Photoacoustic measurement of transparent film stacks

The invention relates to photoacoustic measurement of a transparent film stack. A non-destructive photoacoustic metrology device detects the presence and location of non-uniformities in a film stack comprising a plurality of transparent layers, such as 50 transparent layers or more. A transducer layer at the bottom of the membrane stack generates an acoustic wave in response to an excitation beam. A probe beam is reflected from the layer interface of the membrane stack and the acoustic wave to produce an interference signal that encodes data in the time domain from destructive and constructive interference as the acoustic wave propagates upward in the membrane stack. The data may be analyzed across the time domain to determine the presence and location of one or more non-uniformities in the membrane stack. The acoustic metrology target may be fabricated with a transducer layer configured to generate acoustic waves having a desired acoustic profile based on characteristics of the membrane stack. 本公开涉及透明膜堆叠的光声测