Characterising inhomogeneous and rough materials using a scatterometer
Electrical characterisation of dielectrics, composite resins, ceramics, and printed structures is essential to predict the performance of future space missions.
Materials are characterised in terms of reflection, transmission, and absorption. To measure these, well-known procedures and instruments exist for homogeneous materials with smooth surfaces. For inhomogeneous or rough materials, on the other hand, the accuracy of the measurements is strongly affected by the field which is scattered away from the material. To characterise these materials, it is therefore essential to measure the scattered radiation and use that to correct reflection and transmission measurements, to avoid that scattered radiation is confused with absorption.
Measuring scattered radiation requires an instrument that allows a wide range of incidence and scattering angles.
In 2016, a contract from the European Space Agency – ESA was awarded to Roger Appleby MMW Consulting, Thomas Keating, Ltd, Pixel Analytics Limited and TICRA, with the purpose to design, manufacture and test an instrument for accurate scattering measurements and characterisation of rough and inhomogeneous materials in the 50-750 GHz region.
An initial design of the instrument based on ray tracing techniques and standard ellipsoidal mirrors was available as a starting point. It consisted of two optical benches positioned above each other and linked by a periscope rotating around a vertical axis passing through the material sample.
The full suite of TICRA’s software products was used by TICRA to model in detail the instrument over the full frequency band. Physical Optics was used for all mirrors and smooth samples, while full-wave analysis was considered for rough samples and the periscope support structure. Finally, the power collected by the receiving horn of the instrument was extracted with a coupling analysis. The results from TICRA showed that the initial design suffered from poor beam quality, leading to inaccurate scattering measurements. A number of optical modifications, including the use of polarisation grids and a fully compensated use of the mirrors, were thus proposed and verified by TICRA’s detailed RF simulations, to arrive at a final design that was manufactured and finally installed at the European Space Agency in the The Microwave and (Sub)mm-wave Material RF characterisation facilities.
The instrument is currently in use at this facility in a controlled cleanroom environment for flight hardware testing.
More info on ESA’s Microwave and (Sub)mm-wave Material RF characterisation facilities: https://www.esa.int/Enabling_Support/Space_Engineering_Technology/Antenna_Laboratory