In this paper, one of the major contributions to the OCD metrology error, resulting from
within-wafer variation of the refractive index/extinction coefficient (n/k) of the substrate, is
identified and quantified. To meet the required metrology accuracy for the 65-nm node and beyond,
it is suggested that n/k should be floating when performing the regression for OCD modeling. A
feasible way of performing such regression is proposed and verified. As shown in the presented
example, the measured CDU (3σ) with n/k fixed and n/k floating is 1.94 nm and 1.42 nm,
respectively. That is, the metrology error of CDU committed by assuming n/k fixed is more than
35% of the total CDU.
The advent of integrated metrology (IM) for lithography critical dimension (CD) control has been
widely discussed and debated. A number of factors are pushing chip makers in the direction of
IM implementation, including shrinking line widths and decreasing CD budgets, higher throughput
Litho cells, escalating cost and impracticality of stand-alone CD metrology, and reducing
overhead (or non value-add) time. These factors combine to make the question of IM for CD
control "when" rather than "if".
Scatterometry can provide a wealth of information about structures on a wafer including CD,
sidewall angle, and film thickness for various layers. Although this information unquestioningly
provides additional insight into the lithography process, in the end, the rate of IM implementation
depends on its return on investment (ROI). In this paper, we discuss the implementation of
integrated Optical Digital Profiling (iODPTM) on an advanced lithography track (Tokyo Electron
CLEAN TRACK LITHIUSTM). Included are discussions of lithography trends, metrology
requirements, and IM data flow and analysis. Various strategies for IM implementation are
presented along with their associated ROIs.
In this paper, three different types of spectral scatterometry hardware are compared using Timbre Technologies' Optical Digital Profiler (ODP) as a common software platform. The hardware under consideration includes a spectroscopic reflectometer (R), polarizing spectroscopic reflectometer (RP) and a spectroscopic ellipsometer (SE). Four advanced lithographic applications are evaluated-two from Spansion's 110-nm Flash memory technology line, and two from AMD's 90-nm logic process. ODP models are developed and optimized for each application and each type of hardware. Results include static and dynamic repeatability, throughput, correlation to incumbent metrology and correlation to cross-section. For each application, the authors also attempt to determine the level of model complexity supported by each hardware type, with special attention paid to the relative sensitivity of each system to changes in critical dimension (CD) and resist profile. The results generally indicate that the SE is the most sensitive hardware type while the R is the most stable. The RP occupies some form of middle ground on both counts. These generalizations are largely application dependent and clear differentiations do not always exist. Selecting the right spectral scatterometry hardware, therefore, is a function of one’s application complexity and control objectives.
Timbre's Optical Digital Profilometry (ODP) system is a scatterometry-based metrology. In lithography applications, the critical dimension (CD) is often patterned photoresist (PR) on an anti-reflective coating (ARC). When a patterned PR is exposed to the broadband light of the optical metrology tool, a change in reflectance may occur. For "sensitive" film stacks, the changing optical signals then produce changing ODP CD, sidewall angle, and film thickness measurements. This report summarizes the results of several resist and ARC stacks subjected to the repeated broadband light exposure of a Therma-Wave CCD-i reflectometry system. The purpose is to determine which resist-ARC stacks are significantly affected by repeated measurement exposure, and to quantify these effects. Our analysis shows that very little metrology exposure-induced change occurs for ArF resists. For KrF resists, the change is closely related to the type of KrF resist used; acetal-types incur large spectral changes upon repeated exposure, whereas ESCAP (Environmentally Stable Chemically Amplified Photoreist) resists measurements are very stable. Significant reduction of metrology induced spectral and CD change as achieved by incorporating a long-pass filter into the system. The changes due to a single measurement are negligible, however, they can be substantial for a sensitive material when characterizing metrology repeatability. Thus, it is recommended to use stable materials, such as oxide gratings, for metrology characterization.
The accurate measurement of CD (critical dimension) and its application to inline process control are key challenges for high yield and OEE (overall equipment efficiency) in semiconductor production. CD-SEM metrology, although providing the resolution necessary for CD evaluation, suffers from the well-known effect of resist shrinkage, making accuracy and stability of the measurements an issue. For sub-100 nm in-line process control, where accuracy and stability as well as speed are required, CD-SEM metrology faces serious limitations. In contrast, scatterometry, using broadband optical spectra taken from grating structures, does not suffer from such limitations. This technology is non-destructive and, in addition to CD, provides profile information and film thickness in a single measurement. Using Timbre's Optical Digital Profililometry (ODP) technology, we characterized the Process Window, using a iODP101 integrated optical CD metrology into a TEL Clean Track at IMEC. We demonstrate the Optical CD's high sensitivity to process change and its insensitivity to measurement noise. We demonstrate the validity of ODP modeling by showing its accurate response to known process changes built into the evaluation and its excellent correlation to CD-SEM. We will further discuss the intrinsic Optical CD metrology factors that affect the tool precision, accuracy and its correlation to CD-SEM.
As the semiconductor industry continues the transition to 300mm wafer factories, not only does the cost per wafer increase dramatically, but the number of eligible die (assuming equal die size) more than doubles. Given the parallel transition to design rules of 90nm and below, both the cost of production and the potential revenue from a 300mm wafer are vastly higher than that of a current 200mm wafer. For this reason alone, it is essential that wafer jeopardy, or the
number of wafers processed between metrology events, be reduced dramatically from the levels in a typical 200mm wafer line. The most promising method for achieving this is process tool-integrated metrology. Such systems allow rapid (in some cases near instantaneous) feedback on the process. Such a data stream, as input to an Advanced Process Control (APC) system, provides a volume of data and feedback lag time unparalleled by standalone metrology. In this case, critical dimension (CD) metrology is provided by a scatterometer integrated on a 200mm TEL CLEAN TRACK - ACT 8. The data, available on a wafer-by-wafer basis, is uploaded to the factory host where the APC application can update its state estimation before the entire lot has even completed processing.