LOW-FREQUENCY NORMAL INCIDENCE ULTRASONIC METHOD FOR THIN-LAYER CHARACTERIZATION

A simple and robust on-line low-frequency ultrasonic through transmiss ion method for characterization of thin metal or polymer films at norm al incidence is developed. When the layer (film) thickness h is small compared with the wavelength in the layer material, the reflection/tra nsmission coefficient ratio R/T is proportional to frequency and the s lope of the R/T spectrum carries important information on the layer pr operties. When the layer impedance Z is much higher than that of the i mmersion fluid (air or water) the slope of the R/T spectrum is proport ional to the layer surface density ph. If low-impedance layer is embed ded in a high-impedance medium such as polymer in metal, the slope of the R/T spectrum is proportional to the layer-specific compliance h/C, where C is the elastic modulus of the layer. To obtain the R/T spectr um, only the through-transmission coefficient of the layer needs to be measured with R calculated as [(1 - T)(2)](1/2). Simulations and expe riments show that the proposed double through-transmission method is r obust against misalignment, since the through-transmission coefficient of a thin layer is independent of incident angle over at least +/- 5 degrees of angle range. Immersion measurements have been done on thin metal and ceramic foils with thicknesses ranging from 20 to 200 mu m u sing 2.25 and 5 MHz transducers (h/lambda approximate to 0.05 with lam bda the wavelength in the foil material). Ultrasonic data have been co mpared with direct measurements of the foil thickness-density product showing good agreement. It is also shown that for surface density meas urements of polymer layers air coupling transducers are appropriate.