Abstract:
An apparatus for examining a specimen with a beam of charged particles, where charging of the specimen is avoided or reduced by injecting inert gas onto the sample&#39;s surface. In order to avoid interactions with the electron optics, various embodiments are disclosed for providing a rotationally symmetrical nozzles and/or electrodes. Additionally, embodiments are disclosed wherein a plurality of gas conduits are arranged in a rotationally symmetrical manner. Alternatively, the conduit is incorporated into an element of the electron optics, such as the magnetic lens. Also, in order to reduce or eliminate interaction of the electrons with the gas molecules, embodiments are disclosed wherein the gas is pulsated, rather than continually injected.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a charged particle beam device for the examination of specimen. In particular, this invention relates to the examination of specimen which have the tendency of getting charged while being radiated with a charged particle beam.  
         BACKGROUND OF THE INVENTION  
         [0002]    Negatively or positively charged particles coming from a particle source can be accelerated by a potential of V volts. The direction of travel of such a moving particle is altered either by applying a magnetic or an electric field e.g. a charged particle moving in a magnetic field experiences a force tending to change its direction of motion except when it is traveling parallel to the magnetic lines of force. Suitably shaped magnetic and electric fields can be used to cause charged particles—diverging from a source to converge into a beam, guide the beam along a predetermined path and allowing it to impinge on the surface of a substrate or a specimen.  
           [0003]    The charged particles interact with the atoms of the specimen and cause a number of different effects in the specimen or on its surface. Without limiting the scope of the present invention, the following explanations will primarily concentrate on the use of electrons as charged particles. The impinging electrons, generally called primary electrons (PE), are deflected by collisions with specimen atoms. These collisions may be elastic when the electron is deflected (even up to 180 degrees), but no energy interchange occurs. They may also be inelastic when the primary electron interacts with the atom and supplies energy for a further process to occur. Such a process could result in the emission of an electron, called secondary electron (SE) and/or electromagnetic radiation. Thereby, the primary electron generally experiences only a small deviation of track. After the collision or collisions, the primary electron may re-emerge as backscattered electron (BSE) or as transmitted electron, or may lose all its energy and come to rest in the specimen. There, the primary electron contributes to the specimen&#39;s heating or to the specimen&#39;s absorbed charge.  
           [0004]    The above indicated physical effects can provide much analytical information about the specimen. In the following, the creation of secondary electrons and their informational content of the specimen will be considered in greater detail. An inelastic collision of an incident primary electron having a kinetic energy of e.g. 1 keV, can result in electrons being detached from the specimen atoms. This leaves behind an ionized atom with a positive charge. The dislocated electrons have a low kinetic energy, typically less than 50 eV, and are easily captured by nearby atoms. Some electrons which are created closer to the surface can be emitted from the specimen and can be collected with specific detectors. Consequently, only a small portion of the secondary electrons formed are available for collection. Since the emitted SE originate from a small region very close to the surface of the specimen, they carry corresponding surface information.  
           [0005]    In particular, a surface of the specimen which is tilted relative to the incident beam reacts differently than a surface perpendicular to the incident beam. Compared to a flat surface, the electrons having entered a tilted surface of the specimen propagate a longer distance close to the specimen&#39;s surface. This results in a greater proportion of secondary electrons that are to escape and the electron emission from the surface will increase. The intensity of the secondary electron emission is therefore an indicator of the surface slope and topography. Therefore, the intensity signals collected by secondary electron detectors can be used for high resolution surface imaging. Instruments visualizing these surface effects have become increasingly important for the development of e.g., microelectronic components. They are used to identify deviations from predetermined patterns or to evaluate topographical parameters such as height, width or angle of inclination of the structure under examination. An example of a widely used system is the scanning electron microscope (SEM). A critical dimension SEM (CD-SEM) is routinely used to measure dimensions of elements on semiconductor wafer to a nanometer resolution.  
           [0006]    It should be appreciated that in order to obtain an image using the SEM, the number of electron in the primary beam must be different from the number of electrons emitted from the specimen (i.e., the yield must be different than 1). This imbalance of entering and leaving electrons contributes to the specimen charge. This is especially true for insulators and semiconductors where charge is easily accumulated. The charge can result in a strong electrical field prevailing at the surface of the specimen and substantially altering the image of its surface by, for example, altering the path of PE&#39;s and SE&#39;s. In a semiconductor device, for example, electric insulators such as SiO2 are often deposited on conductors such as Al or semiconductors such as silicon. When a PE beam is directed onto the device, the surface of the insulator is charged. The resulting electric field alters the direction of PE&#39;s and SE&#39;s and results in inaccurate measurement of the features. This problem is even more sever when several lines to be measured are closely positioned, so that many interactions with charge fields occur to cause deviation from the actual measurement. Additionally, such a field at the surface can prevent SE created at the bottom of a contact hole and vias from reaching the detector.  
           [0007]    In the past, a variety of methods have been tried to solve these problems. The approaches included adaptation of the acceleration voltage and the current of the electron beam. Others have altered the scanning speed of the primary electron beam or modulated the primary electron beam before impinging on the specimen. However, these methods have not been satisfactory. In some cases the intensity of the emitted secondary electrons is too low, in other cases the results obtained by comparative measurements are unreliable.  
           [0008]    In an alternative approach, Environmental Scanning Electron Microscopes (ESEM) have been used. Originally these instruments were developed for the examination of specimens which are sensitive to dehydration caused by the vacuum in the specimen chamber. The use of a low pressure environment in the chamber prevented the dehydration. As a secondary effect, the presence of ions in the irradiated gas impeded charging of the specimen. These ESEM systems, however, cause widening of the beam of charged particles due to the scattering of the primary electrons due to the absence of vacuum. Also, the high gas concentration in the electrical fields between the detectors and the specimen can result in arcing. Therefore, the conventional ESEM systems did not lead to satisfactory results either and cannot be used for semiconductor applications since such applications require high vacuum environment.  
           [0009]    Various proposed methods to avoid charging in SEM examination of semiconductors are presented in U.S. Pat. No. 5,869,833. While the focus of that patent is various scanning schematics to prevent charge or cause discharge, there&#39;s also a mention of introducing gas into the vacuum chamber. However, the discussion relating to the introduction of gas is basically limited to a single paragraph and lacks many details needed suggest a working system.  
         SUMMARY OF THE INVENTION  
         [0010]    Accordingly, the present inventors undertook an in-depth study of the subject of semiconductor charging and, in particular, the use of gas discharge in a vacuum chamber. Thus, the present inventors uncovered many of the difficulties needed to be overcome in order to make to such a system workable in practice. The various problems and solutions worked out by the present inventors are detailed herein.  
           [0011]    The present invention intends to provide an improved apparatus and method for examining a specimen with a charged particle beam. Specifically, the various embodiments of the present invention utilize injection of gas into the vacuum chamber to assist in discharge of the specimen.  
           [0012]    According to a further aspect of the present invention, there is provided an apparatus for examining a specimen with a beam of charged particles. The apparatus comprises a particle source for providing a beam of charged particles which propagate along an optical axis and an optical device for directing said beam of charged particles onto said specimen to be examined. Further, the apparatus comprises a gas supply for providing an inert gas to the area of incidence of said beam of charged particles onto said specimen, and a vacuum chamber for loading said specimen. According to one aspect of the present invention, the inert gas is provided in the form of a layer. This layer preferably covers the area of the specimen where the beam of charged particles is incident. In the context of this application “layer” is not understood as a geometrical object with clearly defined limits. Rather, the inert gas molecules forming the layer preferably have a higher concentration in the area of the specimen where the beam is incident. The concentration decreases along the beam with increasing distance to the specimen.  
           [0013]    In a further preferred aspect of the present invention, a nozzle directs the stream of inert gas to the area of incidence. This could preferably be done by either varying the gas flow or by varying the pressure, or by varying both. Instead of a single nozzle two or more can be used to provide the inert gas. In such a case the nozzles are preferably arranged in a symmetrical pattern.  
           [0014]    According to a still further aspect of the present invention, there is provided an electrode in the vicinity of the area of incidence of the charged particle beam. The electrode allows a controlled charging of the specimen. The inert gas will cause an exchange of charge between the specimen and the electrode. Advantageously, the electrode establishes a desired voltage level in a limited region of the specimen. Furthermore, it is preferred to provide means for positioning the electrode e.g. with an x-y-z manipulator (e.g., FIG. 1). Depending on the material or the surface structure of the specimen, the electrode can be placed at a specific location over the specimen with a specific distance from it.  
           [0015]    In another embodiment according to the invention, the nozzle and the electrode are provided as an integral part. This can be achieved either by a common carrier or by attaching the electrode to the nozzle. Preferably the electrode is arranged in front of the nozzle. Both parts, the nozzle and the electrode, advantageously comprise a coupling device. This allows to exchange them with other nozzles or electrodes, having different geometrical dimensions and therefore, different physical properties.  
           [0016]    It is preferred to use an electrode creating an electrostatic field with a rotational symmetry. This symmetry limits the influence on the trajectory of the incident charged particle beam and the secondary electrons. In one such embodiment, the electrode is a circular ring electrode with the charged particle beam being guided through its center.  
           [0017]    Advantageously, the electrode comprises a conductive net. At least one of the net meshes is used to let the particle beam pass. Although such an arrangement will slightly distort the rotational symmetry of the resulting electrostatic field, it improves the efficiency of the electrode in controlling the charging.  
           [0018]    According to another aspect there are provided vacuum means capable of providing a pressure gradient in the specimen chamber. Preferably the pressure gradient is established along the incident particle beam with a higher pressure at the point of incidence and a lower pressure at the aperture for permitting the particle beam enter the specimen chamber. The pressure gradient reduces the number of collisions between the particle beam and any atoms or molecules other than those in the specimen. Preferably the pressure gradient is established in such a way that the particles of the incident beam collide on the average less than once with an atom or molecule in the chamber before hitting the specimen.  
           [0019]    Preferred inert gases used in the context of this application are N 2 , CO 2 , or SF 6  or noble gases such as Argon. The gases used, however, are not limited to those mentioned above. Any other gas which is inert to reacting with the surface of the specimen can be used.  
           [0020]    According to another aspect of the invention, inert gas is provided to the area of incidence in a discontinuous or pulsating manner. This allows reduction of the amount of inert gas in the specimen chamber. Inert gas is primarily guided to the area of incidence when it is needed for decharging.  
           [0021]    According to a further aspect of the invention, the gas conduit is integrated into the objective lens or into any other mechanical parts close to the specimen. Such an arrangement allows for the use of existing set up of a microscope column and maintains the rotational symmetry around the optical axis.  
           [0022]    According to a still further aspect of the present invention, there is provided a charged particle beam apparatus comprising a particle source and an optical device for directing a beam of charged particles onto said specimen to be examined. The apparatus further comprises a gas supply for providing a gas to the area of incidence of said beam of charged particles and a 2-way valve arranged in the gas supply whereby a first port of the 2-way valve is connected with a gas reservoir and a second port of the gas supply is connected with a vacuum reservoir. A control unit switches the 2-way valve.  
           [0023]    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, by any combination of the two or in any other manner. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    Some of the above indicated and other more detailed aspects of the invention will be described in the following description and partially illustrated with reference to the figures. Therein:  
         [0025]    [0025]FIG. 1 is a schematic vertical cross section of an objective lens for a charged particle beam device comprising a gas conduit with a symmetrical electrode, according to one embodiment of the invention.  
         [0026]    [0026]FIG. 2 is a schematic vertical cross section of a second objective lens for a charged particle beam comprising a tube shaped gas conduit, according to another embodiment of the ivention.  
         [0027]    [0027]FIG. 3 shows a simplified cross section of the optical column of a scanning electron microscope with a gas supply attached to the specimen chamber, according to an embodiment of the invention.  
         [0028]    [0028]FIGS. 4 a - 4   c  depict a top view of a plurality of gas nozzles arranged symmetrically around the area of incidence of the primary electron beam, according to various embodiments of the invention.  
         [0029]    [0029]FIGS. 5 a - 5   d  depict a top view of gas nozzles with a reference electrode according to various embodiments of the invention.  
         [0030]    [0030]FIG. 6 shows a possible time correlation between the scan movement of the charged Particle beam and the gas flow.  
         [0031]    [0031]FIG. 7 shows a possible time correlation between a pulsed charged particle beam and the gas flow.  
         [0032]    [0032]FIG. 8 shows an objective lens with an integrated gas conduit whereby the gas conduit is integrated into a pole piece of the magnetic lens.  
         [0033]    [0033]FIG. 9 shows an objective lens with an integrated gas conduit whereby the gas conduct is integrated into an electrode of the electrostatic lens.  
         [0034]    [0034]FIG. 9 a  shows an objective lens with a reference electrode whereby the gas conduit is integrated into the reference electrode arranged below the objective lens.  
         [0035]    [0035]FIG. 10 shows a gas conduit directed to the specimen with a valve arranged between the gas reservoir and the specimen chamber.  
         [0036]    [0036]FIG. 11 shows a gas conduit directed to the specimen with a micro valve arranged at the end of the gas conduit.  
         [0037]    [0037]FIG. 12 shows a gas conduit directed to the specimen with a 2 way valve arranged in the gas conduit. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    An exemplary embodiment of an objective lens unit  1  used in an apparatus for examining a specimen is shown in FIG. 1. This lens is preferably used as a component in the optical column of a scanning electron microscope. A primary electron beam  2  generated by an electron source (shown in FIG. 3) propagates along an optical axis OA and is focused onto a specimen  3 . It is not required that the primary electron beam always propagates along the optical axis. On its way from the source to the specimen the particle beam can be deflected from the optical axis OA or its vicinity and, at a later stage, reflected back to the optical axis. In the shown embodiment the specimen  3  is a microelectronic component with an insulator layer  4  on substrate layer  5 . For examining the specimen, a particular region of the specimen is scanned with the primary electron beam. The scanning or deflection of the beam is done by the scanning coil system which is driven by a scanning generator (both not shown in FIG. 1). The interactions of the beam with the specimen result e.g. in secondary electrons. In the area of incidence, due to the insulating surface  4  of the specimen, the primary electron beam will cause localized charging. The secondary electrons can be measured with an appropriate detector  7 . In the illustrated example the detector is arranged immediately above the lens body symmetrically relative to the optical axis OA. The detector  7  is preferably an annular electron sensitive part that includes a hollow cylinder  16  fitted into a central bore therein. The hollow cylinder  16 , which can be charged with a somewhat lower positive potential compared to the upper electrode  12  of the electrostatic lens, extends into the upper body of the lens body.  
         [0039]    The shown conical magnetic lens having an upper pole piece  8  and a lower pole piece  9  is fed by an excitation coil  11 . The magnetic flux of the magnetic lens is concentrated in the area of the pole piece gap  10  between the upper and lower pole pieces  8 ,  9 . Here, the magnetic field reaches its maximum strength and is nearly rotationally symmetrical about the optical axis OA. The imaging properties of the objective lens unit  1  are also determined by the electrostatic lens. In the embodiment shown, an upper electrode  12  and a lower electrode  13  establish a nearly rotationally symmetrical electric lens field with its principal plane located in between the principal plane of the conical magnetic lens and the specimen  3 . In the illustrated exemplary embodiment the upper electrode  12  is preferably in the form of a truncated cone tapering in the direction of the specimen; its end part is a cylindrical tube projecting into the region of the pole piece gap  10 . The positive potential of this electrode is, for example, 5 to 10 kV. The lower electrode  13  which has a conical shape is for example, grounded. The resulting electric field is a retarding field decelerating the high speed electrons coming through the upper electrode  12 .  
         [0040]    As shown in FIG. 1, a gas conduit  14  is used to supply discharging gas at the vicinity of the sample. However, as noted by the present inventors, presenting the conduit into the vicinity of the primary beam may cause distortions in the electron optics. Thus, the inventors devised a solution as follows.  
         [0041]    Between the specimen  3  and the lower electrode  13  a reference electrode  15  is arranged. In the shown embodiment the electrode has a rotationally symmetrical shape. The reference electrode is arranged at the end of gas conduit  14  which supplies the inert gas to a nozzle integrated into the reference electrode. On one hand, the reference electrode  15  can be used to establish a predetermined voltage in the part of the specimens surface which is located in the direct vicinity of the reference electrode. On the other hand, depending on the voltage applied to it, the reference electrode also assists in sucking secondary electrons from the wafer&#39;s surface to enhance the imaging. Deviations of the electrostatic field caused by the presence of the gas conduit and nozzle are minimized by arranging a symmetrical electrode at the nozzles tip portion. In one embodiment, the nozzle aperture is shaped as an annular opening along the circumference of the reference electrode. Alternatively, such an annular opening of the nozzle aperture can also be used without combining it with a reference electrode. In this case, the circular shaped nozzle attachment  15  acts solely as a circular gas outlet, wherein its circular symmetry minimizes imaging degradation.  
         [0042]    [0042]FIG. 2 shows another objective lens  1 ′ for focusing a beam of primary electrons  2  on the specimen. The lens is essentially composed of a magnetic lens and an electrostatic lens generating fields which are essentially rotationally symmetrical around the optical axis OA of the objective lens. The detector  7  is arranged immediately above the lens body and documents secondary electrons  6  triggered at the specimen by the primary beam of electrons. The magnetic flux generated by an excitation coil  11  is concentrated into a small spatial region around the optical axis OA with the assistance of an upper and a lower pole piece  8 ,  9 . The maximum field strength of the magnetic lens is reached in the pole piece gap  10 . The upper electrode  12  of the electrostatic lens includes a cylindrical portion which is arranged in an insulating fashion within a bore in the upper pole piece  8  of the magnetic lens.  
         [0043]    The lower pole piece  9  of the magnetic lens preferably lies at ground potential and can be lined with a beam guidance tube of magnetic or non magnetic material (not shown in FIG. 2) for protection against contamination. The lower pole piece, thus, forms the lower electrode  13  of the electrostatic lens which establishes a retarding field. A gas conduit  14  is arranged just below the objective lens. The gas conduit is part of a gas supply which further comprises a gas reservoir (not shown in this figure). In the example shown, the gas conduit is provided in form of a tube whose opening for ejecting the gas is directed towards the area of incidence of the primary electron beam. A preferred distance between the lower edge of the outlet opening and the specimen surface is up to 1 mm. In certain applications the distance is about 100 micrometers. In this case, due to the angle of inclination which the tube opening forms with the surface of the specimen, the working distance of the tube opening from the area of the specimen where the beam is incident is about 0.5 mm. Preferably the diameter of the outlet opening is about 100 micrometers. The gas conduit shows a knee portion  17  and the part of the gas conduit which connects the knee portion  17  with the gas reservoir is directed away from the objective lens. The inclination assist in directing the gas towards the specimen, while maintaining the end of the conduit sufficiently away from the vicinity of the primary beam to avoid interaction. If additional protection against interaction with the primary beam is needed, a nozzle with rotational symmetry can be affixed to the end of the conduit  14 .  
         [0044]    An exemplary cross section of a scanning electron microscope is shown schematically in FIG. 3. Electron column  30  can be any conventional column, and includes an electron source  18 , coils, e.g.,  19  and  20 , and an objective lens  21 , all operate to produce a focused primary electron beam. The focused primary electron beam hits the specimen  3  positioned on a specimen holder  27 , located in a vacuum. chamber  26 . Vacuum pump  28  provides a desired vacuum level so that the primary electrons are not deflected from their predicted path by collisions with any atoms or molecules other than those of the specimen. Furthermore, vacuum pump  29  is provided for evacuating the electron column  30 . The aperture  23  separates the optical column from the specimen chamber  26 . An inert gas in a gas reservoir  25  is guided through a valve  24  to the gas conduit  14  which directs the stream of inert gas onto the specimen  3 . A nozzle  15  is provided at the end of the gas conduit  14 . The valve  24  can be used for adjusting the pressure used for providing the inert gas.  
         [0045]    Within the vacuum chamber  26  into which the specimen to be examined is loaded, an exemplary average base pressure of about 5×10 −4  Pa is prevailing. This average base pressure in the chamber increases to about 5×10 −3  Pa if inert gas is directed to the specimen. The pressure at the outlet of the gas conduit is about 10 Pa. The stream of inert gas creates a mini-environment at the point of incidence of the primary electron beam. While significantly reducing the adverse effects of charging close to the surface of the specimen, this mini environment does not substantially affect the average vacuum level in the chamber to an extent where it hinders the transmission of the beam through the optic column. Scattering of the primary electrons with gas molecules in the chamber is thereby kept at an acceptable level. The vacuum gradient is maintained by balancing the gas flow of the inert gas into the specimen chamber with the amount of gas evacuated from that chamber. The exhaust port of the vacuum pump is preferably arranged in alignment with the stream of inert gas.  
         [0046]    [0046]FIGS. 4 a - 4   c  show several symmetrical arrangements of gas conduits  14 , which may obviate the need for a rotationally symmetrical nozzle. The location of the incident primary electron beam is the center of symmetry. FIGS. 5 a - 5   d  show several exemplary combinations of nozzles  15  and gas conduits  14 . In each of these embodiments, the nozzle can also function as an electrode. In FIG. 5 a  the nozzle/electrode is arranged at the tip of the gas outlet or nozzle. In FIG. 5 b  the gas outlet of the conduit projects into the nozzle/electrode. In FIG. 5 c  two gas outlets are connected to the nozzle/electrode and in FIG. 5 d  a conductive net is used to span the rotationally symmetrical electrode. The core mesh of this net is located in the center of the electrode. Alternatively, a rotationally symmetrical conductive net can be used instead of the net shown in FIG. 5 d.    
         [0047]    The inert gas stream directed to the area of incidence of the charged particle beam onto the specimen significantly reduces the adverse effects of charging without substantially affecting the average vacuum level in the chamber. However, depending on the amount of gas present in the specimen chamber, it may hinder the transmission of the beam before it hits the specimen. Accordingly, according to one advantageous embodiment of the present invention, this negative effect is reduced by providing a pulsed gas stream which supplies inert gas only in the amount necessary for reducing charging and cuts out excessive inert gas.  
         [0048]    An example of such a pulse mode is shown in FIG. 6. The upper diagram shows the scanning movement of the charged particle beam. Similar to the scanning of an electron beam on a television screen, the charged particle beam is guided in a first movement across a first line in the area of the specimen to be examined. The scan movement takes place in the time interval t 0  to t 1 . An image of this line is created by the detection of secondary particles created by the impinging primary particles. At the end of this line, the charged particle beam is retraced to just below the beginning of the line. The retrace movement takes place in the time interval t 1  to t 2  and during this time interval no image of the specimen is created. Subsequently, the charged particle beam is guided within the time interval t 2  to t 3  in a second scan movement across the area of the specimen to be examined creating a second line of the respective image. The lower diagram shows the time correlation between the flow of inert gas and the scanning movement of the charged particle beam. In this specific example, the stream of inert gas is directed to the area of incidence during the retrace movement of the charged particle beam i.e. within the time interval t 1  to t 2 . No inert gas is directed to the specimen during the scan movement. This avoids collisions between the primary particles and gas particles. The inert gas particles which are directed to the specimen during the retrace movement remain for some time close to the surface of the specimen thereby reducing the adverse effects of charging.  
         [0049]    In certain applications, it will not be necessary to provide the inert gas during the complete retrace movement. It can be advantageous to stop the gas stream prior to t 2  if inert gas has been provided in a sufficient amount so that it is still capable to reduce charging for the next scanning movement. Alternatively, it can be advantageous to still provide inert gas during the first part of the scanning movement so that there are still enough gas particles present and close to the area of incidence to prevent charging at the end of the scanning interval.  
         [0050]    Depending on the length of the gas conduit, the pressures used for providing the inert gas, and the distances between the end opening of the gas conduit and the area of incidence, the inert gas particles require some time before they reach the area of incidence. This time delay is considered in an alternative embodiment of the invention, where the on-phase of the pulsed gas stream is initiated prior to t 1 , and the gas stream is switched off prior to t 2 . The two time shifts prior to t 1  and t 2  do not necessarily have to have the same length of time.  
         [0051]    A different correlation between the control of the charged particle beam and the pulsed gas stream is shown in FIG. 7. The upper part of the diagram shows a pulsed charged particle beam during the scanning movement. At t 1  the charged particle beam is switched off or blanked and at t 2  it is switched on again. At t 2  the scanning movement of the charged particle beam is continued at the same spot of the specimen where the scanning movement was interrupted at t 1 . The lower part of the diagram shows that the scanning pause is used to direct a stream of inert gas onto the area of incidence. Since there are no charged particles impinging on the specimen during the time interval t 1  to t 2 , the number of collisions between primary and secondary particles on one side and gas particles on the other side is considerably reduced. As explained with respect to FIG. 6, the time interval during which inert gas is provided to the area of incidence can be time shifted with respect to the pulse pauses of the charged particle beam. Also, the time intervals during which inert gas is provided can be longer or shorter than the pulse pauses of the charged particle beam.  
         [0052]    It is important to note that all of the above described pulse modes are only examples of a vast variety of pulse modes which are capable of reducing charging by providing inert gas to the area of incidence and, simultaneously, trying to keep the disturbing effects of the gas particles which collide with primary and secondary particles to a minimum. All of these pulse variations are within the scope of the present invention.  
         [0053]    As noted above, introducing a gas conduit in the vicinity of electrical and magnetic fields influence the trajectory of the primary beam. To avoid disturbing effects, certain embodiments described herein use a nozzle or an electrode having rotational symmetry. In the following, embodiments will be described which incorporate the injection of the gas into lens arrangements.  
         [0054]    In the embodiment shown in FIG. 8, the gas conduit  32  is integrated into the objective lens. A passage  31  in the pole piece  30  of the magnetic lens can either serve directly as a gas conduit or acts as an enclosure for a nozzle or tube implemented into the passage  31 . The opening of the passage, as shown in FIG. 8, ends at the inner cylinder of the pole piece  30 . Alternatively, the open end of the passage can be arranged in the lower end of the pole piece facing the specimen.  
         [0055]    [0055]FIG. 9 shows the electrode of an electrostatic lens  34  arranged between the pole piece  30  of a magnetic lens and the specimen  3 . Here, the gas conduit  31  is integrated into the electrode of the electrostatic lens. A passage  31  serves directly as gas conduit or acts as enclosure for a nozzle or a tube. FIG. 9 a  shows a similar arrangement to that of FIG. 9, whereby an additional reference electrode  15  is placed between the electrode of the electrostatic lens  34  and the specimen  3 . This reference electrode could also be placed between the specimen and the pole piece of the embodiment shown in FIG. 8. By integrating the gas conduit into mechanical parts arranged in the direct vicinity of or closest to the specimen, the set up of the microscope column does not have to be changed and the electric and magnetic fields for directing the primary and secondary particles are hardly altered. The integration of a gas conduit into mechanical parts in the vicinity of and/or closest to the area of impact of the charged particle beam is preferably used in combination with the above described charging phenomenon, however, it can also be used in all kinds of charged particle beam apparatuses where a gas interacts with a charged particle beam.  
         [0056]    The gas flow which is directed to the specimen can be controlled with a continuously adaptable valve for the flow control. Alternatively, it is possible to use a pulsed gas stream with a pulse frequency sufficiently high so that the pulsating gas flow provides a consistent gas layer or environment close to the area of incidence. A fine tuning of the gas environment can be achieved by choosing a proper on/off ratio for the pulse mode.  
         [0057]    [0057]FIG. 10 shows an embodiment in which the valve  24  for manipulating the stream of gas is situated in between the specimen chamber  26  and the gas reservoir  25 . By opening and closing the valve a pulsed gas stream is created and directed through the gas conduit  32  to the specimen  3 . FIG. 11 shows an alternative embodiment in which a micro valve  35  is integrated close to the end of the gas conduit. This has the advantage that after the closing of the micro valve  35 , the time required to evacuate the remaining gas in the gas conduit is considerably reduced. In a further embodiment shown in FIG. 12, a 2 way valve  36  is used. One port of the 2 way valve  36  is connected to the gas reservoir  25  and the other port of the 2 way valve is connected to a vacuum reservoir. Preferably, the volume of the vacuum or evacuated reservoir is much bigger than the volume of the gas conduit  32 . In order to shut off the gas flow, the connection between the gas conduit  32  and the gas reservoir  25  is interrupted and the connection between the gas conduit  32  and the evacuated reservoir  36  is opened. This results in an immediate drop in pressure in the gas conduit. This arrangement can be more advantageous than use of a micro valve. The 2 way valve can not only be used to reduce charging by providing inert gas, but it can be applied in all charged particle apparatuses in which a gas flow of any kind is directed to the area of incidence of a charged particle beam. This allows the gas or gases to interact with the corpuscular beam in defined time intervals. Instead of a 2 way valve a multiple way valve could be used and more than one port connected with different gas reservoirs and/or more than one port could be connected with different vacuum reservoir. Advantageously a gas conduit integrated into mechanical parts of the charged particle beam apparatuses e.g. as shown in FIGS. 8, 9 and  9   a  is used.  
         [0058]    Those skilled in the art will readily appreciate that various modifications and changes may be applied to the preferred embodiments of the invention as hereinbefore described without departing from its scope defined in and by the appended claims.