JPH0372617A - Improvement of characteristic of polycrystalline silicon thin film - Google Patents

Abstract

PURPOSE:To improve the electrical characteristics of a polycrystalline silicon thin film having a large area without forming a discontinuous part on the thin film by a method wherein an energy beam controlled in its energy is scanned on the thin film in such a way that irradiation regions are superposed on the thin film. CONSTITUTION:A beam 7 from an Excimer laser light source 8 is irradiated on a polycrystalline silicon thin film on a wafer placed on a stage 14 through a beam homogenizer 13. At this time, if the stage 14 or the homogenizer 13 is moved and a beam spot 7S is scanned, an annealing of the polycrystalline silicon thin film having a large area can be performed. In the scanning, the beam spot is scanned on a cell 5 (the polycrystalline silicon thin film) in the order of No. 1 to No. 5, for example, then, is scanned in the order of No. 5 to No. 8 and moreover, a central part 9 is irradiated to eliminate completely an unirradiated part. As the energy beam 7, a beam which performs a pulse oscillation in a short wavelength is used and it is desirable that the energy density of the beam 7 is set so as to satisfy the condition that the irradiation temperature of the beam is higher than the melting point of an amorphous silicon film and is lower than the melting point of a single crystal silicon film. Thereby, the crystal of a region 17, where irradiation regions are superposed, is not fused and the microdefect only of the crystal is improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for improving the characteristics of polycrystalline silicon thin films, particularly for manufacturing large-area (large-capacity) LSIs incorporated into liquid crystal display devices, line sensors, etc. The present invention relates to a method for improving the characteristics of a polycrystalline silicon thin film that is suitable for improving the characteristics of a polycrystalline silicon thin film that becomes an active region.

[Existing necessity of invention]

The present invention provides a method for improving the characteristics of a polycrystalline silicon thin film, which is applied, for example, when manufacturing LSIs, etc., in which the energy beam is scanned so that the irradiation area overlaps the polycrystalline silicon thin film. This makes it possible to perform annealing for the purpose of improving the electrical characteristics of a polycrystalline silicon thin film having a large area without forming discontinuous portions.

[Conventional technology]

In general, thin film transistors are made by depositing a semiconductor thin film such as polycrystalline silicon on an insulating substrate such as quartz glass, and this thin film semiconductor layer has an active region where a channel is formed, a low resistance source region, and a drain region. Each of them is adapted to form a field effect transistor.

By the way, silica glass with a high melting point has been generally used as a substrate for thin film transistors, but since the material costs are high and expensive, it is important to use ordinary heat-resistant glass with a lower melting point than quartz glass for the substrate. When using heat-resistant glass with a relatively low melting point as a substrate, a low-temperature process is required to keep the upper limit temperature on the substrate during the manufacturing process of thin film transistors below the strain point of the substrate glass. .

However, in such a low temperature process, it is difficult to obtain an active region with good characteristics. That is, if silicon is simply deposited on a substrate by, for example, CVD (chemical vapor deposition), a polycrystalline silicon layer with a small crystal grain size and high trap density will be formed, which will affect the electrical characteristics, especially the threshold voltage. Voltage vth, subthreshold characteristics,
Good mobility μ and leakage current cannot be obtained.

Therefore, after forming a polycrystalline silicon layer on a substrate, silicon ions S1+ are implanted to make it amorphous, and then low-temperature annealing (approximately 600°C) is performed to obtain a polycrystalline silicon layer with increased crystal grain size. It is considered.

In this case, a relatively high performance thin film transistor can be obtained, but it is not as good as a thin film transistor manufactured by a high temperature process of 1000°C. The cause of this is not the crystal grain size of the polycrystalline silicon layer, but the grain boundary trap density at 600°C.
This is because the improvement is not sufficient.

Therefore, after the above-mentioned low-temperature annealing, it is possible to perform short-time annealing with an Ar laser to reduce the trasop density of the polycrystalline silicon layer, but this Ar laser has a long wavelength and is a continuous wave (CW). Therefore, surface irradiation is difficult, and it is not suitable for polycrystalline silicon thin films having a large area and a film thickness of less than 1,000.

However, recently, pulsed laser annealing using an excimer laser that emits short-wavelength pulses has attracted attention and has been put into practical use.

[Problem to be solved by the invention]

However, since conventional pulsed laser annealing using an excimer laser instantaneously melts the irradiated area, discontinuous surfaces are formed, especially when annealing a large area of polycrystalline silicon thin film, resulting in subsequent device devices. There was an inconvenience in that it affected the manufacturing process.

That is, when an excimer laser beam is irradiated onto a polycrystalline silicon thin film through an optical homogenizer, an irradiation area of 10 mm square can be obtained, and by scanning this, large area annealing is possible. , since scanning is carried out at an energy density (approximately IJ/cnf) that melts the irradiated area, if the irradiated areas are partially overlapped and scanned in order to prevent the formation of unmelted areas between each irradiated area,
The overlapping portions of the irradiated areas are further melted, resulting in the surface of the polycrystalline silicon thin film becoming discontinuous, that is, an uneven surface, which poses an inconvenience in subsequent device fabrication.

The present invention was developed in view of these points, and its purpose is to uniformly increase the trap density in a polycrystalline silicon thin film having a large area without forming discontinuous surfaces. It is an object of the present invention to provide a method for improving the characteristics of a polycrystalline silicon thin film by which annealing can be performed for the purpose of reduction.

[Means to solve the problem]

The method for improving a polycrystalline silicon thin film of the present invention is to apply a controlled energy beam (7) to a polycrystalline silicon thin film (
5), the energy beam (7) is scanned so that the irradiation area (15) overlaps with the energy beam (7).

The energy beam (7) may be one that generates pulses at a short wavelength, such as an excimer laser. The energy density of the energy beam (7) can be set so that the irradiation temperature is higher than the melting point of amorphous silicon and lower than the melting point of single crystal silicon.

[Effect]

According to the method of the present invention described above, the energy density is controlled such that the irradiation temperature of the energy beam (7) is higher than the melting point of amorphous silicon and lower than the melting point of single crystal silicon, for example. (7) to its irradiation area (
15) is scanned over the polycrystalline silicon thin film (5) so that it partially overlaps (17), so the crystalline part in the polycrystalline silicon thin film (5), which has a large area, is not melted and only minute defects are improved. . Therefore, without forming a discontinuous surface (uneven surface) on the surface of the polycrystalline silicon thin film (5) having a large area,
The trap density of the polycrystalline silicon thin film (5) can be reduced.

This makes it possible to achieve annealing for the purpose of improving the film quality of polycrystalline silicon thin films (5) that have large areas, which was previously unachievable. Class D
RAM, SRAM, etc.) can be formed more efficiently and without causing deterioration of characteristics.

〔Example〕

Hereinafter, the present invention will be explained in detail with reference to FIGS. 1 to 4.

FIG. 1 is a process diagram showing a method for improving the characteristics of a polycrystalline silicon thin film according to this embodiment. The steps will be explained step by step below.

First, as shown in FIG.
After forming the film (2), a polycrystalline silicon thin film (3) with a film thickness of about 800 mm is applied to the 3102 film (2) by CVD, for example.
Adhesion is formed using the D method or the like.

Next, as shown in FIG. 1B, a polycrystalline silicon thin film (3
) to transform the polycrystalline silicon thin film (3) into an amorphous silicon thin film (4). The Si+ ion implantation conditions at this time are, for example, an implantation energy of approximately 40 KeV and an implantation dose of approximately 1.5 x 10-'' cm-2.

Next, as shown in Figure C, an amorphous silicon thin film (4
) is subjected to low-temperature heat treatment at, for example, 600° C. for 3 hours to grow crystals, thereby forming a polycrystalline silicon thin film (5) with large crystal grains as shown in the figure. At this time, the grain size of the polycrystalline silicon thin film (5) grows, but the trap density at grain boundaries is poor (high).

Next, as shown in the first diagram, a polycrystalline silicon thin film (5
) on top of the 5102 film (6) with a thickness of about 500 mm
Adhesion is formed using a VD method or the like.

Next, as shown in FIG. 1E, a polycrystalline silicon thin film (5
) to a rare gas halide excimer laser beam (7
) to perform annealing for the purpose of reducing the trap density. In this example, as shown in Figure 2, rare gas
Wavelength 308n as halide excimer laser light source (8)
m Xeα excimer laser light source is used, and the laser light source (
8) onto a stage (14) via a workstation, in the example shown, a mirror (9), an attenuator (10>, mirrors (11) and (12), and a beam homogenizer <13>). The polycrystalline silicon thin film (5) on the mounted wafer (not shown) is irradiated.At this time, the beam (7) irradiated via the beam homogenizer (13) has an extremely uniform beam profile over a wide range. As shown in the enlarged view of FIG. 2, the shape of the beam sub-box (7S) has a rectangular irradiation area of 10 mm square.

The energy density of the beam (7) is such that the irradiation temperature is high enough not to melt the polycrystalline silicon thin film (5), that is, higher than the melting point of amorphous silicon and lower than the melting point of single crystal silicon. Set. This example (800
Thickness of 0 polycrystalline silicon (5), thickness of 500 in
2 membrane (6)), the density is set to closer than 300 mJ/cnt.

In this example, the stage (14) or the beam homogenizer (13) is moved relatively to
(7S) on the stage (■4) with a polycrystalline silicon thin film (
5) Scan. In this way, the laser beam (
7) can be performed over a large area.

Figure 3 shows an example in which a 10 mm square beam spot is scanned on a 2Q mm square rectangular cell (polycrystalline silicon thin film) (5). Spot number 12, 3.4 for cell (5)
scan in this order. At this time, there is still an unirradiated area at the boundary (16) of the irradiated area (15), and the cell (5)
The surface is discontinuous.

Next, as shown in Figure 3B, the beam spot is placed in the cell (
5) for number 5.6. ．． 7. Scan in the order of 8, and further,
The central part of the cell (5) is irradiated as indicated by number 9 in Figure 3C. At this stage, the unirradiated area is completely eliminated and the discontinuous area existing on the surface of the cell (5) disappears. In addition, since the energy density of beam (7) was set as described above, the first irradiation area (area indicated by number 1, 23.4)
(15) and the irradiation area (15) overlap (area indicated by number 5.6.7.8.9)> (1
The crystal part in 7) did not melt and only the micro defects were improved.
No discontinuous surfaces are formed due to melting of the crystals in the polycrystalline silicon thin film (5). In the above example, the beam (7) was irradiated with a constant energy density, but the numbers 5, 6°7, 8.
The energy density when irradiating the region (17) indicated by number 9 may be set lower than the energy density when irradiating the region (15) indicated by numbers 1, 2, 3, and 4. In addition to the above example, the scanning order may be as shown in FIG. 4. Even in this case, the crystal portions in the irradiated areas and the area where the irradiated areas overlap are not melted and only minute defects are improved, and no discontinuous surfaces are formed.

After this step, separation of the element formation region, device fabrication in the element formation region, etc. are performed to obtain an LSI with improved electrical characteristics according to this example.

As described above, according to this example, in FIG. 1E, XeCJ
! During annealing treatment with excimer laser beam (7),
The beam (7) is applied so that the irradiation temperature of the beam (7) is lower than the melting temperature of the crystal in the polycrystalline silicon thin film (5), that is, higher than the melting point of amorphous silicon and lower than the melting point of single crystal silicon. ), and then the beam homogenizer (13) is used to set the energy density of the beam (7).
For example, the shape of S) is made into a rectangular shape of 19 mm square, and this beam spot (7S) is placed on a polycrystalline silicon thin film (5).
However, since we scanned so that some parts overlapped,
Irradiation area (15) and irradiation area (15) of beam (7)
The crystal part in the overlapping region (17) is not melted and only minute defects are improved. As a result, the trap density of the polycrystalline silicon thin film (5) is reduced without forming discontinuous surfaces (uneven surfaces) due to unirradiated and melted crystal parts on the surface of the polycrystalline silicon thin film (5), which has a large area. It is possible to achieve annealing for the purpose of improving the film quality of polycrystalline silicon thin films (5) with large areas, which could not be achieved conventionally. Megabit class DRAM and SRAM
etc.) can be formed more efficiently and without causing deterioration of characteristics.

In addition, the energy density of the beam (7) is set at a temperature that does not melt the crystals in the multilayer 1 2 crystalline silicon thin film (5) as described above, so the burden on laser output can be reduced. In addition, when manufacturing a memory cell with a multilayer structure, the upper transistor can be formed without deteriorating the characteristics of the lower transistor, that is, without unnecessarily diffusing the impurity region.

In addition, when annealing the polycrystalline silicon thin film (5) using the excimer laser beam (7), as shown in FIG. 8102 membrane (6)
becomes a kind of anti-reflection film (i.e., when the polycrystalline silicon thin film (5) is directly irradiated with the XeC# excimer laser beam (7), approximately 70% of it is reflected at the surface, but it is reflected through the 5in2 film (6)). (When irradiated with light, there is almost no reflection), the annealing efficiency of the polycrystalline silicon thin film (5) can be further increased, and the trap density in the film can be further reduced.

〔Effect of the invention〕

In the method for improving the characteristics of a polycrystalline silicon thin film according to the present invention, the energy beam is scanned so that the irradiation area overlaps the polycrystalline silicon thin film, so that the polycrystalline silicon film has a large area. The electrical characteristics of the polycrystalline silicon thin film can be improved without forming discontinuous portions in the thin film.

[Brief explanation of drawings]

Fig. 1 is a process diagram showing a method for improving the characteristics of a polycrystalline silicon thin film according to this embodiment, Fig. 2 is a block diagram showing a laser annealing process according to this embodiment, and Fig. 3 is a scanning order according to this embodiment. FIG. 4 is a process diagram showing another scanning order. (1) is an insulating substrate, (2) and (6) are 5in2 films, (
3), (5) is a polycrystalline silicon thin film, (4) is an amorphous silicon thin film, (7) is a laser beam, (7S) is a beam spot, (8) is a light source, (9), '(11) ,(
12) is a mirror, (10) is an attenuator, (13) is a beam homogenizer, (14) is a stage, (15) is an irradiation area, and (17) is an area where the irradiation areas overlap. 9 4 C Go - 123 - (L 41 S e Introduction 1 Show - 9' Kokyo Interstellar 4th Ward 125

Claims (1)

[Claims] 1. A method for improving the characteristics of a polycrystalline silicon thin film, comprising scanning an energy beam whose energy is controlled so that the irradiation area overlaps the polycrystalline silicon thin film.