Micro-LED Microdisplays Driven by Carbon Nanotube Active-Matrix Backplanes

Area of Science:

• Materials Science
• Electrical Engineering
• Display Technology

Background:

• Micro-light-emitting-diode (μLED) displays promise revolutionary advancements in flat-panel display (FPD) technology, offering superior brightness, contrast, efficiency, and resolution.
• High pixel-per-inch (PPI) μLED microdisplays require advanced thin-film transistor (TFT) backplanes, but current solutions like silicon CMOS, LTPS, and metal oxides face limitations in scalability, performance, and transparency.
• Two-dimensional transition metal dichalcogenides (TMDs) show potential but have been hindered by complex integration and scalability issues, limiting them to semiactive-matrix demonstrations.

Purpose of the Study:

• To develop and demonstrate an optimized carbon nanotube (CNT) thin-film transistor (TFT) backplane for active-matrix (AM) μLED microdisplays.
• To overcome the limitations of existing backplane technologies for high-PPI μLED applications.
• To establish CNT TFTs as a viable and scalable solution for next-generation μLED microdisplays.

Main Methods:

• Fabrication of optimized CNT TFTs utilizing an Al2O3/SiO2 gate dielectric stack and Y2O3/SiO2/polyimide passivation layers.
• Characterization of CNT TFT performance, including driving current and mobility, at varying channel lengths (Lch).
• Development of a heterogeneous integration process using flip-chip eutectic bonding for assembling μLED arrays onto CNT TFT backplanes.
• Design of CNT TFT-based 2T1C pixel circuits and peripheral control circuits supporting PAM and PWM modulation.

Main Results:

• Optimized CNT TFTs with Lch = 3 μm achieved a driving current of ~10 μA/μm and mobility of ~27 cm²/(V·s).
• Scaling Lch to 0.5 μm significantly enhanced performance, yielding a driving current of ~80 μA/μm and mobility of ~40 cm²/(V·s), surpassing previous CNT TFTs for AM displays.
• The heterogeneous integration process achieved ~100% yield for μLED array assembly onto CNT TFT backplanes.
• A 32 × 32-pixel AM-μLED prototype microdisplay with 357 PPI was successfully demonstrated, capable of dynamic image and video display.

Conclusions:

• The developed CNT TFTs provide robust driving capabilities essential for high-PPI AM-μLED microdisplays, enabling resolutions up to 3400 PPI.
• The heterogeneous integration method ensures reliable assembly of μLEDs onto CNT TFT backplanes.
• CNT TFTs represent a promising, scalable solution for advanced μLED microdisplays, paving the way for next-generation AR/VR devices.