JP4570028B2 - Method for forming polysilicon pattern, method for manufacturing thin film transistor, and thin film transistor - Google Patents

Description

  The present invention relates to a method for forming a polysilicon pattern obtained by laser irradiation of amorphous silicon, a method for manufacturing a thin film transistor using the polysilicon pattern, and a thin film transistor obtained thereby.

  In the liquid crystal display, each pixel is driven by a thin film transistor (TFT). In order to obtain a high-definition liquid crystal display, a TFT using polysilicon having higher carrier mobility than amorphous silicon is required. Polysilicon is generally defined as an aggregate of fine crystals having a grain size of 10 to 20 nm or more. As a method of obtaining the polysilicon film, there is a method of laser annealing an amorphous silicon film. As a result, polysilicon TFTs using inexpensive glass substrates (non-alkali glass) have been used in many liquid crystal products without using expensive quartz glass substrates that can withstand high-temperature processing.

Conventionally, as shown in FIG. 5, for example, an SiNx film 2 and an SiO ₂ film 3 are formed on a glass substrate 1 as an underlayer, and an amorphous silicon film 4 is formed on the entire surface, and the amorphous silicon film 4 Then, laser annealing was performed to form polysilicon, and then patterned into a desired pattern. Alternatively, as shown in FIG. 6, there is a method in which a cap layer (for example, SiO ₂ film) 5 is formed on the amorphous silicon film 4 and laser annealing is performed on the amorphous silicon film 4 covered with the cap layer 5 (patent). Reference 1).
JP 2004-64060 A

  In general, the surface of amorphous silicon has high smoothness, but there is a problem that the surface becomes rough due to aggregation of silicon atoms when melted silicon solidifies in the process of transition to polysilicon by laser annealing.

  If the surface irregularities become significant, the irregularities are also reflected in the film stacked on the surface, the film thickness at the stepped portion becomes thin and breakage is likely to occur, and if there are irregularities on the irradiated surface during exposure, the exposure system There is a problem that the lens is partially out of focus, and in particular, the workability of a fine pattern is deteriorated. Furthermore, when a polysilicon pattern is used as an active region of a thin film transistor, a gate insulating film is formed on the polysilicon pattern. Therefore, if the surface of the polysilicon pattern becomes rough, an increase in leakage current, a decrease in withstand voltage, etc. There is also a risk that the characteristics of the thin film transistor may be deteriorated.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to form a polysilicon pattern and a thin film transistor manufacturing method that suppress the roughening of the surface during the transition from amorphous silicon to polysilicon by laser annealing. And providing a thin film transistor.

  The method for forming a polysilicon pattern of the present invention includes a step of forming an island-shaped portion made of amorphous silicon in an insulator, a step of forming a cap layer on the insulator so as to cover the island-shaped portion, and a cap layer. And a step of irradiating the covered island portion with laser to form amorphous silicon into polysilicon.

  According to the present invention, the laser irradiation is performed after the amorphous silicon is first formed into a desired pattern (island-like portion). The island-shaped portion made of amorphous silicon that receives the laser irradiation is covered with a cap layer on the upper surface and the periphery, and the cap layer is firmly fixed to the insulator surface at a portion between the island-shaped portion and the island-shaped portion. Therefore, the island-like portion is irradiated with laser while being constrained by the cap layer. Thereby, melting at the time of laser irradiation and deformation at the time of subsequent recrystallization can be suppressed, and the surface roughness can be reduced.

  The thin film transistor manufacturing method of the present invention includes a step of forming an active region in the polysilicon pattern formed by the above method.

  In this thin film transistor manufacturing method, in particular, if the cap layer is left as it is without removing the cap layer, man-hours and costs can be reduced.

  The thin film transistor of the present invention is obtained by the above-described method for manufacturing a thin film transistor.

  In this thin film transistor, in particular, if there is only one corner portion of the outline of the island-shaped portion that is a polysilicon pattern, crystal growth is likely to proceed from the corner portion at the time of recrystallization by laser annealing. Thus, the crystal grain size can be increased as compared with the case where crystal growth proceeds from a plurality of directions. An increase in crystal grain size leads to an improvement in mobility.

  According to the present invention, the surface of a polysilicon pattern obtained by laser annealing amorphous silicon can be suppressed from becoming rough, and workability and quality can be improved. In particular, if the polysilicon pattern is used as an active region of a thin film transistor, it greatly contributes to improving the characteristics of the thin film transistor.

[First Embodiment]
FIG. 1 shows a method for forming a polysilicon pattern according to an embodiment of the present invention.

First, a precursor film is formed on the substrate 1 as shown in FIG. In the present embodiment, the substrate 1 is, for example, a glass substrate, and the precursor film is composed of, for example, three layers of an SiNx film 2, an SiO ₂ film 3, and an amorphous silicon film 4. The SiNx film 2 is formed on the substrate 1 by, for example, a plasma CVD method. An SiO ₂ film 3 is formed on the SiNx film 2 by, for example, a plasma CVD method. The amorphous silicon film 4 is formed on the SiO ₂ film 3 by a plasma CVD method using silane gas, for example.

As described above, a structure in which the amorphous silicon film 4 is formed on the insulator 8 including the substrate 1, the SiNx film 2, and the SiO ₂ film 3 is obtained. The underlayer composed of the SiNx film 2 and the SiO ₂ film 3 plays a role of relieving stress acting on elements such as various patterns and thin film transistors formed thereon, and insulating and separating patterns and elements. In addition to the configuration shown in FIG. 1, the underlying layer may be a SiNx film 2 only, a SiO ₂ film 3 only, a SiON film only, or a laminate in which at least two of these are combined. Alternatively, the amorphous silicon film 4 may be formed directly on the substrate 1 without forming the underlayer.

Next, the amorphous silicon film 4 formed on the entire surface of the SiO ₂ film 3 is patterned to form island portions 4a as shown in FIG. Specifically, a resist film is applied on the amorphous silicon film 4, a mask on which a desired pattern is formed is adhered thereto, and the resist film is exposed and developed to form a resist pattern. Then, the amorphous silicon film 4 is selectively etched using the resist pattern as a mask.

  In the present embodiment, the island-shaped portion 4a (strictly speaking, the island-shaped portion 6 which is polysiliconized in the process described later) is used as an active region of a thin film transistor for switching each pixel of the liquid crystal display, and has m rows × n columns. Corresponding to the pixels, m × n island-like portions 4a are arranged. The planar shape (both top and bottom) of each island-like portion 4a is a quadrangle as shown in FIG. 3, the cross-sectional shape is trapezoidal, and the thickness is about 50 nm.

After the etching, the resist mask is removed, and the surface of the insulator 8 where the island portions 4a are exposed is cleaned. Thereafter, for example, a cap layer 5 made of a SiO ₂ film is formed on the SiO ₂ film 3 so as to cover the island-like portion 4a by a plasma CVD method using a mixed gas of TEOS (tetra ethoxy silane) and oxygen as a source gas. It forms (refer FIG.1 (c)). The thickness of the cap layer 5 (the thickness of the portion not located on the island-like portion 4a) is about 100 nm to 300 nm.

In addition to the mixed gas of TEOS and oxygen, a mixed gas of SiH ₄ and N ₂ O can also be used as the raw material gas for the SiO ₂ film. In addition to the SiO ₂ film, a SiNx film may be used as the cap layer 5.

  Next, as shown in FIG. 1D, laser irradiation is performed from the cap layer 5 side toward the island-shaped portion 4a. For example, an excimer laser produced based on ultraviolet light emitted when argon gas and fluorine gas are reacted is used. The island-shaped portion 4a that has been irradiated with laser is locally heated to melt the irradiated portion of amorphous silicon, and then recrystallized to form polysilicon. That is, the island-shaped portion 4a made of amorphous silicon becomes the polysilicon-shaped island-shaped portion 6 (see FIG. 3).

  In the present embodiment, as described above, before performing laser irradiation, the amorphous silicon film to be polysiliconized is patterned into an island shape, and the cap layer 5 covers not only from the upper surface but also from the surrounding area. The cap layer 5 filling the space between 4a and the island-like portion 4a is firmly fixed to the surface of the insulator 8. Since the laser irradiation is performed in a state in which the island-shaped portion 4a is constrained by the cap layer 5, the deformation when silicon melted during the transition to polysilicon is recrystallized can be suppressed, and the island after laser annealing can be suppressed. The roughness of the surface of the shape portion 6 can be suppressed. Thereby, the film | membrane piled on the island-shaped part 6 can also obtain favorable smoothness, can prevent the disconnection in a level | step-difference part, and can improve the workability of a pattern.

  In general, it is necessary to scan the laser beam a plurality of times because it is not possible to cover all the surfaces to be polysiliconized by a single laser beam irradiation. In this case, if a portion that is irradiated with a laser beam overlapped by multiple scans is generated, a difference in the degree of crystallization occurs between the portions that are not overlapped and the characteristics of the polysilicon film quality vary. May occur.

  However, in this embodiment, since the amorphous film to be polysilicon is already patterned and reduced to an island shape, the overlapping portion of the laser beam scanning is positioned between the island shape portion and the island shape portion. Thus, it is possible to prevent the overlapping portion of the laser beam scanning from being located on the island-shaped portion. Thereby, it is possible to prevent a difference in the degree of crystallization in each island-shaped portion. Being able to suppress variations in the film quality characteristics of polysilicon can consequently suppress variations in characteristics of thin film transistors that use the polysilicon as an active region.

  After the polysilicon pattern is formed as described above, the thin film transistor manufacturing process is continued as described below.

After the amorphous silicon is converted to polysilicon by laser annealing as described above, the cap layer 5 is usually removed by etching or the like, but in this embodiment, the cap layer, that is, the SiO ₂ film 5 is left as it is and used as the gate insulating film of the thin film transistor. . As a result, the step of forming a separate gate insulating film can be omitted, and man-hours and costs can be reduced.

After the step of FIG. 1D described above, as shown in FIG. 2E, a gate electrode G is formed on a portion of the SiO ₂ film 5 that is located on the polysilicon island 6. To do. Then, impurities are implanted by ion doping using the gate electrode G as a mask (in a self-aligned manner), and a source S and a drain D are formed in the island-shaped portion 6.

Next, as shown in FIG. 2F, an interlayer insulating film 9 is formed on the SiO ₂ film 5 so as to cover the gate electrode G, and a source S and a drain D are formed on the interlayer insulating film 9 and the SiO ₂ film 5. After opening the contact hole reaching to, the electrodes 11 and 12a filling the contact hole are formed. The electrode 11 connected to the source S is connected to a data line (not shown). The gate electrode G is connected to a gate line (not shown). These gate lines and data lines intersect in a matrix. The electrode 12a is connected to the pixel electrode 12b. The thin film transistor is completed as described above.

  The polysiliconized island-like portion 6 functions as an active region of the thin film transistor. The source S and the drain D are formed by the impurity implantation described above, and when a voltage is applied to the gate electrode G, the source S and the drain D are formed. A channel is formed. Since the gate insulating film and further the gate electrode are formed on the island-shaped portion 6 having excellent smoothness, the quality of the thin film transistor can be improved, such as reduction of leakage current and improvement of withstand voltage.

  Next, the result of measuring the surface roughness with respect to the pattern size change of the island-shaped part will be described. The planar shape of the island portion was a square. Then, the one whose length on one side of the upper surface was changed to 3, 5, 10, 20, 50 μm was prepared, and polysilicon was formed by laser annealing using a YAG laser having a wavelength of 543 nm for each of these. Table 1 shows the measurement results of the surface roughness (centerline average roughness) Ra of the island-shaped portion after the formation of polysilicon.

  As is apparent from this result, the surface roughness of all the island-shaped portions having a side length of 50 μm or less is rougher than that obtained by laser annealing in the state of the whole surface film before the island-shaped portion patterning corresponding to the conventional example. Ra can be reduced. In particular, in the case of 25 μm or less, Ra <100%, and a remarkable effect of reducing the surface roughness is obtained.

[Second Embodiment]
In the first embodiment, the planar shape of the island-shaped portion is a quadrangle. However, the direction of crystallization can be controlled by changing the shape. For example, in the present embodiment, the planar island-shaped portion 20 shown in FIG. 4 is formed. The island-shaped portion 20 has an outline formed by two straight portions 21a and 21b that are connected at one end to form a corner portion 20a, and an arc-shaped portion 22 that connects the other ends of the straight portions 21a and 21b. . The inner angle of the corner portion 20a is smaller than 180 °, for example, approximately 90 ° in the illustrated example.

  In the figure, arrows indicate the traveling direction in which small crystal grains grow into large crystal grains. When laser annealing was performed on the island-shaped portion 20 having such a shape, the growth of crystal grains progressed starting from the corner portion 20a, and the crystal grains having a larger grain size than the square pattern of the first embodiment became. In polysilicon, the increase in crystal grain size leads to an increase in carrier mobility and polysilicon quality.

  In the island-shaped portion 20 of the present embodiment, the crystal nucleus was formed into only one corner portion 20a and grew in the direction of the arrow. On the other hand, when there are a plurality of crystal nuclei and the crystal grows from these nuclei as a starting point, a grain boundary is formed where each crystal grain collides, and therefore the crystal grain is difficult to increase. For this reason, it is preferable to use one corner that is likely to be the starting point of crystal growth. In addition, in this island-shaped part 20, it is set as the length of the longer linear part among the linear parts 21a and 21b as a typical length of the planar dimension which is preferably 50 μm or less.

  Further, the planar shape of the island-like portion may be circular. In this case, the representative length of the plane dimension that is preferably 50 μm or less is the diameter. In the case of a square other than a square, the length of the longest side is the representative length.

  As mentioned above, although each embodiment of this invention was described, of course, this invention is not limited to these, A various deformation | transformation is possible based on the technical idea of this invention.

  In the case of a system-on-glass structure in which a peripheral circuit such as a drive circuit and a microprocessor is formed on a glass substrate in addition to the display unit in a liquid crystal display, the polysilicon pattern obtained by the present invention is also formed on the peripheral circuit. Alternatively, a thin film transistor can be applied. The polysilicon pattern is not limited to the thin film transistor, and may be used for a diode, a capacitor, or the like.

  In the above embodiment, the laser irradiation on the island-shaped portion made of amorphous silicon is performed from the cap layer 5 side, but may be performed from the substrate 1 side. In this case, in particular, the unevenness of the upper surface of the island-like portion (the surface opposite to the surface facing the substrate 1) can be suppressed.

  As a method for forming an island-shaped portion made of amorphous silicon, in addition to the method described in the above embodiment, for example, a planar opening corresponding to the planar shape of the island-shaped portion to be obtained is formed on the surface of the insulator 8. Then, an amorphous silicon film is formed on the insulator 8 so as to fill the opening, and the amorphous silicon film is scraped off by CMP (Chemical Mechanical Polishing) or etch back until reaching the surface of the insulator 8. Also good.

It is sectional drawing which shows the formation method of the polysilicon pattern which concerns on embodiment of this invention. It is sectional drawing which shows the method of forming a thin-film transistor in the process following FIG. It is a figure which shows an example of the planar shape of an island-shaped part. It is a figure which shows the planar shape different from FIG. It is sectional drawing of the precursor film | membrane of the prior art example formed on the board | substrate. It is sectional drawing of the precursor film | membrane of another prior art example.

Explanation of symbols

1 ... substrate, 2 ... SiNx film, 3 ... SiO ₂ film, 4 ... amorphous silicon film, an island-shaped portion consisting of 4a ... amorphous silicon, 5 ... cap layer (gate insulating film), 6 ... polysiliconized are islands 8 is an insulator, 9 is an interlayer insulating film, 20 is an island, 20a is a corner, S is a source, D is a drain, and G is a gate.

Claims (6)

Forming an island-shaped portion made of amorphous silicon in the insulator;
Forming a cap layer on the insulator so as to cover the island-shaped portion;
Irradiating the island-shaped portion covered with the cap layer with a laser, and converting the amorphous silicon into polysilicon;
Only including,
A method for forming a polysilicon pattern, wherein the outer shape of the island-shaped portion has only one corner . The step of forming the island-shaped portion includes
Forming an amorphous silicon film on the insulator;
Selectively etching the amorphous silicon film to leave an island shape;
The method of forming a polysilicon pattern according to claim 1, comprising:   3. The method for forming a polysilicon pattern according to claim 1, wherein a representative length of a planar dimension of the island-shaped portion is 50 μm or less.   A method for manufacturing a thin film transistor, comprising a step of forming an active region in a polysilicon pattern formed by the method according to claim 1.   5. The method of manufacturing a thin film transistor according to claim 4, wherein the cap layer is left as a gate insulating film without being removed.   A thin film transistor manufactured by the method according to claim 4 or 5.

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