Abstract
We present the design and application of a 64 × 64 pixel SPAD array to portable colorimetric sensing, and fluorescence and x-ray imaging. The device was fabricated on an unmodified 180 nm CMOS process and is based on a square p+/n active junction SPAD geometry suitable for detecting green fluorescence emission. The stand-alone SPAD shows a photodetection probability greater than 60% at 5 V excess bias, with a dark count rate of less than 4 cps/μm2 and sub-ns timing jitter performance. It has a global shutter with an in-pixel 8-bit counter; four 5-bit decoders and two 64-to-1 multiplexer blocks allow the data to be read-out. The array of sensors was able to detect fluorescence from a fluorescein isothiocyanate (FITC) solution down to a concentration of 900 pM with an SNR of 9.8 dB. A colorimetric assay was performed on top of the sensor array with a limit of quantification of 3.1 μm. X-rays images, using energies ranging from 10 kVp to 100 kVp, of a lead grating mask were acquired without using a scintillation crystal.
Highlights
T HE use of CMOS single photon avalanche diodes (SPADs) in biomedical sensing and imaging has increased as a consequence of their performance in terms of fast acquisition, high sensitivity and portability [1].Typical applications include fluorescence lifetime imaging microscopy (FLIM) [2]–[7], fluorescence [8], point-of-care testing (POCT) [9], Raman spectroscopy [10], positron emission tomography (PET) [11], endoscopic TOF PET [6], x-ray sensing [12], time-resolved spectroscopy [13], 3D vision
In this paper we demonstrate an advance in the use of low-cost legacy CMOS technology and its application in several sensing imaging modalities where a SPAD array is especially well suited
The shallow n-well (S-NW) acts as an electric field enhancer for the active junction [31] whilst the guard ring is formed by implanting a shallow p-well (S-PW) into a deep p-well (D-PW)
Summary
T HE use of CMOS single photon avalanche diodes (SPADs) in biomedical sensing and imaging has increased as a consequence of their performance in terms of fast acquisition, high sensitivity and portability [1].Typical applications include fluorescence lifetime imaging microscopy (FLIM) [2]–[7], fluorescence [8], point-of-care testing (POCT) [9], Raman spectroscopy [10], positron emission tomography (PET) [11], endoscopic TOF PET [6], x-ray sensing [12], time-resolved spectroscopy [13], 3D vision. Very high efficiency CMOS SPAD arrays with extremely complex on chip electronics, resulting in low fill factors have been reported [24]. High DR values, larger than 100 dB, have been achieved using active quenching circuits to prevent saturation at high count rates, but at the expense of fill factor [25] In such devices, microlenses are typically used to improve performance by concentrating light on to the active area, increasing the light capture by up to a factor of 10 [26] enhancing the optical fill factor [27]. In this paper we demonstrate an advance in the use of low-cost legacy CMOS technology and its application in several sensing imaging modalities where a SPAD array is especially well suited.
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