Process and unit for production of polycrystalline silicon film

A process for making a polycrystalline silicon film includes forming, on a glass substrate, an amorphous silicon film having a first region and a second region that contacts the first region, forming a first polycrystalline portion by irradiating the first region of the amorphous silicon film with laser light having a wavelength not less than 390 nm and not more than 640 nm and forming a second polycrystalline portion that contacts the first polycrystalline portion by irradiating the second region and the portion of the region of the first polycrystalline portion that contacts the second region of the amorphous silicon film with the laser light.

TECHNICAL FIELD

The present invention relates to a process and a unit for production of a polycrystalline silicon film as well as a semiconductor device and a process for the same, in particular, to a process and a unit for production of a polycrystalline silicon film with excellent crystallinity, for making a thin film transistor, having high carrier mobility as a semiconductor device, and a process for the making the semiconductor device and in which the polycrystalline silicon film is used.

BACKGROUND ART

At present, the pixel part of a liquid crystal panel forms images through switching of thin film transistors fabricated with an amorphous or polycrystal silicon film on a substrate of glass or synthesized quartz. In the case that it becomes possible to simultaneously form a driver circuit (at present mainly installed independently outside of the panel) to drive pixel transistors on this panel, there would be great merit with respect to production cost, reliability, and the like, of the liquid crystal panel. At present, however, the crystallinity of the silicon film that forms an active layer of transistors is poor and, therefore, the performance of thin film transistors, represented by mobility, is low and it is difficult to fabricate an integrated circuit wherein high speed and high performance are required. As a method of improving the crystallinity of the silicon film for the purpose of implementing a thin film transistor with a high mobility, heat treatment is, in general, carried out by using a laser.

The relationships between the crystallinity of a silicon film and the mobility of a thin film transistor are described in the following. A silicon film gained through laser heat treatment is, in general, polycrystal. Crystal defects locate in crystal grain boundaries of the polycrystal silicon and they block the carrier movement in the active layer of the thin film transistors. Accordingly, the number of times that the carriers cross the crystal grain boundaries while moving through the active layer becomes less and the concentration of crystal defects becomes lower in order to enhance the mobility of the thin film transistors. The purpose of the laser heat treatment is to form a polycrystal silicon film of which the crystal grain diameters are large and wherein there are fewer crystal defects in the crystal grain boundaries.

FIGS. 18to20are cross sectional views for describing a process for a polycrystal silicon film according to a prior art. First, referring toFIG. 18, a silicon oxide film32is formed on a glass substrate31by, for example, carrying out CVD (chemical vapor deposition) on a glass substrate. An amorphous silicon film33is formed on a silicon oxide film32by means of, for example, CVD.

Referring toFIG. 19, an excimer laser (KrF (wavelength: 248 nm)) irradiates an amorphous silicon film33in the direction shown by arrow335. Thereby, the portion irradiated by the excimer laser melts. After that, as the temperature becomes lower, the melted silicon is crystallized so as to form a polycrystal silicon film334.

Referring toFIG. 20, polycrystal silicon film334is patterned so that polycrystal silicon film334only remains in portions. Next, a silicon oxide film and a metal film (low resistance metal film such as Ta, Cr or Al) are formed on polycrystal silicon film334. Gate insulating films36aand36b, as well as gate electrodes37aand37b, are formed by patterning the metal film and the silicon oxide film. Thereby, active regions39aand39bare formed. Next, source and drain regions are formed in a self-aligned manner by means of an ion doping method by using gate electrodes37aand37bas a mask. Thereby, the thin film transistors shown inFIG. 20are completed.

According to a conventional method, as shown inFIG. 19, an amorphous silicon film is polycrystallized by using an excimer laser and, therefore, the mobility of the carriers is low in the transistors formed on the polycrystal silicon film. As a result, a high speed of operation of the transistors is difficult so that it is difficult to achieve a high response of the liquid crystal display device.

In addition, Reference 1 (T. Ogawa, et al., “Thin Film Transistors of Polysilicon Recrystallized by the Second Harmonics of a Q-Switched Nd: YAG Laser” EuroDisplay '99 The 19thInternational Display Research Conference Late-news papers Sep. 6-9, 1999 Berlin, Germany) discloses an amorphous silicon film made polycrystalline by using, for example, the second harmonic of an Nd: YAG laser as a laser light and a thin film transistor formed by using this polycrystalline film wherein the mobility is increased. However, since the output of the second harmonic of the YAG laser is small, an amorphous silicon film having only a small area can be made polycrystalline. Therefore, the manufacturing of a polycrystalline silicon film for manufacturing a liquid crystal display having a large area is difficult.

Therefore, this invention is provided in order to solve the above described problems.

One purpose of this invention is to provide a process and a unit for manufacturing a polycrystal silicon film that is suitable in the fabrication of a thin film transistor of high performance and that has a large area.

In addition, another purpose of this invention is to provide a thin film transistor of high performance and a process for the same.

DISCLOSURE OF THE INVENTION

A process for a polycrystal silicon film according to this invention is provided with the following steps:

(1) the step of forming an amorphous silicon film having a first region and a second region that contacts this first region on a substrate;

(2) the step of forming a first polycrystal portion by irradiating the first region of the amorphous silicon film with a laser of which the wavelength is not less than 390 nm and not more than 640 nm; and

(3) the step of forming a second polycrystal portion that contacts the first polycrystal portion by irradiating the second region of the amorphous silicon film and a portion of the first polycrystal portion that contacts the second region with a laser of which the wavelength is not less than 390 nm and not more than 640 nm.

According to a process for a polycrystal silicon film provided with such steps, first, in the step shown in (2), the first region of the amorphous silicon film is irradiated with a laser so as to form the first polycrystal portion and, after that, the second region of the amorphous silicon film and a portion of the region of the first polycrystal portion are irradiated with a laser so as to form the second polycrystal portion that contacts the first polycrystal portion and, thereby, the first polycrystal portion and the second polycrystal portion can be formed. As a result, the amorphous silicon film can be polycrystallized over a large area so that a polycrystal silicon film having a large area can be formed.

Furthermore, the laser, of which the wavelength is in the above described range, has a large absorption coefficient with respect to amorphous silicon while the absorption ratio with respect to polycrystal silicon is small and, therefore, the amorphous silicon is converted to a polycrystal silicon through the first irradiation and, then, no characteristics are changed due to the second irradiation in the portion that is twice irradiated with the laser. Therefore, there is no difference in the characteristics between the portion irradiated with the laser once and the portion irradiated with the laser twice so that a high quality polycrystal silicon film can be provided.

The reason why the wavelength of the laser is not less than 390 nm is that in the case that the wavelength of the laser is less than 390 nm, the absorption ratio of the polycrystal silicon film exceeds 60% of the absorption ratio of the amorphous silicon film so that the characteristics of the polycrystal silicon film are changed through the second irradiation of the laser, which is not desirable. In addition, the reason why the wavelength of the laser is not more than 640 nm is that in the case that the wavelength of the laser exceeds 640 nm, the absorption ratio of the amorphous silicon becomes 10%, or less, so that the productivity is reduced.

In addition, the step of forming a first polycrystal portion preferably includes the formation of a first polycrystal portion that extends in one direction by scanning the laser on the amorphous silicon film. The step of forming a second polycrystal portion includes the formation of a second polycrystal portion that extends along the first polycrystal portion by scanning the laser in the direction wherein the first polycrystal portion extends.

In this case, since both the first polycrystal portion and the second polycrystal portion are formed by laser scanning, the first and the second polycrystal portions can be formed so as to extend in a predetermined direction. Thereby, the first and the second polycrystal portions can be formed on a substrate of an even larger area.

In addition, the step of forming a first polycrystal portion preferably includes the irradiation of the amorphous silicon film with a first laser of which the wavelength is not less than 390 nm and not more than 640 nm from a first laser light source. The step of forming a second polycrystal portion includes the irradiation of the amorphous silicon film with a second laser of which the wavelength is not less than 390 nm and not more than 640 nm from a second laser light source. In this case, the first polycrystal portion is formed through the irradiation with the laser from the first laser light source while the second polycrystal portion is formed through the irradiation with the laser from the second laser light source and, therefore, the first and the second polycrystal portions can be formed almost simultaneously. Therefore, the productivity of a polycrystal silicon film is increased and a large laser output can be gained in a stable manner.

In addition, the above described laser preferably includes at least one type selected from among a group consisting of the second harmonics of an Nd: YAG laser, the second harmonics of an Nd: YVO4laser, the second harmonics of an Nd: YLF laser, the second harmonics of an Nd: glass laser, the second harmonics of a Yb: YAG laser, the second harmonics of a Yb: glass laser, an Ar ion laser, the second harmonics of a Ti: sapphire laser and a Dye laser. In this case, these lasers can generate a laser of which the wavelength is not less than 390 nm and not more than 640 nm.

In addition, the process for a polycrystal silicon film includes the irradiation with a laser from the second laser light source at an interval of a predetermined period of time after the irradiation with a laser from the first laser light source. In this case, since the laser from the second laser light source can perform irradiation after the laser from the first laser light source has performed irradiation, the first polycrystal portion and the second polycrystal portion can be formed in sequence. As a result, the production efficiency is further increased.

A production unit for a polycrystal silicon film according to this invention is provided with an oscillation means, an irradiation means, a shifting means and a control means. The oscillation means allows a laser, of which the wavelength is not less than 390 nm and not more than 640 nm to oscillate. The irradiation means irradiates an amorphous silicon film formed on a substrate with a laser that is allowed to oscillator by the oscillation means. The shifting means shifts the substrate relative to the irradiation means. The control means controls the shifting means so that the laser carries out scanning so as to form a first polycrystal portion by irradiating the amorphous silicon film with the laser of which the wavelength is not less than 390 nm and not more than 640 nm and so as to form a second polycrystal portion that contacts the first polycrystal portion by irradiating a portion of the amorphous silicon film that partially overlaps the first polycrystal portion with the laser of which the wavelength is not less than 390 nm and not more than 640 nm.

In such a production unit for a polycrystal silicon film, the control means irradiates the amorphous silicon film with the laser so as to form the first polycrystal portion and irradiates the portion that partially overlaps the first polycrystal portion with the laser so as to form the second polycrystal portion and, therefore, the first and second polycrystal portions can be formed over a broad area. Therefore, a polycrystal silicon film with a broad area can be provided. Furthermore, a laser of which the wavelength is in the above described range has a large absorption ratio with respect to amorphous silicon and has a small absorption ratio with respect to polycrystal silicon so that the portion that is irradiated twice does not change. Therefore, a high quality polycrystal silicon film can be provided.

The reason why the wavelength of the laser is not less than 390 nm is that in the case that the wavelength of the laser is less than 390 nm, the absorption ratio of the polycrystal silicon film exceeds 60% of the absorption ratio of the amorphous silicon film so that the characteristics of the polycrystal silicon film are changed through the second irradiation of the laser, which is not desirable. In addition, the reason why the wavelength of the laser is not more than 640 nm is that in the case that the wavelength of the laser exceeds 640 nm, the absorption ratio of the amorphous silicon becomes 10%, or less, so that the productivity is reduced.

In addition, the irradiation means preferably includes the first irradiation means and the second irradiation means. The amorphous silicon film is irradiated with a portion of the laser that is allowed to oscillate by the oscillation means via the first irradiation means. The amorphous silicon film is irradiated with another portion of the laser that is allowed to oscillate by the oscillation means via the second irradiation means. In this case, the amorphous silicon film can be irradiated with the laser that is allowed to oscillate by one oscillation means via the first irradiation means and the second irradiation means and, therefore, the unit can be manufactured at low cost.

In addition, the control means preferably controls the first and second irradiation means, the oscillation means and the shifting means so that the second irradiation means performs the irradiation with the laser at an interval of a predetermined period of time after the first irradiation means has performed the irradiation with the laser. In this case, the second irradiation means performs the irradiation with the laser after the first irradiation means has performed the irradiation with the laser and, therefore, the first polycrystal portion and the second polycrystal portion can be effectively produced. Therefore, a polycrystal silicon film production unit with a high productivity can be provided.

In addition, the irradiation means preferably includes a first irradiation means and a second irradiation means while the oscillation means includes a first oscillation means and a second oscillation means. The amorphous silicon film is irradiated with a laser that is allowed to oscillate by the first oscillation means via the first irradiation means. The amorphous silicon film is irradiated with a laser that is allowed to oscillate by the second oscillation means via the second irradiation means. In this case, since two oscillation means respectively allow lasers to oscillate, lasers of which the outputs are sufficiently large can be allowed to oscillate in a stable manner so that the first and the second polycrystal portions can be effectively produced.

In addition, the control means preferably controls the first and second irradiation means, the first and second oscillation means as well as the shifting means so that the second irradiation means performs the irradiation with the laser at an interval of a period of time after the first irradiation means has performed the irradiation with the laser. In this case, the second irradiation means can perform the irradiation with the laser after the first irradiation means has performed the irradiation with the laser and, therefore, a polycrystal silicon film can be efficiently produced.

In addition, the oscillation means preferably allows the oscillation of a laser that includes at least one type selected from among a group consisting of the second harmonics of an Nd: YAG laser, the second harmonics of an Nd: YVO4laser, the second harmonics of an Nd: YLF laser, the second harmonics of an Nd: glass laser, the second harmonics of a Yb: YAG laser, the second harmonics of a Yb: glass laser, an Ar ion laser, the second harmonics of a Ti: sapphire laser and a Dye laser.

A process for a semiconductor device according to this invention is provided with the step of forming, on a substrate, an amorphous silicon film having a first region and a second region that contacts this first region and the step of forming a polycrystal silicon film by irradiating the amorphous silicon film with a laser. The step of forming a polycrystal silicon film includes the step of forming a first polycrystal portion by irradiating the first region of the amorphous silicon film with a laser of which the wavelength is not less than 390 nm and not more than 640 nm and the step of forming a second polycrystal portion that contacts the first polycrystal portion by irradiating the second region as well as the portion of the first polycrystal portion that contacts the second region of the amorphous silicon film with a laser of which the wavelength is not less than 390 nm and not more than 640 nm.

According to such a process, the first region of the amorphous silicon film is irradiated with the laser so as to form the first polycrystal portion and, after that, the second region and the portion of the region of the first polycrystal portion of the amorphous silicon film are irradiated with the laser so as to form the second polycrystal portion that contacts the first polycrystal portion and, therefore, the first polycrystal portion and the second polycrystal portion can be formed. As a result, the amorphous silicon film can be polycrystallized over a broad area so that a polycrystal silicon film of a large area can be formed.

Furthermore, the laser, of which the wavelength is in the above described range, has a large absorption coefficient with respect to amorphous silicon while the absorption ratio with respect to polycrystal silicon is small and, therefore, the amorphous silicon is converted to a polycrystal silicon through the first irradiation and, then, no characteristics are changed due to the second irradiation in the portion that is twice irradiated with the laser. Therefore, there is no difference in the characteristics between the portion irradiated with the laser once and the portion irradiated with the laser twice so that a high quality polycrystal silicon film can be provided.

The reason why the wavelength of the laser is not less than 390 nm is that in the case that the wavelength of the laser is less than 390 nm, the absorption ratio of the polycrystal silicon film exceeds 60% of the absorption ratio of the amorphous silicon film so that the characteristics of the polycrystal silicon film are changed through the second irradiation of the laser, which is not desirable. In addition, the reason why the wavelength of the laser is not more than 640 nm is that in the case that the wavelength of the laser exceeds 640 nm, the absorption ratio of the amorphous silicon becomes 10%, or less, so that the productivity is reduced.

In addition, a semiconductor device according to this invention uses a polycrystal silicon film produced by means of the above described process as an active region. In this case, since a polycrystal silicon film of which the mobility is large and of which the area is large is used as an active region, a semiconductor device of which the area is large and that is of high performance can be provided

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment

FIGS. 1to6are views showing a process for a polycrystal silicon film according to a first embodiment of this invention. Referring toFIG. 1, a silicon oxide film32is formed on a glass substrate31by means of, for example, a CVD method. An amorphous silicon film33is formed on silicon oxide film32by means of a CVD method. This amorphous silicon film33has a first region33aand a second region33bthat contacts this first region33a.

Referring toFIGS. 2 and 3, first region33aof amorphous silicon film33is irradiated with the second harmonics laser (wavelength 532 nm) of Nd: YAG of the Q switch. Thereby, a portion irradiated with a laser35is polycrystallized so as to form a first polycrystal portion34a. At this time, a unit, as shown inFIG. 3, is used for the irradiation by the laser.

Referring toFIG. 3, a polycrystal silicon film production unit100is provided with a laser oscillator120as an oscillation means for oscillating a laser of which the wavelength is not less than 390 nm and not more than 640 nm, an irradiation means110for irradiating an amorphous silicon film formed on a substrate with a laser oscillated from laser oscillator120, a shifting means130for shifting a substrate relative to the irradiation means and a control means140for controlling the shifting means that forms a first polycrystal portion by irradiating an amorphous silicon film with a laser, of which the wavelength is not less than 390 nm and not more than 640 nm, and that forms a second polycrystal portion that contacts the first polycrystal portion by irradiating the amorphous silicon film with a laser, of which the wavelength is not less than 390 nm and not more than 640 nm, so that the irradiation overlaps a portion of the first polycrystal portion.

Laser oscillator120is a Q switch Nd: YAG laser second harmonics oscillator and oscillates a laser so that amorphous silicon film33formed on glass substrate31is irradiated with this laser via irradiation means110. Here, the silicon oxide film between glass substrate31and amorphous silicon film33is omitted in FIG.3.

Irradiation means110is formed of a mirror111and a beam formation optical system112. Beam formation optical system112forms the laser beam emitted from laser oscillator120into a predetermined form. Then, the laser emitted from beam formation optical system112is reflected by mirror111so as to irradiate amorphous silicon film33. Beam formation optical system112and mirror111are both positioned above amorphous silicon film33.

Shifting means130is formed of a movable stage131and a driving motor132for driving movable stage131. Movable stage131supports glass substrate31and is allowed to shift relative to laser oscillator120and irradiation means110. Therefore, when movable stage131moves, glass substrate31and amorphous silicon film33that are mounted thereon also move.

Movable stage131is connected to driving motor132so that driving motor132drives movable stage131. Here, movable stage131is allowed to shift on a predetermined plane in all directions.

Control means140is connected to driving motor132and laser oscillator120. Control means140sends a signal to driving motor132so that it drives movable stage131at a predetermined time. Driving motor132that has received this signal shifts movable stage131in a predetermined direction. In addition, control means140sends a signal to laser oscillator120and allows laser oscillator120to oscillate a laser.

Control means140sends a signal to laser oscillator120by using such a unit. Laser oscillator120oscillates a laser so as to irradiate first region33aof amorphous silicon film33with this laser via beam formation optical system112and mirror111. Under this condition, control means140sends a signal to driving motor132so that driving motor132shifts movable stage131in the direction shown by arrow131a. Thereby, the portion irradiated with the laser is crystallized so as to form first polycrystal portion34a.

Referring toFIGS. 4 and 5, after first polycrystal portion34ahas been formed, laser oscillator120stops oscillating the laser. Movable stage131is used to shift amorphous silicon film33so that first polycrystal portion34aand second region33bare irradiated with laser35in a line form. Under this condition, laser35is allowed to scan and, thereby, second polycrystal portion34bis formed.

That is to say, the step of forming first polycrystal portion34aincludes the formation of first polycrystal portion34athat extends in one direction by allowing laser35to scan over amorphous silicon film33while the step of forming second polycrystal portion34bincludes the formation of second polycrystal portion34bthat extends along first polycrystal portion34aby allowing laser35to scan in the direction in which first polycrystal portion34aextends.

By repeating this operation, the major portion of amorphous silicon film33is polycrystallized so as to form polycrystal silicon film34.

Referring toFIG. 6, polycrystal silicon film34is patterned, while leaving out a predetermined portion as polycrystal silicon film34, so as to form active regions39aand39b. Next, a silicon oxide film is formed on polycrystal silicon film34. A metal film made of a low resistance metal such as Ta, Cr or Al is formed on the silicon oxide film. By patterning the metal film and the silicon oxide film into a predetermined form, gate insulating films36aand36b, as well as gate electrodes37aand37b, are formed. After that, gate electrodes37aand37bare used as a mask to form sources and drains in a self-aligned manner by means of an ion doping method. Thereby, a thin film transistor is completed.

That is to say, in the process for a semiconductor device according to this invention, an amorphous silicon film is formed on glass substrate31having first region33aand second region33bthat is connected to this first region33aon glass substrate31. Next, amorphous silicon film33is irradiated with a laser so as to form polycrystal silicon film34. The step of forming polycrystal silicon film34includes the steps wherein, first region33aof amorphous silicon film33is irradiated with a laser of which the wavelength is not less than 390 nm and not more than 640 nm so as to form first polycrystal portion34aand, next, wherein second region33bof the amorphous silicon film and a region of a portion of first polycrystal portion.34athat contacts second region33bare irradiated with a laser of which the wavelength is not less than 390 nm and not more than 640 nm so as to form second polycrystal portion34bthat contacts first polycrystal portion34a. In addition, the semiconductor device thus gained uses the polycrystal silicon film fabricated in the above described steps as active regions39aand39b.

FIG. 7is a graph showing the relationships of the wavelength of a laser and the absorption ratio in an amorphous silicon film and in a polycrystal silicon film. Referring toFIG. 7, the absorption ratio of a laser in the amorphous silicon film and in the polycrystal silicon film changes variously depending on the wavelength thereof. Since the wavelength of the laser is not less than 390 nm in the present invention, the absorption ratio of the polycrystal silicon film is 60%, or less, of the absorption ratio of the amorphous silicon film. Therefore, in the case that polycrystal silicon has been formed through laser irradiation of amorphous silicon, such polycrystal silicon does not absorb the energy of the laser even when the polycrystal silicon is irradiated with the laser. As a result, the characteristics of the polycrystal silicon do not vary so that almost the same characteristics can be gained throughout the entirety of the polycrystal silicon film.

In addition, since the wavelength of the laser is not more than 640 nm, the absorption ratio in the amorphous silicon film becomes 10%, or greater. As a result, it becomes easy for the amorphous silicon to absorb the heat from the laser so that the amorphous silicon can be easily polycrystallized.

Here, it is preferable for the wavelength to be not less than 500 nm and not more than 550 nm because the difference in the absorption ratios of the amorphous silicon film and the polycrystal silicon film becomes greater. It is more preferable for the wavelength to be not less than 520 nm and not more than 550 nm because the difference in the absorption ratios of the amorphous silicon film and the polycrystal silicon film becomes particularly great.

FIG. 8is a graph showing the relationships between the silicon film thickness and the absorption ratio with respect to the laser (second harmonics of Nd: YAG (wavelength λ=532 nm)) used in this invention. The absorption ratio of the polycrystal silicon film is smaller than the absorption ratio of the amorphous silicon film in the case that the thickness of the silicon film is set at a variety of values with respect to the laser used in this invention.

In addition, n channel type and p channel type transistors in the structure shown inFIG. 6are fabricated and the mobility and the threshold potential in these transistors are shown inFIGS. 9 and 10.

Referring toFIG. 9, the portion surrounded by solid line201indicates the portion that is twice irradiated with the laser. It can be understood fromFIG. 9that the mobility is maintained at an approximately constant level in the case that either an n channel type transistor or a p channel type transistor is formed. In addition, it can be understood that the mobility in the portion that is twice irradiated with the laser is approximately the same as that in the other portions.

Referring toFIG. 10, the portion surrounded by solid line202is the portion that is twice irradiated with the laser. It can be understood fromFIG. 10that the threshold voltage of either the n channel type transistor or the p channel type transistor is approximately the same in any position. In addition, it can be understood that the threshold potential is approximately the same in the portion that is twice irradiated with the laser and in the portion that is irradiated with the laser only once.

The wavelength of the laser is in an optimal range in the above manner according to the present invention and, therefore, a high quality semiconductor device can be provided wherein the mobility and the threshold potential are constant in both the portion that is once irradiated with the laser as well as in the portion that is twice irradiated with the laser.

That is to say, in the case that a thin film transistor is formed, the thickness of the silicon film is conventionally 100 nm, or less, and in this region the absorption ratios of the amorphous silicon film and of the polycrystal silicon film differ greatly wherein the absorption ratio of the polycrystal silicon film is smaller than the absorption ratio of the amorphous silicon film. As a result, when a polycrystal silicon film is irradiated with a laser having an irradiation energy density that is optimal for amorphous silicon film, the energy absorbed by the polycrystal silicon film is too small to cause the melting of the polycrystal silicon film. That is to say, only the amorphous silicon film portion selectively receives the laser heat processing and, therefore, no difference is caused in the characteristics of the portion that receives laser heat processing twice and the portion that only receives laser heat processing once so that a polycrystal silicon film can be formed of which the characteristics are uniform throughout the region of the substrate. In addition, the same effects can be gained in the case that a polycrystal silicon film that has many crystal defects and of which the absorption high is used as a film that is irradiated with the laser.

FIG. 11is a graph showing the relationships between the film thickness and the absorption ratio of a conventional amorphous silicon film and of a polycrystal silicon film fabricated by using an excimer laser. It can be understood fromFIG. 11that the absorption ratios of the polycrystal silicon film and of the amorphous silicon film are approximately the same. Even in the case that such an amorphous silicon film has been converted to a polycrystal silicon film by being irradiated once with a laser, the polycrystal silicon film is again irradiated with a laser afterwards and, thereby, the polycrystal silicon film absorbs the energy of the laser. Thereby, the polycrystal silicon film is again melted so that the characteristics of the polycrystal silicon film change. Therefore, the portion that is once irradiated with the laser and the portion that is twice irradiated with the laser differ in the characteristics of the polycrystal silicon film and a polycrystal silicon film that has uniform characteristics throughout the film cannot be gained.

That is to say, as shown inFIG. 11, in the case that the amorphous silicon film and the polycrystal silicon film are irradiated with a KrF excimer laser beam (wavelength: 248 nm), the difference in absorption ratios of the amorphous silicon film and the polycrystal silicon film is approximately 7%. At the time of laser heat processing of the amorphous silicon film, the irradiation energy density is set at the optimal value of the amorphous silicon film.

FIG. 12is a graph showing the relationships between the laser energy density and the mobility of the n channel type transistor at the time when the polycrystal silicon film, shown inFIG. 11, is fabricated. As shown inFIG. 12, as for heat processing by means of an excimer laser, the permissible range of the optimal value of the irradiation energy density is very narrow and, therefore, when the absorption ratios differ by 7% it becomes a problem. That is to say, after the polycrystal silicon film portion has been once melted through the irradiation by means of the excimer laser, recrystallization growth occurs and since the irradiation energy density is outside of the permissible range of the optimal value, the region that has undergone a second laser irradiation is converted to a polycrystal silicon film having poor characteristics.

That is to say, the portion that is once irradiated with a laser and the portion that is twice irradiated with a laser in a conventional polycrystal silicon film differ in laser energy density. Therefore, the mobility of the n channel type transistor differs so that uniform characteristics cannot be gained throughout the polycrystal silicon film. As a result, a thin film transistor having the desired characteristics cannot be gained.

Second Embodiment

FIG. 13is a perspective view showing a process for a polycrystal silicon film according to a second embodiment of this invention. The irradiation means of a polycrystal silicon film production unit180, shown inFIG. 13, differs from that of the polycrystal silicon film production unit100. That is to say, in the polycrystal silicon film production unit180, shown inFIG. 13, a first irradiation means110a, a second irradiation means110band a third irradiation means110cexist as the irradiation means. First, second and third irradiation means110a,110band110care, respectively, formed of a mirror111and a beam formation optical system112. Mirror111and beam formation optical system112are similar to those shown inFIG. 3. Alaser oscillator120a, as the first oscillation means, a laser oscillator120bas the second oscillation means and a laser oscillator120cas the third oscillation means are connected to respective beam formation optical systems112. Laser oscillators120a,120band120care, respectively, Q switch Nd: YAG laser second harmonics oscillators. Light emitted from, respectively, laser oscillators120a,120band120cirradiates the beam formation optical system. In addition, laser oscillators120a,120band120care respectively connected to control means140.

A laser oscillated from laser oscillator120airradiates amorphous silicon film33via first irradiation means110a. A laser oscillated from laser oscillator120birradiates amorphous silicon film33via second irradiation means110b. Control means140controls first and second irradiation means110aand110b, laser oscillators120aand120bas well as shifting means130so that second irradiation means110bemits a laser35bafter a predetermined period of time has elapsed since first irradiation means110ahas emitted a laser35a.

That is to say, as shown inFIG. 13, amorphous silicon film33is irradiated with lasers35a,35band35cand, under this condition, shifting means130allows glass substrate31to shift in the direction shown by arrow131aand, thereby, the surface of amorphous silicon film33is irradiated with lasers35a,35band35cso that a polycrystal silicon film can be formed on amorphous silicon film33.

FIG. 14is a view showing the appearance of a laser irradiating the amorphous silicon film. Referring toFIG. 14, the positions of the lasers are arranged so that, first, laser35abecomes the head and is followed by laser35bwhich is, in turn, followed by laser35crelative to the shifting direction of the movable stage shown by arrow131a.

In addition, referring toFIG. 15, lasers35aand35care arranged so as to be positioned in front toward the direction of progression shown by arrow131awhile laser35bmay be arranged so as to be positioned on the rear side. In these methods the positions of respective beams in line forms are parallel to each other and are staggered and, in addition, the edges of the beams in line forms are slightly overlapped with the edges of the adjacent beams so that the heat processed traces of the adjacent beams overlap each other. A plurality of beams in line forms of such a configuration irradiate, simultaneously or in a time staggered manner, the stage while it is being scanned.

Furthermore, as shown inFIG. 16, the beams are arranged so that respective lasers35a,35band35coverlap. InFIG. 16, the respective beams in line forms are connected so as to form a beam in a one line form as shown in the figure. As for the laser irradiation, each of the laser beams irradiates the stage in a time staggered manner so that the adjoining two beams do not simultaneously carry out irradiation while the stage is being scanned.

A semiconductor device that has a polycrystal silicon film can be manufactured by using such a polycrystal silicon film production unit180with the same method as the method of the first embodiment. Furthermore, three laser oscillators are used in this unit and, therefore, throughput is increased and a polycrystal silicon film of a broad area can be effectively produced.

Third Embodiment

FIG. 17is a perspective view of a production unit for a polycrystal silicon film according to a third embodiment of this invention. Referring toFIG. 17, a polycrystal silicon film production unit190according to the third embodiment differs from that of other embodiments in the point that a laser oscillated from one laser oscillator420is used by three irradiation means. That is to say, the irradiation means has a first irradiation means210a, a second irradiation means210band a third irradiation means210c. Irradiation means210a,210band210chave, respectively, a beam formation optical system112and a mirror111that are the same as those in the first embodiment. The mirror114reflects a laser that is oscillated by laser oscillator420of which the wavelength is not less than 390 nm and not more than 640 nm so that amorphous silicon film33is irradiated with this laser in the form of lasers35a,35band35cvia beam formation optical systems112and mirrors111. That is to say, in this polycrystal silicon film production unit190the irradiation means includes first irradiation means210aand second irradiation means210b. Amorphous silicon film33is irradiated with a portion of the laser oscillated from laser oscillator420via first irradiation means210a. Amorphous silicon film33is irradiated with another portion of the laser oscillated from laser oscillator420via second irradiation means210b. In addition, control means140controls first and second irradiation means210aand210b, laser oscillator420as well as shifting means130so that second irradiation means210bcarries out a laser irradiation after a predetermined period of time has passed since first irradiation means210ahas carried out a laser irradiation.

Nd: YAG second harmonics (wavelength: 532 nm) lasers emitted from laser oscillator420are formed to have beam profiles in line forms by means of beam formation optical systems112that correspond to respective lasers.

Such a method also has the same effect as in the first embodiment. Furthermore, the unit can be formed of one laser oscillator420so that the cost of the unit can be reduced.

In addition to the above description of the embodiments of this invention, it is possible to modify the embodiments shown herein in a variety of manners. First, a laser irradiation method as shown inFIGS. 14to16of the second embodiment can be used in the unit shown in FIG.17. Furthermore, in addition to the means for oscillating the second harmonics of an Nd: YAG laser that is shown as a laser oscillator, other laser oscillators such as laser oscillators that oscillate the second harmonics of an Nd: YVO4laser, the second harmonics of an Nd: YLF laser, the second harmonics of an Nd: glass laser, the second harmonics of a Yb: YAG laser, the second harmonics of a Yb: glass laser, an Ar ion laser, the second harmonics of a Ti: sapphire laser or a Dye laser may be used.

Industrial Applicability

This invention can be utilized in the field of a process for thin film transistors that are used in a liquid crystal display.