Optical Critical Dimension Metrology Research Articles

Integrated optical critical dimension metrology with Mueller matrix ellipsometry

Mueller matrix ellipsometry (MME) is commonly applied by the standalone instruments in semiconductor manufacturing process for films and nanostructures characterization. However, MME is rarely used in the integrated metrology optical critical dimension (IM OCD) tools due to the difficulty in extracting the parameters under varied azimuth angle conditions, which is induced by the rotation of R-θ wafer stage adapted to the restricted space. When the measurements on a same wafer are achieved under multiple azimuthal angles, the measured nanostructure parameters usually mismatch the baseline values provided by the manufacturer, and therefore lead to the unacceptable accuracy loss. In this paper, we propose an azimuthal sensitivity analysis and ridge regression algorithm (ASA-RR) to enable the MME-based IM OCD. The sensitivity to variability in azimuth angles is calculated by the local sensitivity analysis algorithm, and the result of which is applied as the weight factor for the spectrum input in the ridge regression. Experiments demonstrate that the ASA-RR algorithm provides more accurate results for the IM OCD, which satisfy the requirements in manufacturing.

Sensitivity enhancement in OCD metrology by optimizing azimuth angle based on the RCWA simulation

Monitoring process using optical critical dimension (OCD) metrology has been performed with a pre-fixed and unoptimized azimuth angle for all modules. Azimuth angle has a great influence on the optical sensitivity and can be easily tuned by changing the wafer rotation. In this study, we suggest the optimal azimuth angle for both FinFET and MOSFET device based on the rigorous coupled wave analysis (RCWA). For FinFET device, the optical sensitivity at the optimal angle was improved around 30% for two modules compared to that of pre-fixed angle. For MBCFET, we demonstrated that the simulated sensitivity is consistent with experiment and confirmed 10% sensitivity enhancement at 25 degree in experiment.

Fast scatterometric measurement of periodic surface structures in plasma-etching processes

To satisfy the continuous demand of ever smaller feature sizes, plasma etching technologies in microelectronics enable the fabrication of device structures in the nanometer range. To control these processes, real-time access to the structure’s dimensions is needed. We develop a special method of optical critical dimension metrology and evaluate the feasibility of reconstructing the etched dimensions from experimental reflectivity spectra of the surface, taken about every second. For a periodic 2D model structure etched into a silicon, we develop and test a fast algorithm.To reduce the computing time, we generate a library of spectra before the etching. We demonstrate that, by replacing the numerically simulated spectra in the reconstruction algorithm by spectra interpolated from the library, it is possible to compute the geometry parameters in times less than a second. Finally, to also reduce memory size and computing time for the library, we reduce the scanning of the parameter values to a sparse grid.

Potential application of far-field superlens in optical critical dimension metrology: a simulation study

Potential application of far-field superlens in optical critical dimension metrology: a simulation study

Effective–CD: a contribution toward the consideration of line edge roughness in the scatterometric critical dimension metrology

We present an overview of our research on the relation between line edge roughness (LER) and optical critical dimension metrology (OCD). Referring to a known fact that LER does have an impact on OCD, we discuss a novel approach that allows for its better understanding. Namely, we show that, in the presence of LER, one can observe a characteristic scatterometry-measured CD offset, which we call effective–CD. The fact that the effective–CD is characteristic renders it to be a good means of accessing the information about LER present on the CD. To assure the completeness of this overview, we begin by reviewing some previously published results, which have drawn our attention and first led us to observing the characteristic influence of LER on a CD measurement. Next, we extend our model-based simulations to confirm the presence of the effective–CD for complex-roughness models, and finally, we demonstrate an experimental verification of our effective–CD hypothesis. We are convinced that our approach will help to better understand the impact of LER on a CD measurement and will be considered a useful contribution to the development of measurement methods for challenging scenarios, in which realistic CD is affected by the presence of LER.

Optical critical dimension metrology for directed self-assembly assisted contact hole shrink

Directed self-assembly (DSA) is a potential patterning solution for future generations of integrated circuits. Its main advantages are high pattern resolution (∼10 nm), high throughput, no requirement of high-resolution mask, and compatibility with standard fab-equipment and processes. The application of Mueller matrix (MM) spectroscopic ellipsometry-based scatterometry to optically characterize DSA patterned contact hole structures fabricated with phase-separated polystyrene-b-polymethylmethacrylate (PS-b-PMMA) is described. A regression-based approach is used to calculate the guide critical dimension (CD), DSA CD, height of the PS column, thicknesses of underlying layers, and contact edge roughness of the post PMMA etch DSA contact hole sample. Scanning electron microscopy and imaging analysis is conducted as a comparative metric for scatterometry. In addition, optical model-based simulations are used to investigate MM elements’ sensitivity to various DSA-based contact hole structures, predict sensitivity to dimensional changes, and its limits to characterize DSA-induced defects, such as hole placement inaccuracy, missing vias, and profile inaccuracy of the PMMA cylinder.

Estimation of the convergence order of rigorous coupled-wave analysis for binary gratings in optical critical dimension metrology

In most cases of optical critical dimension metrology, when applying rigorous coupled-wave analysis to optical modeling, a high order of Fourier harmonics is usually set up to guarantee the convergence of the final results. However, the total number of floating point operations grows dramatically as the truncation order increases. Therefore, it is critical to choose an appropriate order to obtain high computational efficiency without losing much accuracy in the meantime. We show that the convergence order associated with the structural and optical parameters is estimated through simulation. The results indicate that the convergence order is linear with the period of the sample when fixing the other parameters, both for planar diffraction and conical diffraction. The illuminated wavelength also affects the convergence of a final result. With further investigations concentrated on the ratio of illuminated wavelength to period, it is discovered that the convergence order decreases with the growth of the ratio, and when the ratio is fixed, convergence order jumps slightly, especially in a specific range of wavelength. This characteristic could be applied to estimate the optimum convergence order of given samples to obtain high computational efficiency.

Sidewall effect of photomask by scanning electron microscope and optical critical dimension metrology

Sidewall effects on critical dimensions (CD) of photomasks are studied. CDs of patterns are measured by a scanning electron microscope (SEM) and a deep-UV microscope. CDs of the patterns are also calculated by an aerial image simulation. The sidewall effects on the measured CDs are compared with effects on the calculated CD. The results indicate that the sidewall effect on CDs by SEM differs from the effect on the aerial image, and that the effect on CDs by a deep-UV microscope corresponds to the effect on the aerial image. The factor of the sidewall effect on an aerial image is studied using an electric field calculation simulator. The result reveals that the factor is transmissivity. SEMs have no information about transmission, and therefore the effects differ from each other. A deep-UV microscope employs transmitted light just like a wafer aligner, and therefore the effects correspond to each other.

Line-profile and critical-dimension monitoring using a normal incidence optical CD metrology

As lithographic technology drives the minimum integrated circuit feature size toward 0.1 /spl mu/m and below, process tolerances for critical-dimension profile excursion are becoming increasingly demanding. In response, optical critical dimension metrology (OCD), an optical-wavelength light-diffraction technique, is emerging as a fast, accurate, and nondestructive sub-100-nm linewidth and profile monitor. As such, a detailed understanding of the correlation between OCD and existing metrology tools is required. Correlation between CD measurements using OCD and CD-scanning electron microscopy (SEM) techniques is investigated by measuring two types of important structures, e.g., photoresist gratings on a polysilicon gate film stack and shallow trench isolation. Intragrating CD variation is shown to account for scatter in the correlation plot. A qualitative line-profile correlation between cross-section SEM (X-SEM) and OCD is presented for photoresist gratings in a focus exposure matrix. Finally, a summary of the capability of OCD as a monitor for various processing stages is presented.