Abstract:
The present invention is related to a D/A conversion device, it is provided with a first D/A conversion circuit which receives input of digital data composed of plural bits and outputs a corresponding electric output signal, and a second D/A conversion circuit which receives input of a correction code for the digital data and which outputs a corresponding electric correction signal, wherein the first and second D/A conversion circuits are connected to each other at their respective output terminals so that the electric output signal is corrected by the electric correction signal. The D/A conversion device comprises: storing means  105  for storing correction codes each for one bit of the digital data, the correction codes being determined in correlation with the first D/A conversion circuit  11;  and calculating means  107  for performing serial entry and addition of the correction codes each for one bit of the digital data, and outputting a correction code for all bits of the digital data.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based on and claims priority of Japanese Patent Application No. 2006-099222 filed on Mar. 31, 2006, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a D/A (digital-to-analog) conversion device and method which are applicable to a charged particle beam scanning deflector of a charged particle beam exposure apparatus and relates to a charged particle beam exposure apparatus and method.  
         [0004]     2. Description of the Prior Art  
         [0005]     A charged particle beam exposure apparatus requires higher accuracy of exposure position for purposes of finer patterning. The exposure apparatus also has to reduce exposure standby time and thereby improve exposure throughput since it scans a charged particle beam for patterning.  
         [0006]     Incidentally, a D/A conversion circuit is interposed between a control circuit and a deflector for controlling the charged particle beam since a digital signal is used to control the scanning of the charged particle beam. However, the D/A conversion circuit is limited in performance. Deterioration in input-output linearity of the D/A conversion circuit often occurs and thus renders high-precision control difficult.  
         [0007]     A D/A conversion device improved to overcome this difficulty is disclosed in Japanese Patent Application Laid-Open Publication No. Hei 10-290162.  
         [0008]     A D/A conversion device 10A disclosed in FIG. 4 in Japanese Patent Application Laid-Open Publication No. Hei 10-290162 includes a first D/A conversion circuit 11 which receives input of 16-bit digital data and outputs a corresponding electric signal, a memory 13A which stores correction codes for all digital data that can be represented by 16 bits, and a second D/A conversion circuit 12 which receives input of the correction code for the digital data from the memory 13A and outputs a corresponding electric correction signal. Both the first and second D/A conversion circuits 11 and 12 are current output mode circuits. The first D/A conversion circuit 11 is configured of an R-2R ladder resistor network for the low-order 12 bits and a decoder for the high-order 4 bits, as shown in FIG. 14 of Japanese Patent Application Laid-Open Publication No. Hei 10-290162.  
         [0009]     The D/A conversion device 10A is configured so that the electric output signal from the first D/A conversion circuit 11 is corrected by the electric correction signal from the second D/A conversion circuit 12.  
         [0010]     The D/A conversion device performs operation as disclosed in FIG. 5 of Japanese Patent Application Laid-Open Publication No. Hei 10-290162.  
         [0011]     The operation is prepared beforehand by inputting all digital data that can be represented by 16 bits to the first D/A conversion circuit 11, determining measured values corresponding to the digital data, and determining deviations from an ideal curve. The deviations are added, and their sum is stored in the memory 13A as a correction code for the digital data.  
         [0012]     Then, digital data is inputted to the first D/A conversion circuit 11, which in turn outputs an electric output signal in analog form.  
         [0013]     Also, the digital data is used to address the memory 13A that stores the correction codes, and the correction code corresponding to the digital data is read from the memory 13A.  
         [0014]     Then, the read correction code is inputted to the second D/A conversion circuit 12, which in turn outputs an electric correction signal in analog form.  
         [0015]     The electric correction signal is added to or subtracted from the electric output signal from the first D/A conversion circuit 11 to thereby correct the electric output signal.  
         [0016]     However, the D/A conversion device mentioned above requires much time since time for access to the memory 13A is involved in each and every input of the correction code to the second D/A conversion circuit 12. The D/A conversion device also requires a memory having such a large capacity as can store correction codes for all digital data that can be represented by the configuration bits.  
       SUMMARY OF THE INVENTION  
       [0017]     An object of the present invention is to provide a D/A conversion device and method and a charged particle beam exposure apparatus and method, which are capable of achieving both higher accuracy of exposure position and shorter exposure standby time without using a large-capacity memory.  
         [0018]     According to one aspect of the present invention, there is provided a D/A conversion device including a first D/A conversion circuit which receives input of digital data composed of plural bits and which outputs a corresponding electric output signal, and a second D/A conversion circuit which receives input of a correction code for all bits of the digital data and which outputs a corresponding electric correction signal. In the D/A conversion device the first and second D/A conversion circuits are connected at their respective output terminals so that the electric output signal is corrected by the electric correction signal. Therefore, the D/A conversion device can improve the accuracy of the electric output signal.  
         [0019]     In this case, correction codes each for one bit of the digital data, which are preobtained in correlation with the first D/A conversion circuit, are used to generate the correction code for all bits of the digital data. When bit data “1” is displayed by the passage of a current through a corresponding circuit in the first D/A conversion circuit, the correction codes each for one bit may be defined only in a situation where the bit data “1” is set.  
         [0020]     Calculating means performs serial operation on all bits of the digital data by serially adding the correction codes each for one bit in real time in synchronization with the entry of the digital data, and then outputs the correction code for the digital data. Moreover, the correction code outputted by the calculating means is immediately inputted to the second D/A conversion circuit, which in turn outputs the electric correction signal. The electric output signal from the first D/A conversion circuit is immediately corrected by the electric correction signal, so that a desired corrected electric signal can be obtained.  
         [0021]     With a circuit configuration capable of achieving the above functions, such a capacity as can store the correction codes each for one bit of the digital data and the results of calculations is sufficient for storing means, and therefore a small-capacity storage device such as a register can be used as the storing means. This enables high-speed operation.  
         [0022]     According to another aspect of the present invention, expanded correction codes each for plural bits of the digital data, which are preobtained in correlation with the first D/A conversion circuit, are used to generate the correction code for the digital data. This makes it possible to perform serial operation on all bits of input serial data by serially calculating the expanded correction codes each for the plural bits, thus enabling a further reduction in the number of calculations. This enables higher-speed operation.  
         [0023]     Also in this case, such a capacity as can store the expanded correction codes each for plural bits of the digital data and the results of calculations is sufficient for storing means, and therefore a small-capacity storage device such as a register can be used as the storing means. This enables high-speed operation.  
         [0024]     The D/A conversion device of the present invention having the above configuration, as applied to a deflector for charged particle beam scan, can achieve both higher accuracy of exposure position and shorter exposure standby time without using a large-capacity memory. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1  is a schematic diagram showing a charged particle beam exposure apparatus according to a first embodiment of the present invention.  
         [0026]      FIG. 2  is a schematic block diagram showing detailed configurations of a control circuit, a D/A converter, a current-voltage converter, and a DVM, which are included in the charged particle beam exposure apparatus according to the first embodiment of the present invention.  
         [0027]      FIG. 3  is a graph showing input-output characteristics and an ideal output line of a first conversion circuit of the D/A converter included in the charged particle beam exposure apparatus according to the first embodiment of the present invention.  
         [0028]      FIG. 4  is a timing chart showing the flows of signals indicative of digital data (DAT), correction codes (CC), clock pulses (CLK), data stored in a register (REG 1 ), data stored in a register (REG 2 ), output currents I, and correction currents Ic, which are produced in a control section included in the charged particle beam exposure apparatus according to the first embodiment of the present invention.  
         [0029]      FIG. 5  is a flowchart showing operation of the control section included in the charged particle beam exposure apparatus according to the first embodiment of the present invention, particularly operation for generating a correction code CC based on error correcting codes each for one bit of digital data.  
         [0030]      FIG. 6  is a table showing correction codes stored in a correction code table in the control circuit included in the charged particle beam exposure apparatus according to the first embodiment of the present invention.  
         [0031]      FIG. 7  is a schematic block diagram showing detailed configurations of a control circuit, a D/A converter, a current-voltage converter, and a DVM, which are included in a charged particle beam exposure apparatus according to a second embodiment of the present invention.  
         [0032]      FIG. 8  is a flowchart showing operation of a control section included in the charged particle beam exposure apparatus according to the second embodiment of the present invention, particularly operation for generating a correction code CC based on expanded correction codes each for plural bits of digital data.  
         [0033]      FIG. 9  is a table showing expanded correction codes stored in an expanded correction code table in the control circuit included in the charged particle beam exposure apparatus according to the second embodiment of the present invention.  
         [0034]      FIG. 10  is a circuit diagram showing a D/A conversion circuit included in the charged particle beam exposure apparatus according to the embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     Description will be given below with regard to preferred embodiments of the present invention with reference to the accompanying drawings.  
       First Embodiment  
       [0036]      FIG. 1  is a schematic view showing the configuration of a charged particle beam exposure apparatus including a D/A conversion device according to a first embodiment of the present invention.  
         [0037]     The charged particle beam exposure apparatus is configured of an exposure section and a control section as shown in  FIG. 1 .  
         [0038]     The exposure section is configured of a charged particle beam emitting device  20 , a blanking deflector  21 , an aperture plate  22 , an objective lens  23 , and a moving stage  24 .  
         [0039]     A charged particle beam EB emitted from the charged particle beam emitting device  20  is applied through the blanking deflector  21 , the aperture plate  22  and the objective lens  23  to a wafer  25  mounted on the moving stage  24 . A cross section of the charged particle beam EB shaped by the charged particle beam emitting device  20  is reduced and projected onto the wafer  25 . A main deflector  26  of an electromagnetic deflection type and an auxiliary deflector  27  of an electrostatic deflection type, which are disposed in the objective lens  23 , are used to scan the charged particle beam EB over the wafer  25 .  
         [0040]     The control section is configured of a storage device  28 , a control circuit  29   a  or  29   b,  a D/A converter  30  and a current-voltage converter  31  which control the auxiliary deflector  27  in accordance with a signal from the control circuit  29   a  or  29   b,  a digital volt meter (DVM)  32  which measures an output voltage from the current-voltage converter  31  and feeds back a measured value to the control circuit  29   a  or  29   b,  and a D/A converter  33  and an amplifier  34  which control the main deflector  26  in accordance with a signal from the control circuit  29   a  or  29   b.    
         [0041]     The control circuit  29   a  or  29   b  performs control based on pattern data read out from the storage device  28  so that the charged particle beam emitting device  20  operates to shape the cross section of the charged particle beam EB. The control circuit  29   a  or  29   b  also does likewise so that the D/A converter  30  and the current-voltage converter  31  operate to apply a voltage to the auxiliary deflector  27 , and so that the D/A converter  33  and the amplifier  34  operate to feed a current to the main deflector  26 . Thereby, an exposure position is determined on the wafer  25 . The D/A converter  33  that controls the main deflector  26  has the same configuration as D/A conversion circuits disposed in the D/A converter  30  that controls the auxiliary deflector  27 , and the D/A converter  33  is provided with a register REG 5 .  
         [0042]     Description will now be given below with reference to the drawings with regard to detailed configurations of the control circuit  29   a,  the D/A converter  30  that controls the auxiliary deflector  27 , the current-voltage converter  31 , and the DVM  32 .  
         [0043]      FIG. 2  is a block diagram showing relative connections of the control circuit  29   a,  the D/A converter  30  that controls the auxiliary deflector  27 , the current-voltage converter  31 , and the DVM  32 .  
         [0044]     As shown in  FIG. 2 , the control circuit  29   a  is configured mainly of a register (REG 3 )  101 , a serial-parallel conversion circuit  102 , a decision circuit  104 , a correction code table  105 , a pointer  106 , an adder circuit  107 , a register (REG 4 )  108 , and a clock generator circuit  103 . These circuits and the like, and the D/A converter  30  constitute the D/A conversion device.  
         [0045]     The control circuit  29   a  is provided with, besides the above circuits, a circuit which outputs a deflection control code to the D/A converter  33  that controls the main deflector  26 , and others, although they are not shown in  FIG. 2 .  
         [0046]     The register (REG 3 )  101  receives input of plural bits of serial data (or digital data) SDAT as pattern data from the storage device  28 . The serial-parallel conversion circuit  102  converts the serial data SDAT outputted by the register (REG 3 )  101  into parallel data (or digital data), which in turn is outputted to a first D/A conversion circuit  11 .  
         [0047]     Also, the decision circuit  104  determines whether each of the bits of the serial data SDAT outputted by the register (REG 3 )  101  is “0” or “1” in accordance with a signal from the pointer  106 . The correction code table  105  stores correction codes CC corresponding to the respective bits, and outputs the correction codes CC corresponding to the bits judged as “i” by the decision circuit  104 . The adder circuit  107  performs serial addition of the correction codes CC corresponding to the respective bits, and outputs their sum to the register (REG 4 )  108 . The register (REG 4 )  108  serially stores sum data obtained by adding the correction codes CC, and in the end, stores the correction code CC obtained by adding the correction codes CC for every bit of the digital data.  
         [0048]     The D/A converter  30  is configured of a first D/A conversion circuit  11  which converts the digital data outputted by the serial-parallel conversion circuit  102  into an electric output signal I, and a second D/A conversion circuit  12  which converts the correction code CC obtained by adding the correction codes CC for all bits of the digital data into an electric correction signal Ic. Both the first and second D/A conversion circuits  11  and  12  are current output mode circuits. The first D/A conversion circuit  11  is configured of an R-2R ladder resistor network for the low-order 12 bits D 11  to D 0  and a decoder for the high-order 4 bits D 15  to D 12 , as shown in  FIG. 10 . The first D/A conversion circuit  11  has general circuit configuration and functions. Incidentally, the first D/A conversion circuit  11  is configured to adjust the values of resistances  400  to  411  and  500  to  511  and output currents from constant current sources  300  to  315  in accordance with the current output level I as employed in the present invention. The second D/A conversion circuit  12  also has the same circuit configuration and functions as the first D/A conversion circuit  11 . Incidentally, the second D/A conversion circuit  12  is also configured to adjust the values of resistances and the constant current sources in accordance with the correction current value Ic.  
         [0049]     The D/A conversion circuits  11  and  12  are provided with registers REG 1  and REG 2 , respectively. As shown in  FIG. 2 , the registers REG 1  and REG 2  store the digital data and the correction codes CC, respectively, in accordance with a clock from the clock generator circuit  103  of the control circuit  29   a.    
         [0050]     Output terminals of the D/A conversion circuits  11  and  12  are connected to form a common output terminal, which is connected to an input terminal of the current-voltage converter  31 . Under control of the clock from the clock generator circuit  103 , the electric output signal I and the electric correction signal Ic are outputted to produce output of a current J whose errors are corrected, as shown in  FIG. 2 . Then, the current-voltage converter  31  converts the current J into a deflecting voltage, which in turn is outputted.  
         [0051]     An output terminal of the current-voltage converter  31  is connected to an input terminal of the digital volt meter (DVM)  32 , and an output terminal of the digital volt meter  32  is connected to a digital voltage (DV) input terminal of the control circuit  29   a.  This channel can be used to obtain the correction code. The correction code is obtained in a manner as given below. Specifically, the first D/A conversion circuit  11  converts digital data with bits (0 to n) sequentially set to “1” into electric output signals DV 0  to DVn, respectively, which in turn are fed back to the control circuit  29   a.  In this case, variations from an ideal output line, in general, occur due to variations in the resistance values of the D/A conversion circuit and the supply currents therefrom. Deviations Δ 0  to Δn (namely, voltages or currents) of the measured values of the electric output signals DV 0  to DVn from the ideal output line are therefore determined as shown in  FIG. 3 . The deviations Δ 0  to Δn are digitized to form correction codes, which in turn are prestored in the correction code table  105 . The correction codes are given in  FIG. 6 . Incidentally, the ideal output line is set so that the sum of correction codes for data DAT with all bits set to “1,” Δ 0 +Δ 1 + . . . +Δ 10 +Δ 11 + . . . +Δ 26 , is equal to 0.  
         [0052]     Operation of the control section will now be described below with reference to  FIGS. 4 and 5 .  
         [0053]      FIG. 4  is a timing chart showing the flows of signals indicative of digital data (DAT), correction codes (CC), clock pulses (CLK), data stored in the register (REG 1 ), data stored in the register (REG 2 ), output currents I, and correction currents Ic.  FIG. 5  is a flowchart mainly showing a procedure for obtaining correction codes CC for digital data.  
         [0054]     As shown in  FIG. 4 , first digital data is inputted to the control circuit  29   a,  the first digital data (DAT 0 ) is held in the serial-parallel conversion circuit  102 , and a first correction code (CC 0 ) corresponding to the first digital data (DAT 0 ) is held in the register (REG 4 ). Under control of a first clock (CLK), the first digital data (DAT 0 ) is held in the register (REG 1 ) of the D/A conversion circuit  11 , and the first correction code (CC 0 ) is held in the register (REG 2 ) of the D/A conversion circuit  12 . Then, an output current I 0  is outputted by the D/A conversion circuit  11 , and a correction current Ic 0  is outputted by the D/A conversion circuit  12 . After that, the output current I 0  is corrected by the correction current Ic 0 , and the corrected current (J) is converted into a voltage, which in turn is outputted to the auxiliary deflector.  
         [0055]     Before a next clock (CLK), second digital data is inputted to the control circuit  29   a,  the second digital data (DAT 1 ) is held in the serial-parallel conversion circuit  102 , and a second correction code (CC 1 ) corresponding to the second digital data (DAT 1 ) is held in the register (REG 4 ). Under control of a second clock (CLK), the second digital data (DAT 1 ) is held in the register (REG 1 ) of the D/A conversion circuit  11 , and the second correction code (CC 1 ) is held in the register (REG 2 ) of the D/A conversion circuit  12 . Then, an output current I 1  is outputted by the D/A conversion circuit  11 , and a correction current Ic 1  is outputted by the D/A conversion circuit  12 . After that, the output current I 1  is corrected by the correction current Ic 1 , and the corrected current (J) is converted into a voltage, which in turn is outputted to the auxiliary deflector.  
         [0056]     Thereafter, processing is performed on sequentially incoming digital data for deflection control to thereby sequentially convert the digital data into corresponding voltages. The auxiliary deflector is operated under control of the voltages.  
         [0057]     In this case, the correction code (CC) is generated within the control circuit  29   a  through the following steps P 1  to P 8  as shown in  FIG. 5 .  
         [0058]     (P 1 ) First, the correction codes derived from the deviations Δ 0  to Δn of the measured values of the electric output signals DV 0  to DVn from the ideal output line are stored in the correction code table  105 .  
         [0059]     (P 2 ) Then, 0 is assigned to p, where p denotes the value of the pointer  106 .  
         [0060]     (P 3 ) Then, 1 bit of the input serial data SDAT, corresponding to the value p of the pointer  106 , is stored in the register (REG 3 )  101 .  
         [0061]     (P 4 ) Then, the decision circuit  104  determines the bit value of the register (REG 3 )  101 , pointed to by the pointer  106 .  
         [0062]     (P 5 ) Then, when the bit value is “0,” the variable p is incremented by 1, and the processing returns to step P 3 . When the bit value is “1,” the processing proceeds to step P 6 .  
         [0063]     (P 6 ) Then, an error correcting code for the corresponding bit is read by referring to the error correcting code table  105  shown in  FIG. 6 , the error correcting code is added to the previous sum stored in the register (REG 4 )  108 , and the resultant sum is stored in the register (REG 4 )  108 .  
         [0064]     (P 7 ) Then, when p is less than r (p&lt;r), the variable p is incremented by 1, and the processing returns to step P 3 . When p is equal to r (p=r), the processing proceeds to step P 8 . As employed herein, r denotes the number of bits of the data SDAT minus one. As employed in the first embodiment, r is equal to 15 (r=15).  
         [0065]     (P 8 ) Then, the value of the register (REG 4 )  108  is fed to the second D/A conversion circuit  12 , as the correction code CC. In a case where SDAT=“1011010011001011”, for example, the error correcting codes corresponding to bits set to “1” are as follows. 
    Error correcting code for bit 2 15  . . . 1101000110010111     Error correcting code for bit 2 13  . . . 0111100001000001     Error correcting code for bit 2 12  . . . 1010111001100011     Error correcting code for bit 2 10  . . . 0110001111100110     Error correcting code for bit 2 7  . . . 0000110000000010     Error correcting code for bit 2 6  . . . 0000011000111110     Error correcting code for bit 2 3  . . . 1101001110010010     Error correcting code for bit 2 1  . . . 1001010011011001     Error correcting code for bit 2 0  . . . 0100001001110010    
 
         [0075]     All these correcting codes are added, and an MSB (most significant bit) is determined. As a result, the correction code CC is “1000001100100111.” 
         [0076]     As described above, the control circuit  29   a  and the D/A converter  30  for use in the charged particle beam exposure apparatus according to the first embodiment of the present invention are configured so that the control circuit  29   a  is provided with the error correcting code table  105  that stores the error correcting codes corresponding to bits. This eliminates the need for storing the correction codes for all incoming digital data, thus eliminating the need for an expensive memory of wasteful access time. Moreover, the execution of steps P 3  to P 8  makes it possible to determine the correction code for input digital data both at high speed and in real time.  
       Second Embodiment  
       [0077]     Description will now be given with reference to  FIG. 7  with regard to the control circuit  29   b,  the D/A converter  30  that controls the auxiliary deflector  27 , the current-voltage converter  31 , and the DVM  32  for use in a charged particle beam exposure apparatus according to a second embodiment of the present invention.  
         [0078]      FIG. 7  is a block diagram showing the control circuit  29   b,  the D/A converter  30  that controls the auxiliary deflector  27 , the current-voltage converter  31 , and the DVM  32 .  
         [0079]     The control circuit  29   b  and others according to the second embodiment are different from the control circuit  29   a  and others according to the first embodiment shown in  FIG. 2  in that the former uses an expanded correction code table  114  which stores correction codes each for two bits, in place of the error correcting code table  105  that stores the correction codes each for one bit. Thus, the control circuit  29   b  and others are different from the control circuit  29   a  and others in that the former obtains expanded correction codes each for 2 bits of input serial data. Incidentally, as shown in  FIG. 7 , circuits and others in the control circuit  29   b  and the D/A converter  30  constitute a D/A conversion device.  
         [0080]     In this case, the expanded correction codes are obtained in a manner as given below. Specifically, the first D/A conversion circuit  11  converts digital data with bits (0 to n) sequentially set to “1” into electric output signals DV 0  to DVn, respectively, which in turn are fed back to the control circuit  29   b.  In a manner as shown in  FIG. 3 , the deviations Δ 0  to Δn of the measured values of the electric output signals DV 0  to DVn from the ideal output line are determined and then digitized. The deviations Δ 0  to Δn are each processed into a 16-bit digital data. Additions are performed based on the deviations Δ 0  to Δn. Specifically, the deviations Δ 0  to Δn are added for combinations of the low-order or high-order 2 bits, that is, (“0”, “0”), (“0”, “1”), (“1”, “0”) and (“1”, “1”). For example, when the combination of bit  1  and bit  0  is (“1”, “0”), addition is performed to determine the deviation (Δ 1 ) of bit  1  plus the deviation (zero) of bit  0 . When the combination is (“1”, “1”), addition is performed to determine the deviation (Δ 1 ) of bit  1  plus the deviation (Δ 0 ) of bit  0 . Calculations are likewise performed for other combinations. The expanded correction codes are given in  FIG. 9 . The expanded correction codes determined in the manner as above mentioned are prestored in the expanded correction code table  114 .  
         [0081]     Description will now be given below with reference to  FIG. 8  with regard to the operation of the control section and particularly to operation for generating the correction code CC based on the expanded correction codes. Since the general operation of the control section as a whole is the same as the operation shown in the timing chart of  FIG. 4 , description is omitted in this respect.  
         [0082]     (P 11 ) First, the expanded correction codes, each of which is obtained for every 2 bits, based on the deviations Δ 0  to Δn of the measured values of the electric output signals DV 0  to DVn from the ideal line, are stored in the expanded correction code table  114 .  
         [0083]     (P 12 ) Then, the low-order 2 bits of input serial data SDAT are stored in a register (REG 3 )  111 . 1 is assigned to s, where s denotes the value of a pointer  115 .  
         [0084]     (P 13 ) Combinations of the 2 bits in the register (REG 3 )  111  are compared to combinations of the bits in the expanded correction code table  114  pointed to by the pointer  115 , the corresponding expanded correction code is added to a register (REG 4 )  117 , and the resultant sum is stored in the register (REG 4 )  117 .  
         [0085]     (P 14 ) Then, the processing goes to step P 15  when s is less than t (s&lt;t), or the processing goes to step P 16  when s is equal to t (s=t), where t denotes the number of bits of the data SDAT divided by the number of bits, the unit of which undergoes serial arithmetic operation. As employed in the second embodiment, t is equal to 8 (t=8).  
         [0086]     (P 15 ) Then, the next 2 bits of the serial data SDAT are stored in the register (REG 3 )  111 , which in turn is shifted right two bit positions. Also, the variable s is incremented by 1, and the processing returns to step P 13 .  
         [0087]     (P 16 ) Then, the value of the register (REG 4 )  117  as the correction code CC is fed to the second D/A conversion circuit  12 . In a case where SDAT=“0011001101010010”, for example, the expanded correction codes each corresponding to 2 bits, referring to the expanded correction code table  114  shown in  FIG. 9  are as follows. 
    Expanded correction code for bits 2 15  and 2 14  . . . 0000000000000000     Expanded correction code for bits 2 13  and 2 12  . . . 0100100111011110     Expanded correction code for bits 2 11  and 2 10  . . . 0000000000000000     Expanded correction code for bits 2 9  and 2 8  . . . 0111001111101000     Expanded correction code for bits 2 7  and 2 6  . . . 0000011000111110     Expanded correction code for bits 2 5  and 2 4  . . . 0110100001001001     Expanded correction code for bits 2 3  and 2 2  . . . 0000000000000000     Expanded correction code for bits 2 1  and 2 0  . . . 1001010011011001    
 
         [0096]     All these expanded correction codes each for 2 bits are added, and an MSB is determined. As a result, the correction code CC for the digital data is “1110000010010011.” 
         [0097]     As described above, the control circuit  29   b  and the D/A converter  30  for use in the charged particle beam exposure apparatus according to the second embodiment of the present invention are configured so that the control circuit  29   b  is provided with the expanded correction code table  114  that stores the correction codes each corresponding to plural bits. As in the case of the first embodiment, this eliminates the need for holding the correction codes for all incoming digital data and thus eliminates the need for an expensive memory of wasteful access time. Moreover, the execution of steps P 11  to P 16  makes it possible to determine the correction code for input digital data both at high speed and in real time.  
         [0098]     Moreover, serial arithmetic operation is sequentially performed on every plural bits of input serial data, so that the number of calculations can be reduced. This makes it possible to serially calculate the expanded correction codes each for the plural bits on all bits of the input serial data, thus enabling a further reduction in the number of calculations. This leads to higher operation speed.  
         [0099]     Furthermore, the control circuit  29   b  and the D/A converter  30  having the above configuration, as applied to a deflector for charged particle beam scan, can achieve both higher accuracy of exposure position and shorter exposure standby time without using a large-capacity memory.  
         [0100]     While this invention has been described in detail above in connection with certain exemplary embodiments, it is to be understood that the subject matter encompassed by way of this invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of this invention.  
         [0101]     For example, the first D/A conversion circuit  11 , as employed in the above embodiments, is configured of the R-2R ladder resistor network for the low-order  12  bits and the decoder for the high-order 4 bits as shown in  FIG. 10 , but the D/A conversion circuit  11  is not limited to this. The first D/A conversion circuit  11  may be configured of the R-2R ladder resistor network or the decoder for all bits. The same goes for the second D/A conversion circuit  12 .