A detailed understanding of the crystallography of metallic conductors in modern interconnect systems is essential if
we are to understand the influence of processing parameters on performance and reliability. In particular we must be
able to evaluate the grain size, crystallographic orientation and residual elastic stress for interconnect lines having
widths of tens of nm. Transmission electron microscopy might be the obvious choice, but sample preparation and
small sample size make this technique unattractive. On the other hand, electron backscatter diffraction, EBSD, in a
scanning electron microscope provides a very attractive tool. Sample preparation can be relatively simple, especially
if one investigates the structures immediately after CMP; whole wafers may be measured if desired. One limitation
to EBSD is that good diffraction patterns are obtained only from free surfaces and from a limited depth, say a few
hundred nm in copper. Here EBSD will be used to compare structures for the pads and 100-nm lines in two variants
of a commercial copper damascene interconnect structure. EBSD data collection will be discussed as optimized for
characterizing differences in the texture, which were attributed to differences in the processing. By a unique
approach to EBSD mapping we found that neither the texture nor the grain size of the overburden, as represented by
the contact pads, propagated into the 100 nm lines, though they did propagate into some wider lines.
This paper presents recent NIST MicroElectroMechanical Systems fixed-fixed beam test structure data and analysis. These test structures show the most promise in measuring the compressive strain due to simplicity of the test structure design, simplicity of test and analysis, ability to better isolate compressive strain values as a function of geometry, and, most importantly, capability to record process variability data.
Polycrystalline silicon (polysilicon) is widely used as a mechanical layer in MicroElectroMechanical Systems (MEMS). Mechanical elements within MEMS structures are, by design, microscopic in size. Because the thickness of the polysilicon layer is typically around 2 micrometers and the width and length of the freed area is a few to hundreds of micrometers, standard techniques and apparatus for measurements of mechanical properties are not applicable. Furthermore, the deposition techniques for polysilicon cannot be adapted to make specimens big enough to test by conventional techniques. Therefore, special structures were designed to facilitate measurements of Young's modulus and fracture strength: cantilever beams and dog-bone tensile specimens. Here we report first experiences with these structures. These experiences include successes and failures in manipulating and testing the special structures. While no definitive results for either fracture strength or Young's modulus are reported here, some plausible values for both quantities were obtained. Test methods and preliminary results to date are discussed.
A hybrid grid-moiré method for measuring strain is presented. The method employs a line grating attached to a plane surface of the specimen. Images of undeformed and deformed gratings are acquired and stored using a video system and a frame digitizer. The magnification is adjusted to record 10 to 30 lines of a grating with a pitch of 4 μm over the frame width. A fast Fourier transform (FFT) of the intensity data gives a close estimate of the spatial line frequency. A moiré function is formed as the product of a reference sine function and the image intensity-position function. FFTs of the moiré function yield an improved estimate of the line frequency. The line frequency is related to an average strain over the recorded image. The method was demonstrated at a magnification of 1500 x with a grating pitch ps = 4 μm. The accuracy achieved depended on strain. For strains in excess of 0.1%, the errors were usually less than 5% when compared to strain gauge results. For plastic strains, in excess of 1%, the differences between two comparable measurements were less than 3%.
Conference Committee Involvement (1)
Reliability, Packaging, Testing, and Characterization of MEMS/MOEMS VI
23 January 2007 | San Jose, California, United States
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