1. Field of the Invention
The present invention relates to a semiconductor device with an insulation film composed of a transition metal oxide with a high dielectric constant, and to a method of manufacturing such a semiconductor device. Particularly, the present invention relates to an insulation film of a capacitor which can drastically reduce leakage current of the capacitor.
2. Description of the Prior Art
A dynamic random access memory (DRAM) is one of the conventional semiconductor devices. The DRAM carries out information storing operations with the combination of capacitors and transistors. In the DRAM, an SiO.sub.2 film which is generally formed between a capacitor electrode and a semiconductor substrate is used as a capacitor insulation film.
With rapid integration of elements, the SiO.sub.2 film is thinned more and more. For example, an SiO.sub.2 film of a 1-megabit DRAM has the thickness of 100 .ANG. or thinner, and a 4-megabit DRAM employs, instead of conventional plane capacitors, trench capacitors with grooves recessed on the surface of a silicon substrate, or stacked capacitors, to increase effective the capacitor areas.
With further integration of an LSI, the area for capacitors of the LSI is reduced increasingly, while the capacitors are required to have the same capacitance as before. Therefore, only thinning the dielectric SiO.sub.2 film can hardly cope with the requirements for integration.
Instead of employing a single SiO.sub.2 film as an insulation film of the capacitor, there it has been proposed to use a laminated structure of an SiO.sub.2 film and an Si.sub.3 N.sub.4 film whose dielectric constant is larger than that of the SiO.sub.2 film, or to use a laminated structure of an SiO.sub.2 film, an Si.sub.3 N.sub.4 film and SiO.sub.2 film. A more integrated VLSI such as a 16-megabit DRAM may only be realized by developing dielectric materials having higher dielectric constants.
Among such dielectric materials having higher dielectric constants, a tantalum oxide Ta.sub.2 O.sub.5 is most widely studied and developed. The reason of this is because a relative dielectric constant of Ta.sub.2 O.sub.5 is 25 to 30 which is six to eight times larger than that of SiO.sub.2 and three to four times larger than that of Si.sub.3 N.sub.4. Therefore, a film thickness necessary for obtaining the same capacitance may be reduced by such magnitude.
However, a leak current of the Ta.sub.2 O.sub.5 film is several order of magnitude larger than that of the Si.sub.3 N.sub.4 film to cause a problem of drastically deteriorating a dielectric strength and the holding efficiency of a memory element.
The reason why the leak current is high in the Ta.sub.2 O.sub.5 film is generally because transition metal oxide films such as the Ta.sub.2 O.sub.5 film of high dielectric constant inherently have a smaller band gap compared to the SiO.sub.2 film. In addition, the Ta.sub.2 O.sub.5 film for example has a problem of oxygen deficiency, i.e., a deviation from a stoichiometric composition. Due to this, the composition fluctuates from stoichiometric valence. Namely, microstructural defects due to the oxygen deficiency in the film increase a leak current of the film.
To lower the, leak current of the Ta.sub.2 O.sub.5 film, there has been proposed the excessive including oxygen of O.sub.2 in the film to reduce an oxygen deficiency densityl, or adding nitrogen N.sub.2 during the formation of the film according to a sputtering technique. These methods may only slightly improve performance of the film, not provide remarkable improvements.
If the leak current of a dielectric material is large, the charge holding function of a capacitor made from the material deteriorates even if the dielectric material has a large dielectric constant. Then, this dielectric material cannot provide a capacitor suitable for forming a memory cell.
As mentioned in the above, the conventional leak current reduction method do not provide remarkable improving effects. Therefore, a new method is needed to reduce the leak current of metal oxides having high dielectric constants.
To prevent Ta components from exceeding the stoichiometric composition, there is a method of introducing excessive oxygen during the film forming process. For example, in reactive sputtering, a Ta target is sputtered in an atmosphere of excessive oxygen and argon to form a Ta.sub.2 O.sub.5 film. Then, a leak current of the Ta.sub.2 O.sub.5 film may slightly be reduced.
For instance, a Ta.sub.2 O.sub.5 film of 250 .ANG. in thickness is reactively sputtered with excessive oxygen on a P-type (100) silicon substrate having a resistivity of 5 .OMEGA.cm and annealed in nitrogen at 600.degree. C. for 60 minutes. With this film, an electric field strength of a capacitor of 0.1 mm.sup.2 will be 2.1 to 2.2 MV/cm with respect to a current of 10.sup.-11 A (10.sup.-9 A/cm.sup.2), and a breakdown electric field of the capacitor will be 5.5 to 6.2 MV/cm. Here, an upper electrode of the capacitor is pure aluminum.
A reference mark "a" in FIG. 1 is a characteristic curve of this sort of dielectric insulation film. A relative dielectric constant of the film is 9.0. A leak current level required for a DRAM of 4 to 16 megabits is 10.sup.-9 A/cm.sup.2. These properties may be converted into SiO.sub.2 equivalent properties to find an equivalent film thickness and an equivalent electric field strength. Then, 108 .ANG. is the equivalent film thickness and 4.8 to 5.1 MV/cm as the equivalent electric field strength are obtained. From these properties, it is apparent that Ta.sub.2 O.sub.5 is inferior to SiO.sub.2 in terms of the leak current. The electric field strength of SiO.sub.2 is 6 to 7 MV/cm.
For the further increase in the relative dielectric constant of the Ta.sub.2 O.sub.5 film, a 900.degree. C. annealing may be effective. However, the leak current is also increased by the 900.degree. C. annealing. A reference mark "b" in FIG. 1 is an I-V characteristic curve of leak currents of the Ta.sub.2 O.sub.5 film annealed in oxygen of 600.degree. C. for 60 minutes and then in argon of 900.degree. C. for 60 minutes. When the leak current is 10.sup.-9 A/cm.sup.2, the electric field strength is 0.7 MV/cm. The relative dielectric constant is 15. Therefore, an SiO.sub.2 equivalent film thickness is 65 .ANG. and an SiO.sub.2 equivalent electric field is 2.7 MV/cm. Namely, the leak current increases further by raising the annealing temperature.
To improve integration, trench capacitors or stacked capacitors are combined with transition metal oxide films having high dielectric constants. To form such oxide films, a chemical vapor deposition (CVD) method will inevitably be used because the CVD method has excellent step coverage characteristics.
When the CVD method is employed to form a tantalum oxide film, however, a stoichiometric ratio of the film greatly fluctuates from an original stoichiometric ratio of 2:5 toward a direction of causing oxygen deficiency to drastically increase the leak current of the film.
There is another problem. When a Ta.sub.2 O.sub.5 film having a high dielectric constant is formed on the surface of a silicon substrate, a native silicon oxide (SiO.sub.2) film having a small dielectric constant tends to be formed between the high dielectric Ta.sub.2 O.sub.5 film and the silicon substrate. The reason why the SiO.sub.2 film is formed at such an interface is not only because of a native oxide film existing on the surface of the silicon substrate before the formation of the high dielectric insulation film but also because oxygen (inside and outside the film) diffuses to the interface after the formation of the high dielectric insulation film. As a result, a dielectric constant of the capacitor insulation film decreases, and capacitance of the capacitor decreases to an unsatisfactory level.
Existence of such an SiO.sub.2 film of low dielectric constant lowers the capacitance of the capacitor. However, if a material of high dielectric constant is used as a capacitor insulation film in order to compensate the decrease in the capacitance, the leakage current of the film may increase to deteriorate the characteristics of an element formed with the capacitor. This is because materials with high dielectric constants generally have small band gaps.
Therefore, the insulation film of high dielectric constant cannot compensate for the layer of low dielectric constant formed at the interface, because it may increase the leakage current of the capacitor to drastically deteriorate the efficiency of the element.