Dry screw vaccum pump having nitrogen injection

Details Dry screw vaccum pump having nitrogen injection Dry screw vaccum pump having nitrogen injection

2001-11-02

2003-04-29

Vrablik, John J. (Department: 3748)

Rotary expansible chamber devices

Heat exchange or non-working fluid lubricating or sealing

Non-working and working fluids intermix in working chamber

C418S201100, C418S201300

Reexamination Certificate

active

06554593

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a screw rotor-type dry vacuum pump and, more specifically, to a vacuum punp having a corrosion-resistance against gas generated in an apparatus for producing semiconductors, a dry vacuum pump in which a corrosion-resistant nickel alloy is employed as a material of a casing and a screw rotor that come in contact with a corrosive fluid, and a dry vacuum pump in which reaction products of process gas in an apparatus for producing semiconductors are prevented from building up in a path of a blast pipe of the dry vacuum pump.
BACKGROUND ART
A structure of a screw rotor-type dry vacuum pump will be explained with reference to its transverse sectional view shown in
FIG. 1. A
pump casing consists of: a main casing
1
; an inlet-side case
2
attached to a right end face of the main casing
1
; an outlet-side case
3
attached to a left end face of the main casing
1
; and a gear case
4
attached to a left end face of the outlet-side case
3
. A motor
5
is attached to the gear case
4
.
In the main casing
1
, there is provided an inner cylinder
1
a
penetrating through the main casing
1
axially, then an inlet
6
provided in the main casing
1
communicates with the right side of the inner cylinder
1
a
and then, the left side of the inner cylinder
1
a
communicates with an outlet
7
provided in the outlet-side case
3
. An abbreviation numeral
8
denotes a chamber of cooling water.
Two through holes
9
are formed in the inlet-side case
2
and a bearing box
10
containing a bearing
11
therein is attached to each through hole
9
. Two through holes
12
are formed in the outlet-side case
3
and a bearing box
13
containing a bearing
14
therein is attached to each through hole
12
.
Each of two screw rotors
15
consists of: spiral toothlike parts
15
a,
a cross section of each of which is formed by a Quimby curve, a circular arc and a quasi-Archimedean spiral curve; and a shaft
15
b
formed at both sides of each toothlike part
15
a.
The toothlike parts
15
a
are received in the inner cylinder
1
a
intermeshing with each other and each shaft
15
b
is supported by the bearing
11
or bearing
14
.
As to the drive-side screw rotor
15
shown at the lower side in
FIG. 1
out of the two screw rotors
15
, a timing gear
16
is inserted into a left end of the shaft
15
b,
then fixed by a locking mechanism
17
, while the left end of the shaft
15
b
is connected to an output shaft of the motor
5
through a coupling
18
. As to the follower-side screw rotor
15
shown at the upper side in
FIG. 1
out of the two screw rotors
15
, a timing gear
19
that engages with the timing gear
16
is inserted into a left end of the shaft
15
b,
then fixed by the locking mechanism
17
.
As shown in
FIG. 2
, i.e. a partially enlarged view of
FIG. 1
, the locking mechanism
17
consists of a locking member
20
and a tightening member
21
, then a engaging portion
22
for engaging with an outer peripheral surface of the shaft
15
b
is formed at one face of the locking member
20
, then a through hole
24
mating with a screw hole
23
formed on an end face of the shaft
15
b
is formed and then, a pushing projection
25
is formed outside the engaging portion
22
. When the engaging portion
22
of the locking member
20
is inserted into the shaft
15
b,
the locking member
20
is firmly mounted to the shaft
15
b
and the pushing projection
25
abuts on a bottom of a circular groove
26
formed on a side of the timing gear
16
.
The tightening member
21
is a bolt. When its end is screwed into the screw hole
23
through the through hole
24
of the locking member
20
, the pushing projection
25
pushes the timing gear
16
, then the timing gear
16
is pressed between the bearing
14
and the pushing projection
25
and fixed to the shaft
15
b.
When the motor
5
revolves, the coupling
18
and the drive-side screw rotor
15
revolve, then the revolution of the drive-side screw rotor
15
is transmitted to the follower-side screw rotor
15
through the timing gears
16
and
19
, then the two screw rotors
15
revolve in an opposite direction with each other at the same speed so as to transfer the fluid pumped from the inlet
6
to the outlet
7
. During this operation, a portion communicated with the inlet
6
is gradually depressed and the main casing
1
is heated, therefore, the main casing
1
is water-cooled.
As to a conventional vacuum pump for use in an apparatus for producing semiconductors, since corrosive gas is pumped up, a resin coating has been generally performed on surfaces of the inner cylinder
1
a
and the screw rotor
15
. For example, Tefron coating or Defric (polyimide resin) coating has been performed on an inner surface of the inner cylinder
1
a
and a surface of the screw rotor
15
up to the thickness of 25 to 30 &mgr;m.
Recently however, as to the apparatus for producing semiconductors, micro machining employing plasma has been widely used, then fluoride such as CF
4
and C
2
F
6
have been widely employed as to such apparatus for producing semiconductors in order to clean the apparatus during the manufacturing process. Above all, processes of a plasma-induced chemical vapour deposition and plasma etcher have been frequently employed, in which the fluoride such as CF
4
and C
2
F
6
is fed to remove products generated by nitriding, resulting in generation of activated fluorine system F* due to an excitaion by plasma. Since this F* is chemically very active, it reacts with H
2
gas contained in a process gas to generate HF. This very corrosive HF gas corrodes the resin coating and pulverizes them. Above all, since a vacuum pump employed for the process involving the generation of the products generated by nitriding is heated in order to prevent the products from solidifying and piling up in a casing of the vacuum pump, the reaction of HF production is accelerated, resulting in peeling of the resin coating.
When the resin coating performed on an inner surface of the inner cylinder
1
a
and a surface of the screw rotor
15
up to the thickness of 25 to 30 &mgr;m peels off, a gap having a diameter of 100 to 120 &mgr;m is generated between the screw rotor
15
and the inner cylinder
1
a,
causing a severe deterioration in the performance of the vacuum pump. Since the dry vacuum pump does not use a sealing liquid, the enlargement of the gap brings about a serious defect.
As a measure for solving the problem mentioned above, a corrosion-resistant material might be employed for the screw rotor
15
and the main casing
1
without coating them, however, such a corrosion-resistant material, i.e. SUS (stainless steel) is very hard to be machined. Therefore, SUS is not appropriate for the screw rotor
15
that has a complex shape and requires highly dimensional accuracy. In addition, since SUS has a large coefficient of thermal expansion and a drawback that a seizure is easily occurred, SUS can not be employed as a material for the screw rotor
15
and the main casing
1
.
A corrosion-resistant material, in which nickel is added to a spheroidal graphite cast iron having high mechanical strength, has been used to make the screw rotor
15
and the main casing
1
. However, since its coefficient of thermal expansion depends on the added amount of nickel and is different from that of the locking mechanism
17
made of mild steel, the locking mechanism
17
becomes loose, causing a slip for the timing gears
16
and
19
and an undesirable contact between screw rotors
15
with each other.
In addition, a bearing fitting portion between the bearing
14
that supports the shaft
15
b
and the bearing box
13
often suffers a creep phenomenon and the bearing
14
often suffers a damage.
The present invention is to solve the above problems by making a spheroidal graphite cast iron containing nickel, which has the same coefficient of thermal expansion with that of the locking mechanism
17
made of mild steel, taking advantage that its coefficient of thermal expansion can be adjusted by varying the added

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