Flexible Glass

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Flexible Glass

Enabling Thin, Lightweight, and Flexible Electronics

Edited by

Sean M. Garner

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v

Contents

Foreword by Peter L. Bocko xiii

Preface xi

Part I: Flexible Glass & Flexible Glass Reliability

1 Introduction to Flexible Glass Substrates 3

Sean M. Garner, Xinghua Li and Ming-Huang Huang1.1 Overview of Flexible Glass 31.2 Flexible Glass Properties 5

1.2.1 Optical Properties 71.2.2 Surface Attributes 121.2.3 Barrier Properties 151.2.4 Dimensional Stability 161.2.5 Thermal Capability 171.2.6 Electrical Properties 171.2.7 Mechanical Properties 19

1.3 Flexible Glass Web for R2R Processing 211.4 Flexible Glass Laser Cutting 221.5 Summary 23References 24

2 The Mechanical Reliability of Thin, Flexible Glass 35

G. Scott Glaesemann2.1 Introduction 352.2 The Mechanical Reliability of Glass 36

2.2.1 Fatigue in Glass 372.2.2 Managing Fatigue 43

2.2.2.1 Minimum Strength Design 452.2.2.2 Failure Probability Design 46

2.3 Applied Stress 492.3.1 Global Stress Events 502.3.2 Localized Stresses 52

vi Contents

2.4 The Strength of Thin Glass Sheets 522.4.1 Flaws in Glass 532.4.2 Practical Glass Strength 552.4.3 Surface Strength of Thin, Flexible Glass Sheets 562.4.4 Edge Strength of Thin, Flexible Glass Sheets 58

2.5 Summary 60References 60

3 Low Modulus, Damage Resistant Glass for Ultra-Thin Applications 63

Timothy M. Gross and Randall E. Youngman3.1 Introduction 643.2 Young’s Modulus and Basic Fracture Mechanics 64

3.2.1 Young’s Modulus Dependence on Composition 653.2.2 Young’s Modulus Dependence on Fictive

Temperature 743.3 Vickers Indentation Cracking Resistance of Calcium

Aluminoborosilicate Glasses 773.4 Summary 82References 83

Part II: Flexible Glass Device Fabrication

4 Roll-to-Roll Processing of Flexible Glass 87

James C. Switzer III and Mark D. Poliks4.1 Introduction 87

4.1.1 Substrates 884.2 Roll-to-Roll Manufacturing Process Equipment 90

4.2.1 CHA High-Vacuum Roll-To-Roll Sputter System 914.2.2 General Vacuum Equipment Optilab

Roll-to-Roll Sputter Deposition System 934.2.3 R2R Wet Processing Systems 994.2.4 Azores 6600 (Rudolph Technologies) Step

and Repeat Photolithography System 1004.2.5 Other Web Handling and Coating Systems 104

4.3 R2R Deposition and Patterning of ITO on Thin Flexible Glass and Plastic Films 1044.3.1 Room Temperature ITO Depositions on PEN 1064.3.2 Etching of ITO on Flexible Plastic and Glass 1124.3.3 Elevated Temperature Depositions 116

4.4 Conclusions 121

Contents vii

4.5 Future 122Acknowledgements 122References 123

5 Thin-Film Deposition on Flexible Glass by Plasma Processes 129

Manuela Junghähnel and John Fahlteich5.1 Introduction 1305.2 Substrate Requirements for Vacuum Processes 130

5.2.1 Parameters Influencing Film Growth on Glass Surfaces 130

5.2.2 Vacuum-Based Surface Treatment 1315.2.2.1 Inverse Sputter Etching 1325.2.2.2 Ion Surface Treatment 133

5.3 Types of Vacuum Processes 1335.3.1 Overview of Vacuum Coating Technologies 134

5.3.1.1 Thermal and Electron-Beam Evaporation 1355.3.1.2 Magnetron Sputtering 1365.3.1.3 Plasma-Assisted Chemical Vapor Deposition 1425.3.1.4 Atomic Layer Deposition 146

5.3.2 Thin Film Processing on Glass 1505.3.2.1 Sheet-to-Sheet Processing 1505.3.2.2 Roll-to-Roll Processing 1525.3.2.3 In-line Monitoring 154

5.4 Large Area Coatings onto Flexible Glass 1595.4.1 Transparent Conductive Coatings 1595.4.2 Antireflective Coatings 163

5.5 Thermal Pre- and Post-Treatment for Flexible Glass 1675.5.1 Heating of Flexible Glass 1685.5.2 Functionalization of Thin Films by Ultra-Fast

Thermal Annealing 1685.6 Future Trends in Vacuum Processing on Flexible Glass 173References 174

6 Printed Electronics Solutions-Based Processes with Flexible Glass 181

Jukka Hast, Elina Jansson, Riikka Suhonen, Liisa Hakola,

Markus Tuomikoski, Marja Vilkman, Kari Rönkä and

Harri Kopola6.1 Introduction 1816.2 Printing Processes 183

6.2.1 Printed Electronics Background 1836.2.2 Ink Formulations 183

viii Contents

6.2.3 Conventional Printing Processes 1856.2.3.1 Flexography Printing 1856.2.3.2 Gravure Printing 1876.2.3.3 Emerging Printing Techniques 1906.2.3.4 Screen Printing 192

6.2.4 Digital Printing – Inkjet 1956.3 Summary of Different Printing Processes 1986.4 Example – Printed OPV Cell on Ultra-Thin Flexible Glass 1986.5 Future 203References 205

Part III: Flexible Glass Device Applications

7 Flexible Glass in Thin Film Photovoltaics 213

Matthew O. Reese and Teresa M. Barnes7.1 Introduction 2137.2 General Substrate Requirements for

Photovoltaic Applications 2157.3 Requirements for CdTe Superstrates 2337.4 Standard CdTe Device Stack and Processing 2357.5 Flexible CdTe Device Performance 2367.6 Flex and Bend Testing of CdTe 238

7.6.1 TCO Flex Bend/Reliability 2387.6.2 Device Static Bend 241

7.7 Future Trends/Directions 241References 242

8 Ultra-Thin Glass for Displays, Lighting and Touch Sensors 247

Steffen Hoehla and Norbert Fruehauf8.1 Introduction and Overview 247

8.1.1 Different Levels of Flexibility 2488.1.2 Specific Advantages of Ultra Thin Substrates 249

8.2 Ultra Thin Glass Substrates for Flexible Displays 2548.2.1 Specific Substrate Requirements for Flexible

Displays 2548.2.2 Comparison of Common Flexible Substrate

Materials 2558.2.3 Overview – Substrate Requirements for High

Quality Flexible Displays 264

Contents ix

8.3 Thin Film Device Processing on Ultra Thin Glass 2658.3.1 Various Processing Concepts for Ultra Thin Glass 2658.3.2 AMLCD Process on Free Standing Ultra-Thin Glass 268

8.4 Thin Glass Displays 2828.4.1 Thin Glass Display Demonstrators 2828.4.2 Commercially Available Thin Glass Displays 284

References 285

9 Guided-Wave Photonics in Flexible Glass 291

Sheng Huang, Mingshan Li and Kevin P. Chen9.1 Flexible Guided-Wave Photonics 2929.2 Flexible Polymer Passive Waveguide Photonics 2929.3 Flexible Polymer Active Waveguide Photonics 2999.4 Flexible Polymer Waveguides for Electro-Optic

Applications 3019.5 Flexible Glass Optical Substrates 3039.6 Ultrafast-Laser Fabrication of Embedded Waveguides 3059.7 Embedded Waveguides in Flexible Glass 3079.8 Prospective of Thermal Poling in Flexible Glass

Waveguides 3219.9 Summary and Future 324References 325

10 Flexible Glass for Microelectronics Integration 331

Murat Okandan, Jose Luis Cruz-Campa and

Gregory N. Nielson10.1 Introduction 33210.2 Integration Technology Description: Why Flexible

Glass for Electronics/Sensor Integration (3 Dimensional Integrated Circuits – 3DIC) 332

10.3 Example of Microelectronics/Sensor Integration 33310.3.1 Flexible PV 33410.3.2 Sensor Array Example 334

10.4 Fabrication Techniques 33610.4.1 Batch Fabrication (Wafer, Glass Substrate Based) 34010.4.2 Solar Tools 34210.4.3 Continuous (Roll-to-Roll) 34310.4.4 Integration Approaches 34410.4.5 Pick-and-Place 344

x Contents

10.4.6 Monolithic Fabrication 34510.4.7 Hybrid Integration 345

10.5 Future Direction 34510.5.1 Portable/Mobile Electronics Examples 34610.5.2 Space Power Systems 34610.5.3 3DIC, Hybrid Microsystems Integration

for High Functionality, Distributed Systems 346References 347

Index 349

xi

Foreword by Peter L. Bocko

Technological revolutions are often built upon a foundation of self-delusion and naiveté. That bleak statement requires some explanation. While a sci-entific revolution can be nucleated by an individual’s insight, the delivery of a breakthrough technology requires a shared vision of the innovation’s benefit followed by broad and protracted collaboration among materials, process, systems, device and application specialists. And if these collabo-rators realized at the outset the level of resolve and resources ultimately required to deliver a revolutionary technological platform, few would get off the ground.

Fortunately, a revolution in electronics based upon flexible glass has pro-gressed well beyond initial naiveté and subsequent (and periodic) stages of disillusionment. This book is a major milepost in this platform’s develop-ment, documenting over a decade of hard won advances in the flexible glass platform through collaboration across relevant component technolo-gies and applications. As an early promoter, champion and sponsor for the applications of flexible glass, I am excited that the building blocks for broad innovation have achieved critical mass, and for the first time are accessible in one place.

Glass has a capability of being drawn under heat and tension into a film of arbitrary thickness while retaining its desirable surface, mechanical and optical properties. This is simple and intuitive. After explaining to a cus-tomer engineer the process of drawing molten glass into precise sheet for LCDs, he asked “How do you make it thinner?”. “Pull harder.” I answered. Since glass-sheet manufacture has been automated, processes have been pushed to draw glass to the limits of sufficient thinness to achieve flexibil-ity, motivated by the desire to minimize weight, enhance conformability or to enable in-line processing.

While the forming of precise ultra-thin glass has been established across multiple glass manufacturing platforms over the last 20 years, what has been missing were the constellation of enabling component technologies: packaging, handling, deposition, patterning and device design that can be used to transform flexible glass from the glass maker’s forming tool and

xii Foreword by Peter L. Bocko

adapt it to a functional system. This has resulted in skepticism and resis-tance of the electronics industry for commitment to large scale develop-ment of flexible glass platforms.

Things have changed since then, but I expect that it will still take time and much hard work to drive flexible glass to the high-volume applications that fully leverages its potential. The editor of this work as well as chapter author, my erstwhile colleague from Corning, Dr. Sean Garner, is in large part responsible for promoting flexible glass in the technology commu-nity and structuring the collaborations that have brought us to the verge of breakthrough of flexible glass into enabling advanced electronic applica-tions. This book represents a major contribution to the field. The long-incubated flexible glass revolution is upon us.

Peter L. BockoAdjunct Professor of Materials Science & Engineering,

Cornell UniversityFormer Chief Technology Officer, Corning Glass

Technologies, Corning Incorporated

xiii

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 pho-tovoltaics continue to be identified. This is much more than just transition-ing 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 pos-sible or practical with alternative substrate materials and may include elec-tronic 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 advan-tage of the unique properties of flexible glass.

Although there have been previous discussions of flexible glass sub-strates and devices at conferences and in published journals, they have focused on very specific aspects or applications. This book, however, pro-vides a much broader overview as well as detailed descriptions that cover flexible glass properties, device fabrication methods, and emerging applica-tions. This book is not meant to provide a comprehensive, detailed descrip-tion 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 discus-sions 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 vari-ous aspects of flexible glass’ definition, processing, and applications. The

xiv Preface

chapters included here are from some of my more significant collabora-tions 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 flex-ible 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 microelec-tronics 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 mate-rial 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 sig-nificant 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 busi-ness 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 100 s m2 in size and process equip-ment specifically optimized for it are now available, an exciting revolution-ary advancement in electronic device integration has begun.

Sean Garner June 2017

Part I

FLEXIBLE GLASS & FLEXIBLE

GLASS RELIABILITY

3

Sean M. Garner (ed.) Flexible Glass, (3–34) © 2017 Scrivener Publishing LLC

1

Introduction to Flexible Glass Substrates

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

Corning Research & Development Corporation, Corning, NY, USA

AbstractWith the expanding applications and research in flexible electronics, the device

substrate choice is becoming increasingly critical to the overall device functional-

ity and performance. Glass continues to be a crucial substrate material for dis-

play and photovoltaic devices as well as for emerging applications such as OLED

lighting. As the glass thickness is reduced to approximately 200 m 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 high-

light 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 fabrica-

tion 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

*Corresponding author: GarnerSM@Corning.com

4 Flexible Glass

well as its flexural rigidity. Since the flexural rigidity or resistance to bend-ing 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 spe-cific 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 manufactur-ing process. This is mainly driven by the glass flexibility and bend stress enabling practical spool diameters. For discussion purposes, it is conve-nient to refer to glass that is 200 m as flexible. As a comparison, glass single-mode optical fiber used in telecommunication applications, such as Corning SMF-28 , has a diameter of 125 m [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 prac-tical 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 resolu-tion, 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

Introduction to Flexible Glass Substrates 5

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 abil-ity to fabricate electronic and opto-electronic devices along with capa-bilities 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 tak-ing 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 summa-rize 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 histori-cally 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 appli-cation 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 opti-mized 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.

6 Flexible Glass

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 manufactur-ing 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 thick-nesses 200 m, 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 bot-tom 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 abil-ity to manufacture homogeneous ultra-thin glass sheets with dramatically improved surface quality compared to other methods of glass sheet manu-facture, 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 dis-plays. 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 com-position 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 revolu-tionary 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 poly-mer 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 appropri-ate for the material system. The commercially available flexible substrate materials used as reference materials in the following evaluations are listed in Table 1.1.

Introduction to Flexible Glass Substrates 7

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], photo-voltaics, 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 contribut-ing 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 m thick Willow Glass along with glass substrates of similar composition but dif-ferent thicknesses. These other thickness samples were fabricated in small scale sample processes for comparison purposes. A glass thickness of 630 m 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

Table 1.1 Reference flexible substrate materials used for comparison purposes to

100 m-thick Willow Glass.

Abbreviation Material Thickness

COC Cyclic Olefin Copolymer 240 m

PC Polycarbonate 125 m

PEN Polyethylene Naphthalate 125 m

PET Polyethylene Terephthalate 125 m

PI Polyimide 50 m

PMMA Polymethyl Methacrylate 125 m

SS304 Stainless Steel 304 30 m

8 Flexible Glass

substrates. As shown in Figure 1.1a, the optical transmission in the vis-ible 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 inter-faces. This shows that negligible haze or absorption occurs in this wave-length 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

100 300 500 700 900

Wavelength (nm)

Op

tica

l tra

nsm

issi

on

(%

)

1100 1300

25 m

50 m

100 m

630 m

1500 17000%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

200

(b)

(a)

225 250 275 300

Wavelength (nm)

Op

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325 350

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50 m

100 m

630 m

375 400

0%

10%

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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.

Introduction to Flexible Glass Substrates 9

optical properties of the flexible glass are mainly controlled by its mate-rial 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 m, 50 m, 100 m, and 630 m thicknesses occur at wavelengths of 254 nm, 264 nm, 286 nm, and 356 nm, respectively. The 50% value of the maximum trans-mission for the 25 m, 50 m, 100 m, and 630 m 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 transmis-sion 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 spec-trum. The oscillations, which are more pronounced with decreasing glass thickness, are due to light interference effects rather than glass absorption.

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 orienta-tion 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 dis-plays.[42, 75] Figure 1.3b shows continued flexible glass refractive index and optical extinction coefficients in the IR.

1.5 2.0 2.0 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

Wavelength ( m)

Op

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)

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50 m

100 m

630 m

0%

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60%

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

spectrum.

10 Flexible Glass

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 percent-age 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 measure-ments from 100 m 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.

0 5 10 15 20 25 30 35 40

Wavelength ( m)

Wavelength ( m)

Re

fra

ctiv

e in

de

x (

n)

Re

fra

ctiv

e in

de

x (

n)

Ex

tin

ctio

n c

oe

ffici

en

t (k

)

Willow Glass PMMA PET (x) PET (y) PET (z)

1.0

100 300 500

(a)

(b)

700 900 1100 1300 1500 1700

1.45

1.50

1.55

1.60

1.65

1.70

1.2

1.4

1.6

18

2.0

2.2

2.4 1.2

1.0

0.8

0.8

0.4

0.2

0.0

n

k

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.

Introduction to Flexible Glass Substrates 11

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 m thick

Ha

ze (

%)

Willow

Glass

PMMA

Detection noise level

COC PC PEN PET PI0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

9.0

9.1

9.2

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

standard deviation.)

L*

a*,

b*

Willow

Glass

PMMA

L* = 70.5

a* = 16 2.5

2.0

1.5

1.0

0.5

0.0

–0.5

L* a* b*

b* = 98

COC PC PEN PET PI93.0

93.5

94.0

94.5

95.0

95.5

96.0

96.5

97.0

Figure 1.5 Color of flexible glass and polymer film substrates.

12 Flexible Glass

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.

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 screen-ing for comparison purposes, and any specific accelerated testing for tar-geted 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 wave-length and L* values are plotted. This shows a significant decrease in poly-mer 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.

1.2.2 Surface Attributes

Surface attributes have a significant impact on device fabrication and per-formance. 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 m × 300 m win-dow on both surfaces of the substrate. Figure 1.7 shows average surface roughness (R

a) 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 demand-ing 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 R

a 0.5 nm is obtained directly from

Corning’s fusion process for forming glass substrates. There is no need for

Introduction to Flexible Glass Substrates 13

polishing or planarization to achieve the surface quality required for fabri-cation of, for example, thin film semiconductor devices. This surface qual-ity 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 fabrica-tion of devices on both surfaces.

L*

Willow

Glass

PMMA

After

Before

COC PC PEN PET PI

0

10

20

30

40

50

60

70

80

90

100

Tra

nsm

issi

on

(5

50

nm

)

Willow

Glass

PMMA

After

Before

COC

(a)

(b)

PC PEN PET PI

0

10

20

30

40

50

60

70

80

90

100

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*).

14 Flexible Glass

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 sub-strate 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 mea-surement. 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

Ra (

nm

)

Willow

Glass

PMMA Stainless

steel

Side 2

Side 1

COC PC PEN PET PI

0

2

4

6

8

10

12

14

20

40

60

80

120

100

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

flexible electronics.