of 2 pm for 'near normal incidence' on flat samples. With the improved model of our goniometer providing improved angular and positional accuracy, we have achieved a total uncertainty of 0.31 % for peak reflectance and an uncertainty in the center wavelength of 2.6 pm for 'oblique incidence'. Measurements at PTB were used to validate the multilayer coating development at SMT AG16, Fraunhofer IOF Jena17,18 and FOM Rijnhuizen16,19 with regard to the increasingly tight specifications for HVM system components. This investigation also shows ways to further reduce PTB's uncertainties in the characterization of EUV lithography tool optics to keep up with the future needs of the industry.

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