JP2007005508A - Method for manufacturing thin film transistor and for display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve a channel layer of a thin film transistor showing less characteristics change in a threshold voltage, etc., less scattering in electric characteristics, and sufficient breakdown voltage. <P>SOLUTION: A method for manufacturing a reverse staggered thin film transistor includes a process of forming the channel layer 18 of the thin film transistor. The process comprises a step 3 of forming an amorphous silicon film 14, a buffer film 31, and a light-heat conversion film 32 on a gate insulating film 13 in increasing order; a step 4 of emitting semiconductor laser light 16 onto the light-heat conversion film 32 to crystallize the amorphous silicon film 14 into a microcrystalline silicon film 15; a step 5 of eliminating the light-heat conversion film 32 and the buffer film 31; and a step 6 of forming an amorphous silicon film 17 on the microcrystalline silicon film 15. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a thin film transistor having a small characteristic fluctuation and a method for manufacturing a display device capable of maintaining stable display quality for a long period of time.

  Organic electroluminescence (hereinafter, “electroluminescence” is abbreviated as “EL”) The display driving thin film transistor includes: (a) a characteristic variation such as a threshold voltage is small even if the driving is continued for a long period of time; (B) There is little variation in electrical characteristics so that there is no unevenness on the display screen, (c) There is sufficient breakdown voltage even when a multi-stage EL layer such as an MPE (multi-photon emission) structure is driven at a high voltage, etc. Performance is required.

  Thin film transistors (hereinafter abbreviated as “TFT”, abbreviated as “Thin Film Transistor”) that have already been mass-produced for liquid crystal displays can be broadly divided into amorphous silicon TFTs and polycrystals whose channel layers are made of amorphous silicon. There are low-temperature polysilicon TFTs (non-alkali glass substrates) made of silicon and high-temperature polysilicon TFTs (quartz glass substrates).

  When considering an organic EL driving TFT, the conventional amorphous silicon TFT satisfies the performances (b) and (c), but the performance (a) is insufficient. The polysilicon TFT satisfies the performance (a), but the performances (b) and (c) are insufficient. Since the performances of (a) and (b) are not limited to TFTs for organic EL displays but also for TFTs for liquid crystal displays, the following various proposals have been made for the purpose of improving them. ing. However, these methods have not been sufficiently improved, and sufficient performance has not been obtained even when these are used as TFTs for driving an organic EL.

  (1) An inverted staggered thin film transistor composed of a two-layer channel of polycrystalline Si and amorphous Si has been proposed for use in a liquid crystal display device (see, for example, Patent Documents 1 and 2). In this proposal, the characteristic variation of the performance (a) is suppressed by the TFT of the two-layer channel structure, but it is used only as the TFT for the peripheral circuit, and the TFT for the pixel circuit is laminated with amorphous Si. A membrane is used as a channel. This is because the polycrystalline Si channel layer of the two-layer channel is manufactured by excimer laser irradiation with poor output stability, and thus the characteristic variation is large, and the performance point (b) becomes a problem. This is because the TFT cannot be used.

  (2) A structure in which a channel layer has a two-layer structure of microcrystalline silicon or polycrystalline silicon and amorphous silicon hydride, a thin film transistor, and a manufacturing method thereof are disclosed (for example, see Patent Document 3). . Microcrystalline silicon or polycrystalline silicon can be produced by CVD of crystalline silicon on amorphous silicon, or by thermal annealing or flash annealing of the upper surface region of amorphous silicon film. A method for producing a two-layer channel structure for producing a silicon film is disclosed. However, when microcrystalline silicon or polycrystalline silicon is manufactured by the above method, crystalline silicon is formed above amorphous silicon. Therefore, in the case of the staggered thin film transistor according to the present invention, a gate insulating film serving as a channel The nearby semiconductor layer becomes crystalline silicon, and the stability targeted is obtained. However, when the inverted stagger type thin film transistor structure is employed, the vicinity of the gate insulating film serving as the channel becomes amorphous silicon, so that the stability is lowered and the performance of (a) cannot be ensured.

  (3) In a low-temperature polysilicon TFT in which a polysilicon channel layer is formed by laser annealing, a light absorption film is provided on an amorphous or microcrystalline silicon film, and silicon is obtained by irradiating a continuous wave laser with a stable output thereto. This is a technique for producing a large grain silicon layer by melting and recrystallizing the film (see, for example, Patent Documents 4 and 5). In this technique, a continuous-wave laser having a stable output is used to produce large-grain silicon having few defects, and the characteristic variation of (a) and the characteristic variation of (b) are reduced. However, if the TFT is formed using the large-grained silicon or silicon close to a single crystal produced here for the channel layer, it has a feature that high on-current can be obtained with high mobility, but it has low withstand voltage performance and low off-current. The problem that it is high appears, and it is not preferable for a TFT for organic EL that needs to be driven at a high voltage. Furthermore, in the case of an inverted staggered transistor structure, a gate wiring with high thermal conductivity is arranged under the silicon film to be laser-annealed. Therefore, the cooling rate during silicon recrystallization is affected by the wiring pattern and the grain size is reduced. Variations occur and the TFT characteristics vary. As a result, the uniformity of (b) is deteriorated.

  (4) Also in this method, a method has been proposed in which a light absorption layer is provided on a semiconductor film as in (2) above, and the semiconductor film is converted to a large grain silicon film by irradiation with a continuous oscillation semiconductor laser beam. (For example, refer to Patent Document 6). By using a semiconductor laser having a stable output as a light source, a large-grain polysilicon film can be stably produced, and a TFT with high mobility and little variation can be realized at low cost and high throughput. However, in this case as well, since a large grain polysilicon film is used for the channel layer as in (2), the TFT has a high mobility but a low breakdown voltage and a high off current. A multi-gate structure, offset structure, LDD structure, etc. are generally used as a method for reducing the off-current, and the effect has been confirmed, but with regard to the breakdown voltage of the performance of (c), large-grain polysilicon is present in the channel layer. As long as the TFT structure and process conditions are devised, no significant improvement can be expected. Further, as shown in the latter half of the above (3), in the case of the inverted staggered transistor structure, the problem of the performance of (b) also occurs.

Japanese Patent No. 2814319 Japanese Patent No. 3121005 JP-A-6-342909 JP 2002-50576 A Japanese Patent Laid-Open No. 2003-168646 JP 2004-134777 A

  The problem to be solved is that the characteristic variation with respect to long-term driving and the low breakdown voltage cannot be solved simultaneously.

  The present invention improves the channel layer of a thin film transistor so that even if it is driven for a long period of time, there is little variation in characteristics such as threshold voltage, and there is little variation in electrical characteristics so that there is no unevenness in the display screen. It is an object of the present invention to make it possible to have a sufficient breakdown voltage even when a multi-stage EL layer having a photon emission) structure or the like is driven at a high voltage.

  The thin film transistor manufacturing method of the present invention includes a step of forming a gate electrode on a substrate, a step of forming a gate insulating film on the substrate so as to cover the gate electrode, and the gate insulating film on the gate electrode. Forming a channel layer in a method of manufacturing a thin film transistor, comprising: forming a channel layer via a step; and forming a source electrode on one side of the channel layer and forming a drain electrode on the other side. Forming an amorphous silicon film, a buffer film, and a light-to-heat conversion film on the gate insulating film in this order, and irradiating the light-to-heat conversion film with a semiconductor laser beam to form the amorphous silicon film. A step of crystallizing into a microcrystalline silicon film, a step of removing the light-heat conversion film and the buffer film, and a step of forming an amorphous silicon film on the microcrystalline silicon film. Characterized in that it has and.

  The thin film transistor manufacturing method includes the step of irradiating the light-heat conversion film with semiconductor laser light to crystallize the amorphous silicon film into a microcrystalline silicon film. By absorbing much of the irradiation laser light, even if the amorphous silicon film is thin, much of the energy of the irradiation light is absorbed and effectively converted into heat. Then, the amorphous silicon film that has reached the melting point and has melted is cooled and solidified as the irradiation light passes and is changed to microcrystalline silicon. At this time, it functions as a structure that keeps a spatial distance in the film thickness direction (a gap between a buffer film made of a silicon oxide film sandwiching an amorphous silicon film and a silicon oxide film in a gate insulating film) constant. This makes it difficult for the volume change of amorphous silicon to occur, and suppresses the increase in the silicon grain size due to lateral growth, thereby promoting the generation of only microcrystalline silicon. In addition, since semiconductor laser light is used as a light source, high-power and stable laser light irradiation is possible, and the light-heat conversion film can be heated with high uniformity and high accuracy. It becomes easy to produce microcrystalline silicon.

  According to another aspect of the present invention, there is provided a display device manufacturing method including a thin film transistor for driving a pixel of a display panel including a plurality of pixels, wherein the thin film transistor manufacturing step includes a step of forming a gate electrode on a substrate. A step of forming a gate insulating film on the substrate so as to cover the gate electrode, a step of forming a channel layer on the gate electrode through the gate insulating film, and a source on one side of the channel layer Forming a drain electrode on the other side, and forming the channel layer in the order of an amorphous silicon film, a buffer film, and a light-to-heat conversion film on the gate insulating film. A step of irradiating the light-heat conversion film with a semiconductor laser light to crystallize the amorphous silicon film into a microcrystalline silicon film; and the light-heat conversion. And having the removing the buffer layer, and a step of forming an amorphous silicon film on the microcrystalline silicon film.

  In the manufacturing method of the display device, as described above, the microcrystalline silicon film and the amorphous silicon film having a small particle size are adopted by adopting the thin film transistor manufacturing method of the present invention in the manufacturing process of the thin film transistor for driving the pixel. It is possible to manufacture a thin film transistor including a channel layer including:

  In the thin film transistor manufacturing method of the present invention, since the channel layer can be formed in a two-layer structure of a microcrystalline silicon film and an amorphous silicon film, a thin film transistor with small characteristic fluctuations can be manufactured. Even a long display device, such as an organic EL display device, has an advantage that a stable display quality can be maintained for a long time. In addition, since a thin film transistor with small variation in characteristics can be manufactured, there is an advantage that a display device with no uneven display screen, for example, an organic EL display device can be realized. In addition, since a transistor having higher mobility than a conventional amorphous silicon TFT can be obtained, a display device having a small number and high resolution can be realized even in a display device having a large number of transistors in a pixel, for example, an organic EL display device.

  The display device manufacturing method of the present invention can form the driving transistor of the display device by the thin film transistor manufacturing method of the present invention. Therefore, there is an advantage that the operation and effect obtained by the thin film transistor manufacturing method can be obtained. is there. Therefore, it is possible to obtain a display device that maintains stable display quality for a long period of time and has no uneven display screen.

  A first example of an embodiment according to the method for manufacturing a thin film transistor of the present invention will be described with reference to a flowchart showing the manufacturing process of FIG. 1 and a manufacturing process sectional view of FIG.

  Steps 1 to 4 shown in FIG. 1 are performed. First, in step 1, the substrate 11 is cleaned as shown in FIG. As the substrate 11, for example, a glass substrate is used. After cleaning, a gate electrode formation film is formed on the substrate 11. As the gate electrode formation film, a molybdenum film is formed to a thickness of 90 nm by sputtering, for example. Next, through a photolithography process and an etching process, the gate electrode 12 is manufactured by patterning the gate electrode formation film into a predetermined shape.

  Next, step 2 shown in FIG. 1 is performed. In step 2, a gate insulating film 13 is formed on the substrate 11 so as to cover the gate electrode 12, as shown in FIG. For example, the gate insulating film 13 is formed of a laminated film of a silicon nitride film and a silicon oxide film. The thickness of the silicon nitride film is, for example, 50 nm, and the thickness of the silicon oxide film is, for example, 120 nm. Further, as a film for forming a channel layer on the gate insulating film 13, for example, an amorphous silicon film 14 is formed to a thickness of 15 nm, for example. As the film forming method, for example, a plasma enhancement-chemical vapor deposition method (PE-CVD method) can be used.

  Next, step 3 shown in FIG. 1 is performed. In step 3, a buffer film 31 is formed on the amorphous silicon film 14 as shown in FIG. The buffer film 31 is formed of, for example, an insulating film, and is formed by, for example, forming a silicon oxide film with a thickness of, for example, 20 nm. Next, a light-heat conversion film 32 is formed on the buffer film 31. The light-heat conversion film 32 is formed by depositing, for example, molybdenum to a thickness of 100 nm, for example. As this film formation method, a PE-CVD method, a sputtering method, or the like can be used. Here, in the buffer film 31 made of silicon oxide, molybdenum (Mo) of the light-to-heat conversion film 32 that becomes high temperature when irradiated with laser light diffuses into the amorphous silicon film 14 to generate molybdenum silicide. It plays a role to prevent being done.

  Next, step 4 shown in FIG. 1 is performed. In step 4, as shown in FIG. 2A, the light-heat conversion film 32 is irradiated with the laser light 16 to heat the light-heat conversion film 32, and the amorphous silicon film 14 in the lower layer is heated by this heat. Is changed to the microcrystalline silicon film 15.

The laser annealing process at this time will be described in detail. The laser light source used is a broad-area high-power semiconductor laser device with a wavelength of 808 nm, and a light output of about 4 W can be obtained by continuous oscillation. The laser light 16 emitted from the semiconductor laser device (not shown) is passed through a uniform illumination optical system using a microlens array or the like, and is a top hat type with a flat light intensity profile on the long axis side, and on the short axis side. The light intensity profile is shaped into a Gaussian rectangular beam. This beam is condensed to a light intensity of about ² mW / μm ² and irradiated onto the light-heat conversion film 32, and the substrate 11 is moved at a constant speed of about 40 mm / s. The molybdenum film is heated to a high temperature by irradiation with high-intensity semiconductor laser light, and this heat is transferred to the lower buffer film 31 and the amorphous silicon film 14 made of silicon oxide by heat conduction. Reach the melting point. The melted amorphous silicon film 14 is cooled and solidified by passing through the irradiation light and changed to microcrystalline silicon, and the microcrystalline silicon film 15 is formed.

  When the microcrystalline silicon obtained by this recrystallization process was observed with an electron microscope (SEM), the particle size was observed to be approximately 10 nm to 50 nm as shown in FIG. This SEM photograph is observed after the microcrystalline silicon film 15 is subjected to Secco etching. Here, the light-heat conversion film 32 plays two important roles in the silicon crystallization process. One is that the light-to-heat conversion film 32 absorbs much of the irradiation laser light, thereby effectively absorbing much of the energy of the irradiation light even when the amorphous silicon film is thin. Convert to heat. The other is a spatial distance in the film thickness direction when the molten amorphous silicon film is crystallized (the buffer film 31 made of a silicon oxide film sandwiching the amorphous silicon film and the silicon oxide in the gate insulating film 13). It works as a structure that keeps the gap between the films constant. This effect makes it difficult for the volume change of amorphous silicon to occur, and suppresses the increase in the silicon grain size due to lateral growth, thereby promoting the production of only microcrystalline silicon.

  Next, step 5 shown in FIG. 1 is performed. In step 5, the light-heat conversion film 32 and the buffer film 31 that are not necessary for forming the transistor are removed. Molybdenum constituting the light-heat conversion film 32 is removed by being immersed in a phosphonitrate solution. The buffer film 31 made of silicon oxide is removed by being immersed in a hydrofluoric acid solution. In this etching, since the microcrystalline silicon is hardly dissolved in the hydrofluoric acid solution, the microcrystalline silicon film 15 is exposed as shown in FIG. 2 (2), but the microcrystalline silicon film 15 remains crystallized. It remains with a film thickness of.

  Next, step 6 shown in FIG. 1 is performed. In step 6, an amorphous silicon film 17 is formed on the microcrystalline silicon film 15 as shown in FIG. The amorphous silicon film 17 is formed with a thickness of 120 nm, for example. For example, a PE-CVD method can be used as the film formation method. In this manner, a channel layer 18 having a two-layer channel structure composed of the microcrystalline silicon film 15 and the amorphous silicon film 17 which is a feature of the present invention is manufactured.

  Next, steps 7 to 9 shown in FIG. 1 are performed. First, in step 7, as shown in FIG. 2 (4), the same steps as those for manufacturing a general amorphous silicon TFT are performed. A channel protective film is formed on the amorphous silicon film 17. This channel protective film is formed of, for example, a silicon nitride film. As the film formation method, for example, a chemical vapor deposition method can be used. Thereafter, a stopper layer 19 is formed on the channel layer 18 using the channel protective film by a normal photolithography process and etching process.

Next, step 8 shown in FIG. 1 is performed. In step 8, as shown in FIG. 2 (4), an amorphous silicon layer doped with, for example, phosphorus (hereinafter referred to as phosphorus) as an n-type impurity in the region where the source / drain on the amorphous silicon film 17 is formed. (referred to as an n ⁺ a-Si layer). The n ⁺ a-Si layer 20 can be formed by, for example, chemical vapor deposition. Thereafter, an island structure is formed by a photolithography process and a dry etching process. In this dry etching process, for example, a reactive ion etching apparatus can be used.

Next, step 9 shown in FIG. 1 is performed. In step 9, as shown in FIG. 2 (4), an electrode film for forming a source electrode and a drain electrode is formed so as to cover the n ⁺ a-Si layer 20. This electrode film can be formed by sputtering, for example. Then, the source electrode 21 and the drain electrode 22 are formed by patterning the electrode film by a photolithography process and a dry etching process. In this dry etching process, for example, a reactive ion etching apparatus can be used. In this example, a three-layer structure of titanium (Ti) / aluminum (Al) / titanium (Ti) was used as the electrode film. Through the above steps, the inverted staggered thin film transistor 1 in which the channel layer 18 has a two-layer structure of the microcrystalline silicon film 15 and the amorphous silicon film 17 was formed.

  The transistor characteristics of the TFT according to this example were evaluated and compared with those of a conventional TFT. One of the very important characteristics for TFTs for organic EL displays is the variation in current value characteristics. If this variation is large, the image quality becomes uneven and the display quality is deteriorated, so a small value is required. On-current Ion variation (σ of Ion / average value) of 16 adjacent transistors was measured at a plurality of locations in the substrate, and the average value is shown in Table 1.

  As shown in Table 1, the current value variation of the TFT according to this embodiment is as small as 2.0%, which is almost the same as the amorphous silicon TFT normally used in liquid crystal display devices. It was. The variation of the low-temperature polysilicon TFT using excimer laser annealing is as large as 5% or more, and when used for a driving TFT of an organic EL display, streaky unevenness caused by laser annealing exists on the display screen. This is because a pulse laser represented by an excimer laser has a large energy variation for each pulse, and a pulse variation of a normal excimer laser is 5% to 10% in terms of Peak to Peak. Moreover, even a relatively stable YAG laser pumped by a semiconductor laser has a pulse variation of 2% to 3% in Peak to Peak, and the output fluctuation of the semiconductor laser used in this embodiment is Peak to Peak. Compared with 0.5% or less, it has a large fluctuation amount. It can be seen that the organic EL display using the TFT according to the present embodiment has no unevenness in the display screen and has characteristics superior to those of the low-temperature polysilicon TFT in this respect.

  Another important transistor characteristic is variation with time of the threshold voltage (Vth). Table 1 also shows the data obtained when the variation with time of Vth was evaluated.

  There are various evaluation methods depending on the driving method and conditions of the transistor. Here, the amount of change in Vth when a voltage of 15 V is applied to the gate and driving continuously for about 28 hours in an environment of 50 ° C. is shown. The TFT according to this embodiment showed a small value of 0.3 V, and showed high reliability equivalent to low-temperature polysilicon. On the other hand, a normal amorphous silicon TFT shows a large fluctuation amount of 15 V, and it can be easily imagined that if it is continuously driven for a long time, the fluctuation amount of Vth becomes too large to operate normally. Since an organic EL display displays a screen by continuously supplying a current, a transistor having a small Vth variation is required. In this respect, it can be seen that the TFT according to this example has much superior characteristics as compared with the amorphous silicon TFT.

A major feature of the present invention is that the channel layer 18 is composed of two layers of a microcrystalline silicon film 15 and an amorphous silicon film 17. By forming the microcrystalline silicon film 15 on the gate electrode 12 side, the variation in Vth due to voltage application is less likely to occur, and the transistor size can be reduced by increasing the mobility. In this embodiment, a mobility of 2 cm ² / V / s is obtained, and a characteristic four times that of a conventional amorphous silicon TFT having a mobility of about 0.5 cm ² / V / s is obtained.

  Further, the amorphous silicon film 17 stacked on the microcrystalline silicon film 15 has an effect of introducing a uniform high resistance layer between the source and the drain, and exhibits a high withstand voltage characteristic while reducing the off current.

  Next, a second example of the embodiment according to the method for manufacturing a thin film transistor of the present invention will be described.

In this second example, a thin film transistor 1 having a two-layer channel structure made of a microcrystalline silicon film 15 and an amorphous silicon film 17 was fabricated by using a process substantially similar to the first example. The difference from the first example is that a reflection reducing film 33 is formed on the light-heat conversion film 32 in step 3 of FIG. 1 as shown in FIG. The reflection reducing film 33 may be, for example, thickness is formed of silicon oxide (SiO ₂₎ film of 120 nm. With this reflection reducing film 33, the laser light can be efficiently used to raise the temperature of the light-heat conversion film 32 made of a molybdenum film. Therefore, as in the first example, the beam scanning speed when crystallizing the amorphous silicon film 14 can be increased to 120 mm / s. Thereafter, only the process of removing the reflection reducing film is added in Step 5 of FIG. 1, and the other processes can be performed by the same process as in the first example. Various characteristics of the TFT thus obtained were almost the same as those of the TFT of the first example.

  The molybdenum film used as the light-heat conversion film 32 has a reflectance of about 56% with respect to laser light having a wavelength of 808 nm, and the amount of energy absorbed by the film and contributing to annealing is 44%. By using the reflection reducing film 33, the reflectance can be reduced to about 30%. That is, 70% of the incident light quantity can be used for heating the light-heat conversion film 32. When a laser beam having an output of 4 W is used, energy of 1.76 W is used for annealing when the reflection reduction film 33 is not provided. However, by forming the reflection reduction film 33, it is 2.8 W, which is approximately 1.6 times. Can be used.

  When mass-producing a TFT substrate for a display using a large glass substrate, the substrate processing speed of this laser annealing process is an important factor that determines the production amount. As can be seen from the second example, since the beam scanning speed can be significantly increased by using the reflection reducing film, the production amount per apparatus can be greatly increased.

  In the second example, a silicon oxide film is used as the reflection reducing film 33. However, the same effect can be obtained as long as it is a translucent high melting point film having a refractive index of about 1.5 to 2.5. For example, by forming a silicon nitride (SiN) film having a thickness of 90 nm on a molybdenum film, the reflectance can be reduced to about 15%, and the energy utilization efficiency can be further increased than in the case of a silicon oxide film. Can do. Further, as the reflection reducing film 33, a laminated film of a silicon oxide film and a silicon nitride film can be used.

  Next, a third example of one embodiment according to the method for manufacturing a thin film transistor of the present invention will be described.

  In this third example, a thin film transistor 1 having a two-layer channel structure composed of a microcrystalline silicon film 15 and an amorphous silicon film 17 was fabricated using a process substantially similar to that of the first example. The difference from the first example is that the film thickness of the amorphous silicon film 14 formed in step 2 of FIG. 1 is increased to 43 nm, and the light-heat conversion film 32 formed in step 3 of FIG. In step 4 of FIG. 1, the semiconductor laser light used for annealing is not an infrared semiconductor laser light having a wavelength of 808 nm, but a gallium nitride (GaN) blue semiconductor laser having a wavelength of 405 nm. It is a point using light.

  The annealing apparatus used in the third example is obtained by changing the light source unit and the illumination optical system unit of the annealing apparatus used in the first example to a semiconductor laser beam having a wavelength of 405 nm and a uniform illumination optical system corresponding thereto. is there. The crystallization process was performed under the irradiation conditions for adjusting the irradiation light energy intensity and the scanning speed to obtain a suitable microcrystalline silicon film 15. After Step 5 in FIG. 1, a TFT device was manufactured using the same process as in the first example, and the electrical characteristics were evaluated.

  As a result, the electrical characteristics are not significantly different from the results of the first example, the mobility is 1.3 times that of the first example, and the on-current variation between adjacent transistors is 3% on average. Sufficient performance was obtained. A photograph observing the grain size of the microcrystalline silicon film 15 in this device is shown in FIG. The particle diameter was 10 nm to 80 nm, and although the amorphous silicon film 14 was as thick as 43 nm, it was slightly larger than the first example, but no lateral growth was observed. Thus, it can be said that the variation in the on-state current is suppressed by the microcrystalline silicon.

  Next, as a comparative example, a method for manufacturing a thin film transistor in which the buffer film 31 and the light-heat conversion film 32 are not formed will be described.

  In the comparative example, the amorphous silicon film 14 was annealed with 405 nm semiconductor laser light without forming the buffer film 31 and the light-heat conversion film 32 in the third example. The other processes are almost the same as in the third example.

  In this comparative example, as a result of annealing, the amorphous silicon film 14 became a polysilicon film having a grain size greatly grown from about 100 nm to about 1 μm as shown in the electron microscope (TEM) photograph of FIG. Using this polysilicon film, a TFT device was manufactured by performing the same process as in the first example from step 6 onward in FIG. When the characteristics of this device were evaluated, the mobility was as high as about 10 times that of the third example, but the on-current adjacent variation was also as high as about 8%. This variation value is a level that causes unevenness in image quality when used for display applications such as televisions. As described in the problem section of the prior art, when the silicon layer has a large grain size, the breakdown voltage decreases, and the variation in the grain size increases due to the influence of the gate wiring pattern. Arise.

  Table 2 compares the characteristic variation and the crystal grain size in the first example, the third example, and the comparative example.

  As shown in Table 2, in the process in which the crystal grain size grows to 100 nm or more, the characteristic variation, particularly the current value variation (σ of Ion / average value) increases to 6.8%. In the first example, the crystal grain size is 30 nm ± 20 nm, the current value variation (σ of Ion / average value) is 1.3%, and in the third example, the crystal grain size is 50 nm ± 40 nm. The value variation (σ of Ion / average value) was 2.8%. Thus, it can be seen that when the crystal grain size is 100 nm or more, the current value variation becomes too large, and it becomes difficult to use this thin film transistor as a drive transistor of an organic EL display device, for example. On the other hand, the thin film transistor manufactured by the manufacturing method of the present invention has a crystal grain size of 90 nm or less and a current value variation (σ of Ion / average value) of 2.8% or less. It can be seen that the present invention can be applied to the driving transistor.

  Next, the relationship between the on-current and the current value variation in FIG. 7 will be described. In FIG. 7, black diamonds are data of the first example, black squares are data of the third example, and white triangles are data of the comparative example.

  As shown in FIG. 7, in the comparative example having a large crystal grain size, it can be seen that there is a maximum current value variation of 15.5%. On the other hand, when the crystal grain size is 10 nm to 90 nm as in the third example, the maximum value of the current value variation is 4.8%, and when the crystal grain size is 10 nm to 50 nm as in the first example, the current value variation is larger. The maximum value is 3.0%, and it can be seen that the maximum value of the current value variation decreases as the crystal grain size decreases. For this reason as well, when the thin film transistor of the present invention is used as a driving transistor of an organic EL display device, for example, the microcrystalline silicon film 15 having a crystal grain size as small as possible, for example, a crystal grain size close to 10 nm, is formed. It turns out that is effective.

  Next, modified examples of the above embodiments will be described.

  The semiconductor laser device serving as the light source is a continuous oscillation semiconductor laser device having an oscillation wavelength of about 350 nm to 1000 nm. In the above embodiment, a semiconductor laser device using an AlGaAs compound semiconductor as an active layer is used. It is possible to use a semiconductor laser device using a compound semiconductor such as GaN, InGaN, GaAs, AlGaAs, InGaAs, InGaAsP, InGaAlP, ZnSe, ZnS, and SiC as an active layer. In particular, semiconductor laser devices using GaAs or AlGaAs have high output and long life and are commercially available at low cost, and are very suitable as light sources for process devices.

As shown in the relational diagram between the buffer film and the mobility of the thin film transistor in FIG. Can be manufactured. For example, the buffer layer 31 is formed of a silicon oxide film, a film thickness of mobility in 10nm~160nm was ^{1.70cm 2 /V/s~1.77cm 2 / V / s} .

In the above embodiment, the light-heat conversion film 32 uses a silicon oxide film and a molybdenum film. However, a film having heat resistance at a temperature higher than the melting point of silicon (1410 ° C.) is used. be able to. As high melting point metals other than molybdenum (melting point 2623 ° C.), titanium (Ti) (melting point 1666 ° C.), vanadium (V) (melting point 1917 ° C.), chromium (Cr) (melting point 1857 ° C.), zirconium (Zr) (melting point 1852) ), Niobium (Nb) (melting point 2470 ° C.), hafnium (Hf) (melting point 2230 ° C.), tantalum (Ta) (melting point 2985 ° C.), tungsten (W) (melting point 3407 ° C.), High melting point metal compounds such as high melting point metal oxides, nitrides, carbides, oxynitrides, aluminum nitride (AlN), silicon nitride (SiN), silicon oxynitride (SiON), carbide Examples thereof include silicon (SiC), silicon oxide carbide (SiOC), and aluminum oxide (Al ₂ O ₃ ). Alternatively, a stacked film of the metal film, a stacked film of the metal compound film, or a stacked film of the metal film and the metal compound film can be used.

  In addition, for the thin film transistor of the present invention (the thin film transistor of the first example) and the conventional amorphous silicon TFT, the amount of change in threshold voltage depending on the driving time was examined. The result is shown in FIG. In FIG. 9, the vertical axis represents the threshold voltage variation ΔVth (V), and the horizontal axis represents the drive time (s).

  As shown in FIG. 9, the threshold voltage increases as the driving time of the thin film transistor elapses, but the threshold voltage of the thin film transistor of the present invention was 0.07 V even when driven for 100,000 seconds. In the case of the type of amorphous silicon TFT, it was 0.122 V at the time of 10 seconds, and 15.69 V at the driving time of 100,000 seconds. Thus, it can be seen that in the thin film transistor of the present invention, the threshold voltage hardly changes even when the driving time is increased. As described above, the thin film transistor formed by the manufacturing method of the present invention has a feature that a stable display quality can be maintained for a long time. For this reason, by using it as a drive transistor of a display device, especially an organic EL display device, the organic EL display device can be displayed with a stable image quality for a long period of time.

  The thin film transistor of the present invention described above can be used as a driving transistor for driving a pixel of a display panel having a plurality of pixels. Therefore, in the method for manufacturing a display device, in the step of forming a thin film transistor, the above-described method for manufacturing a thin film transistor of the present invention can be employed.

  Therefore, since a thin film transistor with small characteristic fluctuation can be manufactured, there is an advantage that a stable display quality can be maintained for a long time even in a display device with a long driving time. This advantage is particularly effective for a display device such as an organic EL display device having a long driving time of a thin film transistor. In addition, since a thin film transistor with small variation in characteristics can be manufactured, a display device in which the display screen is not uneven can be realized. This advantage is also effective for a display device such as an organic EL display device having a long driving time of a thin film transistor. In addition, since a transistor having higher mobility than a conventional amorphous silicon TFT can be obtained, a display device having a large number of transistors in a pixel, for example, an organic EL display device, can realize a small and high-resolution display device. it can. Furthermore, since the light source used in the process equipment is a small, high-power, inexpensive, and long-life semiconductor laser device, the process equipment itself is inexpensive and highly reliable, keeping the initial investment and maintenance costs of mass production equipment low. As a result, the production cost of the display device can be reduced.

It is the flowchart of the manufacturing process which showed the 1st example of one Embodiment which concerns on the manufacturing method of the thin-film transistor of this invention. It is manufacturing process sectional drawing which showed the 1st example of one Embodiment which concerns on the manufacturing method of the thin-film transistor of this invention. It is an electron microscope (SEM) photograph of the microcrystalline silicon film obtained by the recrystallization process in the first example. It is manufacturing process sectional drawing which showed the 2nd example of one Embodiment which concerns on the manufacturing method of the thin-film transistor of this invention. It is an electron microscope (SEM) photograph of the microcrystalline silicon film obtained by the recrystallization process in the third example. It is an electron microscope (SEM) photograph of the silicon crystal obtained by the recrystallization process in a comparative example. FIG. 4 is a relationship diagram between on-current and current value variation. It is a relationship diagram between the mobility of a buffer film and a thin-film transistor. It is a relationship diagram between the amount of change in threshold voltage and the driving time of a thin film transistor.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Thin film transistor, 11 ... Substrate, 12 ... Gate electrode, 13 ... Gate insulating film, 14 ... Amorphous silicon film, 15 ... Microcrystalline silicon film, 16 ... Semiconductor laser beam, 17 ... Amorphous silicon film, 31 ... Buffer film, 32 ... Light-heat conversion film

Claims (4)

Forming a gate electrode on the substrate;
Forming a gate insulating film on the substrate so as to cover the gate electrode;
Forming a channel layer on the gate electrode via the gate insulating film;
Forming a source electrode on one side of the channel layer and forming a drain electrode on the other side.
The step of forming the channel layer includes:
Forming an amorphous silicon film, a buffer film, and a light-to-heat conversion film in this order on the gate insulating film;
Irradiating the light-heat conversion film with semiconductor laser light to crystallize the amorphous silicon film into a microcrystalline silicon film;
Removing the light-heat conversion film and the buffer film;
And a step of forming an amorphous silicon film over the microcrystalline silicon film. The method for manufacturing a thin film transistor according to claim 1, wherein a crystal grain size of the microcrystalline silicon film is 100 nm or less. Before irradiating the semiconductor laser light after forming the light-heat conversion film, forming a reflection reducing film on the light-heat conversion film,
The method of manufacturing a thin film transistor according to claim 1, wherein the reflection reducing film is removed when the light-heat conversion film and the buffer film are removed. In a method for manufacturing a display device including a thin film transistor for driving a pixel of a display panel including a plurality of pixels,
The manufacturing process of the thin film transistor includes
Forming a gate electrode on the substrate;
Forming a gate insulating film on the substrate so as to cover the gate electrode;
Forming a channel layer on the gate electrode via the gate insulating film;
Forming a source electrode on one side of the channel layer and forming a drain electrode on the other side,
The step of forming the channel layer includes:
Forming an amorphous silicon film, a buffer film, and a light-to-heat conversion film in this order on the gate insulating film;
Irradiating the light-heat conversion film with semiconductor laser light to crystallize the amorphous silicon film into a microcrystalline silicon film;
Removing the light-heat conversion film and the buffer film;
And a step of forming an amorphous silicon film on the microcrystalline silicon film.