A 40 nm CMOS I/O Pad Design With Embedded Capacitive Coupling Receiver for Non-Contact Wafer Probe Test

A receiver for capacitive coupled communication is embedded in a digital input/output pad to add the capacity for non-contact data communication, while maintaining size, ESD protection, and buffering functions unchanged, even in contact mode. The added feature allows non-contact probing of die pads and provides a reliable alternative solution to mechanical probing for electrical wafer sort testing of Systems-on-Chip (SoC) and Systems-in-Package (SiPs) because of elimination of pad damage and reduction of the force required to create stable electrical contacts between probe needles and pads. The proposed receiver detects the displacement current flowing through the capacitive channel created between the connecting probe needle and top metal pad surface when a transition in the input digital stimulus signal occurs. The receiver is designed to work up to 100 Mbit/s data rate with a power of 340 μW in a 40 nm CMOS process. The circuit trade-offs between frequency, amplitude of the step input and distance are discussed. Experimental results show that for a 5 V input voltage amplitude, the receiver allows correct data transmission at a distance up to 5 μm, which increases to 10 μm if the top aluminum layer is divided in two, using a customized I/O pad design. The feasibility of this non-contact testing approach was verified through electrical tests on two IP blocks, an LFSR, and a PLL with a scan chain, using a standard prober and a cantilever probe card designed with 19 needles of different lengths to enable both physical contact connections for power supply and non-contact capacitive coupling data communication for signals.