In this paper, we propose a technique called time-varying frequency scanning (TVFS) to meet the challenges in killer defect inspection. The proposed technique enables the dynamic monitoring of defects by checking the hopping in the instantaneous frequency data and the classification of defect types by comparing the difference in frequencies. The TVFS technique utilizes the bidimensional empirical mode decomposition (BEMD) method to separate the defect information from the sea of system errors. This significantly improve the signal-to-noise ratio (SNR) and moreover, it potentially enables reference-free defect inspection.
By 2017, the critical dimension in patterned wafers will shrink down to 7 nm, which brings great challenges to optics-based defect inspection techniques, due to the ever-decreasing signal to noise ratio with respect to defect size. To continue pushing forward the optics-based metrology technique, it is of great importance to analyze the full characteristics of the scattering field of a wafer with a defect and then to find the most sensitive signal type. In this article, the vector boundary element method is firstly introduced to calculate the scattering field of a patterned wafer at a specific objective plane, after which a vector imaging theory is introduced to calculate the field at an image plane for an imaging system with a high numerical aperture objective lens. The above methods enable the effective modeling of the image for an arbitrary vectorial scattering electromagnetic field coming from the defect pattern of the wafer.
Optical scatterometry is a model based technique, which conventionally requires minimization of a predefined least square function. This minimization relies heavily on the measurement configuration: wavelength, incident angle, azimuthal angle, and sample position, which brings up the question of how to find the configuration that maximizes measurement accuracy. We propose a general measurement configuration optimization method based on error propagation theory and singular value decomposition, by which the measurement accuracy can be approximated as a function of a Jacobian matrix with respect to the measurement configurations. Simulation and experiments for a one-dimensional trapezoidal grating establishes the feasibility of the proposed method.
We recently built a 405nm laser based optical interferometry system for 9nm node patterned wafer defect inspection.
Defects with volumes smaller than 15nm by 90nm by 35nm have been detected. The success of defect detection relied on
accurate mechanical scanning of the wafer and custom engineered image denoising post-processing. To further improve
the detection sensitivity, we designed a higher precision XYZ scanning stage and replaced the laser source with an
incoherent LED to remove the speckle noise. With these system modifications, we successfully detected both defects and
surface contamination particles in bright-field imaging mode. Recently, we have upgraded this system for interferometric
White-light imaging systems are free of laser-speckle. Thus, they offer high sensitivity for optical defect metrology, especially when used with interferometry based quantitative phase imaging. This can be a potential solution for wafer inspection beyond the 9 nm node. Recently, we built a white-light epi-illumination diffraction phase microscopy (epi-wDPM) for wafer defect inspection. The system is also equipped with an XYZ scanning stage and real-time processing. Preliminary results have demonstrated detection of 15 nm by 90 nm in a 9 nm node densely patterned wafer with bright-field imaging. Currently, we are implementing phase imaging with epi-wDPM for additional sensitivity.
In this work, we present recent results on several novel applications including optically monitoring the dissolution of biodegradable materials proposed for use in biological electronic implants, the self-assembly of microtubes during semiconductor etching, and the expansion and deformation of palladium structures for use in hydrogen sensing applications. The measurements are done using diffraction phase microscopy (DPM), a quantitative phase imaging (QPI) technique, which uses the phase of the imaging field to reconstruct a map of the sample’s surface. It combines off-axis and common-path geometries allowing for single-shot, high-speed dynamics with sub-nanometer noise levels.
Even with the recent rapid advances in the field of microscopy, non-laser light sources used for light microscopy have not been developing significantly. Most current optical microscopy systems use halogen bulbs as their light sources to provide a white-light illumination. Due to the confined shapes and finite filament size of the bulbs, little room is available for modification in the light source, which prevents further advances in microscopy. <p> </p>By contrast, commercial projectors provide a high power output that is comparable to the halogen lamps while allowing for great flexibility in patterning the illumination. In addition to their high brightness, the illumination can be patterned to have arbitrary spatial and spectral distributions. Therefore, commercial projectors can be adopted as a flexible light source to an optical microscope by careful alignment to the existing optical path. <p> </p>In this study, we employed a commercial projector source to a quantitative phase imaging system called spatial light interference microscopy (SLIM), which is an outside module for an existing phase contrast (PC) microscope. By replacing the ring illumination of PC with a ring-shaped pattern projected onto the condenser plane, we were able to recover the same result as the original SLIM. Furthermore, the ring illumination is replaced with multiple dots aligned along the same ring to minimize the overlap between the scattered and unscattered fields. This new method minimizes the halo artifact of the imaging system, which allows for a halo-free high-resolution quantitative phase microscopy system.
A highly sensitive laser-based quantitative phase imaging tool, using an epi-illumination diffraction phase microscope, has been developed for silicon wafer defect inspection. The first system used a 532 nm solid-state laser and detected 20 nm by 100 nm by 110 nm defects in a 22 nm node patterned silicon wafer. The second system, using a 405 nm diode laser, is more sensitive and has enabled detection of 15 nm by 90 nm by 35 nm defects in a 9 nm node densely patterned silicon wafer. In addition to imaging, wafer scanning and image-post processing are also crucial for defect detection.
A quantitative phase-shifting Differential Interference Contrast (DIC) system is built using a programmable spatial light modulator (SLM). Our system offers halo-free phase gradient images with low illumination coherence and very good axial sectioning. Results are presented for standard polystyrene micro-beads and live cells.
We provide a quantitative model for image formation in common-path QPI systems under partially coherent illumination. Our model is capable of explaining the phase reduction phenomenon and halo effect in phase measurements. We further show how to fix these phenomena with a novel iterative post-processing algorithm. Halo-free and correct phase images of nanopillars and live cells are used to demonstrate the validity of our method.
In this work, we experimentally determine the transfer function of our recently reported epi-illumination white light diffraction phase microscopy (epi-wDPM) system. The transfer function identifies how the low frequencies below k<sub>0</sub>NA<sub>con</sub> are modified due to the limited spatial coherence and how the high frequencies above k<sub>0</sub>NA<sub>obj</sub> are affected due to the limited objective numerical aperture. Using this transfer function, we perform deconvolution to remove the halo and obtain proper quantitative phase measurements without the need for excessive spatial filtering. The wDPM and epi-wDPM systems are now capable of obtaining halo-free images with proper topography at much higher speeds.
Over the past 2 years, we have developed a common optical-path, 532 nm laser epi-illumination diffraction phase microscope (epi-DPM) and successfully applied it to detect different types of defects down to 20 by 100 nm in a 22nm node intentional defect array (IDA) wafer. An image post-processing method called 2DISC, using image frame 2<sup>nd</sup> order differential, image stitching, and convolution, was used to significantly improve sensitivity of the measured images. To address 9nm node IDA wafer inspection, we updated our system with a highly stable 405 nm diode laser. By using the 2DISC method, we detected parallel bridge defects in the 9nm node wafer. To further enhance detectability, we are exploring 3D wafer scanning, white-light illumination, and dark-field inspection.
We applied epi-illumination diffraction phase microscopy to measure the amplitude and phase of the scattered field from a SEMATECH 22 nm node intentional defect array (IDA) wafer. We used several imaging processing techniques to remove the wafer’s underlying structure and reduce both the spatial and temporal noise and eliminate the system calibration error to produce stretched panoramic amplitude and phase images. From the stretched images, we detected defects down to 20 nm × 160 nm for a parallel bridge, 20 nm × 100 nm for perpendicular bridge, and 35 nm × 70 nm for an isolated dot.
Simulation results for an etched air hole photonic crystal (PhC) vertical cavity surface emitting laser (VCSEL) structure
with various thicknesses of metal deposited inside the holes are presented. The higher-order modes of the structure are
more spread out than the fundamental mode, and penetrate into the metal-filled holes. Due to the lossy nature of the
metal, these higher-order modes experience a greater loss than the fundamental mode, resulting in an enhanced side
mode suppression ratio (SMSR). A figure of merit for determining which metals would have the greatest impact on the
SMSR is derived and validated using a transmission matrix method calculation. A full three-dimensional simulation of
the PhC VCSEL structure is performed using the plane wave admittance method, and SMSRs are calculated for
increasing metal thicknesses. Of the metals simulated, chromium provided the greatest SMSR enhancement with more
than a 4 dB improvement with 500 nm of metal for an operating current of 12 times threshold.
We report on designs for compact VCSEL sensors for use in hydrogen gas detection. We discuss 980nm
VCSELs coated with Pd, which has been proven to react with hydrogen. During this reaction, the device undergoes
hydrogen induced lattice expansion (HILE) which causes two distinct effects that can be detected at the output. These
two effects are a red-shift in the emission wavelength and a decrease in the output power. The compact VCSELs will be
positioned into a 2D array with varying mesa diameters and palladium thickness in order to distinguish and track the
specific level of hydrogen present in the atmosphere.
We present designs and simulations for a highly cascadable, rapidly reconfigurable, all-optical, universal logic gate. We will discuss the gate's expected performance, e.g. speed, fanout, and contrast ratio, as a function of the device layout and biasing conditions. The gate is a three terminal on-chip device that consists of: (1) the input optical port, (2) the gate selection port, and (3) the output optical port. The device can be built monolithically using a standard multiple quantum well graded index separate confinement heterostructure laser configuration. The gate can be rapidly and repeatedly reprogrammed to perform any of the basic digital logic operations by using an appropriate analog electrical or optical signal at the gate selection port. Specifically, the same gate can be selected to execute one of the 2 basic unary operations (NOT or COPY), or one of the 6 binary operations (OR, XOR, AND, NOR, XNOR, or NAND), or one of the many logic operations involving more than two inputs. The speed of the gate for logic operations as well as for reprogramming the function of the gate is primarily limited to the small signal modulation speed of a laser, which can be on the order of tens of GHz. The reprogrammable nature of the universal gate offers maximum flexibility and interchangeability for the end user since the entire application of a photonic integrated circuit built from cascaded universal logic gates can be changed simply by adjusting the gate selection port signals.
We investigated and demonstrated bio-medical imaging using a THz quantum cascade laser. With the THz quantum cascade laser (QCL) at 3.8 THz, we obtained large dynamic range and high spatial resolution in the transmission imaging technique. The various tissues images, such as lung, liver, and brain sections from the laboratory mouse were obtained and studied. The most important factor for this imaging scheme is to obtain high contrast with different absorption characteristics in tissues. We explored distinct images from the fat, muscles and tendon from the freshly cut tissues and investigated absorption coefficient and compared with FTIR measurement. We also demonstrated the image of distinct region of tumors progressed and normal tissues using this technique. The comparison of frequency dependent medical imaging with utilizing different wavelength of QCLs has been addressed.
We achieved 1.5-um CW SQW GaInNAsSb lasers with GaNAs barriers grown by MBE on GaAs substrates with typical room temperature threshold densities below 600A/cm<sup>2</sup>, external quantum efficiencies above 50%, and output powers exceeding 200mW from both facets for 20x1222um devices tested epitaxial-side up. In pulsed mode, 450A/cm<sup>2</sup>, 50%, and 1100mW were realized. Longer devices yielded over 425mW of total CW power and thresholds below 450A/cm<sup>2</sup>. These results are comparable to high quality GaInNAs/GaAs lasers at 1.3um. Z-parameter measurements revealed that these improvements in the performance metrics of approximately 40-60% over previous results are primarily due to reduced monomolecular recombination. The large differential gain of GaInNAsSb/GaNAs/GaAs lasers at 1.5um of approximately 1.2x10-15cm<sup>2</sup> was mostly squandered in previous devices due to large quantities of monomolecular recombination. The characteristic temperatures for threshold current, T0, and for efficiency, T1, were 66K and 132K, respectively. These reduced values, compared to prior measurements of 106K and 208K, respectively, indicate carrier leakage. Since monomolecular recombination is temperature insensitive, the temperature stability of device operation was adversely affected.