05/29/2018

Fast, precise and non-destructive - measurement of thin transparent layers on a wafer

Polymers have become an essential part of modern microelectronics and microsystems technology, microoptics and medical technology. Until now, they have been used almost exclusively as passive materials - for example to protect sensitive components. Recently, the properties of modern polymers have also been used for functional components.

The worldwide development of new types of polymer materials, which open up new fields of application almost every day, is based on one principle: The chosen process must always be easy to handle, easy to integrate into conventional production and, above all, cost-effective. Consisting of crosslinked nanoscale molecules, they offer a broad spectrum of usable material properties that also enable flexible processing with standard technologies.

Under the aspects of process control and optimization, detailed information on the homogeneity of the polymer layer applied to a wafer is required. The hybrid polymers, which by varying the starting materials and process parameters can have different properties for their use as mechanical protective layers or anti-reflective coatings, are first deposited on the wafer as a thin transparent layer. The thickness of this layer has to be measured.

By partially removing the film, the layer thickness can be determined in a measured profile from the height of the top of the film above the wafer surface. The problem is that conventional stylus instruments, which can otherwise be used for such tasks, cannot be used here because they mechanically scratch the soft coating material. Optical distance measuring tools such as confocal sensors, autofocus systems or triangulators also fail when measuring profiles on thin transparent layers because the light reflected on the top and bottom of the layer cannot be evaluated separately.

With the interferometric sensor CWL FT the thickness of the transparent polymer layers can be measured. The CWL FT is based on the spectral evaluation of the superposition of partial waves of a white light source reflected at the interfaces of a transparent layer. At a given layer thickness and refractive index, the intensity of the superimposed partial beams varies with the wavelength. The spectrum then shows a typical interference pattern. From this spectrum and the refractive index of the layer material, the sensor calculates the thickness of thin transparent layers, such as hybrid polymers, which are measured at high resolution. The complete wafer is measured quickly and non-destructively.

The MicroProf® uses the interferometric sensor for highly accurate, locally resolved coating thickness measurements. In a multi-sensor arrangement with the interferometric measuring head and a confocal chromatic distance sensor, the customer has an extremely powerful measuring tool for fast, localized layer thickness and topography measurements.

The analysis program FRT Mark III is used for the evaluation of the measured film thickness. The powerful program is used in the FRT measuring tools for the visualization and evaluation of topography data as well as film thickness measurements.

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