Toshiki Obara

The study of time constant analysis in random telegraph noise at the subthreshold voltage region

We extracted time constants capture and emission of Random Telegraph Noise (RTN), and their dependencies of the gate-source voltage from numerous MOSFETs and discuss the trapping and detrapping processes of carriers at the subthreshold voltage region. The dependence of time to capture on gate-source voltage cannot be determined by the trap depth from the interface and but by the distance between the trap and the carrier to be captured and the trap energy level. On the other hand, it is considered that the dependence of time to emission is determined by the distance between the trap and the Si/SiO2 interface and the trap energy level. It is easy to understand emission processes compared to capture processes. We observed various emission processes caused by tunneling to Si substrate side, tunneling to gate electrode side and tunneling to either Si substrate side or gate electrode side depending on gate-source voltage. Evaluating the time constants individually is indispensable to characterize the trap which causes RTN in subthreshold voltage region.

Extraction of time constants ratio over nine orders of magnitude for understanding random telegraph noise in metal–oxide–semiconductor field-effect transistors

The random telegraph noise (RTN) characteristics in numerous metal–oxide–semiconductor field-effect transistors were evaluated accurately with small floor noise of 59 µV. Because of the small floor noise RTN characteristics with small amplitudes could be measured. The time constants: time to emission (τe) and time to capture (τc) were extracted from measurement results with sampling period of 1 µs and sampling time of 600 s, and they were distributed for over six orders of magnitude. Based on the extracted data of τe and τc, there is no correlation between and . Also, it was clarified that the time constant ratio () were distributed over nine orders of magnitude. Additionally, the gate bias dependence of , , and were analyzed. The distribution of is shifted to larger values with the increase of drain current (ID) and that of is shifted to smaller values with the increase of ID.

Analyzing correlation between multiple traps in RTN characteristics

The correlation between multiple traps in Random Telegraph Noise (RTN) were evaluated by using Time-Lag-Plot (TLP). The correlations between multiple traps were evaluated by transition paths on the TLP and there are two types of RTN. In the 1st case, multiple traps are independent from each other and in the second case the multiple traps have the correlation. We proposed the models for three states RTN to understanding the mechanism of them. These methods help us to understand the RTN mechanism and the correlation between multiple traps.

Atomically flattening of Si surface of silicon on insulator and isolation-patterned wafers

By introducing high-purity and low-temperature Ar annealing at 850 °C, atomically flat Si surfaces of silicon-on-insulator (SOI) and shallow-trench-isolation (STI)-patterned wafers were obtained. In the case of the STI-patterned wafer, this low-temperature annealing and subsequent radical oxidation to form a gate oxide film were introduced into the complementary metal oxide semiconductor (CMOS) process with 0.22 µm technology. As a result, a test array circuit for evaluating the electrical characteristics of a very large number (>260,000) of metal oxide semiconductor field effect transistors (MOSFETs) having an atomically flat gate insulator/Si interface was successfully fabricated on a 200-mm-diameter wafer. By evaluating 262,144 nMOSFETs, it was found that not only the gate oxide reliability was improved, but also the noise amplitude of the gate–source voltage related to the random telegraph noise (RTN) was reduced owing to the introduction of the atomically flat gate insulator/Si interface.

A statistical evaluation of effective time constants of random telegraph noise with various operation timings of in-pixel source follower transistors

We evaluated effective time constants of random telegraph noise (RTN) with various operation timings of in-pixel source follower transistors statistically, and discuss the dependency of RTN time constants on the duty ratio (on/off ratio) of MOSFET which is controlled by the gate to source voltage (VGS). Under a general readout operation of CMOS image sensor (CIS), the row selected pixel-source followers (SFs) turn on and not selected pixel-SFs operate at different bias conditions depending on the select switch position; when select switch locate in between the SF driver and column output line, SF drivers nearly turn off. The duty ratio and cyclic period of selected time of SF driver depends on the operation timing determined by the column read out sequence. By changing the duty ratio from 1 to 7.6 x 10-3, time constant ratio of RTN (time to capture <τc<)/(time to emission <τe<) of a part of MOSFETs increased while RTN amplitudes were almost the same regardless of the duty ratio. In these MOSFETs, <τc< increased and the majority of <τe< decreased and the minority of <τe< increased by decreasing the duty ratio. The same tendencies of behaviors of <τc< and <τe< were obtained when VGS was decreased. This indicates that the effective <τc< and <τe< converge to those under off state as duty ratio decreases. These results are important for the noise reduction, detection and analysis of in pixel-SF with RTN.

Demonstrating individual leakage path from random telegraph signal of stress induced leakage current

Time-dependent characteristics of stress induced leakage current (SILC) for the 5.7 and 7.7 nm oxides were evaluated to detect the random telegraph signal (RTS) of gate leakage current. The leakage current from the RTS of IG was extracted. Each current value extracted from RTS of SILC indicates the absolute current of individual localized leakage spot. The results indicate that the absolute value at individual leakage spot can be analyzed by this method, and this evaluation method is useful for the development of process and design technologies for highly reliable MOS devices.

Extraction of Time Constants Ratio over Nine Orders of Magnitude for Understanding Random Telegraph Noise in MOSFETs

The numerous RTN characteristics in MOSFETs were evaluated accurately for a wide time range by the array test circuit. The time constants were extracted for over six orders of magnitude and there were no correlation between time to emission τe and time to capture τc. The time constant ratios were distributed over nine orders of magnitude. Measuring small amplitude and extracting time constants for wide range are useful to understand the trap properties in MOSFETs.