JP4354015B2 - Manufacturing method of semiconductor device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a semiconductor device, and more particularly, to a method for crystallizing a semiconductor thin film to be an element forming layer of a polycrystalline thin film transistor (TFT).
[0002]
TFTs are used in drive circuits and display matrix circuits built into liquid crystal display (LCD) panels. TFTs require high carrier mobility and low leakage current in the off state, but it is difficult to sufficiently reduce defects in polycrystalline silicon (p-Si) crystals or at grain boundaries. However, practical use has been hindered for a long time.
[0003]
[Prior art]
A TFT for a liquid crystal display is formed on a transparent substrate, and amorphous silicon (a-Si) was conventionally used as the crystal material. By replacing this with polycrystalline silicon (p-Si), carrier Mobility can be improved by several orders of magnitude, resulting in faster TFT operation and reduced device size.
[0004]
A p-Si film on a transparent substrate is often produced by heating an a-Si film at high temperature and crystallizing it. Conventionally, an expensive quartz plate having a high melting point has been used for the transparent substrate. However, an inexpensive glass plate has been used in the future. However, since glass substrates are greatly distorted when heated to 600 ° C or higher, lowering the crystallization temperature is required.
[0005]
An example of the prior art for performing crystallization while maintaining the substrate temperature at 600 ° C. or lower will be described with reference to FIG.
FIGS. 6A to 6D are explanatory views of a conventional example of the crystallization method.
[0006]
In general, pulse 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 beam 4. Here, a silicon dioxide (SiO ₂ ) film 2 is sandwiched between the glass substrate 1 and the Si film 3 in order to suppress impurity diffusion and heat conduction.
[0007]
The laser light 4 is light in the ultraviolet region, which has high light absorption efficiency for the Si film 3 and low for the substrate, and an excimer laser having a large pulse output is suitable. The excimer laser is a short pulse light with a half-width of 10 to 20 ns, and is turned off before heat is transmitted to the substrate 1, so that the substrate temperature is difficult to rise. When the laser is turned on, part or all of the Si film 3 is melted and crystallized upon cooling after the light is turned off.
[0008]
At this time, if the cooling rate is too high, the Si film 3 becomes amorphous or becomes microcrystalline, and even if crystallized, many transitions and stacking faults occur. Therefore, it is necessary to control the cooling rate. In principle, defects are less likely to occur when the cooling rate is low, and the grain size of the polycrystal is likely to increase.
[0009]
As a conventional method for reducing the cooling rate, there is a method in which the substrate temperature is raised by the 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 is about 1/3 that at room temperature, but it is difficult to lower the cooling rate any more.
[0010]
Another method is the 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 while being shifted in time. If one of the lasers 4 is extinguished and the temperature of the Si film 3 decreases, and the laser pulse 7 that is weaker than this is irradiated, the cooling rate can be effectively reduced. This method has problems such as requiring two laser devices, difficulty in matching the timing of turning on and off the laser, and limiting the cooling rate.
[0011]
In addition, as shown in FIG. 6 (d), there is a method of heating with lamp light 8 simultaneously with laser irradiation (Ishimaru et al., JP 06-29212). In this method, crystal nucleation is performed by laser irradiation, and annealing for nucleation is performed by heating with lamp light 8. Compared to heater heating, this case has the advantage that it can be selectively heated using a shading mask. However, lamp irradiation is performed continuously, and no method has been proposed to reduce the cooling rate or to reduce the occurrence of defects. Except for the advantage that local heating is possible, its effect can be reduced. Is similar to heater heating.
[0012]
In addition, an example of crystallizing p-Si for TFT by lamp annealing alone has been reported (I. Yudasaka and H. Ohshima, Extended Abstracts of the 1993 Int. Conf. On Solid 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 down. However, it is extremely difficult to raise the temperature to the crystallization temperature without giving thermal distortion to the substrate by lamp heating. This is because lamp heating takes 3 orders of magnitude longer than laser heating, and the amount of heat transferred from the Si layer to the substrate increases.
[0013]
[Problems to be solved by the invention]
An object of the present invention is to greatly reduce the cooling rate of a crystal after laser extinction, increase the crystal grain size, and suppress the occurrence of defects without complicating the apparatus and the process.
[0014]
[Means for Solving the Problems]
The solution to the above problem is to irradiate a deposition layer made of an amorphous semiconductor layer formed on a substrate with a pulsed laser beam made of excimer laser light, and the deposition layer has a larger light absorption coefficient than the substrate. The lamp light is irradiated to at least the laser light irradiation position of the deposited layer through the slit in synchronization with the laser light irradiation, and the intensity of the lamp light is gradually increased from before the laser light is turned on. A step of gradually decreasing at a gentler slope than immediately before, simultaneously with, or immediately after, and heating the deposited layer at a temperature higher than that of the substrate to crystallize the deposited layer This is achieved by a device manufacturing method. Further, the lamp light is irradiated to the deposited layer through a filter in which one or more plates made of the same material as the substrate are stacked. Further, the substrate is a substrate transparent to visible light, and the lamp light is arc lamp light or halogen lamp light.
[0015]
In the present invention, heating is performed while the intensity of the lamp light is increased and decreased within a predetermined time, together with the heating of the crystal film by the pulse laser.
1A to 1C are explanatory views of the principle of the present invention.
[0016]
As shown in FIG. 1, there are three types of lamp heating: (a) after laser irradiation, (b) simultaneously with laser irradiation, and (c) before laser irradiation. (a) the reduction of defects that occurred after the laser irradiation, (b) a decrease in the cooling rate after the laser irradiation, there is a respective effect on micro-crystallization of (c) is a-Si.
[0017]
The lamp heating at the same time as the laser irradiation in FIG. 1B is the same as that in FIG. 6D in the conventional example, except that the cooling rate is controlled by changing the intensity of the lamp light irradiation. As shown in the lamp light intensity in Fig. 1 (b), the lamp light intensity gradually increases before the laser light is turned on, reaches a maximum when the laser light is irradiated, and gradually decreases after the laser is turned off with a gentler slope than when the laser light is turned on.
[0018]
As a result, the cooling rate of the crystal film after laser irradiation can be greatly reduced with good controllability, so that problems that hinder the operation of the TFT, such as crystal amorphization, microcrystallization, and generation of defects, are prevented. Can be suppressed. It also has the effect of reducing the density of defects remaining after laser irradiation.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1:
2 (a) to 2 (c) are explanatory diagrams of Embodiment 1 of the present invention.
[0020]
In Fig. 2 (a), the substrate 1 is a glass plate (Corning 7059) with a thickness of about 1 mm, and a 200 nm thick SiO ₂ film 2 is deposited on it by sputtering to form the underlying film. Next, an 80 nm thick a-Si film 11 is deposited thereon by plasma vapor deposition (CVD).
[0021]
In FIG. 2B, the a-Si film 11 is crystallized by irradiating it with laser light 4. A KrF excimer laser is used as the laser light source. This is a pulsed light source with an oscillation wavelength of 248 nm and a half width of about 20 ns. The laser beam is irradiated after being shaped into a rectangle using an optical system. As the laser irradiation condition, for example, irradiation is performed with 10 pulses per spot at an energy of 350 mJ / cm ² . By this laser irradiation, the a-Si film 11 is changed to the p-Si film 12.
[0022]
In FIG. 2 (c), the crystallized substrate is transferred to a lamp annealing apparatus. In this apparatus, as shown in FIG. 3, a plurality of linear halogen lamps 21 are arranged at equal intervals, and a reflecting mirror 22 is arranged on the back of each lamp. Nitrogen gas is allowed to flow into the apparatus, and irradiation is performed from the Si layer 12 side. Since it is difficult to directly measure the temperature of the Si layer 12 on the glass substrate 1, the temperature is monitored with a thermochip embedded in a Si chip placed beside the substrate23.
[0023]
The peak wavelength of the lamp 21 varies depending on the magnitude of the energized current, but is around 1 μm and has a wide spectrum. The Si layer 12 is heated by absorbing light having a wavelength of 1 μm or less. The glass substrate 1 is difficult to be heated in the wavelength region below 2 μm, but the temperature rises by absorbing light in the wavelength region longer than that. In order to prevent this, an optical filter 24 is inserted between the light source 22 and the substrate 1.
[0024]
In general, light of 2 μm or more can be cut with an interference filter manufactured by vacuum evaporation, but since it is difficult to manufacture a large-area interference filter, a glass plate made of the same material as the substrate is used here, and the surface is used for antireflection. Apply coating. If the thickness of the glass filter is the same as that of the substrate, the filter itself may be distorted or destroyed by heat. Therefore, if the thickness is smaller than the substrate, for example 0.3 mm, the amount of heat absorption decreases, and the temperature difference in the thickness direction decreases, so that distortion and breakage can be prevented. The amount of infrared light absorbed is increased by placing a plurality of such glass plates. Further, if the thickness of the glass plate is increased as the distance from the light source is increased, the effect of ultraviolet light cut is further increased. You may make it cool by flowing nitrogen along these glass plates. Thus, by using a glass plate made of the same material as the substrate as a filter, the temperature rise of the substrate can be suppressed, and as a result, the lamp light can be strengthened.
[0025]
The lamp heating conditions vary depending on the equipment and need to be optimized. For example, the following procedure is used.
Temperature increased at 50 ° C / second, held at 900 ° C for 10 seconds, temperature decreased to 600 ° C at 50 ° C / second, temperature decreased to 100 ° C at 20 ° C / second. The above process is repeated once or a plurality of times to take out the substrate. The temperature sequence is set by programming the controller 26 of the lamp heating power supply 25. Of course, the fastest control of the temperature rise and fall of the lamp is to increase and decrease the stepped current.
[0026]
The feature of the present invention is to control the temporal change of the lamp light intensity and thereby raise the temperature of the crystal film to a temperature higher than the strain point of the substrate (600 ° C. in the case of a glass substrate). In the conventional example of FIG. 6 (d), the temperature rise, the holding at a constant temperature, the temperature drop and the temperature control are performed, but the present invention differs from this conventional example in the speed of temperature change and the maximum temperature.
[0027]
The effect obtained by this embodiment is shown in FIG.
This figure shows a crystal whose crystal quality was evaluated by measuring Raman scattering. The set temperature on the horizontal axis shows the temperature of lamp heating measured by a Si chip embedded with a thermocouple, and the vertical axis shows the Raman peak wavenumber Fig. 5 (a) and the half-value width Fig. 5 (b). Although it is not possible to measure the temperature of the actual 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 wavenumber and half-value width vary widely before heating, but after heating, they are aligned with the smaller values, indicating that the crystal uniformity has been improved and the defect density has been reduced.
[0028]
Embodiment 2:
In this example, the lamp is supplementarily irradiated to reduce the cooling rate after laser irradiation, which corresponds to FIG. 1 (b). The configuration of the apparatus used for this is shown in FIG.
[0029]
As in the first embodiment, an a-Si layer 3 is deposited on a glass substrate 1, and laser light 4 is irradiated from the a-Si layer 3 side and lamp light 5 is irradiated from the glass substrate 1 side. The laser device consists of d excimer lasers light source 31 and the beam shaping optical system 32 and the mirror 33. The lamp light 5 is irradiated from the glass substrate 1 side through the filter 42, the lens system 43, and the 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 source 25 and an intensity controller 26 are connected to the lamp.
[0030]
Next, the process sequence is explained.
First, the lamp is heated. The temperature rises at 200 ° C / second, and when it reaches 800 ° C, the temperature goes down. At the same time as the temperature drops, the pulse laser is turned on. The energy is smaller than that of the first embodiment, for example, the energy is 280 mJ / cm ² . The ramp cooling rate is 200 ° C / s, and is lowered to 100 ° C. Since there is a step with a fast temperature change and it is difficult to monitor the temperature because of the optical system inserted, the output of the power supply 25 should be programmed by the control device 26. This process is repeated once or several times.
[0031]
Here, the increase / decrease of the lamp intensity and the timing of lighting of the pulse laser are electrically synchronized. There are three types of timing to decrease the lamp intensity: immediately before the pulse laser is lit, simultaneously with the lighting, and immediately after it is lit.
[0032]
When the above steps are repeated, it is necessary to optimize the lamp output and laser output for each cycle. In particular, in the first cycle, the crystal phase changes significantly from a-Si to p-Si, so the optimum conditions are significantly different from the following cycles. In such a case, I.Asai et al., Jpn.J.Appl.Phys.Vol.32 (1993) 474. A method called step scan in which the laser beam is moved in small increments and repeatedly irradiated with pulses. The scan may be performed twice or more, and the intensity may be changed between the first scan and the next scan.
[0033]
In the second embodiment, the laser and the lamp are arranged in three ways: (1) separately from both sides of the substrate, (2) both from the crystal film side, and (3) both from the substrate side. (1) is easy for optical design, and (2) is effective when there is no space on the backside of the substrate or when the substrate is heated with a heater or the like. In this case, the laser device is preferably a long wavelength XeF excimer laser. (351 nm) is used.
[0034]
Embodiment 3:
If lamp heating is performed before pulsed laser irradiation, crystallization of a-Si occurs, which has the effect of assisting crystallization by the laser. This method corresponds to FIG.
[0035]
After a-Si is deposited on the glass substrate, the substrate is placed in a lamp heating apparatus similar to that in Embodiment 1 to heat the crystal layer.
For heating, for example, the temperature is raised to 1000 ° C at 200 ° C / second, and left for 1 second, and lowered to 100 ° C at 200 ° C / second. This step is performed once or a plurality of times.
[0036]
In a similar known process, there is a method in which a-Si is crystallized with a low intensity pulse laser and then recrystallized by increasing the intensity of the laser beam. 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, this is unlikely to occur because lamp cooling has a low cooling rate.
[0037]
Embodiment 4:
The various embodiments described above are not alone, but are more effective by combining the heating methods shown in FIGS. 1 (a) to 1 (c).
[0038]
For example, after controlling the cooling rate of crystallization by laser according to FIG. 1 (b), defects can be reduced only by lamp irradiation according to FIG. 1 (a). In this case, the lamp irradiation is carried out by (1) using the apparatus of FIG. 3, (2) using the apparatus of FIG. 4, (3) replacing the lamp apparatus of FIG. 4 with the apparatus of FIG.
[0039]
The semiconductor layer to be crystallized can be applied to group IV semiconductors such as Ge and SiGe, group II-VI and group III-V semiconductors in addition to Si. In addition to glass, the substrate may be quartz, sapphire, or silicon with an insulating film deposited on the surface. Even if a polymer substrate is used in the future, the present invention can be applied by polarizing a heating condition or an infrared cut filter.
[0040]
【The invention's effect】
According to the present invention, the following effects can be obtained by adding lamp heating whose intensity is increased or decreased within a predetermined time to pulse laser heating, which is difficult to control the crystallization and maintain the stability of the apparatus.
[0041]
(1) By controlling the cooling rate after the laser is extinguished, the generation of defects such as amorphization, microcrystallization, stacking faults and transitions of the crystallized film can be suppressed.
(2) Since the crystal film can be annealed without raising the substrate temperature above the strain point after the laser heating process, defects can be reduced.
[0042]
(3) Since the a-Si can be preliminarily crystallized without raising the substrate temperature above the strain point before laser heating, the efficiency of laser crystallization 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 a diagram illustrating a first embodiment of the present invention. FIG. 3 is a diagram illustrating an apparatus according to a first 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.
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 filter (optical filter)
26 Lamp intensity time change control device
41 Lamp light source (for small area irradiation)

Claims (3)

Irradiating the pulsed laser light comprising an excimer laser beam to the deposition layer formed of an amorphous semiconductor layer formed on the substrate, lamp light towards said deposited layer is the absorption coefficient of light is larger than the substrate the laser beam The at least the laser light irradiation position of the deposited layer is irradiated through the slit in synchronization with the irradiation of the laser light, and the intensity of the lamp light is gradually increased from before the laser light is turned on, immediately before the laser light is turned on, simultaneously, or immediately after, and a step of gradually decreasing loose than when increasing slope, a method of manufacturing a semiconductor device characterized by crystallizing the deposited layer by heating the deposited layer at a temperature higher than the substrate. The process according to claim 1, semiconductor device, wherein the illuminating the lamp light, the deposited layer plates of the same material through one or more stacked filter and the substrate. The substrate is a transparent substrate to visible light, before Symbol lamp light method of manufacturing a semiconductor device according to claim 1, characterized in that the arc lamp light or a halogen lamp light.

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