The rapid development of high frequency devices and IC technologies for wireless communication products has created many opportunities and challenges in RF testing, packaging and modeling. Until recently, the final test of RFICs in production was performed at the package level. In the meantime both the costs of package scrap have become prohibitive and the complexity of troubleshooting faulty chips has increased considerably because of electrical package parasitics. Therefore, complex RFICs are now being tested at the wafer level, prior to packaging. Although on-wafer high-frequency measurements already have an economic impact on chip fabrication costs, there are no methods available for traceable on-wafer measurements in planar transmission lines - a problem which has been long solved in coaxial lines and waveguides. The need for traceable on-wafer scattering parameter measurements up to very high frequencies has also become evident in a recent survey conducted by PTB in industry and academia.
Besides traceable scattering parameter measurements in planar circuits the following topics are currently under investigation:
- wideband characterization of interconnects fabricated on different substrate materials, including silicon
- test concepts with minimum invasiveness (e.g. high-impedance probes)
- numerical methods for network analyzer calibration
In the medium-term research efforts will be undertaken to extend the frequency range up to sub-THz frequencies and to investigate multiport scattering parameter measurements.
All the topics listed above are currently being investigated in a close collaboration with the United States´ national metrology institute National Institute of Standards and Technology (NIST) in Boulder, CO. Research on selected topics is conducted in cooperation with the British national metrology institute National Physical Laboratory (NPL) in Teddington, UK, the University of Hannover, Germany, and the Oregon State University in Corvallis, OR, USA.
The figure on the left shows the current measurement equipment: a semi-automatic wafer prober station and a network analyzer covering frequencies up to 50 GHz. On the right the top view of a test structure for interconnects on lossy silicon together with the touched-down microwave probes is shown. The broadband characterization of electrical interconnect properties is of increasing interest to both microelectronic and RF/microwave industry. Solving this measurement problem involves the development of accurate on-wafer calibrations, broadband planar waveguide characterization methods and multiport scattering parameter measurement techniques. Solutions in one of these areas can immediately lead to progress in an adjacent field, since the topics cannot be treated separately.
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| semi-automatic wafer prober and network analyzer |
interconnect test structure |
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