XEI Scientific, Inc.
The EVACTRON® Anti-Contaminator and De-Contaminator Updated August 2007
US
Patent 6,452,315 issued 9/17/2002
"Compact
RF Plasma Device for Cleaning
ABSTRACT
An
improved apparatus is provided for in-situ cleaning of electron
microscopes and other vacuum chambers. A special RF plasma electrode
is housed in a compact cylinder constructed of standard vacuum
components and a electrical feedthrough. The device allows oxygen
radicals to be generated from air by a low powered RF plasma. The
oxygen radical flow by convection into the electron microscope or
vacuum chamber to be cleaned and react with hydrocarbons to form CO
and H2O vapor which is pumped away.
RELATED
PATENT APPLICATIONS: 09/054,749 filed 4/3/1998 now abandoned. Patent
6,105,589 issued 8/22/200, filed 1/11/1999: "Oxidative Cleaning
Method and Apparatus for Electron Microscopes Using an Air Plasma as
an Oxygen Radical Source" by Ronald Vane and a C.I.P. patent
application of the above 09/552,449 "Low RF Power Electrode for
Plasma Generation of Oxygen Radicals from Air". The present
invention discloses a specific apparatus design for use with the
above method and with the above electrode.
BACKGROUND OF THE INVENTION
Field of Invention
Description Prior Art
It has
been well documented that low temperature (<50o C.) plasmas of
various ionized gases can be used to reactively etch/ash organic
materials found on the surface of materials. As "glow-discharge
cleaning" it has been used by the high-energy physics community
to condition the interiors of large vacuum vessels. Named "plasma
etch" or "plasma ashing", it has been used in the
industrial community to clean and etch semiconductor wafers and other
bulk materials for many years. In the microscopy community RF or DC
plasma, dry-ashing devices are sold by several vendors to clean
electron microscope specimens prior to analysis. In this procedure,
typically the material is placed in an RF cavity or a DC cavity with
a flowing reactive gas. The nature of the gas selected is chosen
based upon the desired effect. Argon, nitrogen, air, oxygen or other
gas mixtures are commonly used, and gases (BCl3, CF4) may be used to
tailor the reaction.
Most of
the current literature and recent patents on glow-discharge cleaning
and plasma etch is concerned with the use of these processes in
semiconductor production. For these processes plasma uniformity,
anisotropic etching, and other highly controlled properties are
important. The geometry of these systems is very carefully designed
for uniform results. A variety of gases can be used for etching and
cleaning. Gases such as Hydrogen, Argon, Nitrogen, Oxygen, CF4 and
gas mixtures such as air and argon/oxygen have successfully been used
for glow-discharge cleaning and plasma etching. Depending on the
process the importance of ion sputtering and reactive ion etching
varies, but in most of processes the neutral free radicals are the
most important reactive species in the plasma. The free radicals,
because they are neutral, are able to leave the electric fields of
the excitation region and travel throughout the chamber by convection.
For the
cleaning and removal of hydrocarbons the reaction with oxygen
radicals to produce CO, CO2 and H2O is the most important. These
reaction products are quickly removed as gases from the vacuum
system. These reactions are the dominant reactions in glow discharge
cleaning methods using oxygen as a reactant gas. The glow discharge
is used to produce oxygen ions that are then transformed into oxygen
radicals by subsequent reactions. The oxygen ions are not needed as
the reactive species for hydrocarbons. In the absence of nitrogen
ions or other reactive species that destroy O radicals, O radicals
are long lived and have the ability to do isotropic cleaning on all
surfaces in the chamber. To prevent the formation of Nitrogen ions or
other active species that destroy O radical a low temperature plasma
is needed. The Ionization potential; of nitrogen is that of oxygen.
In low temperature plasmas in low vacuum air the formation of oxygen
ions is favored and leads to the formation of oxygen radicals in
useful quantities.
Conventional
RF or DC plasma cavities for the production of plasmas are usually
of four different types: parallel plates for DC and RF capacitivly
coupled plasmas, RF inductive coils, RF multiple electrodes, and
hollow cathodes. At high vacuums magnets are often used to confine
the free electrons to keep the plasma ignited. Parallel plates and
inductive coils and their variations are the classical
"textbook" designs and well understood. Designs with
multiple electrodes that are of opposite RF potential have been
devised for filling vacuum cavities with plasmas for etching and
cleaning purposes. The hollow cathode design is efficient because it
traps the electrons between the walls of the cathode and results in
higher free electron densities. Because they are usually purely
capacitive or inductive they require high voltages or power to ignite
the plasma. Typical peak to peak RF voltages are above 400 V and
power is above 100 W. This results in a relatively high energy or
high temperature electrons in the plasma. Many of these sources are
designed to create plasmas under difficult conditions of either high
vacuum or atmospheric pressure where it is harder to ignite or
sustain a plasma. At low vacuum between 0.1 Torr and 2 Torr gases
become highly conductive and plasmas are easy to ignite and sustain.
At these pressures almost any shape of electrode inside a grounded
metal vacuum chamber will ignite and sustain a plasma with the
application of sufficient RF power if the impedance of the RF circuit
is properly matched between the source and the load.
To mount a
plasma device on an electron microscope or other vacuum system the
designer must consider how to fabricate the device economically. The
manufacturing of custom vacuum parts is machine shop is very
expensive. Therefore it is useful to use standard vacuum parts for
assembly of the system. ISO components (a flange system designed in
accordance with ISO, International Standards Organization - standard
2861/1) are an economical, convenient and simplified means of
constructing custom vacuum devices and systems. This flange system
consists of two identical, symmetrical flanges, a centering ring that
supports an elastomer "O" ring and a clamp that compresses
the sealing ring between the two flanges. These ISO fittings have a
variety of designations from their manufacturers as KF, QF, NW, etc.
and are referred to in this specification as ISO KF. An alternate
flange system with copper gaskets is suitable for ultra high vacuum
applications. This system is known as CF or Conflat(r) (a registered
trademark of Varian Associates). The CF system is not used on the
chambers of electron microscopes but is common in other types of
ultra high vacuum instruments. In both the ISO KF and CF flange
system there are available RF coaxial feedthroughs and various sized
adapters and fittings.
SUMMARY OF THE INVENTION
The
present invention is directed to an apparatus for housing a
cylindrical electrode and supplying RF power and reactive gas to a
plasma around the electrode. The apparatus produces oxygen radicals
with a reactive gas of air, with the RF power below 15 Watts, and
with the vacuum between 0.2 Torr and 1 Torr. At these pressures and
power levels sputtering is suppressed which minimizes electrode and
chamber damage and sputter contamination. The apparatus is mounted on
electron microscopes and other vacuum chambers for the purpose of
making oxygen radicals from air for reactively removing hydrocarbons
for the walls of the chamber. It is also useful as a device for
generating low energy active species from a variety of gases at that
uses a low power RF generator and is simple to construct from
commercial parts.
Meeting
the following mechanical and physical requirements is an object of
the present invention:
1.
Mountable with simple adapter flange on a port of most commercial
electron microscopes.
2. No
mechanical interference with other common microscope accessories and detectors.
3. Compact.
4. Simple
to make out of commercial parts with standard sized flanges and
having little custom fabrication.
5. Have a
vacuum gauge and metered gas inlet into the plasma region.
6. Can
operate at low RF power and in low vacuum
The
present invention covers the use of a small cylindrical chamber, a
coaxial electrical feedthrough for RF power, the mounting of the
cylindrical electrode with apertures to the feed through, the feeding
of an reactive gas such as air into to side of the chamber and the
mounting of a pirani gauge to the chamber for measuring the vacuum.
Brief
Description of the Figures
The
present invention together with the above and other objects and
advantages may best be understood from the following detailed
description of the preferred embodiments of the invention illustrated
in the drawing, wherein:
Figure 1
is a cross sectional view of a compact RF plasma device that is
assembled from standard vacuum fittings.
Figure 2
is cross sectional view of an extended RF plasma device the is twice
the length of that in Fig. 1 that utilizes a higher gas flow rate to
make more active species from the gas.
Reference
numbers in drawings:
1 Needle valve
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In
accordance with the invention a plasma generating apparatus has been
developed that is compact and operates in low vacuum at low voltage
and low power that generates active species from air or other gases
for use in cleaning vacuum chambers or other reactive processes. The
device has utility for cleaning electron microscopes and other high
vacuum analytical instruments and high vacuum chambers. It also has
utility as an active species source for a variety of other processes
and process development where a small RF plasma source is desired
The first
preferred embodiment of the invention consists of following parts as
shown in Fig 1. The cylindrical body 12 of the device has ISO KF40
vacuum flanges 11 at both ends. A vacuum seal is made between the ISI
KF flanges 11 by means of an elastomer O-ring 16 that is held in
place by a centering ring 18.The ISO KF 40 flanges 11 are held
together by standard ISO KF clamps which are not shown for clarity.
The inside diameter of the body 12 is about 40 mm. The length of the
body 12 is about 40 mm. Two 1/8 NPT threaded holes 13 enter the body
from the sides. These holes are used for a gas inlet 3 and a pirani
gauge 24 for measuring vacuum. On one end of the cylinder body 12 is
mounded a coaxial feedthrough 22 on a mating ISO KF 40 flange with a
conductor 9 extending coaxially into the middle of the chamber. On
the outside end of the feedthrough a coaxial RF connector 20 allows
RF power to be fed to the conductor 9. The base of the conductor 9
inside the chamber near the flange is covered with a high voltage
ceramic insulator 10. An Aluminum support cross bar 8 that is
1/8" in diameter with a mounting hole in the center is
press-fitted onto the conductor 9 end. This crossbar 8 in turn
supports the cylindrical apertured electrode 6 by being fitted into a
pair of holes in the electrode. The electrode 6 is made of Aluminum
screen that has 1/8" diameter holes punched into it in a regular
pattern. The electrode 6 cylinder has a diameter of about 20 mm. The
conductor 9 is concentric to the electrode cylinder 6.
On the
sides of the device body 12 are mounted the gas inlet 3 manifold and
a pirani gauge 24. The pirani gauge 24 is used to measure the vacuum
inside the chamber 4 while the device is operating. The pressure
range for making oxygen radicals without sputtering from air
efficiently is from 0.2 Torr to about 1 Torr. At these pressures
sputtering is surpressed by the very short mean free paths in the
vacuum. The gas inlet 3 manifold has solenoid valve 2 for opening and
closing the gas flow and a needle valve 1 for controlling the flow of
gas into the chamber. The gas feed port 15 may be left either open to
the air or connected to another gas source.
The device
is mounted to the vacuum chamber or electron microscope by means of
an adapter flange 14 that has ISO KF40 flanges 11 on the end. A
vacuum seal is made between the ISI KF flanges 11 by means of an
elastomer O-ring 16 that is held in place by a centering ring 18.The
ISO KF 40 flanges 11 are held together by standard ISO KF clamps
which are not shown for clarity.
The second
preferred embodiment is shown in FIG 2. In this version of the device
the length of he plasma chamber 4 is extended by clamping two ISO KF
40 cylinders 12 together and making a longer cylindrical electrode 6.
Each segment of the body has a single threaded hole 13 for the gas
inlet and Pirani gauge. The gas inlet 3 is placed closest to the RF
feedthrough 22 and the Pirani gauge 24 is mounted on the other
segment. This extended version is for systems where larger vacuum
pumps allow for high pumping speeds and high gas flow through the
plasma. By making the plasma region longer the gas molecules have a
longer residence time in the plasma and more active species are produced.
The third
embodiment of the device uses CF type flanges with copper gaskets
instead ISO KF40 flanges with O-rings. CF 2 3/4: flanges are also
suitable for cylinders with about 40mm diameter bodies. CF flanges
with copper gaskets allow the device to be mounted on ultra high
vacuum systems.
In the
fourth embodiment of the device ISO KF 50 flanges are used. The use
of ISO KF 50 flanges allows the inside diameter of the chamber
cylinder 4 to be increased to about 50 mm. inches. This in turn
allows the diameter of the electrode 6 to be increased.
The
cylindrical RF electrode 6 has been described in a previous patent
application. In the preferred embodiments of the present invention it
is made of punched Aluminum screen that has been bent into a
cylindrical shape. The punched holes in the screen are about 1/8"
diameter. Because it is cut from sheet screening there are many
sharp edges that form high electric gradient fields to facilitate
plasma ignition. The cylinder forms a hollow cathode surrounded by
many small hollow cathodes. In addition, when RF is supplied to the
electrode, RF eddy currents rotate around the apertures to provide
inductive as well as capacitive coupling of the RF to the plasma.
I claim
1. Apparatus for removing hydrocarbons from Electron Microscopes and other vacuum
chambers by oxidation with RF plasma generated oxygen radicals comprising:
a plasma chamber for generating oxygen radicals (oxygen atoms) from a
variety of oxygen containing gases including air with said radicals
carried by convection into adjoining said vacuum chamber,
said
plasma chamber consisting of a non-ferrous, non-magnetic metal
cylinder between 35 mm and 60 mm inside diameter and between 25mm and
120mm long,
said
plasma chamber mounted on said vacuum chamber with a vacuum tight
seal means,
a gas
inlet means on the side of said plasma chamber that allows a metered
leak of said oxygen containing gas into said plasma chamber to
maintain a predetermined vacuum pressure and to feed said gas through
said RF plasma,
a vacuum
gauge means mounted on the side of said plasma chamber that allows
pressure to be monitored inside said apparatus,
an
electrode means of ignition of said plasma at below 20 Watts of RF
power without magnetic confinement,
an
electrical feedthrough means in a circular flange mounted on the end
of said cylindrical chamber with a vacuum tight seal means,
a coaxial
conductor means extending into said chamber from said feedthrough means,
a
conductive crossbar means connecting said conductor means with said
electrode means to support and to feed RF power to said electrode,
whereby
said oxygen radicals are used to oxidize said hydrocarbons to form
CO, H2O and CO2 gases that may removed from said vacuum chamber by
vacuum pumping means.
2. An
apparatus as described in claim 1 further including an electrode
means that operates at 13.56 MHz in the form of a cylinder with a
diameter between 12 mm and 30 mm containing a multitude of small
apertures between 10 and 25 mm2 size such that the plasma ignites at
low RF power below 20 Watts, and said electrode does not extend
outside said plasma chamber.
RF Plasma Cleaning Systems for Electron Microscopes
and High Vacuum Systems
Stops Artifacts and Removes Hydrocarbons and Organics.
Inventor:
Ronald A. Vane
Electron Microscopes and Vacuum Chambers"
The
present invention relates to a low-vacuum plasma device for cleaning
vacuum chambers and analytical instruments such as Scanning Electron
Microscopes (SEM), Scanning Electron Microprobes, Transmission
Electron Microscopes (TEM) and other charge particle beam instruments
that are subject to contamination problems from hydrocarbons. In
particular it is a novel RF plasma apparatus for generating active
oxygen radicals from air that can be used for cleaning vacuum
chambers such as found in electron microscopes and for other
purposes. The oxygen radicals are used to oxidize the hydrocarbons
and convert them to easily pumped gases. The apparatus can be added
many types of vacuum systems without major modifications and can be
used with other gas mixtures to generate a variety of active species.
The apparatus is designed to be made from standard vacuum components
and electrical feedthroughs.
The
invention described in patent application (09/228,318) a disclosed a
general method and apparatus for cleaning an electron microscope
using air passed through a low powered RF plasma glow discharge to
produce oxygen radicals. Divisional application (09/ ) of the
09/228,318 application disclosed a cylindrical electrode with
multiple apertures that works for generating oxygen radicals from air
that operates at low RF power. A number of arrangements can be used
to mount this electrode assembly in the vacuum chamber of the SEM
that would allow the RF power to be supplied to this cylindrical
electrode assembly and to have the reactive gas pass through the glow
discharge plasma region. The plasma cleaning action of the
cylindrical electrode with apertures of the previous invention was
found to be almost independent of how it was mounted in the chamber
and how it was enclosed.
2 Solenoid valve
3 Gas inlet
4 Plasma chamber
6 Cylindrical electrode with apertures
8 Electrode support cross bar
9 RF conductor (coaxial)
10 Ceramic insulator
11 ISO KF
40 flanges
12 Chamber body with F=KF 40 flanges
13 Threaded Holes
14 KF40 connector flange to main chamber
15 Gas feed port
16 O-ring
18 KF40 centering ring
20 Coaxial RF connector
22 RF feedthrough on KF40 flange
24 Pirani gauge