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Dark-field X-ray microscopy unveils structural origins of material fatigue in copper layers for microelectronics

Dark-field X-ray microscopy was used to observe defect accumulation near the grain boundaries in copper films, resulting in material hardening and embrittlement that can lead to a gradual degradation of electrical conductivity. The results deepen understanding of material fatigue and could help boost the lifetime of microelectronics devices.

With increasing demands in efficiency and power density, ensuring the lifetime of integrated circuits is becoming ever more challenging. In microelectronic devices, metallisation layers are implemented as current and heat conduction pathways and are often manufactured from materials such as copper (Cu), which suffers from low thermal microstructural stability. Especially in high- power applications, devices heat up and cool down cyclically, inducing a cyclical mechanical load that may result in material fatigue and eventually in device failure, thereby effectively limiting device lifetime.

Dark-field X-ray microscopy (DFXM) is a comparatively new technique available at beamline ID06-HXM [1], where a coherent monochromatic X-ray beam is first focused on a sample and an objective compound refractive lens is then placed in an appropriate position to magnify a diffracting feature such as a single grain onto a position-sensitive detector. By tilting the sample very slightly and precisely through various angles while recording the detector images, various quantities such as crystal mosaicity, misorientation, strain and defect concentrations (Figure 33) can be mapped within the individual grain.

In this work, the experiment employed a thermo- mechanical cycling procedure of heating up to 400°C from a baseline temperature of 100°C, with pulses of 200 µs in length and a heating rate of 106 K/s, at a repetition rate of 1 Hz and up to 50 000 times, resulting in significant microstructural changes in the tested samples, consisting of a 20-µm-thick Cu film electro-deposited onto a polysilicon integrated heating structure.

As illustrated by scanning electron microscopy (SEM) images in Figure 34, the surface of the Cu film is roughened significantly as cycling progresses. From the earlier stages, it is evident that roughening is due to accumulated small-amplitude plastic deformation, initially taking the form of slip marks at the film surface, later compounding into protrusions and intrusions over time. At very high cycle numbers, elongated valleys and round-shaped voids become visible. These severe microstructural changes are well-known to be accompanied by a degradation in the Cu film s functional properties [2], such as electrical resistance. It is therefore important to study them more deeply.

Fig. 33: Maps reconstructed from DFXM data showing a Cu grain cycled 50 000 times: (a) depicts the mosaicity of the crystal lattice; in (b) the kernel-average misorientation indicates the diffuse variation of the diffraction vector, (c) shows

the relative axial strain, corresponding to the distribution of second-order elastic strain and (d) presents the diffraction peak breadth (i.e., the concentration of lattice defects). A SEM micrograph of the same grain is shown in (e). The different grain

shapes are due to the fact that in DFXM the entire volume of the grain is analysed, while SEM is surface-sensitive.