Part One Top-Down Strategy

Part One Top-Down Strategy Nanotechnology, Volume 8: Nanostructured Surfaces. Edited by Lifeng Chi Copyright Ó 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31739-4 j3 1 Top-Down Fabrication of Nanostructures Ming Liu, Zhuoyu Ji, and Liwei Shang 1.1 Introduction The top-down approach to nanofabrication involves the creation of nanostructures from a large parent entity. This type of fabrication is based on a number of tools and methodologies which consist of three major steps: 1) The deposition of thin films/coatings on a substrate. 2) Obtaining the desired shapes via photolithography. 3) Pattern transfer using either a lift-off process or selective etching of the films Compared with general chemical fabrication and processing methods, top-down fabrication techniques for the creation of nanostructures are derived mainly from the techniques applied for the fabrication of microstructures in the semiconductor industry. In particular, the fundamentals and basic approaches are mostly based on micro-fabrications. In this chapter, methods of top-down nanofabrication will be discussed, with attention being focused primarily on methods of lithography, especially optical, electron-beam, X-ray and focused ion beam lithography. A brief introduction will also be provided on how to create nanostructures using various methods of thin film deposition and etching materials. Finally, the methods for pattern transfer through etching and lift-off techniques will be discussed. In the past, top-down fabrication techniques have represented an effective approach for nanostructures and, when complemented with bottom-up approaches during the past few decades, have led to amazing progress having been made with a variety of nanostructures. The traditional top-down technology used to create nanostructures and nanopatterns is discussed in the following sections. 1.2 Lithography Lithography, which is also often referred to as photoengraving, was invented in 1798 in Germany by Alois Senefelder. It is the process of defining useful shapes on Nanotechnology, Volume 8: Nanostructured Surfaces. Edited by Lifeng Chi Copyright Ó 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31739-4 4j 1 Top-Down Fabrication of Nanostructures the surface of a semiconductor wafer [1–5]. Typically, it consists of a patterned exposure into some form of photosensitive material that has already been deposited onto the wafer. Many techniques of lithography have been developed during the past fifty years, by using a variety of lens systems and exposure radiation sources that have included photons, X-rays, electrons, ions, and neutral atoms. In spite of the different exposure radiation sources used in the various lithographic methods, and the instrumental details, all of these techniques share the same general approaches and are based on similar fundamentals. Photolithography is the most widely used technique in microelectronic fabrication, particularly for the mass production of integrated circuits (ICs) [2], and has been the driving force behind the miniaturi- zation of such circuits since they were first produced at Fairchild and at Texas Instruments during the early 1960s [6]. 1.2.1 Photolithography Photolithography (also called optical lithography) is simply lithography using a radiation source with wavelength(s) in the visible spectrum. It has served as the dominant patterning technology in the semiconductor industry since the IC was invented almost sixty years ago. From the onset, optical lithography has always managed to keep pace with Moores law [7, 8], including its recent acceleration. In order to keep pace with the shrinking feature size, a steady stream of improvements in the field of resolution, image placement, and pattern transfer have been introduced time after time, and these have enabled optical lithography to hold off the challenges of the competing lithography technologies. The key historical events in photolithography have been as follow: . 1826: Joseph Nicephore Niepce, in Chalon, France, takes the first photograph using bitumen of Judea on a pewter plate, developed using oil of lavender and mineral spirits. 1843: William Henry Fox Talbot, in England, develops dichromated gelatin, patented in Britain in 1852. 1935: Louis Minsk of Eastman Kodak develops the first synthetic photopolymer, poly(vinyl cinnamate), the basis of the first negative photoresists. 1940: Otto Suess of Kalle Division of Hoechst AG, develops the first diazoqui- none-based positive photoresist. 1954: Louis Plambeck, Jr, of Du Pont, develops the Dycryl polymeric letterpress plate. Optical lithography is a process used in microfabrication to selectively remove parts of a thin film (or the bulk of a substrate). It involves the use of an optical technique to produce images at smaller scales, which employs light to transfer a geometric pattern from a photomask to a light-sensitive chemical (photoresist, or simply resist) on the substrate. The steps involved in the photolithographic process include: wafer cleaning; barrier layer formation; photoresist application; soft baking; mask align- ment; exposure and development; and hard-baking. 1.2 Lithography j5 Figure 1.1 Schematic representation of the photolithographic process sequences, in which images in the mask are transferred to the underlying substrate surface. The basic scheme of photolithography (as shown in Figure 1.1) involves three steps [9]: (i) a thin film of resist material is cast over the substrate; (ii) the substrate is then exposed to a pattern of intense light through a mask, during which time the resist material is selectively struck by the light; (iii) the exposed substrate is then immersed into the development solvent. Depending on the chemical nature of the resist material, the photoresist is defined as either a positive or a negative type. For positive resists, the resist is exposed to ultraviolet (UV) light wherever the underlying material is to be removed. In these resists, exposure to the UV light alters the chemical structure of the resist, which causes it to become more soluble in the developing solvent than in the unexposed areas. The resist exposed under the UV light is then washed away by the developer solution. Overall, the process can be described as whatever shows, goes away. In contrast, in the case of a negative resist, the exposed areas may be rendered less 6j 1 Top-Down Fabrication of Nanostructures Figure 1.2 Schematic diagram of working modes of photolithography. soluble in a certain developing solvent; this leads to the production of a negative tone image of the shadow mask, a process described as whatever shows, stays behind. In addition to conventional photoresist polymers, Langmuir–Blodgett films and self-assembled monolayers (SAMs) have also been used as resists in photolithography [10, 11]. In such applications, photochemical oxidation, crosslinking, or the generation of reactive groups are used to transfer patterns from the mask to the mono-layer [12, 13]. A master mask is necessary in the process of photolithography, and in general this is scribed using an optical method and produced by chemical etching. In this case, the light passes through the mask to define the actual structure in the material; according to the position of the mask with respect to the sample, three types of exposure lithography can be defined, namely contact, proximity, and projection lithography (see Figure 1.2). Up until to the early 1970s, most lithography was carried out as either a contact or a close-proximity printing process, in which blue and near-UV light was passed through a photomask directly onto a photoresist-coated semiconductor substrate [14]. This apparently simple shadow imaging process has been described in many research reports and handbooks [15, 16]. 1.2.1.1 Contact Printing In contact-mode photolithography, the resist-coated silicon wafer is brought into intimate physical contact with the glass photomask. For this, the wafer is held on a vacuum chuck and the whole assembly rises until the wafer and mask make contact with each other. The photoresist is exposed to UV light while the wafer is in contact position with the mask, and this allows a mask pattern to be transferred into a photoresist with almost 100% accuracy, as well as providing the highest resolution (e.g., 1 mm features in 0.5 mm of positive resist). Unfortunately, however, the maximum resolution is seldom achieved owing to the presence of dust on the 1.2 Lithography j7 substrates and the nonuniform thicknesses of both the photoresist and the substrate. The main problem with contact printing is that debris, trapped between the resist and the mask, can damage the mask and cause defects in the pattern. 1.2.1.2 Proximity Printing The proximity exposure method is similar to contact printing, and involves intro- ducing a gap about 10–25 mm wide between the mask and the wafer during the exposure stage. This gap minimizes (but may not eliminate) the mask damage. Although a resolution of about 2–4 mm is possible with proximity printing, increasing the gap will reduce the resolution by expanding the penumbral region caused by diffraction. The main difficulties associated with proximity printing include the control of a small and very constant space between the mask and wafer, which can be achieved only by using extremely flat wafers and masks. 1.2.1.3 Projection Printing Generally speaking, projection techniques have a lower resolution capability than that provided by shadow printing. However, unlike shadow printing, in projection printing the lens elements are used to focus the mask image onto a wafer substrate, which is separated from the mask by several centimeters so that damage to the mask is entirely avoided. An image of the patterns on the mask is projected onto the resist- coated wafer, which is located several centimeters away. In order to achieve a high resolution, only a small portion of the mask is imaged, and the small image field is scanned or stepped over the surface of the wafer.

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