JP4239744B2 - Thin film transistor manufacturing method - Google Patents

Description

  The present invention relates to a thin film transistor and a method of manufacturing the same, and more particularly, a thin film transistor using a silicon oxide film formed by plasma CVD as an undercoat film of a semiconductor layer, for example, a thin film transistor constituting each pixel of a liquid crystal display device It relates to the improvement of the method.

  A liquid crystal display device called an active matrix type is formed by forming a large number of thin film transistors in a matrix on a glass substrate. In general, an amorphous silicon film can be manufactured at a lower temperature than a polycrystalline silicon film. Therefore, in a conventional liquid crystal display device, a thin film transistor having an amorphous silicon film as a semiconductor layer, a so-called amorphous silicon TFT (Thin Film TFT). Transistor) is widely used.

  However, the development of a laser annealing technique for irradiating an amorphous silicon film with a laser to locally melt and crystallize it has made it possible to produce a polycrystalline silicon film even at a low temperature. A thin film transistor using a polycrystalline silicon film obtained by laser annealing as a semiconductor layer is called a low-temperature polysilicon TFT. Low temperature polysilicon TFTs have recently attracted attention because of their superior operating characteristics compared to amorphous silicon TFTs.

  While this low-temperature polysilicon TFT has higher operating characteristics and driving capability than amorphous silicon TFTs, it is not easy to control individual TFT characteristics. In view of this, there has been proposed a method for stabilizing TFT characteristics by forming a silicon oxide film as an undercoat film (underlayer film) of a semiconductor layer (for example, Patent Document 1).

  FIG. 8 is a cross-sectional view showing the structure of a conventional low-temperature polysilicon TFT. This low temperature polysilicon TFT is a so-called top gate type thin film transistor. A two-layer insulating film made of a silicon nitride film 2 and a silicon oxide film 3 is formed as an undercoat film on an insulating substrate 1 such as a glass substrate. Polycrystalline silicon (polysilicon) is formed on the undercoat film. A film 4 is formed. The polycrystalline silicon film 4 is obtained by forming an amorphous silicon film and then melting and crystallizing it by irradiating a laser. When the substrate on which the polycrystalline silicon film 4 is formed is patterned by photolithography, an island-shaped semiconductor layer (polycrystalline silicon film 4) is formed. A gate insulating film (silicon oxide film) 5 is formed on the semiconductor layer.

  A conductive metal film such as a chromium film is formed on the gate insulating film 5 by sputtering, and the gate electrode 6 is formed by patterning this conductive metal film by photolithography. This gate electrode 6 is also used as a mask when an impurity such as phosphorus is implanted into the semiconductor layer (polycrystalline silicon film 4). That is, impurities are ion-doped into the polycrystalline silicon film 4 through the gate insulating film 5, and the impurities are activated by the subsequent heat treatment (annealing), so that source / drain regions are formed in the semiconductor layer.

  On the gate electrode 6, a silicon oxide film is deposited by plasma CVD (Chemical Vapor Deposition) to form an interlayer insulating film 7. Thereafter, contact holes are formed in the interlayer insulating film 7 by patterning by photolithography, and a conductive metal film such as a chromium film is formed on the interlayer insulating film 7 by sputtering. By patterning this conductive metal film by photolithography, source and drain electrodes 8 are formed. A passivation film (silicon nitride film) 9 is formed on the source and drain electrodes 8.

  FIG. 9 is a diagram showing an example of a main part of a conventional low-temperature polysilicon TFT manufacturing process, and shows steps until an island-shaped semiconductor layer is formed. First, a silicon nitride film 2 and a silicon oxide film 3 are sequentially formed on the insulating substrate 1 (step of FIG. 9A). The silicon nitride film 2 prevents the diffusion of impurities from the insulating substrate 1 to the semiconductor layer. Further, by providing the silicon oxide film 3 between the silicon nitride film 2 and the semiconductor layer, the semiconductor layer can be kept away from the silicon nitride film 2. For this reason, the influence of the defect level of the silicon nitride film 2 can be suppressed, and the TFT characteristics can be improved. Note that an effect that stress generated between the insulating substrate 1 and the semiconductor layer in the heat treatment such as laser annealing is relaxed by the silicon oxide film 3 is also conceivable. Such a silicon oxide film 3 is formed using, for example, a parallel plate RF plasma CVD apparatus. An amorphous silicon film 10 is formed on the silicon oxide film 3 (step of FIG. 9B).

  Next, a heat treatment for degassing hydrogen in the amorphous silicon film 10 is performed, and laser irradiation is performed after the heat treatment (step of FIG. 9C). For laser irradiation, an excimer laser having a wavelength in the ultraviolet region is used. The amorphous silicon film 10 is melted by laser irradiation and crystallized by subsequent natural cooling. A polycrystalline silicon film 4 is formed by such laser annealing. After laser annealing, the substrate on which the polycrystalline silicon film 4 is formed is patterned by photolithography to form an island-shaped semiconductor layer (polycrystalline silicon film 4) (step of FIG. 9D).

  In general, by using a tetraethoxysilane (TEOS) material gas as a material gas in plasma CVD, the composition is denser than when using another material gas, for example, a monosilane material gas. A good quality silicon oxide film can be formed. That is, it is considered that a silicon oxide film using a TEOS-based material gas has few defects in the crystal structure, and TFT characteristics are improved by using this as an undercoat film.

However, in order to obtain desired TFT characteristics, the film thickness of the silicon oxide film 3 as an undercoat film of the semiconductor layer needs to be a certain value or more, whereas plasma CVD using TEOS as a material gas is The deposition rate (deposition) is slower than that using other material gases. For this reason, when the silicon oxide film 3 is formed by plasma CVD using a TEOS-based material gas, there is a problem that the time required for forming a desired film thickness becomes long and the production efficiency is lowered. .
JP 2000-183360 A

  If a silicon oxide film as an undercoat film of a semiconductor layer is formed using a TEOS-based material gas, TFT characteristics are improved. On the other hand, when the silicon oxide film is formed using a TEOS-based material gas, the conventional thin film transistor manufacturing method described above has a problem that the production efficiency is lowered.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a thin film transistor having improved operating characteristics as a thin film transistor, in particular, a threshold voltage characteristic, and a method for manufacturing the same. It is another object of the present invention to provide a method for manufacturing a thin film transistor that can improve operating characteristics without reducing productivity.

  A method of manufacturing a thin film transistor according to the present invention includes a step of forming a first base film made of silicon oxide on a substrate by plasma CVD using a monosilane material gas, and a plasma CVD using a tetraethoxysilane material gas. , A step of forming a second base film made of silicon oxide on the first base film, and a step of forming a semiconductor film made of amorphous silicon on the second base film.

  According to such a configuration, plasma CVD using a monosilane-based material gas having a relatively high deposition rate and plasma CVD using a tetraethoxysilane-based material gas having a relatively slow deposition rate are sequentially performed. Since the first and second base films are formed, the time required to form a desired film thickness is shortened as compared with the case where the film is formed only by plasma CVD using a tetraethoxysilane-based material gas. Production efficiency can be improved. In addition, since the second base film in contact with the semiconductor film is formed using a tetraethoxysilane-based material gas, the base film in contact with the semiconductor film can have a dense composition and high quality. For this reason, it is possible to improve the operating characteristics of the thin film transistor, particularly the threshold voltage characteristics.

  In addition to the above structure, the method for manufacturing a thin film transistor according to the present invention is configured such that the first base film, the second base film, and the semiconductor film are sequentially formed while maintaining a vacuum state. According to such a structure, the semiconductor film can be formed without exposing the first base film surface and the second base film surface to the atmosphere or the like, so that the operation characteristics as a thin film transistor can be improved. it can.

  In addition to the above configuration, the method for manufacturing a thin film transistor according to the present invention is configured such that the film thickness of the second base film is smaller than the film thickness of the first base film. According to such a configuration, since the film thickness of the second base film having a low film formation rate is smaller than that of the first base film having a high film formation speed, the production efficiency can be further improved. Such an improvement in production efficiency can further increase the thickness of the base film, and an improvement in TFT characteristics can be expected.

  In addition to the above structure, the method of manufacturing a thin film transistor according to the present invention includes a step of forming a third base film made of amorphous silicon on a substrate, and a fourth step made of silicon nitride on the third base film. Forming a first base film, wherein the first base film is formed on the fourth base film, and the third base film, the fourth base film, the first base film, and the second base film are formed. The base film and the semiconductor film are sequentially formed while maintaining a vacuum state. According to such a configuration, by irradiating the substrate with a laser, the first, second, and fourth underlayer heat treatments can be performed simultaneously with the melt crystallization of the semiconductor film. In particular, if laser irradiation is performed with a YAG laser, the semiconductor film and the third base film can be effectively heated. Therefore, the first, second and fourth base films sandwiched between these two films are formed. It can be heated effectively. By performing the heat treatment on the first, second, and fourth base films, defects in the crystal structure are removed, so that the TFT characteristics can be further improved.

  The thin film transistor according to the present invention is formed by plasma CVD using a monosilane-based material gas, the first base film made of silicon oxide formed on the substrate, and the plasma CVD using a tetraethoxysilane-based material gas. A second base film made of silicon oxide formed on the first base film and a semiconductor film made of polycrystalline silicon formed on the second base film.

  In addition to the above structure, the thin film transistor according to the present invention includes a third base film made of silicon formed on the substrate by plasma CVD and a fourth base film made of silicon nitride formed on the third base film. A first base film formed on the fourth base film, and the semiconductor film irradiates an amorphous silicon film formed on the second base film with a YAG laser. It is comprised so that it may be formed. According to such a configuration, the third base film is heated as the semiconductor film is annealed by irradiating the highly transparent YAG laser, so that the first, second and fourth base films are effectively used. Can be heat-treated.

According to the method of manufacturing a thin film transistor motor according to the present invention, on the silicon nitride film on a substrate, a plasma CVD using a material gas of monosilane, by a plasma CVD using a material gas of tetraethoxysilane system, each first and a manufacturing method for forming the Ru performed silicon oxide film as the second underlayer, a second underlayer which is in contact with the semiconductor film that is polycrystalline is formed using a material gas of tetraethoxysilane system, the first base film on the silicon nitride film by using a material gas of monosilane, is formed at a high film forming rate than the deposition rate of the second base film Runode, without reducing productivity, TFT Characteristics can be improved.

Embodiment 1 FIG.
FIGS. 1A to 1D are views showing an example of the main part of the method for manufacturing a low-temperature polysilicon TFT according to the first embodiment of the present invention, and an island-like semiconductor layer is formed. Each process up to is shown.

  The insulating substrate 1 is a transparent substrate having an insulating property such as a glass substrate, and a silicon nitride film 2, a first silicon oxide film 3a, and a second silicon oxide film 3b are sequentially formed on the insulating substrate 1. (Step of FIG. 1A). The three-layer insulating film composed of the silicon nitride film 2, the first silicon oxide film 3a, and the second silicon oxide film 3b is an undercoat film for stabilizing the characteristics of the semiconductor layer, both of which are formed by plasma CVD. It is formed.

  The insulating substrate 1 may contain impurities such as alkali metals. When this impurity diffuses from the surface of the insulating substrate 1 to the semiconductor layer, the characteristics of the semiconductor layer are deteriorated. In order to prevent impurity diffusion, a silicon nitride film is more suitable than a silicon oxide film. For this reason, the silicon nitride film 2 is provided on the insulating substrate 1 side of the undercoat film to stabilize the characteristics of the semiconductor layer. In order to fully exhibit such a barrier function for preventing the diffusion of impurities, the silicon nitride film 2 needs to have a thickness of at least 10 nm.

  By providing a silicon oxide film composed of the first silicon oxide film 3 a and the second silicon oxide film 3 b between the silicon nitride film 2 and the semiconductor layer, the semiconductor layer can be kept away from the silicon nitride film 2. For this reason, the influence of the defect level of the silicon nitride film 2 can be suppressed, and the TFT characteristics can be improved. In addition, stress may be generated between the insulating substrate 1 and the semiconductor layer by heat treatment such as laser annealing described later, which deteriorates the characteristics of the semiconductor layer. In order to relieve stress generated by heat treatment, a silicon oxide film is more suitable than a silicon nitride film. For this reason, a silicon oxide film is provided on the semiconductor layer side of the undercoat film, and the characteristics of the semiconductor layer are stabilized. Since the silicon nitride film 2 has a function of suppressing the influence of the defect level and relieving the stress between the insulating substrate 1 and the semiconductor layer, the first silicon oxide film 3a and the second silicon oxide film 3b It is considered that the thicker the silicon oxide film, the better the characteristics of the semiconductor layer.

The first and second silicon oxide films differ in the type of material gas used in plasma CVD. The first silicon oxide film 3a is formed using a monosilane-based material gas, for example, a mixed gas of SiH ₄ and N ₂ O. On the other hand, the second silicon oxide film 3b is formed using a tetraethoxysilane (TEOS) -based material gas, for example, a mixed gas of Si (OC ₂ H ₅ ) ₄ and O ₂ .

  In general, plasma CVD using a monosilane-based material gas has a higher deposition rate (high deposition rate) than that using a TEOS-based material gas. For this reason, if a silicon oxide film having a desired film thickness is formed by plasma CVD using a TEOS-based material gas and plasma CVD using a monosilane-based material gas, compared to a method using only a TEOS-based material gas. Since the time required for forming the silicon oxide film is shortened, the production efficiency can be improved. In particular, if the film thickness of the second silicon oxide film 3b, which requires a long time for film formation, is smaller than the film thickness of the first silicon oxide film 3a, the production efficiency can be greatly improved.

  In addition, monosilane-based material gas is less expensive than TEOS-based material gas. For this reason, if a silicon oxide film having a desired film thickness is formed by plasma CVD using a TEOS-based material gas and plasma CVD using a monosilane-based material gas, compared to a method using only a TEOS-based material gas. A silicon oxide film can be formed at a low cost.

  The second silicon oxide film 3b formed using the TEOS-based material gas has a finer composition and a higher quality than the first silicon oxide film 3a formed using the monosilane-based material gas. It is. For this reason, when the silicon oxide film in contact with the semiconductor layer is the high-quality second silicon oxide film 3b, characteristics of the semiconductor layer, for example, threshold voltage characteristics and electric field mobility can be improved.

  Further, since the production efficiency is improved, the thickness of the silicon oxide film can be increased, and the characteristics of the semiconductor layer, in particular, the threshold voltage characteristics and the electric field mobility can be improved. For example, in order to obtain desired operating characteristics, it is desirable that the thickness of the silicon oxide film formed of the first silicon oxide film 3a and the second silicon oxide film 3b be 100 nm or more. On the other hand, if the film thickness exceeds 500 nm, improvement in operating characteristics cannot be expected. Therefore, it is sufficient that the silicon oxide film has a film thickness of 500 nm or less. That is, based on production efficiency and operating characteristics, the thickness of the silicon oxide film is preferably set to any one of 100 to 500 nm.

  Next, an amorphous silicon film 10 is formed on the second silicon oxide film 3b (step shown in FIG. 1B). The amorphous silicon film 10 is an amorphous silicon film that is melt-crystallized by laser annealing, which will be described later, and becomes an active semiconductor layer by ion doping, and the film thickness is usually about 50 nm. All of the silicon nitride film 2, the first silicon oxide film 3a, the second silicon oxide film 3b, and the amorphous silicon film 10 are sequentially formed by a CVD apparatus (for example, a parallel plate RF plasma CVD apparatus). . Moreover, all these layers are formed continuously in the vacuum chamber of the CVD apparatus. That is, the processing from the formation of the silicon nitride film 2 to the formation of the amorphous silicon film 10 is performed while keeping the vacuum state without carrying the substrate out of the vacuum chamber. Therefore, it is possible to prevent an oxide film that causes a defect in the crystal structure from being formed at the interface between the layers. In particular, since the interface between the amorphous silicon film 10 as the semiconductor layer and the second silicon oxide film 3b can be prevented from being contaminated by the oxide film, the characteristics of the semiconductor layer can be stabilized. .

  Next, the substrate on which the amorphous silicon film 10 is formed is irradiated with laser, and the amorphous silicon film 10 is melt-crystallized to form the polycrystalline silicon film 4 (step of FIG. 1C). ). For laser irradiation, a YAG (yttrium, aluminum, garnet) laser having a wavelength in the infrared region is used. For example, a second harmonic (wavelength 532 nm) component of a laser having a fundamental wave wavelength of 1064 nm is used.

  This YAG laser is a kind of solid-state laser, and has a wavelength suitable for laser annealing (melt crystallization) of the amorphous silicon film 10 as compared with an excimer laser. That is, the wavelength of the YAG laser is in a wavelength region where the absorption rate by amorphous silicon is higher than the absorption rate by polycrystalline silicon, and amorphous silicon can be selectively annealed. Further, it is difficult to be absorbed by silicon oxide or silicon nitride.

  Further, since the YAG laser has high transparency and can heat the amorphous silicon film 10 substantially uniformly in the laser irradiation direction, it is in a direction perpendicular to the irradiation direction, that is, a direction parallel to the substrate surface. Crystals can be grown. Therefore, if the irradiation light 11 of the YAG laser is scanned in a direction parallel to the substrate surface, the amorphous silicon film 10 is sequentially melted, and then the polycrystalline silicon film 4 is sequentially formed by natural cooling. By moving the irradiation region to the entire surface of the substrate, the entire surface of the amorphous silicon film 10 can be polycrystallized.

  Further, since the semiconductor layer is uniformly heated in the irradiation direction, that is, the depth direction by irradiation with the YAG laser, the layer below the semiconductor layer can be heated by the temperature rise of the semiconductor layer. By this heating, defects in the crystal structure in the undercoat film (silicon nitride film 2, first silicon oxide film 3a, and second silicon oxide film 3b) are removed, so that the defect density is reduced and the operating characteristics are improved. be able to.

  Since the YAG laser has high transparency, it is necessary to consider interference between the irradiation light 11 and the reflected light from the interface. That is, the amorphous silicon film 10 may not be sufficiently heated due to interference between incident light in laser irradiation and reflected light that has been transmitted through each layer and reflected at the interface. For example, in the case of a laser having a wavelength of 532 nm, a standing wave is generated by interference between reflected light reflected from the surface of the silicon nitride film 2 and incident light, and the distance from the silicon nitride film 2 is fixed at a position where the wavelength is a half wavelength. Wave nodes are thought to be formed. In the case of a silicon oxide film, the half wavelength here corresponds to 182 nm because its refractive index is 1.46. However, if the refractive index of the film is different from this depending on the film forming conditions, the refractive index is as follows. It becomes a value according to. For this reason, when the amorphous silicon film 10 is formed at this position, the irradiation light 11 is weakened by interference, so that the amorphous silicon film 10 cannot be heated sufficiently.

  In other words, when a node of a standing wave is formed in the amorphous silicon film 10 formed on the silicon oxide film made up of the first silicon oxide film 3a and the second silicon oxide film 3b, an amorphous state is formed. The quality silicon film 10 cannot be heated sufficiently. Therefore, it is preferable to exclude the integral multiple of 182 nm and 182 nm as the film thickness of the silicon oxide film formed between the silicon nitride film 2 and the amorphous silicon film 10. Note that this value is a value corresponding to the refractive index when the refractive index of the film is different from 1.46 depending on the film forming conditions of the silicon oxide film.

  Next, when the polycrystalline silicon film 4 after laser annealing is patterned by photolithography, an island-shaped semiconductor layer (polycrystalline silicon film 4) is formed (step of FIG. 1 (d)).

  FIG. 2 is a cross-sectional view showing a configuration example of the thin film transistor according to the first embodiment of the present invention. This thin film transistor is formed by forming the island-shaped semiconductor layer (polycrystalline silicon film 4) by the manufacturing process shown in FIG. 1, and then, by the same manufacturing process as the conventional thin film transistor, the gate insulating film 5, the gate electrode 6, An interlayer insulating film 7, a source / drain electrode 8, and a passivation film 9 are formed.

  In this thin film transistor, a silicon nitride film 2, a first silicon oxide film 3a, and a second silicon oxide film 3b exist between an insulating substrate 1 and a semiconductor layer (polycrystalline silicon film 4). The insulating film functions as an undercoat film. That is, the silicon nitride film 2 in contact with the insulating substrate 1 blocks the diffusion of impurities from the insulating substrate 1, while the silicon oxide film in contact with the semiconductor layer reduces the influence of the defect level of the silicon nitride film 2. . Since this silicon oxide film is composed of the high-quality second silicon oxide film 3b and the first silicon oxide film 3a that takes a short time to form, the operation as a thin film transistor can be achieved without reducing the production efficiency. Characteristics can be improved.

  FIG. 3 is a diagram showing an example of operating characteristics of the thin film transistor, and shows the measurement result of the threshold voltage Vth with respect to the film thickness of the undercoat film made of silicon oxide. This measurement was performed on a thin film transistor in which a silicon oxide film was formed by plasma CVD using a TEOS-based material gas as an undercoat film. The threshold voltage Vth of the thin film transistor is a voltage applied between the gate and the source electrode necessary for changing the thin film transistor from the off state to the on state. If this threshold voltage Vth can be reduced, power consumption can be reduced and the operating speed can be improved. Therefore, in order to improve the TFT characteristics, and to form a complicated peripheral circuit on the TFT substrate, it is important to stably obtain a TFT having a low threshold voltage Vth.

  According to the above measurement results, the threshold voltage Vth decreases as the film thickness of the silicon oxide film increases, but this characteristic gradually saturates. If the film thickness exceeds 500 nm, the threshold voltage is changed even if the film thickness changes. It can be seen that Vth hardly decreases. On the other hand, it can be seen that the threshold voltage Vth increases rapidly when the film thickness is smaller than 100 nm. For this reason, in order to obtain a desired threshold voltage characteristic, it is preferable that the thickness of the silicon oxide film be 100 nm or more and 500 nm or less.

  FIG. 4 is a diagram showing a standing wave generated on the substrate by the YAG laser irradiation, and shows the standing wave A generated by the interference between the irradiation light 11 and the reflected light from the surface of the silicon nitride film 2. Yes. When the wavelength of the YAG laser in the silicon oxide film is λ, the node A1 of the standing wave caused by the interference between the irradiation light 11 and the reflected light from the surface of the silicon nitride film 2 on the glass substrate is derived from the silicon nitride film 2. The antinode A2 of the standing wave is formed at a position of (2n−1) λ / 4 (n is a natural number).

Therefore, when the film thickness of the silicon oxide film (the first silicon oxide film 3a and the second silicon oxide film 3b) on the silicon nitride film 2 is D1, and the film thickness of the amorphous silicon film 10 is D2, If both the following expressions (1) and (2) are satisfied, the node A1 of the standing wave is not present in the amorphous silicon film 10 and can be heated effectively, which is desirable.

Furthermore, if the following formula (3) is satisfied, the antinode A2 of the standing wave exists in the amorphous silicon film 10 and can be heated more effectively, which is more desirable.

If the wavelength λ of the YAG laser in the silicon oxide film is 532 / 1.46 = 364 nm, the relationship between D1 and D2 that satisfies the above equation (3) is expressed by the following equation (4). = 2 is considered the most realistic value. For example, if the film thickness D2 of the amorphous silicon film is about 50 nm, the film thickness of the silicon oxide is 223 <D1 <273, and about 250 nm is appropriate.

  Steps S101 to S107 in FIG. 5 are flowcharts showing an example of a main part in the method for manufacturing a low-temperature polysilicon TFT according to the first embodiment of the present invention. First, a silicon nitride film 2, a first silicon oxide film 3a, and a second silicon oxide film 3b are sequentially formed as an undercoat film on the insulating substrate 1 by plasma CVD (steps S101 to S103). The first silicon oxide film 3a is formed using a monosilane-based material gas, and the second silicon oxide film 3b is formed using a TEOS-based material gas.

  Next, an amorphous silicon film 10 is formed on the undercoat film by plasma CVD (step S104). The silicon nitride film 2, the first silicon oxide film 3a, the second silicon oxide film 3b, and the amorphous silicon film 10 are sequentially formed while maintaining a vacuum state in the same vacuum chamber.

  Next, the substrate on which the amorphous silicon film 10 is formed is irradiated with a YAG laser to anneal the amorphous silicon film 10 (step S105). By this laser irradiation, the amorphous silicon film 10 is melted and crystallized, and a polycrystalline silicon film 4 is formed. When all the predetermined irradiation areas in the substrate surface are scanned, the laser irradiation is finished (step S106).

  Thereafter, the polycrystalline silicon film 4 crystallized by laser annealing is patterned by photolithography (step S107). By this patterning, an island-shaped semiconductor layer is formed.

  According to the present embodiment, the formation of the silicon oxide film as the undercoat film is performed by plasma CVD using a monosilane-based material gas having a high deposition rate and plasma CVD using a TEOS-based material gas. Production efficiency can be improved as compared with the case where the silicon oxide film is formed only by plasma CVD using a TEOS-based material gas. In addition, since the second silicon oxide film 3b in contact with the semiconductor layer is formed using a TEOS-based material gas, the operation characteristics as a thin film transistor, in particular, threshold voltage characteristics and electric field mobility are not reduced without reducing the production efficiency. Can be improved.

  In this embodiment, an example in which laser irradiation is performed with a YAG laser has been described, but the present invention is not limited to this. For example, instead of the YAG laser, laser irradiation may be performed using a XeCl excimer laser having a wavelength in the ultraviolet region.

Embodiment 2. FIG.
In the first embodiment, the example in which the amorphous silicon film 10 is formed on the undercoat film has been described. On the other hand, in this embodiment, an amorphous silicon film is newly provided between the insulating substrate 1 and the silicon nitride film 2 as the undercoat film, and the undercoat film is formed by two layers of amorphous silicon films. The annealing effect is improved.

  FIGS. 6A to 6C are diagrams showing an example of a main part of the method for manufacturing a low-temperature polysilicon TFT according to the second embodiment of the present invention. The polycrystalline silicon film 4 is formed by laser irradiation. Each process until it is formed is shown. This embodiment is different from the first embodiment in FIG. 1 in that an amorphous silicon film 12 is provided between the insulating substrate 1 and the silicon nitride film 2.

  First, an amorphous silicon film 12 is formed on the insulating substrate 1 by plasma CVD. Next, the silicon nitride film 2, the first silicon oxide film 3a, and the second silicon oxide film 3b are sequentially formed on the amorphous silicon film 12 (step of FIG. 6A). After the formation of the second silicon oxide film 3b, an amorphous silicon film (semiconductor film) 10 as a second layer is formed (step of FIG. 6B). The amorphous silicon film 12, the silicon nitride film 2, the first silicon oxide film 3a, the second silicon oxide film 3b, and the amorphous silicon film 10 are sequentially formed while maintaining a vacuum state in a vacuum chamber. The Thereafter, the amorphous silicon film 10 is annealed by a YAG laser (step of FIG. 6C).

  Since the YAG laser can selectively heat the amorphous silicon film, a part of the irradiation light 11 transmitted through the amorphous silicon film 10 is under-absorbed with almost no energy absorbed in the undercoat film. The amorphous silicon film 12 is heated by passing through the coating film and reaching the amorphous silicon film 12. In other words, two layers of amorphous silicon film can be heated at the same time by irradiating with YAG laser, so the undercoat film existing between these layers is effectively heated, effectively eliminating defects in the crystal structure. Can be removed.

  FIG. 7 is a cross-sectional view showing a configuration example of the thin film transistor according to the second embodiment of the present invention. In this thin film transistor, after the polycrystalline silicon film 4 is formed by the manufacturing process shown in FIG. 6, the gate insulating film 5, the gate electrode 6, the interlayer insulating film 7, the source, and the like are further manufactured by the same manufacturing process as the conventional thin film transistor. A drain electrode 8 and a passivation film 9 are formed.

  In this thin film transistor, an amorphous silicon film 12 exists between the insulating substrate 1 and the silicon nitride film 2, and the undercoat film is sufficiently heat-treated by two layers of amorphous silicon films.

  According to the present embodiment, by irradiating the substrate with a YAG laser, the silicon nitride film 2, the first silicon oxide film 3a and the second silicon oxide film are simultaneously formed with the melt crystallization of the amorphous silicon film 10. The heat treatment of 3b can be effectively performed. By heat-treating these undercoat films, defects in the crystal structure are removed, so that TFT characteristics can be further improved.

It is the figure which showed an example of the principal part about the manufacturing method of the low temperature polysilicon TFT by Embodiment 1 of this invention. It is sectional drawing which showed one structural example of the thin-film transistor by Embodiment 1 of this invention. It is the figure which showed an example of the operating characteristic in a thin-film transistor. It is the figure which showed the mode of the standing wave which arises on a board | substrate by irradiation of a YAG laser. 5 is a flowchart showing an example of a main part in the method for manufacturing a low-temperature polysilicon TFT according to the first embodiment of the present invention. It is the figure which showed an example of the principal part about the manufacturing method of the low temperature polysilicon TFT by Embodiment 2 of this invention. It is sectional drawing which showed one structural example of the thin-film transistor by Embodiment 2 of this invention. It is sectional drawing which showed the structure of the conventional low-temperature polysilicon TFT. It is the figure which showed an example of the principal part of the manufacturing process of the conventional low temperature polysilicon TFT.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Insulating substrate 2 Silicon nitride film 3a 1st silicon oxide film 3b 2nd silicon oxide film 4 Polycrystalline silicon film (semiconductor layer)
5 Gate insulating film 6 Gate electrode 7 Interlayer insulating film 8 Source and drain electrode 9 Passivation film 10, 12 Amorphous silicon film 11 Irradiation light A of YAG laser A1 Standing wave A1 generated by interference A2 Standing wave node A2 Standing wave Antinode D1 silicon oxide film thickness D2 amorphous silicon film thickness

Claims (5)

Forming a silicon nitride film on the substrate;
Forming a silicon oxide film having a film thickness of at least 500nm or less 100nm on top Symbol silicon nitride film,
Forming a semiconductor film made of amorphous silicon on the silicon oxide film, and irradiating the semiconductor film with a laser to make the semiconductor film a polycrystalline silicon film,
The step of forming the silicon oxide film includes
By plasma CVD using a material gas of monosilane, forming a first base film made of silicon oxide on the upper Symbol silicon nitride film,
By forming a second base film made of silicon oxide on the first base film by plasma CVD using a tetraethoxysilane-based material gas ,
Forming an undercoat film comprising the silicon nitride film, the first base film, and the second base film ;
With
A method of manufacturing a thin film transistor, wherein a film formation speed of the first base film is higher than a film formation speed of the second base film .   2. The method of manufacturing a thin film transistor according to claim 1, wherein the first base film, the second base film, and the semiconductor film are sequentially formed while maintaining a vacuum state.   2. The method of manufacturing a thin film transistor according to claim 1, wherein the film thickness of the second base film is smaller than the film thickness of the first base film. Forming a third underlayer of amorphous silicon on the substrate;
Forming a fourth base film made of silicon nitride on the third base film,
The first base film is formed on the fourth base film,
2. The thin film transistor according to claim 1, wherein the third base film, the fourth base film, the first base film, the second base film, and the semiconductor film are sequentially formed while maintaining a vacuum state. Manufacturing method. The semiconductor film is irradiated with YAG laser substrate formed with the thin film transistor manufacturing method according to claim 1 or 4, comprising the step of annealing the semiconductor film.

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