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. 2021 Apr 16;11(1):8387.
doi: 10.1038/s41598-021-87950-0.

Threshold voltage instability and polyimide charging effects of LTPS TFTs for flexible displays

Hyojung Kim  1   2 , Jongwoo Park  1 , Taeyoung Khim  1 , Sora Bak  1 , Jangkun Song  3 , Byoungdeog Choi  4
Affiliations

Threshold voltage instability and polyimide charging effects of LTPS TFTs for flexible displays

Hyojung Kim et al. Sci Rep. .

Abstract

In this paper, we investigate the Vth shift of p-type LTPS TFTs fabricated on a polyimide (PI) and glass substrate considering charging phenomena. The Vth of the LTPS TFTs with a PI substrate positively shift after a bias temperature stress test. However, the Vth with a glass substrate rarely changed even with increasing stress. Such a positive Vth shift results from the negative charging of fluorine stemmed from the PI under the gate bias. In fact, the C-V characterization on the metal-insulator-metal capacitor reveals that charging at the SiO2/PI interface depends on the applied gate bias and the PI material, which agrees well with the TCAD simulation and SIMS analyses. As a result, the charging at the SiO2/PI interface contributes to the Vth shift of the LTPS TFTs leading to image sticking.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
(a) ID–VG plot of P-type LTPS TFTs before and after the BTS. Note that the P-type LTPS TFTs were fabricated on glass (black filled circle) and PI (blue filled triangle) substrates and (b) ΔVth before/after BTS of TFTs fabricated on glass and PI substrates.
Figure 2
Figure 2
(a) Schematic of 4-pad characterization (floating gate, source, gate and drain) and (b) normalized Vth behaviors of P-type LTPS TFTs fabricated on the on glass (black filled circle) and PI (blue filled triangle) substrates.
Figure 3
Figure 3
Correlation between PI charging induced Vth shift and image sticking for an LTPS TFT with a PI substrate.
Figure 4
Figure 4
Schematics of the vertical structure for the MIM capacitors and SEM image; (a) Ag/SiO2/Ag, (b) Ag/PI/Ag, (c) Ag/SiO2/PI/Ag and (d) the cross-sectional analysis of the MIM capacitor shown in (c).
Figure 5
Figure 5
C–V characterization for 3 different MIM capacitors with increasing voltage; (a) Ag/SiO2/Ag, (b) Ag/PI/Ag, (c) Ag/SiO2/PI/Ag, and (d) the dependency of the MIM capacitors on the applied voltage.
Figure 6
Figure 6
Hydrogen, hydroxy group and oxygen profiles obtained by SIMS measurements of capacitors before/after bias stress; (a) Ag/SiO2/PI-A/Ag and (b) Ag/SiO2/PI-B/Ag.
Figure 7
Figure 7
SIMS characterization of Ag/SiO2/PI/Ag capacitor focused on fluorine profile at the interface; (a, b) with PI-A type and (c, d) with PI-B type. Note that PI-A is less crosslink density than PI-B.
Figure 8
Figure 8
ID–VG plot based on the amount of charge trapped at the SiO2 and PI interface by TCAD simulation.
Figure 9
Figure 9
TCAD simulation for hole concentration (cm−3) versus channel depth (nm) of an LTPS TFT with a PI substrate. Reference represents a LTPS TFT with a PI substrate without charge injection at the SiO2/PI interface.
See this image and copyright information in PMC

References

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