Abstract:
An apparatus for cleaning the specimen and interior specimen chamber of Transmission Electron Microscopes, and similar electron- or charged-particle-beam instruments consisting of a plasma cleaning device mounted on a hollow rod that replaces the stage through the air lock of the instrument by being the same shape and size as the stage support rod. The plasma cleaning device is a small hollow cathode that is excited by RF power. Air or other oxygen containing mixtures is admitted to the plasma through the hollow rod at a pressure below 1 Torr. The plasma creates oxygen radicals from the oxygen containing gas. The oxygen radicals oxidize the hydrocarbons contamination and convert them to easily pumped gases. The apparatus can be attached to the electron microscope whenever cleaning is needed, and then is easily removed to return the instrument to its analytical function.

Description:
TECHNICAL FIELD OF INVENTION 
     The invention relates to cleaning electron microscopes and other charged-beam instruments with a confined space near the specimen analytical area. In particular, it is a method for the production of plasma-activated, gentle chemical etching of hydrocarbon-containing molecules by oxygen radicals in both the sample (or specimen) analysis and sample exchange chambers of the analytical instrument. This method is accomplished by placing the means of generating said oxygen radicals at the distal end of a hollow rod dimensioned to replace the sample rod used to insert samples into the analytical instrument chambers. This method includes sealing and indexing members used to position the sample rod into the analytical instrument chambers and replicate sample rod functions. It is emphasized that said etching process occurs entirely via chemical reactions and not physical mechanisms such as sputtering. 
     BACKGROUND OF THE INVENTION 
     Electron microscopy is used to detect, measure, and analyze features and constituents present in very small areas of material specimens and samples (the terms specimen and sample are used interchangeably). These specimens and samples are placed into the analytical instrument and interact with the electron beam of the electron microscope. These specimens introduced into the analytical instrument have contaminants and other foreign species adsorbed, attached or present on the sample. These contaminant species, upon interaction with the incident electron beam, distort the analytical measurements and detract from the accuracy and quality of the analytical measurements. Most often, the electron beam reacts with the impurity species, forming deposits on the surface of the sample. These deposits in turn interfere with secondary electron emission, X-ray emission or electron image formation in the analytical instrument. Whenever these impurities are present in the analysis chamber, these deposits are produced. The contaminant deposits when present show a demonstrated reduction in contrast and resolution in both transmission electron microscopes (TEM) and scanning electron microscopes (SEM). 
     All modern transmission electron microscopes and some scanning electron microscopes utilize sample introduction and manipulation stages that extend into and between the electron beam lens pole pieces. This is known as “in the lens” imaging. A sample mounting platform is placed at the end of a long, hollow cylindrical “stage” rod. The sample stage extends through the instrument airlock and into the microscope analysis chamber. The length of this rod enables both accurate positioning of the sample at the object plane of the electron microscope and accurate adjustment of the sample, once placed at the correct location. The electron beam is precisely focused on an aperture in the center of the analysis chamber. The sample is located precisely below this aperture. After passing through the aperture and sample, the electrons are collected for subsequent analysis. Adjustment of the sample orientation with respect to the electron beam is achieved with a goniometer connected to the stage mechanism. The goniometer is an angular adjustment mechanism that allows precise positioning of the sample with respect to the electron beam. 
     Samples and specimens for transmission electron microscopy must undergo preparation before they can be analyzed in the instrument. The samples are thinned using various preparation techniques such that the electron beam can pass through the sample, and the TEM imaging optics can properly form a virtual image at the microscope image plane. The electron beam is very tightly focused within this region of the instrument. Electron beam properties at the instrument object plane place great constraints on the sample size and thickness. 
     The electron microscope sample introduction and manipulator stage utilizes a vacuum-airlock mechanism to ensure that the samples are introduced into the analysis chamber with no disruption to the vacuum in the electron microscope. The vacuum airlock mechanism is implemented with multiple O-ring seals positioned at intervals along the hollow stage support rod. The system also uses a location pin or fixture that has a precise relationship to these O-ring seals. Typical operation of the airlock is as follows: Using the stage support rod, the stage is inserted into the airlock until the locating pin reaches a position stop. When the sample stage reaches this stop, it is rotated through a large angle, typically 90 degrees or greater. Rotation of the stage causes the location pin to activate the airlock pump-down cycle. Once an appropriate pressure level is reached in the airlock, an isolation valve between the airlock and the analysis chamber is opened and the sample stage passes into the microscope analysis chamber. This system ensures that sample transfer occurs at the correct time in the evacuation sequence and that the sample is positioned reliably and reproducibly in the microscope analysis chamber. 
     Electron microscopy systems, especially TEM systems, are highly susceptible to chemical contaminants. These contaminants can be introduced into the microscope system by four mechanisms: specimen contamination, stage contamination, airlock contamination, and leaks. These contaminants, once introduced onto system surfaces, including the electron optical system, are only very slowly removed by the inefficient instrument vacuum pumping system. 
     Specimens or samples may carry these contaminants into the chamber. These may be part of the specimen, residues from sample preparation techniques or be caused by improper sample handling or storage techniques. For organic specimens, the contamination may be induced by the exposure of the specimen and degradation of the surface by the high energy electron beam. 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 instrument. 
     Specimens or samples may carry these contaminants into the chamber. These may be part of the specimen, residues from sample preparation techniques or be caused by improper sample handling or storage techniques. For organic specimens, the contamination may be induced by the exposure of the specimen and degradation of the surface by the high energy electron beam. 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 (e.g., solvents and other aromatic compounds). While some of the compounds created in these processes dissipate from surfaces under vacuum and related conditions, other compounds and conditions generally insure residual amounts will remain on surfaces and even minute amounts of this contamination are sufficient to cause problems when the specimen is subsequently examined in the analytical instrument. 
     Plasma cleaning has been shown to be useful for removing hydrocarbon contamination. A device for cleaning electron microscope stages and specimens is described in U.S. Pat. No. 5,510,624 (Zaluzec) for analytical electron microscopes including TEM. That apparatus uses a 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. Several commercial desktop plasma cleaners licensing this patent are sold for cleaning stages and specimen together before inserting into the TEM. 
     Vane disclosed in U.S. Pat. Nos. 6,105,589, 6,452,315 and 6,610,257 the technology used by XEI Scientific, Inc. in the Evactron® De-Contaminator systems that use an air plasma to produce oxygen radicals for downstream cleaning of electron microscopes and other vacuum systems that have been sold since 1999. These patents describe an oxidative cleaning system and apparatus using low powered RF plasma to produce oxygen radicals, an active neutral species, from air to oxidize and remove these hydrocarbons. The device is mounted on the outside of the electron microscope and the excited gas moves into the specimen chamber by convective flow created by the rough vacuum pump. This device works well on SEMs, but on TEMs has achieved only limited success because of the need to add an additional vacuum pump to get enough flow through the chamber. A recent publication describes this technique using the Evactron® De-Contaminator for SEM with an additional pump (Shin Horiuchi et al.,  Contamination - Free Transmission Electron Microscopy for High Resolution Carbon Elemental Mapping of Polymers , ACS Nano, 1297 (2009)). As stated by Horiuchi, “[H]owever, the beam-induced specimen contamination in the TEM cannot be reduced by the simple operation of the plasma generator as for SEMs. The reason is assumed that the specimen chamber and vacuum path of the TEM are considerably narrow, where sufficient oxygen radicals cannot be supplied into the chamber simply by the roughing pump of the microscope.” 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of this invention to reduce the need to add an auxiliary vacuum pump to the transmission electron microscope (or other charged-particle-beam instrument) to increase cleaning speed and to remove hydrocarbons with a plasma device in a reasonable time. It is another object of this invention to bring the production of the excited cleaning gas to a location inside the specimen chamber of the electron microscope to clean the “in the lens” imaging regions. It is another object of this invention to bring the plasma region into the TEM so that the production of radicals is close to the regions to be cleaned. It is another object of the present invention to be able to clean both the specimen exchange chamber and specimen chamber by producing oxygen radicals or other excited species in the hollow cathode generated plasma. It is another object of this invention to clean all of the vacuum chamber by flowing the excited gas out of the plasma region to react with contaminates throughout the chamber. It is another object of this invention to use an oxygen-containing gas such as pure oxygen, air, nitrogen/oxygen mixtures, water vapor, oxygen/argon mixtures and gas mixtures of these to make the excited gas in a plasma. Accordingly, this invention comprises a small hollow cathode plasma electrode mounted on the distal end of a stage rod so it may be inserted into the electron microscope through the specimen exchange chamber, and the electrode is supplied with cleaning gas and plasma power through the rod so that cleaning is done using a hollow cathode generated plasma that excites a cleaning gas within either the specimen chamber or the exchange chamber. The excited gas then flows through the chamber towards the vacuum pump, and the excited gas reacts with hydrocarbons along the way so that they are volatilized to be removed by the pumps. The process is limited by the number of oxygen radicals or other reactive species created in the excited gas mixture. Small TEM chambers can be cleaned with fewer radicals than larger SEM chambers. This allows a smaller source of oxygen radicals to be used in this invention. 
     In U.S. Pat. No. 4,657,616, In-Situ CVD Chamber Cleaner, Benzing describes a method for plasma cleaning of film deposition chambers that are used for repetitive deposition of relatively thick (10,000&#39;s of Angstroms) inorganic films such as silicon, silicon dioxide, and silicon nitride. In Benzing&#39;s method a contaminated, open-ended tube is vented to open air through its open end, then the cleaning apparatus is inserted and sealed against the open end of the tube and the tube is evacuated. A plasma is ignited at pressures of a fraction to several Torr along the length of the electrodes in close proximity to the contaminating film, which then removes the film via chemical and physical means which volatilize the film components. At the end of the cleaning process the CVD chamber is again vented to atmospheric pressure and the cleaning apparatus is removed. It is noteworthy that the Benzing apparati are limited to the single structural form of a cylindrical shape as shown in his high-aspect (Benzing FIGS. 4-10) and medium-aspect (Benzing FIGS. 11,12) electrode designs; this structural limitation is necessitated by industry standards for CVD chambers. (The reduced aspect designs of FIGS. 11 and 12 are simply shorter versions of the designs of FIGS. 4-10 and serve only to limit the cylindrical plasma to a shorter length.) Moreover the structural specifics described by Benzing indicate that the method is limited to the cleaning of cylinder interior walls. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic showing the cleaning device that can be inserted into a TEM in place of a TEM stage. 
         FIG. 2  is a schematic showing the general placement of the cleaning device as it relates to the TEM instrument. 
         FIG. 3  is a schematic of the cleaning device inserted into the TEM specimen chamber. 
         FIG. 4  is a schematic of the cleaning device inserted into the TEM specimen exchange chamber. 
         FIG. 5  is an enlarged view of the small hollow cathode electrode. 
         FIG. 6  is a chart of cleaning rates of the regular “Evactron” hollow cathode electrode and the small electrode of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the Figures, the preferred embodiment of the present invention is shown in  FIG. 1 . It consists of a small hollow cathode  102  mounted onto the distal end of a hollow rod  122  that has the exterior shape of a stage support rod so that it may pass into a Transmission Electron Microscope, or Scanning Transmission Microscope (STEM) or Scanning Electron Microscope that has in-the-lens imaging and small volume vacuum chambers. To enable passage into the instrument through the air lock, the locating pin  28  O-rings  36  and other shapes of the stage support rod are duplicated in exactly the same geometry. The small hollow cathode electrode  102  is mounted on a rigid conductive support  104  that is connected to the electrical conductor  106  that connects to the plasma power source outside the vacuum via the vacuum electrical feedthrough  108  and the electrical power connection utility  100 . In the preferred embodiment, this electrical conductor  106  is a shielded coaxial cable inside the hollow rod  102 . This coaxial cable is connected to an impedance matching network contained within the electrical power connection utility  100 . At the distal end of the rod, an insulating support  104 A centers the rigid support  104  and the hollow cathode  102 . 
     Referring to  FIG. 2 , this figure illustrates the general arrangement of the cleaning device in the context of a charged-particle beam instrument  2  (TEM) of the type with or in which the present invention is used. Such an instrument  2  may be a Transmission Electron Microscope (TEM), a Scanning Transmission Microscope (STEM), a Scanning Electron Microscope (SEM), or any other charged-particle beam instrument that has or uses in-the-lens imaging and a small-volume vacuum chamber. Instrument  2  uses a charged-particle device in the form of an electron gun  4 , which is mounted on a relatively small volume vacuum chamber  6  through a gun valve  8 . Electron gun  4  operates at a higher (lower pressure) vacuum than vacuum chamber  6 , and valve  8  is required to permit the pressure differential. Electron gun  4  and vacuum chamber  6  are evacuated by a series of vacuum pumps  10 . Electron gun  4  emits a beam of electrons (charged particles)  12 , which passes through an objective lens  14  and an aperture  16  formed in an upper of a pair of conical poles  18 . Between the conical poles  18 , a specimen or sample imaging chamber  20  is defined in which the specimen is arranged and manipulated to be impinged upon by the electron beam for analysis. 
     The sample or specimen is inserted into, and manipulated within, this chamber  20  by a stage support rod  22 , which passes into vacuum chamber  6  and specimen chamber  20  through apertures that are sealed and valved  24  against gas flow into and out of vacuum chamber  6 , so that a vacuum can be maintained. Before passing into vacuum chamber  6 , stage support rod  22  passes through a sealed exchange chamber  26 , which functions as an “air lock” to allow samples or specimens to be attached to and removed from stage support rod  22 , while maintaining a vacuum in chamber  6 . Rod  22  is also provided with an indexing member or locating pin  28 , which cooperates with a position sensor  30  to assist in the rotational and axial positioning of rod  22  within chamber  6  and specimen chamber or region  20 . Exchange chamber  26  is evacuated with a vacuum pump acting through a valve  32  and has a pressure gauge  34 . 
     The construction described is almost universal with respect to conventional transmission electron microscopes, especially as concerns stage support rod  22 . Such rods are conventional in such instruments, and, as noted, provided with various sealing  24  and index mechanisms, such as a position sensor and associated instrumentation  28 ,  30 , to insure that vacuum integrity is maintained and to permit careful and precise insertion and manipulation of the sample or specimen within the specimen imaging chamber or region  20  of vacuum chamber  6 . 
     Such sealing and indexing mechanisms vary with the manufacturer and type of instrument. As will be described in greater detail below, an aspect of the invention is to provide oxidative cleaning of the specimen region or area  20  to remove hydrocarbon contaminants from the various surfaces therein. This is in part accomplished by replacing the stage support rod with a similar, if not identically, dimensioned rod that maintains the seal and indexing mechanisms of the stage support rod supplied with the instrument. According to the invention, this replacement stage support rod, referred to as a “hollow rod,” conveys an electrode (cathode) into the vacuum chamber  6  of the instrument  2 , and supplies gas and electric current to the cathode to conduct oxidative cleaning in the specimen imaging region  20 . 
     The preferred and illustrative embodiments of the present invention are shown in  FIGS. 3 and 4 . It comprises a small hollow cathode  102  (positively charged electrode) (described in detail in  FIG. 5 ) mounted onto the distal end of a generally tubular hollow rod  122  that has the exterior shape and dimension of a stage support rod  22  so it may pass into a vacuum  6  and specimen chambers  20  of a charged-particle beam instrument  2  as described above. To enable passage into the vacuum chamber  6  of instrument  2  through the exchange chamber or air lock  26 , the locating pin  28 , O-rings, and other shapes of the stage support rod  122  preferably are duplicated in exactly the same geometry and dimension as the original stage rod  22 . The small hollow cathode electrode  102  is mounted on a rigid conductive support  104  that is connected to an electrical conductor  106  that connects to a plasma power source outside of the vacuum chamber  6  via a coaxial electrical cable feedthrough. In a preferred embodiment, this electrical conductor  106  is a shielded coaxial cable inside the hollow rod  122 , and is electrically connected to an electrical impedance matching network. 
     A cleaning gas is supplied to the interior of the hollow rod  122  to hollow cathode  102  through holes in the insulating support  104 A. The flow of cleaning gas into the hollow rod  122  is controlled by a control valve  110  on the rod support chamber  112  outside the vacuum  6  and exchange chambers  26 . This control valve  110  may be controlled either manually by a needle valve, or electrically by a variable solenoid valve, to vary the flow of gas through hollow rod  122 . A vacuum gauge  114  is used to monitor the pressure and flow through the hollow rod  122 . 
       FIG. 3  shows an embodiment of the instrument  2  of  FIG. 2 , with hollow rod  122  extended into the specimen imaging chamber  20 . In this position, when under vacuum, the assembly may clean the specimen imaging chamber  20  of the instrument. 
     When electrical power is supplied to the hollow cathode electrode  102  while cleaning gas flows through hollow rod  122 , a plasma is created in the plasma region  140 . As previously described, the small hollow cathode electrode  102  is positioned by hollow rod  122  in or proximal to the specimen imaging chamber  20 . Cleaning gas is supplied to the interior of the hollow cathode  102  from the hollow rod  122  through holes in the insulating support  104 A. The flow of cleaning gas into the hollow rod  122  is controlled by a control valve  110  on the rod support chamber  112  outside the vacuum. The cleaning gas moves from the support chamber  112  through the interior of the rod  122 , through the insulating support  104 A into the hollow cathode  102  and into the specimen imaging chamber  20 . 
     In the specimen imaging chamber  20 , the cleaning gas is converted into the excited cleaning gas by powered or energized hollow cathode  102  to form a plasma region  140 . The plasma region  140  may extend “in the lens” or the excited cleaning gas will clean the pole pieces  18  and the aperture  16 . A pressure gauge  114  is used to monitor the pressure and flow through the rod. In a preferred embodiment, this pressure gauge  114  is a pirani-type gauge. The pressure reading from the pressure gauge  114  will read higher than the pressure in the plasma region  140  because of the flow differential caused by the gas flowing out the distal end of rod  122  into the lower pressure found there. The position sensor  30  uses the locating pin  28  on the hollow rod  122  to sense the axial location of the distal end of rod  122  and its rotational orientation so that the air lock valve  24  is opened and closed as appropriate. In this illustration, the locating pin  28  has gone inside the exchange chamber  26  and the inside air lock valve  24  is open with the rod passing through it. 
       FIG. 4  shows another embodiment of the charged-particle-beam instrument  2  with the hollow rod  122  and hollow cathode  102  according to the invention retracted or located inside the specimen exchange chamber/air lock  26 . In this position, when under vacuum, the invention may clean the air lock  26  portion of the instrument. Again, cleaning gas passes through hollow rod  122  and into the specimen exchange chamber  26 . When power is supplied to the hollow cathode electrode  102 , a plasma is created in the plasma region  160 . Plasma region  160  is inside the specimen exchange chamber/air lock  26  and cleans only that region. In the position shown, the locating pin  28  and the position sensor  30  are in contact and the exchange chamber  26  being evacuated by the vacuum pump  10  when vacuum valve  32  is opened. To use the plasma cleaning method of the “Evactron” plasma cleaning device (U.S. Pat. No. 6,105,589 to Vane) without having to add an extra pump to the instrument for rough pumping, a small hollow cathode  102  was developed, as shown in  FIG. 5 . The hollow cathode electrode  102  is a relatively short small-diameter hollow cylinder  130  formed of electrically conductive metal, according to the illustrative embodiment, aluminum. At one end, a disk with an aperture  132  is supported by a pair of radially extending arms  134  in approximately the center of the open end of the hollow cylinder  130 . Aperture  132  receives the bare-wire end of electrical conductor  106 . Aperture  132  may be threaded or crimped onto the end of electrical conductor  106 . 
     The small electrode was needed so that it could be inserted into the specimen exchange chamber/airlock  26  and imaging chamber  20 . To accomplish this, the hollow cathode electrode  102  was made a smaller diameter and length so that it could replace the stage at the end of a hollow rod  122 . As noted, the outer diameter of hollow cathode electrode  102  should be less than or equal to that of hollow rod  122  to permit passage of hollow rod  122  through apertures and seals provided for stage support rod  22 . The plasma cleaning gas could then be produced in-situ by the plasma inside of hollow cathode  102  within the specimen chamber  20  of the instrument  2 . It was determined that plasma within a smaller hollow cathode produces less cleaning gas than the larger “Evactron” hollow cathode electrode described in U.S. Pat. No. 6,610,257 (Vane), as shown by the data in  FIG. 6 . Although a reduced number of radicals are produced, the closer cleaning position and of the electrode in this invention, and the smaller chamber volume of the TEM makes up for the lower production rate of oxygen radicals caused by the smaller electrode size.  FIG. 6  demonstrates the reduced production of the radicals from the smaller sized hollow cathode electrode  102  of the present invention compared to that of the standard Evactron® device. 
     According to the preferred illustrative embodiment of the invention, radio frequency (RF) power at 13.56 MHz or other frequencies is fed to the hollow cathode electrode  102 . If the vacuum pressure is correct, plasma will be created inside the hollow cathode electrode  102  and the plasma will extend into the region surrounding the hollow cathode electrode  102 . The excited space within the plasma sheath is the plasma region  140  or  160 . When air or an oxygen-containing gas mixture is supplied to the plasma region, oxygen radicals and/or other active species can be produced that can oxidize and remove hydrocarbons. In the preferred and illustrative embodiment of the present invention, this reactive cleaning gas is air. The cleaning gas may be any mixture of oxygen with nitrogen, water vapor or argon. These gas mixtures form oxygen radicals in the plasma that remove hydrocarbons by oxidation. Another embodiment uses hydrogen mixtures or ammonia NH 3  gas mixtures as the cleaning gas to form active neutral species in the plasma to destroy contaminants by reduction. Valve  110  controls the cleaning gas flow into the plasma and into the chamber. In the preferred and illustrative embodiment of the present invention, the cleaning gas is fed directly into the hollow cathode electrode  102  through the hollow rod  122  and insulated support  104 A, and oxygen radicals or other active neutral species from the plasma regions  140  or  160  flow into the chambers  20  and  26 , respectively. Pressure gauge  114  is used to monitor the pressure during cleaning. Because the cleaning gas is flowing into the plasma region, an excited gas, including radicals and other active neutral species, will flow out of the plasma region into other interior spaces of the vacuum chamber  6  of instrument  2 . This excited gas flows by the pumping differential and carries radicals and other excited species that chemically react with hydrocarbons to remove contamination molecules as volatile species. 
     Another embodiment of the invention uses high voltage DC or high-frequency AC to create the plasma in the small hollow cathode electrode  102 . The electrical conductor  106  in this embodiment does not need to be a coaxial cable but may be an insulated wire. In this embodiment, an RF impedance matching network is not required. The invention has been described with reference to preferred, but exemplary or illustrative embodiments thereof It is thus not limited, but is susceptible to variation and modification without departing from the scope of the invention.