Superionic conductor metal fluoride solid solution dielectric film and preparation method thereof

Abstract

The invention discloses a super-ionic conductor metal fluoride solid solution dielectric film and a preparation method thereof. The super-ionic conductor metal fluoride solid solution dielectric film is prepared from two or more than two metal fluoride super-ionic conductors by adopting a thermal evaporation method. According to the method, the metal fluoride super-ion conductor is rapidly deposited on the surface of the substrate and crystallized in a thermal evaporation manner, and meanwhile, the prepared fluoride can generate a certain number of fluorine vacancies by controlling a relatively low substrate temperature, so that the ionic conductivity and capacitive coupling of the metal fluoride are improved to a certain extent; therefore, the thermal evaporation metal fluoride super-ionic conductor dielectric film can become a super-ionic conductor fluoride film preparation method with low cost, high efficiency and high compatibility.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor materials, and particularly relates to a superionic conductor metal fluoride solid solution dielectric film and a preparation method thereof. Background Art

[0002] Dielectric materials are one of the core materials in condensed matter physics and advanced electronic device applications. As the miniaturization of traditional semiconductor devices approaches the physical limit, in order to overcome the adverse effects brought by the short-channel effect, an important solution is to find dielectric materials with stronger control ability for channel materials; in the field of condensed matter physics research, the regulation of various strongly correlated electron phenomena also requires the development of dielectric materials with larger capacitive coupling. Traditional low-dielectric-constant SiO2 dielectric materials and subsequently developed high-dielectric-constant dielectric materials such as HfO2 and ZrO2 usually face severe gate leakage problems due to the dielectric breakdown limit, and have gradually been unable to meet the field-effect regulation requirements of electron states in miniaturized semiconductor devices or strongly correlated electron systems. Although researchers have successively developed new dielectrics with larger capacitive coupling, such as ultra-high-dielectric-constant SrTiO3 single-crystal thin-film oxide dielectrics and organic electrolyte material dielectrics. These new dielectrics still have relatively large problems in practical applications. For example, SrTiO3 single-crystal thin-film oxide dielectrics need to be transferred to the target substrate or material surface and are not suitable for wafer-level preparation, while another type of organic electrolyte material, due to its mostly liquid or gel form, is also difficult to be compatible with traditional semiconductor lithography processes. In addition, liquid electrolytes also have electrochemical mechanisms, which seriously affect the stability and repeatability of devices. Therefore, it is very important to find a dielectric material that has both a large coupling capacitance, a wide bandgap, and exists stably in a solid state, is compatible with traditional semiconductor device processes, and can also be grown on a large scale.

[0003] Fluoride solid superionic conductors are a type of solid ionic conductor that lies between crystalline solids with regular structures and liquid electrolytes with freely moving ions. Based on the superionic properties of fluorides, fluoride ions can move rapidly in the crystal, and under the action of an electric field, an electric double layer effect can be formed on the surface of the channel material, generating a huge capacitive coupling. Therefore, such a structural feature makes fluoride materials very suitable as solid ionic dielectrics for various electronic devices. It overcomes the disadvantages of small capacitance and weak gate control ability of traditional oxides, and also overcomes the problem of incompatibility between ionic liquid electrolytes and traditional semiconductor processes. It is a type of semiconductor field-effect transistor dielectric material with great potential to be developed. In the past few decades, the basic properties of fluoride materials have been studied in great depth, but these studies have mainly focused on studying their superionic properties in the form of fluoride powders or bulk solid solutions. In recent years, researchers have gradually used LaF3 single crystal substrates or CaF2 single crystal dielectric films to preliminarily apply fluoride materials as dielectric materials in field-effect transistors. However, fluoride materials in the form of substrates are not compatible with traditional semiconductor processes, and the preparation of fluoride single crystal films requires high-cost techniques such as molecular beam epitaxy, which is also not compatible with traditional semiconductor processes and the crystallization performance is easily restricted by the lattice constants of the substrates. On the other hand, the capacitive coupling of fluoride dielectric materials such as currently reported LaF3 substrates or CaF2 films is still not high, which greatly limits their application in the field of new dielectric materials. Previous studies have shown that in the fluoride bulk solid solution system, the doping engineering of heterovalent cations can directly affect the barrier height of fluoride ion migration, and thereby change the diffusion dynamics of fluoride ions, resulting in higher ionic conductivity, and thus is expected to lead to greater capacitive coupling.

[0004] In summary, the present invention conceives of directly preparing a superionic conductor fluoride film from fluoride powder, and further improving the ionic conductivity and capacitive coupling of fluoride by forming a superionic conductor fluoride solid solution dielectric film through heterovalent and / or isovalent cation doping. Summary of the Invention

[0005] One object of the present invention is to provide a superionic conductor metal fluoride solid solution dielectric film, which is prepared from two or more metal fluoride superionic conductors, and the metal fluoride superionic conductors are selected from scandium fluoride, yttrium fluoride, lanthanum fluoride, cerium fluoride, neodymium fluoride, samarium fluoride, europium fluoride, gadolinium fluoride, holmium fluoride, erbium fluoride, ytterbium fluoride, nickel fluoride, aluminum fluoride, strontium fluoride, calcium fluoride, barium fluoride, manganese fluoride, ferrous fluoride, tin difluoride.

[0006] Furthermore, the conductivity of the superionic conductor metal fluoride solid solution dielectric film is 10 -5 -10 -2 S / cm, and the low-frequency capacitance is 0.1 - 100 μF / cm2 The high-frequency capacitance is 0.1 - 0.4 μF / cm 2 and the leakage current density is less than 10 -5 A / cm 2 , the root mean square of the surface roughness is less than 1 nm, and the fluorine vacancy content is 0.01 - 15%.

[0007] In an embodiment of the present invention, a superionic conductor metal fluoride solid solution dielectric thin film is prepared using lanthanum fluoride (LaF3) and strontium fluoride (SrF2), and the molar ratio of the two is 1:0.001 - 1:1000, preferably 1:0.01 - 1:0.2.

[0008] The second object of the present invention is to provide a method for preparing the above-mentioned superionic conductor metal fluoride solid solution dielectric thin film, and the dielectric thin film is prepared by thermal evaporation.

[0009] Furthermore, the process of the preparation method is as follows: Prepared using a thermal evaporation system. First, the metal fluoride superionic conductor is ground separately, and after mixing, it is placed on the heating column of the thermal evaporation system. When the system vacuum reaches below 10 -5 Pa, evaporation starts, and the mixed fluoride evaporates and deposits on the substrate surface at a rate of until the thickness reaches 10 - 20 nm, then the rate is adjusted to until the target thickness is reached. During the evaporation process, the temperature of the substrate is controlled at 200 - 600 K.

[0010] Furthermore, the metal fluoride superionic conductor is ground to a particle size below 200 mesh.

[0011] Furthermore, the material of the substrate is SiO2 / Si, Si, Ge, mica, sapphire, ruby, indium phosphide, indium arsenide, gallium phosphide, gallium nitride, strontium titanate, zirconia, silicon carbide, quartz glass.

[0012] During the process of thermally evaporating and growing the dielectric thin film, in order to obtain a thin film with a higher density, the evaporation rate selected at the beginning is slower. In the present invention, a rate of is adopted. After the thickness of the thin film reaches a certain value, such as 10 nm - 20 nm, in order to reduce the total evaporation time, the evaporation rate can be increased, such as At the same time, the temperature of the substrate needs to be controlled. Through the water cooling system, the substrate is always controlled at a relatively low temperature (such as between 200 K and 600 K), which can form more fluorine vacancies and help improve the ionic conductivity of the fluoride thin film. In addition, before evaporation, the background vacuum of the vacuum chamber needs to be controlled low enough (<10 -5 Pa magnitude), so that the fluoride superionic conductor solid solution thin film prepared contains fewer impurities, is not easily broken down, and has better insulation performance.

[0013] A third object of the present invention is to provide a method for improving the capacitive coupling ability of metal fluorides. The method is to prepare a dielectric film of a superionic conductor fluoride solid solution doped with hetero-valent and / or cationic ions by thermal evaporation.

[0014] In one embodiment of the present invention, a fluoride superionic conductor with a similar ionic radius and a lower cation valence state, such as SrF2, is selected to perform hetero-valent cation doping on a metal fluoride superionic conductor such as LaF3. Since Sr 2+ can replace La 3+ to occupy the cation framework of the fluoride, and Sr 2+ has a lower valence state than La 3+ , each Sr 2+ ion will in principle introduce one more F - vacancy in LaF3, thereby further improving the ionic conductivity and capacitive coupling of the LaF3 film, making the hetero-valent cation-doped fluoride solid solution dielectric film an effective and universal method for improving the capacitive coupling ability of metal fluoride superionic conductors.

[0015] The method for preparing a superionic conductor metal fluoride solid solution by thermal evaporation in the present invention is suitable for preparing dielectric films of superionic conductor solid solutions composed of any variety of fluorides. Two or more metal fluoride superionic conductors can be selected and mixed to form a solid solution, and hetero-valent charges can be introduced through hetero-valent ion doping and / or lattice vacancies can be formed through homo-valent ion doping, thereby improving the capacitive coupling ability of the metal fluoride superionic conductor.

[0016] The present invention uses thermal evaporation to deposit a dielectric film of a superionic conductor metal fluoride solid solution. Thermal evaporation is a thin film growth method in the prior art that is very simple to operate, low in cost, can form films on a large scale, and the quality and thickness of the thin film are controllable. In the present invention, since the metal cation lattice framework in the metal fluoride allows fluoride ions with a smaller ionic radius to move freely in the crystal, and at the same time the metal fluoride has a large band gap value and good insulation properties, which is beneficial to suppressing the leakage current through the fluoride thin film dielectric. By means of thermal evaporation, the metal fluoride is rapidly deposited on the substrate surface and crystallized. At the same time, by controlling a lower substrate temperature, a certain number of fluorine vacancies can be generated in the prepared fluoride, thereby improving the ionic conductivity and capacitive coupling of the metal fluoride to a certain extent, making the thermal evaporation of metal fluoride dielectric films a low-cost, high-efficiency, and highly compatible method for preparing fluoride thin films. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of the preparation method of the superionic conductor fluoride solid solution film.

[0018] Figure 2 Schematic diagram of the capacitance test geometry for the superionic conductor fluoride solid solution thin film and the capacitance comparison of La 0.95 Sr 0.05 F 2.95 , LaF3, and SrF2 thin films.

[0019] Figure 3 Capacitance coupling variation of La 1-x Sr x F 3-x superionic conductor fluoride solid solution thin films at different doping levels.

[0020] Figure 4 Physical mechanism for the capacitance enhancement of La 1-x Sr x F 3-x superionic conductor fluoride solid solution thin films. Specific implementation manners

[0021] The preferred implementation manners of the present invention will be described in detail below in conjunction with the embodiments. It should be understood that the following embodiments are given only for the purpose of illustration and are not used to limit the scope of the present invention. Those skilled in the art can make various modifications and substitutions to the present invention without departing from the purpose and spirit of the present invention.

[0022] The experimental methods used in the following embodiments are all conventional methods unless otherwise specified.

[0023] The materials, reagents, etc. used in the following embodiments can be obtained from commercial channels unless otherwise specified.

[0024] The thermal evaporation system adopted in the following embodiments is the VZZ-300 model high-vacuum thermal evaporation system of Beijing Micro-Nano Vacuum Company. Other models of high-vacuum thermal evaporation systems can also be used as long as they can control the vacuum regulation of the thermal evaporation process.

[0025] Example 1

[0026] 1. Preparation of La 1-x Sr x F 3-x (x = 0.03, 0.05, 0.09) superionic conductor solid solution dielectric thin films

[0027] First, commercially available LaF3 and SrF2 particles were ground into fluoride powders of about 200 mesh respectively, and the two were fully mixed evenly according to the required ratio of the molar ratio of LaF3 to SrF2 being 0.95:0.05. The mixed powder was loaded into a mold, and a hydraulic device was used to press the mixed powder into a columnar structure. In this embodiment, according to the selected tungsten boat, the mixed powder was pressed into a cylinder with a diameter of 6 mm and a length of 1 cm. Then, the pressed columnar material was placed on a tungsten boat (a molybdenum boat can also be used), and the tungsten boat was transferred to and fixed on the heating column of the thermal evaporation system. At the same time, a high-temperature-resistant tape was used to fix the SiO2 / Si substrate to be deposited (the substrate can be replaced with various common substrates such as mica, sapphire, indium phosphide, strontium titanate, etc.) on the base of the thermal evaporation system, and a substrate baffle was blocked directly below the substrate, and the cavity was sealed. After that, the mechanical pump for pumping rough vacuum and the molecular pump for pumping high vacuum were turned on respectively. After the vacuum degree of the system reached the order of magnitude of 10 -5 Pa, the subsequent thermal evaporation process could be started.

[0028] By slowly increasing the current flowing through the tungsten boat, the heating temperature of the fluoride was increased. When the heating temperature exceeded the melting point of the fluoride, fluoride molecules began to deposit on the pre-placed substrate to form a fluoride solid solution film, as Figure 1 shown. The thickness of the rare earth metal fluoride film on the surface of the SiO2 / Si substrate was monitored by a crystal oscillator film thickness meter. During the evaporation process, first let the fluoride material exhale at a rate of . After the exhaled thickness reached 10 nm, the baffle between the fluoride material and the substrate was opened. Then, the current flowing through the tungsten boat was reduced, and by observing the reading of the crystal oscillator film thickness meter, a slower thermal evaporation rate was maintained, maintaining a rate of until the thickness reached 10 nm. Then, the current flowing through the tungsten boat was increased again, and by observing the reading of the crystal oscillator film thickness meter, the evaporation rate was increased to a faster thermal evaporation rate, maintaining a rate of until the target thickness of 200 nm was reached, obtaining a La Sr F 0.95 superionic conductor dielectric film. 0.05 F 2.95 superionic conductor dielectric film.

[0029] By changing the ratio of LaF3 and SrF2, La 0.97 Sr 0.03 F 2.97 superionic conductor dielectric film and La 0.91 Sr 0.09 F 2.91 superionic conductor dielectric film were prepared according to the same method.

[0030] 2. Preparation of LaF3 superionic conductor dielectric thin film: Grind commercially available LaF3 particles into fluoride powder of about 200 mesh, load the LaF3 powder into a mold, and use hydraulic equipment to press the powder into a columnar structure. Use the same parameters as those for depositing the above La 1- x Sr x F 3-x (x = 0.03, 0.05, 0.09) solid solution dielectric thin films to prepare pure LaF3 thin films of the same thickness.

[0031] Similarly, pure SrF2 thin films of the same thickness can be prepared.

[0032] 3. Dielectric property test of superionic conductor metal fluoride solid solution dielectric thin film

[0033] As Figure 2 shown in a, the capacitance characteristics of the fluoride are tested using a parallel plate capacitor structure, and the substrate used is a silicon substrate with a 300 nm oxide layer. First, prefabricate the pattern of the lower plate electrode on the silicon substrate through photolithography, and deposit a metal electrode as the lower plate electrode of the parallel plate capacitor through an electron beam evaporation system; then deposit a metal fluoride solid solution thin film on the substrate with the prefabricated electrode through a thermal evaporation system; the last step is to prefabricate the pattern of the upper plate electrode on the metal fluoride thin film through photolithography, and also deposit a metal electrode as the upper plate electrode of the parallel plate capacitor through an electron beam evaporation system. Electrochemical impedance spectroscopy (EIS) measurements are carried out on an electrochemical workstation, and the model of the electrochemical workstation used is Zahner, Zennium Pro. Based on the parallel plate capacitor geometric structure of metal electrode / fluoride / metal electrode (with a 200 nm fluoride film), apply an AC voltage of 20 mV as a perturbation signal, and perform EIS measurements in the frequency f range from 0.01 Hz to 1 MHz to obtain the frequency-dependent impedance Z and phase angle θ. The capacitance C per unit area of the fluoride thin film is calculated by the following formula:

[0034] C = 1 / 2πfZ″S.

[0035] where f is the frequency, Z″ is the imaginary part of the impedance, and S is the area of the parallel plate capacitor. Temperature-dependent EIS measurements are carried out in a cryogenic system under vacuum conditions.

[0036] Using hetero-valent cations to form fluoride solid solution thin films can directly affect the barrier height of F - ion migration and generate a large number of F vacancies, thus providing a practical method for improving ionic conductivity and capacitive coupling by inducing lattice defects. Figure 3 Shows the La doped with SrF2 in the LaF3 thin film and formed at different doping levels x 1-x Srx F 3-x Capacitance of the solid solution thin film varying with frequency f at different temperatures. On the one hand, when the value of x increases from 0 to 0.09, the capacitance at f = 0.01 Hz increases from 4.5 μF / cm 2 to 8.8 μF / cm 2 , nearly doubling. This indicates the crucial role of induced lattice defects in F - ion migration and further provides a powerful method to greatly enhance capacitive coupling in such a system. On the other hand, as the temperature rises, the electrical double-layer capacitance of La 1-x Sr x F 3-x increases significantly.

[0037] Figure 3 Shows the capacitance dependence comparison of three dielectric thin films of La 0.95 Sr 0.05 F 2.95 , LaF3, and SrF2. It can be found that for the La 0.95 Sr 0.05 F 2.95 solid solution dielectric thin film compared with the two parent material thin films of LaF3 and SrF2, both its high-frequency capacitance and low-frequency capacitance increase significantly; among them, the low-frequency capacitance increases from 4.5 μF / cm 2 (LaF3) or 6.1 μF / cm 2 (SrF2) to 8.1 μF / cm 2 , and the high-frequency capacitance increases from 0.14 μF / cm 2 (LaF3) or 0.04 μF / cm 2 (SrF2) to 0.27 μF / cm 2 . This shows that doping with heterovalent cations in fluoride to form a solid solution dielectric thin film can significantly improve the capacitive coupling ability of the fluoride thin film.

[0038] To understand this enhanced capacitance value and temperature-dependent capacitive coupling resulting from the increase in the doping level x in the La 1-x Sr x F 3-x thin film, the corresponding frequency-dependent dielectric loss ε″ (the imaginary part of the capacitance) in this dielectric system was subsequently derived, as Figure 4 shown. The ε″-f curve shows a symmetric shape around the central frequency f c , indicating that the charging mechanism of the electrical double-layer capacitance is very consistent with the basic model of the Debye relaxation relationship of static polarization in solid dielectrics. The central frequency decreases significantly as the temperature decreases, which can be explained by τ P = τexp(E a / k Bis explained in the framework of the thermally activated process of (T), where the polarization relaxation time τ P can be derived from the center frequency (τ P = 1 / f c ). The activation energy E a for fluoride ion migration can be derived from the slope in the Arrhenius plot of τ P -T -1 . As the doping level x increases from 0 to 0.09, the activation energy E a significantly decreases from 284 meV to 240 meV, which means that the energy barrier for fluoride ion migration in SrF2-doped LaF3 thin films will be significantly reduced.

Claims

1. A dielectric thin film of a superionic conductor metal fluoride solid solution, characterized in that, Prepared from two or more metal fluoride superionic conductors, and the metal fluoride superionic conductors are selected from scandium fluoride, yttrium fluoride, lanthanum fluoride, cerium fluoride, neodymium fluoride, samarium fluoride, europium fluoride, gadolinium fluoride, holmium fluoride, erbium fluoride, ytterbium fluoride, nickel fluoride, aluminum fluoride, strontium fluoride, calcium fluoride, barium fluoride, manganese fluoride, ferrous fluoride, tin difluoride.

2. The superionic conductor metal fluoride solid solution dielectric film according to claim 1, wherein The conductivity of the thin film is 10 -5 -10 -2 S / cm, the low-frequency capacitance is 0.1 - 100 μF / cm 2 and the high-frequency capacitance is 0.1 - 0.4 μF / cm 2 The leakage current density is less than 10 -5 A / cm 2 , the root mean square of the surface roughness is less than 1 nm, and the fluorine vacancy content is 0.01 - 15%.

3. The preparation method of the superionic conductor metal fluoride solid solution dielectric thin film according to claim 1 or 2, characterized in that, The dielectric film is prepared by thermal evaporation.

4. The preparation method according to claim 3, characterized in that, The process of the preparation method is as follows: It is prepared by using a thermal evaporation system. First, the metal fluoride superionic conductor is ground respectively. After mixing, it is placed on the heating column of the thermal evaporation system. When the system vacuum degree reaches below 10 -5 Pa, evaporation begins. The mixed fluoride evaporates and deposits on the surface of the substrate at a rate of . Until the thickness reaches 10 - 20 nm, the rate is adjusted to until the target thickness is reached. During the evaporation process, the temperature of the substrate is controlled at 200 - 600 K.

5. The preparation method according to claim 4, characterized in that, The metal fluoride superionic conductor is ground to a particle size below 200 mesh.

6. The preparation method according to claim 4, characterized in that, The material of the substrate is SiO2 / Si, Si, Ge, mica, sapphire, ruby, indium phosphide, indium arsenide, gallium phosphide, gallium nitride, strontium titanate, zirconia, silicon carbide, quartz glass.

7. Application of the superionic conductor metal fluoride solid solution dielectric film according to claim 1 or 2 in the preparation of semiconductor devices.

8. The application according to claim 7, characterized in that, The semiconductor device is a field effect transistor, an inverter circuit or a logic gate circuit.

9. A method for improving the capacitive coupling ability of metal fluorides, characterized in that, The method is to prepare a superionic conductor fluoride solid solution dielectric film doped with hetero-valent and / or homo-valent cations by thermal evaporation.

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