Chapter 3 HfTiO MIM Capacitors for DRAM and RF Applications
3.3 Leakage Current Improved by High-Work-Function Electrode…
Fig. 3-6 shows the energy band diagram of the [Pt and Al]/HfTiO/TaN MIM capacitor. The electron affinity of HfTiO dielectric is assumed to be between 2.5~3.9 eV, which corresponds to that of HfO2 and TiO2 are 2.5 and 3.9 eV [3.4], respectively, and is nearly 2.55~4.15 eV reported by K. C. Chiang et al. [3.5]. Since electron affinity of Ta2O5
is 3.2 eV [3.4], the electron affinity of interfacial layer of TaTiO was considered to be approximately 3.4~3.9 eV [3.6] and then the variation of the electron affinity is dependent on film compositions. High work function top electrode such as Pt is expected to achieve low leakage current.
The capacitance densities for Pt and Al top electrodes are 17.5 fF/µm2 and 19.5 fF/µm2, respectively, calculated from the 1 MHz C-V characteristics shown in Fig. 3-7 (a).
The capacitance densities are almost constant with varied bias from -2 to 2 V for Pt top electrode, but larger voltage dispersion for Al top electrode beyond +1.5 V bias is due to larger leakage current. The top electrode area is 3.14×104 µm2 in circlefor all of the capacitors. The difference of capacitance density for HfTiO MIM capacitors with Pt or Al top electrodes may be caused by different metal deposition process or film uniformity.
From the J-V characteristics in Fig. 3-7 (b), the leakage current density for Pt electrode measured at -1 V at room temperature is much lower than that for Al electrode by three orders of magnitude although the capacitance density of Pt electrode is slightly lower than that of Al electrode.
The gate injected leakage currents for both Pt and Al case are contributed from not only high density of traps in dielectric caused by incomplete dielectric activation but also the different Schottky barrier heights between top electrode and dielectric. This may be due to larger work function difference between top Pt and bottom TaN electrodes increasing metal-insulator barrier height to suppress the leakage current from top or bottom carrier injection. Beside this, several phenomena such as Fermi level pinning or the formation of interfacial layer between top metal and insulator as shown in Fig. 3-2 (c) may also affect the leakage current. Therefore, current transport mechanisms should be investigated carefully.
The J-V characteristics of the MIM capacitor with Pt and Al as top electrodes measured at temperatures ranging from 25 to 125oC are shown in Fig. 3-8 (a) and (b), respectively. A small leakage current and weak temperature dependence of 2.4x10-6 and 9.8 x10-6 A/cm2 at 25 and 125oC for Pt electrode are obtained, respectively, at -3 V.
However, much high leakage currents and significant temperature dependence for Al case at 25 and 125oC are 1.01 and 11.3 A/cm2, respectively, at -3 V. The thermal leakage current of high-κ MIM capacitors is very important due to the requirement of a small leakage current at high temperature for both DRAM and nonvolatile memory applications [3.7]. Moreover, it can be observed that the improvement on the leakage current at 125oC by Pt electrode is apparent and the high work function electrode of Ir also had been proved to improve high-temperature leakage current [3.8].
To recognize the leakage current mechanism of the HfTiO MIM capacitors, we take Schottky emission (SE) mechanism into consideration at low field. It is well known that SE mechanism, which the leakage current is electrode-limited and contributed by the carriers that overcome the barrier height between the electrode and the dielectric, has a linear ln(J/T2) - E1/2 relation as depicted by Eq. (3-2) and (3-3) [3.9, 3.10],
where A* denotes the Richardson constant, k is the Boltzmann’s constant, T is the absolute temperature (K), E is the applied external electric field, e is the electron charge, εo is the permittivity in vacuum, is the high-frequency dielectric constant [3.11] (= n
K∞
2, where n is the refractive index) and ϕb is corresponding to the barrier height between metal/dielectric. Compared with the ideal Schottky barrier height (ϕ 0), the actual ϕb usually exhibits a smaller value due to image force, surface states, and external electric field. We have plotted ln(J/T2) versus E1/2 curve for Pt
electrode at 25 and 125oC as shown in Fig 3-9 (a) and extracted the slopes of 0.0069 and 0.0053 eV(m/V)1/2 at low field, respectively. The slopes yield the refractive index of 2.09 and 2.74 from the leakage current of 25 and 125oC, respectively. Since the refractive index of TiO2 [3.12] and HfO2 [3.13] are about 1.7-1.9 and 2.55-2.83, respectively, the refractive index extracted at low field supports that the leakage current mechanism is probably a Schottky emission. Then we use linear extrapolation to extract the Schottky barrier height (SBH) of 0.92 and 0.82 eV at 25 and 125oC, respectively. It is well known that lower SBH at higher temperature would result in increased leakage current.
For the leakage current at very low field, the leakage current increases linearly with the increase of voltage bias as shown in Fig. 3-9 (b). It presents an Ohmic conduction mechanism, which describes the thermal excitation of trapped electrons from one trap to another at low field [3.14]. It is observed that the segment of Ohmic conduction occurred at low electric field become shorter with the temperature increasing.
Fig. 3-10 shows the Schottky emission fitting for Al/HfTiO/TaN MIM capacitors at 25 and 125oC. The linear relationship of ln(J/T2) versus E1/2 curve was obtained for Al electrode, which gives the slope of 0.00672 eV(m/V)1/2 with a refractive index of 2.18 and slope of 0.0102 eV(m/V)1/2 with a refractive index of 1.44 for 25 and 125oC, respectively. The extracted SBH for Al electrode is 0.80 eV (0.67 eV), which is slightly smaller than that of Pt electrode with 0.92 eV (0.82 eV) at 25 oC (125 oC).
For Schottky barrier height, we found that the high-work-function Pt can reduce the barrier height lowering at high temperature.
The work function of Pt is around 5.6 eV and the electron affinity of HfTiO
is 2.5~3.9 eV, which gives the SBH between Pt/HfTiO is 1.7~3.1 eV in theory.
However, the SBH of 0.92 eV at 25 oC for Pt electrode is far smaller than theoretical value and this similar result was also observed from Al case. It is suspected that little difference of SBH compared to work function difference between Pt and Al electrodes may be originated by Fermi level pinning, which describes that the work function of metals on high-κ dielectrics have been observed to differ from their values in vacuum, with the discrepancy depends on the dielectrics used [3.15-3.18].
In addition, it is suspected that the interfacial layer formed between Al electrode and HfTiO as shown in Fig. 3-2 (c) was probably another reason to modulate the SBH and enhance the overall leakage current.
To investigate the leakage current mechanism of the MIM capacitors at high electric field, the ln(J/E) versus E1/2 plots for Pt and Al electrodes are shown in Fig. 3-11 (a) and (b), respectively. The Frenkel-Poole (F-P) conduction mechanism, which is a bulk-limited current and controlled by the detrapping of the electrons from the trap centers to the conduction band of the dielectric. The F-P effect can be described as by Eq.
(3-4) and (3-5) [3.7, 3.8],
Where B is the constant and ϕb is corresponding to the trap energy level. We can extract n values of 3.06 and 2.52 from the slopes for Pt electrode at 25 and 125oC, respectively. For Al electrode, extracted n values of 2.37 and 3.22 are obtained at 25 and
125oC, respectively. The proper n values can explain that the leakage current at high field is the F-P conduction mechanism. To extract the trapping level of HfTiO dielectric, the ln(J/E)-1/KT relationship is plotted in Fig. 3-12 (a) and (b) for Pt and Al electrodes, respectively. Trap energy for Pt and Al electrodes is 0.44 and 0.75 eV, respectively. The trap energies are less than SBH of 0.92 eV and 0.80 eV for Pt and Al cases, respectively, which supports that the conduction mechanism at high electric field were dominated by the F-P rather than SE. Apparent difference exists in the extracted trap potential height with respect to different top electrodes. This indicates that the trap at and around the interface instead of the traps at deep level in the dielectric bulk play the major role to the conduction mechanism [3.19]. It has been observed that by incorporating Al into HfO2
film, shallow trap levels will be eliminated [3.20]. In this thesis, the trapping level for Al case is deeper than that for the Pt case. It is postulated that this phenomenon may be attribute to the incorporation of Al into the HfTiO layer at the Al/HfTiO interface.