JPH08125196A - Semiconductor device, fabrication thereof and doping method therefor - Google Patents

Abstract

PURPOSE: To reduce OFF current of a thin film transistor (TFT) by doping a specified region in the depth direction of a material with one kind of element at a specified concentration thereby implanting impurity ions into a lightly doped region of the TFT within an optimal range thereof. CONSTITUTION: Boron ions are implanted by plasma doping through a silicon nitride 104 using a material gas of diborane. Consequently, a lightly doped region 201 can be doped with boron ions at a concentration of about 10<17> atoms/cm<3> with high controllability. It is an optimal concentration for forming the lightly doped region 201 between a channel forming region 102 and a source or drain region 601. The ratio between ON and OFF currents can be increased within that concentration range and a high performance TFT can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The invention disclosed herein has a low dopant density (for example, approximately 5 × 10 ¹⁶ atoms cm ⁻³⁾ during ion doping of impurity ions into a semiconductor.
˜5 × 10 ¹⁷ atoms cm ⁻³ ).

The invention disclosed in this specification is particularly applicable to LDD.
The present invention relates to a low-density doping method for a light-doped region (described in detail in the related art) of a thin film transistor (TFT) having a structure (described in the related art).

The invention disclosed in this specification is particularly
^The present invention relates to a technique for performing light doping of about ¹⁷ atom cm ⁻³ .

[0004]

2. Description of the Related Art Used for production of semiconductor devices
LDD technology is known. LDD is an abbreviation for Light Dope Drain. The following is a description of the LDD technology and its purpose.

The purpose of doping the semiconductor material (for example, injecting phosphorus or boron into Si) is to create carriers (electrons and holes) in the semiconductor material and control the conductivity type and resistance thereof. By using this doping technique, a semiconductor device having necessary characteristics, such as a thin film transistor (TFT), can be manufactured.

For example, when an attempt is made to form an n-type semiconductor by doping phosphorus into the source and drain regions of a thin film transistor (TFT), phosphorus must be injected into the semiconductor material at a density of 10 ¹⁹ to 10 ²¹ cm ^-3. , TFT does not work.

FIG. 1 is a sectional view showing a schematic structure of a general TFT. FIG. 1 shows a source region 101, a channel formation region 102, and a drain region 103 which are formed in an active layer made of a crystalline thin film silicon semiconductor formed on a glass or quartz substrate. 2 is a schematic cross section of a TFT in which a gate electrode 105 is formed on the channel formation region 102 with a gate insulating film 104 interposed therebetween.

The TFT has a source region 1 as shown in FIG.
01 and the drain region 103, the current flowing in the channel formation region 102 is controlled by the voltage applied to the gate electrode 105. Source at ON operation /
The current flowing between the drains is called the ON current. The ON current is a current flowing when a signal is sent to the channel region so that the current flows.

By the way, there is a current called an OFF current with respect to an ON current. This current is the current that flows when you do not want to apply current to the channel region.
If it exceeds 1/10 ^{5 of the} ON current value, the characteristics as a transistor deteriorate.

Particularly in a thin film transistor arranged in a pixel region in an active matrix type liquid crystal display device,
The OFF current characteristics are extremely important. A thin film transistor arranged in a pixel region of an active matrix liquid crystal display device functions as a switching element for controlling electric charge which goes in and out of a pixel electrode. At this time, since it is necessary to hold the charge in the pixel electrode for a predetermined time, it is necessary that the OFF current of the thin film transistor arranged in the pixel is as small as possible.

In general, in the transistor having the structure shown in FIG. 1, it is difficult to obtain a ratio of ON current and OFF current satisfying the above numerical limits. The reason why the ratio of ON current and OFF current cannot be made large is considered to be due to the following causes.

(1) The source and drain regions doped with a dopant such as phosphorus or boron and the channel formation region not doped with anything have a large difference in physical properties, so that a high electric field is generated at the boundary between them. , The carrier leaks from there, and the OFF current becomes large.

(2) Due to the high electric field generated at the boundary between the doped portion (source / drain region) and the channel formation region described in (1), the carriers strike the atomic lattice and break the carrier, resulting in a defect level. Occurs, carriers leak from it, and the OFF current increases. Lattice destruction is TF
Since it always occurs during T operation, it is also a problem that the deterioration of TFT characteristics is severe. With this deterioration,
The ON current also drops significantly.

The above two causes are considered. One common point of the two causes is that there is a large difference in physical properties between the source / drain region and the channel formation region.

It was the LDD structure that was thought to somehow alleviate this drop. Figure 2 shows the basic structure of the LDD. LD
The feature of D is that a portion 201 called a light-doped region is provided between the source / drain region and the channel formation region. In particular, the light-doped region formed in the region between the channel formation region 102 and the drain region 103 has a large effect, so that this region is referred to as an LDD region (Light D
ope Drain area).

The lightly doped region 201 is doped with phosphorus or boron at a concentration that is two to four orders of magnitude lower than that of the source / drain regions. By providing a region having a property intermediate between the physical properties of the source / drain region and the channel region, one cushion is created due to the difference in the physical properties of the both. As a result, the OFF current is also reduced and deterioration of the TFT characteristics can be suppressed.

[0017]

With the LDD technology described above, it is possible to suppress TFT deterioration and reduce the OFF current. However, there is a problem that it is very difficult to control the density of the dopant doped in the lightly doped region because the density is very small.

It is difficult for the doping apparatus currently on the market to accurately realize a density of 10 ¹⁸ cm ^-3 or less in the case of B (boron) at a desired depth.
The reason is as follows. (All numerical values listed below are values relating to B (boron), and the doping method is the plasma doping method)

In the plasma doping method, a dopant to be implanted is made into ions by high-frequency discharge or the like, and an electric field is further applied to accelerate ionized ions by a potential difference, and the accelerated ions are irradiated to a semiconductor material. Done in. There are three ways to apply the voltage, and there are three methods: deceleration voltage, acceleration voltage, and extraction voltage.
The deceleration voltage plays a role of improving the in-plane uniformity of doping, the acceleration voltage accelerates the ions, and the extraction voltage determines the initial velocity of the ions.

Since the potential difference determines the velocity of the ions, the depth at which the ions are implanted can be changed by changing the potential difference. Further, the density of the dopant is determined by the unit area, the number of ion irradiations per unit time, and the irradiation time. The irradiation number of ions per unit area and unit time can be adjusted by the electric energy (W number), and the irradiation number can be reduced by decreasing the electric energy. However, if the amount of power is reduced too much (generally 5 W or less), the homogeneity of the ion density distribution deteriorates and accurate doping cannot be performed.

Further, if the irradiation time is shortened, the density of the dopant can be reduced. However, if the irradiation time is too short (generally within 4 seconds), the error becomes large and the reproducibility of irradiation is lost. When using the existing apparatus, the dose can be reduced to 10 ¹² cm ^{-2 under} the condition that the number of ions to be irradiated per unit area is 5 W and the irradiation time is 6 seconds. Doping with a dose amount of 10 ¹² cm ⁻² can realize a doping density of the order of 10 ¹⁸ cm ⁻³ .

If the doping density of the order of 10 ¹⁸ cm ^-3 is to be further reduced, the density uniformity of the irradiation is remarkably lowered as described above, and the reproducibility is extremely poor and not practical. Ignoring the numerical limits of the above irradiation conditions and trying to achieve a density of 10 ¹⁷ atoms cm ^{-3 with} an electric power of 5 W, for example, the irradiation time will be about 0.4 seconds, and the doping time will be extremely short. . Generally, in the doping, if the irradiation time is 4 seconds or less, the error is too large and the reproducibility is poor and it is not practical. Therefore, the irradiation time of 0.4 seconds is not realistic. For the above reasons, it is actually difficult to control the dopant density at a density lower than 10 ¹⁸ atom cm ⁻³ .

Then, what is the specific density at which the transistor will have good characteristics? Although it changes depending on the device characteristics to be aimed at, in the following, we will describe the point of determining the dopant density such that the ratio of ON current to OFF current is as large as possible without reducing the ON current as much as possible. (Actually, depending on the length of the LDD area,
The ratio of ON current to OFF current changes, but here we consider the case where this length is fixed at 1 μm and only the dopant density is changed.)

FIG. 3 shows data in which the relationship between the ON / OFF current ratio and the dopant density near the surface of the 1 μm long light-doped region 201 in the TFT structure shown in FIG. 2 is plotted. Show. From this graph, it can be seen that the ON / OFF ratio can be maximized when the density of the dopant (boron) is set to about 10 ¹⁷ atom cm ⁻³ . However, as already mentioned, it is extremely difficult to reproduce this numerical value with good controllability by the existing doping apparatus.

[0025]

The depth profile of the dopant density of a doped semiconductor material has a mountain-like shape having a peak at a certain depth. This is because the velocity of the irradiated ions is changed by colliding with an obstacle such as an atom in the semiconductor material, so that the depth of arrival of the ions has a width.

In general, when a single ion is implanted in a semiconductor material, the dopant distribution becomes Gaussian distribution in the depth direction. Normally, the potential difference and various conditions are adjusted so as to bring the peak portion of this Gaussian distribution to the depth at which the dopant is to be injected. This is because the peak portion has the highest density uniformity in the depth direction, and therefore doping can be performed at a desired depth with high accuracy and high reproducibility. The dopant density discussed in the problem to be solved by the invention generally refers to the density of the peak portion. If the density distribution around the hillside is used, the density of the dopant changes drastically in the depth direction, which makes it difficult to control the density of the dopant, resulting in low practicality.

However, with a dopant having a specific property, the story changes a little. For example, a dopant such as B. In the plasma doping method, the dopant is ionized (for B, diborane (B ₂ H
₆ ) and other gases are ionized and B ions are used.) A voltage is applied. Generally, the vaporized dopant is diluted with hydrogen.

When B is to be doped by the plasma doping method using diborane as a gas source, two types of positive ions represented by BH _x and B ₂ Hy _y are mainly formed. Since these two ions naturally have different weights and sizes, the density distributions in the depth direction formed by the respective ions in the irradiated film also differ.

Although depending on the conditions, when B is doped by the plasma doping method using diborane as a gas source, a density distribution in the depth direction as shown in FIG. 4 can be obtained. FIG. 4 shows the concentration distribution of the boron element in the case where boron ions are implanted from the silicon oxide side by the plasma doping method to the laminated body in which the silicon film and the silicon oxide film are laminated.
It is data measured by MS (secondary ion analysis method).

In FIG. 4, the first peak of high boron density indicated by 401 is present in the silicon oxide film close to the surface, and a low peak of 10 ¹⁷ cm ^{−3 or} less is present in the deeper silicon film. It can be seen that the second peak 402 of the boron element exists in the density.

The lower peak in FIG. 4 can be reproduced sufficiently well with a controlled positive concentration of about 10 ¹⁷ cm ^-3 by adjusting the irradiation conditions. This is because, as indicated by 403, there is a comparatively gentle region of the concentration distribution in the depth direction between the first peak 401 and the second peak 402, and this region is relatively reproducible, and This is because it can be obtained with good controllability.

The above is the solution when boron is used as the dopant. In addition, elements such as boron whose density profile in the depth direction is not divided into two peaks (for example,
Even with phosphorus, it is possible to achieve low density by bringing the density of the middle part of the deep part of the mountain to a desired depth. However, in this case, since the density changes drastically in the depth direction, it becomes difficult to control the density depth.

As another method for lowering the density, it is possible to set the above-mentioned extraction voltage to 0 V or less. By doing so, it is known that the irradiation number of ions per unit time can be reduced. Therefore, the irradiation time of the ions can be lengthened, and the dopant can be controlled at a low density while satisfying the irradiation time limit (4 seconds or more).

Furthermore, by reducing the hydrogen concentration of the dopant, the amount of H + ions can be increased, and the density of the dopant can be lowered without lowering the number of ions irradiated per unit time. This can be said to be an effective means for lowering the density of the dopant, because it is not necessary to reduce the electric power (the electric power limit is 5 W or more).

The structure of the invention disclosed in this specification will be described below. One of the inventions disclosed herein is a method of accelerating and implanting multiple types of ions having different masses and / or ionic radii into a given material,
The plurality of types of ions are composed of a different number of one type of element, utilizing the concentration distribution in the depth direction of the material formed according to each of the plurality of types of ions, in the depth direction of the material It is characterized in that a predetermined region is doped with the one type of element at a predetermined concentration.

In the above structure, as a plurality of types of ions having different masses and / or ionic radii, the ions are BH _x (x is an integer of 1 or more) and B ₂ H _y (y is 1).
The above integers can be given as examples. In this case, diborane gas is ionized to mainly generate ions represented by BH _x and B ₂ H _y and accelerated by an electric field to inject these ions into a predetermined material such as a semiconductor material.

When the predetermined material is a laminated body of a silicon film and a silicon oxide film, as shown in the profile of FIG. 4, the ions represented by BH _x are mainly doped in the silicon film. However, the ions represented by B ₂ H _y can be mainly doped into the silicon oxide film.

This is because the ions represented by B ₂ H _y are heavy and the ionic radius thereof is large, so that the ions represented by BH _x can be doped at a low concentration in the silicon film below the silicon film. This is an example of doping into. Such a structure is extremely effective for forming a low impurity concentration region such as a lightly doped region of a TFT.

If the above structure is adopted, as is apparent from FIG. 4, the boron element should be doped with a boron element in a concentration range of 1 × 10 ¹⁶ atom cm ^{−3 to} 3 × 10 ¹⁷ atom cm ^−3. You can Generally, the above concentration range is 1 × 1.
It is possible to control in the concentration range of 0 ¹⁶ atom cm ^{−3 to} 5 × 10 ¹⁷ atom cm ⁻³ .

Another structure of the invention is a method of manufacturing an insulating gate type field effect semiconductor device having a lightly doped region between a channel forming region and a drain region, wherein an insulating film is formed on the lightly doped region. A step of implanting impurity ions that impart one conductivity type to silicon in the state of
The impurity ions have at least two types of ions having different masses and / or ionic radii, and one of the ions is mainly implanted into the light-doped region.

According to another aspect of the invention, there is provided a laminated body having at least a first thin film material doped with an impurity element and a second thin film material formed on the thin film material.
A step of irradiating a plurality of types of ions composed of a predetermined dopant element and having different masses and / or ionic radii, and the peak of the concentration of the dopant element in the first thin film material and the second thin film material Are respectively present.

When the above structure is adopted, as shown in FIG. 4, peaks of the concentration distribution in the depth direction can be formed in each of the two thin films (the silicon film and the silicon oxide film in the case of FIG. 4). Especially, by utilizing the second small peak or the intermediate region between the two peaks, light doping of about 10 ¹⁷ cm ⁻³ can be enabled.

According to another aspect of the invention, at least a first thin film material doped with an impurity element and a second thin film material formed on the thin film material are provided, and the first thin film contains A region having a peak of the density distribution of the dopant in the thickness direction thereof and having a rate of change of 0 in the density distribution of the dopant in the thickness direction of the second thin film is present in the second thin film.

The above structure includes the case where the peak of the concentration distribution formed in the other thin film is not clear. Of course,
Also in the profile of the concentration distribution as shown in FIG. 4, there is a region where the rate of change of the density distribution of the dopant is 0 as shown by 403 in the thickness direction. In such a region in which the rate of change in the density distribution of the dopant is 0, the change in the concentration distribution in the depth direction is small, and thus it can be obtained with good reproducibility. This is extremely useful for obtaining a semiconductor device having constant characteristics.

Another structure of the invention is an insulating gate type field effect semiconductor device having a lightly doped region between a channel forming region and a drain region, wherein an insulating film is formed on the lightly doped region. In the insulating film and the lightly doped region, there are peaks of the dopant density distribution in the thickness direction.

Another structure of the present invention is an insulating gate type field effect semiconductor device having a lightly doped region between a channel forming region and a drain region, wherein an insulating film is formed on the lightly doped region. , In the insulating film,
A peak of the density distribution of the dopant exists in the thickness direction, and a region in which the rate of change of the density distribution of the dopant is 0 exists in the light-doped region in the thickness direction. .

[0047]

【Example】

[Embodiment 1] In this embodiment, by controlling the density of the dopant (boron) with which the light-doped region of the thin film transistor is doped to about 10 ¹⁷ cm ⁻³ , the OFF current can be reduced without reducing the ON current of the thin film transistor. A technique for reducing the number of digits by one to three digits compared to the conventional method will be described.

First, a thin film transistor which is in the process of being manufactured before being doped is prepared. The specifications of the thin film transistor used in this example are as follows. That is, as shown in FIG. 2, an active layer made of a crystalline silicon film having a thickness of 1200 Å is formed on a glass substrate (Corning 7059) 100, and a silicon oxide film is formed as a gate insulating film 104 on the active layer 1200.
Form a film with a thickness of Å. Then, a gate electrode is formed with a thickness of 500 Å using polycrystalline silicon, silicide or the like as a material. (See Fig. 2) This is a so-called MOS structure. The region to be doped is a specific region of the active layer made of a silicon film.

FIG. 6 shows a schematic cross section of the thin film transistor in this state. In FIG. 6, a region 601 to be a source or drain region (generally, the drain region is preferentially used), a region 201 to be a light doping region (LDD region on the drain region side), and channel formation. Area 102 to be an area, gate insulating film 1
04, the gate electrode 105 is shown.

The gate insulating film 104 is formed on the region 201 to be the lightly doped region, but is not formed on the region 601 to be the source or drain region. This is performed in the state shown in FIG.
This is because in the step of implanting (boron) ions, the region 201 is lightly doped by utilizing the thickness of the gate insulating film (silicon oxide film).

Next, a brief description of the doping apparatus will be given. FIG. 5 shows a schematic view of the doping apparatus. Plasma is generated in the plasma source 501 at the top of FIG. 5, and the ions generated therein are accelerated in the ion acceleration region under the plasma source by applying a voltage to the acceleration region. The plasma source is obtained by ionizing (ionizing) a gas containing a dopant element by high frequency discharge.

As shown in the figure, three ion acceleration regions are provided, and a voltage is applied. In the figure, reference numeral 504 is for applying a deceleration voltage, reference numeral 503 is for applying an acceleration voltage, and reference numeral 502 is for applying an extraction voltage. . When actually doping, voltage is applied in order from the bottom.

The actual doping method is as follows. In this embodiment, B (boron) is used as the dopant. Then, diborane gas (B ₂ H ₆ ) diluted with 5% is used as a source gas of boron. First, diborane gas is injected into the plasma source of the doping apparatus. Then, a thin film transistor (thin film transistor in the state shown in FIGS. 2 and 6) in the process of manufacturing before being doped is set on the substrate stage 505 of FIG. Then, the ion acceleration region is evacuated to inject the diborane gas from the plasma source into the acceleration region, and a deceleration voltage of -1 kV is applied. Next, the acceleration voltage of 27 kV is immediately applied and the state is maintained for 5 seconds. Finally, the extraction voltage of 3kV is applied in analog for 1 second.

By the above process, B ions are implanted with a profile shown in FIG. 4 from the surface of the gate insulating film (silicon oxide film in this embodiment) to the silicon film. FIG. 4 shows the depth from the surface of the silicon oxide film in a laminated film of a 1200 Å-thick crystalline silicon film formed on a glass substrate and a 1200 Å-thick silicon oxide film formed thereon. It is the result of measuring the concentration distribution of boron in the direction by SIMS (secondary ion analysis method).

Looking at the profile from depth 1200 Å to 1700 Å in FIG. 4 (corresponding to a depth of 500 Å from the surface of the silicon film), the dopant enters at a concentration of 1 × 10 ^{17 to} 3 × 10 ¹⁷ cm ^-3. You can see that this is,
It is a profile by BH _x (X is an integer of 1 or more) ion. This means that the concentration of the boron element near the surface of the silicon film can be set to a concentration of 1 × 10 ^{17 to} 3 × 10 ¹⁷ cm ⁻³ .

On the other hand, looking at the profile from the surface to 1200 Å (corresponding to the concentration profile in the film thickness direction of the silicon oxide film), a large mountain-shaped profile 401 is shown, and the maximum concentration is 2 × 10 cm ^-3 . You can see that it contains a dopant. This is a profile due to B ₂ H _y (y is an integer of 1 or more) ions.

The boron concentration distribution in the depth direction shown in FIG. 4 is peculiar to the case of implanting B ions using the plasma doping method. Generally, when a single ion is accelerated and injected into a thin film, the concentration distribution in the depth direction becomes a Gaussian distribution. Therefore, the second small peak as shown at 402 in FIG. 4 is not observed.

FIG. 4 shows a first mountain-shaped peak indicated by 401 and a second small mountain-shaped peak indicated by 402. The peak indicated by 401 in FIG. 4 is
It is due to B ₂ Hy ions. The peak indicated by 402 is due to BHx ions.

As shown in FIG. 4, two peaks appear in the two concentration distributions in the depth direction because the diborane represented by B ₂ H ₆ is plasmatized (ionized).
This is because the ions represented by BHx and the ions represented by B ₂ Hy are generated. And from the ion shown by BHx
Since the proportion of the ions represented by B ₂ Hy existing is larger, a larger peak 401 and a smaller peak 402 are formed as shown in FIG. 4. Also, B ₂ Hy
Since the ion indicated by (4) has a larger mass and ionic radius, it will be more implanted on the surface side as indicated by 401.

In the present embodiment, it is indicated by 403 between the first high concentration peak indicated by 401 (this peak is present in the silicon oxide film) and the low concentration peak indicated by 402. It is characterized in that the lightly doped region is formed by using the relatively gentle concentration distribution in the depth direction in the vicinity. In the vicinity indicated by 403, the concentration distribution in the depth direction is gentle, so
Doping can be performed with extremely good controllability by controlling the concentration to about 10 ¹⁷ atom cm ⁻³ .

The concentration distribution in the thickness direction having two peaks as shown in FIG. 4 is obtained by using a plasma doping method using diborane gas as a source gas, and ions to be doped by mass separation are used. Is a peculiar phenomenon not seen in the method of generating.

Further, the boron element is present at a high concentration of 5 × 10 ¹⁹ atom cm ⁻³ or more in the portion where the peak is indicated by 401, but since this portion is in the gate insulating film, the dopant is used. However, no matter how much it enters, it does not affect the characteristics of the TFT at all.

Incidentally, FIG. 6 corresponding to the outside of the gate insulating film.
The region indicated by 601 is also doped with boron at a high concentration due to the ion represented by B ₂ Hy, but since this region is a region that becomes a source or drain region, a slightly larger amount of dopant may not enter. No problem at all.

As described above, boron ions are implanted through the silicon oxide film 104 by the plasma doping method using diborane as a source gas, so that the boron ion is introduced into the region indicated by 201 at about 10 ¹⁷ atom cm ^−3. It is possible to dope with good controllability at a certain concentration.

As is apparent from FIG. 3, this boron ion concentration is the optimum concentration for forming the lightly doped region 201 formed between the channel forming region 102 and the source or drain region 601. . When the conditions shown in this embodiment are adopted, the boron concentration near the surface of the region 201 is 1 × 10 ^{16 to} 3 × 10 ¹⁶ atoms c.
It becomes m ^-3 . As is clear from FIG. 3, this concentration range is a range in which the ratio of the ON current and the OFF current can be increased, and is a range in which a TFT having extremely high performance can be obtained.

Particularly, the boron concentration in the vicinity of the surface of the light-doped region can be set to a concentration of 1 × 10 to ³ × 10 ¹⁷ cm ⁻³ , and as is clear from FIG. 3, the ON / OFF ratio is large and the characteristics are extremely high. It is possible to obtain an excellent TFT.

[Embodiment 2] In this embodiment, in the step of implanting boron ions shown in Embodiment 1, the extraction voltage applied by 502 is 0 V, the acceleration voltage applied by 503 is 76 kV, and the power amount is 5 W. In this example, the irradiation time is 6 seconds and the ion implantation is performed.

When the extraction voltage is set to 0 V or less, the irradiation number of ions per unit area and unit time can be reduced as compared with the case where a positive voltage is applied. In the present embodiment, the extraction voltage was set to 0 V, but by setting it to be lower than that, it becomes possible to control the dopant of lower density.

[Embodiment 3] In the present embodiment, in the configuration shown in Embodiment 1, a hydrogen cylinder is connected in parallel to a cylinder containing a dopant connected to a plasma source of a doping apparatus to adjust the concentration of the dopant. This is an example of controlling the low density of the dopant.

By controlling the concentration of the hydrogen gas that dilutes the source gas (boron in this case) of the dopant (diborane in this case), the density of the dopant when it is turned into plasma can be controlled. Utilizing this fact, the concentration of the impurity ions implanted in the film can be controlled to some extent.

[Effects of the Invention] By adopting the doping method utilizing the invention disclosed in the present specification, it is possible to implant impurity ions into the lightly doped region of a thin film transistor (TFT) in an optimum concentration range. Yes, TFT ON
Turn OFF current to ON current 6 without reducing the current.
It is possible to reduce the power to 1 or less.

Such a TFT has an optimum characteristic especially for being arranged in a pixel electrode of an active matrix type liquid crystal display device, and is extremely useful.

INDUSTRIAL APPLICABILITY The invention disclosed in the present specification can be effectively utilized when implanting a specific type of ions into a specific material at a predetermined concentration. That is, TF as described above
It can be used not only when forming a lightly doped region of T 2 but also when it is desired to perform ion implantation with adjustment to a predetermined concentration.

[Brief description of drawings]

FIG. 1 shows a schematic configuration of a conventionally known thin film transistor.

FIG. 2 shows a schematic structure of a thin film transistor having a lightly doped region.

FIG. 3 shows the relationship between the ratio of ON current and OFF current of a thin film transistor and the density of boron ions implanted in a lightly doped region.

FIG. 4 shows a concentration distribution in a depth direction when boron ions are implanted into a stack of a silicon film and a silicon oxide film.

FIG. 5 shows a schematic configuration of a plasma doping apparatus.

FIG. 6 shows a schematic state of a thin film transistor during a manufacturing process in a state where boron ions are implanted.

[Explanation of symbols]

 100 substrate (glass substrate or quartz substrate) 101 source region 102 channel formation region 103 drain region 104 gate insulating film 105 gate electrode 201 light doped region 601 source or drain region

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication H01L 21/265 L 9056-4M 29/78 616 L

Claims (13)

[Claims] 1. A method for accelerating and implanting a plurality of types of ions having different masses and / or ionic radii into a predetermined material, wherein the plurality of types of ions are composed of different numbers of one type of element. , Using a concentration distribution in the depth direction of the material formed according to each of the plurality of types of ions, doping a predetermined region in the depth direction of the material with the one type of element at a predetermined concentration A doping method characterized by the above. 2. The doping method according to claim 1, wherein boron is used as one kind of element. 3. The boron according to claim 1, wherein boron is used as one kind of element, the predetermined material has a multi-layer structure of a silicon film and a silicon oxide film, and plural kinds of ions are BH _x (x is 1 or more). Integer) and B ₂
H _y (y is an integer of 1 or more), the ions represented by BH _x are mainly doped in the silicon film, and the ions represented by B ₂ H _y are mainly doped in the silicon oxide film. A doping method characterized by the above. 4. The doping method according to claim 3, wherein the boron element is doped in the silicon film at a concentration of 1 × 10 ¹⁶ atom cm ^{−3 to} 3 × 10 ¹⁷ atom cm ⁻³ . 5. The doping method according to claim 3, wherein the boron element is doped in the silicon film at a concentration of 1 × 10 ¹⁶ atom cm ^{−3 to} 5 × 10 ¹⁷ atom cm ⁻³ . 6. A method for manufacturing an insulating gate type field effect semiconductor device having a lightly doped region between a channel forming region and a drain region, wherein an insulating film is formed on the lightly doped region. Implanting impurity ions imparting one conductivity type, the impurity ions having at least two kinds of ions having different masses and / or ionic radii, and one of the ions is mainly the light. A method for manufacturing a semiconductor device, characterized by being injected into a doped region. 7. The method according to claim 6, wherein the two kinds of ions are BH _x (x is an integer of 1 or more) and B ₂ H.
_A method for manufacturing a semiconductor device, which is represented by _y (y is an integer of 1 or more), in which a boron element is implanted into the lightly doped region. 8. The semiconductor device according to claim 7, wherein the lightly doped region is doped with a boron element at a concentration of 1 × 10 ¹⁶ atom cm ^{−3 to} 3 × 10 ¹⁷ atom cm ^−3. Of manufacturing. 9. The semiconductor device according to claim 7, wherein the lightly doped region is doped with a boron element at a concentration of 1 × 10 ¹⁶ atom cm ^{−3 to} 5 × 10 ¹⁷ atom cm ^−3. Of manufacturing. 10. A stack comprising at least a first thin-film material doped with an impurity element and a second thin-film material formed on the thin-film material, the mass comprising a predetermined dopant element. And / or a step of irradiating a plurality of types of ions having different ionic radii, wherein peaks of the concentration of the dopant element are present in the first thin film material and the second thin film material, respectively. A characteristic doping method. 11. At least a first thin film material doped with an impurity element and a second thin film material formed on the thin film material, wherein the first thin film has a thickness direction in the first thin film material. In the second thin film, there is a region where the rate of change of the dopant density distribution is 0 in the thickness direction of the second thin film. 12. An insulating gate type field effect semiconductor device having a lightly doped region between a channel forming region and a drain region, wherein an insulating film is formed on the lightly doped region. And the lightly doped region has a peak of a dopant density distribution in the thickness direction thereof. 13. An insulating gate type field effect semiconductor device having a lightly doped region between a channel forming region and a drain region, wherein an insulating film is formed on the lightly doped region. Has a peak of the density distribution of the dopant in the thickness direction thereof, and a region in which the rate of change of the density distribution of the dopant is 0 exists in the light doping region. Characteristic semiconductor device.