FEATURE: TEST & MEASUREMENT shows that too many chips would be rejected.


Working together Latkowski and his colleagues are working towards standardising and automating test methods to enable this early validation. For example, TU/e runs the Open Innovation Photonic ICs (OpenPICs), InPulse and OIP4NWE pilot-lines, which are seeking to develop an industrial platorm for PICs. He is also working with the PIXAPP PIC assembly and packaging pilot line, led by the Tyndall Institute in Cork, Ireland. ‘We are starting with small steps, trying to unify chip layouts and implementing test methodology at the design level,’ Latkowski said. ‘Tese allow designers to work on the application-specific photonic integrated circuits, while taking into account testing and also assembly aspects. It also benefits automated test equipment vendors, by helping them to develop modular tools. Such modularity will lead to scalable solutions that will allow for a certain degree of customisation with a minimum cost related to non-recurring engineering.’ But combining electrical and optical


testing is especially challenging, Duis noted. He emphasised that there can be more than 100 critical parameters to test. Tis is difficult in ‘low-volume, high-mix prototypes with different contacting configurations’, he added. ‘Foundries need a minimum amount of runs through the fab with the same design, such that they can build up knowledge on how customer-specific critical parameters behave on a PIC level,’ he said. Foundries and their customers can work


together to optimise PIC designs for testing. For example, they could exploit self- diagnostics, so the foundry can run purely electrical tests on a standard wafer prober. If not, they need to couple in light, using active alignment with fibre-probers on a wafer or chip level. Agreeing this might take years, Duis warned. ‘In order to speed up this process, foundries must have beter statistical process information on all the offered building blocks, reducing the amount of critical test parameters that need to be tested,’ he said.


Complex proposition FiconTEC is involved with many such projects, explained Gregory Flinn, marketing development spokesperson for the firm, which offers automated stand-alone and in-line photonics assembly and testing systems. Te later can validate or verify photonic chip or device performance at any process stage, to check that electrical and optical characteristics are within the specifications. He explained that in these systems, ficonTEC


provides high-resolution mechatronic positioning systems that align pickups to devices or wafers, enabling the connection to test equipment. Yet adequate, dedicated electro-optical probe heads are lacking, he said. Tat’s partly because they are a much more complex proposition than electrical testing probe heads alone, which themselves can have many tens of thousands of pickups.


www.fibre-systems.com @fibresystemsmag ‘Electrical pickups require only metal


contacts, each one comprising a pad that is around 80µm by 80µm,’ Flinn said. ‘Our assembly machines integrate PIC devices together with other micro-optical photonic components to assemble a complete photonic device. Tat device then needs to be tested, and you may be testing for multiple characteristics. ʻIf the plan is to use electro-optical mixed-


signal pickups, then your resolution bounds are entirely defined by the optical side, which can be down to 20nm. In general, the actual alignment takes more time, and places much greater demands on the positioning system, which also means it’s more costly from a capital outlay aspect.’


Time for a compromise To qualify precision in optical fibre alignment to on-wafer devices, ficonTEC has developed and demonstrated systems. For example, one system went to the University of Southampton’s Optoelectronic Research Centre’s Silicon Photonics Group. Te system is intended to have many additional capabilities, said Flinn. A similar demonstration system appeared


at NI Week in Austin, Texas, in May, and at Laser World of Photonics, in June. It aligned dual fibre arrays to couple multiple light signals through multiple on-wafer waveguides. For this purpose, ficonTEC teamed up with optical communication test and measurement equipment supplier Coherent Solutions. In the demo, Coherent Solutions provided the necessary optical input/output test instrumentation to illustrate coupling through the waveguides. Hansjoerg Haisch, program manager,


networks and data centres at Keysight Technologies, agreed that fast, high-precision alignment is needed. However, he also highlighted several other challenges. ‘Coupling light into waveguides through widely-used grating couplers depends strongly on light polarisation and wavelength,’ he stressed. ‘Stability of polarisation state, or good


TYPICALLY, THE TESTING IS FOUR TO EIGHT MINUTES PER PRODUCT IN A VOLUME ENVIRONMENT


knowledge of the state of polarisation, is essential for repeatable and low uncertainty measurements.’ Haisch adds that coupling light into waveguides through the device edge is not usually possible before dicing wafers without using test structures. However, coupling to the edge through etched trenches has recently been developed.


Making a compromise Furthermore, PIC wafers or lone chips cannot always be operated under realistic conditions, as they oſten require amplifiers or lack the heat sinks that will let them run at high power. ‘On-wafer PIC test coverage is thus generally restricted to a few parameters and later module or finished good performance must be predicted based on those few measurements,’ Haisch said. ‘For highly complex PICs, not all functions can be interfaced or accessed independently, without integrating too many monitoring functions, which would need too much chip real-estate. A compromise between test coverage and test time must be found for known good die selection and throughput.’ Reaching higher production volumes helps


make testing equipment more economical, explains Haisch. ‘PIC customers have been reporting increasing volume shipment of PIC- related technology for their datacom products in the last two years,’ he added. Keysight has helped by partnering with PIC industry players to standardise photonic parametric tests. Tis


g


A hybrid laser assembly that PHIX made for Lionix International integrating an InP gain section, with the mirror and long cavity integrated in SiN.


Issue 25 n Autumn 2019 n FiBRE SYSTEMS 11


PHIX


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