Wafer bond characterization
[1] The bond strength can be evaluated using double cantilever beam or chevron respectively micro-chevron tests.
[1] Three additional possibilities to evaluate the bond connection are optical, electron and Acoustical measurements and instrumentation.
The specimen preparation is sophisticated and the mechanical, electronic properties are important for the bonding technology characterization and comparison.
This method gives a rapid qualitative examination[4] and is very suitable due to its sensitivity to the surface and to the buried interface.
Infrared transmitted light is based on the fact that silicon is translucent at wavelength ≥ 1.2 μm.
The IR imaging system enables the analysis of the bond wave and additionally micro mechanical structures as well as deformities in the silicon.
Usually if using monochromatic color IR the center of the wafers is display brighter based on the vicinity.
Particles in the bond interface generate highly visible spots with differing contrast because of the interference (wave propagation) fringes.
[4] The Fourier transform infrared (FT-IR) spectroscopy is a non-destructive hermeticity characterization method.
[6] Ultrasonic microscopy uses high frequency sound waves to image bonded interfaces.
The lateral resolutions depends on the ultrasonic frequency, the acoustic beam diameter and the signal-to-noise ratio (contrast).
[3] The DCB test characterizes the time dependent strength by mechanical fracture evaluation and is therefore well suited for lifetime predictions.
[9] A disadvantage of this method is, that between the entering of the blade and the time to take the IR image, the results can be influenced.
In addition, the measurement inaccuracy increases with a high surface fracture toughness resulting in a smaller crack length or broken wafers at the blade insertion as well as the influence of the fourth power of the measured crack length.
[10] In literature different DCB models are mentioned, i.e. measurement approaches by Maszara, Gillis and Gilman, Srawley and Gross, Kanninen or Williams.
The chevron test uses a special notch geometry for the specimen that is loaded with an increasing tensile force.
The chevron notch geometry is commonly in shape of a triangle with different bond patterns.
The advantage using chevron notch specimen is due to the formation of a specified crack of well-defined length.
[12] The disadvantage of the approach is that the gluing required for loading is time consuming and may induce data scatter due to misalignment.
The reliability characterization is determined based on the fracture mechanical evaluation of critical failure.
The space of the tip of the chevron structure triangle is used as lever arm for the applied force.
[12] The MC test is applied with special specimen stamp glued onto the non-bonded edge of the processed structures.
It is an effective, reliable and precise approach for the development of wafer bonds as well as for the quality control of the micro mechanical device production.
LBI provides nondestructive testing of bonds that were adequately prepared and meet engineering intent.
[16] Measuring bond strength by pull testing is often the best way to get the failure mode in which you are interested.
Additionally, and unlike a shear test, as the bond separates, the fracture surfaces are pulled away from each other, cleanly enabling accurate failure mode analysis.
In these cases, a set of accurately formed and aligned tweezer tips with precision control of their opening and closing is likely to make the difference between success and failure.
[18] White light interferometry is commonly used for detecting deformations of the wafer surface based on optical measurements.
The cavity length d corresponds to the applied pressure and is determined by the spectrum of the reflection of the light of the sensor.
The advantage of using white light interferometry as characterization method is the influence reduction of the bending loss.
