Oxide Semiconductors: Potential Revolutionary AMOLED Fabrication Technology
June 3rd, 2008by Charles Annis, Vice President, Manufacturing Research
SID, the display industry’s leading annual academic conference, is always a great source of information on R&D into future display technologies. Crystallization of a-Si to p-Si and its application to AMOLED fabrication have been a favorite topic of the symposium for several years now. However, looking over the proceedings of this year’s recently held conference, there anecdotally seems to be shift in focus; I could only find one paper on LTPS crystallization for AMOLED backplanes. Instead, a variety of papers promote using oxide semiconductors as a panacea to technical and cost issues that continue to restrict the growth of the AMOLED industry.
Samsung SDI’s development of a 12.1″ WXGA AMOLED (see Figure 1) highlighted the potential of this technology. The company used amorphous indium-gallium-zinc-oxide (a-IGZO) to replace silicon as the semiconducting layer of the TFT backplane.
Figure 1 IGZO based 12.1″ WXGA AMOLED
Source: Samsung SDI, SID 2008
A staggered gate and etch stop layer TFT design (see Figure 2) was employed to prevent degradation of the subthreshold gate swing and uniformity of the threshold voltage.
Figure 2 Staggered gate and etch stop layer TFT design
Source: Samsung SDI, SID 2008
Although IGZO TFTs have much lower mobility than p-Si based TFTs formed by ELA (Excimer Laser Annealing), at around 10 cm2/V-sec, mobility is greater than 10X that of a-Si TFTs and more than sufficient to drive OLEDs.
Advantages of IGZO compared to conventional p-Si TFTs include
- Deposited by PVD at low temperatures that might enable use of flexible or low-cost soda lime substrates
- Can be fabricated on conventional a-Si TFT lines at relatively low cost and scaled to large substrates (i.e., Gen 7 or larger)
- The smooth surface morphology of IGZO enables a clean interface with the gate insulator to provide a higher breakdown field
- Transparency
- Good uniformity and stability
- TFT characteristics are controllable by the metal composition and deposition parameters
In other words, oxide semiconductors are potentially a revolutionary technology that would negate the need for Si crystallization and enable large size, high quality, low-cost AMOLED displays.
Whether or not this will actually happen is still to be determined. Oxide semiconductor technology for display applications is not a mature technology and repeatability is said to be a significant issue. Currently all AMOLEDs in mass production are fabricated with some version of ELA or solid phase crystallization. Sony is investing in a dTLA (Diode Thermal Laser Annealing) pilot line to prove the manufacturability of its µc-Si technology. Others pursuing direct deposit p-Si, RTA (Rapid Thermal Annealing) and other techniques to overcome the continued uniformity, cost and scaling problems related to ELA. But, at least, the strong interest by the SID paper selection committee and the impressive results shown by Samsung SDI and others, suggest that oxide semiconductors are a technology to keep a close eye on when evaluating future AMOLED opportunity.
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