Complete Characterization of Thin- And Thick-Film Materials Using Wideband Reflection Acoustic Microscopy

Abslmcf-A methodology for complete characterization of thinand thick-film materials deposited on a substrate using widehand reflection acoustic microscopy is presented. A wideband reflection acoustic microscope that covers the frequency range of 50 to 175 MHz was constructed to carry out the experimental verification. The amplitude of the reflection coefficient of the film-substrate composite was measured versus the acoustic frequency to identify the resonant frequency of the specimen at which the film thickness is equal to one quarter of the acoustic wavelength. Then both the amplitude and the phase of the reflection coefficient at the resonant frequency were measured. Using the three measured quantities, thickness, acoustic velocity, and mass density of the film material are simultaneously determined without any prior knowledge. Two types of film materials, namely Pyrex glass film sputtered on a sapphire substrate and positive photoresist coated on a Pyrex glass substrate have been characterized using the methodology. The three measured acoustic parameters, film thickness, acoustic velocity, and mass density, agree well with the published values. For the frequency range of the microscope utilized the corresponding range of measurable film thickness is 30 pm to 5 pm for most inorganic materials, and 10 pm to 2 pm for most organic materials. For the case in which the film thickness is less than one quarter of the acoustic wavelength at the highest operation frequency of the microscope, the resonant frequency cannot be identified. However. one or two of the three acoustic parameters can still be determined by measuring the amplitude and the phase of the reflection coefficient a t a fixed acoustic frequency. This capability has been demonstrated using a gold fdm deposited on fused quartz and an aluminum film evaporated on X-cut LiNbOj substrate. Three additional transducer/lens sets with staggered center frequencies are also being assembled to cover the entire frequency range from 175 MHz t o 1000 MHz so that the measurable range of the film thickness can be extended to the submicron region. Consequently, it is concluded that this characterization methodology should he highly useful For nondestructive study of thinand thick-film materials in microelectronics.