IMSE Publications

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Author: Juan M. López Martínez
Year: Since 2002

Journal Papers


Design of High-Efficiency SPADs for LiDAR Applications in 110nm CIS Technology
I. Vornicu, J.M. López-Martínez, F.N. Bandi, R. Carmona-Galán and A. Rodríguez-Vázquez
Journal Paper · IEEE Sensors Journal, vol. 21, no. 4, pp 4776-4785, 2021
abstract      doi      

Single photon avalanche diodes (SPADs) featuring a high detection rate of near-IR photons are much desired for outdoor LiDAR based on direct time-of-flight (ToF). This article presents the complete design flow of a SPAD detector for LiDAR. First, the selection of the emitter wavelength is discussed, considering the maximum allowed power underlying eye safety regulations, solar irradiance, and reflected signal power. Then, the choice of the SPAD structure is discussed based on the TCAD simulation of quantum efficiency and crosstalk. Next, the proposed P-well/Deep N-well SPAD is explained. The electro-optical characterization of the detectors is presented as well. The performance of the time-of-flight image sensors is determined by the characteristics of the individual SPADs. To fully characterize this technology, devices with various sizes, shapes, and guard ring widths have been fabricated and tested. The measured mean breakdown voltage is 18 V. The proposed structure has a 0.4 Hz/µ m2 dark count rate and 0.5% afterpulsing. The FWHM (total) jitter and photon detection probability at 850nm wavelength are of 92 ps and 10%. All figures have been measured at 3 V excess voltage. Finally, the performance of the SPAD detector is analyzed by evaluating the signal-to-noise ratio at different acquisition times. Distance ranging measurements have been performed, achieving a depth resolution of 1 cm up to 6.3 m range.

Conferences


Photon-Detection Timing-Jitter Model in Verilog-A
J.M. López-Martínez, R. Carmona-Galán and A. Rodríguez-Váquez
Conference · IEEE International Symposium on Circuits and Systems ISCAS 2020
abstract      pdf

Single-photon avalanche diodes can be employed to register the arrival of an individual photon. They are biased beyond breakdown voltage, and thus the electron-hole pairs generated by any incident photon is accelerated by the strong electric field triggering an avalanche current. In recent years, there have been attempts to model its characteristics in Verilog-A HDL. However, none of them have modelled its photon-detection timing jitter. This paper explains the mechanism of avalanche triggering and proposes a first approach to model it in Verilog-A. Comparison with experimental data and data reported in literature validates the model.

Limitation of SPADs quantum efficiency due to the dopants concentration gradient
J.M. López-Martínez, R. Carmona-Galán and A. Rodríguez-Váquez
Conference · IEEE International Symposium on Circuits and Systems ISCAS 2020
abstract      pdf

Single-photon avalanche diodes are highly sensitive devices capable of registering the arrival of an individual photon. They are biased beyond breakdown voltage, and thus the electron-hole pairs generated by any incident photon is accelerated by the strong electric field triggering an avalanche current. This paper examines the role of the dopants concentration gradient in the gathering of photons in these devices, and how it can be engineered to maximize quantum efficiency and explain his role to minimize undesirable effects like crosstalk.

An Experimentally-Validated Verilog-A SPAD Model Extracted from TCAD Simulation
J.M. López-Martínez, I. Vornicu, R. Carmona-Galán and A. Rodríguez-Vázquez
Conference · IEEE International Conference on Electronics Circuits and Systems ICECS 2018
abstract     

Single-photon avalanche diodes (SPAD) are photodetectors with exceptional characteristics. This paper proposes a new approach to model them in Verilog-A HDL with the help of a powerful tool: TCAD simulation. Besides, to the best of our knowledge, this is first model to incorporate a trap-assisted tunneling mechanism, a cross-section temperature dependence of the traps, and the self-heating effect. Comparison with experimental data establishes the validity of the model.

Characterization of Electrical Crosstalk in 4T-APS Arrays using TCAD Simulations
J.M. López-Martínez, R. Carmona-Galán and A. Rodríguez-Vázquez
Conference · Conference on Ph.D Research in Microelectronics and Electronics PRIME 2017
abstract     

TCAD simulations have been conducted on a CMOS image sensor in order to characterize the electrical component of the crosstalk between pixels through the study of the electric field distribution. The image sensor consists on a linear array of five pinned photodiodes (PPD) with their transmission gates, floating diffusion and reset transistors. The effect of the variations of the thickness of the epitaxial layer has been addressed as well. In fact, the depth of the boundary of the epitaxial layer affects quantum efficiency (QE) so a correlation with crosstalk has been identified.

TCAD Simulation of Electrical Crosstalk in 4T-Active Pixel Sensors
J.M. López-Martínez, R. Carmona-Galán, J. Fernández-Berni and A. Rodríguez-Vázquez
Conference · Workshop on the Architecture of Smart Cameras WASC 2017
abstract     

CMOS image sensors (CIS) are widely used nowadays in consumer electronics as well as in high-end applications. This is mainly due to their advantages regarding low dark current and low noise characteristics of the pinned photodiode (PPD). Much effort has been put into better understanding key electrical properties of PPDs, like full well capacity, photodiode´s capacitance or pinning voltage. Another important source of sensitivity degradation is crosstalk (CTK). It has been assessed for CCDs and some CMOS devices. However, addressing CTK in CMOS 4T-APS pixels at the design phase is not easy, mainly due to the unavailability of CIS technology parameters.an additional problem is the computational cost of TCAD simulation; e.g., a five pixel linear array like the one shown in Fig. 1, already introduce long periods of computing due to the complexity of the structure. Crosstalk occurs when the charge generated by photon incident on a pixel are finally sensed by a neighboring pixel. CTK degrades performance, cutting down spatial resolution, reducing the overall sensitivity, degrading color separation, and increasing image noise. Crosstalk is defined as the percentage of the total charge generated by incident light that is diverted to non-illuminated pixels in the neighborhood. There are two components in CTK. Optical crosstalk is related to illumination, reflection, refraction and scattering of photons in the different layers of the material that cover the photodiode. This generates stray photons that are absorbed in the neighborhood. The second component is electrical, and it involves the diffusion of photo-generated carriers between adjacent devices. The characterization of electrical CTK in 4T-APS can be achieved using TCAD tools. Particularly, the relation between CKT and quantum efficiency (QE) can be explored and linked to the thickness of the epitaxial layer.

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