Fast ellipsometric measurements based on a single crystal photo-elastic modulator

Abstract

For quality control in high volume manufacturing of thin layers and for tracking of physical and chemical processes, ellipsometry is a common measurement technology. For such kinds of applications we present a novel approach of fast ellipsometric measurements. Instead of a conventional setup that uses a standard photo-elastic modulator, we use a 92 kHz Single Crystal Photo-Elastic Modulator (SCPEM), which is a LiTaO3 crystal with a size of 28 × 9 × 4 mm. This small, simple, and cost-effective solution also offers the advantage of direct control of the retardation via the current amplitude, which is important for repeatability of the measurements. Instead of a Lock-In Amplifier, an automated digital processing based on a fast analog to digital converter controlled by a highly flexible Field Programmable Gate Array is used. This and the extremely compact and efficient polarization modulation allow fast ellipsometric testing where the upper limit of measurement rates is mainly limited by the desired accuracy and repeatability of the measurements. The standard deviation that is related to the repeatability +/-0.002° for dielectric layers can be easily reached.

Microsecond-resolved SDR-based cavity ring down ellipsometry

“…In this case, the accuracy is improved to 0.0026°or 2.7 × 10 −4 Δ 0 with a temporal resolution of 200 μs. This performance compares well to the fastest to date photoelastic-modulator-based ellipsometers [2], which still operate several times slower than our prototype.…”

“…The ellipsometric phase shift and reflectivity measured by ellipsometry can be related to physical properties of the interface such as the refractive index, making it an important tool for the study of thin layers and microelectronics in general. The time resolution of conventional ellipsometry is close to 1 s with high accuracy (10 −3°) and can reach the millisecond range with reduced accuracy [2]. Ultrafast dynamic ellipsometry can extract ellipsometric information in the picosecond time scale; however, this is accomplished by repeating pump-probe measurements and not in a continuous, running fashion [3].…”

Inline imaging-ellipsometer for printed electronics

“…This design does not need any mechanically moving parts and has a very high sampling rate compared to alternative solutions. Furthermore, and most important, it can be implemented as an imaging ellipsometer, which is not the case for the classical PEM based ellipsometry [7]. Figure 1 shows the basic structure of a stroboscopic ellipsometer.…”

Power scaling of AOM-switched lasers with SCPEM-based time-multiplexing

“…The Q-switching is carried out by one AOM in the common part of the resonator. Switching between the two channels is performed with a Single Crystal Photo-Elastic Modulator (SCPEM [14][15][16][17]) that is suitable for high frequency operation and requires just a few volts for driving.…”