Part 7, Metal Interconnects: Electrical Highways

Semiconductors simultaneously have the properties of “conductor,” which allows electricity to pass through, and “insulator,” which does not. The ion implantation process adds impurities to pure silicon to give it conductive properties. We now have a semiconductor on our hands, which can be made to conduct or insulate as needed. Repeating the photolithography, etching, and ion implantation steps creates countless semiconductor circuits on the wafer. For these circuits to work, an electrical signal must be applied from the outside. Metal interconnects are laid along the semiconductor circuit patterns so that the electrical signal is transmitted through the circuits. Metal interconnects: Electrical highways The metal interconnect process uses the conductive properties of metals. In this step, metal lines are drawn over the circuit pattern of the semiconductor. But not all metals can be used in metal interconnects. To quality for use in semiconductor chips, metals must satisfy the following conditions.

Metals which satisfy the conditions above include aluminum (AI), titanium (Ti), and tungsten (W). Alright, so how are the metal interconnects formed?

Aluminum is the most common material for metal interconnects in semiconductor chips. The metal adheres well to the oxide layer (silicon dioxide) and is easily workable. That said, aluminum (Al) and silicon (Si) tend to mix when they meet. This means that when laying aluminum lines over a silicon wafer, fracturing may occur at the junctions. To prevent this from happening, another metal to serve as a barrier between the aluminum and wafer junctions is deposited. This metal is called a barrier metal. By forming a double film, failure of the junctions can be prevented. The laying of metal wires is also carried out using a deposition process. A metal is placed in a vacuum chamber and boiled or electrically shocked at low pressure, which turns the metal into a gas. The wafer is placed in the vacuum chamber, and a thin metal film is formed on the wafer. Semiconductor processes are increasingly refined and evolving through ongoing research and development. For example, a transition to chemical vapor deposition (CVD) is taking place in the metal interconnects process to form more uniform thin films on smaller areas. To date, we’ve looked at the processes involved in fabricating wafers and designing and creating circuit patterns on the way to creating semiconductor chips. Next time, we’ll look into the final steps in the journey to produce flawless semiconductor products: testing and packaging.