The invention relates generally to preventing copper oxidation during a copper anneal by conducting the anneal in a reducing environment.
When fabricating integrated circuits, layers of insulating, conducting and semiconducting materials are deposited and patterned to produce desired structures. Metallization processes include contact formation and metal line or wire formation. Contact formation vertically connects conductive layers through one or more insulating layers. Conventionally, contact vias or openings are formed in an insulating layer. The vias are then filled with conductive material, thereby interconnecting electrical devices and wiring above and below the insulating layers. The layers interconnected by vertical contacts typically include horizontal metal lines running across the integrated circuit.
The interconnects in integrated circuits have traditionally been made of aluminum. Recently, as circuit density has increased, copper has become an attractive material in the field due to its lower resistivity and higher resistance to electromigration. In a typical damascene metallization scheme, trenches and vias are filled with copper by seed layer formation followed by an electroplating process, such as electrochemical deposition (ECD).
Following deposition, the copper layer is preferably annealed. The copper anneal is typically carried out at low temperatures, e.g. below 400xc2x0 C. In the annealing process the copper recrystalizes to form a film with low resistance as compared to the film as deposited. The annealed copper also has an increased grain size and a grain structure that improves the physical properties of the copper layer.
Oxidation of the copper layer during the anneal can be detrimental to subsequent processes. As copper oxidizes rapidly at temperatures above 65xc2x0 C., steps are taken to prevent oxidation of the copper layer during the anneal. For example, the reaction chamber in which the anneal is to take place may be sealed by means of a gate valve and purged with a large flow of inert gas to remove all oxygen and water vapor. Such a reaction chamber is described, for example, in U.S. Pat. No. 6,136,163. Accordingly, the process typically employs a high quality vacuum chamber and large quantities of purge gas to keep the oxygen and water vapor levels sufficiently low. Typically the anneal process is operated at atmospheric pressure, although sub-atmospheric pressure anneals are also possible. H2 may be added to the reaction chamber in an attempt to further inhibit oxidation of the copper layer during annealing. However, at the typical annealing temperatures H2 is not very effective at inhibiting oxidation. This is due at least in part to the low reaction rate of the H2. Further, after completion of the anneal, the wafer needs to be cooled down to a sufficiently low temperature in order to prevent oxidation during removal of the wafer from the reaction chamber.
Accordingly, a need exists for more effective methods of preventing oxidation of deposited copper layers during copper anneal processes.
In accordance with one aspect of the present invention, a process is provided for annealing a metal layer on a substrate. The process comprises heating the substrate to a temperature suitable for annealing the metal while contacting the substrate with one or more organic reducing agents. The metal layer is preferably a copper layer.
According to another aspect, the present invention provides a process for producing an integrated circuit, comprising depositing a copper layer on a substrate and annealing the copper layer in the presence of one or more organic reducing agents.
According to yet another aspect, the present invention provides a process for annealing a copper layer during the production of an integrated circuit. A substrate comprising a copper layer is loaded into a reaction chamber and contacted with one or more organic reducing agents while it is heated to a temperature between about 150xc2x0 C. and about 450xc2x0 C.
The organic reducing agents preferably comprise at least one functional group selected from the group consisting of alcohol (xe2x80x94OH), aldehyde (xe2x80x94CHO), and carboxylic acid (xe2x80x94COOH) and are preferably provided in the vapor phase.