Method for manufacturing tin(II) oxide thin films

Abstract

The object of this invention is to provide a method for further improving the film formation speed and achieving uniform and well-controlled film formation in a method for manufacturing tin(II) thin films using a divalent tin compound as a precursor and atomic layer deposition (ALD). A method for manufacturing a tin(II) thin film involves using a divalent tin precursor, preferably bis(cyclopentadiene)tin(II) or bis(substituted cyclopentadiene)tin(II) represented by the following general formula (1), as a raw material, and sequentially using water and hydrogen plasma as reactants, to form a tin(II) thin film by atomic layer deposition (in general formula (1), R1 and R2 independently represent alkyl groups with 1 to 6 hydrogen atoms, and m and n represent integers from 1 to 5).

Description

Technical Field

[0001] This invention relates to a method for manufacturing tin(II) oxide thin films using a divalent tin compound as a precursor and by atomic layer deposition (ALD). Prior Technology

[0002] Metal oxides such as indium tin oxide (ITO) and zinc oxide (ZnO), known as oxide semiconductors, are widely used as transparent conductive films and thin-film transistors (TFTs) due to their wide bandgap (Eg) and high conductivity. Among them, indium gallium zinc oxide (IGZO) has a very high carrier mobility, enabling the formation of IGZO-TFTs on large-area substrates in a low-temperature, uniform, and stable manner. Therefore, it has been put into practical use in organic EL (electroluminescence) displays, quantum dot displays, and the like.

[0003] These oxide semiconductors are all n-type, but p-type semiconductors, such as tin(II) oxide (SnO), copper(I) oxide (Cu₂O), or silver(I) oxide (Ag₂O), which utilize holes to induce electrical conduction are also being developed.

[0004] In p-type oxide semiconductors, SnO is the ideal metal oxide for fabricating complementary metal-oxide-semiconductor (CMOS) films that combine p-type TFTs and n-channel TFTs. Other metal oxides have low hole mobility and p-type conductivity because their valence band maximum (VBM) is composed of oxygen 2p orbitals. In contrast, SnO has a band gap of 2.7 eV to 3.4 eV, and its VBM is composed of tin 5s and oxygen 2p orbitals, resulting in higher hole mobility. Furthermore, because SnO is in a quasi-stable state, it is easily oxidized to tin oxide (IV) (SnO₂), which presents a challenge. However, heating in an inert atmosphere causes inhomogenization, resulting in metallic tin (Sn) in the SnO film. This metallic Sn functions as a highly mobile phase, with a hole mobility of 18.7 cm² / Vs.

[0005] Incidentally, p-type TFTs can be fabricated using sputtering, thermal evaporation, thermal processing, and pulsed laser deposition. Among these methods, p-type devices fabricated by atomic layer deposition (ALD) exhibit higher on / off ratios and lower sub-threshold coefficients, thus further reductions in power consumption are anticipated. Currently, development of p-type TFTs using ALD is underway.

[0006] Non-Patent Document 1 describes a method for manufacturing a p-type SnO semiconductor thin film that displays thin film transistor (TFT) characteristics by using tin(II) alkoxide as a raw material gas (precursor) and atomic layer deposition (ALD).

[0007] Since tin amino alkoxide complexes with dialkylamine groups coordinated to tin, and complexes with tin as the backbone and having two cyclopentadienyl coordinating groups are both thermally stable liquids with high vapor pressure, they are known as precursors for tin or tin oxide thin films (Patent Documents 1 to 3).

[0008] Patent document 4 reports an example of using tin(II) aminoalkoxides as precursors for atomic layer deposition (ALD). Specifically, tin(II) dialkylaminoethoxides or tin(II) dialkylaminoalkylethoxides are used as precursors, water is used as the reactant, and an SnO thin film is formed by atomic layer deposition (ALD). In this case, the film formation rate (GPC) per cycle is relatively low, at 0.052 nm / 1 cycle (Example 1), 0.010 nm / 1 cycle (Example 2), and 0.014 nm / 1 cycle (Example 3).

[0009] Other examples include: using tin(II)bis(terpentylalkoxide) (Sn(TAA)2), bis(1-dimethylamino-2-methyl-2-propoxide)tin(II) (Sn(dmamp)2), bis(dimethylamino-2-methyl-2-butoxy)tin(II) (Sn(dmamb)2), bis(N-ethoxy-2,2-dimethylpropionic acid)tin (Sn(edpa)2), bis[bis(trimethylsilyl)amino]tin(II) or N,N'-tert-butyl-1,1-dimethylethylenediaminestanene(II) as precursors, water as reactant, and spatial atomic layer deposition (Spatial ALD) to form p-type tin(II) oxide semiconductor thin films (Non-Patent Document 2). According to Non-Patent Document 2, if Sn(TAA)2 is used in the aforementioned precursor for atmospheric pressure spatial atomic layer deposition, the film deposition time can be shortened compared to the general temporal atomic layer deposition (ALD) method (atomic layer deposition method with time cycles). The film deposition rate (GPC) is 0.055 nm / 1 cycle even at a film deposition temperature of 100°C, which is relatively low. As a result, the film formation rate (GPC) of conventional methods for fabricating tin(II) oxide thin films using atomic layer deposition (ALD) with tin compounds as precursors is relatively low, with a maximum of about 0.06 nm / 1 cycle (Non-Patent Literature 2; Table 1), and thus improvement is desired. [Previous Technical Documents] [Patent Literature]

[0010] [Patent Document 1] Japanese Patent No. 5314468. [Patent Document 2] Japanese Patent Application Publication No. 2021-24846. [Patent Document 3] Japanese Patent Application Publication No. 2021-25121. [Patent Document 4] International Publication No. 2020 / 129616. [Non-patent literature]

[0011] [Non-patent document 1] Daisy E. Gomersall, Kham M. Niang, James D. Parish, Zhuotong Sun, Andrew L. Johnson, Judth L. MacManus-Driscoll and Andrew J. Flewitt, Journal of Materials Chemistry C, 2023, 11, 5740-5749. [Non-patent document 2] Alfredo Mameli, James D. Parish, Tamer Dogan, Gerwin Gelinck, Michael W. Snook, Andrew J. Straiton, Andrew L. Johnson and Auke J. Kronemeijer, Advanced Materials Interfaces, 2022, 9, 2101278. Summary of the Invention

[0012] [The problem that the invention aims to solve]

[0013] The purpose of this invention is to provide a method for rapidly increasing the film formation rate and for uniformly and controllably forming a tin(II) oxide thin film by using an atomic layer deposition (ALD) method with a tin(II) oxide compound as a precursor. [Methods used to solve problems]

[0014] The method for manufacturing tin(II) oxide (SnO) thin films of the present invention uses a tin precursor with a tin valence as a raw material and sequentially uses water and hydrogen plasma as reactants to form tin(II) oxide thin films by atomic layer deposition. The aforementioned tin precursors with a valence of 2 are precursors that are adsorbed by reacting with the OH groups on the surface, that is, precursors that can use water as a reactant, and preferably bis(cyclopentadiene)tin(II) or bis(substituted cyclopentadiene)tin(II) represented by the following general formula (1). [Chemical Formula 1] In general formula (1), R1 and R2 independently represent alkyl groups with 1 to 6 hydrogen atoms or carbon atoms, and m and n represent integers from 1 to 5. The optimal growth rate of tin(II) oxide films is 0.08 nm / cycle or higher. [Invention Benefits]

[0015] According to the present invention, a p-type tin(II) oxide thin film can be uniformly formed by atomic layer deposition (ALD) using a divalent tin precursor and sequentially using water and hydrogen plasma as reactants, with a high film formation rate. Implementation

[0016] The following will provide a detailed description of the method for manufacturing the tin(II)(SnO) thin film of the present invention. The method for manufacturing tin(II) oxide (SnO) thin film of the present invention uses a tin precursor with a tin valence as raw material and sequentially uses water and hydrogen plasma as reactants to form a tin(II) oxide thin film by atomic layer deposition (ALD).

[0017] The divalent tin precursor used in this invention is a compound that reacts with water and OH groups on the substrate surface. There are no limitations on the divalent tin precursor as long as it is a divalent precursor that can utilize water as a reactant; precursors from Patent Document 4, Non-Patent Document 1, and Non-Patent Document 2 can also be used. Among the aforementioned divalent tin precursors, precursors that can easily cleave coordination bonds with water are preferred. Precursors with Sn-N bonds generally tend to retain N, therefore organometallic complexes are preferred.

[0018] As the aforementioned organometallic complex, it is preferred to be bis(cyclopentadiene)tin(II) or bis(substituted cyclopentadiene)tin(II) represented by the following general formula (1). [Chemical Formula 2] In general formula (1), R1 and R2 independently represent alkyl groups with 1 to 6 hydrogen atoms or carbon atoms, and m and n represent integers from 1 to 5.

[0019] Examples of alkyl groups having 1 to 6 carbon atoms include: methyl, ethyl, propyl, isopropyl, butyl, isobutyl, dibutyl, tributyl, pentyl, neopentyl, 3-methylbutyl, 1-methylbutyl, 1-ethylpropyl, 1,1-dimethylpropyl, cyclopentyl, hexyl, 3-methylpentyl, 2-methylpentyl, 2,3-dimethylbutyl, 2,2-dimethylbutyl, and cyclohexyl.

[0020] If the number of carbon atoms exceeds 6, the volume of the divalent tin precursor increases, making it less likely to adhere closely to the substrate surface during atomic layer deposition (ALD), resulting in a lower adsorption amount. Therefore, R1 and R2 are preferably alkyl groups with 1 to 4 carbon atoms, and more preferably alkyl groups with 1 to 3 carbon atoms. From the viewpoint of ease of synthesis, R1 and R2 are preferably both hydrogen or have the same number of carbon atoms, more preferably both have 2 or 3 carbon atoms, and even more preferably both are ethyl groups with 2 carbon atoms.

[0021] The divalent tin precursor used in this invention has thermal stability and high vapor pressure even at low temperatures. The aforementioned divalent tin precursor can be either solid or liquid, provided it can be easily vaporized during atomic layer deposition (ALD). From the viewpoint of improving the efficiency of the ALD process, it is preferable to have it as a liquid at room temperature (approximately 23°C).

[0022] From this perspective, since the vapor pressure of bis(cyclopentadiene)tin(II) or bis(substituted cyclopentadiene)tin(II) represented by general formula (1) is 0.05 to 10 torr at 80°C, it is suitable for forming thin films using atomic layer deposition (ALD). Specifically, in general formula (1), bis(cyclopentadiene)tin(II) in which R1 and R2 are hydrogen is a solid at room temperature and has a vapor pressure of 0.1 torr at 80°C. A particularly preferred form of divalent tin precursor, bis(ethylcyclopentadiene)tin(II), is a liquid at room temperature, with a vapor pressure of 1.2 torr at 80°C and 2.3 torr at 100°C.

[0023] The tin oxide (II) thin film of this invention is formed using atomic layer deposition (ALD). ALD is a technique in which a substrate is placed in a vacuum chamber serving as a reaction chamber, and a thin film is formed on the substrate surface by alternating flow of a precursor gas and a reactant. When a precursor gas is introduced, the precursor gas molecules are saturated and adsorbed onto the substrate surface. Then, when the reactant is flowed, it reacts with the saturated adsorbed molecules to form a thin film. By repeatedly performing the above process, the film thickness can be controlled at the atomic layer level, and a very uniform thin film can be formed on the substrate. When forming the tin oxide (II) thin film on the substrate using ALD, water is generally used as the reactant. This is because when using reactants with strong oxidizing power, such as ozone, there is a risk of over-oxidation, resulting in the formation of a tin oxide (IV) thin film. In the formation of the tin(II) thin film of the present invention, water is used as a reactant. However, to improve the gas deposition rate (GPC), a hydrogen plasma is used in conjunction with water. In the present invention, by using hydrogen plasma in conjunction with water, the GPC of the tin(II) thin film formation becomes 0.08 nm / cycle or higher. Preferably, the GPC is 0.1 nm / cycle or higher, more preferably 0.15 nm / cycle or higher, and even more preferably 0.2 nm / cycle or higher. If the hydrogen plasma treatment is insufficient, the GPC cannot be sufficiently improved. On the other hand, if the hydrogen plasma treatment is excessive, a dense film cannot be obtained, and the apparent GPC increases. Therefore, the GPC is preferably 1 nm / cycle or lower, more preferably 0.5 nm / cycle or lower, and even more preferably 0.3 nm / cycle or lower.

[0024] Atomic layer deposition (ALD) is broadly classified into thermal atomic layer deposition (TLD) and plasma atomic layer deposition (PLD). The method for manufacturing tin(II) thin films of this invention uses water and hydrogen plasma sequentially as reactants, thus it is a plasma atomic layer deposition method. In plasma atomic layer deposition, since the tin precursor (II valence) and reactant gas are plasma-excited, the reactivity is high, and the reaction can proceed even at temperatures ranging from room temperature to 100°C. As mentioned above, when using reactants with strong oxidizing power such as ozone, there is a risk of over-oxidation and the formation of tin(IV) thin films. Therefore, in the method for manufacturing tin(II) thin films of this invention, an oxidant with a stronger oxidizing power than water is not used.

[0025] The substrate can be made of materials such as silicon, silicon nitride, zirconium dioxide, titanium(II) oxide, and titanium nitride. The substrate can be in various shapes such as plate, sphere, and flake, and its size is not particularly limited. Preferred materials are silicon wafers and silicon carbide wafers, which are easy to fabricate into semiconductor devices.

[0026] The pressure inside the vacuum container is typically between 1 Pa and 10,000 Pa, preferably between 10 Pa and 1,000 Pa. The reaction temperature is typically from room temperature to 300°C, such as 250°C, 200°C, or 100°C, and can be determined depending on the type of tin precursor with a valence of I, the reactants, etc. The reaction temperature needs to be lower than the thermal decomposition temperature of the tin precursor with a valence of I, and the degree of complete reaction with the reactants is crucial. Furthermore, the reaction temperature is the same as the substrate temperature.

[0027] In the plasma atomic layer deposition method of the present invention, the following steps (1) to (6) are performed as 1 cycle, and the cycle is usually adjusted to be between 10 and 10,000 cycles depending on the thickness of the tin oxide (II) film. (1) The vaporized tin precursor is introduced into a vacuum container and adsorbed onto the substrate. (2) Use inert gas to purge and remove unwanted tin(II) precursors. (3) Water is introduced into a vacuum container and reacted with the tin(II) precursor to deposit tin(II) oxide. (4) Drive inert gas to remove unreacted water and byproduct gas. (5) The hydrogen plasma is introduced into the vacuum container to perform the surface treatment of tin oxide (II). (6) Drive inert gas to remove unreacted hydrogen plasma and byproduct gas.

[0028] In (3) above, water is supplied to the vacuum container to react with the tin precursor of the 2-valent form, and SnO is deposited on the substrate surface. If the tin precursor of the 2-valent form reacts completely with the water, a SnO film is formed. However, since the adsorption force of the tin precursor on the SnO film is much weaker than that on the SnO2 film, it is not easy for the tin precursor to be adsorbed onto the SnO film. This is the reason for the low deposition rate (GPC) in the conventional atomic layer deposition (ALD) method for tin(II) oxide thin films. Therefore, in this invention, after supplying water in (3) above, hydrogen plasma is introduced into the vacuum container in (5) above, thereby making the outermost surface of the SnO film OH-based, which makes it easier to adsorb the tin precursor. That is, by making the outermost surface OH-based, the tin precursor that reacts with the OH-based groups of water reacts with the OH-based groups on the outermost surface and is adsorbed. At this point, the output and duration of the hydrogen plasma are not limited as long as they meet the conditions for OH grouping of the outermost surface of the SnO film, preferably 1 kW to 100 kW and 0.1 seconds to 30 seconds. If the output is low and the duration is short, the surface cannot be sufficiently formed with OH groups; if the output is too high or the duration is too long, plasma damage will occur on the substrate, which is undesirable. Furthermore, the hydrogen plasma is a plasma containing hydrogen gas, which can be a mixture of gas with inert gases such as argon. Within the scope that does not impair the effectiveness of the present invention, it can also be a mixture of gas containing nitrogen such as nitrogen or ammonia, or a mixture of gas containing other elements, preferably hydrogen or a mixture of hydrogen and inert gases. From another perspective, since any divalent tin precursor reacts with water, it is not limited to bis(ethylcyclopentadiene)tin(II) of the examples, and the present invention can be applied to various divalent tin precursors. In the present invention, by using water and hydrogen plasma sequentially as reactants, the growth rate (GPC) per cycle increases dramatically. This is evident from the following results: the maximum growth rate (GPC) of atomic layer deposition (ALD) of divalent Sn complexes described in Non-Patent Document 2 is approximately 0.06 nm / cycle (see Table 1), the growth rate (GPC) of Non-Patent Document 1 is 0.018 nm / cycle (see Figure 3(c) for example), and in contrast, in the embodiments of the present invention, it is approximately 0.3 nm / cycle.

[0029] In steps (2), (4), and (6) above, an inert gas is purged into the vacuum container to remove vapors (gases) or byproduct gases generated in each step. This purging step can be omitted depending on the conditions. The inert gas is argon, helium, or nitrogen, etc. The purging can be carried out under a reduced pressure of 0.01 Pa to 100 Pa.

[0030] Furthermore, the divalent tin precursor and reactant can be supplied as gases from the outside to the vacuum container on which the substrate is mounted, but sublimation or evaporation must be carried out at a temperature lower than the substrate temperature in a manner that prevents condensation on the substrate. In this case, it can be said that if the divalent tin precursor is solid at room temperature, it is difficult to control the sublimation rate; if the divalent tin precursor is liquid, the heat transfer from the container is good, so the evaporation rate can be easily controlled, making it suitable for atomic layer deposition (ALD). [Example]

[0031] The present invention will be further described in detail below based on embodiments, but the present invention is not limited to the following embodiments. [Example 1] Atomic layer deposition (ALD) film formation is performed on silicon wafers coated with a natural oxide film using a showerhead-type ALD apparatus equipped with direct plasma. The silicon wafer in the reaction chamber is set to a temperature of 200°C, and the feedstock is bis(ethylcyclopentadiene)tin(II) (Sn(EtCp)₂), which is vaporized at 70°C and supplied to the reaction chamber. Water and 100W hydrogen plasma are used as reactants, and an ABC-type ALD film formation process is performed with 30 cycles of 10 seconds of feedstock, 30 seconds of water as reactant, and 5 seconds of hydrogen plasma per cycle. The thickness of the obtained tin oxide film was measured using a spectrophotometer and was approximately 8.8 nm. The growth rate (GPC) at this time was approximately 0.29 nm / cycle.

[0032] [Example 2] Except for setting the number of cycles to 15, atomic layer deposition (ALD) was performed in the same manner as in Example 1, resulting in a tin oxide film with a thickness of approximately 4.3 nm. The growth rate at this time was almost the same as in Example 1, at approximately 0.29 nm / 1 cycle, completing ALD without initial nucleus formation.

[0033] [Comparative Example 1] Except that the reaction agent was set to consist only of water for 30 seconds, atomic layer deposition (ALD) was performed in the same manner as in Example 1, resulting in a tin oxide film with a thickness of approximately 1.2 nm. The growth rate at this time was approximately 0.04 nm / cycle, which is very low compared to Example 1.

[0034] [Comparative Example 2] Except for setting the number of cycles to 50, atomic layer deposition (ALD) was performed in the same manner as in Comparative Example 1, resulting in a tin oxide film with a thickness of approximately 1.2 nm. This film thickness is almost the same as that obtained after 30 cycles, and after 30 cycles, the film growth was almost negligible.

Claims

1. A method for manufacturing a tin(II) thin film, wherein a tin precursor of divalent valence is used as a raw material, wherein the tin precursor of divalent valence is bis(cyclopentadiene)tin(II) or bis(substituted cyclopentadiene)tin(II) represented by the following general formula (1), and water and hydrogen plasma are used sequentially as reactants to form a tin(II) thin film by atomic layer deposition; the growth rate of the tin(II) thin film is 0.08 nm / 1 cycle or more; in the general formula (1), R1 and R2 independently represent alkyl groups having 1 to 6 hydrogen atoms or carbon atoms, and m and n represent integers from 1 to 5.

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