<div><div> <h2>CHAPTER 1</h2> <p><b>Nanofabrication Techniques</b></p> <p>JOSEPH W. FREEMAN, LEE D. WRIGHT, CATO T. LAURENCIN, and SUBHABRATA BHATTACHARYYA</p> <br> <p><b>1.1 INTRODUCTION</b></p> <p>Interest in the study and production of nanoscaled structures is increasing. The incredibly small sizes of nanoscaled devices and functionality of nanoscaled materials allow them to potentially change every aspect of human life. This technology is used to build the semiconductors in our computers; nanoscaled materials are studied for drug delivery, DNA analysis, use in cardiac stents, and other medical purposes. Layers of molecules can be placed on machine parts to protect them from wear or aid in lubrication; monolayers of molecules can be added to windows to eliminate glare. Although we are already greatly affected by this technology, new advances in nanofabrication are still being made.</p> <p>Microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) have the potential to perform tasks and study the human body (BioMEMS and BioNEMS) at the molecular level. BioMEMS have existed for decades and were first used in neuroscience. In the 1970s, Otto Prohaska developed the first planar microarray sensor to measure extracellular nerve activity. Prohaska and his group developed probes used for research in nerve cell interactions and pathological cell activities in the cortical section of the brain. In the future, NEMS and other nanoscaled structures may be able to perform more advanced tasks. This technology may allow us to cure diseases or heal tissues at the molecular level. Computers may be even more powerful, while taking up less space.</p> <p>Many of the fabrication methods for nanoscaled devices used today are actually based on previously conceived methods. Others take advantage of new technologies to make nanoscaled structures. Still others combine several different methods to produce new technologies. This section will describe several technologies commonly used in nanofabrication. These methods produce a large variety of structures from fibers, to columns, to layers of materials that are a single molecule thick.</p> <br> <p><b>1.2 PHOTOLITHOGRAPHY</b></p> <p>Originally, lithography was a printing method invented in 1798 by Alois Senefelder in Germany. At that time, there were only two printing techniques: relief printing and intaglio printing. In relief printing, a raised surface is inked and an image is taken from this surface by placing it in contact with paper or cloth. The intaglio process relies on marks engraved onto a plate to retain the ink. Lithography is based on the immiscibility of oil and water. Designs are drawn or painted with an oil-based substance (greasy ink or crayons) on specially prepared limestone. The stone is moistened with water, which the stone accepts in areas not covered by the crayon. An oily ink, applied with a roller, adheres only to the drawing and is repelled by the wet parts of the stone. The print is then made by pressing paper against the inked drawing.</p> <p>Optical lithography began in the early 1970s when Rick Dill developed a set of mathematical equations to describe the process of lithography. These equations published in the "Dill papers" marked the first time that lithography was described as a science and not an art. The first lithography modeling program SAMPLE was developed in 1979 by Andy Neureuther (who worked for a year with Rick Dill) and Bill Oldman.</p> <p>Photolithography is a technique used to transfer shapes and designs onto a surface of photoresist materials. Over the years, this process has been refined and miniaturized; microlithography is currently used to produce items such as semiconductors for computers and an array of different biosensors. To date, photolithography has become one of the most successful technologies in the field of microfabrication. It has been used regularly in the semiconductor industry since the late 1950s; a great deal of integrated circuits have been manufactured by this technology. Photolithography involves several generalized steps, cleaning of the substrate, application of the photoresist material, soft baking, exposure, developing, and hard baking. Each step will be explained briefly below.</p> <br> <p><b>1.2.1 Cleaning of the Substrate</b></p> <p>During substrate preparation, the material onto which the pattern will be developed is cleaned to remove anything that could interfere with the lithography process including particulate matter and impurities. After cleaning, the substrate is dried, usually in an oven, to remove all water.</p> <br> <p><b>1.2.2 Application of the Photoresist Material</b></p> <p>There are two types of photoresist materials, positive and negative. Positive photoresists become more soluble when they are exposed to UV light (Fig. 1.1). So in photolithography, when the mask is laid down onto the positive photoresist, the exposed areas (those not covered by the mask) will be removed by the developing solution leaving only the shape of the mask and the underlying substrate (Fig. 1.1).</p> <p>Negative photoresist materials work in the opposite manner (Fig. 1.1). These photoresist materials polymerize with exposure to UV light, making them less soluble after exposure. Once the mask has been lifted and the material has been washed with developing solution, the photoresist covered by the mask is washed away (Fig. 1.1). Therefore, using a negative photoresist creates the photographic negative of your mask.</p> <p>Photoresists commonly used in the production of microelectronics include ethylene glycol, monoethyl ether propylene glycol and methyl ether acetate.</p> <br> <p><b>1.2.3 Soft Baking</b></p> <p>The soft baking step is used to remove the solvents from the photoresist coating. Soft baking also makes the coating photosensitive.</p> <br> <p><b>1.2.4 Exposure</b></p> <p>In this step, the mask is placed onto or over the substrate so that the pattern can be placed onto the surface of the substrate (Fig. 1.1). There are three different techniques used to position the mask prior to exposing the photoresist to UV light; these techniques are contact printing, proximity printing, and projection printing.</p> <p><b><i>1.2.4.1 Contact Printing</i></b> During contact printing, the substrate surface is covered with the photoresist and the mask physically touches the surface. The substrate is exposed to UV light while it is in contact with the mask. The contact between the mask and the substrate makes micron resolution possible. Unfortunately, if debris is trapped between the mask and the substrate, it can damage the mask and cause defects in the pattern.</p> <p><b><i>1.2.4.2 Proximity Printing</i></b> In proximity printing, the mask and the substrate are separated by a small distance (10-25 µm) before exposure to UV light. This technique protects the mask and the pattern from some debris damage that could occur during contact printing. The distance between the mask and the substrate lowers the resolution to 2-4 µm.</p> <p><b><i>1.2.4.3 Projection Printing</i></b> In projection printing, an image of the mask is projected onto the substrate after it is covered with the photoresist. This method can produce patterns with high resolution (1 µm) by projecting a small section of the mask at a time. Once the mask is in the correct position, the photoresist is exposed to high intensity UV light through the pattern in the mask.</p> <br> <p><b>1.2.5 Developing</b></p> <p>Through developing, the photoresist becomes more soluble (positive photoresists) or less soluble (negative photoresists). When using a positive photoresist, an increase in energy causes an increase in the solubility of the resist until the threshold energy is reached. At this energy, all of the resist is soluble. In negative photoresists, the material becomes less and less soluble with increased energy. At the threshold energy, the material is even less soluble; as the energy is raised above the threshold energy, more of the photoresist is insoluble and more of it will remain after developing. The amount of time and energy necessary to complete the developing step depends on a variety of factors such as the prebaking conditions, amount of photoresist material, and developing chemistry. After the developing, a solvent is used to wash the sample.</p> <br> <p><b>1.2.6 Hard Baking</b></p> <p>This is the final step of photolithography. Hard baking is used to harden the photoresist and improve bonding between the photoresist layer and the substrate underneath.</p> <br> <p><b>1.2.7 Limitations of Photolithography</b></p> <p>Current photolithography techniques used in microelectronics manufacturing use a projection printing system (known as a stepper). In this system, the image of the mask is reduced and projected, via a high numerical aperture lens system, onto a thin film of photoresist that has been spin coated onto a wafer. The resolution that the stepper is capable of is based on optical diffraction limits set in the Rayleigh equation (Eq. 1.1).</p> <p><i>R = k</i><sub>1</sub>λ/<i>NA</i> (1.1)</p> <br> <p>In the Rayleigh equation, <i>k</i><sub>1</sub> is a constant that is dependent on the photoresist, λ is the wavelength of the light source, and NA is the numerical aperture of the lens.</p> <p>The minimum feature size that can be achieved with this technique is approximately the wavelength of the light used, λ; although theoretically, the lower limit is λ/2. So, in order to produce micro- or nanoscaled patterns and structures, light sources with shorter wavelengths must be used. This also makes manufacturing more difficult and expensive.</p> <br> <p><b>1.3 SPECIALIZED LITHOGRAPHY TECHNIQUES</b></p> <p>In order to produce patterns at the nanometer scale, which is necessary for the fabrication of semiconductor integrated circuits, nanoelectromechanical systems, or lab-on-a-chip applications, specialized lithography techniques are used. Some of these techniques involve steps similar to those seen in photolithography; the differences lie in the use of energy sources with smaller wavelengths and smaller masks (both changes are used to produce nanoscaled patterns and structures). Such specialized lithography techniques include electron beam lithography, nanosphere lithography, and focused ion beam lithography (FIB). Other specialized techniques are more reminiscent of original lithography in that they transfer the pattern of molecules directly onto the substrate as a print. These techniques include types of soft lithography (such as microcontact printing, replica molding, microtransfer molding, and solvent-assisted micromolding), nanoimprint lithography, and dip pen lithography (a type of scanning probe lithography). Other techniques, such as LIGA, combine elements from both categories. A few of these techniques will be briefly discussed below.</p> <br> <p><b>1.3.1 Electron Beam Lithography</b></p> <p>Electron beam lithography has been used in the production of semiconductors and the patterning of masks for other types of lithography (such as X-ray and optical lithography). In electron beam lithography, the exposed substrate is modified by the energy from a stream of electrons.</p> <br> <p><b>1.3.2 Nanosphere Lithography</b></p> <p>Nanosphere lithography is similar to other types of lithography. In this type of lithography, the mask is replaced with a layer of nanospheres. After exposure and developing, the uncovered resin is washed away leaving behind nanoscale vertical columns.</p> <br> <p><b>1.3.3 Soft Lithography</b></p> <p>Soft lithography is called "soft" because an elastomeric stamp or mold is the part that transfers patterns to the substrate and this method uses flexible organic molecules and materials rather than the rigid inorganic materials commonly used during the fabrication of microelectronic systems. This process, developed by George Whitesides, does not depend on a resist layer to transfer a pattern onto the substrate. Soft lithography can produce micropatterns of self-assembled monolayers (SAMs) through contact printing or form microstructures in materials through imprinting (embossing) or replica molding.</p> <p>In this process, a self-assembled monolayer is stamped onto the substrate. The molecular impressions left by the monolayer can be used to seed crystal growth or bind strands of DNA for bioanalysis. Soft lithography techniques are not subject to the limitations set by optical diffraction, as discussed earlier (the edge definition is set by van der Waals interactions and the properties of the materials used). They offer procedurally simple, less expensive alternatives to the production of nanoscaled structures through photolithography.</p> <br> <p><b>1.3.4 Dip Pen Lithography</b></p> <p>Dip pen lithography is a type of scanning probe lithography. In this lithographic technique, the tip of an atomic force microscope (AFM) is used to create micro- and nanoscaled structures by depositing material onto a substrate. The AFMtip delivers the molecules to the substrate surface using a solvent meniscus that forms in ambient atmospheres. Structures with features ranging from several hundreds of nanometers to sub-50 nm can be generated using this technique.</p> <p>Dip pen lithography is a direct-write method that yields high resolution and has been used to create microscale and nanoscale patterns with a variety of "inks" (such as biomolecules, organic molecules, polymers, and inorganic molecules) on a number of substrates. The AFMtip was first used to form patterns of octadecanethiols in ethanol on the mica surfaces. The technology can now be used to construct protein arrays for proteomics, pharmaceutical screening processes, and panel immunoassays.</p> <p>Dip pen lithography involves several steps. The first is substrate preparation: the substrate is cleaned and rinsed to remove all impurities and produce a flawless surface. In order to aid in adhesion of the material being deposited, a self-assembled monolayer may be added to the surface. The AFMtip is then coated with the "ink" to be deposited onto the substrate. Finally, the tip is used to produce the desired pattern (Fig. 1.2).</p> <p>The formation, structure, and stability of the deposited material depend on several different variables. The formation and stability of the structure are subject to the strength of the adhesion between the substrate and the deposited material and the amount of adhesion between the material being deposited and the AFMtip. One source of this adhesion is surface charge. The static interaction between a charged substrate surface and oppositely charged nanospecies will lead to the successful deposition onto the substrate surface.</p> <p>If the adhesion between the material and the AFMtip is too strong, it may prevent material deposition; if it is too weak, the material will not stay on the tip long enough to be deposited onto the substrate and the amount on the tip may not be enough to produce the structure. The deposition of material from the tip to the substrate is also influenced by the cohesion between the material already deposited and the material on the tip.</p> <p>Temperature is also an important factor in dip pen lithography. When working with biomolecules and organic molecules, temperature affects the solubility and diffusion rate of the molecules, which influences the size of the nanopatterns. The solvent used is also a factor in stability. The amount of solvent in the material can influence the shape of the deposit. As the solvent evaporates from the material after deposition, the deposits could harden. Temperature can also influence solvent evaporation rates.</p> <p>The speed of tip can change the dimensions of the pattern. Increasing speed causes a decrease in pattern size. Humidity can also affect this process. The proper humidity is necessary for the transfer of the material to the substrate.</p> <p>Progress has been made in using dip pen lithography in nanopatterning biomolecules and organic molecules by modulating interactions between target surfaces and the molecules being deposited. Very few inorganic materials have been successfully patterned with this technique; successes include pure metals and metal oxides.</p> <br> <p><b>1.3.5 LIGA</b></p> <p>LIGA is a German acronym for Lithographie, Galvanoformung, Abformung. This process for creating three-dimensional microstructures was developed in the 1980s by W. Ehrfeld. It is an early technique for producing micro-and nanoscaled structures and involves lithography, electroforming, and plastic molding. LIGA was one of the first techniques used to create microstructures with high aspect ratios and depths of hundreds of nanometers. LIGA is a valuable tool for the production of micro- or nanoscaled molds; these molds can aid in the mass production of micromachine parts. The basic steps of LIGA (Fig. 1.3) are explained below. </div></div><br/> <i>(Continues...)</i> <!-- Copyright Notice --> <blockquote><hr noshade size='1'><font size='-2'>Excerpted from <b>Biomedical Nanostructures</b> by <b>Kenneth E. Gonsalves, Craig R. Halberstadt, Cato T. Laurencin, Lakshmi S. Nair</b>. Copyright © 2008 John Wiley & Sons, Inc.. Excerpted by permission of John Wiley & Sons. <br/>All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.<br/>Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.</font><hr noshade size='1'></blockquote>