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Daesun Kim
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Proceedings Papers
ISTFA2023, ISTFA 2023: Conference Proceedings from the 49th International Symposium for Testing and Failure Analysis, 187-189, November 12–16, 2023,
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As memory devices decrease in dimensions, particles are found to be a major source of defects in unexpected DRAM failures due to their relatively large size. Among many DRAM defects, cross-defects account for the majority of system failures for a long time. Hard cross-defects are frequently observed in the first test step. Most of these defective cells are repaired with row or column redundancy resources. However, after high voltage and temperature stress, some of the cross-defects can additionally spread to adjacent rows or columns with reduced resistance. Recently, a new type of bridge cross-defect accompanied by row failures has emerged. This can be detected electrically through current measurements of 2 wordline (WL) under active mode. But, these defects are not obvious even after both high temperature and voltage stress. The bridge causes intermittent failure in the row-direction during DRAM operation. This soft 2-WL bridge is considered a serious fault source that can cause uncorrectable error (UE) at the system level even though on-die error correction code (ODECC) is introduced. Therefore, it is very important to find, improve, and develop control methods on such defects for future DRAM enhancement. In this paper, 2-WL defects or extended cross-defect were intensively analyzed through electrical failure analysis (eFA) and physical FA (pFA). Results reveal fine particles as the cause.
Proceedings Papers
ISTFA2022, ISTFA 2022: Conference Proceedings from the 48th International Symposium for Testing and Failure Analysis, 362-364, October 30–November 3, 2022,
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DRAM is a type of memory that stores each bit of data in a capacitor cell, leakage current is a very important electrical parameter to retain data. Therefore, larger cell capacitance and smaller leakage current have been regarded as key factors in continuous shrinkage of DRAM. Generally, gate induced drain leakage (GIDL) and junction leakage of cell transistor (CTR) are well-known, but our approach is focused on retention failure by bulk trap. In order to electrically observe the influence of bulk trap, we used the adjacent gate of CTR to control electron migration. Results show that there are many failure cells due to bulk trap, and dimension shrinkage accelerates this failure. Consequently, balancing among electrical bias point and transistor manufacturing process should be carefully considered with bulk traps.
Proceedings Papers
ISTFA2020, ISTFA 2020: Papers Accepted for the Planned 46th International Symposium for Testing and Failure Analysis, 277-279, November 15–19, 2020,
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As dimension shrinkage, uncommon phenomena have been occurring during write and read operation in DRAM. These phenomena are strongly related cell capacitance, and the sensitivity of leakage current increases. Leakage current, especially in cell capacitor or cell transistor, is a major cause of the imbalance between stored charge in write operation and served charge in the read operation. Generally, error induced by leakage current appears data-1 failure, but in our study data-0 failure is observed in the case of extreme low cell capacitance that failure level is ppb (parts per billion). Results show that this phenomenon is influenced by cell capacitance, gate/body voltage of cell transistor, and supplied voltage level of the bitline sense amplifier. Based on various results, the electron loss to form inversion electron channel of cell transistor is regarded as a major factor like Charge Feedthrough [5].
Proceedings Papers
ISTFA2018, ISTFA 2018: Conference Proceedings from the 44th International Symposium for Testing and Failure Analysis, 138-140, October 28–November 1, 2018,
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Reduced noise immunity due to dimensional shrinkage, lower operational voltages and increasing densities results in increased soft or random failures. In practice, noises are generated by complex operation of device. In Dynamic Random Access Memory (DRAM), failures by noise are regarded as either decrease in charge at cell capacitor or increase in systematic interferences. Simple equivalent circuit of One Transistor One Capacitor (1T1C) DRAM and theoretical approach in time-domain are provided for quantitative noise analysis related to sense amplifier circuitries. Results show that local voltage fluctuation reduces sensing margin to judge data-0 or data-1. This phenomenon is easily observed at 1T1C with high resistance because response of voltage generator is comparatively slow.
Proceedings Papers
ISTFA2017, ISTFA 2017: Conference Proceedings from the 43rd International Symposium for Testing and Failure Analysis, 424-426, November 5–9, 2017,
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For fault management, various types of error-correcting codes (ECC) have been widely used for most computers and memory. From a memory perspective, the ECC technique is generally adopted for DRAM modules to correct data corruption among multiple chips, not in-chip level. Recently, increased soft single-bit failures have accelerated introduction of the ECC technique into DRAM components. For reliability, fault generation technique by high voltage at high temperature, also known as burn-in stress, has been widely used in the IC manufacturing process. In DRAM, burn-in stress is also useful to screen latent defects or to predict device lifetime. In this paper, we studied un-correctable errors which occurred due to various types of storage node bridge defects in ECC DRAM. 12 faulty cells among 1,000 cells are observed after burn-in stress. Retention time of each cell is measured with automatic test equipment under the various temperature conditions, and activation energy were extracted from measurement results. Results of activation energy show that there were two types of faults, one was metal-metal hard bridge (0.14eV) and the other was dielectric-dielectric soft bridge (0.35eV), in comparison with normal cells (0.53eV). Moreover, soft bridge was carefully analyzed with TEM and nanoprobing showing that activation energy analysis was well-matched.
Proceedings Papers
ISTFA2016, ISTFA 2016: Conference Proceedings from the 42nd International Symposium for Testing and Failure Analysis, 82-84, November 6–10, 2016,
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System failure due to a progressive defect in memory cell-array of DRAM was studied with automated test equipment. In order to find out relationship correctable single-bit fault and system failure, memory cells with single-bit fault by a cross-defect were selected. After high voltage and temperature stress, a soft cross-defect was changed into a hard cross-defect. Consequentially, invalid operation by a degraded cross-defect causes array-failure. Based on the failure analysis, methods to prevent array-failure are proposed, and applied to DRAM successfully.
Proceedings Papers
ISTFA2016, ISTFA 2016: Conference Proceedings from the 42nd International Symposium for Testing and Failure Analysis, 91-93, November 6–10, 2016,
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Systematic retention failure related on the adjacent electrostatic potential is studied with sub 20nm DRAM. Unlike traditional retention failures which are caused by gate induced drain leakage or junction leakage, this failure is influenced by the combination of adjacent signal line and adjacent contact node voltage. As the critical dimension between adjacent active and the adjacent signal line and contact node is scaled down, the effect of electric field caused by adjacent node on storage node is increased gradually. In this paper, we will show that the relationship between the combination electric field of adjacent nodes and the data retention characteristics and we will demonstrate the mechanism based on the electrical analysis and 3D TCAD simulation simultaneously.
Proceedings Papers
ISTFA2015, ISTFA 2015: Conference Proceedings from the 41st International Symposium for Testing and Failure Analysis, 234-236, November 1–5, 2015,
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As microelectronic feature sizes are scaled down, the characteristics and distribution of DRAM data retention time and write recovery time are getting worse. This degradation is due to the increases in the leakage current and resistance of the cell node and the decrease of cell capacitance in DRAM devices. As the physical distance between storage nodes decreases, node potential is increasingly affected by small potential changes in adjacent storage nodes. In this paper, we will show that the one of the most dominant contributors to failure is the adjacent storage node level, and we will demonstrate how node level affects write time delay. The effect of the adjacent storage node level can be correlated with a change in threshold voltage, much like the MOSFET body effect. We define this phenomenon as the lateral body effect, and propose a model for adjacent potential effect using the Buried Cell Array Transistor (BCAT) structure in sub 20nm DRAM.