Flexible Glass: Enabling Thin, Lightweight, and Flexible Electronics

This book details flexible glass properties that enable use in emerging electronic and opto-electronic applications. Discussion includes flexible glass advantages compared to alternative substrate materials. Examples describe flexible glass in processes such as vacuum deposition, monolithic integration, printing, and roll-to-roll. Flexible glass demonstrations in emerging applications such as photovoltaics, flexible displays, and optical interconnects are also detailed.

 The reader will find in this unique book: 

• Discussion of flexible glass processing and mechanical reliability.
• Demonstration of flexible glass in roll-to-roll (R2R) fabrication processes.
• Flexible glass substrate examples in displays, sensors, and photovoltaics.
• Flexible glass ecosystem description for identification of new applications.

• Language: English
• Publisher: Wiley
• Release date: Aug 4, 2017
• ISBN: 9781118946374

Book preview

Preface

Flexible glass continues to emerge as a significant material component for electronic and opto-electronic applications. Its use goes well beyond earlier capacitor applications. For example, new opportunities in fields of displays, sensors, lighting, backplanes, circuit boards, photonic substrates, and photovoltaics continue to be identified. This is much more than just transitioning the devices that exist currently on thicker rigid glass onto a thinner, flexible substrate. Flexible glass substrates in these applications enable new device designs, manufacturing processes, and performance levels not possible or practical with alternative substrate materials and may include electronic applications such as fully-integrated, large-area, smart surfaces. In addition, these new applications require specifically optimized fabrication processes, manufacturing equipment, and device designs that take advantage of the unique properties of flexible glass.

Although there have been previous discussions of flexible glass substrates and devices at conferences and in published journals, they have focused on very specific aspects or applications. This book, however, provides a much broader overview as well as detailed descriptions that cover flexible glass properties, device fabrication methods, and emerging applications. This book is not meant to provide a comprehensive, detailed description of all attributes and possibilities but rather, it provides the basis for identifying new device designs, applications, and manufacturing processes for which flexible glass substrates are uniquely suited. Information in this book encourages and enables the reader to identify and pursue advanced flexible glass applications that do not exist today and provides a launching point for exciting future directions.

Information in this book is based on over 10 years of valuable discussions and collaborations focused on truly defining what flexible glass means in the context of these emerging electronic and opto-electronic applications. This learning is also built upon decades of previous activities in earlier applications. What started personally for me as an exploratory investigation has occupied most of my career as I collaborated on various aspects of flexible glass’ definition, processing, and applications. The chapters included here are from some of my more significant collaborations meant to provide an overall, well-rounded perspective.

The chapters are grouped into three sections. The first focuses on flexible glass and flexible glass reliability and has three chapters with authors from Corning. The second section focuses on flexible glass device fabrication which includes chapters on roll-to-roll processing, vacuum deposition, and printed electronics. These chapters are authored by established experts in their respective fields that have extensive experience in processing flexible glass substrates in toolsets that range from research to pilot scale. The third section focuses on flexible glass device applications and includes chapters on photovoltaics, displays, integrated photonics, and microelectronics integration. These are authored by experts with direct experience in fabricating and characterizing flexible glass devices. The diverse list of authors and their depth of experience in working with a variety of material systems, processes, and device technologies significantly adds valuable context to the overall flexible glass discussion.

The required ecosystem to truly enable flexible glass device fabrication in sheet and roll-to-roll processes is continuing to emerge. Although a significant element, flexible glass is one technology component required to advance new electronic and opto-electronic applications. Complementary materials and manufacturing equipment are required to bring this into reality. It’s exciting to see reported activities transition from early device-research demonstrations to discussions about process scale-up and business opportunities.

I’ve truly enjoyed my wide-ranging discussions and interactions over the last several years on all aspects of flexible glass and flexible electronic topics. This has included significant interactions with universities, national labs, and corporate collaborators on all aspects of flexible glass properties, processing, and applications. This book highlights the foundational work that new opportunities can be built upon. By transitioning into a flexible substrate, ultra-thin glass enables a complete paradigm shift in flexible electronic applications and high-throughput, roll-to-roll manufacturing. As high-quality, flexible glass substrates 100ʹs m² in size and process equipment specifically optimized for it are now available, an exciting revolutionary advancement in electronic device integration has begun.

Sean Garner

June 2017

Part I

FLEXIBLE GLASS & FLEXIBLE GLASS RELIABILITY

Chapter 1

Introduction to Flexible Glass Substrates

Sean M. Garner*, Xinghua Li and Ming-Huang Huang

Corning Research & Development Corporation, Corning, NY, USA

*Corresponding author: [e-mail]

Abstract

With the expanding applications and research in flexible electronics, the device substrate choice is becoming increasingly critical to the overall device functionality and performance. Glass continues to be a crucial substrate material for display and photovoltaic devices as well as for emerging applications such as OLED lighting. As the glass thickness is reduced to approximately 200 µm or less, the same enabling benefits such as hermeticity, optical quality, surface roughness, and thermo-mechanical stability continue in the glass substrate, but new mechanical behavior arises. Along with the reduced thickness, the glass weight is significantly reduced and flexibility is dramatically increased. This chapter provides an overall description of flexible glass and how its properties enable new device functionality, manufacturing processes, and applications that are not possible or practical with thicker, rigid glass substrates or alterative flexible substrate materials. Comparisons are made to polymer film and metal foil flexible substrate materials that highlight differences in material properties. Laser crack propagation techniques for cutting flexible glass substrates, with the focus on optimizing edge strength, are also described. This basic description of flexible glass enables the device fabrication processes and applications described in subsequent chapters.

Keywords: Flexible substrate, flexible electronics, glass, roll-to-roll, ultra-slim, encapsulation, laser cutting

1.1 Overview of Flexible Glass

With the reduction in glass thickness, associated mechanical properties are likewise affected. For example, the glass substrate weight is reduced as well as its flexural rigidity. Since the flexural rigidity or resistance to bending is proportional to E * t³ [1], (where, E is the Young’s modulus and t is thickness) the glass dramatically becomes more flexible with decreasing thickness. This thickness reduction also results in a decrease of bend stress, which is described in Chapter 2. It is somewhat arbitrary to define a specific thickness value where glass should begin to be referred to as flexible, but it is convenient to use an approximate thickness where it is practical to use continuous spooling or winding operations in the glass manufacturing process. This is mainly driven by the glass flexibility and bend stress enabling practical spool diameters. For discussion purposes, it is convenient to refer to glass that is ≤200 μm as flexible. As a comparison, glass single-mode optical fiber used in telecommunication applications, such as Corning® SMF-28®, has a diameter of 125 μm [2].

Although flexible glass can be used for a variety of applications, the focus of this book will be on use in electronic or opto-electronic device applications. With its reduced thickness but continued intrinsic material properties, flexible glass in general can be used as both a substrate for device fabrication and as a superstrate where it serves as both a substrate and a window to the environment. In addition, flexible glass is an efficient hermetic encapsulating layer. The thickness reduction enables devices that are not only thin but also light weight and conformal or flexible in nature. This resulting flexibility can be utilized in the application after the device has been singulated and packaged, or it can also enable new device manufacturing methods not previously demonstrated with glass substrates such as roll-to-roll (R2R) processing. The unique combination of intrinsic glass material properties with a flexible form factor enable new device designs, applications, and manufacturing processes not practical previously [3].

Flexible glass is compatible with device manufacturing methods not usually associated with glass substrates. These are described in more detail in Chapters 3–6 and include R2R and printed electronic device fabrication methods. These device fabrication methods are optimized for handling and processing flexible glass substrates but still continue to achieve the resolution, registration, performance, and lifetime of devices typically fabricated on thicker, rigid glass substrates.

Emerging flexible glass device and application examples are described in more detail in Chapters 7–10. Application examples include: solar power devices such as photovoltaics and concentrated solar power [4–24], electronic circuit substrates [25–31], antennas [32], integrated optics [33–34], flexible hybrid electronics [15], sensors including touch sensors [21, 31, 35–38], OLED lighting [39–41], and displays and electronic backplanes [31, 36, 42–57]. Each of these application areas can also be further divided, such as displays into LCD [42], OLED display, and e-paper displays [49, 52–53] for example. Also, combining the ability to fabricate electronic and opto-electronic devices along with capabilities of large area lamination, flexible glass enables progression toward large area smart surfaces with integrated display, lighting, sensor, and communication functionality. These applications go beyond simply taking devices that exist today on rigid glass substrates and making them thinner and lighter, but instead opening up new device functionality and application opportunities. The following sections in this chapter summarize the major flexible glass material properties that affect device design and manufacturing processes, as well as providing comparisons to other substrate materials.

1.2 Flexible Glass Properties

In general, a wide variety of thin, flexible glass substrates have historically been produced for applications that have included glass capacitors [58–63], microscope cover slides [58–60], and satellite solar cell cover sheets [64]. These have had their dimensions (thickness, width, length), forming process, and composition optimized specifically for their application requirements. Corning® 0211 Microsheet [65] is an example of a thin, flexible, alkali-containing borosilicate glass primarily used for non-electronic device applications. Corning 0213 [64] and Corning 0214 [66] are examples of a Ce-doped borosilicate glass with UV absorption optimized for satellite solar cell covers. Additionally, examples of flexible silica substrates [67] and flexible ceramic substrates [10, 68–70] have also been demonstrated targeting applications such as high speed circuit boards [29]. Overall, a wide range of flexible inorganic substrate compositions and forming processes have been historically demonstrated, and these were chosen and further optimized based on application requirements.

Over the past 20 years there has been a specific focus on optimizing flexible glass properties specifically for electronic and flexible electronic applications. These emerging applications have new requirements for the glass attributes, and these flexible glass attributes are a combined result of the specific composition and forming process used. Detailed discussions of the glass attributes resulting from specific glass composition or forming process choices are outside the scope of this book since they are covered in detail elsewhere [59–61, 71]. This chapter provides a short overview of representative flexible glass properties.

Throughout this book, Corning® Willow® Glass is used as an example of a flexible glass substrate. It is an alkaline earth boro-aluminosilicate glass composition compatible with semiconductor device manufacturing processes such as those based on silicon, metal oxide, and organic semiconductor materials. Willow Glass is currently manufactured in a continuous fusion draw process and wound directly onto spools in thicknesses ≤200 µm, widths >1 m, and lengths approximately 300 m. The fusion draw process is a glass forming method developed at Corning in the 1960s for the manufacture of thin sheets of glass with pristine surface quality [72]. The process involves flowing molten glass over the walls of both sides of a ceramic isopipe. The two sides of the glass join at the bottom of the isopipe and are drawn into a thin sheet with uniform thickness, where neither side of the glass sheet has come in contact with anything except air. The main advantages of the fusion draw process are the ability to manufacture homogeneous ultra-thin glass sheets with dramatically improved surface quality compared to other methods of glass sheet manufacture, such as the float process used to make glass windows [73]. Besides Willow Glass, the fusion draw process is used to form rigid glass substrates for active matrix flat panel displays such as OLED and liquid crystal displays. An example of these substrates is Corning® Eagle XG® [74] with thicknesses ranging from 0.3 mm to 1.1 mm. Since it is of similar composition as active matrix display glass substrates and also formed using the fusion process, the intrinsic material and surface properties of Willow Glass are similar. The reduction in thickness, though, enables a revolutionary increase in substrate size orders of magnitude larger than what is currently used in display manufacturing. Substrate surface area typically measured in m² for rigid glass sheets has now increased to 100’s m² for spooled glass. The combination of increased substrate size and flexibility enables high throughput manufacturing processes such as R2R as well as very large area device fabrication.

To understand basic similarities and differences of flexible glass to other substrates, this section compares Willow Glass to representative polymer and metal substrates. This is not meant to be a fully comprehensive description of all flexible glass properties and compositional variations, but this section highlights key attributes that could enable new device designs, applications, or manufacturing processes. Since measured values are sensitive to specific metrology and sample prep techniques, this section only reports values measured using similar procedures that are appropriate for the material system. The commercially available flexible substrate materials used as reference materials in the following evaluations are listed in Table 1.1.

Table 1.1 Reference flexible substrate materials used for comparison purposes to 100 µm-thick Willow Glass.

1.2.1 Optical Properties

As a transparent material in the visible to near infrared spectrum, glass is specifically chosen as a component in applications such as displays [42, 49–50, 52–54, 75], sensors including touch sensors [21, 35–38], photovoltaics, transparent antennas [24], photonic integrated circuits [34], and diffractive and lens elements [76–77] where transparency and optical transmission are required. For these applications, in addition to contributing its own optical performance, flexible glass substrates also enable the deposition and coatings of optimized transparent conductors and optical films [9, 14, 20, 24, 78–83]. Vacuum deposition of thin films is discussed in Chapter 5, and some of these applications are discussed in more detail in Chapters 7–9. This section provides basic optical properties of flexible glass that can be used for integrating into optical and photonic device designs and understanding performance. Optical transmission and refractive index data were collected with J.A. Woollam RC2® and IR-VASE® Variable Angle Spectroscopic Ellipsometer systems (courtesy of J.A. Woollam Co., Inc.). More detail about measurements of polymer films that are optically anisotropic can be found in reference [84].

Figure 1.1 shows the measured optical transmission of 100 μm thick Willow Glass along with glass substrates of similar composition but different thicknesses. These other thickness samples were fabricated in small scale sample processes for comparison purposes. A glass thickness of 630 μm was included because it is a typical thickness used in active matrix OLED and liquid crystal displays and serves as a reference for rigid glass substrates. As shown in Figure 1.1a, the optical transmission in the visible to near-IR wavelengths are independent of glass thickness for non-waveguide applications, and the significant factor in the optical loss is from the approximate 4% surface reflection from each of the 2 air-glass interfaces. This shows that negligible haze or absorption occurs in this wavelength range. For optical waveguide applications as discussed in Chapter 9 or applications that require extended optical path length within the glass substrate, the absorption and haze of the flexible glass will have a more significant influence on device performance even in the visible range. The optical properties of the flexible glass are mainly controlled by its material composition. Thickness-dependent losses occur in the UV region due to material absorption, and the absorption loss in this region is linearly dependent on thickness as expected. As shown in Figure 1.1b, the UV cut-off knee of 90% of the maximum transmission for the 25 μm, 50 μm, 100 μm, and 630 μm thicknesses occur at wavelengths of 254 nm, 264 nm, 286 nm, and 356 nm, respectively. The 50% value of the maximum transmission for the 25 μm, 50 μm, 100 μm, and 630 μm thicknesses occur at wavelengths of 218 nm, 224 nm, 240 nm, and 315 nm, respectively. This thickness-dependent UV cut-off enables adjusting of the optical transmission window by optimizing the glass thickness for the application and can be combined with deposited thin film filters as needed. Similarly, Figure 1.2 shows the optical transmission of these glass thicknesses in the IR spectrum. The oscillations, which are more pronounced with decreasing glass thickness, are due to light interference effects rather than glass absorption.

Figure 1.1 Optical transmission of glass substrates of differing thicknesses in the (a) UV to near-IR and (b) UV spectrum. Note that the data was smoothed to reduce significant optical interference fringes in the thinner glass substrates.

Figure 1.2 Optical transmission of glass substrates of differing thicknesses in the IR spectrum.

In terms of optical refractive index, Figure 1.3a shows measured index data for the flexible glass, PMMA, and PET materials. Single curves are shown for the glass and PMMA since they are optically isotropic. The PET sample has 3 index curves due to its biaxial anisotropy caused by orientation during manufacturing. The z data is for out-of-the-plane axis of the PET film, and the x/y data are the 2 in-the-plane axes. The isotropic optical property of glass is important for applications such as liquid crystal displays.[42, 75] Figure 1.3b shows continued flexible glass refractive index and optical extinction coefficients in the IR.

Figure 1.3 (a) Refractive index of Willow Glass and polymer film substrates in the UV to near-IR, and (b) Willow Glass in the IR spectrum.

Transparency is important in applications that require viewing objects through the glass substrate. Alternatively, haze is a measurement of wide angle scattering in which light is diffused in all directions and results in a loss of optical contrast. When passing through the substrate, the percentage of light that deviates from the incident beam greater than 2.5 degrees, on average, is defined as haze [85]. To evaluate optical haze, a Byk-Gardner Haze-Gard LE04 Haze Meter was used. Figure 1.4 compares measurements from 100 μm thick Willow Glass with reference polymer films. Note the broken Y-axis. The haze measurement of the Willow Glass was limited by the detection level of the system.

Figure 1.4 Optical haze of flexible glass and polymer film substrates. (Error bars are standard deviation.)

To evaluate color L*, a*, and b* values, a Filmetrics F10 Spectrometer was used with vertical optical incidence. The color calculation is based on the 1976 CIE system [86–87]. L* is a measure of brightness. a* is a measure along the green (–) to red (+) scale. b* is a measure along the blue (–) to yellow (+) scale. Figure 1.5 compares measurements from 100 μm thick Willow Glass with the reference polymer films. Not shown in the graph is the polyimide film color which had L*, a*, and b* values of 70.5, 16, and 98 respectively.

Figure 1.5 Color of flexible glass and polymer film substrates.

A final topic in this section relates to optical durability and, specifically, UV aging. This is particularly important for outdoor applications such as solar energy and outdoor displays. To compare UV aging characteristics of the flexible substrate materials, samples were exposed for 4000 hours in an Atlas Weather-o-meter. An ASTM G7869 compliant light source was used with a 2.5-sun continuous illumination. The chamber was set for 60 °C and 60% relative humidity. This testing was meant as a material screening for comparison purposes, and any specific accelerated testing for targeted geographic region and use conditions requires a more detailed study. Figure 1.6 shows the effect that UV exposure had on optical transmission and color. In these cases, representative transmission at a 550 nm wavelength and L* values are plotted. This shows a significant decrease in polymer film optical properties due to UV exposure while relatively no change for the Willow Glass substrate. Although not measured in this screening evaluation, the optical change near a wavelength of 400 nm is expected to be more significant. Similar to addressing water vapor transmission rate (WVTR) barrier property concerns in polymer film, achieving polymer durability to UV exposure requires deposition of an additional thin film layer(s) on the polymer surface or use of additives.

Figure 1.6 Optical measurements of Willow Glass and polymer film substrates before and after extended UV exposure. (a) Optical transmission at 550 nm and (b) Color (L*).

1.2.2 Surface Attributes

Surface attributes have a significant impact on device fabrication and performance. For example, thin film devices and printed electronics [83, 88] are affected by surface roughness and surface energy. Chapters 4–8 discuss examples of these devices in more detail.

To evaluate surface roughness, a Zygo NewView 7300 Optical Surface Profiler was used. Measurements were taken over a 300?μm × 300 μm window on both surfaces of the substrate. Figure 1.7 shows average surface roughness (Ra) results for Willow Glass compared to reference polymer film and stainless steel substrates. Note the broken Y-axis with difference scales. For the higher roughness substrates to be used in the more demanding applications, such as active matrix display backplanes, planarization is needed. For example, stainless steel substrates need to go through chemical mechanical polishing to reduce to a level below 1 nm [89], and additional planarizing layers may also be required [90]. It should be noted that the Willow Glass surface roughness of Ra < 0.5 nm is obtained directly from Corning’s fusion process for forming glass substrates. There is no need for polishing or planarization to achieve the surface quality required for fabrication of, for example, thin film semiconductor devices. This surface quality is a direct result of the forming process used. Similar surface quality is routinely achieved in thicker glass substrates, such as Corning® Eagle XG®, that are produced with the same fusion process up to thicknesses of 1.1 mm. It is also important to note that both surfaces of the flexible glass have equivalent high-quality, low surface roughness which enables fabrication of devices on both surfaces.

Figure 1.7 Surface roughness of flexible glass and representative substrates used in flexible electronics.

As another characterization of surface attributes particularly relevant to printed electronics [88], surface energies of the flexible substrates were measured using a Kruss Drop Shape Analysis System DSA 100 with liquids of deionized (DI) water, hexadecane, and diiodomethane. Both substrate surfaces were again measured to observe any differences. Figure 1.8 shows surface energy results for Willow Glass compared to reference flexible substrate materials. Since measurements of surface energy are highly sensitive to the actual surface chemistry of the substrate and any contaminants, all samples underwent the same 10 minute UV-ozone treatment prior to measurement. This evaluation was meant to be used as an initial comparison, and the UV-ozone process is not necessarily optimized for specific glass or polymer film applications. In general, there are many different cleaning procedures that can be used to prepare glass and other surfaces, and the appropriate choice depends on the specific application requirements. Examples of glass cleaning or surface preparation processes range from simple forced air or low-tack adhesive rollers to remove loosely adhered physical contaminants to using plasma, ozone, RCA cleaning, detergent, and/or solvents to remove chemically-adhered contamination. For any of these procedures, flexible glass is compatible with the use of ultrasonics as needed.

Figure 1.8 Surface energy of flexible glass and reference flexible substrates.

1.2.3 Barrier Properties

Establishing a hermetic barrier is a significant requirement for some device applications. Example applications include devices within display (OLED, microplasma) [50, 75, 91–92], lighting (OLED, PLED, EL) [39–40, 93–94], and photovoltaic (organic and perovskite) [6] areas. It is typically reported that these applications require WVTR on the order of 10–6 g/m²/day [91, 95–96]. Rigid glass substrates are typically used as a benchmark that alternative encapsulation methods are compared against [58, 91]. Similar to relatively thick glass substrates, the barrier properties of glass continue as the glass thickness is reduced into the flexible substrate regime [97]. This is because flexible glass substrates are produced in a process similar to rigid glass: they are formed from molten glass. This avoids pinhole defects that might occur in other processes such as thin film vacuum deposition [98]. Multiple methods of measuring the WVTR of Willow Glass have been performed. In each case, the detection limit of the measurement system was reached before an actual WVTR was recorded for the Willow Glass [97]. As an example, Chapter 7 describes a calcium measurement performed that concluded the WVTR of 100 μm glass substrates is below the measurement detection sensitivity of 3 × 10–7 g/m²/day.

With the superior barrier performance of the flexible glass itself, the limitation in hermetically encapsulating devices is really the edge sealing method or other barrier materials used in the design. In general, flexible glass is compatible with a variety of encapsulation approaches [97]. These include thin film deposition, barrier lamination, and edge sealing approaches. In terms of edge sealing, this can be performed with sealing materials such as organic-based adhesives as well as glass frit sealing to achieve the highest performance