Method for forming silicon oxynitride film and method for manufacturing thin film transistor

Abstract

To provide a film formation method for forming a silicon oxynitride film on an oxide semiconductor for improving film formation stability as well as reducing cost.SOLUTION: In a film formation method for a thin film transistor 1 in which a fluorine-containing silicon oxynitride film is formed as a protection layer 8 on a semiconductor layer 5 formed of oxide semiconductor, the fluorine-containing silicon oxynitride film is formed by a plasma CVD method in which SiF4 gas, nitrogen gas, oxygen gas, and hydrogen gas are supplied as process gas. The process gas to be supplied contains the nitrogen gas by 93% or more in flow rate relative to the total flow rate of the nitrogen gas and the oxygen gas.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for forming a silicon oxynitride film and a method for manufacturing a thin film transistor using the method. [Background technology]

[0002] In recent years, thin-film transistors using In-Ga-Zn-O (IGZO) oxide semiconductors as the semiconductor layer (channel layer) have been actively developed. As such thin-film transistors, those in which insulating layers such as a protective layer and a gate insulating layer made of a fluorine-containing silicon oxide film are formed adjacent to the oxide semiconductor are known. For example, Patent Document 1 describes a method in which an insulating layer made of a fluorine-containing silicon oxide film is formed on an oxide semiconductor by a plasma CVD method using SiCl4 gas, SiF4 gas, and oxygen gas as process gases. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2018-195610 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, the manufacturing method disclosed in Patent Document 1 has a problem that the manufacturing cost is high because it uses relatively expensive SiCl4 gas as a process gas. It is therefore conceivable to reduce the manufacturing cost by forming the insulating layer using only SiF4 gas and oxygen gas without using expensive SiCl4 as a process gas, but in this case, there is a problem that the formation of the silicon oxynitride film on the oxide semiconductor becomes unstable (i.e., the film adhesion is poor).

[0005] The present invention has been made to solve the above-mentioned problems at once, and has as its main object to reduce costs and improve film formation stability in a film formation method for forming a silicon oxynitride film on an oxide semiconductor. [Means for solving the problem]

[0006] That is, the film formation method of the present invention is a film formation method for forming a silicon oxynitride film on an oxide semiconductor, and is characterized in that the silicon oxynitride film is formed by a plasma CVD method in which SiF4 gas, nitrogen gas, oxygen gas and hydrogen gas are supplied as process gases, and in the supplied process gases, the ratio of the flow rate of nitrogen gas to the total flow rate of nitrogen gas and oxygen gas is 93% or more. This film formation method uses a relatively inexpensive gas as the process gas in the plasma CVD method, without using expensive SiCl4, and therefore reduces material costs. Furthermore, by setting the ratio of the flow rate of nitrogen gas to the total flow rate of nitrogen gas and oxygen gas supplied as the process gas at 93% or more, and making this ratio very high, a silicon oxynitride film with good insulating properties can be stably formed on an oxide semiconductor. This improves the yield and further reduces the manufacturing cost.

[0007] Although there are still some unclear points as to why the film formation method of the present invention can stably form a silicon oxynitride film on an oxide semiconductor film, the present inventors will explain the mechanism based on the knowledge obtained so far. That is, in the film formation method of the present invention, by setting the ratio of the flow rate of nitrogen gas to the total flow rate of nitrogen gas and oxygen gas supplied as process gas at 93% or more, it is considered that the decomposition of Si-F (binding energy: 5.72 eV), which has a larger binding energy than oxygen gas (binding energy: 5.16 eV) and is difficult to decompose, is promoted, and the formation of a silicon oxynitride film starting from silicon atoms is facilitated. In addition, oxygen gas has a high electronegativity and acts as an additive gas for a gas containing fluorine (F), and it is considered that the outermost surface of the substrate is easily etched. In the CVD process, both surface reactions of film formation and etching proceed in parallel, but by increasing the ratio of nitrogen gas, etching becomes relatively difficult to proceed, and therefore it is considered that a silicon oxynitride film with good insulating properties can be stably formed on an oxide semiconductor. Note that the explanation of this mechanism is not intended to limit the technical scope of the present invention.

[0008] In the film forming method, it is preferable that, in the supplied process gas, a ratio of a flow rate of nitrogen gas to a total flow rate of nitrogen gas and oxygen gas is 96% or more. In this way, the insulating performance of the silicon oxynitride film can be further improved.

[0009] In addition, as a specific embodiment of the silicon oxynitride film formed by the film formation method, the leakage current density is 1×10 -5 A / cm 2 or less, and examples of the dielectric breakdown field strength (maximum field strength) are 3MV / cm or more.

[0010] A specific embodiment of the oxide semiconductor that more significantly exhibits the effects of the film formation method of the present invention is one that is made of In-Ga-Zn-O.

[0011] In an embodiment in which the effect of the film forming method of the present invention is more pronounced, the silicon oxynitride film is formed by plasma CVD at a temperature of 300° C. or lower. At such a low temperature, the film formation method of the present invention can be used to manufacture thin film transistors using a substrate with a low melting point such as a resin, etc. By using the film formation method of the present invention, a thin film transistor having an insulating layer that exhibits good insulating performance even at such a low temperature treatment can be manufactured.

[0012] Furthermore, the method for manufacturing a thin film transistor of the present invention is a method for manufacturing a thin film transistor having an oxide semiconductor made of In-Ga-Zn-O as a channel layer, and is characterized by comprising a semiconductor layer formation step of forming the oxide semiconductor by sputtering, and an insulating layer formation step of forming an insulating layer made of a silicon oxynitride film on the oxide semiconductor by the above-mentioned film formation method. According to such a method for manufacturing a thin film transistor, it is possible to achieve the same effects as those achieved by the above-described film forming method of the present invention. Effect of the Invention

[0013] According to the present invention thus configured, in a method for forming a silicon oxynitride film on an oxide semiconductor, it is possible to reduce costs and improve film formation stability. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a diagram illustrating a configuration of a bottom-gate thin film transistor according to the present embodiment. [Diagram 2] 3A to 3C are diagrams illustrating a manufacturing process of the thin film transistor according to the embodiment. [Diagram 3] FIG. 2 is a diagram showing a schematic configuration of a plasma processing apparatus used in a plasma processing step of a thin film transistor according to the embodiment; [Figure 4] FIG. 11 is a diagram illustrating a configuration of a top-gate type thin film transistor according to another embodiment. [Diagram 5] 13 is a graph showing the relationship between the nitrogen gas flow rate and the stability of film formation in an experimental example. [Figure 6] 13 is a graph showing the relationship between the nitrogen gas flow rate and the film formation rate and refractive index in an experimental example. [Figure 7] 13 is a graph showing the relationship between the nitrogen gas flow rate and insulation properties in an experimental example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] A thin film transistor and a method for manufacturing the same according to one embodiment of the present invention will be described below.

[0016] <1. Thin-film transistor> The thin film transistor 1 of this embodiment is a so-called bottom gate type TFT, and uses an oxide semiconductor for the channel. Specifically, as shown in Fig. 1, it has a substrate 2, a gate electrode 3, a gate insulating layer 4, a semiconductor layer 5, a source electrode 6, a drain electrode 7, and a protective layer 8, which are formed in this order from the substrate 2 side. In this embodiment, the protective layer 8 corresponds to the "insulating layer" as defined in the claims. Each part will be described in detail below.

[0017] The substrate 2 is made of any material that can transmit light, and may be made of, for example, a resin material such as plastics (synthetic resins) such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethersulfone (PES), acrylic, polyimide, etc., or a glass material.

[0018] The gate electrode 3 controls the carrier density in the semiconductor layer 5 by the gate voltage applied to the thin film transistor 1. The gate electrode 3 is made of any material having high conductivity, and may be made of one or more metals selected from, for example, Si, Al, Mo, Cr, Ta, Ti, Pt, Au, Ag, etc. Also, it may be made of a conductive film of a metal oxide such as Al-Nd, Ag alloy, tin oxide, zinc oxide, indium oxide, indium tin oxide (ITO), indium zinc oxide (IZO), In-Ga-Zn-O (IGZO), etc. The gate electrode 3 may be made of a single layer structure of these conductive films or a laminated structure of two or more layers.

[0019] The gate insulating layer 4 is made of any insulating material having high insulating properties, for example, SiO x , SiN x , SiON, Al2O3, Y2O3, Ta2O5, Hf2, etc. The gate insulating layer 4 may be a single-layer structure or a laminate structure of two or more layers of these conductive films.

[0020] The semiconductor layer (channel layer) 5 allows the current flowing between the source electrode 6 and the drain electrode 7 to pass. The semiconductor layer 5 of this embodiment is made of an oxide semiconductor and contains, as a main component, an oxide of at least one element selected from, for example, In, Ga, Zn, Sn, Al, Ti, etc. Specific examples of materials constituting the semiconductor layer 5 include, for example, In-Ga-Zn-O (IGZO), In-Al-Mg-O, In-Al-Zn-O, and In-Hf-Zn-O. This semiconductor layer 5 is composed of an amorphous oxide semiconductor film. The semiconductor layer 5 of this embodiment has a single layer structure, but is not limited thereto, and may have a laminated structure formed by stacking multiple layers having different compositions and crystallinity.

[0021] The source electrode 6 and the drain electrode 7 are formed to be spaced apart from each other so as to partially cover the surface of the semiconductor layer 5. Like the gate electrode 3, the source electrode 6 and the drain electrode 7 are made of a highly conductive material so as to function as an electrode. The source electrode 6 and the drain electrode 7 may have a single-layer structure made of a single material, or may have a laminate structure in which multiple layers made of different materials are stacked on top of each other.

[0022] The protective layer (passivation layer) 8 covers and protects the surface (channel region) of the semiconductor layer 5 exposed between the source electrode 6 and the drain electrode 7, and is made of an insulating material. The protective layer 8 is provided in contact with at least the surface of the semiconductor layer 5. The protective layer 8 of this embodiment is provided so as to further cover the surfaces of the source electrode 6 and the drain electrode 7.

[0023] Specifically, the protective layer 8 is made of a fluorine-containing silicon oxynitride film (SiON:F). The fluorine-containing silicon oxynitride film has a leakage current density of 1×10 -5 A / cm 2 It is preferable that the dielectric breakdown field strength is 3MV / cm or more, and the leakage current density is 1×10 -7 A / cm 2 It is more preferable that the dielectric breakdown field strength is 8 MV / cm or more.

[0024] In addition, a second protective layer made of, for example, a fluorine-containing silicon oxide film (SiN:F), a fluorine-containing silicon oxide film (SiO:F), a silicon nitride film (SiNx), a silicon oxide film (SiOx), or the like may be further provided on the protective layer 8 as necessary.

[0025] <2. Thin-film transistor manufacturing method> Next, a method for manufacturing the thin film transistor 1 having the above-described structure will be described with reference to FIG. The method for manufacturing the thin film transistor 1 of this embodiment includes a gate electrode forming step, a gate insulating layer forming step, a semiconductor layer forming step, a source / drain electrode forming step, a plasma treatment step, and a protective layer forming step. In this embodiment, the protective layer forming step corresponds to the "insulating layer forming step" in the claims. Each step will be described below.

[0026] (1) Gate electrode formation process 2(a), a substrate 2 made of a resin material such as PET is prepared, and a gate electrode 3 is formed on the surface of the substrate 2. The method for forming the gate electrode 3 is not particularly limited, and the gate electrode 3 may be formed by a known method such as a vacuum deposition method.

[0027] (2) Gate insulation layer formation process 2(b), a gate insulating layer 4 is formed so as to cover the surfaces of the substrate 2 and the gate electrode 3. The method for forming the gate insulating layer 4 is not particularly limited, and it may be formed by a known method.

[0028] (3) Semiconductor layer formation process Next, as shown in (c) of FIG. 2, a semiconductor layer 5 is formed on the gate insulating layer 4. This semiconductor layer 5 may be formed by a known method. For example, the semiconductor layer 5 may be formed by sputtering a conductive oxide sintered body such as InGaZnO as a target using inductively coupled plasma. However, the present invention is not limited to this, and the semiconductor layer 5 made of an oxide semiconductor may be formed by other methods.

[0029] (4) Source and drain electrode formation process 2(d), a source electrode 6 and a drain electrode 7 are formed on the semiconductor layer 5. The source electrode 6 and the drain electrode 7 can be formed by a known method using, for example, RF magnetron sputtering. The source electrode 6 and the drain electrode 7 are formed to be spaced apart from each other on the surface of the semiconductor layer 5 and to expose a part of the surface of the semiconductor layer 5.

[0030] (5) Plasma treatment process Here, before forming the protective layer 8 on the surface of the semiconductor layer 5, a plasma treatment (pre-film formation treatment) may be performed on the surface of the semiconductor layer 5. Specifically, this plasma treatment may be performed using an inductively coupled plasma treatment apparatus 100 as exemplified in FIG. 3. Specifically, the plasma treatment apparatus 100 includes a vacuum vessel 20 having a treatment chamber 10 inside which is evacuated to a vacuum and into which a process gas G is introduced, an antenna 30 provided outside the treatment chamber 10, and a high-frequency power supply 40 that applies a high-frequency wave (13.56 MHz) to the antenna 30. When a high-frequency wave is applied from the high-frequency power supply 40 to the antenna 30, a high-frequency magnetic field generated from the antenna 30 is formed in the treatment chamber 10, generating an induced electric field, and thus an inductively coupled plasma P is generated.

[0031] Specifically, in this step, at least nitrogen gas and oxygen gas are supplied as process gas into the processing chamber 10, and in this state, high frequency is applied to the antenna 30 to generate inductively coupled plasma. The process gas supplied here has a ratio of the flow rate of nitrogen gas to the total flow rate of nitrogen gas and oxygen gas (N2 / N2+O2) of preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more. The larger the flow rate ratio of nitrogen gas, the smaller the fixed charge density in the protective layer 8 to be formed later, which is preferable. In addition, this step is preferably performed at a low substrate temperature of 150°C or more and 300°C or less. The processing time for performing the plasma processing is not particularly limited, but is preferably 15 seconds or more and 45 seconds or less from the viewpoint of further reducing the fixed charge density in the protective layer 8. Other factors such as RF power, pressure during film formation, and absolute amount of process gas may be appropriately set. The flow rate of each gas supplied into the processing chamber 10 is controlled by a mass flow controller provided in the gas flow path. That is, the "flow rate" here means the set value in a flow rate control device such as a mass flow controller (the same applies to other processes).

[0032] (6) Protective layer formation process After the plasma treatment step, as shown in (e) of Fig. 2, a protective layer 8 is formed so as to cover the surface of the semiconductor layer 5 exposed between the source electrode 6 and the drain electrode 7. The protective layer 8 is formed by using, for example, the plasma CVD method (chemical vapor deposition method) using the above-mentioned plasma treatment device (hereinafter also referred to as plasma CVD device) 100. Here, the protective layer formation step is started while maintaining the plasma generated in the treatment chamber 10 of the plasma CVD device 100 in the plasma treatment step.

[0033] Specifically, in this protective layer forming step, SiF4 (silicon tetrafluoride) gas, nitrogen gas, oxygen gas, and hydrogen gas are supplied into the processing chamber 10 as process gas, and in this state, high frequency is applied to the antenna 30 to generate inductively coupled plasma. Here, in this embodiment, in the process gas supplied, the ratio of the flow rate of nitrogen gas to the total flow rate of nitrogen gas and oxygen gas (N2 / N2+O2) is 93% or more, and preferably 96% or more. In addition, the upper limit of this ratio is not particularly limited, but is preferably 100%. Note that when the flow rate ratio is 100%, the flow rate of oxygen gas intentionally supplied into the processing chamber 10 is 0 ccm, but even in this case, oxygen gas is inevitably contained in the process gas, so that a fluorine-containing silicon oxynitride film can be formed. In addition, this step is preferably performed at a low substrate temperature of 150°C or more and 300°C or less. In addition, the RF power, the pressure during film formation, the absolute amount of the process gas, etc. may be set appropriately.

[0034] If necessary, a second protective layer made of, for example, a fluorine-containing silicon oxide film (SiN:F), a fluorine-containing silicon oxide film (SiO:F), a silicon nitride film (SiNx), a silicon oxide film (SiOx), or the like may be formed on the protective layer 8. This protective layer can be formed using the plasma CVD apparatus 100, similarly to the protective layer 8.

[0035] (7) Heat treatment process If necessary, the heat treatment may be performed in an atmosphere containing oxygen under atmospheric pressure. The temperature in the furnace during the heat treatment is not particularly limited, and is, for example, 150° C. to 300° C. The heat treatment time is not particularly limited, and is, for example, 1 hour to 3 hours.

[0036] In this manner, the thin film transistor 1 of the present embodiment can be obtained.

[0037] <3. Effects of this embodiment> In the method for manufacturing the thin film transistor 1 of the present embodiment configured as described above, the protective layer 8 is formed by the plasma CVD method using a relatively inexpensive gas as the process gas without using expensive SiCl4 in the protective layer formation step, so that the material cost can be reduced. Furthermore, since the ratio of the flow rate of nitrogen gas to the total flow rate of nitrogen gas and oxygen gas supplied as the process gas is set to 93% or more, which is a very high ratio, the protective layer 8 made of a silicon oxynitride film with good insulating properties can be stably formed on the semiconductor layer 5 made of an oxide semiconductor. This improves the yield and further reduces the manufacturing cost.

[0038] <4. Other Modified Embodiments> The present invention is not limited to the above-described embodiment.

[0039] The thin film transistor 1 in the above embodiment is a bottom gate type in which the gate electrode 3, the gate insulating layer 4, and the semiconductor layer 5 are laminated in this order from the substrate 2 side, but is not limited to this. In another embodiment, as shown in FIG. 4, the thin film transistor 1 may be a top gate type in which the semiconductor layer 5, the gate insulating layer 4, and the gate electrode 3 are laminated in this order from the substrate 2 side. In this case, the gate insulating layer 4 laminated on the semiconductor layer 5 corresponds to the "insulating layer" as referred to in the claims. In this case, the gate insulating layer 4 is made of a fluorine-containing silicon oxynitride film (SiON:F), and has a leakage current density of 1×10 -5 A / cm 2 It is preferable that the dielectric breakdown field strength is 3MV / cm or more, and the leakage current density is 1×10-7 A / cm 2 It is more preferable that the dielectric breakdown field strength is 8 MV / cm or more.

[0040] When the thin film transistor 1 is a top gate type, the manufacturing method thereof includes the steps of forming a semiconductor layer, forming a source / drain electrode, performing a plasma treatment, forming a gate insulating layer, and forming a gate electrode in this order. In this case, the gate insulating layer forming step corresponds to the "insulating layer forming step" in the claims. Therefore, in this embodiment, the gate insulating layer forming process is performed by the plasma CVD method using SiF4 (silicon tetrafluoride) gas, nitrogen gas, oxygen gas and hydrogen gas as process gases. The specific method is the same as that of the protective layer forming process described above.

[0041] In the above embodiment, after the oxide semiconductor layer 5 is formed, the surface of the oxide semiconductor layer 5 is subjected to plasma treatment before the insulating layer (protective layer 8) is formed thereon, but this is not limited to the above. In other embodiments, the insulating layer (protective layer 8) may be formed on the surface of the oxide semiconductor layer 5 without performing plasma treatment on the surface of the oxide semiconductor layer 5.

[0042] In the above embodiment, the semiconductor layer 5 is made of an oxide semiconductor, but is not limited to this. In other embodiments, the semiconductor layer 5 may be made of any semiconductor material, such as amorphous Si or polycrystalline Si.

[0043] Furthermore, the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications are possible without departing from the spirit and scope of the present invention. EXAMPLES

[0044] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples, and can be practiced with appropriate modifications within the scope of the above and below-mentioned aims, and all such modifications are included in the technical scope of the present invention.

[0045] <Experimental Example 1> Relationship between the nitrogen gas flow rate in the process gas and the stability of film formation The relationship between the ratio of the nitrogen gas flow rate to the total flow rate of nitrogen gas and oxygen gas in the process gas in the plasma CVD method (also written as N2 / (N2+O2)) and the stability of film formation on an oxide semiconductor was evaluated.

[0046] Specifically, in this example, the above-mentioned plasma CVD apparatus was used to deposit fluorine-containing silicon oxynitride films on the surfaces of a silicon substrate and an IGZO film by varying the N2 / (N2+O2) ratio in the process gas (from 0% to 100%).Then, the ratio of the film thickness on the IGZO film to the film thickness on the Si substrate (hereinafter also referred to as the film thickness ratio) was measured.

[0047] Specifically, the deposition of each fluorine-containing silicon oxynitride film was performed by supplying SiF, N, O, and H as process gases into the processing chamber of a plasma CVD apparatus, and setting the RF power to 20 kW (0.48 W / cm 2 ), pressure during film formation: 10 Pa, set temperature: 200°C. The flow rates of the gases supplied were SiF4: 500 sccm, H2: 900 sccm, and N2+O2: 3000 sccm. The thickness of the fluorine-containing silicon oxynitride film after film formation was measured using a spectroscopic ellipsometer (Otsuka Electronics, FE-5000S). The measurement results are shown in Figure 5.

[0048] As can be seen from FIG. 5, when the N2 / (N2+O2) ratio in the process gas was in the range of 0% to 50%, the film thickness ratio was less than 0.9 or exceeded 1.1, and the film formation was unstable. On the other hand, when the N2 / (N2+O2) ratio in the process gas was in the range of 93% or more (93.3%, 96.7%, 100%), the film thickness ratio was 0.9 or more and 1.1 or less (i.e., the film thickness difference was within 10%), and a fluorine-containing silicon oxynitride film could be stably formed on the IGZO film.

[0049] <Experimental Example 2> Relationship between the ratio of nitrogen gas flow rate in the process gas and the deposition rate and refractive index Next, the relationship between the ratio of the flow rate of nitrogen gas to the total flow rate of nitrogen gas and oxygen gas in the process gas in the plasma CVD method and the film formation rate and refractive index of the fluorine-containing silicon oxynitride film on the oxide semiconductor was evaluated.

[0050] Specifically, the deposition rate and refractive index were measured for three samples in which films were deposited on an IGZO film in Experimental Example 1 with N2 / (N2+O2) in the process gas set to 93% or more (specifically, 93.3%, 96.7%, and 100%). Specifically, the deposition rate of each sample was calculated by dividing the film thickness measured by the spectroscopic ellipsometer by the deposition time. The refractive index of each sample was also measured by the above spectroscopic ellipsometer. The results are shown in Figure 6.

[0051] As shown in Figure 6, even if the N2 / (N2+O2) ratio in the process gas is set to 93% or more, the deposition rate is good, at 25 nm / min or more. In addition, when the N2 / (N2+O2) ratio in the process gas is set to 93% or more, the refractive index increases significantly as the nitrogen gas flow rate increases, and nitridation proceeds rapidly.

[0052] <Experimental Example 3> Relationship between the ratio of nitrogen gas flow rate in process gas and insulation properties Next, the relationship between the ratio of the flow rate of nitrogen gas to the total flow rate of nitrogen gas and oxygen gas in the process gas in the plasma CVD method and the insulating properties of the fluorine-containing silicon oxynitride film formed on the oxide semiconductor film was evaluated.

[0053] Specifically, in this embodiment, a fluorine-containing silicon oxynitride film was formed on a Si substrate using the above-mentioned plasma CVD apparatus, and an aluminum electrode was formed on the fluorine-containing silicon oxynitride film to create a sample. Here, when forming the fluorine-containing silicon oxynitride film, three types of samples (93.3%, 96.7%, and 100%) were created by changing the N2 / (N2+O2) ratio in the process gas supplied. As shown in FIG. 7, each type of sample was created by changing conditions such as RF power, pressure and temperature during film formation, and process gas flow rate, resulting in a total of six samples. Then, an Al-containing electrode of about 1 mmφ was formed on the film formation surface by resistance heating vacuum deposition, and a current-voltage measurement in the film thickness direction was performed using a digital ultra-high resistance / micro ammeter (Advantest, R8340A) to obtain an IV curve. From the IV curve, the leakage current density (current density at an applied electric field of 3 MV / cm) and dielectric breakdown electric field (leakage current density of 1×10 -5 A / cm 2 The results are shown in Figure 7.

[0054] As shown in Fig. 7, all samples with fluorine-containing silicon oxynitride films in which the N2 / (N2+O2) ratio in the process gas was 93% or more had a dielectric breakdown field of 3 MV / cm or more and a leakage current density of 1×10 -5 A / cm 2 In particular, the samples with fluorine-containing silicon oxynitride films in which the N2 / (N2+O2) ratio in the process gas was 96% or more had a dielectric breakdown field of 8MV / cm or more and a leakage current density of 1×10 -7 A / cm 2 It was confirmed that better insulation properties could be obtained. [Explanation of symbols]

[0055] 1. Thin-film transistor 2. Circuit Board 3. Gate electrode 4 Gate insulating layer 5. Semiconductor layer 6 Source electrode 7 Drain electrode 8...Protective layer

Claims

1. 1. A method for forming a silicon oxynitride film on an oxide semiconductor, comprising the steps of: The silicon oxynitride film is formed by using a SiF ４ The film is formed by a plasma CVD method in which gas, nitrogen gas, oxygen gas, and hydrogen gas are supplied as process gases; A film forming method, characterized in that, in the process gas supplied, the ratio of the flow rate of nitrogen gas to the total flow rate of nitrogen gas and oxygen gas is 93% or more.

2. 2. The film forming method according to claim 1, wherein in the process gas supplied, the ratio of the flow rate of nitrogen gas to the total flow rate of nitrogen gas and oxygen gas is 96% or more.

3. The silicon oxynitride film has a leakage current density of 1×10 －５ A / cm ２ 3. The method for forming a film according to claim 1, wherein the dielectric breakdown field strength is 3 MV / cm or more.

4. 4. The method according to claim 1, wherein the oxide semiconductor is composed of In--Ga--Zn--O.

5. 5. The method according to claim 1, wherein the silicon oxynitride film is formed by the plasma CVD method at a temperature of 300° C. or lower.

6. A method for manufacturing a thin film transistor having an oxide semiconductor made of In-Ga-Zn-O as a channel layer, comprising: a semiconductor layer forming step of forming the oxide semiconductor by sputtering; and forming an insulating layer on the oxide semiconductor layer by the method according to any one of claims 1 to 5.

Images