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C7H8 precursor and ozone
Journal of Alloys and Compounds 701 (2017) 310e315
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ZrO2 film prepared by atomic layer deposition using less viscous cocktail CpZr[N(CH3)2]3/C7H8 precursor and ozone Jong-Ki An a, b, 1, Jin-Tae Kim b, c, 1, Goru Kang b, Nam Khen Oh b, Sung-Ho Hahm a, Geunsu Lee d, In-Sung Park e, **, Ju-Young Yun b, c, * a
School of Electronics Engineering, Kyungpook National University, Daegu 41566, Republic of Korea Vacuum Center, Korea Research Institute of Standards and Science (KRISS), Daejeon 34113, Republic of Korea Department of Nanomaterials Science and Engineering, University of Science and Technology, Daejeon 34113, Republic of Korea d Eugene Technology Materials, Gyeonggi-do 16675, Republic of Korea e Institute of Nano Science and Technology, Hanyang University, Seoul 04763, Republic of Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 October 2016 Received in revised form 30 December 2016 Accepted 30 December 2016 Available online 1 January 2017
The precursor consists of a mixture of CpZr(NMe2)3 (Cp ¼ C5H5, Me ¼ CH3) and cycloheptatriene (C7H8), CpZr(NMe2)3/C7H8, is introduced as a precursor in the atomic layer deposition (ALD) of zirconium oxide (ZrO2). The cocktail CpZr(NMe2)3/C7H8 chemical exhibits a higher vapor pressure of 1.2 torr and a lower viscosity of 7.0 cP at 100 C than the pure CpZr(NMe2)3. In the ALD of ZrO2 films, CpZr(NMe2)3/C7H8 and O3 act as the metal precursor and oxidant, respectively. Self-limited growth of ZrO2 films occurs after a 2 s pulse of CpZr(NMe2)3/C7H8, and a growth rate of 0.8e0.9 Å/cycle is obtained over a wide temperature range of 250e400 C. ZrO2 film formed at 300 C is stoichiometric with a lower impurity level of carbon and shows a tetragonal polycrystalline phase dominantly. The fabricated TiN/ZrO2/TiN capacitors exhibit compatible capacitance and leakage current properties with a quadratic voltage coefficient, inversely proportional to the dielectric film thickness (t) according to a~t1.8. © 2017 Elsevier B.V. All rights reserved.
Keywords: Atomic layer deposition Thin film ZrO2 Cocktail precursor CpZr(NMe2)3 Cycloheptatriene
1. Introduction Insulating dielectric zirconium oxide (ZrO2) thin films have attracted considerable recent attention in a wide range of applications [1]. This has been because of their favorable properties including high dielectric constant (k), wide band gap, suitable band offset on Si, acceptably low leakage current, and good thermal stability [2e4]. These properties of ZrO2 films have allowed them to be applied in microelectronics. Specific applications have included as dielectrics or insulators in metal-insulator-metal (MIM) capacitors of dynamic random access memory (DRAM) [5], in MIM capacitors of radio frequency (RF) and analog/mixed-signal (AMS) integrated circuits [6], in metal-insulator-semiconductor gate capacitors [7], and in resistor unit of resistive non-volatile memory
* Corresponding author. Vacuum Center, Korea Research Institute of Standards and Science (KRISS), Daejeon 34113, Republic of Korea. ** Corresponding author. E-mail addresses: [email protected] (I.-S. Park), [email protected] (J.-Y. Yun). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.jallcom.2016.12.420 0925-8388/© 2017 Elsevier B.V. All rights reserved.
[8]. For example, MIM capacitors containing Al2O3/ZrO2/SiO2/ZrO2/ Al2O3 stacks have achieved desirable electrical properties for RF and AMS circuits, including a high capacitance density and breakdown voltage, and a low leakage current and degree of voltage linearity [9]. Thin ZrO2 films have been prepared by various methods such as atomic layer deposition (ALD) [10,11]. ALD has been widely used to fabricate various metal oxide films and has become one of the leading film deposition techniques [12]. In the ALD process of metal oxide, the film grows via the surface reaction of vaporized metal precursor and oxidant that are alternately supplied to the substrate surface [13]. For the formation of metal oxides such as ZrO2 films, a complete ALD cycle for monolayer growth consists of a pulse of metal precursor (i.e., Zr precursor), a purge of inert gas, a pulse of oxidant, and a purge of inert gas. Each pulse step of precursor and oxidant should be well isolated to prevent any gas phase reaction between them. The significant advantage of the ALD process is the self-limiting growth of the monolayer. Despite its slow growth rate, ALD provides high quality thin films, good control over the thickness at the atomic-layer level, widely uniform coating thickness, and perfect conformal coatings on complex-structured surfaces [14].
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A suitable choice of metal precursor is crucial for a successful ALD process. The metal precursor for film growth requires sufficiently high vapor pressure, high purity, thermal and chemical stability, low viscosity, and effective cost. An appropriate precursor can provide benefits such as allowing film deposition at wide temperature ranges, yielding stoichiometric film composition, allowing a higher growth rate, and generating fewer toxic byproducts [15]. For thermal and plasma-enhanced ALD process, various Zr precursors have been used to grow ZrO2 and Zr-based films. ALD processes using precursors with Cl-, O-, N-, and C-coordinated ligands (and associated heteroleptic ligands) have been reported [16]. ZrCl4 is the representative precursor containing Cl-coordinated ligands. It exhibits high reactivity and thermal stability, so ZrO2 films can be deposited at high deposition temperature with ZrCl4 and possess good electrical properties [17,18]. During the ALD process, the aggressive reaction between Cl in ZrCl4 and Hþ in H2O generates corrosive HCl, which reduces the film growth rate and narrows the ALD window by lowering its upper temperature limit. Furthermore, the incorporation of corrosive Cl-containing contaminants into the ZrO2 film can result in the time-dependent degradation of the film quality and resulting electrical properties. These disadvantages prevent ZrCl4 from being applied in the mass production of ZrO2 films, especially for sensitive microelectronic applications. To avoid Cl contamination, precursors with O-coordinated ligands such as alkoxides have also been used in ALD. The steric hindrance of tert-butoxide ligands of the Zr(OBut)4 precursor prevent its oligomerization during the ALD process. However, alkoxide ligands are easily decomposed by trace water, owing to their catalytic hydrolysis reaction [19]. For example, the self-decomposition of tert-butoxide groups corresponds to a process involving no ALD growth. The growth rate of the ZrO2 film varies from 0.5 to 6.5 Å/ cycle, when the Zr(OtBu)4 pulse length is modulated from 0.2 to 1.5 s [20e22]. N-coordinated alkylamido-type precursors such as tetrakis(ethylmethylamino)zirconium have been developed to yield a high growth rate and high volatility for ALD processes. However, its low thermal stability limits the upper ALD temperature window to 300 C [23,24]. Thermal stability is very important in the ALD process, because film deposition at low temperatures results in high levels of impurities, low film densities, and a high degree of C and eOH bonds [15]. N-coordinated Zr-precursors also exhibit high viscosity. There can be issues with contamination inside the chamber and line, which results in film contamination and necessitates frequent maintenance of instrumentation. The cyclopentadienyl (Cp ¼ C5H5)-based precursors, (CpMe)2ZrMe2 (Me ¼ CH3) [25], (CpMe)2Zr(OMe)Me [25], Cp2Zr(Me)2 [26], and Cp2ZrCl2 [26] have recently been used to achieve increased thermal stability during ALD. The thermal stabilities of Cp-precursors are generally higher than those of N-coordinated alkylamide-based precursors. However, the growth rates of ZrO2 films when using the Cp-based precursors (about 0.5 Å/ cycle) are lower than those when using the alkylamide-based precursor (about 1.0 Å/cycle). Heteroleptic Cp-alkylamide precursors such as (MeCp) Zr(NMe2)3 [24], (EtCp)Zr(NMe2)3 (Et ¼ C2H5) [24], and CpZr(NMe2)3 using ozone as an oxidant have exhibited high growth rates (0.8e0.9 Å/cycle) for ALD-deposited ZrO2 films, and have high thermal stabilities at high temperatures. However, their ALD temperature window is too narrow. There is a very narrow ALD temperature window of 300e325 C, when Zr(NEtMe)2(guan-NEtMe)2 precursor is used for the growth of ZrO2 with a constant growth rate of 1.3 Å/cycle [27]. The narrow ALD windows hinder Cp-based precursors from applying to mass production of ZrO2 films. Tamm
311
et al. and Jaakko et al. respectively developed the CpTi(C7H8) [28] and CpMeZr(C7H8) [29] precursors, where C7H8 exists in ligand form. ZrO2 thin films prepared with using the precursors based on the Cp- and C7H8-ligands yielded high growth rates (0.7e0.8 Å/ cycle), good film purity, and a reasonable ALD temperature window. However, thermogravimetric analysis indicated impurity levels as high as 4.4% at 400 C for CpMeZr(C7H8) [29]. Viscosity is another important precursor property in mass production, because viscous materials remain in the precursor supply line and ALD chamber for longer. Higher viscosity of precursor directly results in the contamination of ALD apparatus. The values of viscosity of Zr precursors are typically higher than those of other precursors containing group IV element, such as precursors based on Ti and Hf. Therefore, the inert gas purge time and downtime of the ALD apparatus for preparing ZrO2 films are longer than those when preparing TiO2 and HfO2 films. The viscosity of precursors have so far received little investigation or discussion. Using the respective advantages of stable CpZr(NMe2)3 and fluent C7H8, their mixture CpZr(NMe2)3/C7H8 was synthesized to get a precursor with lower viscosity. In the current study, the less viscous precursor CpZr(NMe2)3/C7H8 is investigated for its chemical, physical, and electrical properties. This cocktail precursor has a similar form with the reported heteroleptic Cp- and C7H8-based precursors, but exhibits very different properties. The chemical and thermal properties of CpZr(NMe2)3/C7H8 are analyzed. The ALD conditions for fabricating ZrO2 films with this precursor are investigated and optimized. The structural and chemical properties of the resulting ZrO2 films are probed. The electrical properties of ZrO2 films are characterized using MIM capacitors for RF and AMS applications. 2. Experimental 2.1. ALD of ZrO2 films prepared from CpZr(NMe2)3/C7H8 and ozone A cocktail chemical of CpZr(NMe2)3 and C7H8, CpZr(NMe2)3/ C7H8, is used as the metal precursor for the fabrication of ZrO2 films. CpZr(NMe2)3/C7H8 evaporates without any decomposition with a two-step volatilization at about 100 and 150 C and at 204 C. The vapor pressure and viscosity is 3.5 torr and 3.1 cP at 100 C, respectively [Appendix A]. ZrO2 films were deposited on Si(100) substrates by ALD, using a cocktail CpZr(NMe2)3/C7H8 as the metal precursor and O3 as an oxidant. O3 was produced from high quality O2 (>99.999%) by an ozone generator. Prior to the ALD process, Si substrates were dipped in dilute HF solution (~1% HF in deionized water) for 2 min to remove native oxides. Substrates were then rinsed with deionized water, and dried with Ar gas. The cleaned substrates were then introduced onto the heated ALD susceptor and left to stand for 30 min in an inert Ar environment before the ALD process initiated for the temperature stabilization. The CpZr(NMe2)3/C7H8 precursor was vaporized at 65 C, and introduced into the showerhead-type thermal ALD chamber using an Ar carrier gas [11]. To determine the optimal ZrO2 deposition conditions, the substrate temperature range was varied over 200e400 C, and the Zr pulse time was varied over 0.5e5 s. One ALD cycle consisted of four steps: (1) pulsing with CpZr(NMe2)3/C7H8; (2) purging with Ar gas; (3) pulsing with ozone; (4) purging with Ar gas. 2.2. Characterization of deposited ZrO2 films The thicknesses of the ALD-deposited ZrO2 films on Si substrates were measured using ellipsometry. The chemical compositions and impurities in the bulk ZrO2, 18 nm thick, were checked by X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific, Theta
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Probe AR-XPS) after just removing the surface layer contaminated with carbon and/or water using a 10 kV Arþ ion etching for 10 min. Auger electron spectroscopy (AES; PHI, 680) was used to analyze the depth profiles of the films. The crystallinities of the films were investigated using grazing incidence X-ray diffraction (XRD; Rigaku, SmartLab). The roughness and morphology of the film surfaces were probed by atomic force microscopy (AFM; Park system, XE-150). 2.3. Dielectric and electrical properties of ALD-deposited ZrO2 films To evaluate the dielectric and electrical parameters of the ZrO2 films, TiN/ZrO2/TiN MIM capacitors were fabricated for RF and AMS device applications. TiN was deposited on the Si substrate by DC sputtering at room temperature. The top electrodes of the devices were formed using an aligner system with a conventional lift-off method. The ALD-ZrO2 dielectric was prepared from CpZr(NMe2)3/C7H8 and O3 in an optimized process. During ZrO2 deposition, the chamber temperature was maintained at 300 C. Dielectric films with thicknesses of 8e18 nm were grown on the TiN substrates. Capacitance-voltage (C-V) measurements were performed to evaluate the dielectric properties using a HP 4284 LCR meter for MIM capacitors. The capacitance was measured by sweeping the top electrode voltage from þ2 V to 2 V and then sweeping in the reverse direction with a small AC signal at 1 kHz. Current-voltage (I-V) curves were measured using a HP 4155 semiconductor parameter analyzer. 3. Results and discussion 3.1. ALD of ZrO2 films from CpZr(NMe2)3/C7H8 precursor Self-saturation is an important characteristic of ALD. A
saturation curve of a ZrO2 film deposited using the CpZr(NMe2)3/ C7H8 precursor at a substrate temperature of 300 C is shown in Fig. 1(a). The curve shows the growth rate ZrO2 films as a function of Zr-precursor pulse time length. The pulse length of CpZr(NMe2)3/ C7H8 is varied from 0.5 to 5 s, and the subsequent purge length is held constant at 10 s. The pulse length of O3 and its subsequent purge length are 5 and 10 s, respectively. Thus, pulse length saturation occurs after 2 s at 300 C for the CpZr(NMe2)3/C7H8 precursor. The precursor is thermally stable, and induces self-limited growth. The ALD window refers to the temperature range over which a constant growth rate occurs, and is an important characteristic of the ALD process. Fig. 1(b) shows the variation in growth rate as a function of deposition temperatures of 200e400 C, for ZrO2 films prepared from the CpZr(NMe2)3/C7H8 precursor using an optimized pulse length of 4 s for CpZr(NMe2)3/C7H8, and a pulse length of 5 s for O3. There is a minor variation in the growth rate of ZrO2 at 250e400 C, over the range of 0.8e0.9 Å/cycle. A wide ALD window with a high upper temperature limit (i.e., allowing a high decomposition temperature) is required for yielding high quality films. Film growth at high temperature makes the films dense with lower content of C and N impurities [24e29]. Fig. 1(c) shows that the thickness of the ZrO2 film increases linearly with the number of process cycles. The ZrO2 growth rate is 0.9 Å/cycle at 300 C. 3.2. Chemical bonding and elemental analysis of ZrO2 films XPS spectra of the Zr 3d and O 1s regions for a ZrO2 film deposited at 300 C are shown in Fig. 2(a) and (b), respectively. After sufficient surface cleaning with Arþ ion etching, the surface contamination of carbon and hydroxyl is considered to be negligible. Curve fitting of the Zr 3d5/2, Zr 3d3/2, and O 1s regions was conducted after Tougaard background subtraction, using the Voigt
Fig. 1. Growth rate of ALD-deposited ZrO2 thin films using CpZr(NMe2)3/C7H8 as a precursor (a) as a function of its pulse length at 300 C and (b) as a function of deposition temperature. (c) ALD growth linearity with a growth rate of 0.9 Å/cycle on the TiN substrate.
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Fig. 2. XPS spectra of (a) Zr 3d and (b) O 1s regions for the ZrO2 film on a TiN substrate.
Table 1 Atomic percentage of bulk ALD-ZrO2 films fabricated with CpZr(NMe2)3/C7H8 and O3 at 300 C. Zr (at.%)
(Zr)-O (at.%)
(Zr)-OH (at.%)
C (at.%)
O/Zr ratio
31.7
62.1
6.2
<1
1.96
(mixed LorentzianeGaussian 10%) method. The solid line indicates the matched line shapes with the sum of the dot curves. The corresponding Zr 3d5/2 and 3d3/2 peaks appear at 182.15 and 184.53 eV, respectively, and are in good agreement with the XPS spectrum of ZrO2 [30,31]. The intensity ratio of the Zr 3d5/2 and 3d3/2 peaks is around 3:2, which corresponds to the theoretically obtained ratio for Zr4þ. The full width at half maximum values for the Zr 3d3/2 and 3d5/2 peaks are 1.27 and 1.28 eV, respectively. The spin orbital splitting value is 2.38 eV. For the ZrO2 film, the O 1s spectrum in Fig. 2(b) shows two main peaks at 530.03 and 531.50 eV. The more intense peak at 530.03 eV originates from the stoichiometric ZrO2 film and is consistent with the Zr 3d spectrum. The broad weaker peak at 531.50 eV is characteristic of ‒O‒H bonds. The ZrO2 composition results on Zr, O, and C obtained from the quantitative analysis of XPS spectra are summarized at Table 1. The content of C in bulk ZrO2 film is not dominant with the negligible level below the detection limit of XPS. Two types of O are quantified as stoichiometric O ((Zr)-O) and O in hydroxyl ligand ((Zr)-OH). Considering stoichiometric ZrO2 film, the atomic ratio of O to Zr is 1.96 approaching to stoichiometric value of 2. The atomic ratio of O in hydroxyl ligand is 6.2%, which is lower about 2% than the content
Fig. 3. Depth profile of the ZrO2 film on TiN, as measured by AES.
at the XPS spectra obtained from the surface of ZrO2 film (etched for 1 min, not shown). The existence of ‒O‒H ligand in ZrO2 thin film is not surprising since the high atomic contents of H in the zirconium precursor of CpZr(NMe2)3/C7H8. The AES depth profile of the ZrO2/ TiN structure is shown in Fig. 3. The carbon impurity level of the ZrO2 film deposited at 300 C is as low as 0.5%. This result shows the same tendency as the XPS depth profile results. The Ti signal of the underlying TiN substrate begins to appear at a depth of about 18 nm. The ion concentration of Zr and O are constant throughout the full depth of the film, suggesting uniform depth profile of ZrO2 films. 3.3. Crystallinity and morphology of the ZrO2 film Fig. 4 shows the XRD pattern of 20-nm-thick ZrO2 film deposited at 300 C on TiN substrate. The films exhibit a dominant peak at the position of 2q ¼ 30.4 and small peaks at 2q ¼ 28.3 and 35.3 . The main peak corresponds to the cubic or tetragonal phase (JCPDS 491642 and 14-0534, respectively), indexed as (111) and the other peaks mean the presence of monoclinic (111) (JCPDS 36-420) and tetragonal (200). The results indicate that the deposited ZrO2 film at 300 C is polycrystalline. To obtain ZrO2 film with the higher-k value, cubic or tetragonal phase is preferred to amorphous or monoclinic phases [32]. AFM was used to investigate the surface roughness and morphology of the deposited ZrO2 film. The surface
Fig. 4. XRD pattern of ZrO2 film deposited at 300 C on TiN. Each peak is indexed as monoclinic (m, 111), cubic or tetragonal (t, c, 111), and tetragonal (t, 200), indicating a growth of polycrystalline ZrO2 film.
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C(V) ¼ Co(aV2þbVþ1)
Fig. 5. AFM image of a 50-nm-thick ZrO2 film deposited at 300 C from CpZr(NMe2)3/ C7H8 and ozone. The RMS roughness is 2.0 nm.
of a 50-nm-thick ZrO2 film deposited on Si from CpZr(NMe2)3/C7H8 and ozone at 300 C is smooth, with a root mean square (RMS) surface roughness of 2.0 nm (Fig. 5). 3.4. Dielectric and electrical properties of ZrO2 films The dielectric and electrical properties of MIM capacitors containing ZrO2 dielectrics were then investigated. Specifically, C-V and I-V measurements were carried out on TiN/ZrO2/TiN MIM structures containing ZrO2 films with thicknesses of 8, 12, 15, and 18 nm. Fig. 6(a) shows the normalized capacitance of the MIM capacitors, which can be explained by the quadratic equation:
(1)
where a and b are the quadratic and linear voltage coefficients, respectively, and are important parameters in RF and AMS devices [33]. The reference values of Co are 31, 18, 12, and 10 fF/mm2 for ZrO2 films with thicknesses 8, 12, 15, and 18 nm for the forward sweeping from 2 V to þ2 V, respectively. The C-V hysteresis quantified at 0 V is less than 0.1 fF/mm2 and decreases with increasing thickness. Using the capacitance value of Co measured at 0 V, the evaluated capacitance equivalent thickness (CET) for each ZrO2 film is shown in Fig. 6(b). It is also observed that the C-V hysteresis, quantified as CET, is 0.1 nm at most. Being calculated from the slope of the linear relationship between CET and ZrO2 thickness for forward measurement, the dielectric constant of asdeposited ZrO2 film is 17. Fig. 6(c) shows the log-log relationship between a and the capacitance density in the MIM capacitors. As a is inversely proportional to the dielectric thickness, i.e., a~ten~Con, [34], the figure indicates the dependence of dielectric thickness on a. The slope of 1.8 on the log-log plot provides a relationship of a~t1.8. Fig. 6(d) shows J-V curves for MIM capacitors containing ZrO2 films of varying thickness for both positive and negative polarities. Each positive and negative sweeping was conducted separately. The J-V curves are not symmetric with voltage polarities due to the electron ejecting anode effect with the inevitable metal oxide layer on bottom electrode [35], that is, the leakage current level measured at positive voltage were higher than that at negative but the onset of breakdown occurred at the lower positive voltage. Considering this situation, the J-V curve at positive polarity is the characteristic parameter estimating the electrical properties of capacitor. The leakage current at the higher voltage near breakdown may well decrease with increasing the thickness of ZrO2 film, while that at the lower voltage was not depend on the thickness. However, the
Fig. 6. (a) Normalized capacitance of TiN/ZrO2/TiN MIM capacitors as a function of applied voltage measured at a frequency of 1 kHz, and (b) their variation in CET with ZrO2 film thickness. The voltage was swept from positive to negative value (backward) and then reversed (forward). The slope of the linear fitting line obtained forward sweeping in (b) is reciprocally related to the dielectric constant. (c) log-log relationship between the quadratic voltage coefficient (a) and the capacitance density in MIM capacitors, showing that a~Co1.8. (d) I-V curves of MIM capacitors containing ZrO2 films of varying thickness.
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Table 2 Comparison between our MIM capacitor and other ones with as-deposited ZrO2 at 300 C. Electrode (TE/BE)
ZrO2 Thickness [nm]
Growth temperature [ C]
C/A [fF/mm2]
Leakage current [A/cm2]
TiN/TiN Ti/Pt TiN/TiN Pt/W Ti/Pt TiN/TiN TiN/TiN TiN/TiN
5.6 6 8 11e11.5 11 12 15 18
e 300 300 300 300 300 300 300
21.54 18.2 31 16e17 15 18 12 10
2.1 2.0 1.1 1.0 8.0 1.5 6.9 1.0
106 106 107 107 107 107 108 107
at at at at at at at at
2 1 1 1 1 1 1 1
V V V V V V V V
Ref. [36] [27] our work [37] [27] our work our work our work
Remarks: TE e Top electrode, BE e Bottom electrode, C/A e Capacitance density.
leakage current level at þ1 V for all capacitor was near or below 107 A/cm2, which is an acceptable level in device operation. Finally, the dielectric and electrical properties of our MIM capacitors are compared with other reported ones [27,36,37], as shown in Table 2. The compared almost ZrO2 films were deposited at 300 C without any additional annealing process. Considering the following equation at capacitor, C/A f ε/t, where C/A is capacitance density of capacitor, ε and t are the dielectric constant and thickness of dielectric, the dielectric constant could be extracted with the thickness of ZrO2 and capacitance density. From this relationship, it is found that the ZrO2 film fabricated with CpZr(NMe2)3/C7H8 and ozone has a higher-k value compared to other ZrO2 films. Considering the leakage current increases with increasing capacitance density, that is, thing the dielectric, our ZrO2 films show good or compatible leakage barrier properties compared to other ZrO2 films. 4. Conclusions A liquid containing a heteroleptic Zr precursor of CpZr(NMe2)3 (Cp ¼ C5H5) and cycloheptatriene (C7H8) was used as a Zr precursor in ALD process. ZrO2 films were fabricated by ALD, using CpZr(NMe2)3/C7H8 and ozone. Film growth is self-limited at a rate of 0.9 Å/cycle at 300 C, using a precursor pulse length of 2 s. The ALD window for constant growth rate with temperature is 250e400 C, in which the growth rate is 0.8e0.9 Å/cycle. The ZrO2 film deposited at 300 C shows a stoichiometric chemical state, negligible C contamination without additional annealing, and keeps a constant composition throughout the depth of the film. The ZrO2 film is polycrystalline, mainly consisting of a tetragonal phase. The leakage current density of TiN/ZrO2/TiN MIM capacitors with ZrO2 dielectric films of thicknesses of 8e18 nm is approximately 107 A/cm2 at 1 V. The quadratic voltage coefficient is related to the film thickness, according to a~t1.8. These findings provide insight to the performance of ALD-deposited ZrO2 dielectric films in solid electronic devices applications such as DRAM/RF/AMS capacitors. Acknowledgements This research was supported by the Development of Measurement Technology for Advanced Ultra-thin Film Processing in Korea Research Institute of Standards and Science, the R&D Convergence Program of National Research Council of Science and Technology (NST, No. CAP-16-04-KRISS) and Basic Science Research Program (2009-0083540) and Nano$Material Technology Development Program (2016M3A7B4910429) of National Research Foundation of Korea(NRF), funded by the Ministry of Science, ICT & Future Planning. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.12.420.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
ZrO2 film prepared by atomic layer deposition using less viscous cocktail CpZr[N(CH3)2]3/C7H8 precursor and ozone Jong-Ki An a, b, 1, Jin-Tae Kim b, c, 1, Goru Kang b, Nam Khen Oh b, Sung-Ho Hahm a, Geunsu Lee d, In-Sung Park e, **, Ju-Young Yun b, c, * a
School of Electronics Engineering, Kyungpook National University, Daegu 41566, Republic of Korea Vacuum Center, Korea Research Institute of Standards and Science (KRISS), Daejeon 34113, Republic of Korea Department of Nanomaterials Science and Engineering, University of Science and Technology, Daejeon 34113, Republic of Korea d Eugene Technology Materials, Gyeonggi-do 16675, Republic of Korea e Institute of Nano Science and Technology, Hanyang University, Seoul 04763, Republic of Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 October 2016 Received in revised form 30 December 2016 Accepted 30 December 2016 Available online 1 January 2017
The precursor consists of a mixture of CpZr(NMe2)3 (Cp ¼ C5H5, Me ¼ CH3) and cycloheptatriene (C7H8), CpZr(NMe2)3/C7H8, is introduced as a precursor in the atomic layer deposition (ALD) of zirconium oxide (ZrO2). The cocktail CpZr(NMe2)3/C7H8 chemical exhibits a higher vapor pressure of 1.2 torr and a lower viscosity of 7.0 cP at 100 C than the pure CpZr(NMe2)3. In the ALD of ZrO2 films, CpZr(NMe2)3/C7H8 and O3 act as the metal precursor and oxidant, respectively. Self-limited growth of ZrO2 films occurs after a 2 s pulse of CpZr(NMe2)3/C7H8, and a growth rate of 0.8e0.9 Å/cycle is obtained over a wide temperature range of 250e400 C. ZrO2 film formed at 300 C is stoichiometric with a lower impurity level of carbon and shows a tetragonal polycrystalline phase dominantly. The fabricated TiN/ZrO2/TiN capacitors exhibit compatible capacitance and leakage current properties with a quadratic voltage coefficient, inversely proportional to the dielectric film thickness (t) according to a~t1.8. © 2017 Elsevier B.V. All rights reserved.
Keywords: Atomic layer deposition Thin film ZrO2 Cocktail precursor CpZr(NMe2)3 Cycloheptatriene
1. Introduction Insulating dielectric zirconium oxide (ZrO2) thin films have attracted considerable recent attention in a wide range of applications [1]. This has been because of their favorable properties including high dielectric constant (k), wide band gap, suitable band offset on Si, acceptably low leakage current, and good thermal stability [2e4]. These properties of ZrO2 films have allowed them to be applied in microelectronics. Specific applications have included as dielectrics or insulators in metal-insulator-metal (MIM) capacitors of dynamic random access memory (DRAM) [5], in MIM capacitors of radio frequency (RF) and analog/mixed-signal (AMS) integrated circuits [6], in metal-insulator-semiconductor gate capacitors [7], and in resistor unit of resistive non-volatile memory
* Corresponding author. Vacuum Center, Korea Research Institute of Standards and Science (KRISS), Daejeon 34113, Republic of Korea. ** Corresponding author. E-mail addresses: [email protected] (I.-S. Park), [email protected] (J.-Y. Yun). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.jallcom.2016.12.420 0925-8388/© 2017 Elsevier B.V. All rights reserved.
[8]. For example, MIM capacitors containing Al2O3/ZrO2/SiO2/ZrO2/ Al2O3 stacks have achieved desirable electrical properties for RF and AMS circuits, including a high capacitance density and breakdown voltage, and a low leakage current and degree of voltage linearity [9]. Thin ZrO2 films have been prepared by various methods such as atomic layer deposition (ALD) [10,11]. ALD has been widely used to fabricate various metal oxide films and has become one of the leading film deposition techniques [12]. In the ALD process of metal oxide, the film grows via the surface reaction of vaporized metal precursor and oxidant that are alternately supplied to the substrate surface [13]. For the formation of metal oxides such as ZrO2 films, a complete ALD cycle for monolayer growth consists of a pulse of metal precursor (i.e., Zr precursor), a purge of inert gas, a pulse of oxidant, and a purge of inert gas. Each pulse step of precursor and oxidant should be well isolated to prevent any gas phase reaction between them. The significant advantage of the ALD process is the self-limiting growth of the monolayer. Despite its slow growth rate, ALD provides high quality thin films, good control over the thickness at the atomic-layer level, widely uniform coating thickness, and perfect conformal coatings on complex-structured surfaces [14].
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A suitable choice of metal precursor is crucial for a successful ALD process. The metal precursor for film growth requires sufficiently high vapor pressure, high purity, thermal and chemical stability, low viscosity, and effective cost. An appropriate precursor can provide benefits such as allowing film deposition at wide temperature ranges, yielding stoichiometric film composition, allowing a higher growth rate, and generating fewer toxic byproducts [15]. For thermal and plasma-enhanced ALD process, various Zr precursors have been used to grow ZrO2 and Zr-based films. ALD processes using precursors with Cl-, O-, N-, and C-coordinated ligands (and associated heteroleptic ligands) have been reported [16]. ZrCl4 is the representative precursor containing Cl-coordinated ligands. It exhibits high reactivity and thermal stability, so ZrO2 films can be deposited at high deposition temperature with ZrCl4 and possess good electrical properties [17,18]. During the ALD process, the aggressive reaction between Cl in ZrCl4 and Hþ in H2O generates corrosive HCl, which reduces the film growth rate and narrows the ALD window by lowering its upper temperature limit. Furthermore, the incorporation of corrosive Cl-containing contaminants into the ZrO2 film can result in the time-dependent degradation of the film quality and resulting electrical properties. These disadvantages prevent ZrCl4 from being applied in the mass production of ZrO2 films, especially for sensitive microelectronic applications. To avoid Cl contamination, precursors with O-coordinated ligands such as alkoxides have also been used in ALD. The steric hindrance of tert-butoxide ligands of the Zr(OBut)4 precursor prevent its oligomerization during the ALD process. However, alkoxide ligands are easily decomposed by trace water, owing to their catalytic hydrolysis reaction [19]. For example, the self-decomposition of tert-butoxide groups corresponds to a process involving no ALD growth. The growth rate of the ZrO2 film varies from 0.5 to 6.5 Å/ cycle, when the Zr(OtBu)4 pulse length is modulated from 0.2 to 1.5 s [20e22]. N-coordinated alkylamido-type precursors such as tetrakis(ethylmethylamino)zirconium have been developed to yield a high growth rate and high volatility for ALD processes. However, its low thermal stability limits the upper ALD temperature window to 300 C [23,24]. Thermal stability is very important in the ALD process, because film deposition at low temperatures results in high levels of impurities, low film densities, and a high degree of C and eOH bonds [15]. N-coordinated Zr-precursors also exhibit high viscosity. There can be issues with contamination inside the chamber and line, which results in film contamination and necessitates frequent maintenance of instrumentation. The cyclopentadienyl (Cp ¼ C5H5)-based precursors, (CpMe)2ZrMe2 (Me ¼ CH3) [25], (CpMe)2Zr(OMe)Me [25], Cp2Zr(Me)2 [26], and Cp2ZrCl2 [26] have recently been used to achieve increased thermal stability during ALD. The thermal stabilities of Cp-precursors are generally higher than those of N-coordinated alkylamide-based precursors. However, the growth rates of ZrO2 films when using the Cp-based precursors (about 0.5 Å/ cycle) are lower than those when using the alkylamide-based precursor (about 1.0 Å/cycle). Heteroleptic Cp-alkylamide precursors such as (MeCp) Zr(NMe2)3 [24], (EtCp)Zr(NMe2)3 (Et ¼ C2H5) [24], and CpZr(NMe2)3 using ozone as an oxidant have exhibited high growth rates (0.8e0.9 Å/cycle) for ALD-deposited ZrO2 films, and have high thermal stabilities at high temperatures. However, their ALD temperature window is too narrow. There is a very narrow ALD temperature window of 300e325 C, when Zr(NEtMe)2(guan-NEtMe)2 precursor is used for the growth of ZrO2 with a constant growth rate of 1.3 Å/cycle [27]. The narrow ALD windows hinder Cp-based precursors from applying to mass production of ZrO2 films. Tamm
311
et al. and Jaakko et al. respectively developed the CpTi(C7H8) [28] and CpMeZr(C7H8) [29] precursors, where C7H8 exists in ligand form. ZrO2 thin films prepared with using the precursors based on the Cp- and C7H8-ligands yielded high growth rates (0.7e0.8 Å/ cycle), good film purity, and a reasonable ALD temperature window. However, thermogravimetric analysis indicated impurity levels as high as 4.4% at 400 C for CpMeZr(C7H8) [29]. Viscosity is another important precursor property in mass production, because viscous materials remain in the precursor supply line and ALD chamber for longer. Higher viscosity of precursor directly results in the contamination of ALD apparatus. The values of viscosity of Zr precursors are typically higher than those of other precursors containing group IV element, such as precursors based on Ti and Hf. Therefore, the inert gas purge time and downtime of the ALD apparatus for preparing ZrO2 films are longer than those when preparing TiO2 and HfO2 films. The viscosity of precursors have so far received little investigation or discussion. Using the respective advantages of stable CpZr(NMe2)3 and fluent C7H8, their mixture CpZr(NMe2)3/C7H8 was synthesized to get a precursor with lower viscosity. In the current study, the less viscous precursor CpZr(NMe2)3/C7H8 is investigated for its chemical, physical, and electrical properties. This cocktail precursor has a similar form with the reported heteroleptic Cp- and C7H8-based precursors, but exhibits very different properties. The chemical and thermal properties of CpZr(NMe2)3/C7H8 are analyzed. The ALD conditions for fabricating ZrO2 films with this precursor are investigated and optimized. The structural and chemical properties of the resulting ZrO2 films are probed. The electrical properties of ZrO2 films are characterized using MIM capacitors for RF and AMS applications. 2. Experimental 2.1. ALD of ZrO2 films prepared from CpZr(NMe2)3/C7H8 and ozone A cocktail chemical of CpZr(NMe2)3 and C7H8, CpZr(NMe2)3/ C7H8, is used as the metal precursor for the fabrication of ZrO2 films. CpZr(NMe2)3/C7H8 evaporates without any decomposition with a two-step volatilization at about 100 and 150 C and at 204 C. The vapor pressure and viscosity is 3.5 torr and 3.1 cP at 100 C, respectively [Appendix A]. ZrO2 films were deposited on Si(100) substrates by ALD, using a cocktail CpZr(NMe2)3/C7H8 as the metal precursor and O3 as an oxidant. O3 was produced from high quality O2 (>99.999%) by an ozone generator. Prior to the ALD process, Si substrates were dipped in dilute HF solution (~1% HF in deionized water) for 2 min to remove native oxides. Substrates were then rinsed with deionized water, and dried with Ar gas. The cleaned substrates were then introduced onto the heated ALD susceptor and left to stand for 30 min in an inert Ar environment before the ALD process initiated for the temperature stabilization. The CpZr(NMe2)3/C7H8 precursor was vaporized at 65 C, and introduced into the showerhead-type thermal ALD chamber using an Ar carrier gas [11]. To determine the optimal ZrO2 deposition conditions, the substrate temperature range was varied over 200e400 C, and the Zr pulse time was varied over 0.5e5 s. One ALD cycle consisted of four steps: (1) pulsing with CpZr(NMe2)3/C7H8; (2) purging with Ar gas; (3) pulsing with ozone; (4) purging with Ar gas. 2.2. Characterization of deposited ZrO2 films The thicknesses of the ALD-deposited ZrO2 films on Si substrates were measured using ellipsometry. The chemical compositions and impurities in the bulk ZrO2, 18 nm thick, were checked by X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific, Theta
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Probe AR-XPS) after just removing the surface layer contaminated with carbon and/or water using a 10 kV Arþ ion etching for 10 min. Auger electron spectroscopy (AES; PHI, 680) was used to analyze the depth profiles of the films. The crystallinities of the films were investigated using grazing incidence X-ray diffraction (XRD; Rigaku, SmartLab). The roughness and morphology of the film surfaces were probed by atomic force microscopy (AFM; Park system, XE-150). 2.3. Dielectric and electrical properties of ALD-deposited ZrO2 films To evaluate the dielectric and electrical parameters of the ZrO2 films, TiN/ZrO2/TiN MIM capacitors were fabricated for RF and AMS device applications. TiN was deposited on the Si substrate by DC sputtering at room temperature. The top electrodes of the devices were formed using an aligner system with a conventional lift-off method. The ALD-ZrO2 dielectric was prepared from CpZr(NMe2)3/C7H8 and O3 in an optimized process. During ZrO2 deposition, the chamber temperature was maintained at 300 C. Dielectric films with thicknesses of 8e18 nm were grown on the TiN substrates. Capacitance-voltage (C-V) measurements were performed to evaluate the dielectric properties using a HP 4284 LCR meter for MIM capacitors. The capacitance was measured by sweeping the top electrode voltage from þ2 V to 2 V and then sweeping in the reverse direction with a small AC signal at 1 kHz. Current-voltage (I-V) curves were measured using a HP 4155 semiconductor parameter analyzer. 3. Results and discussion 3.1. ALD of ZrO2 films from CpZr(NMe2)3/C7H8 precursor Self-saturation is an important characteristic of ALD. A
saturation curve of a ZrO2 film deposited using the CpZr(NMe2)3/ C7H8 precursor at a substrate temperature of 300 C is shown in Fig. 1(a). The curve shows the growth rate ZrO2 films as a function of Zr-precursor pulse time length. The pulse length of CpZr(NMe2)3/ C7H8 is varied from 0.5 to 5 s, and the subsequent purge length is held constant at 10 s. The pulse length of O3 and its subsequent purge length are 5 and 10 s, respectively. Thus, pulse length saturation occurs after 2 s at 300 C for the CpZr(NMe2)3/C7H8 precursor. The precursor is thermally stable, and induces self-limited growth. The ALD window refers to the temperature range over which a constant growth rate occurs, and is an important characteristic of the ALD process. Fig. 1(b) shows the variation in growth rate as a function of deposition temperatures of 200e400 C, for ZrO2 films prepared from the CpZr(NMe2)3/C7H8 precursor using an optimized pulse length of 4 s for CpZr(NMe2)3/C7H8, and a pulse length of 5 s for O3. There is a minor variation in the growth rate of ZrO2 at 250e400 C, over the range of 0.8e0.9 Å/cycle. A wide ALD window with a high upper temperature limit (i.e., allowing a high decomposition temperature) is required for yielding high quality films. Film growth at high temperature makes the films dense with lower content of C and N impurities [24e29]. Fig. 1(c) shows that the thickness of the ZrO2 film increases linearly with the number of process cycles. The ZrO2 growth rate is 0.9 Å/cycle at 300 C. 3.2. Chemical bonding and elemental analysis of ZrO2 films XPS spectra of the Zr 3d and O 1s regions for a ZrO2 film deposited at 300 C are shown in Fig. 2(a) and (b), respectively. After sufficient surface cleaning with Arþ ion etching, the surface contamination of carbon and hydroxyl is considered to be negligible. Curve fitting of the Zr 3d5/2, Zr 3d3/2, and O 1s regions was conducted after Tougaard background subtraction, using the Voigt
Fig. 1. Growth rate of ALD-deposited ZrO2 thin films using CpZr(NMe2)3/C7H8 as a precursor (a) as a function of its pulse length at 300 C and (b) as a function of deposition temperature. (c) ALD growth linearity with a growth rate of 0.9 Å/cycle on the TiN substrate.
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Fig. 2. XPS spectra of (a) Zr 3d and (b) O 1s regions for the ZrO2 film on a TiN substrate.
Table 1 Atomic percentage of bulk ALD-ZrO2 films fabricated with CpZr(NMe2)3/C7H8 and O3 at 300 C. Zr (at.%)
(Zr)-O (at.%)
(Zr)-OH (at.%)
C (at.%)
O/Zr ratio
31.7
62.1
6.2
<1
1.96
(mixed LorentzianeGaussian 10%) method. The solid line indicates the matched line shapes with the sum of the dot curves. The corresponding Zr 3d5/2 and 3d3/2 peaks appear at 182.15 and 184.53 eV, respectively, and are in good agreement with the XPS spectrum of ZrO2 [30,31]. The intensity ratio of the Zr 3d5/2 and 3d3/2 peaks is around 3:2, which corresponds to the theoretically obtained ratio for Zr4þ. The full width at half maximum values for the Zr 3d3/2 and 3d5/2 peaks are 1.27 and 1.28 eV, respectively. The spin orbital splitting value is 2.38 eV. For the ZrO2 film, the O 1s spectrum in Fig. 2(b) shows two main peaks at 530.03 and 531.50 eV. The more intense peak at 530.03 eV originates from the stoichiometric ZrO2 film and is consistent with the Zr 3d spectrum. The broad weaker peak at 531.50 eV is characteristic of ‒O‒H bonds. The ZrO2 composition results on Zr, O, and C obtained from the quantitative analysis of XPS spectra are summarized at Table 1. The content of C in bulk ZrO2 film is not dominant with the negligible level below the detection limit of XPS. Two types of O are quantified as stoichiometric O ((Zr)-O) and O in hydroxyl ligand ((Zr)-OH). Considering stoichiometric ZrO2 film, the atomic ratio of O to Zr is 1.96 approaching to stoichiometric value of 2. The atomic ratio of O in hydroxyl ligand is 6.2%, which is lower about 2% than the content
Fig. 3. Depth profile of the ZrO2 film on TiN, as measured by AES.
at the XPS spectra obtained from the surface of ZrO2 film (etched for 1 min, not shown). The existence of ‒O‒H ligand in ZrO2 thin film is not surprising since the high atomic contents of H in the zirconium precursor of CpZr(NMe2)3/C7H8. The AES depth profile of the ZrO2/ TiN structure is shown in Fig. 3. The carbon impurity level of the ZrO2 film deposited at 300 C is as low as 0.5%. This result shows the same tendency as the XPS depth profile results. The Ti signal of the underlying TiN substrate begins to appear at a depth of about 18 nm. The ion concentration of Zr and O are constant throughout the full depth of the film, suggesting uniform depth profile of ZrO2 films. 3.3. Crystallinity and morphology of the ZrO2 film Fig. 4 shows the XRD pattern of 20-nm-thick ZrO2 film deposited at 300 C on TiN substrate. The films exhibit a dominant peak at the position of 2q ¼ 30.4 and small peaks at 2q ¼ 28.3 and 35.3 . The main peak corresponds to the cubic or tetragonal phase (JCPDS 491642 and 14-0534, respectively), indexed as (111) and the other peaks mean the presence of monoclinic (111) (JCPDS 36-420) and tetragonal (200). The results indicate that the deposited ZrO2 film at 300 C is polycrystalline. To obtain ZrO2 film with the higher-k value, cubic or tetragonal phase is preferred to amorphous or monoclinic phases [32]. AFM was used to investigate the surface roughness and morphology of the deposited ZrO2 film. The surface
Fig. 4. XRD pattern of ZrO2 film deposited at 300 C on TiN. Each peak is indexed as monoclinic (m, 111), cubic or tetragonal (t, c, 111), and tetragonal (t, 200), indicating a growth of polycrystalline ZrO2 film.
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C(V) ¼ Co(aV2þbVþ1)
Fig. 5. AFM image of a 50-nm-thick ZrO2 film deposited at 300 C from CpZr(NMe2)3/ C7H8 and ozone. The RMS roughness is 2.0 nm.
of a 50-nm-thick ZrO2 film deposited on Si from CpZr(NMe2)3/C7H8 and ozone at 300 C is smooth, with a root mean square (RMS) surface roughness of 2.0 nm (Fig. 5). 3.4. Dielectric and electrical properties of ZrO2 films The dielectric and electrical properties of MIM capacitors containing ZrO2 dielectrics were then investigated. Specifically, C-V and I-V measurements were carried out on TiN/ZrO2/TiN MIM structures containing ZrO2 films with thicknesses of 8, 12, 15, and 18 nm. Fig. 6(a) shows the normalized capacitance of the MIM capacitors, which can be explained by the quadratic equation:
(1)
where a and b are the quadratic and linear voltage coefficients, respectively, and are important parameters in RF and AMS devices [33]. The reference values of Co are 31, 18, 12, and 10 fF/mm2 for ZrO2 films with thicknesses 8, 12, 15, and 18 nm for the forward sweeping from 2 V to þ2 V, respectively. The C-V hysteresis quantified at 0 V is less than 0.1 fF/mm2 and decreases with increasing thickness. Using the capacitance value of Co measured at 0 V, the evaluated capacitance equivalent thickness (CET) for each ZrO2 film is shown in Fig. 6(b). It is also observed that the C-V hysteresis, quantified as CET, is 0.1 nm at most. Being calculated from the slope of the linear relationship between CET and ZrO2 thickness for forward measurement, the dielectric constant of asdeposited ZrO2 film is 17. Fig. 6(c) shows the log-log relationship between a and the capacitance density in the MIM capacitors. As a is inversely proportional to the dielectric thickness, i.e., a~ten~Con, [34], the figure indicates the dependence of dielectric thickness on a. The slope of 1.8 on the log-log plot provides a relationship of a~t1.8. Fig. 6(d) shows J-V curves for MIM capacitors containing ZrO2 films of varying thickness for both positive and negative polarities. Each positive and negative sweeping was conducted separately. The J-V curves are not symmetric with voltage polarities due to the electron ejecting anode effect with the inevitable metal oxide layer on bottom electrode [35], that is, the leakage current level measured at positive voltage were higher than that at negative but the onset of breakdown occurred at the lower positive voltage. Considering this situation, the J-V curve at positive polarity is the characteristic parameter estimating the electrical properties of capacitor. The leakage current at the higher voltage near breakdown may well decrease with increasing the thickness of ZrO2 film, while that at the lower voltage was not depend on the thickness. However, the
Fig. 6. (a) Normalized capacitance of TiN/ZrO2/TiN MIM capacitors as a function of applied voltage measured at a frequency of 1 kHz, and (b) their variation in CET with ZrO2 film thickness. The voltage was swept from positive to negative value (backward) and then reversed (forward). The slope of the linear fitting line obtained forward sweeping in (b) is reciprocally related to the dielectric constant. (c) log-log relationship between the quadratic voltage coefficient (a) and the capacitance density in MIM capacitors, showing that a~Co1.8. (d) I-V curves of MIM capacitors containing ZrO2 films of varying thickness.
J.-K. An et al. / Journal of Alloys and Compounds 701 (2017) 310e315
315
Table 2 Comparison between our MIM capacitor and other ones with as-deposited ZrO2 at 300 C. Electrode (TE/BE)
ZrO2 Thickness [nm]
Growth temperature [ C]
C/A [fF/mm2]
Leakage current [A/cm2]
TiN/TiN Ti/Pt TiN/TiN Pt/W Ti/Pt TiN/TiN TiN/TiN TiN/TiN
5.6 6 8 11e11.5 11 12 15 18
e 300 300 300 300 300 300 300
21.54 18.2 31 16e17 15 18 12 10
2.1 2.0 1.1 1.0 8.0 1.5 6.9 1.0
106 106 107 107 107 107 108 107
at at at at at at at at
2 1 1 1 1 1 1 1
V V V V V V V V
Ref. [36] [27] our work [37] [27] our work our work our work
Remarks: TE e Top electrode, BE e Bottom electrode, C/A e Capacitance density.
leakage current level at þ1 V for all capacitor was near or below 107 A/cm2, which is an acceptable level in device operation. Finally, the dielectric and electrical properties of our MIM capacitors are compared with other reported ones [27,36,37], as shown in Table 2. The compared almost ZrO2 films were deposited at 300 C without any additional annealing process. Considering the following equation at capacitor, C/A f ε/t, where C/A is capacitance density of capacitor, ε and t are the dielectric constant and thickness of dielectric, the dielectric constant could be extracted with the thickness of ZrO2 and capacitance density. From this relationship, it is found that the ZrO2 film fabricated with CpZr(NMe2)3/C7H8 and ozone has a higher-k value compared to other ZrO2 films. Considering the leakage current increases with increasing capacitance density, that is, thing the dielectric, our ZrO2 films show good or compatible leakage barrier properties compared to other ZrO2 films. 4. Conclusions A liquid containing a heteroleptic Zr precursor of CpZr(NMe2)3 (Cp ¼ C5H5) and cycloheptatriene (C7H8) was used as a Zr precursor in ALD process. ZrO2 films were fabricated by ALD, using CpZr(NMe2)3/C7H8 and ozone. Film growth is self-limited at a rate of 0.9 Å/cycle at 300 C, using a precursor pulse length of 2 s. The ALD window for constant growth rate with temperature is 250e400 C, in which the growth rate is 0.8e0.9 Å/cycle. The ZrO2 film deposited at 300 C shows a stoichiometric chemical state, negligible C contamination without additional annealing, and keeps a constant composition throughout the depth of the film. The ZrO2 film is polycrystalline, mainly consisting of a tetragonal phase. The leakage current density of TiN/ZrO2/TiN MIM capacitors with ZrO2 dielectric films of thicknesses of 8e18 nm is approximately 107 A/cm2 at 1 V. The quadratic voltage coefficient is related to the film thickness, according to a~t1.8. These findings provide insight to the performance of ALD-deposited ZrO2 dielectric films in solid electronic devices applications such as DRAM/RF/AMS capacitors. Acknowledgements This research was supported by the Development of Measurement Technology for Advanced Ultra-thin Film Processing in Korea Research Institute of Standards and Science, the R&D Convergence Program of National Research Council of Science and Technology (NST, No. CAP-16-04-KRISS) and Basic Science Research Program (2009-0083540) and Nano$Material Technology Development Program (2016M3A7B4910429) of National Research Foundation of Korea(NRF), funded by the Ministry of Science, ICT & Future Planning. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.12.420.
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