KR100409233B1 - Method of Manufacturing Polycrystalline Silicon Thin Film Transistor using Excimer Laser annealing - Google Patents

Abstract

The present invention relates to a method of manufacturing a polycrystalline silicon thin film transistor using an excimer laser annealing process method, comprising the steps of: (1) forming an amorphous silicon thin film on a wafer or glass substrate on which an oxide film is formed; and (2) using a photo process. Patterning the photoresist film on the amorphous silicon thin film of and implanting impurity ions, (3) depositing an aluminum thin film on the amorphous silicon thin film and the photoresist pattern, and then removing the photoresist film and the aluminum thin film on the photoresist by a lift-off process. Forming an aluminum pattern; (4) irradiating a primary excimer laser on the amorphous silicon thin film to polycrystallize; and (5) removing the aluminum pattern on the top of the polycrystalline silicon thin film, and then performing a secondary excimer laser. Irradiating to activate the ions implanted in the source and drain By providing a method of manufacturing a polycrystalline silicon active layer and a polycrystalline silicon thin film transistor constituted, by increasing the conventional grain size of several hundred nm to about a few μm to significantly reduce the density of grain boundaries in the polycrystalline silicon thin film to facilitate the flow of electrons In addition to improving the mobility characteristics of the device, it is also effective in suppressing leakage current.

Description

Method of Manufacturing Polycrystalline Silicon Thin Film Transistor using Excimer Laser annealing}

The present invention relates to a method for manufacturing a polycrystalline silicon thin film transistor using excimer laser annealing, in particular, by using an excimer laser annealing process method to reduce the density of grain boundaries inherent in the polycrystalline silicon thin film to have high mobility and leakage The present invention relates to a method for manufacturing a polycrystalline silicon thin film transistor capable of suppressing a current.

Amorphous Silicon Thin Film Transistors (a-Si TFTs) have been widely used as switching elements of Active Matrix-Liquid Crystal Displays (AM-LCDs), but current research trends demand high quality LCDs. Accordingly, polycrystalline silicon thin film transistors (poly-Si TFTs), which operate faster than amorphous silicon thin film transistors, are used as switching devices.

The method of fabricating a polycrystalline silicon thin film transistor may include a crystallization process in a method of manufacturing an amorphous silicon thin film transistor, and a heat treatment method and an excimer laser may be used for the crystallization process. Among the above methods, an annealing method using an excimer laser is mainly used because the glass substrate used as the substrate of the LCD cannot withstand the general heat treatment process of 600 ° C. or more. Excimer laser annealing is mainly used to produce a 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 grain form with excellent crystallinity when the amorphous silicon thin film is melted in a liquid state and solidified by a solid by excimer laser irradiation (Appl. Phys. Lett. Vol. 63, no. 14, p. 1969, 1993) The electrical mobility of amorphous silicon thin film transistors is about 1 to 2 cm ² / Vsec, and the electrical mobility of polycrystalline silicon thin film transistors fabricated by general heat treatment is about 10 to 20 cm ² / Vsec. In contrast, the electrical mobility of the polycrystalline silicon thin film transistor fabricated using an excimer laser has a value of more than 100 cm ² / Vsec. (IEEE Trans. Electron Devices, vol. 36, no. 12, p. 2868, 1989)

However, although the electrical characteristics of polycrystalline silicon thin film transistors fabricated using excimer laser annealing are excellent in terms of electrical mobility and driving current, they are only applicable to the switch-on state characteristics, and the leakage current in the switch-off state. Flows high. 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 though a voltage of about 5 to 10V 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 a high electric field between a source and a drain, that is, a polycrystalline silicon channel. 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. (IEEE Trans. Electron Devices, vol. 32, no. 9, p. 1878, 1985)

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 the silicon atoms is broken or incompletely bonded, which is an obstacle to electron movement. do. 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. A common way to increase grain size is to increase laser energy or heat the substrate, but not enough to bring about dramatic improvements in thin film properties.

SUMMARY OF THE INVENTION The present invention has been proposed to solve the above problems of the prior art, and an object of the present invention is to increase the size of grain and order grain boundaries in order to reduce the density of grain boundaries in a channel of a polycrystalline silicon thin film transistor. The present invention provides a method for manufacturing a polycrystalline silicon thin film transistor using an excimer laser annealing method.

1 is a schematic view showing a conventional excimer laser annealing method.

2 is a photograph of a grain structure of a polycrystalline silicon thin film manufactured by a conventional excimer laser annealing method using a high resolution electron microscope.

3 is a schematic view showing an excimer laser annealing method according to the present invention.

4 is a photograph of a grain structure of a polycrystalline silicon thin film manufactured by an excimer laser annealing method according to the present invention under conditions of a substrate having a laser energy density of 400 mJ / cm ² and 200 ° C. using a high resolution electron microscope.

Figure 5 is a graph showing the temperature change inside the amorphous silicon thin film with time after the laser irradiation.

Figure 6 is a laser energy density of 400mJ / cm ² , a photo of the polycrystalline silicon thin film prepared according to the present invention prepared under the conditions of the substrate at room temperature using a high-resolution electron microscope.

7 is a high resolution electron micrograph showing an enlarged interface between an amorphous silicon region and a horizontal growth grain region.

8 is a graph showing the change in the size of the horizontal growth grain with the laser energy and the substrate temperature.

9 is a plan view of a conventional polycrystalline silicon thin film transistor element structure.

10 is a cross-sectional view of a conventional polycrystalline silicon thin film transistor element structure.

11 is a plan view of a polycrystalline silicon thin film transistor element structure according to the present invention.

12 is a cross-sectional view of a polycrystalline silicon thin film transistor element structure according to the present invention.

13 is a process chart showing a method for manufacturing a polycrystalline silicon thin film transistor according to the present invention.

14 is a plan view of a conventional polycrystalline silicon thin film transistor element structure having a long channel structure.

Fig. 15 is a sectional view of a conventional polycrystalline silicon thin film transistor element structure having a long channel structure.

16 is a plan view of a polycrystalline silicon thin film transistor element structure having a long channel structure according to the present invention.

17 is a cross-sectional view of a polycrystalline silicon thin film transistor element structure having a long channel structure according to the present invention.

18 is a process chart showing a method for manufacturing a polycrystalline silicon thin film transistor having a long channel structure according to the present invention.

19 is a graph showing the electrical properties of the polycrystalline silicon thin film transistor device according to the present invention.

In order to achieve the above object, the present invention comprises the steps of (1) forming an amorphous silicon thin film on a wafer or glass substrate on which an oxide film is formed, and (2) a photo process on the channel region of the amorphous silicon thin film formed through the step (1). Implanting impurity ions after patterning the photoresist layer using (3), and depositing an aluminum thin film on the amorphous silicon thin film and the photoresist pattern patterned using the photolithography process and implanted with impurity ions through the step (2). And then removing the photoresist film and the aluminum thin film on the photoresist film by a lift-off process to form an aluminum pattern, and (4) irradiating a primary excimer laser on the amorphous silicon thin film on which the aluminum pattern is formed through (3). Recrystallization to form a polycrystalline silicon thin film having a single grain boundary, and (5) through (4) Irradiating an excimer laser to remove the aluminum pattern on the recrystallized polycrystalline silicon thin film, and irradiating a second excimer laser to activate ions implanted into the source and drain, and (6) through (5) step Forming a gate insulating film by depositing an oxide film or a nitride film on the polycrystalline silicon active layer activated by the implanted ions, and (7) depositing a metal on the gate insulating film formed through the step (6), and then The present invention provides a method of manufacturing a polycrystalline silicon thin film transistor using an excimer laser annealing, characterized in that the patterning and patterning process are performed to form a gate. Forming an amorphous silicon thin film on a glass substrate, and (2) (C) forming a long channel region by implanting impurity ions after patterning the photoresist film in a series form at a predetermined interval on each channel region of the amorphous silicon thin film formed through a photolithography process, and (3) the step (2) Forming an aluminum pattern by depositing an aluminum thin film on the amorphous silicon thin film and the photoresist pattern patterned using a photo process and implanted with impurity ions through a photo process, and then removing the photoresist film and the aluminum thin film on the photoresist, 4) forming a polycrystalline silicon thin film having a single grain between regions implanted with impurities by irradiating a primary excimer laser on the amorphous silicon thin film on which an aluminum pattern is formed through step (3); ) The above polycrystalline silicon recrystallized by irradiating a primary excimer laser through step (4) Removing the aluminum pattern on the top of the thin film and irradiating a second excimer laser to activate ions in the region implanted with the source, the drain, and the impurity; and (6) the activating with the ions implanted through the step (5). Forming a gate insulating film by depositing an oxide film or a nitride film on the polycrystalline silicon active layer, (7) depositing a metal on the gate insulating film formed through the step (6), and then using a gate pattern mask and a photo process Provided is a method of manufacturing a polycrystalline silicon thin film transistor using excimer laser annealing, characterized in that the step of forming a gate by patterning.

Hereinafter, with reference to the accompanying drawings will be described in detail the present invention.

As a method of increasing grain size and ordering grain boundaries in detail, the steps of fabricating a polycrystalline silicon active layer using an excimer laser annealing method for inducing horizontal growth of grains will be described in detail.

FIG. 3 is a schematic diagram of an excimer laser annealing method, in which a laser beam is selectively absorbed into an amorphous silicon thin film by depositing a metal thin film, especially an aluminum thin film, on a amorphous silicon thin film and patterning the photolithography process as shown in FIG. Be sure to The pattern is in the form of a long rectangle. The aluminum pattern formed on the amorphous silicon thin film reflects most laser beams because the reflectivity in the ultraviolet region including the wavelength of the XeCl excimer laser is more than 90%. Therefore, when the substrate is not heated, the internal temperature of the amorphous silicon region covered by the aluminum pattern is maintained at room temperature without change before and after laser irradiation, so that no phase change occurs. On the other hand, in the amorphous silicon region having no aluminum pattern thereon, the laser energy is absorbed to cause melting in the liquid state, and the internal temperature of the liquid amorphous silicon is at a melting point (Tm, about 950 ° C.) or more. At this time, the laser energy to be irradiated should be high enough to completely melt the amorphous silicon thin film to the lower interface, and if the laser energy is insufficient to melt to the bottom of the amorphous silicon thin film, the effect of the present invention cannot be obtained.

The internal temperature change of the amorphous silicon thin film with time after laser irradiation is shown in FIG. 5. From the end of the laser beam irradiation, the internal temperature of the liquid amorphous silicon is cooled by the unmelted amorphous silicon adjacent to both sides, which is solid amorphous silicon on the side and aluminum on the upper side of the lower insulating substrate. This is because the thermal conductivity of the pattern is higher. Therefore, the liquid amorphous silicon reaches the nucleation temperature (Tn, Tn <Tm) preferentially at the interface between the solid phase and the liquid phase on both sides rather than the central portion, and crystal nuclei are formed at this portion. After the nuclei are formed, horizontal growth of grain occurs from the lower temperature to the higher temperature.

4 is a view illustrating grain structure of a polycrystalline silicon thin film according to the present invention by using a high resolution electron microscope, ie, Transmission Electron Microscopy (TEM), manufactured by a conventional general excimer laser annealing method shown in FIGS. 1 and 2. While the average grain size of the polycrystalline silicon thin film is about several hundred nm, the polycrystalline silicon thin film according to the present invention can obtain horizontal growth grain having a length of several micrometers. This is an increase in grain size by several orders of magnitude compared to the conventional method. It is also possible to arrange these horizontal growth grains in a straight line so that the grains growing opposite each other create only one grain boundary. Therefore, when the thin film transistor device is manufactured using the polycrystalline silicon thin film shown in FIG. 4 as a channel, only one grain boundary needs to pass when a current or an electron moves, and thus the characteristics may be remarkably improved. The grain boundaries between the grains growing side by side can be neglected since they do not interfere with the movement of the current or electrons simply by inducing the current or the flow of electrons in parallel with the grain growth direction.

In order to examine the horizontal growth behavior of the grains according to the present invention in more detail, one aluminum pattern was formed, the laser was irradiated, and the grain structure was observed. FIG. 6 is a planar TEM image of a recrystallized polycrystalline silicon thin film according to the present invention. One aluminum pattern was formed to irradiate the laser, and the laser energy density was 400 mJ / cm ² , and the substrate was performed at room temperature without heating. In FIG. 6, an amorphous silicon region is observed, which corresponds to a portion covered by the upper aluminum pattern, and adjacent long growth horizontal growth grains are observed in the polycrystalline silicon region. The average length of the horizontal growth grain is about 1.2 μm in the direction of growth. In addition, polycrystalline silicon regions with dense grains having an average size of about 100 nm adjacent to the horizontally grown grains are observed. This dense grain is not formed by the horizontal growth grain mechanism but is the grain formed by the conventional laser annealing mechanism and is the same as the grain produced by the general laser annealing shown in FIG. In other words, it corresponds to an area where the horizontal growth effect of grain occurring at the edge of the aluminum pattern does not reach.

7 is an enlarged TEM photograph of an interface between an amorphous silicon region and a horizontal growth grain region. In FIG. 7, it can be clearly seen that nucleation of the horizontal growth grain occurred at the interface of the amorphous silicon region covered by the aluminum pattern. In the above experimental results, the size of the horizontal growth grain only occurs for a certain time, and this time is determined by the time when the grain starts to form itself in a region farther away from the edge of the aluminum pattern.

It has been reported that the self nucleation time for growth into grains during normal laser annealing is dependent on the laser energy density and substrate temperature. Therefore, in the proposed process, the size change of the horizontal growth grain with the laser energy and substrate temperature was observed. 8 is shown. As shown in FIG. 8, the grain size increases as the laser energy and substrate temperature increase, and when the laser energy density is 400 mJ / cm ² at a substrate temperature of 200 ° C., which is an appropriate temperature at which the glass substrate can be used, a maximum size of 1.6 μm is obtained. The grain could be obtained. In order to fabricate a polycrystalline silicon thin film having only horizontally grown grains without dense grain regions, the laser energy density is 400 mJ / cm ² , and when performed at room temperature, the pattern spacing of the two aluminum is narrowed to 2 μm. Irradiate the laser. The grain structure at this time is shown in FIG. It can be seen from FIG. 4 that the grains grown from the interface with both amorphous silicon meet at the center of the polycrystalline region. Therefore, there is no region with the dense grain shown in FIG. According to the relationship between the laser energy density and the horizontal growth length of the grain according to the change of the substrate temperature, it is possible to obtain the polycrystalline silicon of the horizontally grown grain having a width of 200 m at the substrate temperature and the laser energy density of 400 mJ / cm ² and a maximum of 3 μm. Can be.

Looking at the manufacturing method of the polycrystalline silicon thin film transistor according to the present invention.

A method of applying a polycrystalline silicon thin film according to the present invention to a thin film transistor device is to form a channel in a polycrystalline silicon region composed of horizontal growth grains, and to arrange the parallel to the grain growth direction. The technique required at this time is to accurately form the aluminum pattern on both edges of the channel of the thin film transistor, that is, the source and drain regions, using a photolithography technique.

The first device structure 1 according to the present invention is shown in FIGS. 11 and 12. In the proposed device, when the gate is switched on and a voltage is applied between the source and the drain, a current flows. At this time, the moving electrons pass through a single grain boundary, so the conventional conventional thin film transistor device shown in FIGS. Compared with this, the electrical mobility and driving current characteristics can be improved. The electrical mobility can be improved compared to the conventional thin film transistor device. The leakage current is also reduced because the density of grain boundaries in the channel is significantly reduced. In the proposed device structure shown in Fig. 11, the channel length can be designed to be equal to or slightly smaller than the sum of the grain lengths grown on both sides.

13 shows a manufacturing process procedure of the element structure 1 according to the present invention.

First, an amorphous silicon thin film having a thickness of 800 kPa was fabricated using plasma enhanced chemical vapor deposition (PECVD) or low pressure chemical vapor deposition (LPCVD) at a substrate temperature of 250 ° C. It deposits on the wafer in which the oxide film was formed, or the glass substrate in which the oxide film of about 5000 micrometers thickness was formed (FIG. 13A). Since a large amount of hydrogen is contained in the amorphous silicon thin film deposited by PECVD, a dehydrogenation process should be performed by annealing at a temperature of about 400 ° C. for 2 hours after deposition.

Since the deposited amorphous silicon thin film is an active layer to be used as a channel, a source and a drain of the thin film transistor, the active region is patterned by a photographic process and an etching process (FIG. 13B). On the amorphous silicon thin film, the photoresist is patterned to cover the channel region by a photolithography process (FIG. 13C). Forming the photoresist pattern on the channel region is to selectively ion implant and dopate only the source and drain regions as shown in FIG. 13D.

As shown in FIG. 13E, an aluminum thin film having a thickness of 2000 μs was deposited by a thermal evaporation process, and as shown in FIG. 13F, the photoresist film and the aluminum thereon were removed by a lift-off process to remove the aluminum pattern. It is formed as shown in Fig. 13F. The method of forming an aluminum pattern using this lift-off process is the main method of the present invention for forming an aluminum pattern on both sides of a channel without misalignment.

Excimer laser annealing is performed as shown in FIG. 13G. By this process, the above-described horizontal growth grains are obtained, and a single grain is formed between the regions into which impurities are implanted. At this time, about 300-450mJ / cm <^2> is suitable for laser energy conditions.

As shown in FIG. 13H, the top aluminum pattern is removed and excimer laser annealing is performed once more. This process is for activating ions implanted in the source and drain. At this time, the laser energy condition is suitably about 150 to 300 mJ / cm ² , and at this energy, the previously formed horizontal growth grain can be maintained. This is because the melting point of the polycrystalline silicon is higher than that of the amorphous silicon, so that the phase change of the polycrystalline silicon does not occur with the laser energy of about 150 to 300 mJ / cm ² .

As shown in Fig. 13I, a gate insulator and a gate are formed. That is, a gate insulating film is formed by depositing an oxide film or a nitride film on the polycrystalline silicon active layer, a metal is deposited on the gate insulating film, and patterned by using a gate pattern mask and a photo process to form a gate.

If it is determined that the length of the channel is short in practical application, it can be manufactured with the device of structure (2). The device structure 2 is shown in FIGS. 16 and 17. As shown in FIG. 17, there is only one grain boundary between regions implanted with impurities, which affects the characteristics of the device, which is different from the polycrystalline silicon thin film transistor fabricated by conventional laser annealing. Corresponding conventional conventional thin film transistor element structures are shown in FIGS. 14 and 15.

The element structure 2 is a form in which several elements of the structure 1 are connected in series. The method of connecting each thin film transistor element in series can be realized by doping the connection region to the same type and concentration as the source and drain injected. The length of the connection region is suitably several μm in length. 18 shows a process sequence for producing the device structure 2. The difference from the manufacturing method of the above-mentioned element structure 1 is only the form of the photosensitive film shown to FIG. 18C. Thus, there is a difference in the region to be ion implanted in FIG. 18D and in the form of the aluminum pattern in FIG. 18F. All other process sequences and conditions are the same.

19 shows the electrical characteristics of the device fabricated using the excimer laser annealing method according to the present invention compared to the device fabricated by a conventional general excimer laser annealing method. Since the effective channel length of the device according to the present invention is 6 μm and the total channel length from the source to the drain is 10 μm, the comparison is made with conventional devices having two respective channel lengths. From the measured results, the electrical mobility of the device according to the present invention showed a high value of 240 cm ² / Vsec, which is about three times higher than that of a conventional device. In addition, the threshold voltage, the slope before the threshold, and the ON / OFF current ratio are also It can be seen that it is superior to the general device of.

As described above, according to the present invention, an ultraviolet reflector such as a metal in a pattern form for inducing horizontal growth of grain can be accurately aligned on the source and drain of the thin film transistor element by using one photo process and a lift-off process. The device can be manufactured easily. In addition, by increasing the grain size of several hundred nm to about several micrometers, the density of grain boundaries in the polycrystalline silicon thin film is remarkably reduced to facilitate the flow of electrons, which not only improves the mobility characteristics of the device but also is effective in suppressing leakage current. .

Claims (6)

(1) forming an amorphous silicon thin film on a wafer or glass substrate on which an oxide film is formed; (2) implanting impurity ions after patterning the photoresist film on the channel region of the amorphous silicon thin film formed through the step (1) by using a photo process; (3) depositing an aluminum thin film on the amorphous silicon thin film and the photosensitive film pattern patterned using the photo process through the step (2) and implanted with impurity ions, and then removing the aluminum thin film on the photosensitive film and the photosensitive film by a lift-off process Forming an aluminum pattern, (4) forming a polycrystalline silicon thin film having a single grain boundary by irradiating a primary excimer laser on the amorphous silicon thin film on which the aluminum pattern is formed through step (3); (5) irradiating the primary excimer laser through step (4) to remove the aluminum pattern on the recrystallized polycrystalline silicon thin film and irradiating the secondary excimer laser to activate ions implanted into the source and drain. Wow, (6) forming a gate insulating film by depositing an oxide film or a nitride film on the polycrystalline silicon active layer activated by the ions implanted through the step (5); (7) polycrystalline by excimer laser annealing, comprising depositing a metal on the gate insulating film formed through the step (6) and then patterning the same by using a gate pattern mask and a photo process. Method of manufacturing a silicon thin film transistor. The method of claim 1, Irradiating the primary excimer laser on the amorphous silicon thin film (4) is a method of manufacturing a polysilicon thin film transistor using an excimer laser annealing, characterized in that performed at 300 ~ 450mJ / cm ² . The method of claim 1, Irradiating the secondary excimer laser on the amorphous silicon thin film (5) is a method of manufacturing a polycrystalline silicon thin film transistor using excimer laser annealing, characterized in that carried out at 150 ~ 300mJ / cm ² . (1) forming an amorphous silicon thin film on a wafer or glass substrate on which an oxide film is formed; (2) forming a long channel region by implanting impurity ions after patterning the photoresist film in series form at a predetermined interval on each channel region of the amorphous silicon thin film formed through the step (1) using a photo process; , (3) depositing an aluminum thin film on the amorphous silicon thin film and the photoresist pattern patterned using the photo process through the step (2) and implanted with impurity ions, and then removing the aluminum thin film on the photosensitive film and the photoresist by the photo process. Forming a pattern, (4) forming a polycrystalline silicon thin film having a single grain between regions implanted with impurities by irradiating a primary excimer laser on the amorphous silicon thin film on which the aluminum pattern is formed through step (3); (5) Irradiating the primary excimer laser through step (4) to remove the aluminum pattern on the recrystallized polycrystalline silicon thin film and irradiating the secondary excimer laser to ion in the region where the source, drain, and impurities are implanted. Activating the (6) forming a gate insulating film by depositing an oxide film or a nitride film on the polycrystalline silicon active layer activated by the ions implanted through the step (5); (7) polycrystalline by excimer laser annealing, comprising depositing a metal on the gate insulating film formed through the step (6) and then patterning the same by using a gate pattern mask and a photo process. Method of manufacturing a silicon thin film transistor. The method of claim 4, wherein Irradiating the primary excimer laser on the amorphous silicon thin film (4) is a method of manufacturing a polycrystalline silicon thin film transistor using an excimer laser annealing, characterized in that carried out at 300 ~ 450mJ / cm ² . The method of claim 4 Irradiating the secondary excimer laser on the amorphous silicon thin film (5) is a method of manufacturing a polycrystalline silicon thin film transistor using excimer laser annealing, characterized in that carried out at 150 ~ 300mJ / cm ² .