Background

Gate dielectrics have been a continuous research interest due to their broad applications in nano- and microelectronics [1,2,3], such as metal oxide films for complementary metal oxide semiconductors (CMOS) [4] and dynamic random access memory (DRAM) [5]. However, the use of the traditional SiO2 comes to its limit due to the scaling of devices, hence it is urgent to develop next generation of gate dielectrics to replace SiO2 in semiconductor industry [6, 7]. Recently, alternative metal oxides have been extensively investigated, such as ZrO2, Ta2O5, HfO2, Nb2O5, and TiO2. In particular, ZrO2 has been considered as an ideal candidate due to its relatively high dielectric constant and wide band gap, as well as excellent thermal and chemical stability [8, 9].

In the past decade, ZrO2 film has been successfully synthesized via vacuum-based vapor-deposition routes [8, 10,11,12] and sol-gel solution-deposition routes [13,14,15]. Among them, atomic layer deposition (ALD) has distinguished advantages over other routes based on saturated self-limiting surface reactions [16, 17], including thickness controllability, large-area uniformity, low deposition temperature, and structure conformality, which makes ALD play an important role in the fabrication of high-quality dielectric films [18, 19].

Several precursors have been successfully applied for deposition of ZrO2 films by ALD processes. Kukli et al. reported that ZrO2 films were grown from ZrI4 and H2O-H2O2 on p-Si(100) substrates using ALD technique. The relative permittivity measured at 10 kHz was 20–24 in the films deposited at 275–325 °C [20]. Kukli et al. reported that ZrO2 films were grown by ALD from ZrCl4 and H2O or a mixture of H2O and H2O2 on Si(100) substrates in the temperature range of 180–600 °C. The films grown at 180 °C contain 5–6 at.% of hydrogen and 4–5 at.% chlorine. The effective permittivity of ZrO2 films is 13–15, when the ZrO2 was deposited at 180–210 °C [21]. Putkonen et al. reported that ZrO2 films were deposited onto (100) silicon substrates by ALD using Cp2Zr(CH3)2 (Cp = cyclopentadienyl) and water as precursors at 200–500 °C. The effective permittivity was 12.5 for the 19.0-nm film. The leakage current density in a 1 MV/cm field was about 2.8 × 10−5 A/cm2 for the 19.0-nm ZrO2 film [22]. Lamperti et al. reported that ZrO2 films were deposited on Si (100) substrates by ALD at 300 °C from (MeCp)2ZrMe(OMe) as Zr precursor and using H2O or O3 as an oxygen source. The measured dielectric constant values after 800 °C annealing are k~ 24 and k~ 30 in the films [23]. Previously, the common precursors were ZrCl4 [8, 24] or ZrI4 [25, 26]. However, low reaction activities of halides lead to high deposition temperatures [27]. Moreover, the formation of corrosive by-products (e.g., HCl) can corrode the films and ALD systems. Recently, metal amides precursors have been regarded as the ideal replacements without corrosive halogen by-products. Moreover, they have much higher reactivity than metal halides, since the metal-nitrogen bond is significantly weaker than the metal-halide bonds. In particular, tetrakis(dimethylamido)zirconium (TDMAZr) has thermal stability [28], sufficient volatility [29], and high reaction activity in the vapor deposition process. Therefore, TDMAZr and H2O as reaction precursors have been used for ALD processes of ZrO2 films [30], Nevertheless, hydrogen and carbon impurities in ZrO2 films have severe effects on the electrical properties of thin films. The “dry” ALD process of metal precursor and ozone (O3) has attracted more and more interest, because O3 as oxidizing agent can avoid introducing hydrogen. Moreover, O3 can remove carbon impurity by forming volatile CO and CO2 [31]. In addition, O3 has been widely used in the fabrication of dielectric layer in order to achieve higher crystallinity accompanied by a higher dielectric constant.

In this work, ZrO2 films were deposited on silicon by ALD technology using TDMAZr as metal precursor and O3 as oxygen precursor. The dependences of growth rate, refractive index, crystal structure, and surface roughness on deposition temperature were investigated in details by optical ellipsometry, glancing angle incidence X-ray diffraction (GAXRD), X-ray photoelectron spectroscopy (XPS), and atomic force microscope (AFM). Post-deposition annealing was used to further improve the crystal quality of ZrO2 films. Most importantly, we evaluated the dielectric constant of ZrO2 film and researched the effect of annealing temperature on the dielectric properties of ZrO2 films by analyzing the capacitive behavior and leakage current of ZrO2-based MIS devices.

Methods

ZrO2 films were grown onto oriented n-type silicon wafers using an ALD reactor (MNT Ltd.). High purity nitrogen gas (99.999%) was used as a carrying and purging gas. TDMAZr as metal precursor was heated to 70 °C and carried into a reactor by carrying gas flowing through the source bottle. O3 as oxidant precursor was generated from oxygen (99.999% purity) by an ozone generator (Newland Ltd.). The delivery lines were remained at 100 °C to keep the gas from condensing. TDMAZr and O3 entry alternately into the reaction chamber to conduct surface gas-solid chemical reactions. We need to ensure that O3 is sufficiently excessive (about 20,000 Pa) and the purging process is long enough. The ZrO2 thin films deposited on silicon were annealed for 2 h under nitrogen atmosphere.

The thickness and refractive index of all samples were measured by an ellipsometer. The crystal structure of ZrO2 film was analyzed by glancing angle incidence X-ray diffraction (GAXRD). The chemical component and chemical state of ZrO2 film were analyzed by X-ray photoelectron spectroscopy (XPS, Sigma Probe, ThermoVG) using a monochromatic Al Ka source (1486.7 eV) to excite the photoelectrons. The positions of all the peaks were calibrated with the C 1s peak assigned at 284.6 eV. The surface morphology and the root-mean-squared (RMS) roughness of ZrO2 film were examined with an atomic force microscope (AFM, JSPM-5200, JEOL Co.). Current-voltage (I–V) measurement was carried out by a Keithley 2410 1100 V source measurement unit (Keithley Instruments Inc., Cleveland, OH, USA) and capacitance-voltage (C-V) measurement was carried out by TH2828S LCR meter (TONGHUI ELECTRONICS). All the measurements were completed at room temperature.

Results and Discussion

Deposition temperature plays a critical role in ALD technology. Figure 1a illustrates the dependences of the deposition rate and refractive index on the deposition temperature. The deposition rate constantly increases with decreasing the deposition temperature below 200 °C, which is due to the physical adsorption between the silicon substrate and the reaction precursors at the low deposition temperatures. From the profiles of refractive index as a function of deposition temperature, the constant decrease of refractive index below 200 °C means that thin films become looser, which also verifies that there is strong physical adsorption during the films’ growth at the low deposition temperatures. The deposition rate of 1.25 Å/cycle can be considered as quite a stable value in the narrow temperature range of 200~250 °C, in which the chemical adsorption plays a dominant role based on saturated self-limiting surface reaction. Then the deposition rate obviously increases with increasing the deposition temperature above 250 °C. These results can be attributed to the thermal decomposition of TDMAZr at the high deposition temperatures, which leads to CVD-like deposition. Figure 1b shows the dependences of the deposition rate and refractive index on the pulse time of TDMAZr. The growth rate at about 1.25 Å/cycle is observed when pulse time is more than 200 ms, which verifies the self-limiting film growth.

Fig. 1
figure 1

The changes of deposition rate and refractive index for ZrO2 thin films as a function of deposition temperature (a) and Zr precursor pulse time (b)

Full size image

Figure 2a depicts the dependence of thickness on the number of ALD cycles in ALD temperature window. The thickness of ZrO2 films on silicon shows a linear relationship with the number of ALD cycles, and the fitted formula is y = − 1.7952 + 0.12806x, R = 0.99938, which demonstrates that the thickness of thin the film can be precisely controlled by regulating the number of ALD cycles in ALD temperature window. Figure 2b shows the dependence of deposition rate on the number of ALD cycles. The deposition rate slightly increases with increasing the number of deposition cycles. Relatively low deposition rate at the beginning of film deposition can be attributed to crystal lattice mismatch and slow nucleation on silicon.

Fig. 2
figure 2

The changes of thickness (a) and deposition rate (b) for ZrO2 thin films as a function of the number of deposition cycles. The deposition temperature is 250 °C

Full size image

The crystallographic structure of the ZrO2 thin films deposited at 150~350 °C was detected by GAXRD. As shown in Fig. 3a, ZrO2 films begin to crystallize at the deposition temperature of 210 °C. The reflections at 30.5° and 35.3° are indexed to the (111) and (200) lattice planes of cubic phase (PDF Card 027-0997), respectively. The reflections at 51° and 60.5° are indexed to the (220) and (311) lattice planes of orthorhombic phase (PDF Card 037-1413). With increasing the deposition temperature to 290 °C, the (111), (− 111), (200), (122) and (− 131) lattice planes of stable monoclinic phase begin to appear at 28.3°, 31.4°, 35.3°, 50.6°, and 60° (PDF card 007-0343), respectively [24]. Moreover, the reflections at 30.5° and 35.3° begin to slightly shift, which indicates the phase change from cubic phases to orthorhombic phase. When the deposition temperature reaches 350 °C, the lattice planes of the cubic phase disappear, and the dominant crystallographic phases in ZrO2 films are both orthorhombic and monoclinic phase. Post-deposition annealing is regarded as a necessary process to eliminate defects and improve the crystal quality. Figure 3b showed the GAXRD patterns of the ZrO2 films (deposited at 250 °C) annealed at 400~1000 °C. It can be found that main crystal structures are both cubic and orthorhombic phases in the ZrO2 film annealed at 400 °C. However, monoclinic phase begins to appear, and the intensities of reflections at 28.3° and 31.4° significantly increase with increasing annealing temperature to 1000 °C. Meanwhile, the phase change from cubic phases to orthorhombic phase can be seen from the shift of the reflection for (111) plane. From the above analysis, both deposition temperature and annealing temperature have significant effects on the crystallographic structure of ZrO2 film.

Fig. 3
figure 3

The GAXRD patterns for (a) ZrO2 films deposited at 150~350°C and (b) ZrO2 films annealed at 400~1000 ℃ (deposited at 250°C)

Full size image

Figure 4 shows the changes of thickness and refractive index for the films deposited from 110 to 350 °C after 400 °C annealing. The thickness of the ZrO2 film deposited below 210 °C significantly decreases after 400 °C annealing, meanwhile, the refractive index correspondingly increases, which demonstrates the thin film becomes denser after post-deposition annealing. However, the thickness and refractive index have no change for the thin film deposited above 210 °C after annealing at 400 °C, which may be because the crystal growth of thin films have been sufficient, namely, the sizes of grains are no longer obviously increase above 210 °C.

Fig. 4
figure 4

The changes of thickness (a) and refractive index (b) for the films deposited at 110~350 °C after post-deposition annealing at 400 °C

Full size image

The chemical component and chemical state of ZrO2 thin film on silicon were carefully investigated by XPS. The thickness of ZrO2 film in XPS measurement is about 50 nm. Moreover, surface cleaning was carried out by etching before XPS measurement. As described in Fig. 5a, a full spectrum result shows the existences of oxygen and zirconium in ZrO2 thin film deposited at 250 °C, with no obvious impurity peak, which confirms the high purity of ZrO2 thin film. Moreover, the ratio of O/Zr in ZrO2 films deposited at 200~250 °C is 1.85–1.9, which is less than the stoichiometric ratio of ZrO2 due to the formation of oxygen vacancy in the ZrO2 film. An asymmetric O 1s XPS spectrum was obtained in this study, shown in Fig. 5b. The O 1s high-resolution spectrum is composed of two overlapping components, with a lower binding energy peak (529.7 eV) and the higher binding energy (531.3 eV), which result from the Zr–O of oxides and O–C of by-product, respectively. In Fig. 5c, the high-resolution spectrum of Zr 3d shows two peaks at 182.4 eV and 184.7 eV, which corresponds with the features of Zr 3d5/2 and 3d3/2, respectively. It should be noted that shoulders of the Zr 3d5/2 peak appear on the lower binding energy side. The Zr 3d spectra are decomposed satisfactorily in terms of four different oxidation states (Zr4+~Zr+) of Zr equally spaced in 1.06 eV (one-fourth of the chemical shift between Zr0 and Zr4+) per oxidation state with respect to the metal [32,33,34]. All the oxidation states are observed in all the films prepared, and the Zr4+ state is the major component in the films due to the effect of suboxide species. In addition, deposition temperature just has little impact on the binding energy of Zr 3d and the shape of the peaks in spectra, which demonstrates that the deposition temperatures within the range of 150~350 °C cannot change the chemical state of Zr 3d.

Fig. 5
figure 5

The full spectrum scan of XPS (a), the fine spectra of O 1 s (b), and Zr 3d (c) for ZrO2 film deposited on Si(100) at 250 °C, and the fine spectra of Zr 3d for ZrO2 films deposited at 150, 250, and 350 °C (d)

Full size image

Roughness and morphology were studied by AFM. Figure 6 displays representative AFM images of ZrO2 thin films, which illustrates that the surfaces of ZrO2 thin films are uniform and smooth. As shown in Fig. 6a and b, the root-mean-square (RMS) roughness for the ZrO2 films deposited at 150 °C and 350 °C are 0.293 nm and 1.718 nm, which are 0.448% and 2.277% of the films aggregate thicknesses, respectively. By comparing, the result suggests that amorphous ZrO2 films are much smoother than crystalline film. Although the RMS roughness does not provide direct evidence of the detailed evolution of crystallographic phases in oxide thin films, the increase in surface roughness could be generally ascribed to the formation and growth of the grain in the thin film. As shown in Fig. 6c and d, ZrO2 films deposited at 250 °C were annealed at 400 °C and 1000 °C, respectively. The RMS roughness has no significant change with the increasing annealing temperature, which indicates that 400 °C annealing has led to sufficient crystal growth and there is no obvious increase in grain size. As shown in Fig. 6e and f, the thickness of ZrO2 film controlled by ALD cycles have an obvious effect on the RMS roughness of the thin film. By comparing, the RMS roughness increases with the increasing number of ALD cycles, which can be explained by that the crystal growth is limited when the thickness of the film is very thin.

Fig. 6
figure 6

AFM surface plots of ZrO2 films deposited at 150 °C (a) and 350 °C (b), ZrO2 films (deposited at 250 °C) annealed at 400 °C (c) and 1000 °C (d), and ZrO2 films (deposited at 250 °C) with 200 ALD cycles (e) and 800 ALD cycles (f)

Full size image

Capacitance-voltage (C-V) and current density-electric field (J-E) measurements were conducted to assess the electrical properties of MIS devices with ZrO2 films as dielectric layers with various annealing temperatures. As shown in Fig. 7a, the capacitance constantly decreases with increasing annealing temperature. Moreover, obvious hysteresis effects between the forward and backward sweep (clockwise) were observed in the samples annealed at 400 °C and 600 °C, and then the hysteresis effect decreases or even disappears with increasing annealing temperature to 1000 °C. The hysteresis effect should be attributed to the charge trap/detrap in the interface between the ZrO2 film and Si substrate [35,36,37]. The oxygen in ZrO2 film diffuses toward the interface to result in the further growth of SiOx layer between ZrO2 and Si substrate with increasing the annealing temperature during post-deposition annealing [37,38,39]. To prove the presence of SiOx interface layer, ultrathin ZrO2 (2.5~3 nm) deposited on Si substrate was used to XPS measurement. As shown in Fig. 8a, in Si 2p spectrum, the signal from the Si substrate (98.81 eV and 99.45 eV) could be clearly observed due to the thinness of the films, and the signal from interface layer (100.38 eV, 100.88 eV, 102.44 eV, and 103.05 eV) indicates the existence of Si sub-oxides. As shown in Fig. 8b, in the O 1s spectrum, the binding energy of 532.72 eV is identified as Si–O in SiOx. These results can prove the formation of SiOX interface layer. The SiOx layer can reduce interface defect state density and improve the interface quality [39]. Therefore, the increase in annealing temperature can reduce the hysteresis effect. Figure 7b depicts J-E curves for ZrO2-based MIS devices. With increasing the annealing temperatures from 400 °C to 800 °C, there is no significant reduction in current densities. However, with increasing the annealing temperature to 1000 °C, the leakage current density decreases to 2.85 × 10−9 A cm−2 at 1 MV cm−1. It can be attributed to the further growth of SiOx intermediate layer with wide band gap during 1000 °C annealing.

Fig. 7
figure 7

The C-V (a) and J-E curves (b) for ZrO2-based MIS devices (with 500 cycles) as a function of annealing temperature

Full size image
Fig. 8
figure 8

The fine spectra of Si 2p and O 2s for the ZrO2 film deposited at 250

Full size image

Figure 9a shows the effect of ZrO2 thickness on the capacitance behavior of ZrO2-based MIS devices. The capacitance constantly decrease with increasing ALD cycles (or thicknesses), which is attributed to the increase in the physical thickness for ZrO2 dielectric layer. To evaluate the dielectric constant of the ZrO2 dielectric layer, the effect of interfacial SiOx layer was eliminated in calculation, as shown in Fig. 10. A similar dielectric constant for ZrO2 films with various thicknesses was obtained in Fig. 9b. The average value of the dielectric constant of ZrO2 film is 32.57. These results indicate that the film quality is reliable for ZrO2 films with various thickness deposited by ALD. Figure 9c and d illustrate that the increase in the physical thickness for ZrO2 films with 400–800 ALD cycles can decrease leakage current densities. However, it should be noted that the leakage current density remains low level when the cycle number are 100 and 200, which is due to less leakage paths in the thin film with limited crystallization. As shown in Fig. 11, the GAXRD patterns for the ZrO2 films with various ALD cycles (or thickness) illustrate the crystallization is inhibited in ZrO2 thin film with 100 and 200 ALD cycles.

Fig. 9
figure 9

The C-V (a) and J-E curves (b) for ZrO2-based MIS devices with 100~800 cycles, the changes of dielectric constant (c), and the current density at 1 MV/cm (d) as a function of the number of deposition cycle

Full size image
Fig. 10
figure 10

Schematic diagram of dielectric constant calculation

Full size image
Fig. 11
figure 11

The GAXRD patterns for the ZrO2 films deposited at 250 °C with various ALD cycles

Full size image

Conclusions

High-quality ZrO2 thin films were deposited on n-type silicon wafers by ALD technology using tetrakis(dimethylamido)zirconium and ozone as precursors. ALD temperature window for ZrO2 film is 200–250 °C, and the deposition rate is a relatively constant at 0.125 nm/cycle. The thickness of the thin film can be precisely controlled by regulating the number of ALD cycles. The ZrO2 films formed at 200~250 °C have an O/Zr atomic ratio of 1.85–1.9 and a low content of carbon impurity. ZrO2 film begins to crystallize in the ALD process above 210 °C, and the crystal structure is changed from cubic and orthorhombic phases to monoclinic and orthorhombic phases with increasing the deposition temperature to 350 °C. Moreover, a similar phase transition also can occur with increasing the annealing temperature from 400 to 1000 °C. The effect of annealing temperature on dielectric properties of ZrO2 film was studied utilizing ZrO2-based MIS device. The growth of the interface layer between ZrO2 and Si substrate leads to the decrease in the capacitance and the leakage current of dielectric layer in the MIS device after 1000 °C annealing. ZrO2 film exhibits the relatively high dielectric constant of 32.57 at 100 kHz and the low leakage current density of 3.3 × 10−6 A cm−2 at 1 MV/cm.