Crystallinity-Controlled Atomic Layer Deposition of Ti-Doped ZrO2 Thin Films

We investigated Ti-doped ZrO2 deposition using a cyclopentadienyl tris(dimethylamino) zirconium (CpZr(NMe2)3) precursor and a titanium tetraisopropoxide (TTIP) precursor using an O3 thermal atomic layer deposition process. In addition, the effect of Ti doping concentration on the chemical bonding and electrical properties of the Ti-doped ZrO2 thin films was studied. O3 was used at a high concentration of 400 g m−3. We varied the Ti doping concentration by controlling the rate of the supercycle process in the ZrO2 process window of 200 °C–300 °C. As a result, the highest dielectric constant was observed at a Ti doping concentration of 2.5% because it enhances the crystallinity of ZrO. Excessive Ti doping hinders crystal formation.

2][3] Research on high-k capacitors for DRAMs has been ongoing for decades; however, reduction in film thickness or increase in effective surface area has reached its limit.As a result, research on high-k materials with sufficient capacitance has prevailed. 4he dielectric constant and band gap energy of a dielectric material are inversely proportional, and a small-band gap material has poor leakage current characteristics because of a low band offset with a metal electrode. 5Therefore, it is necessary to study materials with appropriate dielectric constants and energy band gaps.ZrO 2 has a high dielectric constant (k ∼ 47) and wide band gap (5.5 eV), making it an appropriate solution for applications with dielectric thickness of tens of nanometers. 6,7The dielectric constant of ZrO 2 varies greatly depending on the crystal phase, such as the monoclinic phase (k ∼ 20), tetragonal phase (k ∼ 47), and cubic phase (k ∼ 37). 8][11] When Ti is used as a dopant in ZrO 2 , Ti can substitute Zr to stabilize the tetragonal phase.In this study, we investigate how Ti dopant concentration affects the properties of films. 12

Experiment
A 100-nm-thick TiN film was prepared on a Si substrate as the bottom electrode.The TiN/Si substrate was transferred to an ALD chamber, and Ti-doped ZrO 2 was deposited using ozone generated by an ozone generator as the reactant.The precursor and dopant for ALD of Ti-doped ZrO 2 were tris(dimethylamino) cyclopentadienyl zirconium (CpZr) and titanium isopropoxide (TTIP), respectively.The Zr precursor canister was maintained at 80 °C, and the Ti precursor canister was maintained at 50 °C.The process deposition temperature was 300 °C, and the ozone concentration was 400 g m −3 .
To evaluate the characteristics of Ti-doped ZrO 2 , metal-insulatormetal (MIM) capacitors were fabricated on TiN/Si substrates.TiN was deposited by sputtering as the bottom electrode, and gold (Au) was deposited as the top electrode using an E-beam evaporator through a shadow mask with a radius of 100 μm.
Electrical characterization of the capacitors, such as currentvoltage (I-V) and capacitance-voltage (C-V), was performed at room temperature using an Agilent B1500A semiconductor parameter analyzer.The film thickness was evaluated by a spectroscopic ellipsometer (SE, Nanoview-SG-100).The atomic content of the film was evaluated using Auger electron spectroscopy (AES, PHI-700Xi), and the crystallinity of the film was measured by X-ray diffraction (XRD, smartlab).X-ray photoelectron spectroscopy (XPS, K-Alpha+) was used to analyze the chemical bonds of the film.

Results and Discussion
Figures 1a and 1c shows the growth per cycle (GPC) by deposition temperature when depositing ZrO 2 and TiO 2 films, respectively, at an ozone concentration of 400 g m −3 .The process window of the ZrO 2 film was 200 °C-300 °C, and the process window of the TiO 2 film was 250 °C-300 °C.To check the selflimiting reaction in the process window, the film thickness was measured for each process cycle at a deposition temperature of 300 °C.As shown in Figs.1b, 1d, the film thickness increased in proportion to the number of cycles.Figure 1e shows the GPC of the ZrO 2 film deposited at ozone concentrations of 100 g m −3 to 400 g m −3 .When depositing the ZrO 2 film, the GPC was constant regardless of the ozone concentration.
Figure 2 shows the AES depth profile of the Ti-doped ZrO 2 film according to supercycle ratio.Figure 2a is a 2:1 supercycle process, where Zr has 24.7 at% and Ti has 9.5 at%.And Figs. 2b and 2c show the atomic concentration in 5:1 and 32:1 supercycle processes, respectively.The atomic concentrations for all supercycle processes are summarized in Table I.The carbon in all films was less than 1%.As the number of cycles, x, of the supercycle increases, the Zr at% increases and Ti at% tends to decrease.In the 128:1 supercycle process, only 0.2 at% of Ti is present.
To confirm the doping concentration in the film according to supercycle ratio, the values measured through AES and the theoretical calculation values were determined, as summarized in Table II.The Ti doping concentration is represented by [Ti]/[Ti] +[Zr] and the measured values are indicated in Table I. 13 The theoretical calculation was z E-mail: hjeon@hanyang.ac.kr The ZrO 2 GPC was 0.9 Å/cyc and TiO 2 GPC was 0.7 Å/cyc. 14There was no significant difference between the measured and calculated values.
As shown in Fig. 3, XRD was used to confirm the crystallinity and crystalline phase of the Ti-doped ZrO 2 films.In Figs.3a and 3b,   only the substrate peak appears, indicating that it was amorphous because the TiO 2 solubility limit of tetragonal ZrO 2 was exceeded. 15,16Beyond the solubility limit, the substance failed to stabilize crystals and became amorphous.Because the amorphous structure appeared in the 4:1 supercycle process and crystallinity appeared in the 8:1 supercycle process, the XRD measurement result of the 5:1 supercycle process was also tested and crystallinity was observed.The TiO 2 solubility limit of tetragonal ZrO 2 was 13.2%-16.4% at a deposition temperature of 300 °C.As shown in Fig. 3c, a 30.5°tetragonalphase peak and a 51.0°monoclinic phase peak appeared. 17As shown in Figs.3d to 3f, the peak intensities of t-ZrO 2 and m-ZrO 2 increased, and in Figs.3g and 3h, the intensity decreases slightly compared with Fig. 3f.Nevertheless, Figs. 3f ∼ 3h had higher intensity than Fig. 3i with undoped ZrO 2 .Ti 4+ takes the interstitial sites when its content is below a critical value (2.5%) and then replaces Zr 4+ .When Ti 4+ is substituted, it is known that the tetragonal phase formation becomes easier in the case of Yb 2 O 3 -doped ZrO 2 or another study of ZrO 2 because of its lower crystal energy. 18,19The 32:1 supercycle process, which is close to the Ti 4+ critical value, had the highest peak intensity, and although substitution occurred at Ti 4+ concentrations less than the critical value, the peak intensity was slightly lower than that of the 32:1 supercycle process.
The XPS spectra of Zr 3d, Ti 2p, and O 1 s are depicted in Fig. 4. The Zr 3d peak was separated into Zr 3d 5/2 at 181.9 eV and Zr 3d 3/2 at 184.3 eV. 20All films show the same binding energy, and only a difference in peak intensity is observed.The lower is the Ti%, the higher is the peak intensity, and the higher is the Ti%, the lower is the peak intensity.The Ti 2p peak can be separated into a set of peaks of Ti 2p 3/2 and Ti 2p 1/2 at 458.5 eV and 465.3 eV, respectively. 21,22ontrary to the Zr 3d peak, when Ti% is high, the peak intensity is high, and when Ti% is low, the peak intensity is low.The O 1 s peak shows a binding energy between 530.0 eV and 530.6 eV. 23,24The 128:1 supercycle process film with the lowest Ti% was 530.0 eV, and as Ti% increases, a peak shift occurs with high binding energy.When the Ti% is low, it shows binding energy close to the O-Zr peak; however, as Ti% increases, the spectrum shifts to the higher binding energy because of the formation of the O-Ti bond. 25CS Advances, 2024 3 012002 Undoped ZrO 2 and Ti-doped ZrO 2 were prepared as MIM structures to investigate the effects of Ti doping on ZrO 2 on the electrical properties of the film. 26The C-V and I-V curves of MIM capacitors for various supercycle processes are shown in Fig. 5.As shown in Fig. 5a, the capacitance increased as the supercycle ratio changed from 2:1 to 32:1 and then decreased thereafter.The dielectric constant is calculated as k = (C/ε0) × (d/A), where C, ε0, d, and A are respectively the capacitance, vacuum permittivity (ε0 = 8.854⋅10 −12 F m −1 ), thickness of the film, and electrode surface area. 27The dielectric constants from the above calculations are summarized in Table III.Because the 10-nm film thickness was the same for all samples, the change in dielectric constant was because of the capacitance.The dielectric constant of undoped ZrO 2 was 40, whereas the maximum dielectric constant of Ti-doped ZrO 2 was 45 in a 32:1 supercycle process.Changes in the dielectric constant because of doping can be attributed to changes in the crystalline phase.In this study, Ti 4+ replaced Zr 4+ to find the optimal doping concentration to stabilize the tetragonal phase of ZrO 2 films.However, as in the 2:1 or 4:1 supercycle process, dopants above the solubility limit of tetragonal ZrO 2 for Ti amorphized the film and reduced the dielectric constant.
Figure 5b shows the current density as a function of Ti doping concentration.The leakage current density is observed at 1 V in the I-V measurements and tends to be proportional to the capacitance. 13n other words, the higher is the crystallinity, the higher is the leakage current density; as a result, the 32:1 supercycle process shows the highest leakage current density.As the crystallinity increases, the grain boundaries increase.Grain boundaries are considered important leakage pathways in films. 28Surface depressions along the grain boundaries form and reduce the thickness of the film at the grain boundaries relative to inside the grain area.Therefore, the leakage current increases because of significant electron emissions as the electric field across the film is concentrated at a relatively thin spot.

Summary and Conclusions
We investigated the crystallinity and chemical and electrical properties of undoped ZrO 2 and Ti-doped ZrO 2 films deposited by ozone-based ALD on TiN substrates sputtered on Si.A stoichiometric film with nearly no impurities was deposited using CpZr as a precursor and a high concentration of ozone of 400 g m −3 as a reactant.Films with various Ti% were deposited by controlling the   ECS Advances, 2024 3 012002 ZrO 2 :TiO 2 supercycle ratio from 2:1 to 128:1.The crystallinity of the Ti-doped ZrO 2 film changed depending on Ti%, and the properties in the film, such as chemical bonding, dielectric constant, and leakage current density, also change accordingly.The optimal dopant concentration to improve crystallinity in Ti-doped ZrO 2 was determined.

Figure 1 .
Figure 1.ALD growth of ZrO 2 films deposited at (a) deposition temperature, and (b) cycle number.And ALD growth of TiO 2 films deposited at (c) deposition temperature, and (d) cycle number.

Figure 5 .
Figure 5. (a) C-V measurement and (b) I-V measurement for Ti doped ZrO 2 films in a MIM structure.

Table I .
Real atomic concentrations from the experiment for each supercycle process.

Table II .
Theoretical calculated value and AES analysis value of Ti%. Figure 3. XRD data for Ti doped ZrO 2 films at various supercycle and non-doped ZrO 2 .