New Technology to Make ZnO-Based N-Channel TFTs Key for Driving Future AMOLED Displays

21 Mar

Holst Centre and imec have demonstrated a new technology for fabricating state-of-the-art ZnO-based n-channel thin-film transistors (n-TFTs). The n-TFTs have been processed directly on top of flexible polyethylene naphthalate (PEN) foil by using spin-coating and post-anneal temperatures as low as 160°C. The threshold voltage of 3V makes the technology favorable for hybrid complementary line-drive circuitry at the borders of future flexible active-matrix organic light-emitting diode (AMOLED) displays on PEN foil.

AMOLED technology is currently emerging in handheld devices such as mobile phones and digital cameras and continues to make progress towards low-cost and large-size applications. Simultaneously, researchers work hard to make the displays flexible, thereby creating one of the most promising consumer markets. Imec has now developed a technology that allows integrating the display drive circuitry – which controls the current flowing to each individual OLED pixel – directly onto the flexible substrate. This way, the line drivers no longer restrict the flexibility of the display, and fabrication costs can be further reduced.

Key building block of this technology is the solution-processed ZnO-based metal oxide n-type TFT. The n-TFTs have been processed directly on top of a 25µm thick PEN foil by spin-coating and post-annealing at temperatures as low as 160°C. Such metal oxides processed from solution hold the promise of high-speed operation and low-power driving, and allow a simple and high throughput fabrication on a flexible substrate. PEN foils are preferred over other flexible plastic substrates because they are colorless, cheaper and less water absorbing. So far, however, the high annealing temperature (200 – 400°C) needed to process ZnO-based films was incompatible with using PEN foils.

The new n-TFTs show excellent transistor performances with saturation mobilities µsat of 0.60 up to 1.10cm2/Vs and Ion/Ioff ratios exceeding 107. 24 n-TFTs randomly spread over a 9cm2 area on PEN foil show good electrical uniformity and have promising bias-stress properties. A threshold voltage of 3V makes the technology favorable for hybrid complementary line-drive circuitry. Such a complementary thin-film technology at the edges of the rollable display is preferred because of its high robustness and reliability. The ZnO-based n-TFTs have therefore been integrated with imec’s baseline organic pentacene p-TFT backplane technology. Both complementary inverters and ring-oscillators have been fabricated.  The operating circuitry demonstrates the potential use of this low-temperature, robust and PEN-foil compatible hybrid complementary technology for embedded line drive circuitry at the borders of future AMOLED displays.

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