Method for fabricating a polysilicon thin film transistor

Abstract: In the crystal structure of a polysilicon thin film (3) having a field effect mobility µ FE of about 80 cm²/V·sec, the grain size is about 200 nm and the crystallite size on the (111) plane is about 180 nm. The crystallite size corresponds to the size of a completely monocrystallized portion of a grain. The condition of obtaining a field effect mobility µ FE of about 80 cm²/V·sec is that the crystallite size on the (111) plane is at least 180 nm (measured value). By taking the crystal size into consideration, it becomes possible to achieve a high field effect mobility µ FE which cannot be obtained merely by increasing the grain size.

Description: The present invention relates to a semiconductor device, such as a thin-film transistor, having in its main part a polysilicon thin film for transferring carriers. A technique of forming a thin-film transistor on an insulating substrate is known in a field of active matrix liquid crystal displays (LCDs). The technique allows a semiconductor integrated circuit to be formed on a transparent insulating substrate formed of, for example, glass, or a large insulating substrate which cannot be formed of a monocrystalline semiconductor. At present, a thin-film transistor generally includes an active layer made of amorphous silicon or polysilicon. Since amorphous silicon can be formed at a low temperature, it is applicable to an active matrix LCD which must be formed on a glass substrate having a low melting point. However, since amorphous silicon has a low electron mobility, it has been used only as a switching element for charging a pixel capacitor electrode to apply an electric field to a liquid crystal. An active layer made of polysilicon must be used in a circuit element, e.g., a driver circuit, a ROM, a RAM or a CPU, which must be driven at a high speed. For example, a driver circuit of a high-quality, large-screen liquid crystal TV or a high-definition office automation (OA) liquid crystal display panel is driven at a clock frequency of about 10 MHz. If such a driver circuit is formed of a semiconductor device, a field effect mobility µ FE of 50 cm 2 /V·sec or more, preferably 80 cm 2 /V·sec or more is required. However, the field effect mobility µ FE of a polysilicon thin-film transistor known to the public at present is at most 30 cm 2 /V·sec. In "Manipulation of nucleation sites in solid state Si crystallization", Applied Physics Letters 59(27), 1991, pp. 3565-3567, by H.Kumomi et al., polycrystalline silicon films are disclosed in which the formation of single grains is manipulated so as to control the location of grains and grain boundaries. An experimental procedure is described wherein an amorphous silicon film having a thickness of 100nm is deposited on a thermally oxidized silicon wafer. Document FR-A-2573916 discloses a method of forming a thin semiconductor film that is suitable for a thin film transistor on an insulating substrate. To this end, a polycrystalline film is provided having a (100) orientation, which is transformed into a (111) orientation by annealing with the aid of a silicon dioxide film. Prior art document JP-A- 2148831 describes a method of forming a thin film semiconductor device having a high carrier mobility, wherein a polycrystalline film is obtained by means of laser irradiation. The polysilicon film exhibits a main priority orientation in the (111) plane, wherein an average crystal grain size is approximately 100nm. An internal structure of the crystal grains is not referred to in this document. In IEEE Transactions on Electron Devices 35(7), 1988, pp. 923-928, by T. Katoh, a polysilicon transistor is disclosed having improved characteristics. By means of solid phase crystallization of evaporated amorphous silicon, the grains in the polysilicon film grow to a size of 3.5nm. In "Lateral solid phase epitaxy of Si over SiO 2 patterns and its application to silicon-on-insulator transistors", Applied Pysics Letters 49(7), 1986, pp. 397-399, M. Sasaki et al., MOS transistors are described that are formed on SOI regions, wherein an amorphous silicon film of 600nm is converted into a single crystalline film on the SOI region by means of the lateral solid phase epitaxy. It is accordingly an object of the present invention to provide a semiconductor device which allows the mobility of carriers to greatly increase. To achieve the object, the semiconductor device of the present invention has the features as set forth in claim 1. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figs. 1 to 6 are enlarged cross-sectional views showing the steps of manufacturing a field effect thin-film transistor as an embodiment of a semiconductor device according to the present invention; Fig. 7 is a diagram for explaining the definition of a grain size; Fig. 8 is a diagram for explaining crystallography by X ray Diffractor; Fig. 9A is a graph showing the grain size-field effect mobility characteristic of the thin-film transistor shown in Fig. 6; Fig. 9B is a graph showing the crystal size-field effect mobility characteristic of the thin-film transistor shown in Fig. 6; and Fig. 10 is a graph showing the grain size-crystallite size characteristic of the thin-film transistor shown in Fig. 6. [Basic Concept of the Invention] It has been considered that the field effect mobility µFE of a polysilicon thin-film transistor is determined by grain sizes of silicon crystals constituting an active layer, i.e. a polysilicon thin film. Although there is another determinant such as a trap of carriers which may be caused depending on the state of an interface between the polysilicon thin film and a gate insulating film, it has been considered important to form large and uniform grains, as far as the crystal structure is concerned. Under these circumstances, techniques for an enhancement of the electron mobility by increasing the size and the uniformness of grains have been studied. However, the present inventor discovered throughout his researches that it is difficult to enhance the electron mobility only by increasing the grain size of a polysilicon thin film. According to the inventor's research, the electron mobility was not increased beyond a predetermined value however the grain size is increased, whereas it was satisfactorily increased even when the grain size was not very large. As a result of the research, the inventor found that the crystallite size, as well as the grain size, must be increased to enhance the field effect mobility µ FE of a thin-film transistor having a polysilicon thin film as an active layer. It is desirable that the grain size and the crystallite size are substantially the same. In addition, the mobility was further increased when the grain size and the crystallite size were greater than the thickness of the polysilicon thin film. This appears to result from the monocrystalization effect due to the state that crystallites having a size regarded as a substantially perfect monocrystal region occupy the main region of the grain and decrease the crystal defects in the grain. An embodiment of the present invention will be described in detail. [Embodiment] Figs. 1 to 6 show steps of manufacturing a thin-film transistor according to an embodiment of the present invention. The structure of the thin-film transistor and a method of manufacturing the same will now be described with reference to these drawings in sequence. First, as shown in Fig. 1, an amorphous silicon thin film 2 is deposited on the upper surface of an insulating substrate 1 made of quartz or the like by an LP-CVD (Low Pressure Chemical Vapor Deposition) method at a temperature of 550°C. Thereafter, as shown in Fig. 2, XeCℓ excimer laser beams are applied to the substrate, thereby polycrystallizing the amorphous silicon thin film 2 into a polysilicon thin film 3 by a liquid-phase growth. The crystal structure of the polysilicon thin film 3 in this state will be described later. Then, as shown in Fig. 3, a photoresist film 5 is formed on the upper surface of the polysilicon thin film 3, and patterned not to leave on the regions of film 3 which correspond to source and drain forming regions 4. Using the photoresist film 5 as a mask, impurities such as phosphorus ions or boron ions are injected to a high concentration into the source and drain forming regions 4 of the polysilicon thin film 3, thereby forming impurity-injected regions 6. Thereafter, the photoresist film 5 is removed. Next, as shown in Fig. 4, XeCℓ excimer laser beams are applied to the polysilicon thin film 3 again to activate the impurities injected in the regions 6. Then, as shown in Fig. 5, an unneeded peripheral portion of the polysilicon thin film 3 is removed. A central portion of the polysilicon thin film 3 serves as a channel region 3a, and side portions thereof as source and drain regions 3b formed of activated impurity regions. Subsequently, as shown in Fig. 6, a gate-insulating film 7 formed of a silicon oxide film and the like is formed on the upper surface of the assembly. Thereafter, a gate electrode 8 made of chrome and the like is formed on the upper surface of a portion of the gate insulating film 7 which corresponds to the channel region 3a, by means of a depositing and a patterning. Thereafter, a passivating insulating film 9 made of silicon nitride or the like is formed on the entire upper surface of the assembly. Then, contact holes 10 are formed in those portions of the passivating insulating film 9 and the gate-insulating film 7 which correspond to the source and drain regions 3b. Subsequently, source and drain electrodes 11 made of aluminum and the like are patterned on the passivating insulating film 9 and connected to the source and drain regions 3b through the contact holes 10 are formed on the passivating insulating film 9 and in the holes 10. Thus, a field effect thin-film transistor of a coplanar type is obtained. The crystal structure of the polysilicon thin film 3 shown in Fig. 2 will now be described with reference to Table 1. Sample: LPCVD Thickness of silicon film: 500 A Embodiment of Present Invention Polycrystallizing method Mobility (µFE) Grain size (C) B Crystallite size (hkℓ) Laser anneal XeCℓ excimer laser 250°C, 300mJ/cm 2 [Liquid-phase growth] (111) 202 nm 90 cm 2 /V·sec 246 nm (220) 144 nm (311) 103nm Prior Art Polycrystallizing method Mobility (µ FE ) Grain size (C) B Crystallite size (hkℓ) Heart treatment 600°C 48 hours [Solid-phase growth] (111) 37 nm 30 cm2/V·sec 1.2 µm (220) 23 nm (311) 36 nm As shown in Table 1, a polysilicon thin film according to an embodiment of the present invention was prepared in the following manner: an amorphous silicon thin film having a thickness of about 500 Å was deposited on the upper surface of a quartz substrate, and XeCℓ. excimer laser beams were applied twice to the amorphous silicon thin film at a temperature of about 250°C under an energy density of about 300 mJ/cm 2 , so that the amorphous silicon thin film was polycrystallized in a liquid-phase growth, to obtain a polysilicon thin film. To compare the present embodiment with a conventional device, a polysilicon thin film according to conventional art was prepared in the following manner: an amorphous silicon thin film having a thickness of about 500 A was deposited on the upper surface of a quartz substrate; the substrate was heated in a nitrogen atmosphere at a temperature of about 600°C for 48 hours, so that the amorphous silicon thin film was polycrystallized in a solid-phase growth, to obtain a polysilicon thin film. The crystal structures were analyzed by using a (Transmission Electron Microscope) and an XD (X-ray Diffractor analysis). In case of the TEM, grain size was measured with JEM-2010 of JEOL (acceleration voltage: 200 kV, magnification: 5 × 10 5 to 1.5 × 10 6 ). The grain size of the embodiment was 246 nm, whereas the grain size of the prior art was 1.2 µm, which is 5 times greater than the grain size of the embodiment. The grain size refers to the size of a grain in the top view of the polysilicon thin film, and is represented by an average value of the total of average values c of size of grains obtained by the following equation: c = ( a + b )/2 , where a denotes the length of a grain along the major axis and b denotes the length of the grain along the minor axis, for example, as shown in Fig. 7. Each grain size C indicated in Table 1 is an average calculated based on the measured values at 30 points. The crystallite size by use of the XD was measured by detecting a diffraction intensity by using a low angle incidence method, wherein ' = 1°. RU-200 of Rigaku Denki Company (radiation source: CuKα, maximum output: 12 kW) was used for the measurement. In the measurement, step scanning for 2 seconds was performed three times (30 minutes in total) with a rotational angle of 0.01° within a range of ±1.5° on each of the peak planes, i.e., the (111) plane, the (220) plane, and the (311) plane, under the condition of the incident X-ray intensity 50 kV and the current of 180 mA. The crystallite size was calculated from the peak width at half height by use of the following equation of Scherrer. D hkl = λ/(B·cosB) where D hkl denotes a crystallite size in a direction perpendicular to the (hkl) plane; λ, a wavelength of an X-ray beam; B, a half width; and B, a Bragg's angle. The results of the measurement are shown in Table 1. In the conventional device, the average values of the crystallite size measured in the three-time measurement were 37 nm on the (111) plane, 23 nm on the (220) plane, and 36 nm on the (311) plane, whereas in the present embodiment, the average values were much greater than in the conventional device, i.e., 202 nm on the (111) plane, 144 nm on the (220) plane, and 103 nm on the (311) plane. Coplanar type field effect thin-film transistors as shown in Fig. 6 were manufactured using the thin films of the embodiment and the conventional art, and the field effect mobilities µ FE thereof were measured. As shown in Table 1, the mobility in the conventional thin-film transistor was 30 cm 2 /v·sec, whereas the mobility in the thin-film transistor of the present embodiment was 90 cm 2 /V·sec, which is three times greater than that of the conventional transistor. The measurement result represents that the field effect mobility µ FE is correlated to the crystallite size rather than the grain size. This appears to be based on the fact that a thin-film transistor wherein the grain size is large and the crystallite size is small includes a number of crystal defects, while a thin-film transistor wherein the crystallite size is substantially the same as the grain size includes few crystal defects, even if the grain size is small, since the crystallite is regarded as a complete monocrystal region. To confirm this, similar measurement was performed with respect to a number of samples according to the embodiment. Figs. 9A, 9B, and 10 show results of the measurement. Although the sample indicated in Table 1 includes an amorphous silicon film deposited on the substrate to a thickness of 500 Å by a LPCVD method, some of the samples indicated in Figs. 9A, 9B, and 10 are formed by a plasma CVD method or have various thicknesses (500 to 3000 Å). Fig. 9A shows the relationship between grain size C and field effect mobility µ FE , and Fig. 9B shows the relationship between crystallite size and field effect mobility µ FE on the (111) plane. Fig. 10 shows the relationship between a grain size and a crystallite size on the (111) plane. Although a crystal size measured by a low-angle incidence method must be corrected, the values of the crystallite size indicated in Figs. 9B and 10 are calculated by the equation of Scherrer and uncorrected. As shown in Fig. 9A, there is a specific correlation between the grain size C and the field effect mobility µ FE . Further, as shown in Fig. 9B, there is a specific correlation between the crystallite size on the (111) plane and the field effect mobility µ FE . Conditions for the correlations can be understood from Fig. 10. The broken line in Fig. 10, which makes an angle of 45° with the abscissa, represents a characteristic that the ratio of a crystallite size to a grain size is 1:1. As is obvious from Fig. 10, crystallite size approximates to grain size. In the range that the grain size is 100 nm or smaller, it is substantially the same as the crystallite size. In the range that the grain size is greater than 100 nm, the ratio of the crystallite size to the grain size is slightly smaller than 1. In case that grain size is about 300 nm, the crystallite size is about 60 to 70 % of the grain size. If a thin film having the crystallite size the same as grain size in the range of 200 to 300 nm or greater of the grain size by improving the manufacturing method is obtained, the field effect mobility µ FE can be further increased. It is clear from Figs. 9A and 9B that the crystallite size of about 180 nm (measured value) or greater and the grain size C of about 200 nm or greater suffice to obtain a field effect mobility µ FE of about 80 cm 2 /V·sec. If these conditions are satisfied, a field effect thin-film transistor having a field effect mobility µ FE of about 80 cm 2 /V·sec can be obtained. Accordingly, it is possible to form, using a semiconductor device, a driver circuit which is operated with a clock frequency of about 10 MHz. Conventionally, it has been considered that the field effect mobility µ µ FE increases in proportion to the grain size C . However, the conventional idea, in which the crystallite size is not taken into account, is clearly incorrect in view of Table 1. Further, referring to Table 1, the crystallite sizes on the (111), (220), and (311) planes of the embodiment are respectively 202 nm, 144 nm, and 103 nm, which are all greater than the thickness of the polysilicon thin film, i.e., 500 Å. In contrast, in the conventional device, the crystallite size on the (111) plane, i.e. the maximum peak plane, is 37 nm which is smaller than the thickness of the polysilicon thin film, 500 Å. This proves that the field effect mobility µ FE can be more greatly increased than in the conventional device, if the crystallite size of the polysilicon thin film is greater than the thickness of the polysilicon thin film. According to the embodiment, since the thickness of the polysilicon thin film is 500 to 3000 Å, the field effect mobility µ FE of at least 50 cm 2 /V·sec is ensured. The present invention is not limited to a field effect thin-film transistor, but is applicable to any type of semiconductor device having a polysilicon thin film as its main part to transfer carriers. As has been described above, if a polysilicon thin film has the grain size and the crystallite size substantially the same as each other, the field effect mobility µ FE can be greatly increased, accordingly, a driver circuit which is operated with a clock frequency of about 10 MHz can be formed using a semiconductor device. Note that when the crystallite size is 60 to 70 % or greater of the grain size, these sizes are considered to be "substantially the same".

Claims: A thin film transistor formed on an insulating substrate comprising a polysilicon thin-film having a middle portion for forming a channel, the polysilicon thin film formed by a laser anneal process to obtain liquid phase growth, a gate-insulating film formed on at least the middle portion on the polysilicon thin film, a gate electrode formed on the gate-insulating film, a source electrode connected to a side portion of the polysilicon thin-film, and a drain electrode connected to the other side portion of the polysilicon thin film including a plurality of grains, each of grains having a grain size c = (a + b)/2, where "a" denotes a length of each of the grains along a major axis and "b" denotes a length of each of the grains along a minor axis, the major axis and the minor axis being perpendicular to each other and parallel to the polysilicon thin film, and each grain including a crystallite having a size on the (111)-plane, characterized in that an average value of values representing the sizes on the (111)-plane of said plurality of crystallites is in the range of 60% or greater than 60% of the average c of all the grains included in the polysilicon thin film, said average value being at least 180nm and being greater than a value representing the thickness of said polysilicon thin film. A thin film transistor according to claim 1, characterized in that the average value of the grain size on said (111) plane is at least 200 nm. A thin film transistor according to claim 1 or 2, characterized in that the polysilicon thin-film (3) comprises at least one impurity doped region (3b) in which dopants are diffused and an intrinsic region (3a) which does not include dopants. A thin film transistor according to claim 3, characterized in that the impurity doped regions (3b) are arranged on both sides of the intrinsic region (3a). A thin film transistor according to claim 4, characterized in that the electrodes (8, 11) are respectively connected to the impurity region (3b) and the intrinsic region (3a) directly or indirectly.

Inventors: YAMAGUCHI MICHIYA, YAMAGUCHI, MICHIYA, Yamaguchi, Michiya, Patent Dept., Hamura R&amp;D Ctr
Applicants: CASIO COMPUTER CO LTD, CASIO COMPUTER CO., LTD.
Application year: 1993
Main classification: H01L29
IPC codes: H01L29, H01L21