JP2001230422A - Method of manufacturing thin film transistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a thin film transistor operated at high speeds without a complex process with good reproducibility. SOLUTION: A semiconductor layer 13 having high resistance and a semiconductor layer 12 having low resistance are laminated on a substrate 11 having a pixel electrode 18 and are patterned in the shape of a transistor. Next, a metal film is formed thereon and is patterned to form a source electrode 50 and a drain electrode 51. Here, laser light whose irradiation cross section is shaped like a line is applied thereto to separate the semiconductor layer 12 having low resistance into source and drain connection portions and then laser light is applied to the semiconductor layer 13 having thigh resistance to increase the crystallinity of a channel region. Next, a gate insulating film 16 and a gate electrode 17 are formed to form a substrate in which thin film transistors 10 are aligned.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film transistor (hereinafter also referred to as TFT) using a non-single-crystal semiconductor thin film and a method for manufacturing the same, and particularly to a high-speed response applicable to a liquid crystal display, an image sensor and the like. Related to a thin film transistor.

[0002]

2. Description of the Related Art Recently, a thin film transistor using a non-single-crystal semiconductor thin film manufactured by a chemical vapor deposition method or the like has attracted attention. Since this thin film transistor is formed on an insulating substrate by using the chemical vapor deposition method as described above, its fabrication atmosphere temperature can be formed at a low temperature of about 450 ° C. at maximum, and it is inexpensive soda glass and borosilicate glass. Can be used as a substrate.

This thin film transistor is a field effect type and has a function similar to that of a so-called MOSFET. However, as described above, it can be formed on an inexpensive insulative substrate at a low temperature, and the maximum area of the thin film semiconductor is a thin film semiconductor. It is limited only by the size of the device to be formed, and has an advantage that a transistor can be easily formed over a large-area substrate. For this reason, it is very promising as a switching element of a liquid crystal display having a matrix structure having a large number of pixels or a one-dimensional or two-dimensional image sensor.

In order to fabricate the thin film transistor, photolithography, which is an established technique, can be applied, so-called fine processing can be performed, and integration can be achieved like an IC or the like.

FIG. 2 schematically shows a typical structure of this conventionally known TFT. (20) is an insulating substrate made of glass, (21) is a thin film semiconductor made of a non-single-crystal semiconductor, (22) and (23) are source / drain regions, (24),
(25) is the source / drain electrode, (26) is the gate insulating film, and (27) is the gate electrode. The thin film transistor configured in this way adjusts the current flowing between the source and drain (22) and (23) by applying a voltage to the gate electrode (27).

At this time, the response speed of the thin film transistor is given by the following equation. S = μ · V / L ² where L is the channel length, μ is the carrier mobility, and V is the gate voltage.

The non-single-crystal semiconductor layer used for this thin film transistor contains a large amount of crystal grain boundaries in the semiconductor layer, and due to these, the mobility of carriers is very small as compared with the single-crystal semiconductor, and As can be seen, there has been a problem that the response speed of the transistor is very slow. In particular, when an amorphous silicon semiconductor is used, its mobility is about 0.1 to 1 (cm ² / V · Sec) and almost TF
It did not work as T.

In order to solve such a problem, it is known that the channel length is shortened and the carrier mobility is increased as is apparent from the above equation, and various improvements have been made.

Particularly, when the channel length L is shortened, the response speed is affected by its square, which is a very effective means.
However, when forming an element on a large-area substrate, which is a feature of TFT, using photolithography technology to reduce the distance between the source and drain (corresponding to the approximate channel length) to 10 μm or less requires processing accuracy, Obviously, it is difficult in terms of yield, production cost, etc., and a means that does not use photolithography technology is required as a means to shorten the TFT channel length. As one answer, a TFT having a vertical channel structure as shown in FIG. 3 has been proposed. In this method, a non-single-crystal semiconductor layer comprising a source (30), an active region (31) and a drain (32) is laminated on a substrate, and then a gate insulating film (33) is formed and a gate electrode (34) is formed thereon. It is.

In the case of this structure, the channel length substantially corresponds to the thickness of the active region (31), and the channel length can be easily varied by adjusting the thickness of the active region.

However, since the TFT having this structure is formed by laminating a plurality of non-single-crystal semiconductor layers, it has a large number of interfaces in the direction in which the current flows between the source and the drain.
Good TFT characteristics cannot be obtained. In addition, since the cross-sectional area in the direction in which the current flows is large, there is a problem that the off-state current increases, and the vertical TFT is not an essential solution.

On the other hand, the mobility has been conventionally improved by various methods. Typically, annealing of a non-single-crystal semiconductor to increase single-crystal or polycrystalline grain size has been performed.

In these conventional examples, an expensive heat-resistant substrate must be used in order to anneal at a high temperature, or the semiconductor layer on the entire surface of the substrate is monocrystallized or polycrystallized. There was a problem of becoming longer.

[0014]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has a high reproducibility without a complicated process for TFTs operating at a higher speed than conventionally known TFTs. Its purpose is to provide a way to do so.

[0015]

In order to achieve the above object, the present invention provides a method of forming a plurality of thin film transistors on a substrate by irradiating a laser to form an N or P type conductive film. Cutting a low-resistance non-single-crystal semiconductor having source and drain regions, and selectively irradiating a laser beam, the high-resistance non-single-crystal semiconductor layer was irradiated with a laser beam. It is characterized in that crystallization of a portion is promoted, and the portion is formed to be a channel portion of a plurality of thin film transistors.

In the present invention, in order to cut a low-resistance non-single-crystal semiconductor using a laser beam and form a source / drain region, an optical system for focusing the laser beam is used. (Approximately equivalent to the channel length) can be reduced to about several μm, and the channel length can be shortened which is difficult by the conventional photolithography method.

Further, in order to promote crystallization of a high-resistance non-single-crystal semiconductor layer by laser light irradiation, the carrier mobility of the TFT is increased, and the response speed described above is increased.
As a result, a thin-film transistor element using a non-single-crystal semiconductor can be applied to a liquid crystal display, an image sensor, and the like, which could not be applied conventionally.

Further, in the present invention, a plurality of aligned portions on the substrate are irradiated with laser light in a straight line or in a dot shape. Acceleration of crystallization or cutting of the non-single-crystal semiconductor can be performed at the same time, and crystallization of a plurality of portions of the non-single-crystal semiconductor thin film and cutting of the non-single-crystal semiconductor can be performed in a short time. Also, in the case of irradiating in the form of dots, the program for moving the substrate after irradiating one place is simple because it irradiates the aligned part, and
In the process, crystallization of a plurality of portions of the non-single-crystal semiconductor thin film,
Cutting of the non-single-crystal semiconductor can be performed in a short time.

Further, in the present invention, the portion irradiated with the laser beam is also harder to etch than the portion not irradiated, so that the yield at the time of etching can be increased and the cost can be reduced.

In particular, when the thin film transistor to be manufactured is a coplanar type or an inverted staggered type, the step of cutting the low-resistance non-single-crystal semiconductor thin film and the crystallization of the high-resistance non-single-crystal semiconductor thin film are performed simultaneously. In particular, the time required for the process can be reduced.

Further, for example, when manufacturing a staggered type thin film transistor, an N-type non-single-crystal semiconductor film is formed in a vacuum device, and then a laser is set in the vacuum device with the substrate set in the vacuum device. Guide the light N
The source and drain regions are formed by cutting the semiconductor thin film of the type, a high-resistance (I-type) non-single-crystal semiconductor thin film is formed in that state, and the semiconductor layer of the I-type is formed by irradiating a laser beam again. Is crystallized, and then an insulating film can be formed. That is, the steps from the formation of the N-type semiconductor layer to the formation of the insulating film can be performed without touching the substrate. Therefore, the cut portion of the N-type semiconductor layer and the portion of the I-type semiconductor layer to be crystallized surely match, that is, only the channel region can be crystallized. Furthermore, when a semiconductor thin film with advanced crystallization is produced by irradiating a laser beam while producing an I-type semiconductor thin film, thin film production and crystallization are performed.
The steps that were performed separately can be performed in one step, and the time required for the steps can be reduced. In addition, when combined with the crystallization or cutting of a plurality of portions described above, the process time can be further reduced. Hereinafter, the present invention will be described in detail with reference to examples.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION

[0023]

[Embodiment 1] In this embodiment, fabrication of a coplanar thin film transistor for use in a liquid crystal display will be described. FIGS. 1A to 1G show schematic steps of manufacturing a thin film transistor corresponding to this embodiment.

First, as a substrate (11), a 300 mm × 300 mm soda glass having an ITO electrode (pixel electrode) (18) patterned as a transparent conductive film is used, and a known plasma CVD method is applied on the substrate (11). Then, an I-type non-single-crystal silicon film (13) is formed as a high-resistance semiconductor layer. The manufacturing conditions at this time were as follows. Substrate temperature 250 ° C Reaction pressure 0.05 Torr Rf power (13.56MHz) 150W Gas used SiH ₄ Film thickness 6000Å

Then, similarly, a non-single-crystal silicon film (12) having N-type conductivity is formed as a low-resistance non-single-crystal semiconductor by a plasma CVD method. (FIG. 1A) The manufacturing conditions of this non-single-crystal silicon film (12) are almost the same as those of the non-single-crystal silicon film (13), but the gas used is SiH ₄ + PH ₃ and the film thickness is 2000.
Å

This N-type non-single-crystal silicon film (12)
H at the time of formation_TwoIntroduce a large amount of gas to increase Rf power
It is also possible to use microcrystals that have reduced electrical resistance
No. Next, using known photolithography technology,
These non-single-crystal silicon films (12) and (13) are
Mask the specified external pattern of the area, and _FourMoth
1B is dry-etched, and the state shown in FIG.
I got

Next, a molybdenum thin film was formed by a known sputtering method, and was etched to produce source and drain electrodes (50) and (51). (Fig. 1 (c))

Next, for the non-single-crystal silicon film (12),
An excimer laser beam (15) with a wavelength of 248.7 nm focused by an optical system is irradiated as shown in FIG. The silicon film (12) was cut, and then the crystallinity of the portion of the high-resistance non-single-crystal silicon film (13) irradiated with the laser beam was increased. It should be noted here that the energy of the laser beam is adjusted so as not to cut the non-single-crystal silicon film (13).

Normally, the laser beam is strong at the center and weak at the edges, and exhibits a Gaussian distribution in intensity. Therefore, when irradiation is performed in this light state, crystallization proceeds only in the central portion of the light.
Irradiation was performed using an optical system with uniform light intensity.

Then, the state shown in FIG. 1E was obtained. However,
In FIG. 1 (e), a laser beam is irradiated linearly,
Only the portion where crystallinity is increased is shown. The irradiation conditions of the laser beam in this embodiment are as follows: first, a power density of 1 J / cm ² , a pulse width of 15 μsec, and three pulse irradiations, followed by a power density of 0.3
Two pulses were irradiated at J / cm ² and a pulse width of 12 μsec.

In this embodiment, the first three pulses were irradiated to cut the low-resistance non-single-crystal silicon film, and the last two pulses were irradiated to crystallize the high-resistance non-single-crystal silicon film.
The conditions of the number of irradiations and the laser differ depending on the workpiece, and in the case of this embodiment, a preliminary experiment was conducted to obtain the above-mentioned conditions, and the conditions were used.

Next, a silicon nitride film is formed by a plasma CVD method.
A gate insulating film (16) was formed by forming 0 ° and patterning.

Then, a molybdenum film is formed by a known sputtering method, patterning is performed, a gate electrode (17) is formed, and thin film transistors (10) are aligned as shown in FIG. The placed substrate was completed.
(Fig. 1 (g))

After the formation of the insulating film, the liquid crystal cell was completed through an alignment film application step, a spacer dispersion step, a bonding step, and a liquid crystal injection step.

As described above, the low-resistance non-single-crystal silicon films corresponding to a plurality of thin film transistors can be simultaneously cut using the laser light having a linear cross section using the optical system. The crystallization of a high-resistance non-single-crystal silicon film corresponding to a plurality of thin film transistors can be simultaneously promoted. In addition, since the cutting and crystallization of the two steps can be performed successively, crystallization can be performed only between the source and drain regions, that is, only in the channel portion.
Leakage current can be suppressed to a very small level, and processing can be performed in a short time, which is effective when a plurality of TFTs are arranged on a large substrate such as a liquid crystal display.

[Embodiment 2] In this embodiment, a case where the present invention is used for manufacturing a liquid crystal display as in Embodiment 1 will be described. Note that a case where a staggered thin film transistor is manufactured is described.

First, a molybdenum film is formed on the same substrate as that used in the first embodiment in the same manner as in the first embodiment, and is patterned to form source and drain electrodes.

Next, a non-single-crystal semiconductor thin film having N-type conductivity is formed in the same manner as in the first embodiment.

Then, a laser beam is guided into the vacuum device in which the N-type semiconductor thin film is formed, and the N-type semiconductor thin film is cut.

Then, after cutting the N-type semiconductor thin film, a high-resistance (I-type) non-single-crystal semiconductor layer is formed in this state by the same method as in the first embodiment, and laser light is again irradiated. The I-type non-single-crystal semiconductor layer is crystallized.

In this embodiment, the width is 5 μm and the length is 2.5 μm.
As shown in FIG. 4, a YAG laser beam having a wavelength of 1.06 μm condensed by an optical system so as to have a rectangular irradiation cross section of m is irradiated in a point shape as shown in FIG. Then, the next part was irradiated while moving by a certain length.

At this time, the laser beam was irradiated at a power density of 1 J / cm ^{2 at} a repetition frequency of 10 kHz for 1.5 seconds, followed by a power density of 0.5 J / cm ² and a repetition frequency of 10 kHz.
For 0.5 seconds. In this case, irradiation was performed for the first 1.5 seconds for cutting the N-type semiconductor layer and for the next 0.5 seconds for crystallization of the I-type semiconductor layer.

The number of times of irradiation and the conditions of the laser differ depending on the workpiece. In the case of this embodiment, a preliminary experiment was carried out, and the above-mentioned conditions were obtained.

In this embodiment, similarly to the first embodiment, an optical system was used to make the laser light uniform.

After the irradiation with the laser beam, a silicon nitride film was formed in a thickness of 100 ° in the same vacuum apparatus to form a gate insulating film.

Then, the N-type and I-type semiconductor layers and the gate insulating film were patterned by using a known photolithography technique.

Thereafter, a molybdenum film was formed and patterned to complete a thin film transistor as a gate electrode.

After the insulating film was formed, a liquid crystal cell was completed through a liquid crystal alignment film coating step, a spacer scattering step, a bonding step, and a liquid crystal injection step.

By irradiating only a portion corresponding to the channel portion of the non-single-crystal silicon film of the plurality of thin film transistors formed in a row in this manner with laser light to promote crystallization, the response speed is improved. In addition, a thin film transistor having a large diameter can be manufactured, and further, since the laser light is partially irradiated, crystallization can be performed in a shorter time than a conventional method of irradiating the entire surface.

In this embodiment, since only the portions necessary for the first embodiment and above are irradiated, unnecessary portions are not removed even if a residue is slightly left when etching the non-single-crystal silicon film. Since crystallization has not progressed, leakage current can be reduced.

Further, since the steps from the N-type semiconductor layer preparation to the insulating film preparation were performed in the same vacuum apparatus without moving the substrate even once, the cut portion of the N-type semiconductor and the I-type semiconductor crystal were removed. In addition to the above, the amount of unnecessary leakage current can be reduced, and the time required for the process can be reduced.

Further, since the laser light irradiation is performed in the vacuum apparatus, the gas generated as a result of the N-type semiconductor being vaporized by the laser light is quickly drawn by the vacuum pump, so that the gas once vaporized is adsorbed on the substrate surface again. As a result, the cut surface became very clean, and as a result, the performance of the thin film transistor became very stable.

The present embodiment relates to the production of a staggered thin film transistor. For example, in the case of an inverted staggered type, the steps are as follows: gate electrode production → gate insulating film production → I-type semiconductor layer production → crystallization → N-type semiconductor layer preparation → electrode thin film preparation → N-type semiconductor layer electrode cutting. In this order, since the gate insulating film preparation to the N-type semiconductor layer preparation can be performed without moving the substrate, the N-type semiconductor layer cutting is also performed. The portion and the crystallized portion of the I-type semiconductor layer coincide with each other, so that the effects described above can be obtained. Similar effects can be obtained when other types of thin film transistors are manufactured.

[Embodiment 3] In this embodiment, a case where the present invention is used at the time of manufacturing an image sensor will be described.

First, a molybdenum film is formed on a glass substrate in the same manner as in Embodiment 1, and then a non-single-crystal silicon film having N-type conductivity is formed.

Next, the non-single-crystal silicon film is masked into a predetermined outer shape pattern using a known photolithography technique in the same manner as in the first embodiment, and dry etching is performed using CF ₄ gas.

Next, the width of the non-single-crystal silicon film is 5 μm.
The wavelength focused by the optical system so as to have an approximately linear irradiation section with a length of 230 mm (corresponding to the length of the substrate).
The source and drain regions are formed by irradiating 248.7 nm excimer laser light and cutting the non-single-crystal silicon film in the irradiated portion.

Next, as in the first embodiment, an I-type non-single-crystal silicon film is formed as a high-resistance semiconductor layer.

An excimer laser beam having a wavelength of 248.7 nm focused by an optical system is again irradiated so as to have a substantially linear irradiation section having a width of 5 μm and a length of 230 mm (corresponding to the length of the substrate). The non-single-crystal silicon film of the type was crystallized.

The irradiation conditions of the laser beam so far were as follows: three pulses were irradiated at a power density of 1 J / cm ² and a pulse width of 15 μsec, and then two pulses were irradiated at a power density of 0.5 J / cm ² and a pulse width of 10 μsec. The first three pulses were used for cutting an N-type non-single-crystal silicon film, and the last two pulses were used for cutting an I-type non-single-crystal silicon film.

The conditions of the number of irradiations and the laser differ depending on the workpiece. In the case of the present embodiment, a preliminary experiment was carried out, and the above-mentioned conditions were obtained.

In this embodiment, as in the first embodiment, an optical system is used so that the laser light becomes uniform.

Next, a plasma CV is formed on the I-type silicon film.
A silicon nitride film having a thickness of 100 で was formed by Method D to form a gate insulating film.

After patterning these into a predetermined pattern, a molybdenum film was deposited by a known sputtering method, patterning was performed, a gate electrode was formed, and then an insulating film was formed to complete a thin film transistor.

When a plurality of thin film transistors formed so as to be aligned on a straight line are manufactured as described above, a laser beam having a substantially linear cross section is used, so that the semiconductor layer is cut and crystallized in one step. Could be done with

[0066]

By simultaneously processing a plurality of portions using laser light, the channel length of the thin film transistors formed in alignment and the crystallinity of the channel portion can be increased in a short time. This makes it possible to use a thin film transistor using a non-single-crystal semiconductor which could not be used as a switching element of a display device, an image sensor or the like because of a low carrier mobility.

Further, since laser processing technology is used to increase the crystallinity of the channel portion, there is no problem in processing accuracy even if the area is increased, and a large number of thin film transistors having good characteristics are formed on a large area substrate. It became very easy to form.

Furthermore, since laser processing is performed only on necessary portions such as linear and dot shapes, the yield during etching is increased, and the leak current can be further reduced.

In addition, if the configuration of the present invention is used by guiding the laser beam into the vacuum apparatus, the process time can be further shortened, and the cut portion and the crystallization portion coincide with each other to further reduce the leak current. be able to.

In the embodiments of the present specification, only the n-type semiconductor layer is shown as the low-resistance semiconductor layer. However, from the technical idea of the present invention, the present invention is extremely effective also in the case of a thin film transistor having a p-type semiconductor layer. It is clear that

[Brief description of the drawings]

FIG. 1 shows a manufacturing process of a thin film transistor according to an example of the present invention.

FIG. 2 shows a schematic view of a cross section of a conventional thin film transistor.

FIG. 3 shows a schematic view of a cross section of a conventional thin film transistor.

FIG. 4 shows a manufacturing process of a thin film transistor according to an example of the present invention.

[Explanation of symbols]

 10 Thin film transistor 11 Substrate 12 Low-resistance non-single-crystal semiconductor layer 13 High-resistance non-single-crystal semiconductor layer 14 Part with increased crystallinity 15 Laser light 16 ..Gate insulating film 17 ... Gate electrode 18 ... ITO electrode 50, 51 ... Source and drain electrodes

Claims (10)

[Claims] A non-single-crystal semiconductor film formed on an insulating surface;
The non-single-crystal semiconductor film is crystallized by irradiating a laser beam having a linear irradiation cross section, a gate insulating film is formed on the non-single-crystal semiconductor film, and the gate insulating film is formed above the crystallized portion. A method for manufacturing a thin film transistor, wherein a gate electrode is formed through a film. 2. The method for manufacturing a thin film transistor according to claim 1, wherein the laser light is irradiated under reduced pressure. 3. The method for manufacturing a thin film transistor according to claim 1, wherein the laser light is irradiated with a pulse. 4. The method for manufacturing a thin film transistor according to claim 1, wherein the laser light is an excimer laser light. 5. The method according to claim 1, wherein the laser beam is a YAG laser beam. 6. A thin film transistor manufactured by the method according to claim 1. Description: 7. A display using a thin film transistor manufactured by the method according to claim 1. Description: 8. A display comprising a thin film transistor manufactured by the method according to claim 1 as a switching element of a pixel. 9. A sensor using a thin film transistor manufactured by the method according to claim 1. Description: 10. A sensor, wherein a thin film transistor manufactured by the method according to claim 1 is used as a switching element of a pixel.