JP2594384B2 - Metal oxide thin film, method of manufacturing the same, and electronic device using the metal oxide thin film - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a metal oxide thin film having no defect and excellent insulating properties, a method for manufacturing the same, and an electronic device using the metal oxide thin film.

2. Description of the Related Art Insulating metal oxide thin films have been conventionally used in many electronic devices, and their importance has been further increased, and further higher performance has been demanded. For example, various metal oxide thin films have attracted attention as gate insulating films of MOS transistors and TFTs or as insulating layers of thin-film EL displays (ELDs). TFTs used for switching active matrix liquid crystal displays (LCDs)
In this case, the performance of the gate insulating film affects the on / off property of the TFT, and as a result, the quality of the display display is largely affected. In addition, flat displays such as LCDs and ELDs are required to be formed with a defect-free and high-performance insulating film over a large area.

Conventionally, methods for forming metal oxide thin films include electron beam evaporation, sputtering, and chemical vapor deposition (CV).
D) method is used. However, for example, in a vacuum deposition method, a film having oxygen deficiency is easily formed and a dense film is hardly formed, so that a leak current is large and cannot be used for a TFT gate insulating film or the like. In addition, the sputtering method and the CVD method may cause defects such as pinholes, which may cause variations in characteristics within the film or decrease the withstand voltage due to local electric field concentration on the defects when a device is formed. And other problems.

On the other hand, there is a method in which a metal thin film is first formed on a substrate, and the surface of the metal thin film is oxidized by a method such as anodic oxidation or plasma oxidation to form a metal oxide thin film. In these surface oxidation methods, oxidation proceeds from the surface of the metal thin film. Therefore, even if there is a pinhole in the metal thin film, the pinhole portion is selectively oxidized. There is an excellent advantage that a material film can be formed.

However, it has been newly found that the characteristics of the metal oxide thin film formed by the surface oxidation method as described above strongly depend on the crystal structure of the metal thin film before oxidation. In other words, it has been clarified that even if the crystallinity of the metal thin film is improved, the electric characteristics are not necessarily improved, but the leakage current increases.

In view of the above, an object of the present invention is to provide a metal oxide thin film having excellent electric characteristics.

Means for Solving the Problems The present invention forms a metal thin film having a mixture of two or more different crystal structures on a substrate and having a plurality of crystal grain boundaries in a thickness direction, and at least from the surface of the obtained metal thin film. By oxidizing a part of the metal thin film in a thickness direction, an insulating metal oxide thin film having excellent electric characteristics is formed on the substrate.

Function As described above, the present invention causes many crystal grain boundaries to exist in a metal thin film by mixing different crystal structures. When a metal oxide thin film is formed by oxidizing a metal surface having many such crystal grain boundaries, a low-resistance portion in the metal oxide thin film caused by the crystal grain boundaries before oxidation is finely cut, and as a result, It is believed that the leakage current is greatly reduced.

Further, the metal thin film as described above has many relatively small crystal grains inside. Such a metal thin film is dense and has excellent surface flatness, and the surface of the metal oxide thin film formed by oxidizing the surface, or the flatness of the interface with the metal layer remaining on the base is also excellent, It is considered that local electric field concentration due to unevenness of the electrode interface is unlikely to occur, and the leak current is also reduced.

Example 1 FIG. 1 shows a cross section of a TFT portion of an active matrix type LCD in which the metal oxide thin film of the present invention is applied to a TFT gate insulating film. In FIG. 1, reference numeral 11 denotes a light-transmitting insulating substrate, and reference numeral 12 denotes a gate electrode made of tantalum metal. The gate electrode 12 is formed in a stripe shape by a wet etching method or a dry etching method after a tantalum metal thin film is formed on the entire surface of the insulating substrate 11 by a sputtering method. The gate electrode 12 formed in a stripe shape is oxidized by a predetermined thickness on the surface using an anodic oxidation method to form a gate insulating film 13 made of a tantalum oxide thin film. A second gate insulating film 14 is formed on the gate insulating film 13 by a plasma CVD method in order to increase the adhesion with the semiconductor layer 15, and then a semiconductor layer made of amorphous silicon is formed.
15 is formed into a film. An etching stopper layer 16 made of silicon nitride is formed on the semiconductor layer 15. The etching stopper layer 16 is formed by dry-etching an electrode layer having the same material configuration (in the present embodiment, an ohmic contact layer made of n ⁺ silicon and a laminated film of molybdenum silicide and chromium formed by a plasma CVD method). This is necessary when separating the source electrode 17 and the drain electrode 18 from each other.

FIG. 2 shows an overview of an in-line type sputtering apparatus used for producing a tantalum metal thin film for the gate electrode 12. As shown in FIG. This apparatus includes a load lock chamber 31 for storing a tray 42 on which a substrate 41 before film formation is placed, a sputter chamber 35 for performing film formation, and an unload lock chamber 48 for storing the tray 42 after film formation. It is configured. The sputter chamber 35, the load lock chamber 31, and the unload lock chamber 48 are separated by gate valves 34 and 47, respectively.
This can be performed while maintaining 35 in a vacuum state. A cathode 44 is mounted in the sputtering chamber 35, and a tantalum metal target 43 is bonded on the cathode 44. A permanent magnet is attached inside the cathode 44,
The discharge is a magnetron sputtering method. An argon gas controlled at a predetermined flow rate is flowed from a gas inlet 37 into a sputtering chamber 35 by a flow meter 36 to adjust the pressure to a predetermined pressure. Then, RF power is applied to the cathode 44 to start discharging. After sputtering is performed for about 10 minutes with a predetermined sputtering power in the matching circuit 45, the tray 42 is sent from the load lock chamber 31 into the sputtering chamber 35. While the tray 42 passes over the target 43 at a predetermined speed, the substrate 41 placed on the tray 42
A tantalum thin film is deposited on the surface, sent into the unload lock chamber 48 and stored. The film formation can be performed while the tray 42 is stopped at a desired position in the sputtering chamber 35.

A tantalum metal thin film used for the gate electrode 12 shown in FIG. 1 was prepared using the in-line type sputtering apparatus shown in FIG. 2, and the X-ray (CuKα) diffraction peak intensity was measured. FIG. 7A shows a case where a film is formed while moving the tray 42, and FIG.
7 shows the X-ray diffraction peaks of each tantalum thin film when the film was formed while the film was stopped immediately above the cathode 44. The film formation conditions are the same as those shown in FIGS.
80SCCM, sputtering pressure 10mTorr, RF power 2.0kW, substrate temperature 200
° C. FIG. 3A shows the tray conveyance speed, and FIG. 3B shows the sputtering time by controlling the sputtering time.
In both cases, the thickness of the tantalum thin film was set to 80 nm. As is clear from the figure, the tantalum thin film in FIG. 4B is a crystal consisting of only the β phase (002) plane having a peak near 33.4 degrees, whereas the tantalum thin film in FIG. Degree and 38.2
It is a film in which different crystal structures having two peaks near the degree are mixed. The peak around 38.2 degrees is the tantalum α
Phase (110) plane only, or α phase (110) plane and β
It is considered that the peaks of the planes having different phase orientations overlap.

With respect to the two kinds of tantalum thin films having the different crystal structures described above, a tantalum oxide thin film was formed on each thin film surface by anodization (chemical formation). Anodization is performed using a 0.1N, 50 ° C. oxalic acid solution, a current density of 0.5 mA / cm ² , a constant current formation of a formation voltage of 110 V, and then a constant voltage formation of 110 V to form a 200 nm thick tantalum thin film on the surface. A tantalum oxide thin film was formed. At this time, the thickness of the underlying tantalum thin film is 100 nm. Before forming a TFT, a 100 nm thick aluminum electrode was deposited on the formed tantalum oxide thin film and its current-voltage characteristics were measured in order to examine its basic characteristics as an insulator thin film.
The result is shown in FIG. As is apparent from the figure, the tantalum oxide film formed by anodizing the tantalum metal having only the β phase is different from the same figure (b). The tantalum oxide film formed by anodizing the tantalum metal having different crystal structures. FIG. 3A shows that the leakage current is reduced by three digits or more at an electric field strength of 2 MV / cm.

The result of applying the tantalum oxide thin film obtained by anodizing the above-described tantalum metal having a mixed crystal structure to the gate insulating film 13 shown in FIG. 1 is shown below. First, under the sputtering conditions for forming a tantalum metal film described with reference to FIG. 3A, a 180 nm-thick tantalum metal film having a mixed crystal structure is formed on the insulating substrate 11, and is formed into a stripe shape by wet etching. The gate electrode 12 was formed. Next, after forming a tantalum oxide gate insulating film 13 having a thickness of 200 nm on the surface of the tantalum metal film under the conditions of the anodic oxidation method described above,
As a result of manufacturing the TFT shown in FIG. 1 by the above-described process, a significant improvement in the on / off ratio was observed as compared with the conventional case.

38.2 ° X-ray diffraction peak intensity in the vicinity (and P _h) and 33.4
The ratio P _h / P ₁ of the X-ray diffraction peak intensity in the vicinity of degrees (the P ₁₎ may vary depending on film forming conditions, the method of forming a film while conveying the trays mentioned above, 5 inches × 15 inches square When the target is used, the sputtering power is 2 to 5 kW, the argon gas pressure is 5 to 40 mTorr, and the argon gas flow rate is 30 to 100 SC.
When the CM and the substrate temperature are in the range of room temperature to 350 ° C., a tantalum metal thin film having a mixed crystal structure in which two X-ray diffraction peaks appear as shown in FIGS. 3 (c) and (d) is shown. Obtained. Figure 3 (a) is, diffraction peak intensity ratio P _h / P ₁
Although but has become about 1, FIG. 3 (c), the diffraction peak intensity ratio as shown in (d) of even a metal thin film is not 1, the intensity ratio P _h / P ₁ is equal to 0.5 to 10 The leakage current was reduced by two digits or more.

Example 2 FIG. 5 is a schematic cross-sectional view of a thin film EL (TFEL) device using the metal oxide thin film of the present invention as a first insulator layer. In FIG. 5, reference numeral 61 denotes a light-transmitting insulating substrate, and 62 denotes a transparent electrode in which a transparent conductive film made of tin-added indium oxide (ITO) is formed in a stripe shape. On the transparent electrode 62, a strontium titanate zirconate oxide film (Sr (Ti _x Zr _1-x ) O ₃ ) is formed as a first insulator layer 63 by a sputtering method. EL comprising a manganese-added zinc sulfide thin film formed by a co-evaporation method on the first insulator layer 63
After the light emitting layer 64 is formed and subjected to a heat treatment at 450 ° C. for 1 hour in a vacuum, a second insulator layer 65 is further formed on the EL light emitting layer 64. In this embodiment, the metal oxide thin film according to the present invention is used as the second insulator layer 65. On the second insulator layer 65, a back electrode 66 is formed. Back electrode
Reference numeral 66 denotes an aluminum thin film formed on the entire surface of the second insulator layer 65 by a vacuum evaporation method and processed into a stripe shape in a direction orthogonal to the transparent electrode 62.

The formation of the second insulator layer 65 according to the present embodiment was performed by the following method. First, a tantalum metal thin film having a thickness of 120 nm was formed on the entire surface of the EL light emitting layer 64 by using the in-line type sputtering apparatus described in FIG. 2 in the first embodiment. The sputtering conditions were an argon gas flow rate of 60 SCCM, a sputtering pressure of 20 mTorr, an RF power of 1.5 kW, and a substrate temperature of 200 ° C. By anodizing all of this tantalum thin film, a thickness of 30
A tantalum oxide thin film of 0 nm was formed to form a second dielectric layer 65. Anodizing is performed using a 0.1N, 50 ° C citric acid aqueous solution.
Constant current formation was performed at 170 V, and then constant voltage formation was performed at 170 V. The tantalum oxide thin film thus formed is
When the thin-film EL element used for the insulator layer 65 was driven at a driving voltage of 250 V, good driving without dielectric breakdown could be performed.

As described above, according to the first and second embodiments, the tantalum metal is deposited while the substrate moves in different regions of the plasma state. Therefore, the crystal structure of the obtained tantalum metal thin film has an α phase and a β phase. A mixture of phases and a mixture of β phases having different orientations are observed. In a film in which such different crystal structures are mixed, the crystal growth of each crystal grain hardly occurs.
As schematically shown in FIG. 1A, a structure in which a large number of microcrystals are aggregated and the crystal grain boundaries are complicated and intricate,
Further, the film is dense and has excellent surface flatness. On the other hand, in a film composed of only a single crystal structure with grain growth progressing,
As shown schematically in FIG. 6 (b), the crystal grain boundaries tend to be continuous, and the surface has severe irregularities. When these films are anodically oxidized, a low-resistance surface is generated at a portion where the crystal grain boundaries exist due to impurities precipitated at the crystal grain boundaries or oxides thereof. At this time, in the film obtained by oxidizing the metal film as shown in FIG. 6 (b) in which the crystal grains have grown well, the surface having low resistance generated by the crystal grain boundaries connecting the electrodes sandwiching both surfaces of the oxide film is very small. Due to the narrowing or the large growth of crystal grains, when oxidized, minute cracks (microcracks) occur at the crystal grain boundaries, and as a result, the leak electrode flows through the microcracks. Further, since the surface is poor in flatness, a local electric field concentration at the interface with the electrode causes a leakage current, and dielectric breakdown easily occurs.

On the other hand, in a metal film as shown in FIG. 6 (a) in which different crystal structures are mixed, a large number of microcrystals are aggregated, and the crystal grain boundaries are complicated and complicated. Since the interposed and oxidized surface of the crystal grain boundary connecting the electrodes is very wide and intermittent, it is difficult to form a low resistance surface as in the case of FIG. 6 (b). In addition, since the film is small and has a small crystal grain, microcracks hardly occur even when oxidized. As a result, it is considered that the leak current can be extremely reduced.

In the above embodiments, a tantalum metal oxide thin film was used as the metal oxide thin film, but the main technique in the present invention is to form an insulating metal oxide thin film by oxidizing the surface of the metal thin film. In this case, the present invention uses a metal thin film in which various different crystal structures are mixed. Therefore, the metal to be used is not limited to tantalum metal, and any metal that can form a metal oxide thin film by a surface oxidation method, such as titanium, niobium, aluminum, yttrium, zirconium, and hafnium, can be used. The same effect can be obtained even if there is. In the above embodiments, the sputtering method was used as a method of forming a metal thin film. However, a metal thin film having a mixture of two or more different crystal structures as described above and having a plurality of crystal grain boundaries in the thickness direction was used. It goes without saying that the manufacturing method is not limited to the sputtering method. Further, in the above embodiments, the anodic oxidation method is used as a method for oxidizing the metal thin film, but plasma anodic oxidation in oxygen plasma or the like can also be used. Although oxalic acid, phosphoric acid, and citric acid were used in the examples, the electrolyte solution was not limited to this. Not only an aqueous solution of an acid such as nitric acid, sulfuric acid, and boric acid, but also a neutral salt such as ammonium borate was used. May be used.

Effect of the Invention As described above, according to the present invention, two or more different crystal structures or crystal structures having different orientations are mixed,
By oxidizing at least a part of the metal thin film in the thickness direction from the surface of the metal thin film having a plurality of crystal grain boundaries in the thickness direction to form a metal oxide thin film, there is no defect over a large area and the leakage current is low. , A metal oxide thin film having excellent insulation properties can be provided, and its practical effect is great.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a liquid crystal drive section of an active matrix type LCD according to a first embodiment of the present invention, and FIG. 2 is a schematic view of an in-line type sputtering apparatus used for producing the tantalum metal thin films of the first and second embodiments. FIG. 3, FIG. 3 is an X-ray diffraction pattern diagram of the tantalum metal thin film of the embodiment, FIG. 4 is a current-voltage characteristic diagram of the tantalum oxide thin film formed by the anodic oxidation method, and FIG. FIG. 6 is a sectional view of a tantalum metal thin film. 11 ... insulating substrate, 12 ... gate electrode, 13 ... first layer gate insulating film, 14 ... second layer gate insulating film, 15 ... semiconductor layer, 16 ... etching stopper layer, 17 ... Ohmic contact layer, 18 Source electrode, 19 Drain electrode, 61 Insulating substrate, 62 Transparent electrode, 63 First
Dielectric layer, 64 EL light emitting layer, 65 Second dielectric layer, 66
…… Back electrode.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Koji Matsunaga, Inventor 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. (56) References JP-A-63-185052 (JP, A) JP-A-54-73064 (JP, A) JP-A-58-74079 (JP, A) JP-A-59-141271 (JP, A)

Claims (7)

(57) [Claims] 1. A method in which two or more different crystal structures or crystal structures having different orientations are mixed and at least a part of the metal thin film in the thickness direction is separated from the surface of the metal thin film having a plurality of crystal grain boundaries in the thickness direction. A metal oxide thin film characterized by being oxidized. 2. The metal thin film is a tantalum metal thin film having a plurality of crystal grain boundaries in the thickness direction in which two or more kinds of crystal structures such as β phase and α phase or β phase having different orientations are mixed. The metal oxide thin film according to claim 1, wherein: 3. A step of forming a metal thin film in which two or more different crystal structures or crystal structures having different orientations coexist on a substrate and a plurality of crystal grain boundaries exist in a thickness direction; Anodizing at least a part of the metal thin film in the thickness direction from the surface. 4. The metal thin film is a tantalum metal thin film having a plurality of crystal grain boundaries in a thickness direction in which two or more types of crystal structures such as β phase and α phase or β phase having different orientations are mixed. The method for producing a metal oxide thin film according to claim 3, wherein: 5. A method in which two or more different crystal structures or crystal structures having different orientations are mixed and at least a part of the metal thin film in the thickness direction is separated from the surface of the metal thin film having a plurality of crystal grain boundaries in the thickness direction. An electronic device comprising an oxidized metal oxide thin film. 6. The electronic device according to claim 5, further comprising a structure having a metal oxide thin film and a thin film transistor. 7. The electronic device according to claim 5, further comprising a metal oxide thin film and a thin film EL display element.