High-mobility poly-Si TFTs directly printed on paper

High-mobility poly-Si TFTs directly printed on paper

Technology News |
Printed electronics, hand-in-hand with flexible electronics, is a hot topic where both active materials and substrates are being investigated for their processability as cheap and scalable roll-to-roll alternatives to traditional silicon transistors.
By eeNews Europe


Popular ink materials include organic and metal-oxide semiconductors, though they lag behind silicon in terms of electrons and holes mobility (orders of magnitude lower than poly-silicon) reliability, and energy efficiency. Commonly used substrates found in flexible electronics include thin films of polyethylene naphthalate (PEN), polyethylene terephthalate (PET) and even polydimethylsiloxane (PDMS) used for stretchable or conformable electronics.

Sample of electronics on a paper substrate.

But driving costs further down, a team of researchers from Delft University of Technology (The Netherlands) wants fast electronic circuits to operate on paper (an order of magnitude cheaper than these polymers), without the uneconomical complications of thin-film transfers from a native silicon substrate to paper. 

A couple of years ago, Dr. R. Ishihara, an associate professor in TUDelft’s Department of Microelectronics, lead his team to demonstrate how cyclopentasilane (CPS) could be used as a liquid silicon precursor to synthesized poly-Si at low temperature (under 100ºC). Now Ishihara and his team have successfully fabricated both p-channel and n-channel poly-Si thin-film transistors (TFTs) directly on top of paper, with field-effect mobilities of 6.2 and 2.0 cm2/V s, respectively.

The low temperature and roll-to-roll compatible fabrication method they describe in the npj’s Flexible Electronics journal is an eight-step process, reviewed in detail in a paper titled “Solution-based polycrystalline silicon transistors produced on a paper substrate”.

​The TFT cross-section with a microscope image
of the fabricated TFT (red scale bar is 50μm).

It relies first on blade-coating the CPS liquid silicon ink directly onto paper, under controlled UV light exposure to gradually increase the ink’s viscosity through molecular ring-open polymerization. Under an excimer-laser shone through a shadow mask, the solidified polysilane film is then crystallized into poly-silicon channel patterns, while the unexposed polysilane areas naturally oxidize into electrically insulating SiO2. An extra oxide layer is deposited before a variety of shadow masks are used to deposit the gate, to implant dopant ions and selectively activate them, and finally to etch out the oxide where aluminium contacts will be deposited through evaporation.

In order to guard the paper from degradation, they avoided most liquid chemicals commonly used for etching and cleaning (favouring dry etch processes). Although only a proof-of-concept with much process optimization needed, these transistors on paper offer a much better performance than printed organic devices and could even compete with printed indium–gallium–zinc-oxide (IGZO) TFTs. This research opens the path to very low-cost, low-power and recyclable applications including smart packages, biodegradable health monitoring units, flexible displays, and disposable sensor nodes, conclude the authors.

TFT properties of silicon ink-based PMOS and NMOS TFT fabricated on top of a paper substrate.

TUDelft – https://ectm.tudelft.nl

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