Characterization of Mechanical and Thermal Properties Using Ultrafast Optical Metrology
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Characterization of Mechanical and Thermal Properties Using Ultrafast Optical Metrology G. Andrew Antonelli, Bernard Perrin, Brian C. Daly, and David G. Cahill Abstract Ultrafast lasers have long been used to study the dynamics of fast optical, electronic, and chemical processes in materials. These tools can also be used in a variety of optical pump and probe spectroscopies to generate and detect acoustic signals with frequencies on the order of 100 GHz, and to generate and detect thermal waves with penetration depths on the scale of nanometers. The short wavelengths of these probes make them ideal for the study of the mechanical and thermal properties of thin films, their interfaces, and nanostructures. We describe the picosecond-laser acoustics technique and demonstrate how it can be used to extract the elastic constants and the adhesion of thin films and probe the normal modes of vibration of nanostructures. The thermal properties of thin films are also accessible through time-domain thermoreflectance. Since the mechanical and thermal properties can be obtained quickly on micrometer-scale regions of a sample, spatial mapping of the properties is also possible. Keywords: laser, optical, thin film.
Introduction Thin-film materials can be found everywhere in our daily lives, from the microprocessors in our computers to the cookware in our kitchens. Many of these materials do not exist in a bulk form, and even those that do may have properties that differ dramatically from their thinfilm analogues. For this reason, the characterization and monitoring of the properties of thin films is very important in the manufacture of many products. The mechanical and thermal properties of thin films have become increasingly relevant, particularly for microelectronic devices. Because in most materials acoustic waves travel a few nanometers in a picosecond and heat flows about a hundred nanometers in a nanosecond, very fast instruments are needed to track these changes.
MRS BULLETIN • VOLUME 31 • AUGUST 2006
For this reason, ultrafast laser systems were considered to make such measurements. In only 20 years, we have moved from theory to experimental validation to fully realized instruments that are used in semiconductor manufacturing facilities.
Generating and Detecting Acoustic Excitations The process of generating acoustic excitations with a pulsed laser can be described by macroscopic thermoelastic analysis. Assume that an optical pulse is focused onto an absorbing material. Through optical absorption, the energy of the pulse is deposited in a localized region of the material and creates a localized increase in the temperature. A thermal stress is then generated by thermal expansion of
the material. This stress relaxes by launching an acoustic strain pulse, which propagates according to the elastic properties of the material. When the strain pulse reaches an interface, part of the pulse is transmitted and part is reflected according to the difference in the acoustic impedance (product of density and sound velocity) of the
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