Working with a key manufacturer of vertical-cavity surface-emitting lasers (VCSELs), inspection and metrology equipment maker Unity Semiconductor SAS of Montbonnot-Saint-Martin, Grenoble, France (Unity-SC) has been able to improve their yield by sorting out wafers that presented defects only visible using the LIGHTsEE PSD (Phase Shift Deflectometry) system. If not detected, these defects to failure later in the process or after system delivery.
Over the last decade, VCSELs have attracted increasing interest in many fields for their unique optical and electrical properties, coupled with their advantages in terms of in-line testing and manufacturability, notes Unity-SC. Mostly based on gallium arsenide (GaAs) wafers for red to infrared (IR) wavelengths, they are widely used in light detection & ranging (LiDAR) and 3D recognition. Due to automotive applications, the LiDAR market is growing rapidly with the need for failure-free devices. 3D sensing also represents a very large market, as smartphone makers are now embedding such sensors for face recognition. As such, the need for high-quality and high-reliability GaAs inspection systems is growing constantly, adds the firm.
On top of common surface scanning inspection systems (SSIS) and automatic optical inspection (AOI) systems, Unity has demonstrated what it claims is its unique capacity for detecting otherwise invisible topographic defects. These defects are not visible on standard optical inspection systems, since they do not scatter or absorb light, and can only be revealed by their topography. While they have little impact on the structural quality of the substrate itself, they can to device failure later on in the process. For example, during the production of Bragg grating reflectors, these topographic defects can to the wrong Bragg period or, in some specific cases, to stress-induced cracks. These failures can occur during the process or after system delivery under heat or stress conditions, resulting in system failure.
Due to the phase shift deflectometry technology embedded inside the LIGHTsEE series, these defects can be easily detected, says Unity-SC. PSD provides a full-wafer, non-contact, high-throughput solution with height sensitivity below 5nm. Since acquisition is made without moving the substrate, it is stress-free and compliant with any fragile substrate, says the firm.
Demonstrated with a major VCSEL maker, the technology revealed a yield improvement and a decrease in device failures by systematically suppressing dies impacted by such defects. So, by using PSD in incoming quality control, VCSEL makers can further increase their yield and improve their supply chain, says Unity-SC.
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