We demonstrate a high-efficiency and CMOS-compatible silicon Mach-Zehnder Interferometer (MZI) optical modulator with Cu traveling-wave electrode and doping compensation. The measured electro-optic bandwidth at Vbias = −5 V is above 30 GHz when it is operated at 1550 nm. At a data rate of 50 Gbps, the dynamic extinction ratio is more than 7 dB. The phase shifter is composed of a 3 mm-long reverse-biased PN junction with modulation efficiency (Vπ·Lπ) of ~18.5 V·mm. Such a Cu-photonics technology provides an attractive potentiality for integration development of silicon photonics and CMOS circuits on SOI wafer in the future.
© 2014 Optical Society of America
Silicon photonics devices have a promising future in the application of optical communications, due to their low cost, high performances and compatibility with the existing complementary metal-oxide-semiconductor (CMOS) technology [1–4]. Silicon optical modulator is one key component for data communication related application, and significant progress has been achieved in the field of silicon photonics modulator over the past decade [5–18]. Among all kinds of silicon optical modulators, silicon carrier-depletion-based modulator has proven itself to be the most prevailing solution for optical modulation on silicon because of its high performances, such as high speed and low power consumptions.
In most reported papers, aluminum is usually adopted as electrodes and contact/via plugs material in silicon optical modulators [6–14]. For example, one kind of silicon slot photonic crystal modulator with 0.54 pJ/bit power consumption was formed with Al metal electrode . In 2011, one group from Fujikura achieved a silicon Mach-Zehnder Interferometer (MZI) modulator of extinction ratio beyond 10 dB at 10.0-12.5 Gbps . Also, a silicon MZI modulator with low power and data rate of 12.5 Gbps was reported , and Bell Labs demonstrated a single-drive push-pull silicon MZI modulator with data rate up to 50 Gbps in 2012 . Our group also presented a 50 Gbps silicon MZI modulator last year in which Al was also used as the metal electrode . These silicon modulators with high data rate and low power consumption based on Al electrode have been achieved in the past few years. However, Al material has its shortcomings, such as nano-pores-induced low density and high resistivity, which result in large delay. Large delay impedes further improvement of high speed devices. A good alternative to be used as electrode is Cu, which can replace Al in silicon photonics devices in the future because it has lower resistivity, higher conductivity and lower activation energy than Al [19–21]. In principle, these advantages of Cu contribute to higher speed and lower power consumption for silicon photonics active devices and higher integration intensity for circuits. Currently, few silicon photonics devices with Cu electrode have been reported. IMEC reported one modulator with Cu traveling-wave electrode and contact filling material of W in 2012, which has a data rate up to 40 Gbps . IBM reported a 25 Gbps microring modulator using Cu electrode  and a 90 nm CMOS-photonics technology node for 25 Gbps transceiver  in 2012. Oracle labs reported hybrid-integration of silicon photonics using Cu electrode in 2012 . Luxtera reported a 4 × 25 Gbps transceiver . The stacked Ti/TiN/AlCu/Ti/TiN material utilized as electrodes are also reported to reduce the RF loss [15, 16].
In this paper, we demonstrate a high efficiency PN junction silicon optical modulator with Cu traveling-wave electrode on silicon-on-insulator (SOI) wafer with 220 nm-thick top Si layer. The phase shifter length is 3 mm. To reduce the optical transmission loss of phase shifter caused by ion implantation while keeping the modulation efficiency and switching speed at a high level, a doping compensation method is utilized to optimize the doping level on the depletion region of the phase shift . Since it is difficult to delineate Cu by subtractive etch due to the limited number of volatile Cu compounds, dual-damascene process was utilized to form the Cu traveling-wave electrode and the contact plugs . Cu deposition and chemical-mechanical polishing (CMP) process are included in dual-damascene process. To avoid the dishing caused by CMP process  on Cu surface, a latticed Cu surface pattern is designed. The simulated results show this kind of latticed Cu pattern does not degrade the speed of the modulator in the bandwidth range of 40 GHz. The measured results show the bandwidth of our modulator reaches above 30 GHz. Its modulation efficiency (Vπ·Lπ) is ~18.5 V·mm and the implantation-induced optical loss is ~1.3 dB/mm. The dynamic extinction ratio is 7.08 dB at a bias of −5 V and at a data rate of 50 Gbps, which is also close to the limit of our eye diagram measurement equipment. Cu application can further improve the integration of silicon photonics devices and CMOS circuits in the future.
2. Design and fabrication
2.1 Modulator design
Figure 1 shows the microscope image of the modulator. The silicon MZI modulator is based on a 3 mm-long PN junction phase shifter on a SOI wafer with 220 nm-thick top Si and 2 µm-thick buried oxide (BOX). The waveguide width is 500 nm and the slab height of the ridge waveguide of the phase shifter is 100 nm. A 1 × 2 multimode interference (MMI) structure is used as the splitter and the combiner. It is an asymmetrical MZI structure and the arm length difference ∆L is 300 µm. The inset (a) of Fig. 1 shows the pattern of implantations. The implantation compensations are designed on both areas of the ridge corners for lower implantation-induced optical loss and higher modulation speed. The implantation compensation design is similar to our reported modulator .
In our design, the Cu thickness is 2 µm. In order to reduce the dishing on Cu surface caused by CMP process [26–28], a latticed Cu pattern was used as Cu traveling-wave electrode of our silicon modulator. HFSS, a commercial simulation software, was used to evaluate the RF loss of the latticed Cu electrode. The inset (b) of Fig. 1 shows the latticed Cu pattern. The size of each dielectric slot pattern is 3 µm × 8 µm. The maximum Cu unit size in the electrode is 15 µm × 15 µm, which can effectively reduce the Cu dishing. Three kinds of metal electrodes are simulated, including the normal Al electrode, the normal Cu electrode and the latticed Cu electrode. Coplanar waveguide (CPW) models were adopted in this simulation. All models are with a Si substrate, which has permittivity of εr = 11.9 and resistivity of ρ = 1000 Ω·cm. Between the Si substrate and the CPW layer, there was an oxide layer with 4 µm-thick. The thickness of the CPW layer is 2 µm. To obtain a 50-Ω impedance match, the width of the central signal CPW was set to be 10 µm, and the gap between the signal and ground was set to be 6.4 µm. Assuming that both materials do not have any defects, and the simulated result of insertion loss S21 and return loss S11 are shown in Fig. 2. The insertion loss and the return loss of the electrical signal in the latticed Cu pattern are quite close to that in the normal Cu electrode at 40GHz, which are both smaller than that in the normal Al electrode for different electrode lengths. These are caused by the lower resistivity of Cu (ρCu = 1.72e-6 Ω·cm) than Al (ρAl = 2.63e-6 Ω·cm). The RF 6.4-dB bandwidth is related to the electro-optic (EO) 3-dB bandwidth [29, 30]. The insertion loss of the latticed Cu electrode is less than 6.4 dB, and the return loss is less than −10 dB within 40 GHz when the electrode length is no more than 5 mm, which is longer than the length of latticed Cu electrode of our modulator. Therefore, this kind of latticed Cu electrode does not degrade the speed of our modulator within the range of 40 GHz.
2.2 Modulator fabrication
This silicon modulator was fabricated on an 8-inch SOI wafer with top Si layer of 220 nm and BOX of 2 μm. After P and N compensation implantations were done, the waveguide was formed by double silicon dry etching processes. Figure 3 shows the scanning electron microscope (SEM) images of the silicon waveguide. Four more implantations and a rapid thermal annealing were performed for the formation of PN junction. Based on the actual implantation condition, the P and N doping levels in the PN junctions are estimated as ~4e17 cm−3. And, the P and N doping levels in both compensation areas are estimated as ~3e16 cm−3. Dual-damascene process was utilized to form the Cu traveling-wave electrode and the contact connection. After SiO2 dielectric layer was deposited and polished, the trench of Cu electrode and the contact hole were formed in sequence.
To avoid the diffusion of Cu into the Si/SiO2 layer, a 250 Å-thick TaN barrier layer was deposited first. A 1500 Å-thick Cu seed layer was next deposited by physical vapor deposition (PVD) followed by 6 µm-thick Cu layer by electrochemical-plating (ECP). After removing the excess Cu by CMP, the Cu electrode and contact plugs were finally formed after annealing. The structures of the Cu electrode are presented in Fig. 4, with Fig. 4(a) showing the image of the Cu electrode surface after Cu CMP. A 5000 Å-thick SiO2 was deposited as a dielectric layer over the Cu electrode subsequently. After the opening of the bond-pad, a thin Al layer was formed on the bond-pad pattern to avoid the oxidation of Cu electrode. Finally, more than 100 μm-deep Si trench was etched to hold optical lensed fiber for coupling with the nano-taper of Si waveguide. Figure 4(b) shows the transmission electron microscope (TEM) image of the phase shifter cross-section with Cu electrode and contact plugs. The inset shows the cross-section of the silicon ridge waveguide and the silicon slab height is ~100 nm.
3. Characterization results and discussion
3.1 Cu contact and Cu traveling-wave electrode characterization
Small signal microwave performance in the latticed Cu electrode with 3 mm-long phase shifter was measured through Agilent N4373C Lightwave Component Analyzer (LCA) which has a maximum bandwidth of 40 GHz. The signal is dependent on the PN junction of phase shifter by which the ground and signal Cu electrodes are connected. After the EE calibration of measurement setup, EE S21 signals of Cu electrode are measured with different DC biases which are used to avoid the PN junction effect, shown in Fig. 5(a). The EE bandwidth increases with the bias. This is caused by the carrier depletion out of the PN junction area under a reversed bias voltage, and the effect of PN junction on the Cu electrode reduces with increasing bias. The 6.4-dB bandwidth is more than 40 GHz when the bias is −18 V or more. This result proves that this latticed Cu electrode transmission speed is beyond 40 GHz and it does not degrade the speed of modulator within the range of 40 GHz. The Al traveling-wave electrode, which is laid over the same implanted 3 mm-long phase shifter, is also characterized to compare the bandwidth. The 6.4-dB bandwidth of microwave transmission in the Al electrode increases from 9.7 GHz at Vbias = 0 V to 21.1 GHz at Vbias = −18 V. It is verified that the Cu traveling-wave electrode can provide a higher bandwidth than Al.
Two lensed fibers with 2.5 µm focal-length were used to characterize the optical performance of the modulator. Figure 5(b) shows the result of the Cu-induced propagation loss of silicon waveguide. When the Cu-to-waveguide distance is more than 1 µm, the Cu-induced optical loss is less than 0.25 dB/cm. In our design, the Cu-to-waveguide distance is 4 µm as seen in Fig. 4(b). Therefore, the Cu-induced propagation loss in our modulator is negligible. The 2 µm-thick Cu sheet resistance is shown in Table 1 (left). It is 18.3 ± 0.3 mΩ/square, lower than Al sheet resistance of 25.3 ± 0.4 mΩ/square. It also reveals that Cu is better than Al as the electrode and contact material of silicon modulator for higher modulation speed. The Cu-to-Si contact resistivity was also measured and shown in Table 1 (right). The contact size of our modulator is 4 × 3000 µm2 for both N- and P-contact. Based on the Cu-to-Si contact resistivity, the contact resistances of the modulator for both N- and P-contact are 15 mΩ and 19 mΩ, respectively.
3.2 DC measurement of silicon optical modulator
The measured output spectra of the silicon optical modulator under different reversed bias voltages are shown in Fig. 6(a). The bias is applied on one arm of the modulator. The free spectrum range (FSR) of the asymmetric MZI is 1.85 nm, which is dependent on ∆L. Without any bias, the optical extinction ratio of this modualtor is ~28 dB. With the reversed bias, the carrier is pumped out of the waveguide and the optical loss reduces. Thus, the optical extinction ratio decreases due to the unbalance of optical power in two modulator’s arms with the increase of the reversed bias. The measured insertion loss of the modulator is ~9 dB as shown in Fig. 6(a), while the dynamic loss is shown in the inset of Fig. 6(a), which is the average measurement result. Based on the waveguide loss of 1.2 dB (undoped waveguide propagation loss ~0.2 dB/mm), 2 MMI loss of 0.6 dB and double fiber-to-waveguide coupling loss of 3.2 dB, the optical loss caused by implantation is 1.3 dB/mm. In Fig. 6(b), a π-phase shift can be realized under 6.0 V reversed voltage for a 3 mm-long phase shifter, which corresponds to a modulation efficiency (Vπ·Lπ) = 18.5 V·mm. With an increase in the applied reversed voltage from −2 V to −10 V, the efficiency is reduced from 11.1 V·mm to 21.5 V·mm, which is caused by the depletion of free carriers in the PN junction. In the deep depletion region, the modulation efficiency becomes lower because there are fewer free carriers left in the depletion region. The efficiency is improved compared with our previous Al-modulator  mainly due to the sheet resistance of Cu is 28% smaller than Al as shown in Table 1. Under the same DC bias measurement condition, the Cu-modulator PN junction experiences a higher DC voltage compared with the Al-modulator, therefore Cu-modulator has a larger phase shift.
3.3 AC measurement of silicon optical modulator
The small signal response of the silicon optical modulator with 3 mm-long phase shifter was measured using Agilent N4373C LCA. The input signal was adopted by a 67 G probe which was pinned on one end of Cu electrode. The 50-Ω matching impedance as a terminator was connected on the other end of Cu electrode by another 67 G probe to reduce the signal reflection. The measured EO bandwidth of silicon modulator is shown in Fig. 7(a). Under a Vbias of −5 V, the 3-dB bandwidth of this modulator is up to 37 GHz. In order to get the eye diagram results, a high speed electrical signal coming from a 50/56-Gbps Anritsu Pattern Generator MP1822A was firstly amplified through a 67 G high speed driver. It was applied to the modulator through a 60 G DC bias tee and the input 67 G probe. A continuous-wave light coming from the 1550 nm tunable laser was firstly amplified through an erbium doped fibre amplifier (EDFA), and then it was modulated by adding a non-return-zero pseudorandom binary sequence (PRBS) 231−1 signal under Vbias = −5.0 V with Vpp = 3.5 V. The output optical signal was amplified again and collected by an Agilent Digital Communications Analyzer (DCA) after the optical filter. The data rate of the eye diagram reaches 50 Gbps with a dynamic extinction ratio of 7.08 dB, as shown in Fig. 7(b). A performance comparison of this work to other MZI modulators is shown in Table 2.
We have demonstrated a CMOS-compatible silicon MZI optical modulator enabled by Cu traveling-wave electrode and doping compensation. Cu electrode with low resistance provides higher bandwidth than Al electrode and does not show any undesirable influence on the EO performance of silicon modulator within the range of 40 GHz. The phase shifter of the modulator is composed of a 3 mm-long reverse-biased PN junction with modulation efficiency (Vπ·Lπ) of ~18.5 V·mm. The eye diagram of 50 Gbps data rate with dynamic extinction ratio of 7.08 dB is reached under Vbias = −5.0 V with Vpp = 3.5 V. The measured EO bandwidth is up to above 30 GHz at Vbias = −5.0 V when it is operated at 1550 nm. Such a Cu-photonics application can further improve the integration of silicon photonics devices and CMOS circuits in the future.
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