CMOS-based avalanche photodiodes for direct particle detection

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Abstract

Active Pixel Sensors (APSs) in complementary metal-oxide-semiconductor (CMOS) technology are augmenting Charge-Coupled Devices (CCDs) as imaging devices and cameras in some demanding optical imaging applications. Radiation Monitoring Devices are investigating the APS concept for nuclear detection applications and has successfully migrated avalanche photodiode (APD) pixel fabrication to a CMOS environment, creating pixel detectors that can be operated with internal gain as proportional detectors. Amplification of the signal within the diode allows identification of events previously hidden within the readout noise of the electronics. Such devices can be used to read out a scintillation crystal, as in SPECT or PET, and as direct-conversion particle detectors. The charge produced by an ionizing particle in the epitaxial layer is collected by an electric field within the diode in each pixel. The monolithic integration of the readout circuitry with the pixel sensors represents an improved design compared to the current hybrid-detector technology that requires wire or bump bonding. In this work, we investigate designs for CMOS APD detector elements and compare these to typical values for large area devices. We characterize the achievable detector gain and the gain uniformity over the active area. The excess noise in two different pixel structures is compared. The CMOS APD performance is demonstrated by measuring the energy spectra of X-rays from 55Fe.

Introduction

Complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) imaging devices provide a low-cost alternative to charge-coupled devices (CCDs); however, the readout noise in the CMOS devices frequently limits their performance. Fabricating CMOS APS devices with pixels capable of amplifying the signal allows the identification of events previously hidden within the readout noise of the electronics. The internal gain of avalanche photodiode (APD) devices enables the amplification signals at the pixel.

APD pixels have been successfully fabricated in commercially available CMOS processes. The diodes have diameters from 5 μm up to 150 μm. When bias is applied to the device, charge pairs generated in the pixel are accelerated by the electric field and multiplied by impact ionization, increasing the signal amplitude. We have recorded pulse height gains of over 1000 from light-emitting-diode (LED) signals. Fig. 1 shows a cartoon of the multiplication process in the CMOS APD pixel.

When operated below the reverse-bias breakdown voltage, the CMOS APD pixels produce an output signal that is proportional to the input signal while providing internal gain. We refer to this traditional APD operating mode as “proportional mode”. Uses of diodes operated in this mode include direct particle detection, as in high-energy physics experiments. The extremely small size of the pixels allows high precision position determination.

Section snippets

Experimental

Quantum efficiency (QE) as a function of wavelength is measured against a calibrated photodiode (Hanamatsu S1336-8BQ) using incandescent light filtered through a monochromator. We measure the QE profile by scanning a focused 632-nm HeNe laser across the surface of the device. A picoammeter measures the photocurrent as the position of the laser focus is scanned. The proportional-mode gain is measured by illuminating the pixel with a pulsed 470-nm LED. Unity gain is assumed when the diode is

Characterization of CMOS APD pixels

To characterize the proportional-mode performance of the device, we measure the QE for detecting optical photons, at unity gain, as a function of the wavelength. Fig. 2 compares the QE of two different pixel designs, labeled as “Design 4” and “Design 12”.

The p-on-n design, type 12, isolates the avalanche junction from the substrate, resulting in a shallower device that loses QE at long wavelengths (>700 nm). Fig. 3 provides a cross-sectional illustration of these diode designs.

In the design-4

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There are more references available in the full text version of this article.

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