Optical annealing method for semiconductor layer and method for producing semiconductor device employing the same semiconductor layer

Abstract: An optical annealing method for a semiconductor layer provided on a substrate comprises irradiating a base member provided with a semiconductor layer and an absorbing layer for incoherent light across an insulating layer, with incoherent light.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an optical annealing method for obtaining a semiconductor layer of high quality, and a method for producing a semiconductor device utilizing said semiconductor layer. Related Background Art In the field of crystal growing technology for growing a thin crystalline film on an amorphous substrate, there has been proposed a solid phase growing method consisting of heat treating an amorphous thin film, formed in advance on a substrate, at a temperature lower than the melting point, thereby transforming said film to a crystalline state. For example, it was reported that large lamified polycrystals of the order of a micron could be formed by applying heat treatment to a thin silicon film, formed on an amorphous insulating substrate and made amorphous by ion implantation, for several tens of hours at 600 °C in an N 2 atmosphere and transistors of satisfactory characteristics could be formed thereon (T. Noguchi, H. Hayashi & H. Oshima, 1987 Mat. Res. Soc. Symp. Proc., 106, Polysilicon and Interfaces, 293, Elsevier Science Publishing, New York, 1988). Also for improving the characteristics of the thin film and the device, there have been proposed methods of irradiation with an ultraviolet laser and with an infrared lamp (T.Noguchi et at., Japanese Laid-Open Patent No. 61-289620) (Ino and Tani, Richo Technical Report No. 14, p. 4, November 1985). These two methods with lamp heating are to deposit a thin Si film on a quartz substrate, to superpose an Si wafer in contact with said Si film, to cause said Si wafer to absorb the light of the lamp transmitted by said quartz substrate and said thin Si film, and to heat said thin Si film on said quartz substrate by conduction thus generated heat. These methods are associated with drawbacks of insufficient or uneven contact in microscopic scale, contamination, fusing etc. resulting from the superposing of a heat absorbing member. For example it is well known that the silicon wafer has modulations of several microns, even in the smoothest state. It is therefore impossible to obtain complete contact by superposing said Si wafer on the thin Si film so that the heat received by said thin Si film is inevitably uneven. As a result, the heated thin Si film generates a temperature distribution therein, and such unevenness in temperature becomes an obstacle in obtaining a uniform structure in the crystal, and eventually affects the characteristics of the device in undesirable manner. Also if the temperature is raised close to the melting point, there may result fusion of the thin Si film and the Si wafer serving to absorb the irradiating light, so that the device manufacturing process cannot be initiated. EP-A-0225592 relates to a method of recrystallizing a region of a nonmonocrystalline conductive layer comprising: forming a thermal-conduction-controlling layer forming an energy-absorbing cap layer above the thermal-conduction-controlling layer; and irradiating the energy absorbing cap layer; the energy-absorbing cap layer absorbs energy from the beam and, as a result, heats up, the thermal-conduction-controlling layer conducting heat from the cap layer to the nonmonocrystalline layer in a selective fashion owing to the presence of the increased-thickness portion, such that material of the nonmonocrystalline layer in the said region is melted and provided with a temperature distribution such that it recrystallizes, on cooling, in a grain-boundary-free condition. Reference is made to Patent Abstracts of Japan Vol. 10, No. 213 and JP-A-61051874 which discloses a method of producing a MOS transistor in which laser beams are projected onto a lower silicon layer to effect recrystallisation thereof. An object of the present invention is to provide an optical annealing method free from the aforementioned drawbacks of the conventional methods and capable of applying a uniform heat treatment over a wide area, thereby obtaining a semiconductor layer with uniform semiconductive characteristics. The method may serve to form a semiconductor device with uniform semiconductive characteristics over a wide area. According to the invention there is provided a method for producing a thin film semiconductor device comprising the steps of: (i) providing a substrate comprising a base plate, a semiconductor layer, an insulating layer, and a silicon layer doped with an impurity to absorb light, wherein said semiconductor layer and said light absorbing layer are disposed one on each side of the insulating layer, said base plate and said semiconductor layer comprising materials capable of transmitting incoherent light without absorbing incoherent light; (ii) irradiating all regions of said substrate simultaneously with incoherent light from a lamp for annealing said semiconductor layer to achieve a uniform crystalline structure; (iii) removing, subsequent to the step of irradiating, a patterned portion of said light absorbing layer, to form a gate electrode comprising the remaining portion of said light absorbing layer; (iv) removing a patterned portion of said insulating layer to form a gate insulating film comprising the remaining portion of said insulating layer; and (v) forming a source region and a drain region in said semiconductor layer. In the accompanying drawings: Fig. 1 is a chart showing the relation between the absorption coefficient of silicon and the wavelength of light; Fig. 2 is a schematic view for explaining the present invention; Fig. 3 is a schematic view of a semiconductor device; and Figs. 4 to 6 are schematic views showing the steps of forming a semiconductor device according to the present invention. Now there will be detailedly explained the function and structure of the present invention, together with the findings which have lead thereto. Fig. 1 shows the relationship among the black body radiation spectrum of an ordinary tungsten halogen lamp (2200 K), absorption coefficient of silicon and of free carriers therein, and wavelength of irradiating light. As shown in Fig. 1, the absorption coefficient of silicon at the basic absorption end rapidly increases at a wavelength of 1.2 µm and shorter. On the other hand, the black body radiation spectrum of the tungsten halogen lamp at 2200 K extends over a wide infrared range from 0.4 µm to 3.5 µm which is the cut-off wavelength of the quartz glass. Also the absorption of free carriers extends to the longer wavelength side from 1.0 µm, and becomes stronger as the carrier display becomes higher. The above-explained relationship clearly indicates following three facts: 1) The quartz substrate can be scarcely heated by infrared irradiation; 2) Si layer absorbs light of a wavelength region below about 1 µm, but such light constitutes only a very small portion of the light emitted by the lamp; and 3) An increase in the number of carriers in the Si layer increases the light absorption, thus allowing effective utilization of the light of the lamp. In addition, the light absorption of Si layer is dependent on the thickness thereof. The thickness of Si layer, when used as a semiconductor layer for a transistor, preferably do not exceed 0.1 µm in consideration of electrical characteristics such as the leak current. However an Si layer of thickness of the order of 0.1 µm formed on a quartz substrate scarcely absorbs but almost transmits the light from the lamp. Therefore, incoherent lamp light of an irradiation intensity capable of heating an Si wafer of a thickness of 7,500 µm even to 1,200 °C can only heat an Si layer of a thickness of 0.1 µm on a light-transmitting quartz substrate to several hundred degrees. According to the present invention, in order to achieve heating sufficient for annealing a thin Si layer provided on a light-transmitting substrate, a light-absorbing layer is deposited on said Si layer, across an insulating layer, as indicated by a basic structure shown in Fig. 2. As shown in Fig. 2, on a light-transmitting substrate 21 for example of quartz, there are provided in succession a semiconductor layer 22 for example of silicon, an insulating layer 23, a light-absorbing layer 24, and a protective layer 25. For forming a base member of the layer structure shown in Fig. 2, a semiconductor layer 22 for example of Si, SiGe, GaAs or InP is at first formed by an ordinary film depositing method, on a light-transmitting substrate. For forming a thin film electronic device utilizing the semiconductor layer annealed according to the present invention, the thickness of the semiconductor layer 22 is preferably in a range from 0.05 to 0.3 µm, more preferably from 0.06 to 0.2 µm. Subsequently an insulating layer 23, serving as a separating layer, is formed by an ordinary film depositing method. Said insulating layer 23 can be composed, for example, of silicon oxide, silicon nitride, SiO x N y or tantalum oxide. For the purpose of separating the semiconductor layer 22 and the light-absorbing layer 24 and being used as a part of the structural components of the electronic device, said insulating layer 23 has a thickness preferably in a range from 0.05 to 1 µm, more preferably from 0.05 to 0.5 µm, and most preferably from 0.05 to 0.1 µm. Then a light-absorbing layer 24 is formed on said insulating layer 23. Said light-absorbing layer 24 is composed, for example, of polycrystalline Si, amorphous Si, polycrystalline SiGe etc, doped by the presence of free carriers, such as P-doped amorphous Si, B-doped amorphous Si or As-doped amorphous Si. For example, a polycrystalline Si layer of a thickness of 0.5 µm doped with P at a concentration of 10 20 /cm 3 is heated about 100 °C higher than the undoped layer under same light irradiating condition. A protective layer 25 may be provided for preventing the surface coarseness or evaporation of the light-absorbing layer 24, appearing under a high light intensity. Said protective layer 25 may be composed of a light-transmitting material capable of withstanding a high temperature, such as SiO 2 , Si 3 N 4 or a SiO 2 /Si 3 N 4 bilayer structure. When the above-explained multi-layered structure is irradiated with the light of lamp from above, said light is transmitted by the protective layer 25 and is absorbed by the absorbing layer 24 thereby elevating the temperature thereof. As a result, the semiconductor layer 22 of a thickness insufficient for direct heating by light absorption can be sufficiently heated by conduction through the separating layer 24. Also in case the light irradiation is conducted from below, the light of the lamp is transmitted by the quartz substrate 21, further by the semiconductor layer 22 and the separating layer 23, and absorbed by the light-absorbing layer 24 thereby elevating the temperature thereof. The heating effect for the light-absorbing layer 24 is enhanced by light irradiations from above and below. For sufficiently absorbing the incident light and heating the semiconductor layer, the thickness of the light-absorbing layer is selected preferably in a range from 0.5 to 2 µm, more preferably from 0.5 to 1.5 µm and most preferably from 0.5 to 1 µm. If the starting material for the semiconductor layer 22 is for example amorphous Si or polycrystalline Si with small grain-size, there will result a growth of grains to the order of a micron, with an improvement in the performance of the device due to a reduced influence of the grain boundaries. Also if the aforementioned large-sized lamified polycrystals of the order of a micron are employed as the starting material for the semiconductor layer 22, there is obtained a significant decrease in the twin crystal defects present at a high density in the semiconductor layer 22. Particularly if the annealing of the crystals by light irradiation is conducted at a high temperature from 1,000 °C to 1,420 °C, there can be achieved efficient annihilation of lamination defects and transitions within the semiconductor layer 22. The heating of the thin Si film, formed on a light-transmitting substrate, can be enhanced with improved control and uniformity, by forming a semiconductive light-absorbing layer by deposition on or under said Si film across an insulating layer, or such light-absorbing layer on and under said Si film respectively across insulating layers, thereby securing satisfactory adhesion between the light-absorbing layer(s) and the Si film, and giving a light irradiation thereto. Also the process for producing the semiconductor device can be simplified by utilizing said light-absorbing layer as a part of the structure of said device. Also the use of incoherent light prevents unevenness in the thermal treatment temperature resulting from the light interference at the irradiation, thereby achieving uniform heat treatment over the entire film. For achieving collective heat treatment of a large area within a short time, the intensity of light irradiation is preferably selected within a range from 0.1 to 300 W/cm 2 , more preferably from 10 to 150 W/cm 2 . Also for obtaining a semiconductor layer of a high quality, the heat treatment of the semiconductor layer by light irradiation is preferably conducted within a temperature range from 1,200 °C to 1,400 °C, more preferably from 1,250 °C to 1,400 °C and most preferably from 1,300 °C to 1,380 °C. In the following there will be explained a case of utilizing the Si film with the above-explained multi-layered structure in a field effect transistor. In an ordinary semiconductor process, the semiconductor layer 22 is used as a channel 31, the separating layer 23 as a gate insulating layer 38, and the light - absorbing layer 24 as a gate electrode layer 37, as shown in Fig. 3. Highly doped source 32 and drain 33 shown in Fig. 3 may be formed by forming apertures in the separating layer 23 shown in Fig. 2, corresponding to the areas of said source 32 and drain 33, utilizing doped polycrystalline Si as the light-absorbing layer 24 and effecting thermal diffusion at the heating with the light of lamp, or by effecting ion implantation in said apertures by an ordinary semiconductor process. For electrical contact with said source 32, drain 33 and gate electrode layer 37, there are formed a gate electrode 34, a source electrode 35 and a drain electrode 36 of a conductive material such as aluminum. The thin film field effect transistor thus formed exhibit improved characteristics according to the heating temperature by the lamp. In the following there will be explained examples of the present invention for the purpose of further clarification, but it is to be understood that the present invention is by no means limited in any way by such examples. Example 1 On a quartz glass plate 21 of a size of 10 x 10 cm and a thickness of 500 µm, a polycrystalline Si layer (active layer) 22 of a thickness of 0.1 µm, having a small average crystal size of about 50 nm (500 Å), was formed by reduced pressure CVD, thermally decomposing SiH 4 under a pressure of 40 Pa (0.3 Torr) and with a substrate temperature of 620°C. Then, on said polycrystalline Si layer 22, a SiO 2 layer 23 of a thickness of 50 nm (500 Å) was formed by thermal CVD, introducing SiH 4 gas and O 2 gas under a pressure of 101300 Pa (760 Torr) and with a substrate temperature of 400°C. Subsequently, on said SiO 2 layer 23, a polycrystalline Si layer of a thickness of 500 nm (5,000 Å) by reduced pressure CVD, thermally decomposing SiH 4 gas under a pressure of 40 Pa (0.3 Torr) and with a substrate temperature 620°C, and phosphor ions were implanted with a concentration of 10 20 ions/cm 3 to obtain a highly P-doped light-absorbing layer 24. Further, a protective SiO 2 layer 25 of a thickness of 1 µm was formed thereon by thermal CVD, introducing SiH 4 gas and O 2 gas under a pressure of 101300 Pa (760 Torr) and with a substrate temperature of 400°C. The obtained based member, of the multi-layered structure shown in Figure 2, was irradiated with the light of tungsten halogen lamps from above and below for 3 minutes, with an intensity of 78 W/cm 2 ion a N 2 atmosphere. The temperature, measured with an optical pyrometer on a substrate Si water provided as a temperature monitor, reached 1,380°C after irradiation for 3 minutes. The thermal energy generated by the irradiating light absorbed by the n + polycrystalline Si layer 24 of a thickness of 0.5 µm was transferred by conduction through the SiO 2 layer 23 to the undoped polycrystalline Si layer 22 of a thickness of 0.1 µm, thus applying a heat treatment thereto and increasing the grain size several hundred times to the order of several microns. The surface of the Si layer was not made coarse by the capping effect of the protective layer 25. The obtained base member, thus anealed by the light irradiation explained above, was used in the preparation of a field effect transistor by an ordinary IC process. In the following there will be explained the process for producing said field effect transistor, with reference to Figs. 4 to 6. At first the protective layer 25 was removed by etching with buffered hydrofluoric acid. Subsequently a gate area was formed by etching off the light-absorbing layer 24 except for the area of the gate electrode layer 37, forming a gate electrode layer, forming apertures in the separating layer 23 corresponding to the drain and source, and forming a gate insulating layer 38 (Fig. 4). Then the source 32 and the drain 33 were formed by implanting P ions with a dose of 5 x 10 15 ions/cm 2 into the semiconductor layer 22 exposed in said apertures (Fig. 5). Subsequently a protective SiO 2 layer 39 of a thickness of 1 µm was formed by CVD, introducing SiH 4 gas and O 2 gas under a pressure of 101300 Pa (760 Torr) and with a substrate temperature of 400 °C, and contact holes were formed for electrical contact with the gate electrode layer 37, source 32 and drain 33 (Fig. 6). Then aluminum was deposited as the electrode material, and the gate electrode 34, source electrode 35 and drain electrode 36 were formed by patterning. Thus the field effect transistor was completed (Fig. 3). The transistor thus formed showed satisfactory transistor characteristics with a carrier mobility at least equal to 100 cm 2 /V·sec, a threshold value of about 1.5 V, and a sub threshold characteristic of 300 mV/decade. Stated differently, the semiconductor layer 22 annealed according to the present example was proved to have satisfactory electrical properties for use as a functional area of a semiconductor device. Also in the present example, the annealed base member is provided with a light absorbing layer 24 consisting of polycrystalline Si containing a large amount of phosphor atoms, a semiconductor layer 22 consisting of large-sized Si grains with excellent semiconductive characteristics, and a separating layer 23 consisting of an insulating material for separating said light absorbing layer 24 and semiconductor layer 22, so that the source 32 and the drain 33 could be formed in said semiconductor layer 22, the gate insulating layer 38 in said separating layer 23, and the gate electrode layer 37 in said light absorbing layer 24. Thus, according to the present invention, after the annealing treatment for obtaining a semiconductor layer of a high quality, a semiconductor device can be produced with a fewer number of steps, excluding steps for forming an insulating layer for the gate insulating layer and a highly doped layer for the gate electrode layer. Example 2 On a quartz substrate (100 mm), an Si layer of a thickness of 0.1 µm was deposited by vacuum evaporation with a deposition rate of 10 -1 nm/sec (1Å/sec.) under a pressure of 1.33 x 10 -6 Pa (1 x 10 -8 Torr) and with a substrate temperature of 200°C. Said Si layer, deposited in amorphous state, was transformed into a large-sized lamified polycrystalline layer, by a heat treatment for 50 hours at 600 °C in an N 2 atmosphere. Then, on said polycrystalline layer, a separating layer of Ta 2 O 5 with a thickness of 50 nm (500 Å) was deposited by sputtering, and a light absorbing layer was formed by depositing polycrystalline Si, added with P with a concentration at least equal to 10 20 atoms/cm 3 , into a thickness of 0.5 µm. Subsequently a protective SiO 2 layer of a thickness of 1.0 µm was deposited by CVD, thereby completing the base member. Said base member was irradiated for 3 minutes from both sides with the light of tungsten halogen lamps of an intensity of 72 W/cm 2 . A temperature monitoring wafer reached a temperature of 1,345 °C after light irradiation for 3 minutes. A field effect transistor was prepared in a similar manner as in the Example 1, utilizing the light absorbing polycrystalline Si layer for the gate, and there were obtained satisfactory characteristics comparable to those of a field effect transistor prepared on a silicon wafer. Observation under a transmissive electron microscope proved the defects, present in the active layer close to the interface were significantly decreased. Example 3 On a quartz glass substrate (100 mm), a polycrystalline Si layer (semiconductive layer) with fine crystal grains of an average size of about 50 nm (500 Å) was deposited with a thickness of 0.1 µm by reduced pressure CVD by thermally decomposing SiH 4 under a pressure of 40 Pa (0.3 Torr) and with a substrate temperature of 620°C, and a SiO 2 layer of a thickness of 50 nm (500 Å) was formed by thermal oxidation. Then a polycrystalline Si layer of a thickness of 500 nm (5,000 Å) was again deposited by reduced pressure CVD similar to the foregoing one, and phosphor was added to said polycrystalline Si layer with a high concentration of 10 21 atoms/cm 3 by diffusion from phosphor glass (light absorbing layer). The base member of the multi-layered structure thus formed was irradiated for 3 minutes from both sides by the light of tungsten halogen lamps with an intensity of 75 W/cm 2 , in an N 2 atmosphere. The temperature, measured with an optical pyrometer on a temperature monitoring Si wafer reached 1,380°C after irradiation for 3 minutes. The thermal energy, generated by the light absorbed in the n + polycrystalline Si layer of a thickness of 0.5 µm, was transferred by conduction, through said SiO 2 layer, to the undoped polycrystalline Si layer of the thickness of 0.1 µm, thereby heating said semiconductor layer. This heat treatment increased the grain size of said polycrystalline Si layer several hundred times up to 3 µm. After the light irradiation, a field effect transistor was prepared in a similar manner as in the Example 1, with an ordinary IC process, utilizing the light absorbing n + polycrystalline Si layer as the gate, and there were obtained satisfactory characteristics.

Claims: A method for producing a thin film semiconductor device comprising the steps of: (i) providing a substrate comprising a base plate (21), a semiconductor layer (22), an insulating layer (23), and a silicon layer (24) doped with an impurity to absorb light, wherein said semiconductor layer (22) and said light absorbing layer (24) are disposed one on each side of the insulating layer (23), said base plate (21) and said semiconductor layer (22) comprising materials capable of transmitting incoherent light without absorbing incoherent light; (ii) irradiating all regions of said substrate simultaneously with incoherent light from a lamp for annealing said semiconductor layer (22) to achieve a uniform crystalline structure; (iii) removing, subsequent to the step of irradiating, a patterned portion of said light absorbing layer (24), to form a gate electrode comprising the remaining portion of said light absorbing layer; (iv) removing a patterned portion of said insulating layer (23) to form a gate insulating film comprising the remaining portion of said insulating layer; and (v) forming a source region (32) and a drain region (33) in said semiconductor layer (22). A method as claimed in claim 1, characterised in that said substrate has a protective layer (25) at the outermost surface. A method as claimed in claim 1, characterised in that said semiconductor layer (22) is composed of a silicon-derived semiconductive material. A method as claimed in any one of claims 1-3, characterised in that a part of said insulating layer is removed by etching. A method as claimed in any one of claims 1-4, characterised in that a part of said remaining portion of said insulating layer comprises said gate insulating film.

Inventors: YONEHARA TAKEO, YONEHARA, TAKEO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L21
IPC codes: H01L21