Ryota Kudo

Height measurement of single nanoparticles based on evanescent field modulation

We propose a novel height measurement method for single nanoparticles illuminated by an evanescent field in the total internal reflection microscopy scheme. The method utilises the scattered light intensity response to incident angle modulation. We introduced a physical model to derive a height measurement formula, and confirmed its validity through numerical simulations based on Maxwells equation. We also verified the practical feasibility of the proposed method and confirmed that under an incident angle error of less than 0.1°, height measurement error of less than 10 nm could be achieved.

Lateral resolution improvement of laser-scanning imaging for nano defects detection

Abstract Demand for higher efficiency in the semiconductor manufacturing industry is continually increasing. In particular, nano defects measurement on patterned or bare Si semiconductor wafer surfaces is an important quality control factor for realizing high productivity and reliability of semiconductor device fabrication. Optical methods and electron beam methods are conventionally used for the inspection of semiconductor wafers. Because they are nondestructive and suitable for high-throughput inspection, optical methods are preferable to electron beam methods such as scanning electron microscopy, transmission electron microscopy, and so on. However, optical methods generally have an essential disadvantage about lateral spatial resolution than electron beam methods, because of the diffraction limit depending on the optical wavelength. In this research, we aim to develop a novel laser-scanning imaging method that can be applied to nano-/micro manufacturing processes such as semiconductor wafer surface inspection to allow lateral spatial super-resolution imaging with resolution beyond the diffraction limit. In our proposed method, instead of detecting the light intensity value from the beam spot on the inspection surface, the light intensity distribution, which is formed with infinity corrected optical system, coming from the beam spot on the inspection surface is detected. In addition, nano scale shifts in the beam spot are applied for laser spot scanning using a conventional laser-scanning method in which the spots are shifted at about a 100 nm pitch. By detecting multiple light intensity distributions due to the nano scale shifts, a super-resolution image reconstruction with resolution beyond the diffraction limit can be expected. In order to verify the feasibility of the proposed method, several numerical simulations were carried out.

Multiple image reconstruction for high-resolution optical imaging using structured illumination

The resolving power in optical imaging is limited not only by optical diffraction but also by the sampling size, which in turn is determined by the optical magnification and pixel size of the imaging devices such as CCD or CMOS. In order to exceed these limits, we propose a method for improving the optical resolving power by using structured illumination shift and multiple image reconstruction. Theoretical and experimental verifications reveal that the use of structured light illumination together with successive approximation (which provides the extrapolation effect) causes the resolving power of the proposed method to exceed the optical diffraction limit. Furthermore, we focused on subpixel sampling using the structured illumination shift method. Subpixel image processing can improve the resolving power without narrowing the visual field of the imaging optics. In addition, the proposed method can provide subpixel resolving power without necessitating the mechanical displacement of the CCD camera. We investigated the relationship between the CCD pixel size and the resolving power provided by the proposed method. We found that the subpixel spatial shift of the structured illumination not only improves the optical resolving power but also enables sub-pixel sampling for optical imaging.

Influence of standing wave phase error on super-resolution optical inspection for periodic microstructures

The miniaturization of microfabricated structures such as patterned semiconductor wafers continues to advance, thereby increasing the demand for a high-speed, nondestructive and high-resolution measurement technique. We propose a novel optical inspecting method for a microfabricated structure using the standing wave illumination (SWI) shift as such a measurement technique. This method is based on a super-resolution algorithm in which the inspection systems resolution exceeds the diffraction limit by shifting the SWI. Resolution beyond the diffraction limit has previously been studied theoretically and realized experimentally. The influence of various experimental error factors needs to be investigated and calibration needs to be performed accordingly when actual applications that utilize the proposed method are constructed. These error factors include errors related to the phase, pitch and shift step size of the standing wave. Identifying the phase accurately is extremely difficult and greatly influences the resolution result. Hence, the SWI phase was focused upon as an experimental error factor. The effect of the phase difference between the actual experimental standing wave and the computationally set standing wave was investigated using a computer simulation. The periodic structure characteristic of a microfabricated structure was analyzed. The following findings were obtained as a result. The influence of an error is divided into three modes depending on the pitch of the periodic structure: (1) if the pitch is comparatively small, the influence of the error is cancelled, allowing the structure of a sample to be resolved correctly; (2) if the pitch of the structure is from 150 to 350 nm, the reconstructed solution shifts in a transverse direction corresponding to a phase gap of SWI; and (3) if it is a comparatively large pitch, then it is difficult to reconstruct the right pitch. Verification was experimentally attempted for mode (2), and the same result as that for the simulation was obtained.

Simulation-Based Analysis of Influence of Error on Super-Resolution Optical Inspection

Microfabricated structures such as semiconductors and MEMS continue shrinking as nanotechnology expands, demand that measures microfabricated structures has risen. Optics and electron beam have been mainly used for that purpose, but the resolving power of optics is limited by the Rayleigh limit and it is generally low for subwavelength-geometry defects, while scanning electron microscopy requires a vacuum and induces contamination in measurement. To handle these considerations, we propose optical microfabrication inspection using a standing-wave shift. This is based on a super-resolution algorithm in which the inspection resolution exceeds the Rayleigh limit by shifting standing waves with a piezoelectric actuator. While resolution beyond the Rayleigh limit by proposed method has been studied theoretically and realized experimentally, we must understand the influence of experimental error factors and reflect this influence in the calibration when actual application is constructed. The standing-wave pitch, initial phase, and noise were studied as experimental error factors. As a result, it was confirmed that super-resolution beyond the Rayleigh limit is achievable if (i) standingwave pitch error was 5% when standing-wave pitch was 300 nm or less and (ii) if initial phase error was 30◦ when standing-wave pitch was 300 nm. Noise accumulation was confirmed in studies of the noise effect, and a low-pass filter proved effective against noise influence.