JPH11145484A - Manufacture of thin-film transistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a thin-film transistor on which electric characteristics can be improved by reducing the grain boundary trap in a polycrystalline semiconductor thin film. SOLUTION: An SiO2 insulating film 2 of about 1,000 Å in thickness is formed on a substrate 1 as an insulated substrate, and a polycrystalline silicon film 3 is deposited thereon in a thickness of 400 Å using a CVD method (chemical vapor deposition) at a temperature of 610 deg.C, for example. Then, silicon ions (Si <+> ) of 1×10<14> /cm<2> in dosage is made into an amorphous state and is (low temperature annealed at 600 deg.C for 40 hours (low temperature annealing). Moreover, the annealing is stopped at the state, wherein crystallization of the polycrystalline silicon film 3 is not saturated. Then, the material is heated up at a temperature between the melting point temperature of amorphous silicon and the melting point temperature of single-crystal silicon by projecting an excimer laser of 220 mj/cm<2> , an amorphous part 3A is formed, it is changed into a single-crystal part 3B, and a polycrystalline silicon film 3 having large grain diameter of 0.02 μm is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a thin film transistor having a polycrystalline semiconductor thin film in an active layer.

[0002]

2. Description of the Related Art In recent years, amorphous silicon has been actively used in liquid crystal displays, LSIs and the like. As a device using this amorphous silicon, there is a thin film transistor (TFT).

A method of forming a semiconductor thin film for forming such a thin film transistor is disclosed in Japanese Patent Application No. 61-78.
A technique according to Japanese Patent Publication No. 120 is known. In this prior art, a thin film semiconductor layer on an insulating substrate is irradiated with a laser and melted, and then cooled and solidified to form a thin film single crystal. The particle size is made uniform.

More recently, liquid crystal displays, LS
For example, polycrystalline silicon thin films, which are expected to have high mobility, have been actively studied.

[0005]

However, when a thin film transistor is formed using a polycrystalline silicon thin film, traps are generated at grain boundaries, and the trap density is increased.

When the trap density (N _t ) increases, the following equation is used: S〜kT / q · ln10 · (1 + qN _t / C _ox ) (1) S: voltage swing value q: electric charge C _ox : insulating film capacity μ to μ ₀ · exp (−ΔE / kT) (2) μ: mobility ΔE: energy for overcoming the grain boundary μ ₀ ∝L (3) L: particle size ΔE∝N ^{_{t 2 ... (4) J r}} = (qN t kTπ) δv th n i / 2W R (V R) ... (5) (pn
Junction) J: current density N _i: the intrinsic carrier density V _R: with reverse bias mobility μ is small, a large swing of the voltage, also has a problem that the leakage current increases. In particular, the grain boundary portion of polycrystalline silicon can be regarded as fine amorphous, and it can be said that dangling bonds exist and the trap density is high.

The present invention has been made in view of such a conventional problem, and a polycrystalline semiconductor thin film having improved electrical characteristics by reducing the trap density of grain boundaries is used as an active layer. And a method of manufacturing a thin film transistor.

[0008]

(1) A step of forming a polycrystalline semiconductor thin film on an insulating substrate, and melting at least grain boundaries of crystals constituting the polycrystalline semiconductor thin film, The method is characterized by including a step of performing an annealing process on the polycrystalline semiconductor thin film without changing the thickness, and a process of forming a thin film transistor having the polycrystalline semiconductor thin film subjected to the annealing process in an active layer.

Further, the polycrystalline semiconductor thin film is a thin film deposited by a chemical vapor deposition method,
The polycrystalline semiconductor thin film forms an amorphous semiconductor thin film on the insulating substrate, and anneals the amorphous semiconductor thin film to such an extent that crystallization is not saturated, leaving an amorphous portion in part. It is a polycrystalline semiconductor thin film including a crystallized region, and the annealing step is a step of melting the amorphous portion.

(2) The grain boundaries and minute defects of the polycrystalline semiconductor thin film are formed by heat treatment at a temperature higher than the melting point of the semiconductor in an amorphous state and lower than the melting point of the semiconductor in a single crystal state. Only the boundaries are melted, the unpaired atomic layer is reduced, the trap density is reduced, and the electrical properties of the thin film are improved.
Therefore, the thickness of the polycrystalline semiconductor thin film does not change and flatness can be maintained.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for manufacturing a thin film transistor according to the present invention will be described below with reference to the drawings.

(First Embodiment) First, as shown in FIG. 1, a substrate 1 made of, for example, heat-resistant glass is used as an insulating base.
An SiO ₂ insulating film 2 having a thickness of about 1000 ° is formed thereon, and a polycrystalline silicon film 3 is further formed on the SiO ₂ insulating film 2 by, for example, a CVD method (vapor phase growth method) at a temperature of 610 ° C.
To a thickness of 400 °.

Next, the polycrystalline silicon film 3 is irradiated with an excimer laser at a temperature of about 1300.degree.
Irradiation at mJ / cm ² melts only the grain boundaries of polycrystalline silicon. The thickness of the polycrystalline silicon film 3 after the irradiation with the excimer laser does not change at 400 °.

The irradiation temperature of the excimer laser is:
It can be set within a range higher than the melting point temperature of amorphous silicon (1100 ° C.) and lower than the melting point temperature of single crystal silicon (1415 ° C.).

(Second Embodiment) In the present embodiment, first, as in the first embodiment, SiO _{2 is}
After sequentially forming an insulating film 2 and a polycrystalline silicon film 3, FIG.
As shown in (a), the dose of silicon ion (S _i ⁺ ) is 1
Ion implantation is performed at × 10 ¹⁴ / cm ² to form a polycrystalline silicon film 3.
Is made amorphous, and annealing (low-temperature annealing) is performed at 600 ° C. for 40 hours. This annealing is stopped in a state where the crystallization of the polycrystalline silicon film 3 is not saturated.

FIG. 2B shows a cross-sectional state in which the polycrystalline silicon film 3 has been changed into an amorphous portion 3A which has been made amorphous by the above process and a single crystal portion 3B which has been crystallized. That plane.

Next, as shown in FIG. 2C, an excimer laser is irradiated at 220 mJ / cm ² , and the melting point temperature of the amorphous silicon and the melting point temperature of the single crystal silicon are changed in the same manner as in the first embodiment. By heating at an intermediate temperature, the amorphous portion 3
A is melted and changed to a single crystal part 3B, and the grain size is large (~
0.02 μm) A polycrystalline silicon film 3 is formed. Also in this embodiment, the thickness of the polycrystalline silicon film 3 did not change.

Next, a channel length of 1 μm and a channel width of 1
FIGS. 4 to 7 show the characteristics of a thin film transistor (TFT) having a thickness of 0 μm and a thin film transistor having a similar structure formed using a silicon thin film in which an excimer laser irradiation step is omitted.
Compare based on the graph.

The graph shown in FIG. 4 shows the relationship between the gate voltage and the drain current of the thin film transistor irradiated with the excimer laser. The graph shown in FIG. By comparison, the swing is large and the rising characteristics are good.

FIGS. 6 and 7 are graphs showing the relationship between the electron mobility and the gate voltage. The electron mobility of the thin film transistor irradiated with the excimer laser shown in FIG. 6 is shown in FIG. The electron mobility of the thin film transistor not subjected to laser irradiation is significantly improved.

As can be seen from the comparison of the characteristics described above, it is considered that the amorphous portion serving as the grain boundary is melted by the irradiation of the excimer laser, the trap density is reduced, and various characteristics are improved.

The embodiments have been described above.
The present invention is capable of various other modifications.

For example, in each of the above embodiments, an excimer laser is used as the heating means. However, any other heating means can be used as long as the heating temperature is equal to or higher than the melting point of amorphous silicon and lower than the melting point of single crystal silicon. May be used.

[0024]

As is apparent from the above description, the method of manufacturing a thin film transistor according to the present invention has the effect of reducing grain boundary traps in a polycrystalline semiconductor thin film and improving electrical characteristics.

Further, since only the vicinity of the grain boundary is melted,
This has the effect of maintaining the flatness of the thin film. Further, since the basic crystal grain size is maintained, there is an effect of improving the controllability of the grain size.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a first embodiment of a method for manufacturing a thin film transistor according to the present invention.

FIG. 2 is a sectional view of a second embodiment of the present invention.

FIG. 3 is a plan view of a second embodiment of the present invention.

FIG. 4 is a graph showing a swing of a transistor using a thin film irradiated with an excimer laser.

FIG. 5 is a graph showing a swing of a transistor using a thin film which is not irradiated with an excimer laser.

FIG. 6 is a graph showing mobility of a transistor using a thin film irradiated with an excimer laser.

FIG. 7 is a graph showing the mobility of a transistor using a thin film which is not irradiated with an excimer laser.

[Explanation of symbols]

3 ... polycrystalline silicon film, 3A ... amorphous part, 3B ... single crystal part.

Claims (4)

[Claims] 1. A step of forming a polycrystalline semiconductor thin film on an insulating substrate, and an annealing treatment for melting at least grain boundaries of crystals constituting the polycrystalline semiconductor thin film while not changing the thickness of the polycrystalline semiconductor thin film To the polycrystalline semiconductor thin film, and a step of forming a thin film transistor having the annealed polycrystalline semiconductor thin film in an active layer. 2. The method according to claim 1, wherein the polycrystalline semiconductor thin film is a thin film deposited by a chemical vapor deposition method. 3. The polycrystalline semiconductor thin film is partially amorphous by forming an amorphous semiconductor thin film on the insulating substrate and annealing the amorphous semiconductor thin film to such an extent that crystallization is not saturated. The method according to claim 1, wherein the thin film is a polycrystalline semiconductor thin film including a crystallized region while leaving a quality part. 4. The method according to claim 3, wherein the annealing is a step of melting the amorphous portion.