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When it comes to dielectric materials characterization, manufacturers routinely rely on cavity perturbation for measurements. However, the dimensions of the cavity are inversely proportional to the resonance frequency used to measure a material’s dielectric response. As 5G technologies push toward higher frequencies, the dimensions of the cavity and the sample size (especially the thickness) must shrink, which increases the fractional uncertainties and sensitivity to measurement error.

The material sets generally used to fabricate HDI (high-density interconnect) and organic package substrates are provided as either curable liquids or curable thin film sheets. Often the materials’ electrical characteristics are dependent on the details of the curing process. Because of this, large, thick, machinable material samples are generally not available and any suitable metrology has to be able to work with thin (10 um - 250 um) sheets that are frequently fragile, flexible, and brittle. Furthermore, the thickness uniformity of these samples may not be ideal and metrologies that can tolerate or incorporate non-uniformity of the samples into the measurement may have advantages.

Many of the techniques used for material characterization heavily rely on knowledge of the sample thickness. Although the typical samples available are not generally soft, they are frequently compliant under pressure. Contact thickness measurements like hand-held micrometers can lead to significant measurement uncertainties due to operator techniques, particularly for more compliant materials.

With large flexible samples, precise placement in measurement rigs can be difficult because the middle region of the sample may bow or warp out of plane. This introduces errors and/or limits repeatability. Often the amount of bowing is difficult to measure, particularly if the samples are thin.  For example, a 50 um 100 mm x 100 mm sample could easily bow 100 um against the force of gravity and not be visually obvious.

As part of the 5G/6G MAESTRO project, work on this page is supported by the Office of Advanced Manufacturing in the National Institute of Standards and Technology (NIST), under the Federal Award ID Number 70NANB22H050.

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