The article presents a novel optical metrology method for accurate critical dimension (CD) measurement of sub-micrometer structures with high spatial resolution and light efficiency. The proposed method takes advantage of the spatially coherent nature of the supercontinuum laser to detect submicron-scale structures with high aspect ratios. By using the method, CD measurement of individual microstructures such as vias and redistribution layers (RDL) becomes achievable when a high magnification optical configuration is incorporated. Proved by a test run on measuring submicron structures with linewidths as small as 0.7 μm and an aspect ratio over 4, the measurement precision of the depth can be kept within a few nanometers.
A new optical surface measuring method based on correlation-based diffractive image profilometry (DIP) is developed for accuracy enhancement by introducing external optical aberration to the microscope. According to the diffraction theory, the diffractive images formed in the microscope mainly depend on the microscopic optical system and the surface features of the tested object. The most critical issue affecting the measurement accuracy of the DIP is that the uniqueness of the diffractive images corresponding to various surface geometric parameters such as different heights and orientations cannot be always guaranteed. This situation can bring undesired uncertainties in surface measurement since undesired ambiguity in image correlation or model estimation may be introduced. To resolve this, a designed foreign aberration is introduced into the microscopic optical system to develop the feature variance of diffractive images for significantly increasing the degree of the image variance, therefore the risk of ambiguity is effectively avoided. Proved by some experimental tests, with this method, the accuracy in measuring height, tilting angle, and tilting direction can be achieved to a level of sub-micrometer and less than 0.01 degrees, respectively.
A new full-field profilometry based on diffraction image correlation (DIC) was developed in which the technique bases on calibrated database of reference diffractive images (RDIs) to estimate surface depth information. This technique has the advantage of removing vertical scanning for achieving high measurement efficiency in microscopic surface profilometry. However, as the diffractive images not only depend on the tested height but also the local surface tilt. Thus, the pre-built image database may not match the measured diffractive image and lead to a systematic measured error incurred by the surface tilting condition. Thus, in the article, the influence of surface tilt to the diffraction images is investigated and analyzed to understand the relationship between the surface tilt and the image variation. A lateral drifting phenomenon depends on the tilt angle and tilt direction was also quantified to estimate the impact of the tested surface height. Meanwhile, a detection algorithm to determine the center of diffractive image was developed to position the shifting quantity of the image. Moreover, the information coupling problem between height and tilting parameters, such as the tilt angle and tilt direction was studied to decouple these parameters from the tested height, so the tested surface can be reconstructed accurately. To realize the proposed method, some data processing strategies were also proposed to decouple the depth information from multi-surface parameters such as surface tilt angles (pitch and yaw) and direction effectively.
This work presents a novel microscopic profilometry using diffraction image correlation, which is appropriate for in-situ automated optical inspection (AOI). In contrast to the traditional confocal microscopy, the developed technique replaces the detector pinhole by observing and matching the diffraction patterns. Thus accurate surface depth detection can be achieved by eliminating time-consuming vertical scanning operation. The development is the first attempt in optics to use image correlation between the pre-calibrated database of diffraction images and the measured one of the detecting object surface. The feasibility of the method has been theoretically verified by scalar diffraction theory, then verified by experimental testing. Also, a depth response curve with the physical meaning of similarity is introduced and interpreted. Meanwhile, multi-point lateral scanning in one field of view (FOV) is achieved by quickly switching micromirrors on the digital micromirror device (DMD), thus the quasi full-field 3-D reconstruction can be acquired by combining tens of captured images. Verified experimentally, a 3-D reconstruction with sub-micrometer vertical resolution can be realized, with a tunable lateral resolution.
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