Scanning Probe Lithography: Fundamentals, Materials, and Applications 9781032122144, 9781032122151, 9781003223610

Citation preview

Scanning Probe Lithography The most complete book available on scanning probe lithography (SPL), this work details the modalities, mechanisms, and current technologies, applications, and materials on which SPL can be performed. It provides a comprehensive overview of this versatile and cost-effective technique, which does not require clean room conditions and can be performed in any lab or industry facility to achieve highresolution and high-quality patterns on a wide range of materials: biological, semiconducting, polymers, and 2D materials. • Introduces historical background of SPL, including evolution of the technique and tools • Explains the mechanism of sample modifcation/manipulation, types of AFM tips, technical parts of the experimental setup, and materials on which the technique can be applied • Shows the different types of devices and structures fabricated by SPL, together with the processing steps • Contains a complete and state-of-the art package of examples and different approaches, performed by different international research groups • Summarizes strengths, limitations, and potential of SPL This book is aimed at advanced students, technicians, and researchers in materials science, microelectronics, and others working with lithographic techniques and fabrication processes.

Emerging Materials and Technologies Series Editor: Boris I. Kharissov The Emerging Materials and Technologies series is devoted to highlighting publications centered on emerging advanced materials and novel technologies. Attention is paid to those newly discovered or applied materials with potential to solve pressing societal problems and improve quality of life, corresponding to environmental protection, medicine, communications, energy, transportation, advanced manufacturing, and related areas. The series takes into account that, under present strong demands for energy, material, and cost savings, as well as heavy contamination problems and worldwide pandemic conditions, the area of emerging materials and related scalable technologies is a highly interdisciplinary feld, with the need for researchers, professionals, and academics across the spectrum of engineering and technological disciplines. The main objective of this book series is to attract more attention to these materials and technologies and invite conversation among the international R&D community. Scanning Probe Lithography Fundamentals, Materials, and Applications Yu Kyoung Ryu and Javier Martinez Rodrigo Engineered Nanoparticles as Drug Delivery Systems Nahid Rehman and Anjana Pandey MXene Filled Polymer Nanocomposites Edited by Soney C George, Sharika T. Nair and Joice Sophia Ponraj Polymeric Biomaterials Fabrication, Properties and Applications Edited by Pooja Agarwal, Divya Bajpai Tripathy, Anjali Gupta and Bijoy Kumar Kuanr Innovations in Green Nanoscience and Nanotechnology Synthesis, Characterization, and Applications Edited by Shrikaant Kulkarni Sustainable Nanomaterials for the Construction Industry Ghasan Fahim Huseien and Kwok Wei Shah 4D Imaging to 4D Printing Biomedical Applications Edited by Rupinder Singh For more information about this series, please visit: www.routledge.com/EmergingMaterials-and-Technologies/book-series/CRCEMT

Scanning Probe Lithography Fundamentals, Materials, and Applications

Yu Kyoung Ryu and Javier Martinez Rodrigo

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

MATLAB® is a trademark of The MathWorks, Inc. and is used with permission. The MathWorks does not warrant the accuracy of the text or exercises in this book. This book’s use or discussion of MATLAB® software or related products does not constitute endorsement or sponsorship by The MathWorks of a particular pedagogical approach or particular use of the MATLAB® software. First edition published 2023 by CRC Press 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487–2742 and by CRC Press 4 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN CRC Press is an imprint of Taylor & Francis Group, LLC © 2023 Yu Kyoung Ryu and Javier Martinez Rodrigo Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microflming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, access www. copyright.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978–750–8400. For works that are not available on CCC please contact [email protected] Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identifcation and explanation without intent to infringe. ISBN: 978-1-032-12214-4 (hbk) ISBN: 978-1-032-12215-1 (pbk) ISBN: 978-1-003-22361-0 (ebk) DOI: 10.1201/9781003223610 Typeset in Times by Apex CoVantage, LLC

Dedication Yu Kyoung Ryu “That’s what all we are: amateurs. We don’t live long enough to be anything else.” Charlie Chaplin To my mother and Víctor To Ricardo and Armin, my mentors in SPL Javier Martinez Rodrigo To my mother Aurora, to my sister Aurori and to my father Alfonso

Contents Acknowledgements ...................................................................................................xi Author Biographies ................................................................................................ xiii Acronyms................................................................................................................. xv Chapter 1

Historical Background and Place in the Lithography Roadmap...............................................................................................1 1.1. Introduction to Nanolithography by AFM ................................ 1 1.2. Historical Background...............................................................2 1.3. AFM: The Most Versatile Tool at the Nanoscale ......................5 1.4. Book Scheme .............................................................................6 Chapter References...............................................................................7

Chapter 2

Basic Concepts and Modalities ............................................................9 2.1. 2.2. 2.3.

The Tool: An Atomic Force Microscope...................................9 An Atomic Force Microscope for Lithography ....................... 11 Preparing a Scanning Probe Lithography Experiment .............................................................................. 12 2.4. “How to Name the Technique?” .............................................. 13 Chapter References.............................................................................13 Chapter 3

Mechanical Scanning Probe Lithography .......................................... 15 3.1. 3.2. 3.3. 3.4. 3.5.

Fundamentals .......................................................................... 15 Parameters for the Lithographic Process................................. 16 Manipulation of Nano-Objects ................................................ 18 Cleaning of 2D Materials Post-Processing..............................20 Applications and Proof of Concepts ........................................ 22 3.5.1. Lift-Off and Pattern Transfer Processes .................... 22 3.5.2. Templates.................................................................... 22 3.5.3. Charge Modulation in Quantum Devices...................24 3.5.4. 2D Materials Based Devices and Patterns .................25 3.5.5. Other Proof-of-Concept Devices and Nanostructures.....................................................26 Chapter References.............................................................................27 Chapter 4

Dip Pen Nanolithography ................................................................... 33 4.1.

Fundamentals .......................................................................... 33 4.1.1. Type of Inks................................................................ 33 4.1.2. Material Transport Models.........................................34 vii

viii

Contents

(a) Molecular Diffusion in the Case of Diffusive Inks ................................................34 (b) Mass Fluid Flow in the Case of Liquid Inks .........................................................34 4.1.3. Derivatives of Dip Pen Nanolithography Method ....................................................................... 35 4.1.4. Developments on the Tool .......................................... 35 4.2. Parameters for the Lithographic Process................................. 36 4.2.1. Tip .............................................................................. 36 4.2.2. Surface........................................................................ 37 4.2.3. Ink .............................................................................. 38 4.2.4. Writing Conditions ..................................................... 38 4.3. Electrochemical and Thermal Dip Pen Nanolithography ...................................................................... 39 4.4. Applications .............................................................................40 4.4.1. Etching Masks and Chemical Templates ................... 41 4.4.2. Biomolecular or Organic Molecule Patterns .............. 43 4.4.3. Inorganic Patterns ......................................................44 Chapter References.............................................................................46 Chapter 5

Field Emission Scanning Probe Lithography ..................................... 53 5.1. Fundamentals .......................................................................... 53 5.2. Parameters for the Lithographic Process................................. 53 5.3. Towards Single-Digit Nanometer Lithography ....................... 55 5.4. Other Processes Driven by fe-SPL .......................................... 58 Chapter References.............................................................................59

Chapter 6

Oxidation Scanning Probe Lithography............................................. 61 6.1.

6.2. 6.3. 6.4.

Fundamentals .......................................................................... 61 6.1.1. Oxide Growth Kinetics .............................................. 63 6.1.2. Composition of the Oxides Fabricated by o-SPL ..................................................................... 63 6.1.3. Growth of Oxides over and under the Substrate Surface ........................................................64 Parameters for the Lithographic Process................................. 65 Tunability of o-SPL Processes: Polarity and Atmosphere.............................................................................. 67 Applications and Proof of Concepts ........................................ 68 6.4.1. Lift-Off and Pattern Transfer Processes .................... 68 6.4.2. Templates.................................................................... 68 6.4.3. Barriers for Quantum Devices ................................... 71 6.4.4. Control over Metallic/Insulating State Transitions .................................................................. 72 6.4.5. 2D Materials Based Devices and Patterns ................. 73

Contents

ix

6.4.6.

Other Proof-of-Concept Devices and Nanostructures ........................................................... 73 Chapter References.............................................................................75 Chapter 7

Thermal Scanning Probe Lithography ............................................... 85 7.1. 7.2.

Fundamentals and Components of the Tool ............................ 85 From the Millipede to the NanoFrazor ................................... 85 7.2.1. The Cantilever ............................................................ 86 7.2.2. The Tool and the Technique ....................................... 86 7.2.3. The Resist ................................................................... 87 7.2.4. Markless Lithography ................................................ 88 7.3. Parameters for the Lithographic Process................................. 89 7.4. Applications and Proof of Concepts ........................................90 7.4.1. Lift-Off and Pattern Transfer Processes ....................90 7.4.2. Etching Masks and Templates ....................................90 7.4.3. Three-Dimensional Structures ................................... 93 7.4.4. 2D Materials ............................................................... 95 7.4.5. Physical and Chemical Conversion ............................96 Chapter References.............................................................................97

Chapter 8

Lithography Using a Scanning Tunneling Microscope .................... 103 8.1. 8.2.

Fundamentals and Parameters............................................... 103 Manipulation of Atoms and Molecules ................................. 104 8.2.1. Parallel Processes ..................................................... 104 8.2.2. Perpendicular Processes ........................................... 106 8.2.3. Hydrogen Depassivation Lithography and Atomically Precise Manufacturing ................... 107 8.3. Scanning Proximal Probe Lithography ................................. 110 Chapter References...........................................................................110

Chapter 9

High-Throughput Strategies ............................................................. 115 9.1. Array of Cantilevers .............................................................. 115 9.2. Soft/Hard Stamps .................................................................. 116 9.3. Mix and Match Lithography.................................................. 120 Chapter References...........................................................................120

Index...................................................................................................................... 125

Acknowledgements I fell in love—professionally speaking—with scanning probe lithography (SPL) during the development of my PhD under the supervision of Prof. Ricardo Garcia at Instituto de Ciencia de Materiales de Madrid (Consejo Superior de Investigaciones Científcas). Then, I had the opportunity to make a postdoctoral stay at IBM Research Zurich under the supervision of Dr. Armin Knoll, which reinforced my love for SPL further. By the way, Prof. Garcia and Dr. Knoll are some of the all-stars in the SPL feld. During my PhD thesis and my participation in a European project called “Single Nanometer Manufacturing for beyond CMOS devices” (project code: 318804), I had the opportunity to learn about the state-of-the-art works and advances in several lithographic techniques and enter into contact with experts on the SPL feld such as Prof. Jacob Sagiv, Prof. Francesc Pérez-Murano, and Prof. Ivo Rangelow. Then, I started to work with Prof. Javier Martinez Rodrigo, who also had experience in SPL and likes the technique a lot. That’s why, when I told him that we had an opportunity to write a book on SPL, he supported me immediately and agreed to participate in this adventure. I would like to acknowledge the editors for all the help they have given us so quickly and kindly so many times. I would like to acknowledge one of the reviewers of our book proposal, Dr. Jean-Francois de Marneffe. He provided very valuable comments about how to improve the content and the scope of the book. If he reads the book one day, I hope he considers that his opinion was seriously taken into account as much as possible. I would also like to acknowledge my colleagues from Instituto de Ciencia de Materiales de Madrid and IBM Research Zurich, with whom I spent so many hours, days, fghting for achieving the best patterns with an AFM tip.

OFFICIAL ACKNOWLEDGEMENTS The authors would like to thank the research project Regraph-2D from Ministry of Science and Innovation (PID2020–114234RB-C22//AEI/10.13039/501100011033) and project REACT UE Sarsno CM from the Comunidad de Madrid and Unión Europea, Fondo Europeo de Desarrollo Regional, Una manera de hacer Europa, Financiado como parte de la respuesta de la Unión a la pandemia de COVID-19.

xi

Author Biographies Yu Kyoung Ryu received a PhD in physics from the Universidad Complutense de Madrid and realized her PhD thesis at Instituto de Ciencia de Materiales de Madrid (CSIC). She made a postdoctoral stay at IBM Zurich Laboratory (Switzerland). Since 2020, she has worked as a postdoctoral researcher at the Technical University of Madrid in the Institute of Optoelectronic Systems and Microtechnology (ISOM). She has developed an expertise in scanning probe lithography. Her current research focuses on the application of different lithographic techniques for the fabrication of energy storage devices based on graphene and other 2D materials. ResearcherID: V-8224–2017. ORCID: 0000-0002-5000-2974. Javier Martinez Rodrigo received a PhD in physics and a graduate degree in electronic engineering from the University of Valladolid (Spain). His postdoctoral stage was at the Lawrence Berkeley National Laboratory (USA) working in nanotechnology. Since 2011, is a professor I3 at the Technical University of Madrid in the Institute of Optoelectronic Systems and Microtechnology (ISOM). He is also the coordinator of the network of clean rooms called ICTS Micronanofabs. His research interest is focused on the development of nanoelectronic devices fabricated with graphene and 2D materials for energy applications. ResearcherID: B-5803–2013, ORCID:0000-0002-5912-1128.

xiii

Acronyms 2DES AFM APTES DPN EBL EUV FET FIB HDMS MHA, MHDA NEMS NP(s) ODT OTS PDMS PEG PMMA PPA PPL PPV PS RH RIE SAM SEM SET SPL STEM STM TEM TMD(s) UHV

Two-dimensional electron system Atomic force microscope/microscopy Aminopropyltriethoxysilane Dip pen nanolithography Electron beam lithography Extreme ultraviolet lithography Field-effect transistor Focused ion beam Hexamethyldisilizane 16-mercaptohexadecanoic acid Nanoelectromechanical systems Nanoparticle(s) 1-octadecanethiol Octadecyltrichlorosilane Polydimethylsiloxane Poly(ethylene glycol) Polymethyl methacrylate Polyphthalaldehyde Polymer pen lithography Poly(p-phenylene vinylene) Polystyrene Relative humidity Reactive ion etching Self-assembled monolayer Scanning electron microscope/microscopy Single-electron transistor Scanning probe lithography Scanning transmission electron microscopy Scanning tunneling microscope/microscopy Transmission electron microscope/microscopy Transition metal dichalcogenide(s) Ultrahigh vacuum

xv

1

Historical Background and Place in the Lithography Roadmap

1.1 INTRODUCTION TO NANOLITHOGRAPHY BY AFM In this book, you will be introduced to all the existing nanofabrication techniques using atomic force microscopy (AFM) and their potential when making nanometric devices. To get a global idea of them, one must frst look at the front cover of the book. On it, the reader can see four fgures that represent the different types of nanolithography possible by atomic force microscopy. Let’s explain one by one from left to right and from top to bottom. The frst of these is by making grooves on the surface using a sharp tip with a radius of a few nanometers. It is a very invasive technique and only useful for soft surfaces. In addition, the tip suffers a lot and quickly ends up being destroyed. It was the frst technique to be developed, and this type of technology can be traced back even to the prehistoric era, in which humans modifed their utensils by rubbing sharp stones to create small grooves on their surface. The second image is nanolithography by dip pen. In it, the sharp tip has been immersed in an ink with certain molecules, and when it is brought closer to the surface a liquid meniscus forms and the molecules are deposited on the surface by capillarity. To understand this process again, you can remember the cave dweller with a fnger dipped in some liquid and drawing animals on the walls of the caves, like in Altamira (Spain). Or a more recent equivalent would be the writer Miguel de Cervantes dipping his pen in ink and later writing Don Quixote. The third image is a water meniscus that forms between the surface and the tip due to the application of an electric feld between them. Due to the very small dimensions of the tip and its proximity to the surface, the electric feld in that area is very high and allows a chemical modifcation of the surface. The way to describe this process, in an informal way, is like the discharge of electric lightning on the surface of the earth. This type of nanolithography produces patterns that grow both on the outside of the surface and on the inside. In recent years, a new technique has appeared that can carve different types of resists in three dimensions. It is called thermal probe nanolithography. In this case, the tip is at a high temperature, and when it approaches the resist, it sublimates. That is, it evaporates without a trace. The patterns made in the resist can later be transferred to the substrate through other attack processes in a clean room. The analogy to this technique in the macroscopic world could be found on the island of Murano DOI: 10.1201/9781003223610-1

1

2

Scanning Probe Lithography

near Venice, where glass artisans generate all kinds of motifs by bringing a hot iron to the glass. Once the reader already has in mind what the possible nanofabrication processes are like, it is essential to know the origins of these techniques. In Figure 1.1, one can observe how nature has created more and more sophisticated systems just starting from atoms and small molecules. This is called the bottom-up approach. Instead, humans have generally developed their technology by reducing its size through a top-down approach. Over the last decades, however, researchers have been developing bottom-up technology as well. Although nanofabrication techniques using an atomic force microscope began shortly after its discovery in 1985 at the IBM Zurich Laboratories by researchers Binnig, Quate, and Gerber [2], it is interesting to look back and understand frst, how it has been possible to achieve nanometric motifs with lithography techniques.

1.2

HISTORICAL BACKGROUND

We have started this book with references to prehistoric humans, and it is not by chance. This is because since that time, the human being has been interested in drawing as accurately as possible. At that time, the writing instrument would be the fngers of the hands, and later a stick would be used to enable the drawing of bison in the caves with greater precision. At the end of reading the book, the reader will realize the great evolution that human beings have made throughout history in the use of their tools to create smaller functional devices. It is not known specifcally when humans began writing. What is known is “verba volant scripta manent”, which translated from Latin means “the words fy, what is written remains.” Therefore, humanity has always tried to leave its mark through written messages. Ivory plaques have been found in Egypt dating back to 5,400 years BC, but there are also important fndings in China and Mesopotamia. The neighboring countries copied these writing techniques, which spread throughout the different regions. On the American continent, the Mayans were the frst to make writing motifs. For many centuries, human beings went from writing on stone, such as hieroglyphics, to writing on papyrus or paper using ink. One of the most important aspects of the evolution during all these centuries was the reduction of the size of the writing utensil, since it allows greater resolution and improves portability. In the same way, the brushes with which the painters made their best portraits of the royal families evolved, each time fner. For centuries, written documents or books were replicated one by one by copyist monks, and this work required a lot of patience and a lot of time. It was not until 1440 that Johannes Gutenberg developed the printing press. This invention, which enabled the reproduction of documents simply by having metal stamps and impregnating them with ink to stamp them on a paper, meant historical, cultural, and intellectual change. In 1796 the lithography technique appeared. Lithography comes from the Greek word “lithos” (stone) and “graphein” (to write). This technique was discovered by the German playwright Alos Senefelder. At that time, engraving was performed using

Historical Background and Place in the Lithography Roadmap

Source: Biological and technological scales compared-en.svg, Wikimedia Commons, Creative Commons Attribution-Share Alike 2.5 Generic license.

3

FIGURE 1.1 Scale comparison between elements done by nature and technology done by humans. The image is reproduced with permission from [1].

4

Scanning Probe Lithography

copper as a printing medium, but it was too expensive for him. Instead, he used a smooth, soft stone and succeeded in etching the stone with acid, which could later be used as an impression seal. In 1820, a new engraving technique using light appeared. The pioneer of this technique was Nicéphore Niépce, who used a photoresist substance to make a single camera photograph. This technique was a great turning point since it made it possible to generate motifs using light. It was not until 1947 that a remarkable event occurred that would allow humanity to move from vacuum tubes made of glass and of considerable volume and high consumption to a small device made of solid-state materials. In December of that year, in the Bell Labs of Murray Hill (New Jersey), researchers John Bardeen, Walter Brattain, and William Shockley invented the frst feld-effect transistor in 1947 and were awarded the Nobel Prize in Physics in 1956. The electronics era began, and human beings could make devices of much smaller size that had the same functionalities [3]. Shockley went to the West Coast and founded the company Shockey Semiconductors with the idea of becoming a millionaire by selling transistors, but he failed in his attempt. In addition, his behavior as an unfriendly boss caused the best eight engineers in his company to leave the company and founded Fairchild Semiconductors. Each transistor has three terminals, which means that a kit with 100 transistors would need 300 hand solder points. That was a big problem that needed to be fxed as soon as possible. In order to solve that problem, another giant step was taken in 1958, when a new hire at Texas Instruments named Jack S. Kilby was assigned to work in the summer while the rest of the staff was on vacation. In those weeks, he wrote the frst patent for the integrated circuit. He was the frst person to integrate up to six transistors with its connections on a single piece of germanium. However, it took a long time to recognize his great contribution since he received the Nobel Prize in Physics in 2000. Few months later, Robert Noyce, one of the founders of Fairchild Semiconductors, fled another patent for an integrated circuit, but in this case in silicon [4]. Throughout the years, this company improved the way of making integrated circuits through photolithographic techniques. He demonstrated that smaller and smaller devices could be made using a photoresist and a light source. Later he, together with Gordon Moore, founded the Intel company in 1968 and was known as “The Mayor of Silicon Valley”. Also in 1959, Richard Feynman delivered his acclaimed lecture “Plenty of Room at the Bottom” at the American Physical Society [5], in which he described the possibility of making ever smaller and more effcient devices. He was awarded the Nobel Prize for Physics in 1965 and is considered the father of nanotechnology. Throughout the 70s, 80s, and 90s, the famous Moore’s law was fulflled, in which the microelectronics industry would be proftable as long as the number of transistors per area doubled every 18 months. This law led to the enormous development of photolithography techniques, allowing motifs of less than one micron to be made. Today these techniques have advanced enormously and immersion lithography approaches have been developed, in which a liquid with a certain refractive index is

Historical Background and Place in the Lithography Roadmap

5

introduced between the photomask and the sample in order to reduce the size of the motifs to be made. Or even smaller motifs can be made by exposure with extreme ultraviolet light (EUV). All these techniques allow the fabrication of transistors of around 22 nanometers in order to continue to comply with Moore’s law. Although photolithography techniques have developed industrially over the years to make smaller and smaller parallel patterns, they have increased exponentially in price, and today an extreme ultraviolet lithography system can be the size of a trailer truck and cost millions of euros. They also have the drawback that once the photomask is designed, it is not possible to make any changes.

1.3 AFM: THE MOST VERSATILE TOOL AT THE NANOSCALE Despite being the workhorse of microelectronics, the aforementioned limitations of photolithography motivated the development of several lithographic techniques to fabricate devices and patterns at the nanoscale. These techniques include direct writing techniques with high-resolution capabilities, such as electron beam lithography (EBL) and focused-ion beam related schemes. They also include techniques that are a compromise between high-throughput, low-cost, and high-resolution capabilities, such as nanoimprint lithography and soft lithography. Where should we place scanning probe lithography (SPL) among these techniques? If we take a look on the graph shown in Figure 1.2, where the resolution versus areal throughput of different lithographic techniques is displayed [6, 7], we observe that the STM lithography owns the highest resolution on lithography, but it has the disadvantages of ultrahigh vacuum (UHV) and very low temperatures below 5 Kelvin.

FIGURE 1.2 Graph with different lithographic techniques, representing resolution versus areal throughput.

6

Scanning Probe Lithography

The throughput is so small that its use outside very specifc scientifc problems and quantum device fabrication is limited. Being fair, the resolution achievable by SPL, EBL, nanoimprint lithography, and EUV falls within the same range. However, the throughput of SPL is smaller than the one achievable by the other methods. Then, why did we dare to call this section “AFM: The Most Versatile Tool at the Nanoscale”? Being aware of the main disadvantages of SPL, which are low throughput and tip patterning lifetime, let’s enumerate the many virtues of SPL: • • • • • • • •

•

• •

Sub-10 nm resolution Maskless, direct writing Nondestructive/noninvasive Absence of proximity effect Capability to write arbitrary-shaped patterns with nanometric overlay and stitching accuracy Nanometric vertical resolution to write three-dimensional patterns Relatively simple tool setup: most of the experiments can be done in any laboratory, under ambient conditions and using a commercial AFM tool The lithography, the inspection of the fabricated patterns, and the correction of the writing parameters can be done in situ using the same tool. This allows the implementation of closed-loop lithography schemes and the interruption of the lithographic process if it is not being successful, in order to reduce the time consumption Somehow related to the previous point, with the same AFM, after fabricating a structure or device, its material properties can be characterized as electrical, mechanical, magnetic, chemical, and so on through the multiple AFM modalities SPL can be performed in all kind of materials: polymer, biological, semiconductors, insulators, metallic, magnetic, 2D materials With a commercial AFM, through some electronics implementation, multiple modifcation processes can be applied to different samples: scratching a polymer, generating oxides under the application of voltage pulses between the tip and the sample, and wetting the AFM tip in a solution with molecules to deposit them on a substrate

If the reader is not convinced yet about the incredible versatility of scanning probe lithography, then we kindly invite them to read the book.

1.4 BOOK SCHEME The different techniques offered by scanning probe lithography will be described in detail throughout the chapters of the book. It has been divided into nine chapters for a better understanding by the reader. It will begin in the second chapter describing the basic elements of atomic force microscopy and the modes of operation to be able to do nanolithography. In the third chapter, the techniques used to make nanometric patterns mechanically with the tip will be described. The fourth chapter will describe the method called dip pen nanolithography, in which the tip is immersed

Historical Background and Place in the Lithography Roadmap

7

in a liquid with molecules that are subsequently deposited by capillarity on specifc areas of the surface. Chapter 5 will describe the effect of an electric feld applied at the end of the probe for making feld emission nanolithography. Also using another electric feld over the tip, in Chapter 6 will be described the local oxidation lithography technique. Both techniques, described in Chapters 5 and 6, will chemically change the surface giving rise to nanometric patterns of another material. Chapter 7 will be devoted to describing thermal probe nanolithography. In other words, the tip is heated by the passage of an electric current, and the high temperature reached at the tip will make it possible to carve certain types of resists that have previously been deposited on the surface of the sample. Finally, two additional chapters have been added that allow the reader to learn about other aspects of this exciting world of nanofabrication. On one hand, there is the most extreme nanofabrication, in conditions of ultrahigh vacuum and temperatures of a few degrees Kelvin. Thus, in Chapter 8, we will describe the great achievements made by scanning tunneling microscopes, which are the tools used by the nanolithographic technique with the highest resolution. STM lithography enables the manipulation of single atoms to create quantum structures such as quantum corrals and qubit systems. And fnally, future developments will be discussed for using these nanofabrication techniques in industrial applications. That is why in Chapter 9 we will provide examples in which high-throughput strategies were pursued to produce more devices in the shortest possible time. Some of these approaches involve the translation of scanning probe lithography processes into nanoimprint or soft lithography schemes, which would be similar to Gutenberg’s printing press but on the nanometric scale. A solid development of parallel fabrication techniques could mean an expansion in the use of scanning probe lithography in upcoming years.

CHAPTER REFERENCES 1. Paumier, G., Ronan, P., NIH, Jan Fijałkowski, A., Walker, J., Jones, M. D., Heal, T., Ruiz, M., Science Primer (National Center for Biotechnology Information), Liquid_2003, Nordmann, A., and The Tango! Desktop Project. https://commons.wikimedia.org/wiki/ File:Biological_and_technological_scales_compared-en.svg 2. Binnig, G., Quate, C. F., and Gerber, C. “Atomic force microscope”. Physical Review Letters, 56(9), 930 (1986) 3. Horowitz, P., and Hill, W. The Art of Electronics (Third edition). Cambridge University Press. ISBN-10: 9780521809269 (2015) 4. Berlin, L. The Man Behind the Microchip: Robert Noyce and the Invention of Silicon Valley. Oxford University Press US, ISBN 0-19-516343-5 (2005) 5. Feynman, R. P. “Plenty of Room at the Bottom”. American Physics Society (1959) 6. Radha, B., and Kulkarni, G. U. “Direct write nanolithography”. Chapter 3 in Nanoscience: Volume 2. Ed: O’Brien, P., and Thomas, P. J. Royal Society Chemistry Publishing (pp. 58–80). ISBN: 978-1-84973-582-7 (2013) 7. Garcia, R., Knoll, A. W., and Riedo, E. “Advanced scanning probe lithography”. Nature Nanotechnology, 9, 577 (2014)

2

Basic Concepts and Modalities

2.1 THE TOOL: AN ATOMIC FORCE MICROSCOPE The invention of the atomic force microscope (AFM) by Binnig, Quate, and Gerber [1] in 1985 meant a scientifc revolution, since it was one of the key instruments to unravel the world at the nano- and atomic scale. It was developed shortly after the invention of the scanning tunneling microscope (STM) [2], having the advantage of working on insulating substrates and under ambient conditions. In AFM, a mechanical minute probe, typically with a radius apex smaller than 10 nm, scans the surface of a material. The interaction forces between the tip and the sample are used to take the images and extract the information of interest. Beyond the topography of a sample surface, any kind of physical and chemical properties can be characterized using and AFM: compositional [3], mechanical [4, 5], electrical [6], magnetic [7], or biological [8]. There are several available commercial tools, to name a few: Park Systems AFMs, (www.parksystems.com), Asylum Research Cypher Family of AFMs (afm.oxinst. com), JPK NanoWizard BioScience series and Dimension Icon AFM (www.bruker. com), and NTEGRA series (www.ntmdt-si.com). The user will choose an AFM with the required features to measure the targeted properties. In some cases, a research group will adapt an AFM in a home-built fashion according to its needs. However, several main components are common to all AFMs, described in the scheme of Figure 2.1. The probe consists of a cantilever with a sharp tip at the free end. It can be made of silicon, silicon nitride, or silicon with a metallic coating and has different geometries in order to have a spectrum of resonant frequencies, f 0, and force constants, k. The chosen f 0 and k will depend on the sample material characteristics and the environment under which the experiment will take place (ambient conditions, vacuum, liquid). A piezoelectric scanner controls the movement in z (vertical) to approach/ retract the tip to/from the sample surface, respectively and while the tip is scanning over the surface, the movement in x and y (in-plane). The displacement of the cantilever is tracked by the optical beam defection (OBD) system [9]. A laser beam emitted from a solid-state diode refects from the backside of the cantilever, being collected by a position sensitive photodiode (PSPD). This device consists of a foursegmented detector that measures the defection of the cantilever as a function of the positions of the laser spot with respect to the center of the photodiode. The control electronics of the AFM processes the signals collected by the detection system. A closed-loop system corrects the drift that originates from the nonlinear behavior of the piezoelectric. Depending on the chosen operation mode, a feedback loop system keeps constant the defection, amplitude, or frequency of the cantilever during the DOI: 10.1201/9781003223610-2

9

10

Scanning Probe Lithography

FIGURE 2.1 Scheme of an atomic force microscope and its principal components. The components are not to scale.

scanning. Through the computer and the corresponding software program, the user controls the measuring parameters, acquires the raw images, and processes them. Normally, the AFM is placed on an anti-vibration table or in an isolation chamber to minimize the noise during the acquisition of the images. An AFM can be operated choosing several modes [10–13]. We describe briefy the ones that are selected to perform the vast majority of scanning probe lithography experiments. a. In contact mode AFM (Figure 2.2(a)), the tip is very close to the surface of the sample, being the dominant force repulsive. The topography of the surface is measured by the defection of the cantilever, which represents the interaction forces between the tip and the sample. The feedback loop unit keeps constant the position of the cantilever during the scanning. b. In dynamic mode AFM (Figure 2.2(b)), an actuator excites the tip to make it oscillate over the surface of the sample with a determined amplitude and at or near its free resonance frequency. The free oscillation amplitude (or resonance frequency) changes when the tip scans the sample surface due to the interaction forces between them. If the AFM works in the amplitude modulation (also called usually tapping) mode (AM-AFM), the feedback loop readjusts the tip height to keep constant the oscillation amplitude. Depending on the free amplitude and other adjustments, the AFM can work in a noncontact (attractive forces regime) or in an intermittent contact (repulsive forces regime) mode. In the frequency modulation mode (FM-AFM) case, the feedback loop works to keep constant the frequency shift (Δf).

Basic Concepts and Modalities

11

FIGURE 2.2 Scheme of atomic force microscopy operating modes. (a) Contact mode, (b) dynamic mode.

2.2 AN ATOMIC FORCE MICROSCOPE FOR LITHOGRAPHY A few years after the development of the AFM, the interaction between the tip and the sample started to be exploited not only to image the structure of the matter, but also to manipulate and modify it. That is how scanning probe lithography was born. Some of the pioneers were Majumdar et al (1992) [14], Mamin et al (1992) [15], and Day and Alle (1993) [16]. In the case of [14], a voltage bias was applied between a gold-coated silicon nitride tip and a polymethyl methacrylate (PMMA) thin layer to modify the resist chemically through an electron exposure dosage, analogue to the principle of electron beam lithography. Mamin et al [15] heated an AFM tip by focusing an infrared laser on it. The hot tip was used to indent PMMA to explore data storage applications. Finally, Day and Alle [16] applied a voltage bias between a gold-coated silicon nitride tip and Si to form silicon oxide by local anodization. Therefore, from our point of view, we defne scanning probe lithography as a technique where the interaction between an AFM tip and the surface of a material is used to change this material physically or/and chemically from an initial state in order to produce new functional patterns or devices at the nanoscale. SPL comprises a wide variety of processes: placement of nanoentities in preferential sites, scratching of soft/hard substrates, control in the orientation of magnetic domains, sublimation/ ablation of a polymer resist, electrochemical reaction, and beyond. All these ways to transform materials have been developed over the years within different modalities, which build up the family of SPL techniques. They are described shortly hereafter in order to provide the readers a general view of them. Thereafter, from Chapters 3 to 7, each modality has a whole chapter devoted to explain in detail its fundamentals, the main parameters to control the lithographic process in each case, and the applications. To provide the readers a fuid immersion in the book, these modalities have been ordered as a function of the increase in the tip-sample interaction complexity: a. Chapter 3 is devoted to mechanical SPL (m-SPL). In this modality, the AFM tip is used to displace physically a nano-object or scratch a sample surface to defne trenches. b. Chapter 4 is devoted to dip pen nanolithography. In this modality, the AFM tip is wet with a solution that contains molecules or nanoparticles. In this

12

Scanning Probe Lithography

way, the tip acts as a “pen” and the solution as “ink”. By scanning the surface, these molecules or nanoparticles are deposited locally. c. Chapter 5 and Chapter 6 deal with the application of a potential between the tip and the sample to originate an electric feld that will drive different modifcations. Due to a well-defned differentiation in the mechanism and used parameters to control the lithographic process, two modalities were developed independently: feld emission SPL (fe-SPL, Chapter 5) and oxidation SPL (o-SPL, Chapter 6). d. Chapter 7 is devoted to thermal SPL. In this modality, a hot AFM tip is required to trigger different responses on the material: polymer sublimation, polymer carving, magnetic domain orientation, and phase change. In the central chapters of the book, we hope to convey two aspects of scanning probe lithography to readers: a. Its versatility and its ability to be integrated in the leading topics through the decades: semiconductors industry, 2D materials, and quantum computing. b. It is a technique that is used to fabricate structures and devices in all the important research felds: biomedicine, electronics, magnetism, optics, and spintronics. We described in section 2.1 the atomic force microscope because it is the tool used to perform SPL. In the case of m-SPL, the indentation of the surfaces is carried out without any modifcation of the AFM, just by controlling the load force with the correct cantilever. On the other hand, most of the companies have already implemented in their microscopes the module and the electronics to perform the lithographic technique and its corresponding software. Some examples are NanoMan and NanoPlot (www.bruker.com), MicroAngeloTM (afm.oxinst.com), and SmartLithoTM (www. parksystems.com) [17]. In some cases, instead of using the commercial systems, the user can perform SPL by modifying an AFM with additional electronics. Finally, some groups have developed novel AFM confgurations to optimize the features of the technique in specifc modalities such in the case of feld emission and thermal SPL. The components and characteristics of these devoted tools will be described in the corresponding chapters. One of the main advantages of SPL compared to other lithographic techniques is the possibility of monitoring the patterning process in situ, which allows for interrupting the writing if it is not producing the targeted structure in order to optimize the time consumption. Some incidents that could happen during the writing of a pattern are (a) the degradation of the tip (attachment of undesired particles, wear: a blunt tip is not useful anymore to achieve high-resolution feature sizes), and (b) the tip has encountered a barrier (particle, wrinkle, crack on the material) that has destroyed the programmed structure.

2.3

PREPARING A SCANNING PROBE LITHOGRAPHY EXPERIMENT

A standard AFM tip has a nominal radius apex