Characterization of Thin Films

Scientifíc disciplines are identified and differentiated by the experimental equipment and measurement techniques they employ. The same is true of thin-film science and technology. For the first half of this century, interest in thin films centered around optical applications. The role played by films was largely a utilitarian one, necessitating measurement of film thickness and optical properties. However, with the explosive growth of thin-film utilization in microelectronics, there was an important need to understand the intrinsic nature of films. With the increasingly interdisciplinary nature of applications, new demands for film characterization and other property measurements aróse. It was this necessity that drove the creativity and inventiveness that culminated in the development of an impressive array of commercial analytical instru-ments. These are now ubiquitous in the thin-film, coating, and broader scientifíc communities. In many instances, it was a question of borrowing and modifying existing techniques employed in the study of bulk materials (e.g., X-ray diffraction, microscopy, mechanical testing) to thin-film applications. In other cases well-known physical phenomena (e.g., electrón spectroscopy, nuclear scattering, mass spectroscopy) were exploited. A partial list of themodern techniques employed in the characterization of electronic thin-film materials and devices is given in Table 6-1. Among their characteristics are the unprecedented structural resolution and chemical analysis capabilities over small lateral and depth dimensions. Some techniques only sense and provideinformation on the first few atom layers of the surface. Others probé more deeply, but in no case are depths much beyond a few microns accessible for analysis. Virtually all of these techniques require a high or ultrahigh vacuum ambient. Some are nondestructive, others are not. In common, they all utilize incident electrón, ion, or photon beams. These interact with the surface and excite it in such a way that some combination of secondary beams of electrons, ions, or photons are emitted, carrying off valuable structural and chemical information in the process. A rich collection of acronyms has emerged to differentiate the various techniques. These abbreviations are now widely employed in the thin-film and surface science literature.

General testing and analysis of thin films is carried out with equipment and instruments which are wonderfully diverse in character. For example, consider the following extremes in their attributes:

1. Size— This varies from a portable desktop interferometer to the 50-ft long accelerator and beam line of a Rutherford backscattering (RBS) facility.

2. Cosí—This ranges from the modest cost of test instruments required to measure electrical resistance of films to the approximate $1 million price tag of a commercial SIMS spectrometer.

3. Operating Environment—This varies from the ambient in the measure ment of film thickness to the 10"^l0-torr vacuum required for the measure ment of film surface composition.

4. Sophistication—At one extreme is the manual scotch-tape film peel test for adhesión, and at the other is an assortment of electrón microscopes and surface analytical equipment where operation and data gathering, analysis, and display are essentially computer-controlled.

What is remarkable is that films can be characterized structurally, chemi-cally, and with respect to various properties with almost the same ease and precisión that we associate with bulk measurement. This despite the fact that there are many orders of magnirude fewer atoms available in films. To appreciate this, consider AES analysis of a Si wafer surface layer containing 1 at% of an impurity. Only the top 10-15 A is sampled, and since state-of-the-art 

systems have a lateral resolution of 500 A, the total measurement volume corresponds to (ir/4)(500)²(15) = 3 X 10⁶Á³. In Si this corresponds to about 150,000 matrix atoms, and therefore only 1500 impurity atoms are detected in the analysis! Such measurements pose challenges in handling and experimental techniques, but the problems are usually not insurmountable.

This chapter will only address the experimental techniques and applications associated with determination of

1.          Film thickness

2.          Film morphology and stmcture

3.          Film composition

These represent the common core of information required of all films and coatings irrespective of ultímate application. Within each of these three cate-gories, only the most important techniques will be discussed. Beyond these broad characteristics there are a host of individual properties (e.g., hardness, adhesión, stress, electrical conductivity, reflectivity, etc.), that are specific to the particular application. The associated measurement techniques will there-fore be addressed in the appropriate context throughout the book.

 Film Thickness

The thickness of a film is among the first quoted attributes of its nature. The reason is that thin-film properties and behavior depend on thiclcness. Histori-cally, the use of films in optical applications spurred the development of techniques capable of measuring film thicknesses with high accuracy. In contrast, other important film attributes, such as structure and chemical composition, were only characterized in the most rudimentary way until relatively recently. In some applications, the actual film thickness, within broad limits, is not particularly crucial to function. Decorative, metallurgical, and protective films and coatings are examples where this is so. On the other hand, microelectronic applications generally require the maintenance of precise and reproducible film thicknesses as well as lateral dimensions. Even more stringent thickness requirements must be adhered to in optical applications, particularly in multilayer coatings.

The varied types of films and their uses have generated a multitude of ways to measure film thickness. A list of methods mentioned in this chapter is given in Table 6-2 together with typical measurement ranges and accuracies. In-cluded are destructive and nondestructive methods. The overwhelming major-ity are applicable to films that have been prepared and removed from the deposition chamber. Only a few are suitable for real-time monitoring of film thickness during growth. We start with optical techniques, a subject that is covered extensively in virtually every book and reference on thin films.

Optical Methods for Measuring Film Thickness

Optical techniques for film thickness determinations are widely used for a number of reasons. They are applicable to both opaque and transparent films, yielding thickness valúes of generally high accuracy. In addition, measure-ments are quickly performed, frequently nondestructive, and utilize relatively inexpensive equipment. The single basic principie which most optical tech niques rely on is the interference of two or more beams of light whose optical path difference is related to film thickness. The details of instrumentation differ, depending on whether opaque or transparent films are involved.

Ronellys Flores---CRF---Libro the materials science  of thin films

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