Patent Number: 
Section: claims

1. A method for correcting drifts of an electron beam, comprising:periodically correcting drift of the electron beam once per time period while varying the time period in length;measuring a value of a specified disturbance factor; andcorrecting, in addition to the correction per time period, the drift of the electron beam regardless of elapse of said time period when a change in value of the specified disturbance factor occurs with a prespecified change amount,wherein the specified disturbance factor is at least one of a temperature, a magnetic field, a flow rate of coolant water, and a power supply voltage,a drift correcting interval for the correction per time period is kept shorter soon after a startup of pattern writing and the drift correcting interval is made longer as the pattern writing proceeds with time. 2. The method according to claim 1, wherein a relation of said specified disturbance factor and the drift of the electron beam is measured beforehand off-line, and said prespecified change amount is determined as an amount of change of said specified disturbance factor occurring when the drift reaches a permissible limit. 3. The method according to claim 1, wherein said method is used as one of processes to be performed by a pattern writing apparatus using the electron beam to write a pattern and wherein when correction is done by a change in the value of said disturbance factor, the electron beam drift correction is done after completion of writing of a certain unit part. 4. The method according to claim 1, wherein during the periodical correction per unit of the time period, said time period is shortened upon occurrence of the change of the disturbance factor value with the prespecified change amount. 5. An electron beam drift correction method comprising:(a) periodically correcting drift of an electron beam once per elapse of a time period while varying the time period in length;(b) measuring a value of a trigger; and(c) providing the trigger for start-up of drift correction of the electron beam in a way independent of step (a),wherein at least one of values of a temperature, a magnetic field, a flow rate of coolant water, and a power supply voltage is used as the trigger,a drift correcting interval for the correction per time period is kept shorter soon after a startup of pattern writing and the drift correcting interval is made longer as the pattern writing proceeds with time. 6. A method for correcting drift of an electron beam after elapse of a specified time period from startup of irradiation of the electron beam, said method comprising:periodically correcting drift of the electron beam once per time period while varying the time period in length;measuring a value of a certain disturbance factor;when the measured value of said disturbance factor reaches a predetermined change amount, measuring a drift degree of the electron beam; andusing the measured drift degree of said electron beam to correct a deflection amount thereof,wherein the disturbance factor is at least one of a temperature, a magnetic field, a flow rate of coolant water, and a power supply voltage,a drift correcting interval for the correction per time period is kept shorter soon after a startup of pattern writing and the drift correcting interval is made longer as the pattern writing proceeds with time. 7. An electron beam writing method comprising:writing using an electron beam deflectable by a deflector;periodically correcting drift of the electron beam once per time period while varying the time period in length;during the writing, measuring a value of certain disturbance factor;when the measured value of the disturbance factor reaches a predefined change level, measuring a drift amount of the electron beam;correcting a deflection degree of said electron beam based on the measured drift amount of the electron beam;dividing said predefined change level into first and second sublevels, the second sublevel being greater than the first sublevel;when a change in the value of the disturbance factor reaches said first sublevel, correcting a drift amount of the electron beam after having written a unit part of said pattern; andwhen the disturbance value change reaches said second sublevel, immediately finishing the pattern writing,wherein the disturbance factor is at least one of a temperature, a magnetic field, a flow rate of coolant water, and a power supply voltage, anda drift correcting interval for the correction per time period is kept shorter soon after a startup of pattern writing and the drift correcting interval is made longer as the pattern writing proceeds with time. 8. The method according to claim 7, further comprising:when a change in value of the disturbance factor reaches said predefined change level, temporarily pausing the pattern writing after having written a unit part of a writing area; andafter the disturbance change value returns to fall within a preset range, measuring the drift amount of said electron beam. 9. An electron beam writing method comprising:writing by use of an electron beam being deflected by a deflector;periodically correcting drift of the electron beam once per time period while varying the time period in length;during the writing, measuring a value of a chosen disturbance factor;correcting, in addition to the correction per time period, the drift of the electron beam regardless of elapse of said time period when a change in value of the disturbance factor occurs with a prespecified change amount;when the measured value of the disturbance factor reaches a predefined change level, measuring a deflection position of the electron beam within a deflection range of said deflector;dividing said predefined change level into first and second sublevels, the second sublevel being greater than the first sublevel;when a change in the value of the disturbance factor reaches said first sublevel, measuring the deflection position of the electron beam after having written a unit part of said pattern; andwhen the disturbance value change reaches said second sublevel, immediately terminating the writing,wherein the disturbance factor is at least one of a temperature, a magnetic field, a flow rate of coolant water, and a power supply voltage, anda drift correcting interval for the correction per time period is kept shorter soon after a startup of pattern writing and the drift correcting interval is made longer as the pattern writing proceeds with time. 10. The method according to claim 9, wherein the deflection position of said electron beam is measured after completion of the writing of a unit part of a writing area. 11. The method according to claim 9, further comprising:when a change in the value of said disturbance factor reaches said predefined change level, temporarily stopping the writing after having written a unit part of a writing area; andafter said value of the disturbance factor returns to fall within a preset value range, measuring a deflection position of said electron beam. 12. The method according to claim 9, wherein the value of said disturbance factor is at least one of values of an outside atmospheric pressure, a temperature, a magnetic field, a flow rate of coolant water and a power supply voltage. 13. The method according to claim 9, further comprising:modifying a deflection coefficient of said deflector based on the measured deflection position of said electron beam. 14. The method according to claim 13, wherein when a deviation of the measured deflection position of said electron beam exceeds a predefined value, correction is made to the deflection coefficient of said deflector. 15. An electron beam writing method comprising:writing by an electron beam on a workpiece with its position in a Z direction being detected by a Z sensor;periodically correcting drift of the electron beam once per time period while varying the time period in length;during the writing, measuring a value of a disturbance factor;correcting, in addition to the correction per time period, the drift of the electron beam regardless of elapse of said time period when a change in value of the disturbance factor occurs with a prespecified change amount;when the value of the disturbance factor thus measured reaches a predetermined change level, using a mark as provided on or above a stage with said workpiece being placed thereon to measure a position of the mark in the Z direction;correcting an offset value of said Z sensor based on the measured position in said Z direction;dividing said predefined change level into first and second sublevels, the second sublevel being greater than the first sublevel;when a change in the value of the disturbance factor reaches said first sublevel, measuring the position of the mark in the Z direction after having written a unit part of a writing area; andwhen the disturbance value change reaches said second sublevel, immediately terminating the writing,wherein the disturbance factor is at least one of a temperature, a magnetic field, a flow rate of coolant water, and a power supply voltage, anda drift correcting interval for the correction per time period is kept shorter soon after a startup of pattern writing and the drift correcting interval is made longer as the pattern writing proceeds with time. 16. The method according to claim 15, wherein the value of said disturbance factor is at least one of values of a temperature, a magnetic field, a flow rate of coolant water and a power supply voltage.