Microwave Annealing Boosts Dielectric Performance of Al₂O₃/ZrO₂/Al₂O₃ MIM Capacitors

Abstract

Metal‑insulator‑metal (MIM) capacitors are pivotal in RF, DRAM, and mixed‑signal ICs, where ever‑increasing capacitance density is required as device dimensions shrink. In this study, microwave annealing (MWA) is employed to enhance the dielectric properties of Al₂O₃/ZrO₂/Al₂O₃ (A/Z/A) MIM capacitors. A 1400 W, 5‑min MWA treatment raises the permittivity of ZrO₂ to 41.9—a 40 % improvement—while maintaining a substrate temperature below 400 °C, compatible with back‑end‑of‑line (BEOL) processes. Leakage current densities remain comparable (1.23 × 10⁻⁸ A cm⁻² for as‑deposited vs. 1.36 × 10⁻⁸ A cm⁻² for 1400 W), and the conduction mechanism is confirmed as field‑assisted tunneling.

Background

MIM capacitors are essential components in RF, DRAM, and analog/mixed‑signal integrated circuits. Scaling down device feature sizes demands higher capacitance density; for instance, the 2020 ITRS node requires >10 fF µm⁻². High‑κ dielectrics such as HfO₂, ZrO₂, Ta₂O₅, and TiO₂ have been extensively investigated. ZrO₂, with a monoclinic κ of 16–25 and a 5.8 eV bandgap, can reach κ ≈ 37–47 when crystallized into cubic or tetragonal phases, offering a route to superior capacitance density. Microwave annealing has proven effective for dopant activation and silicide formation in silicon, providing lower process temperatures than conventional thermal treatments. Recent work has shown MWA can improve key MOS capacitor parameters—equivalent oxide thickness, interface state density, and leakage current density—by selectively heating high‑dielectric‑loss materials. Here we investigate the effect of MWA on TaN/Al₂O₃/ZrO₂/Al₂O₃/TaN (TaN/A/Z/A/TaN) MIM capacitors. MWA significantly enhances ZrO₂ permittivity and slightly increases leakage current density, with the underlying conduction mechanism explored in detail.

Methods

A 500‑nm SiO₂ layer was grown on Si by PECVD, followed by sputtering 20 nm TaN and 100 nm Ta. The wafer was then transferred to an ALD chamber where a 2 nm Al₂O₃ / 20 nm ZrO₂ / 2 nm Al₂O₃ stack was deposited at 250 °C using Al(CH₃)₃/H₂O and [(CH₃)₂N]₄Zr/H₂O precursors. An ultrathin Al₂O₃ interlayer was inserted beneath the ZrO₂ to suppress interfacial layer growth. MWA was performed in a DSGI octagonal chamber at 5.8 GHz. Samples were positioned at the field‑uniform centre, and the in‑situ temperature was monitored by a Raytek XR infrared pyrometer. Annealing powers ranged from 700 W to 1400 W with a fixed 5‑min duration. Finally, a 100‑nm TaN top electrode was defined by reactive sputtering, lithography, and RIE. Film thicknesses were measured by ellipsometry (SOPRA GES 5E) and confirmed by TEM. Capacitance–voltage (C‑V) characteristics were obtained with an Agilent 4294A impedance analyzer (50 mV AC). Current–voltage (I‑V) measurements were performed with an Agilent B1500 device analyzer in a dark box, bias applied to the top electrode.

Results and Discussion

The A/Z/A MIM capacitor structure and the MWA chamber are illustrated in Fig. 1a and b, respectively. Figure 1c shows a cross‑sectional TEM image of a capacitor annealed at 1400 W for 5 min, revealing a fully crystallized ZrO₂ layer and well‑defined stack layers. Figure 2a presents the cumulative probability of capacitance density for the four annealing powers. At 50 % probability, capacitance densities are 7.34, 8.87, 8.96, and 9.06 fF µm⁻² for 0, 700, 1050, and 1400 W, respectively. The narrow distribution indicates excellent annealing uniformity. The inset displays typical C‑V curves for each sample. Excluding the Al₂O₃ contribution (κ ≈ 8), the extracted ZrO₂ dielectric constants are 28.3, 40.1, 41.0, and 41.9 for 0, 700, 1050, and 1400 W, respectively. A 40 % increase at 1400 W is attributed to high‑degree crystallization. XRD analysis (Fig. 2b inset) shows a tetragonal (111) peak at ~30.7°, confirming the transition to the tetragonal phase, which is known to yield κ values up to 46.6. Process compatibility is critical for BEOL integration. Temperature curves (Fig. 3) show peak substrate temperatures of 260, 350, and 400 °C for 700, 1050, and 1400 W, respectively—well below the 400 °C BEOL limit. Compared with 10‑min RTA at 420 °C (which also yields κ ≈ 40), MWA offers a significantly lower thermal budget and selective heating of high‑loss materials. Leakage characteristics (Fig. 4a) exhibit a two‑region behavior for all samples, indicating distinct conduction mechanisms. Leakage densities at ±4 V rise from 1.06 × 10⁻⁷ A cm⁻² (as‑deposited) to 1.92 × 10⁻⁵ A cm⁻² (1400 W), a two‑order‑of‑magnitude increase due to grain‑boundary‑induced leakage paths. However, at a practical 2 V operating point, leakage densities remain low (1.23 × 10⁻⁸ A cm⁻² vs. 1.36 × 10⁻⁸ A cm⁻²). Breakdown voltage slightly decreases from 9.8 V to ~9 V with MWA, likely reflecting microstructural changes. Conduction mechanisms were examined by fitting the high‑field J–E data to the field‑assisted tunneling (FAT) model. Accurate electric fields across the ZrO₂ layer were derived using the Gauss and Kirchhoff laws. Linear fits of ln(J/E_Z²) versus 1/E_Z (Fig. 5) confirm FAT dominance at high fields. Extracted trap barrier heights (φ_t) decrease from 0.73 eV (as‑deposited) to 0.35 eV (1400 W) for bottom‑injection, indicating the introduction of shallow traps likely associated with grain boundaries. At low fields, trap‑assisted tunneling (TAT) is the prevailing mechanism.

Conclusions

Al₂O₃/ZrO₂/Al₂O₃ nano‑stack MIM capacitors fabricated by ALD exhibit significant dielectric improvement when subjected to 1400 W, 5‑min MWA. Capacitance density rises to 9.06 fF µm⁻² (a 23.4 % gain), and the ZrO₂ dielectric constant reaches 41.9—an approximate 40 % increase—thanks to high‑degree crystallization into the tetragonal phase. The process remains fully BEOL‑compatible with substrate temperatures below 400 °C. Leakage currents at 2 V are virtually unchanged, and the dominant high‑field conduction remains FAT, while low‑field transport follows TAT. MWA therefore represents a promising, low‑thermal‑budget technique for enhancing MIM capacitor performance in CMOS integration.

Abbreviations

A/Z/A:

Al₂O₃/ZrO₂/Al₂O₃

ALD:

Atomic layer deposition

BEOL:

Back end of line

C‑V:

Capacitance‑voltage

DRAM:

Dynamic random access memory

FAT:

Field‑assisted tunneling

ITRS:

International Technology Roadmap for Semiconductors

I‑V:

Current‑voltage

MIM:

Metal‑insulator‑metal

MWA:

Microwave annealing

PECVD:

Plasma enhanced chemical vapor deposition

RF:

Radio frequency

RTA:

Rapid thermal annealing

TAT:

Trap‑assisted tunneling

TEM:

Transmission electron microscope