Compact mass spectrometer for plasma discharge ion analysis

A mass spectrometer and methods for mass spectrometry which are useful in characterizing a plasma. This mass spectrometer for determining type and quantity of ions present in a plasma is simple, compact, and inexpensive. It accomplishes mass analysis in a single step, rather than the usual two-step process comprised of ion extraction followed by mass filtering. Ions are captured by a measuring element placed in a plasma and accelerated by a known applied voltage. Captured ions are bent into near-circular orbits by a magnetic field such that they strike a collector, producing an electric current. Ion orbits vary with applied voltage and proton mass ratio of the ions, so that ion species may be identified. Current flow provides an indication of quantity of ions striking the collector.

BACKGROUND OF THE INVENTION 
Knowledge of ion species and their concentrations in plasmas is necessary 
to the conduct of research and to improve manufacturing processes which 
utilize plasmas. The mass spectrometers currently used for this purpose 
are complex, expensive, and usually too large to properly characterize a 
plasma. These spectrometers can be used only for edge studies, in which 
measurements are made at the edges of a plasma rather than at locations 
where articles are placed for processing. Edge spectroscopy and 
theoretical modeling provide useful qualitative information on plasma 
composition, but normally do not provide quantitative information. The 
present invention provides an inexpensive and less complex mass 
spectrometer which is sufficiently small that it is capable of use in 
characterizing the entire volume of a plasma without perturbing it, that 
is, without significantly changing the characteristics of a plasma by use 
of the spectrometer. It is capable of use for performing in-situ mass 
spectroscopy of plasmas. Quadrupole mass spectrometers are widely used for 
characterization of plasmas at their edges. The cost of a quadrupole mass 
spectrometer and necessary accessories suitable for such use is in the 
range of $30,000 to $100,000.00. An instrument of the present invention 
can be fabricated and the required power supply and signal processing 
apparatus purchased for less than $1,000.00 (the cost will increase with 
increased sophistication of the electronics). 
A plasma is comprised of atoms and molecules in a gaseous state having no 
electrical charge, ions formed from the gas by providing energy to the 
gas, and electrons. The un-ionized atoms and molecules are termed 
neutrals. The ions are normally positive, but attachment of electrons to 
neutrals can take place so that negative ions are present in the plasma. A 
plasma may be produced in a chamber maintained at low pressure (normally, 
at pressures in the vacuum range) by introducing a gas into the chamber 
and providing energy to the gas by such means as an arc discharge or 
radio-frequency induction fields. 
Plasma processing is currently used primarily in manufacture of 
microelectronics components and is expected to be used more extensively in 
the future. Wet chemical processes are used to etch (remove material from) 
surfaces of wafers which are an intermediate product in the manufacture of 
integrated circuit devices, or chips. Waste products from such wet 
chemical processing are hazardous and expensive to reclaim. Plasma 
processing may be substituted for wet chemical processing. An oxygen 
plasma may be used for removal of photoresist mask material from 
microelectronics components after the process requiring masking is 
accomplished. Cleaning of surfaces may be done by plasma processing. For 
example, cutting fluids, oils, and greases may be removed from machined 
parts by means of an oxygen plasma, thus avoiding use of solvents. When a 
part is exposed to the plasma, oxygen ions strike the surface of the part, 
breaking chemical bonds between atoms of the hydrocarbons and creating 
reactive sites. Also, the plasma provides energy to the near-surface 
region which assists desorption of contaminants and enhances the chemical 
reactions. The products of this cleaning process are carbon dioxide and 
water vapor. Plasma processing is used to deposit material on a surface. 
For example, a mirror may be coated with a reflecting layer of gold or 
aluminum. Glass may be coated with a material which prevents ultraviolet 
light from passing through the glass. An oxygen plasma may be used to 
produce a thin layer of silicon oxide on a silicon surface. Diamond-coated 
objects may be produced by subjecting the object to carbon ions produced 
in a plasma. implantation of ions in a surface to modify its properties 
may be accomplished by plasma processing. For example, surfaces may be 
hardened by implanting nitrogen ions (nitriding) or boron atoms 
(boriding). The mass spectrometer of this invention provides means to 
monitor such processes in a production facility and to study such 
processes. 
SUMMARY OF THE INVENTION 
This invention is a mass spectrometer and methods for mass spectrometry 
which are useful in characterizing a plasma. This mass spectrometer for 
determining type and quantity of ions present in a plasma is simple, 
compact, and inexpensive. It accomplishes mass analysis in a single step, 
rather than the usual two-step process comprised of ion extraction 
followed by mass filtering. Ions are captured by a measuring element 
placed in a plasma and accelerated by a known applied voltage. Captured 
ions are bent into near-circular orbits by a magnetic field such that they 
strike a collector, producing an electric current. Ion orbits vary with 
applied voltage and proton mass ratio of the ions so that ion species may 
be identified. Current flow provides an indication of quantity of ions 
striking the collector. 
It is an object of this invention to provide an apparatus and method for 
determining the quantity and mass of ions produced in a plasma containing 
a single species or multiple species of ions. 
It is also an object of this invention to provide a mass spectrometer which 
is sufficiently small that it can be introduced at any desired location in 
a plasma for use in completely characterizing a plasma without 
significantly perturbing it. 
Another object of the invention is to provide an inexpensive mass 
spectrometer useful in characterizing a plasma. 
In one embodiment, the invention includes a mass spectrometer comprising in 
combination a shield enclosure of an electrically conductive material; a 
measuring enclosure of a magnetic material which is located inside the 
shield enclosure and is spaced apart from the shield enclosure by 
electrically insulating material; an aperture in the shield enclosure and 
a foil which covers the shield aperture, where the shield foil has an ion 
pathway; an entrance aperture in the measuring enclosure and an entrance 
foil which covers the entrance aperture, where the entrance foil has an 
ion pathway, and where the shield ion pathway and said entrance ion 
pathway are in register with one another; an exit aperture in the 
measuring enclosure and an exit foil which covers the exit aperture, where 
the exit foil has an ion pathway, and where the exit aperture and the exit 
foil are disposed such that the axial centerline of the exit ion pathway 
and the axial centerline of the entrance ion pathway are located in a 
single plane; one or more permanent magnets disposed inside the measuring 
enclosure to form a magnetic field, where magnetic lines of force of the 
magnetic field are normal to the plane of the ion pathways; a collector 
which is disposed adjacent to the exit foil in such manner that ions 
passing out of the measuring enclosure through the exit ion pathway will 
strike said collector; an electrical path from the collector to a 
reference location; means for establishing a known applied electrical 
potential between the measuring enclosure and the reference location, 
where said applied potential has a value effective for determination of 
proton mass ratio of the ions which strike the collector; and means for 
measuring electric current flowing along said electrical path, where said 
the electric current results from the ions striking the collector. 
In another embodiment, the invention is a method for determining mass and 
quantities of ions comprising the steps of maintaining a shield enclosure 
at an electrical potential relative to a reference location, where the 
electrical potential is termed the floating potential and is established 
by means of placing the shield enclosure in plasma, where the shield 
enclosure has an ion pathway, and where the difference in magnitude 
between plasma potential and said floating potential induces ions to enter 
the shield enclosure through said shield ion pathway and accelerates the 
entering ions; providing a measuring enclosure having an entrance ion 
pathway, where the measuring enclosure is located inside the shield 
enclosure and is spaced apart from the shield enclosure by electrically 
insulating material, and where the entrance ion pathway is in register 
with the shield ion pathway; establishing a known electrical potential, 
termed the applied potential, between the measuring enclosure and the 
reference location, where the applied potential induces ions passing 
through the shield ion pathway to enter the measuring enclosure through 
the entrance ion pathway and accelerates the entering ions; providing a 
magnetic field in the measuring enclosure having magnetic lines of force 
which are normal to the paths of the ions entering the measuring 
enclosure, where the magnetic field causes the ions which enter the 
measuring enclosure to travel in a non-linear orbit; providing an exit ion 
pathway in the measuring enclosure, where the axial centerline of the exit 
ion pathway and the axial centerline of the entrance ion pathway are in a 
single plane, and where the exit ion pathway is located such that a 
portion of the ions entering the measuring enclosure will pass through the 
exit ion pathway; providing a collector disposed outside the measuring 
enclosure in such manner that the ions passing through the exit ion 
pathway will strike said collector; providing an electrical path from the 
collector to the reference location; measuring electric current flow in 
said electrical path, where said electric current results from the ions 
striking the collector; and determining the proton mass ratio of ions 
striking the collector by reference to the values of the applied potential 
and the plasma potential.

DETAILED DESCRIPTION OF THE INVENTION 
Following is a description of a prototype mass spectrometer and an 
exemplary embodiment of the invention which is depicted in FIGS. 1 and 2. 
Measuring element 45 is comprised of measuring enclosure 3 and shield 
enclosure 2. The measuring element is placed in a plasma. The measuring 
enclosure is located within the shield enclosure. Shield enclosure 
aperture entrance foil 32, or shield foil 32, covers shield enclosure 
aperture 1. Measuring enclosure aperture entrance foil 9, or entrance foil 
9, covers measuring enclosure entrance aperture 34. Reference number 4 
depicts the paths of ions passing into the shield enclosure through a hole 
in shield foil 32, that is, along shield ion pathway 33, and then into the 
measuring enclosure through a hole in entrance foil 9, that is, along 
entrance ion pathway 10. Ion pathways 10 and 33 are in register with one 
another, that is, the axial centerlines of the two ion pathways are on a 
single line. Reference numbers 5, 6, and 7 show paths which ions may take 
after they enter the measuring enclosure; the path which an ion follows 
depends on its energy. The two enclosures are spaced apart from one 
another by electrically insulating material, as depicted by exemplary 
spacer blocks 37. 
Ions which enter the measuring enclosure assume non-linear orbits which are 
near circular because they are acted upon by a near-constant magnetic 
field produced by permanent magnets 11 and 12. Arrow 13 shows the 
direction of the magnetic lines of force of the magnetic field which is 
normal, or perpendicular, to a plane which contains the axial centerlines 
of entrance ion pathway 10 and exit ion pathway 36. Ions traveling along 
ion path 6 pass through exit ion pathway 36 in exit foil 25, which covers 
exit aperture 35. These ions then strike collector 8 which is connected to 
a reference location at a constant electrical potential by means of an 
electrical path represented by wires 17 and 22. The reference location is 
normally the chamber in which the plasma producing the ions is located, 
referred to as the chamber ground. The chamber is usually grounded to the 
earth. Reference number 18 shows means for measuring and recording 
electric current flowing in wires 17 and 22 which results from ions 
striking collector 8. 
Reference number 19 depicts means for applying, recording and varying an 
electrical potential between measuring enclosure 3 and the reference 
location. Wire 16 connects this power supply means to the enclosure. Wires 
16 and 17 pass through shield 20, which is located inside of stem 15. Stem 
15 is an elongated hollow rod of an electrically insulating material which 
is suitable for service in the vicinity of a plasma, such as a ceramic. 
Shield 20 is a hollow rod of an electrically conductive material which 
surrounds wires 16 and 17. Plug 21 provides a vacuum seal at one end of 
stem 15 through which the wires pass. Stem 15 is connected to shield 
enclosure 2. Conduit 14 communicates with the interior of the shield 
enclosure via stem 15 and with vacuum pump 38 which is capable of reducing 
the pressure inside of the shield enclosure. Aperture 39 is provided in 
measuring enclosure 3 to ensure that a path exists for removal of gas from 
the measuring enclosure. 
FIG. 3 depicts apparatus for providing an ion pathway into the measuring 
enclosure. This exploded drawing is a detail of FIG. 1. A similar 
arrangement may be used for the shield ion pathway and there are other 
ways known to those skilled in the art to provide an ion pathway effective 
in the practice of this invention. The wall of the measuring enclosure, 
denoted by reference number 29, contains aperture 34. Entrance foil 9, 
having ion pathway 10, rests in recess 40 of wall 29. Washer 31 is placed 
in recess 40, with an interference fit, in order to hold foil 9 in place. 
Wire mesh 30 is sandwiched between the foil and the wall of the enclosure. 
The purpose of the wire mesh is to prevent voltage shielding by the 
plasma. The mesh used in the exemplary apparatus is of copper with square 
openings of 64 .mu.m and 95% transparency. It is desirable that the size 
of the mesh be about equal to or smaller than typical plasma Debye 
lengths. 
FIG. 4 depicts apparatus for providing an ion pathway to the collector. 
Other arrangements may be used. Exit foil 25 rests in recess 41 of 
measuring enclosure wall 28, covering aperture 35. Exit foil 25 has exit 
ion pathway 36 and is retained in recess 41 by washer 26. Collector 8 is a 
disk of copper which rests, in an interference fit, in recess 42 of 
retainer 27. Retainer 27 is of an electrically insulating material and 
rests inside of washer 26 in an interference fit. Wire 17 passes out of 
the enclosure to current measuring means 18. 
The measuring element is placed in a plasma. Ions of the plasma pass into 
the measuring enclosure by means of the ion pathways in the shield foil 
and the entrance foil. The difference in magnitude between plasma 
potential, V.sub.p, and floating potential, V.sub.f, induces ions to enter 
the shield enclosure, and some ions enter simply because their paths 
happen to coincide with the shield ion pathway. The ions are accelerated 
from the electrical potential of the plasma, V.sub.p, to the floating 
potential. Floating potential is the electrical potential, relative to the 
reference location, which is assumed by the shield enclosure as a result 
of its presence in the plasma. Ions which enter the shield enclosure are 
induced to enter the measuring enclosure and are further accelerated by 
V.sub.a, the electrical potential applied between the measuring enclosure, 
and the reference location. Ions are acted upon by the magnetic field 
created by the permanent magnets in the measuring enclosure and their 
paths, or orbits, are bent into a near circular configuration. Orbit 
radius refers to the path which an ion follows after entering the 
measuring enclosure. Ion paths 5, 6, and 7 of FIG. 1 are examples of 
different orbit radii. Ion mass and quantities may be determined as 
follows. 
Energy possessed by an ion, E (in volts), is 
EQU E=V.sub.p V.sub.a. 
Orbit radius, .rho., of an entering ion, in mm, can be calculated by the 
equation 
EQU .rho.=0.144 (EA/Z).sup.0.5 /B. 
Transforming the equation: 
EQU A/Z=48.2253 B.sup.2 .rho..sup.2 /E. 
B is the average magnetic field strength in Tesla at the location of the 
ion path. Z is the charge state of the ion. A is the proton mass ratio of 
the ion, which is ion mass divided by mass of a proton. 
.rho..sub.s is the orbit radius of an ion entering the measuring enclosure 
which passes through the exit foil and strikes the collector. .rho..sub.s 
is established by the dimensions of the measuring enclosure. At a single 
value of V.sub.a, only ions of a single proton mass ratio will strike the 
collector. Ions having other mass ratios will assume orbits having radii 
less than .rho..sub.s, as shown by ion path 5 of FIG. 1, or greater than 
.rho..sub.s, as shown by ion path 7. Determination of proton mass ratio of 
ions striking the collector by means of the above equations serves to 
identify the species of the ions. Those skilled in the art normally deal 
with the quantity A/Z. Ions striking the collector cause an electric 
current, termed ion current, to flow in an electrical path between the 
collector and the reference location. Ion current provides an indication 
that ions are striking the collector. When ion current is zero, no ions 
are striking the collector. V.sub.a may be varied while ion current flow 
is simultaneously measured. This is termed a voltage scan or sweep. A plot 
of applied voltage versus ion current may be used to identify values of 
V.sub.a for use in calculating A/Z and A. The proton mass ratio is 
calculated for peak values of ion current and provides the atomic weight 
of ions which strike the collector at those points, or peaks, on a voltage 
scan plot. 
FIG. 5 is a simulated plot of ion current flow versus V.sub.a obtained when 
a voltage scan was done with a measuring element placed in a pure oxygen 
plasma formed in a vacuum chamber maintained at about 1.0 mTorr, where the 
plasma was formed by energy supplied by an induction coil operated at 500 
W. The peak indicated by reference number 43 is at about minus 42 V, and 
the peak indicated by reference number 44 is at about minus 13 V. Using 
the above equations, A for peak 43 is 16 and A for peak 44 is 32, 
indicating that the first peak is due to O.sup.+ ions striking the 
collector and the second peak is due to O.sub.2.sup.+ ions striking the 
collector, since the molecular weight of oxygen is 16 and that of an 
O.sub.2 molecule is 32. The valleys at a current flow of zero on the X 
axis indicate that no ions were striking the collector at the 
corresponding applied voltage values. In a plasma containing other ions, 
peaks would occur at other voltages and calculation of proton mass ratios 
for those voltages would indicate the species of ion present. Peak heights 
indicate the relative amounts of ions present. In FIG. 5, peak heights 
were 1.35 nA and 1.52 nA, showing that O.sub.2.sup.+ was the more 
populous species and that the ratio of O.sub.2.sup.+ to O.sup.+ is 1.13. 
Calibration of a system may be accomplished by feeding gases into a plasma 
chamber one at a time. Investigators are normally interested in relative 
quantities of ion species rather than absolute numbers of ions, though 
absolute quantities may be calculated by those skilled in the art. 
The outside dimensions of the shield enclosure of the prototype instrument 
are 58 mm.times.47 mm.times.38 mm high. The outside dimensions of the 
measuring enclosure are 36 mm.times.28 mm.times.25 mm high. .rho..sub.s is 
16 mm and the linear distance between the shield foil and the entrance 
foil is 11 mm. A pumped measuring element having outside dimensions of 
40.times.32.times.32 mm and d=.rho..sub.s =16 mm has been designed. 
Referring to FIG. 1, .rho..sub.s =b=c and d=11 mm. The shield enclosure 
shields the measuring enclosure from the plasma and serves to prevent a 
large extracted current flow. The prototype shield enclosure is of 
aluminum; however it may be comprised of any conductive material suitable 
for exposure to the operational environment. The shield enclosure is 
present because, if it were not, electron current would collect on the 
measuring enclosure when applied potential is greater than plasma 
potential. This would perturb the plasma and cause a large current flow in 
the wire to the power supply, probably causing the current flowing from 
the power supply to be greater than its rating. A shield enclosure may be 
of a non-conductive material, such as quartz, with a metal lining. The 
prototype measuring enclosure is of steel; any magnetic material may be 
used. Teflon (registered trademark) was used for spacer blocks to provide 
electrical separation between the two enclosures and for the collector 
retainer since it has good high temperature and vacuum properties, 
remaining dimensionally stable and not outgassing while in the vicinity of 
a plasma; other insulating materials, such as a machinable ceramic, may be 
used. The shield foil, entrance foil, and exit foil of the prototype 
measuring element are of 50 .mu.m thick stainless steel. The shield ion 
pathway, entrance ion pathway, and exit ion pathway were drilled through 
the foils using a 1/32 in. drill. The foils may be of any conductive 
material. 
The permanent magnets of the prototype instrument are samarium/cobalt and 
provide an average magnetic field strength of about 0.28 T. Other types of 
magnets which provide magnetic fields of appropriate strengths to match 
the parameters of the above equation may be used in determining proton 
mass ratio. Location and quantities of magnets may be varied as long as 
magnetic field direction is as specified herein. The collector of the 
prototype instrument was a copper disc of 6 mm diameter and 1.6 mm thick 
having a wire soldered to it. The collector may be of any convenient size 
such that ions leaving the measuring enclosure will strike it. It may be 
of any conductive material which is resistant to ion impact and which will 
provide low secondary electron emissions. In applications where ion 
impacts might cause spallation of the collector surface, a more resistant 
material than copper, such as tungsten or molybdenum, may be used. It is 
preferable to use wire mesh (to prevent voltage shielding) of tungsten 
rather than copper in order to avoid, when plasma ions strike the mesh, 
creation of copper ions and a small peak attributable to copper. The plug, 
or seal, at one end of the stem may be of any suitable material. The 
washers for retaining the foils may be of any convenient material which is 
resistant to conditions near the plasma being studied. Aluminum and Teflon 
were used in the prototype. 
The axial centerlines of the ion pathways in the foils should be normal to 
the foils. It is desirable that the foils be thin so that ions having 
paths which are not parallel to the axial centerlines are less likely to 
strike the foil as they pass through, that is, so that clipping is 
avoided. It is desirable that the ions passing through the exit foil 
travel along paths normal to the foil, that is describe an arc of 
90.degree. between the entrance foil and the exit foil. Referring to FIG. 
1, this may be achieved by setting the c dimension equal to the b 
dimension. Providing a measuring enclosure with b equal to c reduces the 
possibility that ions passing through the exit foil will strike the edge 
of the foil before they leave the ion pathway. In order to achieve good 
mass resolution, it is highly desirable that the d dimension be about 
equal to the b dimension and that the d dimension be as large as possible 
without causing the measuring element to be so large as to significantly 
perturb the plasma. To achieve this, the d dimension will be about 100 to 
200 plasma Debye lengths. Also, resolution improves with decreasing ion 
pathway diameter. Resolution is also affected by orbit radius, ion pathway 
alignment, ion-neutral collisions, plasma potential fluctuations, and 
plasma temperature. High resolution provides a spike rather than a broad 
peak, that is, the voltage range over which an ion current relating to a 
particular ion species is seen is small. 
In experimentation conducted with the prototype instrument, V.sub.p was in 
the range of 10 to 20 V and V.sub.f was in the range of 0 to 4 V. These 
are the ranges normally seen in experimentation. The applied potential may 
be provided with a direct current power supply and varied manually in 
steps. In the first experiments, steps of 2 to 5 V were used and ion 
currents were measured with a Fluke Model 77 multimeter, using the 
10.sup.7 .OMEGA. resistor of the meter. Typical ion currents were 1 to 10 
nA. In later experimentation, voltage scans from plus 100 V to minus 100 V 
were accomplished with circuitry similar to that used for Langmuir probes. 
Data was taken with a personal computer-based Langmuir probe data 
acquisition system, where a plus or minus 5 V range is amplified to plus 
or minus 50 V and a 100 V voltage sweep is obtained with an additional 
fixed series voltage. Slow sweeps of 15 seconds duration over the 200 V 
range (250 samples with 60 ms/sample and 0.4 V increments) were used to 
avoid lead capacitance effects. The collected ion currents were measured 
with a 10.sup.8 .OMEGA. resistor connected to the data acquisition system 
via an operational amplifier (Tektronix AM 501). Typically, 3 to 5 
successive sweeps were done and then averaged in order to improve the 
signal to noise ratio. The data were very repeatable. 
Use of a variable extraction voltage and a fixed magnetic field has a 
potential danger of introducing variations in extracted current. Extracted 
current is ion current (current due to ions striking the collector) plus 
current due to ions which strike the measuring enclosure at locations on 
its inner surface. Varying extracted current will cause variations in peak 
heights, causing ratios of ion species quantities to be inaccurate. 
However, both theoretical analysis and experimental data show that a 
constant extracted current can be maintained, provided that dimension d of 
FIG. 1 is at least about 100 to 200 plasma Debye lengths; this can be seen 
from FIG. 6. FIG. 6 presents calculated normalized ion currents as a 
function of normalized ion energy for four different ratios of aperture 
separation, d, to bulk plasma Debye length, .lambda.. Normalized current 
equals extracted current divided by plasma current. Plasma 
current=en.sub.o V.sub.o, where e=electron charge, n.sub.o =bulk plasma 
density, and V.sub.o =(2eT.sub.e /m).sup.0.5, where T.sub.e =electron 
temperature and m=ion mass. Normalized ion energy is E/T.sub.e. As d is 
increased, normalized ion current remains flat at a value of one for 
larger ranges of normalized ion energy. Thus, establishing d at an 
appropriate value removes the danger of introducing variations in 
extracted current. Also, accuracy is assured by limiting V.sub.a to a 
range of from -100 V to +100 V. Experiments to demonstrate that extracted 
current remains sufficiently constant within the 200 V range of applied 
voltage were conducted. A collector was placed inside the measuring 
enclosure in a location close to the entrance ion pathway such that 
substantially all ions entering the measuring enclosure struck the 
collector. Data was gathered using an oxygen plasma formed by a 400 W 
inductive source at 1 mTorr. Before this invention, it was not known by 
those skilled in the art that constant extracted current was obtainable in 
the manner of the invention. 
The shield enclosure and measuring enclosure may be pumped, that is; a 
vacuum pump may be connected to the stem or shield enclosure to remove gas 
from the measuring element in order to reduce ion scattering. It is 
desirable that the mean free path, that is, the average length of a path 
followed by a particular ion between collisions by the ion with neutrals, 
be larger than the length of the ion path from entrance foil to exit foil. 
Pumping is not required if neutral pressure is less than about 2 to 3 
mTorr, since ion/neutral mean free paths are about 50 mm at 1 mTorr. 
Pumping can be accomplished by connecting the conduit of FIG. 2 to a 
channel through the shield enclosure, though it may not be desirable to 
locate the conduit such that it may perturb the plasma. The stem may be 
utilized as a handle, in that the measuring element may be moved across a 
plasma chamber by inserting or withdrawing the stem. In this case, the 
stem passes out of the chamber through a port having a sealing mechanism 
around the stem. The measuring enclosure and shield enclosure shown in 
FIGS. 1 and 2 are of rectangular geometry. However, the enclosures may be 
of any convenient geometry. This invention can be used in characterizing 
plasmas having densities in the range of about 10.sup.15 to 10.sup.18 
m.sup.-3 and electron temperatures of a few eV. 
FIG. 7 shows data from experimentation with an argon plasma. Six different 
Ar.sup.+ (A=40) peaks are shown for 6 different pressures in the measuring 
enclosure established by pumping. The pressures are 0.25, 0.5, 1, 3, 5, 
and 10 mTorr. The Ar.sup.+ peak shifts to high voltage values as pressure 
is decreased. V.sub.p can be determined from peak locations. 
In order to detect light ions without using large values of applied 
voltage, a secondary collector may be used. Light ions may be defined as 
those having proton mass ratios in the range of 1 to 10. The secondary 
collector will be located such that ions having a small orbit radius 
(relative to ions which strike the primary collector) will strike it. A 
secondary collector 52 is depicted in FIG. 1. Ions following the path 5 
pass through secondary exit ion pathway 51, which is located in secondary 
exit foil 50. Ion current flows to a reference location, passing through 
wire 53, which is routed through shield 20 in stem 15 and is measured as 
described above in a similar manner to the current appearing on collector 
8 by electric current measuring and recording means 54. The features of 
the secondary collection system are the same as those described above in 
regard to the primary collection system. Of course, ions striking the 
secondary collector pass through a 180.degree. arc inside the measuring 
enclosure. 
Prior to fabrication of the prototype described herein, a measuring element 
having d=5 mm and a different exit aperture was tested. The exit aperture 
was located on the wall of the measuring enclosure parallel to the wall 
containing the entrance aperture, but lower down, near the floor of the 
measuring enclosure. This device provided data of poor quality.