Silicon photonics technology is regarded as a future technology for computing and communication systems. Recently, considerable progress has been made on silicon photonic devices such as modulators and photodetectors based on CMOS compatible processes [1–32]. Especially when these devices are successfully monolithic integrated into major silicon chips in computing systems as chip-level optical I/O, it can bring innovative changes including computer architecture [1–10].

The silicon optical modulator, a key device for transmitting optical data in optical interconnection, is usually based on the refractive index change of a silicon waveguide by either carrier injection in a PIN diode or carrier-depletion effect in a PN diode. Intensive work has been done in this area in recent years and has made remarkable progress [10–32]. However, further improvement is desired in the silicon modulator for the realization of practical chip-level optical I/O applications. The most commonly investigated silicon modulator is a modulator based on a Mach–Zehnder interferometer (MZI), which has a broad spectral response and high thermal tolerance. MZ modulators (MZMs) operated in a depletion mode have demonstrated characteristics of high speed up to $60\mathrm{Gb}/\mathrm{s}$ with various effective modulation efficiencies [11–32]. However, most of the reported silicon PN depletion-mode MZMs require a relatively long phase-shifter length in order to achieve sufficient modulation depths, which make them less favorable for chip-level optical interconnect applications. MZMs capable of high-speed operation with effective modulation depth, small footprint, and low power/energy consumption are desired for chip-level integration. Previously, we reported high-performance $30\mathrm{Gb}/\mathrm{s}$ silicon photonic circuits (PICs) defined for off- or on-chip photonic interconnects, where 1 mm long lateral-depletion-junction phase-shifter Mach–Zehnder modulators and evanescent-coupled waveguide Ge-on-Si photodetectors were monolithically integrated [24]. In this Letter, we have investigated compact carrier-depletion-type silicon Mach–Zehnder modulators with considerably short phase shifters, which can be utilized in future chip-level integration. Devices were fabricated based on a CMOS compatible process using I-line lithography with the elaborated line-narrowing process to achieve the required pattern widths narrower than 180 nm. In the following, device design, fabrication, and performance characterizations are described.

The proposed scheme for a silicon MZ modulator entails a narrow slit-type electrical terminal formed on the top of a phase-shifter waveguide, parallel to the wave propagation axis, to control at least one vertically dipped PN depletion junction (VDJ) beneath. Introduction of an electrically controlled vertically dipped junction into a phase-shifter waveguide arm, which has two lateral interfaces and a bottom interface, can give an increased depletion area and results in enhanced index changes with a shorter phase shifter under reverse-biased-mode operation. This can provide effective increase of the modulation efficiency and frequency response of the device with reduced size and resistance. Figure 1(a) shows a schematic diagram of a vertically dipped junction structure into the N-doped base waveguide by P-doping into a slit-shaped window. This method can be also extended to the case where the multiple VDJs controlled through an electrical terminal are embedded in a phase-shifter arm, which can be possible by utilizing fine-scaled lithography node fabrication. Left inset of Fig. 1(a) shows the schematic example for the case of double VDJs dipped into a phase-shifter waveguide. By simply switching dopant types for a base waveguide and an embedded-junction region, the reversed PN configuration, that is, an N-doped VDJ into a P-type base phase-shifter waveguide is possible. In this Letter, we present the device based on a P-doped VDJ into an N-type base waveguide region.

 

Fig. 1. (a) Schematic diagram for the cross section of a silicon MZ modulator phase shifter arm with a vertically dipped depletion junction (VDJ) structure. Left inset shows a schematic example for the case of a phase-shifter waveguide with double VDJs. Right inset shows a side-view schematic diagram of the VDJ structure. (b) TEM cross-sectional image of a silicon MZM phase-shifter arm with a VDJ structure. Right inset shows the calculated beam-propagation profile through a VDJ phase-shifter waveguide arm. (c) Top views of SEM and microscopic images of the fabricated silicon MZ modulator with a 100 μm long VDJ phase-shifter.

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The modulators based on an asymmetric MZI with $2\times 1$ multimode interferometers (MMIs) and VDJ structures embedded in both phase-shifter arms were fabricated with a CMOS compatible process. Ridge waveguides were defined by I-line lithography and a high dense-plasma dry-etching process on a 6 in. (15 cm) SOI wafer with a top-Si thickness of 260 nm and a buried oxide thickness of 1 μm. The width of the ridge waveguide is 600 nm with 130 nm slab height. Two silicon grating couplers (GCs) with 315 nm pitch were defined at the ends of the waveguides with a target etch depth of 70 nm, for surface normal coupling to fibers. The length of the phase shifter is 100 or 250 μm. N-implantation with phosphorus was performed into the phase-shifter waveguide as a base doping, followed by the P-implantations with Boron and ${\mathrm{BF}}_2$ into the 180 nm wide 100 μm long (or 250 μm long) slit-shape window region to achieve a vertical depletion junction dipped into the N-type base waveguide. The target doping levels were $\sim 2\times {10}^{18}{\mathrm{cm}}^{-3}$ for N-type base doping, and $\sim 1\times {10}^{18}{\mathrm{cm}}^{-3}$ for P-type dopings. The ${\mathrm{N}}^{++}$ implants for both sides of the active waveguides were performed to the doping concentration level of $\sim 1\times {10}^{20}{\mathrm{cm}}^{-3}$, followed by an activation process at 900°C for 30 s. On top of the vertically P-doped region, in situ $P^{++}$-doped polysilicon plug (poly-Si) was formed for electrical contact. Due to the 350 nm linewidth limit in our fabrication node, laborious linewidth narrowing process of slit-shape patterns to 180 nm for P-implantation and polysilicon selective-epitaxial-growth processes were performed with extra passivation and etching processes repeated until patterns with 180 nm width were achieved. Metallization with 950 nm thick Ti/TiN/Al_1%Si/TiN with an alloying process were performed. Figure 1(b) shows a high-resolution-focused ion beam (FIB) transmission electron microscope (TEM) cross-sectional image of a phase-shifter active region in the fabricated MZ modulator, where a VDJ structure is defined. Right inset of Fig. 1(b) shows the calculated beam profile propagating through a VDJ phase-shifter arm, which exhibits considerable extra propagation loss through the polysilicon plug structure. Reduction of this extra loss can be achieved by altering the width or shape of a poly-Si. For instance, simulation predicts the reduction of the optical propagation loss through the poly-Si considerably by simply narrowing the poly-Si width to a half ($\sim 90\mathrm{nm}$), which can be easily done in 90 nm lithography or below. Furthermore, this type of narrowing can increase the modulation efficiency simultaneously, since it causes a more confined beam profile in the silicon waveguide region, so that the effective overlap between the optical mode and the depletion region for index changes can be increased. Figure 1(c) shows scanning electron microscope (SEM) top-view images of a fabricated MZM with a 100 μm long phase shifter.

For device characterization, continuous wave (CW) light from a tunable laser source was coupled to the GC on a chip through a polarization controller to feed a modulator (TE mode), and the electrical signal from an external driver was connected to drive a MZM. Figure 2(a) shows the measured optical transmission spectra of the MZM with a 100 μm long VDJ phase-shifter at various DC biases. The free spectral range (FSR) is $\sim 5.4\mathrm{nm}$. Here only one phase-shifter arm is biased. The black solid curve in Fig. 2(a) represents the transmission spectrum of the unbiased modulator. The net insertion loss of an MZM with 100 μm long VDJ phase-shifter was measured to be $\sim 3\mathrm{dB}$ at the maximum transmission, which is extracted by normalizing to the waveguide with the same length. This includes $\sim 0.8\mathrm{dB}/2*\mathrm{MMI}$ loss, $\sim 0.2\mathrm{dB}/\mathrm{mm}$ waveguide loss, and $\sim 1.88\mathrm{dB}/100\mathrm{\mu m}$ phase-shifter propagation loss, which includes the carrier-induced loss of $\sim 0.35\mathrm{dB}/100\mathrm{\mu m}$ in the active silicon waveguide [24]. The estimated reflective loss between simple Si waveguide and the phase-shifter region is more than 0.3 dB. The considerable portion of the beam profile is in the poly-Si region, as seen on the right inset of Fig 1(b). This gives $\sim 1.5\mathrm{dB}$ of extra loss, which is the major source for a 100 μm VDJ phase-shifter loss and is the reason for decrease of index-change based on the depletion region of the active waveguide beneath the poly-Si. Narrowing the poly-Si width can result in more than 1 dB improvement in optical loss easily by utilizing finer-scaled lithography nodes. The red solid curve represents transmission spectrum of the modulator biased at $-5\mathrm{V}$, showing a voltage-induced wavelength shift, $\mathrm{\Delta }\lambda$ of $\sim 0.15\mathrm{nm}$ in Fig. 2(a). An avalanche breakdown of the device occurs around $-5.6\mathrm{V}$. The voltage-induced phase shifts, $\mathrm{\Delta }\varphi =2\pi \mathrm{\Delta }\lambda /\mathrm{FSR}$ for a 100 μm long VDJ MZM shows a nonlinear characteristic in Fig. 2(b). The modulation efficiency, $V_{\pi }{L}_{\pi }$, the applied voltage and length required to obtain $\mathrm{\Delta }\varphi =\pi$ shows variation with respect to the applied bias in Fig. 2(b) and is low to $\sim 0.6\mathrm{V}·\mathrm{cm}$ at $-2\mathrm{V}$ bias.

 

Fig. 2. Optical transmission spectra of a MZM with a 100 μm long VDJ phase shifter at various DC biases. (b) Resulting voltage-induced phase shifts.

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The high-speed performance of a MZM was characterized by measuring the 3 dB bandwidth and eye diagrams at high transmission rates. The series resistance is $\sim 5.6\mathrm{\Omega }$ for a 100 μm long VDJ diode, and the capacitance (C) is measured $<550\mathrm{fF}$ near $-2{\mathrm{V}}_{\mathrm{DC}}$. The predicted RC limit bandwidth, $f_{-3\mathrm{dB}}=1/(2\pi \mathrm{RC})$, is $\sim 51.6\mathrm{GHz}$. The frequency-response measurement was carried out using an Agilent 67 GHz N4373C light wave component analyzer (LCA). The high-speed electrical signal and DC bias voltage were applied to the modulator through a 50G bias-tee and a 40G RF probe. The modulated output signal was amplified using an EDFA and measured by LCA. The frequency responses of a 100 μm phase-shifter MZM with varying reverse bias on one phase-shifter arm are shown in Fig. 3(a). The measured bandwidth of a modulator is $\sim 43.7\mathrm{GHz}$ at $0{\mathrm{V}}_{\mathrm{DC}}$ bias, and above 50 GHz beyond $-2{\mathrm{V}}_{\mathrm{DC}}$ bias, as shown in Fig. 3(b).

 

Fig. 3. (a) Measured frequency response of a 100 μm VDJ phase-shifter MZ modulator. (b) Measured 3 dB bandwidth ($f_{-3\mathrm{dB}}$) is $\sim 43\mathrm{GHz}$ at $0{\mathrm{V}}_{\mathrm{DC}}$ bias and greater than 50 GHz beyond $-2{\mathrm{V}}_{\mathrm{DC}}$ bias.

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On-wafer measurements of eye diagrams were performed at various bit rates up to $50\mathrm{Gb}/\mathrm{s}$. The non-return-to-zero pseudorandom bit sequence (PRBS) $2^{31}-1$ of the Anritsu MP1758A pulse pattern generator (PPG) was combined with a DC bias using a bias-tee and applied to the modulator. The input CW beam from a tunable laser was passed through a polarization controller and coupled to the input GC to feed the modulator. The modulated output signal from the chip was coupled to fiber probe aligned with the output GC. An EDFA was used to boost the modulated output signal, and a tunable wavelength filter was used before a light signal was detected with Agilent DCA-X 86100D/86109A module.

Figure 4 shows measured eye diagrams of a fabricated 100 μm VDJ phase-shifter MZM driven differentially with $2.5{\mathrm{V}}_{\mathrm{pp}}$ up to $50\mathrm{Gb}/\mathrm{s}$ at $-2{\mathrm{V}}_{\mathrm{DC}}$ bias. The measured ER is $\sim 6.0\mathrm{dB}$ at $40\mathrm{Gb}/\mathrm{s}$ data transmission. Here the optical loss of $\sim 9.3\mathrm{dB}$ is measured at the ‘1’ level of the signal compared to the maximum transmission case. For $50\mathrm{Gb}/\mathrm{s}$ data transmission, the measured ER is $\sim 5.3\mathrm{dB}$. The power consumption of this device without considering the optical energy is estimated as $0.86\mathrm{pJ}/\text{bit}$, using the equation of $E_{\text{bit}}=(1/4){\mathrm{CV}}_{\mathrm{pp}}^2$. For $1.2{\mathrm{V}}_{\mathrm{pp}}$ differential drives, the same device exhibits eye openings up to $45\mathrm{Gb}/\mathrm{s}$ operation with an ER of $\sim 2.2\mathrm{dB}$ at $-0.4{\mathrm{V}}_{\mathrm{DC}}$ bias.

 

Fig. 4. Measured eye diagrams of the MZ modulator with a 100 μm long VDJ phase shifter show ERs of (a) $\sim 6\mathrm{dB}$ at $40\mathrm{Gb}/\mathrm{s}$ operation and (b) $\sim 5.3\mathrm{dB}$ at $50\mathrm{Gb}/\mathrm{s}$ operation at $-2{\mathrm{V}}_{\mathrm{DC}}$ with $2.5{\mathrm{V}}_{\mathrm{pp}}$ differential voltage driving.

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Figure 5(a) shows an eye diagram of the MZM modulator with a 250 μm long VDJ phase shifter driven differentially with $1.2{\mathrm{V}}_{\mathrm{pp}}$ swing up to $40\mathrm{Gb}/\mathrm{s}$ $2^{31}-1$ PRBS signal at $-0.4{\mathrm{V}}_{\mathrm{DC}}$ bias. As seen in the figure, the measured eye diagram exhibits eye opening with 2.7 dB ER. The optical loss of $\sim 8\mathrm{dB}$ is measured at the “1” level of the signal compared to the maximum transmission case. Also Fig. 5(b) shows the measured $50\mathrm{Gb}/\mathrm{s}$ eye diagram of the same device for $2.5{\mathrm{V}}_{\mathrm{pp}}$ differential drives at $-2{\mathrm{V}}_{\mathrm{DC}}$ bias, which shows an ER of $\sim 5.6\mathrm{dB}$. Here the additional optical loss of $\sim 6\mathrm{dB}$ for the “1” level of the signal compared to the maximum transmission was measured. As mentioned before, a relatively wide ($\sim 180\mathrm{nm}$) pattern for a polysilicon-plug structure for electrical contact has contributed as a major optical phase-shifter loss. Also the resulting less-confined beam in the active waveguide region has led to lower modulation efficiency than expected, and this in turn resulted in relatively large additional optical loss to get larger ERs in the modulation measurements. By narrowing the width of a polysilicon structure to bring down the beam profile more to the VDJ region of the active silicon waveguide, we expect meaningful improvement such as low insertion loss below 1.5 dB with better modulation efficiency. Further doping optimization or multiple VDJs embedding in a phase-shifter arm can also improve the modulation efficiency. Our experimental results suggest that together with mature compact Ge-on-Si photodetectors [24], small-sized VDJ phase-shifter MZ modulators with further optimization can provide implementable silicon photonic integrated circuits of high-data-rate optical I/O for future inter/intra-chip interconnect applications.

 

Fig. 5. High-speed operations of a 250 μm long VDJ phase-shifter MZM for (a) $40\mathrm{Gb}/\mathrm{s}$ with $1.2{\mathrm{V}}_{\mathrm{pp}}$ differential drive at $-0.4{\mathrm{V}}_{\mathrm{DC}}$ and (b) $50\mathrm{Gb}/\mathrm{s}$ with $2.5{\mathrm{V}}_{\mathrm{pp}}$ differential drive at $-2{\mathrm{V}}_{\mathrm{DC}}$ with same wavelength. Measured ERs are (a) $\sim 2.7\mathrm{dB}$ for $1.2{\mathrm{V}}_{\mathrm{pp}}$ drive and (b) 5.6 dB for $2.5{\mathrm{V}}_{\mathrm{pp}}$ for $50\mathrm{Gb}/\mathrm{s}$ modulation.

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In conclusion, we presented small-sized depletion-type silicon MZ modulator based on a vertically dipped depletion junction in each phase-shifter waveguide. The silicon modulator with a 100 μm VDJ phase shifter, fabricated by CMOS compatible process with I-line lithography node has demonstrated a modulation efficiency of $V_{\pi }{L}_{\pi }\sim 0.6\mathrm{V}·\mathrm{cm}$, with a 3 dB bandwidth over 50 GHz. The device showed an ER of $\sim 6\mathrm{dB}$ for $40\mathrm{Gb}/\mathrm{s}$ and an ER of $\sim 5.3\mathrm{dB}$ for $50\mathrm{Gb}/\mathrm{s}$ modulation for differential drive of $2.5{\mathrm{V}}_{\mathrm{pp}}$. The VDJ propagation loss was $\sim 1.88\mathrm{dB}/100\mathrm{\mu m}$ for maximum optical transmission, which is mostly contributed from the extra beam-propagation loss through polysilicon-plug structure for electrical contact. Also a silicon modulator with a 250 μm VDJ phase-shifter exhibited $\sim 2.7\mathrm{dB}$ ER for $40\mathrm{Gb}/\mathrm{s}$ modulation with $1.2{\mathrm{V}}_{\mathrm{pp}}$ differential drive and $\sim 5.6\mathrm{dB}$ ER for $50\mathrm{Gb}/\mathrm{s}$ with $2.5{\mathrm{V}}_{\mathrm{pp}}$ differential drive. Further improvements in the insertion loss and modulation efficiency can be expected by utilization of advanced lithography nodes. Our experimental result indicates the possibility of compact-sized depletion-type MZ modulators capable of high-performance such as simultaneous high modulation efficiency, high speed, low power/energy, and small device footprint for future chip-level integration.