JPH02224346A - Formation of thin film transistor - Google Patents

Abstract

PURPOSE:To form a TFT, which operates at high speed, without applying a complicated process and with good reproducibility by a method wherein a low-resistance unsingle crystal semiconductor layer is cut using a laser beam and an optical system for narrowing a laser beam is used for forming source and drain regions. CONSTITUTION:In case a plurality of pieces of thin film transistor elements are formed in an alignment on a substrate, a low-resistance unsingle crystal semiconductor layer having an N-type or P-type conductivity type is out by irradiating a laser beam to form source and drain regions. Moreover, a laser beam is selectively applied to enhance the crystallization of a part, which is irradiated with the laser beam, of a high-resistance unsingle crystal semiconductor layer to contrive so that the part becomes a channel part of a plurality of pieces of the thin film transistor elements. Thereby, a reduction of a channel length can be made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION INDUSTRIAL APPLICATION FIELD 1 The present invention relates to a thin film transistor (hereinafter also referred to as TPT) using a non-single crystal semiconductor thin film and a method for manufacturing the same, and in particular to a liquid crystal display.

The present invention relates to a thin film transistor with high-speed response that can be applied to image sensors and the like.

rPrior Art 1 Recently, thin film transistors using non-single crystal semiconductor thin films produced by chemical vapor deposition or the like have been attracting attention.

Since this thin film transistor is formed on an insulating substrate using a chemical vapor phase method as mentioned above, it can be formed at a low atmosphere temperature of about 450°C at maximum, and it can be formed using inexpensive soda glass or borosilicate glass. etc. can be used as a substrate.

This thin film transistor is a field effect type, so-called M
O5Ff! It has the same function as T, but as mentioned above, it can be formed at low temperature on an inexpensive insulating substrate, and the maximum area that can be formed is limited only by the dimensions of the device that forms the thin film semiconductor. It had the advantage that transistors could be easily fabricated on large-area substrates. Therefore, it is extremely promising as a switching element for matrix-structured liquid crystal displays having a large number of pixels, one-dimensional or two-dimensional image sensors, and the like.

In addition, photolithography, which is an already established technology, can be applied to fabricate this thin film transistor.
So-called microfabrication was possible, and it was also possible to integrate it like ICs and the like.

A typical structure of this conventionally known TPT is schematically shown in FIG.

(20) is an insulating substrate made of glass, (21)
are thin film semiconductors made of non-single crystal semiconductors, (22), (2
3) is the source/drain region, (24) and (25) are the source/drain electrodes, (26) is the gate insulating film, and (27)
is the gate electrode.

A thin film transistor configured in this way has a gate electrode (
By applying a voltage to source-drain (27),
It adjusts the current flowing between 2) and (23).

At this time, the response speed of this thin film transistor is given by the following equation.

S=β・V/L" Here, L is the channel length and μ is the carrier mobility.

■ is the gate voltage.

The non-single-crystal semiconductor layer used in this thin-film transistor contains a large number of crystal grain boundaries, etc., and due to these, the carrier mobility is extremely small compared to a single-crystal semiconductor, as can be seen from the above equation. There was a problem that the response speed of the transistor was extremely slow. In particular, when an amorphous silicon semiconductor is used, its mobility is approximately 0.
It was about 1 to 1 (cta''/V-Sec), which was such that it could hardly function as a TPT.

It is known that in order to solve such problems, as is clear from the above equation, it is necessary to shorten the channel length and increase carrier mobility, and various improvements have been made.

In particular, if the channel length is shortened, the response speed will be affected by the square of the length, so this is a very effective means.

However, when forming an element on a large-area substrate, which is a feature of TPT, 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 of 1. It is clearly difficult in terms of yield, production cost, etc., and TP
As a means for shortening the channel length of T, a means that does not use photolithography technology is required.

One answer to this question is the vertical channel, as shown in Figure 3.
A TPT with a double structure has been proposed. This is done by laminating a non-single crystal semiconductor layer consisting of a source (30), an active region (31), and a drain (32) on a substrate, and then a gate insulating film (33).
) and has a gate electrode (34) thereon.

In this structure, the channel length is approximately equal to the active region (31
), the channel length could be easily varied by adjusting the thickness of the active region.

However, since TPT with this structure has multiple non-single-crystal semiconductor layers stacked, it has many interfaces in the direction in which current flows between the source and drain, resulting in a good TPT.
T 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-state current increases.
Vertical TPT does not essentially solve the problem.

On the other hand, various methods have been used to improve mobility. Typically, non-single crystal semiconductors are annealed to increase the grain size of single crystals or polycrystals.

In these conventional methods, an expensive heat-resistant substrate must be used because the annealing is carried out at high temperatures, and the processing time becomes longer because the semiconductor layer on the entire surface of the substrate is made into single crystal or polycrystal. A problem was occurring.

Object of the Invention The present invention solves the above-mentioned problems, and provides a method for manufacturing a TPT that operates at higher speed than conventionally known TPTs without complicated processes and with good reproducibility. Its purpose is to.

rStructure of the Invention In order to achieve the above object, the present invention provides the following features when a plurality of thin film transistor elements are arranged and formed on a substrate.
A process of cutting a low-resistance non-single-crystal semiconductor having an N or P-type conductivity by irradiating a laser to produce a source and drain region; The method is characterized by promoting crystallization of a portion of the non-single-crystal semiconductor layer irradiated with laser light, and manufacturing the non-single crystal semiconductor layer so that the portion becomes the 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 laser light to create source and drain regions,
By using an optical system to focus the laser beam, it is possible to reduce the distance between the source and drain regions (corresponding to the channel length) to about several micrometers, which is difficult to achieve with conventional photolithography methods. This makes it possible to shorten the channel length.

In addition, laser light irradiation promotes crystallization of the high-resistance non-single crystal semiconductor layer, increasing the carrier mobility of TPT and increasing the response speed mentioned above. This makes it possible to apply thin film transistor elements using non-single crystal semiconductors to sprays, image sensors, and the like.

Furthermore, in the present invention, in order to irradiate a plurality of aligned portions on the substrate with laser light in a straight line or dot shape,
Compared to conventional methods, when irradiating in a straight line, it is possible to promote crystallization of a straight part or cut a non-single crystal semiconductor at the same time, and it is possible to crystallize multiple parts of a non-single crystal semiconductor thin film, Non-single crystal semiconductors can be cut in a short time.

Furthermore, even in the case of dot-shaped irradiation, the program for moving the substrate after irradiating one spot is simple because it irradiates aligned parts, and in terms of the process, multiple non-single-crystal semiconductor thin films can be irradiated. It is possible to crystallize the part and cut the non-single crystal semiconductor in a short time.

Furthermore, in the present invention, during etching, the portions irradiated with laser light are less likely to be etched than the portions not irradiated, so the yield during etching can be increased and costs can be reduced.

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

Furthermore, when manufacturing a staggered thin film transistor, for example, after manufacturing an N-type non-single crystal semiconductor film in a vacuum apparatus, a laser beam is guided into the vacuum apparatus with a substrate set in the vacuum apparatus. The source and drain regions are created by cutting the N-type semiconductor thin film, and in this state, a high-resistance (I-type) non-single-crystal semiconductor thin film is formed, and by irradiating it with laser light again, a ■-type semiconductor thin film is formed. A semiconductor layer can be crystallized, and then an insulating film can be formed. In other words, the steps from forming an N-type semiconductor layer to forming an insulating film can be performed without touching the substrate. Therefore, it is ensured that the cut portion of the N-type semiconductor layer and the crystallized portion of the ■-type semiconductor layer coincide with each other, that is, only the channel region can be crystallized. Furthermore, if a semiconductor thin film with advanced crystallization is produced by irradiating laser light while producing a ■-type semiconductor thin film, the process that used to be performed in two steps, thin film production and crystallization, can be completed in one step. This process can be carried out in several steps, and the time required for the process can be shortened. Moreover, if this is combined with the crystallization or cutting at a plurality of locations as described above, the process time can be further shortened.

The present invention will be explained in detail below using examples.

Example 11 In this example, the production of a coplanar thin film transistor for use in a liquid crystal display will be described.

FIGS. 1(a) to 1(g) schematically show the manufacturing process of a thin film transistor corresponding to this example.

First, as a substrate (11), a 300 nm x 300 mm soda glass having an ITO electrode (@element electrode) (1B) patterned as a full-time conductive film was used.
On this substrate (11), an I-type non-single crystal silicon film (13) is formed as a high-resistance semiconductor layer by a known plasma CVD method. The manufacturing conditions at this time were as follows.

Substrate temperature 250°C Reaction pressure 0.05TorrRf power (13.56MHz) 150W Gas used
5iHa Film thickness: 6,000 people Similarly, a non-single crystal silicon film (12
) to form. (Figure 1(a)) This non-single crystal silicon film (
The manufacturing conditions for 12) were almost the same as for the non-single crystal silicon film (13), except that the gas used was 5iHn+PHs and the film thickness was 2.
000 people.

This N-type non-single crystal silicon film (12) is formed by H
It is also possible to introduce a large amount of two gases, increase the Rf power, microcrystallize the material, and lower the electrical resistance.

Next, using a known photolithography technique, the non-single crystal silicon films (12) and (13) are masked into a predetermined external pattern of the source and drain 'pUjA.
Dry etching was performed using CF4 gas, as shown in Figure 1 (
The state b) was obtained.

Next, a molybdenum thin film is formed using a known sputtering method, and etched to form source and drain electrodes (50 mm).
) and (51) were prepared. (FIG. 1(C)) Next, the non-single crystal silicon film (12) is
Excimer laser light (15) with a wavelength of 248.7 nm focused by an optical system is irradiated as shown in Fig. 1(d) so that the irradiation cross section is a long rectangle with a width of 2.5 μm. The single crystal silicon film (12) was cut, and then 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 adjusting the energy of the laser beam so as not to cut the non-single crystal silicon film (13).

Normally, laser light is strong at the center and weak at the edges, exhibiting a Gaussian intensity distribution. Therefore,
If the light is irradiated in this state, crystallization will proceed only in the center of the light, so in this example, an optical system was used to uniformize the intensity of the light and perform the irradiation.

Then, the state shown in FIG. 1(e) was obtained. However, in Figure 1 (e
), laser light is irradiated in a straight line to show only the areas with increased crystallinity.

The laser beam irradiation conditions in this example were: first, 3 pulses were irradiated with a power density of IJ/cm" and a pulse width of 15 p sec, followed by 2 pulses of irradiation with a power density of 0.3 J/cm" and a pulse width of 12 μsec. did.

In the case of this example, the first three pulses were used to cut the low-resistance non-single crystal silicon film, and the latter two pulses were used to crystallize the high-resistance non-single crystal silicon film. The number of irradiations and laser conditions vary depending on the workpiece, and in this example, preliminary experiments were conducted to determine the conditions described above, and those conditions were used.

Next, 100 silicon nitride films were formed by plasma CVD and patterned to form a gate insulating film (16).

Then, a molybdenum film is deposited and patterned using a known sputtering method to form a gate electrode (17), and a thin film transistor (
10) was completed. (FIG. 1(g)) After forming the insulating film, the liquid crystal cell was completed through an alignment film coating process, a spacer dispersion process, a bonding process, and a liquid crystal injection process.

As described above, it is possible to simultaneously cut a low-resistance non-single-crystal silicon film corresponding to multiple thin film transistors using a laser beam with a linear cross section using an optical system, and further At the same time, the crystallization of a high-resistance non-single-crystal silicon film corresponding to the above can be promoted. In addition, since the cutting and crystallization of the above two steps can be performed consecutively, crystallization can be performed only between the source and drain regions, that is, the channel region, and leakage current can be suppressed to a very low level. This method is particularly effective in producing a plurality of TPTs aligned on a large substrate such as that used in a liquid crystal display because processing can be performed in a short time.

rExample 2J In this example, similar to Example 1, a case where the present invention is used in manufacturing a liquid crystal display will be described. However, the case where a staggered thin film transistor is manufactured will be described.

First, Example 1 was prepared on the same substrate as used in Example 1.
A molybdenum film is formed in the same manner as above and 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 Example 1.

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

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 laser light was irradiated again to form an I-type semiconductor layer. Crystallize the non-single crystal semiconductor layer.

In this example, YAG laser light with a wavelength of 1.06 μm is focused by an optical system so as to have a rectangular irradiation cross section with a width of 5 μm and a length of 2.5 μm, and is irradiated in a dotted manner as shown in FIG. Each time one spot was irradiated, the substrate was moved by a fixed length in the X1 or Y direction, and the next spot was irradiated.

The laser light irradiation conditions at this time are power density IJ/cm
”, after irradiating for 1.5 seconds at a repetition frequency of 10kHz, power density 0.5J/C1I2, repetition frequency 10
The irradiation length is 0.5 seconds at kHz. In this case, the first 1.5
The irradiation was performed for 1 second to cut the N-type semiconductor layer, and for the next 0.5 seconds to crystallize the I-type semiconductor layer.

The number of irradiations and laser conditions vary depending on the workpiece, and in this example, preliminary experiments were conducted to determine the conditions described above, and those conditions were used.

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

After irradiation with laser light, 100 people formed a silicon nitride film in the same vacuum apparatus to form a gate insulating film.

Then, using a known photolithography technique, N
The semiconductor layer of type I and type I, and further the gate insulating film were patterned.

After that, a molybdenum film was fabricated and patterned to serve as a gate electrode, and a thin film transistor was completed.

After forming an insulating film, a liquid crystal cell was completed through a liquid crystal alignment film coating process, a spacer dispersion process, a bonding process, and a liquid crystal injection process.

In this way, by irradiating laser light only on the part corresponding to the channel part of the non-single crystal silicon film of the plurality of thin film transistors formed in an array and promoting crystallization, thin film transistors with high response speed can be formed. Furthermore, since the laser beam is irradiated partially, crystallization can be achieved in a shorter time than in the conventional method of irradiating the entire surface.

In this example, since only the necessary parts are irradiated compared to Example 1, even if a slight residue remains during etching of a non-single crystal silicon film, unnecessary parts will not be crystallized. Since the leakage current is not advanced, the leakage current can be reduced.

Furthermore, since the steps from N-type semiconductor layer fabrication to insulating film fabrication were performed in the same vacuum device without moving the substrate even once, the cut portion of the N-type semiconductor and the crystallized portion of the ■-type semiconductor This makes it possible to reduce excess leakage current and also shorten the time required for the process.

Furthermore, since the laser beam irradiation was performed in a vacuum device, the gas generated as a result of the N-type semiconductor being vaporized by the laser beam irradiation was quickly pulled out by the vacuum pump, so that the vaporized gas could be adsorbed onto the substrate surface again. As a result, the performance of the thin film transistor became extremely stable.

This example relates to the production of a staggered thin film transistor, but for example, in the case of an inverted staggered type, the steps include gate electrode production, rumored gate insulating film production, nI
The order is: Preparation of type semiconductor layer → Crystallization → Preparation of N type semiconductor layer Preparation of facing electrode thin film 4 Cutting of N type semiconductor layer electrode. Of these, the steps from fabrication of gate insulating film to fabrication of N type semiconductor layer can be performed without moving the substrate. After all, the cut portion of the N-type semiconductor layer and the crystallized portion of the summer-type semiconductor layer coincide, and the above-mentioned effect can be obtained. Further, similar effects can be obtained when manufacturing other types of thin film transistors.

Example 3 In this example, a case where the present invention is used in manufacturing 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, using a known photolithography technique, the non-single crystal silicon film is masked into a predetermined external pattern in the same manner as in Example 1, and dry etching is performed using CF and gas.

Next, for this non-single crystal silicon film, a width of 5 μm and a length of 230
The wavelength of 248 mm is focused by the optical system so that the irradiation cross section is approximately linear (corresponding to the length of the substrate).
．． A 7 nm excimer laser beam is irradiated to cut the non-single-crystal silicon film in the irradiated portion to produce source and drain regions.

Next, as in Example 1, a summer type non-single crystal silicon film is formed as a high resistance semiconductor layer.

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

The laser light irradiation conditions up to this point are the power density IJ/a
m”, 3 pulses with a pulse width of 15 μsec, then 2 pulses with a power density of 0.5 J/cm” and a pulse width of 10 μsec.
Pulse irradiation was performed. The first three pulses were used to cut an N-type non-single crystal silicon film, and the latter two pulses were used to cut a summer-type non-single crystal silicon film.

The number of irradiations and laser conditions vary depending on the workpiece, and in this example, preliminary experiments were conducted to determine the conditions described above, and those conditions were used.

In this example, as in Example 1, an optical system is used to make the laser beam uniform.

Next, 100 silicon nitride films were formed on this summer-type silicon film by plasma CVD to serve as gate insulating films.

After patterning these into a predetermined pattern, a molybdenum film was deposited and patterned using 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 manufacturing a plurality of thin film transistors that are aligned in a straight line, the cutting and crystallization of the semiconductor layer is performed in one step because a laser beam with a nearly linear cross section is used.
I was able to do it in one step.

r effect j By processing multiple parts simultaneously using laser light, it was possible to shorten the channel length of thin film transistors formed in alignment and increase the crystallinity of the channel part in a short time. This has made it possible to use thin film transistors using non-single crystal semiconductors, which conventionally could not be used as switching elements in display devices, image sensors, etc. due to low carrier mobility.

In addition, since we used laser processing technology to increase the crystallinity of the channel part, there is no problem with processing accuracy even when the area is increased, and it is possible to form a large number of thin film transistors with good characteristics on a large area substrate. has become very easy.

Furthermore, since laser processing is performed only on necessary portions such as straight lines and dots, the yield during etching is increased and leakage current can be further reduced.

Furthermore, if the configuration of the present invention is used by guiding a laser beam into a vacuum device, the process time can be further shortened.
In addition, the cut portion and the crystallized portion coincide to further reduce leakage current.

In addition, in the examples of this specification, only N-type is shown as a low-resistance semiconductor layer, but based on 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. is clear.

[Brief explanation of the drawing]

FIGS. 1(a) to (g) and FIG. 4 show the manufacturing process of a thin film transistor according to an embodiment of the present invention. FIGS. 2 and 3 show schematic cross-sectional views of conventional thin film transistors. 12 ・ ・ ・ 13 ・ ・ 14 ・ ・ 15 ・ ・ 16 ・ ・ 17 ・ ・ 18 ・ ・ 50. 51 Partial laser light gate insulating film gate electrode ITO electrode...source, drain electrode

Claims (1)

[Claims] 1. A step of forming a low-resistance non-single-crystal semiconductor layer having N or P type conductivity to serve as source and drain regions when forming a plurality of thin film transistor elements in alignment; The method includes a step of forming a high-resistance non-single crystal semiconductor layer, a step of forming a gate insulating film, and a step of forming a gate electrode. A step of cutting a low resistance non-single crystal semiconductor layer having a shape to produce a source and drain region, and selectively irradiating the high resistance non-single crystal semiconductor layer with laser light. promotes crystallization of parts,
A method for manufacturing a thin film transistor element, characterized in that the part is manufactured so as to become a channel part of a plurality of thin film transistors. 2. In claim 1, the step of cutting the low resistance non-single crystal semiconductor layer having N or P type conductivity, and selectively irradiating the high resistance non-single crystal semiconductor layer with a laser beam. A method for manufacturing a thin film transistor, characterized in that a step of promoting crystallization of a portion of a semiconductor layer irradiated with laser light is performed at the same time.