KR20010087667A - Method for fabricating active layer for TFT type array substrate - Google Patents

Abstract

PURPOSE: A method for manufacturing an active layer of a TFT type array substrate is provided to form an active layer by using a polysilicon layer with a fine surface. CONSTITUTION: A polysilicon layer is formed on a silicon layer(111) located on a drive portion(A) of a gate drive circuit region(115) and a data drive circuit region(113) by using an SLS(Sequential Alteral Solidification) method. The whole region of the silicon substrate(111) is crystallized by using a MPGG(Multiple Pulse Grain Growth) method. The crystallization is spread to a part crystallized by the SLS method when a pixel region of an LCD panel is crystallized by the MPGG method.

Description

Method for fabricating active layer for TFT type array substrate

The present invention relates to a method for manufacturing a thin film transistor array substrate, and more particularly, to a method for manufacturing a thin film transistor array substrate having an active layer formed of polysilicon.

In general, a thin film transistor is composed of a multilayer and divided into a semiconductor layer, an insulating layer, a protective layer, and an electrode layer.

Each element of the thin film transistor will be described in more detail. Amorphous silicon or polysilicon may be used as a semiconductor layer, and a silicon nitride film (SiN _X ) or silicon may be used as an insulating layer. An oxide film (SiO ₂ ), aluminum oxide (Al ₂ O ₃ ), tantalum oxide (TaO _X ), and the like are used, and a transparent organic insulating material or an insulating material is used as a passivation layer, and an electrode layer is used as an electrode layer. Metal conductive materials such as aluminum (Al), chromium (Cr) and molybdenum (Mo) are generally used.

The materials according to each of these elements are deposited using a deposition apparatus, i.e., a sputtering apparatus, a chemical vapor deposition (CVD) apparatus, and then subjected to lithography. Each element is formed.

The semiconductor layer of each component layer configured as described above serves as a conducting channel through which electrons flow, and the electrode layer is composed of a source electrode, a drain electrode, and a gate electrode. do.

In addition, the source electrode is a means for emitting a signal voltage to the drain electrode through the semiconductor layer.

The gate electrode is a means for switching the flow of current from the source electrode to the drain electrode.

Therefore, the thin film transistor is used as a switching element and is applied as a switching element for an active matrix liquid crystal display device (AMLCD).

The active matrix liquid crystal display device uses a thin film transistor in which cadmium serenide (CdSe), hydrogenated amorphous silicon (a-Si: H), and poly crystalline silicon (poly-Si) are used as a semiconductor layer. Successful configuration is possible.

As described above, amorphous silicon is a material used as the semiconductor layer of the thin film transistor, and thus, since the process is simple and can be processed at low temperature, it is already used for manufacturing a large area device such as a solar cell.

In addition, the manufacturing process of the device using amorphous silicon is convenient because the maximum temperature can be performed alone in a low temperature processing system of about 350 ℃.

However, in practice, low electron mobility (<2cm2 / Vsec) in amorphous silicon acts as a barrier to switching characteristics of thin film transistors, and also drives circuitry and thin films that control thin film transistors at high speed. Makes the integration of transistors difficult.

On the other hand, a thin film transistor using polysilicon as a semiconductor layer is suitable for an active matrix liquid crystal display device.

Thin-film transistors made of polysilicon require a new processing step, but instead have a response speed several times faster than amorphous silicon as a switching element in an active matrix liquid crystal display.

In addition, the greatest advantage of polysilicon over the widely used amorphous-thin film transistor is that it has a high field effect mobility of about 20 to 550 cm 2 / Vsec.

Field effect mobility determines the switching speed of thin film transistors and is several times faster than amorphous silicon.

This difference is due to the fact that polysilicon is composed of several grains and has fewer defects than amorphous silicon.

Accordingly, polysilicon is expected to be a device capable of integrating a driving circuit as well as switching for a next generation liquid crystal display device having a large area screen.

Crystallization of such polysilicon includes the SPC method, MIC method, excimer laser annealing method and the like.

The solid phase crystallization (SPC) method is a solid phase crystallization method and is a method of crystallizing amorphous silicon at a high temperature (600 degrees). In this method, since crystallization takes place in a solid phase, crystal grains are poor due to many defects (micro-twins, dislocations) in the grains, and a high temperature (˜1000 degrees) thermal oxide film is used as a gate insulating film to compensate for this. Therefore, there is a disadvantage that must use a high-priced material such as crystals that can withstand above 1000 ℃.

The metal induced crystallization (MIC) method is a metal-induced crystallization method, in which metal is deposited on amorphous silicon to be crystallized by applying heat. At this time, the metal serves to lower the enthalpy of amorphous silicon to be crystallized.

Therefore, it is possible to process at low temperatures of about 500 ° C., but the surface condition is not good and the electrical properties are degraded by the metal. In addition, since this method is also solid phase crystallization, there are many defects in the grains.

Another method is to use a laser, and this method is excellent in terms of price competitiveness because low-temperature processing can be used and low-cost glass substrates can be used.

In particular, the thin film transistor manufactured by the excimer laser annealing method (Excimer laser annealing method) is able to have a moving speed of 100 cm 2 / Vsec or more, the operation characteristics of the device is good.

Polysilicon crystallized by the above-mentioned methods is obtained in the early stage of crystallization, the liquid silicon is cooled from the silicon seed (silicon seed) to obtain a good grain (grain), the silicon crystal growth in the lateral growth (lateral growth) Large grains can be obtained.

In general, if the silicon seed spacing is larger than the maximum growth distance of silicon grain, the silicon crystal that grows laterally around the silicon seed is grown to the maximum and then nucleated by super-cooling in the remaining liquid phase. This happens and small grains are formed. However, if the seed spacing is smaller than the maximum growth distance, lateral growth occurs around the seed, and each grain forms grain boundaries, thereby forming a large grain of poly-silicon (poly-Si) thin film.

As described above, the crystal of large silicon must be uniformly arranged on the substrate while forming a boundary to obtain a thin film transistor (TFT) device having excellent performance.

Therefore, the silicon seed distribution is less than the maximum crystal growth distance, but should be evenly spaced at the largest possible interval.

As shown in FIGS. 1A to 1C, the grains 13 of silicon that are laterally grown around the silicon seed 11 are laterally grown with liquid silicon, and each of the grains 13 is grain boundary. Crystal growth ends, forming a boundary 15.

Here, the crystallization step of the conventional polysilicon using the excimer laser will be described.

2 is a perspective view showing the configuration of an optical system for polysilicon crystallization using an excimer laser.

As shown, in order to crystallize amorphous silicon deposited on a substrate using a laser beam, a laser beam device (not shown), a mask 33, and a projection lens 35 are simply required.

The projection lens 35 is positioned on the substrate 31, and a mask is positioned on the projection lens 35. At this time, when the laser beam 37 is projected on the mask 33, the laser beam is incident according to the mask pattern, and the incident light has a pattern of 4 to 6 times through the projection lens 35. As it shrinks, silicon crystallization is performed on the substrate according to the fine mask pattern.

At this time, the growth of grains of polysilicon to be crystallized can be controlled by the shape and energy density of the laser beam and the temperature and cooling rate of the substrate.

In more detail, the relationship between grain size and energy density can be explained by explaining the energy density of the crystal of the silicon thin film in three regions.

First, a low energy density region where the lower silicon thin film is not melted (low energy density regime-partial melting regime), an area near the complete melting where only some seeds of the lower silicon remain and the remaining silicon is completely melted, The high energy density region can be divided into a region in which the silicon thin film is completely melted down to the lower interface (Higyh energy density regime-complete melting regime), and in the low energy density region, the silicon melting depth is less than the silicon thickness, and the unmelted lower silicon Vertical growth occurs from the seed of the layer so that the grain diameter is smaller than the film thickness of the semiconductor layer.

The near complete melting regime is a region in which all silicon films are melted, leaving only a part of the lower silicon seeds of the semiconductor layer, and are capable of lateral growth around the seeds.

In this case, the polysilicon crystallization method using the energy of the complete melting region band is called a sequential lateral solidification (SLS) crystallization method, and the polysilicon crystallization method using an energy band close to the complete melting area is MPGG (multiple pulse grain growth) Called the crystallization method.

Hereinafter, the SLS crystallization method will be described with reference to FIG. 2.

As shown, a mask 33 for forming a laser beam pattern on the substrate 31 and a projection lens 35 for reducing the pattern of the mask 33 to be exposed on the substrate 31 are configured. The laser annealing can be started.

Referring to the polysilicon crystallization process using such a configuration, first, the laser beam 37 is uniformized by a predetermined means.

Next, the shape of the beam to be formed on the substrate 31 through the mask 33 is determined. Next, a beam having a beam width of several μm is formed through the projection lens 35 having a reduced magnification.

Next, crystallization is performed by the laser beam while the substrate 31 placed on the X-Y stage moves at a submicron / pulse.

3 is a plan view showing a mechanism for crystallizing amorphous silicon using the laser beam.

As shown, amorphous silicon can be crystallized to polysilicon by exposing to the pulse of the laser beam through each of the divided slits A, B, and C, and at this time, the primary exposure 45 Lateral growth takes place from the solid-state silicon seeds at both ends to the liquid silicon in the laser beam, forming a boundary 41 in the middle. At this time, the energy density uses a region where the silicon thin film is fully melted, and the beam width is also made (2 times less than the maximum lateral growth distance).

In the secondary exposure 47, crystals formed in the primary exposure are grown continuously.

Thus, after the N-th exposure, the polysilicon crystallized by lateral growth is formed by continuous growth of the grains 43 by the distance between the slits A, B, and C. In addition, the portion where each of the slit regions A, B, and C meet is a grain boundary 41a region of polysilicon.

As described above, the SLS crystallization method can form polysilicon having a large grain size by continuous laser beam irradiation in one direction.

Therefore, the active layer manufactured by the SLS method has a characteristic of excellent charge mobility.

The MPGG method is a method of forming polysilicon using a laser, in which silicon is dissolved and recrystallized, leaving only a seed.

Unlike the SLS method, the laser beam used in the MPGG method has a size of several millimeters, and grains grow around a plurality of seeds during melting and recrystallization using silicon.

The MPGG method has a certain limitation on the size of the grain compared to the SLS method, but has an advantage of fast process time.

Therefore, in the conventional art, when the active layer of the thin film transistor formed in the driving circuit region is formed, polysilicon is formed by the SLS method, and the active layer of the thin film transistor positioned in the pixel portion of the array substrate uses the MPGG method. To form polysilicon.

Hereinafter, a method of configuring an active layer of a conventional thin film transistor array substrate will be described with reference to FIG. 4.

As illustrated, the polysilicon type liquid crystal display device 61 includes a switching element of the gate driver circuit 63 and the data driver circuit 65 formed simultaneously with the thin film transistor formed in the pixel region of the array substrate. The circuit unit 63 and the data driving circuit unit 65 are connected to one printed circuit board 69 (PCB) to receive an input signal.

In such a configuration, the gate driving circuit and the data driving circuit require fast switching characteristics.

In general, what influences the operating characteristics of the switching element is related to the mobility of the semiconductor layers constituting the switching element.

In other words, if the mobility of the semiconductor layer is fast, the operation characteristics of the switching device are good. On the other hand, if the mobility of the semiconductor layer is slow, the operation property of the switching device is not good.

Therefore, in the related art, the pixel region B of the liquid crystal panel is configured with an active layer by using the MPGG method which can form polysilicon in a short time, and the active layer of each of the driving circuit regions A is the aforementioned SLS. By using the method to increase the grain size in one direction, a switching speed of about 100 to 120 cm 2 / Vsec was obtained.

However, the polysilicon crystallized by the SLS method has an advantage that the grain width can be formed to be large, whereas a large number of defects are present on the surface during the crystallization process.

5 is a photograph showing the surface of polysilicon crystallized by the SLS crystallization method.

As shown, there are many low angle defects 51 in the grain boundaries of polysilicon crystallized by the SLS crystallization method.

The defect is formed by melting the silicon and then blocking the laser beam so that the heat present in the silicon is released through the lower substrate.

As such, silicon starts to crystallize by the cooling process, and in particular, crystals close to the surface are abnormally grown by sudden cooling.

Polysilicon having such a surface state is subjected to a process of laminating an insulating layer later.

At this time, by depositing an insulating layer on the polysilicon layer having a large number of defects, trap states for electrons are generated by mismatches occurring at the interface between the polysilicon semiconductor layer and the insulating layer. For this reason, the mobility of electrons flowing through the surface of the polysilicon layer is remarkably lowered, and there are many limitations on the operation characteristics of the device.

The switching speed of a thin film transistor including an active layer formed of polysilicon crystallized by the SLS method is about twice as fast as that of a general polysilicon thin film transistor, but it is necessary to establish a further speed.

Accordingly, an object of the present invention is to propose a method for manufacturing a polysilicon type liquid crystal display device having a semiconductor layer formed of a polysilicon film having a fine surface.

1A to 1C are schematic plan views showing a general polysilicon forming mechanism,

2 is a perspective view showing a configuration for silicon crystallization,

3 is a plan view illustrating a silicon crystallization process;

4 is a schematic plan view of a polysilicon liquid crystal display device;

5 is an enlarged photograph of the surface of a conventional polysilicon layer crystallized using only the SLS method,

6 is a schematic plan view of a thin film transistor array substrate showing a configuration of an active layer according to the present invention;

7 is an enlarged photograph of the surface of the polysilicon layer according to the present invention recrystallized by using the SLS method and the MPGG method continuously,

8 is a graph illustrating current characteristics versus voltage of a conventional polysilicon layer and a polysilicon layer crystallized according to the crystallization method of the present invention.

<Brief description of the main parts of the drawing>

121: Solid line showing the relationship of the current to the threshold voltage of the polysilicon layer by the SLS crystallization method.

123: dotted line which shows the relationship of the electric current with respect to the voltage of the polysilicon layer recrystallized by carrying out the SLS determination method and the MPGG determination method continuously.

A semiconductor layer forming method of an array substrate for a liquid crystal display device according to the present invention for achieving the above object comprises the steps of preparing a substrate having a first region and a second region; Depositing amorphous silicon on the entire surface of the substrate to form an amorphous silicon film; Irradiating an amorphous silicon film on the first region of the substrate with a laser beam having an energy density of a complete melting regime to form polysilicon; Recrystallizing the polysilicon film formed in the first region by irradiating the polysilicon and the amorphous silicon film positioned on the first region and the second region of the substrate, respectively, with an energy beam having an energy density of almost complete melting region; And forming the amorphous silicon located in the second region from polysilicon having a plurality of grains grown about the seed.

According to an aspect of the present invention, a switching device is formed by irradiating an amorphous silicon having a seed with a laser beam having an energy density of a complete melting area capable of melting the seed to form a first polysilicon, and then forming a surface of the first polysilicon. An active layer formed of a second polysilicon obtained by irradiating a laser beam having a weaker energy density to the first polysilicon; Source and drain electrodes forming a channel of the active layer; And a gate electrode for switching the active layer.

According to the present invention, the active layer of the driving circuit region is first crystallized from polysilicon using the SLS method, and the entire region including the SLS region is crystallized using the MPGG method again to form a surface defect of the polysilicon formed by the SLS method. By eliminating, the switching speed was further improved.

6 is a schematic plan view of a thin film transistor array substrate showing the configuration of an active layer according to the present invention.

First, a polysilicon layer is formed by using an SLS crystallization method of irradiating a laser beam having an energy density of a complete melting region to a silicon film located in the driving portion A of the gate driving circuit region 115 and the data driving circuit region 113. do.

Next, the entire region B of the substrate 111 on which the silicon layer including polysilicon crystallized by the SLS method is formed is recrystallized by using the MPGG method.

That is, while crystallizing the `pixel region of the liquid crystal panel by the MPGG method, the crystallization is enlarged to the portion crystallized by the SLS method.

At this time, since the MPGG method uses a laser beam having an energy density near zero melting, the surface of the polysilicon crystallized by the SLS crystallization method is recrystallized after a predetermined depth.

At this time, the surface of the recrystallized polysilicon is in a state where a large number of defects existing in the grains have been removed.

FIG. 7 is an enlarged photograph of the surface of the polysilicon layer of each driving circuit region recrystallized by the method of FIG. 6.

As shown, unlike the conventional art, it can be seen that no defects are found in the grains 125.

Therefore, it is obvious that a semiconductor layer having a much higher charge mobility than the conventional one can be obtained.

Hereinafter, referring to FIG. 8, the mobility of the polysilicon layer crystallized only by the general SLS method and the SLS method and the MPGG method according to the present invention are sequentially used to compare the mobility of the recrystallized polysilicon.

8 is a graph comparing the mobility of the P-type polysilicon layer according to the crystallization method with respect to the voltage.

In the graph shown, the solid line 121 shows the result value of the polysilicon layer crystallized by the conventional SLS determination method, and the dotted line shows the result of the polysilicon layer recrystallized successively in the SLS method and the MPGG method according to the present invention. Values are shown.

Mobility can be obtained by differentiating the current value against voltage, and the threshold voltage driving the polysilicon layer crystallized by the conventional SLS determination method is about -3.3V, and the mobility at this time is about 107cm2 / sec. In accordance with the present invention, the threshold voltage driving the continuously recrystallized polysilicon layer is -1.7V and the mobility is obtained at the result of 145cm 2 / sec.

Therefore, it has been concluded that the switching element including the polysilicon layer recrystallized according to the present invention can be driven at a lower voltage and has a faster switching characteristic than in the prior art.

Accordingly, in the polysilicon type thin film transistor type liquid crystal display device according to the present invention, the semiconductor layer of the gate driving circuit portion and the data driving circuit portion is formed of polysilicon crystallized by using the SLS crystallization method and the MPGG crystallization method in succession, and thus the semiconductor layer is formed in the grain. By forming a polysilicon semiconductor layer in which no defects are present, there is an effect that a driving circuit having high mobility can be configured.

In addition, due to the fast operating characteristics of the driving circuit, there is an effect of increasing the current driving capability of the pixel switch.

In addition, it is advantageous for high opening ratio and high resolution of a large area liquid crystal display device.

Claims (2)

Preparing a substrate having a first region and a second region; Depositing amorphous silicon on the entire surface of the substrate to form an amorphous silicon film; Irradiating an amorphous silicon film on the first area of the substrate with a laser beam having an energy density of a complete melting regime to form polysilicon; Recrystallizing the polysilicon film formed in the first region by irradiating the polysilicon and the amorphous silicon film positioned on the first region and the second region of the substrate, respectively, with an energy beam having an energy density of almost complete melting region; Forming the amorphous silicon located in the second region with polysilicon having a plurality of grains grown around the seed. Method for forming a semiconductor layer of the array substrate for a liquid crystal display device comprising a. After irradiating the amorphous silicon having the seed with a laser beam having an energy density of the complete melting area to melt the seed, the first polysilicon was formed, and then the laser beam having a weaker energy density on the surface of the first polysilicon. An active layer formed of second polysilicon obtained by recrystallizing the first polysilicon; Source and drain electrodes forming a channel of the active layer; A gate electrode for switching the active layer Switching device comprising a.