Enabling Simultaneous RF And DC Measurements During Wafer Probing
Traditionally, probe cards for parametric testers have been designed for either RF signals or low-level DC measurements, but not both. This was acceptable for semiconductor wafers containing only RF test circuits or those designed for DC operation. However, as digital switching speeds have increased to RF frequencies, and mixed RF and DC test devices are designed into wafers, separate RF and DC probing has become very costly. It requires a changeover from one type of probe card to another, recalibration of the parametric tester and its interconnects, and then re-probing the wafer for a second set of measurements.
In addition, on-wafer testing of semiconductor devices often utilizes the interfacing between test instruments and the test points and structures located within wafer scribe lanes. As semiconductor dies and scribe lanes decrease in size, it becomes more difficult to connect test instruments to the device under test (DUT). This problem is compounded when both RF and ultra-low current DC measurements are desired. In RF measurements, an important consideration is minimizing RF signal losses through a variety of techniques, such as maintaining the desired characteristic impedance to minimize RF energy reflections along the signal path. In ultra-low current measurements (for example, sub-nanoamp magnitudes), it is important to minimize the effects of extraneous voltage potentials in the vicinity of the test point.
To help alleviate this situation, some probe cards have recently been designed for a few RF connections, with the remaining probes calibrated for DC. This helps to some extent but limits the effectiveness of parametric test systems, most of which could be programmed to conduct either RF or DC measurements on any pin. Still, parametric testers themselves need to be designed so that interconnections on any signal path can handle either type of signal. Until recently, this type of system has not been available.
Keithley Instruments and Mesatronic Group (Voiron, France) have formed a technical alliance to address this need for small-geometry mixed-RF and DC testing. Engineers from the two companies are working together to create probe cards for parametric test systems that take RF and low-level DC measurements on any combination of wafer prober pins. Project objectives include: easier layout and manufacture of probe cards, shorter production times, and ultimately lower cost to parametric test users. In addition, the companies hope to reduce the user's probe card inventory by utilizing one-card design for many different applications.
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