However, their characterization is challenging even if contact with the sample is acceptable, and is extremely difficult if sample delicacy or cleanliness, processing speed, or other considerations rule out contact. The development of a real-time, noncontact, nondestructive in situ thin film characterization technique by researchers at MIT promises to dramatically simplify thin film evaluation. The method has been used for evaluation of thin films composed of organic polymers, metals, ceramics, diamond, and many other materials. It can be used during or after film fabrication and cure. Most of the films examined have been in the 1-30 micron thickness range, but films as thin as 40 nm as well as thick films and bulk materials have been characterized.
Light absorption leads to sudden, mild heating and thermal expansion at the grating peaks.
This launches coherent acoustic waves whose wavelength and orientation match those of the grating pattern. The data provide quantitative values for the acoustic frequency, velocity, and damping rate. In addition, after the acoustic damping is complete, steady-state thermal expansion at the heated regions continues to modulate the film surface and to give rise to a diffracted signal, which persists until thermal diffusion equalizes the temperature at the grating peaks and nulls. Thermal diffusivity is determined from the data shown in Figure 1b. To summarize, the data from a single measurement yield the acoustic frequency, velocity, damping rate, and the thermal diffusivity.