JP3413710B2 - 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 method for manufacturing a top gate type thin film transistor.

[0002]

2. Description of the Related Art As a thin display device, a liquid crystal display device using a thin film transistor as a pixel switch is rapidly prevailing as a monitor for OA, TV and the like. The thin film transistors used in these liquid crystal display devices are formed by using a semiconductor thin film made of amorphous silicon that can be formed on an insulating substrate having a large area at a low temperature without deforming the insulating substrate. Was there. However, since such a thin film transistor has a low driving capability, not only is a large element area required, but it cannot be used for anything other than a pixel switch. Therefore, in the liquid crystal display device using the thin film transistor as a pixel switch, there are problems that a light transmission area becomes small and a driver needs to be separately provided in an IC shape, resulting in an increase in cost.

Therefore, a technique for polycrystallizing a semiconductor thin film made of amorphous silicon by laser annealing has attracted attention. A thin film transistor using this technique is manufactured as follows. First, in the first manufacturing method, an insulating film is formed on a semiconductor thin film made of amorphous silicon, and laser annealing is performed on the insulating film to polycrystallize the semiconductor thin film. Next, after peeling off the insulating film, a new gate insulating film is formed on the semiconductor thin film by the CVD method, and a gate electrode is formed on the gate insulating film. Thereafter, by ion implantation using the gate electrode as a mask, impurities for forming source / drain are introduced into the semiconductor thin film by self-alignment.

According to the above-mentioned first manufacturing method, the semiconductor thin film is polycrystallized by laser annealing from above the insulating film, so that the surface of the semiconductor thin film is prevented from being roughened by the irradiation of the laser beam, and at the same time the laser is irradiated. The reflection of light is prevented. Further, since the semiconductor thin film is polycrystallized, a thin film transistor having a high driving ability can be obtained as compared with the case where a semiconductor thin film made of amorphous silicon is used.

In the second manufacturing method, a semiconductor thin film made of amorphous silicon is directly laser-annealed to polycrystallize the semiconductor thin film, and then a gate insulating film is formed on the semiconductor thin film by a CVD method. A film is formed and a gate electrode is formed on this gate insulating film. Then, the first
In the same manner as the above method, impurities for forming source / drain are introduced into the semiconductor thin film.

According to the second manufacturing method, the semiconductor thin film is roughened by the laser annealing. However, when a large amount of hydrogen remains in the semiconductor thin film, there is an advantage that the hydrogen is easily desorbed during laser annealing.

[0007]

However, since the gate insulating film is formed by the CVD method in the above-described two thin film transistor manufacturing methods, the gate insulating film is formed by thermal oxidation as compared with the case where the gate insulating film is formed by thermal oxidation. The quality of the insulating film is inferior and the amount of fixed charges in the film increases. Further, in the thin film transistor having such a gate insulating film, the flat band voltage becomes large, and thus the bias of the threshold voltage becomes large. Then, this is a factor that prevents a circuit from being formed by a thin film transistor formed by applying a technique of polycrystallizing a semiconductor thin film made of amorphous silicon by laser annealing. Therefore,
In a liquid crystal display device in which it is necessary to form a semiconductor thin film on a large-area insulating substrate, it is difficult to use a thin film transistor as a component (eg driver) other than the pixel switch.

[0008]

The present invention is a method of manufacturing a thin film transistor, which has been made to solve the above problems. That is, in the manufacturing method of the present invention, first, an insulating film is formed on an amorphous or microcrystalline semiconductor thin film, and then the semiconductor thin film is polycrystallized by laser annealing. Next, after forming a gate electrode on the insulating film, impurities for forming a source / drain are introduced into the semiconductor thin film by ion implantation using the gate electrode as a mask.

In the present invention, the gate electrode may be formed on the insulating film after the surface layer of the insulating film on the polycrystallized semiconductor thin film is removed by etching.

In the above manufacturing method, when the semiconductor thin film is polycrystallized by laser annealing, the heat of the semiconductor thin film is transferred to the insulating film on the semiconductor thin film. Then, in the insulating film portion on the interface side with the semiconductor thin film, heat is transferred from the semiconductor thin film to repair defects in the insulating film.
After that, a gate electrode is formed on this insulating film, and a part of the insulating film is used as it is as a gate insulating film.
Therefore, in the gate insulating film, the defects on the interface side with the semiconductor thin film are repaired and the fixed charges are reduced. Therefore, a thin film transistor having a small flat band voltage and a small deviation of the threshold voltage is formed.

When the surface layer of the insulating film is removed by etching, only the surface layer of the insulating film that is contaminated during laser annealing is removed while leaving the interface with the semiconductor thin film in which the defect is repaired. To be done. Therefore, the surface of the insulating film exposed after the etching is cleaned, and the gate electrode is formed above the surface of the cleaned insulating film.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments to which the method for manufacturing a thin film transistor according to the present invention is applied will be described below with reference to FIGS.
It will be described based on.

This manufacturing method is a method for manufacturing a so-called top gate type thin film transistor. First, Fig. 1
As shown in (a), an insulating substrate 11 is formed by laminating a silicon nitride film with a thickness of about 150 nm and a silicon oxide film with a thickness of about 200 nm on the surface of a glass substrate by plasma CVD. prepare. The silicon nitride film and the silicon oxide film serve as barrier layers.
Then, a semiconductor thin film 12 made of amorphous silicon and having a film thickness of about 40 nm is formed on the insulating substrate 11 by the plasma CVD method. The semiconductor thin film 12 may be amorphous silicon containing crystalline material or microcrystalline silicon.

Next, a dehydrogenation annealing process is performed at 400 ° C. for about 1 hour in a nitrogen atmosphere, and then the semiconductor thin film 12 is patterned into an island shape if necessary. Then, an insulating film 13 made of silicon oxide and having a film thickness of about 50 nm is formed on the insulating substrate 11 by plasma CVD so as to cover the semiconductor thin film 12.

Next, as shown in FIG. 1B, the insulating film 1
The semiconductor thin film 12 is removed by laser annealing from above.
Polycrystal. At this time, using an excimer laser as a light source,
Irradiation energy density of polycrystal of amorphous silicon
And perform the above laser annealing.
The irradiation energy density here is 350 mJ / cm. ²
Set to a degree.

By performing the laser annealing as described above, the semiconductor thin film 12 is heated and polycrystallized, and the heat of the semiconductor thin film 12 is transferred to the insulating film 13. Then, the defect of the insulating film 13 is repaired at least in the portion on the interface side with the semiconductor thin film 12. In addition, the insulating film 13
Becomes a capping insulating film of the semiconductor thin film 12, so that the surface of the semiconductor thin film 12 is prevented from being roughened by laser annealing.

When the semiconductor thin film 12 is made of microcrystalline silicon, the laser annealing causes the crystals of the semiconductor thin film 12 to grow. Then, the semiconductor thin film 12
Becomes polycrystal. At this time, the defects of the insulating film 13 are repaired and the surface of the semiconductor thin film 12 is prevented from being roughened in the same manner as described above.

After that, the surface layer of the insulating film 13 is removed by wet etching with hydrofluoric acid (0.3%) for 30 seconds. Here, only the surface layer of the insulating film 13 contaminated during the laser annealing is removed, and the lower layer portion of the insulating film 13, that is, the portion of the insulating film 13 in which the defect is repaired during the laser annealing is left. To

Next, as shown in FIG. 1C, a silicon oxide film 14 having a thickness of about 50 nm is formed on the insulating film 13 by the plasma CVD method, and 5 is formed on the upper surface.
A silicon nitride film 15 having a film thickness of about 0 nm is continuously formed. As a result, the lower insulating film 1 having a three-layer structure including the insulating film 13, the silicon oxide film 14, and the silicon nitride film 15 is formed.
6 is formed. A part of the lower insulating film 16 serves as a gate insulating film.

If the insulating film 13 alone is sufficient as a gate insulating film, it is not necessary to form the silicon oxide film 14 and the silicon nitride film 15 on the insulating film 13.

Next, as shown in FIG. 1D, the gate electrode 17 is formed on the lower insulating film 16 on the semiconductor thin film 12 (on the insulating film 13 when only the insulating film 13 is provided). Here, the lower insulating film 1 is formed by the sputtering film forming method.
A molybdenum (Mo) film having a thickness of about 200 nm is formed on the insulating film 6 (insulating film 13), and the Mo film is patterned to form the gate electrode 17 made of Mo.

Next, the gate electrode 17 and the illustration here are shown.
Ion implantation using a non-resist pattern as a mask
Thus, impurities are introduced into the semiconductor thin film 12. Guide here
Impurities that enter may be, for example, N-channel transistors.
In the semiconductor thin film 12 in the formed region, PH₃ ⁺Ion
10 with injection energy of 140 keV¹⁵Pieces / cm ²
Introduce a degree. Also, a P-channel transistor is formed
B in the semiconductor thin film 12 in the region₂H₆ ⁺Ion
8 × 10 with implantation energy of about 30 keV¹⁴Pieces / cm
²Introduce a degree.

Then, activation annealing of the impurities is performed using an excimer laser. At this time, the irradiation energy density of the excimer laser is set to about 260 mJ / cm ² . As described above, the source 12a and the drain 12b are formed in the semiconductor thin film 12 portions on both sides of the gate electrode 17.

Next, as shown in FIG. 1E, the lower insulating film 1 is formed by the CVD method while covering the gate electrode 17.
A silicon oxide film 18 having a film thickness of about 100 nm is formed on 6 (insulating film 13), and a silicon nitride film 19 having a film thickness of about 200 nm is further formed on the upper surface thereof. Thereby, the upper insulating film 20 including the silicon oxide film 18 and the silicon nitride film 19 is formed.

Thereafter, the film is further exposed to hydrogen plasma for 30 minutes and then furnace-annealed at 350 ° C. for about 1 hour to supply hydrogen to the polycrystallized semiconductor thin film 12 so that the gap between the crystals of the semiconductor thin film 12 is increased. Stabilize.

Next, the upper insulating film 20 and the lower insulating film 16
A contact hole 21a reaching the source 12a and the drain 12b is formed in the gate electrode 17 and the gate electrode 17 is formed in the upper insulating film 20.
To form a contact hole 21b.

Thereafter, by sputtering, an aluminum film is formed on the upper insulating film 20 in a state of filling the contact holes 21a and 21b, and the aluminum film is patterned to form the sources 12a,
An aluminum wiring 22 connected to the drain 12b and the gate electrode 17 is formed. After that, sintering is performed by performing heat treatment at 300 ° C. for 1 hour to complete the thin film transistor 1.

In the above manufacturing method, the insulating film 13 in which defects are repaired when the semiconductor thin film 12 is polycrystallized by laser annealing is used as it is as the gate insulating film 13. Moreover, the silicon oxide film 14 is laminated on the surface of the insulating film 13 cleaned by the etching of the surface layer, or the gate electrode 17 is directly formed on the insulating film 13, so that the defect is repaired. The surface of the insulating film 13 can be cleaned while leaving the above. From the above, a thin film transistor having a small flat band voltage and a small deviation of the threshold voltage is formed.

FIG. 2 is a graph showing current-potential characteristics in an N-channel thin film transistor. As shown in this graph, as compared with the thin film transistor manufactured by the conventional method, in the thin film transistor manufactured by the method of the above-described embodiment, the threshold voltage shifts to the enhancement type by about 1 V and the deviation thereof is reduced. Was confirmed.

FIG. 3 is a graph showing current-potential characteristics in a P-channel thin film transistor. As shown in this graph, as compared with the thin film transistor manufactured by the conventional method, in the thin film transistor manufactured by the procedure of the above embodiment, the threshold voltage shifts to the depletion type by about 1 V and the deviation thereof is reduced. Was confirmed.

Next, FIG. 4 shows a manufacturing process diagram (2) of a thin film transistor to which the present invention is applied, as a modified example of the above embodiment. In the manufacturing procedure shown in FIG.
The insulating film 13 is formed on the semiconductor thin film 12 as shown in FIG. 4A, and the semiconductor thin film 12 is polycrystallized by laser annealing from the insulating film 13 as shown in FIG. As shown in FIG. 4C, the semiconductor thin film 12 together with the insulating film 13 is patterned into an island shape as needed. When the thin film transistor is manufactured by such a procedure, the surface layer of the insulating film 13 is removed by etching after the semiconductor thin film 12 is patterned into an island shape. And, FIG. 4 (d), (e)
The process shown in (1) is performed in the same manner as described with reference to FIGS. 1D and 1E in the above embodiment. Thereby, the thin film transistor 1 is completed.

Although not shown in the drawings, in addition to the above modification, a patterned insulating film is formed on the semiconductor thin film, and the semiconductor thin film is polycrystallized by laser annealing from the insulating film. The thin film may be patterned. When a thin film transistor is manufactured by such a procedure, the surface layer of the insulating film is removed by etching after the semiconductor thin film is patterned. Then, the subsequent steps are performed in the same manner as in the above embodiment, thereby completing the thin film transistor.

The same operational effects as those of the above-mentioned embodiment can be obtained by the above-mentioned respective procedures.

[0034]

As described above, according to the method of manufacturing a thin film transistor of the present invention, when the amorphous or microcrystalline semiconductor thin film is polycrystallized by laser annealing, the insulating film capped on the semiconductor thin film. By using as a gate insulating film as it is, it becomes possible to repair defects in the gate insulating film by the heat transmitted from the semiconductor thin film during the laser annealing and to reduce fixed charges. Therefore, the deviation of the threshold voltage of the thin film transistor formed by applying the technique of polycrystallizing the semiconductor thin film made of amorphous silicon by laser annealing can be reduced. Then, for example, in a liquid crystal display device in which it is necessary to form a semiconductor thin film on an insulating substrate having a large area, the thin film transistor can be used as an element constituting a driver other than a pixel switch.

[Brief description of drawings]

FIG. 1 is a manufacturing process diagram (No. 1) for explaining an embodiment when the present invention is applied to a method for manufacturing a top-gate thin film transistor.

FIG. 2 is a graph showing current-potential characteristics of an N-channel thin film transistor.

FIG. 3 is a graph showing current-potential characteristics of a P-channel thin film transistor.

FIG. 4 is a manufacturing process diagram (2) for explaining the embodiment of the invention.

[Explanation of symbols]

1 Thin Film Transistor 12 Semiconductor Thin Film 12a
Source 12b Drain 13 Insulating film 17 Gate electrode

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ identification code FI H01L 29/78 617U (58) Fields investigated (Int.Cl. ⁷ , DB name) H01L 21/336 H01L 21/265 H01L 21 / 268 H01L 29/786

Claims (2)

(57) [Claims] 1. A first step of polycrystallizing the semiconductor thin film by performing laser annealing after forming an insulating film on an amorphous or microcrystalline semiconductor thin film, and a surface layer of the insulating film. Second step of forming a gate electrode on the insulating film after etching removal, and ion implantation using the gate electrode as a mask to form a source and a drain in the semiconductor thin film under both sides of the gate electrode. And a third step of introducing an impurity for achieving the above. 2. The manufacture of the thin film transistor according to claim 1.
In the method Prior to the first step, silicon nitride was formed on the surface of the glass substrate.
Insulating substrate formed by stacking a silicon film and a silicon oxide film
And forming the semiconductor thin film on the insulating substrate.
Of thin film transistor characterized by carrying out the steps
Method.