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We present a silicon microring resonator with a lithium niobate top cladding and integrated tuning electrodes. Submicrometer thin films of z-cut lithium niobate are bonded to silicon microring resonators via benzocyclobutene. Integrated electrodes are incorporated to confine voltage controlled electric fields within the lithium niobate thin film. The electrode design utilizes thin film metal electrodes and an optically transparent electrode wherein the silicon waveguide core serves as both an optical waveguide medium and as a conductive electrode medium. The hybrid material system combines the electro-optic functionality of lithium niobate with the high index contrast of silicon waveguides, enabling compact low tuning voltage microring resonators. Optical characterization of fabricated devices results in a measured loaded quality factor of 11,500 and a free spectral range of 7.15 nm in the infrared. The demonstrated tunability is 12.5 pm/V, which is over an order of magnitude greater than electrode-free designs.
© 2013 Optical Society of America
1. Introduction
Voltage controlled silicon microring resonators enable electrically tunable optical filters [1], switches [2], and modulators [3] in compact integrated photonics for applications such as on-chip wavelength-division multiplexing (WDM) optical networks. Resonance tuning of a ring resonator can be achieved by changing the effective index of the optical mode in the ring waveguide. Electro-refraction based resonance tuning mechanisms in silicon microring resonators include tuning the refractive index of silicon by the thermo-optic effect and the plasma dispersion effect [4–6]. Alternatively, modifying the refractive index of integrated materials, such as polymers [7] and III-V semiconductors [8] in optical proximity to the silicon can also be exploited for tuning. Due to inversion symmetry, unstrained bulk silicon does not exhibit second-order susceptibility. Therefore, hybrid material systems consisting of silicon and other materials have been extensively studied to enhance the versatility and functionality of silicon photonics [9–11].
Single crystalline thin films of lithium niobate (LiNbO3) are attractive for compact integrated optical devices because of the high optical confinement and the well-understood optical properties of LiNbO3 [12]. A LiNbO3-on-insulator system consisting of ion-sliced LiNbO3 thin films bonded to a LiNbO3 substrate with a silicon dioxide or benzocyclobutene (BCB) bonding layer has been proposed as a platform for integrated optics [13]. Furthermore, LiNbO3 thin films have been integrated on silicon substrates by wafer bonding and polishing, or by ion slicing. Mode confinement is achieved by patterning the LiNbO3 thin film via dry etching [14] or selective oxidation of refractory metals on top of the LiNbO3 thin film [15]. Recently, a hybrid silicon-on-insulation (SOI) and LiNbO3 material system has been introduced to combine the benefits of high optical confinement in silicon waveguides and the second order susceptibility of LiNbO3 [16,17]. In the hybrid system, a thin film of LiNbO3 is bonded to the top of a silicon waveguide to serve as a portion of the top cladding of an optical waveguide mode by direct [16] or indirect [17] bonding. Compared to direct bonding, indirect bonding requires a lower processing temperature and is relatively insensitive to surface topography [18]. Modulation of optical resonances at DC [16] and RF [17] has been demonstrated in the hybrid system. However, no design to date includes integrated electrodes, limiting the tunability (shift of the optical resonance with unit voltage) and their applications in integrated photonics. By incorporating integrated electrodes in the hybrid silicon and LiNbO3 material system, voltage induced electric fields are locally confined to the LiNbO3 thin film thereby enabling large tunability. In contrast to the polymer-on-silicon platform that requires in-device poling of electro-optic polymers [7,19], the hybrid silicon/LiNbO3 system has immediate access to the second order susceptibility of the LiNbO3 thin film. The use of BCB for indirect bonding in the hybrid silicon/LiNbO3 system takes advantage of its high thermal stability, having a glass transition temperature of 350 °C [18].
In this paper, we present a low tuning voltage hybrid silicon and LiNbO3 optical microring resonator with integrated electrodes. The device is fabricated by indirectly bonding an 800 nm thick z-cut LiNbO3 ion-sliced thin film to serve as a portion of the top cladding of a 15 μm radius silicon microring resonator using BCB. The design utilizes thin film metal electrodes and an optically transparent electrode wherein the silicon waveguide core serves as both an optical waveguide medium and as a conductive electrode medium. The demonstrated tunability from fabricated devices is 12.5 pm/V, which is over an order of magnitude greater than electrode-free designs.
This paper is organized as follows. Section two describes the design of the tunable hybrid microring resonator. Section three conveys the fabrication details of the integrated electrodes and hybrid integration with indirect bonding. Section four describes the experimental setup and characterization results of the device. Finally, conclusions are given in section five.
2. Design
A schematic of the tunable hybrid silicon and LiNbO3 microring resonator is shown in Fig. 1(a). The resonator consists of a silicon rib waveguide microring and a LiNbO3 thin film bonded to the silicon resonator as a portion of the top cladding via BCB. The silicon rib waveguides are 500 nm wide with a 70 nm slab thickness and 180 nm rib height. A bottom metal electrode is formed around and exterior to the microring on the silicon slab to provide an electric path to the silicon core. Contact resistance is reduced by ion implantation and nickel silicidation processes. A top metal electrode is aligned to the microring on top of the LiNbO3 thin film and a SiO2 buffer layer that is deposited by plasma enhanced chemical vapor deposition (PECVD). Figure 1(b) shows a schematic of the cross-section of the device structure.
Fig. 1 (a) Schematic of a tunable hybrid silicon and LiNbO3 microring resonator with integrated electrodes. For clarity, the PECVD SiO2 top-cladding layer and electrical contact pads are not shown. (b) Schematic of the cross-section of the device structure along the dashed line shown in (a). Both schematics are not drawn to scale.
The LiNbO3 thin film and the PECVD SiO2 buffer layer serve as the top cladding. On the cross-section of the microring waveguide, a portion of the optical guided-mode is within the LiNbO3 thin film. Figure 2(a) shows the fraction of the optical mode power within the LiNbO3 thin film as a function of the thickness of the LiNbO3 thin film for the transverse electric (TE) and transverse magnetic (TM) modes, at 1550 nm optical wavelength, calculated using the semi-vector beam propagation method (BPM). In the simulation, the PECVD SiO2 layer below the aluminum electrode is set to be 1 µm thick. The refractive indices of silicon, SiO2, LiNbO3 and BCB are set to 3.48, 1.44, 2.14 and 1.54, respectively, in the simulation. The maximum fraction of the optical mode power in the LiNbO3 approaches 42% for the TM mode and 11% for the TE mode as the LiNbO3 thickness approaches 1 µm. The larger optical mode field overlap with the LiNbO3 for the TM mode is desirable for greater tuning efficiency. Moreover, the TM mode accesses the r33 electro-optic coefficient in LiNbO3, whereas the TE mode accesses the r13 electro-optic coefficient (r33 = 31 pm V−1, r31 = 8 pm V−1 in bulk LiNbO3) [20,21]. Therefore, the device is designed for TM mode. Furthermore, the fraction of the optical mode power in the LiNbO3 for the TM mode is marginally improved for LiNbO3 thicknesses greater than 600 nm. The thickness of the LiNbO3 is therefore chosen to be 800 nm.
Fig. 2 (a) BPM calculations of the optical mode power in LiNbO3 versus the thickness of LiNbO3 for the TM mode and the TE mode in the hybrid Si/LiNbO3 structure. (b) Calculations of the optical loss (blue) induced by the top aluminum electrode and the voltage induced vertical electric field in LiNbO3 (red) versus the PECVD silicon dioxide thickness. The LiNbO3 thin film thickness is set to be 800 nm and the applied voltage is 1 V.
The use of the silicon microring as a transparent conductor minimizes the dielectric layer thickness required to isolate the optical mode from the electrodes [22]. As a result, the voltage induced electric field inside the LiNbO3 cladding layer is enhanced thereby enabling a larger change in the refractive index of the LiNbO3 via the linear electro-optic effect. The PECVD SiO2 cladding layer thickness significantly influences the electric field intensity in the LiNbO3 (εz = 29.1 [20]) thin film since the permittivity of LiNbO3 is much higher than that of PECVD SiO2 (relative permittivity εr = 4.2 [23]). The red curve in Fig. 2(b) shows the vertical (perpendicular to the surface of the substrate) electric field intensity (Ez) of the applied DC field in the LiNbO3 thin film as a function of the PECVD SiO2 thickness simulated using the finite element method (FEM) electro-static solver. The electric field distribution within LiNbO3 is not uniform due to the rib topology of the silicon waveguide. Therefore, the electric fields are evaluated at the center of the LiNbO3 thin film directly above the center of the silicon waveguide. For an 800 nm thick LiNbO3 thin film and an applied voltage of 1 V between the top electrode and the silicon core, Ez in the LiNbO3 thin film decreases from 1.15 V/µm to 0.45 V/µm as the PECVD SiO2 layer increases from 0 nm to 300 nm.
A thinner PECVD SiO2 layer is desirable for achieving higher electric field in LiNbO3. Conversely, reducing the PECVD SiO2 thickness leads to optical loss from the optical mode interacting with the top aluminum electrode. The blue curve in Fig. 2(b) shows the TM mode optical loss caused by the aluminum electrode at 1550 nm wavelength, as a function of the PECVD SiO2 thickness, calculated by BPM. In the simulation, the imaginary part of the refractive index of aluminum is set to 16 [24]. The thickness of the PECVD SiO2 layer is chosen to be 125 nm. Relatively large electric fields in the LiNbO3 and metal-induced optical loss less than 0.2 dB/cm are expected.
Compared to fabricated devices, the simulations shown in Fig. 2 neglect several tens of nanometers of BCB that resides on the top surface of the silicon core. We have previously demonstrated that the BCB layer between the top of the silicon core and the bottom of the LiNbO3 thin film can be as thin as approximately 20~30 nm [17]. The presence of the BCB layer affects both the optical mode overlap in the LiNbO3 thin film and the bending loss of the microring resonator. The optical mode power decreases in the LiNbO3 as the thickness of the BCB on top of the silicon core increases. The result is lower tuning efficiency. Figure 3(a) shows the TM mode optical bending loss as a function of ring radius for BCB thicknesses of 0 nm, 20 nm, and 40 nm. As the BCB thickness increases, the optical mode becomes less confined and the bending loss increases. Considering fabrication tolerances, the ring radius is chosen to be 15 µm. For a BCB layer of 20 nm on top of the silicon core, the bending loss for a 15 µm radius ring is 0.02 dB/cm.
Fig. 3 (a) Calculated TM mode optical bending loss versus the ring radius for BCB thickness of 0 nm, 20 nm, and 40 nm between the top of the silicon core and the bottom of the LiNbO3 thin film. (b) Calculated optical electric field distribution of the hybrid silicon and LiNbO3 structure for the fundamental TM mode at 1550 nm wavelength (Ez component).
Figure 3(b) shows the cross-section electric field distribution of the final design for the fundamental TM mode at 1550 nm optical wavelength calculated using BPM. Material boundaries are indicated by the white dashed lines and the material regions are indicated in the insets. The BCB thickness is 0 nm in the simulation. The effective index is calculated to be 2.3.
3. Fabrication
The fabrication process is shown schematically in Fig. 4. The process begins with a silicon-on-insulator (SOI) wafer with a p-type background doping of 1015 cm−3, a buried oxide (BOX) thickness of 1 μm, and a silicon device layer thickness of 250 nm. Silicon rib microring waveguides and bus waveguides with inverse width tapers are defined in hydrogen silsesquioxane (HSQ) resist using electron-beam lithography (EBL) and inductively coupled plasma reactive ion etching (ICP-RIE) using Cl2/O2 chemistry. The silicon rib waveguides are 500 nm wide with a 70 nm slab thickness and 180 nm rib height. The radii of the ring resonators are 15 μm and the coupling gaps are 350 nm.
Fig. 4 Fabrication process of the device: (a) Silicon rib waveguide ring resonator patterned on SOI wafer using electron beam lithography and plasma etch, (b) BF2+ ion implantation, (c) nickel silicidation, (d) 100 nm aluminum bottom electrode deposition, (e) spin-coat of BCB, (f) indirect bonding of LiNbO3 thin film, (g) plasma etch of BCB, (h) deposition of 125 nm PECVD SiO2 (illustrated on top of the LiNbO3 only for simplicity), (i) deposition of 250 nm top aluminum electrode, (j) deposition of 900 nm PECVD SiO2, (k) patterning of via and top aluminum pad, (l) fabrication of cantilever couplers.
The bottom electrode fabrication process consists of ion implantation, nickel silicidation, and aluminum evaporation, as shown in Figs. 4(b)–4(d). The silicon slab area around and exterior to the microring is doped with BF2+ ions with a fluence of 2 × 1015 cm−2, resulting in a doping level of 8 × 1019 cm−3, for contact with the bottom metal electrode. Twenty five nanometers of nickel silicide is formed in the doped region before 100 nm of aluminum is deposited in order to reduce contact resistance.
Thin films of LiNbO3 are obtained from a bulk single crystal z-cut LiNbO3 wafer using helium ion implantation, thermal treatment, and chemical etch [15]. The wafer is implanted on the + z side of the wafer with an implantation energy of 342 keV and a fluence of 4 × 1016 ions cm−2. After implantation, the wafer is annealed using rapid thermal annealing (RTA) at 300 °C and then etched in 10% hydrofluoric acid (HF) solution. Non-uniform stress produces 900 nm thick LiNbO3 thin films in the areal shape of strips and triangles whose edges are formed along crystal planes. The exfoliated thin films are directly bonded to an unpolished silicon carrier and annealed by RTA at 1000 °C for 30s to repair the crystal lattice and restore the electro-optic properties [25]. The surface roughness of the exfoliated side of the LiNbO3 thin film is reduced to 4 nm by ICP etching with Ar chemistry resulting in a final film thickness of 800 nm. Next, BCB is spin-coated from solution onto the silicon rib waveguides and bottom electrode as shown in Fig. 4(e). LiNbO3 thin films are transferred to the top of the SOI ring resonators with the + z side facing down using a fiber tip on a probe station. After transfer, the sample is annealed to 250 °C to cure the BCB. The LiNbO3 thin film does not crack after annealing despite the large mismatch in thermal expansion coefficients between silicon and LiNbO3. BCB that is not covered by the LiNbO3 thin film is etched via ICP-RIE with CF4/O2 chemistry as shown in Fig. 4(g). A 125 nm thick PECVD SiO2 layer is then blanket deposited to provide optical isolation between the optical mode and the top metal electrode. A 250 nm thick top aluminum electrode is patterned on top of the PECVD SiO2 isolation layer. A final 900 nm thick PECVD SiO2 film is deposited as a capping layer. Electrical interconnects through the SiO2 capping layer are etched and aluminum contact pads are patterned to allow access to the top and bottom electrodes. Finally, cantilever couplers are patterned for fiber-to-chip optical coupling [26,27]. Throughout the fabrication process, EBL with alignment markers is used to generate multi-layer patterns with layer-to-layer misalignment less than 100 nm. Positive tone resist poly-methyl methacrylate (PMMA) and negative tone resist HSQ are used to generate the masks.
4. Measurements
The ability to tune the hybrid microring resonator is characterized by optical transmission measurements. The experimental setup is shown in Fig. 5 along with a top-view optical micrograph of the fabricated device. An infrared continuous-wave laser source is connected to a polarization controller which outputs linearly polarized TM mode light. Tapered optical fibers with tip diameters of approximately 2 μm are butt-coupled to the input and output cantilever couplers of the device. The output light is detected using a photodetector and measured by a power meter. Bias voltage is applied to the device through the integrated electrodes using a voltage source and DC probes. DC bias is applied to the top aluminum electrode and the bottom aluminum electrode is grounded. The measured transmission near 1552 nm as a function of applied voltage is given in Fig. 6(a). By changing the DC bias between −10 V and 10 V, the resonance position linearly shifts by 250 pm between 1552.195 nm and 1551.945 nm. The measured shift in resonance position yields a tunability of 12.5 pm/V. The blueshift (redshift) of the resonance with positive (negative) applied voltage is consistent with the orientation of the LiNbO3 c-axis.
Fig. 6 (a) Measured optical transmission of a single resonance as a function of applied voltage. The measured resonance tunability is 12.5 pm/V. (b) Measured optical transmission spectrum of two consecutive resonances.
The optical transmission of two consecutive resonances with varying DC bias is shown in Fig. 6(b). The measured quality factor for both resonances is approximately 11,500 and the free spectral range (FSR) is 7.15 nm, indicating a group index of 3.57. The ripple between resonances is due to Fabry-Pérot fringes created between the input and output fiber-to-chip coupling facets. Resonance tunability for the resonance near 1559 nm is again measured to be 12.5 pm/V. For the measured rings, 10.8 V applied bias is required for tuning of 135 pm which is equal to the full-width-half-maximum (FWHM) of the measured resonance. The demonstrated tunability is equivalent to a VπL value of 2.63 V cm.
The hybrid silicon/LiNbO3 microring device with integrated electrodes is attractive for tunable on-chip filters, switches and modulators. The tunability demonstrated in this paper is over an order of magnitude greater than the metal electrode-free hybrid silicon/LiNbO3 microring resonator design discussed in reference 16 (~0.6 pm/V) and the LiNbO3 thin film microring resonator discussed in reference 12 (~1 pm/V). Furthermore, the demonstrated tunability is comparable to state-of-the-art tunable silicon microring resonators based on reverse biased PN junctions (~20 pm/V at the optimal bias condition) [28,29] and hybrid silicon-polymer slot waveguide microring resonators (16.5 pm/V) [7]. Finally, resonance tuning of the hybrid silicon/LiNbO3 microring resonator is linear for both forward and reserve bias with little effect on the quality factor and extinction ratio.
5. Conclusion
A low tuning voltage hybrid silicon and LiNbO3 microring resonator with integrated electrodes is designed, fabricated, and characterized. The hybrid device combines an ion-sliced LiNbO3 thin film with an SOI rib waveguide ring resonator using BCB as an intermediate bonding layer. The tunability of the resonator is enhanced by optimizing the electrode design to increase the electric field intensity in the thin film. The electrode design utilizes thin film metal electrodes and an optically transparent electrode wherein the silicon waveguide core serves as both an optical waveguide medium and as a conductive electrode medium. The resonator has a ring radius of 15 μm, measured loaded quality factor of 11,500, and FSR of 7.15 nm at infrared wavelengths for the TM mode. A tunability of 12.5 pm/V is demonstrated, which is over an order of magnitude greater than electrode-free designs. The large tunability of the hybrid silicon and LiNbO3 device is attractive for compact integrated photonics. Future work involves device optimization for high electrical frequency operation.
Acknowledgments
This work was supported by the Army Research Office (ARO) under grant number W911NF-12-1-0488.
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