Microsphere-assisted, nanospot, non-destructive metrology for semiconductor devices

Near-field terahertz time-domain spectroscopy for in-line electrical metrology of semiconductor integration processes for memory

Monitoring electrical properties in semiconductor integration processes is crucial in identifying electrical defects that determine the reliability and performance of metal oxide semiconductor field-effect transistors. A non-destructive in-line metrology technique using terahertz (THz) waves was developed to observe electrical properties between semiconductor integration processes. By combining near-field microprobes with THz time-domain spectroscopy (TDS), sub-10 μm resolution was achieved, enabling the measurement of on-chip micro-patterns. The system was integrated into a memory production line and demonstrated consistent results with conventional destructive methods. The TDS signal correction method effectively suppressed signal variations in unwanted layers. The results of non-invasive THz TDS measurements of tungsten films deposited by three different processes were consistent with those obtained by four- point probe method. We also non-destructively detected differences in THz transmission at the gate-oxide/Si-substrate interface due to the infiltration of nitrogen species after the thermal nitridation process at nitridation temperatures ranging from 670 to 730 °C, which were consistent with the results of secondary ion mass spectrometry. Our in-line near-field THz TDS will predict electrical performance immediately after the process, allowing for rapid correction of production conditions.

Ultrafast laser-enabled optoacoustic characterization of three-dimensional, nanoscopic interior features of microchips

The recent advances in micromanufacturing have been pushing boundaries of the new generation of semiconductor devices, which, in the meantime, brings new challenges in the material and structural characterization - a key step to ensure the device quality through the micromanufacturing process. An ultrafast laser-enable optoacoustic characterization methodology is developed, targeting in situ calibration and delineation of the three-dimensional (3-D), nanoscopic interior features of opaque semiconductor chips. With the guidance of ultrafast electron-phonon coupling effect and velocity-perturbated optical interference, a femtosecond-laser pump-probe set-up based on Sagnac interferometer is configured to generate and acquire picosecond ultrasonic bulk waves (P-UBWs) traversing the microchips. The interior features of the microchips shift the phase of acquired P-UBW signals, reflected in the perturbed probe laser beam. The phase shifts are calibrated to compute signal correlation of P-UBW signals between different acquiring positions, whereby to delineate the interior features in an intuitive manner. The approach is experimentally validated by characterizing nanoscopic, invisible interior aurum(Au)-gratings with periodically varied depths in typical microchips. Results highlight that the 3-D nanoscopic features of the microchips can be revealed with a microscopic and a nanoscopic spatial resolution, respectively along the transverse and depth directions of the chip, where the Au-gratings become "visible" with a depth variance of a few tens of nanometers only. This proposed approach has provided a fast, nondestructive approach to "see" through an opaque microchip with a nanoscopic resolution.

Microsphere-assisted super-resolution polarized light microscopy for polarization-sensitive materials

Polarization sensitivity is a fundamental phenomenon observed in nature, and its application is vital for advancing scientific discoveries. Here, we present a microsphere-assisted polarized light microscopy method that enables high-quality, label-free super-resolution imaging of polarization-sensitive materials. We investigated the imaging performance of this method using various samples, including colloidal crystals and grating structures. Both simulations and experiments were performed to explore the underlying physical mechanisms driving the enhancement in imaging quality. The results of this work have potential values for the fields of polarized light microscopy and super-resolution imaging.

Nanophotonic inspection of deep-subwavelength integrated optoelectronic chips

Artificial nanostructures with ultrafine and deep-subwavelength features have emerged as a paradigm-shifting platform to advanced light-field management, becoming key building blocks for high-performance integrated optoelectronics and flat optics. However, direct optical inspection of integrated chips remains a missing metrology gap that hinders quick feedback between design and fabrications. Here, we demonstrate that photothermal nonlinear scattering microscopy can be used for direct imaging and resolving of integrated optoelectronic chips beyond the diffraction limit. We reveal that the inherent coupling among deep-subwavelength nanostructures supporting leaky resonances allows for the pronounced heating effect to access reversible nonlinear modulations of the confocal reflection intensity, yielding optical resolving power down to 80 nm (~λ/7). The versatility of this approach has been exemplified by imaging silicon grating couplers and metalens with minimum critical dimensions of 100 nm, as well as central processing unit chip with 45-nm technology, unfolding the long-sought possibility of in situ, nondestructive, high-throughput optical inspection of integrated optoelectronic and nanophotonic chips.

Reflectance mapping with microsphere-assisted white light interference nanoscopy

The characterisation of novel materials presents a challenge that requires new and original developments. To face some of these demands for making measurements at the nanoscale, a new microsphere-assisted white light interference nanoscope performing local reflectance mapping is presented. This technique presents the advantages of being non-destructive, full-field and label-free. A 145 μm diameter microsphere, glued to the end of an optical fiber, is inserted inside the white light interference microscope to improve the lateral resolution from 940 nm to 520 nm. The acquisition and the Fourier transform processing of a stack of interference images superimposed on the virtual image produced by the microsphere allows the extraction of the local reflectance over a wavelength range of 460 nm to 900 nm and a field of view of 8 μm in diameter. The enhancement in the lateral resolution of the reflectance is demonstrated through the spectral distinction of neighboring ripples on a laser-textured colored stainless-steel sample that cannot be resolved without the microsphere, on regions with a surface of 279 × 279 nm2 horizontally spaced 279 nm apart. Future improvements could potentially lead to a lateral resolution of reflectance measurement over a 100 nm diameter area in air, paving the way to sub-diffraction reflectance mapping.

Intensity-Product-Based Optical Sensing to Beat the Diffraction Limit in an Interferometer

The classically defined minimum uncertainty of the optical phase is known as the standard quantum limit or shot-noise limit (SNL), originating in the uncertainty principle of quantum mechanics. Based on the SNL, the phase sensitivity is inversely proportional to K, where K is the number of interfering photons or statistically measured events. Thus, using a high-power laser is advantageous to enhance sensitivity due to the K gain in the signal-to-noise ratio. In a typical interferometer, however, the resolution remains in the diffraction limit of the K = 1 case unless the interfering photons are resolved as in quantum sensing. Here, a projection measurement method in quantum sensing is adapted for classical sensing to achieve an additional K gain in the resolution. To understand the projection measurements, several types of conventional interferometers based on N-wave interference are coherently analyzed as a classical reference and numerically compared with the proposed method. As a result, the Kth-order intensity product applied to the N-wave spectrometer exceeds the diffraction limit in classical sensing and the Heisenberg limit in quantum sensing, where the classical N-slit system inherently satisfies the Heisenberg limit of π/N in resolution.

Microsphere-assisted hyperspectral imaging: super-resolution, non-destructive metrology for semiconductor devices

As semiconductor devices shrink and their manufacturing processes advance, accurately measuring in-cell critical dimensions (CD) becomes increasingly crucial. Traditional test element group (TEG) measurements are becoming inadequate for representing the fine, repetitive patterns in cell blocks. Conventional non-destructive metrology technologies like optical critical dimension (OCD) are limited due to their large spot diameter of approximately 25 μm, which impedes their efficacy for detailed in-cell structural analysis. Consequently, there is a pressing need for small-spot and non-destructive metrology methods. To address this limitation, we demonstrate a microsphere-assisted hyperspectral imaging (MAHSI) system, specifically designed for small spot optical metrology with super-resolution. Utilizing microsphere-assisted super-resolution imaging, this system achieves an optical resolution of 66 nm within a field of view of 5.6 μm × 5.6 μm. This approach effectively breaks the diffraction limit, significantly enhancing the magnification of the system. The MAHSI system incorporating hyperspectral imaging with a wavelength range of 400-790 nm, enables the capture of the reflection spectrum at each camera pixel. The achieved pixel resolution, which is equivalent to the measuring spot size, is 14.4 nm/pixel and the magnification is 450X. The MAHSI system enables measurement of local uniformity in critical areas like corners and edges of DRAM cell blocks, areas previously challenging to inspect with conventional OCD methods. To our knowledge, this approach represents the first global implementation of microsphere-assisted hyperspectral imaging to address the metrology challenges in complex 3D structures of semiconductor devices.

Snapshot Mueller spectropolarimeter imager

We introduce an imaging system that can simultaneously record complete Mueller polarization responses for a set of wavelength channels in a single image capture. The division-of-focal-plane concept combines a multiplexed illumination scheme based on Fourier optics together with an integrated telescopic light-field imaging system. Polarization-resolved imaging is achieved using broadband nanostructured plasmonic polarizers as functional pinhole apertures. The recording of polarization and wavelength information on the image sensor is highly interpretable. We also develop a calibration approach based on a customized neural network architecture that can produce calibrated measurements in real-time. As a proof-of-concept demonstration, we use our calibrated system to accurately reconstruct a thin film thickness map from a four-inch wafer. We anticipate that our concept will have utility in metrology, machine vision, computational imaging, and optical computing platforms.

Wide-Field and Real-Time Super-Resolution Optical Imaging By Titanium Dioxide Nanoparticle-Assembled Solid Immersion Lens

Super-resolution optical imaging techniques can break the optical diffraction limit, thus providing unique opportunities to visualize the microscopic world at the nanoscale. Although near-field optical microscopy techniques have been proven to achieve significantly improved imaging resolution, most near-field approaches still suffer from a narrow field of view (FOV) or difficulty in obtaining wide-field images in real time, which may limit their widespread and diverse applications. Here, the authors experimentally demonstrate an optical microscope magnification and image enhancement approach by using a submillimeter-sized solid immersion lens (SIL) assembled by densely-packed 15 nm TiO₂ nanoparticles through a silicone oil two-step dehydration method. This TiO₂ nanoparticle-assembled SIL can achieve both high transparency and high refractive index, as well as sufficient mechanical strength and easy-to-handle size, thus providing a fast, wide-field, real-time, non-destructive, and low-cost solution for improving the quality of optical microscopic observation of a variety of samples, including nanomaterials, cancer cells, and living cells or bacteria under conventional optical microscopes. This study provides an attractive alternative to simplify the fabrication and applications of high-performance SILs.

Self-Sensing Scanning Superlens for Three-Dimensional Noninvasive Visible-Light Nanoscale Imaging on Complex Surfaces

Microsphere-assisted super-resolution imaging technology offers label-free, real-time dynamic imaging via white light, which has potential applications in living systems and the nanoscale detection of semiconductor chips. Scanning can aid in overcoming the limitations of the imaging area of a single microsphere superlens. However, the current scanning imaging method based on the microsphere superlens cannot achieve super-resolution optical imaging of complex curved surfaces. Unfortunately, most natural surfaces are composed of complex curved surfaces at the microscale. In this study, we developed a method to overcome this limitation through a microsphere superlens with a feedback capability. By maintaining a constant force between the microspheres and the sample, noninvasive super-resolution optical imaging of complex abiotic and biological surfaces was achieved, and the three-dimensional information on the sample was simultaneously obtained. The proposed method significantly expands the universality of scanning microsphere superlenses for samples and promotes their widespread use.

Fourier ptychographic topography

Topography measurement is essential for surface characterization, semiconductor metrology, and inspection applications. To date, performing high-throughput and accurate topography remains challenging due to the trade-off between field-of-view (FOV) and spatial resolution. Here we demonstrate a novel topography technique based on the reflection-mode Fourier ptychographic microscopy, termed Fourier ptychograhpic topography (FPT). We show that FPT provides both a wide FOV and high resolution, and achieves nanoscale height reconstruction accuracy. Our FPT prototype is based on a custom-built computational microscope consisting of programmable brightfield and darkfield LED arrays. The topography reconstruction is performed by a sequential Gauss-Newton-based Fourier ptychographic phase retrieval algorithm augmented with total variation regularization. We achieve a synthetic numerical aperture (NA) of 0.84 and a diffraction-limited resolution of 750 nm, increasing the native objective NA (0.28) by 3×, across a 1.2 × 1.2 mm² FOV. We experimentally demonstrate the FPT on a variety of reflective samples with different patterned structures. The reconstructed resolution is validated on both amplitude and phase resolution test features. The accuracy of the reconstructed surface profile is benchmarked against high-resolution optical profilometry measurements. In addition, we show that the FPT provides robust surface profile reconstructions even on complex patterns with fine features that cannot be reliably measured by the standard optical profilometer. The spatial and temporal noise of our FPT system is characterized to be 0.529 nm and 0.027 nm, respectively.

Surface plasmon-enhanced dark-field microsphere-assisted microscopy

We present for the first time a surface plasmon-enhanced dark-field microsphere-assisted microscopy in imaging both low-contrast dielectric objects and metallic ones. We demonstrate, using an Al patch array as the substrate, the resolution and contrast in imaging low-contrast dielectric objects are improved compared to that of the metal plate substrate and a glass slide in dark-field microscopy (DFM). 365-nm-diameter hexagonally arranged SiO nanodots assembled on the three substrates can be resolved, with the contrast varied from 0.23 to 0.96, and the 300-nm-diameter hexagonally close-packed polystyrene nanoparticles can only be discerned on the Al patch array substrate. The resolution can be further improved by using the dark-field microsphere-assisted microscopy, and an Al nanodot array with a nanodot diameter of ∼65 nm and a center-to-center spacing of 125 nm can be just resolved, which cannot be distinguished in a conventional DFM. The focusing effect of the microsphere, as well as the excitation of the surface plasmons, provides evanescent illumination with enhanced local electric field (E-field) on an object. The enhanced local E-field acts as a near-field excitation source to enhance the scattering of the object, resulting in the improvement of imaging resolution.

Mapping spectroscopic micro-ellipsometry with sub-5 microns lateral resolution and simultaneous broadband acquisition at multiple angles

Spectroscopic ellipsometry is a widely used optical technique in both industry and research for determining the optical properties and thickness of thin films. The effective use of spectroscopic ellipsometry on micro-structures is inhibited by technical limitations on the lateral resolution and data acquisition rate. Here, we introduce a spectroscopic micro-ellipsometer (SME), capable of recording spectrally resolved ellipsometric data simultaneously at multiple angles of incidence in a single measurement of a few seconds, with a lateral resolution down to 2 μm in the visible spectral range. The SME can be easily integrated into generic optical microscopes by the addition of a few standard optical components. We demonstrate complex refractive index and thickness measurements by using the SME, which are in excellent agreement with a commercial spectroscopic ellipsometer. The high lateral resolution is displayed by complex refractive index and thickness maps over micron-scale areas. As an application for its accuracy and high lateral resolution, the SME can characterize the optical properties and number of layers of exfoliated transition-metal dichalcogenides and graphene, for structures that are a few microns in size.

Super-Resolution Fluorescence Imaging for Semiconductor Nanoscale Metrology and Inspection

The increase in the number and complexity of process levels in semiconductor production has driven the need for the development of new measurement methods that can evaluate semiconductor devices at the critical dimensions of fine patterns and simultaneously inspect nanoscale contaminants or defects. However, conventional optical inspection methods often fail to resolve device patterns or defects at the level of tens of nanometers required for device development owing to their diffraction-limited resolutions. In this study, we used the stochastic optical reconstruction microscopy (STORM) technique to image semiconductor nanostructures with feature sizes as small as 30 nm and detect individual 20 nm-diameter contaminants. STORM imaging of semiconductor nanopatterns is based on the development of a selective labeling method of fluorophores for a negative silicon oxide surface using the charge interaction of positive polyethylenimine molecules. This study demonstrates the potential of STORM for nanoscale metrology and in-line defect inspection of semiconductor integrated circuits.

Microspheres give improved resolution in nondestructive examination of semiconductor devices

The minimum spatial resolution of typical optical inspection systems used in the microelectronics industry is generally governed by the classical relations of Ernst Abbe. Kwon et al. show in a new Light: Science and Applications article that using an additional glass microsphere in the optical path can improve the resolution significantly.