To achieve high accuracy and precision in optical metrology for advanced semiconductors, it is crucial to identify and compensate for errors from optical components and environmental perturbations. In this study, we investigated the sources of the errors in the interferometric ellipsometer developed for next-generation OCD. The objective lens and beam splitters, the critical optical components of the system, are intensively investigated. The system errors induced by temperature fluctuation, wavelength inaccuracy, and defocus were quantitatively examined. We also proposed methods for compensating individual errors and analyzed the effect of the compensation. As a result of error compensation, the accuracy and precision of the system is improved by 6.9 times and 2.3 times, respectively. Although the investigation was conducted based on our interferometric ellipsometry system, the finding is not limited to this system, as these errors are commonly found in most optical metrology systems. The proposed method for error compensation will be essential strategies for various ellipsometry systems suffering from a low level of accuracy and precision.
An innovative metrology technique has been devised to address current limitations of optical critical dimension (OCD) in advanced semiconductor metrology. This technique is based on multiple self-interferometric pupil imaging, called Mueller matrix self-interferometric pupil ellipsometry (M-SIPE). The system integrates an innovatively designed interference generator in both illuminating and imaging optics, allowing for the massive acquisition of full polarization information across entire angles around the device. The vast amount of information can offer fully comprehensive structural analysis, accomplishing enhanced sensitivity and the ability to break the well-known parameter correlation issues. The system employs a single-shot holographic measurement technique on the pupil plane, enabling rapid acquisition of three-dimensional spectral information, such as wavelengths, incidence angles, and azimuth angles. Thus, unlike conventional OCD tools, M-SIPE can obtain multi-angular and full polarization information without any mechanical movements. We verified the performance of M-SIPE by the experiment of non-patterned wafers of various conditions using an optical testbed. Our results confirmed good agreement between the experiment and theoretical simulations across all angular ranges. Furthermore, the actual device simulation was conducted to show sensitivity enhancement and ability for breaking the parameter correlation issues. The results confirmed that the large amount of angular information from M-SIPE technique could overcome current metrological challenges.
We present advanced application of novel ellipsometry technique, referred to as self-interference pupil ellipsometry (SIPE), integrating self-interference and pupil microscopy to overcome the sensitivity limitations raised from the conventional spectroscopic ellipsometry. We investigated various samples including a SiO2 monolayer, grating patterned wafers, and DRAM wafers to demonstrate outstanding capability of SIPE for metrology. The angular range corresponds to approximately 5,000 acquisition of conventional ellipsometry tools with 2º angular step scanning. From the experimental results and simulation, we expect the sensitivity of SIPE for structure metrology is at least 0.15 nm at a single wavelength and even better for multispectral measurements.
A recently introduced novel concept ellipsometry, characterized by its unique derivation process of directly extracting the polarization information from the hologram image on pupil plane, has been evaluated experimentally targeting for the application to OCD and overlay tools. With an improvement of splitting the hologram on the pupil, this self-interferometric pupil ellipsometry (SIPE) has gained the capability of measuring all of Stokes parameters (S0-S3) throughout the incident angle of 0 to 72 degrees with omnidirectional orientation. A prototype system incorporating SIPE technology has been fabricated to conduct the performance test with patterned test samples for overlay and CD evaluation, the result of which exhibits the sufficient sensitivity to dimension variation and superior repeatability for practical use. The measurement of thousands of ellipsometric data on pupil only takes tens of milliseconds at the most, realized by leveraging the advantages of self-interferometry that does not have any rotating elements in optics. The experimental result demonstrates the consistency with the simulation results based on TEM data within entire pupil. In the front-line of advanced semiconductor manufacturing, the main obstacles to OCD application, low sensitivity and parameter coupling, have been evaluated, which indicates good prospects with SIPE technology.
An innovative self-interferometric pupil ellipsometry(SIPE) has been demonstrated to overcome the spectral sensitivity and throughput limitations for optical critical dimensions (OCD) metrology in the advanced semiconductor devices. The two orthogonally polarized lights from the target structure on wafer were combined through suitably devised polarization state analyzer to generate an interferometric fringe pattern on the pupil surface of the SIPE optical system. The measured fringe pattern was processed with our novel holographic reconstruction algorithm to extract the ellipsometric information (Ψ and Δ) with the entire incident angles 0 to 70º and azimuthal angle 0 to 360º separately. In contrast to conventional ellipsometry tools, no mechanical movements were required to obtain the multi-angular information. To verify the usefulness of SIPE system and the algorithms, both experimental and theoretical validation have been performed for patterned wafers as well as for SiO2 mono-layered wafers. We first measured the non-patterned wafers of various different thicknesses, and found that the obtained values from SIPE, commercial ellipsometry tool, and theoretical simulation present a good agreement for wide spectral and angular ranges. Furthermore, we show that the large amount of angle resolved information from SIPE technique can greatly enhance the ability to overcome the OCD ellipsometry’s recent challenges such as spectral sensitivity issues, parameter correlation and structural asymmetry problems, etc. In short, the proposed system and algorithms, which are completely new approaches, show a capability to overcome current metrology challenges and we strongly believe that the SIPE is a promising metrology solution that can be eventually replacing the traditional OCD metrology tools..
An innovative self-interferometric pupil ellipsometry (SIPE) technique has been demonstrated to overcome the accuracy and throughput limitations raised from the conventional spectroscopic ellipsometry (SE) tools to precisely measure the optical critical dimensions (OCD) in the advanced semiconductor devices. The proposed SIPE technique will be extremely powerful, because key ellipsometric parameters, Ψ and Δ, from all possible incident angles can be obtained simultaneously from the single measurement, while the conventional SE technique needs to collect several hundreds of measurements to get the identical information. By employing a Nomarski prism, one can angularly separate the reflected light from the wafer into two orthogonally polarized lights. Then, the self-interference pupil ellipsometer could interfere those two beams without an additional reference beam path. The interfered fringe includes rich ellipsometric information at incident angles from -70º to 70º with 0-360º azimuthal directions, where those Ψ and Δ information can be extracted by the novel holographic algorithm we proposed. To verify the usefulness of SIPE system and the algorithms, both experimental and theoretical validation have been performed for the patterned wafers. In short, the proposed system and algorithms, which are completely new concept, show a capability to overcome current metrology challenges by breaking multiple parameter correlations between various structural parameters, eventually resulting in the improved metrology sensitivity and precision. Based on the results presented here, we strongly believe the SIPE is a promising metrology solution that can be eventually replacing the traditional OCD tools.
Lensfree digital holographic technique can become a powerful microscopic solution by adequately adapting a super resolution(SR) method together with an advanced phase retrieval algorithm. However, it comes at the cost of acquiring multiple images as well as processing large volume of data. Here, we present a multi-height based SR technique that can maximize the signal to noise ratio and the resolution, approximately 2.5 times over the actual pixel size of an image sensor, while minimizing computational cost by utilizing the much less set of the sub-pixel shifted images compared to the conventional SR methods.
We demonstrate a high resolution lens-free holographic microscopy in reflection geometry based on a pixel super resolution (SR) method. The lens-free microscopy uses a novel Michelson geometry suitable to image reflective samples with the large field of view, while the Fourier domain SR technique is applied to obtain the high resolution hologram, achieving the sub-pixel resolution of 1.2 μm in the USAF reflection target by utilizing the randomly shifted low resolution images. The proposed compact microscopy technique enables to provide high resolution amplitude and phase imaging, those are suitable for biology and semiconductor imaging applications.
Imaging brain tissues is an essential part of neuroscience because understanding brain structure provides relevant information about brain functions and alterations associated with diseases. Magnetic resonance imaging and positron emission tomography exemplify conventional brain imaging tools, but these techniques suffer from low spatial resolution around 100 μm. As a complementary method, histopathology has been utilized with the development of optical microscopy. The traditional method provides the structural information about biological tissues to cellular scales, but relies on labor-intensive staining procedures. With the advances of illumination sources, label-free imaging techniques based on nonlinear interactions, such as multiphoton excitations and Raman scattering, have been applied to molecule-specific histopathology. Nevertheless, these techniques provide limited qualitative information and require a pulsed laser, which is difficult to use for pathologists with no laser training.
Here, we present a label-free optical imaging of mouse brain tissues for addressing structural alteration in Alzheimer’s disease. To achieve the mesoscopic, unlabeled tissue images with high contrast and sub-micrometer lateral resolution, we employed holographic microscopy and an automated scanning platform. From the acquired hologram of the brain tissues, we could retrieve scattering coefficients and anisotropies according to the modified scattering-phase theorem. This label-free imaging technique enabled direct access to structural information throughout the tissues with a sub-micrometer lateral resolution and presented a unique means to investigate the structural changes in the optical properties of biological tissues.
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