JP5099740B2 - Thin film transistor - Google Patents

Thin film transistor Download PDF

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JP5099740B2
JP5099740B2 JP2006314818A JP2006314818A JP5099740B2 JP 5099740 B2 JP5099740 B2 JP 5099740B2 JP 2006314818 A JP2006314818 A JP 2006314818A JP 2006314818 A JP2006314818 A JP 2006314818A JP 5099740 B2 JP5099740 B2 JP 5099740B2
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thin film
insulating film
oxide semiconductor
semiconductor thin
formed
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JP2007194594A (en
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守 古田
孝 平尾
寛 古田
時宜 松田
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財団法人高知県産業振興センター
カシオ計算機株式会社
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  The present invention relates to a thin film transistor, and more particularly to a thin film transistor having an oxide semiconductor thin film layer containing zinc oxide as a main component in an active layer.

It has long been known that oxides such as zinc oxide or magnesium zinc oxide exhibit excellent semiconductor (active layer) properties, and in recent years, such as thin film transistors (hereinafter abbreviated as TFTs), light-emitting devices, transparent conductive films, etc. Aiming at device application, research and development of semiconductor thin film layers using these compounds has been activated.
TFTs using zinc oxide or magnesium zinc oxide as semiconductor thin film layers are electrons compared to amorphous silicon TFTs using amorphous silicon (a-Si: H), which is mainly used in conventional liquid crystal displays, as semiconductor thin film layers. Active development is underway, with advantages such as high mobility, excellent TFT characteristics, and the expectation of high mobility by obtaining a crystalline thin film even at low temperatures near room temperature.

  As a TFT using an oxide semiconductor thin film layer, a bottom gate type and a top gate type structure have been reported.

  As an example of a bottom gate structure, a structure is known in which a gate electrode and a gate insulating film are formed in order from the substrate, and an oxide semiconductor thin film layer mainly composed of zinc oxide is formed covering the upper surface. Yes. This structure is similar to the structure of bottom gate type amorphous silicon TFTs that are currently commercialized as driving elements for liquid crystal displays, and can be applied to the process of manufacturing the amorphous silicon TFTs, so it is often used for zinc oxide TFTs. ing.

  As an example of the top gate structure, a structure in which a pair of source / drain electrodes, an oxide semiconductor thin film layer, a gate insulating film, and a gate electrode are stacked in this order from the substrate can be exemplified.

  A bottom-gate thin film transistor has a structure in which an oxide semiconductor thin film layer is stacked over a gate insulating film, and thus an area at an early stage of film formation with insufficient crystallinity must be used as an active layer. There is a problem that mobility cannot be obtained. On the other hand, a top-gate thin film transistor has a structure in which a gate insulating film is provided above an oxide semiconductor thin film layer. Therefore, a region having favorable crystallinity above the oxide semiconductor thin film layer can be used as an active layer. It is more effective than a bottom-gate thin film transistor in that it can be made.

However, zinc oxide does not have sufficient heat resistance, and it is known that oxygen and zinc are desorbed and form lattice defects by heat treatment during the TFT manufacturing process. The lattice defect forms a shallow impurity level electrically and causes a reduction in resistance of the oxide semiconductor thin film layer. For this reason, when zinc oxide is used for the active layer of a thin film transistor, the drain current flows without applying a gate voltage, that is, a normally-on type operation, that is, a depletion type operation. As the defect level increases, the threshold voltage decreases. , Leakage current increases.
In addition, lattice defects prevent the induction of carriers in zinc oxide serving as an active layer and reduce the carrier concentration. The decrease in the carrier concentration lowers the conductivity of the active layer and affects the electron mobility and current transfer characteristics (eg, subthreshold characteristics and threshold voltage) of the thin film transistor.

As described above, the lattice defects affect the current transfer characteristics of the transistor.
On the other hand, hydrogen has been reported as an element that forms a shallow impurity level in zinc oxide in addition to the lattice defects (for example, see Non-Patent Document 1 below). Therefore, it is considered that elements such as hydrogen introduced in the TFT manufacturing process other than lattice defects affect the characteristics of the zinc oxide TFT.
However, there are no documents including the non-patent document 1 that describe the influence of contained elements in oxides such as zinc oxide on TFT characteristics.

Cetin Kilic et al., "N-type doping of oxides by hydrogen", APPLIED PHYSICS LETTERS, July 1, 2002, Vol. 81, No. 1, p. 73-75

  The present invention has been made in view of the above situation, and in a thin film transistor having an oxide semiconductor thin film layer containing zinc oxide as a main component, the hydrogen concentration in the oxide semiconductor thin film layer forming the channel of the thin film transistor is controlled. Accordingly, an object of the present invention is to provide a thin film transistor that controls the conductivity of an oxide semiconductor thin film layer and exhibits effects such as suppression of leakage current, reduction of threshold voltage, and improvement of electron mobility.

The invention according to claim 1 includes at least an oxide semiconductor thin film layer mainly composed of zinc oxide formed as a channel on a substrate, a gate insulating film, and a gate electrode. In the thin film transistor, the hydrogen concentration in the oxide semiconductor thin film layer is 5 × 10 20 cm −3 or more and 7 × 10 21 cm −3 or less .

According to a second aspect of the present invention, the thin film transistor includes the oxide semiconductor thin film layer formed on the substrate, and the gate insulating film formed so as to cover an upper surface and a side surface of the oxide semiconductor thin film layer. A top gate type thin film transistor having the gate electrode stacked on the gate insulating film, wherein the gate insulating film is a silicon oxide film, a silicon oxynitride film, a silicon nitride film, or silicon nitride with oxygen or oxygen 2. The thin film transistor according to claim 1 , wherein the thin film transistor is an insulating film using at least a part of a film doped with oxygen by using a compound containing as a constituent element.

According to a third aspect of the present invention, the thin film transistor includes the gate electrode formed on the substrate, the gate insulating film formed to cover the gate electrode, and the gate insulating film formed on the gate insulating film. A bottom-gate thin film transistor having an oxide semiconductor thin film layer and a protective insulating film formed on the oxide semiconductor thin film layer, wherein the protective insulating film is a silicon oxide film, a silicon oxynitride film, a silicon nitride film, 2. The thin film transistor according to claim 1 , wherein the thin film transistor is an insulating film using at least a part of a film doped with oxygen by using oxygen or a compound containing oxygen as a constituent element in silicon nitride.

  According to the invention of claim 1, an oxide semiconductor in a thin film transistor having at least an oxide semiconductor thin film layer mainly composed of zinc oxide formed as a channel on an insulating substrate, a gate insulating film, and a gate electrode. By containing at least hydrogen in the thin film layer, the electric conductivity of the oxide semiconductor thin film layer, that is, the carrier concentration can be controlled, and a thin film transistor in which the conductivity in the channel is controlled is obtained.

According to the first aspect of the present invention, the hydrogen concentration contained in the oxide semiconductor thin film layer forming the channel of the thin film transistor is 5 × 10 20 cm −3 or more and 7 × 10 21 cm −3 or less. Thus, a thin film transistor having a low threshold voltage and a high electron mobility, in which sufficient carriers are induced in the active layer and leakage current is suppressed.

According to the invention of claim 2 , an oxide semiconductor thin film layer in which a thin film transistor is formed on a substrate, a gate insulating film formed so as to cover an upper surface and a side surface of the oxide semiconductor thin film layer, and the gate By being a top-gate thin film transistor including a gate electrode stacked over an insulating film, a thin film transistor using a region with favorable crystallinity above the oxide semiconductor thin film layer as an active layer can be provided.
Further, at least a part of the gate insulating film is any one of a silicon oxide film, a silicon oxynitride film, a silicon nitride film, or a film in which silicon nitride is doped with oxygen or a compound containing oxygen as a constituent element. By using the insulating film used in the above, lattice defects due to desorption of oxygen or zinc at the time of forming the gate insulating film can be prevented. Therefore, the amount of hydrogen diffused into the oxide semiconductor thin film layer can be controlled more strictly, and the thin film transistor can exhibit more reliably the effects of suppressing leakage current, lowering the threshold voltage, and improving electron mobility.

According to the invention of claim 3 , the gate electrode on which the thin film transistor is formed on the substrate, the gate insulating film formed so as to cover the gate electrode, and the oxide semiconductor thin film formed on the gate insulating film A bottom gate type thin film transistor having a layer and a protective insulating film formed on the oxide semiconductor thin film layer, thereby producing a bottom gate type amorphous silicon TFT currently commercialized as a driving element of a liquid crystal display The process can be applied, new equipment investment can be reduced, and a thin film transistor having an oxide semiconductor thin film layer mainly composed of zinc oxide can be commercialized.
Further, at least a part of the protective insulating film is a silicon oxide film, a silicon oxynitride film, a silicon nitride film, or a film in which silicon nitride is doped with oxygen or a compound containing oxygen as a constituent element. The use of the insulating film can prevent lattice defects due to desorption of oxygen or zinc when forming the protective insulating film. Therefore, the amount of hydrogen diffused into the oxide semiconductor thin film layer can be controlled more strictly, and the thin film transistor can exhibit more reliably the effects of suppressing leakage current, lowering the threshold voltage, and improving electron mobility.

  Embodiments of a thin film transistor according to the present invention will be described below with reference to the drawings.

  FIG. 1 is a sectional view showing the structure of a thin film transistor according to a first embodiment of the present invention. The thin film transistor 100 includes a substrate 1, a pair of source / drain electrodes 2, an oxide semiconductor thin film layer 3, a first gate insulating film 4, a contact portion 5a, a pair of source / drain external electrodes 2a, a second gate insulating film 6, and a gate. It has an electrode 7 and a display electrode 8, and is formed as a top gate type in which these components are stacked.

  The thin film transistor 100 is formed on the substrate 1 as shown in FIG.

  A pair of source / drain electrodes 2 are stacked on the substrate 1. The pair of source / drain electrodes 2 are arranged on the upper surface of the substrate 1 with a gap.

The oxide semiconductor thin film layer 3 is stacked on the substrate 1 and the pair of source / drain electrodes 2.
The oxide semiconductor thin film layer 3 is disposed so as to form a channel between the pair of source / drain electrodes 2, and is formed of an oxide semiconductor containing zinc oxide as a main component. Here, the oxide semiconductor containing zinc oxide as a main component includes intrinsic zinc oxide, p-type dopants such as Li, Na, N, and C, and n-type dopants such as B, Al, Ga, and In, and Mg. And zinc oxide doped with Be or the like.

  The first gate insulating film 4 is formed so as to cover only the upper surface of the oxide semiconductor thin film layer 3. The first gate insulating film 4 is provided as a part of the gate insulating film, and also serves as a protective film for protecting the oxide semiconductor thin film layer 3 from a resist stripping solution in the manufacturing process.

  The second gate insulating film 6 is laminated so as to cover the pair of source / drain electrodes 2, the side surfaces of the oxide semiconductor thin film layer 3, and the entire surface of the first gate insulating film 4. Thus, by laminating the second gate insulating film 6, the surface of the oxide semiconductor thin film layer 3 can be completely covered with the first gate insulating film 4 and the side surface can be completely covered with the second gate insulating film 6. it can.

The first gate insulating film 4 and the second gate insulating film 6 are composed of silicon oxide (SiOx) film, silicon oxynitride (SiON) film, silicon nitride (SiNx) film, or silicon nitride (SiNx) with oxygen or oxygen as a constituent element. By using the compound contained in the film, a film doped with oxygen is formed. As the first gate insulating film 4 and the second gate insulating film 6, a compound having a dielectric constant larger than that of silicon oxide (SiOx) or silicon oxynitride (SiON), SiNx containing oxygen or oxygen as a constituent element, For example, a film doped with oxygen by using N 2 O is preferably used.
The first gate insulating film 4 and the second gate insulating film 6 are formed by plasma chemical vapor deposition (PCVD), for example. At this time, it is desirable to perform film formation by plasma enhanced chemical vapor deposition (PCVD) at 250 ° C. or lower, which is a substrate temperature at which reduction of the oxide semiconductor thin film layer or desorption of oxygen and zinc does not occur.

  The pair of source / drain external electrodes 2a are connected to the corresponding source / drain electrodes 2 via the contact portions 5a.

  The gate electrode 7 is formed on the second gate insulating film 6. The gate electrode 7 serves to control the electron density in the oxide semiconductor thin film layer 3 by a gate voltage applied to the thin film transistor.

  The display electrode 8 is formed in order to apply a voltage to the liquid crystal used for the liquid crystal display via a thin film transistor. Since this electrode requires high transmittance for visible light, an oxide conductive thin film using indium tin oxide (ITO) or the like is formed. Although omitted in FIG. 1, the display electrode 8 is formed to extend rightward on the second gate insulating film.

After the manufacturing process of the thin film transistor is completed, the oxide semiconductor thin film layer 3 includes at least hydrogen. The manufacturing conditions of each step are adjusted so that the hydrogen concentration in the oxide semiconductor thin film layer 3 is preferably 5 × 10 20 cm −3 or more and 7 × 10 21 cm −3 or less. When the hydrogen concentration is lower than 5 × 10 20 cm −3 , sufficient carriers are not induced in the active layer of the thin film transistor and the leakage current is reduced, but the electron mobility is not improved. On the other hand, when the hydrogen concentration is higher than 7 × 10 21 cm −3 , the resistivity of the active layer decreases, the thin film transistor operates in a normally-on type, that is, a depletion type, and the leakage current increases. Therefore, in any case, the hydrogen concentration is outside the range of 5 × 10 20 cm −3 or more and 7 × 10 21 cm −3 or less.

  A manufacturing method of the first embodiment of the thin film transistor (TFT) according to the present invention will be described below with reference to FIG.

  First, as shown in FIG. 2A, after a metal thin film is formed on the entire surface of the substrate 1, a pair of source / drain electrodes 2 is formed by subjecting this thin film to photolithography.

Next, as shown in FIG. 2 (2), an oxide semiconductor thin film layer 3 is formed on the entire surface of the substrate 1 and the pair of source / drain electrodes 2, and an oxide semiconductor containing zinc oxide as a main component is about 50 to 100 nm, for example. It is formed with a film thickness. When the oxide semiconductor thin film layer is intrinsic zinc oxide (ZnO), the semiconductor thin film is formed by a magnetron sputtering method using a high-purity zinc oxide ceramic as a target, and a mixed gas of argon (Ar) and oxygen (O 2 ) A semiconductor thin film is formed on the substrate by plasma discharge. Before plasma discharge, the vacuum vessel is evacuated until the degree of vacuum is 1 × 10 −4 Pa or less, and then a mixed gas of argon and oxygen is introduced. Since the pressure at the time of sputtering is about 1 to 10 Pa, the proportion of hydrogen due to residual moisture in the vacuum chamber can be suppressed to 1/10 4 or less of zinc oxide in terms of atomic concentration. In this case, the atomic concentration of hydrogen after forming the oxide semiconductor thin film is 5 × 10 19 cm −3 or less. The hydrogen concentration can be determined by secondary ion mass spectrometry (SIMS) method.

Next, the first gate insulating film 4 is formed on the ZnO by a method and conditions that do not reduce the resistance. When performing with NH 3 and a mixed gas of SiH 4 to form the first gate insulating film, by changing the flow ratio of NH 3 and SiH 4, by controlling the hydrogen concentration in the oxide semiconductor thin film layer (See the test example below).
As the first gate insulating film 4, a silicon oxide (SiOx) film, a silicon oxynitride (SiON) film, a silicon nitride (SiNx) film, or a silicon nitride (SiNx) containing oxygen or a compound containing oxygen as a constituent element is used. It is desirable to use a silicon insulating film such as a film doped with oxygen. In particular, a film obtained by doping oxygen with SiNx using oxygen or a compound containing oxygen as a constituent element, for example, N 2 O, is desirable. This is because these components have a high dielectric constant and are excellent from the viewpoint of preventing the elimination of zinc and oxygen from the oxide semiconductor thin film layer 3.
Note that as a method for controlling the hydrogen concentration in the oxide semiconductor thin film layer, in addition to the above-described method, a method of changing the substrate temperature, gas type, plasma treatment conditions, etc. during the formation of the gate insulating film can also be exemplified. . Atomic layer deposition (ALD), which introduces moisture during the formation of oxide semiconductor thin film layers, specifically introduces water vapor during sputtering or alternately introduces diethyl zinc (DEZ) and water vapor. ) Can also be mentioned.

  As shown in FIG. 2C, a photoresist is coated on the first gate insulating film 4, the first gate insulating film 4 is dry-etched using the patterned photoresist 4a as a mask, and then the oxide semiconductor Wet etching is performed on the thin film layer 3.

  FIG. 2 (4) shows a cross section in which the photoresist 4a is removed after wet etching of the oxide semiconductor thin film layer 3, and the TFT active layer having the first gate insulating film 4 having the same shape as the oxide semiconductor thin film layer 3 is shown. A region is formed. In addition to forming an interface with the oxide semiconductor thin film layer 3, the first gate insulating film 4 also plays a role of protecting the oxide semiconductor thin film layer 3 when patterning the active region. That is, when the resist stripping solution used for stripping the photoresist 4a after the active layer patterning contacts the surface of the oxide semiconductor thin film layer 3, the surface of the thin film and the crystal grain boundary are roughened by etching. The presence of the film 4 on the surface of the oxide semiconductor thin film layer 3 serves as a protective film against various chemicals such as a resist stripping solution in a photolithography process, and can prevent surface roughness of the oxide semiconductor thin film layer 3.

  After the patterning of the TFT active layer region, as shown in FIG. 2 (5), the substrate 1, the pair of source / drain electrodes 2, the oxide film so as to cover the first gate insulating film 4 and the pair of source / drain electrodes 2. A second gate insulating film 6 is formed on the entire surface of the physical semiconductor thin film layer 3 and the first gate insulating film 4, and then contact holes 5 are opened on the source / drain electrodes. In this case, the second gate insulating film 6 is desirably formed under the same conditions as the first gate insulating film 4 (interface control type insulating film).

  Finally, as shown in FIG. 2 (6), a gate electrode 7 made of a metal film is formed on the second gate insulating film 6, and a pair of source / drain external electrodes 2 a are made of the same material as the gate electrode 7 and contact parts 5 a. It connects so that it may connect with a pair of corresponding source / drain electrodes 2 via. Thereafter, the display electrode 8 is formed to complete the TFT of the first embodiment.

  Next, the structure of the thin film transistor according to the second embodiment of the present invention will be described with reference to FIG. The structure of the second embodiment is similar to the structure of the bottom-gate type amorphous silicon TFT that is currently commercialized as a driving element for liquid crystal displays, and can be applied to the amorphous silicon TFT manufacturing process. This is effective in that it can reduce the amount of zinc oxide and commercialize zinc oxide TFTs.

  FIG. 3 is a cross-sectional view showing the structure of the thin film transistor 101 according to the second embodiment of the present invention. The thin film transistor 101 includes a substrate 9, a gate electrode 10, a gate insulating film 11, an oxide semiconductor thin film layer 12 mainly composed of zinc oxide, a protective insulating film 13, a first overcoat insulating film 14, and a pair of source / drain electrodes 15. The second overcoat insulating film 16 is provided, and as shown in FIG.

  The thin film transistor 101 is formed on the substrate 9 as shown in FIG.

  A gate electrode 10 is formed on the substrate 9.

  The gate insulating film 11 is laminated on the entire surface of the substrate 9 so as to cover the gate electrode 10.

  The oxide semiconductor thin film layer 12 is formed so as to cover a part of the gate insulating film 11 across the gate electrode 10. The oxide semiconductor thin film layer 12 is formed of an oxide semiconductor containing zinc oxide as a main component.

The protective insulating film 13 is laminated so as to cover the upper surface of the oxide semiconductor thin film layer 12. The protective insulating film 13 is provided to protect the oxide semiconductor thin film layer 12 made of zinc oxide from damage and reductive desorption. As a protective film that protects the oxide semiconductor thin film layer 12 from the resist stripping solution in the manufacturing process. Also plays a role.
The protective insulating film 13 is formed by using a silicon oxide (SiOx) film, a silicon oxynitride (SiON) film, a silicon nitride (SiNx) film, or silicon nitride (SiNx) by using oxygen or a compound containing oxygen as a constituent element. It is formed by a doped film. As this protective insulating film 13, a compound having a dielectric constant larger than that of silicon oxide (SiOx) or silicon oxynitride (SiON) and containing oxygen or oxygen as a constituent element in SiNx, such as N 2 O, is used. A film doped with oxygen is preferably used.
The protective insulating film 13 is formed by, for example, a plasma chemical vapor deposition (PCVD) method. At this time, it is desirable to perform film formation by plasma enhanced chemical vapor deposition (PCVD) at 250 ° C. or lower, which is a substrate temperature at which reduction of the oxide semiconductor thin film layer or desorption of zinc or oxygen does not occur.

The first overcoat insulating film 14 is provided for the purpose of device protection of the thin film transistor 101 and is laminated so as to cover the entire surface of the protective insulating film 13 and the side surface of the oxide semiconductor thin film layer 12.
By providing the first overcoat insulating film 14, the side surface of the oxide semiconductor thin film layer 12 that is not covered with the protective insulating film 13 can be reliably covered.

  The pair of source / drain electrodes 15 are formed at a distance from each other so as to be in contact with the oxide semiconductor thin film layer 12 through contact holes opened in the protective insulating film 13 and the first overcoat insulating film 14.

The second overcoat insulating film 16 is provided for the purpose of protecting the device of the thin film transistor 101 and is laminated so as to cover the entire surface of the thin film transistor.
By providing the second overcoat insulating film 16, the entire device of the thin film transistor 101 can be more reliably protected.

After the manufacturing process of the thin film transistor is completed, the oxide semiconductor thin film layer 12 includes at least hydrogen as in the TFT according to the first embodiment. The manufacturing conditions of each step are adjusted so that the hydrogen concentration in the oxide semiconductor thin film layer 3 is preferably 5 × 10 20 cm −3 or more and 7 × 10 21 cm −3 or less.

  Next, a method for manufacturing the bottom gate type TFT according to the second embodiment of the present invention will be described below with reference to FIG.

  As shown in FIG. 4A, a gate electrode 10 is formed on the entire surface of the substrate 9 made of glass or the like by a magnetron sputtering method or the like, and by photolithography.

Next, as shown in FIG. 4B, a gate insulating film 11 is formed on the entire surface of the substrate 9 so as to cover the gate electrode 10.
The method for forming the gate insulating film 11 is not particularly limited, but it is preferable to use a plasma enhanced chemical vapor deposition (PCVD) method capable of forming a film over a large area substrate.
After the gate insulating film 11 is formed, the substrate surface is preferably cleaned with plasma using an oxidizing gas such as oxygen (O 2 ) or nitrous oxide (N 2 O). In particular, when oxygen is used as the oxidizing gas, the amount of oxygen radicals generated is increased by using a plasma in which a rare gas such as Ar, Xe, He, or Kr is added to oxygen, and the surface of the oxide semiconductor thin film layer is increased. The cleaning efficiency for the adsorbed organic component and moisture is increased, and at the same time, metal impurities on the surface of the oxide semiconductor thin film layer can be removed by the sputtering effect of the additive gas, which is more preferable.

  After the formation of the gate insulating film 11, as shown in FIG. 4C, the oxide semiconductor thin film layer 12 is formed on the entire surface of the gate insulating film 11 with a film thickness of, for example, about 50 to 100 nm. Similar to the TFT of the first embodiment, the oxide semiconductor thin film layer is formed by a magnetron sputtering method using a high-purity zinc oxide ceramic as a target, and plasma discharge using a mixed gas of argon (Ar) and oxygen. A semiconductor thin film is formed thereon.

After the oxide semiconductor thin film layer 12 is formed, as shown in FIG. 4D, a protective insulating film 13 that covers the entire surface of the oxide semiconductor thin film layer is formed.
The protective insulating film 13 is formed by using a silicon oxide (SiOx) film, a silicon oxynitride (SiON) film, a silicon nitride (SiNx) film, or silicon nitride (SiNx) by using oxygen or a compound containing oxygen as a constituent element. It is desirable to use a silicon-based insulating film such as a doped film. In particular, a film in which oxygen is doped with SiNx by using oxygen or a compound containing oxygen as a constituent element, such as N 2 O, is desirable. This is because these components have a high dielectric constant and are excellent from the viewpoint of preventing the elimination of zinc and oxygen from the oxide semiconductor thin film layer 3.
In forming the protective insulating film 13, it is preferable to use a plasma enhanced chemical vapor deposition (PCVD) method.

  After the formation of the protective insulating film 13, a photoresist is coated on the upper surface of the protective insulating film 13, the protective insulating film 13 is etched using the patterned photoresist as a mask, and then the patterned protective insulating film 13 is used as a mask. Wet etching is performed on the oxide semiconductor thin film layer 12. As a result, a structure in which the protective insulating film 13 having the same planar shape is laminated on the channel region, that is, the oxide semiconductor thin film layer 12 having the outer shape of the device is formed.

  As described above, after the shape processing is performed on the oxide semiconductor thin film layer 12 and the protective insulating film 13, the photoresist is peeled off, and as shown in FIG. 4 (5), the protective insulating film 13, the oxide semiconductor thin film A first overcoat insulating film 14 is formed so as to cover the entire surface of the layer 12 and the gate insulating film 11. By forming the first overcoat insulating film 14, the oxide semiconductor thin film layer 12 is completely covered with the first overcoat insulating film 14 while maintaining the function of the oxide semiconductor thin film layer 12 as the channel layer. Can be realized.

After the first overcoat insulating film 14 is formed, a photoresist is coated on the first overcoat insulating film 14 and patterned (not shown). Then, as shown in FIG. 4 (6), two contact holes are formed at intervals as contact portions between a pair of source / drain electrodes 15 and an oxide semiconductor thin film layer 12 to be described later.
The contact hole penetrates the protective insulating film 13 and the first overcoat insulating film 14 by forming an opening in the photoresist coated on the first overcoat insulating film 14 and etching through the opening. Then, the oxide semiconductor thin film layer 12 is formed to a depth reaching the surface.

After the contact hole is formed, the photoresist is peeled off to form a pair of source / drain electrodes 15.
The pair of source / drain electrodes 15 are filled with contact holes and spaced apart.

  Finally, the second overcoat insulating film 16 is formed on the thin film transistor, thereby completing the TFT of the second embodiment.

(Test example)
Hereinafter, the effect of the present invention will be made clearer by comparing test examples of the thin film transistor according to the present invention.

A thin film transistor (see FIG. 1) according to the first embodiment of the present invention was prepared by the following method (see FIG. 2).
A pair of source / drain electrodes 2 made of indium tin oxide (ITO) was formed to a thickness of 40 nm on a substrate 1 made of alkali-free glass mainly composed of SiO 2 and Al 2 O 3 .
Intrinsic zinc oxide was formed as an oxide semiconductor thin film layer 3 to a thickness of 50 nm on the entire surface of the substrate 1 and the pair of source / drain electrodes 2 by magnetron sputtering.
After the oxide semiconductor thin film layer 3 was formed, a first gate insulating film 4 made of SiNx was formed to a thickness of 50 nm on the entire upper surface of the oxide semiconductor thin film layer 3. The first gate insulating film was formed by plasma enhanced chemical vapor deposition (PCVD) using SiH 4 + NH 3 + N 2 gas at 250 ° C.
When forming the first gate insulating film using a mixed gas of NH 3 and SiH 4 , the NH 3 / SiH 4 gas flow ratio was changed from 1 to 20 in order to change the hydrogen concentration in the zinc oxide active layer. It was. Increasing the NH 3 / SiH 4 gas flow ratio increases the concentration of hydrogen taken into the oxide semiconductor thin film layer during the formation of the gate insulating film.
Next, a photoresist was coated on the first gate insulating film 4, and the first gate insulating film 4 was dry-etched using CF 4 + O 2 gas using the patterned photoresist 4 a as a mask.
After etching the first gate insulating film 4, wet etching is performed on the oxide semiconductor thin film layer using a 0.2% HNO 3 solution to remove the photoresist, and the substrate 1 and the pair of source / drain electrodes 2. A second gate insulating film 6 made of SiNx was formed to a thickness of 300 nm over the entire surface of the oxide semiconductor thin film layer 3 and the first gate insulating film 4.
The second gate insulating film 6 was formed at 250 ° C. using a plasma enhanced chemical vapor deposition (PCVD) method using SiH 4 + NH 3 + N 2 gas. The NH 3 / SiH 4 ratio during the formation of the second gate insulating film was set to be the same as that of the first gate insulating film.
After the second gate insulating film 6 was formed, a contact hole 5 was opened on the pair of source / drain electrodes 2 by photolithography and dry etching using CF 4 + O 2 gas.
Finally, a gate electrode 7 made of Cr is formed on the second gate insulating film 6 to a thickness of 100 nm, and a pair of source / drain external electrodes 2a are made of the same material via the contact portions 5a, respectively. A display electrode 8 made of indium tin oxide (ITO) was formed on a part of the second gate insulating film 6 to a thickness of 100 nm so as to be connected to the electrode 2 to produce a thin film transistor.
This time, the gas flow rate ratio was changed to control the hydrogen concentration in the oxide semiconductor thin film layer. However, the method is not limited to this method. Even if the conditions are changed, the hydrogen concentration of the oxide semiconductor thin film layer can be similarly controlled. Alternatively, water is introduced when forming an oxide semiconductor thin film layer. Specifically, water vapor is introduced during sputtering, or diethyl zinc (DEZ) and water vapor are alternately introduced (Atomic Layer Deposition: ALD). The hydrogen concentration in zinc oxide can also be controlled by such a method.

(Transfer characteristics evaluation test)
The hydrogen concentration contained in the oxide semiconductor active layer of each prepared thin film transistor was measured with a secondary ion mass spectrometry (SIMS) apparatus, and the transfer characteristics of the thin film transistor were evaluated.
The result is shown in FIG.

As apparent from FIG. 5, when the NH 3 / SiH 4 gas flow rate ratio is lowered and the hydrogen concentration in the oxide semiconductor thin film layer is lowered, sufficient carriers are not induced in the active layer, and the thin film transistor Although the leakage current is small, the mobility is also small. When the NH 3 / SiH 4 gas flow rate ratio is 1.5 or more and the hydrogen concentration in the active layer is 5 × 10 20 cm −3 or more, the mobility starts to improve while the leakage current is kept low. Furthermore, increasing the NH 3 / SiH 4 gas flow ratio increases the hydrogen concentration in the active layer. In this test example, the mobility was maximized when the hydrogen concentration in the oxide semiconductor layer was 1 × 10 21 cm −3 . When the NH 3 / SiH 4 gas flow ratio is made higher than 10.0 and the hydrogen concentration is made higher than 7 × 10 21 cm −3 , the resistivity of the active layer decreases, and the normally-on type or depletion type operation is achieved. As a result, the leakage current increased and the thin film transistor was not turned off.
From the above results, the thin film transistor used as the driving element of the liquid crystal display is required to have a small leakage current, so that the hydrogen concentration in the active layer is 5 × 10 20 cm −3 or more and 7 × 10 21 cm −3 or less. As a result, it was found that good transistor characteristics can be obtained while maintaining a low leakage current.

  As described above, the thin film transistor using the zinc oxide according to the present invention for the semiconductor thin film layer has excellent performance and can be suitably used as a driving element for a liquid crystal display device or the like.

It is sectional drawing which shows the form of the thin-film transistor (TFT) based on the 1st Example of this invention. BRIEF DESCRIPTION OF THE DRAWINGS It is sectional drawing which shows one form of the manufacturing method of the thin-film transistor (TFT) based on 1st Example of this invention over time, (1) Sectional drawing of the structure which formed the source / drain electrode on the board | substrate (2) Oxide Sectional view of structure coated with semiconductor thin film layer and first gate insulating film (3) Sectional view of structure coated with photoresist (4) Sectional view of structure patterned with oxide semiconductor thin film and first gate insulating film (5) ) Cross-sectional view of the structure in which the second gate insulating film and the contact hole are formed. (6) The cross-sectional view of the structure in which the gate electrode, contact portion, source / drain external electrode, and display electrode are formed. It is sectional drawing which shows the form of the thin-film transistor (TFT) based on the 2nd Example of this invention. It is sectional drawing which shows one form of the manufacturing method of the thin-film transistor (TFT) based on 2nd Example of this invention over time, (1) Sectional drawing of the structure in which the gate electrode was formed on the board | substrate (2) Gate insulating film Cross-sectional view of coated structure (3) Cross-sectional view of structure coated with oxide semiconductor thin film layer (4) Cross-sectional view of structure coated with protective insulating film (5) Shape processing of oxide semiconductor thin film layer and protective insulating film Thereafter, a cross-sectional view of a structure in which a first overcoat insulating film is formed (6) A cross-sectional view of a structure in which a source / drain electrode and a second overcoat insulating film are formed. It is a figure which shows the transfer characteristic of the transistor of a test example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Source / drain electrode 3 Oxide semiconductor thin film layer 4 First gate insulating film 6 Second gate insulating film 7 Gate electrode 9 Substrate 10 Gate electrode 11 Gate insulating film 12 Oxide semiconductor thin film layer 13 Protective insulating film 14 First Overcoat insulating film 15 Source / drain electrode 16 Second overcoat insulating film 100 Top gate type thin film transistor 101 Bottom gate type thin film transistor

Claims (3)

  1. An oxide semiconductor thin film layer mainly composed of zinc oxide formed as a channel on the substrate; a gate insulating film; and a gate electrode, wherein the oxide semiconductor thin film layer contains at least hydrogen , A thin film transistor, wherein the concentration of hydrogen contained in the oxide semiconductor thin film layer is 5 × 10 20 cm −3 or more and 7 × 10 21 cm −3 or less .
  2. The thin film transistor is stacked on the gate insulating film, the oxide semiconductor thin film layer formed on the substrate, the gate insulating film formed to cover an upper surface and a side surface of the oxide semiconductor thin film layer A gate oxide film comprising a silicon oxide film, a silicon oxynitride film, a silicon nitride film, or a silicon nitride compound containing oxygen or oxygen as a constituent element 2. The thin film transistor according to claim 1 , wherein the thin film transistor is an insulating film using at least a part of any of the films doped with oxygen.
  3. The thin film transistor includes the gate electrode formed on the substrate, the gate insulating film formed to cover the gate electrode, the oxide semiconductor thin film layer formed on the gate insulating film, A bottom-gate thin film transistor having a protective insulating film formed over an oxide semiconductor thin film layer, wherein the protective insulating film contains oxygen or oxygen in a silicon oxide film, a silicon oxynitride film, a silicon nitride film, or silicon nitride. 2. The thin film transistor according to claim 1 , wherein the thin film transistor is an insulating film using at least a part of a film doped with oxygen by using a compound containing a constituent element.
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