TFT Devices Research Articles

An efficient device model for ferroelectric thin-film transistors

Ferroelectric thin-film transistors (Fe-TFTs) have promising potential for flexible electronics, memory, and neuromorphic computing applications. Here, we report on a physics-based efficient device model for Fe-TFTs that effectively describes memory switching and device I–V characteristics. This model combines a stochastic multi-domain description of FE switching dynamics with a virtual source treatment of TFT device characteristics. It demonstrates that the memory window of Fe-TFTs depends on the amplitude and duration of the applied voltage pulses, thus suggesting quantitative means of programming and control. Additionally, we introduce a machine-learning-enabled method to automatically generate optimal voltage pulses for accurately programming multiple intermediate FE states, which is crucial for multi-bit memory and neuromorphic computing applications. To showcase the model’s applications, we simulate a 4×4 crossbar array circuit based on Fe-TFTs, highlighting its utility in performing multiply-accumulate computing operations. This small array can achieve a high speed of ∼1.28 tera operations per second (OPS) and a power efficiency of ∼0.43 W/PetaOPS. The model developed here is valuable for exploring the capabilities of Fe-TFTs in future flexible memory and computing technologies.

IGZO TFT Backplane Integration for µLED Flat-Panel Display

The focus of this work is the process integration of Indium Gallium Zinc Oxide (IGZO) transistors as a µLED backplane for row/column addressing. A single pixel is composed of a µLED driven by an arrangement of two transistors and a storage capacitor. The pixels are then arrayed on a glass substrate to support active-matrix control of monochrome and full color (RGB) displays from 1x1cm (50 x 50 pixels) up to 7.6 x 7.6 cm (380 x 380 pixels). Optimization of circuit parameters considering size and scan frequency was modeled using existing TFT and µLED electrical device compact models. New process parameters and procedures were determined for the proper integration of µLEDs in an existing TFT fabrication process. Red, green, and blue µLED devices were fabricated at Tyndall National Institute on native substrates and were transferred to TFT pixels using an X-Display MTP-1003 micro-transfer printer. The performance of individual test cells was assessed using an Agilent B1500, revealing a voltage transfer characteristic indicating the ability to modulate and control µLED current.

9‐3: The Latest Technology Breakthroughs for 31” 4K Flexible Printed OLED TV Display Technology

The flexible printed OLED TV display technology will be able to create new application market, such as rollable/foldable TV, in the future. It's TV Mobiles. Since the TCL (TCL CSOT, Guangdong Juhua Printed Display Technology Co. Ltd. & TCL Corporate Research) has successfully demonstrated the 31” FHD flexible printed OLED TV, there are a lot of technology breakthroughs for improving the panel performance and cost reduction. The co‐planar top gate Oxide TFT device architecture, printing OLED top emission device architecture with new OLED materials, MURA free ink jet printing technology with unique bank design, advanced encapsulation technology with new inorganic material for panel performance improvement are developed. Especially innovative mechanical release technology without using laser lift off technology is developed for cost reduction and production yield up in flexible printed OLED TV MP line. Based on these technology breakthroughs, 31” 4K flexible printed OLED TV with high resolution(142ppi) are fabricated.The performance of 31” flexible printed OLED TV is, 4K high resolution (3840x2160, 142ppi), luminance: FW 150nits, excellent C/R; > 1M:1, color gamut: DCI‐P3: >99%, wide viewing angle (Δu'v' < 0.02): ≥ +/‐ 45°, A/R: 47.75%, and curvature radius: 20mmIn this paper, the panel performance of 31” 4K high resolution flexible printed OLED TV and the latest technology breakthroughs are discussed in details.

14‐1: A Study on Flexibility Improvement of AMOLED Back Plane and Mask Reduction Process Architecture Using Photo‐sensitive Organic Insulation Films

In this work, the method of patterning inter‐layer deposition (ILD) and passivation layer (PVX) of the AMOLED back plane structure with an organic film was studied. By replacing the inorganic film of SiNx + SiO2 with an organic film Poly‐imide, back plane suitable for flexible AMOLED was secured. It was possible to secure equivalent TFT transfer characteristics of device compared to inorganic inter layer structure by applying low‐temperature buffer and 2nd gate insulator SiNx to the organic inter layer TFT device. Also, VIA + passivation layer 2 mask photo process was replaced with 1 mask photo process using a VIA Half‐tone mask. Both processes were able to secure a process architecture that can be applied to flexible and rollable displays in the future by using photo‐sensitive organic insulation films.

P‐168: Development of Conformal Anti‐diffraction Transparent LTPS TFT Pixel Arrays on Two‐axis Surface

This study shows the electrical and optical results of anti‐diffraction and high aperture ratio low‐temperature poly‐silicon (LTPS) TFT on bending surface. The resistance maintenance rate of the signal metal wiring is up to 90.4%. Our work gives rise to the results of stretchable LTPS TFT device, and indicates the excellent electrical maintenance stability of the metal wiring on a two‐axis bending surface with radius of Rx/Ry=600 mm/800 mm. The results also imply the high‐transparent anti‐diffraction pixel design on two‐axis surface could show clearer background image than traditional pixel design, which can be used as a basis for future development of different applications with conformal stretchable transparent displays.

Flexible TFT backplane development for extremely small bending radius with organic ILD and novel TFT structures

AbstractA new flexible low‐temperature polycrystalline silicon thin film transistors (LTPS TFTs) on PI substrate with organic ILD layer, island TFT, and TFT channel perpendicular to stress direction was proposed. AMOLED display panel manufactured with organic ILD did not show any visible degradation of panel image after outward rolling cycles of 200,000 at rolling radius of 4 mm or inward folding cycles of 200,000 at folding radius of 1 mm. Another novel structure of TFT with channel perpendicular to stress direction significantly reduced change of threshold voltage of −0.03 V compared with change of threshold voltage of −0.6 V for TFT with channel parallel to stress direction for reliability test of high drain current (HDC) after 100,000 outward‐bending cycles at bending radius of 3 mm. After inward‐bending cycles of 200,000 at bending radius of 1 mm, reliability test results for TFT device with organic ILD layer showed that change of on‐current is 10.07% for hot carrier instability (HCI) test, change of threshold voltage −0.31 V for negative bias thermal instability (NBTI) test, change of threshold voltage −0.24 V for hysteresis test, and breakdown voltage of gate insulator 7.73 MV/cm, respectively, and these performances are similar to those of TFT device with inorganic ILD (SiNx).

Microstructure, dielectric and optical properties of transparent flexible high-k Bi1.5MgNb1.5O7 thin films

High-k insulation materials can effectively reduce the operating voltage and leakage current of TFT devices, improving device performance owing to their high dielectric constant and insulation properties. Transparent flexible Bi1.5MgNb1.5O7 (BMN) dielectric thin films were synthesized via RF magnetron sputtering at room temperature. The impact of argon to oxygen ratio (Ar:O2) on the microstructure, elemental composition, optical, and dielectric properties of the BMN dielectric films was systematically explored. X-ray photoelectron spectroscopy, scanning electron microscopy, and optical property tests revealed that the proportion of metallic Bi and grain size play crucial roles in the optical transmittance and optical bandgap of the BMN films. Moreover, the dielectric properties were significantly influenced by the space charge polarization resulting from the percolation capacitance model formed by metallic Bi in the samples. To further optimize the dielectric properties of BMN films, sandwich structured OAO (BMNO/BMNAr/BMNO) composite films were designed. The OAO composite film exhibited the best overall performance, with an average transmittance of 92.27 %, a dielectric constant of 84, a dielectric loss of 0.00672, and a leakage current density of approximately 1.4 × 10−8 (5 V) when the thickness of the BMNO layer was 15 nm. These results highlight the significant potential of BMN as a transparent flexible high-k dielectric material.

Effect of Barrier Layer Properties of LTPS TFT Devices on AMOLED Short-term Image Sticking Performance

Effect of Barrier Layer Properties of LTPS TFT Devices on AMOLED Short-term Image Sticking Performance

P‐13.6: The investigation on high‐temperature color deviation of flexible AMOLED display

As a new type of display technology, organic light‐emitting diode display technology (OLED) is widely used in vehicle, tablet, industrial control medical, mobile phone, wearable and other fields, and it is described as the future mainstream display technology after the liquid crystal display technology. However, when the AMOLED display is working for a long time, the display screen will heat up, the temperature of the screen will rise. And as the temperature rises, the performance of the TFT and OLED devices will be affected. The investigation on high temperature color deviation is studied in detail, we compared and analyzed the change of CIE‐x and CIE‐y color point, I‐V characteristic curve, efficiency and recombination center at different temperatures. The results show that the recombination center of OLED device changes with the temperature increasing, leads to the changes of the efficiency. The difference of red, green and blue efficiency leads to the color deviation, and the green pixels is the most sensitive to the change of current due to its highest efficiency, which is easier to show greenish phenomenon.

P‐7.2: Suppression of Ion Drop of ITO‐Decapped a:Si‐H TFT for ADS Pro Display

The decay of Ion of ADS Pro‐type TFTs experienced 1ITO decap often brings about a lot of TFT devices scrapped. It is necessary and valuable to identify the reasons and to solve the Ion drop problem induced by decap. In the present work, the difference in procedure and electronic performance between the normal and the decap TFT process was first compared, then a series of controlled and confirmation experiments to get the mechanism of Ion drop were designed and carried out. It is found that not the extra etching, but the additional ITO film deposition procedure in the decap process is the cause of Ion degradation; during ITO deposition process, the plasma cleaning would lead to the plasma damage to the silicon island, and the In and O atoms in ITO target would intrude into the silicon to form p‐type doping or SiOx oxide, giving rise to the decrease of the carrier concentration and the increase of the resistance, which eventually account for the Ion drop. Finally, the measures to suppress the Ion degradation caused by 1ITO decap were proposed and verified, it shows that the decay percentage of Ion will decline from 15% in the normal decap process to 8.9%, 12.2% and 10.6% by turning off the plasma cleaning, cutting down the deposition power, and reducing the ITO annealing temperature, respectively. And when these three above methods are combined introduction together, the Ion drop after decap can be further lower to less than 5%, which meets the requirements of mass production. This study provides a way to suppress Ion degradation and a reference to optimize TFT properties and improve product yield.

Performance Enhancement of Solution-Processed SixSnyO TFTs using Solution Combustion Synthesis

Amorphous oxide semiconductors thin-film transistors (AOs TFTs) have gained interest over the years because of their advantages over a-Si and poly-Si transistors.In particular, tin oxide is used as a semiconductor material because of its abundance, low toxicity, and high intrinsic mobility. Performance-wise, tin oxide TFTs need improvement because the reported average mobility remains < 10 cm2/Vs.1 Previous research used silicon doping to reduce tin oxide’s oxygen vacancies and improve its performance as a semiconductor channel.Currently, SixSnyO TFTs are fabricated using vacuum process, but solution processing is important to realize large-scale and high-throughput electronics. Sol-gel method is one of the conventional routes to produce solution-processed metal oxides. However, it requires high temperature (≤500°C) to facilitate film densification which is not compatible with flexible substrates having low degradation temperatures. Among the approaches for low-temperature TFT fabrication, solution combustion synthesis (SCS) has garnered significant attention because of its simplicity and versatility.2 SCS uses the energy released by combustion to help the formation of more M-O-M networks in the film. Thus, there is no need to use high temperature to condense and densify the film. This study attempts to produce SixSnyO TFTs through solution processing for the first time. We investigated the effect of solution combustion synthesis to the quality of SixSnyO films. Next, we optimized the aging time and oxidizer to fuel ratio (ammonium nitrate to urea) to determine the films with better quality. For aging time optimization, SixSnyO precursors were stirred at RT for 24-h, 48-h, and 72-h. Lastly, we compared the TFT performance in terms transfer characteristics of SixSnyO TFTs fabricated by sol-gel and SCS annealed at the same temperature. Secondary ion mass spectrometry (SIMS) analysis was used to compare the carbon impurities present in both SixSnyO films. The sol-gel film contained higher amounts of carbon impurities, about an order of magnitude higher as shown in Figure 1. This proves that SCS provides extra energy to help degrade the carbon-containing compounds in the film. Carbon atoms are considered impurities that can introduce defects in the SixSnyO film. These defects are detrimental to the performance characteristics of the fabricated TFT device. In addition, the thickness of the films was determined using X-ray reflectivity. The SCS film is 15.70 nm-thick while the sol-gel film is 18.88-nm thick. Furthermore, optimization of the SixSnyO films via SCS revealed that oxidizer to fuel ratio affects the roughness and amount of M-O bonds formed in the film. The 3:1 ratio failed to produce a film while other ratios were successful in producing films. Although the film with 1:1 ratio has root mean square roughness (Rq) value of 0.380 nm, it contains depressions and voids. On the other hand, the film using a 1:3 ratio showed a uniform and even surface with Rq value of 0.377 nm. In addition, characterization of the two films through X-ray photoelectron spectroscopy (XPS) showed higher percentage of M-O bonds in the O1s spectrum (530.5 eV) for the film with 1:3 ratio. The values are 45.9% and 59.3% for 1:1 and 1:3 ratios, respectively. The amount of oxygen vacancy and H- containing compounds also decreased in the 1:3 film.SixSnyO TFTs were fabricated from both SCS and sol-gel methods. Both TFTs showed high off-current and highly negative Vth, as shown in Figure 1. Therefore, both TFTs still contain several defects which suggests that 3% Si doping may be insufficient and must be increased to produce a significant effect. The sol-gel fabricated TFT also showed lower drain current. Therefore, the low annealing temperature at 300 ˚C is insufficient to densify the film and reduce impurities. On the other hand, SCS-assisted fabrication showed higher mobility at 3.14 cm2/Vs – confirming that SCS provided additional energy to degrade impurities and improve the M-O network in the SixSnyO channel. In conclusion, we were able to produce SixSnyO TFTs with improved mobility using solution combustion synthesis. However, the high V th and high off-current issues need to be addressed by increasing silicon dopant addition and exploring other fuel options. But overall, we can confirm that SCS is beneficial in improving the quality of SixSnyO films annealed at 300 ˚C. In the future, this low temperature fabrication is important to realize flexible SixSnyO TFT devices.AcknowledgementsThis research was supported by NAIST foundation grant.References Chang, H., et al. ACS Applied Electronic Materials, 3(11), 4943–4949.Carlos, E., et al. Chemistry - A European Journal, 26(42), 9099–9125. Figure 1

(Digital Presentation) High Performance Back-Channel-Etch Igzo TFT with Self-Passivate HfO2 By Backside Exposure Technology

Recently, amorphous oxide semiconductor (AOS)-based TFTs have drawn much attention for the backplane technologies of the ultra-high resolution TFT-LCDs and AMOLEDs [1-2]. To overcome the degradation of electrical characteristics caused by the ambient, a passivation layer capsulation is proposed to solve the problem. In this work, a capping layer of HfO2 film is studied acting as both a passivation layer and an etch stop layer for TFT device structure with the back channel etching (BCE) process. The BCE type of indium-gallium-zinc-oxide (IGZO) TFT is successfully demonstrated with a self-aligned backside exposure method for the photo-mask reduction, which not only decreases the complexity of the device fabrication but also lowers the manufacturing cost [3-4].ExperimentAmorphous IGZO TFTs with a bottom gate staggered structure were fabricated in this work. A layer of metallic Mo thin film was first deposited on a glass substrate by DC sputtering and patterned as the gate electrode. It was followed by the deposition of HfO2 film as gate insulator and a-IGZO as channel layer by RF sputtering. Besides, a HfO2 capping layer was deposited on top of IGZO channel layer as the passivation and etch stop layer (ESL). The self-aligned backside exposure process was then implemented to pattern the IGZO channel layer and ESL simultaneously. And Mo source/drain (S/D) electrodes were formed to finish the BCE a-IGZO TFT manufacture. The IGZO TFT was also fabricated with a lift-off S/D process. Firstly, the exposure of the S/D area patterns was executed by removing the photoresist covered on the top of the S/D in the development process. Then, a layer of Mo film was deposited by DC sputtering, and followed by performing the lift-off process in acetone for the photoresist stripping. Thus, the open hole on S/D pattern areas will be filled with Mo and form S/D electrodes. The manufacture of lift-off S/D IGZO TFT was completed, and it will be compared with the characteristics of BCE a-IGZO TFT.Result and DiscussionThe experimental results have shown that the HfO2 capping layer can keep IGZO channel from exposing to oxygen and hydrogen in the atmosphere. Also, HfO2 capping layer could be acting as etching stop layer to protect IGZO channel from the damage during BCE process. As compared with a lift-off S/D process without BCE damage, the nearly non-degraded BCE a-IGZO TFT in this work demonstrates the compatible performance such as threshold voltage of 0.03 V, an approximately ideal sub-threshold swing of 65.8 mV/dec., field-effect mobility of 9.91 cm2/V·s, on/off current ratio of 6×107 (at VD=0.1 V), and extremely low electrical hysteresis loop of ~ 0 mV was obtained successfully. Furthermore, the self-aligned process with backside exposure could lower the cost for device fabrication by decreasing the number of photomasks, and the BCE process have the ability for channel size scaling. With the excellent characteristics mentioned above, the BCE a-IGZO TFT is highly applicable to the emerging high-resolution display technologies.Reference[1] T. Kamiya, K. Nomura , and H. Hosono, Science and Technology of Advanced Materials, vol. 11, no. 4, pp. 044305, 2010.[2] P. Y. Kuo, C. M. Chang, I. H. Liu, and P. T. Liu, Scientific reports, vol. 9, no. 1, pp. 1-7, 2019.[3] Y. Kuo, Journal of The Electrochemical Society, vol. 138, no. 2, pp. 637, 1991.[4] N. Münzenrieder, P. Voser, L. Petti, C. Zysset, L. Büthe, C. Vogt, G. A. Salvatore, and G. Tröster, IEEE Electron Device Letters, vol. 35, no. 1, pp. 69, 2014. Figure 1

Intrinsic Channel Mobility Associated with Extended State Transport in IGZO TFTs

There is wide variation in the understanding of the interaction and/or independence of channel mobility and charge trapping in IGZO TFTs. A new interpretation of transport behavior proposes an intrinsic channel mobility with distinctive temperature dependence. Bottom-gate TFTs were fabricated with a 50 nm IGZO film sandwiched between a 50 nm gate oxide and a 50 nm passivation oxide layer. The working metal was molybdenum for the gate electrode and source/drain contacts. A passivation anneal was done and a capping layer was added to promote electrical stability; full process details are provided in a previous report. Devices were tested using a Lakeshore cryogenic probe station, with transfer characteristic measurements taken from room temperature to below 100 K.Electrical measurements made on long-channel devices (L = 12 µm) has revealed temperature-dependent behavior that is not explained by existing TCAD models employed for defect states and carrier mobility. An IGZO TFT device model has been recently developed using Silvaco Atlas, which accounts for the role of donor-like oxygen vacancy defects, acceptor-like BTS, acceptor-like interface traps, and a temperature-dependent intrinsic channel mobility. The model demonstrates a remarkable match to transfer characteristics measured at T = 150 K to room temperature. The temperature-dependent mobility follows a power-law relationship resembling behavior consistent with ionized defect scattering; the source of which is proposed. Details of the material and device model, and associated defect distribution parameters will be presented. Figure 1

P‐6.3: The Analysis and Improvement of TFT Vth Shift Effect on AMOLED Panel Lifetime Test†

In order to solve the problem in quick drop of luminance on the initial stage of AMOLED lifetime test, from experiment results, the quick drop of luminance is caused by TFT Vth shift. Present actions include the TFT device stability improvement and aging the TFT device before lifetime test to stable state which can increase the panel lifetime.

Intrinsic Channel Mobility Associated with Extended State Transport in IGZO TFTs

There is wide variation in the interpretation of the interaction and/or independence of channel mobility and charge trapping in IGZO TFTs. Electrical measurements reveal temperature-dependent behavior that is not explained by existing TCAD models employed for defect states and carrier mobility. An IGZO TFT device model has been recently developed using Silvaco Atlas, which accounts for the role of donor-like oxygen vacancy defects, acceptor-like BTS, and acceptor-like interface traps, thus properly regulating the amount of free channel charge. The channel mobility is defined as only a function of temperature, and reflects a thermally-activated diffusive mobility with a distinct activation energy Ea = 40 meV. This thermally activated process is clearly intrinsic, as it is constant over all bias conditions and is not dependent on the steady-state trapped charge level. The model demonstrates a remarkable match to transfer characteristics measured at T = 150 K to room temperature.

Electrical Characteristics Enhancement for P-Type SnOx Thin Film Transistors with Furnace and Microwave Annealing

Since the first n-type TFT was introduced in 1962, it was made of polycrystalline cadmium sulfide (CdS) as the active layer, silicon dioxide (SiO2) and gold (Au) as the insulator electrodes. Now, n-type TFT is still the most applications in display products. In order to make display products more energy-efficient, complementary MOS circuit design is considered as a promising selection, and P-type TFT devices play an important role in it. This work focuses on how the oxygen ambient annealing process affects SnOx TFTs device electrical characteristic. In the oxygen ambient, both furnace and microwave annealing (MWA) are used to anneal P-type SnOx TFTs to realize its P-type property and enhance its electrical characteristics. The best results are reached at 300 °C, 30 min of furnace annealing and 1200 W, 100 sec of MWA. The fieldeffect mobility are 0.2596 cm2/V * s and 0.1581 cm2/V * s, threshold voltage are −2.65 V and −3.28 V, sub threshold swing are 0.485 V/decade and 0.93 V/decade, current ratio Ion/Ioff are 3.07×104 and 1.65×104, respectively. The experiment results show that both furnace annealing and MWA in oxygen ambient could effectively enhance P-type SnOx TFTs.

P‐14: Effects of Film Density on IGZO Based TFT Device Reliability

Density effects of IGZO films were studied based on DFT calculations, and the electrical and physical properties between the high‐density film and the conventional film were compared. As the density increased, the carrier concentration, hole mobility, and the optical energy band gap of the IGZO film increased. Additionally, micro‐pore defects were not observed in the high‐density film unlike the conventional film through HR‐TEM cross‐sectional analysis. Also, we fabricated coplanar TFT devices with different density IGZO films and conducted PBTS and NBTiS reliability tests. The device with high‐density IGZO film was showed superior reliability characteristics

P‐19: Performance Development of Oxide Semiconductor Photodiode with High Work Function Electrode Suitable for Mass Production

IGZO as the typical representative of oxide semiconductor used for TFT devices, has been widely used in display panel foundary, such as large size OLED, LTPO technology direction. To develop oxide based photodetector (sensor), not only can expand the application direction of oxide semiconductor material, moreover the oxide photodetector integrated with oxide TFT is more convenient. We developed several oxide based vertical MSM photodiode with schottky junction, the IGZO MSM can respond to ultra violet(UV), while new oxide material S1 and S2 can response to blue light. In order to form a schottky junction, metal electrodes materials should have high work function, thus we developed a high work function electrode suitable for mass production, and of witch work function is ~4.7eV. In addition, in order to gain the characteristics of diode, the ACT film need be deposited in high oxygen process to prevent form ohm contact. Assisted by TCAD simulation, we researched how to control the oxide schottky junction, including schottky junction, barrier tunneling, to instruct the material selection and electrical properties of control.

P‐17: Outstanding Image Sticking Performance via L‐SWTFT Channel Ā Tuning in AMOLED Display Application

This paper introduces a convenient technique for improving AMOLED image sticking performance which is currently a tough problem in the display industry. Based on our experimental results together with TFT device simulation outcome, by changing the L‐SWTFT (Dual‐gate SWTFT, T2) gate size in our 7T1C circuit, the outstanding voltage‐writing ratio in charging phase is realized, so as to get better image sticking performance. Different from the conventional image sticking improvement method, this technique provides a more feasible design‐related change proposal to replace the process optimization of traditional TFT‐related film quality and interface. Only by reducing the channel length of L‐SWTFT, the driver TFT gate working voltage can be easier to be written to the target value right after the L‐SWTFT Ion increases, so as to refresh the influence of residual electrical charges due to picture switching from the previous frame. Our image sticking evaluation results reveal the average ISFOM score for T0 is increased by about 14.3% with the maximum score increased by about 28.1% and the average ISFOM score for T300 is improved by 23.4% with the best value of 42.2% when the L‐SWTFT length is reduced, which shows a significant improvement for image sticking performance.

P‐27: A Novel PWM Driving Pixel Circuit with Metal‐Oxide TFTs for MicroLED Displays

A pixel circuit for micro‐LED display has been developed with IGZO TFT device. By using pulse width modulation method, the pixel circuit can operate at high peak current and low duty ratio to keep micro‐LED at high emission efficiency and stable color performance.