Dielectric Enhancement of Atomic Layer-Deposited Al2O3/ZrO2/Al2O3 MIM Capacitors by Microwave Annealing
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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.
KeywordsMicrowave annealing Atomic layer deposition Al2O3/ZrO2/Al2O3 MIM capacitors
Atomic layer deposition
Back end of line
Dynamic random access memory
International Technology Roadmap for Semiconductors
Plasma enhanced chemical vapor deposition
Rapid thermal annealing
Transmission electron microscope
Metal-insulator-metal (MIM) capacitors have been widely used in the fields of radio frequency (RF), dynamic random access memory (DRAM), and analog/mixed-signal integrated circuits. With the scaling down of the device feature size, it is desirable to obtain an ever higher capacitance density. For example, the capacitance density is required to be greater than 10 fF/μm2 according to the 2020 node of the International Technology Roadmap for Semiconductors (ITRS) . As a consequence, a large number of high-κ materials have been investigated, such as HfO2 [2, 3, 4, 5, 6], ZrO2 [7, 8, 9, 10, 11, 12, 13, 14], Ta2O5 [15, 16, 17, 18], and TiO2 [19, 20, 21, 22, 23, 24]. Among these high-κ materials, ZrO2 has a dielectric constant (κ) of 16~25 (monoclinic phase) and a bandgap of 5.8 eV. However, the κ value of ZrO2 can be enhanced to 36.8 and 46.6 when it is crystallized into cubic and tetragonal phase, respectively . Hence, the capacitance density can be further increased. The microwave annealing (MWA) technique has been tremendously explored for the dopant activation in silicon [26, 27, 28] and the silicide formation [29, 30] due to its lower process temperature compared with conventional thermal processing techniques. In addition, Shih et al.  investigated the effect of MWA on electrical characteristics of TiN/Al/TiN/HfO2/Si MOS capacitors. Some key parameters such as equivalent oxide thickness, interface state density, and leakage current density were all improved.
In this work, the effect of MWA on electrical properties of TaN/Al2O3/ZrO2/Al2O3/TaN (TaN/A/Z/A/TaN) MIM capacitors is investigated. With the usage of MWA, the permittivity of ZrO2 is remarkably enhanced and the leakage current density is slightly increased. Moreover, the underlying conduction mechanism is also studied.
Firstly, a 500-nm-thick SiO2 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 N2/Ar plasma. Subsequently, the Si wafer coated with the TaN/Ta films was transferred into the ALD chamber, and the nano-stack of Al2O3 (2 nm)/ZrO2 (20 nm)/Al2O3 (2 nm) were deposited at 250 °C. Al2O3 and ZrO2 films were grown from Al (CH3)3/H2O and [(CH3)2N]4Zr/H2O, respectively. It is worth mentioning that an ultrathin Al2O3 layer between the bottom TaN electrode and the ZrO2 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 leakage current density (J) at ± 4 V for all the samples
J@4 V (A/cm2)
1.06 × 10−7
6.68 × 10−7
7.63 × 10−6
1.92 × 10−5
J@-4 V (A/cm2)
3.41 × 10−8
3.30 × 10−7
1.20 × 10−6
3.48 × 10−6
where A is a constant, E is the electric field, q is the electronic charge, m* represents the effective electron mass (about 0.25 m0, where m0 is the free electron mass), k is the Boltzmann constant, φt is the energy barrier separating traps from the conduction band, and h is the Planck’s constant.
Atomic layer-deposited Al2O3/ZrO2/Al2O3 nano-stack is used as the insulator of the MIM capacitors. With the effect of MWA at 1400 W for 5 min, the capacitance density is increased to 9.06 fF/μm2, approximately 23.4% of capacitance enhanced. Decoupling the influence of Al2O3, the dielectric constant is deduced as 41.9 for 1400 W sample (~ 40% of permittivity increased). Such enhancement of the permittivity is originated from a high crystallization of the ZrO2 film. In addition, the substrate temperature is lower than 400 °C, which enables MWA compatible with the BEOL process. This lower substrate temperature can be attributed to the selective heating on the materials of MWA. In terms of a working voltage of 2 V, 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. 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.
There is no acknowledgement.
This work was supported by the National Key Technologies R&D Program of China (2015ZX02102–003), the National Natural Science Foundation of China (61474027) and the Project funded by China Postdoctoral Science Foundation.
Availability of Data and Materials
All datasets are presented in the main paper and freely available to any scientist wishing to use them for non-commercial purposes, without breaching participant confidentiality.
BZ carried out the main part of fabrication and analytical works. XW and WJL participated in the sequence alignment and drafted the manuscript. SJD, DWZ, and ZF conceived the study and participated in its design. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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