he thin-film transistor (TFT) became commercially available slightly more than 30 years ago in the form of a switch for the Liquid Crystal Display. It all started with an amorphous silicon (a-Si) TFT. Compared to the traditional crystalline silicon CMOS transistor, the a-Si TFT can be produced on large substrates and at low processing temperatures, below 300 °C, enabling integration on glass substrates and even flexible substrates.
A-Si TFTs are mainly implemented as simple pixel switches due to their low charge carrier mobility (0.5-1 cm2/Vs). An alternative semiconductor on glass substrates is low-temperature polycrystalline silicon (LTPS), outperforming a-Si TFTs by a 100x larger mobility (50-100 cm2/Vs) and often used for high-end displays and imagers. Despite the advantages, fabrication of an LTPS TFT takes more process steps, is limited in substrate size, and requires a larger process temperature. Oxide-based semiconductors as indium-gallium-zinc-oxide (IGZO) fill this gap between a-Si and LTPS nicely, exhibiting low processing temperatures and a decent charge carrier mobility of 10 up to 40 cm2/Vs [1].
With such characteristics, the IGZO TFTs can be used to fabricate relatively complex circuits on flexible substrates. Consequently, IGZO TFTs are evolving beyond displays and entering the fields of wearable devices and the Internet of Things (IoT). Some highlights include an ultra-flexible circuit for recording electrocardiograms [2], radiofrequency identification (RFID) tags and near-field communication (NFC) tags [1]. Even the memory field has noticed IGZO and its extremely low OFF current and recently demonstrated a capacitor-less IGZO-based DRAM cell with a retention time longer than 400 seconds [3]. We can expect the first IGZO products beyond display applications to emerge in the near future.
Perhaps ESD protection is not a great concern for wireless products, for example, NFC and RFID tags which have no wired input and output ports. Here the sensitive electronic parts will not be exposed to the user as they are electrically insulated. In this case, like for displays [4,5], a good ESD control program might be enough to protect the IGZO components during assembly. The full display will, in any case, include ESD protection circuits at the system level, even perhaps at the peripherals where the electrical connections leave the display, but not necessarily at the IGZO component level. On the other hand, some wearables like the electrocardiogram patch, might not be able to afford system-level ESD protection circuits fabricated in a different technology other than IGZO. Since the electrodes have direct contact with the end-user, they may require ESD protection circuits at the device level. The same will be true for displays and imagers if their peripheral circuits are implemented using IGZO – to enable a fully flexible display, for example.
The second challenge of the IGZO TFT technology for ESD circuit design is that there are usually no diodes available. Even though it is possible to make diodes in the IGZO TFT technology, it also increases the process cost and is not a commodity in this industry. Therefore, the best next choice would be a diode-connected transistor. The technology options to improve the ESD performance of the diode-connected transistor would be to smartly use the back-gate or to optimize the channel material resulting in larger mobility.
Nevertheless, given the limited IGZO TFT conductivity, to achieve a product-worthy ESD protection level, the ESD circuits will have to be in the millimeter or even centimeter size range [5][2]. Given the IGZO TFT technology is optimized for large areas, spending these kinds of areas for ESD protection should not be a showstopper.
Unlike the silicon integrated circuit technology, there is a very limited choice of devices in IGZO, which also limits the possible solutions of ESD protection circuits. Passive devices could help with that. Inductors, capacitors, and resistors could be used as ESD protection or to complement the active devices. A spark gap could be a compact ESD protection option too.
This work received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation program under grant agreement no. 716426 (FLICs project).
This work was financed by the Flexlines project within the Interreg V-program Flanders-The Netherlands, a cross-border cooperation program with financial support from the European Regional Development Fund, and co-financed by the Province of Noord-Brabant, The Netherlands.
- K. Myny, “The development of flexible integrated circuits based on thin-film transistors,” Nature Electronics, vol. 1, no. 1, p. 30, 2018.
- M. Sugiyama, T. Uemura, M. Kondo, M. Akiyama, N. Namba, S. Yoshimoto, Y. Noda, T. Araki, and T. Sekitani, “An ultraflexible organic differential amplifier for recording electrocardiograms,” Nature Electronics, vol. 2, no. 8, pp. 351–360, 2019.
- A. Belmonte, H. Oh, N. Rassoul, G. L. Donadio, J. Mitard, H. Dekkers, R. Delhougne, S. Subhechha, A. Chasin, M. J. van Setten, L. Kljucar, M. Mao, H. Puliyalil, M. Pak, L. Teugels, D. Tsvetanova, K. Banerjee, L. Souriau, Z. Tokei, L. Goux, and G. S. Kar, “Capacitor-less, Long-Retention (>400s) DRAM Cell Paving the Way towards Low-Power and High-Density Monolithic 3D DRAM,” in 2020 IEEE International Electron Devices Meeting (IEDM), 2020, pp. 28.2.1–28.2.4.
- ESD Association, “ESD TR21.0-01-18 Technical Report for Challenges in Controlling ESD in the Manufacturing of Flat Panel Display,” 2018.
- Joshua (Yong Hoon) Yoo, “ESD Issues for Flat Panel Displays”, In Compliance Magazine, February 2021.
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