JPH08241872A - Laser annealing method - Google Patents

Abstract

PURPOSE: To make uniform anneal effects by emitting laser beams by a method wherein an energy profile of laser beams is made trapezoidal and pulse lasers are scanned in one direction relative to an object to be emitted while emitting. CONSTITUTION: Laser beams oscillated from an oscillator 2 are amplified by an amplifier 3 via total reflection mirrors 5, 6, and further introduced into an optical system 4 via total reflection mirrors 7, 8. Beams of laser beams are processed to be linear beams by an optical system 4. Further, an energy density distribution in a width direction of linear laser beams after passed through the optical system 4 is trapezoidal. After the linear beams are output from the optical system 4, they are emitted to a sample 11 via total reflection mirrors 9, but as a width of beams is longer than that of the sample 11, the laser beams can be emitted to the sample 11 by moving the sample 11 in one direction. Thus, it is possible to make uniform anneal effects by emitting of the laser beams.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention disclosed in this specification relates to a technique of irradiating a semiconductor material with a laser beam to anneal it. In addition, the present invention generally relates to a technique of performing various kinds of processing and alteration of an irradiated object by irradiating a laser beam.

[0002]

2. Description of the Related Art In recent years, much research has been conducted on lowering the temperature of semiconductor device processes. The big reason is
This is because it is necessary to form a semiconductor element on an insulating substrate made of glass or the like which is inexpensive and highly workable. Furthermore, when manufacturing an active matrix type liquid crystal display device,
This is because it is necessary to form several hundreds × several hundreds or more thin film transistors on the glass substrate. In addition, there are demands for miniaturization of elements and multilayering of elements.

In the manufacturing process of a semiconductor device, an amorphous component contained in a semiconductor material or an amorphous semiconductor material is crystallized, or although it is originally crystalline, it is irradiated with ions for impurity implantation. It may be necessary to recover the crystallinity of the semiconductor material whose crystallinity has been lowered, or to improve the crystallinity, although it is crystalline. Conventionally, thermal annealing has been used for this purpose. When silicon is used as a semiconductor material, it is annealed at a temperature of 600 ° C. to 1100 ° C. for 0.1 to 48 hours or more to crystallize amorphous, recover the crystallinity, Have been improved.

[0004] In such thermal annealing, in general, the higher the temperature, the shorter the processing time may be, but at a temperature of 500 ° C or lower, there is almost no effect. Therefore, from the viewpoint of reducing the temperature of the process, it is necessary to replace the step conventionally performed by thermal annealing with another means. Especially when a glass substrate is used as the substrate, it is required that the temperature of the thermal annealing be 600 ° C. or lower. Moreover, it is required to shorten the heating time as much as possible. This is because the glass substrate is deformed by heating for a long time. In liquid crystal display devices, several μm
Since the liquid crystal is held between the pair of glass substrates having the gap, the deformation of the substrate greatly affects the display of the liquid crystal display device.

As a process that replaces thermal annealing, a technique of performing various types of annealing by irradiation with laser light is known. Laser light has a feature that high energy comparable to thermal annealing can be limitedly applied only to a required portion, and it is not necessary to expose the entire substrate to a high temperature.

With respect to the irradiation of laser light, two methods are roughly classified and proposed.

The first method uses a continuous wave laser such as an argon ion laser and irradiates a semiconductor material with a spot-like beam. This is a method of crystallizing the semiconductor material by melting the semiconductor material and then slowly solidifying it due to the difference in energy distribution inside the beam and the movement of the beam.

The second method is to crystallize the semiconductor material by irradiating the semiconductor material with a high energy laser pulse by using a pulsed laser such as an excimer laser, instantaneously melting and solidifying the semiconductor material. Is.

[0009]

The problem of the first method is that the processing takes a long time. This is because the maximum energy of the continuous wave laser is limited and the size of the beam spot is at most mm ² unit. On the other hand, in the second method, the maximum energy of the laser is very large, and therefore, it is possible to improve the mass productivity by using a large spot of several cm ² or more.

However, in the case of a square or rectangular beam which is usually used, it is necessary to move the beam vertically and horizontally to process one large area substrate, and there is still room for improvement in terms of mass productivity. was there.

This point can be greatly improved by deforming the beam linearly so that the width of the beam exceeds the substrate to be processed and scanning the beam. The term "scanning" as used herein refers to irradiating linear lasers while gradually shifting and overlapping.

What remains to be solved as a problem is the uniformity of the laser irradiation effect. The following measures have been taken in order to improve this uniformity. As one measure, there is a method in which the shape of the distribution of the beam is made as close as possible to a rectangle by using a slit to reduce the variation in the linear beam.

In the above technique, in order to further reduce the nonuniformity, prior to the irradiation with the strong pulse laser light (hereinafter referred to as main irradiation), the preliminary irradiation with the weaker pulse laser light (hereinafter referred to as the preliminary irradiation) is performed. It is reported that the homogeneity is improved.

This effect is very high, and the characteristics of the semiconductor device can be remarkably improved. This is because by dividing the irradiation energy into two stages, crystallization of the semiconductor film can be performed in stages, and non-uniformity in crystallinity and generation of grain boundaries, and even stress due to abrupt phase change occur. This is because problems such as concentration can be alleviated. In addition, this stepwise irradiation can be increased by increasing the number of times,
The effect can be further enhanced. By these two measures, the uniformity of the laser irradiation effect can be considerably improved. However, when the two-step irradiation method as described above is used, the laser processing time is doubled, so that the throughput is reduced. Further, even if the uniformity of the laser irradiation effect could be considerably improved, it was not yet sufficient.

[0015]

In the invention disclosed in this specification, this problem is solved by devising the distribution of the linear laser beam. In the invention disclosed in this specification, the distribution of the linear laser beam described above is trapezoidal in the narrow width direction (the scanning direction of the laser beam). Then, the semiconductor material is scanned with a laser beam having such an energy distribution. Then, in the laser irradiation method that performs the preliminary irradiation and the main irradiation described above, the role of the weak laser energy of the preliminary irradiation is played by the portion with the energy gradient (tail portion) of the energy distribution (trapezoidal distribution) described above. The same effect as when the irradiation energy is changed stepwise can be obtained.

The main inventions disclosed in this specification will be described below. One of the main inventions disclosed in the present specification is a linearly processed pulsed laser beam,
The energy profile of the laser beam is trapezoidal, and the pulsed laser is irradiated while scanning the irradiation target object in one direction relatively.

FIG. 3 shows how the laser beam is irradiated with the energy beam profile as shown in the above structure.
FIG. 3 shows a beam profile (energy density distribution state in the beam) of energy density in the width direction (scanning direction) of the laser beam linearly processed. The beam profile of the energy density shown in FIG. 3 has a trapezoidal shape in the scanning direction of the laser beam.

According to another aspect of the invention, in the step of irradiating a linearly processed pulse laser while displacing it in one direction, when one point on the irradiation object is focused, the pulse laser irradiates the point a plurality of times. It is characterized by overlapping and striking the laser beam.

When a pulsed laser beam having a beam profile having an energy density as shown in FIG. 3 is moved little by little and the beams are irradiated so as to partially overlap each other, in a specific linear region, A pulse that is irradiated multiple times, and a pulse whose irradiation energy density is gradually increased during the irradiation of these multiple pulses is irradiated at the first stage, and then a pulse whose irradiation energy density is gradually decreased. Will be irradiated.

According to another aspect of the invention, in the step of irradiating a linearly processed pulse laser while shifting it in one direction, when one point on the irradiation target is focused, the pulse laser is 3 to 100 points at that point. It is characterized by overlapping and striking the laser beam so that it is irradiated twice.

According to another aspect of the invention, in the step of irradiating a linearly processed pulse laser while displacing it in one direction, when one point on the irradiation object is focused, the pulse laser is 10 to 20 points at that point. It is characterized by overlapping and striking the laser beam so that it is irradiated twice.

[0022]

When a pulsed laser beam processed into a linear beam is scanned and irradiated in the width direction of the line, a trapezoidal energy distribution is provided in the width direction of the linear laser beam. A region with a low energy density at the skirt of the trapezoidal energy distribution is irradiated to the irradiated region, and the laser beam is subsequently scanned, so that irradiation is performed with a gradually higher energy density.
Further, the laser beam is irradiated with the energy density of the upper side (maximum value) of the trapezoidal distribution, and finally the energy density is gradually reduced. In this way, the irradiation of the laser beam is performed on a certain irradiation target area at an energy density that continuously changes corresponding to the trapezoidal energy distribution.

By irradiating the laser light in the above-mentioned state, when focusing on a certain irradiation area, the weak laser light is first irradiated, the strong laser light is gradually irradiated, and the laser light is gradually further irradiated. The irradiation energy density of light weakens and the irradiation ends. When such laser light irradiation is performed, the energy supplied to the irradiation region does not change rapidly, and thus a rapid phase change in the irradiation region can be prevented. For example, when an amorphous semiconductor is crystallized by irradiation with laser light, it does not undergo abrupt phase change, so it does not have a rough surface or internal stress accumulation, and it must have uniform crystallinity. You can That is, the annealing effect can be made uniform.

[0024]

EXAMPLE In this example, a silicon film is used as a semiconductor material. Then, a structure in which the crystallinity of the silicon film is increased by irradiation with laser light will be described.

First, the device will be described. FIG. 1 shows a conceptual diagram of a laser annealing apparatus used in this embodiment.
In the laser annealing apparatus shown in FIG. 1, the main components are arranged on a table 1. In the configuration shown in FIG.
The laser light is oscillated by the oscillator 2. The laser light oscillated by the oscillator 2 is a KrF excimer laser (wavelength 24
8 nm, pulse width 25 ns). Of course, other excimer lasers or lasers of other types can be used. However, it is necessary to use pulsed laser light.

The laser light oscillated by the oscillator 2 is amplified by the amplifier 3 via the total reflection mirrors 5 and 6, and is further introduced into the optical system 4 via the total reflection mirrors 7 and 8.
The beam of laser light just before entering the optical system is 3 × 2
It is a rectangle with a size of about cm ² , but it has a length of 1 due to the optical system 4.
It is processed into an elongated beam (linear beam) having a width of 0 to 30 cm and a width of 0.1 to 1 cm. The energy density distribution in the width direction of the linear laser beam after passing through this optical system has a trapezoidal shape as shown in FIG. The energy of the laser beam that has passed through this optical system 4 is 1000 m at maximum.
J / shot.

The reason why the laser beam is processed into such an elongated beam is to improve the workability. That is, the linear beam exits the optical system 4, passes through the total reflection mirror 9, and is irradiated on the sample 11. However, since the width of the beam is longer than the width of the sample, the sample should be moved in one direction. Thus, the entire sample can be irradiated with laser light. Therefore, the sample stage and the driving device 10 have a simple structure and are easy to maintain. In addition, the positioning operation (alignment) when setting the sample is easy.

The stage 10 of the substrate (sample) irradiated with the laser light is controlled by a computer and is designed to move in a direction perpendicular to the linear laser light. Furthermore, it is convenient to change the scanning direction of the laser beam by providing the table on which the substrate is placed with a function of rotating within the table surface. Further, a heater is built in below the stage 10 so that the sample can be kept at a predetermined temperature when the laser beam is irradiated.

The optical path inside the optical system 4 is shown in FIG. The laser light incident on the optical system 4 passes through a cylindrical concave lens A, a cylindrical convex lens B, and lateral fly-eye lenses C and D, and further passes through cylindrical convex lenses E and F, and a mirror G (corresponding to the mirror 9 in FIG. 1). )
Is focused by the cylindrical lens H via
The sample is irradiated. By moving the lens H up and down relatively to the irradiation surface, the shape of the laser beam distribution on the irradiation surface can be changed from a shape close to a rectangle to a shape close to a normal distribution.

Using the invention disclosed in the present specification,
An example of forming a crystalline silicon film on a glass substrate by irradiation with laser light will be described. First, a 10 cm square glass substrate (for example, Corning 7959 glass substrate) is prepared. Then, a silicon oxide film of 2000 Å is formed on this glass substrate by a plasma CVD method using TEOS as a raw material.
To the thickness of. This silicon oxide film functions as a base film that prevents impurities from diffusing into the semiconductor film from the glass substrate side.

Next, an amorphous silicon film (amorphous silicon film) is formed by the plasma CVD method. Although a plasma CVD method is used here, a low pressure thermal CVD method may be used. The thickness of the amorphous silicon film is 500
Å. Of course, this thickness may be the required thickness.

Next, in a nitrogen atmosphere, the temperature of 450 ° C. is maintained for 1 hour to release hydrogen in the amorphous silicon film. This is because by intentionally forming dangling bonds in the amorphous silicon film, the threshold energy at the time of subsequent crystallization is lowered.

Next, a metal element that promotes crystallization of silicon is introduced. Here, nickel is used as the metal element, and a nickel acetate solution is applied onto the amorphous silicon film in order to introduce the nickel element. Then, the nickel element is held in contact with the surface of the amorphous silicon film.
Then, the amorphous silicon film is crystallized by performing heat treatment at 550 ° C. for 4 hours in a nitrogen atmosphere.

Thus, a crystalline silicon film can be obtained on the glass substrate. However, the crystalline silicon film thus obtained contains a large amount of amorphous components inside, and such a state causes deterioration or change of electric characteristics. Therefore, in this embodiment, in addition to crystallization by the above heat treatment, laser light irradiation is performed to improve the crystallinity.

Here, using the apparatus shown in FIG.
Excimer laser (wavelength 248nm, pulse width 25n
s) is applied to the crystalline silicon film. By irradiating with this laser light, the crystallinity can be further enhanced.

The laser beam is shaped into a rectangle by passing through the optical system 3, and the beam area at the irradiated portion is 125.
mm × 1 mm. The beam profile of the linear laser has a characteristic that the end of the beam is unclear. Therefore, in this specification, a portion having an energy of 5% or more of the maximum energy in the beam profile is defined as a beam. The linear energy profile (energy distribution) in the width direction has a trapezoidal distribution as shown in FIG.

The sample is placed on the stage 10, and the entire surface of the sample is irradiated by moving the stage at a speed of 2 mm / s. The irradiation condition of the laser light is such that the energy density of the laser light is 100 to 500 mJ.
/ Cm ² and the number of pulses is 30 pulses / s. The energy density here means the density of the upper bottom portion (portion having the maximum value) of the trapezoidal beam.

When laser irradiation is performed under the above-mentioned conditions, if one point on the sample is focused, the laser irradiation is 1
It will be a 5-step irradiation. This is 0.5s per beam pass
Because of this, irradiation of 15 pulses is performed at one location by one irradiation while scanning the beam. In this case, in the above 15 irradiations, the irradiation of the first several times is the irradiation whose irradiation energy density is gradually increased, and the irradiation of the last several times is the irradiation whose energy density is gradually decreased. .

This state is schematically shown in FIG. The laser energy gradually increases in the first half of the fifteen stages (note A in FIG. 3), and gradually decreases in the latter half (note B in FIG. 3). The number of times of 15 can be easily calculated from the beam width of the laser, the speed of the stage, and the number of pulses of the laser. According to our experiments, a 3 to 100 step irradiation, preferably 10 to 20 step irradiation gave a silicon film having the best crystallinity.

The substrate temperature is 550 during the laser irradiation.
It is kept at ℃. This is done in order to moderate the rate of rise and fall of the substrate surface temperature by the laser.
It is generally known that rapid changes in the environment impair the uniformity of the substance, but by keeping the substrate temperature high, deterioration of the uniformity of the substrate surface due to laser irradiation is suppressed as much as possible. In the present embodiment, the substrate temperature is set to 500 degrees, but in actual implementation, the optimum temperature for laser annealing is selected from 450 ° C. to the strain point of the glass substrate. Irradiation is performed in the atmosphere without any particular atmosphere control.

[0041]

According to the laser light irradiation technique disclosed in this specification, mass productivity can be improved and uniformity of a film to be a semiconductor device can be improved. INDUSTRIAL APPLICABILITY The invention disclosed in the present specification can be used for all laser processing processes used for semiconductor device processes, but when a TFT is taken as a semiconductor device, the uniformity of the threshold voltage of the TFT is improved, Can improve the uniformity of properties. Further, when the invention disclosed in this specification is used in the activation process of the impurity element of the source / drain of the TFT, the field effect mobility of the TFT or the uniformity of the on-current can be increased.

[Brief description of drawings]

FIG. 1 shows an outline of a laser annealing apparatus.

FIG. 2 shows an outline of an optical system that processes laser light into a linear shape.

FIG. 3 schematically shows a state in which a laser having a trapezoidal energy profile is irradiated.

[Explanation of symbols]

 1 unit 2 Laser oscillator 3 Amplifier 4 Optical system 5, 6, 7, 8, 9 Total reflection mirror 10 Driving device 11 Sample

Claims (7)

[Claims] 1. A linearly processed pulsed laser beam, wherein the energy profile of the laser beam is trapezoidal, and the pulsed laser is irradiated while scanning the object in one direction relatively. A laser annealing method characterized by the above. 2. A laser beam for irradiating a linearly processed pulse laser while displacing it in one direction, and when focusing on one point on an object to be irradiated, the pulse laser is irradiated to the point a plurality of times. A laser annealing method characterized in that a plurality of layers are hit. 3. The laser annealing method according to claim 2, wherein the energy profile of the beam is trapezoidal. 4. A step of irradiating a linearly processed pulse laser while displacing it in one direction, and when focusing on one point on an object to be irradiated, the pulse laser is 3-10 at that point.
A laser annealing method characterized in that a laser beam is overlapped and hit so as to be irradiated 0 times. 5. The laser annealing method according to claim 4, wherein the energy profile of the laser beam is trapezoidal. 6. A step of irradiating a linearly processed pulse laser while shifting it in one direction, and when focusing on one point on an object to be irradiated, the pulse laser is 10-2 at that point.
A laser annealing method characterized in that a laser beam is overlapped and hit so as to be irradiated 0 times. 7. The laser annealing method according to claim 6, wherein the energy profile of the laser beam is trapezoidal.