JPH09162121A - Method for manufacturing semiconductor device - Google Patents

Abstract

(57) [Abstract] [Problem] Crystallization without complicating the device or process,
The crystal cooling rate after turning off the laser is greatly reduced to increase the crystal grain size and suppress the generation of defects. SOLUTION: A step of irradiating a deposition layer formed on a substrate with a laser beam, and a step of irradiating the deposition layer with light having a larger light absorption coefficient than that of the substrate, and the intensity of the light Is gradually increased or decreased, and the deposited layer is heated at a temperature higher than that of the substrate to crystallize the deposited layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of crystallizing a semiconductor thin film which is an element forming layer of a polycrystalline thin film transistor (TFT).

TFTs are used in drive circuits and display matrix circuits incorporated in liquid crystal display (LCD) panels. TFTs are required to have high carrier mobility and a small leak current in the off state, but it is difficult to sufficiently reduce defects in the crystal of polycrystalline silicon (p-Si) or at grain boundaries. However, its practical use has been hindered for a long time.

[0003]

2. Description of the Related Art A TFT for a liquid crystal display is formed on a transparent substrate, and amorphous silicon (a-Si) has been used as the crystal material for the TFT, but it has been changed to polycrystalline silicon (p-Si). By replacing them, the carrier mobility can be improved by several orders of magnitude, and as a result, the operating speed of the TFT can be increased and the device size can be reduced.

A p-Si film on a transparent substrate is often produced by heating an a-Si film at high temperature and crystallizing it. In the past, an expensive quartz plate with a high melting point was used for the transparent substrate, but in the future, an inexpensive glass plate will be used. However, when the glass substrate is heated above 600 ° C, it is significantly distorted, so the crystallization temperature must be lowered.

An example of a conventional technique for performing crystallization while keeping the substrate temperature at 600 ° C. or lower will be described with reference to FIG. 6 (a) to 6 (d) are explanatory views of a conventional example of the crystallization method.

In general, pulsed laser irradiation as shown in FIG. 6 (a) is the most effective method. According to this, only the Si film 3 on the glass substrate 1 can be selectively heated by the laser light 4. Here, in order to suppress impurity diffusion and heat conduction between the glass substrate 1 and the Si film 3, silicon dioxide (SiO 2
₂ ) The membrane 2 is sandwiched.

The laser light 4 is light in the ultraviolet region whose light absorption efficiency is higher than that of the Si film 3 and lower than that of the substrate, and an excimer laser having a large pulse output is suitable. The excimer laser is a short pulsed light with a full width at half maximum of 10 to 20 ns.
Since the light is turned off before heat is transferred to the substrate, the substrate temperature does not rise easily. When the laser is turned on, part or all of the Si film 3 melts, and it crystallizes when cooled after turning off.

At this time, if the cooling rate is too high, the Si film 3
Will become amorphous or become microcrystalline, and even if crystallized, many dislocations and stacking faults will occur. Therefore, it is necessary to control the cooling rate. In principle, the smaller the cooling rate, the less likely defects will occur, and the larger the grain size of the polycrystal.

As a conventional method for reducing the cooling rate, there is a method in which the substrate temperature is raised by a heater 6 as shown in FIG. 6 (b). H. Kuriyama et al., Jan. J.
Appl.Phys.Vol.30 (1991) 3700. There is a calculation example that when the substrate temperature is raised to 400 ° C, the cooling rate of the Si film 3 becomes about 1/3 of that at room temperature, but it is difficult to reduce the cooling rate further.

As another method, there is a double pulse / dual beam excimer laser method (Ishihara et al., 1995 Spring Applied Physics Society 29p-Q-4). In this method, as shown in FIG. 6 (c), two laser beams having different intensities are irradiated at different times. Observe the timing when the temperature of the Si film 3 drops after turning off one laser 4,
When irradiated with, the cooling rate can be effectively reduced. In this method, there are problems that two laser devices are required, it is difficult to match the timing of turning on and off the laser, and there is a limit to lowering the cooling rate.

As shown in FIG. 6 (d), there is also a method of heating with lamp light 8 at the same time as laser irradiation (Ishimaru et al., Japanese Patent Laid-Open No. 06-29212). In this method, crystal nucleation is performed by laser irradiation, and annealing for nucleus growth is performed by heating with lamp light 8. Compared with heater heating, this case has the advantage that a light-shielding mask can be used for selective heating. However, the lamp irradiation is carried out continuously, and no method for reducing the cooling rate or a method for reducing the occurrence of defects has been presented. Except for the advantage that local heating is possible, its effect Is similar to heater heating.

Also, an example in which p-Si for TFT is crystallized only by lamp annealing has been reported (I. Yudasaka and H. Oh.
shima, Extended Abstracts of the 1993 Int. Conf. o
nSolid State Dev. and Mater, 1993, pp.1005.). In this method, the temperature is raised to 600 to 700 ° C in 50 seconds and then the power is turned off to cool. However, it is extremely difficult to raise the crystallization temperature without heating the substrate by heating the lamp. This is because lamp heating has an irradiation time that is three orders of magnitude longer than laser heating, and the amount of heat transferred from the Si layer to the substrate is large.

[0013]

SUMMARY OF THE INVENTION According to the present invention, the crystal cooling rate after the laser is turned off is greatly reduced to increase the crystal grain size and to suppress the occurrence of defects without complicating the apparatus and the process. The purpose is to

[0014]

To solve the above problems, 1) a step of irradiating a deposition layer formed on a substrate with laser light, and the deposition layer has a larger light absorption coefficient than the substrate. Irradiating the deposited layer with light and gradually increasing or decreasing the intensity of the light, and heating the deposited layer at a temperature higher than that of the substrate to crystallize the deposited layer, Or 2) the method of manufacturing a semiconductor device according to the above 1, wherein the intensity of the light is increased and decreased once or more after the irradiation of the laser light, or 3) the intensity of the light is irradiated before the irradiation of the laser light. 2. The method for manufacturing a semiconductor device according to 1 above, wherein the rising and falling of the light is performed once or more, or 4) the light is deposited through a filter in which one or more plates having the same material or light absorption characteristics as the substrate are stacked. Of the semiconductor device according to the above 1 for irradiating a layer Manufacturing method, or 5) The substrate is a transparent substrate for visible light, the deposited layer is a semiconductor layer, the laser light is excimer laser light, and the light is arc lamp light or halogen lamp light. 2. The method for manufacturing a semiconductor device as described in 1 above. Is achieved by

In the present invention, the heating of the crystal film by the pulse laser is carried out while the lamp light intensity is increased and decreased within a predetermined time. 1 (a) to 1 (c) are explanatory views of the principle of the present invention.

The heating of the lamp is performed as shown in FIG.
Is before laser irradiation, (b) is simultaneous with laser irradiation, and (c) is before laser irradiation. (a) Reduction of defects generated after laser irradiation, (b) Reduction of cooling rate after laser irradiation,
Each of (c) has an effect on the microcrystallization of a-Si.

While heating the lamp at the same time as the laser irradiation of FIG. 1 (b) is the same as that of FIG. 6 (d) of the conventional example, the point that the cooling rate is controlled by changing the intensity of the lamp light irradiation. different. As shown in the lamp light intensity in Fig. 6 (b), the lamp light intensity gradually increases before the laser light is turned on, reaches the maximum when the laser light is irradiated, and gradually decreases after the laser light is turned off with a gentler slope than when the laser light is gradually increased.

As a result, the cooling rate of the crystal film after laser irradiation can be greatly reduced with good controllability, so that amorphization of crystals, microcrystallization, generation of defects, etc.
It is possible to suppress the occurrence of obstacles that hinder the operation of T. Also,
It also has the effect of reducing the density of defects remaining after laser irradiation.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 FIGS. 2 (a) to 2 (c) show Embodiment 1 of the present invention.
FIG.

In FIG. 2 (a), the substrate 1 has a thickness of about 1 m.
Using a m-thick glass plate (Corning 7059), a SiO ₂ film 2 with a thickness of 200 nm is deposited on this by sputtering to serve as the base film. Then, an a-Si film 11 with a thickness of 80 nm is deposited on this by plasma vapor deposition (CVD) method.

In FIG. 2B, the laser light 4 is applied to the a-Si film 11.
To crystallize. A KrF excimer laser is used as the laser light source. This has an oscillation wavelength of 248 nm and a half width of about 20 ns.
Pulsed light source. The laser beam is shaped into a rectangle using an optical system and is irradiated. Laser irradiation conditions are, for example,
Irradiate 10 pulses per site with energy of 350 mJ / cm ² . This laser irradiation changes the a-Si film 11 into a p-Si film 12.

In FIG. 2 (c), the crystallized substrate is transferred to a lamp annealing apparatus. In this device, a plurality of linear halogen lamps 21 are arranged at equal intervals as shown in FIG. 3, and a reflecting mirror 22 is arranged on the back surface of each lamp. Nitrogen gas is flown into the device, and irradiation is performed from the Si layer 12 side. Since it is difficult to measure the temperature of the Si layer 12 on the glass substrate 1 directly, the temperature is monitored by the one 23 with the thermocouple embedded in the Si chip placed beside the substrate.

The peak wavelength of the lamp 21 is around 1 μm, although it varies depending on the magnitude of the current to be passed, and has a broad spectrum. The Si layer 12 is heated by absorbing light having a wavelength of 1 μm or less. The glass substrate 1 is hard to be heated in the wavelength range of 2 μm or less, but absorbs light in the wavelength range of more than that and the temperature rises. To prevent this, the light source 22 and the substrate 1
The optical filter 24 is inserted between and.

Generally, light of 2 μm or more can be cut by an interference filter manufactured by vacuum deposition, but it is difficult to manufacture a large area interference filter. Therefore, a glass plate made of the same material as the substrate is used here, and the surface is not reflected. Apply a protective coating. If the glass filter has the same thickness as the substrate, the filter itself may be distorted or destroyed by heat. Therefore, if the thickness is smaller than that of the substrate, for example 0.3 mm, the amount of heat absorption will decrease and the temperature difference in the thickness direction will decrease, so that distortion and damage can be prevented. By arranging and placing a plurality of such glass plates, the absorption amount of infrared light is increased. In addition, the effect of blocking UV light is further increased by increasing the thickness of the glass plate as the distance from the light source increases. You may make it cool by making nitrogen flow along these glass plates. In this way, by using the glass plate of the same material as the substrate as a filter, the temperature rise of the substrate is suppressed,
As a result, the effect of increasing the lamp light can be obtained.

The heating conditions of the lamp need to be optimized because they vary depending on the device, but the following procedure is used, for example.
Temperature rising at 50 ℃ / sec, holding at 900 ℃ for 10 seconds, 600 ℃ at 50 ℃ / sec
Cool down to 100 ℃ at 20 ℃ / sec. The above process 1
Take out the substrate repeatedly or multiple times. The temperature sequence is set by programming the controller 26 of the lamp heating power supply 25. The fastest control of the temperature rise and fall of the lamp is, of course, the stepwise increase and decrease of the current.

A feature of the present invention is to control the time change of the lamp light intensity and thereby raise the temperature of the crystal film to a strain point of the substrate (600 ° C. in the case of a glass substrate) or higher. In the conventional example shown in FIG. 6 (d), the temperature is raised, the temperature is maintained at a constant temperature, the temperature is lowered, and the temperature is controlled. However, the present invention is different from the conventional example in the speed of temperature change and the maximum temperature.

The effects obtained by this embodiment are shown in FIG. This figure shows the crystal quality of the obtained crystal film evaluated by measuring Raman scattering. The set temperature on the horizontal axis shows the temperature of lamp heating measured by a Si chip with a thermocouple embedded, and the vertical axis shows the Raman peak wavenumber.
And the half width is shown in Fig. 5 (b). Although it is not possible to measure the actual temperature of the crystal film by measuring 5 points per sample, it is considered that the temperature is lower than this because the light absorption rate is small. The Raman peak wave number and the half-width have large variations before heating, but after heating, they are aligned to the smaller values, indicating that the crystal uniformity is improved and the defect density is reduced.

Embodiment 2 This example is intended to reduce the cooling rate after laser irradiation by irradiating the lamp with auxiliary light, and corresponds to FIG. 1 (b). The structure of the apparatus used for this is shown in FIG.

As in the first embodiment, on the glass substrate 1,
The a-Si layer 3 is deposited, and the laser light 4 is irradiated on the a-Si layer 3 side and the lamp light 5 is irradiated on the glass substrate 1 side. The laser device includes an excimer laser light source 31, a beam shaping optical system 32, and a mirror 33 for laser light. Lamp light 5 is a filter
Irradiation from the glass substrate 1 side through 42, lens system 43, and slit 44. The shape of the lamp light on the a-Si layer 3 is the same as or larger than the laser beam. A power supply 25 and an intensity control device 26 are connected to the lamp.

Next, the sequence of processes will be described. First, the lamp is heated. Raise the temperature at 200 ° C / sec, and when it reaches 800 ° C, move to the temperature lowering stage. The pulse laser is turned on at the same time when the temperature is lowered. Energy is smaller than in the first embodiment,
For example, the energy is 280 mJ / cm ² . The ramp temperature is 200 ℃ / sec, and it is lowered to 100 ℃. Since there are steps in which the temperature changes rapidly and it is difficult to monitor the temperature due to the insertion of the optical system, it is advisable to program the output of the power supply 25 by the controller 26. This process is repeated once or several times.

Here, the increase and decrease of the lamp intensity and the lighting timing of the pulse laser are electrically synchronized. There are three types of timing to reduce the lamp intensity: immediately before lighting the pulse laser, at the same time as lighting, and immediately after lighting.
Normally it is the same time.

When the above steps are repeated, it is necessary to optimize the lamp output and the laser output for each cycle.
Especially, in the first cycle, the crystal phase changes significantly from a-Si to p-Si, so the optimum conditions differ greatly from the subsequent cycles. In such a case, a method called step scanning, in which a laser beam is moved in small steps and a repetitive pulse is emitted, I. Asai et al., Jpn.J.Appl.Phys.
In Vol.32 (1993) 474., the scan may be performed twice or more, and the intensity may be changed between the first scan and the next scan.

In the second embodiment, lasers and lamps are separately arranged from both sides of the substrate, both from the crystal film side, and both from the substrate side. The optical design is easy, and is effective when there is no space on the backside of the substrate or when the substrate is heated by a heater, etc. In this case, the laser device uses a long-wavelength XeF excimer laser (351 nm).

Embodiment 3: If lamp heating is carried out before pulse laser irradiation, crystallization of a-Si occurs, and there is an action of assisting crystallization by laser. This method corresponds to FIG. 1 (c).

After depositing a-Si on the glass substrate, the substrate is placed in the same lamp heating device as in Embodiment 1 to heat the crystal layer. For heating, for example, increase the temperature to 200 ℃ / sec up to 1000 ℃,
Leave it for 1 second, and lower the temperature to 100 ℃ at 200 ℃ / second. This step is performed once or plural times.

In a similar known process, there is a method in which after a-Si is crystallized by a pulsed laser having a small intensity, the intensity of laser light is increased and recrystallization is performed. In this case, since the cooling rate is high, all or part of the deposited film may return to a-Si. On the other hand, lamp heating has a low cooling rate, and this is unlikely to occur.

Embodiment 4: The above-mentioned various embodiments are not independent, but the effect is further enhanced by combining the heating methods shown in FIGS. 1 (a) to 1 (c).

For example, after controlling the cooling rate of crystallization by laser as shown in FIG. 1 (b), defects can be reduced only by irradiation with a lamp as shown in FIG. 1 (a). The lamp irradiation in this case uses the apparatus of FIG. 3 and the apparatus of FIG.
The lamp device of FIG. 4 is replaced with the device of FIG.

The semiconductor layer to be crystallized is not only Si but also Ge, SiGe
It is also applicable to group IV semiconductors such as, group II-VI, group III-V semiconductors, etc. In addition to glass, the substrate is made of quartz, sapphire,
It may be silicon whose surface is coated with an insulating film. Further, even if a polymer substrate is used in the future, the present invention can be applied by polarizing the heating condition or the infrared cut filter.

[0040]

EFFECTS OF THE INVENTION According to the present invention, the following effects can be obtained by adding lamp heating of which the intensity is increased or decreased within a predetermined time to pulsed laser heating in which it is difficult to control crystallization and maintain the stability of the apparatus. Can be obtained.

By controlling and reducing the cooling rate after turning off the laser, it is possible to suppress the occurrence of defects such as amorphization, microcrystallization, stacking faults and dislocations of the crystallized film. After the laser heating process, the crystal film can be annealed without raising the substrate temperature above the strain point, so that defects can be reduced.

Before laser heating, a-Si can be preliminarily crystallized without raising the substrate temperature above the strain point, so that the efficiency of crystallization by laser is improved. As a result,
A TFT with high carrier mobility and low leakage current can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is an explanatory diagram of Embodiment 1 of the present invention.

FIG. 3 is an explanatory diagram of the device according to the first embodiment of the present invention.

FIG. 4 is an explanatory diagram of an apparatus according to a second embodiment of the present invention.

FIG. 5 is an explanatory diagram of the effect of the present invention.

FIG. 6 is an explanatory diagram of a conventional example.

[Explanation of symbols]

1 Glass substrate 2 SiO ₂ film 3 Si film 4 Laser light 11 a-Si film 12 Crystallized p-Si film 21 Lamp light source (for large area irradiation) 24 Infrared light cut filter (optical filter) 26 Lamp intensity Time change control device 41 Lamp light source (for small area irradiation)

Claims (5)

[Claims] 1. A step of irradiating a deposition layer formed on a substrate with laser light, and irradiating the deposition layer with light having a larger light absorption coefficient than that of the substrate, and A step of gradually increasing or decreasing the strength, and heating the deposited layer at a temperature higher than that of the substrate to crystallize the deposited layer. 2. The method for manufacturing a semiconductor device according to claim 1, wherein the intensity of the light is increased and decreased once or more after the irradiation of the laser light. 3. The method of manufacturing a semiconductor device according to claim 1, wherein the intensity of the light is increased and decreased at least once before the irradiation of the laser light. 4. The semiconductor device according to claim 1, wherein the light is applied to the deposited layer through a filter in which one or more plates having the same material as the substrate or having the same light absorption characteristics are stacked. Production method. 5. The substrate is a transparent substrate for visible light, the deposited layer is a semiconductor layer, the laser light is excimer laser light, and the light is arc lamp light or halogen lamp light. The method for manufacturing a semiconductor device according to claim 1, wherein