Development of double twin path laser interferometer for thin film thickness measurement

We describe a novel laser interferometer which measure thin film thickness automatically with simultaneous reference level control. The system use a He-Ne laser as light source and a grating as beam splitter-recombiner. The 0 and -1 order beam diffracted by the grating are reflected at the substrate and the +1 order beam is reflected at the film. These beams again pass through the grating and diffracted to interfere. The interference between the diffracted components of 0 and -1 order beam is used to control the reference level, and the interference between the diffracted components of 0 and +1 order beam is usad to measure the film thickness. Rising in the two pairs of twin path we call this interferometer as a double twin path laser interferometer. Theoretical estimations give the high accuracy of better than ±0.1 nm. A mechanically stable and compact system has been constructed. The experimental results show that the performance of system is fairly satisfactory. Alminium films of thickness ranging from 70 nm to 300 nm were measured with a accuracy of ±20 nm. Additionally we discuss the problems in the first trial construction and how to improve the accuracy and stability in this system. The second trial construction including the improvements is attempted.