The invention relates. to semiconductor devices, and, more particularly, to integrated circuit interconnects and methods of fabrication.
Integrated circuits typically include field effect transistors with source/drains formed in a silicon substrate and insulated gates on the substrate together with multiple overlying metal (or polysilicon) interconnects formed in levels. An insulating layer lies between the gates/sources/drains and the interconnects formed from the first metal level (premetal dielectric) and also between successive metal levels (intermetal-level dielectric). Vertical vias in the insulating layers filled with metal (or polysilicon) provide connections between interconnects formed in adjacent metal levels and also between the gate/source/drain and the first metal level interconnects.
Typically the metal interconnects are made of aluminum due to its high conductivity and ease of fabrication. However, poor electromigration and stress voiding characteristics have driven the need to improve the reliability of aluminum interconnects. Alloying, dopant precipitation and intermetallic reaction layer formation affect electromigration, stress-migration robustness and mechanical properties of aluminum films. The simplest approach is to dope the aluminum with small amounts (0.5-4% by weight) of copper. Alloys with high copper content are difficult to etch and also have a tendency for whisker formation depending on the nature of the intermetallic precipitates that are formed. Excessive copper leads to etch residues and an increased tendency for metallic corrosion. These typically constrain the copper doping to below 2%.
Reliability of aluminum interconnects is improved by sandwiching the aluminum between other materials such as TiN and Ti. TiN reduces the tendency for hillock/void formation, serves as an anti-reflection coating to ease patterning, provides a current shunting path if voids are indeed formed, improves the texture of the aluminum, prevents stress migration at vias, and acts as a barrier to prevent interaction between the aluminum and other materials in the device structure. Titanium improves the texture of the aluminum and reacts with aluminum to form an intermetallic with improved electromigration characteristics. Preferred crystallographic orientation has long been recognized to playt an important role in interconnect reliability (i.e., thermal hillocks, grain collapse, stress voiding, and electromigration). A strong (111) texture improves electromigration lifetime. Deposition conditions and the presence of underlayers have the greatest influence on texture. As linewidths are scaled to below 0.5 xcexcm, the percentage of Al surface area that is encapsulated with Ti and TiN gets smaller, and the effectiveness of this approach diminishes. For a 0.35 xcexcm wide line, only the top and bottom surfaces can be protected with Ti/TiN layers, while the sides (0.6 xcexcm tall) remain bare. Metal stacks are also harder to etch since a multiple step etch. with high selectivity to the underlayers is necessary.
Deposition conditions can also improve interconnect reliability. Higher deposition temperatures incrase grain size (electromigration performance is typically superior for larger grain sizes), but contact/via reliability can degrade. for high thermal budget processes. Thus there is a need for a method that provides complete Al encapsulation. One approach treats aluminum lines with a wet chemical bath of H2O2 to oxidize the surfaces for passivation.
In general, addition of dopants (e.g., arsenic, antimony, metals, oxygen, nitrogen, and fluorine) into aluminum can modify electrical and mechanical properties. For example, arsenic improves elecrotromigration, and fluorine reduces stress-induced hillock formation.
The present invention provides doping of interconnect surface regions to encapsulate the interconnect and improve properties such as electromigration and corrosion resistance and also serves as a barrier. In particular, preferred embodiments include implants of dopants into the sidewalls of clad or unclad aluminum interconnects and dopant implants into sidewalls of trenches formed in dielectrics followed by diffusion into copper interconnects filling the trenches.