An AlN electro-optic phase shifter with a parallel plate capacitor structure is fabricated on Si using the back-end complementary metal-oxide-semiconductor technology, which is feasible for multilayer photonics integration. The modulation efficiency (Vπ⋅Lπ product) measured from the fabricated waveguide-ring resonators and Mach-Zehnder Interferometer (MZI) modulators near the 1550-nm wavelength is ∼240 V⋅cm for the transverse electric (TE) mode and ∼320 V⋅cm for the transverse magnetic (TM) mode, from which the Pockels coefficient of the deposited AlN is deduced to be ∼1.0 pm/V for both TE and TM modes. The methods for further modulation efficiency improvement are addressed.
© 2016 Optical Society of America
Phase shifter, which enables electrically tuning the effective refractive index of a waveguide, is an essential component for photonic circuits. For deposited photonics such as Si3N4, the thermo-optic (TO) effect is commonly utilized by employing an external metal heater above the waveguide. However, the TO modulation is inherently power hungry and slow . Moreover, a relatively thick cladding layer is required in order to reduce the thermal crosstalk, making it difficult to integrate multiple photonic layers on silicon.
Recently, deposited aluminum nitride (AlN) has emerged as an alternative material for deposited photonics because of its excellent properties such as low optical loss over broad wavelength spectrum, similar refractive index as Si3N4, small stress, and CMOS compatibility [2–5]. Moreover, it offers large piezoelectric effect and nontrivial Pockels and Kerr coefficients, thus enabling for realization of various active optical devices [4–7]. Especially, an AlN ring modulator utilizing its Pockels effect has been demonstrated [4, 5]. However, the reported modulator has both electrodes placed above the AlN waveguide. Such a co-planar electrode structure requires a non-planar process for fabrication and the electro-optic overlap is relatively small because the maximum electric field created by a voltage applied on the electrodes does not go through the AlN waveguide.
Here, a straightforward structure is proposed as shown in Fig. 1(a) schematically. It is a standard parallel plate capacitor by introducing a thin metallic bottom electrode such as TiN below the AlN waveguide. Such a vertical double plate electrode structure can be fabricated by the standard planar complementary metal-oxide-semiconductor (CMOS)-compatible process and is feasible for integration of multiple photonic layers on Si. Upon applying a voltage, an almost constant electric field parallel to the y-axis (i.e. ) is created between the bottom and upper electrodes, as shown in Fig. 1(b). The electric field inside the AlN waveguide equals approximately to , where V is the applied voltage, tupper, tbottom, and tAlN are thicknesses of the upper SiO2, the bottom SiO2, and the AlN core, respectively, and εSiO2 ( = 3.9) and εAlN (≈10 ) are dielectric constants of SiO2 and AlN, respectively. The electro-optic overlap approximately equals to the ratio of optical power confined in the AlN core.
To reach a large electric field at a given voltage, one need reduce the cladding SiO2 thickness. However, it will increase the propagation loss because the metallic electrode is usually a highly lossy material. For a typical 1-µm × 0.4-µm AlN waveguide, Fig. 1(c) plots the calculated Ey at 40-V bias and the metal-induced propagation loss as a function of the cladding SiO2 thickness. With the tradeoff between the electric field and metal-induced loss, the optimized cladding SiO2 thickness is ∼1.4 µm for the transverse electric (TE) mode and it is >2 µm for the transverse magnetic TM mode because the TE mode can be confined within the AlN core more tightly than the TM mode.
2. Fabrication and measurement
The proposed devices were fabricated on Si using the standard CMOS back-end process. The starting were SiO2-covered Si wafers. First, 120-nm TiN and 50-nm Si3N4 were deposited and patterned to form the bottom electrode, followed by 2-μm SiO2 deposition and chemical mechanical polishing (CMP) to planarize the surface. Second, 400-nm AlN was deposited by sputtering using a pure Al target, followed by 200-nm SiO2 deposition. The AlN layer was patterned by dry etch down to the SiO2 layer by using the 200-nm SiO2 layer as the hard mask. Third, 2-μm SiO2 was deposited, followed by CMP to planarize the surface. After opening the contact holes to the TiN electrode, 2-µm Al was deposited and patterned. Here, the upper electrode is Al itself for simplification. For integration of multiple photonic layers, the upper electrode can be also a thin TiN layer. Since the standard planar processes with temperature not exceed 400°C were used, it is feasible to fabricate other photonic layer either beneath or above this AlN photonic layer for multilayer photonics integration. Moreover, some wafers before the Al deposition were additionally annealed at temperatures ranging from 500 to 1000°C for 5 min. The TiN/SiO2/AlN structure remains stable up to 900°C annealing, whereas the 1000°C anneal damages the TiN film seriously. To converter phase variation into intensity modulation, both add-drop rings and asymmetric MZI modulators were fabricated, as shown in Figs. 2(a) and 2(b) respectively. Figure 2(c) shows the cross sectional transmission electron microscopy (XTEM) image of the fabricated phase shifter. The cladding SiO2 thickness is ∼1.5 µm. Figure 2(d) is a zoom-in XTEM image of the AlN core. One sees it has a columnar micro-grain structure with c-axis orientation, in consistence with our previous X-ray diffraction (XRD) measurement.
Numerical simulation shows that the effective mode index (neff) of the fabricated AlN waveguide at 1550 nm is 1.62 for TE and 1.54 for TE, as shown in Figs. 2(e) and 2(f) respectively. The electrode-induced absorption is calculated to be 0.1 dB/cm for TE and 5.6 dB/cm for TM. The dneff/dnAlN value is calculated to be 0.7 for TE and 0.43 for TM, close to the ratio of optical power confined in the AlN core. Therefore, the fabricated devices are more suitable for the TE mode. For usage at TM mode, the AlN thickness should be increased.
After dicing, the chips were measured using the conventional fiber-waveguide-fiber method by edge coupling with inverted couplers. Devices with and without the additional annealing were measured. They exhibit similar performances. Therefore, only the results measured from the devices without additional annealing are presented here. The results for the TM mode are also presented to extract the AlN material properties at the TM mode although its performance is poor due to the large metal-induced loss.
3. Results and discussion
The propagation losses are measured from the straight waveguides with different lengths using the cutback method. The results are plotted in Figs. 2(a) and 2(b) for the waveguides without the electrodes. The propagation loss is ∼2.0 dB/cm for TE and ∼3.3 dB/cm for TM, close to the previous results . The waveguides with the electrodes exhibit similar loss for TE whereas much larger loss for TM due to the electrode-induced absorption. Figures 3(c) and 3(d) plot spectra measured from add-drop ring resonators at the TE and TM modes, respectively. The Q-value is ∼1.1 × 104 for TE and ∼2.0 × 103 for TM. The group indices (ng) are extracted to be 2.33 for TE and 2.12 for TM. The insertion loss (excludes the coupling loss, which is ∼2 dB per facet) is ∼0.5 dB for TE and ∼5 dB for TM.
When a voltage is applied, the spectrum of the ring shifts, as shown in Figs. 4(a) and 4(b) for TE and TM respectively. The shift of the resonant wavelength (λr) depends on voltage almost linearly from −40 V to + 40 V, as shown in Fig. 4(c). The dλr/dV value is deduced to be ∼0.26 pm/V for TE and ∼0.14 pm/V for TM, which corresponds to dng/dV of ∼3.9 × 10−7 V−1 for TE and ∼2.0 × 10−7 V−1 for TM.
Figures 5(a) and 5(b) plot spectra measured from an asymmetric MZI modulator with 1.4-cm long arm under −40, 0, and 40 V for the TE and TM modes. The TE spectrum exhibits typical behavior as it shifts almost linearly with the voltage increasing from −40 V to 40 V, from which the product of Vπ⋅Lπ is extracted to be ∼240 V⋅cm. The insertion loss (excludes the coupling loss) is ∼3 dB, which is contributed by the waveguide loss and the splitting/combining loss. The TM spectrum exhibits abnormal behavior because of the large electrode-induced loss. The measured insertion loss is ∼8 dB. Nevertheless, we can still estimate the product of Vπ⋅Lπ to be ∼320 V⋅cm approximately from the voltage-induced spectrum shift. The dneff/dV value is estimated from the Vπ⋅Lπ values to be ∼3.2 × 10−7 V−1 for TE and ∼2.4 × 10−7 V−1 for TM, which is consistent with that extracted from the ring resonators after taking the difference in neff and ng into account. The dnAlN/dV value can be extracted by taking the calculated dneff/dnAlN value into account. Both TE and TM modes give a similar dnAlN/dV value of ∼5.0 × 10−7 V−1. Based on the equation , where r13 (r33) is the AlN’s Pockels coefficient for TE (TM) mode and E is the electric field within the AlN waveguide (which can be read from Fig. 1(b)), the r13 and r33 values can be extracted. Our experimental results show that the deposited AlN has similar r13 and r33 values to be ∼1.0 pm/V, and they are stable up to 900°C annealing.
It is expected that the EO phase shifter offers high speed because it is only limited by the resistance-capacitance (RC) delay. To estimate the transient property, a 0–80-V pulsed voltage with 1 MHz frequency is applied on the MZI modulator. The input light is a 1549-nm TE light from a tunable laser source and the output light is recorded by a photodetector as a function of time. The result is plotted in Fig. 6. Both the rise time (τr) and fall time (τf) is read to be ∼0.005 µs, then, the cutoff frequency defined as is estimated to be ∼30 MHz. However, this speed is mainly limited by the switching time of our slow voltage supplier. It is expected that the fabricated MZI modulator should have the cutoff frequency much larger than the above apparent value.
The main issue of the present AlN phase shifter is its low modulation efficiency due to the relatively small Pockels coefficient of AlN. A method to improve the modulation efficiency is to increase the Pockels coefficient of AlN. It is argued that the Pockels coefficient of AlN may depend on its crystallinity degree . Therefore, the Pockels coefficient may be improved by optimizing the AlN deposition process such as additional high temperature annealing and/or adding a thin buffer layer. Since our experimental result shows that 900°C annealing does not improve the Pockels coefficient, the annealing temperature for the possible crystallinity improvement needs to be larger than 900°C.
The other method is to increase the electric field inside the AlN waveguide at a given voltage. From Fig. 1(b), we can see that the electric field within the AlN waveguide is much smaller than that within the cladding SiO2 layer because AlN has much larger dielectric constant than SiO2. Therefore, one solution is to adopt a dielectric with large dielectric constant but small refractive index as the cladding dielectric. One possible candidate is Al2O3, which has dielectric constant of ∼10  and refractive index of ∼1.7 . Another possible solution is to employ a subwavelength grating (SWG) structure for the cladding layer . The optical mode can be isolated by such a SWG structure whereas the overall equivalent oxide thickness (EOT) can be reduced so that the electric field within the AlN waveguide at a given voltage can be increased significantly.
In conclusion, an AlN EO phase shifter with a parallel plate capacitor structure is demonstrated on Si using the planar process, which is suitable for multilayer integration. Some new evidences are experimentally clarified: (1) the deposited AlN has similar Pockels coefficient of ∼1.0 pm/V for TE and TM modes, (2) the Pockels coefficient of the deposited AlN keeps stable up to 900°C annealing, and (3) the refractive index of AlN depends on the applied voltage almost linearly over a large range at least from −40 V to 40 V. Although the present phase shifter suffers from low modulation efficiency due to the relatively small Pockels coefficient, approaches for further improvement exist. Owing to the extremely low DC power consumption, high modulation speed, and feasibility for multilayer integration, the proposed phase shifter will play a key role for deposited photonics.
This work was supported by the Singapore A*STAR-MINDEF Joint Funding Programme 1423300091.
References and links
1. A. Masood, M. Pantouvaki, D. Goossens, G. Lepage, P. Verheyen, J. Van Campenhout, P. Absil, D. Van Thourhout, and W. Bogaerts, “Fabrication and characterization of CMOS-compatible integrated tungsten heaters for thermo-optic tuning in silicon photonics devices,” Opt. Mater. Express 4(7), 1383–1388 (2014). [CrossRef]
2. S. Zhu and G. Q. Lo, “Vertically stacked multilayer photonics on bulk silicon toward three-dimensional integration,” J. Lightwave Technol. 34(2), 386–392 (2016). [CrossRef]
4. C. Xiong, W. H. Pernice, and H. X. Tang, “Low-loss, silicon integrated, aluminum nitride photonic circuits and their use for electro-optic signal processing,” Nano Lett. 12(7), 3562–3568 (2012). [CrossRef] [PubMed]
5. C. Xiong, W. H. P. Pernice, X. Sun, C. Schuck, K. Y. Fong, and H. X. Tang, “Aluminum nitride as a new material for chip-scale optomechanics and nonlinear optics,” New J. Phys. 14(9), 095014 (2012). [CrossRef]
8. X. Li, Z. Xu, Z. He, H. Cao, W. Su, Z. Chen, F. Zhou, and E. Wang, “On the properties of AlN thin films grown by low temperature reactive r.f. sputtering,” Thin Solid Films 139(3), 261–274 (1986). [CrossRef]
9. M. C. Larciprete, A. Bosco, A. Belardini, R. Li Voti, G. Leahu, C. Sibilia, E. Fazio, R. Ostuni, M. Bertolotti, A. Passaseo, B. Potì, and Z. Del Prete, “Blue second harmonic generation from aluminum nitride films deposited onto silicon by sputtering technique,” J. Appl. Phys. 100(2), 023507 (2006). [CrossRef]
10. R. D. Clark, “Emerging applications for high k materials in VLSI technology,” Materials (Basel) 7(4), 2913–2944 (2014). [CrossRef]
12. R. Halir, P. J. Bock, P. Cheben, A. Ortega-Moñux, C. Alonso-Ramos, J. H. Schmid, J. Lapointe, D.-X. Xu, J. G. Wangüemert-Pérez, Í. Molina-Fernández, and S. Janz, “Waveguide sub-wavelength structures: a review of principles and applications,” Laser Photonics Rev. 9(1), 25–49 (2015). [CrossRef]