Flexible Stainless-Steel Substrates

Stainless steel may not be the obvious choice as a substrate for flexible displays and electronics, but the more it is considered, the more attractive it becomes.

by Vincent Cannella, Masat Izu, Scott Jones, Sigurd Wagner, and I-Chun Cheng

IN RECENT INITIATIVES to develop flexible displays and microelectronics, much of the industry has focused on plastic substrates such as PET, polycarbonates, PEN, and polyimide.

There are, however, at least three serious problems with plastic substrates for these applications. First, the lack of stability at high temperatures (greater than 300°C) limits the subsequent processes that can be used to fabricate devices, and thus limits the ability to achieve high device performance. A second and even greater shortcoming is the poor dimensional stability of plastic substrates compared to glass, for example. This limits pattern resolution for multi-step photolithography and makes it very difficult to maintain the device resolution necessary for display driver circuits.

Third, the permeability of plastics to the diffusion of oxygen and water vapor presents a major problem for organic light-emitting-diode (OLED) displays, which degrade when exposed to even low levels of oxygen and water vapor. The use of plastic substrates consequently requires the addition of complex and costly multilayer barrier coatings to limit water-vapor diffusion to less than 10^-5gm/ cm²/day. However, thin, less than 5 mils, stainless-steel substrates can provide an attractive alternative substrate for emissive or reflective displays for which transparency is not a requirement.

These stainless-steel substrates tolerate high-temperature processes (up to 1000°C), have much better dimensional stability than plastic substrates, and present a perfect diffusion barrier to oxygen and water vapor, in addition to being tougher and more durable than plastic. The stability at high-temperatures not only enables a full temperature range for high-quality amorphous-silicon (a-Si) and thin-film-transistor (TFT) gate-insulator deposition, but also permits the growth of high-quality large-grain polycrystalline silicon (poly-Si) without the uniformity problems typically observed when using laser crystallization at lower temperatures.

Stainless steel also has excellent dimensional stability, which allows multi-photostep microprocessing at device resolutions comparable to those of Corning 1737 display glass, for example. This dimensional stability is much higher than that of a plastic substrate because of stainless steel's much higher elastic modulus, lower coefficient of thermal expansion, zero coefficient of hydrolytic expansion, and negligible mechanical hysteresis. An added advantage of mechanical stability is resistance to the curling caused by film stresses in plastic substrates, which require special control.

Another advantage is the chemical stability of stainless-steel substrates, which suffer no mechanical degradation from exposure to UV radiation or ozone and are not affected by organic solvents. Finally, the impermeability of stainless-steel substrates to oxygen or water vapor provides ideal diffusion barriers for front-emitting OLED displays, assuring longer display lifetimes.

The fabrication of TFT circuits, in both a-Si and poly-Si, on stainless-steel substrates for displays has been well demonstrated on the R&D scale.^1–3 In particular, Wu and his colleagues at Princeton University have fabricated high-quality TFTs – μ_e greater than 60 cm²/V-sec – on flexible SiO₂-coated stainless-steel substrates from crystallized a-Si at process temperatures from 600 to 950°C.⁴

Active-matrix OLED displays on stainless-steel substrates have also been demonstrated by Wu and his colleagues at Princeton University⁵ and Troccoli and his group at Lehigh University,⁶ and with two- and four-TFT a-Si active-matrix arrays. E Ink Corp. has reported⁷ and demonstrated product prototypes of its active-matrix electrophoretic display on stainless-steel substrates. In other words, the ability to fabricate TFT arrays on flexible stainless-steel webs has been well demonstrated and documented.

The use of flexible stainless-steel substrates in large-scale semiconductor manufacturing is also well established. Large-area solar modules, up to 16 square feet, and products have been fabricated using roll-to-roll stainless-steel processes for both a-Si and copper indium gallium diselenide (CIGS) technologies. In particular, United Solar Ovonic Corp. (Energy Conversion Devices' photovoltaics unit) manufactures and sells a-Si-based solar cells on 5-mil-thick stainless steel using a roll-to-roll manufacturing process. In that process, triple-junction a-SiGe solar-cell structures having nine semiconductor layers are simultaneously fabricated on six 1.5-mile-long stainless-steel rolls (Fig. 1).

Comparing Substrates for Flexible Backplanes

Several properties distinguish stainless-steel foil from organic polymer ("plastic") or glass foils. Steel is tougher than plastic, and glass foil is brittle even when passivated with a plastic coating. Steel and plastic can be bent to similar radii, which are much smaller than that possible for glass. Being opaque, stainless steel cannot be used in transmissive displays or as a substrate for bottom-emitting devices.

Stainless steel will withstand high process temperatures, in contrast to glass and even more to plastics. A critical thermal issue is the strain produced by differential thermal expansion between the substrate and device films. Amorphous-silicon processes can tolerate a substrate coefficient of thermal expansion up to about 20 ppm/°C, which steel meets, as does glass and a number of plastics. Steel has exceptional dimensional stability and requires neither a pre-bake nor a temporary carrier to stabilize it during thermal processing.

However, stainless-steel substrates tend to have a rough surface that must be planarized. In addition, control of gray scale in a display requires excellent electrical insulation below the pixel, and video scan rates require low capacitive coupling to the substrate. These requirements can be met by a planarization film of high-temperature plastic or spin-on-glass and a good electrical insulation layer (Fig. 2).

 (a)  (b) United Solar Ovonic Corp.

Fig 1: (a) Flexible stainless-steel substrates are currently used in photovoltaic panels. (b) United Solar Ovonic Corp. currently makes such panels with a triple-junction a-SiGe solar-cell structure having nine semiconductor layers simultaneously fabricated on six 1.5-mile-long stainless-steel rolls on this 300-ft.-long roll-to-roll manufacturing line.

Fig. 2: When back-channel passivated a-Si:H TFTs for display applications are fabricated on different types of substrates, different combinations of passivation and planarization layers must be applied to the substrates. (a) glass, (b) plastic, and (c) stainless steel.

The properties of 100-μm-thick flexible substrates of stainless steel compare favorably to the properties of plastic and glass substrates of the same thickness (Table 1). Stainless steel's tightest safe bending radius is much better than that of glass and comparable to plastics. The "tightest safe bending radius" is defined as that bending radius which produces a strain in the surface of 0.1% for steel and plastic and 0.01% for glass, with the glass not being passivated with plastic, to permit processing up to 600°C.

Through proper encapsulation, the display electronics can be placed in the mechanically neutral plane where, theoretically, it will not experience any bending strain. The required thickness of such a plastic encapsulant is listed in Table 1 in multiples of the thickness of the substrate. But only displays made on plastic can be conformably shaped – to an automobile's dashboard, for example – without causing damage to the devices.

Preparation of Steel Substrates for Displays

Beginning with stainless-steel-foil material currently available in commercial volumes, the group at Princeton University developed the following process to prepare the foil for use as display substrates, the process being divided into three major steps.

First, the substrate is cleaned with a solvent (acetone, methanol, and/or isopropanol rinses) or a hot aqueous detergent. Next, a planarization layer is applied to minimize the surface roughness that is typically created by the final rolling of the foil. Third, an insulating layer is applied to provide good electrical passivation of the conducting substrates. Different coating structures are required for glass, plastic, and stainless-steel substrates in display applications (Fig. 2).

For stainless steel, depending on subsequent process steps, it may be possible to apply the planarization and electrical insulating layers only to the device side of the substrate [Fig. 2(c)]. But under many processing circumstances, a double-sided coating is preferred. The back-side coat serves as a protection layer against the processing chemicals, as a diffusion barrier against contaminants released from the steel substrates in high-temperature processing, and as a stress-balancing layer when the substrate is very thin.

Planarization Layer

Unlike glass or plastic substrates, a stainless-steel foil typically comes with rolling marks that have sharp profiles, and these usually cause degradation or failure of the devices fabricated on top of them. Therefore, the foil must either be well polished or planarized with a film.

The planarization layer can be organic, inorganic, or a mixture of the two, and commercially available planarization materials are available (Table 2). In general, the higher the organic content of the material, the thicker the film that can be applied crack-free and the smoother the resulting surface. This is particularly important in OLED-display applications, which require a surface roughness of less than 5 nm rms. However, the organic content imposes a ceiling on the subsequent process temperatures.

Typical process steps for applying the planarization layer include spin-on, hot-plate bake, and high-temperature curing. As a rule of thumb, the final curing temperature sets the upper limit on the subsequent device process temperature. In addition to planarization, the incorporation of getter materials such as phosphorus into these spin-on silicate films provides a diffusion barrier against possible contaminants from the steel substrate.

Passivation

Stainless-steel substrates are usually encapsulated with SiN_x or SiO₂ layers. The passivation layer provides electrical insulation, adhesion to subsequent device layers, and a barrier against process chemicals. SiN_x is a standard material in the a-Si:H TFT process and is also an excellent diffusion barrier against contaminants. But the hydrogen content in SiN_x may cause the film to crack or ablate during subsequent processing at high temperature or by laser-annealing. In such cases, a deposited SiO₂ film is preferred. Another advantage of SiO₂ is its low dielectric constant, which reduces the capacitive coupling from the substrate compared to the same thickness of SiN_x. A barrier layer of 0.2 μm suffices for electrical insulation, but in an active-matrix circuit a thicker layer may be required to minimize capacitive coupling.

Is Stainless Steel Viable?

In summary, stainless-steel substrates planarized by a compatible high-temperature spin-on-glass and capped with a plasma-enhanced chemical-vapor-deposition (PECVD) insulating material can provide multipurpose substrates for flexible electronics and displays. These are an attractive substrate alternative to plastics for flexible-display applications because stainless steel has

• Thermal stability compatible with both a-Si TFT and poly-Si processing,

• Excellent mechanical stability for multi-photostep processing, and

• Perfect barrier properties against the diffusion of oxygen and water vapor.

Stainless-steel substrates are more rugged and robust than plastic substrates and are well suited to applications that require mechanical strength as well as flexibility and resilience. Stainless steel has the added advantage of chemical stability when exposed to organic solvents and to UV radiation and ozone. Fabrication of active-matrix circuits on stainless-steel substrates has been demonstrated, using both a-Si and high-temperature poly-Si TFTs.

Stainless-steel foil is currently available commercially with a surface finish compatible with planarization layers at prices less than $10 per square meter. This is one-third to one-quarter the cost equivalent of polyimide substrates and about the same as high-quality PEN substrates supplied in moderate volumes. The spin-on-glass planarization layer for stainless steel will not be spun on in production but applied in a process with much higher utilization, so the materials cost for double-sided passivation will be less than $2 per square meter.

Since stainless-steel substrates require only one PECVD insulator deposition, while the plastic substrates require two, the insulator-deposition costs will also be less. Plastic substrates require multilayer barrier coatings to prevent oxygen and water-vapor diffusion through plastic for OLED displays, and the cost of these is currently about $75 per square meter. The conclusion is that the bifacially planarized stainless-steel substrates described above will be economically very attractive compared to plastic substrates.

Up to now, the development of stainless-steel substrates for flexible displays has been restricted to low-volume R&D and prototype samples. Based on the technical advantages discussed in this article, particularly for OLED displays, there is substantial interest from display companies in the viability of the technology for display production and whether there can be an economically feasible volume supply. As a result, the U.S. Display Consortium (USDC) has funded a cost-shared contract to develop a production-capable design and process for flexible stainless-steel substrates. The contractor for this program is Energy Conversion Devices, Inc., Rochester Hills, Michigan. The work is scheduled to be completed in October 2006.

References

¹S. D. Theiss and S. Wagner, "Amorphous silicon thin-film transistors on steel foil substrates," IEEE Electron Dev. Lett. 17, 578–580 (1996).

²R. S. Howell et al., "Poly-Si thin-film transistors on steel substrates," IEEE Electron Dev. Lett. 21, 70–72 (2000).

T. Serikawa and F. Omata, "High-quality polycrystalline Si TFTs fabricated on stainless-steel foil by using sputtered Si films," IEEE Trans. Electron Dev. 49, 820–825 (2002).

⁴M. Wu, X. Z. Bo, J. C. Sturm, and S. Wagner, "Complementary metal-oxide-semiconductor thin-film transistor circuits from a high-temperature polycrystalline silicon process on steel foil substrates," IEEE Trans. Electron Dev. 49, 1993–2000 (2002).

⁵C. C Wu et al., "Organic LEDs integrated with a-Si TFTs on lightweight metal substrates," SID Symposium Digest Tech. Papers, 28, 67–70 (1997).

⁶M. Troccoli et al., "AMOLED TFT Pixel Circuitry for Flexible Displays on Metal Foils," MRS Symposium Proc. 769, 93–99 (2003).

⁷Y. Chen, K. Denis, P. Kazlas, and P. Drzaic, "A Conformable Electronic-Ink Display Using a Foil-Based a-Si TFT Array," SID Symposium Digest Tech. Papers 32, 157-159 (2002). •

*Laser-annealing requires an approximately 2-μm-thick thermal barrier layer on top of an organic-polymer planarization layer.