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Using Pattern-Quality Confirmation to Control a Metal-level DUV Process with a Top-down CD SEM Chien-Sung Liang, Haiqing Zhou, Mark Boehm, Ricky Jackson, KFAB Photolithography Module, Texas Instruments Chih-Yu Wang, Mike Slessor, KLA-Tencor
As critical-feature patterning processes increase in complexity and sensitivity, conventional critical-dimension (CD) mea surements may not afford the level of process control required for effective device production. By comparing recorded top-down scanning-electron-microscope (SEM) images to a predefined reference image, Pattern Quality Confirmation (pQC) enables a more detailed analysis of measurements captured by KLA-Tencor 8xxx CD-SEMs. An example of the utility of this additional information is discussed for a metal interconnect level patterned with a deep-ultraviolet (DUV) photolithography process. In particular, we demonstrate that, for certain ranges of focus-exposure conditions, conventional post-develop CD measurements remain well within specification; however, when etched, the resulting metal-line CDs are significantly below the lower specification limit. The pQC image analysis results predict the observed post-etch CD variations and, consequently, offer sensitivity to yield-limiting focus drifts and excursions, enabling effective product-dispositioning (rework) decisions.
Introduction
At virtually every processing step, modern semiconductor manufacturing relies on highly automated metrology for process control. For lithography and etch processes, automated critical-dimension scanningelectron microscopes (CD SEMs) fill this role; however, there exists a set of yieldrelevant excursions, e.g., profile changes, scumming, etc., which can elude detection by standard CD measurements. The contraction of multi-dimensional image information into a single CD value can obscure such subtle effects, which are evident in the images themselves. Pattern Quality Confirmation (pQC) is a method that utilizes some of this image information to enable detection of subtle changes as part of a fully-automated CD SEM metrology step. In effect, a comparison is 56
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made between a known “good� template, and the image obtained during measurement of the feature of interest. A correlation score provides the metric of comparison; a perfect match returns a score of 100, no match returns a score of zero. On KLA-Tencor 8xxx CD SEMs, this measurement option is available in two flavors: a onedimensional or linescan pQC correlation score, and a two-dimensional or image-based pQC correlation score. Both one- and two-dimensional pQC measurements have been demonstrated to provide additional value to top-down CD SEM measurements; 1,2,3 in this work, we explore the use of two-dimensional pQC in monitoring an aluminum-interconnect patterning process. The process of interest employs deep-ultraviolet (DUV) photoresist application and exposure techniques along with a conventional metal-etch process. For this process and layer, a thick layer of photoresist is required to pattern the desired interconnect structure, and the lithography process is found to be relatively sensitive to variations in exposure-tool focus and energy. In turn,
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etch processes in general are sensitive to photoresist sidewall profile, providing a challenge to overall etched-linewidth control and associated interconnect performance and reliability. Method
For this work, a conventional patterning process currently in production use was employed, allowing us to directly assess the effectiveness of pQC in a productmonitoring scenario. Metallized wafers (AlCu+Ti/TiN) were coated with an 890 nm thick film of commerciallyavailable deep-ultraviolet (DUV) photoresist, exposed using a binary reticle in a Nikon NSR2205EX14D step-and-repeat KrF exposure tool, and developed using a 60-second puddle process with a 0.26N (2.38 percent TMAH) developer. This process has a (known) nominal best focus of 0.1 µm and a nominal best exposure of 48 mJ/cm2. For the purposes of this study, photoresist linewidth and profile were intentionally modulated using a standard focus-exposure matrix centered at the nominal best values. Focus values were varied from -0.9 µm to 0.9 µm in increments of 0.2 µm, with exposures from 40 mJ/cm2 to 56 mJ/cm 2 in increments of 2 mJ/cm2. Both conventional CDs (linewidths) and pQC correlation scores were recorded for both isolated and dense features; in addition, sidewall angles were inferred using an edge-width algorithm. Wafers were then etched using a standard metal-etch process, and postetch CDs were then measured. All measurements were recorded using standard algorithms available on the KLA-Tencor 8100XP CD SEM. Linewidth and pitch algorithms are standard practice and are not described here; a brief discussion of pQC and edge-peak width algorithms is presented here for completeness. Detailed discussions are available elsewhere.4,5 The pQC algorithms are available in both one- and two-dimensional varieties; the two-dimensional version was employed for this work, applied to quasi-onedimensional structures (lines). This measurement reports a pixel-by-pixel correlation with a reference, or “golden” image recorded from a successfully patterned feature. Here, we recorded reference images at a magnification of 25kX, in fields printed at the known best exposure of 48 mJ/cm 2 and a focus value of -0.1 µm. The selection of these values will be discussed in the Results section. The images are shown in Figure 1, for both isolated and dense features.
F i g u re 1. Reference, or “golden ”, images re c o rded fo r i sola ted (l eft) an d dense (right) features. These image d ata are used by the p QC al gorithm for compari son wi th images re c o rded for the measure m e n t si te of interest.
Edge-peak width measurements employ CD SEM linescans to infer the effective width of the transition (sidewall) region evident in image data by computing the distance between a predefined rising-edge threshold value and an accompanying falling-edge threshold value. To further understand the correlation between sidewall profile changes and image-based pQC scores, we also employed atomic force microscopy (AFM) to directly probe the sidewall angle. Of course, this technique is impractical for production line monitoring, however, it provides a direct, high-resolution measurement of the sidewall profile. Klarity ProDATA, a commercially available software package, was used for data analysis and presentation throughout this work. This package enables rapid, automated analysis of conventional critical-dimension and pQC data to produce results such as Bossung curves, process windows, and overlap process windows. Examples of each are found in the Results section, below. Results
The post-develop response of isolated and dense line critical dimensions, with respect to changes in focus and exposure, is shown in Bossung curves in Figure 2. These particular scribe-line structures have CD targets of 0.33 µm and 0.22 µm for isolated and dense linewidths, respectively; specification limits of ±10 percent are imposed on the process. As seen in Figure 2, each feature exhibits a reasonable range of focus and exposure values for which CDs are printed within specifications. Individual process windows computed for these two features, along with the overlap process window, are shown in Figure 3. Individually, each feature exhibits an acceptably large depth of focus and exposure latitude, Fall 2001
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F i g u re 2. Boss ung cur ves for isolated (left ) a nd dense ( righ t) lines, with CD specificat ion (target (10%) lines superimposed. Bo th axes re p o r ted i n un its of micro n s .
in addition, both windows are centered at approximately the same focus and exposure values. The overlap process window is determined primarily by the behavior of the isolated line; Klarity ProDATA process-window overlap analysis indicates a best focus of 0.08 µm with a depth of focus of 0.77 µm, and a best exposure of 45.4 mJ/cm2 with an exposure latitude of 4.5 mJ/cm2. These values compare favorably with the known best values, indicating that the lithographic process is behaving normally. The next step in this process is pattern transfer from the photoresist to the metal film, producing the metal lines for back-end interconnect. Representative results for isolated-line CDs, measured after completion of the etch process, are shown in Figure 4. These post-etch
linewidths are measured for metal lines printed on the same wafer at best exposure (48 mJ/cm2) with various values of focus. For relatively small values of positive defocus, we observe a precipitous decrease in etched width of the isolated line, well below the imposed lower specification limit of 0.55 µm. Of course, an unintended linewidth (cross-sectional area) change can result in associated effects on device performance and reliability, such as changes in interconnect resistance, power dissipation, and probability of electromigration failure.6 There is no such decrease observed for negative focus values, and the near-symmetry of the isolated-line Bossung curve for post-develop data (Figure 2, left) has
F i g u re 3. Individual process windows computed for each feature, along
F i g u re 4. Criti cal dimensio n ( wi dth) of isol ated metal line f ollowing
with overlap process window. Overlap window describes focus-exposure
et ch. Note t he decrease i n l inewi dth for moderat ely positi ve focus
region for wi thin-s pecification pri nting of both fea tures simultaneous ly.
deviations. Imposed lower specification limit of 0.55 µm is also shown.
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F i g u re 5. CD-SEM images an d s uperimposed electr on-i ntensity profiles of isolated li ne print ed at vari ous focus values and best d ose (48mJ/c m2). F rom left to right, the nomina l f ocus varies fr om –0.5 µm to +0.5 µm in 0. 2 µm i ncre m e n t s .
been lost in the etch process. Interestingly, this postdevelop CD data exhibited a rather moderate decrease with increasing positive focus and gave no indication of this post-etch behavior. This is problematic in the context of product dispositioning. In this positive-focus regime, post-develop CD measurements do not allow accurate prediction of post-etch metal linewidths, substantially decreasing the effectiveness of productrework decisions. Examination of post-develop SEM image data (Figure 4) provides an indication of the root cause of the observed reduction in post-etch linewidth. In particular, a continuous qualitative change in the image of the sidewall regions can be observed with increasing focus; we infer from these images that the photoresist profile is becoming more rounded or sloped for positive focus values. Such a resist profile would be less resistant to etch at the edges of the line, and would be expected to
produce narrower lines than a profile with a closer-tovertical sidewall. Given the image-to-image comparison performed by the pQC algorithm, the qualitative changes in image data such as those observed in Figure 5, it is reasonable to expect these changes to be reflected in pQC correlation scores. “Bossung-style” plots of post-develop pQC scores, with the corresponding conventional CD-based Bossung curves are show in Figures 6 below for the isolated line. Qualitatively, the isolated-line pQC correlation score data exhibits a more rapid decrease for increasing positive focus than does the CD data, reflecting the change in sidewall behavior inferred from the SEM images of Figure 5. This positive-focus decrease is reminiscent of post-etch CD data of Figure 4. As described in the Method section, pQC template images were recorded at best exposure, but with a focus of
F i g u re 6. Isolat ed-l ine Bossung cur ves fo r p QC correlation scores (left), an d CD measurements (right). Corre lation scor es exhib it a rapid decre a s e with i ncreasi ng fo cus, tr acking the (quali tative) pos t-d evelop imag es.
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(48 mJ/cm2) and focus values ranging from -0.5 µm to +0.5 µm. To enable this comparison, each value was normalized by the maximum value attained by that data type (e.g., post-develop CD) over the focus range. In addition, SEM images are included for reference. Here, it is clear that post-develop CD has failed to capture the sidewall variations qualitatively observed in the images. As discussed above, these sidewall variations are the root cause of etched-linewidth reduction associated with positive focus values; pQC scores are in agreement with both the qualitative changes in the sidewall images and the reduction in etched linewidth. Post-develop pQC correlation scores are seen to provide a F i g u r e 7. Comparison of isol ated-li ne post -develop and post -etch C D mea suremen ts with pQC quantitative, predictive metric of post2 c o rrelation scores, as a function of focus. All features printed at a constant exposure of 48 mJ/cm . etch CD variation—a variation that Post -develo p SEM i mages are a lso shown for each f ocus va lue. Values for ea ch quantity are p o s t -develop CD measurements were n o rma lized by the maxi mum value for that quantity t o allow di rect compar ison of all three unable to predict. The additional m e a s u rements. Note the devia tion between post-d evelop and post-etch CD measurement s f or information provided by pQC analysis, positive val ues of focus. using automated top-down CD SEM image data alone, is seen to enable -0.1 µm, an offset of (negative) 0.2 µm from the known quantitative and reliable product-dispositioning best focus value. This value was chosen to enhance sen(rework) decisions. As discussed above, such decisions sitivity of the reported correlation score to changes in were not possible on the basis of conventional linewidth the sidewall region, while minimizing its sensitivity to measurements. changes in absolute feature size in the region of interest. The SEM images presented in Figure 5 are indicative A direct comparison of post-develop CD and postof a change in sidewall profile with changing focus; a develop pQC data with post-etch CD data is found result mirrored in the image-based pQC results. To in Figure 7 for features patterned at best exposure confirm these SEM image results, booted-tip atomic-
F i g u r e 8. Inferred post-develop photoresis t s idewa ll ang les from AFM and top-down CD SEM measur ements. Sidewall angles measured by both techniques as a function of focus (left) corrobora te concl usions made on the b asi s of SEM image data. R easonab le a greement between the di rect AFM techn ique and t he indirect SEM i mage measuremen t i s a lso ind icated (right).
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result not surprising in light of the etch behavior in this regime. Conclusions
F i g u re 9. Isolated-lin e process wind ows for both critical di mension and pQC correl ation score, a s well as t he overlap process wind ow for both. Note th at the positive f ocus limit of the win dow is d etermined by the pQC score, reflecting the i ncreased detection sensitivity of sid ewall angle excursions aff o rded by pQC.
force microscope (AFM) measurements were used to directly measure post-develop photoresist sidewall angle as a function of focus. As can be seen in Figure 8, the AFM results corroborate the inference of a changing sidewall angle. In addition, there is reasonable agreement between directly-measured angles (AFM) and angles inferred from a calculation using edge-peak width measurements and the known photoresist thickness. Interestingly, the dense line data showed much less modulation of sidewall angle with focus, and all metrics discussed for the isolated line were seen to be much more stable with changing focus. Consequently, there is little to be gained by monitoring the dense line alone; in fact, significant risks would be incurred by implementing a control scheme that ignored isolatedline behavior. Finally, we can exploit the increased sensitivity to the observed focus offered by the pQC measurement to revise the usable lithographic process window that results in successfully (within-specification) etched lines. In particular, we define a pQC correlation-score specification of 55 percent, a value chosen to correspond to the level at which etched lines were observed to fall below their lower specification limit (Figure 5). This specification is analogous to the more-familiar 10 percent CD specification, and allows definition of a pQCbased process window in focus-exposure space. As seen in Figure 9, the pQC-based process window allows a significantly smaller range of positive focus values, a
In this work, we investigated the patterning of metal lines using both conventional critical-dimension measurements and pQC image analysis. We find regimes where measured post-develop CDs are still well within specification, but image-based post-develop pQC correlation scores have dropped to less than 55 percent of the nominal value. Examination of SEM images through the focus-exposure range indicates modulation of sidewall angle that has been captured by the (image-based) pQC correlation score, but has eluded detection by conventional CD measurements. This sidewall modulation is also corroborated by atomic-force microscope (AFM) measurements, which show good agreement with image-based measurements. This sidewall angle modulation has a significant effect on post-etch linewidths; for positive focus values, a reduction in post-etch CDs to values well below the specification limits is observed. Conventional postdevelop CD measurements were unable to predict this behavior, however, post-develop pQC correlation scores were an effective predictor of the post-etch linewidth variation. The capability to quickly and accurately measure these additional profile characteristics lends a powerful tool in detecting a variety of excursion types that further reduce already challenging depths-of-focus. The results presented here illustrate some of the shortcomings of conventional CD measurements in monitoring this process, however, they also illustrate the richness of top-down SEM image data beyond these conventional measurements. Acknowledgements
We wish to thank Vladimir Ukraintsev, Raymond Yip, and Sunil Desai for their expert assistance with the measurements. References 1 . B. Choo, T. Riley, B. Schulz, and B. Singh; “Automated P rocess Control Monitor for 0.18 Âľm Technology and Beyond,â€? M e t ro l o g y, Inspection, and Process Control for Microlithography XIV, Neal T. Sullivan, Editor, Proceedings of SPIE Vol. 3998, pg. 218-226, SPIE, Bellingham, WA, 2000.
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2 . J. Allgair, G. Chen, S. Marples, D. Goodstein, J. Miller, and F. Santos; “Feature Integrity Monitoring for Pro c e s s C o n t rol Using a CD SEM,” Metro l o g y, Inspection, and Process Control for Microlithography XIV, Neal T. Sullivan, E d i t o r, Proceedings of SPIE Vol. 3998, pg. 227-231, SPIE, Bellingham, WA, 2000. 3. D. M. Goodstein, B. Choo, B. Singh, “Correlation Flagging of i-Line Lithographic Process Drift,” KLA-Tencor CD SEM Users Group Meeting, Santa Clara, CA, 1999. 4. 8xxx Series CD SEM Operation Manual (v3.1.X), KLA-Tencor Corporation, 2000.
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5. 8xxx Series CD-SEM Application Note, “Measuring Edge Peak Widths for Process Characterization,” KLA-Tencor Corporation, 1999. 6. S.M. Sze, Editor, VLSI Te c h n o l o g y, Second Edition, McGraw Hill, 1988.
A version of this article was originally published in SPIE Proceedings Vol. 4344, paper 108, entitled “Using Pattern Quality Confirmation to Control a Metal-level DUV Process with a Top-down CD-SEM” by Chien-Sung Liang; Haiqing Zhou. Mark Boehm, Ricky Jackson (KFAB Photolithography Module, Texas Instruments); Chih-Yu Wang, Mike Slessor (KLA-Tencor Corporation).
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