1. A computer-implemented image pattern matching method for wafer alignment, for determining an overall similarity value and an overall geometry relationship between a target wafer image and a model wafer image, the method comprising: determining a plurality of model patterns in the model wafer image; searching the target wafer image to identify a plurality of target patterns, one of the plurality of target patterns matching a corresponding one of the plurality of model patterns, thereby generating a plurality of matches each including a respective target pattern and model pattern; selecting, using multiple threshold values, ones of the plurality of matches according to a plurality of similarity values, each one of the plurality of matches having one of the plurality of similarity values indicating a similarity between the target pattern and the model pattern in the one of the plurality of matches; and determining, using a predetermined algorithm and the selected ones of the matches, the overall similarity value and the overall geometry relationship between the target wafer image and the model wafer image, wherein if a largest one of the plurality of similarity values is greater than or equal to a first threshold value, the selecting comprises selecting a first match of the plurality of matches that has the largest one of the plurality of similarity values, and the determining of the overall similarity value and the overall geometry relationship comprises determining the similarity value and a geometry relationship between the target pattern and the model pattern in the first match as the overall similarity value and the overall geometry relationship between the target wafer image and the model wafer image, respectively; and wherein if each of the plurality of similarity values is smaller than the first threshold value, the selecting comprises selecting multiple matches of the plurality of matches that each have a similarity value greater than a different threshold value that is smaller than the first threshold value; and the determining of the overall similarity value and the overall geometry relationship comprises determining the overall similarity and the overall geometry relationship using: ones of the selected multiple matches that have relatively large similarity values among the selected multiple matches, or ones of the selected multiple matches that correspond to a relatively small difference in pattern distance among the selected multiple matches.