Microelectronic Engineering 88 (2011) 1986–1991

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Microelectronic Engineering

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Electrostatic clamping with an EUVL mask chuck: Particle issues

Gerhard Kalkowski a,⇑, Jacob R. Zeuske b, Stefan Risse a, Sandra Müller a, Thomas Peschel a,Mathias Rohde a

a Fraunhofer Institut für Angewandte Optik und Feinmechanik, 07745 Jena, Germanyb Computational Mechanics Center, University of Wisconsin, Madison, WI 53706, USA

a r t i c l e i n f o a b s t r a c t

Article history:Available online 17 February 2011

Keywords:Electrostatic chuckPin chuckParticlesEUVLMaske-Beam

0167-9317/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.mee.2011.01.063

⇑ Corresponding author.E-mail address: gerhard.kalkowski@iof.fraunhofer.

Particle and defect issues related to electrostatic chucking with an ultra-planar, pin-structured maskchuck for EUVL application were addressed. By mapping particles/defects on the backside of 8 inchSi-wafers before and after chucking, particle transport from the chuck to the wafer was studied atapplication relevant electrostatic forces. Particles were detected by analysis of stray light intensitieson the wafer side. Investigations were performed under ambient conditions at high chucking voltageson a bipolar chuck. Particle transport from the chuck surface to the wafer surface was mapped andfound to concentrate at the pin sites. Successively lower particle counts with increasing number ofchucked wafers were observed, indicating a ‘‘cleaning effect’’ on the chuck’s surface induced by theelectrostatic chucking procedure. No influence of electric field direction (polarity) on particle countwas discernable.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

For lithography at the 22 nm node and below, extreme ultra-violet lithography (EUVL) at a wavelength of approximately13 nm is considered for mass production. This technology requiresa high vacuum environment and clamping of the mask with anelectrostatic chuck in close analogy to e-beam lithography. Thechuck design is based solely on ultra low thermal expansion mate-rials (ULE). Challenging flatness conditions are imposed on thechuck surface to minimize in-plane distortions from out-of-planedeviations under non-telecentric illumination conditions [1]. Anupdated EUVL Mask Chucking Standard (SEMI 4584) is planned,which would tighten chuck flatness within the central quality areaof 142 � 142 mm2, from about 50 nm peak-to-valley (PV) cur-rently, to 32 nm in the future [2]. Particles might spoil flatness ran-domly and irreproducibly and might deteriorate exposureperformance. Currently, particles of size >1 lm are considered crit-ical for the mask backside [2], but impact of smaller particles is un-clear and the full specification is still under debate. So anygeneration of particles is crucial at the lower lithography nodes,even if confined to the mask backside (which is still arguable).Yet, understanding of defect and particle generation in the chuck-ing process is poor, since inspection results are scarce and onlylimited information is available about the surfaces of the chucksinvolved [3,4].

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de (G. Kalkowski).

In this paper, we report on investigations of defect generationand particle transport related to the chucking of Si wafers on anEUVL mask chuck prototype, which has been detailed in severalprevious papers [5–7]. Si wafers instead of mask blanks were cho-sen for their ease of availability in well established surface quality,as well as excellent optical visibility of surface defects on thesesubstrates. Due to the high index of refraction of Si, the limit of vis-ibility of small particles is improved as compared to mask surfaces.Although Young’s modulus is lower for Si (�140 GPa) than for thetypical backside thin Cr metallisation film (�250 GPa [8]) of a typ-ical ULE mask blank (�70 GPa), the results should be indicative ofbasic defect and transport mechanisms.

2. Experimental

A pin-structured, bipolar EUVL mask chuck prototype was usedto clamp Si-wafers at voltages of up to ±3 kV. The chuck design iscircular with a diameter of �200 mm and height of �25 mm. Asquare array of circular pins – each �2.4 mm in diameter and<3 lm high – covers the quality area at a pitch of about 10 mm[5,6]. The actual flatness of the pin-surface (with the surface nor-mal oriented horizontally) was evaluated to 47 nm PV with a ZYGOinterferometer of the Fizeau type, using a reference flat of k/30accuracy @k = 632 nm. Surface roughness of the pins amounts to�1 nm Root-Mean-Square (RMS) typically, as measured with aZYGO New View white–light interferometer. Double side polishedh1 0 0i Si-wafers of 200 mm diameter and 720 lm thickness wereused for the chucking experiments, with the wafers selected for

Fig. 2. Wafer bending at ±3 kV after P5 min.

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Warp <10 lm and Total Thickness Variation (TTV) <2 lm. Initialchuck cleaning was performed by carefully wiping the surface witha clean-room tissue using an organic solvent.

All wafer handling and chucking was performed in a clean-roomclass 10 environment in intimate neighbourhood of the particlemeasurement tool, a KLA-Tencor particle detector of type CandelaCS10. The samples are rotated in this device and scanned with ablue laser to measure stray light intensity variations from defectson the surface. Particles (bumps) and defects (pits) are detectedas scattering intensity increases and decreases above background,respectively. This is true in general but not always, since a lightabsorbing particle will also decrease intensity, making our ‘‘parti-cle’’ assignment prone to fault to some extent. However, from sev-eral additional measurements after wet cleaning (not shown here)the mostly ‘‘particle nature’’ of the defects was confirmed. Particlesize detectability amounted to about 80 nm Poly-Styrene Latex(PSL) equivalent size (on polished Si substrates) from available cal-ibration data on various particle sizes ranging up to 2 lm PSL.

Wafers were manually picked from a 25 unit storage box with avacuum gripper, measured for their initial particle count, depositedonto the mask chuck and finally clamped at symmetric +/� volt-ages for a given period of time (typically 5 min) at ambient atmo-sphere. So the wafer was kept at �0 potential all the time,experiencing opposite electric field polarities on each half of itssurface when chucking. (Compare the ‘‘double D’’ bipolar electrodepattern [5,6]) Electrode polarities were alternated with eachchucking cycle, to avoid long term polarization effects. Generally,the next wafer was already scanned for its initial particle countduring the ongoing chucking process. So, immediately after liftingoff the previously chucked wafer for particle inspection, the nextone could be placed onto the chuck to avoid uncontrolled particledeposition from atmosphere.

2.1. Chucking force measurements

Since our chucking experiments were performed at ambientatmosphere, discharge or humidity influences might be suspectedto decrease forces [9,10]. To verify the chucking forces, the localbending deformations of an 8-inch Si-wafer (720 lm thick) acrossthe pin pattern was measured with our Fizeau interferometer. Inthese measurements, Si h1 1 0i was pointing along the diagonal

Fig. 1. Wafer bending at ±3 kV after 630 s.

of the pin pattern. Figs. 1 and 2 illustrate the induced deformations(in the central wafer area) for a chucking voltage of ±3 kV afterabout 30 s and 5 min, respectively. Clearly, no decline of forces isdetectable over a period of 5 min.

The height profiles along the pin diagonals through the center ofFig. 2 were extracted and are shown in Fig. 3. The green line corre-sponds to the diagonal from lower left to upper right (throughpin#3 and pin#1), the blue line to the diagonal from upper leftto lower right (through pin#2 and pin#4). By Finite Element mod-elling, expected wafer bending between adjacent pins was calcu-lated for chucking voltages of 3, 2.5, and 2 kV, corresponding toabout 20 (18), 14 (13) and 9 (8) kPa electrostatic pressure, respec-tively, on (off) the chuck pins. Fig. 4 shows the resulting height pro-files for going from the center of a pin half distance to the next onealong the pin pattern diagonal. Calculated height profiles for 3 kVchucking voltage predict a peak to valley deformation of about200 nm. As can be inferred from Fig. 3, measured and calculatedamplitudes match quite well, indicating that full chucking forceis reached (and maintained) under atmospheric conditions.

2.2. Particle measurements

Although the chuck was essentially unused (i.e. had only expe-rienced a few high voltage characterization tests) and thoroughlycleaned with inorganics as described above, the initial cleanlinessof the chuck was poor, as seen after 1st chucking. Particle mapsover an inspection diameter of 170 mm for a Si-wafer before andafter 1st chucking, respectively, are displayed in Figs. 5 and 6. De-tected particle sizes are classified into six groups, ranging from>80 nm (blue) to >2 lm (green). Note the colour scale for particlesizes defined in Fig. 5, which is the same throughout the rest ofthe paper (Figs. 6–12). The outer circumference of the wafer ismasked in these figures, to discriminate against particle countsfrom the chuck’s surface outside of the pin-structured mask area,which is irrelevant for EUVL application. Unfortunately, a manualtouch of the back-side by mistake partly affects the initial particlecount of wafer#1. Yet, the large increase of particle count uponchucking and the concentration of these particles around the pinsites are evident.

Similar particle maps after performing a sequence of ninechucking cycles are illustrated in Figs. 7 and 8.

The initial (random) particle count on the wafer still increaseswith chucking, but now shows only about 1000 ‘‘adders’’, which

Fig. 3. Height profiles across diagonals in Fig. 2.

Fig. 4. FE modelling results for wafer bending.

1988 G. Kalkowski et al. / Microelectronic Engineering 88 (2011) 1986–1991

is two orders of magnitude less than at the beginning. The pin pat-tern is still visible, i.e. the particle transfer takes place at the pin

Fig. 5. Wafer#1 before chucking (1

sites predominantly. Indentation of particles into the wafer at thesites of mechanical contact might be expected. However, the differ-ence in Young’s modulus between the chuck’s surface material(ULE �70 GPa) and the wafer (Si �140 GPa) would rather suggestthe impression of particles into the chuck surface. Investigationsinto the chuck’s surface are planned for the near future.

The decrease in particle count with increasing number of chuck-ing cycles has been traced in detail on four pin sites (located on thecorners of a square parallel to the quality area) at a distance of3 � 10

ffiffiffi

2p

mm from the center. The cumulative particle count inthese four areas after chucking (±3 kV for 5 min) is displayed inFig. 9. The numbers on the scale for chucking sequence indicatethe wafer numbers in Figs. 5–12. Note the dominance of particlesizes in the class 100–270 nm PSL.

Detailed particle maps from one of these pins (Pin#2) afterchucking are given in Figs. 10 and 11. Note, that the initial particlemaps (before chucking) showed zero particle count in this area forall four wafers. After chucking, the initial wafer is covered by manyparticles across the pin area (Fig. 10). As the sequence proceeds,

5 300 particles per 227 cm2).

Fig. 6. Wafer#1 chucked (5 min ± 3 kV) (270 000 particles per 227 cm2).

Fig. 7. Wafer#9 before chucking (76 particles per 299 cm2).

Fig. 8. Wafer#9 chucked (5 min ± 3 kV) (1084 particles per 227 cm2).

Fig. 9. Cumulated particles on four pins.

G. Kalkowski et al. / Microelectronic Engineering 88 (2011) 1986–1991 1989

particle counts decrease, with still an area coverage apparent(Fig.11), indicating that not just the rims of the pins (where com-pression stress is highest [7]) are printing. Apparently, also someparticles outside of the pin area are attached to the wafer afterchucking (top left in Fig.10, bottom right in Fig.11). Whether thesewere lifted from the chuck surface to the wafer during the chuck-ing process, or moved later during handling, is not clear at present.

Still rather unclear – yet noticeable – is a local change in dielec-tric properties of the wafer surface at the pin locations after chuck-ing. This effect is detectable from polarization changes when theincident light is (circularly) polarized and is known in thin filmcharacterization as Ellipsometry technique. Fig. 12 displays the

corresponding signal variation (QAbsPhase) in the area aroundpin#4 (opposite pin#2) of wafer#2 after chucking. Exactly the fullpin surface is imaged as a dark circle on a light background. Appar-ently, changes in the surface properties (presumably the surfaceoxide) of the Si-wafer occur during chucking. For comparison, thescattering signal (SSc) used for particle mapping is also given.The changes in the surface properties cannot be seen in the SScsignal.

3. Conclusions

We have investigated particle contamination of Si-wafers froman EUVL mask chuck under application relevant conditions of elec-trostatic chucking. Particle transfer from the chuck surface to thewafer surface during the chucking process is apparent. The grossmultitude of particles is observed at the pin locations, where thechuck contacts mechanically to the wafers. Yet, some particles also

Fig. 12. Scattering and QAbsPhase signals after chucking.

Fig. 10. Particles near Pin#2 at beginning of sequence.

Fig. 11. Particles near Pin#2 at end of sequence.

1990 G. Kalkowski et al. / Microelectronic Engineering 88 (2011) 1986–1991

appear at intermediate locations. Successively lower particlecounts with increasing number of chucking processes indicate a‘‘cleaning effect’’ obtained with chucking. The effect is clearly visi-

ble from a series of nine successive chuckings, but statistics isinsufficient to estimate the limit value after a large number ofchucking processes.

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Acknowledgement

We greatly acknowledge assistance in sample characterizationand data reduction by R. Bauer of Fraunhofer IOF, as well as discus-sion and encouragement in this work by R. L. Engelstad of Univer-sity of Wisconsin (USA) and M. Sogard of Nikon (USA).

References

[1] SEMI P40-1103 Specification for EUV Mask Substrate Chucking,www.sematech.org.

[2] SEMI 4584, Draft Document, Specification for EUV Mask Substrate Chucking,priv. communication, J. Sohn, SEMATECH (2009).

[3] M. Amemiya, K. Ota, T. Taguchi, O. Suga, Experimental Study of Particle-freeMask Handling Techniques Using the MPE Tool, 2008 Internat. Sympos. onEUVL, Sept. 28–Oct. 1, Lake Tahoe, CA (USA), 2008.

[4] A.V. Hayes, R. Randive, I. Reiss, J. Menendez, P. Kearney, T. Sugiyama,Evaluation of backside particle contamination and electrostatic chuck designon the cleanliness of EUV reticle mask blanks in a Mo/Si ion beam depositionsystem, Proceedings of SPIE, 7122 (2008) 71223O.

[5] G. Kalkowski, S. Risse, S. Müller, G. Harnisch, Microelectron. Eng. 83 (2006)714–717.

[6] G. Kalkowski, T. Peschel, S. Risse, S. Müller, R.L. Engelstad, J.R. Zeuske, P.Vukkadala, Microelectron. Eng. 87 (2010) 1287–1289.

[7] G. Kalkowski, C. Semmler, S. Risse, T. Peschel, C. Damm, S. Müller and R. Bauer,Electrostatic chucking of EUVL masks: coefficients of friction, Proceedings ofSPIE 7636 (2010) 76362J.

[8] M. Sogard, V. Ramaswamy, A.R. Mikkelson, R.L. Engelstad, J. Micro/Nanolithography, MEMS & MOEMS 8 (2009) 041506.

[9] G. Kalkowski, S. Risse, V. Guyenot, Microelectron. Eng. 61–62 (2002) 357–361.[10] J. VanElp, T.M. Giesen, A.M. DeGroof, Microelectron. Eng. 73–74 (2004) 941–

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