A nondestructive technique is demonstrated for the metrology of adhesive layers that are considerably thinner than a single wavelength of ultrasound in that material. The acoustic microprobe technique discussed here is capable of measuring the thickness and elastic properties of a thin layer sandwiched between two relatively thicker layers. Real time measurements over a 50 #mu#m diameter spot are possible with this technique. The particular case of a silicone adhesive layer between an aluminum lid and die in a flip chip package is discussed here and layer thickness was measured down to 5 #mu#m. The minimum measurable thickness limit of the acoustic microprobe technique is several times better (smaller) than that of time based techniques using the same transducer.
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