What Is Aluminum?

Aluminum is a lightweight, silvery-white metal with atomic number 13 and atomic symbol Al, widely used in electrical and electronic engineering for its combination of low density, high electrical conductivity, corrosion resistance, and low cost. In electrical applications, aluminum serves as a conductor in power transmission cables, a metallization layer in integrated circuits, a structural material in electronic packaging, and an alloying component in compound semiconductors such as aluminum gallium arsenide. Its electrical resistivity of approximately 2.65 × 10⁻⁸ ohm-meters is about 60 percent higher than that of copper, but its lower density makes it the preferred conductor wherever weight is a constraint. The study of aluminum in electronics spans materials science, solid-state physics, device fabrication, and reliability engineering.

Aluminum became the dominant interconnect metal in silicon integrated circuits from the 1960s onward because it could be deposited by evaporation or sputtering, patterned by wet or dry etching, and bonded using established wire bonding techniques. Its role in IC fabrication has since been partially replaced by copper at the sub-100-nanometer node, but aluminum remains in use for upper metal layers, bond pad metallization, and specialty processes where copper's higher process complexity is uneconomical.

Aluminum as an Integrated Circuit Conductor

In microelectronic devices, aluminum thin films are deposited on dielectric layers to form the wiring that connects transistors within and across chips. Typical aluminum interconnect lines are 0.3 to 2 micrometers thick and alloyed with 0.5 to 2 percent copper, and sometimes silicon, to improve reliability. The addition of copper was driven by the need to suppress electromigration, a failure mode in which the momentum of conducting electrons displaces aluminum atoms along the direction of current flow, creating voids that eventually open the circuit. Research on electromigration failure modes in aluminum metallization established that electromigration rates follow an Arrhenius relationship with temperature and increase steeply with current density, setting the reliability limits for aluminum interconnect design. Barrier layers of titanium or titanium nitride between the aluminum and the adjacent dielectric further improve electromigration lifetime and adhesion.

Aluminum in Compound Semiconductors

Aluminum appears as an alloying element in several compound semiconductor systems used for high-frequency and optoelectronic devices. In the aluminum gallium arsenide (AlₓGa₁₋ₓAs) alloy system, varying the aluminum mole fraction x tunes the bandgap from 1.42 eV (pure GaAs, x = 0) to 2.16 eV (pure AlAs, x = 1) without significantly changing the lattice constant. The near-perfect lattice match between AlGaAs and GaAs is the basis for high electron mobility transistors (HEMTs): a thin AlGaAs layer grown on a GaAs substrate creates a quantum well at the heterojunction interface where a two-dimensional electron gas forms, providing very high electron mobility and enabling transistor operation at millimeter-wave frequencies. AlGaAs/GaAs HEMTs are used in satellite communications, radar, and low-noise amplifier applications.

Aluminum Sheaths and Structural Uses

In electrical power engineering, aluminum serves as the sheath and armor layer in power cables, enclosing and protecting the insulation from moisture ingress and mechanical damage. Aluminum sheaths are extruded or corrugated around the cable core and also provide a ground return path in high-voltage cables. Aluminum conductor steel-reinforced (ACSR) cables use aluminum strands for the current-carrying outer layers and a high-strength steel core for mechanical support, covering the majority of overhead high-voltage transmission lines worldwide. The Stanford EE311 interconnect notes covering aluminum metallization technology detail the material properties, deposition methods, and failure mechanisms relevant to both IC and power applications.

Applications

Aluminum has applications in a wide range of electrical and electronic fields, including:

• Overhead power transmission lines in ACSR and all-aluminum conductor configurations
• Integrated circuit bond pads and upper-level metal interconnect layers
• AlGaAs/GaAs HEMT devices for microwave and millimeter-wave amplification
• Cable sheaths and armor in underground and submarine power cables
• Aluminum electrolytic capacitors in power supplies and motor drives
• Heat sinks and electronic chassis for passive thermal management