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Home > Nanotechnology Columns > UAlbany College of Nanoscale Science and Engineering > Materials Characterization and Nanoscale Materials
Professor of Nanoscale Science
UAlbany College of Nanoscale Science and Engineering
Nanoscale materials have opened a rich new world of possibilities for science and engineering. In a discussion of nanoscale materials it is useful to divide them into ultra-thin films, ultra thin wires, and nano scale dots. These nano films, wires and dots all exhibit new phenomena which is the origin of the richness.
April 6th, 2010
Materials Characterization and Nanoscale Materials
Nanoscale materials have opened a rich new world of possibilities for science and engineering. In a discussion of nanoscale materials it is useful to divide them into ultra-thin films, ultra thin wires, and nano scale dots. These nano films, wires and dots all exhibit new phenomena which is the origin of the richness. Research, development, and manufacture of nanoscale materials all require advances in materials characterization including microscopy and physical/electrical characterization. In order to perform these measurements, one must both improve the measurement and understand how to interpret the impact of nanoscale phenomena on the measurement. The goal of this article is to provide a brief discussion of some of the key methods used to characterize nanoscale materials. All of these methods work together to provide a complete picture of nanoscale materials and their properties.
In addition to their size, nanoscale materials all have a more surface area than bulk samples. When nanoscale materials are assembled into a composite material, interfacial layers can change their properties. Today's nanoscale transistors provide a well known example of the impact of interfacial layers. In a transistor, the gate starts at the silicon wafer and includes the transistor channel, interfacial oxide, high k, and then the metal gate electrode (often called the gate stack). Reducing the transistor dimensions required replacing silicon dioxide (actually silicon oxynitride) with a high k film. Experience has shown that an interfacial oxide layer was required before depositing the high k film. Because the interfacial oxide is only a few monolayers thick, characterizing it is very challenging. It is important to note that despite the nanoscale size of this layer, it has a very big impact on the electrical properties of the transistor. Thus characterizing and controlling it is critical.
Several key materials characterization methods and their relationship to nanoscale thin films are discussed below. The College of Nanoscale Science and Engineering (CNSE) routinely uses these methods in academic research and for the R&D it does with its partners such as IBM, TEL, Applied Materials, and ASML.
Nanoscale Metrology for Nanoscale Films
As mentioned above, nanoscale films are often one film in a stack of thin film layers. Often, some of the films are opaque or the films stack complicated enough to require characterizing these films through a variety of methods. Below, we discuss optical, X-Ray, and Ion Beam methods that one typically uses to characterize the transistor gate stack.
A variety of optical measurements are useful for nanoscale films. Ellipsometry, photoreflectance, and second harmonic generation have all been used to characterize nanoscale materials. Ellipsometry is frequently used to measure the thickness of dielectric, semiconductor and thin metal films. It can also determine the dielectric function of new materials when the thickness is known. The dielectric function of the high k films changes with the composition of the film. For example, the amount to silicate in hafnium silicate and the process conditions both have a strong impact on the dielectric function and thus optical response as shown in figure 1. Ellipsometry is an excellent means of controlling manufacturing processes such as those used to fabricate the gate stack. Photoreflectance provides a means of characterizing the stress in single crystal semiconductor films. This method is well suited to characterizing silicon-germanium films used as the transistor channel. Second harmonic generation is a useful method of characterizing interfacial properties especially ones between a dielectric and semiconductor.