Method of forming polycrystalline semiconductor thin film

Abstract: Method of forming a high-quality polycrystalline semiconductor thin film having large grain sizes by laser annealing. First, a film formation step is carried out to grow a semiconductor layer on an insulating substrate under certain film formation conditions, thus forming a precursory film. This precursory film comprises clusters of microscopic crystal grains. Then, an irradiation step is carried out. That is, the precursory film is irradiated with a laser beam such as an excimer laser pulse. The crystal sizes are increased to change the precursory film into a polycrystalline semiconductor thin film. During the film formation step, a precursory film having a crystal grain size of more than 3 nm is formed at a temperature of 500 to 650°C, for example, by LPCVD or APCVD. Under these conditions, the resulting polycrystalline precursory film is substantially free from hydrogen. During the irradiation step, a single pulse of excimer laser radiation is emitted.

Description: FIELD OF THE INVENTION The present invention relates to a method of forming a polycrystalline semiconductor thin film. More particularly, the invention relates to techniques for increasing the grain size of a polycrystalline semiconductor thin film by irradiating it with an energy beam. The invention also relates to a method of fabricating a semiconductor device in which TFTs (thin-film transistors) are formed at a high density using a polycrystalline semiconductor thin film as an active layer. Furthermore, the invention relates to a method of fabricating an active matrix liquid crystal display incorporating semiconductor devices as an active matrix array substrate. BACKGROUND OF THE INVENTION The TFT semiconductor device has used a heatproof insulating substrate made of quartz or the like. TFTs have been fabricated at a high density by performing a high-temperature process above 1000°C. Such TFT semiconductor devices have been earnestly developed, for example, as active matrix array substrates for active matrix liquid crystal displays. To realize wider application of liquid crystal displays, there is a demand for reductions in the cost of fabricating TFT semiconductor devices. Low-temperature processes with which cheap glass substrates can be used have been discussed. Especially, in order to fabricate a large-sized, high-information content liquid crystal display, low-temperature processes capable of utilizing cheap glass substrates have been earnestly developed. In this connection, a technique consisting of forming a film of amorphous silicon on a low-melting point glass substrate and irradiating the film with a laser beam to convert the film into high-quality polysilicon has been studied. Since polysilicon has a larger carrier mobility than that of amorphous silicon, high-performance TFTs can be formed at a high density. However, where amorphous silicon is used as a precursory film of clusters, even if the silicon is irradiated with a laser beam, it is impossible to obtain sufficiently large grain size. Rather, only crystallites are obtained. With silicon thin films in the state of crystallites, it is difficult to obtain TFTs of high performance. However, if the energy of the irradiating laser beam is optimized to some extent, it is possible to convert amorphous silicon into polysilicon. Also in this case, the range of the irradiating energy enabling satisfactory grain sizes is very narrow and so it is difficult to stably mass-produce high-quality polysilicon thin films in practice. It is common practice to form amorphous silicon thin films by plasma-assisted CVD. However, the films contain a large amount of hydrogen. For this reason, the contained hydrogen is caused to bump by the laser irradiation, and other problems occur. These make it difficult to put this technique into practical use. In the publication of Troxell et al. in I.E.E.E. Electron Device Letters, vol. EDL-8, no.12, 1 December 1987, pages 576-578, a fabrication method of silicon thin-film transistors is described wherein a silicon film was deposited by low-pressure chemical vapor deposition (LPCVD) at 625°C on a low-cost glass substrate and recrystallization of the entire film was accomplished thereafter by impinging radiation from an argon ion laser onto the sample. According to the publication of Sameshima in Japanese Journal of Applied Physics, vol.32, no.10B, 15 October 1993, pages L 1485-1488, a polycrystalline silicon film was deposited by LPCVD on a quartz substrate and the film was melted thereafter completely by irradiation of a XeCl excimer laser. The EP 0 681 316, which is prior art according to Art. 54(3) EPC, discloses a method of manufacturing semiconductor chips for display, wherein a semiconductor thin film is first formed on an insulating substrate, and then a series of processes containing a heat-treatment process for the semiconductor thin film are carried out to form integrated thin film transistors on a sectioned area for one chip. Thereafter, pixel electrodes for one picture (frame) are formed within the sectioned area. During the series of processes, a laser pulse is irradiated onto the sectioned area by one shot to perform a heat treatment on the semiconductor thin film for one chip collectively and simultaneously (i.e., perform a batch heat treatment on the semiconductor thin film). Through the batch heat treatment the crystallization of the semiconductor thin film is promoted. In addition, after the semiconductor thin film is doped with impurities, the activation of impurities doped in the semiconductor thin film can be performed by the batch heat treatment. SUMMARY OF THE INVENTION In view of the foregoing problems with the prior art techniques, it is an object of the present invention to provide high-quality polycrystalline semiconductor thin films. To achieve this object, there is provided a method of forming a polycrystalline semiconductor thin film according to the features of the appended claim 1. The method of the invention of forming a polycrystalline semiconductor thin film is applicable to a method of fabricating TFT semiconductor devices according to the features of the appended claim 4. The aforementioned method of fabricating the TFT semiconductor devices can be especially preferably applied to manufacture of active matrix liquid crystal displays according to the features of the appended claim 6. According to the present invention, a precursory cluster film consisting, for example, of polycrystalline silicon is first formed on an insulating substrate by low-pressure chemical vapor deposition (LPCVD) or atmospheric-pressure chemical vapor deposition (APCVD). This cluster film is irradiated with an energy beam such as a laser beam to crystallize the film. At this time, the polysilicon formed as a film by LPCVD or APCVD is substantially free from hydrogen. Since the hydrogen content of the precursory cluster film is low, it is easy to increase the grain sizes. By these methods, the thin-film semiconductor formed on the insulating substrate can be converted into a high-quality polycrystalline semiconductor having large grain sizes by the use of the energy beam. Other objects and features of the invention will appear in the course of the description thereof, which follows. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1(A) is a view illustrating a method of forming a polycrystalline semiconductor thin film; Fig. 1(B) is a view illustrating another method of forming a polycrystalline semiconductor thin film; Fig. 2 is a schematic diagram illustrating a method of calculating crystal grain diameters; Fig. 3 is a diagram illustrating the laser irradiation step according to an embodiment of the present invention; Fig. 4 is a graph showing a cross-sectional intensity distribution of laser radiation according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention are hereinafter described in detail with reference to the accompanying drawings. Fig. 1(A) is a view illustrating a method of forming a polycrystalline semiconductor thin film. Fig. 1(B) illustrates another method of forming a polycrystalline semiconductor thin film, for comparison purposes. According to the invention, as shown in Fig. 1(A), a film formation step is first carried out to grow a semiconductor layer on an insulating substrate (not shown) under certain film formation conditions so as to form a precursory film 1. This precursory film 1 consists of clusters of microscopic crystal grains 2 and is polycrystalline. Then, an irradiation step is performed. An energy beam such as a laser beam is directed at the film. The film is subjected to thermal hysteresis. The grain sizes are increased to convert the precursory film into a thin film 3 of polycrystalline semiconductor. Specifically, polysilicon having grain sizes of more than 3 nm is formed as the precursory film 1 on the insulating substrate at a film formation temperature of 500 to 650°C by LPCVD or APCVD. Where this film formation method is applied to the manufacture of an active matrix liquid crystal display, the insulating substrate is made of a transparent material such as glass. The crystal grains 2 contained in the polysilicon have grain sizes of at least 3 nm. If the grain sizes are less than this value, the state is close to an amorphous state rather than a polycrystalline state. Preferably, polysilicon having grain sizes of 30 to 50 nm is formed as a film by LPCVD. In LPCVD, 100% SiH 4 , for example, is used as a gaseous raw material. The polysilicon is grown by CVD, using the direct decomposition of this silane. Desired grain sizes can be accomplished at a film formation temperature of 500 to 600°C. Glass material or the like which can withstand this film formation temperature may be used as the material of the insulating substrate. Since the film is formed by making use of direct decomposition of silane, the polysilicon is substantially free from hydrogen. If a trace amount of hydrogen is contained, it is released during the film formation step, because the film formation temperature is higher than 400°C. In the next irradiation step, the film is irradiated, for example, with excimer laser radiation having a wavelength of 308 nm, an energy density of 150 to 450 mJ/cm 2 , and a pulse duration time of about 100 to 1000 ns such that a substrate temperature of about 20 to 450°C is obtained. The polysilicon is once melted by single-shot irradiation with an excimer laser pulse and then recrystallizes. The crystal grains increase in size because of this thermal hysteresis. On the other hand, in the reference example shown in Fig. 1(B), amorphous silicon is formed as a film by plasma-assisted chemical vapor deposition (PCVD). This film is used as a precursory film 1. In the PCVD, a mixture gas of silane and hydrogen is used as a gaseous raw material, and amorphous silicon is grown by plasma decomposition. The substrate temperature is about 180 to 350°C. Because the gaseous raw material contains a large amount of hydrogen, and because the film formation temperature is below 450°C, the amorphous silicon contains, for example, approximately 15-20% hydrogen. If this precursory film 1 consisting of the amorphous silicon is irradiated with laser radiation, a polycrystalline semiconductor thin film 3 is obtained. Crystallization of both polysilicon and amorphous silicon is made to progress by performing laser annealing as described above. As shown in Fig. 1(A), where polysilicon is used as the precursory film, the grain sizes of the crystal grains 2 not yet having undergone laser irradiation have been increased greatly by the laser irradiation. For instance, the grain sizes obtained after the laser irradiation reach 500 nm, for example. On the other hand, as shown in Fig. 1(B), where the amorphous silicon is used as the precursory film 1, the laser irradiation may result in a semiconductor thin film 3 but the crystal grain sizes are smaller than the sizes obtained where polysilicon is used as the precursory film. Table 1 below shows the relation between the average crystal grain size of the polycrystalline semiconductor thin film obtained in this way and the energy density. For comparison, results obtained by laser annealing amorphous silicon similarly are also shown. The data on the crystal grain sizes listed in Table 1 was calculated by the method illustrated in Fig. 2. In particular, crystal grain sizes were found by actually measuring the longer diameter a and the shorter diameter b of each individual crystal grain by transmission electron microscopy (TEM) and calculating the square of the product of both diameters. As can be seen from the results of measurements listed in Table 1, where the amorphous silicon is used as the precursory film, the average crystal grain diameters depend heavily on the energy density. The range of conditions providing large grain sizes is narrow. On the other hand, where polysilicon is used as the precursory film, the hydrogen content of the film is lower than the content in the case of the amorphous silicon. Therefore, the average crystal grain diameters obtained after the laser irradiation depend to a lesser extent on the energy. Large grain sizes can be obtained over a wide range of energies. Furthermore, the crystal grain sizes are larger than the sizes obtained where amorphous silicon is employed. Large grain sizes amounting to 500 nm are obtained. energy density (mJ/cm 2 ) amorphous silicon (nm) polysilicon (nm) 100-200 72 467 200-300 308 523 300-400 34 486 Fig. 3 shows a specific example of laser irradiation as one embodiment of the present invention. In this example, an insulating substrate 11 consisting of a large wafer is irradiated with a single shot of laser pulse 12. This irradiated region is a square area region 13, for example, of 5 × 5 cm 2 . The wafer is partitioned into such area regions 13, thus obtaining desired thin-film semiconductor devices. That is, each area region 13 corresponds to one chip. Of course, before the laser irradiation, a precursory film consisting, for example, of polysilicon is formed on the surface of the insulating substrate 11. The distribution of cross-sectional intensities of the laser pulse 12 along the diagonal line A-A across the area region 13 is represented by the graph of Fig. 4. As shown, the laser beam intensity is so set that it is lower at the center of the area region and higher in the peripheral regions. More specifically, the laser beam intensity in the peripheral regions is set higher than that at the center by about 1 to 70%. The graph of Fig. 4 shows the distribution of the cross-sectional intensities along the diagonal line A-A shown in Fig. 3. Similar intensity distribution is observed in other orientations. Thus, it follows that the shape obtained by rotating the profile shown in Fig. 4 about a normal line passing through the midpoint of the diagonal line A-A shows a three-dimensional energy distribution of the laser pulse. By establishing the energy distribution in this way, temperature drop due to heat dissipation from the peripheral portions can be prevented. Hence, a uniform crystal grain diameter can be obtained over the whole area region irradiated with the laser radiation. This cross-sectional intensity distribution of the laser pulse can be obtained by increasing the relative beam intensity in the peripheral regions with a simple optical system and reducing the relative beam intensity at the center.

Claims: A method of forming a polycrystalline semiconductor thin film, comprising the steps of growing a semiconductor layer on an insulating substrate to form a precursory polycrystalline film comprising clusters of microscopic crystal grains wherein said semiconductor layer growing step is carried out at a temperature of 500 - 650°C by CVD; and irradiating a single-shot of a laser pulse energy beam to the precursory film to increase grain size of said crystal grains, thereby converting said precursory film into a polycrystalline semiconductor thin film, wherein the cross section of the laser pulse energy beam has the shape of a square so as to irradiate a square area region of the precursory film, and wherein the intensity distribution of the energy beam in the plane of the square is such that the beam intensity in a central portion of the square is lower than in a peripheral portion of the square. A method of forming a polycrystalline semiconductor thin film as set forth in claim 1, wherein said CVD is low-pressure CVD or atmosphere pressure CVD. A method of forming a polycrystalline semiconductor thin film as set forth in claim 1, wherein said irradiating step is conducted by irradiating with an excimer laser pulse energy beam. A method of fabricating thin-film semiconductor devices, comprising the steps of: growing a semiconductor layer on an insulating substrate to form a polycrystalline film comprising clusters of microscopic crystal grains wherein said step of growing said semiconductor layer is carried out at a film formation temperature of 500 - 650°C by CVD; irradiating a single-shot of a laser pulse energy beam to the polycrystalline film to increase grain sizes of said crystal grains and to convert said polycrystalline film into a polycrystalline semiconductor thin film, wherein the cross section of the laser pulse energy beam has the shape of a square so as to irradiate a square area region of the precursory film and the intensity distribution of the energy beam in the plane of the square is such that the beam intensity in a central portion of the square is lower than in a peripheral portion of the square; and forming integrated thin film transistors, each of the thin film transistors having the polycrystalline semiconductor thin film as an active layer. A method of fabricating thin-film semiconductor devices as set forth in claim 4, wherein said irradiating step is conducted by irradiating with an excimer laser pulse energy beam. A method of fabricating a liquid crystal display, comprising the steps of: growing a semiconductor layer on an insulating substrate to form a polycrystalline film comprised of clusters of microscopic crystal grains wherein said step of growing said semiconductor layer is carried out at a film formation temperature of 500 - 650°C by CVD; irradiating a single-shot of a laser-pulse energy beam to said polycrystalline film to increase grain sizes of said crystal grains and to convert said polycrystalline film into a polycrystalline semiconductor thin film, wherein the cross section of the laser pulse energy beam has the shape of a square so as to irradiate a square area region of the precursory film and the intensity distribution of the energy beam in the plane of the square is such that the beam intensity in a central portion of the square is lower than in a peripheral portion of the square; performing a first working operation in such a way that TFTs are formed while using said polycrystalline semiconductor thin film as an active layer of the TFT; performing a second working operation in such a way that pixel electrodes connected with said TFTs are formed; bonding a counter substrate having a counter electrode formed thereon to said insulating substrate via a gap; and then injecting a liquid crystal material into said gap, thus assembling said liquid crystal display. A method of fabricating a liquid crystal display as set forth in claim 6, wherein said irradiating step is conducted by irradiating with an excimer laser pulse energy beam.

Inventors: HAYASHI HISAO, INO MASUMITSU, KUNII MASAFUMI, MINEGISHI MASAHIRO, URAZONO TAKENOBU, HAYASHI, HISAO, INO, MASUMITSU, KUNII, MASAFUMI, MINEGISHI, MASAHIRO, URAZONO, TAKENOBU, HAYASHI, HISAO, C/O SONY CORPORATION, Ino, Masumitsu, c/o Sony Corporation, Kunii, Masafumi, c/o SONY CORPORATION, Minegishi, Masahiro, c/o Sony Corporation, Urazono, Takenobu, c/o Sony Corporation
Applicants: SONY CORP, SONY CORPORATION
Application year: 1995
Main classification: H01L21
IPC codes: G02F1, H01L27, H01L21, H01L29