In the race to improve transistor performance as well as to reduce the size of transistors, much research has been done on the contacts that connect a transistor's source and drain regions to the various layers of metallization over the transistor. In the search for better contacts, copper has been researched as a potential method to improve the resistance of the contact, as well as the overall performance of the device as a whole. However, the use of copper has some inherent problems that must be overcome, such as copper's propensity to migrate and cause unwanted reactions with other materials.
FIG. 1 illustrates one attempt to overcome some of these problems of using copper as a contact. FIG. 1 illustrates a substrate 101 with shallow trench isolation (STI) 103, and a device 100 formed on the substrate 101. The device 100 comprises a gate dielectric 105, gate electrode 107, spacers 109, source/drain regions 111, and silicide contacts 113. An inter-layer dielectric (ILD) 115 covers the device 100. Openings are formed in the ILD 115, and contact barrier layers 117, made of a material such as cobalt tungsten phosphide (CoWP), are formed on the silicide contacts 113. A barrier layer 119 is then formed in the openings and the openings are overfilled with copper 121, planarized by a method such as CMP, and connected to later formed metal lines 123 within a dielectric layer 125. The contact barrier layers 117 help to reduce the contact resistance and prevent the copper 121 from migrating and reacting with the silicide contacts 113.
However, this solution, while an improvement over previous methods, does not by itself solve the problems with using copper as a contact. Certain properties of the contact barrier layers 117 need further improvement for its full application in devices. For example, the contact barrier layers 117 formed as described above have grain boundaries at their interfaces with other materials that are unfilled. These unfilled grain boundaries increase the potential contact resistance, making this solution less desirable.
Because of these and other problems associated with the current methods, what is needed is a contact barrier that has improved properties to lower the contact resistance of the contact and work to prevent copper from migrating and having unwanted reactions with adjacent layers.