Abstract:
One embodiment of the present invention is an electron microscope that includes: (a) a main vacuum chamber housing a stage therein and connected to a vacuum pump; (b) a load lock for loading a specimen into said main vacuum chamber; (c) a minicolumn non-translatably positioned inside said main chamber; and (d) a vacuum pump situated inside the main vacuum chamber and external to and connected to the minicolumn.

Description:
This is a continuation of a patent application entitled “Charged Particle Beam Microscope with Minicolumn” having Ser. No. 09/162,103 that was filed on Sep. 28, 1998 now U.S. Pat. No. 6,740,889. 

   TECHNICAL FIELD OF THE INVENTION 
   The invention relates to charged particle beam microscopes and, particularly, to arrangements for equipping such a microscope with a minicolumn. 
   BACKGROUND OF THE INVENTION 
   Charged particle beam microscopes, such as an electron microscope, are well known in the art, and are widely used during the manufacture of semiconductor wafers. For ease of discussion, the remaining disclosure makes reference to electrons as the charged particles; however, it should be appreciated that the discussion is equally applicable to other charged particles. The elements of a conventional electron microscope which are of particular relevance here are depicted in  FIG. 1 . Specifically, a vacuum chamber  10  houses an x-y stage  20  upon which the wafer  40  is placed by a robot (not shown). The chamber  10  is evacuated via outlet  70 . The wafer  40  is introduced into the chamber  10  via a load lock  30  so as to avoid having to evacuate the chamber  10  each time a wafer is loaded 
   An electron column  50  is hermetically attached to the chamber  10 . The column  50  houses the electron source and all the necessary electron optics (not shown). The column  50  is evacuated via outlet  60 . The diameter of a conventional column is roughly 6–10 inches, while its height is roughly 15–30 inches. The conventional column is capable of providing an electron beam of sufficiently small diameter for wafer and reticle inspection, review and metrology. 
   One disadvantage of the prior art design is that whenever the column requires a repair which necessitates its removal from the chamber or breaking the vacuum in the column, the vacuum of the chamber is also broken. Breaking the vacuum in the chamber necessarily means that the microscope will be out of service for several hours. Another disadvantage is the requirement for separate vacuum systems for the column and the chamber, which increases the complexity and price of the system, while adversely affecting its reliability and stability. 
   Recently, a new type of column has been developed, and is generally referred to as a “minicolumn.” A cross section of a minicolumn investigated by the current inventors is depicted in  FIG. 2 . In  FIG. 2 , element  200  is the electron source (preferably a shottky emitter),  210  is an aperture (suppressor), and  220  generally designates the lens arrangement. More specifically, lens arrangement  220  comprises three lenses  230  made of conductive material and insulating spacers  240  interposed between the lenses  230 . Ordered from the emitter, the lenses  230  comprise an extraction lens, a focusing lens, and an acceleration lens, respectively. 
   Notably, the diameter and height of such a column is measured in single-digit centimeters. More specifically, the diameter of the lens arrangement depicted in  FIG. 2  is on the order of 3 centimeters, while its height is on the order of 1 centimeter. While this column is remarkably smaller than the conventional column, it provides an electron beam which has small diameter and was determined by the present inventors to be suitable for use in electron microscopes. Further information regarding the study of the minicolumn is presented in an article entitled “Novel high brightness miniature electron gun for high current e-beam applications” by F. Burstert, D. Winkler and B. Lischke,  Microelectronic Engineering  31 (1996) pp. 95–100; and in an article entitled “Miniature electrostatic lens for generation of a low-voltage high current electron probe,” by C.-D. Bubeck, A. Fleischmann, G. Knell, R. Y. Lutsch, E. Plies and D. Winkler,  Proceedings of the Charged Particle Optics Conference , Apr. 14–17, 1998. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention provides arrangements for installing minicolumns onto charged particle microscopes, especially electron microscopes, while providing synergistic advantages over prior art column arrangements. That is, the disclosed arrangements provide advantages in addition to the advantages of the minicolumn per se. 
   According to one set of embodiments of the invention, a second load lock is provided on the microscope&#39;s vacuum chamber. The second load lock is used to introduce the minicolumn into the chamber without having to break the vacuum in the chamber. Thus, a technician can replace the minicolumn without having to break the vacuum in the chamber. 
   According to another set of embodiments, the minicolumn is situated inside the microscope&#39;s vacuum chamber. While this arrangement necessitates breaking the vacuum for each minicolumn service, it is still advantageous in that there is no need for a separate vacuum system for the column. This is advantageous especially if more than one minicolumn is used inside the chamber. 
   Another advantage of the invention is that it provides arrangements for more than one minicolumn per microscope. Such arrangements are especially advantageous for taking multiple perspectives simultaneously or increasing the throughput. 

   
     BRIEF DESCRIPTION OF THE FIGURE 
       FIG. 1  depicts relevant elements of a conventional electron microscope. 
       FIG. 2  depicts a lens arrangement of a minicolumn investigated by the present inventors. 
       FIG. 3A  depicts an embodiment of an isolation valve arrangement in a closed position, while  FIG. 3B  depicts the arrangement of  FIG. 3A  in an open position. 
       FIG. 4A  depicts another embodiment of an isolation valve in a closed position, and  FIG. 4B  depicts the embodiment of  FIG. 4A  in an open position. 
       FIG. 5A  depicts an embodiment of a minicolumn within the microscope chamber, while  FIG. 5B  depicts an embodiment of a plurality of minicolumns arranged inside the chamber at different angles. 
       FIGS. 6A and 6B  depict embodiments using a turntable stage for reduced footprint. 
       FIG. 7A  depicts an arrangement of an arm having a plurality of minicolumns situated at different angles that is advantageous for defect review, while  FIG. 7B  depicts an arm having a plurality of minicolumns at a single angle that is advantageous for sector-wise inspection. 
   

   DETAILED DESCRIPTION 
     FIGS. 3A and 3B  depict a first embodiment of an isolation valve for the minicolumn  300 . Specifically, the vacuum chamber  310  is equipped with a valve  320  capable of hermetically sealing chamber  320  in the closed position. Minicolumn  300  is position inside a mini-environment chamber  330 , which can be evacuated via outlet  335 . In the exemplified embodiment, the mini-environment chamber  330  has collapsible walls  340 , which are actuated by bellows  345 . However, it should be appreciated that other solutions having rigid walls with means for elevating and lowering the columns are also workable. 
   During maintenance, the mini-environment chamber  330  is in its closed position. In the closed position, exemplified in  FIG. 3A , the bellows  345  are extended so as to raise the walls  340  to an upright position. In this upright position, the minicolumn is extruded from the chamber  310  and valve  320  is closed to maintain the vacuum level inside the chamber  310 . When maintenance is completed, the mini-environment chamber  330  can be evacuated via outlet  335  and, when the evacuation is completed, the valve  320  can be opened and the minicolumn lowered to the chamber  310 . 
   Specifically,  FIG. 3B  exemplifies the situation during operation of the microscope. When the mini-environment chamber  330  has been evacuated and the valve  320  opened, the bellows collapse the walls  340  so as to introduce the minicolumn into the chamber  310 , close to the stage  315 . Unless the minicolumn malfunctions, there is no need to revert to the position shown in  FIG. 3A , and the microscope can be maintained in the position shown in  FIG. 3B . However, if the minicolumn requires maintenance or replacement, the bellows  345  are extended to raise the walls  340  and extrude the minicolumn  300  from chamber  310 ; the valve  320  is closed; and the mini-environment chamber  330  is brought to atmospheric pressure via outlet  335 . 
   Another embodiment for isolation valve is depicted in  FIGS. 4A and 4B . Minicolumn  400  is situated inside a mini-environment chamber  430  that is open at its bottom to chamber  410 . Mini-environment chamber  430  has an outlet  435  which, in this example, is connected to the outlet  445  via vacuum valve  440 . Thus, mini-environment chamber  430  and chamber  410  can be connected to the same vacuum pump (not shown). However, it should be appreciated that outlet  435  can be connected independently to a separate vacuum pump. Isolation valve  450  is pivoted on shaft  455 , which is capable of elevation motion, i.e., in the Z direction. 
   During operation (depicted in  FIG. 4B ), isolation valve  450  is swiveled away from the opening of mini-environment chamber  430 , and the shaft  455  is in its uppermost position so as to place the isolation valve  450  out of the working area of the microscope. Preferably, stage  415  is equipped with actuators for Z motion so that during operation the distance between the minicolumn and the specimen can be adjusted for proper imaging. Such stages are well known in the art and will not be described here. If the outlet arrangement depicted in  FIG. 3B  is used, then during operation valve  440  can be maintained open so that vacuum pump operation maintains vacuum in both chambers  410  and  430 . 
   When access to the minicolumn is required, the stage is lowered and the valve  450  is brought to its closed position. For that operation, preferably the shaft  455  is lowered to its lowest position, the valve  450  is swiveled to its close position and the shaft  455  is elevated sufficiently to cause a hermetic seal between the valve  450  and the opening of the mini-environment chamber  430 . Then valve  440  can be moved to the open position so that mini-environment chamber  430  is bought to atmospheric pressure. Then the back plate  460  can be removed for access to the minicolumn. Preferably, the minicolumn itself is secured to the back plate  460  so that it is removed together with the back plate  460 . 
     FIG. 5A  depicts an arrangement of a minicolumn enclosed within the microscope chamber. Specifically, minicolumn  500  is positioned completely inside the chamber  510  so that no separate evacuation is necessary for the minicolumn  500 . Preferably, the stage  515  is capable of elevation motion to control the distance between the minicolumn  500  and the specimen. Here again, it is preferred that the minicolumn be attached to a back plate  560 , so that removal of the back plate  560  would remove the minicolumn  500  as well. Such an arrangement is particularly useful for metrology, such as for critical dimension (CD) measurement microscopes. Also exemplified in  FIG. 5A  is an in-chamber integrated vacuum pump, which controls the vacuum inside the minicolumn  500 . 
   Electron microscopes can also be used for review of locations on wafers which are suspected of having defects thereupon. In such application, it is particularly useful to be able to scan the suspect area at different angles. A particularly elegant way of doing so using a conventional column is described in U.S. Pat. No. 5,329,125 to Feuerbaum. In that patent, a system is disclosed which is capable of placing the column at any tilt between 0–45 degrees, without breaking the vacuum inside the column or the microscope chamber. Thus, one can take a picture at 0 tilt, and then tilt the column to a desired position and take another picture for added information. Notably, pictures taken at a tilt tend to have more topographic information than those taken without tilt. 
     FIG. 5B  exemplifies a system having a plurality of minicolumns, and particularly suitable for an electron microscope review station. As shown in the Figure, a first minicolumn  500  is situated inside the chamber at zero tilt. A second minicolumn  520  is positioned at a first tilt θ and a third minicolumn is positioned at a second tilt φ. In the preferred embodiment, the tilt angles θ and φ are fixed and different from each other. Preferably, the tilts are fixed at 30 and 60 degrees, respectively, or 30 and 45 degrees, respectively. However, as shown in  FIG. 5B , the tilts can be variable by, for example, pivoting the columns  520  and  525  about pivots  530  and  535 , respectively. 
   It is well known that chamber size directly affects the quality of the vacuum maintained within the chamber, and consequently, can affect the reliability and “cleanliness” of the equipment. Additionally, large chambers require large footprint, which is of paramount consideration for fabrication plants, wherein clean room real estate is at a premium. However, x-y stages generally require large chambers since they require motion space that is at least twice the size of the largest specimen to be inspected. Considering that the semiconductor industry is moving towards 300 mm wafers, an x-y stage for such wafers can dictate a very large footprint. 
     FIGS. 6A and 6B  depict embodiments which are particularly advantageous for reducing the footprint of the microscope. Specifically,  FIGS. 6A and 6B  depict a minicolumn  600  attached to an arm  620 , which is situated inside the chamber  610 . Rather than a x-y stage, a turntable stage  615  is used. In  FIG. 6A , the arm  620  is pivoted about pivot  625 , while in  FIG. 6B  the arm is attached to a linear carriage  635 . In both cases, the arms  620  are capable of moving the minicolumn  600  through the entire radius of the stage  615 . Through the rotational motion of the turntable stage  615 , and the motion of the arm  630  (whether radial or linear), every location on the specimen can be reached in polar (r,θ) coordinates. 
   As noted above, it is desirable to be able to obtain images of the same spot using tilt.  FIG. 7A  depicts a turntable arrangement similar to that depicted in  FIG. 6B , except that the arm  720  carries two minicolumns  700  and  705 . In the exemplified embodiment, minicolumn  700  is situated with zero tilt, while minicolumn  705  is situated with a fixed tilt, preferably of 30 or 45 degrees. However, it should be appreciated that more than two minicolumns can be provided, and that the tilt can be variable rather than fixed. 
   It is also well known to use electron microscopes to inspect wafers and reticles for defects. An exemplary system is disclosed in U.S. Pat. No. 5,502,306 to Meisburger et al. That system uses a single conventional column to scan the entire wafer/reticle for defects. The system is sold under the name of SEMSpec by KLA of San Jose Calif. and is known to have a very slow throughput. 
     FIG. 7B  depicts an arm  740  structured to support a plurality of columns  745  positioned with zero tilt. The arm  740  is attached to a linear carriage  755 . Such an arm can be installed in a chamber having a turntable stage for inspecting an entire wafer for defects. Specifically, the wafer is divided into concentric sectors corresponding to the number of minicolumns  745  attached to arm  740 . Thus, as the wafer is rotated, the carriage  755  need travel only a length equal to the radial length of one sector. During such motion, each minicolumn  745  would scan its corresponding sector, thereby covering the entire wafer. Of course, a small overlap may be provided to ensure complete coverage. 
   While the invention has been described with reference to particular embodiments thereof, it should be appreciated that other embodiments and modification can be implemented without departing from the spirit and scope of the invention as defined by the appended claims.