KR101052982B1 - Silicon Crystallization Method to Improve Flatness - Google Patents

Abstract

The present invention relates to a sequential horizontal crystallization method, and more particularly, to a sequential horizontal crystallization method with improved flatness. The present invention comprises the steps of sequential horizontal crystallization of amorphous silicon; And recrystallizing the grain boundary region generated in the sequential horizontal crystallization step, and adjusting the intensity of the laser energy used in the recrystallization step by applying a mask including an opening and melting the grain boundary completely. Crystallization in the near region can yield crystalline silicon with improved flatness.

Sequential Horizontal Crystallization, Mask, SLS, ELA

Description

Silicon crystallization method to improve flatness {CRYSTALLIZATION METHOD OF SILICON FOR IMPRUVING FLATNESS}

1 is a graph showing the relationship between laser energy and the size of particles to be crystallized.

2 is a plan view showing the principle of sequential horizontal crystallization;

Top view of a sequential horizontally crystallized silicon layer.

4 is a cross-sectional view of a sequential horizontally crystallized silicon layer.

5 is a schematic configuration diagram of a laser optical system of the present invention.

6A-6D are flowcharts showing the crystallization method of the present invention.

7A is a schematic diagram of a first mask used for first crystallization of the present invention.

Fig. 7B is a plan view showing the state of crystallization by the first crystallization of the present invention.

8A to 8B are plan views showing a part of the substrate to be crystallized by the first crystallization of the present invention.

9 is a schematic diagram of a second mask used for second crystallization of the present invention;

***** Explanation of symbols for main parts of the present invention *****

601: amorphous silicon layer 602a: first mask

610: protrusions 620a, 620b: openings                 

602b: second mask 630: exposure area

640: non-exposed area 701: first mask

710a, 710b, 710c: Opening part 901: Second mask

902: opening

TECHNICAL FIELD The present invention relates to an amorphous silicon crystallization method, and more particularly, to a method for preparing polysilicon by improving flatness and sequential lateral solidification (SLS).

Until now, amorphous silicon thin film transistors (a-Si TFTs) have been widely used as switching elements in video display devices such as liquid crystal displays (LCDs). Accordingly, researches using polycrystalline silicon thin film transistors (poly-Si TFTs), which have faster operating speed than amorphous silicon thin film transistors, are being conducted.

As a method for fabricating a polycrystalline silicon thin film transistor, a method of directly forming a polycrystalline silicon film and a method of crystallizing an amorphous silicon film by heat treatment are mainly used. As a heat treatment method for crystallizing the amorphous silicon film, a method using an excimer laser can be used.                         

In the case of LCDs, an annealing method using an excimer laser is mainly used because the glass substrate used as the substrate cannot withstand a general heat treatment process of 600 ° C. or higher. An excimer laser annealing method uses an amorphous silicon laser beam having a high energy. By irradiating the thin film, since crystallization occurs by instantaneous heating of about several tens of nsec, there is an advantage of not damaging the glass substrate.

In addition, the electrical properties of the polycrystalline silicon thin film manufactured using the excimer laser are superior to the polycrystalline silicon thin film manufactured by the general heat treatment method. This is because the silicon atoms are rearranged in a crystalline form having excellent crystallinity when the amorphous silicon thin film is melted in a liquid state by the excimer laser irradiation and solidified into a solid, so that the electrical mobility of the amorphous silicon is 0.1 to 0.2 cm ^2. / Vsec and the electrical mobility of the polycrystalline silicon produced by the general heat treatment method is about 10 to 20cm ² / Vsec, whereas the electrical mobility of the polycrystalline silicon produced using the excimer laser exceeds 100cm ² / Vsec. Have

However, polysilicon thin film transistors fabricated using excimer laser annealing (ELA) are excellent in terms of electrical mobility and driving speed, but only in the switch-on state. At high leakage current flows. Ideally, in the switched-off state, the leakage current should not flow at all, which is a problem to be solved in the polycrystalline silicon thin film transistor.                         

The causes of leakage current of polycrystalline silicon thin film transistors are as follows. In the switch-off state, even when a voltage of about 5 to 10 V is applied across the transistor channel, that is, between a source and a drain, by a switch-off action of a gate, between the source and the drain. As a current does not flow in the high current, a high electric field is formed between the source and the drain.

In such a high electric field, electron-hole pairs are generated that act as a source of current at grain boundaries where the bonds between silicon atoms are relatively weak, and these electron-hole pairs generate leakage currents.

In addition, the grain boundary inside the polycrystalline silicon thin film causes deterioration of the electrical mobility of the device even in the switched-on state. The grain boundary is a state in which the bond between silicon atoms is broken or incompletely bonded. It acts as a disorder.

The device made of polycrystalline silicon has higher electrical mobility than the device made of amorphous silicon, but the electrical mobility is lower than that made of monocrystalline silicon because of the grain boundaries present at high density inside the polycrystalline silicon.

A fundamental way to solve the problem of polycrystalline silicon described above is to increase the grain size to lower the density of grain boundaries in question. Common methods of increasing grain size include increasing laser energy or heating the substrate.

The relationship between the grain size and the laser energy intensity will be described with reference to FIG. 1.                         

1 is a graph showing the relationship between the laser energy density irradiated to amorphous silicon and the size of particles to be crystallized.

As shown in the graph of FIG. 1, the crystallization of amorphous silicon may be divided into first and second regions according to the intensity of laser energy to be irradiated.

The first region is a partial melting region, in which laser energy is irradiated to the amorphous silicon layer at an intensity such that only the surface of the amorphous silicon layer is melted. In the first region, only the surface of the amorphous silicon is partially melted by laser irradiation, and small crystal particles are formed on the surface of the amorphous silicon layer through a solidification process.

The second region is a near-complete melting region, which irradiates the amorphous silicon with laser energy stronger than the first region, thereby causing the amorphous silicon layer to almost melt. However, although not completely melted, small nuclei that remain unmelted act as seeds and grow crystals, thereby obtaining large crystal grains compared to the first region. However, crystals growing in the second region are not uniform and the second region is considerably smaller in width than the first region.

The third region is a complete melting region, in which the intensity of the irradiated laser energy is higher than that of the second region to melt all the amorphous silicon layers. The fully molten silicon layer undergoes cooling and undergoes solidification. The crystals formed are homogeneous nucleation, but the particles are very fine.                         

Hereinafter, the principle of the crystallization in the complete melt near region and the complete melt region in more detail as follows.

When the laser is irradiated to the amorphous silicon film, the amorphous silicon film is melted. The surface of the amorphous silicon film directly exposed to the laser beam is irradiated with strong laser energy and the relatively weak laser energy is irradiated to the lower part of the amorphous silicon film so that the surface is completely melted. The bottom is not a complete melt. However, since the grain grows around the seed, amorphous silicon particles, which are not completely melted at the bottom, act as the seed, and the grain grows around the seed to make a large size grain. The method of crystallizing using the energy density of the second region is called an excimer laser annealing (ELA) method.

On the other hand, when the crystallization proceeds in the third region where the silicon energy is completely melted, the amorphous silicon is melted and there is no nucleus for grain growth.

Subsequently, the amorphous silicon irradiated with a strong laser is randomly formed in the cooling process, and crystallization proceeds around the nucleus.

The size of the crystal grains generated at this time is very small, as shown in the third region of FIG. 1.

If only a part of the amorphous silicon layer is melted by the laser, immediately after the irradiation of the laser energy on the amorphous silicon is finished, the amorphous silicon is cooled through both sides, that is, the amorphous silicon layer not irradiated with the laser. This is because the amorphous silicon layer on the side has a greater thermal conductivity than the insulating layer below the amorphous silicon layer.

After the irradiation of the amorphous silicon, crystallization proceeds by using the amorphous silicon, which is not irradiated on the side, as a nucleus, and the crystallization takes place with a constant pattern in the horizontal direction.

The molten amorphous silicon layer, which is not adjacent to the solid amorphous silicon layer to be used as a nucleus, contains a randomly distributed nucleus during cooling, and grains grow around the nucleus.

FIG. 2 is a diagram showing the structure of amorphous silicon in which crystallization is performed by the above crystallization method. FIG.

First, a part of the amorphous silicon is covered with a mask and irradiated with a laser.

Amorphous in the masked area is not melted, the amorphous silicon remains intact, and the laser irradiated amorphous silicon is completely melted and then cooled.

In the cooling process, the molten amorphous silicon is crystallized horizontally using the solid amorphous silicon on the side as a nucleus, and the molten amorphous silicon in the region not in contact with the solid amorphous silicon undergoes a natural cooling process, so the grain has a small size of several hundred nm. To grow. In this case, the exposed laser energy should be the energy intensity of the third region for completely melting the silicon layer to be crystallized.

As shown in FIG. 2, the crystallization process is called sequential horizontal crystallization (SLS) because grains grow horizontally. The grains growing horizontally usually have a size of 1 ~ 2㎛.                         

The grain size through general laser annealing shows several hundred nm, but the grain size obtained through the SLS crystallization method reaches several micrometers. Therefore, when the polycrystalline silicon having the grain size is applied to the device, large mobility can be obtained. Can be implemented.

In particular, considering the maximum size of grain that can be grown by sequential horizontal crystallization, if sequential horizontal crystallization proceeds from both sides, crystallization has only one grain boundary and crystallization occurs.

3 is a view showing a state of grain grown by the above method.

When sequential horizontal crystallization is performed using a mask including an opening having a width of about 2 μm, grains may be horizontally grown crystals having a grain boundary in the middle as shown in FIG. 3. When the horizontally grown polycrystalline silicon is applied as a channel of the thin film transistor, it is possible to implement a switching device having high speed operation characteristics.

However, as described above, the crystalline grown by the sequential horizontal crystallization method causes a problem that crystal growth protrudes to the top while grains growing horizontally on both sides meet in the center, thereby weakening the flatness of the crystallized polysilicon.

The problem will be described with reference to FIG. 4.

As shown in FIG. 4, the crystals grow horizontally by a sequential horizontal crystallization method, so that the crystallites protrude upward from the point where the crystallites of both sides meet each other. The reason why the crystalline protrudes upward is hypothesized to be due to thermal oxidation of the silicon layer to be crystallized.

The height of the protruding crystalline silicon can be as large as 500Å, and when the liquid crystal display device is manufactured using crystalline silicon in which the step is generated, the film is not well formed on the crystalline silicon due to the step and the exposure process. Optical nonuniformity may occur in the

Therefore, an object of the present invention is to improve the generation of steps on the upper surface of the crystalline silicon by crystalline protruding upward at the grain boundary in the sequential horizontal crystallization process. In addition, an object of the present invention is to fabricate a liquid crystal display device having enhanced bonding characteristics between an active layer and a gate insulating layer using the polysilicon layer having improved step height.

The present invention proposes a silicon crystallization method of exposing twice using a first exposure mask including a plurality of openings and a second exposure mask having an opening width smaller than that of the opening of the first exposure mask.

The laser optical system applied to the crystallization method includes a laser generator, an attenuator for attenuating the energy intensity of the laser light source generated from the laser generator to a predetermined level, and an aiming mirror for adjusting the laser light source traveling from the attenuator in the longitudinal direction and the width direction. And a homogenizer for modifying a cross-sectional profile of the laser beam traveling from the collimator, a laser mask for filtering the homogenized laser, and a projection lens for irradiating the laser light processed from the laser mask to a target.

The method of crystallizing a silicon layer using the laser optical system comprises the steps of forming an amorphous silicon layer on a substrate, applying sequential horizontal crystallization by applying a first mask to the amorphous silicon layer, and forming a sequential horizontal crystallized silicon layer. And performing second crystallization by applying a second mask.

Hereinafter, a laser optical system applied to crystallization of the present invention will be described with reference to FIG. 5.

The laser optical system of the present invention includes a laser generator 501, a collimator 502 for collecting the laser light generated by the laser generator in a horizontal direction and a vertical direction, and an homogenizer 503 for modifying the cross-sectional profile of the laser light collected by the collimator. ), A mask 504 for sequential horizontal crystallization using a laser beam traveling from the homogenizer, and a projection lens 504 for irradiating a laser beam passing through the mask 504 to a target material 505. do. In particular, the present invention is characterized in that the laser optical system includes a mask for sequentially horizontally crystallizing the amorphous silicon. The sequential horizontal crystallization method irradiates the amorphous silicon layer by selectively irradiating the amorphous silicon through the mask to divide the amorphous silicon into the molten part and the non-melting part and to crystallize the amorphous silicon of the non-melting part as a seed of crystal growth. A mask is needed to filter the lasers that are being used.

The present invention uses two kinds of masks for the filtration of the laser. The first mask is for sequentially horizontally crystallizing the amorphous silicon layer by primary crystallization and has a predetermined width and width. The second mask is for secondary crystallization of the protrusion protruding to the upper surface of the horizontally crystallized silicon layer, the second mask having a width smaller than the width of the first mask and configured to be equal to or larger than the width of the protrusion, and the width of the opening is the first mask. It is comprised like the opening part of one mask.

In the laser generator of the present invention, the energy intensity may be adjusted by a mask that filters the laser light in a predetermined form before the laser light is irradiated onto the specimen. For example, the larger the opening of the mask through which the laser beam passes, the stronger the laser intensity irradiated onto the specimen, i.e., when the laser beam passes through a wide window (mask opening), the laser intensity at the top of the mask is the same, forming a straight line. While having a profile (top hat profile), when passing through a narrow window, the top profile is transformed into a Gaussian distribution, which results in a change in energy intensity.

Therefore, the intensity of the laser light irradiated onto the specimen through the mask can be adjusted.

Hereinafter, the gist of the present invention will be described with reference to FIGS. 6A and 6B.

The present invention filters the laser beam generated in the laser generator 501 and processed into a uniform shape through the homogenizer 503 through the first mask 602, and then irradiates the amorphous silicon layer 601.

The first mask 602a has a plurality of openings 620, the width of the openings 620a being limited to a constant length for sequential horizontal crystallization.                     

Since the length of the sequential horizontal crystallization is generally known to be 1 to 2 μm, the opening 620a is configured to be twice the length of the horizontal crystallization possible. Therefore, the width of the opening 620a is set to 2 μm for convenience of description.

The laser beam irradiated to the amorphous silicon layer 601 through the opening 620a completely melts the amorphous silicon of the exposure area 630. The molten silicon particles of the exposed region 630 undergo crystallization horizontally using amorphous silicon of the non-exposed region 640 as a seed for crystal growth during cooling. As a result, as shown in FIG. 6A, the crystallization proceeds horizontally and a grain boundary is formed in the middle of the exposure area 630.

In general, sequential horizontal crystallization is performed in an atmospheric pressure environment. In the sequential horizontal crystallization process, the grain boundary reacts with oxygen in the atmosphere and thermally oxidizes to protrude upward. The protrusion 610 formed by thermal oxidation causes a problem of weakening the contact characteristics of the crystallized silicon layer and the thin film formed thereon by generating a step. In particular, when a liquid crystal display device is manufactured using the crystalline silicon layer formed by the above method, there is a problem of weakening the contact characteristic with the gate insulating film formed on the crystalline silicon layer to generate a leakage current.

Therefore, in the present invention, after the sequential horizontal crystallization using the first mask is completed, the second laser crystallization is performed using the second mask 602b having a width smaller than the width of the first mask 602a.

Referring to Figure 6b looks at the second laser crystallization of the present invention.                     

As briefly described above, the intensity of the laser light irradiated onto the specimen may be adjusted by the aperture ratio of the mask for filtering the laser beam, and the present invention may apply the second mask 602b having the aperture ratio adjusted to sequential horizontally. The laser beam is irradiated onto the protrusion 610 of the crystallized crystalline silicon layer. According to the present invention, the intensity of the laser beam passing through the opening 620b of the second mask having a narrower width than that of the first mask 601a is passed through the first mask 602a. It becomes smaller than the intensity of the laser beam.

The width of the opening 620b of the second mask of the present invention may be equal to or larger than the width of the protrusion 610 occurring in the grain boundary region formed by sequential horizontal crystallization.

In addition, the opening 620b of the second mask is characterized in that the intensity of the laser beam irradiated to the protrusion 610 by the opening ratio.

That is, by the second laser exposure to which the second mask of the present invention is applied, the protrusion 610 is almost completely melted and crystallization proceeds. Since the sequential horizontal crystallization proceeds again after the complete melting under atmospheric pressure, the second crystallization proceeds like the excimer laser solidification (ELS) method in which the crystallization proceeds in a nearly complete melting state. The secondary crystallization is performed in the near complete melting region of the laser energy intensity for crystallization, the present invention is to filter the laser without adjusting the intensity of the excimer laser irradiated to the protrusion 610 The laser intensity is controlled by adjusting the aperture ratio of the mask. As a result, the laser energy can be irradiated to the protrusion 610 with the intensity of the laser energy controlled by the second mask 602b of the present invention.

As a result, as shown in FIG. 6C, the protrusion 610 is almost completely melted, and crystallization takes place using seeds of crystalline silicon formed on the side and bottom of the cooling process.

As a result, crystallization by the second laser irradiation proceeds with crystallization close to the excimer laser annealing (ELA) method rather than sequential horizontal crystallization. Since the crystalline silicon formed by the ELA crystallization method has more uniform flatness than the crystalline silicon formed by the SLS crystallization method, the crystallization of the protrusion 610 formed by the second laser irradiation is performed while maintaining a flat surface shape. . The result is shown in FIG. 6D.

Therefore, according to the present invention, the amorphous silicon layer is crystallized by a sequential horizontal crystallization method (SLS) by applying a first mask having an opening having a predetermined width, and the protrusion is formed at a grain boundary generated by the SLS crystallization method. By applying a second mask having an opening width smaller than the opening of the first mask, ELA crystallization is performed by secondary crystallization while adjusting the laser energy intensity irradiated to the protrusion to obtain crystalline silicon having improved flatness.

However, since ELA and SLS crystallization use excimer lasers in which the size of the cross-sectional area of the irradiated laser is limited, it takes a long time to crystallize all the amorphous silicon layers formed on a very large substrate as compared to the excimer laser generator.

Therefore, a separate method is needed to shorten the crystallization time.

Hereinafter, a method of crystallizing by the SLS method by applying the first mask of the present invention will be described with reference to FIGS. 7A to 8B.

7A illustrates a portion of a first mask according to an embodiment of the present invention. The first mask 701 has a plurality of openings with the first, second and third openings 710a, 710b and 710c as one unit. In this embodiment, an SLS crystallization process will be described using three openings constituting one unit of the opening pattern as an example.

Each opening of the first mask 701 according to the present exemplary embodiment may have a width of 2.5 μm and a predetermined length. In addition, as shown in FIG. 7A, second and third openings 710b and 710c are symmetrically arranged at a lower end of the first opening 710a and spaced apart from each other by a predetermined distance.

In this embodiment, the separation distance is 1.5 μm. As a result, the second and third openings 710b and 710c are configured to overlap the first opening 710a by 0.5 μm at the left and right sides.

When the first laser shot is performed using the first mask 701 having the above configuration, the silicon layer exposed through the opening is sequentially horizontally crystallized.

Subsequently, the laser generator or the substrate is moved to be equal to or smaller than the length of the opening and a second laser shot is performed. Then, the non-exposure portion 720 between the first laser sash unexposed second opening 710b and the third opening 710c is exposed through the first opening 710a.

In this case, since the width of the non-exposed part 720 is 1.5 μm, the non-exposed part 720 is crystallized by overlapping each of the crystallization regions existing on the left and right sides of the second laser chassis by 0.5 μm. In this case, as shown in FIG. 7B, horizontal crystallization is performed using the horizontally crystallized region on the left and right sides of the non-exposure unit 720 as a seed for crystallization. As a result, recrystallization proceeds following the crystallization horizontally crystallized by the first laser shot, thereby obtaining larger horizontally crystallized grain. In addition, the size of the recrystallized grain may have a uniform size.

In the present embodiment, since the horizontal crystallization length by the first laser shot is 1.25 μm and the length crystallized by recrystallization is 0.75 μm, a total grain thickness of 2 μm in total can be obtained. 7b shows horizontal crystallized grains having a width of 2 μm.

A plurality of openings as shown in FIG. 7A are repeatedly formed in the mask. If a mask of 125 mm in width and length is used, a plurality of openings are repeatedly formed in the mask of the size.

FIG. 8A shows the crystallized silicon region after applying the first mask and making the first laser shot, and FIG. 8B shows the second shot after moving the substrate or laser generator to the length of the opening or less. The grain state of the crystallized silicon layer is then shown. The size of the horizontally crystallized unit grain is 2 mu m as described above.

Subsequently, after sequential horizontal crystallization is completed by applying the first mask, second crystallization is performed to recrystallize the protrusion formed at the grain boundary.

In the second crystallization, laser crystallization is performed by applying a second mask including a plurality of openings having an opening width smaller than the width of the opening formed in the first mask.

The opening pattern of the second mask will be described with reference to FIG. 9. The second mask is used to recrystallize the protrusion formed using the first mask, and the opening formed in the second mask may be formed to be larger than or equal to the size of the protrusion. In this embodiment, since the distance between the grain boundaries formed by the first mask is 2 μm and the width of the protrusion is less than or equal to half of the sequential horizontal crystallization length, the width of the opening formed in the second mask is 1 μm and the distance between the openings is 1 μm. To form a second mask.

As shown in FIG. 9, a second mask 901 having a plurality of openings having a width of the opening 902 having a width of 1 μm and a space between the openings having a thickness of 1 μm repeatedly is prepared and second crystallization is performed.

The second crystallization is performed by second laser exposure after sequential horizontally crystallized crystalline silicon with the second mask 901 correctly arranged in a vacuum chamber.

Since the laser intensity generated by the laser generator during the second laser exposure is the same as the laser intensity during the first laser exposure, no separate laser intensity adjustment is necessary. That is, since the second mask is applied in which the aperture ratio is reduced than the first mask, the laser intensity used for the second crystallization can be automatically adjusted.

The laser light irradiated through the opening 902 of the second mask 901 by the second laser exposure, as described above with reference to FIG. 6, makes the protrusions almost completely melted, and then recrystallizes there is no step difference. Crystalline silicon can be formed.

The second crystallization is repeated for the entire substrate to obtain a crystalline silicon layer having improved flatness throughout the substrate.

As described above, the present invention obtains crystalline silicon having improved flatness by sequentially crystallizing the grain boundary region of the sequential horizontally crystallized crystalline silicon. When the crystalline silicon is flattened and the crystalline silicon is applied as an active layer to form a liquid crystal display device, the coupling property between the gate insulating layer and the active layer formed on the active layer is improved to prevent leakage current and a thinner liquid crystal of thinner film. The display element can be configured. In addition, since the flatness is improved, a precise exposure process is possible in the exposure process, thereby enabling the formation of fine patterns.

Claims (11)

Forming an amorphous silicon layer on the substrate; The first crystallization, which is a sequential horizontal crystallization, is applied by applying a first mask having a first opening on the amorphous silicon layer and a plurality of sub-openings having a separation space smaller than the width of the first opening, and irradiating with a first laser. Proceeding; Moving the substrate so that the first opening corresponds to the separation space, and performing recrystallization by irradiating a second laser; And, A second crystallization by laser irradiation by applying a second mask having a second opening having a width smaller than the first opening to a grain boundary of the recrystallized silicon layer, The second crystallization step is Matching a grain boundary of the sequential horizontally crystallized silicon with the center of the second opening; And the width of the second opening is less than or equal to half the width of the first opening. delete delete The method of claim 1, wherein said second crystallization step is performed in a chamber of vacuum. The method of claim 1, wherein the intensity of laser energy irradiated in the second crystallization step is controlled by the second mask to be a laser intensity of a near complete melting region. delete 2. The method of claim 1 wherein the crystalline region comprising grain boundaries formed by the sequential horizontal crystallization in the second crystallization step is recrystallized. delete delete delete delete

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