JPH08247084A - Turbo-molecular pump - Google Patents

Abstract

PURPOSE: To design a blade cascade of a turbo-molecular pump excellent in exhaust performance in an extensive area capable of providing exhaust speed in the same degree as in a free molecule flow region even in an intermediate flow region (0.1 to 1Pa) as well as in the free molecular flow region where suction port side pressure is high vacuum. CONSTITUTION: A spacing (s) of cascades 5a, 6a is made 20% or less than 20% of an average free path h of gas molecule, a spacing code ratio s0 of the cascades 5a, 6a is found by theoretical computation, and the cascades 5a, 6a are formed so as to provide a high compression ratio (k) in the region that the spacing code ratio s0 is s0 =0.5 to 1.5 in a turbo-molecular pump consisting of the multiple stage axial flow cascades 5a, 6a.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a turbo molecular pump used in an ultra-high vacuum device and a vacuum generator required for manufacturing ICs, semiconductors and the like.

[0002]

2. Description of the Related Art A turbo-molecular pump has a pump function section consisting of a rotor having a multi-stage blade row similar in shape to an axial compressor and a stator having multi-stage blade rows alternately arranged with the blade row. have.

As a conventional turbo-molecular pump consisting of multi-stage axial flow blades, in order to achieve a target exhaust velocity and compression ratio with a minimum number of blade rows and to reduce the manufacturing cost, the rotor blades and the stationary blades are the blade angle of each stage while successively smaller extent going downstream toward the exhaust port side from the intake port side, blades and spacing code ratio s _o of each stage of the stationary blade also known is a method of sequentially smaller extent go downstream ing. (For example, Japanese Examined Patent Publication No. 50-27204).

[0004] Here spacing code ratio s _o = s /
b and s are the spacing (mm) of the blade row opening, and b is the blade cord length (mm).

[0005]

In the conventional turbo-molecular pump, the range of the inlet pressure that operates effectively is limited to the high-vacuum free molecular flow region (the Knudsen number Kn is 10 or more), and the Knudsen number Kn Since the exhaust speed of the conventional turbo-molecular pump is remarkably reduced in the intermediate flow region where the value of 0.1 to 10, the suction pressure range of the intermediate flow is 0.1 to 1 Pa as in the IC or semiconductor manufacturing apparatus. There is a problem that the conventional turbo-molecular pump cannot cope with the application in which a large flow rate needs to be exhausted in a region corresponding to.

The present invention applies a new theory to the design of a cascade of turbo molecular pumps, and has effective compression performance and pumping speed not only in the free molecular flow region but also in the intermediate flow region, and the axial dimension. Realizes medium and large size turbo molecular pumps with small external dimensions, and high compression that can obtain exhaust speed equivalent to high molecular weight gas such as nitrogen gas even for low molecular weight gas such as hydrogen. The objective is to realize medium to large size turbo molecular pumps with high ratio, wide range and high flow rate.

[0007]

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, the present invention provides a turbo molecular pump comprising a multi-stage axial flow blade row with a spacing s of the blade row equal to 20 of the mean free path λ of gas molecules. % with the following, characterized in that the formation of the wings string to a value in the range of wings row spacing code ratio s _o value = 0.5 to 1.5. Mean free path here λ is the gas molecules (mm), s the blade row opening spacing (mm), s _o is a spacing code ratio of blade _{rows, s o = s / b,} b is the wing The code length (mm) is shown.

[0008]

In the turbo molecular pump of claim 1, the blade row spacing and the spacing code ratio are designed by a new theory. Therefore, in the intermediate flow region, the compression performance and the pumping speed are higher than those of the turbo molecular pump according to the conventional design method. Can be obtained.

[0009] In the turbo molecular pump according to claim 2, since the spacing code ratio s _o of the cascade was set to be Hododai go downstream, because it reduces the axial dimension of the subsequent blade row, the conventional design A turbo molecular pump having an axial dimension shorter than that of the turbo molecular pump manufactured by the method can be used.

In the turbo molecular pump according to the third aspect, the exhaust speed of the turbo molecular pump in the free molecular flow region can be increased.

In the composite molecular pump according to the fourth aspect, it is possible to obtain the same exhaust speed for hydrogen gas as for nitrogen gas.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIGS.

FIG. 1 is a partial cross-sectional view of a turbo molecular pump, in which 1 is a casing of the turbo molecular pump, 2 is an intake port side thereof, and 3 is an exhaust port side thereof.

Reference numeral 4 denotes a rotor rotatably attached to the casing 1, and the rotor 4 has a plurality of blade blades 5a formed by radially projecting a plurality of blades 5 made of flat blades arranged in multiple stages.
Further, in the casing 1, there is provided a stator in which a large number of blades 6a, each of which is also a flat blade, protrudes inwardly and are installed in multiple stages, and the blade rows 5a and 6a have a narrow interval therebetween. Are arranged.

FIG. 2 shows the blade rows 5a and 6a by expanding the XX circumferential cross section in FIG. 1. The moving blade 5 and the stationary blade 6 are shown in FIG.
The wing angles α of are opposite to each other.

A _o is a blade row stage on the upstream side of the turbo molecular pump, B _o is a blade row stage on the middle flow side, and C _o is a blade row stage on the downstream side.

[0017] In the design method of the blade row according to the invention, the value of the spacing cord ratio s _o = s / b is determined in the range of s _o = 0.5 to 1.5 by the theoretical formula shown below.

The theoretical formula will be described below with reference to FIGS. 3 to 4b.

That is, FIG. 3 is an enlarged sectional view of the moving blade 5. The moving blade 5 has a cord length b, a blade angle α, and a spacing s.
To form the blade row 5a.

Further, D indicates the inlet side of the blade row 5a and E indicates the outlet side thereof, and the blade row 5a rotates at a peripheral speed V in the direction of arrow V.

The exhaust flow rate of the blade row 5a (product of volumetric exhaust velocity S and inlet side pressure P) is Q, and the compression ratio by the blade row 5a is K.

As shown in FIG. 3, the center position of the wall surface of the blade is
Assuming that the positions x and y are determined along the blade wall surface, the following equation (1) is established by stochastically treating the movement of gas molecules.

[0023]

[Equation 1]

Where M ₁₂ is the spacing s from region D
It is the probability that the gas molecules incident on the (blade row opening) finally pass through to the region E (including intermolecular collision and collision with the wall).

M ₁₂ is the probability that gas molecules that have entered the spacing s (blade row opening) from the region D will pass directly to the region E without causing intermolecular collision or collision with the wall.

M _x2 is the probability that the gas molecules reflected at the dx portion of the blade wall will eventually escape to the region E including collisions with the walls without causing intermolecular collisions.

Similarly, m _y2 is the probability that the gas molecules reflected on the other dying wall surface dy will finally escape to the region E without causing intermolecular collision.

U _x · d _x is the number of molecules that reach the wall surface dx without experiencing intermolecular collision and collision with the wall among all gas molecules incident on the spacing s (blade row opening) from the region D. Percentage.

Similarly, u _y · d _y is also the ratio of molecules that reach the wall surface dy without experiencing intermolecular collision and collision with the wall.

Dτ is a replacement of dx and dy corresponding to x and y.

The gas molecules colliding with each other in the passage between the blades are equally divided into a region D and a region E.

The above-mentioned m ₁₂ , m _x2 , m _y2 , u _x · d _x , u _y ·
d _y, the speed of the gas molecules v, the mean free path of the molecule λ
_v , the maximum probability velocity a of the molecule, the peripheral velocity V of the blade row, the blade angle α,
The angles θ and r _xy , r _xs' , determined by the cross-sectional shape of the passage between the blades
As a function of r _{ys ′} and the spacing s of the blade row, it can be obtained as follows.

Here, a = (2RT / M) ^1/2 , R is the gas constant, T is the absolute temperature, M is the molecular weight of the incident gas, and c is the speed ratio of the blade row c = V / a.

_{M x2} and m _y2 are obtained by numerically solving the following simultaneous equations.

[0035]

[Equation 2]

[0036]

(Equation 3)

Here, D _x2 is a region E in which gas molecules reflected at the dx portion of the wall surface do not collide with each other and collide with the wall.
Probability of reaching.

Similarly, D _y2 is the probability that the gas molecules reflected at the dy portion of the other wall surface reach the region E without causing intermolecular collision and collision with the wall.

F (x, y) dy is the proportion of the gas molecules reflected on the dx portion of the wall surface that reach the wall surface dy without experiencing intermolecular collision and collision with the wall.

Similarly, F (y, x) dx is the ratio of the molecules reaching the wall surface dx without experiencing intermolecular collision and collision with the wall among the gas molecules reflected at the dy portion of the wall surface. Etc. are expressed as the following equations.

[0041]

[Equation 4]

[0042]

(Equation 5)

[0043]

(Equation 6)

Here, r _xy , r _{xs '} , and r _ys' are those shown in FIGS. 4a and 4b, and are (x, y), (x,
θ), (y, θ).

Θ _ox and θ _oy are also the angles shown in FIGS. 4a and 4b, which are functions of x and y, respectively.

From the symmetry, F (x, y) = F (y,
x).

Further, as described above, λ _v is the mean free path of the gas molecule having the velocity v, which can be replaced by the mean free path λ.

Here, the Knudsen number Kn is introduced.

The Knudsen number Kn is expressed by the following equation by the mean free path λ and the spacing s which is the representative dimension of the passage.

[0050]

(Equation 7)

In the above equations (4), (5) and (6),
If the shapes of the blade rows 5a and 6a, that is, x, y, θ, and α are given, a functional expression of the velocity v of the gas molecule and the mean free path λ _v is obtained.
By expressing v and λ _v by the Knudsen number Kn, the above equations (4), (5), and (6) can be made into the functional equation of Kn, and the simultaneous equations (2) and (3) can be expressed by the Knudsen equation. With the number Kn as a parameter, the values of m _x2 and m _y2 can be given for each x and y. Thus, M _{12 of the} equation (1) is obtained.

Contrary to M ₁₂ , the probability that gas molecules incident from the outlet side E of the blade row 5a will finally pass through the region D (including intermolecular collisions and wall collisions) M ₂₁ Then, M ₂₁ becomes the value of M ₁₂ obtained by setting the peripheral velocity V of the blade row in FIG. 3 to −V.

That is,

[0054]

(Equation 8)

It is

On the other hand, regarding the relationship between the exhaust flow rate Q of the blade row 5a and the compression ratio K, the compression ratio K reaches the maximum value Kmax when the exhaust flow is zero (Q = 0), and Since the maximum value Qmax of the flow rate is when there is no compression (K = 1), the above equations (1) and (8) and these Kma
The relationship between x and Qmax can be expressed by the following equation.

[0057]

[Equation 9]

[0058]

[Equation 10]

[0059] M ₁₂ and M ₂₁ are those determined a Knudsen number Kn as the as the parameter, and x, the value of y also defines the spacing code ratio s _o, FIGS. 5 to these relationship graph Shown in 10.

FIGS. 5 to 7 are graphs showing results obtained by calculating the maximum compression ratio Kmax at various spacing code ratios _{o using} the equation (9) with the reciprocal number (1 / Kn) of the Knudsen number Kn as a parameter. .

Further, FIGS. 8 to 10 use the spacing code ratio s _o = s / b as a parameter, and express the maximum exhaust quality Qmax with respect to the reciprocal (1 / Kn) of the Knudsen number Kn by (1
The graph of the result calculated | required by 0) formula is shown.

[0062] Using this like Figure 5 to Figure 10, it is possible to select the spacing code ratio s _o obtained the pumping speed S excellent in the intermediate flow region in the following manner.

As is apparent from FIGS. 8 to 10, in the region where the value of the reciprocal (1 / Kn) of the Knudsen number Kn is 0.2 or less, the maximum exhaust flow rate Qmax is uniform and the 1 / K
When the value of n exceeds 0.2, the maximum exhaust flow rate Qmax suddenly increases.
Is reduced.

Therefore, first, the reciprocal of the Knudsen number Kn (1
/ Kn) is set to 0.2 or less, 1 / Kn = s /
Make λ <0.2. That is, the spacing s of the blade row opening is 20% or less of the mean free path λ of gas molecules.

Here, the mean free path λ of the gas molecule is a value determined by the kind of the gas and the temperature / pressure, and if the intake port 2 is in the intermediate flow region, it is λ corresponding to this.

Next, referring to FIGS. 5 to 7, the maximum compression ratio K
spacing code ratio s _o that maximum value is obtained in the max is indicated by a dotted line a _o, the figure 5 to 7, the spacing code obtained a high maximum compression ratio K ratio s _o = s /
b can be obtained.

[0067] Incidentally, compatible spacing code ratio s _o in this condition, in a range of between about 0.5 to 1.5, can be seen from figure 5 to 7.

According to FIGS. 5 to 7, the maximum compression cost K
The dotted line a ₀ that gives the maximum value of max is the Knudsen number Kn.
It is shown that the spacing code ratio s ₀ becomes larger as the reciprocal of (1 / Kn) increases. However, since the internal pressure of the multi-stage turbo molecular pump increases toward the latter stage, Since the Knudsen number Kn of the internal gas decreases and the reciprocal number (1 / Kn) of the Knudsen number Kn increases in the subsequent blade row, it is desirable to increase the spacing code ratio s ₀ in the latter stage.

According to the conventional blade cascade design method for a turbo molecular pump, the blade angle α and the spacing code ratio s ₀ are increased toward the front stage closer to the intake port side 2 and toward the rear stage toward the exhaust port side 3. The blade angle α and the spacing code ratio s ₀ are made small (for example, Japanese Patent Publication No. 50-27204), and in this respect also, the blade row designing method according to the present invention is based on a new concept different from the conventional one. It indicates that there is.

Next, an example of the performance of the turbo molecular pump using the cascade of blades according to the design method of the present invention is shown by the S-Ps graph in FIG.

Here, S is the exhaust port 3 of the turbo molecular pump.
Is the exhaust speed (m ³ / s), and Ps is the intake pressure (Pa) at the intake port 2 of the turbo molecular pump.

In FIG. 11, a curve CD shows an example of the performance of the turbo molecular pump according to the first embodiment of the present invention, and a curve AB has a rotor diameter substantially equal to that of the first embodiment. 1 shows an example of the performance of a turbo molecular pump according to US Pat.

In FIG. 11, the performance curve AB of the medium-sized machine according to the conventional design method shows that the exhaust speed S is as high as about 0.6 m ³ / s in the high vacuum free molecular flow region, but the intake port pressure Ps
In the intermediate flow region on the higher pressure side around 0.1 Pa, the exhaust speed S rapidly decreases and becomes unusable.

On the other hand, the performance curve CD of the first embodiment of the present invention having the same rotor diameter as the example by the conventional design method.
According to the equation, the exhaust velocity S is 0.3 m ^{3 in the} free molecular flow region.
/ S, which is half the performance of the conventional design method, but the exhaust velocity S does not decrease sharply even in the intermediate flow region, and the inlet pressure Ps does not decrease to around 1 Pa and extends.

Further, comparing the maximum flow rates of the two turbo molecular pumps, the maximum flow rate in the conventional design method is 0.08 Pa · m ³ / s as shown by G in FIG. The maximum flow rate of 0.25 is 0.25 as shown in F of FIG.
It is Pa · m ³ / s, which is about three times as high as that of the conventional design method.

A second embodiment of the present invention is also shown in FIGS.
1 will be described.

In the second embodiment, the spacing s is set on the upstream side A ₀ of the turbo molecular pump of the first embodiment, corresponding to the mean free path λ of gas molecules corresponding to the pressure on the inlet side 2. And disk stages 5a, 6a of a blade row having a large cord length b
Is added.

An example of the performance of the turbo molecular pump of the second embodiment is shown by the curve AED in FIG.

According to this performance curve AED, the exhaust velocity s in the high-vacuum free molecular flow region is 0.6 m ³ / s, which is the same as in the conventional design method, and the maximum flow rate is also shown in FIG.
As shown by F above 1, 0.25 Pa · m ³ / s is obtained.

A third embodiment of the present invention will be described with reference to FIG.

The third embodiment is a composite molecular pump in which a thread groove pump section is added to the rear of the turbo molecular pump of the first or second embodiment, 7 is the turbo molecular pump section, and 8 is a screw. It is a groove pump section.

In the conventionally designed composite molecular pump, the exhaust speed S _H for a gas having a small molecular weight such as hydrogen is about 60% of the exhaust speed S _N for a gas having a large molecular weight such as nitrogen. In the embodiment, the exhaust rate S _H for a gas having a small molecular weight such as hydrogen can be made comparable to that of a gas having a large molecular weight such as nitrogen, while keeping the entire length to the same level as that of a conventionally designed composite molecular pump.

[0083]

As described above, according to the present invention, not only in the free molecular flow region where the intake pressure of the turbo molecular pump is high vacuum, but also in the intermediate flow region where the intake pressure is 0.1 to 1 Pa, It is possible to design a cascade of turbo-molecular pumps that can achieve an exhaust velocity equivalent to that in the molecular flow region,
Further, since the spacing code ratio s ₀ = s / b in the latter stage is made large, the axial dimension in the latter stage blade row can be made small, and the total length of the rotor of the turbo molecular pump is shortened. It is extremely advantageous in terms of rotor balance and vibration prevention, and when using a magnetic bearing for the rotor,
This is advantageous in making the size of the magnetic bearing compact, and further, since the thickness of the blade disk can be made small, the thickness of the material of the disk becomes small, which is also effective in terms of material cost and processing cost.

[Brief description of drawings]

FIG. 1 is a partial vertical cross-sectional view of a turbo molecular pump.

FIG. 2 is a development view of an XX section of FIG.

FIG. 3 is a partially enlarged sectional view.

4a is an explanatory diagram of FIG. 3. FIG.

4b is an explanatory diagram of FIG. 3. FIG.

FIG. 5: Maximum compression ratio Kmax and spacing code ratio s
₉ is a graph showing a relationship of ₀ .

FIG. 6 shows the maximum compression ratio Kmax and the spacing code ratio s.
₉ is a graph showing a relationship of ₀ .

FIG. 7: Maximum compression ratio Kmax and spacing code ratio s
₉ is a graph showing a relationship of ₀ .

FIG. 8 is a graph showing the relationship between the maximum exhaust flow rate Qmax and the reciprocal (1 / Kn) of the Knudsen number Kn.

FIG. 9 is a graph showing the relationship between the maximum exhaust flow rate Qmax and the reciprocal number (1 / Kn) of the Knudsen number Kn.

FIG. 10 is a graph showing the relationship between the maximum exhaust flow rate Qmax and the reciprocal number (1 / Kn) of the Knudsen number Kn.

FIG. 11 is a graph showing a performance example of a turbo molecular pump.

FIG. 12 is a vertical sectional view of a composite molecular pump.

[Explanation of symbols]

 2 Inlet port 3 Exhaust port 4 Rotor 5 Moving blade 6 Stator blade 5a 6a Blade row b Code length s Spacing α Blade angle

Claims (4)

[Claims] 1. A turbo molecular pump comprising a multi-stage axial flow blade row, wherein the blade row spacing s is 20% or less of the mean free path λ of gas molecules, and the spacing code ratio s of the blade row is s. A turbo-molecular pump, wherein the blade row is formed so that the value of _{o is in} the range of s ₀ = 0.5 to 1.5. Where λ is the mean free path of the gas molecule (mm), s is the spacing of the blade row opening (mm), s
_o is the spacing code ratio of the blade _{cascade, so} = s / b, b
Indicates the cord length (mm) of the blade. Wherein in said turbo-molecular pump, the spacing code ratio s _o for each blade row, the spacing s and as s _o enough to go to the subsequent stage to the exhaust port side from the intake port side is large The turbo molecular pump according to claim 1, wherein a code length b of the blade is formed. 3. A disk stage of a blade row having a large spacing s and a large chord length b is added on the upstream side of the rotor of the turbo molecular pump in correspondence with the mean free path λ of gas molecules corresponding to the suction pressure. The turbo-molecular pump according to claim 1 or 2, characterized in that. 4. The turbomolecular pump section of the hybrid molecular pump uses the turbomolecular pump according to claim 3, and a thread groove pump section is provided behind the turbomolecular pump section.