PROCESS FOR FORMING A POLYCRYSTALLINE SILICON THIN FILM AT LOW TEMPERATURE

Abstract: A polycrystalline silicon thin film, which can be formed at a temperature below 400 °C on a cheap glass base plate (3) such as a blue plate glass, or on a glass base plate (3) provided with metallic electrodes or transparent electrodes, and whose hydrogen content is less than 5 atom %. The polycrystalline silicon thin film can be formed at such a temperature by repeating several times the steps of forming an amorphous silicon film on the glass plate by, e.g., a CVD technique and then exposing the film to a hydrogen plasma for a predetermined time.

Description: The present invention relates to a polycrystalline silicon (polysilicon) thin film fabricated on an inexpensive glass substrate and large-area low-temperature fabrication method thereof, having potential application for thin film transistors and thin-film solar cells. Since conventional methods for the fabrication of a polysilicon thin film require a high temperature of more than 650 °C, a glass substrate with a softening point of less than 650 °C cannot be used. In order to avoid diffusion of impurities, it is necessary to use a highly pure quartz glass substrate among glass substrates with a softening point above 650 °C. Recently, a low temperature producing method using an excimer laser has been intensively researched. In this method, however, small area such as 5 mm squares is able to be crystallized, so that it is necessary to move the substrate in order to obtain complete crystallization over a large area. When the substrate is moved, nonuniformity of crystallization arises in the boundary region depending upon the moving speed of the substrate, so that it is difficult to uniformly form the polysilicon thin film on a large area. The present invention is made to overcome the above-mentioned disadvantages of the conventional art, namely, it is an object of the present invention to provide a polysilicon thin film and a low temperature fabrication method thereof in which an inexpensive glass substrate such as a soda glass is used as a substrate. The object of the invention is solved by the method according to claims 1 to 4. The polysilicon thin film of the present invention is preferably fabricated over the surface of a metal electrode or transparent electrode, wherein both said metal or transparent electrodes are fabricated on the surface of a glass substrate. Furthermore, the thin polysilicon film is preferably fabricated using repetitions comprising the successive fabrication of amorphous silicon film by the CVD method or PVD method, exposure of the film to hydrogen plasma for a set period of time, and further fabrication of amorphous silicon film. Fig. 1 is a schematic diagram of a film forming apparatus applied to the film fabrication method of the present invention; Fig. 2 is the Raman spectrum of an embodiment of the present invention; and Fig.3 is the X-ray diffraction spectrum of an embodiment of the present invention. Hereinafter the present invention is described on the basis of an embodiment. As a substrate for fabrication of the thin polysilicon film, insulating substrates such as glass, quartz, sapphire and so on, or these substrates on which transparent electrodes such as ITO, SnO 2 and so on are formed can be used. However, of these substrates, a glass substrate is preferable because of its lower cost, and in particular, an inexpensive soda glass substrate or this substrate having deposited thereon transparent electrodes or metal electrodes. The most important consideration is that since the fabrication temperature of the thin polysilicon film is not more than 400 °C, the diffusion of alkali metals, such as Na, and alkaline earth metals, such as Mg, which are present in the glass substrate, is prevented. Examples 1∼2 and Comparative Example Using a CVD apparatus shown in Fig.1, a polysilicon thin film was deposited onto a glass substrate 3. In the fabricating procedure, firstly an a-Si:H is deposited onto the glass substrate 3 to 20 Å thickness by RF discharge; and secondly the film on substrate 3 is exposed for about 20∼about 30 seconds to an ECR plasma generated by a ECR plasma apparatus 5 constructed with permanent-magnets 5a. By repeating this procedure about 300 times, a polysilicon thin film of about having 6,000 Å thickness is obtained (Example 1). In the drawing, number 1 denotes the chamber, number 2 denotes a substrate rotating device, and number 4 denotes the RF electrodes. The condition at which the a-Si:H film was formed. were the substrate temperature of 350 °C, the pressure of 67 Pascal (0.5 Torr). RF power density of 0.4 W/cm 2 , SiH 4 gas flow rate of 40 SCCM, and H 2 gas flow rate of 200 SCCM. Further the ECR conditions used were: a pressure of 2,7 Pascal (0.02 Torr), a microwave electrical power of 400 W, and an H 2 gas flow rate of 200 SCCM. The Raman spectrum of the fabricated film is shown in Fig.2, and the X-ray diffraction results in Fig.3. From the X-ray diffraction of Fig.3, the polysilicon film is seen to have a strong (110) orientation. In addition, a value of mobility of 20 cm 2 /Vs was measured for this film using the Van der Pauw method. Similarly, in the case where the substrate temperature was 250 °C, a polysilicon film was also obtained using the above-described method (Example 2). A charactristic feature of the use of the permanent magnet 5a is that under the influence of the magnetic field, the ions are not accelerated towards the substrate 3 as occurs in the case of a standard ECR apparatus, but instead are accelerated towards the center of the diameter of the field. As a result, both the amount and the energy of ions influxing onto the substrate 3 are small compared with general ECR sources using electromagnets. On the other hand, since radicals diffuse in random directions, a similar amount of radicals is produced as with general ECR sources. In order to observe the effect of varying the H 2 plasma power, namely, the effect varying the density of Hydrogen radicals, a film was made setting microwave power of 100 W(Comparative Example). In the comparative example, however, no crystallization is observed. Industrial Applicability As is mentioned above, since the polysilicon thin film of the present invention is formed onto inexpensive glass substrates, the manufacturing cost is low. Furthermore, according to the fabrication method of the present invention, polysilicon films can be fabricated at low temperature. Therefore, inexpensive glass substrates can be adopted as a substrates.

Claims: A method of forming a polysilicon thin film on a substrate, said film having a hydrogen content of not more than 5 atomic % wherein the polysilicon thin film is oriented to (110), comprising the steps of: a) depositing an amorphous silicon film on a substrate by the CVD method or PVD method and b) exposing the film to a hydrogen plasma generated by ECR discharge using permanent magnets, wherein the pressure of the plasma is not more than 100 mTorr, the microwave electrical power is 400 W, and the H 2 gas flow rate is 200 SCCM. The method according to claim 1, wherein the amorphous silicon film is deposited at a temperature of not more than 400 °C. The method according to Claim 1, wherein said manufacturing method comprises multiple repetitions of steps a) and b). The method according to Claim 3, wherein the CVD method or PVD method and the exposure to a hydrogen plasma are carried out in the same chamber.

Inventors: OKAMOTO KEISHI, YAMAMOTO KENJI, OKAMOTO, KEISHI, YAMAMOTO, KENJI, OKAMOTO, KEISHI 6-31-17, SHIOYA, YAMAMOTO, KENJI 1-3-504, YOSHIDA-CHO 1-CHOME
Applicants: KANEGAFUCHI CHEMICAL IND, KANEGAFUCHI CHEMICAL INDUSTRY CO., LTD.
Application year: 1992
Main classification: H01L21
IPC codes: H01L21