Bond characterization

The wafer bond characterization is based on different methods and tests. Considered a high importance of the wafer are the successful bonded wafers without flaws. Those flaws can be caused by void (composites) formation in the interface due to unevenness or impurities. The bond connection is characterized for wafer bond development or quality assessment of fabricated wafers and sensors.

Wafer bonds are commonly characterized by three important encapsulation parameters: bond strength, hermeticity of encapsulation and bonding induced stress.

The bond strength can be evaluated using double cantilever beam or chevron respectively micro-chevron tests. Other pull tests as well as burst, direct shear tests or bend tests enable the determination of the bond strength. The packaging hermeticity is characterized using membrane, He-leak, resonator/pressure tests.

Three additional possibilities to evaluate the bond connection are optical, electron and Acoustical measurements and instrumentation. At first, optical measurement techniques are using an optical microscope, IR transmission microscopy and visual inspection. Secondly, the electron measurement is commonly applied using an electron microscope, e.g. scanning electron microscopy (SEM), high voltage transmittance electron microscopy (HVTEM) and high resolution scanning electron microscopy (HRSEM). And finally, typical acoustic measurement approaches are scanning acoustic microscope (SAM), scanning laser acoustic microscope (SLAM) and C-mode scanning acoustic microscope (C-SAM).

The specimen preparation is sophisticated and the mechanical, electronic properties are important for the bonding technology characterization and comparison.

Infrared (IR) void imaging is possible if the analyzed materials are IR transparent, i.e. silicon. This method gives a rapid qualitative examination and is very suitable due to its sensitivity to the surface and to the buried interface. It obtains information on chemical nature of surface and interface.

Infrared transmitted light is based on the fact that silicon is translucent at wavelength ≥ 1.2 µm. The equipment consists of an infrared lamp as light source and an infrared video system (compare to figure "Schematic infrared transmission microscopy setup").

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