Impact of Chamber/Annealing Temperature on the Endurance Characteristic of Zr:HfO₂ Ferroelectric Capacitor

Abstract

The endurance characteristic of Zr-doped HfO₂ (HZO)-based metal–ferroelectric–metal (MFM) capacitors fabricated under various deposition/annealing temperatures in the atomic layer deposition (ALD) process was investigated. The chamber temperature in the ALD process was set to 120 °C, 200 °C, or 250 °C, and the annealing temperature was set to 400 °C, 500 °C, 600 °C, or 700 °C. For the given annealing temperature of 700 °C, the remnant polarization (2P_r) was 17.21 µC/cm², 26.37 µC/cm², and 31.8 µC/cm² at the chamber temperatures of 120 °C, 200 °C, and 250 °C, respectively. For the given/identical annealing temperature, the largest remnant polarization (P_r) was achieved when using the chamber temperature of 250 °C. At a higher annealing temperature, the grain size in the HZO layer becomes smaller, and thereby, it enables to boost up P_r. It was observed that the endurance characteristics for the capacitors fabricated under various annealing/chamber temperatures were quite different. The different endurance characteristics are due to the oxygen and oxygen vacancies in ferroelectric films, which affects the wakeup/fatigue behaviors. However, in common, all the capacitors showed no breakdown for an externally applied pulse (up to 10⁸ cycles of the pulse).

1. Introduction

Ferroelectric materials have been widely used/adopted for various types of sensors and devices. Among various ferroelectric materials, HfO₂-based ferroelectric devices have attracted great interest [1]. A HfO₂-based ferroelectric film with fluorite structure solved the drawbacks in the conventional perovskite–structure ferroelectrics. They have extraordinary compatibility with complementary metal-oxide semiconductors (CMOS) and excellent ferroelectricity at ultra-thin (<10 nm) thickness [2,3]. The ferroelectric properties of HfO₂-based film originated from the non-centrosymmetric orthorhombic phase (o-phase), and the stabilization of the o-phase enhances the ferroelectric behavior [4]. The ferroelectric phase can be stabilized through annealing, and it can be characterized differently by various factors, such as dopant, thickness of ferroelectric film, and deposition temperature [5,6].

Various dopants with HfO₂ have been studied, and among them, zirconium (Zr) was chosen as the most promising material for memory and logic devices [6,7]. Unlike other dopants remaining stable at much lower concentration, Zr dopants can be stable with the same percentage as Hf in HZO film. Moreover, ferroelectric properties using Zr dopants can be obtained in much lower annealing temperature (T_A) than other dopants.

As a memory device, the ferroelectric films require good endurance properties. The main factor affecting the endurance is the oxygen vacancy (V_O) in ferroelectric films. During the electric field cycling, V_O is redistributed, which results in the uniform distribution of V_O in the bulk region of the ferroelectric layer [8]. This phenomenon increases the P_r value by decreasing the built-in field and then decreases the P_r value with additional cycling. This increase in P_r is called the “wake-up effect” and the decrease in P_r is called the “fatigue effect” [9,10]. The breakdown of the films is observed when V_O forms a filament, which results as a leakage path. Therefore, it is very important to control the amount of V_O for the reliability of memory devices.

The characteristics of ferroelectric films can vary through various methods such as adjusting the doping effect and the chamber temperature during ALD [11,12]. Adjusting the temperature of the chamber during ALD changes the deposition rate and the average grain size of the film [6,13]. These factors change the distribution of V_O, which can significantly affect the ferroelectricity and the endurance properties. However, studies on the relationship of the effects of chamber temperature and the endurance performance are still lacking.

In this study, the ferroelectric properties of TiN/HZO/TiN capacitors with different chamber temperatures were investigated. The electrical characteristics of each capacitor were analyzed through polarization–voltage (P-V) curves and leakage current–voltage (I-V) curves. Moreover, the endurance performance related to the amount of V_O was investigated under different chamber temperatures.

2. Fabrication

The illustrated cross-sectional view and fabrication flow of TiN/HZO/TiN capacitors are shown in Figure 1a,b, respectively. First, a p⁺-doped silicon wafer was cleaned by a SPM cleaning, which was followed by the conventional RCA method (i.e., SC-1 cleaning and SC-2 cleaning). Then, the 50 nm-thick TiN bottom electrode was deposited on the Si substrate by using DC sputtering. The 10 nm-thick HZO thin film was deposited by thermal atomic layer deposition (ALD). The tetrakis (ethylmethylamino) hafnium (TEMAH), tetrakis (ethylmethylamino) zirconium (TEMAZ), and H₂O source precursor were used for the ALD process to deposit the HZO film. The Hf and Zr was deposited using an ALD supercycle [14]. The aforementioned fabrication was identically completed but at three different chamber temperatures (T_CH) of 120 °C, 200 °C, and 250 °C. As shown in Figure 1c, the growth per (super)cycle of HZO film decreased as the chamber temperature (T_CH) increased. The total number of supercycles to make 10 nm-thick HZO thin film was set to 56, 61, and 68 for T_CH of 120 °C, 200 °C, and 250 °C, respectively. The 50 nm-thick TiN top electrode on the HZO layer was deposited using the DC sputtering used for bottom electrodes. Then, the HZO capacitors were patterned to have the electrode area of 6400 µm². Finally, the post-metallization annealing (PMA) was completed by rapid thermal annealing (RTA) at 400, 500, 600, and 700 °C for 30 s in N₂ atmosphere to crystallize the HZO films.

To investigate the electrical characteristics of HZO capacitors, P-V curves and I-V curves were measured using semiconductor parameter analyzer (Keithley 4200-SCS). The X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS) were used to characterize the HZO capacitor.

3. Results and Discussion

3.1. Electrical Characteristics

Figure 2a–c show the measured P-V curves of the TiN/HZO/TiN capacitors with a few annealing temperatures (i.e., 400, 500, 600, and 700 °C) at T_CH of 120 °C, 200 °C, and 250 °C, respectively. The measured P_r was increased at a higher T_A for all the capacitors. This is mainly because the ratio of o-phase in the HZO layer was increased at the higher T_A [2,10]. For a given T_A, a higher P_r was implemented at a higher T_CH. This achievement was physically originated from a few reasons, as follows: (1) The number of supercycles increases at a higher T_CH because the deposition rate of HZO film is decreased at a higher T_CH (see Figure 1c). This leads to increasing the number of HfO₂/ZrO₂ nanolaminates, resulting in the improved P_r [15]. (2) The average grain size of HZO film becomes smaller at a higher deposition temperature, if H₂O is used as the oxygen source in ALD [13]. The smaller average grain size enables us to boost up the ferroelectric polarization [10,16]. This is primarily originated from the tetragonal phase (t-phase) and the orthorhombic phase (o-phase), which are easily stabilized in smaller grain regions. (3) Because defects (noted as V_O) are likely to be accumulated along the grain boundaries, the smaller grain size is likely to build more grain boundaries for a given volume. This would distort the distribution of V_O. Herein, it is noteworthy that the well-balanced distribution of V_O in HZO films should help stabilize the ferroelectric phase, and thereby, the ferroelectric polarization would be enhanced [10,17].

In Figure 2d, the leakage current of TiN/HZO/TiN capacitors fabricated at different T_A and T_CH was measured. It turned out that for a given T_CH (T_A), the leakage current increases at a higher T_A (T_CH). In the MFM ferroelectric capacitor, the leakage current is mostly originated from V_O (i.e., the higher V_O is, the more the leakage current flows [15]). In Section 3.2, with TEM, XPS, and EDS data, it is investigated how V_O is varied at different T_CH.

3.2. XPS, TEM, and EDS

Figure 3 shows the XPS depth profile of the TiN/HZO/TiN capacitor at T_A of 700 °C under three different T_CH values. The atomic percent clearly reveals the TiN/HZO/TiN device structure. Among many atoms in the profile, the O 1s in the HZO layer is deconvoluted to figure out the oxygen bonds in great detail (see Figure 4). Notice that the intensities of each net peak were normalized to the same scale. The O 1s was divided into two peaks (i.e., lattice oxygen peak and sub-oxide peak). The lattice oxygen peak describes the bonds such as Hf-O and Zr-O, of which the binding energy is located in ≈532 eV. The sub-oxide peak indicates oxygen in a lattice, which does not contain its full complement of oxygen (and therefore, indicating the presence of vacancies). The peak describes the bonds such as oxygen interstitial and oxygen vacancy (V_O), of which the binding energy locates at ≈534 eV.

The amount of O 1s in Figure 3 and Figure 4a confirms that there was a difference in the ratio of O 1s in HZO film, depending on three different T_CH. The intensity of peak amplitude is decreased at a higher T_CH. In addition, the atomic percent of Ti and N is decreased at a lower T_CH (see Figure 3). In other words, the formation of the dead layer (i.e., TiO₂ and TiON) between the HZO layer and the TiN electrode was suppressed at a lower T_CH, which reduces the amount of V_O in the HZO layer. It was confirmed that the ratio of V_O increases if the dead layer between the HZO layer and the TiN electrode is formed [18].

It was observed that the percentage of sub-oxide bonding is increased at a higher T_CH (see Figure 4a). This also indicates that the ratio of V_O increases. In other words, the oxygen should move from the HZO layer to the electrodes to make the interfacial layer, and thereby, the V_O in HZO film is increased [17]. Hence, the increased V_O means that the larger amount of oxygen has moved toward the interfacial layer. In addition, in the XPS for Hf 4f spectra, it was confirmed that the sub-oxide means the oxygen vacancy in the HZO layer [19]. As shown in Figure 4b, it was observed that the ratio of sub-oxide in Hf 4f spectra is increased when T_CH is increased, which leads to a higher V_O. In summary, the ratio of oxygen in the HZO layer becomes smaller at a higher T_CH, which is closely associate with the amount of V_O. A higher T_CH would make the ratio of V_O higher. This is well agreed to the increase in leakage current at a higher T_CH because of the increased V_O (see Figure 2d).

Figure 5a–c show the TEM image of TiN/HZO/TiN capacitors fabricated at three different chamber temperatures (T_CH) with T_A of 500 °C. It was observed that the thicknesses of the HZO films were all the same: 10 nm. Figure 5d shows the EDS image of the TiN/HZO/TiN capacitor fabricated with T_A of 700 °C. The ratio of elements in each layer can be determined through the EDS analysis (see

Table 1. Energy-dispersive spectrometer (EDS) quantification of elements in HZO layer.

Chamber Temperature (TCH) (°C)	Atomic Percent (%)
	Ti	N	Hf	Zr	O	Si
120	4.50	9.76	20.90	18.16	30.44	16.24
200	9.20	4.49	24.95	21.00	23.32	17.03
250	10.74	10.80	24.75	21.18	19.38	13.15

). For example, the largest (smallest) atomic rate of oxygen in HZO films was implemented with T_CH of 120 °C (250 °C), which was consistent with the XPS analysis. The amount of oxygen vacancy (V_O) was increased, as the oxygen was decreased with increasing T_CH. However, the discrepancy of the ratio of elements exists between the XPS and EDS analysis. Note that the purpose of the EDS analysis in this work was to confirm/verify the change of oxygen atoms.

3.3. Endurance

The endurance (especially, affected by an electric field cycling) in TiN/HZO/TiN capacitors was investigated. Figure 6 shows the pulsing scheme for evaluating the endurance of the capacitors. The electric field cycling was completed with using a trapezoidal pulse, and the number of cycling was applied up to 10⁸. After the cycling pulses were applied, a triangular pulse was applied to measure the P-V characteristic of the capacitor. Note that both pulses have the same peak amplitude of 3 V. The P-V characteristic of each capacitor was compared to each other (see Figure 7).

The remnant polarization (P_r) of each HZO capacitor in pristine state is as follows: In case of T_CH of 120 °C, P_r of 0.28 µC/cm², 1.17 µC/cm², 2.68 µC/cm², and 17.21 µC/cm² was observed for T_A of 400 °C, 500 °C, 600 °C, and 700 °C, respectively. In case of T_CH of 200 °C, P_r of 5.66 µC/cm², 6.88 µC/cm², 13.43 µC/cm², and 26.37 µC/cm² was observed for T_A of 400 °C, 500 °C, 600 °C, and 700 °C, respectively. Finally, in case of T_CH of 250 °C, P_r of 16.38 µC/cm², 20.46 µC/cm², 25.3 µC/cm², and 31.8 µC/cm² was observed for T_A of 400 °C, 500 °C, 600 °C, and 700 °C, respectively. Regardless of T_CH, it turned out that P_r was increased with increasing T_A. For a given/identical T_A, P_r can be improved with higher T_CH. This is primarily because the amount of ferroelectric phase was increased with increasing either T_A or T_CH [2,9].

As shown in Figure 8, all the capacitors did not show any breakdown up to 10⁸ cycles. However, depending on T_CH, the number of cycles at which the fatigue begins (i.e., P_r is about to decrease) is varied. Regardless of T_CH, higher P_r was observed with higher T_A. This is primarily originated from a well-distributed V_O in bulk region of HZO film at a higher T_A [10].

It is known that the ferroelectric characteristics (notably, represented by P_r) can be improved as the number of cycles increases (a.k.a., wake-up effect). However, the ferroelectric characteristics should not be ever enhanced because of fatigue. The maximum value of 2P_r and the number of cycles at which fatigue begins (a.k.a. critical number of cycles) are summarized in Figure 9. It turned out that the value of 2P_r increases with increasing T_A. (However, only for T_CH of 120 °C, T_A of 400 °C shows insufficient ferroelectricity, which results in a low 2P_r.) Compared to the other T_CH of 200 °C and 250 °C, the measured P_r was significantly degraded for T_CH of 120 °C. When H₂O reactant is used as an oxygen source in ALD, the lowest T_CH (i.e., 120 °C) forms the largest average grain size. This causes more formation of monoclinic phase (m-phase), which is non-ferroelectric phase, and it makes the depolarization field stronger [20,21]. Therefore, for T_CH of 120 °C, the sudden decrease in P_r in the HZO capacitor was understood with less m-phase.

In Figure 9, it was also observed that the critical number of cycles (i.e., the number of cycles at which fatigue begins) decreases at a higher T_CH for the same T_A. As shown in Figure 2d and Figure 4, more V_O in the ferroelectric layer was observed at a higher T_CH, and thereby, so is the leakage current in HZO capacitor (this is because the excessive amount of V_O makes undesirable conducting paths in the HZO film [10,22]). The more V_O in the ferroelectric film should cause the ferroelectricity of the film to be fatigued at a rapid pace.

4. Conclusions

In this work, the impact of chamber/annealing temperatures in the atomic layer deposition (ALD) process on the ferroelectric property of the HZO layer in a TiN/HZO/TiN capacitor was investigated. Regardless of the chamber temperature (T_CH), a higher remnant polarization (P_r) was achieved with a higher annealing temperature (T_A). For a given T_A, P_r was increased with increasing T_CH. This ferroelectricity was well retained up to the cycles of 10⁸. However, the capacitors fabricated under the three different T_CH showed different endurance performances in terms of the critical number of cycles. This is due primarily to the oxygen and oxygen vacancies in the HZO layer (which was quantitatively analyzed and confirmed by XPS and EDS analysis). The more oxygen vacancies (V_O) at a higher T_CH enabled for P_r to be improved, but they made an undesirable conducting path in the HZO film, resulting in decreasing the critical number of cycles.