JP3213528B2 - Method for manufacturing polycrystalline semiconductor film - 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 polycrystalline semiconductor film, and a thin film transistor (hereinafter referred to as a TFT) used for a driving device or the like using the polycrystalline semiconductor film formed by the method. And a display device using the thin film transistor.

[0002]

2. Description of the Related Art In recent years, with the aim of realizing high image quality and high definition, a driving device for driving pixels or peripheral circuits thereof has been developed.
Various high performance technologies of FT have been developed. For example,
As a technique for improving the quality of an active layer material that affects device characteristics, a technique of forming a thin polycrystalline silicon film by an excimer laser annealing method using an amorphous silicon film as a starting material has been developed.

A method for forming the polycrystalline silicon film will be described with reference to FIG. First, as shown in FIG. 12A, an amorphous silicon film 112 is formed on an insulating substrate 111 made of glass or the like by a chemical vapor reaction (CVD) method. Next, as shown in FIG. 12B, using the resist pattern 113 as a mask on the amorphous silicon thin film 112, as shown in FIG. Island 116 is formed. Subsequently, as shown in FIG. 12D, after removing the resist 113, the amorphous silicon thin film is irradiated with an excimer laser 117 in vacuum to melt and recrystallize the amorphous silicon thin film. A crystalline silicon film 118 is formed.

According to the above method, Jpn.
J. Appl. Phys. Vol. 32 (1993) p
p. L1485-L1488 "Self Organ"
sized grain growth laser
han 1μm through Pulsed-La
ser-Induced Melting of Si
As described in “Licon Films”, there is a problem that as the crystal grain size increases, the surface irregularities also increase.

The reason will be described with reference to FIG.
FIG. 13 is a schematic diagram showing a state of recrystallization by excimer laser annealing in a vacuum. In addition, FIG.
-(A-3) show the recrystallized state, (b-1)-(b)
-3) shows the temperature distribution in that state. Since the surface of the amorphous silicon irradiated with the laser is flat as shown in FIG. 1A, the temperature immediately after the laser irradiation uniformly rises as shown in FIG. The temperature becomes higher than the melting temperature ( _TM ), and the amorphous silicon is melted. At this time, since the substrate is held in a vacuum, the heat of the molten silicon escapes mainly toward the substrate. Therefore, FIG.
As shown in 2), the center of the island conducts heat in the left-right direction and in the direction of the substrate, so that the heat quickly drops and becomes low. At the end, the heat is transmitted only in the direction of the substrate and becomes higher than in the center. At the same time, due to the wettability between the molten silicon and the substrate, the molten silicon gathers at the end portion, and FIG.
As shown in the figure, the central portion is concave and the end portion is raised. After a further time, as shown in FIG. 3B-3, the temperature difference between the central part and the end part becomes large, and the thermal distortion between them becomes large. And the state where the melting temperature (T _M ) or more continues at the end for a long time due to the large thermal distortion,
The growth of crystal grains is promoted.

As a result, as shown in FIG. 1 (a-3), a film is formed in which the central portion is depressed and the surface becomes uneven, and the crystal grain size at both ends is large. In the above method, the crystal grain size at both ends is large, but the crystal grain size at the central portion is small, and the grain size is not uniform as a whole. For this reason, characteristics such as mobility have been greatly varied.

[0007]

As described above, in a polycrystalline semiconductor thin film which is recrystallized by laser annealing after normal islanding, surface irregularities increase.
Since a uniform particle size cannot be obtained in the entire film, there is a problem that characteristics such as mobility largely vary. As a result, in a display device using a TFT in which the ordinary island-formed polycrystalline semiconductor thin film is used as an active layer, there is a problem that good display cannot be obtained.

Further, when a TFT having uniform characteristics is formed using the islands, it is necessary to perform flattening after recrystallization, which complicates the manufacturing process, lowers the yield, and lowers the cost associated therewith. There was a problem of rising. The present invention has been made to solve the conventional problems described above, Tayui the surface can be achieved is uniform characteristics by performing flat to and uniformity of crystal grain size
It is an object of the present invention to provide a method for manufacturing a crystalline semiconductor film .

[0009]

According to a first aspect of the present invention, there is provided a method of manufacturing a polycrystalline semiconductor film, comprising: forming an inclined portion at an end of an island-shaped amorphous semiconductor thin film provided on a substrate; Irradiating and crystallizing the amorphous semiconductor thin film to form a polycrystalline semiconductor film, wherein the cross-sectional shape of the amorphous semiconductor thin film is such that both upper corners of a square are the inclined portions.
Presenting a hexagon cut out by providing
The ratio of the uppermost side to the lowermost side of the hexagon is 0.8 or less, and the output of the laser beam is 200 mJ / cm ^{2 to} 400 m
The gist is to control to J / cm ² .

According to a second aspect of the present invention, there is provided a method for manufacturing a polycrystalline semiconductor film.
The invention according to claim 1, wherein the output of the laser beam is 2
To control the 10mJ / cm ² ~380mJ / cm ²
This is the gist.

[0011]

That is , by forming a slope at the end of the island and irradiating the surface with a laser beam,
In the inclined portion, the amount of incident energy per unit area becomes smaller than that in the flat portion, and the surface temperature becomes lower at the end portion. After melting, heat is more likely to escape in the center, so that the temperature difference between the center and the ends is smaller, and thermal distortion is smaller. Therefore, crystallization proceeds uniformly throughout the film.

Further, the molten silicon extruded to the edge due to the wettability with the substrate is absorbed by the inclined portion, and the surface after crystallization becomes flat. Furthermore, according to the present invention, the surface of the polycrystalline semiconductor thin film becomes flat and a uniform grain size can be obtained throughout the film, so that the mobility and the off-current of the TFT become uniform. This also makes the writing voltage to the pixel electrode and the image signal holding time uniform, thereby improving the contrast of the display device and obtaining a good display.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described.
This will be described with reference to FIGS. As shown in FIG. 1 (a), on a conductive substrate 1 of SiO _2, SiN _X or the like 5nm~1
The insulating layer 2 is formed by forming a film having a thickness of μm. An amorphous silicon film (hereinafter abbreviated as an a-Si film) is formed on the substrate 1 on which the insulating layer 2 is formed by CVD using silane (SiH ₄ ), disilane (Si ₂ H ₆ ), or a silane compound. 4) is formed. The reaction temperature at this time is 300 to 60
At 0 ° C., the thickness of the a-Si film 4 is several% smaller than the target thickness.
A thick film is formed (about 20 nm to 100 nm).

Next, as shown in FIG. 1B, an island pattern 5 made of a resist is formed, and an island 6 made of a-Si is formed by wet or dry etching using the island pattern 5 as a mask.
And, in order to form an inclined portion in this island 6,
Removing the resist 5 'of the end portions of the island pattern 5, by wet or dry etching to form an inclined portion 6a to the island 6, the cross-sectional shape of the island 6 is six
The etching control is performed so as to form a square or a dome.
In this embodiment, islands are formed so that the cross-sectional shape becomes hexagonal . In order to form the island 6 by dry etching, the sectional shape as shown in the figure can be easily controlled by selecting the type, flow rate and pressure of the etching gas. Specifically, using a resist,
After island formation by IE (CF ₄ ), isotropic etching is performed under conditions (selective reaction pressure is increased and oxygen is introduced) so that the selectivity between the resist and a-Si becomes, for example, 2: 1. , Can be formed.

Subsequently, as shown in FIG.
The surface of the island 6 made of the i film is irradiated with the laser beam 7 to recrystallize the a-Si film, thereby forming the polycrystalline silicon thin film 8. At this time, a short pulse laser having a high energy density such as F ₂ , ArF, KrF, X
Throughput is improved by using an excimer laser such as eF.

FIG. 2 shows a state of recrystallization by excimer laser annealing in this embodiment. Figure (a-1)
To (a-3) show the recrystallized state, and (b-1) to (b-
3) shows the temperature distribution in that state. In recrystallization by the laser beam, FIG. 2 (a-
As shown in FIG. 1B , since the island shape is hexagonal by laser beam irradiation, as shown in FIG.
The surface temperature exceeds the melting temperature (T _M ), and a convex distribution with a low temperature at the end can appear. In other words, the surface of the portion where heat concentration occurs due to the laser beam has a slope with respect to the irradiation direction of the laser beam, so that the surface temperature exceeds the melting temperature (T _M ) and the end portion has a low temperature. Appears.

Then, due to heat conduction, a temperature distribution is generated such that thermal distortion is small as shown in FIG. Simultaneously, the molten silicon extruded to the end due to wettability with the substrate is absorbed by the inclined portion of the end, and the substrate surface is flattened as shown in FIG. Then, FIG.
As shown in -3), the crystallization rapidly proceeds from the point where the temperature has become equal to or lower than the melting temperature, but since the temperature is uniform as a whole, the whole is liable to become large, and the average crystal diameter is improved. At the same time, as shown in FIG. 2A-3, unevenness on the surface can be suppressed.

FIG. 3 shows the surface irregularities after recrystallization when the island made of a-Si has a hexagonal cross section, as shown in FIG. 1, and the top and bottom sides of the hexagon . 3 shows the results of measuring the dependence on the ratio. In FIG. 1, the uppermost side is l _O ′, the lower side is l _O , the film thickness at the highest position of the island before recrystallization is h _O , and the film thickness at the same highest position in the recrystallization is h ′, and the respective ratios were measured.

From FIG. 3, the surface irregularities are the uppermost side and the lowermost side.
Depending upon the ratio of the sides, l _O '/ l _O rapidly it can be seen that the irregularities increases from about 0.8. In the case of the conventional island (l _O ′ / l _O = 1), the surface irregularities are about 4
On the other hand, when l _O ′ / l _O = 0.5,
h ′ / h _O = 1, which indicates that there is almost no unevenness on the surface and a flat polycrystalline silicon film can be obtained.

FIG. 4 shows an island having a conventional shape,
The results obtained by measuring the dependence of the surface irregularities and the energy density of the laser beam on the island-shaped one in this embodiment in which l _O ′ / l _O was set to 0.5 are shown. From FIG.
In a conventional island, surface irregularities also depend on the energy density of the irradiating laser beam, and are about 350 mJ / cm.
It found to have a steep peak at ^2. In contrast, in the case of this example, no remarkable surface unevenness was observed within the measurement energy range.

FIG. 5 shows the relationship between the energy beam and the average grain size when the conventional example having the same shape as that shown in FIG. 4 is recrystallized by a laser beam using the island in this embodiment. Show. The average particle diameter is an average of the entire crystallized polycrystalline silicon film. In FIG. 5, the maximum average particle diameter is 1 and the ratio is shown.

As shown in FIG. 5, in the conventional island shape, the average grain size increases with an increase in the energy density corresponding to the energy density of the laser beam to be irradiated.
It has a steep peak at 0 mJ / cm ^2, and sharply decreases at higher energies. On the other hand, in the case of this embodiment, a broad peak is obtained at about 200 to 400 mJ / cm ² . Especially about 210 to 380 mJ /
In the range of cm ² , the average particle size is almost constant.

In the above-described embodiment, a substrate in which an insulating layer is formed on a conductive substrate is used.
The same effect can be obtained by using an insulating substrate such as quartz glass or low melting point glass. Here, a polycrystalline silicon TFT manufactured by the method of manufacturing a polycrystalline silicon film of the present invention as described above and a method of manufacturing a pixel portion of a transmissive LCD (Liquid Crystal Display) using the TFT as a pixel driving element will be described. Description will be made with reference to the drawings.

FIG. 6 is a specific plan structure diagram around the pixel portion, and FIG. 7 is a cross-sectional structure diagram taken along a cutting line AA in FIG. The pixel part is a TFT as a driving element
And a liquid crystal cell and an auxiliary capacitor CS. The gate G of the TFT is connected to the gate wiring Gm, and the drain D of the TFT is connected to the drain wiring Dn. The display electrode 22 of the liquid crystal cell and the storage capacitor CS are connected to the source of the TFT. The liquid crystal cell and the storage capacitor constitute a signal storage element.

As shown in FIG. 7A, an insulating film 2 is formed on the entire surface.
A hexagonal polycrystalline silicon film 8 serving as an active layer of a TFT is formed on the substrate 1 on which is formed by the present method. Furthermore, as shown in FIG. 7 (b), the polycrystalline silicon film
8 , a normal pressure CVD (AP-CVD) method, a low pressure CVD (L
A gate insulating film 9 is formed by using a P-CVD method or the like, and a polycrystalline silicon film 10 is formed thereon by using a thermal CVD method.

After that, as shown in FIG. 7C, a resist 11 is patterned on the polycrystalline silicon film 10 and the polycrystalline silicon film is etched to form a gate electrode 12.
To form The gate electrode 12 may be formed of a metal, for example, aluminum, chromium, or the like by an evaporation method or a sputtering method. The method for forming the gate insulating film 9 includes a normal pressure CVD (AP-CVD) method,
A VD (LP-CVD) method or the like is used. As a material of the gate insulating film, a silicon oxide film, a silicate glass, a silicon nitride film, or the like is used.

Then, as shown in FIG. 7D, patterning 13 is performed on the gate insulating film 9, an opening 14 is formed in the gate insulating film using anisotropic etching, and an ion shower doping method or the like is performed. Doping 15 with an n-type impurity such as phosphorus. Further, as shown in FIG. 8E, an n-type drain region 16 and a source region 17 are formed in the polycrystalline silicon film. At the same time, the gate electrode is also doped with an n-type impurity such as phosphorus. Thereby, the resistance of the gate electrode is reduced.

As shown in FIG. 8F, indium tin oxide (ITO) is formed on the pixel region of the substrate.
e) An auxiliary capacitance electrode 18 made of ITO or the like is formed.
Further, a gate wiring 19 made of a metal such as molybdenum, a metal silicide, or a polycrystalline silicon film is formed on the gate electrode by a sputtering method. Further, as shown in FIG. 8G, an interlayer insulating film 20 made of silicon nitride or the like is formed on the entire surface of the substrate. And
The interlayer insulating film 20 is partially removed by etching, and a contact hole 21 is formed above the drain region 16 and the source region 17.

Then, as shown in FIG. 8 (h), an IT is formed on the interlayer insulating film located in the pixel portion by sputtering.
A display electrode 22 made of O is formed. A part of the display electrode 22 is electrically connected to the source region through the contact hole 21. Further, after a conductive material is formed on the entire surface, patterning is performed to form a drain electrode 23 and a source electrode 24 connected to the drain region 16 and the source region 17, respectively.

Through the above steps, a TFT using a polycrystalline silicon film as an active layer is completed. As shown in FIG. 9, a pixel portion of an LCD using the above-described TFT as a pixel driving element has a transparent insulating substrate 25 on which a polycrystalline silicon TFT is formed and a transparent insulating substrate 25 on a surface of which a common electrode 26 is formed. 27 are opposed to each other, and a liquid crystal is sealed between the substrates to form a liquid crystal layer 28.

FIG. 10 shows an active matrix type LCD block configuration of this embodiment. Gn, Gn + 1... Gm and data lines (drain wires) D1... Dn, Dn + 1. ing. Each gate line and each drain line are orthogonal to each other, and a pixel 30 is provided at the orthogonal portion. Each gate wiring is connected to a gate driver 31 so that a gate signal (scan signal) is applied. Each drain wiring is connected to a drain driver (data driver) 32 so that a data signal (video signal) is applied. A peripheral drive circuit 33 is configured by these drivers. And
At least one of the drivers is connected to the pixel unit 29.
LCDs formed on the same substrate are generally referred to as driver-integrated (driver-embedded) LCDs. Note that the gate driver 31 may be provided on both sides of the pixel unit 29 in some cases. Further, the drain driver 32 may be provided on both sides of the pixel unit 29 in some cases.

FIG. 11 shows a gate wiring Gn and a drain wiring Dn.
2 shows an equivalent circuit of a pixel provided in a portion orthogonal to FIG.
Pixels include a TFT as a pixel driving element, a liquid crystal cell LC,
It is composed of auxiliary capacity. The gate wiring Gn has a TFT
, And the drain of the TFT is connected to the drain wiring Dn. The display electrode (pixel electrode) of the liquid crystal cell LC and the auxiliary capacitance (storage capacitance or additional capacitance) are connected to the source of the TFT. The liquid crystal cell LC and the storage capacitor constitute the signal storage element. The voltage Vcom is applied to the common electrode (the electrode on the opposite side of the auxiliary capacitance electrode) of the liquid crystal cell LC. On the other hand, in the auxiliary capacitance, a constant voltage VR is applied to an electrode on the opposite side of the electrode connected to the source of the TFT. The common electrode of the liquid crystal cell LC is an electrode that is literally common to all pixels. Further, a capacitance is formed between the display electrode and the common electrode of the liquid crystal cell LC. In the auxiliary capacitance, the electrode on the opposite side of the electrode connected to the source of the TFT may be connected to the adjacent gate line Gn + 1.

In the pixel thus configured, when the gate line Gn is set to a positive voltage and a positive voltage is applied to the gate of the TFT, the TFT turns on. Then, the capacitance and the auxiliary capacitance of the liquid crystal cell LC are charged by the data signal applied to the drain wiring Dn. Conversely, the gate wiring Gn
When a negative voltage is applied to the TFT gate by setting
The TFT is turned off, and the voltage applied to the drain wiring Dn at that time is held by the capacitance and the auxiliary capacitance of the liquid crystal cell LC. In this manner, by supplying a data signal to be written to a pixel to the drain wiring Dn and controlling the voltage of the gate wiring Gn, an arbitrary data signal can be held in the pixel. The transmittance of the liquid crystal cell LC changes according to the data signal held by the pixel, and an image is displayed.

[0035]

As described above, according to the present invention, a polycrystalline semiconductor film having a flat surface and a uniform crystal grain size can be formed with good reproducibility. In addition, since a large margin of laser beam energy density can be obtained, the cost of an apparatus used for crystallization can be reduced.

Further, since the occurrence of unevenness on the surface can be suppressed, the flattening step can be omitted, the manufacturing cost can be reduced, and the yield can be improved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a method of manufacturing a polycrystalline semiconductor film according to the present invention for each process.

FIG. 2 is a schematic diagram showing the relationship between the state of recrystallization and the temperature distribution according to the present invention.

FIG. 3 is a characteristic diagram showing a relationship between an island cross-sectional shape and surface irregularities.

FIG. 4 is a diagram showing the results of measuring the dependence of surface irregularities on the basis of the energy densities of an example of the present invention and a conventional example.

FIG. 5 is a cross-sectional view showing a relationship between an energy density and an average particle diameter of an example of the present invention and a conventional example.

FIG. 6 is a specific plan structure diagram around a pixel unit.

FIG. 7 is a sectional structural view taken along a cutting line AA in FIG. 6;

FIG. 8 is a sectional structural view taken along a cutting line AA in FIG. 6;

FIG. 9 is a cross-sectional view of a pixel portion of an LCD using the above-described TFT as a pixel driving element.

FIG. 10 illustrates an active matrix type L according to the present embodiment.
FIG. 3 is a block diagram of a CD.

FIG. 11 is an equivalent circuit diagram of a pixel provided at a portion orthogonal to a gate line Gn and a drain line Dn.

FIG. 12 is a cross-sectional view showing a conventional method of manufacturing a polycrystalline semiconductor film for each process.

FIG. 13 is a schematic diagram showing a relationship between a conventional recrystallization state and a temperature distribution.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 4 a-Si film 6a Inclined part 7 Laser beam 8 Polycrystalline silicon film

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-6-314698 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 29/786 H01L 21/20 H01L 21 / 336

Claims (2)

(57) [Claims] An amorphous semiconductor thin film provided on a substrate is provided with an inclined portion at an end thereof, and then irradiated with a laser beam to crystallize the amorphous semiconductor thin film to form a polycrystalline semiconductor. Forming a film, wherein the cross-sectional shape of the amorphous semiconductor thin film is such that both upper corners of a square form the inclined portion.
It has a hexagon cut out by providing
The ratio between the uppermost side and the lowermost side of the square is 0.8 or less, and the output of the laser beam is 200 mJ / cm ^{2 to} 400 mJ.
/ Cm ² . 2. The output of the laser beam is 210 mJ /
The method for producing a polycrystalline semiconductor film according to claim 1, wherein the control is performed at cm ^{2 to} 380 mJ / cm ² .