Device and method for computing amount of drift and charged particle beam system

1. A drift amount computing device comprised of hardware executing software programs and, optionally, at least in part by dedicated circuitry for computing an amount of drift between a first image and a second image, said drift amount computing device comprising:

a correlation function computing section for calculating a correlation function between the first and second images;

a local maximum position searching section for searching a range of positions of the correlation function for local maximum positions at which the correlation function assumes its local maxima;

a local maximum position determining section for assigning weights to intensities of the local maximum positions searched for by the local maximum position searching section according to the distance from the center of the correlation function, comparing the weighted intensities, and determining one of the local maximum positions which corresponds to the amount of drift; and

a drift amount computing section for computing the amount of drift, based on the local maximum position corresponding to the amount of drift,

wherein said plural local maximum positions searched for by said local maximum position searching section are two in number and include a first local maximum position and a second local maximum position which have intensities A and B, respectively, and

wherein said local maximum position determining section takes the first local maximum position as a candidate which is selected from the local maximum positions and which corresponds to the amount of drift if the distance between the first local maximum position and the center is greater than the distance between the second local maximum position and the center and, at the same time, a relationship given by B×α<A (where α>1) is satisfied, and takes the first local maximum position as a candidate which is selected from the local maximum positions and which corresponds to the amount of drift if the distance between the first local maximum position and the center is smaller than the distance between the second local maximum position and the center and, at the same time, a relationship given by B<A×α is satisfied.

2. The drift amount computing device as set forth in claim 1 , wherein said correlation function is a cross-correlation function or a phase-only correlation function.

3. The drift amount computing device as set forth in claim 1 , wherein said local maximum position searching section searches for the local maximum positions in turn first from the position least distant from said center of the correlation function, and wherein said local maximum position determining section assigns weights to the intensities of the local maximum positions in turn in the order in which the local maximum positions were searched for by the local maximum position searching section according to the distance from said center and compares the weighted intensities.

4. The drift amount computing device as set forth in claim 1 , further comprising a drift correction amount computing section for calculating an amount of drift correction by which to correct the drift between the first and second images, based on the amount of drift.

5. A method of computing an amount of drift between a first image and a second image, comprising the steps of:

calculating a correlation function between the first and second images;

searching a range of positions of the correlation function for local maximum positions at which the correlation function assumes its local maxima;

assigning weights to intensities of the local maximum positions searched for by the step of searching a range of positions according to the distance from the center of the correlation function, comparing the weighted intensities of the local maximum positions, and determining one of the local maximum positions which corresponds to the amount of drift; and

computing the amount of drift, based on the local maximum position corresponding to the amount of drift,

wherein said plural local maximum positions searched for by the step of searching a range of positions are two in number and include a first local maximum position and a second local maximum position which have intensities A and B, respectively, and

wherein during the step of determining a local maximum position, if the distance between the first local maximum position and the center is greater than the distance between the second local maximum position and the center and, at the same time, a relationship given by B×α<A (where α<1) is satisfied, then the first local maximum position is taken as a candidate which is selected from the local maximum positions and which corresponds to the amount of drift and if the distance between the first local maximum position and the center is smaller than the distance between the second local maximum position and the center and, at the same time, a relationship given by B<A×α is satisfied, then the first local maximum position is taken as a candidate which is selected from the local maximum positions and which corresponds to the amount of drift.

6. The method of computing an amount of drift as set forth in claim 5 , wherein said correlation function is a cross-correlation function or a phase-only correlation function.

7. The method of computing an amount of drift as set forth in claim 5 , wherein during the step of searching a range of positions, the local maximum positions are searched for in turn first from the position least distant from the center of the correlation function, and wherein during the step of determining a local maximum position, weights are assigned in turn to the intensities of the local maximum positions according to the distance from said center in the order in which the local maximum positions were searched for during the step of searching a range of positions and the weighted intensities of the local maximum positions are compared.

8. The method of computing an amount of drift as set forth in claim 5 , further comprising the step of calculating an amount of drift correction by which to correct the drift between the first and second images, based on said amount of drift.

9. A charged particle beam system including a drift amount computing device as set forth in claim 1 .