HP Picks Rollable Process for Cheaper Displays
Chris Edwards, Contributing Editor -- Semiconductor International, 4/8/2008
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Speaking at the Printed Electronics Europe conference held this week in Dresden, Germany, Hewlett-Packard (HP) Program Manager Carl Taussig said the company’s aim in developing a roll-to-roll process was different to most other companies working on polymer-based displays. “The aim was not to create a flexible display, but to create a cheap display.”
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| 1. The imprint process can be limited by yield issues. |
Some of the techniques were reused from existing roll-to-roll processes. HP opted to base the material deposition and etching steps used in its process around techniques developed by PowerFilm Solar (Ames, Iowa) that have been used for the production of photovoltaic cells based on amorphous silicon (Fig. 1).
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| Carl Taussig, HP Program Manager |
Taussig said the team performed an analysis of the relative costs of roll-to-roll and more conventional batch production; roll-to-roll processing looked to be “massively cheaper.” The main reduction in cost came from the move to nanoimprint lithography from conventional photolithography techniques used in most polymer displays now going onto the market. “We looked at what kind of patterning would be appropriate in the roll-to-roll environment. And we kept coming back to find that imprint lithography looked to be the best candidate,” he explained.
The switch to imprint allowed HP to produce finer geometries than are feasible today with photolithography on polymer substrates (Fig. 2). The polymer substrate stretches, particularly when heated, making it hard to accurately register shapes on different layers.
“Our self-aligned imprint lithography puts down all of the information in a single process. We can go through the entire process with no distortion. The mask effectively stretches with the substrate,” Taussig said. In addition, the single lithography step reduces cost over photolithographic processes.
| 2. The advantages of self-aligned imprint lithography over conventional lithography in roll-to-roll applications. |
Taussig claimed HP has produced 40-nm-thick traces on a polyimide substrate, although thin-film transistors made using the process had channel lengths of 2 µm with device widths of 40 µm.
Yield is an issue for the imprint technology, Taussig conceded. “Early on, we found that we were susceptible to particle contamination,” he said, with some of the particles coming from the way a roller geared for solar cell production wore down the substrate used for HP’s prototype displays. Altering that design improved yield, but Toussing added, “You deal with one yield problem, then the next rises to the top.”
The CTO of Polymer Vision (Eindhoven, Netherlands), Edzer Huitema, said the process described by Taussig was impressive, adding, “Roll-to-roll is probably the way for the future. But it could be 20 years away.”
Huitema said his company is using photolithography because it has 40 years of experience behind it — something that imprint lithography has yet to build up.
Michel Jongerius, manager of advanced competence development at Philips Applied Technologies (Eindhoven, Netherlands), has produced cost models of printed electronics technologies to guide development at the company. He said, “Roll-to-roll provides an opportunity to get to low cost. But it only works if you increase the speed of your process dramatically. And yield is very important. If you have a loss of yield, in roll-to-roll you have to take the faulty product to the end of the line, and so you lose some of the cost benefits.”
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