Mass production of photonic integrated circuits requires high-throughput
wafer-level testing. We demonstrate that optical probes equipped with
3D-printed elements allow for efficient coupling of light to etched facets of
nanophotonic waveguides. The technique is widely applicable to different
integration platforms.Comment: Accepted for presentation at European Conference on Optical
  Communications (ECOC) 201

Billah, Muhammad Rodlin

Blaicher, Matthias

Dietrich, Philipp-Immanuel

Freude, Wolfgang

Hoose, Tobias

Koos, Christian

Trappen, Mareike

Xu, Yilin

English

arXiv

KITopen

3D-Printed Optics for Wafer-Scale Probing Mareike Trappen (1, 2, a), Matthias Blaicher(1, 2), Philipp-Immanuel Dietrich(1, 2, 3), Tobias Hoose(1, 2),  Yilin Xu(1, 2), Muhammad Rodlin Billah(1, 2), Wolfgang Freude(1) and Christian Koos (1,2,3,b) (1) Institute of Photonics and Quantum Electronics (IPQ), Karlsruhe Institute of Technology (KIT),     Engesserstraße 5, 76131 Karlsruhe, Germany  (2) Institute for Microstructure Technology (IMT), Karlsruhe Institute of Technology (KIT),     Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany (3) Vanguard Photonics GmbH, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen,          Germany (a) mareike.trappen@kit.edu, (b) christian.koos@kit.edu Abstract Mass production of photonic integrated circuits requires high-throughput wafer-level testing. We demonstrate that optical probes equipped with 3D-printed elements allow for efficient coupling of light to etched facets of nanophotonic waveguides. The technique is widely applicable to different inte-gration platforms. Introduction Scalable production of photonic integrated cir-cuits (PIC) requires high-throughput wafer-level testing to evaluate device performance early in the fabrication process, prior to entering costly and complex chip separation and packaging steps. To this end, light must be efficiently cou-pled from optical probes to in-plane waveguides with high reproducibility. Presently, optical wafer-scale probing predominantly relies on grating structures1,2 to couple light from in-plane nano-photonic waveguides to single-mode fibers (SMF) that approach the chip from an essentially surface-normal direction. While such grating cou-plers (GC) have been widely used for silicon pho-tonic2 (SiP), or silicon nitride (SiN) waveguides3, the concept is inherently limited to typical 3-dB bandwidths of 50 nm and cannot be transferred to other photonic integration platforms such as in-dium phosphide (InP) or low index-contrast silica waveguides. In addition, future PIC designs in-creasingly rely on broadband edge-coupling4,5 (EC) via vertical waveguide facets, which can be fabricated on wafer level by deep etching of dic-ing trenches into the wafer surface. Coupling to and from vertical waveguide facets represents a major challenge of wafer-level testing, which was recently addressed by optical probes that exploit total internal reflection at a polished angled facet of a planar lightwave circuit (PLC)6. In this exper-iment coupling to planar SiN waveguides with comparatively large mode field diameter (MFD) of approximately 10 μm was demonstrated, leading to a total insertion loss of 5.7 dB per coupling in-terface. Transferring this approach to probing of high index-contrast waveguides with smaller cross sections is challenging, since the PLC probe always emits a divergent beam, which does not match the mode field of small in-plane waveguides.   In this paper we present a novel approach to op-tical probes, that allows for low-loss coupling be-tween etched facets of in-plane waveguides and standard single-mode fibers approaching the wa-fer from a surface-normal direction. The concept relies on in-situ 3D-printing of periscope-like freeform coupling structures to the SMF end-faces. These SMF-lens-periscopes (SMF-LP) combine a total internal reflection (TIR) mirror for redirecting the light with a refractive optical sur-face for focusing the emitted beam. SMF-LP       Fig. 1: Wafer-level probing of a SiN waveguide using a standard single-mode fiber (SMF) array with 3D-printed “lensed peri-scopes” (SMF-LP, working distance 50 μm), which fit into a standard dicing trench. (a) Schematic of probing arrangement for a loop-back transmission measurement with a typical mode field size of 10 μm. All dimensions are indicated in μm. Inset: SEM image of TriPleX chip facet. (b) Scanning electron microscope (SEM) image of 3D-printed SMF-LP. (c) Coupling efficiency be-tween a SMF-LP and test SMF as a function of lateral offset. 100μm20DicingtrenchLong focuslensxzyTriPleXchipSMFarrayLowcontrastwaveguide>653D-printedperiscopeSMF4090 10 μmMFD(a) (b) (c)SMF5 μmSurfaceFacetSMFSMF-2 -1 0 1 22.52.01.5  x direction y directionLoss [dB]Lateral axis offset [μm]Port BPort Axzy Port A Port B70stand out due to their small size, which easily fits into standard dicing trenches and allow to adapt the mode fields to a wide variety of in-plane waveguide cross sections. We demonstrate the viability of the concept by probing of waveguides on the SiN (TriPleX) and SiP platform, which fea-ture mode field sizes of 10 μm and 1.5 μm and lead to coupling losses of 3.1 dB and 5.7 dB, re-spectively. Probing Concept The concept of wafer-level probing is illustrated in Fig. 1 (a). The optical probes access the verti-cal facets of on-chip waveguides from top via the chip edges or the dicing trenches. For efficient coupling, a vertically positioned array of SMF is supplemented by a TIR mirror to redirect the beam horizontally, Fig. 1(a). A focusing lens al-lows to adapt the spot size of the emitted beam to the mode field of the on-chip waveguide. We exploit direct-write two-photon laser lithography for in-situ structuring of the SMF-LP at the SMF end face with high alignment accuracy in respect to the fiber core7, Fig. 1(b). The height of the fiber array endface over the wafer surface is chosen to be 70 μm to prevent mechanical damage while aligning the probe. The position and the shape of the lens surface is chosen to match the mode field of the waveguide facet while maintaining a working distance of at least 10 μm between the apex of the lens and the vertical facet. The SMF-LP fit into standard dicing trenches with minimum accessible trench widths of 65 μm for the TriPleX interface, Fig. 1, and 45 μm for the SiP interface, Fig. 2. The minimum trench depth (20 μm) is de-termined by the lens diameter. To simplify align-ment of the probe with respect to the on-chip waveguides, we print alignment markers to end-faces of the SMF array, Fig. 2, which can be used for automated alignment by machine vision in fu-ture implementations. The marker tips point along the SMF-LP optical axis and allow to coarsely align the probe, before active alignment tech-niques are used for refined positioning.    Fabrication  The SMF-LP structures were in-situ printed into the commercially available photoresist IP-Dip us-ing a customised 3D two-photon lithography sys-tem (Nanoscribe Photonic Professional GT, 40? objective lens with NA = 1.4), which is comple-mented by a proprietary high precision auto-mated alignment software. Slicing and hatching writing distances were both set to 100 nm. The mode field diameter amounts to approximately 1.5 μm (10 μm) for the SiP (TriPleX) waveguide. For optimum coupling to these mode fields, the aspherical lens surfaces are optimised using ei-ther the physical-optics module of Zemax Optic-Studio (TriPleX)  or an in-house wave-propaga-tion8 software (SiP). Demonstrators  To prove the viability of the concept, we designed two different SMF-LP probes for coupling to Tri-PleX and SiP waveguides at a wavelength of 1550 nm. Both SMF-LP were positioned at the edge of a single photonic chip or in front of a SMF by use of a 6 axis alignment stage. For characterizing the TriPleX SMF-LP, we first measure the coupling to a test SMF as a function of lateral offset, see Fig. 1(c) and Tab. 1, Experi-ment 1. For an SMF optimally aligned with re-spect to the SMF-LP we find a coupling loss of 1.6 dB between the input of the SMF-LP-Tab. 1: Overview of coupling experiments and measured losses. “Loss A to B“ and “Loss A to C” denote the transmission losses between the respective reference points A, B, and C as indicated in the second column.   Experiment no. Schematic of Coupling Experiment Loss  A to B Loss  A to C 1   1.6 dB - 2  6.2 dB 3.1 dB 3  10.3 dB 5.1 dB 4   5.7 dB - 5  9.0 dB 4.5 dB     Fig. 2: Wafer-level probing of a silicon photonic (SiP) wave-guide using a standard SMF array with 3D-printed SMF-LP (working distance 10 μm). The alignment markers help in po-sitioning the SMF array relative to the on-chip waveguides. The chip waveguide is terminated with a Ge photodiode. For clarity, only one SMF is shown although the SMF-LP were fabricated on a V-groove array of 8 fibers.  Inset: SEM image of 3D-printed periscope. All dimensions are indicated in μm.  >452010903D-printedperiscopeWG3D-printedalignment markerSMFxzy ViewSOI chipTaper20 μmDicingtrenchShortfocuslensPhotodiodePort APort B70equipped fiber (“port A”) and the output of the test SMF (“port B”). The 1-dB lateral alignment toler-ance amounts to ? 2.3 μm. The excess loss of 1.6 dB is attributed to the loss of the TIR reflec-tion mirror, to Fresnel reflections at the lens sur-face and to fabrication imperfections, with signifi-cant potential for further improvement7. For esti-mation of the coupling efficiency to the TriPleX chip, we use a loop-back waveguide with 250 μm spacing between the input and output facet and measure the fiber-to-fiber loss between to be 6.2 dB, see Fig. 1(a) and Tab.1, Experiment 2. Assuming identical coupling interfaces and a lossless waveguide loop, the attenuation per cou-pling amounts to 3.1 dB. For comparison we measured the fiber-to-fiber loss for coupling into the TriPleX chip with a SMF array, see Tab. 1, Experiment 3, leading to an overall loss of 10.3 dB. Assuming again the waveguide to be lossless we calculate an attenuation of 5.1 dB per coupling interface. The high coupling loss are at-tributed to the high surface roughness of the chips facet, see Inset of Fig. 1 (a). Note that, in contrast to previous demonstrations6, the cou-pling losses of the SMF-LP-based optical probe are smaller than those obtained for direct cou-pling to SMF. We attribute this to the ability of the SMF-LP to focus the light to a distant working spot (working distance: 40 μm), hence allowing to obtain best coupling efficiency without bringing the facet into direct physical contact, which al-ways bears the risk of damaging the optical sur-face. In contrast to this, the standard SMF array in Experiment 3 needs to be in direct contact with the etched surface.  For characterising the SiP SMF-LP we measured the coupling to a tapered on-chip waveguide fea-turing a MFD of about 1.5 μm, see Fig. 2 and Tab.1, Experiment 4. The waveguides were fab-ricated through the SiP foundry process offered by the A*Start Institute of Microelectronics (IME) in Singapore. The waveguide was terminated by an on-chip Ge photodiode, which was electrically contacted to measure the photocurrent. Neglect-ing the on-chip propagation losses, the attenua-tion loss attributed to the SMF-LP interface is 5.7 dB. In the experiment, the photodiode re-sponsivity was determined using another photo-diode, nominally identical to the one used in the coupling experiment, which was connected to waveguides terminated by GC. Moreover, we de-termine the GC losses by measuring the trans-mission through a SiP reference structure, Tab. 1, Experiment 5. The used SIP chips show GC losses of 4.5 dB. Note that direct interfacing of a standard SMF to a SiP device facet would not lead to conclusive results due to the large MFD mismatch. This emphasizes once more the im-portance of shaping the mode field by a refractive surface of the SMF-LP.  In comparison, the TriPleX SMF-LP show lower loss than the SIP SMF-LP. This result was ex-pected from simulations, for which the TriPleX SMF-LP showed an optimized coupling efficiency of 99% (10 μm MFD) compared to 69% (1.6 dB) for the SIP SMF-LP (1.5 μm MFD). Note that these simulations do not account for Fresnel re-flection at the lens surface, which causes addi-tional loss. Still, these findings leave room for fur-ther improving the coupling efficiency of the opti-cal probe.  Summary  We have introduced and experimentally demon-strated a technique for high-precision wafer-scale probing of edge-coupled PIC. The concept relies on compact 3D-printed coupling structures, which can be introduced into standard dicing trenches etched into the wafer surface. We demonstrate losses of 3.1 dB and 5.7 dB when coupling to silicon nitride (TriPleX SiN) and silicon photonic on-chip waveguides. The concept is ap-plicable to SMF arrays, which allow for simulta-neous readout of multiple channels and therefore enable parallel measurements. The fabrication process of direct laser writing allows high flexibil-ity in designing the coupling elements, making the concept applicable to a wide variety of optical integration platforms. Acknowledgements We thank K. Wörhoff and A. Leinse, both at LioniX BV, for providing TriPleX chips. This work was supported by the Bun-desministerium für Bildung und Forschung (BMBF) joint pro-ject PRIMA (13N14630), the Helmholtz International Re-search School for Teratronics (HIRST), the European Re-search Council (ERC Starting Grant ‘EnTeraPIC’, # 280145; ERC Consolidator Grant ‘TeraSHAPE’, # 773248), by the H2020 Photonic Packaging Pilot Line PIXAPP (# 731954), by the Deutsche Forschungsgemeinschaft (DFG) through CRC WavePhenomena (# 1173), and by the Karlsruhe Nano-Micro Facility (KNMF).  References [1] D. Thomson et al., “Roadmap on silicon photonics”, J. Opt., vol. 18, Art. no. 073003 (2016) [2] D. Taillaert et al., "Compact efficient broadband grating coupler for silicon-on-insulator waveguides," Opt. Lett. 29, 2749-2751 (2004) [3] G. Maire et al. "High efficiency silicon nitride surface grating couplers," Opt. Express 16, 328-333 (2008) [4] J. H. Song et al., “Practical design of lensed fibers for semiconductor laser packaging using laser welding tech-nique”, J. Lightw. Technol. 27, 1533–1539 (2009). [5] C. Kopp et al., “Silicon photonic circuits: on-CMOS inte-gration, fiber optical coupling, and packaging.” IEEE J. Sel. Top. Quantum Electron. 17, 498–509 (2010) [6] R. Polster et al., "Wafer-scale high-density edge cou-pling for high throughput testing of silicon photonics," OFC M3F.2, San Diego (2018) [7] P.-I. Dietrich et al., ”In situ 3D nanoprinting of free-form coupling elements for hybrid photonic integration”, Nat. Photon. 12, 241–247 (2018) [8] S. Schmidt et al., "Wave-optical modeling beyond the thin-element-approximation," Opt. Express 24, 30188-30200 (2016)  

3D-Printed Optics for Wafer-Scale Probing

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3D-Printed Optics for Wafer-Scale Probing

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