Scanning Acoustic Microscopy versus Colored Picosecond Acoustics to detect interface defects in hybrid wafer bonding

Abstract

Nowadays, microelectronics is making progress in miniaturization and diversification of chips by 3D integration: dies are stacked together to gain space and performances. The stacking used here is called hybrid bonding [1], as stacked surfaces are heterogeneous: part Cu and part oxide. After the bonding, it exists some defects at the interface which size can be milimetric to nanometric. Acoustic waves are very useful for detecting defects. In the field, the most used technique is the Scanning Acoustic Microscopy (SAM) which makes an image of the interface using ultrasonics (10-1000 MHz) with a XY resolution of 20 �m [2]. To detect smaller defects, Transmission Electron Microscopy (TEM) is used. Its resolution is about 0,1 nm, making this technic very local compared to SAM. Like the SAM, Colored Picosecond Acoustics (APiC) is based on acoustic waves but at higher frequencies (10-500 GHz) using of a femtosecond laser in a pump-probe scheme [3,4]. Such a technique is closer to TEM resolution. In this work, both acoustic techniques (SAM and APiC) are applied to the characterization of the same samples made of bonded wafers. APiC is first used to characterize the stack (thickness, velocity). Then, the interface contributions is focused on: Cu/Cu, Cu/oxide, oxide/oxide. As the technic makes a local measurement (typically 1-2 �m), the process is repeated at different points of the surface to get an interface image comparable to SAM result. In this work will be presented the APiC results obtained on wafers also studied by SAM to compare both approaches, especially about the resolution enhancement ApiC brings. The minimal size of the defects that APiC is able to detect will be explored.