Patent ID: 12253089

DETAILED DESCRIPTION

Referring to the drawings, a vacuum pump according to an example of the present disclosure is now described.FIG.1shows a turbomolecular pump100as a vacuum pump according to an example of the present disclosure. The turbomolecular pump100is to be connected to a vacuum chamber (not shown) of a target apparatus such as a semiconductor manufacturing apparatus.

FIG.1is a vertical cross-sectional view of the turbomolecular pump100. To avoid complicating the drawing,FIG.1schematically shows the internal structure of the turbomolecular pump100. In particular, the turbomolecular pump100of this example has many major characteristic structures in a groove exhaust mechanism portion in an exhaust mechanism portion. For this reason, the illustration of the groove exhaust mechanism portion is simplified inFIG.1, and the basic configuration from suction to exhaust of the turbomolecular pump100is shown. The specific structure and function of the groove exhaust mechanism portion are shown inFIG.5and the subsequent figures, and a detailed description of the groove exhaust mechanism portion is provided following the overall description of the turbomolecular pump100.

As shown inFIG.1, the turbomolecular pump100has a circular outer cylinder127having an inlet port101at its upper end. A rotating body103in the outer cylinder127includes a plurality of rotor blades102(102a,102b,102c, . . . ), which are turbine blades for gas suction and exhaustion, in its outer circumference section. The rotor blades102extend radially in multiple stages. The rotating body103has a rotor shaft113in its center. The rotor shaft113is supported and suspended in the air and position-controlled by a magnetic bearing of 5-axis control, for example.

Upper radial electromagnets104include four electromagnets arranged in pairs on an X-axis and a Y-axis. Four upper radial sensors107are provided in close proximity to the upper radial electromagnets104and associated with the respective upper radial electromagnets104. Each upper radial sensor107may be an inductance sensor or an eddy current sensor having a conduction winding, for example, and detects the position of the rotor shaft113based on a change in the inductance of the conduction winding, which changes according to the position of the rotor shaft113. The upper radial sensors107are configured to detect a radial displacement of the rotor shaft113, that is, the rotating body103fixed to the rotor shaft113, and send it to the controller200.

In the controller200, for example, a compensation circuit having a PID adjustment function generates an excitation control command signal for the upper radial electromagnets104based on a position signal detected by the upper radial sensors107. Based on this excitation control command signal, an amplifier circuit150(described below) shown inFIG.2controls and excites the upper radial electromagnets104to adjust a radial position of an upper part of the rotor shaft113.

The rotor shaft113may be made of a high magnetic permeability material (such as iron and stainless steel) and is configured to be attracted by magnetic forces of the upper radial electromagnets104. The adjustment is performed independently in the X-axis direction and the Y-axis direction. Lower radial electromagnets105and lower radial sensors108are arranged in a similar manner as the upper radial electromagnets104and the upper radial sensors107to adjust the radial position of the lower part of the rotor shaft113in a similar manner as the radial position of the upper part.

Additionally, axial electromagnets106A and106B are arranged so as to vertically sandwich a metal disc111, which has the shape of a circular disc and is provided in the lower part of the rotor shaft113. The metal disc111is made of a high magnetic permeability material such as iron. An axial sensor109is provided to detect an axial displacement of the rotor shaft113and send an axial position signal to the controller200.

In the controller200, the compensation circuit having the PID adjustment function may generate an excitation control command signal for each of the axial electromagnets106A and106B based on the signal on the axial position detected by the axial sensor109. Based on these excitation control command signals, the amplifier circuit150controls and excites the axial electromagnets106A and106B separately so that the axial electromagnet106A magnetically attracts the metal disc111upward and the axial electromagnet106B attracts the metal disc111downward. The axial position of the rotor shaft113is thus adjusted.

As described above, the controller200appropriately adjusts the magnetic forces exerted by the axial electromagnets106A and106B on the metal disc111, magnetically levitates the rotor shaft113in the axial direction, and suspends the rotor shaft113in the air in a non-contact manner. The amplifier circuit150, which controls and excites the upper radial electromagnets104, the lower radial electromagnets105, and the axial electromagnets106A and106B, is described below.

The motor121includes a plurality of magnetic poles circumferentially arranged to surround the rotor shaft113. Each magnetic pole is controlled by the controller200so as to drive and rotate the rotor shaft113via an electromagnetic force acting between the magnetic pole and the rotor shaft113. The motor121also includes a rotational speed sensor (not shown), such as a Hall element, a resolver, or an encoder, and the rotational speed of the rotor shaft113is detected based on a detection signal of the rotational speed sensor.

Furthermore, a phase sensor (not shown) is attached adjacent to the lower radial sensors108to detect the phase of rotation of the rotor shaft113. The controller200detects the position of the magnetic poles using both detection signals of the phase sensor and the rotational speed sensor.

A plurality of stator blades123(123a,123b,123c, . . . ) are arranged slightly spaced apart (by predetermined gaps) from the rotor blades102(102a,102b,102c). Each rotor blade102(102a,102b,102c, . . . ) is inclined by a predetermined angle from a plane perpendicular to the axis of the rotor shaft113in order to transfer exhaust gas molecules downward through collision.

The stator blades123are also inclined by a predetermined angle from a plane perpendicular to the axis of the rotor shaft113. The stator blades123extend inward of the outer cylinder127and alternate with the stages of the rotor blades102. The outer circumference ends of the stator blades123are inserted between and thus supported by a plurality of layered stator blade spacers125(125a,125b,125c, . . . ).

The stator blade spacers125are ring-shaped members made of a metal, such as aluminum, iron, stainless steel, or copper, or an alloy containing these metals as components, for example. The outer cylinder127is fixed to the outer circumferences of the stator blade spacers125with a slight gap. A base portion129is located at the base of the outer cylinder127. The base portion129has an outlet port133providing communication to the outside. The exhaust gas transferred to the base portion129through the inlet port101from the chamber (vacuum chamber) is then sent to the outlet port133.

According to the application of the turbomolecular pump100, a threaded spacer131may be provided between the lower part of the stator blade spacer125and the base portion129. The threaded spacer131is a cylindrical member made of a metal such as aluminum, copper, stainless steel, or iron, or an alloy containing these metals as components. The threaded spacer131has a plurality of helical thread grooves131ain its inner circumference surface. When exhaust gas molecules move in the rotation direction of the rotating body103, these molecules are transferred toward the outlet port133in the direction of the helix of the thread grooves131a. In the lowest section of the rotating body103below the rotor blades102(102a,102b,102c, . . . ) (more specifically, below rotating discs220ato220cof a Siegbahn type exhaust mechanism portion201described below), a cylindrical portion102dextends downward. The outer circumference surface of the cylindrical portion102dis cylindrical and projects toward the inner circumference surface of the threaded spacer131. The outer circumference surface is adjacent to but separated from the inner circumference surface of the threaded spacer131by a predetermined gap. The exhaust gas transferred to the thread groove131aby the rotor blades102and the stator blades123is guided by the thread groove131ato the base portion129.

The base portion129is a disc-shaped member forming the base section of the turbomolecular pump100, and is generally made of a metal such as iron, aluminum, or stainless steel. The base portion129physically holds the turbomolecular pump100and also serves as a heat conduction path. As such, the base portion129is preferably made of rigid metal with high thermal conductivity, such as iron, aluminum, or copper.

In this configuration, when the motor121drives and rotates the rotor blades102together with the rotor shaft113, the interaction between the rotor blades102and the stator blades123causes the suction of exhaust gas from the chamber through the inlet port101. The exhaust gas taken through the inlet port101moves between the rotor blades102and the stator blades123and is transferred to the base portion129. At this time, factors such as the friction heat generated when the exhaust gas comes into contact with the rotor blades102and the conduction of heat generated by the motor121increase the temperature of the rotor blades102. This heat is conducted to the stator blades123through radiation or conduction via gaseous molecules of the exhaust gas, for example.

The stator blade spacers125are joined to each other at the outer circumference portion and conduct the heat received by the stator blades123from the rotor blades102, the friction heat generated when the exhaust gas comes into contact with the stator blades123, and the like to the outside.

In the above description, the threaded spacer131is provided at the outer circumference of the cylindrical portion102dof the rotating body103, and the thread groove131ais engraved in the inner circumference surface of the threaded spacer131. However, this may be inversed in some cases, and a thread groove may be engraved in the outer circumference surface of the cylindrical portion102d, while a spacer having a cylindrical inner circumference surface may be arranged around the outer circumference surface.

According to the application of the turbomolecular pump100, to prevent the gas drawn through the inlet port101from entering an electrical portion, which includes the upper radial electromagnets104, the upper radial sensors107, the motor121, the lower radial electromagnets105, the lower radial sensors108, the axial electromagnets106A,106B, and the axial sensor109, the electrical portion may be surrounded by a stator column122. The inside of the stator column122may be maintained at a predetermined pressure by purge gas.

In this case, the base portion129has a pipe (not shown) through which the purge gas is introduced. The introduced purge gas is sent to the outlet port133through gaps between a protective bearing120and the rotor shaft113, between the rotor and the stator of the motor121, and between the stator column122and the inner circumference cylindrical portion of the rotor blade102.

The turbomolecular pump100requires the identification of the model and control based on individually adjusted unique parameters (for example, various characteristics associated with the model). To store these control parameters, the turbomolecular pump100includes an electronic circuit portion141in its main body. The electronic circuit portion141may include a semiconductor memory, such as an EEPROM, electronic components such as semiconductor elements for accessing the semiconductor memory, and a substrate143for mounting these components. The electronic circuit portion141is housed under a rotational speed sensor (not shown) near the center, for example, of the base portion129, which forms the lower part of the turbomolecular pump100, and is closed by an airtight bottom lid145.

Some process gas introduced into the chamber in the manufacturing process of semiconductors has the property of becoming solid when its pressure becomes higher than a predetermined value or its temperature becomes lower than a predetermined value. In the turbomolecular pump100, the pressure of the exhaust gas is lowest at the inlet port101and highest at the outlet port133. When the pressure of the process gas increases beyond a predetermined value or its temperature decreases below a predetermined value while the process gas is being transferred from the inlet port101to the outlet port133, the process gas is solidified and adheres and accumulates on the inner side of the turbomolecular pump100.

For example, when SiCl4is used as the process gas in an Al etching apparatus, according to the vapor pressure curve, a solid product (for example, AlCl3) is deposited at a low vacuum (760 [torr] to 10−2[torr]) and a low temperature (about 20 [° C.]) and adheres and accumulates on the inner side of the turbomolecular pump100. When the deposit of the process gas accumulates in the turbomolecular pump100, the accumulation may narrow the pump flow passage and degrade the performance of the turbomolecular pump100. The above-mentioned product tends to solidify and adhere in areas with higher pressures, such as the vicinity of the outlet port133and the vicinity of the threaded spacer131.

To solve this problem, conventionally, a heater or annular water-cooled tube149(not shown) is wound around the outer circumference of the base portion129, and a temperature sensor (e.g., a thermistor, not shown) is embedded in the base portion129, for example. The signal of this temperature sensor is used to perform control to maintain the temperature of the base portion129at a constant high temperature (preset temperature) by heating with the heater or cooling with the water-cooled tube149(hereinafter referred to as TMS (temperature management system)).

The amplifier circuit150is now described that controls and excites the upper radial electromagnets104, the lower radial electromagnets105, and the axial electromagnets106A and106B of the turbomolecular pump100configured as described above.FIG.2is a circuit diagram of the amplifier circuit150.

InFIG.2, one end of an electromagnet winding151forming an upper radial electromagnet104or the like is connected to a positive electrode171aof a power supply171via a transistor161, and the other end is connected to a negative electrode171bof the power supply171via a current detection circuit181and a transistor162. Each transistor161,162is a power MOSFET and has a structure in which a diode is connected between the source and the drain thereof.

In the transistor161, a cathode terminal161aof its diode is connected to the positive electrode171a, and an anode terminal161bis connected to one end of the electromagnet winding151. In the transistor162, a cathode terminal162aof its diode is connected to a current detection circuit181, and an anode terminal162bis connected to the negative electrode171b.

A diode165for current regeneration has a cathode terminal165aconnected to one end of the electromagnet winding151and an anode terminal165bconnected to the negative electrode171b. Similarly, a diode166for current regeneration has a cathode terminal166aconnected to the positive electrode171aand an anode terminal166bconnected to the other end of the electromagnet winding151via the current detection circuit181. The current detection circuit181may include a Hall current sensor or an electric resistance element, for example.

The amplifier circuit150configured as described above corresponds to one electromagnet. Accordingly, when the magnetic bearing uses 5-axis control and has ten electromagnets104,105,106A, and106B in total, an identical amplifier circuit150is configured for each of the electromagnets. These ten amplifier circuits150are connected to the power supply171in parallel.

An amplifier control circuit191may be formed by a digital signal processor portion (not shown, hereinafter referred to as a DSP portion) of the controller200. The amplifier control circuit191switches the transistors161and162between on and off.

The amplifier control circuit191is configured to compare a current value detected by the current detection circuit181(a signal reflecting this current value is referred to as a current detection signal191c) with a predetermined current command value. The result of this comparison is used to determine the magnitude of the pulse width (pulse width time Tp1, Tp2) generated in a control cycle Ts, which is one cycle in PWM control. As a result, gate drive signals191aand191bhaving this pulse width are output from the amplifier control circuit191to gate terminals of the transistors161and162.

Under certain circumstances such as when the rotational speed of the rotating body103reaches a resonance point during acceleration, or when a disturbance occurs during a constant speed operation, the rotating body103may require positional control at high speed and with a strong force. For this purpose, a high voltage of about 50 V, for example, is used for the power supply171to enable a rapid increase (or decrease) in the current flowing through the electromagnet winding151. Additionally, a capacitor is generally connected between the positive electrode171aand the negative electrode171bof the power supply171to stabilize the power supply171(not shown).

In this configuration, when both transistors161and162are turned on, the current flowing through the electromagnet winding151(hereinafter referred to as an electromagnet current iL) increases, and when both are turned off, the electromagnet current iL decreases.

Also, when one of the transistors161and162is turned on and the other is turned off, a freewheeling current is maintained. Passing the freewheeling current through the amplifier circuit150in this manner reduces the hysteresis loss in the amplifier circuit150, thereby limiting the power consumption of the entire circuit to a low level. Moreover, by controlling the transistors161and162as described above, high frequency noise, such as harmonics, generated in the turbomolecular pump100can be reduced. Furthermore, by measuring this freewheeling current with the current detection circuit181, the electromagnet current iL flowing through the electromagnet winding151can be detected.

That is, when the detected current value is smaller than the current command value, as shown inFIG.3, the transistors161and162are simultaneously on only once in the control cycle Ts (for example, 100 μs) for the time corresponding to the pulse width time Tp1. During this time, the electromagnet current iL increases accordingly toward the current value iLmax (not shown) that can be passed from the positive electrode171ato the negative electrode171bvia the transistors161and162.

When the detected current value is larger than the current command value, as shown inFIG.4, the transistors161and162are simultaneously off only once in the control cycle Ts for the time corresponding to the pulse width time Tp2. During this time, the electromagnet current iL decreases accordingly toward the current value iLmin (not shown) that can be regenerated from the negative electrode171bto the positive electrode171avia the diodes165and166.

In either case, after the pulse width time Tp1, Tp2 has elapsed, one of the transistors161and162is on. During this period, the freewheeling current is thus maintained in the amplifier circuit150.

In the turbomolecular pump100with the basic configuration described above, the upper side as viewed inFIG.1(the side including the inlet port101) serves as a suction portion connected to the target apparatus, and the lower side (the side including the base portion129in which the outlet port133protrudes leftward as viewed in the figure) serves as an exhaust portion connected to an auxiliary pump (a roughing back pump) or the like (not shown). The turbomolecular pump100can be used not only in an upright position in the vertical direction shown inFIG.1, but also in an inverted position, a horizontal position, and an inclined position.

Also, in the turbomolecular pump100, the above-mentioned outer cylinder127and the base portion129are combined to form a single case (hereinafter, they may be collectively referred to as a “main body casing” or the like). The turbomolecular pump100is electrically (and structurally) connected to a box-shaped electrical case (not shown), and the above-mentioned controller200is incorporated in the electrical case.

The configuration within the main body casing (the combination of the outer cylinder127and the base portion129) of the turbomolecular pump100may be divided into a rotation mechanism portion, which rotates the rotor shaft113and the like with the motor121, and an exhaust mechanism portion, which is rotationally driven by the rotation mechanism portion. The exhaust mechanism portion may be divided into a turbomolecular pump mechanism portion, which includes the rotor blades102, the stator blades123, and the like, and a groove exhaust mechanism portion (described below), which includes the cylindrical portion102d, the threaded spacer131, and the like.

The above-mentioned purge gas (protection gas) is used to protect components such as the bearing portions and the rotor blades102, prevents corrosion caused by the exhaust gas (process gas), and cools the rotor blades102, for example. This purge gas may be supplied by a general technique.

For example, although not illustrated, a purge gas flow passage extending linearly in the radial direction may be provided in a predetermined section of the base portion129(for example, at a position approximately 180 degrees apart from the outlet port133). The purge gas may be supplied to the purge gas flow passage (specifically, a purge port serving as a gas inlet) from the outside of the base portion129via a purge gas cylinder (e.g., N2 gas cylinder), a flow rate regulator (valve device), or the like.

The protective bearing120described above is also referred to as a “touchdown (T/D) bearing”, a “backup bearing”, or the like. In case of any trouble such as trouble in the electrical system or entry of air, the protective bearing120prevents a significant change in the position and orientation of the rotor shaft113, thereby limiting damage to the rotor blades102and surrounding portions.

In the figures showing the structure of the turbomolecular pump100(such asFIGS.1and5), hatch patterns indicating cross sections of components are omitted to avoid complicating the drawing.

The above-described groove exhaust mechanism portion is now described with reference toFIG.5and the subsequent figures.FIG.5shows the same turbomolecular pump100schematically shown inFIG.1but, unlikeFIG.1, specifically shows the groove exhaust mechanism portion (formed by a Siegbahn type exhaust mechanism portion201and a Holweck type exhaust mechanism portion301) and its surrounding portion in order to illustrate the specific structure and the function of the groove exhaust mechanism portion, as described above.

As shown inFIGS.5and6(a), the groove exhaust mechanism portion of the present example includes a Siegbahn type exhaust mechanism portion201and a Holweck type exhaust mechanism portion301. Of these, the Siegbahn type exhaust mechanism portion201is in the stage following (immediately downstream of) the turbomolecular pump mechanism portion, which includes the above-described rotor blades102(102a,102b,102c, . . . , each including blade row) and the stator blades123(123a,123b,123c, . . . ) for example, and is formed to be spatially continuous with the turbomolecular pump mechanism portion. The Holweck type exhaust mechanism portion301is in the stage following (immediately downstream of) the Siegbahn type exhaust mechanism portion201and formed to be spatially continuous with the Siegbahn type exhaust mechanism portion201.

The Siegbahn type exhaust mechanism portion201is formed such that gas is transferred in the radial directions with respect to the axis of the rotor shaft113. In contrast, the Holweck type exhaust mechanism portion301is formed such that gas is mainly transferred in the axial direction of the rotor shaft113.

The Holweck type exhaust mechanism portion301of the present example is configured to transfer gas in the radial direction with respect to the axis of the rotor shaft113and to transfer gas in the axial direction of the rotor shaft113. However, the section that transfers gas in the radial direction may be classified as a part of the Siegbahn type exhaust mechanism portion201, and only the section that transfers gas in the axial direction of the rotor shaft113may be classified as the Holweck type exhaust mechanism portion301. Details of the Holweck type exhaust mechanism portion301according to the present example will be described below.

The above-mentioned Siegbahn type exhaust mechanism portion201is a Siegbahn type exhaust mechanism and includes stator discs219aand219band rotating discs220ato220c. The rotating discs220ato220cand the stator discs219aand219bare made of a metal, such as aluminum, iron, stainless steel, or copper, or a metal such as an alloy containing these metals as components.

The stator discs219aand219bare integrally coupled to the main body casing (the combination of the outer cylinder127and the base portion129). A stator disc (219a,219b) of one stage is inserted between two stages of upper and lower rotating discs (220ato220c) arranged in the axial direction of the rotor shaft113.

The rotating discs220ato220care formed integrally with the cylindrical rotating body103, and rotate in the same direction as the rotor shaft113and the rotating body103as the rotating body103rotates. That is, the rotating discs220ato220crotate integrally also with the rotor blades102(102a,102b,102c, . . . ).

In the present example, the Siegbahn type exhaust mechanism portion201has two stator discs219aand219band three rotating discs220ato220c. Also, the stator discs219aand219band the rotating discs220ato220care arranged alternately in the axial direction of the rotor shaft113from the side including the suction portion (the side including the inlet port101) in the order of the rotating disc220a, the stator disc219a, the rotating disc220b, the stator disc219b, and the rotating disc220c.

As shown enlarged inFIG.6(a), a large number of ridges261having a rectangular cross-sectional shape protrude between the stator discs219aand219band the rotating discs220ato220c. Also, Siegbahn spiral groove portions262, which are spiral groove flow passages, are formed between adjacent ridges261.

In the following description, with reference toFIGS.5and6(a) for example, the side including the suction portion (the side including the inlet port101) located on the upper side as viewed in the figures may be referred to as an “upstream side”, while the side including the exhaust portion (the side including the outlet port133) on the lower side as viewed in the figures may be referred to as a “downstream side”.

FIG.6(a)is an enlarged view of a section on the right side of the rotor shaft113as viewed inFIG.5(inside the frame L indicated by the dashed double-dotted line) of the groove exhaust mechanism portion. The groove exhaust mechanism portion has a line-symmetrical structure (left-right symmetrical as viewed inFIG.5) with respect to the axis of the main body casing (combination of the outer cylinder127and the base portion129) or the rotor shaft113, for example. As such, only the right section inFIG.5is shown enlarged here, and the illustration of the left section is omitted.

As shown inFIG.6(a), each of the stator discs219aand219bhas the above-mentioned ridges261formed integrally on both plate surfaces266and267. In the following description, the plate surfaces266and267of the stator discs219aand219bare designated by the same reference numerals, and the common reference numerals (reference numerals266and267in this example) are used for the different stator discs219aand219b.

As for the ridges261, regardless of the difference between the stator discs219aand219b, and regardless of the difference between the plate surfaces266and267, all the ridges are designated by common reference numeral261in the description. Furthermore, inFIG.6(a), to avoid complicating the drawing, reference numerals are mainly indicated for the upstream stator disc219aof the stator discs219aand219b. The indication of the same reference numerals for the downstream stator disc219bis omitted.

The stator disc219a,219bhas a disc-shaped main body portion268having a through hole270(also shown inFIG.6(b)) in the center. The upstream plate surface266of the upstream stator disc219aas viewed inFIG.6(a)is inclined toward the downstream plate surface267from the central side (the side including the through hole270) toward the outer circumference side, which is the proximal end side, of the main body portion268.

In contrast, the downstream plate surface267is formed to be substantially horizontal as viewed in the figure. In other words, the downstream plate surface267of the upstream stator disc219ais formed to be substantially perpendicular to the axis of the rotor shaft113. The thickness of the main body portion268of the upstream stator disc219ais not constant and changes to gradually decrease from the inner circumference side, which is the central side, toward the outer circumference side, which is the proximal end side.

In contrast, the main body portion268of the downstream stator disc219bis formed to have a substantially uniform thickness from the central side to the outer circumference side, which is the proximal end side.

As used herein, the “outer circumference side” refers to the outer side of the stator discs219aand219bin the normal direction (radial direction) of the main body portion268, and the “inner circumference side” refers to the inner side in also the normal direction (radial direction) of the main body portion268.

The outer circumference edge portions of the main body portions268of the stator discs219aand219bare processed to have a substantially uniform and equal thickness, and inserted between and supported by multiple stator disc spacers269, which are stacked in stages.

As shown schematically inFIG.6(b)in addition toFIGS.5and6(a), each of the plate surfaces266and267of the stator discs219aand219bhas a plurality of ridges261as described above. The ridges261are formed in a spiral shape around the center of the main body portion268on the plate surfaces266and267of the main body portion268. Each ridge261extends along a smooth curve from the circumference edge of the through hole270(inner circumference edge) to the outer circumference edge (a section located near the stator disc spacers269).

FIG.6(b)generally (schematically) shows, as an example, a state of the downstream stator disc219bas viewed in the axial direction from the side corresponding to the upstream plate surface266. InFIG.6(b), the ridges261formed on the upstream plate surface266are indicated by solid lines, and the ridges261formed on the downstream plate surface267are indicated by relatively thin broken lines. Also, inFIG.6(b), illustration of the stator disc spacer269is omitted. Furthermore, inFIG.6(b), the rotating body103and the rotor shaft113are indicated by imaginary lines (dashed double-dotted lines).

In each stator disc219a,219b, each ridge261protrudes from each plate surface266,267of the disc-shaped main body portion268at a predetermined angle. In the present example, as described above, the upstream plate surface266of the upstream stator disc219ais inclined toward the downstream plate surface267from the central side to the outer circumference side, which is the proximal end side, of the main body portion268.

Accordingly, on the upstream plate surface266of the upstream stator disc219a, the ridges261protrude obliquely with respect to the plate surface266.

Additionally, on the upstream plate surface266of the upstream stator disc219a, the ridges261have different protruding amounts depending on the position (phase), but their distal ends (the upper ends as viewed inFIG.6(a)) reach the same height and are located on the same plane perpendicular to the axis of the rotor shaft113.

In contrast, on the downstream plate surface267of the upstream stator disc219aand on both the plate surfaces266and267of the downstream stator disc219b, the ridges261protrude substantially perpendicularly with respect to the plate surfaces266and267. On these three plate surfaces267,266, and267, the protruding amounts of the ridges261are substantially uniform regardless of the position (phase).

In the present example, to avoid complicating the description, the plate surfaces266and267each have nine ridges. However, the number of ridges is not limited to this, and may be eight or less or ten or more. Also, the stator discs219aand219band the plate surfaces266and267do not have to have the same number of ridges and may have different numbers of ridges.

The above-mentioned Siegbahn spiral groove portions262are now described. For the Siegbahn spiral groove portions262, regardless of the difference between the stator discs219aand219band the plate surfaces266and267, all the groove portions are also designated by common reference numeral262in the description. However, some Siegbahn spiral groove portions262may be designated by different reference numerals (such as262a) and distinguished from other Siegbahn spiral groove portions262depending on the situation.

Each Siegbahn spiral groove portion262is spirally formed between two adjacent ridges261on each of the plate surfaces266and267. The Siegbahn spiral groove portion262is partitioned and defined by the ridges261. The Siegbahn spiral groove portions262are formed, together with the ridges261, on the upstream plate surface266and the downstream plate surface267of each of the stator discs219aand219bso as to be mutually at the same phase, with the respective starting points (starting portions) as the origins. Each Siegbahn spiral groove portion262is a space having a relatively wide outer circumference side (with a wide opening width) and a relatively narrow inner circumference side (with a narrow opening width).

The rotating discs220ato220care now described. In this example, the thickness of each of the rotating discs220ato220cis substantially uniform in the area from the central side near the rotating body103to the outer circumference side. Also, the rotating discs220ato220chave substantially the same (common) thickness. Furthermore, the protruding amounts of the rotating discs220ato220cfrom the rotating body103are substantially the same (common), and the end surfaces of the outer circumferences of the rotating discs220ato220care aligned in the axial direction along the entire circumference.

Also, the rotating discs220ato220cface the distal end portions (protruding end portions) of the ridges261and partition the Siegbahn spiral groove portions262with slight gaps of about 1 mm, for example. As described above, the upstream plate surface266of the upstream stator disc219ais inclined toward the downstream plate surface267from the central side to the outer circumference side, which is the proximal end side, of the main body portion268. Each Siegbahn spiral groove portion262between the most upstream rotating disc220a(the uppermost stage inFIG.6(a)) and the upstream plate surface266of the upstream stator disc219ais a space that gradually narrows from the outer circumference side toward the inner circumference side.

As described above, the Siegbahn spiral groove portions262formed on the upstream plate surface266of the upstream stator disc219amay be designated by reference numeral262aand distinguished from the other Siegbahn spiral groove portions262in the following description.

The depth of an opening281on the upstream side (outer circumference side) of each Siegbahn spiral groove portion262ais defined as H1, and the depth of an opening282on the downstream side (inner circumference side) is defined as H2. The “depth” as used herein is the depth in the axial direction, which is the up-down direction as viewed inFIG.6(a)(which coincides with the axial direction of the rotor shaft113). These depths H1and H2are the distances between a plate surface (reference numeral omitted) of the rotating disc220aand the upstream plate surface266of the stator disc219ain the axial direction.

The Siegbahn spiral groove portions262aform a section serving as a gas inlet of the groove exhaust mechanism portion, as will be described below. As such, the Siegbahn spiral groove portions262amay be hereinafter referred to as a “groove exhaust mechanism portion inlet portion” or a “Siegbahn exhaust flow passage inlet portion.”

Then, turning portions286and287are formed between the rotating discs220ato220cand the stator discs219aand219b. The turning portions286and287are sections with spatial turning structures relating to the gas flow passage.

That is, as described above, the ridges261and the Siegbahn spiral groove portions262extend from the respective origins (starting points) and are spatially continuous with one another in the same phase on both plate surfaces266and267of the stator discs219aand219b. Accordingly, at the inner circumference side of each of the stator discs219aand219b, a turning portion286is formed to spatially connect the Siegbahn spiral groove portions262on the upstream plate surface266to the Siegbahn spiral groove portions262on the downstream plate surface267.

Additionally, at the outer circumference side of each of the rotating discs220ato220c, a turning portion287is formed to spatially connect the Siegbahn spiral groove portions262on the upstream plate surface (reference numeral omitted) to the Siegbahn spiral groove portions262on the downstream plate surface (reference numeral omitted). The Siegbahn spiral groove portions262and the turning portions286and287form spatially continuous gas flow passages. Hereinafter, this series of flow passages is referred to as a “Siegbahn exhaust flow passage” and designated by reference numeral291as shown inFIG.6(a).

Regarding this Siegbahn exhaust flow passage291, the dimension of the distance between the inner circumference end surface284of the stator disc219a,219band the outer circumference surface285of the rotating body103is defined as depth H3. This H3is larger than the above-mentioned H2(the opening dimension of the opening282on the downstream side (inner circumference side) of the Siegbahn spiral groove portion262a).

Also, the dimension of the distance between the outer circumference surface285of each of the rotating discs220ato220cand the stator disc spacers269is defined as depth H4. This H4is larger than the above-mentioned H2(the opening dimension of the opening282on the downstream side (inner circumference side) of the Siegbahn spiral groove portion262a). Furthermore, in the present example, this H4is set slightly smaller than the depth H3, which is the dimension of the distance between the stator discs219aand219band the rotating body103. However, H4is not limited to this and may be set larger than H3, for example.

Also, the downstream plate surface267of the upstream stator disc219aand the upstream plate surface (reference numeral omitted) of the second rotating disc220bfrom the upstream side face each other and are substantially parallel. The distance (the depth of the gas flow passage) between the downstream plate surface267of the upstream stator disc219aand the second rotating disc220bis set to be the same as H2described above from the inner circumference side to the outer circumference side (from the inlet to the outlet of the Siegbahn spiral groove portion262).

Similarly, the upstream plate surface266of the downstream stator disc219band the downstream plate surface (reference numeral omitted) of the second rotating disc220bfrom the upstream side face each other and are substantially parallel. The distance (the depth of the gas flow passage) between the upstream plate surface266of the downstream stator disc219band the second rotating disc220bis set to be the same as H2described above from the outer circumference side to the inner circumference side (from the inlet to the outlet of the Siegbahn spiral groove portion262).

Similarly, the downstream plate surface267of the downstream stator disc219band the upstream plate surface (reference numeral omitted) of the third rotating disc220cfrom the upstream side face each other and are substantially parallel. The distance (the depth of the gas flow passage) between the downstream plate surface267of the downstream stator disc219band the third rotating disc220cis set to be the same as H2described above from the inner circumference side to the outer circumference side (from the inlet to the outlet of the Siegbahn spiral groove portion262).

That is, the depth of the flow passage of the Siegbahn exhaust flow passage291gradually narrows from H1to H2in the most upstream Siegbahn spiral groove portions262aserving as the “Siegbahn exhaust flow passage inlet portion”. The depth of the flow passage in the Siegbahn exhaust flow passage291is H2, which is a constant dimension, in each Siegbahn spiral groove portion262except for the turning portions286and287. As such, in the Siegbahn exhaust flow passage291, the section in which the flow passage depth is a constant value (H2) may be referred to as a “constant flow passage depth portion of the Siegbahn exhaust flow passage291”, for example.

In the present example, the value of the depth H2of the flow passage is Ha [mm]. H2is set to Ha [mm] for the reason described below. The term “constant” for the depth H2means that when the unit of dimension is mm (millimeter), the dimension is the same to at least one decimal place without rounding off. As such, when the depth H2(=Ha) is several [mm], for example, any variations within the range of less than 10% (=±0.1 [mm]) are considered as “constant”.

The starting position of the above-described “constant flow passage depth portion of the Siegbahn exhaust flow passage291” (predetermined position at which the region that is continuously constant at a predetermined depth starts) is at the end portion (inlet) on the inner circumference side between the upstream stator disc219aand the second rotating disc220b. The “constant flow passage depth portion of the Siegbahn exhaust flow passage291” is the region that is continuously constant at a predetermined depth.

In the Siegbahn type exhaust mechanism portion201with such a structure, the rotating discs220ato220crotate when the above-mentioned motor121is driven. Relative rotational displacement then takes place between the stator discs219aand219band the rotating discs220ato220c. Also, as indicated by a large number of arrows Q (only some of which are designated by reference numerals) inFIGS.5,6(b), and7, the gas transferred by the turbomolecular pump mechanism portion (including the rotor blades102and the stator blades123, for example) reaches the Siegbahn type exhaust mechanism portion201of the groove exhaust mechanism portion.

The gas reaching the Siegbahn type exhaust mechanism portion201flows into the most upstream Siegbahn spiral groove portions262aserving as the “Siegbahn exhaust flow passage inlet portion” and passes through the flow passage that gradually narrows in the depth direction (the axial direction of the rotor shaft113). Then, the gas flows through the turning portions286and287and Siegbahn spiral groove portions262of a constant depth, and then flows into the Holweck type exhaust mechanism portion301, which will be described below.

The direction of relative rotation between the stator discs219aand219band the rotating discs220ato220cmay be referred to as a ‘tangential direction’ in terms of straight line and a ‘circumferential direction’ in terms of curve.

The Siegbahn type exhaust mechanism portion201may be broken down into further details for explanation. For example, the exhaust flow passage formed between the most upstream first rotating disc220aand the upstream plate surface266of the upstream stator disc219amay be referred to as a “flow passage of a first Siegbahn type exhaust mechanism”.

Also, the exhaust flow passage formed between the second rotating disc220band the downstream plate surface267of the upstream stator disc219amay be referred to as a “flow passage of a second Siegbahn type exhaust mechanism”. The exhaust flow passage formed between the second rotating disc220band the upstream plate surface266of the downstream stator disc219bmay be referred to as a “flow passage of a third Siegbahn type exhaust mechanism”.

Furthermore, the exhaust flow passage formed between the third rotating disc220cand the downstream plate surface267of the downstream stator disc219bmay be referred to as a “flow passage of a fourth Siegbahn type exhaust mechanism”.

When the Siegbahn type exhaust mechanism is divided into multiple parts in this manner, the Siegbahn type exhaust mechanism portion201may be considered as having Siegbahn type exhaust mechanisms in multiple stages. In this case, the “fourth Siegbahn exhaust mechanism” is the Siegbahn exhaust mechanism in the lowest stage.

The above-mentioned Holweck type exhaust mechanism portion301is now described. As shown inFIGS.5and6(a), the Holweck type exhaust mechanism portion301is mainly formed by the threaded spacer131described above. The threaded spacer131is a cylindrical member and has a plurality of helical thread grooves131aengraved in its inner circumference surface.

The upper surface302of the threaded spacer131extends in the radial direction (the direction substantially perpendicular to the axial direction of the rotor shaft113). The upper surface302of the threaded spacer131faces and is substantially parallel to the downstream plate surface (reference numeral omitted) of the rotating disc220cin the lowest stage in the Siegbahn type exhaust mechanism portion201.

Furthermore, on the upper surface302of the threaded spacer131, ridges303and spiral groove portions304are formed in the same manner as the stator discs219aand219bin the Siegbahn type exhaust mechanism portion201. Of these, the ridges303are formed integrally with the upper surface302of the threaded spacer131and protrude.

Furthermore, the ridges303are formed in a spiral shape around the center on the upper surface302of the threaded spacer131. Each ridge303extends along a smooth curve from the circumference edge (inner circumference edge) of the threaded spacer131to the outer circumference edge. The ridges303protrude substantially perpendicularly with respect to the upper surface302, and the protruding amounts of the ridges261are substantially uniform regardless of the position (phase).

The number of the ridges303may be nine in the same manner as the Siegbahn type exhaust mechanism portion201, for example. However, the number of ridges303is not limited to this and may be eight or less or ten or more.

On the upper surface302of the threaded spacer131, the above-mentioned spiral groove portions304are each formed between two adjacent ridges303in a spiral shape. Hereinafter, these spiral groove portions304are referred to as “Holweck spiral groove portions304” to distinguish them from the Siegbahn spiral groove portions262.

As with the Siegbahn spiral groove portions262, the Holweck spiral groove portions304are partitioned and defined by ridges303. Also, the Holweck spiral groove portions304are arranged to form, together with the ridges303, a turning portion287between the Holweck spiral groove portions304and the downstream plate surface267of the downstream stator disc219bof the Siegbahn type exhaust mechanism portion201. Each Holweck spiral groove portion304is a space having a relatively wide outer circumference side (with a wide opening width) and a relatively narrow inner circumference side (with a narrow opening width).

The Holweck spiral groove portions304are also partitioned by the third rotating disc220cfrom the upstream side in the Siegbahn type exhaust mechanism portion201. The distance between the upper surface302of the threaded spacer131and the third rotating disc220cis set to be the same as the above-mentioned H2from the inner circumference to the outer circumference (from the inlet to the outlet of the Holweck spiral groove portion304).

Also, in the Holweck type exhaust mechanism portion301, the inner circumference surface306of the threaded spacer131has the helical thread grooves131adescribed above. This inner circumference surface306faces the outer circumference surface307of the cylindrical portion102dof the rotating body103. The distance (depth) between the inner circumference surface306of the threaded spacer131and the outer circumference surface307of the cylindrical portion102dof the rotating body103is constant over the entire axial length of the inner circumference surface306(from the upper end to the lower end of the inner circumference surface306as viewed in the figure). The value of the distance (depth) matches the H2described above.

The helical thread grooves131aare spatially continuous with the Holweck spiral groove portions304. The connecting portions between the Holweck spiral groove portions304and the thread grooves131amay be referred to as “bent portions”, for example. The helical thread grooves131areach the lower end of the inner circumference surface306, and the lower end of the inner circumference surface306extends to approximately the same position as the lower end of the outer circumference surface307of the cylindrical portion102d.

That is, between the threaded spacer131and the rotating body103, a gas flow passage, which has an L-shaped cross section as viewed inFIG.6(a)(inverted L shape inFIG.6(a)), is formed between the upper surface302of the threaded spacer131and the outer circumference surface307of the cylindrical portion102dof the rotating body103. This gas flow passage is hereinafter referred to as a “Holweck exhaust flow passage” and designated by reference numeral321as shown inFIG.6(a).

The Holweck exhaust flow passage321is continuous with the Siegbahn exhaust flow passage291described above, and receives the gas that has passed through the Siegbahn exhaust flow passage291. The Holweck exhaust flow passage321guides the gas received in the Holweck spiral groove portions304from the outer circumference side to the inner circumference side, and introduces it into the thread grooves131athrough the bent portions. The gas introduced in the thread grooves131ais then guided to the downstream side along the thread grooves131aat the rotating body103rotates.

The Holweck exhaust flow passage321has a constant depth H2. The depth H2of the Holweck exhaust flow passage321matches the depth H2of the constant flow passage depth portion of the Siegbahn exhaust flow passage291in the Siegbahn type exhaust mechanism portion201(the section excluding the Siegbahn exhaust flow passage inlet portion (Siegbahn spiral groove portions262a) and the turning portions286and287).

In other words, in the turbomolecular pump100, the depth of the Holweck exhaust flow passage321, which is the flow passage of the Holweck type exhaust mechanism portion301, is continuously constant at a predetermined depth (H2), and the Siegbahn type exhaust mechanism portion201has a region that is continuously constant at the predetermined depth (H2) from a predetermined middle position (end portion of the Siegbahn exhaust flow passage inlet portion (Siegbahn spiral groove portions262a)).

As described herein, the depth of the flow passage of the Siegbahn type exhaust mechanism portion201(Siegbahn exhaust flow passage291) and the depth of the flow passage of the Holweck type exhaust mechanism portion301(Holweck exhaust flow passage321) are constant (H2), excluding the turning portions286and287in the Siegbahn exhaust flow passage291.

However, the depths H3and H4of the turning portions286and287may be narrowed to H2. In this case, the flow passage of the groove exhaust mechanism portion of the turbomolecular pump100has a region that is continuously constant at the predetermined depth (H2) over the entire region from a predetermined middle position (the end portion of the Siegbahn exhaust flow passage inlet portion (the Siegbahn spiral groove portions262a).

Furthermore, when the Siegbahn type exhaust mechanism portion201is considered as having multiple stages of the first Siegbahn type exhaust mechanism to the fourth Siegbahn type exhaust mechanism as described above, of the multiple Siegbahn type exhaust mechanisms in the turbomolecular pump100, at least the Siegbahn type exhaust mechanism in the lowest stage connected to the Holweck type exhaust mechanism portion301(the fourth Siegbahn type exhaust mechanism in this example) is continuously constant at the predetermined depth (H2).

In the present example, the term “Siegbahn type exhaust mechanism” may refer to a unit of a single Siegbahn spiral groove portion262on one plate surface266,267of a stator disc219a,219b, and may also refer to a unit of Siegbahn spiral groove portions262.

Additionally, the term “Siegbahn type exhaust mechanism” may also refer to an exhaust mechanism formed by a flow passage extending across both the upstream and downstream plate surfaces266and267of each stator disc219a,219b.

Also, in the present example, the Holweck type exhaust mechanism portion301is described as being configured to transfer gas in the radial direction with respect to the axis of the rotor shaft113and to transfer gas in the axial direction of the rotor shaft113. The Holweck exhaust flow passage321is also described as having an L-shaped cross section as shown inFIG.6(a)(inverted L shape inFIG.6(a)).

However, the Holweck type exhaust mechanism portion301may include only the section that transfers gas in the axial direction of the rotor shaft113, and the section that transfers gas in the radial direction may be classified as a part of the Siegbahn type exhaust mechanism portion201. In this case, the Siegbahn type exhaust mechanism portion201may be considered as having not only the first to fourth Siegbahn type exhaust mechanisms but also a fifth Siegbahn type exhaust mechanism. In this case, the fifth Siegbahn type exhaust mechanism is the Siegbahn exhaust mechanism in the lowest stage.

The turbomolecular pump100of the present example described above is structured so that the flow passage depth of the Siegbahn type exhaust mechanism portion201and the flow passage depth of the Holweck type exhaust mechanism portion301are set to a common constant value (H2), thereby achieving the back pressure characteristic as shown inFIGS.8(a) and8(b). The back pressure characteristic of the turbomolecular pump100of the example is described below.

First, the indexes relating to the performance characteristics of vacuum pumps including the turbomolecular pump100include the above-mentioned “back pressure characteristic”. The indexes relating to this “back pressure characteristic” include “back pressure dependence”. This “back pressure dependence” is an index based on the relationship with the above-mentioned auxiliary pump (back pump) installed downstream of the vacuum pump, and indicates susceptibility to the back pressure (represents the back pressure characteristic from a different point of view).

More specifically, for example, when a back pump (not shown) is placed downstream of the turbomolecular pump100, the exhaust of the turbomolecular pump100is performed under the influence of the exhaust performed by the back pump. Also, the performance of the back pump combined with the turbomolecular pump100is not uniform, and may vary depending on the selection by the user of the turbomolecular pump100. Additionally, the exhaust of the turbomolecular pump100is affected by the thickness and layout of piping from the turbomolecular pump to the back pump. The compression ratio, which indicates the compression performance of the turbomolecular pump, is defined as outlet pressure/inlet pressure, and the achievable pressure at the inlet port101of the turbomolecular pump100(inlet pressure) may change with the gas pressure at the outlet port133of the turbomolecular pump100(outlet pressure).

However, as for the side including the inlet port101of the turbomolecular pump100, a change in gas pressure at the inlet port101(inlet pressure) caused by the back pump or the like that is combined on the downstream side causes the exhaust target apparatus of the turbomolecular pump100to be also affected by the back pump or the like. This is not desirable.

As described above,FIGS.8(a) and8(b)show examples of the relationship between the outlet pressure (Pb) and the inlet pressure (Ps) of the turbomolecular pump100of the present example. In the graphs ofFIGS.8(a) and8(b), the horizontal axis represents outlet pressure (Pb), and the vertical axis represents inlet pressure (Ps), both in logarithmic scales. Furthermore, the unit of the outlet pressure (Pb) is [Torr] (same as [torr] described above), and the unit of the inlet pressure (Ps) is [mTorr].

InFIGS.8(a) and8(b), as the back pressure characteristic, a change in the inlet pressure (Ps) on the vertical axis with respect to the outlet pressure (Pb) on the horizontal axis is referred to as the “back pressure dependence of the inlet pressure”.FIG.8(a)shows the back pressure dependence of the inlet pressure in a situation in which the gas being exhausted is of a certain gas type (gas A), andFIG.8(b)shows the back pressure dependence of the inlet pressure in a situation in which the gas to be exhausted is of another gas type (gas B). Hereinafter, “back pressure dependence of the inlet pressure” may be simply referred to as “back pressure dependence”.

InFIG.8(a), reference numerals S1to S7indicate curves representing back pressure dependence with different flow rates. The flow rates of S1to S7are a predetermined flow rate of 1 sccm, a predetermined flow rate of 2 sccm, a predetermined flow rate of 3 sccm, a predetermined flow rate of 5 sccm, a predetermined flow rate of 7 sccm, a predetermined flow rate of 9 sccm, and a predetermined flow rate of 10 sccm, respectively. As for the magnitude relationship of these flow rates, the flow rate increases in the order of predetermined flow rate1to predetermined flow rate10.

Reference numerals T1to T3inFIG.8(b)also indicate the back pressure characteristic (back pressure dependence) with different flow rates. The flow rates of T1to T3are a predetermined flow rate of 2 sccm, a predetermined flow rate of 7 sccm, and a predetermined flow rate of 10 sccm, respectively.

InFIG.8(a), assuming that the origin of the graph is a reference value (Pb=Ps=1 [Torr] in this example) for example, the curve S1at the bottom shows that the inlet pressure (Ps) is substantially constant at a value at substantially the midpoint between the lines of 2 [Torr] and 3 [Torr] when the outlet pressure (Pb) is from 6 [Torr] to a value slightly above 200 [Torr]. Similarly, the other curves S2to S7show substantially constant values from the left end positions of curves S2to S7to positions where the outlet pressures (Pb) are slightly above 200 [Torr].

InFIG.8(b), assuming that the origin of the graph is a reference value (Pb=Ps=1 [Torr] in this example) in the same manner asFIG.8(a)for example, the curve T1at the bottom shows that the inlet pressure (Ps) is substantially constant at a value exceeding 2 [Torr] when the outlet pressure (Pb) is from2[Torr] to around 200 [Torr]. Similarly, the other curves T2and T3show substantially constant values from the left end positions of curves T2and T3to positions where the outlet pressure (Pb) is near 200 [Torr] (for T2) or around 20 [Torr] (for T3).

That is,FIGS.8(a) and8(b)show the presence of outlet pressures (Pb) with which the inlet pressure (Ps) hardly changes regardless of variations of the gas type or flow rate. As such, when the range of outlet pressure (Pb) that allows the inlet pressure (Ps) to be constant and allows the curve to assume a horizontal line is larger, the inlet pressure is considered less susceptible to a change in the outlet pressure (Pb).

In other words, for example, the larger the pressure range of outlet pressure (Pb) up to the point at which a gradient starts and the inlet pressure (Ps) starts to increase as in the right end portion of each of curves S1to S7for gas A inFIG.8(a), the smaller the possibility that the inlet pressure is affected by a change in the outlet pressure (Pb).

In contrast to the turbomolecular pump100having the structure according to the present example,FIGS.13(a) and13(b)show examples of the back pressure characteristic of a turbomolecular pump having a conventional structure in semi-logarithmic scales.FIGS.13(a) and13(b)show, as back pressure characteristic, the back pressure dependence of inlet pressure (Ps) in situations in which different gas types are used.

Of these, curves U1to U8inFIG.13(a)indicate, for a certain gas type (gas1), the back pressure dependence in situations in which the flow rate is a predetermined flow rate of 1 sccm, a predetermined flow rate of 3 sccm, a predetermined flow rate of 5 sccm, a predetermined flow rate of 6 sccm, a predetermined flow rate of 7 sccm, a predetermined flow rate of 8 sccm, a predetermined flow rate of 10 sccm, and a predetermined flow rate of 11 sccm in this order from the bottom in the figure. Here, predetermined flow rate11is a flow rate larger than predetermined flow rate10.

Curves U11to U17inFIG.13(b)indicate, for a certain gas type (gas2) different from the gas type ofFIG.13(a), the back pressure dependence in situations in which the flow rate is a predetermined flow rate of 1 sccm, a predetermined flow rate of 2 sccm, a predetermined flow rate of 4 sccm, a predetermined flow rate of 5 sccm, a predetermined flow rate of 6 sccm, a predetermined flow rate of 7 sccm, and a predetermined flow rate of 8 sccm in this order from the bottom in the figure.

With the gas type shown inFIG.13(a), the range of the substantially flat portion starting from the left end of each of curves U1to U8is shorter with a greater flow rate. Also, with curves U1to U8, the outlet pressure (Pb) at which the inlet pressure starts to increase as shown in the right portion is lower with a greater flow rate.

As for the gas type shown inFIG.13(b), curves U11to U17do not have flat portions on the graph, and the inlet pressure increases in the manner of cubic parabola as the outlet pressure increases.

That is, in the conventional structure shown inFIGS.13(a) and13(b), the rise of the inlet pressure (Ps) occurs with a lower outlet pressure (Pb) than in the structure used in the turbomolecular pump100of the present example. Also, depending on the gas type, the obtained curve may not have a flat portion.

As described above, in the conventional structure, it is difficult to obtain a back pressure characteristic (back pressure dependence in this example) that achieves a curve with a flat portion. Also, depending on the gas flow rate, it is difficult to obtain a curve of back pressure characteristic having a large flat range. However, as illustrated inFIGS.8(a) and8(b), the turbomolecular pump100of the present example can obtain a curve of back pressure characteristic having a large flat range regardless of the gas type and flow rate.

In the turbomolecular pump100of the present example, the “predetermined depth” (=H2(constant value)) relating to the depth of the flow passage described above is determined based on the following concept.FIG.9shows the relationship between the inlet depth and inlet pressure (Pin) of the thread groove exhaust mechanism.

In the turbomolecular pump100of the present example, the flow passage depth of the Holweck exhaust flow passage321is constant (H2) from the inlet to the outlet based on the concept described below. Accordingly, the “inlet depth” is equal to the flow passage depth of the continuous section from the inlet to the outlet of the Holweck exhaust flow passage321. The relationship of “inlet depth”=“outlet depth” is therefore established.

Also, the gas in the Holweck exhaust flow passage321is compressed while being transferred, and the “inlet depth” is preferably set so as to increase the compression efficiency in the Holweck exhaust flow passage321. According to a simulation experiment conducted by the inventor, the “inlet depth” with which the value of the pressure Pin [Torr] on the vertical axis inFIG.9is low is considered as the “inlet depth” that achieves high compression efficiency.

In the simulation experiment conducted by the inventor, as shown inFIG.9illustrating a general tendency, the pressure Pin gradually decreased at first as the “inlet depth” of the experimental model increased. However, when the “inlet depth” value of the experimental model was set to Ha [mm], the pressure P reached the lowest point, and then the pressure P increased as the value of the “inlet depth” increased.

Based on the result of this experiment, the constant value Ha is determined to be the value with which the pressure Pin [Torr] is lowest. This Ha is used as the common depth (H2) for the entire Holweck exhaust flow passage321and the section of the Siegbahn exhaust flow passage319after the inlet portion.

The optimal constant value (H2) of the flow passage depth may vary depending on factors including the number of revolutions of the turbomolecular pump100during operation and the diameter dimensions of relevant components (such as the stator discs219aand219band the rotating discs220ato220c). Thus, the optimal flow passage depth (H2) with which the peak of the exhaust performance (including compression performance) is achieved is preferably determined taking into account the above factors. The flow passage depth is usually designed in the range of at least 2 mm to 10 mm (more preferably 3 mm to 5 mm) approximately.

With the turbomolecular pump100of the present example, the reasons for the improved back pressure characteristic shown inFIGS.8(a) and8(b)are yet to be fully analyzed, but the following explanation may be given through the modeling shown inFIG.10.

FIG.10is a diagram illustrating the characteristics of a general groove exhaust mechanism portion. In the description of the present example herein, the groove exhaust mechanism portion (FIG.6(a)) of the turbomolecular pump100is modeled. As described above, the groove exhaust mechanism portion according to the present disclosure includes the Siegbahn type exhaust mechanism portion201and the Holweck type exhaust mechanism portion301. The inlet portion of the groove exhaust mechanism portion (Siegbahn exhaust flow passage inlet portion) is formed by the Siegbahn spiral groove portions262a, which narrow as they extend further into the flow passage to have the flow passage depth of H2.

In the model shown inFIG.10, the section corresponding to the groove exhaust mechanism portion is designated by reference numeral321, and its one end portion (upper end portion in the figure) is designated by “262a”, which is the same reference numeral as the Siegbahn spiral groove portions that serve as the Siegbahn exhaust flow passage inlet portion, for convenience.

In the model shown inFIG.10, reference numeral322indicates a stator model formed by combining and then halving the stator disc219a,219bforming the Siegbahn exhaust flow passage291and the threaded spacer131forming the Holweck exhaust flow passage321. Reference numeral323indicates a rotating model formed by halving the rotating body103including the rotating discs220ato220cof the Siegbahn exhaust flow passage291.

Symbol K in the figure indicates the rotation axis, and arrow J indicates that the rotating model323rotates around the rotation axis K. Reference numeral H1indicates the depth (flow passage depth) of the opening281on the upstream side (outer circumference side) of the Siegbahn spiral groove portions262a, as described above. Reference numeral H2indicates the constant flow passage depth portion of the Siegbahn exhaust flow passage291described above and a constant value as the flow passage depth of the Holweck exhaust flow passage321.

FIGS.11(a) and11(b)are graphs illustrating the exhaust performance of the model shown inFIG.10corresponding to the flow passage depth. Of these, the horizontal axis in the graph ofFIG.11(a)represents “flow passage position”, and the vertical axis represents “flow passage depth”. The “flow passage position” on the horizontal axis represents the position in the groove exhaust mechanism portion311. Moving the observation point from the inlet (upper end inFIG.10) toward the outlet (lower end inFIG.10) of the groove exhaust mechanism portion311is expressed here as “the flow passage position increases”.

InFIG.11(a), solid line V1indicates the relationship between the flow passage position and the flow passage depth in the model shown inFIG.10. Broken line W1indicates the relationship between the flow passage position and the flow passage depth according to a conventional structure.

The conventional structure as used herein refers to a structure in which, as indicated by broken line W1, the flow passage depth changes slowly and gradually as the flow passage position increases so that the flow passage depth decreases. In contrast, with the model shown inFIG.10, as the flow passage position increases in the inlet portion262a(Siegbahn exhaust flow passage inlet portion) of the groove exhaust mechanism portion311, the flow passage depth decreases sharply as compared with the conventional structure, as indicated by solid line V1.

However, when the flow passage position further increases and the observation point is positioned in the constant flow passage depth portion of the Siegbahn exhaust flow passage291beyond the inlet portion262aof the groove exhaust mechanism portion311, the flow passage depth becomes a constant value (H2). Then, the flow passage depth maintains the constant value (H2) even when the flow passage position increases (even in the Holweck exhaust flow passage321).

The conventional structure in which the flow passage depth gradually decreases from the inlet to the outlet of the groove exhaust mechanism portion311has a potential for improving the exhaust performance such as “exhaust speed” and “compression performance”. Thus, it is relatively easy to improve the exhaust performance. However, the structure may increase the possibility of gas backflow and therefore should continuously and smoothly exhaust (transfer) the incoming gas.

In contrast, by maintaining the flow passage depth constant as indicated by solid line V1obtained by modeling the turbomolecular pump of the present example, backflow can be easily prevented with a simple design.

The horizontal axis in the graph ofFIG.11(b)represents “flow passage position”, and the vertical axis represents “pressure”. The “flow passage position” on the horizontal axis is the same as inFIG.11(a). The “pressure” on the vertical axis indicates the pressure of the gas in the flow passage.

InFIG.11(b), broken line W2indicates a certain pressure change that is deemed ideal. With the pressure change indicated by broken line W2, the pressure increases at a constant rate of change as the flow passage position increases. Broken line W3indicates a pressure change in a situation in which gas backflow described above or the like occurs and lowers the exhaust performance. With the pressure change indicated by broken line W3, the pressure increases with a gradient less steep than W2described above as the flow passage position increases.

In contrast, solid line V2indicates the pressure change according to the model ofFIG.10. In the model ofFIG.10, at the inlet portion of the groove exhaust mechanism portion (groove exhaust mechanism portion inlet portion, Siegbahn spiral groove portions262a), as the flow passage position increases, the pressure rises sharply as compared with W2and W3. This portion efficiently increases the degree of gas compression.

Subsequently, although the rate of change decreases, the pressure gradually increases as the flow passage position increases. When the observation point is positioned in the constant flow passage depth portion of the Siegbahn exhaust flow passage291beyond the inlet portion262aof the groove exhaust mechanism portion311, the flow passage depth becomes the constant value (H2). Then, the pressure at the outlet of the groove exhaust mechanism portion311becomes a value between W2and W3described above.

In other words, as in the model ofFIG.10, when the depth of the flow passage of the groove exhaust mechanism portion311is set constant (H2) from a middle point (a middle flow passage position), the compression performance is limited and not significantly improved. However, the backflow of gas is less likely to occur, and the pressure from the middle stage to the end stage of the groove exhaust mechanism portion311can be closer to W2, which is the ideal pressure.

It is clear that the compression performance can be improved by further increasing the length of the flow passage depth H2.

The region in which the flow passage depth is a constant value (H2) (constant region) is preferably determined so that gas backflow is minimized (less likely to occur) in the flow passage, even if the peak of the compression performance is not obtained.

The above-mentioned gas backflow can be explained as follows.FIG.12(a)shows a model for Couette-Poiseuille flow between parallel flat plates. First, a steady flow between two parallel flat plates is considered. One of the plates is stationary and the other is moving at a velocity of u. Thus, the Navier-Stokes equation is simplified, and the following expression (Expression 1) is obtained.

∂p∂x=μ⁢∂2u∂y2[Math.1]

Here, in Expression 1, u is a function of y only, and p is a function of x only. Thus, Expression 1 can be expressed as an ordinary differential equation (Expression 2).

dpdx=μ⁢d2⁢udy2[Math.2]

The boundary conditions are y=0: u=0, y=h: u=U.

The solution can be easily obtained by integration and expressed by the following expression (Expression 3).

u⁡(y)=U⁢yh+h22⁢μ⁢(-dpdx)[yh⁢(1-yh)][Math.3]

This solution is a superposition of a simple shear flow (first term, Couette flow) and a parabolic velocity profile (second term, Poiseuille flow).

Dividing both sides of Expression 3 by U gives the following expression (Expression 4).

u⁡(y)U=yh+h22⁢μ⁢U⁢(-dpdx)[yh⁢(1-yh)]=yh+h22⁢μ⁢U⁢(-dpdx)[y⁡(h-y)h]=yh+h22⁢μ⁢U⁢(-dpdx)[y⁡(h-y)][Math.4]

Here, the shape changes depending on the positive or negative sign of the dimensionless pressure gradient (Expression 5) of the second term on the right side of Expression 4. As shown in the graph ofFIG.12(b), when P is less than −1, a backflow part is present in which u/U is negative.

P=h22⁢μ⁢U⁢(-dpdx)[Math.5]

As can be identified from Expressions 4 and 5, a larger h increases the backflow component. In other words, it can be considered that a greater flow passage depth tends to increase the possibility of backflow.

As described above, according to the turbomolecular pump100of the present example, the flow passage depth of the groove exhaust mechanism portion is continuously constant (H2) from a middle section of the Siegbahn type exhaust mechanism portion201to the outlet of the Holweck type exhaust mechanism portion301. This achieves excellent back pressure characteristic as shown inFIGS.8(a) and8(b). As a result, the present example can provide the turbomolecular pump100with excellent exhaust performance.

Moreover, as shown inFIGS.5and6(a), the Siegbahn type exhaust mechanism portion201and the Holweck type exhaust mechanism portion301are continuously formed in the groove exhaust mechanism portion. The Siegbahn type exhaust mechanism portion201and the Holweck type exhaust mechanism portion301form an exhaust flow passage in the groove exhaust mechanism portion. As such, as compared to a configuration that includes only one of the Siegbahn type exhaust mechanism portion201and the Holweck type exhaust mechanism portion301, a long exhaust flow passage can be easily ensured. This also contributes to provide the turbomolecular pump100with excellent exhaust performance.

Furthermore, in the Siegbahn type exhaust mechanism portion201, a plurality of flow passages (flow passages of the first Siegbahn type exhaust mechanism to the fourth Siegbahn type exhaust mechanism) are spatially connected via the turning portions286and287to form the Siegbahn exhaust flow passage291. The Siegbahn type exhaust mechanism portion201has a meandering flow passage as shown inFIGS.5and6(a). Thus, the long Siegbahn exhaust flow passage291can be easily ensured. This also contributes to provide the turbomolecular pump100with excellent exhaust performance.

There may be a possibility that the presence of the turning portions286and287causes backflow or stagnation of the gas, resulting in a decrease in performance. However, since the gas flow passage is lengthened as much as possible, it is assumed that backflow and stagnation are prevented as much as possible. Also, at the turning portions286and287, due to the drag (drag force) effect acting when the gas flows, a pressure drop does not occur, or a pressure drop is not excessive, if any.

As shown inFIGS.5and6(a), the Holweck exhaust flow passage321in the Holweck type exhaust mechanism portion301is formed to have an L-shaped cross section. As such, as compared to a configuration that has an exhaust flow passage only on the inner circumference surface306of the threaded spacer131, the exhaust flow passage can be extended by the length of the Holweck spiral groove portion304. This also contributes to provide the turbomolecular pump100with excellent exhaust performance.

Furthermore, in the present example, as shown inFIGS.5and6(a), the groove exhaust mechanism portion is in the stage following (downstream of) the turbomolecular pump mechanism portion, which includes the rotor blades102(102a,102b,102c, . . . ), the stator blades123(123a,123b,123c, . . . ), and the like, and is formed so as to be spatially continuous with the turbomolecular pump mechanism portion. As a result, a longer exhaust flow passage can be easily formed by the groove exhaust mechanism portion and the exhaust flow passage of the turbomolecular pump mechanism portion. This also contributes to provide the turbomolecular pump100with excellent exhaust performance.

The turbomolecular pump100of the present example may also be described as follows. When a long gas flow passage is ensured as in the turbomolecular pump100, provided that the opening width and depth are common, the capacity of the space for the flow of gas (the space that contains gas per unit time) generally increases. This is considered to be one of the reasons that ensuring a long gas flow passage improves the back pressure characteristic.

That is, as indicated by broken line W1inFIG.11(a), when the flow passage depth changes from the inlet to the outlet of the groove exhaust mechanism portion, the exhaust performance relating to “exhaust speed” and “compression performance” can be improved as described above. However, as for the “back pressure characteristic”, ensuring a long flow passage alleviates the influence of changes in the flow passage depth from the inlet to the outlet of the groove exhaust mechanism portion. According, a longer flow passage length of the groove exhaust mechanism portion can moderately improve the exhaust performance, resulting in satisfactory “back pressure characteristic”.

It is also conceivable that one of the factors behind the excellent back pressure characteristic as shown inFIGS.8(a) and8(b)is that the ultimate pressure is kept low by the Siegbahn spiral groove portions262a(the inlet portion of the groove exhaust mechanism portion) serving as the inlet portion of the groove exhaust mechanism portion.

That is, the ultimate pressure is a factor concerning the compression ratio, and, in general, the higher the compression ratio, the lower the ultimate pressure. Providing the Siegbahn spiral groove portions262aas the inlet portion of the groove exhaust mechanism portion allows the opening of the inlet portion to be larger than the constant value (H2) of depth, increases the compression ratio, and keeps the ultimate pressure low.

It is also conceivable that one of the factors behind the excellent back pressure characteristic as shown inFIGS.8(a) and8(b)is that the turning portions286and287are formed in the Siegbahn exhaust flow passage291, in addition to that the flow passage depth is constant (H2) and that the opening of the inlet portion of the Siegbahn spiral groove portion262ais large.

That is, it is conceivable that the pressure distribution at the turning portions286and287in the above configuration advantageously allows the gas in the Siegbahn exhaust flow passage291to be less susceptible to stagnation or backflow.

Stagnation and backflow of gas can lower the exhaust performance. Causes of stagnation (such as local stagnation in the flow passage) include a reduced diameter (narrowing) of the flow passage and a decrease in conductance. Causes of backflow include a negative pressure gradient.

In the turbomolecular pump100of the present example, the Siegbahn exhaust flow passage291is formed in multiple stages that lie on top of one another in the axial direction (the axial direction of the rotor shaft113) with the turning portions286and287interposed therebetween. Also, in the Holweck type exhaust mechanism portion301, the Holweck exhaust flow passage321is formed to have an L-shaped cross section.

For this reason, even though the Siegbahn type exhaust mechanism portion201and the Holweck type exhaust mechanism portion301are arranged in the axial direction, the overall size (height dimension) of the turbomolecular pump100in the axial direction is limited as much as possible.

As for the Siegbahn spiral groove portion262and the Holweck spiral groove portion304, it is desirable to select appropriate widths and areas of the flow passages because an excessively wide flow passage increases the possibility of backflow.

The present disclosure is not limited to the example of the present disclosure described above, and various modifications are possible. For example, the number of stator discs is not limited to two, and the number of rotating discs is not limited to three.

Also, the objects that include the ridges261or the groove portions262are not limited to the stator discs219aand219b, and may be the rotating discs220ato220c. Furthermore, it is possible to combine a stator disc and a rotating disc having ridges261or groove portions262. For example, ridges261or groove portions262may be formed on one of the plate surfaces of a rotating disc and one of the plate surfaces of a stator disc. Also, ridges261or groove portions262may be provided only on one side facing a rotating disc of each of the upper and lower (upstream and downstream) stator discs on opposite sides of the rotating disc.

It should be noted that the present disclosure is not limited to the above-described examples, and various modifications can be made by the ordinary creative ability of those skilled in the art within the scope of the technical idea of the present disclosure.