Ultra-thin glass for tomorrow’s microelectronics From chip packaging to protection for flexible electronics

Ultradünne Gläser mit Dicken von 100 bis zu 25 µm 
eröffnet neue in Zukunft vielfältige Möglichkeiten für die Elektronik- und Halbleiterindustrie. Der heute absehbare Einsatz hauchdünner Gläser reicht vom Chip-Packaging über (Touch-)Sensoren und Dünnschichtbatterien bis hin zu Konzepten für biegsame Smartphones.
(Bild: Schott)
Ultra-thin glasses that are 25 µm to 100 µm thick open up all kinds of new opportunities for the electronics and semiconductor industries in the future. Current foreseeable uses of extremely thin glasses range from chip packaging through (touch) sensors and thin film batteries right up to concepts for flexible smartphones.

Ultra-thin, flexible glasses with thicknesses under 50 µm provide multiple opportunities as an enabler for the development of new electronic solutions of the future. Their uses range from interposer in highly-integrated component packages through to new display solutions, or very compact solid-state batteries and biotech-applications in the field of microfluidic systems.

In electronics, glass is until now primarily viewed as a material for displays, or also as a starting material for implementing high-performance, glass fiber solutions. This will change in future with the development of ultra-thin glasses. Ultra-thin glass, as manufactured today for example by Schott, is highly transparent, hence hardly visible and moreover is also extremely thin.

For some years now, Schott provides such glasses, with a thickness of up to 50 µm in the form of so-called sheets, to customers. In the meantime, glasses with a thickness of only 25 µm are used in research co-operations with partners. Researchers in laboratories at Schott are currently working on ultra-thin glass with a thickness of only 10 µm.

The possibilities that ultra-thin glass, due to its properties, offers for use in the electronics sector are made clear by a key difference to the conventional semiconductor material silicon: Glass is a significantly better electrical isolator than silicon. Due to this fact, chips and conducting paths on glass can be arranged much closer together, and operated at higher frequencies than on a silicon substrate. In this way, the performance of smartphones, tablets and cameras can be significantly increased still further. In order to drill ultra-thin glass in such a way that it can be equipped with printed circuit boards, hole diameters from 1 µm to 50 µm are needed. At Schott, ultra-short pulse lasers are used, with which few thousandths of a millimeter small areas can be melted in the glass, or even evaporated, without touching the surrounding material.

Processed in this way, ultra-thin glass can be used, for example, as an interposer. Unlike organic substrate materials, such as polymers or composites, ultra-thin glass exhibits an extraordinarily good mechanical stiffness with highly integrated packaging concepts under the influence of heat over a wide temperature range. It does not bend and therefore enables the thinnest form factors for very flat, slim device constructions such as, in the meantime, not only required in the smartphone or tablet sectors. 
Furthermore, glass has a much higher electrical isolation than the standard semiconductor material silicon, and can transport high-frequency data streams with low power loss via metal feedthroughs. As a result, processor performances with up to eight times higher data transfer rates than the previous technology are possible.

In a joint project with Georgia Tech (Georgia Institute of Technology, USA), Schott has already produced prototypes of interposes from 30 µm thin glass. With partners from the semiconductor industry, the company is now working on using ultra-thin glasses in interposer applications and IC packaging in mass production.

In future, thin-film batteries or solid-state batteries can also benefit from ultra-thin glass. Because the cathode and anode materials of these batteries are deposited in a high temperature vacuum process directly on the ultra-thin glass substrate, such batteries can be considerably miniaturized in the future. As the quality of the active battery materials is influenced by thermomechanical, such as expansion coefficient and glass transformation temperature, as well as chemical interactions with the substrate material, the use of the appropriate ultra-thin glass contributes directly to increased performance of the battery.

In other sectors, ultra-thin glass also provides a replacement for existing solutions in electronics. Schott also offers chemically hardened, ultra-thin glass in the form of D263T eco. It offers four times greater strength compared to non-hardened basic glass. Hence, ultra-thin glass can also be used under rough conditions, such as for touch displays in smartphones. Functions such as fingerprint sensors with an extremely high degree of recognition accuracy can be realized with ultra-thin glass. However, the resolution of a (capacitive) fingerprint sensor depends on the thickness of the glass protecting it. Furthermore, a relative dielectric constant, as provided by D263T eco, is important in this application.

The three technology companies Schott AG, tesa SE and von Ardenne GmbH are working in the consortial project KONFEKT on another use of ultra-thin glass in electronics. The German Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung - BMBF) is supporting this development for a period of three years with a total of EUR 5.6 million. KONFEKT is intended to drive forward the development of ultra-thin glass on roll for use in application fields such as organic electronics, for example, in the production of future generations of organic light-emitting diode (OLED) applications. The goal of the consortium is to refine windable glass through lamination with functional adhesive tapes and by applying special functional layers. This will hopefully result in a process-ready rolled substrate that offers unique properties for many applications.

In subproject one; Schott and tesa have been occupied with the development of a laminate made of ultra-thin glass with a barrier adhesive tape that will serve as a hermetic encapsulation of electronic components. Specifically, this involves protecting sensitive electronic components such as OLEDs from humidity and oxygen by using ultra-thin glass. Reliable encapsulation will protect the sensitive components from aging. Flexible glass is well suited as a top ultra-barrier (z barrier) because it forms a chemically impermeable layer that is impenetrable to water vapor and oxygen, even at a thickness of 10 micrometers. Furthermore, in contrast to coating solutions, it does not exhibit any pinholes.

tesa’s expertise as a developer of specialty adhesive tapes comes into play in lateral sealing. The ultra-thin glass will be delivered to the user laminated with a special adhesive layer. This adhesive layer ensures that the components are not only sealed hermetically by the glass on their surface, but also experience no lateral diffusion of liquids and gases (x/y barrier). The combination of ultra-thin specialty glass and barrier tape provides complete protection, thanks to the functional x/y/z barrier. This roll application will provide processing companies with a high-quality and cost-effective sealing process. In the second subproject, von Ardenne is developing a vacuum coating system specifically for roll-to-roll (R2R) coating of flexible glasses that will meet the special handling requirements of such glasses. Thin glass can thus be used as a functional substrate in sophisticated electronic applications. For example, a conductive transparent conductive oxide (TCO) layer such as indium tin oxide (ITO) is applied in a special vacuum-based physical vapor deposition (PVD) coating process in the same manner as in the manufacture of OLEDs or organic photovoltaic cells. “We expect the consortium to play an important role in the next three years in the development of a new production platform based on glass on roll for innovative use in manufacturing electronic components,” said Dr. Ruediger Sprengard, Director of Business Development for Ultra-Thin Glass at Schott.