We demonstrate high-speed silicon modulators based on carrier depletion in interleaved pn junctions fabricated on 300mm-SOI wafers using CMOS foundry facilities. 950µm-long Mach Zehnder (MZ) and ring resonator (RR) modulator with a 100µm radius, were designed, fabricated and characterized. 40 Gbit/s data transmission has been demonstrated for both devices. The MZ modulator exhibited a high extinction ratio of 7.9 dB with only 4 dB on-chip losses at the operating point.
© 2013 Optical Society of America
The ongoing growth of data traffic for numerous applications from long-haul telecommunications to on-chip interconnects continuously drives the need for high bandwidth communications. Silicon photonics has the potential to solve the electrical interconnect bottleneck [1,2]. As one of the key optoelectronic building blocks, high-speed silicon optical modulators have been extensively studied in the past few years. 40 Gbit/s up to 50 Gbit/s data transmissions have been demonstrated with various phase shifter configurations [3–11]. Going from the improvement of stand-alone devices, the current important challenge is the integration of both photonic and electronic circuits. Before this final integration, a crucial intermediate step is the fabrication of high-performance building blocks in large-scale microelectronic foundry. Such demonstrations pave the way of achieving further technological nodes, targeting high-performance and low power consumption of microelectronic chips.
In this paper, interleaved pn junctions were used to demonstrate high-performance modulators. Those structures were processed on 300mm SOI wafers, available in large-scale microelectronic foundries and allowing higher volume production but also an improved Si thickness and lithography overlay control compared to standard processes available in 200mm fabs. Modulation efficiencies, optical loss and high-speed capabilities were evaluated using both Mach Zehnder (MZ) and ring resonator (RR) modulators.
1. Device design, fabrication and experimental set-up
A schematic representation of the phase shifter cross-section is shown in Fig. 1. A 400nm-wide, 220nm-thick silicon rib waveguide was used with an etching depth of 120 nm. The phase shifter is based on interleaved PN junctions all along the waveguide. Light crosses successively doped and depleted regions so that there is a large overlap between the depletion region and the optical mode allowing for the maximization of the phase shift. The design is similar to the one used in , where the P and N regions were 400nm- and 300nm-long, respectively. In both cases, moderate doping concentrations were used in the waveguide to avoid extra losses while larger doping concentrations far from the guided mode region reduced the access resistance. The targeted doping concentrations were 5 × 1017 cm−3 and 1018 cm−3 for p-doped and n-doped regions respectively. The p + + and n + + doping concentrations are larger than 1019 cm−3.
Ring resonators and asymmetric MZ modulators are used to convert phase modulation into intensity modulation. Lumped electrodes were used for the rings while Coplanar Waveguide Electrodes (CPW) were used for MZ modulator.
The fabrication of the silicon modulators was carried out in 300 mm silicon-on-insulator (SOI) wafers with 2 μm-thick buried oxide (BOX), and 220 nm-thick silicon film on top. A SiO2 layer deposited on the silicon substrate was used as a hard mask. Deep-UV optical lithography and reactive ion etching were used to define the waveguides, followed by four optical lithography and ion implantation steps to create the diodes as shown. Finally, a W/Cu/Al metal stack was used to take the electrical contacts and define the electrodes.
To characterize the modulator, a linearly TE-polarized light beam was coupled in/out of the waveguide using grating couplers. Electrical probes were used to bias the diode and measure the static response of the modulator. To evaluate its high-speed performance electro-optic bandwidths as well as eye diagrams were measured. A sinusoidal electrical signal generated by the Agilent 86030A opto-RF vector network analyzer or pseudo random binary sequence (PRBS) electrical signal from a Centellax TG1P4A 40 Gbit/s source with a 215-1 pattern length, was coupled to DC bias using a bias tee to drive the modulator. The modulated optical signal was fed through an Erbium Doped Fiber Amplifier (EDFA) and then through a tunable wavelength filter to eliminate a large part of the amplified spontaneous emission noise from the EDFA, before being coupled back to the opto-RF vector network analyzer or to a 32 GHz Agilent photodiode connected to an Infiniium Agilent oscilloscope. It can be mentioned that grating couplers with losses lower than 3 dB ensured large signal at the receptor stage, reducing the contribution of the residual spontaneous emission of the EDFA. In all cases, to ensure correct electrical impedance at the output of the amplifier, a 50 ohm load was connected through a DC block, either at the output electrode of the MZ modulator or in parallel to the ring modulator.
2. DC characterization
A 0.95µm-long MZ modulator and a ring resonator modulator were characterized to evaluate the modulators’ figures of merit. The static optical transmissions of these devices are reported in Fig. 2. MZ on-chip loss of 3.5 dB, with extinction ratio contrasts of more than 25 dB were measured, demonstrating a good balance of the 1 × 2 Multi-Mode Interference (MMI) couplers (Fig. 2(a)). The ring modulator radius was 100µm, and the distance from waveguide to ring was 150 nm with a phase shifter integrated all along the ring waveguide except in the coupling region. A high Q factor of around 20 000 was obtained. It can be seen from Fig. 2(b) that critical coupling could be obtained at slightly lower wavelengths than 1505 nm (Fig. 2(b)).
Optical transmission comparisons of passive waveguides and MZ modulators with different active regions lengths from 0.95 to 4.7 mm were used to dissociate loss contribution of the different parts of the modulator (splitter, phase shifter, propagation loss, insertion loss). From these analyses, an active region loss of 2.1 dB/mm was obtained. A precise control of the doping concentrations is required for further reduction of optical losses and increase of modulation efficiency. However the phase shifter length of the high speed Mach Zehnder modulator in the following is as short as 0.95 mm, ensuring low loss and good efficiency. MMI loss from 0.7 to 0.9 dB and waveguide loss lower than 1 dB/cm have been estimated.
Finally wavelength resonance shift as a function of applied bias voltage was used to measure phase shifter efficiencies (Fig. 3.), from which a VπLπ of 2.4 V.cm was deduced. These results are reported in Table 1.
3. High speed performance
The measured optical response of the MZ modulator as a function of electrical signal frequency is shown in Fig. 4. A 3 dB optical bandwidth of 20 GHz was obtained for a DC bias of −3 V, which indicates a proper diode (RC contribution) and RF electrodes design, as well as a minimized electrical loss contribution from SOI substrate, due to the use of 2µm-thick buried oxide and high-resistivity silicon substrates.
40 Gbit/s eye diagrams of both the ring and the MZ modulators are reported in Fig. 5. It has to be noted that the EDFA required for high-speed measurements operate between 1530 and 1560 nm. At these wavelengths, the ring modulators do not operate in critical coupling condition, leading to a reduced extinction ratio (ER). However, more than 3 dB ER was obtained as seen in Fig. 5(a), while a higher ER of 7.9 dB was obtained using the MZ modulator (Fig. 5 (b)). For this last measurement the maximum eye diagram level was 0.5 dB below the maximum of transmission level of the device. In other words, to achieve such extinction ratio the total on chip loss was 4 dB. These results, summarized in Table 2, can be compared favorably with current state of the art. Indeed, comparing the results obtained with the previous demonstrations reported in the literature, only few devices achieved optical loss lower than 5 dB at high-speed operating point, and these devices presented generally limited ER of less than 5dB .
In summary, 40Gbit/s silicon optical modulators based on reverse biased diodes embedded in MZ and ring resonators were demonstrated using 300 mm platform. Such a platform allows mass production of silicon photonic circuits with a compatibility with advances sub-65nm CMOS node, the uses of state of the art 193nm photolithography with sub-50nm resolution and a better SOI thickness uniformity in comparison with 200mm wafer. The MZ modulator delivered a high ER of 7.8 dB at 40 Gbit/s with low optical loss of only 4 dB. Further phase shifter loss reduction is still considered. Ring modulator was also fabricated and characterized at high-speed, exhibiting 40 Gbit/s operation with 3 dB ER. This result is mainly limited by the EDFA operating wavelength required for the high-speed measurements. Further optimization of the ring modulator can be implemented to increase this extinction ratio.
The research leading to these results has received funding from the European Community's under project Plat4m
References and links
1. B. Jalali and S. Fathpour, “Silicon Photonics,” J. Lightwave Technol. 24(12), 4600–4615 (2006). [CrossRef]
2. K.-H. Koo, P. Kapur, and K. C. Saraswat, “Compact performance models and comparisons for gigascale on-chip global interconnect technologies,” IEEE Trans. Electron. Dev. 56(9), 1787–1798 (2009). [CrossRef]
3. L. Liao, A. Liu, D. Rubin, J. Basak, Y. Chetrit, H. Nguyen, R. Cohen, N. Izhaky, and M. Paniccia, “40 Gbit/s silicon optical modulator for high speed applications,” Electron. Lett. 43(22), 1196–1197 (2007). [CrossRef]
4. D. J. Thomson, F. Y. Gardes, Y. Hu, G. Mashanovich, M. Fournier, P. Grosse, J.-M. Fédéli, and G. T. Reed, “High contrast 40Gbit/s optical modulation in silicon,” Opt. Express 19(12), 11507–11516 (2011). [CrossRef] [PubMed]
6. M. Ziebell, D. Marris-Morini, G. Rasigade, J.-M. Fédéli, P. Crozat, E. Cassan, D. Bouville, and L. Vivien, “40 Gbit/s low-loss silicon optical modulator based on a pipin diode,” Opt. Express 20(10), 10591–10596 (2012). [CrossRef] [PubMed]
7. T. Baba, S. Akiyama, M. Imai, N. Hirayama, H. Takahashi, Y. Noguchi, T. Horikawa, and T. Usuki, “50-Gb/s ring-resonator-based silicon modulator,” Opt. Express 21(10), 11869–11876 (2013). [CrossRef] [PubMed]
8. H. Xu, X. Xiao, X. Li, Y. Hu, Z. Li, T. Chu, Y. Yu, and J. Yu, “High speed silicon Mach-Zehnder modulator based on interleaved PN junctions,” Opt. Express 20(14), 15093–15099 (2012). [CrossRef] [PubMed]
12. H. Yu, M. Pantouvaki, J. Van Campenhout, D. Korn, K. Komorowska, P. Dumon, Y. Li, P. Verheyen, P. Absil, L. Alloatti, D. Hillerkuss, J. Leuthold, R. Baets, and W. Bogaerts, “Performance tradeoff between lateral and interdigitated doping patterns for high speed carrier-depletion based silicon modulators,” Opt. Express 20(12), 12926–12938 (2012). [CrossRef] [PubMed]
13. M. Ziebell, D. Marris-Morini, G. Rasigade, P. Crozat, J.-M. Fédéli, P. Grosse, E. Cassan, and L. Vivien, “Ten Gbit/s ring resonator silicon modulator based on interdigitated PN junctions,” Opt. Express 19(15), 14690–14695 (2011). [CrossRef] [PubMed]