Meaning
A thin-film metal layer deposited onto a semiconductor wafer’s bond pads serves as the structural and electrical interface between the chip’s internal circuitry and the external solder bumps. This critical multi-layer structure, known as under bump metallization, provides a low-resistance electrical path while protecting the underlying aluminum or copper circuits from the solder alloys. In advanced microelectronic packaging, the direct contact of solder with the chip’s metal pads can cause rapid interdiffusion and corrosion, destroying the electrical connection.
Under bump metallization acts as a barrier to prevent this interdiffusion, as an adhesion layer to secure the solder bump to the pad, and as a wettable surface that allows the molten solder to flow and form a strong mechanical bond during the reflow process. The design and composition of this metal stack are essential for ensuring the mechanical strength and electrical reliability of flip-chip and wafer-level packages.
Layer Stack
The metallization structure is typically composed of three or four distinct sub-layers, each designed to perform a specific function within the interconnect. The first sub-layer is the adhesion layer, often made of titanium or chromium, which bonds strongly to both the chip’s dielectric passivation layer and the metal pad. The second is the diffusion barrier layer, usually consisting of nickel or platinum, which blocks the migration of solder metals into the underlying chip circuitry.
The third is the wettable layer, often made of copper, which dissolves partially during reflow to form a stable intermetallic compound with the tin in the solder, creating the mechanical joint. In some designs, a thin protective layer of gold or silver is deposited on top of the stack to prevent the wettable layer from oxidizing before the solder is applied, ensuring a clean and reliable interface.
Deposition Technique
The application of the metal layers to the semiconductor wafer is performed using highly controlled manufacturing processes to ensure uniform thickness and adhesion across all bond pads. The primary deposition methods used in the industry are physical vapor deposition, such as sputtering, and electrochemical plating. Sputtering is typically used to deposit the thin adhesion and barrier layers because of its high precision and excellent film quality.
Plating is then used to build up the thicker copper and nickel layers required for high-current applications, providing a cost-effective method for high-volume manufacturing. After deposition, the metal stack is patterned using photolithography and wet etching to define the individual pads, ensuring that the under bump metallization is perfectly aligned with the chip’s bond pads and microbumps.
Reliability Role
The structural integrity and electrical performance of the metallization stack are major determinants of the long-term reliability of the packaged semiconductor device. During operation, the under bump metallization must withstand the mechanical stresses generated by the thermal expansion mismatch between the silicon die and the substrate, preventing the solder joints from delaminating or cracking. A well-designed barrier layer also limits the growth of brittle intermetallic compounds, which can weaken the joint and lead to premature failure under mechanical shock.
Furthermore, the metallization layers must distribute the high current densities passing through the microbumps to prevent localized hot spots and electromigration, which would otherwise cause rapid void growth and open circuits. By optimizing the material selection and layer thicknesses of the metal stack, engineers can maximize the lifespan and durability of the high-performance package.
