UNCONVENTIONALNANOPATTERNINGTECHNIQUES AND

APPLICATIONS

John A. RogersHong H. Lee

A John Wiley & Sons, Inc., Publication

InnodataFile Attachment9780470405772.jpg

UNCONVENTIONALNANOPATTERNINGTECHNIQUES AND

APPLICATIONS

UNCONVENTIONALNANOPATTERNINGTECHNIQUES AND

APPLICATIONS

John A. RogersHong H. Lee

A John Wiley & Sons, Inc., Publication

Copyright C© 2009 by John Wiley & Sons, Inc. All rights reserved.

Published by John Wiley & Sons, Inc., Hoboken, New Jersey.Published simultaneously in Canada.

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CONTENTS

PREFACE xv

I NANOPATTERNING TECHNIQUES 1

1 INTRODUCTION 3

2 MATERIALS 72.1 Introduction / 72.2 Mold Materials and Mold Preparation / 8

2.2.1 Soft Molds / 82.2.2 Hard Molds / 192.2.3 Rigiflex Molds / 19

2.3 Surface Treatment and Modification / 21References / 23

3 PATTERNING BASED ON NATURAL FORCE 273.1 Introduction / 273.2 Capillary Force / 28

3.2.1 Open-Ended Capillary / 293.2.2 Closed Permeable Capillary / 313.2.3 Completely Closed Capillary / 403.2.4 Fast Patterning / 433.2.5 Capillary Kinetics / 45

3.3 London Force and Liquid Filament Stability / 483.3.1 Patterning by Selective Dewetting / 493.3.2 Liquid Filament Stability: Filling and Patterning / 51

3.4 Mechanical Stress: Patterning of A Metal Surface / 56References / 63

4 PATTERNING BASED ON WORK OF ADHESION 674.1 Introduction / 674.2 Work of Adhesion / 68

v

vi CONTENTS

4.3 Kinetic Effects / 714.4 Transfer Patterning / 744.5 Subtractive Transfer Patterning / 794.6 Transfer Printing / 82References / 91

5 PATTERNING BASED ON LIGHT: OPTICALSOFT LITHOGRAPHY 955.1 Introduction / 955.2 System Elements / 96

5.2.1 Overview / 965.2.2 Elastomeric Photomasks / 965.2.3 Photosensitive Materials / 99

5.3 Two-Dimensional Optical Soft Lithography (OSL) / 1005.3.1 Two-Dimensional OSL with Phase Masks / 1005.3.2 Two-Dimensional OSL with Embossed Masks / 1045.3.3 Two-Dimensional OSL with Amplitude Masks / 1055.3.4 Two-Dimensional OSL with Amplitude/Phase Masks / 109

5.4 Three-Dimensional Optical Soft Lithography / 1105.4.1 Optics / 1115.4.2 Patterning Results / 112

5.5 Applications / 1175.5.1 Low-Voltage Organic Electronics / 1175.5.2 Filters and Mixers for Microfluidics / 1185.5.3 High Energy Fusion Targets and Media for

Chemical Release / 1185.5.4 Photonic Bandgap Materials / 120

References / 122

6 PATTERNING BASED ON EXTERNAL FORCE:NANOIMPRINT LITHOGRAPHY 129L. Jay Guo

6.1 Introduction / 1296.2 NIL MOLD / 133

6.2.1 Mold Fabrication / 1336.2.2 Mold Surface Preparation / 1376.2.3 Flexible Fluoropolymer Mold / 137

6.3 NIL Resist / 1386.3.1 Thermoplastic Resist / 1396.3.2 Copolymer Thermoplastic Resists / 1416.3.3 Thermal-Curable Resists / 1426.3.4 UV-Curable Resist / 1466.3.5 Other Imprintable Materials / 148

6.4 The Nanoimprint Process / 1496.4.1 Cavity Fill Process / 149

CONTENTS vii

6.5 Variations of NIL Processes / 1526.5.1 Reverse Nanoimprint / 1526.5.2 Combined Nanoimprint and Photolithography / 1556.5.3 Roll-to-Roll Nanoimprint Lithography (R2RNIL) / 156

6.6 Conclusion / 159References / 160

7 PATTERNING BASED ON EDGE EFFECTS:EDGE LITHOGRAPHY 167Matthias Geissler, Joseph M. McLellan, Eric P. Lee and Younan Xia

7.1 Introduction / 1677.2 Topography-Directed Pattern Transfer / 169

7.2.1 Photolithography with Phase-Shifting Masks / 1707.2.2 Use of Edge-Defined Defects in SAMs / 1727.2.3 Controlled Undercutting / 1757.2.4 Edge-Spreading Lithography / 1767.2.5 Edge Transfer Lithography / 1787.2.6 Step-Edge Decoration / 180

7.3 Exposure of Nanoscale Edges / 1817.3.1 Fracturing of Thin Films / 1827.3.2 Sectioning of Encapsulated Thin Films / 1827.3.3 Thin Metallic Films along Sidewalls of

Patterned Stamps / 1847.3.4 Topographic Reorientation / 186

7.4 Conclusion and Outlook / 187References / 188

8 PATTERNING WITH ELECTROLYTE: SOLID-STATESUPERIONIC STAMPING 195Keng H. Hsu, Peter L. Schultz, Nicholas X. Fang, and Placid M. Ferreira

8.1 Introduction / 1958.2 Solid-State Superionic Stamping / 1978.3 Process Technology / 1998.4 Process Capabilities / 2038.5 Examples of Electrochemically Imprinted Nanostructures

Using the S4 Process / 208Acknowledgments / 211References / 211

9 PATTERNING WITH GELS: LATTICE-GAS MODELS 215Paul J. Wesson and Bartosz A. Grzybowski

9.1 Introduction / 2159.2 The RDF Method / 218

viii CONTENTS

9.3 Microlenses: Fabrication / 2189.4 Microlenses: Modeling Aspects / 220

9.4.1 Modeling Using PDEs / 2209.4.2 Modeling Using Lattice-Gas Method / 221

9.5 RDF at the Nanoscale / 2229.5.1 Nanoscopic Features from Counter-Propagating

RD Fronts / 2229.5.2 Failure of Continuum Description / 2259.5.3 Lattice-Gas Models at the Nanoscale / 227

9.6 Summary and Outlook / 229References / 230

10 PATTERNING WITH BLOCK COPOLYMERS 233Jia-Yu Wang, Wei Chen, and Thomas P. Russell

10.1 Introduction / 23310.2 Orientation / 235

10.2.1 Self-Assembling / 23510.2.2 Self-Directing / 247

10.3 Long-Range / 25410.3.1 Solvent Annealing / 25410.3.2 Graphoepitaxy / 25610.3.3 Sequential, Orthogonal Fields / 260

10.4 Nanoporous BCP Films / 26210.4.1 Ozonolysis / 26410.4.2 Thermal Degradation / 26410.4.3 UV Degradation / 26710.4.4 Selective Extraction / 27110.4.5 “Soft” Chemical Etch / 27210.4.6 Cleavable Junction / 27210.4.7 Solvent-Induced Film Reconstruction / 274

References / 276

11 PERSPECTIVE ON APPLICATIONS 291

II APPLICATIONS 293

12 SOFT LITHOGRAPHY FOR MICROFLUIDICMICROELECTROMECHANICAL SYSTEMS (MEMS)AND OPTICAL DEVICES 295Svetlana M. Mitrovski, Shraddha Avasthy, Evan M. Erickson,Matthew E. Stewart, John A. Rogers, and Ralph G. Nuzzo

12.1 Introduction / 29512.2 Microfluidic Devices for Concentration Gradients / 297

CONTENTS ix

12.3 Electrochemistry and Microfluidics / 30012.4 PDMS and Electrochemistry / 30212.5 Optics and Microfluidics / 30612.6 Unconventional Soft Lithographic Fabrication

of Optical Sensors / 314Acknowledgments / 317References / 318

13 UNCONVENTIONAL PATTERNING METHODS FOR BIONEMS 325Pilnam Kim, Yanan Du, Ali Khademhosseini, Robert Langer, and Kahp Y. Suh

13.1 Introduction / 32513.2 Fabrication of Nanofluidic System for

Biological Applications / 32613.2.1 Unconventional Methods for Fabrication of

Nanochannel / 32613.2.2 Application of Nanofluidic System / 332

13.3 Fabrication of Biomolecular Nanoarrays for BiologicalApplications / 33813.3.1 DNA Nanoarray / 33813.3.2 Protein Arrays / 34013.3.3 Lipid Array / 345

13.4 Fabrication of Nanoscale Topographies for Tissue EngineeringApplications / 34713.4.1 Nanotopography-Induced Changes in Cell

Adhesion / 34713.4.2 Nanotopography-Induced Changes in Cell

Morphology / 348References / 349

14 MICRO TOTAL ANALYSIS SYSTEM 359Yuki Tanaka and Takehiko Kitamori

14.1 Introduction / 35914.1.1 Historical Backgrounds / 359

14.2 Fundamentals on Microchip Chemistry / 36114.2.1 Characteristics of Liquid Microspace / 36114.2.2 Liquid Handling / 36214.2.3 Concepts of Micro Unit Operation and Continuous-Flow

Chemical Processing / 36214.3 Key Technologies / 365

14.3.1 Fabrication of Microchips / 36514.3.2 Patterning for Fluid Control / 36614.3.3 Detection / 366

14.4 Applications / 36814.4.1 Synthesis / 368

x CONTENTS

14.4.2 Cell Adhesion Control / 36914.4.3 Liquid Handling: Valve Using Wettability / 370

References / 372

15 COMBINATIONS OF TOP-DOWN AND BOTTOM-UPNANOFABRICATION TECHNIQUES AND THEIRAPPLICATION TO CREATE FUNCTIONAL DEVICES 379Pascale Maury, David N. Reinhoudt, and Jurriaan Huskens

15.1 Introduction / 37915.2 Top-Down and Bottom-Up Techniques / 380

15.2.1 Top-Down Techniques / 38015.2.2 Bottom-Up Techniques / 38315.2.3 Mixed Techniques / 384

15.3 Combining Top-Down and Bottom-Up Techniques for HighResolution Patterning / 38515.3.1 Top-Down Nanofabrication and

Polymerization / 38615.3.2 Top-Down Nanofabrication and Micelles / 38715.3.3 Top-Down Nanofabrication and Block Copolymer

Assembly / 38715.3.4 Top-Down Nanofabrication and NP Assembly / 38915.3.5 Top-Down Nanofabrication and Layer-by-Layer

Assembly / 39215.4 Applicaion of Combined Top-Down and Bottom-Up

Nanofabrication for Creating Functional Devices / 39715.4.1 Photonic Crystal Devices / 39715.4.2 Protein Assays / 400

References / 406

16 ORGANIC ELECTRONIC DEVICES 41916.1 Introduction / 41916.2 Organic Light-Emitting Diodes / 42016.3 Organic Thin Film Transistors / 429References / 439

17 INORGANIC ELECTRONIC DEVICES 44517.1 Introduction / 44517.2 Inorganic Semiconductor Materials for Flexible

Electronics / 44617.2.1 “Bottom-Up” Approaches / 44717.2.2 “Top-Down” Approaches / 449

CONTENTS xi

17.3 Soft Lithography Techniques for Generating InorganicElectronic Systems / 45217.3.1 Micromolding in Capillaries / 45317.3.2 Imprint Lithography / 45417.3.3 Dry Transfer Printing / 454

17.4 Fabrication of Electronic Devices / 45917.4.1 Transistors on Rigid Substrates via

MIMIC Processing / 45917.4.2 Flexible Inorganic Transistors / 45917.4.3 Flexible Integrated Circuits / 46317.4.4 Heterogeneous Electronics / 46617.4.5 Stretchable Electronics / 469

References / 475

18 MECHANICS OF STRETCHABLE SILICON FILMS ONELASTOMERIC SUBSTRATES 483Hanqing Jiang, Jizhou Song, Yonggang Huang, and John A. Rogers

18.1 Introduction / 48318.2 Buckling Analysis of Stiff Thin Ribbons on

Compliant Substrates / 48418.3 Finite-Deformation Buckling Analysis of Stiff Thin Ribbons

on Compliant Substrates / 48818.4 Edge Effects / 49518.5 Effect of Ribbon Width and Spacing / 49818.6 Buckling Analysis of Stiff Thin Membranes on Compliant

Substrates / 50218.6.1 One-Dimensional Buckling Mode / 50418.6.2 Checkerboard Buckling Mode / 50618.6.3 Herrington Buckling Mode / 506

18.7 Precisely Controlled Buckling of Stiff Thin Ribbonson Compliant Substrates / 507

18.8 Concluding Remarks / 512Acknowledgments / 512References / 512

19 MULTISCALE FABRICATION OF PLASMONIC STRUCTURES 515Joel Henzie, Min H. Lee, and Teri W. Odom

19.1 Introduction / 51519.1.1 Brief Primer on Surface Plasmons / 51719.1.2 Conventional Methods to Plasmonic Structures / 518

19.2 Soft Lithography and Metal Nanostructures / 51819.3 A Platform for Multiscale Patterning / 520

xii CONTENTS

19.3.1 Soft Interference Lithography: Patterns on a NanoscalePitch / 520

19.3.2 Phase-Shifting Photolithography: Patterns on aMicroscale Pitch / 520

19.3.3 PEEL: Transferring Photoresist Patterns to PlasmonicMaterials / 521

19.4 Subwavelength Arrays of Nanoholes: PlasmonicMaterials / 52219.4.1 Infinite Arrays of Nanoholes / 52319.4.2 Finite Arrays (Patches) of Nanoholes / 525

19.5 Microscale Arrays of Nanoscale Holes / 52619.6 Plasmonic Particle Arrays / 528

19.6.1 Metal and Dielectric Nanoparticles / 52819.6.2 Anisotropic Nanoparticles / 53119.6.3 Pyramidal Nanostructures / 531

Acknowledgments / 533References / 533

20 A RIGIFLEX MOLD AND ITS APPLICATIONS 539Se-Jin Choi, Tae-Wan Kim, and Seung-Jun Baek

20.1 Introduction / 53920.2 Modulus-Tunable Rigiflex Mold / 54020.3 Applications of Rigiflex Mold / 544

20.3.1 From Nanoimprint to Microcontact Printing / 54420.3.2 Rapid Flash Patterning for Residue-Free

Patterning / 54720.3.3 Continuous Rigiflex Imprinting / 54920.3.4 Soft Molding Application / 55320.3.5 Capillary Force Lithography Applications / 55620.3.6 Transfer Fabrication Technique / 558

References / 561

21 NANOIMPRINT TECHNOLOGY FOR FUTURE LIQUIDCRYSTAL DISPLAY 565Jong M. Kim, Hwan Y. Choi, Moon-G. Lee, Seungho Nam, Jin H. Kim,Seongmo Whang, Soo M. Lee, Byoung H. Cheong, Hyuk Kim,Ji M. Lee, and In T. Han

21.1 Introduction / 56521.2 Holographic LGP / 569

21.2.1 Design and Properties of Holographic LGP / 57021.2.2 NI Technology for the Holographic LGP / 572

21.3 Polarized LGP / 57321.3.1 Design and Properties of Polarized LGP / 574

CONTENTS xiii

21.3.2 Fabrication of the Polarized LGP / 57521.3.3 Optical Performance of the Polarized LGP / 576

21.4 Reflective Polarizer: Wire Grid Polarizer / 57921.4.1 Design and Properies of WGP / 58021.4.2 Fabrication and Applications / 581

21.5 Transflective Display / 58521.5.1 Design and Optical Properties of Reflecting Pattern / 58721.5.2 Fabrication of the Reflecting Pattern / 588

References / 592

INDEX 595

PREFACE

The area of nanofabrication is a dynamic and rapidly growing field that is sometimesdominated by activity focused on the development of systems for fabrication in mi-croelectronics. In the mid-1990s a growing appreciation of the value of alternativemethods, often driven primarily by materials and chemistry rather than by optics andphysics, led to the formation of a separate field of study on unconventional or alter-native techniques for nanofabrication. Early demonstrations of various forms of softlithography by George Whitesides (Harvard University), nanoimprint lithography byStephen Chou (then at University of Minnesota, and presently at Princeton Univer-sity), and polymer phase separation by Richard Register and Paul Chaikin (PrincetonUniversity) were among the important events that catalyzed these developments. Oneof us (John A. Rogers) was in the Whitesides group as a Harvard Junior Fellow dur-ing this time, and has remained active in the field ever since. The interest of the other(Hong H. Lee) derived from a nanoproject on tera bit level memory device. Takentogether, we have published more than 250 papers on various aspects of unconven-tional nanofabrication and its application to diverse device classes, and they havetrained more than 80 students in these areas. The gradual maturing of the field andthe emergence of meaningful applications provide the motivation for assembling abook at this time. We hope that the outcome will be useful as a reference text forpractitioners and developers alike.

HHL is thankful to Audrey Lee for assistance. We both are very grateful toMs Hyewon Kang, who took care of the details of editing for the whole book.

John A. RogersChampaign, USA

Hong H. LeeSeoul, Korea

xv

INANOPATTERNING

TECHNIQUES

Unconventional Nanopatterning Techniques and Applications by John A. Rogers and Hong H. LeeCopyright C© 2009 John Wiley & Sons, Inc.

1

1INTRODUCTION

Tools for nanofabrication are central to every field of nanoscience and nano-technology. For research and initial development purposes, nanofabrication typicallyinvolves the use of specialized techniques to fabricate small collections of nanoscaledevices, in processes that resemble a form of craftsmanship. Discoveries that emergefrom such work will only yield valuable nanotechnologies, however, when they canbe implemented with techniques that can be scaled for cost-effective manufactur-ing. As a result, the success of nanotechnology depends not only on versatile nano-fabrication techniques for discovery in nanoscience, but also on approaches that offerlow cost operation and high throughputs, suitable for mass production. In some cases,the techniques might rely on adapted versions of methods whose origins are in themicroelectronics industry, such as photolithography and electron-beam lithography.In many others, including certain areas of photonics, microfluidics, biotechnology,and flexible electronics, new approaches are required, either to facilitate commercial-ization, to allow manipulation of unusual materials, or to enable challenging featuressizes and structure geometries.

The need for unconventional nanofabrication techniques was recognized broadlyin the early 1990s, even before nanotechnology was generally recognized as a sep-arate field of study. During this time, new areas of research emerged around softlithography, imprint lithography, and various types of self-assembly and scanningprobe based patterning methods. The interest in these approaches is driven by theirdiverse, underlying scientific content, their conceptual novelty, and their technicalcapabilities for nanofabrication. Their ultimate success, however, is measured firstby the extent of their adoption for research purposes and then by their use in man-ufacturing. Self-assembly and scanning probe techniques will be useful for someapplications, but their limited patterning versatility (i.e., materials and geometries)and modest throughput, represent significant disadvantages. Soft lithography and

Unconventional Nanopatterning Techniques and Applications by John A. Rogers and Hong H. LeeCopyright C© 2009 John Wiley & Sons, Inc.

3

4 INTRODUCTION

1994 1996 1998 2000 2002 2004 20060

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Figure 1.1. Numbers of scientific publications (bottom) and citations (top) for the fields of softlithography and imprint lithography, since 1993.

imprint lithography avoid these problems and, in our view, have significant poten-tial both for research and for realistic implementations in wide-ranging classes ofapplications. The growth of research in these areas has been explosive, starting withthe introduction of microcontact printing, the first form of soft lithography, in 1993and then imprint lithography in 1995. Figure 1.1 shows the numbers of papers in softand imprint lithography, and citations of these papers as a function of time. Thesedata indicate clearly the value of these methods for laboratory scale applications andresearch. This growth and the substantial development work on these methods atsmall and large companies also suggest an expansion of their use to prototyping andmanufacturing.

This book covers unconventional methods for nanofabrication, with a focus onsoft lithographic and related imprint lithographic methods, but also with a sum-mary of some of the most promising self-assembly methods. The content is orga-nized in two separate parts. The first deals with the principles and underlying science

INTRODUCTION 5

associated with a range of different techniques. In particular, the first chapter coversthe classes of materials and surface chemistries that are most commonly used for thestamps, molds, and conformable photomasks of soft lithography. The next severalchapters review established and new strategies for using these and analogous “hard”elements in procedures that range from transfer of solid materials to control of theflow of photons to molding of liquid or softened polymers to control of diffusionof chemicals or ions into and out of the substrate. The final chapter in this sectiondemonstrates the power of self-assembly in procedures that rely on polymer phaseseparation. The second part of the book focuses on applications of these techniquesin some of the most promising areas, as outlined in more detail in Chapter 11.

The content is intended for practitioners, for researchers developing new methods,and for students in specialized courses in chemistry, physics, biology, chemical en-gineering, materials science, electrical engineering, or mechanical engineering. Thesizes of these communities are growing rapidly, due to the high level of importanceof the methods to broad areas of nanotechnology, information technology, biotech-nology, and related fields. We hope that this book will help expand even further thereach of these methods and that this expansion will facilitate their further develop-ment, potentially culminating with their broad-based use in bridging the gap betweennanoscience and nanotechnology.

2MATERIALS

2.1 INTRODUCTION

The unconventional nanopatterning techniques treated in this book are based pri-marily on the use of molds or stamps. The features of these techniques are largelydetermined by the properties of the materials used. These properties include surfaceenergy, Young’s modulus, transparency to light, and compliance or flexibility. Forexample, the mold material should have a low surface energy for the mold to bereleased cleanly and easily from the material being patterned. Similarly, if the ma-terial to be patterned is an ultraviolet (UV) curable prepolymer, the mold should betransparent to the light.

Feature resolution and large area applicability are two main items of interest inany patterning technique. The smallest feature size that can be patterned with a moldis largely determined by the rigidity of the mold, a measure of which is its Young’smodulus, E. Generally, a more rigid mold, or a mold with a higher Young’s modulus,permits a better resolution. The large area applicability is mainly dependent on theextent of compliance of the mold to substrate surface, or mold flexibility. If the extentto which a sheet deflects when it is subjected to a load is used as a measure offlexibility, then the flexibility is determined by Et3, where t is the sheet thickness (seeSection 4.4). Any rigid mold can be made flexible by making the sheet sufficientlythin.

In terms of Young’s modulus, there are two extremes for techniques that are inwidespread use: molds made from poly(dimethylsiloxane) (PDMS) and those madefrom silicon. The Young’s modulus of typical PDMS is less than 2 MPa, whereas thatof silicon is around 130 GPa. Soft molds, such as those made with PDMS, are usedfor soft lithography [1] and hard molds, such as those made with silicon, are usedfor imprint lithography [2]. On the other hand, a mold can have a Young’s modulus

Unconventional Nanopatterning Techniques and Applications by John A. Rogers and Hong H. LeeCopyright C© 2009 John Wiley & Sons, Inc.

7

8 MATERIALS

between the two extremes, which is more than tens of MPa but less than a few GPa.A mold with a Young’s modulus in this range is rigid enough for nanoscale pattern-ing but at the same time flexible enough in its film form for large area applications.As opposed to soft and hard molds, these molds that are rigid yet flexible are oftenreferred to as “rigiflex” molds [3]. Polymers are typically used for soft and rigiflexmolds, whereas silicon, quartz or metals are used for hard molds. Polymer molds aremost often prepared by casting a liquid prepolymer onto a master or template andthen curing it either photochemically or thermally. The master is typically fabricatedby photolithography or electron-beam lithography. While polymer molds can be pro-duced from the master, as many times as desired, the master itself typically becomesthe mold for patterning when a hard mold is used.

This chapter reviews materials used for molds, beginning with soft molds, andthen following with hard molds and rigiflex molds. The surface of a mold is oftentreated with a material of low surface energy to ensure clean and easy release ofthe mold from the material being patterned. The need for surface treatment becomesmore acute for smaller feature size and more densely populated pattern features. Thelatter part of the chapter covers this subject of surface treatment.

2.2 MOLD MATERIALS AND MOLD PREPARATION

Nanopatterning can be accomplished with hard, soft or rigiflex molds. The choice ofmold depends on the requirements of the application. A soft mold is typically usedfor soft lithography, whereas a hard mold is generally used for imprint lithography. Arigiflex mold can be used in place of a hard or soft mold in most cases. This sectioncovers materials and preparation methods for these three types of molds.

2.2.1 Soft Molds

The most representative of soft molds are made from Sylgard 184 (DowCorning). Molds of this type date back to the first reports of microcontact printing,the first type of soft lithographic technique, in 1993 [4]. Such molds are fabricatedby casting a mixture of prepolymer and cross-linker at a recommended ratio of 10:1against a master with relief structures that correspond to the desired pattern. A cur-ing time of 4–6 h and a curing temperature of 60◦C should be used. A lower ratio ofcross-linker leads to a stickier mold surface.

The Sylgard PDMS, sometimes referred to as soft PDMS (s-PDMS), has a num-ber of characteristics and physical properties that are well suited for soft lithography.Its surface energy is low at 21 dyn cm−1 and it is transparent in the UV and visi-ble regions. In addition, it has high gas permeability. For example, the permeabilityfor O2 is 10−11 cm3 cm (cm2 s Pa)−1. Its flexibility and tackiness allow conformalcontact of the mold with the underlying surface. The thermal expansion coefficientis, however, relatively high at 310 μm (m ◦C)−1 such that a linear shrinkage on theorder of 1.5% occurs when cooled after curing at 60◦C [5]. The mold also has thedisadvantage that it swells in many organic solvents as summarized in Table 2.1 [6].

2.2 MOLD MATERIALS AND MOLD PREPARATION 9

Table 2.1. Solubility Parameters, Swelling Ratios, and Dipole Moments of VariousSolvents Used in Organic Synthesis

Solvent δa Sb μ(D)

Perfluorotributylamine 5.6 1.00 0.0Perfluorodecalin 6.6 1.00 0.0Pentane 7.1 1.44 0.0Poly(dimethylsiloxane) 7.3 ∞ 0.6–0.9Diisopropylamine 7.3 2.13 1.2Hexanes 7.3 1.35 0.0n-Heptane 7.4 1.34 0.0Triethylamine 7.5 1.58 0.7Ether 7.5 1.38 1.1Cyclohexane 8.2 1.33 0.0Trichloroethylene 9.2 1.34 0.9Dimethylethoxyethane (DME) 8.8 1.32 1.6Xylenes 8.9 1.41 0.3Toluene 8.9 1.31 0.4Ethyl acetate 9.0 1.18 1.8Benzene 9.2 2.28 0.0Chloroform 9.2 1.39 1.02-Butanone 9.3 1.21 2.8Tetrahydrofurane (THF) 9.3 1.38 1.7Dimethyl carbonate 9.5 1.03 0.9Chlorobenzene 9.5 1.22 1.7Methylene chloride 9.9 1.22 1.6Acetone 9.9 1.06 2.9Dioxane 10.0 1.16 0.5Pyridine 10.6 1.06 2.2N-Methylpyrrolidone (NMP) 11.1 1.03 3.8tert-Butyl alcohol 10.6 1.21 1.6Acetonitrile 11.9 1.01 4.01-Propanol 11.9 1.09 1.6Phenol 12.0 1.01 1.2Dimethylformamide (DMF) 12.1 1.02 3.8Nitromethane 12.6 1.00 3.5Ethyl alcohol 12.7 1.04 1.7Dimethyl sulfoxide (DMSO) 13.0 1.00 4.0Propylene carbonate 13.3 1.01 4.8Methanol 14.5 1.02 1.7Ethylene glycol 14.6 1.00 2.3Glycerol 21.1 1.00 2.6Water 23.4 1.00 1.9

Source: Reprinted with permission from [6]. Copyright 2003 American Chemical Society.aSolubility parameter δ in units of cal1/2 cm−3/2.bS denotes the swelling ratio that was measured experimentally; S = D/D0, where D is the length ofPDMS in the solvent and D0 is the length of the dry PDMS, and μ denotes the dipole moment.

10 MATERIALS

A larger swelling ratio indicates more solvent-induced swelling. A solvent with asolubility parameter close to that of PDMS is a good solvent. For example, triethy-lamine with the solubility parameter of 7.5 is a better solvent than benzene for whichthe parameter is 9.2 since the solubility parameter of PDMS is 7.3 cal1/2 cm−3/2.

The modulus of elasticity or Young’s modulus of s-PDMS (Sylgard 184) is around2 MPa, depending on the mixing method, curing time, and temperature. This lowmodulus limits the fabrication of features with high aspect ratios due to collapse,merging, and buckling of the structures of relief [7–9]. These deformation modeshave been examined both theoretically [8, 9] and experimentally [10]. The theoreticalcriteria [8–10] that can be used for dimensional stability and conformity of a moldare summarized in Table 2.2. Conformity here means full contact of the mold featurewith the underlying substrate surface. The criteria given in the table are such that ifthey are satisfied then the mold avoids the associated deformation. For instance, ifthe equation in the first entry of the table is satisfied, then the mold will not undergoroof collapse.

The results suggest that roof collapse, buckling, and lateral collapse (merging) canbe avoided by increasing E∗ or Young’s modulus. On the other hand, the conformitydecreases when Young’s modulus increases. Deformations other than those relatedto conformity can prevent accurate patterning. It is instructive to examine the criteriafor an equal line and space pattern, for which a = w or a/w = 1. For a given load, theroof collapse is determined by 1/(EA), where A is the aspect ratio given by h/a or h/w.Roof collapse, therefore, is less likely to occur for a mold with a higher aspect ratioand higher Young’s modulus. On the other hand, buckling is more likely to occur fora mold with a higher aspect ratio but less likely for a mold with a higher Young’smodulus because buckling is determined by A/E. Lateral collapse is determined byA/(Ea)1/4. Therefore, lateral collapse is more likely to occur if the aspect ratio ishigher, the Young’s modulus is lower, or the feature size is smaller.

The conformity is determined by EA, meaning that conformal contact improves asYoung’s modulus and the aspect ratio decrease. As the feature size is reduced, the lat-eral collapse problem becomes more acute. This is one of the reasons why a s-PDMSmold cannot be used for feature sizes smaller than several hundred nanometers forequal line and space patterns with an aspect ratio larger than unity.

An obvious way to overcome unwanted deformations is to use a mold with ahigher Young’s modulus. Therefore, there have been attempts to use materials ofhigh Young’s modulus for soft molds. The earliest example used alternative siloxanepolymers [5] having a Young’s modulus of around 9 MPa, known as hard PDMS(h-PDMS). To overcome the shortcomings of h-PDMS, such as its brittleness, andthe need to apply pressure to achieve conformal contact with a substrate, a compos-ite mold of PDMS was introduced [7], in which a thin h-PDMS layer with reliefstructure is supported by a thick layer of s-PDMS. This composite design combinesthe advantages of both a more rigid layer (to achieve high resolution patterning) anda more flexible support (to facilitate handling and the establishment of conformalcontact) [7]. Figure 2.1 illustrates the procedure for preparing such composite ele-ments. The h-PDMS is formed [7] by mixing and degassing for 1–2 min 3.4 g of avinyl PDMS prepolymer (VDT-731, Gelest Corp.), 18 μL of a Pt catalyst (platinum

2.2 MOLD MATERIALS AND MOLD PREPARATION 11

Table 2.2. Dimensional Stability and Conformity Criteria

Roof collapse:unwanted contact

σ∞−4σ∞w

π E∗h

(1 + a

w

)cosh−1

−[(

cos

(wπ

2(w + a)))−1]

< 1

Buckling

σ∞

−12σ∞h2

π 2 E∗a2<

1

1 + (w/a)

Lateral collapseh

2a

(2γs

3E∗a

)1/4<

√w

a

Smooth surfaceasperities(conformity)

−π E∗h2σ∞L

< 1 + wa

Radius of an edgerounded by surfacetension

R R ∼ γ2E

Source: Reprinted with permission from [8]. Copyright 2002 American Chemical Society.E∗ = E/(1/ν2): E—Young’s modulus; ν—Poisson ratio; γ —surface tension; L—period (2(a+w));σ—remote stress (load).

divinyltetramethyldisiloxane, SIP 6831.1, Gelest Corp.), and one drop (approxi-mately 0.1 wt%) of a moderator (2,4,6,8-tetramethyl-tetravinylcyclotetrasiloxane,87927, Sigma-Aldrich). Then, 1 g of a hydrosilane prepolymer (HMS-301, GelestCorp.) is stirred into this mixture. Within 3 min, a thin layer (30–40 μm) of h-PDMSis spin coated onto a master and cured for 30 min at 60◦C. Then, a liquid polymerlayer (∼3 mm) of Sylgard 184 PDMS is poured onto the h-PDMS layer and cured forat least an hour at 60◦C. The composite mold is released from the surface by cuttingand peeling the mold from the surface while warm.

12 MATERIALS

Master 500 nm

Spin coat h-PDMS

Cast 184 PDMS

Release from master

Compositestamp

3 mm

40 μm

184 PDMS

184 PDMS

h-PDMS

h-PDMS

h-PDMS

Si

Si

SiSi

Si

Si

Figure 2.1. Procedure for fabricating a two-layer composite stamp. h-PDMS is spin coated ontoa master and cured at 60◦C for 30 min (after this process, the layer is still tacky). PDMS 184prepolymer is poured on top of this h-PDMS layer and cured at 60◦C for at least 1 h. Thecomposite stamp is released from the master by (i) cutting around the patterned areas witha razor blade and (ii) removing the stamp from the surface using tweezers. (Reprinted withpermission from [7]. Copyright 2002 American Chemical Society.)

In preparing molds with casting and curing procedures such as those in Figure 2.1,the mode of interaction between the mold material and the master is extremely im-portant. The role of fluid dynamics and wettability in preparing the PDMS mold areusually ignored. One of the most common reasons for the unsuccessful fabrication ofa mold, particularly at small feature sizes, is the inability of the liquid prepolymer topenetrate into and fill up the holes and grooves in the master template [11]. The mainfactor that governs flow in this process is viscosity. The viscosity can be loweredby simply mixing the prepolymer with a solvent. It has been shown for a polymersolution that the viscosity at a polymer volume fraction α, η(α), is related to that atanother volume fraction β, η(β), as follows [12]:

η(α)

η(β)=(

α

β

)5.1(2.1)

The relationship indicates that a considerable reduction in viscosity can be real-ized by introducing a solvent. When PDMS oligomers are diluted with toluene to69% by weight, the measured viscosity decreases from 362 to 50 cP. Although theweight fraction is different from the volume fraction unless the density is unity, areduction to 69 wt% should lead to a viscosity of approximately 55 cP according toequation 2.1, which compares well with the measured value of 50 cP [11].

The rate of curing of the PDMS is also important. For example, the curing pro-cess should be delayed until after the solvent is completely volatilized and onlythe prepolymer fills up the void space in the master. Otherwise, incomplete fillingand/or pattern shrinking will result. The moderator used in preparing the PDMS mold