CMOS Geiger photodiode array with integrated signal processing for imaging of 2D objects using quantum dots. Christopher J. Stapels, William G. Lawrence, Rajan S. Gurjar, Erik B. Johnson, James F. Christian Radiation Monitoring Devices, 44 Hunt St. Watertown, MA 02472 ABSTRACT Geiger-mode photodiodes (GPD) act as binary photon detectors that convert analog light intensity into digital pulses. Fabrication of arrays of GPD in a CMOS environment simplifies the integration of signal-processing electronics to enhance the performance and provide a low-cost detector-on-a-chip platform. Such an instrument facilitates imaging applications with extremely low light and confined volumes. High sensitivity reading of small samples enables two- dimensional imaging of DNA arrays and for tracking single molecules, and observing their dynamic behavior. In this work, we describe the performance of a prototype imaging detector of GPD pixels, with integrated active quenching for use in imaging of 2D objects using fluorescent labels. We demonstrate the integration of on-chip memory and a parallel readout interface for an array of CMOS GPD pixels as progress toward an all-digital detector on a chip. We also describe advances in pixel-level signal processing and solid-state photomultiplier developments. Keywords: GPD, APD, integrated signal processing, fluorescence imaging, SSPM, avalanche diode 1. INTRODUCTION Low light imaging of 2D objects requires sensitive detectors and flexible electronics. Digital output detector systems can greatly facilitate the development of new instruments and allow for dramatic flexibility in experimental design and actual product implementation. The bulky supporting electronics normally associated with photon counting devices can be replaced with a simple PC interface. Some applications include low light scanning of biological samples, scintillation readout, focal plane arrays, single molecule detection and others. The prototype array presented here consists of sixteen Geiger-mode avalanche photodiode pixels (GPD) with integrated active quenching and a sixteen-bit counter at each pixel. The actively quenched GPD essentially digitizes the single photon event at the pixel. Subsequent on-chip digital components record the light intensity on the pixel as a photon count rate. The prototype module includes power supplies, and digital readout circuitry connected to a USB port, which also provides all power for the device. This paper presents a collection of developments in CMOS light sensor technology and investigates some of the initial imaging properties of a prototype detector and supporting electronics. 1.1 Features Several features of the prototype detector make it useful and applicable to multiple applications. First, the ability to detect single photons allows for sensing in extremely low light applications. Single photon counting, especially in pulsed applications can be achieved over a large array of CMOS pixels. The large amplification, on the order of 10 6 , depending on the operating bias, provided by the GPD pixels ensures that the signal to noise presented to the sensing comparator is extremely high, and the logic output from the pixel level sensing circuitry has virtually no readout noise for false signals above the logic threshold. The possibility of lock-in signal processing is provided by an external connection for the counter direction. If a signal with a 50% duty cycle is set to trigger the counter direction, then the average dark counts in each integration period will be added and subtracted from the counter value for alternating integration periods, resulting in subtraction of the average dark counts without the need for additional signal processing. In addition, the illumination signal can be modulated with the counters to remove other sources of background. The ability to print arrays of pixels with independent or multiplexed outputs provides imaging capabilities. In the present design a linear array can be scanned in one dimension to create a 2D image, with a resolution limited by the pixel spacing. Tilting the detector at an angle reduces the spacing between pixels and allows for resolution at the scale of the pixel size, which is 30 microns in this module. Functioning pixels with diameters as small as 10 microns have been fabricated. Infrared Systems and Photoelectronic Technology III, edited by Eustace L. Dereniak, John P. Hartke, Paul D. LeVan, Randolph E. Longshore, Ashok K. Sood, Proc. of SPIE Vol. 7055, 70550S, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.796329 Proc. of SPIE Vol. 7055 70550S-1 2008 SPIE Digital Library -- Subscriber Archive Copy