移相掩膜的一点疑问，请指教

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移相掩膜PSM it has its own problem--phase conflict.这里的phase conflict 是什么？中文好像是相冲突？为什么会有这个问题呢？
to solve such a problem, there are many variations of PSM available today. Amany these alternatives , Complementary phase shift mask technology has demonstrated the power to improve exposure windows significantly.
互补移相掩膜技术解决了这个问题。什么是互补移相掩膜呢？百度了一下没有。。