Ferroelectricity of HfxZr1−xO2 thin films fabricated by 300 °C low temperature process with plasma-enhanced atomic layer deposition

Microelectronic Engineering 215 (2019) 111013

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Research paper

Ferroelectricity of HfxZr1−xO2 thin films fabricated by 300 °C low temperature process with plasma-enhanced atomic layer deposition

T

Takashi Onayaa,b,c, , Toshihide Nabatameb, , Naomi Sawamotoa, Akihiko Ohib, Naoki Ikedab, Takahiro Nagatab, Atsushi Oguraa ⁎

⁎⁎

a

Department of Electrical Engineering, Graduate School of Science and Technology, Meiji University, 1-1-1 Higashimita, Tama-ku, Kawasaki, Kanagawa 214-8571, Japan International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan c Research Fellow of Japan Society for the Promotion of Science (JSPS), 5-3-1 Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan b

ARTICLE INFO

ABSTRACT

Keywords: Plasma-enhanced atomic layer deposition Ferroelectric HfxZr1-xO2 thin film Low temperature fabrication process High-k material

We investigated the characteristics of 10-nm-thick ferroelectric Hf0.43Zr0.57O2 (HZO) thin films fabricated using plasma-enhanced atomic layer deposition (PE-ALD) at 300 °C with plasma O2 gas and a post metallization annealing (PMA) process at 300–400 °C. The as-grown HZO film had a nanocrystalline structure (5–10 nm) with ferroelectric orthorhombic, tetragonal, and cubic (O/T/C) phases, and the PMA process subsequently led to the crystal growth of O/T/C phases along the nanocrystals in the as-grown film. As a result, the 300 °C-PMA-treated HZO film exhibited excellent remanent polarization (2Pr = Pr+ − Pr−) and dielectric constant (k) of 34 μC/cm2 and 39, respectively. These results suggest that the formation of the nanocrystal grains in as-grown HZO film could play an important role in the fabrication of HZO films with the metastable O/T/C phases during a low temperature fabrication process at 300 °C to obtain a superior ferroelectricity.

1. Introduction Recently, HfxZr1-xO2 (HZO) films have attracted a great deal of attention for use in ferroelectric random access memory, tunnel field effect transistors (FETs), and negative-capacitance FETs, due to their large bandgap (> 5 eV), stable ferroelectricity even at < 10 nm, and compatibility with complementary metal-oxide-semiconductor (CMOS) technology [1–11]. It has been reported that HZO films fabricated using various deposition methods, such as atomic layer deposition (ALD) [12–18], sputtering [19–21], metal organic chemical vapor deposition (MOCVD) [22,23], and chemical solution deposition (CSD) [24], and an annealing process at 400–700 °C showed stable ferroelectricity. However, for HZO films to be used in the next generation devices mentioned above, a low process temperature and method of achieving conformal coverage on complicated three-dimensional structures with well-controlled thin films are required. Therefore, an ALD method is absolutely necessary for the deposition of HZO thin films. The thermal ALD (TH-ALD) method with H2O or O3 as an oxidant gas has been typically employed for the fabrication of HZO films. Asgrown HZO films fabricated using TH-ALD generally have an amorphous structure, and crystallized HZO films with ferroelectricity are

subsequently obtained by annealing with TiN electrodes, because the TiN electrodes induce tensile stress to the HZO film, promoting the formation of the metastable ferroelectric orthorhombic (O) phase (Pca21) [12,25–29]. On the other hand, we focused on plasma-enhanced ALD (PE-ALD) with plasma O2 gas, because PE-ALD has stronger oxidizing power than TH-ALD [30]. It is expected that asgrown HZO films deposited by the PE-ALD process can be crystallized. In our previous study on ferroelectric HZO films, HZO film were crystallized along a polycrystalline ZrO2 seed layer with the metastable O, tetragonal, and cubic (O/T/C) phases, inserted between the HZO film and the TiN electrode, resulting in a higher 2Pr value than films without a seed layer [31,32]. We expect that as-grown HZO films will act as a nucleation layer like the ZrO2 seed layer if the as-grown HZO film can form nanocrystal grains with O/T/C phases for the ferroelectric HZO film fabrication. In this paper, we investigated the crystallinity and ferroelectricity of HZO thin films fabricated using the PE-ALD method at 300 °C and a low-temperature annealing process at 300–400 °C.

Correspondence to: T. Onaya, Department of Electrical Engineering, Graduate School of Science and Technology, Meiji University, 1-1-1 Higashimita, Tama-ku, Kawasaki, Kanagawa 214-8571, Japan. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (T. Onaya), [email protected] (T. Nabatame). ⁎

https://doi.org/10.1016/j.mee.2019.111013 Received 15 April 2019; Received in revised form 23 May 2019; Accepted 25 May 2019 Available online 29 May 2019 0167-9317/ © 2019 Elsevier B.V. All rights reserved.

Microelectronic Engineering 215 (2019) 111013

T. Onaya, et al.

2. Experimental

(a) As-grown

Metal–ferroelectric–metal (MFM) capacitors of TiN/HZO/TiN were fabricated as follows: A 10-nm-thick HZO film was deposited on a 15nm-thick TiN bottom-electrode (BE-TiN) by PE-ALD at 300 °C using (Hf/Zr)[N(C2H5)CH3]4 (Hf:Zr = 1:1) as a cocktail precursor and O2 plasma as an oxidant gas. A 100-nm-thick TiN top-electrode with an area of approximately 1.5 × 10−4 cm2 was then fabricated by directcurrent sputtering. After fabrication of the TiN/HZO/TiN capacitor, post metallization annealing (PMA) was carried out at 300–400 °C for 1 min in a N2 ambient atmosphere. An as-grown HZO film deposited by TH-ALD with H2O gas was also prepared as a reference. The thicknesses of the HZO films were measured by spectroscopic ellipsometry and cross-sectional transmission electron microscopy (TEM). The crystal structures of the HZO films were determined using X-ray diffraction (XRD). The crystallinity and morphology were observed using TEM. Polarization–voltage (P–V) and current–voltage (I–V) measurements were carried out using a Keithley 4200-SCS semiconductor characterization system at room temperature under ambient conditions. Capacitance–voltage (C–V) measurements were also carried out at room temperature under ambient conditions using an Agilent B1500A semiconductor device analyzer.

O/T/C(111)

Intensity (arb. unit)

370

320

PE-ALD

270

220

170

TH-ALD 120

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20

25 26 27 28 29 30 31 32 33 34

2θ (degree) (b) PMA O/T/C(111)

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3. Results and discussion

1450

Intensity (arb. unit)

3.1. Characteristics of PE-ALD HZO films The Hf/Zr ratio in the HZO films after the deposition of PE-ALD at 300 °C was estimated by X-ray photoelectron spectroscopy (XPS) analysis to be 0.43/0.57 [31,32]. Fig. 1(a) shows the XRD patterns of 10nm-thick HZO films by the TH- and PE-ALD methods at 300 °C. The HZO film deposited by TH-ALD had an amorphous structure, as no notable peak was confirmed in the pattern. On the other hand, for the as-grown HZO film deposited by PE-ALD, a small diffraction peak from the (111) plane of O/T/C phases appeared at 2θ ≈ 30.5°. It is difficult to separate the peak at 2θ ≈ 30.5° into separate peaks for the O, T, and C phases because the peak positions are extremely close. The XRD pattern shows that the PE-ALD HZO film could be crystallized and consisted dominantly of O/T/C phases. After PMA at 300–400 °C, the peak originating from the metastable O/T/C (111) was clearly observed while those from the stable monoclinic (M) phase with a paraelectric property, which has two obvious peaks from the (−111) and (111) planes at 2θ ≈ 28.6° and 31.7°, were fully suppressed, as shown in Fig. 1(b). Figs. 2(a) and (b) show cross-sectional TEM images of as-grown PEALD and its 300 °C-PMA-treated 10-nm-thick HZO films, respectively. For the as-grown HZO film, lattice fringes with different orientations were partially observed, while the other part of the film remained an amorphous structure. Based on this and the XRD pattern shown in Fig. 1(a), we found that the as-grown HZO film was partially crystallized with a grain size of ~5 nm and consisted mainly of O/T/C phases. Furthermore, the HZO film after PMA at 300 °C had a polycrystalline structure, which was fully crystallized with a grain size of 10–20 nm, determined from Figs. 1(b) and 2(b).

1350

400°C 1250

1150

350°C 1050

950

300°C 850

750

25 26 27 28 29 30 31 32 33 34

2θ (degree) Fig. 1. XRD patterns of (a) as-grown HZO films deposited by TH- and PE-ALD at 300 °C, and (b) PMA-treated HZO films deposited by PE-ALD. The thickness of the HZO film was 10 nm. The annealing temperature was varied from 300 to 400 °C.

as shown in Figs. 3(a) and (b). Note that the PMA capacitor, even at 300 °C, showed excellent 2Pr and k values of 34 μC/cm2 and 39, respectively. These high 2Pr and k values result from the formation of ferroelectric O, T, and C phases which have higher k values than the M phase according to the Clausius–Mossotti relation [33–36]. Moreover, the 2Pr value of the PMA capacitors increased as the annealing temperature increased, while these films showed almost the same k. This might be because of the larger mechanical stress to the HZO films from the TiN electrodes, leading to the formation of ferroelectric O phase, as the annealing temperature increased. The leakage current density–electric field (J–E) properties of asgrown and PMA capacitors are shown in Fig. 5. The as-grown capacitor showed a remarkably low J value especially in a low E region compared to PMA capacitors. In addition, the PMA capacitors showed slightly higher J when the PMA temperature was increased. All capacitors exhibited a breakdown E of 3–4 MV/cm, which matched reported values in previous studies [19]. The breakdown E of the PMA capacitors gradually decreased from 3.7 to 3.2 MV/cm with increasing annealing temperature. The leakage current generally passed through grain

3.2. Electrical properties of PE-ALD HZO films Figs. 3(a) and (b) show the polarization–electric field (P–E) and capacitance–electric field (C–E) characteristics of as-grown and PMAtreated MFM capacitors, respectively. The remanent polarization (2Pr = Pr+ − Pr−) and k values were also estimated from the P–E and C–E properties, respectively, and are shown in Fig. 4. The as-grown capacitor exhibited small hysteresis loops with 2Pr and k values of ~2 μC/cm2 and 27, respectively, due to the existence of the amorphous region, which typically has a lower k value than the crystal phase. On the other hand, the PMA capacitors clearly exhibited hysteresis loops, 2

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(a) As-grown

P (µC/cm2)

TE-TiN HZO (10 nm)

30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -35 4.0

BE-TiN C (µF/cm2)

3 nm (b) PMA 300°C

(a) P-E

As-grown 300°C 350°C 400°C

C-E-2.0 -1.0 0.0 1.0 2.0 3.0 -4.0(b) -3.0

E (MV/cm)

3.0 2.0 As-grown 300°C 350°C 400°C

1.0

TE-TiN

0.0

HZO (10 nm)

-4

-3

-2

-1

0

1

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4

E (MV/cm) Fig. 3. (a) P-E and (b) C-E properties of as-grown and PMA-treated 10-nm-thick HZO films which were deposited by PE-ALD.

50

70

BE-TiN

60

40

Fig. 2. Cross-sectional TEM images of (a) as-grown and (b) 300 °C-PMA-treated 10-nm-thick HZO films fabricated using PE-ALD.

boundaries of crystalline HZO films [37,38]. Thus, we assumed that J value increased because the grain boundaries are more clearly formed for increasing annealing temperature. Therefore, we found that a superior J property of HZO film was obtained using an annealing process with a low temperature of 300 °C. The relationship between the process temperature and the 2Pr value of the ferroelectric HfO2-based films is summarized in Fig. 6. The HZO films generally exhibited high 2Pr values in the low process temperature range of 400–700 °C while the data of Si- [39], Al- [40,41], and Ladoped [42] HfO2 films were located above 600 °C. This is considered to be related to the formation temperature of ferroelectric O/T/C phases, and the process temperature became high in the HfO2 film with doping because the crystallization temperature of SiO2, Al2O3, and La2O3 is higher than that of ZrO2 [35–37,43–47]. Furthermore, in case of the HZO film, the large fluctuation of 2Pr value was observed, suggesting that the difference of the amount of the ferroelectric O phase occurs. It is clear that the 2Pr value of our HZO films shifted to a lower process temperature below 400 °C while maintaining a high 2Pr value. As explained in Figs. 1 and 2, the HZO films could be formed a polycrystalline structure with dominantly O/T/C phases even at a low temperature range 300–400 °C. At first, we paid attention to preferential growth of O/T/C phases in ZrO2 films without doping because of favorable surface energy effect, while no HfO2 film without doping formed O/T/C phases [46]. Next, it

50 30

40

20

30

k

2Pr (µC/cm2)

3 nm

20

2Pr k

10

As-grown PMA

0

10 0

300

350

400

Process temperature (°C) Fig. 4. 2Pr and k values of HZO films as a function of the annealing temperature from 300 to 400 °C.

is considered that PE-ALD process must be related to the formation of O/T/C phases of ZrO2 film. ZrO2 is known to be candidate material as capacitor insulator for dynamic random access memory and its structure change under ALD condition has been aggressively studied to obtain high-k. Actually, as-grown ZrO2 films deposited by ALD using O3 at 270–350 °C had a polycrystalline structure with O/T/C phases [35–37,46,47]. The extent of crystallization and the morphology of asgrown ZrO2 films is known to be influenced by the oxidizing strength (H2O < O3 < plasma O2) of oxidant gas during ALD process. The structure of the as-grown ZrO2 film was changed from amorphous structure to the metastable T phase by increasing O3 concentration during ALD process [48]. Therefore, like our study, the HZO film with polycrystalline with the metastable O/T/C phases could be easily formed by using PE-ALD process with strong oxidizing gas (plasma O2). 3

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exhibited a superior J property compared to that after PMA at 400 °C. Therefore, nanocrystals with the metastable O/T/C phases in the asgrown HZO film could play an important role as nuclei of the crystal growth of ferroelectric HZO film during PMA process. Based on these results, a low temperature fabrication process 300 °C is found to be achieved using PE-ALD technique for next-generation flexible electric device applications with HZO film.

100

J (A/cm2)

10-2

10-4

Acknowledgments

10-6 As-grown 300°C 350°C 400°C

10-8 10-10 0

1

2

3

This study was partially supported by JSPS KAKENHI (JP18J22998) and Japan Science and Technology Agency (JST) CREST (JPMJCR13C3). The authors wish to thank all the staff members of the Nanofabrication Group of NIMS for their support in fabricating the MFM capacitors.

4

E (MV/cm)

Declaration of competing interests

Fig. 5. J-E characteristics of as-grown and PMA-treated 10-nm-thick HZO films which were deposited by PE-ALD.

None. References

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2Pr (µC/cm2)

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40

HZO Si doped HfO2 Al doped HfO2 La doped HfO2

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[17]

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30 [13] [40]

[14]

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[42]

[15] [41] [39]

10 [22] [20]

0 300 350 400 450 500 550 600 650 700 750 800 850 900

1000

Process temperature (°C) Fig. 6. Relationship between the process temperature from 300 to 1000 °C and the 2Pr value of the ferroelectric HfO2-based thin films.

Next, we discuss the crystal growth of the HZO films during the PMA process. We previously reported a new fabrication technique of HZO films using ZrO2 seed layers [31,32]. In this method, epitaxial-like grain growth of the HZO film was observed from crystallized ZrO2 layers with O/T/C phases during the annealing process, resulting in high 2Pr value. Considering these dates, therefore, the HZO film could be crystallized through crystal grains of the as-grown HZO film as nuclei during PMA treatment even at a temperature range under 400 °C, leading to the metastable O/T/C phase formation, as shown in Figs. 1(b) and 2(b). This indicates that the primitive crystal structure (O/T/C phases) of the as-grown HZO film is significantly important in determining the crystal structure of the HZO film after the PMA process. Based on these experimental results, PE-ALD process can be considered a candidate deposition method of HZO thin films to achieve superior ferroelectricity for a low temperature fabrication process below 300 °C. 4. Conclusion The crystallization and characteristics of MFM capacitors with HZO films fabricated by PE-ALD and a low temperature PMA process of ≤400 °C, were studied systematically. The as-grown HZO film deposited by PE-ALD at 300 °C was partially crystallized with an average grain size of ~5 nm and consisted mainly of O/T/C phases. Consequently, excellent 2Pr and k values of 34 μC/cm2 and 39, respectively, of the HZO film were achieved using the PMA process at 300 °C due to the formation of O/T/C phases without the paraelectric M phase of the HZO film. Furthermore, the HZO film after PMA at 300 °C 4

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