Impulsive Stimulated Thermal Scattering

Abstract

Impulsive stimulated thermal scattering (ISTS) is a purely optical, noncontact method for characterizing the acoustic behavior of surfaces, thin membranes, coatings, and multilayer assemblies, as well as bulk materials. The method has emerged as a useful tool for materials research in part because (1) it enables accurate, fast, and nondestructive measurement of important acoustic (direct) and elastic (derived) properties that can be difficult or impossible to evaluate in thin films using other techniques; (2) it can be applied to a wide range of materials that occur in microelectronics, biotechnology, optics, and other areas of technology; and (3) it does not require specialized test structures or impedance‐matching fluids which are commonly needed for conventional mechanical and acoustic tests. Further, recent advances in experimental design have simplified the ISTS measurement dramatically, resulting in straightforward, low‐cost setups, and even in the development of a commercial ISTS photoacoustic instrument that requires no user adjustments of lasers or optics. With this tool, automated single‐point measurements, as well as scanning‐mode acquisition of images of acoustic and other physical properties, are routine. ISTS, which is based on transient grating (TG) methods, is a spectroscopic technique that measures the acoustic properties of thin films over a range of acoustic wavelengths. This article focuses only on ISTS measurement of acoustic motions in thin films. It begins with an overview of other related measurement techniques. It then describes the ISTS acoustic data and demonstrates how it can be used to determine (1) the stress and flexural rigidity in thin membranes; (2) the elastic constants of membranes and supported films; and (3) the thicknesses of single or multiple films in multilayer stacks.

Quantifying Mechanical Properties of Molecular Crystals: A Critical Overview of Experimental Elastic Tensors

“…Noncontact experimental methods include light scattering from ultrasonic waves (the Schaefer–Bergmann method [24] ) and Brillouin scattering, [25] which measures the scattering of light from acoustic modes of a crystal, and can be applied to quite small samples. Impulsive stimulated thermal scattering (ISTS) [26] is an all‐optical noncontact method suitable for much smaller samples. For example, ISTS measurements have been performed on polished thin slabs 600–800 μm thick for the explosives PETN and RDX, [27] enabling a critical comparison with previous ultrasonic, RUS and Brillouin scattering results, and this comparison is discussed in detail in Section 5.14.…”

Review—Management of Copper Damascene Plating

“…The signal waveforms are averaged over approximately 1000 excitation pulses, which takes approximately 1 s. The measurement apparatus is described in more detail elsewhere. 60,61 It was found that the ISTS technique can be applied for the following in copper damascene copper plating management: PVD monitoring of the barrier/ Cu seed process, ECD thickness and uniformity monitoring, edge control, and Cu fill control. The merits of this technique for these applications include thickness sensitivity, good precision, small spot size capability for wafer edge (see Figure 12) as well as small feature characterization, good throughput, capability to determine other materials parameters for the Cu films, including sheet resistivity, capability to measure the thickness of the underlying barrier metal, and nondestructive nature which allows the wafer to be used for subsequent processing and/or measurements.…”

Photoacoustic Tomography System for Roughly Painted Micro Objects

“…Photoacoustic (PA) methods have overcome the challenges found in acoustic coupling by using laser light to excite and probe acoustic disturbances without contacting the sample [9,10]. Moreover, the laser-driven piezoelectric technology with femtosecond laser pulse has successfully generated acoustic waves in order of terahertz [11].…”