Analytical techniques combining focused ion beam milling with scanning electron microscopy allow for the three dimensional mapping of internal stresses within microelectronic components. This fib sem strain analysis is a specialized form of failure analysis used to identify the root cause of cracks or delamination in semiconductor packages. By removing thin layers of material with a high energy beam of ions, the instrument can expose the internal structure of a solder joint or a copper interconnect.
The scanning electron microscope then captures high resolution images of the exposed surface, which are used to measure the displacement of the crystal lattice. This data is processed using digital image correlation software to visualize the distribution of strain across the sample. The method provides a level of detail that is not possible with traditional cross sectioning techniques, making it indispensable for the development of new packaging materials.
Removal of material at the nanometer scale requires extreme precision to avoid introducing new stresses that could distort the results of the study. During fib sem strain analysis, the sample is placed in a vacuum chamber where the ion beam is used to cut a small trench or a thin foil from the area of interest. This process must be conducted at low currents to minimize the heating of the sample and the potential for structural changes.
A protective layer of platinum or tungsten is often deposited on the surface before milling to prevent the top of the sample from being eroded by the beam. The resulting cross section must be perfectly flat and free of artifacts to allow for accurate measurement of the internal features. This preparation phase is the most time consuming part of the analysis and requires a high level of skill from the operator.
Any error during the milling process can render the entire sample useless for strain measurement.
Detection of the changes in the spacing of atoms within the material provides the raw data needed to calculate the internal stress. Fib sem strain analysis uses electron backscatter diffraction to determine the crystal orientation and the degree of lattice distortion at thousands of points across the sample. As the electron beam hits the surface, the backscattered electrons form a diffraction pattern that is unique to the crystal structure of the material.
By comparing these patterns to a reference state, the software can calculate the elastic strain tensor at each measurement point. This allows researchers to see how the stress is concentrated around defects, grain boundaries or interfaces between different materials. The resolution of this technique is sufficient to detect strain levels as low as point zero one percent, which is critical for understanding the early stages of fatigue.
This detailed mapping helps to identify the specific locations where failure is most likely to occur under operating conditions.
Visualization of the strain data in three dimensions allows for a better understanding of how internal forces interact within a complex microelectronic assembly. The results of the fib sem strain analysis are often presented as color coded maps that show the magnitude and direction of the stress. These maps can be overlaid on the structural images of the device to correlate the strain with specific features like microvias or intermetallic layers.
This information is used to validate the results of finite element models and to refine the design of the package. For example, if the analysis shows a high level of strain at the corner of a chip, the engineer might decide to change the underfill material or the geometry of the solder bumps. The ability to see the stress distribution in 3d is particularly useful for analyzing modern stacked die architectures where the interactions between layers are complex.
The final outcome of the analysis is a more durable and reliable product that can withstand the rigors of thermal cycling and mechanical shock.
Coupled hydrostatic stress models predict thermal shock vacancy voiding across ternary intermetallic joints before thermal cycling failure occurs.
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