웨이퍼 정렬을 위한 움직임 벡터 기반의 오버레이 계측 알고리즘
Motion Vector Based Overlay Metrology Algorithm for Wafer Alignment
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초록
반도체 제품의 높은 수율을 달성하기 위해서는 정확한 오버레이 계측이 필수적이다. 오버레이 계측 성능은 오버레이 타깃 설계와 측정 방법에많은 영향을 받는다. 따라서 오버레이 타깃은 성능 개선을 위해 다양한 타깃에 적용할 수 있는 측정 방법들이 요구된다. 본 연구는 이미지 기반의오버레이를 측정할 수 있는 새로운 알고리즘을 제안한다. 제안하는 측정 알고리즘은 움직임 벡터를 이용하는 방법으로 서브 픽셀 단위의 위치를추정할 수 있다. 움직임 벡터는 선택된 영역의 픽셀들을 이용하여 다항식 전개를 통해 2차 방정식의 모델을 생성한다. 그 후 모델을 이용하여서브픽셀 단위의 위치를 추정할 수 있다. 움직임 벡터를 활용한 측정방법은 X축, Y축의 적층 오류를 각각 계산하는 기존 상관계수 기반의 측정방법과는 달리 한 번에 모든 방향의 적층 오류를 계산할 수 있다. 따라서 X축과 Y축의 관계를 반영하여 보다 정확한 오버레이 측정이 가능하다. 하지만기존 상관계수 기반의 알고리즘보다 계산량이 증가하기 때문에 더 많은 연산시간이 사용될 수 있다. 본 연구에서는 기존 방법보다 개선된 알고리즘을제시하는 것이 아닌 새로운 측정 방법의 방향을 제안하는 것에 목적을 두고 있다. 실험 결과를 통해 기존 방법과 유사한 정밀도의 측정 결과를얻을 수 있음을 확인하였다.
Abstract
Accurate overlay metrology is essential to achieve high yields of semiconductor products. Overlay metrology performance is greatlyaffected by overlay target design and measurement method. Therefore, in order to improve the performance of the overlay target,measurement methods applicable to various targets are required. In this study, we propose a new algorithm that can measure image-basedoverlay. The proposed measurement algorithm can estimate the sub-pixel position by using a motion vector. The motion vector mayestimate the position of the sub-pixel unit by applying a quadratic equation model through polynomial expansion using pixels in theselected region. The measurement method using the motion vector can calculate the stacking error in all directions at once, unlike theexisting correlation coefficient-based measurement method that calculates the stacking error on the X-axis and the Y-axis, respectively. Therefore, more accurate overlay measurement is possible by reflecting the relationship between the X-axis and the Y-axis. However,since the amount of computation is increased compared to the existing correlation coefficient-based algorithm, more computation timemay be required. The purpose of this study is not to present an algorithm improved over the existing method, but to suggest a directionfor a new measurement method. Through the experimental results, it was confirmed that measurement results similar to those of theexisting method could be obtained.
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