Formation of magnesium vapor with high evaporation speed

Abstract: A Mg source 3 is received in a vessel 1 having an opening 8, and the vessel 1 is heated at 670 °C or higher to effuse Mg vapor through the opening 8. Since the vessel 1 is filled with Mg vapor, Mg is evaporated from molten state under stable condition without fluctuations in evaporation speed. The Mg vapor is effectively consumed for vapor deposition. A reflector plate may be provided at the outlet of the opening 8, or a duct for introducing Mg vapor from the vessel to the surface of a substrate sheet may be provided. In order to evaporate Mg from stabilized molten state, operational conditions are preferably determined so as to satisfy the relationships of W 1 /W 2 < 0.6 × (P Mg /V 3 ) and W 1 /W 2 < 0.04 × P Mg , wherein W 1 represents the area (mm 2 ) of the opening 8, W 2 represents the evaporation surface area (mm 2 ) of the Mg source 3, P Mg represents Mg vapor pressure (torr.) and V represents the degree of vacuum.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of evaporating magnesium which is volatile in a vacuum atmosphere for the use of vapor Mg deposition or the like. A Mg source has been evaporated so far using the heating means such as an electric heater or electron beam which has been used for the evaporation of unvolatile metal such as Al or Cu. For instance, in the case of evaporation in small amount, a Mg source is put on a boat made of Mo or W, and the boat is resistance heated to evaporate Mg. In the case of evaporation in large amount, a Mg source is put in a crucible, and the crucible is heated by an electric heater or electron beam. The material Mg is the metal which is volatile in a vacuum atmosphere. Due to this property, Mg is sublimated from solid state without passing through liquid state. Therefore, the Mg source changes its shape in the solid state with the lapse of time when it is heated on the boat or in the crucible. The shape changing means the change of surface area and the change of evaporation speed. The evaporation is abruptly changed and difficult to control. Even when the temperature of the boat or crucible is controlled by adjusting the out put power of the electric heater or electropn beam, the temperature control can not follow the abrupt changing of the evaporation speed. In order to control the thickness of a vapor deposition film and/or the speed of vapor deposition with high accuracy, the evaporation speed of Mg can be kept at a constant value or controllled properly by sputtering process. However, the sputtering process is inferior in productivity, since Mg is evaporated at a slow speed. In addition, the efficiency of input energy consumed for the evaporation is very low in the sputtering process. For instance, the ratio of input energy consumed for the evaporation is only 10% or lower. If Mg can be evaporated from a liquid phase, the evaporation at a high speed under constant condition would be estimated. JP-A-02073960 discloses a method of depositing Mg vapor on a substrate wherein Mg is evaporated from a Mg melt in a vessel having a narrow opening. US-A-2 793 609 discloses an apparatus for deposition of vaporised material upon a surface in a vacuum, comprising a container, an outlet passage having an elongated cross-section and a cylindrical rod at the entrance to the passage wherein the rod completely interrupting all straight line paths through the passage from the material in the container. In the case of the material such as Al or Cu which is evaporated from liquid state, the evaporation source is melted on the way. Due to this molten state, an evaporation surface is kept constant by the inner face of a vessel such as a crucible. Consequently, the evaporation speed can be controlled at a constant level or properly changed with high accuracy, by adjusting the output power of an electric heater or electron beam so as to hold the vessel at a predetermined temperature. If such the evaporation condition is applicable for Mg evaporation, Mg can be evaporated at a high speed with high accuracy. The present invention is accomplished to overcome the afore-mentioned problems. One object of the present invention is to control the evaporation amount of Mg with high accuracy by effusing Mg vapor through the opening of a vessel filled with Mg vapor. The other object of the present invention is to evaporate Mg in controlled amount under stabilized condition without splashing by controlling the opening ratio of the opening in relation with operational conditions such as Mg vapor pressure, the degree of vacuum and evaporation surface area in an evaporation vessel. These objects are achieved with the methods of claims 1 and 2. According to the present invention, a Mg source is received in a vessel having a narrow opening, said vessel is heated at a temperature of 670-770°C, and formed Mg vapor is effused through said opening. Since the vessel is filled with the Mg vapor at the pressure enough to hold the Mg source in molten state, the efflux of Mg vapor from the vessel can be easily controlled with high accuracy. A reflector plate is provided at the outlet of the opening outside the vessel. Since the reflector plate is heated at 500°C or higher, the evaporation amount of Mg is controlled with much higher accuracy. A duct is provided in the state extending from the outlet of the vessel to the position where a substrate sheet is located, for introducing Mg vapor to the substrate sheet. The Mg evaporation is further performed under the conditions satisfying the relationships of W 1 /W 2 < 10665 × (P Mg /V 3 ) and W 1 /W 2 < 0.0003 × P Mg , wherein W 1 represents the area (mm 2 ) of the opening, W 2 represents the evaporation surface area (mm 2 ) of the Mg source, P Mg represents Mg vapor pressure (Pa), and V represents the degree of vacuum (Pa). Fig. 1 is the sectional view of a vessel having a reflector plate used for Mg evaporation according to the present invention. Fig. 2 is the sectional view of a vessel having a duct for introducing Mg vapor to a substrate sheet. Fig. 3 is the sectional view of a vessel having the duct in which a current plate is disposed for making vapor deposition uniform. According to the present invention, Mg source is received in a vessel having a narrow opening, and the vessel is heated at 670-770°C. The Mg source is evaporated by said heating, and the vessel is filied with formed Mg vapor, so that the internal pressure of the vessel is held at the same level not lower than Mg vapor pressure. This internal pressure assures the presence of a thick gaseous layer having the same pressure as Mg vapor pressure above the surface of the Mg source and suppresses sublimation even in a vacuum atmosphere. Consequently, Mg can be evaporated from molten state. The high - pressure gaseous layer of 1 mm or more in thickness seems to be necessary in order to hold the Mg source in stable molten state. Since Mg is evaporated from the molten state, the evaporation is continued at a constant speed under stabilized condition. Fig. 1 shows a Mg source 3 received in a vessel 1 having a narrow opening 8. When the vessel 1 is heated at 670-770°C by a heater 2, the vessel 1 is filled with formed Mg vapor until the internal pressure of the vessel 1 reaches Mg vapor pressure. As a result, sublimation is suppressed even in a vacuum atmosphere, and the Mg source 3 is held in molten state. Mg vapor pressure is approximately 933 Pa (7 torr). at 700 °C. The vessel 1 is substantially filled with Mg vapor during the Mg evaporation, even when there are other gasses in the vessel 1. The opening 8 is properly determined in dimensions under the condition that the internal pressure of the vessel 1 is nearly equal to Mg vapor pressure. The opening 8 may have a circular or slitted shape. The opening 8 may be provided at any position, i.e. not only at the upper part of the vessel but also at the side, as far as the opening 8 is located at the level higher than the surface of the molten Mg source 3. The Mg source 3 in the vessel 1 shall be held at a temperature of 670-770°C in order to inhibit the solidification which decelerates the Mg evaporation due to the fluctuation of temperature. There are no restrictions on the degree of vacuum in the vessel. When Mg is evaporated in a decompressed gas atmosphere, inert gas shall be used, and the pressure of said inert gas shall be held lower than Mg vapor pressure. If gas active to Mg is used, the gas flowing into the vessel 1 would react with the Mg source 3 and form compound films as reaction products on the surface of the Mg source 3 even when the amount of the gas flowing into the vessel 1 is very small. The compound films often impede the Mg evaporation. If the pressure of the inert gas exceeds Mg vapor pressure, Mg vapor would not be effused through the opening 8. When Mg vapor is effused from the vessel 1 through the opening 8 to the outside, Mg vapor is adiabatically expanded since the outside is in the vacuum or decompressed atmosphere. The adiabatic expansion causes the temperature falling of Mg vapor and the bonding of Mg vapor, i.e. the formation of clusters. The formation of clusters is promoted as the increase of Mg vapor effused through the opening 8. The clusters sometimes grow up to 100 µm or more in size and put harmful influences on the formation of a vapor deposition film of a few µm in thickness. When the vessel 1 is over-heated, the molten Mg source 3 is likely to bump and splash. Some of splashes are a few µm in size. If such splashes pass through the opening 8 and reach a substrate sheet, significant defects would be formed in a vapor deposition film. Such clusters or splashes are smashed against a heated reflector plate 4 provided at the outlet of the opening 8, as shown in Fig. 1. The clusters or splashes are heated again on the reflector plate 4 and re-evaporated to atomic vapor by the collision. As a result, it is possible to inhibit the formation of defects in the vapor deposition film caused by the clusters or splashes. The reflector plate 4 may be made of carbon or steel. The reflector plate 4 may have any configuration which does not permit the flowing of clusters or splashes. The reflector plate 4 is heated at 500°C or higher in order to re-evaporate all Mg deposited on it without accumulation. A duct 5 for introducing Mg vapor to a substrate sheet 6 is provided in the state extending from the outlet of the opening 8 to the position where the substrate sheet 6 is located, as shown in Fig. 2. Proper heating means may be attached to the duct 5. When the duct 5 is heated at 500°C or higher, the inner wall of the duct 5 is protected from Mg vapor deposition and its accumulation. The duct 5 has the same effect as the reflector plate 4 for eliminating clusters or splashes. However, when the duct 5 has big sectional area with respect to its length, some clusters and splashes pass through the duct 5 to the substrate sheet 6. In this case, another reflector plate similar to the reflector plate 4 shown in Fig. 1 is additionally provided in the duct to inhibit the pass of clusters or splashes. A current plate 7 may be provided in the duct 5, as shown in Fig. 3, to make vapor Mg deposition uniform on the substrate sheet 6. By the way, when H 2 O or O 2 remains in the vacuum atmosphere for Mg evaporation, H 2 O or O 2 invades through the opening 8 into the vessel 1 and reacts with highly-active Mg to form the films of oxide or other reaction products on the surface of the Mg source 3. Such the films decelerates Mg evaporation. Especially, H 2 O in large amount is adsorbed on the wall of a vacuum chamber, when the vacuum chamber is released to the open atmosphere. After the chamber is evacuated to a vacuum atmosphere, the adsorbed H 2 O is slowly desorbed from the wall surface, but the desorption of H 2 O from the wall surface to the vacuum atmosphere continues for a long time. Consequently, H 2 O naturally exists in the atmosphere at any degree of vacuum and causes the formation of oxide films on the surface of Mg source 3. Nitride would act as a harmful reaction product. The nitride is formed by the reaction between highly-reactive Mg and relatively inactive N 2 remaining in large amount in the vacuum atmosphere. The films of oxide or other reaction products formed on the surface of the Mg source 3 promotes the splashing of the Mg source 3, too. Even when there are neither oxides nor reaction products on the surface of the Mg source 3, bubbles are formed in the molten Mg source 3 heated at a higher temperature. The bubbles go up and erupt on the surface of the molten Mg source 3. The eruption causes the splashing of the Mg source 3, so as to fluctuate evaporation speed and to make the control of vapor deposition difficult. The suppression of such splashes for stabilized Mg evaporation is assured by using the vessel 1 having the opening 8 made small in size, as shown in Fig.1. When the opening 8 of the evaporation vessel 1 is small compared with the surface area of Mg evaporation in the vessel 1, Mg vapor in the vessel 1 is held at high pressure. The high pressure assures the effusion of Mg vapor in large amount at high speed through the opening 8 and inhibits the invasion of gasses from the atmosphere into the vessel 1. Since the Mg vapor pressure in the vessel 1 becomes bigger as the temperature-rising of the Mg source, the same effect is obtained by heating the Mg source. When the interior of the cell 3 is held at a high degree of vacuum, the pressure difference between the outside and inside of the vessel 1 becomes bigger, so that Mg vapor is effused at high speed through the opening 8 while inhibiting the invasion of atmospheric gasses. As afore-said, the area ratio of the opening 8 to the surface of Mg evaporation in the vessel 1, Mg vapor pressure and the degree of vacuum have mutual relationships and put influences on Mg evaporation. According to our researches and examinations, the condition of W 1 /W 2 < 10665 × (P Mg /V 3 ) is effective for inhibiting the invasion of atmospheric gas and for inhibiting the formation of oxide films or other reaction product films on the surface of the molten Mg source 3. Consequently, Mg is evaporated at a constant speed with high accuracy. Since there are neither oxide films nor other reaction product films, splashing is inhibited, and the deviation of evaporation speed is suppressed. Consequently, Mg deposition is performed with uniform thickness, even when Mg is vapor deposited on a long substrate such as steel sheet or coil. When the opening is made smaller according to W 1 /W 2 < 0.0003 × P Mg so as to inhibit the reduction of inner pressure in the vessel 1, otherwise Mg would be solidified. The condition of W 1 /W 2 < 0.0003 × P Mg is effective for holding the inner pressure in the vessel 1 above a predetermined value so as not to solidify Mg, even when the inner pressure in the vessel 1 is likely to be reduced by changing operational conditions such as the reduction of the degree of vacuum, too. In addition, since the interior of the vessel 1 is held above a predetermined pressure, the formation of splashes is inhibited, too. The formation of bubbles in the molten Mg source 3 is inhibited by holding the molten Mg source 3 at a temperature of 770 °C or lower. As a result, Mg evaporation is continued under stable condition without the formation of splashes. The solidification of Mg is inhibited regardless a little bit deviation in temperature, by holding the molten Mg source 7 at a temperature of 670°C or higher, so as to evaporate Mg under stable condition. When the conditions for evaporation is controlled according to the present invention as afore-mentioned, Mg evaporation is stabilized while inhibiting the formation of oxide films on the surface of the molten Mg source 3 and the formation of splashes. Consequently. Mg is evaporated from molten state as such under stable condition. According to the present invention as afore-mentioned, Mg is evaporated from molten state so that vapor Mg deposition can be performed under stabilized condition without fluctuations in evaporation speed. The clusters or splashes which would cause defects in vapor deposition films are suppressed by providing a reflector plate at a Mg receiving vessel and a duct extending from the vessel to the surface of a substrate sheet. Since the duct directly introduces Mg vapor toward the substrate sheet and eliminates the wasteful stream of Mg vapor, Mg vapor is efficiently consumed for vapor deposition, and the position to locate the substrate sheet is determined with the high degree of freedom. When the opening area of the vessel is adjusted in relation with other operational conditions such as the evaporation surface of the Mg source, vapor pressure and the degree of vacuum, the stabilized evaporation of Mg from molten state is assured without the formation of oxide films or reaction products on the surface of the Mg source or the formation of splashes. Accordingly, vapor Mg deposition is performed under stable condition without fluctuations in evaporation speed.

Claims: A Mg evaporating method comprising the steps of: a) receiving a Mg source in a vessel having a narrow opening and a reflector plate provided at the outlet of the opening outside the vessel, b) heating said vessel at a temperature of 670 - 770°C so as to fill said vessel with formed Mg vapor at a pressure enough to hold the Mg source in molten state, c) evaporating Mg from molten state, while holding said vessel filled with said Mg vapor, d) effusing said Mg vapor through said opening, and clusters or splashes in the Mg vapor being smashed against said reflector plate which is heated at a temperature of 500°C or higher, and e) carrying said Mg vapor through a duct from the outlet of the vessel to a substrate sheet, said duct being heated at 500°C or higher. A Mg evaporation method comprising the steps of: locating an evaporation vessel having a narrow opening and a reflector plate at the outlet of the opening outside the vessel in a vacuum atmosphere or a decompressed N 2 atmosphere, determining operational conditions so as to satisfy the relationships of W 1 /W 2 < 10665x (P Mg /V 3 ) and W 1 /W 2 < 0.0003xP Mg , wherein W 1 represents the area (mm 2 ) of the opening, W 2 represents the evaporation surface area (mm 2 ) of the Mg source, P Mg represents Mg vapor pressure (Pa) and V represents the degree of vacuum (Pa), heating said vessel at a temperature of 670 - 770°C so as to fill said vessel with formed Mg vapor at a pressure enough to hold the Mg source in molten state, evaporating Mg from molten state, while holding said vessel filled with said Mg vapor, effusing said Mg vapor through said opening, and clusters or splashes in the Mg vapor being smashed against said reflector plate which is heated at a temperature of 500°C or higher, and carrying said Mg vapor through a duct from the outlet of the vessel to a substrate sheet, said duct being heated at 500°C or higher.

Inventors: FUKUI YASUSHI, MIONO TADAAKI, SAITO MINORU, FUKUI, YASUSHI, MIONO, TADAAKI, SAITO, MINORU, Fukui, Yasushi, S. and T. Dev. Laboratories, Miono, Tadaaki, S. and T. Dev. Laboratories, Saito, Minoru, S. and T. Dev. Laboratories
Applicants: NISSHIN STEEL CO LTD, NISSHIN STEEL CO., LTD.
Application year: 1996
Main classification: C23C14
IPC codes: C23C14