Traditional ellipsometry can measure the thickness and refractive index of isotropic thin films layered on isotropic substrates. But in some cases, the films or substrates may be anisotropic, meaning that the real or imaginary parts of the refractive index are different along different directions. When dealing with anisotropic films or substrates, Generalized Ellipsometry (GE) must be used.  Generalized Ellipsometry is an extension of traditionally ellipsometry where Berreman equations are used instead of Fresnel equations in the analysis, and a more sophisticated set of measurements is required.

Because the AxoScan system measures the full Mueller matrix (all 16 elements with no approximations nor assumptions) in only 30 ms, it is the ideal system for GE measurements.

AxoScan MMSP-VRTF-1-MSRS system for generalized ellipsometry (GE) measurements.

The measurement technique typically involves measuring the full Mueller matrix of the sample in reflection, and then rotating the sample to a new orientation and repeating the measurement. This process is repeated for a large number of rotation angles, and our EllipsoView software analyzes the results to determine the refractive indices and thicknesses of the various thin films.

The GE technique involves testing a wide range of polarization states while the sample is rotated.

Some common applications for GE include:

• Polymerized liquid crystal films (LCPs)
• Photo-aligned polyimide (PI) alignment layers used as alignment layers in liquid crystal displays
• Thin films deposited on anisotropic crystalline substrates

Example - Photo-aligned polyimide (PAPI) testing

In a liquid crystal display (LCD), the TFT and color filter glass substrates are both coated with a thin PI film which is then rubbed or brushed to create anisotropy. When the cell is filled, the liquid crystal molecules at the PI interface will orient themselves to be aligned with the induced anisotropy. The harder the PI is rubbed, the larger the anisotropy will become, and the stronger the anchoring energy between the LC and the PI will become.

In recent years, as the pixel size of LCD's has become smaller and smaller, microscopic defects caused by individual hairs of the rubbing brushes began causing visible defects on displays and the industry has evolved towards using polarized UV light exposure to create the anisotropy in the PI.  Because the photo-aligned PI process was quite new, LCD manufacturers needed a way to monitor the anisotropy and orientation of the PI early in the manufacturing process. Axometrics worked with leading manufacturers to develop a technique based on our AxoScan measurement system.

Using our R&D systems, manufacturers test samples exposed with different UV energies to determine optimum exposure energy to maximize anisotropy, and thereby maximize the LC anchoring energy.

Finding the UV exposure energy that maximizes anisotropy.

Using our large production systems, manufacturers can spot check panels inline during the manufacturing process, or they can perform offline high-resolution scans across the LCD substrate to look for variations in their manufacturing process.  In the example below, a serious misalignment of the polarizers in the UV photoalignment machine cause the PI anisotropy orientation to vary by almost 1° across the panels.

High-resolution line scan across 3 LCD panels on a Gen 4 substrate reveals misalignment in the anisotropy orientation.