1.  Technical Field
The present disclosure relates to the fabrication of integrated circuit transistors, and in particular, devices that are fabricated using a replacement metal gate process. 
2.  Description of the Related Art
An integrated circuit chip typically includes precision resistors that can be accessed by circuit designers for use by various devices on board the chip.  For example, resistors may be needed to regulate power supplied on board the chip, or to fulfill impedance matching requirements for communications functions that may need certain input or output resistances.  In the past, when transistor gates were made of polysilicon, it was straightforward to pattern precision resistors as part of the polysilicon mask layer, thus integrating the fabrication of the resistors with that of the active devices. 
With the development of metal gate transistors, there has arisen an incompatibility between metals that are suitable for use as transistor gates and metals that are suitable for use as precision metal resistors.  For example, aluminum, often used for metal gates, lacks the resistivity needed for precision metal resistors.  Materials such as tantalum nitride (TaN), tungsten (W), titanium nitride (TiN), titanium aluminum (TiAl) or other metals having a similar work function have been substituted to form the integrated resistors.  When material properties are not interchangeable, however, it is generally not feasible to deposit and pattern precision resistors as part of the metal gate mask layer.  A drawback to building resistors out of such metals is that the use of these additional materials necessitates extra equipment and process steps, and generally increases the complexity, and therefore the cost, of the semiconductor manufacturing process.  Generally, it is desirable to fabricate precision resistors from a material that is already available in the existing process flow for metal gate transistors.