JP2940051B2 - Method of forming insulating thin film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] The present invention relates to a method for forming an insulating thin film used in an active matrix type display device and the like, which has good film quality, is dense, has good adhesion to a base, and has good coverage of a base step. A method of forming a compound insulating film on the surface of a sample by an atomic layer epitaxy method in which a thin film is formed by an ALE method and the sample is alternately exposed to a plurality of different types of source gas atmospheres a plurality of times in an atmosphere. The gas atmosphere has a pressure in the molecular flow region, and the atmosphere has a vapor pressure of 1 to several tens mTorr of the source gas.

[Industrial applications]

 The present invention relates to a method for forming an insulating thin film.

A thin film transistor matrix used for driving an active matrix type liquid crystal display device or the like includes a gate insulating film, an interlayer insulating film, a channel protective film, a protective film as an ion barrier for preventing intrusion of ions from outside, and an auxiliary film. Many insulating thin films such as a capacitor inter-electrode insulating film are used.

The gate insulating film and the interlayer insulating film are dense, have high withstand voltage, have no cracks and pinholes, and have good adhesion to the base, in order to eliminate point defects and line defects of the display device.
Further, it is strongly required that the coatability of the underlayer is good.

In addition, the protective film disposed on the upper layer of the operation semiconductor layer has an ion barrier property to prevent the property deterioration of the thin film transistor, and is dense and free from cracks and pinholes.
Good adhesion to the base is required.

Such a demand is a problem common to not only a thin film transistor matrix for driving a liquid crystal display device but also an insulating thin film used for various electronic devices.

[Conventional technology]

The insulating thin film is generally formed by a plasma chemical vapor deposition (P-CVD) method in a thin film transistor matrix for driving a liquid crystal display device using an operation semiconductor layer made of amorphous silicon (a-Si). For example, by using silicon nitride or silicon nitride oxynitride for the gate insulating film, the gate insulating film, the active semiconductor layer, and the channel protective insulating film can be continuously formed in the same vacuum chamber without breaking vacuum.

Therefore, although the manufacturing process has the advantage of being simple,
It is difficult to eliminate pinholes in the insulating film, the step coverage of the step of the underlying electrode is not sufficient, and the mechanical strength of the gate insulating film is not sufficient. It is often difficult to obtain withstand voltage or insulation resistance.

As described above, it has been difficult to say that the P-CVD method has always provided an insulating film that is satisfactory in terms of denseness, adhesion to a base, and step coverage.

In recent years, instead of the above-mentioned P-CVD method, Japanese Patent Publication No. 56-351
No. 58, Japanese Patent Publication No. 60-21955, an atomic layer epitaxy [ALE: Atomic Layer Epitaxy] in which an atomic layer is deposited one by one by exposing a sample substrate to a predetermined source gas atmosphere many times.
taxy] law has been proposed. Further, Japanese Patent Application Laid-Open No. 1-179423 discloses an ALE method using a metal alcoholate.
No. 1 proposes an ALE method using a hydrogen compound containing a silicon element or a radical thereof.

For example, in JP-A-1-179423, the step of exposing a sample substrate to a metal alcoholate atmosphere is defined as a first step,
Subsequent to the first step, the step of exposing the sample substrate to a H ₂ O.O ₂ mixed gas atmosphere decomposed and radicalized by high-frequency discharge is referred to as a second step, and the two steps are referred to as one cycle. A technique of forming a metal oxide film on a sample substrate by repeating the technique is disclosed.

In the ALE method, a sample substrate is repeatedly exposed to a plurality of different types of source gas atmospheres over a plurality of cycles to form an adsorption layer containing a desired atom, and reacting a gas containing another element with the adsorption layer. Then, compound films are deposited one by one.

The film formed by this ALE method was considered to be dense, had good adhesion to the base, and had good step coverage.

[Problems to be solved by the invention]

However, when a compound insulating film is formed by the above conventional ALE method, the pressure of the source gas atmosphere is 1 Torr to several Torr.
It is performed under the condition of relatively low vacuum of r.

In the vapor phase growth method, the higher the reaction pressure is, the higher the vacuum is, the more the straightness of the flying particles increases, and when there is a step on the base, the thickness of the inclined surface is smaller than that of the flat surface. Therefore, when the underlying surface has irregularities, in order to improve the coverage of the steps, it is general to reduce the straightness by setting the pressure condition at the time of film formation to a low vacuum, and to use an isotropic film formation condition. is there.

When the ALE method is performed under such conditions, the mean free path of the source gas molecules becomes short, and the source gas flow becomes viscous. Therefore, in the case of a compound having a high condensing property, the raw material gas molecules collide with each other in the course of the process and form clusters. It contains a large number of large cluster molecules of several hundred moles or more, and these clusters adhere to the surface of the sample substrate.

If the surface of the substrate is severely uneven or foreign particles are attached, the decomposition of clusters on the substrate will not proceed, and will remain in the film as stacking faults or abnormal growth nuclei, and the resulting insulating film itself will have defects. In the case of a channel protective film, the shielding property is reduced.

When the above stacking faults and abnormal growth nuclei are present, the portion does not become a dense film, and the adhesion and insulation are not sufficient,
Further, when this occurs at the step, the coverage is not good.

As described above, in the conventional ALE method, it is difficult to say that the quality of the obtained insulating film is necessarily satisfactory.

The present invention provides an insulating thin film that has good film quality, is dense, has good adhesion to the base, and has good coverage of the base step.
It is intended to be formed by the ALE method.

[Means for solving the problem]

The above object is to provide a method for forming a compound insulating film on the surface of a sample by an atomic layer epitaxy method in which a sample is alternately exposed to a plurality of different types of source gas atmospheres a plurality of times, wherein the source gas atmosphere is 1 to several tens mTorr. The problem is solved by a method for forming an insulating thin film having a vapor pressure.

Further, the sample is a thin film transistor in which a gate electrode is formed on a substrate in advance, the compound insulating film is a gate insulating film on the gate electrode, and the sample is a thin film transistor in which a gate bus line is formed in advance on the substrate. A matrix substrate, wherein the compound insulating film is an interlayer insulating film with a drain bus line formed on the gate bus line, and the sample is a thin film transistor formed in advance on the substrate, and the compound insulating film is A protective film covering at least a channel upper layer portion of the transistor;
The problem is solved by a method of forming an insulating film in which the sample is a thin film transistor matrix substrate with an auxiliary capacitor and the compound insulator is an inter-electrode insulating film forming an auxiliary capacitor.

[Action]

The present invention has been made as a result of various studies for the purpose of reducing the occurrence of stacking faults due to condensation of source gas molecules, which is a drawback of conventional atomic layer epitaxy.

That is, the pressure in the reaction chamber in which the ALE method is performed is set to a high vacuum of 10 mTorr or less, and as a condition that the source gas flow becomes a molecular flow, the mean free path of the source gas molecules is lengthened, so that collision between the source molecules is reduced. Therefore, no giant cluster molecules are generated in the feed gas stream.

Therefore, the clusters do not adhere to the substrate surface, and the raw material gas molecules are adsorbed one atomic layer at a time in one treatment, and there are no stacking faults or abnormal growth nuclei, and the substrate is dense and has good adhesion to the substrate. And a defect-free and highly insulating thin film can be formed.

In addition, despite the conditions in which the source gas flow forms a molecular flow as described above, a uniform film was formed even when there was a step on the base, and the coverage of the step on the base was extremely good.

The reason is probably that the ALE method does not adsorb more than one atomic layer, no matter how many source molecules reach the substrate surface. It is understood that the flat surface naturally adheres to one atomic layer.

It has also been confirmed that when an alumina film is formed by the ALE method in a molecular flow region, a film having a high insulating property can be formed even at a lower temperature than the conventional formation temperature.
Film formation far beyond the common sense of the law is possible.

Note that, since the name of the ALE method is generalized as the name of the film forming method, this name is also used in the present specification. However, since the formed film is not a single crystal, it is rather an atomic layer deposition ( ALD: Atomic Layer Depositio
n) It is appropriate to call the method.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to an example in which the present invention is applied to an active matrix type liquid crystal display panel.

First, an example in which a gate insulating film of a thin film transistor, an interlayer insulating film between bus lines, and a protective film are formed on a thin film transistor matrix substrate will be described with reference to FIGS. FIG. 1 is an enlarged plan view of one pixel of the thin film transistor matrix substrate, and FIGS. 2 and 3 are cross-sectional views taken along the lines II and IV-IV of FIG. In FIG. 1, the protective film in FIGS. 2 and 3 is omitted.

In this embodiment, as shown in FIGS. 1 to 3, for example, a gate electrode G made of a metal film such as a titanium (Ti) film 11 and a gate bus line are formed on a transparent insulating substrate such as a glass substrate 1. A lower layer film of GB is formed. Then, after an upper layer film of the gate bus line is formed by an aluminum (Al) film 12, Al ₂ O 3 is formed by an ALE method as a lower layer film of the gate insulating film 2 and the interlayer insulating film 8 which respectively cover the gate electrode and the gate bus line. An O ₃ (alumina) thin film 21 is formed, and a silicon nitride (SiN) film 22 is further laminated thereon by a P-CVD method.

In forming the Al ₂ O ₃ thin film 21 by the ALE method, it is possible to use a thin film forming apparatus by the ALE method proposed by the present inventors in Japanese Patent Application No. 63-227118 (hereinafter simply referred to as an ALE apparatus). it can.

As shown in FIG. 4, the ALE apparatus forms an Ar gas barrier 31 in the center of the fan-shaped reaction chamber 30 with an argon (Ar) gas as an inert gas introduced from a gas inlet Nc. Source gas inlets Na and Nb are disposed one by one at symmetrical positions with respect to the Ar gas barrier 31 as a center, and an intake port of an exhaust turbo molecular pump Vp is disposed at a main portion of the fan. The raw material gas is supplied from the raw material gas inlets Na and Nb into the reaction chamber 30.
And a thin film formation region 3 on both sides of the Ar gas barrier 31.
Form 2,33.

Next, a method of forming the gate insulating film 2 and the interlayer insulating film 8 made of the Al ₂ O ₃ thin film 21 using this ALE device will be described.
Although this embodiment shows an example in which the gate insulating film and the interlayer insulating film are formed simultaneously in the same step, they may be formed in different steps.

The glass substrate 1 to be processed has two thin film formation regions 3
Place on a mechanism that can move between 2,33. First, the glass substrate 1 is heated to about 300 ° C., and the inside of the reaction chamber 30 is evacuated to about 5 × 10 ⁻⁷ Torr by the turbo molecular pump Vp. Next, the orifice valve V0 is closed, Ar gas is flowed at about 500 sccm, and the indoor pressure is adjusted to about 0.01 Torr.

Next, open the orifice valve V1 and allow Ar gas to flow, then heat the aluminum chloride container to about 110 ° C to generate aluminum chloride (AlCl ₃ ) vapor and form a thin film of aluminum chloride vapor from the raw material gas inlet Na. Send to area 32.

Next, a water container (not shown) is kept at about 20 ° C., and the orifice valve V2 is opened to flow steam. Blocked by the Ar gas barrier 31 created by the steady flow of Ar gas, the aluminum chloride vapor of the source gas and the water vapor do not mix. Reaction chamber 30 at this time
The degree of vacuum inside is maintained at about 0.01 Torr.

The moving mechanism is operated at a speed that does not disturb the steady flow thus produced, for example, at a cycle of three seconds reciprocation, to reciprocate the glass substrate 1 between the thin film formation regions 32 and 33, Expose to a steam atmosphere and a steam atmosphere alternately. In one reciprocation under this condition, an Al ₂ O ₃ thin film of one atomic layer is formed only on a part of the surface, and one or less layers are formed as an average film thickness on the entire surface, and this is repeated about 6,000 times. Then, an Al ₂ O ₃ thin film 21 having a thickness of about 4000 mm is formed over the entire surface.

Next, a silicon nitride thin film 22 having a thickness of about 200 ° is formed thereon by a P-CVD method. In the SiN film, the a-Si layer provided on the film in the next step is normally grown, and the interface state is improved by forming the a-Si layer in the same chamber.

In the present embodiment, the gate insulating film 2 and the interlayer insulating film 8 are constituted by the Al ₂ O ₃ thin film 21 and the silicon nitride thin film 22 thus laminated. The interlayer insulating film 8 is the gate insulating film 2
When it is formed in another process, a single layer structure of an Al ₂ O ₃ film can be used.

Thereafter, the process may proceed according to a normal manufacturing method. That is, the a-Si layer 3 is used as an active semiconductor layer of the thin film transistor.
Is continuously formed by a P-CVD method. This film is also formed on the SiN film 22 of the interlayer insulating film 8.

Next, the n ⁺ a-Si layer of the contact layer of the thin film transistor
4. A channel protection film 6 made of a SiO ₂ film is formed. In addition,
The SiO ₂ film of the channel protective film 6 is also formed on the surface of the a-Si layer 3 on the gate bus line GB. Here, the interlayer insulating film 8
Is completed by the Al ₂ O ₃ thin film 21, SiN layer 22, a-Si layer 3 and SiO ₂ film 6. Next, a metal film such as a Ti film 5 is formed, a source electrode S, a drain electrode D, and a drain bus line DB are formed.
A display electrode E made of a film is formed. Finally, AL on the entire surface of the substrate
The surface protection film 7 of the Al ₂ O ₃ film is formed by the method E, and the thin film transistor matrix substrate according to the present embodiment is completed.

In the step of forming the protective film on the surface, the temperature cannot be set too high because the thin film transistor has already been formed. If the temperature is too high, the voltage-current characteristics of the thin film transistor deteriorate, and the rise voltage increases. Conventionally, to avoid this, SiN films and SiO ₂ films were formed by the P-CVD method with the formation temperature kept low, but the denseness of the film was poor and the protection effect of the element was insufficient. . Further, AL using alcoholate as a raw material described in JP-A-1-179423 described above is used.
The purpose of the E method is to form a thin film at a low temperature. However, even with this method, a film formed at a low temperature has poor denseness, and it cannot be said that a sufficient protective effect was obtained.

Therefore, in this embodiment, the heating temperature of the sample substrate is set to about
The temperature is set to 200 ° C., and the Al ₂ O ₃ thin film 7 is formed by the same manufacturing method as that for the Al ₂ O ₃ thin film 21 described above. That is, the sample substrate is placed between the aluminum chloride vapor atmosphere and the water vapor atmosphere for about 60 hours.
By reciprocating 00 times, an Al ₂ O ₃ thin film 7 having a thickness of about 4000 ° is formed.

Or a thin film transistor matrix substrate manufactured by this example as a gate insulating film 2, an Al ₂ O ₃ film of the underlying Al ₂ O ₃ thin film 21 and the surface protective film 7 of the interlayer insulating film 8, the feed gas stream Was formed by the ALE method under the condition of forming a molecular flow atmosphere, so that a dense and highly insulative film having good adhesion to the base and good coverage of the steps of the base was obtained. Further, since the formation temperature of the surface protection film 7 can be set low, a stable thin film transistor having no change in operation characteristics can be obtained.

FIG. 5 is a diagram showing the relationship between the pressure in the reaction chamber and the defect of the film when a thin film is formed using the above-mentioned ALE apparatus, with the flow rate of the barrier gas relative to the flow rate of the source gas as a parameter.

As can be seen from the figure, the higher the degree of vacuum in the reaction chamber, the fewer the defects in the film. This indicates that the magnitude of the mean free path of the source gas molecules strongly affects the film quality, as described above. Incidentally, when the pressure in the reaction chamber is set to 0.01 Torr, the number of film defects is improved by about three digits as compared with the case where the chamber pressure is set to 1 Torr. However, if the room pressure is 1 mTorr or less, the film forming rate becomes large and is not practical. Therefore,
In this embodiment, a room pressure in the range of 1 to several tens mTorr is desirable.

Furthermore, as the ratio r of the flow rate of the barrier gas to the flow rate of the source gas increases, the number of defects in the film decreases. This indicates the importance of preventing the mixing of two source gases because the barrier effect increases as the flow rate of the barrier gas increases.

Next, another embodiment of the present invention will be described with reference to FIGS. 6, 7, and 8. FIG. In this embodiment, an auxiliary capacitor is manufactured by using the ALE method according to the present invention. FIG. 6 shows an equivalent circuit of an active matrix type liquid crystal display device having an auxiliary capacitor, and a thin film transistor matrix substrate is realized. FIG. 7 (a) and FIG.

In order to improve the display quality of an active matrix type liquid crystal display device, it is effective to insert an auxiliary capacitance in parallel with the liquid crystal cell capacitance.

The method shown in the equivalent circuit of FIG. 6 is such that the display electrode E of the liquid crystal cell LC and the extension of the earth line EB provided separately from the gate bus line GB and the drain bus line DB are opposed to each other via an insulating film. This is an example in which the auxiliary capacitance Cs is configured.

FIG. 7 (a) uses a gate insulating film 2 as an insulating film 9 for insulating between two opposing electrodes of the storage capacitor Cs.
In FIG. 2B, a dedicated inter-electrode insulating film is formed separately from the gate insulating film 2.

When providing the auxiliary capacitance Cs, it is necessary that interlaced driving be possible in order to be able to cope with a large area panel, and therefore, one electrode of the auxiliary capacitance Cs can be independently set to an arbitrary potential Vc. Has been requested.

7 (a) and 7 (b) show that interlaced driving is possible and the potential of the ground line EB can be set independently. However, when the screen size becomes large, the configuration of FIG. There is a limit to lowering the resistance.
Therefore, in order to reduce the resistance, an independent earth line must be installed in a mesh form to take the configuration shown in FIG.
Although not shown in FIG. 6 when using a mesh-like ground line, adjacent ground lines are connected at many places in the display area, and the number of intersections with the gate bus lines increases by that number. For this reason, the area requiring the electrode-to-electrode insulation increases dramatically in FIG.

In addition, in the configuration shown in (b), the ground line EB must be a transparent electrode such as an ITO film having a relatively high resistivity in order to secure the aperture ratio of the pixel. Since the ITO film has a relatively high resistivity, in order to form a low-resistance line using the ITO film, the thickness of the ITO film must be increased.

Due to such a problem, the insulating film for insulating the electrodes of the auxiliary capacitor needs to be a film having good coverage with respect to the underlying surface having a large step and having no defect and having a dense insulating property.

This embodiment satisfies such a demand,
This is an example in which an inter-electrode insulating film of the storage capacitor Cs is formed by the ALE method according to the present invention. Also in this embodiment, the thin film forming apparatus shown in FIG. 4 is used.

First, as shown in FIG.
The earth line EB is formed using the ITO film.

Next, an ALE method is performed at a substrate temperature of about 300 ° C. to form an Al ₂ O ₃ thin film 91 having a thickness of about 4000 ° as shown in FIG. The film forming conditions at this time may be exactly the same as in the above-described embodiment.

Next, as shown in FIG. 4C, a gate electrode G made of a Ti film having a thickness of about 800 ° and a gate bus line (not shown) connected thereto are formed, and further heated at about 100 to 400 ° C. An oxygen plasma treatment is performed.

Next, a gate insulating film 2 made of SiO ₂ having a thickness of about 1000 ° and a SiN film having a thickness of about 2000 ° is formed. The SiO ₂ film and the SiN film of the gate insulating film are also formed on the Al ₂ O ₃ film 91 and become a part of the inter-electrode insulating film 9.

Next, an a-Si film (operating semiconductor layer) having a thickness of about 150 °
Approximately 1400 mm thick SiO ₂ film or SiN film (channel protective film)
6 is continuously formed by a P-CVD method, and further, an n ⁺ a-Si film (contact layer) 4, a source electrode S and a drain electrode D composed of a Ti film 5, and a display electrode E composed of an ITO film are formed. Then, as shown in FIG. 4D, the thin film transistor matrix substrate of this embodiment is completed.

Note that a series of steps from the heating oxygen plasma treatment to the formation of the SiO ₂ film 6 as a channel protective film can be continuously performed without breaking a vacuum by using a load-lock type CVD apparatus.

In the thin film transistor matrix obtained as described above, the Al ₂ O ₃ thin film 9 for interelectrode insulation can be formed by molecular flow using the ALE method even if the thickness is increased in order to reduce the resistance of the ground line EB. Since it is formed by performing in a region, the coverage with respect to the earth line EB is sufficiently satisfactory, and the Al ₂ O ₃ thin film 9 is dense, has good adhesion to the base, and exhibits high insulation properties. Therefore, defects such as a short-circuit between electrodes are greatly reduced even though the laminated structure of the electrodes is dramatically increased.

Therefore, according to the present embodiment, it is possible to manufacture a large-screen active matrix with high display quality and capable of interlaced driving at a high yield.

The method of forming an insulating thin film according to the present invention can be used to form an inter-electrode insulating film regardless of the structure of the auxiliary capacitor.

For example, in the structure of FIG. 6B, it can be formed simultaneously with the gate insulating film 2 by the ALE method described above.

As described above, by the method for forming an insulating thin film according to the present invention, various kinds of insulating thin films of a thin film transistor matrix can be formed, and the obtained insulating thin film is dense, has good adhesion to a base, and has pinholes and cracks. And an insulating film with good film quality. In addition, according to the present invention, since the film can be formed at a relatively low temperature, the quality of the film formed before that is not deteriorated.

In the above description, an example is shown in which the present invention is applied to an inverted staggered thin film transistor matrix. However, similar effects can be obtained by applying the present invention to a staggered thin film transistor matrix.

〔The invention's effect〕

As described above, according to the method of forming an insulating thin film of the present invention, it is possible to form an insulating film having good film quality, few defects, and good coverage on irregularities at a base at a relatively low temperature. it can. This method for forming an insulating thin film can be used in any process for forming an insulating film constituting a thin film transistor matrix. By using the present invention, the breakdown voltage characteristics, display quality, reliability, and manufacturing yield of a thin film transistor matrix substrate can be obtained. Is improved.

[Brief description of the drawings]

1 to 3 are explanatory views of an embodiment in which the present invention is applied to a thin film transistor, an interlayer insulating film and a surface protective film on a thin film matrix substrate, FIG. 4 is a perspective view of a thin film forming apparatus, and FIG. FIG. 6 is a diagram showing an equivalent circuit of an active matrix type liquid crystal display device with an auxiliary capacitor. FIG. 7 is a cross-sectional view of a main part of the thin film transistor matrix substrate of FIG. 6 according to another embodiment of the present invention. FIG. 8 is a view for explaining the embodiment of FIG. 7 (b) in the order of steps. In the figure, 1 is a transparent insulating substrate (glass substrate), 2 is a gate insulating film, 3 is an operating semiconductor layer (a-Si layer), 4 is a contact layer (n ⁺ a-Si layer), and 5 is a metal film ( Ti film), 6 is a channel protective film (SiO ₂ film), 7 is a surface protective film (SiO ₂ film)
₂ ), 8 is an interlayer insulating film, 9 is an inter-electrode insulating film (Al ₂ O ₃ thin film), 21 is an Al ₂ O ₃ thin film (thin film formed by the ALE method), 22 is Si
N film (film formed by the P-CVD method), 30 is a reaction chamber, 31 is an Ar gas barrier, 32 and 33 are thin film formation regions, 91 is an Al ₂ O ₃ thin film (ALE
G is a gate electrode, E is a display electrode,
GB indicates a gate bus line, DB indicates a drain bus line, and EB indicates an earth line.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-55-130896 (JP, A) JP-A-1-157518 (JP, A) JP-A-2-246161 (JP, A) JP-A-2- 74029 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) H01L 21/316 H01L 21/318

Claims (5)

(57) [Claims] 1. A method of forming a compound insulating film on a surface of a sample by an atomic layer epitaxy method in which a sample is alternately exposed to a plurality of different types of source gas atmospheres a plurality of times, wherein the source gas atmosphere is 1 to several tens mTorr. A method for forming an insulating thin film, comprising: 2. The method according to claim 1, wherein the sample is a thin film transistor having a gate electrode formed on a substrate in advance, and the compound insulating film is a gate insulating film on the gate electrode. Method. 3. The method according to claim 1, wherein the sample is a thin film transistor matrix substrate having a gate bus line formed on the substrate in advance, and the compound insulating film is an interlayer insulating film with a drain bus line formed on the gate bus line. The method for forming an insulating thin film according to claim 1. 4. The insulating thin film according to claim 1, wherein said sample is a thin film transistor formed in advance on a substrate, and said compound insulating film is a protective film covering at least a channel upper layer portion of said transistor. Forming method. 5. The semiconductor device according to claim 1, wherein the sample is a thin film transistor matrix substrate having an auxiliary capacitor, and the compound insulator is an inter-electrode insulating film forming an auxiliary capacitor.
The method for forming an insulating thin film according to the above.