Diffraction-based overlay metrology using angular-multiplexed acquisition of dark-field digital holograms

Multi-spectral snapshot diffraction-based overlay metrology

Diffraction-based overlay (DBO) metrology has been successfully introduced to deal with the tighter overlay control in modern semiconductor manufacturing. Moreover, DBO metrology typically needs to be performed at multiple wavelengths to achieve accurate and robust measurement in the presence of overlay target deformations. In this Letter, we outline a proposal for multi-spectral DBO metrology based on the linear relation between the overlay errors and the combinations of off-diagonal-block Mueller matrix elements ΔM = M_ij - ( - 1)^jM_ji (i = 1, 2; j = 3, 4) associated with the zeroth-order diffraction of overlay target gratings. We propose an approach that can realize snapshot and direct measurement of ΔM over a broad spectral range without any rotating or active polarization component. The simulation results demonstrate the capability of the proposed method for multi-spectral overlay metrology in a single shot.

Designing Highly Precise Overlay Targets for Asymmetric Sidewall Structures Using Quasi-Periodic Line Widths and Finite-Difference Time-Domain Simulation

The present study introduces an optimized overlay target design to minimize the overlay error caused by asymmetric sidewall structures in semiconductor manufacturing. To achieve this goal, the overlay error formula was derived by separating the asymmetric bottom grating structure into symmetric and asymmetric parts. Based on this formula, it was found that the overlay target design with the linewidth of the bottom grating closed to the grating period could effectively reduce the overlay error caused by the sidewall asymmetry structure. Simulation results demonstrate that the proposed design can effectively control the measurement error of different wavelengths within ±0.3 nm, even under varying sidewall angles and film thicknesses. Overall, the proposed overlay target design can significantly improve the overlay accuracy in semiconductor manufacturing processes.

Field-position dependent apodization in dark-field digital holographic microscopy for semiconductor metrology

Measuring overlay between two layers of semiconductor devices is a crucial step during electronic chip fabrication. We present dark-field digital holographic microscopy that addresses various overlay metrology challenges that are encountered in the semiconductor industry. We present measurement results that show that the point-spread function of our microscope depends on the position in the field-of-view. We will show that this novel observation can be explained by a combination of the finite bandwidth of the light source and a wavelength-dependent focal length of the imaging lens. Moreover, we will also present additional experimental data that supports our theoretical understanding. Finally, we will propose solutions that reduce this effect to acceptable levels.

Multi spectral holographic ellipsometry for a complex 3D nanostructure

We present an innovative ellipsometry technique called self-interferometric pupil ellipsometry (SIPE), which integrates self-interference and pupil microscopy techniques to provide the high metrology sensitivity required for metrology applications of advanced semiconductor devices. Due to its unique configuration, rich angle-resolved ellipsometric information from a single-shot hologram can be extracted, where the full spectral information corresponding to incident angles from 0° to 70° with azimuthal angles from 0° to 360° is obtained, simultaneously. The performance and capability of the SIPE system were fully validated for various samples including thin-film layers, complicated 3D structures, and on-cell overlay samples on the actual semiconductor wafers. The results show that the proposed SIPE system can achieve metrology sensitivity up to 0.123 nm. In addition, it provides small spot metrology capability by minimizing the illumination spot diameter up to 1 µm, while the typical spot diameter of the industry standard ellipsometry is around 30 µm. As a result of collecting a huge amount of angular spectral data, undesirable multiple parameter correlation can be significantly reduced, making SIPE ideally suited for solving several critical metrology challenges we are currently facing.

Dark-field scattering image compression using a sparse matrix

Dark-field scattering imaging is an imaging method with high contrast and high sensitivity. It has been widely employed in optical components evaluation, biomedical detection, semiconductor manufacturing, etc. However, useless background information causes data redundancy, which increases unnecessary time-space costs in processing. Furthermore, the problem is particularly serious in high-resolution imaging systems for large-aperture components. The dark-field scattering image compression (DFSIC) based on the compressed sparse row is proposed to solve this problem. The compression method realizes local data access for a sparse matrix. The result of the experiments shows that the average time-space consumption of the DFSIC is reduced to less than 2%, compared with the raw image structure, and is still kept below 68% in dense cases. This method provides a more efficient program implementation for the dark-field scattering imaging and exhibits potential in the application of the optical detection with large scale.

Aberration calibration and correction with nano-scatterers in digital holographic microscopy for semiconductor metrology

Overlay metrology measures pattern placement between two layers in a semiconductor chip. The continuous shrinking of device dimensions drives the need to explore novel optical overlay metrology concepts that can address many of the existing metrology challenges. We present a compact dark-field digital holographic microscope that uses only a single imaging lens. Our microscope offers several features that are beneficial for overlay metrology, like a large wavelength range. However, imaging with a single lens results in highly aberrated images. In this work, we present an aberration calibration and correction method using nano-sized point scatterers on a silicon substrate. Computational imaging techniques are used to recover the full wavefront error, and we use this to correct for the lens aberrations. We present measured data to verify the calibration method and we discuss potential calibration error sources that must be considered. A comparison with a ZEMAX calculation is also presented to evaluate the performance of the presented method.