As a non-volatile memory which may freely perform rewriting and which retains data even if the supply of electricity is stopped, a ferroelectric random access memory (FeRAM) has been known. Since being superior to a flash memory in terms of low-voltage operation, low power consumption, and high-speed writing, the ferroelectric random access memory is a very promising memory device.
As with other semiconductor devices, the ferroelectric random access memory described above is also required to have a higher degree of integration and a higher degree of performance.
In order to improve the degree of integration of the ferroelectric random access memory, reduction in cell area is desired. For the reduction in cell area, it has been known that a stack type cell structure is advantageous compared to a planar type cell structure. In a stack type memory cell, an oxygen barrier film, a lower electrode, a ferroelectric film, and an upper electrode are sequentially formed over a conductive plug connected to a diffusion layer of a transistor to form a ferroelectric capacitor. The oxygen barrier layer functions to prevent oxidation of the conductive plug. In general, a conductive film functioning as both the oxygen barrier layer and the lower electrode is formed, and a laminate film including a titanium nitride film, an iridium film, and an iridium oxide film sequentially laminated to each other has been known to be used for such a conductive film.
In addition, in order to improve the degree of performance of the ferroelectric random access memory, improved electric properties of a ferroelectric capacitor are desired. In order to improve the electric properties of the ferroelectric capacitor, it is desirable to improve the crystalline orientation of its ferroelectric film. The crystalline orientation of the ferroelectric film is influenced by the film quality and the flatness of an underlayer provided under the ferroelectric film.
In order to improve the properties of the ferroelectric film, as a technique to improve the film quality of a lower electrode, a technique has been proposed in which a conductive film composed of a material having self-orientation properties, such as titanium, is formed on a conductive plug and on an interlayer insulating film so as to be used as an underlayer for the lower electrode.
In addition, as a technique to improve the flatness where a conductive plug is formed, a technique has been proposed in which a tungsten plug buried in an interlayer insulating film is etched back, and a recess portion formed by this etching back is filled with titanium nitride for planarization.
However, since titanium nitride has a higher electrical resistance than that of titanium, when the recess portion over the plug is filled with titanium nitride, the electrical resistance increases, and as a result, the electrical properties of a ferroelectric capacitor are degraded.