JP3426255B2 - Active matrix substrate - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the structure of a thin film transistor used in a liquid crystal display.

[0002]

2. Description of the Related Art A conventional thin film transistor is, for example, "1".
989 International Electron
Device Meeting (IEDM) Tec
h-nical Digest, p. p. 157-16
0, H. Ohshimaet. al. , ”
Basically, the structure is as shown in FIG.
1 or a silicon thin film 10 including a drain electrode 102, a source electrode 103, and a channel region 104 on an insulating film
5, a gate insulating film 106 covering the silicon thin film 105, a gate electrode 107 facing the channel region 104 through the gate insulating film 106, an interlayer insulating film 108 covering these.
Then, a drain electrode wiring 109 and a source electrode wiring 110 which are respectively connected to the drain electrode 102 and the source electrode 103 of the silicon thin film 105 through contact holes formed in the gate insulating film 106 and the interlayer insulating film 108 are formed.

In such a thin film transistor, the silicon thin film 105 is roughly classified into amorphous silicon and polycrystalline silicon, and polycrystalline silicon is preferable in terms of performance such as carrier mobility.

Several techniques are used to improve the performance of polycrystalline silicon thin film transistors, that is, increase the ON current by increasing the mobility of carriers in semiconductor silicon. For example, by irradiating a silicon thin film with high-energy laser light to promote crystallization of silicon, or by melting and recrystallizing it to reduce carrier traps, mobility can be improved compared to the case without laser light irradiation. A very large and high-performance thin film transistor can be obtained. In addition, passivation of traps in silicon by hydrogen plasma technology and so-called solid phase growth method in which heat treatment is performed for a long time are also effective for increasing mobility of thin film transistors.

[0005]

By improving the performance of the thin film transistor by the above method, a larger current (ON current) can be passed through the thin film transistor. However, along with this, deterioration of the thin film transistor itself due to a current flowing through the thin film transistor becomes a problem. One of the problems is deterioration due to injection of so-called hot electrons into the gate insulating film, and the other is deterioration due to heat generation of the thin film transistor itself. The former has been improved by adopting a so-called LDD structure in which the impurity concentration in the portion of the drain of the thin film transistor near the gate electrode is reduced.

In a transistor formed on a silicon substrate, the heat conductivity of the silicon substrate is relatively large, and the heat of each transistor is immediately diffused into the substrate. On the other hand, in a thin film transistor used for an active matrix type liquid crystal display or the like, since the thin film transistor is formed on a substrate having a low thermal conductivity such as glass, the heat generated by each thin film transistor is difficult to diffuse. In such a case, when the current flowing through the thin film transistor increases and the heat generation increases as described above, the crystal state of the silicon thin film forming the thin film transistor changes due to the heat and a new defect occurs, or the hydrogen plasma technology Due to the re-desorption of hydrogen in the silicon thin film introduced by the method, carrier traps increase, the characteristics of the thin film transistor change, and the initial characteristics as designed cannot be maintained for a long time, resulting in lack of reliability.

[0007]

An active matrix substrate of the present invention is formed on an insulating substrate, has a thermal conductivity higher than that of the insulating substrate, and is made of a metal serving as a wiring layer. And a thin film transistor having an LDD structure in which hydrogen is injected into silicon by hydrogen plasma treatment. Further, the active matrix substrate of the present invention is formed on an insulating film formed on an insulating substrate, has a thermal conductivity higher than that of the insulating film, and is made of a metal that also serves as a wiring layer. A thin film transistor having an LDD structure, which is formed on a conductive layer via an insulating film and in which hydrogen is injected into silicon by hydrogen plasma treatment, is provided. Further, the heat conducting layer is characterized by forming a chromium layer having a thickness of about 2000 angstroms.

[0008]

EXAMPLE An example of the thin film transistor of the present invention will be specifically described with reference to FIG.

The glass substrate 201 may be the glass as it is, but it may be used after forming an insulating film such as SiO _{2 on} the surface of the glass. Here, the glass is used as it is. The heat conductive layer 2 is formed on the glass substrate 201.
As a No. 02, a Cr thin film of about 2000 Å is formed by the sputtering method.
Of SiO _{2 and} SiO ₂ as an insulating film 203 thereon.
A thin film with a thickness of about 2000Å is prepared by chemical vapor deposition (CVD
Method).

A silicon thin film 204 made of polycrystalline silicon is formed on this by a low pressure CVD method, and a laser beam is applied to this, followed by patterning by a photolithography technique.
An iO ₂ thin film was formed on the entire surface by the ECR plasma CVD method. Irradiation of the laser beam to the silicon thin film may be performed after the patterning of the silicon thin film 204. Further, amorphous silicon may be deposited as the silicon thin film 204 by a plasma CVD method and then crystallized by laser light irradiation. The gate insulating film 205 can also be formed by thermal oxidation of the silicon thin film 204.

After further depositing a silicon thin film, patterning was performed to form a gate electrode 206. The photoresist 207, which is a mask when patterning the silicon thin film 204, has a so-called LDD structure (Lightly Doped Dra) in which the thin film transistor has a low impurity concentration in the portion near the gate electrode of the drain electrode.
In structure), it will be necessary in a later step, so it is left. Next, the gate electrode 206 and the photoresist 207
With the mask as a mask, phosphorus ions or boron ions as impurities are introduced into the silicon thin film 204 as the first ion shower 2
No. 08, the source electrode 209 and the drain electrode 210 were formed in a high concentration by self-alignment.

After that, the gate electrode 206 is further etched by the plasma etching method. Since the photoresist 207 was left in the previous step, the gate electrode 206
Is thinned by etching only both ends. Here, the photoresist 207 is removed, and the second ion shower 211 is removed.
Then, the same ions as in the first ion shower 208 were implanted at a lower concentration than in the first ion shower 208. The LDD portion 212 is the portion where the ions of low concentration are implanted, and the channel region 213 is the portion under the gate electrode 206 where the ions are not implanted. Source electrode 209
There is also a region where ions are implanted at a low concentration on the side,
This is also called the LDD portion 212. After that, the ions implanted by heating or laser light irradiation were activated.

On top of this, an SiO ₂ film is formed as an interlayer insulating film 214.
Is deposited by a CVD method of about 5000 Å, contact holes are formed in the gate insulating film 205 and the interlayer insulating film 214 on the source electrode 209 and the drain electrode 210, and the source electrode wiring 21 is formed.
5 and the drain electrode wiring 216 were formed, and further exposed to hydrogen plasma to passivate the carrier trap in the silicon thin film 204 to complete the thin film transistor.

In the present embodiment, metal chromium (Cr) is used for the heat conducting layer between the thin film transistor and the insulating substrate or the insulating film, and this dissipates the heat emitted to the silicon thin film of the thin film transistor through the insulating film. However, the metal used for the heat conductive layer may not be Cr as long as it can withstand the heat in the subsequent steps. When the heat conducting layer is a metal, for example, when the thin film transistor of the present invention is applied to an active matrix substrate, the heat conducting layer also serves as some wiring layer or can be formed at least in the same layer. .

Further, when an insulator is used for the heat conducting layer, it is not necessary to interpose an insulating film between the thin film transistor and the thin film transistor as in the case of using a metal, so that heat can be more effectively dissipated and the process is simplified. Can be converted. As the insulator or a material close to the insulator that can be used for the heat conduction layer, for example, a ceramic thin film or a diamond thin film can be considered. In particular, the diamond thin film is transparent, and when the thin film transistor of the present invention is applied to a liquid crystal display or the like, it is not necessary to perform processing such as patterning, and the number of steps is not significantly increased compared with the conventional thin film transistor.

One example of the deterioration of the thin film transistor as described in the [Prior Art] and [Problems to be Solved by the Invention] is shown in FIG. Shown in. In a thin film transistor having a channel width of 10 μm and a channel length of 5 μm, a gate-source voltage (Vg
In the graph of (s) -drain source current (Ids), changes in the rising potential of the current and an overall decrease in the current value were observed. Particularly in a thin film transistor in which a carrier trap in silicon is passivated by hydrogen plasma technology, when the temperature rises to about 400 ° C. or more, hydrogen injected into silicon starts to be desorbed by hydrogen plasma, and the characteristics of the thin film transistor deteriorate. And the higher the temperature, the more severe the deterioration. In addition, phosphorus ions or boron ions implanted as impurities into the drain electrode or the source electrode of the thin film transistor,
Diffusion into the channel region due to heat generation in the channel region is also considered to be the cause of the characteristic change. As is conceivable from such circumstances, the thin film transistor of the present invention manufactured through the above-described steps is formed directly on a substrate having a small thermal conductivity such as glass or only through a SiO ₂ film. Compared with the conventional thin film transistor described above, since the ability to dissipate heat generated by the thin film transistor is high, deterioration of the characteristics is small.
Further conceivable as the effect of the heat conduction layer is the effect of suppressing the diffusion of harmful impurities from the glass substrate or the like due to heat. Although the diffusion of impurities can be suppressed by using a SiO ₂ thin film, etc., considering that the diffusion of impurities is faster at higher temperatures, the diffusion of impurities can be further delayed by providing a heat conduction layer, and the reliability of the thin film transistor for a long time can be improved. It is effective in securing.

[0017]

As described above, the thin film transistor for a liquid crystal display according to the present invention has a large heat dissipation ability against its own heat generation as compared with the conventional thin film transistor, and has improved performance so that a large current can flow. In particular, it contributes to the improvement of reliability in the thin film transistor for display.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a structure of a conventional thin film transistor.

FIG. 2 is an explanatory view showing a process and a structure of a thin film transistor of the invention.

FIG. 3 is a graph of gate-source potential (Vgs) -drain-source current (Ids) showing deterioration of characteristics of a conventional thin film transistor.

[Explanation of symbols]

101 insulating substrate 102 210 drain electrode 103 209 Source electrode 104 channel region 105 204 Silicon thin film 106 205 gate insulating film 107 206 Gate electrode 108 Interlayer insulating film 109 216 Drain electrode wiring 110 215 Source electrode wiring 201 glass substrate 202 heat conductive layer 203 insulating film 207 photoresist 208 First Ion Shower 211 Second ion shower 212 LDD part 213 channel region 214 Interlayer insulation film

Continued front page       (56) References JP-A-56-126956 (JP, A)                 JP 62-254459 (JP, A)                 JP 63-142851 (JP, A)                 JP 62-16509 (JP, A)                 JP-A-63-10516 (JP, A)                 JP-A-58-142566 (JP, A)                 JP-A-58-204570 (JP, A)                 JP-A-59-204275 (JP, A)                 JP 64-61062 (JP, A)                 Japanese Patent Laid-Open No. 3-4214 (JP, A)                 JP 61-51973 (JP, A)

Claims (3)

(57) [Claims] 1. A hydrogen conductive layer formed on an insulating substrate, having a thermal conductivity higher than that of the insulating substrate, and made of a metal which also serves as a wiring layer, and a hydrogen formed on the thermal conductive layer via an insulating film. An active matrix substrate, comprising: a thin film transistor having an LDD structure in which hydrogen is injected into silicon by plasma treatment. 2. A heat conductive layer formed on an insulating film formed on an insulating substrate, having a thermal conductivity higher than that of the insulating film, and made of a metal also serving as a wiring layer, and insulating on the heat conductive layer. An active matrix substrate comprising: a thin film transistor having an LDD structure, which is formed through a film and in which hydrogen is injected into silicon by hydrogen plasma treatment. 3. The active matrix substrate according to claim 1, wherein the heat conducting layer forms a chromium layer having a thickness of about 2000 angstroms.