3D integrated hybrid silicon laser

A laser is realized by flip-chip bonding an indium phosphide reflective semiconductor optical amplifier with a turning mirror to a silicon photonic circuit with a surface grating coupler. An external cavity laser is formed and single-mode CW lasing is demonstrated.


Introduction
Although silicon photonics (SiPh) has undergone significant maturation [1][2][3] , integrated laser sources still remain an open problem. Monolithic integration techniques include germanium on silicon 4 and quantum dot on silicon [5][6] . These approaches, based on direct epitaxial growth, are promising but are in the research stage. To date, only discrete lasers on silicon have been realized with these approaches.
Heterogeneous techniques include the wafer-bonded silicon evanescent laser, which has demonstrated a host of novel active photonic integrated circuits on silicon [7][8][9] . These approaches require sensitive wafer bonding steps and co-fabrication of incompatible materials (indium phosphide (InP) and silicon). Also, the InP gain material is bonded to the silicon on insulator (SOI) layer, therefore the gain medium is thermally isolated from the substrate. Lasers realized with this approach exhibit high thermal impedance limiting their efficiency and potential for high-temperature operation, the latter of which is critical for onchip applications. InP and silicon also exhibit different coefficients of thermal expansion, therefore, reliability is a concern for devices realized by direct bonding of these materials.
Lastly, hybrid integration generally involves butt coupling of InP chips to silicon waveguides. Coupling to silicon nanowires is challenging due to the significant mode mismatch of the waveguides 10 . Precise alignment is required in all dimensions and the vertical alignment is especially critical since typical InP lasers exhibit large divergence in this direction. Large (micronscale) silicon waveguides can alleviate the mode mismatch somewhat 11 ; however, such waveguides are not conducive to sharp bend radii, which is a principal benefit of SiPh. Also, it is desirable to realize lasers on a 220-nm SOI platform, which is somewhat standard for SiPh.
Previously, we proposed a novel 3D integration technique for realizing lasers on silicon 12 . This technique, which is a hybrid approach, is based on flip-chip bonding of an InP laser or gain chip to a SiPh chip. The InP chip contains a monolithic turning mirror for vertical emission 13 . Light is coupled from this chip to a silicon waveguide through a surface grating coupler. We previously demonstrated an approach to optimize the design parameters, which included the angle of the turning mirror, the vertical and in-plane position of the InP chip with respect to the silicon chip, and the period and fill factor of the grating coupler. We also proposed a novel silicon photonic external cavity laser (SPECL) based on this 3D integration approach. The SPECL consists of an InP reflective semiconductor amplifier (RSOA) with integrated turning mirror coupled to a SiPh circuit comprising a ring resonator filter and a distributed Bragg reflector (DBR). In this work, we have experimentally demonstrated the 3D integration technique and SPECL for the first time.
Our 3D integration technique has many advantages for laser integration. Firstly, the InP chip can be flip-chip bonded P-side (epi-side) down to the silicon substrate. This allows for effective heat removal from the active region and low thermal impedance, which facilitate high laser efficiency and high-temperature operation.
Integration with standard 220-nm SOI is possible, whereas this is not the case with some other approaches. The InP and silicon frontend processes are carried out separately with little to no change to their inherent processes -the 3D integration approach does not require cofabrication of these materials. The attachment of the InP RSOA chips, which is carried out in a backend step, is accomplished with standard flip-chip bonding techniques that are used in the integrated circuit industry. The metal used for bonding alleviates thermal expansion coefficient mismatch. And lastly, the integration can be carried out at wafer level, meaning that InP chips can be attached within die on a full SiPh wafer. The 3D integration approach is also as scalable as any other laser integration approach.

Device design and fabrication
A schematic and photograph of the SPECL realized by 3D hybrid integration are shown in Fig. 1(a) and (b), respectively. The InP RSOA provides gain for the laser, the ring resonator provides filtering, and the DBR mirror closes the cavity and provides additional filtering. InP RSOA arrays (with four RSOAs) with different gain section lengths (L Gain ) were fabricated and bonded. The DBR mirrors were realized with edge-corrugated gratings and were designed for a center wavelength of 1550 nm. For the SPECL characterized and reported here, L Gain was 500 µm and the number of DBR grating periods was 80. The 3D integration approach is illustrated in the sideview schematic in Fig. 1(c). The turning mirror angle, position of the InP RSOA with respect to the grating coupler, and the grating coupler parameters were designed in our previous work 12 . Unfortunately, due to a fabrication error, the coupling efficiency demonstrated experimentally is lower than it could be.
The InP RSOAs were fabricated with an etched-facet process that is conducive to lowcost manufacturing 13 . Both the back facet and the turning mirror were formed by chemicallyassisted ion beam etching. A high-reflection coating was applied to the back facet and an anti-reflection (AR) coating was applied to the exit surface of the vertical emitting device. Pmetal contacts were formed on the topside and N-metal contacts on the backside. InP chips with four RSOAs were singulated following on-wafer testing and coating. These chips are approximately 1.5 x 1.0 mm 2 in size. The lateral spacing of the RSOAs was made conservatively large. We estimate that we could incorporate at least 12 RSOAs on a chip of the same size.
The silicon chips were fabricated at the Interuniversity Microelectronics Centre (IMEC) on 200-mm silicon on insulator (SOI) wafers 14 .
The silicon layer is 220-nm thick and the process incorporates a poly-silicon overlay and patterning to increase the silicon thickness for the grating couplers. Although the primary silicon components for the SPECL are grating couplers, ring resonators, and DBR mirrors, the chips also contain PN-junction modulators and germanium (Ge) photodetectors. In fact, the emission from the 3D integrated hybrid lasers can couple directly to high-speed Mach-Zehnder modulators. For our experiments, the Ge photodetectors were used primarily for calibrating fiber coupling loss.
The InP RSOA chips were flip-chip bonded, P-side down, to the silicon chips using thermocompression bonding. The flip-chip bonding tool utilized is capable of 1-µm alignment accuracy. Both chips were cleaned prior to bonding. The bonding temperature was 350ºC and the force applied was approximately 20 N. After bonding, devices were mounted to a temperaturecontrolled stage for measurements.

Measurement results
The current-voltage (IV) characteristics were measured for a standalone InP RSOA and for a 3D integrated laser. The gain length L Gain was 1 mm for both devices. As shown in Fig. 2, the IV characteristics are nearly identical indicating that the metal bond makes a proper electrical connection. To determine the amount of light  coupled from the InP chip to the silicon chip, an InP chip was bonded with a front AR coating with high enough reflectivity to support lasing at low threshold current. This chip has a gain length L Gain of 500 µm and operates as a multimode Fabry-Perot laser. Light coupled to the silicon chip through the turning mirror (in the InP chip) and the grating coupler (in the silicon chip) propagates in a single-mode silicon waveguide, and then is coupled to an optical fiber through a second grating coupler. The light-current-voltage (LIV) characteristic for this device is shown in Fig. 3. The peak lasing wavelength was 1565 nm and the chip power at a current of 100 mA was 0.75 mW. From this measurement, the coupling efficiency (from the laser chip to the silicon waveguide) was estimated to be 3.9%. This low coupling efficiency is due to the foundry error discussed earlier. Simulations with this error incorporated yield similar coupling efficiency, therefore substantial improvements are expected in future experiments, as will be discussed further. An InP RSOA chip was bonded to another silicon chip to construct a 3D integrated SPECL. The parameters for the SPECL were described in the previous section. The output of this laser structure (the SPECL) was coupled to an optical fiber through a second grating coupler. The lasing spectrum was measured with an optical spectrum analyzer and the results are shown in To motivate future work, we performed 3D finite difference time domain simulations to optimize the 3D integrated hybrid silicon laser. The waveguide structures were optimized and an apodized (nonuniform) grating was incorporated. As shown in Fig. 5, a coupling efficiency of 85% (corresponding to a coupling loss of <0.7 dB) was demonstrated. This would represent the highest coupling efficiency achieved for any hybrid integration approach.

Conclusions
We have experimentally demonstrated a novel 3D hybrid integration technique for realizing lasers in silicon. The laser was constructed by flip-chip bonding an InP RSOA with a turning mirror to a SiPh circuit with a surface grating coupler. The laser demonstrates single-mode operation at room temperature and under CW conditions. Although the demonstrated coupling efficiency is low, significantly higher coupling efficiency (85%) is expected in future work.