Manufacturing method for semiconductor device

Abstract: A manufacturing method for a semiconductor device is preferably used for a semiconductor device using SOI (Silicon on Insulation) technology. At minimum, the method includes the following steps: the step of forming a gate electrode (5) on a substrate (1) by using a light-intercepting material; of forming a gate insulating film (4) on the substrate (1) including the gate electrode (5); of forming a semiconductor layer (3) on the gate insulating film (4); and of forming a source region (2a) and a drain region (2b) by virtue of the fact that the light (hD), having a wavelength such that the light is absorbed into the semiconductor layer (3) while not being absorbed into the substrate (1), is irradiated from the back of the substrate (1), before supplying impurities into the semiconductor layer (3).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a manufacturing method for a semiconductor device, and more particularly to a manufacturing method preferably used for a semiconductor device using SOI (Silicon on Insulation) technology. Related Background Art Conventionally, SOIs are known in a broad sense as technology, such as single crystal SOIs, used mainly in high-speed or three-dimensional ICs, and TFTs (Thin Film Transistors) which use thin film semiconductor layers, such as poly-Si or a-Si, and are mainly and preferably used as driving transistors for contact reading sensors or liquid crystal displays. In the above technology, TFTs have conventionally gained attention in terms of low costs and the fact that they can be manufactured on large area substrates. However, recent demands for high performance and high precision contact reading sensors and the like have required TFTs with superior characteristics. As a method of accomplishing such superior characteristics, the quality of a semiconductor layer of the TFT is improved to approach a single crystal. TFTs using poly-Si, with mobility higher than that of a-Si, have been recently realized. Although endeavors have been made to improve the performance of the conventional TFT, the current conventional TFT, having a semiconductor layer formed by the poly-Si, is behind the single crystal SOI in terms of gain and leakage current characteristics. Furthermore, in contrast to the above single crystal SOI, in which seeds (seed crystals) are obtained from a single crystal substrate called SEG (Selective Epitaxial Growth) or LSPE, it is difficult for the TFT to grow a single crystal layer on the entire face of a glass substrate. For these reasons, as a method of improving the TFT characteristics, improvements in a device structure based on the manufacturing process have gained attention. For example, in the conventional stacking process, as illustrated in Fig. 1, which is a schematic cross-sectional view, the semiconductor device in accordance with the conventional SOI is constructed in the following manner: A source region 2a as well as a drain region 2b are previously formed on an insulating film. A semiconductor layer 3 and a gate insulating film 4 are then formed on the upper layer of the regions, followed a gate electrode 5 being formed on the upper layer of the gate insulating film 4. The construction such as the above one, however, poses problems in that variations in the effective gate length increase, and that the amount of overlapping between the gate and the drain also increases. For these reasons, in a semiconductor device in which an active element and a drive element are integrated with each other, when the manufacturing process inevitably generates an overlapped portion between the semiconductor layer 3 and the gate electrode 5, as in the TFT construction shown in Fig. 1, this overlapped portion assumes a parasitic capacity in driving the elements, thereby hindering high-speed driving. To overcome such problems as the above ones, it is possible to adopt a process, such as a self alignment, which is utilized as a conventional IC process. In this case, the structure of a semiconductor device is, for example, like the one illustrated in Fig. 2, which is a schematic cross-sectional view. Direct introduction of the conventional IC process, such as the self alignment, however, causes the process to be complicated, and is not suitable for a glass substrate because of the need for a high temperature process. This leads to the foremost problem of increased cost of the semiconductor. In addition, as illustrated in Fig. 2, a problem exists in that since an ion implantation and heat diffusion are performed on the semiconductor layer 3 in order to form the source region 2a and the drain region 2b, diffusion takes place in a horizontal direction, not allowing the elimination of the overlapped parasitic capacity. JP-A-62 14472 discloses a manufacturing method for producing a semiconductor device as outlined in the preamble of claim 1. SUMMARY OF THE INVENTION An object of the present invention is to propose a manufacturing method for a semiconductor device which provides inexpensive TFTs enabling high-speed operations. An object of the present invention is to propose a manufacturing method for a semiconductor device, as outlined in claim 1. Other features will become apparent from the following Description of the Preferred Embodiments when read with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 are schematic cross-sectional views explaining a semiconductor device according to the conventional SOI technology; Figs. 3 through 5 are respectively schematic cross-sectional views explaining the manufacturing process of a semiconductor device in accordance to the present invention; DESCRIPTION OF THE PREFERRED EMBODIMENTS To achieve the above object of the present invention, the invention provides a manufacturing method for a semiconductor device, as outlined in the claims. In order to supply impurities to the above semiconductor layer, a gas containing impurities, which is used as an impurity source, may be introduced during light irradiation. The wavelength of light to be irradiated is chosen according to the band gap of the semiconductor being employed, such as semiconductor Si or GaAs. For instance, when Si is employed for the semiconductor layer, light with a wavelength not longer than that of blue light preferably may be used, or more preferably ultraviolet rays may be used. The essential point regarding the above light is to use light with a wavelength such that the light is mainly absorbed in the semiconductor layer, and impurities are diffused into the part which has been irradiated by the light. In addition, though light may be irradiated continuously, irradiating the light in a so-called pulse-like manner (i.e.intermittently) is a more preferable irradiating method, since it does not adversely affect a substrate with insufficient heat resistance, such as, in particular, a glass substrate. Materials which do not absorb the above light and which are capable of functioning as a gate electrode, may be utilized as a gate electrode. For example, metals such as Aℓ and W, or alloys containing such metals, may be utilized. As described above it is possible to form a TFT by the self alignment process without being accompanied by a high temperature process. This is because the source and drain regions are formed by virtue of the fact that light, with a wavelength such that it is absorbed into the above semiconductor layer while not being absorbed into the substrate, is irradiated from the back of the substrate, while at the same time impurities are supplied from the above-described surface of the semiconductor. Thus, in accordance with the present invention, since impurity diffusion in a horizontal direction hardly takes place, it is possible to eliminate the occurrence of a parasitic capacity, thereby allowing high drive speeds. Moreover, since the high temperature process is not required for the entire device, the use of a commonly employed glass substrate, serving as a transparent substrate, permits the semiconductor device to be manufactured at a lower cost. The preferred embodiments of the present invention will be described hereinafter with the accompanying drawings. According to the present invention, a description will be given of a method in which a gas containing impurities is introduced together with light irradiation in order to supply the impurities from the side of a semiconductor surface. An embodiment will now be described according to a manufacturing process with reference to Fig. 3 through 5. (1) First, as illustrated in Fig. 3, a spattering method is utilized to deposit 100nm WSi₂ on a quartz substrate 1. A gate electrode 5 is then formed by patterning the WSi₂ with the aid of commonly employed photolithography etching. (2) Next, SiH₄ is allowed to react with N₂O by using a plasma CVD method to deposit a 50nm SiO₂ film, which will serve as a gate insulating film 4. (3) A semiconductor layer 3 is formed by depositing 100 nm thick polycrystal silicon, whose impurity atom concentration is 1 x 10¹⁵ atms/cm³. The deposition is made by reacting SiH₄ with PH₃ at 650°c using a vacuum CVD method. (4) The above substrate 1 is arranged in a vacuum container in which the pressure is adjusted to 5 torrs by introducing B₂H₆. Eximer laser light (wavelength 193 nm) using an ArF gas is then collected into a 10 x 10 mm area. The collected light is irradiated from the back of the substrate 1 through a quartz window of the vacuum container (the irradiation is in the direction indicated by an arrow in the middle of Fig. 4). This light irradiation is performed by scanning the light across the entire face of the substrate 1. This heats the part of the semiconductor layer which has been irradiated by the light, and boron (B) is impurity-diffused into only a source region 2a as well as a drain region 2b. Such conditions are shown in Fig. 4. (5) As illustrated in Fig. 5, insulating films between layers 8 are formed by an ordinary IC process, and contact holes are formed in the source and drain regions of the insulating films between layers 8. (6) After Aℓ-Si is deposited to a thickness of 1000 nm by means of the sputtering method, a source electrode 9 and a drain electrode 9 are formed by patterning the Aℓ-Si. (7) Lastly, as a protective film 10, 700 nm of PSG (phosphosilicate glass) is deposited, thus completing a TFT. On the thus formed source and drain regions of the TFT, the impurity atom concentration is 1 x 10²¹ atms/cm 3, and the sheet resistance is 30 Ω/□. Furthermore the Aℓ-Si forming the electrodes exhibits favorable ohmic junction characteristics. As has been described, according to the present invention, it is possible to form the source and drain regions of TFT by the self alignment, owing to the fact that the regions are formed by the so-called laser doping, in which light is irradiated from the back of the substrate. Also, it is possible to manufacture TFTs with no parasitic capacity which permits high speed operations, because of no impurity diffusion into a horizontal direction. Furthermore, the manufacturing process in which temperature may be lowered is capable of being simplified, since a heat treatment, required for activation after impurities are injected, is not required, as compared with a method which employes the ion implantation, which does require a heat treatment. In addition, the manufacturing process can be further simplified, because etching is not necessary for the gate insulating film, as opposed to the laser doping in which light is irradiated from the surface of the substrate. A semiconductor layer onto which light is irradiated is not necessarily polycrystalline, but it may be amorphous or single crystalline, so long as the impurity doping is carried out by irradiating light as desired.

Claims: A manufacturing method for producing a semiconductor device, which method comprises the following steps of: forming a gate electrode (5) of light-intercepting material on a surface of a substrate (1); forming a gate insulating film (4) upon said substrate (1) and said gate electrode (5); forming a semiconductor layer (3) upon said gate insulating film (5); exposing said semiconductor layer (3) to a source of dopant impurity; and irradiating said semiconductor layer (3) from the substrate side thereof with light, while said semiconductor layer (3) is exposed, and forming impurity doped source and drain regions (2a,2b) therein, this step being performed using light of a wavelength which is such that said light is absorbed mainly in said semiconductor layer (3) rather than in said substrate, so that regions of said semiconductor layer (3), unshielded from said light by said gate electrode (5), are heated and impurity dopant from said source is thermally diffused therein to form said source and drain regions (2a, 2b), which method is characterised in that: said step of exposing said semiconductor layer (3) is performed by exposing said semiconductor layer (3) to a gaseous source of said dopant impurity. A method according to claim 1, wherein said semiconductor layer (3) comprises silicon. A method according to claim 2, wherein said silicon is in polycrystalline form. A method according to any one of the preceding claims 1 to 3, wherein said semiconductor layer (3) comprises gallium arsenide. A method according to any preceding claim, wherein said step of irradiating is performed intermittently. A method according to any preceding claim, wherein said wavelength is not longer than blue wavelength. A method according to any one of the preceding claims, wherein said substrate (1) is of a transparent glass material. A method according to claim 7, wherein said glass material is quartz glass. A method according to any preceding claim, wherein said gate electrode (5) is of either aluminium (Al) or tungsten (W), or an alloy or a metallic compound comprising either aluminium or tungsten. A method according to any preceding claim, wherein said gaseous source is B₂H₆.

Inventors: SHINDO HITOSHI C O CANON KABUS, SHINDO, HITOSHI, SHINDO, HITOSHI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: H01L31
IPC codes: H01L29, H01L21