Random telegraph noise (RTN), which occurs in in-pixel source follower (SF) transistors, has become one of the most critical problems in highsensitivity CMOS image sensors (CIS) because it is a limiting factor of dark random noise. In this paper, the behaviors of RTN toward changes in SF drain current conditions were analyzed using a low-noise array test circuit measurement system with a floor noise of 35 µV rms . In addition to statistical analysis by measuring a large number of transistors (18048 transistors), we also analyzed the behaviors of RTN parameters such as amplitude and time constants in the individual transistors. It is demonstrated that the appearance probability of RTN becomes small under a small drain current condition, although large-amplitude RTN tends to appear in a very small number of cells.

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We investigated the impact of the interface roughness in W-CoFeB-MgO-CoFeB-W structures on MgO reliability using the atomic force microscopy images of a MgO underlayer surface and I-V measurements. As a result, the electric current in the MgO film increased as the MgO underlayer surface roughness increased. We assumed that this phenomenon is caused by the local electric field concentration, and verified it by comparing the two electric field concentration values simulated from the AFM images and calculated from the I-V measurements. These two electric field concentrations calculated from the different results have a strong correlation to each other. It is concluded that the current of the MgO film is increased by the large roughness between MgO and the electrodes. This causes the large current variation in memory cells in magnetic random access memory (MRAM), then it leads to a small margin in memory operation.

In this work, the impact of pattern edge of MgO films due to electrode separation process on its electrical reliability was investigated by measuring and analyzing I–V and time-dependent dielectric breakdown (TDDB) characteristics of MgO films with different pattern edge length. The measured TDDB lifetime at 63% drastically decreased from 1440 to 2 s when the pattern edge length increased from 32 to 320 μm, thus the electric reliability of MgO films can be significantly degraded due to the pattern edge. It was found that this issue can be solved by improving the process condition of ion bean etching (IBE), by turning the IBE angle and the IBE angle divergence. These findings can lead to high reliability magnetic tunnel junction fabrication in spin-transfer torque magnetic random access memory.

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