Fall 2001 Yield Management Solutions32

Lithography

IntroductionIt is well known that 193 nm resist featureschange size permanently during CD-SEMmeasurements.1-5 The size of the shrinkage,often up to 40 nm, should be compared tothe CD metrology budget of 1 nm for fea-tures in the 100 nm design rule node, when193 nm lithography is expected to enterproduction for critical layers.1 The severalclasses of 193 nm resist chemistry (COMA,acrylate, cyclo-olefins, VEMA) and layerschemes (single, thin imaging layer andhybrid) all exhibit shrinkage to varyingdegrees depending on their formulations,process history and measurement conditions.Shrinkage is observed to progress in a non-linear way with applied e-beam dose andunderstanding the mechanisms that con-tribute to this shrinkage is complex. Severalstudies2, 3 have been reported the attempt toimprove this understanding as a basis toimprove the resist materials. As yet, completeelimination of e-beam-initiated shrinkagehas yet to be achieved. This effect has larg e l ybeen overcome in 248 nm resist metrology,but we may expect similar or worse effectsin some 157 nm materials. It is, therefore,important to understand and minimize resistshrinkage in order to be able to meet thechallenges for production worthy CD-SEMmetrology of advanced materials. This

paper discusses investigations of shrinkage effects car-ried out in joint work between IMEC and KLA-Tencorin a study to develop recommendations for CD-SEMconditions that can minimize shrinkage.

ExperimentsThis study investigates a 193 nm resist exhibitingabove-average e-beam shrinkage. Wafers were uniformlyexposed several days prior to CD-SEM measurementsusing an ASML 5500/900 argon fluoride scanner atIMEC. Tr e n c h e s with a nominal CD of 150 nm weremeasured using fiv e to ten fresh sites for each experi-ment. It should be noted that resist shrinkage cause thereported trench CD measurement values to increase. Afirst set of CD-SEM measurements were carried out atK L A - Te n c o r, San Jose on an 8200-R CD-SEM andthese were repeated and extended in IMEC on 8100-XP and 8100-ER systems.

It has already been widely reported that e-beam expo-sure of the measurement position must be minimized.Therefore, a standard 193 nm resist measurementrecipe was created with a low-magnification patternrecognition step (magnification of 6.25kX, 24 µm fieldof view (FOV)) to identify the region to be measured.The e-beam spot size was then automatically focused ina region away from the measurement site. A higher-magnification pattern recognition step (magnification12.5kX, 12 µm FOV) was then used to identify theexact area of the trench to be measured.

S P E C I A L F O C U S

Investigation of 193 nm ResistShrinkage During CD-SEMMeasurements

Thomas Hoffmann, Greet Storms, Monique Ercken, Mireille Maenhoudt, Ivan Pollentier, Kurt Ronse, IMEC, BelgiumFranck Felten, Evelyn Wong, Jonathan England, KLA-Tencor, Europe

193 nm resists are known to shrink during CD-SEM measurements. The large size and non-linear behavior of this shrinkagemust be characterized and understood if CD-SEM metrology is to be correctly applied in advanced lithography processing.This paper describes a study in which recommendations for the best measurement conditions were developed and speculationson possible models for the observed shrinkage mechanisms could be made.

Fall 2001 Yield Management Solutions 33

CD-SEMs have traditionally carried out measurementsby analyzing high-magnification images of the featureto be measured. However, KLA-Tencor CD-SEMsdirectly collect the linescan (the intensity of the detectedelectrons signal as the electron beam is scanned acrossthe feature) from the measurement location using anelectron beam which is scanned at 120 Hz, four timesthe industry-standard TV rate. In this application, thistechnique has the advantages of being faster than whenhaving to acquire complete images, and, more impor-tantly, minimizes the total sample dose. For the mea-surements reported in this study, 768 linescans werecollected at 128 locations equally spaced over 720 nmof the feature. The reported CD measurements werecalculated from the average of these linescans using a50-percent derivative algorithm. Experiments were carried out to investigate the effect of beam conditionsand recipe parameters on shrinkage. Early measurementsconsidered 10 static measurements (the sample is notmoved between repeated measurement cycles), but thenumber of measurements was later extended, up to1500 in some cases, to investigate more fully the variousshrinkage mechanisms.

ObservationsFigure 1 shows the increase in trench CD over 250 staticmeasurements made using a 600 eV, 10pA beam.Three regimes of shrinkage can be identified as report-ed elsewhere.3 Each regime can be fitted by an expo-nential term, each with a characteristic half dose analo-gous to a half-life in radioactive decay. In the data ofFigure 1, there can be seen:

i) an initial fast-shrinkage, with a half-dose of ninemeasurements;

ii) an intermediate-term shrinkage with a half-dose of55 measurements;

iii) a long-term shrinkage with a half-dose of 540 measurements.

Variation with Landing EnergyShrinkage has previously been reported to change in anabsolute way, rather than as a percentage of feature size.1

This implies that the shrinkage is a surface effect, whichis easily understood due to the limited penetrationdepth of the electrons from the CD-SEM. In this study,decreasing the electron-beam energy reduced the size ofall the shrinkage mechanisms. This is demonstrated inFigure 2, which shows comparative data to Figure 1 formeasurements taken with a 400 eV beam. This depen-dency can be understood because the interaction volumeis smaller and less energy is deposited in the resist asthe energy decreases. Estimates of the range taken frompublished range tables6 show that expected electron-penetration depths are consistent with energy dependenceseen in the data. It should be noted that the lower energ ydata shows greater scattering because the smaller numberof secondary electrons emitted from the sample hasreduced the signal-to-noise ratio of the linescan signals.

S P E C I A L F O C U S

F i g u re 1a. The trench CD variation when using a 600 eV, 10pA beam.

The intermediate and slow contributions are shown below the data.

F i g u re 1b. The first 100 points of the trench CD cur ve. The fas t and

i n t e rmedia te cont ributions are shown below the data.

F i g u re 2. The equiva lent trench CD curve to Figure 1a, but measure d

using a 400 eV, 10pA beam.

Fall 2001 Yield Management Solutions34

Dose DependencyBeam Current —First experiments on ten static mea-surements indicated that beam current had little effecton the shrinkage. This surprising result has beenreported in work elsewhere.4 Figure 3 shows shrinkagemeasured for three beam currents over a larger range ofmeasurements. It must be pointed out that the inter-pretation of the data in this study is complicated bythe fact that we do not know the size of the undosedfeature being measured. The first static measurementalready includes some unknown amount of shrinkage.Fresh samples have to be used for each experiment, andthe CD control across the wafer (measured to be ±9 nm3σ) does not allow data from each experiment to becompared without having to consider an offset betweenthe collected data sets. The offsets between the sets ofdata in Figure 4 have been made so that the intermedi-ate shrinkage region for all the beam currents overlap,in agreement with the early observations that thisregime is independent of beam current. Under thisinterpretation, the fast shrinkage mechanism is observedto increase with beam current. The long-term shrinkagemechanism also changes with beam current. At 40pA, ahigher than normal beam current, the trench can be seento narrow once the other mechanisms have stabilized.An alternative analysis of the beam current data withdifferent applied offsets could lead to the conclusion thatall the shrinkage regimes depended on beam current.The precision of the data did not vary greatly until thebeam current was reduced to 5pA. This reflects thereduced signal-to-noise at this low beam current, analogous to the trend with beam energy.

Effect of Scan Overlay after Each Measurement —Anearly experiment attempted to determine if a time delayplaced between successive static measurements would

change the rate of shrinkage by altering the inducedtemperature of the resist. A variable time delay betweenstatic measurements was introduced by using an optionknown as scan overlay. In this option, an image of themeasured feature was acquired after each measurementat the magnification at which the measurement wasdefined (75kX, 2 µm FOV in this case). The measuredlinescan was then displayed over this image. Ch a n g i n gthe time could be used to delay the period between suc-cessive static measurements. No difference in the inter-mediate shrinkage was observed when this delay waschanged between one and five seconds, but it soonbecame apparent that the image acquisition itself wascausing a difference. Figure 4 shows the overlap ofshrinkage curves for the first 100 measurements withscan overlay, compared to the first 500 measurementswithout scan overlay. For clarity, only the fitted trendfor the measurements without scan overlay is shown. Inthis figure it has been assumed that the scan-overlaystep creates the same shrinkage as four measurementonly sequences. Therefore, the horizontal scale for thedata for measurement plus scan overlay has been multi-plied by a factor of five. The dose applied to the waferduring imaging is different from the dose during ameasurement by a factor of two. This implies that dosesapplied in different timescales have caused differentamounts of shrinkage.

Proposed Model for 193 nm ShrinkageUsing the above interpretation of the data, it is possibleto speculate on what processes might be occurring inthe resist during electron bombardment. Confirmationof this model will require further experiments, includingthe use of complementary techniques to those used inthis study, and it is hoped that the suggestions below

S P E C I A L F O C U S

F i g u re 3. The variation of the trench shrinkage with beam current.

F i g u re 4. The shrinkage over the fi rst 100 measurements with scan

overlay (red dots) compared to the trend (black line) of the first 500

m e a s u rements without scan over lay. The beam conditions were 600eV,

1 0 p A .

Fall 2001 Yield Management Solutions 35

might stimulate such further investigations and discussions.

The fast mechanism appears to change with energy andbeam current, suggesting that it is related to the inci-dent power. The mechanism may be a short-lived, ther-mally activated process such as the release of certainmolecules, perhaps solvent, from near the surface of theresist before the surface has stabilized. This mechanismcan be reduced after UV treatment of the surface.3

The intermediate mechanism is saturated with current,and has a lifetime of tens of microseconds. Perhaps thisis cross-linking. When an electron impacts the resist, itwill undergo many interactions with molecules as itslows down. Some of these interactions create radicalson the resist molecules. The process appears to be soefficient that, in the range of beam currents used in aCD-SEM, all possible radicals are created. The radicalsmay form cross links before they decay. After the firstfew dose events (approximately 20 in this study), thesurface is cross-linked, and so the fast mechanism issuppressed. Once all the cross-links within the e-beaminteraction zone have been made (after approximately200 measurements in this study), the intermediateshrinkage mechanism stops. Electrons in the e-beamhit the sample on the tens of nanoseconds scale.Therefore, altering the beam current changes events inthis timescale. The proposal that beam current does notchange the intermediate shrinkage mechanism suggeststhat the intermediate mechanism is saturated andlonger-lived than tens of nanoseconds. During a mea-surement, the beam returns to the same spot on thesample approximately every ten microseconds. If theradicals have not decayed in this time, the returningbeam cannot produce more radicals. During an imageacquisition, the beam returns to the same spot on thesample at a slower rate, approximately every ten mil-liseconds. If the radicals have now decayed, the return-ing beam will now be able to re-create them. Thereforethe increased shrinkage induced during imaging com-pared to measurement suggests the radicals have a life-time longer than tens of microseconds, but shorter thanten milliseconds.

The slow shrinkage mechanism also proceeds at thesame time the above two mechanisms are progressing.This mechanism may be mass loss. There are sugges-tions that this could be molecular scission or solventremoval.2, 3 The mass loss gives slower shrinkage, whichonly becomes apparent after the medium mechanismhas finished, but continues for a longer dose. When the

resist has stabilized, carry-over can also become evident.This is presumably due to the same mechanism (carbon-ization or “charging”) seen in 248 nm metrology. Atthe lower beam currents typically used for metrology,the trench continues to widen as mass loss dominatesover carry-over. At extreme beam currents, such as 40pAinvestigated in this study, excessive carry-over canactually dominate over shrinkage. Under the normalbeam current conditions in the KLA-Tencor CD-SEM,the amount of carry-over is low and hard to observe.

Best Measurement Conditions Irrespective of the explanation for the different mecha-nisms occurring in the resist, the above work can beused to make recommendations for 193 nm resist mea-surements. Lithographic performance is best character-ized by measuring feature dimensions before inducedshrinkage. In production, after-develop inspection(ADI) is used to control and predict the after-etchinspection (ACI) feature size. The etch environmentmay quickly cause the resist to shrink in a similar wayto which it shrinks in the CD-SEM. It is tempting tosuggest that, under these conditions, measuring thefully shrunken dimension at ADI might give a reason-able prediction of the ACI dimension. In a relatedtheme, suggestions have been made that resists couldbe stabilized, presumably both against e-beam-and etch-induced shrinkage, by introducing a pre-conditioningprocess such as UV irradiation, e-beam cure, or thermalprocessing.2, 4 However, measuring the un-dosed featuresize does not require the assumption of systematicprocess offsets that are well controlled under all manu-facturing conditions and does not incur an increasedprocess cost.

In determining zero-dose dimensions, it is vital to consider sources of random and systematic error in themeasurements. We can attribute random errors to vari -ations in linescans caused by the usual effects that con-tribute to static and dynamic precision in a CD-SEM.Systematic errors may be attributed to uncertainties inthe fits of successive measurements leading to the esti-mate of the CD of the undosed feature. In 248 nmresist metrology, systematic errors could largely beignored, and the best conditions chosen to optimizedynamic precision. For 193 nm resist metrology, thesystematic errors can no longer be ignored. To reducesystematic errors, multiple measurements should betaken in a dose regime where the medium term mecha-nism dominates. An e-beam current of 10pA will allowreduced contributions to the systematic errors from the

S P E C I A L F O C U S

Fall 2001 Yield Management Solutions36

fast shrinkage mechanism and still allow good signal-to-noise to be obtained. A beam energy of around 500eV should be the optimum balance point between“dynamic” and “systematic” errors. Choosing 400 eVwould not give enough signal-to-noise ratio for goodstatistics on the linescans, and challenges the creationof truly robust production recipes. 600 eV may be tol-erable, but higher energies would cause greater system-atic uncertainties. Manual measurements cannot beused because the uncontrolled dosing of samples wouldlead to variations in shrinkage. Pattern recognition andfocus steps can be set up in remote locations and at lowmagnifications to avoid shrinkage at the measurementsite. Image refresh at the measurement magnificationmust be avoided at all costs. Once collected, the trendof the data has to be corrected for shrinkage. A linearfit (such as that used in 248 nm resist metrology)would no longer be sufficient, as the fast shrinkage hasto be accounted for. Accurate correction of this fastshrinkage is likely to give the most problems in futuremetrology. The coefficients of the fit would depend onthe resist and measurement conditions.

It is interesting to note that the early literature of 193 nm metrology includes several studies in whichthe intermediate regime has quickly been exceeded dueto the high doses applied to samples. While this allowsmeasurements to be taken in the region of slow shrink-age and would lead to measurements with low randomerrors, correction of systematic errors would be diffi-cult. The use of non-image based metrology and fourtimes TV rate scanning in KLA-Tencor CD-SEMsallows collection of many measurements before theintermediate regime is exceeded. Care must be takenwhen benchmarking the capabilities of different CD-SEMs. By overdosing the sample and choosing a beamcurrent at which carry over balances mass loss, it wouldbe possible to show 193 nm measurements that appearto exhibit little initial shrinkage and then a low carry-over regime over a long set of measurements. The pre-cision would look very good, but there would be apenalty in accuracy.

Summary and Future WorkThis work has shown that three regimes have to beaccounted for in the shrinkage of a particular 193 nmresist. A fast regime is the most difficult to account forbecause it is so short-lived and uncertain in magnitude.This creates difficulties in both interpreting the data ofthis study and for the corrections in metrology. Basedon one interpretation of the data, it has been possible

to speculate on possible mechanisms that could occurin the resist during e-beam exposure, but further workis required to refute or confirm this model.Independent of the mechanisms, recommendationshave been made for the best conditions to use for 193nm resist metrology in which the balance between sys-tematic and random error contributions has been con-sidered. The above measurement conditions will beapplied to automated focus exposure measurements of193 nm resists and then in investigations of early 157 nm resists.

AcknowledgementsThe authors would like to acknowledge the help ofDiziana Vangoidsenhoven, Myriam Moelants, NadiaVandenbroeck, and Christie Delvaux (IMEC) for waferprocessing and exposure, and the many people at IMECand KLA-Tencor for their useful discussions on thiswork, in particular Rob Watts, Amir Azordegan, GianLorusso, and Gianni Leonarduzzi.

References1. I. Pollentier, M. Ercken, A. Eliat, C. Delvaux, P. Jaenen, K.

Ronse, “Front-end of line development using 193 nm lith-ography”, Proceedings SPIE Micro e l e c t ronic and MEMSTechnology Conference 2001

2. M. Neisser , T. Kocab, B. Beauchemin, T. Sarubbi, S.Wong, W. Ng, “Mechanism Studies of Scanning Electro nM i c roscope Measurement Effects on 193 nm Photore-sists and the Development of Improved Linewidth Mea-s u rement Methods”, Proceedings Interface2000, p. 43-5 2

3. T. Kudo, J. Bae, R. Dammel, W. Kim, D. McKenzie, M.Rahman, M. Padmanaban, W. Ng, “CD Changes of193 nm Resists During SEM Measurement”, Pro c e e d i n g sSPIE Microlithography Conference 2001

4. L. Pain, N. Monti, N. Martin, V. Ti r a rd, A. Gandolfi, M.Bollin, M. Vasconi, “Study of 193 nm Resist BehaviorUnder SEM Inspection : How to Reduce Line-width Shrink-age Effect ?”, Proceedings Interface2000, p. 233-248

5. B. Su, A. Romano, ‘Study on 193 nm Photoresist Shrink-age After Electron Beam Exposure”, Proceedings Inter-face2000, p. 249-264

6. L. Reimer, “Image Formation in Low-Voltage ScanningE l e c t ron Microscopy”, SPIE (1993) p52

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