Recently, precise film structures have become essential for integrated circuit (IC) and flat panel display (FPD) products because of their multi-functional purposes and compactness. In these products, thin films are deposited on a silicon or glass substrate with various materials and thicknesses for conductivity, insulation and packaging. For the successful operation and performance, film structure should be manufactured as designed. In addition, the measurement or inspection of the geometrical dimensions and material properties of film layers becomes an important issue in order to confirm their functional uniformity. One approach is using an interferometer to obtain surface profiles of the film layers. Low coherence interferometry (LCI) can detect the film layers with the low coherence characteristic of the optical source and scanning mechanism in the time domain and spectral domain, which enables to extract their optical thicknesses.
On the other hand, specimens such as a secondary battery cell are manufactured with a thin metal substrate and non-conducting dielectric films. On both sides of the substrate, the films are deposited to prevent the link between the battery cells, which are piled up to build the whole secondary battery. In this case, the thicknesses of the films and the substrate need to be measured to assure the safety of the battery.

In this investigation, we propose a dual interferometric probe to measure the film and substrate thicknesses at once. In the optical probe, low coherence scanning interferometry (LCSI) and spectrally-resolved interferometry are adopted to measure the film thickness. LCSI has been an attractive tool for measuring 3D surface profile of a sample because it can avoid the well-known 2-pi ambiguity problem. By using low coherence characteristic of a light source, LCSI can obtain a localized correlogram when the optical path difference (OPD) between reference and measurement arms becomes near zero by adjusting one of two path lengths. Spectrally-resolved interferometry (SRI) is the well-known technique to measure absolute distances and topographic or tomographic surface profiles based on the analysis of the spectral interferogram. Although phase shifting techniques can be involved in SRI to improve the measurement accuracy in some cases, the most attractive feature of SRI is to obtain the spectral phase to extract the measuring distance at once without any scanning mechanism opposed to the white light scanning interferometry. Based on these two interferometric principles, the proposed system can analyze the film structure of the specimen. The whole system is built with optical fiber structure, and the optical switch is used for changing the channel of the measurement probes. The measurement probes are located on both sides of the specimen, and the foci of the probes are coincided with each other. From the geometry of the measurement probes, the thickness of the substrate can be obtained.

In the experiments, we measured several specimens such as bare silicon wafer, pellicle BS, flexible film specimens to verify the performance of the system. In addition, it was confirmed that the proposed system was able to measure the film and substrate thicknesses of the secondary battery cell.