Bottom-Metal Induced Leakage Current of LTPS Diode for ESD Protection
Authors: Hwang Han Wook , Kim Ki Woo , Ha Yong Min , Kim Hyun Jae 
Published: February 2020

Leakage current behavior was investigated for diode-connected LTPS TFTs (LTPS diode) for electrostatic discharge (ESD) protection in display product with high luminance backlight system. In this study, the bottom metal layer was adopted to suppress leakage current caused by light coming from backlight system. Different off-current behavior as each other electrical connection of metal layer was observed, source-contacted or floating bottom metal showed the high leakage current by increasing current flow through back-channel at off states while gate-contacted bottom metal have comparable low leakage current with non-bottom metal layer. This leakage current characteristic was verified by technology computer-aided design (TCAD) simulation.

Objectives and Background

A low temperature polycrystalline silicon (LTPS) have been widely used for in active matrix liquid crystal displays (AMLCDs) and active matrix organic light emitting diode displays (AMOLEDs) as thin film transistor (TFT) backplane and system on panel (SOP), combined with controller, memory, and driver circuits, due to excellent electrical characteristics. The LTPS TFT based displays can achieve high brightness, high aperture ratio, high resolution, and low power consumption due to excellent electrical characteristics. On the other hand, in display product with high luminance backlight system, such as auto and mobile products, the LTPS TFTs require a bottom metal layer to reduce leakage current caused by light coming from backlight unit (BLU). This bottom metal layer in contacted with gate or source electrodes is applied under the poly-Si layer. However, this bottom metal layer may also affect seriously the leakage current of diode-connected LTPS TFTs (LTPS diode) for electrostatic discharge (ESD) protection in SOP.
In this study, the leakage current of LTPS diode for ESD protection was investigated with gate-contacted bottom metal, source-contacted bottom metal, and floating bottom metal compared with non-bottom metal. We measured the electrical characteristics with variation of electrical connection of bottom metal layer. Also, the technology computer-aided design (TCAD) simulation was performed to analyze the physical mechanism. From analysis results, optimized bottom metal design of LTPS diode was proposed for ESD production.

Experiment

Figure 1 shows the top-gate coplanar n-channel LTPS TFT structure with bottom metal layer under poly-Si. Gate and Drain electrodes were connected to common electrode in LTPS TFTs structure for LTPS diode. This LTPS diode was fabricated at low temperature of less than 450°C on glass substrate. The standard commercial NMOS processes, including the ion implantation, the excimer laser annealing (ELA), and the rapid thermal annealing (RTA), were employed in this experiment. To observe the electrical characteristics of LTPS diode, I-V characteristics were measured by using an Agilent B1500A semiconductor analyzer at room temperature.

Results and Discussion

Figure 2 shows I-V characteristics of LTPS diodes with gate-contacted bottom metal, source-contacted bottom metal, and floating bottom metal compared with non-bottom metal. The leakage current with source-contacted and floating bottom metal increased considerably compared with non-bottom metal. Especially, the leakage current with source-contacted bottom metal increased more than with floating bottom metal. On the other hand, the leakage current with gate-contacted bottom metal was low enough to be comparable with non-bottom metal. To analyze this phenomenon, we observed the electron concentration distribution within poly-Si at VGS= 0V, 10V, and -10V region using technology computer-aided design (TCAD) simulation as shown figure 3. For source-contacted bottom metal and floating bottom metal, at VGS= -10V, the electron was accumulated in the back-channel of poly-Si since the positive voltage is applied to back-channel by voltage difference between drain and bottom metal even though drain (=gate) electrode is applied to negative voltage. Therefore, at VGS= -10V, the leakage current flows through the back-channel in LTPS diodes with source-contacted bottom metal and floating bottom metal. But, for LTPS diode with gate-contacted bottom metal, when drain electrode was applied to negative voltage, there is no different voltage bottom metal and gate electrode because these electrodes were linked in common. Therefore, LTPS diode with gate-contacted bottom metal has low leakage current as comparable with non-bottom metal.

Conclusion

In summary, the leakage current of LTPS diode was strongly dependent on electrical connection of bottom metal layer. The electrical connection of bottom metal affects back-channel region within poly-Si due to the difference between bottom metal and drain electrodes at off region. Especially, LTPS diode with source-contacted bottom metal and floating bottom metal increased considerably the leakage current, while, gate-contacted bottom metal decreased leakage current. Therefore, bottom metal layer should be connected by gate electrode in LTPS diode for ESD protection.

Figure 1