Method for measuring dimensions and optical system using the same

1. A method of measuring dimensions for an optical system so as to measure a critical dimension of a sample object comprising:

preparing a plurality of standard objects, each of said plurality of standard objects having a grating-shaped standard pattern with a predetermined pitch and line width;

selecting a predetermined focus metric algorithm;

performing an analyzing process on each of said plurality of standard objects comprising:

moving the standard object to a plurality of predetermined off-focus positions with respect to an on-focus position of the optical system;

capturing a plurality of off-focus images of the standard object on the predetermined off-focus positions using the optical system; and

analyzing the off-focus images using the predetermined focus metric algorithm to generate a plurality of distributions of focus metric values on the predetermined off-focus positions, each of said plurality of distributions of focus metric values having at least one extreme value;

analyzing said plurality of distributions of focus metric values to determine target orders from the extreme value and a plurality of characteristic values corresponding to the target orders of said plurality of distributions of focus metric values;

generating a reference relation correlating the characteristic values with the line widths of the standard objects;

measuring the sample object to acquire a measured characteristic value; and

determining the critical dimension of the sample object based on the measured characteristic value and the reference relation.

2. The method of measuring dimensions of claim 1 , wherein the step of generating a reference relation comprises curve fitting to generate a relation curve correlating the characteristic values with the line widths of the standard objects.

3. The method of measuring dimensions of claim 1 , wherein the step of generating the reference relation further comprises correlating the characteristic values with the pitches of the standard objects.

4. The method of measuring dimensions of claim 1 , wherein the target order is one of a zero order peak, a positive first order peak, and a negative first order peak.

5. The method of measuring dimensions of claim 1 , wherein the characteristic values are obtained by the focus metric algorithm and correspond to the extreme values of the target orders.

6. The method of measuring dimensions of claim 1 , wherein the characteristic values are obtained by the focus metric algorithm and correspond to one of the predetermined off-focus positions in the target order.

7. The method of measuring dimensions of claim 1 , wherein the predetermined off-focus positions use the on-focus position as a reference position.

8. The method of measuring dimensions of claim 1 , wherein the target order is determined by the steps of:

normalizing the focus metric distribution based a zero order peak thereof; and

selecting a peak or a valley other than the zero order peak as the target order.

9. The method of measuring dimensions of claim 8 , wherein the characteristic values are obtained by the focus metric algorithm and correspond to peak values of the target orders.

10. The method of measuring dimensions of claim 8 , wherein the focus metric algorithm is one of a gradient energy method, a Laplacian method, a standard deviation method, and a contrast method.

11. A method of measuring dimensions for an optical system to measure a critical dimension of a sample object comprising:

using the optical system to capture a plurality of off-focus images from a plurality of objects, each object having a grating-shaped standard pattern with a predetermined pitch and a plurality of line widths;

using an algorithm to analyze the off-focus images to generate a plurality of characteristic distributions corresponding to the line widths;

determining characteristic values for the plurality of characteristic distributions to generate a relation curve of the characteristic values and the line widths;

measuring the sample object to acquire a measured characteristic value; and

determining a critical dimension of the sample object based on the measured characteristic value and the relation curve.

12. The method of measuring dimensions of claim 11 , wherein the plurality of characteristic distributions are obtained by a focus metric algorithm.

13. The method of measuring dimensions of claim 11 , wherein the algorithm is one of a gradient energy method, a Laplacian method, a standard deviation method, and a contrast method.

14. The method of measuring dimensions of claim 11 , wherein each characteristic distribution includes a plurality of peaks and valleys.

15. The method of measuring dimensions of claim 14 , wherein the characteristic values for the characteristic distribution correspond to one of peaks and valleys.

16. The method of measuring dimensions of claim 14 , wherein the characteristic values are the peaks or the valleys other than a zero order peak of the plurality of characteristic distributions after normalizing the plurality of characteristic distributions.

17. The method of measuring dimensions of claim 14 , wherein the characteristic values for the plurality of characteristic distributions are peaks or valleys of the characteristic distributions before a normalizing process.

18. An optical system of measuring dimensions of a sample object comprising:

a database configured to store at least one characteristic relation curve of a pitch, the characteristic relation curve correlating a characteristic value with the pitch; and

a measuring unit configured to acquire a measured characteristic value of the sample object, wherein a critical dimension of the sample object is determined based on the measured characteristic value and the characteristic relation curve;

using the optical system to capture a plurality of off-focus images from a plurality of objects, each object having a grating-shaped standard pattern with a predetermined pitch and a plurality of line widths; and an algorithm to analyze the off-focus images to generate a plurality of characteristic distributions corresponding to the line widths.

19. The optical system of measuring dimensions of claim 18 , wherein the characteristic value is an extreme value obtained by a focus metric algorithm.

20. The optical system of measuring dimensions of claim 18 , wherein the characteristic value corresponds to one of peaks or valleys of the characteristic relation curve.