Wireless CMOS TSV

Short-rage wireless communication technology has been widely applied. In the ubiquitous era, applications for connecting things have been expanding. When communication range is shorter than 1/10 of its wave length, the electromagnetic field shows unique characteristics of near field. For instance, inter-chip wireless communication of chips that are stacked in a package is the near-field communication. Wireless TSV employing inductive coupling bares comparison with TSV in performance with much inexpensive cost (Fig. 1). Application researches with industry have started after fundamental research in academia. For instance, SSD (Solid-State Drive) in a package can be realized by stacking NAND Flash memories. By changing number of processor chips in stacking according to required performance, one design can cover wide ranges of applications. A high-speed low-power link between a processor and a memory can be implemented. Inductive coupling inter-poser for rewiring coils with offset in position is also investigated. Extension of communication rages to millimeter ranges makes it possible to probe internal bus data through package for debugging and to perform non-contact wafer simultaneous testing. Wireless power delivery by using inductive coupling will further widen the applications. In this paper technology and applications of the wireless CMOS TSV are presented. Chip performance improves 70% per year by device scaling. In order to fully utilize the performance improvement, signal bandwidth to/from a chip should be improved by 44% annually. Device scaling, however, provides with only 28% increase in the signal bandwidth. Innovations in communication schemes and circuits have kept the 44% improvement, but this approach is facing limitation. Brute-force acceleration of circuits have resulted in rapid increase of power dissipation in I/O [8]. TSV (Through Silicon Via) that can utilize area, not just periphery, is receiving attractions. TSV, however, is expensive and leaves reliable issues in manufacturing. A proposal of replacing TSV with a wireless transceiver, and moving from mechanical to electrical approach was made [8]. Near-field wireless communication is employed. Regions that are shorter than a distance of wavelength divided by 2πis called near field [13-15]. In the near field electromagnetic waves receive strong attenuation that is in proportion to power of 3 of the distance. Therefore, when arranging communications channel in high density, there is very little crosstalk [3]. By this feature, inter-chip data link in SiP can be suitably made. Especially, wireless TSV by using inductive coupling is attracting attention. We have been investigating wireless TSV and presenting research achievements every year in ISSCC since 2004 [1, 4, 6, 12, 20, 24, 25, 26], in the Symposium on VLSI Circuits [2, 7, 27, 28, 29, 30], and in SSDM [5, 9, 10, 16, 21]. It bares comparison with TSV in performance. Data rate per coil is 11Gb/s/ch [20]. Energy consumption is 0.14pJ/b [12], which is two orders of magnitude smaller than that of the conventional high-speed memory links such as DDR. Layout area is small and chip stacking is short. Reliability of communication is as high as wired communications. By enlarging coil size, communications range extends [8]. Eddy current reduces strength of the inductive coupling when power mesh lines are placed in between coils [19], but that can be compensated by increasing transmission power [19]. The coil can be placed above SRAM memory cell array without problem [19]. Since inductive coupling provides with AC coupling, chips can be connected even under different DC voltages [8]. Furthermore, by thinning chip thickness, changes of magnetic field in the inductive-coupling link can be Performance C os t Wire Bonding Wireless TSV TSV >20¢/chip Cost Down High Power Delivery Analog Communication Applications High-End Low-End Graphics Server Super Computer Cellular Phone Mobile Terminal Automotive PC, Storage Multimedia, Game Digital Consumer