Orthogonal polarization Mirau interferometer using reflective-type waveplate

Abstract

This work proposes an orthogonal polarization Mirau interferometry using a reflective-type waveplate to generate different polarization orientations for broadband white light interferometry. The reflective-type half-waveplate is employed as the reference arm of the Mirau interferometer to convert polarization and it generates a reference light with an orientation orthogonal to the object light. An advantage of the proposed interferometer is its ability to control the ratio of light intensity between the object and reference arms to maximize the interferometric fringe contrast. Better, more accurate calibration of standard step height has been achieved by the developed interferometer, which also can measure solder bumps that traditional Mirau interferometers usually cannot measure.

An insight on optical metrology in manufacturing

“…WLI microscopes [140][141][142][143][144][145] have been widely used for measuring film thickness and monitoring surfaces with discontinuities of more than several wavelengths. In contrast to laser phaseshifting interferometry (PSI) employing a light source with a long coherence length, WLI uses a broadband light source with a short coherence length of approximately 1 µm [146]. Therefore, the interference pattern acquired by WLI is a superposition of the interference patterns of many wavelength components.…”

“…Apart from these common errors, the optical properties of the surface to be measured are another important uncertainty source in SWLI. This type of error commonly occurs when the measured surface is made of materials exhibiting different phase changes on reflection or multi-reflection issues [159]. Continuous efforts have been made to minimize these potential errors in SWLI so that the measurement performance can be enhanced to satisfy even higher manufacturing demands.…”

“…A difference in intensity between the reference arm and the object arm is one of the major limitations of SWLI. This limitation makes it difficult to measure objects with low surface reflectivities such as biological samples or highly scattering surfaces, in which the light reflected from the object is much weaker than that reflected from the reference mirror [159]. In the literature, the concept of polarization-based Mirau interferometry was first introduced by Massig [187] in 1992, for detecting changes in the distance of a small reflective measurement surface.…”

“…More recently, another interesting approach proposed by Chen [159] for balancing the two-arm light difference is a new type of Mirau interferometer which separates light rays in the reference and object arm using orthogonal polarization. Figure 33 presents a schematic of the proposed design of the Mirau interferometric objective [146,159]. The design involves polarization separation using an optical thin-film coating, with so-called silver nanorod arrays (NRA) and a reflective wave plate.…”

“…The beam splitter can separate light polarization, and the reference arm is employed to increase the polarization separation of the light. Figure 34 shows a scanning electron microscope (SEM) image of the silver NRA coating used in the reference arm of the interferometer [146,159]. Figure 35 shows the anisotropic thin film developed as the reflective wave plate [146,159].…”