Patent Number: 
Section: claims

1. A charged particle beam writing apparatus which divides a predetermined region on which writing is effected by a charged particle beam into a plurality of region groups and writes a pattern on the predetermined region while compensating the CPB drift at regular time intervals in accordance with a time-profile, said time interval being determined for each region group, said charged particle beam writing apparatus comprising:grouping means for dividing said predetermined region into smaller regions each consisting of one or the same number of frames, obtaining data indicative of the areal density of a pattern to be written on each smaller region, determining the amount of change in pattern areal density between each two adjacent smaller regions, and grouping said smaller regions into region groups depending on whether or not said amount of change is greater than a predetermined value. 2. The charged particle beam writing apparatus according to claim 1, further comprising:areal density determining means for dividing said predetermined region into smaller regions each consisting of one or the same number of frames, and determining the areal density of a pattern to be written on each smaller region. 3. The charged particle beam writing apparatus according to claim 1, wherein:said predetermined region includes a first predetermined region and a second predetermined region, and said writing is to be performed on said first and second predetermined regions in that order; andthe amounts of change in pattern areal density determined by said grouping means include the amount of change in pattern areal density between the last smaller region of said first predetermined region on which said writing is to be performed and the first smaller region of said second predetermined region on which said writing is to be performed. 4. The charged particle beam writing apparatus according claim 3, further comprising:areal density determining means for dividing said predetermined region into smaller regions each consisting of one or the same number of frames, and determining the areal density of a pattern to be written on each smaller region.