A low-temperature polycrystalline oxide or LTPO display is a specific type of backplane technology used as a primary thin-film transistor material. Apple Inc. developed and patented this technology for use in devices equipped with OLED displays. However, it is also applicable to liquid crystal display or LCD technologies such as in-plane switching or IPS LCD, as well as emerging display technologies such as quantum dot and microLED displays.
Understanding Low-Temperature Polycrystalline Oxide
What exactly is low-temperature polycrystalline oxide? What is an LTPO display? Note that an LTPO is a semiconductor that combines the properties of low-temperature polycrystalline silicon or LTPS and oxide-based materials such as indium gallium zinc oxide or IGZO. Hence, a backplane technology based on LTPO has the characteristics of an LTPS TFT and an IGZO TFT.
More specifically, in an LTPS display, switching circuits use LTPS material while the driving TFT uses IGZO material. Combining both silicon-based and oxide-based semiconducting materials provide notable advantages. Apple developed this material to maximize the functional and aesthetical characteristics of its smaller devices such as the Apple Watch.
The Pros: Advantages and Applications of LTPO Display
Power Efficiency for Portable Devices
Efficiency in power consumption is one of the major advantages of LTPO display over other displays using other backplane technologies. The material has a power saving of between 5 to 15 percent when compared to LTPS. Power efficiency has key applications on smaller devices.
Remember that Apple developed and patented this material for the Apple Watch and other portable devices in the future. The technology has allowed the company to use an ultra-low power display to offset the tradeoff between battery capacity and screen size.
Furthermore, Apple has managed to include additional hardware capabilities and features with newer generations of the Apple Watch due to its power efficient display. These include an integrated power management circuit, additional sensors based on LEDs and photodiodes for measuring blood oxygen level and heart rate, and better processors, among others.
Advantages of LTPS and IGZO Materials
Because an LTPO is fundamentally a hybrid of low-temperature polycrystalline silicon and oxide-based materials such as indium gallium zinc oxide, an LTPO display features the advantages of an LTPS display and an IGZO display.
One of these advantages is better electron mobility when compared to amorphous silicon or a-Si TFT display. Better electron flow allows the manufacturing of display panels with higher resolutions and faster pixel response time. In addition, it enables manufacturers to produce small display panels with high pixel density.
Note that low-temperature polycrystalline oxide is an intermittent type of semiconductor. More specifically, because the driving TFT uses IGZO material, an LTPO display does not need a continuous drive, unlike silicon-based backplane technologies.
This material is key to the always-on display feature first showcased on the Apple Watch Series 5, and allows the device to change the refresh rate from 60Hz down to 1Hz when not in use. The material allows a display panel to operate at a low refresh rate without visible artifact.
The Cons: Disadvantages and Limitations of LTPO Display
The fact that low-temperature polycrystalline oxide combines LTPS and IGZO means that it also shares some of the drawbacks and limitations of both materials. For example, LTPS is more complicated and costlier to manufacture than a-Si, while IGZO uses rare-earth materials that can be difficult to procure.
Producing displays based on LTPO is inherently more taxing because it requires attending to the manufacturing challenges of both LTPS and IGZO. Note that low-temperature polycrystalline oxide is more expensive to produce than IGZO alone. The cost implication affects the overall cost and market price of end-user devices.
The metal oxide component of this material also has a high reactivity to oxygen, thereby making it susceptible to low voltage. It may also age faster than LTPOS or a-Si due to oxidation, as demonstrated by diminishing voltage sensitivity or capacity to become responsive to the application of voltage.
On the other hand, low-temperature polycrystalline silicon is sensitive to high temperatures and susceptible to overheating. Exposure to heat can breakdown the silicon-hydrogen bond on the material and degrade the overall backplane component.
FURTHER READINGS AND REFERENCES
- Chang, T-K., Lin, C.-W., & Chang, S. 2019. “39‐3: Invited Paper: LTPO TFT Technology for AMOLEDs.” SID Symposium Digest of Technical Papers. 50(1): 545-548. DOI: 1002/sdtp.12978
- Chang, T-C., Tsao, Y-C., Chen, P-H., Tai, M-C., Huang, S-P., Su, W-C., and Chen, G-F. 2020. Flexible Low-Temperature Polycrystalline Silicon Thin-Film Transistors. Materials Today Advances. 5: 100040. DOI: 1016/j.mtadv.2019.100040
- Chung, U-J., Choi, S-C., Noh, S. Young, Kim, K-T., Moon, K-J., Kim, J-H., Park, K-S., Choi, H-C., & Kang, I-B. 2020. “15‐1: Invited Paper: Manufacturing Technology of LTPO TFT.” SID Symposium Digest of Technical Papers. 51(1): 192–195. DOI: 1002/sdtp.13835