Ultrathin HfO2 dielectric film and preparation method and application thereof

Abstract

The invention provides an ultrathin HfO2 dielectric film as well as a preparation method and application thereof. The solvent used for preparing the precursor solution is the mixed solvent of ethanol and water, the precursor solution of the ethanol-water mixed system can effectively reduce defects caused by violent hydrolysis compared with a precursor solution of a full-water system, and meanwhile, the precursor solution of the ethanol-water mixed system is easier to obtain complete HfO2 conversion compared with the precursor solution of the full-ethanol system; therefore, a more uniform and compact HfO2 dielectric film can be formed after annealing treatment; by regulating and controlling the concentration of the solvent and the precursor and the annealing process, the ultra-thin HfO2 film with the thickness of less than 7nm is prepared by adopting the mixed solvent of ethanol and water in a ratio of 1: 1 and the precursor concentration of 0.18 M under the annealing condition of 400 DEG C, and the ultra-thin HfO2 film shows excellent properties, especially dielectric properties.

Description

Technical Field

[0001] This invention relates to the field of dielectric thin film preparation technology, and in particular to an ultrathin HfO2 dielectric thin film, its preparation method and application. Background Technology

[0002] With the rise of mobile electronic devices and the concept of the metaverse, thin-film electronic devices such as TFTs (thin-film transistors) are developing towards low power consumption, low cost, and miniaturization. The performance of the gate dielectric layer plays a key role in the stability and power consumption of TFTs. When the thickness of the traditional silicon dioxide (SiO2) dielectric layer is reduced to a critical value, the tunneling effect leads to a sharp increase in leakage current, which seriously affects the standby power consumption and stability of TFTs and may even damage TFT devices. Numerous studies have shown that when the thickness of SiO2 is less than 1.5 nm, its leakage current is already difficult to meet the performance requirements of microelectronic devices. To address this issue, researchers have noted that high-k dielectric (high-κ) materials can effectively reduce leakage current while maintaining a large capacitance per unit area. For thin-film electronic devices such as metal-dielectric-metal (MIM) devices, high-κ dielectric materials, due to their high dielectric constant, have a small equivalent oxide thickness, enabling them to provide the same capacitance as SiO2 while maintaining a thicker dielectric layer. This results in strong capacitive coupling between the gate and channel layers, effectively suppressing the increase in leakage current. At the same time, a high dielectric constant also means a higher gate capacitance, which helps to improve the driving capability and switching speed of transistors.

[0003] Among numerous high-k dielectric materials such as HfO2, ZrO2, Al2O3, Y2O3, and La2O3, HfO2 stands out for its superior dielectric properties. Its high theoretical dielectric constant allows it to provide higher capacitance density under the same electric field conditions; its large band gap (5.3–5.7 eV) effectively blocks the penetration of electrons and holes, thereby reducing leakage current and improving device reliability and efficiency; its high thermal and chemical stability (up to 700℃) makes it resistant to decomposition or phase transitions and compatible with various fabrication processes; and its large band compensation and ability to form a good interface with silicon substrates are particularly important for TFT device performance. Due to its high dielectric constant, large band gap, high thermal stability, and good interface compatibility, HfO2 is considered an ideal material for TFT gate dielectric layers.

[0004] Currently, most studies employ multiple solution spin-coating methods to prepare HfO2 thin films. This can lead to numerous defects at the film interfaces, affecting device performance. Furthermore, the prepared films are typically thicker than 10 nm, and the properties of ultrathin (below 10 nm) films have not been explored. Secondly, while solution methods have shown great potential in preparing ultrathin HfO2 films, several challenges remain in practical application. Firstly, ultrathin HfO2 films are prone to crystalline structures, and the increased grain boundaries in crystalline films typically result in higher leakage currents, making it difficult to achieve the desired high dielectric properties. Secondly, reduced film thickness leads to higher density of interface defects and trapped states, which further increases leakage current and can even cause device failure under high electric fields. Therefore, to ensure the excellent performance of ultrathin HfO2 films, structural and interface optimization is necessary during the preparation process. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide an ultrathin HfO2 dielectric film, its preparation method, and its applications. This invention utilizes a solution method to prepare an ultrathin HfO2 dielectric film. The precursor solution is prepared using a mixture of ethanol and water. Compared to a purely aqueous precursor solution, the ethanol-water mixture effectively reduces defects caused by severe hydrolysis and facilitates complete HfO2 conversion. Therefore, after annealing, a more uniform and dense HfO2 dielectric film can be formed. This invention demonstrates that by controlling the solvent, precursor concentration, and annealing process, using a 1:1 ethanol-water mixture, a precursor concentration of 0.18 M, and annealing at 400°C, an ultrathin HfO2 film of <7 nm can be prepared, exhibiting excellent properties, especially dielectric characteristics.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing an ultrathin HfO2 dielectric film, comprising the following steps:

[0008] Hafnium salt is added to a solvent to obtain a precursor solution;

[0009] The precursor solution is coated onto the substrate surface to form an HfO2 thin film;

[0010] The HfO2 thin film was annealed to obtain an HfO2 dielectric thin film;

[0011] The solvent is a mixture of ethanol and water.

[0012] Preferably, the volume ratio of ethanol to water is 1:1.

[0013] Preferably, the concentration of hafnium salt in the precursor solution is 0.045–0.36 mol / L.

[0014] Preferably, the hafnium salt includes at least one of hafnium tetrachloride, hafnium oxychloride, and hafnium nitrate.

[0015] Preferably, the HfO2 thin film is annealed to obtain an HfO2 dielectric thin film, specifically including:

[0016] The HfO2 film was pre-annealed at 180–220 °C for 10–20 min;

[0017] The pre-annealed HfO2 film was then annealed at 350–600°C for 1 hour.

[0018] Preferably, the substrate is a silicon substrate;

[0019] The process before coating the precursor solution onto the substrate surface also includes:

[0020] The substrate was ultrasonically cleaned sequentially with isopropanol, ethanol and deionized water, and then dried with nitrogen.

[0021] The cleaned substrate is placed in a plasma cleaner, oxygen is introduced, and the surface is activated for 10-20 minutes.

[0022] Preferably, in the step of coating the precursor solution onto the substrate surface, the coating method is spin coating, the spin coating speed is 4500 rpm, and the spin coating time is 30 s.

[0023] Preferably, the solvent is a mixture of ethanol and water, wherein the volume ratio of ethanol to water is 1:1;

[0024] The concentration of hafnium salt in the precursor solution is 0.18 mol / L;

[0025] The HfO2 thin film is annealed to obtain an HfO2 dielectric thin film, specifically including:

[0026] The HfO2 film was pre-annealed at 200℃ for 10 min;

[0027] The pre-annealed HfO2 film was then annealed at 400℃ for 1 hour.

[0028] Secondly, the present invention also provides an ultrathin HfO2 dielectric film, which is prepared by the aforementioned preparation method.

[0029] Thirdly, the present invention also provides an ultrathin HfO2 dielectric film prepared by the preparation method described above, or the application of the ultrathin HfO2 dielectric film in the preparation of thin-film electronic devices.

[0030] The ultrathin HfO2 dielectric film, its preparation method, and its application of the present invention have the following advantages compared with the prior art:

[0031] 1. The method for preparing the ultrathin HfO2 dielectric film of the present invention involves adding hafnium salt to a solvent to obtain a precursor solution; coating the precursor solution onto the surface of a substrate to form an HfO2 film; and annealing the HfO2 film to obtain the HfO2 dielectric film. The solvent used in the preparation of the precursor solution in the present invention is a mixed solvent of ethanol and water. Compared with the precursor solution of the all-water system, the ethanol-water mixed system precursor solution can effectively reduce defects caused by violent hydrolysis, and at the same time, it is easier to obtain complete HfO2 conversion than the precursor solution of the all-ethanol system. Therefore, a more uniform and dense HfO2 dielectric film can be formed after annealing.

[0032] 2. This invention systematically explores the key parameters affecting the quality of ultrathin HfO2 dielectric films during the solution preparation process, mainly including three aspects: precursor solvent, precursor concentration, and annealing temperature.

[0033] (1) Through comparative studies of different solvents, this invention found that optimizing the solvent system can effectively control the hydrolysis rate of the precursor solution and the film formation process. In the film prepared using a 1:1 mixed solvent system of ethanol and water, a smooth HfO2 film with a roughness of less than 1 nm was obtained, while significantly improving the density and uniformity of the film.

[0034] (2) The present invention regulates the precursor concentration, which not only changes the solute behavior, but also affects the continuity and thickness of the HfO2 film. As the concentration decreases, the film thickness gradually decreases, which is in line with the basic rules of solution preparation process. When the concentration is controlled at 0.18M, ​​an ultrathin HfO2 film of 6.4nm is obtained, and the film exhibits the highest capacitance per unit area and the lowest leakage current. This shows that an appropriate concentration can maintain good dielectric properties, but too high or too low concentration will lead to a decrease in dielectric properties, which is manifested as a significant increase in leakage current.

[0035] (3) This invention investigated the properties of HfO2 dielectric films annealed at temperatures ranging from 350℃ to 600℃ and found that the films prepared under annealing conditions at 400℃ exhibited the best overall performance. The HfO2 dielectric films were in a completely amorphous state, possessing not only the lowest surface roughness and the highest film density, but also the lowest leakage current of 6.52 × 10⁻⁶. -7 A / cm 2 (@6.0MV / cm), capacitance per unit area at 1000Hz is 673.75nF / cm. 2 This further proves its excellent dielectric properties, making it an ideal choice for gate dielectric layer applications;

[0036] In summary, this invention demonstrates that optimizing the precursor solvent, rationally adjusting the precursor concentration, and selecting an appropriate annealing temperature can significantly improve the overall performance of HfO2 dielectric films. In a system using a 0.18M ethanol-water 1:1 mixture as the precursor solvent, ultrathin HfO2 films <7nm obtained by solution annealing at 400℃ exhibit the best comprehensive dielectric properties, providing solid technical support for their application in high-κ dielectric films and related electronic devices. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a schematic diagram of the preparation method of the ultrathin HfO2 dielectric film of the present invention;

[0039] Figure 2 This is a schematic diagram of a metal-dielectric-metal (MIM) device.

[0040] Figure 3 Thermogravimetric curves of the precursor solutions prepared using different solvents in Example 1 and Comparative Examples 1-2 are shown.

[0041] Figure 4 Atomic force microscopy images of HfO2 dielectric films prepared using different solvents in Example 1 and Comparative Examples 1-2;

[0042] Figure 5 The X-ray reflectivity (XRR) and XRD (X-ray diffraction) patterns of HfO2 dielectric films prepared using different solvents in Example 1 and Comparative Examples 1-2 are shown.

[0043] Figure 6 The SE diagrams (i.e., ellipsometric spectra measured by an ellipsometer) of HfO2 dielectric films prepared using different solvents in Examples 1 and Comparative Examples 1-2, the Fourier infrared curves of the precursor powder, the light transmittance diagrams of the HfO2 dielectric films, and the light absorption coefficient spectra of the HfO2 dielectric films are shown.

[0044] Figure 7 The leakage current characteristic curves and capacitance-frequency characteristic curves of HfO2 dielectric films prepared using different solvents in Example 1 and Comparative Examples 1-2 are shown.

[0045] Figure 8XRR curves and thicknesses of HfO2 dielectric films prepared with different concentrations of hafnium tetrachloride in the precursor solutions of Examples 2-5;

[0046] Figure 9 AFM images and roughness of HfO2 dielectric films prepared with different concentrations of hafnium tetrachloride in the precursor solutions of Examples 2-5;

[0047] Figure 10 XRD patterns, SE patterns, UV-Vis transmittance curves (inset shows the band gap), and UV-Vis absorption curves of HfO2 dielectric films prepared with different concentrations of hafnium tetrachloride in the precursor solutions of Examples 2-5.

[0048] Figure 11 The leakage current characteristic curves and capacitance-frequency characteristic curves of HfO2 dielectric films prepared with different concentrations of hafnium tetrachloride in the precursor solutions of Examples 2-5 are shown.

[0049] Figure 12 AFM images and roughness variation diagrams of HfO2 dielectric films prepared at different annealing temperatures in Examples 6-10 are shown.

[0050] Figure 13 GIXRD patterns of HfO2 dielectric films prepared at different annealing temperatures in Examples 6-10, and TEM images of films annealed at 400℃, 450℃, and 500℃.

[0051] Figure 14 The UV transmittance curves, SE curves, XRR curves, and FTIR curves are for the HfO2 dielectric films prepared at different annealing temperatures in Examples 6-10.

[0052] Figure 15 XPS images of HfO2 dielectric films prepared at different annealing temperatures in Examples 6-10;

[0053] Figure 16 The capacitance-frequency characteristic curves, annealing temperature-capacitance diagrams, and annealing temperature-dielectric constant diagrams of HfO2 dielectric films prepared at different annealing temperatures in Examples 6-10 are shown.

[0054] Figure 17 The leakage current characteristic curves are for HfO2 dielectric films prepared at different annealing temperatures in Examples 6-10. Detailed Implementation

[0055] The technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0056] It should be noted that the order of description of the following embodiments is not intended to limit the preferred order of embodiments. Furthermore, in the description of this application, the term "comprising" means "including but not limited to". Various embodiments of the present invention may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a rigid limitation on the scope of the invention; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single digits within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Additionally, whenever a numerical range is indicated herein, it means including any referenced number (fraction or integer) within the indicated range. The application principles of the present invention will now be described in detail with reference to the accompanying drawings.

[0057] This invention provides a method for preparing an HfO2 dielectric thin film, comprising the following steps:

[0058] S1. Add hafnium salt to the solvent to obtain a precursor solution;

[0059] S2. The precursor solution is coated onto the substrate surface to form an HfO2 thin film;

[0060] S3. Anneal the HfO2 film to obtain the HfO2 dielectric film;

[0061] The solvent is a mixture of ethanol and water.

[0062] The method for preparing the HfO2 dielectric film of the present invention includes: adding hafnium salt to a solvent to obtain a precursor solution, coating the precursor solution onto a substrate surface to form an HfO2 film, and then annealing the HfO2 film to obtain the HfO2 dielectric film. The HfO2 dielectric film of the present invention is prepared by a solution method, which does not require expensive equipment such as magnetron sputtering, resulting in low preparation cost and facilitating large-scale production. At the same time, the solvent used in the preparation of the precursor solution in the present invention is a mixed solvent of ethanol and water. The ethanol-water mixed system precursor solution can effectively reduce defects caused by violent hydrolysis compared with the all-water system precursor solution, and it is easier to obtain complete HfO2 conversion than the all-ethanol system precursor solution. Therefore, a more uniform and dense HfO2 dielectric film can be formed after annealing.

[0063] In some embodiments, the volume ratio of ethanol to water is 1:1.

[0064] In some embodiments, the concentration of hafnium salt in the precursor solution is 0.045–0.36 mol / L.

[0065] In some embodiments, the hafnium salt includes at least one of hafnium tetrachloride, hafnium oxychloride, and hafnium nitrate.

[0066] In some embodiments, annealing an HfO2 thin film to obtain an HfO2 dielectric thin film specifically includes:

[0067] The HfO2 film was pre-annealed at 180–220 °C in air for 10–20 min.

[0068] The pre-annealed HfO2 film was then annealed at 350–600°C in air for 1 hour.

[0069] In some embodiments, the substrate is a silicon substrate;

[0070] The process before coating the precursor solution onto the substrate surface also includes:

[0071] The substrate was ultrasonically cleaned sequentially with isopropanol, ethanol and deionized water (each cleaning for 10-15 minutes), and then dried with nitrogen.

[0072] The cleaned substrate is placed in a plasma cleaner, oxygen is introduced, and the surface is activated for 10-20 minutes. This significantly reduces the contact angle and improves wettability by enhancing the surface energy of the silicon substrate.

[0073] Specifically, a radio frequency (RF) power source (commonly 13.56MHz) or microwave source excites an electric field in the cavity, ionizing oxygen molecules and generating plasma. The plasma contains active oxygen species (such as oxygen free radicals O·, atomic oxygen O, and ozone O3). The oxygen plasma can also slightly etch the surface, forming hydrophilic groups such as hydroxyl groups (-OH), increasing the surface energy and enhancing the adhesion of subsequent thin film deposition or bonding (i.e., "surface activation").

[0074] In some embodiments, in the step of coating the precursor solution onto the substrate surface, the coating method is spin coating, the spin coating speed is 4500 rpm, and the spin coating time is 30 s.

[0075] In some embodiments, the volume ratio of ethanol to water is 1:1;

[0076] The concentration of the hafnium salt solution is 0.18 mol / L;

[0077] The HfO2 thin film is annealed to obtain an HfO2 dielectric thin film, specifically including:

[0078] The HfO2 film was pre-annealed at 200℃ for 10 min;

[0079] The pre-annealed HfO2 film was then annealed at 400℃ for 1 hour.

[0080] In some embodiments, the precursor solution is stirred at room temperature for 6 hours under atmospheric conditions and aged for 24 hours to eliminate bubbles and stabilize dispersibility before use. Before spin coating, the prepared precursor solution is stirred for 40 minutes before use.

[0081] In some embodiments, spin coating specifically includes: turning on the spin coater, introducing nitrogen gas to protect the motor, and then turning on the vacuum pump; precisely placing the silicon wafer into the center area of ​​the spin coater's suction cup using tweezers, activating the vacuum adsorption system to stabilize the substrate, filtering the precursor solution through a 0.22μm filter membrane, using a microsyringe to transfer the precursor solution, and uniformly dropping it onto the silicon wafer surface; after the self-flowing and spreading process forms a homogeneous liquid film interface, starting the spin coating process, setting the spin coater speed to 4500rpm and the spin coating time to 30s, thus completing the thin film spin coating step.

[0082] For details, please refer to Figures 1-2 As shown, this is a schematic diagram of the preparation method of the HfO2 dielectric film of the present invention. After the precursor solution is drawn up with a syringe, it is filtered and flows onto the substrate surface. The spin coater is turned on for spin coating. After annealing, an HfO2 dielectric film is obtained on the substrate. Then, using a mask, the source and drain electrodes are deposited on the surface of the HfO2 dielectric film. The source and drain electrodes can be Au electrodes. In this way, a metal-dielectric-metal (MIM) device is obtained for subsequent performance testing.

[0083] like Figure 2 As shown, it is a schematic diagram of the structure of a metal-dielectric-metal (MIM) device, including: a substrate 1, an HfO2 dielectric film 2 (as a gate dielectric layer), a source 3 and a drain 4 located on the surface of the HfO2 dielectric film 2 (the material of the source and drain is Au).

[0084] In some embodiments, an HfO2 dielectric film is obtained on a substrate according to the above method; then an In2O3 film is deposited on the surface of the HfO2 dielectric film. Specifically, an In2O3 film with a thickness of 2-5 nm is prepared by solution spin coating; then, source and drain electrodes are deposited on the surface of the In2O3 film using a mask. Specifically, Au electrodes can be used for the source and drain electrodes, thus obtaining a TFT device.

[0085] Based on the same inventive concept, the present invention also provides an ultrathin HfO2 dielectric film, which is prepared by the above-described preparation method.

[0086] Based on the same inventive concept, the present invention also provides an ultrathin HfO2 dielectric film prepared by the above-described preparation method or the application of the above-described ultrathin HfO2 dielectric film in the preparation of thin-film electronic devices.

[0087] Specifically, thin-film electronic devices include, but are not limited to, metal-dielectric-metal (MIM) devices, TFT devices, etc.

[0088] The following specific embodiments further illustrate the HfO2 dielectric thin film, its preparation method, and its applications of the present invention. This section further explains the content of the present invention in conjunction with specific embodiments, but should not be construed as limiting the present invention. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in the art.

[0089] The following embodiments use the following methods to test dielectric properties such as capacitance-frequency characteristics, leakage current characteristics, and breakdown electric field strength:

[0090] After preparing HfO2 dielectric films in the examples and comparative examples, source and drain electrodes were deposited on the surface of the HfO2 dielectric films using a mask. Both source and drain electrodes were Au electrodes, resulting in metal-dielectric-metal (MIM) devices. The dielectric properties of the MIM devices, such as capacitance-frequency characteristics, leakage current characteristics, and breakdown electric field strength, were tested.

[0091] Specifically, the capacitance-frequency characteristic characterizes the evolution of charge storage capacity with the frequency of an alternating electric field. This invention uses a Keysight-E4980AL high-precision impedance analysis system to test the capacitance-frequency characteristics of dielectric thin films. This test was performed on metal-dielectric-metal (MIM) devices prepared from different HfO2 dielectric thin films at ambient temperature. In the actual test, one probe was connected to the silicon substrate, and the other probe was connected to the fabricated drain, forming a vertical capacitor plate structure. The test results are presented in the form of C (capacitance)-F (frequency) curves. The dielectric constant can be calculated from the obtained CF curves for further analysis of its dielectric properties.

[0092] This invention employs a dual-channel digital source meter, Keysight-2612B, to characterize dielectric properties of dielectric films, such as leakage current and breakdown electric field strength. This invention tests the leakage current characteristics and breakdown electric field strength of metal-dielectric-metal (MIM) devices. The testing of leakage current and breakdown electric field strength of MIM devices is conducted at both ends, with one probe connected to the gate and the other probe connected to the drain.

[0093] Example 1

[0094] This embodiment provides a method for preparing an ultrathin HfO2 dielectric film, including the following steps:

[0095] S1. Add hafnium tetrachloride (HfCl4) to a solvent to obtain a precursor solution; the solvent includes a mixture of ethanol and water in a volume ratio of 1:1; the concentration of hafnium tetrachloride in the precursor solution is 0.3 M (mol / L);

[0096] S2. The silicon substrate is sequentially ultrasonicated in isopropanol, ethanol, and deionized water (15 min for each ultrasonication), and then dried with nitrogen gas.

[0097] The cleaned substrate was placed in a plasma cleaner, oxygen was introduced, and the surface was activated for 10 minutes.

[0098] S3. Spin-coat the precursor solution onto the surface of the activated silicon substrate at a spin speed of 4500 rpm for 30 s to form an HfO2 thin film.

[0099] S5. Pre-anneal the HfO2 film at 200℃ in air for 10 min.

[0100] The pre-annealed HfO2 film was then annealed at 400°C in air for 1 hour to obtain the HfO2 dielectric film.

[0101] Comparative Example 1

[0102] This comparative example provides a method for preparing an HfO2 dielectric thin film, comprising the following steps:

[0103] S1. Add hafnium tetrachloride (HfCl4) to a solvent to obtain a precursor solution; the solvent is ethanol; the concentration of hafnium tetrachloride in the precursor solution is 0.3 M (mol / L);

[0104] S2. The silicon substrate is sequentially ultrasonicated in isopropanol, ethanol, and deionized water (15 min for each ultrasonication), and then dried with nitrogen gas.

[0105] The cleaned substrate was placed in a plasma cleaner, oxygen was introduced, and the surface was activated for 10 minutes.

[0106] S3. Spin-coat the precursor solution onto the surface of the activated silicon substrate at a spin speed of 4500 rpm for 30 s to form an HfO2 thin film.

[0107] S5. Pre-anneal the HfO2 film at 200℃ in air for 10 min.

[0108] The pre-annealed HfO2 film was then annealed at 400°C in air for 1 hour to obtain the HfO2 dielectric film.

[0109] Comparative Example 2

[0110] This comparative example provides a method for preparing an HfO2 dielectric thin film, comprising the following steps:

[0111] S1. Hafnium tetrachloride (HfCl4) is added to a solvent to obtain a precursor solution; the solvent is deionized water; the concentration of hafnium tetrachloride in the precursor solution is 0.3 M (mol / L);

[0112] S2. The silicon substrate is sequentially ultrasonicated in isopropanol, ethanol, and deionized water (15 min for each ultrasonication), and then dried with nitrogen gas.

[0113] The cleaned substrate was placed in a plasma cleaner, oxygen was introduced, and the surface was activated for 10 minutes.

[0114] S3. Spin-coat the precursor solution onto the surface of the activated silicon substrate at a spin speed of 4500 rpm for 30 s to form an HfO2 thin film.

[0115] S5. Pre-anneal the HfO2 film at 200℃ in air for 10 min.

[0116] The pre-annealed HfO2 film was then annealed at 400°C in air for 1 hour to obtain the HfO2 dielectric film.

[0117] The solution-based preparation process begins with a liquid-phase dispersion stage of the metal precursor. Water or alcohols (e.g., H₂O or ROH) in the liquid medium gradually convert metal cations into hydroxyl (-OH) or oxo groups (O₂) complexes through coordination. In the subsequent polymerization stage, these metal hydroxide units undergo dehydration condensation to form a three-dimensional MOM network framework. Therefore, Examples 1 and Comparative Examples 1-2 of this invention aim to explore the crucial role of precursor solvent selection in the solution-based preparation of HfO₂ dielectric films. Using HfCl₄ as the solute, 0.3M solutions were prepared using different solvent systems (all water, all ethanol, and a 1:1 volume mixture of water and ethanol). Solvent regulation was used to optimize the dissolution behavior, and TGA was used to investigate its thermal decomposition and hydrolysis processes. HfO₂ films annealed in air at 400°C were prepared by spin-coating using the solution method, and the physical properties of the films were investigated using various characterization techniques.

[0118] Figure 3 Thermogravimetric curves of the precursor solutions prepared using different solvents in Example 1 and Comparative Examples 1-2 are shown. Figure 3 The left vertical axis, Weight (%), represents the percentage of the sample's remaining mass relative to its initial mass (Weight% is the mass retention rate). The right vertical axis, Deriv.Weight (%), represents the rate of weight change.

[0119] Figure 3 (a) shows the precursor solution prepared using deionized water (DI-Water) as a solvent in Comparative Example 2; (b) shows the precursor solution prepared using ethanol (Ethonal) as a solvent in Comparative Example 1; and (c) shows the precursor solution prepared using a mixed solvent of ethanol and water (Ethonal:DI-Water = 1:1) in Example 1. Before the specific test, the precursor solution was dried in an oven at 80°C for 8 hours to obtain a precursor solution dry gel. The dry gel was then tested at 10°C for 1 minute. -1 The heating rate was used to control the test temperature, and the dry gel was heated from room temperature to 800°C in a nitrogen atmosphere.

[0120] As can be seen from the thermogravimetric curves, in Comparative Example 2, the all-water system undergoes violent and instantaneous hydrolysis of HfCl4 in water, releasing a large amount of heat. According to the hydrolysis reaction, hafnium hydroxychloride is formed. This stage corresponds to the low-temperature stage (50-200℃) where a severe mass loss occurs.

[0121] HfCl4 + H2O → Hf(OH)Cl3 + HCl↑

[0122] Unstable hafnium hydroxychloride will further decompose in water, as shown in the following reaction equation:

[0123] Hf(OH)Cl3→HfOCl3+HCl↑

[0124] When the precursor solution is spin-coated onto the substrate, the deposited film consists of HfOCl2·nH2O. After thermal annealing, HfOCl2 decomposes and transforms into HfO2. During the dehydration process, HfOCl2 has a loose structure, which may affect the compactness of the final film. The decomposition process is shown in the following formula:

[0125] HfOCl2·nH2O→HfO2+2HCl↑+(n–1)H2O

[0126] In Comparative Example 1, the all-ethanol system effectively inhibited the hydrolysis reaction due to the complexing effect of ethanol on HfCl4. This was achieved by the hydroxyl group (-OH) of ethanol replacing the chloride ion (Cl) in HfCl4 through coordination. - This process generates stable tetraethoxyhafnium (Hf·(OCH2H3)). n This makes the precursor more stable, resulting in the formation of Hf·(OCH2H3). n Further hydrolysis and condensation occur in the presence of trace amounts of water. At 50-300℃, the main reactions are organic solvent volatilization and residual alcohol ligand decomposition, resulting in relatively small TGA mass loss and a relatively gradual decomposition process. The specific reaction process is as follows:

[0127] HfCl4+nC2H5OH→Hf·(C2H5OH)n+nHCl↑

[0128] Hf·(OCH2H3) n +2H₂O→HfO₂↓+4C₂H₅OH↑

[0129] The water-ethanol mixture in Example 1 (ethanol added first, then water) combines the characteristics of both, where HfCl4 is converted to Hf·(OCH2H3) under the action of ethanol. n The complex, upon the addition of water, undergoes further mild hydrolysis, promoting the reaction of Hf·(OCH2H3). n With sufficient hydrolysis, the mass loss of TGA falls between that of the all-water and all-ethanol systems, and the thermal decomposition process is relatively uniform. The specific dissolution process is as follows:

[0130] HfCl4 + nC2H5OH → Hf·(OCH2H3) n +nHCl

[0131] Hf·(OCH2H3) n +mH2O→HfO2↓+nC2H5OH↑(m>2)

[0132] In summary, appropriate control of the precursor solvent composition plays a crucial role in optimizing the stability of the precursor solution. Compared to the all-water system, the ethanol-water mixed system of the precursor solution can effectively reduce defects caused by severe hydrolysis, and it is also easier to obtain complete HfO2 conversion than the all-ethanol system. Therefore, it is expected to form a more uniform and dense HfO2 film after annealing.

[0133] To verify the influence of different solvent systems on the film morphology and surface roughness, HfO2 films with three different solvent systems were prepared by spin coating at 400℃, such as... Figure 4 As shown. Figure 4 Atomic force microscopy (AFM) images of HfO2 dielectric films prepared using different solvents in Example 1 and Comparative Examples 1-2. Figure 4 (a) shows the HfO2 dielectric film prepared using ethanol as a solvent in Comparative Example 1, (b) shows the HfO2 dielectric film prepared using water as a solvent in Comparative Example 2, and (c) shows the HfO2 dielectric film prepared using a mixed solvent of ethanol and water in Example 1.

[0134] from Figure 4 As can be seen in Figure (a), the HfO2 dielectric film obtained in the all-ethanol system of Comparative Example 1 has the highest surface roughness (RMS = 22.40 nm), and the surface exhibits a clear stacked structure. Due to the low polarity of ethanol, it has strong solubility for HfCl4, but it is difficult to provide a sufficient hydrolysis environment, resulting in Hf... 4+ The main process involves alcoholysis to form a complex with ethanol, leading to incomplete decomposition during subsequent film growth and resulting in a non-uniform granular structure, increasing surface roughness. Figure 4 As can be seen in Figure (b), the surface roughness (RMS = 6.72 nm) of the HfO2 dielectric film prepared in the all-water system in Comparative Example 2 is significantly reduced, but a relatively obvious rod-like structure still exists. This is because although the all-water system provides sufficient hydrolysis conditions, allowing HfO2 to form a more robust hydrolytic structure, the surface roughness of the HfO2 dielectric film is significantly reduced. 4+ It can rapidly hydrolyze to generate Hf(OH)Cl3 and related hydrated oxides, but the excessively rapid hydrolysis rate leads to localized agglomeration of the precursor during film formation, resulting in large-sized hydrated hafnium oxide particles. During subsequent annealing, the dehydration process of these hydrated hafnium oxide particles leads to significant porosity, and the film surface still exhibits a certain degree of inhomogeneity. In contrast, from... Figure 4 As can be seen in (c), the HfO2 dielectric film obtained from the ethanol-water mixed system in Example 1 has the lowest surface roughness (RMS = 0.48 nm), and unlike the rod-like structure of the all-water system and the stacked morphology of the all-ethanol system, the film surface is relatively uniform and smooth. This is because the ethanol-water mixed system, while providing a moderate hydrolysis environment, mitigates the HfO2 degradation through the complexing effect of ethanol.4+ The hydrolysis rate allows the precursor to be deposited more uniformly during spin coating and to form a denser and smoother film during subsequent heat treatment.

[0135] Figure 5 The X-ray reflectivity (XRR) of the HfO2 dielectric films prepared using different solvents in Example 1 and Comparative Examples 1-2 is shown below. Figure 5 As shown in (a), the XRR ordinate has been normalized and the XRD (X-ray diffraction) plot is shown. Figure 5 (as shown in (b)); Figure 5 In this context, Ethanol represents the HfO2 dielectric film prepared in Comparative Example 1, DI-Water represents the HfO2 dielectric film prepared in Comparative Example 2, and Ethanol:DI-Water represents the HfO2 dielectric film prepared in Example 1.

[0136] Specifically, XRR, as a non-destructive testing method, measures the reflectivity of X-rays on the surface of a thin film with extra spaces as a function of the incident angle, enabling the determination of parameters such as thickness and roughness without contact with the sample. When the incident angle is less than the critical angle, total internal reflection of X-rays occurs; beyond the critical angle, the reflectivity decreases rapidly with increasing angle and exhibits periodic oscillations (Kiessig fringes). The oscillation period is inversely proportional to the film thickness, and its quantitative relationship can be expressed as:

[0137]

[0138] In the formula, d is the film thickness, λ is the X-ray wavelength, and Δθ is the angle difference between adjacent oscillation peaks. According to the formula for calculating the thickness using XRR, the thinner the film, the larger the oscillation period (i.e., the number of peaks per unit angle is reduced).

[0139] Figure 5 The XRR measurement results in (a) indicate that the film prepared in Example 1 using a 1:1 mixture of ethanol and water exhibits the best compactness and smoothness. Its XRR curve shows typical film peaks, and the attenuation amplitude is smaller compared to the other two groups, indicating that this group of films has lower surface roughness. Comparing the other two groups of film samples based on attenuation amplitude, the film with the all-ethanol system in Comparative Example 1 has the highest surface roughness, followed by the all-water system in Comparative Example 2. This trend is consistent with the AFM results. Furthermore, due to the high roughness and inhomogeneity of the film samples in Comparative Examples 1 and 2, no obvious film peaks were observed, further leading to the unsuccessful calculation and fitting of film thickness using XRR.

[0140] Figure 5 (b) XRD patterns of the three groups of samples prepared in Example 1 and Comparative Examples 1-2, from... Figure 5As can be seen from the XRD pattern in (b), no obvious crystalline phase diffraction peaks were observed in any of the three groups of samples, indicating that the prepared HfO2 film is in an amorphous state and that different solvent systems have no effect on the crystalline phase of the film.

[0141] Figure 6 SE images of HfO2 dielectric films prepared using different solvents in Example 1 and Comparative Examples 1-2 ( ) Figure 6 In (a), the SE curve usually refers to the curve of optical parameters as a function of wavelength (or incident angle) measured by ellipsometry (SE), with the ordinate being the refractive index, and the Fourier transform infrared curve of the precursor powder. Figure 6 (b) Light transmittance diagram of HfO2 dielectric film ( Figure 6 (c) The test was performed using a UV-Vis-NIR spectrometer, with the inset showing the light absorption coefficient spectrum of the HfO2 dielectric film corresponding to the band gap. Figure 6 (d) The test was conducted using the transmission-reflection method based on an ultraviolet-visible-near-infrared spectrometer. Figure 6 In this context, Ethanol represents the HfO2 dielectric film prepared in Comparative Example 1, DI-Water represents the HfO2 dielectric film prepared in Comparative Example 2, and Ethanol:DI-Water represents the HfO2 dielectric film prepared in Example 1.

[0142] Figure 6 The SE results in (a) show that the refractive indices of HfO2 films prepared with different solvent systems differ significantly in the visible light range. The refractive index is mainly related to the film's density and porosity. In Example 1, the ethanol-water mixed film has the highest refractive index, approaching 2.680 at 550 nm, and gradually decreases with increasing wavelength. The refractive index of the all-water system is approximately 1.560, while the refractive index of the all-ethanol system is the lowest at 1.180. In Comparative Example 1, the all-ethanol system exhibits a lower hydrolysis rate, resulting in higher refractive indices of HfO2 films. 4+ The complexes with ethanol are difficult to completely decompose during film formation, resulting in residual organic phases or amorphous structures inside the film, thus leading to a low refractive index. In contrast, although the all-water system in Comparative Example 2 provides a sufficient hydrolysis environment, the rapid hydrolysis rate may cause the precursor to form some nanoparticles or aggregates in the solution, making it difficult to form a uniform and dense film during spin coating, ultimately reducing the refractive index. The ethanol-water mixed system effectively modulates the Hf value through the complexing effect of ethanol. 4+The hydrolysis rate of the ethanol-water mixture allows for more uniform deposition of the precursor on the substrate surface, resulting in a uniform and dense hafnium oxide film during subsequent heat treatment. The mixed solvent system forms a more regular network structure, reducing internal defects and porosity, thereby increasing the refractive index and improving the film uniformity. Combining the AFM and XRR results mentioned above, the film prepared by the ethanol-water mixture has the smoothest surface and the highest density, which corroborates the refractive index measurement results.

[0143] Figure 6 (b) The FTIR test results of powders after drying and annealing with different precursor solutions are shown, further revealing the influence of precursor solvents on the chemical composition of thin films, 500-700 cm⁻¹. -1 The peak represents the Hf-O bond. 1400-1500 cm⁻¹ -1 The peak originates from chemically bonded water molecules remaining in the hydrate or gel network. Due to the rapid hydrolysis rate of the all-aqueous system, a large amount of hydrated hafnium oxide (HfO2·XH2O) is easily formed, resulting in a strong absorption peak only in this region of the all-aqueous system. 1600cm -1 The peak at 3200-3600 cm⁻¹ represents a C=O bond. -1 The wavelength bands exhibit characteristic broad peaks associated with hydroxyl (-OH) groups. The vibrational peak is strongest in the all-ethanol system, indicating the formation of more -OH groups. The -OH vibrational peak intensity is second strongest in the all-water system, while the characteristic absorption peak is weakest in the ethanol-water mixture. Excessive -OH groups lead to an increase in mobile hydrogen ions, resulting in increased capacitance frequency dependence at low frequencies, which is detrimental to achieving high-performance oxide thin-film transistors. FTIR analysis of three different precursor solvents shows that the appropriate amount of water involved in the reaction helps to balance the hydrolysis and complexation processes of the precursors, thereby optimizing the film formation quality. In conclusion, the ethanol-water mixture system can effectively control the hydrolysis behavior of HfCl4, forming a denser and more uniform HfO2 film.

[0144] Figure 6 (c) and (d) show the ultraviolet-visible transmission, absorption spectrum, and band gap. Figure 6As shown in the inset (c), the standard Tauc plot method was used for fitting. The results show that HfO2 films prepared by different solvent systems have significant differences in optical properties. The film prepared by the all-ethanol system has stronger absorption in the short-wavelength region, resulting in lower transmittance in the visible light region and a slightly smaller band gap (4.36 eV). This may be because ethanol has a strong inhibitory effect on the hydrolysis reaction of the precursor, resulting in more residual organic matter and defect centers in the film. The all-water system provides an excessively fast hydrolysis rate, and some areas are prone to local aggregation or oxygen-deficient regions. The band gap is slightly increased compared to the all-ethanol system (5.73 eV). The 1:1 ethanol-water mixture balances the decomposition rate of the precursor under moderate hydrolysis and complexation effects, which improves the density and chemical homogeneity of the film. It exhibits relatively low absorption in the ultraviolet region and has a larger optical band gap (5.87 eV). This indicates that this system can effectively remove structural water and chlorides while reducing defect centers and residual organic matter, thereby obtaining better transmittance and a wider band gap. Based on the previous analysis, it can be inferred that the ethanol-water mixed solvent system achieves a better balance between the microstructure and chemical composition of the thin film through reasonable control of hydrolysis kinetics, thus exhibiting significant advantages in optical properties.

[0145] The refractive index, roughness, thickness, and dielectric properties of HfO2 thin films with different precursor solvent systems in Examples 1 and Comparative Examples 1-2 are shown in Table 1 below.

[0146] Table 1 - Performance of HfO2 dielectric films in Example 1 and Comparative Examples 1-2

[0147]

[0148] Figure 7 The leakage current characteristic curves of the HfO2 dielectric films prepared using different solvents in Example 1 and Comparative Examples 1-2 are shown. Figure 7 (a) and the capacitance-frequency response curve ( Figure 7 (b) Figure 7 In this context, Ethanol represents the HfO2 dielectric film prepared in Comparative Example 1, DI-Water represents the HfO2 dielectric film prepared in Comparative Example 2, and Ethanol:DI-Water = 1:1 represents the HfO2 dielectric film prepared in Example 1.

[0149] from Figure 7 As can be seen, the dielectric properties of the HfO2 dielectric films prepared using different solvents in Example 1 and Comparative Examples 1-2 are significantly different; among them, the HfO2 dielectric film prepared using ethanol-water as a precursor solvent in Example 1 has outstanding performance.

[0150] Figure 7As shown in Figure (a), the HfO2 dielectric film in Example 1 at 2.0 MV cm⁻¹ -1 The electric field is as low as 3.64 × 10⁻⁷ A / cm. -2 The lower leakage current density indicates better compactness and fewer defects. In contrast, the HfO2 dielectric films prepared in the all-water system of Comparative Example 2 and the all-ethanol system of Comparative Example 1 exhibit leakage current densities as high as 4.65 × 10⁻⁶. -2 and 11.15Acm -2 Furthermore, the breakdown field strength of HfO2 dielectric films prepared in all-water and all-ethanol systems is significantly lower than that of HfO2 dielectric films prepared in ethanol-water mixed systems. Based on the aforementioned characterization of the HfO2 dielectric films, this is due to the presence of numerous defects and cracks in the films, i.e., excessive surface roughness leading to severe leakage current.

[0151] Figure 7 In (b), the capacitance per unit area represents the combined effect of dielectric constant and film thickness. In Example 1, the ethanol-water mixture has the highest capacitance per unit area, indicating that the film has a high dielectric constant and good compactness. The capacitance values ​​of the all-water system and the all-ethanol system are extremely low, mainly because the film is too porous and has large pores, failing to form an effective insulating layer, which leads to a significant reduction in capacitance. This is consistent with the aforementioned characterization analysis of the films in the three systems.

[0152] Example 2

[0153] This embodiment provides a method for preparing an ultrathin HfO2 dielectric film, including the following steps:

[0154] S1. Hafnium tetrachloride (HfCl4) is added to a solvent to obtain a precursor solution; the solvent includes a mixture of ethanol and water in a volume ratio of 1:1; the concentration of hafnium tetrachloride in the precursor solution is 0.36 M (mol / L);

[0155] S2. The silicon substrate is sequentially ultrasonicated in isopropanol, ethanol, and deionized water (15 min for each ultrasonication), and then dried with nitrogen gas.

[0156] The cleaned substrate was placed in a plasma cleaner, oxygen was introduced, and the surface was activated for 10 minutes.

[0157] S3. Spin-coat the precursor solution onto the surface of the activated silicon substrate at a spin speed of 4500 rpm for 30 s to form an HfO2 thin film.

[0158] S5. Pre-anneal the HfO2 film at 200℃ in air for 10 min.

[0159] The pre-annealed HfO2 film was then annealed at 400°C in air for 1 hour to obtain the HfO2 dielectric film.

[0160] Example 3

[0161] The preparation method of the ultrathin HfO2 dielectric film provided in this embodiment is the same as that in Example 2, except that the concentration of hafnium tetrachloride in the precursor solution is 0.18 M (mol / L), and the other process parameters are the same as those in Example 2.

[0162] Example 4

[0163] The preparation method of the ultrathin HfO2 dielectric film provided in this embodiment is the same as that in Example 2, except that the concentration of hafnium tetrachloride in the precursor solution is 0.09 M (mol / L), and the other process parameters are the same as those in Example 2.

[0164] Example 5

[0165] The preparation method of the ultrathin HfO2 dielectric film provided in this embodiment is the same as that in Example 2, except that the concentration of hafnium tetrachloride in the precursor solution is 0.045 M (mol / L), and the other process parameters are the same as those in Example 2.

[0166] In solution-based spin coating for thin film preparation, the precursor concentration directly affects not only the viscosity and film-forming kinetics of the sol during the spin coating step, but also the crystallization behavior during subsequent heat treatment. During the experiment, the thickness and morphology of the thin film can be controlled by adjusting the spin coating rate and solution concentration. Specifically, the film thickness formula is shown below:

[0167] T h =A(ν / w) 1 / 2 , among which, T h ν represents the boundary layer thickness, ν represents the solution viscosity, w represents the substrate rotation angular frequency, and A represents the fitting coefficient.

[0168] Based on a fluid dynamics model, reducing the precursor concentration or increasing the spin-coating rate can induce a thinning of the fluid boundary layer, thereby suppressing cation deposition and ultimately achieving controllable preparation of ultrathin oxide layers. Experimental results verify that this mathematical model can effectively guide the precise nanoscale control of film thickness. After spin-coating, the sample is placed on a hot plate annealing stage in a non-vacuum environment. As the substrate temperature rises, the solvent evaporates, and the solution reaches a saturated to supersaturated state, thus nucleating on the substrate surface. The solute concentration at the solid-liquid interface decreases to a saturation level, forming a concentration gradient that provides the driving force for ion and molecular diffusion. With prolonged annealing time, the film continues to grow, the solid-liquid interface gradually moves, and finally, a complete film is formed on the entire substrate surface.

[0169] In Examples 2-5, 0.36M, 0.18M, ​​0.09M, and 0.045M solutions were prepared by spin-coating HfO2 dielectric films using a solvent of ethanol and water in a 1:1 ratio. The films were then air-annealed at 400°C to study the effect of different precursor concentrations on the HfO2 dielectric films. Figures 8-11 0.36M corresponds to Example 2, 0.18M corresponds to Example 3, 0.09M corresponds to Example 4, and 0.045M corresponds to Example 5.

[0170] Figure 8 XRR curves and thicknesses of HfO2 dielectric films prepared with different concentrations of hafnium tetrachloride in the precursor solutions of Examples 2-5 are shown (the inset shows the thickness measured by AFM step test).

[0171] like Figure 8 As shown, XRR and AFM step thickness tests were performed on the film thickness at different precursor concentrations. The results showed that as the precursor concentration (positively correlated with solution viscosity) decreased from 0.36 M to 0.045 M, the film thickness gradually decreased from 13.4 nm to 1.824 nm, satisfying the boundary layer thickness T of the film thickness formula. h The relationship with solution viscosity; specifically, in Example 2, the precursor concentration was 0.36 M and the HfO2 dielectric film thickness was 13.4 nm; in Example 3, the precursor concentration was 0.18 M and the HfO2 dielectric film thickness was 6.4 nm; in Example 4, the precursor concentration was 0.09 M and the HfO2 dielectric film thickness was 3.4 nm; in Example 5, the precursor concentration was 0.045 M and the HfO2 dielectric film thickness was 1.824 nm; in Example 2, the precursor concentration was 0.36 M and the HfO2 dielectric film thickness was the largest (1 The oscillation peaks were most concentrated at 3.4 nm, while in Example 5, the precursor concentration was 0.045 M, and the HfO2 dielectric film thickness was the smallest (1.824 nm). Its XRR curve did not show oscillation peaks. The possible reasons are: (1) The thinner the film, the larger the oscillation period, which causes the oscillation peak spacing to exceed the instrument's angular resolution range, making it impossible to effectively detect the signal; (2) The electron density difference between the ultrathin film and the substrate (silicon wafer) is small, and the reflected signal is masked by the substrate noise, making it difficult to distinguish the interface between the film and the substrate; (3) The interface roughness of the ultrathin film may significantly broaden the oscillation peaks and reduce the signal-to-noise ratio. Therefore, XRR has limitations in measuring the thickness of films close to 2 nm. Therefore, by using direct morphology characterization methods such as AFM step testing, an effective and distinct step can be prepared between the substrate and the film, and the step height difference can be directly measured as the film thickness. Its principle relies on the mechanical probe reaching a vertical resolution of 0.1 nm and being unaffected by the optical properties of the material or substrate interference. In the experiment, the 0.045 M sample was measured to have a step height of 1.824 nm by AFM.

[0172] In solution-based spin coating for thin film preparation, the precursor concentration determines the effective content of metal ions in the solution. Higher concentration solutions result in more sol deposition per unit area during spin coating, leading to a thicker gel layer after heat treatment for dehydration and condensation. Furthermore, the solution viscosity increases with increasing concentration, further suppressing the leveling effect of the sol during spin coating, resulting in a thicker film retained on the substrate after spin coating. This phenomenon can also be explained by the film thickness formula, which shows a positive correlation between film thickness and solution viscosity. When the precursor concentration decreases from 0.36 M to one-quarter of that (0.09 M), the thickness decreases simultaneously to one-quarter (from 13.4 nm to 3.4 nm), exhibiting an approximately linear relationship. This indicates that the sol deposition efficiency is stable within this concentration range, and the critical saturation value of the solution viscosity has not been reached. However, when the concentration is too low to 0.045M, the thickness deviates slightly from the linear trend. This may be because (1) when the precursor concentration is reduced to 0.045M, the amount of sol deposition is close to the critical state of monolayer coverage. At this time, the solution viscosity and substrate wettability together limit the film thickness; (2) the accuracy of AFM step test is affected by the radius of curvature of the probe tip and the surface cleanliness. If there is diffusion or contamination at the edge of the prepared step, the actual thickness may be overestimated.

[0173] Figure 9 AFM images and roughness of HfO2 dielectric films prepared with different concentrations of hafnium tetrachloride in the precursor solutions of Examples 2-5.

[0174] The surface roughness of HfO2 films prepared with different precursor concentrations was further investigated. AFM test results showed that... Figure 9 As shown, from Figure 9 As can be seen, as the precursor concentration decreased from 0.36 M to 0.045 M, the root mean square roughness (RMS) of the prepared HfO2 dielectric film surface were 6.690 nm, 0.190 nm, 0.186 nm, and 0.234 nm, respectively. For the HfO2 dielectric film prepared with a hafnium tetrachloride concentration of 0.36 M, the solution viscosity was high and the tendency of sol particles to aggregate was enhanced. The sol flow was restricted during spin coating, resulting in uneven solvent evaporation during film formation. Local stress accumulation led to microcracks or island structures, such as... Figure 10The AFM image shown in (a) reveals isolated protrusions, significantly increasing surface undulations. During high-temperature annealing, the film growth rate accelerates, and the difference in volume shrinkage of the HfO2 dielectric film further amplifies surface irregularities, resulting in a high roughness of 6.690 nm. When the hafnium tetrachloride concentration decreases to 0.18 M to 0.045 M, the sol viscosity decreases and dispersibility improves. During spin coating, the solution flows more easily to form a continuous and uniform wet film. During annealing, the densification of sol particles increases, and surface undulations are suppressed, thus reducing the RMS value to approximately 0.2 nm, approaching atomic-level smoothness. The HfO2 dielectric film prepared with a hafnium tetrachloride concentration of 0.045 M exhibits a slightly higher roughness than those prepared with 0.18 M and 0.09 M. This may be due to the reduced ability of the sol to cover the substrate at extremely low concentrations, insufficient critical wettability, and the reduced number of precursor molecules per unit area caused by excessively low sol concentrations, making it difficult to form a completely continuous film. Isolated nanoparticles or micropores may remain on the surface.

[0175] Figure 10 XRD patterns of HfO2 dielectric films prepared with different concentrations of hafnium tetrachloride in the precursor solutions of Examples 2-5 Figure 10 (a)), SE diagram ( Figure 10 (b) and UV-Vis transmittance curves ( Figure 10 (c) Inset shows the band gap and UV-Vis absorption curves. Figure 10 (d)

[0176] XRD such as Figure 10 As shown in (a), no crystallization peaks were found in any of the films, indicating that no HfO2 grains were induced under the experimental conditions. The amorphous structure avoids scattering loss caused by differences in grain size between grain boundaries and grains, which is beneficial to optical uniformity. Figure 10 (b) shows the SE curves of HfO2 films with different precursor concentrations. The refractive index directly reflects the material density. Experimental data show that when the concentration decreases from 0.36M to 0.045M, the refractive indices (@550nm) are 2.070, 2.194, 2.206, and 2.184, respectively, corresponding to a calculated density (the relative density of HfO2 dielectric films is calculated using the Yoldas formula. The Yoldas method is a classic method for calculating the relative density of films using X-ray diffraction (XRD) technology, proposed by BE. Yoldas in the 1970s) of 9.82 g cm⁻¹. -3 10.56g cm -3 10.63g cm -3 10.50g cm -3 The density is relatively close to the theoretical density value of HfO2 thin film material, which is 9.68–10.5 g / cm³. -3This trend is consistent with the roughness changes observed in AFM testing: the 0.36M film sample, due to its high sol viscosity and uneven film formation, has a surface roughness as high as 6.69 nm, and high internal porosity, resulting in lower density and refractive index; while the 0.18M and 0.09M samples have more uniform and denser films, with RMS values ​​all less than 0.2 nm. Figure 9 As shown in the figure, the porosity is reduced, therefore its refractive index and calculated density are slightly higher than the theoretical values ​​of HfO2 thin film materials. When the concentration is further reduced to 0.045M, the refractive index and density decrease slightly, possibly due to the excessively thin film thickness, which enhances the signal interference at the substrate interface during SE fitting, leading to errors, or the reduced sol coverage at extremely low concentrations, resulting in discontinuities in local areas of the film, introducing micropores, and reducing the overall density. Furthermore, through the analysis of... Figure 10 In section (b), the SE curve was further fitted to obtain the film thickness (specifically, the film thickness was obtained by fitting the "SE curve" (i.e., the curve of the measured polarization parameter as a function of wavelength or incident angle) using ellipsometry). The thicknesses of the HfO2 film corresponding to 0.36M to 0.045M were 12.60nm, 6.50nm, 3.91nm, and 1.72nm, respectively, which are not much different from the thicknesses measured by XRR and AFM step test. The specific film information is shown in Table 2.

[0177] Table 2 - Refractive index, thickness, and density of HfO2 thin films prepared with different precursor concentrations in Examples 2-5

[0178]

[0179] Figure 10 In the middle (c), the ultraviolet transmission spectra of HfO2 films with different precursor concentrations at room temperature are shown. The transmittance of the samples decreases with increasing concentration. The inset represents the band gap, with band gap values ​​of 5.36 eV, 5.52 eV, 5.61 eV, and 5.57 eV from high concentration to low concentration. Figure 10 In the figure (d), the ultraviolet absorption curve of the HfO2 film is shown. As the concentration of the precursor decreases, the ultraviolet absorption edge shifts to a shorter wavelength.

[0180] Figure 11 The leakage current characteristic curves of HfO2 dielectric films prepared with different concentrations of hafnium tetrachloride in the precursor solutions of Examples 2-5 are shown. Figure 11 (a) and the capacitance-frequency response curve ( Figure 11 (b)

[0181] like Figure 11 Table 3 shows the (a) leakage current characteristic curves and (b) capacitance-frequency characteristic curves of HfO2 thin films with different precursor concentrations, and the calculated dielectric properties are shown in Table 3. Figure 11 (a) Leakage current characteristic curve. This parameter reflects the electrical loss and breakdown characteristics of the thin film. The lower the leakage current density, the better the insulation performance of the thin film. The leakage current of the 0.18M sample is as low as 8.23 ​​× 10⁻⁶. -7 A cm -2 (@3.0MV / cm), significantly better than other samples. The 0.36M sample had a larger leakage current, which was due to insufficient density and excessive roughness leading to increased leakage. The 0.045M sample had the highest leakage current of 1.01×10. -2 Acm -2 This may be due to tunneling caused by the film being too thin, thus failing to provide effective electrical isolation. Regarding the breakdown field strength, the 0.18 M film exhibits a significantly higher breakdown field strength than other concentrations. For example... Figure 11 (b) shows the capacitance-frequency characteristic curve of the thin film. The highest value appears at 0.09 M, followed by 0.18 M, and finally 0.36 M. The 0.045 M film is too thin to form an effective insulating layer, and its capacitance value is almost zero due to environmental influences during capacitance testing. This indicates that appropriately reducing the precursor concentration may help improve the capacitance per unit area, but excessively low concentrations lead to poor film performance. The instability of the film capacitance value at low frequencies (<1000 Hz) in Example 5 has been explained in detail above and will not be repeated here. The dielectric constant (κ value) measures the dielectric energy storage capacity of a material. A higher κ value means better capacitor performance. The dielectric constant is highest at 0.18 M (5.959), followed by 0.09 M (5.648). The dielectric constant is lower at 0.36 M (5.261) due to the thicker film. The 0.045 M film is not included in the calculation because the measured capacitance value is almost zero. Based on the above characterization, a suitable precursor concentration (0.18M) can yield better dielectric film properties and superior dielectric performance.

[0182] The properties of HfO2 dielectric films prepared using hafnium salt solutions of different concentrations according to the methods in Examples 2-5 are shown in Table 3 below.

[0183] Table 3 - Properties of HfO2 dielectric films in Examples 2-5

[0184]

[0185] Specifically, the method for calculating the average equivalent oxide thickness (EOT) is as follows:

[0186] in, This represents the relative permittivity of SiO2 (usually taken as 3.9);

[0187] ε matThe relative permittivity of the dielectric material is shown in Table 3 (HfO2 is shown in Table 3 after testing);

[0188] d represents the physical thickness of the actual dielectric material (unit: nm).

[0189] Table 3 shows the dielectric constant (κ value), which measures the dielectric energy storage capacity of a material. A higher κ value means better capacitor performance. The dielectric constant (κ value) of HfO2 dielectric films was tested using the capacitance method (parallel plate model).

[0190] Example 6

[0191] This embodiment provides a method for preparing an HfO2 dielectric thin film, including the following steps:

[0192] S1. Hafnium tetrachloride (HfCl4) is added to a solvent to obtain a precursor solution; the solvent includes a mixture of ethanol and water in a volume ratio of 1:1; the concentration of hafnium tetrachloride in the precursor solution is 0.18 M (mol / L);

[0193] S2. The silicon substrate is sequentially ultrasonicated in isopropanol, ethanol, and deionized water (15 min for each ultrasonication), and then dried with nitrogen gas.

[0194] The cleaned substrate was placed in a plasma cleaner, oxygen was introduced, and the surface was activated for 10 minutes.

[0195] S3. Spin-coat the precursor solution onto the surface of the activated silicon substrate at a spin speed of 4500 rpm for 30 s to form an HfO2 thin film.

[0196] S5. Pre-anneal the HfO2 film at 200℃ in air for 10 min.

[0197] The pre-annealed HfO2 film was then annealed at 350°C in air for 1 hour to obtain the HfO2 dielectric film.

[0198] Example 7

[0199] This embodiment provides a method for preparing an HfO2 dielectric film, which is the same as in Embodiment 6, except that the pre-annealed HfO2 film is annealed at 400°C in air for 1 hour; the other process parameters are the same as in Embodiment 6.

[0200] Example 8

[0201] This embodiment provides a method for preparing an HfO2 dielectric film, which is the same as in Embodiment 6, except that the pre-annealed HfO2 film is annealed at 450°C in air for 1 hour; the other process parameters are the same as in Embodiment 6.

[0202] Example 9

[0203] This embodiment provides a method for preparing an HfO2 dielectric film, which is the same as in Embodiment 6, except that the pre-annealed HfO2 film is annealed at 500°C in air for 1 hour; the remaining process parameters are the same as in Embodiment 6.

[0204] Example 10

[0205] This embodiment provides a method for preparing an HfO2 dielectric film, which is the same as in Embodiment 6, except that the pre-annealed HfO2 film is annealed at 600°C in air for 1 hour; the other process parameters are the same as in Embodiment 6.

[0206] Annealing is a crucial step in solution-based thin film preparation. Annealing can control the lattice construction process of the material. As the annealing temperature increases, it promotes the densification of the film structure, effectively reducing the defect density and thus achieving synergistic optimization of the optical and electrical properties of the thin film material. In Examples 6-10, based on the above exploration of the precursor solvent system and precursor concentration, a 0.18M precursor solution prepared with a 1:1 mixture of ethanol and water was selected to prepare HfO2 thin films. The effect of different annealing temperatures on the film properties was studied. By analyzing the film structure, optical properties, and electrical properties, the HfO2 dielectric thin film with optimal performance was obtained. Figures 12-17 In the text, 350℃ represents Example 6, 400℃ represents Example 7, 450℃ represents Example 8, 500℃ represents Example 9, and 600℃ represents Example 10.

[0207] Figure 12 AFM images and roughness variation diagrams of HfO2 dielectric films prepared at different annealing temperatures in Examples 6-10 are shown.

[0208] To investigate the roughness variation of HfO2 dielectric films at different annealing temperatures, AFM roughness tests were performed on HfO2. Figure 12The experimental data show that as the annealing temperature increases from 350℃ to 600℃, the surface roughness of the film exhibits a non-monotonic trend of first decreasing and then increasing: the RMS value at 350℃ and 400℃ decreases from 0.199nm to 0.190nm, then increases slightly to 0.195nm at 450℃. When the temperature exceeds 500℃, the RMS value increases to 0.219nm at 500℃ and 0.225nm at 600℃. The surface roughness of the film annealed at all temperatures is controlled in the sub-nanometer range, indicating that the surface of HfO2 is very smooth. The AFM roughness reaches its lowest value of 0.190nm at 400℃, at which point the amorphous densification is complete and the surface chemical homogeneity is optimal. This nanoscale smooth surface can effectively reduce the carrier scattering probability at the interface and suppress charge trap formation, thereby enhancing the carrier migration capability in the thin-film transistor and playing a key role in improving the electrical performance of the device. When the temperature rises to 450℃, the roughness gradually increases because grain nucleation begins. The orientation differences between different grains lead to anisotropic growth rates, resulting in topological undulations of surface protrusions (grains) and depressions (grain boundaries). At high temperatures, grains coarsen, with smaller grains being engulfed by larger grains, further amplifying surface undulations and increasing roughness. This surface morphology evolution directly affects the interface characteristics of thin-film transistors: increased roughness enhances carrier scattering and reduces field-effect mobility.

[0209] Figure 13 GIXRD patterns of HfO2 dielectric films prepared at different annealing temperatures in Examples 6-10 Figure 13 (a) , 400℃ Figure 13 (b) ), 450℃ Figure 13 (c) ), 500℃ Figure 13 TEM image of (d) annealed.

[0210] like Figure 13 The films annealed at different temperatures were subjected to GIXRD (e.g., Figure 13 The test shown in (a) was performed, and high-resolution TEM (e.g., at 400℃, 450℃, and 500℃) was conducted. Figure 13In tests (b) to (d), it can be seen that GIXRD did not detect a crystallization peak in the HfO2 film annealed at 350℃ to 500℃, indicating that the overall state is still predominantly amorphous. However, high-resolution TEM shows the presence of localized micrograins at 450℃ and 500℃, while no grains were observed at 400℃. This indicates that after 450℃, the film begins to transform from amorphous to crystalline, forming nanoscale grains. However, the grain size is small and sparsely distributed, not reaching the detection threshold of GIXRD. When the temperature rises to 600℃, the grains further grow and form a long-range ordered structure. At this point, GIXRD detects a significant crystallization peak. Simultaneously, grain coarsening and increased grain boundary density lead to a significant increase in surface roughness, such as... Figure 12 During AFM testing, the RMS increased from 0.199 nm at 500 °C to 0.225 nm. For high-k dielectric materials with crystalline structures, the voids and grain boundaries between internal grains can provide channels for electron conduction, leading to increased leakage current. Simultaneously, these structural defects also provide channels for the diffusion of impurity ions, further exacerbating impurity ion diffusion. In contrast, amorphous structures, lacking defined grain boundaries, can effectively suppress charge leakage and impurity ion migration, thus exhibiting superior interface and reliability in devices such as metal-oxide-semiconductors and thin-film transistors.

[0211] Figure 14 The ultraviolet transmittance curves of HfO2 dielectric films prepared at different annealing temperatures in Examples 6-10 are shown. Figure 14 (a) and SE curve ( Figure 14 (b) and XRR plot ( Figure 14 (c) FTIR plot ( Figure 14 (d)

[0212] Figure 14Figure (a) shows the UV-Vis transmittance curves of HfO2 films annealed at different temperatures. As can be seen from the figure, the transmittance of the films in the visible light range is close to or exceeds 80%, indicating that the visible light transmittance of HfO2 films annealed at different temperatures is relatively high. The fitting results show that the band gaps corresponding to the films annealed from 350℃ to 600℃ are 5.21eV, 5.36eV, 5.17eV, 5.11eV, and 5.01eV, respectively, showing a trend of first increasing and then decreasing. In the amorphous stage at 350℃ and 400℃, appropriate annealing temperatures promote the improvement of short-range atomic order, eliminate structural defects such as dangling bonds and micropores in the amorphous network, and effectively reduce the density of band tail states. At this time, the band gap increases from 5.21 eV to 5.36 eV. When the temperature exceeds the crystallization critical point (450℃) and above, the formation of crystal nuclei triggers the reconstruction of long-range atomic order. However, new oxygen vacancy defect states are formed at the grain boundaries. At the same time, the formation of the crystal phase leads to the Brillouin zone folding effect, and the overlap between the localized states at the bottom of the conduction band and the extended states at the top of the valence band increases, resulting in a decrease in the band gap. The optical band gaps measured at all temperatures are much larger than the photon energy threshold (3.1 eV) in the visible spectrum region (380-780 nm), indicating that the HfO2 film has intrinsic photoinertia characteristics under visible light irradiation. This non-photosensitive property provides a key guarantee for the long-term working stability of metal-dielectric-metal (MIM) display devices under ambient light conditions by suppressing photoinduced leakage current.

[0213] Figure 14 (b) shows the SE curves at different annealing temperatures. The refractive indices (@550nm) from 350℃ to 600℃ are 2.180, 2.229, 2.225, 2.217, and 2.112, respectively; the measured fitted thicknesses are 6.52nm, 6.47nm, 6.33nm, 6.22nm, and 6.19nm, respectively; and the calculated density is 10.46g / cm³. 3 10.77 g / cm 3 10.74 g / cm 3 10.70 g / cm 3 10.07 g / cm 3 Within the temperature range of 350℃-400℃, the refractive index of the HfO2 thin film increased from 2.180 to 2.229, and the calculated density also increased from 10.46 g / cm³. 3 Increased to 10.77 g / cm³ 3 This is mainly attributed to the densification process of the amorphous structure. As the annealing temperature increases, atomic migration is activated, promoting the optimization of the short-range ordered structure and eliminating micropores and dangling bond defects in the amorphous network. However, when the temperature exceeds 400℃, the refractive index and density begin to decrease monotonically. This is closely related to the formation of the crystalline phase. During grain nucleation and growth, intrinsic defects such as oxygen vacancies are generated at grain boundaries. These defect states introduce local electronic states, which manifest as a decrease in refractive index.

[0214] Figure 14 Table (c) shows the XRR test results for HfO2 films at different annealing temperatures. The film thicknesses obtained from the XRR test and the fitted thicknesses from the SE method are shown in Table 4. Both show a monotonically decreasing trend with increasing temperature, and the thicknesses obtained from the two methods are not significantly different, indicating the stability of the HfO2 film prepared by the solution method. The SE-fitted thickness decreased from 6.52 nm to 6.19 nm with increasing temperature, and the XRR-calculated thickness decreased from 6.91 nm to 5.98 nm with increasing temperature. This is mainly due to the combined effect of annealing-driven film densification and defect elimination. At lower temperatures (350℃~400℃), thermal energy activates atomic diffusion, leading to a denser amorphous network, and the SE-calculated density increases to 10.77 g / cm³. 3 In the vertical direction, shrinkage manifests as a decrease in thickness. When the temperature exceeds 400℃, XRR results show that the rate of thickness reduction slows down, indicating that the material is approaching a densification saturation state. At this point, the formation of crystalline phases dominates the structural evolution. Although grain growth continues to eliminate residual defects, the disordered atomic arrangement at grain boundaries may partially offset the shrinkage effect.

[0215] Figure 14 (d) shows the FTIR curves of the HfO2 thin film under different annealing temperatures. It was found that the heat treatment temperature significantly affects the chemical structure of the material. The film at 350℃ exhibits distinct characteristic peaks in multiple wavelength bands, while the peak intensity decreases sharply after annealing at temperatures above 400℃, particularly in the 3200-3770 cm⁻¹ range. -1 The broad, gentle absorption band in the region corresponds to the stretching vibration peak of the hydroxyl group (-OH), at 1600 cm⁻¹. -1 The peak at that point represents the C=O bond. Increasing the annealing temperature leads to complete decomposition of the organic matter, and the response amplitude of the organic group vibration peaks exhibits a regular decay. This not only removes the restriction of organic chemical bonds on the gelation and dehydration chelation reactions between metal cations and oxygen atoms, but also promotes the reconstruction of the metal oxide network and reduces the defect concentration. This plays a crucial role in the leakage current characteristics and capacitance-frequency dependence characteristics of the HfO2 thin film.

[0216] Table 4 - Refractive index, thickness, density, and roughness data of HfO2 films annealed at different temperatures in Examples 6-10

[0217]

[0218] Figure 15 The images shown are XPS images of HfO2 dielectric films prepared at different annealing temperatures in Examples 6-10. Figure 15 (a) shows the O1s spectrum and peak fitting results, (b) shows the Hf4f spectrum and peak fitting results, and (c) shows the O1s spectrum and peak fitting results. I O II The change in content, (d) is Hf5 / 2 With Hf 7 / 2 The area ratio of the peaks.

[0219] To further investigate the temperature-dependent changes in the metal-oxygen coordination structure in HfO2 thin films, XPS was used to characterize the chemical states of the samples, such as... Figure 15 As shown, all spectra were charged based on the C1s peak (binding energy 284.8 eV). Figure 15 The O1s spectrum shown in (a) was analyzed by peak fitting to reveal two characteristic peaks at 530.4 eV and 531.8 eV. I The main peak at 530.4 eV represents oxygen ions (O3) bonded to metal ions. 2- (Hf-O bonding), characterizing the main phase structure of the metal oxide network; O II The 531.8 eV peak represents oxygen vacancies and oxygen molecules bonded to nonmetallic ions (including surface-adsorbed -OH, water molecules (H2O), or peroxide groups (O2)). Figure 15 In the middle (c), the red line represents O. I / (O I +O II ), that is, O I The percentage, the black line represents O. II / (O I +O II ), that is, O II The proportion of O. It can be seen that as the annealing temperature increases, O... I The proportion of Hf-O bonds is continuously increasing, reaching 74.53%, 79.54%, 81.57%, 82.15%, and 92.71% respectively. The proportion of Hf-O bonds increases with rising temperature. The mechanism lies in the thermal decomposition of organic components within the film or their reaction with atmospheric oxygen to produce byproducts such as carbon dioxide and water. 4+ The ions then combine with oxygen or oxygen ions to form more Hf-O-Hf bonds.

[0220] like Figure 15 As shown in (b), the Hf4f spectrum of the HfO2 thin film has two main peak positions: Hf 7 / 2 The peak is located at 17.05 eV, Hf 5 / 2 The peak is located at 18.74 eV. When the annealing temperature increases from 350℃ to 500℃, Hf... 5 / 2 With Hf 7 / 2 Peak area ratio, Figure 15 As shown in Figure (d), Hf gradually increases from 0.53 at 350℃ to 0.81 at 600℃. This change indicates that as temperature increases, Hf... 5 / 2 The enhanced binding state is related to changes in the structure or chemical environment, indicating that more HfO2 is formed inside the film.

[0221] Figure 16 The dielectric properties of HfO2 dielectric films prepared at different annealing temperatures in Examples 6-10 are shown. Figure 16 (a) is the capacitance-frequency response curve; (b) is the annealing temperature-capacitance diagram; (c) is the annealing temperature-dielectric constant diagram.

[0222] Figure 16 The figure shows the variation of capacitance per unit area of ​​HfO2 thin films with frequency at different annealing temperatures (a), and statistical graphs of the dispersion of capacitance (b) and dielectric constant (c), covering a frequency range from 20 Hz to 300 kHz. The capacitance values ​​of HfO2 thin films annealed at 350℃, 400℃, 450℃, 500℃, and 600℃ are 694.45 nF·cm. -2 673.75 nF·cm -2 695.22 nF·cm -2 726.91nF·cm -2 and 796.79 nF·cm -2 It is evident that the capacitance decreases significantly when the temperature rises from 350℃ to 400℃, primarily due to a substantial reduction in the number of hydroxyl groups (-OH) within the film. As the temperature increases, the volatilization of -OH reduces the number of polarizable molecular groups, weakening the dielectric polarization capability of the film and leading to a decrease in capacitance per unit area. When the annealing temperature exceeds 400℃, the capacitance gradually increases, reaching its maximum value at 600℃. This phenomenon is related to the fact that high-temperature annealing promotes the repair of film defects and optimization of crystal structure: Firstly, at high temperatures, defects such as oxygen vacancies are gradually repaired, reducing interface scattering and leakage losses, thus improving dielectric properties; secondly, at higher temperatures, the metal oxide framework forms and densifies, transforming the HfO2 film from amorphous to a more ordered nanocrystalline or partially crystalline structure, making the polarization response more stable and increasing the dielectric constant; finally, according to the area capacitance formula and the phenomenon of film thickness reduction, as the temperature increases, the film density increases, and the overall thickness decreases, resulting in an enhanced equivalent electric field under the same applied voltage, thereby increasing the capacitance per unit area.

[0223] like Figure 16As shown in (b) and (c), among the films prepared at all annealing temperatures, the film annealed at 400℃ exhibits smaller dispersion and higher stability. Specifically, within the test frequency range, its capacitance density and corresponding dielectric constant show a smaller variation range, indicating that the film at this temperature forms a uniform oxide network with lower interface state density and better electrical uniformity. The thicknesses of the HfO2 films annealed at temperatures from 350℃ to 600℃ are 6.52nm, 6.47nm, 6.33nm, 6.22nm, and 5.77nm, respectively. Combining the capacitance formula, the dielectric constants of the films at 1000Hz at the corresponding annealing temperatures are 5.49, 4.57, 4.99, 5.09, and 5.22, respectively. At low annealing temperatures, the dielectric constant of the HfO2 film is unusually high. This is mainly because the residual impurity groups in the film absorb moisture from the environment, and since the dielectric constant of water is 80, the dielectric constant of the film is abnormally increased. Substituting the test data into the formula for calculating the equivalent oxide thickness (EOT), the EOT of HfO2 films annealed at temperatures from 350℃ to 600℃ can be calculated to be 4.92nm, 5.71nm, 5.00nm, 5.03nm, and 4.54nm, respectively.

[0224] In the dielectric layer replacement scheme of metal-dielectric-metal (MIM) devices, the SiO2 replacement process imposes dual performance requirements on candidate high-κ materials: on the one hand, the capacitance density per unit area needs to be significantly increased to meet the miniaturization requirements of the device; on the other hand, the gate leakage current must be strictly controlled within a reasonable range. If the dielectric layer cannot meet these two performance indicators, it will lead to a serious degradation of the gate control capability, which in turn will cause an exponential increase in the static power consumption of the device, and in extreme cases, it may trigger irreversible dielectric breakdown failure.

[0225] Figure 17 The leakage current characteristic curves are for HfO2 dielectric films prepared at different annealing temperatures in Examples 6-10.

[0226] Figure 17 The experimental data shown reveal the current conduction characteristics of HfO2 films under different annealing temperature conditions. By comparing the electric field strength and leakage current density (J / L), the current conduction characteristics of HfO2 films are further analyzed. leakThe curves clearly show that the annealing temperature parameter has a significant modulation effect on the conductivity mechanism of the HfO2 dielectric layer, providing experimental basis for further optimization of the dielectric properties of high-κ materials. The leakage current density of the HfO2 film shows a trend of first decreasing and then increasing with the annealing temperature, reaching a minimum at 400℃, while the leakage current density is the maximum after annealing at 600℃. At lower annealing temperatures (350℃), the HfO2 film still contains a large amount of -OH and adsorbed moisture. These impurities are easily formed into conductive channels under the action of an electric field, resulting in a high leakage current density. When the temperature rises to 400℃, a large amount of -OH volatilizes, the oxygen vacancy concentration in the film decreases, and the structure becomes more compact, significantly improving the insulation of the dielectric layer, thereby increasing the leakage current density. leak The leakage current is reduced to a minimum, and as the aforementioned TEM results show, HfO2 is still in an amorphous state at this point, with a low interface state density, few defects, and a relatively high barrier height, making it more difficult for electrons to pass through the dielectric layer, thus significantly reducing leakage current. When the annealing temperature continues to rise to 600℃, the film structure transforms from amorphous to partially crystalline, leading to an increase in the number of grain boundaries. Grain boundary regions are often rich in oxygen vacancies and other structural defects, which easily become carrier transport channels, enhancing leakage current. Moreover, high temperature may cause HfO2 to react with the Si interface, increasing the interface state density and further reducing the barrier height, making it easier for electrons to be injected, ultimately manifesting as J leak The sharp increase in J; on the other hand, high-temperature annealing densifies the film while reducing its thickness. Under the same applied voltage, the reduced film thickness enhances the equivalent electric field. According to Poisson's equation, an increased electric field increases the probability of electron tunneling, thereby increasing the leakage current density. Therefore, although the HfO2 film annealed at 600℃ increases the unit capacitance, the higher electric field strength and larger structural defects lead to a sharp increase in J. leak The highest breakdown field strength of HfO2 films at different annealing temperatures reaches 10.51 MV / cm at 400℃. 2 This is because the HfO2 film is in its amorphous state at this temperature, exhibiting the lowest defect density and the most uniform stress distribution. Above this temperature, the benefits of structural optimization are offset by the negative effects of grain formation and coarsening, and increased defects, leading to performance degradation. In summary, the leakage current density of the HfO2 film is affected by factors such as hydroxyl volatilization, defect reduction, crystal phase change, and film thickness. At 400℃, due to fewer defects, a denser structure, and a higher barrier height, J... leak The lowest; as the temperature continues to rise, the increase in defects caused by crystallization and the decrease in film thickness work together to cause a significant increase in leakage current density after annealing at 600℃.

[0227] HfO2 dielectric films were prepared using hafnium salt solutions of different concentrations according to the methods in Examples 6-10, and their performance results are shown in Table 5 below.

[0228] Table 5 - Dielectric properties of HfO2 films at different annealing temperatures

[0229]

[0230] In summary, this invention systematically explores the key parameters affecting the quality of ultrathin HfO2 dielectric films during the solution preparation process, mainly including three aspects: precursor solvent, precursor concentration, and annealing temperature.

[0231] (1) Through comparative studies of different solvents, it was found that optimizing the solvent system can effectively control the hydrolysis rate of the precursor solution and the film formation process. In the film prepared by using a 1:1 mixed solvent system of ethanol and water, a smooth HfO2 film with a roughness of less than 1 nm was obtained, and the density and uniformity of the film were significantly improved.

[0232] (2) Adjusting the precursor concentration not only changes the solute behavior, but also affects the continuity and thickness of the HfO2 film. As the concentration decreases, the film thickness gradually decreases, which is in line with the basic rules of solution preparation process. When the concentration is controlled at 0.18M, ​​an ultrathin HfO2 film of 6.4nm is obtained, and the film exhibits the highest capacitance per unit area and the lowest leakage current. This shows that an appropriate concentration can maintain good dielectric properties, but too high or too low concentration will lead to a decrease in dielectric properties, which is manifested as a significant increase in leakage current.

[0233] (3) The study of the properties of HfO2 dielectric films annealed at temperatures ranging from 350℃ to 600℃ revealed that the films prepared under annealing at 400℃ exhibited the best overall performance. The HfO2 dielectric films were in a completely amorphous state, possessing not only the lowest surface roughness and the highest film density, but also the lowest leakage current of 6.52 × 10⁻⁶. -7 A / cm 2 (@6.0MV / cm), capacitance per unit area at 1000Hz is 673.75nF / cm. 2 This further demonstrates its excellent dielectric properties, making it an ideal choice for gate dielectric layer applications.

[0234] This invention demonstrates that optimizing the precursor solvent, rationally adjusting the precursor concentration, and selecting an appropriate annealing temperature can significantly improve the overall performance of HfO2 dielectric films. In a system using a 0.18M ethanol-water 1:1 mixture as the precursor solvent, ultrathin HfO2 films <7nm obtained by solution annealing at 400℃ exhibit optimal comprehensive dielectric properties, providing solid technical support for their application in high-κ dielectric films and related electronic devices.

[0235] It is understood that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0236] The above are merely preferred embodiments of this application, and only specifically describe the technical principles of this application. These descriptions are only for explaining the principles of this application and should not be construed as limiting the scope of protection of this application in any way. Based on this explanation, any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application, as well as other specific embodiments of this application that can be conceived by those skilled in the art without creative effort, should be included within the scope of protection of this application.

Claims

1. A method for preparing an ultrathin HfO2 dielectric film, characterized in that, Includes the following steps: Hafnium salt is added to a solvent to obtain a precursor solution; The precursor solution is coated onto the substrate surface to form an HfO2 thin film; The HfO2 thin film was annealed to obtain an HfO2 dielectric thin film; The solvent is a mixture of ethanol and water.

2. The method for preparing the ultrathin HfO2 dielectric film as described in claim 1, characterized in that, The volume ratio of ethanol to water is 1:

1.

3. The method for preparing the ultrathin HfO2 dielectric film as described in claim 1, characterized in that, The concentration of hafnium salt in the precursor solution is 0.045–0.36 mol / L.

4. The method for preparing the ultrathin HfO2 dielectric film as described in claim 1, characterized in that, The hafnium salt includes at least one of hafnium tetrachloride, hafnium oxychloride, and hafnium nitrate.

5. The method for preparing the ultrathin HfO2 dielectric film as described in claim 1, characterized in that, The HfO2 thin film is annealed to obtain an HfO2 dielectric thin film, specifically including: The HfO2 film was pre-annealed at 180–220 °C for 10–20 min; The pre-annealed HfO2 film was then annealed at 350–600°C for 1 hour.

6. The method for preparing the ultrathin HfO2 dielectric film as described in claim 1, characterized in that, The substrate is a silicon substrate; The process before coating the precursor solution onto the substrate surface also includes: The substrate was ultrasonically cleaned sequentially with isopropanol, ethanol and deionized water, and then dried with nitrogen. The cleaned substrate is placed in a plasma cleaner, oxygen is introduced, and the surface is activated for 10-20 minutes.

7. The method for preparing the ultrathin HfO2 dielectric film as described in claim 1, characterized in that, In the step of coating the precursor solution onto the substrate surface, the coating method is spin coating, the spin coating speed is 4500 rpm, and the spin coating time is 30 s.

8. The method for preparing the ultrathin HfO2 dielectric film as described in claim 1, characterized in that, The solvent is a mixture of ethanol and water, with a volume ratio of ethanol to water of 1:1; The concentration of hafnium salt in the precursor solution is 0.18 mol / L; The HfO2 thin film is annealed to obtain an HfO2 dielectric thin film, specifically including: The HfO2 film was pre-annealed at 200℃ for 10 min; The pre-annealed HfO2 film was then annealed at 400℃ for 1 hour.

9. An ultrathin HfO2 dielectric film, characterized in that, It is prepared by any one of the preparation methods described in claims 1 to 8.

10. The application of an ultrathin HfO2 dielectric film prepared by any one of the preparation methods described in claims 1 to 8 or the ultrathin HfO2 dielectric film described in claim 9 in the preparation of thin-film electronic devices.

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