High flux ion gun apparatus and method for enhancing ion flux therefrom

The ion flux obtainable from an otherwise conventional ion source, to project a controlled ion beam at a target contained within an evacuated target chamber, is significantly enhanced by providing a flow of an ionizable gas directly into the ion source canister instead of being supplied into the target chamber. Flux enhancement values exceeding an order of magnitude may thus be obtained with ionizable gases such as argon, helium and neon. The highly enhanced ion flux is particularly advantageous for applications such as sputtering and ion scattering spectroscopy (ISS).

FIELD OF THE INVENTION 
This invention relates to an ion gun that provides a controlled beam of 
ionized gas particles directable to a target and, more particularly, to an 
ion gun with a high ion flux suitable for applications involving ion 
sputtering and to a method for enhancing the ion flux from an otherwise 
conventional ion source. 
FIELD OF THE INVENTION 
There are numerous manufacturing processes, particularly in the manufacture 
of solid state circuits and electronic components, in which a carefully 
controlled beam of ionized particles, preferably positively charged ions 
of a selected gas, is directed to the surface of a target of a selected 
material, e.g., for cleaning the same. In another common application the 
ion beam may be directed at the target, now acting as a source of a 
selected material, to cause atoms of that material, from the 
source/target, to be released for deposition elsewhere. The latter 
process, commonly referred to as sputtering, generally involves a low 
pressure gas, generally selected from a group of gases including argon, 
helium and neon, directed through an ion source to be ionized therein, 
after which the charged gas ions are electrostatically accelerated by an 
electric field toward the target. 
Preferred source gases for generating such an ionized beam, sometimes 
referred to as a plasma, include normally nonreactive gases such as argon, 
helium and neon. In the sputtering process, when such gas ions impact on 
the target or source, they dislodge atoms off the source material and 
these may be further accelerated by appropriately designed electrodes 
toward the surface to be coated, typically a substrate in the formation of 
a solid state element or circuit. Alternatively, the sputtered atoms may 
be formed into a separate focused beam for particular use. The target or 
source is often made a cathode in a circuit and may be heated to further 
assist the release of target, species atoms when exposed to the gas plasma 
or ion beam 
For sputtering and other similar applications, an evacuated target chamber 
is provided to enclose therewithin a target or source suitably located 
with respect to an ion source from which the ion beam is projected on to 
the target. Various electrical connections are made, in conventional 
manner, to appropriately charge the target chamber (which is normally 
grounded) with respect to the target or source (also normally grounded) 
and various portions of the ion source. 
Among the numerous devices and techniques taught in the relevant prior art, 
U.S. Pat. No. 4,692,230 to Nihei et al discloses a sputtered coating 
device including a differential exhausting device for maintaining a 
desired pressure differential between a main portion of a target chamber 
and the interior of a canister surrounding the ion source. This reference 
does not appear to teach any specifics regarding the relative gas 
pressures within the ion source and target chamber in the volume outside 
the ion source. 
U.S. Pats. No. 4,250,009 by Cuomo et al, No. 4,486,286 by Lewin, and No. 
4,491,735 by Smith, all disclose ion sources that include means for 
directly introducing the ionizable gas into a discharge chamber where the 
ions are to be formed therefrom. Differences between the teaching of these 
references and the current invention include the mode of ion formation, 
the corresponding ion source gas pressure required for ion formation, and 
the size of the orifice through which ions flow between the ion source 
canister and the target chamber. 
In all these known devices, the ions are formed by an RF or DC discharge, 
rather than by electron impact from an electron source filament as in the 
present invention. A well-known advantage of the electron impact method 
for generating ions is in relative simplicity and low cost of construction 
of the total ion source (including its electronics). In order to maintain 
these discharges according to the above-cited references, a relatively 
high pressure of the gas is required in the discharge region where the gas 
is to be ionized (.about.10.sup.-3 -10 torr). This is a much higher 
pressure than is required or typically employed in the source region of 
devices which utilize electron impact ionization (10.sup.-8 
-5.times.10.sup.-5 torr) Because of the higher pressures required, devices 
based on RF or DC discharges frequently employ a very small orifice 
(.about.1-1000 micrometers) through which the ions pass between the 
discharge region where they are formed and the target at which they are 
aimed. The target chamber is separately pumped, so that a substantial 
pressure gradient develops between the ion source canister and the target 
chamber due to the constriction of gas flow by the very small orifice 
therebetween. However, as is well known, the pressure gradient across any 
such orifice decreases drastically as the orifice diameter increases and 
as the pressure in the source region decreases. See, for example, 
"Scientific Foundations of Vacuum Techniques", by Dushman, S., Lafferty, 
J. M., ed. (2d. ed.), John Wiley and Sons, N.Y., 1962). 
By contrast, ion sources based on electron impact from a hot filament 
require a much larger orifice (.about.1 cm) and a lower pressure in the 
ionization region (.about.10.sup.-8 -10.sup.-4 torr) than described in 
these references. With the electron impact ionization technique, a useful 
pressure gradient can be readily maintained between the ion source 
canister and a typical target chamber. In the present invention employing 
impact ionization, the gas is introduced directly into the source canister 
where ionization is provided by impact of electrons produced at a filament 
with neutral source gas atoms, while a useful pressure gradient is 
maintained between the source canister and the target. 
In apparatus of the type described in the above-cited references, and as is 
common in utilizing ion beams or electron beams, it is also known to 
provide a beam-focusing lens that is generally of cylindrical form and is 
suitably charged, as well as beam deflection plates usually disposed in 
two orthogonally disposed pairs each member of which is separately charged 
to generate a composite electrostatic field capable of rapidly deflecting 
the charged particle beam that is to be controlled. 
For certain applications, sputtering being one, it is highly desirable, 
having selected an ionizable gas, to obtain a relatively high flux, i.e., 
a high rate of transfer of ionized gas particles from the ion source to 
the target surface per square area thereof per unit of time with a low 
rate of consumption (load) of the gas. The improvement taught and claimed 
herein is intended to and has been shown to enhance the ion beam flux from 
otherwise conventional ion sources by more than an order of magnitude over 
known techniques through the provision of a simple inlet tube for 
controllably introducing an ion source gas into an electron impact type of 
ion source canister which is mounted as usual within a target chamber. 
SUMMARY OF THE DISCLOSURE 
Accordingly, it is an object of this invention to provide an improved 
filament type electron impact ion source for obtaining a high flux ion 
beam therefrom. 
It is another object of this invention to provide an ion source apparatus 
provided with a controlled flow of a gas for ionization therein in a 
manner that promotes the generation of a high flux ion flow therefrom. 
It is a related object of this invention to provide a method for enhancing 
the ion flux of an ion beam emanating from an otherwise conventional ion 
source apparatus. 
It is a further related object of this invention to provide a method for 
sputtering selected atoms from a source thereof by directing thereto a 
high flux ion beam utilizing a selected gas to generate a beam of 
controllably directed ions. 
These and other related objects of the present invention are realized by 
providing apparatus and a method for directing a flow of a selected 
ionizable gas directly into a filament-type electron impact ion source, 
contained within an evacuated chamber, to generate an ion flux therefrom 
and regulating this flow of ionizable gas such that a partial pressure of 
the ionizable gas inside the ion source is maintained higher than a 
partial pressure of the ionizable gas outside the ion source but still 
within the evacuated chamber while the ion flux is being generated. 
Focusing of the enhanced ion flux into an ion beam and rastering thereof 
across a target surface are accomplished by known means and techniques. 
Still other objects and advantages of the present invention will become 
readily apparent to those skilled in this art from the following detailed 
description, wherein only the preferred embodiments of the invention are 
shown and described, simply by way of illustration of the best modes 
contemplated of carrying out 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 obvious respects, all 
without departing from the invention. Accordingly, the drawing and 
description hereof are to be regarded merely as illustrative in nature and 
not as restrictive, the invention being defined solely by the claims 
appended hereto.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The typical ion gun is a device for producing an ion beam from a selected 
gas and for directing the ion beam to a selected target. In normal use, 
such an ion gun is placed within the same evacuated volume as the target, 
within what is commonly known as a target chamber. The shape and the size 
of this target chamber, as well as that of the target, will necessarily 
vary with the application at hand. Referring now to FIG. 1, a typical ion 
gun 10, such as for example the Perkin Elmer 04-162, includes a hot 
filament 20 which acts as an electron source for electron-impact 
ionization of selected gas molecules housed within a barrel-shaped ion 
source canister 12 that is insulatedly mounted on a base or flange 14. An 
ion exit aperture 16 is normally provided in an end wall of the source 
canister 12 between the ionizing means and the target 18, so that a 
controlled flow of ionized gas atoms or molecules may pass therethrough 
from the source canister 12 to the target 18 across the volume of space 
therebetween. This is better understood with reference to FIG. 2, in which 
target 18 is seen disposed in known manner at a predetermined distance 
from aperture 16. 
The ionizing means of electron impact from a hot filament 20 has been 
widely used for many decades in ion guns, mass spectrometers and ion 
gauges. The generally known structure also involves the filament 20, 
commonly comprising thoria-coated iridium, mounted in a circular geometry 
so thaty it is concentric with and surrounding a metallic wire grid 
generally formed as a barrel shaped ion cage 22. This filament is 
generally concentric with and inside the source canister as indicated in 
FIG. 2. 
The individual electrical voltages applied to the source canister 12, to 
the two ends of filament 20, i.e., to electrical leads 24 and 26, as well 
as to the ion cage 22 are all separately controllable. They require four 
electrical connections insulatedly guided through a wall of a target 
chamber 28 enclosing the ion source and the target and connected to an 
external power supply (not shown) in known manner. 
The entire ion gun 10 and the various electrical connections are easily 
mounted, as best seen in FIG. 1, by one or more supports 29 extending from 
a typical copper gasket sealed flange 42 bolted to the bottom of target 
chamber 28. 
The process of generating gas ions with this type of ion source has been 
well known for decades. Electrons are generated at the hot filament 20 
which is heated by applying an AC or DC voltage across the ends of 
filament 20. Electrons are released from the thoria-covered surface of 
filament 20 and become available to be directed electrostatically in a 
manner described more fully hereinafter. The target 18 and the target 
chamber 28 walls are all typically maintained at ground potential, i.e., 0 
volts. Commensurately, ion cage 22 is maintained at a relatively large 
positive DC voltage, preferably in the range 400 to 5000 volts, via 
electrical lead 23, with respect to ground. 
The filament 20 is maintained at a nominal negative DC voltage, preferably 
in the range 50 to 200 volts, with respect to the ion cage 22, so that 
electrons generated at the hot filament 20 are accelerated toward and into 
the more positively charged ion cage which has an open grid structure and 
permits free entry and exit of such electrons. The ion source canister 
walls, i.e., 12, are maintained at a nominal negative DC voltage, 
preferably in the range 50 to 200 volts, with respect to filament 20. 
Therefore, the electrons on passing completely through the ion cage 22 are 
repelled by the (relatively) negatively charged canister walls 12 and 
return through the openings in ion cage 22. In this manner, they can make 
many passes through the ion cage 22. 
As persons skilled in the art will appreciate, this highly energetic motion 
of negatively charged electrons within ion source canister 12 is bound to 
result in collisions between the electrons and neutral gas particles 
present therein. Therefore, any neutral gas atoms that are present in the 
ion cage 22 have a significant probability of encountering one of these 
highly energetic, fast-moving electrons and thereby becoming ionized by 
interaction therewith. In this manner, numerous neutral gas particles 
become positively charged ions intermixed with the moving electrons and, 
likewise, becoming amenable to guided motion by the imposition of 
electrical fields provided by carefully selected electrical voltage 
differences in or near the source of such ionized gas atoms. 
The negative voltage applied on the source canister 12 (relative to the ion 
cage) is felt attractively by the positively charged ions that are present 
near the open end 30 of the ion cage 22. These ions are therefore 
accelerated out of open end 30 of the ion cage 22 and toward the ion exit 
aperture 16 in the end wall of source canister 12. Aperture 16 preferably 
has its center coaxial with that of the barrel-shaped ion cage 22. Some of 
the positively charged ions produced in the ion cage 22 will thus pass 
through exit aperture 16 and continue their trajectory along the axis of 
the ion cage 22. Target 18 is normally located so that its 
ion-intercepting surface is substantially intersected by the ion cage 
axis. As best seen with reference to FIG. 2, and indicated therein by a 
succession of paired arrows, a thus directed stream of ions forms an ion 
beam 32 directed toward the target. This ion beam 32 is accelerated by the 
relatively positive DC voltage between the ion source canister 12 and the 
target 18. Note that the source canister 12 was relatively positively 
biased with respect to the grounded target 18 for this purpose In known 
manner, the magnitude of this voltage difference can be controlled to vary 
the electrostatic acceleration applied to enhance the kinetic energy 
contained in the ions by the time they actually strike the target 18. Such 
an ion extractor system has been used with many ion sources and is well 
known in the art. 
In typical ion guns, the gas to be ionized normally enters the source 
canister 12 by effusion from the target chamber 28 through the aperture 16 
in the end wall of the source canister A valve (not shown) is normally 
provided in the wall of the target chamber for introducing the gas to be 
ionized from an external reservoir (not shown) into the target chamber. In 
this way, the respective partial pressures of the gas in the target 
chamber and the ion source are approximately equal, and can be 
conveniently monitored with any pressure gauge or mass spectrometer (not 
shown) available in the target chamber. Since the ion flux produced at the 
target by such a device increases in a reproducible way as this partial 
pressure increases up to .about.5.times.10.sup.-5 torr, the ion flux to 
the target can be reproducibly controlled by regulating the flow (and 
therefore the pressure) of the ionizing gas into the target chamber. 
Also well known and generally used are means such as a focusing lens 34, 
that often has the form of an electrically charged cylinder, for the 
purpose of focusing the ion beam 32 to a narrow spot onto target 18. In 
addition, the ion beam 32 may be rastered in known manner, i.e., its 
impact point on the target may be controllably traversed, across the 
target by means of paired deflection plates 36 and 38 in known manner. 
These deflection plates are therefore connected to external sources of 
electrical voltage to influence motion of the ion beam impact point at 
target 18 in known manner. 
Coming now to the specific improvement offered by the present invention, 
most conveniently understood with reference to FIGS. 1 and 2, a 
significant enhancement in the ion flux is obtained according to the 
preferred embodiment by providing a regulated flow of the gas that is to 
be ionized directly into the ion canister 12 via a tube 38 and a control 
valve 40. The partial pressure of that gas which is present in the 
canister 12 is maintained by gas flow dynamics through aperture 16 to be 
higher than the pressure of the gas that is in the target chamber 28 in 
the volume outside the canister 12. This gas flow is delivered 
conveniently through a small bore tube directed into the interior of 
canister 12, as best seen with reference to FIG. 2. 
Notice that the only possibility for this gas to then leave canister 12 is 
either as ions or as unionized gas, in both cases through the exit 
aperture 16. Notice specifically that there are no additional tubes 
provided on this canister for attaching a separate pump for removing this 
gas, as is the case in several commercial so-called 
"differentially-pumped" ion guns based on RF or DC discharge ionization. 
The gas then enters the target chamber 28, from which it exits through a 
tube 50 (not shown to scale, but much smaller than in typical 
applications) connected to a vacuum pump (as indicated by an arrow marked 
by the letter "V"). Therefore, an element of the improvement according to 
this invention is the use of the exit aperture 16 of appropriately chosen 
diameter as a gas flow restriction means as well as an aperture for 
extracting the ion flux. The pressure differential between the source 
canister 12 and the target chamber 28 will be directly related to the 
ratio of the pumping speed of the target chamber to the flow conductance 
of the gas through aperture 16, according to known principles of gas flow 
(see the previous cited reference of Dushman et al.). This flow 
conductance is, in turn, directly related to the diameter of the exit 
aperture. 
Details of entrance tube 38 and control valve 40, which are elements of the 
improvement according to this invention and as those used in the 
experiment for regulating delivery of the ionizable gas into the ion 
source canister 12, are best understood with reference to both FIGS. 1 and 
2. In the prototype apparatus, a hole was drilled into the mounting flange 
42 of the ion source and a narrow bore stainless steel tube 38 was 
attached to a standard ultra high vacuum leak valve 40 and welded or 
brazed to mate with this hole. A gas supply (not shown) containing the 
selected gas that was to be ionized was connected to an inlet port of the 
valve 40. A length of this stainless steel tube 38 which passed through 
flange 42 was then extended up to the base 14 of the ion source. The 
tubing 38 was mated into a small hole 46 in the canister base 14 through a 
ceramic adapter 44, made of a commercially available machinable ceramic 
known as Macor (DM). This adapter 44, which resembles a ceramic shoulder 
washer for a 1/16 inch bolt, serves as an electrical insulator since the 
ion source canister 12 typically operates at a relatively high positive 
voltage, typically around 4000-5000 volts. In this way, the gas which is 
leaked through valve 40 flows through the flange 42 via the tube 38 and 
into the canister 12 via the adapter 44 and hole 46. 
In experiments conducted with the prototype of the preferred embodiment of 
this invention, the ion exit aperture 16 on the ion source canister 12 was 
formed as a circular hole of diameter 0.6 cm. The ion source was mounted 
in a pre-existing target chamber such that the aperture 16 was 
approximately 15 cm away from a copper target (diameter=1.2 cm), and 
aligned such that the axis of the source canister 12 was aimed at the 
target through this aperture 16. The target chamber was pumped by a 
conventional ion pump attached to tube 50 such that the target chamber had 
a typical overall pumping speed for argon of approximately 150 liters/s. 
See the previously cited reference of Dushman et al for a definition and 
calculations of pumping speed. 
To test this ion gun, it was used to sputter the target with argon ions. 
Argon gas was introduced directly into the ion source canister 12 through 
the tube 38. With optimum voltages on the various components, a 3 KV argon 
ion beam of flux 3.times.10.sup.12 ions/cm.sup.2 /s at the target was thus 
generated with a pressure rise of only 1.times.10.sup.-6 torr of argon in 
the target chamber (for a nominal gun to target distance of 14 cm). In 
order to achieve this same flux by leaking the argon directly into the 
target chamber as in conventional designs (via a valve not shown), a 
pressure rise of 5.times.10.sup.-5 torr of argon was required. This 
implies that, when the gas was controllably leaked directly into the 
canister 12 the local argon pressure inside the canister was approximately 
fifty-fold higher than in the target chamber. Thus, the present invention, 
according to such a preferred embodiment thereof, leads to a reduction in 
the amount of argon gas required for the same amount of scattering by a 
factor of approximately fifty. Similarly, the time required to pump the 
target chamber back down to a routine operating pressure of 
.about.3.times.10.sup.-10 torr was decreased by a factor of about ten due 
to this decreased gas load, thus improving greatly the system recovery 
time after argon sputtering. For any fixed argon pressure in the target 
chamber which was below .about.10.sup.-6 torr, the ion flux at the target 
was a factor of approximately fifty larger when the argon was leaked 
directly into the source canister via the tube 38 than when leaked 
directly into the target chamber. 
When helium (He) was used as the ionizable gas, the pressure rise in the 
target chamber was 17 times less for the same He ion flux at the sample 
when the He gas was leaked directly into the ion source via tube 38 as 
compared to the conventional method of leaking the helium directly into 
the target chamber 12. This is understood to mean that good quality ion 
scattering spectroscopy (ISS) spectra can be collected in a few minutes 
with a helium supply pressure (P.sub.T) of only 3.times.10.sup.-8 torr, 
while a pressure greater than 5.times.10.sup.-7 torr would have been 
required without the improvement according to the present invention. Since 
ion energy analyzers are typically recommended not to be operated above 
about 2.times.10.sup.-7 torr, this turns out to be a critical difference 
and a very significant advantage over the known art. The energy resolution 
of the ions from such sputter guns is also quite sufficient for use in 
most ISS applications. Their energy spread of less than a few electron 
volts is considerably narrower than the ISS peaks, the widths of which, 
typically 30 electron volts, are usually determined by the physics of the 
ion scattering process. The improvement, as taught herein, therefore 
enhances the range and scope of use of otherwise conventional ion sources 
with only very minor structural modification in any easy-to-control 
manner. 
In this disclosure, there are shown and described only the preferred 
embodiments of the invention, but, as aforementioned, it is to be 
understood that the invention is capable of use in various other 
combinations and environments and is amenable to changes or modifications 
within the scope of the inventive concept as expressed herein.