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Collapse of the Deep-UV and 193 nm Lithographic Focus Window Yield Impact of Cross-Field and Cross-Wafer CD Spatial Uniformity Kevin Monahan, Pat Lord, Waiman Ng, Hubert Altendorfer, George Kren, and Scott Ashkenaz KLA-Tencor Corporation

© 1999 IEEE. Reprinted, with permission, from the Proceedings of the 1999 IEEE Symposium on Semiconductor Manufacturing; 1999; pgs 115-118.

The 0.13 µm semiconductor manufacturing generation, shipping as early as 2001, will have transistor gate structures as small as 100 nm, creating a demand for sub-10 nm gate linewidth control. Linewidth variation consists of cross-chip, cross-wafer, cross-lot, and run-to-run components. In this work, we explore spatial dependencies across the lithographic field due to reticle error and across the wafer due to wafer and chuck nanotopography. Both sources of spatial variation can cause collapse of the lithographic focus window near the limits of resolution, resulting in CD excursions for gate structures in high-performance microprocessors. Our work supports the contention that photolithography-induced defects may become the primary source of yield loss for the 0.13 µm generation and beyond.

As an extension of previous work on temporal variation1, we are currently exploring spatial dependencies across the lithographic field due to reticle and lens error and across the wafer due to wafer nanotopography and chuck flatness. The new study uses data from a comprehensive set of measurement technologies, including reticle and wafer CD SEM metrology, phase-shift focus metrology, cross-wafer interferometry, differential interference contrast metrology, and macro defect inspection. We have found that, as in the case of temporal variation, spatial variation can cause collapse of the common CD-defocus window near the limits of lithographic resolution, particularly for the gate and contact structures in high-performance devices.

There are many sources of spatial variation that contribute to process window collapse.

These include overlay error, reticle error, lens error, and focus errors. To predict yield, we treat each of them as defects with a specific “kill potential”. One example is the recent use of logistic regression to correlate overlay error with the probability of device failure2. In such a unified defect scenario, yield may be expressed as a product of survival probabilities given by N

Y=

∏(1 – λ )n

i

i

i=1

where lambda is the defect kill potential for defects of type i, n is the number of defects per die of type i, and N is the total number of defect types. In the case of parametric defects, the kill potentials may be functions of measured parameters such as overlay and critical dimension (CD) or unknown parameters, such as exposure variation and local defocus, which are observed indirectly in the form of CD excursions. Generally, we Summer 2000

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200

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% Dose

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60 70 80 90 100 110 120 130 140 150

140 Gate CD (nm)

Raleigh Dof (nm)

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200 150 100

120 100 80 60

50 40

0

248

193 157 Exposure Wavelength (nm)

EUV

20 0 -3

2

Figure 1. Collapse of the 0.5λ/(NA) Raleigh focus window as litho-

-2

-1

0 Defocus Units

1

2

3

graphic exposure wavelength is decreased. Values for the numerical apertures are assumed to be 0.7, 0.7, 0.7, and 0.25, respectively.

Figure 2. CD-defocus response surface (equation below) for isolated lines in negative resist (raw data not shown). Underexposure was used

The EUV wavelength is 13 nm.

to get 130 nm gate structures (90 percent exposure dose, E) at the expense of low focus latitude and high sensitivity to exposure variation

improve the accuracy of the lithographic yield model by identifying those defects with the highest kill potential, or even those that pose a quantifiable economic risk by affecting bin yield3.

Lithographic defocus is likely to be an indirect source of CD defects with high kill potential, particularly as exposure wavelengths decrease from 248 nm to 193 nm, 157 nm, and even 13 nm. This is primarily due to the reduction of the Raleigh focus window with shorter wavelength. CD and defocus are highly interactive, as shown in the example of Figure 2. Reticle CD errors or overlay errors that force reduction of the overall CD error budget will have a negative effect on the allowable range of defocus.

Results and discussion

A comprehensive “systems approach” was used to analyze complex spatial uniformity data from reticles and wafers. At least five state-of-the-art methodologies were applied to the problem:

collapse of the focus window as the CD tolerance is tightened by 50 percent.

1 2 y(E,D) = (b0 = b1D + b2D 2) + — (b3 + b4D + b5D ) E

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Wafer surface nanotopography using differential interference contrast metrology

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Cost-effective screening using darkfield and brightfield macro inspection technology

Our results show that the sources of CD error due to lithographic defocus can be de-confounded using this comprehensive approach. Reticle CD error, for example, can be stripped out using CD SEM measurements.

Confounded lens, wafer, and chuck components can be separated using a phase-shift focus monitor, combined with double-sided, wafer-scale interferometry and singlesided, differential interference contrast metrology.

Reticle and Wafer CD SEM Metrology

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Lithographic reticle and optical characterization using reticle and wafer CD SEM metrology

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Phase-shift focus measurement using optical overlay metrology and model-based analysis

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Wafer thickness metrology using wafer-scale transmission interferometry

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under conditions of defocus (D). The outer and inner boxes show the

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Focus errors create spatial CD non-uniformity. These can be due to the reticle, the projection optics, and the wafer/chuck surface upon which the pattern is printed.

CD SEMs can be used to map cross-field and cross-wafer errors, creating model-based CD uniformity maps and generating feedback to the stepper/track systems for correction of systematic spatial variation. Examples of cross-field CD error, measured using a specially adapted

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Figure 3. Cross-field CD SEM data on a reticle, showing radial

Figure 4. Cross-field CD SEM data on a wafer showing apparent

dependence of the CD values in nanometers.

“tilt” in the DUV optics. Reticle CD error has been removed. Scale is in nanometers.

CD SEM, are shown in Figure 3 (reticle) and Figure 4 (wafer). In this case, the total CD variation on the wafer is due to reticle error, lens error, and nanotopography of the chucked wafer. Since the reticle and wafer CDs are measured in the same SEM, the reticle error is removed from the wafer data without heterogeneous tool matching.

Data from a phase-shift focus monitor characterizes focal plane deviations within the field and from field to field across the wafer. A monitor reticle with asymmetrically phase-shifted overlay targets is used in conjunc-

tion with a high-speed overlay tool to generate artificial registration errors that are a linear function of lithographic defocus. Fitting the data to a model enables the quantitative assessment of lens tilt, field curvature, astigmatism, scan errors, wafer/chuck flatness, lens heating, barometric effects, and other lithographic focus anomalies. The cross-field mapping capability of the phase-shift focus monitor is shown in Figure 5.

Figure 6 shows the same capability across a wafer. The total range is ±200 nm, including top surface nanotopography, wafer thickness variation, and chuck nonuniformity.

Figure 5. Cross-field response surface generated with data from a

Figure 6. Cross-wafer phase-shift focus data includes top surface

phase-shift focus monitor. The root-cause of CD error is often traced to

nanotopography, wafer thickness variation, and chuck non-uniformity.

cross-field defocus effects arising from wafer/chuck nanotopography.

Range is ±200 nm.

Registration-Based Phase-Shift Focus Metrology

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Figure 7. Cross-wafer interferometry isolates top surface nanotopography and thickness variation from chuck non-uniformity. Range is ±200 nm on 300 mm wafer.

Cross-Wafer Transmission Interferometry Cross-wafer interferometry can be used to separate thickness variation and surface nanotopography from chuck-induced deformation. In this case, monochromatic light is projected through the wafer and interference from the top and bottom surfaces of the wafer is used to determine thickness. Data for a 300 mm wafer polished on both sides is shown in Figure 7. The total range is ±200 nm. Measured in this way, the thickness data is confounded with top surface nanotopography.

In general, nanotopography with spatial periods below 5 mm (the slit-width of a scanner) and amplitudes in the hundred-nanometer range can create significant focusing errors in scanning lithography. A 193 nm scanner with 0.7-N.A. optics will have a theoretical depth-of-focus of about ±200 nm for dense lines, and less for isolated features.

Nanotopography at high spatial frequencies can exceed the dynamic range of a scanner’s in-situ focusing subsystems. If the focus errors are large, the resulting CD variations can create severe device yield and speed binning excursions, particularly in high-performance microprocessors.

Differential Interference-Contrast (DIC)Metrology Differential interference-contrast metrology uses the phase response of light reflected from the top surface of the wafer. Along with micro-tilt sensing, it can be used to separate top surface nanotopography from wafer thickness variation. In our case, the DIC metrology is 42

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Figure 8. Differential interference metrology isolates top surface nanotopography. Range is ±30 nm.

implemented on a high-speed, unpatterned-film inspection tool. A high-resolution profilometer is used for height calibration. As shown in Figure 8, DIC metrology responds to the higher spatial frequencies that could be missed by cross-wafer transmission interferometry. The total range of the top surface nanotopography is ±30 nm, much of which is due to “polishing chatter” arising from a process excursion that could have gone unnoticed without the DIC monitor. In some cases, we have observed top surface nanotopography with ranges below ±3 nm. This level of wafer surface quality is costly, but it may become critical for chemical-mechanical planarization (CMP) used in advanced shallow-trench isolation (STI) technologies.

Whole-Wafer Macro-Defect Inspection Gross defocus on patterned wafers is generally visible as a “hot spot” during macro-defect inspection. Hot spots can sometimes be seen in brightfield illumination, but they are much more visible in darkfield illumination due to scattering from pattern defects. Hot spots often result from extreme nanotopography caused by particles on the backside of a wafer, a problem that could become worse as the industry makes the transition to double-sided polishing on 300 mm wafers.

These gross focus excursions can be detected using high-speed, macro-defect inspection tools as monitors.

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The wavelength reduction strategy for extending optical lithography will have the following consequences:

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Reticle error will become a larger part of the CD error budget, forcing narrower lithographic process windows for focus and exposure.

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Focus windows will dwindle as the CD error budgets shrink and theoretical depth-of-focus drops in proportion to exposure wavelength.

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The need for monitors may increase as spatial CD variation and nanotopography effects detract from the focus window and impact yield.

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The kill potential of both direct (CD) and indirect (defocus) parametric defects will need to be quantified more accurately using robust statistical methods4.

Figure 9. Whole-wafer, darkfield macro inspection data showing

Acknowledgements

focus “hot spot” in the upper left quadrant.

We are in debt to Ken Schroeder, Robert Lee, Jan Waluk, and many others at KLA-Tencor for their encouragement and contributions to this work.

Since backside particles produce pattern defects over relatively large areas, the 50-micrometer sensitivity of a macro inspector is more than sufficient to detect this form of wafer contamination. A typical darkfield image of a hot spot is shown in Figure 9.

Summary

Wavelength reductions and increases in numerical aperture have extended the life of optical lithography, but the improvements in resolution come at the cost of reduced depth-of-focus, as shown by the Raleigh equation below:

λ NA2 where λ is the exposure wavelength and NA is the numerical aperture of the optics.

DOF = 0.5

References 1. K. M. Monahan and P. Lord, “Lithographic focus stabilization for model-based gate CD control systems”, Proc.

ISSM, Tokyo, October 7-9, 1998, pp. 347-350.

2. M. E. Preil, J. McCormak, “A new approach to correlating overlay and yield”, Proc. SPIE, Vol. 3677, 1999.

3. K. M. Monahan, P. Lord, C. Hayzelden, and W. Ng, “An application of model-based, lithographic process control for cost-effective IC manufacturing at 0.13 micron and beyond”, Proc. SPIE, Vol. 3677, p. 435 (1999).

4. R. Martin, X. Chen, and I. Goldberger, “Measuring fab overlay programs”, Proc. SPIE, Vol. 3677, p. 64 (1999).

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