2018 年 67 巻 9 号 p. 829-833
We developed a heterodyne interferometry system cancelling the noise caused by a light source using a pair of heterodyne interferometers. The interferometry is one of the powerful tools to precisely measure the flatness of a polished semiconductor wafer. Generally, the noise caused by a light source affects the change of distance measured by interferometers, and it is difficult to avoid the noise by filtering a lock-in-amplification process. Here, we demonstrate that a pair configuration of interferometers synchronizing each other dramatically cancels the noises. The noise cancelling has been conducted by replacing the arrangement of the modulation frequency of two light waves producing an interference signal in one and the other interferometer each other. Utilizing these noise-cancelling heterodyne interferometers, we characterized the flatness of a polished silicon wafer. The precision has been improved 16% by the noise cancelling, and the system has enabled the measurement of the wafer flatness with repeatability of 0.32 nm.