This paper discusses a novel methodology of material characterization that directly utilizes the scatterometry targets on
the product wafer to determine the optical properties (n&k) of various constituent materials. Characterization of optical
constants, or dispersions, is one of the first steps of scatterometry metrology implementation. A significant benefit of
this new technique is faster time-to-solution, since neither multiple single-film depositions nor multi-film depositions on
blanket/product wafers are needed, making obsolete a previously required-but very time-consuming-step in the
scatterometry setup. We present the basic elements of this revolutionary method, describe its functionality as currently
implemented, and contrast/compare results obtained by traditional methods of materials characterization with the new
method. The paper covers scatterometry results from key enabling metrology applications, like high-k metal gate (postetch
and post-litho) and Metal 2 level post-etch, to explore the performance of this new material characterization
approach. CDSEM was used to verify the accuracy of scatterometry solutions. Furthermore, Total Measurement
Uncertainty (TMU) analysis assisted in the interpretation of correlation data, and shows that the new technique provides
measurement accuracy results equivalent to, and sometimes better than, traditional extraction techniques.
Proc. SPIE. 6922, Metrology, Inspection, and Process Control for Microlithography XXII
KEYWORDS: Signal to noise ratio, Calibration, Scanning electron microscopy, Photoresist materials, Scatterometry, Process control, Line edge roughness, Structural design, Scatter measurement, Edge roughness
Fluctuations in the line edge of lithographic features, termed line edge roughness (LER) always
exist. At 32 nm line width (and below), LER can be a significant fraction of the feature
dimensions. LER can be simply detected by AFM or SEM techniques, however, fast and
nondestructive optical techniques should be developed in order to enable effective process
control. Optical scatterometry is preferable over other existing measurement techniques, due to
the relatively simple implementation in production and lower photoresist damage.
In this article we show simulations of LER by 3-dimensional Rigorous Coupled Wave Theory
(RCWT) calculations. The prediction of tool capabilities was done using simulations. The
outcome of these simulations results where analyzed and used for the basic design of photoresist
structures. The conclusions from sensitivity and correlation analysis of the simulation data were
verified against measured scatterometry data. Well-defined features with controlled LER, in the
range of 2.5 to 15nm, were fabricated by e-beam direct write technique (IMEC, Belgium). The
photoresist features we created were a large matrix of different scatterometry targets with varying
parameters of CD, Period, LER level, and LER frequency. These features were characterized by
electron microscopy and AFM in order to verify the LER values and a NovaScan 3090 system
and NovaMARS modeling software were used for the Scatterometry characterization.
To achieve better sensitivity to the lower roughness dimensions, we used an option of Effective
Medium Approximation (EMA) modeling for spectra analysis. Based on this reference data and
the scatterometry measurements we have developed a novel scatterometry method that is
sensitive to very low level of LER. This method is based on design of a special test structure
which can show better sensitivities than the basic noise levels of the tool. The basic idea in this
design is the calibration of the scatterometry measurement on a series of sites with LER steps. It
will be shown that LER changes of about 1 nm can be detected based on these designed test
structures. This is well below the normal capabilities of current optical tools.
Scatterometry is anew optical technique for profile measurement, capable of providing 2 or 3 dimensional profile information. The scatterometry optical system can be made small and economic enough to enable integration into each production tool and is therefore ideally suited for integrated process control. In addition, the range of applications for which this method applicable is wide: lithography photoresist profiles, etch structures, and even measurements of transparent layer thickness above arrays after CVD or CMP steps. Scatterometry is base don the measurement of the optical diffraction characteristics of aperiodic structure. Interpretation is done by evaluating the expected spectrum for a given profile, comparing the result to the measurement, and continuously fitting the profile parameters until a best fit is achieved. The leading physical model used in scatterometry is the Rigorous Coupled-Wave Analysis (RCWA). It is based on dissecting the profile into several layers and treating each as a waveguide with a fixed cross-section. The accuracy of the model is determined by the resolution of the dissection, and the number of Fourier modes retained in the calculation. RCWA gives very accurate results provided sufficient number of modes and layers are used. As calculation time scales linearly with number of layers and roughly cubic with number of modes, this usually leads to long calculation time, thus inhibiting real time interpretation which is especially required for integrated metrology. One possible solution is to consider alternative physical models to replace RCWA, such as Green's Function Integral Formulation. The selection between calculation methods should be determined by the calculation time per given spectrum accuracy. The latter should be determined for each application, considering the required parameters to be measured. In this paper we examine the use of two different models for several applications, which will be briefly described. It will be shown that is some important applications use of GFI may be preferable for real time calculations relative to RCWA.
Scatterometry is a promising method, capable of providing accurate profile information for a large range of applications. However, applying scatterometry to the production environment and applying it to APC is still difficult. In this paper we propose an alternative approach in which we apply a Neural Network to directly correlate scatterometry raw data and the lithography process control parameters. The proposed method is much easier to use than normal scatterometry, and can therefore be applied to APC much faster.
The common baseline for lithography process control is the assumption of the presence of a uniform substrate. This assumption may lead one to disregard variations introduced by the substrate thickness and optical properties. With the reduction of lithographic feature size, the impact of the substrate optical parameters, neglected until now, increases and reaches a level that has a tangible contribution to the CD budget. Non-uniformity of substrate and/or ARC create CD variations. A first step should be the measurement of the actual reflectivity variations, then compare the process reflectivity variations relative to the exposure latitude. The reason is the impact of the reflectivity of the substrate/photoresist interface on the amount of absorbed energy inside the photoresist itself. In this paper we present measurement data from the NovaScan 420 Integrated Thickness Monitoring (ITM) system that has been gathered during CMP process steps of several 0.18 and 0.25 technologies. Data was extracted from a large database collected worldwide. We analyzed the actual thickness scatter of the top polished oxide layer as well as measurements of the thickness variations of layers below the top oxide (e.g. TiN, Oxynitrides). These data were used for the analysis of the substrate/photoresist interface reflectivity variations at lithography steps. The analysis enabled simulation of 'across the wafer' reflectivity profiles at the DUV wavelengths. It appeared that some applications (e.g. photoresists on dielectrics) presented 20% 'across the wafer' variations in substrate reflectivity while others (e.g. photoresist on metal) did not show any variations. 'Wafer-to-wafer' trends reveal that the average values of the wafer indicate small changes, while site-to-site differences are consistent and non-negligible. Usually the tests that investigate the CD sensitivity to ARC or other substrate layers reflectivity do not reveal the expected correlation since the substrate optical reflectivity is not the dominant factor. PEB temperature, develop uniformity, dose control, focus control and metrology noise may have similar contributions that hide the CD correlation to reflectivity. Estimations for the actually achieved improvements for each application will be presented, together with the data collection requirements necessary to show the reduction of CD variations (such as: large sampling, integrated tool, angstrom level repeatability). Using the measured ITM system data, we suggest to use 'wafer-to-wafer' trends and wafer reflectivity maps to create compensation for the reflectivity variations by adjusting the coating thickness or the exposure dose respectively. In some cases the 'across the wafer' and 'wafer to wafer' data will illustrate the need for Integrated Metrology. This will enable the monitoring of the ARC thickness, refractive index, optical extinction coefficient, and also other layers that have an important contribution to the reflectivity.