A hafnium oxide-based ferroelectric thin film with a gradient structure, a ferroelectric capacitor and a preparation method

Abstract

This invention discloses a gradient structure hafnium oxide-based ferroelectric thin film, a ferroelectric capacitor, and a method for its preparation. The gradient structure hafnium oxide-based ferroelectric thin film comprises, from bottom to top, a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer, and a fourth ferroelectric layer, wherein the first and fourth ferroelectric layers have the same composition, both being Hf. x Zr (1‑x) O2, the second ferroelectric layer and the third ferroelectric layer have the same composition, both being Hf (1‑x) Zr x O2, where 0

Description

Technical Field

[0001] This invention belongs to the field of ferroelectric capacitor technology, specifically relating to a gradient structure hafnium oxide-based ferroelectric thin film capacitor and its preparation method. Background Technology

[0002] Ferroelectric memories, based on the bistable positive and negative polarization of ferroelectric materials, possess characteristics such as fast read / write speeds, low power consumption, and non-volatility, showing great potential for application in information storage technology. However, the ferroelectricity of traditional perovskite ferroelectrics, such as barium titanate (BaTiO3) and lead zirconate titanate (Pb(Zr,Ti)O3), exhibits a significant size effect (i.e., after shrinking to approximately 100 nanometers, ferroelectric performance significantly decreases with further reduction in thickness), and suffers from poor compatibility with Si-based standard complementary metal-oxide-semiconductor (CMOS) processes, severely restricting the miniaturization, integration, and high-performance development of ferroelectric memories. In 2011, German scientists observed ferroelectricity in a 10 nm thick doped hafnium oxide film, a phenomenon that quickly attracted the attention of the academic and research communities both domestically and internationally. Compared to traditional perovskite ferroelectric materials, hafnium oxide-based ferroelectric films not only have strong scalability but also excellent compatibility with Si-based standard CMOS processes, making them considered ideal materials for next-generation high-performance ferroelectric memories. However, existing hafnium oxide-based ferroelectric thin films usually exhibit a significant wake-up effect and are prone to fatigue failure under the action of an electric field. Their fatigue resistance is significantly different from that of traditional perovskite-type ferroelectric materials, which severely limits the application of hafnium oxide-based thin films in ferroelectric memory.

[0003] Currently, methods to improve the wake-up effect and fatigue failure of hafnium oxide-based ferroelectric thin films mainly include elemental doping, oxygen source control, and stacked structure design. Among these, the most similar implementation scheme to this invention is the stacked structure. First, a superlattice structure composed of hafnium oxide and zirconium oxide is prepared using atomic layer deposition (ALD). TiN is selected as the bottom electrode material, the thickness of the hafnium oxide-zirconia superlattice thin film is 12 nm, the ALD temperature is 300 °C, and 100 nm thick Pt is used as the top electrode material. Rapid annealing is performed at 600 °C in an N2 atmosphere, with top heating and bottom heating as the annealing methods. Although a hafnium oxide-based ferroelectric thin film with almost no wake-up effect is prepared, the remanent polarization intensity (2P) is still low. r Only 27.4 μC / cm 2 Furthermore, fatigue resistance was not considered; secondly, a 1nm thick Hf layer was used. 1-x Zr xA stacked design is carried out with O₂ and a 1 nm thick ZrO₂ layer, the nano-stacked sample is prepared by an atomic layer deposition process, and rapid annealing treatment is performed at 500°C under N₂ atmosphere. When x=0.5, the maximum dielectric constant can reach 60. However, this stacked structure design does not pay attention to the influence of the stacked structure on the wake-up effect and fatigue characteristics of ferroelectric properties.

[0004] The currently reported stacking technologies for hafnia-based ferroelectric thin films have not achieved significant simultaneous improvement of the wake-up effect and fatigue failure phenomenon of their ferroelectric properties, and the annealing temperature is generally above 450°C, which is difficult to meet the temperature requirement of the back-end process of integrated circuits, and severely restricts the application of hafnia-based ferroelectric thin films in high-performance and highly integrated ferroelectric memories. Contents of the Invention

[0005] In order to solve the above-mentioned problems existing in the prior art, the present invention provides a gradient-structure hafnia-based ferroelectric thin film capacitor and a preparation method thereof. The technical problem to be solved by the present invention is realized through the following technical solutions:

[0006] In a first aspect, the present invention discloses a gradient-structure hafnia-based ferroelectric thin film. The structure of the gradient-structure hafnia-based ferroelectric thin film sequentially comprises, from bottom to top: a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer and a fourth ferroelectric layer, and the first ferroelectric layer and the fourth ferroelectric layer have the same composition, both of which are Hf x Zr (1-x) O₂, the second ferroelectric layer and the third ferroelectric layer have the same composition, both of which are Hf (1-x) Zr x O₂, where 0<x<1 and x≠0.5; the composition of Hf element decreases or increases gradually from the middle layer to both sides, and the composition of Zr element increases or decreases gradually from the middle layer to both sides.

[0007] In a second aspect, the present invention discloses a gradient-structure hafnia-based ferroelectric thin film capacitor. The structure of the gradient-structure hafnia-based ferroelectric thin film capacitor sequentially comprises, from bottom to top: a substrate, a metal bottom electrode, a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer, a fourth ferroelectric layer and a metal top electrode, and the first ferroelectric layer and the fourth ferroelectric layer have the same composition, both of which are Hf x Zr (1-x) O₂, the second ferroelectric layer and the third ferroelectric layer have the same composition, both of which are Hf (1-x) Zr x O₂, where 0<x<1 and x≠0.5; the composition of Hf element decreases or increases gradually from the middle layer to both sides, and the composition of Zr element increases or decreases gradually from the middle layer to both sides.

[0008] In a third aspect, the present invention discloses a method for preparing a hafnium oxide-based ferroelectric thin film capacitor with a gradient structure, comprising the steps of:

[0009] preparing a substrate;

[0010] preparing a metal bottom electrode on the upper surface of the substrate;

[0011] preparing a hafnium oxide-based ferroelectric thin film with a gradient structure on the upper surface of the metal bottom electrode, wherein the structure of the hafnium oxide-based ferroelectric thin film sequentially comprises from bottom to top: a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer and a fourth ferroelectric layer, and the first ferroelectric layer and the fourth ferroelectric layer have the same composition, both are Hf x Zr (1-x) O2, the second ferroelectric layer and the third ferroelectric layer have the same composition, both are Hf (1-x) Zr x O2, where 0<x<1 and x≠0.5; the composition of Hf element decreases or increases progressively from the middle layer to both sides, and the composition of Zr element increases or decreases progressively from the middle layer to both sides.

[0012] preparing a metal top electrode on the upper surface of the hafnium oxide-based ferroelectric thin film to obtain the hafnium oxide-based ferroelectric capacitor with a gradient structure;

[0013] subjecting the hafnium oxide-based ferroelectric capacitor to an annealing treatment to obtain the hafnium oxide-based ferroelectric thin film capacitor with a gradient structure.

[0014] In one embodiment of the present invention, the method for preparing the metal bottom electrode is a vacuum magnetron sputtering method, and the material of the prepared metal bottom electrode includes TiN, TaN or W.

[0015] In one embodiment of the present invention, the thickness of the metal electrode is 30-50nm.

[0016] In one embodiment of the present invention, the method for preparing the first ferroelectric layer, the second ferroelectric layer, the third ferroelectric layer and the fourth ferroelectric layer is an atomic layer deposition process, the adopted metal sources are TDMA-Hf and TDMA-Zr, the adopted oxygen source is H2O, O3 or oxygen plasma, the adopted carrier gas is 99.999% high-purity nitrogen, and the deposition temperature is 250-270°C.

[0017] In one embodiment of the present invention, the metal source pulse period of the first ferroelectric layer to the fourth ferroelectric layer is 10-80 cycles.

[0018] In one embodiment of the present invention, the method for preparing the metal top electrode comprises:

[0019] A circular mask pattern with a radius of 50-100 μm is covered on the upper surface of the hafnium oxide-based ferroelectric thin film, and the metal upper electrode is prepared by magnetron sputtering to obtain the hafnium oxide-based ferroelectric capacitor with a gradient structure.

[0020] In one embodiment of the present invention, the material and thickness of the metal upper electrode are respectively the same as those of the metal lower electrode.

[0021] In one embodiment of the present invention, the specific method of the annealing treatment includes:

[0022] Placing the hafnium oxide-based ferroelectric capacitor in a rapid annealing furnace, introducing high-purity nitrogen with a purity of 99.999%, raising the temperature from room temperature to 350-450°C at a heating rate of 20-200°C / s, holding the temperature for 30-300s, then lowering the temperature to room temperature at a cooling rate of 100-500°C / s, to obtain the hafnium oxide-based ferroelectric capacitor with a gradient structure.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] The gradient-structured hafnium oxide-based ferroelectric thin film of the present invention sequentially includes, from bottom to top: a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer and a fourth ferroelectric layer, and the first ferroelectric layer and the fourth ferroelectric layer have the same composition, both being Hf x Zr (1-x) O₂, the second ferroelectric layer and the third ferroelectric layer have the same composition, both being Hf (1-x) Zr x O₂, where 0<x<1 and x≠0.5; through the design that the composition of Hf element decreases or increases from the middle layer to both sides, and the composition of Zr element increases or decreases from the middle layer to both sides, the hafnium oxide-based ferroelectric thin film presents a symmetrical structure / composition feature in the thickness direction, which reduces the difference of the built-in electric field under positive and reverse applied voltages. The finally obtained ferroelectric thin film exhibits excellent ferroelectricity and fatigue resistance, which can improve the accuracy and reliability of information storage in ferroelectric memories. In addition, the gradient-structured hafnium oxide-based ferroelectric thin film capacitor prepared by the preparation method of the present invention not only has high polarization intensity, low operating voltage, excellent stability and reliability, but also has improved wake-up effect and fatigue resistance. Brief Description of the Drawings

[0025] Figure 1 is a schematic structural diagram of a gradient-structured hafnium oxide-based ferroelectric thin film provided by an embodiment of the present invention;

[0026] Figure 2 is a schematic structural diagram of a gradient-structured hafnium oxide-based ferroelectric thin film capacitor provided by an embodiment of the present invention;

[0027] Figure 3 is a flowchart of a method for fabricating a gradient-structure hafnium oxide-based ferroelectric thin film capacitor provided by an embodiment of the present invention;

[0028] Figure 4 is the hysteresis loop of the gradient-structure hafnium oxide-based ferroelectric thin film capacitor of the first embodiment measured at a voltage of 3V before wake-up;

[0029] Figure 5 is the hysteresis loop of the gradient-structure hafnium oxide-based ferroelectric thin film capacitor of the first embodiment measured at a voltage of 3V after wake-up;

[0030] Figure 6 shows the variation of remanent polarization intensity of the gradient-structure hafnium oxide-based ferroelectric thin film capacitor of the first embodiment under different cycle numbers. DETAILED DESCRIPTION

[0031] The present invention is further described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto.

[0032] Please refer to Figure 1 , Figure 1 which is a schematic structural diagram of a gradient-structure hafnium oxide-based ferroelectric thin film provided by an embodiment of the present invention. The gradient-structure hafnium oxide-based ferroelectric thin film of the embodiment of the present invention sequentially comprises, from bottom to top: a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer and a fourth ferroelectric layer, wherein the first ferroelectric layer and the fourth ferroelectric layer have the same composition, both are Hf x Zr (1-x) O2, the second ferroelectric layer and the third ferroelectric layer have the same composition, both are Hf (1-x) Zr x O2, where 0<x<1 and x≠0.5; the content of Hf element in the composition decreases or increases progressively from the intermediate layer to both sides, and the content of Zr element in the composition increases or decreases progressively from the intermediate layer to both sides.

[0033] The specific structural design of the present invention is described below from the perspective of the storage principle of the ferroelectric memory prepared from the ferroelectric thin film material:

[0034] The ferroelectric memory prepared from the ferroelectric thin film material mainly utilizes the positive remanent polarization intensity (+P r ) and the negative remanent polarization intensity (-P r) to realize the information storage function. When the symmetry difference of the internal composition or macroscopic structure of the ferroelectric thin film in the thickness direction (the direction of the applied electric field) is large, an asymmetric built-in electric field is easily generated under the action of external forward and reverse voltages, which affects the accuracy of information storage. In the ferroelectric thin film layer of the present invention, the composition of Hf element decreases or increases from the middle layer to both sides, and the composition of Zr element increases or decreases from the middle layer to both sides. The purpose of such a design is to make the hafnium oxide-based ferroelectric thin film exhibit symmetrical structure / composition characteristics in the thickness direction, reduce the difference of the built-in electric field under forward and reverse applied voltages, and the finally obtained ferroelectric thin film exhibits excellent ferroelectricity and fatigue resistance, which can improve the accuracy and reliability of information storage.

[0035] Please refer to Figure 2 , Figure 2 which is a schematic structural diagram of a hafnium oxide-based ferroelectric thin film capacitor with a gradient structure provided by an embodiment of the present invention. The hafnium oxide-based ferroelectric thin film capacitor with a gradient structure according to the embodiment of the present invention sequentially includes from bottom to top: a substrate, a metal bottom electrode, a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer, a fourth ferroelectric layer, and a metal top electrode, and the first ferroelectric layer and the fourth ferroelectric layer have the same composition, both are Hf x Zr (1-x) O₂, the second ferroelectric layer and the third ferroelectric layer have the same composition, both are Hf (1-x) Zr x O₂, where 0<x<1 and x≠0.5; the composition of Hf element decreases or increases from the middle layer to both sides, and the composition of Zr element increases or decreases from the middle layer to both sides.

[0036] Based on the same principle, in the capacitor including the hafnium oxide-based ferroelectric thin film with a gradient structure of the present invention, through the design that the composition of Hf element decreases or increases from the middle layer to both sides and the composition of Zr element increases or decreases from the middle layer to both sides, the hafnium oxide-based ferroelectric thin film exhibits symmetrical structure / composition characteristics in the thickness direction, the difference of the built-in electric field under forward and reverse applied voltages is reduced, and the finally obtained ferroelectric thin film exhibits excellent ferroelectricity and fatigue resistance.

[0037] In addition, existing hafnium oxide-based ferroelectric thin film capacitors usually adopt a buffer layer between the bottom electrode layer and the ferroelectric thin film layer, the purpose of which is to increase the content of the orthorhombic phase in the ferroelectric thin film and improve the ferroelectric performance by introducing the buffer layer, but the introduction of the buffer layer virtually increases the preparation cost. However, the capacitor structure of the present invention, through the structural design of the hafnium oxide-based ferroelectric thin film with a gradient structure and the preparation method of the ferroelectric thin film capacitor of the present invention, does not require a buffer layer, which can not only reduce the preparation cost, but also significantly improve the wake-up effect and fatigue failure phenomenon of the hafnium oxide-based ferroelectric thin film. The obtained ferroelectric thin film capacitor can achieve excellent ferroelectric performance, and the remanent polarization intensity can reach 24 μC / cm 2or more.

[0038] Please refer to Figure 3 , Figure 3 which is a flowchart of a method for fabricating a hafnium oxide-based ferroelectric thin film capacitor with a gradient structure provided by an embodiment of the present invention. The method for fabricating a hafnium oxide-based ferroelectric thin film capacitor with a gradient structure according to the embodiment of the present invention includes the following steps:

[0039] S1. Preparing a substrate.

[0040] The substrate in this step may be any one of silicon, germanium, gallium arsenide, gallium nitride and gallium oxide, and heavily doped monocrystalline silicon is preferred.

[0041] As a preferred solution, a cleaning operation may be performed on the substrate. The specific process includes: placing the substrate in a mixed solution of concentrated sulfuric acid and hydrogen peroxide for cleaning for 10-30 minutes to remove metal particles and organic contaminants on the surface of the substrate, then sequentially placing the substrate in ultrapure water and hydrofluoric acid with a concentration of 0.5-1.5% for cleaning for 10-30 minutes, and repeating the above operations 2-3 times to avoid the influence of defects and impurities as much as possible.

[0042] S2. Fabricating a bottom metal electrode on the substrate.

[0043] Specifically, the fabrication method of the bottom metal electrode in this step is a vacuum magnetron sputtering method, the material of the fabricated bottom metal electrode includes TiN, TaN or W, and the thickness of the bottom metal electrode is 30-50 nm.

[0044] The use of TiN, TaN or W electrode materials in the present invention is fully compatible with existing CMOS (Complementary Metal-Oxide-Semiconductor) processes.

[0045] S3. Fabricating a hafnium oxide-based ferroelectric thin film with a gradient structure on the upper surface of the bottom metal electrode, wherein the hafnium oxide-based ferroelectric thin film sequentially comprises, from bottom to top: a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer and a fourth ferroelectric layer, and the first ferroelectric layer and the fourth ferroelectric layer have the same composition, both of which are Hf x Zr (1-x) O₂, the second ferroelectric layer and the third ferroelectric layer have the same composition, both of which are Hf (1-x) Zr x O₂, where 0<x<1 and x≠0.5; the composition of the Hf element decreases or increases from the intermediate layer to both sides, and the composition of the Zr element increases or decreases from the intermediate layer to both sides.

[0046] In this step, the first ferroelectric layer Hf x Zr (1-x) O₂, the second ferroelectric layer Hf (1-x) Zr x O₂, the third ferroelectric layer Hf (1-x) Zr xO2 and the fourth ferroelectric layer Hf x Zr (1-x) O2 was prepared using atomic layer deposition (ALD) with TDMA-Hf and TDMA-Zr as the metal sources, H2O, O3, or oxygen plasma as the oxygen source, and 99.999% high-purity nitrogen as the carrier gas. The deposition temperature was 250-270℃. The first ferroelectric layer was Hf. x Zr (1-x) O2 to the fourth ferroelectric layer Hf x Zr (1-x) The metal source pulse period for O2 is 10-80 cycles.

[0047] It should be noted that alternating the number of Hf and Zr sources during atomic layer deposition is to control the Hf and Zr ratios, resulting in ferroelectric layers with varying Hf and Zr proportions. For example, alternating between two cycles of Hf and three cycles of Zr source in the second and third ferroelectric layers constitutes one cycle, resulting in ferroelectric layers with a composition of Hf... 0.4 Zr 0.6 O2; the first and fourth ferroelectric layers are alternately fed with one cycle of Hf source and one cycle of Zr source as one cycle, resulting in a ferroelectric layer with the composition of Hf. 05 Zr 05 O2. The larger the number of cycles, the thicker the ferroelectric layer. Therefore, the thickness and composition of the ferroelectric layer can be precisely controlled by controlling the cycle period of the metal source and the ratio of Hf and Zr sources.

[0048] S4. Prepare a metal top electrode on the surface of a hafnium oxide-based ferroelectric thin film to obtain a hafnium oxide-based ferroelectric capacitor.

[0049] Specifically, the methods for fabricating the metal upper electrode include:

[0050] A circular mask pattern with a radius of 50-100 μm was covered on the surface of a hafnium oxide-based ferroelectric thin film, and a metal top electrode was fabricated by magnetron sputtering to obtain a hafnium oxide-based ferroelectric capacitor with a gradient structure.

[0051] Preferably, the material and thickness of the upper metal electrode are the same as those of the lower metal electrode. That is, the material of the upper metal electrode includes TiN, TaN, or W, and the thickness of the upper metal electrode is 30-50 nm.

[0052] S5. Anneal the hafnium oxide-based ferroelectric capacitor to obtain a gradient structure hafnium oxide-based ferroelectric thin film capacitor.

[0053] Specifically, the annealing process includes:

[0054] A hafnium oxide-based ferroelectric capacitor is placed in a rapid annealing furnace, and high-purity nitrogen gas with a purity of 99.999% is introduced. The temperature is increased from room temperature to 350-450℃ at a heating rate of 20-200℃ / s, held at that temperature for 30-300s, and then cooled to room temperature at a cooling rate of 100-500℃ / s to obtain a hafnium oxide-based ferroelectric capacitor with a gradient structure.

[0055] It should be noted that the heating and cooling rates directly affect the thermal budget of the thin film preparation process. Slow heating and cooling rates increase the thermal budget, making it difficult to precisely control the holding time. Furthermore, slow cooling rates reduce the content of the orthorhombic phase within the film, weakening its ferroelectric properties. Conversely, excessively fast heating and cooling rates impose higher requirements on the film preparation conditions, increasing equipment costs. Considering both preparation costs and the performance requirements of ferroelectric thin films, this invention employs a heating rate of 20-200℃ / s and a cooling rate of 100-500℃ / s. The annealing temperature of this invention does not exceed 450℃, meeting the temperature requirements of integrated circuit back-end processes and promoting the application of hafnium oxide-based ferroelectric thin films in high-performance and highly integrated ferroelectric memories. Furthermore, in the annealing process of this invention, the temperature is increased from room temperature to 350-450°C at a heating rate of 20-200°C / s, held for 30-300s, and then cooled to room temperature at a cooling rate of 100-500°C / s. By adopting the above-mentioned heating rate, cooling rate, and corresponding temperature control range and holding time, combined with the structural characteristics of the gradient structure hafnium oxide-based ferroelectric thin film of this invention, the resulting gradient structure hafnium oxide-based ferroelectric thin film capacitor not only has high polarization intensity, low operating voltage, and excellent stability and reliability, but also its wake-up effect and fatigue resistance are improved, thereby enhancing the accuracy and reliability of information storage in the ferroelectric memory.

[0056] The effects of the gradient structure hafnium oxide-based ferroelectric thin film and the gradient structure hafnium oxide-based ferroelectric thin film capacitor prepared by the present invention will be further explained below with reference to specific preparation methods.

[0057] Example 1

[0058] The method for preparing the gradient structure hafnium oxide-based ferroelectric thin film capacitor in this embodiment includes the following steps:

[0059] (1) Using heavily doped single-crystal silicon as the substrate material, it was placed in a mixed solution of concentrated sulfuric acid and hydrogen peroxide for 10 min to remove metal particles and organic contaminants on the surface of the single-crystal silicon. Then it was placed in ultrapure water and hydrofluoric acid with a concentration of 1.5% for 10 min in sequence. The above operation was repeated 3 times.

[0060] (2) A W metal lower electrode was prepared on a cleaned single-crystal silicon substrate by magnetron sputtering. The thickness of the metal lower electrode was 50 nm.

[0061] (3) A gradient-structured HfO2-based ferroelectric thin film was prepared on the upper surface of the W metal lower electrode by atomic layer deposition.

[0062] The structure of the HfO2-based ferroelectric thin film, from bottom to top, includes a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer, and a fourth ferroelectric layer, with chemical compositions of HfO2, HfO2, and HfO2, respectively. 0.4 Zr 0.6 O2, Hf 0.6 Zr 0.4 O2, Hf 0.6 Zr 0.4 O2 and Hf 0.4 Zr 0.6 O2. The metal sources used were TDMA-Hf and TDMA-Zr, the oxygen source was H2O, the carrier gas was 99.999% high-purity nitrogen, and the deposition temperature was 250℃. The first ferroelectric layer alternately used two cycles of Hf source and three cycles of Zr source as one cycle, for a total of 5 cycles; the second ferroelectric layer alternately used three cycles of Hf source and two cycles of Zr source as one cycle, for a total of 5 cycles; the third ferroelectric layer alternately used three cycles of Hf source and two cycles of Zr source as one cycle, for a total of 5 cycles; the fourth ferroelectric layer alternately used two cycles of Hf source and three cycles of Zr source as one cycle, for a total of 5 cycles.

[0063] (4) A circular mask pattern with a radius of 100 μm is covered on the upper surface of the gradient thin film prepared above, and a W metal top electrode with a thickness of 50 nm is prepared by magnetron sputtering to obtain an HfO2-based ferroelectric capacitor with a gradient structure.

[0064] (5) The prepared ferroelectric capacitor was placed in a rapid annealing furnace, and high-purity nitrogen gas with a purity of 99.999% was introduced. The temperature was increased from room temperature to 450℃ at a heating rate of 20℃ / s, held for 30s, and then cooled to room temperature at a cooling rate of 100℃ / s to obtain a HfO2-based ferroelectric thin film capacitor sample with a gradient structure.

[0065] Example 2

[0066] The method for preparing the gradient structure hafnium oxide-based ferroelectric thin film capacitor in this embodiment includes the following steps:

[0067] (1) Using heavily doped single-crystal silicon as the substrate material, it was placed in a mixed solution of concentrated sulfuric acid and hydrogen peroxide for 10 min to remove metal particles and organic contaminants on the surface of the single-crystal silicon. Then it was placed in ultrapure water and hydrofluoric acid with a concentration of 1.0% for 10 min in sequence. The above operation was repeated 3 times.

[0068] (2) A TiN metal bottom electrode was prepared on a clean single-crystal silicon substrate by magnetron sputtering. The thickness of the metal bottom electrode was 30 nm.

[0069] (3) A gradient-structured HfO2-based ferroelectric thin film was prepared on the upper surface of the TiN metal lower electrode by atomic layer deposition.

[0070] The structure of the HfO2-based ferroelectric thin film, from bottom to top, includes a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer, and a fourth ferroelectric layer, with chemical compositions of HfO2, HfO2, and HfO2, respectively. 0.2 Zr 0.8 O2, Hf 0.8 Zr 0.2 O2, Hf 0.8 Zr 0.2 O2 and Hf 0.2 Zr 0.8 O2. The metal sources used were TDMA-Hf and TDMA-Zr, the oxygen source was H2O, the carrier gas was 99.999% high-purity nitrogen, and the deposition temperature was 270℃. The first ferroelectric layer alternately used one cycle of Hf source and four cycles of Zr source as one cycle, for a total of 5 cycles; the second ferroelectric layer alternately used four cycles of Hf source and one cycle of Zr source as one cycle, for a total of 5 cycles; the third ferroelectric layer alternately used four cycles of Hf source and one cycle of Zr source as one cycle, for a total of 5 cycles; the fourth ferroelectric layer alternately used one cycle of Hf source and four cycles of Zr source as one cycle, for a total of 5 cycles.

[0071] (4) A circular mask pattern with a radius of 50 μm is covered on the gradient thin film prepared above, and a TiN metal top electrode is prepared by magnetron sputtering. The thickness of the metal top electrode is 30 nm, thus obtaining an HfO2-based ferroelectric capacitor with a gradient structure.

[0072] (5) The prepared ferroelectric capacitor was placed in a rapid annealing furnace, and high-purity nitrogen gas with a purity of 99.999% was introduced. The temperature was increased from room temperature to 350℃ at a heating rate of 200℃ / s, held for 300s, and then cooled to room temperature at a cooling rate of 500℃ / s to obtain a HfO2-based ferroelectric thin film capacitor sample with a gradient structure.

[0073] Example 3

[0074] The method for preparing the gradient structure hafnium oxide-based ferroelectric thin film capacitor in this embodiment includes the following steps:

[0075] (1) Using heavily doped single-crystal silicon as the substrate material, it was placed in a mixed solution of concentrated sulfuric acid and hydrogen peroxide for 10 min to remove metal particles and organic contaminants on the surface of the single-crystal silicon. Then it was placed in ultrapure water and hydrofluoric acid with a concentration of 0.5% for 30 min in sequence. The above operation was repeated twice.

[0076] (2) A TaN metal bottom electrode was prepared on a clean single-crystal silicon substrate by magnetron sputtering. The thickness of the metal bottom electrode was 40 nm.

[0077] (3) A gradient-structured HfO2-based ferroelectric thin film was prepared on the upper surface of the TaN metal lower electrode by atomic layer deposition.

[0078] The structure of the HfO2-based ferroelectric thin film, from bottom to top, includes a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer, and a fourth ferroelectric layer, with chemical compositions of HfO2, HfO2, and HfO2, respectively. 0.7 Zr 0.3 O2, Hf 0.3 Zr 0.7 O2, Hf 0.3 Zr 0.7 O2 and Hf 0.7 Zr 0.3 O2. The metal sources used were TDMA-Hf and TDMA-Zr, the oxygen source was H2O, the carrier gas was 99.999% high-purity nitrogen, and the deposition temperature was 260℃. The first ferroelectric layer alternately used 7 cycles of Hf source and 3 cycles of Zr source as one cycle, for a total of 4 cycles; the second ferroelectric layer alternately used 3 cycles of Hf source and 7 cycles of Zr source as one cycle, for a total of 4 cycles; the third ferroelectric layer alternately used 3 cycles of Hf source and 7 cycles of Zr source as one cycle, for a total of 4 cycles; the fourth ferroelectric layer alternately used 7 cycles of Hf source and 3 cycles of Zr source as one cycle, for a total of 4 cycles.

[0079] (4) A circular mask pattern with a radius of 60 μm is covered on the upper surface of the gradient thin film prepared above, and a TaN metal top electrode is prepared by magnetron sputtering. The thickness of the metal top electrode is 40 nm, thus obtaining an HfO2-based ferroelectric capacitor with a gradient structure.

[0080] (5) The prepared ferroelectric capacitor was placed in a rapid annealing furnace, and high-purity nitrogen gas with a purity of 99.999% was introduced. The temperature was increased from room temperature to 400℃ at a heating rate of 100℃ / s, held for 100s, and then cooled to room temperature at a cooling rate of 200℃ / s to obtain a HfO2-based ferroelectric thin film capacitor sample with a gradient structure.

[0081] The hysteresis loops of the gradient hafnium oxide-based ferroelectric thin-film capacitor prepared in Example 1 were tested using a ferroelectric analyzer before and after wake-up, and the residual polarization intensity (±P) was summarized. r The changes with fatigue testing. Figure 4 The hysteresis loop of the gradient structure hafnium oxide-based ferroelectric thin film capacitor of Example 1 was measured at 3V voltage and before wake-up. Figure 5 The hysteresis loop of the gradient structure hafnium oxide-based ferroelectric thin film capacitor of Example 1 was measured at 3V voltage and after wake-up. Figure 6 This shows the change in residual polarization intensity of the gradient structure hafnium oxide-based ferroelectric thin film capacitor in Example 1 at different cycle numbers. Figure 4 , Figure 5 and Figure 6 It can be seen that the hysteresis loop exhibits a large remanent polarization intensity both before and after wake-up, especially the remanent polarization intensity (±P) after wake-up. r It can reach 24μC / cm 2 That's all. Although the residual polarization intensity decreases with increasing cycle number, after 10... 9 The fact that the values ​​remain high even after cycling indicates that the gradient structure hafnium oxide-based ferroelectric thin film capacitor prepared in this invention exhibits excellent ferroelectric performance.

[0082] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A gradient structure hafnium oxide-based ferroelectric thin film, characterized in that, The structure of the gradient-structured hafnium oxide-based ferroelectric thin film sequentially includes from bottom to top: a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer and a fourth ferroelectric layer, and the first ferroelectric layer and the fourth ferroelectric layer have the same composition, both are Hf x Zr (1-x) O₂, the second ferroelectric layer and the third ferroelectric layer have the same composition, both are Hf (1-x) Zr x O₂, where 0<x<1 and x≠0.5; the composition of the Hf element decreases or increases from the middle layer to both sides, and the composition of the Zr element increases or decreases from the middle layer to both sides.

2. A gradient structure hafnium oxide-based ferroelectric thin-film capacitor, characterized in that, The gradient-structure hafnium oxide-based ferroelectric thin film capacitor sequentially comprises, from bottom to top: a substrate, a metal bottom electrode, a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer, a fourth ferroelectric layer, and a metal top electrode, and the first ferroelectric layer and the fourth ferroelectric layer have the same composition, both being Hf x Zr (1-x) O2, the second ferroelectric layer and the third ferroelectric layer have the same composition, both being Hf (1-x) Zr x O2, where 0<x<1 and x≠0.5; the composition of Hf element decreases or increases gradually from the intermediate layer to both sides, and the composition of Zr element increases or decreases gradually from the intermediate layer to both sides.

3. A method for preparing a gradient structure hafnium oxide-based ferroelectric thin-film capacitor, characterized in that, Including the following steps: Prepare the substrate; A metal lower electrode is fabricated on the substrate; Preparing a hafnium oxide-based ferroelectric thin film with a gradient structure on the upper surface of the metal lower electrode, wherein the structure of the hafnium oxide-based ferroelectric thin film sequentially includes, from bottom to top: a first ferroelectric layer, a second ferroelectric layer, a third ferroelectric layer, and a fourth ferroelectric layer, and the first ferroelectric layer and the fourth ferroelectric layer have the same composition, both being Hf x Zr (1-x) O2, the second ferroelectric layer and the third ferroelectric layer have the same composition, both being Hf (1-x) Zr x O2, where 0<x<1 and x≠0.5; the composition of the Hf element decreases or increases progressively from the intermediate layer to both sides, and the composition of the Zr element increases or decreases progressively from the intermediate layer to both sides; A metal top electrode is fabricated on the upper surface of the hafnium oxide-based ferroelectric thin film to obtain a hafnium oxide-based ferroelectric capacitor; The hafnium oxide-based ferroelectric capacitor is annealed to obtain the gradient structure hafnium oxide-based ferroelectric thin film capacitor.

4. The preparation method according to claim 3, characterized in that, The method for preparing the metal lower electrode is vacuum magnetron sputtering, and the material of the prepared metal lower electrode includes TiN, TaN or W.

5. The preparation method according to claim 3, characterized in that, The thickness of both the upper and lower metal electrodes is 30-50 nm.

6. The preparation method according to claim 3, characterized in that, The first ferroelectric layer, the second ferroelectric layer, the third ferroelectric layer and the fourth ferroelectric layer are prepared by atomic layer deposition process, using TDMA-Hf and TDMA-Zr as metal sources, H2O, O3 or oxygen plasma as oxygen sources, 99.999% high-purity nitrogen as carrier gas, and a deposition temperature of 250-270 ºC.

7. The preparation method according to claim 3, characterized in that, The metal source pulse period for the first ferroelectric layer to the fourth ferroelectric layer is 10-80 cycles.

8. The preparation method according to claim 3, characterized in that, The method for preparing the metal upper electrode includes: A circular mask pattern with a radius of 50-100 μm is covered on the upper surface of the hafnium oxide-based ferroelectric thin film, and the metal top electrode is prepared by magnetron sputtering to obtain a hafnium oxide-based ferroelectric capacitor with a gradient structure.

9. The preparation method according to claim 3, characterized in that, The material and thickness of the upper metal electrode are the same as those of the lower metal electrode.

10. The preparation method according to claim 3, characterized in that, The specific methods for the annealing process include: The hafnium oxide-based ferroelectric capacitor was placed in a rapid annealing furnace, and high-purity nitrogen gas with a purity of 99.999% was introduced. The temperature was increased from room temperature to 350-450 ºC at a heating rate of 20-200 ºC / s, held at that temperature for 30-300 s, and then cooled to room temperature at a cooling rate of 100-500 ºC / s to obtain a hafnium oxide-based ferroelectric capacitor with a gradient structure.

Images