Pressure Measurement

Chapter 3

Pressure Measurement 3.1 INTRODUCTION As the range of vacuum used is widened so too has the number of types of gauges that can be used to measure the vacuum achieved. Although there are many types of gauges available, some are more commonly used than others and it is these that will be described. The more complex the vacuum process and/or the more precise the coating produced has to be, then more attention has to be paid to the gauging. It can be important to choose the correct gauge for the process. The aim is to have a gauge with the optimal combination of sensitivity and stability that operates without drift over the pressure range used in the process. If the gauge is to be used as part of the process control system it is important that the gauge is regularly maintained and calibrated. It is pointless to buy a high-quality gauge and then abuse it on the system or never check the calibration. It is also a waste of money to buy a gauge more sophisticated than the process requires or to spend too much time cleaning, checking, and calibrating the gauge. It is also important to site the gauge in a suitable position within the vacuum system. Positioning the gauge directly in line with the gas input will give a false, high reading. Hiding the gauge away in the corner of the vessel will give a lower reading and also will give a slow response to any pressure changes. Placing the gauge close to the pumping orifice may also result in a reading lower than is representative of process area that is of most interest. If there are different zones within the system it will be preferable to measure the pressure in each zone independently and not rely on one gauge and infer the pressure in the other. It is also worth being aware that the pressure reading can be different at different points, even within the same zone, depending upon the components that fill the space. Ideally, critical areas will have a backup gauge available. It is invariably cheaper to have a second, backup gauge than to have to stop the process to replace a gauge because of a gauge failure. This is more critical with filament-type gauges that can burn out and have a shorter lifetime than some other gauges. Vacuum Deposition onto Webs, Films and Foils. DOI: http://dx.doi.org/10.1016/B978-0-323-29644-1.00003-7 © 2015 Elsevier Inc. All rights reserved.

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This duplication of gauges can have advantages when things go wrong. If a gauge tells you there is no vacuum you then have the dilemma, is it the gauge that is correct, or, is it the gauge that has stopped working? Having a second gauge aids troubleshooting and confirms either the loss of vacuum or failure of the gauge. If the duplication of gauges is regarded as too costly, then having isolation valves that allow the gauges to be exchanged, whist the whole system is kept under vacuum, may be a suitable alternative solution. Table 3.1 gives a quick summary of some of the most widely used gauges with the range they cover and some comments about their dependence, or not, on gas species.

TABLE 3.1 A Table of Gauge Types, Measurement Technique, and Range of Measurement Gauge Type

Quantity Measured

Approximate Range

Comments

U-tube manometer

Height of a liquid column usually mercury column

101 0.133 kPa

Independent of gas species

Mechanical manometer Bourdon, diaphragm, capsule

Mechanical deflection of diaphragm or thin wall

101 0.133 kPa

Capacitance manometer

Capacitance change from diaphragm position

101 kPa 13 mPa

Thermocouple gauge

Filament temperature change with pressure

1.33 kPa 133 mPa

Filament temperature change with pressure

133 Pa 133 mPa

Glow discharge current

1.3 Pa 1.3 mPa

Pirani gauge

Penning-cold cathode ionization gauge

760 1 Torr

760 1 Torr

760 1024 Torr

10 Torr 1 mT

1 Torr 1 mT

1022 1025 Torr

Independent of gas species

Independent of gas species

Dependent on ambient temperature and gas species Dependent on ambient temperature and gas species Dependant on gas species (Continued )

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TABLE 3.1 (Continued) Gauge Type

Quantity Measured

Approximate Range

Comments

Bayard-Alpert hot ionization gauge

Ion current from constant electronemission current

133 mPa 1.3 3 1029 mPa

Dependant on gas species

Spinning rotor

Spinning ball slows by molecular drag by gas

1023 10210 Torr 13 1.3 3 1025 Pa 21

10

27

10

Torr

Dependant on gas species pressure, temperature, and ball

Bourdon tube

Gears and levers to amplify tube movements

Connection to vacuum vessel where pressure is to be measured FIGURE 3.1 A schematic of a Bourdon gauge.

3.2 BOURDON GAUGE On many large vacuum systems where there is a motor used to close the vacuum vessel door it is common to see a capsule gauge. This gauge uses a tube that is sealed at one end and open to the vacuum. As the system is evacuated the tube distorts. This movement is converted via levers and gears into a dial movement. Often the tube is in a spiral configuration (Figure 3.1). The diaphragm version of this gauge has the lever mechanism welded to a diaphragm instead of the Bourdon tube. When the diaphragm distorts as the vacuum is applied, the lever is moved and the pressure indicated.

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The main purpose of this gauge is to check that the door is sufficiently closed and the pumps are pulling vacuum. If the gauge does not show an immediate movement away from atmospheric pressure it usually means that the door needs to be closed further. The accuracy of the gauge is poor but adequate for the purpose.

3.3 PIRANI AND THERMOCOUPLE GAUGES The backing lines and roughing gauges require something more accurate when reaching a lower pressure than the capsule gauges. The thermocouple or Pirani gauge fits this requirement. These gauges use a current to heat a wire. The heated wire loses heat to the gas molecules that collide with the wire. Thus the higher the pressure the greater the number of molecules colliding with the wire and hence more heat is lost and the temperature of the wire is reduced. In the Pirani gauge, the wire is part of a bridge circuit and a change in the wire resistance can be measured. Similarly, if the pressure decreases the temperature of the wire will increase and again the changed resistance can be measured. The resistance is converted into a reading of pressure. In the thermocouple gauge, a thermocouple is spot welded directly to the resistance wire and so a direct temperature measurement is taken and converted into a pressure measurement. The gauges are sensitive to the type of gas in the system. The different gases have different masses and hence will take different amounts of heat out of the wire. However, this does mean that if the gauge is calibrated using air and is then used to measure a process gas such as Argon, there will be an error in the pressure measurement. The error is reproducible and so in most cases the pressure is accepted as being a relative, but reproducible, measure of pressure (Figures 3.2 3.4). 10

Xenon

Argon Dry air Helium

True pressure (mbar)

1

0.1

The values shown are approximate — the plots show the trends

Hydrogen

0.01

0.001 0.001

0.01 0.1 1 Pirani gauge actual readings (mbar)

10

FIGURE 3.2 A graph showing the differences between the true and the actual readings for a gauge calibrated for dry air.

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Gauge head

Sensing wire

Power supply

Bridgebalancing resistor

Reference or compensation head

FIGURE 3.3 A schematic of a Pirani gauge.

Connection to vacuum system

Thermocouple

Hot wire FIGURE 3.4 A schematic of a thermocouple gauge.

3.4 CAPACITANCE MANOMETER A tube has a flexible diaphragm separating a vacuum at one end of the tube and the vacuum system at the other end of the tube. When the diaphragm deflects, due to there being a different pressure on either side of the diaphragm, the capacitance between the diaphragm and electrodes on one or both sides of the diaphragm changes. These changes can be detected.

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Diaphragm

Permanent reference vacuum

Connected to system where vacuum is to be measured

One or two electrodes to measure capacitance FIGURE 3.5 A schematic of a capacitance manometer.

The deflection is purely a measure of the pressure differential and does not depend upon the gas species. This is an absolute measure of pressure. In early gauges, there was some drift with temperature but more recent gauges have temperature compensation. One other area of drift or variation derives from some hysteresis in the gauge that can be caused from leaving the gauge at atmospheric pressure for a long time. The diaphragm can take some time to relax back to its optimum measurement state. As with many other items the gauge works best if kept under vacuum. The absolute pressure measurement makes this an increasingly popular type of gauge for active process control. These gauges are commonly referred to by the generic name Baratron; this is a trade name from the manufacturer that first made them widely available (Figure 3.5).

3.5 PENNING OR COLD CATHODE IONIZATION GAUGE The Penning gauge is named after Penning who utilized the current produced by a glow discharge as a measure of the pressure. The basis of the gauge is to have an anode and cathode within a magnetic field. A potential of around 2 kV is applied and in its usable range a self-sustaining glow discharge is produced. The electrons produced in the glow discharge spiral round in the magnetic field and increase the chance of undergoing further ionizing collisions. The positive ions that are less affected by the magnetic field reach the cathode and produce a current. The ion and electron-emission currents are measured and used to indicate pressure. The geometry of the gauges has varied with early gauges having the cathodes as two flat plates with the anode as a loop of wire between the cathodes and the magnet surrounding both. The design was then changed to have a central anode and the cathode as a cylinder around the anode and the magnet concentric with both (Figure 3.6). The gauge has its limitations. A glow discharge must be struck and if the pressure is too high it will not strike because too many collisions take place

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Magnet S

N

Cathode

Hot wire Connection to Vacuum system

S

Magnet

N

FIGURE 3.6 A schematic of a coaxial or inverted Penning gauge.

and the energy is lost and the glow will not self-sustain. If the pressure is too low there are not enough collisions to sustain the glow discharge and again the gauge will not operate. In the range where the gauge will work is species dependent. Different gases have different ionization efficiencies and at worst the gauge can be in error by as much as 50%. In its favor, the gauge has no fragile filament and this makes it very robust and easily cleaned. With care it can be reliable and robust.

3.6 ION OR HOT-CATHODE IONIZATION GAUGE This gauge uses a hot wire to produce electrons by thermionic emission. The electrons are attracted to an anode grid. This grid is of an open structure such that many of the electrons overshoot the grid and oscillate past the grid a number of times before hitting the grid. The electrons in oscillating around the grid increase their chances of undergoing ionizing collisions. The ions that are produced are attracted to a third electrode producing a measurable current. The pressure is related to the ion current and the electron-emission current. The ion gauge has the advantage that as the electrons are produced by thermionic emission the gauge can operate at lower pressures than the Penningtype gauge. The accuracy of the gauge is also much better than the penning gauge. The disadvantage is that the hot-wire filament can be damaged by thermal shock, mechanical shock, and chemical attack. Thus it is essential to take great care to switch off the gauge when bringing the system back up to atmospheric pressure. Also if reactive gases are being used in the process, such as oxygen, it can be better to switch off the gauge to preserve the filament. The gauge still has the problem of being species dependent. Table 3.2 shows the sensitivity of ion gauges to different gases or vapors (Figure 3.7).

TABLE 3.2 A Table Showing Sensitivity of Different Gases Normalized to Nitrogen Gas or Vapor

Sensitivity Normalized to Nitrogen

Acetone

4

Argon

1.19

18

Carbon dioxide

1.37

22

Carbon monoxide

1.07

14

Helium

0.15

2

Hydrogen

0.46

2

Krypton

1.86

36

Mercury

3.44

80

Neon

0.24

10

Nitrogen

1

14

Oxygen

0.84

16

Water

0.89

8

Xenon

2.73

54

Amplifier

Number of Electrons

Gauge

Fine wire cylindrical electron collector Connection to vacuum system

Hot wire electron emitter

Fine wire ion collector

FIGURE 3.7 A schematic of a hot ionization gauge.

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FURTHER READING [1] Harris NS. Modern vacuum practice. New York: McGraw-Hill; 1989, ISBN 0 07 707099 2. [2] Basic vacuum practice. 3rd ed. Lexington: Varian Associates Inc.; 1992 [Varian Part No. 900-0085]. [3] Hill RJ, editor. Physical vapor deposition. Temascal; 1986 [Part No. T-0186-6001-1]. [4] Roth A. Vacuum technology. North Holland Publishing Co.; 1979, ISBN 0 444 10801 7. [5] Maissel LI, Glang R. Handbook of thin film technology. New York: McGraw-Hill; 1983 [reissue] ISBN 0 07 039742 2. [6] O’Hanlon JF. A users guide to vacuum technology. New York: Wiley Interscience; 1980. [7] Guthrie A. Vacuum technology. New York: John Wiley & Sons Wiley Interscience; 1963. [8] Weissler GL, Carlson RW, editors. Vacuum physics and technology, vol. 14. New York: Academic Press; 1979. [9] AVS recommended practices on vacuum measurements & techniques. American Vacuum Society; vol.1, 1992 [Recommended practices & standards series. S-2]. [10] Utterback NG, Griffith Jr. T. Reliable submicron pressure readings with capacitance manometer. Rev Sci Inst 1966;37(7):866. [11] Sullivan JS. Advances in capacitance manometer systems. Res Dev 1976;42 4. [12] Sullivan JS. Modern capacitance manometers. Transducer Technol 1979;1(5):22 7. [13] Mitchell AR. Pressure and vacuum standards review. Transducer Technol 1982;4(2):9 [continued in Transducer Technol 1982; 4(4):8]. [14] Tompkins HG. Monograph Series HB-1 Vacuum technology: a beginning. New York: American Vacuum Society; 2002. [15] Lewin G. An elementary introduction to vacuum technique, Monograph Series M, vol. 8. New York: American Vacuum Society; 1987. [16] Tompkins HG. Vacuum gauging and control, Monograph Series M, vol. 12. New York: American Vacuum Society; 1994.