Method for producing tin(II) oxide thin film

Abstract

To provide a method for producing a tin(II) oxide thin film by atomic layer deposition (ALD) using a II-valent tin compound as a precursor, which further improves the film formation rate and uniformly forms the film with high controllability.SOLUTION: This method for producing a tin(II) oxide thin film is characterized by forming a tin(II) oxide thin film by atomic layer deposition using, as raw material, a II-valent tin precursor, preferably biscyclopentadienyltin(II) or bis-substituted cyclopentadienyltin(II) represented by general formula (1), and using water and hydrogen plasma as reactants in the stated order. (In general formula (1), R1 and R2 each independently represent a hydrogen atom or a C1-6 alkyl group, and m and n each represent an integer of 1-5.)SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing tin(II) oxide thin films by atomic layer deposition (ALD) using divalent tin compounds as precursors. [Background technology]

[0002] Metal oxides called oxide semiconductors, such as indium tin oxide (ITO) and zinc oxide (ZnO), are widely used as transparent conductive films and thin film transistors (TFTs) due to their wide band gap (Eg) and high conductivity. Indium gallium zinc oxide (IGZO) has a very high carrier mobility, and IGZO-TFTs can be formed uniformly and stably at low temperatures on large-area substrates, so it has been put to practical use in organic electroluminescence displays and quantum dot displays.

[0003] All of these oxide semiconductors are n-type, but p-type oxides, such as tin(II) oxide (SnO), copper(I) oxide (CuO) and silver(I) oxide (AgO), that use holes to conduct electricity have also been developed.

[0004] Among p-type oxide semiconductors, SnO is a promising metal oxide for fabricating p-type TFTs and complementary metal oxide semiconductors (CMOS) combined with n-channel TFTs. Other metal oxides have low hole mobility and p-type conductivity because the top of the valence band (VBM) is composed of oxygen 2p orbitals. However, SnO has a band gap of 2.7 to 3.4 eV, but the VBM of SnO is composed of tin 5s and oxygen 2p orbitals, so it has a relatively high hole mobility. In addition, since SnO is in a metastable state, it is easily oxidized to tin (IV) oxide (SnO2), which can be said to be a drawback. However, when heated in an inert atmosphere, disproportionation occurs and metallic tin (Sn) is contained in the SnO thin film. This metallic Sn acts as a high mobile phase, and the hole mobility is 18.7 cm 2 It can also be / Vs.

[0005] By the way, p-type TFTs can be fabricated by sputtering, thermal evaporation, thermal processing, pulsed laser deposition, etc. Among these, p-type devices fabricated by ALD show a high on-off ratio and a low subthreshold coefficient, so in the hope of further reducing power consumption, p-type TFTs using ALD have recently been developed.

[0006] Non-Patent Document 1 describes a method for producing a p-type SnO semiconductor thin film exhibiting thin film transistor (TFT) characteristics by atomic layer deposition (ALD) using di(t-butoxy)tin(II), which is a tin(II) alkoxide, as a raw material gas (precursor).

[0007] Tin aminoalkoxide complexes in which a dialkylamino group is coordinated to tin, and complexes having a tin skeleton and two cyclopentadienyl ligands are both thermally stable liquids with high vapor pressures, and are therefore known as precursors for tin or tin oxide thin films (Patent Documents 1 to 3).

[0008] Patent Document 4 reports an example in which tin(II) aminoalkoxide is used as a precursor for ALD. Specifically, a SnO thin film is formed by ALD using tin(II) dialkylaminoethoxide or tin(II) dialkylaminoalkylethoxide as a precursor and water as a reactant. In this case, the film formation rate (GPC) per cycle is low, at 0.052 nm / cycle (Example 1), 0.010 nm / cycle (Example 2), and 0.014 nm / cycle (Example 3).

[0009] There is also an example where a p-type tin(II) oxide semiconductor thin film was formed by Spatial ALD using tin(II) bis(t-amyl oxide) (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-dimethylpropanamido)tin (Sn(edpa)2), bis[bis(trimethylsilyl)amino]tin(II) or N,N'-t-butyl-1,1-dimethylethylenediaminestannylene(II) as a precursor and water as a reactant (Non-Patent Document 2). According to Non-Patent Document 2, when spatial ALD is performed at atmospheric pressure using Sn(TAA)2 among the precursors, the film formation time can be shortened compared to general temporal ALD (ALD that cycles by time), but the GPC is low at 0.055 nm / cycle even when the film formation temperature is 100°C. Thus, in the conventional method of producing a tin(II) oxide thin film by atomic layer deposition (ALD) using a divalent tin compound as a precursor, the GPC is low at a maximum of about 0.06 nm / cycle (Non-Patent Document 2, Table 1), and improvement is desired. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Patent No. 5314468 [Patent Document 2] Patent Publication No. 2021-24846 [Patent Document 3] Patent 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 [Problem to be solved by the invention]

[0012] The present invention aims to provide a method for producing a tin(II) oxide thin film by atomic layer deposition (ALD) using a divalent tin compound as a precursor, which dramatically improves the film formation rate and forms a uniform film with good controllability. [Means for solving the problem]

[0013] The method for producing a tin(II) oxide (SnO) thin film of the present invention uses a divalent tin precursor as a raw material and water and hydrogen plasma, in that order, as reactants to form a tin(II) oxide thin film by atomic layer deposition. The divalent tin precursor is a precursor that reacts with and is adsorbed onto OH groups on the surface, i.e., a precursor in which water can be used as a reactant, and is particularly preferably biscyclopentadienyltin(II) or bis-substituted cyclopentadienyltin(II) represented by the following general formula (1): [ka] In general formula (1), R 1 and R 2 each independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, and m and n each represent an integer of 1 to 5. The growth rate of the tin (II) oxide thin film is preferably 0.08 nm / cycle or more. Effect of the Invention

[0014] According to the present invention, a divalent tin precursor is used, and a p-type tin(II) oxide thin film can be formed uniformly at a high deposition rate by atomic layer deposition (ALD) using water and hydrogen plasma as reactants, in that order. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The method for producing a tin(II) oxide (SnO) thin film of the present invention will be described in detail below. The method for producing a tin(II) oxide (SnO) thin film of the present invention uses a divalent tin precursor as a raw material and water and hydrogen plasma, in that order, as reactants to form a tin(II) oxide thin film by atomic layer deposition (ALD).

[0016] The divalent tin precursor used in the present invention is a compound that reacts with water or OH groups on the substrate surface. The divalent tin precursor is not limited as long as it is a divalent precursor that can use water as a reactant, and the precursors in Patent Document 4, Non-Patent Document 1, and Non-Patent Document 2 can also be used. Among the divalent tin precursors, a precursor whose coordinate bond is easily broken by water is preferred. In general, a precursor having a bond between Sn and N is preferred as an organometallic complex because N is likely to remain.

[0017] The organometallic complex is preferably biscyclopentadienyltin(II) or bis-substituted cyclopentadienyltin(II) represented by the following general formula (1). [ka] In general formula (1), R 1and R 2 each independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, and m and n each represent an integer of 1 to 5.

[0018] Examples of the alkyl group having 1 to 6 carbon atoms include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a s-butyl group, a t-butyl group, a pentyl group, a neopentyl group, a 3-methylbutyl group, a 1-methylbutyl group, a 1-ethylpropyl group, a 1,1-dimethylpropyl group, a cyclopentyl group, a hexyl group, a 3-methylpentyl group, a 2-methylpentyl group, a 2,3-dimethylbutyl group, a 2,2-dimethylbutyl group, and a cyclohexyl group.

[0019] If the number of carbon atoms exceeds 6, the divalent tin precursor becomes bulky, making it difficult for it to adhere to the substrate surface during ALD film formation, resulting in a small amount of adsorption. 1 and R 2 is preferably an alkyl group having 1 to 4 carbon atoms, and more preferably an alkyl group having 1 to 3 carbon atoms. R 1 and R 2 From the viewpoint of ease of synthesis, it is preferable that all of are hydrogen or have the same number of carbon atoms, more preferably all have 2 or 3 carbon atoms, and particularly preferably all are ethyl groups having 2 carbon atoms.

[0020] The divalent tin precursor used in the present invention has thermal stability and a high vapor pressure even at low temperatures. The divalent tin precursor may be solid or liquid as long as it can be easily vaporized during ALD, but from the viewpoint of the efficiency of the ALD process, it is preferable that the divalent tin precursor is liquid at room temperature (about 23° C.).

[0021] In this respect, the vapor pressure of biscyclopentadienyltin(II) or bis-substituted cyclopentadienyltin(II) represented by the general formula (1) is 0.05 to 10 torr at 80° C., and therefore it is suitable for thin film formation by ALD. 1 and R 2Biscyclopentadienyltin(II), where R is 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(ethylcyclopentadienyl)tin(II), is a liquid at room temperature and has a vapor pressure of 1.2 torr at 80° C. and 2.3 torr at 100° C.

[0022] The tin(II) oxide thin film of the present invention is formed by atomic layer deposition (ALD). ALD is a technology in which a substrate is placed in a vacuum vessel, which is a reaction chamber, and a precursor gas and a reactant are alternately flowed to form a thin film on the substrate surface. When a precursor gas is introduced, the precursor gas molecules are saturated and adsorbed on the substrate surface, and then when a reactant is flowed, it reacts with the saturated and adsorbed molecules to form a thin film. By repeating this process, a thin film can be formed on the substrate very uniformly while controlling the film thickness at the atomic layer level. When a tin(II) oxide thin film is formed on a substrate by ALD, water is generally used as a reactant. This is because if a highly oxidizing reactant such as ozone is used, there is a risk that it will be excessively oxidized and a tin(IV) oxide thin film will be formed. The tin(II) oxide thin film formation of the present invention also uses water as a reactant, but is characterized in that hydrogen plasma is used after water to improve GPC. In the present invention, by using hydrogen plasma after water, the GPC of the tin(II) oxide thin film formation is 0.08 nm / cycle or more. The GPC at this time is preferably 0.1 nm / cycle or more, more preferably 0.15 nm / cycle or more, and particularly preferably 0.2 nm / cycle or more. If the hydrogen plasma treatment is insufficient, the GPC is not sufficiently improved. On the other hand, if the hydrogen plasma treatment is performed excessively, a dense film cannot be obtained, and the apparent GPC becomes large. For this reason, the GPC is preferably 1 nm / cycle or less, more preferably 0.5 nm / cycle or less, and particularly preferably 0.3 nm / cycle or less.

[0023] ALD is broadly classified into thermal ALD and plasma ALD, but the method for producing a tin(II) oxide thin film of the present invention is plasma ALD because it uses water and hydrogen plasma in this order as reactants. In plasma ALD, divalent tin precursor and reactant gas are plasma-excited and used, so the reactivity is high and the reaction proceeds even at room temperature to 100°C. As described above, when a reactant with a strong oxidizing power such as ozone is used, there is a risk of excessive oxidation resulting in the formation of a tin(IV) oxide thin film, so an oxidizing agent with a stronger oxidizing power than water is not used in the method for producing a tin(II) oxide thin film of the present invention.

[0024] The substrate may be made of, for example, silicon, silicon nitride, zirconium dioxide, titanium (II) oxide, titanium nitride, etc. The substrate may have various shapes such as a plate, a sphere, a scale, etc., and is not particularly limited in size, but is preferably a silicon wafer or a silicon carbide wafer, which is easy to fabricate a semiconductor device on.

[0025] The pressure inside the vacuum chamber is usually 1 Pa to 10,000 Pa, and preferably 10 Pa to 1,000 Pa. The reaction temperature is usually room temperature to 300°C, and is determined depending on the type of divalent tin precursor, the reactant, etc., such as 250°C, 200°C, 100°C, etc. The reaction temperature must be lower than the thermal decomposition temperature of the divalent tin precursor and high enough to sufficiently react with the reactant. The reaction temperature is the same as the substrate temperature.

[0026] In the plasma ALD of the present invention, the following steps (1) to (6) constitute one cycle, which is usually repeated within a range of 10 to 10,000 cycles depending on the thickness of the tin(II) oxide thin film. (1) A gasified divalent tin precursor is introduced into a vacuum chamber and adsorbed onto a substrate. (2) Purge with an inert gas to remove unwanted tin(II) precursor. (3) Water is introduced into the vacuum chamber and reacted with the tin(II) precursor to deposit tin(II) oxide. (4) Purge with an inert gas to remove unreacted water and by-product gases. (5) Hydrogen plasma is introduced into the vacuum chamber to perform surface treatment of tin(II) oxide. (6) Purge with inert gas to remove unreacted hydrogen plasma and by-product gases.

[0027] In the above (3), water is supplied into the vacuum chamber, and SnO is deposited on the substrate surface by reacting with the divalent tin precursor. When the divalent tin precursor and water completely react, they become an SnO film, but 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 to adsorb the tin precursor on the SnO film. This is the reason why the GPC is low in the conventional ALD of tin (II) oxide thin films. Therefore, in the present invention, after supplying water in the above (3), hydrogen plasma is introduced into the vacuum chamber in the above (5) to convert the outermost surface of the SnO film into OH, making it easier to adsorb the tin precursor. That is, by converting the outermost surface into OH groups, the tin precursor that reacts with the OH groups of water reacts with the OH groups on the outermost surface and is adsorbed. At this time, there is no limitation on the power and time of the hydrogen plasma as long as the outermost surface of the SnO film can be converted into OH, but it is preferably 1 to 100 kW and 0.1 to 30 s. If the output is low and the time is short, the surface cannot be sufficiently converted into OH groups, and if the output is too high or the time is too long, plasma damage occurs to the substrate, which is not preferable. The hydrogen plasma is a plasma of a gas containing hydrogen gas, and may be a mixed gas with an inert gas such as argon, or may be a mixed gas with a gas containing N such as nitrogen or ammonia, or a gas containing other elements, as long as it does not impair the effects of the present invention, but hydrogen gas or a mixed gas of hydrogen and an inert gas is preferable. Considering this in reverse, the divalent tin precursor may be any precursor that reacts with water, and therefore the present invention can be applied to various divalent tin precursors, not limited to bis(ethylcyclopentadienyl)tin(II) in the examples. In the present invention, the deposition rate (GPC) per cycle is dramatically increased by using both water and hydrogen plasma as reactants in this order. This is evident from the result that the growth rate (GPC) of the ALD of the divalent Sn complex described in Non-Patent Document 2 is at most about 0.06 nm / cycle (see Table 1), whereas the GPC described in Non-Patent Document 1 is 0.018 nm / cycle (see, for example, FIG. 3(c)), and the GPC in the examples of the present invention is about 0.3 nm / cycle.

[0028] In the above (2), (4) and (6), the vacuum vessel is purged with an inert gas to remove the vapor (gas) or by-product gas generated in each process, but this purging process may be omitted depending on the conditions. The inert gas may be argon, helium, nitrogen, etc. Purging may be performed under a reduced pressure of 0.01 to 100 Pa.

[0029] The divalent tin precursor and reactant are supplied as gas from the outside to the vacuum chamber in which the substrate is placed, but they need to be sublimated or evaporated at a temperature lower than the substrate temperature so as not to condense on the substrate. If the divalent tin precursor is a solid at room temperature, it is difficult to control the sublimation rate, but if the divalent tin precursor is liquid, the heat transfer from the chamber is good, so the evaporation rate can be easily controlled, making it suitable for ALD. EXAMPLES

[0030] The present invention will now be described in more detail with reference to examples, but the present invention is not limited to the following examples. [Example 1] ALD film formation was performed on silicon wafers with native oxide films using a showerhead-type ALD device equipped with direct plasma. The temperature of the silicon wafer placed in the reaction chamber was set to 200°C, and the raw material used was bis(ethylcyclopentadienyl)tin(II) (Sn(EtCp)2), which was maintained at 70°C and vaporized before being supplied to the reaction chamber. Water and 100W hydrogen plasma were used as reactants, and 30 cycles of ABC-type ALD film formation were performed, with one cycle consisting of 10 seconds of raw material, 30 seconds of water as reactant, followed by 5 seconds of hydrogen plasma. The thickness of the resulting tin oxide thin film was measured by a spectroscopic ellipsometer to be about 8.8 nm, with a growth rate (GPC) of about 0.29 nm / cycle.

[0031] [Example 2] ALD film formation was performed in the same manner as in Example 1, except that the number of cycles was set to 15, and a tin oxide thin film with a thickness of about 4.3 nm was obtained. The growth rate at this time was about 0.29 nm / cycle, which was almost the same as in Example 1, and ALD without initial nucleation was performed.

[0032] [Comparative Example 1] ALD film formation was performed in the same manner as in Example 1, except that the reactant was water for 30 seconds, and a tin oxide thin film with a thickness of about 1.2 nm was obtained. The growth rate at this time was about 0.04 nm / cycle, which is significantly lower than that of Example 1.

[0033] [Comparative Example 2] ALD film formation was performed in the same manner as in Comparative Example 1, except that the number of cycles was 50. A tin oxide thin film with a thickness of about 1.2 nm was obtained. This thickness was almost the same as the thickness after 30 cycles, and almost no film grew after the 30th cycle.

Claims

1. A method for producing a tin(II) oxide thin film, comprising the steps of: forming a tin(II) oxide thin film by atomic layer deposition using a divalent tin precursor as a raw material and water and hydrogen plasma, in that order, as reactants.

2. 2. The method for producing a tin(II) oxide thin film according to claim 1, wherein the divalent tin precursor is biscyclopentadienyltin(II) or bis-substituted cyclopentadienyltin(II) represented by the following general formula (1): 【Chemistry 1】 (In general formula (1), R 1 and R 2 each independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, and m and n each represent an integer of 1 to 5.

3. 3. The method for producing a tin (II) oxide thin film according to claim 1, wherein the growth rate of the tin (II) oxide thin film is 0.08 nm / cycle or more.

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