Invited Paper

Review of photoresist based lens evaluation methods

Joseph P. Kirk IBM Microelectronics, Hopewell Jet., NY 12533

ABSTRACT

Optical lithography is moving into an era of refinement where lenses are continuously improving but must be superb to utilize the resolution enabled by new complex retitles. Phase Measuring Interferometry used during lens fabrication is not available to the lithographer who must rely on ingenuity in reticle design to form measurable, aberration sensitive images in photoresist. This ingenuity, as reported in the papers that follow, is enabling lithographers to measure lens aberrations on their tools with a precision nearly that of a PMI. The principles behind these photoresist methods are reviewed in this paper to help enable critical review of those that follow.

Keywords: Aberrations, lens evaluation, photoresist lens testing

1. INTRODUCTION

Perusal of session titles in Conference 4000, opticaZMicroZ~thogr~~~~y1L?II strongly suggests that lens quality is moving into an era of refinement. Session titles such as: “RET Integration with Assist/OPC”, “Strong PSM Implementation “, “Optical Proximity Correction”, and “Attenuated PSM” suggest applications only possible with availability of low aberration lenses . This low kl value lithography requires lenses with-total RMS wavefront error less than 0.025h and lithographers want to know that their lenses are of this quality at time of use. This concern is growing as evidenced by the increase in papers from last year’s 5 to this year’s 16 dedicated to insitu measurement of lens qua.lit$]. With this information and a good simulator, the lithographer is able to predict a process window for any combination of devices required for production on the installed tools.

Lens quality is simply indicated by how well the lens converts a spherical wavefront emerging from the reticle into a spherical wavefront converging to the wafer surface, Figure 1. A plot of a typical set of deviations from spheric@ of a good 1999 era microlithography lens is shown in Figure 2, and is conveniently expressed as a Zernike polynomU2J

w~,8)=aw@,e)=o+al2pc0se+a22psine+a3~(2p2-i)+a4~~2sin2e+as~ COSTS+ PI a&(3p3 -2)sinB+a&(3p 3-2)eosB+asJirp3sin36+~&p3cos38 + . . . . . .

where: p=Jsq @=tan-‘$ are pupil coordinates. It is the coefficients of this polynomial that guide the lens fabricator in building the lens. The lithographer needs to know if the lens is in the same state as when fabricated and if not, what are the current coefficients of the Zemike polynomial so performance is correctly predicted.

The gold standard for specification of lens quality is the Phase Measuring Interferometer[3] used by the lens fabricator during the manufacture of the lens; but this is not available once the lens is installed in the lithography tool. The lithographer is restricted to using photoresist images to confirm that lens performance is the same as when last measured by PMI. It seldom is!

The purpose of this paper is to categorize photoresist methods of lens characterization and point out what to look for in the papers that follow.

Proc. of SPIE Vol. 4000, Optical Microlithography XIII, ed. C. Progler (Mar, 2000) Copyright SPIE

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2. CATEGORIES OF PHOTORESIST BASED EVALUATIONS

There are two general categories of photoresist based evaluations: 1) Meets Specifications, 2) Aberration Estimation. At one extreme, the lithographer’s only interest is lens performance on specific features and aberrations are of no interest. At the other extreme, aberrations are known exactly and the lens performance is accurately predicted for all device choices. Lithographers lives would be simplified if photoresist techniques could record aberrations exactly and we see in this conference that they are pushing toward this goal.

2.1. Meets specification --- Who cares about aberrations?

This usually takes the form of measuring L/W vs Exposure and Dose , measuring the difference in isolated vs. grouped line widths, measuring across chip line width variation, etc. While this testing is an essential part of any tool acceptance, it is impossible to test all combinations of partial coherence, NA, photoresists, image enhancements, etc. Furthermore, the lens fabricator in the interest of meeting specification, tries to tune the lens, possibly at the sacrifice of features not included in the specification. The litho-engineer who certified a lens meeting specifications that then produced the contact hole pattern[41 shown in Figure 3. This is the person who cares about aberrations.

2.2. Aberration estimation

Insitu testing of lenses using photoresist does not directly measure the wavefront, but rather attempts to infer its shape from measurement of photoresist images. This measurement leads to an estimation that a particular set of aberrations caused the observed image. The degree to which this estimate approaches a correct apprasial of the aberrations depends on what portions of the wavefront are used to form the measured image. This is where the lithographers’ ingenuity is demonstrated in design of aberration sensitive patterns and an analysis that separates contributions of particular aberrations. Mask features such a phase edge@], phase dot@, line end shortening[q are applied and more are reported in this conference.

2.2.1. Distributed pupil sampling

Device patterns typically are formed using a large partial coherence and they diffract a small portion of the light over a large portion of the pupil. A 5 bar pattern, Figure 4, forms an image using the pupil in a way making it difficult to identify the aberration; in this case pure coma.

A phase dot is a particularly interesting test feature in its way of distributing light in the pupil. Its size and phase is adjusted so that the quantity of undiffracted light and light difRacted into an annular ring are approximately equal. The image of this structure, Figure 5, is much more sensitive to the same aberration as the 5 bar pattern of Figure 4. Since light is uniformly distributed as an annulus in the pupil, the azimuthal structure in the image must be that imposed by the aberrations and appropriate analysis can estimate the components.

2.2.2 Resonant pupil sampling:

Sensitivity of the test image is enhanced by designing the test feature to resonate with particular aberrations. Examples are shown in Figures 6,7, & 8. . The images of these features are measured and input aberrations to a simulator are varied until it matches the observation. The accuracy of aberration estimation is increased by matching a range of different conditions and limiting the possible choices by considering what is likely for the lens type under consideration. The match to simulation, even then, may not find a unique set of aberrations that correctly appraise the lens. On the other hand, this technique is very effective in pointing out problems in a lens and if there have been any changes.

It should be noted that these resonate structures are becoming more common with the introduction of image enhancement technology. The Levenson phase shift for equal line/space, Figure 6, will be particularly sensitive to variations in astigmatism and 4 foil. This is also true for the closely spaced attenuated phase contacts of Figure 7.

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It is easy to see why the contact pattern of Figure 3 was affected by 3 foil aberration because it illuminated the pupil as shown in Figure 7. The lithography engineer would do well to include such patterns as part of a lens performance specification.

2.2.3. Discrete pupil sampling

Discrete areas of the pupil are sampled by measuring the images formed by each of those areas. The PMI, while not a photoresist technique, directly measures the phase point by point over the pupil.

2.2.3.1. Focal variation’81

A blazed reticle illuminates the pupil in two regions and forms a two beam interference image, Figure 9. The zero order always passes through the lens center and the first order samples different regions of the wavefront as the grating period and orientation are varied. Focal positions giving greatest photoresist modulation as a function of pupil position are recorded, this performance is matched by simulation and the lens aberrations are estimated to be those used to make the match.

2.2.3.2 NJ@, @)I& aW@, 0)/i@ , Slope of the wavefront[gl

Individual images are formed, each using a small region of the pupil and consequently being displaced proportional to the average slope of the wavefront in that local region. This is achieved by placing a limiting aperture between the reticle and the lens, Figure 10, that is large enough to allow light to form an image of a relatively large feature and small enough to limit the image formation to a small region of the pupil. The resulting image displacements, Figure 11, are proportional to the average slope of the wavefront in the region used to form the image. The samples of the wavefront slope are used to solve for the Zernike coefficients.

dW@,8)/&=~~2(ysinO+xcos8)+p +a22(xsin&-yc0sO)sp +a&iiLx

+tkJCi(~sin2t?-yc0s2t?) . . . . . .

aW~,0)ldy=a120,c0sO-.~sin8)~p +a220,sini?+xcos8)+p +aJiFy

+ a4 JZ (X cos 28 -y sin 28) . . . . . .

Care must be taken[*“] because the first derivative of equation [l] does not form an orthogonal set over the circular wavefront. This method of lens quality measurement is reported in papers that follow.

2.2.3.3. W@, 9, Phase of the wavefront

Phase Measuring Interferometery is not a photoresist technique, but is the gold standard in aberration measurement and is usually implemented on a test stand using photoelectric detection on a CCD array of the interference between a reference wavefront and the wavefront from the lens under test. A 128x 128 CCD array can give approximately 11,000 phase measurements over the wavefront, Figure 12. These phase deviations can be used directly in a simulator but usual practiceI” is to fit a Zemike polynomial, equation [ 11, to the wavefront and then use the polynomial as input to a simulator. This high density of PM1 phase measurements over the wavefront enable the most accurate estimation of lens performance currently available.

3. SUMMARY: WHAT TO LOOK FOR IN THE PAPERS THAT FOLLOW

A limitation with current photoresist testing of lens quality is that the rapid phase variations are averaged over areas of the wavefront used to form the measured images. Even with this limitation, the high sensitivity and robustness of many of these tests enable the lithographer to identify problems, track and confirm the condition of a

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lens that is in use on a tool. In the papers that follow look for: 1) How the wavefront is sampled, 2) How accurately the estimates match the know PMI, 3) How sensitive is an evaluation to process variations.

Perhaps someday photoresist lens evaluations will be equivalent to that of a PM1 but that day requires continued technical innovation of the types reported in this conference.

ACKNOWLEDGMENTS

Thanks to Chris Progler and Alfred Wong for discussions that clarified the authors understanding of the use of aberrations in lithography simulation. Discussions with Nigel Farr of Cymer Corp. and Kafai Lai of IBM helped in understanding the application of first derivative wavefront measurements. The author appreciates the patience and help of Tim Brunner and of those whose lenses were measured.

REFERENCES

[l] SPIE Conference 4000, Optical Microlithography XIII, papers [4000 - 02,03,04, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 27, 291 [2]V.N. Mahajan, “Zernike Circle Polynomials and Optical Aberrations of Systems with Circular Pupils”, Engineering & Laboratory Notes, Vol. 17 No.3 August 1994, Optical Society of America.: V. N. Mahajan, Aberration Theory Made Sitnple, SPIE Optical Engineering Press, 199 1 [ 3]D. Malacara, ed., Optical Shop Testing, Wiley, New York (1977) [4]J. Preuninger, S. Bukofsky, G. Kunkel, “High Order Lens Aberration Monitor” MNE 199, Rome

[ 51 T. Brunner, T.A. Martin, A.L. Martino, R.M. Ausschnitt, C.P. Newman, T.H. Hibbs, M.S. “Quantitative stepper metrology using the focus monitor test mask’, SPIE Vol.2197 1994 P54 1-9: Nakatani, M. Kojima, Y. Nakano, H. Kamon, K. Sato, K. Takano, H. Ishihara, “Effect of lens aberration on resist pattern profiles in edge line phase shift method “ 0. Jpn. J. Appl. Phys. 1, Regul. Pap. Short Notes (Japan) Vo1.34, No.9A Sept. 1995 P5043-8 [6]~. Dirksen, et al, “Novel aberration monitor for optical lithography”, SPIE, March 1999, Vol. 3679, p77 [7] Ausschnitt, C.P. Lagus, M.E., “Seeing the forest for the trees: a new approach to CD control” Proc. SPIE - Int. Sot. Opt. Eng. (USA) Vol.3332 1998 P212-20 [ 8) J.P. Kirk, C. J. Progler, “Application of blazed gratings for determination of equivalent primary aberrations” SPIE, March 1999, Vol. 3697, p70 [9]K. Rebitz, A. Smith, Characterizing exposure tool optics in the fab”, Microlithography World, Summer 1999: Patent # US5828455 “Apparatus, method of measurement, and method of data analysis for correction of optical system.” [ Io]Fisher, Stall, “Mechanism for surface fitting interferometric slope data”, SPIE Vol. 2003 Interferometry VI (1993)/23 1: W. H. Southwell, “Wave-Front estimation from Wave-Front Slope Measurements”, J.Opt.Soc.Am., vol.70,No. 8, Aug. 1980, pp. 998-1006: E. Wallner, “Optimal Wave-Front Correction using Slope Measurements”, J.Qpt.Soc.Am., vol. 73, No. 12, Dec. 1983, pp 1771-1775 [ 11IC.J. Progler, “Zernike coefIicients: are they really enough”, [4OOO-051, SPIE Optical Microlithography XIII, March 1,200O

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tens :a, >:,:

r&de :I:;>* .:.:.:.I . . . . . .;.:.:.;.

/ .~~ ~~~, vcrilfer

w ~ toimage

:Ijl(d

w!Qvefrotlt 2;;: .I... :.:.:.: >:.: .a.*. ::

Figure 1. Spherical wavetiont emerging for object Figure 2, Typical wavefront deviation of a good 1999 point, passing through lens and emerging as a spherical wavefront distorted by the lens aberrations.

era microlithugraphy lens. The plot is a 37 term Zemikc polynomial that was fitted to PM1 measurements.

Figure 3. Equal size contact holes, distorted by the presence of 3-foil aberration in the lens.

Figure 4. 5 bar pattern imaged by lens with a small amount of coma. a) pupil illumination with 0.5 partial coherence, b) Intensity distribution in image

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Figure 5. Phase dot imaged with the same amount of coma as Figure 4. a) The annular pupil illumination achieved by adjustment of size and phase of the dot. b) Intensity distribution in the image showing the exaggerated response to coma. Note: the dot size is near the resolution limit and must be measured by an SEM.

Figure 7. Attenuated phase shift contacts on square grid. a) 4 pole diffraction illuminating the pupil. b) the image will resonate with 4 foil aberration

Figure 6. Equal line/space Levenson phase shift. a) The pupil is symmetrically illuminated by two diffraction orders. b) An astigmatic wavefront will resonate with this pattern.

Figure 8. Three fold symmetric object. a) Pupil illumination. b) The image will resonate with 3 foil aberration ---- as the pattern of Figure 3.

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reticle with

Figure 9. Blazed grating that diffracts Figure 10. Pin hole limits image formation of only a +1 order (no - 1 order) into the pupil each object point to a discrete portion of the lens.

Figure 11. Two discrete regions of the lens form images in the presence of coma. Images using diametrically opposite pupil areas are displaced in the same direction. a) The two image forming regions of the lens. b) Distortion map of image displacements proportional to the slope of the wavetiont.

Figure 12. Example set of PM1 data that was fitted with a 37 term Zemike polynomial to give Figure 2.

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