JPH04307727A - Formation method of silicon semiconductor layer - Google Patents

Abstract

PURPOSE:To form a uniform crystal silicon layer on the whole of a substrate by devising a method in which a silicon layer is irradiated with a laser beam. CONSTITUTION:An amorphous silicon layer whose hydrogen content is at 1% or lower is formed on a silicon substrate or an insulating substrate, irradiated with an excimer laser whose energy intensity distribution has been improved to be rectangular. A laser beam which radiated from a laser source 201 and whose energy intensity is set to a pseudo-Gaussian distribution is improved to a laser beam whose energy intensity distribution is trapezoidal by using a special optical system such as a fly-eye lens 204 or the like. Then, the laser beam is improved to a laser beam 208 whose energy intensity distribution is rectangular through a convex lens 205 and a concave lens 206 and through a mask 207 composed of a high-melting-point metal. The silicon layer is irradiated with the laser beam. Even when an annealing operation is performed by overlapping the improved beam, the edge part of the beam does not become uneven. As a result, it is possible to form the uniform and high-quality silicon layer on the whole of the substrate.

Description

[Detailed description of the invention]

0001

[Industrial Field of Application] The present invention relates to semiconductor integrated circuits, SO
I. Concerning the structure of an active matrix type thin film transistor and a tertiary element.

0002

2. Description of the Related Art Conventionally, semiconductor thin films on single crystal insulating substrates have been known to have the following advantages over bulk semiconductors, as seen in SOS (silicon on sapphire).

(2) When cutting into islands or performing dielectric isolation, isolation between elements can be easily and reliably achieved. (2) Stray capacitance can be reduced by reducing the PN junction area.

Furthermore, since single-crystal insulating substrates such as sapphire are expensive, as alternatives, fused quartz plates, amorphous SiO2 films formed by oxidizing Si substrates at temperatures above 1000°C, and Si Amorphous S deposited on the substrate
A method has been proposed in which an iO2 film or an amorphous SiN film is used and a semiconductor thin body is formed thereon. However, since these SiO2 films and SiN films are not single crystals, when a silicon layer is deposited thereon and crystallized in a process at a temperature of 1000 DEG C. or higher, polycrystals grow on the substrate. The grain size of this polycrystal is several tens of nanometers, and in addition, M
Even if an OS transistor is formed, its carrier mobility is about a fraction of that of a MOS transistor on bulk silicon.

[0005] Furthermore, for MOS transistors on inexpensive glass substrates with strain points of 850°C or less for active matrix substrates of liquid crystal displays, it is not possible to use processes at temperatures above 1000°C. Even if a silicon layer is deposited using a growth method, the grain size of the polycrystal is only a few nanometers at most, so even if a MOS transistor is formed on it,
Its carrier mobility is about a few tenths of that of a MOS transistor on bulk silicon.

[0006]Recently, therefore, a method of scanning a silicon thin film with a laser beam, an electron beam, etc. and melting and re-individuating the thin film to increase the crystal grain size and make it monocrystalline or polycrystalline has been studied. ing. According to this method,
It is possible to form a high quality silicon single crystal phase or high quality polycrystal on an insulating substrate, and the characteristics of devices fabricated using it are also improved to the same extent as the characteristics of devices fabricated in bulk silicon. Furthermore, this method allows elements to be stacked, making it possible to realize a so-called three-dimensional IC. As a result, circuits with characteristics such as high density, high speed, and multifunction can be obtained.

As an example of manufacturing a thin film transistor by forming a high quality silicon layer using a laser beam, Ext.
Ended Abstracts of the
22nd (1990 International)
Conference on Solid St
ate Devices and Materi
als, Sendai, 1990, pp. 967-97
0"XeCl Excimer Laser-In
reduced amorphization and
Crystallization of Sil
icon Films”.

[0008]

However, it is difficult to uniformly crystallize a silicon layer over the entire substrate by laser beam irradiation. In order to obtain a high quality silicon layer formed by PECVD or low pressure chemical vapor deposition using a laser beam, a certain amount of appropriate energy is required. As in the conventional example above, in the case of an amorphous silicon layer containing hydrogen formed by decomposing monosilane by glow discharge, a polycrystalline silicon film with a large grain size can be formed at a certain moderate beam energy. However, if the energy is lower than that, a silicon layer with microcrystalline grains will result.

In the excimer laser used in the above conventional example, the energy distribution of the beam in a cross section perpendicular to the beam traveling direction has a pseudo Gaussian distribution as shown in FIG. 4, and is not necessarily constant. It is not a beam with energy distribution. Then, depending on the energy intensity of the beam, portions of the silicon layer with large grain size or the silicon layer with microcrystals are formed, making it impossible to obtain a silicon layer of uniform quality. Therefore, in the conventional example, in order to obtain a uniform polycrystalline silicon layer over a large area, a method of annealing while shifting the position of a laser beam at minute intervals has been attempted. However, this method has the drawback of very low efficiency because the silicon layer at the same point must be irradiated several dozen times. In addition, in this method, the laser beam is irradiated with a higher intensity than the energy required to obtain a silicon layer with large particles, so the surface of the silicon layer may become rough or the silicon layer may adhere. There was a drawback that damage occurred to the thin film.

On the other hand, attempts have been made to uniformize the energy distribution of the beam by providing a special optical system between the laser beam oscillation source and the silicon layer that is the sample.

However, the result of improving the beam intensity distribution by this special optical system is not uniform over the entire beam, and non-uniformity is still observed at the edges of the beam. In the case of a silicon layer formed by PECVD, a silicon layer formed by PECVD becomes a microcrystalline silicon layer in areas where the energy intensity of the beam is insufficient, and then this microcrystalline silicon layer is replaced with large grains from the initial silicon layer. Even when irradiated with the energy necessary to form a silicon layer with particles, it remains in a microcrystalline state and does not change. Therefore, in a conventional energy beam with a non-uniform energy intensity distribution,
Pulsed laser irradiation has the disadvantage that it is not possible to uniformly improve the quality of a silicon layer over a wider area than the pulsed laser beam.

0012

[Means for Solving the Problems] In view of the above problems, the present invention provides a method for forming a silicon semiconductor layer that uniformly improves the quality of a silicon layer over a wider area than the pulsed laser beam by irradiation with a pulsed laser. This is what we provide.

[0013]

Embodiments The embodiments will be described in detail below with reference to the drawings.

An example is shown in FIG.

First, as shown in FIG. 1A, a silicon dioxide film 102 with a thickness of 200 nm is formed on a transparent insulating substrate 101 by electron cyclotron resonance sputtering. Instead of the silicon dioxide film, SiNx, SiON,
An insulating film such as PSG may also be used.

Next, an amorphous silicon layer 103 with a thickness of 50 nm is formed at a temperature of 550° C. by, for example, low pressure chemical vapor deposition. As described above, the silicon layer formed by low pressure chemical vapor deposition has a hydrogen content of 1% or less in terms of atomic ratio.

Next, as shown in FIG. 1b, the surface of the silicon layer 103 is irradiated with a laser beam 104. The laser beam is a XeCl excimer laser and has a wavelength of 308 nm. Since the amorphous silicon layer has a large absorption coefficient of 106 cm at 308 nm, most of the energy of the laser beam is absorbed by the silicon layer. absorbed into. The laser beam conditions are that the half width is 5
0ns, energy density is 200-400mJc
m-2, and the geometric size of the beam irradiation surface is a square with one side of 10 mm. The geometric dimensions of this beam can be varied as required.

Next, improvement of the energy distribution of the laser beam 104 will be explained with reference to the drawings. Figure 2
The energy distribution of the laser beam 202 emitted from the laser source 201 shown in FIG. 2 has a pseudo Gaussian distribution as shown in FIG. The laser beam 202 passes through an attenuator 203 and a special optical lens 204, such as a fly's eye lens. The attenuator 2
At step 03, the energy of the laser beam is attenuated as necessary. With the special optical lens 204, the energy distribution of the laser beam can be changed from a pseudo Gaussian distribution to that shown in FIG.
The peak EMAX with a constant energy density as shown in
The energy distribution is improved to a quasi-trapezoidal energy distribution. next,
The laser beam whose energy distribution has been improved to have a quasi-trapezoidal shape is passed through a convex lens 205. Next, the laser beam that has passed through the convex lens 205 is passed through a concave lens 206. Next, by changing the distance between the convex lens 205 and the concave lens 206, the size of the beam can be changed. Further, the laser beam passing through the concave lens 206 becomes a parallel light beam. The laser beam that has passed through the concave lens 206 is passed through a mask 207. The mask 207
The material of the substrate is high quality quartz, and as shown in FIG. 4, the drawn portion 401 of the mask that blocks laser light is made of a high melting point metal such as tungsten. Alternatively, a metal thin film having a large reflection coefficient for light with a wavelength of 308 nm may be used. By passing through the mask 207, a laser beam 208 with a rectangular energy distribution is obtained, as shown in FIG. 5, with extremely steep edges and uniform energy density in areas other than the edges.

The atmosphere around the silicon layer 103 when beam annealing the silicon layer 103 is a vacuum, an inert gas atmosphere, a nitrogen atmosphere, or an air atmosphere.

[0020] Furthermore, the intensity of the laser beam 104 is
It can be adjusted as appropriate by changing the distance between the attenuator 203, the convex lens 205, and the concave lens 206.

As shown in FIG. 2, a substrate 209 on which the silicon layer 103 is formed is placed on a stage 210, and a drive system 211 installed perpendicularly or obliquely to the beam traveling direction is connected to the substrate 209. The irradiation position of the pulse beam can be changed while the stage 210 is controlled by a computer 212 that controls the laser oscillation frequency of the laser source 201, the timing of laser emission, and the operation of the drive system.

When annealing the silicon layer 103 having an area larger than the geometric size of the laser beam 104, it is necessary to irradiate the silicon layer surface with a pulse beam multiple times at different irradiation positions.

FIG. 6 shows how the silicon layer crystallizes when the irradiation position is changed.

The example of FIG. 6a shows that the special optical lens 204
An example is shown in which the silicon layer is annealed by changing the irradiation position of a laser beam that passes through the convex lens 205 and the concave lens 206 but does not pass through the mask 207. This laser beam has a trapezoidal energy distribution as shown in Figure 3a.

FIG. 3b shows the state of the silicon layer when irradiated with the pulse beam having the trapezoidal energy distribution. As shown in FIG. 3a, when the energy density E of the pulsed laser is in the range O≦E≦E1, the silicon layer 10
3 does not change. Energy density E of the pulsed laser
When E2≦E≦E3, the silicon layer 103 changes into a high-quality polycrystalline silicon layer 301 having large grain size crystals. Further, when the energy density E of the pulsed laser is in the range of E1≦E≦E2, the silicon layer 103 is a silicon layer 302 having microcrystalline particles.
Changes to Further, if the energy density E of the pulsed laser is in the range of E3≦E, the energy density is too large, and the silicon layer changes to an amorphous silicon layer. Therefore, FIG. 3b shows a case where the maximum energy density EMAX of the pulsed laser is smaller than E3. As shown in FIG. 3B, a polycrystalline silicon layer is formed in the central portion 301 of the beam irradiated portion, while a silicon layer having microcrystalline particles is formed in the peripheral portion 302.

Next, as shown in FIG. 6A, the irradiation position is changed and a second pulse beam is irradiated so as to form a portion 603 overlapping the first irradiation portion. Maximum energy EM of the first pulse beam and second pulse beam
AX If the energies are equal, area 601 and area 60
The two silicon layers are in the same crystalline state. In addition, as mentioned above, in the case of an amorphous silicon layer with a low hydrogen content formed by low-pressure chemical vapor deposition, etc., the laser beam 1
A crystal in the same state can be obtained regardless of the number of times the pulse beam of 04 is irradiated. Therefore, the silicon layers in region 601, region 602, and region 603 are in the same crystalline state. However, even if the silicon layer having microcrystalline particles in the peripheral portion 302 is irradiated with a laser beam having an energy E within the above range of E2≦E≦E3, the silicon layer having microcrystalline particles remains unchanged. Further, even if the polycrystalline silicon layer in the central portion 301 is irradiated with a pulsed laser having an energy within the above range of E1≦E≦E2, no change is observed. In FIG. 6a, the irradiation area where the energy of the first and second pulse beams satisfies E1≦E≦E2 is indicated as 604.

Therefore, when the silicon layer is annealed by changing the irradiation position with the pulsed beam having the energy distribution shown in FIG. 3a, a region 606 of the silicon layer having microcrystalline particles is formed as shown in FIG. 6b for the reason shown above. Ru.

That is, E1≦E as shown in FIG. 3a.
When the silicon layer 103 is irradiated multiple times with a trapezoidal or similar energy distribution pulse beam having an energy of ≦E2 and changing the irradiation position, a silicon layer with a non-uniform crystal state is formed. The characteristics of a thin film transistor made of a silicon layer with a non-uniform crystalline state are unevenly distributed over the entire substrate.

FIG. 6b shows a schematic diagram of the distribution of the crystalline state of the silicon layer when irradiated as shown in FIG. 6a. A region 605 of a high quality silicon layer having large grain size particles and a silicon layer 606 having microcrystalline grains are formed.

The example in FIG. 7 shows the state of the silicon layer when the energy distribution of the laser beam is improved by the above method. The first pulse beam irradiation transforms the silicon layer 103 into a polycrystalline silicon layer 701. Next, change the irradiation position and overlap the first irradiation area 70
The second pulse beam is irradiated so as to form 3. If the energy of the first pulse beam and the second pulse beam are equal, the silicon layers 701 and 702 are in the same crystal state.

In addition, as mentioned above, in the case of an amorphous silicon layer with a low hydrogen content formed by low pressure chemical vapor deposition, etc., the same state is obtained regardless of the number of pulses irradiated with the laser beam 104. Crystals are obtained. Therefore, the crystalline state of region 703 shown in FIG. 7a is the same as region 701 and region 702. Furthermore, as mentioned above, since the laser beam has a rectangular energy distribution, the boundary 7
Since no microcrystalline particles are formed in 04, it is essentially region 7.
A uniform polycrystalline silicon layer with no boundary state is formed between the regions 01, 702, and 703. FIG. 7b shows a schematic diagram of the distribution of the crystalline state of the silicon layer when irradiated as shown in FIG. 7a. Only a region 705 of a high-quality silicon layer having large grain size particles is formed, and a silicon layer having microcrystalline grains is not formed. Therefore, with a pulse beam of moderate energy having a rectangular energy distribution shown in FIG. 5, the irradiation position is changed so that the silicon layer 103 of the entire substrate is
When annealed, a good silicon layer 10 with large grains uniform over the entire substrate is formed, as shown in Figure 1c.
You can get 5. Therefore, by irradiating the silicon layer 103 with the laser beam 208, that is, the laser beam 104, a uniform polycrystalline silicon layer 105 can be obtained over the entire substrate, thereby providing uniform characteristics over the entire substrate. A high performance thin film transistor with high mobility can be manufactured.

Although the above embodiment described crystallization of a silicon layer formed by low pressure chemical vapor deposition,
Even with a silicon layer formed by PECVD, 450
When annealing is performed for 60 minutes at .degree. C. in a nitrogen atmosphere, the hydrogen content becomes 1% or less, so the present invention can also be applied to a silicon layer formed by PECVD. Of course, the present invention can also be applied to a silicon layer formed by sputtering.

Although the above embodiment describes the crystallization of a silicon layer, the present invention is not limited to the above embodiment, and can also be used when uniformly modifying a thin film or a substrate over a large area using a pulsed laser beam. is applicable.

[0034]

Effects of the Invention As explained above, according to the present invention, it is possible to uniformly crystallize a silicon layer over a wider area than the geometrical shape of the laser beam. By irradiation, a high-quality silicon layer with a large crystal grain size can be uniformly formed. Therefore, high-performance thin film transistors with high mobility can be formed on the entire surface of the substrate by laser annealing at room temperature, so active matrix type flat displays with built-in drive circuits can be manufactured on glass substrates instead of quartz. can do. As a result, the cost of the flat display can be reduced because it can be formed on an inexpensive glass substrate instead of the more effective quartz substrate.

Furthermore, since the quality of the silicon layer can be improved by using a pulsed excimer laser having a wavelength with a large absorption coefficient of the silicon layer, it is also possible to form a three-dimensional semiconductor integrated circuit.

[Brief explanation of drawings]

FIG. 1 is a process diagram of an embodiment of forming a silicon semiconductor layer of the present invention.

FIG. 2 is a diagram of an optical system of the method of improving the energy distribution of a pulsed laser beam into a rectangular shape according to the present invention.

FIG. 3 is a diagram showing the energy intensity distribution of a laser beam before improving the energy distribution and the change in the silicon layer with respect to the energy intensity.

FIG. 4 is a diagram of a mask for improving the energy intensity distribution of a pulsed laser beam into a rectangular shape according to the present invention.

FIG. 5 shows an improved energy intensity distribution of a pulsed laser beam according to the present invention.

FIG. 6 is a diagram showing changes in a silicon layer when a pulsed laser beam having a conventional energy intensity distribution is irradiated multiple times.

FIG. 7 shows the change in a silicon layer upon multiple irradiations with the improved pulsed laser beam according to the present invention.

[Explanation of symbols]

101 Transparent insulating substrate 102 Silicon dioxide film 103 Silicon layer 104 Laser beam 105 Polycrystalline silicon layer 201 Laser source 202 Laser beam before improvement 203 Attenuator 204 Special optical lens 205 Convex lens 206 Concave lens 207 Mask

Claims (2)

[Claims] 1. A step of forming a silicon layer on a substrate, a step of irradiating the silicon layer with a beam of short wavelength light that is partially blocked by a shielding plate, and a step of irradiating the silicon layer with the beam multiple times. A method for forming a silicon semiconductor layer, comprising the steps of: and a step in which the irradiation surfaces of the respective beams on the silicon layer overlap. 2. The method of forming a silicon semiconductor layer according to claim 1, wherein the short wavelength light beam is pulsed light.