EBSD Applications in the Electronics Industry

Electron backscatter diffraction (EBSD) has been used for many years to improve our understanding of the complex microstructures within microelectronics samples, including the successful characterisation of many key structures such as interconnects, solders and thin films. However, the demand to keep up with Moore's Law has resulted in continuous miniaturisation at the integrated circuit level as well as increased vertical stacking of chips, making routine analysis using EBSD more challenging.

The recent development of the world's first fibre-optic coupled complementary metal-oxide semiconductor (CMOS) based EBSD detector, Oxford Instruments' Symmetry, has significantly improved the capability of EBSD in the electronics industry. The latest Symmetry S2 detector, capable of speeds in excess of 4500 patterns per second (pps) without requiring high electron beam currents, now enables the effective characterisation of nanostructured devices in a matter of minutes without compromise.

EBSD has many benefits over alternative analytical approaches, including high spatial resolution (below 20 nm), high speed acquisition and the completeness of microstructural information. No other technique can provide data on grain size, boundary characteristics, texture and phase distribution on the all-important sub-µm scale in a single analysis. The ability to collect these data in just a few minutes ensures that EBSD, together with complementary chemical data from energy dispersive X-ray spectrometry (EDS), is the ideal technique for routine characterisation of electronics packaging and semiconductor samples.

In this short application note several example case studies are provided, highlighting the power of the latest EBSD systems from Oxford Instruments to solve key microstructural problems in the electronics industry.

Example 1: Solder Micro-bumps

Micro-bumps, along with through silicon vias (TSVs), are key components for enabling the vertical stacking of chips and the development of 3D integrated circuits, in the drive to improve electrical performance with smaller footprints. The necessity to eliminate Pb from electronic components has led to the extensive use of Sn-based alloys, such as the Sn-Ag-Cu (SAC) system, resulting in complex microstructures and the formation of undesirable intermetallic compounds (IMCs). Understanding the identity, distribution and characteristics of these IMCs is critical for evaluating the long-term performance and stability of the micro-bumps.

Here a micro-bump has been sectioned and analysed using combined EDS-EBSD, allowing a detailed examination of the IMCs and the grain characteristics of the constituent layers. The total analysis time was only 12 minutes, and the results showed the formation of a number of IMCs (Cu₃Sn, Ni-stabilised η-(Cu,Ni)₆Sn₅ and Ag₃Sn) between alternating Cu and highly textured, deformed Ni layers.

Backscattered electron image of the solder micro-bump, showing the EBSD-EDS analysis area in red

Combined EDS element map indicating the presence of several IMCs

EBSD phase map

EBSD orientation map

Sample courtesy of AMD, Singapore

Example 2: Wire Bonds

Wire bonding is a critical process in the manufacture of electronic components, with the microstructure of both the bonding to the semiconductor device and the bond wire itself influencing the bond strength and electrical performance. Effective analysis using EBSD requires high speed characterisation across a range of length scales, providing details on the wire texture, grain size and localised deformation as well as on the formation of IMCs at the wire-device interface.

In this first example, we present a large-scale analysis of a wire bond that demonstrates how the bonding process is potentially impacting the lifetime of the wire bond, due to an increase in grain size and localised deformation close to the interface. A higher resolution analysis of the wire-device interface highlights details of the plastic deformation and indicates possible delamination that could result in failure due to bond lifting.

Orientation map (IPF-X colouring) of the Cu wire bond. Dominant red colour indicates strong <111> fibre texture parallel to the wire long axis. Scale bar marks 200 um.

Map showing the mean grain orientation spread (i.e. extent of plastic deformation) highlighting increased deformation close to the wire-device interface to the right.

Higher resolution scan of the wire-device interface, with kernel average misorientation values highlighting the extensive deformation and potential delamination along one side of the interface (to the right). Scale bar marks 50 um.

Backscattered electron image of a Cu-Si bond. Scale bar marks 10 um.

Higher magnification forescattered electron image of the 2Cu IMC phase. Cu-Si interface. Red box marks the EBSD analysis region. Scale bar marks 5 um.

In a second example, a Cu wire is bonded to a Silicon device, using a thin Al bond pad. A detailed EBSD map (with a resolution of 20 nm) across the interface characterises the formation of 2 intermetallic phases, Al₂Cu and Al₄Cu₉, that form sub-µm wide layers between the Al and Cu. The EBSD data also provide clear evidence for delamination between the bond pad and the IMC phases, a likely initiation point for bond failure.

Example diffraction patterns (as collected and indexed) for the 5 phases present across the interface. Colour scheme corresponds to the EBSD phase map, below.

EBSD phase map across the interface, showing 3D unit cell orientations for the individual grains. Note the delamination between the Al bond pad and the Al2Cu IMC phase.

Summary

Since the introduction of high-speed, high-sensitivity CMOS-based EBSD detectors, EBSD has become a powerful technique for the rapid characterisation of a range of samples in the electronics and semiconductor industries.

The new Symmetry S2 EBSD detector, with unparalleled sensitivity due to its unique fibre optic lens system, can analyse samples in just a few minutes with acquisition speeds of >4500 pps. This enables large sample areas to be measured with high resolution and minimises any charging issues that may be caused by insulating materials. This application note highlights just a few examples in which EBSD can solve specific problems associated with the manufacture and design of electronics components, including solder micro-bumps, wire bonds and the all-important formation of intermetallic compounds at key interfaces.