Patent Number: 
Section: claims

1. An electron beam writing apparatus which has a holding mechanism for holding a mask at a back surface thereof and applies an electron beam onto a surface of the mask held by the holding mechanism by electron beam applying means thereby writing a desired pattern, comprising:an XY stage movable in a direction orthogonal to the direction of an optical axis of the electron beam;an electron-beam focal adjusting mark table fixed onto the XY stage;a Z stage mounted onto the XY stage in avoidance of an area to which the mark table is fixed, and movable in the optical axis direction; andmeasuring means for measuring a height of the mark table and a height of the mask placed on the holding mechanism,wherein the electron beam applying means includes a focal adjustment mechanism for varying a focal height of the electron beam within a predetermined adjustable range and a middle value of the adjustable range coincides with the height of the mark table,wherein the mask holding mechanism includes difference calculating means placed on the Z stage for calculating a difference between the height of the mark table measured by the height measuring means and the height of the mask placed on the holding mechanism, and Z stage control means for moving the Z stage based on information about the calculated difference in such a manner that the height of the mask coincides with the height of the mark table, andwherein the height of the mask used in the calculation of the difference calculating means is a middle value between highest and lowest values of heights of a plurality of measurement points, which are obtained by measuring the plurality of measurement points on the mask by the height measuring means. 2. The electron beam writing apparatus according to claim 1, wherein the measurement points on the mask measured by the height measuring means are at least five spots corresponding to four corners of the mask and the center thereof. 3. The electron beam writing apparatus according to claim 1, further including inclination calculating means for calculating an inclination of the mask placed on the holding mechanism,wherein the Z stage includes inclination correcting means for correcting the inclination of the mask based on information about the calculated inclination in such a manner that the surface of the mask becomes orthogonal to the optical axis of the electron beam. 4. The electron beam writing apparatus according to claim 2, further including inclination calculating means for calculating an inclination of the mask placed on the holding mechanism,wherein the Z stage includes inclination correcting means for correcting the inclination of the mask based on information about the calculated inclination in such a manner that the surface of the mask becomes orthogonal to the optical axis of the electron beam. 5. The electron beam writing apparatus according to claim 3, wherein the Z stage equipped with the inclination correcting means comprises three or more support mechanisms movable independently with respect to the optical axis direction. 6. The electron beam writing apparatus according to claim 4, wherein the Z stage equipped with the inclination correcting means comprises three or more support mechanisms movable independently with respect to the optical axis direction. 7. The electron beam writing apparatus according to claim 1, wherein the holding mechanism comprises an electrostatic chuck for adsorbing the back surface of the mask. 8. An electron beam writing method for applying an electron beam to a surface of a mask held by a holding mechanism for holding the mask at a back surface thereof by electron beam applying means by writing a desired pattern, comprising the steps of:fixing an electron beam focal adjusting mark table to an XY stage movable in a direction orthogonal to the direction of an optical axis of the electron beam, mounting a Z stage movable in the optical axis direction on the XY stage in avoidance of an area to which the mark table is fixed, and placing the holding mechanism on the Z stage;adjusting the electron beam applying means in such a manner that a middle value of a predetermined adjustable range of a focal height of the electron beam, said range being varied by a focal adjustment mechanism provided in the electron beam applying means, coincides with a height of the mark table; andmeasuring the height of the mark table, measuring heights of a plurality of measurement points on the mask placed on the holding mechanism, setting a middle value between highest and lowest values of the heights of the measurement points as a measured height of the mask, comparing the measured height of mark table and the measured height of mask to calculate a difference therebetween, and movably controlling the Z stage based on information about the calculated difference in such a manner that the height of the mask coincides with the height of the mark table. 9. The electron beam writing method according to claim 8, wherein the plurality of measurement points are at least five spots corresponding to four corners of the mask and the center thereof. 10. The electron beam writing method according to claim 8, further including an inclination correcting step for calculating an inclination of the surface of the mask from the heights of the plurality of measurement points on the mask placed on the holding mechanism and controlling inclination correcting means provided in the Z stage, based on information about the calculated inclination in such a manner that the surface of the mask becomes orthogonal to the optical axis of the electron beam. 11. The electron beam writing method according to claim 9, further including a step for calculating an inclination of the surface of the mask from the heights of the plurality of measurement points on the mask placed on the holding mechanism, and controlling inclination correcting means provided in the Z stage, based on information about the calculated inclination in such a manner that the surface of the mask becomes orthogonal to the optical axis of the electron beam. 12. The electron beam writing method according to claim 10, wherein the inclination correcting step is performed after the height adjusting step. 13. The electron beam writing method according to claim 11, wherein the inclination correcting step is performed after the height adjusting step. 14. The electron beam writing method according to claim 10, wherein the height adjusting step is performed after the inclination correcting step. 15. The electron beam writing method according to claim 11, wherein the height adjusting step is performed after the inclination correcting step.