Non-contact scanning probe station for advanced wafer scale testing of photonic integrated circuits

Integrated photonics manufacturing is rapidly becoming a mature multi-billion pound global industry as photonics is underpinning an very wide range of applications aligned with UK's industrial strategy in AI and Web 5.0, the Future of Mobility, Healthcare, and Future Sensors. Test and measurement forms a critical part of most fabrication workflows both in industry and in research, for its potential of picking up deviations at the earliest stage possible and certainly before any expensive packaging and integration steps. On-wafer testing provides an early opportunity which can potentially save significant costs by preventing malfunctioning devices to continue in the processing workflow thus avoiding unnecessary waste of resources, tooling and energy. Importantly, wafer testing allows to feed back any deviations from the original design caused by failures in the fabrication process and smaller drifts exceeding the manufacturing tolerances. The semiconductor industry's leading IRDS Roadmap 2020 has identified the importance of photonics in next generation computing but has pointed out critical bottlenecks in available testing tools for addressing challenges in yield, variability, precision, and tunability of photonic chips. Fabrication imperfections are currently amongst the main limiting factors for achieving reliable high-volume photonics manufacturing.

With the appearance of new photonic probe extensions to commercially available wafer probers commonly used in semiconductor electronics manufacturing, a range of sophisticated end-to-end characterisations is now available. This is ideal for a range of tests verifying performance and validating the entire circuit response against the expected output and identifying the critical outliers which are responsible for failure at the systems level. However the current generation of tools lack the capability of extracting information on what happens inside the photonic circuit. As integrated circuits become more and more complex, the lack of intermediate probe points in the circuit becomes an ever more pressing issue. Indeed this issue was addressed recently in other research projects, where groups have proposed erasable output couplers in the circuit as an option for more in-depth testing of intermediate probe points.

However a more general approach is within reach as shown by us in a number of proof of principle studies leading to this project. It turns out that the semiconductors used in these photonic circuits are responsive to short-wavelength UV light, in fact responsive enough that illumination of a small microscopic point in the device gives rise to a traceable signal at the output of the circuit. By scanning this spot through the device, we can build up a detailed image of where the light is in both time and space. We can even resolve this map in wavelength, to build up a complete picture of device performance far beyond the capabilities of the commercial probe stations.

While this so far has remained a basic research topic, we propose here to push this approach forward as a versatile tool for wafer-scale photonics testing. For this we need to make the techniques much faster, robust and reliable for use in a manufacturing workflow, and aligned with the actual requirements of the end users on different platforms. The majority of the project is therefore focused on developing this instrumentation, operating this with an open source Python data acquisition framework for interoperability and user customization, and generate a convincing set of tests and demonstrators for each platform that will be used to leverage the capabilities of this platform for different application areas. At the end of the project we expect that we have developed a self-contained instrument that will find use in a wide range of research and manufacturing environments around the world.