Abstract:
The invention concerns a column for producing a focused particle beam comprising: a device ( 100 ) focusing particles including an output electrode ( 130 ) with an output hole ( 131 ) for allowing through a particle beam (A); an optical focusing device ( 200 ) for simultaneously focusing an optical beam (F) including an output aperture ( 230 ). The invention is characterized in that said output aperture ( 230 ) is transparent to the optical beam (F), while said output electrode ( 130 ) is formed by a metallic insert ( 130 ) maintained in said aperture ( 230 ) and bored with a central hole ( 131 ) forming said output orifice.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
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   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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   INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
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   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   This invention relates that to an optical column for simultaneously focusing an ion beam and a photon beam onto the same region. 
   The invention is particularly useful in the field of analysis and repair and manufacture of integrated circuits. 
   (2) Description of Related Art 
   Focused ion beams such as ion or electron beams are currently widely used for various types of integrated circuit analysis and manufacturing operations, notably characterisation, identification, design and failure analysis, depassivation, vapor phase deposition, micro-machining, etc. These operations are performed using a particle beam production column designed to be focused onto the integrated circuit at the place intended for the desired intervention. 
   Such a column typically comprises a source of ions such as Ga+ produced from liquid metal which, after extraction, form an ion beam, which is then manipulated by a focusing device comprising a certain number of electrodes operating at determined potentials so as to form an electrostatic lens system adapted to focus the ion beam onto the integrated circuit. Each electrode of the focusing device, notably the output electrode, consists of a series of metallic electrodes having an aperture for passage of the particle beam. It should be noted here that the shape of the various electrodes as well as the aperture diameter plays a determining part in aberrations, notably spherical and chromatic aberration, of the particle focusing device. 
   One of the limits of applying focused ion beams is the impossibility of employing them to provide an in-depth image of a solid. Only surface images can be obtained. In the case of passivated and planarized integrated circuits, a surface image gives no information on the underlying layers and circuits, which has the disadvantage of making any intervention in the depth of the circuit extremely difficult such as, in particular, the cutting or breaking of buried metal tracks made necessary by design and failure analysis. To overcome this disadvantage, we employ an auxiliary light (photon) beam simultaneously and coaxially focused with the particle beam. In effect, using the light beam to obtain images in the thickness of the circuits, it is possible to visualize layers and tracks in depth and explore them, in real time, using the ion beam. It will now be understood that associating two types of beam, an ion and a photon beam, allows the operator to bring the ion beam exactly to the desired point on the object by means of the image supplied by the light beam. 
   Certain ion beam production columns also include an optical focusing device, a Cassegrain-Schwartzfeld (C-S) mirror objective lens for example, terminating at an outlet aperture placed close to the surface of a sample subjected to the ion beam. 
   French patent 2,437,695 discloses an emission ion lens associated with a C-S type mirror objective lens. 
   In this system, the ionic lens part, the elements of which consist of two perforated electrodes and of the sample itself, is located between the object and the mirror objective lens. 
   In this configuration, the apertures in the ion focusing device electrodes must simultaneously be sufficiently large to provide a geometrical expanse for the optical beam allowing sufficient sample illumination, and, relatively small so as not to deteriorate ion beam quality through excessive aberrations. The final diameter chosen for the outlet aperture is consequently a trade-off which is not satisfactory either for the optical beam extent or for ion beam focusing. 
   Secondly, the system disclosed in French patent 2,437,695 necessitates a very small (a few millimetres) working distance and the submitting of the sample to an electrical field. These two constraints are unacceptable in focused ion beam technology applied to integrated circuits: the danger of destroying the circuits by micro-electrostatic breakdown, impossibility of slanting the sample, difficulty in collecting secondary electrons, and the practical impossibility, through lack of space, of associating the system with a capillary tube for injecting pre-cursor gas which is an essential accessory in focused ion beam technology. 
   BRIEF SUMMARY OF THE INVENTION 
   Thus, the technical problem to be resolved by the subject matter of this invention is to provide a focused particle beam production column comprising: 
   a device for focusing said particles carrying an output electrode having an outlet aperture for the passage of said particle beam, 
   an optical focusing device for simultaneously focusing a light beam, carrying an outlet opening, 
   such column making it possible to associate: 
   a comfortable working distance of the order of 15 to 20 mm; 
   a final ionic lens having chromatic and spherical aberration coefficients of the order of magnitude of aberration coefficients encountered in conventional ionic lenses; 
   a sufficient numerical aperture for the mirror optics, of the order of 0.3; and 
   zero electric field on the object. 
   The solution to the technical problem posed consists, according to this invention, in that the outlet opening is transparent to said light beam, said output electrode being formed by a metal insert held in said opening and carrying a central aperture forming said outlet aperture. 
   Thus, the column of the invention introduces independence between outlet aperture diameter of the particle focusing device and outlet aperture diameter of the optical focusing device. It is thus possible to adjust central aperture diameter of the metal insert to an optimum value for reducing output electrode aberrations, without this in any way prejudicing optical beam numerical aperture, the latter being determined by the diameter of the aperture transparent to the optical beam. 
   According to one embodiment of the invention, provision is made for the particle focusing device, with said particle focusing device including an intermediate electrode, for the metal insert to project from the opening towards the intermediate electrode. In this way, if electrical breakdown were to accidentally occur between the output electrode and the intermediate electrode, this has maximum probability of occurring at the metal insert, thereby protecting the means for supporting said metal insert, notably the surface treatment of a transparent window of the outlet aperture. 
   The particle production column of the invention is suited to a great number of applications including: 
   treatment of a sample with a charged particle beam using information supplied by the optical beam and, in particular, precise investigation of the effects of a particle beam on an integrated circuit by means of information supplied by the optical beam, 
   treatment of a sample requiring use of a laser focused onto said sample and, in particular, removal of integrated circuit layers by laser with or without chemical assistance, allowing etching or milling at a finer and more local scale, assisted deposition, or electron or ion beam analysis, 
   integration of electron or ion beams with infra-red microscopy for integrated circuit analysis, 
   laser chemical etching allowing milling of integrated circuits by ionic beam or electron beam probing, 
   display of optical transitions created, for example, by the effect of ion beams or other light phenomena appearing on a sample, 
   laser marking of integrated circuits, 
   electron beam probing of diffusion in integrated circuits or other samples, 
   cancelling of the effects of static charges by UV photons when performing focused electron or ion treatment, 
   spectroscopic micro-analysis of photons emitted under particle impact. 
   The description that follows with reference to the attached drawings, provided by way of non-limiting example, will lead to a better understanding of the invention and how it may be carried out. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       FIG. 1  is a partial side view in section of a particle beam production column according to one first embodiment of the invention. 
       FIG. 2  is a partial side view in section of a particle beam production column according to a second embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 1 , we have partially shown, in section, a particle beam production column for focusing onto an integrated circuit  1 . The particle beam axis which coincides with the column axis is identified by reference letter A. Although the column in  FIG. 1  applies to all sorts of charged particles, electrons or ions, we shall take below the example of an ion beam. 
   Only the downstream part of the column is shown in  FIG. 1 , the ion source and the means for extracting and conditioning the ion beam which are known per se, not being shown. 
   The part of the column shown in  FIG. 1  essentially comprises a device  100  for focusing the ion beam onto integrated circuit  1 . This device  100  carries three electrodes, specifically an input electrode  110  which is grounded, an intermediate electrode  120  brought to a nonzero potential V which may be positive or negative for example of 20 Kev, and an output electrode  130  also grounded. These electrodes  110 ,  120 ,  130  are contained between lateral walls  140  of the column, the latter being grounded. 
   In fact, on  FIG. 1  it can be seen that intermediate electrode  120  is a complex two-part electrode made up by a first intermediate electrode  121  arranged close to input electrode  110  and by a second intermediate electrode  122  arranged close to output electrode  130 . These electrodes together form an electrostatic lens of the thick, geometrically asymmetric but electrically symmetric type. 
   It can be seen on  FIG. 1  that an optical focusing device  200  designed to focus an optical beam F simultaneously and coaxially with the particle beam on axis A is located between the two intermediate electrodes  121 ,  122 . This device  200  allows both optical beam F to be focused onto sample  1  thereby forming an enlarged image of the sample as well as collection of light radiation emitted by said sample or by sputtered atoms following ionic bombardment. Optical beam F is obtained from a non-illustrated light source generally arranged laterally with respect to the column with the light being re-directed parallel to axis A and by a mirror at 45° located on said axis and including an aperture for passage of the ion beam. 
   In the embodiment of  FIG. 1 , optical focusing device  200  is a Cassegrain-type mirror objective lens comprising a first convex spherical mirror  210  located in optical beam path F and a second concave spherical mirror  220  focusing onto integrated circuit  1  the beam coming from first mirror  210 . The latter includes an aperture  211  for allowing the ion beam to pass through the second intermediate electrode  122 , the assembly formed by the first mirror  210  and said second intermediate electrode  122  being held at the centre of the column by a metal tripod  212  providing a high degree of transparency to the light beam. 
   As can be seen in  FIG. 1 , optical focusing device  200  also carries an outlet aperture  230  itself including a window  240  that is transparent to photons of optical beam F, held by its edges to the outer housing of the grounded column. Output electrode  130  is formed by a metal insert passing through a window  240 , and which is retained by said window  240  and including a central aperture at its middle  131  for the output of electrode  130 . In order to ground said output electrode  130 , transparent window  240  is electrically conducting. In particular, it can be glass-plated covered with at least one conducting layer  241  such as indium and/or tin oxide. It is thus possible to select a small diameter outlet for aperture  131 , compatible with the resolution desired for the ion beam, while maintaining, in an independent fashion, a larger diameter opening  230 , providing a geometrical expanse for the optical beam ensuring sufficient numerical aperture and thereby obtaining a high quality optical image of the sample  1  observed. Clearly, outlet window  240  could just as well be made of any bulk material transparent to photons, and electrically conducting. 
   In  FIG. 1  it can be seen that metal insert  130  projects from the surface of window  240  towards second intermediate electrode  122 , thereby protecting said window in the case of electrical breakdown, the latter occurring between insert  130  and the second electrode  122 . 
   Like the embodiment shown in  FIG. 1 , optical focusing device  200  of the embodiment of the invention shown in  FIG. 2  is a Cassegrain-type objective lens with mirrors  210 ,  220  brought to a high-voltage comprised, for example, between 10 and 20 keV. 
   However, a first mirror  300  is located on ion beam axis A between the first intermediate electrode  121  and the second intermediate electrode  122  and, more precisely, between first intermediate electrode  121  and the Cassegrain-type objective lens with mirrors  210 ,  220 . This mirror  300  carries an aperture  310  for passage of the ion beam. It is inclined substantially at 45° with respect to axis A in order to deflect optical beam F through about 90° laterally towards a second mirror  320  arranged in the space comprised between the lateral walls  140  of the column and part  120 . This second mirror  320  is itself angled at 45° with respect to axis A. It deflects beam F through 90 degrees in the same direction as axis A, parallel to the latter. 
   Thus, the diameter of aperture  111  provided at the extremity of input electrode  110 , designed to allow passage of ion beam A but the function of which is not, contrary to the embodiment of  FIG. 1 , to allow passage of the optical beam, can be reduced to values of millimetric scale order. Further, deflector plates  10  located upstream of input electrode  110  no longer require the anti-reflection treatment needed for good conduction of the optical beam. Finally, artefacts due to the light beam interacting with the walls of the ionic optical elements which did exist upstream of first mirror  300 , in particular at deflector plates  10  of the embodiment in FIG.  1  and which notably decrease quality of interpretation of the images obtained, are eliminated. 
   Further, in the embodiment of  FIG. 2 , aperture  230  does not carry a window  240  but rather a set of metallic or, at the least electrically-conducting, tabs or legs. There are for example three such tabs forming a metallic tripod  250  which is retained by the edges of the outer housing of the grounded column, delimiting aperture  230 . They ensure good retention of insert  150  while ensuring aperture  230  is kept transparent for the optical beam. Thus, like in the embodiment of  FIG. 1 , it is possible to choose, for output aperture  131 , a small value of diameter compatible with the resolution required for the ionic beam, while maintaining, completely independently, a larger diameter aperture  230  offering the light beam a geometrical expanse allowing sufficient illumination of the observed sample  1 . 
   Finally, in the embodiment of  FIG. 2 , the tabs or legs of metal tripod  212  designed to hold the unit formed by mirror  210  and the second intermediate electrode  122  are curved so as to increase their spacing from the legs of tripod  250  and output electrode  130 . Thanks to this, risks of spark-over are limited as are distortions of the electrical field due to the tripod.