JP3484411B2 - Method for manufacturing thin film transistor - Google Patents

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 a TFT) using a non-single-crystal semiconductor thin film and a manufacturing method thereof, and particularly to a high-speed response applicable to a liquid crystal display, an image sensor and the like. The present invention relates 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 produced by a chemical vapor deposition method or the like has been receiving attention. Since this thin film transistor is formed on the insulating substrate by the chemical vapor deposition method as described above, it can be formed at a low production ambient temperature of about 450 ° C., and is inexpensive soda glass or borosilicate glass. Etc. can be used as the substrate.

This thin film transistor is of the field effect type and has a function similar to that of a so-called MOSFET, but as described above, it can be formed on an inexpensive insulating substrate at a low temperature, and the maximum area to be formed is a thin film semiconductor. The size of the device to be formed is limited, and it has an advantage that a transistor can be easily formed on a large-area substrate. Therefore, it is extremely promising as a switching element for a liquid crystal display having a matrix structure having a large number of pixels and a switching element for a one-dimensional or two-dimensional image sensor.

Further, in order to manufacture this thin film transistor, photolithography, which is a well-established technique, can be applied, so-called microfabrication is possible, and it is possible to achieve integration like an IC or the like.

A typical structure of this conventionally known TFT is schematically shown in FIG. (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 a source / drain electrode, (26) is a gate insulating film, and (27) is a gate electrode. The thin film transistor thus configured 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 this 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 in 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 a single crystal semiconductor, and the above formula As can be seen, there was a problem that the response speed of the transistor was very slow. Especially when amorphous silicon semiconductor is used, its mobility is about 0.1 to 1 (cm ² / V · Sec), which is almost TF.
It didn't 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 clear from the above equation, and various improvements have been made.

In particular, when the channel length L is shortened, the square of the channel length affects the response speed, which is a very effective means.
However, when forming a device on a large-area substrate, which is a feature of TFT, it is important to set the distance between the source and drain (corresponding to the approximate channel length) to 10 μm or less by using photolithography technology. It is obviously difficult in terms of yield and production cost, and there is a need for a method that does not use photolithography technology as a means of shortening the TFT channel length. As one answer, a TFT with a vertical channel structure as shown in Fig. 3 has been proposed. This is one in which a non-single-crystal semiconductor layer consisting of a source (30) active region (31) and a drain (32) is laminated on a substrate, then a gate insulating film (33) is formed and a gate electrode (34) is formed thereon. 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 changed by adjusting the thickness of the active region.

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

On the other hand, improving the mobility has been conventionally performed by various methods. Typically, a non-single crystal semiconductor is annealed to increase the grain size of single crystal or polycrystal.

In these conventional examples, an expensive heat-resistant substrate must be used for annealing 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 long.

[0014]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, and produces a TFT that operates at a high speed with high reproducibility compared with a conventionally known TFT, without complicated steps. Its purpose is to provide a method of doing so.

[0015]

In order to achieve the above object, the present invention provides an N or P type conductivity type by irradiating a laser when a plurality of thin film transistor elements are aligned and formed on a substrate. A step of cutting a low-resistance non-single-crystal semiconductor having a source and a drain region, and selectively irradiating laser light to the high-resistance non-single-crystal semiconductor layer It is characterized in that the crystallization of a portion is promoted and that portion is formed so as to serve as a channel portion of a plurality of thin film transistors.

In the present invention, the source and drain regions are formed by cutting the low-resistance non-single-crystal semiconductor with laser light and using an optical system for focusing the laser light to form the source and drain regions. Can be set to about several μm (approximately equivalent to the channel length), and the channel length can be shortened, which was difficult with the conventional photolithography method.

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

Further, in the present invention, since a plurality of aligned portions on the substrate are irradiated with the laser beam in a straight line or in a dot shape, in the case of linear irradiation, a straight line portion is irradiated as compared with the conventional method. The crystallization can be promoted or the non-single-crystal semiconductor can be cut at the same time, and a plurality of portions of the non-single-crystal semiconductor thin film can be crystallized and the non-single-crystal semiconductor can be cut in a short time. In addition, when irradiating in a dot shape, the program for moving the substrate after irradiating in one place is simple for irradiating the aligned portions, and
Even in the process, crystallization of multiple parts of the non-single crystal semiconductor thin film,
The non-single-crystal semiconductor can be cut in a short time.

Further, in the present invention, the portion irradiated with the laser beam is less likely to be etched than the unirradiated portion even during etching, so that the yield at the time of etching may be increased and the cost may be reduced.

Particularly 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. It is possible to shorten the time required for the process.

Further, for example, in the case of manufacturing a staggered thin film transistor, after manufacturing an N-type non-single crystal semiconductor film in a vacuum device, a laser is set in the vacuum device with the substrate set therein. Guide the light N
Type semiconductor thin film is cut to form source and drain regions, a high resistance (I type) non-single-crystal semiconductor thin film is formed in that state, and laser light is irradiated again to form an I type semiconductor layer. Can be crystallized, and then an insulating film can be formed. That is, it is possible to perform the process from the N-type semiconductor layer production to the insulating film production without touching the substrate. Therefore, the cut portion of the N-type semiconductor layer and the crystallized portion of the I-type semiconductor layer are sure to coincide with each other, that is, only the channel region can be crystallized. Further, when a semiconductor thin film with advanced crystallization is produced by irradiating laser light while forming an I-type semiconductor thin film, thin film production, crystallization and
The steps that have been performed separately can be performed in one step, and the time required for the steps can be shortened. In addition, if combined with the crystallization or cutting at a plurality of points described above, the process time can be further shortened. The present invention will be described in detail below with reference to examples.

[0022]

DETAILED DESCRIPTION OF THE INVENTION

[0023]

EXAMPLES Example 1 In this example, production of a coplanar type thin film transistor for use in a liquid crystal display will be described. Schematic manufacturing steps of a thin film transistor corresponding to this example are shown in FIGS.

First, as the 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 used on the substrate (11). The 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 ℃ Reaction pressure 0.05Torr Rf Power (13.56MHz) 150W Working gas SiH ₄ film thickness 6000Å

Similarly, a non-single crystal silicon film (12) having N-type conductivity is formed as a low resistance non-single crystal semiconductor by plasma CVD. (FIG. 1 (a)) The conditions for manufacturing the non-single crystal silicon film (12) are almost the same as those for 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) is
H when formed₂Introduce a large amount of gas to increase the Rf power
Therefore, it is possible to use one that has been microcrystallized to reduce the electrical resistance.
Yes. Next, using a known photolithography technique,
This non-single crystal silicon film (12), (13)
Mask the specified external pattern of the area and use CF _FourMoth
Dry etching is performed using
Got

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

Next, with respect to the non-single crystal silicon film (12),
Irradiation of excimer laser light (15) with a wavelength of 248.7 nm focused by an optical system as shown in Fig. 1 (d) to form an elongated rectangular irradiation cross section with a length of 300 mm and a width of 2.5 µm, and non-single crystal. The silicon film (12) was cut, and subsequently the crystallinity of the portion of the high resistance non-single crystal silicon film (13) irradiated with laser light was increased. What must be noted here is to adjust the energy of the laser light so as not to cut the non-single crystal silicon film (13).

Usually, the laser light is strong at the center and weaker at the edges, and exhibits a Gaussian distribution in intensity. Therefore, if irradiation is performed in this light state as it is, crystallization proceeds only in the central portion of the light.
Irradiation was performed by using an optical system to make the light intensity uniform.

Then, the state of FIG. 1 (e) was obtained. However,
In FIG. 1 (e), laser light is radiated linearly,
Only the portion of increased crystallinity is shown. In the present embodiment, the laser light irradiation conditions are as follows: power density is 1 J / cm ² , pulse width is 15 μsec, and 3 pulses are irradiated, and then power density is 0.3.
Two pulses were irradiated with J / cm ² and a pulse width of 12 μsec.

In the case of this embodiment, the first three pulses were used for cutting the low resistance non-single crystal silicon film, and the latter two pulses were used for crystallizing the high resistance non-single crystal silicon film.
The number of times of irradiation and the conditions of the laser differ depending on the object to be processed, and in the case of this example, a preliminary experiment was conducted to obtain the above-mentioned conditions and used the conditions.

Next, a silicon nitride film is formed by plasma CVD to a thickness of 10 nm.
It was formed as 0Å and patterned to form a gate insulating film (16).

Then, a molybdenum film is formed and patterned by a known sputtering method to form a gate electrode (17), and a thin film transistor (10) as shown in FIG. 1 (f) is aligned. The arranged substrate is completed.
(Fig. 1 (g))

After forming the insulating film, the liquid crystal cell was completed by passing through an alignment film coating process, a spacer spraying process, a bonding process and a liquid crystal injecting process.

As described above, it is possible to simultaneously cut a low-resistance non-single-crystal silicon film corresponding to a plurality of thin film transistors by using a laser beam having a linear cross section by using an optical system. Crystallization of a high-resistance non-single-crystal silicon film corresponding to a plurality of thin film transistors can be promoted at the same time. In addition, since the cutting and crystallization of the above two steps can be continuously performed, it is possible to perform crystallization only between the source and drain regions, that is, the channel portion.
The leak current can be suppressed to a very low level, and it is particularly effective because it can be processed in a short time when a plurality of TFTs are aligned on a large substrate used for a liquid crystal display.

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

First, a molybdenum film is formed on the same substrate as that used in Example 1 by the same method as in Example 1 and patterned to form source and drain electrodes.

Next, a non-single-crystal semiconductor thin film having N-type conductivity is formed by the same method as in Example 1.

Then, a laser beam is introduced into the vacuum device in which the N-type semiconductor thin film is produced, 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 was formed in the same manner as in Example 1 and irradiated with laser light again. The I-type non-single-crystal semiconductor layer is crystallized.

In this embodiment, the width is 5 μm and the length is 2.5 μ.
A YAG laser beam with a wavelength of 1.06 μm focused by an optical system so as to form a rectangular irradiation cross section of m is irradiated pointwise as shown in FIG. 4, and the substrate is irradiated in the X or Y direction at each irradiation. It was moved for a certain length to irradiate the next part.

The irradiation conditions of the laser beam at this time are as follows: irradiation with a power density of 1 J / cm ² and a repetition frequency of 10 kHz for 1.5 seconds, and then 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 cutting the N-type semiconductor layer for the first 1.5 seconds, and for crystallizing the I-type semiconductor layer for the second 0.5 seconds.

The number of times of irradiation and the conditions of the laser differ depending on the object to be processed, and in the case of this example, a preliminary experiment was conducted to obtain the above-mentioned conditions and the conditions were used.

Also in this example, as in Example 1, an optical system was used to make the laser light uniform.

After the laser irradiation, a silicon nitride film having a thickness of 100 Å was formed in the same vacuum device as 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.

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

After forming the insulating film, a liquid crystal cell was completed through a liquid crystal alignment film coating process, a spacer spraying process, a bonding process and a liquid crystal injecting process.

As described above, 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 alignment with the laser light to promote crystallization, the response speed is increased. It is possible to manufacture a thin film transistor having a large size, and since the laser light is partially irradiated, crystallization can be performed in a short time as compared with the conventional method of irradiating the entire surface.

In the present embodiment, since the irradiation is performed only on the portions necessary for the first and above embodiments, unnecessary portions are left unetched even if a slight residue remains when the non-single crystal silicon film is etched. Since crystallization has not progressed, the leak current can be reduced.

Further, since the steps from the N-type semiconductor layer production to the insulating film production were performed in the same vacuum apparatus without moving the substrate even once, the cut portion of the N-type semiconductor and the crystal of the I-type semiconductor were performed. In addition to being able to reduce the excess leak current, the time required for the process can be shortened.

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

This example relates to the production of a staggered type thin film transistor. For example, in the case of the inverted staggered type, the steps are: gate electrode production → gate insulating film production → I-type semiconductor layer production → crystallization → N-type semiconductor layer production → electrode thin film production → N-type semiconductor layer electrode cutting are performed in this order. However, since it is possible to perform the steps from gate insulating film production to N-type semiconductor layer production without moving the substrate, cutting of N-type semiconductor layer is also required. The part and the crystallized part of the I-type semiconductor layer coincide with each other, and the effect described above is obtained. Further, similar effects can be obtained when manufacturing other types of thin film transistors.

[Embodiment 3] In this embodiment, the case where the present invention is used in the production of an image sensor will be described.

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

Next, a known photolithography technique is used to mask the non-single-crystal silicon film into a predetermined outer shape pattern in the same manner as in Example 1, and dry etching is performed using CF ₄ gas.

Next, a width of 5 μm is applied to this non-single-crystal silicon film.
m wavelength of 230 mm (corresponding to the length of the substrate)
Irradiation with 248.7 nm excimer laser light is performed to cut off the non-single-crystal silicon film in the light-irradiated portion to form source and drain regions.

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

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

The irradiation conditions of the laser light so far were such that three pulses were irradiated with a power density of 1 J / cm ² and a pulse width of 15 μsec, and then two pulses were irradiated with a power density of 0.5 J / cm ² and a pulse width of 10 μsec. The first three pulses were used for cutting the N-type non-single-crystal silicon film, and the latter two pulses were used for cutting the I-type non-single-crystal silicon film.

The number of times of irradiation and the conditions of the laser differ depending on the object to be processed, and in the case of the present embodiment, the above-mentioned conditions were obtained by performing a preliminary experiment and used.

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

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

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

In this way, when a plurality of thin film transistors formed in a straight line are manufactured, a semiconductor layer is cut and crystallized in one step because a laser beam having a substantially linear cross section is used. Could be done in.

[0066]

By simultaneously processing a plurality of portions using laser light, it is possible to shorten the channel length and increase the crystallinity of the channel portions of thin film transistors formed in alignment. As a result, it has become possible to use a thin film transistor using a non-single-crystal semiconductor, which has been conventionally unusable as a switching element of a display device, an image sensor or the like because of its low carrier mobility.

Since the laser processing technique 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 provided on a large area substrate. Very easy to form.

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

Moreover, if the laser light is guided into the vacuum apparatus and the structure of the present invention is used, the process time can be further shortened, and further, the cut portion and the crystallized portion coincide with each other to further reduce the leak current. be able to.

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

[Brief description of drawings]

FIG. 1 shows a manufacturing process of a thin film transistor according to an embodiment 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 for 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 ・ ・ ・ Parts with increased crystallinity 15 ... laser light 16 ... Gate insulating film 17 ... Gate electrode 18 ... ITO electrode 50, 51 ... Source and drain electrodes

─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-60-245124 (JP, A) JP-A-63-299174 (JP, A) JP-A-61-105884 (JP, A) JP-A-59- 195871 (JP, A) JP 58-127318 (JP, A) JP 63-25913 (JP, A) JP 55-68652 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01L 21/336 G02F 1/1368 H01L 21/20 H01L 29/786

Claims (10)

(57) [Claims] 1. A gate electrode is formed on an insulating surface, a gate insulating film is formed on the gate electrode, an I-type first non-single-crystal semiconductor film is formed on the gate insulating film, N-type or P-type second on the non-single-crystal semiconductor film of No. 1
Non-single crystal semiconductor film is formed, by irradiation sectional illuminates the linear laser beam, Seo
The second region to form a drain region and a drain region .
Cutting the non-single-crystal semiconductor film, and subsequently cutting the second non-single-crystal semiconductor film in the first non-single-crystal semiconductor film to form a channel portion whose surface is exposed. A method for manufacturing a thin film transistor, which comprises crystallizing a region. 2. The method of manufacturing a thin film transistor according to claim 1, wherein the laser light is applied under reduced pressure. 3. The method for manufacturing a thin film transistor according to claim 1 or 2, wherein the laser light is pulse-irradiated. 4. The method of manufacturing a thin film transistor according to claim 1, wherein the laser light is excimer laser light. 5. The method for manufacturing a thin film transistor according to claim 1, wherein the laser light is YAG laser light. 6. A thin film transistor manufactured by the method according to any one of claims 1 to 5. 7. A display using a thin film transistor manufactured by the method according to claim 1. Description: 8. A display using a thin film transistor manufactured by the method according to claim 1 for 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 using a thin film transistor manufactured by the method according to claim 1 for a switching element of a pixel.