Abstract
In the displacement measurement of the wafer stage in lithography machines, signal quality is affected by the relative angular position between the encoder head and the grating. In this study, a two-degree-of-freedom fiber-coupled heterodyne grating interferometer with large operating range of rotation is presented. Fibers without fiber couplers are utilized to receive the interference beams for high-contrast signals under the circumstances of large angular displacement and ZEMAX ray tracing software simulation and experimental validation have been carried out. Meanwhile, a reference beam generated inside the encoder head is adopted to suppress the thermal drift of the interferometer. Experimental results prove that the proposed grating interferometer could realize sub-nanometer displacement measurement stability in both in-plane and out-of-plane directions, which is 0.246 nm and 0.465 nm of 3σ value respectively within 30 s.
Highlights
In lithography machines for semiconductor fabrication, the laser interferometer and grating interferometer have been extensively applied due to their nanometer, even sub-nanometer, precision measurement ability [1,2,3]
The grating interferometer, known as interferometric encoder, is less subject to those impacts owing to the stability of grating period and short fixed optical length [5,6,7]
In the mainstream of 28 nm to 10 nm lithography equipment, the encoder head is fixed at each corner of the wafer stage, combined for the measurement of six-degree-of-freedom displacement relative to the grating as Figure 1 shows
Summary
In lithography machines for semiconductor fabrication, the laser interferometer and grating interferometer have been extensively applied due to their nanometer, even sub-nanometer, precision measurement ability [1,2,3]. In the mainstream of 28 nm to 10 nm lithography equipment, the encoder head is fixed at each corner of the wafer stage, combined for the measurement of six-degree-of-freedom displacement relative to the grating (including a ±1 mm range in out-of-plane direction and ±1.5 mrad angular range in rotation) as Figure 1 shows. For ultra-precision measurement of the wafer stage, in-plane and out-of-plane displacement measurement ability in the motion range and sub-nanometer stability over the exposure time of a single wafer (given a throughput of more than 200 wafers per hour, this time is typically less than about half minutes) are demanded of a single interferometric encoder [2,3].
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