Three-dimensional integrated circuits (3D ICs) hold promise for high performance system applications, as a single integrated circuit can be connected to multiple device levels using through-silicon vias (TSV). This approach avoids the need for edge wiring requirements, thereby reducing the electrical path-length and power consumption and providing rapid device operation.
The reliability of copper TSVs is based on the thermal loading and deposition conditions during the 3D IC manufacturing process. In order to improve the service life of the device, it is necessary to optimize the loading, thermal annealing and deposition conditions. This article describes how electron backscattered diffraction (EBSD) can be used to improve the TSV reliability in 3D circuits.
Existing Solutions for Improving TSV Reliability
Analysis of microstructural changes occurring due to processing conditions at the time of device fabrication can indicate the reliability of TSVs in 3D ICs. These microstructural features can be observed using the following characterization techniques:
- Focused ion beam (FIB) - FIB images provide qualitative information on TSV deposition, grain size and filling quality via crystallographic channelling imaging. FIB imaging does not provide direct quantitative grain size determination or crystallographic orientation information. Crystal orientation can provide information on the deposition procedure and grain size provides information on the annealing procedure.
- Transmission electron microscopy (TEM) - TEM offers crystallographic-based imaging of grains and defects within a TSV. The obtained images provide qualitative microstructural details. The determination of the crystallographic orientation in the TEM is generally carried out manually. This makes statistically reliable sampling complex for preferred orientation and grain size determination.
- Nanoindentation - This characterization is carried out to provide information on elastic moduli and yield strength, providing in-depth insight into the plastic strain within the TSV and grain size. The strain values and local strength measured can be changed using microstructural features like composition and crystal orientation, however the variation in determined values cannot be precisely explained without knowing this microstructure completely.
Electron Backscattered Diffraction
EBSD offers a fast and automated solution to characterize the microstructure of copper TSVs and facilitates direct measurement of grain size using crystallographic orientation measurements.
The technique measures copper grain size after deposition and thermal cycling, meaning the manufacturing parameters can be adjusted to control grain size distribution and achieve complete TSV fill.
It also provides measurement of crystallographic orientation and texture, which helps in understanding the copper film deposition process. Parameters such as deposition rate, voltage and bath additives are used for the determination of preferred orientation that influences the probability of void formation and fill rate.
The intergranular misorientations formed within the TSV copper can also be measured using EBSD. Such misorientations show evidence of plastic deformation during thermal cycling. The occurrence of plastic deformation in turn shows that copper protrusions may be created, causing consistency concerns such as delamination and cracking.
Results of Microanalysis Using EBSD
EBSD data was obtained for a 6 µm x 40 µm copper TSV sample following deposition and thermal cycling which simulate back-end-offline (BEOL) processing. Figure 1 shows an orientation map with colored orientations corresponding to the sidewall growth direction. The data shows a recrystallized structure consisting of numerous twin boundaries without any significant preferred orientation development.
The average grain size was observed to be 978 nm. Further, good TSV fill was also observed. Twin boundaries offer considerably slower diffusion pathways via the TSV with regards to random high-angle grain boundaries. If twin boundaries are not included in the grain determination algorithm, the twin adjusted grain size measured is 2.72 µm. The consistency will be better forecast with this adjusted grain size.
Figure 1. Orientation map of copper throughsilicon via showing no preferred orientation.
Figure 2 shows the grain structure with and without twin boundaries, where the grains are randomly colored to reveal the grain size and morphology. The grain structure without twin boundaries was observed to be closer to the desired “bamboo” structure, in that the high-angle boundaries are at right angles to the TSV length. This grain structure limits the probable grain boundary diffusion paths via the TSV and will offer higher resistance to electromigration failure.
Figure 2. Grain maps of copper TSV with twin boundaries included and excluded from grain
A Kernel average misorientation (KAM) map with coloring based on the plastic deformation level in the copper TSV is shown in the Figure 3. The coefficient of thermal expansion varies between the copper and surrounding silicon wafer during thermal cycling, resulting in the formation of stresses.
Permanent plastic deformation tends to occur when these stresses exceed the elastic limit of copper. The KAM map reveals the plastic deformation region at the bottom of the TSV. Thermal cycling variables need to be modified in such cases to reduce the applied stress and improve reliability.
Figure 3. Kernel average misorientation map showing the plastic strain developing after thermal cycling, which can reduce reliability
About EDAX Inc.
EDAX is the global leader in Energy Dispersive X-ray Microanalysis, Electron Backscatter Diffraction and Micro X-ray Fluorescence systems. EDAX manufactures, markets and services high-quality products and systems for leading companies in semiconductors, metals, and geological, biological, material and ceramics markets.
Since its founding in 1962, EDAX has utilized its knowledge and expertise to develop ultra-sensitive silicon radiation sensors, digital electronics and specialized application software that facilitate solutions to research, development and industrial requirements.
EDAX is a unit of AMETEK Materials Analysis Division. AMETEK, Inc. is a leading global manufacturer of electronic instruments and electric motors with annualized sales of more than $1.8 billion.
This information has been sourced, reviewed and adapted from materials provided by EDAX Inc.
For more information on this source, please visit EDAX Inc.