John A. Woollam

Overview of variable-angle spectroscopic ellipsometry (VASE): I. Basic theory and typical applications

Variable angle spectroscopic ellipsometry (VASE) is important for metrology in several industries, and is a powerful technique for research on new materials and processes. Sophisticated instrumentation and software for VASE data acquisition and analysis is available for the most demanding research applications, while simple to use software enables the use of VASE for routine measurements as well. This article gives a basic introduction to the theory of ellipsometry, references “classic” papers, and shows typical VASE applications. In the following companion paper, more advanced applications are discussed.

Overview of variable-angle spectroscopic ellipsometry (VASE): II. Advanced applications

A preceding companion paper provides a general introduction to Variable Angle Spectroscopic Ellipsometry (VASE), and also describes many typical applications of the technique. In this paper, more advanced VASE applications are discussed. These applications rely on recent advances in ellipsometric hardware, which allow extremely accurate ellipsometric data to be acquired over a broad spectral range, from the IR to VUV. This instrumentation can also quantitatively measure the optical response of nonisotropic samples. Advanced data analysis techniques are also presented.

Recent developments in spectroscopic ellipsometry for in situ applications

The in situ measurement capabilities and advantages of recently developed spectroscopic ellipsometry (SE) instrumentation, which covers wide spectral ranges (190-1700 nm, or 0.73-6.5 eV) and is based on rotating-compensator technology, are described. A technique which can quantitatively correct for window birefringence is presented. Current in situ SE deposition monitoring and control applications in the compound semiconductor, display, and optical coatings industries are also presented.

Variable angle spectroscopic ellipsometry in the vacuum ultraviolet

Optical properties of thin films and bulk materials at short wavelengths, including 157 nm and shorter, are needed for development of new lithographic processes, new fundamental science, and new metrology in the semiconductor, optical and data storage industries. Variable angle spectroscopic ellipsometry offers non-destructive and precise measurement of thin film thickness and refractive index in the wavelength range from 140 nm to 1700 nm (0.73 eV to 8.9 eV). The addition of short wavelengths allows analysis of multilayer dielectric stacks, often difficult to do using visible spectroscopy alone. Another major application is in study of wide bandgap materials such as SiC and GaN related compound semiconductors for blue lasers and detectors. This paper reviews the present status of spectroscopic ellipsometry applications in the vacuum ultraviolet.

Characterizing antireflection coatings on textured monocrystalline silicon with Spectroscopic Ellipsometry

Mono-crystalline silicon is chemically textured and coated with a silicon nitride surface anti-reflection (AR) film to improve light trapping and device efficiency in photovoltaic applications. The thickness and optical properties of the AR coating determine the effective suppression of reflected light. However, optical characterization of films on chemically textured surfaces is challenging due to the low reflectance. We present new measurement geometries and modeling methodology using Spectroscopic Ellipsometry (SE) to determine film thickness and optical properties of thin AR coatings on textured mono-crystalline silicon. Special measurement geometries are used to collect specular reflected light from the etched silicon pyramid facets. Both apexial and lateral measurement geometries are demonstrated, where the latter requires a special sample stage to tilt and rotate the sample to detect the specular reflected light from the pyramid facets. Measurement considerations are discussed including probe-beam incident angle, sample tilt and rotational angles. Effects of pyramid surface-coverage are also discussed in relation to the proposed measurement geometries. Previous modeling attempts using Effective Medium Approximation (EMA) theory showed inconsistent results when comparing apexial and lateral measurements of the same sample surface. In this work, results from a scattering-corrected modeling approach provide improved consistency for a series of SiNx coatings on textured monocrystalline silicon for both lateral and apexial measurement geometries.

Real-time monitoring and control during MBE growth of GaAs/AlGaAs Bragg reflectors using multiwave ellipsometry

New multi-wavelength in-situ ellipsometer acquiring accurate ellipsometric data at 44 wavelength form 415 to 750 nm in less than 1s is directly mounted on a MBE growth system. Compared to single wavelength ellipsometers enough measured data are available to have access to layer thickness, composition, temperature and exact angle of incidence. In- situ monitoring and real time analysis was used to control the process of GaAs/AlGaAs Bragg reflectors with center wavelength of 1000 nm. The layer thickness is controlled very accurately even though ellipsometric data was acquired only every 3 seconds. The accuracy of shutter timing can be made very precisely even for slow ellipsometric acquisition rates and substrate wobble due to MBE substrate rotation. The control algorithm for two reflectors did not attempt to control the Al composition of an individual AlGaAs layer, but the measured composition was used to adjust the Al cell temperature for the next AlGaAs layer. In comparison for another reflector, the FastDyn fitting routine were used to simultaneously control the thickness and surface composition of the AlGaAs layers. An overview about the hardware and software integration on the MBE system will be given. The in-situ measurements during the growth control were later compared with ex-situ measurements made with spectroscopic ellipsometer system VASE.

Spectroscopic ellipsometry and band structure of Si{sub 1{minus}y}C{sub y} alloys grown pseudomorphically on Si (001)

The authors have measured the dielectric functions of three Si{sub 1{minus}y}C{sub y} alloys layers (y {le} 1.4%) grown pseudomorphically on Si (001) substrates using molecular beam epitaxy at low temperatures. From the numerical derivatives of the measured spectra, they determine the critical point energies E{sub 0}{prime} and E{sub 1} as a function of y (y {le} 1.4%) using a comparison with analytical line shapes and analyze these energies in terms of the expected shifts and splittings due to negative hydrostatic pressure, shear stress, and alloying. Their data agree well with the calculated shifts for E{sub 1}, but the E{sub 0}{prime} energies are lower than expected. They discuss their results in comparison with recent tight-binding molecular dynamics simulations by Demkov and Sankey predicting a total breakdown of the virtual-crystal approximation for such alloys.