Density, Low-Power WDM Optical Interconnects

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Extensive uniformity data is presented for ring modulators, Ge photodetectors .... In order to thermally tune the ring, a W heater is implemented on top of the ring ...
Highly Uniform 28Gb/s Si Photonics Platform for HighDensity, Low-Power WDM Optical Interconnects P. Verheyen1, M. Pantouvaki1, P. De Heyn1, H. Chen1,2, G. Lepage1, J. De Coster1, P. Dumon , A. Masood1,2, D. Van Thourhout1,2, R. Baets1,2, W. Bogaerts1,2, P.Absil1 and J. Van Campenhout1 1,2

1 Imec, Kapeldreef 75, Leuven B-3001, Belgium Photonics Research Group , Dept. of Information Technology, Ghent University – imec, St. Pietersnieuwstraat 41, 9000 Ghent, Belgium Author e-mail address:[email protected]

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Abstract: We report on electro-optical device performance in a fully integrated 28Gb/s Si photonics platform running on a 130-nm CMOS toolset. Extensive uniformity data is presented for ring modulators, Ge photodetectors and compact ring-based WDM filters. OCIS codes: (200.4650) Optical interconnects; (230.4110) Modulators; (230.0250) Optoelectronics; (230.5160) Photodetectors.

1. Platform description The photonics platform discussed in this paper uses 200mm SOI wafers with a nominal Si thickness of 220nm on top of a buried oxide of 2µm are used as starting substrates. The top Si layer is etched to 3 different depths using three 193nm litho levels: full etch, a 70nm deep etch, and a 150nm deep etch. After these patterning steps, a high density plasma (HDP) oxide is deposited, which is capable of filling narrow trenches. This HDP oxide is polished using chemical-mechanical polishing (CMP), selective to the hard-mask used to pattern the three levels. This creates a flat starting surface for the advanced passives levels. The advanced passives module enables advanced optical devices including raised grating couplers with 14nm at less than 0.5pJ/bit, to compensate either for wafer-scale variations of absolute channel wavelength, or for temperature variations.

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Figure 2: 5 channel WDM performance metrics: (c) 1dB bandwidth, (d) worst-case crosstalk collectively tuned, and (e) worst-case insertion loss collectively tuned. (f) Peak wavelength of channel 1 across the wafer. -15

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Figure 3: (a) Microscope image of the ring modulator, (b) typical trsnmission spectrum, (c) modulation efficiency and (d) Q-factor variation over a full wafer.

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Figure 4: (a) Modulator Resistance and (b) Capacitance as extracted from the S11 measurement. (c) Heater Efficiency, (d) Eye diagram at 28Gb/s.

Ring Modulator Performance: Figure 3 (b) shows a microscope picture of the ring modulator structure reported in this work [3]. The modulator has a radius of 7.5µm and is formed by the 150nm deep etch. The modulator implants are done over the full ring circumference forming a lateral PN diode. A typical spectrum is given in Figure 2(b). The wafer-scale distribution of the modulation efficiency and the Q factor measured at 0V bias is given in Figure 3 (c) and (d) respectively, showing average values and standard deviations of respectively 35±3 pm/V and 4800±160. At the optimum operation wavelength, typical values for extinction ratio and insertion loss are 4.3dB and 4.1dB, when driven with a 1.5Vpp drive signal. In order to thermally tune the ring, a W heater is implemented on top of the ring as shown in Fig. 2a. In order to improve heater efficiency, thermal isolation trenches and a partial removal of the Si substrate underneath the ring using the UCUT module. Wafer-scale heater tuning performance is shown in Fig. 2. A uniform efficiency of 260±7pm/mW is demonstrated, which can be further improved by shrinking the ring size and

optimizing the UCUT regions. This ring modulator has an electro-optical bandwidth of 22GHz, limited both by the RC time constant and cavity lifetime. The wafer-scale distributions of R and C values extracted from S11 measurements are shown in Figure 4(a) and (b) , and reveal wafer-scale averages and standard deviations of respectively 35.6±1.2 fF and 57.2±2.4 Ohm. Finally, a 28Gb/s eye diagram is shown in Figure 4 (d), using a 1.5Vpp differential drive signal. Such ring modulators can be implemented in a cascaded configuration to enable WDM transmitters [4].

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Figure 5: (a) x-section of a vertical Ge device. (b) Responsivities for the three types of devices measured on 150 dies. (c) Dark current for the same devices. All measurements are done using a 1V reverse bias.

Ge PD performance: The Ge photodetectors reported in this paper are Ge/Si vertical PIN diodes, with a fixed length of 15µm. They have a p-type contact in the Ge and a n-type contact in the Si. Figure 5(a) shows a microscope picture of such a structure. We present three different Ge PD designs. These designs have different contacting strategies, resulting in a different responsivity versus bandwidth trade-off. Figure 5(b) and (c) show respectively the wafer scale measurements of the responsivity and the dark current obtained on these structures at -1V reverse bias, and 1555nm wavelength. The average responsivities are 0.58A/W, 0.85A/W and 0.97A/W, with a standard deviation of 0.03A/W. The average dark current at -1V is similar for the three designs, with values between at 10nA-20nA. Figure 6 (a) shows a contour plot of the responsivity measured for design 2. The devices show a yield of about 97%. The wafer scale plot of the responsivity shown in figure 2(a) does not show any distinctive feature. The S-parameter extractions depicted in Figure 6 (b), and show average f3dB of 23GHz for design 1, and >50GHz for design 2 and 3 (50GHz is the instrument limitation). Figure 6 (c) shows clear open eye diagram obtained at 28Gb/s.

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Figure 6: (a) Uniformity plot of responsivity for the device 2 (0.85A/W). (b) f3dB extracted from S parameter measurement at 1V reverse bias, values are based on curve fit and extrapolated >50GHz. (b) eye diagram at 28Gb/s operation speed.

4. Conclusion In conclusion, we have reported on the wafer-scale electro-optical device performance for various key building blocks required for high-density, low-power WDM optical interconnects in silicon at 28Gb/s and beyond. The reported devices were all co-integrated in a single platform with state-of-the art performance and excellent yield. The demonstrated devices and integration platform show good potential to further scale the capacity, bandwidth density and power efficiency of Si photonics transceivers. This work was supported by imec’s CORE partner program. [1] D. Vermeulen, et al, Opt. Express 18, 18278-18283 (2010) [2] J. Van Campenhout, et al " in OFC Conference, OSA Technical Digest (Optical Society of America, 2012), paper OM2E.4. [3] P. De Heyn, et al in OFC Conference, OSA Technical Digest (online) (Optical Society of America, 2014), paper Th4C.5. [4] M. Pantouvaki, et al in OFC Conference, OSA Technical Digest (online) (Optical Society of America, 2014), paper Th1C.5.