Abstract:
A vacuum processing chamber preventing corrosion of a magnetic seal means and removing the heavy metals, organic matter, etc. produced by the corrosion, including a vacuum processing chamber, a rotated member (for example, a susceptor and a wafer holder), a rotary shaft penetrating one side of the vacuum chamber and rotating the rotated member, and a cavity being defined between an inner wall of the magnetic seal means and the rotary shaft, said magnetic seal means maintaining the air-tightness in the vacuum processing chamber by using a magnetic fluid in the cavity; a gas inlet introducing a barrier gas (for example, an inert gas like N 2 , Ar, or He or an inert gas having a slight amount of O 2  added) to the cavity; and a gas outlet extracting a gas inside the cavity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a magnetic seal bearing unit used for example for a wafer rotation mechanism, wafer transfer mechanism, etc. of an oxidation-diffusion furnace, chemical vapor deposition (CVD) apparatus, etc. for rotatably holding a rotary shaft while maintaining air-tightness of a vacuum processing chamber and to a vacuum processing apparatus. 
     2. Description of the Related Art 
     In the past, for example, an oxidation-diffusion furnace or CVD apparatus or other semiconductor processing apparatus has been improved in the uniformity of the processing by rotating the susceptor, wafer holder, etc. holding the wafer inside the vacuum processing chamber kept at a high vacuum. 
     Also, a system called “cluster tools” is used to successively perform different processing while holding a high vacuum and thereby improve the throughput. In this system, a plurality of processing chambers are provided around a common transfer chamber. After a processed wafer is taken out from a processing chamber by an arm of the wafer transfer mechanism provided in the transfer chamber, the wafer is transferred to another processing chamber. 
     A rotation mechanism for high vacuum required for these apparatuses is required to have a high air-tightness and not to produce any wafer contaminants. 
     For example, Japanese Unexamined Publication (Kokai) No. 7-169706 discloses an upright heat treatment apparatus having a rotation mechanism providing a drive side magnet at an outer casing secured to a pulley connected to a motor shaft by a belt and providing a driven side magnet at a rotary shaft and causing rotation of a wafer holder connected to the rotary shaft by the repulsion force of the two magnets along with the rotation of the pulley. 
     In this rotation mechanism, an inner casing secured to the bottom surface of the furnace is interposed between the outer casing and the rotary shaft at a predetermined distance from the two. Ceramic bearings having ceramic rollers are provided at a total of four locations above and below the two magnets between the inner casing and the outer casing and between the inner casing and the rotary shaft. 
     Accordingly, particles are generated from the ceramic bearings or due to damage of magnets made of brittle materials or corrosion due to gas from the furnace. There is therefore the problem that the wafer is contaminated by heavy metals and organic substances. 
     Accordingly, the upright heat treatment apparatus disclosed in this publication is designed with a feed pipe and an exhaust pipe for an inert gas provided between the furnace and the rotation mechanism and with the inert gas fed and discharged by a control unit for example for 10 seconds before evacuation in the furnace. At this time, in the rotation mechanism having the above structure, it is necessary to discharge the air around the magnets and around the ceramic bearings beneath them. A communicating pipe is formed in the rotary shaft to discharge the air inside the shaft space. 
     In this rotation mechanism, just the use of bearings using mutual contact of mechanical parts, such as ceramic bearings, becomes a cause for the generation of particles. Furthermore, it is necessary to provide a large number of ceramic bearings to obtain a high air-tightness. This makes it easier for particles to accumulate in the shaft space. 
     Also, since the communicating pipe for discharging the particles generated from the ceramic bearings and the above magnets to the outside is formed by working the rotary shaft, the mechanical strength of the rotary shaft is reduced. 
     Furthermore, looking at the control of the introduction of the inert gas, since the inert gas is fed and discharged only before evacuation in the furnace, contamination of the wafer due to the generation of particles during the following processing is unavoidable. 
     On the other hand, in place of such bearings using ceramic or other rollers, frequent use is now being made of magnetic seal bearings using magnetic fluid (magnetohydrodynamic) since the air-tightness is high and almost no particles are generated by contact of mechanical parts. 
     FIG. 8A shows the overall configuration of a magnetic seal bearing unit of the related art, and FIG. 8B shows the schematic structure of the magnetic seal portion. 
     The magnetic seal bearing unit  100  of the related art is, as shown in FIG. 8A, provided with a seal cap  102 , a magnetic seal portion  104 , and a ball bearing portion  106  inside its not illustrated unit housing from the vacuum processing chamber side. A rotary shaft  108  is provided penetrating through the center. One end of the vacuum processing chamber side of the rotary shaft  108  is provided with a susceptor or a wafer holder etc., while the other end is connected to a drive. 
     The magnetic seal  104  comprises, as shown in FIG. 8B, a member called a pole piece  110  and a permanent magnet  112 . The pole piece  110  has a plurality of ridges  110   a . A plurality of spaces called magnetic seal gaps G 1  to G 3  are formed therebetween. 
     The spaces between the ridges  110   a  of the pole piece and the rotary shaft  108  are filled with a magnetic fluid  114  made by mixing iron oxide-based fine particles in a solvent composed of vacuum oil. The magnetic fluid  114  has a high viscosity because of its being an oil base and accumulates at the tips of the ridges  110   a  so as to fill the spaces between the rotating rotary shaft  108  and the ridges  110   a . Also, the magnetic fluid  114  is influenced by the magnetic field resulting from the permanent magnetic  112  due to the intermixture of the magnetic material. As a result, the magnetic fluid  114  is prevented from concentrating at the lower vacuum side due to the evacuation. 
     FIG. 9 shows the step-shaped changes of an inner pressure of the magnetic seal gap due to the evacuation. The abscissa of FIG. 9 indicates a distance x in the axial direction of the rotary shaft, and the ordinate indicates a pressure P. 
     As the evacuation proceeds, air bubbles start to move in the part of the magnetic fluid closest to the vacuum chamber due to the pressure difference between the two sides and an inner pressure P 1  of the magnetic seal gap G 1  gradually falls. When the inner pressure P 1  of the magnetic seal gap G 1  becomes lower to a certain degree, air bubbles start to move in the magnetic fluid of the part second closest to the vacuum processing chamber. In the same way, air bubbles start to move in the magnetic fluid of the part third closest in a chain reaction. As a result, a step shape difference is created in the inner pressures P 1  to P 3  of the magnetic seal gaps G 1  to G 3 . 
     Finally, as shown in FIG. 9, the inner pressure P 1  of the magnetic seal gap G 1  closest to the vacuum processing chamber becomes the lowest, the inner pressure P 3  of the magnetic seal gap G 3  closest to the drive side becomes the closest value to the air pressure Pa, and the inner pressure P 2  of the magnetic seal gap G 2  becomes a value between the two. 
     In the magnetic seal portion  104 , by providing a large number of contact points with the magnetic fluid  114 , the pressure difference at the two sides of the parts of the magnetic fluid  114  becomes smaller and breakage of the seal is prevented. In other words, in order not to break the seal of the parts of the magnetic fluid even when evacuating quickly from the air pressure Pa, the volumes of the seal gaps G 1  to G 3  and the number of the contact points are determined in advance according to the maximum exhaust capability etc. of the usable vacuum pump. 
     A bearing using such a magnetic seal has a ball bearing portion  106  for a mechanical support. The air-tightness is maintained at a level higher than that of the magnetic seal portion  104 . Also, the particles from the ball bearing portion  106  are blocked by the magnetic seal portion  104  and not introduced into the vacuum processing chamber, therefore there is the advantage that wafers are not contaminated by the particles generated from mechanical members contacting each other. 
     In a bearing using a magnetic seal of the related art, however, when using dichlorosilane (SiH 2 Cl 2 ) or another gas which generates the corrosion product HCL such as in a CVD apparatus for forming a silicon nitride film, the material of the pole piece of the magnetic seal, that is, stainless steel, is corroded and sometimes ends up generating metal particles comprised of iron, chrome, nickel, etc. 
     Also, the vacuum oil serving as the solvent of the magnetic fluid deteriorates. When the temperature of the magnetic seal portion rises to tens of degrees to 150° C. due to, for example, the heating during the processing, the vacuum oil easily becomes volatile. Therefore, the vacuum oil itself and the decomposed matter and other organic substances fly about as well. At this time, the iron oxide, manganese oxide, etc. in the magnetic fluid are fly about together with the organic matter. 
     The degree of contamination of a wafer by such organic matter and metals is quite low comparing with that by particles generated from mechanical bearings. Furthermore, the contamination is, in many cases, limited to the case where the reaction gas is repeatedly used. 
     However, due to the recent miniaturization of semiconductor devices, it has been known that even a slight amount of contaminant from the magnetic seal portion can affect the device characteristics. 
     For example, when forming a silicon nitride film for prevent oxidization during LOCOS formation, even a slight amount of iron contamination (1×10 13  atoms/cm 3 ) ruins the quality of the gate oxide film formed at the next step. As a result, the step stress time zero dielectric breakdown (TZDB) withstand voltage of a 9 nm gate oxide film sometimes declines. 
     Also, even slight contamination by organic matter and metals on the bear silicon region after removing the silicon nitride film leads to deterioration of the withstand voltage of the pn-junction. 
     As a result, it was learned that even chemical contamination of a level hardly considered a problem before could cause a drop in the yield of the devices. 
     On the other hand, there is an apparatus which is configured to continuously pass an inert gas to the vacuum reaction chamber (processing chamber) side of the magnetic seal. portion, however, not only is this not a fundamental solution to the wafer contamination, but also conversely spreads the contamination wider and can shorten the cycle of maintenance for cleaning and exchanging members inside the processing chamber due to the contamination spreading to the entire processing chamber. 
     The above disadvantages of corrosion due to a reactive gas are also seen in a wafer transfer arm operating a high vacuum. For example, Japanese Unexamined Patent Publication (Kokai) No. 9-131680 discloses the technique of protecting the mechanical portion of the wafer transfer arm used in a cluster tool etc. from the chemical atmosphere by covering the wafer transfer arm with a cover and by adjusting the pressure of the inert gas to be introduced inside the cover to, for example, higher than the surroundings to prevent entry of the reactive gas from the surroundings inside the cover. 
     In this publication, a magnetic seal is used as a specific example of a sealing means for increasing the air-tightness of the rotary shaft portion. 
     However, in the technique disclosed in this publication, the mechanism for adjusting the pressure after filling the inert gas in the cover in a wide range is complex. 
     Also, the higher the pressure inside the cover, the better the corrosion of the wafer transfer arm can be prevented, however, the amount of the inert gas becomes larger and the degree of vacuum of the surroundings ends up being lowered so a longer time ends up being taken for evacuation. 
     Furthermore, although a pressure difference is created between the inside and outside of the cover, if the set pressure inside the cover is kept down a little when it is not desired to reduce the degree of vacuum of the surroundings and lower the throughput, there is a possibility that the pressure inside the cover will become lower than the outside, for example, at the arm portion at the distal end from the inlet of the inert gas. In such a case, the magnet seal portion is exposed to the reactive gas since it is exposed to the surroundings, thus the magnetic seal portion is corroded or the vacuum oil is degraded, which results in the same disadvantage as explained above that wafer contamination spreads from the magnetic seal portion. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a magnetic seal bearing unit which can prevent wafer contamination due to impurities generated from a magnetic seal portion effectively and by a simple configuration and a vacuum processing apparatus using the same. 
     According to a first aspect of the present invention, there is provided a vacuum processing apparatus comprising: a vacuum processing chamber; a rotated member rotated inside of the vacuum chamber; 
     a rotary shaft penetrating one side of the vacuum chamber and rotating the rotated member; a magnetic seal means provided at the outside of the vacuum processing chamber through which the rotary shaft is penetrated and around the rotary shaft of the outside of the vacuum processing chamber, a cavity being defined between an inner wall of the magnetic seal means and the rotary shaft, the magnetic seal means maintaining the air-tightness in the vacuum processing chamber by using a magnetic fluid in the cavity; 
     a gas inlet introducing a barrier gas to the cavity; and 
     a gas outlet extracting a gas inside the cavity. 
     Preferably, the gas inlet and the gas outlet are arranged close to each other. 
     Alternatively, preferably, a plurality of the gas inlets and the gas outlets are provided and the gas inlets and gas outlets are arranged alternately. 
     Alternatively, preferably, the gas inlet and the gas outlet are arranged facing each other across the rotary shaft. 
     Preferably, further provision is made of at least one of a gas introduction pressure control means for controlling an introducing pressure of the barrier gas and a gas extraction pressure control means for controlling a extracting pressure of the gas outlet in a pipe communicating with the gas inlet or the gas outlet. 
     Preferably, the gas introduction pressure control means controls the flow rate of the barrier gas to 1 ml/min to 10 l/min. 
     Alternatively, preferably the gas introduction pressure control means and/or the extraction pressure control means controls the pressure in the cavity to 0.1 Pa to 100 Pa so as not to influence the vacuum processing. 
     Preferably, further provision is made of a housing secured air-tightly sealing the surroundings of a hole formed at one side of the vacuum processing chamber and a bearing means provided in the housing and rotatably supporting the rotary shaft passing through the hole and extending inside the vacuum processing chamber, the magnetic seal means being arranged between the hole and the bearing means. 
     Preferably, the gas inlet and the gas outlet are formed at the rotary shaft supporting housing. 
     Preferably, the barrier gas comprises an inert gas or an inert gas plus 100 ppm to 10% of oxygen. 
     According to a second aspect of the present invention, there is provided a magnetic seal rotary bearing unit comprising: a unit housing which flange portion is fixed on outside surrounding of a hole formed at one side of a vacuum processing chamber and penetrated by a rotary shaft of a vacuum processing apparatus; a bearing means provided at the inside of the unit housing for rotatably supporting the rotary shaft; a magnetic seal means provided at the inside of the unit housing and arranged between the bearing means and the flange portion, a cavity being defined between an inner wall of the magnetic seal means and rotary shaft, the magnetic seal means maintaining the air-tightness in the vacuum processing chamber by using a magnetic fluid in the cavity; a gas inlet introducing a barrier gas to the cavity; and a gas outlet extracting a gas inside the cavity. 
     Preferably, the gas inlet and the gas outlet are arranged close to each other. 
     Alternatively, preferably a plurality of the gas inlets and the gas outlets are provided and the gas inlets and gas outlets are arranged alternately. 
     Alternatively, preferably the gas inlet and the gas outlet are arranged facing each other across the rotary shaft. 
     In the vacuum processing apparatus and the magnetic seal bearing unit of the present invention configured as above, for example, while introducing the process gas to the vacuum processing chamber or during the processing, the gas inside the vacuum processing chamber (reactive gas etc.) is blocked by the barrier gas introduced from the gas inlet and does not reach the magnetic seal means. Therefore, the magnetic seal means is not corroded and no contaminants are generated from it either. 
     Also, even if contaminants are generated from the magnetic seal means, they are exhausted from a very close gas outlet to the outside. 
     Furthermore, by adjusting the introducing pressure of the barrier gas from the gas inlet and/or the extracting pressure of the gas outlet, almost all of the barrier gas can be exhausted to the outside while maintaining the effect of protection of the magnetic seal means by the barrier gas. As a result, the contaminants are no longer carried by the barrier gas to the inside of the vacuum processing chamber. 
     When there are several magnetic seal bearings, the introduction and exhaust of the barrier gas can be optimized in the individual magnetic seal bearings. Note that if a barrier gas comprised of an inert gas plus a slight amount of O 2  is used, it has a cleaning effect on the piping etc. so is further preferable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of the present invention will become clearer from the following description of the preferred embodiments given with reference to the accompanying drawings, in which: 
     FIG. 1 is a view of the configuration of the core part of a CVD apparatus according to an embodiment of the present invention; 
     FIG. 2 is a view of the configuration of the core part of an upright oxidation-diffusion apparatus according to an embodiment of the present invention; 
     FIG. 3 is a schematic view of Arrangement Example 1 of the components in a magnetic seal rotary bearing unit according to an embodiment of the present invention; 
     FIG. 4 is a schematic view of Arrangement Example 2 of the components in a magnetic seal rotary bearing unit according to an embodiment of the present invention; 
     FIG. 5 is a schematic view of Arrangement Example 3 of the components in a magnetic seal rotary bearing unit according to an embodiment of the present invention; 
     FIG. 6 is a sectional view of an example of the structure of the magnetic seal rotary bearing unit of Arrangement Example 3; 
     FIG. 7A is an enlarged view of the magnetic seal means, and FIG. 7B is a view of the step-shaped changes in the inner pressure of the magnetic seal gap due to evacuation; 
     FIG. 8A is a view of the overall configuration of a magnetic seal rotary bearing unit of the related art, and FIG. 8B is a view of the schematic structure of the magnetic seal means of a magnetic seal rotary bearing unit of the related art; and 
     FIG. 9 is a view of the step-shaped changes of the inner pressure of the magnetic seal gap due to evacuation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Below, preferred embodiments of a vacuum processing apparatus and a magnetic seal rotary bearing unit according to the present invention will be described with reference to the accompanying drawings. Note that the present invention may be applied to, for example, a rotary bearing of a wafer transfer arm or other wafer transfer mechanism, but the case of application of the present invention to a wafer rotation mechanism will be explained here. 
     FIG. 1 is a view of the configuration of the core part of a CVD apparatus to which the present invention is applied. FIG. 2 is a view of the configuration of the core part of an upright oxidation-diffusion apparatus to which the present invention is applied. 
     The CVD apparatus  1  shown in FIG. 1 is a parallel plate type plasma CVD apparatus wherein two electrodes are provided inside a reaction chamber  2 . The lower electrode  4  serving also as a susceptor has a built-in heater etc. and is cylindrically shaped. A wafer W is placed over the same. The upper electrode  6  is connected to a power supply unit  8  secured to the top of the reaction chamber. An inlet  10  and an outlet  12  of a process gas are formed at the lower part of the reaction chamber  2 . 
     A predetermined process gas is introduced from the inlet  10  to the reaction chamber  2 , and high frequency power is applied between the two electrodes  4  and  6  from a not illustrated high frequency power source to generate a plasma of a low pressure reactive gas between the electrodes and thereby deposit a thin film of silicon oxide, silicon nitride, etc. on the wafer. 
     In order to improve the uniformity of thin film formation, a rotation mechanism is provided to rotate the susceptor  4 . That is, a hole  2   a  is formed in the bottom plate of the reaction chamber  2 , a shaft (rotary shaft)  14  is provided for supporting the susceptor  4 , and a rotary bearing unit  16  is attached outside of the reaction chamber  2  connected to the rotary shaft  14 . 
     The rotary bearing unit  16  of the present example is a magnetic seal rotary bearing unit which secures the air-tightness of the reaction chamber  2  by using the magnetic seal. The configuration of the inside of the magnetic seal rotary bearing unit  16  will be explained later. 
     In the present embodiment, a feed pipe  18  and an exhaust pipe  20  for the inert gas are connected to the magnetic seal rotary bearing unit  16 . An introduction pressure control means for adjusting the introducing pressure of the barrier gas, for example, a mass flow controller  22 , is provided in the feed pipe  18 . An extraction pressure control means for adjusting an extracting pressure, for example, a flow control valve  24 , is provided in the exhaust pipe  20 . 
     A barrier gas comprised of an inert gas such as N 2 , Ar, or He or an inert gas plus a slight amount of 100 ppm to 10% of dry O 2  is used controlled to a suitable flow rate within the range of 1 ml/min to 10 l/min. Also, the introduction and/or extraction pressure is controlled to a range of 0.1 Pa to 100 Pa so as not to influence the vacuum processing. The flow rate of the gas and the pressure are controlled by the mass flow controller  22  and the flow control valve  24 . 
     The upright oxidation-diffusion apparatus  30  shown in FIG. 2 has a reaction tube  32  having a two-layer structure in consideration of the uniformity of the thin film to be formed and the impurity diffusion area. The reaction tube  32  is comprised of an outer tube  32   a  at the outside and an inner tube  32   b  at the inside. 
     At the outside of the reaction tube  32  is provided a heater  34  which can control the temperature divided into, for example, four levels as shown by “U”, “CU”, “CL”, and “L” in the figure. The top of the heater  34  is covered by an insulating material  36 . 
     Wafers W are set laterally at predetermined intervals in the perpendicular direction on a wafer holder  38  inside the inner tube  32   b . A quartz cap  40  is provided at the lower portion of the inner tube  32   b.    
     The reaction tube  32  itself is secured to a flange  42  made of stainless steel. A feed pipe  44  for process gas is inserted from one end of the flange  42  to inside the inner tube  32   b . Also, an outlet  46  is provided at the other end of the flange  42 . 
     To improve the uniformity of processing of the oxidation-diffusion apparatus  30 , a rotation mechanism is provided for rotating the wafer holder  38 . Specifically, in this example, an elevator  48  having a rotation mechanism is provided which moves the wafer holder  38  upward and downward along with the quartz cap  40 . The reaction tube  32  has a bottom plate  50  having a hole formed at its center through which a rotary shaft  52  is passed for supporting the wafer holder  38  and the quartz cap  40 . A rotary bearing unit  54  and the elevator  48  are provided beneath the bottom plate  50  connected to the rotary shaft  52 . The elevator  48  has a stroke sufficient to shift all of the wafers W to the bottom outside of the reaction tube  32  and serves as a wafer transfer unit. 
     The rotary bearing unit  54  of the present example is a magnetic seal rotary bearing unit which secures air-tightness of the reaction tube  32  by using the magnetic seal. The configuration of the inside of the magnetic seal rotary bearing unit  54  will be explained below. 
     In the present embodiment, a feed pipe  56  and an exhaust pipe  58  are connected to the magnetic seal rotary bearing unit  54 . An introduction pressure control means for adjusting the introducing pressure of the barrier gas, for example, a mass flow controller  60 , is provided in the feed pipe  56 . An extraction pressure control means for adjusting the extracting pressure, for example, a flow control valve  62 , is provided in the exhaust pipe  58 . 
     FIGS. 3 to  5  are schematic views of examples of the arrangement of the components in the above magnetic seal rotary bearing units  16  and  54 . 
     As shown in FIGS. 3 to  5 , a ball bearing means  70  for mechanically supporting the rotary shaft and a magnetic seal means  80  are arranged on the drive side (air side) of the rotary shafts  14  and  52  inside the magnetic seal rotary bearing unit. Also, a suction head  90  and a purge head  92  for discharging the barrier gas are provided on the vacuum processing chamber (reaction chamber  2  or reaction tube  32 ) side of the magnetic seal means  80 . The ball bearing means  70  and the magnetic seal means  80  are arranged to surround the rotary shaft in the circumferential direction. 
     In Arrangement Example 1 shown in FIG. 3, at least one suction head  90  and purge head  92  are provided at the circumference of the rotary shaft. Preferably, more than one are provided at predetermined intervals. 
     The purge head  92  communicates with the feed pipes  18  or  56  in FIG. 1 or FIG.  2  and discharges a barrier gas, for example, an inert gas of N 2  or Ar, from the inlet  92   a  of the top of the purge head  92  to the open end side of the magnetic seal means  80 . 
     The suction head  90  communicates with the exhaust pipe  20  or  58  in FIGS. 1 or  2 . Barrier gas (and part of the process gas) is extracted from the outlet  90   a  of the top of the suction head  90  and exhausted to the outside. 
     Note that the purge head  92  is provided on the magnetic seal means  80  side and the suction head  90  is provided on the vacuum processing chamber side in the illustrated example, but it is also possible to arrange them in reverse with the purge head  92  provided on the vacuum processing chamber side and the suction head  90  on the magnetic seal means  80  side. 
     In Arrangement Example 2 shown in FIG. 4, a plurality of suction heads  90  and purge heads  92  are alternately arranged in the rotary shaft direction. 
     In Arrangement Example 3 shown in FIG. 5, the suction head  90  and the purge head  92  are arranged facing each other across the rotary shaft. By this, it is possible to make the barrier gas flow as an air curtain. 
     FIG. 6 is a sectional view of an example of the configuration of the magnetic seal rotary bearing unit in Arrangement Example 3, for example. 
     In the magnetic seal rotary bearing unit, the ball bearing means  70  and the magnetic seal means  80  are placed in the internal space inside a cylindrical-shaped unit housing  64  and a flange portion  66  provided on the end surface of the vacuum processing chamber side of the housing  64 . The ball bearing means  70  rotatably supports a rotary shaft by rollers  70   a  provided therein. A cooling water path  64   a  is provided in the unit housing  64 . 
     In the present example of the configuration, air paths  66   b  and  66   c  are formed in the flange  66  as the purge head  92  and the suction head  90  in FIG.  5 . 
     The air path  66   b  is connected to the feed pipe  18  or  56  and the air path  66   c  is connected to the exhaust pipe  20  or  58 . 
     The magnetic seal rotary bearing unit configured as above has the flange portion  66  air-tightly sealed on the outer surface side by an O-shaped ring  66   a  and is attached around the hole of the vacuum processing chamber. 
     FIG. 7A is an enlarged view of the magnetic seal means. 
     The magnetic seal means  80  comprises, as shown in FIG. 8B, pole pieces  82  and permanent magnets  84 . The pole pieces  82  have ridges  82   a  and a plurality of magnetic seal gaps G 1  to G 5  formed therebetween. As the material for the pole pieces  82 , stainless steel including a variety of metal elements such as Cr, Ni, Cu, Nb, Mo, Ti, Mn, and S is used. 
     The spaces between the ridges  82   a  of the pole pieces and the rotary shaft  14  or  52  are filled with magnetic fluid  86  comprised of a solvent made of vacuum oil in which are mixed iron oxide-based fine particles. As the solvent of the magnetic fluid  86 , perfluoropolyether (CF 3 —(C 3 F 5 O) x —(CF 2 O) y —CF 2 ) or other complete fluorinated oil comprised of the three elements of carbon, fluorine, and oxygen is used. Also, as the iron oxide-based fine particles, for example, manganese zinc ferrite (Mn.Zn.Fe 2 O 3 ) is used. 
     The magnetic fluid  86  has a high viscosity because it is oil-based and accumulates at the tips of the ridges  82   a  so as to fill the spaces between the rotating rotary shaft and the ridges  82   a . Also, the magnetic fluid  86  is influenced by the magnetic field created by the permanent magnets  84  due to the intermixture of the magnetic material therein. As a result, the magnetic fluid  86  is prevented from concentrating at the high vacuum side due to evacuation. 
     FIG. 7B shows the step-shaped changes of the inner pressure of the magnetic seal gaps due to evacuation. The abscissa of FIG. 7B indicates a distance x in the axial direction of the rotary shaft, and the ordinate indicates a pressure P. 
     As the evacuation proceeds, air bubbles start to move in the part of the magnetic fluid closest to the vacuum chamber due to the pressure difference between the two sides and an inner pressure P 1  of the magnetic seal gap G 1  gradually falls. When the inner pressure P 1  of the magnetic seal gap G 1  becomes lower to a certain degree, air bubbles start to move in the magnetic fluid of the part second closest to the vacuum processing chamber. Then, in the same way, air bubbles start to move in a chain reaction in other parts of the magnetic fluid. As a result, step-shaped differences are created in the inner pressures P 1  to P 5  of the magnetic seal gaps G 1  to G 5 . 
     Finally, as shown in FIG. 7B, the inner pressure P 1  of the magnetic seal gap G 1  closest to the vacuum processing chamber becomes the lowest, the inner pressure P 5  of the magnetic seal gap G 5  closest to the drive side becomes a value closest to the air pressure Pa, and the inner pressures P 2  to P 4  of the magnetic seal gaps G 2  to G 4  become values changing in a step-shape. 
     In the magnetic seal means  80 , by providing a large number of contact points with the magnetic fluid  86 , the pressure difference at the two sides of the parts of the magnetic fluid  86  becomes smaller and breakage of the seal is prevented. In other words, in order not to break the seal of the parts of the magnetic fluid  86  even when exhausting quickly from the air pressure Pa, the volumes of the seal gaps G 1  to G 5  and the number of the contact points of the magnetic fluid  86  are determined in advance according to the maximum exhaust capability etc. of the usable exhaust pump. The structure of the magnetic seal means  80  of the present example corresponds to an exhaust apparatus (dry pump etc.) having an exhaust capability of a high vacuum of a vacuum degree of about 1×10 −5  to 1×10 −3  Pa from the atmosphere. 
     Note that in the above embodiments, the above introduction pressure control means (mass flow controllers  22  and  60 ) and/or the extraction pressure control means (flow control valves  24  and  62 ) may be controlled individually, however preferably control is performed linked with the vacuum degree of the vacuum processing chamber due to the above exhaust apparatus. This is for control so as not to allow the barrier gas to be discharged inside the vacuum processing chamber as much as possible within the range where the protective effect of the magnetic seal means  80  due to the discharge of the barrier gas can be obtained. Note that the barrier gas may be an inert gas plus a slight amount of O 2  of about 100 ppm (atmospheric concentration) to 10%. This is because the piping etc. is cleaned. 
     Each of the magnetic seal bearing units  16 ,  54  of the above embodiments has a ball bearing means  70  for mechanically supporting the rotary shaft. The air-tightness is maintained at a high level by the magnet seal means  80 . Also, since particles from the ball bearing means  70  are blocked by the magnetic seal means  80  and not introduced into the vacuum processing chamber, a wafer is not contaminated by particles generated by contact of mechanical parts. 
     Also, for example, while introducing the process gas to the vacuum processing chamber or during the processing, the gas inside the vacuum processing chamber (process gas or reaction gas etc.) is blocked by the barrier gas discharged from the inlet  92   a  of the purge head and does not reach the magnetic seal means  80 . Therefore, the stainless steel portion of the magnetic seal means  80  is not corroded. 
     Even if the magnetic seal means  80  is corroded and elements constituting the stainless steel and their oxides are produced from it, they are sublimed as low level oxides when a slight amount of O 2  is added. Then the sublimed oxides are immediately drawn from the gas outlet  90   a  of the suction head  90  and exhausted to the outside. 
     Furthermore, in the above embodiments, a mass flow controller  22  or  60  is provided as an introduction pressure control means of the inert gas and a flow control valve  24  or  62  is provided as an extraction pressure control means. Therefore, by adjusting an introducing pressure of the barrier gas from the gas inlet  92   a  and/or a suction pressure from the gas outlet  90   a , almost all of the barrier gas can be discharged to the outside while maintaining the protective effect of the magnetic seal means  80  by the barrier gas. As a result, the contaminants are no longer carried by the barrier gas to inside the vacuum processing chamber. Even if contaminants are produced from the magnetic seal means  80 , a wafer will not be contaminated by them. 
     Note that a variety of modifications can be made to the above embodiments. 
     For example, the mass flow controllers  22  and  60  and other introduction pressure control means and flow control valves  2  and  62  and other extraction pressure control means are both provided to facilitate the pressure control for exhausting almost all of the barrier gas to the outside while maintaining the protective effect of the magnetic seal means  80  by the barrier gas, however it is also possible to provide only one of them. Note that when the introducing pressure and the extracting pressure of the barrier gas are constant, the pressure may be controlled structurally by the design of the pipe diameter, the constricted structure of the gas inlet or outlet, etc. 
     Also, the permanent magnets  84  may be arranged for every other ridge  82   a  of the pole pieces or for every three ridges. Also, the shape of the gaps can be variously modified. 
     Furthermore, the configuration of the magnetic seal rotary bearing units  16  and  54  are not limited to the illustrated ones. Further, the present invention may be applied to sputtering apparatuses, dry etching apparatuses, and other vacuum processing apparatuses. 
     Summarizing the advantageous effects of the invention, according to the vacuum processing apparatus and magnetic seal rotary bearing unit according to the present invention, it is possible to effectively prevent wafer contamination due to corrosion of the magnetic seal means and heavy metals and organic matter from the magnetic seal means. Further, contamination can be eliminated by a simple structure of just providing a gas inlet and gas outlet. 
     While the invention has been described with reference to specific embodiments chosen for purpose of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.