A low-temperature polycrystalline oxide or LTPO display is a specific type of backplane technology used as a primary thin-film transistor material. Apple developed and patented this technology for use in devices equipped with OLED displays. However, beyond OLED, it is also applicable to liquid crystal display or LCD technologies such as in-plane switching or IPS LCD and 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? 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. A backplane technology based on LTPO has the characteristics of an LTPS TFT and an IGZO TFT.
An LTPS display uses switching circuits made of LTPS material and a driving TFT made of IGZO material. The combination of both silicon-based and oxide-based semiconducting materials provides notable advantages. Apple developed this material to maximize the functional and aesthetical characteristics of its smaller devices such as the Apple Watch.
Pros: Advantages of LTPO Display and Applications
Power Efficiency for Portable Devices
Efficiency in power consumption is one of the major advantages of LTPO display over other displays based on 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 in which battery life and physical dimension are important design and functional factors
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.
Apple has also 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
Remember that low-temperature polycrystalline oxide or LTPO is a hybrid material made of low-temperature polycrystalline silicon and oxide-based materials such as indium gallium zinc oxide. This means that another advantage of an LTPO display is that it has the properties and specific advantages unique to both 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. A better electron flow allows the design and production of display panels with higher resolutions and faster pixel response time. The same benefit also enables manufacturers to produce small display panels with high pixel density and high resolution.
It is also important to underscore the fact that low-temperature polycrystalline oxide is an intermittent type of semiconductor. To explain further, because the driving TFT uses IGZO material, a particular LTPO display panel does not need a continuous drive, unlike silicon-based materials such as amorphous silicon and derivative backplane technologies.
This material is key to the always-on display feature first showcased on the Apple Watch Series 5 and it allows the device to change the refresh rate from 60 Hz to 1 Hz when not in use. It also enables a display panel to operate at a low refresh rate without visible artifacts.
Cons: Disadvantages of LTPO Display and Limitations
The fact that low-temperature polycrystalline oxide combines the properties and advantages of LTPS and IGZO means that it also shares some of the drawbacks and limitations unique to each material. Take note that an LTPS is more complicated and costlier to manufacture than a-Si and an 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. LTPS is more expensive to produce than IGZO alone. The cost 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. This makes it susceptible to low voltage. It also has a shorter lifespan because it ages faster than LTPS and amorphous silicon due to oxidation. This is demonstrated through diminishing voltage sensitivity or instances of becoming responsive to the application of voltage.
It is also worth mentioning that low-temperature polycrystalline silicon is sensitive to high temperatures and is more susceptible to overheating. Heat can break down 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