Spectroscopic Ellipsometers

Light can be described as an electromagnetic wave traveling through space. For purposes of ellipsometry, it is adequate to discuss the waves’s electric field behavior in space and time, also known as polarisation. The electric field of a wave is always orthogonal to the propagation direction. Therefore, a wave traveling along the z-direction can be described by its x- and y- components. When the light has completely random orientation and phase, it is considered unpolarised. For ellipsometry, however, we are interested in the kind of electric field that follows a specific path and traces out a distinct shape at any point. This is known as polarised light. When two orthogonal light waves are in-phase, the resulting light will be linearly polarised. The relative amplitudes determine the resulting orientation. If the orthogonal waves are 90° out-of-phase and equal in amplitude, the resultant light is circularly polarised. The most common polarisation is “elliptical”, one that combines orthogonal waves of arbitrary amplitude and phase. This is where ellipsometry gets its name.

The film thickness is determined by interference between light reflecting from the surface and light traveling through the film. Depending on the relative phase of the rejoining light to the surface reflection, interference can be defined as constructive or destructive. The interference involves both amplitude and phase information. The phase information from Δ is very sensitive to films down to sub-monolayer thickness. The figure below compares reflected intensity and ellipsometry for the same series of thin SiO₂layers on Si. There are large variations in Δ, while the reflectance for each film is nearly the same.

(left) Reflected intensity and (right) ellipsometric delta for two thin oxides on silicon show the high sensitivity of Delta to nanometer scale films not observable with the intensity measurement.

Ellipsometry is typically used for films whose thickness ranges from sub-nanometers to a few microns. As films become thicker than several tens of microns, interference oscillations become increasingly difficult to resolve, except with longer infrared wavelengths. Other characterisation techniques are preferred in this case.

Thickness measurements also require that a portion of the light travel through the entire film and return to the surface. If the material absorbs light, thickness measurements by optical instruments will be limited to thin, semi-opaque layers. This limitation can be circumvented by targeting measurments to a spectral region with lower absorption. For example, an organic film may strongly absorb UV and IR light, but remain transparent at mid-visible wavelengths. For metals, which strongly absorb at all wavelengths, the maximum layer for thickness determination is typically about 100 nm.