Due to the ongoing development of autonomous acting units for mobile autonomous manufacturing, highly dynamic, efficient and reliable sensors for three-dimensional detection of the environment are increasingly needed. The CSPAD detectors developed by Fraunhofer IMS combine highly sensitive and fast 3D-vision with reliable algorithms for high-performance use in LiDAR applications. The central goal here is to achieve application-specific ranges even at high sunlight intensity while keeping system costs low.
CSPAD technology is based on the integration of SPAD (single photon avalanche diode) detectors in a 0.35 µm CMOS process and optimized for optoelectronic applications. This allows SPAD detectors and evaluation electronics to be accommodated on the same chip. An example of this is the SPADeye2 double line sensor with 2x192 pixels and one time-to-digital converter per pixel. In addition, backside-illuminated CSPAD detectors (BSI CSPADs) can be realized using 3D integration in wafer-to-wafer and chip-to-wafer bonding processes. The stacked arrangement of 3D-vision sensor and readout electronics makes the detector even more efficient and compact. In this way, CSPAD area sensors with thousands of pixels can be realized for use in solid-state LiDAR systems with high resolution.
The CSPAD detectors developed at Fraunhofer IMS are characterized in particular by the adaptive photon coincidence circuits integrated into the pixels for background light suppression. This is the only way to ensure an increase in range in high sunlight and reliable distance measurement even in changing weather conditions.
An important milestone in the development of intelligent CMOS photodetectors has been reached. With the now new generation of CSPADs, Fraunhofer IMS demonstrates new forms of integration of Single-Photon Avalanche Diodes (SPAD) into CMOS for 3D-vision applications.
The CSPAD αlpha combines for the first time the highly sensitive photodetectors with their electronics in the form of a three-dimensional chip stack. The wafer-to-wafer interconnect technology developed in-house allows high-resolution pixel matrices to be integrated with fast readout electronics in a very small space. CSPAD αlpha demonstrates the quality of this new interconnection method with its LiDAR capabilities and forms the basis for further chip designs.
With 32 x 24 pixels, distance measurements can be performed using only one illumination (Flash LiDAR) at a readout rate of 26 kHz. The pixel individual TDCs (Time to Digital Converter) are integrated on-chip in the readout electronics and enable a time resolution of 312.5 ps or a distance resolution of about 4.6 cm.
Apart from the typical application area LiDAR, the CSPAD αlpha can also be used in the fields of spectroscopy, quantum technology and data encryption, among others.
In addition to the development and production of new, also customer-specific, 3D sensors, the Fraunhofer IMS provides a wide range of services in the field of LiDAR. Thus, evaluation boards for CSPAD chips as well as complete LiDAR cameras with corresponding software are offered, with which the sensors can be evaluated in realistic measurement scenarios. Furthermore, system simulations are carried out at the Fraunhofer IMS and new methods for signal processing are constantly being investigated with which an additional increase in range can be achieved.
The Fraunhofer IMS CSPAD technology can also be combined with other Fraunhofer technologies such as MEMS micro mirrors to realise scanning systems. More information on this can be found on the pages of the Forschungsfabrik Mikroelektronik Deutschland: