PRECISE - New miniature high frequency probes for precision on-wafer microwave measurements

While significant progress has been made in semiconductor technology and high speed integrated-circuit (IC) fabrication, the measurement infrastructure for characterizing such ICs remains largely underdeveloped especially for on-wafer measurements. In particular, commercial ground-signal-ground (GSG) probes for applications below 110 GHz use technologies that have been developed and patented about 15 to 20 years ago [1-4] with patents having nearly expired. Moreover, these commercial technologies are very expensive since they rely typically on manual assembly of several components. Furthermore, this manual assembly leads to large dispersion from product to product as depicted in Fig. 1. This type of assembly also limits the possible geometry down-scaling of the probe, which is required to improve their performance. From an electrical performance point of view, commercial probes are not sufficiently down-scaled and not sufficiently shielded, thus inducing a high coupling between the substrate and/or adjacent circuits, and the probes, and/or a high coupling from probe to probe. This leads to unreliable measurement results above 40/50 GHz with return losses greater than one for simple open circuits and also large dispersion between probes, as already mentioned above [5] (see Fig. 2). Another and most prominent example, in the framework of the semiconductor companies competition, which is based on figure of merit (FOM) measurement, is the measurement of silicon HBT technology having fMAX world record of 720 GHz. This one was measured in two different laboratories, giving similar results below 40 GHz and showing large deviation above [6]. This is due to the fact that, as the transistors get higher in frequency, weaker and weaker elements have to be measured and thus the impact of the probe couplings becomes quasi-dominant over the intrinsic device at very high frequency. We have observed that this problem becomes even more important while measuring FET in 28-nm FDSOI technology especially for the smallest geometry.