Drying and gas or vapor contact with solids – Apparatus – With automatic control
Reexamination Certificate
1999-08-25
2001-02-27
Wilson, Pamela (Department: 3749)
Drying and gas or vapor contact with solids
Apparatus
With automatic control
C034S092000, C034S242000, C310S090000
Reexamination Certificate
active
06192603
ABSTRACT:
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 con
Kananen Ronald P.
Rader Fishman & Grauer
Sony Corporation
Wilson Pamela
LandOfFree
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