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

Abstract

Background A key challenge in semiconductor 3D integration is the occurrence of small voids with diameters ranging from a few nanometers to several micrometers. Depending on the chip/package design and production process, these voids may occur across a large depth range (1 to 1000 μm)—e.g., the typical bond interface depth in wafer-to-wafer bonding lies at a depth of 775 μm. The standard tools to detect these deep defects are infrared microscopy, scanning acoustic microscopy (SAM), and X-ray. However, they all have limitations in resolution or penetration.

Aim Here, we propose an acoustical technique named half-wavelength contact acoustic microscopy (HaWaCAM®) to sidestep this problem. HaWaCAM® consists of a 4 GHz transducer integrated on top of a custom-designed probe and uses solid-to-solid contact to couple waves into the sample, enabling a higher detectability and resolution compared with SAM.

Approach In this work, we investigate the suitability of HaWaCAM® to detect deeply buried voids. We used the HaWaCAM® setup on a silicon wafer sample containing programmed voids of different sizes buried below 49 μm of silicon.

Results We show that HaWaCAM® successfully detects voids down to a width of 0.75 μm at a depth of 49 μm. The signal-to-noise ratio varied from 30 to 45 dB, 20 to 30 dB, and 15 to 30 dB for void widths of 2.5, 1, and 0.75 μm, respectively.

Conclusions We demonstrate the capability of HaWaCAM® to detect voids buried at large depths. We expect the current HaWaCAM® setup to detect voids with 2.5 μm widths with 6 dB SNR buried below >800 μm of silicon.