Letter pubs.acs.org/NanoLett
Ultralow-Loss CMOS Copper Plasmonic Waveguides Dmitry Yu. Fedyanin,*,† Dmitry I. Yakubovsky,† Roman V. Kirtaev,† and Valentyn S. Volkov†,‡ †
Laboratory of Nanooptics and Plasmonics, Moscow Institute of Physics and Technology, 9 Institutsky Lane, Dolgoprudny 141700, Russian Federation ‡ Centre for Nano Optics, University of Southern Denmark, Campusvej 55, Odense M DK-5230, Denmark ABSTRACT: Surface plasmon polaritons can give a unique opportunity to manipulate light at a scale well below the diffraction limit reducing the size of optical components down to that of nanoelectronic circuits. At the same time, plasmonics is mostly based on noble metals, which are not compatible with microelectronics manufacturing technologies. This prevents plasmonic components from integration with both silicon photonics and silicon microelectronics. Here, we demonstrate ultralow-loss copper plasmonic waveguides fabricated in a simple complementary metal-oxide semiconductor (CMOS) compatible process, which can outperform gold plasmonic waveguides simultaneously providing long (>40 μm) propagation length and deep subwavelength (∼λ2/50, where λ is the free-space wavelength) mode confinement in the telecommunication spectral range. These results create the backbone for the development of a CMOS plasmonic platform and its integration in future electronic chips. KEYWORDS: Copper plasmonics, plasmonic nanocircuitry, CMOS plasmonics, hybrid plasmonic waveguide, near-field optical microscopy
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materials,6,10 such as titanium nitride, can be perspective for epsilon near-zero and hyperbolic metamaterials, but they are too lossy at telecom wavelengths (e.g., εTiN = −75 + 23i at λ = 1.55 μm)11 and cannot compete with gold and silver (εAu = −115 + 11i and εAg = −129 + 3.3i)12 in deep-subwavelength plasmonic devices.13,14 Another material, which is even more import for modern microelectronics than aluminum, copper, is known to have a plasma frequency of about12 1.33 × 1016 s−1, which is very similar to that of gold. However, so far implementation of copper in plasmonic circuits was seriously restrained by high ohmic losses, resulting in very short SPP propagation lengths in deep-subwavelength waveguides.8,15,16 Here, we report the fabrication and characterization of ultralow-loss copper hybrid plasmonic waveguides, which ensure long propagation length along with strong mode confinement. Furthermore, we demonstrate for the first time that by using a relatively simple and CMOS compatible fabrication process, it is possible to outperform gold waveguides, which is crucially important for the design and development of nanophotonic circuits and their integration with electronic logic on a chip. The proposed waveguide fabrication process (Figure 1a) starts with deposition of a 170 nm-thick polycrystalline copper thin film on the (100)-oriented silicon wafer with a 2 nm-thick native oxide layer at a deposition rate of ∼1 Å/s. At room temperature under high-vacuum conditions (at a pressure of 3
lasmonics is widely considered to be the most promising candidate for the next generation of chip-scale technology.1−3 Surface plasmon polaritons (SPPs), which are collective electromagnetic excitations at the interface between a metal and an insulator, give an opportunity to overcome the diffraction limit, decrease the size of optical component down to the size of an electronic transistor, and route optical signals at dimensions well below the light wavelength.3−5 At the same time, implementation of plasmonic components fundamentally requires a low-loss material with a negative dielectric function. To date, characteristics acceptable for practical implementation have been demonstrated only with noble metals, gold and silver, which combine a significantly high real part ε′ of permittivity and a low imaginary part ε″ ≪ |ε′| responsible for SPP losses.6 However, real-world practical applications of plasmonic devices require them to be compatible with industry standard fabrication processes, such as the complementary metal-oxide-semiconductor (CMOS) technology, which allows low-cost fabrication of large-scale plasmonic structures and their integration with electronic logic and on-chip photonics. Aluminum is CMOS compatible and is widely used in microelectronics. For decades it was believed to be very lossy for practical applications, however it was recently demonstrated that, thanks to the high free-electron density and consequently very high plasma frequency, it is superior to noble metals in ultraviolet7 and shows decent SPP propagation lengths in nanometer-scale metal-semiconductor waveguides at telecom wavelengths.8 Nevertheless, for the same reason, in the infrared, aluminum cannot provide the same level of SPP mode confinement as gold and silver.8,9 Many alternative plasmonic © XXXX American Chemical Society
Received: September 28, 2015 Revised: December 1, 2015
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DOI: 10.1021/acs.nanolett.5b03942 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 1. (a) Schematic illustration of the fabrication process. (b) Scanning electron microscopy (SEM) image of the fabricated waveguides with taper couplers. The width of each waveguide is measured to be 410 nm, and the taper angle is equal to 13°. (c) SEM image of a lateral cross-section of the fabricated CMOS plasmonic waveguide showing the SiN and Cu layers, separated by a 10 nm-thick layer of SiO2 highlighted in green.
× 10−6 Torr), 99.999% pure metal was deposited via electron beam evaporation. Formation of the thin copper film was immediately followed by the deposition of a 10 nm-thick silicon dioxide layer in the same evaporation chamber without exposing the sample to ambient conditions. On top of the insulator, a silicon-rich silicon nitride (SiN) film was deposited by plasma-enhanced chemical vapor deposition (PECVD) at a temperature of 660 K. The obtained SiN layer was measured to be 410 nm thick, and its refractive index was about 2.30 at telecommunication wavelength. To define the waveguide pattern, we used electron-beam lithography, lift-off techniques, and a plasma etching process. At the first step, a copolymer/ PMMA bilayer resist stack was formed and patterned by the electron-beam direct writing lithography system, which was followed by development in methyl isobutyl ketone. Thereafter, a 15 nm-thick hard chromium mask was created using electronbeam evaporation and lift-off processes. At the next step, the SiN layer was dry etched with SF6 plasma down to the bottom SiO2 layer. Finally, the chromium mask was removed using Ar plasma etching. In the last fabrication step, the sample was cleaved perpendicular to the waveguide direction (Figure 1b). Figure 1b,c illustrates an array of fabricated copper hybrid plasmonic waveguides. The waveguides were extended by funnel structures, which have been used to couple laser radiation into the waveguide mode. The fabricated CMOS hybrid plasmonic waveguides were characterized with a collection scanning near-field optical microscope (SNOM) having an uncoated fiber tip used as a probe. The SPP mode was excited via end-fire excitation by accurate positioning a tapered-lensed polarization-maintaining single-mode fiber, which focused the p-polarized laser radiation on the funnel tapering structure of the plasmonic waveguide. The far-field image (Figure 2a) clearly shows that the track of the propagating radiation completely vanishes at a distance of >40 μm from the taper coupler, while a very bright spot is observed at the waveguide termination. This proves that only a guided mode was efficiently excited and that scattering due to waveguide imperfections (e.g., sidewall roughness) was negligibly small. Following the far-field adjustment, the whole fiber-sample arrangement was moved under the SNOM head for near-field measurements. An uncoated fiber tip was raster
scanned across the sample surface at a constant distance of a few nanometers maintained by the shear-force feedback, and the radiation collected by the SNOM probe was detected with a femtowatt InGaAs photoreceiver (New Focus 2153), whereby both topographic and near-field optical images were recorded simultaneously. The near-field images (Figure 2b,c) obtained at a distance of about 100 μm from the in-coupling waveguide edge, which is done to decrease the influence of the stray light (i.e., the light that was not coupled into the SPP mode), demonstrate strong mode confinement: the cross-section of the SNOM optical image perpendicular to the waveguide yields a full width at halfmaximum of less than the waveguide width. We have also found that the signal does not go to zero outside the waveguide, indicating the presence of a homogeneous background at a level of
