JP2004088118A - Method for manufacturing thin film transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a thin film transistor uniform in crystalline quality by turning an amorphous silicon thin film into polycrystalline form by the scan irradiation of linear excimer laser beams. <P>SOLUTION: If the position of the focal point of the excimer laser beam is set to a position 1 to 4 mm above the surface of the amorphous silicon thin film, the beam intensity distribution in the beam width direction on the surface of the amorphous silicon film exhibits a hill-shaped curve extended in the beam width direction, the high-temperature region near the central part in the beam width direction decreases in width and the low-temperature regions on both sides thereof considerably increase in width, compared with a case where the position of the focal point is set on the surface of the amorphous silicon thin film. After a polycrystalline silicon thin film uniform in crystalline quality is obtained by scanning such excimer laser beams in an overlapping manner in the beam width direction, the thin film transistor uniform in characteristic is formed by using the polycrystalline silicon thin film. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to a method for manufacturing a thin film transistor for forming a thin film transistor by crystallizing a semiconductor thin film such as an amorphous silicon thin film.

For example, in a method of manufacturing a thin film transistor by polycrystallizing an amorphous silicon thin film, an amorphous silicon thin film is formed on an upper surface of a glass substrate, and the amorphous silicon thin film is irradiated with an excimer laser beam, thereby increasing the amount of the amorphous silicon thin film. There is a method in which a large number of thin film transistors are formed by crystallizing a polycrystalline silicon thin film and separating the polycrystalline silicon thin film into elements. In this case, there is a method in which the beam size of the excimer laser beam is made linear by an optical system, and the linear excimer laser beam is scanned and irradiated while overlapping in the beam width direction.

By the way, in such a conventional crystallization method, first, a linear excimer laser beam is irradiated with its focal position as the surface of the amorphous silicon thin film. The reason is that by making the beam intensity distribution in the beam width direction of the excimer laser beam into a trapezoidal shape close to a rectangle, the uniform intensity irradiation area in the beam width direction is increased, and the crystal quality (crystal This is because the degree of conversion and the crystal grain size) are made uniform. However, when the beam intensity distribution in the beam width direction has a trapezoidal shape close to a rectangle, the temperature of the amorphous silicon thin film at the end in the scanning direction in one pulse irradiation region rapidly rises. As a result, there is a problem that the crystal quality of the polycrystalline silicon thin film varies, and the characteristics of the thin film transistor also vary. Second, when scan irradiation is performed while linear excimer laser beams are overlapped in the beam width direction, the overlap ratio may be within a range of about 50 to 97%. As the height increases, the crystal quality of the polycrystalline silicon thin film can be improved. For this reason, it is preferable that the overlap ratio is high, but on the other hand, there is a problem that the throughput is reduced. Third, when high energy is applied to an amorphous silicon thin film formed by, for example, a plasma CVD method by irradiating an excimer laser beam, hydrogen in the amorphous silicon thin film is bumped to generate defects.
Therefore, in order to avoid this, the formed amorphous silicon thin film is subjected to a dehydrogenation treatment by performing a heat treatment at a temperature of about 450 ° C. for about 2 hours in a nitrogen gas atmosphere. The dehydrogenation process is performed by scanning and irradiating an excimer laser beam with low energy density while overlapping in the beam width direction. Therefore, a step dedicated to the dehydrogenation treatment is required, and there is a problem that the number of steps is increased.
An object of the present invention is to make the crystal quality of a semiconductor thin film such as a polycrystalline silicon thin film more uniform.

According to the first aspect of the present invention, the semiconductor thin film is crystallized by irradiating a linear laser beam with scanning while overlapping in the beam width direction, and a gate is formed on the crystallized semiconductor thin film. A method of manufacturing a thin film transistor, comprising forming an insulating film, forming a gate electrode on the gate insulating film, and forming source / drain electrodes connected to source / drain regions of the semiconductor thin film via the gate insulating film. Irradiating the semiconductor thin film to crystallize the semiconductor thin film by shifting a focal position of the laser beam from a surface of the semiconductor thin film, and forming a beam intensity distribution in a beam width direction on the surface of the semiconductor thin film into a mountain shape. It is characterized.
In one embodiment of the present invention, the focal position of the laser beam may be shifted by 1 to 4 mm forward from the surface of the semiconductor thin film.
Further, as one embodiment of the invention according to claim 1, the beam intensity distribution in the beam width direction of the laser beam on the surface of the semiconductor thin film is different from the case where the focal position coincides with the surface of the semiconductor thin film. It is preferable that the peak shape is increased by 80% in the half width and reduced by 30% in the highest region.
In one embodiment of the present invention, the laser beam applied to the semiconductor thin film has a beam width of 0.15 mm on the surface of the semiconductor thin film and an energy density of 277 mJ / cm. When one pulse is applied to the semiconductor thin film as ^two or more, it is preferable that the width of the irradiation mark appearing on the semiconductor thin film is 220 μm or more.
In one embodiment of the present invention, an energy density of the laser beam is 277 mJ / cm ^{2 to} 308 mJ / cm ² , and an irradiation mark that appears on the semiconductor thin film by one pulse irradiation of the laser beam. Has a width of 220 to 238 μm, and the width of the crystallized structure appearing in the irradiation trace is 65 μm or less.
Further, in any one of the first and second embodiments, the semiconductor thin film before crystallization is an amorphous silicon thin film, and the laser thin film is irradiated with the laser beam to perform dehydration without dehydrogenating the amorphous silicon thin film. Elementary treatment and crystallization can be performed collectively.

As in the first aspect of the present invention, when the focal position of the laser beam for irradiating the semiconductor thin film to crystallize the semiconductor thin film is shifted from the surface of the semiconductor thin film and the beam intensity distribution in the beam width direction is formed into a mountain shape. However, the beam intensity distribution in the beam width direction of the laser beam on the surface of the semiconductor thin film becomes an intensity distribution in which the width of the low-temperature region existing on both sides of the high-temperature region near the center of the beam width direction is widened, and as a result, When the thin film is microcrystallized, not only can the temperature of the scanning direction end of the semiconductor thin film in the one pulse irradiation region not be rapidly increased, but also the scanning direction end can be microcrystallized. As a result, the crystal quality can be further uniformed. In this case, the crystal quality can be made uniform irrespective of the laser beam overlap rate, so that even if the laser beam overlap rate is low, the crystal quality is almost the same as when the laser beam overlap rate is high. And thus the throughput can be increased.

Next, a method for crystallizing a semiconductor thin film according to an embodiment of the present invention will be described with reference to FIGS. 1 (A) and 1 (B). First, as shown in FIG. 1A, an underlayer 2 made of silicon oxide and having a thickness of about 1000 ° is formed on the upper surface of a glass substrate 1 having a plane size of 320 mm × 340 mm by a high frequency sputtering method. Next, an amorphous silicon thin film 3 having a thickness of about 500 ° is formed on the upper surface of the underlayer 2 by a plasma CVD method or a low pressure CVD method. Next, dehydrogenation treatment is performed by heat treatment or irradiation with an excimer laser beam having a low energy density. Next, as shown in FIG. 1B, the amorphous silicon thin film 3 is irradiated with an excimer laser beam as described later, thereby polycrystallizing the amorphous silicon thin film 3 to form a polycrystalline silicon thin film 4.

Next, irradiation of an excimer laser beam will be described. First, the beam size of the KrF excimer laser beam is set to a linear shape of 360 mm × 0.15 mm by an optical system, and the focal position thereof is set at a position about 1 to 4 mm above (forward) the surface of the amorphous silicon thin film. I made it. In this case, the beam size of 360 mm × 0.15 mm means that the focal position of the KrF excimer laser beam is set to the surface of the amorphous silicon thin film. The beam width of 0.15 mm is a half-width of the beam intensity because the beam intensity distribution of the KrF excimer laser beam in the beam width direction has a mountain shape as shown in FIG. Furthermore, the reason why the beam length is set to 360 mm is to make the beam length larger than the length of a predetermined side of the glass substrate (in this case, 320 mm).

By the way, when the KrF excimer laser beam is irradiated such that its focal position is located at a position of about 1 to 4 mm away (forward) from the surface of the amorphous silicon thin film, the beam intensity distribution in the beam width direction is energy density Is 150 mJ / cm ² , as shown in FIG. Hereinafter, such irradiation is referred to as out focus. On the other hand, when the focal position is set to the surface of the amorphous silicon thin film, it becomes as shown in FIG. Hereinafter, such irradiation is called just focus. Due to such a difference in irradiation, in the case shown in FIG. 3, as compared with the case shown in FIG. 4, the mountain-shaped beam intensity distribution spreads in the beam width direction, and the half-value width increases by 80%. At 30%.

The amorphous silicon thin film was irradiated with a KrF excimer laser beam by gradually increasing the energy density one pulse at a time by out-focusing and just-focusing. In this case, for example, in the case of an out-of-focus of about 2 mm, when the energy density is 277 mJ / cm ² , as shown in FIG. 5A, an irradiation trace having a width of 220 μm appears for the first time, and when the energy density is 308 mJ / cm ² . For the first time, an irradiation trace as shown in FIG. In the case of FIG. 5B, the width of the irradiation trace was 238 μm, and the width of the crystallized structure indicated by hatching (oblique lines) that appeared in the irradiation trace was 65 μm. In the case of just focus, when the energy density is 200 mJ / cm ² , an irradiation trace having a width of 101 μm appears for the first time as shown in FIG. 6A, and when the energy density is 250 mJ / cm ² , FIG. Irradiation traces as shown appear for the first time. In the case of FIG. 6B, the width of the irradiation trace was 115 μm, and the width of the crystallized structure indicated by hatching that appeared in the irradiation trace was 42 μm.

From the above, the threshold value of the energy density at which the crystallized structure appears exists between 277 and 308 mJ / cm ^{2 in} the case of out-focus, and 200 to 250 mJ / cm ² in the case of just-focus. ^It can be said that it exists between ^two . The appearance of this crystallized structure can be said to be attributable to the high-temperature region indicated by hatching near the center in the beam width direction, as inferred from the mountain-shaped beam intensity distributions shown in FIGS. Incidentally, for example, first it appears energy density irradiation traces, if the out-of-focus than just focus (200mJ / cm ²⁾ is more of (277mJ / cm ²⁾ larger. Considering this, for example, in the case shown in FIG. 3 and the case shown in FIG. 4, the crystal structure appearance threshold has the same beam intensity. However, in the case shown in FIG. 3, as compared with the case shown in FIG. 4, the mountain-shaped beam intensity distribution spreads in the beam width direction, increases by 80% at the half-value width, and decreases by 30% at the height. As a result, the width of the low-temperature region on both sides in the beam width direction is considerably increased. Therefore, when the energy densities are the same, the substantial energy density per unit area on the irradiated surface is lower in the case shown in FIG. 3 than in the case shown in FIG. Therefore, seems to compensate for the decrease in the substantial energy density, the energy density in the case of out-of-focus (277mJ / cm ²⁾ is assumed to be larger than the energy density when the just-focus (200mJ / cm ²⁾ .

Next, a KrF excimer laser beam is applied to the amorphous silicon thin film by out-focusing and just-focusing while gradually increasing the energy density one pulse at a time. As a result, the result shown in FIG. 7 was obtained. In FIG. 7, a white square indicates the irradiation trace width in the case of out-of-focus, a black square indicates the crystallized structure width in the case of out-of-focus, a white circle indicates the irradiation trace width in the case of the just-focus, and a black circle indicates the just-in-focus. The width of the crystallized structure in the case of focus is shown.

As is clear from FIG. 7, the width of the irradiation mark indicated by a white circle and the width of the crystallization structure indicated by a black circle in the case of just focus both gradually increase with an increase in the energy density, whereas the width in the case of out focus Both the irradiation trace width indicated by the white square and the crystallized structure width indicated by the black square rapidly increase with an increase in the energy density. In addition, the difference between the irradiation trace width and the crystallization structure width at the same energy density is considerably larger in the case of out-focus indicated by white and black squares than in the case of just focus indicated by white and black circles. Therefore, the ratio of the width of the crystallized structure to the width of the irradiation trace in the case of out-focus is considerably smaller than that in the case of just-focus. From this, it was confirmed that the low-temperature region in the case of out-focus was considerably wider than that in the case of just-focus.

Next, the amorphous silicon thin film after the dehydrogenation treatment is irradiated with a KrF excimer laser beam in the beam width direction at a scan pitch of 0.015 mm (overlap rate of 90%) by just focusing, and the laser beam is output under the same conditions. Scan irradiation was performed by focusing. The reciprocal number (crystal grain) of the integral width of the (111) peak of the polycrystalline silicon thin film was obtained by an X-ray diffraction method in which an X-ray parallel beam was incident on the polycrystalline silicon thin film thus formed at θ = 1.00 °. When the energy density dependence of (corresponding to the size) was examined, the result shown in FIG. 8 was obtained. In FIG. 8, a black square indicates a reciprocal of the integration width in the case of out-focus, and a black circle indicates a reciprocal of the integration width in the case of just-focus.

Looking at the black square (crystallized structure width in the case of out-of-focus) in FIG. 7, a crystallized structure appears at an energy density of 277 mJ / cm ² , while the black square in FIG. Looking at the reciprocal of the integral width), the reciprocal of the integral width dramatically increases at an energy density of 277 mJ / cm ² . In the case of just focus, the same can be said at an energy density of 250 mJ / cm ² . From this, it can be said that the appearance of the crystallized structure has dramatically increased the crystal grain size.

Next, the sheet resistance of the same polycrystalline silicon thin film as in the case where the result shown in FIG. 8 was obtained was measured, and the dependence of the standard deviation (corresponding to the uniformity of the crystal grain size) on the energy density was examined. The result shown in FIG. 9 was obtained. In FIG. 9, a black square indicates the standard deviation value of the sheet resistance in the case of out focus, and a black circle indicates the standard deviation value of the sheet resistance in the case of just focus. As is apparent from FIG. 9, the standard deviation value of the sheet resistance in the case of the just focus indicated by the black circle rapidly increases at the energy density of 250 mJ / cm ² or more due to the appearance of the crystallized structure indicated by the black circle in FIG. I'm On the other hand, the standard deviation value of the sheet resistance in the case of out-of-focus indicated by the black square is an energy density of 277 mJ / cm ² or more, and is substantially the same even when the crystallized structure indicated by the black square in FIG. 7 appears. . Thus, in the case of just-focus, the appearance of the crystallized structure considerably deteriorates the uniformity of the grain size, whereas in the case of out-of-focus, the appearance of the crystallized structure depends on the uniformity of the grain size. Can be said to have no adverse effect on

Next, FIG. 10 is a view for explaining a case where a KrF excimer laser beam is scanned and irradiated by out-of-focus by overlapping a high-temperature region in a beam width direction indicated by an arrow, and (a) shows a beam intensity spectrum. (B) shows the polycrystalline state of the amorphous silicon thin film. The hatched portion in FIG. 10A is, for example, a region where the high-temperature regions shown by hatching in FIG. In this case, a portion without hatching in the direction opposite to the arrow direction of the first pulse on the right side and a portion without hatching in the arrow direction on the left side of the last one pulse are, for example, a low-temperature region indicated by a portion without hatching in FIG. It is. Therefore, the portion indicated by hatching in FIG. 10B is a polycrystalline region.

Now, in FIG. 10A, the portion without hatching in the arrow direction of the last one pulse on the left side is a wide low-temperature region. Therefore, when scan irradiation is performed in the arrow direction, this wide low-temperature region always precedes. . Thus, a linear region of the amorphous silicon thin film will be illuminated first by the preceding wide low temperature region and then by the subsequent high temperature region. In this case, the irradiation with the preceding wide low-temperature region gradually raises the temperature of the amorphous silicon thin film, so that the temperature at the end of the amorphous silicon thin film in the scanning direction in the one-pulse irradiation region can be prevented from rising rapidly. . Further, the amorphous silicon thin film is microcrystallized by the irradiation in the preceding wide low temperature region. Then, the subsequent irradiation in the high-temperature region promotes the growth of the crystal grains and polycrystallizes. Therefore, the crystal quality of the polycrystalline silicon thin film can be made more uniform.

The dehydrogenation treatment can be performed in a wide, low-temperature region that precedes it. That is, if the dehydrogenation process is first performed in the preceding wide low-temperature region and then the crystallization is performed in the subsequent high-temperature region, the dehydrogenation process and the crystallization can be performed collectively. Therefore, a step dedicated to the dehydrogenation treatment becomes unnecessary, and the number of steps can be reduced.

Next, a KrF excimer laser beam having an energy density of 277 mJ / cm ² is applied to the dehydrogenated amorphous silicon thin film by out-focusing in a scan pitch of 0.075 mm, 0.06 mm, 0 mm in the case of just-focusing in the beam width direction. Scan irradiation was performed at 0.045 mm, 0.03 mm, and 0.015 mm (overlap ratio 50%, 60%, 70%, 80%, and 90%). Then, the integrated intensity of the polycrystalline silicon thin film 4 (111) peak (in terms of crystallinity) was determined by an X-ray diffraction method in which an X-ray parallel beam was incident on the polycrystalline silicon thin film thus formed at θ = 1.00 °. When the dependence on the overlap ratio of the reciprocal of the integral width (corresponding to the crystal grain size) and the reciprocal of the integral width (corresponding to the crystal grain size) was examined, the results shown in FIG. 11 were obtained.

に お い て In FIG. 11, a black square indicates the integral intensity, and a black circle indicates the reciprocal of the integral width. As is apparent from FIG. 11, regardless of the overlap ratio of the KrF excimer laser beam, the integral intensity indicated by a black square and the reciprocal of the integral width indicated by a black circle are almost the same. Therefore, it can be said that the crystal quality (crystallinity and crystal grain size) of the polycrystalline silicon thin film can be made uniform regardless of the overlap ratio of the KrF excimer laser beam. As a result, even when the overlap ratio of the KrF excimer laser beam is reduced to, for example, 50%, the crystal quality of the polycrystalline silicon thin film is made almost uniform as in the case where the overlap ratio of the KrF excimer laser beam is as high as 90%. And thus the throughput can be increased.

Next, consider that the crystal quality of the polycrystalline silicon thin film can be made uniform regardless of the overlap ratio of the KrF excimer laser beam. As described above, in the case shown in FIG. 3, the mountain-shaped beam intensity distribution spreads in the beam width direction and the width of the low-temperature region existing on both sides of the high-temperature region is considerably wider than in the case shown in FIG. Has become. Then, in the case shown in FIG. 3, the substantial energy density per unit area on the irradiation surface is lower than that in the case shown in FIG. To compensate for the decline. That is, in the case of out-of-focus, the mountain-shaped beam intensity distribution expands in the beam width direction, and the substantial decrease in energy density accompanying the expansion is compensated for. This means that even if the overlap ratio of the KrF excimer laser beam is reduced to, for example, 50%, the crystal quality of the polycrystalline silicon thin film is made substantially the same as when the overlap ratio of the KrF excimer laser beam is as high as 90%. It seems to be able to.

Here, an example of a structure of a thin film transistor formed using the polycrystalline silicon thin film 4 shown in FIG. 1B will be described with reference to FIG. First, after the polycrystalline silicon thin film 4 is element-separated as is well known, a gate insulating film 5 and a gate electrode 6 are formed, and impurities are diffused into the polycrystalline silicon thin film 4 using the gate electrode 6 as a mask. Interlayer insulating film 7 is formed on gate electrode 6, and source / drain electrode 9 connected to impurity diffusion layer 4 a of polycrystalline silicon thin film 4 via contact hole 8 formed in gate insulating film 5 and interlayer insulating film 7. Is formed, a thin film transistor is completed. In this thin film transistor, the characteristics can be made more uniform.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a cross-sectional view showing a state in which an amorphous silicon thin film is formed, and FIG. 1B is a cross-sectional view showing a state in which an amorphous silicon thin film is formed according to an embodiment of the present invention. FIG. 3 is a cross-sectional view of a state in which a polycrystalline silicon thin film is formed by polycrystallizing the silicon. FIG. 4 is a view for explaining a beam width of a KrF excimer laser beam. FIG. 4 is a diagram illustrating a beam intensity distribution in a beam width direction in the case of out-of-focus. FIG. 5 is a diagram illustrating a beam intensity distribution in a beam width direction in the case of just focus. FIGS. 4A and 4B are diagrams illustrating an irradiation trace and a crystallized structure in the case of out-of-focus. FIGS. 3A and 3B are diagrams shown for explaining an irradiation trace and a crystallized structure in the case of just focus. The figure which shows the energy density dependence of irradiation trace width | variety and crystallization structure width | variety in the case of out focus and just focus. The figure which shows the energy density dependence of the reciprocal of the integral width of a (111) peak of a polycrystalline silicon thin film in the case of an out focus and a just focus. FIG. 9 is a diagram showing the energy density dependence of the standard deviation value of the sheet resistance of the polycrystalline silicon thin film in the case of out-focus and just-focus. FIGS. 7A and 7B are diagrams for explaining a case where a KrF excimer laser beam is scanned and irradiated by out-of-focus, in which FIG. 7A is a diagram illustrating a beam intensity spectrum, and FIG. 8B is a diagram illustrating a polycrystalline state of an amorphous silicon thin film. The figure which shows the overlap ratio dependence of the reciprocal of the integral intensity and integral width of the (111) peak of a polycrystalline silicon thin film in the case of an out-of-focus. FIG. 2 is a cross-sectional view illustrating an example of a structure of a thin film transistor including the polycrystalline silicon thin film illustrated in FIG.

Explanation of reference numerals

Reference Signs List 1 glass substrate 3 amorphous silicon thin film 4 polycrystalline silicon thin film

Claims (6)

The semiconductor thin film is crystallized by scanning and irradiating a linear laser beam while overlapping in the beam width direction, thereby crystallizing the semiconductor thin film, forming a gate insulating film on the crystallized semiconductor thin film, and forming the gate. Forming a gate electrode on an insulating film and forming a source / drain electrode connected to a source / drain region of the semiconductor thin film via the gate insulating film; A method of manufacturing a thin film transistor, wherein a focus position of the laser beam for crystallizing the semiconductor thin film is shifted from a surface of the semiconductor thin film, and a beam intensity distribution in a beam width direction on the surface of the semiconductor thin film is formed into a mountain shape. . The method according to claim 1, wherein a focal position of the laser beam is shifted by 1 to 4 mm forward from a surface of the semiconductor thin film. In the invention described in claim 1, the beam intensity distribution in the beam width direction of the laser beam on the surface of the semiconductor thin film has a half-value width of 80% with respect to the case where the focal position is aligned with the surface of the semiconductor thin film. A method of manufacturing the thin film transistor, wherein the peak shape increases by 30% and decreases by 30% in the highest region. 2. The semiconductor laser according to claim 1, wherein the laser beam irradiated on the semiconductor thin film has a beam width of 0.15 mm on the surface of the semiconductor thin film and an energy density of 277 mJ / cm ² or more. A method of manufacturing a thin film transistor, wherein the width of an irradiation mark appearing on the semiconductor thin film when the thin film is irradiated with one pulse is 220 μm or more. In the invention according to claim 1, the energy density of the laser beam is 277 mJ / cm ^{2 to} 308 mJ / cm ² , and the irradiation mark that appears on the semiconductor thin film by the irradiation of one pulse of the laser beam has a width. A method for producing a thin film transistor, wherein the width of a crystallized structure appearing within the irradiation trace is 65 μm or less, which is 220 to 238 μm. The invention according to any one of claims 1 to 5, wherein the semiconductor thin film before crystallization is an amorphous silicon thin film, and the laser beam is irradiated to perform dehydrogenation without performing a dehydrogenation process on the amorphous silicon thin film. A method for manufacturing a thin film transistor, wherein the treatment and the crystallization are performed collectively.

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