Hwan Hur

Spectrally encoded common-path fiber-optic-based parallel optical coherence tomography

We demonstrate a fiber-optic-based parallel optical coherence tomography (OCT) using spectrally encoded extended illumination with a common-path handheld probe, where the flexibility and robustness of the system are significantly improved, which is critical in the clinical environment. To the best of our knowledge, we present the first parallel OCT based on fiber optics including a fiber coupler with a sensitivity of 94 dB, which is comparable to that of point-scanning OCT. We also investigated the effect of the phase stability of the fiber-based interferometry on the parallel OCT system by comparing the common-path OCT with two-arm OCT. Using the homemade common-path handheld probe based on a Mirau interferometer, the phase stability was 32 times better than that of the two-arm OCT. The axial resolution of the common-path OCT was measured as 5.1±0.3  μm. To demonstrate the in vivo imaging performance of the fiber-optic-based parallel OCT, human skin was imaged.

Thermal Resolution Analysis of Lock-in Infrared Microscope

Abstract In this study, we analyzed and showed the enhanced thermal resolution of a lock-in infraredthermography system by employing a blackbody system and micro-register sample. The noise level or thermalresolution of an infrared camera system is usually expressed by a noise equivalent temperature difference (NETD),which is the mean square of the deviation of the different values measured for one pixel from its mean valuesobtained in successive measurements. However, for lock-in thermography, a more convenient quantity in thephase-independent temperature modulation amplitude can be acquired. On the basis of results, it was observed thatthe NETD or thermal resolution of the lock-in thermography system was significantly enhanced, which we considerto have been caused by the averaging and filtering effects of the lock-in technique. Keywords: Lock-in Thermography, Noise Equivalent Temperature Difference, Infrared Microscope, Thermal Resolution[Received: September 23, 2014, Revised: October 20, 2014, Accepted: October 29, 2014] *한국기초과학지원연구원첨단장비개발사업단, ** IT , Corresponding Author: Center for Analytical한국과학기술원스마트 융합시스템연구단 ✝Instrumentation Development, Korea Basic Science Institute, 169-148 Gwahak-ro, Yuseong-gu, Daejeon 305-806, Korea(E-mail: ksc@kbsi.re.kr)

High speed parallel spectral-domain OCT using spectrally encoded line-field illumination

We report parallel spectral-domain optical coherence tomography (OCT) at 500 000 A-scan/s. This is the highest-speed spectral-domain (SD) OCT system using a single line camera. Spectrally encoded line-field scanning is proposed to increase the imaging speed in SD-OCT effectively, and the tradeoff between speed, depth range, and sensitivity is demonstrated. We show that three imaging modes of 125k, 250k, and 500k A-scan/s can be simply switched according to the sample to be imaged considering the depth range and sensitivity. To demonstrate the biological imaging performance of the high-speed imaging modes of the spectrally encoded line-field OCT system, human skin and a whole leaf were imaged at the speed of 250k and 500k A-scan/s, respectively. In addition, there is no sensitivity dependence in the B-scan direction, which is implicit in line-field parallel OCT using line focusing of a Gaussian beam with a cylindrical lens.

3D Defect Localization on Exothermic Faults within Multi-Layered Structures Using Lock-In Thermography: An Experimental and Numerical Approach

Micro-electronic devices are increasingly incorporating miniature multi-layered integrated architectures. However, the localization of faults in three-dimensional structure remains challenging. This study involved the experimental and numerical estimation of the depth of a thermally active heating source buried in multi-layered silicon wafer architecture by using both phase information from an infrared microscopy and finite element simulation. Infrared images were acquired and real-time processed by a lock-in method. It is well known that the lock-in method can increasingly improve detection performance by enhancing the spatial and thermal resolution of measurements. Operational principle of the lock-in method is discussed, and it is represented that phase shift of the thermal emission from a silicon wafer stacked heat source chip (SSHSC) specimen can provide good metrics for the depth of the heat source buried in SSHSCs. Depth was also estimated by analyzing the transient thermal responses using the coupled electro-thermal simulations. Furthermore, the effects of the volumetric heat source configuration mimicking the 3D through silicon via integration package were investigated. Both the infrared microscopic imaging with the lock-in method and FE simulation were potentially useful for 3D isolation of exothermic faults and their depth estimation for multi-layered structures, especially in packaged semiconductors.

Laser Scanning Confocal Thermoreflectance Microscope for the Backside Thermal Imaging of Microelectronic Devices

In this paper, we report on a confocal thermoreflectance imaging system that can examine the thermal characteristics of microelectronic devices by penetrating the backside of a device through the substrate. In this system, the local reflectivity variations due to heat generation in the device are measured point by point by a laser scanning confocal microscope capable of eliminating out-of-focus reflections and the thermoreflectance is extracted via Fourier-domain signal processing. In comparison to the conventional widefield thermoreflectance microscope, the proposed laser scanning confocal thermoreflectance microscope improves the thermoreflectance sensitivity by ~23 times and the spatial resolution by ~25% in backside thermoreflectance measurements.

Preliminary Study on Lock-In Thermography Using an Inverse Dynamic System Design

We adopted an inverse dynamical system(IDS) into the Lock-in Thermography(LIT) to increase the sensitivity of the semiconductor inspection system. IDS was designed to obtain the maximum signal to noise ratio(SNR) of LIT image by applying an optimized signal to the semiconductor chip to be inspected using a programmable voltage generator. In addition, we introduce some user friendly features such as monitoring the state of the system and the flexible control of Lock-in frequency and source voltage.

Study on Nonlinear Heat Transfer Effect in the Lock-in Thermography for Defects Characterization on a Three-Dimensional Integrated Circuit

The use of the lock-in thermography (LIT) as a non-destructive evaluation technique is becoming increasingly attractive for detecting and characterizing the defects such as the shorts and resistive opens in a 3D package or stacked IC [1,2]. According to this trend, the study for improving the performance of the LIT system is required such as synchronous undersampling method [3]. One of the performance degradation factors of the LIT system is nonlinear distortion effects. Although the heat action is applied periodically, the resulting surface temperature is not synchronized with the periodic heating due to nonlinear heat transfer. A new approach to overcome unsynchronized result is proposed in one-dimensional systems [4]. In this study, we developed the 3D FE model of TSV structure with silicon chip integration and experimentally validated our FE model using measured current–voltage (I–V) and lock-in thermography (LIT) measurements in order to analysis nonlinear heat transfer. From the finite element analysis, the thermal distributions including nonlinear heat transfer in the TSV model and phase angle for the lock-in frequency are calculated and compared with experimental results of a TSV-based 3D IC sample.

NDE of Low-Velocity Impact Damage in GFRP Using Infrared Thermography Techniques

In this study, low-velocity impact damage (LVID) in glass fiber reinforced plastic (GFRP) was investigated using pulse thermography (PT) and lock-in thermography (LIT) techniques. The main objective of this study was to evaluate the detection performance of each technique for LVID in GFRP. Unidirectional and cross-ply GFRPs were prepared with four energy levels using a drop weight impact machine and they were inspected from the impact side, which may be common in actual service conditions. When the impacted side was used for both inspection and thermal loading, results showed that the suggested techniques were able to identify the LVID which is barely visible to the naked eye. However, they also include limitations that depend on the GFRP thickness at the location of the delamination produced by the lowest impact energy of five joule.

Side-emitting optical probe on beam expanding coreless silica fiber for biomedical applications

We fabricate a compact all-optical side-emitting probe on beam expanding hybrid fiber using high precision femtosecond laser ablation process. The illuminating properties of the side-emitting fibers are experimentally investigated and compared with numerical simulations.