Abstract:
A mechanical scanning stage for high speed image acquisition in a focused beam system. The mechanical scanning stage preferably is a combination of four stages. A first stage provides linear motion. A second stage, above the first stage, provides rotational positioning. A third stage above the rotational stage is moveable in a first linear direction, and the fourth stage above the third stage is positionable in a second linear direction orthogonal to the first direction. The four stages are responsive to input from a controller programmed with a polar coordinate pixel addressing method, for positioning a specimen mounted on the mechanical stage to allow an applied static focus beam to irradiate selected areas of interest, thereby imaged by collecting signals from the specimen using a polar coordinate pixel addressing method.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
     ThisThe present application is a reissue application of U.S. Pat. No. 6,911,656, issued on Jun. 28, 2005 and filed as U.S. patent application Ser. No. 10/884,698 on Jul. 1, 2004, and entitled “Rotational Stage for High Speed Large Area Scanning in Focused Beam Systems”, which is a continuation of U.S. Pat. Ser. No. 10/245,865 filed on Sep. 16, 2002 now U.S. Pat. No. 6,777,688, issued on Aug. 17, 2004 and filed as U.S. patent application Ser. No. 10/245,865 on Sep. 16, 2002, the disclosures of which are incorporated by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to moveable stages for use in focused beam systems; and more particularly to a high speed rotational stage in conjunction with a linear stage to form a high speed scanning system without scanning the beam, allowing a large area specimen to be imaged with a substantially higher speed using an optimally focused beam. 
     2. Description of the Prior Art 
     In the context of scanning microscopy, the most common image formation systems in the prior art make use of Cartesian raster scanning to form an image. In a typical imaging system, a source of energy such as an electron beam, ion beam or photon beam is used to irradiate a specimen. The interaction between the source beam and the specimen produces a signal that can be detected which corresponds to the signal intensity at the interaction point. There are two Cartesian raster scanning mechanisms commonly used to form an image: (1) a beam scanning system wherein the source beam is Cartesian raster scanned over the area of interest of a static specimen; (2) a stage scanning system wherein the specimen is mounted on a mechanical Cartesian scanning stage, and the stage is scanned with respect to the static source beam to cover the area of interest. 
     In a beam scanning system, the source beam is typically scanned from left to right in a raster manner, pixel by pixel, before ‘flying back’ to the beginning of the next line. This process repeats from the top to the bottom for a complete image acquisition before returning to the top of the scan again. In a stage scanning system, the mechanical scanning stages scan from left to right in a raster manner using stepper motors, servo motors or voice coils. These two methods impose significant problems and limitations. Firstly, both methods need a fly-back at the end of each line scan, which slows down the image acquisition. For the stage scanning system, the relatively large mass of the mechanical stage needs significant settling time, which further slows down the rate of image acquisition. In addition to this, the beam scanning system suffers from aberrations when the beam is deflected from the optical axis while scanning a relatively large area. This is a serious drawback of the beam scanning system when scanning a large area. 
     In conclusion, a mechanical scanning stage with high speed capability for large area specimen scanning would have advantages in many applications. 
     SUMMARY OF INVENTION 
     It is, therefore, an object of the present invention to provide a mechanical scanning stage for high speed image acquisition in a focused beam system. 
     It is another object of the present invention to provide a mechanical scanning stage that can achieve high speed image acquisition of a large area specimen. 
     Briefly, a preferred embodiment of the present invention includes a mechanical scanning stage for high speed image acquisition in a focused beam system. The mechanical scanning stage preferably is a combination of four stages. A first stage provides linear motion. A second stage, above the first stage, provides rotational positioning. A third stage above the rotational stage is moveable in a first linear direction, and the fourth stage above the third stage is positionable in a second linear direction orthogonal to the first direction. The four stages are responsive to input from a controller programmed with a polar coordinate pixel addressing method, for positioning a specimen mounted on the mechanical stage to allow an applied static focus beam to irradiate selected areas of interest, thereby imaged by collecting signals from the specimen using a polar coordinate pixel addressing method. 
    
    
     
       IN THE DRAWING 
         FIG. 1  is a three dimensional view illustrating a preferred embodiment of the present invention; 
         FIG. 2  is a plot illustrating a typical scanning and acquisition pattern using a concentric circular polar coordinates addressing method as applied by the embodiment of  FIG. 1 ; and 
         FIG. 3  is a plot illustrating a scanning and acquisition pattern using a spiral polar coordinates addressing method as applied by the embodiment of  FIG. 1 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring now to  FIG. 1  of the drawing, a preferred embodiment of the present invention includes a mechanical scanning stage  10  constructed as a combination of four stages  12 - 18  for positioning a specimen  20 . The specimen  20  is placed on specimen mounting apparatus, such as a plate  21  attached to the uppermost stage, which as illustrated in  FIG. 1  is stage  18 . The first stage  12  provides linear motion along a direction/axis  22 . The second stage  14  is configured to rotate the specimen  20  in its plane, around an axis  24  orthogonal to the specimen. This is indicated by rotational arrow  26 . The third stage  16 , also designated as a Y position stage, is for moving the specimen linearly in a first (Y) direction indicated by arrow  28 , and the fourth stage  18  (X positioning stage) provides linear motion in a second (X) direction  30 , orthogonal to the first (Y) direction  28 . All of the four stages  12 - 18  are configured to be responsive to direction from a controller  32 . Communication for direction of all four stages is symbolically illustrated by arrow/bus  34 . The actual connection/communication method can be either wired or wireless, which will be apparent to those skilled in the art. 
     The arrangement of the four stages in  FIG. 1  is given to illustrate a particular embodiment of the present invention. Various alternate embodiments will be apparent to those skilled in the art upon reading the present disclosure, and these are to be included in the present invention. For example, stages  12  and  14  can be reversed in their vertical placement in the stack of stages. As will be discussed in the following, a further alternate embodiment includes the third and fourth stages eliminated. A still further alternate embodiment is the elimination of either, but not both of the third and fourth stages. For example, placing a second linear stage such as the third stage immediately above the first stage, in an orthogonal arrangement followed by the rotational second stage, provides much of the flexibility of the four stages. These and other variations are to be included in the spirit of the present invention. In addition, the various stages can be stacked vertically in any order, and these variations are all included in the present invention. 
     As illustrated in  FIG. 1 , the specimen  20  to be observed is mounted on the stage  18 , or i.e. on a plate  21  attached to stage  18 , and a static source (energy) beam  36  is applied to form a focused spot  38  on the specimen  20 . The beam  36  can be any type of energy beam as required for the particular imaging operation. For example, it can be an electron beam, ion beam or photon beam. The axis of rotation  24  of the rotational stage  14  is preferably aligned with the optical axis  40  of the focused beam system at the beginning of a scanning procedure. Other starting points are also included as alternate embodiments of this invention. 
     An area of interest on the specimen  20  is moved to the static source beam  36  spot  38  by moving the X positioning stage  18  and the Y positioning stage  16 . The X and Y positioning stages  18  and  16  may be activated by wireless control or other means of remote activation, symbolically represented by controller  32  and arrow  34 . The scanning motion of the mechanical stage  10  includes rotational motion provided by the rotational stage  14  such that the specimen  20  rotates in the rotational direction  26 , and linear motion provided by the linear scanning stage  12 , scanning in the direction  22 . The rotational stage  14  may be rotated in the clockwise or counter clockwise direction. The source beam  36  remains stationary, directed along the axis  40 . The scanning operations are performed by the stage  10  elements  12 - 18 . As referred to above for one embodiment, the mechanical stage can be positioned initially so as to place the rotational axis  24  in alignment with the beam axis  40 . Starting in this position, a movement of the linear stage  12  one unit along the axis  22  moves the axis of rotation  24  of the rotational stage one unit away from the axis  40  of the beam  36 . As a result, the static source beam  36  can be activated to irradiate specimen areas along a circular path on the rotating specimen  36  as the rotational stage  14  is rotated. After the rotational stage  14  has rotated one revolution, the linear scanning stage  12  moves a pre-programmed distance, enabling the static source beam  36  to address areas on another concentric path as the stage  14  is rotated. As the linear scanning stage  12  moves further, the source beam  36  addresses a point further from the axis of rotation of the specimen stage  14 . This linear movement of the linear scanning stage  12  is preferably stopped when the source beam  36  reaches the edge of an area of interest on the specimen  20 . The linear movement may reverse its direction until the source beam  36  addresses the starting point again and vice versa. The above description details how image pixels corresponding to areas of interest on the specimen are addressed using concentric circular polar coordinates. This is further as depicted in  FIG. 2 .  FIG. 2  illustrates a pattern of specimen areas shown as dots  42 , that are accessed by simply stepping the linear first stage  12  an increment/step equal to “W” from the center  44 , and the measurement of signal is acquired at time increments/steps “U” while the rotational stage  14  rotates at constant speed. When the stage  10  moves the specimen so as to position a desired area  42  in line with the beam  36 , the beam is activated, and the measurement is acquired. 
     In the above described example of operation of the mechanical stage  10 , the third and fourth stages  16  and  18  are used to initially position the beam  36  at a required central location of an area of interest on the specimen. Subsequent to this positioning, the stages  16  and  18  preferably remain in a fixed position relative to the rotational stage axis  24 , serving no further purpose. 
     In another embodiment of operation of the mechanical stage  10 , the linear scanning stage  12  moves simultaneously and concurrently with the rotational stage  14 . The image pixels can then be addressed using spiral polar coordinates as depicted in  FIG. 3 .  FIG. 3  illustrates areas on the specimen shown as dots  46 , wherein similar to the process described in reference to  FIG. 2 , the mechanical stage is directed by a controller to bring the desired areas  46  in line with the beam  36 . When an area  46  is in line with the beam, the beam source (not shown) is activated to apply the energy (beam), irradiating the area, resulting in the system acquiring the desired signal/measurement. The equipment for detecting and displaying such signals is well known to those skilled in the art, and need not be described herein in order for someone skilled in the art to reproduce the present invention. 
     Another embodiment of the mechanical stage  10  of the present invention includes only the linear stage  12  and rotational stage  14 , omitting the X and Y stages  16  and  18 . Operation in this embodiment requires manual alignment of a specimen orthogonal to the direction  22  of the linear stage  12 . The specimen is mounted on the upper stage, for example on the rotational stage  14  if the rotational stage is above the linear stage. The initial position of the specimen is then adjusted either manually, or manually and in combination with the linear stage  12 . 
     The apparatus of the present invention operated as described above, minimizes or eliminates linear stop and start motions, and totally avoids the settling down and “fly back” involved in the prior art line scanning systems which are responsible for the long image acquisition times of the prior art. The operation of the present invention illustrated in  FIG. 3  and described above, eliminates all stop and start operations in image acquisition. The method described above employing concentric circles of acquisition as shown in  FIG. 2 , minimizes the magnitude of linear stop and start movements and avoids the prior art requirement of “fly back”. In the method of  FIG. 2 , the source beam irradiates a full circular path after each unit movement of the linear scanning stage  12 , and the complete circular images require the linear scanning stage  12  to travel a total distance of only one half the image diameter. The static, non-moving, focused beam of the present invention avoids the problems associated with moving a beam. Prior art systems that required scanning the beam, for example, have an undesirable characteristic known as beam aberration. The apparatus of the present invention has the additional advantage of making it practical to scan a larger area of the specimen compared to a scanning beam system. In other words, the apparatus of the present invention can scan a large area without sacrificing image quality. This is not possible with a beam scanning system. Prior art mechanically scanned stages have the disadvantage of being very slow. 
     Although the present invention has been described above in terms of a specific embodiment, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modifications as fall within the true spirit and scope of the invention.