Photolithography Fabricated Broadband Waveguide Grating Couplers With 1 dB Bandwidth Over 100 nm

In this paper, we propose and validate a new approach for increasing the optical bandwidth of waveguide grating couplers by using a center-symmetric grating structure. Our results demonstrate state-of-the-art performance in increasing the 1 dB optical bandwidth to 108 nm and experimentally validated coupling efficiency of −3.0 dB. The fabrication uses standard MPW fabrication service offered by commercial foundry with a minimum feature size above 181 nm. Such a flat wideband and high-efficiency performance is suitable for large-volume manufacturing and will enable the use of grating couplers in wideband applications such as wavelength division multiplexed communications, broadband applications of optical frequency combs and optical sensors.

Photolithography Fabricated Broadband Waveguide Grating Couplers With 1 dB Bandwidth Over 100 nm Xuetong Zhou , Student Member, IEEE, and Hon Ki Tsang , Fellow, IEEE Abstract-In this paper, we propose and validate a new approach for increasing the optical bandwidth of waveguide grating couplers by using a center-symmetric grating structure.Our results demonstrate state-of-the-art performance in increasing the 1 dB optical bandwidth to 108 nm and experimentally validated coupling efficiency of −3.0 dB.The fabrication uses standard MPW fabrication service offered by commercial foundry with a minimum feature size above 181 nm.Such a flat wideband and high-efficiency performance is suitable for large-volume manufacturing and will enable the use of grating couplers in wideband applications such as wavelength division multiplexed communications, broadband applications of optical frequency combs and optical sensors.
Index Terms-Integrated optics, diffraction gratings, grating coupler, silicon photonics, silicon on insulator technology.

I. INTRODUCTION
W AVEGUIDE grating couplers (GCs) have been widely used in silicon photonics for coupling light to optical fibers because they have a number of attractive properties, including large fabrication tolerance, freedom to be positioned anywhere on the chip surface, compatibility with wafer-scale testing, and relaxed tolerances for fiber alignment because they can interface directly to regular single mode fibers rather than the small core fibers needed for inverse-taper edge couplers.We previously reported photolithography patterned grating couplers with measured insertion loss below 1 dB [1] using the optimized shift pattern overlay method [2], [3], but it had a relatively narrow 1 dB optical bandwidth of about 35 nm.In some applications, broadband operation is needed.However, the inherent wavelength dependence of Bragg diffraction has meant that grating couplers have a much smaller optical bandwidth than edge couplers [4].Inverse design algorithms have been used to design GC with wide optical bandwidth and improved tolerance to the spectral shift caused by temperature or fabrication variations [5].Previously reported approaches to improve the bandwidth of grating couplers include reducing the index of the grating region [6], [7], [8] by using low index material platform [9], using subwavelength structures to reduce the average index of the grating region [10], [11], modifying the optical dispersion [12] with a high index overlay of 2D materials, and the use of high numerical aperture optical fibers [13].Reducing the index of the GC region introduces additional requirements on the grating, such as requiring multiple device layers or operating with a much larger angle of incidence [14], [15].One of the disadvantages of using subwavelength gratings is the small minimum feature sizes needed and the difficulty in fabricating subwavelength structures with typical photolithography systems employed by silicon photonics foundries, particularly when it is desired to operate at shorter wavelengths in photonic integrated circuits.Compared with two-dimensional subwavelength gratings, onedimensional gratings are easier to design and fabricate.Based on the inverse design method, broadband grating couplers based on one-dimensional subwavelength gratings have also received research attention [5], [14], [15].
To the best of our knowledge, the best 1 dB optical bandwidth previously published is about 100 nm, but it required the fabrication of subwavelength structures and achieved only relatively low coupling efficiencies as summarized in Table I [5], [10], [15].An impressive 1-dB bandwidth of almost 158 nm was predicted recently with a novel structure [16] and required the use of an ultra-high numerical aperture fiber, and minimum feature size of only 40 nm, which is smaller than the minimum feature size supported by many commercial foundries.With standard SMF-28 fiber, an excellent 1-dB bandwidth of over 90 nm was achieved with a tilted prism coupling structure [17] combined with subwavelength grating for engineering the index and dispersion, and a minimum feature size of 100 nm was needed.The reported experiment results suffered from reflections which introduced a very large ripple of about 5 dB [17].Wide bandwidth grating couplers are easier to realize on the low index platform, such as silicon nitride grating.Doerr et al. demonstrated a wideband grating coupler in the silicon nitride platform with coupling efficiency of −4.2 dB and 1 dB bandwidth of 67 nm [18].To enhance the coupling efficiency further, Sacher et al. used an optimized silicon layer as a back reflector below the silicon nitride layer to enhance the directionality of the silicon nitride grating coupler, and achieved a coupling efficiency of −1.3 dB with 1 dB bandwidth of 80 nm [9].However, the fabrication of substrate mirrors requires additional processes that are not commonly available in commercial foundries.
In this paper, we propose a new conceptual approach for the design of wideband grating couplers.We demonstrate our approach in a commercial silicon foundry multi-project wafer process, and our new design is fully compatible with large volume manufacturing.The new approach improves significantly on the 30 nm 1 dB bandwidth of the foundry's library grating coupler design, to 108 nm 1 dB bandwidth.The demonstrated grating coupler has a high efficiency when compared with previously reported approaches for making wideband grating couplers, and the transmission spectrum is very flat, making it suitable for CWDM communication systems and frequency comb generation.Besides, the wideband and flat-top transmission gives the proposed grating coupler a large tolerance against random fabrication-related variations in dimensions, thus enabling high yield in large volume production.In this paper, we describe the new basic approach to achieve wideband operation of the grating coupler.This design approach can also be used in other platforms, such as the low index silicon nitride platform, to further improve the bandwidth of the silicon nitride grating coupler.
Based on the Resonant-cavity-enhanced photodetector and utilizing the critical coupling concept, high performance photodetector has been proposed [19], which predicts near unity quantum efficiency under the critical coupling condition.Critical coupling in a cavity has also been proposed for high coupling efficiency grating coupler [20].Resonant-cavity-enhanced broadband photodetector has also been proposed [21].Here, we applied a resonant-cavity-enhanced structure with dispersion engineering together to realize a high coupling efficiency and broadband grating coupler.
Our proposed structure is center symmetric.The grating can be regarded as the combination of many cavities around the center of symmetry at the middle of the structure.We attain high coupling efficiency by using the approach similar to the critical coupling concept [20], in which by careful design of the cavities, light will be trapped in the cavity structure, with only out of plane first order diffraction being available as the main mechanism for light to escape the cavity.We engineer the diffraction mode profile to match the fiber mode, thus enabling high coupling efficiency.As for the wideband operation, the cavity used here will enable a double pass effect [20], which means that the cavity shortens the effective length of grating, thus increasing its bandwidth since the optical bandwidth scales inversely with the number of periods in a grating.Also, with careful design, different cavities can be designed to have different resonant wavelengths to enhance the grating coupler overall bandwidth.We can also design the spectral notches of this structure to be unevenly spaced [22], which means that the dispersion of this structure can be flexibly engineered to compensate the waveguide dispersion [12] and thus reduce the whole dispersion of the grating, and thus realize a broadband or colorless structure that facilitates wideband operation [21].
We present both simulations and experimental results that demonstrate this concept for the wide broadband grating coupler (WBGC).The 3D finite difference time domain simulations of the numerically optimized grating predict a coupling efficiency of −2.91 dB with a 1-dB bandwidth of 109 nm, while the proof-of-concept experimental demonstration achieved experimentally measured best coupling efficiency of −3.0 dB with 1-dB bandwidth of 108 nm.We fabricated different devices with the largest 1-dB bandwidth measured to be 112 nm for a device with a coupling efficiency of −3.59 dB, and the smallest 1-dB bandwidth measured to be 96 nm with a coupling efficiency of −3.28 dB.The mean of the experimental results is −3.34 dB coupling efficiency with 1-dB bandwidth of 105.6 nm.The results advance the state-of-the-art in the bandwidth of silicon waveguide grating couplers for standard single mode fiber.The designed grating has good fabrication tolerance with a minimum feature size larger than 181 nm.

II. GRATING COUPLER DESIGN AND OPTIMIZATION
The conceptual operation of the WBGC coupler is illustrated in Fig. 1: the structure has mirror symmetry about the central cavity.Intuitively, this center symmetric structure can be regarded as many cavities combined together, and these cavities have different cavity lengths with each other.Therefore, the spectral resonance of each cavity is different, and these spectral resonances can be individually tuned, and when combined together, they give broadband operation.By engineering the coupling strength between different cavities, the flat-top and broad pass band can be realized [23].Besides, these multiple cavities will have interaction or intercoupling between each other, and by carefully designing the intermediate coupling strength, the proportion of light can be flexibly regulated between different cavities.This way, both their resonant wavelengths and intercoupling strength can be flexibly designed, and this carefully designed intercoupling between different wavelength cavities will enable the generation of a flatter topped passband [24], [25].The multiple cavity structure will also enable the light passing through the grating diffraction to have a multiple pass effect, and this will help shorten the length of grating, which will increase the optical bandwidth since the longer the grating, the more well defined is the wavelength selectivity of the grating and the narrower is its optical bandwidth.By carefully engineering the center symmetric structure, the multiple cavities can enable the scattered beam power profile of the grating to have a good mode overlap with the fiber mode for high coupling efficiency and a broadband response.
Based on the above center symmetric structure as the starting point, we used a numerical optimization method, the genetic algorithm [26], to engineer the individual lengths of each period of the grating structure.We applied the algorithm that we previously employed for a grating coupler design, as detailed in our previous published paper [27], but with three key differences: i) we constrained the algorithm to produce a grating that has mirror symmetry about the center; ii) we modified the fitness parameter that was evaluated in determining whether the random change in the genetic algorithm was fit or not by defining the fitness parameter to be the product of the grating coupler coupling efficiency with the 1 dB bandwidth of the grating coupler.The fitness parameter was evaluated by 2-dimensional finite time domain (2D-FDTD) simulations in order to reduce the simulation time.
The optimization was for a silicon-on-insulator wafer with 220 nm thick top silicon layer and a 2 µm thick buried-oxide layer.The thickness of the polysilicon layer is 160 nm.The etch depth is 230 nm with top 160 nm polysilicon layer fully etched and down crystalline silicon waveguide layer 70 nm shallowly etched.All of these were standard MPW fabrication processes [28], [29] without any process customization for this design.Structural parameters including the width of groove and teeth of every grating period were optimized by the genetic algorithm.To fulfill the design rule in the foundry, we set the minimum feature size to be 181 nm in the design presented in this paper.Due to the mirror symmetry of the structure, it is only necessary to optimize half of the parameters compared to a conventional waveguide grating coupler design.
The final optimized structural parameters of the wideband grating coupler are shown in Fig. 2, which plots the etch width and silicon width (i.e., the width of the etched groove and the width of the silicon grating tooth), respectively, for each period in the designed structure.The minimum feature size of 181 nm was present only in the first, eighth, and eleventh periods.Most of the other used feature sizes are much larger than 181 nm, and the structure can be fabricated robustly using deep UV photolithography.The wideband grating coupler was designed for coupling to the transverse-electric mode and is designed for interfacing with standard SMF-28 single-mode fiber with a core diameter of 9 µm and mode field diameter of 10.4 µm at 1550 nm.
The final-optimized waveguide grating design was simulated by 3-D FDTD simulation as presented in Fig. 3.The final optimized wideband grating coupler has a simulated coupling efficiency of −2.91 dB at 1565 nm and a 1 dB bandwidth of 109 nm.We employed a focusing grating design with elliptical grating lines to reduce the footprint of the taper from the single mode waveguide to the wide width needed to match the fiber mode field diameter [30].The 3D FDTD simulation includes the losses from the grating region to the waveguide.The designed wideband grating coupler is suitable for large-array photonic integration, considering its compact footprint of 42 µm × 24 µm.The cross-sectional view and top view of the |E| 2 profiles are shown in Fig. 4.  Before submitting the tapeout of the design for fabrication at a commercial foundry (IMEC), we also carried out the analysis of the fabrication tolerances of the WBGC structure.We introduced random variations on the structure dimensions, including the thickness of the standard polysilicon overlay layer, the shift between the upper polysilicon layer with the lower silicon layer, etch depth and width of the grating groove.Fig. 5(a)-(d) show the transmission spectra of the wideband grating coupler against structural parameter fluctuations in fabrication.From Fig. 5  of the grating do introduce a wavelength shift because of the changes in the grating effective refractive index.These results show that the broadband grating coupler is robust to fabrication fluctuations in volume production.
The wideband grating coupler was fabricated using 193 nm deep-ultraviolet (DUV) lithography in the multi-project wafer fabrication service provided by IMEC.We received about 20 dies and measured the coupling efficiency of different chips.The basic test structure comprised two focusing broadband grating couplers connected with a straight waveguide.The experiment test results of single wideband grating coupler is shown in Fig. 6.The measured coupling loss also includes the losses from the grating region to the waveguide, and the width of the waveguide is 450 nm.We measured eight different chips.Fig. 6(a) shows the simulation result and two typical chip experiment test results for the broadband grating coupler.Fig. 6(b) shows the experiment test coupling efficiency and bandwidth for the eight different chips in the different positions in the wafer.The best coupling efficiency in this batch of devices was −3.0 dB (measured without any index matching gel between the fiber and the grating), and the grating has a 1 dB bandwidth of 108 nm.The best 1-dB bandwidth we measured was 112 nm with a coupling efficiency of −3.59 dB, and the worst 1-dB bandwidth measured was 96 nm with a coupling efficiency of −3.28 dB.The results are the averaged response of two gratings (one input and one output).The mean peak coupling efficiency in the set of sixteen gratings couplers was −3.35 dB, and the mean 1 dB optical bandwidth was 105.6 nm.In these measurements, we neglect the losses of the waveguide and assume equal performance of the losses and bandwidths for the input and output grating couplers.The actual coupling efficiency and bandwidth for the grating couplers may be higher than the presented result if the losses are included and/or there is an unequal split in the losses and bandwidth between input and output grating couplers.
There appears to be about 10-15 nm shift to shorter wavelengths in the experiment results compared with the simulation result in Fig. 6(a).The shift is likely caused by small differences in effective refractive index (possibly introduced by variations in the thickness of the silicon or polysilicon layers) that were fabricated compared with what were used in the simulations.The coupling efficiency and bandwidth also differ among different chips taken from different positions in the wafer, as shown in Fig. 6(b), and this may be due to nonuniform (nanometer level variations) in the nominally 220 nm top silicon thickness across the 200 mm diameter silicon on insulator wafer.
The microscope images of one pair of the fabricated focusing broadband grating coupler are presented in Fig. 7(a) and (b).The demonstrated focusing broadband grating coupler has a compact footprint of 42 µm × 24 µm and can be placed with a minimum pitch of about 24 µm, making it suitable for high-density multicore fibers with the typical channel-to-channel spacing ≥31 µm.Compared with previously reported broadband grating couplers, as shown in Table I, our proposed compact focusing broadband grating coupler has the best bandwidth despite not using the low effective index method with subwavelength structures [7], [8] to increase bandwidth, and it has a high efficiency with good fabrication-tolerance.The minimum feature size of the device is 181 nm, and the device is compatible with large volume manufacturing using 193 nm DUV lithography.

III. CONCLUSION
We proposed and demonstrated a new conceptual approach for significantly enhancing the optical bandwidth of integrated waveguide grating couplers, designed with mirror symmetry about the central period.The concept was validated by experimental implementation using a commercial multi-project wafer fabrication service provided by IMEC.Our design had a minimum feature size of 181 nm, and the device has the best combination of high measured coupling efficiency of −3.0 dB and wide 1 dB bandwidth of 108 nm, from 1519 nm to 1627 nm.In this paper, we verified our approach using devices fabricated at a deep UV photolithography foundry for coupling to standard SMF-28 fiber.The demonstrated wideband high efficiency and flat transmission spectrum grating coupler was implemented by a commercial foundry and may therefore find use in large volume manufacturing for wideband coarse wavelength division multiplexing transceivers, integrated optical frequency comb sources, wideband integrated optical spectrometers and any other application that requires wideband operation.

Fig. 4 .
Fig. 4. Intensity distribution, proportional to |E| 2 distribution, of the focusing WBGC simulated at 1550 nm.(a)(b) Input from waveguide and coupling into the fiber.(c)-(d) Input from fiber and coupling into the waveguide.Note the fiber intensity is much lower than the Si waveguide because of the larger crosssectional area of the fiber.
(a)-(d), we can see that the coupling efficiency and bandwidth of the wideband grating coupler can be well guaranteed despite thickness fluctuation of +/− 60 nm, overlay shift errors variations of +/− 80 nm, etch depth fluctuation of +/− 50 nm and width variations of +/− 60 nm.The changes in dimensions

Fig. 6 .
Fig. 6.(a) Simulated and experiment test transmission spectra of the WBGC, inset shows the measured FP ripples.(b) The coupling efficiency and bandwidth test results for different chips.

TABLE I COMPARISON
OF FIGURES OF MERITS OF THE BROADBAND GRATING COUPLER