JP2012015140A - Thin film transistor manufacturing method, thin film transistor and display device - Google Patents

Abstract

A method of manufacturing a thin film transistor capable of improving reliability and improving electrical characteristics is provided.
A polycrystalline silicon film is formed in a vertical growth mode by irradiating an amorphous silicon film formed on a glass substrate with laser light from the back side of the glass substrate. The polycrystalline silicon film 30 is formed by solidifying the seed crystal formed at a high density on the surface of the molten semiconductor toward the glass substrate 11 side. As a result, an incomplete crystal layer 32 including many microcrystalline silicon regions is formed near the surface of the polycrystalline silicon film 30. Therefore, the incomplete crystal layer 32 is removed by etching to form a polycrystalline silicon film 33, and a TFT having the polycrystalline silicon film 33 as an active layer is manufactured.
[Selection] Figure 4

Description

  The present invention relates to a method for manufacturing a thin film transistor, a thin film transistor, and a display device. More specifically, the present invention relates to a method for manufacturing a thin film transistor, a thin film transistor, and a display device that are suitably used for an active matrix display device.

  In recent years, active matrix liquid crystal display devices and organic EL (Electro Luminescence) display devices driven by thin film transistors (hereinafter referred to as “TFTs”) have attracted attention. In order to improve display quality in such a display device, a TFT having an active layer made of polycrystalline silicon instead of an active layer made of amorphous silicon is used. A TFT using polycrystalline silicon can greatly improve mobility compared to a TFT using amorphous silicon. For this reason, TFTs using polycrystalline silicon have come to be used not only as switching elements in pixel forming portions but also as TFTs constituting peripheral circuits such as gate drivers and source drivers. As a method for forming such polycrystalline silicon, a solid phase growth method, a laser annealing method, and the like are known. In particular, a laser annealing method using an excimer laser is the most preferable method for forming polycrystalline silicon having high crystallinity. Many are used.

  However, when the laser annealing method is used, the temperature gradient between the molten silicon (hereinafter referred to as “molten silicon”) and the substrate is increased by irradiating the laser beam, so the molten silicon solidifies quickly and becomes polycrystalline silicon. become. For this reason, the crystal grain of the polycrystalline silicon formed by the laser annealing method becomes small.

  Patent Document 1 discloses a laser annealing method for forming polycrystalline silicon having large crystal grains. Specifically, first, an amorphous silicon film is formed on the surface of the base coat film formed on the glass substrate. Next, XeCl excimer laser (hereinafter referred to as “laser light”) is irradiated from the back side of the glass substrate to melt the amorphous silicon film, and then solidify. At this time, since the glass substrate and the amorphous silicon film are simultaneously heated by the laser beam, the temperature gradient between the glass substrate and the amorphous silicon film is reduced. As a result, the molten silicon solidifies slowly, so that a polycrystalline silicon film having a large crystal grain size is formed.

  In addition, when a polycrystalline silicon film is formed by irradiating an amorphous silicon film with laser light from the surface side of the glass substrate, solidification proceeds from the back surface to the surface, so that unevenness having a large step of about 50 nm, for example. Is formed on the surface of the polycrystalline silicon film. If a gate insulating film is formed on the surface of such a polycrystalline silicon film, the thickness of the gate insulating film is locally reduced at the convex portions. For this reason, dielectric breakdown is likely to occur at the thinned portion of the gate insulating film during the operation of the TFT, and the reliability of the TFT is lowered.

  Therefore, as disclosed in Patent Document 1, the amorphous silicon film is irradiated with laser light from the back side of the glass substrate. The amorphous silicon film becomes molten silicon, and the back surface temperature is higher than the surface temperature. For this reason, solidification of molten silicon starts from the front surface and proceeds toward the back surface. In this case, unlike the case where the laser beam is irradiated from the surface side of the glass substrate, the uneven step formed on the surface of the polycrystalline silicon film is reduced to, for example, about 30 nm. Even if a gate insulating film is formed on the surface of such a polycrystalline silicon film, the thickness of the gate insulating film is not easily locally reduced at the convex portion. As a result, the gate insulating film is difficult to break down, and the reliability of the TFT can be improved.

JP-A-9-330879

  However, when the electrical characteristics of a TFT having a polycrystalline silicon film formed by using the laser annealing method described in Patent Document 1 as an active layer were measured, the mobility was 184 cm 2 / V · s, and the S coefficient ( subthreshold voltage swing) was 0.32 V / deg. On the other hand, the mobility in a TFT having a polycrystalline silicon film formed by irradiating laser light from the surface side of a glass substrate as an active layer was 220 cm 2 / V · s, and the S coefficient was 0.25 V / deg. From this, it was found that when the laser beam was irradiated from the back side of the glass substrate, each electrical characteristic was deteriorated by about 20% compared to the case of irradiation from the front side of the glass substrate.

  Accordingly, an object of the present invention is to provide a method of manufacturing a thin film transistor capable of improving reliability and improving electrical characteristics. Another object of the present invention is to provide a thin film transistor and a display device manufactured by such a manufacturing method.

The first invention is a method of manufacturing a thin film transistor formed on an insulating substrate transparent in the visible light region,
Forming a semiconductor film on the surface of the insulating substrate;
Irradiating the semiconductor film with a laser beam from the back side of the insulating substrate to form a first polycrystalline semiconductor film having an incomplete crystal layer including a microcrystalline semiconductor region on a surface in a vertical growth mode;
Removing the incomplete crystalline layer from the first polycrystalline semiconductor film to form a second polycrystalline semiconductor film;
Patterning the second polycrystalline semiconductor film to form an active layer;
Forming a gate insulating film so as to cover the active layer;
Forming a gate electrode on the surface of the gate insulating film.

According to a second invention, in the first invention,
The incomplete crystal layer is removed by removing the incomplete crystal layer from the first polycrystalline semiconductor film by using a dry etching method.

According to a third invention, in the first invention,
The incomplete crystal layer is removed by removing the incomplete crystal layer from the first polycrystalline semiconductor film using a wet etching method.

According to a fourth invention, in any one of the first to third inventions,
The semiconductor film is an amorphous semiconductor film.

According to a fifth invention, in any one of the first to fourth inventions,
A part of the laser light is light having a wavelength that is absorbed by the insulating substrate.

According to a sixth invention, in the fifth invention,
The laser beam is a pulsed XeCl excimer laser.

  A seventh invention is a thin film transistor manufactured by the method of manufacturing a thin film transistor according to any one of the first to sixth inventions.

An eighth invention is a display device comprising the thin film transistor according to the seventh invention and an image display unit,
The thin film transistor is used as a switching element of the image display unit.

In a ninth aspect based on the eighth aspect,
A peripheral circuit for driving the image display unit;
The peripheral circuit includes the thin film transistor.

  According to the first aspect of the present invention, the first polycrystal is formed in the vertical growth mode by irradiating the semiconductor film formed on the transparent insulating substrate in the visible light region with the laser beam from the back surface side of the insulating substrate. A semiconductor film is formed. The first polycrystalline semiconductor film is formed by solidifying the seed crystal formed at a high density on the surface of the molten semiconductor toward the insulating substrate. A region containing a microcrystalline semiconductor is formed in the vicinity of the surface of the first polycrystalline semiconductor film thus formed. Therefore, the region including the microcrystalline semiconductor is removed by etching to form a second polycrystalline semiconductor film. As a result, the unevenness of small steps formed on the surface of the first polycrystalline semiconductor film is transferred to the surface of the second polycrystalline semiconductor film while maintaining the uneven steps, thereby improving the reliability. Thin film transistors can be manufactured. Furthermore, since the crystallinity of the second polycrystalline semiconductor film is increased, it is possible to manufacture a thin film transistor with improved electrical characteristics, such as an increase in carrier mobility and a decrease in S coefficient. it can.

  According to the second aspect, since the incomplete crystal layer formed on the surface of the first polycrystalline semiconductor film is removed by the dry etching method, the etching conditions are adjusted and the in-plane variation in film thickness is small. A second polycrystalline semiconductor film can be formed. Thereby, a thin film transistor having stable electrical characteristics can be manufactured.

  According to the third aspect, the incomplete crystal layer formed on the surface of the first polycrystalline semiconductor film can be easily removed by wet etching. Thereby, the manufacturing method of a thin-film transistor can be simplified.

  According to the fourth aspect, since the amorphous semiconductor film is a semiconductor film that can be easily formed, the method for manufacturing the thin film transistor can be simplified.

  According to the fifth aspect, a part of the laser light is absorbed by the insulating substrate, so that the temperature of the insulating substrate is increased and the temperature gradient between the insulating substrate and the molten semiconductor is decreased. For this reason, the melted semiconductor is slowly solidified to become a first polycrystalline semiconductor film having large crystal grains. Thus, a thin film transistor with improved electrical characteristics can be manufactured.

  According to the sixth aspect of the invention, by using the XeCl excimer laser as the laser light, the ratio of the laser light absorbed by the insulating substrate is increased, and the temperature gradient between the insulating substrate and the molten semiconductor is further reduced. Can do. In addition, since the laser light is pulse-oscillated, the entire semiconductor film is not melted. Thereby, since the crystal grains of the first polycrystalline semiconductor film can be made larger, a thin film transistor with improved electrical characteristics can be manufactured.

  According to the seventh aspect, the uneven step formed on the surface of the second polycrystalline semiconductor film is small and its crystallinity is high. By using such a second polycrystalline semiconductor film as an active layer, it is possible to make it difficult for dielectric breakdown of the gate insulating film to occur and to increase crystallinity. Thus, a thin film transistor with improved reliability and improved electrical characteristics can be obtained.

  According to the eighth aspect, when a thin film transistor is used as a switching element in a pixel formation portion of a display device, the mobility of the thin film transistor is large and the S coefficient is small, so that the switching operation can be performed at high speed. As a result, the thin film transistor can charge the pixel capacitor with the video signal supplied from the source wiring in a short time, so that the number of the pixel forming portions can be increased and the image display portion can be made high definition.

  According to the ninth aspect, if the peripheral circuit is configured using thin film transistors, the operation speed of the peripheral circuit can be increased. As a result, the circuit scale of the peripheral circuit is reduced, so that the frame portion of the display panel on which the image display unit is formed can be reduced, and the display device can be reduced in size. In addition, the display device can have high performance and high image quality.

(A) is sectional drawing of the glass substrate which formed the amorphous silicon film through the basecoat film, (b) is a laser to an amorphous silicon film from the back surface side of the glass substrate shown to (a). It is sectional drawing which shows the laser annealing method which irradiates light and forms a polycrystalline-silicon film. FIG. 2 is a cross-sectional view of a polycrystalline silicon film formed by the laser annealing method shown in FIG. FIG. 3 is a cross-sectional view of the polycrystalline silicon film from which the incomplete crystal layer shown in FIG. 2 is removed. (A)-(d) is process sectional drawing which shows each manufacturing process of the thin-film transistor which concerns on embodiment of this invention. (A)-(c) is process sectional drawing which shows each manufacturing process of the thin-film transistor which concerns on embodiment of this invention. (A) is a perspective view which shows the liquid crystal panel of an active-matrix liquid crystal display device, (b) is a perspective view which shows the TFT substrate contained in the liquid crystal panel shown to (a).

<1. Basic study>
After an amorphous silicon film is formed on the surface of the base coat film formed on the glass substrate which is a transparent insulating substrate in the visible light region, the amorphous silicon film is irradiated with laser light from the back side of the glass substrate ( Hereinafter, a problem and a method for solving the problem in forming a polycrystalline silicon film will be examined.

  FIG. 1A is a cross-sectional view of a glass substrate 11 on which an amorphous silicon film 20 is formed via a base coat film 15, and FIG. 1B is a cross-sectional view from the back side of the glass substrate 11 shown in FIG. 4 is a cross-sectional view showing a laser annealing method for forming a polycrystalline silicon film 30 by irradiating a crystalline silicon film 20 with laser light. FIG. As shown in FIG. 1A, a base coat film 15 made of silicon nitride (SiNx) or the like is formed on a glass substrate 11, and an amorphous silicon film 20 is formed on the surface of the base coat film 15.

  As shown in FIG. 1B, the amorphous silicon film 20 is irradiated with a laser beam on the back surface (irradiated from the lower side of FIG. 1B). At this time, the energy density and scanning speed of the laser beam to be irradiated are adjusted so that the amorphous silicon film 20 does not melt (full melt) in all layers. The amorphous silicon film 20 becomes molten silicon 21 when irradiated with laser light, and further crystal grows in a vertical growth mode to become a polycrystalline silicon film 30.

A TFT having the polycrystalline silicon film 30 formed by such a method as an active layer was manufactured, and its mobility and S coefficient were measured. As a result, the mobility of the TFT was 150 cm ² / V · s, and the S coefficient was 0.39 V / deg.

On the other hand, a polycrystalline silicon film is formed by irradiating the amorphous silicon film with laser light (hereinafter referred to as “surface irradiation”) from the surface side of the glass substrate. A TFT having this polycrystalline silicon film as an active layer was manufactured, and its mobility and S coefficient were measured. As a result, the mobility of the TFT was 220 cm ² / V · s, and the S coefficient was 0.25 V / deg. The S coefficient is defined as a gate voltage required to change the drain current by one digit in a state where the gate voltage is equal to or lower than the threshold voltage. The smaller the S coefficient, the faster the on / off switching. .

  From the above results, the TFT using the polycrystalline silicon film 30 formed by the backside irradiation as the active layer has an electrical characteristic of 20% as compared with the TFT using the polycrystalline silicon film formed by the surface irradiation as the active layer. It turned out to be worse.

  FIG. 2 is a cross-sectional view showing the structure of the polycrystalline silicon film 30 formed by backside irradiation. As shown in FIG. 2, the amorphous silicon film 20 (semiconductor film) is not crystallized on the surface of the polycrystalline silicon film 30 and remains from the amorphous silicon or small crystal grains of 0.1 μm or less. There is a region containing microcrystalline silicon (hereinafter referred to as “microcrystalline silicon region 31”).

  The microcrystalline silicon region 31 is considered to be formed on the surface of the polycrystalline silicon film 30 as follows. When the amorphous silicon film 20 becomes the molten silicon 21 by the back surface irradiation, seed crystals are formed at a high density on the surface whose temperature is lower than the back surface of the molten silicon 21 (the surface on the glass substrate 11 side). Note that the seed crystal is not shown in FIG. 2 because it is so small that it cannot be seen with the naked eye. The molten silicon 21 gradually grows from the seed crystal formed on the surface thereof toward the back surface, and becomes a polycrystalline silicon film 30. At this time, since the crystallization on the surface side at a low temperature does not proceed sufficiently, the microcrystalline silicon region 31 is likely to be formed near the surface. In particular, the microcrystalline silicon region 31 is easily formed between the surface of the polycrystalline silicon film 30 and a depth of 10 to 20 μm. Therefore, in this specification, a layer that is formed near the surface of the polycrystalline silicon film 30 and includes a large amount of the microcrystalline silicon region 31 is referred to as an incomplete crystal layer 32.

  On the other hand, in the polycrystalline silicon film formed by surface irradiation, the seed crystal is formed on the back surface (glass substrate side) of the molten silicon. The molten silicon gradually grows from the seed crystal formed on the back surface toward the front surface side to become a polycrystalline silicon film. For this reason, a polycrystalline silicon film formed by front surface irradiation has many microcrystalline silicon regions near the back surface thereof.

  When an active layer of a TFT is formed using the polycrystalline silicon film 30, a large number of microcrystalline silicon regions 31 are present on the surface of the polycrystalline silicon film 30 on the gate electrode side through which an on-current flows, so that the number of crystal grain boundaries increases. The mobility becomes small. On the other hand, when a TFT active layer is formed using a polycrystalline silicon film formed by surface irradiation, there are almost no microcrystalline silicon regions on the surface of the polycrystalline silicon film on the gate electrode side, so there are fewer grain boundaries. , Mobility increases.

  Further, when the TFT active layer is formed using the polycrystalline silicon film 30, since there are many microcrystalline silicon regions 31 on the surface of the polycrystalline silicon film 30 on the gate electrode side, the gate voltage is reduced when the TFT is turned off. The reduction rate of the off-current due to the decrease decreases, and the S coefficient increases. On the other hand, when the active layer of the TFT is formed using a polycrystalline silicon film formed by surface irradiation, since there is almost no microcrystalline silicon region on the surface of the polycrystalline silicon film on the gate electrode side, the reduction rate of off-current is The S coefficient becomes small as it increases. Therefore, a TFT having an active layer made of a polycrystalline silicon film formed by front surface irradiation is a TFT having an active layer made of a polycrystalline silicon film 30 having a microcrystalline silicon region 31 on the surface formed by back surface irradiation. It can be seen that better electrical characteristics are exhibited.

  On the other hand, as described above, in the polycrystalline silicon film 30 formed by the backside irradiation, the uneven step formed on the surface is reduced. For this reason, in the TFT having the polycrystalline silicon film 30 formed by the backside irradiation as the active layer, the dielectric breakdown of the gate insulating film hardly occurs, and the reliability is improved.

  Therefore, it can be understood that the above-described problems can be solved by removing the incomplete crystal layer 32 on the front surface after forming the polycrystalline silicon film 30 by backside irradiation. FIG. 3 is a cross-sectional view of the polycrystalline silicon film 33 with the incomplete crystal layer 32 removed. As shown in FIG. 3, the incomplete crystal layer 32 on the surface of the polycrystalline silicon film 30 formed by backside illumination is removed by etching, leaving a polycrystalline silicon film 33 (second polycrystalline silicon film). In the TFT using the polycrystalline silicon film 33 thus obtained as an active layer, reliability is improved and electrical characteristics are improved. The unevenness formed on the surface of the polycrystalline silicon film 30 when the molten silicon 21 is solidified by backside irradiation is transferred to the surface of the polycrystalline silicon film 33 even after the incomplete crystalline layer 32 is removed by etching, and the same step is formed. It is left in a state of maintaining.

<2. Thin Film Transistor Manufacturing Method>
4 and 5 are process cross-sectional views showing respective manufacturing processes of the TFT 10 using the polycrystalline silicon film 30 crystallized by backside irradiation as an active layer. The same components as those shown in FIGS. 1 to 3 will be described with the same reference numerals.

As shown in FIG. 4A, a base coat film 15 is formed on a glass substrate 11 which is an insulating substrate transparent in the visible light region. The base coat film 15 is made of silicon nitride or silicon oxide (SiO ₂ ), and is formed by a plasma enhanced chemical vapor deposition (plasma CVD) method or the like. An amorphous silicon film 20 having a thickness of 30 to 70 nm is formed on the surface of the base coat film. The amorphous silicon film 20 is formed by a low pressure chemical vapor deposition (CVD) method using hydrogen (H ₂ ) gas and monosilane (SiH ₄ ) gas as source gases.

As shown in FIG. 4B, the amorphous silicon film 20 is annealed by irradiating a laser beam from the back side of the glass substrate 11 (the lower side of FIG. 4B). The laser beam to be irradiated is a pulsed XeCl excimer laser with a wavelength of 308 nm. In order to form the polycrystalline silicon film 30 by crystallization in the vertical growth mode, it is necessary to adjust the energy density and scanning speed of the laser beam so that the amorphous silicon film 20 does not melt all layers. In this embodiment, for example, the energy density is 200 to 500 mJ / cm ² and the scan speed is 5 to 10 mm / sec. 2B is also formed in the incomplete crystal layer 32 shown in FIG. 4B, the illustration of the incomplete crystal region 31 is omitted in FIG. 4B. Further, in this embodiment, if the amorphous silicon film 20 is irradiated with a continuous wave laser instead of the pulsed laser, the amorphous silicon film 20 is entirely melted. For this reason, in this embodiment, a continuous wave laser cannot be used.

  About 50% of the laser beam having a wavelength of 308 nm is absorbed by the glass substrate 11 and used to heat the glass substrate 11. The remaining approximately 50% of the laser light that has not been absorbed by the glass substrate 11 is used to heat the amorphous silicon film 20 and convert it into molten silicon 21. Thus, the amorphous silicon film 20 and the glass substrate 11 can be heated simultaneously by irradiating the laser beam once. Thereby, since the temperature gradient between the glass substrate 11 and the molten silicon 21 is reduced, the polycrystalline silicon film 30 having a large crystal grain size can be formed.

  Further, since the temperature of the molten silicon 21 is high on the back surface side and low on the front surface side, a large number of seed crystals are formed on the surface of the molten silicon 21. Solidification of the molten silicon 21 proceeds gradually from the seed crystal toward the back surface. Of the polycrystalline silicon film 30 formed in this way, the portion from the surface to a depth of 10 to 20 nm is an incomplete crystal layer 32 in which many microcrystalline silicon regions 31 exist. Further, since the solidification proceeds from the surface of the molten silicon 21 toward the back surface, the uneven step formed on the surface of the incomplete crystal layer 32 is small.

As studied in the basic study, the incomplete crystal layer 32 on the surface of the polycrystalline silicon film 30 is removed by a dry etching method to form a polycrystalline silicon film 33 (see FIG. 4C). As an etching gas, a fluorine-based gas such as carbon tetrafluoride (CF ₄ ) gas or sulfur hexafluoride (SF 6) gas, or a chlorine-based gas such as carbon tetrachloride (CCl ₄ ) gas is used. For example, when CF4 gas is used, it is preferable to perform etching under the conditions that the gas flow rate is 100 to 300 sccm, the RF power is 1000 to 5000 W, the temperature in the chamber is 30 to 50 ° C., and the pressure in the chamber is 3 to 50 Pa. . In this case, the etching rate of the polycrystalline silicon film is 0.5 to 2 nm / sec, and the in-plane variation of the polycrystalline silicon film 33 after etching can be suppressed to about ± 3 nm.

  As described above, when the dry etching method is used, the thickness of the polycrystalline silicon film 33 remaining after etching the incomplete crystal layer 32 becomes substantially uniform in the plane. Therefore, the TFT 10 using the polycrystalline silicon film 33 as an active layer. The electrical characteristics of can be further improved. Even if the incomplete crystal layer 32 is removed by etching, the unevenness formed on the surface thereof is transferred to the surface of the polycrystalline silicon film 33 while maintaining the level difference. However, since the uneven step formed on the surface of the imperfect crystal layer 32 by the backside irradiation is smaller than the uneven step formed by the front surface irradiation, the uneven step transferred to the surface of the polycrystalline silicon film 33. Becomes smaller.

Note that the incomplete crystal layer 32 may be removed using a wet etching method instead of the dry etching method. As an etchant for wet etching the incomplete crystal layer 32, for example, a mixed solution containing hydrofluoric acid (HF), nitric acid (HNO ₃ ), and acetic acid (CH ₃ COOH) is used.

  As shown in FIG. 4D, a resist pattern 35 having a desired shape is formed on the surface of the polycrystalline silicon film 33 from which the incomplete crystal layer 32 has been removed. Next, the polycrystalline silicon film 33 is etched using the resist pattern 35 as a mask to form an island-shaped active layer 40.

  Further, as shown in FIG. 5A, a gate insulating film 50 is formed so as to cover the entire surface of the glass substrate 11 including the active layer 40. The gate insulating film 50 is made of, for example, silicon oxide having a thickness of 100 nm, and is formed by a plasma CVD method or the like. Next, a tantalum (Ta) film 60 is formed on the surface of the gate insulating film 50 by a sputtering method, and the tantalum film 60 is patterned by using a photolithography method. Thereby, the gate electrode 70 is formed at the position of the gate insulating film 50 corresponding to the central portion of the active layer 40. Instead of the tantalum film 60, a metal film such as a tungsten (W) film, a titanium (Ti) film, a molybdenum (Mo) film, or a stacked metal film formed by stacking them may be formed.

  Using gate electrode 70 as a mask, active layer 40 is doped with n-type impurity ions such as phosphorus (P) ions by ion implantation or ion doping. Thereafter, the n-type impurity ions doped in the active layer 40 are activated by irradiating laser light from above the gate electrode 70. As a result, a source region 41 and a drain region 42 composed of high-concentration n-type regions are formed at both ends of the active layer 40, respectively, and a region sandwiched between the source region 41 and the drain region 42 becomes a channel region 43.

As shown in FIG. 5C, an interlayer insulating film 80 is formed so as to cover the entire surface of the glass substrate 11 including the gate electrode 70. The interlayer insulating film 80 is made of silicon oxide, and is formed using a plasma CVD method using, for example, TEOS (Tetra Ethyl Ortho Silicate: Si (OC ₂ H ₅ ) ₄ ).

  Contact holes 81 and 82 reaching the source region 41 and the drain region 42 and contact holes (not shown) reaching the gate electrode 70 are opened in the interlayer insulating film 80. A metal film such as an aluminum (Al) film (not shown) is formed on the surface of the interlayer insulating film 80 including the insides of the contact holes 81 and 82 by sputtering. Next, by patterning the aluminum film using a photolithography method, the source electrode 91 electrically connected to the source region 41 through the contact hole 81 and the drain region 42 through the contact hole 82 are electrically connected. A drain electrode 92 connected in a connected manner is formed. Thereafter, heat treatment is performed in a hydrogen atmosphere to complete the n-channel TFT 10.

<2.2 Effect>
According to the present embodiment, since the uneven step on the surface of the polycrystalline silicon film 30 formed by backside illumination can be reduced, the TFT 10 with improved reliability can be manufactured. Furthermore, since the crystallinity of the active layer 40 of the TFT 10 is high, the electrical characteristics of the TFT 10 can be improved, for example, the carrier mobility can be increased and the S coefficient can be decreased.

  If the dry etching method is used to remove the incomplete crystal layer 32 in which many microcrystalline silicon regions 31 are present from the polycrystalline silicon film 30, the incomplete crystal layer 32 can be uniformly etched. As a result, the polycrystalline silicon film 33 having a substantially uniform in-plane film thickness can be easily formed, and the TFT 10 with stable electrical characteristics can be manufactured.

    If the wet etching method is used to remove the incomplete crystal layer 32 in which many microcrystalline silicon regions 31 are present from the polycrystalline silicon film 30, the incomplete crystal layer 32 can be easily removed. The method can be simplified.

<2.3 Modification>
In this embodiment, a XeCl excimer laser is used as a laser beam used for laser annealing. However, the laser is not limited to the XeCl excimer laser, and any laser having a wavelength with a high absorption rate by the glass substrate 11 may be used.

  In the present embodiment, the amorphous silicon film 20 and the polycrystalline silicon film 30 have been described as examples of the amorphous semiconductor film and the polycrystalline semiconductor film, respectively. However, the present embodiment can be similarly applied to semiconductor films such as an amorphous silicon germanium film and a polycrystalline silicon germanium film.

  In the present embodiment, the polycrystalline silicon film 30 is formed by melting and solidifying the amorphous silicon film 20 by laser annealing. However, a polycrystalline silicon film may be formed instead of the amorphous silicon film 20, and the polycrystalline silicon film may be melted and solidified by a laser annealing method. In this case, a polycrystalline silicon film with higher crystallinity can be formed.

  In this embodiment, phosphorus ions that are n-type impurities are implanted to form the source region 41 and the drain region 42. However, boron (B) ions that are p-type impurities may be implanted instead of phosphorus ions. In this case, the TFT is a p-channel TFT.

<3. Liquid crystal display>
FIG. 6A is a perspective view showing a liquid crystal panel 100 of an active matrix liquid crystal display device, and FIG. 6B is a perspective view showing a TFT substrate 120 included in the liquid crystal panel 100 shown in FIG. As shown in FIG. 6 (a), the liquid crystal panel 100 includes two glass substrates 120 and 140 disposed opposite to each other and a liquid crystal layer (not shown) sandwiched between the two glass substrates 120 and 140. It is a full monolithic panel including a sealing material 150 for sealing. Of the two glass substrates 120 and 140, a glass substrate in which a plurality of pixel forming portions including TFTs are formed in a matrix is called a TFT substrate 120, and is arranged to face the TFT substrate 120. ) Or the like is referred to as a CF substrate 140.

  As shown in FIG. 6B, the TFT substrate 120 includes an image display unit 130 in which a plurality of pixel formation units 131 are arranged. In the pixel formation portion 131, a switching element 132 and a pixel electrode 133 connected to the switching element 132 are formed. Peripheral circuits such as a source driver 121 and a gate driver 122 are provided on the outer frame portion of the image display unit 130. The gate driver 122 outputs a control signal for controlling the timing for turning on / off the switching element 132 to the gate wiring GL, and the source driver 121 outputs an image signal for displaying an image or an image signal on the pixel formation unit 131. A control signal for controlling is output to the source line SL.

  By sequentially activating the gate wiring GL and turning on the switching element 132 connected to the activated gate wiring GL, the image signal applied to the source wiring SL is connected to the pixel electrode via the switching element 132. 133. The pixel electrode 133 forms a pixel capacitance together with a common electrode (not shown) formed on the CF substrate 140 and holds a given image signal. Backlight emitted from a backlight unit (not shown) provided on the lower surface of the TFT substrate 120 is transmitted through the pixel forming unit 131 according to an image signal, and an image is displayed on the image display unit 130 of the liquid crystal panel 100. Is done.

  In such a liquid crystal panel 100, when the TFT 10 is used as the switching element 132 of the pixel formation portion 131, the TFT 10 has a high mobility and a small S coefficient, and therefore a switching operation can be performed at high speed. As a result, the TFT 10 can charge the video signal supplied from the source wiring SL to the pixel capacitor in a short time, so that the number of the pixel forming portions 131 is increased and the image display portion 130 of the liquid crystal panel 100 is made high definition. be able to.

  Further, if the peripheral circuit is configured using the TFT 10 having high mobility, the operation speed of the peripheral circuits such as the source driver 121 and the gate driver 122 can be increased. As a result, the circuit scale of the peripheral circuit is reduced, so that the frame portion of the liquid crystal panel 100 can be reduced and the liquid crystal panel 100 can be downsized. In addition, the liquid crystal display device can have high performance and high image quality.

  Note that a liquid crystal display device has been described as an example of a display device to which the TFT 10 can be applied. However, the TFT 10 can also be applied to a display device such as an organic EL (Electro Luminescence) display device or a plasma display device.

10 ... TFT (Thin Film Transistor)
11 ... Glass substrate 20 ... Amorphous silicon film (semiconductor film)
30. Polycrystalline silicon film (first polycrystalline silicon film)
31 ... Microcrystalline silicon region (region including microcrystalline silicon)
32 ... Incomplete crystal layer 33 ... Polycrystalline silicon film (second polycrystalline silicon film)
DESCRIPTION OF SYMBOLS 40 ... Active layer 50 ... Gate insulating film 70 ... Gate electrode 100 ... Liquid crystal panel 121 ... Source driver 122 ... Gate driver 130 ... Image display part 132 ... Switching element

Claims (9)

A method of manufacturing a thin film transistor formed on an insulating substrate transparent in the visible light region,
Forming a semiconductor film on the surface of the insulating substrate;
Irradiating the semiconductor film with a laser beam from the back side of the insulating substrate to form a first polycrystalline semiconductor film having an incomplete crystal layer including a microcrystalline semiconductor region on a surface in a vertical growth mode;
Removing the incomplete crystalline layer from the first polycrystalline semiconductor film to form a second polycrystalline semiconductor film;
Patterning the second polycrystalline semiconductor film to form an active layer;
Forming a gate insulating film so as to cover the active layer;
And a step of forming a gate electrode on the surface of the gate insulating film.   2. The method of manufacturing a thin film transistor according to claim 1, wherein the incomplete crystal layer is removed by removing the incomplete crystal layer from the first polycrystalline semiconductor film using a dry etching method.   2. The method of manufacturing a thin film transistor according to claim 1, wherein the incomplete crystal layer is removed by removing the incomplete crystal layer from the first polycrystalline semiconductor film using a wet etching method.   4. The method of manufacturing a thin film transistor according to claim 1, wherein the semiconductor film is an amorphous semiconductor film. 5.   5. The method of manufacturing a thin film transistor according to claim 1, wherein a part of the laser light is light having a wavelength that is absorbed by the insulating substrate. 6.   6. The method of manufacturing a thin film transistor according to claim 5, wherein the laser beam is a pulsed XeCl excimer laser.   A thin film transistor manufactured by the method for manufacturing a thin film transistor according to any one of claims 1 to 6. A display device comprising the thin film transistor according to claim 7 and an image display unit,
The display device, wherein the thin film transistor is used as a switching element of the image display unit. A peripheral circuit for driving the image display unit;
The display device according to claim 8, wherein the peripheral circuit includes the thin film transistor.

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