Optical edge coupling method for fully automated PIC wafer-level testing

As the field of silicon photonics applications expands, so does the demand for high throughput optical on-wafer testing of photonic integrated circuits (PICs). The well-known grating couplers used for many years for on wafer testing have many drawbacks such as: high polarization dependence, reduced bandwidth and relatively low coupling efficiency.
Recently, the edge coupler has become a prominent candidate for photonic foundries as a coupling interface to the chip. The low polarization dependence, low coupling loss and compatibility with advanced packaging solutions make this interface very attractive for many applications. However, testing on wafer level using edge coupling has lacked behind grating coupler based probing due to the complexity of the method and problems
that need to be overcome. The calibration automation was always complex and heavily dependent on the skills of the operator. In recent years, there have been some concepts and experiments in the literature regarding edge coupling on wafer level. To date, though, a fully automated solution has not been demonstrated. In this presentation we would like to demonstrate the fully automated turn key solution that has been developed by FormFactor Inc. We will provide an overview of the system and show repeatability results obtained on a 200 mm photonic wafer. We propose an alternative optical probing technique suitable for automated on-wafer measurements based on edge coupling, but that can also be used for surface coupling. Here, we evaluate a solution based on FormFactor’s CM300-SiPh probe station and the Pharos Probe. A brief overview of the system is given and the repeatability of the coupling across the wafer is investigated. The solution is suitable for a wide range of photonics test requirements. The overall system is based on fully-automated Probe Station (FormFactor CM300xi) with an advanced eVue machine vision based microscope system and 6-axis optical positioner (based on PI HexaPod H-811 and NanoCube P-611.3). The optical probe is a fiber array with optical lenses and will be described more in the presentation. The automated algorithm calibrates and moves the optical probe in Y and Z directions to find the maximum signal intensity and thus the best coupling point.