Basic Characteristics of HfO 2 Dielectrics Deposited on Ta

Figure 2-3 reveals The C-V characteristics of the as-deposited HfO2 gate dielectrics for 5nm, 6nm, and 9nm were deposited difference thickness. The capacitance density deposited at 9nm of film thickness was lower than 5nm deposited sample with similarly condition. When gate oxide film reduces, the capacitor density significantly shows high desired results. The Figure 2-4 clearly display the capacitance density at 5nm film thickness has 25.67fF/cm². The capacitor dielectrics

with higher dielectric constant κ offer an attractive path to achieving enhanced capacitance per area. It should be pointed out here that high-κ use as a gate oxide is distinct from high-κ use in DRAM capacitors. As mentioned previously, HfO2 gate dielectrics exhibit high capacitance density and replace conventional SiO2.

Since thinner HfO2 dielectric films would expect to have higher capacitance density than thick dielectric films. The corresponding capacitance-voltage (C-V) characteristics at the frequencies from 1 kHz to 1 MHz curves were presented in Figure 2-5 (a) (b). Figure 2-5 (a) (b) shows the frequency-dependence of the voltages of HfO2 dielectric films at 5nm samples. The capacitance densities reduced from 40.65fF/cm² at 1 kHz to 25.67fF/cm² at 100 kHz. However, the capacitance densities are very low at 1 MHz. The capacitance density decreases to about 0.47 fF/µm² at 1 MHz compared to 40.7 fF/µm² at 100 kHz. However, more serious frequency dispersion effect is shown in MIM. The capacitance densities decrease with increasing frequency in the range from 100 Hz to 1 MHz. It can be explained that at lower frequencies, we have different types of polarizations such as electronic polarization, orientation polarization, space charge polarization, and atomic polarization. Figure 2-2 shows the four types of polarizations.

These four compositions are illustrated as following.

(1) Electronic polarizability, αe.

Electronic polarization occurs in all dielectric materials. The electrons surrounding each nucleus are shifted very slightly in the direction of the positive electrode and the nucleus is very slightly shifted in the direction of the negative electrode. As soon as the electric field is removed, the electrons and nuclei return to their original distributions and the polarization disappears. The effect is

analogous to elastic stress and strain. The displacement of charge is very small for electronic polarization, so the total amount of polarization is small compared to the other mechanisms of polarization.

(2) Orientation polarizability, αo.

Orientation polarization involves nonsymmetrical molecules that contain permanent electric dipoles. An example is H2O. The covalent bonds between the hydrogen and oxygen atoms are directional such that the two hydrogens are on one side of the oxygen. The hydrogen side of the molecule has a net positive charge and the oxygen side has a net negative charge. Under an electric field, the molecules will align with the positive side facing the negative electrode and the negative side facing the positive electrode. Orientation polarization results in a much higher degree of polarization than electronic polarization. This is because large charge displacement is possible in the relatively large molecules compared to the spacing between the electrons and nucleus in individual atoms.

(3) Space charge polarizability, αs.

Space charges are random charges caused by cosmic radiation, thermal deterioration, or are trapped in the material during the fabrication process.

(4) Atomic or ionic polarizability, α_i.

It involves displacement of atoms or ions within a crystal structure when an electric field is applied. A wide range of polarization effects is possible through this mechanism, depending on the crystal structure, the presence of solid solution, and other factors. Examples include pyroelectricity, piezoelectricity, and ferroelectricity, Figure 2-6 (a) shows the four types of polarizations. At higher frequencies, the capacitance densities have main contribution from the electronic polarization [21].

Just as we have a relaxed and an unrelaxed elastic modulus, we have a dependence of the capacitance densities on frequency which shown in Figure 2-6 (b). The electronic

polarization is the only process sufficiently rapid to follow alternative fields in the visible part of the spectrum. Ionic polarization processes are able to follow an applied high-frequency field and contribute to the capacitance densities at frequencies up to the infrared region of the spectrum. Orientation and space charge polarization have relaxation times corresponding to the particular system and process but, in general, participate only at lower frequencies. At Figure 2-5 (a) (b), the capacitance densities decrease with increasing frequency in the range from 100 Hz to 1 MHz. It can be explained that at lower frequencies, we have different types of polarizations such as electronic polarization, orientation polarization, space charge polarization, and atomic polarization. At higher frequencies, the dielectric constant has main contribution from the electronic polarization. Refer to Figure 2-6 (b), the capacitance densities decreases in the frequency ranging from 100 Hz to 1 MHz may be attributed to the decrease of space charge polarization.

Figure 2-7 (a) shows the normalized C-V curves (△C/Co) of MIM structure (Ta/HfO2/Ta). Voltage coefficient of capacitance (VCC) is one of the important parameters of MIM structure. It has been demonstrated that pure SiO2 MIM structures show negative parabolic curves in C-V relationship, but high-κ MIM structures exhibit strong positive parabolic curves in C-V relationship [40]. The mechanism of nonlinearity of C-V curves is unclear. It is supposed to relate with E-field polarization, carrier injections [23], high-κ thickness [24、25], frequency [26] and leakage current [27]. Theoretically, VCC decreases with measured frequency increases [24]. It is believed that the carrier mobility becomes smaller with increasing frequency, which leads to a higher relaxation time and a smaller capacitance variation [23]. From the equation below, where the voltage coefficients of capacitance (VCC) values of α and

β are listed in Table 2.2. The requirement of the quadratic coefficient of capacitance α is smaller than 100 ppm/V², and the requirement of the linear coefficient of capacitance β is below 1000 ppm/V according to the ITRS roadmap [28].

) 1

Normalized capacitances (△C/Co) as a function of voltage with different thickness are shown Figure 2-7 (b) and Table 2.3. The decreasing of α with thickness is slower. This thickness effect is due to E-field reduction with increased thickness [47]. Thickness effect of the VCC is a negative impact for thinner film dielectric.

2.3.2 Thermal Stress on the MIM Capacitors

Figure 2-8 (a) depicts Capacitance density of the MIM capacitor with 5nm thickness at 100 kHz from 25°C to 125°C. Figure 2-8 (b) depicts Capacitance density of the MIM capacitor with 5nm thickness as a function of frequency after thermal stress from 25°C to 125°C. After thermal stress, the capacitance density decreases with temperatures at 100 kHz. The major reason is considered to be the interface defect density increasing during the thermal stress process. Moreover, the capacitance density decreases with frequency at all thermal stress. Especially, the capacitance density at 1 MHz is very lower than other frequency. As mentioned previously, the poorer frequency dispersion for MIM is probably decreased with space charge polarization. Figure 2-9 clearly confirms the results of Figure 2-8 (a) and (b).