JPH0732253B2 - Method for manufacturing field effect transistor - Google Patents

Description

TECHNICAL FIELD The present invention relates to an enhancement mode field effect transistor (hereinafter abbreviated as an enhancement mode FET device), and more particularly to a method for manufacturing an organic semiconductor device utilizing a field effect. Is.

[Conventional technology]

Conventional FET elements generally use an inorganic semiconductor such as Si or Ge or an inorganic compound semiconductor such as GaAs or InP as a main constituent material. However, since these are expensive, cheaper organic semiconductors, that is, organic substances having electrically semiconducting electrical characteristics, specifically, FET devices using polyacetylene have been reported. .

Fig. 1 is a cross-sectional view of a typical FET device, for example, Journal of Applied Physics, Volume 54, No. 6
Page 3255-Page 3259 (F. Ebisawa et al. Journal of App
lied Physics Vol.54 No.6 pp 3255-3259) describes the case of using polyacetylene for the semiconductor layer.

In the figure, (1) is a glass serving as a substrate, (2) is an aluminum film serving as a gate electrode, (3) is a polysiloxane film serving as an insulating film, (4) is a polyacetylene film serving as a semiconductor layer, (5) and (6) is a gold film which becomes a source electrode and a drain electrode, respectively.

Next, the operation will be described. When a voltage is applied between the source electrode (5) and the drain electrode (6), the source electrode (5) and the drain electrode (6) are passed through the polyacetylene film (4).
An electric current flows between them. At this time, the polyacetylene film (4) is provided on the glass substrate (1) and is formed by the insulating film (3).
When a voltage is applied to the gate electrode (2) which is separated from the gate electrode (2), the electric conductivity of the polyacetylene film (4) can be changed by the electric field effect, so that the current between the source and the drain can be controlled. This is because the width of the depletion layer in the polyacetylene film (4) close to the insulating film (3) changes depending on the voltage applied to the gate electrode (2), and the effective channel cross-sectional area of holes changes. It is believed that.

In this case, the polyacetylene film must have semiconductor-like electrical characteristics, and it must be in ohmic contact with the source electrode (5) and the drain electrode (6). Furthermore, it is necessary that the polyacetylene film (4) and the gate electrode (2) sandwich the insulating film (3) to form a MIS junction.

In the conventional FET device using this polyacetylene, the polyacetylene film (4) has a polymer journal of Volume 2, No. 2, pp. 231 to 244 (H. SHIRAKAWA et al.
mer Journal Vol.2 No.2 pp 231-244), that is, a method of polymerizing acetylene gas with a Ziegler-Natta catalyst.

[Problems to be solved by the invention]

In the FET element using the conventional organic semiconductor as described above,
Since polyacetylene is used, if left in the air, polyacetylene with many unsaturated bonds is easily attacked by oxygen and water, and deteriorates relatively quickly. Therefore, the FET device using polyacetylene has the problems of poor stability, short life, and poor electrical characteristics. Also, from the viewpoint of the production method, the method of polymerizing acetylene gas with a Ziegler-Natta catalyst to form a polyacetylene film is relatively complicated, and the catalyst used at the time of synthesis remains in polyacetylene for practical use. Many problems remain to be solved.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a method for manufacturing a semiconductor device that is stable, has a long life, and has excellent electrical characteristics.

[Means for Solving the Problems] A semiconductor element targeted by the present invention is a π-conjugated polymer having a five-membered heterocyclic ring whose conductivity is adjusted as a current path between a source electrode and a drain electrode. The conductivity of a semiconductor layer is controlled by a gate electrode.

[Action]

Since the semiconductor layer in this invention is easily produced using a π-conjugated polymer having a hetero five-membered ring,
A FET element that is homogeneous and has excellent electrical characteristics can be obtained. or,
It is cheap because an inexpensive organic compound is used.

〔Example〕

If the semiconductor layer (4) in the cross-sectional view of the general FET device shown in FIG. 1 has a conductivity adjusted according to the present invention and a π-conjugated polymer having a five-membered heterocyclic ring is used, one embodiment of the present invention can be realized. An example enhancement mode FET device is obtained.
Further, FIGS. 2, 3, and 4 show another embodiment of the present invention.
A sectional view of a FET element is shown. In the figure, (1) to (3)
And (5) and (6) are the same as those in FIG. 1, and (4) is a π-conjugated polymer film having a five-membered heterocyclic ring, the conductivity of which acts as a semiconductor layer is adjusted.

Here, the materials used in the present invention are as follows.

Glass is generally used as the substrate, but an insulating polymer film such as a polyester film or a polyimide film can also be used.

As the gate electrode, metals such as gold, platinum, chromium, palladium, aluminum, and indium, tin oxide, indium oxide, indium tin oxide (ITO), and the like are generally used, but needless to say, are limited to these materials. However, it is not necessary to use two or more kinds of these materials as the gate electrode. In the enhancement mode FET element of the present invention shown in FIGS. 1 and 2, p-type silicon or n-type silicon is used as the gate electrode (2).
And the substrate (1) can also be used. In this case, the substrate (1) can be omitted. Further, in this case, it is practically preferable that the volume resistivity of p-type silicon or n-type silicon is smaller than that of the π-conjugated polymer having a complex five-membered ring used as the semiconductor layer. Furthermore, a conductive organic polymer may be used as the gate electrode.

As the insulating film, any insulating material can be used, including inorganic and organic materials. Generally, silicon oxide (SiO ₂ ), silicon nitride, aluminum oxide, polyethylene, polyvinylcarbazole, polyphenylene Ruffid, polyparaxylene, etc. are used. As the source electrode and the drain electrode, the same metal and tin oxide, indium oxide, indium tin oxide (ITO), etc. as in the case of the gate electrode (2) can be used, but it has a five-membered heterocyclic ring. A metal having a large work function capable of making ohmic contact with a π-conjugated polymer, such as gold,
Platinum, chromium, palladium, etc. are preferably used. Further, even if a conductive organic polymer is used as the source electrode (5) and the drain electrode (6), there is no problem in terms of characteristics. The π-conjugated polymer having a five-membered heterocyclic ring, which acts as the semiconductor layer (4), is usually an insulator by itself but is a suitable electron acceptor, for example, perchlorate ion, tetrafluoroborate ion, sulfonate ion. Etc. and electron donors, for example
By doping with Na, K, Li, amine, etc., a p-type or n-type semiconductor can be obtained, and its conductivity can be controlled in a wide range from the insulator region to the metal region.
Control the electric conductivity so that an enhancement FET device can be obtained. In the present invention, a π-conjugated polymer film having a p-type doped five-membered heterocyclic ring is preferably used from the viewpoint of stability and characteristics.

A π-conjugated polymer having a five-membered heterocyclic ring has the general formula (However, X is one of S and O atoms, R ₁ , and
R ₂ is one of -H, -CH ₃ , -OCH ₃ , -C ₂ H ₅ and -OC ₂ H ₅ groups, n is an integer), and a general formula (However, R ₁ and R ₂ are one of -H, -CH ₃ , -OCH ₃ , -C ₂ H ₅ and -OC ₂ H ₅ groups, and R ₃ is -H, -CH ₃ , -C ₂ H. ₅ , −C ₃
H ₇ , One of the NO ₂ groups, n is an integer). (Hereinafter, simply referred to as π-conjugated polymer) These π-conjugated polymers are extremely excellent materials from the viewpoint of stability and characteristics of the enhancement mode FET device.

When forming the thin film of the π-conjugated polymer, it is suitable to form it by an electrochemical polymerization method (electrolytic polymerization method).

For example, in order to form the π-conjugated polymer film by an electrolytic polymerization method, a monomer corresponding to the π-conjugated polymer and a supporting electrolyte are dissolved in an organic solvent or water, or a mixed solvent of water and an organic solvent. A reaction solution is used, and in the fabrication of the enhancement mode FET element of the present invention shown in FIGS. 2 and 3, at least one of the source electrode (5) and the drain electrode (6) is used as a working electrode, for example, a counter electrode such as platinum. A desired π-conjugated polymer is deposited on and around the working electrode by causing an electric current to flow through the electrode, and the source electrode (5) and the drain electrode (6) are connected by the π-conjugated polymer and deposited. The π-conjugated polymer film is thoroughly washed and then dried in a nitrogen atmosphere. In this case, the deposited π-conjugated polymer film is doped with anions of the supporting electrolyte during the reaction to become a p-type organic semiconductor, and the distance between the source electrode (5) and the drain electrode (6) is sufficiently short so that both electrodes are The insulating film in between is also completely covered with the π-conjugated polymer film, and both electrodes are partly or completely covered with the p-type organic semiconductor film. Also this p
The type organic semiconductor film can be appropriately dedoped after electrolytic polymerization so as to have a conductivity suitable for an enhancement mode FET device. The polythiophene and poly (3-methylthiophene) synthesized by this method have particularly excellent characteristics as a semiconductor layer of an enhancement mode FET device, and the electrolytic polymerization method is suitable for synthesizing both polymers.
Here, as the organic solvent, any solvent that can dissolve the supporting electrolyte and the above-mentioned monomer may be used, and examples thereof include acetonitrile, nitrobenzene, benzonitrile, nitromethane,
N, N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), dichloromethane, tetrahydrofuran,
A polar solvent such as ethyl alcohol, methyl alcohol, or water is used alone or as a mixed solvent of two or more kinds.
The supporting electrolyte has a high oxidation potential and a high reduction potential, and does not undergo oxidation or reduction reaction itself during electrolytic polymerization, and is a substance capable of imparting conductivity to a solution by being dissolved in a solvent, for example, Perchloric acid tetraalkylammonium salt, tetraalkylammonium tetrafluoroborate salt, tetraalkylammonium hexafluorophosphate salt, tetraalkylammonium paratoluenesulfonate salt, sodium hydroxide and the like are used, but of course two or more kinds may be used in combination. I do not care.

Next, in order to form the π-conjugated polymer film by the chemical oxidative polymerization method, a predetermined amount of an oxidizing agent is dissolved as an initiator in deionized water or an organic solvent, or a mixed solvent of water and an organic solvent, After preparing a solution sufficiently deoxidized, a predetermined amount of a monomer corresponding to the π-conjugated polymer is added to the solution to polymerize the monomer. At this time, for example, in the fabrication of the FET element shown in FIG. 2, the substrate (1), that is, the intermediate member, on which the gate electrode (2), the insulating film (3), the source electrode (5) and the drain electrode (6) are provided in advance. Is soaked in this solution to form a semiconductor layer (4) composed of a polymerized film of π-conjugated polymer on the substrate (1). At this time, a small amount of oxidizing agent or anion is doped into the π-conjugated polymer film (4). Further, in order to adjust the conductivity suitable for the enhancement mode FET device, doping from the gas phase or electrochemical doping is continuously performed.

The substrate (1) may be dipped in the solution immediately after or simultaneously with the addition of the monomer to the solution. This method is excellent in film thickness controllability and film uniformity, and in many cases, the conductivity suitable for enhancement mode FET can be obtained at the same time as film formation. Here, ferric chloride, potassium ferricyanide, or the like is used as the initiator, but the initiator is not limited to these. Any oxidizing agent can be used in which the redox potential of the initiator is nobler than that of the monomer.

On the other hand, the enhancement mode FET device of one embodiment of the present invention, which uses a π-conjugated polymer having a five-membered heterocyclic ring whose conductivity is adjusted in the semiconductor layer (4) in FIG.
The stability as a FET device is remarkably increased as compared with the conventional FET device using polyacetylene, and as is apparent from FIGS. 5 and 6 below, for example, when the gate voltage is applied only when the drain current is desired to flow. It is possible to obtain an element having extremely excellent characteristics that the enhancement mode has. In the FET element configured as described above, its operating mechanism is not clear yet,
It can be considered as follows. That is, the width of the depletion layer formed on the π-conjugated polymer film (4) side at the interface between the π-conjugated polymer film (4) and the insulating film (3) is the width of the gate electrode (2) and the source electrode (5). It is considered that the current flowing between the source electrode (5) and the drain electrode (6) is changed due to the change of the effective channel cross-sectional area of the hole, which is controlled by the voltage applied between the two. At this time, usually, as the π-conjugated polymer film (4), a film having a p-type semiconducting property with low electric conductivity is preferably used. Further, an organic polymer having higher conductivity than the π-conjugated polymer film (4) may be used for the source electrode (5) and the drain electrode (6), which is π.
It is considered that this is because the holes are effectively injected into the conjugated polymer film (4) and the holes are removed from the π-conjugated polymer film (4).

Further, in the enhancement mode FET device shown in FIGS. 2 and 3, as the gate electrode (2), a p electrode having higher conductivity than the π-conjugated polymer film (4) is used as well as the metal electrode.
Even if type silicon, n-type silicon, or an organic polymer is used, a depletion layer having a sufficiently large width is formed in the π-conjugated polymer film (4) and characteristics as an enhancement mode field effect transistor appear. Conceivable.

Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.

Example 1 A 3,000 Å thick silicon oxide film was provided on both surfaces of an n-type silicon plate (3.0 cm × 3.0 cm) having a conductivity of 6 S / cm and a thickness of 380 μm by a thermal oxidation method. Next, using a positive resist on one side, a pattern to be a source electrode and a drain electrode (effective area 0.2 cm x 0.4 cm; gap to be a channel: 5
μm) and then deposit a chrome film 200
After providing Å and further providing 300 Å of a gold film thereon, the resist was removed to form a source electrode and a drain electrode. Lead the source and drain electrodes at a silver pace,
The contact part was fixed with epoxy resin.

2.2'-dithiophene (0.
36 g) and tetraethylammonium perchlorate (0.73 g) were dissolved, and this was used as reaction dissolution. The source and drain electrodes on the silicon plate are used as working electrodes, a platinum plate (1 cm x 2 cm) is used as the counter electrode (counter electrode), and SCE (saturated calomel electrode) is used as the reference electrode. Soaked. A constant current (100 μA / cm ² ) is passed between the counter electrode and the working electrode as an anode in a nitrogen gas atmosphere for 8 minutes to completely complete the working electrode, that is, the source and drain electrodes and the silicon oxide between both electrodes. Was coated with polythiophene.

Next, the potential of the working electrode was set to +0.2 V with respect to SCE for 4.5 hours with a potentiostat, and the polythiophene in the p-type doped state was electrochemically partially dedoped and then twice with acetonitrile. After washing, it was dried under a nitrogen gas atmosphere. Part of the silicon oxide on the other surface of the silicon plate not covered with polythiophene thus provided (0.5 cm ² ) was removed with a paper file and indium-gallium was used to make ohmic contact with the n-type silicon. The lead was taken out and fixed at the contact point with an epoxy resin, and the n-type silicon acted as a gate electrode through this lead wire.

In this embodiment in which the enhancement mode FET element of the embodiment of the present invention having the structure shown in FIG. 2 is manufactured as described above, (1) and (2) in FIG. And (3) a silicon oxide that functions as an insulating film, (4) a polythiophene film that is a semiconductor layer, and (5) and (6) a source electrode made of a chromium film coated with a gold film, respectively. It is a drain electrode. The electrical conductivity of the polythiophene is calculated as about 5 × 10 ⁻⁷ Ω ⁻¹ cm ⁻¹ from FIG. 5 below.

Example 2 A chromium film having a thickness of 200 Å is provided in a ribbon shape near the center of a 3.0 cm × 3.0 cm glass substrate by photolithography and a vacuum evaporation method, and a gold film is further vacuum evaporated to a thickness of 500 Å on the chromium film. It was provided by the method and used as a gate electrode (effective electrode area: 10 mm × 10 μm).

Further, a silicon oxide film having a thickness of 5000 Å was provided on the substrate by a CVD method to form an insulating film. On top of that, a 200 Å-thick chrome film with a channel length of 3 μm, and a 500 Å gold film on the chrome film with a gate electrode sandwiched in two places were vacuumed using the same lift-off method as in Example 1. Provided by vapor deposition method and used these as a source electrode and a drain electrode (each effective area: 1 mm × 10 mm) Leads were taken from the gate electrode, source electrode, and drain electrode with silver paste, and then contact points were made with epoxy resin. It was fixed and used for the subsequent experiments.

N-methylpyrrole (0.4
g) and a solution in which tetraethylammonium perchlorate (0.7 g) was dissolved was used as a reaction solution. The electrodes corresponding to the above-mentioned source electrode and drain electrode were used as working electrodes, a platinum plate (1 cm × 2 cm) was used as the counter electrode, and SCE was used as the reference electrode, and these were immersed in the reaction solution. A constant current (120 μA / cm ² ) is applied between the working electrode and the counter electrode under a nitrogen gas atmosphere.
After flowing for 15 minutes, poly (N-methylpyrrole) was completely coated on the working electrode, that is, on the source electrode and the drain electrode and 3 μm between both electrodes. Next, set the potential of the working electrode to + 0.3V with respect to SCE with a potentiostat for 4 hours,
Poly (N-methylpyrrole) in p-type doping state
Was subjected to electrochemical dedoping, washed twice with acetonitrile, and then dried under a nitrogen gas atmosphere.

As described above, in this embodiment in which the enhancement mode FET element of the embodiment of the present invention having the structure shown in FIG. 2 is prototyped, in FIG. 2, (1) is a glass substrate and (2) is a gold film. A gate electrode made of a chromium film coated with, (3) silicon oxide as an insulating film, (4) poly (N-methylpyrrole) acting as a semiconductor layer, and (5) and (6) coated with a gold film. A source electrode and a drain electrode made of a chromium film. The electric conductivity of the poly (N-methylpyrrole) is calculated to be about 2 × 10 ⁻⁶ Ω ⁻¹ cm ⁻¹ from FIG. 6 below.

Example 3 A chromium film having a thickness of 500 Å was formed in a ribbon shape near the center of a 3.0 cm × 3.0 cm glass substrate by photolithography and a vacuum evaporation method, and a gold film was further vacuum evaporated to a thickness of 1000 Å on the chromium film. Method, and this was used as a gate electrode (effective electrode area: 10 mm × 10 μm). Further, a silicon nitride film having a thickness of 5000 Å was formed on the substrate by a CVD method to form an insulating film.

High purity nitrogen gas is bubbled through a solution of ferric chloride (FeCl ₃ · 6H ₂ O, 2.7g) dissolved in 100 ml of pure water for 30 minutes, then the above substrate is dipped and the high purity nitrogen gas is bubbled. While adding 1 ml of N-methylpyrrole to this solution. As soon as N-methylpyrrole is added, a chemical oxidative polymerization reaction starts, and a poly (N-methylpyrrole) film starts to form on the silicon nitride film, and after 3 hours, the substrate is taken out from the solution and water and ethanol are sufficient. After washing, vacuum drying was performed for 3 hours.

Poly (N-methylpyrrole) obtained as described above
A 1000Å gold film was provided on the upper part of the gate electrode by a vacuum evaporation method so as to serve as a source electrode and a drain electrode. Then, the lead was taken out from the source electrode, the drain electrode, and the gate electrode by using a silver paste, the contact portion was fixed with an epoxy resin, and subjected to the subsequent electrical measurement.

Thus, the enhancement mode FET element of the embodiment of the present invention shown in FIG. 1 was manufactured as a prototype. In this embodiment, (1) in FIG. 1 is a glass substrate, (2) is a gate electrode made of a chromium film coated with a gold film, (3) is an insulating film of silicon nitride, and (4) serves as a semiconductor layer. Poly (N-
Methylpyrrole), (5) and (6) are gold films which are the source electrode and the drain electrode, respectively.

Example 4 500Å by vacuum deposition method over the entire surface of a 3.0 cm × 3.0 cm glass substrate
A chromium film was formed, and a gold film having a thickness of 1000 Å was further formed thereon by a vacuum evaporation method. A source electrode and a drain electrode facing each other with a width of 10 μm were formed on this by photolithography using a negative resist (each effective area:
0.2 cm × 0.4 cm) Using the source electrode and the drain electrode as working electrodes, the electrochemical polymerization method similar to that in Example 1 was performed to completely remove poly (3-) between the source electrode and the drain electrode and between the source and drain electrodes. Methylthiophene). However, in this case, 3-methylthiophene was used as a monomer instead of 2.2'-dithiophene, and a constant current (0.5 mA / cm ² ) was passed between the working electrode and the counter electrode for 10 minutes. The poly (3-methylthiophene) thus obtained is in a highly p-type doped state, and the potential of the working electrode is potentiostated to electrochemically dedope it, and SCE It was set to -0.1V for 4 hours. After that, it was washed twice with acetonitrile and then dried under a nitrogen atmosphere.

Next, after depositing 5000 Å of CVD silicon nitride on poly (3-methylthiophene), a 1000 Å gold film as a gate electrode was provided on the silicon nitride by a vacuum deposition method between the source and drain electrodes. In this way, the enhancement mode FET device according to the embodiment of the present invention shown in FIG. 3 was prototyped. In this embodiment, (1) in FIG. 3 is a glass substrate,
(2) is a gate electrode made of a gold film, (3) is a silicon nitride film which is an insulating film, (4) is a poly (3-methylthiophene) film which is a π-conjugated polymer film, (5) and (5) 6)
Are a source electrode and a drain electrode, each of which is a chromium film coated with a gold film.

FIG. 5 and FIG. 6 show source-drain current (10 ⁻¹ μA) -source-at each gate voltage of the enhancement mode FET devices produced in Examples 1 and 2, respectively.
FIG. 3 is a characteristic diagram showing a drain-to-drain voltage (V) characteristic, in which the horizontal axis represents the source-drain voltage (V) and the vertical axis represents the source-drain current (10 ⁻¹ μA). According to it, both elements,
It is an enhancement mode in which the drain current increases greatly as a negative voltage is applied as the gate voltage.
Excellent characteristics, which cannot be obtained by the conventional FET device (depletion mode) using polyacetylene as the semiconductor layer, are obtained, in which the gate voltage is applied only when the drain current is desired to flow. In addition, the characteristics of both devices remained almost unchanged after being left in the air for one month.

The characteristics of the enhancement mode FET element manufactured in Example 3 were almost the same as the characteristics of the element manufactured in Example 2. Enhancement mode produced in Example 4
The FET element showed characteristics equal to or higher than those of the element manufactured in Example 1.

〔The invention's effect〕

As described above, by adopting the process of the present invention, it becomes possible to use a π-conjugated polymer material having a five-membered heterocyclic ring as an actual semiconductor device material, and to obtain a semiconductor device having excellent electrical characteristics. Obtainable.

[Brief description of drawings]

FIG. 1 is a sectional view of a general FET element, and FIGS. 2, 3, and 4 are sectional views of an FET element of an embodiment of the present invention.
5 and 6 are characteristic diagrams of source-drain current-source-drain voltage (V) at each gate voltage according to the embodiment of the present invention. In the figure, (2) is a gate electrode, (3) is an insulating film,
(4) is a semiconductor layer, and (5) and (6) are a source electrode and a drain electrode, respectively. In the drawings, the same reference numerals indicate the same or corresponding parts.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Torahiko Ando, 8-1-1 Tsukaguchihonmachi, Amagasaki City, Hyogo Sanryo Electric Co., Ltd. Materials Research Laboratory (56) Reference JP-A-58-114465 (JP, A)

Claims (1)

[Claims] 1. A substrate on which source and drain electrodes are formed and a counter electrode in a solution prepared by dissolving a monomer of a π-conjugated polymer having a five-membered heterocyclic ring and a supporting electrolyte in water or an organic solvent. And at least one of the source and drain electrodes is used as a working electrode in this solution, and a current is passed between the working electrode and the counter electrode to form a complex on the source and drain electrodes and between the source and drain electrodes. The first step of forming a π-conjugated polymer film having a five-membered ring, and then a predetermined potential is set to the working electrode to dedope the π-conjugated polymer film having the five-membered heterocyclic ring. And a second step of controlling the electrical conductivity of the π-conjugated polymer film having the above-mentioned five-membered heterocyclic ring, the method for producing a field effect transistor.