For many years, lithographic resolution has been the main obstacle for keeping the pace of transistor densification to
meet Moore's Law. The industry standard lithographic wavelength has evolved many times, from G-line to I-line, deep
ultraviolet (DUV) based on KrF, and 193nm based on ArF. At each of these steps, new photoresist materials have been
used. For the 45nm node and beyond, new lithography techniques are being considered, including immersion ArF
lithography and extreme ultraviolet (EUV) lithography. As in the past, these techniques will use new types of
photoresists with the capability of printing 45nm node (and beyond) feature widths and pitches.
This paper will show results of an evaluation of the critical dimension-scanning electron microscopy (CD-SEM)-based
metrology capabilities and limitations for the 193nm immersion and EUV lithography techniques that are suggested in
the International Technology Roadmap for Semiconductors. In this study, we will print wafers with these emerging
technologies and evaluate the performance of SEM-based metrology on these features. We will conclude with
preliminary findings on the readiness of SEM metrology for these new challenges.
The need for 3D metrology is becoming more urgent to address critical gaps in metrology for both lithographic and etch
processes. Current generation lithographic processing (ArF source, where λ=193 nm) sometimes results in photoresist
lines with re-entrant profiles or T-topping, as do many etch processes. A re-entrant profile misleads top-down metrology
into reading the critical dimension (CD) as too large. Recent advances in gate process technology also raise challenges to
traditional top-down metrology. One such example is the FinFET, which is truly a 3D device with 3D metrology needs.
The ability to measure the bottom width of a profile is crucial for process control. Recently, tilt-beam critical dimension-scanning
electron microscopy (CD-SEM) applications have been developed to measure the bottom CD of such features,
using the tilted-view to "see" the bottom, avoiding the feature's larger top. This is an important achievement, as the
bottom of a profile is the main feature of interest in many processes.
Estimation of sidewall angle (SWA) is also important. For several years, tilt-beam CD-SEM has been an available
technique for this measurement, with limited adoption by the litho-metrology community. However, in this paper we
will explore another method to use the tilt feature to measure average sidewall angle, based on edgewidth measurement
and the assumption of basic trapezoidal profile and known height and combined with the ability to sample multiple-features.
While it will not provide exact profile shape, this technique can be quite useful in providing average profile
information and will definitely exhibit good throughput. Samples used will be photoresist and etched FinFET structures
to measure sidewall angles. Correlations of the results to a traceable CD-atomic force microscopy (AFM) reference
measurement system are provided. Conclusions will show preliminary findings of the readiness of tilt-beam CD-SEM
for measuring profile and, by extension, the status of measuring 3D structures such as FinFETs, and using CD-SEM as a
direct control of lithographic tooling for T-topped photoresist profiles.
One goal of CD metrology is to monitor lithographic process control and how it relates to post-etch results. At present, in-fab process control for this purpose is achieved through top-down CD measurements. To acquire profile information requires destructive cross-section SEM measurements or time-consuming AFM measurements. To find height and profile information about a resist or etched structure directly on a CD-SEM, techniques using the combination of in-column beam tilt and stereographic imaging have been developed, implemented and improved on the Applied Materials NanoSEM-3D. This work is an extension of results previously published, although the tool used is greatly improved and the target feature stacks more thorough. The column of the NanoSEM-3D is designed to be able to electronically tilt the incident beam at small angles as it approaches the sample, through bending the beam within the column. Two images can be captured of the sidewall of the feature target, one at a smaller tilt angle and one at a larger tilt angle. Through matching common features between these two images, a reconstructed profile can be mathematically generated. A feature height and sidewall angle can be calculated, and general shape information such as top-rounding, footing and undercutting can also be displayed. To benchmark the effectiveness of this new technique, an experiment has been conducted to quantify the repeatability and reproducibility of height and sidewall angle measurements of lines of resist-on-poly and the resulting etched-poly lines, and correlate these to measurements of the these same profiles using XSEM to determine the accuracy of the technique. This study will span a reasonable lithographic process window. We hope to demonstrate the necessary precision and accuracy capability to non-destructively replace some cross-section work. In applying these techniques to a common etch bias problem, we also hope to demonstrate a strong correlation which can be used to directly predict post-etch behavior and serve as a model for other etch processes, work with which XSEM cross-sections have typically been used until now. The profile measurement technique is also applied to other CMOS features such as etched STI trench, resist and etched contact holes, and resist and etched damascene trenches and vias, with calculated tool precisions for feature height and sidewall angle.
One goal of CD metrology is to monitor lithographic process control and how it relates to post-etch results. At present, in-fab process control for this purpose is achieved through top-done CD measurements. To acquire profile information requires destructive cross-section SEM measurements or time- consuming atomic force microscope (AFM) measurements. To find height and profile information about a resist or etched structure directly on a CD-SEM, new techniques using the combination of in-column beam tilt and stereo graphic imaging have been developed and implemented on the Applied Materials VeraSEM-3D.