KR20030088089A - Vacuum gauge of heat capacity type - Google Patents

Abstract

PURPOSE: A vacuum gauge for measuring heat capacity is provided to precisely measure a vacuum degree in a vacuum vessel with a high sensitivity by installing a heat capacity measurement driving circuit in a vacuum gauge sensor. CONSTITUTION: A vacuum gauge measures a vacuum degree of a vacuum chamber(3). The vacuum gauge includes a signal source(1) for generating an AC heat generating signal. A bridge circuit is connected to the signal source(1). The bridge circuit has a filament in a form of a wire. The filament is inserted into the vacuum chamber(3). A lock-in amplifier(2) is connected to both terminals of the bridge circuit so as to detect the vacuum degree in the vacuum chamber(3) by amplifying voltage detected by the filament. A variable resistor is provided in the bridge circuit so as to remove ohm decrease voltage caused by voltage of the signal generator.

Description

Vacuum gauge of heat capacity type {Vacuum gauge of heat capacity type}

Vacuum degree is a quantification of the number of gas molecules per unit volume at sub-atmospheric pressure.To summarize the phenomena related to vacuum degree, 1) force due to pressure difference, 2) rapid evaporation, 3) blocking sound and heat, 4) kinetic resistance Reduction, 5) discharge, 6) oxidation prevention, 7) clean surface, and so on. With the development of vacuum technology, the range of vacuum degree currently applied is very wide at 10 ^-13 Torr at atmospheric pressure (760 Torr), and the vacuum degree is measured by using a vacuum gauge that operates on different principles depending on the application area. Vacuum measurement of about 10 ^-3 Torr at atmospheric pressure uses thermal conductivity gauges and capacitive diaphragm gauges .

In the case of a thermally conductive vacuum gauge, an object (filament) heated in a vacuum loses heat by conducting thermal conduction through a conductor, thermal conduction through gas molecules, and thermal radiation.As a result, the thermal conduction through gas molecules varies depending on the vacuum degree. The thermal conductivity vacuum gauge used wasPirani vacuum gauge,Thermocouple Vacuum Gauge Etc. are widely used. The Pirani gauge measures the temperature change of the filament by the degree of vacuum by the change of the electrical resistance. The thermocouple vacuum gauge has the same measuring principle as the Pirani gauge, but the thermocouple is used to directly measure the temperature of the filament. More than 10 vacuum gauges are mainly^-3Sensitivity is good from Torr to 1 Torr, but 10^-3Below Torr, the thermal conductivity due to thermal radiation is better than that due to gas molecules, so its sensitivity is significantly lowered. Above 1 Torr, the mean free path of gas molecules is shortened, resulting in a hot airsheath around the filament. ) Is also produced, which interferes with the smooth thermal conduction of gas molecules, which also lowers its sensitivity. Convective vacuum gauges enhance the sensitivity above 1 Torr by enhancing the convective cooling effect of the Pirani vacuum gauge.

On the other hand, the diaphragm type vacuum gauge is a device for measuring the degree of vacuum by placing a thin diaphragm between the reference vacuum degree and the vacuum vessel to be measured and converting the mechanical strain caused by the pressure difference into an electrical signal. 1) Absolute pressure can be measured. 2) It has high resolution in the desired narrow vacuum region (about 3 decades) and is used for accurate gas flow control of semiconductor processes. However, it is expensive and the measurement pressure range is limited, so several diaphragm gauges for the pressure range must be used simultaneously to measure a wide range of vacuum levels. In addition, the diaphragm welding technology is difficult in the manufacturing process, and it is difficult to localize the technology because some foreign manufacturers monopolize the technology.

1) The first objective is to provide a gauge that accurately measures the degree of vacuum with high sensitivity at 10 Torr to atmospheric pressure (760 Torr), which cannot be measured by existing TC and Pirani vacuum gauges.

2) Furthermore, by applying the non-thermal measurement type driving circuit according to the present invention to the existing thermal conductivity type vacuum gauge sensor, it is aimed at greatly improving the performance by simply constructing the non-thermal measurement type vacuum gauge.

1 is a block diagram of a non-thermal measuring vacuum gauge according to the present invention.

* Explanation of code for main part of drawing

1: Signal Source-Generates a sinusoidal wave of angular frequency ω

2: Lock-in Amplifier-Measures signals with triplet frequency 3ω.

3: Vacuum Chamber

4: R1, R2-resistors making up Wheatstone Bridge

5: Rv-Variable resistor for bridge balance

6: R-filament resistance in flat or wire form

(1) Specific heat and pressure of gas

Specific heat is an amount that describes the change in temperature due to the amount of microheat applied to a sample per unit mass or unit volume. In the case of gas molecules, if there are N molecules in a volume V vacuum vessel, the specific heat of the gas is expressed as shown in equation (1).

.

Where C is the specific heat of the gas, dQ is the amount of heat applied to the sample, dT is the temperature change, and n is the molecular density. The molecular density n is from the state equation of the ideal gas

,

It is an amount directly proportional to the pressure P at a constant temperature T ( k is Boltzmann's constant). Therefore, the specific heat of the gas is

,

It can be seen that the amount is proportional to the pressure, that is, the degree of vacuum, and the degree of vacuum can be obtained from the specific heat measurement.

(2) Signal Characteristics of Flat Film Filaments

When a filament in the form of a flat film is placed in a vacuum vessel and an alternating current (angular frequency ω ) is flowed, the temperature vibration (δ T ) of the filament is determined by the filament's geometry, current, frequency, specific heat of gas molecules, and thermal conductivity. Assuming the mass of the filament is small enough, the magnitude of the temperature vibration is

Where A is a constant determined by the geometry of the filament and the magnitude of the current flowing, C is the specific heat of the gas, and κ is the thermal conductivity of the gas. In Eq. (3), C is proportional to pressure, κ is constant at vacuum of 1 Torr to atmospheric pressure, and is proportional to pressure at 10 ^-3 Torr to 1 Torr. Therefore, the dependence of the temperature vibration of the filament on the degree of vacuum,

Ⓛ 1 Torr ~ Atmospheric Pressure:

② 10 ^-3 Torr ~ 1 Torr:

This is distinguished from the conventional thermally conductive vacuum gauge which has no temperature dependency in the range of 1 Torr to atmospheric pressure.

(3) signal characteristic of wire filament

When an alternating current (angular frequency ω ) flows through a conventional Piranha / thermocouple vacuum gauge sensor (wire-shaped microwire), the temperature vibration of the filament (δ T ) is

Expressed asP ₀ Is the power per unit length by the current,K ₀ Is the Bessel function,aIs the radius of the wire,kIs the thermal wavelengthk = to be. The temperature oscillation dependence on the degree of vacuum is more complicated than the dependence of the film form, but since it can measure the specific heat of the gas due to the alternating current drive, it also has a significant sensitivity at a vacuum degree of 1 Torr to atmospheric pressure. Therefore, if the AC non-thermal measurement driving circuit is applied to the existing thermally conductive vacuum gauge sensor, the operation area can be significantly expanded.

(4) Measurement of temperature vibration through 3ω method

The 3ω method for measuring the temperature oscillation (δ T ) discussed in 2) Signal Characteristics of Planar Film Filaments and 3) Signal Characteristics of Wire Filaments is briefly described. The 3ω method is a general method of determining the thermal properties of a material surrounding a resistor by using a metal resistor having a large resistance coefficient as a hot wire and a temperature sensor. When the target material is a gas, the thermal properties measured have a direct relationship with the pressure of the gas. Thus, the non-thermal measurement vacuum gauge according to the present invention measures the degree of vacuum in the vacuum vessel through the 3ω method. Current of frequency ω to filament, When driven by, the filament vibrates at 2ω due to Joule heating, so the filament has a temperature resistance coefficient ( When) is constant, the vibration of resistance is

Is given by Where φ represents the relative phase difference between the vibration of the resistance and the current drive.

On the other hand, the voltage across the filament

Where the second and third terms contain the signal related to the temperature oscillation (δ T ), or pressure, of the filament. Since α = 0.004 K ^-1 and | δ T | are usually less than 1 degree, these terms are much smaller than the first term, but the third term is the third harmonic frequency and if we use Wheatstone Bridge If so, you can measure this signal.

(5) Detailed description of the drawings

1 is a non-thermal measuring vacuum gauge is composed of an AC signal generator (Signal Source), a fixed-frequency amplifier (Lock-in Amplifier), a Wheatstone bridge containing a filament. The filament is a flat film or wire-type resistor, which has both a role of hot wire and a temperature sensor.

Frequency in the signal generator Current flows through the bridge up and down to ground, and the lock-in amplifier is the point a (between resistor R1 and resistor Rv in FIG. 1) and point b of the Wheatstone Bridge. The voltage difference between the resistor R2 and the resistor R in FIG. 1 is read in the complex space. As shown in [Equation 7], the voltage read by the lock-in amplifier reads not only the voltage caused by the temperature vibration of the filament but also the difference of ohmic drop across the Wheatstone bridge. Since the voltage caused by the temperature oscillation of is the third harmonic signal (3ω) and the ohmic voltage drop voltage is the driving frequency (ω) signal, if the reference of the fixed frequency amplifier is fixed to the three times frequency, the temperature vibration of the filament Only voltage by can be obtained. However, if the ohmic voltage drop voltage is much larger than the multiplied frequency voltage caused by the temperature vibration of the filament, the signal-to-noise ratio (S / N ratio) becomes worse, so that the multiplied frequency voltage cannot be accurately obtained. This can be solved by eliminating the ohmic voltage drop voltage. On the other hand, although the size of the resistors R1, R2, Rv, etc. is small, but the multiplication frequency voltage is generated due to the temperature vibration, these resistors have a negligible resistance of the temperature resistance coefficient, for example, Manganin Wire Consists of.

1) Specific heat measurement vacuum gauge measures the specific heat of the gas in proportion to the pressure along with the thermal conductivity of the gas, so it has good sensitivity even over 1 Torr.

2) Its operation principle is similar to that of thermally conductive vacuum gauges, so it can have a high price competitiveness compared to performance.

3) By using a non-thermal measurement type driving circuit for an existing thermally conductive vacuum gauge sensor, a non-thermal measurement type vacuum gauge can be simply configured.

Claims (3)

In the apparatus for measuring the degree of vacuum of a vacuum vessel, A signal generator for generating an alternating heat generating signal; A bridge circuit comprising a filament in the form of a flat film connected to the signal generator and inserted into the vacuum container; And And a fixed frequency amplifier connected to both terminals of a symmetry point of the bridge circuit to amplify a voltage due to temperature vibration detected by the filament to detect a vacuum degree of the vacuum vessel. In the apparatus for measuring the degree of vacuum of a vacuum vessel, A signal generator for generating an alternating heat generating signal; A bridge circuit composed of a wire filament connected to the signal generator and inserted into the vacuum container; And And a fixed frequency amplifier connected to both terminals of a symmetry point of the bridge circuit to amplify a voltage due to temperature vibration detected by the filament to detect a vacuum degree of the vacuum vessel. The method of claim 1, wherein the bridge circuit is And a variable resistor is added to the bridge circuit to cancel the ohmic drop voltage generated by the voltage of the signal generator.