Instrumentation

Four common types of ellipsometers include the rotating compensator, the rotating analyzer, the phase modulated, and the liquid crystal phase modulated. Each varies in the way a known phase shift is introduced. For example, a phase shift is introduced into the rotating compensator system by adding in a rotating compensator just before the sample, whereas for the phase modulated ellipsometer, the phase shift is introduced by a photoelastic modulator placed after the sample. HORIBA Scientific specializes in both phase modulated and liquid crystal phase modulated ellipsometers.

HORIBA Scientific products manufactures on phase modulated ellipsometry. Currently, we offer two scanning instruments that incorporate a photoelastic modulator — the UVISEL PLUS and UVISEL 2 — and two CCD-based instruments that incorporate liquid crystal modulation — the Auto-SE and the Smart-SE. The UVISEL PLUS and UVISEL 2 instruments are designed for highest spectral resolution, precision and sensitivity; and the Smart-SE and Auto-SE are designed for quick, easy, high throughput measurements.

Phase modulated ellipsometry provides excellent precision on the ellipsometric parameter, Δ, resulting in a high sensitivity to ultrathin films on transparent substrates. It also uses fixed elements, so there is no sensitivity to mechanical noise, resulting in a precise and stable signal. Phase modulated ellipsometry can cover a wide spectral range from 190-2100 nm, with a high polarization modulation rate of 50,000 Hz, compared to tens Hz for other rotating element systems.

This higher modulation rate allows for quick measurements with unsurpassed signal to noise. Also, a phase modulated ellipsometer allows for the measurement of ψ and Δ over their full ranges (ψ from 0° to 90° and Δ from 0° to 360°).

Laser ellipsometers measure ψ and Δ at only one wavelength, which means they can only determine two properties, such as thickness of a single transparent layer, and its refractive index at the laser wavelength. If the layer is semi-transparent, the absorption cannot be determined. An ellipsometer, however, measures ψ and Δ at each wavelength, resulting in an accurate thickness and optical properties over a spectral range.

A laser ellipsometer also cannot be used to determine thicknesses of films in a multi-layer stack, whereas, Spectroscopic ellipsometry works well for this. One other problem with laser ellipsometry is that ψ and Δ at one wavelength can represent multiple thicknesses, so it is hard to determine which is correct.

The vision system allows the user to see exactly where the spot is hitting the sample for accurate measurements. Our patented imaging system, as used on the Auto SE, Smart SE and UVISEL 2, gives you clear visualization on practically any surface. The microspot helps reduce the size of the incoming light beam in order to measure small features on patterned samples. When used together, the vision system allows the user to see exactly where the microspot is placed on the patterned sample for precise measurements.

The microspots are very useful for patterned or heterogeneous samples which may have small regions of different materials. With the microspot, you can accurately measure small patterned regions of your thin film sample without overlapping other areas of the sample.

You get up to 8 computer selectable spot sizes on our ellipsometers.There are no fibers or optics for the operator to manually change, and no alignment or recalibration is needed.

This is instrument dependent. Currently, the smallest microspot available on the Auto-SE is 25 μm x 60 μm. The smallest microspot for the UVISEL PLUS is 50 μm and the smallest microspot for the UVISEL 2 is 35 μm both at normal incidence.

In general, you should use the largest spot size possible for the structure being measured so as to maximize the light intensity and increase signal.

A CCD-based system collects all wavelengths at once, whereas a monochromator/PMT- based system is a scanning system, collecting one wavelength at a time, sequentially. CCD-based systems acquire data quickly, but the monochromator/PMT-based systems provide measurements with significantly higher precision, sensitivity and spectral resolution. A CCD-based system will introduce more noise into the measurement since the gain is set for all wavelengths. The monochromator-based system, however, adjusts the gain for each wavelength individually, maximizing the signal and reducing the noise.

The Photoelastic Modulator (PEM) is an SiO₂ bar which exhibits isotropic behavior when no stress is applied. Once stress is applied, via a piezoelectric transducer, the bar becomes birefringent. This means the bar has different optical properties in different directions, or that light travels faster along one axis than the other. Because of this difference in phase velocities, a modulation is introduced.

The UVISEL system has no moving optical components, making the measurements accurate, precise, and stable. It also provides excellent precision on delta, a very fast acquisition rate (>100 ms/point), excellent repeatability, a high polarization modulation rate resulting in unsurpassed signal to noise, and it can measure ψ and Δ over their full range.

Because the UVISEL system is a scanning system, it provides high resolution data when compared to the CCD-based systems, but the overall measurement (depending on the wavelength range and increments) can take longer than for a CCD-based system.

The Auto-SE/Smart-SE systems also have no moving components, resulting in a stable signal, and they also offer very quick measurements over a full spectral range. The Auto-SE/Smart-SE systems come with an additional component of the DeltaPsi 2 software which has a very intuitive graphical user interface (GUI). This software component makes it easy to measure routine samples simply and quickly, and can be very useful for QA/QC needs.