JP5148104B2 - Vacuum measuring method and vacuum gauge - Google Patents

Description

  The present invention relates to a vacuum gauge which is a pressure gauge for measuring a vacuum, and more particularly to a method for obtaining pressure from the number of ions generated by ionizing gas molecules and a vacuum gauge using the method.

There are various types of vacuum gauges, and they are properly used depending on the purpose.
For example, an ionization vacuum gauge is known as one means for measuring the pressure inside a vacuum chamber such as a vacuum processing apparatus.
Further, there is known a mass spectrometer having a structure similar to that of an ionization vacuum gauge and capable of knowing a partial pressure by detecting only ions having a specific mass / charge ratio.

  An ionization vacuum gauge usually has a cathode electrode that is an electron source, an anode electrode that captures electrons, an ion collecting electrode that captures ions, an ion current detector that detects an ion current due to the captured ions, a control device that controls them, an ion It comprises an arithmetic unit that calculates pressure from an ionic current obtained by a current detector, a storage device that stores various parameters necessary for calculating pressure, and a display that displays the calculated pressure.

  When electrons emitted from the cathode electrode toward the anode electrode controlled to a positive potential collide with gas molecules, the gas molecules are ionized, and some of them reach the ion collecting electrode, and the ionic current Will occur. Since the ion current is detected by an ion current detector and the magnitude thereof is usually compared with the gas molecule density, the ion current is obtained between the ion current Ii (A) and the pressure P (Pa) obtained at the gas molecule density. Is known to hold the relationship of the following formula 1 (for example, see Non-Patent Document 1).

Ii = S × Ie × P (Formula 1)


Ii: Ion current (A)
S: Sensitivity of ionization gauge (Pa ^-1 )
Ie: Electron current (A)
P: Pressure (Pa)

  Based on the relationship of Equation 1, the indication value Pint of the conventional ionization gauge has been calculated by Equation 2 from the obtained ion current Ii.

Pint = Ii / (S × Ie) (Formula 2)

  According to Equation 1, if the sensitivity S and the electron current Ie are always constant, the ion current Ii is proportional to the pressure, and Pint = P. However, in practice, as the pressure increases, interactions such as collision between ions and gas molecules cannot be ignored, and the mean free path of electrons emitted from the ions and the cathode electrode is shortened. For this reason, the ratio of ions reaching the ion collecting electrode with respect to the pressure decreases, and the sensitivity of the ionization vacuum gauge decreases. For this reason, the indication value Pint calculated by the conventional calculation formula 2 has a problem that it deviates from the true pressure value as the pressure increases.

Therefore, the upper limit of the measurement pressure range of a general ionization vacuum gauge is about 10 ⁻³ Torr (≈0.13 Pa) (for example, see Non-Patent Document 2). FIG. 2A shows an example in which the indication value of the ionization gauge is actually recorded using the indication value of the diaphragm gauge, which is said to have relatively high reliability, as the reference pressure value. It can be seen that there is a deviation from the indicated value of the diaphragm vacuum gauge as the pressure rises from around 0.5 Pa.

  Therefore, for example, in a conventional ionization vacuum gauge, “a survey is performed in advance in several pressure states, and the data group obtained by the survey is compared with the ion current at the time of measurement. The method for calculating the sensitivity from the above "is presented (see Patent Document 1).

Similar to the pressure dependence of the sensitivity of the ionization vacuum gauge, the pressure dependence of the partial pressure sensitivity of the mass spectrometer has been pointed out as a problem in the partial pressure measurement.
Therefore, for example, in a conventional mass spectrometer, “in a quadrupole mass spectrometer having means for measuring total pressure ion current and determining the total pressure, the partial pressure instruction value of the quadrupole mass spectrometer is obtained by the total pressure obtained. Is proposed "(see Patent Document 2).

JP-A-5-203524 Japanese Patent Application No. 11-31473 "Experimental Physics Course 4 Vacuum Technology" Kyoritsu Shuppan 1985 P349 "Vacuum Technology Practical Reader" Ohmsha 1983 P118

  As described in Patent Document 1, “Study in advance in several pressure states, and the data group obtained by the survey is compared with the ion current at the time of measurement. According to the “method for calculating sensitivity”, it is assumed that the sensitivity between two points has a linear relationship.

  However, in practice, as can be seen from the uncorrected indicated value graph shown in FIG. 2A, the relationship between the sensitivity and pressure of the conventional vacuum gauge is non-linear. Therefore, in order to calculate the sensitivity with higher accuracy, the linearity between the data can be obtained by conducting a sensitivity survey under more pressure conditions and recording the result in a storage device built in the vacuum gauge. It was necessary to increase. Therefore, there is a problem that the storage capacity of the storage unit 50 in FIG. 4 of Patent Document 1 has to be increased as the accuracy of the calculated sensitivity is increased.

Further, as described above, the pressure dependence of the partial pressure sensitivity of the mass spectrometer has been pointed out as a problem in the partial pressure measurement as well as the pressure dependence of the sensitivity of the ionization vacuum gauge.
Therefore, as described in Patent Document 2, in the quadrupole mass spectrometer having means for measuring the total pressure ion current and determining the total pressure, the partial pressure instruction value of the quadrupole mass spectrometer is obtained by the total pressure obtained. According to the method for correcting the above, it is premised that a highly accurate total pressure instruction value can be obtained.

  However, in this method, when the total pressure command value is calculated according to the conventional formula (Formula 2), the higher the pressure, the more the total pressure command value deviates from the true pressure. There was a problem that the accuracy decreased.

  A correction method with practical accuracy even under a relatively high pressure within a pressure range in which an ionization vacuum gauge or mass spectrometer is used, and a storage device for the ionization vacuum gauge or mass spectrometer using the same What does not increase the storage capacity is desired.

  An object of the present invention is to correct the indicated value of the vacuum gauge in consideration of the fact that the sensitivity of the vacuum gauge also changes as the vacuum pressure changes.

  Specifically, the ion generated near the anode electrode of the vacuum gauge is corrected in the direction of increasing the ion current value, which is the output of the ion detector, based on the rate at which the ions reach the collector electrode.

  According to the ionization vacuum gauge to which the present invention is applied, it is possible to indicate a pressure with higher accuracy than before even in a relatively high pressure range of vacuum.

  In addition, according to the mass spectrometer to which the present invention is applied, the accuracy of the partial pressure indication value can be improved more than in the past even in a relatively high vacuum range.

Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings. FIG. 1 shows a vacuum gauge according to an embodiment of the present invention.
The vacuum gauge according to the embodiment of the present invention includes an ionization vacuum using a cathode electrode 1 that is an electron source, an anode electrode 2 that captures electrons, a collector electrode 3 that captures ions, and a control device 21 that controls them. It functions as a total of 20.

  The control device 21 is connected to the cathode electrode 1, the anode electrode 2, and the ion collection electrode 3, and performs overall control thereof. Specifically, the applied voltage between the cathode electrode 1 and the anode electrode 2 is controlled to be constant. Further, an ion current that is an output of the ion collection electrode 3 is received.

  More specifically, the control device 21 controls the ion current detector 4 that detects the ion current Ii that is the output of the collector electrode 3 and the applied voltage between the cathode electrode 1 and the anode electrode 2 to be constant. The control device 5 is an input / output interface that receives data corresponding to the ion current obtained by the ion current detector 4, the calculator 6 that calculates the vacuum pressure by calculating according to a program built in advance, and calculates the vacuum pressure. It has a storage device 7 for storing various parameters required at the time, and a display 8 for displaying the calculated vacuum pressure.

  The control device 5 and the arithmetic unit 6, the arithmetic unit 6 and the storage device 7, and the arithmetic unit 6 and the display unit 8 are connected by a data bus. Data corresponding to the ion current received by the control device 5 is sent to the computing unit 6 through the data bus 15a. The computing unit 6 takes out various parameters necessary for calculating the vacuum pressure from the storage device 5 through the data bus 15b in accordance with a program built in advance. Further, the arithmetic unit 6 sends data corresponding to the corrected ion current Ii obtained by calculation according to a program built in advance to the display device 8 through the data bus 15c, and the display device 8 displays the indication value of the vacuum gauge. Display as.

  Electrons emitted from the cathode electrode 1 serving as the cathode are emitted toward the anode electrode 2 serving as the anode, but some of them collide with gas molecules while flying through the ionization chamber formed by the anode electrode 2. At this time, the gas molecules are ionized and some of them reach the ion collecting electrode 3. Since the ions that have reached the ion collection electrode 3 have a charge, an ion current Ii is generated. This ion current Ii is converted into an indicated value by the ion current detector 4 connected to the collector electrode 3. This instruction value is taken into the control device 21 and used for calculation.

The control device 21 performs calculations according to the following steps and displays the results on the display.
(A) A step of obtaining pressure (conventional indication value) proportionally from ion current;
(B) transforming a function indicating the pressure dependence of sensitivity;
(C) Substituting measurement conditions into a function indicating the pressure dependence of the deformed sensitivity to obtain a pressure correction function;
(D) calculating the flight distance of the ions and substituting them into the pressure correction function;
(E) Substituting the pressure (conventional indication value) obtained proportionally from the ionic current into a correction function to obtain a corrected pressure (indication value in which the pressure dependency of sensitivity is corrected);
To perform the calculation.

That is, the output from the ion current detector 4 that detects the ion current Ii is in the relationship of Equation 1 as described above.

Ii = S × Ie × P (Formula 1)


Ii: Ion current (A)
S: Sensitivity of ionization gauge (Pa ^-1 )
Ie: Electron current (A)
P: Pressure (Pa)

The pressure (conventional indication value) is proportionally obtained from the ionic current, which is obtained by Equation 2.
This corresponds to the process (A) of the calculation process of the control device 21.

Pint = Ii / (S × Ie) (Formula 2)

Here, when considering the phenomenon that the sensitivity S also changes with the change in pressure, the relationship between the ion current Ii ′ and the pressure can be expressed as follows.

Ii ′ = S (P) × Ie × P
= S ₀ × F (P) × Ie × P (Formula 3)

S (P): Pressure dependent sensitivity (= S ₀ × F (P))
S ₀ : Sensitivity when the pressure is sufficiently low (for example, 1 × 10 ⁻⁴ Pa or less)
F (P): function indicating pressure dependence of sensitivity

In the present embodiment, instead of the 1 × 10 ^-4 Pa or less sensitivity S _0, the pressure range to be measured shall be appropriately selected S _0. To apply the present invention, as S _0, preferably 1 × 10 ^-4 Pa or less, then the adoption in the range of 1 × 10 ^-4 Pa~0.05Pa may.

The indication value Pint of the conventional ionization vacuum gauge is regarded as a constant value (= S ₀ ) without considering the pressure dependence of the sensitivity S. Therefore, the following expression 4 is expressed from Expression 2 and Expression 3.

Pint = Ii ′ / (S ₀ × Ie)
= S ₀ × F (P) × Ie × P / (S ₀ × Ie)
= F (P) × P (Formula 4)

Equation 4 shows that the indicated value Pint of the conventional ionization gauge is a value obtained by multiplying the true pressure P by the pressure dependency F (P) of sensitivity.
By transforming Equation 4, the true pressure P can be expressed as Equation 5.

P = Pint / F (P) (Formula 5)

When the factor that the sensitivity depends on the pressure is considered to be only due to the collision between the ion and the gas phase molecule, the function F (P) that represents the pressure dependency of the sensitivity is the ion generated near the anode 2. This is considered to be the same as the change in the rate of reaching the ion collection electrode 3 and is given by the following equation.
This corresponds to the process (B) of the calculation process of the control device 21.

F (P) = exp (−L / λ) = (− LπPD ² / kT) (Formula 6)

Where λ = kT / πPD ²
L: Flight distance of ions
λ: ion mean free path (m)
D: Ion diameter (m)
k: Boltzmann constant (J / K)
T: Absolute temperature (K)

When the ion diameter D is considered to be 3.78 × 10 ⁻¹⁰ (m), which is the same as nitrogen used for calibration of an ionization vacuum gauge, at room temperature T = 300 (K), the arrival of the ion according to Equation 6 The rate F (P) is calculated as follows:

F (P) = exp (−108 × L × P) (Expression 7)

In addition, although the diameter D of ion was considered to be the same as nitrogen (N ₂ ) that is usually used for calibration of an ionization vacuum gauge, it differs depending on the type of gas in the vacuum atmosphere. This amount of calibration is known as a relative sensitivity coefficient, and is performed by having a data table in the storage device in advance. For example, data on helium (He), water (H ₂ O), oxygen (O ₂ ), etc. may be incorporated.

  In the range of 10 Pa or less, since the difference between Pint and P is small in a general ionization gauge, P in Equation 7 is approximated by a pressure value Pint obtained by a conventional calculation method. Since the difference between P and Pint is generally small, the effect of this approximation on correction is small. The pressure dependence F (P) of the sensitivity after approximation is shown in the following equation.

F (P) ≈F (Pint) = exp (−108 × L × Pint) (Formula 8)

The pressure instruction value Pcor obtained by applying the equation 8 to the equation 5 and correcting the pressure dependency of the sensitivity can be expressed by the following equation.
This corresponds to the step (C) of the calculation step of the control device 21.

Pcor = Pint / F (Pint)
= Pint × exp (108 × L × Pint) (Formula 9)

  If the ion flight distance L is determined by the above modification, the pressure command value Pcor corrected for pressure dependency of sensitivity can be easily derived from the pressure value (conventional command value) obtained proportionally from the ion current Ii. I understand.

Here, the flight distance L of the ions is determined, but various methods for determining L are conceivable, so that they are appropriately determined according to the applied environmental conditions.
This corresponds to the step (D) of the calculation step of the control device 21.

  The actual ion flight distance L has a distribution depending on the difference in the location where ions are generated and the flight trajectory. Since the distribution varies depending on the shape of the anode electrode of the ionization vacuum gauge and the geometrical structure such as the positional relationship with the ion collection electrode, it is not easy to accurately grasp the distribution. However, the average is expected to be about the same as the distance between the anode electrode and the collector electrode (a few to several times) in a general ionization gauge. An example of sensitivity correction when L is estimated to be the same as the distance between the anode electrode 2 and the collector electrode 3 will be shown in Example 1 later.

  Further, as another effective means for obtaining L, there is a method of calculating from the relationship between the pressure indication value of the ionization vacuum gauge investigated in advance and the reference pressure. In a relatively high pressure situation where the decrease in sensitivity is significantly observed (0.5 Pa or more in the vacuum gauge used in the embodiment of the present invention), the ratio between the pressure Pint obtained by the conventional calculation method and the reference pressure value Pref Is considered to be equivalent to the arrival rate F (Pint) of ions, and therefore can be expressed as Equation 10.

Pint / Pref = F (Pint)
= Exp (-108 * L * Pint) (Formula 10)

  If the values of Pint and Pref are known from Equation 10, L can be estimated by calculation. An example of sensitivity correction in this case is shown in Example 2 later.

As described above, by using the formula 9 or the formula 10, it is possible to correct the indication value of the vacuum gauge in consideration that the sensitivity of the vacuum gauge also changes with the change of the vacuum pressure.
Thereby, based on the rate at which ions generated near the anode electrode 2 of the vacuum gauge reach the collector ion electrode 3, the ion current value that is the output of the ion current detector 4 is corrected in the increasing direction, Even in the relatively high pressure range of the vacuum, the effect that the pressure can be indicated with higher accuracy than before can be obtained.

  In the following Examples 1 and 2, the pressure obtained by proportionally calculating (E) the ionic current of the calculation of the control device 21 (conventional indication value) was substituted into the correction equation and corrected (the pressure dependency of sensitivity was corrected). This corresponds to the step of obtaining the indicated value. That is, two examples of different methods for determining the actual flight distance L of ions are respectively illustrated.

In the ionization vacuum gauge used in this example, the distance between the anode electrode 2 and the collector electrode 3 was 2 mm. From this, when L is estimated to be the same as the distance between the anode electrode 2 and the collector electrode 3, L in Equation 9 is set to 2 × 10 ⁻³ (m), and Pcor is as shown in Equation 11.

Pcor = Pint / F (Pint)
= Pint × exp (0.216 × Pint) (Formula 11)

  FIG. 2B shows an example in which the instruction value is actually corrected using the expression 11 according to this embodiment. It can be seen that the reference value of the reference vacuum gauge matches well with the conventional instruction value (a) which is not corrected.

In the ionization vacuum gauge 20 used in the present example, the pressure value indicated by the conventional calculation method was 1.4 Pa with respect to the indicated value 2 Pa of the reference vacuum gauge. When L is calculated from the relationship between the pressure indication value of the ionization gauge and the reference pressure that has been investigated in advance, if Pint = 1.4 and Pref = 2 are substituted into Equation 10, L = 2.36 × 10 ⁻³ (m) Is calculated. Applying this to L in Equation 9 yields Equation 12.

Pcor = Pint / F (Pint)
= Pint × exp (0.254 × Pint) (Formula 12)

  FIG. 2C shows an example in which the instruction value is actually corrected using Expression 12 according to this embodiment. It can be seen that the reference value of the vacuum gauge referred to is in good agreement with the graph of FIG.

In summary, according to the present invention, the function F (Pint) indicating the pressure dependency of the sensitivity is obtained as a specific function, and the sensitivity S that has been regarded as constant without depending on the pressure in the past is the sensitivity S when the pressure is sufficiently low. _By multiplying ₀ and the sensitivity correction function F (Pint), the sensitivity that changes with the pressure change is appropriately corrected, and in the pressure region of 0.5 Pa or higher, with higher accuracy than the conventional ionization gauge It is possible to indicate the pressure.

Next, a mass spectrometer 30 which is another vacuum gauge according to the embodiment of the present invention will be described.
As shown in FIG. 3, the vacuum gauge of the present embodiment has a cathode electrode 1 that is an electron source, an anode electrode 2 that captures electrons, a collector electrode 3 that captures ions, and a specific mass / charge ratio. A mass spectrometer including an analyzer 23 that selectively distributes only the ions that it has, a collection ion electrode 25 for measuring partial pressure for capturing ions distributed by the analyzer 23, and a control device 31 that controls them. 30. The analysis unit 23 was a quadrupole type analysis unit.

  The control device 31 is connected to the cathode electrode 1, the anode electrode 2, the ion collection electrode 3, the analysis unit 23, and the ion collection electrode 25, and performs overall control thereof. Specifically, the applied voltage between the cathode electrode 1 and the anode electrode 2 is controlled to be constant. Further, by applying a DC voltage and an alternating current of a specific frequency to the analyzing unit 23 together, control is performed so that only ions having a specific mass / charge ratio are selectively distributed. Further, an ion current that is an output of the ion collection electrode 25 is received.

  More specifically, the control device 31 controls the ion current detector 4 that detects the ion current Ii that is the output of the collector electrode 3 and the applied voltage between the cathode electrode 1 and the anode electrode 2 to be constant. In addition, the analyzer 23 is controlled to selectively distribute only ions having a specific mass / charge ratio, and is a control that is an input / output interface that receives data corresponding to the ion current obtained by the ion current detector 4. A device 5 ', a calculator 6 for calculating a vacuum pressure by calculating according to a program built in advance, a storage device 7 for storing various parameters necessary for calculating the vacuum pressure, and the calculated vacuum pressure are displayed. Display 8.

  The control device 5 ′ and the computing unit 6, the computing unit 6 and the storage device 7, and the computing unit 6 and the display unit 8 are connected by a data bus. Data corresponding to the ion current received by the control device 5 ′ is sent to the computing unit 6 through the data bus 15 a. The computing unit 6 takes out various parameters necessary for calculating the vacuum pressure from the storage device 5 through the data bus 15b in accordance with a program built in advance. Further, the calculator 6 sends data corresponding to the corrected ion current obtained by calculation according to a program built in advance to the display device 8 through the data bus 15c to display the indicated value.

  Electrons emitted from the cathode electrode 1 serving as the cathode are emitted toward the anode electrode 2 serving as the anode, but some of them collide with gas molecules in a vacuum during the flight to the anode electrode 2. At this time, the gas molecules are ionized, and some ions reach the ion collection electrode 25 by selectively distributing only ions having a specific mass / charge ratio in the analysis unit 23. Other part of the ions reaches the ion collection electrode 3. Since the ions that have reached the collection electrode 25 and the collection electrode 3 have a charge, a collection ion current is generated. This collected ion current is taken into the control device 31 connected to the collected ion electrode 25 and the collected ion electrode 3 and used for calculation.

  That is, the mass spectrometer according to the embodiment of the present invention has an analysis unit 23 that selectively distributes only ions having a specific mass / charge ratio between the cathode electrode 1 and the ion collection electrode 25 of FIG. Thus, the partial pressure of the gas in the vacuum can be measured.

  Here, the ion current of ions having a specific mass / charge ratio, which is the output of the ion current detector 27, is increased in a direction based on the total pressure obtained by the first or second embodiment of the present invention. to correct. Thereby, an accurate partial pressure instruction value can be obtained. As a calculation formula for this correction, a method disclosed in Patent Document 2 is known.

  As described above, according to the mass spectrometer 30 of the embodiment of the present invention, an indication value of partial pressure can be obtained with higher accuracy than in the past even in a vacuum range of relatively high pressure.

  The ionization vacuum gauge 20 used when correcting the partial pressure instruction value of the mass spectrometer 30 of the present invention is also corrected using the ionization vacuum gauge 20 with the sensitivity corrected as in the present invention. Thus, sensitivity correction of the mass spectrometer 30 can be performed more appropriately, and the accuracy of the partial pressure instruction value of the mass spectrometer 30 in a high pressure environment can be improved.

  In addition, the mass spectrometer of the present invention is not limited to the quadrupole type in its analysis unit, but may be another type such as a magnetic field deflection type. It can be applied if the rate of reaching the ion collection electrode is known.

  As described above, the embodiment of the present invention has been described by taking the ionization vacuum gauge and the mass spectrometer as examples. Of course, the present invention is not limited to these, and various modifications can be made based on the technical idea of the present invention. Can be modified.

  For example, the vacuum gauge of the present invention uses an arithmetic unit having a configuration capable of performing both of the methods described in the first embodiment and the second embodiment, and depending on the environment, either the method of the first embodiment or the method of the second embodiment. Either one may be selected and correction may be performed.

  In addition, the third embodiment of the present invention can realize both functions of an ionization vacuum gauge and a mass spectrometer as one apparatus as shown in FIG. 3, but it is configured as a separate apparatus and used in combination. Also good. For example, an interface circuit is provided in both the control device 31 of the mass spectrometer 30 and the control device 21 of the ionization vacuum gauge 20 so that the data of the ionization vacuum gauge 20 can be transferred to the mass spectrometer 30. Communication between the control device 5 ′ and the control device 5 of the ionization vacuum gauge 20 may be performed.

It is the schematic of the structure of the vacuum gauge of embodiment of this invention. FIG. 2 is a graph showing an example of the indicated value of the ionization vacuum gauge. (A) shows the instruction value before correcting the sensitivity calculated by the conventional method. (B) shows the indicated value after the sensitivity is corrected by the first embodiment to which the present invention is applied. (C) shows the indicated value after the sensitivity is corrected by the second embodiment to which the present invention is applied. The horizontal axis represents the indicated value of the reference diaphragm gauge. It is the schematic of the structure of the vacuum gauge of other embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Cathode electrode, 2 ... Anode electrode, 3 ... Ion collection electrode, 4 ... Ion current detector, 5 ... Control apparatus, 5 '... Control apparatus, 6 ... Arithmetic unit, 7 ... storage device, 8 ... display, 9 ... analysis unit, 10 ... vacuum gauge, 12a ... power line, 12b ... power line, 12c ... power line, 13a ... Signal line, 13b ... Signal line, 14a ... Signal line, 14b ... Signal line, 15a ... Data bus, 15b ... Data bus, 15c ... Data bus, 20. ..Ionization gauge, 21 ... control device, 23 ... analyzer, 25 ... collection electrode, 27 ... ion current detector, 21 ... control device, 30 ... mass spectrometry Total, 31 ... Control device,

Claims (5)

A vacuum measurement method in which an electron current is applied to ionize gas molecules in a vacuum to generate ions, the generated ions are captured to detect the ion current, and a pressure indication value is calculated from the detected ion current. And
Using the pressure P as the electron current Ie, the ion current Ii, and the sensitivity S ₀ of vacuum measurement of 1 × 10 ⁻⁴ Pa or less that can be regarded as a constant,
P = Ii / (S ₀ × Ie)
And calculating
The pressure indication value Pcor is calculated using the pressure P and a function F (P) of the pressure P, and F (P) = exp (α × L × P) (α depends on the measuring element, temperature, and gas type. A constant to be determined, L represents the flight distance of ions)
Pcor = P / F (P)
The vacuum measurement method characterized by calculating. 2. The vacuum measurement according to claim 1 , wherein a flight distance of the ions is a distance between an anode electrode facing an electron source of electrons that ionize gas molecules and an ion collecting electrode that traps ions. Method. The vacuum measurement method according to claim 1 , wherein the flight distance of the ions is calculated from a ratio between a pressure indication value of an ionization vacuum gauge and a pressure indication value of a reference vacuum gauge. An electron source of electrons for ionizing gas molecules in the vacuum chamber; an ionization chamber formed by an anode electrode facing the electron source; and an ion collection electrode for capturing ions of the gas molecules generated in the ionization chamber; A vacuum gauge comprising a calculation means for calculating a pressure indication value of the vacuum chamber from the captured ions,
The arithmetic means measures the pressure P, the electron current Ie applied to the electron source, the ion current Ii detected from the ion collecting electrode, and a vacuum measurement sensitivity S _{0 of} 1 × 10 ⁻⁴ Pa or less that can be regarded as a constant. And
P = Ii / (S ₀ × Ie)
And calculating
The pressure indication value Pcor is calculated using the pressure P and a function F (P) of the pressure P, and F (P) = exp (α × L × P) (α depends on the measuring element, temperature, and gas type. A constant to be determined, L represents the flight distance of ions)
Pcor = P / F (P)
A vacuum gauge comprising means for calculating. The vacuum gauge according to claim 4 , further comprising mass separation means for mass-separating ions generated in the ionization chamber.

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