How to make the world’s fastest flexible silicon transistor

Engineers fabricate high-performance transistors with wireless capabilities using a radical fabrication method based on huge rolls of flexible plastic. "We don’t want to make them the way the semiconductor industry does now."
April 21, 2016

World’s fastest silicon-based flexible transistors, shown here on a plastic substrate (credit: Jung-Hun Seo/UW–Madison)

A team headed by University of Wisconsin—Madison engineers has fabricated a flexible transistor that operates at a record 38 gigahertz, but may be able to operate at 110 gigahertz.

The process could allow manufacturers to easily and cheaply fabricate high-performance transistors with wireless capabilities, using a radical fabrication method based on huge rolls of flexible plastic.

The new transistor can also transmit data or transfer power wirelessly, which could unlock advances in a whole host of applications ranging from wearable electronics to sensors.

Low-cost radical method uses less energy, achieves higher transistor density

The researchers’ nanoscale fabrication method (based on a simple, low-cost process called nanoimprint lithography) replaces conventional lithographic approaches — which use light and chemicals to pattern flexible transistors — overcoming such limitations as light diffraction, imprecision that leads to short circuits of different contacts, and the need to fabricate the circuitry in multiple passes.

The researchers — led by Zhenqiang (Jack) Ma, the Lynn H. Matthias Professor in Engineering and Vilas Distinguished Achievement Professor in electrical and computer engineering, and research scientist Jung-Hun Seo —  published details of the advance Wednesday April 20 in an open-access paper in the journal Scientific Reports.

With a unique, three-dimensional current-flow pattern, the high-performance transistor consumes less energy and operates more efficiently. And because the researchers’ method enables them to slice much narrower trenches than conventional fabrication processes can, it also could enable semiconductor manufacturers to squeeze an even greater number of transistors onto an electronic device.

Ultimately, says Ma, because the mold can be reused, the method could easily scale for use in a technology called roll-to-roll processing (think of a giant, patterned rolling pin moving across sheets of plastic the size of a tabletop), and that would allow semiconductor manufacturers to repeat their pattern and mass-fabricate many devices on a roll of flexible plastic.

“Nanoimprint lithography addresses future applications for flexible electronics,” says Ma, whose work was supported by the Air Force Office of Scientific Research. “We don’t want to make them the way the semiconductor industry does now. Our step, which is most critical for roll-to-roll printing, is ready.”

The process

  1. Using low-temperature processes, the researchers patterned the transistor circuitry using nanoimprint lithography.
  2. Using selective doping, the researchers introduced impurities into materials in precise locations to enhance their properties — in this case, electrical conductivity. Currently, the dopant sometimes merges into areas of the material it shouldn’t, causing what is known as the “short channel” effect. The researchers took an unconventional approach: They blanketed their single crystalline silicon with a dopant, rather than selectively doping it.
  3. They added a light-sensitive material, or photoresist layer, and used a technique called electron-beam lithography — which uses a focused beam of electrons to create shapes as narrow as 10 nanometers wide — on the photoresist to create a reusable mold of the nanoscale patterns they desired. They applied the mold to an ultrathin, very flexible silicon membrane to create a photoresist pattern.
  4. They finished with a dry-etching process — essentially, a nanoscale knife — that cut precise, nanometer-scale trenches in the silicon following the patterns in the mold, and added wide gates, which function as switches, atop the trenches.

Additional authors are at UW–Madison, the University of Michigan, the University of Texas at Arlington, and the University of California, Berkeley.

Abstract of Fast Flexible Transistors with a Nanotrench Structure

The simplification of fabrication processes that can define very fine patterns for large-area flexible radio-frequency (RF) applications is very desirable because it is generally very challenging to realize submicron scale patterns on flexible substrates. Conventional nanoscale patterning methods, such as e-beam lithography, cannot be easily applied to such applications. On the other hand, recent advances in nanoimprinting lithography (NIL) may enable the fabrication of large-area nanoelectronics, especially flexible RF electronics with finely defined patterns, thereby significantly broadening RF applications. Here we report a generic strategy for fabricating high-performance flexible Si nanomembrane (NM)-based RF thin-film transistors (TFTs), capable of over 100 GHz operation in theory, with NIL patterned deep-submicron-scale channel lengths. A unique 3-dimensional etched-trench-channel configuration was used to allow for TFT fabrication compatible with flexible substrates. Optimal device parameters were obtained through device simulation to understand the underlying device physics and to enhance device controllability. Experimentally, a record-breaking 38 GHz maximum oscillation frequency fmax value has been successfully demonstrated from TFTs with a 2 μm gate length built with flexible Si NM on plastic substrates.