Aerial image based lens metrology for wafer steppers

Transcription

1 Aerial image based lens metrology for wafer steppers P. Dirksen*, J.J.M. Braat**, A.J.E.M. Janssen*, T. Matsuyama***, T. Noda*** *Philips Research Europe, Belgium **Delft University of Technology, The Netherlands ***Nikon Corporation, Japan

2 Introduction: lens metrology Tool of choice: Phase Measurement Interferometer (PMI), is the benchmark PMI measures the wave front aberrations of the lens It achieves a very high accuracy and reproducibility of 1 mλ

3 Introduction: lens metrology But, PMI requires an at-wavelength coherent source, with a coherence length of a few meters PMI demands the insertion of special optical components such as beam splitters and mirrors This presentation discusses an alternative method, based on an aerial image measurements

4 Benefits aerial image measurements May use incoherent source Accounts for chromatic errors Retrieves not only phase but also the pupil transmission errors Achieves a good match to PMI and high reproducibility of 1 mλ Advantage for certain lenses

5 Overview Introduction Experimental set-up: PMI and aerial image Aerial image analysis by ENZ-theory Experimental results Summary

6 PMI set-up Interferogram Prototype DUV lens, NA =.75 Fizeau surface Laser with long coherence length Spherical mirror Beam splitter Object plane Image plane A prototype lens is chosen to demonstrate aberration retrieval capabilities. The lens shows significant aberrations and does not represent the state of the art.

7 Analysis interferogram Zernike coefficients of the projection lens: mλ mλ Spherical Coma Astigmatism file # 1, Spherical Coma Thre-foil Four-foil Astigmatism Three-foil Four-foil Five-foil Five-foil (Subtract contribution spherical mirror)

8 Aerial image measurement set-up Stepper illuminator Spherical wave Pinhole Projection lens Aberrated image Objective lens CCD-camera Point-spread function (PSF)

9 Aerial image analysis by ENZ theory Zernikes of the lens: mλ Spherical Coma Astigmatism file # 1, Spherical Coma Astigmatism mλ 15 5 Three-foil Four-foil Five-foil Thre-foil Four-foil Five-foil (Subtract contribution objective lens, see conference paper)

10 Overview Introduction Experimental set-up: PMI and aerial image Aerial image analysis by ENZ-theory Experimental results Summary

11 Extended Nijboer-Zernike theory Arbitrary defocus, general retrieval ENZ is born 1942 Nijboer-Zernike THE DIFFRACTION THEORY OF ABERRATIONS 2 4 High-NA, Full vectorial ENZ for lens metrology 6 Chromatic errors and vibrations PROEFSCHRIFT TER VERKRIJGING VAN DEN GRAAD VAN D OCTOR IN DE W IS- EN NATU URKUND E AAN DE RIJKS-UNIVERSITEIT TE GRONINGEN, OP GEZAG VAN DEN RECTOR MAGNIFICUS Dr. J. M. N. KAPTEYN, HOIOGLEERAAR IN DE FACULTEIT DER LETTEREN EN W IJSBEGEERT E, T EGEN D E BED EN KIN GEN VA N D E FACULTEIT D ER W IS- EN NATU URKUND E TE VERDEDIGEN OP MAANDAG 1 JUNI 1942, DES NAMIDDAGS OM 4.15 UUR PRECIES DO OR BERNARD ROELOF ANDRIES NIJBOER GEBOREN TE MEPPEL I (r, f ) = V (Cc λ) 2 V V + Re(V4V *)

12 Aerial image analysis by ENZ theory Focus F = -.9 [um] F = -.6 [um] F = -.3 [um] F = [um] F =.3 [um] F =.6 [um] F =.9 [um] The extended Nijboer-Zernike (ENZ) theory gives an analytical expression for the through-focus complex PSF: = U ( r, f ) = 2 β V Cos( mθ ), V nm ( r, f ) nm exp( if ) nm l= 1 nm ( 2if ) J r p l m+ l+ 2 j ( ) 1 vlj l j= lr Observed intensity PSF = β nm Basic functions ( V nm )

13 Retrieve transmission and phase F = -.9 [um] F = -.6 [um] F = -.3 [um] F = [um] F =.3 [um] F =.6 [um] F =.9 [um] Focus Match Retrieved Retrieved Retrieved Retrieved Retrieved Retrieved Retrieved Experiment Theory β nm : Transmission + Phase

14 .8 Impact transmission error CD [nm] nm Cr-line, λ=193, NA= nm +3 nm -3 nm Horizontal Vertical Simulated CD for a % transmission error. HV-line width difference Unlike astigmatism this HV-difference is also present in best focus Pitch [nm] 49

15 Overview Introduction Experimental set-up: PMI and aerial image Aerial image analysis by ENZ-theory Experimental results Summary

16 PMI to Aerial Image Measurement Compare the phase errors Φ(ρ,θ)

17 PMI to Aerial Image Measurement mλ Spherical PMI ENZ Three-foil Coma Four-foil Astigmatism Five-foil mλ

18 Aerial image [mλ] PMI vs Aerial image 5-5 Spherical Other PMI = Aerial mλ mλ Correlation (1σ): All Zernikes: 5 mλ Excl. 2 spherical: 3 mλ Offset: Spherical Three-foil 15 5 PMI ENZ Coma Four-foil Astigmatism Five-foil <1 mλ PMI [mλ]

19 Reticle height detuning experiment Spherical aberration terms are relative difficult to measure A change in conjugates, i.e. a non ideal wafer-reticle height position, induces spherical. Reticle height detuning experiment to induce spherical in a controlled manner to check linearity and offset of the aerial image measurement method.

20 Reticle height detuning experiment A change in conjugates, i.e. a shift in reticle and wafer position, induces low and high order spherical aberration. z Reticle For an ideal stepper lens: Z Z 9 16 = = c c 9 16 z z c9 c 16 Wafer

21 Reticle height detuning experiment Spherical [mλ] - - Z 9 Z 16 Nominal position Z 9 and Z 16 linear in reticle position Proves the capability to retrieve large aberrations Crossing close to nominal Offsets < 5 mλ Reticle position [µm]

22 Spherical [mλ] Repeatability For all Zernikes the repeatability was within 1 mλ Measurement # Z 9 Z 16 Example: spherical [mλ] Z 9 : 1σ =.94 Z 16 : 1σ =.73 Z 25 : 1σ =.85 Z 36 : 1σ =.68

23 Summary We have discussed an aerial image based lens metrology method. The method uses the ENZ theory for analysis and provides a: Good match to PMI High reproducibility of 1 mλ Additional benefits: Accounts for chromatic errors Determines transmission errors

24 Acknowledgement The authors wish to thank Hiroshi Ooki from Nikon Corporation, Japan and Martin McCallum from Nikon Precision, Europe as well as Yuri Aksenov from Philips Research Europe, Belgium for their helpful discussions. More info: