Abstract:
The invention provides a miniaturized optical column for a charged particle beam apparatus for examining a specimen ( 14 ). The column is constituted by, among other things, a charged particle source ( 2 ) for providing a beam of charged particles ( 10 ); a lens system for guiding the beam of charged particles ( 10 ) from the source ( 2 ) onto the specimen ( 14 ); and a housing ( 40 ) which, during operation, is set on beam boost potential.

Description:
This is a National stage entry under 35 U.S.C. §371 of Aaplication No. PCT/EP01/00931 filed Jan. 29, 2001; the disclosure of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The invention relates to an apparatus for the examination of specimen with a beam of charged particles. In particular, this invention relates to a miniaturized optical column for a charged particle beam device. 
   BACKGROUND OF THE INVENTION 
   Charged particle beam devices such as scanning and transmission or microprobe apparatuses, to quote only a few, are powerful instruments which permit the observation and characterization of heterogeneous organic and inorganic materials and their surfaces. In these instruments, the area to be examined is irradiated with a charged particle beam, which may be static or swept in a raster across the surface of the specimen. Depending on the specific application, the charged particle beam is more or less focused and the kinetic energy of the particles can vary considerably. 
   The types of signals produced when the charged particles impinge on a specimen surface include secondary electrons, backscattered electrons, Auger electrons, characteristic x-rays, and photons of various energies. These signals are obtained from specific emission volumes within the sample and can be used to examine many characteristics of the sample such as composition, surface topography, crystallography, etc. 
   Lately, attempts have been made to miniaturize charged particle beam devices. Several of these devices could then be grouped together to simultaneously examine larger areas of the specimen or they could be installed in process lines with tight space restrictions. Furthermore, since spherical and chromatic aberrations of particle beam devices scale proportional to their geometrical dimensions, as long as the potential remains constant, miniaturized devices would be able to deliver higher spatial resolution and high beam current in a given spot size. 
   In general, most of the present charged particle devices are between 0.5 and 1.2 meters high with an average diameter of about 15 cm -40 cm. Distinct from that, developers are aiming at producing beam devices which are smaller than 10 cm with an average diameter of about 4 cm. However, since modem charged particle beam apparatuses are complex technical instruments with sophisticated vacuum systems, alignment mechanism and electronic control units, their geometrical dimensions can not simply be shrinked proportionally, even though this is attempted wherever possible. 
   In low voltage applications, the performance of standard charged particle beam devices can considerably be increased by using a so called beam booster. It accelerates the charged particles within the optical column of the microscope to high kinetic energy and decelerates them before hitting the specimen. The deceleration step is mostly carried out when the charged particles pass the objective lens. 
   In general, the beam boost is achieved by surrounding the beam in the column with electrodes being shifted to a high accelerating potential. In microscopes with isolation valves positioned between the charged particle source and the specimen chamber or with any other conductive part arranged in the vicinity of the charged particle beam column, these parts are also shifted on the accelerating potential. The corresponding shielding electrodes, the valves or other conductive parts require insulation against the grounded column housing. Usually ceramic is used as insulating material. The end edges of the insulators are folded or provided with grooves to increase the creepage path. In miniaturized charged particle beam columns, however, this kind of isolation is not satisfactory. 
   SUMMARY OF THE INVENTION 
   The present invention intends to provide an improved apparatus for examining a specimen with a beam of charged particles. According to one aspect of the present invention, an apparatus is provided as specified in independent claim  1 . 
   By setting the housing of the miniaturized optical column on beam boost potential a complicated arrangement of insulators between parts being set during operation either on ground or on beam boost potential can be omitted. Complications arise from the reduced space within the column which obliges to miniaturize the insulators in addition of having to form long creepage paths at their ends. Another advantage is that special electrodes for shielding the beam path (e.g. from the potential of the grounded housing) are not required anymore. 
   Furthermore, in state of the art devices, some insulators have to be soldered to metal to vacuum seal the column. The sealing between insulator and metal has to be mechanically weak because it has to compensate for mechanical tensions resulting from the thermal treatment during the bake out of the column which could affect the stability of the column. 
   Further advantages, features, aspects and details of the invention are evident from the dependent claims, the description and the accompanying drawings. The claims are intended to be understood as a first non-limiting approach of defining the invention in general terms. 
   A beam boost potential, in general, is an accelerating potential applied somewhere between the source and the specimen. The absolute value of the potential of the components used for the beam boost is only of secondary importance. For a beam boost, it is primarily important to establish a potential difference between the source of the charged particles and the components responsible for the beam boost so that the particles get accelerated to high speed on their way through the column to the specimen. The housing, in general, is used to accommodate optical components for guiding and shaping the beam of charged particles. Usually, it is made of electrically and sometimes magnetically conductive material e.g. mu-metal to shield the interior of the optical column from disturbing electric and/or magnetic fields. 
   According to a further aspect of the present invention, the housing comprises a valve, deflection units, detectors, or other parts and components which are arranged in the vicinity of the beam path. Which of these parts and components needed and their specific set up within the column, greatly depends on the application for which the device is intended to be used. Should these parts be arranged in the vicinity of the beam path so that their electric fields could have a negative influence on the beam trajectory then they are preferably set on beam boost potential. Naturally, the potential of these parts might slightly deviate from the beam boost potential; electric deflection units for example require these differences in potential in order to deflect the beam. 
   By keeping as many parts as possible together with the housing on beam boost potential, the number of insulators used within the miniaturized optical column can be significantly reduced. Thereby, it is still within the scope of the invention if certain parts within the column are nevertheless isolated from the housing e.g. the charged particle source. 
   The present invention is advantageously used in combination with miniaturized electro- and magnetostatic (with permanent magnets) columns. This avoids putting the power supply units which drive the electric coils on beam boost potential and allows for more stable operation of the column. 
   The invention is also directed to methods by which the described apparatus operates. It includes method steps for carrying out every function of the apparatus. Furthermore, the invention is also directed to apparatus for carrying out the disclosed methods and including apparatus parts for performing each described method step. These method steps may be performed by way of hardware components, a computer programmed by appropriate software, any combination of the two or any other manner. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some of the above indicated, as well as more detailed aspects of the invention, will be described in the following description and partially illustrated with reference to the figures. Therein: 
       FIG. 1  shows an optical column of a charged particle device of the present invention whereby the housing is set on a beam boost potential. 
       FIG. 2  shows an optical column of a charged particle device of the state of the art. 
       FIG. 3  shows a further embodiment of a charged particle beam device according to the present invention. 
       FIG. 4  shows the specimen chamber of a charged particle device of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A charged particle beam device of the state of the art is schematically shown in FIG.  2 . The charged particle beam device  1  comprises a top cover plate  20  to which a charged particle source  2  is attached. The wires  3  for providing the source  2  with power go through the top cover plate to a power source. In electron beam devices, electron sources such as Tungsten-Hairpin guns, Lanthanum-Hexaboride Guns, Field emission guns etc. can be used. The invention, however, is not limited to electron sources; it can be used together with all kinds of charged particle sources. In the state of the art embodiment shown in  FIG. 2  an extractor and a suppressor  4  is arranged below the particle source  2 . The extractor which is, for example, set on an accelerating potential below the beam boost potential attracts the charged particles coming from the source. Contrary to that, the suppressor, which is arranged between the accelerator and the source, is set on a suppression potential to limit the number of particles getting pulled towards the extractor. This way the beam current can be regulated and prevented from becoming to high. 
   After the beam of charged particles  10  has been formed and left the source, a system of electromagnetic lenses, deflection coils, and stigmators is involved in the control and refinement of the charged particle beam before it hits the specimen. The particular arrangement of these components depends strongly on the specific applications of the charged particle device used. In many devices, a series of two or three condenser lenses (not shown) are used to successively demagnify the beam spot which is essential for better resolutions at high magnifications. A stigmator (not shown) is a component that is used to control any distortions in the roundness of the spot formed by the beam that is scanned over the specimen. Beam spots that are not round will generate an image on the viewing monitor that is smeared in one direction. If required, a scan unit (not shown) is arranged in the optical column. For example, a set of small coils of wire are energized by a varying voltage produced by the scan generator and create a magnetic field that deflects the beam of charged particles back and forth in a controlled pattern called a raster. The raster is very similar to the raster in a television receiver. 
   The final lens in or below the column, called the objective lens, focuses the image by controlling the movement of the crossover along the optical axis (z-axis) of the column. In  FIG. 2  an electrostatic lens  12  is arranged below the column. In many applications, however, a magnetic lens with a controllable current I through its windings is used. Due to the large control current cooling is sometimes required. 
   When the particles of the beam  10  strike the surface of specimen  14 , they undergo a series of complex interactions with the nuclei and electrons of the atoms of the sample. The interactions produce a variety of secondary products, such as electrons of different energy, X rays, heat, and light. Many of these secondary products are used to produce the images of the sample and collect additional data from it. Detectors for collecting the secondary products can either be circularly arranged around the optical axis or at the side.  FIG. 2  shows a disk shaped collector  16  with a bore at its center to let the primary beam pass on its way from source  2  to specimen  14 . In this embodiment, the detector is attached to a light guide  17  which goes through housing  18  and connects the detector with an imaging electronic (not shown). The light pulses created by the secondary products impinging onto detector  16  are guided outside the housing  18  and used for forming the final image. 
   Particularly in low voltage applications, the performance of a charged particle beam device can be considerably increased by using a beam booster. The beam booster causes the charged particles to travel at high speed through the optical column and then slows them down before they hit the specimen. The particles in the beam have different kinetic energies and thus different wavelength. The differing wavelengths cause the particles to be brought to focus at different points which blurs the image. The ratio of the difference in kinetic energy to the mean kinetic energy of the charge particles ΔE/E influences the extent to which the image is out of focus. Since the variation in kinetic energy ΔE is largely independent with respect to the total kinetic energy of the particle E, the lens defect can be reduced by letting the charged particles enter the objective lens at a relative high speed and subsequently decelerate them before they hit the specimen. 
   A beam boost in a typical optical column of a charge particle device is realized by letting the charged particles on their way from the source to the objective lens see a high potential. This is achieved by shielding the beam path from all conductive parts having a potential considerably different to the beam boost potential. In the embodiment shown in  FIG. 2 , shielding electrodes  30  shield the beam path from the grounded housing  18  and, if required, from beam alignment coils which, in general, are also put on ground potential. For shielding electrodes  30 , a so called liner tube is often used. In the lower part of the column, shielding electrodes  32  shield, for example, the beam path from the grounded housing  18 . In the central part of the column a valve unit  6 , 8  is arranged. It separates the vacuum in the gun chamber from the vacuum prevailing in the specimen chamber (see e.g. FIG.  4 ). Furthermore, it allows replacement of the specimens without breaking the vacuum of the gun chamber by closing the sliding valve  8  which is attached to the holding unit  6 . The sliding valve  8  and holding unit  6  are also put on beam boost potential so that they do not adversely effect the electrical field in the vicinity of the beam path. It should be noted, however, that depending on the particular setup of the optical column or the application used, a valve unit is not always required. 
   In certain devices used in the state of the art, the shielding electrodes and all other parts which are set on beam boost potential are connected to each other to form a single inner channel around the beam path. This results in very good shielding of the beam path from disturbing fields. 
   In general, the above mentioned components, shielding electrodes  30  (or liner tube), valve unit  6 , 8 , shielding electrodes  32  need to be fixed to the grounded housing  18 . This requires an isolation between the housing and these components. The difference in potential used to boost the beam is in general higher than 1 kV. Therefore, the insulators  34 ,  36 ,  38  have to be adapted to the high voltage difference. 
   The housing  18  is connected to the walls of the specimen chamber  22  which are only partially shown in FIG.  2 . For security reasons the housing is grounded. The materials used for the housing are usually electrically and magnetically conductive to shield the inside from disturbing electric and magnetic fields. 
     FIG. 1  shows an embodiment of the present invention. Thereby, the same reference numbers denote corresponding parts. The charged particle beam device  1  with its housing  40  being put on beam boost potential during operation of the device comprises a charged particle source  2  with an accelerator and a suppressor  4 . The power supply wires  3  for particle source  2  are laid through top cover plate  20 . Below the particle source  2 , a suppressor and accelerator unit regulates the beam current. A lens system for guiding the beam, stigmators, deflection coils etc. are not shown in FIG.  1 . Only valve unit  6 ,  8  for dividing the optical column into two sections which can be held at different vacuum levels, (if necessary) is connected to housing  40  and put on beam boost potential. Consequently, valve unit  6 ,  8  does not require insulators and the electrical fields stemming from holding unit  6  and sliding valve  8  do not disturb the beam boost potential prevailing in the column. 
   Detector  16  is positioned in beam path  10  to collect secondary products. It is maintained at beam boost potential either with a conductive connection along light guide  17  or with any other conductive connection arranged between the detector and the housing or other conductive parts being put on beam boost potential. 
   It should be noted that not all optical columns need a valve unit  6 ,  8 . Additionally, the detector does not always need to be located in the beam path. It can be positioned at the side or even outside and below the optical column e.g. in transmission microscopes. If a particular application necessitates an optical component to be arranged close to the beam path then it is preferred to keep it a beam boost potential. The possible disturbing influence of the electrical field of a component depends on its size, its distance to the beam path, the materials used, and its potential during operation of the charged particle beam device. This allows one to estimate to what extend it is necessary to put the respective part on beam boost potential. It is most preferred, however, to keep all possibly disturbing components or parts on beam boost potential. 
     FIG. 1  shows a electrostatic objective lens  12  with three electrodes below the housing  40  of the charged particle beam device. In a preferred embodiment, the upper electrode is maintained at beam boost potential. The middle electrode is used for focusing the charged particles coming from the source onto the specimen. Simultaneously, this electrode decelerates the charged particles before they hit the specimen. The lower electrode is primarily used for controlling, e.g. it adjusts the extracting field for secondary particles coming from the source. The use of three electrodes is not a necessity, in principal, only one electrode would suffice. However, this limits for example the options of regulating the focusing properties of the objective lens. 
   Top cover plate  20  is connected to housing  40  and therefore on beam boost potential. The wires  3  which supply charged particle source  2  with power and the source itself are in general not on beam boost potential. Isolation is provided between the top cover plate  20  on one side and wires  3  and source  2  on the other side. Alternatively, it is possible to maintain the top cover plate  20  at the potential of the charged particle source  2 , but then the isolation needs to be provided between housing  40  and top cover plate  20 . 
   Housing  40  and top cover plate  20  are surrounded by a cover  42  and top cover  46  which are put on ground potential. In between housing  40  and cover  42  and in between top cover plate  20  and top cover  46  an insulating layer  48  is put. The insulating layer could be a dielectric material of varying thickness or a sufficiently large air gap. Particularly in the latter case, it is preferred to round the edges of top cover plate  20  to reduce the likely hood of electric discharge. In addition, insulation is provided between wire  3  and top cover  46 . 
     FIG. 3  shows the prior art charged particle beam device of  FIG. 2 , however, with omitted insulators. Shielding electrodes  30  and  32  are directly connected with housing  40  and on beam boost potential. Depending on the application, stigmators, deflection coils or any other components could be arranged behind them. It should be noted, however, that it is not mandatory to have these components arranged behind shielding electrodes. The optical column is surrounded by cover  42  and cover plate  46 . Advantageously, all parts and components located between source  2  and objective lens are put on beam boost potential. 
   It is possible to place getter material close to the charged particle source to evacuate the gun chamber even more. Depending on the exact location, the getter material is either put on beam boost potential or on the potential of the charged particle source. Alternatively, a getter pump can be arranged outside the column and a tube connects the pump with the gun chamber. Naturally, the tube end extending into the gun chamber is preferably put on beam boost potential during operation. The parts of the tube which either go through the housing  40  or through cover  42  need to be insulated accordingly. This could be done with insulating layers and/or with flanges having insulating material at their interfaces. 
     FIG. 4  shows the lower part of the optical column connected to the specimen chamber  22 . In general, the specimen chamber has a separate vacuum pump (not shown) for evacuating the chamber. To avoid leakage, a sealing  44  e.g. o-ring is put between cover  42  and specimen chamber. Depending on the specific set up of the wall, an alternative or additional sealing can be put between housing  40  and cover  42 .