Apparatus for and method of ion detection using electron multiplier over a range of high pressures

Ions in a chamber or space are detected using an electron multiplier operating at relatively low gain. The electron multiplier is placed in communication with the chamber, such as a chamber of a mass spectrometer, such that ions from the chamber enter the electron multiplier. A bias voltage applied to the multiplier sets the gain of the multiplier. By setting the gain at a relatively low value, the gain of the multiplier remains independent of chamber pressure, such that an accurate pressure measurement is obtained without calibration at a particular pressure or as a function of pressure.

FIELD OF THE INVENTION 
The present invention relates generally to electron multipliers, and more 
particularly to an electron multiplier for measuring the pressures (and 
thus the volumetric number densities) of gases over a large range of 
pressures. 
BACKGROUND OF THE INVENTION 
In many applications, it is desirable to detect the presence of ions in a 
chamber or space. For example, in a mass spectrometer, ions of the various 
gas constituents are detected to determine the partial pressure of each 
gas constituent in a chamber and compared to the detected total pressure 
of the gas within the chamber. By detecting the partial pressure of each 
particular gas constituent, as well as the total pressure of the combined 
gases within the chamber, useful information can be acquired. For example, 
both total and partial pressures are proportional to the corresponding 
volumetric number density, respectively, of the total and constituent 
gases, thus providing information of the quantity of each gas constituent 
that is present. Knowledge of total and partial pressures is useful, for 
example, for detecting leaks in a system. For this and other reasons, it 
is highly desirable to measure both total and partial pressures as 
accurately and precisely as possible. 
In conventional mass spectrometers and other systems, measurement of the 
partial and total pressures of the gases is based upon the probability of 
an electron colliding with a neutral atom or molecule, and thereby 
creating a positive ion. The probability is proportional to the volume 
number density of the neutral atom or molecule along the electron flight 
path. The probability is a function of the partial and total pressures, 
with the probability increasing with increasing pressure. Ions thus are 
measured within the chamber. In quadrupole mass spectrometers, partial 
pressures are measured using a quadrupole mass filter assembly and an ion 
current measurement device, positioned at the output of the filter 
assembly within the chamber, having a surface which (a) is exposed to the 
ions exiting the filter, and (b) generates a current when positively 
ionized particles contact a surface of the device. A current measurement 
instrument is used to measure the current which is proportional to the 
total volumetric number density of the neutral atoms or molecules of the 
gas constituent being measured, and therefore is proportional to the 
partial pressure of the neutral atoms or molecules of that gas. Thus, 
knowledge of the current due to ions contacting the surface provided with 
the current measurement instrument provides knowledge of the partial 
pressure of each constituent gas. Typically, the surface provided with the 
current measuring instrument is an ion detector which includes a device 
commonly known as a Faraday plate or cup. Charged ions strike the Faraday 
plate causing an ion current to be generated in the plate. 
The Faraday plate is useful for detecting ions at relatively high chamber 
pressures, and in fact a second Faraday plate or cup can be used in the 
chamber to measure the total pressure in the chamber by continually 
detecting positive ions created in the chamber from all of the constituent 
gases. However, at low pressures, where the ion current is low, it is 
often desirable to enhance the sensitivity of the ion detector. One 
solution is to detect the ions with an electron multiplier. Electrons 
produced by the multiplier are collected by an anode or electron 
collector. Current at the anode is measured to quantify the electrons and 
to indicate the input ion current. 
More specifically, an electron multiplier typically includes an 
ion/electron converter typically comprising a layer of doped resistive 
material. Electrons emitted from the converter in response to detected 
ions are increased (or multiplied) by a predetermined factor so as to 
create additional or secondary electrons measured through a more easily 
detected dynamic range. The space in which the number of electrons are 
multiplied is typically subjected to a bias voltage applied across the 
length of the multiplier space. The bias voltage creates an electric field 
gradient. Ions from the chamber enter the multiplier and strike the 
surface of the ion/electron converter, resulting in the release of 
electrons from the surface. Additional or secondary electron generating 
surfaces are provided within the field gradient so that when an electron 
travels through the field gradient and strikes one of these surfaces, 
there is a high probability that multiple secondary electrons are 
generated from the surface for each electron that strikes the surface. 
These secondary electrons are accelerated by the electric field such that 
they in turn strike another internal surface to cause the release of more 
secondary electrons, and so on. Finally, the secondary electrons exit the 
multiplier and strike the anode. The current at the anode is measured to 
quantify the electrons exiting the multiplier. In principle, since the 
gain of the multiplier, i.e., the number of electrons exiting the 
multiplier for each ion entering, is known, the number of electrons 
measured provides a determination of the number of ions and, therefore, 
the measured pressure. The predetermined factor or gain of typical 
electron multipliers used in presently available mass spectrometers 
typically varies from as low as 1000 to as high as 10,000,000. 
The gain of the multiplier is determined by several of its characteristics 
and operating parameters, including the multiplier geometry and 
composition and the applied bias voltage level creating the electric field 
gradient. Given a particular multiplier, the gain is controllable by 
varying the bias voltage so as to vary the electric field gradient, 
although in the prior art it is assumed that the gain remains fixed during 
operation of the electron multiplier. Ideally, the gain of the multiplier 
is independent of pressure in the chamber. However, certain phenomena that 
occur within the multiplier cause the gain to vary with chamber pressure. 
One such phenomenon is referred to as ion feedback, which causes the gain 
to increase rapidly with increased pressure, particularly at high gain. 
Ion feedback occurs when one or more of the secondary electrons inside the 
multiplier strike gas molecules with sufficient energy to ionize them. The 
resulting ions and electrons are accelerated by the electric field within 
the multiplier until they collide with an internal surface, causing more 
secondary electrons to be released and to produce still more secondary 
electrons. The result is more electrons exiting the multiplier for a given 
gain (bias voltage). 
At low pressures, very few gas molecules are present in the multiplier and, 
therefore, the relatively small effects of ion feedback are negligible. 
However, at higher pressures, many more gas molecule collisions take 
place, and the gain of the multiplier varies rapidly with chamber 
pressure. Significant ion feedback typically occurs when the pressure at 
the electron multiplier is above 1.0 millitorr. As a result, the electron 
current measurement taken at the output end of the multiplier no longer 
provides a reliable measurement of the number of ions entering the 
multiplier, and inaccuracies are introduced into the pressure measurement. 
Further, operating at very high gains and high pressures increases the 
chances of voltage discharge and/or breakdown, as well as decreases the 
useful life of the multiplier by increasing the number of collisions with 
the doped inner surfaces of the multiplier. For this reason, electron 
multipliers of the prior art typically are not operated when the pressure 
at the electron multiplier is above 0.5 millitorr. 
Presently, there are quadrupole mass spectrometers designed to operate at 
pressures up to about 20 mtorr. At least one of these spectrometers uses a 
Faraday cup ion detector, which as described above, does not have good 
performance at very low pressures. At least another of these prior art 
spectrometers includes both an electron multiplier with a collection anode 
and a Faraday cup to detect ions in a mass spectrometer. As a solution to 
the dependence of gain on gas pressure, this prior art system uses the 
electron multiplier for low pressures and the Faraday cup at high 
pressures. Specifically, at low pressures, ions entering the electron 
multiplier are multiplied as described above, and the electrons produced 
thereby are collected by the anode. The anode current is measured. As the 
pressure increases beyond a predetermined threshold (the threshold being 
equal to or less than 1.0 mtorr), within the 1.0-20.0 mtorr range, the 
multiplier is not used, but instead the ion current is measured directly 
with the Faraday cup. In such a system, the low-noise amplification 
benefits of the electron multiplier are forfeited at these higher 
pressures. 
OBJECTS OF THE INVENTION 
It is a general object of the present invention to provide an improved 
electron multiplier which substantially overcomes or reduces the 
above-identified problems of the prior art. 
Another, more specific object of the present invention is to improve the 
small-signal detection capabilities of a mass spectrometer operating at 
relatively high pressure. 
And another object of the present invention is to provide an electron 
multiplier ion detector having improved ion detection capabilities through 
a broader range of pressures including pressures where in the prior art 
devices, described above, ion feedback can be significant, i.e., above 1.0 
mtorr. 
Yet another object of the present invention is to provide an improved 
electron multiplier ion detector useful in detecting ions up to 100 mtorr 
or greater without the need to calibrate the gain as a function of gas 
pressure. 
Still another object of the present invention is to provide the benefits of 
the low-noise amplification of an electron multiplier while eliminating 
the high-gain nonlinearities found in prior systems at high pressures. 
And yet another object of the present invention is to operate an electron 
multiplier at relatively low gain so as to eliminate the possibility of 
voltage discharge and/or breakdown that can become likely at high bias 
voltages (high gain) and high pressures, as well as increasing the useful 
life of the multiplier by reducing collisions with the doped inner 
surfaces of the multiplier. 
SUMMARY OF THE INVENTION 
These and other objects are achieved by an ion detection system and method 
used to measure a pressure in a space or chamber which eliminate the 
drawbacks associated with the variation in electron multiplier gain at 
high pressure. The measured pressure can be a total chamber pressure or 
one or more partial pressures associated with particular constituents of 
the contents of the space or chamber. In the method of the invention, an 
electron multiplier is immersed in the space or chamber having a 
relatively high total pressure. In one embodiment, the total pressure is 
within the range of about 0.1 to about 100 millitorr. The gain of the 
multiplier is adjusted to a relatively low level, i.e., a level at which 
the gain is substantially constant with respect to total pressure. In one 
embodiment, at this low gain setting, the output electron current from the 
electron multiplier varies linearly with the total pressure and/or the 
partial pressure of a constituent gas. An ion is received at a receiving 
end of the multiplier. The resulting electrons exiting the multiplier are 
detected to determine the measured pressure within the space. 
In one embodiment, the system and method of the invention are used to 
measure pressure in a mass spectrometer. In one particular embodiment, the 
mass spectrometer is a quadrupole mass spectrometer. In that embodiment, 
the ions entering the electron multiplier are taken from the output of a 
quadrupole mass filter in the mass spectrometer. 
In one embodiment, the system and method of the invention are used to 
measure the total pressure within the chamber of the mass spectrometer. 
The invention can also be used to measure partial pressures of particular 
constituent gases and is therefore applicable to measurement of gases 
introduced into a process chamber during semiconductor processes such as 
phase vapor deposition (PVD) and is also applicable to residual gas 
analysis (RGA) in which the amounts of low-pressure residual gases in a 
chamber are measured. 
In one embodiment, an adjustable voltage source is connected across the 
electron multiplier to apply the bias voltage. The source can be adjusted 
to set the gain of the multiplier at a desired level. In accordance with 
the present invention, the bias voltage is set to adjust the gain to a 
relatively low level to maintain a near constant gain with pressure. In 
one embodiment, the gain is adjusted to a value below 1000. In one 
particular embodiment, the gain is adjusted to a value between about 10 
and about 100. 
The invention is applicable to any type of electron multiplier. 
Specifically, the multiplier can be a discrete dynode type, a continuous 
channel electron multiplier (CEM) type, a continuous microchannel plate 
(MCP) type or other type of multiplier. 
Still other objects and advantages of the present invention will become 
readily apparent to those skilled in the art from the following detailed 
description wherein several embodiments are shown and described, simply by 
way of illustration of the best mode of the invention. As will be 
realized, the invention is capable of other and different embodiments, and 
its several details are capable of modifications in various respects, all 
without departing from the invention. Accordingly, the drawings and 
description are to be regarded as illustrative in nature, and not in a 
restrictive or limiting sense, with the scope of the application being 
indicated in the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a schematic functional diagram which illustrates operation of a 
discrete dynode electron multiplier 10 in accordance with one embodiment 
of the present invention. The multiplier 10 includes multiple dynodes 
12a-12e separated from each other by a resistance, indicated in FIG. 1 as 
discrete resistors 14. The multiplier includes an input end 20 which can 
be placed in communication with a space or chamber whose pressure is to be 
measured. For example, the input end 20 can be connected to the output end 
of a mass filter in a mass spectrometer. Ions enter the multiplier 10 
through the input end 20 and cause electrons to exit the multiplier 10 
through the output end 22. 
A voltage source 16 is connected across the multiplier 10 as shown to 
generate an electric field within the interior 18 of the multiplier 10. In 
one embodiment, the electric field is characterized by a potential which 
increases in the direction from the input end 20 to the output end 22. 
Electrons 28 exiting the multiplier 10 can be collected by an anode or 
collector 24. The anode 24 is connected by a line 30 to a current 
measuring device 26 such as an electrometer. 
In operation, an ion 25 enters the multiplier 10 at the input end 20 and 
strikes the first dynode 12a. The first dynode functions as an 
ion-to-electron converter. The collision thus causes multiple electrons 28 
to be emitted from the dynode 12a. These "secondary" electrons are 
accelerated by the electric field toward the output end 22 of the 
multiplier 10. They collide with the next dynode 12b, causing more 
secondary electrons to be released into the interior of the multiplier 10. 
These new secondary electrons 28 accelerate to the next dynode where they 
cause still more electrons to be released. 
This multiplication process continues to the output end 22 of the 
multiplier 10. The electrons 28 exiting the multiplier and striking the 
anode 24 induce in the line 30 a current which is measured by the 
electrometer 26. The measured current is used to quantify the ions 
entering the multiplier 10. 
Ion feedback occurs when one or more of the electrons 28 strike gas 
molecules within the interior 18 of the multiplier. If an electron strikes 
a molecule with sufficient energy to ionize it, a positively charged ion 
and one or more electrons can be produced in the multiplier 10. They can 
strike the dynodes 12 with sufficient energy to cause additional secondary 
electrons to be released and multiplied by the process described above. 
These additional electrons can adversely affect the current measurement 
taken at the output end of the multiplier. 
The gain of the multiplier 10 is a measure of the number of electrons 28 
produced at the output of the multiplier for each ion 25 entering the 
multiplier. In general, it is dependent upon the number of stages in the 
multiplier and the number of electrons produced by each collision. In one 
embodiment, the gain is given by G=n.sup..gamma., where G is the gain, 
.gamma. is the number of stages and n is the number of electrons released 
per collision. For a given multiplier configuration, at low pressures, the 
gain is constant with respect to the pressure within the multiplier. 
In general, n is dependent upon the applied bias voltage V. Therefore, the 
gain G is actually also a function of bias voltage V. That is, 
G(V)=n(V)!.sup..gamma.. 
FIG. 2 indicates that at high gain and high pressure, the gain G is also 
dependent upon pressure. FIG. 2 is a graphic representation of electron 
current measured at the output of the multiplier as a function of chamber 
pressure. In the curve labeled 50, the gain is set at a relatively high 
value, e.g., 10.sup.4. In the curve labeled 52, in accordance with the 
present invention the gain is set to a relatively low value, e.g., 100. 
FIG. 2 illustrates that at high gain (curve 50), the measured multiplier 
output current varies approximately linearly with the chamber pressure at 
relatively low pressures, i.e., below about 10.sup.-3 torr. Therefore, at 
these pressures, the gain is constant with pressure. However, as the 
pressure increases above 10.sup.-3 torr, the response becomes nonlinear. 
The output current begins to rise rapidly with increasing pressure, due to 
the increasingly prevalent effects of ion feedback in the multiplier. As a 
result, the gain of the multiplier increases with increasing pressure. 
Because of this variation in gain, it becomes difficult to characterize 
the chamber pressure using the electron current measurement without some 
additional operation such as a calibration at the particular gain setting 
and pressure being used. 
However, if the system is operated at a lower gain, the nonlinearity and 
its associated effects, namely, the variation in gain with pressure, can 
be eliminated. As shown by curve 52 in FIG. 2, at lower gain, e.g., 
between 10 and 100, the variation in output current with pressure remains 
linear, even through high pressures above 10.sup.-1 torr (i.e., 100 
mtorr). The multiplier gain remains constant with pressure; therefore, the 
measured output current can be readily related to the chamber pressure to 
produce a more accurate pressure measurement. 
The invention is also applicable to electron multipliers that are different 
from the discrete dynode type referred to above to illustrate the 
principles of the invention. For example, the invention is applicable to 
continuous channel electron multipliers (CEMs) such as those manufactured 
and sold by Galileo Electro-Optics Corporation of Sturbridge, Mass. 
FIG. 3 is a schematic partially cut-away functional block diagram which 
illustrates the invention applied to a typical CEM 100. The input end 104 
of the multiplier tube 100 is coated with a conductive electrode 108, and 
the output end 106 is coated with a conductive electrode 110. The voltage 
source 102 is connected across the multiplier tube 100 at the electrodes 
108, 110 to apply the multiplier bias voltage. 
Ions enter the input end 104 of the tube 100 and collide with the inner 
wall of the tube resulting in the emission of electrons. The inner wall 
thus functions as an ion-to-electron converter. The resulting secondary 
electrons are accelerated down the tube by the bias voltage. The electrons 
collide with the inner wall to release more electrons. The process repeats 
itself until the secondary electrons exit the tube 100 at the output end 
106 where they are collected by the anode 24. The resulting current in 
line 30 is measured by the current measuring device 26. 
Tests have shown that using a Galileo CEM at pressures between about 
10.sup.-4 and 10.sup.-1 torr, the invention yields accurate measurements. 
With the gain set below 1000, particularly, between about 10 and about 
100, the system response remains linear, the gain remains constant with 
pressure, and output current from the CEM is adequately high to permit 
pressure measurements at desired sensitivity and accuracy. 
The invention is also applicable to microchannel plate (MCP) electron 
multipliers such as those also manufactured and sold by Galileo 
Electro-Optics. FIG. 4 is a schematic functional diagram which illustrates 
the present invention applied to a MCP 200. The MCP 200 is made from a 
wafer 201 which can be a lead silicate glass wafer. The wafer 201 includes 
multiple holes or channels 208 formed through the wafer, each of which 
serves as a channel electron multiplier as described above in connection 
with FIG. 3. In one embodiment, the channels are on the order of 5-25 
.mu.m in diameter, are separated by a distance between centers on the 
order of 6-32 .mu.m and have a length-to-diameter ratio of between 40:1 
and 60:1. In one embodiment, the density of channels on the surface of the 
wafer is between 10.sup.5 and 10.sup.7 channels/cm.sup.2. 
The top surface 204 of the wafer 201 forms the input ends of the channels 
208, and the bottom surface 206 forms the output ends of the channels 208. 
The top surface 204 and bottom surface 206 are coated with conductive 
material which serves as the electrodes to which the bias voltage source 
202 is connected. As in the previously described embodiments, the bias 
voltage sets the gain of the channels 208. 
In operation, ions enter the channels 208 at the top surface 204 as shown. 
The resulting multiplied output electrons exit the channels 208 at the 
bottom surface of the wafer 201. The output electrons are collected by the 
anode 24, and the current in line 30 is measured by the current measuring 
device 26. 
Once again, by setting the bias voltage at source 202 to a sufficiently low 
level, the gain of the device is maintained at a relatively low level. The 
gain of the channels remains independent of pressure at high pressures and 
the device operates linearly to provide an accurate pressure measurement. 
FIG. 5 is a schematic functional block diagram of one embodiment of a mass 
spectrometer 300 using ion detection in accordance with the present 
invention. The mass spectrometer 300 includes an ion source 307 which 
directs ions into a mass filter 305. In this embodiment, ions exiting the 
mass filter 305 can enter the curved channel electron multiplier 301 or 
can be collected directly by the plate portion 325 of electrode 324, 
depending upon the state of switch 304. 
If switch 304 is closed, the bias voltage at source 302 is applied across 
the multiplier 301. Positive ions exiting the mass filter 305 are 
attracted into the multiplier 301. The resulting electrons are collected 
by the anode portion 326 of the electrode 324, and the resulting current 
in line 330 is measured by the electrometer 26 to provide a pressure 
measurement, which can be a partial pressure measurement of a particular 
constituent gas or a total pressure measurement or any other pressure 
measurement. 
If the switch 304 is open, the multiplier 301 is not activated, and 
positive ions exiting the mass filter 305 bypass the multiplier 301 and 
are collected by the plate portion 325 of the electrode 324. The resulting 
current in line 330 is measured by the electrometer 26. 
FIG. 5 depicts one exemplary embodiment in which the Faraday plate 325 used 
to collect positive ions and the anode 326 used to collect multiplied 
electrons are parts of the same electrode 324. In addition, only a single 
electrometer 26 is used to measure current induced in the electrode 324. 
In another embodiment, a separate Faraday plate and anode, each with its 
own electrometer, can be used. 
The method and system of the invention provide numerous advantages over 
prior approaches. For example, the small-signal detection capabilities of 
a mass spectrometer operating at relatively high pressure is improved. In 
addition, the method and system of the present invention provide an 
electron multiplier ion detector having improved ion detection 
capabilities through a broader range of pressures including pressures 
where in the prior art devices, described above, ion feedback can be 
significant, i.e., above 0.1 mtorr, and in fact the improved electron 
multiplier ion detector is useful in detecting ions up to 100 mtorr or 
greater without the need to calibrate the gain as a function of gas 
pressure. The invention provides the benefits of the low-noise 
amplification of an electron multiplier while eliminating the high-gain 
nonlinearities found in prior systems at high pressures. Because the 
behavior of the multiplier is linear and therefore readily characterized 
and predictable, the measurement of output current provides a more 
reliable indication of input ion quantities at high pressures than was 
possible with prior approaches. Also, by operating at relatively low gain, 
the invention eliminates the possibility of voltage discharge and/or 
breakdown that can become likely at high bias voltages (high gain) and 
high pressures. The invention also increases the useful life of the 
multiplier by operating at low gain and, as a result, reducing collisions 
with the doped inner surfaces of the multiplier. Finally, in the present 
invention, there is no appreciable variation in gain with pressure. As a 
result, there is no need for calibration as a function of pressure or at 
the operating pressure. 
While this invention has been particularly shown and described with 
references to preferred embodiments thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made therein without departing from the spirit and scope of the invention 
as defined by the following claims.