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Lithography S

“The MEEF Meter”: a Realistic Approach for Lithography Process Monitoring by Frank Schellenberg, Pat LeCour and Olivier Toublan, Mentor Graphics Geoffrey Anderson and Raymond Yip, KLA-Tencor Corporation

With the advent of sub-wavelength lithography, process control has taken on a whole new meaning. This article discusses a practical process monitor target for the low k1 lithography regime, labeled the MEEF meter. Its purpose is to allow accurate determination of the MEEF effect and a consistent monitor for any changes in the lithography process that can impact this effect. The investigation and characterization were followed from the design phase, through reticle fabrication and finally onto the wafer.

Introduction

CD metrology using top-down scanning electron microscopes (SEM) is a routine technique for monitoring IC wafer processes.1 When process CDs vary beyond established control limits, corrective action can be taken. Test structures which fit in the scribe lines between chips are routinely introduced to allow the metrology on periodic samples of production wafers or on all production wafers if so desired. However, with most contemporary processes (i.e., those targeted with minimum features at 180 nm or smaller) the combination of process factors is typically tuned to reduce any variation in CD, at least until the process fails catastrophically. Illumination conditions, reticle techniques, resist technologies, post-exposure baking processes, etch recipes, etc., are all chosen to work in tandem to reduce the variation of target CDs as much as possible.2 Many recent papers have discussed the mask error enhancement factor, or MEEF.3, 4, 5, 6, 7, 8, 9, 10, 11, 12 This represents an

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“amplification” of reticle errors by wafer process phenomena, producing changes larger than expected on the wafer. Perfect linearity in a process would give a MEEF of 1; but when process conditions significantly deviate from linearity (usually when attempts are made to create sub-wavelength lithography features), larger values are observed. In highly nonlinear conditions, MEEF values as large as 8 have been reported.8 Typical sub-wavelength MEEF values for binary lithography are in the range of 1.5-2.5.8, 11 On the other hand, MEEF values for special phase-shifting cases, or for certain dimensions of dense lines or with assist bars, have been predicted to be significantly less than 1.7, 12 Understanding MEEF for a process is, therefore, very important to any resolution enhancement technologies, such as OPC and PSM, that compensate for predicted wafer effects. What we propose here is that the MEEF, being essentially a derivative measurement of linearity conditions, may actually serve as a sensitive monitor for process variations. MEEF has been predicted to be a strong function of defocus,7 so this is something to examine under experimental conditions. To this end, we have created a MEEF meter and observed its performance under conditions of focus and exposure.

MEEF and lithography

Experimental technique

MEEF is defined as the ratio of the CD measured on the wafer to the (adjusted) CD measured on the mask. In a perfect world, the CD on the wafer directly maps to the CD reticle, and the MEEF is unity.

To measure the CDs and MEEF, a tool such as KLATencor’s reticle CD SEM, the 8100 XP-R, is ideal. This tool allows initial measurement of the reticle CDs in the MEEF meter; it can then be immediately switched to wafer mode to allow measurement of the exact wafer sites corresponding to the wafer measurement sites under the same conditions and calibrations.13

Mathematically, MEEF can be expressed: MEEF =

∂CDwafer ∂(CDreticle/M)

where M is the stepper reduction ratio (typically M=4 in DUV steppers.) To adequately measure the MEEF, a change in the CD on a reticle must be programmed in order to observe its effect on the printed wafer. Since prior research has shown the MEEF values for isolated and dense lines (with their difference called the “MEEF Gap”), any test needs to take this effect into account. Enter the MEEF meter

The test pattern scheme is illustrated in Figure 1. On the left, a programmed change in isolated CD around a target dimension is shown. On the right, the same programmed change in CD is reproduced, but now in the context of dense lines with a 1:1 duty cycle centered around the target dimension. The change in CD is approximately 20 nm (wafer dimensions). By measuring each of these six CD features on the reticle and then measuring the corresponding features on the wafer, both isolated and dense MEEF can be calculated.

First, a GDSII layout of the MEEF meter was prepared. A jobdeck for a reticle was created placing the MEEF meter throughout the field. A reticle was then fabricated on an ALTA 3500 mask writer using a wet-etch fabrication process at the Reticle Technology Center (RTC). Several MEEF meters were included on the reticle layout, including designs for 250 nm, 180 nm, 150 nm, 130 nm and 100 nm. Only the 180 nm results at center of field are reported here. Wafers were then exposed on an ASML PAS 5500/300 DUV 4x reduction stepper using this reticle. Focus/ exposure matrices were produced using NA=0.63 and conventional illumination with σ= 0.5. Ultraflat silicon wafers were used to minimize focus distortion effects. The resist process used was a Shipley UV110 resist with thickness 450 nm and a Shipley ARII anti-reflective coating of thickness 66 nm. All exposures were carried out at ASM Lithography’s Tempe lab facility. SEM measurements using the KLA-Tencor 8100 XP-R were made on the wafer resist structures after development.

+20

Target CD

180

-20

a) Isolated MEEF

b) Dense MEEF

Figure 1: Schematic of the layout for the MEEF meter. A line with the nominal target CD (in this case, 180 nm) is made larger and smaller in an isolated context (left) and a dense context (right). All other features in the dense context are at the target CD. The CD that varies is shaded for ease of visibility in the figure; the features are all conven-

Figure 2: MEEF meter features as fabricated on the reticle and the

tional chrome lines on the reticle.

resulting image in resist on the wafer. MEEF structures are shown for both isolated lines (left) and dense lines (right).

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300

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Wafer CD (nm)

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Isolated CD (Target 180 nm)

Dense CD (Target 172 nm)

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Defocus (µm)

Exposure (m J)

Figure 3: Wafer CD results as a function of defocus (left) at constant exposure and exposure (right) at constant defocus for isolated and dense lines. The “target CD” is the measured reticle CD/4. The process is clearly tuned to produce minimal variation with focus, but CD does var y with exposure, especially for isolated lines.

After initial measurement of the reticle CDs for a particular test site, an automated metrology job was set up and run to gather data across the focus/exposure matrix for those identical sites. Final data analysis was carried out using a combination of Klarity ProDATA from KLA-Tencor and Microsoft Excel for final calculation of MEEF values.

curve near 180 nm provides the local MEEF value. This is plotted in Figure 4. As the varying line of the MEEF meter approaches a target CD of 100 nm, it vanishes on the wafer for both the dense and isolated cases. Features that are this small simply cannot be reliably produced without the aid of phase-shifting or another resolution enhancement

Nominal CD behavior

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This indicates that direct CD measurements can, therefore, be a good indicator of exposure drift; however, CDs are a poor indicator of process drift for defocus variation (see Figure 3).

Bridging 250

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Wafer CD (nm)

First, the behavior of the target CD with focus and exposure was investigated. This represents how process monitoring may typically be measured today. The results for this process, fairly typical of a well-controlled process, are shown in Figure 3. Variation of exposure dose causes the final resist line to grow thicker or thinner in dense and isolated cases, while the process has been tuned to be fairly insensitive to defocus—until the imaging fails entirely.

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Isolated CD (Target 180 nm)

Dense CD (Target 172 nm)

Feature vanishes 0 0

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Measured Reticle CD/4(nm)

MEEF behavior

Figure 4: Linearity plot for the var ying features of the MEEF meter plot-

Very different results were found with examination of the MEEF. To calculate the MEEF, we must first measure the CD linearity from the MEEF meter. The wafer CDs are plotted against the reticle CDs, and the slope of the

ted against the measured reticle CD/4. The isolated line presents a

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classic linearity cur ve, while the var ying center line in a dense context has a higher slope and is susceptible to scumming and bridging as it grows.

Once the MEEF values are calculated, we can plot them as a function of focus and exposure. These are shown in Figure 5. Although the MEEF for the isolated feature does not vary much with defocus, the MEEF for dense lines appears to vary parabolically with defocus. The increasing and decreasing MEEF values also follow the trend, with increasing MEEF always slightly larger and decreasing MEEF always slightly smaller than the total MEEF. These MEEF values represent concave curvature of the linearity plot around 180 nm. This defocus dependence, especially for the dense MEEF, provides a strong indication of a focus drift even though the target CD itself (see Figure 3) is nearly constant. By comparison, however, the exposure dependence of the MEEF, also shown in Figure 5, appears essentially constant with exposure and is also fairly noisy. This actually reflects the linear dependence of the exposure data in Figure 3. All lines in the MEEF meter grow and shrink together as the exposure changes; so the MEEF, representing their relative changes, remains roughly constant. Given that the exposure changes can induce scumming and bridging at extreme values, the additional noise at the exposure extremes is also expected. In summary, the MEEF seems to be a good indicator of focus changes, while it is fairly insensitive to exposure changes, indicating that all the lines are growing and shrinking together. For defocus variations, MEEF is a good indicator of process drift, while for exposure variations, MEEF is a poor indicator of process drift. A numerical comparison between the two monitor techniques for exposure and defocus is presented in Tables I and II. Clearly, the magnitude of the change,

3.0

technology. As the varying feature becomes larger, however, the isolated line simply grows in proportion, while scumming and then bridging begin to occur on the wafer for the dense case.

2.5

MEEF

As can be seen in this figure, there is clearly a difference in slope for the dense and isolated MEEF meter features near the target value of 180 nm, indicating a “MEEF Gap.” It should also be noted that, due to curvature in the linearity plot, the “decreasing” MEEF (i.e., slope when the reticle CD is getting smaller) and the “increasing” MEEF (i.e., slope when the reticle CD is getting larger) do not always match. Typically, we actually observe that the increasing MEEF is slightly larger than the decreasing MEEF, and the “total” MEEF around 180 nm is an average of the two values.

1.5

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Isolated CD (Target 180 nm)

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Dense CD (Target 172 nm)

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Defocus (µm) Figure 5: MEEF for isolated and dense lines as a function of defocus and exposure. Although the MEEF appears to var y parabolically with defocus, the MEEF appears constant for exposure; the exposure sensitivity is fairly noisy due to the increased susceptibility to bridging.

visible by comparing Figures 3 and 5, is confirmed by the numerical comparisons. For exposure changes, CD variations are three to ten times larger than the corresponding MEEF changes. For defocus changes, on the other hand, MEEF variations are three to ten times larger than the corresponding CD changes. The two metrics, therefore, appear to present an independent technique to monitor defocus and exposure changes, as processes drift. A suitable system incorporating these metrics would then be able to dictate corrective action before the process failed. A “Process MEEF” variation

These results offer an interesting methodology for use as a process monitor. However, a variation on the MEEF meter that is easier to simulate using commercial simulators has been the structure more typically examined when MEEF for dense lines is discussed.9, 10 This is the case where it is not just a single line changing, such as would be found in a mask-writing error, but when all the lines change together, as might be found in a mask-processing error. We have called this the “Process MEEF” Meter, to distinguish it from the single feature MEEF Meter above. “Process” MEEF meters were also included in the test patterns used in these experiments. The schematic for the layout, as well as SEM images of the reticle and wafer, are shown in Figure 6. Autumn 2000

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Conclusions

This article has shown the potential use of direct CD measurements in combination with calculations of MEEF as a potential process monitor. Results suggest the use of direct CD measurement can be a sensitive indicator of exposure changes, while the MEEF measurement can detect defocus changes and the two measurement techniques appear to be independent.

Figure 6: “Process” MEEF meter features in schematic (left, not to scale), as formed on the reticle (center) and as they print on the wafer (right). The target dimension on this case was 180 nm. Although MEEF can still be calculated in some cases, increasing the mask feature sizes here tends to cause scumming or bridging on the wafer, especially at lower exposure doses.

A linearity plot for both dense and “process” MEEF features targeted around 180 nm is shown in Figure 7. Fundamentally, the “process” MEEF meter data overlays the dense MEEF meter data, so the MEEF values in general will be very close to those generated for the dense MEEF values presented above. However, for increasing MEEF, an increased tendency toward scumming and bridging causes far more noise in the measurements. This then becomes a less reliable measure of MEEF and, therefore, process changes, than the dense MEEF meter. 300

Bridging 250

Although these are believed to be representative data sets, further measurement of a process systematically over time using the MEEF meter is the only way to determine whether the MEEF meter performs reliably over time as a process monitor. Additional variants on the MEEF meter using assist bars or phase-shifting designs would also be useful to confirm their predicted mitigating effects.7, 12 Future studies would require a more detailed examination of potential variations and noise sensitivity. Other possible sensitivities to be investigated would be the sensitivity to NA and coherence variations, as well as possible sensitivity to aberrations (although this may be best aided by a phase-shifted MEEF meter). Acknowledgements

This work could not have been completed without the help of many friends and colleagues. We are especially grateful to Susan MacDonald and Craig West of DuPont Photomask for their help in creating the reticle layout, Greg Hughes of the RTC for fabricating the reticle, Luigi Capodieci and Bob Socha of ASML Masktools for coordinating the wafer exposures, Mohan Ananth and Waiman Ng of KLA Tencor’s metrology division for their help in making the reticle and wafer CD measurements, and Moshe Preil of KLA Tencor for helpful discussions.

Wafer CD (nm)

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Process MEEF Meter

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Measured Reticle CD/4 (nm) Figure 7: Linearity comparison of the Dense MEEF meter and the “Process” MEEF meter. General behavior is identical, except for a propensity for scumming and bridging in the “increasing” MEEF direction.

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“Handbook of critical dimension metrology and process control,” K. M. Monahan, ed. Proc SPIE CR52, SPIE Press, Bellingham, WA, 1994. M. D. Levenson, “Wavefront engineering for photolithography,” Physics Today, (July, 1993), p. 28 ff. W. Maurer and D. Samuels, “Masks for 0.25-micron lithography,” Photomask and X-Ray Mask Technology, HideoYoshihara; Ed. Proc. SPIE vol. 2254, (1994), pp. 26-35. W. Maurer, “Mask specifications for 193 nm lithography,” 16th Annual BACUS Symposium on Photomask Technology and Management, G. V. Shelden and J. A. Reynolds, Eds. Proc SPIE vol. 2884, (1996), pp. 562-571.

A. Wong, R. Ferguson, L. Liebman, S. Mansfield, A. Molless, and M. Neisser, “Lithographic effects of mask critical dimension error,” in Optical Microlithography XI, Luc Van den Hove; Ed, Proc. SPIE vol. 3334, (1998), pp. 106-116. J. Randall, A. Tritchkov, R. Jonckheere, P. Jaenen, and K. Ronse, “Reduction of mask induced CD errors by optical proximity correction,” in Optical Microlithography XI, Luc Van den hove, ed. Proc. SPIE vol. 3334, (1998), pp. 124-130. J. Randall and A. Tritchkov, “Optically induced mask critical dimension error magnification in 248 nm lithography,” J. Vac. Sci. Technol. B16 , (1998) pp. 36063611. F.M. Schellenberg, V. Boksha, N. Cobb, J. C. Lai, C. H. Chen, and C. A. Mack, “Impact of mask errors on full chip error budgets,” Optical Microlithography XII, Luc Van den Hove; Ed, Proc. SPIE vol. 3679, (1999) pp. 261275.

Defect at Flat view 0º Tilt

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F. M. Schellenberg and C. Mack, “MEEF in theory and practice”, 19th Annual BACUS Symposium on Photomask Technology, F. Abboud and B. Grenon, ed. Proc SPIE vol. 3873, (1999) pp 189-202. C. Mack, “Mask linearity and the mask error enhancement factor,” Microlithography World, Winter 1999 p. 11-12. A. Vacca, B. Eynon, and S. Yeomans, “Killer defects caused by localized sub-100-nm critical dimension reticle errors,” in Optical Microlithography XI, Luc Van den hove, ed. Proc SPIE vol. 3334, (1998), p. 642-648. M. D. Levenson “Can phase shift save the semiconductor industry?,” Proceedings of the 1998 Interface Conference, (Olin Chemical, 1998), pp 165-177. W. Ng, G. Anderson, H. Villa, and F. Kalk, “A study of CD SEM suitability for CD metrology of modern photomasks,” Photomask and X-Ray Mask Technology VI, Hiroaki Morimoto; Ed. Proc SPIE vol. 3748, (1999) pp 585-591. circle RS#050

Defect at Flat view 45º Tilt

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