Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – With means applying electromagnetic wave energy or...
Reexamination Certificate
2000-04-18
2003-08-26
Mayekar, Kishor (Department: 1741)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
With means applying electromagnetic wave energy or...
C422S186050, C422S186070
Reexamination Certificate
active
06610257
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a new device for the plasma generation of oxygen radicals from air for use in 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.
2. Description of Prior Art
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.
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 (BCI
3,
CF
4
) may be used to tailor the reaction.
Most designs for electrodes for generating RF plasmas for cleaning employ either capacitive coupling the plasma or inductive coupling to the plasma. Parallel plates for capacitive coupling and helical coils for inductive coupling are the textbook methods for generating plasmas and glow discharges. They are easily modeled mathematically and popular. These have the disadvantage of having to operate at relatively high power to ignite a plasma. However at low vacuum between 0.1 Torr and 5 Torr most gases are very conductive and it easy to produce a glow discharge plasma. Devices that can mix capacitive and inductive coupling can operate at lower ignition power at low vacuum since mixed power modes are used for plasma distribution to the plasma. Hollow cathode designs are effective for forming low energy plasmas. Hollow cathodes act as a electron trap. Trapped between the cathode walls free electrons are accelerated by the RF fields but repelled by the plasma sheaths at the walls to remain within the hollow cathode area. These entrapped electrons cause a very high level of ionization of the gas and a very dense plasma. This effect is what is known as classically as Hollow cathode glow. In the RF mode, this plasma is characterized by a very low impedance, allowing high ionization at relatively low power levels. The Bumble et al U.S. Pat. No. 4,637,853 describes a hollow cathode device for high rate plasma etching and deposition.
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, CF
4
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
LandOfFree
Low RF power electrode for plasma generation of oxygen... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Low RF power electrode for plasma generation of oxygen..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Low RF power electrode for plasma generation of oxygen... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3127893