Real-time optical characterization of surface acoustic modes of polyimide thin-film coatings

Characterization of acoustic and thermal properties of thin ( 1 pm) polymeric films either free standing or used as’ coatings is -of great technological importance for the polymer, microelectronics, and biomedical industries. The ideal method for doing this would be noninvasive and would be applicable to films in their ha1 configurations or during fabrication and curing. A rapid, noninvasive characterization method could be used for feedback and control over thin-film fabrication and curing. Impulsive stimulated thermal scattering (ISTS) is a time-resolved optical spectroscopy method that has been used extensively for characterization of bulk samples.“2 In ISIS, absorption of crossed picosecond excitation pulses in a sample gives rise to spatially periodic heating and thermal expansion which launches counterpropagating acous-e tic waves. The acoustic oscillations and decay, and finally thermal diffusion, are monitored through measurement of the time-dependent diffraction of a third probe beam. ISTS has been used for the determination of elastic constants and thermal diffusion rates in bulk samples.3 ISTS has previously been applied to probe the acoustic responses of samples on the order of micrometer thickness by Meth, Marshall, and Fayer4 for thin flakes of naphthalene and by Rao et aL5 for thin polystyrene layers doped with naphthalene to induce optical absorption. In the earlier bulk and thin-film studies, data acquisition involved many repetitions (typically thousands) of an excitation-probe pulse sequence as described below. This leads to long data collection times and in some cases to cumulative sample heating and damage. In a previous study of a polymer film5 the sample was translated during data collection to avoid excessive heating and damage. Such a scheme is not feasible when data is collected in reflection mode (for opaque samples as described below) since slight sample warping leads to movement of the signal beam upon translation. In a recent preliminary report,’ we described a modification of the ISTS technique that allows real-time characterization of the acoustic modes of thin supported films. Data can be collected with minimal signal averaging and even a single excitation-probe pulse sequence can yield adequate signal. In this paper, we describe these modifications in more detail and demonstrate the utility of the technique in analyzing the pseudo-Rayleigh or Sezawa acoustic waves propagating on a thin polyimide film attached to a silicon substrate. We also present a detailed analysis of these modes. The main purpose of the treatment is to determine which modes are excited most effectively by the crossed excitation pulses and which contribute most to the diffraction of probe light. It will be shown that this determination can simplify the extraction of the elastic properties of the system from ISTS data. We note that Brillouin scattering offers an alternative optical method for the characterization of thin-film acoustic modes.’ However, extensive practical applications are unlikely at present since data acquisition times for these studies range from 20-30 h.