High Resolution Sem Imaging of Multilayer Thin Films
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J. R. Kingsley, I. D. Ward, J. P. Vitarelli Charles Evans & Associates. Redwood City, CA 94063
ABSTRACT
The high resolution capability of the JEOL JSM-890 In Lens Field Emission Scanning Electron Microscope (ILFESEM) is used for the routine determination of both film thicknesses and surface morphologies from a single sample. This single sample approach is possible because of the 0.7 nm resolution of the instrument, and the simple sample preparation. In many cases, the desired information can be obtained from a simple cleave of the sample. INTRODUCTION
There exists a need to study the morphology, integrity and uniformity of thin films produced for a wide range of industrial applications. Scanning electron microscopy, (SEM), has aidgd this quest since its inception, evaluating materials as both a research and development tool and as a quality control, failure analysis tool [1]. A typical SEM sample chamber permits samples a few centimeters in size to be observed, if necessary at a long working distance to maximize the available depth of field. Subsequent development of the field emission source has greatly improved the imaging resolution of the basic SEM to the I to 2 nm range. An in-lens configuration places the sample in the pole piece gap of the final probe forming lens. This severely restricts the size of the sample, but enhances imaage resolution, (sub nm), by virtue of a shorter working distance. In addition, the lens field surrounding the sample provides a filtering effect, discriminating against one of the sources of spurious secondary electrons [2]. The differences between the two configurations is shown in fig. i. A
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Figure 1. (a)Layout of a conventional SEM, (b)Layout of an in lens SEM. 207 Mat. Res. Soc. Symp. Proc. Vol. 382 01995 Materials Research Society
Sample preparation considerations
The micrographs presented here were obtained using a JEOL JSM-890 ILFESEM. For this instrument the sample size must be restricted to 15mm long by 4mm wide by 2mm thick. Only the central 7mm by 3mm region is available for imaging. Appropriate sample handling systems were developed to reduce the typical starting material to meet this restraint, without compromising the surfaces and cross sections under investigation. For example, a sample carrier was designed, fig. 2, to fit both the sample polishing system and the microscope stage. Once mounted in the carrier, the sample is never again directly handled. A thin conductive coating is usually used to dissipate charge when imaging poorly conducting samples. Typically a gold palladium alloy is used for this because of its high secondary electron yield and very small grain size. A modem high reolution SEM is quite capable of imaging these grains which can therefore obscure some real sample details. Figure 3a is an example of the grain structure of a thin platinum film which has a finer grain structure than gold palladium. This film was deposited using a triode sputter
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