| 摘要 |
The invention provides a reference assembly that is mounted coaxially with an axis of a projection unit used in lithographic equipment to form semiconductor devices and LCDs. The reference assembly is made of a material having a low coefficient of thermal expansion, and the reference assembly has one or more reference features such as a mirror or off-axis alignment system that represent the axis of the projection unit. The reference assembly is attached to a housing of the projection unit by expansion joints such as flexures, which absorb the thermal expansion and contraction of the housing without significantly affecting the relationship of the reference features to the axis of the projection unit. The invention also provides a projection unit incorporating a reference assembly as well as a method of providing reference features and a method of making a semiconductor device.
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