Temperature‑Dependent Evolution of HfO₂/Si Interfaces: Crystallization Mechanisms and Optimal Annealing Conditions

Introduction

Hafnium oxide (HfO₂) thin films are prized for their high density, high refractive index, wide bandgap, and thermal stability, making them suitable for multilayer optical coatings, protective layers, gate dielectrics, and passivation layers. Conventional deposition techniques—including electron beam evaporation, chemical solution deposition, reactive sputtering, MOCVD, MBE, and ALD—each present trade‑offs in thickness control and uniformity. Atomic layer deposition, particularly remote‑plasma ALD (RP‑ALD), offers atomic‑scale thickness precision and exceptional uniformity, but its properties are highly sensitive to post‑deposition annealing. Previous work has shown that HfO₂ films crystallize above 500 °C, with the crystalline phase directly affecting optical and electrical parameters: the refractive index shifts from 1.7 to 2.09, the optical gap from 5.75 to 6.13 eV, and the dielectric constant from 24.5 to 14.49. For films deposited on silicon, an interfacial SiO₂ layer typically forms, which can reduce the effective dielectric constant. While the crystallization behavior of the film itself has been studied, the evolution of the HfO₂/Si interface under varying annealing temperatures remains less explored. This research addresses that gap by systematically varying the annealing temperature and characterizing both film and interface structure.

Method

We used double‑sided polished 2‑inch, 250‑µm thick, p‑type Czochralski Si wafers (resistivity 30 Ω cm). Wafers were cleaned with a standard RCA process followed by a 2‑min dip in 5 % diluted HF to remove native oxides, then dried with high‑purity N₂. Approximately 15 nm of HfO₂ (168 ALD cycles) was deposited at 250 °C via RP‑ALD (Picosun R‑200) using TEMAH and O₂ plasma. Each cycle comprised a 1.6 s TEMAH pulse, 10 s N₂ purge, 10 s O₂ plasma pulse, and 12 s N₂ purge. Post‑deposition, the samples were annealed in N₂ for 10 min at temperatures ranging from 400 to 600 °C to probe crystallization effects. Detailed deposition and annealing parameters are summarized in Table 1. AFM was performed in tapping mode (2 µm × 2 µm scan, 256 × 256 resolution) to assess surface morphology. GIXRD (Rigaku TTRAXIII, Cu λ = 0.154 nm, 50 kV, 300 mA, incident angle 0.5°) provided diffraction patterns over 2θ = 20–60°. XPS (Thermo Fisher K‑alpha, Al Kα, 1486.6 eV) used a 100‑µm spot and 45° take‑off angle. Cross‑sectional TEM samples were prepared by focused ion beam lift‑out (Hitachi NX2000) and examined with a field‑emission HR‑TEM (JEM‑2100F).

Results and Discussion

Figure 1 displays AFM images of HfO₂ films annealed at 400, 500, and 600 °C. The root‑mean‑square roughness (RMS) rises from 0.44 nm (as‑deposited) to 0.47 nm at 500 °C, then to 0.69 nm at 600 °C, indicating a gradual structural transition. The average roughness (Ra) follows a similar trend.

AFM images of a as‑deposited, b 400 °C‑annealed, c 500 °C‑annealed, and d 600 °C‑annealed HfO₂ films

Figure 2 shows the temperature‑dependent GIXRD spectra. The as‑deposited film is amorphous, and remains so after annealing at 400 and 450 °C. Crystalline peaks emerge above 500 °C, revealing monoclinic (–111, 111) and orthorhombic (111) planes. As the annealing temperature rises, the monoclinic phase diminishes while the orthorhombic phase dominates, with the (111) peak shifting to higher d‑spacing, reflecting lattice contraction.

GIXRD spectra of HfO₂ thin films annealed at different temperatures

Depth‑profiled XPS reveals the O/Hf ratio decreases from 1.60 to 1.29 as temperature increases, confirming oxygen depletion in the film during nitrogen annealing. This loss of oxygen leads to reduced d‑spacing, as noted in the XRD data.

Atomic ratio of oxygen to hafnium for HfO₂ thin films annealed at different temperatures

High‑resolution HR‑TEM cross‑sections (Figure 4) confirm the progression from amorphous to polycrystalline HfO₂. At 400 °C, the film is largely amorphous with isolated nanocrystalline domains (d ≈ 2.82 Å and 3.12 Å, corresponding to monoclinic (111) and (–111)). Between 500–550 °C, the film exhibits a mixture of monoclinic and orthorhombic phases. At 600 °C, the orthorhombic (111) phase dominates while monoclinic features vanish. Concomitantly, the interfacial SiO₂ layer transitions from amorphous to crystalline cubic (220), with thickness increasing from 1.0 to 1.6 nm. The rise in d‑spacing of the SiO₂ layer (2.48→2.56 Å) indicates increased oxygen incorporation from the HfO₂ film.

Cross‑sectional TEM images of a as‑deposited, b 400 °C‑annealed, c 450 °C‑annealed, d 500 °C‑annealed, e 550 °C‑annealed, and f 600 °C‑annealed HfO₂/Si

Figure 5 summarizes the mechanistic evolution. Below 400 °C the film is amorphous and the interfacial layer is a mixed a‑HfO₂/a‑SiO₂. Between 450–550 °C, the film crystallizes into a multi‑phase polycrystalline structure while a crystalline SiO₂ layer forms at the interface, driven by oxygen diffusion from HfO₂ into the silicon. Above 550 °C, the film becomes single‑phase orthorhombic, and the interfacial layer fully crystallizes. Although annealing enhances dielectric properties, excessive crystallization can degrade performance. The dielectric constant peaks at 17.2 when annealed at 500 °C, balancing phase purity and interfacial quality.

Diagrams of the crystallization mechanism of HfO₂ films and the interfacial layer across three temperature ranges: a as‑deposited to 400 °C, b 450–550 °C, and c >550 °C. D‑spacing values and crystalline orientations are indicated.

Conclusion

Using RP‑ALD followed by nitrogen RTA, we systematically mapped the crystallization of HfO₂ thin films on p‑type Si. As‑deposited and <400 °C‑annealed films remain amorphous, while the interfacial layer is a mixed a‑HfO₂/a‑SiO₂. Increasing temperature reduces the orthorhombic d‑spacing and increases the SiO₂ d‑spacing, evidence of oxygen migration from HfO₂ to the interface. Above 550 °C, the film becomes a single‑phase orthorhombic crystal and the interface is fully crystalline SiO₂. Although annealing is essential for high dielectric constant and passivation, excessive crystallization can be detrimental. An annealing temperature of 500 °C delivers the best compromise, yielding optimal dielectric constant and Si wafer passivation.

Abbreviations

AFM

Atomic force microscopy

a‑HfO₂

Amorphous hafnium oxide

ALD

Atomic layer deposition

a‑SiO₂

Amorphous silicon dioxide

c‑SiO₂

Crystalline silicon dioxide

GIXRD

Grazing‑incident X‑ray diffraction

HfO₂

Hafnium oxide

HR‑TEM

High‑resolution transmission electron microscopy

N₂

Nitrogen

O₂

Oxygen

RMS

Root‑mean‑square

RP‑ALD

Remote plasma atomic layer deposition

RTA

Rapid thermal annealing

TEMAH

Tetrakis (ethylmethylamino) hafnium

XPS

X‑ray photoelectron spectroscopy