Abstract
On-line thickness monitor of thin film deposition by thermal evaporation is demonstrated using femtosecond pump-probe photoacoustics. A Ti:sapphire femtosecond laser (wavelength 800 nm, pulse width ~110 fs, pulse energy ~1.3 nJ) is split into a pump and a probe beams. The pump beam is used to instantaneously heat the illuminated area of an aluminum thin film during its deposition on a silicon substrate, whereas the probe beam is used to investigate the change in the transient thermoreflectance ΔR/R. The thermal expansion, by the laser heating, creates stress in the aluminum surface region that generates an acoustic wave of ultrasonic frequency. The travel time of the optically-induced acoustic wave normal to the surface of the thin film is measured to evaluate the thickness of the aluminum in the range of ~32 to 388 nm during the film deposition.
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