Scatterometry is a fast, precise and low cost way to determine the mean pitch and dimensional parameters of periodic structures with lateral resolution of a few nanometer. It is robust enough for in-line process control and precise and accurate enough for metrology measurements. Furthermore, scatterometry is a non-destructive technique capable of measuring buried structures, for example a grating covered by a thick oxide layer. As scatterometry is a non-imaging technique, mathematical modeling is needed to retrieve structural parameters that describe a surface. In this review, the three main steps of scatterometry are discussed: the data acquisition, the simulation of diffraction efficiencies and the comparison of data and simulations. First, the intensity of the diffracted light is measured with a scatterometer as a function of incoming angle, diffraction angle and/or wavelength. We discuss the evolution of the scatterometers from the earliest angular scatterometers to the new imaging scatterometers. The basic principle of measuring diffraction efficiencies in scatterometry has remained the same since the beginning, but the instrumental improvements have made scatterometry a state-of-the-art solution for fast and accurate measurements of nano-textured surfaces. The improvements include extending the wavelength range from the visible to the extreme ultra-violet range, development of Fourier optics to measure all diffraction orders simultaneously, and an imaging scatterometer to measure area of interests smaller than the spot size. Secondly, computer simulations of the diffraction efficiencies are discussed with emphasis on the rigorous coupled-wave analysis (RCWA) method. RCWA has, since the mid-1990s, been the preferred method for grating simulations due to the speed of the algorithms. In the beginning the RCWA method suffered from a very slow convergence rate, and we discuss the historical improvements to overcome this challenge, e.g. by the introduction of Li's factorization rules and the introduction of the normal vector method. The third step is the comparison, where the simulated diffraction efficiencies are compared to the experimental data using an inverse modeling approach. We discuss both a direct optimization scheme using a differential evolution algorithm and a library search strategy where diffraction efficiences of expected structures are collected in a database. For metrology measurements two methods are described for estimating the uncertainty of the fitting parameters. The first method is based on estimating the confidence limits using constant chi square boundaries, which can easily be computed when using the library search strategy. The other method is based on calculating the covariances of all the free parameters using a least square optimization. Scatterometry is already utilized in the semiconductor industry for in-line characterization. However, it also has a large potential for other industrial sectors, including sectors making use of injection molding or roll-2-roll fabrication. Using the library search strategy, the comparison can be performed in ms, making in-line characterization possible and we demonstrate that scatterometry can be used for quality control of injection molded nano-textured plastic samples. With the emerging methods of highly parallel manufacturing of nano-textured devices, scatterometry has great potential to deliver a characterization method for in-line quality control and metrology measurements, which is not possible with conventional characterization techniques. However, there are some open challenges for the scatterometry techniques. These include corrections for measuring on non-ideal samples with a large surface roughness or line-edge roughness and the path towards performing traceable scatterometry measurements.

http://iopscience.iop.org/article/10.1088/2051-672X/4/2/023003/pdf

Scatterometry—fast and robust measurements of nano-textured surfaces

We present an innovating instrument based on optical Fourier transform (OFT) capable to measure simultaneously the specular and non specular diffraction pattern of sub-micronic periodic structures. The sample is illuminated at fixed wavelength (green laser) versus a large angular aperture both in incidence (0 to 80°) and azimuth (0 to 180°). In the present paper we focus on the possibility to measure line edge roughness (LER) and line width roughness (LWR) using this new technique. To understand the problem, different gratings with artificial periodic LER and LWR roughness have been fabricated and characterized precisely by atomic force microscopy (AFM). Different light scattering measurements have been performed using the OFT instrument with different illuminations in order to understand precisely the optical behavior of these systems. We show that we can distinguish LER and LWR by measuring simultaneously the diffracted contributions coming from the grating and from the periodic roughness. In phase LER with small LWR does not give first order diffraction contribution for the periodic roughness. In contrast, LER in opposite phase with large LWR gives a strong contribution for the first order of diffraction of the periodic roughness. In any case, the sensitivity to LER and LWR is better than 5nm for 500nm period gratings measured at 532nm. This result can be extended to samples with real LER and LWR. It shows without ambiguity that simultaneous measurement of the specular and diffracted light diffraction patterns is necessary to extract separately the two parameters.

Optical Fourier transform scatterometry for LER and LWR metrology

Line edge roughness (LER) has been identified as a potential source of uncertainty in optical scatterometry measurements. Characterizing the effect of LER on optical scatterometry signals is required to assess the uncertainty of the measurement. However, rigorous approaches to modeling the structures that are needed to simulate LER can be computationally expensive. In this work, we compare the effect of LER on scatterometry signals computed using an effective medium approximation (EMA) to those computed with realizations of rough interfaces. We find that for correlation lengths much less than the wavelength but greater than the rms roughness, an anisotropic EMA provides a satisfactory approximation in the cases studied.

/pdf/effective-medium-approximations-for-modeling-optical-2i025cpmzs.pdf

Effective medium approximations for modeling optical reflectance from gratings with rough edges.

We demonstrate optical critical dimension measurement of lines in silicon grating targets using back focal plane scatterfield icroscopy. In this technique, angle-resolved diffraction signatures are obtained from grating targets by imaging the back focal plane of a brightfield microscope that has been modified to allow selection of the angular distribution and polarization of the incident illumination. The target line profiles, including critical dimension linewidth and sidewall angle, are extracted using a scatterometry method that compares the diffraction signatures to a library of theoretical signatures. Because we use the zero-order component of the diffraction, the target features need not be resolved in order to obtain the line profile. We extracted line profiles from two series of targets with fixed pitch but varying linewidth: a subresolution 300-nm-pitch series, and a resolved 600-nm-pitch series. Linewidths of 131 nm to 139 nm were obtained, with nanometer-level sensitivity to linewidth, and a linear relationship of linewidth obtained from scatterfield microscopy to linewidth measured by scanning electron microscopy was demonstrated. Conventional images can be easily collected on the same microscope, providing a powerful tool for combining imaging metrology with scatterometry for optical critical dimension measurement.

/pdf/optical-critical-dimension-measurement-of-silicon-grating-2fqed3ues5.pdf

Optical critical dimension measurement of silicon grating targets using back focal plane scatterfield microscopy

Supported by the European Commission and EURAMET, a consortium of 10 participants from national metrology
institutes, universities and companies has recently started a joint research project with the aim of overcoming current
challenges in optical scatterometry for traceable linewidth metrology and to establish scatterometry as a traceable and
absolute metrological method for dimensional measurements. This requires a thorough investigation of the influence of all significant sample, tool and data analysis parameters, which affect the scatterometric measurement results. For this purpose and to improve the tool matching between scatterometers, CD-SEMs and CD-AFMs, experimental and
modelling methods will be enhanced. The different scatterometry methods will be compared with each other and with
specially adapted atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurement systems.
Additionally novel methods for sophisticated data analysis will be developed and investigated to reach significant
reductions of the measurement uncertainties in critical dimension (CD) metrology. To transfer traceability to industrial
applications of scatterometry an important step and one final goal of this project is the realisation of different waferbased
reference standard materials for calibration of scatterometers. The approaches to reach these goals and first design
considerations and preliminary specification of the scatterometry standards are presented and discussed.

First steps towards a scatterometry reference standard

We present an innovating method to measure the overlay by scatterometry using an optical Fourier transform (OFT) based system. In order to measure the overlay of patterned layers α and β, one line grating is placed in layer α and another in layer β. The two gratings have the same pitch and their lines are parallel. The whole scattering pattern of the double grating structure is then measured at fixed wavelength in a large range of incidence (0 to 80°) and for all the azimuth angles. This measurement is very rapid thanks to the OFT and not sensitive to vibration. The main advantage of OFT compared to standard OCD techniques like normal incidence reflectometry or spectroscopic ellipsometry is that the scattering pattern is more sensitive to overlay at an azimuth depending on the pitch value which is never parallel or perpendicular to the grooves of the gratings. In addition, the optical response is also sensitive to the sign of the overlay in addition to its amplitude. In a second method, we propose to measure the overlay simultaneously along the two directions of the plane using two bi-periodic structures patterned in layer α and β. By using OFT it is possible to deduce directly from the whole diffracted pattern, the overlay signs and amplitudes along both directions of the plane. The paper presents some simulations and some experimental results to illustrate this new method.

Improved CD and overlay metrology using an optical fourier transform instrument

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Optical Fourier transform scatterometry for LER and LWR metrology

Improved CD and overlay metrology using an optical fourier transform instrument