Interview: Flexible Display Manufacturing by the Meter

Ralph Delmdahl during the interview with LTJ editor Anna-Lena Idzko (Source: Wiley-VCH)

Ralph Delmdahl during the interview with LTJ editor Anna-Lena Idzko (Source: Wiley-VCH)

The new laser system UVblade enables high volume flexible display manufacturing. It provides a meter-long, thin blade of UV laser light which smoothly separates a flexible polymer layer from a rigid glass carrier in the lift-off process. It is capable of separating hundreds of flexible displays every minute. Especially industrial sectors which focus on mobile displays as central feature will benefit from the innovation. First and foremost, these are smartphone, tablet and wearable display industries.

Laser Technik Journal: Congratulation to the nomination and the third prize. Four years ago, your colleague received the second prize also with an excimer laser (recrystallization of silicon films with excimer laser annealing). Is the UVblade based on this technology?
Ralph Delmdahl: There are commonalities in the process: also a thin blade of laser light is used, also an excimer laser with the same wavelength (λ = 308 nm) and it is used in the display market. But we work with another process window. We use a smaller energy density (200 mJ/cm2) and we have an even more price sensitive process. For the silicon annealing process, higher laser energies are necessary. Therefore, several laser beams – now we use up to four – need to be bundled and overlaid with optical systems. That is not necessary with UVblade. Another very important difference is that UVblade is a single-shot process. For excimer laser annealing we need twenty pulses per area. This is very important for speed, costs and lifetime since every laser pulse is money.

LTJ: UVblade enables a cost-efficient volume production of flexible displays. What is its working principle?
Delmdahl: The process we use is called laser lift-off. To fabricate flexible displays, a glass carrier is coated with a polymer; on top of the polymer the display/OLED layers are fabricated with high accuracy. The glass is only used as a temporary carrier and needs to be separated again after the process. Since the display/OLED layers are very thin, it is not possible to pull off the foil mechanically or to use chemicals without damaging them. During the lift-off process, we use a blade of UV laser light to evaporate the polymer where it touches the glass carrier. After that, the foil can be removed from the glass resulting in a paper-thin and flexible display.
With this process, a “rigid fab” can be easily switched to a flexible production since it is possible to use the existing production line. It is only necessary to add an UVblade and a polymer spin coating process to the established processes.

LTJ: What is unique about the UVblade process?
Delmdahl: The laser lift-off process is the fastest excimer process in the world. Since there is no need for masks or other structures, we have a very high efficiency. The beam utilization factor is about 85 %.
UVblade consists of an excimer laser and a line beam optical system. For the laser we developed a new platform: LEAP. The LEAP laser is designed cost-efficiently and we also improved the gas composition with additives we did not use before. The big advantage is that no cryostatic gas cleaning during operation is necessary. Anyway, the laser features long lifetimes.
The line beam optics have the big advantage of a large depth of field with about +/- 200 µm. Our customers work on larger and larger glass substrates and need highest accuracy and precision. So with a large depth of field we provide a large process window to maximize yield. And yield is extremely important.
The glass substrates are half a millimeter thick, the polymer is about some ten micrometers thin and the OLED or LCD layers are also very thin. Everything is very close together. And now I want to evaporate the polymer with a laser through the glass but without heat transfer. Therefore, we need an excimer laser with a short wavelength. A penetration depth of only some ten nanometers ensures that enough polymer remains which is protecting the light-generating front-plane layers. I think the process is fascinating because you merge a macroscopic with a microscopic process. We are working with an accuracy of sub 100 nm and produce macroscopic output.
With a UVblade system it is possible to separate 30.000 m2 of flexible displays every month. This corresponds to about five football fields. High accuracy combined with speed – that is what the flat screen industry needs.

LTJ: Besides the penetration depth, why are you using an excimer laser?
Delmdahl: An advantage of the excimer laser is the energy stability – which is also important for the yield. Since the laser itself generates the wanted wavelength, there is no need for wavelength conversion and that’s why we gain high laser pulse stabilities (about 0.5 % rms).
Another advantage is that we have a single pulse process. Using a solid-state laser, you have temperature effects during switching on or off and not all pulses can be used. Solid-state lasers use up about a hundred times more laser pulses than the UVblade. With UVblade, the pulses can be applied very precisely – keyword: digital photonics. With the pulse-on-demand principle, the laser fires pulses only when and where it is necessary. During panel replacement, the pulses stop and start immediately and the pulses can be controlled in a way, that every single pulse has the same energy. No matter if it is the first or the last one. That brings a lot of savings.

LTJ: Are there any limitations?
Delmdahl: Up to now, there are no performance limitations for the UVblade. Actually, we could process even bigger panels. The driver for flexible displays are OLEDs. But their process scale-up is not as far developed as the process for TFT displays. Therefore, the panel size for OLEDs is not as big as that for TFTs. When OLED processes move to larger glass panel generations, it is no problem for UVblade to follow – the necessary power and the length of the laser line is already used for the TFT annealing process.

LTJ: The produced displays are mainly used for smartphones. Are there other applications?
Delmdahl: Markets which are starting to roll are OLED lighting as well as automotive. In a car, you have a display for the navigation system and a lot of buttons. The display is getting larger but the buttons still exist. This reminds me of the former cell phones: The displays were enlarged until finally the key pad itself was integrated into the display. I think this is coming up for the automotive sector, too – and those displays will not be flat, they will adapt flexibly onto the center console. Already, the first rear lights are made from OLEDs, allowing new shapes of the lights.
Another classical flexible market is wearables, but it is not that big. I think the smartphone market is still very important. A flexible basis is a game changer. Maybe, we will see the first foldable smartphone next year – a prerequisite is a flexible display.

The Interview was conducted by Anna-Lena Idzko. (Source: Laser Technik Journal, Wiley-VCH)

Links: International Laser Technology Congress AKL’16, • Innovation Award Laser Technology • Coherent Laser-Systems GmbH & Co. KG, Göttingen, Germany


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