TaO_xN_y: A Promising Alternative to Ta₂O₅

		At a Glance
	Although tantalum pentoxide (Ta2O5) is a promising material for the high dielectric constants that will be required in next-generation memories, there is room for improvement. We have developed a process for producing TaOxNy high-k dielectric materials that demonstrate characteristics that are superior to those of traditional Ta2O5.

Next-generation memory devices will require materials with higher dielectric constants. Tantalum pentoxide (Ta₂O₅) is one of the most promising materials because it has a high dielectric constant (~25), meets the requirements for conventional VLSI processes, and has comparatively low leakage current.^1-3

However, compared with some types of ferroelectric dielectrics, such as (Ba, Sr)TiO₃, the dielectric constant of Ta₂O₅ films is modest. If the dielectric constant of Ta₂O₅ films can be physically improved, their integration into next-generation ULSI devices may be more readily achievable.

It is known that a plasma anneal in a nitrogen ambient can significantly reduce leakage current and trap density in the film.⁴ In addition, it has been reported that the dielectric constant of Ta₂O₅ films deposited on Ru can be significantly improved by a rapid thermal nitridation.⁵ In this article, we describe how we deposited TaO_xN_y films using NH₃ and Ta(OC₂H₅)₅ source materials, analyzed the impact of nitrogen on the Ta₂O₅ film, and carefully compared the electrical characteristics of TaO_xN_y to traditional Ta₂O₅ films.

Experiment

Research was performed using Jusung's EUREKA2000 CVD tool, which is distinguishable from conventional single-wafer CVD reactors in that it utilizes a gas boundary layer. It uses an injector system to obtain good film uniformity without a showerhead. It has a warm wall heating method that uses two heat sources to maximize the heat capacity and thermal stability in the reactor. Therefore, the incoming gases are preheated to minimize heat loss at the wafer surface, providing optimal control of temperature uniformity.

The reactor can also perform plasma-enhanced CVD (PECVD) and plasma-enhanced dry cleaning. It is equipped with remote and afterglow plasma discharge technology to optimize the PECVD, in situ dry wafer cleaning, and in situ chamber cleaning processes.

1. RBS spectra of 30 nm-thick TaOxNy film deposited with 25 sccm of NH3.

The precursor for Ta₂O₅ deposition, Ta(OC₂H₅)₅, is a liquid source. Thus, we had to convert liquid states of Ta(OC₂H₅)₅ into gas states for the Ta₂O₅ deposition process, requiring a liquid delivery system (LDS). We used an LDS designed by Jusung that features an independent dry pump in to ensure the repeatability of liquid flow and wafer-to-wafer thickness uniformity. The LDS also contains ethanol and gas purge lines for cleaning.

TaO_xN_y-based capacitors were fabricated using phosphorus-doped polycrystalline silicon as a bottom electrode. As a first step, the wafers were cleaned using 100:1 DHF.

Next, 1.2 nm-thick nitride films were grown by NH₃ plasma at 430°C to prevent oxidation from forming at the interface during the Ta₂O₅ deposition or during post-treatment. After nitridation, 8 nm-thick Ta₂O₅ films were deposited using Ta(OC₂H₅)₅ and O₂.

2. Capacitance (a) and leakage current (b) of Ta2O5 and TaOxNy films are compared relative to bias voltage.

Separately, for purposes of comparison, 8 nm-thick TaO_xN_y films were deposited using Ta(OC₂H₅)₅ and NH₃. For RBS (rutherford backscattering spectroscopy) measurements, 30 nm-thick Ta₂O₅, TaO_xN_y films were also deposited.

After deposition, wafers were annealed with an O₂ plasma at 430°C and 700°C. In the case of the wafers annealed at 430°C, an additional annealing process and wet reoxidation were performed at 450°C for 10 min to improve film quality. This also reduced the leakage current.

Finally, Pt or TiN films were deposited by evaporation or by CVD to form the top electrode of the MOS capacitor.

Results and discussion

Figure 1 shows the atomic composition (measured by RBS) of the 30 nm-thick TaO_xN_y film deposited with 25 sccm of NH₃. As expected, nitrogen incorporation was detected in the TaO_xN_y films. The Table shows that the composition of nitrogen increases, and the composition of oxygen relatively decreases in the TaO_xN_y films with an increasing NH₃ flow rate. Therefore, we find a possibility that nitrogen substitutes for oxygen sites.

Table. Composition of As-Deposited TaOxNy Films
NH3(sccm)	Ta (at%)	O (at%)	N (at%)	O/Ta
0	30.4	69.6	—	2.29
15	22.0	56.0	22.0	2.55
25	22.0	52.0	26.0	2.36
35	22.0	51.0	27.0	2.32

In addition, compared with the O/Ta ratio of as-deposited Ta₂O₅ and TaO_xN_y, the O/Ta ratio of TaO_xN_y is higher than that of Ta₂O_5. Now we can think of a model in which nitrogen plays the role of a catalyst, with oxygen being filled to the vacant sites of tantalum pentoxide films.

However, it has been reported that nitrogen in films disappears by outgassing, and then the TaO_xN_y phase changes to the Ta₂O₅ phase with less than 3% of nitrogen when TaO_xN_y films are annealed at high temperature.⁶ If the nitrogen makes a stable chemical bonding with tantalum, it is possible to make phases like Ta₃N₅, TaN or TaON. If the TaO_xN_y films change, even a little, into conductors such as Ta₃N₅ or TaN, the films will have a higher leakage current than Ta₂O₅. But, as shown in Figures 2b and 3, the leakage current of TaO_xN_y films is lower than that of Ta₂O₅. So we believe that TaO_xN_y films will have the structure of nitrogen-doped Ta₂O₅ involving nitrogen in interstices or voids.

3. Leakage current behavior of 8 nm-thick Ta2O5 and TaOxNy films annealed with O2 plasma at 700°C.

As shown in Figure 2a, after deposition, anneal with O₂ plasma at 430°C, wet reoxidation at 450°C and then deposition of the Pt electrode, the capacitance of TaO_xN_y films is slightly higher than that of Ta₂O₅. As shown in Figure 2b, the leakage current of TaO_xN_y is lower than that of Ta₂O₅. Therefore, we believe that the better leakage current of TaO_xN_y film is due to the lower oxygen vacancy.