edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 4 4 EDFAAO (2025) 4:4-11 1537-0755/$19.00 ©ASM International® INNOVATIVE THERMAL APPROACHES TO FAULT ISOLATION IN THREE-DIMENSIONAL SEMICONDUCTOR STRUCTURES Norimichi Chinone1 and Shimpei Tominaga2 1Hamamatsu Corp., San Jose, California 2Hamamatsu Photonics KK, Japan nchinone@hamamatsu.com INTRODUCTION Semiconductor devices have been on a continuous march to deliver higher performance, which is great for consumers, but creates ongoing challenges for fault isolation. Device improvement has been achieved by scaling design rules and by extending the device structure into the z-axis.[1] 3D structures have a variety of types and a range of thicknesses. Stacked-die is one type of 3D structure, where multiple dies (i.e., memory dies) are stacked. High bandwidth memory (HBM) is a another leading example of stacked-die with a thickness of 700 µm.[2] In HBM, each die is connected by through-silicon vias (TSVs) and micro-bumps.[2] 3D NAND flash memories became a major category of 3D structures in the last decade, displacing planar NAND flash memories.[3] A 3D NAND memory cell has hundreds of layers of word-line/insulator stacks that form vertically connected non-volatile memory cells, of which the total thickness can range from single digits to 10 µm.[3,4] Moreover, 3D NAND flash memories have increased the number of layers and stacked the cell on peripheral control logic,[5] which is referred to as cellover-peri (COP), CMOS under array (CUA), or 4D-NAND, depending on the manufacturer. Metal interconnects in Si wafers also form 3D structures with total thicknesses of single digits to 10 µm but they used to be on one side of the wafer only. Recent developments suggest backside metallization is going to be introduced, where power delivery network (PDN) and I/O network would be migrated to the backside (opposite of the active layer).[1] This novel structure is expected to provide relief in the metal interconnect density and voltage IR-drop.[6] However, this adds another challenge to FA because it blocks direct optical access to the active layer. To localize defects in these 3D-structured devices, the FA community has devoted much effort by using a variety of techniques including optical fault isolation (OFI), x-ray,[7] ultrasonic,[8] electron beam (EB),[9] and tera-hertz techniques.[10] As for OFI, techniques that rely on heat diffusion and magnetic field have gained attention because of their penetrative features. Thermal lock-in (LIT)[11] and optical-beam induced resistance change (OBIRCH)[12,13] or thermal induced voltage alteration (TIVA)[13] are major heat-relying OFI techniques. Because the latter two techniques are similar, this article focuses on OBIRCH. LIT images use modulated heat sources on the sample with a focal plane array that is sensitive to mid-infrared (MIR) and software lock-in operation.[14] The resulting images are generally used to find abnormal hotspots that may indicate short defect location. Because the LIT phase image contains in-depth information, studies have been done to localize the defect in the xyz space.[15] Generally, modulation frequency of LIT is less than 1 kHz and according to Nyquist-Shannon sampling theorem, it should be less than half of the frame rate. In this frequency range, relatively thick 3D structures such as stacked chips are the target. The theoretical background of this technique is discussed later. OBIRCH focuses a laser beam on a sample that is operated at constant voltage or current. The laser spot locally heats the sample, which causes a small perturbation in current or voltage if the heated location is temperature sensitive.[12,13] A raster scan of laser spots generates a map of temperature sensitive locations, which tends to indicate a short defect or abnormal metal trace confinement locations.[12] Lock-in (LI) OBIRCH is a variant of this technique, where the stimulation laser beam is pulsed at certain frequency and the resulting current/voltage perturbation is lock-in detected.[16,17] This technique improves the signal-to-noise ratio by an order of magnitude. In the Hamamatsu tool, laser modulation frequency covers sub-kHz to over 100 kHz, which can correspond to
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