Selection of optimized electron beam parameters for in-line monitoring is necessary to eliminate false signals. Application of electron beam to detect electrical defects, particularly leakages, for static random access memory (SRAM) cells poses a great challenge as it requires current measurement tool with nanometer resolution to complement it. By correlating the brightness intensity or the gray-level value to the measured current values, we have shown that conductive atomic force microscopy (C-AFM) can overcome this obstacle and can be used to verify the validity of the voltage contrast (VC) captured by HMI eScan3xx Ebeam inspection tool.
IC functional failure is always a challenge for failure analysis engineers since it needs test pattern to access the defect location and electrically trigger it. Dynamic failure analysis is the only way to be used to do this kind of analysis. But it's time consuming and complex to employ dynamic analysis, so the success rate is lower and cycle time is longer. Static failure analysis is impossible to apply on these kinds of analysis since the test pattern and design information is needed. However, in this paper, the application of advanced FIB circuit edit (CE) was employed to isolate the suspected function block, and make it accessible to the DC bias. With static FA analysis, the defect location was successfully localized. Nanoprobing was employed on the further electrical analysis, and abnormal electrical performance was successfully observed. Combined with the device physics analysis, the suspected process was identified. Further PFA Wright etch was applied to visualize the defect which was a soft failure of Bipolar Junction Transistor (BJT) device. Failure mechanism was built up to explain the electrical and physical phenomena successfully.
Comparing with much valuable research on vibrational spectroscopy on low-k dielectrics in different substrates, this paper investigates the vibrational spectroscopy of low-k and ultra-low-k dielectric materials on patterned wafers. It is found that both Raman and FTIR spectroscopy are necessary as complement to characterize low-k and ultra-low-k dielectric materials on patterned wafers. Significant differences in the Raman and FTIR spectra between low-k and ultra-low-k dielectric materials are also observed. Moreover, Raman spectroscopy has an advantage in analyzing the mixed structure of low-k/ultra-low-k and Cu at nanometer-scaled sizes. The results in this paper show that Raman combined with FTIR spectroscopy is an effective tool to characterize dielectric thin film properties on patterned wafers.
Abstract Top-down, layer-by-layer de-layering inspection with a mechanical polisher and serial cross-sectional Focused Ion Beam (XFIB) slicing are two common approaches for physical failure analysis (PFA). This paper uses XFIB to perform top-down, layer-by-layer de-layering followed by Scanning Electron Microscope (SEM) inspection. The advantage of the FIB-SEM de-layering technique over mechanical de-layering is better control of the de-layering process. Combining the precise milling capability of the FIB with the real-time imaging capability of the SEM enables the operator to observe the de-layering as it progresses, minimizing the likelihood of removing either too much or too little material. Furthermore, real time SEM view during top-down XFIB de-layering is able to provide a better understanding of how the defects are formed and these findings could then be feedback to the production line for process improvement.
Abstract The case study in this paper describes how collaboration between customer design and test teams and a thorough FAB investigation triggered by a detailed electrical analysis using the Atomic Force Nanoprober (AFP) resulted in the effective resolution of a challenging implant related issue on LDMOS structure that caused yield loss. The quick success in this case has led to a shorter yield ramp cycle on this new product for mass production.
Abstract In this work, we present two case studies on the utilization of advanced nanoprobing on 20nm logic devices at contact layer to identify the root cause of scan logic failures. In both cases, conventional failure analysis followed by inspection of passive voltage contrast (PVC) failed to identify any abnormality in the devices. Technology advancement makes identifying failure mechanisms increasingly more challenging using conventional methods of physical failure analysis (PFA). Almost all PFA cases for 20nm technology node devices and beyond require Transmission Electron Microscopy (TEM) analysis. Before TEM analysis can be performed, fault isolation is required to correctly determine the precise failing location. Isolated transistor probing was performed on the suspected logic NMOS and PMOS transistors to identify the failing transistors for TEM analysis. In this paper, nanoprobing was used to isolate the failing transistor of a logic cell. Nanoprobing revealed anomalies between the drain and bulk junction which was found to be due to contact gouging of different severities.
Abstract Conductive-Atomic Force Microscopy (C-AFM) is a popular failure analysis method used for localization of failures in Static Random Access Memory (SRAM) devices [1-4]. The SRAM structure has a highly repetitive pattern where any abnormality in a failed cell compared to neighboring cells could be easily identified from its current image [5-7]. Unlike topographical imaging, the C-AFM requires the probe tip to be coated with a conductive layer in order to pick up the electrical signals from the device under test. The coating needs to be sufficiently thick as it would wear off after a certain amount of physical scanning. This additional coating on the AFM tip is essential but poses a limit to the tip radius curvature. The commercially available tip radius is approximately 35nm (DDESP-10 from Bruker) and the dimension is too large for imaging of 20nm technology device. However, the limitation could be alleviated by subjecting the sample surface to treatment prior to C-AFM imaging. The aim of this surface treatment is to ensure C-AFM tip maintains sufficient scanning contact with the tiny conductive (tungsten) structure of the sample in order to achieve distinct current image. The surface treatment is done by creating a receding Inter-Layer Dielectric (ILD) from its neighboring tungsten contact. The creation of the receding depth could be achieved by either wet etching or dry etching (Reactive Ion Etching, RIE). In this work, the surface treatments by these two methods have been investigated and the recipe is optimized to obtain a clear current image. The optimized recipe is then applied on actual failure analysis where three cases are studied.
Abstract This paper places a strong emphasis on the importance of applying Systematic Problem Solving approach and use of appropriate FA methods and tools to understand the “real” failure root cause. A case of wafer center cluster RAM fail due to systematic missing Cu was studied. It was through a strong “inquisitive” mindset coupled with deep dive problem solving that lead to uncover the actual root cause of large Cu voids. The missing Cu was due to large Cu void induced by galvanic effects from the faster removal rate during Cu CMP and subsequently resulted in missing Cu. This highlights that the FA analyst’s mission is not simply to find defects but also play a catalyst role in root cause/failure mechanism understanding by providing supporting FA evidence (electrically/ physically) to Fab.
The combined use of scanning probe microscope based techniques, namely conductive atomic force microscopy (C-AFM) and tunneling atomic force microscopy (TUNA), and nanoprobing technique is presented. In 90nm process and below, C-AFM identifies leakage by current mapping, while TUNA measures the current-voltage (I-V) curves of different contacts to study the integrity of individual contacts. Nanoprobing is used to obtain and compare the I-V characteristics of good and leaky transistors.
Recently, scanning probe microscope (SPM) has become a promising technique for nanofabrication. In this paper, we present a novel method of nano-fabrication, namely, nano-fabrication by atomic force microscope (AFM) tips under laser irradiation. The SPM was operated as an AFM. During imaging and nano-fabrication, the AFM is in constant force mode. The tip is fixed with the sample moving via a tube scanner. Nano-lithography software controls the scanner motion in x and y directions. The SPM has an open architecture allowing an external laser beam incident on the tip at an incident angle between 0 to 45°. A vertically-polarized Nd:YAG pulsed laser with a pulse duration of 7 ns was focused on the tip. An electrical shutter was introduced to switch the laser irradiation. Alignment between the laser beam and the tip was performed under a high-power charge coupled device (CCD) microscope. The kinetics of the nanostructure fabrication has been studied. Craters were created in air ambient under different laser pulse numbers, pulse energies and tip force. The feature size of the craters, which are in the nanometer scale, increases with the pulse number, pulse energy and the tip force. This technique has potential applications in nano-lithography and high-density data storage.
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