XEI Scientific, Inc.
The EVACTRON® Anti-Contaminator and De-Contaminator Updated August 2007
Excerpts
from US Patent 6,105,589
"Oxidative
Cleaning Method and Apparatus for Electron Microscopes Using an Air
Plasma as an Oxygen Radical Source"
ABSTRACT
An
improved method and apparatus are provided for cleaning the specimen
and interior specimen chamber of Electron Microscopes, and similar
electron beam instruments. The apparatus consists of a
glow-discharge, oxygen-radical generator placed on a specimen chamber
port with an excitation source to create a low-power glow-discharge
plasma inside the generator. Air or other oxygen and nitrogen mixture
is admitted to the generator at a pressure between 0.3 Torr and 5
Torr. The low power glow discharge is used to disassociate oxygen
preferentially over nitrogen to create the oxygen radicals. The
oxygen radicals then disperse by convection throughout the chamber to
clean hydrocarbons from the surfaces of the chamber, stage and
specimen by oxidation to CO and H2O gases. The excitation power of
the plasma is limited to limit the nitrogen ion production that
destroys the oxygen radicals and to limit the projection of the
electrically active plasma into the specimen chamber. The optical
emission or color of the plasma is observed for the selection of the
correct power level for maximum oxygen radical production.
BACKGROUND OF THE INVENTION
1. Field of Invention
2. Description of Prior Art
Another
problem is the condensation of pump oils on the windows of the x-ray
and electron detectors distorting results. The most serious problem
of this type is the absorption of low-energy x-rays from Be, C, N, O
and F by oil films which can prevent measurement of these elements by
X-ray emission spectroscopy.
Contaminants
typically are introduced by one of four ways including the specimen,
the specimen stage, carried into the chamber by the evacuation
system, or are present on the internal components of the instrument.
Contaminants introduced from the evacuation system can be reduced by
trapping, by purging, or by using cleaner pumps. Once present inside
the chamber these contaminants reside on the chamber surfaces, and
can be removed only slowly and with low efficiency by the high vacuum pump.
Inorganic
specimens (metals, ceramics, semiconductors, etc.) may carry
contaminants into the chamber. These may be part of the specimen,
residues from sample preparation techniques or be caused improper
sample handling or storage techniques. In addition, clean surfaces
will accumulate a surface film of hydrocarbon scum if left exposed to
ordinary room air for any length of time. The sources of these
hydrocarbons are most any living thing, organic object, or other
source of hydrocarbon vapors in the vicinity. While the part of the
films created in these processes dissipate under vacuum conditions, a
small amount generally remains on surfaces and is sufficient to cause
problems when the specimen is subsequently examined in the analytical
instruments listed.
These
residues are widely distributed and generally are at low
concentrations on the various surfaces. Some of the contaminant
molecules become mobile in the vacuum environment. At high vacuum the
mean free path of molecules once vaporized is comparable to or longer
than the dimensions of the vacuum chamber of these instruments. The
contaminants move in the vapor phase from surface to surface in the
vacuum environment and are attracted to any focused electron probe
beam, forming deposits through an ionization and deposition process.
Since these contaminants can travel large distances within the vacuum
chamber and over the surface of a specimen, it is important to remove
or immobilize these species as much as possible prior to an analysis
without disturbing the microstructure of the specimen.
Ronald Vane, Dba XEI Scientific, has sold a nitrogen purge system for
cleaning SEM chambers since 1991. Operating at a pressure of
approximately 1 Torr in the chamber, this system uses viscous flow
vacuum dynamics to carry contaminants from the chamber to the
roughing pumps. This system is operated every night and needs at
least 40 hours a week of operation to keep the chamber clean. It is
not fast, it does not reactively clean, and cannot be used where 24
hr instrument availability is needed for the electron microscope..
Another problem for the purge technique is the changing design of
electron microscope vacuum systems. The latest design pumping systems
use turbo molecular pump without a valve between the chamber and the
pump. To vent the chamber the turbo pump is stopped and gas admitted
to the chamber. During the pump down cycle, roughing takes place
through the turbo molecular pump while it is accelerating. Any leak
of gas into the chamber into the chamber during rough will result in
an overheated turbo pump. Thus a continuous purge is not possible for
this type of vacuum system.
It has
been well documented that low temperature (<50 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. CF4 or other fluorine containing gases are
sometimes added to oxygen containing plasmas to speed the oxidation
of hydrocarbons by performing hydrid extraction on the base molecules
to make them more susceptible to oxygen attack. This mechanism is
important in the oxidation and removal of polymers such as photoresist.
The
Sakai et al patent describes a method where electrically neutral
active species are formed outside of the chamber in a plasma
discharging gas and passed through a selection device which stops the
ions and electrons from entering the chamber. The plasma described is
generated in a microwave cavity and then carried to the chamber in a
tube. This method destroys some of the active neutral species by wall
collisions on the way to the chamber whereby its effectiveness is
reduced. The preferred embodiment of the Sakai et al method also uses
a oxygen and CF4 gas mixture as the reactive gas. This mixture or
pure oxygen can cause explosive conditions in vacuum pumps using
conventional hydrocarbon pump oil. Electron microscopes commonly use
this oil and its replacement involves an expensive rebuild and
cleaning of the pumps to accept non-reactive fluorocarbon oil. For
oxygen to be used as a cleaning gas in electron microscope it must be
diluted with a inert gas such the noble gases or nitrogen to avoid
this explosion hazard.
A device
for cleaning electron microscope stages and specimens is described in
Patent 5,510,624 (Zaluzec) for analytical electron microscopes. That
apparatus uses an plasma generating chamber and an airlock to allow
the specimen and stages to be placed into the plasma chamber for
cleaning. It may be connected with the analytical chamber of the
analytical electron microscope. A refinement of this device is
described in Patent 5,633,503 (Fichone). This device cleans by the
use of sputter etching and fragmentation when argon is used as the
reactive gas. When oxygen is used reactive ion etching adds to the
cleaning effect. Sputter etching and reactive ion etching are
anisotropic processes. This has two effects, cleaning is not evenly
distributed but is concentrated to where there is ion bombardment of
surfaces, and the ion bombardment may cause unwanted of sputter
deposition and etching within the analytical chamber resulting in
damage to the instrument and specimen.
The use of
glow discharge cleaning within the specimen chamber of the electron
microscope using a plasma generator in-situ within the specimen
chamber was found to have several disadvantages in practice as follows:
a) The use
of argon and other noble gases is not good for the ion pumps used on
the electron columns of many electron microscopes. Noble gases are
not pumped well by dipole ion pumps.
b)
Generating a free plasma inside the specimen chamber may subject the
walls and surfaces within the chamber to electron and ion bombardment
which may cause damage.
c) Some
ion species in some gas-plasmas will polymerize the surface
hydrocarbons to inhibit further oxidation and cleaning.
It is an
object of the present invention to provide an improved method for
cleaning the specimen chamber, specimen stage and a specimen in the
vacuum system of an electron microscope that uses air passed through
a low powered glow-discharge or gas-plasma as source of oxygen
radicals to oxidize the hydrocarbon contaminants in the specimen
chamber and convert them to easily pumped gases. It is another object
of the present invention to minimize contact and ion bombardment of
the specimen and specimen stage by the electrically active gas plasma
containing ions and electrons that may cause damage. It is another
object of the present invention to provide a cleaning system that is
small and that can be mounted on a standard chamber port of the
electron microscope without mechanical interference from other
devices and parts of the electron microscope.
SUMMARY OF THE INVENTION
An
improved method and apparatus for oxidative cleaning the specimen
chamber, the specimen, and the specimen stage of electron microscopes
and other charged beam instruments are disclosed. The invention
covers the use of oxygen radical generator that uses air and a glow
discharge that is mounted on a port of the specimen chamber of the
electron microscope whereby the oxygen radicals enter the chamber by
convection and remove hydrocarbons by oxidation. The invention also
covers the operation and design of the oxygen radical generator in
such a way that allows it to generate oxygen radicals from air and
other nitrogen/oxygen mixtures without the production of large
numbers of nitrogen and NO+ ions. This generator contains a gas
plasma and apparatus for RF or DC excitation of a gas-plasma or
"glow-discharge " to create oxygen radicals for cleaning
the interior walls and surfaces, specimen stage and specimen. The
invention also covers a control method and arrangement for operating
the evacuation system of the electron microscope, the RF plasma or DC
plasma, and the admission of gas into the chamber. The invention also
covers the use of the change of optical emission or color of the
plasma to select the proper power or voltage for exciting the plasma
to maximize oxygen radical production. The invention also discloses a
glow discharge electrode that is effective for generating oxygen
radicals from air.
DETAILED DESCRIPTION OF THE INVENTION
In
accordance with the invention, a technique has been developed which
allows simultaneous cleaning of the interior, a specimen, and a
specimen stage of a scanning electron microscope which minimizes and
in some cases eliminates contamination and films from the surface of
inorganic specimens during analysis by Scanning Electron Microscopes.
The invention also has utility for other analytical instruments such
as Transmission Electron Microscopes, Scanning Electron Microprobes
and other charged particle beam instruments that have a vacuum
environment and provide analytical information from emitted electrons
and x-rays from the specimen. The specimen need not be present during
chamber and stage cleaning. The procedure, which involves subjecting
the specimen chamber, specimen, and stage to oxygen radicals for
oxidation and removal of hydrocarbons, is carried out prior to
analysis. The oxygen radicals are generated by a glow discharge in
low-pressure air or other nitrogen and oxygen mixture. The
glow-discharge is generated inside an apparatus mounted on a chamber
port on the specimen chamber of the electron microscope or similar
electron beam instrument. The apparatus is subject to the same vacuum
as the chamber and is either within the chamber or in an extension of
the chamber.
The method
of the present invention is supported by the invention of an
apparatus to produce a low powered glow discharge in air at stated
vacuum conditions that generates oxygen radicals when operated at low
RF power or DC voltages in the 250 V to 450 V range.
The
present invention controls the temperature of the plasma as an
important part of the method for generation the oxygen radicals from
air or other oxygen and nitrogen gas mixtures. When oxygen is ionized
a series of reactions leads to the formation of oxygen radicals:
O2
+ O+ —> O2+ + O
O2+
+ e- —> O + O
Compared
to the ions these radicals are long-lived species and may leave the
plasma region if they don't react with other ion species or the walls.
The
ionization potential of oxygen is 12.1 eV and nitrogen is 15.6 eV.
Thus oxygen ionization takes place in a lower temperature or lower
energy plasma than nitrogen. By lowering the average temperature of
the electron energy distribution oxygen ionization is favored. When
nitrogen ions are produced in an air plasma they react with O
radicals by the following fast reactions:
N2+
+ O —> NO+ + N
N +
O —> NO+ + e-
Thus
two oxygen radicals are destroyed by every N2+ ion produced. Because
nitrogen is the major constituent of air, this destruction takes
place quickly once large scale nitrogen ionization begins. In
addition NO+ is a stable ion with a low ionization potential (9.5
eV). It is unable to react with the neutral diatomic gases in air. It
has weak ability to perform hydride extraction on the surface
hydrocarbons to degrade them, but is more likely to form a nitrogen
oxide hydrocarbon polymer that is resistant to further oxidation and
removal. The transition from an oxygen dominated plasma to nitrogen
dominated plasma is easily seem by optical emission. With some glow
discharge electrode configurations the plasma suddenly changes color
as power is raised by as little as 1 watt. The exact wattage or
voltage of this transition depends on the glow discharge means
electrode configuration but for a typical RF (13.56 MHz) excited glow
discharge the transition was found to lie between 10 and 30 Watts of
input power. This transition is also a function of pressure. Higher
pressures shorten the mean free path for electrons and they less able
to pick up energy from the electric field between collision with
molecules. But higher pressures also have the effect of shortening
the life times of the oxygen radicals due to increased collision
frequency. Thus in the present invention an operating pressure is
chosen such that net oxygen radical flux to the surfaces is
maximized. The plasma power is adjusted in the present invention to a
point below this transition at a selected pressure to obtain
oxidative cleaning action. This power level may be found just once
for each gas-plasma oxygen-radical generator configuration, and then
this level is used for all subsequent operation of the device.
The method
in the present invention keeps the plasma temperature low by
minimizing the RF power or DC voltage generating the glow discharge,
resulting in minimized nitrogen ionization and maximized oxygen
radical production from air. The method also uses the changes is
optical emission or color of the plasma as an aid to select the
proper power or excitation voltage for the plasma excitation that
achieves this state.
DETAILS OF
FIGURES ARE OMITTED
Primary Claims
I claim:
1. A
method for simultaneous specimen chamber, specimen and stage cleaning
for Scanning Electron Microscope, Transmission Electron Microscope,
Scanning Electron Microprobe or other charged particle beam
instruments by generating oxygen radicals from air or other
nitrogen/oxygen gas mixtures under vacuum conditions produced within
said specimen chamber of said instrument,
Comprising the steps of:
a)
providing means for producing said oxygen radicals by a
glow-discharge plasma,
b)
providing means for temperature or power control of said
glow-discharge plasma, and
c) using
said temperature and power control to minimize the ionization of
nitrogen and maximize oxygen radical production in said plasma
whereby
said oxygen radicals are used to remove hydrocarbon contaminants by oxidation.
Apparatus for simultaneous specimen chamber, specimen and stage cleaning for Scanning Electron Microscope, Analytical Electron Microscope,Scanning Electron Microprobe or other charged particle beam instruments by generating oxygen radicals from air or other nitrogen/oxygen gas mixtures under vacuum conditions within said specimen chamber of said instrument comprising:
RF Plasma Cleaning Systems for Electron Microscopes
and High Vacuum Systems
Stops Artifacts and Removes Hydrocarbons and Organics.
Ronald A. Vane, Issued Aug 22, 2002
The
present invention relates to cleaning 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 method and apparatus for
cleaning the specimen chamber, specimen stage, and specimen in-situ
inside the vacuum system of these instruments with oxygen radicals
that uses air passed through a glow discharge as an oxygen radical
source. The oxygen radicals are used to oxidize the hydrocarbons and
convert them to easily pumped gases. The method and apparatus can be
added to the analytical instrument with no change to its analytical
purpose or design.
Electron
microscopy is used to detect, measure, and analyze constituents
present in very small areas of materials. Contaminants adsorbed on
the surface or surface films interacting with the incident electron
probe beam can distort the results. Deposits created by the
interaction of the probe beam with the surface specimen also may
interfere with the probe bean or emitted electrons and x-rays and
thus adversely affect accurate analysis. Deposits also add
uncertainty to SEM measured line widths for semiconductor device
critical dimension metrology.