Suitability of half-wavelength contact acoustic microscopy to detect deeply buried voids

A key challenge in semiconductor 3D-integration is the occurrence of small voids (O(0.01 – several μm). Depending on the chip/package design and process these voids may occur across a large depth range (1 – 1000 μm) – e.g. the typical bond interface depth in wafer-to-wafer bonding lies at a depth of 775 μm. The small void size, large depth and increasingly common use of optically opaque layers means that the optics-based industry metrology workhorses struggle with the detection of these defects. One option is Scanning Acoustic Microscopy (SAM). However, this requires a liquid couplant to couple acoustic waves into the sample and has a lower resolution limit of several μm. A new technique entitled Half-Wavelength Contact Acoustic Microscopy (HaWaCAM) sidesteps this problem. It uses a 4 GHz transducer integrated on top of a custom designed probe and solid-solid contact to couple waves into the sample (no liquid needed) enabling higher frequencies (resolution) compared to SAM. Here, we investigate the suitability of HaWaCAM to detect deeply buried voids. For this we used a silicon wafer sample containing air voids (matrix of square voids, 40 μm pitch, void: height 75 nm, width 10 nm – 20 μm) buried below 49 μm of silicon. HaWaCAM could successfully detect voids down to a width of 0.75 μm at a depth of 49 μm. The signal-to-noise-ratio varied from 25 – 40 dB, 15 – 30 dB, 10 – 25 dB for void widths of 2.5, 1 and 0.75 μm, respectively. We expect the current setup version to be able to detect 2.5 μm void widths with 6 dB SNR buried below >800 μm of silicon. This shows the capability of HaWaCAM to detect voids buried at large depths.