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.