Structure and electrical properties of sputtered TiO2/ZrO2 bilayer composite dielectrics upon annealing in nitrogen
© Dong et al; licensee Springer. 2012
Received: 20 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
The high-k dielectric TiO2/ZrO2 bilayer composite film was prepared on a Si substrate by radio frequency magnetron sputtering and post annealing in N2 at various temperatures in the range of 573 K to 973 K. Transmission electron microscopy observation revealed that the bilayer film fully mixed together and had good interfacial property at 773 K. Metal-oxide-semiconductor capacitors with high-k gate dielectric TiO2/ZrO2/p-Si were fabricated using Pt as the top gate electrode and as the bottom side electrode. The largest property permittivity of 46.1 and a very low leakage current density of 3.35 × 10-5 A/cm2 were achieved for the sample of TiO2/ZrO2/Si after annealing at 773 K.
High dielectric constant [high-k] materials have been researched for a few years in material science and have been applied firstly in Intel's 45 nm MOSFET in 2007. Nowadays, for the demand of the next generation devices for sub-22 nm technology nodes, expect that high-k materials such as HfO2, ZrO2, Ta2O5, and rare earth oxides are extensively researched, and binary oxides of high-k materials become more attractive and are expected to be utilized in the future ultra large scale integrated circuit [1–8]. Among them, ZrO2 has a relatively high permittivity, large band gap, and good thermal and chemical stabilities. TiO2 is a high-k material with a very high permittivity of about 80 . In order to improve the permittivity of ZrO2, the feasible way is to fabricate ZrO2-TiO2 composite films. Meanwhile, as a composite thin film, the addition of TiO2 can improve the crystallization temperature [10, 11]. As ZrO2-TiO2 binary oxides, a nanolaminate structure which can tailor the electrical properties of dielectric stacks has many applications such as MIM diodes, storage capacitors, non-volatile memories, and transparent thin film transistors; thus, the nanolaminated ZrO2-TiO2 high dielectric constant thin film is worth studying
Concerning high-k stacks on silicon, the interface has an important role to influence the device. Normally, it is often thought that TiO2 is easier to react with the Si substrate which may deteriorate the property of the device, and thus, TiO2/ZrO2/Si stacks may have better electrical characterization [12–14]. In the present work, metal-oxide-semiconductor [MOS] capacitors with high-k gate dielectric TiO2/ZrO2/p-Si were fabricated using Pt as the top gate electrode and as the bottom side electrode. The structure and electrical property of the TiO2/ZrO2/Si stack are studied.
ZrO2 and TiO2 thin films were grown onto p-type (100) Si (P~1015 cm-3) to fabricate TiO2/ZrO2/Si stacks by radio frequency magnetron sputtering at room temperature. Pure ZrO2 (99.999%) and TiO2 (99.999%) ceramic targets (50 mm in diameter) were used as the sputtering targets. The sputtering power of ZrO2 and TiO2 are 60 W and 30 W, respectively. Pure argon (99.999%) with 30 cm3/min flow rate controlled by a mass flow controller was used as sputtering gas, and the base pressure of the vacuum chamber is about 3 × 10-5 Pa. Sputtering was carried out at a pressure of 0.3 Pa. As for the deposited TiO2/ZrO2/Si stacks, post annealing of 573 K, 773 K, and 973 K in N2 for 30 min was performed.
The structural characteristics of the films were investigated by X-ray diffraction [XRD] (Bruker D8, Bruker, Billerica, MA, USA) and transmission electron microscopy [TEM] (FEI Tecnai G20, FEI Co., Hillsboro, OR, USA). Film thickness was determined by an ex situ phase-modulated spectroscopic ellipsometry [SE] (Model Jobin Yvon, HORIBA Jobin Yvon Inc., Edison, NJ, USA) over the spectral range of 1.5 to 6.5 eV at an angle of incidence of 70°. For the purpose of exploring electrical properties, a Pt/TiO2/ZrO2/p-Si MOS capacitor was fabricated by sputtering a Pt top electrode with an area of 1.96 × 10-7 m2 through a shadow mask. The back side of the wafer was HF-cleaned, and the Pt thin film was deposited. The MOS capacitors were electrically characterized using a Radiant Precision Premier (Radiant Technologies Inc., Albuquerque, NM, USA) tester system to obtain current-voltage [I-V] curves. Capacitance-voltage [C-V] measurements were performed by a precision LCR meter (Agilent 4294A; Agilent Technologies Inc., Santa Clara, CA, USA).
Results and discussion
The chemical composition of the TiO2/ZrO2/Si film can be measured by XRF, and all samples have nearly the same atomic Ti content of 21%, which indicates that the annealing process did not change the composition. Concerning the Ti content in the TiO2-ZrO2 binary system, the optimal content of about 21% has been verified in our previous work .
Spectroscopic ellipsometry was employed to measure the film thickness. The Tauc-Lorentz model which is especially suitable for an amorphous material was adopted to characterize the dielectric function of the TiO2/ZrO2 bilayer composite film [15–17]. In order to get the best fitting of SE data, different models were built due to the structure change of the TiO2/ZrO2 bilayer composite film. For the as-deposited thin film, a double layer optical model was built on Si (100) substrate, i.e., ZrO2 layer (L1) and TiO2 layer (L2), while for the annealed one, only one layer of the ZrO2-TiO2 composite thin film was built. Lastly, we can obtain the thickness of the as-deposited thin film with a ZrO2 layer (L1) of 27.639 ± 0.521 nm and TiO2 layer (L2) of 10.077 ± 0.627 nm. For the sample annealed at 773 K, the total thickness is 28.149 ± 1.102 nm. This result indicates that annealing makes the film denser and decreases the thickness.
The high-k dielectric TiO2/ZrO2 bilayer composite film was prepared on a Si substrate by radio frequency magnetron sputtering and post annealing in N2 at various temperatures in the range of 573 K to 973 K. The bilayer film fully mixed together to become a composite single layer and has good interfacial property after annealing at 773 K. The largest property permittivity of 46.1 and a low leakage current density of 3.35 × 10-5A/cm2 were achieved for the sample of Pt/TiO2/ZrO2/Si/Pt after annealing at 773 K.
This work is supported in part by the National Nature Science Foundation of China (No. 51072049), STD and ED of Hubei Province (Grant Nos. 2009CDA035, 2008BAB010, 2010BFA016, and Z20091001).
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