Hoffman D.M., Singh B., Thomas J.H. (Eds). Handbook of Vacuum Science and Technology

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270

Fig. 13.

Chapter

3.1:

The

Measurement

of Low

Pressures

4A CONVECTION GAUGE

VOLTS

y\\

.01

^ ^ .1 ^ ^ 1 ^ ^ 10 ^ ' 100 ^ ^ 1000

PRESSURE (Torr)

Output Curve for Convection Gauge

thermocouple, and the temperature is measured between heating pulses. The sec-

ond (upper) thermocouple measures convection effects and also compensates for

ambient temperature. At pressures below ~2 torr (270 Pa) the temperature in the

upper thermocouple is negligible. The gauge tube operates as a typical thermo-

couple in the constant temperature mode. Above 2 torr, convective heat transfer

causes the upper thermocouple to be heated. The voltage output is subtracted

from that of the lower, requiring more current to maintain the wire temperature.

Consequently, the range of pressure that can be measured (via current change)

is extended to atmospheric pressure (see Figure 13). Orientation of the sensor is

with the axis vertical.

The use of convection gauges with process control electronics allows for auto-

matic pumpdown with the assurance that the system will neither open under vac-

uum or be subject to overpressure during backfill down to air conditions. These

gauges with their controllers are relatively inexpensive. In oil-free systems, they

afford long life and reproducible results.

3,1.3.2 Hot-Cathode Ionization Gauges

Hot-cathode ionization gauges include the simple triode as well as Bayard-Alpert

and other designs.

3.1.3 Indirect Reading Gauges

271

Rg.l4.

250V

Typical Triode Connection

TRIODE HOT CATHODE IONIZATION GAUGES

The triode electron tube—a device consisting of a thermionic emitting cathode

(in this case a filament), a grid, and an anode (old-timers call it a plate)—has

been used as an indirect vacuum gauge for at least 80 years [15]. In fact this appli-

cation is an extension of the diagnostic "gas test" used to this day by gauge tube

manufacturers

[16].

In the gas test

triode, or any thermionic gauge tube with three

or more elements, is connected as an amplifier (see Figure 14 for a typical triode

connection). The voltages are arranged so that the grid is at —10 volts or more

with respect to the cathode. A fairly high voltage (+250 volts) is placed on the

anode or plate, and the filament voltage is adjusted to allow 10 mA to flow from

cathode to anode. Under these conditions the current flowing to the grid is clearly

caused by positive ions formed by collision of the electron stream with gas mole-

cules in the cathode to plate space. If the triode has cylindrical symmetry, the

ion/collector current can easily be estimated [17]. Assuming the filament to plate

spacing is

cm, the pressure in the tube is

10 ~^

torr and the electron (anode) cur-

rent, the current flowing in the cathode to plate (anode) circuit, is 10 mA, the

mean free path (mfp) of an electron in air at

"^ torr is 4 X 10^ cm and the prob-

ability of ionization is 10%. Using Equation (1), we can then say the ion current

collected by the grid is 2.5 X

10""^.

Ii = ih)iD,_,)(R,/L,)

(1)

272

Fig.

15.

Chapter

3.1:

The Measurement of Low Pressures

lOV

Alternate Triode Connection

where I^ - ion current (A)

/e = electron current (A)

Ri = ionization probability (decimal) [ions/cm-^/torr]

= electron mfp (cm)

Df_p = filament-to-plate distance (cm)

A more rigorous treatment of triode gauge performance is given by Morgulis [18]

and Reynolds [19] as recorded by Leek [20].

It was discovered that if the triode were connected as in Figure 15 so that the

grid is positive and the plate is negative all with respect to the filament, the ion

current collected by the plate for the same electron current to the grid was greatly

increased [21]. The ion current increases because the electron path is much longer

as electrons miss the grid wires, turn around, and miss again. The electrons oscil-

late in this fashion until they strike the grid.

It is in this second higher-sensitivity mode that the triode gauge is used today.

The linearity of ion current to molecular density (pressure) has been demon-

strated by several investigations [15,21,22]. This linearity allows one to define a

sensitivity factor S such that

I, = SXI^XP (2)

where /} = ion current (A)

/g = electron current (A)

P = pressure

S = sensitivity (in units of reciprocal pressure)

3.1.3 Indirect Reading Gauges 273

Another, and perhaps more confusing sensitivity that was used by the industry in

the past, was to quote the sensitivity in microamps per micron of Hg at a certain

electron current. In this manner, the micrometer in the ion collector circuit is able

to read directly in torr with a suitable range multiplier. For example, the VGl A,

perhaps the most common triode gauge of its time, had a sensitivity to dry air of

100 microamps/micron at an electron current of 5.0 milliamps.

The triode gauge with suitable controller providing regulated grid and collector

voltages together with a servo circuit to ensure constant electron current and a

sensitive current meter at the collector is capable of measuring pressures from

10 ""^

to 10"^ torr. Schulz-Phelps triode gauges allow a pressure range of 10"^

to 10"^ torr [23]. The stability and linearity of triode gauges are such that they

have been used as a secondary standard [24], however as with all indirect reading

gauges, the triode sensitivity is dependent on the gas being measured [25].

In all but special applications that require operation at relatively high pressures,

the triode gauge has been replaced by the Bayard-Alpert gauge.

BAYARD-ALPERT HOT-CATHODE IONIZATION GAUGES

In the 1940s the gauge capable of measuring the lowest pressure was the triode

gauge. It soon became apparent that the so-called pressure barrier at 10"^ torr

was caused by a failure in measurement rather than pumping [26]. Nottingham

correctly deduced that this artificial barrier was caused by electrons striking the

grid, causing low-energy X-rays, which in turn struck the ion collector (plate),

emitting photoelectrons. The electron emission is indistinguishable from positive

ion collection and was calculated to be of the order of 10"^ torr for the triode

gauges then in use [27]. In 1950 a simple solution to this problem was proposed

by Bayard and Alpert [28]. This is now the most widely used gauge for general

UHV measurement.

The Bayard-Alpert gauge is essentially a triode gauge reconfigured so that only

a small amount of the internally generated X-rays strike the collector. Figure 16

shows the essential features of the gauge and a simple circuit. One can see that the

cathode has been replaced by a thin collector located at the center of the grid. The

cathode filament is now outside the grid and spaced several millimeters from it.

One advantage to the Bayard-Alpert design was its ability to use the same con-

troller as the triode gauge taking into account any sensitivity differences. The nude

version of the Bayard-Alpert gauge is shown in Figure 17. At about the same time

the Bayard-Alpert gauge was gaining popularity, Weinriech offered a solution to

the burnout of the tungsten filaments then prevalent [29,30]. The use of platinum

metals coated with refractory oxides allowed the gauge to withstand sudden ex-

posure to atmosphere with the filament hot. The common combinations used are

either thoria or yttria on iridium. It was soon noticed that Bayard-Alpert and even

triode gauges of identical structure and dimensions but with different filaments,

e.g. tungsten vs. thoria iridium, had different sensitivities, tungsten filament ver-

274

Fig.

16.

Chapter

3.1:

The Measurement of Low Pressures

Bayard-Alpert Hot Cathode Ionization Gauge

sions being 20 to 40% more sensitive than the iridium of the same construction.

As discussed earlier, low-energy X-rays striking the ion collector and emitting

electrons set a limit to the lowest pressure that can be measured. A variety of in-

novative schemes to reduce this X-ray limit have been proposed. Special designs

with extremely fine collectors have been made that extend the high-vacuum end

of readability to 10"^^ torr but at the expense of the high-pressure end

[31,32].

3.1.3 Indirect Reading Gauges

275

Fig. 17.

GRID SUPPORT

CDLLECTDR

(CENTERED INSIDE GRID)

NOT SHOWN

FILAMENT

Nude Ionization Gauge

Redhead [33] designed a modulated gauge with an extra electrode near the ion

collector. Measuring the ion current at two modulator potentials allows the X-ray

current to be subtracted increasing the range to 5 X

~ ^^.

Other gauges use sup-

pressor electrodes in front of the ion collector [34,35]. However, at this time the

most widely used UHV hot-cathode gauge for people who need to measure

10 ~^^

torr is the extractor [36]. This gauge is shown schematically as Figure 18. In this

gauge, the ions are extracted out of the ionizing volume and deflected or focused

onto a small collector. More recent designs have been developed [37,38] and a

variant using a channel electron multiplier [39] has reduced the low-pressure

limit to 10"^^ torr.

Although the Bayard-Alpert gauge is the most widely used in its pressure

range, it does suffer from some disadvantages. Inspection of Equation (1), which

holds in a general way for Bayard-Alpert gauges, indicates that the ion current, as

well as being pressure dependent, is geometry dependent and changes in geome-

try, tube to tube, or during the life of

the

tube, will change the sensitivity. Redhead

[40] has reported on these sensitivity changes, as has Ohsako [56]. Other investi-

gators have reported on the inaccuracy and instability of Bayard-Alpert gauges,

with widely differing results [41,42,43,44,45,46]. Designs have been put forth

that reduce or eliminate these problems [47,48]. As mentioned earlier, the sensi-

tivity of the Bayard-Alpert gauge is dependent on the nature of the gas being mea-

276

Chapter

3.1:

The Measurement of Low Pressures

Fig.

18.

FILAMENT

GRID

SHIELD

= IDN

REFLECTOR

= IDN

CDLLECTDR

TDP VIEW

^ h-

SIDE VIEW

Extractor Ionization Gauge

viy^

sured. Flaim and Owenby [49] presented data on the relative sensitivity of various

gases that are presented by O'Hanlon [50]. These data, with additions from com-

mercial sources, are shown as Table 2.

3.1.3.3 Cold-Cathode Ionization Gauges

In 1936 Penning [51] invented the cold-cathode discharge gauge as a method to

measure pressures below

10 ~^

torr. In a Crookes tube below 10"^ torr, the mean

free path is so great that little ionization takes place. Penning increased the prob-

ability of ionization by placing a magnetic field parallel to the paths of ions and

electrons, which forces both particles into helical trajectory.

In one configuration of this gauge (see Figure 19), an anode is placed midway

between two parallel-connected cathodes. The anode is a loop of metal wire whose

plane is parallel to that of the cathodes. A potential difference of a few kilovolts

is maintained between the anode and the cathodes. In addition, a magnetic field

is applied between the cathodes by a permanent magnet, usually external to the

3.1.3 Indirect Reading Gauges

277

Table

Average Ionization Gauge Sensitivity Ratios

"r"

for Various Gases Normalized for Nitrogen

Gas

Dry Air

CO2

H2O

SF,

Reynolds'^

0.16

0.23

1.30

1.00

2.75

Dushman &

Young^^

0.15

0.24

1.19

1.86

2.73

0.46

1.00

3.44

Wegener &

Johnson ^^

0.52

1.00

0.85

1.07

1.37

0.89

Riddiford^8

0.26

1.06

0.38

1.00

1.14

0.81

Shultz^^

0.21

0.33

1.49

0.42

1.00

2.54

O'Hanlon'

0.18

0.25

1.20

0.48

1.00

0.85

3.50

0.48

0.90

ETI80

0.13

1.47

0.42

1.00

0.77

0.90

1.01

1.09

gauge body. Electrons emitted from either of the two cathodes must travel in heli-

cal paths due to the magnetic field, eventually reaching the anode, which carries a

high positive charge. During the course of this long electron path, many of the

electrons collide with the molecules of residual gas, creating positive ions, which

Fig. 19.

TUBE BODY

Penning Gauges

278 Chapter

3.1:

The Measurement of Low Pressures

travel more directly to the cathodes. The ionization current thus produced is read

out on a sensitive current meter in units of pressure.

This gauge, with its increased ruggedness as compared to the glass envelope

hot-cathode ionization gauge, has found favor in recent years for industrial appli-

cations. Applications include use in leak detectors, vacuum furnaces, and elec-

tron beam welders. The cold-cathode gauge is also known as the Penning gauge

or the Phillips ionization gauge (PIG).

The Penning gauge is rugged, simple, and inexpensive. However, the current is

not a linear function of the pressure. The range of the Penning gauge is only

10 ~^

torr, and there is some instability and lack of accuracy. This gauge does

not have the accuracy and stability of hot-cathode gauges. Penning and Nienhauis

[52] improved performance by using a cylinder for the anode with cathode plates

at each end and fitted with a cylindrical magnet. This slightly more complex gauge

operates in the range of

10 "-^

to 10"^ torr (1 to 10"^ Pa). Even with this im-

provement, the Penning designs have a problem: At pressures below 10"^ or

10"^

torr, the discharge-initiating operation may fail to strike. Applying higher

voltage does assist starting, but leads to field emission from the ion collector,

which then sets the limit of low-pressure measurement.

Redhead extended the range with a magnetron design [53] and later with

Hobson by using an inverted magnetron design [54]. These designs extended the

low-pressure range to 10"^^ torr [55] or better. With all these improvements,

problems with instability, hysteresis, and starting, though improved, still exist

[57,58].

Modern magnetrons use a more simplified design without an auxiliary

cathode.

More recently a double inverted magnetron was introduced [59]. This gauge

has greater sensitivity (amp/torr) than the other types. It has been operated suc-

cessfully at «1 X 10"^^ torr. The gauge (see Figure 20) consists of two axially

magnetized annular-shaped magnets (1) placed around a cylinder (2) so that the

north pole of one magnet faces the north pole of the other one. A nonmagnetic

spacer (3) is placed between the two magnets, and thin shims (4) are used to focus

the magnetic fields. This gauge has to date been operated at

—10"^^

torr, stays

ignited and reignites quickly when power is restored at this pressure. Essentially

instantaneous reignition has been demonstrated by use of radioactive trigger-

ing [60].

3.1.3.4 Resonance Gauges

One example of a resonance-type vacuum gauge is the quartz friction vacuum

gauge. Although not widely and commonly used for general vacuum measure-

ment, one such example is reviewed [61].

3.1.3 Indirect Reading Gauges

279

Rg.20.

ffi

Double Inverted Magnetron

A quartz oscillator can be built to measure pressure by a shift in resonance fre-

quency caused by static pressure of the surrounding gas or by the increased power

required to maintain a constant amplitude. Used in this way, it is effective in mea-

suring from near-atmospheric pressure to about 0.1 torr. A second method is

to measure the resonant electrical impedance of a tuning fork oscillator. This

method is shown to be proportional to the friction force applied by the ambient

gas.

Test results for this device show an accuracy within ±

10%

for pressure from

~ho

10 ^

torr. The lengths of the tuning fork oscillator varied from

2.5

to 20 mm.

3.1.3.5 Molecular Drag Gauges (spinning rotor) Gauges

The idea of measuring pressure by means of the molecular drag of rotating de-

vices was introduced by Meyer [62] and Maxwell [63] in 1865. In their devices

the rotors were tethered to a wire or thin filament. Holmes [64] introduced the

concept of the magnetic rotor suspension, leading to the spinning rotor gauge.

Beams [65] et al. disclosed the use of a magnetically levitated, rotating steel ball

to measure pressure at high vacuum. Fremerey [66] has traced the interesting his-

torical development of this gauge.

The spinning rotor

gauge,

more usually called the molecular drag gauge (MDG),

has become more popular since its commercial introduction in 1982 [67]. The

useful range of the MDG overlaps the low-pressure range of the capacitance

manometer and the high-pressure range of hot-cathode ion gauges, thus filling an