Semiconductor nanowires are important materials for quantum transport experiments and are used in research on qubits. Extended arrays of nanowires can be grown bottom-up by Molecular Beam Epitaxy (MBE). The full process involves several steps. When fabricating nanowires, a common practice is to follow a well-established recipe and only characterize the finalized materials. If the final wires are found to be flawed, the process must be repeated with new parameters. It is therefore desirable to have a characterization method to monitor the process before and after each fabrication step. Conventional characterization techniques such as SEM are time-consuming and, in some cases, damage the samples, e.g. before and after an electron beam lithography process. Scatterometry is fast, accurate, non-destructive and is already used in the semiconductor industry. In this work, it is demonstrated that the imaging scatterometry technique is capable of monitoring the MBE fabrication process of InAs-nanowire arrays during the different process steps. Relevant parameters such as thin film thickness, hole depth, and diameter, etc., are found with nm precision for a macroscopic area in a few minutes. Using this approach, we demonstrate that errors can be caught early in the process and ultimately save resources while assuring a high quality of the final material.
Accurate scatterometry and ellipsometry characterization of non-perfect thin films and nanostructured
surfaces are challenging. Imperfections like surface roughness make the associated modelling and
inverse problem solution difficult due to the lack of knowledge about the imperfection on the surface.
Combining measurement data from several instruments increases the knowledge of non-perfect
surfaces. In this paper we investigate how to incorporate this knowledge of surface imperfection into
inverse methods used in scatterometry and ellipsometry using the Rigorous Coupled Wave Analysis.
Three classes of imperfections are examined. The imperfections are introduced as periodic structures
with a super cell periods ten times larger than the simple grating period. Two classes of imperfections
concern the grating and one class concern the substrate. It is shown that imperfections of a few
nanometers can severely change the reflective response on silicon gratings. Inverse scatterometry
analyses of gratings with imperfection using simulated data with white noise have been performed. The
results show that scatterometry is a robust technology that is able to characterize grating imperfections
provided that the imperfection class is known.
We report a correlation between the scattering value “Aq” and the ISO standardized roughness parameter Rq. The Aq value is a measure for surface smoothness, and can easily be determined from an optical scattering measurement. The correlation equation extrapolates the Aq value from a narrow measurement range of ±16° from specular to a broader range of ±80°, corresponding to spatial surface wavelengths of 0.8 μm to 25 μm, and converts the Aq value to the Rq value for the surface.
Furthermore, we present an investigation of the changes in scattering intensities, when a surface is covered with a thin liquid film. It is shown that the changes in the angular scattering intensities can be compensated for the liquid film, using empirically determined relations. This allows a restoration of the “true” scattering intensities which would be measured from a corresponding clean surface. The compensated scattering intensities provide Aq values within 5.7 % ± 6.1 % compared to the measurements on clean surfaces.
High quality scatterometry standard samples have been developed to improve the tool matching between different scatterometry methods and tools as well as with high resolution microscopic methods such as scanning electron microscopy or atomic force microscopy and to support traceable and absolute scatterometric critical dimension metrology in lithographic nanomanufacturing. First samples based on one dimensional Si or on Si3N4 grating targets have been manufactured and characterized for this purpose. The etched gratings have periods down to 50 nm and contain areas of reduced density to enable AFM measurements for comparison. Each sample contains additionally at least one large area scatterometry target suitable for grazing incidence small angle X-ray scattering. We present the current design and the characterization of structure details and the grating quality based on AFM, optical, EUV and X-Ray scatterometry as well as spectroscopic ellipsometry measurements. The final traceable calibration of these standards is currently performed by applying and combining different scatterometric as well as imaging calibration methods. We present first calibration results and discuss the final design and the aimed specifications of the standard samples to face the tough requirements for future technology nodes in lithography.
Scatterometry is a common technique for dimensional characterisation of nanostructures in the semiconductor industry. Currently this technique is limited to relative measurements for process development and process control. Although the high sensitivity of scatterometry is well known, it is not yet applied for absolute measurements of critical dimensions (CD) and quality control due to the lack of traceability. Thus we aim to establish scatterometry as traceable and absolute metrological method for dimensional measurements. Suitable high quality calibrated scatterometry reference standard samples are currently developed as one important step to enable traceable absolute measurements in industrial applications. The reference standard materials will base either on Si or on Si3N4. A traceable calibration of these standards will be provided by applying and combining different scatterometric as well as imaging calibration methods. First Silicon test samples have been manufactured and characterised for this purpose. The etched Si gratings have periods down to 50 nm and contain areas of reduced density to enable AFM measurements for comparison. We present the current design and first characterisations of structure details and the grating quality based on AFM measurements, optical, EUV and X-Ray scatterometry as well as spectroscopic ellipsometry. Finally we discuss possible final designs and the aimed specifications of the standard samples to face the tough requirements for future technology nodes in lithography.
Hollow-core photonic bandgap fibers guide light using diffraction rather than total internal reflection as is the case with
normal single- mode communications fibers. The fibers consist of a hollow capillary (~19 micrometers in diameter)
surrounded by capillary (~4 micrometers in diameter) arranged in a honey-comb like structure. The honey-comb
structure scatters light in the core such that light within the bandgap wavelengths cannot escape from the core. However,
the bandgap properties greatly depend on the accuracy with which the microstructures can be controlled during the
fabrication process. For measuring the geometrical properties of hollow core photonic crystal fibers with a honeycomb
cladding structure we use an angular scatterometric setup. For analyzing the experimentally obtained data we rigorously compute the scattering signal by solving Maxwell's equations with finite-element methods. This contribution focuses on the numerical analysis of the problem. A convergence analysis demonstrates that we reach highly accurate solutions. Our results show very good qualitative agreement between experimental and numerical results. We furthermore demonstrate concepts for accurately monitoring dimensional parameters in the fiber manufacturing process.
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.
We present an innovative method Optical Diffraction Microscopy (ODM). for the simultaneous measurement of specular and non-specular diffraction patterns of sub-micron periodic structures. A sample is illuminated with broadband light and the diffraction pattern is collected by using a pair of ellipsoidal mirrors, optical fibers and a spectrometer. This method allows for rapid measurements and makes used of the Rigorous Coupled Wave algorithm for data analysis. In the present work the method has been applied to binary and multi-layer sub-micron gratings. A series of binary gratings with periods of 318 nm and 360 nm with different exposure levels of the photoresist were investigated. We succeded in characterize underexposed, ideally exposed and overexposed photoresist grating profiles. The measurements are well-suited to determine the delivered exposure energy density to photoresist gratings. The ODM technique may thus be applied to specify the exposure window and as a feedback in order to adjust the exposure energy density on-line. The homogeneity of a grating on multi-layered substrate has been investigated. Heights and duty cycles ranging from 50 nm to 55 nm and 0.25 to 0.97, respectively, have been found. AFM measurements of the gratings verify the ODM results and demonstrate that the ODM technique can be used to determine grating topology.
Atomic force microscopy (AFM) and optical diffraction microscopy (ODM) are used to measure the profiles of grating grooves with depths much larger than their widths. Gratings with these features are essential in numerous optical devices such as spectrometers, monochromators and for the production of many fibre Bragg gratings. However, measurement of the physical shape is inherently difficult but necessary for the understanding of their function and in order to improve the manufacturing process. After a thorough calibration of an AFM and by tilting the plane of the grating by up to 17° relative to the symmetry axis of the sensing probe we measured accurately and traceably the sidewall angle and the sidewall profile in a non-destructive way. ODM is a new method where the intensity of the optical field diffracted is measured as a function of the frequency and an inverse algorithm is used to reconstruct the surface profile. It is fast, non-destructive, and it gives height and filling degree of a grating very accurately. As example a high aspect ratio grating with period p of 220 nm, depths d of ≈300 nm, and sidewall angles
γ of approximately ≈90° and filling degree f of ≈40 % were examined. Standard uncertainties as low as u(d) = 3 nm, u( α) = 0.4° and u(f) = 3.1 % were achieved. Despite the fact that the AFM responds to the physical surface and ODM responds to the optical
properties of the material we find that the results are in very good agreement and consistent with (destructive) scanning electron microscopy measurements of the filling degree.