Abstract:
An e-beam system generates a set of massively parallel beams of order of magnitude 1,000 by employing a flash eprom to store calibration data and to receive on/off signals directed through the address system of the memory array, the individual electron sources being mounted above the memory array in a geometric array that tracks the structure of the memory array.

Description:
TECHNICAL FIELD  
         [0001]    The field of the invention is that of electron beam systems using many beams in parallel.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the field of integrated circuits, the cost of reticles has become a significant portion of the total manufacture cost.  
           [0003]    As circuit feature sizes become smaller, the number of pixels in a circuit increases and the cost of making reticles increases. The trend toward system on a chip ASICs and other forms of custom or semicustom chips has caused production runs to decrease in magnitude, so that the cost of a reticle set must be spread over a smaller number of chips sold.  
           [0004]    The conventional form of electron beam, in which a single beam traces out the circuit elements while following a raster pattern has a cost structure that increases with the number of pixels.  
           [0005]    A number of attempts have been made to employ parallel electron beams, typically limited to a small number (e.g. less than ten) because of the difficulty in providing a high quality magnetic field to provide the conventional demagnification ratio of four. Highly parallel configurations have been discussed, but have the obstacle that providing a way to focus, collimate, etc. the beams takes up space and that conflicts with the goal of putting a number of beams close together.  
           [0006]    Attempts to produce massively parallel (meaning more than one thousand beams) systems have not resulted in commercially available systems. One problem that has not been solved in the art is that of adjusting individual beams to compensate for individual variations between beams and for the effects of use on the parameters of a beam, both of which are inherent in all known systems.  
         SUMMARY OF THE INVENTION  
         [0007]    The invention relates to an e-beam system employing a massively parallel array of electron sources that are assembled by integrated circuit techniques together with systems for each beam to store calibration data and to control the individual beams using that calibration data.  
           [0008]    A feature of the invention is the use of non-volatile memory to store calibration data.  
           [0009]    Another feature of the invention is controlling individual electron sources using current sources controlled by the stored calibration data.  
           [0010]    Yet another feature of the invention is the fabrication of a beam cell containing the data and the current sources, the beam cell having microscopic dimensions that fit within the transverse dimensions of an individual electron source within the array of electron sources. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 shows an overall view of a system according to the invention.  
         [0012]    [0012]FIG. 2 shows a portion of a pattern written by the system.  
         [0013]    [0013]FIG. 3 shows an overlapping region between the patterns written by adjacent sources.  
         [0014]    [0014]FIG. 4A shows a partially schematic, partially pictorial view of an individual electron beam cell.  
         [0015]    [0015]FIG. 4B shows a block diagram of the cell of FIG. 4A.  
         [0016]    [0016]FIG. 5 shows a block diagram of a current source for use with the invention.  
         [0017]    [0017]FIG. 6 shows a view of a prior art system.  
         [0018]    [0018]FIG. 7 shows a perspective view of the connection between the emitting tips and current sources. 
     
    
     DETAILED DESCRIPTION  
       [0019]    [0019]FIG. 1 shows a perspective view of an example of the invention. At the top of the figure, integrated circuit  10  as explained in more detail later, contains a set of memory cells  20  that store data for adjustment of the parameters of individual e-beam sources. Below circuit  10 , a set of tungsten points  60  emit individual e-beams (under control of controller  100 ) that travel downward in the figure toward photo resist  260  that coats a workpiece, such as a reticle blank  250 . Workpiece  250  may also be a wafer in a version of the invention used for direct-write applications.  
         [0020]    It is an advantageous feature of the invention that the dimensions of an apparatus according to the invention are extremely small compared to the dimensions of conventional e-beam systems, such as that shown in FIG. 6, where a single beam travels through a pair of lenses  4 , being defined by an aperture in a plate  5 . The geometrical constrains of the lenses and aperture require that there be only one or a few beams in the system. In contrast, The transverse dimensions of a write head according to the invention will be comparable in size to the reticle of chip being written. Since the calibration data are stored within the unit that contains the write head, the head, once calibrated, can be stored waiting for the next time that a batch of reticles needs to be written, the calibration data being stored along with the head itself.  
         [0021]    The workpiece is mounted on a precision movable stage  200  that can travel in both the x and y directions. On the right of the figure, box  300  represents schematically an optional magnetic lens system for focusing the set of beams and/or a set of electrostatic grids that control the potential distribution. The magnetic and electrostatic fields are options and not required. In the preferred embodiment, the distance between the tips  60  and the workpiece is as small as possible—illustratively between 10 micrometers and  1  or so millimeters, so that beam divergence is negligible and the expense of a magnetic lens field may be avoided.  
         [0022]    As an example, the electrostatic field near the tips of sources  60  may be maintained by grid  410  at a value suitable for emitting electrons. Another value suitable for attracting the electrons to the workpiece may be imposed with grid  420  at the top surface of the photo resist. Illustratively, the total potential difference is in the range of 15-100V, typical of a voltage range used for flash eproms.  
         [0023]    In operation, the set of sources will emit a set of parallel beams under control of a control system  100 , which may be a general purpose computer, and the stage moves beneath it. As a simplified example, if the distance between sources  60  is 1 micrometer, there will be 1,000 sources per 1 mm of length.  
         [0024]    One row of sources could be used, but preferably, there will be a convenient number of rows extending in the direction of the stage travel. This permits the pattern to be written more than once to average out location errors and/or to increase the speed by having each row write a separate portion of the circuit. In an extreme case, where the dimension of circuit  10  is the same as that of the pattern being written, the stage would step once over the distance between sources (e.g. 500 nm). If a pixel is 10 nm on a side, there will be 50 pixels in the 500 nm range covered by a source or row of sources.  
         [0025]    In FIG. 2, bracket  320  represents the range of the ith source. Horizontal lines  310  illustrate the raster pattern within which the data is written. The stage may scan to the left and right to traverse the width of the workpiece and also scan downward in the figure to traverse the length. Since reticles are conventionally four times the size of the final chip, the embodiment illustrated has the capability of scanning in both directions. As a design choice, the chip could be made the full width of the workpiece, so that only a vertical (in the drawing) scan is required. Such a dimensional arrangement realizes to the full the advantage of parallelism—the great increase in speed possible from writing thousands of pixels at the same time, rather than sequentially.  
         [0026]    Illustratively, the data storage circuity associated with the set of sources is organized as a memory array. In that case, control of the beams may simply be effected by sending signals through the conventional memory address of the individual beam in question, thus taking advantage of the engineering that has previously been done to create memory chips. Referring again to FIG. 2, edges  322  on the left of the shape being written indicate distances where the beams start writing. This is effected by sending an “on” signal to the relevant addresses of the individual sources. Likewise, edges  324  where the pattern stops are defined by sending on “off” signal to the relevant addresses. Thus, to write the pattern illustrated, the workpiece is scanned vertically in the drawing. When the top edge of the shape is reached, the addresses in the range between 322 and 324 are turned on and the beams within that range start writing. When the edge of the wide portion of the shape is reached, the beams outside the sub-range  322 ′ to  342 ′ are turned off and the beams within the subrange remain on.  
         [0027]    In such a parallel system, there is always an alignment problem at the join where one range meets the adjacent one. Referring now to FIG. 3, the overlap between brackets  320  and  325  indicates such a join area, also indicated by the oval. In that area, illustratively, the beams are controlled so that each gives half the usual dose, thus providing the photo resist in that area with the correct dose for exposure. Various mechanisms for delivering such a dose will be discussed below.  
         [0028]    [0028]FIG. 4A shows in partially schematic, partially pictorial form an individual cell in circuit  10 . The ith cell illustratively contains one or more flash memory cells  30 - i  that store data resulting from calibration tests on the properties of the individual cell e.g. the amount of current that it emits during a “flash” of standard duration. The ith current source is controlled by the stored data to send the correct amount of current to the emitting tip  60 - i . Tip  60 - i  is illustratively deposited on a set of electrodes  50 - i  that are formed in the top layer of integrated circuit  10  by conventional techniques. As discussed below, several electrodes, each connected to a source of current, are connected to a single tip  60 . The several electrodes are denoted in the Figure by letters a-n This permits variation in the amount of current fed through a tip in a flash by simply turning on or off as many current sources as are required. In addition, there may be spare current sources in order to provide redundancy for replacement of defective cells and/or current supplies.  
         [0029]    Tips  60  are illustratively formed from a layer of material, e.g. Tungsten that is 5 μm to 50 μm thick and are shaped by a conventional process of erosion etching to have a tip emitting surface  62 . Tip  62  is drawn as flat to illustrate that a perfect point is impossible to manufacture and that there will be a finite width to the tips. The areas  64  at the top and bottom of the figure illustrate insulators that isolate adjacent cells. Dotted lines  20  indicate the boundary of the ith cell. FIG. 7 shows an alternate view of this arrangement, in which a set of electrodes  50  are driven by currents sources  40  and are in mechanical and electrical contact with tip  60 . The number of electrodes  50  per cell will be set by the designer as discussed elsewhere.  
         [0030]    In construction, the current module will be formed by standard integrated circuit techniques in a semiconductor substrate  10 . The memory cells and current sources will be formed from a set of flash memory cells required to store the data for that current module and a set of current sources arranged within the boundaries of the current module. Conductive members will extend vertically from substrate  10  (to the right in the Figure) to the plane of electrodes  50 - i . Conventional back end dielectric layers will be put down to provide support for tips  60  and vias will be etched through the dielectric to provide the conductive paths, using conventional integrated circuit techniques.  
         [0031]    [0031]FIG. 4B shows a block diagram of the ith cell. At the top, controller  100 , which is common to all the cells (and may be a general purpose CPU), sends signals to the flash array  30  and the current sources  40 . For example, the ith cell could contain a set of one-bit flash memory cells sufficient to store the data.  
         [0032]    Advantageously, the magnitude of the voltages required to write in a flash memory cell are the same as those for the beam acceleration, so that the technology of handling voltages in the rages of tens to 100 volts can be used for both the data storage and for the beams.  
         [0033]    In operation the cells  30  contain data on the magnitude-of the charge to be delivered and, at the correct time for the pattern being written, controller  100  sends on (and, optionally, off) signals to the current sources  40 , using the address technology for the flash eprom structure that the cells are contained within.  
         [0034]    As a design choice, controller  100  may send signals indicating when a pixel comes within range and whether the full charge (or a one half charge in an overlap region) is to be delivered. The individual modules then control the current magnitude and/or duration to deliver the adjusted charge, adjusted for individual variation between current modules and for aging.  
         [0035]    Those skilled in the art will readily be able to design control systems that use stored data to control the duration of a standard-magnitude current to achieve the result of adjusting the total number of electrons delivered to a pixel; i.e. if the ith current module is to deliver Q i  microcoulombs, the controller sends an “on” signal, the current sources  40  are turned on for a length of time controlled by a timing circuit within the cell that reads the stored data to determine when to turn the current sources within the module off. Illustratively, the timing circuit is contained within the box labeled  40  representing the current source and controls for each source.  
         [0036]    Referring now to FIG. 5, there is shown a simple example of a current control. Two brackets  142  and  144  denote two sets of current sources  42  that are all turned on or off at the same time. A dotted line  130  surrounds individual controllable current sources. In operation, an on signal from controller  100  turns on all the sources  142  and  144  and those of sources  130  that have on signals stored in the memory cells. This set of sources delivers a current that varies from one module to another until an off signal is received (or a standard time has elapsed).  
         [0037]    As a specific example, each of groups  142  and  144  contain four sources, so the range of current available is eight from groups  142  and  144  plus 0 to 4 from group  130 . Thus, the range is 8 to 12 or 10 plus or minus 20%. Calibration data permits the adjustment of individual beam within that plus or minus 20% range, by means of stored data in the flash memory cells. Those skilled in the art can readily increase the number of current sources to provide finer control and/or to provide redundancy.  
         [0038]    The requirement of reducing current in the overlap region where beam spatial ranges overlap is addressed by turning off one of groups  142  or  144  in the overlap range.  
         [0039]    Another method of charge control is the use of a multi-bit (having several voltage levels in a cell, not just two) flash memory that is commercially available. A circuit within the current module reads the stored data and either uses analog techniques to control current or duration of converts the stored analog data to digital data and uses one of the preceding examples.  
         [0040]    While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.