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This article is our ninth in a series intended as an overview for those who are not technical specialists in their field. We will discuss metallization in semiconductor processing. 02
During the medium-scale integration (MSI) era, metallization was straightforward, requiring only a single metal process. Contact holes were etched through the surface layers to the device/circuit component parts. As chip density has increased, more components are on the wafer surface, which has lead to decreased area available for surface wiring. This issue has lead to multilevel metallization schemes. Below we will discuss the primary materials used for metal interconnection layers.
Before the development of very large-scale integration (VLSI), pure aluminum was used as the primary metallization material. Some early metallization schemes used gold, but gold has a high contact resistance with silicon and required a top layer of molybdenum to over come its softness. Copper was occasionally used as a replacement for aluminum, but it has a high contact resistance with silicon and raises problems with device performance if it gets into the device area. Aluminum became the preferred metal because these potential problems were avoided. Its resistivity is low enough, and possesses a good current-carrying density. Aluminum has superior adhesion to silicon dioxide, is available in high purity, has a naturally low contact resistance with silicon, and is relatively easy to pattern.
Issues with Aluminum
One of the first issues with pure aluminum leads was shallow junctions in the wafer. The problem occurred with the need to bake aluminum-silicon interfaces to stabilize the electrical contact. The aluminum and silicon dissolved into each other however, due to a eutectic formation point. A eutectic formation point exists when two materials heated in contact with each other melt at temperatures lower then their individual melting temperatures. Two solutions to this problem are possible. A barrier metal layer can be used, that separates the aluminum and silicon and prevents the eutectic alloy from forming. Or, an alloy of aluminum with 1 to 2% silicon can be used. During the contact heating process, the aluminum alloys more with the silicon in the alloy and less with the silicon from the wafer. Other drawbacks of aluminum alloys include increased complexity for the deposition equipment and process, different etch rates, and an increase in film resistivity.
As individual devices get smaller and circuit operate faster, signals must move fast enough through the metal system to prevent processing delays. Circuit speeds can become restricted with aluminum. The small contract resistance between aluminum and silicon surfaces add up to a restrictive level as the number of circuit components grow. Even though aluminum provides a workable resistance, it is difficult to deposit in high aspect ratio vias/plugs. Copper is a better conductor than aluminum with a resistance of 1.7 micro-ohm cm compared to the 3.1 micro-ohm cm of aluminum. Copper is also resistant to electromigration and can be deposited at low temperatures.
Issues with Copper
Some drawbacks of copper include etching problems, vulnerability to scratching, corrosion, and the requirement of barrier metals to keep the copper out of the silicon.
As mentioned earlier, barrier metals are often used to prevent the eutectic alloying of silicon and aluminum metallization. Metals that are often used as barriers aluminum metallization include titanium-tungsten (TiW) and titanium nitride (TiN). TiW is sputter-deposited onto the wafer into the open contacts before the aluminum or aluminum alloy deposition takes place. The TiW deposited on the field oxide is removed from the surface during the aluminum etch step. Titanium nitride layers can be placed on the wafer using various deposition techniques, such as evaporation, sputtering, and CVD. A layer of titanium is required under TiN films to provide a high conductivity intermediate with silicon substrates. Barriers are also necessary for copper metallization, as copper inside the silicon ruins device performance. Metals used for barriers in copper metallization include TiN, tantalum (Ta) and tantalum nitride (TaN).
We hope that you found this review of metallization helpful. Please feel free to comment below and let the bloggers at Glew Engineering know if there is a specific topic you’d like us to blog about in the future.
Van Zant, P. (2000). Microchip fabrication, a practical guid to semiconductor processing. (4th ed.). New York, NY: McGraw-Hill.