Dielectric Enhancement of Atomic Layer-Deposited Al2O3/ZrO2/Al2O3 MIM Capacitors by Microwave Annealing

For metal-insulator-metal (MIM) capacitors applicated in the fields of RF, DRAM, and analog/mixed-signal integrated circuits, a high capacitance density is imperative with the downscaling of the device feature size. In this work, the microwave annealing technique is investigated to enhance the dielectric characteristics of Al2O3/ZrO2/Al2O3 based MIM capacitors. The results show that the permittivity of ZrO2 is increased to 41.9 (~ 40% enhanced) with a microwave annealing at 1400 W for 5 min. The substrate temperature is lower than 400 °C, which is compatible with the back end of line process. The leakage current densities are 1.23 × 10−8 and 1.36 × 10−8 A/cm2 for as-deposited sample and 1400 W sample, respectively, indicating that the leakage property is not deteriorated. The conduction mechanism is confirmed as field-assisted tunneling.

In this work, the effect of MWA on electrical properties of TaN/Al 2 O 3 /ZrO 2 /Al 2 O 3 /TaN (TaN/A/Z/A/TaN) MIM capacitors is investigated. With the usage of MWA, the permittivity of ZrO 2 is remarkably enhanced and the leakage current density is slightly increased. Moreover, the underlying conduction mechanism is also studied.

Methods
Firstly, a 500-nm-thick SiO 2 film was grown onto Si substrate by PECVD, followed by deposition of TaN (20 nm)/ Ta (100 nm) films, and TaN was grown by sputtering Ta target in N 2 /Ar plasma. Subsequently, the Si wafer coated with the TaN/Ta films was transferred into the ALD chamber, and the nano-stack of Al 2 O 3 (2 nm)/ZrO 2 (20 nm)/Al 2 O 3 (2 nm) were deposited at 250°C. Al 2 O 3 and ZrO 2 films were grown from Al (CH 3 ) 3 /H 2 O and [(CH 3 ) 2 N] 4 Zr/H 2 O, respectively. It is worth mentioning that an ultrathin Al 2 O 3 layer between the bottom TaN electrode and the ZrO 2 layer was inserted to restrain the formation of interfacial layer during ALD and post-deposition annealing. Afterwards, the samples were subject to the microwave annealing. MWA was performed in a DSGI octagonal chamber at 5.8 GHz. During annealing, the samples were placed at the middle of the chamber, where the electromagnetic field is most uniform. The in situ temperature of the samples was monitored by a Raytek XR series infrared pyrometer facing the backside of the samples. The power was varied from 700 W to 1400 W with a fixed annealing time of 5 min. Finally, a 100-nm-thick TaN top electrode was formed in turn by reactive sputter, lithography, and reactive ion etching.
The ALD film thicknesses were measured with an ellipsometer (SOPRA GES 5E) and confirmed by transmission electron microscope (TEM). Capacitance-voltage (C-V) was measured by a precision impedance analyzer (Agilent 4294A) with a 50 mV AC amplitude. Current-voltage (I-V) measurements were performed with a semiconductor device analyzer (Agilent B1500) in a dark box. The bias was applied to the top electrode.

Results and Discussion
The schematic structures of the A/Z/A based MIM capacitor and the MWA chamber are shown in Fig. 1a and b, respectively. Figure 1c exhibits the cross-sectional TEM image of the A/Z/A-based MIM capacitor which is subject to the MWA at 1400 W for 5 min. It is observed that the ZrO 2 layer is fully crystallized and the stacked layers can be distinguished clearly, see the inset. Figure 2a shows the cumulative probability plot of the capacitance density at different annealing power. The results show that the capacitance densities of the MIM capacitors are 7.34, 8.87, 8.96, and 9.06 fF/μm 2 respectively for 0, 700, 1050, and 1400 W at a 50% cumulative probability. Therefore, the capacitance   Fig. 2b. Regarding the microwave power of 1400 W, the dielectric constant of the ZrO 2 film increases by 40% compared with the as-deposited sample. The significant enhancement of the permittivity of ZrO 2 can be ascribed to the high-degree crystallization during the microwave annealing, shown in Fig. 1c. As mentioned above, the dielectric constant of ZrO 2 can be enhanced to 36.8 and 46.6 when it is crystallized into cubic and tetragonal phase, respectively [25]. Hence, the XRD measurement was performed to further investigate the mechanism of the dielectric constant enhancement. As exhibited in the inset of Fig. 2b, a peak existed at~30. 7°after the MWA processing at 1400 W, indicating the appearance of the tetragonal phase (111) in ZrO 2 [32,33]. The presence of this tetragonal phase is responsible for the enhancement of the dielectric constant from 28.3 to over 40.
Since the MIM capacitors are fabricated in the back end of line (BEOL) of integrated circuits, the process temperature must be lower than 400°C [34]. As shown in Fig. 3, the temperature curves of MWA indicate that the highest temperatures of the substrate are 260, 350, and 400°C for 700, 1050, and 1400 W, respectively. Therefore, MWA is compatible with the CMOS process from the viewpoint of process temperature. Furthermore, in the previous work [13], Al 2 O 3 (2 nm)/ZrO 2 (20 nm)-based MIM capacitors were subject to rapid thermal annealing (RTA) at 420°C for 10 min in N 2 /H 2 ambient and the resulting dielectric constant of ZrO 2 was evaluated as 40. For RTA, the annealing time was kept constant at 420°C for 10 min, so the thermal budget was much larger compared with MWA. For MWA [35,36], dipole polarization is thought to be the most important mechanism for energy transfer at the molecular level. When materials in contact have different dielectric properties, microwaves will selectively couple with the higher dielectric loss materials. In contrast, conventional RTA transfers heat most efficiently to materials with high conductivity.
Leakage current is another important parameter for MIM capacitors. As shown by Fig. 4a, the leakage current curve can be divided into two sections for all the samples since there is an obvious turning point, indicating different electron conduction mechanisms. As for the samples with MWA processing, the voltage corresponding to the turning point is smaller compared with the as-deposited sample. Table 1 lists the leakage current density at ± 4 V for all the samples. Take 4 V for example, the leakage current density is increased from 1.06 × 10 −7 to 1.92 × 10 −5 A/cm 2 , i.e., two orders of amplitude enhanced when the microwave power is augmented from 0 to 1400 W. Due to a high crystallization of the ZrO 2 film, a large number of grain boundaries will appear and serve as the leaky path, thus  Obviously, the microwave annealing has little effect on the leakage performance under a low electric field. Furthermore, the breakdown voltage was extracted from the I-V test and plotted in Fig. 4b. For the as-deposited sample, the breakdown voltage is about 9.8 V at a 50% cumulative probability. With the application of MWA, the breakdown voltage is reduced to~9 V. This reduction of breakdown voltage could be related to the change of the ZrO 2 microstructure.
In order to further understand the effect of MWA on the leakage current, the conduction mechanisms of the MIM capacitors are investigated. Based on the previous research on Al 2 O 3 (2 nm)/ZrO 2 (20 nm)-based MIM capacitor [13,14], the dominant conduction mechanism in a high electric field was confirmed as field-assisted tunneling (FAT). For FAT which is trap-related tunneling, electrons are captured by the traps in the insulator firstly and then tunnel to the conduction band of the insulator directly [37]. In the current work, the Al 2 O 3 and ZrO 2 films in the A/Z/A-based MIM capacitors were deposited by the same conditions, so the leakage current is probably predominant by FAT as well. The FAT model can be expressed by Eq. (1) [37] where A is a constant, E is the electric field, q is the electronic charge, m* represents the effective electron mass (about 0.25 m 0 , where m 0 is the free electron mass), k is the Boltzmann constant, φ t is the energy barrier separating traps from the conduction band, and h is the Planck's constant.
In terms of the stacked dielectrics, the electric field applied to each layer differs from each other because of different permittivity and thickness. Hence, using the average electric field across the entire stack will bring about severe errors while discussing the conduction mechanism. As a consequence, the electric field across the ZrO 2 layer must be extracted accurately. The electric fields across ZrO 2 are 3.125 × 10 7 × V stack , 2.5× 10 7 × V stack , 2.47×10 7 × V stack , and 2.44 × 10 7 × V stack respectively for as-deposited, 700 W, 1050 W, and 1400 W sample according to the Gauss law and Kirchhoff voltage law [38,39]: where k A and κ Z represent the dielectric constants of Al 2 O 3 and ZrO 2 , respectively; E A and E Z denote the electric fields across Al 2 O 3 and ZrO 2 , respectively; d A and d Z equal the thicknesses of Al 2 O 3 and ZrO 2 , respectively; and V stack is the voltage applied to the stack. Accordingly, Ln (J/E Z 2 ) versus 1/E Z was arbitrarily plotted in Fig. 5, where a straight line fitting was achieved in the high field region for each sample under electron bottom-injection (see Fig. 5a) or electron top-injection (see Fig. 5b). This means that the FAT mechanism is dominated at high electric fields. The extracted φ t is 0.73, 0.51, 0.38, and 0.35 eV respectively for as-deposited, 700 W, 1050 W, and 1400 W sample under electron bottom-injection. In terms of electron top-injection, the corresponding φ t is 0.82, 0.53, 0.47, and 0.43 eV, respectively. Therefore, some shallow traps are induced by MWA. The shallow traps are reported to arise from the grain boundary defects that can introduce additional electronic states near the conduction band [40]. In addition, the conduction mechanism at low fields is most likely trap-assisted tunneling (TAT). The dominated conduction mechanism in the high electric fields is confirmed as a FAT process. The leakage current in the low electric fields is likely dictated by TAT. Based on the above facts, the microwave annealing is a promising technique used in the CMOS process to enhance the dielectric performance of the MIM capacitors.