Patent Description:
The present invention relates generally to optical communications, and particularly to devices and methods for high-speed modulation of optical signals.

High-speed optical communications require modulation of light sources at very high frequencies. The fastest optical interconnects that have been implemented to date are capable of operating at <NUM> Gb/s per lane, and higher speeds are in the planning stages.

Plasmonic modulators have been proposed as a possible solution to the need for higher modulation speed. Modulators of this sort are based on the interaction between surface plasmon polaritons (SPPs) and externally applied electric fields. Surface plasmon polaritons are generated at the interface between a dielectric material and a metal, and can be directly excited by light beams. Application of a rapidly-varying electric field to the metal causes a corresponding modulation of the SPPs, which in turn translates into modulation of the light beam at the end of the plasmonic regime. The term "plasmonic modulator" is used in the present description and in the claims to refer to devices that apply an electric field to modulate surface plasmons polaritons, which are then converted back to light beams at the end of the plasmonic regime. The term "light" is used in this context to optical radiation in any of the visible, infrared and ultraviolet ranges.

Plasmonic modulators based on the above principles have been demonstrated experimentally and described in the patent literature, but they are still far from commercial deployment. For example, <CIT> describes a metal-oxide-semiconductor plasmonic slot waveguide, which includes a silicon layer, a silicon oxide layer laterally disposed next to a first side wall of the silicon layer, a first metal layer laterally disposed next to the silicon oxide layer, and a second metal layer laterally disposed next to a second side wall of the silicon layer, wherein the second side wall is opposite to the first side wall. A plasmonic mode can propagate along the slot waveguide, and the propagating characteristics can be adjusted by the voltage applied on the metal layers. A metal-oxide-semiconductor plasmonic modulator includes first and second metal-oxide-semiconductor plasmonic slot waveguides of this sort.

<CIT> describes an electro-optic modulator is provided which comprises an electro-optic substrate, a Mach-Zehnder Interferometer optical waveguide structure, and at least two electrodes. In an embodiment of the invention, the Mach-Zehnder Interferometer optical waveguide structure has a mode transition section for reducing the optical mode size and bringing the optical mode center closer to the upper surface of the electro-optic substrate. In another embodiment, there is a laterally recessed adhesion layer disposed between the electrodes and the electro-optic substrate. The recess in this adhesion layer minimizes the dissipative effect that the adhesion layer would ordinarily have on the propagation of RF energy along the electrode. In yet another embodiment, a dual drive arrangement is described for driving an electro-optic modulator. Finally, an integrated modulator array is described.

<CIT> describes Methods and systems for hybrid integration of optical communication systems which include receiving continuous wave (CW) optical signals in a silicon photonics die (SPD) from an optical source external to the SPD. The received CW optical signals are processed based on electrical signals received from an electronics die bonded to the SPD via metal interconnects. Modulated optical signals are received in the SPD from optical fibers coupled to the SPD. Electrical signals are generated in the SPD based on the received modulated optical signals and communicated to the electronics die via the metal interconnects. The CW optical signals are received from an optical source assembly coupled to the SPD and/or from one or more optical fibers coupled to the SPD. The received CW optical signals are processed utilizing one or more optical modulators, which comprise Mach-Zehnder interferometer modulators.

Embodiments of the present invention that are described hereinbelow provide improved devices and methods for high-speed modulation of light beams.

There is therefore provided, in accordance with the invention, an optoelectronic device, including a semiconductor substrate and thin film structures disposed on the substrate and patterned to define components of an integrated drive circuit, which is configured to generate a drive signal. A back end of line (BEOL) stack of alternating metal layers and dielectric layers disposed over the thin film structures. The metal layers include a modulator layer, which contains a plasmonic waveguide and is patterned to define a plurality of electrodes, which are configured to apply a modulation to surface plasmons polaritons (SPPs) propagating in the plasmonic waveguide in response to the drive signal applied to the electrodes. A plurality of interconnect layers are patterned to define electrical traces, which are connected by vias to the thin film structures on the substrate and to the electrodes in the modulator layer so as to interconnect the components of the integrated drive circuit and to apply the drive signal generated thereby to the electrodes. An optical input coupler is configured to couple light into the modulator layer, whereby the light is modulated by the modulation of the SPPs. An optical output coupler is configured to couple the modulated light out of the modulator layer.

In one embodiment, the plasmonic waveguide is configured as a Mach-Zender interferometer, having first and second parallel legs, and the electrodes includes at least first and second electrodes, which are configured to apply the modulation to the SPPs with different, respective phases to the first and second parallel legs.

In another embodiment, the electrodes are patterned to define a ring modulator.

In some embodiments, at least one of the optical couplers is disposed in a plane of the modulator layer. Alternatively or additionally, at least one of the optical couplers is formed on the modulator layer and is configured to couple the light between the modulator layer and a propagation direction that is not parallel to a plane of the modulator layer.

In a disclosed embodiment, the modulator layer is a final, outer layer of the BEOL stack.

In some embodiments, the device includes an electro-optical layer disposed over the modulator layer and within the plasmonic waveguide. Additionally or alternatively, a transparent conductive oxide is disposed over the modulator layer and within the plasmonic waveguide.

There is also provided, in accordance with the invention, a method for fabrication of an optoelectronic device. The method includes forming and patterning thin film structures on a semiconductor substrate so as to define components of an integrated drive circuit, which is configured to generate a drive signal. A back end of line (BEOL) stack of alternating metal layers and dielectric layers is deposited and patterned over the thin film structures. The metal layers include a modulator layer, which contains a plasmonic waveguide and is patterned to define a plurality of electrodes, which are configured to apply a modulation to surface plasmon polaritons (SPPs) propagating in the plasmonic waveguide in response to the drive signal applied to the electrodes. A plurality of interconnect layers are patterned to define electrical traces, which are connected by vias to the thin film structures on the substrate and to the electrodes in the modulator layer so as to interconnect the components of the integrated drive circuit and to apply the drive signal generated thereby to the electrodes. An optical input coupler is formed below, in, or over the modulator layer for coupling light into the modulator layer, whereby the light is modulated by the modulation of the SPPs, and an optical output coupler is formed below, in, or over the modulator layer for coupling the modulated light out of the modulator layer.

Plasmonic modulators are physically capable of modulating light at rates well in excess of <NUM> Gb/s. Furthermore, due to the increased light-matter interaction exhibited by surface plasmon polaritons (SPPs), plasmonic devices can apply deep modulation to an incident light beam over very short interaction lengths, while requiring only moderate excitation voltages. To realize these advantages in practical devices, however, it is also necessary to generate and apply the required high-speed electrical drive signals to the plasmonic modulator efficiently, over a link that is as short as possible, while minimizing parasitic capacitance and power loss.

Embodiments of the present invention that are described herein address this need by integrating a plasmonic modulator into the back end of line (BEOL) stack of the same integrated circuit (IC) device that generates the drive signal. As is well known in the art, IC fabrication starts with front end of line (FEOL) steps, in which thin film structures are deposited on a semiconductor substrate, such as a silicon wafer, and are patterned to define the components of the IC, such as transistors, diodes, capacitors and resistors. The BEOL stack is then formed by depositing alternating metal layers and dielectric layers over the thin film structures. The metal layers are patterned to define electrical traces, which are connected by vias to the thin film structures on the substrate so as to interconnect the components of the IC and thus create a functional device.

In the present embodiments, one of the BEOL layers serves as a modulator layer. An optical input coupler couples light into the modulator layer. This layer contains a plasmonic waveguide, with metal patterned to define electrodes in contact with the waveguide. The FEOL and the remaining, interconnect layers of the BEOL stack form a high-speed integrated drive circuit, which generates a drive signal for the modulator. Vias between the interconnect layers and the modulator layer supply this drive signal to the electrodes, which thus modulate the plasmons in the waveguide. This modulation is translated into optical modulation when the plasmons are converted back to light at the output of the plasmonic modulator. An optical output coupler couples the modulated light out of the modulator layer.

This use of a BEOL layer as an optical modulator differs from usual IC fabrication practices, in which the active components of the IC are in the FEOL layers, and the BEOL provides only passive interconnects. BEOL processes, however, are well suited to etching the electrical structures and plasmonic waveguide of the modulator and can be adapted to receive and transmit the light beam that is to be modulated, using optical coupling techniques that are known in the art. The three-dimensional integration of the modulator with its drive circuit in the same IC chip that is provided by the present embodiments achieves high integration density, as well as very short interconnects, with low parasitic capacitance, between the drive circuit and the modulator. Modulators in accordance with the present embodiments are thus capable of ultrahigh-speed modulation with high electrical efficiency and low heat dissipation.

Reference is now made to <FIG>, which schematically illustrate an integrated plasmonic modulation device <NUM>, in accordance with an embodiment of the invention. <FIG> is a top view, while <FIG> and <FIG> are pictorial and side views, respectively, showing details of the internal structure of the device. The upper layer of device <NUM> comprises a plasmonic modulator <NUM>, which receives an input beam of light via an input optical waveguide <NUM>, and outputs a modulated beam to an output optical waveguide <NUM>. Waveguides <NUM> and <NUM> may comprise optical fibers, for example. Alternatively, modulator <NUM> may receive and transmit beams of light through free space.

Modulator <NUM> in this embodiment has the form of a Mach-Zender interferometer, comprising a slot waveguide <NUM> for surface plasmon polaritons (SPPs) within a metal modulator layer <NUM>. Waveguide <NUM> splits into two parallel legs at a Y-junction at one end of modulator <NUM>, which then rejoin at another Y-junction at the other end. The slots of waveguide <NUM> are defined by a common central electrode <NUM> and excitation electrodes <NUM> and <NUM> on opposing sides of the modulator. In a typical implementation, the slots are about <NUM>-<NUM> deep, <NUM>-<NUM> wide, and <NUM>-<NUM> long; but these dimensions are presented solely by way of example, and larger or smaller dimensions may alternatively be used. Electrode <NUM> may be grounded, for example, while electrodes <NUM> and <NUM> are driven by signals with different respective phases, such as a drive signal S on electrode <NUM> and its inverse S̃ on electrode <NUM>. The drive signals are typically in the range of a few volts peak-to-peak, but larger or smaller voltages may alternatively be used depending on application requirements. An electro-optical layer <NUM> is deposited over modulator layer <NUM>, as shown in <FIG>, and fills the slots of waveguide <NUM>. Layer <NUM> may comprise any suitable sort of electro-optical material, for example a monolithic chromophore, such as DLD164, or a suitable ceramic, such as barium titanate (BaTiO3).

Optical couplers <NUM>, parallel to the plane of modulator layer <NUM>, couple the light into and out of modulator <NUM>. In the pictured example, couplers <NUM> comprise tapered dielectric waveguides, for example SiN waveguides, which are formed by deposition and etching in a dielectric layer <NUM> below modulator <NUM>. Alternatively, couplers <NUM> may comprise other suitable optical materials, such as silicon, and may be formed in or over the plane of modulator <NUM>. Couplers <NUM> in the present example taper adiabatically, so that light propagates into and out of waveguide <NUM> in a single mode, without substantial reflection or energy transfer into higher order-modes, thus exhibiting low optical loss. Alternatively, other suitable sorts of couplers may be used. Although modulator layer <NUM> is shown in <FIG> as being the top metal layer in device <NUM>, the modulator layer may alternatively be an internal layer within the IC device.

As shown in <FIG> and <FIG>, device <NUM> comprise FEOL layers <NUM>, overlaid by a BEOL stack <NUM>, which includes modulator layer <NUM>. FEOL layers <NUM> typically comprise a semiconductor substrate <NUM>, such as a silicon wafer substrate. Thin film structures <NUM> are formed on substrate <NUM> by processes of doping, thin film deposition, and etching, for example, with an overlying dielectric layer <NUM>, comprising SiO<NUM>, for example. BEOL stack <NUM> comprises a lower metal layer <NUM>, followed by alternating dielectric layers <NUM> and metal interconnect layers <NUM>. Lower metal layer <NUM> and interconnect layers <NUM> are patterned to define electrical traces, which are interconnected by vias <NUM> through dielectric layers <NUM> and by vias <NUM> through dielectric layer <NUM> to thin film structures <NUM>.

The patterned metal layers <NUM>, <NUM> and vias <NUM>, <NUM> thus interconnect the components of FEOL layers <NUM> to create an integrated drive circuit, and may comprise associated logic circuits, as well. Any suitable IC technology that is known in the art may be used for this purpose. For example, the drive circuit may be implemented using standard complementary metal-oxide-semiconductor (CMOS) technology. Alternatively, for higher speed, the drive circuits may be implemented using BiCMOS technology, which combines CMOS transistors with bipolar junction transistors.

In any case, the drive signal generated by this drive circuit is applied between electrodes <NUM>, <NUM> and <NUM> through vias <NUM>, which connect modulator layer <NUM> to the next metal layer <NUM> down BEOL stack <NUM>. (In an alternative embodiment, not shown in the figures, the modulator layer can be located at an intermediate level, as noted earlier, with interconnecting vias above and below. ) Layer <NUM> may comprise any suitable metal, and not necessarily one of the metals normally used with the IC technology of the drive circuit.

<FIG> is a schematic pictorial view of an integrated plasmonic modulation device <NUM>, in accordance with another embodiment of the invention. Device <NUM> is substantially similar to device <NUM>, as described above, except that in device <NUM>, an electro-optical layer <NUM> is deposited only over the part of the modulator layer where it is actually required, for example over the parallel legs of waveguide <NUM>. This approach leaves the rest of the upper surface of device open for other features, such as optical couplers (not shown in this figure). Alternatively, the electro-optical material may be deposited only within the slots of the waveguide, particularly when the modulator layer is not the outer layer of the BEOL stack.

<FIG> is a schematic pictorial view of an integrated plasmonic modulation device <NUM>, in accordance with yet another embodiment of the invention. Device <NUM> also resembles device <NUM>, with modulator <NUM> in the final, outer layer of BEOL stack <NUM>. Instead of the in-plane optical coupling in device <NUM>, however, device <NUM> comprises out-of-plane optical couplers <NUM>, for example grating couplers, as are known in the art. Couplers <NUM> are formed on the modulator layer and couple light into and out of waveguide <NUM> from and to propagation directions that are not parallel to the plane of the modulator layer. The light may be transmitted to and from couplers <NUM> through free space or through waveguides, such as suitably positioned optical fibers.

Although device <NUM> is shown in <FIG> as comprising both input and output couplers <NUM> of this sort, in an alternative embodiment, the modulator device may comprise one in-plane coupler and one out-of-plane coupler.

<FIG> is a schematic detail view of a plasmonic ring modulator <NUM>, in accordance with still another embodiment of the invention. Ring modulator <NUM> may be used in the sorts of plasmonic modulation devices described above in place of Mach-Zender modulator <NUM>, mutatis mutandis. Alternatively, other plasmonic modulator configurations (not shown in the figures) that are suitable for integration in the BEOL stack may be used, as well.

In the example shown in <FIG>, a dielectric waveguide <NUM>, comprising SiN, for example, is deposited over or formed within a dielectric (typically SiO<NUM>) layer <NUM> in the BEOL stack. Alternatively, other types of waveguides may be used, such as a silicon waveguide. A disk electrode <NUM> and a surrounding ring electrode <NUM> are patterned in the overlying metal layer to define the ring modulator, with a circular slot waveguide <NUM> between the electrodes. The slot is typically about <NUM>-<NUM> deep, <NUM>-<NUM> wide, and <NUM> in diameter, and is filled with an electro-optical material, as in the preceding embodiment. (These dimensions are again presented solely by way of example, and larger or smaller dimensions may alternatively be used. ) Electrical contacts <NUM> and <NUM>, which are typically formed as part of the BEOL metallization, apply the drive signal between the electrodes, across waveguide <NUM>, thus modulating the light passing through waveguide <NUM>. Although contact <NUM> is shown in <FIG> as a bridge extending above ring electrode <NUM>, this contact may alternatively be made from below disk electrode <NUM>, within the BEOL stack.

<FIG> is a schematic detail view of a plasmonic slot modulator <NUM>, in accordance with still another embodiment of the invention. Slot modulator <NUM> may be used in the sorts of plasmonic modulation devices described above in place of Mach-Zender modulator <NUM>, mutatis mutandis.

In the example shown in <FIG>, a slot <NUM> is etched through a metal layer of the BEOL stack, thus defining electrodes <NUM> on one or both sides of the slot. The metal layer may comprise gold, for example, with a thickness (and thus slot depth) of <NUM>, which is deposited over a dielectric layer <NUM>, such as SiO<NUM>. The width of the slot is typically in the range of <NUM>, for example. A thin dielectric layer <NUM>, for example Al<NUM>O<NUM>, with a thickness of about <NUM>, is deposited over electrodes <NUM>. A transparent conductive oxide (TCO) layer <NUM>, such as indium tin oxide (ITO) is then deposited over and within slot <NUM>.

Application of a voltage V to electrodes <NUM> causes charges <NUM> to accumulate in slot <NUM>, thus changing the permittivity and hence the absorption of plasmons in in the TCO within the slot. Modulating the voltage results in a corresponding modulation of the absorption. Modulator <NUM> is thus capable of high-frequency plasmonic modulation by electro-absorption, without requiring the sort of interferometric structures used in the preceding embodiments.

<FIG> is an electrical schematic diagram of an integrated drive circuit <NUM> for use in a plasmonic modulation device, in accordance with an embodiment of the invention. A drive circuit of this sort can be implemented in FEOL layers <NUM> of the plasmonic modulation devices described above, with interconnects through BEOL stack <NUM>. Drive circuit <NUM> is suitable for implementation using BiCMOS technology, in order to reach a drive bandwidth that fully exploits the available modulation bandwidth of plasmonic modulator <NUM>.

Drive circuit <NUM> uses a power multiplexer (PMUX) approach, in which multiple low-speed tributaries are combined through a number of multiplexing stages to create a high-speed signal. In other words, drive circuit <NUM> receives several input data signals (four signals in the pictured example) via respective buffer amplifiers <NUM>. A clock divider <NUM> splits an input clock at the desired drive frequency f into component clocks of frequency f/<NUM> with phases <NUM>° apart. Individual clock dividers <NUM> split these clock signals again into four input clocks with frequency f/<NUM> and different, respective phases for the four input data channels. Two multiplexers <NUM> each combine a pair of the input signals, and a multiplexer <NUM> combines these paired signals to generate the drive signal to modulator <NUM> at the drive clock rate f, which is four times the input clock rate of each of the four data channels.

Alternatively, a smaller or larger number of data inputs and multiplexing stages can be used to generate drive signals at a smaller or larger multiple of the input clock rate. For example, the outputs of two <NUM>:<NUM> multiplexers with the topology shown in <FIG> may be multiplexed together with different, respective phases in order to multiplex eight data channels into a single output.

The final multiplexer <NUM> directly drives plasmonic modulator <NUM>, without requiring an additional buffer or driver amplifier. The signal modulation format in the embodiment shown in <FIG> is non-return-to-zero (NRZ), but alternatively, the drive circuit can be adapted to other modulation schemes, such as four-level pulse amplitude modulation (PAM4).

In alternative embodiments, other sorts of drive circuits can be used to drive plasmonic modulators in accordance with the present embodiments, even if not at the maximal data rate supported by the modulator. For example, plasmonic modulator <NUM> can be integrated in the BEOL stack of a CMOS IC, such as a CMOS switching circuit. In this case the "drive circuit" of the modulator could simply be the SerDes (serializer/deserializer) at the output of the switching circuit.

In any of the above embodiments, because of the small dimensions of the plasmonic modulator, the modulator is seen by the drive circuits as a small, lumped capacitive load, typically on the order of a few femtofarads. Because the modulator is so closely coupled to the FEOL layers of the IC, there is little or no parasitic capacitive or inductive loss in the circuit, and no need for <NUM> ohm termination. Thus, the plasmonic modulator can fully exploit the available data rate and driving power of the drive circuit, regardless of the IC technology - whether high-speed BiCMOS as in <FIG>, CMOS, or any other suitable IC type.

Claim 1:
An optoelectronic device (<NUM>), comprising:
a semiconductor substrate (<NUM>);
thin film structures (<NUM>) disposed on the substrate and patterned to define components of an integrated drive circuit (<NUM>), which is configured to generate a drive signal;
a back end of line, BEOL, stack (<NUM>) of alternating metal layers (<NUM>, <NUM>) and dielectric layers (<NUM>) disposed over the thin film structures, the metal layers comprising:
a modulator layer (<NUM>), which contains a plasmonic waveguide (<NUM>, <NUM>, <NUM>) and is patterned to define a plurality of electrodes (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), which are configured to apply a modulation to surface plasmons polaritons, SPPs, propagating in the plasmonic waveguide in response to the drive signal applied to the electrodes; and
a plurality of interconnect layers (<NUM>), which are patterned to define electrical traces, which are connected by vias to the thin film structures on the substrate and to the electrodes in the modulator layer so as to interconnect the components of the integrated drive circuit and to apply the drive signal generated thereby to the electrodes;
an optical input coupler (<NUM>, <NUM>), which is configured to couple light into the modulator layer, whereby the light is modulated by the modulation of the SPPs; and
an optical output coupler (<NUM>, <NUM>), which is configured to couple the modulated light out of the modulator layer.