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Pyramid Probe: RF Calibration and Probe Aging Considerations in HVM High Speed IO Devices

Presented at SWTest 2023
The growth in AI (such as ChatGPT and BING AI) is requiring large investments into the expansion of data centers, driving higher and higher data rates in IO devices.  To reach these data rates, wafer test is moving to bandwidth > 60 GHz.  Pyramid Probes are widely used for wafer test at up to 81 GHz and in high volume manufacturing (HVM).  These probes feature micro-strip or CPW transmission lines that provide controlled impedance and low loss (<6 dB at 67 GHz) for the industry leading performance at wafer test.  However, in the standard layout, the signal trace faces the wafer and is only 70 um away, so that it can sometimes couple to structures on the wafer.  This coupling can have significant effects in some cases, in both DUT performance and RF calibration of the probe head. Further, one expects the coupling to increase as the probe ages and the tips become shorter, bringing the microstrip closer to the wafer.  To reduce this coupling, Pyramid Probes using a microstrip with the ground between the transmission line and the wafer (inverted microstrip) are available.  In this study, we will use probes with standard and inverted microstrips to measure TIA (transimpedance amplifier) performance.  Measurements before and after aging the probes will be compared to characterize these effects.  In addition, we will look at different calibration methods (SOLR, SOLT, SOL de-embed) and compare them to provide recommendations for calibration.

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