'3D Wafer Bonding with a Silicon Germanium HBT BiCMOS Wafer for Compact Fast Light Modulator for Photonic Interconnect'

摘要

It is now widely acknowledged that interconnections are having an increasingly adverse impact on the speed of computers through wiring delay. Electronic interconnections suffer from resistance, capacitance and inductance. Additionally special considerations apply when frequencies exceed 5-10 GHz due to skin effects, silicon substrate losses, and dielectric losses. Additional problems occur due to transmission line effects at millimeter wave frequencies. Optical interconnections are often considered as an alternative for metallic interconnections for these reasons. Although optical interconnections are limited to width and thickness dimensions on the order of a wavelength, they can avoid many of the aforementioned metallic parasitics. Recently INTEL demonstrated that it is possible to modulate light in Silicon with a Mach-Zender interferometer employing a voltage controlled delay element consisting of a MOS Capacitor. The technique is unusual in that it employs only pre-quantum mechanics concepts of electromagnetic wave interaction with charge carriers in metals (extended to semiconductors). The voltage-controlled delay is derived from the Drude Effect as the light passes through the carrier plasma under the gate electrode induced during accumulation or depletion. Modulation up to 40 Gb/s has now been attained. However, the W dimension of the "gate" electrode of the MOS Capacitor was reported as 2.4mm, which is too long for practical use in driving intra-chip wiring and demands large drivers for the electrode charging. Additionally the L dimension of the "gate" must be wide enough to accommodate the optical wavelength and this therefore limits the speed with which carriers can perfuse into the region under the gate. To enhance the efficiency of such a device one can redirect the light repeatedly back and forth through the plasma, effectively reusing these carriers. Optical resonators have been employed to accomplish this. But the optical cavity has a long "ring down" time, limiting the bandwidth of such a device. In this paper we consider the introduction of an augmenting structure called a "photonic crystal" in modern parlance [1]. However, in one important aspect, the concept of photonic crystal is well known to microwave engineers as a slow wave structure. By slowing the optical wave's propagation through the plasma the interaction may be greatly enhanced [2], while the lengthy ring down time of the resonator can be avoided. When this is employed, the FET-capacitor largest dimension shrinks to perhaps 100 microns. The conclusion is that the reuse of the plasma can shrink the size of the device. Additionally the current required to move the charge into the optical beam and back out is reduced if the total volume of the charge carriers can be reduced. Hence this enhancement can both reduce total power and physical dimension, while controlling the ring down time that plagues the resonator. But the frequency limitation implied by the short dimension of the device having to be larger than a wavelength limits its ultimate speed.