Why wafer bonding

Skip to main content Skip to table of contents. This service is more advanced with JavaScript available. Encyclopedia of Microfluidics and Nanofluidics Edition. Editors: Dongqing Li. Contents Search. Wafer Bonding. Authors Authors and affiliations Zheng Cui. This method is most commonly used for highly standardized products like stacked memories. It is generally not suitable for applications with a wide mix of package components. Finally, laser debonding is emerging as a cost-effective and versatile solution.

Light shining through a transparent carrier decomposes the bonding material, causing the bond to fail. Laser debonding is attractive in part because of its digital character. The adhesive is permanent until the laser decomposes it completely.

When more than one of these options can be used, cost is usually the deciding factor. The cost of a debonding process depends on its throughput and the amount of energy — whether supplied by heat, light, or a mechanical tool — needed to drive the process. Mechanical debonding might require less energy than thermal debonding, for example, but be slower than laser debonding. Any additional steps needed to remove adhesive residue will also contribute to the total cost. What about the adhesive?

Chemical and thermal debonding typically use a single layer adhesive, which also serves as a compliance layer, planarizing the bonded surface of the wafer and accommodating any thermal expansion mismatch between the wafer and the carrier. This material must be able to tolerate whatever process steps the wafer will see. Delamination, whether due to stress or chemical decomposition, can cause local deformation of the wafer, misalignment, and other issues.

After debonding, the adhesive should be cleanly removable without leaving residue on either the wafer or the carrier. Mechanical and laser debonding, in contrast, frequently use a dual layer temporary adhesive.

The first layer, applied to the wafer, is a planarizing adhesive. The second layer, in contact with the carrier, is a release layer. In laser debonding, the laser decomposes the release layer without affecting the adhesive, which can be removed by conventional de-taping methods.

These defects appear to occur if a pocket within the adhesive layer decomposes, producing gases that can push the release layer aside. If the adhesive contacts the carrier directly, the debonding laser will be unable to release that area.

The debonding front propagates along the clearly defined interface between the two materials. Depending on the packaging scheme, permanent bonding might involve either complete wafers or singulated dice, connected either to other wafers or to redistribution layers or interposers. Wafer-to-wafer bonding is preferable to chip-to-chip or chip-to-wafer bonding because it offers high throughput and simplifies alignment between the layers of the structure. Generally speaking, the bonding surfaces need to be planar and clean.

Topography can cause voids or misalignment, while adhesive residue and metallic oxides can degrade electrical conductivity. Aluminum metallization is appealing because of its compatibility with CMOS processes, but surface roughness and rapid oxidation are significant obstacles. The IHP group used a vacuum bonding process, etching the bond pads to produce a clean surface.

Wafer bonding processes based on copper metallization can use BEOL inlaid copper damascene methods to form the bond pads. Either the dielectric must resist diffusion — BCB resin, for instance — or a separate diffusion barrier, typically SiC, is needed.

While CMP is a well-established and mature process for BEOL metallization, wafer-to-wafer bonding can present dramatic topography variations with scribe lanes and large copper bond pads. Moreover, the wafers being bonded might come from completely different fabs, with different processes and specifications.

The package assembly process must be able to account for these variations. Uhrmann described a hybrid bonding process developed in collaboration with Imec. It depends on van der Waals forces to draw the two wafers together, facilitated by a thin layer of moisture at the interface.

Wafer bonding has various applications: packaging e. The System Packaging department offers standard wafer level bonding technologies such as silicon direct, anodic, glass frit and adhesive bonding. These technologies can be adapted according to the specific requirements of target applications. Other bonding techniques including thermos-compression, solid-liquid inter-diffusion, eutectic, surface activated and reactive bonding are also available in the department and can be optimized for different requirements e.

Other approaches for wafer bonding are primarily driven by the emergence of new materials and processes in micro systems technology.