Smartphones should be robust and even foldable. This is made possible by the extreme properties of glass at the limit of what is technically feasible. Examples include ultra-stable and ultra-flexible cover glasses.

When Apple introduced the first generation of its iPhone back in 2007, a glass cover protected the novel touch display from the daily challenges of life for the first time. Where protective plastic covers were once used, glass now fills users with pride. But why glass? Transparent polymer covers that were used in the many smartphone predecessors of well-known brands such as Nokia, Palm and BlackBerry were inappropriate for bare-finger navigation and made the use of tactile input aids, such as the stylus, necessary. The displays also proved to be very susceptible to scratches. Glass, which is brittle and not shatterproof by any means, offered clear advantages over polymers in terms of brilliance, transparency and scratch resistance – especially with regard to the high quality standards in the premium segment.

The first generation of the iPhone was followed by new smartphones, hardware and software approaches from other players – in 2008, for example, with the now omnipresent Google Android. As the industry flourished, SCHOTT also invested in the development of modern cover glass materials. Between 2010 and 2012, the company introduced several cover glass types, marketed under the Xensation® brand. The 0.5 to 2-millimeter- thick aluminosilicate glass (AS) was manufactured in Jena, Germany, using the microfloat process and met the industry’s requirements. It was brilliant, thin, strong and scratch-resistant. Due to its high share of aluminum and sodium, it perfectly met the requirements of chemical strengthening – a process that makes the glass extremely strong.

The combination of aluminum and sodium allows for a highly effective ion exchange. The raw glass is immersed in a salt bath, whereby smaller sodium ions are exchanged with larger potassium ions. This post-processing gives the glass increased resistance to impact, bending and scratching.

Chemical Preload

Chemical strengthening involves the transfer of ions. In AS glasses, the non- crystalline network structure is modified in such a way that smaller sodium (Na+) ions are exchanged by larger potassium (K+) ions. This is made possible by immersing the glass in a potassium nitrate bath (KNO3). Due to their chemical composition, LAS glasses allow for a two-stage strengthening process. In the first step, sodium ions are exchanged by lithium ions; in the second step, sodium ions are exchanged by potassium ions. This creates compressive stress in the surface of the glass, and an opposite tensile stress as a counterforce inside the material. The special feature of LAS glass is that high compressive stress on the immediate surface can be combined with deeply located compressive stress, which creates an additional mechanism that increases the breaking strength when hitting rough, hard surfaces.

Flexible cover glasses are revolutionizing the smartphone market and opening up completely new possibilities for developers.

Shortly after the introduction of the first Xensation® generation, SCHOTT was the first glass manufacturer in the world to develop an aluminosilicate-based cover glass that added lithium, a lithium aluminosilicate (LAS) glass. This type of glass allows for even deeper strengthening thanks to multi-ion exchange and, as a result, increases the breaking strength of the glass when it is dropped onto rough, hard surfaces.

Right cover glass for every trend

SCHOTT addressed current market trends with the first generation of LAS glasses. Over time, smartphone designs showed fewer and fewer frames – the display surface dominated. A full-surface, seamlessly integrated touchscreen posed new challenges. After all, the display could now bounce against the floor without any protective casing. Based on positive experiences regarding cover glass development, the next evolutionary leap followed with Xensation® Up. a further developed high-performance LAS glass.

“With Xensation® Up., we have been selling large quantities in the premium segment since 2018 and are involved in various high-end devices from the Chinese manufacturer vivo,” Dr. Lutz Klippe, Product Group Manager for Cover Glass at SCHOTT, is delighted to report. “Its breaking strength under real conditions is a major advantage of LAS glass. The device drop test is thus the measure of all things, as it comes much closer to everyday life than laboratory tests. Xensation® Up. is ten times more resistant to a fall than AS glass.”

Higher breaking resistance

What is so special about LAS glass? The composition of the ingredients enables much deeper chemical strengthening. Temperature, time and the composition of the glass have a direct influence on the depth strength. The ion exchange takes place at a temperature that is considerably lower than the glass transition temperature (Tg), so the original network structure of the glass is preserved. Potassium ions, which are about 1.3 times larger than sodium ions, take the place of the sodium ions and cause a “stuffing effect.” This creates compressive stress in the surface of the glass, with the positive effect of higher breaking resistance. The tempering depth for AS glass is about 40 μm, whereas it is up to 200 μm for LAS glass, facilitated by a twostage strengthening process.

LAS glass is an important factor on the journey to indestructible glass: “We are currently already working on the next cover glass generation. Our research makes us confident that we are a big step closer to our vision of unbreakable glass,” Klippe adds.

Flexible ultra-thin glass for foldable phones

Cover glasses do not always have to be as break-resistant as possible, however. New trends and designs of foldable smartphones – “foldables” or “flip phones” – rather demand cover glasses that are flexible and offer a very small bending radius. Here too, SCHOTT is presenting a groundbreaking product in this area. Xensation® Flex is the first ultra-thin cover glass on the market that enables a bending radius of less than two millimeters after processing and chemical strengthening.

In the spring of 2020, Samsung introduced the first foldable smartphone with a display cover window that features foldable ultra-thin glass. High-tech glass from SCHOTT was part of the success story. “We are proud that Xensation® Flex is used in a new generation of foldable smartphones,” says Dr. Jack Ju, Product Group Manager for UTG Cover Glass at SCHOTT. “Our technology helps premium foldable displays achieve unprecedented quality.”

SCHOTT has produced UTG since the 1990s using a down-draw production process. There are several different types of ultra-thin glass. With continuous development, a UTG thickness of 16 μm has already been achieved in the laboratory. By comparison, one red blood cell is 8 μm thick!

Down-Draw

With the down-draw process, a glass ribbon is simply pulled down. The rule here is the faster the glass ribbon is drawn, the thinner the end product – and the more challenging the production. In addition to seamless adjustment of the thickness, the process offers an advantage in that the glass does not need secondary slimming, thus eliminating downstream etching processes that are harmful to the environment. Glass produced by the down-draw process also shines with a low thickness variation and a flawless surface (surface roughness ~1 nm).

Speed is crucial in the down-draw process for the production of ultra-thin glass: the faster, the thinner.
Speed is crucial in the down-draw process for the production of ultra-thin glass: the faster, the thinner.

March 26nd, 2021

Contact

Michael Matthias Müller
Marketing & Communication
SCHOTT AG

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