Patent Publication Number: US-7589335-B2

Title: Charged-particle beam pattern writing method and apparatus and software program for use therein

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2006-194570 filed on Jul. 14, 2006, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to charged-particle beam lithography technologies and, more particularly, to a method and apparatus for writing a pattern on a workpiece by use of an electron beam while performing correction of a deflection position of the beam on a real-time basis. 
     2. Description of Related Art 
     Lithography techniques indispensable for growing miniaturization of semiconductor devices are to produce patterns unlike other semiconductor fabrication processes and, for this reason, are very important processes. In recent years, as LSI chips further increase in integration density, circuit line widths required for semiconductor devices are becoming smaller year by year. To form a desired circuit pattern on these semiconductor devices, it becomes necessary to use a high-accuracy original image pattern (also called the reticle or photomask). Note here that electron beam (EB) lithography techniques offer inherently excellent image resolutions and are used for production of such high-precision original pattern. 
       FIG. 13  shows schematically a perspective view of an electron beam optics in prior known variable-shaped electron beam (EB) lithographic apparatus. 
     As shown herein, the EB lithography tool includes a first aperture  410  having a rectangular opening or hole  411  for shaping an electron beam  330 . The EB tool also includes a second aperture  420  having a variable shaping hole  421  for reshaping the electron beam  330  that passed through the hole  411  into a desired rectangular cross-sectional shape. The electron beam  330  that was emitted from a charged particle source  430  and then passed through the hole  411  is deflected by a deflector to penetrate part of the variable shaping hole  421  to thereby fall onto a workpiece  340 , which is situated on a stage structure that is continuously movable in a prespecified one direction (e.g., X direction). In brief, only a beam with its rectangular shape capable of penetrating both the hole  411  and the variable shaping hole  421  is permitted to reach a pattern write area of the workpiece as mounted on the stage that continuously moves in the X direction, followed by pattern writing thereon. The scheme for creating any given shape by guiding the beam to pass through the holes  411  and  421  is called the variable-shaped beam (VSB) lithography. 
     Note here that in the EB lithographic tool, its pattern writing chamber can vary in shape with a change in atmospheric air pressure. This deformation affects a relative distance between an electron lens barrel overlying the writing chamber and the surface of a workpiece such as a photomask disposed within the chamber. If the relative distance is kept out of alignment due to a change in atmospheric pressure, appreciable aberration can occur in position of a pattern to be written and also in focus point of an electron beam, resulting in the lack of an ability to perform highly accurate pattern writing. In particular, while extra-high accuracy is required with growth in miniaturization of on-chip circuit linewidths in recent years, the risk of a decrease in pattern writing accuracy becomes no longer negligible, which is occurrable due to such atmospheric pressure variation-caused relative-distance/focus-point deviations. 
     A technique adapted for use in ultraviolet (UV) exposure apparatus for exposing a mask image onto wafers is disclosed, for example, in JP-A-7-211612, although it is not specifically directed to EB lithography. This Japanese patent bulletin involves the teaching as to an approach to obtaining the amount of curvature or “warp” of an image plane due to a change in atmospheric air pressure and then driving a stage to move to an optimal position in Z-axis direction. 
     As previously stated, while higher accuracy is required with further miniaturization of onchip circuit linewidth in recent years, the risk of a decrease in pattern writing accuracy becomes no longer negligible, which is occurrable due to the atmospheric pressure change-caused relative-distance/focus-point deviations. Additionally, a pattern writing position on the workpiece surface is defined two-dimensionally in x- and y-axis directions. Usually the electron beam&#39;s deflection position also is corrected two-dimensionally in the x and y directions. However, relative displacements due to atmospheric pressure variation take place three-dimensionally in x, y and z directions, respectively. Thus, it is required to achieve a three-dimensional (3D) correction scheme with handleability of these phenomena. Unfortunately, a technique for correcting deflection position deviations occurring due to atmospheric pressure variations has not yet been established in the prior art. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a charged-particle beam pattern writing method and apparatus capable of correcting deviations of a pattern writing position and beam focus position occurring due to a change in atmospheric air pressure. 
     In accordance with one aspect of this invention, a charged particle beam pattern writing apparatus includes an air pressure measuring unit operative to measure a value of an outside air pressure, a coordinate value correcting unit operative to correct a set of three-dimensional (“3D”) coordinate values by use of the value of the outside air pressure measured, a deflection amount computing unit operative to calculate a deflection amount of a charged particle beam by using the 3D coordinate values corrected, an irradiator unit for irradiation of the charged particle beam, and a deflector unit for deflection of the charged particle beam based on the deflection amount. 
     In accordance with another aspect of the invention, a charged particle beam writing method includes the steps of measuring a value of an atmospheric air pressure, correcting 3D coordinate values by use of the atmospheric air pressure value measured, computing a deflection amount of a charged particle beam by using the 3D coordinate values corrected, and irradiating the charged particle beam as deflected based on the deflection amount to thereby write a desired pattern on a workpiece. 
     In accordance with a further aspect of the invention, a computer-readable recording medium is provided which stores therein a software program for causing a logic operation device to perform a procedure including the steps of reading, from a storage device storing therein a first coordinate correction value and a second coordinate correction value plus a third coordinate correction value for correction of first, second and third coordinate values based on a value of an atmospheric air pressure, the first, second and third coordinate correction values to thereby use the first, second and third coordinate correction values thus read to correct the first to third coordinate values for storage in a storage device, reading from the storage device the third coordinate value for using this value to perform conversion of one or more coefficients of a prespecified formula for calculation of a deflection amount of a charged particle beam, and using the first and second coordinate values and the coefficients of the formula to determine the deflection amount of the charged particle beam and then outputting a result thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing an overall configuration of a pattern writing apparatus in accordance with one embodiment of this invention. 
         FIG. 2  is a diagram showing major process steps of an electron beam pattern writing apparatus also embodying the invention. 
         FIG. 3  is a diagram showing a perspective view of a workpiece which is mounted on a movable XY stage structure. 
         FIG. 4  is a plan view of the XY stage. 
         FIG. 5  is a pictorial representation of the workpiece having a main deflection area and sub deflection area. 
         FIG. 6  is a diagram for explanation of a way of measuring a mark position in the embodiment. 
         FIG. 7  is a diagram for explanation of a way of measuring the height of the mark in the embodiment. 
         FIG. 8  is a diagram showing some occurrable errors in rotation and magnification of a focused electron beam depending upon the height of the workpiece. 
         FIG. 9  is a graph showing rotation/magnification errors of a focused electron beam depending on the height of the workpiece. 
         FIG. 10  is a diagram for explanation of a way of occurring distortion in the pattern writing apparatus of  FIG. 1  when the ambient air pressure increases. 
         FIG. 11  is a diagram showing an exemplary pattern writing operation flow along with the timing of air pressure correction operation. 
         FIG. 12  illustrates, in cross-section, a structure of main part of a pattern writing apparatus also embodying the invention. 
         FIG. 13  depicts schematically a perspective view of an electron beam optics in prior known variable-shaped electron beam lithographic apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment 1 
     In embodiments below, an arrangement will be set forth which uses an electron beam as one example of the charged particle beam. The charged particle beam is not limited to only the electron beam and may alternatively be other similar energy beams, such as an ion beam or else. Additionally, an explanation will be given of a charged-particle beam pattern writing apparatus—particularly, a variable-shaped electron beam lithographic tool—as one example of the charged particle beam pattern writing apparatus as claimed. 
       FIG. 1  is a diagram showing an overall configuration of a pattern writing apparatus in accordance with one embodiment of this invention. 
     As shown in  FIG. 1 , the pattern writing apparatus  100  is generally made up of a pattern writing unit  150  and a control unit  160 . The pattern writing apparatus  100  is one example of that is one example of the electron beam apparatus or “the electron beam lithographic apparatus”. The writing apparatus  100  writes or “draws” an integrated circuit pattern on a workpiece  101 . A typical example of the workpiece  101  is a photomask for use in the manufacture of semiconductor devices, such as LSI chips. The pattern writing unit  150  includes a writing chamber  103  and an electron lens barrel  102  disposed above the chamber  103 . Provided in the electron lens barrel  102  are an electron gun  201 , illumination lens  202 , first aperture  203 , projection lens  204 , deflector  205 , second aperture  206 , objective lens  207 , sub-deflector  212 , and main deflector  214 . An XY stage  105  is disposed within the pattern writing chamber  103 . On this XY stage  105 , a workpiece  101  is mounted, which is a pattern writing object. A mirror  192  is situated on the XY stage  105 . A z-axis position sensor module is disposed on the upper surface side of the writing chamber  103 , which sensor has a light projector  532  and a photodetector  534  for detection of a position or “height” in z-axis direction of the workpiece  101 —i.e., the direction at right angles to the workpiece  101 &#39;s x-y plane defining its pattern writing surface. The light projector  532  may illustratively be a light irradiator, such as a light-emitting diode (LED). The photodetector  534  is preferably a position sensitive device (PSD). 
     The control unit  160  includes a pattern writing control circuit  110 , a deflection control circuit  140 , an air pressure-measuring device, i.e., barometer  170  for measuring an atmospheric pressure in the installation environment of electron lens barrel  102 , a digital-to-analog converter (DAC)  172 , an amplifier  174 , a DAC  182 , an amp  184 , a DAC  192 , an amp  194 , and a laser-assisted length-measuring device  190 . The pattern write control circuit  110  has a control computer with its central processing unit (CPU)  120 , and a data storage or memory device  130 . The deflection control circuit  140  has a control computer (CPU)  142 , and storage device  144 . The control circuit  110  is connected to the barometer  170 , deflection control circuit  140 , photodetector  534  and laser length meter  190  via a bus or buses (not shown). The laser length meter  190  is connected at its output to the deflection control circuit  140  via a bus (not shown). The deflection control circuit  140  generates an output signal (DAC value) for sub-deflection use, which signal is converted into an analog signal by DAC  172  and amplified by amp  174  for output to the sub-deflector  212 . This output value&#39;s voltage potential is used to deflect an electron beam  200  within a subdeflection plane. An output signal (DAC value) for main deflection use which is generated from the deflection control circuit  140  is analog-converted by DAC  182  and amplified by amp  184  for output to the main deflector  214 . Using this output value&#39;s voltage potential, let the electron beam  200  deflect in a main deflection plane. An output signal (DAC value) from the deflection control circuit  140  for shaping/deflection use is analog-converted by DAC  192  and amplified by amp  194  for output to the deflector  205 . By this output value&#39;s voltage potential, the electron beam  200  is shaped and deflected. The CPU  120  has respective internal functional modules, such as an outside air pressure measuring unit  122  and a coordinate correction value computing unit  124 . Any one of information to be input to CPU  120  and information in the process of computation processing and after completion of processing is stored in the storage device  130  in an event-sensitive manner. The other CPU  142  has respective internal function modules, such as a coordinate value correction unit  126 , a coefficient conversion unit  128 , a deflection voltage computing unit  146 , a corrected value verify unit  147  and an update unit  148 . Any one of information being input to CPU  142  and information being presently processed and having already been processed is stored in the storage device  144  in an event-sensitive way. Although in  FIG. 1  several constituent elements other than those necessary for explanation of Embodiment 1 are eliminated in illustration, it readily occurs to a person skilled in the art that the pattern writing apparatus  100  is usually arranged to include other necessary configurations. 
     Also note that although in  FIG. 1  the CPU  120  that is one example of the computer is arranged to execute the processing of respective functions of the outside air pressure measuring unit  122  and coordinate correction value computing unit  124 , this design is not an exclusive one and may alternatively be modified so that these functions are implemented by hardware configurations using electrical or electronic circuitry. Alternatively, the same is arrangeable by using combinations of electric/electronic circuit-based hardware and software programs. Still alternatively, similar results are obtainable by any possible combinations of hardware and firmware configurations. Similarly, although the CPU  142  that is one example of the computer is arranged to execute the processing of respective functions of the coordinate value correction unit  126 , coefficient converter unit  128 , deflection voltage computing unit  146 , corrected value verify unit  147  and update unit  148 , this design is not exclusive and may alternatively be modified so that these functions are implemented by hardware configurations by means of electrical or electronic circuitry. Alternatively, the same is arrangeable by using combinations of electric/electronic circuit-based hardware and software programs. Still alternatively, similar results are obtainable by possible combinations of hardware and firmware configurations. 
     The electron beam  200  which was emitted from the electron gun  201  that is one example of the irradiation unit is guided by the illumination lens  202  to illuminate an entire surface area of the first aperture  203  having a rectangular opening or hole. At here, the electron beam  200  is shaped to have a rectangular shape in cross-section. Then, the electron beam  200  of a first aperture image which passed through the first aperture  203  is projected onto the second aperture  206  by the projection lens  204 . A position of the first aperture image on the second aperture  206  is controlled by the deflector  205  to enable the beam to change in shape and size. Then, the electron beam  200  of a second aperture image which passed through the second aperture  206  is subjected to focussing by the objective lens  207  and deflected by the sub-deflector  212  and main deflector  214 , which make up the deflector unit as claimed, to fall onto the workpiece  105  on the movably disposed XY state  105  at a desired position on the workpiece surface. Positions in x and y directions in parallel with the workpiece surface (pattern writing plane) of XY stage  105  are detected and measured in a way such that the laser length meter  109  is activated to emit laser light, which reaches the mirror  192  and is reflected therefrom, resulting in production of reflected light which is received and sensed by the photodetector  534  for length/distance measurement. Although in  FIG. 1  only a one pair of mirror  192  and laser length meter  190  is illustrated, the reality is that more than two pairs are laid out to enable length measurement of coordinate positions in the x direction and y direction. 
       FIG. 2  is a diagram showing major process steps of an electron beam writing method also embodying the invention. 
       FIG. 2  shows processing procedures to be internally executed in the pattern writing control circuit  110  and the deflection control circuit  140 , which are necessary for correction of a pattern write position deviable due to atmospheric pressure variations. As the internal processing of the pattern writing control circuit  110 , a series of processes are performed, including an outside air pressure measurement step S 102  and a coordinate correction value calculation step S 104 . The internal processing of the deflection control circuit  140  employs what is called the “dual task” scheme, which permits two different tasks—e.g., a barometric pressure correcting task and a pattern writing task—to get started simultaneously. The pattern writing task includes a series of steps, i.e., a coordinate value correction step S 302 , coefficient conversion step S 304 , deflection voltage computing step S 306 , pattern writing step S 308 , settling step S 310 , and decision step S 312 . The air pressure correcting task includes in succession a corrected value verify step S 202  and update step S 204 . 
       FIG. 3  is a diagram showing a perspective view of workpiece  101  mounted on the movable XY stage  105 . 
     When writing a circuit pattern on the workpiece  101 , the XY stage  105  is driven by a driver unit (not shown) to continuously move in the x direction. Simultaneously, the electron beam  200  emitted is guided to fall onto the workpiece  101  to scan one of a plurality of beam-deflectable long and narrow strip-like regions which are virtually divided from a pattern writing area of workpiece  101 . While the XY stage  105  moves along the x direction, a present shot position of the electron beam  200  is controlled to perform tracking of such stage motion. Letting it move continuously makes it possible to shorten a total length of time as taken to write the pattern. After having written one strip region, the XY stage  105  is forced to perform step shifting in the y direction and then move in the x direction (this time, in the reverse direction) for execution of pattern writing of the next strip region. By progressing respective strip regions&#39; pattern writing operations in a serpentine manner, it is possible to shorten the total moving time of the XY stage  105 . 
     Note here that when performing such electron-beam pattern writing, a need is felt to perform in advance the measurement (calibration) of the electron beam optics. This measurement (calibration) is performed for an entirety of the electron beam optics inside of the pattern writing unit  150  shown in  FIG. 1 . With the measurement and adjustment, one or more correction coefficients are calculated, which are needed for beam deflection sensitivity adjustment and beam deflection. These measurement and adjustment are regarded as a beam adjustment process, which is performed in a way separate from the pattern writing process, as prearrangement of the writing process of writing a desired pattern(s) on the workpiece. 
     By the beam adjustment process, the deflection sensitivity of a deflector, e.g., main deflector  214 , is measured (calibrated). A digital-to-analog converted (DAC) value to be corrected by such correction is defined by using an X value and a Y value which become DAC value data, wherein the DAC value is output, for example, to DAC  182  which determines a voltage value that is set in the main deflector  214 . As an example, these X and Y values are obtainable by Equations (1) and (2) below. Here, these equations are called the beam deflection sensitivity correction functions. 
     
       
         
           
             
               
                 
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     To obtain these equations, what is needed to be done first is to determine through measurement an error between the original or “ideal” position designed and an actual position to be measured by the laser length meter  190 . 
       FIG. 4  is an upper plan view of the XY stage  150  for explanation of a mark position measurement process in the EB lithographic tool  100  shown in  FIG. 1 . 
     As shown in  FIG. 4 , a mark  152  for measurement of the deflection sensitivity of the electron beam  200  is provided on the XY stage  105  with the workpiece  101  mounted thereon. This mark&#39;s position coordinates are searchable by stage movement and electron beam deflection. Reflected electrons occurring upon irradiation of the electron beam  200  to mark  152  are measured, and a certain set of coordinates with the measurement value maximized is determined to be the coordinates of the mark position. This mark&#39;s position coordinates are calculated by use of the deflection of electron beam  200  so that an atmospheric pressure value-dependent variation or fluctuation can be seen therein. One or several correction coefficients for such atmospheric pressure variation are derivable from the relativity of this atmospheric pressure value and mark position coordinate deviation. 
     Here, an explanation will be given as to areas or regions for deflection of the main deflector  214  and sub-deflector  212 . 
       FIG. 5  is a pictorial representation of the workpiece having its main deflection area and sub-deflection area. 
     As shown in  FIG. 5 , in the case of a specified circuit pattern being written or “drawn” in the EB lithography tool  100 , the pattern write region of a photomask that is an example of the workpiece  101  is subdivided along the y direction into a plurality of strip-like unit regions for pattern writing, each of which regions has a width that is deflectable by the main deflector  214 . In each strip, a region deflectable by the main deflector becomes a main deflection area. This main deflection area is further divided into fine regions, which become sub-deflection regions, also called the sub-fields. 
     The sub-deflector  212  is used to accurately control a per-shot position of the electron beam  200  at high speeds. To this end, a deflection range is limited to a sub-field as shown in  FIG. 5 . For pattern writing exceeding this region, the subfield&#39;s position is moved or shifted by the main deflector  214 . In view of the fact that this main deflector  214  is used to control the subfield position and that the XY stage  105  is continuously moving in the x direction during pattern writing, the subfield&#39;s origin for pattern writing is caused by the main deflector  214  to move (perform tracking) whenever the need arises, thereby causing it to follow the movement of the XY stage  105 . 
       FIG. 6  is a diagram for explanation of a way of measuring a mark position in the embodiment apparatus. 
     As shown in  FIG. 6 , the XY stage  105  is driven to move so that the mark  152  moves to each position within the main deflection region  10 . Then, the electron beam  200  is deflected at each position in the main deflection region  10  to thereby measure the position of mark  152  and then obtain a residual difference thereof. Here, such operation is performed with respect to a total of twenty fine (25) locations in the main deflection area  10 —i.e., a matrix of five rows and five columns of points. Then, the residual difference obtained is subject to execution of the fitting by a third-order or tertiary function equation with the x and y of the above-stated Equations (1) and (2) being as variables thereof whereby it is possible to obtain respective coefficients a 0  to a 9  and b 0 -b 9  of the x-y function equation. 
     Then, the deflection control circuit  140  inputs from the pattern writing control circuit  110  shot data (not shown) along with the position information and obtains an X value that becomes one of the DAC value data by Equation (1) while obtaining a Y value that is another one of the DAC value data by Equation (2). Next, the electron beam  200  is deflected by using an amplified value of a digital-to-analog converted version of a DAC value (deflection voltage) with the X value and Y value being as its parameters. For example, in the case of using an eight-pole electrostatic deflector having eight separate electrodes No.  1  to No.  8  which are laid out on the main deflector  214  in a clockwise direction when being looked at from its upper side, the DAC value setting may be performed in a way which follows. To deflect the beam in a predetermined direction of the x and y directions, a value Y is set to the electrode # 1 ; a value (X+Y)/2 1/2  is set to electrode # 2 ; X is set to electrode # 3 ; (X−Y)/2 1/2  is set to electrode # 4 ; −Y is set to electrode # 5 ; (−X−Y)/2 1/2  is set to electrode # 6 ; −X is set to electrode # 7 ; and, (−X+Y)/2 1/2  is set to electrode # 8 . Then, a voltage with its potential equivalent to each amplified value of a digital-to-analog converted version of each DAC value thus set is applied to its corresponding one of the electrodes. Here, in light of the fact that the write beam can experience unwanted distortion and/or deviance of its focusing point in a way depending upon the workpiece  101 &#39;s present position in the z direction, it is preferable to convert the coefficients of Equations (1) and (2) at a z-axis position that was detected by the z sensor. This coefficient conversion is definable by Equation (3) to be presented later. 
       FIG. 6  is a diagram for explanation of a process of calculating the coefficients in Equations (1) and (2). As shown in  FIG. 6 , the XY stage  105  is driven to move so that the mark  152  moves to each position within the main deflection region. Then, after having determined a mark that becomes the reference, the electron beam  200  is deflected to each position in the main deflection region to thereby measure the mark position and then obtain a residual difference between a moved degree of laser coordinates and a deflection distance. Here, this operation is performed with respect to a total of twenty fine (25) locations in the main deflection area—i.e., a matrix of five rows and five columns of points. Then, the residual difference obtained is subject to execution of the fitting by the tertiary function equation with the x and y of the above-stated Equations (1) and (2) being as its variables to thereby calculate respective coefficients a 0 -a 9  and b 0 -b 9  of the deflection sensitivity correction function (i.e., x-y function equation). 
       FIG. 7  is a diagram for explanation of a way of measuring the height of the mark in the embodiment. 
       FIG. 8  is a diagram showing rotation and magnification errors in out-of-focus of an electron beam depending upon the height of the workpiece surface in this embodiment. 
       FIG. 9  is a graph schematically showing rotation and magnification errors in out-of-focus of the electron beam depending on the height of the workpiece plane in the embodiment. 
     Using  FIGS. 7-9 , an explanation will be given of a way of creation of deviations of electron beam focus point and deflection sensitivity in accordance with the height of the workpiece surface to be processed. As shown in  FIG. 7 , in case the electron beam  200  is irradiated onto the mark  152  having height differences with a level of Z=0 being as a reference point, the electron beam  200  exhibits unwanted rotation and size change such as shrinkage or expansion of the deflection region as shown in  FIG. 8 . When the beam&#39;s focus point is deviated or offset, mismatching takes place among the deflection region&#39;s magnification component and rotation component as well as shift component. Accordingly, in the case of correction of the beam&#39;s focus point in accordance to the height of the workpiece surface, it becomes inevitable to perform the magnification correction and rotation correction plus shift correction of the deflection region. However, if the current center of a correction-use lens is adjusted properly, the shift correction is no longer required. Here, upon determination of a given height reference point, rotation and magnification errors from such reference point are each representable by a linear function of the height (Z) such as shown in  FIG. 9 . And, these magnification, rotation and shift corrections are achievable with sufficiently high accuracy by use of a linear expression such as indicated by Equation (3) below.
 
 a   0   =a   0   +a   0   ·z, a   1   =a   1   +a   1   ·z, a   2   =a   2   +a   2   ·z b   0   =b   0   +b   0   ·z, b   1   =b   1   b   1   ·z, b   2   =b   2   +b   2   ·z    (3)
 
     In this equation, a 0  and b 0  represent axis deviation correction, a 1  and b 2  indicate magnification correction, and a 2  and b 1  represent rotation correction, which are substituted into Equations (1) and (2), respectively. In addition, substituted into z in Equation (3) is a value that was measured by the z sensor prior to pattern writing. By dividing the workpiece surface into a matrix of mesh-like portions and then measuring the grid height of each mesh by the z sensor, it is possible to perform mapping of the height (z) in the workpiece surface. This makes it possible to obtain the z value of any given position. By putting this z value into Equation (3), it becomes possible to perform adequate beam focussing in a way pursuant to the height of the workpiece surface in the main deflection area. 
     In this way, by obtaining respective coefficients a 0 -a 2  and b 0 -b 2  from the relationship shown in  FIG. 9 , it is possible to achieve the intended correction of about-the-z-axis rotation, magnification and shift of the deflection region even in cases where focus-point correction is implemented in deference to a deviation in height of the workpiece surface. 
     Note here that the discussion above works out under an assumption that the outside air pressure is kept constant. If the atmospheric pressure varies, further errors occur inevitably. 
       FIG. 10  is a diagram for explanation of an exemplary way of distortion occurring in the EB lithographic apparatus  100  when its ambient air pressure increases. 
     An example is that when the outside air pressure P rises up, a ceiling on the top plate side of the pattern writing chamber  103  with its interior space being evaluated by a vacuum pump (not shown) is pushed or “pressed” by the atmospheric pressure P and thus is deformed toward the inside of chamber  103 . Such deformation is typically on the order of magnitude of nanometers (nm). In such case, the electron lens barrel  102  overlying the writing chamber  103  can deviate in its position, resulting in the relative position between the optics within barrel  102  and the surface of workpiece  101  being displaced three-dimensionally in x-, y- and z-axis directions, respectively. Regarding the z direction, for example, the value z is changed to a value z′. Additionally, an error can take place in the z sensor&#39;s measurement value per se. This can be said because the chamber ceiling&#39;s dishing results in the light projector  532  and its associated photosensor  534  being deviated in installation positions. To avoid this risk, this embodiment employs a process of amending such deviations as shown in the flow shown in  FIG. 2 . 
     At step S 102 , an outside air pressure is measured. To do this, the atmospheric pressure measurement unit  122  inputs from the barometer  170  a value of atmospheric air pressure P and measures a present atmospheric pressure. 
     At step S 104  which is for calculation of coordinate correction values, the coordinate correction value computing unit  124  uses the measured value of atmospheric air pressure value P to calculate coordinate correction values Δx, Δy and Δz (first, second and third coordinate correction values) for correction of coordinate values (x,y,z) (first, second and third coordinate values) and then stores these values in the storage device  130 . The coordinate values are defined by an x-coordinate value which indicates a position in first direction (x direction) in parallel with the pattern writing surface of workpiece  101 , a y-coordinate value indicating a position in second direction (y direction) in parallel with the pattern writing surface and at right angles to the x direction, and a z-coordinate value indicative of a position (height) of the pattern writing surface in third direction (z direction) perpendicular to the pattern write surface. Then in this coordinate correction value calculation step, there are calculated as the coordinate correction values an x-coordinate correction value Δx for correction of the x-coordinate value, a y-coordinate correction value Δy for correction of the y-coordinate value, and a z-coordinate correction value Δz for correction of the z-coordinate value. 
     Firstly, the x-coordinate correction value Δx is obtainable by Equation (4) given below, by using a coefficient c 1  of proportionality in x-axis direction (its unit is typically nanometers per hectopascal or “nm/hPa”) and an offset value P 1  of the atmospheric pressure P (e.g., unit is hPa).
 
Δ x=c   1 ·( P−P   1 )   (4)
 
     The y-coordinate correction value Δy is definable by Equation (5) below, by using a proportionality coefficient c 2  in y-direction (its unit is typically “nm/hPa”) and the above-stated offset value P 1  of the atmospheric pressure P.
 
Δ y=c   2 ·( P−P   1 )   (5)
 
     The z-coordinate correction value Δz is definable by Equation (6) below, by using a proportionality coefficient c 3  in z-direction (its unit is “nm/hPa”) and the above-stated offset value P 1  of the atmospheric pressure P.
 
Δ z=c   3 ·( P−P   1 )   (6)
 
     The coordinate correction values Δx, Δy, Δz obtained in the way stated above are temporarily stored in the storage device  130 . This series of steps S 102  to S 104  will be repeated at a prespecified time interval to update the coordinate correction values Δx, Δy, Δz to the latest values. The interval is set at a one minute, by way of example. Alternatively, it may be a shorter period. Alternatively it may be a longer period, although in this case the real-time property is somewhat degraded. Additionally the coordinate correction value calculation step is a process independent of the pattern writing step or the beam adjustment step and is out of sync therewith. 
     On the other hand, a pattern writing task gets started in the deflection control circuit  140 . In this case the latest version of coordinate correction values Δx, Δy, Δz are stored in the storage device  144  whenever the need arises in a way as will be described later. 
     At step S 302 , coordinate value correction is performed. More specifically, the coordinate value correction unit  126  reads the coordinate correction values Δx, Δy and Δz out of the storage device  144  and uses the coordinate correction values Δx, Δy, Δz thus read to correct the set of coordinate values (x, y, z). Then, the coordinate value correction unit  126  computes a set of corrected coordinate values (x′, y′, z′). In the coordinate value correction step, the x-coordinate correction value Δx is used to correct the x-coordinate value; the y-coordinate correction value Δy is used to correct the y-coordinate value; and, the z-coordinate correction value Δz is used to correct the z-coordinate value. 
     The corrected coordinate value x′ (one example of the first coordinate value) is obtainable by Equation (7) below, which is for adding the x-coordinate correction value Δx to the x-coordinate value.
 
 x′=x+Δx    (7)
 
     The corrected coordinate value y′ (one example of the second coordinate value) is obtainable by Equation (8) below, which is for adding the y-coordinate correction value Δy to the y-coordinate value.
 
 y′=y+Δy    (8)
 
     The corrected coordinate value z′ (one example of the third coordinate value) is obtainable by Equation (9) below, which is for adding the z-coordinate correction value Δz to the z-coordinate value.
 
 z′=z+Δz    (9)
 
     At step S 304 , coefficient conversion is performed in a way which follows. The coefficient converter unit  128  reads from the storage device  144  the corrected z-coordinate value z′ (third coordinate value) and use this value to correct the coefficients of Equations (1) and (2). Of the coefficients of Equations (1) and (2), specific coefficients a 0 -a 2  and b 0 -b 2  of first-degree and less degree terms therein are calculated, which are indicated by Equation (3) that is under the influence of the z value. More specifically, when obtaining these coefficients a 0 -a 2  and b 0 -b 2  in Equation (3), substitute thereinto the corrected z-coordinate value z′ in place of the z value. By doing this, it is possible to obtain the intended coefficients a 0 -a 2  and b 0 -b 2  with a variation of the atmospheric pressure P being taken into consideration. 
     At step S 306 , beam deflection voltage computing is performed in a way which follows. The deflection voltage computing unit  146  uses the amended version of coordinate values obtained at the previous step to determine through computation an optimal deflection voltage of the electron beam  200 . 
     Then at step S 308 , pattern writing is performed in a way which follows. The electron beam  200  which was emitted from the electron gun  201  that is one example of the irradiator unit as claimed is deflected by the main deflector  214  that is one example of the deflector unit in such a way as to irradiate the electron beam  200  that was deflected using the obtained deflection voltage to thereby draw a desired circuit pattern on the workpiece  101 . In other words, an operation is performed for correcting the x-coordinate value and y-coordinate value based on the atmospheric air pressure by causing the main deflector  214  to apply electrostatic deflection to the electron beam  200 . Additionally, by changing magnetic excitation to the objective lens  207  based on the corrected z-coordinate value z′ to thereby correct the beam&#39;s focus position, it is also possible to perform correction of the z-coordinate value based on the atmospheric pressure. 
     Then at step S 310 , settling is performed in a way which follows. After completion of pattern writing per prespecified drawing unit region, data is set to DAC of deflector; here, a time period (called the settling time) is provided for waiting for potential stabilization of its output. An example is that in case an attempt is made to change the main deflector  214 &#39;s deflection position to the next sub-field (SF) after completion of pattern writing of one SF, the system routine goes into a wait state for the settling time after having set the data in DAC  182  for the main deflection use. This settling time may be about  20  microseconds (us) in this embodiment. 
     On the other hand, in an atmospheric pressure correction task as internally executed by the deflection control circuit  140  that was commanded from CPU  120  in the pattern writing control circuit  110 , the following processing is executed. 
     At step S 202 , corrected value verifying is performed in such a way that the corrected value verify unit  147  in deflection control circuit  140  is operatively responsive to receipt of a command from CPU  120  in pattern writing control circuit  110  for verifying whether the atmospheric pressure variation-based coordinate correction values Δx, Δy and Δz are changed or not at the time the pattern writing task is in the settling time period. To determine whether such value change is present or absent, an attempt may be made to obtain information from CPU  120  in pattern writing control circuit  110 . 
     At step S 204 , updating is performed. To do this, the update unit  148  in deflection control circuit  140  operates in such a way that when a result of the above-stated verifying process indicates the presence of a value change(s), it reads the coordinate correction values Δx, Δy and Δz from the storage device  130  in the deflection control circuit  140  and performs updating by overwriting the read values on the previously obtained data being stored in the storage device  144  in deflection control circuit  140 . By inputting the data only when value change(s) is/are found, it is possible to lessen the workload for data transfer. 
     As apparent from the foregoing description, regarding first the task to be executed by the CPU  142  in deflection control circuit  140 , the intended data update is achievable on a real-time basis by receiving one-minute interval access from the execution process of CPU  120  in pattern writing control circuit  110 , which becomes the atmospheric pressure correction process. Here, it is considered that the updating from the atmospheric pressure correction process can experience confliction or “batting” with a beam adjustment process and/or a pattern writing process being presently executed in the deflection control circuit  140 . In light of this risk, this embodiment employs the so-called “dual task” configuration, which permits the task being executed in the deflection control circuit  140  to be of two parts. Doing so makes it possible to avoid the process batting. More precisely, with a one task scheme, a problem occurs as to a must to perform a troublesome and time-consuming procedure for receiving a command from the pattern writing process, for pausing the inherent pattern write task in order to provide access to the atmospheric pressure correction process during pattern writing, and for forcing a task for the atmospheric pressure correction use to get started. On the contrary, using the dual-task scheme makes it possible to permit startup of the atmospheric pressure correcting task in a parallel way without having to pausing the pattern writing task. 
     Also note that in the pattern writing task, a session for writing strip regions high in shot density can sometimes exceed one minute. The atmospheric pressure correction process is such that access is given to inside of the deflection control circuit  140  in any events at a preset length of time intervals (e.g., one-minute intervals) as stated previously. It is a must for the deflection control circuit  140  to monitor or “watchdog” asynchronous access from this atmospheric pressure correction process, so it takes a certain length of time for completion of this processing. In view of this, by performing calling of the atmospheric pressure-use task in a mid course of the pattern writing task, it is possible to complete the data update within the settling time period; for example, one minute-interval real-time correction is enabled. Additionally, in the pattern write processing task, the settling of the main deflection DAC amplifier having DAC  182  and amp  184  takes 20 μs, or more or less. Accordingly, by temporarily switching to the atmospheric pressure correction task at an idle time of the core during the settling time, it becomes possible to achieve real-time communications with the atmospheric pressure correction task. 
     At decision step S 312 , the CPU  142  of deflection control circuit  140  determines whether the pattern writing is completed or not. If it is completed, then quit the pattern writing task; otherwise, return to step S 301 . 
     Then, in the pattern writing task, the real-time corrected coordinate correction values Δx, Δy, Δz are used to compute coordinate values x′, y′ and z′ at step S 302 . Then, use z′ 0  to convert the coefficients at step S 304 . Next at step S 306 , use the converted coefficients along with the corrected coordinate values x′ and y′ to compute DAC value data serving as the original of the next deflection voltage. Practically, at the deflection voltage computing step, respective coefficients a 0 -a 2  and b 0 -b 2  which have been computed by Equation (3) with its coefficients having been calculated using the z′ value are substituted as variables the corrected x′ and y′ values into Equations (1) and (2) that are deflection sensitivity correction functions (x-y function formulas), thereby to obtain X value and Y value which become DAC value data. Then, use these X and Y values to compute DAC value (deflection voltage) used for each electrode of the deflector  214 . Then at step S 308 , proper pattern writing is performed with the use of a value that is a DA-converted and amplified version of the DAC value that was corrected relative to a change in atmospheric air pressure. 
     It has been stated that the focussing correction and the correction of a beam position with the atmospheric pressure variation-caused coordinate position errors (Δx, Δy, Δz) being involved therein are performed by a process having the steps of obtaining respective coefficients a 0 -a 2  and b 0 -b 2  of the deflection sensitivity correction functions for calculation of the optimum deflection voltage and inputting the corrected values x′ and y′ and then obtaining DAC value data on a real-time basis and finally performing pattern writing processing while performing correction using these values, thereby making it possible to form a pattern on the workpiece  101  at a pattern writing position therein while at the same time increasing or maximizing the accuracy thereof. 
       FIG. 11  is a diagram showing an exemplary pattern writing operation flow along with the timing of an air pressure correction operation. 
     In  FIG. 11 , there are shown a pattern writing operation in a one strip and one sub-field and an atmospheric pressure correction operation in sync with the former operation. The one-strip writing operation is performed by repeated execution of any given number of subfield writing operations. The one-subfield write operation starts with reading of the main deflection coordinate value(s), followed by main deflection amount calculation and repeated execution of a given number of shots. The deflection control circuit  140  performs main deflection amount computation by substituting the read main deflection coordinate value (x,y) into the deflection sensitivity functions (i.e., Equations (1) and (2)) in the deflection voltage computing unit  146 . In the case of the atmospheric pressure correction being executed, in this process, the coordinate value correction unit  126  adds thereto a corrected amount calculated by the coordinate correction value computing unit  124 . Then, the coefficient converter unit  128  further uses the correction-completed z′ value to perform coefficient conversion. The coordinate correction value computing unit  124  is operating in async with the pattern writing operation and the beam adjustment process, and calculates, at one-minute intervals as an example, those correction amounts Δx, Δy and Δz of 3D coordinate values of x, y and z, which will be passed to the deflection control circuit  140 . This control circuit  140  performs readout of an atmospheric pressure correction amount from the coordinate correction value computing unit  124  within a wait time period other than its active time for computing the main deflection amount—here, within the main deflection settling time period in a subfield writing operation, by way of example. Thus it is possible to permit synchronization of the atmospheric pressure correction in units of subfield writing operations. In addition, owing to this operation principle, it becomes possible to achieve the atmospheric pressure correction on a real-time basis. In this way, the addition control is performed in a way that it operates independently of the pattern writing process and the beam adjustment process while causing the coordinate correction values Δx, Δy and Δz to be in sync with 3D coordinate values in any one of the pattern writing process and the beam adjustment process in units of subfield operations. 
     An explanation will here be given of a method for management of the pattern writing process and the beam adjustment process. The beam adjustment process is executed according to a preset processing menu in units of adjustment time periods. In this embodiment, an explanation will be given of a case where focussing or else is performed once per week. If a focussing error occurrable due to changes in outside air pressure is not corrected, the magnetic excitation current of an optimal focus point is variable depending on an atmospheric pressure. Consequently, in case the focussing is done at the time of a low pressure, a pattern is to be drawn in the state that a beam is out of focus in ordinary weather. Additionally, at a stage of beam calibration for correction of a deflection error of the beam, rotation of deflection and magnification errors can take place in accordance with an atmospheric air pressure. For this reason, the beam calibration performed in the state of atmospheric depression would result in unwanted execution of pattern writing with deviation of deflection sensitivity in ordinary weather, which poses a problem as to an accidental increase in batting errors. 
     In view of this risk, this embodiment is arranged to correct atmospheric pressure variation-dependent errors in 3D beam positions in x-, y- and z-axis directions, thereby enabling the beam&#39;s focus point to stay constant in any events while at the same time making it possible to suppress fluctuations of beam deflection errors. Thus it becomes possible to achieve high-accuracy pattern writing while preventing beam focussing offsets and deflection sensitivity errors from increasing depending on the timing of a change in atmospheric pressure. 
     Embodiment 2 
     In Embodiment 1, the case has been described where a distribution of height values of a workpiece surface is measured in advance as a z-axis position map or “z-map.” In this case, an atmospheric pressure change-dependent variation component Δz is added to the premeasured z-map data to obtain a newly calculated z′ value, which is used to write a pattern after correction, as stated previously. This is done because the z-axis sensor&#39;s measurement value indicative of the height (z) of workpiece surface deviates unintentionally depending upon a change in atmospheric air pressure. However, this z-sensor may also be reduced to a commercial product with a structure being lessened in variability of the atmospheric pressure. Embodiment 2 is arranged to employ such z-sensor with enhanced robustness against atmospheric pressure variations. Its other structures and configurations are similar to those of Embodiment 1. In the case of using such z-sensor with enhanced robustness against atmospheric pressure variations, it is preferable to employ a system design which performs z-axis position correction during pattern writing on a real-time basis, rather than the z-map scheme. More specifically, read the height (z) data out of the z sensor on a per-subfield basis. Then, calculate Equation (3) to obtain the coefficients of Equations (1) and (2). By inputting atmospheric pressure change-dependent variable position error-corrected position information (x′, y′) to the deflection sensitivity correction functions of Equations (1) and (2) to thereby perform pattern writing, similar results are obtainable. 
     Embodiment 3 
     In Embodiments 1 and 2, the main deflector  214  that is an electrostatic deflector was used to correct the beam&#39;s position by means of electrostatic deflection. In other words, the main deflector  214  electrostatically deflect the electron beam  200  to thereby perform correction of the x-coordinate value and y-coordinate value based on an outside air pressure while at the same time causing the objective lens  207  to perform correction of the z-coordinate value based on the atmospheric pressure. However, the beam correction should not exclusively be limited to this approach. In Embodiment 3, a case will be explained where another configuration is used to correct the beam&#39;s deflection position and focussing position. 
       FIG. 12  illustrates, in cross-section, a structure of main part of a pattern writing apparatus also embodying the invention. 
     An electron beam optics shown in  FIG. 12  is similar to that shown in  FIG. 1  with alignment coils  215 ,  216  and  217  and an electrostatic lens  218  being added to the former. Its other arrangements also are similar to those of Embodiment 1, except for those points to be described below. At the alignment coils  215 - 217  which are situated over the objective lens  207 , beam deflection is performed by use of a magnetic field(s) as derived by an excitation current(s). The beam position may be corrected by using these coils. In other words, it is also preferable to perform correction of the x-coordinate value and y-coordinate value based on an outside air pressure by causing at least one of the alignment coils  215 - 217 , in place of the main deflector  214 , to electrostatically deflect the electron beam  200 . 
     Regarding the correction of z-coordinate value based on an atmospheric pressure, it is also permissible to use either the main deflector  214  or the independently disposed electrostatic lens  218  (second electrostatic deflector) in place of the objective lens for correction. For example, the beam focussing is corrected by additionally applying a constant potential level of voltage to the main deflector  214 , whereby it is possible to perform the correction of the z-coordinate value based on the atmospheric pressure. Similarly, by correcting the beam focussing position by applying for superposition a constant voltage to the electrostatic deflector  218 , it is possible to perform the correction of the z-coordinate value based on the atmospheric pressure. 
     It should be noted that in the description above, the processing contents or operation contents of those recited as “. . . units” or “. . . steps” are arrangeable by software programs which are executable by electronic arithmetic processing apparatus, such as digital computers or else. Alternatively, these processing/operation contents may be implemented not only by software programs but also by any possible combinations of hardware and software configurations. Still alternatively, similar results are obtainable by arrangement combined with firmware configurations. In the case of a software program being used for the intended arrangement, this program is typically installed and stored in an adequate recording media, such as a magnetic disk device, magnetic tape device, floppy diskettes (FDs), read-only memory (ROM), or nonvolatile programmable memory—e.g., “Flash” memory. Regarding the embodiments as disclosed herein, the program is stored in the storage device  130  or  144 . 
     Also note that in the embodiment shown in  FIG. 1 , the CPU  120  and/or CPU  142  for use as computer equipment may be connected via an internal data transmission bas(es) to a random access memory (RAM) for use as the storage device, ROM, large-capacity hard disk drive (HDD), data entry device such as a keyboard with or without a pointing device called the “mouse,” data output device such as a monitor display or printer, input/output interface (IO-I/F) equipment, and external storage device such as an FDD, a digital versatile disk (DVD) drive, a compact disc (CD) device, etc. 
     While the invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. 
     Although the description excludes explanations as to those parts or components which are deemed unnecessary for explanation of this invention such as apparatus arrangements and control schemes, such components are employable through appropriate choices on a case-by-case basis. For example, the configuration of the control unit for control of the pattern writing apparatus  100  may be designed to include necessary components other than those discussed and illustrated herein. 
     Any other possible charged-particle beam lithographic methods and apparatuses comprising the subject matter of this invention and being design-changeable by a technician in the semiconductor lithography art with software programs for use therein are all interpreted to be included in the scope and coverage of the invention. 
     Additional advantages and modification will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.