The Characterization of Bonded Wafers

The quality of the fabrication process and the quality of finished devices depends on the characterization of the bonded wafers. In addition to destructive methods, such as tensile or shear testing, optical measurements are frequently the method of choice when it comes to evaluating bond strength.

The Characterization of Bonded Wafers

Image Credit: FormFactor Inc.

Critical elements of semiconductor-based devices include adhesives. On printed circuit boards, the glues are used to attach, touch, and encapsulate wafers, chips, as well as other microelectromechanical components.

Glue bonding, also known as adhesive bonding, is a wafer bonding technique used in the semiconductor industry. It involves applying an intermediary glue layer to join substrates made of various materials.

The quality of the fabrication process or the finished devices depends on the classification of the bonded wafers.

In addition to destructive methods, such as tensile or shear testing, optical measurements are frequently used to evaluate bond strength.

These non-destructive optical techniques identify bond layer (adhesive) homogeneity and defects such as voids or fissures in various layers.

The FormFactor FRT IRT sensors were created to measure the thickness of materials that are transparent to near-infrared light. The IRT is an infrared light source and an interferometric film thickness sensor.

Measurements can be made at a single spot, along a profile, or by mapping the film thickness over a broader area. The primary function of these sensors is to measure wafer thicknesses accurately. However, single levels of multi-layer systems can also be defined.

Measurement of Bond Layer Thickness.

Measurement of Bond Layer Thickness. Image Credit: FormFactor Inc.

Measurement of Wafer thickness and Membrane thickness.

Measurement of Wafer thickness and Membrane thickness. Image Credit: FormFactor Inc.

Significant benefits of the FormFactor FRT IRT 800 sensor’s most recent generation include:

More room in the tool due to the smaller size of the sensor electronics. The lateral resolution has enhanced up to 5.5 μm, enabling the measurement of tiny objects like scratches or voids.

Detection of voids in the bond layer.

Detection of voids in the bond layer. Image Credit: FormFactor Inc.

The possibility of quantifying 55 μm thicker layers, thanks to the expanded film thickness measuring range of up to 3200 μm. Silicon can currently be measured up to a maximum thickness of 850 μm.

The measurement range must be divided by the material’s refractive index in the calculation, e.g., standard silicon wafers have a refractive index of n ≈ 3.6 (3200 μm/ 3.6 ~880 μm).

The FRT IRT 80 sensor is available from FormFactor for thin films (4-300 um/n).

The IRT 800 sensor now measures film thickness five times quicker than before, at 20.000 measurements per second (20 hHz), which is the ideal combination.

The table below displays all of the parameters.

The Characterization of Bonded Wafers

Image Credit: FormFactor Inc.

The video below demonstrates how to measure bonded wafers:

FRT Metrology - Bonded Wafer Measurement

Video Credit: FormFactor Inc.

This information has been sourced, reviewed and adapted from materials provided by FormFactor Inc.

For more information on this source, please visit FormFactor Inc.

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