Cyclic Chemical-Vapor-Deposited TiO[sub 2]∕Al[sub 2]O[sub 3] Film Using Trimethyl Aluminum, Tetrakis(diethylamino)titanium, and O[sub 2]

Abstract

Titanium aluminum oxide films have been studied as potential alternative gate dielectrics. However, most studies have focused on sputtered films. In this study, we demonstrate that a combination of tetrakis͑diethylamino͒titanium, trimethyl aluminum, oxygen, and cyclic chemical vapor deposition ͑CVD͒ is a promising approach for laminated TiO 2 /Al 2 O 3 films with low impurities and high thermal stability even at low temperatures. The growth of the films is carried out in a cold-wall CVD chamber at 300°C and 0.7 Torr. Our studies show that the properties of TiO 2 improve with the addition of even a few percent of Al 2 O 3 . X-ray diffraction analyses indicate that as-deposited TiO 2 /Al 2 O 3 films have amorphous structure. Upon annealing as-deposited films in Ar at 700°C for 5 min, TiO 2 /Al 2 O 3 films maintain their amorphous structure, while pure TiO 2 films crystallize at these conditions. Atomic force microscopy shows that the surfaces of TiO 2 /Al 2 O 3 films are smoother than those of TiO 2 films deposited at the same conditions. Even though annealing increases the roughness of the TiO 2 /Al 2 O 3 films, film roughness is still significantly lower than that of as-deposited TiO 2 films. Moreover, Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy show that there is no detectable formation of interfacial silicon oxide and negligible carbon impurity in as-deposited TiO 2 /Al 2 O 3 films.The continuous decrease of transistor feature sizes and related concerns of high tunneling leakage current and low gate capacitance of ultrathin SiO 2 have been driving the development of alternative dielectric materials with higher permittivity. 1 Alumina ͑Al 2 O 3 ͒ is a promising gate dielectric material. It is stable on Si and remains amorphous up to high temperatures. Also, it has the largest bandgap ͑8.8 eV͒ next to SiO 2 and has high band offset with respect to Si ͑2.8 eV for conduction band offset and 4.9 eV for valence band offset͒. 1 However, its dielectric constant, k, ͑in the range of 8-10͒ makes it a relatively short-term solution for industry needs. 1 TiO 2 films have attracted a lot of attention due to their higher dielectric constant, up to 80. 2 However, issues such as several stable oxidation states, 1 low bandgap ͑3.0-3.5 eV͒, 3 low crystallization temperature ͑around 300-400°C͒, 4 and instability on Si 5 induce high leakage current and impede the application of TiO 2 as gate dielectric.Recently, many studies focus on heterogeneous materials, especially silicates and aluminates, 6 which may modify film properties and overcome the limitations of TiO 2 films. Among these composite films, titanium aluminum oxide thin films have emerged as one of the promising alternative high-k materials. It has been reported that alloying titanium oxide and aluminum oxide yields TiAlO materials that retain a high permittivity close to that of TiO 2 7,8 and also have excellent thermal stability like that of Al 2 O 3 . 8,9 In addition, the high oxygen affinity of Al 2 O 3 decreases the reduction reaction...

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