Indirect contact probing method for characterizing vertical interconnects

Abstract

Recently, vertical interconnects in wafer-level are used to achieve system integration with stacked chips. Although the wafer-level vertical interconnects provide smaller interconnection delay and lower power consumption, popularizing the technology is difficult due to testing issues. A main difficulty in testing vertical interconnects comes from that possible damages caused by the direct-contact probing. Therefore, an indirect contact probing method is presented for safe characterization in waver level. The proposed method is based on the capacitive coupling method. Utilizing a dielectric contactor, the sensitivity of capacitive coupling can be improved with ensuring the protection of vertical interconnects. In addition, extra probe control module and sensor electronics are not required since the dielectric contactor maintains the constant gap. The proposed method is verified in both cases of a single-pair via and multiple vias. The procedure of the proposed method for a single-pair vias starts with one-port calibration. To apply one-port calibration, we have measurements on three different calibration vias by the indirect and the direct-contact probing. From the measurement data, the characteristic of dielectric contactor is fully characterized. After the dielectric contactor is mounted on the DUT containing vertical interconnects, the DUT is measured by the indirect-contact probing manner. Finally, de-embedding the dielectric contactor portion, we can obtain the characteristic of a single-pair via. The proposed method is verified in printed circuit board (PCB) level. The extracted via impedances show a good agreement with the direct-contact probing in frequency ranges 0.8 GHz to 30 GHz and 2.5 GHz to 18 GHz by simulations and measurements, respectively. In the case of multi-via testing, the procedure is similar to a single-pair via extraction but additional fixtures are required. By adopting the socket and calibration substrates, the dielectric contactor consisting of multiple pads can be characterized. From dielectric contactor characteristics corresponding to each via, multiple vias can be extracted based on the reference plane. The extracted impedances of multiple vias show a good agreement with the direct-contact probing up to 24 GHz by simulations and 22 GHz by measurements. From the extracted impedances, we can diagnose all defects among multiple vias. Since the proposed method for multi-via test is limited to testing, the indirect contact probing method for the multi-port characterization is also proposed. It characterizes a multi-port network of a DUT by de-embedding the multi-port characteristics of the dielectric contactor, hence we can also capture inter-via couplings from a multi-port network. Based on simulations, a two-port network is successfully characterized in the range of 0.8 GHz to 24 GHz.